(582bp) Effects of Surfactant Concentration and Cosolvent on the Morphology of Hexagonal Mesoporous Silica

A neutral templating route to mesoporous molecular sieves has been reported by Pinnavaia based on hydrogen bonding interactions and self-assembly between neutral primary amine micelles and neutral inorganic precursors. Most of previous studies yielded nonuniform aggregates of very small, distinct particles. However, for many applications in catalysis, chemical sensoring or as optical devices, defined morphologies are required. The effect of cosolvent on the morphology of HMS is still not clear. To investigate the morphological change of this HMS material, the samples were prepared at various concentrations of dodecylamine (DDA), variety of alcohol to water (R/H) ratios, and different adding rate of tetraethyl orthosilicate (TEOS) in this study.

Hexagonal mesoporous silica (HMS) was prepared at ambient temperature by using DDA as the template and TEOS as a silica derivative in this study. The properties of HMS were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and N₂adsorption/desorption. The morphology of HMS is usually irregular and small particle. To investigate the morphological change of this HMS material, the samples prepared at various TEOS adding rate, solvent compositions and compositions were studied.

In the synthesis of HMS materials, DDA was not only as a surfactant but also as a catalyst. Hence, the concentration of DDA was adjusted to obtain various shapes of HMS silicate. In order to decrease the parameter of experiment, the DDA/TEOS molar ratio and ethanol/H₂O volume ratio were kept at 0.27 and 1, respectively. The concentration of DDA varied from 0.1268 M to 3.963×10^-3 M. At slightly lower concentration ([DDA]= 0.1268~0.0317 M), HMS silicate formed various size particles having crack on surface. At [DDA]= 0.0159 M, SEM image showed uniform and isolated sphere. At very lower concentration ([DDA]= 7.926×10^-3 and 3.963×10^-3 M), the uniform spheres were shrank. Overall, the shape of mesoporous molecular sieves became gradually large and spherical as the concentration of DDA decreased.

With decreasing TEOS adding rate, the sizes of HMS particles increase and the textural porosity reduces. With the concentration of DDA reduces, HMS particles become crackled bigger particles from small particles and the pore size reduces from 2.32nm to 1.72 nm. As [DDA]= 0.0159 M, the spherical particles of HMS silicas with uniform size and isolated are obtained. Besides, TEOS adding rate also affected the size of the spheroids. The noodle-like structures are prepared in HMS silicas prepared at R/H ratio=0. These noodle-like structures were about 0.1μm in diameter and about submicrometer to several micrometers in length. In a series of HMS silicas synthesized by various [TEOS], the irregular trends indicated that it is required to prepare the samples under non-alcoholic solution to have non-spherical particles. However, some other parameters may also play a role.