To realize practical applications of the photocatalysis technique, it is necessary to synthesize semiconductor photocatalysts with specific facets that induce high reactive activities and high reactive selectivity. However, a challenge lies in the synthesis of metal oxides containing more than one type of metal with specific facets. Usually, surfactants are used to control the crystal morphology, which induces surface contamination for the final products. Here, using the GaOOH nanoplate as precursor, ZnGa2O4 nanocubes with exposed {100} facets are synthesized by an hydrothermal ion-exchange route without requiring the introduction of morphology controlling agents. These ZnGa2O4 nanocubes exhibit improved performance in the photoreduction of CO2 into CH4 or water splitting into hydrogen. Theoretical calculations indicates that the light-hole effective mass on the {100} facets of ZnGa2O4 corresponds to the high hole mobility, which contributes to the efficient water oxidation to offer the protons for promoting CO2 photoreduction into hydrocarbon fuels.