Delafossite-alloy photoelectrodes for PEC hydrogen production: A density functional theory study

Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectrodes to have certain electronic properties. Unfortunately, at present available semiconductors do not meet all these criteria. So, a thorough understanding of band-engineering for mixed alloys is necessary to successfully design these photoelectrodes. Among the semiconductors, transition metal oxides are of particular interest due to their low cost and relatively high stability in aqueous media. Here, we will present a theoretical study of delafossite-alloys for PEC photo-electrodes. Previous studies have indicated that the group IIIA delafossite family (CuMO₂, M = Al, Ga, In) do not exhibit direct band gaps. Their fundamental band gaps are significantly smaller than their reported optical band gaps. On the other hand group IIIB delafossite family (CuMO₂, M = Sc, Y, La) in general show direct band gaps and, except for CuLaO₂, band gaps are above 3.00 eV. However, both of these two families exhibit p-type conductivity. We will show that by appropriate alloying of these two delafossite-families we can tune their band gaps and other opto-electronic properties. These types of alloying are desirable, as these introduce no localized impurity states in the band gap due to isovalent alloying. Also, the electronic effective masses can be lowered by selective doping of main group elements. Finally, it will be discussed that, lowering the symmetry constraints of these alloys would enhance their optical absorption properties. We'll also discuss that alloying with other 3d metal elements may decrease the band gap, but would increase the effective masses of the photo-electrons.