Work Function Investigations on Nickel Surfaces in Dynamic Equilibrium with Water Vapor Covering the Pressure Range 10−3 — 10 Pa

The interaction of water vapor with clean and oxidized surfaces of polycrystalline Nickel was studied at temperatures from 300 K to 430 K. — The changes of the work function Δθ were recorded as a function of the interaction time of water with these surfaces. Immediately after starting water exposure a steep decrease of the work function was observed at all temperatures studied. The amount of this initial decrease depends on the H2O partial pressure. The subsequently observed increase of the work function was found to be mainly temperature dependent with a typical time constant of a few minutes. — Determining the relative stability of the H2O molecules and their dissociation products the influence of the substrate temperature must be emphasized. Size and sign of Δθ in the equilibrium state change due to variation of the substrate temperature from 320 K to 435 K. — The observed time dependence of the work function was attributed to a two step dissociation process. Its slight increase was related to the dissociation of OHad and to the beginning of oxide layer formation. This work function increase is a thermally activated process with an activation energy of 61.8 kJ/mol and preexponential factor of 6.3 · 106 s−1.