Graphitic carbon nitride modified by methyl viologen and an I3−/I− carrier relay for improved photocatalytic hydrogen evolution

Abstract

Graphitic carbon nitride (g-C₃N₄, known as CN) has attracted interest as an unassuming polymer semiconductor capable of harnessing visible light for hydrogen (H₂) production. However, the fast recombination of photoinduced electrons and holes (e⁻/h⁺) remains a substantial problem. Here, we develop an innovative strategy involving the introduction of iodine pairs (I₃⁻/I⁻) as a hole relay in the CN structure (CN-KI₃-KI). In addition, a built-in electric field is introduced by integrating cationic methyl viologen ions (1,1′-dimethyl-4,4′-bipyridinium dichloride hydrate, MV²⁺) to establish an auxiliary electron-transfer channel over CN-KI₃-KI, named CN–I₃⁻/I⁻-MV²⁺ nanocomposites, for the efficient generation of H₂. The MV²⁺ molecules serve as a hot electron acceptor and function as an effective mediator for electron migration, finally transferring the photogenerated electrons from MV⁺˙ to platinum for H₂ production from H⁺. The introduction of I³⁻/I⁻ and MV²⁺ redox mediators leads to an efficient spatial separation of e⁻/h⁺ in the CN–I₃⁻/I⁻-MV²⁺ composite photocatalyst, significantly boosting photocatalytic hydrogen production. The H₂ production rate of CN–I₃⁻/I⁻-MV²⁺ rises to 136.67 µmol h⁻¹ upon the addition of 1 wt% MV²⁺, surpassing 31.63 and 4.1 times the rate over CN (4.32 µmol h⁻¹) and CN-KI₃-KI (33.33 µmol h⁻¹), respectively. A minimal decline in photocatalytic activity is observed after continuous illumination for up to 16 hours, indicating its excellent stability. The combination of two emergent functional molecules as an electron–hole relay provides a straightforward yet cost-effective and potentially effective method for excellent photocatalytic hydrogen generation under visible light irradiation. This study presents an innovative methodology for developing economically feasible materials that significantly improve solar-to-fuel conversion applications.