No abstract

We report on the mechanism of growth of mesoporous silica (SBA-15, plane group p6m). In situ studies of the formation using ultrasmall angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS) covering length scales from 5 to 10 000 Å, complemented with UV−vis and transmission electron microscopy (TEM), provide unique data on particle growth coupled with information regarding the progression of the mesostructure formation and the micellar evolution.

We present a model that explains the morphology of mesoporous SBA-15 particles based on the relative surface energies of the defining faces. We also describe how the formation process influences the morphology and hence the surface energies. The model is compared to experimental observations, made primarily with scanning and transmission electron microscopy. Some materials were also examined in more detail with scanning transmission electron microscopy tomography. Materials were synthesized as a function of synthesis temperature, silica source and content of sodium chloride. The model describes the observed change in aspect ratio of the particles with respect to temperature. Silica source and addition of sodium chloride also affect the morphology. Under certain conditions the particle morphology is the result of an aggregation step that has an orientational character. Under the lower synthesis temperatures this association leads to flake-like particles whereas at higher temperatures rods of particles associated end-to-tail will result. At intermediate temperatures the aggregation is non-specific. The presented model could be used as a means for controlling particle morphology and size.

The influence of salts (NaCl, NaBr, and NaI) on the formation of mesoporous silica SBA-15 was studied in situ by small-angle X-ray scattering and diffraction. Pluronic P104 was used as structure director. The micellar properties and the dynamics of formation were clearly dependent on the presence of salt. It was also shown that the kinetics of mesophase formation, the initial value of the cell parameters, and the extent of long-range order were all influenced by salt additions. The observations are explained to primarily originate from the influence of the anions on the ethylene oxide part of the polymer, i.e., the corona region of the Pluronic micelles. Two effects are identified: a general ion effect causing dehydration of the ethylene oxide part and consequently inducing micellar growth, and a specific ion effect that counterbalances this. The study provides the basis for understanding the means by which addition of simple Na-salts influence the formation of mesoscopically ordered silicas synthesized using nonionic surfactants as structure directors, hence advancing the knowledge base toward a more rational design of mesoporous 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.