Boron doping induced charge transfer switching of a C3N4/ZnO photocatalyst from Z-scheme to type II to enhance photocatalytic hydrogen production

Heterojunction photocatalysts are very promising for solar hydrogen production due to their high efficiency in photo-driven charge generation and separation. A C3N4/ZnO heterostructure nanocomposite harvests a wide range of solar light from the UV and visible regions and retains a high redox potential due to its Z-scheme band structure. However, since both C3N4 and ZnO have sufficiently high conduction band energies to drive hydrogen photoreduction, a type II heterojunction is more beneficial for enhancing the hydrogen production efficiency in the current system. In this study, we first demonstrated the charge transfer mechanism switching from the Z-scheme to type II by simple boron (B) doping of C3N4/ZnO. The doping of C3N4 with low-electronegativity boron increases its Fermi level by 0.4 V, making it even higher than that of ZnO. As a result, the Fermi level alignment of B-doped C3N4 with ZnO causes a reversed band bending direction at the C3N4/ZnO junction. The resultant charge transfer switching from the Z-scheme (C3N4/ZnO) to type II (B-doped C3N4/ZnO) was confirmed by UPS and ESR analysis. Type II B-doped C3N4/ZnO shows a stable, drastic increase in the photocatalytic hydrogen evolution rate, approximately 2.9 times higher than that of undoped C3N4/ZnO. The decreased bandgap energy of B-doped C3N4/ZnO also contributes to an additional improvement in efficiency through enhanced light harvesting. Our work presents a simple but effective strategy to design highly capable heterojunction photocatalysts via charge transfer switching with a doping method.