Insight into the key role of proton-functionalization in metal-free C3N5 activated visible light system for efficient hydrogen evolution reaction

As a novel metal-free photocatalyst, g-C3N5 exhibits the narrower bandgap energy and wider visible light response range, which is beneficial for hydrogen evolution reaction compared to g-C3N4. However, the intrinsic features including the lower the separation rate of photogenerated carriers inhibit the further application for photocatalysis water splitting. Herein, the novel protonated-g-C3N5 was fabricated via the reaction between pristine g-C3N5 and nitric acid and further explore the optimal protonation time. The experimental results demonstrate that after the treatment of nitric acid, the specific area surface of g-C3N5 was significantly increased due to the etching effect of acids, which is contributed to provide more reactive sites for enhanced the photocatalytic reaction rate. The superior photocatalytic hydrogen evolution efficiency belonging to the protonated g-C3N5 was exhibited 3.68 times than that of pristine g-C3N5 under the visible light irradiation due to the introduction of protons. The increased photocatalytic activity was attributed to the promoted reduction potential and carrier separation efficiency. Based on five consecutive cycling tests, the prepared samples display the excellent recyclability and stability and the features of the obtained samples were maintained after the cycling experiments. This work provided a facile and low-cost functionalized modification approach for graphitic carbon nitride to achieve higher hydrogen evolution driven by visible light.