Hybrid and nanocomposite concepts: a driving force for novel materials

The general concepts of hybrid and nanocomposite were used not only to classify but also to drive the synthesis optimizing the materials for the different applications. The common denominator of each material was the titanium dioxide as photoreactive material, chapters 3 and 4, and dielectric material in chapter 5. Because both the morphology and the crystal phase of the titanium dioxide play a crucial role on the material performances the used TiO2 was always synthesized ex situ by hydrothermal synthesis allowing to control the morphological characteristics. In the chapter 3 and 4 anatase phase was used for its photocatalytic ability in presence of oxygen and water while in the chapter 5 the rutile phase was used because its highest dielectric constant compare anatase. In chapter 3 a polyacrilate composite material prepared by mechanical mixing of nanocrystalline titania with acrylate oligomers shows that, without titania surface functionalisation, the oxide forms micrometric aggregates reducing the exposed surface of the TiO2. Despite the filler aggregation the material preserves its photocatalytic properties. As drawback of the photocatalytic activity in phenol photomineralisation (use to simulate pollutants in water) some photooxidative degradation phenomena involve the organic matrix. Hence the necessity to have a stable material was the driving force to create a new material containing titania as photoactive material while embedded into an inorganic matrix. In the chapter 4 this porous and UV transparent inorganic-inorganic nanocomposite material is described. In order to obtain the desired porosity of the final material the silica sol-gel solution was mixed with PEG obtaining a class I hybrid material. During the silica formation PEG segregates in warm-like polymeric phase that, once the material is calcinated, leaves the voids conferring the desired macroporosity to the material. The photoactive oxide, previously functionalized on the surface with organic molecules, migrates in the polymeric phase during silica precursors hydrolysis and condensation. After calcination the titania nanocrystallites decorate the wall of the channels leaved by the organic species remotion. The molecules functionalizing the catalyst surface induce the TiO2 migration into the PEG phase because of their more affinity with polymer instead with silica. The exposed titania is then able to freely react with pollutants while the silica matrix provides the UV transparency and macroporosity for the photocatalytic reactions. The abatement efficiency of the material is comparable with slurry TiO2. The material is not affected by the photocatalyst leaching demonstrating that it is suitable for an industrial application. The nanocomposite material was tested for NOx degradation too using P25 commercial titania instead of home made one demonstrating the generality of the preparation method. The abatement efficiency of the NOx was comparable with the DENOX technology currently used for industrial applications. In chapter 5 the same reaction technique used to functionalize the nanoparticles in the chapter 4 was used to functionalize rutile titania nanoparticles with a RAFT reagent. After the styrene “polymerization from” reaction polystyrene chains were obtained. The brush like conformation of the chains justifies the high polymer surface density. The functionalized nanoparticles (class II material) are mixed in different concentrations with commercial polystyrene. The different concentration materials present good dispersion because of the high compatibilization properties of the surface functionalisation. At high concentrations the material shows a percolative behavior ascribed to the formation of chestnut like aggregates which increase the relative dielectric constant. Despite the charges percolation trough the material the polymeric surface layer acts as an insulating layer which contributes to mitigate the charge mobility and consequently the conductivity of the material. The low conductivity of the material allows to obtain low tanδ values. The low tanδ values in a large range of frequencies allows to candidate the material for radio frequency (RF) applications where very low dissipation factor is desired to avoid signal losses. In conclusion the present work, despite it covers three different materials, demonstrates how it is possible to create and optimize a material modifying the surface of the nanoparticles in order to confer them peculiar properties which drive the final material morphology. The final material morphology is then able to combine the properties both of the active material and of the matrix giving a new optimized material for a specific application.