Low-temperature elimination of organic components from mesostructured organic-inorganic composite films using vacuum ultraviolet light

A novel and simple procedure named photocalcination has been developed for removing organic components from mesostructured organic-inorganic composite films. This procedure employs an excimer lamp radiating vacuum ultraviolet (VUV) light of 172 nm in wavelength. Organic molecules are removed through two distinct photochemical reactions proceeding simultaneously at room tem perature: the photocleavage of C-R and C-C bonds in the organic molecules b y direct photoexcitation and their subsequent oxidation with activated oxyg en species generated by the photoexcitation of atmospheric oxygen molecules . in this study, a composite of cetyltrimethylammonium chloride-silica (CTA C-silica) was used. Thin films of this organic-inorganic composite were pre pared on Si substrates coated with a self-assembled monolayer of octadecylt rimethoxysilane. The films were irradiated with VUV light under pressures o f 10, 10(2), 10(3), or 10(5) Pa. Although, in each case, the CTAC molecules in the composite films were completely oxidized and removed, the eliminati on rate increased with an increase in the photocalcination pressure. While 3 h of irradiation was necessary to completely remove the CTAC molecules at 10 Pa, only 0.5 h was required at 105,Pa. However, the periodic mesostruct ures of the film photocalcined at 10(5) Pa distorted significantly because of the rapid photooxidation. As a control experiment, identical CTAC-silica mesocomposite films-were calcined thermally. The films were heated in air at temperatures from 373 to 773 K. Although the CTAC molecules could be eli minated completely at temperatures higher than 573 K, the periodic mesostru ctures of the thermocalcined films were much more distorted than those of t he photocalcined films.