Preparation of Ga2O3 photocatalyst highly active for CO2 reduction with water without cocatalyst

[Display omitted] •Ga2O3 photocatalysts are prepared by calcining Ga(NO3)3·8H2O at 673 1173 K in air.•The Ga2O3 photocatalysts promote photocatalytic CO2 reduction and H2O splitting.•CO2 reduction and H2O splitting proceed independently.•H2 production rate increases with Ga2O3 surface area but CO production rate does not.•γ, β mixed phase Ga2O3 shows the highest activity, even higher than Ag loaded ones. Ga2O3 samples were synthesized by calcination of gallium nitrate powder at a given temperature ranging from 673 K to 1173 K (denoted as T-Ga2O3 with T, calcination temperature). XRD and XAFS structural analyses revealed that samples calcined at temperatures below 673 K were consisted of defective ε phase, whereas those calcined above 773 K of β and γ phases, and β phase dominated above 923 K. All calcined Ga2O3 samples promoted photocatalytic CO2 reduction with water to produce CO, H2 and O2. As for the H2 production by water splitting, the H2 production rates were well correlated with the surface areas of calcined Ga2O3 samples. It was also found that the water splitting and the CO2 reduction proceeded fully independently. 823-Ga2O3 showed the highest activity for the CO2 reduction, suggesting that the coexistence of β and γ phases plays a key role on the photocatalytic CO2 reduction.