Controlling particle morphology and size in the synthesis of mesoporous SBA-15 materials.

Plate-like particles of SBA-15 form from smaller units (primary particles) that aggregate in an oriented manner. In this report we influence this aggregation by adding salt to the ongoing synthesis, generating well-ordered hexagonal p6m structure (SBA-15), with varying particle diameters. The additions, with either NaCl or NaI, were made at a time corresponding to the onset of oriented aggregation. Both salts promote the aggregation and particles with diameters of up to 5 microm were obtained. The thickness, about 0.4 microm, of the particles was retained, in accordance with the proposed growth mechanism. The aggregation characteristic was dependent on the salt used and the concentration of the salt. Both the cation and the anion were found to influence the aggregation. Having salt present already at the start of the synthesis gave a different product. The particle formation process depends on an interplay between attractive and repulsive forces between aggregates. We interpret the observed effect of salt addition to a medium which already has a very high ionic strength (1.6 M HCl) as due to ion binding to the combined polyethylene oxide-silica brush at the particles surface.