Hierarchically helical mesostructured silica nanofibers templated by achiral cationic surfactant

Recently, ordered chiral mesoporous silica with a twisted hexagonal rod-like morphology and hexagonally ordered chiral channels has been synthesized by using chiral anionic surfactants as a liquid crystal template ( S. Che, Z. Liu, T. Ohsuna, K. Sakamoto, O. Terasaki and T. Tatsumi, Nature, 2004, 429, 281). In this work, we report an observation of hierarchically helical mesoporous silica nanofibers organized by the achiral cationic surfactant cetyltrimethylammonium bromide ( CTAB). These nanofibers ( diameter ranging around 100 - 300 nm) grew from a two-phase system (H2O, CTAB, HCl for the aqueous phase and tetraethylsiloxane (TEOS) in hexane for the oil phase). SEM and TEM characterizations were performed and the results indicate that these nanofibers possess rope-like twisted hexagonal morphology and helical ( chiral) mesoporous channels running inside winding around the fiber axis. These twisted hexagonal nanofibers could further curve spirally to form a second-level helical morphology ( hierarchically helical morphology). As no chiral molecules are used in the synthesis, the hierarchically helical morphology of nanofibers could be explained by the different kinds of topological defects existing in the silicate liquid crystal seeds formed in a diffusion-controlled kinetic process, and these defects could initiate and direct the growth of particular forms of mesostructured silica. Formation of the ordered chiral mesorporous silica would be expected to be a general phenomenon in the cooperative assembly between amphiphilic organic molecules ( templates) and inorganic species, no matter whether the templates are chiral or achiral.