Understanding the Role of Cyclodextrins in the Self-Assembly, Crystallinity, and Porosity of Titania Nanostructures

A series of mesoporous titania photocatalysts with tailorable structural and textural characteristics was prepared in aqueous phase via a colloidal self-assembly approach using various cyclodextrins (CDs) as structure-directing agents. The photocatalysts and the structure-directing agents were characterized at different stages of the synthesis by combining X-ray diffraction, N-2-adsorption, field emission scanning electron microscopy, transmission electron microscopy, UV-visible spectroscopy, dynamic light scattering, and surface tension measurements. The results demonstrate that the cyclic macromolecules efficiently direct the self-assembly of titania colloids, resulting in a fine-tuning of the crystal phase composition, crystallite size, surface area, particle morphology, pore volume, and pore size. Depending on the chemical nature of the substituents in the cyclodextrin ring, synergistic or competitive effects arising from the adsorption capacity of these cyclic oligosaccharides onto titania surface, surface-active properties, and ability to aggregate in water by intermolecular interactions were found to substantially impact the characteristics of the final material. We propose that, in contrast to the native cyclodextrins, which tend to favor the local agglomeration of titania nanoparticles due to the strong intermolecular interactions, the substitution of hydroxyl groups by a relatively large number of methoxyl or 2-hydropropoxyl ones in the beta-CD derivatives allows for creating smoother interfaces, thus facilitating the self-assembly of the colloids in a more homogeneous network. The photocatalytic activity of those titania materials was evaluated in the photodegradation of a toxic herbicide, phenoxyacetic acid, and was correlated to the structural and textural characteristics of the photocatalysts.