Hexagonal Mesostructure in Powders Produced by Evaporation-Induced Self-Assembly of Aerosols from Aqueous Tetraethoxysilane Solutions

Spherical submicron mesoporous particles possessing two-dimensional hexagonal order have been produced by evaporation-induced self-assembly in aerosols generated from acidic (pH ≈ 2) aqueous solutions of tetraethoxysilane (TEOS) and the surfactant cetyltrimethylammonium bromide (CTAB). Particle internal structure consists of multiple regions of hexagonally ordered tubular pore bundles with various orientations. Exploration of a broad range of CTAB and TEOS content showed that hexagonally ordered particles with Brunauer−Emmett−Teller surface areas of 700−1300 m2/g are produced when the CTAB/Si ratio is in the range 0.09 < CTAB/Si < 0.28. The mean pore diameter in ordered material is nearly constant at 2.8−3.0 nm, while wall thickness decreases from approximately 1 to 0.6 nm as the CTAB/Si ratio increases over that range. Hexagonal order is lost very abruptly for CTAB/Si < 0.09, with a corresponding rapid loss of surface area. Similar results were found for a solution pH of 1−3; however, a solution pH above 3.5 led to particles that appeared to be agglomerates of precipitated nanoparticles. Aerosol reactor temperatures from 30 to 550 °C were explored, with ordered particles produced under appropriate conditions at all temperatures. Lower reactor temperatures produced more highly ordered particles; however, reactor temperatures below approximately 125 °C can lead to particle coalescence on the collection filter. Coalescence occurs when silica condensation reactions are not sufficiently promoted to fix the individual particle structure before collection.