Crystalline-amorphous homojunction engineering in carbon nitride for solar-driven nitrate into ammonia conversion

Photocatalytic nitrate to ammonia conversion offers a promising route to close the nitrogen cycle, yet is limited by inefficient charge separation and poor reaction selectivity. Here, crystalline-phase-engineered oxygen-doped carbon nitride homojunctions are constructed by integrating poly(heptazine-triazine) imides (PHTI), poly(heptazine imide) (PHI) and poly(triazine imide) (PTI) into oxygen-doped carbon nitride microspheres (OCNMs), forming S-scheme, type-II and type-I architectures. Among them, the urchin-like S-scheme PHTI/OCNMs homojunction achieves an ammonia formation rate of 893 µmol g_cat ^-1 h^-1 with an apparent quantum efficiency of 6.28% at 400 nm, far exceeding its type-II and type-I analogues. In situ probe techniques and theoretical calculations demonstrate that a strong built-in electric field enforces S-scheme charge separation, retaining high-energy electrons on OCNMs for nitrate reduction and high-energy holes on PHTI for oxidation. The synergistic effects of crystalline phase alignment, oxygen-doped sites, and S-scheme interfacial recombination enhance nitrate adsorption, stabilize *HNO intermediates and lower the free-energy barriers toward *NH₃, enabling NH₄ ⁺ selectivity above 94%. The PHTI/OCNMs catalyst remains active under unconcentrated natural sunlight and in real water matrices, highlighting the practical potential of crystalline homojunction engineering for solar-driven nitrate valorization.