Silica nanofibers from electrospinning/sol-gel process

Abstract

The fabrication of materials with one-dimensional nanoscale structures is of great promise for the fundamental understanding of the roles of dimensionality and size in an optical, electrical, and mechanical properties with reference to applications in semiconductor mechanical and chemical industries. Polymer nanofibers are of considerable interest for various kinds of applications including filters, reinforcing agents, biomedical materials, and fiber templates to prepare nanotubes [1–6]. Recently, there has been a growing interest in one-dimensional, inorganic nanosized materials such as carbon nanotubes, carbide nanorods, silica and titania nanofibers/nanotubes [7–11]. These one-dimensional nanomaterials exhibit some novel physical and chemical properties due to their peculiar structure and size effect, and are of great importance in nanodevices and mesoscopic theoretical research. Electrospinning technique is an effective method to produce nanofibers [12–16]. The electrospinning process involves the application of a strong electrostatic field to a capillary connected with a reservoir containing a polymer solution or melt. Under the influence of the electrostatic field, a pendant droplet of the polymer solution at the capillary tip is deformed into a conical shape (Taylor cone). If the voltage surpasses a threshold value, electrostatic forces overcome the surface tension, and a fine charged jet is ejected. The jet moves towards a ground plate acting as counter electrode. Due to the extensional viscosity of the polymer solution and the presence of entanglements, the jet remains stable and does not transform into spherical droplets as expected for a liquid cylindrical thread. The solvent begins to evaporate immediately after the jet is formed. The result is the deposition of a thin polymer fiber on a substrate located above the counter electrode. The sol-gel method has widely been used as an alternative technology for the preparation of a wide variety of forms including monoliths, powders, coatings, and fibers [17–20]. The typical sol-gel method is hydrolysis and condensation of tetraethyl orthosilicate (TEOS), Si(OCH2CH3)4. In recent years, there have been efforts to synthesize metal oxide (silica or titania) nanofibers and nanotubes by the sol-gel template method [11, 21, 22]. Meanwhile, micron-scale silica fibers have been achieved by extruding the spinnable sol through an orifice [23–25]. In the present work, we study formation of silica nanofibers using the sol-gel method and electrospinng technique. We note that the TEOS solution used in this study does not contain any gelator or binder to help spinnability. Kobayashi and coworkers synthesized titania fibers via the sol-gel method from a physical gel of titanium tetraisopropoxide by a low molecular weight organogelator [10]. Zhang and coworkers synthesized silica and titania nanorods with the sol-gel method and anodic alumina template membrane [11, 21]. The silica sol was prepared from tetraethyl orthosilicate (TEOS), distilled water, ethanol, and HCl. The sol composition in molar ratio was 1:2:2:0.01 (TEOS:ethanol:water:HCl). First, TEOS was mixed with ethanol in a beaker. The HCl/water solution was then added drop by drop to the TEOS/ethanol solution under vigorous stirring. The solution was heated at 80 ◦C for 30 min and then cooled down to room temperature. The silica sol was placed in a pasteur pipet and the electrode was directly connected with the solution. A tubular shaped counter electrode with a diameter of 22 cm was located below the reservoir. The winding drum was rotated at speed of 10 rpm during the electrospinning. The fibers were collected on aluminum foil covered the tubular layer. The distance between the tip of the capillary and the counter electrode (tip-tocollector distance, TCD) was 10 cm and the applied voltages ranged from 10 kV to 16 kV. The morphology and diameter of silica fiber were measured with SEM (S-2350 of Hitachi). The composition of silica fiber was determined with FTIR (Travel IR of SensIR Technol.). The structure of silica fiber was analyzed with XRD (DMAX 2000 of Rigaku Denki). The thermal property was analyzed with a thermogravimetric analyzer of TGA 2050 of TA Instrument. TGA analysis was performed at 50–800 ◦C with 20 ◦C/min in air. It has been known that silica fibers obtained by the conventional technique through the sol-gel process are affected by composition of sol and ripening condition [20, 24]. In the current study, silica nanofibers were obtained successufully by electrospinning technique and