Coral-Shaped TiO_2−δ Decorated with Carbon Quantum Dots and Carbon Nanotubes for NO Removal

Coral-like TiO2−δ microstructures with high surface area were synthesized by a simple hydrothermal method, followed by thermal treatment in a N2 atmosphere. The introduction of oxygen vacancies (OVs) to TiO2 significantly improved light absorption and inhibited the recombination of photogenerated charge carriers. Then, multiwalled carbon nanotubes (CNTs) and N-doped carbon quantum dots (N-CQDs) were decorated on the surface of coral-like TiO2−δ microstructures. The CNTs could greatly improve the separation and transfer efficiency of photogenerated charge carriers, while the N-CQDs could further extend light absorption to longer wavelengths, including the IR region. According to the results of electron spin resonance (ESR) spectroscopy, the introduction of OVs to TiO2−δ and surface modification with CNTs and N-CQDs promoted the generation of •O2– and •OH active species. The formation of •O2– and •OH active species on the surface of TiO2−δ/CNTs/N-CQDs played an important role in the deep oxidation and selective conversion of NO into nitrate. Density functional theory calculations revealed the adsorption and reaction sites for H2O and O2 separately in space over the surface of TiO2−δ/CNTs/N-CQDs. Namely, the reaction sites of H2O and O2 are on the surface of TiO2−δ and CNTs, respectively. The electron transfer from TiO2−δ to CNTs was further verified through the differential charge density. This study demonstrates a straightforward approach for designing full-spectrum-responsive photocatalysts with high efficiency, stability, and selectivity for NO removal.