| AVS 58th Annual International Symposium and Exhibition | |
| Surface Science Division | Tuesday Sessions |
| Session SS-TuP |
| Session: | Surface Science Poster Session |
| Presenter: | Masamitu Takahasi, Japan Atomic Energy Agency |
| Authors: | M. Takahasi, Japan Atomic Energy Agency S. Fujikawa, Japan Atomic Energy Agency W. Hu, Japan Atomic Energy Agency H. Tajiri, Japan Synchrotron Radiation Institute |
| Correspondent: | Click to Email |
Absorption of hydrogen into a substrate begins with dissociated adsorption of hydrogen molecules. The mechanism of the transition from adsorption to absorption is an important knowledge for improving the performance of hydrogen storage materials. The aim of the present study is to verify the hydrogen absorption model in atomic scale through quantitative determination of structures of hydrogen-adsorbed Pd(110) by synchrotron X-ray diffraction.
Experiments were performed at a synchrotron beamline 13XU at SPring-8 using a surface X-ray diffractometer integrated with a UHV chamber equipped with a cryostat. The sample was Pd(110) single crystal which was 10 mm in diameter and 3 mm in thickness. The clean Pd(110)-(1x1) surface was prepared by electrochemical etching in HCl solution and cycles of Ar-sputtering and annealing at ca. 900 K in UHV. For the hydrogen adsorption experiments, the hydrogen pressure was carefully controlled with a needle valve and a nude ion gauge.
We measured five crystal truncation rod (CTR) profiles with increasing substrate temperature to room temperature from 57 K after the substrate was exposed to 10-6 Torr s hydrogen. Before and after hydrogen adsorption at 57 K, the CTR profiles changed only slightly. More distinctive changes were observed when the sample temperature reached 180 K. At room temperature, the CTR profiles were accounted for by a structure model with disordering of surface atoms. According to past Low-energy electron diffraction, He diffraction and thermal desorption spectroscopy studies, hydrogen adsorption below a substrate temperature of 120 K induced the (1x2) surface reconstruction with 1.5 monolayer (ML) hydrogen through the (2x1) structure at a hydrogen coverage of 1 ML. When the temperature is raised to 200 K, the (1x2) reconstruction returns to a low-coverage 2x1 phase without hydrogen desorption. A comparison with our X-ray diffraction results and these past studies shows that the change of the CTR profiles at 180 K corresponds to the transition from the (2x1) to (1x2) reconstructions and concomitant incorporation of hydrogen into subsurface. These results provide us with an atomic-scale picture that explains how adsorbed hydrogen is absorbed into the Pd bulk.