Surfactant-Assisted Synthesis of Uniform Titania Microspheres and Their Clusters
Authors: HK Yu, G Yi, J Kang, Y Cho, VN Manoharan, DJ Pine, and S Yang.
Publication: HK Yu, G Yi, J Kang, Y Cho, VN Manoharan, DJ Pine, and S Yang. (2008) "Surfactant-assisted synthesis of uniform titania microspheres and their clusters." Chem. Mater. 20 pp. 2704-2710.
This work focuses on creating uniform microspheres of titania using a surfactant-assisted synthesis method. The motivation for making clusters has to do with their unique optical properties. The spheres can also be doped with a Europium compound to create phosphorescence sources.
Colloids with sizes comparable to the wavelength of light interact strongly with light, and thus exhibit interesting and potentially useful optical properties. One such use could be as light scatterers in displays, etc. Titania is the material used in this study, because it has a relatively high refractive index. Titania microspheres are hard to create with uniformity and repeatability, so a new synthesis route was developed using surfactants to stabilize growing microspheres.
Explanation of Experimental Methods
The mechanism of this sort of colloidal growth is shown in Figure 2, from sources at Wikipedia  and Washington University . In this study , the solvent phase is acetone, and the active "monomer" and initiator is chelated titanium butoxide in ethylene glycol. The surfactant forms micelles of a certain size, and the colloidal particles grow within these micelles, producing colloids of uniform size and shape.
In this study , the titania microspheres were made from a precursor solution of 0.46% titanium butoxide, ethylene glycol, a stabilizing surfactant, and acetone.
Various surfactants were experimented with, including Tween 20, SDS, CTAB, and Pluronic P123.
To create monodispersed 650nm micropheres, 2.03mM Tween 20 was added to 12mL of acetone. Then 1.2mL of a mixture of 0.46% titanium butoxide in ethylene glycol was added. The titanium butoxide concentration in acetone was then 1.46mM. After one day, precipitate formed. Unreacted precursors and acetone were removed using solvent exchange. The precipitate was redispersed in ethanol.
The sizes of microspheres could be controlled by changing the concentration of titanium butoxide or by changing the concentration of surfactant.
Figure 1 shows that monodisperse microspheres of titania can be produced using Tween 20 surfactant. The scale bars are all the same size, and particle diameters range from 300nm to 650nm.
Colloid sizes can be carefully controlled in two ways, shown in Figure 3. First, the micelle size can be controlled using different concentrations of surfactant. However, these changes are small, and the effect on colloid size is correspondingly small (Figure 3b). It would seem that higher concentrations of surfactant lead to smaller micelles. Secondly, the size of the finished colloids can be controlled by changing the amount of titania precursor dissolved in the emulsified phase. Higher concentrations lead to significant increases in colloid size, as would be expected (Figure 3a).
Figure 4 shows experimentation with different surfactants other than Tween 20 (which was the most successful). SDS, CTAB, and Pluronic P123 were all tried, with limited success. The resulting titania particles were not spherical colloids and many agglomerates were produced, including rod-like structures with Pluronic P123 (Figure 4c). These effects are attributed to the fact that the ionic surfactants SDS and CTAB were not as good as the nonionic Tween 20 in acetone. The rod-like structures grown using the triblock copolymer surfactant Pluronic P123 are attributed to the shape of the templating micelles being rod-like as well.
As a further step, colloidal clusters were created from monodisperse colloids using hexane in water emulsions. The colloids were dispersed in the hexane phase, which was encapsulated in Pluronic P123 micelles in water. By evaporating the hexane phase from this emulsion, the micelles slowly shrank and the colloids would be pulled together, creating colloidal clusters, as shown in the scheme in Figure 5.
Though the paper did not go into these details, the mechanism of surfactant action in stabilizing microspheres is probably by adsorbing at the acetone / growing titania interface, producing micellar structures . The size of the micelles controls the size of the finished colloids. The micelle size can be controlled by varying the surfactant concentration, and the size of the colloids can also be varied by increasing or decreasing the concentration of active titania precursor inside the micelle emulsion phase.
The colloidal titania particles and clusters produced in this work  could have interesting optical applications due to their relatively high refractive index (measured at 2.2) and their comparable size to the wavelength of light. It will be interesting to see what applications arise.
 HK Yu, G Yi, J Kang, Y Cho, VN Manoharan, DJ Pine, and S Yang. (2008) "Surfactant-assisted synthesis of uniform titania microspheres and their clusters." Chem. Mater. 20 pp. 2704-2710.