XPS study of hydrated MgCl2 impregnated on flat SiO2/Si(1 0 0) Mo and Au substrates

Hydrated MgCl2 was impregnated, on polycrystalline Mo and Au foils and on a Si(1 0 0) wafer covered with a 3 nm thick hydroxylated silica layer. Impregnation by spin-coating offers control over the loading and the structure of the deposit. X-ray photoelectron spectroscopy was used for the surface chemistry investigation of these systems at a molecular level. MgCl2 in aqueous solutions forms neutral as well as charged species. The nature of the substrate determines the kind of species attached, through exchange and condensation reactions or via electrostatic adsorption. At room temperature, MgCl2−x · nH2O, with x and n depending on the nature of the substrate, was detected on all the substrates as a result of condensation. On the negatively charged silica support, XPS indicated the existence of positively charged complexes. Weakly adsorbed species such as Mg(Cl)x(OH)2−x were identified on the gold surface whereas MgO was formed on the more reactive molybdenum surface.