Novel N/Carbon Quantum Dot Modified MIL-125(Ti) Composite for Enhanced Visible-Light Photocatalytic Removal of NO

Highly efficient and stable photocatalysts were synthesized at room temperature by modifying MIL-125(Ti) with N-doped carbon quantum dots (N/CM(Ti)). The N/CM(Ti) with 2.5 vol % N doped carbon quantum dots (N/CQDs) had the best light absorption and visible-light photocatalytic nitrogen oxide (NO) removal efficiency (approximately 49%). It was found through X-ray photoelectron spectroscopy analysis that a N–Ti–O bond was formed in the 2.5 vol % N/CM(Ti), which is more conducive to charge transfer. Photocurrent and electrochemical impedance data also showed that the carrier separation efficiency of 2.5 vol % N/CM(Ti) was significantly superior to that of MIL-125(Ti). In addition, the TiIII–TiIV of MIL-125(Ti) acts as the active center for photocatalytic removal of NO. Two possible electron migration paths were proposed: electron transfer from N/CQDs to TiIII–TiIV center of MIL-125(Ti) due to the photoinduced electron transfer property of N/CQDs, and absorption of UV light generated from the N/CQDs by the terephthalic acid ligand followed by electron transfer to metal active sites for photocatalytic removal of NO.