Silica glassceramics containing zinc oxides nanoparticles by solgel derived bottom-up approach

The typical synthetic route of glassceramics involves the controlled crystallization of molten glasses through tailored thermal treatments. The resulting material is an amorphous phase which contains one or more crystalline phases. Glassceramics share peculiar characteristics of both their constituents. Thus, they show workability and machinability of glasses plus the functional and structural properties imparted by ceramic crystals. Among the many fields of application, glassceramics used as optical materials have more stringent requirements. A key feature in this type of products is to maintain a good transparency. Therefore, it is necessary to reduce crystal dimensions down to the nanometre scale and to minimize refractive index mismatch between host and guest so as to reduce scattering losses. Nonetheless the realization of glassceramic-based optical materials represents a fascinating way for the design of novel lasing medium, special optical fibres, and photo-responsive devices. In this context, solgel has been proved to be a valid method for the controlled precipitation of crystal nanoparticles (NPs) in amorphous pure silica matrix [3]. Here we present a bottom-up approach to produce glassceramics embedding prototypical wide bandgap semiconductors. Briefly, a ZnO NP suspension in an alcoholic solution is used as precursor for silica solgel. ZnO NPs were prepared by standard air-free Schlenk technique starting from zinc acetate as precursors and capped with oleic acid obtaining NPs of about 5 nm in size [2]. The ZnO capping was then stripped and exchanged with (3-Aminopropyl)triethoxysilane and suspended in ethanol. To start the gelification process, water and tetraethyl orthosilicate were added to the alcoholic suspension following well-established silica synthesis [3]. Finally, after complete gelification at 40°C for two weeks, xerogels underwent to thermal treatment up to ≈900 °C to obtain fully densified silica glassceramic containing dispersed nanoparticles. Transparent bulk samples with up to 1% mol of ZnO were obtained and characterized by UV-VIS and Raman spectroscopy, X-ray diffraction and electron microscopy. The results demonstrate that part of ZnO acted as nucleating agent for willemite (Zn2SiO4) and that, during gelation, there is a slight accumulation of NPs at the bottom of the xerogel originated by gravitational sedimentation. Figure 1. Raman spectra of bottom (red line) and top (blue line) side of the synthetized glassceramic with nominal composition of 1% mol ZnO in SiO2. The peaks marked with an asterisk are compatible with Zn2SiO4 in the willemite phase. Further studies will be carried out to extend this general synthesis strategy to other NPs and to establish the relationship between NP load and scattering losses.