This study investigated the best phases of cobalt oxide for the photochemical and photo-electrochemical (PEC) water-splitting reaction. Cobalt oxide was produced via a hydrothermal process of cobalt nitrate hexahydrate and then annealed at different temperatures from 450 degrees C to 950 degrees C. The Co3O4 phase was produced during pre-annealing and annealing at 450 degrees C. The mixed phase of Co3O4 and CoO was produced during annealing at 550 degrees C and 650 degrees C, and pure CoO was produced during annealing from 750 degrees C to 950 degrees 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 similar to 272.6 mu mol h(-1) g(-1) after 8 h photocatalytic process. The amount of collected hydrogen was stable until 12 h of the process. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.