Ultra-high vacuum deposition of Na on SnS2 in single crystal and powder forms:: evidence of a decomposition reaction

The influence of particle shape on the reactivity of SnS2 towards Na atoms under ultra-high vacuum (UHV) conditions and at room temperature was investigated by X-ray photoelectron spectroscopy (XPS) in combination with atomic force microscopy (AFM). The study was carried out on single-crystal and powdered samples. Na core-level spectra indicate that the alkali metal is deposited as a highly ionic single species onto the surface. However, shifts in the photoelectron binding energies of the substrates upon Na deposition are insignificant. In both substrates, deposited Na on the crystal surface migrates to the bulk material; however, Na diffusion in polycrystalline SnS2 is enhanced by the smaller particle size of microcrystals and their random orientation. Also, the relative intensities of S and Sn peaks undergo major changes in polycrystalline SnS2. The most widely accepted model for the reaction between the alkali metal and the layered sulfide, an intercalation reaction, call only be assumed for the single-crystal sample. By contrast, a decomposition reaction prevails in the polycrystalline compound. AFM images clearly show that Na deposition on single crystals results in significant deterioration of the surface upon contact with the atmosphere. (C) 1999 Elsevier Science B.V. All rights reserved.