Unveiling enhanced dark photocatalysis: Electron storage-enabled hydrogen production in polymeric carbon nitride

Abstract

Researchers have successfully replicated light-driven reactions of natural photosynthesis on semiconductors. However, an important problem limiting practical applications of these reactions is their dependence on light. Catalytic activity of the semiconductors is lost once light ceases, as the generation of charge carriers (i.e., electron-hole pairs) stops. In light of this problem, photosynthetic reactions in the dark are worthy of extra attention. Despite several reports on dark photocatalysis, the energy conversion efficiency remains low, and the mechanism is unclear. In this study, we developed an artificial photocatalytic system capable of decoupling the light and dark hydrogen production reactions. The system is composed of polymeric carbon nitride (PCN), in which electron storage sites are deliberately created. By tuning the number of the electron storage sites in the system, a record hydrogen production rate of 1480 μmol g−1 h−1 was achieved after termination of the visible-light (λ > 420 nm) illumination. In-situ spectroscopic techniques reveal that these electron reservoirs are composed of cyanamide groups capable of storing electrons. Moreover, these electron reservoirs can be excited and show surface plasmon resonance (SPR) effects, leading to the enhanced optical absorption. Importantly, the oxidation side of the dark photocatalytic reaction is found to stem from a cascade reaction involving hole-derived radicals rather than electron-generated oxygenic species reported in literature. We established a complete and stepwise dark photocatalytic reaction process as well as distinguished a rate-determining step.