Titanium-oxide-hydrates based organic/inorganic molecular hybrids produced via sol-gel processing

Abstract

The titanium oxide hydrate / poly(vinyl alcohol) hybrid produced from TiCl4 is an exceptional example of taylor-made organic / inorganic hybrid. This material combines remarkable optical properties (i.e. tunable refractive index and low optical loss) with a simple synthetic process and advantageous processing versatility. Recent studies have demonstrated the potential of its application for the realisation of some outstanding solution processed thin film based and micro-patterned optical structures. This work reviews the chemistry of the titanium oxide hydrate / poly(vinyl alcohol) hybrid and provides means to chemically modify its properties both altering the synthesis in situs and by means of post deposition chemical treatments. In this work we elucidate the sol-gel process that fully explains the dynamic of the hybrid formation and the response to thermal treatments. Two innovative post deposition treatments (i.e. the water washing and the ammonia washing) are introduced that are able to promote crosslinking reactions. These treatments produce alterations of the optical properties that are comparable to those seen after annealing treatments. Remarkably, these washing treatments allow for the realisation of entirely room temperature processed titanium oxide hydrate / poly(vinyl alcohol) hybrids with record refractive index values and improved thermal stability. A new titanium oxide hydrate / poly(vinyl alcohol) hybrid is introduced, the TiBr4 based hybrid, realised substituting the conventional titanium precursor, the TiCl4, with TiBr4. The halogen species play an important role in regulating the kinetic of the sol-gel process in titanium oxide hydrate / poly(vinyl alcohol) hybrids. The crosslinking is less thermodynamically favoured in TiBr4 hybrids leading to less densely cross-linked materials, thereby affecting both optical and mechanical properties. Our findings are strongly supported by a combination of optical, mechanical and thermal characterisations that were obtained using investigation techniques including: UV-vis spectroscopy, nanoindentation, thermogravimetric analysis (TGA), dynamic scanning calorimetry (DSC) and contact angle.