SXPS analysis of passivation and complexation on the CdS (101¯ 0) surface

CdS(101¯ 0) surfaces were chemically modified by oxidation in air or UV/ozone, sulfurization after treatment with a Br₂-methanol solution and reaction with the multidendate organic reactant 2-mercaptobenzothiazole (MBT). Surface sensitive photoelectron spectra from tuned excitation using synchrotron radiation (SXPS) show that initial oxidation by air gives monolayer coverages of sulfate (with minor contributions of sulfite) and chemisorbed polysulfur species passivating the surface. In contrast to this behavior the oxidation by UV/ozone proceeds very fast, giving thick sulfate layers. Thick layers of elemental sulfur are created during Br₂-methanol treatment. SXPS results show that a monolayer of chemisorbed polysulfide between the sulfur coverage and the sulfide is only removed by annealing the substrate up to 500 K. The reaction of MBT with the CdS surface is photochemically activated giving a submonolayer of MBT/adsorbate complex covered by bis(2-benzothiazolyl)disulfide (BBTD). A scheme of the photoinduced oxidation mechanism creating the disulfide is proposed.