Surfactant-Assisted Synthesis of Uniform Titania Microspheres and Their Clusters

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Original Entry: Stephen Fleming, AP 225, Fall 2012

Under Construction 11/28/12

Figure 1, reproduced from [1]. SEM images of titania colloids made using different concentrations of titanium butoxide precursor and Tween 20 surfactant. Diameters are (a) 650nm, (b) 460nm, (c) 330nm, and (d) 300nm.
Figure 2, reproduced from [3]. Emulsion polymerization. The reactive monomers and initiator molecules are trapped inside micellar emulsion droplets dispersed in a solvent phase. The polymerization takes place inside the micelle, and is limited by the amount of material present inside. This scheme is similar to what takes place in the research in [1], except instead of polymeric colloids, titania colloids are produced.
Figure 3, reproduced from [1]. Explanation.
Figure 4, reproduced from [1]. Explanation.
Figure 5, reproduced from [1]. Explanation.


General Information

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.

Access to paper

Keywords: Surfactant, Self assembly, Colloid

Summary

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.


Background

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

From a source at Wikipedia [3] and Washington University [2], the mechanism of this sort of colloidal growth is shown in Figure 2. 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 [1], 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.


Results

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.


Discussion

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 [2].


Reference

[1] 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.

[2] Washington University colloids and emulsions

[3] Wikipedia emulsion polymerization