Heterogeneous photosynthesis is a promising route for sustainable ammonia production, which can utilize renewable energy and water as the hydrogen source under ambient condition. In this study, a series of Bi₅O₇I (BOI) nanosheets and nanotubes are synthesized, and the surface tensile strain is formed by curling the nanosheets into nanotubes to tune the concentration and location of dynamic vacancies. Scanning transmission electron microscopy (STEM) with spherical aberration correction confirms the presence of intrinsic areal defects on the surface of the BOI nanotube resulted from surface tensile strain. The presence of areal defects lowers the formation energy of I vacancies (IV) at step edge site, thus the IV with higher concentration would be favorably generated under visible light. Rapid scan in situ Fourier transform infrared (FT-IR) analysis in the aqueous media reveals that the IV promotes photocatalytic N₂ activation and reduction, and proceeds through an associative alternating mechanism. Specially, after turning off the light, the surface vacancy sites can be reoccupied by I⁻ ions, which enables the protection and regeneration of photocatalyst surface in an aerobic and dark environment. This work provides an innovative strategy to tune concentration and location of dynamic surface vacancies on photocatalysts by building surface tensile strain for advancing sustainable ammonia production.

Photocatalytic CO₂ reduction is mainly inspired by natural photosynthesis, which could convert CO₂ into high value-added fuels or chemicals through the role of catalysts. However, the photocatalysis efficiency of the currently developed catalysts is far from meeting the actual needs due to the low efficiency of charge separation and energy transfer, and the poor adsorption and activation of CO₂ by catalyst surface. Single-atom catalysts (SACS) show an excellent activity, selectivity and stability in many important reactions, and exhibit great potential in photocatalytic reduction of CO₂ owing to their high atomic utilization and controllability of active sites. In the current review, recent progresses and challenges on SACs for photocatalytic CO₂ conversion systems are presented. The key fundamental principles and reaction mechanisms focusing on charge separation/transfer and molecular adsorption/activation on single-atom photocatalysts for CO₂ reduction are systemically explored. We outlined how single-atom active sites promote the photogenerated carriers separation/transfer and enhance molecular photoactivation. Besides, we put forward some challenges and prospects for the future development of single-atom photocatalysts in CO₂ reduction.