NANO LETTERS
Fabrication of Titania Nanofibers by Electrospinning
2003 Vol. 3, No. 4 555-560
Dan Li and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received January 20, 2003; Revised Manuscript Received February 20, 2003
ABSTRACT This paper describes a procedure based on electrospinning for generating nanofibers of anatase with controllable diameters and porous structures. When an ethanol solution containing both poly(vinyl pyrrolidone) (PVP, Mw ≈ 1 300 000) and titanium tetraisopropoxide was injected through a needle under a strong electrical field, composite nanofibers made of PVP and amorphous TiO2 were formed (with lengths up to several centimeters) as a result of electrostatic jetting. These nanofibers could be subsequently converted into anatase without changing their morphology via calcination in air at 500 °C. The average diameter of these ceramic nanofibers could be controlled in the range from 20 to 200 nm by varying a number of parameters such as the ratio between PVP and titanium tetraisopropoxide, their concentrations in the alcohol solution, the strength of the electric field, and the feeding rate of the precursor solution. Both supported and free-standing mats consisting of anatase nanofibers have been successfully fabricated.
One-dimensional (1D) nanostructures of metal oxides and related materials have been a subject of intense research because of their potential applications in many areas that include electronics, photonics, mechanics, and sensing.1 A number of synthetic methods for generating nanoscale wires, belts, and tubes from various metal oxides have been demonstrated; notable examples include those based on the vapor-solid (VS),2 vapor-liquid-solid (VLS),3 solutionsolid,4 and solvothermal routes.5 New strategies are still being developed. Here we demonstrate that titania nanofibers can be conveniently prepared by electrospinning an alcohol solution that contains a polymer of high molecular weight and a titanium alkoxide precursor. Calcination in a subsequent step leads to the formation of polycrystalline nanofibers made of anatase TiO2 with controllable diameters and interesting porous structures. One of the attractive features associated with this method is that the ceramic nanofibers can be readily prepared as mats (both supported and freestanding) that are sought for applications related to sensing, catalysis, and membrane-based separation or purification. Titania was selected as our primary target to demonstrate the concept because of its important role in applications related to environmental cleaning and protection, photocatalysis, gas sensing, and fabrication of solar cells and batteries.6 We note that titania whiskers, nanorods, and nanotubes have been synthesized using sol-gel and electrochemical methods with channels in anodic alumina membranes, self-assembled supramolecular structures, and patterned polymer grooves as the templates.7 Most recently, * To whom correspondence should be addressed. E-mail: xia@chem. washington.edu. 10.1021/nl034039o CCC: $25.00 Published on Web 03/13/2003
© 2003 American Chemical Society
oxide-coated polymer nanofibers and submicrometer-sized tubes of titania (and other materials) have also been fabricated by dip-coating the surfaces of electrospun, polymeric fibers with sol-gel precursors, followed by postsynthesis treatments.8 Although these template-directed methods provide a straightforward and reliable strategy for processing titania into nanostructures with 1D morphologies, all of them involve multiple (at least three) steps: for example, the fabrication of templates, the filling or coating of the surfaces of templates with titania (or a precursor to this material), and finally the selective removal of the templates. The quantity of nanostructures that can be obtained in each processing run is often limited. It will clearly be an advantage if one can develop a simpler method that is capable of generating titania nanofibers in fewer steps and in more copious quantities. Electrospinning represents a relatively simple and versatile method for generating fibular mesostructures.9 In a typical process, a polymer solution or melt is injected from a small nozzle under the influence of an electric field as strong as several kV/cm. The build up of electrostatic charges on the surface of a liquid droplet induces the formation of a jet, which is subsequently stretched to form a continuous ultrathin fiber. In the continuous-feeding mode, numerous copies of fibers can be formed within a period of time as short as a few seconds. These fibers are often collected on the surface of a conductor to form nonwoven mats that are characterized by high surface areas and relatively small pore sizes. In the past several decades, more than 20 different types of organic polymers have been successfully processed as ultrathin fibers using the electrospinning technique, with typical examples
Figure 1. (A) SEM image of TiO2/PVP nanofibers that were electrospun from an ethanol solution containing Ti(OiPr)4 (0.1 g/mL) and PVP (0.03 g/mL). The electric field strength was 1 kV/cm. (B) SEM image of the same sample after it had been calcined in air at 500 °C for 3 h. (C) Histogram showing the size distribution of nanofibers contained in the calcined sample. The size distribution was obtained from the SEM images of ∼100 nanofibers. (D) XRD pattern of the same calcined sample. All diffraction peaks can be indexed to those of the anatase phase of titania.
including various engineering plastics, biopolymers, electrically conductive polymers, and fluorescent polymers.10 These fibers are of great interest for applications that range from texturing, composite reinforcement, sensing, enzyme immobilization, and tissue engineering to membrane separation. To our knowledge, electrospinning has been mainly applied to pure organic polymers. There were only a few reports that dealt with electrospinning of precursor solutions that could lead to the formation of composite fibers.8b,11 Here we demonstrate that titanium tetraisopropoxide (a sol-gel precursor to titania) can be directly added to an alcohol solution containing high-molecular-weight poly(vinyl pyrrolidone) (PVP) to prepare TiO2/PVP composite nanofibers. We selected PVP as the base polymer because of its good solubility in alcohols and water and because of its compatibility with some titania precursors even when its molecular weight is as high as 1 300 000. Yashida et al. reported that titanium tetraisopropoxide could be mixed with PVP in 2-propanol to form a stable solution, from which TiO2/PVP composite films and optical waveguiding structures could be fabricated.12 In addition to PVP, we also found that acetic 556
acid had to be added to stabilize the solution and to control the hydrolysis reactions of the sol-gel precursor. In a typical procedure, 1.5 g of titanium tetraisopropoxide (Ti(OiPr)4, Aldrich) was mixed with 3 mL of acetic acid and 3 mL of ethanol in a glovebox. After 10 min, this solution was removed from the glovebox and added to 7.5 mL of ethanol that contained 0.45 g of PVP (Aldrich, Mw ≈ 1 300 000), followed by magnetic stirring for ∼1 h (with the solution held in a capped bottle). The mixture was immediately loaded into a plastic syringe equipped with a 23 gauge needle made of stainless steel. The needle was connected to a high-voltage supply (ES30P-5W, Gamma High Voltage Research Inc., Ormond Beach, FL) that is capable of generating DC voltages up to 30 kV. The feeding rate for the precursor solution was controlled using a syringe pump (KDS-200, Stoelting Co., Wood Dale, IL). A piece of flat aluminum foil was placed ∼5 cm below the tip of the needle to collect the nanofibers. The electrospinning process was conducted in air. The as-spun nanofibers were left in air for ∼5 h to allow the hydrolysis of Ti(OiPr)4 to go to completion. Finally, the PVP was selectively removed from Nano Lett., Vol. 3, No. 4, 2003
Figure 2. Plots showing the dependence of nanofiber diameter on various processing parameters: (A) concentration of PVP; (B) electric field strength; (C) feeding rate of the ethanol solution; and (D) concentration of Ti(OiPr)4. The solid and open dots represent measurements taken from nanofibers before and after they had been calcined (in air at 500 °C for 3 h), respectively. When one parameter was varied, all other parameters were maintained as the following: PVP concentration (0.03 g/mL); Ti(OiPr)4 concentration (0.1 g/mL), feeding rate (0.2 mL/h); and electric field strength (1 kV/cm). The distance between the injection orifice and the collector was kept at 5 cm for all experiments.
these nanofibers by treating them in air at 500 °C for 3 h. Samples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were directly prepared by placing silicon wafers or gold grids on the aluminum foil during electrospinning. The composite nanofibers were calcined on silicon wafers and then transferred onto copper grids. SEM images were taken using a field-emission scanning electron microscope (Sirion, FEI; Portland, OR) operated at an accelerating voltage of 5 kV. The diameters of these fibers were quantitatively evaluated using their high-magnification SEM images. TEM images were taken using a Philips EM-430 microscope operated at 80 kV. The X-ray diffraction (XRD) pattern was recorded using a Philips PW-1710 diffractometer (Cu KR radiation) at a scanning rate of 0.02°/s in 2θ ranging from 20 to 80°. Figure 1A shows the SEM image of TiO2/PVP composite nanofibers electrospun from an ethanol solution containing 0.03 g/mL PVP and 0.1 g/mL Ti(OiPr)4. Each individual nanofiber was uniform in cross section, and the average diameter of this sample was 78 ( 9 nm. Since Ti(OiPr)4 can be rapidly hydrolyzed by moisture in the air, continuous networks (gels) of TiO2 sols were able to form in the nanofibers once they had been ejected from the orifice. As the PVP was selectively removed by burning the sample in air at 500 °C, the nanofibers remained as continuous structures (Figure 1B), and their average diameter was reduced to 53 ( 8 nm. (See Figure 1C for the size distribution.) This size reduction could be accounted for by Nano Lett., Vol. 3, No. 4, 2003
considering the loss of PVP from the nanofibers and the crystallization of titania. Figure 1D shows an XRD pattern of the calcined sample indicating the formation of a pure anatase phase after calcination at 500 °C in air for 3 h. This result also confirmed that Ti(OiPr)4 was trapped in the nanofibers during electrospinning instead of being electrosprayed out in the form of droplets (and thus as discrete nanoparticles), a process that was observed to occur in the absence of PVP. Similar to the situation with systems based on pure organic polymers,9,10 the thickness of both TiO2/PVP composite and anatase nanofibers could be varied by controlling a number of electrospinning parameters. As shown in Figure 2, the nanofibers increased in diameter as the PVP concentration was increased. When the Ti(OiPr)4 concentration was fixed at 0.1 g/mL and an electric field of 1 kV/cm was applied, the average diameter of TiO2/PVP composite fibers could be tuned in the range of 52 ( 11 to 374 ( 66 nm, and the average diameter of anatase fibers accordingly varied from 33 ( 6 to 192 ( 69 nm. The strength (E) of the electric field was another key factor that determined the morphology and diameter of the electrospun fibers. If the electric field was lower than 0.6 kV/cm, then no stable liquid jets were observed. When E was increased, thinner nanofibers were obtained as the final product. At 1.6 kV/cm, anatase nanofibers as thin as 30 ( 7 nm were generated. However, when E was greater than 1.6 kV/cm, the spinning jets became unstable, and the average diameter was found to increase 557
Figure 3. (A) TEM image of TiO2/PVP composite nanofibers fabricated by electrospinning an ethanol solution that contained 0.03 g/mL PVP and 0.1 g/mL Ti(OiPr)4. (B) TEM image of the same sample after it had been calcined in air at 500 °C for 3 h. (C, D) TEM images of nanofibers made of anatase that were prepared under the same conditions except that the precursor solution contained (C) 0.025 g/mL and (D) 0.15 g/mL Ti(OiPr)4, respectively. (E, F) High-magnification SEM images taken from the samples shown in C and D, respectively. No gold coatings were applied to the samples for all SEM studies.
slightly with increasing E. The feeding rate of the PVP solution also influenced the diameter of the fibers. Faster feeding rates often resulted in thicker fibers (see Figure 2C), but the jets became unstable if the feeding rate exceeded 0.5 mL/h. The concentration of Ti(OiPr)4 in the precursor solution had a minor impact on the diameter of TiO2/PVP composite fibers but played a significant role in determining the size of the calcined anatase nanofibers. In general, the use of Ti(OiPr)4 at lower concentrations led to the formation of thinner ceramic nanofibers. Figure 3A shows the TEM image of several nanofibers that were electrospun from a solution containing 0.03 g/mL PVP and 0.1 g/mL Ti(OiPr)4. The uniformity in contrast indicates that TiO2 was uniformly dispersed in the PVP matrix. Figure 3B-D gives the TEM images of samples with different TiO2 content after they had been calcined in air at 500 °C. These images clearly indicate that lower concentrations of Ti(OiPr)4 in the electrospinning solutions resulted in the formation of TiO2 nanofibers with thinner diameters. The thinnest nanofiber was around 20 nm in diameter. Figure 3E and F shows the corresponding SEM images taken from samples C and D, respectively. Both images indicated that each nanofiber was formed through the sintering of TiO2 558
nanoparticles that were ∼10 nm in diameter, and voids existed between adjacent nanoparticles. Our preliminary results suggest that the porous structures on the surfaces of titania nanofibers can be manipulated by controlling the hydrolysis rate of Ti(OiPr)4 in the fibers (e.g., through controlling the humidity in the atmosphere,) and a systematic investigation on this effect is underway. Previous work by other groups has demonstrated that the sol-gel process can be adopted to prepare mesostructured thin films or colloidal particles of titania by modifying the precursor solution with various additives such as tetrabutylammonium hydroxide and amines.13 All of these demonstrations imply that it will be possible to fine tune the secondary porous textures on individual nanofibers by carefully controlling a number of parameters. The composite nanofibers electronspun from mixtures of PVP and sol-gel precursors could stretch up to several centimeters in length. Such long nanofibers tended to form loops and to be randomly oriented when they were deposited on the surface of a collector electrode. As a result, they usually entangled to form a thin mat characterized by a nonwoven textile structure. The mat could be directly deposited on a variety of substrates (e.g., aluminum foil, Nano Lett., Vol. 3, No. 4, 2003
lable diameters and two modes of porous structures. Although the present work has been mainly focused on titania, we believe that the electrospinning procedure described here could be extended to provide a generic route to nanofibers made of other oxides such as SnO2, SiO2, Al2O3, and ZrO2. In addition, nanofibers with diameters thinner than and porous structures different from those reported here also seem to be achievable by carefully controlling the electrospinning parameters and the composition of the precursor solution. Acknowledgment. This work has been supported in part by an AFOSR-DURINT subcontract from SUNY-Buffalo, a Career Award from the NSF (DMR-9983893), and a Fellowship from the David and Lucile Packard Foundation. Y.X. is an Alfred P. Sloan Research Fellow (2000) and a Camille Dreyfus Teacher Scholar (2002). We thank Dr. Y. Sun, Dr. X. Jiang, T. Herricks, and Dr. K. Kamata for their help with the TEM and XRD studies. References
Figure 4. (A) Low- and (B) high-magnification SEM images of a free-standing mat made of anatase nanofibers. A gold layer ∼25 nm thick was sputtered on the surface of this sample to reduce electrostatic charging during SEM imaging.
silicon wafers, and glass slides or polymer sheets whose surfaces had been patterned with metal electrodes) and could be immediately used for applications related to sensing and electronic packaging. Free-standing mats of TiO2/PVP fibers could also be obtained by peeling off relatively thick films from the aluminum foil. Calcination can be subsequently used to convert them into ceramic membranes without destroying their porous structures. During calcination, the nanofibers in physical contact might be slightly fused at their interfaces to enhance the mechanical strength associated with these membranes further. Figure 4 shows two SEM images of such a self-supporting mat after it had been calcined at 500 °C in air for 3 h. The macroporous structures resulting from the intrinsic entanglement of nanofibers could be clearly observed in Figure 4B. Combined with the secondary porous structures on the surfaces of individual nanofibers, these ceramic membranes should be particularly useful as supports for applications (such as catalysis, filtration, sensing, and solar cells) where surface area plays a key role in determining their performance. In summary, we have demonstrated a simple and versatile approach to the fabrication of titania nanofibers with controlNano Lett., Vol. 3, No. 4, 2003
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