Crystalline Triazine/Heptazine‐Based Carbon Nitride With Built‐in Electric Field for Synergistic Photoproduction of Hydrogen Peroxide and Xylose Oxidation

ABSTRACT Photocatalytic coupling of biomass oxidation with H 2 O 2 production offers a sustainable route for solar‐to‐chemical conversion, but its efficiency is limited by slow charge separation and surface reactions. Herein, we report a KCl‐assisted co‐precipitation–calcination method to synthesize highly crystalline carbon nitride (MCAK) featuring a triazine/heptazine‐coupled framework and surface‐enriched atomic K dispersion. This unique structure introduces cyano defects and creates a strong built‐in electric field that reduces the exciton binding energy from 116.50 meV (bulk carbon nitride, BCN) to 74.57 meV, thereby promoting charge separation. As a result, MCAK achieves an exceptional H 2 O 2 production rate of 12470.4 µmol/(g × h) under white light in O 2 —50.6 and 37.8 times higher than that of BCN and supramolecular carbon nitride (MCA), respectively—with negligible decay in air. Simultaneously, it oxidizes xylose to xylonic acid with 73.3% selectivity. Mechanism studies reveal that the performance originates from enhanced O 2 adsorption and an optimized two‐electron oxygen reduction pathway driven by the built‐in electric field. This work demonstrates a synergistic design strategy that integrates crystallinity control, intramolecular heterojunction engineering, and built‐in field intensification for efficient bifunctional photocatalysis.