Bi2O3-sensitized hierarchically mesoporous ZnO nanoparticles for Hg(II) reduction

Herein, an affordable and novel approach to design Bi₂O₃-sensitized hierarchically mesoporous ZnO nanoparticles (NPs) with a variety of Bi₂O₃ contents is achieved for Hg(II) reduction upon visible light exposure. TEM images of both ZnO and 3% Bi₂O₃/ZnO samples exhibit nanoscale spherical-like structures with a regular shape and a particle size of ~30 nm. The incorporation of Bi₂O₃ on hierarchically mesoporous ZnO networks allows visible light to be harvested in a broad range, and the mesoporous 3% Bi₂O₃/ZnO heterostructure demonstrates the best photocatalytic efficiency for Hg(II) reduction with a value of ~100% after 60 min. The photoreduction rate over the 3% Bi₂O₃/ZnO heterostructure is enhanced 10 and 20 times more than that of TiO₂-P25 and ZnO NPs. The rate constant of the 3% Bi₂O₃/ZnO heterostructure is 16.8 and 33.6 fold larger than that of TiO₂-P25 and ZnO NPs. The superior Hg(II) photoreduction performance could be ascribed to the synergistic effect, excellent visible-light harvest, large surface area, and pore volume provided by incorporating Bi₂O₃ and the heterojunction design between p-type Bi₂O₃ and n-type ZnO. This alignment of the electronic bands provides charge carrier separation, thereby decreasing the recombination rate. Finally, the mechanisms and kinetics for the photocatalytic reduction of Hg(II) are proposed.