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Materials Molecular Science Yokoyama Group

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Location: Myoudaiji, Main Laboratory Bldg. Room319
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X-Ray Absorption Spectroscopy, Surface & Thin Film Magnetism, Ambient Pressure Hard X-Ray Photoelectron Spectroscopy

Exploitation of Novel Spectroscopic Methods for Material and Surface Science

For the developments of novel functional materials, it is quite important to exploit simultaneously new analytical methods based on advanced technology. Novel materials and devices often require spatial and/or time resolved analysis to optimize their qualities. In our group, we have been exploiting spectroscopic methods for material and surface science using mainly synchrotron radiation (SR) and partly lasers.

The present first subject is the exploitation of ambient pressure hard x-ray photoelectron spectroscopy (HAXPES) and its application to polymer electrolyte fuel cells (PEFC) under working conditions. In 2017, we succeeded in the HAXPES measurement under real ambient pressure of 105 Pa for the first time in the world. We are investigating working PEFC by the technique for the elucidation of degradation and/or poisoning mechanisms of PEFC to improve its performance with longer life time.

The second subject is the fast time resolved x-ray absorption fine structure (XAFS) measurements of short-lived photoexcited states in photocatalysts in order to understand detailed mechanisms o f photocatalytic reactions such as water photodecomposition to yield hydrogen and to improve their functionalities and efficiencies. Time resolving power is <100 ps using the laser-pump and SR-probe method. Recently, we successfully determined the quantitative local structure for the photoexcited polaron state of a visible-light active photocatalyst of tungsten(VI) oxide.

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Ambient pressure hard x-ray photoelectron spectroscopic system.
(a) Apparatus installed at SPring-8 Beamline 36XU.
(b) Front corn with a 30-μm aperture of the electron energy analyzer, made by Equipment Development Center, IMS.
(c) Au 4f HAXPES of Au(111) at a real ambient pressure of 105 Pa.
(d) Hydrogen pressure dependent Pd 3d5/2 HAXPES of Pd black. An abrupt change is found at ~3 kPa due to the transformation between metallic Pd and Pd hydride states.

Selected Publications

  1. “Operando Observation of Sulfur Species Poisoning Polymer Electrolyte Fuel Cell Studied by Near Ambient Pressure Hard X‑ray Photoelectron Spectroscopy” L. Yu, Y. Takagi, T. Nakamura, T. Sakata, T. Uruga, M.Tada, Y. Iwasawa, S. Masaoka, and T. Yokoyama, J. Phys. Chem. C 123 , 603 (2019).
  2. “X-ray photoelectron spectroscopy under real ambient pressure conditions” Y. Takagi, T. Nakamura, L. Yu, S. Chaveanghong, O. Sekizawa, T. Sakata, T. Uruga, M. Tada, Y. Iwasawa and T. Yokoyama, Appl. Phys. Exp10, 076603 (2017).
  3. “Dynamics of Photoelectrons and Structural Changes of Tungsten Trioxide Observed by Femtosecond Transient XAFS” Y. Uemura, D. Kido, Y. Wakisaka, H. Uehara, T. Ohba, Y. Niwa, S. Nozawa, T. Sato, K. Ichiyanagi, R. Fukaya, S. Adachi, T. Katayama, T. Togashi, S. Owada, K. Ogawa, M. Yabashi, K. Hatada, S. Takakusagi, T. Yokoyama, B. Ohtani, and K. Asakura, Angew. Chem. Int. Ed55, 1364 (2016).
  4. “Anharmonicity and Quantum Effects in Thermal Expansion of an Invar Alloy” T. Yokoyama and K. Eguchi, Phys. Rev. Lett. 107, 065901 (2011).
  5. “Magnetic circular dichroism near the Fermi level” T. Nakagawa and T. Yokoyama, Phys. Rev. Lett96, 237402 (2006).