The kinetics of hydrogen interaction with TiH x films, 0≤x≲2

Abstract

The kinetics of hydrogen adsorption on and desorption from TiH x films at room temperature was investigated by quartz crystal microbalance (QCM) measurements. The Ti samples were prepared by evaporation under UHV conditions and exposed to hydrogen at pressures 5 × 10 -8 – 1 × 10 -5 Torr. The absorption kinetics is characterized by three reaction regions. For H/Ti ratios ≲ 0.3 (region I) the sticking coefficient decreases monotonically from an initial sticking coefficient on the clean surface of 0.17. In region II (0.3 ≲ H/Ti ≲ 1.8) the absorption rate is almost constant. When the hydrogen gas is removed, desorption of H 2 is observed and the desorption kinetics is well described by a simple associative desorption model. The desorption rate and the total amount of desorbing H 2 is increasing with increasing pressure during the H 2 exposure, but are relatively insensitive to the H/Ti atom ratio in region II. Only a small fraction of the total hydrogen content in the sample desorbs. A kinetic model is proposed for region II where it is assumed that a surface hydride many atomic layers thick is formed on the sample surface during reaction in region I. The observed desorption is attributed to a decrease in the hydrogen binding energy in the hydride when the H/Ti ratio approaches two. The rate limiting steps in the model are surface adsorption and desorption and diffusion over the surface hydride-bulk interface. In region III the absorption rate decreases and ceases when TiH 2 is formed. Addition of monolayer amounts of CO or O 2 strongly influences the absorption and desorption kinetics. Oxidation of partly hydrided samples virtually passivates the surface against further hydrogen absorption, but addition of small amounts of Ti on top of the oxide results in a recovery of the absorption capability. The oxide obviously strongly reduces the hydrogen dissociation rate but does not act as a diffusion barrier for atomic hydrogen.