Designing delafossite CuAl1-xTMxO2 solid solutions: the role of 3d transition metal spin states in photo(electro)catalytic performance

This study delves into the effects of 3d transition metal (TM) spin states on the structural and electronic properties of CuAlTMO solid solutions, focusing on their implications for photo(electro)catalytic performance. The research reveals that the Jahn–Teller distortion, associated with TM, is a critical factor in determining the lattice parameters and photo(electro)catalytic performances. Solid solutions without Jahn–Teller distortion adhere to Vegard's law, whereas those with strong distortion exhibit deviations, indicating the influence of TMO octahedral distortion on solubility and lattice parameters. The electronic structure of solid solutions with weak Jahn–Teller distortion is governed by the O–Cu–O and TMO crystal fields, which leads to a narrowed bandgap and reduced conduction band minimum (CBM), impacting the hydrogen evolution potential. In particular, the CuAlCrO shows a significant enhancement in photocurrent density and hydrogen production rate due to its balanced light absorption and effective charge carrier separation. In contrast, the weak Jahn–Teller distortion in CuAlFeO results in localized electronic states at CBM, leading to diminished carrier mobility. Solid solutions with strong Jahn–Teller distortion, such as CuAlTMO (TM = Mn and Ni), display a range of electronic properties from semiconductor to semimetallic, with the semimetallic CuAlMnO capable of infrared light absorption and efficient photocatalytic hydrogen and oxygen production.