Microwave-assisted molten-salt rapid synthesis of isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic hydrogen evolution performance

Rapid synthesis and construction of graphitic carbon nitride (g-C3N4) based heterojunctions, cost-effective metal-free photocatalysts for hydrogen evolution reaction (HER) under solar irradiation, is of highly practical significance. This work reports a one-pot microwave-assisted molten-salt (mw-ms) process to rapidly synthesize isotype triazine-/heptazine based g-C3N4 heterojunctions with highly enhanced photocatalytic HER performance using melamine as the single-source precursor. The typical sample (mw-ms-g-C3N4) was obtained by thermally polymerizing melamine molecules at 550 °C for 30 min in the media of eutectic KCl/LiCl salts under microwave irradiation in air. The analyses of phases, chemical compositions and microstructures indicate that the mw-ms-g-C3N4 sample consists of an isotype triazine-/heptazine based g-C3N4 heterojunction, taking on a plate-like morphology with a specific surface area (SBET) of 25.7 m2 g−1. Comparatively, the g-C3N4 sample synthesized via an electric-resistance molten-salt (er-ms) process at 550 °C for 240 min is composed of a triazine-based g-C3N4 phase with a SBET of 58.1 m2 g−1, whereas the samples obtained by electric-resistance heating (er, at 550 °C for 240 min) and microwave heating (mw, at 550 °C for 30 min) processes consist of a heptazine-based g-C3N4 phase. The mw-ms-g-C3N4 sample shows a photocatalytic HER rate of 1480 μmol g−1 h−1, which is 5 times that (300 μmol g−1 h−1) of the er-ms-g-C3N4 sample, 15 times that (95 μmol g−1 h−1) of the er-g-C3N4 sample and 23 times that (63 μmol g−1 h−1) of the mw-g-C3N4 sample, under the similar visible-light (λ ≥ 420 nm) irradiation. The typical apparent quantum yield of the mw-ms-g-C3N4 sample at 420 nm is up to 10.7%. The UV–vis DR spectra suggest that both the triazine-based g-C3N4 and heptazine-based g-C3N4 phases have a similar bandgap of ∼2.66 eV, whereas the Mott-Schottky analysis indicates that the triazine-based g-C3N4 phase has a more positive flat conductive potential (−0.90 V) than the triazine-based g-C3N4 phase (−1.22 V). Due to the suitable alignment of their energy bandgap structures, the isotype triazine-/heptazine based g-C3N4 hybrids in the mw-ms-g-C3N4 sample form a type II heterojunction of semiconductor/semiconductor, which provides a convenient carrier transfer path and leads to more efficient separation of photo-generated electron-hole pairs than the other samples. The synergistic effects of microwave heating and molten-salt liquid polycondensation provide a robust platform for rapid and large-scale construction of isotype g-C3N4/g-C3N4 heterojunctions as metal-free high-performance HER photocatalysts using a simple single-source precursor.