Positioning cyanamide defects in g-C3N4: Engineering energy levels and active sites for superior photocatalytic hydrogen evolution

g-C3N4 has recently emerged as a promising photocatalyst for solar energy conversion. Nonetheless, attempts to enhance its inherently low activity are rarely based on precise molecular tunability strategy. In this study, two type cyanamide defects-grafting g-C3N4 (CCN) was prepared through the thermal polymerization of thiourea in the presence of KCl. Stable potassium isothiocyanate (KSCN) was in situ generated via thiourea isomerization and then reacted with different amino groups (-NH2 and =NH) in tri-s-triazine rings to obtain two-type cyanamide defects. Theoretical calculations and experiment results confirm that the ratio of the two-type cyanamide defects could be adjusted by KCl dosage, accompanying tunable energy levels of CCN. The charge carrier transfer and separation of CCN was greatly improved. Furthermore, the existence of cyanamide defects hindered the formation of intermolecular hydrogen bonds among g-C3N4, which facilitated the formation of porous structure and exposed more active sites for photocatalytic hydrogen evolution reaction (HER). As a result, the optimized photocatalyst (CCN-0.03) showed a high HER rate of 4.0 mmol g(-1)h(-1), which was 5 times higher than 0.8 mmol g(-1) h(-1) for pristine g-C3N4. And the apparent quantum efficiency reached up to 14.65% at 420 +/- 10 nm. The findings deepen the understanding on precise molecular tuning of g-C3N4.