Effect of adsorbed hydrogen on the stability of titanium atoms on aluminum surfaces

Abstract

The thermal stability of Ti atoms evaporated onto polycrystalline Al and Al(100) surfaces was studied using ion scattering spectroscopy. Measurements were made in vacuum and in the presence of deuterium. When heated in vacuum, Ti begins to disappear from the surface at substrate temperatures below $100\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and is not detected on the surface at temperatures above $250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. When measurements are made while the sample is being exposed to atomic D, Ti persists on the surface to noticeably higher temperatures. Diffusion coefficients for surface Ti moving into the Al substrate were obtained at temperatures in the range $25--150\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The experimental results are compared to density-functional theory calculations, which indicate that direct and Al mediated H-Ti interactions stabilize surface Ti. At H coverages around 1 ML (monolayer), the most stable structure has 1/2 ML Ti in the second layer and 1 ML H on specific bridge sites on the top Al layer. Ti directly exposed to the surface is less stable and thus likely to be less active for Ti catalysis of H sorption in alanate H storage materials.