Abstract
Mesoporous MMT-1 silica nanoparticles with c2mm symmetry and helical morphology were synthesized by a "pH-jump" method using sodium silicate as silica source and ethyl acetate (and other acetate esters) as a pH modifier. In the alkaline synthesis mixture containing both cationic and nonionic surfactants at low concentration, acetate esters hydrolyzed rapidly and caused drastic drop in solution pH to initiate the cooperative assembly of silicate/surfactant mesophase and the condensation of silicate species. When ethyl acetate (EA) was used, nanoparticles with ordered c2mm mesostructure and helical morphology could be formed with an EA-to-silicate ratio (x) of 4.0 ∼ 9.0. As x was increased from 4.0 to 9.0, the structural order was decreased yet the degree of silica condensation was enhanced. As a result, the materials exhibited much better hydrothermal stability than those synthesized from tetraalkoxysilanes (such as tetraethoxysilane). Moreover, the increase in x caused changes in particle morphology to long-pitch helical rods with increased length and increased extent of void defects that were irregular in shape and size. The differences in structural and textural properties of the MMT-1 materials should be associated with the pH drop rate and profile during the synthesis with varied x. In-situ small-angle X-ray scattering measurements were performed and analyzed to elucidate the formation mechanism of the materials. The results showed that the MMT-1 materials derived from the pH-jump method are promising for catalytic and other applications. © 2014 Elsevier Inc. All rights reserved.