(739c) Scalable Flame Synthesis and Growth of SiO2 Nanowire Films On Plain Glass as Anti-Fogging Coatings

Pure SiO2 nanowire arrays were grown directly onto glass substrates by scalable flame spray pyrolysis (1) of organometallic solutions (hexamethyldisiloxane or tetraethyl orthosilicate). Silicon dioxide films consisted of a network of interwoven nanofibers or nanowires from few to several hundred nm long (depending on process conditions) and 10 - 15 nm thick, as determined by microscopy. These films were formed by chemical vapor deposition (surface growth) on the glass substrate, for the first time to our knowledge, by combustion of precursor solutions at ambient conditions, as determined by thermophoretic sampling of the flame aerosol and microscopy. Replacing the glass substrate by metal resulted in a highly porous film. In contrast, titanium dioxide films on glass made also by direct FSP and in-situ annealing (2) consisted of nanoparticles 3 - 5 nm in diameter that were formed in the flame and deposited onto the glass substrate resulting in highly porous, lace - like nanostructures. Mixed SiO2 - TiO2 films (40 mol% SiO2) had similar morphology to pure TiO2 films though the titania particles appeared spiky like. Under normal solar radiation, all such films having a minimal thickness of about 300 nm completely prevented fogging of the glass substrates. These anti - fogging properties were attributed to inhibition of water droplet formation by these super - hydrophilic coatings as determined by wetting angle measurements. Deactivated (without UV radiation) pure TiO2 coatings lost their super - hydrophilicity and anti - fogging properties even though their wetting angle was reduced by their nanowicking. In contrast, SiO2 - TiO2 coatings exhibited the best anti - fogging performance at all conditions taking advantage of the high surface coverage by TiO2 nanoparticles and the super - hydrophilic properties of SiO2 on their surface.

(1) R. Mueller, L. Maedler, S.E. Pratsinis, ?Nanoparticle Synthesis at High Production Rates by Flame Spray Pyrolysis?, Chem. Eng. Sci., 58, 1969-1976 (2003).

(2) A. Tricoli, M. Graf, F. Mayer, S. Kühne, A. Hierlemann, S.E. Pratsinis ?Micropatterning Layers by Flame Aerosol Deposition - Annealing?, Adv. Mater., 20, 3005-10 (2008).