Structure Of Transition Metal Oxides Research Articles

NiFe2O4@NiCo2O4 hollow algae-like microspheres enabled by Mott-Schottky for electrochemical energy storage

Designing hybrid structure of transition metal oxides (TMOs) with controlled morphology can adjust the electronic structure and create more kinetics reactions. Herein, a new kind of NiFe2O4@NiCo2O4 nanocomposite was synthesized with one pot hydrothermal route. The unique hollow algae-like microspheres with plenty nanowires and the synergistic effect factors promote high electroactive sites and generate a built-in electric field. The constituted Mott-Schottky heterostructure creates strong interfacial interaction, paves the way for the electrolyte ions diffusion and results high charges transfer abilities by reducing the energy barriers in the heterointerfaces. As cathode material, the NiFe2O4@NiCo2O4 presents satisfactory specific capacity of 996C·g−1 at 0.5 A·g−1 and maintains ultrahigh capacity retention of 81 % at 30 A·g−1 with marvellous stability of only 6 % loss during 20,000 cycles. Interestingly, asymmetric hybrid supercapacitor device based NiFe2O4@NiCo2O4//active carbon (AC) provides tremendous energy density of 117.6 Wh·kg−1 at 438 W·kg−1 power density. What’s more, all-solid-state device offers ultra-low self-discharge process of 9.5 % through 24 h. Therefore, the combination of NiFe2O4@NiCo2O4 to form Mott-Schottky heterojunction was a promising strategy to develop electrode material with high energy storage performance.

Crystal Orientation Regulation of Spin-Orbit Torque Efficiency and Magnetization Switching in SrRuO<sub>3</sub> Thin Films

Spintronic devices utilize the spin property of electrons for the storage, transmission, and processing of information, inherently possessing advantages such as low power consumption and non-volatility, thus attracting widespread attention from both academia and industry. Spin-orbit torque(SOT) is an efficient method of manipulating magnetic moments using electric current for writing, harnessing the spin-orbit coupling (SOC) effect within materials to achieve the mutual conversion between charge current and spin current. Enhancing the efficiency of charge-spin conversion is a critical issue in the field of spintronics. Strontium ruthenate (SRO) in transition metal oxides(TMO) has attracted significant attention as a spin source material in SOT devices due to its large and tunable charge-to-spin conversion efficiency. However, current research on SOT control in SRO primarily focuses on utilizing substrate strain, with limited exploration of other control methods. Crystal orientation can produce various novel physical properties by affecting material symmetry and electronic structure, is one of the important means to control the properties of TMO materials. Given the close correlation between the SOT effect and electronic structure as well as surface states, crystal orientation is expected to affect SOT properties by adjusting the electronic band structure of TMO. This work investigates the effect of crystal orientation on the SOT performance of SrRuO<sub>3</sub> films and develops a novel approach for SOT control. (111)-oriented SRO/CoPt heterostructures and SOT devices were prepared using pulse laser deposition, magnetron sputtering, and micro-nano processing techniques. Through harmonic Hall voltage(HHV) measurements, we found that the SOT efficiency reached 0.39, and the spin Hall conductivity reached 2.19×10<sup>5</sup><i>ħ</i>/2e Ω<sup>−1</sup> m<sup>−1</sup>, which were 86% and 369% higher than those of the (001) orientation, respectively. Furthermore, current-driven perpendicular magnetization switching was achieved in SrRuO<sub>3</sub>(111) devices at a low critical current density of 2.4×10<sup>10</sup> A/m<sup>2</sup>, which was 37% lower than that of the (001) orientation. These results demonstrate that crystal orientation is an effective approach to significantly enhance the comprehensive performance of SrRuO<sub>3</sub>-based SOT devices, providing new insights for developing high-efficiency spintronic devices.

Eliminating the Mn 3d Orbital Degeneracy to Suppress the Jahn–Teller Distortion for Stable MnO2 Cathode

AbstractThe cycle stability of manganese‐based oxides cathode for sodium‐ion supercapacitor is impeded by severe structural collapse due to the Jahn–Teller (J–T) distortion of [MnO6] octahedra as Mn4+ is reduced to Mn3+. Herein, MnO2 cathode is stabilized by Mg2+ ions confinement between transition metal slabs. The Mg2+ confinement induces the transformation from original cubic [MnO6] octahedra to compressed configuration (com‐MnO2), resulting in the elimination of Mn 3d orbital degeneracy, thereby alleviating the structural deformation of [Mn3+O6] octahedra by suppressing J–T distortion during the reduction of Mn4+. As a result, the prepared com‐MnO2 exhibits greatly improved cycling stability with 99.1% retention of its initial capacity after 20 000 cycles due to inhibited J–T distortion. Additionally, an enhanced capacity of 434 F g−1 at 1 A g−1, a rate capacity of 275 F g−1 at 20 A g−1 can be achieved by com‐MnO2. The in situ XRD, Raman spectra reveal the reversible structural transformation during a cycle. It is concluded that the elimination of Mn 3d orbital degeneracy plays a crucial role in suppressing J–T distortion, which is conducive to in‐depth understanding of electron structure of transition metal oxides toward cycling performance.

Hole Selectivity of n‐Type Molybdenum Oxide Carrier Selective Layer for Commercial and Emerging Thin‐Film Photovoltaics: A Critical Analysis of Interface Energetics and Ensuant Device Physics

Energetics at the carrier selective layer/absorber interface plays a subtle, yet crucial role in influencing the performance of any photovoltaic technology. This review focuses on the interface energetics and resulting aspects of device physics associated with n‐type molybdenum oxide, which is adopted as a hole selective layer over a diverse class of photovoltaic technologies ranging from silicon heterojunction, organic, and perovskite solar cells. As existing literatures provide a very inconsistent picture of the electronic structure of transition metal oxides, specifically in the case of molybdenum oxide (MoOx), this review makes a first‐of‐its‐kind attempt to connect the scattered reports and thus highlight the ways in which device performance varies for a diverse class of photon absorbers used in conjunction with n‐type MoOx hole selective layer. In spite of the interfacial energy requirements and in turn fundamental working mechanism of varied photovoltaic technologies being the same, an attempt to compare and collate the contributions of MoOx as a charge selective layer for each of these technologies is yet to be done. Herein, the role played by MoOx/absorber interface engineering on the performance of varied photovoltaic technologies, which remains a fundamental and yet unaddressed theme till date, is focused on.

Structure sensitivity in the photocatalytic reduction of CO2 with Co3O4 catalysts

Transition metal oxides (TMOs) have been proposed as potential candidates for CO2 photoreduction (CO2RR). Nonetheless, they often exhibit low efficiency and poor selectivity. Understanding how structural surface features influence photocatalytic reactions is crucial for the development of efficient catalysts. Unfortunately, the intricate structure of TMOs often hinders the thorough understanding of their structure–activity relationships. In this study, three spinel Co3O4 nanocrystals with different planes are used as model catalysts to systematically investigate the structure sensitivity for CO2RR. According to both experimental results and theoretical calculations, Co3O4 with a higher exposure of Co2+ is more effective in the photoreduction of CO2 to CO. This can be attributed to a combination of factors including improved separation of charge carriers, O-defects that promote CO2 activation and the Co2+ sites that facilitate the desorption of CO product. Moreover, the CO2RR performance over Co3O4 catalysts can be controlled by the surface atomic arrangement of Co2+ and Co3+, which through affecting a tradeoff between activating the CO2 reactant and releasing the CO product. Therefore, the proper control of Co2+/Co3+ ratio on the surface of Co3O4 can optimize the reaction performance. This study provides guidance for designing highly active and cost-effective TMO-based catalysts for CO2RR.

Promoting the activation of H2O via vacancy defects over metal-organic framework-derived cobalt oxide for enhanced oxygen evolution

Engineering the electronic structures of transition metal oxides via vacancy defects is an efficient approach to enhance their catalytic performances, while facilely creating vacancy defects and deeply exploring the effects still remain challenging. Herein, a facile reduction method was applied to introduce oxygen vacancy defects into metal-organic frameworks (MOFs)-derived cobalt oxide (Co3O4) polyhedron. Using zeolitic imidazolate framework-67 (ZIF-67) as Co source and self-sacrificial template, Co3O4 with polyhedron morphology and porous structure was synthesized, and then oxygen vacancy defects were introduced into the as-prepared Co3O4 through one-step NaBH4 reduction treatment. Strikingly, the obtained electrocatalyst (denoted as Co3O4-x) exhibited a larger current density (112.9 mA cm−2 vs. 1.75 V), a lower overpotential (335 mV at 10 mA cm−2) and a smaller Tafel slope (83 mV dec−1) than the Co3O4, which is even superior to the state-of-the-art IrO2 catalyst. Moreover, experiment results combined with density-functional theory (DFT) calculations revealed that the existence of oxygen vacancies could improve the conductivity, promote the activation of H2O and reduce the potential of potential-limited step (PLS), thus greatly boosting electrocatalytic oxygen evolution reaction (OER) performances. This work reported a strategy for preparing defect-rich OER electrocatalysts, and also provided a deep understanding of the defect effects on catalytic performances.

Regulating Spin States in Oxygen Electrocatalysis.

Developing efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate the d orbitals occupancy, which influence the strength of metal-ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.

Regulating Spin States in Oxygen Electrocatalysis

AbstractDeveloping efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate thedorbitals occupancy, which influence the strength of metal‐ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.

A Brand-New Hybrid Structure with Advantageous Electron State for Ultrahigh Energy Density Asymmetric Supercapacitors

Rational design and preparation for improving surface chemical properties and electron structure of transition metal oxides to enhance their energy storage capacity are highly desired. Herein, a brand-new organic–inorganic hybrid structure (NH2-MIL-125/ZnMn2O4) consisting of a metal–organic framework and polynary transition metal oxide was synthesized successfully and further modified by Ag decoration engineering. In detail, the hybrid structure can provide abundant dual metal reactive sites (Zn and Mn), and based on the Ag decorating, the electron structure of the hybrid structure presents an advantageous state, which means high chemical activity for electrochemical redox reactions. Benefiting from the above and additional outstanding advantages of good electrical conductivity, complex valence states, high specific surface area (SSA), and well-developed micro-morphology, the targeted sample shows an extraordinary energy density of 204.31 Wh kg–1 at power density of 3.15 kW kg–1, which is advanced among related electrode materials. In addition, density functional theory (DFT) calculations confirm the pseudocapacitance improvement mechanism by Ag decorating. In summary, this work can not only provides some new thoughts for the design and preparation of electrode materials but also encourages more profound modification paths in the area of electron structure and reaction mechanism.

Persona of Transition Metal Ions in Solids: A Statistical Learning on Local Structures of Transition Metal Oxides.

The local structure of a transition metal (TM) ion is a function of cation elements and valence states. More than that, in this work, by employing a trove of first‐principles data of TM oxides, the local structures of TM cations are statistically analyzed to extract detailed information about cation site preference, bond length, site structural distortion, and cation magnetization. It is found that cation radius alone poorly describes the local structure of a transition metal oxide, while the statistics of coordination number as well as the TM—O bond length distribution, especially that of the 3d TMs, can provide comprehensive knowledge for understanding the behavior of TM elements. Based on these statistics, the interplay of site distortion due to the Jahn–Teller effect, cation site similarity, and a new set of ionic radii are all obtained to chart the “persona” of transition metal ions in solids.

The interplay between selective etching induced cation defects and active oxygen species for volatile organic compounds degradation

Surface electronic structure of transition metal oxides plays a vital role in determining the catalytic performance. Herein, we present a selective etching strategy to tune the surface cation defect of the CuWO4 (CW) catalyst for improving the catalytic activity of volatile organic compounds (VOCs). HRTEM, SEM-EDS, EPR, and XPS show that the chelation of metal ions in acetic acid and ammonium hydroxide can help to remove a small number of surface cations in CW to form suitable W defects. Cu L-edge and O K-edge XAS, Raman, and O 1s XPS spectrum illustrate that cation defects can improve the hybrid orbits of metal-oxygen bonds, which increases the activity of surface lattice oxygen and metal sites. In-situ DRIFTS spectra reveal that CW with cation defects can easily adsorb toluene, cleave and oxidize benzene ring, and desorb CO2 because of more surface dangling bonds and active oxygen species. Therefore, the toluene conversion rates of CW-Aci and CW-Alk are much higher than CW in VOCs degradation and the catalytic performance improved 33 times and 22 times at 200 °C, respectively. This study offers a new pathway in engineering surface electronic structure and highlights the interplay between cation defects and active oxygen species.

Metal Substitution in Rutile TiO2: Segregation Energy and Conductivity

State-of-the-art acidic oxygen evolution reaction catalysts are mainly based on mixed oxides of titanium and iridium or ruthenium. Because of its high chemical corrosion resistance and stability in acid, $${\hbox {TiO}_2}$$ is a promising support material, yet with poor electrical conductivity. We herein theoretically examined the stability and the electronic properties of substituted rutile-phase titania, in which Ti is partly replaced by elements of the fourth, fifth and sixth period. For negative values of the calculated segregation energy, we conclude that the heteroelement M forms a solid solution for 25% or 50% substitution. For these heteroelements, we calculate the electrical conductivity. Since it is known that the electronic structure of transition metal oxides strongly depends on the applied theoretical method, we performed self-consistent optimizations of the Hartree-Fock contribution to the exchange functional in a hybrid functional for every compound. Experimentally, two favorable and two unfavorable elements were synthesized with 25% foreign metal concentration. The most promising candidates to improve the electronic properties of titania are Ta and Nb.

Breaking of Thermopower-Conductivity Trade-Off in LaTiO3 Film around Mott Insulator to Metal Transition.

Introducing artificial strain in epitaxial thin films is an effective strategy to alter electronic structures of transition metal oxides (TMOs) and to induce novel phenomena and functionalities not realized in bulk crystals. This study reports a breaking of the conventional trade‐off relation in thermopower (S)–conductivity (σ) and demonstrates a 2 orders of magnitude enhancement of power factor (PF) in compressively strained LaTiO3 (LTO) films. By varying substrates and reducing film thickness down to 4 nm, the out‐of‐plane to the in‐plane lattice parameter ratio is controlled from 0.992 (tensile strain) to 1.034 (compressive strain). This tuning induces the electronic structure change from a Mott insulator to a metal and leads to a 103‐fold increase in σ up to 2920 S cm−1. Concomitantly, the sign of S inverts from positive to negative, and both σ and S increase and break the trade‐off relation between them in the n‐type region. As a result, the PF (=S 2 σ) is significantly enhanced to 300 µW m− 1K−2, which is 102 times larger than that of bulk LTO. Present results propose epitaxial strain as a means to finely tune strongly correlated TMOs close to their Mott transition, and thus to harness the hidden large thermoelectric PF.

The electronic structure of transition metal oxides for oxygen evolution reaction

The electronic structure of transition metal oxides for oxygen evolution reaction

Hydrogen in tungsten trioxide by membrane photoemission and density functional theory modeling

The measurement of hydrogen-induced changes in the electronic structure of transition metal oxides by x-ray photoelectron spectroscopy is a challenging endeavor, since no photoelectron can be unambiguously assigned to hydrogen. The H-induced electronic structure changes in tungsten trioxide have been known for more than 100 years but are still controversially debated. The controversy stems from the difficulty in disentangling effects due to hydrogenation from the effects of oxygen deficiencies. Using a membrane approach to x-ray photoelectron spectroscopy, in combination with tunable synchrotron radiation, we measure simultaneously core levels and the valence band up to a hydrogen pressure of 1000 mbar. Upon hydrogenation, the intensities of the ${\mathrm{W}}^{5+}$ core level and a state close to the Fermi level increase following the pressure-composition isotherm curve of bulk ${\mathrm{H}}_{x}{\mathrm{WO}}_{3}$. Combining experimental data and density functional theory, the description of the hydrogen-induced coloration by a proton polaron model is corroborated. Although hydrogen is the origin of the electronic structure changes near the Fermi edge, the valence band edge is now dominated by tungsten orbitals instead of oxygen as is the case for the pristine oxide, having wider implications for its use as a (photoelectrochemical) catalyst.

A full configuration interaction quantum Monte Carlo study of ScO, TiO, and VO molecules.

Accurate ab initio calculations of 3d transition metal monoxide molecules have attracted extensive attention because of their relevance in physical and chemical science as well as theoretical challenges in treating strong electron correlation. Meanwhile, recent years have witnessed the rapid development of the full configuration interaction quantum Monte Carlo (FCIQMC) method to tackle electron correlation. In this study, we carry out FCIQMC simulations to ScO, TiO, and VO molecules and obtain accurate descriptions of 13 low-lying electronic states (ScO 2Σ+, 2Δ, 2Π; TiO 3Δ, 1Δ, 1Σ+, 3Π, 3Φ; VO 4Σ-, 4Φ, 4Π, 2Γ, 2Δ), including states that have significant multi-configurational character. The FCIQMC results are used to assess the performance of several other wave function theory and density functional theory methods. Our study highlights the challenging nature of the electronic structure of transition metal oxides and demonstrates FCIQMC as a promising technique going forward to treat more complex transition metal oxide molecules and materials.

The electronic structure of transition metal oxides for oxygen evolution reaction

In this review article, we summarise the key electronic features of transition metal oxides that govern their OER catalytic properties, and how such electronic descriptors are applied for OER electrocatalysts design.

Construction of hierarchical structure of Co3O4 electrode based on electrospinning technique for supercapacitor

The design and synthesis of hierarchical micro-nano structures of transition metal oxides have played an essential role in the supercapacitor field. In this work, in situ three-dimensional construction of nanoporous cobalt oxide (Co3O4) has been derived from the metal-organic framework (MOF) distributed evenly in electrospun polyacrylonitrile nanofibers. Due to large specific surface area and network architectures, the as-synthesized Co3O4 electrode notably presents a high specific capacitance of 970 F/g at a current density of 1 A/g. Besides, the as-obtained electrode exhibits a high energy density of 54.6 Wh/kg at a power density of 360.6 W/kg and maintains a capacitance retention of 77.5% after 5000 cycles at 6 A/g. Therefore, this method paves a way to produce the nanoporous MOF-derived Co3O4 network architecture as advanced electrodes materials, which shows an application potential for the energy storage industry.

Universality in the electronic structure of 3d transition metal oxides

Electronic structure of strongly correlated transition metal oxides (TMOs) is a complex phenomenon due to competing interaction among the charge, spin, orbital and lattice degrees of freedom. Often individual compounds are examined to explain certain properties associated with these compounds or in rare cases few members of a family are investigated to define a particular trend exhibited by that family. Here, with the objective of generalization, we have investigated the electronic structure of three families of compounds, namely, highly symmetric cubic mono-oxides, symmetry-lowered spinels and less explored asymmetric olivine phosphates, through density functional calculations. From the results we have developed empirical hypotheses involving electron hopping, electron-lattice coupling, Hund's rule coupling, strong correlation and d-band filling. These hypotheses, classified through the point group symmetry of the transition metal - oxygen complexes, can be useful to understand and predict the electronic and magnetic structure of 3d TMOs.

Strong spin-orbit effects in transition metal oxides with tetrahedral coordination

To prove that spin-orbit coupling can play a relevant role in determining the magnetic structure of transition metal oxides with tetrahedral coordination, we investigate the d1 Mott insulator KOsO4, combining density functional theory calculations and the exact diagonalization approach. We find that the interplay between crystal field, strong spin-orbit coupling, electronic correlations and structural distortions brings the system towards an antiferromagnetic phase, characterized by a non-vanishing orbital angular momentum and anisotropy among the in-plane and the out-of-plane antiferromagnetic correlations. We also show that, due to the peculiar interplay between spin-orbit coupling, Hund's coupling and hopping connectivity the system is on the verge of developing short range ferromagnetic correlations marked by strong directionality.