Bandgap Reduction and Enhanced Photoelectrochemical Water Electrolysis of Sulfur-doped CuBi2O4 Photocathode

Abstract

As interest in hydrogen energy grows, eco-friendly methods of producing hydrogen are beingexplored. CuBi2O4 is one of the p-type semiconductor cathode materials that can be used forphotoelectrochemical hydrogen production via environment-friendly water electrolysis. CuBi2O4 has abandgap of 1.5 – 1.8 eV which allows it to photogenerate electrons and holes from the absorption of visiblelight. This study investigated the effect of sulfur doping on the bandgap and photoelectrochemical waterreduction properties of CuBi2O4. Sulfur-doped CuBi2O4 thin films were electrochemically synthesized usinga nitrate-based precursor solution with thiourea. This was followed by two-step annealing in an Aratmosphere, which effectively prevented the oxidation of sulfur. Sulfur doping up to 0.1 at% led to expansionof the lattice volume of the CuBi2O4. The bandgap was reduced from 1.9 eV to 1.5 eV with increasing dopingconcentration, which resulted in the enhancement of photoelectrochemical current density by ~240%. X-rayphotoelectron spectroscopy showed that sulfur-doping reduced oxygen vacancies with increasing dopingconcentration, confirming that the enhanced photoelectrochemical properties resulted from the reduction inbandgap, not from any extrinsic factor such as oxygen vacancies. Further studies of sulfur-doped CuBi2O4 toimprove surface coverage are expected to lead to a more promising photoelectrochemical cathode material.