Defect and Crystallinity-Mediated Charge Separation in Carbon Nitride for Synergistically Boosted Solar-Driven Hydrogen Evolution

\Defect design and crystallinity engineering can greatly promote the charge carrier transfer and separation of carbon nitride (CN). Herein, we develop an efficient strategy for synthesizing a defect-rich crystalline CN with both improved surface and bulk carrier separation for superior solar-driven hydrogen evolution. This process is realized by the rationally designed CN photocatalyst with high bulk crystallinity and abundant surface nitrogen defects via a two-step alkali treatment coupled to a molten salt method. Molten salt synthesis yields highly crystalline carbon nitride and substantially promotes bulk photocarrier separation. The following KOH treatment creates abundant nitrogen vacancies and cyano groups on the surface of CN while preserving high crystallinity in the bulk, which act as effective electron sinks to trap photogenerated electrons, leading to promoted surface and bulk carrier separation and accelerated charge migration from the bulk to surface. Through tailoring the KOH concentration, the levels of nitrogen defects can be flexibly tuned. As a result, the overall photocarrier separation is promoted by the synergistic action of high crystallinity and a surface defect structure. The optimally modulated photocatalyst achieves a remarkably boosted hydrogen production rate by 4.7 times compared to pristine bulk CN under visible light irradiation and maintains high activity after a prolonged reaction of 35 h. This study provides a facile and efficient route to facilitate carrier dynamics of CN and helps us to understand the relationship between defect, crystallinity, and photocatalytic activity.