Photocatalytic properties of atomic layer deposited TiO2 inverse opals and planar films for the degradation of dyes

The pollution of waste water due to organic dyes used in textile and chemical industries is an important environmental issue. Inverse opals (IO) offer a great potential for increasing the efficiency of semiconductor photocatalysts as TiO2 by the synergy of high specific surface and photonic crystal properties. In this work, we report the synthesis of both IO and planar TiO2 films by Atomic Layer Deposition and a comparative study of their photocatalytic activity for the degradation of methylene blue in water under UV irradiation. The porosity of planar TiO2 films was modified by a pre-treatment of the substrate to analyze its effect on the photocatalytic activity. A rutile single-crystal was also used for comparison. The kinetics of the MB degradation process was studied for long times to investigate the eventual effect of the progressive increase of degradation products in the solution. A degradation percentage about 90% was obtained after 10 h using IO films, and only about 60% by using planar and dense films. A first-order reaction kinetics was shown in the case of IO films. For the other catalysts, a slowing-down of the reaction kinetics was shown above 8 h. The adsorption of the degradation products at the catalyst surface was addressed to explain this effect. The results highlight the potential of IO films synthesized by ALD for photocatalytic applications.