Molten salt-assisted synthesis of nitrogen-vacancy crystalline graphitic carbon nitride with tunable band structures for efficient photocatalytic overall water splitting

Nitrogen-vacancy crystalline g-C 3 N 4 nanosheets with tunable band structures were successfully prepared by alkali-molten salt-assisted method. The excellent and stable photocatalytic overall water splitting activity with H 2 and O 2 evolution rate of 49.60 μmol g -1 h -1 and 24.71 μmol g -1 h -1 was obtained over g-C 3 N 4 -D2 which loaded Pt and Co 3 O 4 nanorods as cocatalysts (Pt/g-C 3 N 4 -D2/3%Co 3 O 4 NRs) under AM 1.5G simulated light irradiation. In the half-reaction experiments, the maximum H 2 evolution rate of Pt/g-C 3 N 4 -D2/3%Co 3 O 4 NRs is 3.78 mmol g -1 h -1 with a notable quantum efficiency of 11.94% at 400 nm, and the solar-hydrogen conversion efficiency (STH) is 1.48%. • Nitrogen-vacancy crystalline g-C 3 N 4 was prepared via molten-salt-assisted method. • Vacancy and highly-crystalline can efficiently promote charge separation. • Matching of HER/OER via Pt and Co 3 O 4 NRs further improve charge carrier separation. • g-C 3 N 4 -D2/3%Co 3 O 4 has high H 2 evolution rate with marked AQE(11.94%) and STH(1.48%). • Pt/g-C 3 N 4 -D2/3%Co 3 O 4 possesses outstanding overall water splitting activity. The photocatalytic overall water splitting performance of graphitic carbon nitride (g-C 3 N 4 ) has still been restricted by its inefficient charge separation efficiency. Herein, nitrogen-vacancy crystalline g-C 3 N 4 nanosheets (g-C 3 N 4 -D) with tunable band structures were successfully prepared by the alkali-molten salt-assisted method for efficient photocatalytic overall water splitting. Among them, g-C 3 N 4 -D2 not only combines the advantages of the amorphous and crystalline state of two-dimensional g-C 3 N 4 nanosheets, but also introduces nitrogen vacancy to adjust the bandgap structures of crystalline g-C 3 N 4 by an alkali etching, thus improving the light energy utilization and charge separation efficiency. TEM and EPR spectrum prove the existence of N defects in the crystalline g-C 3 N 4 . The excellent and stable photocatalytic overall water splitting activity with H 2 and O 2 evolution rate of 49.60 μmol g -1 h -1 and 24.71 μmol g -1 h -1 was obtained over g-C 3 N 4 -D2 with Pt and Co 3 O 4 nanorods as cocatalysts (Pt/g-C 3 N 4 -D2/3%Co 3 O 4 NRs) under AM 1.5G simulated light irradiation. In the half-reaction experiments, the maximum H 2 evolution rate of Pt/g-C 3 N 4 -D2/3%Co 3 O 4 NRs is 3.78 mmol g -1 h -1 with a significant quantum efficiency of 11.94% at 400 nm, and the solar-hydrogen conversion efficiency (STH) is 1.48%. The photocatalytic water oxidation activity of Pt/g-C 3 N 4 -D2/5%Co 3 O 4 NRs is 42.34 μmol g -1 h -1 . Additionally, the potential mechanism of photocatalytic overall water splitting was testified by in-situ XPS. This work provides a simple strategy for further advancing the potential application of g-C 3 N 4 by molten salt-assisted alkali etching to introduce N vacancies to regulate the band gap of crystalline g-C 3 N 4 for photocatalytic overall water splitting.