Local structures of cobalt in Co-doped TiO2 by synchrotron x-ray absorption near edge structures (original) (raw)
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Nanomaterials, 2022
We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co3O4). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O 1s to O 2p states hybridized with the unoccupied 3d states of cobalt atoms. Also, since Co3O4 contains two types of Co atoms, i.e., Co3+ and Co2+, we find that contribution of Co2+ ions to the pre-edge peak is solely due to single spin-polarized t2g orbitals (dxz, dyz, and dxy) while that of Co3+ ions is due to spin-up and spin-down polarized eg orbitals (dx2−y2 and dz2). Furthermore, we deduce the magnetic moments on the Co3+ and Co2+ to be zero and 3.0...
Physical Review B, 2021
Cobalt oxides with multiple local CoO coordination environments such as octahedral, tetrahedral, and tetragonal networks display versatile electronic and magnetic properties which have attracted great interest in many fields. Understanding the ground-state properties and determining the fundamental band gap remain challenges in cobalt-based compounds and thin films, which have been investigated here using synchrotron-based Co L 23-edge x-ray absorption measurements followed by configuration interaction cluster calculations. We focus on the detailed Co L 23-edge absorption spectral variations in different octahedral crystal fields as well as in the less investigated tetrahedral and tetragonal systems, taking into account Co ions with different valence states. From a quantitative comparison between the simulated spectrum and an accurately measured absorption spectrum of a specified compound, the crystal field value can be extracted from the Co L 23-edge absorption spectrum, which is complementary to the results obtained in optical measurements and other calculations. Furthermore, Co L 23-edge x-ray linear dichroism shows the same spectral evolutions as a result of either local CoO 6 cluster with tetragonal symmetry or the magnetic exchange field, whereas both coexist in most antiferromagnetic cobalt oxide thin films. Detailed temperature and polarization-dependent Co L 23-edge absorption measurements have been proposed to distinguish both contributions, which show different spectral variations due to the specified modifications of the ground and final states at different temperatures. Our results offer theoretical guidance for understanding the multiplet structure of Co L 23-edge absorption spectrum, obtaining the precise crystal field value for cobalt oxides with versatile coordinations, and explaining the underlying mechanism of x-ray linear dichroism, as well as understanding the fundamental physical properties and their potential applicability of cobalt oxides and their thin films.
Structural determination of Co/TiO2 nanocomposite: XRD technique and simulation analysis
Materials Science-Poland, 2016
Synthesis and complex theoretical and experimental studies of Co/TiO2 anatase have been reported. The preparation of Co/TiO2 was carried out by sol-gel method. Distribution of cations among the two tetrahedral and octahedral sites was estimated by analyzing the powder X-ray diffraction patterns by employing Rietveld refinement technique, and the results revealed the existence of tetragonal structure. Band structure and density of states calculations were performed using the first-principles methods. The structural and electronic properties of Co/TiO2 were calculated in the general gradient approximation (GGA). An additional comparison with pure TiO2 anatase allowed us to clarify cobalt doping effect on the electronic structure and the band gap. The band gap of Co/TiO2 was decreased by broadening the valence band as a result of the overlap among Co 3d, Ti 3d, and O 2p states, which made it respond better to visible and solar light.
We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O 2 , with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O 2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co 3 O 4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.
Electronic structure investigation of CoO by means of soft x-ray scattering
2002
The electronic structures of transition-metal compounds have been extensively investigated due to the discovery of exotic properties such as high-Tc superconductivity, giant magnetoresistance, and insulating behavior. For a theoretical description in ordinary band theory, an understanding of the electronic structure of transition-metal oxides represents a fundamental problem since the metal ions show a more or less atomiclike behavior.
The microstructure and chemistry of 2 mol.% and 5 mol.% cobalt-oxide-doped Ce 0.8 Gd 0.2 O 1.9 sintered at different temperatures were examined by a combination of electron energy-loss spectroscopy and energy-filtering and high-resolution transmission electron micros-copy. Co grain boundary excess was evaluated. It is found that Co solubility in Ce 0.8 Gd 0.2 O 1.9 is low at temperatures between 800 and 1150 °C, resulting in a large number of Co precipitates at grain boundaries. With increasing sintering temperature, precipitates grow, influencing the Co redistribution and further altering the segregation amount in the grain boundary. The Co grain boundary concentration is shown to increase with the increase of sintering temperature from 890 to 1050 °C, which is suggested to be due to grain growth. It is found that Co grain boundary segregation induces a detectable variation in the ELNES of Ce-M 4,5 and O-K absorption edges, indicating a reduction of Ce atoms in the grain boundary region. The phase of the precipitates was identified as CoO at temperatures between 890 °C and 1150 °C. HRTEM reveals that grain boundaries are less disordered after prolonged sintering time at higher temperature. At a dopant level of 5 mol.% Co oxide in Ce 0.8 Gd 0.2 O 1.9 , the grain boundaries become more disordered, and exhibit a high amount of Co segregation.
Solvation and speciation of cobalt(II). A theoretical X-ray absorption and RIXS study
Chemical Physics, 2020
The X-ray spectroscopic signatures of solvated Co 2+ ions mimicking the aqueous solution of CoCl2 are investigated accounting for multiconfigurational as well as spin-orbit coupling effects. To this end the RASSCF/RASSI methodology with second order corrections due to dynamical correlation (RASPT2) is employed. Emphasis is put on the identification of spectral signatures of different species in octahedral, [Co(H2O)6-xClx] (2-x)+ , and tetrahedral, [Co(H2O)4-xClx] (2-x)+ , coordination. X-ray absorption spectra show distinct differences in the L3 band only. Here, the best agreement between theory and experiment is obtained for the hexaaqua complex [Co(H2O)6] 2+. For better identification of particular species it is proposed to use RIXS spectroscopy, which shows pronounced species-dependent inelastic features.
In-situ near edge X-ray absorption fine structure spectroscopy was performed to monitor the oxidation states of Co and CoPt nanoparticles (NPs) of 4 nm size in the presence of H 2 and O 2 in the pressure range of 1 bar and 36 Torr respectively. Platinum helps the rapid reduction of cobalt oxides in hydrogen at a rather low temperature (38°C). In addition, reversible changes of the oxidation states of cobalt in the Co and CoPt NPs as a function of cycling oxygen pressure (in the range of millitorr to 36 Torr) are quantified and compared. The role of Pt in the process of Co reducing and oxidizing was explored. Our findings permit the prediction of the cobalt oxidation states as the reaction conditions are altered. The experimental results also suggest the presence of tetrahedral structure of Cobalt oxide that differs from the Co 3 O 4 spinel structure.
X-ray absorption studies of the structure of nanocrystalline oxides
Solid State Ionics, 2006
X-ray absorption spectroscopy is a prime tool in probing the local structure in condensed matter. This contribution describes some of the applications of this technique to probe the microstructure of nanocrystalline oxides. By the use of specific examples four areas will be covered, namely the effect of grain size and preparation route on microstructure, the locating of dopant sites and the chemical nature of metal/oxide catalysts. The materials involved are tin oxide, lithium niobate, titanium dioxide and copper/ceria.