ZnO with Controllable Oxygen Vacancies for Photocatalytic Nitrogen Oxide Removal

Semiconductor-based photocatalysis is an ideal method for air purification by eliminating nitrogen oxide (NO). However, sluggish carrier separation, photocatalysts deactivation and incomplete oxidation are significant bottlenecks for photocatalytic treatment of indoor pollutant NO. Herein, ZnO with assorted structures is fabricated and undergoes further modification for deliberate surface defect constructions. Utilized flux agents during the synthesis provide a more feasible reducing atmosphere, under which spontaneous generations of the surface vacancies become easier, and gradient concentrations are precisely controlled. Photocatalyst characterizations affirm the successful creation of surface defects, which are further evaluated by solar-light-driven NO (ppb level) removal investigations. Results showed that ZnO rich in oxygen vacancies (VO-rich ZnO) exhibited 5.43 and 1.63 times enhanced NO removal with fewer toxic product NO2 formations than its counterparts pristine and VO-poor ZnO, respectively. Importantly, with higher VO on the unusual nonpolar facets, VO-rich ZnO does not only display enhanced NO conversion, but also shows the unselective NO removal process by producing NO3–. The plausible reaction mechanisms of promoted NO conversions are further investigated based on the surface VO, well-positioned band structures, and enhanced carrier separations. Results showed that the surface VO with gradient concentrations are not only promoted carrier separation, but also facilitate molecular oxygen activation, leading to the generations of strong oxidant superoxide radicals (·O2–), and contributing to the enhanced improved efficiency. Adsorption of small molecules (O2, H2O and NO) on the defective surface was further investigated by density functional theory (DFT) calculations, which validated the successful adsorption/activation of NO and O2, further contributed to the improved NO conversions.