Enhancing photothermal VOCs removal in ZnO-based catalysts via transition metal-based band edge tuning and oxygen vacancy engineering

In this study, the band edge tuning and oxygen vacancy optimization of ZnO-based catalysts were explored via transition metal doping for application to photothermal aromatic and halogenated VOCs degradation. Initial minute doping (0.5 wt %) experiments were conducted to determine the dopant with the highest potential, and it was revealed that Zr4+ doping was able to improve chlorobenzene removal to 74.14 % after 4 h of simulated solar light irradiation at 320 K. Further optimization of Zr (Zr0.05ZnOx; 5 % Zr doping) resulted to even greater reduction of toluene and chlorobenzene, from 300 ppmv to 0.4 ppmv (99.9 %) and 52.9 ppmv (82.4 %), respectively. Additional comparative removal experiments also showed the superior performance of Zr0.05ZnOx in contrast to ZnO. The extent of removal highly depended on the functional group attached, with the removal efficiency following the trend: o-xylene ≥ toluene > benzene > bromobenzene > chlorobenzene. Based from the comprehensive analysis of the EPR, XPS, and UV-DRS results, the observed boost in the photocatalytic activity from pristine ZnO can be attributed to 1) the increase in the concentration of oxygen vacancies, resulting in improving the hole-carrier separation, and 2) the valence band shifts associated to the doping of Zr4+ into the ZnO lattice for an enhanced potential difference towards ·OH generation. Comparison with commercially available photocatalysts (TiO2 and ZnO) also revealed that the VOC removal over sol-gel derived Zr0.05ZnOx was much higher, suggesting immense potential for industrial application.