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Identifying real active sites and understanding the mechanism of oxygen evolution reaction (OER) are still a big challenge today for developing efficient electrochemical catalysts in renewable energy technologies. Here, using a combined in situ/operando experiments and theory, the catalytic mechanism of the ordered OER active Co and Ir ions in Sr2CoIrO6−δ is studied, which exhibits an unprecedented low overpotential 210 mV to achieve 10 mA cm–2, ranking the highest performance among perovskite‐based solid‐state catalysts. Operando X‐ray absorption spectroscopies as a function of applied voltage indicates that Ir4+ ion is gradually converted into extremely high‐valence Ir5+/6+, while the part of Co3+ ion is transferred into Co4+ under OER process. Density functional theory calculations explicitly reveal the order Co‐O‐Ir network as an origin of ultrahigh OER activity. The work opens a promising path to overcome the sluggish kinetics of OER bottleneck for water splitting via proper arrangements of the multi‐active sites in catalyst. [ABSTRACT FROM AUTHOR]

Confusion has arisen about the Co spin‐state transition under high pressure for multiferroic BiCoO3. The crystal structure analysis indicates a high‐spin (HS) to low‐spin (LS) state transition with increasing pressure, while Co Kβ X‐ray emission spectra present a high‐spin to intermediate‐spin (IS) state transition. Herein, the spin state of Co3+ in BiCoO3 is investigated utilizing soft X‐ray absorption spectroscopy (XAS) at the Co‐L2,3 and O‐K edges at room temperature under ambient pressure. Furthermore, the spin state transition of Co3+ under high pressure is investigated by Co Kβ X‐ray emission spectra. A pure HS‐Co3+ ground state is found in the BiCoO3 sample prepared from the precursor and oxidizer mixed method—and the existence of a small part of LS‐Co3+ in the BiCoO3 sample prepared from the precursor and oxidizer sandwiched method. A complete HS‐Co3+ to LS‐Co3+ transition is observed under high pressure up to 32.3 GPa. The results clarify the debate on the spin‐state transition of BiCoO3 under high pressure and will deepen our understanding of the intrinsic physical properties on it. [ABSTRACT FROM AUTHOR]

A negative-thermal-expansion-like volume contraction in LaCu 3 Fe 4 O 12-δ was proposed to result from an inter-site charge transfer between Fe and Cu, namely from Cu3+-Fe3+ at room temperature to Cu2+-Fe3.75+ above 393 K (T C , charge-transfer temperature). Although the partial valence transition of the Fe valence state was observed by Mössbauer spectroscopy and the BVS method, there is no direct observation of the electron transfer to the Cu site in this material until now. In this work, we have studied the valence state of Cu ions in LaCu 3 Fe 4 O 12-δ as a function of temperature across T C using soft x-ray absorption spectroscopy at the Cu- L 3 and the O K edges. We indeed observed a sharp decrease in the valence state of Cu across the phase transition temperature T C. Nevertheless, Cu is 2.2 + and 2.7 + at 440 K and 80 K, respectively, indicates an incomplete inter-site charge transfer across T C. The temperature-dependence Cu- L 3 XAS spectra indicate that there is merely partial electron transfer from Fe to Cu. This is different from the proposed charge transfer from Cu3+-Fe3+ to Cu2+-Fe3.75+ since Cu is not pure 3 + at 80 K far below the phase transition temperature (T C = 393 K) and on the other hand not pure 2 + at 440 K far above T C. Image 1 [ABSTRACT FROM AUTHOR]

This study examines Al-doped ZnO ( AZO) transparent conductive thin films prepared on glass substrate using the pulsed laser deposition method with an expanding magnetic field perpendicular to the sample surface. O K-, Zn L3-, and Al K-edge X-ray absorption near-edge structure ( XANES) and X-ray emission spectroscopy ( XES) were used to investigate the relationship between the effects of magnetic field size and the electronic structure of AZO thin films. Analysis of the XANES spectra showed increasing O 2 p states as film resistance decreases, suggesting that the enrichment of the O 2 p-dangling bond along the c-axis is the main factor affecting the electric performance of AZO thin films. Magnetic field size affects electrons itinerating from Zn atoms to Al sites through O 2 p-Zn 3 d and O 2 p-Al 3 sp hybridization and consequently weakens O 2 p-Zn 3 d hybridization with the downsizing of the crystallite size. XES and XANES spectra of O 2 p states at the O K-edge exhibit that the conduction-band minimum affects Eg and the valence-band maximum is nearly unaffected by changes in magnetic field size. Factors affecting the resistance/electric conductivity of AZO thin films are the preferential orientation of (002) along the c-axis, the thermal/annealing effect accompanied by the changes of magnetic field size, the surface effect, energy bandgap, and the density of unoccupied O 2 p-derived states. [ABSTRACT FROM AUTHOR]

Confusion has arisen about the Co spin‐state transition under high pressure for multiferroic BiCoO3. The crystal structure analysis indicates a high‐spin (HS) to low‐spin (LS) state transition with increasing pressure, while Co Kβ X‐ray emission spectra present a high‐spin to intermediate‐spin (IS) state transition. Herein, the spin state of Co3+ in BiCoO3 is investigated utilizing soft X‐ray absorption spectroscopy (XAS) at the Co‐L2,3 and O‐K edges at room temperature under ambient pressure. Furthermore, the spin state transition of Co3+ under high pressure is investigated by Co Kβ X‐ray emission spectra. A pure HS‐Co3+ ground state is found in the BiCoO3 sample prepared from the precursor and oxidizer mixed method—and the existence of a small part of LS‐Co3+ in the BiCoO3 sample prepared from the precursor and oxidizer sandwiched method. A complete HS‐Co3+ to LS‐Co3+ transition is observed under high pressure up to 32.3 GPa. The results clarify the debate on the spin‐state transition of BiCoO3 under high pressure and will deepen our understanding of the intrinsic physical properties on it. [ABSTRACT FROM AUTHOR]

The nature of the non-magnetic to paramagnetic transition of Co3+ oxide LaCoO 3 was strongly disputed in the literature for many decades from a low-spin (LS) state below 20 K and to a mixed LS state and high-spin (HS) state or an intermediate-spin (IS) state above 100 K. In this context, the layered perovskite LaSrCoO 4 is more favorable for a Jahn-Teller-active IS state because of an elongated distortion, but has been scarcely studied with experimental X-ray spectroscopies as a function of temperature or external pressure. Here, our Co-L 2,3 X-ray absorption spectroscopic study indicates a mixture of 40% HS-Co3+ and 60% LS-Co3+ for LaSrCoO 4 against 25% HS-Co3+ and 75% LS-Co3+ for LaCoO 3 at 300 K and ambient pressure (AP). At 10 K, we observed a sizable magnetic-circular-dichroism signal and a clear HS state of the magnetic Co3+ ion from the Co-L 2,3 edge of LaSrCoO 4. This result demonstrates that the HS state is already populated in the ground state versus a pure LS ground state in LaCoO 3. A quantitative change of quantum number of the spin of the Co3+ ion of LaSrCoO 4 as a function of pressure and temperature investigated systematically with Co-Kβ X-ray emission experiments firmly demonstrates not only a mixed state of LS/HS at 300 K and AP but also a presence of the pure LS-Co3+ and HS-Co3+ states only under high pressure and high temperature, respectively. • Co3+ ion in LaSrCoO 4 with an elongated distortion has a mixed spin state rather than a disputable intermediate spin state. • It is demonstrated that 2:3 ratio of HS:LS for LaSrCoO 4 against 1:3 for LaCoO 3 at 300 K and ambient pressure. • The scenario of intermediate spin state of Co3+ ion in LaSrCoO 4 is safely excluded based on the theoretical calculations. [ABSTRACT FROM AUTHOR]

Abstract: Ni and Ni–NiO core–shell nano-arrays were fabricated by means of electroless deposition, where the latter was covered by a NiO shell of ∼10nm by annealing the former at 350°C for 30min in an atmospheric condition. HRTEM showed that the NiO shell was developed at the expense of Ni at the array''s surface. Ferromagnetic ordering of the Ni–NiO arrays was found to be suppressed compared with those of the less oxidized reference Ni arrays. This is attributed to the screening effect of the NiO shell, and weak ferromagnetism of inner Ni arrays resulted from the development of the NiO. X-ray absorption spectrum reveals that the reference Ni is partially oxidized. Also, X-ray magnetic circular dichroism suggests that the magnetic suppression of the Ni–NiO arrays is associated with a reduced spin moment. [Copyright &y& Elsevier]