Theoretical and Experimental Study of Cobalt Spinel Oxides for Solar Driven Hydrogen Production

Using sunlight and semiconductors to split water in a photoelectrochemical (PEC) process to generate hydrogen provides a viable way to produce renewable fuel. Metal oxide photoelectrodes are of particular interest due to their low cost and relatively high stability in aqueous media. Spinel Co-Fe-Al oxide system has shown the potential for PEC hydrogen production.  The electronic and optical properties of Co based spinel oxides, CoX₂O₄  (X =Al, Ga, In) are calculated using first-principle density functional theory. We show that the desirable properties for solar photoconversion can be obtained by controlling the alloy compositions. The ability to manipulate the electronic and optical properties is attributed to the different s-orbital energies and sizes of the cations. Our calculated bandgaps as function of composition provide detailed practical guidance for synthesizing Co based spinel oxides with electronic and optical properties necessary to achieve high efficiency PEC decomposition of water for hydrogen production by sunlight. Our experimental work demonstrated the tunable band gaps and the PEC responses of the Co based spinel oxides.