Crystalline isotype heptazine-/triazine-based carbon nitride heterojunctions for an improved hydrogen evolution

The establishment of a built-in electric field between different phases of the same material is one of the most effective techniques to promote photocatalytic activity. Crystalline graphitic carbon nitride (g-C3N4) based on the heptazine or triazine structure has recently attracted extensive attention because of its excellent photocatalytic activity. However, the framework of the crystalline isotype heptazine-/triazine-based g-C3N4 (HTCN) heterojunctions remains ambiguous. Here, we developed an easy and reliable approach to prepare crystalline HTCN heterojunction by using preheated melamine as precursor and molten salts as the liquid reaction media. We investigated for the first time the interfacial interactions of the HTCN heterojunction by first principles calculations. The energy band structures show that the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the heptazine-based g-C3N4 (HCN) are more negative than those of the triazine-based g-g-C3N4 (TCN) after their close contact. Therefore, the HTCN heterojunction displays remarkable charge transfer from HCN to TCN, which is beneficial to enhance photocatalytic activity. Moreover, the phase ratio of TCN and HCN was studied from 77:23 to 31:69 by changing the preheating temperature. The optimal HTCN-500 samples show the highest photocatalytic hydrogen production rate at 890 mu mol h(-1)g(-1) with apparent quantum yield of 26.7 % at 420 nm, which is higher than those of BCN, TCN, and HCN by 15, 8 and 1.6 times, respectively. The phase content of HTCN-500 is 16 % TCN and 84 % HCN. The appropriate phase ratio between HCN and TCN increases the formation of photogenerated electrons and ameliorates the inability of the bridged nitrogen atoms in the heptazine frameworks to transfer the photogenerated electrons. This work presents a novel method to prepare a HTCN heterojunction and provides a new pathway for designing the heterostructure between different phases of the same material.