Electron transfer reactions on Cs/MoS2(0002) with chlorine, oxygen, and water: High resolution x-ray photoelectron spectroscopy and theoretical study

Abstract

Supra-valence electron transfer from surface Cs-doped MoS2(0002) to electron acceptor adsorbates was investigated by high resolution x-ray photoelectron spectroscopy (HRXPS) in the valence band region and above the valence band maximum (VBM). Deposition of a sub-monolayer amount of Cs onto the basal plane of MoS2 introduced a new electron density of states at ca. 1.25 eV above VBM. Angle-resolved HRXPS and theoretical analysis located this electron density in the MoS2 layer. Upon the reaction with Cl2, this Cs-induced photoelectron almost completely disappeared and the Cs 3d and Cl 2p core levels indicated the formation of a surface Cs-chloride species. The Cs-covered MoS2(0002) surface also reacted with O2 to form surface peroxides and superoxides, as evidenced by two distinct binding energies of the O 1s core level peaks. However, the reaction with water proved to be more difficult: Exposure of the Cs-covered MoS2(0002) surface to H2O at 10−5 Torr did not result in electron transfer reaction, but the Cs/MoS2(0002) surface exposed to H2O at 1 Torr showed a substantial decrease in the density of states above VBM as well as formation of a surface-hydroxide, indicated by the O 1s core level position. Theoretical calculations using a full-potential linearized augmented plane wave density functional theory (FLAPW-DFT) confirm the conclusion based on experimental intensity anisotropy of the new peak: the Cs 6s electron transfers into the MoS2 substrate, forming the Cs/MoS2 electron donor–acceptor complex with Csδ+. In addition, all phenomena observed during the adsorption of electron donor–acceptor molecules are quantitatively accounted by the theory.