Effect of Dissolved O2 on the Photochromism of Suspended WO3 Nanoparticles

We report the synthesis by a polyol route of close-to-WO₃ nanoparticles (NPs), in the sense that the as-prepared NPs have a deficiency in oxygen stoichiometry (a WO_3–x formula is therefore more accurate), as a key point in addition to their photochromic properties. Their surface modification with 1-butylphosphonic acid was shown to prevent their hydrolysis when dispersed in water. The aqueous suspensions of the functionalized NPs do not exhibit any photochromism under UV irradiation, unlike those in ethanol, which have a strong and reversible photochromic contrast. This difference has been attributed to the dissolved oxygen content, which is 16.5 times higher in ethanol than in water. This high amount of dissolved oxygen allows ethanol to act not only as an oxidizing agent (in bleaching) but also as a reducing agent (in coloring), whereas the water’s low affinity for oxygen makes it a very poor redox solvent. The effect of oxygen on the photochromism (in terms of characteristic coloring and bleaching times) of the WO₃ NPs was further evidenced through the UV irradiation of suspensions in butanone, octanol, and dimethylformamide, all chosen because their dissolved oxygen content is much higher than that of water. Photochromism is all the more rapid (and therefore “effective”) the greater the affinity of the chosen solvents with molecular oxygen.