Alkali-assisted deep-deamination to improve the crystallinity of poly(heptazine imide) for boosted photocatalytic H2 evolution

The poly(heptazine imide) (also known as heptazine-based crystalline carbon nitride, abbreviated as PHI) has drawn wide attention in solar energy conversion, which is profited from its improved photoinduced charge separation compared with melon-based carbon nitride. Nevertheless, the crystallinity of the samples synthesized by direct heat-treating precursors is low and further impede electron transport. This situation is especially prominent if precursors are used in large quantities and the reason is also unclear at present. Herein, this crucial issue was preliminarily revealed: the by-product HCl formed in the synthesis process has seriously restricted the deamination of PHI if it cannot be removed in time. Based on this, an original alkali-assisted method was proposed and demonstrated to realize deep-deamination for improving the crystallinity and extending π-conjugation of PHI. Benefit from this, alkali-assisted synthesized PHI illustrates significantly enhanced electron transport performance and further improved photocatalytic H2 evolution activity. The optimized PHI exhibits highly boosted photocatalytic H2 evolution activity of 2223 μmol h−1, which is 18 times that of original PHI synthesized without alkali-assistance. Additionally, the mechanism was further verified by density-functional theory (DFT) computations and the universality of alkali-assisted method was verified by using different kinds of alkali.