Cobalt oxide as photocatalyst for water splitting: Temperature-dependent phase structures

This study investigated the best phases of cobalt oxide for the photochemical and photoelectrochemical (PEC) water-splitting reaction. Cobalt oxide was produced via a hydrothermal process of cobalt nitrate hexahydrate and then annealed at different temperatures from 450 °C to 950 °C. The Co3O4 phase was produced during pre-annealing and annealing at 450 °C. The mixed phase of Co3O4 and CoO was produced during annealing at 550 °C and 650 °C, and pure CoO was produced during annealing from 750 °C to 950 °C. The Co3O4 phase produced the highest photocurrent density with a value of 1.15 mA cm−2 at a −0.4 V potential bias vs. Ag/AgCl. This value two times higher than that reported by other researchers at the same potential bias. Furthermore, the highest rate of hydrogen collected by Co3O4 was ~272.6 μmol h−1 g−1 after 8 h photocatalytic process. The amount of collected hydrogen was stable until 12 h of the process.