Early Transition Metal Oxides Research Articles

Early transition metal oxides with oxygen vacancies as new thermodynamically stable materials for electrocatalysis

Currently developed electrocatalysts are difficult to achieve high thermodynamic stability at each active site. Early transition metal oxides are promising electrocatalysts with excellent thermodynamic stability and water dissociation ability for alkaline hydrogen evolution reaction (HER). However, they have not been developed as direct-electrocatalysts for water splitting. Here, we introduce oxygen vacancies (Vo) by electrodeposition to convert early transition metal oxides (scandium oxide (Sc2O3), yttrium oxide (Y2O3), and zirconium oxide (ZrO2)) into active electrocatalysts. By optimizing the metal element species and Vo content, the screened Y2O2.13@GP shows the lowest overpotential (266 mV at 500 mA cm−2. Moreover, Y2O2.13@GP is able to operate stably for over 1000 h at a current density of 500 mA cm−2 under harsh industrial conditions of 6.0 M KOH and 80°C without dissolution and reconstruction. Experimental studies and theoretical calculations indicate that the introduction of Vo can enhance the conductivity of the catalyst, increase the exposure of the active sites, modulate the electronic structure of Y2O2.13@GP, and decrease the *H adsorption and water dissociation energy barriers.

Redox Mechanisms upon the Lithiation of Wadsley-Roth Phases.

The Wadsley-Roth family of transition metal oxide phases are a promising class of anode materials for Li-ion batteries due to their open crystal structures and their ability to intercalate Li at high rates. Unfortunately, most early transition metal oxides that adopt a Wadsley-Roth crystal structure intercalate Li at voltages that are too high for most battery applications. First-principles electronic structure calculations are performed to elucidate redox mechanisms in Wadsley-Roth phases with the aim of determining how they depend on crystal structure. A comparative study of two very distinct polymorphs of Nb2O5 reveal two redox mechanisms: (i) an atom-centered redox mechanism at early stages of Li intercalation and (ii) a redox mechanism at intermediate to high Li concentrations involving the bonding orbitals of metal-metal dimers formed by edge-sharing Nb cations. Our study motivates several design principles to guide the development of new Wadsley-Roth phases with superior electrochemical properties.

Size Effects in Gas-phase C-H Activation.

The gas-phase clusters reaction permits addressing fundamental aspects of the challenges related to C-H activation. The size effect plays a key role in the activation processes as it may substantially affect both the reactivity and selectivity. In this paper, we reviewed the size effect related to the hydrocarbon oxidation by early transition metal oxides and main group metal oxides, methane activation mediated by late transition metals. Based on mass-spectrometry experiments in conjunction with quantum chemical calculations, mechanistic discussions were reviewed to present how and why the size greatly regulates the reactivity and product distribution.

High-Rate Electrochemical Lithium Cycling and Structure Evolution in Mo4O11

Shear-phase early transition metal oxides, mostly of Nb, and comprising edge- and corner-shared metal–oxygen octahedra have seen a resurgence in recent years as fast-charging low-voltage electrodes for Li–ion batteries. Mo oxides, broadly, have been less well studied. Here we examine the reduced Mo oxide Mo4O11 that has a structure comprising only corner-connected MoO4 tetrahedra and MoO6 octahedra. Here we show that an electrode formed using micron sized particles of Mo4O11 as the active material can function as a high-rate Li–ion electrode against Li metal, with a stable capacity of over 200 mAh g-1 at the high rate of 5C. Operando X-ray diffraction (XRD), Raman spectroscopy, and entropic potential measurements are employed to understand the nature of charge storage. It is found that the structure dramatically changes upon the first lithiation, and subsequent cycling is completely reversible with low capacity fade. It is the newly formed, and potentially more layered structure that demonstrates high-rate cycling and small voltage polarization. This finding expands the scope of candidate high-rate electrode materials to those beyond the expected Nb-containing shear-phase oxide materials. Figure 1

High-Rate Lithium Cycling and Structure Evolution in Mo4O11

Shear-phase early transition metal oxides, mostly of Nb, and comprising edge- and corner-shared metal–oxygen octahedra have seen a resurgence in recent years as fast-charging, low-voltage electrodes for Li+-ion batteries. Mo oxides, broadly, have been less well studied as fast-charging electrodes. Here we examine a reduced Mo oxide, Mo4O11, that has a structure comprising only corner-connected MoO4 tetrahedra and MoO6 octahedra. We show that an electrode formed using micrometer-sized particles of Mo4O11 as the active material can function as a high-rate Li+-ion electrode against Li metal, with a stable capacity of over 200 mAh g–1 at the high rate of 5C. Operando X-ray diffraction (XRD), entropic potential measurements, and ex situ Raman spectroscopy are employed to understand the nature of the charge storage. The crystal structure dramatically changes upon the first lithiation, and subsequent cycling is completely reversible with low capacity fade. It is the newly formed and potentially more layered structure that demonstrates high-rate cycling and small voltage polarization. A space group and unit cell for the new structure is proposed. This finding expands the scope of candidate high-rate electrode materials to those beyond the expected Nb-containing shear-phase oxide materials.

Eliminating Delocalization Error to Improve Heterogeneous Catalysis Predictions with Molecular DFT + U

Approximate semilocal density functional theory (DFT) is known to underestimate surface formation energies yet paradoxically overbind adsorbates on catalytic transition-metal oxide surfaces due to delocalization error. The low-cost DFT + U approach only improves surface formation energies for early transition-metal oxides or adsorption energies for late transition-metal oxides. In this work, we demonstrate that this inefficacy arises due to the conventional usage of metal-centered atomic orbitals as projectors within DFT + U. We analyze electron density rearrangement during surface formation and O atom adsorption on rutile transition-metal oxides to highlight that a standard DFT + U correction fails to tune properties when the corresponding density rearrangement is highly delocalized across both metal and oxygen sites. To improve both surface properties simultaneously while retaining the simplicity of a single-site DFT + U correction, we systematically construct multi-atom-centered molecular-orbital-like projectors for DFT + U. We demonstrate this molecular DFT + U approach for tuning adsorption energies and surface formation energies of minimal two-dimensional models of representative early (i.e., TiO2) and late (i.e., PtO2) transition-metal oxides. Molecular DFT + U simultaneously corrects adsorption energies and surface formation energies of multilayer models of rutile TiO2(110) and PtO2(110) to resolve the paradoxical description of surface stability and surface reactivity of semilocal DFT.

N-doped ZrO2 nanoparticles embedded in a N-doped carbon matrix as a highly active and durable electrocatalyst for oxygen reduction

Fabricating highly efficient and robust oxygen reduction reaction (ORR) electrocatalysts is challenging but desirable for practical Zn-air batteries. As an early transition-metal oxide, zirconium dioxide (ZrO2) has emerged as an interesting catalyst owing to its unique characteristics of high stability, anti-toxicity, good catalytic activity, and small oxygen adsorption enthalpies. However, its intrinsically poor electrical conductivity makes it difficult to serve as an ORR electrocatalyst. Herein, we report ultrafine N-doped ZrO2 nanoparticles embedded in an N-doped porous carbon matrix as an ORR electrocatalyst (N-ZrO2/NC). The N-ZrO2/NC catalyst displays excellent activity and long-term durability with a half-wave potential (E1/2) of 0.84 V and a selectivity for the four-electron reduction of oxygen in 0.1 M KOH. Upon employment in a Zn-air battery, N-ZrO2/NC presented an intriguing power density of 185.9 mW cm−2 and a high specific capacity of 797.9 mA h gZn−1, exceeding those of commercial Pt/C (122.1 mW cm−2 and 782.5 mA h gZn−1). This excellent performance is mainly attributed to the ultrafine ZrO2 nanoparticles, the conductive carbon substrate, and the modified electronic band structure of ZrO2 after N-doping. Density functional theory calculations demonstrated that N-doping can reduce the band-gap of ZrO2 from 3.96 eV to 3.33 eV through the hybridization of the p state of the N atom with the 2p state of the oxygen atom; this provides enhanced electrical conductivity and results in faster electron-transfer kinetics. This work provides a new approach for the design of other enhanced semiconductor and insulator materials.

Radical abstraction vs. oxidative addition mechanisms for the activation of the S-H, O-H, and C-H bonds using early transition metal oxides.

Quantum chemical calculations are performed to study the S-H, O-H, and C-H bond activation of H2S, H2O, and CH4 by bare and ligated ZrO+ and NbO+ units. These representative oxides bear low energy oxo and higher energy oxyl units. S-H and C-H bonds are readily activated by metal oxyl states (radical mechanism), but the O-H bond is harder to activate with either the oxyl or oxo states. Our results suggest that known practices for the C-H bond activation can be applied to S-H, but not to O-H bonds. The identified trends are rationalized in terms of the HS-H, HO-H, and H3C-H dissociation energies to the homolytic or heterolytic fragments. We also found that these dissociation energies drop to about half after coordination of H2S or H2O to the metal oxide unit. In addition, chlorine ligands are shown to stabilize the higher energy oxyl states of the transition metal oxygen unit enhancing the reactivity of the formed complexes.

Synthesis of early transition metal oxide nanomaterials and their conversion to nitrides

The synthesis of nanorods and nanowires of early transition metal oxides by hydrothermal methods and the conversion of some bulk oxides to nitrides in NH3 flow are both well established. However, the conversion of such oxide nanostructures to nitrides is not as well explored but could be a valuable method of producing nanowires, nanorods, and other asymmetric or high aspect ratio nanoparticles of the highly conductive metal nitrides. In this work, nanostructures of TiO2, V2O5, MoO3, and WO3 and mixtures thereof were synthesized by hydrothermal reactions, and conversion into nitride was attempted by heating the oxide material under NH3 flow with or without N2 flow. It was found that slow heating produces the best shape retention, but the nitridized products are porous in even the most favorable cases. The products were characterized by X-ray powder diffraction and scanning electron microscopy. Two-point conductivity measurements were done on the nitride materials, and optical measurements to characterize the plasmon absorption were done in favorable cases.

Hole antidoping of oxides

In standard doping, adding charge carrier to a compound results in a shift of the Fermi level towards the conduction band for electron doping and towards the valence band for hole doping. We discuss the curious case of antidoping, where the direction of band movements in response to doping is reversed. Specifically, $p$-type antidoping moves the previously occupied bands to the principal conduction band resulting in an increase of band gap energy and reduction of electronic conductivity. We find that this is a generic behavior for a class of materials: early transition and rare-earth metal (e.g., Ti, Ce) oxides where the sum of composition-weighed formal oxidation states is positive; such compounds tend to form the well-known electron-trapped intermediate bands localized on the reduced cation orbitals. What is less known is that doping by a hole annihilates a single trapped electron on a cation. The latter thus becomes electronically inequivalent with respect to the normal cation in the undoped lattice, thus representing a symmetry-breaking effect. We give specific theoretical predictions for target compounds where hole antidoping might be observed experimentally: Magn\'eli-like phases (i.e., $\mathrm{Ce}{\mathrm{O}}_{2\text{\ensuremath{-}}x}$ and $\mathrm{Ti}{\mathrm{O}}_{2\text{\ensuremath{-}}x}$) and ternary compounds (i.e., $\mathrm{B}{\mathrm{a}}_{2}\mathrm{T}{\mathrm{i}}_{6}{\mathrm{O}}_{13}$ and $\mathrm{B}{\mathrm{a}}_{4}\mathrm{T}{\mathrm{i}}_{12}{\mathrm{O}}_{27}$), and note that this unique behavior opens the possibility of unconventional control of materials conductivity by doping.

Fabrication of cobalt doped titania for enhanced oxygen evolution reaction

Due to poor electron mobility, early transition metal oxides typically, are not known to catalyze water electroxidation. Here we report the modulation of electrocatalytic properties of pristine anatase by the doping of an earth-abundant metal (Co). So obtained hybrid materials show enhanced catalytic activity for oxygen evolution reaction (OER). It was noted that the retention of free carbon during the synthesis of such hybrid materials further enhances their OER activity bringing it at par with the benchmark IrO2 catalyst. Evaporation induced self-assembly (EISA) was used to prepare mesoporous titania using a triblock copolymer (P123) as structure directing agent. Nanostructured titania was first impregnated with a cobalt complex followed by thermal treatment at 500, 700 and 900 °C under air, argon and hydrogen atmosphere to obtain a series of catalysts. The effect of metal content and annealing atmosphere was studied on the composition and OER activity of the catalysts. The best OER activity (η: 440mV@10 mA.cm−2), comparable to benchmark IrO2 and decent stability was noted for the material annealed under argon and containing 15 wt % Co. Improved OER activity was attributed to better water adsorption, charge transfer and conductivity of titania upon Co doping and carbon retention.

Weak-field ligands enable inert early transition metal oxides to convert methane to methanol: the case of ZrO.

Zirconium monoxide, ZrO, was studied by multi-reference configuration interaction (MRCI) and coupled cluster methods using large basis sets in conjunction with effective core potentials. Complete potential energy curves were constructed and bonding patterns are proposed for several electronic states. Numerical results include accurate equilibrium bond lengths, harmonic vibrational frequencies, anharmonicities, excitation energies, dipole moments, and binding energies for both ground and excited states. The application of a ZrO unit as the catalytic center for methane activation is explored through the reaction ZrO + CH4→ Zr + CH3OH. Optimal density functional structures combined with single-point MRCI energy calculations are obtained for the complete reaction pathway. It is found that the lower energy singlet and triplet multiplicities (oxo states) favor the [2+2] mechanism and the higher energy quintets (oxyl states) favor the radical mechanism, which is overall more efficient in producing methanol. We finally suggest proper ligands that stabilize the oxyl states. These include halogens or other weak-field ligands, which finally convert the inert early transition metal oxide units to efficient methane-to-methanol catalysts.

µ-PES Studies on TiNCl and Quasi-two-dimensional Superconductor Na-intercalated TiNCl

Electron-doped TiNCl is a superconductor, for which exotic mechanisms of the superconductivity have been discussed. However, difficulty in preparing large single-phase samples has prevented the direct observation of its electronic structure and how that changes with electron doping. In this study, micro-photoemission spectroscopy ({\mu}-PES) was used to reveal the electronic structures of TiNCl and Na-intercalated TiNCl (Na-TiNCl). Comparison of Ti 2p core-level spectra shows the enhancement of a spectral feature in Na-TiNCl that suggests an increase in its metallic character. This indicates the introduction of electron carriers to the TiNCl layers. Although the overall valence band electronic structure of parent TiNCl could be reproduced by first-principle calculations, that of Na-TiNCl showed a marked deviation from the rigid band model near the Fermi level (EF). The spectral shape observed near EF of Na-TiNCl was found to be similar to a result in early transition metal oxides that has been attributed to the effect of strong electron correlation. The present study also demonstrated the usefulness of {\mu}-PES to obtain reliable electronic structure data for samples that are difficult to make at large sizes.

Strain tuning of plasma frequency in vanadate, niobate, and molybdate perovskite oxides

A novel approach for finding new transparent conductors involves taking advantage of electronic correlations in metallic transition metal oxides, such as SrVO$_3$, to enhance the electronic effective mass and suppress the plasma frequency ($\omega_P$) to infrared. Success of this approach relies on finding a compound with the right electron effective mass and quasiparticle weight $Z$. Biaxial strain can in principle be a fruitful way to manipulate the electronic properties of materials to tune both of these quantities. In this study, we elucidate the behavior of the electronic properties of early transition metal oxides SrVO$_3$, SrNbO$_3$, and SrMoO$_3$ under strain, using first principles density functional theory and dynamical mean field theory. We show that strain is not an effective way to manipulate the plasma frequency, but dimensionality of the crystal structure and origin of electronic correlations strongly affect the trends in both $\omega_P$ and $Z$.

Photophysical and Catalytic Properties of Silica Supported Early Transition Metal Oxides Relevant for Photocatalytic Applications

In this paper the applicability of selected silica-supported early transition metal oxides in photocatalysis is discussed. The catalytic properties of highly dispersed transition metal oxides often differ strongly from their corresponding bulk material. The structural and photophysical properties can often be linked to those changes.

Assisting the Effective Design of Polar Iodates with Early Transition-Metal Oxide Fluoride Anions.

Polar materials are of great technical interest but challenging to effectively synthesize. That is especially true for iodates, an important class of visible and mid-IR transparent nonlinear optical (NLO) materials. Aiming at developing a new design strategy for polar iodates, we successfully synthesized two sets of polymorphic early transition-metal (ETM) oxide-fluoride iodates, α- and β-Ba[VFO2(IO3)2] and α- and β-Ba2[VO2F2(IO3)2]IO3, based on the distinct structure-directing properties of oxide-fluoride anions. α- and β-Ba[VFO2(IO3)2] contain the trans-[VFO2(IO3)2]2- polyanion and crystallize in the nonpolar space groups Pbcn and P212121. In contrast, α- and β-Ba2[VO2F2(IO3)2]IO3 contain the cis-[VO2F2(IO3)2]3- Λ-shaped polyanion and crystallize in the polar space groups Pna21 and P21, respectively. Detailed structural analyses show that the variable polar orientation of trans-[VFO2(IO3)2]2- polyanions is the main cause of the nonpolar structures in α- and β-Ba[VFO2(IO3)2]. However, the Λ-shaped configuration of cis-[VO2F2(IO3)2]3- polyanions can effectively guarantee the polar structures. Further property measurements show that polar α- and β-Ba2[VO2F2(IO3)2]IO3 possess excellent NLO properties, including the large SHG responses (∼9 × KDP), wide visible and mid-IR transparent region (∼0.5-10.5 μm), and high thermal stability (up to 470 °C). Therefore, combining cis-directing oxide-fluoride anions and iodates is a viable strategy for the effective design of polar iodates.

Towards higher zT in early transition metal oxides: optimizing the charge carrier concentration of the WO3-x compounds

Thermoelectric devices are believed to play an important role in the energy research for the next decades. Thanks to their low costs coupled with high stability and sustainability, metal oxides are very promising materials even if their efficiencies still need improvements to ensure a wide applicability. Slightly reduced early transition metal oxides show intrinsic defects in the crystal structure which guarantee very low values of the thermal conductivity. The challenge to fulfil the “phonon-glass electron-crystal” concept is to decouple the optimization of the electronic properties from the thermal transport properties. In this contribution we report the optimization of the charge carrier concentration in the Magnéli phase WO2.90 through the manipulation of the oxygen content. Keeping the thermal conductivity constant and increasing the electronic transport we manage to rise the figure of merit of a 30% compared to the non-optimized sample.

Theoretical Investigation of Ta2O5, TaON, and Ta3N5: Electronic Band Structures and Absolute Band Edges

Early transition metal oxides, nitrides, and oxynitrides have attracted a great deal of interest because of their potential applications in photovoltaics and photocatalysis. In this work, a systematic investigation is conducted of the electronic band structures of the Ta2O5 polymorphs, β-Ta3N5 and β-TaON, which are crucial for the understanding of their photocatalytic properties, based on state-of-the-art first-principles approaches. The calculated results imply that many-body perturbation theory in the GW approximation can overcome the severe underestimation of the band gap caused by standard density functional theory (DFT) in the local and semilocal approximations and provide a quantitative agreement with experiment. The effects of the electron–phonon coupling on the electronic band structure are considered by the Frolich model, and especially for ϵ-Ta2O5, a strong electron–phonon coupling is predicted as a result of small high-frequency dielectric constants and large effective masses. Based on an analy...

Polymer-containing Organo-Polyoxometalate: Towards New Catalytic Object.

Polyoxometalates (POMs) are early transition metal oxide clusters, which form a diverse family of compounds with highly tunable shapes, sizes, charges, etc. Generating hybrid structures is an attractive path to increase the structural diversity of POMs. The present article is a personal account that retraces how we inserted polyoxometallic platforms onto polymeric chains. We focused on two main families of functionalized organo-POMs: the organotin-substituted Dawson polyoxotunstates α1 and α2-[P(2)W(17)O(61)(SnR)](7-) and the Dawson vanadotungstates [P(2)V(3)W(15)O(59){(OCH(2))(2)-C(Et)NHC(=O)R}](5-). Herein we discuss how the connection of POMs to polymers affects the self-assembly of the polymers and the catalytic properties of the polyoxometalates.

A chemists view: Metal oxides with adaptive structures for thermoelectric applications

Thermoelectric devices can help to tackle future challenges in the energy sector through the conversion of waste heat directly into usable electric energy. For a wide applicability low‐cost materials with reasonable thermoelectric performances and cost‐efficient preparation techniques are required. In this context metal oxides are an interesting class of materials because of their inherent high‐temperature stability and relative high sustainability. Their thermoelectric performance, however, needs to be improved for wide application. Compounds with adaptive structures are a very interesting class of materials. A slight reduction of early transition metal oxides generates electrons as charge carriers and crystallographic shear planes as structure motif. The crystallographic shear planes lead to a reduction of intrinsic thermal conductivity. At the same time, the electronic transport properties can be tuned by the degree of reduction. So far only a few transition metal oxides with adaptive structures have been investigated with respect to their thermoelectric properties, leaving much room for improvement. This review gives an overview of thermoelectric oxides, highlights the structural aspects of the crystallographic shear planes and the resulting thermoelectric properties.