Photoelectrochemical properties of p-type CuBi2O4 prepared by spray pyrolysis of carbon-free precursor aqueous solution combined with post-annealing treatment

Energy conversion using semiconductor-based photoelectrodes for photoelectrochemical (PEC) water splitting is a promising technology that contributes to an environmentally friendly society. CuBi2O4 is a p-type semiconductor material that can be used in visible-light-responsive photocathodes. To date, the fabrication of CuBi2O4 has been examined using wet processes such as spray pyrolysis of precursor solutions containing organic solvents and organic additives. Spray pyrolysis causes problems such as CO2 emission during the decomposition of the precursor solution. In this study, we established a carbon-free method for the fabrication of CuBi2O4 using a precursor solution comprising Cu(NO3)2 and Bi(NO3)3 dissolved in a dilute nitric acid aqueous solution. The aqueous precursor solution was sprayed onto F-doped tin oxide-coated glass substrates, followed by post-annealing treatment in the temperature range of 400–700 °C for 1 h in air. An annealing temperature over 500 °C provides CuBi2O4 as revealed by the results of X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy characterization. The PEC properties of the CuBi2O4 photoelectrodes were investigated to determine the optimal post-annealing conditions for maximizing the photocurrent density under AM 1.5G solar illumination. The optimized CuBi2O4 photoelectrodes generated a photocurrent density of −0.94 mA cm−2 at 0.6 V vs. a reversible hydrogen electrode in a potassium phosphate aqueous solution with the H2O2 sacrificial reagent. This study provides rational guidelines for visible-light-absorbing p-type metal oxide-based photoelectrodes utilizing a carbon-free preparation process.