Formation Mechanism for Hexagonal-Structured Self-Assemblies of Nanocrystalline Titania Templated by Cetyltrimethylammonium Bromide

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Hexagonal-structured self-assemblies of nanocrystalline (anatase) titania templated by cetyltrimethylammonium bromide (C16H33N(CH3)3Br; CTAB) (Hex-ncTiO2/CTAB Nanoskeleton) were formed after mixing of aqueous solutions containing CTAB spherical micelles and titanium oxysulfate acid hydrate (TiOSO4·xH2SO4·xH2O) as a titania precursor in the absence of any other additives. Formation mechanism of the Hex-ncTiO2/CTAB Nanoskeleton was examined in terms of the reaction temperature, titania precursor/CTAB mixing ratio, surfactant type, electrostatic interaction, micelle formation and molecular component. We found that crystal growth of crystalline (anatase) titania (polymorphic crystallization) was promoted with higher temperature and lower titania precursor content in aqueous solutions. In addition, we revealed that the crystalline (anatase) titania was formed in polycation, poly(allylamine hydrochloride ([CH2CH(CH2NH2)HCl]n; PAH), and formamide (HCONH2) solutions. On the other hand, no titania formation was observed in anionic systems such as sodium dodecyl sulfate (CH3(CH2)11OSO3Na; SDS) and poly(sodium 4-styrenesulfonate ([C8H7SO3Na]n; PSSS). This indicates that hydrolysis reaction of the titania precursor is initiated by not only cations but also nitrogen atoms in molecules and polymers. Hexagonally structure was formed in only cationic surfactant micellar solutions but not in polycation solutions and formamide.
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