Oxygen vacancy regulating transition mode of MIL-125 to facilitate singlet oxygen generation for photocatalytic degradation of antibiotics

Efficient yield of 1O2 determines the photocatalytic degradation rate of antibiotics, but the regulatory mechanism for 1O2 selective generation in O2 activation is still lacking exploration. Herein, oxygen vacancy (OV) modification strategy of MIL-125 was successfully practiced to promote the selective generation of 1O2. Multiple characterizations including extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance spectra (EPR) confirmed the formation of oxygen vacancy in OV-MIL-125. The synthesized OV-MIL-125 exhibited greatly enhanced 1O2 selective (∼90%) and antibiotics removal rate in water with high mineralization rate. Dynamics analysis of excitons by transient-steady state fluorescence and phosphorescence, transient absorption spectra (TAS) revealed that oxygen vacancy greatly enhanced the intersystem crossing (ISC) of singlet exciton, promoting triplet exciton generation. Density functional theoretical (DFT) calculation also proved the reduced gap of intersystem (ΔEST) and the modulated highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) population which was conducive to intersystem crossing process. Calculation of transition state further confirmed the lower energy barrier for π* orbital spin flip of O2 adsorbed on OV-MIL-125. The Dexter energy transfer involving triplet annihilation dominated the O2 activation mechanism to generate 1O2 instead of the charge transfer to generate O2•− which happened in MIL-125. This study provides new thinking for photocatalytic activation of molecular oxygen and is expected to guide the design of MOF-based catalysts for water treatment.