Cu K-edge x-ray-absorption spectroscopic study on the octahedrally coordinated trivalent copper in the perovskite-related compounds La2Li0.5Cu0.5O4 and LaCuO3

Tuning the Electronic Structures of Multimetal Oxide Nanoplates to Realize Favorable Adsorption Energies of Oxygenated Intermediates

Highly active oxygen evolution reaction (OER) electrocatalysts are important to effectively transform renewable electricity to fuel and chemicals. In this work, we construct a series of multimetal oxide nanoplate OER electrocatalysts through successive cation exchange followed by electrochemical oxidation, whose electronic structure and diversified metal active sites can be engineered via the mutual synergy among multiple metal species. Among the examined multimetal oxide nanoplates, CoCeNiFeZnCuO_x nanoplates exhibit the optimal adsorption energy of OER intermediates. Together with the high electrochemical active surface area, the CoCeNiFeZnCuO_x nanoplates manage to deliver a small overpotential of 211 mV at an OER current density of 10 mA cm^-2 (η₁₀) with a Tafel slope as low as 21 mV dec^-1 in 1 M KOH solution, superior to commercial IrO₂ (339 mV at η₁₀, Tafel slope of 55 mV dec^-1), which can be stably operated at 10 mA cm^-2 (at an overpotential of 211 mV) and 100 mA cm^-2 (at an overpotential of 307 mV) for 100 h.

Interplay between local structure, vibrational and electronic properties on CuO under pressure

The electronic and local structural properties of CuO under pressure have been investigated by means of X-ray absorption spectroscopy (XAS) at Cu K edge and ab initio calculations, up to 17 GPa. The crystal structure of CuO consists of Cu motifs within CuO₄ square planar units and two elongated apical Cu-O bonds. The CuO₄ square planar units are stable in the studied pressure range, with Cu-O distances that are approximately constant up to 5 GPa, and then decrease slightly up to 17 GPa. In contrast, the elongated Cu-O apical distances decrease continuously with pressure in the studied range. An anomalous increase of the mean square relative displacement (EXAFS Debye-Waller, σ²) of the elongated Cu-O path is observed from 5 GPa up to 13 GPa, when a drastic reduction takes place in σ². This is interpreted in terms of local dynamic disorder along the apical Cu-O path. At higher pressures (P > 13 GPa), the local structure of Cu²⁺ changes from a 4-fold square planar to a 4+2 Jahn-Teller distorted octahedral ion. We interpret these results in terms of the tendency of the Cu²⁺ ion to form favorable interactions with the apical O atoms. Also, the decrease in Cu-O apical distance caused by compression softens the normal mode associated with the out-of-plane Cu movement. CuO is predicted to have an anomalous rise in permittivity with pressure as well as modest piezoelectricity in the 5-13 GPa pressure range. In addition, the near edge features in our XAS experiment show a discontinuity and a change of tendency at 5 GPa. For P < 5 GPa the evolution of the edge shoulder is ascribed to purely electronic effects which also affect the charge transfer integral. This is linked to a charge migration from the Cu to O, but also to an increase of the energy band gap, which show a change of tendency occurring also at 5 GPa.

High pressure synthesis of a quasi-one-dimensional GdFeO3-type perovskite PrCuO3 with nearly divalent Cu ions

A new perovskite-type cuprate PrCuO₃ with unusual oxidation state and quasi-one-dimensional Cu–O chains has been synthesized by high-pressure oxygen annealing.

Structural Reversibility of LaCo1-x Cux O3 Followed by In Situ X-ray Diffraction and Absorption Spectroscopy

Combinations of perovskite-type oxides with transition and precious metals exhibit a remarkable self-regenerable property that could be exploited for numerous practical applications. The objective of the present work was to study the reversibility of structural changes of perovskite-type oxides under cyclic reducing/oxidizing atmosphere by taking advantage of the reducibility of LaCoO3 . LaCoO3±δ and LaCo0.8 Cu0.2 O3±δ were prepared by ultrasonic spray combustion and were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). XRD and XAS data confirmed that copper adopted the coordination environment of cobalt at the B-site of the rhombohedral LaCoO3 under the selected synthesis conditions. The structural evolution under reducing atmosphere was studied by in situ XRD and XANES supporting the assignment of the observed structural changes to the reduction of the perovskite-type oxide from ABB'O3 (B'=Cu) to B'0 /ABO3 and to B'0 B0 /A2 O3 . Successive redox cycles allowed the observation of a nearly complete reversibility of the perovskite phase, i. e. copper was able to revert into LaCoO3 upon oxidation. The reversible reduction/segregation of copper and incorporation at the B-site of the perovskite-type oxides could be used in chemical processes where the material can be functionalized by segregation of Cu and protected against irreversible structural changes upon re-oxidation.

An efficiently tuned d-orbital occupation of IrO2 by doping with Cu for enhancing the oxygen evolution reaction activity

Tuning Ir d-orbital occupation via doping Cu into the IrO₂ lattice to prepare a highly efficient oxygen evolution reaction catalyst, Cu_0.3Ir_0.7O_δ.

The oxygen evolution reaction (OER) has been regarded as a key half reaction for energy conversion technologies and requires high energy to create O Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O bonds. Transition metal oxides (TMOs) seem to be a promising and appealing solution to the challenge because of the diversity of their d-orbital states. We chose IrO₂ as a model because it is universally accepted as a current state-of-the-art OER catalyst. In this study, copper-doped IrO₂, particularly Cu_0.3Ir_0.7O_δ, is shown to significantly improve the OER activity in acidic, neutral and basic solutions compared to un-doped IrO₂. The substituted amount of Cu in IrO₂ has a limit described by the Cu_0.3Ir_0.7O_δ composition. We determined that the performance of Cu_0.3Ir_0.7O_δ is due primarily to an increase in the Jahn–Teller effect in the CuO₆ octahedra, and partially to oxygen defects in the lattice induced by the IrO₆ octahedral geometric structure distortions, which enhance the lift degeneracy of the t_2g and e_g orbitals, making the d_z² orbital partially occupied. This phenomenon efficiently reduces the difference between ΔG2 and ΔG3 in the free energy from the density functional theoretical (DFT) calculations and can yield a lower theoretical overpotential comparable to that of IrO₂. The proposed method of doping with foreign elements to tune the electron occupation between the t_2g and e_g orbital states of Ir creates an opportunity for designing effective OER catalysts using the TMO groups.

Valence properties of Cu and Ru in titanium-substituted LnCu3Ru4O12+δ (Ln = La, Pr, Nd) investigated by XANES and TGA

In La_yCu₃Ru_xTi_4−xO_12+δ ruthenium remains as Ru⁴⁺, while the Cu valence decreases from +2 to +1.6 with increasing x.

Influences of Alkaline Earth Metal Substitution on the Crystal Structure and Physical Properties of Magnetic RuSr1.9A0.1GdCu2O8 (A = Ca, Sr, and Ba) Superconductors

We have investigated the effect of alkaline earth metal substitution on the crystal structure and physical properties of magnetic superconductors RuSr(1.9)A(0.1)GdCu(2)O(8) (A = Ca, Sr, and Ba) in order to probe an interaction between the magnetic coupling of the RuO(2) layer and the superconductivity of the CuO(2) layer. X-ray diffraction and X-ray absorption spectroscopic analyses demonstrate that the isovalent substitution of Sr ions with Ca or Ba ions makes it possible to tune the interlayer distance between the CuO(2) and the RuO(2) layers. From the measurements of electrical resistance and magnetic susceptibility, it was found that, in contrast to negligible change of magnetization, both of the alkaline earth metal substitutions lead to a notable depression of zero-resistance temperature T(c) (DeltaT(c) approximately 17-19 K). On the basis of the absence of a systematic correlation between the T(c) and the interlayer distance/magnetization, we have concluded that the internal magnetic field of the RuO(2) layer has insignificant influence on the superconducting property of the CuO(2) layer in the ruthenocuprate.

Evolution of the crystal and electronic structures of magnetic RuSr(2-x)La(x)GdCu2O8 superconductors upon La substitution

We have investigated systematically the effects of La substitution on the chemical bonding nature and physical properties of magnetic RuSr(2-x)La(x)GdCu2O8 superconductors. X-ray diffraction and energy dispersive spectroscopic microprobe analyses reveal that a fraction of Sr ions can be successfully replaced by La ions with the contraction of unit cell volume. According to electrical resistance and dc magnetization measurements, the La substitution gives rise to a significant reduction of superconducting transition temperature (T(c)) but to an increase of magnetic ordering temperature with depressed remanent magnetization. Ru K- and Cu K-edge X-ray absorption spectroscopic results clarify that average Ru and Cu oxidation states decrease upon the La substitution. On the basis of the spectroscopic evidences presented here, we are able to attribute the T(c) reduction upon the La substitution to the depletion of the hole density in CuO2 layers and the accompanying variation of magnetic coupling behavior to the change of Ru oxidation state.

High-Tc superconductors in the two-dimensional limit:

The free modulation of interlayer distance in a layered high-transition temperature (high-Tc) superconductor is of crucial importance not only for the study of the superconducting mechanism but also for the practical application of high-Tc superconducting materials. Two-dimensional (2D) superconductors were achieved by intercalating a long-chain organic compound into bismuth-based high-Tc cuprates. Although the intercalation of the organic chain increased the interlayer distance remarkably, to tens of angstroms, the superconducting transition temperature of the intercalate was nearly the same as that of the pristine material, suggesting the 2D nature of the high-Tc superconductivity.

Copper K-Edge X-ray Absorption Studies of La(2)(-)(x)()K(x)()CuO(4) Superconductors, Ba(4)NaCuO(4)(CO(3))(2), and NdCu(2)O(4)

X-ray near-edge absorption spectroscopy (XANES) measurements are reported at the Cu K-edge for La(2)(-)(x)()K(x)()CuO(4) superconductors with x = 0.1 and 0.2. Comparisons of the data were made with the absorption spectra of a Cu(II) standard (La(2)CuO(4)), a new Cu(III) standard (Ba(4)NaCuO(4)(CO(3))(2)), and a new mixed-valent Cu(II)/Cu(III) phase (NdCu(2)O(4)). There is a consistent increase in the energy of the main Cu absorption edge in the following series: La(2)CuO(4) < La(1.9)K(0.1)CuO(4) < La(1.8)K(0.2)CuO(4) approximately NdCu(2)O(4) < Ba(4)NaCuO(4)(CO(3))(2). This trend can be attributed to an increase in the average Cu valence and to a decrease in bond length and an increase in covalency. The magnitude of the energy shift from Cu(II) to Cu(III) for the main absorption edge is approximately 3 eV. There is no measurable change in the energy of the pre-edge feature irrespective of Cu valence. Furthermore, no new features appear at the edge for samples with formal oxidation state between 2 and 3, even for NdCu(2)O(4), which contains both Cu(II) and Cu(III) in crystallographically distinct sites.