Carbon incorporated isotype heterojunction of poly(heptazine imide) for efficient visible light photocatalytic hydrogen evolution†

Clean hydrogen production using renewable solar energy is an important aspect in the development of a sustainable society. The premise of developing highly efficient photocatalysts for hydrogen production relies on achieving smooth charge carrier kinetics with efficient visible light absorption. Constructing isotype heterojunctions with structural or compositional similarity can enhance charge carrier separation at the interface, leading to improved utilization of light energy. However, this approach is often constrained by the availability as well as intrinsic properties of monomers. Herein, carbon facilitated in situ fabrication of an isotype heterojunction based on a poly(heptazine imide) (PHI) structure with high crystallinity and extended π-conjugation was proposed by calcinating carbon-modified melon in the “semi-liquid” NaCl/KCl salt. The heterojunction effect induced by the visible light responsive Na–PHI and K–PHI, as well as the strong charge coupling between heptazine and carbon ring in the covalent interface forms multi-directional built-in electric field and effectively promotes the separation of charge carriers. Together with the visible light absorption extension by simultaneous carbon ring decoration, C@Na–PHI/K–PHI shows superior photocatalytic hydrogen evolution activities under visible light irradiation and the apparent quantum efficiencies reach 29.3% and 3% under 420 and 550 nm, respectively. This study pioneers the idea and provides a useful reference for the design of PHI isotype heterojunctions for the effective utilization of solar energy.