2D Metal Oxide Research Articles

Enhanced water splitting photocatalyst enabled by two-dimensional GaP/GaAs van der Waals heterostructure

Two-dimensional (2D) photocatalysts are promising alternatives to traditional 3D metal oxides. However, it is still a great challenge to surmount the limitation restricted by the 2D photocatalytic bandgap together with the band edge position. Herein, we have newly designed 2D GaP/GaAs van der Waals (vdW) heterostructure-based photocatalyst for water splitting and examined the corresponding operating mechanism. Firstly, the tunable electronic, higher carrier mobilities, and excellent optical properties of XP/XAs (X = Ga, In) vdW heterostructures and single-layer counterparts are introduced based on density functional theory. Specially, the GaP/GaAs vdW heterostructure with direct bandgap exhibits more noticeable optical absorption than the single-layer counterparts. Secondly, intrinsic electric field can be obtained in GaP/GaAs vdW heterostructure and the excited electrons and holes of GaP/GaAs vdW heterostructure are present in opposite layers, demonstrating that the GaP/GaAs vdW heterostructure can spontaneously generate the electron-hole separators. Thirdly, the thermodynamic stability of GaP/GaAs vdW heterostructure has been confirmed, which shows high potential for experimental implementation. Finally, due to the superior characteristics, GaP/GaAs vdW heterostructure has been constructed for the photocatalytic water-splitting. Importantly, the photoexcited electrons of GaP/GaAs vdW heterostructure can spontaneously induce hydrogen half reaction without sacrificial reagents. Moreover, the driving force in oxidation half reaction of GaP/GaAs vdW heterostructure can be greatly boosted under light illumination. Our constructed 2D-vdW-semiconductors-based devices provide a promising strategy to achieve high efficiency water-splitting photocatalyst.

Controllable Transformation of Metal-Organic Framework Nanosheets into Oxygen Vacancy NixCo3-xO4 Arrays for Ultrahigh-Capacitance Supercapacitors with Long Lifespan.

The amino-functionalized bimetal NH2-NiCo-MOF nanosheet array is first fabricated on Ni foam substrates and then controllably transformed into oxygen vacancy bimetal oxide arrays by simply thermal annealing in air. This NiCo-based oxide array (NixCo3-xO4/NF) achieves high capacitance (2484 F g-1 at 1 A g-1), excellent rate performance (91.4%), and long cycling life when assessed as promising electrode material for supercapacitors. Notably, the existing oxygen vacancy in NixCo3-xO4 promotes the electrochemical performance of NixCo3-xO4/NF due to the enhancement of electrical conductivity and capture capability for OH-. In addition, the assembled asymmetric supercapacitor (ASC) device exhibits an excellent energy density of 39.3 W h kg-1 at a power density of 800.2 W kg-1, which still remains 32.2 W h kg-1 even at a high power density of 7994.5 W kg-1. Furthermore, a light-emitting diode can be lightened for more than 6 min, demonstrating a great potential for practical application of ASC devices. This work knocks on the door of a feasible strategy for designing and synthesizing 2D metal oxide nanosheet arrays with excellent electrochemical properties.

Modulation and Modeling of Three‐Dimensional Nanowire Assemblies Targeting Gas Sensors with High Response and Reliability (Adv. Funct. Mater. 10/2022)

Gas Sensors In article number 2108891, Harry L. Tuller, Yeon Sik Jung, and co-workers systematically engineer 3D metal oxide nanowire assemblies to achieve superior gas response, conductance, and signal stability compared to random nanowire aggregates. The 3D geometric variations and their modeling provide important insights into the electrical conduction and electrochemical responses of the assemblies, revealing the critical role of wire-to-wire junction points and their arrangement.

Surfactant‐Free and Microporous AlOOH/Al2O3 Nanosheets on TiO2‐Based Nanofibers: A Sustained‐Release Dominated Topotactic Transformation

AbstractUp to now, the synthesis of non‐ligand‐capped 2D transition‐metal nanostructures remained big synthetic challenges. Herein, ultrathin and microporous AlOOH/Al2O3 nanosheets were topologically grown on the surface of TiO2‐based nanofibers by a water‐only hydrothermal strategy starting from electrospun Al2O3/TiO2 nanofibers. The TiO2 nanofibers lowered the surface energy for the sustained‐released aluminum species and thereby allowed them in situ hydrated to laminar AlOOH. The metastable AlOOH tended to dehydrate to Al2O3 simultaneously, leaving a well‐maintained 2D structure with sharp corners and steps, as well as enriched micropores. The nanosheets‐on‐nanofibers exhibited 2856–3370 mg/g of adsorption capacity toward RhB and an efficient removal efficiency over 70% at high throughput (30 mL/h). The promoted light‐harvesting by the perpendicularly attached nanosheets further endowed a significant photodegradation for RhB within 1 h. The presented green and facile approach toward the fabrication of 2D metal oxide nanostructures paves the way to a new variety of oxide nanomaterials.

High performance and illumination stable In2O3 nanofibers-based field effect transistors by doping praseodymium

Although different kinds of one dimensional (1D) metal oxide nanofibers (MO-NFs) based field effect transistors (FETs) have been developed in recent years, their intrinsic deficiency in illumination instability are not addressed yet, which further prevented their application in practical life. In this work, due to the outstanding performance of suppressing oxygen vacancies and the blue light down-conversion with low charge transfer transition energy, praseodymium (Pr) is demonstrated an ideal candidate to dope into In2O3 NFs for both transferring the depletion mode to the enhancement mode and enhancing the illumination stability of the FETs. The optima In2O3 NFs-based FET was achieved by doping with 3 mol% Pr and exhibited an excellent electrical performance with a carrier mobility of 6.92 cm2V−1s−1, an on/off current ratio of ∼5.4 × 107, threshold voltage of 5.2 V, and subthreshold swing of 0.37 V/dec. Moreover, the FETs exhibited good illumination stability under 3600 s negative gate bias stress test with illumination (NBIS). Finally, the FET was also used as a switch to bright the LED in this work, demonstrating its great promise of Pr-doped In2O3 NFs in future one-dimensional electronic devices.

Structural Properties and Magnetic Ground States of 100 Binary d-Metal Oxides Studied by Hybrid Density Functional Methods.

d-metal oxides play a crucial role in numerous technological applications and show a great variety of magnetic properties. We have systematically investigated the structural properties, magnetic ground states, and fundamental electronic properties of 100 binary d-metal oxides using hybrid density functional methods and localized basis sets composed of Gaussian-type functions. The calculated properties are compared with experimental information in all cases where experimental data are available. The used PBE0 hybrid density functional method describes the structural properties of the studied d-metal oxides well, except in the case of molecular oxides with weak intermolecular forces between the molecular units. Empirical D3 dispersion correction does not improve the structural description of the molecular oxides. We provide a database of optimized geometries and magnetic ground states to facilitate future studies on the more complex properties of the binary d-metal oxides.

Two-dimensional copper oxide/zinc oxide nanoflakes with three-dimensional flower-like heterostructure enhanced with electrocatalytic activity toward nimesulide detection

The strategy of building two-dimensional (2D) metal oxide nanoflakes has inspired several innovations in different fields of application on account of its tremendous significance. It includes ultrathin planar surface, large charge carrier mobility, and tunable band structures, providing unprecedented features for sensing. Moreover, the intercalation of 2D dimensions to 3D superstructures will result in improved and dual advantages of both 2D/3D. The construction of 2D/3D copper oxide zinc oxide nanocomposite as electrode material for specific detection of nimesulide (NMS) is herein reported. The conversion of 2D CZ nanoflakes to 3D CZ microflowers was possibly achieved via the self-assembly process. This simple and cost-effective development of the CZ composite was characterized for evaluating the physical, chemical, and morphological properties. The highly crystalline nature of CZ was observed from powder X-ray diffraction and X-ray photoelectron spectroscopy analysis. The formation of 2D nanoflakes of CZ was strongly confirmed from field emission scanning electron microscopy and high-resolution transmission electron microscopy images. To verify the strong attachments, Fourier transforms infrared spectroscopy spectra were analyzed. Electrochemical sensing of NMS at CZ fabricated glassy carbon electrode reflects higher electrocatalytic activity with a linear range of NMS addition from 0.299 μM to 319.15 μM. The lower detection limit was about 0.005 μM with a sensitivity of 7.152 μAμM−1 cm2. The CZ nanocomposite will be more applicable for sensing several drugs with enriched active sites, higher conductivity, and large surface area raised from low-cost metal oxides when compared with highly conducting materials.

Designing 3d metal oxides: selecting optimal density functionals for strongly correlated materials.

Transition metal oxides (TMOs) have remarkable physicochemical properties, are non-toxic, and have low cost and high annual production, thus they are commonly studied for various technological applications. Density functional theory (DFT) can help to optimize TMO materials by providing insights into their electronic, optical and thermodynamic properties, and hence into their structure-performance relationships, over a wide range of solid-state structures and compositions. However, this is underpinned by the choice of the exchange-correlation (XC) functional, which is critical to accurately describe the highly localized and correlated 3d-electrons of the transition metals in TMOs. This tutorial review presents a benchmark study of density functionals (DFs), ranging from generalized gradient approximation (GGA) to range-separated hybrids (RSH), with the all-electron def2-TZVP basis set, comparing magneto-electro-optical properties of 3d TMOs against experimental observations. The performance of the DFs is assessed by analyzing the band structure, density of states, magnetic moment, structural static and dynamic parameters, optical properties, spin contamination and computational cost. The results disclose the strengths and weaknesses of the XC functionals, in terms of accuracy, and computational efficiency, suggesting the unprecedented PBE0-1/5 as the best candidate. The findings of this work contribute to necessary developments of XC functionals for periodic systems, and materials science modelling studies, particularly informing how to select the optimal XC functional to obtain the most trustworthy description of the ground-state electron structure of 3d TMOs.

Combustion Synthesized Electrospun InZnO Nanowires for Ultraviolet Photodetectors

AbstractUltraviolet (UV) photodetectors play an important role in numerous commercial and scientific applications. The UV photodetectors based on binary‐cation indium zinc oxide (InZnO) thin films exhibit great performance enhancement, compared with their single‐cation counterparts. However, UV photodetectors based on 1D InZnO nanowires could potentially exhibit more superior optoelectrical performance, due to the large surface‐to‐volume ratio and favorable carrier transport characteristics of nanowires. This work has combined combustion synthesis with electrospinning technique to efficiently fabricate InZnO nanowire‐based UV photodetectors. At the annealing temperature of 375 °C, the newly designed InZnO nanowire photodetectors exhibit excellent photoelectric performance under the irradiation of 310 nm UV light, including a photo‐to‐dark current ratio of 1.2 × 104, a photo responsivity of 2.8 × 103 A W–1, and a high detectivity of 2.4 × 1016 Jones. This study not only demonstrates the opportunity to construct new‐generation transparent electronics based on 1D metal oxide nanowires but also sheds new light on how to further decrease the annealing temperature of metal oxide nanowire devices for low‐temperature fabrication processes.

Anomalous Optoelectric Properties of an Ultrathin Ruthenium Film with a Surface Oxide Layer for Flexible Transparent Conducting Electrodes

AbstractThe growing industrial demand for flexible optoelectric devices has led to intensive researches on highly flexible transparent electrode materials such as graphene, reduced graphene‐oxide (r‐GO), Ag‐nanowire, and 2D metal oxides. However, except Ag‐nanowire, transparent electrode materials having optoelectric properties comparable to that of indium–tin–oxide (ITO) have not yet been developed. In this study, an ultrathin ruthenium film with a ruthenium oxide (RuO2) subsurface layer has been introduced as a flexible transparent electrode. The metallic Ru thin film is fabricated from a RuO2 nanosheet using a layer‐by‐layer coating technique, followed by thermal reduction. The thin film (≈6 nm) reveals comparable sheet resistance and transmittance as that of conventional ITO electrodes. The high transmittance (≈79%) of the metal thin film in the visible range is attributed to the presence of an oxide subsurface layer which acts as antireflection. The Ru film (with oxide subsurface layer) with figure‐of‐merit ≈3.4 × 10−4 Ω−1 shows the best performance among the thin films fabricated using a wet‐chemistry process with 2D nanosheets including graphene, r‐GO, and other metal oxides. In addition, the high mechanical flexibility of Ru thin film makes it next‐generation flexible transparent conducting electrodes, beyond graphene, r‐GO, and 2D metal oxides.

Development of 2D based ZnO–MoS2 nanocomposite for photodetector with light-induced current study

Composites of 2D transition metal dichalcogenides and metal oxides give suitable UV–Visible photoresponse properties. Here, in our research work, the preparation of ZnO–MoS2 nanocomposite is done via a two-step sol-gelassisted cost-effective, and facile synthesis method. The optical bandgap of the as-prepared nanocomposite was found to be 3.68 eV. X-ray diffractogram revealed the crystalline nature of the as-prepared nanocomposite. The optical properties of the material were investigated using Ultraviolet–Visible spectroscopy (UV–Visible) and Fourier Transform Infrared (FTIR) spectroscopy. Surface morphological studies were performed by Scanning electron microscopy (SEM) and the elemental investigations were carried out by the Energy Dispersive X-ray spectroscopy (EDS). For the as-prepared nanomaterial, photoresponsivity was found to be 34.50 mA/W, specific detectivity was 2.36 × 1010 Jones, and response time was found to be 2.46 s at the applied drift potential of 5 V.

Assembling and Regulating of Transition Metal‐Based Heterophase Vanadates as Efficient Oxygen Evolution Catalysts

Developing efficient, durable, and low-cost earth-abundant elements-based oxygen evolution reaction (OER) catalysts by rapid and scalable strategies is of great importance for future sustainable electrochemical hydrogen production. The earth-abundant high-valency metals, especially vanadium, can modulate the electronic structure of 3d metal oxides and oxyhydroxides and offer the active sites near-optimal adsorption energies for OER intermediates. Here, the authors propose a facile assembling and regulating strategy to controllably synthesize a serial of transition metal (CoFe, NiFe, and NiCo)-based vanadates for efficient OER catalysis. By tuning the reaction concentrations, NiFe-based vanadates with different crystallinities can be facilely regulated, where the catalyst with moderate heterophase (mixed crystalline and amorphous structures) shows the best OER catalytic activity in terms of low overpotential (267mV at the current density of 10mA cm-2 ), low Tafel slope (38mV per decade), and excellent long-term durability in alkaline electrolyte, exceeding its noble metal-based counterparts (RuO2 ) and most current existing OER catalysts. This work not only reports a facile and controllable method to synthesize a series of vanadates-based catalysts with heterophase nanostructures for high-performance OER catalysis, but also may expand the scope of designing cost-effective transition metal-based electrocatalysts for water splitting.

High‐Speed Ionic Synaptic Memory Based on 2D Titanium Carbide MXene

AbstractSynaptic devices with linear high‐speed switching can accelerate learning in artificial neural networks (ANNs) embodied in hardware. Conventional resistive memories however suffer from high write noise and asymmetric conductance tuning, preventing parallel programming of ANN arrays. Electrochemical random‐access memories (ECRAMs), where resistive switching occurs by ion insertion into a redox‐active channel, aim to address these challenges due to their linear switching and low noise. ECRAMs using 2D materials and metal oxides however suffer from slow ion kinetics, whereas organic ECRAMs enable high‐speed operation but face challenges toward on‐chip integration due to poor temperature stability of polymers. Here, ECRAMs using 2D titanium carbide (Ti3C2Tx) MXene that combine the high speed of organics and the integration compatibility of inorganic materials in a single high‐performance device are demonstrated. These ECRAMs combine the speed, linearity, write noise, switching energy, and endurance metrics essential for parallel acceleration of ANNs, and importantly, they are stable after heat treatment needed for back‐end‐of‐line integration with Si electronics. The high speed and performance of these ECRAMs introduces MXenes, a large family of 2D carbides and nitrides with more than 30 stoichiometric compositions synthesized to date, as promising candidates for devices operating at the nexus of electrochemistry and electronics.

Two-Dimensional Mesoporous WO3 Nanosheets for Detection of Dimethyl Trisulfide

Two-dimensional (2D) metal oxides semiconductor is considered as an important candidate material in the field of gas sensing. Here, fabrication of 2D ultra-thin mesoporous WO3 nanosheets (WO3NS) was demonstrated by hydrothermal method and morphology-inheritance route. The thickness and specific surface area of the as-prepared 2D WO3NS were 5 nm and 57 m2/g, respectively. It was found that 2D ultrathin mesoporous WO3 NS exhibited outstanding enhanced sensing properties for ppb-level C2H6S3 detection, which could be ascribed to ultrathin structure and edge effect of mesoporous structure. The response of sensors based on 2D ultrathin WO3 NS towards 5 ppb was 1.6, which is very helpful for the identification of biomarkers. In addition, the 2D ultrathin WO3 NS show rapid response, and high selectivity to ppb-level C2H6S3.

Modulation and Modeling of Three‐Dimensional Nanowire Assemblies Targeting Gas Sensors with High Response and Reliability

AbstractDespite improved sensitivity, simple downsizing of gas‐sensing components to randomly arranged nanostructures often faces challenges associated with unpredictable electrical conduction pathways. In the present study, controlled fabrication of three‐dimensional (3D) metal oxide nanowire networks is demonstrated that can greatly improve both signal stability and sensor response compared to random nanowire arrays. For example, the highest ever reported H2S gas response value, and a 5 times lower relative standard deviation of baseline resistance than that of random nanowires assemblies, are achieved with the ordered 3D nanowire network. Systematic engineering of 3D geometries and their modeling, utilizing equivalent circuit components, provide additional insights into the electrical conduction and gas‐sensing response of 3D assemblies, revealing the critical importance of wire‐to‐wire junction points and their arrangement. These findings suggest new design rules for both enhanced performance and reliability of chemical sensors, which may also be extended to other devices based on nanoscale building blocks.

2D layered Mn and Ru oxide nanosheets for real-time breath humidity monitoring

Collecting real-time breath humidity data is important for calibrating gas sensors from interfering signals in breath components, ultimately for accurately monitoring a patient’s physiological information for applications in non-invasive and point-of-care diagnostics. In this work, atomically thin 2D metal oxide nanosheets (NSs) were synthesized by a liquid phase exfoliation process and their humidity sensing properties were investigated. Interestingly, Opposite humidity sensing responses (RD/RH) were observed between semiconducting oxide and metallic oxide NSs. For the semiconducting manganese (Mn) oxide NSs, decreasing resistance transitions were obtained with the response of 24.01 at 44.5% RH at low humidity levels (i.e., 6.1–45% RH), which was governed by proton (H+) conduction. On the other hand, the metallic ruthenium (Ru) oxide NSs exhibited increasing resistance transitions with the response of 0.28 at 96.3% RH at a high humidity range (i.e., 50–99.9% RH) as a result of proton trapping by accepting electrons upon the exposure to excess water molecules. Ru oxide NSs exhibited the response and recovery times of 68 sec and 8 sec, respectively, at 96.3% RH. Real-time breath humidity monitoring is demonstrated by integrating Ru oxide NSs with a wristband-type wireless sensing module, which can transmit the sensing data to a mobile device.

Construction of Multilayer Films and Superlattice- and Mosaic-like Heterostructures of 2D Metal Oxide Nanosheets via a Facile Spin-Coating Process.

This study reports a design of a variety of nanostructured films of 2D oxide nanosheets. We systematically examined the deposition of perovskite-type Ca2Nb3O10- nanosheets by spin-coating their dimethyl sulfoxide dispersion. Neat and homogeneous monolayer tiling was attained on various substrates by selecting an optimum rotation speed, which was dependent on the nanosheet concentration. Repeating the optimized spin-coating process allowed for layer-by-layer deposition of the nanosheets into multilayer films with a designed layer number. Vertical superlattice heterostructures could also be assembled by alternately spin-coating the suspensions of Ca2Nb3O10- and Ti0.87O20.52- nanosheets. Furthermore, spin-coating of a mixed suspension of Ca2Nb3O10- and Ti0.87O20.52- nanosheets led to a mixed mosaic-like monolayer of these two nanosheets. The present study thus demonstrated spin-coating as a facile and powerful route to construct various nanostructures based on 2D oxide nanosheets.

Facile tree leaf-templated synthesis of mesoporous CeO2 nanosheets for enhanced sensing detection of p-xylene vapors

The fabrication of nano-structured 2D metal oxide materials using simple, low-cost, pollution-free and general method has been still challenging. Herein, mesoporous CeO2 nanosheets were prepared by simple cerium nitrate impregnation and calcination using tree leaves as template. Meanwhile, the influence of calcination temperature on microstructure and gas-sensing properties was also investigated. The CeO2-550 nanosheets calcinated at 550 °C are assembled by cross-linking nanoparticles with small size, which possess homogeneously mesoporous distribution, relatively large surface area and oxygen vacancies originated from Ce3+ ions, as well as the synergism of surface coordination unsaturated lattice oxygen and imprinting effect from bio-template. The CeO2-550 sensor exhibits high response (S = 16.5) and selectivity towards 100 ppm p-xylene vapors at 170 °C, and also presents low detection limit of 1 ppm, which are all significantly better than those of reported CeO2-based sensors. Therefore, mesoporous CeO2-550 nanosheets could be utilized as candidate for detecting trace p-xylene vapors.

Low-Dimensional Nanostructures Based on Cobalt Oxide (Co3O4) in Chemical-Gas Sensing

Highly sensitive, stable, low production costs, together with easy maintenance and portability, sensors are ever most demanded nowadays for monitoring and quantification of hazardous chemicals/gases in the environment. The utilization of one dimensional (1D) metal oxide nano structured chemical/gas sensors for environmental monitoring is vastly investigated because of their superior surface to volume ratio, stability, and low production costs, to provide information on the presence of chemical species. Several outstanding attempts have been pursued investigating 1D nano structures of Co3O4 over the past decades as an active material for chemical analytes detection owing to its superior catalytic effect together with its excellent stability. This article reviews the state-of-the-art of growth and characterization of Co3O4 1D nano structures and their functional characterization as chemical/gas sensors. Moreover, fundamental concepts and characteristic features, that enhance the key performances of chemical/gas sensors, are discussed. Finally, challenges and prospective for growth and fabrication of 1D Co3O4 chemical/gas sensors are discussed.

Construction of IrO2@Mn3O4 core-shell heterostructured nanocomposites for high performance symmetric supercapacitor device

In the present work, we have designed and synthesized nanocomposite of IrO2@Mn3O4 with two-step simple and scalable chemical routes. In this route, nanofibers of IrO2 were synthesized by a single nozzle electrospinning technique onto which Mn3O4 was overlaid by a simple SILAR route. The ratio of Mn3O4 and IrO2 was varied by varying the SILAR cycles onto electrospun IrO2 thin film as 20, 40, 60, and 80 cycles. The structural, morphological, and energy storage performance of IrO2@Mn3O4 composite electrodes were investigated. A 2 V kinetic potential with a rectangular-shaped cyclic voltammogram was observed for the IrO2@Mn3O4 electrodes. Moreover, the specific capacitance of 1027 F/g at 1 mA/cm2 was observed for the optimized electrode which is superior as compared with other electrodes. The optimized electrode showed better current and voltage than the individual compounds which might be due to the synergic effect of IrO2 and Mn3O4. Finally, a PVA-LiClO4 gel electrolyte-based solid-state IrO2@Mn3O4//IrO2@Mn3O4 symmetric device was fabricated. The symmetric device possessed an energy density of 81 Wh/kg with a power delivery of 714 W/kg which was capable to light up a green LED. Hence, the 2D transition metal oxides laminated on 1D metal oxides with high conductivity can be promising electrodes for future research.