Investigation of the Photochromism of WO₃, TiO₂, and Composite WO₃-TiO₂ Nanoparticles

We report the synthesis of WO₃, TiO₂, and TiO₂-WO₃ 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 W⁶⁺ concentrations and high ripening temperatures allow the preparation of WO₃ nanoparticles with high photochromic efficiency. WO₃-TiO₂ nanocomposites (NCs) prepared by the introduction of a TiO²⁺ solution in a WO₃ nanoparticle suspension exhibit a strong coloring photochromism, which is attributed to the TiO₂ coating of the WO₃ nanoparticles as it involves the formation of W-O-Ti oxo-bonds in place of W⁵⁺-ν_O defects. Especially, after an oxidative treatment in order to obtain an initial pale-yellow material, such WO₃-TiO₂ 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 WO₃-TiO₂ 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.