Enhanced photocatalytic performance of g-C3N4 by introducing gradient energy band structure and activated n-π* electronic transition for visible light hydrogen generation

Abstract

The g-C3N4-based materials (CNs) are a promising hydrogen generation photocatalyst for solar energy conversion. To further improve visible light photocatalytic activity of the CN photocatalysts, this study proposes a multi-step thermal decomposition process to design the optimized CN photocatalysts. The nanostructure, photoelectric behavior, band energy structure, and corresponding visible light photocatalytic activity of CN photocatalysts were systematically investigated. To enhance visible light response, the inactive n-π* electronic transitions were activated by breaking the symmetrical planar structure of the heptazine units using a two-step thermal decomposition process. The doping K+ into the CN structure resulted in a redshift of π-π* electronic transitions by co-thermal treatment of the CN with KCl, which expands the visible light response range by enhancing the planar delocalization of π electrons in the CN structure. A gradient K+ doped CN was prepared by controlling the K+ doping reaction conditions, which introduces a gradient band energy structure. The gradient band energy structure significantly improves the separation efficiency of photogenerated charge carriers and charge mobility to the photocatalyst surface. Therefore, the optimized photocatalyst was designed by combining the synergistic effect of the activated n-π* electronic transitions, the redshift of π-π* electronic transitions, and charge separation effect of the gradient band energy structure. An excellent CN photocatalyst with a 70 times higher visible light photocatalytic activity for the H2 production (382 μmol/L) than that prepared by traditional thermal decomposition process was achieved by using the synergistic effect. This study provides a new approach to optimize the light absorption and charge carrier separation for the CN photocatalysts.