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

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The photocatalytic overall water splitting performance of graphitic carbon nitride (g-C3N4) has still been restricted by its inefficient charge separation efficiency. Herein, nitrogen-vacancy crystalline g-C3N4 nanosheets (g-C3N4-D) with tunable band structures were successfully prepared by the alkali-molten salt-assisted method for efficient photocatalytic overall water splitting. Among them, g-C3N4-D2 not only combines the advantages of the amorphous and crystalline state of two-dimensional g-C3N4 nanosheets, but also introduces nitrogen vacancy to adjust the bandgap structures of crystalline g-C3N4 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-C3N4. The excellent and stable photocatalytic overall water splitting activity with H2 and O2 evolution rate of 49.60 μmol g−1 h−1 and 24.71 μmol g−1 h−1 was obtained over g-C3N4-D2 with Pt and Co3O4 nanorods as cocatalysts (Pt/g-C3N4-D2/3%Co3O4 NRs) under AM 1.5G simulated light irradiation. In the half-reaction experiments, the maximum H2 evolution rate of Pt/g-C3N4-D2/3%Co3O4 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-C3N4-D2/5%Co3O4 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-C3N4 by molten salt-assisted alkali etching to introduce N vacancies to regulate the band gap of crystalline g-C3N4 for photocatalytic overall water splitting.