Low energy electron diffraction and electron spectroscopic studies of the oxidation and sulphidation of Pb(100) and Pb(110) surfaces

Abstract

The interaction of oxygen and hydrogen sulphide with Pb(100) and Pb(110) surfaces has been studied by X-ray photoelectron spectroscopy (X.p.s.) and low energy electron diffraction (l.e.e.d.). In the temperature range 90-295 K, oxygen interaction with both crystal planes gave rise to an O(1s) peak of binding energy 529.5 eV and a full width at half maximum height of only 1.4 eV. Both were essentially invariant during oxidation. The Pb 4f7/2 peak on the other hand increased in width after an oxygen exposure of only 100LJ and with further oxygen exposure a distinct and new peak developed at 1 eV higher binding energy. This is indicative of oxide formation. The l.e.e.d data indicate the growth of orthorhombic PbO on both Pb(100) and Pb(110), the oxide being formed as four domains with dimensions very close to that of bulk PbO ortho , the essential difference being that the domains are not precisely rectangular (α = 89°). This structure does not change during oxide growth. A similar analysis accounts for the diffraction data observed with Pb(110). A nucleation model is pro¬posed to account for the particular growth behaviour observed. Water vapour does not interact with atomically clean Pb(110) surfaces at pressures up to 10 -3 Pa at 295 K. Hydrogen sulphide does not adsorb on Pb(100) at 295 K but prior oxidation of the surface results in facile dissociation of the molecule with subsequent surface sulphidation. We suggest that the activation energy for sulphidation is lowered due to the strong oxygen-hydrogen interaction; the oxygen is subsequently removed and desorbs as water. The sulphide structure is similar to that observed when the atomically clean Pb(100) surface is sulphided at 420 K. The particular orientation at which the two square domains grow, ±3.5° with respect to the [011] direction of the Pb(100) surface, is explained in terms of nucleation of the (100) plane of PbS at steps in the Pb(100) surface.