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

Abstract

Molybdenum-rich solid solutions of CuWO₄ and CuMoO₄ (i.e., CuW_1−xMo_xO₄, x > 0.4) having a wolframite structure were prepared as thin-film type electrodes using a new electrochemical route. The synthesis of Mo-rich CuW_1−xMo_xO₄ was not previously achieved because the CuMoO₄ phase that is isostructural to CuWO₄ is not thermodynamically stable. The resulting solid solution, CuW_0.35Mo_0.65O₄, exhibited a significantly reduced optical bandgap (E_g = 2.0 eV), compared to CuWO₄ (E_g = 2.3 eV). Since both CuW_0.35Mo_0.65O₄ and CuWO₄ are n-type semiconductors, their photoelectrochemical properties were compared for possible use as photoanodes in water splitting photoelectrochemical cells. CuW_1−xMo_xO₄ 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.