Photochromism and Size Effect of WO₃and WO₃−TiO₂Aqueous Sol

The chromism of tungsten oxides has been investigated for a long time because of its academic and application importance. In this article the photochromism and its dynamics for a WO3 aqueous sol and WO3-TiO2 mixed sols are studied in detail. The chromic performance of the WO3 aqueous sol is improved significantly by adding TiO2 aqueous sol, and shows a clear size effect. The intrinsic absorption peak at around 777 run from W5+ in WO3 nanocluster, which generally appears at around 1000 nm in a WO3 film, shifts with cluster size because of the quantum confinement effect of delocalized carriers in WO3 nanoclusters. Besides the 777 nm band from W5+ in WO3, a new extrinsic 640 nm band, due to the transient formation of WOClx species in a WO3 sol in the presence of Cl- and TiO2 nanoclusters, appears after adding TiO2 sol. These results are helpful to the understanding of the chromism of tungsten oxides.