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 being explored. CuBi<sub>2</sub>O<sub>4</sub> is one of the p-type semiconductor cathode materials that can be used for photoelectrochemical hydrogen production via environment-friendly water electrolysis. CuBi<sub>2</sub>O<sub>4</sub> has a bandgap of 1.5 – 1.8 eV which allows it to photogenerate electrons and holes from the absorption of visible light. This study investigated the effect of sulfur doping on the bandgap and photoelectrochemical water reduction properties of CuBi<sub>2</sub>O<sub>4</sub>. Sulfur-doped CuBi<sub>2</sub>O<sub>4</sub> thin films were electrochemically synthesized using a nitrate-based precursor solution with thiourea. This was followed by two-step annealing in an Ar atmosphere, which effectively prevented the oxidation of sulfur. Sulfur doping up to 0.1 at% led to the expansion of the lattice volume of the CuBi<sub>2</sub>O<sub>4</sub>. The bandgap was reduced from 1.9 eV to 1.5 eV with increasing doping concentration, which resulted in the enhancement of photoelectrochemical current density by ~240%. X-ray photoelectron spectroscopy showed that sulfur-doping reduced oxygen vacancies with increasing doping concentration, confirming that the enhanced photoelectrochemical properties resulted from the reduction in bandgap, not from any extrinsic factor such as oxygen vacancies. Further studies of sulfur-doped CuBi<sub>2</sub>O<sub>4</sub> to improve surface coverage are expected to lead to a more promising photoelectrochemical cathode material.