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 approximate to 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 m(2)/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 550degreesC 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 125degreesC 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.