Nozomu Hiraoka

We investigated the size dependence of the metal-insulator transition (MIT) in Cr-doped V 2 O 3 nanocrystals by photoemission spectroscopy using complementary probing depths, together with magnetic susceptibility measurements. Photoemission spectra show that MIT signatures persist down to an average particle size of 5.6 nm, and magnetic susceptibility measurements exhibit a nearly size-invariant transition onset. The contrast between surface-sensitive and deeper-probing photoemission spectra reveals that the transition survives in the nanocrystal interior. At the same time, the spectra indicate a systematic suppression of coherent quasiparticle weight with decreasing size, pointing to the growth of an insulating surface dead layer. These results demonstrate that nanoscaling does not intrinsically eliminate the MIT itself, but progressively enhances the influence of surface-driven insulating behavior, thereby providing insight into the practical limits of miniaturizing Mott-based devices.

High‐resolution structural analysis of heterogeneous catalysts under reaction conditions is essential for understanding reaction mechanisms, active‐site dynamics, and structure–activity relationships. High‐energy‐resolution fluorescence‐detected X‐ray absorption near‐edge structure (HERFD‐XANES) spectroscopy has recently emerged as a powerful method for probing local structures with enhanced energy resolution. This advantage of HERFD‐XANES becomes particularly important at high‐energy absorption edges, such as the K ‐edges of 4d transition metals, where severe core–hole lifetime broadening limits conventional XANES analysis. In the present study, taking advantage of this feature, a Rh/ γ ‐Al ₂ O ₃ catalyst under reaction conditions was analyzed using in situ Rh K ‐edge HERFD‐XANES spectroscopy to elucidate surface adsorbates on Rh. By combining in situ HERFD‐XANES measurements with spectral simulations based on Rh(111) slab models, bare, CO‐, H‐, and O‐adsorbed Rh surfaces were successfully distinguished in the experimental spectra. In addition, under CO‐rich NO─CO reaction conditions (e.g., NO/CO = 0.67), the Rh surface was identified as consisting of CO‐adsorbed and vacant sites. These results demonstrate that Rh K ‐edge HERFD‐XANES, and more generally 4d transition‐metal K ‐edge HERFD‐XANES, extend conventional XANES analysis beyond oxidation‐state evaluation, enabling identification of adsorbate‐specific surface states and providing direct insight into reactions on active sites.

Strong correlations and ferromagnetic Hund's coupling lead to diverse electronic phenomena in transition-metal oxides that sensitively depend on the d-orbital electron filling. Fullerides, their p-electron counterparts, exhibit effective antiferromagnetic Hund's coupling in a different energy range. At half-filling (n = 3, three electrons in triply degenerate orbitals), both d- and p-electron systems are Mott insulators due to strong correlations and Hund's coupling. Away from half-filling, in single-electron/hole (n = 1,5) d-orbital systems, Hund's coupling opposes the correlations, reducing the Mott gap and allowing survival of metallicity. Here we report a single-hole multiorbital correlated p-electron system, orthorhombic-structured Yb₂CsC₆₀ comprising pentavalent C₆₀^5- anions, which also exhibits a robust metallic state with no Mott transition, just like in the metastable single-electron cubic-structured CsC₆₀. We assert that particle-hole symmetry holds well in (n = 1,5) fullerides and that their p-electron-derived states are analogous to those in d-orbital solids, providing impetus for further study of these correlated systems.

Electronic and crystal structures of Ce(Ru_{1-x}Rh_{x})_{2}Al_{10} have been studied using x-ray emission spectroscopy, photoelectron spectroscopy (PES), and x-ray diffraction. No structural phase transition was observed up to x=0.1, while the x-ray absorption spectra showed a Ce valence transition between x=0.03 and 0.05, accompanied by an increase in c-f hybridization strength, suggesting a Lifshitz transition. Angle-resolved PES conducted on the x=0.1 sample revealed stronger hybridization along the b axis compared to the other axes. Direction-dependent hybridization introduces complexity in applying conventional phase diagrams. For the first time, we propose a direction-dependent phase diagram for Ce(Ru_{1-x}Rh_{x})_{2}Al_{10}, advancing beyond the traditional Doniach phase diagram.

Although numerous techniques are developed to enable real‐time understanding of dynamic interactions at the solid–liquid interface during electrochemical reactions, further progress in the development of these methods over the last several decades has faced challenges. With the rapid development of high‐brilliance synchrotron sources, operando X‐ray spectroscopies have become increasingly popular for revealing interfacial features and catalytic mechanisms in electrocatalysis. Nevertheless, the resulting spectra are highly sensitive to factors such as X‐ray radiation, reaction environment, and acquisition procedures, all of which may potentially introduce artifacts that are often overlooked, leading to misinterpretations of electrocatalytic behaviors. In this perspective, several emerging hard X‐ray spectroscopies used in electrocatalysis research are reviewed, highlighting their electronic transition processes, detection modes, and functional complementarity. Significantly, based on a case study of operando X‐ray absorption spectroscopy at various beamlines, potential artifacts generated by X‐ray irradiation are systematically investigated through photon‐flux density‐, dose‐, and time‐dependent studies of typical copper electrocatalysts. Accordingly, a practical protocol for conducting reliable X‐ray spectroscopic measurements in operando electrocatalytic studies to minimize potential artifacts that can affect the resulting X‐ray spectra, thereby ensuring accurate interpretation and a deeper understanding of interfacial interactions and electrocatalytic mechanisms, is established.

The performances of a spherically bent Bragg analyzer and a cylindrically bent Laue analyzer in an X-ray Raman/emission spectrometer are compared. The reflectivity and energy resolution are evaluated from the intensity of the elastic scattering and the width of the energy distribution on a SiO₂ glass sample. Widely used, Bragg analyzers display excellent performance at the photon energy E ≤ 10 keV. However, at higher E, the reflectivity and the resolution gradually deteriorate as E increases, showing poor performance above 20 keV. On the other hand, the reflectivity of the Laue analyzer gradually increases at E > 10 keV, displaying excellent reflectivity and good resolution around 20 keV. The Laue analyzer is suitable for X-ray absorption spectroscopy in high-energy-resolution fluorescence-detection mode or X-ray emission spectroscopy on 4d transition metal compounds. Furthermore, the X-ray Raman features of the lithium K-edge in LiF and the oxygen K-edge feature in H₂O, measured by nine Bragg analyzers (2 m radius) at E ≃ 9.9 keV and by five Laue analyzers (1.4 m radius) at E ≃ 19.5 keV, have been compared. Similar count rates and resolutions are observed.