Intramolecular S-scheme of g-C3N4 to boost photocatalytic hydrogen evolution

• An intramolecular S-scheme junction was created in carbon nitride via co-polycondensation of ethylenediamine-mediated precursor. • Aromatic ring-modified melon (A-domain) and pristine melon (P-domain) in the product established a molecular-level built-in electric field to promote carriers transfer and separation. • Aromatic substitution triggered n→π transitions and broadening visible-light absorption. • The product achieved a superior H₂ evolution rate that is 39 times higher than that of pristine g-C 3 N 4 . Graphitic carbon nitride (g-C 3 N 4 ) has emerged as a promising metal-free photocatalyst for solar driven hydrogen evolution. However, the rapid recombination of photogenerated electron-hole pairs in pristine g-C 3 N 4 severely limits its opportunities for practical application. Herein, we developed an intramolecular S-scheme with an endogenous built-in electric field (BIEF) via simply ethylenediamine-mediated two-step thermal polycondensation, producing an heteroaromatic-modified g-C 3 N 4 (HA-CN). The experimental results showed this structural innovation achieves dual optimization: (1) molecular-level integration of aromatic ring-modified melon (A-domain) and pristine melon-skeleton (P-domain) establish a BIEF to promote carriers transfer and separation; (2) the aromatic substitution extends π-electron delocalization of the adjacent melon domains, enhancing n→π* transitions and broadening visible-light absorption. The optimized HA-CN exhibited exceptional enhancement of hydrogen evolution (5.99 mmol g⁻¹ h⁻¹, λ ≥ 420 nm), surpassing pristine g-C 3 N 4 by 39-fold, with an apparent quantum efficiency (AQE) of 3.93 % at 420 nm. Time-resolved spectroscopy and electrochemical analysis confirm the suppressed charge recombination and reduced interfacial charge-transfer resistance, corresponding to BIEF-driven directional carrier migration. This research offers valuable insights into junction engineering strategy from molecular level for designing efficient photocatalysts.