Defect Engineering in 0D/2D S-Scheme Heterojunction Photocatalysts for Water Activation: Synergistic Roles of Nickel Doping and Oxygen Vacancy

Photocatalytic hydrodechlorination (HDC) is a promising method for eliminating chlorinated organic compounds (COCs) from water, but it requires catalysts with excellent water activation ability. Defect engineering is a feasible way to enhance the catalytic performance of photocatalysts by improving light adsorption, charge carrier dynamics, and surface reactions. Herein, a well-designed 0D/2D S-scheme heterojunction with favorable band structures and defective interfaces was constructed via defect tailoring on TiO₂ quantum dots (QDs) and the interface structure. The optimized catalyst Ni-TiO_2-x/g-C₃N₄ with 1% Ni doping after thermal treatment at 300 °C under nitrogen resulted in superior visible-light-driven activity in trichloroethylene (TCE) photocatalytic HDC, approximately an 18.2-fold increase as compared with g-C₃N₄. Ni doping and thermal-induced oxygen vacancies were verified to synergistically endow the catalyst with improved visible-light absorption efficiency, ameliorated charge separation and migration, and enhanced redox potential. Experimental and theoretical results showed that the synergy of multifold defects in promoting visible-light harvesting was mainly due to the characteristic multiple midgap states, in terms of different intermediate energy levels and narrowed bandgap. Furthermore, the contradicting effects of midgap states on photogenerated charge carrier dynamics were mediated by the defective S-scheme heterojunction, where the detrimental charge recombination relating to excessive defects was considerably inhibited via superior spatial charge separation and promoted surface redox reactions. The design of defect-engineered heterojunctions and the role of controlled defects in adjusting band structures provide valuable insights for creating highly efficient artificial photosystems in the visible region.