Enhanced visible light hydrogen production via a multiple heterojunction structure with defect-engineered g-C₃N₄ and two-phase anatase/brookite TiO₂

Polymeric g-C₃N₄ is a promising candidate for solar hydrogen production. However, its hydrogen production rate is low when used alone due to fast recombination of photogenerated electron–hole pairs. In this paper, we report much improved hydrogen production by coupling g-C₃N₄ with two-phase anatase/brookite TiO₂ nanoparticles to form multiple heterojunctions. Results have shown that under visible light illumination, photogenerated electrons transfer from g-C₃N₄ to TiO₂. In addition, systematic comparison was carried out among different type of heterojunctions, viz., g-C₃N₄ coupled with a single phase of TiO₂ (anatase or brookite), dual-phase TiO₂ (anatase/brookite or anatase/rutile), or a three-phase TiO₂ (anatase/brookite/rutile) mixture. g-C₃N₄ with two-phase anatase/brookite TiO₂ produces the largest amount of hydrogen under visible light illumination. The comparison reveals two important factors behind photocatalytic hydrogen generation: effective charge transfer and the conduction band potential position. The band edge positions of all the constituent phases of the heterojunction have to be more cathodic than the hydrogen reduction potential in order to realize the full benefit of effective charge separation.