Review—Solution Electrochemical Process for Fabricating Metal Oxides and the Thermodynamic Design

Abstract

Electrochemical processes in aqueous solutions are widely used for preparation of metals, alloys, composites, metal oxides and compounds. For understanding and designing an electrochemical process, it is crucial to study its soluble chemical species, potential-pH diagram, and solubility curves drawn based on thermodynamics. In this review, equilibrium electrode potentials, critical pH values, and dissolved chemical species related to the oxidation-reduction, acid-base, and ligand-exchanging reactions, in addition to the calculation based on standard Gibbs free energy are first briefly mentioned. This is followed by the description of the change in equilibrium electrode potentials of metal and metal compounds as demonstrated in the electrochemical preparation of the Cu–In–Se precursor for the Cu(In,Ga)Se2 solar cell application. Additionally, the advantages and usefulness of soluble chemical species, potential-pH diagram, and solubility curves are discussed, by giving examples of direct electrodepositions of metal oxides, the chemical introduction of impurities into ZnO enabling characteristic control, the chemical bath deposition process (CBD) for Zn(S,O,OH) buffer layer in Cu(In,Ga)Se2 solar cell, and lastly, the design of the electrochemical process for fabricating CuO/Cu2O bilayers.