The process that converts CO₂ to value-added chemical fuels or industrial feedstocks is called the electrochemical carbon dioxide reduction reaction (CO₂RR). When used in combination with renewable energy resources such as solar or wind, it represents one of the most promising strategies for transforming the intermittent renewable energy to chemical energy. However, because CO₂ molecules are thermodynamically stable, their electrochemical reduction is kinetically challenging. CO₂RR also has several different reaction pathways with a large spectrum of reduction products, making its selectivity problematic. It often requires the assistance of highly effective electrocatalysts with excellent activity, selectivity, and durability. Recently, palladium (Pd)-based nanomaterials have attracted considerable attention for CO₂RR. They can enable the selective production of formic acid or formate (HCOOH or HCOO^-) at near the theoretical equilibrium, as well as CO at a more negative potential. Unfortunately, the strong surface affinity of Pd toward CO often results in the deactivation of catalytic activity in the electrocatalytic process, in particular for formate production. Over recent years, extensive research effort has been invested into enhancing the electrochemical performances of Pd-based electrocatalysts. By controlling the size, morphology, and crystal surfaces of Pd nanocrystals, the distribution and structure of the atoms on the catalyst surface can be carefully engineered. For example, reducing the size of Pd nanoparticles has been found to significantly enhance the reaction activity and selectivity for the production of both CO and formate. The high-index crystal surfaces of Pd nanocrystals with low coordination numbers also generally show higher electrocatalytic activities. The design of Pd-based alloy nanostructures with tunable electronic structures represents another effective way to improve the electrochemical performance. Incorporation of non-precious metals can not only reduce the cost, but also effectively weaken the surface binding of CO. In addition, dispersing Pd nanoparticles on high-surface-area supports can increase the surface exposure of active sites and facilitate the formation of the electrochemical active phase. In this perspective, we provide an overview of the recent progress on nanostructured Pd-based catalysts for electrochemical CO₂ reduction. First, we briefly introduce the CO₂RR fundamentals as well as the reaction mechanism on Pd-based nanostructures. We then review a number of strategies to promote CO₂RR performance, including utilizing the size effect, morphology effect, alloy effect, core-shell effect, and support effect. Finally, we conclude with a perspective on the future prospects of Pd-based CO₂RR electrocatalysts, providing readers a snapshot of this rapidly evolving field.

Direct methanol fuel cells (DMFCs), as one of the important energy conversion devices, are of great interest in the fields of energy, catalysis and materials. However, the application of DMFCs is presently challenged because of the limited activity and durability of cathode catalysts as well as the poisoning issues caused by methanol permeation to the cathode during operation. Herein, we report a new class of Rh-doped PdCu nanoparticles (NPs) with ordered intermetallic structure for enhancing the activity and durability of the cathode for oxygen reduction reaction (ORR) and achieving superior methanol tolerance. The disordered Rh-doped PdCu NPs can be prepared via a simple wet-chemical method, followed by annealing to convert it to ordered phases. The results of transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), power X-ray diffraction (PXRD) analysis and high resolution TEM (HRTEM) successfully demonstrate the formation of near-spherical NPs with an average size of 6.5 ± 0.5 nm and the conversion of the phase structure. The complete phase transition temperatures of Rh-doped PdCu NPs and PdCu are 500 and 400 ℃, respectively. The molar ratio of Rh/Pd/Cu in the as-synthesized Rh-doped PdCu NPs is 5/48/47. Benefitting from Rh doping and the presence of the ordered intermetallic structure, the Rh-doped PdCu intermetallic electrocatalyst achieves the maximum ORR mass activity of 0.96 A·mg^-1 at 0.9 V versus reversible hydrogen electrode (RHE) under alkaline conditions—a 7.4-fold enhancement compared to the commercial Pt/C catalyst. For different electrocatalysts, the ORR activities follow the sequence, ordered Rh-doped PdCu intermetallics > ordered PdCu intermetallics > disordered Rh-doped PdCu NPs > disordered PdCu NPs > commercial Pt/C catalyst. In addition, the distinct structure endows the Rh-doped PdCu intermetallics with highly stable ORR durability with unaltered half-wave potential (E_1/2) and mass activity after continuous 20000 cycles, which are higher than those of other electrocatalysts. Furthermore, the E_1/2 of the Rh-doped PdCu intermetallics decreases by only 5 mV after adding 0.5 mol·L^-1 methanol to the electrolyte, while the commercial Pt/C catalyst negatively shifts by 235 mV and a distinct oxidation peak can be observed. The results indicate that the ORR activity of the Rh-doped PdCu intermetallic electrocatalyst can be well maintained even in the presence of poisoning environment. Our results have demonstrated that Rh-doped PdCu NPs with ordered intermetallic structures is a potential electrocatalyst toward the next-generation high-performance DMFCs.

Nanoparticles (NPs) are ideal building blocks for constructing functional materials and devices due to their unique optical, electronic, magnetic, and mechanical properties. The precise assembly and patterning of NPs to obtain ordered structures are vital to explore the special properties of NPs. The specific configurations of large-scale NP assemblies from two-dimensional (2D) NP patterns to one-dimensional (1D) NP arrays on substrates are considered the ideal platform for many technological devices, such as solar cells, magnetic memory, switching devices, and sensing devices, due to their unique transport phenomena and the cooperative properties of NPs in assemblies. Regulation with high-precision control over the orientation and spatial arrangement of nanoarchitecture is required to achieve the coupling and collecting between NPs and thereby translate the properties of the individual NPs to the functions of the macroscopic materials. Therefore, the development of effective methods to build and implement ordered nanocomposites has been accelerated considerably over the last decade. However, due to the complex physics and thermodynamics of the NP assembly, precise control over the orientation and spatial distribution of nanoassemblies with a large area and high homogeneity remains a challenge. In order to tune the position and shape of the NPs into desired structures, a series of strategies and methods have been proposed and developed. These strategies include manipulation of interparticle physical interactions, modification of NP surface chemistry, effect of external fields, utilization of physically or chemically patterned templates, and application of an inkjet printing technique to achieve the desired level of spatial and orientational control over the assembly of NPs. In this paper, we summarized the typical morphologies and the precise control of the architectures prepared by the NPs self-assembly. The particle density, particle size, and interparticle distance of the NP assemblies were strongly controlled. Then the bottom-up strategies for positioning NPs into desired structures with high resolution and considerable throughput were shown. In addition, we discussed the unique functions and diverse applications of the ordered NP assemblies. Both the strong surface plasmon resonance coupling and directional electron transport between particles were studied, which was of highly significance in the development of many technological devices and of great scientific interest. Finally, we investigated the challenges and opportunities of the precise assembly of NPs, which could provide insight and guidance for the future development of functional nanoassembly devices.

Using solar energy to power evaporative processes has found various applications in desalination, wastewater treatment, and power generation among other fields. Due to its green-energy sources and energy-efficient conversions, it has gained increasing attention. However, as one of the oldest approaches, the application of solar evaporation is still limited by issues such as low evaporation efficiency, fouling, and the rapid degradation of solar absorbers. During solar evaporation processes, the solar absorbing materials are directly heated by sunlight and the generated heat is transferred to the water around the material. Within this process, in situ photo-thermal conversion is realized by absorber materials at the air-water interface. After the water is heated, it vapors continuously. Therefore, the material for solar absorption and photo-thermal conversion is key to improving the efficiency of solar evaporation processes. Currently, many approaches are being developed to achieve high-efficient solar evaporation, such as the photon management, nano-scale thermal regulation, the development of new photo-thermal conversion materials, and the design of efficient light-absorbing solar stiller. Carbon-based materials including carbon nanotubes, graphene, carbon black, graphite, etc. have broad light-absorption profiles over the entire solar spectrum, which makes them the outstanding photo-thermal conversion materials. Herein, we design a new structure and house-like solar still on the basis of carbon-based materials to achieve high light absorption, efficient photo-thermal conversion, and continuous desalination. We use the chemical vapor deposition (CVD) technique to fabricate a reticulated carbon-nanotube solar evaporation membrane. Stainless steel mesh (SSM) is used as a reticulated skeleton, providing porous structures and increasing the mechanical strength of the membrane. Then the carbon nanotubes (CNTs) are grown on the reticulated skeleton to function as solar conversion structures due to their wide range of light absorption capacities. The CVD grown CNTs reticulated membrane (CGRM) is fixed in a house-like device with a sloped ceiling to condense and collect water vapors ensuring the continuous desalination of water. Our experiments show that the fabricated CGRM is hydrophobic with an average contact angle of 133.4° for a 100.0 g·L^-1 NaCl solution, only allowing water vapors to pass through while rejecting salts. When the light intensity was 1 kW·m^-2, the surface temperature of the membrane increased rapidly and stabilized at 84.37 ℃. The salt rejection rate of the system could reach up to 99.92%.To perform a comparative study, we also prepared a mechanically-filled CNTs reticulated membrane (MFRM1, MFRM2) for solar evaporation tests, which showed an inferior performance to that of the growing structure of the CGRM. Therefore, it was determined that our system might provide a potential way to harvest freshwater readily with portable-type equipment.

Selective hydrogenation of dienes and alkynes to monoenes is an important topic of research in the fields of pharmacology and organic synthesis. Catalyst design plays a key role in this process, where a general principle involves controlling the steric diene adsorption by modifying the surface of the metal nanoparticles. For example, upon introducing Bi species into Rh nanoparticles, the resulting RhBi/SiO₂ showed 90% selectivity to 2-hexene, with 95% conversion of 1, 4-hexadiene under ambient conditions, because of the suppressed adsorption of the internal C＝C bond. However, the catalyst activity decreased remarkably; that is, the activity of the unmodified Rh/SiO₂ was about 27 times higher than that of RhBi/SiO₂. Controlled steric adsorption of the diene molecules could also be achieved by the constructing porous channels around the metal nanoparticles. For example, metal-organic framework (ZIF-8) or mesoporous silica (MCM-41) encapsulated noble metals showed high selectivity for the hydrogenation of terminal C＝C bonds. However, these catalysts had poor durability under the thermal/hydrothermal reaction/regeneration conditions. In contrast, zeolites have superior durability under harsh reaction conditions, but they are rarely used in semi-hydrogenation reactions. We recently found that metal nanoparticles fixed within zeolite crystals (e.g., ZSM-5 and Beta) efficiently catalyze the selective hydrogenation of molecules bearing multiple reducible groups. Thus inspired, we developed a catalyst by fixing Rh nanoparticles within zeolite crystals via an inter-zeolite transformation method. The Rh@CHA catalyst was synthesized by introducing Rh species into the parent Y zeolite (Rh@Y) and transformation of the Y zeolite to chabazite (CHA zeolite) under hydrothermal conditions. X-ray diffraction patterns, N₂ sorption isotherms, scanning/transmission electron microscopy images, and model reactions (hydrogenation of probe molecules) confirmed the successful fixation of the Rh nanoparticles inside the CHA zeolite crystals. As expected, the Rh@CHA catalyst was highly selective for the hydrogenation of dienes. For example, Rh@CHA showed a 2-hexene selectivity of 86.7%, with 91.2% conversion of 1, 4-hexadiene. In contrast, the generally supported Rh nanoparticle catalyst (Rh/CHA) showed a low 2-hexene selectivity of 37.2% under identical reaction conditions. Considering that Rh@CHA and Rh/CHA comprise the same CHA zeolite crystals and have similar Rh nanoparticle sizes, the remarkably high selectivity of Rh@CHA is assigned to the steric adsorption of dienes on the Rh surface controlled by the micropores of the CHA zeolite. This work demonstrates that a zeolite-fixed metal core-shell structure is a powerful tool for developing efficient catalysts to be used in diene hydrogenation.

As a type of layered material, layered double hydroxides (LDHs) exhibit high development potential and application prospects, and have been used widely in adsorbents, catalysts, ion exchangers, flame retardants, biology, sensing, medicine, and other fields. With the continued development in nanoscience and nanotechnology, it has been established that monolayer LDHs contain an abundance of exposed highly unsaturated coordination sites, and so display unexpected functionality. However, due to the higher charge density of the LDHs layers, the strong interactions between the layers, and the hydroxyl groups on the surface of the layers, the result is a compact stacking of the layers. Consequently, it is still a great challenge to synthesize high-quality monolayer LDHs. Despite various methods of preparing monolayer LDHs having been developed, which can generally be divided into top-down and bottom-up strategies, most of these approaches have used organic solvents, which take a long time to achieve the exfoliation of LDHs, or require special equipment. Furthermore, high costs and the low yields have prevented large-scale production of monolayer LDHs. With the rapid development of the national economy, the industrial preparation of monolayer LDHs has become an inescapable trend. The separate nucleation and ageing method for the preparation of nanostructured LDHs is a feasible method, the key features of which are a very rapid mixing and nucleation process in a colloid mill, followed by a separate ageing process. This method has been successfully applied to a pilot plant in China for the industrial-scale synthesis of LDHs materials. It should be noted that the particle size distribution of LDHs obtained by this method can be well controlled. Moreover, the synthesis operation is simple, and quick (with a short duration of only several minutes). Through new in-depth technology studies on two-dimensional layered materials, large-scale preparation, and industrial application of monolayer LDHs will certainly be increasingly realized, and ultimately transformed into economic benefits. In this review, we summarize the synthesis method of monolayer LDHs, describe the necessary characterization technologies that have been used to study monolayers LDHs nanosheets, such as X-ray diffraction, transmission electron microscopy, and atomic force microscopy. Then we discuss the applications in various fields, such as photocatalysis, electrocatalysis, batteries, supercapacitors, membrane materials, and biomedical fields. We further discuss the recent breakthroughs in the synthesis of monolayer and ultrathin LDHs and the advance of production scale-up of LDHs. Finally, the performance of monolayer/ultrathin LDHs is summarized to provide a basis for the ensuing design of high-performance monolayer LDHs.

Gold nanoparticles (AuNPs) have been widely applied in the biomedical field due to their tunable localized surface plasmon resonance (LSPR) properties, versatile surface modifiability, and favorable biocompatibility. With the in-depth study, individual AuNPs cannot meet the requirements for multifunctional biological applications. By combining AuNPs with other inorganic materials, material scientists and chemists have successfully prepared various gold-based nanocomposites (AuNCs), such as Au-metals, Au-semiconductors, Au-magnetic nanocomposites, etc. Furthermore, despite their compositions being the same, products with different topological morphologies (i.e. core@shell, yolk@shell, core-satellite, and Janus) can be fabricated by sophisticated chemistry. Based on their special collective properties and synergetic effects, multifunctional AuNCs have been proposed for tumor theranostic applications; their parts exhibit impressive imaging/therapeutic performances and present tremendous application perspectives for basic research and at a pre-clinical stage. For example, when Au is combined with porous drug carriers, its photothermal properties can promote drug release, which can be useful for developing intelligent drug delivery platforms. In addition, Au and magnetic material hybrids can be used in multimodal imaging and combination therapy, depending on the integration of its optical and magnetic properties. Moreover, when Au is combined with semiconductor material, either LSPR coupling effects or nonradiative energy transfer occurs between them, causing enhanced photothermal or photodynamic therapy. Therefore, in this review, we have highlighted the fabrication approaches of AuNCs including the one-step method, seed-mediated growth method, ex situ assembly method, etc. For the one-step method, we emphasize that Au and other components nucleate and grow concurrently, which is frequently employed for the preparation of Au based alloys. The key to synthesizing AuNCs by seed-mediated growth method is the degree of lattice matching between the different components and the interactions of their heterogeneous interfaces; if the crystal lattices are well-matched, AuNCs can be obtained by direct epitaxial growth. Furthermore, approaches including the introduction of a ligand/surfactant, intermediate layer, and molecular weak interactions into the heterogeneous interfaces can regulate the two components' interaction and fabricate various AuNCs. For the ex situ assembly method, prefabricated AuNPs and other nanoparticles can be assembled into AuCNs by electrostatic attraction, host-guest reaction, bio-recognition, covalent binding, etc. We also summarize the recent achievements of typical AuNCs (i.e. Au-MOF, Au-Fe₃O₄, and Au-Cu_2–xS) in tumor theranostic applications based on their collective properties and/or synergetic effects, respectively. Finally, the main problems and the future developments of this research field are also discussed.

As opposed to nanoparticles, atomically precise metal clusters possess a well-defined surface and crystal structure, which aids in understanding the relationship between the structure and chemical reactivity at the atomic level. As an interesting subgroup of metal cluster compounds, heterometallic lanthanide-titanium oxo clusters (LnTOCs) have attracted extensive attention due to their interesting chemical properties. However, the controlled precise synthesis of LnTOCs remains a great challenge because of the intense hydrolysis of Ti⁴⁺ ions and the competitive coordination of Ln³⁺ ions. Owing to this synthetic difficulty, high-nuclearity LnTOCs are very rare, which obstructs further studies on their properties. Choosing the appropriate chelating ligands should be an effective strategy to synthesize LnTOCs because chelating ligands can reduce the degree of hydrolysis of Ti⁴⁺ ions. Herein, four new LnTOCs, formulated as [EuTi₆(μ₃-O)₃(OC₂H₅)₈(dtbsa)₆(Hdtbsa)]·(C₂H₅OH) (1), [EuTi₇(μ₃-O)₃(μ₂-OH)₂(OⁱPr)₉(dtbsa)₆(Hdtbsa)Cl]·(HOⁱPr)₃ (2), [EuTi₇(μ₃-O)₃(μ₂-OH)₂(OⁱPr)₈(dtbsa)₇(Hdtbsa)]·(HOⁱPr)₃ (3), and [LaTi₇(μ₃-O)₃(μ₂-OH)₂(OC₂H₅)₈(dtbsa)₇(Hdtbsa)]₂·(C₂H₅OH)₄ (4), were prepared by a solvothermal method via the reaction of 3, 5-di-tert-butylsalicylic acid (H₂dtbsa), rare-earth salts, and Ti(OⁱPr)₄. Single-crystal analysis showed that the heptanuclear compound 1 contains a EuTi₆ metal core featuring a trigonal prismatic structure, wherein Eu³⁺ is located at the center of the prism formed by six Ti⁴⁺ ions. The metal core structure of octanuclear compounds 2 and 3 can be viewed as the EuTi₆ unit in 1 connected to another Ti⁴⁺ on one side of the triangular prism. The metal framework of Ln₂Ti₁₄ in 4 can be regarded as a dimer of EuTi₇ in 2. UV-Vis diffuse reflectance spectra revealed that the band gaps of 1, 2, and 3 (2.35, 2.07, and 2.16 eV, respectively) are significantly smaller than that of anatase (3.2 eV). The results of photoelectric tests indicated that the three clusters show an obvious photoelectric response, and the charge separation efficiency of 1 and 2 was better than that of 3. In order to explore the applications of these compounds to photocatalysis, H₂ production by light-driven water splitting under irradiation by a 300 W Xe lamp (300–800 nm) in an aqueous methanol solution (20 mL, 10%) was attempted. The H₂ production rates for 1, 2, and 3 were 112, 106, and 87 μmol∙h^-1∙g^-1, respectively, which were higher than that obtained with the commercial P25. Powder X-ray diffraction (PXRD) spectra and thermogravimetry (TGA) profiles confirmed the optical and thermal stability of the three clusters. This work not only provides a chelating ligand strategy for the synthesis of LnTOCs but also reveals their light-driven photocatalytic activity stemming from the small band-gap.

Solid materials containing frustrated Lewis pairs (FLPs) as active sites have attracted much attention due to their ability to activate and transform small molecules. However, it is still highly challenging to precisely construct FLP sites on the surfaces of nanomaterials, thereby limiting the applications of these materials. Nanostructured ceria (CeO₂) is commonly employed as a catalyst or functional support, and exhibits both Lewis acid and basic properties as well as abundant and easily regulated surface defects, which originate from the reversible Ce³⁺/Ce⁴⁺ redox pair. When the Lewis acid and base sites of CeO₂ are independent of each other, the combined Lewis acid-base sites play a similar role to that of homogeneous FLP sites. Thus, the rich surface properties of nanostructured CeO₂ provide significant potential for the construction of solid FLPs.

Herein, we demonstrate that solid FLP sites can be successfully constructed on the surface of CeO₂(110) via the regulation of surface defect clusters, which can be used to create new Lewis acid sites composed of two adjacent Ce³⁺ atoms on the surface. Novel interfacial FLP sites can then be formed by combining these Lewis acid sites with neighboring surface lattice oxygens, which act as Lewis base sites. Porous CeO₂ nanorods (PN-CeO₂) with boundary surface defects were prepared by a special two-step hydrothermal process, and exhibited remarkable catalytic FLP properties. Hydrogen molecules could be effectively activated on the surface of PN-CeO₂ with a low activation energy of 0.17 eV via a heterolytic cleavage process. Hydrogenation of alkenes and alkynes to alkanes could then be realized by the activated hydrogen under mild reaction conditions.

PN-CeO₂ nanorods with FLP active sites were also able to activate CO₂ molecules effectively. Unlike in other solid FLP sites, CO₂ molecule activation was realized via a Lewis base site binding with the C atom while two Lewis acid sites bound the two O atoms, owing to the unique configuration of the FLP sites in PN-CeO₂. When combined with the epoxidation of olefins by "isolated" Ce³⁺ sites in PN-CeO₂, the FLP-inspired activated CO₂ could be used to transform olefins and CO₂ to cyclic carbonates through a selective tandem transformation route. In addition, density functional theory studies indicate that the FLP sites on CeO₂(110) can activate the C―H bond of CH₄ with activation energies as low as 0.63 eV, which can be attributed to the enhanced acidity and basicity of the FLP sites.

With this improved understanding of solid FLP sites constructed on ceria, we have also been able to summarize the challenges and prospects in this field, including their construction, characterization, and mechanism analysis.

Fuel cells, whose energy source can be hydrogen, formic acid, methanol, or ethanol, have received considerable attention in recent years because of their environmentally friendly characteristics. A high Pt loading is often required to achieve a practical power density in fuel cells, thus leading to high costs and limited applications. Meanwhile, the high Pt loading promotes aggregation during cycling under harsh electrocatalytic conditions, which reduces the surface area of the catalyst and leads to a decrease in catalytic activity. The formation of alloy or intermetallic nanocrystals via the addition of non-precious metals along with precious metals is one strategy to effectively reduce the cost. Due to the electronic and geometric effects introduced by the non-precious metals, the catalytic performance of these bimetallic nanocrystals can be retained or even improved. Compared to the metallic alloy nanocrystals, the intermetallic ones are more stable in critical catalytic conditions. Due to their highly ordered structures, Pt-based intermetallic nanocrystals are widely used as electrode materials for various electrocatalytic reactions in fuel cells, and they show high stability against oxidation and etching. PtCo intermetallic nanocrystals have attained performances that exceed the 2020 target of the U.S. Department of Energy (DOE) for Pt activity and stability for the cathode reaction of fuel cells (oxygen reduction reaction). Decreasing the size of intermetallic compounds to the nanometer scale can significantly increase their active site densities due to the large specific surface area. However, the preparation of intermetallic nanocrystals is more complicated than that of alloys. Therefore, to further improve the electrocatalytic properties of intermetallic nanocrystals, an in-depth study of the factors affecting the electrocatalytic properties of nanocrystals is necessary. This review summarizes recent advances in Pt-based intermetallic nanocrystals. First, we highlight the controlled synthesis strategies, including direct liquid-phase synthesis, the thermal annealing approach, and chemical vapor deposition. Of these strategies, direct liquid-phase synthesis is the most common approach to prepare the intermetallic nanocrystals. Second, the diverse potential applications of different electrocatalytic reactions are summarized. The reactions include the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and hydrogen oxidation reaction (HOR), as well as the oxidation reactions of formic acid (FAOR), methanol (MOR), and ethanol (EtOR). Of these reactions, ORR is the most important, and it has been widely studied. Some advanced characterization techniques and machine learning research based on density functional theory (DFT) are also mentioned. Finally, the challenges and the future perspectives of intermetallic nanocrystals are outlined.

The assembly of two-component nanocrystals (NCs) such as metals, magnets, and semiconductors into binary nanocrystal superlattices (BNSLs) provides a fabrication route to novel classes of materials. BNSLs with certain structures can exhibit the combined and collective properties of their building blocks and are widespread in the fields of electronics and magnetic devices. As most studies have focused on combined two-component NCs of different sizes for self-assembling BNSLs, there are a few studies on single-component NCs of different sizes for the construction of BNSLs; this is especially true for Au NCs. Noble metallic Au NCs are an excellent candidate material because of their exceptional chemical stability, catalytic activity, process ability, and metallic nature; these characteristics provide them unique size-dependent optical and electronic properties as well as a wide variety of applications in sensing, imaging, electronic devices, medical diagnostics, and cancer therapeutics owing to their strong interactions with external electromagnetic fields. Therefore, it is important to develop a simple and efficient procedure to build BNSLs with different sizes of Au NCs. In our study, we synthesized monodispersed (size distribution < 10%) 6.0, 7.3, and 9.6 nm Au NCs using dodecanethiol-stabilized 3.7 nm Au NCs as seeds through a seed-growth method in oleylamine. The obtained Au NCs exhibited morphology and nanocrystallinity (single-domain and polycrystalline) similar to those of Au seeds. As the size of Au NCs increased from 3.7 to 6.0, 7.3, and 9.6 nm, the surface plasmon resonance peaks narrowed and indicated a red shift. The oleylamine-functionalized 6.0, 7.3, and 9.6 nm Au NCs were mixed with 3.7 nm Au NCs at certain concentration ratios. Au BNSLs with AB₂ (hexagonal AlB₂ structure), AB₁₃ (NaZn₁₃ structure), and AB (cubic NaCl structure) type were obtained through the solvent evaporation method. The (001) plane of the AlB₂-type structure, (001) plane of the NaZn₁₃-type structure, and (100) plane of NaCl-type structure superlattices were observed through transmission electron microscopy (TEM). The effective particle size ratios (γ= D_small/D_large) serve as the critical determining factor in the formation of the BNSLs. The effective particle size of NCs is equal to the sum of the metal core diameter and twice the thicknesses of the surface ligand. In our study, the effective particle size D_small (Au seed) is 5.7 nm; the effective particle sizes D_large (6.0, 7.3, and 9.6 nm Au NCs) are 9.0, 10.3, and 12.6 nm, respectively. The effective particle size ratios γ were therefore calculated to be 0.63, 0.55, and 0.45, respectively. The relevant space filling principle predicted the stability of the AlB₂, NaZn₁₃, and NaCl-type structures in the range of 0.482 < γ < 0.624, 0.54 < γ < 0.625, and γ < 0.458, respectively; the experimental results adequately matched the relevant space filling principle. The investigation of such a single nanocomponent as a building block is noteworthy with regard to the structures and properties of BNSLs as well as the potential development of novel meta-materials.

Dynamic regulation of self-assembly is of vital importance in chemistry, biology and material science thanks to its great potential for development of smart materials and devices. Polyoxometalates (POMs) are a class of functional inorganic nanoclusters, which has become one of the excellent building blocks for supramolecular self-assemblies, especially when covalently or non-covalently modified by organic species. As typical stimuli-responsive functional clusters, the POMs could be photochemically or electrochemically reduced to mixed-valence states, of which the structural integrity remains even after encountering stepwise multi-electron redox process. The intriguing photochromism of the POMs in different states exhibits distinct photophysical properties, which motivates us to exploit the dynamic self-assemblies of POM-based complexes. The divalent Lindqvist-type hexamolybdate cluster [Mo₆O₁₉]^2- is one of the least negative-charged POMs, which is the ideal building blocks to construct novel assembly structures. Based on this motivation, herein, a single chain surfactant-encapsulated polyoxometalate (POM) complex (ODTA)₂[Mo₆O₁₉] was prepared by simple counterion replacement of Lindqvist-type (TBA)₂[Mo₆O₁₉] with octadecyltrimethylammonium (ODTA) in acetonitrile solution. The structure of the POM complex was confirmed by ¹H nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) and elemental analysis. The solution of complex (ODTA)₂[Mo₆O₁₉] in the mixed solvents of acetonitrile and isopropanol with the volume ration of 4 to 1 exhibited reversible photochromism upon alternate UV light irradiation and air exposure. Upon UV light irradiation, the light yellow transparent solution of (ODTA)₂[Mo₆O₁₉] turned into blue quickly. The new broad absorption band appearing at ca.751 nm assigned to the Mo^V → Mo^VI intervalence charge-transfer (IVCT) transition, indicated the formation of reduced POM, as revealed by UV-Vis absorption spectra. After exposed to air, the blue solution was bleached. The alternate photochromism could be conducted for multiple cycles. Helical self-assembled morphology of (ODTA)₂[Mo₆O₁₉] was formed in acetonitrile/isopropanol, characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. More interestingly, morphology transformation of the complex from helical strips to spherical assemblies occurred accompanied by photochromism occurrence. The morphology evolution during the photochromism process experienced from shortened helical strips through sea urchin-like aggregates to spherical assemblies. Most significantly, the helical assemblies could be recovered again after air oxidation, implying the reversible morphology transformation driven by redox stimulus. The redox-modulated reversible self-assembly is driven by the variation of electrostatic attraction between organic cations and inorganic anions as well as the electrostatic repulsion between inorganic ionic clusters, proved by X-ray photoelectron spectroscopy (XPS) and ¹H NMR spectra. The results will contribute to better understanding the mechanism of dynamic assemblies and inspire the precise fabrication of advanced smart materials.