A new application of UV-ozone treatment in the preparation of substrate-supported, mesoporous thin films

A nominally room temperature photochemical method, simply employing ultravi olet light(187-254 nm) generated ozone environment, is shown to provide an efficient alternative for the removal of surfactant templates for a routine production of mesoporous silica thin films at low temperatures. The treatm ent concomitantly strengthens the silicate phase by fostering the condensat ion of unreacted silanols leading to mesoporous thin films with well-define d mesoscopic morphologies. The surfactant/silicate thin film mesophases wer e prepared onto a polycrystalline Au surface by dip-coating or spin-casting methods using sub-critical micelle concentration (cmc) nonionic ethylene o xide surfactant in an oligomeric silica sol mixture. The structures and com positions of the thin film mesophases before and after exposure to UV/ozone were determined using a combination of reflection-absorption Fourier trans form infrared spectroscopy, transmission electron microscopy, and thin film X-ray diffraction measurements. The pore characteristics of the UV/ozone-t reated films were determined using nitrogen adsorption/desporption isotherm measurements. Results presented here clearly establish that the UV/ozone p rocessing leads to complete removal of the surfactant template; strengthens the inorganic skeleton by fostering silica condensation; and renders the m esophase thin film surfaces highly hydrophilic. Two of the most attractive features of the method developed here, namely its usefulness in application s for temperature intolerant substrates (e.g., thin metal films) and in spa tially selective removal of the surfactant templates to create patterns of mesoporous thin films, are also illustrated. Finally, the mechanistic impli cations of these observations are also discussed.