Dynamics of Photogenerated Holes in TiO₂-Polyheptazine Hybrid Photoanodes for Visible Light-Driven Water Splitting

Abstract

The dynamics of visible-light photogenerated holes in nanocrystalline TiO2-polyheptazine (TiO2-PH) hybrid photoelectrodes for water photooxidation was investigated by polychromatic and wavelength-resolved photocurrent measurements. The evaluation of the hole reactivity was addressed by direct comparison to photoelectrodes based on pristine TiO2. The visible-light generated holes in TiO2-PH are located in the thin polyheptazine ("graphitic carbon nitride") layer at the surface of TiO2 and possess a lower oxidation potential (by ∼0.9 V) as compared to UV light-photogenerated holes in pristine TiO2. Due to their slow water oxidation kinetics, the photoholes accumulate at the surface, which leads to negligible oxygen evolution and increased recombination. This problem can be overcome by introducing a suitable co-catalyst (IrO2 nanoparticles), as evidenced by dioxygen evolution under visible light (λ > 420 nm) irradiation.