Complex Metal Oxides Research Articles

In operando‐formed interface between silver and perovskite oxide for efficient electroreduction of carbon dioxide to carbon monoxide

AbstractElectrochemical carbon dioxide (CO2) reduction (ECR) is a promising technology to produce valuable fuels and feedstocks from CO2. Despite large efforts to develop ECR catalysts, the investigation of the catalytic performance and electrochemical behavior of complex metal oxides, especially perovskite oxides, is rarely reported. Here, the inorganic perovskite oxide Ag‐doped (La0.8Sr0.2)0.95Ag0.05MnO3–δ (LSA0.05M) is reported as an efficient electrocatalyst for ECR to CO for the first time, which exhibits a Faradaic efficiency (FE) of 84.3%, a remarkable mass activity of 75 A g−1 (normalized to the mass of Ag), and stability of 130 h at a moderate overpotential of 0.79 V. The LSA0.05M catalyst experiences structure reconstruction during ECR, creating the in operando‐formed interface between the perovskite and the evolved Ag phase. The evolved Ag is uniformly distributed with a small particle size on the perovskite surface. Theoretical calculations indicate the reconstruction of LSA0.05M during ECR and reveal that the perovskite–Ag interface provides adsorption sites for CO2 and accelerates the desorption of the *CO intermediate to enhance ECR. This study presents a novel high‐performance perovskite catalyst for ECR and may inspire the future design of electrocatalysts via the in operando formation of metal–metal oxide interfaces.

Enhancing OER Activity of Ni/Co Oxides via Fe/Mn Substitution within Tailored Mesoporous Frameworks

Non-noble mixed metal oxides are promising electrocatalysts for water splitting reactions in alkaline media. The synthesis of complex mixed metal oxides with the desired composition is challenging due to the formation of phase impurities when synthesis is performed via classical approaches. In this work, we applied the nanocasting technique combined with low-temperature calcination (200 °C) in a quasi-sealed container to obtain highly ordered mesoporous mixed metal (Mn/Fe/Ni/Co) oxides. This procedure provides electrocatalysts with distinctive physicochemical characteristics and improved electrocatalytic properties. Partial Ni substitution in Ni0.5Co2.5O4 by Fe and Mn in combination with the highly ordered mesostructure offers a large number of active sites and enhances the performance of the catalyst toward the oxygen evolution reaction (OER) in the alkaline electrolyte. The Mn/Fe/Ni/Co oxide calcined at 200 °C outperforms all other synthesized oxides (current density of 262 mA cm–2 at 1.7 V versus RHE and overpotential of 363 mV at 10 mA cm–2) due to synergistic effects. Moreover, this catalyst exhibits significantly higher activity compared to the oxide of identical composition calcined at higher temperature. The results presented in this work show that tuning the synthesis conditions and composition of mixed metal oxides is a simple way to tailor their surface chemistry, mesoporous structure, and catalytic performance for the OER.

A REVIEW OF RECENT DEVELOPMENT OF THE SOLVENT-DEFICIENT METHOD

The challenges of modern society and the requirements for high-tech materials have led to a new stage in the development of chemical synthesis methods. This stage is demanding a reliable, controllable, and green approach to the production of necessary materials. One of the recently developed approaches for metal oxide nanoparticles synthesis is the solvent deficient method. A cutting edge of this synthetic method is a synthesis with increased control throughout chemical transformation resulting in optimal conditions for materials production. As a result, oxide nanoparticles are produced fast, environmentally friendly, cost-effective, and with low energy consumption. On the other hand, a complex reaction mechanism is presenting a challenge for reliable kinetic studies. The present review follows the recent trends in the development of the solvent deficient method concerning the binary, ternary, and complex metal oxides. Special attention is given to considering approaches responsible for desirable mechanisms of catalytic applications. In future development, the reported synthesis method is expected to be employed in a high entropy oxides formation. Due to simple preparation steps, it will be possible to automatize the reaction procedure and investigate a large number of possible compositions which is imperative for new complex materials. Finally, precursors heating could be transformed from conventional to microwave. Solvent-deficient method deals with precursors sensitive to microwave irradiation which could ensure rapid heating and transformation to desirable oxide nanomaterials.

(Invited) Extracting Photoelectrochemical Phenomena from Transient X-Ray Experiments

To date, ultrafast X-ray spectroscopy has been used to measure electron and hole energies, mid-gap state recombination, small polaron formation, and transport of carriers in multi-layer junctions – explaining its appeal for studying photocatalysis and photoelectrochemistry. While ultrafast x-ray spectroscopy can measure both structural and electronic dynamics, the resulting transient plots are complex and hard to interpret because of how photoexcitation perturbs the core-level transition. We will discuss progress in theory and experiment that now allows routine interpretation of photoexcited X-ray edges using a customized DFT-GW-Bethe Salpeter Equation approach. In addition to interpreting excited state phenomenon, the code can project the X-ray peak structure and the relevant X-ray Hamiltonian components - such as angular momentum coupling, screening, and state-filling – onto the band structure to help give physical intuition as to why the measured spectra appear the way they do. The talk will cover the general principles of how photoexcitation modifies transient X-ray spectra rather than detailed theory. Example experiments will include photoelectrodes ranging from ZnTe to complex transition metal oxides.

CH4 Activation over Perovskite Catalysts: True Density and Reactivity of Active Sites

The high thermal stability of perovskites has drawn attention toward their applications for catalytic CH4 activation and conversion, typically occurring at high temperatures. The reaction rates of perovskite catalysts for CH4 combustion, however, trail behind those of noble metal catalysts. Ways to optimize the performance of perovskite catalysts are destined to trial-and-error approaches unless their complex reconstructed surfaces are correlated with fundamental kinetic parameters. Discerning the intrinsic activity of surface catalytic sites and the density of those sites is crucial to rationally envision complex metal oxides with enhanced catalytic performance. The present work presents a detailed kinetic analysis of catalytic CH4 combustion over a set of seven perovskites (SrTiO3, SrZrO3, SrFeO3, LaFeO3, LaInO3, LaCoO3, LaMnO3) with various surface terminations. Steady-state isotopic transient kinetic analysis was employed to measure turnover frequency (TOF) and density of surface intermediates (N) under operando conditions. Top surface characterization elucidated performance-structure relationships between near-monolayer surface composition and intrinsic reactivity of the catalysts. By using a chemical etching procedure to expose Fe-sites at the top surface of LaFeO3 (LaFeO3,HNO3), its TOF was increased 4-fold, compared with the unmodified sample, although N on the surface of LaFeO3,HNO3 decreased. Density functional theory simulations corroborated that surface Fe-termination and La-Fe termination offer lower energetic barriers for CH4 activation when compared with La-termination. In general, surface reconstruction is shown as a tool to tune TOF and N to improve reaction rates. This work fills a gap in current kinetic studies of perovskites through a careful assessment and discussion of the density and intrinsic reactivity of active sites for methane combustion over well-characterized reconstructed perovskite surfaces.

Defect-induced magnetism in homoepitaxial SrTiO3

Along with recent advancements in thin-film technologies, the engineering of complex transition metal oxide heterostructures offers the possibility of creating novel and tunable multifunctionalities. A representative complex oxide is the perovskite strontium titanate (STO), whose bulk form is nominally a centrosymmetric paraelectric band insulator. By tuning the electron doping, chemical stoichiometry, strain, and charge defects of STO, it is possible to control the electrical, magnetic, and thermal properties of such structures. Here, we demonstrate tunable magnetism in atomically engineered STO thin films grown on STO (001) substrates by controlling the atomic charge defects of titanium (VTi) and oxygen (VO) vacancies. Our results show that the magnetism can be tuned by altering the growth conditions. We provide deep insights into its association to the following defect types: (i) VTi, resulting in a charge rearrangement and local spin polarization, (ii) VO, leading to weak magnetization, and (iii) VTi–VO pairs, which lead to the appearance of a sizable magnetic signal. Our results suggest that controlling charged defects is critical for inducing a net magnetization in STO films. This work provides a crucial step for designing magnetic STO films via defect engineering for magnetic and spin-based electronic applications.

(Invited) Engineering Challenges in Scaling-up Solar Water Splitting Devices

The last decade has witnessed significant progress in the development of solar water splitting devices, with solar-to-hydrogen (STH) efficiency as high as 30% already demonstrated. However, two major challenges remain. First, the high-efficiencies (> 15%) have only been achieved using devices based on expensive and non-scalable III-V semiconductors. On the other hand, low-cost metal-oxide based devices, mainly using BiVO4 as the absorber, have only achieved STH efficiency of < 10%. Due to stability limitations, many of these metal-oxide based devices are operated in near-neutral pH electrolytes, which presents an additional mass transport challenge. Second, the majority of the demonstrated devices are still at the laboratory scale. Reports on large-area devices start to emerge, but they typically show much lower efficiencies. This is best illustrated in a recent review:[1] even when III-V semiconductor-based devices are considered, there is no report of devices with a semiconductor absorber area larger than 10 cm2 and STH efficiency > 10%.In this talk, we will discuss the scale-up of our photoelectrochemical water splitting devices based on a complex metal oxide photoabsorber. Factors other than the semiconductor photoabsorber itself are found to be responsible for a total voltage loss of > 500 mV and therefore limit the overall performance of the large-area device.[2] To properly address this limitation, we quantify and break down the different loss mechanisms associated with the device scale-up and the practical operational conditions.[3] Concentration overpotential due to pH gradient is found to be a major contributor to the performance loss, and we show using multiphase multiphysics simulations and in-situ fluorescence measurements that careful control of natural and forced convection can overcome this limitation.[3-5] In addition, we also explore the possibility to achieve efficient product separation in devices with and without separators. The product crossover, optical and Ohmic losses are quantified using a combination of experiments and simulations, and the optimization of the device working parameters and/or separator properties to achieve the minimum overall loss will be discussed.[6,7] References J. H. Kim et al., Chem. Soc. Rev. 48, 2019, 1908I. Y. Ahmet et al., Sust. Energy Fuels 3, 2019, 2366F. F. Abdi et al. Sust. Energy Fuels 4, 2020, 2734K. Obata et al. Energy Environ. Sci. 13, 2020, 5104K. Obata & F. F. Abdi, Sust. Energy Fuels, 5, 2021, 3791K. Obata et al. Cell Rep. Phys. Sci. 2, 2021, 100358C. Özen et al. in revision

Incorporating Metal Precursors towards a Library of High-resolution Metal Parts by Stereolithography

Additive manufacturing (AM) of three-dimensional (3D) metals with intricate fine features and complex geometry at the macro- and micro- scale for energy generation and storage has attracted great interest recently. Fabrication of metals using state-of-art powder-based AM technology with high resolution and tunable microstructure remains a challenge. In this work, a new method for achieving sophisticated metallic structure via the digital light process (DLP) of metal precursors containing photo-resin followed by thermal heating and chemical reduction process is presented. Thermal decomposition of 3D-printed metal compound and chemical reduction makes it promising to synthesize a morphology-tunable and geometrically complex metal, alloy, metal oxide, or multi-material component. Two potential applications of 3D-printed copper using this method are demonstrated in the electrochemical field as oxygen evolution reaction (OER) catalyst support and current collector. NiFe hydroxide@3D-printed copper can drive a 100 mA/cm2 stably with a low energy barrier in an alkaline solution. Galvanostatic cycling profile of DLP Copper/electrolyte/Li at current densities of 2 mA/cm2, 4 mA/cm2 and 10 mA/cm2 can be conducted stably for a capacity of 20 mA·h/cm2. This work is the first to realize metal-based printing with ultra-high resolution thorough photopolymerization.

Phase transition of SrCo0.9Fe0.1O3 electrocatalysts and their effects on oxygen evolution reaction

AbstractComplex metal oxide has emerged as an important class of electrocatalyst for oxygen evolution reaction (OER) because of its flexibility in the crystal phase and relative ease in the control of defect structures. While different crystalline phases and solid structures may be produced from the same reactant precursors, their phase transitions and OER performances are not well understood. In this work, we present the preparation of different phases of SrCo0.9Fe0.1O3‐δ (nominal ratio) solids using a sol–gel method and the structure–catalytic property relationship of both crystalline and amorphous structures formed at different temperatures. We observed as synthetic temperature increased, phase and composition transitions occurred from the amorphous phase to strontium carbonate and cobalt oxide, to oxygen‐deficient perovskite, and finally to perovskite. The OER activity depended highly on crystal structures. The oxygen‐deficient perovskite prepared at 750°C exhibited the highest catalytic activity, while the perovskite showed the lowest activity. X‐ray photoelectron spectroscopy analysis reveals the change of oxidation state occurred during the phase transition. This work provides a useful guideline for the design of OER electrocatalysts through controlling the crystal structures and defects in complex metal oxides, such as perovskite.

The Nature of Interface Interactions Leading to High Ionic Conductivity in LiBH4/SiO2 Nanocomposites.

Complex metal hydride/oxide nanocomposites are a promising class of solid-state electrolytes. They exhibit high ionic conductivities due to an interaction of the metal hydride with the surface of the oxide. The exact nature of this interaction and composition of the hydride/oxide interface is not yet known. Using 1H, 7Li, 11B, and 29Si NMR spectroscopy and lithium borohydride confined in nanoporous silica as a model system, we now elucidate the chemistry and dynamics occurring at the interface between the scaffold and the complex metal hydride. We observed that the structure of the oxide scaffold has a significant effect on the ionic conductivity. A previously unknown silicon site was observed in the nanocomposites and correlated to the LiBH4 at the interface with silica. We provide a model for the origin of this silicon site which reveals that siloxane bonds are broken and highly dynamic silicon–hydride–borohydride and silicon–oxide–lithium bonds are formed at the interface between LiBH4 and silica. Additionally, we discovered a strong correlation between the thickness of the silica pore walls and the fraction of the LiBH4 that displays fast dynamics. Our findings provide insights on the role of the local scaffold structure and the chemistry of the interaction at the interface between complex metal hydrides and oxide hosts. These findings are relevant for other complex hydride/metal oxide systems where interface effects leads to a high ionic conductivity.

Nanotoxicity of Emerging NMC-Family Energy Storage Nanomaterials

The increasing use of metal oxide materials has met with equal concern as regards associated health risks and environmental transference. NMC-family (LiNi_xCo_yMn_zO₂, where x+y+z=1) energy storage materials are complex transition metal oxides that are being rapidly deployed as replacements to the conventional lithium-ion batteries (LiBs) in the electric vehicle industry. Sustainable catalysis is another area of increasing study since multi-component oxides have a high propensity for cleaving strong chemicals such as the carbon-fluorine bond. NMC materials are mostly synthesized through different thermomechanical processes, with the final composition depending on the required properties in the derived nanostructures. The biological impact of NMC-type materials has been found to be dependent not just on the overall mass, but also on the mass distribution which could be in the form of micro or nanostructures. This study therefore assesses the toxicity effect of NMC-family nanoscale material for energy storage and catalysis, with a view to determining possible safety levels as regards the composition of constituent materials. Preliminary toxicity studies were conducted on inoculated S. oneidensis, B. subtilis and D. magna substrates exposed to NMC materials for time intervals ranging from 0 to 72h and found a direct relationship between the ease of release of Ni and Co ions and cellular respiration and growth. The chemical transformation of NMC nanomaterials was however found to favor an ion release of the order Li > Ni > Co > Mn, although this is an entirely slow process. Altering the configuration of NMC materials through Mn enrichment was also found to significantly reduce its dissolution and could be an important approach to preventing its release into the environment.

Oxidative stress, antioxidant status and lung function test among iron welders in Sokoto, Nigeria

Welding poses potential and chemical hazards because its fumes consist of a wide range of complex metal oxide particles which can be deposited in all regions of the respiratory tract. We aimed in the present study to determine lipid peroxidation and antioxidant status of Iron welders in Sokoto. The study involves 54 active welders and 20 age matched control males. Their lung function test (LFT) was carried out using spirometry and their blood was collected by venipuncture and analyzed for oxidative stress, antioxidant enzymes, vitamins and minerals via spectrophotometric methods. Serum levels of malondialdehyde (MDA), Cu-Zn superoxide dismutase (Cu-ZnSOD), glutathione peroxidase (GPx) and catalase (CAT) were significantly higher in welders (p &lt; 0.05) when compared to non-welders. Peak Expiratory Flow Rate (PEFR) is significantly higher in welders (P &lt; 0.05) than in non-welders. Across duration of occupational age exposure, levels of MDA,Vitamin A and GPx were significantly higher (p&lt;0.05) when compared to other parameters. Pearson correlation indicated strong negative correlation between markers of oxidative stress and lung function test. The important findings among the sample of welders in comparison to controls are higher lung function test, antioxidant enzymes and vitamins.

新規固定化金属錯体・金属ナノ粒子・金属酸化物触媒の創製とオペランド放射光X線解析

新規固定化金属錯体・金属ナノ粒子・金属酸化物触媒の創製とオペランド放射光X線解析

Multiple Factors on Catalytic Activity for Oxygen Evolution Reaction in Magnetoplumbite Fe–Co Oxide BaFe12–xCoxO19

Complex metal oxides consisting of multiple transition-metal elements exhibit a higher electrocatalytic performance than simple transition-metal oxides. In this study, we demonstrate the physical and electrochemical properties of BaFe12–xCoxO19 (x = 0–12), a solid solution of magnetoplumbite-structured BaFe12O19 and BaCo12O19. Single-phase samples in the whole composition range are successfully obtained by using high-pressure and high-temperature conditions of 6.5 GPa and 1373 K. Co- and Fe-doping into BaFe12O19 and BaCo12O19, respectively, efficiently increases the catalytic activity for the oxygen evolution reaction (OER). The OER activity of BaFe12–xCoxO19 is maximized at two compositions of x = 5 and 10, whereas the nonsystematic change in the OER activity for the intermediate compositions between x = 5 and 10 indicates a competition of positive and negative factors on the OER activity such as the dopant as an active site, electrical conductivity, spin polarization, and charge-transfer resistance. We eventually found a characteristic relationship between specific activity and charge-transfer resistance reflecting the mechanistic difference for BaFe12–xCoxO19.

Metal Release Mechanism and Electrochemical Properties of Lix(Ni1/3Mn1/3Co1/3)O2

Complex metal oxides (CMOs) are used broadly in applications including electroreactive forms found in lithium-ion battery technology. Computational chemistry can provide unique information about how the properties of CMO cathode materials change in response to changes in stoichiometry, for example, changes of the lithium (Li) content during the charge–discharge cycle of the battery. However, this is difficult to measure experimentally due to the small cross-sectional area of the cations. Outside of operational conditions, the Li content can influence the transformations of the CMO when exposed to the environment. For example, metal release from CMOs in aqueous settings has been identified as a cross-cutting mechanism important to CMO degradation. Computational studies investigating metal release from CMOs show that the thermodynamics depend on the oxidation states of lattice cations, which is expected to vary with the lithium content. In this work, computational studies track changes in metal release trends as a function of Li content in Lix(Ni1/3Mn1/3Co1/3)O2 (NMC). The resulting dataset is used to construct a random forest tree (RFT) machine learning (ML) model. A modeling challenge in delithiation studies is the large configurational space to sample. Through investigating multiple configurations at each lithium fraction, we find structural features associated with favorable energies to chemically guide the identification of relevant structures and adequately predict voltage values.

Polyoxometalate Functionalized Sensors: A Review.

Polyoxometalates (POMs) are a class of metal oxide complexes with a large structural diversity. Effective control of the final chemical and physical properties of POMs could be provided by fine-tuning chemical modifications, such as the inclusion of other metals or non-metal ions. In addition, the nature and type of the counterion can also impact POM properties, like solubility. Besides, POMs may combine with carbon materials as graphene oxide, reduced graphene oxide or carbon nanotubes to enhance electronic conductivity, with noble metal nanoparticles to increase catalytic and functional sites, be introduced into metal-organic frameworks to increase surface area and expose more active sites, and embedded into conducting polymers. The possibility to design POMs to match properties adequate for specific sensing applications turns them into highly desirable chemicals for sensor sensitive layers. This review intends to provide an overview of POM structures used in sensors (electrochemical, optical, and piezoelectric), highlighting their main functional features. Furthermore, this review aims to summarize the reported applications of POMs in sensors for detecting and determining analytes in different matrices, many of them with biochemical and clinical relevance, along with analytical figures of merit and main virtues and problems of such devices. Special emphasis is given to the stability of POMs sensitive layers, detection limits, selectivity, the pH working range and throughput.

Interface matters: Design of an efficient α-Ag2WO4/Ag3PO4 photocatalyst

Heterojunction engineering of complex metal oxides is an active area of research that addresses fundamental questions in solid-state systems with broad technological applications. In this work, α-Ag2WO4/Ag3PO4 heterojunctions with different amounts of α-Ag2WO4 (12, 24, and 36 wt%) were synthesized by the coprecipitation method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy, and photoluminescence. The α-Ag2WO4/Ag3PO4 heterojunction containing 24% wt of α-Ag2WO4 showed the most enhanced photocatalytic activity for the degradation of Rhodamine B, being much higher than Ag3PO4 and α-Ag2WO4. Trapping experiments revealed that the holes and superoxide radical, in minor extent, were the main active species in the photocatalytic degradation. Such enhanced photocatalytic performance was explained by the surface plasmon resonance effect associated with the presence of metallic Ag at the interface and the formation of a type I heterojunction between α-Ag2WO4 and Ag3PO4 semiconductors.

Charge Localization in Defective BiVO4

We study the native defects in bismuth vanadate using hybrid density functional theory. We pay special attention to where excess charges localize by considering different polaronic distortions and find that charge localization has a profound effect on the local chemical environment around certain defects. In particular, oxygen dimerization may occur in the presence of acceptor defects. On the basis of Fermi level pinning due to compensation between donors and acceptors we additionally find that intrinsic p-type conductivity is difficult to achieve in BiVO4, in good agreement with experimental observations. Our results give new insights into the defect chemistry of bismuth vanadate and act as a guide for future studies on defects in complex metal oxides.

On the Choice of Some Modifiers when Creating Composite Nickel-phosphorus Coatings

The article considers the issues of choosing some modifiers of composite nickel-phosphorus coatings (CNiPC) using the method of crystal-chemical design in order to create high reliability and durability in the friction units of machines and mechanisms of tribological systems (TS). The paper shows the possibility of improving the tribological and anti-corrosion properties of NiP coatings without modifiers by introducing modifying additives into the coatings – ultradisperse powders of simple and complex metal oxides, oxides with corundum and rutile structures, simple substances with magnesium and tungsten structures, and polytetrafluoroethylene

Epitaxial-strain–dependent reorientation of oxygen octahedral tilting axis in manganite films

As one of the most ubiquitous lattice distortions in complex transition metal oxides, oxygen octahedral rotation/tilting plays an important role in their physical properties. In this paper, we report the evolution of oxygen octahedral tilting in La2/3Ca1/3MnO3 films grown on NdGaO3 substrates with different orientations by using an aberration-corrected scanning transmission electron microscope. Different epitaxial strains depending on the substrate orientation lead to different orientation relationships between the films and substrates. Unlike conventional coherent growth, our experimental results demonstrate that the reorientation of the oxygen octahedral tilting axis, depending on epitaxial strains, would occur at the interface between the film and substrate for the (010)-oriented substrate or inside the film for the (1-12)-oriented substrate, as well supported by theoretical calculations via density functional theory method and electron energy loss spectroscopy measurements. Considering the universality of interfacial oxygen octahedral coupling in oxide heterostructures, these experimental findings offer great insights in understanding thickness- and orientation-dependent physical properties of thin films.