Preparation of Hard Mesoporous Silica Spheres

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Original entry: Ian Bruce Burgess Fall 2009


References

Q. Huo, J. Feng, F. Schuth, G.D. Stucky, Chemistry of Materials 9, 14-17 (1997).


Keywords

emulsions, mesoporous materials, surfactant, calcination

Summary

This paper describes the synthesis of small spheres (0.1-2mm in diameter) made of a mesoporous silica from a surfactant-stabilized emulsion chemistry. What is remarkable about this technique is that the spheres, with hierarchical, levels of ordering on multiple length scales, are synthesized in a one step process. The spheres are synthesized as follows: The silicon ester tetrabutyl orthosilicate (TBOS, <math>Si(OC_{4}H_{9})_{4}</math>) is used as both the silica precursor and the organic phase in a slightly basic aqueous emulsion. The surfactant, cetyltetramethylammonium bromide (CTABr), serves a dual purpose. It acts as a stabilizer for the emulsion, whose droplet size determines the size of the spheres, and forms nanoscale micelles in the organic phase which template the final porous structure. The TBOS reacts with water molecules (catalyzed in basic conditions) forming silica with butanol acting as the leaving group. The butanol plays an auxiliary key role of enhancing the penetration of water into the TBOS phase through hydrogen bonds, enabling the hydrolysis of the TBOS. The reaction proceeds relatively slowly with the emulsion stirring for 15-30h. The stirring speed is also shown to influence the sphere size. The spheres are formed and isolated with surfactant micelles embedded inside. The surfactant is removed by calcination, leaving behind an ordered lattice of air pores. The figure below shows the final product. The spheres (left) are infiltrated with a highly ordered lattice of pores, shown in the TEM image on the right.

Iantopic9.jpg

Soft-Matter Discussion

This paper shows how surfactants can be used as a powerful engineering tool for structuring materials on multiple scales. In this experiment, the surfactant provided stability to the water/organic emulsion, allowing control of micron-size features in the products (the sphere size) while also forming a tightly packed array of micelles in the silica precursor, templating the nanoscale porosity in the silica. These two scales can be independently tuned: while the micelle size is relatively fixed, the lattice spacing of the pores can be tuned by varying the micelle concentration in the TBOS phase and is done by varying the relative concentration of [TBOS:CTABr]. The droplet size in the emulsion can be tuned independently of concentrations by varying the degree of agitaiton of the emulsion via the stir speed during the reaction. A second useful property of surfactant-based templating of inorganic materials is that the surfactant can be removed by heat, leaving behind a multi-scale ordered structure that has single component. Thus the surface chemistry can be redesigned after removal of the template.

Mesoporous materials have many useful properties for a variety of applications. They have a high surface-area/volume ratio and can have very high porosities, with air occupying the majority of the volume. These type of structures have applications as low-index materials for photonics, low dielectric constant materials for electronics, strong light-weight materials, and high S.A./volume materials for catalysis.