Kiyofumi Nitta

Hierarchy of bond stiffnesses within icosahedral-based gold clusters protected by thiolates

Unique thermal properties of metal clusters are believed to originate from the hierarchy of the bonding. However, an atomic-level understanding of how the bond stiffnesses are affected by the atomic packing of a metal cluster and the interfacial structure with the surrounding environment has not been attained to date. Here we elucidate the hierarchy in the bond stiffness in thiolate-protected, icosahedral-based gold clusters Au25(SC2H4Ph)18, Au38(SC2H4Ph)24 and Au144(SC2H4Ph)60 by analysing Au L3-edge extended X-ray absorption fine structure data. The Au–Au bonds have different stiffnesses depending on their lengths. The long Au–Au bonds, which are more flexible than those in the bulk metal, are located at the icosahedral-based gold core surface. The short Au–Au bonds, which are stiffer than those in the bulk metal, are mainly distributed along the radial direction and form a cyclic structural backbone with the rigid Au–SR oligomers.

Insulator-to-Superconductor Transition upon Electron Doping in a BiS2-Based Superconductor Sr1−xLaxFBiS2

Effects of electron doping on the BiS2-based superconductors Sr1−xLaxFBiS2 (0 ≤ x ≤ 0.6) have been investigated using the systematically synthesized polycrystals by means of x-ray diffraction, x-ray absorption spectroscopy, transport and thermodynamic measurements. The pristine compound is a band insulator with the BiS2 layer, which accommodates electron carriers through the La substitution for Sr, as evidenced by the change in x-ray absorption spectra reflecting the occupancy of Bi 6p orbitals. With increasing the carrier density, the resistivity progressively decreases and a bad metallic state appears for x ≥ 0.45, where bulk superconductivity manifests itself below approximately 3 K. The value of Tc gradually increases with decreasing x from 0.6 to 0.45 and immediately decreases down to zero at the critical concentration of x ∼ 0.4, resulting in an insulator–superconductor transition highly sensitive to the carrier density. Thermodynamic measurements furthermore have revealed the possible enhancement o...

Structural kinetics of a Pt/C cathode catalyst with practical catalyst loading in an MEA for PEFC operating conditions studied by in situ time-resolved XAFS

The structural kinetics of surface events on a Pt/C cathode catalyst in a membrane electrode assembly (MEA) with a practical catalyst loading (0.5 mgPt cm(-2)) for a polymer electrolyte fuel cell were investigated by in situ time-resolved X-ray absorption fine structure analysis (XAFS; time resolution: 100 ms) for the first time. The rate constants of structural changes in the Pt/C cathode catalyst in the MEA during voltage cycling were successfully estimated. For voltage-cycling processes, all reactions (electrochemical reactions and structural changes in the Pt catalyst) in the MEA were found to be much faster than those in an MEA with a thick cathode catalyst layer, but the in situ time-resolved XAFS analysis revealed that significant time lags similarly existed between the electrochemical reactions and the structural changes in the Pt cathode catalyst. The time-resolved XAFS also revealed differences in the structural kinetics of the Pt/C cathode catalyst for the voltage-cycling processes under N2 and air flows at the cathode.

New XAFS beamline for structural and electronic dynamics of nanoparticle catalysts in fuel cells under operating conditions

We are currently constructing a new X-ray absorption fine structure (XAFS) beamline BL36XU at SPring-8 dedicated for the structural and electronic analysis of the dynamic events on polymer electrolyte fuel cell (PEFC) cathode catalysts for the development of next-generation PEFCs. To investigate the cathode catalyst nanoparticles in PEFCs under the operating conditions, the beamline is designed to provide time- and spatially resolved XAFS techniques having a time resolution of 100 μs, spatial resolution of 200 nm, and depth resolution of 1 μm. We report the outline and design of the new beamline.

In situ observation of carrier transfer in the Mn-oxide/Nb:SrTiO3 photoelectrode by X-ray absorption spectroscopy

The Mn-oxide/Nb:SrTiO3 photoelectrode for oxygen evolution reaction was investigated by in situ Mn K-edge XAFS spectroscopy under UV irradiation. The oxidization of the Mn oxide was observed via photoexcited carrier transfer, which results in the positive potential shift of the Mn oxide cocatalyst toward oxygen evolution reaction.

Network topology for the formation of solvated electrons in binary CaO–Al2O3 composition glasses

Glass formation in the CaO–Al2O3 system represents an important phenomenon because it does not contain typical network-forming cations. We have produced structural models of CaO–Al2O3 glasses using combined density functional theory–reverse Monte Carlo simulations and obtained structures that reproduce experiments (X-ray and neutron diffraction, extended X-ray absorption fine structure) and result in cohesive energies close to the crystalline ground states. The O–Ca and O–Al coordination numbers are similar in the eutectic 64 mol % CaO (64CaO) glass [comparable to 12CaO·7Al2O3 (C12A7)], and the glass structure comprises a topologically disordered cage network with large-sized rings. This topologically disordered network is the signature of the high glass-forming ability of 64CaO glass and high viscosity in the melt. Analysis of the electronic structure reveals that the atomic charges for Al are comparable to those for Ca, and the bond strength of Al–O is stronger than that of Ca–O, indicating that oxygen is more weakly bound by cations in CaO-rich glass. The analysis shows that the lowest unoccupied molecular orbitals occurs in cavity sites, suggesting that the C12A7 electride glass [Kim SW, Shimoyama T, Hosono H (2011) Science 333(6038):71–74] synthesized from a strongly reduced high-temperature melt can host solvated electrons and bipolarons. Calculations of 64CaO glass structures with few subtracted oxygen atoms (additional electrons) confirm this observation. The comparable atomic charges and coordination of the cations promote more efficient elemental mixing, and this is the origin of the extended cage structure and hosted solvated (trapped) electrons in the C12A7 glass.

Visualization of the Heterogeneity of Cerium Oxidation States in Single Pt/Ce2Zr2Ox Catalyst Particles by Nano‐XAFS

The cerium oxidation states in single catalyst particles of Pt/Ce2Zr2O(x) (x=7 to 8) were investigated by spatially resolved nano X-ray absorption fine structure (nano-XAFS) using an X-ray nanobeam. Differences in the distribution of the Ce oxidation states between Pt/Ce2Zr2O(x) single particles of different oxygen compositions x were visualized in the obtained two-dimensional X-ray fluorescent (XRF) mapping images and the Ce L(III)-edge nano X-ray absorption near-edge structure (nano-XANES) spectra.

Multilayer formation of the fluoroalkanol-ω-hydrogenated fluorocarbon mixture at the hexane/water interface studied by interfacial tensiometry and X-ray reflection

Novel multilayer formation of fluorocarbon compounds at the hexane/water interface was investigated from the viewpoint of intermolecular interaction and miscibility of molecules in the adsorbed film. The two kinds of mixed systems were employed: 1H,1H,2H,2H-perfluorododecanol (FC12OH)-1H-perfluorodecane (HFC10) (System A) and 1-icosanol (C20OH)-HFC10 (System B). The interfacial tension γ between the hexane solution and water was measured as a function of total concentration m and the composition of HFC10 in the mixture X(2) at 298.15 K under atmospheric pressure. X-ray reflectivity (XR) measurement was performed at BL37XU in SPring-8 as a function of scattering vector Q(z). In both systems, the γ vs m curves except for the pure HFC10 system have a break at low concentrations, which corresponds to the gaseous-condensed monolayer transition for System A and the expanded-condensed monolayer for System B. The remarkable difference between the two systems was that the curves in a limited bulk composition range (0.45 ≤ X(2) ≤ 0.9) of System A show another break at high concentrations close to the solubility limit. The total interfacial density above this break point was around 7-11 μmol m(-2), suggesting the spontaneous molecular piling to form a multilayer. The phase diagrams of adsorption in the condensed monolayer indicated that the film composition of HFC10 is negative in System B but definitely positive above X(2) ≥ 0.45 in System A. This clearly shows that HFC10 molecules are miscible with FC12OH but immiscible with C20OH in the condensed monolayer. Thus, it is likely that the mixing of HFC10 with FC12OH in the condensed monolayer induces multilayer formation. The X-ray reflectivity normalized by Fresnel reflectivity R/R(F) vs Q(z) plot in the condensed monolayer of System A was fitted by a one-slab model with uniform electron density and thickness. The electron density profile was almost the same as that of the pure FC12OH system. The plot in the multilayer, on the other hand, was fitted well by the two-slab model with different electron densities and thicknesses. The electron density profile showed that the multilayer consists of two layers, one of which has slightly higher electron density than the bulk hexane phase and piles on the lower layer with almost the same electron density as the condensed FC12OH monolayer.

Local Static Distortion of Rare Earth Ions in ROs4Sb12 (R = La, Ce, Pr, Nd, Sm) by Extended X-ray Absorption Fine Structure

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on ROs 4 Sb 12 (R = La, Ce, Pr, Nd, Sm) to study the motion of rare earth ions accommodated in the Sb icosahedron cage. The temperature dependences of the thermal (Debye–Waller) factors were analyzed from 15 to 300 K. The Einstein temperature obtained from EXAFS analysis of SmOs 4 Sb 12 is the lowest of the compounds, while there are no significant differences among the other compounds. The local static disorder obtained from the temperature dependence of EXAFS shows good correlation with the space inside the Sb icosahedron cage. Effective static distortion is expected to occur when the cage space is larger than about 0.5 A.

Atomic scattering factor of the ASTRO-H (Hitomi) SXT reflector around the gold's L edges.

The atomic scattering factor in the energy range of 11.2-15.4 keV for the ASTRO-H Soft X-ray Telescope (SXT) is reported. The large effective area of the SXT makes use of photon spectra above 10 keV viable, unlike most other X-ray satellites with total-reflection mirror optics. Presence of golds L-edges in the energy band is a major issue, as it complicates the function of the effective area. In order to model the area, the reflectivity measurements in the 11.2-15.4 keV band with the energy pitch of 0.4 - 0.7 eV were made in the synchrotron beam-line Spring-8 BL01B1. We obtained atomic scattering factors f1 and f2 by the curve fitting to the reflectivities of our witness sample. The edges associated with the L-I, II, and III transitions are identified, of which the depths are found to be roughly 60% shallower than those expected from the Henkes atomic scattering factor.