A photoemission study of solute-solvent interaction: Coadsorption of Na and H2O on WSe2 (0001)

Semiconductor/electrolyte model interfaces are prepared in UHV by adsorbing and coadsorbing the electrolyte species Na and H2O onto chemically inert v an der waals (0001) surfaces of WSe2 at low temperatures (100K). The state of the adsorbed species is monitored by soft X-ray photoelectron spectrosco py (SXPS) and the surface potentials, i.e., band bending eV(bb), surface ph otovoltage SPV, electron affinity chi, and work function Phi by SXPS and ul traviolet photoelectron spectroscopy (UPS). H2O coadsorbed to metallic Na c lusters is dissociated and electron emission lines due to Na+ and OH- ions are observed. Coadsorption of Na to condensed ice layers preserves the H2O molecules. The 3s electrons of the adsorbed Na atoms are transferred to the p-WSe2 substrate as indicated by a depletion space charge layer on p-WSe2. With increasing exposition to SX radiation, the 2p emission of the Na+ ion s is stepwise shifted to lower binding energy. We relate this successive sh ift to an increasing number of reorganized H2O molecules from the adsorbate layer in its ice configuration to the hydration shell of Na+.