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

A nominally room temperature photochemical method, simply employing ultraviolet 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 treatment concomitantly strengthens the silicate phase by fostering the condensation of unreacted silanols leading to mesoporous thin films with well-defined mesoscopic morphologies. The surfactant/silicate thin film mesophases were prepared onto a polycrystalline Au surface by dip-coating or spin-casting methods using sub-critical micelle concentration (cmc) nonionic ethylene oxide surfactant in an oligomeric silica sol mixture. The structures and compositions of the thin film mesophases before and after exposure to UV/ozone were determined using a combination of reflection-absorption Fourier transform infrared spectroscopy, transmission electron microscopy, and thin film X-ray diffraction measurements. The pore characteristics of the UV/ozone-treated films were determined using nitrogen adsorption/desporption isotherm measurements. Results presented here clearly establish that the UV/ozone processing leads to complete removal of the surfactant template; strengthens the inorganic skeleton by fostering silica condensation; and renders the mesophase thin film surfaces highly hydrophilic. Two of the most attractive features of the method developed here, namely its usefulness in applications for temperature intolerant substrates (e.g., thin metal films) and in spatially selective removal of the surfactant templates to create patterns of mesoporous thin films, are also illustrated. Finally, the mechanistic implications of these observations are also discussed.