Efficient solar photocatalytic hydrogen production using direct Z-scheme heterojunctions

We report the preparation of a series of heterojunctions made of Ta₃N₅ and TiO₂ nanoparticles that show good properties for photocatalytic hydrogen production. The composite photocatalyst with a light-response range up to 620 nm shows a hydrogen evolution rate of 250 μmol h^-1. The apparent quantum efficiency at 330 nm can be as high as 46%. Particularly, normalized spectral studies indicate that the heterojunction is more active upon full-spectrum (without using optical filters) irradiation, and its activity is even superior  to the total activity exhibited upon UV-light irradiation (λ ≤ 420 nm) and visible-infrared light irradiation (λ ≥ 420 nm). Moreover, in situ photodeposition of platinum nanoparticles on the surface of the photocatalyst as well as the band alignment analysis demonstrate the Z-scheme mechanism associated with the photocatalytic process. Specifically, photogenerated electrons from TiO₂ will rapidly combine with the photogenerated holes from Ta₃N₅ through interfacial charge transfer, leaving the more active electrons and holes in Ta₃N₅ and TiO₂, respectively, to facilitate redox reactions. Basically, TiO₂ is only UV-light active, while Ta₃N₅ can be activated under visible-light irradiation. In this case, a synergy effect, upon simultaneous UV-light excitation and visible-light excitation, can be achieved by full-spectrum irradiation, leading to a much higher photocatalytic activity. This work thus provides a favorable and upward direction for the establishment of heterojunctions for high-efficiency hydrogen production and solar energy applications.