X-ray photoelectron spectroscopic study of a pristine millerite (NiS) surface and the effect of air and water oxidation

Millerite. NiS, fractured under high vacuum and reacted with air and water has been analyzed by X-ray photoelectron spectroscopy (XPS). The pristine millerite surface gives rise to photoelectron peaks at binding energies of 853.1 eV (Ni 2p(3/2)) and 161.7 eV (S 2p), thus resolving ambiguities concerning binding energies quoted in the literature. Air-reacted samples show the presence of NiSO4 and Ni(OH)(2) species. There is evidence for polysulfide species (S-n(2-), where 2 less than or equal to n less than or equal to 8) on air-oxidized surfaces. These may occur in a sub-surface layer or may be intermixed with the Ni(OH)(2) in the oxidized layer. The NiSO4 species at the millerite surface occur as discrete crystallites whereas the Ni(OH)(2) forms a thin veneer covering the entire millerite surface. The NiSO4 crystallites form on the surface of millerite but not on surfaces of adjacent minerals. Surface diffusion of Ni2+ and SO42- across the millerite surface is thought to be responsible for the transport and subsequent growth of NiSO4 crystallites developed on millerite surfaces. Although it is clear that Ni and SO42- does not diffuse onto surfaces of adjacent minerals in sufficient quantity to form crystallites, the explanation is uncertain. XPS results for water-reacted surfaces show little difference from the vacuum fractured surfaces with the exception that minor amounts of polysulfide and hydroxy nickel species are present. Similar reaction products to those formed in air [NiSO4 and Ni(OH)(2)] are believed to be produced, but these are removed from the millerite surface by dissolution, leaving behind a sulfur-enriched surface (polysulfide) and hydroxyl groups chemisorbed to nickel ions at the millerite surface.