Water adsorption on polycrystalline vanadium from ultra-high vacuum to ambient relative humidity

We have studied the reaction of water vapor with a polycrystalline vanadium surface using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) which allows the investigation of the chemical composition of the vanadium/water vapor interface at p(H2O) in the Torr range. Water dissociation on the vanadium surface was studied under isobaric conditions at p(H2O) ranging from 0.01 to 0.50 Torr and temperatures from 625 K to 260 K, i.e. up to a relative humidity (RH) of ~ 15%. Water vapor exposure leads to oxidation and hydroxylation of the vanadium foil already at a pressure of 1 × 10- 6 Torr at 300 K (RH ~ 4 × 10- 6%). The vanadium oxide layer on the surface has a stoichiometry of V2O3. Initial adsorption of molecular water on the surface is observed at RH > 0.001%. Above a RH of 0.5% the amount of adsorbed water increases markedly. Experiments at increasing temperatures show that the water adsorption process is reversible. Depth profile measurements show a thickness for the vanadium oxide layer of 3-5 mono layers (ML) and for vanadium hydroxide of 1-1.5 ML over the whole RH range in the isobar experiments. The thickness of the adsorbed water layer was found to be in the sub-ML range for the investigated RH's.