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‐C 3 N 4 . Herein, an alternating hydrogen‐oxygen‐plasma treatment is proposed to endow the g‐C 3 N 4 with enriched vacancy defect sites for the subsequent immobilization of carboxyl groups, thus overcoming the problem of lacking of covalent binding sites in g‐C 3 N 4 in developing carboxyl defective g‐C 3 N 4 photocatalysts. The alternating hydrogen‐oxygen‐plasma treatment does not only influence the defect structure of g‐C 3 N 4 , but also changes its morphology, optimizes the electronic distribution, and increases the separation efficiency of photogenerated electrons and holes, thereby increasing photocatalytic H 2 evolution by 7.91 times. Density functional calculations and electrochemical characterization suggest that the carboxyl defects generated by the fast H 2 ‐O 2 plasma modification lead to a local asymmetric electron environment, which enhances carrier separation capability and significantly improves H 2 generation activity. This study provides a new insight into the rational design and fabrication of defect‐containing photocatalysts, carbon materials, and polymers.