Tracing the slow photon effect in a ZnO inverse opal film for photocatalytic activity enhancement

Highly ordered, dense and continuous ZnO inverse opal (ZnO-IO) films with different air sphere sizes have been successfully prepared by a metal salt-based sol-gel infiltration method and used to prove and demonstrate the slow photon effect occurrence to enhance the photocatalytic activity. The obtained ZnO-IO films have a pure wurzite structure with similar crystallite size according to the XRD experiments and show very ordered macroporous structures from SEM and TEM analyses. The ZnO-IO films present a photoinduced surface wettability conversion phenomenon and the wettability of the ZnO film can be tuned from superhydrophobicity to hydrophilicity after UV-vis irradiation. Compared with the ZnO film without inverse opal structure, both ZnO-IO films demonstrate highly enhanced photocatalytic activities due to the hierarchically macro-mesoporous structure and particularly the slow photon enhanced light absorption. The synergy of the slow photon effect and hierarchically porous structure of inverse opal itself results in the highest photocatalytic activity at an incident light angle of theta = 40 degrees. Moreover, our results suggest that the slow photon effect occurring at the red edge of PBG exhibits a higher photocatalytic reaction rate than that at the blue edge of PBG. The extraordinary enhancement of the photocatalytic activity via changing the incident light angle reveals that the slow photon effect does take place and further dramatically enhance the photocatalytic activity of ZnO-IO films. This work may open an exciting door to all the fields related to light absorption, such as solar cells, and optical and electrooptical devices.