Construction of Type-II Heterojunctions in Crystalline Carbon Nitride for Efficient Photocatalytic H-2 Evolution

As an encouraging photocatalyst, crystalline carbon nitride (CCN) exhibits unsatisfactory photocatalytic activity and stability due to its rapid recombination of photo-generative carriers. Herein, high-crystalline g-C3N4 was prepared, including CCN obtained in KCl (K-CCN), LiCl-KCl mixture (Li/K-CCN), and LiCl-NaCl-KCl mixture (Li/Na/K-CCN), via the molten salt strategy using pre-prepared bulk carbon nitride (BCN) as a precursor. The obtained BCN sample was formed by heptazine-based units, which convert into triazine-based units for K-CCN. Heptazine and triazine are two isotypes that co-exist in the Li/K-CCN and Li/Na/K-CCN samples. Compared with BCN and other CCN samples, the as-prepared Li/Na/K-CCN sample exhibited the optimal photocatalytic hydrogen evolution rates (3.38 mmol center dot g(-1)center dot h(-1) under simulated sunlight and 2.25 mmol center dot g(-1)center dot h(-1) under visible light) and the highest apparent quantum yield (10.97%). The improved photocatalytic performance of the Li/Na/K-CCN sample is mainly attributed to the construction of type-II heterojunction and the institution of the built-in electric field between triazine-based CCN and heptazine-based BCN. This work provides a new strategy for the structural optimization and heterostructure construction of crystalline carbon nitride photocatalysts.

Automated summary

In this study, high-crystalline g-C3N4 samples, including K-CCN, Li/K-CCN, and Li/Na/K-CCN, were prepared via the molten salt strategy using pre-prepared BCN as a precursor. The Li/Na/K-CCN sample exhibited the highest photocatalytic hydrogen evolution rates and the highest apparent quantum yield compared to other CCN samples. The improved photocatalytic performance of Li/Na/K-CCN is attributed to the construction of type-II heterojunction and the institution of the built-in electric field between triazine-based CCN and heptazine-based BCN.