Nanomaterials and nanoparticles are nowadays used in a wide range of applications; from sunscreens, paint and food industry to catalytic processes, lighting, TV's and displays, their presence is percolating in our daily life. New applications rise every day, pushing forward the technology limits with increasing efforts and funding for nanomaterial science and applications. Among different nanomaterials, titanium dioxide (TiO2) continues to attract considerable theoretical and experimental attention because of its technological relevance in applications such as environmental purification, solar energy harvesting, memristors, catalysis, advanced ceramic materials and more. Different allotropes and morphologies of the compound are being examined, in particular nanoparticles and nanofilaments (nanowires and nanotubes) in titanate (H2Ti3O7), anatase or rutile phase.
In this dissertation, we have deeply investigated the behavior of titanium-oxide based nanowires (TiO2 NWs), which, thanks to their larger specific surface area than bulk or polycrystalline counterparts, are very promising for photocatalysis. The TiO2 NWs are very suitable for such nanofilamentary applications since they have an ideal form for assembling themselves into nanoporous aerogel membranes. Our nanowires can be used directly in both 2D and 3D architecture, and they can serve as ideal support (reducing Ostwald ripening and coarsening in general) for various metal nanoparticles.
H2Ti3O7 NWs are equally important, indeed, both as active components, for example in case of very sensitive humidity sensors and efficient catalysts and as reinforcement fibers in composites or as metal nanoparticles support for advanced CO oxidation catalysis.
The NW behavior in titanate, anatase and rutile phase has been studied and the phase transition temperature determined, finding out that the strong surface effects, in the 2D system, result in unexpected stabilization of the metastable anatase phase and the absence of rutile phase above 900 °C. However, the classical Ostwald ripening particle coarsening is observed in sintered pellets, accompanied by a typical anatase to rutile phase transition at 900 °C.
Mechanical measurements and nanoscopic observations both on bulk and on individual nanowire have allowed determining the mechanical moduli and the hardness. In particular, for the first time, the shear modulus of individual nanowire in titanate phase has been directly measured and its relatively low value of 1.5±0.8 GPa confirms the layered structure with a relatively weak bond between layers. The elastic modulus is within the expected range of about 70 GPa for titanate and above 100 GPa for anatase.
Functionality and potential impact of TiO2 NWs for a few specific catalytical applications have been explored. The photocatalytic effectiveness of titania nanowires for destroying DNA chains after a suitable exposition to UV light (elimination of pollutants) was established. Finally, I investigated the application of our TiO2 NWs in the form of paper filters for viruses and bacteria, like the current SARS-CoV-2. I demonstrated the concept of novel personal protective equipment (PPE), that TiO2 NWs based-facial mask is may be very effective stopping the virus or bacteria, but also that it is easily regenerated with simple UV exposition, cutting down the economical and environmental cost of a future product.
EPFL2020
