A nine-fold enhancement of visible-light photocatalytic hydrogen production of g-C3N4 with TCNQ by forming a conjugated structure

Abstract

Photocatalytic hydrogen evolution by water splitting has become a very effective way to solve the energy crisis. For use in that process, graphitic carbon nitride (g-C₃N₄) has drawn much attention for its response in the visible region. However, its insufficient sunlight absorption efficiency and easy recombination of photoinduced carriers restrict its photocatalytic activity. Herein, we demonstrate a two-step liquid ultrasonic method in water to synthesize a series of tetracyanoquinodimethane (TCNQ)–C₃N₄ photocatalysts aiming to form a conjugated structure by 7,7,8,8-TCNQ. g-C₃N₄ was treated with APTES firstly on its surface in order to give a better interface contact with TCNQ. Benefiting from the conjugation effect between TCNQ and g-C₃N₄, the separation and transport efficiency of photogenerated carriers were significantly improved. Besides, introducing TCNQ also broadened the absorption region. Both of these points lead to the enhancement of photocatalytic H₂ production rate, with the optimized 5% TCNQ–C₃N₄ giving a rate nearly 9.48 times that of pure g-C₃N₄. Also, 5% TCNQ–C₃N₄ (U) was prepared with unmodified g-C₃N₄, which exhibited a rate only 6.87 times that of pure g-C₃N₄, thus validating the necessity of surface modification. Our work reveals that the rational conjugated structure could modulate the electrical and optical properties of g-C₃N₄, yielding an improvement of photocatalytic activities.