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In this study hybrid silica gels were doped by adding gold citrate nanospheres. The interface of the two components of this material and the structural differences between doped and non-doped melting gels were studied. Melting gels are a class II subgroup of Organically Modified Silicates (ORMOSILs). This type of gels has been named a melting gel (MG) after its property to soften when heated up at a certain temperature and become rigid when cooled down. This repeatable process can be performed many times until the hybrid silica network is fully crosslinked and results into a glass material.
The melting gels studied in this project were synthesized by the organoalkoxysilanes methyltriethoxysilane (MTES) and dimethyldiethoxysilane DMDES). MTES and DMDES were combined in a sol gel synthesis. Manipulating of the mono-substituted and di-substituted organoalkoxysilanes ratio is a way to control the amount of organic matter being introduced in the hybrid organic-inorganic silica network. For the preparation of the melting gel samples the molar ratios of precursors used were 65% MTES – 35% DMDES, 70% MTES – 30% DMDES, 75% MTES – 25% DMDES. During the synthesis process of MG, gold citrate nanospheres (Au-nsps) were added in the melting gel transforming colorless gel into purple transparent gels. Gold nanospheres were added in five different concentrations as they were synthesized.
With an interest in the modifications that the nanoparticles caused to the hybrid silica network a series of analyses were performed. Vibrational spectroscopies, microscopy and small angle X-ray scattering (SAXS) was utilized to decipher how the MGs are altered by the addition of the nanoparticles. Fourier transform infrared spectroscopy (FT-IR) was used to monitor the consolidation process of both doped and non-doped melting gels. This analysis showed how the melting gel transitions from gel to glass. Raman as a complementary method to FT-IR was employed to study differences between hybrid glasses of the three different aforementioned precursor ratios. TEM was used to image the doped melting gels. Micrographs of microtomed consolidated MGs showed that the gold nanoparticles were distributed across the melting gel network. The size and shape of the monodispersed nanoparticles was not changed during the incorporation process, while some agglomerations were noticeable probably around defects of the network. SAXS performed on doped hybrid glasses confirmed the monodispersity of the Au-nsps, maintaining the size and shape, and the appearance of few agglomerations.
All the compositions exhibited different behavior indicating the formation of distinctive networks and the comparison among different compositions of Au-nsps was held with UV-Vis spectroscopy, rotational rheometry, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). UV-Vis spectroscopy was the first analysis to confirm the maintenance of plasmonic behavior of the nanocomposite. The nanoparticles continued to absorb in UV-Visible range after their incorporation in the melting gel media. An effect to the glass transition temperature (Tg) was confirmed by both rotational rheometry and DSC, as the concentration of Au-nsps the Tg and viscosity of 65% MTES – 35% DMDES and 75% MTES – 25% DMDES went through a minimum. Another noticeable characteristic was that non-doped melting gels in TG-DTA showed more mass loss than doped melting gels.
In conclusion, a new material was created, combining melting gels and gold citrate nanospheres. Doped melting gels inherited the main characteristics of both components, such as thermal and plasmonic properties. Gold nanospheres did affect hybrid silica network both in gel and in glass form. The study of the consolidation process gave substantial information how this transition is carried and what aspects of the hybrid silica network are changing. Lastly, incorporation of Au-nsps was depicted via TEM and confirmed by SAXS. The analysis of this unique material revealed a lot of information for glasses doped by metallic nanoparticles which will be useful in the research of doped hybrid glasses and their applications such as corrosion protection coatings.

ETD_10155

Ph.D.

Includes bibliographical references

doi:10.7282/t3-yb3m-4207

ETD doctoral

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