抄録
Diffusion of Hydrogen and its dynamics in metals have been in focus owing to their rich quantum behavior [1]. Despite several studies, a physical understanding of the H diffusion in metals is still lacking. Platinum being known for its catalytic properties has been less understood for its interaction with H. Moreover, the studies on the PtHx system are limited to specific surfaces in the thermal regime (>100K) [2,3]. Here, we present the H diffusion in PtHx of different H concentrations (x) over a wide range of temperatures (5 – 200K).
Hydrogen diffusion in a thin polycrystalline Pt (25 nm) film has been investigated through resistance relaxation experiments at temperatures in the range (5 – 200 K). The film is hydrogenated by a low-energy ion-irradiation technique at an energy of 500 eV at 25 K, resulting in metastable PtHx. Arrhenius analysis of the H-hopping rate extracted from the exponential fitting of the relaxation data (Fig.1) shows a leveling off to a weak temperature-dependent regime below 140 K, commensurate with a mechanism dominated by quantum-rich processes. In the classical regime (>140 K), the H hopping rate shows a H concentration dependence (x) whereas this dependence becomes negligible in the quantum regime. The activation energy EA extracted from the Arrhenius slope shows a decrease from 115 meV to 70 meV when x in PtHx sample rises from 0.23 to 0.45 H/Pt, indicating modification of energy level of H-atoms at the metastable sites in the PtHx by a repulsive H-H interaction at higher x.
References:
[1] K. Christmann, Surf. Sci. Rep. 9 (1988) 1.
[2] A. P. Jardine et al., Phys. Rev. Lett. 105, (2010) 136101.
[3] G. Kallen et al., Phys. Rev. B, 65, (2001) 033406.
