Room-temperature epitaxial welding of 3D and 2D perovskites.

Formation of epitaxial heterostructures via post-growth self-assembly is important in the design and preparation of functional hybrid systems combining unique properties of the constituents. This is particularly attractive for the construction of metal halide perovskite heterostructures, since their conventional solution synthesis usually leads to non-uniformity in composition, crystal phase and dimensionality. Herein, we demonstrate that a series of two-dimensional and three-dimensional perovskites of different composition and crystal phase can form epitaxial heterostructures through a ligand-assisted welding process at room temperature. Using the CsPbBr3/PEA2PbBr4 heterostructure as a demonstration, in addition to the effective charge and energy transfer across the epitaxial interface, localized lattice strain was observed at the interface, which was extended to the top layer of the two-dimensional perovskite, leading to multiple new sub-bandgap emissions at low temperature. Given the versatility of our strategy, unlimited hybrid systems are anticipated, yielding composition-, interface- and/or orientation-dependent properties.

Study on Room-Temperature Wet Oxidation of Silicon Catalyzed by Copper.

The anomalously fast growth of the silicon oxide layer at room temperature has been reported for the Cu/Si system. However, the systematical exploration of such a reaction under humidity conditions has not yet been carried out. Through one combination of the experiments and first-principle density functional theory (DFT) simulations, here, we investigate the influence of the imparted Cu atoms in Cu/Si on the oxidation of Si with the presence of H2O. The Cu addition causes the geometric distortion of the Si lattice, which alters the charge transfer to absorbed H2O and decreases its dissociation energy. This results in the experimental formation of much defective SiOx for the Cu/Si system than bare Si under humidity conditions. Furthermore, the presence of such an oxide structure and the catalytic effect of Cu provide the suitable diffusion channels and adsorption sites for the H2O transport and its dissociation. This enhances the oxidation rate of Si consequently and results in the fast growth of the oxide layer on Cu/Si at room temperature.

Paving Metal-Organic Frameworks with Upconversion Nanoparticles via Self-Assembly.

The combination of metal-organic frameworks (MOFs) and luminescent nanomaterials with upconversion characteristics could enable the development of new nanomaterials and applications in information security, optical sensing, and theranostics. However, currently available methods are not ideally suitable for fabricating composites of MOF and upconversion nanomaterial, and incorporating upconversion nanomaterials with MOFs in a controllable manner remains challenging. Here, we demonstrate an in situ self-assembly route to the nanocomposites in which MOFs are homogeneously paved with upconversion nanoparticles. Without additional assistance, this strategy, mainly driven by electrostatic interactions, can be used to incorporate different upconversion nanoparticles with diverse MOFs. The as-synthesized composites can be further used to construct composites with unique structures, such as MOF@upconversion nanoparticles@MOF sandwiched nanocomposites, and would be useful for applications including luminescence-monitored drug delivery, anticounterfeiting, and photodynamic therapy. These findings should shed light on new avenues for fabricating multifunctional composites of MOF and upconversion nanomaterials for varied applications.

Parallel near-field photolithography with metal-coated elastomeric masks.

Developing a cost-effective nanolithography strategy that enables the production of subwavelength features with various shapes over large areas is a long-standing goal in the nanotechnology community. Herein, an inexpensive nanolithographic technique that combines the wafer-scale production capability of photolithography with the subwavelength feature size controllability of near-field photolithography was developed to fabricate centimeter-scale up to wafer-scale sub-100-nm variously shaped nanopatterns on surfaces. The wafer-scale elastomeric trench-based photomasks with subwavelength apertures created at the apexes were compatible with mask aligners, allowing for the production of wafer-scale subwavelength nanopatterns with adjustable feature sizes, shapes, and periodicities. The smallest feature sizes of 50 and 80 nm were achieved on positive tone and negative tone photoresist surfaces, respectively, which could be ascribed to a near-field optical effect. The fabricated centimeter-scale nanopatterns were functionalized to study cell-matrix adhesion and migration. Compared to currently developed nanolithographic methods that approach similar functionalities, this facile nanolithographic strategy combines the merits of low cost, subwavelength feature size, high throughput, and varied feature shapes, making it an affordable approach to be used in academic research for researchers at most institutions.

Hybrid crystals comprising metal-organic frameworks and functional particles: synthesis and applications.

Hybrid crystals containing encapsulated functional species exhibit promising novel physical and chemical properties. The realization of many properties critically depends on the selection of suitable functional species for incorporation, the rational control of the crystallinity of the host materials, and the manipulation of the distribution of the encapsulated species; only a few hybrid crystals achieve this. Here, a novel synthetic method enables the encapsulation of functional species within crystalline metal-organic frameworks (MOFs). Various kinds of single-crystalline MOFs with incorporated particles are presented. The encapsulated particles can be distributed in a controllable manner, and the hybrid crystals are applied to the heterogeneous catalysis of the reduction of nitroarenes. These findings suggest a general approach for the construction of MOF materials with potential applications; by combining species and MOFs with suitable functionalities, new properties--not possible by other means--may arise.

Self-assembled metal-organic frameworks crystals for chemical vapor sensing.

A 3D metal-organic frameworks (MOFs) crystals film is obtained via Langmuir-Blodgett technique and used as a photonic sensor for chemical vapor detection. The MOFs crystals film exhibits both acute responses towards various chemical vapors and high controllability in terms of peak intensity and position. The method represents a general, facile and flexible strategy for the fabrication of MOFs-based photonic sensors.

Facile synthesis of Cu-based catalysts from Cu3Si and their catalysis properties study.

Supported copper species are well-known for their remarkable catalytic properties across numerous reactions. However, the current preparation methods pose challenges for large-scale production. In this study, we present a cost-effective method for the facile preparation of a series of copper-silicon composites using Cu3Si@Si particles as precursors. We evaluate the catalytic properties of these composites in the conversion of 4-nitrophenol to 4-amionphenol. Notably, the Cu@SiOx/Si composite exhibits exceptional catalytic performance, attributed to the synergy effect between Cu and Si, and the formation of a metastable Si-H2 complex that enhances the reaction kinetics. This research introduces a novel approach for creating efficient and stable catalysts for hydrogenation reactions.

Vapor-liquid-solid growth of endotaxial semiconductor nanowires.

Free-standing and in-plane lateral nanowires (NWs) grown by the vapor-liquid-solid (VLS) process have been widely reported. Herein, we demonstrate that the VLS method can be extended to the synthesis of horizontally aligned semiconductor NWs embedded in substrates. Endotaxial SiGe NWs were grown in silicon substrates by tuning the directional movement of the catalyst in the substrates. The location of the SiGe NWs can be controlled by the SiO(2) pattern on the silicon surface. By varying the growth conditions, the proportion of Ge in the obtained NWs can also be tuned. This approach opens up an opportunity for the spatial control of the NW growth in substrates and can potentially broaden the applications of NWs in new advanced fields.

Surface-functionalized boron nanosheets and their assembled suprastructures with unprecedented proton-transport properties.

Two-dimensional (2D) boron nanomaterials have received considerable attention due to their distinct physicochemical properties in contrast to bulk boron. However, the susceptibility to oxidation in air has limited their practical applications. In this study, we synthesize an environmentally stable bifunctionalized boron nanosheet via a wet chemical route. By lyophilization, we have hierarchically assembled the boron nanosheets into various well-ordered macroscopic forms, which exhibit unique structural features, such as stacking-induced nanochannels for proton transport. The resulting suprastructures show exceptionally high proton conductivity (∼90 mS cm-1 at 85 °C) and humidity sensitivity (response >40 000% at 97% RH). These findings demonstrate the immense potential of boron nanomaterials in electrochemical applications.

Scalable synthesis of hydroxyl-functionalized boron nanosheets for high ion-conductive solid-state electrolyte applications.

A hydroxyl-functionalized boron nanosheet is developed as the filler material for the solid-state electrolyte (SSE) of lithium batteries. The nanosheet exhibits good oxidation resistance and thermal stability. Its composite SSE shows high ionic conductivity, and the resulting batteries present much enhanced capacities, rate capability and cycling performance, proving the electrochemical advances of the boron nanosheet.

Realization of Oriented and Nanoporous Bismuth Chalcogenide Layers via Topochemical Heteroepitaxy for Flexible Gas Sensors.

Most van der Waals two-dimensional (2D) materials without surface dangling bonds show limited surface activities except for their edge sites. Ultrathin Bi2Se3, a topological insulator that behaves metal-like under ambient conditions, has been overlooked on its surface activities. Herein, through a topochemical conversion process, ultrathin nanoporous Bi2Se3 layers were epitaxially deposited on BiOCl nanosheets with strong electronic coupling, leading to hybrid electronic states with further bandgap narrowing. Such oriented nanoporous Bi2Se3 layers possessed largely exposed active edge sites, along with improved surface roughness and film forming ability even on inkjet-printed flexible electrodes. Superior room-temperature NO2 sensing performance was achieved compared to other 2D materials under bent conditions. Our work demonstrates that creating nanoscale features in 2D materials through topochemical heteroepitaxy is promising to achieve both favorable electronic properties and surface activity toward practical applications.

A solvent decomposition and explosion approach for boron nanoplate synthesis.

A new approach was applied to synthesize boron nanoplates based on layer exfoliation of etched MgB2 particles through a controlled decomposition and explosion of a widely used dipolar aprotic solvent: dimethyl sulfoxide (DMSO). These nanoplates are environmentally stable, surface-functionalized, and reveal excellent electrochemical performance as an anode material for lithium-ion batteries.

Surface-induced synthesis and self-assembly of metal suprastructures.

A systematic study on the synthesis of a series of self-assembled suprastructures, such as cubes, stars, belts, and microspheres, of Ag nanoparticles (AgNPs) in borosilicate glassware heavily cleaned with aqua regia is presented. These self-assembled structures are mostly formed from the crystallographically iso-oriented AgNPs, and exhibit well-defined shapes. In regular washed glassware, only Ag nanowires are synthesized. The formation mechanisms of these self-assembled Ag structures, based on monitoring of their structural evolution in glassware decorated with different molecules, are proposed. This work not only demonstrates that the surface energy of glassware can affect chemical synthesis, but also provides an interesting approach to the shape-controlled synthesis of novel self-assembled suprastructures of AgNPs, which could be potentially used as synthesis templates, drug vessels, and microreactors.

Imparting Boron Nanosheets with Ambient Stability through Methyl Group Functionalization for Mechanistic Investigation of Their Lithiation Process.

Although ultrahigh theoretical capacity has long been predicted for boron-based lithium-ion battery anodes, experimentally, boron has exhibited only limited performance and its lithiation process remains elusive. The two-dimensional (2D) form of boron is believed to be an ideal model system to investigate the lithiation behavior of boron; however, unfortunately, most reported 2D boron structures are prone to oxidation under ambient conditions. In this contribution, through a simultaneous etching and in situ functionalization process, we synthesized for the first time methyl-functionalized boron nanosheets, which remain stable up to 250 °C. Combining experiments and theoretical calculations, we found that lithiation of boron is realized through the formation of alloys such as LiB3 and Li3B14, while alloys with higher Li content such as Li5B are thermodynamically less favored. In addition, detailed electrochemical analysis reveals that side reactions on the boron surface may also contribute to the unsatisfactory performance of boron-based electrodes. Our findings suggest that reducing the enthalpy of formation of high Li content alloys and the choice of a less nucleophilic electrolyte are key to developing high-performance anodes based on novel boron materials. Our demonstration of stable 2D boron structures also paves the way for their fundamental study and practical applications.

Realization of vertical metal semiconductor heterostructures via solution phase epitaxy.

The creation of crystal phase heterostructures of transition metal chalcogenides, e.g., the 1T/2H heterostructures, has led to the formation of metal/semiconductor junctions with low potential barriers. Very differently, post-transition metal chalcogenides are semiconductors regardless of their phases. Herein, we report, based on experimental and simulation results, that alloying between 1T-SnS2 and 1T-WS2 induces a charge redistribution in Sn and W to realize metallic Sn0.5W0.5S2 nanosheets. These nanosheets are epitaxially deposited on surfaces of semiconducting SnS2 nanoplates to form vertical heterostructures. The ohmic-like contact formed at the Sn0.5W0.5S2/SnS2 heterointerface affords rapid transport of charge carriers, and allows for the fabrication of fast photodetectors. Such facile charge transfer, combined with a high surface affinity for acetone molecules, further enables their use as highly selective 100 ppb level acetone sensors. Our work suggests that combining compositional and structural control in solution-phase epitaxy holds promises for solution-processible thin-film optoelectronics and sensors.

Composition- and phase-controlled synthesis and applications of alloyed phase heterostructures of transition metal disulphides.

The facile tuning of the composition and structures of two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets is essential for improving their performance in various applications, but remains difficult to realize via a direct solution process. Here, we report the one-step liquid-phase preparation of alloyed Mo1-χWχS2 nanosheets with tunable 1T/2H phase ratios. These alloyed nanosheets showed composition- and phase-dependent electrochemical and electronic properties. By tuning the Mo/W ratio, an optimized combination of high-density active sites for the hydrogen evolution reaction (HER) and low charge transfer resistance can be achieved. Additionally, due to the formation of 1T/2H (metal/semiconductor) heterojunctions, the alloyed TMD nanosheets with an optimized 1T concentration exhibited much enhanced gas sensing capability compared to the highly metallic nanosheets or the annealed semiconducting nanosheets with the same chemical composition. Our findings suggest that the ability to tune the composition and crystal structures of 2D materials via a facile one-pot solution process may provide more opportunities to control their functional properties and thus widens their range of practical applications.

Interdiffusion Reaction-Assisted Hybridization of Two-Dimensional Metal-Organic Frameworks and Ti3C2Tx Nanosheets for Electrocatalytic Oxygen Evolution.

Two-dimensional (2D) metal-organic framework (MOF) nanosheets have been recently regarded as the model electrocatalysts due to their porous structure, fast mass and ion transfer through the thickness, and large portion of exposed active metal centers. Combining them with electrically conductive 2D nanosheets is anticipated to achieve further improved performance in electrocatalysis. In this work, we in situ hybridized 2D cobalt 1,4-benzenedicarboxylate (CoBDC) with Ti3C2Tx (the MXene phase) nanosheets via an interdiffusion reaction-assisted process. The resulting hybrid material was applied in the oxygen evolution reaction and achieved a current density of 10 mA cm-2 at a potential of 1.64 V vs reversible hydrogen electrode and a Tafel slope of 48.2 mV dec-1 in 0.1 M KOH. These results outperform those obtained by the standard IrO2-based catalyst and are comparable with or even better than those achieved by the previously reported state-of-the-art transition-metal-based catalysts. While the CoBDC layer provided the highly porous structure and large active surface area, the electrically conductive and hydrophilic Ti3C2Tx nanosheets enabled the rapid charge and ion transfer across the well-defined Ti3C2Tx-CoBDC interface and facilitated the access of aqueous electrolyte to the catalytically active CoBDC surfaces. The hybrid nanosheets were further fabricated into an air cathode for a rechargeable zinc-air battery, which was successfully used to power a light-emitting diode. We believe that the in situ hybridization of MXenes and 2D MOFs with interface control will provide more opportunities for their use in energy-based applications.

Preparation and applications of novel composites composed of metal-organic frameworks and two-dimensional materials.

Metal-organic frameworks (MOFs), an emerging type of porous crystalline materials, have received increasing attention in recent years due to their compositional, structural and chemical versatility. Moreover, great progress has been made in the fundamental study and technological development of two-dimensional (2D) materials, such as graphene and metal dichalcogenide nanosheets, which exhibit a number of unique and attractive properties for wide applications. Recently, the smart integration of the aforementioned two types of functional materials, i.e. MOFs and 2D materials, has led to improved performance in molecular absorption, separation and storage, and shown promise in selective catalysis and biosensing. This feature article aims at providing a brief introduction to the composites composed of MOFs and 2D materials, focusing mainly on their preparation methods and applications. Finally, technical challenges and future opportunities in this field will also be discussed.

Metal-organic framework composites: from fundamentals to applications.

Metal-organic frameworks (MOFs) are a class of crystallized porous polymeric materials consisting of metal ions or clusters linked together by organic bridging ligands. Due to their permanent porosity, rich surface chemistry and tuneable pore sizes, MOFs have emerged as one type of important porous solid and have attracted intensive interests in catalysis, gas adsorption, separation and storage over the past two decades. When compared with pure MOFs, the combination of MOFs with functional species or matrix materials not only shows enhanced properties, but also broadens the applications of MOFs in new fields, such as bio-imaging, drug delivery and electrical catalysis, owing to the interactions of the functional species/matrix with the MOF structures. Although the synthesis, chemical modification and potential applications of MOFs have been reviewed previously, there is an increasing awareness on the synthesis and applications of their composites, which have rarely been reviewed. This review aims to fill this gap and discuss the fabrication, properties, and applications of MOF composites. The remaining challenges and future opportunities in this field, in terms of processing techniques, maximizing composite properties, and prospects for applications, have also been indicated.