Two-dimensional Transition Metal Oxides Research Articles

Remarkable enhancement of the adsorption and diffusion performance of alkali ions in two-dimensional (2D) transition metal oxide monolayers via Ru-doping

Transition metal oxides (TMO) are the preferred materials for metal ion battery cathodes because of their high redox potentials and good metal-ion intercalation capacity, which serve as an outstanding replacement for layered sulphide. In this work, using first-principles calculations based on Density functional theory approach, we explored the structural and electronic properties which comprise of adsorption and diffusion behaviour along with the analysis of voltage profile and storage capacity of Ru doped two-dimensional transition metal oxide MnO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$MnO_{2}$$\\end{document}, CoO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$CoO_{2}$$\\end{document}, and NiO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$NiO_{2}$$\\end{document} monolayers. The adsorption of alkali ions (Li, Na) to the surface of TMOs is strengthened by Ru-atom doping. Ru doping enhanced the adsorption energy of Li/Na-ion by 25%/11% for MnO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$MnO_{2}$$\\end{document}, 8%/13% for CoO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$CoO_{2}$$\\end{document}, and 10%/11% NiO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$NiO_{2}$$\\end{document} respectively. The open circuit voltage (OCV) also increases due to the high adsorption capacity of doped Monolayers. Ru doping makes the semiconducting TMOs conduct, which is suitable for battery application. As alkali ion moves closer to the dopant site, the adsorption energy increases. When alkali ions are close to the vicinity of doping site, their diffusion barrier decrease and rises as they go further away. Our current findings will be useful in finding ways to improve the storage performance of 2D oxide materials for application in energy harvesting and green energy architecture.

CrO<sub>2</sub> monolayer: a two-dimensional ferromagnet with high Curie temperature and half-metallicity

Owing to the complete spin-polarization of electronic states near Fermi energy, half-metallic ferromagnets, especially two-dimensional half-metallic ferromagnets, have garnered significant attention in the field of spintronics. However, the practical applications of these materials are greatly hindered by their low Curie temperatures. Therefore, the exploration of high Curie temperature half-metallic ferromagnets poses a necessary and challenging task. In this study, we predict a two-dimensional transition metal oxide, CrO<sub>2</sub> monolayer, and employ first-principles calculations to investigate the crystal structure, electronic properties, magnetic ground state, and ferromagnetic phase transition. The calculations of phonon spectrum, elastic constant, and molecular dynamics simulations indicate that CrO<sub>2</sub> monolayer is dynamically, mechanically, and thermally stable. The convex hull diagram of Cr-O systems shows that the hull energy of the predicted CrO<sub>2</sub> layer is only 0.18 eV, further confirming the structural stability and large possibility for experimental fabrication. More importantly, the electronic and magnetic properties of CrO<sub>2</sub> monolayer demonstrate that it is a two-dimensional ferromagnetic half-metal with wide band gap. Five d suborbitals are divided into E<sub>g</sub> and T<sub>2g</sub> orbitals because of the crystal field of Cr atom in the center of O tetrahedron, and the spin-polarizations of E<sub>g</sub> orbitals make a major contribution to the moment around Cr atom. The ferromagnetic coupling along Cr-O-Cr chain is dominated by the superexchange interaction bridged by O 2p orbitals, similar to the typical Mn-O-Mn superexchange model. The magnetic behavior of the Cr spin lattice in a CrO<sub>2</sub> monolayer is described by a two-dimensional Heisenberg model, in which the exchange coupling anisotropy is ignored and the single ion anisotropy is the main consideration. By solving the Heisenberg model through using the Monte Carlo simulation method, the Curie temperature is determined to be over 400 K. The high Curie temperature ferromagnetism is rare in two-dimensional ferromagnetic materials and even rarer in semi-metallic materials, which makes it an ideal material for fabricating spintronic devices and studying spin quantum effects.

Printable 2D Materials for Thin Film Batteries and Supercapacitors

Two-dimensional transition metal oxide (TMO) nanosheets are the oxide equivalents of the well-known 2D graphene phase. Compared with graphene, a much wider range of compositions, structures and properties can be realized from TMOs. 2D metal oxides typically have thicknesses of 0.45-2.5 nm, and lateral dimensions between 50 nm and tens of micrometers. Interestingly, although they are oxides, they are very flexible and bendable owing to their thinness. So far, several dozen nanosheet phases have been reported in literature, but many more compositions are possible.Dispersions of TMO nanosheets can be made following one of several solution-phase strategies. Often, they are prepared by chemical delamination of isomorphous layered metal oxide parent phases. In a few cases, the 2D TMO phase can be grown directly bottom-up from an aqueous chemical solution. Either way, the resulting colloidal solutions of 2D single crystal nanosheets can serve in subsequent process step as the basis of a chemical ink suitable for ink jet printing on arbitrary substrates like paper, silicon or flexible polymers. Redox-active nanosheets may also be used as active electrode elements for micro-supercapacitors and flexible thin film batteries.For example, 2D MnO2 nanosheets that are isomorphous to the layered δ-MnO2 (birnessite) phase exhibit high pseudocapacance owing to their beneficial redox properties, relatively high in-plane conductivity and very large specific surface area. Ink-jet printed 2D MnO2 based thin film microsupercapacitors were realized that showed comparable performance with other state of the art systems. However, undoped MnO2 nanosheets exhibit relatively low sheet conductivity. Substitutional doping of 3d metal ions (Co, Fe and Ni) into MnO2 nanosheets improved the volumetric energy density to 1.13 mWh cm−3 at a power density of 0.11 W cm−3. The dopants introduced new electronic states near the Fermi level which enhanced the electronic conductivity within the nanosheets and contributed to the further formation of redox-active 3d surface states. Among these, Fe-doped MnO2 exhibited a significant increase in the carrier density and conductivity, while doping with Ni or Co had a much more limited effect on conductivity. These Fe-doped MnO2 nanosheet-based microsupercapacitors show long cycle life and bendability without performance loss on flexible plastic substrates [1].Following a similar inkjet printing approach, thin film cathodes for Li ion batteries were made using V2O5 nanosheets as charge-storage medium. In this case the conductivity of the electrode was improved by addition of highly conductive 2D MXene (Ti3C2) platelets derived from the parent MAX phase Ti3AlC2. Using LiPF6 in EC/DMC as electrolyte, the hybrid V2O5-MXene electrodes showed a high capacity of 321 mAh g-1 at 1C, 112 mAh g-1 at 10.5C and 92% capacity retention after ~700 cycles [2].As third example, an all inkjet-printed MXene/graphene oxide (GO)/MXene thin film supercapacitor with GO as solid electrolyte phase will be discussed. Here, free water molecules trapped between GO layers enabled proton transport through the electrolyte. The as-made capacitor showed high areal capacitance, good cyclability and power densities comparable with state of the art printed supercapacitors. The specific capacitance could be increased further by addition of liquid electrolyte [3].[1] Y. Wang, Y.-Z. Zhang, Y.-Q. Gao, G. Sheng, J.E. ten Elshof, Nano Energy 68 (2020) 104306.[2] Y. Wang, T. Lubbers, R. Xia, Y.-Z. Zhang, M. Mehrali, M. Huijben, J.E. ten Elshof, J. Electrochem. Soc. 168 (2021) 020507[3] Y. Wang, M. Mehrali, Y.-Z. Zhang, M.A. Timmerman, B.A. Boukamp, P.-Y. Xu, J.E. ten Elshof, Energy Storage Mater. 36 (2021) 318-325.

Ultrathin 2D nanosheets of transition metal (hydro)oxides as prospective materials for energy storage devices: A short review

The ultrathin two-dimensional (2D) transition metal oxides and hydroxides (TMO and TMH) nanosheets are attractive for creating high-performance energy storage devices due to a set of unique physical and chemical properties. Flat 2D structure of such materials provides a sufficient number of active adsorption centers, and the ultra-small thickness, on the order of several nanometers, provides fast charge transfer, which significantly improves electronic conductivity. This brief review summarizes recent progress in the synthesis of materials based on ultrathin 2D nanosheets for energy storage applications, including pseudocapacitors, lithium-ion batteries, and other rechargeable devices. The review also presents examples of representative work on the synthesis of ultrathin 2D nanomaterials based on TMO and TMH for various power sources. In conclusion, the article discusses possible prospects and directions for further development of methods and routes for the synthesis of ultrathin two-dimensional transition metal oxides and hydroxides.

Interplay between singlet and triplet pairings in multiband two-dimensional oxide superconductors

We theoretically study the superconducting properties of multi-band two-dimensional transition metal oxide superconductors by analyzing not only the role played by conventional singlet pairings, but also by the triplet order parameters, favored by the spin-orbit couplings present in these ma- terials. In particular, we focus on the two-dimensional electron gas at the (001) interface between LaAlO3 and SrTiO3 band insulators where the low electron densities and the sizeable spin-orbit couplings affect the superconducting features. Our theoretical study is based on an extended su- perconducting mean-field analysis of the typical multi-band tight-binding Hamiltonian, as well as on a parallel analysis of the effective electronic bands in the low-momentum limit, including static on-site and inter-site intra-band attractive potentials under applied magnetic fields. The presence of triplet pairings is able to strongly reduce the singlet order parameters which, as a result, are no longer a monotonic function of the charge density. The interplay between the singlet and the triplet pairings affects the dispersion of quasi-particle excitations in the Brillouin zone and also induces anisotropy in the superconducting behavior under the action of an in-plane and of an out- of-plane magnetic fields. Finally, non-trivial topological superconducting states become stable as a function of the charge density, as well as of the magnitude and of the orientation of the magnetic field. In addition to the chiral, time-reversal breaking, topological superconducting phase, favored by the linear Rashba couplings and by the on-site attractive potentials in the presence of an out- of-plane magnetic field, we find that a time-reversal invariant topological helical superconducting phase is promoted by not-linear spin-orbit couplings and by the inter-site attractive interactions in the absence of magnetic field.

Structural, electronic, and optical properties of two-dimensional hafnium monoxide nanosheets

Ultrathin two-dimensional transition metal oxides are highly desirable as promising materials for energy storage, gas sensors, photonic and electronic devices. In this work, we studied the structural stability, electronic structure and optical properties of two-dimensional hafnium monoxide with hexagonal and tetragonal lattice by the first principles calculations. Phonon dispersion reveals the lattice dynamic stability of hexagonal structures. Results of band structure showed that both hexagonal and tetragonal HfO nanosheets had highly anisotropic metallic behavior. The optical properties showed strong light absorption and anisotropy.

A universal strategy for the synthesis of porous two-dimensional transition metal oxide nanosheets based on chemical topology transformation

Two-dimensional (2D) transition metal oxides (TMOs) have attracted much attention for various applications, owing to the abundance of active sites, rapid ion transmission speed, and short carrier migration distance. However, the current preparation strategies are usually limited to producing intrinsically layered compounds or sacrificing template. Here, we report a universal strategy for preparing ultrathin porous 2D TMO nanosheets by chemical topological transformation of the corresponding transition metal selenides. We observed that the as-prepared 2D TMO nanosheets not only perfectly inherit the transition metal selenides’ 2D topological structure, but also possess numerous pore structures formed as a way to release the structural stress associated with lattice growth. As a proof of concept, ultrathin porous WO3, MoO3, and Co3O4 nanosheets were successfully prepared based on the in-situ oxidation of the corresponding ultrathin WSe2, MoSe2, and Co0.85Se, respectively. The outstanding sensing properties and photodetector performance displayed by the as-prepared porous 2D WO3 nanosheets further indicate the promising prospects of topology transformation for the preparation of porous 2D TMO nanosheets.

Recent advance in biosensing applications based on two-dimensional transition metal oxide nanomaterials

In recent years, two-dimensional transition metal oxide nanomaterials (2D TMONs) have drawn increasing attention due to their various functionalization, tunable electronic characteristics, unique optical properties, excellent chemical and thermal stabilities, large surface area and strong oxidation ability. The metal ions of 2D TMONs usually possessed the unfilled d-orbital. Furthermore, 2D TMONs contained oxygen ion in comparison with other 2D nanomaterials. Thus, 2D TMONs has a series of features which included reactive electronic transitions, high dielectric constants, wide bandgaps and excellent electrical property. They could act as quencher to quench the fluorescence intensity of fluorescent sensor or electrochemiluminescence. Recently, they have been demonstrated both excellent biological compatibility and good dispersion for the oxygen ions. These properties endow 2D TMONs could be used in optic, electronic, catalytic, energy technology, biosensing to biomedical diagnosis and therapy. In this review, we provide a brief overview regarding the progress of 2D TMONs based biosensors that function through various analytical methods including fluorescence, chemiluminescence, electrochemical and colorimetric in recent five years. The review may do some help to the researchers who are interested in 2D TMONs based biosensors.

Two-Dimensional Transition Metal Oxide and Hydroxide-Based Hierarchical Architectures for Advanced Supercapacitor Materials.

The supercapacitor has been widely seen as one of the most promising emerging energy storage devices, by which electricity is converted from chemical energy and stored. Two-dimensional (2D) metal oxides/hydroxides (TMOs/TMHs) are revolutionizing the design of high-performance supercapacitors because of their high theoretical specific capacitance, abundance of electrochemically active sites, and feasibility for assembly in hierarchical structures by integrating with graphitic carbon, conductive polymers, and so on. The hierarchical structures achieved can not only overcome the limitations of using a single material but also bring new breakthroughs in performance. In this article, the research progress on 2D TMOs/TMHs and their use in hierarchical structures as supercapacitor materials are reviewed, including the evolution of supercapacitor materials, the configurations of hierarchical structures, the electrical properties regulated, and the existence of advantages and drawbacks. Finally, a perspective covering directions and challenges related to the development of supercapacitor materials is provided.

Exfoliation in a low boiling point solvent and electrochemical applications of MoO3.

MoO3 is a versatile two-dimensional transition metal oxide having applications in areas such as energy storage devices, electronic devices and catalysis. To efficiently utilize the properties of MoO3 arising from its two-dimensional nature exfoliation is necessary. In this work, the exfoliation of MoO3 is carried out in 2-butanone for the first time. The achieved concentration of the dispersion is about 0.57 mg·mL−1 with a yield of 5.7%, which are the highest values reported to date. These high values of concentration and yield can be attributed to a favorable matching of energies involved in exfoliation and stabilization of MoO3 nanosheets in 2-butanone. Interestingly, the MoO3 dispersion in 2-butanone retains its intrinsic nature even after exposure to sunlight for 24 h. The composites of MoO3 nanosheets were used as an electrode material for supercapacitors and showed a high specific capacitance of 201 F·g−1 in a three-electrode configuration at a scan rate of 50 mV·s−1.

Theoretical Exploration of Single-Layer Tl2O as a Catalyst in Lithium–Oxygen Battery Cathodes

Two-dimensional transition metal oxides have been widely explored as a catalyst in the high capacity non-aqueous lithium-oxygen batteries due to their excellent electrochemical performance in the o...

Recent progress in self-supported two-dimensional transition metal oxides and (oxy)hydroxides as oxygen evolution reaction catalysts

The synthesis, activity enhancement methods and applications of 2D-SS-TMO OER catalysts are reviewed.

Formation of two-dimensional transition metal oxide nanosheets with nanoparticles as intermediates.

Two-dimensional (2D) materials have attracted significant interest because of their large surface-to-volume ratios and electron confinement. Compared to common 2D materials such as graphene or metal hydroxides, with their intrinsic layered atomic structures, the formation mechanisms of 2D metal oxides with a rocksalt structure are not well understood. Here, we report the formation process for 2D cobalt oxide and cobalt nickel oxide nanosheets, after analysis by in situ liquid-phase transmission electron microscopy. Our observations reveal that three-dimensional (3D) nanoparticles are initially formed from the molecular precursor solution and then transform into 2D nanosheets. Ab initio calculations show that a small nanocrystal is dominated by positive edge energy, but when it grows to a certain size, the negative surface energy becomes dominant, driving the transformation of the 3D nanocrystal into a 2D structure. Uncovering these growth pathways, including the 3D-to-2D transition, provides opportunities for future material design and synthesis in solution.

Facile mass production of self-supported two-dimensional transition metal oxides for catalytic applications.

We report a novel metal-corrosion route beyond traditional top-down or bottom-up strategies for the mass production of 2D TMOs with a self-supported structure. The self-supported 2D Co3O4, a typical TMO, exhibits excellent electrocatalytic activity and stability in the oxygen evolution reaction surpassing those of commercial precious metal RuO2 catalysts at high currents.

Co3O4 Nanosheets with In-Plane Pores and Highly Active {112} Exposed Facets for High Performance Lithium Storage

Recently, two-dimensional transition metal oxide nanomaterials have been extensively investigated as promising candidates for the lithium-ion battery anode materials due to their elastic volume change, efficient ion/electrical pathways, and additional interfacial lithium storage sites. Herein, we report a simple wet-chemical method followed by thermal treatment to synthesize Co3O4 nanosheets with the in-plane pores. The as-prepared nanosheets are found to selectively expose the highly active {112} facets as the dominant surfaces. When fabricated into the anode configuration, a specific capacity of 1717 mA h g–1 can be reliably retained after 100 cycles at a current density of 200 mA g–1. While increasing the current density to 1 A g–1 and prolonging the cycle life to 400 cycles, the nanosheets can still deliver a capacity of 1090 mA h g–1 with a Coulombic efficiency of 99.5%. This excellent electrochemical performance can be attributed to the unique morphological structures of our porous nanosheets for th...

Holey two-dimensional transition metal oxide nanosheets for efficient energy storage

Transition metal oxide nanomaterials are promising electrodes for alkali-ion batteries owing to their distinct reaction mechanism, abundant active sites and shortened ion diffusion distance. However, detailed conversion reaction processes in terms of the oxidation state evolution and chemical/mechanical stability of the electrodes are still poorly understood. Herein we explore a general synthetic strategy for versatile synthesis of various holey transition metal oxide nanosheets with adjustable hole sizes that enable greatly enhanced alkali-ion storage properties. We employ in-situ transmission electron microscopy and operando X-ray absorption structures to study the mechanical properties, morphology evolution and oxidation state changes during electrochemical processes. We find that these holey oxide nanosheets exhibit strong mechanical stability inherited from graphene oxide, displaying minimal structural changes during lithiation/delithiation processes. These holey oxide nanosheets represent a promising material platform for in-situ probing the electrochemical processes, and could open up opportunities in many energy storage and conversion systems.

Prediction of high-temperature quantum anomalous Hall effect in two-dimensional transition-metal oxides

Quantum anomalous Hall (QAH) insulator is a topological phase which exhibits chiral edge states in the absence of magnetic field. The celebrated Haldane model is the first example of QAH effect, but difficult to realize. Here, we predict the two-dimensional single-atomic-layer V2O3 with a honeycomb-Kagome structure is a QAH insulator with a large band gap (large than 0.1 eV) and a high ferromagnetic Curie temperature (about 900 K). Combining the first-principle calculations with the effective Hamiltonian analysis, we find that the spin-majority dxy and dyz orbitals of V atoms on the honeycomb lattice form a massless Dirac cone near the Fermi level which becomes massive when the on-site spin-orbit coupling is included. Interestingly, we find that the large band gap is caused by a cooperative effect of electron correlation and spin-orbit coupling. Both first-principle calculations and the effective Hamiltonian analysis confirm that 2D V2O3 has a non-zero Chern number (i.e., one). Our work paves a new direction towards realizing the QAH effect at room temperature.

Scalable salt-templated synthesis of two-dimensional transition metal oxides.

Two-dimensional atomic crystals, such as two-dimensional oxides, have attracted much attention in energy storage because nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. However, current strategies are largely limited to intrinsically layered compounds. Here we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to not only layered compounds but also various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is hypothesized to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional hexagonal-MoO3 exhibits high pseudocapacitive performances (for example, 300 F cm−3 in an Al2(SO4)3 electrolyte). The synthesis of various two-dimensional transition metal oxides and the demonstration of high capacitance are expected to enable fundamental studies of dimensionality effects on their properties and facilitate their use in energy storage and other applications.

Generalized Self-Assembly of Two-Dimensional Transition Metal Oxide Nanosheets on Flexible Graphene

Two-dimensional (2D) transition metal oxide systems present exotic electronic properties and high specific surface areas, and also demonstrate promising applications ranging from electronics to energy storage. Yet, in contrast to other types of nanostructures, the question as to whether we could assemble 2D nanomaterials with an atomic thickness from molecules in a general way, which would give them some amazing properties such as those of graphene, still remains unresolved. Herein, we report a generalized and fundamental approach to molecular self-assembly synthesis of ultrathin 2D nanosheets of transition metal oxides by rationally employing lamellar reverse micelles. It is worth emphasizing that the synthesized crystallized ultrathin transition metal oxide nanosheets possess confined thickness, high specific surface area, and chemically reactive facets, so that they could have promising applications in nanostructured electronics, photonics, sensors, and energy conversion and storage devices.

Generalized self-assembly of scalable two-dimensional transition metal oxide nanosheets.

Two-dimensional (2D) transition metal oxide systems present exotic electronic properties and high specific surface areas, and also demonstrate promising applications ranging from electronics to energy storage. Yet, in contrast to other types of nanostructures, the question as to whether we could assemble 2D nanomaterials with an atomic thickness from molecules in a general way, which may give them some interesting properties such as those of graphene, still remains unresolved. Herein, we report a generalized and fundamental approach to molecular self-assembly synthesis of ultrathin 2D nanosheets of transition metal oxides by rationally employing lamellar reverse micelles. It is worth emphasizing that the synthesized crystallized ultrathin transition metal oxide nanosheets possess confined thickness, high specific surface area and chemically reactive facets, so that they could have promising applications in nanostructured electronics, photonics, sensors, and energy conversion and storage devices.