Cathodic electrodeposition of mixed molybdenum–selenium oxides

Molybdenum–selenium oxides were electrochemically deposited onto indium–tin oxide (ITO) coated glass substrates from aqueous solutions containing molybdate (MoVIO42-), selenate (SeIVO32-), and dimeric and tetrameric peroxo-polymolybdate (i.e., [Mo2O3(O2)4(H2O)2]2−, [Mo4O9(O2)4]2−) anions. Electrodeposition mechanisms were elucidated using chronocoulometry, cyclic voltammetry, spectroelectrochemistry, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electrochemical quartz crystal nanogravimetry (EQCN). At relatively positive deposition potentials from −0.1 V to −0.4 V (versus Ag/AgCl) a substoichiometric molybdenum oxide phase, Mo3O8, co-deposits with an insulating phase of Se0. At more negative potentials, mixed molybdenum–selenium oxides (MoxSe1−xOy, 0 < x < 0.4) are deposited. Competing side reactions involving hydrogen and hydrogen selenide influence the structure, composition, and morphology of the mixed molybdenum–selenium oxide deposits.