Formation of thin oxide and sulphide films on polycrystalline molybdenum foils: characterization by XPS and surface potential variations

Abstract

By combining XPS and measurements of surface potential variation ΔΦ, the surface evolution of polycrystalline Mo sample has been studied during several processes of oxidation and sulphidation from room temperature up to 400°C. The chemical states of Mo, O and S have been identified from their core level binding energies, whereas the ΔΦ variation reflects changes in the work function during formation of the films. Oxidation by O2 at 400°C produces an oxygen-deficient MoO3 film which is further transformed by immersion in water with the formation of lower Mo oxidation states (mainly MoIV and MoIII). Direct immersion of the metal in water results in the progressive growth of a highly hydroxylated and hydrated oxide film with the predominance of a MoO2 structure after prolonged immersion. Thick Mo sulphide films (thickness >; 112 Å) are obtained by immersion of the foil in a thiomolybdate solution and further UHV annealing or by H2S sulphidation of the preoxidized metal. In the sulphided films, the presence of both S2− and S22− species has been detected, the distribution of which influences the ΔΦ variation according to: ΔΦ (mV) = 550 − 13.75[S22−(%)]. It has also been found that the ΔΦ values of the films decrease in the order: ΔΦ[MoO3] > ΔΦ[MoS2] > ΔΦ[MoS3] > ΔΦ[Mo0].