Silver-based photochromic coating obtained using SiO₂@TiO₂ doped hybrid phosphotungstate-ormosils matrix

Full text: For the last few years, our group has been involved in the development of different coating materials for photochromic devices [1,2] such as customized UV sensors, smart windows and photochromic lenses. These coatings employ an ormosil (organically modified silicates) matrix to homogeneously immobilize the photochromic component, the phosphotungstate ([PW₁₂O₄₀] ^3-), a Keggin polyoxometalates. Once irradiated, the phosphotungstate undergoes photo-reduction resulting in the formation of blue coloured species called heteropolyblues. In previous studies, we have demonstrated an enhancement in the photochromic response of phosphotungstate-ormosils films by doping with TiO₂ [3] nanoparticles and ZnO/ZnCl₂ [4]. Similar results were obtained in layer-by-layer coating of titania and phosphotungstates formed on bacterial cellulose [5]. TiO₂ helps in the photo-reduction of phosphotungstate due to a transfer of electron from the conduction band of photo-excited TiO₂ to phosphotungstate [5]. The reduced phosphotungstate can then reduce metal ions. The photoreaction of phosphotungstate by TiO₂ becomes energetically more favourable if TiO₂ with small particle size (<10 nm) and wider band-gap (E_g >3.2 eV) are used. Thus, we doped the ormosil-phosphotungstate films with SiO₂@TiO₂ particles in which the shell is made of well-dispersed and tiny TiO₂ nanoparticles (~5 nm). To improve further the photochromic response and sensitivity toward UV, we have doped the materials with Ag nanoparticles, which are obtained by photoreduced of Ag¹⁺ by phosphotungstate and TiO₂ to metallic nanoparticles. The resulting silver nanoparticles lead to photochromic coating with yellowish coloration and high UV sensitivity. The XRF results of the Ag¹⁺ uptake by the ormosils films. A higher amount of Ag¹⁺ ions are adsorbed during the first 10 minutes followed by low adsorption rate and a plateau is seen after 10 min. The FEG-SEM image of SiO₂@TiO₂ doped ormosils-phosphotungstate film shows the formation of Ag nanoparticles of around 30 nm size as confirmed by their EDX analysis. The Ag-ormosil-phosphotungstate films turns yellow after exposure to UV light (Fig 2, left). The UV spectra of the irradiated films with different immersion time in silver solution is shown in Fig. 2 (right). The characteristic surface plasmon resonance band of Ag at around 420 nm can be seen for samples with immersion times longer than 10 min. It turns out that 10 minutes immersion time which correspond to 2% (w:w) of Ag⁺ ions is necessary to accumulate enough Ag⁺ ions for the formation of the Ag nanoparticles. The nanoparticle formation at lower Ag loading (<1.5%) is not observed probably due to their higher dispersion at such lower concentrations. The plasmonic band at 420 nm and the FEG-SEM image allow us to estimate the Ag nanoparticle average size as 20-25nm. [6] Therefore, new silver-based photochromic coating with higher UV sensitivity were prepared using using phosphotungstate-ormosils as the hybrid matrix which helps in adsorption of Ag¹⁺ and their subsequent photoreduction into Ag nanoparticles of around 20-25 nm. References [1] Ferreira-Neto, E.P., Surface structure and reactivity study of phosphotungstic acid nitrogenated ormosils. J.Sol-Gel.Sci.Technol. 66(2013), 363-371. [2] de Oliveira, M., et al. Chem.Mater., 23 (4), 953-963, 2011. [3] Gonçalves, L.P. Master Thesis, Nanopartículas de dióxido de titânio como aditivos em materiais híbridos orgânico-inorgânico fotocrômicos baseados em fosfotungstatos. São Carlos, SP, 2011 [4] Ferreira-Neto, E.P. et al, J Sol-Gel Sci Tecnol., accepted [5] S. Ullah, et al., Photoactive layer-by-layer films of cellulose phosphate and titanium dioxide containing phosphotungstic acid, Appl. Surf. Sci., 277(2013),111-120. [6] Baida H, et al.; Quantitative Determination of the Size Dependence of Surface Plasmon Resonance Damping in Single Ag@SiO₂ Nanoparticles. Nano Letters, V9, n10, p 3463-3469, 2009. (author)