Defect Engineering in g‐C ₃ N ₄ for Enhanced Photocatalytic H ₂ Production

The development of efficient photocatalytic hydrogen production systems is crucial for sustainable energy conversion. As a compelling two‐dimensional semiconductor, graphitic carbon nitride (g‐C 3 N 4 ) exhibits excellent visible‐light absorption and robust chemical stability; however, its catalytic performance is frequently limited by rapid charge recombination and a scarcity of surface active sites. Defect engineering, particularly the rational introduction of vacancies has emerged as a transformative strategy to fundamentally modulate the electronic band structure, accelerate charge carrier spatial separation, and enrich catalytic active centers. This review systematically summarizes the formation mechanisms and specific photocatalytic roles of intrinsic carbon and nitrogen vacancies within the g‐C 3 N 4 framework, as well as heterophase oxygen vacancies in coupled composite systems. Specifically, carbon vacancies break local structural symmetry to induce paramagnetic centers for electron trapping, while nitrogen vacancies primarily broaden light absorption and serve as high‐activity adsorption sites. Furthermore, oxygen vacancies in heterojunctions function as vital interfacial bridges, driving efficient S‐scheme or Z‐scheme charge transfer. Building upon these mechanistic insights, we particularly highlight the cutting‐edge synergy between vacancy engineering and single‐atom catalysis, wherein vacancies serve as robust physical anchors and electronic modulators to optimize the local coordination microenvironment of isolated metal atoms. Finally, the review concludes by outlining the grand challenges and future opportunities in designing atomic‐precision, vacancy‐engineered g‐C 3 N 4 photocatalysts for next‐generation solar energy conversion.