High photocatalytic activity enhancement of titania inverse opal films by slow photon effect induced strong light absorption

The photonic effect on the photocatalytic activity of the continuous titania inverse opal (TiO₂-IO) films differing only by the air sphere size (185 and 165 nm) and prepared by a colloid crystal template approach and annealing at different temperatures (700 and 800 °C) has been investigated by aqueous solution degradation of dye pollutants using mesoporous TiO ₂ thin films as the reference. The high quality of TiO₂ inverse opal films has been confirmed by a blue shift of the Photonic Band Gap (PBG) with increasing light incident angle. Excellent agreement was found between theoretical and experimental reflectance spectra, confirming the photonic crystal structure of the samples. The slow photon light absorption enhancement effect inducing highly improved photocatalytic degradation of dye pollutants has been revealed in an aqueous reaction system. When compared with the mesoporous (m-TiO₂) films obtained under the same conditions, all the TiO₂-IO films demonstrate a much higher photocatalytic activity. At a light incident angle of 0°, the TiO₂-IO-700 film (air sphere size: 185 nm) showed a better photocatalytic activity than that of TIO₂-IO-800 (air sphere size: 165 nm). Most importantly, with increasing light incident angle, the photocatalytic activity of the TiO ₂-IO-700 film decreases whilst that of the TiO₂-IO-800 film increases sharply due to the enhancement of light absorption related to a slow photon effect, generating more electron-holes. The present work revealed that photocatalytic activity can be dramatically enhanced by utilizing slow photons located at the PBG edges with energies close to the electronic bandgap of the semiconductor. The study using the slow-photon effect on the basis of photonic crystals to improve the photocatalytic activity by enhancing the light absorption could be an important future research direction. The slow-photon effect can open a new exciting avenue to all the fields related to light absorption including solar cells, optical telecommunications and optical computing.