Electron spectroscopic studies of galena and its oxidation by microwave-generated oxygen species and by air

Abstract

The reaction of (100) surfaces of PbS crystals (natural galena) with reactive oxygen species generated by microwave discharge in NO has been compared with aerial oxidation in studies using angle-resolved X-ray and ultraviolet photoelectron spectroscopies. Reaction with the discharge products was much faster, facilitating the study of this process in a controlled environment. The final products of the two processes were similar. Initial attack by atomic oxygen at Pb sites led to the formation of oxide species with the liberation of free sulphur, most of which was lost from the surface. Some, however, remained trapped under an oxidized layer. Sulphate species were also formed, probably by direct attack on PbS. Surfaces of good crystallinity (assessed via their photoelectron diffraction patterns) and chemical purity could be regenerated from oxidized surfaces by 800 eV argon-ion bombardment followed by annealing at ca. 350 °C. Core-level binding energies (E_b) for bulk PbS, PbSO₄, PbO, PbCO₃, PbS₂O₃ and 5Pb(OH)₂· Pb(NO₃)₂ were obtained via gold decoration and confirmed by reference to adventitious carbon. The E_b of the surface sulphate and oxide differed from those of the individual bulk lead salts, and they were inferred to coexist in an intimate mixture which probably also contained hydroxide. After the oxidized surfaces were heated to ca. 350 °C only the sulphate and some of the oxide remained.