La-Doped BiOI Microspheres for Efficient Photocatalytic Oxidation of NO under Visible Light Illumination

Photocatalytic oxidation has been widely acknowledged as an economical and effective technology for the treatment of low- concentration NO. Three-dimensional (3D) BiOI microspheres, which are typical visible-light responsive semiconductor photocatalysts, often suffer from quick recombination of photogenerated carriers and unsatisfactory electrical conductivity when applied in NO photocatalytic oxidation reactions. However, owing to their micro-sized structures, they are usually difficult to couple with other semiconductors and co-catalysts because of their incompact interfaces that provide insufficient contact. In this study, a rare-earth metal (La) doping strategy was first adopted to modify BiOl microspheres via a simple one-step solvothermal method; subsequently, the photocatalytic NO oxidation performance under visible light illumination was systematically investigated. Further, the La precursors and doping contents were optimized. It was found that La(NO3)(2) was the best precursor when compared to LaCl3 and La(AC)(3). Moreover, 0.3%La/BiOI exhibited the best NO photocatalytic conversion efficiency of up to 74%, which was significantly higher than that of the pure BiOI benchmark (44%). It also exhibited excellent stability during the continuous 5-cycle experiments. Analysis of the physicochemical properties revealed that La doping facilitated the crystallization of BiOI without altering its morphology and structure. La3+ may enter the BiOI lattice by substituting Bi3+ or forming La2O3 nanoclusters that homogeneously scatter in the mesopores of BiOI microspheres. The analysis of the underlying mechanism further revealed that La doping not only enhanced the light harvesting properties by decreasing the bandgaps of BiOI and accelerating the charge separation and transfer dynamics, but also introduced more oxygen vacancies and facilitated the formation of more OH radicals by dissociating the water molecules. All these factors co-contributed to the promotion of NO photocatalytic oxidation activities. Furthermore, NO was mainly oxidized to NO2 over La/BiOI, and the formed NO2 tended to desorb from the catalyst surface, which not only maintained the intactness of active sites and facilitated the sustainable occurrence of NO photocatalytic oxidation reactions, but also prevented the photocatalysts from frequent washing-regeneration; therefore, these factors account for the superior photocatalytic stability of La/BiOI and its long-term operation. The formed NO2 could be easily and totally absorbed by the tail alkaline liquid, thereby effectively avoiding secondary pollution. Therefore, this study elucidates that doping is indeed a feasible and effective approach for the modification of 3D BiOI microspheres, while providing inspiration for the rational design and modification of other 3D semiconductor materials for various photocatalytic applications.