Tailoring the electronic properties of g-C3N4 via D-π-A framework for metal-free photocatalytic CO2 conversion and H2 evolution

Tuning the optical and electronic properties of graphitic carbon nitride (g-C 3 N 4 ) is an effective strategy for improving solar-light utilization and charge carrier dynamics. Herein, sulfur self-doping was employed to systematically modulate the optoelectronic characteristics of g-C 3 N 4 yielding sulfur-doped carbon nitride (SCN) using thiosemicarbazide as a sulfur source. Subsequently, 2,6-diaminopyridine (DAP) was molecularly incorporated into SCN to construct a donor−π−acceptor (D−π−A) type architecture. DAP incorporation induces an asymmetric electronic environment, promotes charge separation, and suppresses charge-carrier recombination, as indicated by photoluminescence analysis. It also enhances the π-conjugation and broadens the optical absorption range, facilitating improved solar harvesting. The optimized 7 mg DAP-doped SCN (7 DSCN) achieved a CH 4 production rate of 0.13686 μmol⋅g −1 ⋅h −1 , through photocatalytic CO 2 reduction, approximately 5-fold and 1.5-fold higher than bulk g-C 3 N 4 (BCN) and the SCN respectively. Under outdoor solar light, 7 DSCN delivers an H 2 evolution rate of 169 μmol⋅g −1 ⋅h −1 representing 8.4- and 5.5-fold enhancements over BCN and SCN, respectively. Moreover, 7 DSCN exhibits exceptional photostability, producing 8092 μmol⋅g −1 of H 2 accumulated across 13 consecutive days under mid-day peak-hour solar-light irradiation for 4 h per day. This study demonstrates molecular engineering as a promising strategy for developing stable carbon nitride photocatalysts for solar-driven CO 2 conversion and H 2 production.