Titanium oxide hydrates: optical properties and applications

Abstract

TiO2 has been extensively studied in the last decades due to its interesting optical and electronic properties, which, combined with low fabrication costs, renders this material very attractive for applications in photovoltaic and photocatalysis. However, the performances of titania in specific device applications were found to be strongly dependent on the synthetic methods selected for its production. The majority of such synthetic procedures rely on the hydrolysis of suitable precursors and often produce an amorphous solid, generally referred as the “amorphous” titanium oxide beside the crystalline titania. In this thesis, we thus set out to investigate amorphous materials produced by the hydrolysis of titanium tetrachlorides and tetraisopropoxide. We show that these amorphous products consists of titanium oxide hydrates, which are relatively stable at room temperature and fully convert into crystalline titania only after extended temperature treatments. We also find that titanium oxide hydrates may display highly desirable characteristic such as a strong photochromic response – especially when placed in a suitable chemical environment. In the following chapter, we then show 3 that hybrid systems can be readily prepared of titanium oxide hydrates with, for instance, macromolecular materials such as poly(vinylalcohol). The amorphous nature of the titanium oxide hydrates allows to introduce more than 90 vol.% of the inorganic species into such systems – compared to 15 vol.% or less when producing hybrids comprising, e.g., crystalline nanoparticles of TiO2. Therefore, materials can be realized that display a refractive index n of at least 2.1, without compromising transparency of the resulting structures. Remarkably, n can not only be adjusted by varying the content of the inorganic species, but also through suitable heat treatments and/or irradiation with UV-light. Potential applications for such new, versatile and tunable optical systems are also discussed in this thesis.