Enhanced photocatalytic NO removal and toxic NO2 production inhibition over ZIF-8-derived ZnO nanoparticles with controllable amount of oxygen vacancies

The controlled introduction of oxygen vacancies (OVs) in photocatalysts has been demonstrated to be an efficient approach for improving the separation of photogenerated charge carriers, and thus, for enhancing the photocatalytic performance of photocatalysts. In this study, a two-step calcination method where ZIF-8 was used as the precursor was explored for the synthesis of ZIF-8-derived ZnO nanoparticles with gradient distribution of OVs. Electron paramagnetic resonance measurements indicated that the concentration of OVs in the samples depended on the temperature treatment process. Ultraviolet–visible spectra supported that the two-step calcined samples presented excellent light-harvesting ability in the ultraviolet-to-visible light range. Moreover, it was determined that the two-step calcined samples presented superior photocatalytic performance for the removal of NO, and inhibited the generation of NO2. These properties could be attributed to the contribution of the OVs present in the two-step calcined samples to their photocatalytic performance. The electrons confined by the OVs could be transferred to O2 to generate superoxide radicals, which could oxidize NO to the final product, nitrate. In particular, the NO removal efficiency of Z 350-400 (which was a sample first calcined at 350 °C for 2 h, then at 400 °C for 1 h) was 1.5 and 4.6 times higher than that of Z 400 (which was one-step directly calcined at 400 °C) and commercial ZnO, respectively. These findings suggested that OV-containing metal oxides that derived from metal-organic framework materials hold great promise as highly efficient photocatalysts for the removal of NO.