Mixed phases of GaOOH/beta-Ga2O3 and alpha-Ga2O3/beta-Ga2O3 prepared by high energy ball milling as active photocatalysts for CO2 reduction with water

The photocatalytic activity of mixed phases of GaOOH/beta-Ga2O3 and alpha-Ga2O3/beta-Ga2O3 for CO2 reduction with water is investigated for the first time. GaOOH/beta-Ga2O3 is synthesized by high energy ball milling of beta-Ga2O3 in water, and alpha-Ga2O3/beta-Ga2O3 are prepared by the calcination of GaOOH/beta-Ga2O3 at 773 K. Both show higher activity for both CO2 reduction and the accompanying H-2 evolution than the original material of beta-Ga2O3. This is the first observation that the mixed phase of GaOOH/beta-Ga2O3 has high photocatalytic activity. The H-2 production rate is correlated to the specific surface area irrespective of the GaOOH/beta-Ga2O3 and alpha-Ga2O3/beta-Ga2O3 samples. The CO production rate for the mixed phase of alpha-Ga2O3/beta-Ga2O3 is high, agreeing with the previous observation that the mixed phases or phase boundaries enhance the photocatalytic activity, and the CO production rate increases with the abundance rate of the alpha-Ga2O3 phase. Although the selectivity of the CO production remains low, it could be enhanced if an Ag cocatalyst was used. As an alternative mechanism of H-2 production (H-2 evolution) over the samples of GaOOH/beta-Ga2O3 and alpha-Ga2O3/beta-Ga2O3, a redox type reaction mechanism is proposed, in which the H-2 evolution proceeds with the reduction of GaOOH to alpha-Ga2O3 emitting H-2 and O-2 by UV illumination, while alpha-Ga2O3 returned to GaOOH in water without illumination. Although some of the H-2 thus evolved would cause CO2 reduction, CO2 reduction to CO requires specific active sites on the alpha-Ga2O3 surface.