Preparation of Hard Mesoporous Silica Spheres

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Original entry: Ian Bruce Burgess Fall 2009


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


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, 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 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.


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.