Hydrothermal Alkali‐Assisted Growth of Highly In‐Plane Crystallized Poly(Heptazine Imide) for Photocatalytic <scp> H ₂ O ₂ </scp> Production

Photocatalysis is emerging as a promising alternative for H 2 O 2 production to the current energy‐intensive anthraquinone process. However, developing the catalysts with efficient charge transfer and robust proton extraction kinetics is critical but quite challenging. Herein, we present a new re‐crystallization synthesis strategy for polymeric semiconductors with a highly crystallized poly(heptazine imide) photocatalyst designed. Pre‐alkali hydrothermal treatment enhances the structural disorder of the precursor with hydrophilic active sites introduced, facilitating the formation of the fully extended conjunction structure during the subsequent molten salted growth. Benefiting from the structural advantages including high in‐plane crystallinity and sufficient active sites from introduced functional groups, the synthesized photocatalyst exhibits record‐level visible‐light‐responsive H 2 O 2 production of 486.00 μM at λ &gt; 500 nm and 1946.96 μM at λ &gt; 420 nm, achieving an apparent quantum yield of 13.75%. Notably, a rarely reported high piezo‐photocatalytic H 2 O 2 production of 19.11 μM in pure water was also achieved. This work provides new insights into the design of high‐performance polymeric photocatalysts for sustainable H 2 O 2 production.