Slow photons in TiO2 inverse opals: optical amplification and effect of disorder on the photocatalytic efficiency

Nanocrystalline TiO2 (nc-TiO2) in the anatase phase is widely used for photo-degrading organic pollutants in a variety of environmental applications. Herein we show that slow photons in photonic crystals fashioned from nc-TiO2 can optically amplify the photocatalytic efficiency as a result of the longer path length of light and increased probability in anatase absorption, and that the optical amplification is tolerant to some degree of disorder. We investigated the photodegradation of adsorbed methylene blue on inverse TiO2 opals (i-nc-TiO2-o) with different stop-band energies under monochromatic and white light irradiation. By using template spheres with different diameters, the energy of the slow photons was tuned in-and-out of the anatase electronic absorption, thereby allowing the systematic study of the effects of photonic structure on the photo-degradation efficiency of TiO2. Under monochromatic irradiation at 370 nm, a remarkable twofold enhancement was observed for i-nc-TiO2-o with stop-band at 345 nm, as a result of slow photon coupling at 370 nm. Under white light (>300 nm) irradiation, an increase in the photo-degradation efficiency was observed when the stop-band moves from 370 to 300 nm, as a result of slow photon coupling and the suppression of stop-band reflection by the anatase absorption. By optimizing the energy of the photonic stop-band with respect to the semiconductor electronic band gap, we effectively harvested slow photons in the dielectric part of the material to give optically amplified photochemistry. Furthermore, we studied the effect of structural disorder on the photocatalytic efficiency of the inverse opals by introducing different fractions and sizes of guest spheres into the opal template. We found that half of the enhancement originally achieved by the inverse opal made from monodispersed spheres is conserved when the domain size of the host spheres remains above a critical threshold. Such a high tolerance to structural disorder provides strong support for the potential use of inverse TiO2 opals in environmental cleanup and water treatment applications.