Core-shell double doping of Zn and Ca on β-Ga₂O₃photocatalysts for remarkable water splitting

Photocatalytic water splitting reaction attracts considerable attention owing to their potential application to generate H2 gas from H2O by using solar energy. However, further activity enhancement is indispensable for industrial use. Various approaches have been adopted to improve its activity; however, as these defects and impurities are the main obstacles that reduce quantum efficiency (QE), the fabrication of fine crystals with low defects and impurities has been essential for activity improvement. Here, we found that Zn and Ca core-shell double doping in polycrystalline β-Ga2O3 photocatalysts is very effective to enhance the photocatalytic activity, and QE reaches 71% under 254 nm illumination. Time-resolved IR absorption spectroscopy and first-principles calculations revealed that Zn and Ca create shallow mid-gap states, and electron trapping at these states prevent the electron-hole recombination. STEM-EDS mapping analysis demonstrate that Ca is doped uniformly in the bulk, but Zn is doped on the surface. These findings herein indicate that the induced concentration gradient of the dopants effectively inhibit the recombination in the bulk and at the surface and assist the diffusion of trapped electrons from the bulk to the surface, thereby accelerating reactions at the surface. These cooperative effects provide an attractive strategy to enhance the photocatalytic activity, which can be applied to many other photocatalysts including rough polycrystalline powders. This method requires neither the fabrication of fine single crystals nor the precise control of the cocatalyst loading.