Melon-derived carbon nitride photocatalysts are a kind of star layered materials applied in solar energy conversion. With in-plane π orbitals of the heptazine subunits and their overlap along the melon chains being the most distinctive feature, the condition of melon chains is of great importance for the atomic and energy band structures of carbon nitride photocatalysts as well as their photo-activities. In principle, fragmentized melon chains in practical carbon nitride would lead to unfavorable structure disorder both in longitudinal and vertical directions, thus inhibiting the efficient transfer for photo-induced electrons and holes, respectively. Here, with a facile regrowth approach, that is to treat carbon nitride under the atmosphere containing C/N species, the melon chains in carbon nitride were experimentally lengthened, which was reflected by the regularly fraction variation of different nitrogen species derived from X-ray photoelectron spectroscopy (XPS) analysis. The prolonged melon chains led to dramatically improved in-plane structure order and boosted transfer of photo-induced electrons and holes, which were confirmed by the spontaneous photo-deposition of oxidants and reductants. The combination of this regrowth approach with homogenously distributed nitrogen vacancies resulted in much enhanced visible-light-responsive photoactivities. Besides, control experiments using nitrogen-vacancy-free carbon nitride and different C/N-contained precursors showed the compatibility as well as the critical factors for the lengthening effects of the regrowth approach. We hope that the facile but efficient regrowth approach could be widely adopted in melon-derived carbon nitride photocatalysts used for various applications.

Graphitic carbon nitride (g-C₃N₄) is viewed as a promising visible-light photocatalyst for industrialization due to its low processing temperature and high chemical stability. However, serious charge recombination caused by incomplete polymerization during direct calcination of nitrogen-rich precursors significantly limits its photocatalytic performances. To boost charge separation, herein, we propose a rational strategy by constructing a crystalline g-C₃N₄/g-C₃N_4−xS_x isotype heterostructure through the molten salt method. Theoretical calculation reveals that apparent charge-transfer channels are formed between g-C₃N₄ and S-doped g-C₃N₄ layers in the heterostructure. Owing to high crystallinity for decreasing charge recombination and isotype heterostructure for efficient charge transfer, the as-prepared g-C₃N₄/g-C₃N_4−xS_x showed remarkable photocatalytic performances with the hydrogen production rate elevated by up to 12.3 times of its singular components. Another novelty of this work is we investigated for the first time the piezocatalytic activity of crystalline g-C₃N₄ by characterizing its performance for H₂O₂ generation and KMnO₄ reduction. Strikingly, its superior piezocatalytic performance over components can be further improved by NaBH₄ treatment, which is uncovered to enhance the asymmetric structure of crystalline g-C₃N₄ by introducing extra cyano groups and removing partial NH_x species in its tri-s-triazine layer structure. This work opens up new strategies for the design of highly efficient polymeric photocatalysts and highlights the piezocatalytic studies of g-C₃N₄.