Harnessing atomic-scale defect engineering in 2D photocatalysts: synergistic integration of nanocomposite architectures for bandgap tuning and charge transfer optimization.

The urgent global need for sustainable energy and effective environmental remediation has propelled photocatalysis to the forefront of materials research. Two-dimensional (2D) photocatalysts, empowered by atomic-scale defect engineering, have revolutionized this field by enabling precise bandgap tuning, enhanced charge separation, and the creation of abundant active sites. This review systematically explores the synergistic integration of defect-rich 2D materials into advanced nanocomposite architectures such as Type-II, Z-scheme, and Schottky heterojunctions, highlighting how these strategies overcome the intrinsic limitations of pristine 2D systems. We detail the roles of vacancies, dopants, edge sites, and grain boundaries in modulating the electronic structure and reactivity and examine how rational composite design further amplifies these effects through optimized interfacial charge transfer and band alignment. Comparative analyses of recent open-access studies underscore the superior photocatalytic efficiencies achieved by heterojunction-engineered 2D photocatalysts and membranes in applications ranging from solar fuel generation (H evolution and CO reduction) to environmental remediation (dye degradation and heavy metal removal) and emerging fields such as NOx abatement and green organic synthesis. This review also addresses key challenges, including the scalable and green synthesis of defect-engineered materials and their long-term stability, toxicity, and integration with circular-economy principles. We conclude by outlining a roadmap for their commercialization and envisioning future directions, including bioinspired systems and space applications. This synthesis establishes atomic-scale defect engineering, in concert with nanocomposite design, as a paradigm-shifting approach for next-generation photocatalytic technologies with transformative real-world impact.