g‑C₃N₄ Hydrogen-Bonding Viologen for Significantly Enhanced Visible-Light Photocatalytic H₂ Evolution

Graphitic carbon nitride (g-C₃N₄) has recently emerged as a promising metal-free photocatalytic material for the conversion of solar energy into chemical energy under visible-light irradiation. Unfortunately, the photocatalytic activity of g-C₃N₄ is still unsatisfactory due to the serious recombination of photogenerated electron–hole pairs. Here, we develop a strategy to construct a type of g-C₃N₄-based composite photocatalyst (C₃N₄/CBV²⁺), a g-C₃N₄ surface coupled with a viologen redox mediator (1,1′-bis­(4-carboxylatobenzyl)-4,4′-bipyridinium dichloride, denoted as CBV²⁺) through hydrogen bonds, for enhanced H₂ production from water under visible-light irradiation. The CBV²⁺ molecules not only provide sites for metal particle formation but also act as an efficient electron transfer mediator to transfer the photoinduced electrons from g-C₃N₄ to platinum nanoparticles (Pt NPs). The vectorial charge transfer results in an efficient spatial separation of electrons and holes in the C₃N₄/CBV²⁺ composite photocatalyst and facilitates the photogenerated charge carriers for direct photocatalytic water splitting. When 1 wt % CBV²⁺ is introduced, the hydrogen production rate of C₃N₄/CBV²⁺ dramatically increases up to 41.57 μmol h^–1, exceeding 85 times the rate over unmodified g-C₃N₄ (only 0.49 μmol h^–1). It is noted that a negligible loss of photocatalytic activity was observed over continuous irradiation up to 20 h, demonstrating its good stability. The combination of the two emerging functional materials represents a simple but economical and powerful approach for highly effective photocatalytic hydrogen production under visible light irradiation. This study opens a window to rationally develop cost-acceptable materials for more efficient solar energy conversion applications