Sustainable ammonia-driven large-scale synthesis of low-defect poly(heptazine imide) for boosted photocatalytic hydrogen evolution

The synthesis of crystalline poly(heptazine imide) (PHI) represents a promising approach to enhance the photocatalytic activity of carbon nitride. However, achieving high yields of PHI has been a persistent challenge in the field. Herein, we introduced a novel one-step strategy for large-scale synthesis of PHI by thermal heating a mixture of melamine, KSCN and NH₄Cl. Benefitting from the dual effects of the by-product precursor NH₄SCN, produced from the reaction of KSCN and NH₄Cl, as well as the maintenance of a sustainable NH₃ atmosphere, the yields of the resultant PHIx samples can reach up to 100.5 % relative to 2 g of melamine. Among these samples, PHI2 stands out due to its superior crystallinity and minimal defect density compared to conventional PHI. As a result, the photocatalytic H₂ evolution reaction (HER) rate of PHI2 is 7.0 times than that of PHI. Furthermore, PHI2 demonstrates an exceptional apparent quantum efficiency (AQE) of 30.7 % at a wavelength of 420 nm. The enhanced HER rate and AQE of PHI2 are attributed to the reduced defect density in its conjugated heptazine skeleton, which facilitates efficient charge carrier separation and transfer. This work not only achieves high-yield production of PHI but also paves the way for the large-scale application of carbon nitride photocatalysts.