Investigation of the Photochromism of WO 3 , TiO 2 , and Composite WO 3 -TiO 2 Nanoparticles.

We report the synthesis of WO ₃ , TiO ₂ , and TiO ₂ -WO ₃ nanoparticles by a polyol route, with the objective of studying the influence of the preparation method on their photochromic properties. By combining transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and diffuse reflectance experiments, we show that low W ⁶⁺ concentrations and high ripening temperatures allow the preparation of WO ₃ nanoparticles with high photochromic efficiency. WO ₃ -TiO ₂ nanocomposites (NCs) prepared by the introduction of a TiO ²⁺ solution in a WO ₃ nanoparticle suspension exhibit a strong coloring photochromism, which is attributed to the TiO ₂ coating of the WO ₃ nanoparticles as it involves the formation of W-O-Ti oxo-bonds in place of W ⁵⁺ -ν _O defects. Especially, after an oxidative treatment in order to obtain an initial pale-yellow material, such WO ₃ -TiO ₂ NCs exhibit a fully reversible photochromism with a large contrast between the colored and bleached state. They could therefore be incorporated in hybrid smart films for solar control on building window glasses. On the other hand, while the WO ₃ -TiO ₂ NCs are functionalized with DPA (n-dodecyl phosphonic acid), the as-prepared nanocomposites exhibit exacerbated coloring contrast but with a nearly nonreversible photochromism (very limited bleaching), which makes them good candidates for the fabrication of smart UV-sensor devices that can indicate the cumulative UV dose which is received.