Crystal defect engineering to construct oxygen vacancies in MXene-derived TiO₂ nanocomposites for boosting photocatalytic degradation of 2,4,6-trichlorophenol

Chlorophenolic pollutants pose serious threats to the aquatic ecosystem. Photocatalysis technology has broad prospects for decontamination. In this work, MXene-derived TiO₂ nanocomposites with mixed-crystal characteristics and oxygen vacancies (OVs) (E–TiO₂@rGO) were successfully synthesized by atmosphere modulation. Compared with commercial TiO₂ and N₂–TiO₂@rGO (anatase phase), the rate constants of E–TiO₂@rGO (0.06874 min⁻¹) over 2,4,6-trichlorophenol (2,4,6-TCP) degradation increased 4 and 1.3 times, respectively. The enhanced photocatalytic performance was attributed to the co-existence of anatase and rutile phases. On the one hand, the mixed-crystal phase narrowed the band gap, broadening the light absorption range. On the other hand, it induced the generation of OVs, which has been demonstrated by DFT calculation. The presence of OVs improved the adsorption and activation of O₂ and H₂O molecules, beneficial for the formation of active free radicals. In addition, the incorporation of rGO promoted the separation of electron-hole pairs. The Gibbs free energy of 2,4,6-TCP degradation revealed that the OVs and mixed-crystal could reduce the energy barrier of the rate-determining step, thereby boosting the dichlorination efficiency of 2,4,6-TCP. The superoxide free radical ([rad]O₂⁻) was determined as the dominant active species. Finally, a possible mechanism of 2,4,6-TCP degradation with E–TiO₂@rGO was clarified clearly. The proposed mixed-crystal defect engineering represents a novel strategy in the design and synthesis of photocatalysts.