Carbon quantum dot-modified UiO-66-NH2 hybrid materials: visible-to-ultraviolet upconversion and enhanced visible-light photocatalysis

Efficient utilization of long-wavelength visible light remains a key challenge in developing metal-organic-framework-based photocatalysts for solar-driven pollutant removal. To address this issue, carbon quantum dot (CQD)-modified UiO-66-NH 2 hybrid materials with different nominal CQD loadings were constructed through induced loading and secondary thermal fixation to combine spectral conversion with interfacial photophysical regulation. Structural and textural characterizations showed that CQD incorporation preserved the crystalline and porous framework of UiO-66-NH 2 while modifying its surface composition, pore characteristics, and optical response. The CQDs displayed visible-to-ultraviolet anti-Stokes emission centered at approximately 340 nm under 600 nm excitation. CQD incorporation also enhanced visible-light absorption and induced photoluminescence quenching consistent with interfacial photophysical coupling. Transient-current measurements showed reproducible light-switching responses, while EIS revealed a lower impedance response for CU-3 under illumination; together, these results are consistent with facilitated interfacial charge migration. Under visible-light irradiation in the presence of FeCl 3 as an electron scavenger, CU-3, containing 3 wt% CQDs, achieved a background-corrected net methyl orange removal of 67% within 90 min. Its apparent rate constant was 1.203 × 10 − 2 min − 1 , approximately 6.6 times that of pristine UiO-66-NH 2 . Tert-butanol inhibition and DMPO spin-trapping ESR measurements jointly supported the involvement of hydroxyl radicals, with CU-3 exhibiting a stronger light-induced DMPO–·OH signal than pristine UiO-66-NH 2 . The effect of EDTA-2Na could not be assigned exclusively to photogenerated holes because of its complexation with Fe 3+ . These findings demonstrate the importance of optimizing CQD loading and suggest that spectral conversion and interfacial photophysical coupling contribute to the enhanced apparent visible-light activity.