Synthesis of Self‐Supported Ordered Mesoporous Cobalt and Chromium Nitrides

Herein, we demonstrate an ammonia nitridation approach to synthesize self‐supported ordered mesoporous metal nitrides (CoN and CrN) from mesostructured metal oxide replicas (Co₃O₄ and Cr₂O₃), which were nanocastly prepared by using mesoporous silica SBA‐15 as a hard template. Two synthetic routes are adopted. One route is the direct nitridation of mesoporous metal oxide nanowire replicas templated from SBA‐15 to metal nitrides. By this method, highly ordered mesoporous cobalt nitrides (CoN) can be obtained by the transformation of Co₃O₄ nanowire replica under ammonia atmosphere from 275 to 350 °C, without a distinct lose of the mesostructural regularity. Treating the samples above 375 °C leads to the formation of metallic cobalt and the collapse of the mesostructure due to large volume shrinkage. The other route is to transform mesostructured metal oxides/silica composites to nitrides/silica composites at 750–1000 °C under ammonia. Ordered mesoporous CrN nanowire arrays can be obtained after the silica template removal by NaOH erosion. A slowly temperature‐program‐decrease process can reduce the influence of silica nitridation and improve the purity of final CrN product. Small‐angle XRD patterns and TEM images showed the 2‐D ordered hexagonal structure of the obtained mesoporous CoN and CrN nanowires. Wide‐angle XRD patterns, HRTEM images, and SAED patterns revealed the formation of crystallized metal nitrides. Nitrogen sorption analyses showed that the obtained materials possessed high surface areas (70–90 m² g^?1) and large pore volumes (about 0.2 cm³ g^?1).     

Ultrathin Amorphous Silica Membrane Enhances Proton Transfer across Solid‐to‐Solid Interfaces of Stacked Metal Oxide Nanolayers while Blocking Oxygen

A large jump of proton transfer rates across solid‐to‐solid interfaces by inserting an ultrathin amorphous silica layer into stacked metal oxide nanolayers is discovered using electrochemical impedance spectroscopy and Fourier‐transform infrared reflection absorption spectroscopy (FT‐IRRAS). The triple stacked nanolayers of Co₃O₄, SiO₂, and TiO₂ prepared by atomic layer deposition (ALD) enable a proton flux of 2400 ± 60 s^?1 nm^?2 (pH 4, room temperature), while a single TiO₂ (5 nm) layer exhibits a threefold lower flux of 830 s^?1 nm^?2. Based on FT‐IRRAS measurements, this remarkable enhancement is proposed to originate from the sandwiched silica layer forming interfacial SiOTi and SiOCo linkages to TiO₂ and Co₃O₄ nanolayers, respectively, with the O bridges providing fast H⁺ hopping pathways across the solid‐to‐solid interfaces. Together with the complete O₂ impermeability of a 2 nm ALD‐grown SiO₂ layer, the high flux for proton transport across multi‐stack metal oxide layers opens up the integration of incompatible catalytic environments to form functional nanoscale assemblies such as artificial photosystems for CO₂ reduction by H₂O.     

Nanocasting of Mesoporous In‐TM (TM = Co,Fe, Mn) Oxides: Towards 3D Diluted‐Oxide Magnetic Semiconductor Architectures

Transition metal (Co, Fe, Mn)‐doped In₂O_3?y mesoporous oxides are synthesized by nanocasting using mesoporous silica as hard templates. 3D ordered mesoporous replicas are obtained after silica removal in the case of the In‐Co and In‐Fe oxide powders. During the conversion of metal nitrates into the target mixed oxides, Co, Fe, and Mn ions enter the lattice of the In₂O₃ bixbyite phase via isovalent or heterovalent cation substitution, leading to a reduction in the cell parameter. In turn, non‐negligible amounts of oxygen vacancies are also present, as evidenced from Rietveld refinements of the X‐ray diffraction patterns. In addition to (In_1?xTM_x)₂O_3?y, minor amounts of Co₃O₄, α‐Fe₂O₃, and Mn_xO_y phases are also detected, which originate from the remaining TM cations not forming part of the bixbyite lattice. The resulting TM‐doped In₂O_3?y mesoporous materials show a ferromagnetic response at room temperature, superimposed on a paramagnetic background. Conversely, undoped In₂O_3?y exhibits a mixed diamagnetic‐ferromagnetic behavior with much smaller magnetization. The influence of the oxygen vacancies and the doping elements on the magnetic properties of these materials is discussed. Due to their 3D mesostructural geometrical arrangement and their room‐temperature ferromagnetic behavior, mesoporous oxide‐diluted magnetic semiconductors may become smart materials for the implementation of advanced components in spintronic nanodevices.     

A General Synthesis of Crumpled Metal Oxide Nanosheets as Superior Chemiresistive Sensing Layers

Metal oxide nanosheets having high mesoporosity, grain size distribution of 5–10 nm, and ultrathin thickness have attracted much attention due to their intriguing properties such as high surface‐to‐volume ratio and superior chemical activities. However, 2D nanostructures tend to restack, inducing a decrease in accessible surface area and a number of pores. To solve this problem, herein, a unique synthetic method of crumpled metal oxide nanosheets using spray pyrolysis of metal ion–coated graphene oxide, followed by heat treatment, is reported. This method is applicable not only to single‐component metal oxides but also to heterogeneous multicomponent metal oxides in which composition can be controlled. Crumpled SnO₂, ZnO, and Co₃O₄ as well as SnO₂/ZnO and SnO₂/Co₃O₄ nanosheets with heterogeneous interfaces are successfully synthesized and used as superior gas sensing layers. Because of the abundant reaction sites, well‐developed porosity for high gas accessibility, the formation of heterojunctions, the crumpled SnO₂/ZnO and SnO₂/Co₃O₄ nanosheets exhibit outstanding sensing performance (R_air/R_gas = 20.25 toward 5 ppm formaldehyde, and R_air/R_gas = 14.13 toward 5 ppm acetone, respectively). This study can contribute to the realization of a family of heterogeneous crumpled metal oxide nanosheets that can be applied to various research fields.     

Optimal Doping for Enhanced SnO2 Sensitivity and Thermal Stability

Tin oxide nanocrystals (5–10 nm) doped with silica (0–15 wt %) were made by flame‐spray‐pyrolysis direct deposition onto the sensing electrodes and in situ stabilization by rapid flame annealing. Although increased SiO₂‐doping reduced the SnO₂ crystal and grain size, its sensing performance to ethanol vapor (0.1–50 ppm) exhibited an optimum with respect to SiO₂ content. The thermal stability and morphology of SiO₂‐doped SnO₂ nanoparticles were evaluated by sintering at 200–900 °C for 4–24 h in air. At low SiO₂ content, sintering of SnO₂ was prevented only partially resulting in small sinter necks (bottlenecks) between SnO₂ primary particles (smaller than twice the Debye length). This morphology drastically enhanced the sensitivity toward the analyte by maintaining a thermally stable high surface area and fully depleted connections at the primary particle necks. This enhancement is attributed mostly to the decreasing neck size of the SnO₂ SiO₂ heterojunctions rather than the decreasing SnO₂ crystallite and grain sizes with increasing SiO₂ doping. At high SiO₂ contents, SnO₂ sintering was inhibited as its grains were separated effectively by dielectric SiO₂; this resulted in isolated SnO₂ nanocrystals with drastically reduced sensitivity, thereby effectively being insulators.     

Novel Nanostructures of Functional Oxides Synthesized by Thermal Evaporation

Functional oxides are the fundamentals of smart devices. This article reviews novel nanostructures of functional oxides, including nanobelts, nanowires, nanosheets, and nanodiskettes, that have been synthesized in the authors’ laboratory. Among the group of ZnO, SnO₂, In₂O₃, Ga₂O₃, CdO, and PbO₂, which belong to different crystallographic systems and structures, a generic nanobelt structure has been synthesized. The nanobelts are single crystalline and dislocation‐free, and their surfaces are atomically flat. The oxides are semiconductors, and have been used for fabrication of nanodevices such as field‐effect transistors and gas sensors. Taking SnO₂ and SnO as examples, other types of novel nanostructures are illustrated. Their growth, phase transformation, and stability are discussed. The nanobelts and related nanostructures are a unique group that is likely to have important applications in electronic, optical, sensor, and optoelectronic nanodevices.     

General Synthesis of N‐Doped Macroporous Graphene‐Encapsulated Mesoporous Metal Oxides and Their Application as New Anode Materials for Sodium‐Ion Hybrid Supercapacitors

A general method to synthesize mesoporous metal oxide@N‐doped macroporous graphene composite by heat‐treatment of electrostatically co‐assembled amine‐functionalized mesoporous silica/metal oxide composite and graphene oxide, and subsequent silica removal to produce mesoporous metal oxide and N‐doped macroporous graphene simultaneously is reported. Four mesoporous metal oxides (WO_{3? x} , Co₃O₄, Mn₂O₃, and Fe₃O₄) are encapsulated in N‐doped macroporous graphene. Used as an anode material for sodium‐ion hybrid supercapacitors (Na‐HSCs), mesoporous reduced tungsten oxide@N‐doped macroporous graphene (m‐WO_{3? x} @NM‐rGO) gives outstanding rate capability and stable cycle life. Ex situ analyses suggest that the electrochemical reaction mechanism of m‐WO_{3? x} @NM‐rGO is based on Na⁺ intercalation/de‐intercalation. To the best of knowledge, this is the first report on Na⁺ intercalation/de‐intercalation properties of WO_{3? x} and its application to Na‐HSCs.     

Analysis of specific growth features of zinc oxide nanocrystals from aqueous solutions to fabricate thin-film solar cells

Specific features of the chemical deposition of zinc oxide nanocrystals from aqueous solutions have been studied. A correlation between geometric parameters of zinc oxide nanocrystals and technological modes of their chemical deposition from aqueous solutions has been found. The efficiency of using an array of zinc oxide nanocrystals in SnO₂:F/ZnO/In₂S₃/CuSCN thin-film heterostructured solar cells is demonstrated.     

Synthesis of New RuO2@SiO2 Composite Nanomaterials and their Application as Catalytic Filters for Selective Gas Detection

RuO₂@SiO₂ nanomaterials are prepared using hybrid mesostructured silica (EtO)₂P(O)(CH₂)₃SiO_1.5/x SiO₂ (x = 9, 16) by anchoring the metal precursor [Ru(COD)(COT)] (COD is 1,3‐cyclooctadiene, COT is 1,3,5‐cyclooctatriene) inside the pores of the organized silica matrix through the phosphonate moieties. Following this task, the nanoparticles are fabricated by i) decomposing the metal precursor with hydrogen at room temperature in tetrahydrofuran to achieve ruthenium nanoparticles and ii) thermally treating the ruthenium particles in silica at 450 °C in air to fabricate RuO₂. The materials containing Ru and RuO₂ nanoparticles are characterized by elemental analysis, transmission electron microscopy (TEM), X‐ray diffraction (XRD), nitrogen sorption measurements, and ³¹P and ¹³C NMR. The obtained RuO₂@SiO₂ nanomaterials are evaluated as catalytic filters when deposited onto gas sensors for the preferential detection of propane in the multicomponent gas mixture propane/carbon monoxide/nitrogen dioxide.     

Ternary Oxide Nanocrystals: Universal Laser‐Hydrothermal Synthesis,Optoelectronic and Electrochemical Applications

Ternary oxide nanocrystals (TONs) have received growing attention for their great potential applications in optoelectronics and electrochemistry despite the current scarcity of universal, facile, and green synthesis methods. Here, we introduce a universal laser‐hydrothermal approach for various TONs and demonstrate their potential for high‐performance photodetectors (PDs) and pseudocapacitors. The obtained clean surface is derived by laser ablation in liquid (LAL) and subsequent hydrothermal growth. The LAL‐generated precursors contain many kinds of highly reactive species, including H⁺, OH^?, metal ions, and clusters, which facilitate the fast and facile formation of various TONs in the subsequent hydrothermal process. The universality of the method is systematically proven by the synthesis of a series of TONs, including Zn₂GeO₄, NiCo₂O₄, Zn₂SnO₄, ZnFe₂O₄, ZnMnO₃, and Fe₂GeO₄. Significantly, the absence of chemical additives, such as surfactants, guarantees highly clean surfaces, which further benefits the electron transport through the nanocrystals, and thus in the resultant devices. This is also exemplified by a Zn₂GeO₄‐nanorod‐based, deep‐ultraviolet PD and NiCo₂O₄ nanocrystal supercapacitors.     

Graphene‐Based Mesoporous SnO2 with Enhanced Electrochemical Performance for Lithium‐Ion Batteries

Graphene‐based metal oxides generally show outstanding electrochemical performance due to the superior properties of graphene. However, the aggregation of active metal oxide nanoparticles on the graphene surface may result in a capacity fading and poor cycle performance. Here, a mesostructured graphene‐based SnO₂ composite is prepared through in situ growth of SnO₂ particles on the graphene surface using cetyltrimethylammonium bromide as the structure‐directing agent. This novel mesoporous composite inherits the advantages of graphene nanosheets and mesoporous materials and exhibits higher reversible capacity, better cycle performance, and better rate capability compared to pure mesoporous SnO₂ and graphene‐based nonporous SnO₂. It is concluded that the synergetic effect between graphene and mesostructure benefits the improvement of the electrochemical properties of the hybrid composites. This facile method may offer an attractive alternative approach for preparation of the graphene‐based mesoporous composites as high‐ performance electrodes for lithium‐ion batteries.     

Hierarchical Co3O4 Nanofiber–Carbon Sheet Skeleton with Superior Na/Li‐Philic Property Enabling Highly Stable Alkali Metal Batteries

Uncontrollable dendritic behavior and infinite volume expansion in alkali metal anode results in the severe safety hazards and short lifespan for high‐energy batteries. Constructing a stable host with superior Na/Li‐philic properties is a prerequisite for commercialization. Here, it is demonstrated that the small Gibbs free energy change in the reaction between metal oxide (Co₃O₄, SnO₂, and CuO) and alkali metal is key for metal infusion. The as‐prepared hierarchical Co₃O₄ nanofiber–carbon sheet (CS) skeleton shows improved wettability toward molten Li/Na. The 3D carbon sheet serves as a primary framework, offering adequate lithium nucleation sites and sufficient electrolyte/electrode contact for fast charge transfer. The secondary framework of Co/Li₂O nanofibers provides physical confinement of deposited Li and further redistributes the Li⁺ flux on each carbon fiber, which is verified by COMSOL Multiphysics simulations. Due to the uniform deposition behavior and near‐zero volume change, modified symmetrical Li/Li cells can operate under an ultrahigh current density of 20 mA cm^?2 for more than 120 cycles. When paired with LiFePO₄ cathodes, the Li/Co–CS cell shows low polarization and 88.4% capacity retention after 200 cycles under 2 C. Convincing improvement can also be observed in Na/Co–CS symmetrical cells applying NaClO₄‐based electrolyte. These results illustrate a significant improvement in developing safe and stable alkali metal batteries.     

Exploring the Leidenfrost Effect for the Deposition of High‐Quality In2O3 Layers via Spray Pyrolysis at Low Temperatures and Their Application in High Electron Mobility Transistors

The growth mechanism of indium oxide (In₂O₃) layers processed via spray pyrolysis of an aqueous precursor solution in the temperature range of 100–300 °C and the impact on their electron transporting properties are studied. Analysis of the droplet impingement sites on the substrate's surface as a function of its temperature reveals that Leidenfrost effect dominated boiling plays a crucial role in the growth of smooth, continuous, and highly crystalline In₂O₃ layers via a vapor phase‐like process. By careful optimization of the precursor formulation, deposition conditions, and choice of substrate, this effect is exploited and ultrathin and exceptionally smooth layers of In₂O₃ are grown over large area substrates at temperatures as low as 252 °C. Thin‐film transistors (TFTs) fabricated using these optimized In₂O₃ layers exhibit superior electron transport characteristics with the electron mobility reaching up to 40 cm² V^?1 s^?1, a value amongst the highest reported to date for solution‐processed In₂O₃ TFTs. The present work contributes enormously to the basic understanding of spray pyrolysis and highlights its tremendous potential for large‐volume manufacturing of high‐performance metal oxide thin‐film transistor electronics.     

Metastable Hexagonal In2O3 Nanofibers Templated from InOOH Nanofibers under Ambient Pressure

Single‐crystal, metastable, hexagonal In₂O₃ (H‐In₂O₃) nanofibers with an average diameter of 80 nm and length of up to several micrometers were synthesized on a large scale, for the first time under ambient pressure, by annealing InOOH nanofibers at 490 °C. The InOOH nanofibers were prepared by a controlled hydrolysis solvothermal reaction, using InCl₃·4H₂O as the starting material and ether as the solvent, in the temperature range of 190–240 °C. The solvent has significant effects on the formation of the metastable phase and the morphology of the In₂O₃ nanocrystals during the synthesis of the precursor InOOH. Room‐temperature optical absorption spectra of the hexagonal In₂O₃ nanofibers showed strong absorption peak located at 325 nm (3.83 eV) with a slight blue‐shift compared with that of bulk In₂O₃ (3.75 eV). The H‐In₂O₃ nanofibers photoluminesce at room temperature with emission peaks at 378 nm, 398 nm, and 420 nm. The successful production of metastable hexagonal In₂O₃ nanofibers in large scale under mild conditions could be of interest both for applications and fundamental studies.     

Inorganic Macroporous Films from Preformed Nanoparticles and Membrane Templates: Synthesis and Investigation of Photocatalytic and Photoelectrochemical Properties

Colloidal dispersions of titania, zirconia, tin oxide, indium oxide, and ceria have been successfully used to impregnate membrane templates and form the respective metal oxide (MO) porous films. The use of alumina and iron oxide sols in the same procedure, however, resulted in compact structures. By mixing different nanoparticle solutions before impregnation, final inorganic films containing two metal oxides, of variable metal oxide ratios, were obtained. The porous inorganic materials were analyzed in terms of surface area, pore size, film thickness, and crystallinity. The mechanism of nanoparticle infiltration and particle adsorption to the template walls is proposed based on the stability of the inorganic film and a study of the influence of either the sol concentration or washing times on the amount of inorganic substance incorporated in the hybrid material. The photocatalytic decomposition of an organic pollutant, 2‐chlorophenol, was demonstrated for the porous titania material along with the structures containing mixtures of titania with zirconia, indium oxide, and tin oxide. A ratio of 9:1 TiO₂/MO gave the highest photocatalytic activity, which was higher than the activity of Degussa P25 for the TiO₂/In₂O₃ and TiO₂/SnO₂ systems under the same conditions. The titania films have also been attached to substrates—glass or indium tin oxide (ITO) surfaces—and the photoelectrochemical properties of the porous film attained. A comparison with a spin‐coated titania film (prepared from the same colloidal dispersion) showed that the structured porous inorganic film has two times the photoelectrochemical efficiency as the spin‐coated film.     

Nanocrystalline Metal Oxides from the Injection of Metal Oxide Sols in Coordinating Solutions: Synthesis,Characterization, Thermal Stabilization,Device Processing,and Gas‐Sensing Properties

Metal oxide (SnO₂, TiO₂, In₂O₃, ZnO) sols are prepared by various sol–gel processes in such a way as to hinder the condensation reactions. The obtained sols are injected at 160 °C into a solution of tetradecene and dodecylamine, and kept under heating for different periods of time. Depending on the starting sol, variously crystallized oxide nanoparticles are obtained, whose phase compositions and chemical structure have been studied by X‐ray diffraction (XRD) and Fourier transform IR spectroscopy. The elimination of the organic residuals has been carried out by thermal treatment, and the thermal evolution of the nanoparticles has been studied by thermal analyses and Raman spectroscopy. High‐resolution transmission electron microscopy studies coupled with XRD measurements show that the thermal treatment does not markedly affect the particle size, which remains in the nanometer‐sized regime (from 3.5 to 8.5 nm, depending on the system), except in the case of ZnO. The thermally purified and stabilized powders, drop‐coated onto alumina substrates with pre‐deposited electrical contacts, have been tested as gas‐sensing devices, displaying outstanding sensing properties even at room temperature.     

Nanoflake Arrays of Lithiophilic Metal Oxides for the Ultra‐Stable Anodes of Lithium‐Metal Batteries

A molten lithium infusion strategy has been proposed to prepare stable Li‐metal anodes to overcome the serious issues associated with dendrite formation and infinite volume change during cycling of lithium‐metal batteries. Stable host materials with superior wettability of molten Li are the prerequisite. Here, it is demonstrated that a series of strong oxidizing metal oxides, including MnO₂, Co₃O₄, and SnO₂, show superior lithiophilicity due to their high chemical reactivity with Li. Composite lithium‐metal anodes fabricated via melt infusion of lithium into graphene foams decorated by these metal oxide nanoflake arrays successfully control the formation and growth of Li dendrites and alleviate volume change during cycling. A resulting Li‐Mn/graphene composite anode demonstrates a super‐long and stable lifetime for repeated Li plating/stripping of 800 cycles at 1 mA cm^?2 without voltage fluctuation, which is eight times longer than the normal lifespan of a bare Li foil under the same conditions. Furthermore, excellent rate capability and cyclability are realized in full‐cell batteries with Li‐Mn/graphene composite anodes and LiCoO₂ cathodes. These results show a major advancement in developing a stable Li anode for lithium‐metal batteries.     

Generation of Self‐Assembled 3D Mesostructured SnO2 Thin Films with Highly Crystalline Frameworks

Crack‐free, mesoporous SnO₂ films with highly crystalline pore walls are obtained by evaporation‐induced self‐assembly using a novel amphiphilic block‐copolymer template (“KLE” type, poly(ethylene‐co‐butylene)‐block‐poly(ethylene oxide)), which leads to well‐defined arrays of contracted spherical mesopores by suitable heat‐treatment procedures. Because of the improved templating properties of these polymers, a facile heat‐treatment procedure can be applied whilst keeping the mesoscopic order intact up to 600–650 °C. The formation mechanism and the mesostructural evolution are investigated by various state‐of‐the‐art techniques, particularly by a specially constructed 2D small‐angle X‐ray scattering setup. It is found that the main benefit from the polymers is the formation of an ordered mesostructure under the drastic conditions of using molecular Sn precursors (SnCl₄), taking advantage of the large segregation strength of these amphiphiles. Furthermore, it is found that the crystallization mechanism is different from other mesostructured metal oxides such as TiO₂. In the case of SnO₂, a significant degree of crystallization (induced by heat treatment) already starts at quite low temperatures, 250–300 °C. Therefore, this study provides a better understanding of the general parameters governing the preparation of mesoporous metal oxides films with crystalline pore walls.     

Atomic force microscopy and photoluminescence analysis of porous metal-oxide materials

The results of studies of porous materials based on various metal oxides (Fe₂O₃, ZnO, In₂O₃, etc.) by photoluminescence analysis and atomic force microscopy are reported. The influence of the composition and structure of metal-oxide nanocomposites on features of the photoluminescence and optical transmittance spectra is considered.     

A Universal Scheme for Patterning of Oxides via Thermal Nanoimprint Lithography

Direct patterning of oxides using thermal nanoimprint lithography is performed using either the sol‐gel or methacrylate route. The sol‐gel method results in resists with long shelf‐life, but with high surface energy and a considerable amount of solvent that affects the quality of imprinting. The methacrylate route, which is limited to certain oxides, produces polymerizable resists, leading to low surface energy, but suffers from the shorter shelf‐life of precursors. By combining the benignant elements from both these routes, a universal method of direct thermal nanoimprinting of oxides is demonstrated using precursors produced by reacting an alkoxide with a polymerizable chelating agent such as 2‐(methacryloyloxy)ethyl acetoacetate (MAEAA). MAEAA possesses β‐ketoester, which results in the formation of environmentally stable, chelated alkoxide with long shelf‐life, and methacrylate groups, which provide a reactive monomer pendant for in situ copolymerization with a cross‐linker during imprinting. Polymerization leads to trapping of cations, lowering of surface energy, strengthening of imprints, which enables easy and clean demolding over 1 cm × 2 cm patterned area with ≈100% yield. Heat‐treatment of imprints gives amorphous/crystalline oxide patterns. This alliance between two routes enables the successful imprinting of numerous oxides including Al₂O₃, Ga₂O₃, In₂O₃, Y₂O₃, B₂O₃, TiO₂, SnO₂, ZrO₂, GeO₂, HfO₂, Nb₂O₅, Ta₂O₅, V₂O₅, and WO₃.