Assembly of g-C₃N₄-based type II and Z-scheme heterojunction anodes with improved charge separation for photoelectrojunction water oxidation

Graphitic carbon nitride (g-C₃N₄) has been widely studied as a metal-free photocatalyst, leading to some excellent results; however, the rapid recombination of photogenerated charge carriers substantially limits its performance. Here, we establish two types of g-C₃N₄-based heterojunction (type II and nonmediator assisted Z-scheme) photoanodes on a transparent conducting substrate via coupling with rod-like and nanoparticulate WO₃, respectively. In these composites, g-C₃N₄ film grown by electrophoretic deposition of exfoliated g-C₃N₄ serves as the host or guest material. The optimized type II WO₃/g-C₃N₄ composite exhibits an enhanced photocurrent of 0.82 mA cm^-2 at 1.23 V vs. RHE and an incident photo-to-current conversion efficiency (IPCE) of 33% as compared with pure WO₃ nanorods (0.22 mA cm^-2 for photocurrent and 15% for IPCE). Relative to pure g-C₃N₄ film (with a photocurrent of several microampere and an IPCE of 2%), a largely improved photocurrent of 0.22 mA cm^-2 and an IPCE of 20% were acquired for the Z-scheme g-C₃N₄/WO₃ composite. The enhancement can be attributed to accelerated charge separation in the heterointerface because of the suitably aligned band gap between WO₃ and g-C₃N₄, as confirmed by optical spectroscopy and ultraviolet photoelectron spectroscopy (UPS) analysis. The photocatalytic process and mechanism of the g-C₃N₄-based heterojunctions are proposed herein, which potentially explain the origin of the enhanced photoelectrochemical performance. This achievement and the fundamental information supplied here indicate the importance of rationally designing heterojunction photoelectrodes to improve the performance of semiconductors. This is particularly important for materials such as pure g-C₃N₄ and WO₃, as their photoactivities are strongly restricted by high recombination rates.