Crystalline Titania Nanorods for Colloidal Liquid Crystal Based Switchable Optics

Abstract

This thesis deals with the synthesis, characterization, and optimization of titania nanorods colloidal dispersions. Two polymorphs of titania nanorods are developed in the form of highly stable colloidal dispersions. We show that by engineering the ligand density at the surface of titania nanorods and optimizing experiment conditions, relatively monodisperse titania nanorods with tunable aspect ratios and lengths less than 100 nm can be achieved at a high yield at the colloidal level. These NRs are small enough not to strongly scatter light, but their high refractive indexes and anisotropies can be utilized to manipulate (polarized) light. Self-assembled structures of the brookite nanorods are qualitatively similar to the nematic and smectic liquid crystalline phases predicted for hard rod-shaped particles. Interestingly, it is also observed that the ordering within the smectic layers can be tuned from hexagonal to tetragonal reflected by the shape of the brookite nanorods’ hard cores. Furthermore, in our preliminary results, it is observed that both of these isotropic dispersions of titania nanorods at relatively high volume fractions can be aligned into the direction of the applied electric field within ca. 250 microseconds switching times in various field strengths, one order of magnitude faster than commercial Freedericksz-based liquid crystals. Possible mechanisms contributing to the reorientation of titania NRs are discussed and experimental observations are confirmed with theories. Moreover, refractive indexes of anatase and brookite colloidal dispersions at various volume fractions are measured and subsequently compared with effective medium models such as Maxwell-Garnett and Bruggeman theories. The largest estimated birefringence for utilized anatase and brookite NRs dispersions are approximated to be 0.025 and 0.034, respectively. Finally, the possibility of charging such colloidal nanorods in apolar and low-polar media is shown. Our results confirm that both titania NRs are charged positively in toluene (as an apolar solvent) and cyclohexylbromide (as a low-polar solvent) if Span80 (an acidic nonionic surfactant) is used and are charged negatively if OLOA1200 (basic nonionic surfactant) is present. The ion formation and sustaining electric charges are believed to be caused by introducing reverse micelles formed by added surfactants. Although electrophoretic measurement does not provide any insights in apolar solvent without the addition of surfactant, we think there may be some charge on these NRs in ‘pure’ toluene but it is too small to measure, since we could still observe reliably small charges on the NRs in CHB which has similar refractive index contrast. Nevertheless, we believe this charge has been promoted and sustained in the bulk solution via the reverse micelles formed by the existence of desorbed ligand molecules in the solution. Finally, we believe that our demonstrated results provide experimental supports for the well-known theory of acid-base charging in the nonaqueous media. Considering high colloidal stability together with the large optical birefringence and fast switching dynamics in titania nanorods dispersions, we believe they are promising candidates for many fields in the optical industry.