Transition Metal Oxide Surfaces Research Articles

Novel P2-Type Na0.66Fe0.5-2xMn0.5TixVxO2 Cathode for High-Capacity

Necessity of energy storage and battery production is soaring up day by day due to the growth of portable electronic devices and surging evolution of electric vehicles (EVs), plug-in-hybrid electric vehicles (PHEVs) and renewables. Lithium-ion battery (LIB) technology has been the primary choice for such applications due to its high-energy-density, high-stability, and longer cycle life. Albeit with all merits of LIB, the prime concern has been the scarcity of lithium in earth’s crust and depletion of lithium reserve due to its wide usage in batteries, resulting in a higher production cost. This has prompted battery researchers to search for new alternatives to LIB for energy storage applications. Sodium, in this respect, can be a viable solution as it is the sixth most abundant element and shares the same group with lithium in the periodic table. Sodium has similar structural and electrochemical working mechanisms of lithium. The pivotal factors hindering the deployment of the laboratory-based sodium-ion battery (SIB) technologies into the commercial energy storage market are low energy density compared to that of LIB, lower stability, lower ion-transport mechanism, and lower operating voltage. Improvement of overall electrochemical performance of cathode materials can be a game-changer as it affects the energy density, lifespan, and tolerance of batteries. In this research, a cobalt-free novel P2-type transition metal cathode Na0.66Fe0.5-2xMn0.5TixVxO2 was synthesized by doping NaFeMnO2 with vanadium and titanium for SIB battery applications. The cathode was synthesized based on a sol-gel reaction where citric acid was used as a chelating agent. A set of physicochemical analysis, including Field-Effect Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDAX) analysis, as well as electrochemical analysis like Cyclic Voltammetry and Galvanometric Charge/ Discharge were performed to understand the correspondence between layered transition metal oxide chemistry, surface morphology and electrochemical competence of the new material. The crystal structures of the pristine material and cathodes containing different percentages of doped vanadium and titanium were examined and lattice parameters were refined through X-ray Diffraction (XRD) and Rietveld analysis. These exhaustive structural and morphological comparisons were performed between the pristine and modified NaFeMnO2 (NFM) cathode structures, which provided insights on the effects of vanadium and titanium doping on stabilizing surface structures, reducing Jahn-Teller distortion, enhancing stability and capacity retention and promoting Na+ carrier transport mechanism.

Resolving the Two-Track Scaling Trend for Adsorbates on Rutile-Type Metal Oxides: New Descriptors for Adsorption Energies

Identifying the trends in the adsorption energy (AE) of a given intermediate is of fundamental importance theoretically and also vital for rational catalyst design but is far less known on important transition metal oxide (TMO) catalysts. Herein, we report adsorption trends of a wide range of adsorbates on a variety of rutile-type TMO surfaces. We show that all the adsorbates on surface metallic sites (Mcus) can be surprisingly classified into two cross-uncorrelated groups, which scale well with either oxygen AE (EO@M) or hydrogen AE (EH@M). Two new descriptors, that is, the dz2 electrons and valence electrons of the Mcus site, are identified for EO@M and EH@M with R2 = 0.95 and 0.91, respectively. Their different bonding natures are demonstrated to be the chemistry origin of these two adsorption classes. In contrast, AEs of all the adsorbates on the lattice Obri sites can simply correlate with EO@M. These results may have far-reaching implications in TMO catalysis.

Coherent Acoustic Interferometry during the Photodriven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate (Ti-OH*).

The oxygen evolution reaction (OER) from water requires the formation of metastable, reactive oxygen intermediates to enable oxygen-oxygen bond formation. Conversely, such reactive intermediates could also structurally modify the catalyst. A descriptor for the overall catalytic activity, the first electron and proton transfer OER intermediate from water, (M-OH*), has been associated with significant distortions of the metal-oxygen bonds upon charge-trapping. Time-resolved spectroscopy of in situ, photodriven OER on transition metal oxide surfaces has characterized M-OH* for the charge trapping and the symmetry of the lattice distortions by optical and vibrational transitions, respectively, but had yet to detect an interfacial strain field arising from a surface coverage M-OH*. Here, we utilize picosecond, coherent acoustic interferometry to detect the uniaxial strain normal to the SrTiO3/aqueous interface directly caused by Ti-OH*. The spectral analysis applies a fairly general methodology for detecting a combination of the spatial extent, magnitude, and generation time of the interfacial strain through the coherent oscillations' phase. For lightly n-doped SrTiO3, we identify the strain generation time (1.31 ps), which occurs simultaneously with Ti-OH* formation, and a tensile strain of 0.06% (upper limit 0.6%). In addition to fully characterizing this intermediate across visible, mid-infrared, and now GHz-THz probes on SrTiO3, we show that strain fields occur with the creation of some M-OH*, which modifies design strategies for tuning catalytic activity and provides insight into photo-induced degradation so prevalent for OER. To that end, the work put forth here provides a unique methodology to characterize intermediate-induced interfacial strain across OER catalysts.

Analysis of the limitations in the oxygen reduction activity of transition metal oxide surfaces

Analysis of the limitations in the oxygen reduction activity of transition metal oxide surfaces

Detection of a real heterogeneous catalyst with an inactive oxygen-covered surface: Au/Li4Ti5O12

In the investigation of the catalytic mechanism, O-covered transition-metal oxide surfaces with concealed transition-metal cations are occasionally employed as model catalyst support structures. These are the most extreme models for investigating the importance of O species on the support surface. There are few cases in which a catalytic metal is supported on O-covered surfaces, and its catalytic activity is confirmed. In this study, we confirmed that the most exposed surface of Li4Ti5O12 is the O-covered (Ti-sublayer) (111) surface, and we prepared and evaluated a real catalyst using Li4Ti5O12 as the support. Au nanoparticles were used as the catalytic metal, and the interface orientation was determined to be Au(111)//Li4Ti4O12(111). Furthermore, the interface comprising the inactive O-covered (Ti-sublayer) surfaces of transition-metal oxides exhibited catalytic activity. These are important for examining the catalytic mechanism using the surface O species of the support by theoretical calculations, thereby bridging the gap between theory and experiments.

CO adsorption on MnO(100): Experimental benchmarks compared to DFT

CO adsorption on the MnO(100) surface was studied using temperature programmed desorption (TPD) and density functional theory (DFT). TPD results show that CO is weakly-bound on MnO(100), with an experimental adsorption energy of -35.6 ± 2.1 kJ/mol at terrace sites in the low coverage limit. PBE simulations suggest that CO adsorption causes an implausible (2 × 2) surface reconstruction. PBE+U simulations show no signs of surface reconstruction, and provide an accurate estimate of the adsorption energy (-36.4 kJ/mol) when combined with the DFT-D3 method with Becke-Jonson damping to correct for van der Waals interactions. This simulation also shows that CO adsorbs C-down onto the Mn2+ terrace site in a tilted geometry, which is also observed experimentally and computationally on the similarly-structured NiO(100) transition metal oxide surface. TPD results for large doses show a plateauing of the coverage at about 0.4 monolayers of CO at 85 K, with a defect coverage equivalent to 0.08 monolayers. Adsorption associated with defect sites is indicated by a high-temperature desorption tail which is not satisfactorily explained by DFT simulations of simple step or oxygen vacancy defects.

Determination of rutile transition metal oxide (110) surface terminations by scanning tunneling microscopy contrast reversal

The surfaces of rutile transition-metal oxides $({\mathrm{TMO}}_{2})$ are widely investigated for catalysis, photoelectrochemical solar cells, memristors, and supercapacitors, but their structures have remained controversial. Here we employ density functional theory to predict that a universal behavior of metallic ${\mathrm{TMO}}_{2}$ surfaces, i.e., the stoichiometric ${\mathrm{TMO}}_{2}$ surfaces, exhibit a contrast reversal in simulated scanning tunneling microscopy (STM) images at different scanning biases. The predictions are verified by experimental STM imaging of ${\mathrm{RuO}}_{2}(110)$ surfaces and this feature is shown to enable accurate determinations of the ${\mathrm{TMO}}_{2}(110)$ surface structures under various conditions. This work provides different insights into the electronic properties of ${\mathrm{TMO}}_{2}(110)$ surfaces and offers an effective method to directly map the surface structure and point defects using bias-dependent STM.

Electrically Induced Manipulation of the Au Nanoclusters on the Oxidized Rutile TiO2(110) Surface by Atomic Force Microscopy at 78 K

Noble metal nanoclusters supported on a transition metal oxide surface are always regarded as the prototypical catalysts, and the controllable manipulation of the metal catalysts provides an effici...

Accuracy in Resolving the First Hydration Layer on a Transition-Metal Oxide Surface: Experiment (AP-XPS) and Theory

Understanding the equilibrium conditions at the metal oxide/aqueous interface is a key component toward visualizing the structure of water in confined environments and differentiating the catalytic...

Dynamics of water trapped in transition metal oxide-graphene nano-confinement

Motivated by practical implementation of transition-metal oxide-graphene heterostructures, we use all atom molecular dynamics simulations to study dynamics of water in a nano slit bounded by a transition metal oxide surface, namely, TiO2 termination of SrTiO3, and graphene. The resultant asymmetric, strong confinement produces square ice-like crystallites of water pinned at TiO2 surface and drives enhanced hydrophobicity of graphene via the proximity effect to the hydrophilic TiO2 surface. This importantly brings in dynamic heterogeneity, both in translational and rotational degrees of freedom, due to coupling between the slow relaxing, strongly adsorbed water layer at the hydrophilic oxide surface, and faster relaxation of subsequent water layers. The heterogeneity is signalled in the ruggedness of the effective free energy landscapes. We discuss possible implications of our findings in drug delivery.

Role of magnetism on transition metal oxide surfaces in vacuum and solvent

The transition metal oxides MnO, FeO, NiO, and CoO are essential materials systems for catalysis applications and energy technologies. These materials exhibit a magnetic phase transition from paramagnetism to antiferromagnetism upon cooling. In this work, we show that an accurate treatment of the surfaces requires a description of the disordered local magnetic moments of the paramagnetic phase. We determine how the magnetic phase transition and the presence of solvent affect the surface energies using density-functional theory and a solvation model. To accurately account for the correlations in the $d$-electron system, and to match the observed magnetic order and band gaps of the room-temperature phases, we include the Hubbard-$U$ correction. In the case of MnO, FeO, and CoO, which are paramagnetic semiconductors or insulators at room temperature, we demonstrate the importance of the local magnetic moments and model the materials using special quasirandom structures. To determine the equilibrium shape of MnO, FeO, NiO, and CoO nanocrystals, we calculate the surface energies of their low-energy (100), (110), and (111) facets and perform the Wulff construction. For the (111) facet, we consider various reconstructions that remove the polar nature of the unreconstructed surface. The processing conditions of these oxide nanoparticles, in most cases, involve a solvent. To analyze the influence of the solvent environment on the surface energies of different facets and thereby the crystal shape, we calculate the surface energies of these oxides in water using the continuum solvation model VASPsol. We find that the surface energies decrease due to the dielectric screening. However, as the ratio of surface energies remains sufficiently similar, we predict that the equilibrium crystal shape is only weakly affected by the presence of the solvent.

Electrolytic-anion-redox adsorption pseudocapacitance in nanosized lithium-free transition metal oxides as cathode materials for Li-ion batteries

Abstract Conventional batteries (except redox flow batteries) react by involving only their electrode redox centers, with electrolyte ions shuttled between the cathode and the anode. However, this chemistry causes large electronic and structural changes in the bulk of the electrode materials, leading to performance degradation. Herein, we report that electrolytic-anion-redox adsorption pseudocapacitance, relayed by lithium intercalation in lithium-free transition metal oxide electrodes, can be exploited to boost the total energy density of a battery. Focused on the study of Mn3O4, we show that this pseudocapacitance is processed by the reversible adsorption/desorption of PF6− on the surface of transition metal oxides, with charge transfer between Mn3O4 and PF6− through Mn–F bond while fluoride ions acting as redox centers. This new electrolytic-anion-redox chemistry provides greater design freedom for high-capacity energy storage devices since it only involves a specific electrode surface and the electrolyte anion, rather than specially crystallized compounds for bulk reactions.

Probing the Surface of La0.6Sr0.4MnO3 in Water Vapor by In Situ Photon-In/Photon-Out Spectroscopy

Resonant inelastic X-ray scattering (RIXS) is a promising method for elucidating detailed electronic structure of materials in a broad range of chemical and physical applications. Here, we use the fine fluorescence energy resolution of a RIXS spectrometer to obtain various proxies of the Mn-L edge X-ray absorption spectra (XAS) of the perovskite La0.6Sr0.4MnO3 (LSMO) as a model catalyst for the oxygen evolution reaction (OER) and evaluate the suitability for in situ surface studies of this electrocatalyst. We conclude that the inverse partial fluorescence yield (IPFY) of the O 2p–1s transition and the partial fluorescence yield of the 3s–2p transition (3s-PFY) are most suitable for determining changes at the surface of the perovskite because distortions at grazing incidence measurements are low. In particular, the negligible angular dependence of the 3s-PFY spectra can be perfectly simulated by using a fluorescence model in the thin sample limit which is justified by low reabsorption of 3s photons. Remarkably, the 3s-PFY reveals an influence of water vapor on the electronic reconstruction of the LSMO surface. Thus, our work paves the road for quantitative distortion-free X-ray spectroscopy of transition metal oxide surfaces under in situ conditions, which is needed to understand fundamental chemical processes such as corrosion and catalysis.

Water adsorption on vanadium oxide thin films in ambient relative humidity.

In this work, ambient pressure x-ray photoelectron spectroscopy (APXPS) is used to study the initial stages of water adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and valence band XPS spectra were collected as a function of relative humidity in a series of isotherm and isobar experiments. Experiments were carried out on two VO2 thin films on TiO2 (100) substrates, prepared with different surface cleaning procedures. Hydroxyl and molecular water surface species were identified, with up to 0.5 ML hydroxide present at the minimum relative humidity, and a consistent molecular water adsorption onset occurring around 0.01% relative humidity. The work function was found to increase with increasing relative humidity, suggesting that surface water and hydroxyl species are oriented with the hydrogen atoms directed away from the surface. Changes in the valence band were also observed as a function of relative humidity. The results were similar to those observed in APXPS experiments on other transition metal oxide surfaces, suggesting that H2O-OH and H2O-H2O surface complex formation plays an important role in the oxide wetting process and water dissociation. Compared to polycrystalline vanadium metal, these vanadium oxide films generate less hydroxide and appear to be more favorable for molecular water adsorption.

A general doping rule: rational design of Ir-doped catalysts for the oxygen evolution reaction.

Revealing how to rationally select a suitable dopant or the host counterpart is greatly important for optimizing the catalytic activity of transition metal oxides (TMOs). We systematically report the adsorption trends of atomic O and H, two selected representative adsorbates, on numerous doped rutile-type TMO surfaces under two different doping modes, aiming to demonstrate adsorption energy (AE) variations of different adsorbates across doped TMO surfaces. A "host-guest interaction" induced adsorption tuning rule for the ternary doping systems is identified, which rationally directs the proposal of cost-effective Ir-doped CrO2 or MnO2 catalysts for the electrocatalytic oxygen evolution reaction compared with the common IrO2.

Effects of a conductive support on the bonding of oxygen containing molecules to transition metal oxide surfaces

Conventional oxygen electrocatalysts are expensive for industrial use. Transition metal oxides (TMOs), as a more economical option, have emerged as an alternative to potentially replace conventional precious metal catalysts. However, many experimental studies have suggested that although a few of the TMOs supported by conductive substrates are stable under electrocatalytic conditions, their performances are far from the industrial level, especially in the acidic oxygen reduction reaction (ORR). At present, their ORR and also oxygen evolution reaction (OER) performances are still not well understood. In this study, we analyze the effects of the support on ORR/OER adsorbate binding to TMO catalysts. We show that for wide bandgap TMOs (e.g., ZrO2 and HfO2), the use of a metal support leads to a marked enhancement of the adsorbate binding strengths due to a significant induced electron charge gain in the adsorbates, and a considerable up-shift in the ORR/OER adsorbate binding scaling relation. Meanwhile, these support-induced effects are significant even with relatively thick TMO layers on a thin metal substrate, requiring a large thickness cutoff to eliminate the influence. In contrast, the metal-like TMOs (e.g., PdO2 and SnO2) are less affected by the metal support. This study suggests that the thickness of the TMO layer can be used to tune the adsorption properties of electronegative adsorbates and thus provides an interesting new design option for oxygen electrocatalysis.

Integrated Experimental-Theoretical Approach To Determine Reliable Molecular Reaction Mechanisms on Transition-Metal Oxide Surfaces.

By combining experimental and theoretical approaches, we investigate the quantitative relationship between molecular desorption temperature and binding energy on d and f metal oxide surfaces. We demonstrate how temperature-programmed desorption can be used to quantitatively correlate the theoretical surface chemistry of metal oxides (via on-site Hubbard U correction) to gas surface interactions for catalytic reactions. For this purpose, both CO and NO oxidation mechanisms are studied in a step-by-step reaction process for perovskite and mullite-type oxides, respectively. Additionally, we show solutions for over-binding issues found in COx, NOx, SOx, and other covalently bonded molecules that must be considered during surface reaction modeling. This work shows the high reliability of using TPD and density functional theory in conjunction to create accurate surface chemistry information for a variety of correlated metal oxide materials.

Catalytic Activity of Nitrides and Oxides for Electrochemical Ammonia Synthesis: Computational Guides and Experimental Insights

By mimicking the naturally occurring enzymatic process, ammonia can be produced electrochemically from nitrogen and water at ambient conditions where protons are solvated in the electrolyte by water splitting and electrons are driven to the surface of catalyst via electricity supplied by renewable sources of energy. In this manner, the ammonia synthesis becomes a zero-carbon emitting process and more decentralized compared to the Haber-Bosch. Here we present the result of comprehensive density functional theory (DFT) calculations on the surface of transition metal nitrides and transition metal oxides as two different cathode materials for catalyzing nitrogen conversion to ammonia electrochemically in aqueous solutions and at ambient conditions. The aim was to find candidates that were stable and active toward ammonia formation while simultaneously suppressing the competing reaction of hydrogen evolution. Among the nitrides, the most promising candidates are found to be VN, ZrN, NbN, and CrN1-4, which are predicted to catalyze ammonia formation at overpotentials of 0.50 to 1.40 V versus the standard hydrogen electrode (SHE). However, the most promising oxide candidate is found to be the (110) facets of rutile NbO2, which is anticipated to produce ammonia at relatively low overpotentials of 0.57 V versus SHE5. IrO2 was actually predicted to be the most active oxide catalyst for synthesis of ammonia with only an overpotential of 0.36 V, but this catalyst might contribute a portion of the supplied electrical potential to formation of hydrogen gas too, and thus yields lower rate of ammonia formation. Following these computational guidance, experimentalists investigated the catalytic activity of VN nanoparticles and measured 6% faradaic efficiency (FE)6 towards ammonia synthesis but with significant suppression of hydrogen evolution. Nonetheless, engineering the catalyst surface (as elaborated by the DFT results) is a key factor for enhancement of performance and efficiency on these materials that should be the scope of future works. Also Huang et al. 7got inspiration from our theoretical predictions on oxides and thus tested NbO2 experimentally where they reported 32% FE towards ammonia formation at ambient conditions and in aqueous electrolyte. This is currentlya world record in the field. Both these theoretical and experimental investigations have therefore opened up new avenues for the possibility of improved activity and selectivity of cathode material toward electrochemical ammonia synthesis.

Surface Energy and Reconstruction of Li and Na Based Transition Metal Oxides: A Computational Study

Interfaces between cathode particles and electrolyte have significant influence on performance and longevity of Li-ion batteries. Therefore, for further battery improvement deep understanding of cathode-electrolyte interface structure and its relationship with electrochemical processes inside the battery is strongly required. During the last few years, more and more studies are focused on the interfaces and surfaces of cathode particles, especially for layered transition metal oxides (TMO). For example, it was established that TMO surfaces experience reconstruction from layered to spinel and cubic structures, considerably affecting charge transfer through cathode/electrolyte interface and electrolyte and cathode stability [1]. Though initially reconstruction was considered mostly as a detrimental effect, resulting in increased barriers for Li insertion and cathode degradation, new studies show that reconstructed layer can serve as a natural protection from oxygen loss, preventing decomposition of electrolyte and insertion of water in cathode structure [2]. Though it is evident from experimental studies that the reconstructed layer resembles spinel and rock-salt structures, the exact ordering of atoms is controversial. More importantly, the governing factors for reconstruction are still not fully clear: in some cases, reconstruction occurs during synthesis and in others it proceeds under exposure to electrolyte and/or during cycling. Finally, surface reconstruction remains almost unexplored in promising Na-based TMO cathodes, where its role can be highly important, serving as a protective layer from water insertion [2]. In this work, we develop an algorithm for finding reconstructed structures of surface for transition metal oxides. The algorithm is based on the Monte-Carlo method used for a random swap of alkali atoms, transition atoms, and vacancies, while the density functional theory is used for atomic optimization after each Monte-Carlo step and accurate calculation of energies. The Metropolis method allows to take into account the influence of temperature and predict kinetically controlled surface structures. The algorithm is implemented in our SIMAN package for high-throughput materials modelling and available for free use [3]. Employing the developed algorithm, we consider several oxide compounds, such as LiNiO2, NaNiO2 and NaMnO2 used for pragmatic applications for rechargeable Li-ion and Na-ion batteries. We found that in LiNiO2 and NaNiO2 the formation of surface and subsurface anti-site defects is much easier than that in the bulk region. By making simulation at real synthesis temperatures, we observed kinetically controlled formation of spinel layers for both LiNiO2 and NaNiO2. Under oxidative conditions the transformation of the LiNiO2 surface to a rock-salt structure is thermodynamically favorable. The obtained results explain experimentally observed surface reconstruction to spinel and rock-salt structures in layered oxides. The undergoing study will identify the reasons that control the type of reconstruction depending on the chemical composition and synthesis conditions, providing useful guidelines for improving cathode materials.

Anisotropic magnetoresistance in multiband systems: Two-dimensional electron gases and polar metals at oxide interfaces

Low density two-dimensional electron gases (2DEGs) with spin-orbit coupling are highly sensitive to an in-plane magnetic field, which impacts their Fermi surfaces and transport properties. Such 2DEGs, formed at transition metal oxide surfaces or interfaces, can also undergo surface phase transitions leading to polar metals which exhibit electronic nematicity. Motivated by experiments on such systems, we theoretically study magnetotransport in $t_{2g}$ orbital systems, using Hamiltonians which include atomic spin-orbit coupling (SOC) and broken inversion symmetry, for both square symmetry (001) and hexagonal symmetry (111) 2DEGs. Using a numerical solution to the full multiband matrix-Boltzmann equation, together with insights gleaned from the impurity scattering overlap matrix, we explore the anisotropic magnetoresistance (AMR) in the presence of impurities which favor small momentum scattering. We find that transport in the (001) 2DEG is dominated by a single pair of bands, weakly coupled by impurity scattering, one of which has a larger Fermi velocity while the other provides an efficient current-relaxation mechanism. This leads to strong angle-dependent current damping and a large AMR with many angular harmonics. In contrast, AMR in the (111) 2DEG typically features a single $\cos(2\vartheta)$ harmonic, with the angle-averaged magnetoresistance being highly tunable by a symmetry-allowed trigonal distortion. We also explore how the (111) 2DEG Fermi sufaces are impacted by electronic nematicity via a surface phase transition into a 2D polar metal for which we discuss a Landau theory, and show that this leads to distinct symmetry components and higher angular harmonics in the AMR. Our results are in qualitative agreement with experiments from various groups for 2DEGs at the SrTiO$_3$ surface or the LaAlO$_3$-SrTiO$_3$ interface.