Unraveling Structural Carboxyl Defects in g‐C₃N₄ for Improved Photocatalytic H₂ Evolution via Alternating Hydrogen‐Oxygen‐Plasma Treatment

Abstract Structural defect‐endowed photocatalysts are being increasingly recognized due to the enhanced catalytic activity of multiple defect sites (e.g., vacancies or functional groups). However, because of the excessive destruction effect of conventional chemical oxidation methods toward carboxyl defects engineering, the mechanism is still unclear and practice is rare in developing high‐quality structural carboxyl defect‐involved g‐C₃N₄. Herein, an alternating hydrogen‐oxygen‐plasma treatment is proposed to endow the g‐C₃N₄ with enriched vacancy defect sites for the subsequent immobilization of carboxyl groups, thus overcoming the problem of lacking of covalent binding sites in g‐C₃N₄ in developing carboxyl defective g‐C₃N₄ photocatalysts. The alternating hydrogen‐oxygen‐plasma treatment does not only influence the defect structure of g‐C₃N₄, but also changes its morphology, optimizes the electronic distribution, and increases the separation efficiency of photogenerated electrons and holes, thereby increasing photocatalytic H₂ evolution by 7.91 times. Density functional calculations and electrochemical characterization suggest that the carboxyl defects generated by the fast H₂‐O₂ plasma modification lead to a local asymmetric electron environment, which enhances carrier separation capability and significantly improves H₂ generation activity. This study provides a new insight into the rational design and fabrication of defect‐containing photocatalysts, carbon materials, and polymers.