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

Abstract

The photocatalytic activity of mixed phases of GaOOH/β-Ga₂O₃ and α-Ga₂O₃/β-Ga₂O₃ for CO₂ reduction with water is investigated for the first time. GaOOH/β-Ga₂O₃ is synthesized by high energy ball milling of β-Ga₂O₃ in water, and α-Ga₂O₃/β-Ga₂O₃ are prepared by the calcination of GaOOH/β-Ga₂O₃ at 773 K. Both show higher activity for both CO₂ reduction and the accompanying H₂ evolution than the original material of β-Ga₂O₃. This is the first observation that the mixed phase of GaOOH/β-Ga₂O₃ has high photocatalytic activity. The H₂ production rate is correlated to the specific surface area irrespective of the GaOOH/β-Ga₂O₃ and α-Ga₂O₃/β-Ga₂O₃ samples. The CO production rate for the mixed phase of α-Ga₂O₃/β-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 α-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 H₂ production (H₂ evolution) over the samples of GaOOH/β-Ga₂O₃ and α-Ga₂O₃/β-Ga₂O₃, a redox type reaction mechanism is proposed, in which the H₂ evolution proceeds with the reduction of GaOOH to α-Ga₂O₃ emitting H₂ and O₂ by UV illumination, while α-Ga₂O₃ returned to GaOOH in water without illumination. Although some of the H₂ thus evolved would cause CO₂ reduction, CO₂ reduction to CO requires specific active sites on the α-Ga₂O₃ surface.