Gradient Zn-Doped Poly Heptazine Imides Integrated with a van der Waals Homojunction Boosting Visible Light-Driven Water Oxidation Activities

Ionothermal synthesis of amorphous melon in a ternary salt melt (ZnCl2–LiCl–KCl) with lower melting points could promote the polycondensation process and improve the degree of crystallinity. As Zn2+ ions tend to incorporate with edge nitrogen of carbon nitride via Lewis acid–base interactions, a series of crystalline hybrid poly heptazine imides (PHI), namely, Zn-PHI/PHI, integrated by van der Waals interactions, were generated as main products. Interestingly, the contents of doped Zn2+ ions of the as-synthesized Zn-PHI/PHI gradually decrease from the bulk interior to surfaces, due to the evaporation of ZnCl2 during thermal heating processes. Accordingly, a built-in electric field would be created in the catalyst, which largely accelerates a separation of the photogenerated charge carriers at the interfaces. Owing to the fast charge carrier separation and transfer, this hybrid polymer is active for photocatalytic water oxidation with an apparent quantum yield of up to 3.6% at 420 nm. This work offers a strategy for the rational design and synthesis of polymeric photocatalysts for water oxidation reaction.