With the increasing energy demands and the limited petroleum reserves, it is highly desirable to produce fuels and chemicals from non-petroleum feedstocks, such as coal, natural gas and biomass. Catalytic conversion of C1 resources (CO, CO2, CH3OH, CH4, etc.) affords various products and attracts increasing attention from both academia and industries. Methane and methanol are important C1 feedstocks in the production of fuels and chemicals. In order to obtain high selectivity for the target product, it is necessary to control the activation of C―H bonds in methane and methanol. However, this remains a great challenge. Although the traditional thermal catalytic conversion of methane and methanol has been developed over decades, there are still some disadvantages associated with the catalytic process, such as harsh reaction conditions, high energy consumption, and low selectivity. Photocatalysis, which is driven by photoenergy, can compensate for the Gibbs free energy. In the photocatalytic reactions, semiconductor photocatalysts absorb photons and generate electrons and holes in their conduction and valence bands, respectively, to accelerate the reaction rate. The position of the conduction band determines the oxidation capacity, and the bandgap determines the light absorption property. Normally, the oxidation capacity of photocatalysts is regulated by choosing semiconductors with a suitable bandgap or anions/cations doping. Fabrication of heterojunction and loading metalsare recognized as effective methods to promote the separation of electron-hole pairs and improve the photocatalytic efficiency. In contrast to thermal catalysis, photocatalysis can be carried out under mild reaction conditions with low energy consumption. Recently, photocatalysis has been considered an attractive route for the efficient conversion of methane and methanol to fuels and chemicals. Partial oxidation of methane, which is necessary to avoid the formation of byproducts, can be achieved by adjusting the wavelength and intensity of the light and the oxidation capacity of the photocatalysts. In addition, light-induced plasmon resonance improves the efficiency of methane conversion by forming an intrinsic high-energy magnetic field that can polarize methane. In methanol conversion, the C―H bond can be selectively activated, instead of the O―H bond, by light irradiation. Therefore, C―C coupling can be realized for the production of various value-added chemicals from methanol. This review summarizes the recent advances in the photocatalytic conversion of methane and methanol including the reactions of reforming, oxidation, and coupling. Perspectives and challenges for further research on the photocatalytic conversion of methane and methanol are also discussed.
Normal alkyl sp3C―H bonds are ubiquitous in compounds such as methane, linear alkanes, and cycloalkanes that are not linked directly to heteroatoms or other functional groups. These unactivated bonds are not broken readily under mild conditions because their bond dissociation energy values are high and acidity values are low. Moreover, in the radical processes at high temperatures, reaction selectivity is not good for an alkane substrate with various alkyl sp3C―H bonds, which is commonly methyl < 1° < 2° < 3°. In the past five decades, C―H activation by transition-metal species to give C-metal bonds under mild conditions was intensively studied; all efforts were undertaken to provide new methods that can be applied in both chemical synthesis and chemical industry. However, the effective transformations of inert C―H bonds, particularly alkyl sp3C―H bonds, without the assistance of directing groups have been rarely investigated. This review focuses on the functionalization of normal alkyl sp3C―H bonds, such as methyl and primary sp3C―H bonds, via electrophilic activation or oxidative addition by using homogenous transition-metal catalysts, which are two main strategies in the study of inert C―H activation. The selectivity on C―H bond is methyl > 1° > 2° > 3° in both the reactions. Neither heterogeneous catalysis nor biocatalysis is mentioned in this review. Some remarkable progress is described on the study of reaction mechanisms and the establishment of novel reactions. For example, several selective oxidations of methane or linear alkanes have been introduced to afford new C―O, C―Cl, or even C―C bonds in the presence of Pt or Pd catalysts. The Shilov chemistry, which combines electrophilic activation of the C―H bond by the transition-metal complex, oxidation of the transition-metal intermediate, and nucleophilic substitution of organometallic species, has been emphasized in these reactions. Other transition-metal catalysts including Rh, Ir, Re, and W have been employed successfully in the carbonylation, borylation, and dehydrogenation of alkanes at moderate temperatures. The reaction pathways normally involve oxidative addition of the C―H bond with the transition-metal complex followed by insertion-elimination, reductive elimination, or β-H elimination. In the cascade reactions consisting of dehydrogenation of alkanes and addition of alkenes, new C―C or C―Si bonds can also be formed at terminal sites of linear alkanes. However, most of the above-mentioned reactions are still under investigation because of limited scope of the substrate, excess loading of the alkane, low efficiency of the catalyst, and high cost of the reaction operation. Breakthroughs in this promising field of alkane functionalization are possible when new concepts and technology are realized and applied.
Na-ion batteries are currently an emerging and low-cost energy storage technology, which have attracted enormous attention and research due to its promising potentiality for large-scale energy storage applications. As the key electrode materials for Na-ion batteries, non-graphite carbonaceous materials have been regarded as the best choice for practical application due to its high sodium storage activity, low-cost and non-toxicity. According to the current research, graphite materials are not suitable to be anode materials of Na-ion batteries for practical application due to its low sodium storage capacity in carbonate electrolytes. Hard carbons have a high capacity of ~300 mAh·g-1 with low sodium storage potential and thus are suitable for practical applications. Soft carbons have a sodium storage capacity about 200 mAh·g-1 with sodium storage potential below 1 V vs. Na+/Na. Soft carbons usually exhibit excellent rate performances and thus are suitable to be used as anode materials for power Na-ion batteries. Reduced graphene oxide (rGO) has a sodium storage capacity of about 220 mAh·g-1 and excellent rate performances. A high sodium storage capacity can be obtained by doping heteroatoms and introducing defect sites in rGO. However, the low material density, high sodium storage potential and large irreversible capacity of rGO will restrict its practical application. Porous carbons have high capacities of 300-450 mAh·g-1 with excellent rate performances because their developed porous structure can provide more defects as the active sites for sodium storage and shorten the diffusion path of Na+ to improve rate performances. Carbon nanowires/fibers have good flexibility due to their unique one-dimensional feature and stable sodium storage reversible capacity with good rate performance. These materials have advantages to be flexible electrodes for sodium-based flexible energy storage devices. By introducing N, S and other heteroatoms, heteroatom-doped carbons have more active sites for sodium storage and thus achieve higher sodium storage capacity. In summary, carbon materials with low graphitization degree are important development directions for anode materials of low cost Na-ion batteries. New carbon materials with unique microstructure and morphology have higher sodium storage capacity and rate capability, so they can be used as high power anode materials for sodium storage. Considering many factors, such as cycle life, energy density, power density and manufacturing cost, of practical application, hard carbon anodes is currently the best choice for practical application of Na-ion batteries. In the future, improving SEI stability, increasing Coulombic efficiency and improving electrical conductivity of hard carbon are urgent problems to be solved for practical application. Herein, the recent progress of carbonaceous anode materials is reviewed. The sodium storage mechanism and characteristics of carbon materials are summarized and discussed. Furthermore, the relationship between micro-structures and electrochemical performances, and remained problems of carbon anodes are discussed. This review will promote the development and understanding of carbon anode materials for sodium storage.
Micelles, a kind of surfactant aggregate formed in water, may be swollen to a generally limited extent upon addition of a liquid hydrophobic compound. Swollen micelles have attracted considerable research attention because they can enhance the solubility of the said hydrophobic compound. The development of swollen micelles is of significant interest in terms of both scientific and industrial applications, such as drug delivery, oil recovery, and soil remediation. While there have been many studies focusing on micellar solubilization, several questions remain unanswered: the capacity to quantitatively solubilize the drug in drug delivery, the interaction between micelles and non-polar oil when microemulsions are not formed, and the differences and similarities between swollen micelles and microemulsions. Comprehensive understanding of and insight into swollen micelles will be helpful to tailor surfactants for industrial applications. Herein, we reviewed the recent progress in the field of swollen micelles in terms of solubilization capacity, solubilization site, micellar morphology, etc. First, the UV spectrophotometry results demonstrate that the solubilization capacity of micelles is related to their molecular structures and surfactant properties. The solubilization is also dependent on the composition and nature of the hydrophobic compounds, the presence of electrolytes, temperature, etc. Second, the solubilization site may be located in the micellar core, the palisade layer of the micelle, the micelle surface, or the hydrophilic shell of the micelle, depending on the property of the solubilized compounds and the morphology of the micelles. In general, the micellar aggregation number increases with increasing oil concentration; high concentration of oil causes the formation of spherical micelles, while high concentration of oil results in ellipsoidal micelles. Furthermore, the micellar size increased gradually with increasing oil concentrations. Finally, the differences and similarities between swollen micelles and microemulsions were clarified. It is believed that microemulsions can be considered as swollen micelles, but there has been some strong evidence that differentiates swollen micelles and microemulsions. Based on our results, we believe that microemulsions can be considered as swollen micelles, but all micellar solutions cannot be swollen to the extent of microemulsions, unless the specific structural requirements and conditions are satisfied. Overall, understanding the properties of swollen micelles and how they transform to microemulsions not only provide theoretical support for practical applications of surfactants, but can also be used to design new surfactants.
Two-dimensional (2D) layered materials have garnered increasing interest in the past few years due to their unique structures and novel properties. These 2D layered materials with atomic thicknesses cover metals, semiconductors, and insulators, including graphene, black phosphorus (BP), transition metal dichalcogenides (TMDs) and hexagonal boron nitride (BN). Their bandgaps are usually tunable by changing the number of layers and the thicknesses. These 2D material are also sensitive to changes in the surrounding environment, e. g. changes in temperature, pressure, and illumination. Particularly, most 2D materials have high absorption coefficients. Owing to their excellent performance in electronics and optoelectronics and their potential for further development, many optoelectronic devices based on 2D materials, such as photodetectors, have been manufactured and widely used. In this paper, the latest progress of photodetectors based on 2D materials has been outlined. We introduce some 2D materials and their preparation methods, and the mechanisms of photodetectors based on 2D materials, i. e. photovoltaic effect, photoconductive effect, photogating effect, photothermoelectric effect and bolometric effect, have been discussed. Next, we summarize the parameters used to evaluate the performance of photodetectors, including photoresponsivity, external quantum efficiency, internal quantum efficiency, photoconductive gain, signal-to-noise ratio, noise-equivalent power, response time, cutoff frequency, linear dynamic range, and specific detectivity. We also report some recent studies on photodetectors based on 2D materials; among the 2D materials used in these studies, graphene, TMDs, and BP are the most widely used. Many methods have been proposed to improve the performances of photodetectors based on 2D materials, such as doping, designing novel structures, changing the dielectric layer, modifying the contact between channel and electrodes, controlling the surface and the interface, etc. Compared to single 2D materials, heterostructures composed of different 2D materials are more promising for use in photodetectors because they combine materials with different properties, which makes it possible to obtain photodetectors with desired and enhanced performances. Thus, we present some van der Waals heterojunctions and their applications in photodetectors. Finally, we provide a brief summary of the full article and an outlook for future development.
Flexible electronic devices have attracted immense attention in recent years. Conventional electronics that are predominantly fabricated with rigid metallic materials demonstrate poor flexibility. Compared to traditional electronic devices, flexible electronic devices with better flexibility can adapt to different working environments. Consequently, they fit perfectly with different systems with minimal rejections. However, such flexible electronic devices need to achieve good extensibility and flexibility without compromising on their electronic properties. Therefore, new challenges and requirements arise while fabricating conductive materials. Manufacturing of flexible metal electrodes for flexible electronic devices include strategies such as reducing the thickness of the electrodes and designing electrodes with unique structures. However, these technologies are complex and expensive. Carbon nanotube (CNT) films exhibit good flexibility, excellent conductivity, good chemical and thermal stability, as well as good optical transparency, making them ideal candidates for flexible electronics. Therefore, the preparation and application of CNT films for the development of next generation flexible electronics have been extensively studied. In this review, we summarize the recent advances in the preparation of CNT films and their application in flexible electronic devices. Initially, the two main kinds of preparation methods for CNT films—dry and wet methods—are introduced. The dry methods for CNT film preparation include the membrane extraction method based on a vertical array of CNTs and the floating catalytic chemical vapor deposition method. Moreover, the wet methods predominantly discussed include vacuum filtration method, impregnation method, electrodeposition method, self-assembly method, and spraying method. Subsequently, the latest research advancements in assembly techniques, their performance and applications in various flexible electronics are discussed. This review primarily introduces the application of CNT films in the fields of flexible sensors, flexible energy devices, flexible transistors, and flexible display screens. The fundamentals of typical flexible sensors, such as strain sensors, pressure sensors, gas sensors, temperature sensors, and humidity sensors are presented. Besides, flexible lithium-ion batteries, flexible nanogenerators, and flexible thermoelectric devices based on CNT films are also investigated. Moreover, other flexible electronic devices, such as flexible transparent conductive film, flexible transistor, and flexible photodetector, based on CNT films are briefly described. Finally, advanced flexible electronics based on CNT films are summarized. The challenges and future prospects of these films are also discussed.
Since the rediscovery of black phosphorus as a fascinating two-dimensional material, other two-dimensional materials comprising group VA elements have attracted tremendous interest, such as antimonene. Since 2015, besides intensive research efforts on the atomic structures, electronic properties and synthesis methods of antimonene, scientists have conducted applied researches on semiconductor and nonlinear optical devices, molecular adsorption and thermoelectric applications based on antimonene. In addition, antimonene quantum dots (SbQDs) as derivatives of antimonene, have also been studied recently, and their potential applications in photothermal therapy have been reported. To further explore the unique properties and potential applicationsof SbQDs, it is important tosynthesize large amounts of high-quality SbQDs. In this work, antimonene samples were prepared by sonication-assisted liquid exfoliation method. Antimony powders (200 mg) were dispersed in 200 mL water, C2H5OH and 1-methyl-2-pyrrolidone (NMP) solvents separately and sonicated for 10 h at a power of 180 W. Thereafter, the suspensions were centrifuged at 6000 r∙min-1 for 20 min, and the supernatant containing antimonene samples were decanted and characterized. The dispersion concentration of antimonene samples in the three solvents (water, C2H5OH and NMP) were measured as 0.57, 1.04, and 4.27 µg∙mL-1, respectively. However, the antimonene concentrations in water, C2H5OH and NMP dropped by 73.7%, 30.8% and 10.5%, respectively, after standing for 96 h. Thus, antimonene dispersed in NMP demonstrated the highest concentration and best stability, which indicates that NMP is more suitable for antimonene exfoliation. Furthermore, transmission electron microscopy (TEM) studies revealed that only the samples prepared in NMP were morphologically quantum dots, while antimonene samples obtained in the other two solvents were mainly nanosheets. The obtained SbQDs in NMP had a lateral size of approximately 3.0 nm. High-resolution transmission electron microscope (HRTEM) also confirmed the good crystal quality of theobtained SbQDs. In addition, we measured the turbidities of antimonene dispersed in those three solvents at various concentrations. As theoretically predicted, the turbidity of antimonne dispersions linearly depends on the concentraion; thus, the antimonene concentrations can be calculated by measuring the turbidity through an optical method. Thus, this study provides a high-throughput, nondestructive method for determining antimonene dispersion concentration, which will faciliate further research in this area.
With the rapid development of science and technology, various nanomaterials have continually emerged to meet human needs. As a newly emerging class of nanomaterials, two-dimensional (2D) materials have received wide attention recently in energy storage, catalysis, sensing and biomedicine due to their unique features such as good mechanical property, high specific surface area, excellent thermal and electrical conductivity. Biomacromolecules are the special organic molecules with various biological activities which exist extensively in every aspect of human life. When 2D materials meet biomacromolecules to display their own unique advantages, more opportunities and challenges have arisen for the exploitation and fabrication of novel nanomaterials with unique electrical, mechanical, biological properties and specific functions. In recent years, extensive research has been carried out with outstanding achievement thus the combination of 2D materials and biomacromolecules becomes a new hotspot. There were generally two binding interactions between 2D materials and biomacromolecules, namely non-covalent binding (electrostatic interaction, hydrophobic effect, π–π stacking, van der Waals interaction) and covalent binding (special chemical reactions between the functional groups of 2D materials and biomacromolecules). In addition, due to the excellent photothermal conversion performance, 2D materials could exhibit a non-contact interaction to biomacromolecules through the photo-thermal effect which has greatly broadened their applications. Up to now, numerous studies have clearly revealed the binding and effect mechanism and the research will be more focused on expanding the scope and application. Currently, the combination of 2D materials and biomacromolecules has widely involved in many cutting-edge applications such as flexible device, biosensor, smart skin, drug delivery, antibacterial, disease therapy and so on. Although a lot of progress has been made, several highlight open questions still need to be urgently addressed, such as the production cost of 2D materials, biological activity of biomacromolecules, stability and biocompatibility of 2D/biomacromolecule nanomaterials. This review summarizes the interactions between some typical 2D materials (i.e. graphene, graphene oxide, nitrogen-doped graphene, molybdenum disulfide, phosphorene, silylene and germanene) and biomacromolecules (i.e. silk protein, lysozyme, bovine serum albumin, bovine hemoglobin, ovalbumin, villin, bovine fibrinogen, DNA/RNA, glucose oxidase and chitosan) and focuses on the recent progress of some typical applications (i.e. engineering application, disease therapy and antibacterial). The non-covalent and covalent bindings of 2D materials and biomacromolecules are discussed in detail, and the applications of the combination of 2D materials and biomacromolecules in engineering and bioscience have been reviewed. Finally, the challenges for the future development of 2D materials and biomacromolecules are also briefly proposed.
Supercapacitors have been widely used in various fields because of their high power density, long cycle life, and cost-effectiveness. Plant-based porous carbon continues to be the most suitable alternative for manufacturing the commercial electrode materials of supercapacitors because of its good electrochemical performance, simple preparation process, high availability, and low cost. Although plant-based porous carbon prepared using physical activation has been widely used in commercial supercapacitors, its performance is severely restricted because of its low value of specific surface area and highly microporous structure. With a view to achieving high values of specific gravimetric/volumetric capacitances and outstanding rate performance in supercapacitors, this review summarizes the recently developed methods for preparing plant-based ultrahigh specific surface area porous carbon materials, mesoporous carbon materials, hierarchical porous carbon materials, and nitrogen-doped porous carbon materials. The factors affecting the electrochemical performance of plant-based porous carbon are also discussed. We also summarize some novel strategies to improve the volumetric electrochemical performance of plant-based porous carbon materials, such as preparing dense and porous carbon materials, performing heteroatom doping, and combining the carbon with pseudocapacitive materials (conductive polymers or metal oxides). Finally, the challenges and perspectives of using plant-based porous carbon in supercapacitors are also proposed. In brief, when used as the electrode material for supercapacitors, the ultrahigh surface area porous carbon prepared by KOH activation shows high value of specific capacitance at low current densities. However, the tortuous and deep micropores in the plant-based porous carbon result in its sluggish ion-transport kinetics and high value of equivalent series resistance, which, in turn, result in poor rate performance. To improve the rate performance, tremendous efforts have been made to introduce mesopores in the carbon as ion-transport channels. However, this strategy usually involves the coalescence of a large number of micropores, resulting in the reduced surface area as well as energy storage ability of the carbon. Hence, many researchers have utilized the inherent porous structure and inorganic templates of plants to prepare hierarchical porous carbon both with high specific surface area and high mesopore volume for use in devices with high capacitance and power. In addition to altering the surface area and pore structure of the carbon, doping with nitrogen is another promising approach to enhance the capacitance and electronic conductivity of the plant-based porous carbon. Surface nitrogen can be introduced by the direct carbonization/activation of nitrogen-rich plant precursors or by the reaction of the carbon with nitrogen-containing reagents. Porous carbon with large specific area and with developed mesoporous structure may exhibit superior gravimetric capacitance but inferior volumetric capacitance because of the trade-off between its well-developed microporous structure and packing density. To improve the volume performance, some methods, such as preparing dense and porous carbon with reasonably porous structure, using heteroatom-doped carbon, and incorporating the carbon with pseudocapacitive materials, have been developed. Although the electrochemical performance of plant-based porous carbon has been significantly improved using the aforementioned methods, yet issues such as the lack of green methods and low-cost activation methods to prepare large surface area porous carbon, the design and controlled modulation of carbon micro-structures, the influence of heteroatom doping on pseudocapacitance, and weak interaction between pseudocapacitive components and plant-based porous carbon still need to be resolved. We hope that this review may provide the necessary background and ideas to develop more effective preparation methods for high-performance plant-based porous carbon.
Molecular electronics has been the subject of increasing interest since 1974. Although it describes the utilization of single molecules as active components of electrical devices, molecular electronics remains a fundamental subject to date. Considering that the length of a single molecule is typically several nanometers, the electrical characterization of a probe molecule is a significant experimental challenge. A metal/molecule/metal junction can bridge the gap between nanometer-sized molecules and the macroscopic measuring circuit and is, thus, generally considered as the most common prototype in molecular electronics. For the fabrication and characterization of single-molecule junctions, break junction methods, which include the mechanically controllable break junction (MCBJ) technique and the scanning tunneling microscopy-break junction (STM-BJ) technique, were proposed at the turn of the century and have been developed rapidly in recent years. These methods are widely employed in the experimental study of charge transport through single-molecule junctions and provide a platform to investigate the physical and chemical processes at the single-molecule level. In this review, we mainly focus on MCBJ and STM-BJ techniques applicable for single-molecule conductance measurement and highlight the progress of these techniques in the context of identification and modulation of chemical reactions and evaluation of their reaction kinetics at the single-molecule level. We begin by presenting the operation principles of MCBJ and STM-BJ and stating their brief comparison. Subsequently, we summarize the recent advances in modulating single-molecule chemical reactions. In this regard, we introduce several examples that involve changing the environmental solution, applying an external electrical field, and resorting to electrochemical gating. Next, we overview the application of the break junction techniques in the investigation of reaction kinetics at the single-molecule level. In this section, we also present a brief introduction to studies on single-molecule reaction kinetics using graphene-based nanogaps, wherein conventional metallic electrodes were replaced by graphene electrodes. Furthermore, we discuss the combination of break junction techniques and surface-enhanced Raman spectroscopy for detecting single-molecule reactions occurring at nanometer-scale separation. We discuss the historical development of this combined method and present the latest advancement explaining the origin of the low conductance of 1, 4-benzenedithiol, which is a topic of significant concern in single-molecule electronics. Finally, we discuss some future issues in molecular electronics, including the expansion from simple molecules to complex molecular systems and the introduction of multi-physical fields into single-molecule junctions. Moreover, we provide a list of critical characterization tools in molecular electronics and discuss their potential applications.
Owing to their high power density, excellent rate performance, and good cycle performance, supercapacitors are widely used for energy storage applications. Two-dimensional (2D) layered materials are structural compounds having a layered host structure with a layer thickness of nanometers and a lateral dimension of several micrometers. Their layer spacing can be controlled by changing the interaction between the layers. 2D layered materials can be delaminated into nanosheets. Exfoliated nanosheet materials provide a new strategy for improving the performance of supercapacitors. Unlike bulk layered materials, the exfoliated nanosheets not only provide a unique nano-scale reaction space for electrochemical reactions, but also offer the possibility for improving the specific capacitance and storage rate of the supercapacitor. However, in 2D layered materials, the ion or electron transport in the vertical direction is obstructed, despite their fast ion and electron transport in the horizontal direction. As a result, the occurrence of electrode reactions, and hence the realization of rapid storage become difficult. It is detrimental to the power and energy densities and rapid energy storage of supercapacitors. Therefore, it is imperative to develop novel electrode materials in order to fabricate supercapacitors showinghigh energy densities at high power densities.
Porous 2D layered electrode materials offer two major advantages. First, the porous structure can alleviate the problems caused by the stacking of nanosheets during the assembly process. Second, the porous structure can effectively promote the electrolyte penetration of the electrode, which alleviates the volume changes of the electrode material during the charging-discharging process and releases the structural strain. Hence, such materials facilitate ion or electron transport, thus increasing the specific capacitance of the supercapacitor. Over the past few years, 2D nanosheet holeization has evolved as a promising approach to improve the energy density of supercapacitors at high power densities. Various porous 2D layered supercapacitor electrode materials have been developed. This paper reviews the exfoliation of 2D layered materials, nanosheet holeization strategy, and the application of assembled porous layered electrode materials in supercapacitors. In this paper, we have reviewed the exfoliation of 2D layered materials with different electric properties and the performance of 2D nanosheets. Different methods used for the preparation of holey 2D nanosheets have also been discussed. We prepared holey MnO2 nanosheets and reduced graphite oxide via redox holeization mechanism, and 2D porous nanomaterials were also prepared by using suitable templates such as hard or self-sustaining templates. These holey 2D nanosheets were used to prepare porous 2D layered electrode materials such as holey graphene/manganese dioxide composite fibers and holey graphene/polypyrrole hybrid aerogels. The capacitance of these electrode materials was investigated systematically. Finally, the prospects for the development of porous 2D layered electrode materials such as the optimization of theirrate performance, flexibility, and energy density were discussed. Novel holeization methods should be developed in order to prepare metal oxide nanosheets with controllable hole sizes. In addition, other 2D materials such as MXene should be explored.
Two-dimensional (2D) materials, led by graphene, have emerged as nano-building blocks to develop high-performance membranes. The atom-level thickness of nanosheets makes a membrane as thin as possible, thereby minimizing the transport resistance and maximizing the permeation flux. Meanwhile, the sieving channels can be precisely manipulated within sub-nanometer size for molecular separation, such as gas separation. For instance, graphene oxide (GO) channels with an interlayer height of about 0.4 nm assembled by external forces exhibited excellent H2/CO2 sieving performance compared to commercial membranes. Cross-linking was also employed to fabricate ultrathin (< 20 nm) GO-facilitated transport membranes for efficient CO2 capture. A borate-crosslinked membrane exhibited a high CO2 permeance of 650 GPU (gas permeation unit), and a CO2/CH4 selectivity of 75, which is currently the best performance reported for GO-based composite membranes. The CO2-facilitated transport membrane with piperazine as the carrier also exhibited excellent separation performance under simulated flue gas conditions with CO2 permeance of 1020 GPU and CO2/N2 selectivity as high as 680. In addition, metal-organic frameworks (MOFs) with layered structures, if successfully exfoliated, can serve as diverse sources for MOF nanosheets that can be fabricated into high-performance membranes. It is challenging to maintain the structural and morphological integrity of nanosheets. Poly[Zn2(benzimidazole)4] (Zn2(bim)4) was firstly exfoliated into 1-nm-thick nanosheets and assembled into ultrathin membranes possessing both high permeance and excellent molecular sieving properties for H2/CO2 separation. Interestingly, reversed thermo-switchable molecular sieving was also demonstrated in membranes composed of 2D MOF nanosheets. Besides, researchers employed layered double hydroxides (LDHs) to prepare molecular-sieving membranes via in situ growth, and the as-prepared membranes showed a remarkable selectivity of ~80 for H2-CH4 mixture. They concluded that the amount of CO2 in the precursor solution contributed to LDH membranes with various preferred orientations and thicknesses. Apart from these 2D materials, MXenes also show great potential in selective gas permeation. Lamellar stacked MXene membranes with aligned and regular sub-nanometer channels exhibited excellent gas separation performance. Moreover, our ultrathin (20 nm) MXene nanofilms showed outstanding molecular sieving property for the preferential transport of H2, with H2 permeance as high as 1584 GPU and H2/CO2 selectivity of 27. The originally H2-selective MXene membranes could be transformed into membranes selectively permeating CO2 by chemical tuning of the MXene nanochannels. This paper briefly reviews the latest groundbreaking studies in 2D-material membranes for gas separation, with a focus on sub-nanometer 2D channels, exfoliation of 2D nanosheets with structural integrity, and tunable gas transport property. Challenges, in terms of the mass production of 2D nanosheets, scale-up of lab-level membranes and a thorough understanding of the transport mechanism, and the potential of 2D-material membranes for wide implementation are briefly discussed.
With the continuous miniaturization and integration of electronic and optoelectronic nanodevices, Moore's Law faces huge challenges from the demands of devices with multifunctional and high-performance characteristics. With several recent reports of the successful synthesis of nanomaterials such as nanoparticles, quantum dots, nanowires, and two-dimensional layered materials, the utilization of such materials for the fabrication of electronic and optoelectronic nanodevices has demonstrated potential for realizing multifunctional and high-performance nanodevices in the future. In particular, owing to their excellent electrical, thermal, mechanical, and optical properties, atomically two-dimensional layered materials have emerged as the most promising materials for nanodevices to solve the bottleneck problems of traditional silicon-based devices. Two-dimensional semiconductor materials have been widely applied in many aspects of functional modules, including pn junctions, field effect transistors, rectifiers, photodetectors, and even solar cells. To provide a strong foundation for the development of high-performance and multifunctional nanodevices in the future, this review summarizes the recent advances in electronic and optoelectronic nanodevices based on novel two-dimensional semiconductor materials. We begin the review with a brief introduction of existing two-dimensional materials, including graphene, transition-metal dichalcogenide materials, black phosphorus, hexagonal boron nitride, and van der Waals heterostructures. The atom structure features, electronic and optical properties, and major applications in devices are discussed. The semiconductor materials are suitable for device channels, while graphene and hexagonal boron nitride can be used as electrodes, encapsulating materials, and components of van der Waals heterostructures for channel of field effect transistors. Next, we mainly discuss the advances in electronic and optoelectronic nanodevices based on transition-metal dichalcogenide materials, black phosphorus, and van der Waals heterostructures. In the context of electronic nanodevices, we introduce field effect transistors and other important functional devices, such as sensors, memristors, and integrated circuits. The mobility, on-off ratio, rectification ratio, and other properties of electronic devices are mentioned. In addition, we describe the potential applications of optoelectronic nanodevices for photodetectors, lasers, light-emitting diodes, photovoltaic devices, and so on. The metrics of devices performance such as responsivity, response time, and spectrum response range are compared. Finally, we summarize and compare the advantages and disadvantages of nanodevices based on different materials. Manufacturing comprehensive and high-performance nanodevices will be a promising direction in the future. In addition, the methods for improving the performance of devices are classified. This review will serve as an important reference for the development of future multifunctional and high-performance nanodevices.
Carbon materials can offer various micro- and nanostructures as well as bulk and surface functionalities; hence, they remain the most popular for manufacturing supercapacitors. This article critically reviews recent developments in the preparation of carbon materials from new precursors for supercapacitors. Typical examples are activated carbon (AC) and graphene, which can be prepared from various conventional and new precursors such as biomass, polymers, graphite oxide, CH4, and even CO2 via innovative processes to achieve low-cost and/or high specific capacitance. Specifically, when producing AC from natural biomasses or synthetic polymers, either new, spent, or waste, popular activation agents, such as KOH and ZnCl2, are often used to process the ACs derived from these new precursors while the respective activation mechanisms always attract interest. The traditional two-step calcination process at high temperatures is widely employed to achieve high performance, with or without retaining the morphology of the precursors. The three-step calcination, including a post-vacuum treatment, is also the preferred choice in many cases, but it can increase the cost per capacity (kWh∙g−1). More recently, one-step molecular activation promises a better and more economical approach to the commercial application of AC, although further increase of the yield is necessary. In addition to activation, graphitization, N doping, and template control can further improve ACs in terms of the charging and discharging rates, or pseudocapacitance, or both. Considerations are also given to material structure design, and carbon regeneration during activation. Metal-organic frameworks, which were initially used as templates, have been found to be good direct carbon precursors. Various graphene structures, including powders, films, aerogels, foams, and fibers, can be produced from graphite oxide, CO2, and CH4. Similar to AC, graphene can possess micropores by activation. Self-propagating high-temperature synthesis and molten salt processing are newly-reported methods for fabrication of mesoporous graphene. Macroporous graphene hydrogels can be produced by hydrothermal treatment of graphite oxide suspension, which can also be transferred into films. Hierarchically porous structures can be achieved by H2O2 etching or ZnCl2 activation of the macroporous graphene precursor. Sponges as templates combined with KOH activation are applied to create both micro- and macropores in graphene foams. Graphene can grow on fibers and textiles by electrodeposition, dip-coating, or filtration, which can be woven into clothes with a large area or thick loading, illuminating the potential application in flexible and wearable supercapacitors. The key obstacles in AC and graphene production are high cost, low yield, low packing density, and low working potential range. Most Carbon materials derived from new precursors work very well with aqueous electrolytes. Charge storage occurs not only in the electric double layer (i.e., the "carbon | electrolyte" interface), but also via redox activity in association with the bulk and surface functionalities, and the resulting partial delocalization of valence electrons. The analysis of the capacitive electrode has shown a design defect that prevents the working voltage of a symmetrical supercapacitor from reaching the full potential window of the carbon material. This defect can be avoided in AC-based supercapacitors with unequal electrode capacitances, leading to higher cell voltages and hence higher specific energy than their symmetrical counterparts. There are also emerging ways to raise the energy capacity of AC supercapacitors, such as the use of redox electrolytes to enable the Nernstian charge storage mechanism, and of the three dimensional printing method for a desirable electrode structure. All these developments are promising carbon materials from various precursors of new and waste sources for a more affordable and sustainable supercapacitor technology.
The world is currently facing a series of energy-related problems and challenges. In response, scientists are committed to seeking green high-performance energy storage devices to meet the demands of long-term, sustainable, and innovative development in the future. As a new type of green energy storage device, the supercapacitor has the advantages of high power density, high theoretical specific capacitance, fast charge and discharge speed, long cycle life, high safety, environmental friendliness, and economy to help people cope with the energy crisis. In addition, energy storage devices including Li-ion batteries and supercapacitors are being transformed from heavy, rigid, and bulky devices into light, flexible, and small units to fulfill the needs of the next generation. Among these energy storage systems, the electrode material is an important factor affecting the performance of supercapacitors. In recent years, supercapacitors based on manganese dioxide have been widely studied owing to their high theoretical specific capacitance, good chemical stability, and environmental friendliness. At the same time, a variety of two-dimensional materials are also used as supercapacitor electrode materials after graphene. Two-dimensional structural features play an important role in improving the energy density of electric double-layer capacitors and improving the pseudocapacitance of capacitors. To achieve high specific capacitance and high rate of performance, combining manganese dioxide with two-dimensional materials is a promising option. In this paper, we systematically introduce the application of composites that combine two-dimensional materials represented by graphene and manganese dioxide in supercapacitors, and considers the electrochemical properties of these composites. However, there is still a long way to go in order to fabricate a suitable hierarchical structure consisting of two-dimensional materials and manganese dioxide. For example, a suitable two-dimensional material must be chosen and combined with manganese dioxide to form composites that possess excellent electrochemical properties. In addition, the fabrication methods for these composites are a principal factor that affects their performance. Thus, there are reasons for us to strongly believe that if these key issues are resolved, the properties of these composites consisting of manganese dioxide and two-dimensional materials will make great progress. Overall, this paper only points out some general directions for these kinds of composites in the future, such as principles for choosing the two-dimensional materials to combine with manganese dioxide, and the composite methods which have been reported previously. We are pleased that other researchers are being inspired by our work, and we are looking forward to seeing better studies in this field.
Graphene is one of the most promising materials in nanotechnology and has attracted worldwide attention and research interest owing to its high electrical conductivity, good thermal stability, and excellent mechanical strength. Perfect graphene samples exhibit outstanding electrical and mechanical properties. However, point defects are commonly observed during fabrication which deteriorate the performance of graphene based-devices. The transport properties of graphene with point defects essentially depend on the imperfection of the hexagonal carbon atom network and the scattering of carriers by localized states. Furthermore, an in-depth understanding of the effect of specific point defects on the electronic and transport properties of graphene is crucial for specific applications. In this work, we employed density functional theory calculations and the non-equilibrium Green's function method to systematically elucidate the effects of various point defects on the electrical transport properties of graphene, including Stone-Waals and inverse Stone-Waals defects; and single and double vacancies. The electrical conductance highly depends on the type and concentration of point defects in graphene. Low concentrations of Stone-Waals, inverse Stone-Waals, and single-vacancy defects do not noticeably degrade electron transport. In comparison, DV585 induces a moderate reduction of 25%–34%, and DV55577 and DV5555-6-7777 induce significant suppression of 51%–62% in graphene. As the defect concentration increases, the electrical conductance reduces by a factor of 2–3 compared to the case of graphene monolayers with a low concentration of point defects. These distinct electrical transport behaviors are attributed to the variation of the graphene band structure; the point defects induce localized states near the Fermi level and result in energy splitting at the Dirac point due to the breaking of the intrinsic symmetry of the graphene honeycomb lattice. Double vacancies with larger defect concentrations exhibit more flat bands near the Fermi energy and more localized states in the defective region, resulting in the presence of resonant peaks close to the Fermi energy in the local density of states. This may cause resonant scattering of the carriers and a corresponding reduction of the conductance of graphene. Moreover, the partial charge densities for double vacancies and point defects with larger concentrations exhibit enhanced localization in the defective region that hinder the charge carriers. The electrical conductance shows an exponential decay as the defect concentration and energy splitting increase. These theoretical results provide important insights into the electrical transport properties of realistic graphene monolayers and will assist in the fabrication of high-performance graphene-based devices.
The increased demand for high-performance supercapacitors has fueled the development of electrode materials. As an important part of supercapacitors, the electrochemical performance of the supercapacitor is directly affected by the specific surface area, conductivity, electrochemical activity, and stability of electrode materials. In the traditional manufacturing method, a binder must be added to powdered electrode materials to enhance their combination with the current collector, which could lead to morphology damage, pore blockage, and reduced conductivity of active materials that will adversely affect their electrochemical behavior. Thus, research on binder-free electrode materials has attracted significant interest. Recently, electrospun nanofibers have been widely used as supercapacitor electrode materials because of their advantages such as large specific surface area, high porosity, and easy preparation. The attainable continuity and flexibility endow electrospun nanofiber membranes outstanding performance among large numbers of binder-free materials. This review considers recent studies on electrospun nanofiber-based binder-free electrode materials for supercapacitors, including carbon nanofibers, carbon-based composite nanofibers, conductive polymer-based composite nanofibers, and metal oxide nanofibers. These studies demonstrate that pore structure construction, activation treatment, and nitrogen doping can improve the specific surface area, electrochemical activity, wettability, and graphitization degree of carbon nanofibers to enhance their electrochemical properties. Moreover, combining carbon nanofibers with metal oxides, metal sulfides, metal carbides, and conductive polymers by methods such as blending, chemical deposition, electrochemical deposition, etc., can improve their capacitance, rate performance, and cycling stabilities, which complement the advantages of different materials and proves that the performance of multicomponent materials is better than that of single-component materials. In particular, conductive polymers based on composite nanofibers and metal oxide nanofibers can be used as binder-free materials by electrospinning technology, but their dependence on other substances as well as fragile fiber membrane limit their widespread application. Therefore, in order to ensure the continuity or flexibility of fiber membranes, carbon-based composite nanofibers with multicomponent and hierarchical structure could potentially be used/constructed as binder-free electrode materials. Combinations with new types of electrode materials such as metal-organic frameworks (MOF), covalent organic frameworks (COF), MXenes, metal nitride, metal phosphide, etc., and the preparation of materials with novel structures have also been attempted. In order to realize the practical application of eletrospun nanofiber-based binder-free electrode materials, more attention should be given to improving their mechanical properties, production efficiency, and research on the application of flexible devices. We hope that this review can broaden ideas for improving the development and application of electrospun nanofiber-based binder-free electrode materials for high-performance supercapacitors.
Supercapacitors have attracted considerable attention as new-generation energy storage devices because of their high charge-discharge rate, ultralong lifetime, and high power density. However, the performance of supercapacitors is severely restricted by either the low intrinsic capacitance of porous carbons or the poor conductivity and sluggish electrochemical kinetics of pseudocapacitive components. Therefore, high-performance electrode materials integrated with high gravimetric and volumetric capacitances, high rate capability, and superb cycling stability are urgently needed. As emerging carbon nanomaterials, carbon-/graphene quantum dots (CDs/GQDs) have uniquely small particle sizes, abundant edge sites, and various functional groups, thus endowing them with great potential for developing high-performance electrode materials for supercapacitors. With the purpose of identifying the application advantages and critical problems of CDs/GQDs for supercapacitor electrodes, this review summarizes the development of CDs/GQDs, quantum dots (QDs)/conductive carbon, QDs/metal oxides, QDs/conductive polymer composites, and QD-derived carbon materials in recent years. In each section of this paper, we introduce the typical and updated studies from corresponding fields in terms of novel preparation routes, the crucial roles of CDs/GQDs in composite materials, and the electrochemical performance of electrode materials and assembled devices. Finally, the advantages and limitations of CD/GQD electrode materials are described, and the future development of QD-based materials is discussed. In general, the previous studies have shown that when directly used as electrode materials for micro-supercapacitors, CDs/GQDs performed at an ultrahigh charge-discharge rate of up to 1000 V∙s−1. However, because of the discontinuous conductive network and good dispersibility in electrolytes, they are not suitable for use in common devices because of poor cycle stability. One of the most promising means for fully realizing active ion storage sites for portable devices is to strongly anchor CDs/GQDs onto conductive carbon scaffolds such as activated carbon, graphene nanosheets, and carbon nanofibers. QDs simultaneously improve the capacitive and rate performance owing to the active sites and improved surface wettability of the composite materials. To further improve the capacitance and cycle stability of electrode materials, different types of QDs/pseudocapacitive material composites (metal oxides or conductive polymers) have been developed. QDs have been shown to increase entire conductivity, accelerate ion transport, and depress the volume expansion of metal oxides and conductive polymers. Benefiting from their small-sized structure and outstanding reactivity, CDs/GQDs can also be used as emerging precursors for constructing advanced carbon electrode materials. Several forward-looking studies have shown that QDs can be converted to carbon nanosheets, heteroatom-doped carbon, and dense and porous carbon blocks with high gravimetric/volumetric capacitances and remarkable rate performance for fast and compact energy storage. Although related fields have rapidly developed in recent years, several critical problems including those related to green and effective methods to prepare CDs/GQDs, construction of entire conductive networks without active sites being sacrificed, the synergistic effect, and the underlying mechanism of QDs in composite materials still must be solved. We hope that this review can provide inspiration and references for further investigation in this promising field.
With the ongoing depletion of fossil fuels, the exploration of sustainable energy resources and advanced energy technologies is necessary and the development of clean and sustainable energy storage devices has become an important topic worldwide. In this regard, rechargeable batteries and supercapacitors (SCs) are currently considered to be promising electrochemical energy storage systems for widespread applications in electronic devices, electric vehicles, and smart-grid energy storage stations. Batteries typically exhibit high energy densities but are limited by their low power density and relatively poor cycling performance. In contrast, SCs exhibit high power density, stable cyclability, and good safety, but the energy densities of SCs are generally inferior to those of batteries, which hinders their widespread application. A reliable approach to addressing this issue is to fabricate hybrid supercapacitors (HSCs) composed of battery-type and capacitive electrodes. This device configuration enables the direct integration of the high energy densities of batteries and high power densities of SCs, making HSCs a promising class of energy storage devices. However, the mismatch of capacity and rate performance between the battery-type and capacitive electrodes hinders the widespread applications of HSCs. A key challenge for the development of high-performance HSCs is to optimize the balance between both electrodes. Recently, tremendous efforts have been focused on the search for suitable electrodes and considerable progress has been achieved. Nevertheless, in traditional electrodes, binders are commonly used to combine individual active materials with conductive additives. Unfortunately, these binders are generally electrochemically inactive and insulating, reducing the overall specific capacity/capacitance and deteriorating the charge/mass transport. Recently, binder-free nanoarray electrodes have provided a promising opportunity for designing effective HSCs owing to the merits of their direct electron transport pathway, short ion diffusion length, and ordered-structure-enabled abundant reaction sites. This review briefly addresses the energy storage mechanism of HSCs and the advantages of array electrodes, and subsequently reviews the recent advances in emerging HSCs developed by our group. The performance-electrode structure relationship is discussed from the perspective of devices featuring different electrolytes, including organic, aqueous neutral and aqueous alkaline electrolytes. Moreover, some solutions are put forward to solve the existing issues of HSCs, and the potential applications of array electrode-based HSCs in flexible/wearable electronics are envisioned. Finally, the challenges and future development trends of HSCs are proposed.
The strategy of transition-metal-catalyzed C―H activation has been greatly developed in recent years. Direct transformations of inert C―H bonds undoubtedly provide powerful ways to construct various C―C and C―X (X = heteroatom) bonds, with enhanced atom- and step-economy. Impressive efforts have been devoted to this research all along. However, concerns about reactivity and selectivity remain to be tackled, due to their strong dependence on directing groups and acidic reactive sites. In this regard, more effective catalytic systems are of great importance and therefore in high demand. Bimetallic C―H activation, by virtue of the cooperative effect, has emerged as a promising solution to this issue. The intriguing interactions between two metals with substrates afford exceptional reaction efficiency and selectivity. Intensive interest in both experimental and computational studies has been recently triggered. In this minireview, diverse bimetallic catalytic reactions are summarized into three categories according to the initiator in the C―H activation step, namely, bimetallic catalyses based on palladium, nickel, and other metals. Experimental results as well as density functional theory (DFT) calculations are invoked in the plausible mechanistic considerations. In the first part, collaborative modes based on palladium are described, in which magnesium, chromium, cobalt, and silver are successfully engaged as accessory partners. Most of them stabilize the C―H activation transition states by decreasing the energy, thus facilitating the cleavage of C―H bonds. Notably, some reactions previously reported as examples of monomeric palladium catalysis are now reinvestigated as bimetallic scenarios, in light of computational discussions. In the second part, reactions based on the synergy of nickel, and zinc or aluminum, are generalized, in which zinc or aluminum acts as a Lewis acid to increase the acidity of C―H bonds. It has been shown that the choice of different kinds of Lewis acids and ligands has a great influence on the reaction chemo-, regio-, and stereoselectivity. Gratefully, even enantioselective transformations can be achieved using the cooperation of nickel and aluminum. Moreover, a key reaction intermediate in the bimetallic C―H activation by nickel and aluminum has been isolated, providing guidance for this bimetallic catalytic system in further mechanistic studies and applications. In the last part, synergetic catalysis based on various other metals is presented. Bimetallic regimes of ruthenium/copper, rhodium/bismuth, iridium/aluminum, manganese/zinc, and zirconium/aluminum have been elegantly applied to C―H activation reactions. Multifarious action modes are proposed on account of the mechanistic research.
Controllable synthesis of MoS2 with desired number of layers via chemical vapor deposition (CVD) remains challenging. Hence, it is highly desirable to develop a theoretical model that can be used to predict the single- and multilayer growth of MoS2 quantitatively, and provide guidelines for experimental fabrication. Herein we have established a kinetic Monte Carlo (kMC) model to predict the CVD growth of mono- and bilayer MoS2. First, we proposed that the growth rates of layer 1 and layer 2 were governed by the distribution of the adatom concentration, and the growth kinetics of compact triangular MoS2 followed the kink nucleation-propagation mechanism. The adatom concentration was formulated in terms of adatom flux, effective lifetime of adatoms, growth temperature, binding energies, edge energies, and nucleation criterion. The kink nucleation and propagation were determined by energy barriers of the adatom attachments to the zigzag and armchair edges. We then employed an analytic thermodynamic criterion to extract these parameters. Using the calibrated model, we found that the growth rate of layer 2 strongly depended on the size of layer 1 and decreased monotonically with increasing size of layer 1, and might even become prohibited at the maximum size of layer 1. Furthermore, we analyzed the size and morphology evolutions of bilayer MoS2 at different growth temperatures and adatom fluxes. Throughout the growth processes of bilayer MoS2, the morphologies of layers 1 and 2 maintained triangular shapes with compact edges, consistent with the kink nucleation-propagation growth mechanism. Our simulations revealed that the growth of bilayer MoS2 was promoted by increasing the growth temperature or decreasing the adatom flux, which corroborated the experimental observations. The increase in growth temperature led to reduced adatom concentration at the edge of layer 2 in accordance with the adatom concentration far from the edge of layer 2, resulting in a consistent difference in the adatom concentration to promote the growth of bilayer MoS2. Similarly, the decrease in adatom flux lowered the difference between the adatom concentrations far from the edge and at the edge of layer 1, decelerating the growth of layer 1. The decelerated growth of layer 1 reduced the difference between the adatom concentrations far from the edge and at the edge of layer 2 to zero, permitting the growth of bilayer MoS2. To guide the experimental synthesis, we constructed a phase diagram to delineate the permitted or prohibited growth of bilayer MoS2 at different growth temperatures and adatom fluxes. Hence, this work not only unveils the conditions for the growth of mono- and bi-layer MoS2, but also provides guidelines for controllable synthesis of MoS2 with the desired number of layers.
As a new 2D material with excellent chemical stability, good electric conductivity, and high specific surface area, graphene has been widely used in energy storage and conversion devices. However, 2D graphene layers are easily stacked, which may significantly reduce the surface area and degrade the excellent electrical properties of graphene. To avoid this, one of the most effective methods is to construct 3D graphene (3DG) with specific porous microstructures. Chemical vapor deposition (CVD) is an important method for the synthesis of high-quality 3DG, where templates play a defining role in controlling the structure and cost of 3DG. Metallic materials with 3D microstructures, such as nickel foam, have proven to be useful as substrates for the growth of high-quality 3DG. However, metal substrates are usually expensive, and the pickling solution generated after etching may cause environmental problems. Therefore, non-metallic substrate materials with lower costs have been investigated for the preparation of 3DG. Herein, we developed a novel template material, mammal bone ashes, for the CVD preparation of 3DG. Mammal bone ash is an inexpensive and abundant biomass hydroxyapatite. During the high-temperature CVD reaction, the bone ash powders were slightly sintered to form a continuous porous structure with graphene coating. The morphology of 3DG is inherited from the microstructure of bone ash templates. After removing the bone ash template with hydrochloric acid, the template-grown 3DG was obtained with a unique bicontinuous structure, i.e. both the graphene framework and the void space were continuous. In addition, the pickling solution of the bone ash templates after etching was exactly the same as that for the raw materials for the production of phosphoric acid to achieve high atom utilization. We further optimized the graphitization degrees, layer number, and porous morphology of 3DGs. The microstructure evolution of 3DG is highly relevant to the layer thickness and uniformity of graphene layers. A short growth time would lead to a non-uniform and thin layer of graphene, which is not able to support a complex 3D porous structure. In contrast, a uniform graphene layer with proper thickness is capable of forming a robust 3D architecture. In addition, the facile CVD method can be extended to a series of metal phosphate templates, including tricalcium phosphate [Ca3(PO4)2], trimagnesium phosphate [Mg3(PO4)2], and aluminum phosphate [AlPO4]. 3DG with bicontinuous morphology is promising as a conductive frame material in electrochemical energy storage devices. As an illustration, high-performance Li-S batteries were fabricated by the uniform composition of an S cathode on 3DG. In comparison with heavily stacked 2D graphene sheets in reduced graphene oxide / S composite, the non-flat structure of 3DGs remained unchanged even after the harsh melt-diffusion process of high-viscosity liquid sulfur. The resulting 3DG/S cathode delivered a high specific capacity of ~550 mAh∙g-1 at a high current rate (2C). Our work opens an avenue to the low-cost and high-utility production of 3D graphene, which could be integrated with the well-developed phosphorus chemical industry.
Recently, ferroelectric materials have attracted considerable research attention. In particular, two dimensional (2D) ferroelectric materials have been considered as most crucial for next-generation circuit designs because of their application as novel electric memory devices. However, a 2D ferroelectric material is very rare. The ferroelectric materials with the form ABP2X6 (A = Ag, Cu; B = Bi, In; X = S, Se) are of interest because of their ferroelectric property maintained in their ultrathin structures. Within the ABP2X6 monolayer, the P―P bonds form the pillars that hold the top and bottom X planes, while the off-center A―B atoms between the X layers induce a spontaneous ferroelectric polarization. If the two off-center A―B sites are equally aligned, this would lead to the appearance of the paraelectric state. Such intriguing structures must impart novel mechanical properties to the materials. Until now, there has been no report on the mechanical properties of monolayer ABP2X6. Based on ﬁrst-principles calculations, we studied the structural, electronic, mechanical as well as the electromechanical coupling properties of monolayer ABP2X6 (A = Ag, Cu; B = Bi, In; X = S, Se). We found that they are all semiconductors with wide bandgaps of 2.73, 2.17, 3.00, and 2.31 eV for CuInP2Se6, CuBiP2Se6, AgBiP2S6, and AgBiP2Se6, respectively, which are calculated based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional model. The conduction band minimum is mainly from p orbitals of X and B atoms, whereas the valence band maximum is due to the hybridization of the p orbital of X atoms and the d orbital of A atoms. Moreover, there are three short and three long A/B―X bonds due to the A―B off-center displacement. Together with the d-p orbital hybridization, the main reason for the distorted ferroelectric structure in ABP2X6 monolayers is the Jahn-Teller effect. ABP2X6 monolayers are predicted to be a new class of auxetic materials with an out-of-plane negative Poisson's ratio, i.e., the values of the negative Poisson's ratio are in the order AgBiP2S6 (−0.805) < AgBiP2Se6 (−0.778) < CuBiP2Se6 (−0.670) < CuInP2S6 (−0.060). This is mainly due to the tensile strain applied in the x/y direction enlarging the angle between P―P bonds and top layer X atoms, thereby enhancing the bucking height of monolayer ABP2X6. Moreover, external strain has a significant impact on the A―B off-center displacement, rendering an out-of-plane piezoelectric polarization. The values of e13 for CuInP2S6, CuBiP2Se6, AgBiP2S6, AgBiP2Se6 monolayers are calculated to be −3.95 × 10−12, −5.68 × 10−12, −3.94 × 10−12, −2.71 × 10−12 C∙m−1, respectively, which are comparable to the only experimentally confirmed 2D out-of-plane piezoelectric Janus system (piezoelectric coefficient = −3.8 × 10−12 C∙m−1). This unusual auxetic behavior, ferroelectric polarization, and the electromechanical coupling in monolayer ABP2X6 could potentially lead to enormous technologically important applications in nanoelectronics, nanomechanics, and piezoelectrics.
The photocatalytic hydrogen evolution reaction (PHER) has gained much attention as a promising strategy for the generation of clean energy. As opposed to conventional hydrogen evolution strategies (steam methane reforming, electrocatalytic hydrogen evolution, etc.), the PHER is an environmentally friendly and sustainable method for converting solar energy into H2 energy. However, the PHER remains unsuitable for industrial applications because of efficiency losses in three critical steps: light absorption, carrier separation, and surface reaction. In the past four decades, the processes responsible for these efficiency losses have been extensively studied. First, light absorption is the principal factor deciding the performance of most photocatalysts, and it is closely related to band-gap structure of photocatalysts. However, most of the existing photocatalysts have a wide bandgap, indicating a narrow light absorption range, which restricts the photocatalytic efficiency. Therefore, searching for novel semiconductors with a narrow bandgap and broadening the light absorption range of known photocatalysts is an important research direction. Second, only the photogenerated electrons and holes that migrate to the photocatalyst surface can participate in the reaction with H2O, whereas most of the photogenerated electrons and holes readily recombine with one another in the bulk phase of the photocatalysts. Hence, tremendous effort has been undertaken to shorten the charge transfer distance and enhance the electric conductivity of photocatalysts for improving the separation and transfer efficiency of photogenerated carriers. Third, the surface redox reaction is also an important process. Because water oxidation is a four-electron process, sluggish O2 evolution is the bottleneck in photocatalytic water splitting. The unreacted holes can easily recombine with electrons. Sacrificial agents are widely used in most catalytic systems to suppress charge carrier recombination by scavenging the photogenerated holes. Moreover, the low H2 evolution efficiency of most photocatalysts has encouraged researchers to introduce highly active sites on the photocatalyst surface. Based on the abovementioned three steps, multifarious strategies have been applied to modulate the physicochemical properties of semiconductor photocatalysts with the aim of improving the light absorption efficiency, suppressing carrier recombination, and accelerating the kinetics of surface reactions. The strategies include defect generation, localized surface plasmon resonance (LSPR), element doping, heterojunction fabrication, and cocatalyst loading. An in-depth study of these strategies provides guidance for the design of efficient photocatalysts. In this review, we focus on the mechanism and application of these strategies for optimizing light absorption, carrier separation and transport, and surface reactions. Furthermore, we provide a critical view on the promising trends toward the construction of advanced catalysts for H2 evolution.
The rapid development of electronic products has increased the demand for safe, low-cost, and high-performance energy storage devices. Lithium-ion batteries have been commercialized owing to their high energy density. However, the limited lithium resources and their uneven distribution have triggered the search for alternative energy storage systems. In this context, rechargeable aqueous zinc-ion batteries have gained immense attention owing to their low cost and environmental friendliness. Nevertheless, it is highly challenging to develop zinc-ion battery cathode materials with both high capacity and long cycle life. Hence, in this study, we prepared three-dimensional porous activated carbon (3DAC) with high specific surface area by using ethylenediaminetetraacetic acid (EDTA) tetrasodium salt hydrate as the raw material. We developed a zinc-ion hybrid capacitor (ZIHC) in 1 mol∙L−1 ZnSO4 using 3DAC as the cathode and a zinc foil as the anode. The ZIHC stored charge by the reversible deposition/dissolution of Zn2+ on the zinc anode and rapid reversible adsorption/desorption of ions on the 3DAC cathode. Owing to the large specific surface area and highly porous structure of the 3DAC cathode, the assembled ZIHC exhibited excellent electrochemical performance. It worked well over the voltage range of 0.1–1.7 V, providing a high specific capacitance of 213 mAh·g−1 at the current density of 0.5 A·g−1 (the highest value reported till date). The ZIHC showed specific capacities of 182, 160, 139, 130, 127, 122, and 116 mAh·g−1 at the current densities of 1, 2, 4, 6, 8, 10, and 20 A·g−1, respectively. Meanwhile, it exhibited the highest energy density of 164 Wh·kg−1 (at a power density of 390 W·kg−1) and still delivered the highest power density of 9.3 kW·kg−1 with a high energy density of 74 Wh·kg−1. In addition, our ZIHC also exhibited excellent cycling stability. After 20000 cycles at 10 A·g−1, it retained 90% of its initial capacity and exhibited high Coulombic efficiency (≈100%). In order to investigate the causes of capacity decay, we examined the cycled zinc foil by scanning electron microscopy and X-ray diffraction. The results showed that a large number of Zn4SO4(OH)6⋅3H2O disordered dendrites were formed on the surface of the zinc foil. These dendrites inhibited the reversible deposition/dissolution of zinc ions, resulting in the capacity decay of the ZIHC during the cycling process. This study will be helpful for developing next-generation high-performance energy storage devices.
Transition-metal-catalyzed C―H bond activation, which has been widely applied to construct new covalent bonds, has emerged as one of the most effective strategies in synthetic chemistry due to atom economy and simple procedure. In this review, we have summarized the recent reports on the theoretical mechanistic study of transition-metal-catalyzed C―H bond cleavage. Based on these comprehensive theoretical studies, we have systematically discussed the general modes of C―H bond activation, which involves oxidative addition, base-assisted deprotonation, σ-metathesis, Friedel-Crafts-type electrophilic aromatic substitution, α- or β-hydrogen elimination, and hydrogen atom abstraction. From a mechanistic point of view, C―H bond activation by oxidative addition generally involves a zero-valent transition metal catalyst with strong reducibility, which requires a low activation barrier. The concerted metalation-deprotonation (CMD)-type C―H bond cleavage often occurs via a six-membered cyclic transition state using transition metal carboxylate as the catalyst with a directing group, which is a common mechanism for transition metals with high oxidation states. Base-assisted internal electrophilic substitution (BIES)-type C―H bond activation is commonly performed in the presence of cationic transition metal catalysts, in which electron-rich arenes react preferentially compared to electron-deficient arenes. In some other cases, outer-sphere base-assisted deprotonation can also result in C―H activation, which is dependent on the strength of the base used. The stronger the base used, the lower the energy barrier, and thus, the easier it is to protonate. The σ-metathesis pathway, which could occur via a four-membered cyclic transition state, is often considered an alternative for concerted metalation-deprotonation. If the aromatic hydrocarbon is attacked by electrophiles, the C―H bond can be activated by Friedel-Crafts-type electrophilic aromatic substitution. Elimination of α- or β-hydrogen is also frequently proposed for transition-metal-catalyzed C―H functionalization. Hydrogen atom abstraction could achieve C―H bond activation via a free radical process. Moreover, the C―H bonds of hydrocarbons can be considered weak nucleophiles because the electronegativity of carbon is higher than that of hydrogen, and they could be converted to strong nucleophiles (C―M) in the presence of transition metal catalysts via the different pathways mentioned above. It enables further functionalization with electrophiles or nucleophiles to construct complex molecular skeletons. Summarizing the general modes of C―H bond activation will increase our understanding of the associated chemical mechanism and will pave the way for new synthetic strategies. This review aims to offer theoretical guidance for experimental studies and inspire new reaction design by summarizing the modes of transition-metal-catalyzed C―H bond activation.
Driven by the wide-scale implementation of intermittent renewable energy generating technologies, such as wind and solar, sodium-ion batteries have recently attracted attention as an inexpensive energy storage system due to the abundance, low cost, and relatively low redox potential of sodium. However, in comparison with lithium-ion batteries, which are known for long cycle life, sodium-ion batteries usually suffer from significant capacity fading during long-term cycling due to the large volume expansion/contraction of the electrode active materials caused by insertion/extraction of the large sodium ion. In recent years, intense effort has been focused on the search for high performance electrode materials and electrolytes to improve the cyclability of sodium-ion batteries, and some progress has been achieved. The incorporation of additives into the electrolyte is a simple and efficient method of improving the cycle stability of sodium-ion batteries. Fluoroethylene carbonate (FEC) is generally considered to be a suitable additive for the formation of the anode solid electrolyte interphase (SEI), due to a relatively low-lying lowest unoccupied molecular orbital (LUMO). However, it is suggested that FEC it will not be oxidized on the cathode since it also has a relatively low highest occupied molecular orbital (HOMO). In this study, we investigated the effect of FEC as an additive on the cycle life of a sodium-ion battery with a P2-NaxCo0.7Mn0.3O2 (x ≈ 1) layered sodium transition metal oxide as the cathode active material, a sodium metal foil anode, a glass fiber separator, and an electrolyte composed of NaClO4 and a varying mass content of FEC dissolved in propylene carbonate (PC). We analyzed the effect of the FEC additive on the morphology and chemical composition of the separator and cathode electrode surface using scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS), and studied the evolution of the crystalline structure of the cathode active material during charge and discharge using in situ X-ray diffraction (XRD). We found that an appropriate amount of FEC additive significantly suppressed the decomposition of the PC solvent, and assisted the formation of a NaF-rich protective layer on the cathode surface, which helped to maintain the structural stability of the cathode material, thereby improving the cycle stability of the sodium-ion battery. Density functional theory (DFT) calculations showed that FEC coordinates more readily with the ClO4- anion on the cathode surface than does the PC solvent. This drives the formation of the NaF-rich protective layer on the cathode surface. We believe these results could provide inspiration in the design of electrolyte additives for protection of the sodium cathode during cycling, thus improving the cycling performance of sodium-ion batteries.
Pyridones represent an important family of heterocycles that exhibit a wide range of biological activities. They are often found in pharmaceutical agents and biomolecules. Several transition-metal-catalyzed transformations have been developed to access this family of heterocycles. Among them, C―H bond activation has recently emerged as a general strategy for the construction of substituted pyridones. In most cases, the core nitrogen-containing heterocycle is assembled via the dehydrogenative annulation of α, β-unsaturated amides and alkynes. Such processes involve a cascade sequence of N―H cleavage, sp2 C―H activation, and annulation. Despite this progress, the more readily available α, β-saturated amides are rarely used. Ideally, tethering the direct dehydrogenation of an amide with the above-mentioned C―H annulation cascade would give a more practical synthesis of pyridones. Nevertheless, the dehydrogenation of amides under mild conditions is a synthetic challenge due to their intrinsic weak α-acidity. Recently, we have reported a general protocol for the aerobic dehydrogenation of γ, δ-unsaturated amides, acids, and ketones. A key Ir―allyl intermediate was believed responsible for enhancing the α-acidity of the amides studied, which enables the dehydrogenation step to occur under mild reaction conditions. Herein, we describe a new method for the synthesis of polysubstituted pyridones using γ, δ-unsaturated amides and alkynes. In the presence of [RhCp*Cl2]2, the dehydrogenation step occurs via β-C―H bond activation. The resulting π-allyl―Rh intermediate undergoes an accelerated dehydrogenation reaction to afford the doubly unsaturated amide. This in-situ generated dienamide undergoes sp2 C―H activation at the β-position and a subsequent alkyne insertion/cyclization reaction to yield the target heterocycle. Regeneration of the Rh catalyst is accomplished using an external oxidant and completes the streamlined double C―H activation and double dehydrogenation catalytic cycle. Various functional groups are well tolerated. The γ-alkenyl moiety not only facilitates the direct dehydrogenation of amides, but also serves as a handle for further derivatization of the as-obtained products. To gain a mechanistic insight into the reaction cascade, a set of control experiments were carried out. The results demonstrate that the dienamide is one of the key reaction intermediates. NMR experiments confirmed that the fast dehydrogenation process occurs during the early stage of the reaction. The alkyne insertion is believed to be the rate-determining step in the reaction cascade, as suggested by competition experiments.
Converting solar light into chemical energy is currently a hot topic for addressing the worldwide energy and environmental crises. However, the utilization of solar energy greatly suffers from its low energy flow density and discontinuous space-time distribution, which are essential for a reasonable energy conversion strategy toward effective storage and utilization. To this end, photocatalytic water splitting is a promising method for utilizing solar light to produce environmentally friendly hydrogen energy; yet, the efficiency needs to be improved. Generally, such processes can be divided into three elementary steps: light absorption, charge separation and migration, and surface redox reaction. The overall performance is determined by the cumulative efficiencies of the above three steps. The construction of cocatalysts is among the extensive efforts taken to improve the solar conversion efficiency. First, the cocatalysts possess higher work function than the semiconductors, and the photogenerated electrons migrate from semiconductor to cocatalysts, thereby promoting the charge separation. Second, cocatalysts usually lower the activation energy and provide abundant surface reactive sites. Particularly, the addition of cocatalysts can remarkably accelerate the four-electron transfer O2 evolution kinetics, which usually requires much higher overpotential and is often considered as the bottleneck for water splitting. Third, cocatalysts can timely remove the photogenerated charges from the surface of the semiconductor and subsequently inhibit the photocorrosion and improve the stability of the photocatalysts. Moreover, the cocatalysts also retard the backward recombination of H2 and O2. In general, cocatalysts for water splitting can be classified into three categories: H2 evolution cocatalysts, O2 evolution cocatalysts, and dual cocatalysts. The H2 evolution cocatalysts mainly contain noble metals such as Pt, Au, and other transition metals such as Co, Ni, and Cu and their phosphides or sulfides, which are capable of trapping electrons and promoting proton reduction. The O2 evolution cocatalysts are often noble metal oxides and transition metal (hydro)oxides and corresponding phosphates, which are always efficient in adsorbing and dissociating water molecules. To realize the overall water splitting, H2 evolution cocatalysts and O2 evolution cocatalysts are often integrated on one photocatalyst, which results in the so-called dual cocatalyst system. Furthermore, the performance of cocatalysts can be improved by modulating the loading amount, morphology, particle size, etc. In addition, composites such as Pt/Ni(OH)2 cocatalyst can not only provide both H2 and O2 evolution sites but also accelerate the intrinsic surface redox kinetics by promoting H2O activation, thus being much more active than the conventional dual cocatalyst system. This review summarizes the important role and design principle of cocatalysts in photocatalytic systems. The construction and functional mechanism of H2 evolution cocatalyst, O2 evolution cocatalyst, and dual cocatalysts in overall water splitting photocatalysts are discussed in detail, and the design strategy of new cocatalysts toward water activation is proposed.
The discoveries about the functions of biomolecular liquid-liquid phase separation in cell have been increased rapidly in the past decade. Condensates produced by phase separation play key roles in many cellular curial events. These biological functions are based on the physicochemical properties of phase separation. This review discusses the recent progress in understanding the physical and chemical mechanisms of biological liquid-liquid phase separation. (1) We summarized the basic properties and experimental characterization methods of phase separation droplets, including the morphology, fusion, and wetting, along with the dynamic properties of molecules in droplets, which are usually described by diffusion coefficients or viscosity and permeability. (2) We discussed the conditions affecting the liquid-liquid phase separation of biological molecules, including concentration, temperature, ionic strength, pH, and crowding effects. A database for liquid-liquid phase separation, LLPSDB, was introduced, and three types of nucleic acid concentration effects on the phase separation of protein molecules are discussed. These effects depend on the relative interaction strengths of protein-nucleic acid and protein-protein interactions. The major driving force of phase separation is multivalent interactions, and molecular flexibility is necessary for the dynamic properties. We summarized the diverse sources of multivalence, including multiple tandem repetitive domains, regular oligomerization, low-complexity domains (usually intrinsically disordered with repeat motifs for binding), and nucleic acid molecules via the main chain phosphates or repeat sequences. (3) We reviewed the statistical thermodynamics theories for describing the macromolecular liquid-liquid separation, including the Flory-Huggins theory, Overbeek-Voorn correction, random phase approximation method, and field theory simulation method. We discussed the experimental and simulation methods for studying the physiochemical mechanism of liquid-liquid phase separation. Model systems with simplified sequences for experimental studies were listed, including systems for studying the effects of charge properties, residue types, sequence length, and other properties. Molecular simulation methods can provide detailed information regarding the liquid-liquid phase separation process. We introduced two coarse-grain methods, the slab molecular dynamic simulation and Monte Carlo simulation using the lattice model. (4) The physiochemical properties of liquid-liquid phase separation govern the diverse functions of reversible phase transitions in a cell. We collected and analyzed important cases of biomolecular phase separation in cell activities. These biological functions were classified into five categories, including enrichment, sequestration, biological switching cooperation, localization, and mechanical force generation. We linked these functions with the physiochemical properties of liquid-liquid phase separation. To understand the specific phase-separation processes in biological activities, three types of related molecules must be studied: scaffold molecules mainly contributing to aggregate formation, recruited functional client molecules, and molecules that regulate the formation and disassembly of aggregates. We reviewed four regulation methods for the phase separation process, including changing the charge distribution by post-translational modification, changing the molecular concentration by gene expression or degradation regulation, changing the oligomerization state, and changing the cell solution environment (such as pH). Designing compounds for phase separation regulation has attracted significant attention for treating related diseases. Methods for discovering molecules that can regulate post-translational modifications or inhibit interactions in the droplets are emerging. The recently discovered phase separation phenomena and molecules in living organisms represent only the tip of the iceberg. In the future, it will be necessary to systematically examine liquid-liquid phase separation events and related molecules in all phases of biological processes.
Methane, the most abundant constituent of natural gas, is a potential substitute for the dwindling petroleum resources for the chemical industry as a carbon-based feedstock. Over the last two decades, global research endeavors have focused on the development of more efficient and selective catalysts for the conversion of ubiquitous but inert methane. In addition, the transportation of gaseous methane in pipelines is unavoidably accompanied by leakage, and methane is recognized as a potent greenhouse gas (20 times more powerful than carbon dioxide per molecule). Thus, the conversion of methane into heavier derivatives is also of crucial environmental concern. Unfortunately, there is still a lack of economical and practical routes for methane conversion. Currently, the major route for methane conversion is the steam reforming of methane into synthetic gases, which is a multistep and energy-consuming route. Another option is to use photoenergy to drive the conversion of methane, which has significant advantages such as the capacity to minimize coking by running at room temperature. A promising approach to photocatalytic methane conversion is the photo-powered direct coupling or oxidation of methane to form ethane, methanol and hydrogen. The ethane or methanol produced can, in turn, be converted into ethene or liquid fuels through metathesis or dehydrogenation, respectively. Furthermore, the direct dehydrogenation of methane is the best way to produce clean H2 energy from fossil fuels since methane has the highest H/C ratio among hydrocarbons. However, the methane conversion efficiency of previously reported photocatalysts is low. Furthermore, the wavelength of light used in previously reported photocatalytic systems usually needs to be less than 270 nm, which is beyond the range of the solar spectrum (wavelength λ > 290 nm) reaching the Earth's surface. To achieve substantial yield and selectivity, and to exploit solar energy effectively, the development of photocatalytic systems with distinctly higher activity, higher selectivity, and lower photon energy threshold is desired. Over the past decades, many efforts have been made to activate the strong C―H bond in methane by light at room temperature. Based on the current state of research on photocatalytic methane conversion, we have focused our review on the following aspects: non-oxidative coupling of methane, dehydroaromatization of methane, and total and partial oxidation of methane. Finally, we summarize the difference between photocatalysis and thermal catalysis in the methane conversion reaction.
Carbon dioxide is the most common compound. As a potential source of carbon, it can be used to prepare a variety of high value-added chemicals, such as carbon monoxide, methane, methanol, and formic acid. The traditional method of thermal catalytic conversion of CO2 requires high energy consumption and harsh reaction conditions. Therefore, the efficient conversion of CO2 to value-added chemicals under mild conditions has long been an area of great interest in the field of catalysis. Photocatalysis usually takes place under mild reaction conditions and is environmentally friendly. However, pure photocatalytic reactions generally have a limited solar energy utilization efficiency and low separation efficiency of photogenerated charge carriers. In view of the above problems, the introduction of electrocatalysis on the basis of photocatalysis can improve the charge separation efficiency. At a lower overpotential, multi-electrons and protons can be transferred to CO2, thus improving the catalytic reaction efficiency. Photoelectrochemical catalysis combines the advantages of photocatalysis and electrocatalysis to improve the efficiency of the catalytic reduction of CO2, offering a new method for the clean utilization of CO2. According to the principle of photocatalysis, the absorption capacity of a semiconductor is governed by its band structure. Optimization of the band structure is a major strategy to enhance the absorptivity of photocatalysts. In addition, the loading of light-absorbent materials on photocatalysts is an effective way to enhance the photocatalytic absorption of a photocatalytic system. During photoelectrocatalytic CO2 reduction, numerous photogenerated charge carriers recombine in bulk and on the surface of the catalyst, greatly reducing the efficiency of the catalytic reaction. Therefore, increasing the separation efficiency of charge carriers is an important means to improve the photoelectrocatalytic efficiency. In photoelectrocatalytic CO2 reduction, heterojunction construction and electric field formation often lead to the efficient separation of charge carriers. The interfacial reaction is a crucial step in the photoelectrocatalytic process. After generation, the photogenerated charge carriers need to migrate to the surface of the catalyst to participate in the redox reaction. In photoelectrocatalytic CO2 reduction, electrons participate in the reduction of CO2, while holes participate in the oxidation of water. Studies show that acceleration of the interfacial reaction process is of paramount importance for improving the efficiency of the photoelectrocatalytic reduction of CO2. This review summarizes the basic enhancement strategies of photoelectrocatalytic CO2 reduction from three aspects: light absorption, charge separation, and surface reaction, based on the basic mechanism of the reduction. The future prospects and research areas are also proposed.
Thioesters, which are essential sulfur-containing organic molecules, are indispensable in natural products, pharmaceuticals, and organic light-emitting materials. Efficient synthesis of thioethers has received considerable attention due to the widespread applications of these compounds, and many fundamental approaches for C―S bond formation have been proposed. However, most of them construct C―S bonds by employing organic halides/organic boronic acid. These methodologies generally suffer from a pre-functionalized starting material. Recently, selective C―H functionalization emerged as a powerful tool for the synthesis of C―N, C―O, C―C, and C-halogen bonds. Nevertheless, C―S bond formation via C―H functionalization has only recently been given more importance because organosulfur compounds are believed to inactivate catalysts. In contrast to traditional cross-coupling reactions, direct functionalization of C―H bonds for the synthesis of thioethers can shorten the reaction steps and minimize the amount of waste formed. In this review, which is divided into several parts, we describe C―H functionalization strategies for the construction of thioethers. In Part Ⅰ, we introduce the importance and widespread applications of thioethers in daily life. For example, Lissoclibadin 6 is a polysulfur aromatic alkaloid that shows antimicrobial activity. Seroquel is an antipsychotic medicine. It is used to treat bipolar disorder and schizophrenia in adults, and children who are at least 10 years old. Tazarotene is approved for the treatment of psoriasis, acne, and sun-damaged skin. Furthermore, a comparison between conventional synthesis methods and C―H thiolation is discussed. In Part Ⅱ, we introduce copper-catalyzed or copper-mediated C―H thiolation. Along with the direct functionalization of sp2 and sp C―H for the synthesis of aryl sulfides, some significant and challenging thiolations of sp3 C―H are included. In addition to copper, palladium is an excellent catalyst for C―H functionalization. In Part Ⅲ, we elucidate palladium-catalyzed C―H thiolation and discuss many proposed mechanisms. Nickel, which is a first-flow, low-cost, and earth-abundant metal catalyst, has increasingly gained attention. In contrast to copper and palladium, despite its late start, several remarkable reports on nickel-catalyzed C―H thiolation were published by several groups. Rhodium plays a key role in selective C―H functionalization. Some published results proved the capacity of rhodium catalysts to promote C―S construction via C―H functionalization. In Part Ⅳ, we introduce rhodium-catalyzed C―H thiolation. In recent years, metal-free C―H functionalization has been quite attractive. In Part Ⅴ, some C―S construction strategies via metal-free C―H functionalization are presented. In the last part, the conclusion discusses the limitations and possible development directions of these advances in the construction of thioethers.
Sodium batteries have drawn increasing attention from multiple researchers owing to the abundant reserves and low cost of sodium resources. However, traditional sodium batteries based on organic solvent electrolyte systems have safety risks. Thus, the utilization of solid electrolyte materials instead of organic electrolytes could effectively resolve safety issues and ensure the safe performance of the battery. Solid sodium-ion battery is a promising energy storage device. The sodium ion solid-state electrolytes mainly includes Na-β-Al2O3, Na super ionic conductor (NASICON), sulfide, polymer, and borohydride. Inorganic solid electrolytes have the advantage of ionic conductivity compared with polymer solid electrolyte. This paper summarizes the research progress on three common inorganic sodium ion solid electrolytes: Na-β-Al2O3, NASICON, and sulfide. Research efforts have mainly focused on increasing ionic conductivity and interface stability. Na-β-Al2O3 has been successfully commercialized in high-temperature Na-S and ZEBRA batteries with molten electrodes. Pure β″-Al2O3 is difficult to prepare owing to its low thermodynamic stability. The synthesized β″-Al2O3 based on traditional solid-state reaction generally contains impurities such as β-Al2O3 and NaAlO2 (around the boundaries). Further improvements are required to develop favorable methods for fabricating pure β″-Al2O3 with high production yield, low cost, and well-controlled microstructure. NASICON, one of the most promising ionic conductors for solid sodium-ion batteries, has attracted considerable attention for its high ionic conductivity at room temperature. The general method to enhance ionic conductivity is to increase the bottleneck size by introducing proper substituents. However, the substitution of synthetic elements could result in different optimal calcination temperatures, which would lead to a change in the density of ceramic sintering. β″-Al2O3 and NASICON have higher ionic conductivity at room temperature but cannot achieve good performance in the field of high power densities and long-term cycling owing to the poor interface contact with electrode materials. Because the high polarizability and large ionic radius of sulfur atoms weaken the interaction between skeleton and sodium ions, sulfide solid electrolytes often provide higher ionic conductivity at room temperature than analogous oxides. At the same time, sulfide solid electrolytes can be easily pressed into a mold at room temperature. However, sulfide electrolytes have low chemical stability in air because of hydrolysis by water molecules with the generation of H2S gas, which should be handled in inert gas atmosphere. In conclusion, this review discusses the recent progress in different aspects of ionic conductivity and interface stability.
Since Fujishima and Honda demonstrated the photoelectrochemical water splitting on TiO2 photoanode and Pt counter electrode, photocatalysis has been considered as one of the most promising technologies for solving both the problems of environmental pollution and energy shortage. This process can effectively use solar energy, the most abundant energy resource on the earth, to drive various catalytic reactions, such as water splitting, CO2 reduction, organic pollutant degradation, and organic synthesis, for energy generation and environmental purification. Except for the various metal-based semiconductors, such as metal oxides, metal sulfides, and metal oxynitrides, developed for photocatalysis, graphitic carbon nitride (g-C3N4) has attracted significant attention in the recent years because of its earth abundancy, non-toxicity, good stability, and relatively narrow band gap (2.7 eV) for visible light response. However, g-C3N4 suffers from insufficient absorption of visible light in the solar spectrum and rapid recombination of photogenerated electrons and holes, thus resulting in low photocatalytic activity. Until now, various strategies have been developed to enhance the photocatalytic activity of g-C3N4, including element doping, nanostructure and heterostructure design, and co-catalyst decoration. Among these methods, element doping has been found to be very effective for adjusting the unique electronic and molecular structures of g-C3N4, which could significantly expand the range of photoresponse under visible light and improve the charge separation. Especially, non-metal doping has been well investigated frequently to improve the photocatalytic activity of g-C3N4. The non-metal dopants commonly used for the doping of g-C3N4 include oxygen (O), phosphorus (P), sulfur (S), boron (B), and halogen (F, Cl, Br, I) and also carbon (C) and nitrogen (N) (for self-doping), as they are easily accessible and can be introduced into the g-C3N4 framework through different physical and chemical synthetic methods. In this review article, the structural and optical properties of g-C3N4 is introduced first, followed by a brief introduction to the modification of g-C3N4 as photocatalysts. Then, the progress in the non-metal doped g-C3N4 with improved photocatalytic activity is reviewed in detail, with the photocatalytic mechanisms presented for easy understanding of the fundamentals of photocatalysis and for guiding in the design of novel g-C3N4 photocatalysts. Finally, the prospects of the modification of g-C3N4 for further advances in photocatalysis is presented.
The research in two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs) and black phosphorus, has been further flourished with the recent emergence of heterostructures composed of dissimilar 2D materials. The interfacing/coupling between different constituent components in a heterostructure has given rise to interesting phenomena and useful properties. For example, depending on the type of 2D materials, the distance and the kind of bonding between them, as well as the crystalline property of the hetero-interface, the interface may provide charge traps, exciton recombination centers, or bridges for effective charge/energy transfer. It has also been found that the spatial arrangement in addition to the composition of the constituents is an important factor influencing the overall properties of the heterostructures. Although many methods, such as dry transfer and vapor-phased growth are able to yield heterostructures from pristine or highly crystalline 2D crystals with spatial control, such as vertical heterostructures and lateral heterostructures, these methods are generally not scalable, which has restricted the use of the obtained heterostructures mostly to fundamental studies. The solution-phased synthesis methods, such as solvothermal/hydrothermal synthesis, electrochemical deposition and hot-injection method, may be more suitable for mass production of functional heterostructures despite the relatively low product quality. In the past couple of years, a diverse kinds of hetero/hybrid structures of 2D materials have been prepared successfully in wet-chemical processes. However, precise control over the geometric arrangement of the constituent components has been challenging in solution. Currently, four types of heterostructures including 2D crystals grown on a larger 2D template, vertical heterostructures, lateral heterostructures, and core-shell heterostructures have been prepared in solution. For the first type, flexible 2D nanosheets such as graphene and monolayer TMDs are used as synthesis templates to support the nucleation and growth of other 2D crystals. For vertical heterostructures, relatively rigid nanoplates are used to allow continuous deposition of 2D layers of other materials to form sandwich-like structures. The formation of lateral heterostructures requires edge growth on existing 2D materials without basal deposition, and therefore other methods such as cation exchange can be used as alternative routes. The preparation of core-shell 2D heterostructures generally involves both epitaxial edge growth and basal deposition and has been realized in both metallic and semiconductor structures. In this review, these kinds of heterostructures based on 2D materials will be discussed in terms of their synthesis methods, properties and possible applications. In addition, we will discuss the challenges and possible opportunities in this research direction.
The activation of methane (CH4) is a key step in its conversion to more valuable products. The activation mechanisms of CH4 on catalyst surfaces have been widely studied using gas-phase cluster models, which can be operated on systems with a precise number of atoms and determined structures. Herein, we have used MV3Oyq (M = Au/Ag, y = 6–8, q = 0 or ±1) clusters, in which a single Au or Ag atom was supported on vanadium oxide clusters, as simple models to mimic the properties of newly developed single-atom catalysts. The adsorption and activation of CH4 on these MV3Oyq clusters were systematically studied via density functional theory calculations at the B3LYP/Def2-TZVP level, which provided insights into the geometric structures, adsorption energies, and charge distributions of the adsorption systems. Five Au-containing clusters, AuV3O6, AuV3O7, AuV3O8, AuV3O6+, and AuV3O7+, were able to activate CH4, while other clusters, including all Ag-containing clusters, were inert. In the active clusters, all Au atoms were adsorbed on the O-atom sites of the supporting V3Oyq cluster and served as the active sites for CH4 activation. The activation of CH4 was characterized by the lengthened C―H bond (approximately 115 pm), short distances between CH4 and Au (approximately 184 pm), relatively high adsorption energies of CH4 (~0.590–1.145 eV), and significant electron transfer from CH4 to the clusters (above 0.08e). In particular, AuV3O8, which is a neutral cluster with a close-shell electronic state, can activate CH4 with a C―H bond length of 115 pm, Au―H bond length of 183 pm, the adsorption energy of CH4 of 0.853 eV, and the charge on CH4 of +0.088e. The charge state of the cluster has a significant effect on the activation ability: cationic clusters are the most active, followed by neutral clusters, while anionic clusters have the lowest activities toward CH4. Consistently, the local charge on the M atom has a positive correction with the activation ability of MV3Oyq clusters with a certain M. However, as compared to Au-containing clusters, Ag-containing clusters have lower activities despite the higher local charges on Ag in each MV3Oyq cluster. The results indicate that the inclusion of D3 dispersion correction has a small effect on structures and energies. This study may serve as a foundation for further research on the activation of CH4 on single-atom catalysts and provides useful information on rational designing of single-atom catalysts for CH4 conversion at low temperatures.
The rapid economic development has necessitated increased environmental protection and tremendous efforts have been devoted to designing and preparing environmentally friendly energy storage and conversion devices, including batteries, supercapacitors (SCs), and solar cells. As useful energy storage and conversion devices, SCs have received significant attention due to their high power density, long cycle stability, and rapid charging rate. The performance of SCs largely depends on the electrode materials, which can be composed of metal oxides, conducting polymers, carbon materials, and their composites. Carbon materials, including carbon nanotubes, carbon fibers, porous carbons, template carbons, and graphene-like carbon nanosheets (GCNSs), have attracted significant attention owing to their tailorable pore size and excellent physiochemical stability. GCNSs are considered to be outstanding carbon materials for use in high-performance SC because of their large specific surface area and high electrical conductivity. To date, many carbon precursors have been used to synthesize GCNSs for SCs such as coal, biomass, and chemical by-products. In particular, cheap and abundant chemical by-products for the synthesis of carbon materials can reduce preparation costs and environmental pollution as well as achieve high value-added chemicals. Coal tar, a by-product of the coal coking process, is rich in aromatic polycyclic hydrocarbon molecules, which can be polymerized, carbonized, and activated to synthesize GCNSs for SCs. In addition, MgO particles can be used as templates due to their stable properties and low-cost compared with other metal oxide templates (e.g. Fe2O3, NiO, or CuO), imparting space confinement and structure guidance during the preparation of GCNSs. Herein, we report a facile method for the preparation of interconnected graphene-like nanosheets (IGNSs) from coal tar by MgO templating combined with in-situ KOH activation. The IGNSs were obtained after the impurity removal by repeated washing with dilute acid. The as-synthesized IGNSs feature high specific surface area of up to 2887 m2∙g−1 and abundant hierarchical short pores, which provide abundant active sites for ion adsorption, supply plentiful channels for fast ion transport and boost electrical conductivity. As electrodes for SCs, IGNSs manifest high specific capacitance value of up to 313 F∙g−1 at 0.05 A∙g−1, good rate capability of 261 F∙g−1 at 20 A∙g−1, and excellent cycle stability with 92.7% of initial capacitance retained after 10000 cycles in 6 mol∙L−1 KOH aqueous electrolyte. This study provides a facile method for large-scale production of IGNSs from aromatic hydrocarbon molecules for use in high-performance energy storage devices.
The sol-gel method, developed for over 150 years, is a conventional route for designing and preparing various kinds of metal oxide materials. In the sol-gel method, different chemical agents are homogenously mixed together in aqueous or organic solutions. During the evaporation of the solvents, the solution transforms to sol and gel through polycondensation or polyesterification reaction, and the dried gel is obtained after the complete evaporation of the solvent. Then, the dried precursor is often heat-treated in air at high temperature to induce the formation of oxide materials, especially the multi-component oxide materials that are difficult to prepare using other methods. Recently, new developments have been achieved in the sol-gel method. The application of the sol-gel method has been extended to the preparation of metallic nanomaterials, especially the alloy nanocrystals. For instance, the sol-gel method can be used to prepare CoPt and FePt hard magnetic alloy nanocrystals; CoCrCuNiAl high-entropy alloy nanocrystals; Ni3Fe and Cu3Pt alloy nanocrystals with equilibrium-ordered crystalline phases; and Ni, Cu, Bi, Sb, Te, Ag, Pt, and Pd monometallic nanocrystals. This article reviews the recent progresses in the sol-gel method for designing and preparing metallic and alloy nanocrystals, as well as the detailed experimental procedures and the different metallic nanocrystals that can be obtained by the sol-gel method. The crystalline phase formed in the final calcined products can be determined from the thermodynamic calculations of the sol-gel method. The thermodynamic model involves the calculation of the Gibbs free energy change of the reaction between the metallic oxide and reducing gases, such as hydrogen. A negative change and a positive change in the Gibbs free energy of the reaction correspond to the formation of metallic and alloy crystalline phases, or oxide crystalline phase, respectively. Based on the thermodynamic calculations and the relationship between the Gibbs free energy and standard electrodynamic potential of the chemical reaction, a new parameter, metal oxide standard electrode potential, was proposed. This electrode potential is different from the conventional standard metal electrode potential. A metallic crystalline phase is obtained if the electrode potential of the corresponding metal oxide is positive, while a metal oxide crystalline phase is obtained if the electrode potential of the metal oxide is negative. We also discuss the possible applications, including the magnetic and electrocatalytic applications, of the metallic and alloy nanocrystals that have been obtained by the sol-gel method. Finally, the future prospects of the application of the sol-gel method in designing metallic and alloy nanocrystals are discussed.
Lithium-ion batteries (LIBs) are widely used in cellphones, laptops, and electric cars owing to their high energy density and long operational lifetime. However, their further deployment in large-scale energy storage systems is restricted by the uneven distribution of lithium resources (~0.0017% (mass fraction, w) in the Earth's crust). Therefore, alternative energy storage systems composed of abundant elements are of urgent need. Recently, sodium-ion batteries (SIBs) have attracted significant attention and are considered to be a potential alternative for next-generation batteries owing to abundant sodium resources (~2.64% (w) of the Earth's crust), suitable potential (−2.71 V), and low cost. SIBs are similar to LIBs in terms of their physical and electrochemical properties. Previous studies have mainly focused on SIB storage materials, including hard carbon, alloys, and hexacyanoferrate, while the safety of SIBs remains largely unexplored. Similar to LIBs, the current electrolytes used in SIBs are mainly composed of flammable organic carbonate solvents (or ether solvents), sodium salts, and functional additives, which pose possible safety issues. Moreover, the chemical activity of sodium is much higher than that of lithium, leading to a higher risk of fire, thermal runaway, and explosion. To overcome this problem, herein we propose a fluorinated non-flammable electrolyte composed of 0.9 mol∙L−1 NaPF6 (sodium hexafluorophosphate) in an intermixture of di-(2, 2, 2 trifluoroethyl) carbonate (TFEC) and fluoroethylene carbonate (FEC) in a 7 : 3 ratio by volume. Its physical and electrochemical properties were studied by ionic conductivity, direct ignition, cyclic voltammetry, and charge/discharge measurements, demonstrating excellent flame-retarding ability and outstanding compatibility with sodium electrodes. The electrochemical tests showed that the Prussian blue cathode retained a capacity of 84 mAh∙g−1 over 50 cycles in the prepared electrolyte, in contrast to the rapid capacity degradation in a flammable conventional carbonate electrolyte (74 mAh∙g−1 with 57% capacity retention after 50 cycles). To test the practical application of the proposed electrolyte, a hard carbon anode was used and exhibited exceptional performance in this system. The enhancement mechanism was further verified by Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning emission microscopy (SEM) investigations. Polycarbonate on the surface of the cathode played an important role for the studied electrolyte system. The polycarbonate may originate from FEC decomposition, which can enhance the ionic conductivity of the solid electrolyte interface (SEI) layer and reduce impedance. Hence, we believe that this proposed electrolyte may provide new opportunities for the design of robust and safe SIBs for next-generation applications.
Organic field effect transistors (OFETs) have great potential in flexible sensor and display driver applications. However, there are immense challenges in the development of large-area and high-quality thin-film fabrications. In this article, we introduce a method to fabricate patterned organic semiconductor films by oxygen plasma treatment and the synergy of Marangoni and coffee-ring effects. The procedure is as follows: First, we spin-coated the cyclic transparent amorphous fluoropolymers (CYTOP) on the substrate in the form of a hydrophobic layer. Then, parts of the substrate surface were treated with plasma and modified to make them hydrophilic. By comparing the water contact angle on the plasma treated surface with that on the untreated surface, we optimized the treating time to get a relatively uniform water contact angle on a different region of the substrate surface. The plasma treated substrate was dipped into 2, 7-dioctylbenzothieno[3, 2-b]benzothiophene (C8-BTBT) solution with methylbenzene and carbon tetrachloride as a mixed solvent, and then lifted from it. So the mixed solution flowed down rapidly on the hydrophobic portion of the surface, leaving droplet on the hydrophilic portion. Subsequently, the droplet started evaporating under the synergy of Marangoni and coffee-ring effects. Based on the difference between the hydrophilic and hydrophobic portions on the substrate surface, we successfully obtained the patterned C8-BTBT thin films on the substrate. Furthermore, the solvent ratio was optimized while growing the C8-BTBT film to adjust the boiling point of the solution, which was due to a fully covered surface was obtained. From the grazing-incidence X-ray diffraction (GIXRD) measurement of the films with three different concentrations, we observed that increasing in the concentration of the solution yielded different molecular orientations. Based on the three films, OFETs with bottom gate and top contact structure were fabricated. Moreover, the mobility and the on/off current ratio became more uniform with the progressive increase in the concentration of the solution. This may be attributed to the increase in the number of different molecular orientations and charge transfer channels. Although the increase in the number of different molecular orientations might lead to the decrease in mobility, it could improve the alignment of the electric field and also increase π–π stacking direction of the molecules, which promote highly uniform device performance distribution. Since uniform distribution of device performance is significant for practical applications, we believe the transistors that are fabricated at the highest concentration are better than those generated at lower concentrations. Thus, on the 5 cm × 5 cm substrate, it is observed that the average mobility of the transistors is 7.9 cm2·V-1·s-1, and all the devices have threshold voltages less than -2 V with the on/off current ratio of 104. This work is significant for the fabrication of large-area and high-performance thin films and transistors.
Polymers are widely used advanced materials composed of macromolecular chains, which can be found in materials used in our daily life. Polymer materials have been employed in many energy and electronic applications such as energy harvesting devices, energy storage devices, light emitting and sensing devices, and flexible energy and electronic devices. The microscopic morphologies and electrical properties of the polymer materials can be tuned by molecular engineering, which could improve the device performances in terms of both the energy conversion efficiency and stability. Traditional polymers are usually considered to be thermal insulators owing to their amorphous molecular chains. Graphene-based polymeric materials have garnered significant attention due to the excellent thermal conductivity of graphene. Advanced polymeric composites with high thermal conductivity exhibit great potential in many applications. Therefore, research on the thermal transport behaviors in graphene-based nanocomposites becomes critical. Vacancy defects in graphene are commonly observed during its fabrication. In this work, the effects of vacancy defects in graphene on thermal transport properties of the graphene-polyethylene nanocomposite are comprehensively investigated using molecular dynamics (MD) simulation. Based on the non-equilibrium molecular dynamics (NEMD) method, the interfacial thermal conductance and the overall thermal conductance of the nanocomposite are taken into consideration simultaneously. It is found that vacancy defects in graphene facilitate the interfacial thermal conductance between graphene and polyethylene. By removing various proportions of carbon atoms in pristine graphene, the density of vacancy defects varies from 0% to 20% and the interfacial thermal conductance increases from 75.6 MW·m−2·K−1 to 85.9 MW·m−2·K−1. The distinct enhancement in the interfacial thermal transport is attributed to the enhanced thermal coupling between graphene and polyethylene. A higher number of broken sp2 bonds in the defective graphene lead to a decrease in the structure rigidity with more low-frequency (< 15 THz) phonons. The improved overlap of vibrational density states between graphene and polyethylene at a low frequency results in better interfacial thermal conductance. Moreover, the increase in the interfacial thermal conductance induced by vacancy defects have a significant effect on the overall thermal conductance (from 40.8 MW·m−2·K−1 to 45.6 MW·m−2·K−1). In addition, when filled with the graphene layer, the local density of polyethylene increases on both sides of the graphene. The concentrated layers provide more aligned molecular arrangement, which result in better thermal conductance in polyethylene. Further, the higher local density of the polymer near the interface provides more atoms for interaction with the graphene, which leads to stronger effective interactions. The relative concentration is insensitive to the density of vacancy defects. The reported results on the thermal transport behavior of graphene-polyethylene composites provide reasonable guidance for using graphene as fillers to tune the thermal conduction of polymeric composites.
Organic solar cells (OSCs) are a promising next-generation photovoltaic technology that can be used to harvest clean and renewable solar energy. OSCs are typically composed of donor:acceptor blends as photo-active materials. Compared to the conventional inorganic silicon solar cells, OSCs are suitable for large-scale production using roll-to-roll technology, promising low-cost and the potential to avoid environmental pollution. The last few years have witnessed the rapid development of OSCs. The power conversion efficiencies (PCEs) of OSCs have surpassed ~14%–16%, benefiting from the design of novel materials, optimization of blend morphology, and deep understanding of the charge generation mechanism. Currently, the most widely used processing solvents for preparing high-efficient OSCs are chlorinated or aromatic solvents including chlorobenzene, dichlorobenzene, and chloroform, which are highly detrimental to the environment and human health, and may not be utilized for future in industry. Thus, replacing these highly toxic solvents with environmentally friendly alternatives called "green solvents" is an important topic in OSC research. Herein, poly[(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1, 2-b:4, 5-b′]dithiophene)-co-(1, 3-di(5-thiophene-2-yl)-5, 7-bis(2-ethylhexyl)benzo[1, 2-c:4, 5-c′]dithiophene-4, 8-dione)] (PBDB-T) was used as a reference material to design and synthesize a novel conjugated polymer (PBDB-DT) by extending the alkyl side chains and enlarging the conjugated side groups. The thermal stability of the polymer donor was examined via thermogravimetric analysis, showing that the polymers exhibit very good stability at > 400 ℃. Importantly, PBDB-DT exhibits good solubility in low-toxic solvent tetrahydrofuran (THF) due to its longer alkyl side chains, and shows a strong aggregation effect in THF due to the larger conjugated side groups. A favorable PCE of 10.2% was achieved for the THF-processed PBDB-DT:IT-M based OSC device. In contrast, PBDB-T has limited solubility in THF. The solar cell device based on PBDB-T:IT-M delivered a moderate PCE of 6.41%. The investigation of blend morphology via atomic force microscope suggested that the PBDB-DT:IT-M has a smooth surface, which is favorable for charge generation and transport. These results demonstrate that molecular optimization is a promising strategy to modulate the solubility and achieve high efficiency for organic photovoltaic materials processed using green solvents.
Transition-metal-catalyzed C―H functionalization reactions, assisted by directing groups (DGs), have become some of the most powerful strategies to form C―C and C―X (X = O, N, S, etc.) bonds. It has brought about a revolution in the synthesis of drugs and natural products, and the method is widely applicable in the fields of material chemistry and pharmaceutical industry. This strategy has mainly focused on regioselective C―H functionalization of amides, esters, carbamates, and enamides with DGs to form C―C and C―X bonds. Since these DGs are relatively stable, they must be removed by other methods when the reaction is completed. Therefore, the use of a traceless DG is one of the important challenges for transition-metal-catalyzed C―H functionalization. Recently, N-phenoxyamide has been attracting significant research attention as a versatile DG. Oxyacetamide (O―NHAc) is one of the most versatile functionalities for directed C―H functionalization cascades, such as the internal oxidation with N―O bond cleavage. The O―NHAc has been reported as a superb DG for redox-neutral C―H activation/annulation cascade reactions to synthesize phenol and complex heterocyclic scaffolds by coupling with alkynes, alkenes, heteroarenes, and diazo compounds. However, for the external oxidation with preservation of the N―O bond, e.g. when a stoichiometric external oxidant is present, N-phenoxyamides could react with aldehydes or α, β-unsaturated aldehydes. In addition, the solvent can control the chemoselectivity. In this minireview, the C―H bond functionalization of N-phenoxyamide is divided into five categories according to the different substrates, viz. alkenes, alkynes, diazo, and other compounds and intramolecular C―H bond activation reactions. Based on experimental and theoretical research results, the reaction mechanism was discussed. In the first part, we summarize the ortho-alkylation, alkenylation, and cyclization of N-phenoxyamide with olefins. In the second part, we present the Rh- and Ir-catalyzed C―H activation or cyclization of N-phenoxyamide with alkanes to synthesize phenol or benzofuran compounds. In the third part, we describe the synthesis of phenolic compounds functionalized by Rh-catalyzed diazo compounds by carbene intermediates and N-phenoxyamides. The forth part summarizes the C―H activation/annulation reaction using aldehydes, heterocyclic aromatic, and sulfur reagents as substrates. The last part of the paper generalizes the intramolecular ortho-hydroxylation and ortho, para-amidation reactions.
Sodium-ion batteries (SIBs) are promising candidates to replace lithium-ion batteries (LIBs) to meet the emergent requirements of various commercial applications. SIBs and LIBs are similar in many aspects, including their reduction potentials, approximate energy densities, and ionic semidiameters. Analogously, safety issues, including liquid leakage, high flammability, and explosiveness limit the usage of SIBs. All-solid-state batteries have the potential to solve the aforementioned problems. However, polycarbonates as promising solid electrolytes have been rarely exploited in all-solid-state SIBs. In addition, organic electrode materials, including non-conjugated redox polymers, carbonyl compounds, organosulfur compounds, and layered compounds, have been intensively investigated as part of various energy storage systems owing to their low cost, environmental friendliness, high energy density, and structural diversity. Nevertheless, the dissolution of small organic compounds in organic-liquid electrolytes has hindered its further applications. Fortunately, the utilization of solid polymer electrolytes combined with organic electrode materials is a promising method to prevent dissolution into the electrolyte and improve the cycling performance of SIBs. Thus, we proposed the utilization of a poly(propylene carbonate) (PPC)-based solid polymer electrolyte and cellulose nonwoven with a 3, 4, 9, 10-perylene-tetracarboxylicacid-dianhydride (PTCDA) cathode in an all-solid-state sodium battery (ASSS). The solid electrolyte significantly enhanced the safety of the SIB and was successfully synthesized via a facile method. The morphology of the as-prepared solid electrolyte was examined by electron scanning microscopy (SEM). Furthermore, the electrochemical performances of the PTCDA/Na battery with organic-liquid and solid electrolytes at room temperature were compared. The SEM results demonstrated that the solid polymer electrolyte and sodium bis(fluorosulfonyl)imide (NaFSI) were evenly distributed inside the pores of the nonwoven cellulose. The ionic conductivity of the composite solid polymer electrolyte (CSPE) at room temperature was 3.01 × 10-5 S·cm-1, suggesting that the CSPE was a promising candidate for commercial applications. In addition, the ASSS showed significantly improved cycling performance at a current density of 50 mAh·g-1 with a high capacity retention of 99.1%, whereas the discharge capacity of the liquid PTCDA/Na battery was only 24.6mAh·g-1 after 50 cycles. This indicated that the cycling performance of the PTCDA cathode in the SIB was largely improved by preventing the dissolution of the PTCDA cathode material in the electrolyte. Electrochemical impedance spectroscopy results demonstrated that the CSPE was compatible with the organic cathode electrode.
Ca2+ and Mg2+ ions are the main divalent cations in living cells and play vital roles in the structure and function of biological membranes. To date, the differences in the effects of these two ions on the Escherichia coli (E. coli) inner membrane at various concentrations remain unknown. Here, the effects of Ca2+ and Mg2+ ions on a mixed lipid bilayer composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) in a 3 : 1 ratio (mol/mol), which mimics the E. coli inner membrane, were quantitatively differentiated at different concentrations by dynamic light scattering (DLS), zeta potential measurements and all-atom molecular dynamics (AA-MD) simulations. The DLS results demonstrated that the POPE/POPG liposomes were homogeneous and monodisperse in solutions with Ca2+ or Mg2+ ion concentrations of 0 and 1 mmol∙L-1. As the Ca2+ or Mg2+ ion concentration was increased to 5-100 mmol∙L-1, lipid aggregation or the fusion of unilamellar liposomes occurred in the ion solutions. The zeta potential measurements showed that both the Ca2+ and Mg2+ ions had overcharging effects on the negatively charged POPE/POPG liposomes. The AA-MD simulation results indicated that the Ca2+ ions irreversibly adsorbed on the membranes when the simulation time was longer than 100 ns, while the Mg2+ ions were observed to dynamically adsorb on and desorb from the membranes at various concentrations. These results are consistent with the DLS and zeta potential experiments. The average numbers of Ca2+ and Mg2+ ions in the first coordination shell of the oxygen atoms of the phosphate, carbonyl and hydroxyl groups of POPE and POPG (i.e., the first coordination numbers) in the pure membrane and membranes containing 5 and 100 mmol∙L-1 ions were calculated from the radial distribution functions. The results indicated that the primary binding site of these two ions on POPE and POPG at the concentrations studied was the negatively charged phosphate group. Thus, these results might explain the overcharging effects of both the Ca2+ and Mg2+ ions on the POPE/POPG liposomes. Moreover, as the Ca2+ concentration increased, the area per lipid of the lipid bilayers decreased, and the membrane thickness increased, while the Mg2+ ions had negligible effects on these membrane parameters. In addition, these ions had different effects on the orientation of the lipid head groups. These simulation results may be used to provide the possible explanations for the differences between Ca2+ and Mg2+ ions in DLS and zeta potential measurements at the atomic level. The experimental results and MD simulations provide insight into various biological processes regulated by divalent cations, such as membrane fusion.
Lithium metal is the most promising anode material for Li (ion) batteries from the viewpoint of energy density because of its high theoretical specific capacity (3860 mAh∙g-1, 2061 mAh∙cm−3) and low reduction potential (−3.04 V vs standard hydrogen electrode (SHE)). Lithium has been used as an anode material for lithium metal batteries since the 1970s. However because of the serious reaction between Li and non-aqueous electrolytes, the large volume expansion during Li plating, and the formation of Li dendrites during cycling, Li batteries with Li metal anodes show very low Coulombic efficiency (CE) and are easily short-circuited. This limits the widespread commercialization of Li metal anodes for Li batteries. Motivated by our previous study on the development of a Li carbon nanotube (Li-CNT) composite anode material, in this study, we prepared a Si-loaded Li carbon nanotube composite (Li-CNT-Si) via a facile molten impregnation method. The introduction of Si nanoparticles increased the Li content of the composite, thus increasing its specific capacity (the specific capacity of the Li-CNT composite increased from 2000 mAh∙g-1 to 2600 mAh∙g-1 with the addition of 10% Si (mass fraction)). Moreover, Si nanoparticles decreased the polarization for Li plating/stripping, resulting in an improved electrochemical performance. The Li-CNT-Si composite showed the merits of the Li-CNT composite with the advantages of limited electrode volume expansion and negligible Li dendrite formation during cycling. Furthermore, the Si nanoparticles filled the pores inside the Li-CNT microspheres, thus preventing the electrolyte from flowing into the microspheres to corrode the Li metal present inside them. Hence, the incorporation of Si nanoparticles improved the CE of the composite anode. When the 10% Si-loaded Li-CNT-Si composite was used as an anode and coupled with a commercial LiFePO4 cathode, the resulting battery showed more than 900 stable cycles in an ether-based electrolyte at a charge/discharge rate of 1C (0.7 mA∙cm-2) corresponding to a CE of 96.7%, which is considerably higher than those of the Li-CNT (90.1%) and Li metal foil (79.3%) anodes obtained under the same conditions. We believe that the Li-CNT-Si composite prepared in this study is a promising anode material for Li secondary batteries having high energy density, particularly for those employing Li-free cathodes, e.g., Li-sulfur and Li-oxygen batteries.
Compressible supercapacitor is a promising flexible energy storage device in view of its excellent capacitive performance, which is recoverable at different compression states. The compressible electrode constitutes the core component that largely determines the performance of a compressible supercapacitor. Commercial polymer sponges are highly compressible materials because most of them are composed of elastic and interconnected polyurethane fibers. However, polymer sponges cannot be directly used as supercapacitor electrodes due to their non-conductive polymer framework. In contrast, carbon sponge (CS) derived from melamine sponge has superior compressible property and exhibits substantially improved conductivity compared to commercial polymer sponge. However, the low specific surface area of CS leads to low specific capacitance, which severely limits its application as compressible supercapacitor electrodes. Currently, pseudocapacitive materials are grown on the conductive CS framework to form hybrid electrodes with improved specific capacitance. Among various pseudocapacitive electrode, iron oxides have attracted considerable attentions due to their natural abundance, high theoretical specific capacitance, and negative working potential. Moreover, the much higher specific capacitance than that of carbon electrodes makes iron oxides one of promising negative candidates for configuring an asymmetric supercapacitor. Herein we report the successful growth of α-Fe2O3 nanosheets on CS by electrodeposition followed by low-temperature thermal annealing. The α-Fe2O3 on CS displays typical nanosheet morphology with mass loading ranging from 3.4 to 6.7 mg·cm-3 that can be facilely controlled by extending the deposition time from 4 to 16 h. The CS-Fe2O3 electrode retains 90% of its geometric height even after manual compression for 100 cycles. Moreover, the CS-Fe2O3 can withstand 60% strain even at Fe2O3 mass loading as high as 6.5 mg·cm-3. The performance of the CS-Fe2O3 electrode at different strains was systematically investigated in 3.0 mol·L-1 KOH aqueous electrolyte by cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) in a three-electrode system. Our results show that the CS-Fe2O3 composite electrode produces lower specific capacitance at lower strain. The EIS characterization and IR drop results indicate that this is due to the larger internal resistance arising from looser contact of electrode with the current collector and longer ion diffusion length. Particularly, the CS-Fe2O3-12 electrode delivers a maximum specific capacitance of 294 F·g-1 at a current density of 1.0 A·g-1, 1.7-times higher than that of CS substrate. Assuming that the specific capacitance of CS-Fe2O3-12 is derived from the double-layer capacitance of CS and the pseudocapacitance of Fe2O3, the capacitance of Fe2O3 nanosheets in the hybrid is calculated to be as high as 421 F·g-1, much higher than most of recently reported results, showing that the sheet-like structure with more exposed active sites and short ion pathways could dramatically improve the utilization efficiency of the electrode for reversible faradaic reactions. More importantly, the CS-Fe2O3-12 electrode retains 87% of its initial capacitance after continuous charge-discharge at 5.0 A·g-1 for 10000 cycles, showing promising application as a stable and compressible supercapacitor electrode.
All-solid-state sodium ion batteries (ASIBs) are important for future large-scale energy storage applications. ASIBs have come to occupy an important position in research on advanced secondary batteries in recent years owing to their advantages of abundance in resources, low cost, long lifetime, and high safety. As the key to the success of ASIBs, solid-state electrolytes such as polymers, oxide ceramics, and sulfide glass-ceramics have always attracted immense interest. Chalcogenide electrolytes for ASIBs have high room-temperature conductivity, high elastic modulus, and can be easily pressed into a mold at room temperature; hence, they are the research focus in ASIBs. This paper summarized recent studies on the structure and properties of chalcogenide electrolytes for ASIBs. These studies demonstrate the relationship between the phase structure and ionic conductivity of sulfide-based electrolytes and selenide-based electrolytes. Besides, arguments that the sodium vacancy in the crystal structure dominates ionic conduction, and creating a sodium vacancy via cation substitution is the principal strategy to increasing ionic conduction, are discussed. Further, the intrinsic chemical stability and interface stability between the electrode and electrolyte are highlighted. Based on the soft and hard acid and base theory, some studies adopted various anion/cation ion substitution strategies to improve the chemical stability of chalcogenide electrolytes in humid air. Particularly, the inconsistency in the electrochemical stability window of a representative chalcogenide electrode, Na3PS4, as measured by a semi-blocking electrode and calculated by first-principles, is compared. Additionally, to develop all-solid-state Na-S and Na-O2 batteries with high capacity, the nonnegligible interface instability of the sulfide electrode against the sodium metal anode and feasible solutions are summarized. Next, the research progress on ASIBs using chalcogenide electrolytes is reviewed. Chalcogenide electrolytes are restricted by the electrochemical stability window and chemical compatibility with electrode materials; hence, they are expected to only be applicable to ASIBs using sulfur, sulfide, and organic matter as the cathode and Na-Sn alloy as the anode. However, these ASIBs have long cycling life (> 500 cycles), illustrating their potential applications in large-scale energy storage power stations. Finally, we comprehensively evaluate the ionic conductivity, stability against humid air, stability of the interface, electrochemical stability window, and ease of preparation of typical chalcogenide electrolytes, including Na3PS4, Na3PSe4, Na3SbS4, Na3SbSe4, Na10SnPS12, and Na11Sn2PS12. Moreover, we highlight the challenges and propose possible solutions toward the development of chalcogenide electrolytes in future. Advanced technologies in fine synthesis, in situ characterization, and surface/interface modification are essential to overcome existing challenges and promote the development of chalcogenide-electrolyte-based ASIBs.
In this study, a novel silicon carbide/platinum/cadmium sulfide (SiC/Pt/CdS) Z-scheme heterojunction nanorod is constructed using a simple chemical reduction-assisted hydrothermal method, in which Pt nanoparticles are anchored at the interface of SiC nanorods and CdS nanoparticles to induce an electron-hole pair transfer along the Z-scheme transport path. Multiple characterization techniques are used to analyze the structure, morphology, and properties of these materials. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results show that the SiC/Pt/CdS materials with good crystal structure are successfully synthesized. Transmission electron microscopy reveals that Pt nanoparticles grow between the interfaces of SiC nanorods and CdS nanoparticles. UV-Vis diffuse reflectance spectroscopy shows that the as-prepared Z-scheme heterojunction samples have a wider light absorption range in comparison with pristine CdS materials. Photoluminescence spectroscopy and the transient photocurrent response further demonstrate that the SiC/Pt/CdS nanorod sample with an optimal molar ratio possesses the highest electron-hole pair separation efficiency. The loading amount of CdS on the surface of SiC/Pt nanorods is effectively adjusted by controlling the molar ratio of SiC and CdS to achieve the optimal performance of the SiC/Pt/CdS nanorod photocatalysts. The optimal H2 evolution capacity is achieved at SiC : CdS = 5 : 1 (molar ratio) and the maximum H2 evolution rate reaches a high value of 122.3 µmol·h−1. In addition, scanning electron microscopy, XRD, and XPS analyses show that the morphology and crystal structure of the SiC/Pt/CdS photocatalyst remain unchanged after three cycles of activity testing, indicating that the SiC/Pt/CdS nanocomposite has a stable structure for H2 evolution under visible light. To prove the Z-scheme transfer mechanism of electron-hole pairs, selective photo-deposition technology is used to simultaneously carry out the photo-reduction deposition of Au nanoparticles and photo-oxidation deposition of Mn3O4 nanoparticles in the photoreaction. The experimental results indicate that during photocatalysis, the electrons in the conduction band of CdS participate mainly in the reduction reaction, and the holes in the valence band of SiC are more likely to undergo the oxidation reaction. The electrons in the conduction band of SiC combine with the holes in the valence band of CdS to form a Z-scheme transport path. Therefore, a possible Z-scheme charge migration path in SiC/Pt/CdS nanorods during photocatalytic H2 production is proposed to explain the enhancement in the activity. This study provides a new strategy for synthesizing a Z-scheme photocatalytic system based on SiC nanorods. Based on the characterization results, it is determined that SiC/Pt/CdS nanocomposites are highly efficient, inexpensive, easy to prepare, and are stable structures for H2 evolution under visible light with outstanding commercial application prospects.
Because of their high energy density and long cycle life, lithium-ion batteries (LIBs) have dominated the portable electronics market for over 20 years. However, with the increasing demand for large-scale energy storage systems for grid applications, the price of Li resources has increased owing to the low abundance of Li in Earth's crust and non-uniform distribution on the planet. Because Na has similar physical and chemical properties as Li and is an abundant natural resource, room-temperature sodium-ion batteries (SIBs) are expected to be among the most promising next-generation large grid energy storage devices. It is known that the cathode, anode, separator and electrolyte materials are the main components of batteries. Among these, Na-containing cathode materials are of critical importance. As a cathode material for SIBs, polyanion-type compounds have become a hot research topic owing to their versatile structural frameworks, high thermal stabilities, high ambient stabilities even in the charging state, small volume changes, tunable operating voltage by tuning the chemical environment of the polyanions, and high operating voltages owing to the inductive effects of the polyanionic groups (PO43−, SO42−, SiO44−, etc.). In particular, for Earth's abundant resources and inherent stability, polyanion-based compounds are suitable for large-scale stationary energy storage. Taking grid balancing into account, batteries with fast charge rates are in demand, which requires cathodes having high rate capability. However, despite the presence of ion diffusion channels in polyanion compounds, the electronic transport channels are blocked owing to the separation of the metal polyhedral and the strong electronegativity of the anions, leading to poor electron conductivity, which largely limits the rate capability of polyanion compounds. Therefore, it is crucial to understand the inherent limitation of the kinetics in terms of the structural aspects and to determine strategies for improving the rate capability. This review discusses the intrinsic reasons for the factors impacting ion diffusion based on the different structures of polyanion-type cathodes. From the perspectives of surface modification and morphology, strategies for enhancing the transport of sodium ions and electrons at the surface and interface are summarized and discussed. Then, from the standpoint of the hierarchical structures of materials to the design of a structural framework, which have been rarely reported, this review proposes schemes that intrinsically enhance the rate capability of polyanion compounds and provides a perspective on developments that can further improve the rate capability of cathode materials. This review provides suggestions for designing and optimizing high-rate polyanion-type and other kinds of cathodes from both academic and practical viewpoints.
Nuclear magnetic resonance (NMR) is an effective and widely adapted technique that can be used for medical diagnosis and chemical analysis. However, its application has been limited by low sensitivity originating from the extremely low polarization of nuclear spins that follow a typical Boltzmann distribution. In principal, it is possible to break this Boltzmann distribution using different physical or chemical mechanisms to generate hyperpolarization and increase NMR sensitivity by several orders of magnitude. The crucial point of hyperpolarization is to transfer the polarization from highly polarized systems to nuclear spins. For example, the unpaired electrons in organic radicals or other systems exhibit much higher polarization than that of nuclear spins (~660 times higher than 1H) under the same magnetic field. The high polarization of electrons at thermal equilibrium can be transferred to nuclear spins via microwave irradiation and hyperfine coupling. This hyperpolarization method is called dynamic nuclear polarization (DNP) and has been successfully and widely applied for the evaluation of the protein structure and the examination of nanomaterial surface chemistry. Electron spins can also be hyperpolarized using circularly polarized light (CPL) or nonpolarized light in some systems, and this polarization can be transferred to nuclear spins as well. These hyperpolarization methods are referred to as optical pumping (OP) and optical nuclear polarization (ONP), respectively. A common application of OP is the production of hyperpolarized noble gases, including hyperpolarized xenon-129, which can be used in magnetic resonance imaging of lungs or evaluation of porous structures. For ONP, the nitrogen-vacancy center in diamond is the most promising system that has demonstrated the ability to track the precession of a single spin. In addition, electrons can be polarized by certain chemical reactions as used in chemically induced dynamic nuclear polarization (CIDNP). CIDNP can be used to study the active sites of proteins and identify low-concentration intermediates that are formed during chemical processes. In addition to electrons, hydrogen molecules with unique spin state, i.e., parahydrogen, can be converted to hyperpolarized NMR signals via hydrogen addition reactions, which is known as parahydrogen induced polarization (PHIP). PHIP was originally used to understand the mechanisms of hydrogenation processes, but has recently become a promising hyperpolarization technique via the protocols of signal amplification by reversible exchange (SABRE). Herein, the basic mechanisms and potential applications of DNP, OP, CIDNP, and PHIP techniques are reviewed. These emerging hyperpolarization techniques have the potential to push the limits of NMR beyond current conceptions.
Two-dimensional transition metal disulfides (TMDs) have recently attracted significant research attention due to their rich physical and chemical properties. Graphene has also been studied intensively due to its high electron mobility of ~200000 cm2·V−1·s−1. Since there is no band gap, it is difficult for a graphene-based device to achieve high current on/off ratio. For TMDs, such as MoS2, MoSe2, WSe2, and WS2, the band gaps of these materials can be adjusted according to the number of layers. Since TMD has the advantage of suppressing source-drain tunneling current in an ultra-short transistor and offering superior immunity to short-channel effects, it is also attractive for use as a channel material in Si complementary metal oxide semiconductor (CMOS) devices larger than 22 nm. Among them, MoS2 in single-layer and multi-layer films have been intensively researched for many years. MoS2-based field effect transistors (FETs) with excellent electrical properties have been reported. WS2 has lower in-plane electronic mass than MoS2, MoSe2, and MoTe2, and therefore has potential for higher carrier mobility or higher output current for WS2-based FETs. Experimental research on WS2 is limited compared to MoS2, and more work is needed to further exploit the full potential of WS2-based FETs. Therefore, the electron-phonon interaction and vibration properties of WS2 used in nano-electronic applications and FETs must be investigated. To this end, mono-layer (1L), few-layer (FL), and bulk WS2 films were prepared using mechanical exfoliation from a WS2 crystal. 3M scotch-tape was used for transferring the WS2 films. Detailed temperature-dependent Raman study on 1L, FL, and bulk WS2 films has been conducted using a 514-nm excitation laser. Raman spectroscopy, as an effective and non-destructive approach for phonon vibration study, has been used to evaluate TMDs. The Raman spectra reveal much useful information on the test sample in terms of peak position and spectral shape change. With the film thickness increasing to bulk, the A1g(Γ) and E2g1(Γ) modes show blue-shift and red-shift, respectively, with respect to 1L WS2. Moreover, when the dominant Raman vibration modes swaps between E2g1(Γ) and A1g(Γ), the "cross-over" temperature was identified for 1L, FL, and bulk WS2 films. WS2 shows smaller frequency change Δ between the E2g1(Γ) and A1g(Γ) modes than MoS2, with varying film thickness. The temperature coefficient of the Raman peak position was one magnitude lower for WS2 than MoS2, implying that WS2 has better thermal stability than MoS2. The results of this systematic study provide a physical guidance for WS2-based device design.
Lithium-ion batteries have been widely used in portable electronic devices and electric vehicles because of their high energy density and long cycle life. Sodium-ion batteries have broad application prospects in the areas of large-scale electrochemical energy storage systems and low-speed electric vehicles because of their abundant raw materials, low resource cost, safety, and environmental friendliness. However, the development of sodium-ion batteries has been hindered by the low reversibility, sluggish ion diffusion, and large volume variations of the host materials. Suitable electrode materials with decent electrochemical performance must be primarily explored for the successful use of sodium-ion batteries. Since the electrochemical potential and specific capacities of cathode materials have a major impact on the energy densities of sodium-ion batteries, the development of cathode materials is critical. To date, various Na-insertable frameworks have been proposed, and some cathode materials have been reported to deliver reversible capacities approaching their theoretical values. Among them, transition metal oxides show a high reversible capacity and high working potential, but most of them still possess problems such as irreversible phase transition, air instability, and insufficient battery performance. Another type is the Prussian blue analogs. These materials exhibit a favorable operating voltage, cycling stability, and rate capability; however, the main obstacles to their practical application are the control of lattice defects, thermal instability, and low tap density. Polyanionic phosphates are the most promising cathode materials for sodium-ion batteries and have great research value and application prospects because of their stable framework structure, suitable operating voltage, and fast ion diffusion channels. However, their inherent defects, such as poor electronic conductivity and low theoretical energy density, considerably limit their practical applications. Researchers have conducted modification studies through bulk structure adjustment and micro-nano structural control with the goal of improving the performance of phosphate cathode materials and promoting the research and development of sodium-ion energy storage systems. This study reviews the recent advances in phosphate cathode materials for sodium-ion batteries, including orthophosphates, pyrophosphates, fluorophosphates, and mixed phosphate compounds. In this study, the intrinsic relationships among material composition, structure, and electrochemical properties are identified through analyses of the crystal structures, sodium storage mechanisms, and modification strategies of phosphate materials, thereby providing a reference for the continuous modification of polyanion phosphate cathode materials and exploration of high-voltage phosphate cathode materials. Some directions for future research and possible strategies for building advanced sodium-ion batteries are also proposed.
Methane is a promising energy source with vast reserves, and is considered one of the promising alternatives to nonrenewable petroleum resources because it can be converted into valuable hydrocarbon feedstocks and hydrogen through appropriate reactions. Recently, the conversion of CH4 into other high-value-added products has received increasing attention because of their sustainability for energy and the environment. However, methane has a tetrahedral geometry with four equivalent C―H bonds due to the sp3 hybridization of the central carbon atom, with a C―H bond length of 0.1087 nm and an H-C―H bond angle of 109.5°. The absence of a dipole moment and the small polarizability (2.84 × 10−40 C2·m2·J−1) imply that methane requires a high local electric field for polarization and for nucleophilic or electrophilic attack. Nevertheless, it is believed that an effective method to activate CH4 would be available, so that not only methanol, formaldehyde, and ethylene but also other industrially valuable raw materials can be obtained. On the other hand, the conversion of this combustible gas into the corresponding liquid fossil fuel proceeds via secondary chemical conversion, and it can greatly reduce transportation costs. From the economic viewpoint, this can still provide considerable benefits. Homogeneous catalysts have been reported to catalyze methane, but most of them operate at high pressures (2–7 MPa), or in strongly acidic media and at high temperatures (up to 500 K). Heterogeneous catalysts reported in the literature are also active only at high temperatures. Therefore, finding an efficient method to active methane has become a hot research topic. Photocatalysis technology is recognized as the optimal solution for the conversion of CH4 since solar energy is by far the largest exploitable resource of energy. In the past years, much effort has been undertaken for the conversion of CH4 under light at low temperature. In this regard, several photocatalysts, including silica-alumina-titania, silica-supported oxides, and ceria- and zeolite-based materials, have been developed. In photocatalytic methane conversion, the C―H bond can be selectively activated by adjusting the wavelength and intensity of the incident light and the oxidation capacity of the photocatalysts, thereby avoiding the formation of byproducts. This review summarizes a series of photocatalytic direct methane conversion systems developed in recent years, including methane oxidation and coupling processes. The effects of the catalyst composition and structure, oxidant, and electron transfer on the activation of the C―H bond of methane are detailed. Finally, future perspectives and challenges for the photocatalytic conversion of methane are discussed.
In order to meet high-performance propulsion system requirements for aerospace technology and severe future restrictions on hydrazine use, research on non-toxic, high-performance, and low-cost propulsion technology is urgently needed. The N2O-C2 hydrocarbon monopropellant NOFBX (Nitrous Oxide Fuel Blend) provides significant benefits for meeting these criteria and has become a focus of increased research in recent years. In this study, a chemical kinetic model for NOFBX combustion that integrates the reduced C2 sub-mechanism, the N2O sub-mechanism in the literature, and the N2O/CH species reaction mechanism has been developed. The present mechanism consists of 52 species and 325 elementary reactions. For better predictions of ignition and combustion characteristics, the kinetic parameters of the sensitive reactions with comparatively high rate constant uncertainties have been revised. The present model has been validated against published experimental data, including flow reactor results on N2O/H2O/N2 mixture decomposition, shock tube ignition delay times on N2O/C2 hydrocarbons diluted with N2 or Ar mixtures, heat flux of flat flame laminar flame speeds on N2O/C2H2 diluted with N2 mixtures, and Bunsen flame laminar flame speeds on N2O/C2H4 diluted with N2 mixtures. Additionally, this study compares the new model to other published small hydrocarbon fuel kinetic models with a NOx sub-mechanism. The experimental validations show that the present model accurately captures the nitrous oxide decomposition process and precisely predicts N2O, O2, NO, and NO2 vital species concentration distributions. For all N2O-C2 hydrocarbon fuel systems (ethane-, ethylene-, and acetylene-nitrous oxide), the ignition delay times predicted by the present model are in good agreement with the experimental data. Furthermore, at a wider range of initial temperatures (1100-1700 K), initial pressures (0.1-1.6 MPa), and equivalence ratios (0.5-2.0) for the ignition delay times of ethylene-nitrous oxide, the present model exhibits improved predictions of experimental data. For the laminar flame speeds of N2O-C2H2 and N2O-C2H4 mixtures, the present model generally exhibits satisfactory predictions of the experimental data over the whole range of equivalence ratios (0.6-2.0). However, at initial pressure 0.1 MPa and equivalence ratios of 1.0-1.6 for N2O-C2H4 laminar flame speeds, the present model slightly underestimates experimental data. Considering the much higher uncertainty of the measured laminar flame speeds by the Bunsen flame method, this discrepancy is acceptable. Due to the small scale, full experimental validations and good applicability, the present model can be used to further research on multi-dimensional combustion simulation in NOFBX engine combustors.
In the past few decades, new energy industries have developed rapidly due to the threat of the depletion of non-renewable resources. Among them, the lithium-ion battery has attracted significant attention of various researchers. However, lithium-ion batteries are limited by the uneven distribution of lithium resources and high cost. Sodium, which is in the same periodic group as Lithium, can help alleviate the problems related to the limited development of lithium ion batteries owing to the shortage of lithium resources. Sodium ion batteries are cheap, with varying choice of electrolytes, and have relatively stable electrochemical performances. However, the radius of a sodium ion is larger than that of a lithium ion, leading to slow ion transportation as well as changes in the volume of the host material during the charging and discharging processes. Therefore, compared with existing lithium ion batteries, sodium ion battery anode materials are very limited. Moreover, most sodium ion battery electrode materials have low specific capacities and poor cycle retention rates. Among these, ternary metal oxides, which have two different cations and can reversibly react with sodium ions are promising high-capacity anode materials for sodium ion batteries. In this study, Ti2Nb2O9 nanosheets are obtained by ion-exchange and chemical-delaminate methods. The carbon-coated Ti2Nb2O9 nanosheets are obtained after hydrothermal coating with sucrose and calcination. From the thermogravimetric analysis (TG) curve, the carbon content in the composites is calculated to be approximately 8.0%. Owing to the rich reactive sites and a short ion transport pathway, the Ti2Nb2O9/C electrode delivers a high reversible capacity of 265.2 mAh·g-1 at a current density of 50 mA·g-1. Even at a high current density of 500 mA·g-1, the electrode exhibits an excellent electrochemical performance with a reversible capacity of 160.9 mAh·g-1 after 200 cycles (capacity retention of 75.3%). Additionally, the Ti2Nb2O9/C nanosheets exhibit high reversible capacities of 251.3, 224.6, 197.4, 176.3, and 156.5 mAh·g-1 at the current densities of 100, 200, 500, 1000, and 2000 mA·g-1, respectively. It is demonstrated through the use X-ray photoelectron spectroscopy (XPS) that the following process involving the transfer of four electrons occurs: Ti4+/Ti3+, Nb5+/Nb4+, during the charging and discharging process of the Ti2Nb2O9 electrode in the voltage range of 0.01–3.0 V. The theoretical specific capacity of Ti2Nb2O9 in this process is calculated to be 252 mAh·g-1, corresponding to the electrochemical data. Overall, this study demonstrates that the Ti2Nb2O9/C anode nanosheets have an excellent charge-discharge performance, cycle stability, and rate performance in sodium ion batteries, thereby providing a feasible choice for sodium ion battery anode materials.
The fabrication of compact, continuous, and large-scale metal organic framework (MOF) membranes with high permeability and H2/CO2 selectivity remains challenging because of the wake interaction between the MOF membrane and the substrate. In addition, substrates with smooth and plain surfaces and suitable pore size are required to prepare high-quality MOF membranes because it is difficult to obtain dense and continuous MOF membranes on a substrate with large pores and rough surfaces. To overcome these challenges, numerous MOF membrane growth methods have emerged, including in situ (direct) growth, secondary (seeded) growth, and layer-by-layer growth methods as well as electrostatic spinning and the chemical modification of the substrate. Among these methods, usage of substrates suitable for surface-functionalization is a promising technique. Herein, Al2O3 was selected as the substrate and was coated with PIM-1 (one polymer of intrinsic microporosity), followed by carboxylation of PIM-1 to furnish a large number of carboxyl groups on the surface. In situ growth of the MOF membrane using the interactions between the carboxyl group and the metal yielded two types of compact, continuous, and large-scale polymer-supported MOF membranes (PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1). Furthermore, the fabricated polymer-supported MOF membrane structures were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Gas separation experiments were performed to explore the gas permeability and selectivity of the prepared MOF membranes. The XRD characterization confirmed the pure phase and high crystallinity of the MOF membranes. The SEM images showed that the MOF membranes were compact and continuous with a tight combination between the MOF crystal membrane and the substrate. Gas separation measurements showed that both MOF membranes exhibited high H2 permeability and selectivity for H2/CO2. For the PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1 membranes, the 1 : 1 binary mixtures gas separation factors of H2/CO2 calculated as the gas molar ratios in the permeate and retentate side 7.32 and 9.69, respectively, at room temperature and atmospheric pressure. The H2/CO2 mixture separation factors of the two MOF membranes exceeded the corresponding Knudsen constants (4.7), with H2 permeances higher than 3.16 × 10-6 and 1.14 × 10-6 mol·m-2·s-1·Pa-1, respectively. The ideal separation factors of H2/CO2 of both MOF membranes calculated as the ratio of single gas permeances were 7.70 and 12.04, respectively, with the respective H2 permeances of up to 3.73 × 10-6 and 3.86 × 10-6 mol·m-2·s-1·Pa-1. Because of their outstanding characteristics, these novel MOF membranes can be widely used in the fields of H2 purification and separation.
Multilayer phosphorescent organic lighting-emitting diodes (PHOLEDs) with complicated device configurations have greatly increased the complexity of manufacturing and the fabrication cost. Therefore, there is strong incentive to develop simplified OLEDs, such as a single-layer device that has the structure of anode/hole injection layer (HIL)/emissive layer/electron injection layer/cathode. However, because of the absence of a carrier transport layer, the single-layer device suffers from severe charge injection difficulties and unbalanced carrier transport. Hence, the performances of single-layer devices reported so far have not been satisfactory. It has been proved that the modification of the electrode/organic interface could influence carrier injection to improve the device performance in multilayer PHOLEDs. Modification of the electrode/organic interface is more essential for achieving high-performance single-layer OLEDs. In this work, efficient green phosphorescent single-layer OLEDs based on the structure of indium tin oxide (ITO)/C60 (1.2 nm):MoO3 (0.4 nm)/1, 3, 5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi):fac-tris(2-phenylpyridine)iridium [Ir(ppy)3]/LiF (0.7 nm)/Al (120 nm) were fabricated. C60, MoO3, and C60:MoO3 were applied as the HILs, respectively, for comparison. The layer of TPBi played a dual role of host and electron-transporting material within the emission layer. Thus, the properties of the HILs play an important role in the adjustment of electron/hole injection to attain transport balance of the charge carriers in single-layer OLEDs with electron-transporting hosts. It is found that appropriate adjustment of the HIL is a key factor to achieve high-efficiency single-layer OLEDs. The large affinity of MoO3 (6.37 eV), inducing electron transfer from the highest occupied molecular orbital of C60 to MoO3, results in the formation of C60 cations and induces the decrease of the valence from Mo+6 to Mo+5; therefore, C60:MoO3 can adjust the hole injection properties well. Finally, a single-layer OLED with a maximum current efficiency of 35.88 cd∙A−1 was achieved. Compared with devices with MoO3 (28.99 cd∙A−1) or C60 (10.46 cd∙A−1) as HILs, the device performance was improved by 24% and 243%, respectively. Overall, a novel and effective method of using different mixed ratios of C60 and MoO3 as the HIL to realize effective charge carrier regulation is proposed, and it is of great significance for fabricating high-performance single-layer OLEDs.
Increasing climate change and environmental pollution caused by the excessive use of fossil fuels have prompted intensive research into clean and efficient renewable sources as a substitute for traditional fossil fuels. A very promising approach is to mimic the water splitting process that occurs in plants during photosynthesis, in order to convert solar energy into chemical energy. A successful water splitting reaction, which comprises two half reactions (water oxidation and the reduction of protons), can generate H2 and O2 from water. Hydrogen is a promising renewable energy carrier because of its clean combustion and high calorific value. Light-driven water splitting is considered to be a feasible way to transform water and solar energy into hydrogen energy. However, water oxidation is considered to be the bottleneck process of water splitting because it advances in a thermodynamically uphill manner with the involvement of 4e− and 4H+. Inspired by the nature of Mn4CaO5 in photosystem Ⅱ (PS Ⅱ), the comprehensive understanding of its key features for use in active molecular water oxidation catalysts (WOCs) remains challenging. Extensive effort has been devoted to researching and manufacturing the structure and biomimicking the catalytic activity of Mn4CaO5 clusters that contain the Mn3CaO4 cubane structure, for the construction of low-cost and robust WOCs. WOCs can be divided into heterogeneous and homogeneous catalysts. Although heterogeneous WOCs are convenient for recycling and are easily prepared on a large scale, homogeneous WOCs, especially complexes based on organic ligands or polyoxometalates (POMs), have more advantages owing to their catalytic efficiency, structural modifications, and mechanistic understanding. Thus, recently, some molecules with an M4O4 (M = transition metals, mainly Mn, Co, Ni, and Cu) cubic structure have been reportedly used as photocatalytic WOCs. In this review, we present an overview of the most important and recent advances based on M4O4 cubic WOCs that contain first-row transition metal cubanes for visible light-driven water oxidation. Our main focus is on the structure of cubane catalysts, including metal complexes, POMs, and a system containing BiVO4 or polymeric carbon nitride (PCN) as a photosensitizer, and cubic complexes as WOCs. Results have shown that the activity and stability of the catalyst can be tuned by the ligand stability, metal center, coordination environment, and other factors. This review will be helpful for designing new cubane catalysts for photocatalytic water oxidation that are highly efficient and stable.
Thermal analysis (TA) is a technology that can be applied to evaluate the relationship between the physical properties of substances and temperature changes under programmed temperature control. It has been widely used in many fields and is particularly useful for determining the thermal stability and service life of polymers and other materials, the stability of drugs, and the danger of flammable and explosive materials. Simultaneously, the mechanism of dehydration, decomposition, and degradation of inorganic materials or dissociation of complexes can be studied and the decomposition rates of environmental pollutants can be estimated. Recently, TA kinetics has become the most extensively studied topic in TA research. The main purpose of kinetic analysis is to obtain the three kinetic triplets of a reaction process, namely, activation energy Ea, pre-exponential factors A, and and mechanism function f(a). For a solid-state reaction, many mathematical models and corresponding data processing methods can be used for the study of TA kinetics. These methods can be classified as either isothermal or non-isothermal methods and further divided into integral and differential methods in the form of the kinetic equation. These equations can be divided into a single scanning rate and multiple scanning rate methods (isoconversion method) by the operation method. The isoconversion method can calculate activation energies without the mechanism function, and the complexity of the reaction can be determined by the change in activation energy as a function of conversion rate. Therefore, the International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommends the isoconversion method for processing TA data. Because of the limitation of traditional isoconversion methods, novel isoconversion methods have been proposed over the past 10 years. The relationship among the existing dynamic analysis methods must be complementary, instead of competitive, because the reliability of the analysis results can be improved only through complementarity. Further efforts to popularize modern integral and differential methods with equal conversion rates are essential. Herein, the progress in isoconversion method development is briefly introduced. A novel kinetic equation and seven new isoconversion methods are reviewed, and the characteristics and limitations of these methods are discussed. In addition, the development trends and prospects of TA kinetics research methods are highlighted. We suggest that the Arrhenius formula should be modified on the basis of the relationship between the rate constant and temperature. The rate equation that is more suitable for non-isothermal and heterogeneous reactions should be used. The mechanism of multi-step solid-state reactions should be studied in depth, and unified standards must be adopted for the study of thermal decomposition kinetics. This represents imminent and important progress in the study of TA kinetics.
Sodium has the advantages of being an abundant resource and having a low cost; thus, sodium ion batteries are considered as one of the best candidates for replacing lithium ion batteries in the future. However, the radius of the sodium ion is larger than that of the lithium ion, and the de-intercalation of the sodium ion will seriously damage the crystal structure of most electrode materials during the charging and discharging process, which considerably limits its charge-discharge specific capacity, cycle performance, and rate performance. However, finding appropriate electrode materials is one of the difficulties in fabricating high-performance sodium ion batteries. Among the many candidate materials, vanadate materials can improve the stability of material structures by introducing cations to increase the coordination numbers of vanadium, thus improving the electrochemical performance of sodium ion batteries. In this paper, an in situ phase separation method to fabricate V2O5/Fe2V4O13 nanocomposite materials is reported. First, we synthesized hydrated crystalline Fe5V15O39(OH)9·9H2O nanomaterials using a water bath heating method; then, we in situ constructed two-phase nanocomposite V2O5 and Fe2V4O13 from the single phase by further high-temperature treatment. The morphology, composition, and structure of the electrode materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy(FTIR), as well as other methods. The V2O5/Fe2V4O13 nanocomposite materials were found to have a more stable structure, higher initial discharge capacity (342 mAh·g-1 at a current density of 0.1 A·g-1), longer cycle life, and better rate performance than V2O5 nanowires. Therefore, this research on V2O5/Fe2V4O13 nanocomposite materials has broadened ability to develop new high-performance anode materials for sodium ion batteries.
Propylene is widely used as a raw material for producing polypropylene, acrylonitrile, propylene oxide, etc. Typical manufacturing processes for propylene (steam cracking and FCC process) are over-reliant on petroleum resources and cannot meet the rapidly growing global demands. New routes for producing propylene from non-oil resources, particularly methanol-to-propylene (MTP) technology, have attracted increasingly more attention, where a fixed-bed reactor is used and ZSM-5 zeolite is the best alternative catalyst. However, structural optimization of ZSM-5 to enhance the lifetime and propylene selectivity and a deep understanding of the mechanism of the MTP reaction are still considerable challenges. For the conventional ZSM-5 zeolite, carbon deposition preferentially occurs near the outer surface of the zeolite particles because of the high acid density on the external surface, which accelerates the deactivation by blocking the outer pore openings, especially in a long-term MTP reaction. Large amounts of external strong acids also promote secondary reactions, such as hydrogen transfer reactions, resulting in a decrease in propylene selectivity. To study the effects of strong and weak acid distributions of ZSM-5 zeolite on the MTP reaction, two series of boron-modified ZSM-5 zeolites were designed: B-Al-ZSM-5 zeolites by one-step synthesis and Al-ZSM-5@B-ZSM-5 core-shell zeolites by two-step synthesis. These were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) mapping, N2 physical adsorption-desorption, temperature-programmed desorption of ammonia (NH3-TPD) and 1, 3, 5-triisopropylbenzene (TIPB) cracking, and B1-Al-ZSM-5 and Al@B1-ZSM-5, B2-Al-ZSM-5 and Al@B2-ZSM-5, and B3-Al-ZSM-5 and Al@B3-ZSM-5 samples in the two series were found to have similar texture properties, acid amounts and acid strengths, but different B and Al elemental distributions and acid distributions. We used these two sets of samples to compare the effect of different strong and weak acid distributions—a uniform distribution and a gradient distribution of strong and weak acids on the performance of the MTP reaction. The results showed that samples with a uniform distribution of strong and weak acids have higher propylene selectivity due to lower strong and weak acid densities, whereas samples with a gradient acid distribution have a longer catalytic lifetime in the MTP reaction due to the absence of strong acid density and higher weak acid density on the outer surface. The different acid distributions lead to two different carbon deposition modes. Carbon deposition of the sample with the uniform acid distribution preferentially formed on the outer surface, resulting in rapid deactivation by blocking external micropores and leaving the internal active centers not fully utilized. However, for the sample with the gradient acid distribution, the carbon-blocking rate of the external surface considerably decreased, which increased the time that the reactant molecules had to enter the internal micropores. Thus, the utilization rate of the active centers and the catalytic lifetime of the Al-ZSM-5@B-ZSM-5 core-shell sample considerably increased.
With the increasing energy demands for electronic equipment, numerous studies have been conducted to achieve higher energy conversion and develop storage devices such as metal-air batteries, water splitting devices, and fuel cells. All these devices are related to the oxygen evolution reaction (OER) and/or oxygen reduction reaction (ORR). Currently, platinum group metals (PGMs) or their oxides are the most active electrocatalysts for OER and ORR. However, the high cost and scarcity of these noble metals hinder their widespread application. Therefore, the development of a low-cost electrocatalyst that exhibits catalytic performance comparable to or better than that of PGMs is essential.
Metal-organic frameworks (MOFs) are a new class of porous materials constructed from metal ions and organic linkers. MOF materials have diverse metal centers. In addition, organic ligands containing various heteroatoms can change the microenvironment of these metal centers. Moreover, the size, morphology, and porosity of MOF materials can be precisely tuned. These advantages of MOF are beneficial for electrocatalytic reactions. However, MOF is generally considered to be a poor electrocatalyst and is rarely used in the field of electrocatalysis because of its low electrical conductivity. To increase the electrical conductivity of MOF, high-temperature calcination or hybridization with conductive supports is necessary. However, high-temperature calcination may sacrifice the intrinsic molecular metal active sites of MOFs, whereas hybridization with conductive supports may block their inherent micropores. The development of MOF materials with high electrical conductivity is vital for electrocatalysis.
Herein, we report a two-dimensional conductive MOF based on copper foam growth (Cu3HITP2/CF, where HITP = 2, 3, 6, 7, 10, 11-hexaaminotriphenylene hexahydrochloride, CF = copper foam), which has high electrical conductivity and excellent catalytic stability and can be used as a bi-functional electrocatalyst in OER and ORR. In addition, this catalyst does not require heat treatment or the addition of a conductive agent. We first electroplated needle-shaped Cu(OH)2 nanowires onto the surface of a blank copper foam, and then immersed it in a solution of HITP to convert it into Cu3HITP2 at 65 ℃. To confirm its physicochemical properties, the as-synthesized Cu3HITP2/CF was characterized and analyzed by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The morphology was characterized by scanning and transmission electron microscopy. The as-synthesized Cu3HITP2/CF maintained a two-dimensional needle-like morphology during the reaction and could be stably operated in an alkaline solution. The overpotential at 10 mA·cm-2 in the OER was only 1.53 V, and the current density did not decrease significantly after 24 h. The Faraday efficiency was as high as 96.84%, and only 1.57% of the by-product H2O2 was produced. In addition, during the ORR, the half-wave potential of Cu3HITP2/CF reached 0.75 V and its activity did not decrease significantly after 2000 cycles of voltammetric scanning. Moreover, its electron transfer number was 3.85, with 5.7% H2O2 generation. Comparative experiments with powder Cu3HITP2 showed that Cu3HITP2 grown on copper foam had a larger electrochemical specific surface area and exhibited superior OER and ORR properties, which was due to its two-dimensional needle-like morphology. In general, this study not only provides a method for in-situ growth of MOF materials on copper foam but also provides new ideas for developing two-dimensional conductive MOF materials in the field of electrocatalysis.
Methane activation by transition metal species has been extensively investigated over the past few decades. It is observed that ground-state monocations of bare 3d transition metals are inert toward CH4 at room temperature because of unfavorable thermodynamics. In contrast, many mono-ligated 3d transition metal cations, such as MO+ (M = Mn, Fe, Co, Cu, Zn), MH+ (M = Fe, Co), and NiX+ (X = H, CH3, F), as well as several bis-ligated 3d transition metal cations including OCrO+, Ni(H)(OH)+, and Fe(O)(OH)+ activate the C―H bond of methane under thermal collision conditions because of the pronounced ligand effects. In most of the above-mentioned examples, the 3d metal atoms are observed to cooperate with the attached ligands to activate the C―H bond. Compared to the extensive studies on active species comprising of middle and late 3d transition metals, the knowledge about the reactivity of early 3d transition metal species toward methane and the related C―H activation mechanisms are still very limited. Only two early 3d transition metal species HMO+ (M = Ti and V) are discovered so far to activate the C―H bond of methane via participation of their metal atoms. In this study, by performing mass spectrometric experiments and density functional theory calculations, we have identified that the diatomic vanadium boride cation (VB+) can activate methane to produce a dihydrogen molecule and carbon-boron species under thermal collision conditions. The strong electrostatic interaction makes the reaction preferentially proceed the V side. To generate experimentally observed product ions, a two-state reactivity scenario involving spin conversion from high-spin sextet to low-spin quartet is necessary at the entrance of the reaction. This result is consistent with the reported reactions of 3d transition metal species with CH4, in which the C―H bond cleavage generally occurs in the low-spin states, even if the ground states of the related active species are in the high-spin states. For VB+ + CH4, the insertion of the synergetic V―B unit (rather than a single V or B atom) into the H3C―H bond causes the initial C―H bond activation driven by the strong bond strengths of V―CH3 and B―H. The mechanisms of methane activation by VB+ discussed in this study may provide useful guidance to the future studies on methane activation by early transition metal systems.
Functionalized graphene has attracted significant interest over the past decade due to its unique physical properties and potential applications. Graphene oxide (GO), a readily scaled-up product, is a basic material for further functionalization. Using reductive processes, highly conductive reduced graphene oxide (RGO) can be obtained, which exhibits electrical and optical properties analogous to those of graphene. Moreover, due to the presence of oxygen-containing functional groups, its chemical reactivity and electronic properties can be easily tailored by chemical doping with nitrogen. However, developing strategies for doping graphene is challenging and the fundamental roles of the doping atom configuration and its environment on the resulting properties of graphene remain poorly understood. These properties are important for electrical and catalytic applications of graphene. Thus, synthesizing specific configurations of nitrogen-doped graphene and consequently investigating the electrical and catalytic properties of the product is imperative. Herein, we demonstrate an approach that allows for successful production of nitrogen-functionalized RGO using Schiff base condensation between the amino groups in an o-aryl diamine compound and the carbonyl groups in GO. Three typical nitrogen-containing species including o-phenylenediamine (OPD), 2, 3-diaminopyridine (23DAP), and bis(trifluoromethyl)-1, 2-diaminobenzene (BTFMDAB) were used for functionalizing the GO samples, and the corresponding RGO derivatives (OPD-RGO, 23DAP-RGO, and BTF-RGO) were obtained by thermal annealing. Pyrazine nitrogen was successfully introduced into graphitic framework, as confirmed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, thermal gravimetric analysis (TGA), Raman, and X-ray photoelectron spectroscopy (XPS). Field-effect transistors (FETs) based on the BTF-RGO exhibited hole-dominated ambipolar field-effect behavior with a Dirac point at a 9 V gate voltage and hole mobilities up to 2.5 times that of RGO. The weak p-type doping effect originated from the strongly electron-withdrawing trifluoromethyl groups. By studying the OPD-RGO and 23DAP-RGO-based FETs, containing pyrazine nitrogen and mixed pyrazine/pyridine nitrogen, respectively, we found that pyrazine nitrogen provided weak n-type doping effects, while pyridine nitrogen exhibited weak p-type doping effects due to its electron-withdrawing ability. Enhanced p-type doping effect was accompanied by the introduction of groups with stronger electron-withdrawing ability into the graphitic framework. Impressively, pyridine nitrogen in the pyrazine nitrogen-doped RGO yielded a weak p-type doped graphene due to the electron-withdrawing effect of the pyridine nitrogen. Nitrogen-doped graphene can be finely tuned from weak n-type to weak p-type doping by adjusting the electron-withdrawing ability of o-aryl diamine compounds. This study demonstrates the effect of nitrogen configuration and its surrounding environment on the electrical properties of RGOs, providing additional possible applications.
Solar energy, which is clean, affordable and reliable, can help alleviate the current environmental pollution and energy crisis efficiently. In the past few decades, great progress has been made in harvesting and converting solar energy into chemical energy. Among various technologies, plasmon-induced photoelectrochemistry has been proposed as a promising alternative for solar energy conversion. The hot electrons generated from plasmon excitation and transfer from metal nanostructures to semiconductors is a potential new paradigm for solar energy conversion. However, the ultrafast decay of the hot carriers is unfavorable for the improvement of photocatalytic efficiency. Therefore, finding more efficient photocatalysts, with enhanced light absorption and a longer carrier lifetime, is of paramount importance for improving the conversion efficiency of solar energy, but their fabrication is challenging. In this work, a plasmonic metal/semiconductor heterostructure based on Ag nanoparticles embedded in two-dimensional (2D) amorphous sub-stoichiometric tungsten trioxide (a-WO3−x), followed by annealing, was successfully fabricated. Firstly, the peculiar nanostructure of 2D a-WO3−x was successfully constructed from WS2 nanosheets with supercritical CO2 (SC CO2) at 200 ℃. Secondly, the Ag/a-WO3−x heterostructure was synthesized using an in situ reduction method. Finally, the obtained 2D heterostructure of Ag/WO3−x was annealed at 400 ℃ in N2 to further improve its stability and conductivity. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure, morphology, and composition of the material, respectively. UV-Vis spectra were also measured to evaluate light adsorption. Characterization results show that the amorphous structure can effectively anchor metal nanoparticles, and the metal nanoparticles are uniformly dispersed in the amorphous region and have a small size. The as-prepared nanocomposites showed efficient photoelectrochemical (PEC) water splitting when serving as photoelectrode materials, and efficient PEC activity towards photo-oxidation degradation currents under excitation of Ag localized surface plasmon resonance (LSPR). The photocurrent response of the Ag/WO3−x heterostructure was approximately five times greater than that of a-WO3−x. Moreover, the PEC degradation efficiency of Ag/WO3−x reached 96.7% for MO under Vis light illumination (after reaction for 120 min), while the PEC degradation efficiency of WO3−x was only 63.6%. The high PEC performance of the composite photoanode can be ascribed to the local surface plasmon resonance (LSPR) effect of the Ag nanoparticles, which can enhance the light absorption and hot electron transformation. Moreover, the construction of local crystalline-amorphous interfaces can further promote the separation efficiency of the photogenerated electron-hole pairs, and thus increase conductivity. This work provides a positive strategy for the fabrication of advanced photocatalysts, and a new perspective on understanding of the synergistic effects of structural and electronic regulations.
In recent years, photocatalytic degradation of organic pollutants has attracted considerable attention because of its potential application for solving environmental problems. Among various semiconductor photocatalysts, TiO2 is considered a promising candidate due to its excellent structural stability. Many researchers have focused on improving the visible-light catalytic efficiency of TiO2, because the large band gap of TiO2 limits its utilization of visible light energy. Recently, it has been proved that intrinsic defects like oxygen vacancies in TiO2 can trigger the visible light activity. TiO2 hollow microspheres with large surface areas have shown high photocatalytic efficiencies in the degradation of organic pollutants. To date, the photocatalytic performance of TiO2-x hollow microspheres has not been investigated. The kinetics of photocatalytic degradation of organic dyes is usually depicted by the pseudo-first-order kinetic equation. However, a few studies have demonstrated the impact of light absorption by the dye itself on photocatalytic performance in terms of the rate equation. In this study, defective TiO2-x hollow microspheres were prepared by the hydrogen reduction process to effectively promote photocatalytic activity under visible light irradiation. The structure and properties were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), electron spin-resonance (ESR), Raman spectrometry, ultraviolet-visible diffuse-reflectance spectroscopy (UV-Vis DRS), and electrochemical tests. The photocatalytic performance was evaluated based on the photocatalytic degradation of methylene blue (MB) solution under visible light irradiation. The mechanism underlying the enhancement of photocatalytic activity was also discussed. The results show that the visible-light photocatalytic activity of TiO2-x, and TiO2-x hollow microsphere benefit from the presence of oxygen vacancies on the surface. The photocatalytic activity of TiO2-x hollow microspheres is better than that of TiO2-x, attributed to the formation of hollow structures with higher specific surface areas. The mechanism of MB degradation occurring on the TiO2-x hollow microsphere surface was also investigated. The results show that the MB molecules are photodegraded by the photogenerated hole (h+), reactive superoxide radical (•O2-), and hydroxyl radicals (•OH), and that the •OH radicals, produced only by photogenerated holes, play an essential role in the degradation of MB. Based on the discussion of the effect of initial concentration of MB on the degradation process, a new kinetic model was proposed for the photocatalytic degradation of dye, considering the effect of visible light absorbed by MB molecules, because the data estimated by pseudo-first-order kinetic equation do not fit well with the experimental data. The Runge-Kutta method was used to obtain the numerical solution of the kinetic model. The results show that the kinetic model proposed for photocatalytic dye degradation gives a more realistic description of the photocatalytic degradation of MB because the calculated results fit better with the experimental data. The rate constant (kapp) of the pseudo-first-order kinetic equation decreases with increasing initial concentration of MB, indicating that kapp is affected by the light absorption properties of MB, because an increase in the initial concentration of MB will lead to increased absorption of visible light by MB molecules rather than by TiO2-x hollow microsphere. Unlike the rate constant kapp, the rate constant ka in the proposed model describes the process of photocatalytic dye degradation more effectively because it does not depend on the initial dye concentration.
As a unique two-dimensional material, graphitic carbon nitride (g-C3N4) has received significant attention for its particular electronic structure and chemical performance. Its instinctive defect can provide a stable anchoring site for metals, potentially improving the surface reactivity. Ni-based catalysts are economical but their activity for CO2 methanation is lower than that of noble metal catalysts. Ni nanoparticles (NPs) supported on a substrate can further enhance the stability and activity of catalysts. Based on the principles of strong metal-support interaction (SMSI) and the synergistic effect on an alloy, MNi12/g-C3N4 composites as novel catalysts are expected to improve stability and catalytic performance of Ni-based catalysts. The configurations are established with core-shell structures of MNi12 (M = Fe, Co, Cu, Zn) nanoparticles (NPs) supported on g-C3N4 in this work. In the CO2 methanation reaction, the reactivity of CO on slab (ECO) is a critical factor, which is relative to the catalytic activity. Thus, the catalytic reactivity of these complexes via CO adsorption were explored using density functional theory (DFT). The values of cohesive energy (Ecoh) for MNi12 NPs range from -39.90 eV to -34.82 eV, suggesting that the formation of these NPs is favored as per thermodynamics, and Ecoh and partial density of state (PDOS) reveal that the central M atom with the less filled d-shell interacts more strongly with surface Ni atoms. Therefore, ZnNi12 is the most unstable structure among all the studied alloy, and the synergistic effect is also the weakest among them. When MNi12 NPs are supported on the g-C3N4 substrate, the binding energies (Eb) vary from -9.40 eV to -8.39 eV, indicating that g-C3N4 is indeed a good material for stabilizing these NPs. The PDOS analysis of pure g-C3N4 suggests the sp2 dangling bonds of N atoms in g-C3N4 can stabilize these transition metal NPs. Furthermore, the results of CO adsorbed on MNi12 NPs and MNi12/g-C3N4 composites show that ECO and dCO reduced with the introduction of g-C3N4. According to the results of the analysis of the Hirshfeld charges and electrostatic potential (ESP), the reason is that CO obtains less electrons from MNi12 NPs after deposition on the g-C3N4 substrate, which lowers the reactivity of CO on catalysts. Additionally, the deformation charge density is analyzed to investigate the interaction between the NPs and g-C3N4. With the introduction of g-C3N4, charge redistribution indicates the strong metal-support interaction, which further reduces the CO adsorption energy. In summary, MNi12 supported on g-C3N4 exhibit not only high stability but also tunable reactivity in CO2 methanation. These changes are beneficial for CO2 methanation reaction.
Sodium-ion batteries (SIBs) have recently garnered considerable attention because of the greater abundance, wider distribution, and lower cost of Na compared to Li. However, the investigation is insufficient, mainly because Na+ is larger and heavier than Li+, thereby limiting the Na+ insertion and extraction ability from the host materials. Anodes with alloying reactions such as Sn, Ge, and Sb have been considered for SIBs owing to their high gravimetric and volumetric specific capacities. In this study, we devised a one-pot reaction strategy for the in-situ fabrication of a spherical porous nano-SnSb/C composite by employing aerosol spray pyrolysis, and subsequently applied it as an anode in SIBs. The products of spray pyrolysis generally feature three-dimensional spherical hierarchical structures, which are considered to be relatively stable and also act as high-packing-density electrode materials. Additionally, they can be easily handled during the fabrication of the electrode. By adjusting the precursor concentration of SnCl2·2H2O and SbCl3, different sizes for SnSb nanoparticles (10 and 20 nm) were obtained. The crystal structures and morphologies of the as-prepared samples were characterized using X-ray diffraction, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. Thermal gravimetric analysis was carried out to analyze the carbon content of SnSb/C composites by using a TG-DSC analyzer with a heating rate of 5 ℃·min-1 in air from 25 ℃ to 600 ℃. The specific surface areas of the microspheres were determined by Brunauer-Emmett-Teller analysis. X-ray photoelectron spectroscopy and Raman spectroscopy were used to investigate the studied materials. The micro-nanostructured composite is composed of SnSb nanoparticles (10 and 20 nm); moreover, the carbon content and size of SnSb nanograins could be controlled by altering the reaction conditions. Owing to its unique structure, the obtained nano-composite displays stable cycle performance and high rate capability as the anode for SIBs. The specific capacity of 10-SnSb/C was 722.1 mAh·g-1 at the first cycle, and the coulombic efficiency of the first cycle was 86.3%. The 10-SnSb/C was stable at different current densities of 100, 1000, and 3000 mA·g-1, and exhibited specific capacities of 607.7, 645.4 and 452.2 mAh·g-1, respectively. The reversible capacity reached 623 mAh·g-1 after 200 cycles at a current density of 1000 mA·g-1, and the capacity retention rate was 95%. The outstanding performance of SnSb/C was due to its distinctive nanostructure, which could effectively improve the utilization rate of active materials, facilitate the transportation of Na+ ions, and prevent the nanoparticle pulverization/agglomeration upon prolonged cycling. The facile synthesis technique and good performance would shed light on the practical development of SnSb/C nanocomposites as high rate capability and long cycle life electrodes for SIBs.
The broad existence of the biaryl linkage in bioactive organic molecules and functional materials makes it an attractive synthesis target via construction of aryl-aryl carbon bonds. Transition metal catalyzed cross-coupling reactions of two pre-functionalized aryl partners, e.g., Suzuki-Miyaura cross-coupling and Negishi cross-coupling reactions, are the main methods typically used for the construction of biaryl linkages. Since the end of the last century, transition metal catalyzed direct C―H arylation of unactivated arenes has emerged as a practical alternative to the well-established cross-coupling strategies. However, the use of transition metal catalysts and/or organometallic reagents would lead to problems, such as the disposal of waste from large-scale syntheses and the removal of heavy metal contaminants from pharmaceutical intermediates. In this regard, the base-promoted homolytic aromatic substitution (BHAS) reaction of aryl halides with unactivated arenes offers a simpler strategy for the synthesis of biaryl scaffolds, and avoids the use of transition metals. Although the BHAS reaction can proceed to a small extent without any additives, particularly at elevated temperatures, the addition of organic promoters would significantly accelerate the reaction rate and improve the overall efficiency of the process. Over the past ten years, a wide variety of N- and O-based organic promoters have been developed to promote the BHAS reaction in the presence of the tert-butoxide base. The mechanism of the BHAS reaction has been studied extensively, and is accepted as occurring via a radical chain process involving an aryl radical. However, the role and mode of initiation of most organic promoters studied remain controversial. The development of more and varied organic promoters will surely promote the mechanistic understanding and further development of the BHAS reaction. Herein, we report that 1, 1'-bis(diphenylphosphino)ferrocene (dppf, or DPPF) can act as a P-based promoter to facilitate the direct arylation of unactivated arenes with aryl iodides using potassium tert-butoxide as the base and electron donor. A broad range of aryl iodides and arenes reacted smoothly under the optimized reaction conditions, giving arylated products in good yields and with high regio-selectivity. Intramolecular C―H arylation also worked well following a sequence of single electron transfer (SET)/initiation, 5-exo-trig aryl radical addition, ring expansion, deprotonation, and re-aromatization/propagation. A mechanistic study indicated that the diphenylphosphino group of dppf played a vital role in the initiation step by enhancing the SET-inhibiting ability of the tert-butoxide anion. A primary kinetic isotope effect was observed in the parallel reactions between 4-methoxy-iodobenzene with benzene and deuterated benzene, implying that the deprotonation of the cyclohexadienyl radical intermediate by tert-butoxide was the rate-determining step in the radical chain pathway.
The selective oxidation of methane to basic petrochemicals (ethylene and ethane) is desirable and has attracted extensive research attention. The oxidative coupling of methane (OCM) is considered a promising one-step route for the production of C2 compounds (ethylene and ethane) from methane, and has been the focus of industrial and fundamental studies. It is widely accepted that the composition is a crucial factor governing the activity of a catalyst system. It was found that the phase structures, basicity, existing status and distribution of the active components, oxygen species, and chemical states of the catalyst were influenced by the composition and ratio, resulting in different catalytic performances for the OCM. In this study, a series of solid acid WO3/TiO2-supported lithium-manganese oxide catalysts for OCM were synthesized via the impregnation method. The impacts of diverse compositions, such as the individual contents (Li and Mn) and dual contents (Li-Mn), on the OCM were investigated in detail, using inductively coupled plasma optical emission spectrometry, X-ray diffraction, high-resolution transmission electron microscopy, CO2-temperature-programmed desorption, O2-temperature-programmed desorption, H2-temperature-programmed reduction, Raman spectroscopy, X-ray photoelectron spectroscopy, and CH4-temperature-programmed surface reaction. The addition of Li content to the catalyst not only led to the anatase-to-rutile crystal structure transformation of TiO2, and the reduction of the high-valence-state Mn species to low-valence-state Mn, but also increased the content of surface lattice oxygen and decreased the surface basicity. The observed effects on the structures and catalytic performance suggest that the Li content is helpful in suppressing the formation of completely oxidized CO2, and increases the C2 selectivity. Moreover, increasing the Li content of the catalyst facilitated the mobility of the lattice oxygen, which triggered the promotion of CH4 activation, thereby enhancing the OCM catalytic performance. The Mn content acted as the active sites for OCM; therefore, the performance of the catalyst was closely related to the Mn concentration and valence state. However, the WO3/TiO2-supported catalyst with excessive Mn content exhibited a high surface basicity, high valence state of Mn, and low abundant lattice oxygen, which was unfavorable for C2 selectivity. The Raman spectroscopy results revealed that MnTiO3 was formed due to the co-existence of Li and Mn on WO3/TiO2, and played an essential role in improving the low-temperature OCM performance. There was a synergic effect of the Li and Mn components on the OCM. The optimal performance (16.3% C2 yield) was achieved over the WO3/TiO2-supported lithium-manganese catalyst with n(Li) : n(Mn) = 2 : 1 at 750 ℃.
In situ liquid cell transmission electron microscopy (LCTEM) was used to observe the dynamic self-assembly behavior of gold nanorod (AuNR)/graphene (G) composites in real-time. Many important reactions in chemistry, physics, and biology occur in solution and real-time imaging of the reaction objects in a liquid medium can further our understanding of the reaction at the nanoscale. Observations of liquid samples using transmission electron microscopy (TEM) have historically been challenging due to issues with evaporation and difficulty in forming thin liquid layers that are suitable for election beam transmission. In situ LCTEM, as an emerging technology, provides novel opportunities for the real-time and high-resolution observation of dynamic processes in solution. In this communication, we report the use of in situ LCTEM to study the assembly behavior of graphene and AuNRs. By tracking and recording the changes in the positions and shapes of the AuNRs and graphene over time, novel composite formation mechanisms between AuNRs and graphene were observed. The AuNRs tended to approach the graphene edges tip-first due to charge attraction. After the assembled structures were formed, the AuNRs could rotate with the graphene edges, among which the edge-to-edge structure was more stable, without angle changes between the AuNR and graphene edge. Drifting motions of the self-assembled structures were observed. And compared with smaller self-assembled structures, the larger structures seem more effectively resisted pushing by liquid flow. In addition, the motions of the larger structure were more easily slowed due to the drag from the liquid cell window substrate. Graphene folding structures were also observed with LCTEM, suggesting that the folding structure can open and close in the liquid, causing apparent relative position changes between Au and graphene for a fixed AuNR on the graphene layer. Overall, the self-assembled structures are very stable and did not show any disassembly behavior in the liquid. Moreover, the AuNR/graphene composites were used as catalysts and showed improved catalytic performance compared to that of bare AuNRs in 4-nitrophenol reduction experiments. The self-assembled catalyst with a mass composite 1 : 5 AuNRs/G ratio exhibited the best performance with a kapp value of 0.5570 min−1, 8 times that of the bare AuNRs. This significant improvement is closely related to the optimized and stable structure of the AuNR/graphene composites. In situ LCTEM provided a powerful characterization method for analyzing the complex self-assembly behavior of the composites in a liquid and will be useful for the development of high performance composite catalyst materials.
Exploring economical and efficient photocatalysts for hydrogen production is of great significance for alleviating the energy and environmental crisis. In this study, 3D In2O3 nanostructures with appropriate self-assembly degrees were obtained using a facile hydrothermal strategy. To study the significance of 3D In2O3 nanostructures with appropriate self-assembly degrees in photocatalytic hydrogen production, the photocatalytic performances of samples were evaluated based on the amount of hydrogen gas release under visible-light irradiation (λ > 400 nm) and simulated solar light illumination. Interestingly, the 3D In2O3-150 nanostructured photocatalyst (hydrothermal temperature was 150 ℃, denoted as In2O3-150) exhibited extremely superior photocatalytic hydrogen evolution activity, which may have been caused by their unique structure to improve light reflection and gas evolution. The special structure can enhance light harvesting and induce more carriers to participate in photocatalytic hydrogen production. Despite possessing similar 3D nanostructures, the In2O3-180 photocatalyst exhibited poor photocatalytic activity. This may have been caused by the high self-assembly degree, which can hinder light irradiation and isolate a portion of the water. In addition, the 3D nanostructures could effectively make uniform the carrier migration direction, which is from the interior to the rod end. However, the direction of carrier migration of the In2O3-110 photocatalyst could transfer in various directions, whereas the In2O3-130 photocatalyst could transfer to both ends of the rod. This might cause partial migration to counteract each other. The compact cluster rod-like structure of In2O3-180 might prevent the light from exciting the carrier effectively. Through a photocatalytic recycling test, the 3D In2O3-150 nanostructured photocatalyst exhibited outstanding photochemical stability. This work highlights the importance of controlling the self-assembly degree of 3D In2O3 nanostructures and explores the performances of 3D In2O3 nanostructured photocatalysts in hydrogen production under visible light and simulated solar light.
As an important component in electrodes, the choice of an appropriate binder is significant when fabricating lithium-ion batteries (LIBs) with good cycle stability and rate capability, which are used in numerous applications, especially portable electronics and eco-friendly electric vehicles (EVs). Semi-crystalline poly(vinylidene fluoride) (PVDF), which is a traditional and widely used binder, cannot efficiently accommodate the volume changes observed in the anode during the charge-discharge process while binding all the components in the electrode together, which results in increased internal cell resistance, detachment of the electrode components, and capacity fading. Herein, we have investigated a highly polar and elastomeric polyacrylonitrile-butadiene (NBR) rubber for use as a binder in LIBs, which can accommodate graphite particles of different shapes compared to semi-crystalline PVDF. Prior to our electrochemical tests, NBR was analyzed using thermogravimetric analysis (TGA) and X-ray diffraction (XRD), showing good thermal stability and an amorphous morphology. NBR is more conformable to irregular surfaces, which results in the formation of a homogeneous passivation layer on both spherical and flaky graphite particles to effectively suppress any electrolyte side reactions, further allowing more uniform and fast Li ion diffusion at the electrolyte/electrolyte interface. As a result, the electrochemical performance of both spherical and flaky shape graphite electrodes was significantly improved in terms of their first cycle Coulombic efficiency (CE) and cycle stability. With comparative specific capacity, the first cycle CE of the NBR-based spherical and flaky graphite electrodes were 87.0% and 85.5%, compared to 85.3% and 82.6% observed for their corresponding PVDF-based electrodes, respectively. After 1000 discharge-charge cycles at 1C, the capacity retention of the NBR-based graphite electrodes was significantly higher than that of PVDF-based electrodes. This was attributed to the good stability of the solid electrolyte interphase (SEI) formed on the graphite electrodes and the high stretching ability of the elastomeric NBR binder, which help to accommodate the repeated volume fluctuation of graphite observed during long-term charge-discharge cycling. Electrochemical impedance spectroscopy (EIS) and microscopic analysis (SEM and TEM) were carried out to investigate the formation and evolution of the SEI layers formed on the spherical and flaky graphite electrodes. The results show that thin, homogeneous, and stable SEI layers are formed on the surface of both spherical and flaky graphite electrodes prepared using the NBR binder. When compared to the PVDF-based graphite electrodes, the graphite electrodes constructed using NBR showed decreased resistance in the SEI layer and faster charge transfer, thus enhancing the electrode kinetics for Li ion intercalation/deintercalation. Our study shows that the electrochemical performance of spherical and flaky graphite electrodes prepared using the NBR binder is significantly improved, demonstrating that NBR is a promising binder for these electrodes in LIBs.
As a new fluorescent nanomaterial, carbon dots (CDs) have many advantages, such as uniform particle size distribution, good light stability, adjustable excitation-emission wavelength, and surface modification. Moreover, one of the fascinating characters of CDs is that they are considered to be low-toxic and eco-friendly alternatives in chemical and biological analyses. They have exhibited broad application prospects in the fields of analysis, detection, and bioimaging. Silkworm excrement is dried and easily available. A large number of hydroxyl and carboxyl compounds in silkworm excrement can be used as ideal starting materials for the preparation of CDs. Also, compounds containing nitrogen and sulfur in silkworm excrement can be used as nitrogen and sulfur sources; thus, when used in the preparation of CDs, silkworm excrement can impart many more unique properties to CDs. Nitrogen-containing CDs prepared by microwave synthesis have an average hydration diameter of 4.86 nm. Elemental analysis data show that the prepared CDs contained 59.84% carbon, 5.46% nitrogen, and 2.32% sulfur. XPS spectra reveals sulfur (2p), carbon (1s), nitrogen (1s), and oxygen (1s) in CDs. FTIR data demonstrate that the prepared CDs may contain hydroxyl, amino, carbonyl, sulfonic, ester, and ether functional groups as well as carbon-nitrogen structures. The XRD pattern of the CDs has a broader peak of the amorphous carbon phase at approximately 2θ = 24.6°, and only D bands (at ~1400 cm-1) can be obviously detected in the Raman spectra of CDs. The intensity of fluorescence emission peak of CDs increases first and then decreases with the increase in excitation wavelength. The maximum intensity of fluorescence emission shifts gradually with the red shift of the excitation wavelength, and the relationship between excitation and emission wavelengths is exponential. In the pH ranging from 2.18 to 10.24, the fluorescence emission intensity of CDs decreases gradually with the increase in pH, and the maximum fluorescence emission intensity shifts gradually with the increase in pH. There is a linear relationship between pH and maximum emission wavelength. The fluorescence emission intensity of CDs decreases gradually with the increase in metal ion concentration. Under neutral conditions, CDs can selectively detect Cu2+. Under acidic conditions, CDs can detect Cu2+, Fe3+, Al3+, Ni2+, and Fe2+ separately without interference from other ions. There is a Stern-Volmer linear relationship between metal ion concentration and fluorescence intensity. The intensity of the fluorescence emission peak of CDs decreases with the increase in temperature, which may be due to the non-radiative transition process caused by molecular thermal motion. There is a linear relationship between temperature and fluorescence intensity. The maximum fluorescence emission intensity of CDs gradually shifts with the increase in polarity of the dispersed solvents. There is a linear relationship between fluorescence intensity and empirical constant ET of solvent polarity. Compared with the reported CDs prepared from natural products, silkworm-excrement-based CDs have abundant surface groups although they do not have an obvious crystal structure, which makes them have excellent response to various environmental factors (pH, temperature, ion concentration, temperature, solvent polarity, etc.) in a wide range. Above all, the fluorescence property changes with multiple environmental parameters will facilitate a broad application of silkworm-excrement-based CDs in biodetection and imaging.
Over the past decades, advances in science and technology have greatly benefitted the society. However, the exploitation of fossil fuels and excessive emissions of polluting gases have disturbed the balance of the normal carbon cycle, causing serious environmental issues and energy crises. Global warming caused by heavy CO2 emissions is driving new attempts to mitigate the increase in the concentration of atmospheric CO2. Significant efforts have been devoted for CO2 conversion. To date, the electroreduction of CO2, which is highly efficient and offers a promising strategy for both storing energy and managing the global carbon balance, has attracted great attention. In addition, the electrosynthesis of value-added C2+ products from CO2 addresses the need for the long-term storage of renewable energy. Therefore, developing catalysts that function under ambient conditions to produce C2 selectively over C1 products will increase the utility of renewable feedstocks in industrial chemistry applications. Recently, great progress has been made in the development of materials for electrocatalytic CO2 reduction (ECR) toward C2+ products; however, some issues (e.g., low selectivity, low current efficiency, and poor durability) remain to be addressed. In addition, the elementary reaction mechanism of each C2+ product remains unclear, contributing to the blindness of catalyst design. In this regard, the development of proposed mechanisms of ECR toward C2+ products is summarized herein. The key to generating C2+ products is improving the chances of C―C coupling. Test conditions significantly influence the reaction path of the catalyst. Thus, three different paths that that are most likely to occur during ECR to C2+ products are proposed, including the CO, CO-COH, and CO-CO paths. In addition, typical material regulatory strategies and technical designs for ECR toward C2+ products (e.g. crystal facet modulation, defect engineering, size effect, confinement effects, electrolyzer design, and electrolyte pH) are introduced, focusing on their effects on the selectivity, current efficiency, and durability. The four strategies for catalyst design (crystal facet modulation, defect engineering, size effect, and confinement effect) primarily affect the selectivity of the ECR via adjustment of the adsorption of reaction intermediates. The last two strategies for technique design (electrolyzer design and electrolyte pH) contributing greatly toward improving the current efficiency than selectivity. Finally, the challenges and perspectives for ECR toward C2+ products and their future prospects are discussed herein. Therefore, breakthroughs in the promising field of ECR toward the generation of C2+ products are possible when these catalyst design strategies and mechanisms are applied and novel designs are developed.
Hydrogen (H2), a clean and sustainable energy carrier, is regarded as one of the most promising alternatives to carbon-based fuels. Hydrogen can be generated in a more sustainable way from renewable energy sources via electrocatalytic water splitting. However, the high cost and low abundance of the benchmarking platinum-based hydrogen evolution reaction (HER) catalysts hinder their widespread applications. Thus, developing highly efficient, stable, and low-cost electrocatalysts to replace platinum for HER is imperative, but remains a challenging task. Recently, efforts have been devoted to developing non-noble HER electrocatalysts, including transition metal carbides, oxides, phosphides, and sulfides. However, traditional synthetic strategies cannot effectively control active sites and the catalysts tend to aggregate under high temperature. Recently, polyoxometalates (POMs) have been applied as precursors for the preparation of non-noble HER electrocatalysts as they contain discrete metal-oxygen clusters with well-defined structures. POMs are typically composed of oxygen ligands and high-valent metal ions such as Ⅴ(Ⅴ), Mo(Ⅵ), and W(Ⅵ), which can serve as Ⅴ, Mo, and W sources to produce the corresponding metal carbides, oxides, phosphides, and sulfides by pyrolysis at high temperature. Some POMs may also contain a series of redox-active heteroatoms, which are usually named hetero-polyoxometalates. These can serve as precursors to electrocatalysts with uniform heteroatom doping. Moreover, direct applications of POMs as molecular catalysts in HER have, in recent years, received rapidly growing attention. This is because POMs not only serve as mediators or molecular catalysts to facilitate the HER, but can also be deposited on the electrode surface to catalyze the HER. However, the interpretation that HER catalytic activity enhancement is due to the intrinsic catalytic properties of the electrodeposited polyoxometalate or the deposition of small amounts of platinum has been highly debated. Reviewing these studies may help us understand the intrinsic active sites as well the intrinsic HER mechanism of POMs and POMs-derived catalysts, and thus design more efficient HER catalysts. This review, therefore, focuses on recent progress in the applications of POMs and their derivatives in electrocatalytic HER. Firstly, basic HER mechanisms for common metal catalysts and POMs molecular catalysts are discussed along with challenges in the field of HER. Next, applications of POMs molecular catalysts and POMs-derived catalysts in HER are summarized. Finally, some perspectives of POMs-based catalysts/pre-catalysts for electrocatalytic HER are proposed.