Kinetic Evidence for Type-II Heterojunction and Z-Scheme Interactions in g-C₃N₄/TiO₂ Nanotube-Based Photocatalysts in Photocatalytic Hydrogen Evolution

Composite materials based on g-C 3 N 4 and TiO 2 nanotubes have been synthesized as environmentally friendly photocatalysts with heterojunctions suitable for enhanced photocatalytic hydrogen production. Composites were prepared with various ratios ( x = 0–1) of g-C 3 N 4 and after chemical modification and exfoliation of bulk g-C 3 N 4 . Contact formation between g-C 3 N 4 and TiO 2 generally enhanced photoactivity, which caused the x -dependent changes in the photocatalytic hydrogen evolution rates of the g-C 3 N 4 /TiO 2 compounds to vary following a volcano-shaped curve with the maximum rate at x ∼ 0.6 for all the compounds regardless of the pretreatment (bulk or modified) of g-C 3 N 4 . The modified g-C 3 N 4 -based composites showed higher photoactivities than the unmodified bulk g-C 3 N 4 due to the high surface area. The major reason for the enhanced photoactivity with the volcano shape was attributed to the Z -scheme interaction at the heterojunction. Interestingly, detailed analysis of the kinetic H 2 evolution rates of the composites with Pt cocatalysts only on TiO 2 nanotubes further showed that the dominant type of interaction at the heterojunctions changed from the type-II heterojunction to the Z -scheme at x ∼ 0.1. It is inferred that structural diversity at the g-C 3 N 4 /TiO 2 interfaces is the origin of the changes in the dominant type of interaction in the composites with increasing ratios of g-C 3 N 4 .