One-pot molten salt method for co-engineering nitrogen vacancy and elements dopants in crystalline PHI for efficient photocatalytic hydrogen evolution

Graphitic carbon nitride (g-C₃N₄) always suffers from its innately low photocatalytic activity due to the serious charge carrier recombination caused by incomplete polymerization. To promote charge carrier separation, herein, poly heptazine imide (PHI) was successfully prepared via a facile molten salt-assisted heat treatment of bulk g-C₃N₄ (BCN) with binary eutectic mixture of CaCl₂-NaCl, in which the molten salt CaCl₂-NaCl serves as a propellant for accelerating the polymerization and deamination processes. The mole ratio of this molten salt and the calcination temperature were investigated in detail for modification of the BCN. The comprehensive characterization results reveal the successful introduction of nitrogen vacancies and elements dopants (Ca²⁺ and oxygen) into the framework of g-C₃N₄, along with enhanced crystallinity. As a result, the optimized PHI-Ca_1.5Na-600 exhibited a hydrogen evolution rate of 3.84 mmol∙g^-1∙h^-1 under visible light, which was 28.4-fold higher than that of BCN (0.135 mmol∙g^-1∙h^-1). The substantially boosted photocatalytic activity of PHI-Ca_1.5Na-600 is ascribable to its relatively high crystallinity compared to that of BCN, as well as to the engineered electronic band structure and enhanced charge carrier migration efficiency. This work not only provides a facile strategy for further advancing the potential application of g-C₃N₄ by molten salt-assisted method that simultaneously introduces vacancies and elemental dopants to regulate the electronic band structure, but also sheds light on the role of molten salts in designing and fabricating highly efficient photocatalysts for solar energy conversion.