Photocatalytic properties of SrTiO3 doped with Na ion (Na+-SrTiO3) were investigated to develop the photocatalyst exhibiting high photocatalytic performances, not only the activity but also the durability for overall H2O splitting. The photocatalytic activity was remarkably improved by the introduction of a small amount of Na ion into SrTiO3 photocatalyst using rhodium and chromium mixed oxide (Rh0.7Cr1.3O3) as the co-catalyst. Moreover, the photocatalytic activity was influenced by the purity of the raw materials used for preparing Na+-SrTiO3 photocatalyst rather than the properties that originated with the preparation methods. The apparent quantum yield (AQY) of overall H2O splitting was 28% under the irradiation at 365 nm over the optimal Rh0.7Cr1.3O3/Na+-SrTiO3 photocatalyst. The photocatalytic performances of Rh(2-y)CryO(3)/Na+-SrTiO3 were confirmed to be further improved by separating the hydrogen evolution reaction (HER) site and the oxygen evolution reaction (OER) site over the surface by co-loading CoOx co-catalyst as OER sites. The states of Na+ doped SrTiO3 was investigated to make clear the key factors in the enhancement of the photocatalytic activity by the doping of aliovalent ions. (C) 2020 Elsevier Inc. All rights reserved.