Synthesis, photoelectrochemical properties, and first principles study of n-type CuW1-xMoxO4 electrodes showing enhanced visible light absorption

Molybdenum-rich solid solutions of CuWO4 and CuMoO4 (i.e., CuW1-xMoxO4, x > 0.4) having a wolframite structure were prepared as thin-film type electrodes using a new electrochemical route. The synthesis of Mo-rich CuW1-xMoxO4 was not previously achieved because the CuMoO4 phase that is isostructural to CuWO4 is not thermodynamically stable. The resulting solid solution, CuW0.35Mo0.65O4, exhibited a significantly reduced optical bandgap (E-g = 2.0 eV), compared to CuWO4 (E-g = 2.3 eV). Since both CuW0.35Mo0.65O4 and CuWO4 are n-type semiconductors, their photoelectrochemical properties were compared for possible use as photoanodes in water splitting photoelectrochemical cells. CuW1-xMoxO4 showed enhanced photon absorption not only in the 2.0 eV <= E <= 2.3 eV region but also above 2.3 eV, compared to CuWO4, which directly resulted in enhanced photocurrent generation. Ab initio calculations were performed to understand the origin of the bandgap reduction by Mo incorporation. The calculations showed that the conduction band edge of CuWO4 is mainly composed of W 5d and O 2p hybrid orbitals. Therefore, when Mo atoms with 4d orbitals, which are lower in energy than W 5d orbitals, occupy W sites, the conduction band edge is shifted to lower energy. These results suggest that there may be many unexplored new compositions in ternary oxide systems that possess ideal bandgap energies for solar energy conversion.