Synthesis of Mesoporous Silica Nanofibers with Controlled Pore Architectures

A one-phase route has been developed for the synthesis, under strongly acidic conditions with cationic surfactants, of mesoporous silica nanofibers with controlled internal pore architectures. The diameters of the fibers range from 50 to 250 nm, and the lengths are up to millimeters. The nanofibers can have either a longitudinal pore architecture, in which the pore channels are aligned parallel to the fiber axis, or a circular pore architecture, in which the pore channels are wound circularly around the fiber axis. The pore channels in both types of nanofibers are hexagonally packed. The cross sections of the nanofibers with longitudinal pore architectures are hexagonal and those of the nanofibers with circular pore architectures are circular. Systematic experiments were carried out in which the fraction of the nanofibers with circular pore architectures among the overall nanofibers was determined as a function of the growth temperature for hexadecyltrimethylammonium chloride (C16TMAC), hexadecyltrimethylammonium bromide (C16TMAB), hexadecylpyridinium chloride (C16PC), and hexadecylpyridinium bromide (C16PB) surfactants. Generally, lower temperatures produce nanofibers with longitudinal pore architectures and higher temperatures produce nanofibers with circular pore architectures. The transition temperatures for C16TMAC and C16TMAB surfactants are around 75 °C and those for C16PC and C16PB surfactants are around 55 °C. In addition, the one-phase synthesis was carried out with cationic surfactants of different tail lengths, including dodecyltrimethylammonium bromide (C12TMAB), tetradecyltrimethylammonium bromide (C14TMAB), and octadecyltrimethylammonium bromide (C18TMAB). No nanofibers are obtained from the synthesis with C12TMAB or C14TMAB surfactants, while greater than 95 wt % of the products from the synthesis with C18TMAB surfactant are nanofibers with longitudinal pore architectures in the temperature range of 40−90 °C. Furthermore, rhodamine 6G (Rh6G) dye molecules were incorporated during growth to make mesostructured nanofibers that are millimeters long and fluoresce uniformly along the entire length of the individual fiber.