Research on two-dimensional (2D) materials has been explosively increasing in last seventeen years in varying subjects including condensed matter physics, electronic engineering, materials science, and chemistry since the mechanical exfoliation of graphene in 2004. Starting from graphene, 2D materials now have become a big family with numerous members and diverse categories. The unique structural features and physicochemical properties of 2D materials make them one class of the most appealing candidates for a wide range of potential applications. In particular, we have seen some major breakthroughs made in the field of 2D materials in last five years not only in developing novel synthetic methods and exploring new structures/properties but also in identifying innovative applications and pushing forward commercialisation. In this review, we provide a critical summary on the recent progress made in the field of 2D materials with a particular focus on last five years. After a brief background introduction, we first discuss the major synthetic methods for 2D materials, including the mechanical exfoliation, liquid exfoliation, vapor phase deposition, and wet-chemical synthesis as well as phase engineering of 2D materials belonging to the field of phase engineering of nanomaterials (PEN). We then introduce the superconducting/optical/magnetic properties and chirality of 2D materials along with newly emerging magic angle 2D superlattices. Following that, the promising applications of 2D materials in electronics, optoelectronics, catalysis, energy storage, solar cells, biomedicine, sensors, environments, etc. are described sequentially. Thereafter, we present the theoretic calculations and simulations of 2D materials. Finally, after concluding the current progress, we provide some personal discussions on the existing challenges and future outlooks in this rapidly developing field.

The increasing development of society has resulted in the ever-growing demand for energy storage devices. To satisfy this demand, both energy density and safety performance of lithium batteries must be improved, which is challenging. Solid-state lithium batteries are promising in this regard because of their safe operation and high electrochemical performance. In recent years, intense effort has been devoted toward the exploration of materials with high ionic conductivity for room-temperature solid-state batteries. Among several types of solid-state electrolytes, Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP), an inorganic NASICON-type electrolyte, has drawn considerable attention because of its high ionic conductivity, wide electrochemical window, and environmental stability. However, the formation of lithium-ion-conducting networks within the electrode and between the electrode-LAGP interface is limited because of high interfacial resistance caused by the direct contact and volume expansion between the electrode and electrolyte. Thus, the application of LAGP in the fabrication of solid-state batteries is limited. Moreover, the occurrence of the unavoidable side reaction because of the direct contact of LAGP with the lithium metal anode shortens battery life. In addition, the rigid brittle nature of the LAGP electrolyte leads to the limits the facile fabrication of solid-state batteries. To overcome these limitations, herein, a novel strategy based on in situ polymerization of a vinylene carbonate solid polymer electrolyte (PVC-SPE) was proposed. The in situ formed PVC-SPE can effectively construct ion-conducting pathways within the cathode and on the interfaces of the LAGP electrolyte and electrodes. Furthermore, the PVC-SPE can significantly inhibit the side reaction between the lithium anode and LAGP electrolyte. The electrochemical performances of Li | LAGP | Li and Li | LAGP | Li with in situ PVC-SPE modified interface symmetrical solid-state batteries were compared. The in situ modified Li | LAGP | Li symmetrical solid-state battery exhibited stability toward plating and stripping for over 2700 h and a low overpotential (34 mV) at room temperature. Moreover, a Li | LAGP | LiFePO₄ solid-state battery exhibited a capacity retention of 94% at 0.2 C after 200 cycles with a capacity of 158 mAh·g^-1. In addition, high rate capability (72.4% capacity retention at 3 C) was achieved at room temperature. Therefore, the proposed in situ modification strategy was found to resolve the interface-related problem and facilitated the construction of the ion-conducting network within the electrode; thus, it can be a promising approach for the fabrication of high-performance solid batteries.

Semitransparent organic solar cells (ST-OSCs) have attracted attention for use in building integrated photovoltaics because of their large range tunability in colors, transparency, and high efficiency. However, the development of semitransparent devices based on fullerene acceptors remained almost stagnant in the early period. This was due to the weak absorption of fullerene small molecules in the visible and near-infrared regions as well as the large non-radiative energy loss, resulting in drastic open-circuit voltage loss. In addition, the energy level and chemical structure of fullerene molecules cannot be easily regulated, and the strong aggregation characteristics of fullerenes greatly limit the development of OSCs. In contrast, the designability of the chemical structures and controllability of the energy levels of non-fullerene electron acceptors has encouraged researchers to explore high-performance organic solar cells while and simultaneously stimulating the development of ST-OSCs. In this review, the recent progress in non-fullerene small molecule acceptors for ST-OSCs is summarized. The article focuses on ST-OSCs from the aspects of device structures and active layers. In view of the semitransparent device structure, except for replacing the traditional electrodes with semitransparent electrodes, researchers have introduced suitable interface layers to regulate the absorption and reflection of sunlight. The interface layers mainly contain a reflective layer (evaporated on the top electrode to reflect near-infrared light); an anti-reflection layer (located below ITO (indium tin oxide)) to mitigate light reflection at the air-glass interface and thus enhance the absorption of sunlight); and an optical outcoupling layer (simultaneously increasing reflection and transmission). From the active layer, it is mainly divided into two categories. First, researchers have optimized the photovoltaic performance of semitransparent devices from the perspective of molecular structures, mainly by broadening the absorption window of non-fullerene small molecule acceptors, thus improving the crystallinity and charge mobility of small molecules, and regulating the stacking behavior and orientation of molecules in the films. Second, regarding the active layer processing, much effort has been undertaken to optimize the light absorption, morphology, and charge carrier transport channels of blended films.

The growing demand for electric vehicles, communication devices, and grid-scale energy storage systems urgently calls for the development of rechargeable batteries. Although lithium-ion batteries have dominated the new energy market for decades, there are challenges limiting their development, such as the high cost of lithium, as well as the toxicity and flammability of the organic electrolyte. In recent years, aqueous zinc-ion batteries (ZIBs) have gained much attention due to their advantages of high safety, high capacity, low cost, and nontoxicity. Materials based on multivalent vanadium and manganese have shown great potential for application as cathodes that are compatible with the metallic zinc anode in ZIBs. However, the commercialization of ZIBs has been hindered by the choice of cathodes, since the cathode materials show unsatisfactory energy densities and suffer from severe structural collapse, dissolution of the electrode components, sluggish reaction kinetics and detrimental side reactions during cycling. This stalemate was broken when a Zn²⁺/H₂O co-inserted V₂O₅ (Zn_0.25V₂O₅·nH₂O) material was first reported in 2016, and it showed much higher cycling stability and capacity than those of V₂O₅. The Zn²⁺ and water molecules pre-intercalated into the interlayer served as pillars to maintain the crystal structure and increase the interplanar spacing, leading to high structural stability and fast Zn²⁺ diffusion. Since then, several guest ions (Li⁺, Na⁺, K⁺, Ca²⁺, NH₄⁺, PO₄^3-, N^3-, etc.) and molecules (H₂O, polyethylene dioxythiophene (PEDOT), polyaniline (PANI, etc.) have been widely used to improve the electrochemical performance of aqueous ZIB cathodes, especially with manganese-based and vanadium-based materials. It is demonstrated that pre-intercalation of the guest ions or molecules can effectively optimize the electronic structure, regulate the interplanar spacing, and improve the reaction kinetics of the host. The local coordination structure of the host with pre-intercalated guest ions/molecules directly influences the zinc-ion storage performance. For example, sodium vanadates with a tunneled structure generally show better cycling stability than those with a layered structure due to their stronger Na-O bonds, since the O atoms on their layer surfaces are only single-connected. Manganese dissolution could be greatly suppressed by intercalation of the large potassium ions into tunneled α-MnO₂, where solid K-O bonds act as pillars to be connected with Mn polyhedrons, and thus strengthen the structure. New mechanisms underlying reduction/displacement reactions could also be revealed in vanadates upon the introduction of Ag⁺ and Cu²⁺. Thus, we believe that guest pre-intercalation is a promising method for optimizing the zinc-ion storage performance of the appropriate cathodes and is worthy for further exploration. Here we have reviewed the recent advances in manganese-based and vanadium-based cathodes via the guest pre-intercalation strategy, discussed the related advantages and challenges. The future research direction for these two kinds of aqueous ZIB cathodes is also prospected.

The electrocatalytic activity of commercial Pt/C for ethanol oxidation is relatively low, and the C―C bond is difficult to break. Thus, the complete oxidation process is not easy, and the fuel utilization efficiency becomes considerably reduced. Increasing the temperature can increase the reaction rate and enhance the mass transport; therefore, a temperature-controlled electrode was used during our in situ FTIRS (Fourier Transform Infrared Spectroscopy) investigation. The temperature sensor was placed at a certain distance from the surface of the electrode; thus, the surface temperature needed to be corrected. The temperature was calibrated using the "potentiometric" measurement method, which was because the potential-temperature coefficient of the redox couple is constant under certain conditions, and the electrode surface temperature was obtained by potential conversion at different temperatures during the experiment. The experimental results showed that the relationship between the heating temperature, T_h, and the surface temperature, T_S, was T_S = 0.57T_h + 7.71 (30 ℃ < T_h ≤ 50 ℃) and T_S = 0.62T_h + 5.12 (50 ℃ < T_h ≤ 80 ℃), and according to error analysis, the maximum error was 1 ℃. The temperature-controlled electrode was applied to investigate the electrooxidation of ethanol using both in situ FTIRS and cyclic voltammetry using a commercial Pt/C catalyst at different temperatures. Clearly, based on the CV curve for the oxidation of ethanol, with increasing temperature, the overall oxidation current increased, and the onset potential and peak potential both negatively shifted, indicating that thermal activation allows the oxidation reaction to proceed easier. Electrooxidation of ethanol showed two positive oxidation peaks, and the ratio of the first peak current to the second peak current was used to qualitatively evaluate the selectivity of CO₂. Compared with at 25 ℃, the first peak current increased by 30% at 65 ℃, indicating that the high temperature was conducive to C―C bond cleavage. Comparing the in situ FTIRS recorded at 50 ℃, 35 ℃, and 25 ℃, we found that the onset potential of CO₂ on the commercial Pt/C catalyst was lower by 200 mV, indicating that Pt/C can provide oxygen-containing species at lower potentials at high temperature; however, the onset potentials of CH₃CHO and CH₃COOH did not change with temperature. The CO₂ selectivity was semi-quantitatively calculated by the area of CO₂ compared with the area of CH₃COOH from the FTIRS data. It was found that CO₂ had the highest selectivity at high temperature and low potential, indicating that high temperature is conducive to complete ethanol oxidation during CO₂ formation, possibly because both the ethanol bridge adsorption pattern and adsorbed OH (OH_ad) increased with temperature, enhancing subsequent CO_ad and OH_ad oxidation reactions. The low selectivity of CO₂ at the high potential was due to the adsorption of oxygen-containing species that occupied the surface-active site, blocking the adsorption of ethanol.

It is important to determine the effects of misfit dislocations and other defects on the domain structure, ferroelectricity, conductivity, and other physical properties of ferroelectric thin films to understand their ferroelectric and piezoelectric behaviors. Much attention has been given to ferroelectric PbTiO₃/SrTiO₃ or PbZr_0.2Ti_0.8O₃/SrTiO₃ heterointerfaces, at which improper ferroelectricity, a spin-polarized two-dimensional electron gas, and other physical phenomena have been found. However, those heterointerfaces were all (001) planes, and there has been no experimental studies on the growth of (010) PbTiO₃/SrTiO₃ heterointerface due to the 6.4% misfit between two materials. In this study, we selected an atomically flat (010) PbTiO₃/SrTiO₃ heterointerface grown using a two-step hydrothermal method as the research subject, and this is the first experimental report on that interface. Interfacial dislocations can play a significant role in causing dramatic changes in the Curie temperature and polarization distribution near the dislocation cores, especially when the size of a ferroelectric thin film is scaled down to the nanoscale. The results of previous studies on the effects of interfacial dislocations on the physical properties of ferroelectric thin films have been contradictory. Thus, this issue needs to be explored more deeply in the future. This study used aberration corrected scanning transmission electron microscopy (STEM) to study the atomic structure of a (010) PbTiO₃/SrTiO₃ heterointerface and found periodic misfit dislocations with a Burgers vector of a[001]. The extra planes at the dislocation cores could relieve the misfit strain between the two materials in the [001] direction and thus allowed the growth of such an atomically sharp heterointerface. Moreover, monochromated electron energy-loss spectroscopy with an atomic scale spatial resolution and high energy resolution was used to explore the charge distribution near the periodic misfit dislocation cores. The fine structure of the Ti L edge was quantitatively analyzed by linearly fitting the experimental spectra recorded at various locations near and at the misfit dislocation cores with the Ti³⁺ and Ti⁴⁺ reference spectra. Therefore, the accurate valence change of Ti could be determined, which corresponded to the charge distribution. The probable existence of an aggregation of electrons was found near the a[001] dislocation cores, and the density of the electrons calculated from the valence change was 0.26 electrons per unit cell. Based on an analysis of the fine structure of the oxygen K edge, it could be argued that the electrons aggregating at the dislocation cores came from the oxygen vacancies in the interior regions of the PbTiO₃. This aggregation of electrons will probably increase the electron conductivity along the dislocation line. The physics of two-dimensional charge distributions at oxide interfaces have been intensively studied, however, little attention had been given to the one-dimensional charge distribution. Therefore, the results of this study can stimulate research interest in exploring the influence of the interfacial dislocations on the physics of ferroelectric heterointerfaces.

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.

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.

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-dioctyl[1]benzothieno[3, 2-b][1]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.

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 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.

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.

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.

The global energy shortage has led to vigorous research for the development of new energy technologies in all countries to cope with the energy crisis and to meet the demand for green energy. In this regard, the development of new battery systems with high energy densities has become the current research hotspot. Lithium-sulfur battery is considered as a promising candidate due to its high energy density and low cost. However, it suffers from the insulating nature of sulfur and the shuttle effect of polysulfide, which hinder its practical application. Selenium (Se), as a congener element of sulfur, possesses electrochemical properties similar to sulfur. Lithium-selenium batteries have attracted considerable research attention due to the high electronic conductivity of selenium and high volumetric capacity. The conductivity of Se (1 × 10^-3 S·m^-1) is ~24 orders of magnitude higher than that of sulfur (5 × 10^-28 S·m^-1), providing much better electron transportation in cathode, leading to fast electrochemical reaction and high utilization of Se. Moreover, Se is compatible with cheap carbonate-based electrolytes, which could greatly reduce the cost. Lithium-selenium batteries also have much higher theoretical volumetric and gravimetric capacities (3253 mAh·cm^-3 and 675 mAh·g^-1, respectively) than those of commercial lithium-ion batteries such as LiCoO₂/graphite and LiFePO₄/graphite systems. Volumetric capacity is a more important requirement than gravimetric capacity for a battery, especially for applications in electric vehicles and portable electronic devices, because they have limited space for accommodating batteries. Thus, research on lithium-selenium batteries with high volumetric capacities is of great significance for these volume-sensitive applications. However, the electrochemical performance of current lithium-selenium batteries is not satisfactory. Several key problems must be solved, including shuttle effect, electrolyte compatibility, volume change during cycling, capacity fading, and low coulombic efficiency. In recent years, widespread research has been conducted to address these problems and considerable progress has been made. This review summarizes the status of research on lithium-selenium batteries, and focuses on the recent progress in the research of selenium-carbon cathode materials. The advantages and disadvantages of lithium-selenium batteries are discussed in detail. The performance-structure relationship is analyzed from the perspective of different dimensional structures, including one-dimensional nanofibers and nanowires, two-dimensional nanosheets, composite films and scaffolds, three-dimensional hollow structures, solid structures, and freestanding structures (spheres, nanobelts, nanotubes, microcubes etc.). Other types of selenium-based cathodes, including metal selenides and heterocyclic selenium-sulfur (Se_xS_y), are also described. The compatible electrolytes and functional interlayers for lithium-selenium batteries are also discussed. The reaction mechanisms of lithium-selenium batteries and their relationship with various electrolytes are summarized. Finally, the future perspective of lithium-selenium batteries is proposed.

Interactions between surfactants and small organic molecules not only enhance the surface activity of the surfactants and induce aggregate transitions in them, but also improve the solubility and stability of the organic molecules. Understanding the interaction between surfactants and small molecules will help in widening the scope of application of surfactants. Folic acid, a member of the vitamin B family, has a pteridine ring, para-aminobenzoic acid, and glutamic acid, and is crucial for many reactions inside the human body. The unique structure of folic acid also facilitates the preparation of functional materials such as liquid crystals and gels. However, the poor solubility and precipitation of folic acid limit its applications. Therefore, it is essential to improve the solubility and stability of folic acid. Surfactants are efficient in solubilizing and stabilizing small molecules. The interactions of folic acid with four types of surfactants, namely, an anionic surfactant, sodium dodecyl sulfate (SDS); a cationic surfactant, dodecyl trimethylammonium bromide (DTAB); a cationic ammonium gemini surfactant, 12-6-12; and a cationic ammonium trimeric surfactant, 12-3-12-3-12; have been investigated at pH 7.0 by surface tension measurements, ultraviolet-visible (UV) absorption spectroscopy, dynamic light scattering, isothermal titration calorimetry, and nuclear magnetic resonance spectroscopy. At pH 7.0, the carboxylic acid groups of folic acid are deprotonated, so each folic acid molecule carries two negative charges. The addition of a small amount of folic acid sharply reduces the critical micelle concentration (CMC) of cationic surfactants and their surface tension at the CMC. However, the surface activity and aggregation of SDS show only minimal changes with the introduction of folic acid. In addition, the photodegradation of folic acid in the presence of different surfactants is studied by fluorescence and UV absorption spectroscopy. When irradiated with UV light, folic acid undergoes rapid degradation in aqueous solution, in the absence of any surfactants. In contrast, the degradation is greatly suppressed in the presence of surfactants. The extent of suppression by cationic surfactants is more significant than that by the anionic surfactant. The residual folic acid concentration increases from nearly 0 in the absence of any surfactant to 43%, 89%, 96%, and 96% in the presence of SDS, DTAB, 12-6-12, and 12-3-12-3-12, respectively, in the concentration range studied. The amount of surfactant required to prevent the degradation decreases with an increase in the degree of oligomerization of the cationic surfactants. The greater number of binding sites and hydrophobic tails in the gemini and oligomeric surfactants result in much stronger electrostatic and hydrophobic interactions with folic acid. In addition, the close and compact packing in these surfactant molecules prevents folic acid from coming in contact with oxygen, thereby retaining its stability and preserving its properties. This work provides a new methodology for regulating the surface activity of the surfactants and their aggregation in the presence of small functional molecules, which in turn improves the stability of the small molecules that are otherwise unstable.

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-Na_xCo_0.7Mn_0.3O₂ (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.

Borophene, a boron analogue of graphene, exhibits a rich variety of chemical and physical properties. Here, we provide an intensive overview of recent progress in theoretical modeling and experimental synthesis of borophene. In particular, we analyze the influence of substrate, growth temperature, and precursor on the selectivity of boron nucleation. While three-dimensional (3D) bulk boron is more stable than a two-dimensional (2D) boron sheet, the nucleation barrier determined by the growth process controls the formation of the material and it depends on the specific growth environment. Theoretical studies have shown that a metal substrate can play an important role in stabilizing 2D boron clusters over their 3D form, resulting in the kinetically favored growth of 2D boron on the substrate even though the 2D boron clusters will be overwhelmingly less stable than the 3D form with increasing cluster size. Ag and Cu substrates have proven to be particularly suitable for achieving this preference. Guided by theoretical works and perhaps original insights, experimentalists from two independent groups have successfully synthesized 2D boron sheets on silver substrates by depositing ultra-high purity boron onto a clean Ag (111) surface under high vacuum conditions. Moreover, the borophene samples were found to exhibit the same atomic structure previously predicted to be preferred on this substrate. Besides the substrate, the growth temperature is also key to the final product. When the temperature is too low, boron growth cannot overcome the nucleation barrier of the 2D structure. As a result, boron clusters or amorphous boron structures are likely to be formed. In contrast, an excessively high growth temperature will steer the growth to overcome the nucleation barrier of 3D boron, possibly yielding boron nanofilms with finite thickness. Therefore, the growth temperature needs to be carefully controlled, so that the free energy of boron growth will be located between the nucleation barriers of the 3D and 2D forms. Some impurity elements found in synthetic source materials, such as hydrogen and oxygen, can also impact boron nucleation. The existence of these elements may alter the competition between 2D and 3D structures during the nucleation process. More importantly, hydrogen and oxygen can passivate the dangling bonds on the surface of a 3D boron structure, lowering its surface energy, and therefore, impairing the nucleation of 2D boron structures. At present, molecular beam epitaxy (MBE) is the only method with which borophene has been successfully synthesized. Yet this method is very expensive, suffers from low yield, and is constrained to small sample sizes. Thus, exploring the growth of borophene via chemical vapor deposition (CVD) on different substrates is critically important for realizing the great potential of borophene in various applications. By discussing possible growth conditions and atomistic mechanisms of borophene nucleation as well as theoretical methods for modeling and simulations, we suggest prospects for chemical vapor deposition growth of borophene on selected substrates. This work aims to offer useful guidance for chemical synthesis of large-area, high-quality borophenes and promote their practical applications.

Carbon atoms can bond together in different molecular configurations leading to different carbon allotropes including diamond, fullerene, carbon nanotubes, graphene, and graphdiyne that are widely used or explored in a number of fields. Carbon dots (CDs), which are generally surface-passivated carbon nanoparticles less than 10 nm in size, are other new members of carbon allotropes. CDs were serendipitously discovered in 2004 during the electrophoresis purification of single-walled carbon nanotubes. Similar to their popular older cousins, fullerenes, carbon nanotubes, and graphene, CDs have drawn much attention in the past decade and have gradually become a rising star because of the advantages of chemical inertness, high abundance, good biocompatibility, and low toxicity. Interestingly, CDs typically display excitation-energy- and size-dependent fluorescent behavior. Depending on their structures, the fluorescence from CDs is either attributed to the quantum-confinement effect and conjugated π-domains of the carbogenic core (intrinsic states), or determined by the hybridization of the carbon skeleton and the connected chemical groups (surface states). Compared with the traditional semiconductors, quantum dots, and their organic dye counterparts, fluorescent CDs possess not only excellent optical properties and small-size effect, but also the advantages of low-cost synthesis, good photo-bleaching resistance, tunable band gaps, and surface functionalities. For these reasons, CDs are considered to be emergent nanolights for bio-imaging, sensing, and optoelectronic devices. Additionally, CDs feature abundant structural defects at their surface and edges, excellent light-harvesting capability, and photo-induced electron-transfer ability, thus facilitating their applications in photocatalysis and energy storage and conversions. To date, remarkable progress has been achieved in terms of synthetic approaches, properties, and applications of CDs. This review aims to classify the different types of CDs, based on the structures of their carbogenic cores, and to describe their structural characteristics in terms of synthesis approaches. Two well-established strategies for synthesizing CDs, the top-down and bottom-up routes, are highlighted. The diverse potential applications, in the bio-imaging and diagnosis, sensing, catalysis, optoelectronics, and energy-storage fields, of CDs with different structures and physicochemical properties, are summarized, covering the issues of surface modification, heteroatom doping, and hybrids made by combining CDs with other species such as metals, metal oxides, and biological molecules. The challenges and opportunities for the future development of CDs are also briefly outlined.

The self-assembly of nanoparticles can effectively control morphology and surface exposure during materials processing and has promising potential applications in the synthesis and exploration of different types of materials. As the process often occurs in liquid solvents, it is difficult to observe and study the dynamic process and mechanism of the self-assembly of nanoparticles in situ. With the recent development of environmental transmission electron microscopy (TEM), researchers have been able to observe and study the dynamic processes of various chemical reactions in liquid environments in real time on the nanometer- and atomic scale. In this paper, we used advanced in situ liquid environmental TEM to characterize the self-assembly process of Co₃O₄ nanorods in water and study the mechanism of self-assembly. A self-assembly phenomenon of the Co₃O₄ nanorods in water was discovered under the influence of electron beam irradiation that caused a change in the dielectric constant and increased the electrical conductivity in the irradiated water. The movement of the nanorods was initiated and energized in the irradiated water. As the distance between the nanorods decreased, the drift velocity of the nanorods and the interaction force between them increased. By analyzing the nanorods' movement (including the distance and the rotation angle) as a function of time, the variation of the mean square displacement with respect to time was obtained. Based on the Stokes-Einstein equation, the driving force was estimated, and we found that the driving force increases with the decreasing distance and that the maximum value of the driving force is approximately 12 pN. By characterizing the morphology of the Co₃O₄ nanorods, we found that the exposed crystal facets of the nanorods are mostly in the {100}, {110}, and {111} planes. As Co₃O₄ is a polar metal oxide, these crystalline planes tend to carry certain electric charge according to the terminative Co or O atoms. In a conductive aqueous environment, it is because of these easily exposable surfaces with different surface charges that the Co₃O₄ nanorods will attract each other under the driving force of opposite electric charges. Furthermore, self-assembly of nanorods with a complementary morphology can be driven quickly. The polarity of the residual surface charge plays a decisive role in the effective assembly of nanorods. It is already known that surface charge compensation takes place on polar crystal surfaces; however, this work provides a deeper understanding of the incomplete nature of this surface charge compensation. The results presented here may provide important experimental data and theoretical reference for artificially regulating the metal oxide self-assembly process.

Benefitting from high-temperature operating conditions, solid oxide fuel cells (SOFCs) exhibit high electricity efficiency and can be coupled with conventional engines such as gas turbines, to achieve cascaded energy utilization. However, high temperature inevitably accelerates material deterioration, and simultaneously complicates the on-line diagnosis of SOFCs. Electrochemical impedance spectroscopy (EIS) is a mature on-line testing technology that is been widely used in SOFC research. Using EIS analyses, researchers can clearly determine the ohmic resistance of pure ion/electron conduction and the magnitude of the polarization impedance of electrochemical processes or diffusion. However, the lack of decomposition of polarization impedance constitutes a limitation for the deeper understanding of SOFC operation. To better utilize information-rich EIS, this work used a typical Ni-Y₂O₃ stabilized zirconia (Ni-YSZ) anode-supported SOFC and designed a complete set of impedance tests by modifying the test temperature, anode operating atmosphere, and cathode operating atmosphere. In addition, this study combined two advanced impedance spectrum analysis methods: the analysis of differences in impedance spectra (ADIS) and distribution of relaxation time (DRT) methods to allow for a deeper comprehension of the impedance spectrum and obtain the characteristic frequency of each process in the SOFC. Based on the characteristic frequency obtained by the spectral decomposition, this study analyzed and explained the physical or (electro-) chemical origins of the impedance in each frequency band. In addition, the equivalent circuit method (ECM) was adopted to fit the SOFC impedance spectra by 6 (RQ) parallel circuits to summarize the impedance distribution. The ADIS method was more convenient than the DRT method and could be used to initially determine the range of characteristic frequencies. However, it failed to provide a clear decomposition for the high-frequency region of the impedance spectrum. The DRT method can yield a good decomposition of a single impedance spectrum, and the characteristic frequency of Ni-YSZ SOFC was separated into ~2, ~20, ~30, 1 × 10³-1.5 × 10³, 2 × 10³-4 × 10³, and ~4 × 10⁴ Hz regions. The highest frequency (~4 × 10⁴) region was dominated by charge transfer impedance at the YSZ/GDC (gadolinia doped ceria) or LSCF/GDC interface. Peaks at 2 × 10³-4 × 10³ and 1 × 10³-1.5 × 10³ Hz correspond to the H₂ electrochemical reaction at the Ni/YSZ/H₂ boundary and O₂ reduction at the LSCF/O₂ boundary, respectively. The impedance at lower frequencies can be explained by diffusion polarization impedance. In this study, ~20 Hz corresponds to the cathode diffusion impedance, and ~30 Hz corresponds to the anode diffusion resistance. At a frequency of ~2 Hz, the impedance magnitude is influenced by both cathode and anode diffusion. The results of this study combined three methods (ADIS, DRT, and ECM) for SOFC impedance investigations, and lays a foundation for future studies regarding SOFC performance degradation and degradation characteristics based on DRT analysis.

Non-fullerene electron acceptors have attracted enormous attention of the research community owing to their advantages of optoelectronic and chemical tunabilities for promoting high-performance polymer solar cells (PSCs). Among them, fused-ring electron acceptors (FREAs) are the most popular ones with the good structural planarity and rigidity, which successfully boost the power conversion efficiencies (PCEs) of PSCs to over 14%. In considering the cost-control of future scale-up applications, it is also worthwhile to explore novel structures that are easy to synthesize and still maintain the advantages of FREAs. In this work, we design and synthesize a new electron acceptor with an unfused backbone, 5, 5'-((2, 5-bis((2-hexyldecyl)oxy)-1, 4-phenylene)bis(thiophene-2-yl))bis(methanylylidene)) bis(3-oxo-2, 3-dihydro-1H-indene-2, 1-diylidene))dimal-ononitrile (ICTP), which contains two thiophenes and one alkoxy benzene as the core and 2-(3-oxo-2, 3-dihydroinden-1-ylidene) malononitrile (IC) as the terminal groups. The synthetic route to ICTP involves only three steps, with high yields. Density functional theory calculations indicate that the non-covalent interactions, O…H and O…S, help reinforce the space conformation between the central core and the terminals. ICTP shows broad and strong absorption in the long-wavelength range between 500 and 760 nm. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of ICTP were measured to be -5.56 and -3.84 eV by cyclic voltammetry. The suitable absorption and energy levels make ICTP a good acceptor candidate for medium bandgap polymer donors. The best devices based on PBDB-T:ICTP showed a PCE of 4.43%, with an open circuit voltage (V_OC) of 0.97 V, a short circuit current density (J_SC) of 8.29 mA∙cm^-2, and a fill factor (FF) of 0.55, after adding 1% 1, 8-diiodooctane (DIO) as the solvent additive. Atomic force microscopy revealed that DIO could ameliorate the strong aggregation in the blended film and lead to a smoother film surface. The hole and electron mobilities of the optimized device were measured to be 9.64 and 2.03 × 10^-5 cm²∙V^-1∙s^-1, respectively, by the space-charge-limited current method. The relatively low mobilities might be responsible for the moderate PCE. Further studies can be performed to enlarge the conjugation length by including more aromatic rings. This study provides a simple strategy to design non-fullerene acceptors and a valuable reference for the future development of PSCs.

Recently, non-fullerene polymer solar cells (NPSCs) have been developed rapidly because of the flexible energy-level variability and excellent optical absorption properties of non-fullerene electron acceptors. Among them, fused-ring electron acceptors (FREAs) with acceptor-donor- acceptor (A-D-A) structures have been extensively exploited in high-performance NPSCs. These FREAs often employ central aromatic fused rings attached to several rigid side-chains and flanked by two electron-deficient terminals. Many efforts have focused on the modification of the central flat conjugated backbone in order to gain broad and strong absorption and dense stacking. However, the preparation of such FREAs is relatively complex, especially for large fused-ring structures. In a previous work, we provided a simple and useful method to extend the effective conjugation length and broaden the absorption spectrum of the acceptor by noncovalent intramolecular interactions. On this basis, in this work, we have designed and synthesized a new A-D-A-type FREA (ITOIC-2Cl) that uses 4, 9-dihydro-s-indaceno[1, 2-b:5, 6-b']dithiophene (IDT) as a central donor unit, bis(alkoxy)-substituted thiophene rings as conformational locking π-bridges between the donor and acceptor units, and cyanoindanones modified with two high-electron-affinity chlorine atoms as end-capping acceptor units. On one hand, we can attain good backbone planarity in the solid state via the noncovalent conformational locking induced by sulfur−oxygen (S···O) and oxygen−hydrogen (CH···O) interactions, which are not strong enough to lock the coplanar conformation in solution, thus simultaneously endowing ITOIC-2Cl with good solubility. On the other hand, we can enhance the intramolecular charge transfer by enhancing the electron deficiency of the terminal groups. The optical and electrochemical properties of ITOIC-2Cl were systematically explored. Moreover, in combination with the donor polymer of [(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1, 2-b:4, 5-b']dithiophene))-alt-(5, 5-(1', 3'-di-2-thienyl-5', 7'-bis(2-ethylhexyl)benzo[1', 2'-c:4', 5'-c']dithiophene-4, 8-dione))] (PBDB-T), the photovoltaic performances of the devices and the corresponding blend morphologies were studied. ITOIC-2Cl exhibited a broad absorption spectrum up to 900 nm, which is beneficial for broad harvesting of photons across the visible and NIR region. The PBDB-T:ITOIC-2Cl-based blend films exhibited favorable fibrous nanostructures with appropriate nanoscale phase separation, verified by atomic force microscopy and transmission electron microscopy characterizations. This morphology is beneficial for charge transport. Through the space-charge-limited current measurement, the PBDB-T:ITOIC-2Cl-based device exhibited the high hole/electron mobility of 1.85 × 10⁻⁴/1.19 × 10⁻⁴ cm²∙V⁻¹∙s⁻¹. The PBDB-T:ITOIC-2Cl-based devices obtained a high power conversion efficiency of 9.37%, with an open-circuit voltage (V_oc) of 0.886 V, short-circuit current (J_sc) of 17.09 mA cm⁻², and a fill factor (FF) of 61.8%. These results thus demonstrate the efficacy of the proposed strategy for designing high-performance non-fullerene FREAs.

Recently, non-fullerene small molecular acceptors (NFSMAs) have received great attention because of their broad and strong absorption spectra and stable active layer morphology when compared with traditional fullerene acceptors. The most widely used strategy to design NFSMAs is through A-D-A type molecules, in which an electron-rich core unit (D) is flanked by two electron-deficient units (A). In order to fine-tune the absorption spectra, energy levels, and photovoltaic properties of NFSMAs, great efforts have been made to modify the conjugated backbone of A-D-A type molecule acceptors. In a previous work, we developed a small molecular electron acceptor, namely MBN-Ph, with an A-D-A structure and an organoboron core unit. MBN-Ph exhibited distinctive absorption spectra with two absorption bands in short- and long-wavelength regions. It is known that side chains or substituents on small molecular electron acceptors can also play an important role in the molecular properties and photovoltaic performance of bulk heterojunction organic solar cells (OSCs). In this work, we report an A-D-A type organoboron compound (MBN-Th) bearing a thienyl substituent on the boron atom, which can be used as an electron acceptor for OSCs. The lowest unoccupied molecular orbital (LUMO) of MBN-Th delocalized on the entire backbone, while the highest occupied molecular orbital (HOMO) localized on the core unit. The unique electronic structure of MBN-Th resulted in two strong absorption peaks at 490 and 726 nm, which indicate a wide absorption spectrum and superior sunlight harvesting capability. Compared with the phenyl substituent, the thienyl group led to an unchanged LUMO energy level, low-lying HOMO energy level by 0.1 eV, and blue-shifted absorption spectrum by 20 nm. OSCs with MBN-Th as an electron acceptor showed a power conversion efficiency of 4.21% and a wide photoresponse from 300 to 850 nm. Our results indicate that the substitution of the boron atom with a thienyl group is an effective strategy to tune the electronic structure of organoboron compounds for applications as electron acceptors in OSCs.

Ternary blends have been considered as an effective approach to improve power conversion efficiency (PCE) of organic solar cells (OSCs). Among them, the fullerene-containing ternary OSCs have been studied extensively, and their PCEs are as high as over 14%. However, all non-fullerene acceptor ternary OSCs are still limited by their relatively lower PCEs. In this work, we used wide-bandgap benzodithiophene-difluorobenzotriazole copolymer FTAZ as the donor, low-bandgap fused-ring electron acceptor (FREA), fused tris(thieno- thiophene) end-capped by fluorinated 1, 1-dicyanomethylene-3-indanone (FOIC) as acceptor, and two medium-bandgap FREAs, indaceno-dithiophene end- capped by 1, 1-dicyanomethylene-3-indanone (IDT-IC) and indacenodithiophene end-capped by 1, 1-dicyanomethylene-3-benzoindanone (IDT-NC), as the third components to fabricate the ternary blends FTAZ:FOIC:IDT-IC and FTAZ:FOIC:IDT-NC, and investigated the effects of the third components on the performance of ternary OSCs. Both IDT-IC and IDT-NC are based on the same indacenodithiophene core but contain different terminal groups (phenyl and naphthyl). Relative to IDT-IC with phenyl terminal groups, IDT-NC with naphthyl terminal groups has extended π-conjugation, down-shifted lowest unoccupied molecular orbital (LUMO), red-shifted absorption and higher electron mobility. The binary devices based on the FTAZ:FOIC, FTAZ:IDT-IC and FTAZ:IDT-NC blends exhibit PCEs of 9.73%, 7.48% and 7.68%, respectively. Compared with corresponding binary devices, both ternary devices based on FTAZ:FOIC:IDT-IC and FTAZ:FOIC:IDT-NC exhibit better photovoltaic performances. When the IDT-IC weight ratio in acceptors is 50%, the FTAZ:FOIC:IDT-IC ternary devices exhibit the best PCE of 11.2%. The ternary-blend OSCs yield simultaneously improved open-circuit voltage (V_OC), short-circuit current density (J_SC) and fill factor (FF) compared with the binary devices based on FTAZ:FOIC. The higher V_OC is attributed to the higher LUMO energy level of IDT-IC compared with FOIC. The improved J_SC is attributed to the complementary absorption of FOIC and IDT-IC. The introduction of IDT-IC improves blend morphology and charge transport, leading to higher FF. The FTAZ:FOIC:IDT-NC system yields a higher PCE of 10.4% relative to the binary devices based on FTAZ:FOIC as the active layer. However, the PCE of the FTAZ:FOIC:IDT-NC-based ternary devices is lower than that of the FTAZ:FOIC:IDT-IC-based ternary devices. Compared with the binary devices based on FTAZ:FOIC, in FTAZ:FOIC:IDT-NC-based ternary devices, as the ratio of the third component increases, the V_OC increases due to the higher LUMO energy level of IDT-NC, the FF increases due to optimized morphology and improved charge transport, while the J_SC decreases due to the overlapped absorption of FOIC and IDT-NC. The terminal groups in the third components affect the performance of the ternary OSCs. The lower LUMO. energy level of IDT-NC is responsible for the lower V_OC of the FTAZ:FOIC:IDT-NC devices. The red-shifted absorption of IDT-NC leads to the overlapping of the absorption spectra of IDT-NC and FOIC and lower J_SC. On the other hand, replacing the phenyl terminal groups by the naphthyl terminal groups influences the π-π packing and charge transport. The FTAZ:FOIC:IDT-NC blend exhibits higher electron mobility and more balanced charge transport than FTAZ:FOIC:IDT-IC, leading to a higher FF.

The development of the photocatalytic production of hydrogen from water splitting has attracted immense attention in recent years. CdS is a potential photocatalyst with a visible light response, though it still suffers from a limited activity for hydrogen production due to the fast recombination of photo-induced electron/hole pairs and the low reaction rate of hydrogen evolution on the surface. Studies on the effect of CdS surface structure and properties on hydrogen production are still very limited. In this work, we prepared three CdS nanocrystals with different morphologies: long rod, short rod, and triangular plate. The prepared samples were well characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area analysis, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). From the results of TEM, XRD and XPS, we find that the three CdS nanocrystals with different morphologies were successfully synthesized. From the PL spectra, we conclude that the area of exposed nonpolar surface and degree of surface defects increase with an increase in aspect ratio. We also performed the photocatalytic hydrogen production reaction using the three CdS crystals. Long rod-like CdS (lr-CdS) exhibits the highest photocatalytic activity, with a hydrogen production rate of 482 μmol·h^-1·g^-1, which is 2.6 times that of short rod-like CdS (sr-CdS) (183 μmol·h^-1·g^-1) and 8.8 times that of triangular plate-like CdS (tp-CdS, 55 μmol h^-1·g^-1). It is found that lr-CdS shows a higher hydrogen production rate than sr-CdS and tp-CdS. We find that the hydrogen production rate is related to the degree of surface defects. Surface defects can trap the photo-induced electrons/holes, thus decreasing their probability of recombination. In addition, these defects can be used to anchor Pd particles to form a heterojunction structure that facilitates the separation of photo-induced charges. Therefore, we also compared three CdS/Pd nanocrystals synthesized with the three abovementioned morphologies with respect to hydrogen production. With 1% (w, mass fraction) Pd, the hydrogen production rate was greatly enhanced compared to all the CdS catalysts. Compared to the unpromoted CdS, the reaction rate is enhanced 43.1, 10.7 and 6.0 times over those of sr-CdS, lr-CdS and tp-CdS, respectively. Notably, the hydrogen production rate with short rod-like CdS/Pd reaches 7884 μmol·h^-1·g^-1, which can be favorably compared with the ever-increasing values reported in the literature. Hopefully, this work provides knowledge on the effect of crystal surface structure and properties on photocatalysis.

Over the past decade, significant progress has been made in theoretical and experimental research in the field of chemical reaction dynamics, moving from triatomic reactions to larger polyatomic reactions. This has challenged the theoretical and computational approaches to polyatomic reaction dynamics in two major areas: the potential energy surface and the dynamics. Highly accurate potential energy surfaces are essential for achieving accurate dynamical information in quantum dynamics calculations. The increased number of degrees of freedom in larger systems poses a significant challenge to the accurate construction of potential energy surfaces. Recently, there has been substantial progress in the development of potential energy surfaces for polyatomic reactive systems. In this article, we review the recent developments made by our group in constructing highly accurately fitted potential energy surfaces for polyatomic reactive systems, based on a neural network approach. A key advantage of the neural network approach is its more faithful representation of the ab initio points. We recently proposed a systematic procedure, based on neural network fitting, for the construction of accurate potential energy surfaces with very small root mean square errors. Based on the neural network approach, we successfully developed potential energy surfaces for polyatomic reactions in the gas phase, including the reactive systems OH₃, HOCO, and CH₅, and the dissociation of gas-phase molecules on metal surfaces, such as H₂O on the Cu(111) surface. These potential energy surfaces were fitted to an unprecedented level of accuracy, representing the most accurate potential energy surfaces calculated for these systems, and were rigorously tested using quantum dynamics calculations. The quantum dynamics calculations based on these potential energy surfaces produce accurate results, which are in good agreement with experiments. We have also proposed a new method for developing permutationally invariant potential energy surfaces, named fundamental-invariant neural networks. Mathematically, fundamental invariants are used to finitely generate the permutation-invariant polynomial ring; thus, fundamental-invariant neural networks can approximate any function to arbitrary accuracy. The use of fundamental invariants minimizes the size of the input permutation-invariant polynomials, which reduces the evaluation time for potential energy calculations, especially for polyatomic systems. Potential energy surfaces for OH₃ and CH₄ were constructed using fundamental-invariant neural networks, with their accuracies confirmed by full-dimensional quantum dynamics and bound-state calculations. These developments in the construction of highly accurate potential energy surfaces are expected to extend the theoretical study of reaction dynamics to larger and more complex systems.

The structure of low-temperature α-Cu₂Se, which is of great importance for understanding the mechanism of the significant increase in thermoelectric performance during the α-β phase transition of Cu₂Se, has still not been fully solved. Because it is restricted by the quality of polycrystal and powder specimens and the accuracy of characterization methods such as the conventional transmission electron microscopy (TEM) and X-ray diffraction (XRD), direct observation with atomic-scale resolution to reveal the structural details has not been realized, although electron diffraction and high-resolution transmission electron microscopy (HRTEM) studies have indicated the complexity of the α-Cu₂Se layered structure. Owing to developments in the focused ion beam (FIB) milling preparation method, high-quality single crystalline specimens with specific crystallographic orientations can be prepared to ensure that atomic-resolution images along a specific orientation can be acquired. Furthermore, the developments in aberration correction technology in TEM and scanning transmission electron microscopy (STEM) allow us to observe the subtle details of structural variation and evolution. Herein, we report, for the first time, the atomic-resolution high-angle annular dark field (HAADF) images acquired along the $ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$ axis of α-Cu₂Se using spherical-aberration (C_s)-corrected STEM from FIB-prepared single crystalline specimens. The observations revealed that the complex structure is generated by ordered fluctuations of Se atoms with various forms, including that some of the Se atoms on the two sides of the Cu deficiency layer get closer to each other than the others and the neighboring Cu deficiency layers have different forms of ordered Se fluctuations. These characteristics can only be observed along the $ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$ axis, while these details were not visible in a previous study along the $ <\bar{1}10{{>}_{\text{c}}} $ axis or in our results obtained along the ${{\left[ \bar{1}01 \right]}_{\text{c}}} $ axis. By combining the electron diffraction patterns, several models of the unit cell variants were established, including the two-layer and four-layer cells (both have two different shapes) and the two-layer variants with and without central symmetry. These variants can also transform into each other, and an α-Cu₂Se crystal can be formed through the random assembly of these variants. Using the program QSTEM, the corresponding HAADF images of these variants were simulated. The simulation results were similar to the experimental HAADF images and reflected most of the observed details, including the different forms of the ordered fluctuations of Se atoms and the dispersion of Cu atoms, which indicates that our structure models of α-Cu₂Se are reasonable. This work provides new critical information for thoroughly understanding the structure of α-Cu₂Se and the α-β phase transition of Cu₂Se.

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.

In recent years, organic solar cells (OSCs) have attracted increasing attention, and the power conversion efficiency (PCE) of OSCs has markedly improved. To enhance the photovoltaic properties of OSCs, it is important to develop the donor materials in the light-harvesting layer, which mainly include conjugated polymers and small molecules (SMs). Compared with polymeric materials, small-molecule materials have been widely investigated for their superior characteristics, such as well-defined molecular structures that can provide good batch-to-batch reproducibility. In this work, we synthesized three SM donor materials with theacceptor-donor-acceptor (A-D-A) structure by employing the trialkylthienyl-substituted benzodithiophene (TriBDT-T) unit as the D-core unit, and rhodanine (RN), cyano-rhodanine (RCN), and 1, 3-indanone (IDO) as the A end groups, respectively. The optical properties, molecular energy levels, and thermogravimetic characteristics of the three SMs were studied; moreover, the blend morphologies and photovoltaic properties of the devices by employing the non-fullerene (NF) acceptor, IT-4F, were systematically investigated. The results showed that 1) the three SMs exhibit good thermal stabilities as evinced by thermogravimetric analysis (TGA), and all decomposition temperatures exceeded 410 ℃; 2) They all exhibit strong and broad absorption in the visible light range (300–700 nm), and show similar molar extinction coefficients; 3) the HOMO levels are -5.47 eV, -5.54 eV, and -5.44 eV for RN, RCN, and IDO, respectively, implying the clear influence of the different end groups for the energy levels of the A-D-A-type SMs; the slight differences in the optical and electrochemical properties of the corresponding donor material could be attributed to the different electron-withdrawing ability of the A-type end groups. When studying the photovoltaic properties, interestingly, the RN:IT-4F blend was found to form fibrillar-like aggregates with appropriate size, and the corresponding devices exhibited desirable short circuit current (J_sc) and thus the highest PCE value of 9.25%; however, large-size aggregates were formed in the RCN:IT-4F and IDO:IT-4F blend films, resulting in a much lower J_sc and fill factor (FF), and the PCE of the corresponding devices were only around 6%. In summary, by introducing RN, RCN, and IDO as the A units, we synthesized a class of TriBDT-T based A-D-A type SMs. This study shows that terminal A units exert influence on the absorption spectra, molecular energy level, and morphologies after blending with the acceptor material, and hence, the corresponding devices exhibit significant differences in photovoltaic performance. This work also provides useful information for the molecular design of SM donor materials.

Nonadiabatic processes widely exist in photochemistry and photophysics. The theoretical treatment of nonadiabatic processes is an important challenge in theoretical chemistry. In nonadiabatic dynamics, the well-known "Born-Oppenheimer approximation" breaks down due to the involvement of strong nuclear-electronic coupled motions. Hence, the development of a theoretical framework is required for the proper treatment of nonadiabatic dynamics. The fewest-switches trajectory surface-hopping method developed by Tully is one of the most widely used methods in the treatment of nonadiabatic processes because of its rather simple numerical implementation. In this approach, the nuclear degrees of freedom are propagated on the potential energy surface of an electronic state using the classical equations of motion, while the electronic degrees of freedom are propagated along the same trajectory according to the time-dependent Schr?dinger equation. Nonadiabatic effects are included by allowing sudden hops between different potential energy surfaces. After averaging over many trajectories, a reasonable description of nonadiabatic dynamics is achieved at low computational cost. Particularly, when we combine the trajectory surface-hopping dynamics with on-the-fly molecular dynamics, it is possible to describe the nonadiabatic dynamics of realistic polyatomic molecular systems at a fully atomic level with all degrees of freedom included. After the introduction of hybrid multiscale methods, the simulation of photochemistry in solutions and in biological systems becomes possible. The simulation results provide important information concerning the nonadiabatic dynamics of realistic polyatomic systems, such as excited-state lifetime, major active molecular motions, reaction channels, and branching ratio. This review article summarizes some progresses in this field. After briefly introducing the basic theory of widely used fewest switches surface-hopping dynamics methods, we mainly focus on several numerical details in the implementation of on-the-fly fewest switches surface-hopping dynamics. The seamless combination of surface-hopping dynamics and electronic-structure calculations is emphasized in this review, rather than an exhaustive discussion of rigorous nonadiabatic dynamics theories. Numerical methods to estimate nonadiabatic coupling terms are discussed, which allow us to perform the trajectory surface-hopping calculations when the nonadiabatic coupling vectors are not available in the electronic structure calculations. Several important issues, such as decoherence corrections, diabatic or adiabatic representations, and initial sampling methods are discussed in detail. We also summarize the theoretical treatment of the nonadiabatic dynamics of some interesting molecular systems, which include the photostability of DNA, photo-isomerization of organic systems, photochemistry of transition metal complexes, and photovoltaics. Finally, we discuss the theoretical challenges of this direct dynamics approach and provide an outlook of this field from our personal perspective.