Stimulating Protonation Capability by Eliminating Detrimental Defects in Crystalline Carbon Nitride for Photocatalytic Hydrogen Evolution

Abstract Ionothermal synthesis method often results in significant structural defects in the prepared crystalline carbon nitride due to insufficient penetration of the molten salt. Herein, a strategy involving the addition of a foaming agent (NH₄Cl) to the precursor is adopted, which significantly enhances the penetration of the molten salt during the ionothermal synthesis of crystalline carbon nitride. This results in a more uniform internal structural unit and fewer detrimental structural defects (terminal amine groups and hydrogen bonds formed by these groups) in crystalline carbon nitride due to the transformation from the triazine‐heptazine mixed phase to the heptazine phase. This improvement enables the photocatalyst, upon loading of cocatalysts, to maximally utilize photogenerated electrons and holes, establishing a smooth pathway for surface protonation and electron‐proton coupling. The results show that the photocatalytic hydrogen production rate reaches 8.67 mmol g ⁻¹ h ⁻¹ and exhibits a high apparent quantum efficiency of 22.1% (λ = 400 nm) for hydrogen evolution. This study elucidates the relationship between molten salt penetration and the crystal structure of carbon nitride during the ionothermal synthesis process. It also reveals the impact of these factors on photocatalytic hydrogen production from the perspectives of photogenerated carrier behavior and photocatalytic reaction kinetics.