Speciation Model of the Mo(VI)-Ni(II)-Citrate-S(VI)-N(III) Aqueous System for the Study of the Electrodeposition of Molybdenum and Nickel Oxides Films

A speciation model is proposed for the determination of the concentration of different species formed in an aqueous solution containing Mo(VI), Ni(II), citrate, ammonia and sulfate at 298 K, 105 Pa and variable pH and Mo(VI)/Ni(II) activity ratios. This solution is to be used as electrolyte in the electrodeposition process of thin films of Mo and Ni oxides, which appear to be a promising material for the fabrication of photo-anodes for water splitting in photo-electrochemical cells. The speciation model comprises 53 species and their stability constants, which are related through molar and charge balances to estimate the composition of the solution given a pH value and formal concentrations. As a result, predominance and distribution diagrams are produced, based on which electrolyte conditions (pH and central species concentrations) are recommended to maximize the availability of desired species for the electrodeposition process. Eh-pH diagrams for molybdenum and nickel species at the recommended activities (10−2 for Mo and Ni species, and 10−1 for citrate, ammonia and sulfate species) are also produced, to determine the adequate potential range to be applied for the electrodeposition of Mo and Ni oxides films.