Interaction of hydrogen and deuterium with CoTi surfaces.

Abstract

The interaction of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ with polycrystalline CoTi in the annealed and unannealed states, and at 300 and 80 K, has been studied using ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy, electron-energy-loss spectroscopy (EELS), thermal desorption spectroscopy, and measurement of work function. The two surface conditions, annealed and unannealed, were found to be slightly enriched in Ti and Co, respectively. Both ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ were found to react readily with CoTi at both 300 and 80 K, causing changes in the UPS spectra that could be explained from density-of-states calculations on CoTi and (by extension) on ${\mathrm{FeTiH}}_{\mathrm{x}}$. The surface stoichiometry was unaltered by the interaction, indicating no adsorbate-induced segregation, and there was no evidence from EELS of the formation of a separate hydride phase. However, EELS was able to demonstrate the presence of ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ at the surface by virtue of the rapid quenching of a surface loss feature on initial exposure. On heating above \ensuremath{\approxeq}500 K ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ were desorbed, the desorption threshold temperature increasing progressively with successive cycles of heating. For both ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ interaction the work function increased on exposure, but reached different saturation levels at 300 and 80 K, respectively. All these results are consistent with a mechanism of ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ chemisorption followed by inward diffusion and the formation of an \ensuremath{\alpha}-phase solid solution, but not with formation of a hydride phase.