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Composite Polymer Nanofibers with Carbon Nanotubes and Titanium Dioxide Particles Shahar Kedem, Judith Schmidt, Yaron Paz, and Yachin Cohen* Department of Chemical Engineering, TechnionsIsrael Institute of Technology, Haifa 32000, Israel Received January 27, 2005. In Final Form: April 5, 2005 Composite nanofibers containing nanometric TiO2 particles and multiwalled carbon nanotubes dispersed in poly(acrylonitrile) (PAN) were prepared by the electrospinning technique. The structure and quality of the precursor dispersions were evaluated by cryo-transmission electron microscopy. The fabricated nanofibers, the diameters of which were in the 20-200 nm range, contained well-oriented nanotubes and spherical TiO2 nanoparticles in close proximity. Such nanofibers are under investigation as new photocatalytic reactor elements.
Introduction Heterogeneous photocatalysis, using titanium dioxide as the photocatalyst of choice, is a useful technique for the degradation of many contaminants in air,1 in water,2 and on solid surfaces.3,4 To enhance the activity of the photocatalyst, nanoparticles with high surface area, on the order of 50 m2/g are often used. Unfortunately, this small size creates several practical problems such as the separation of nanometric catalyst particles from the suspension once the reaction is complete and possible aggregation of suspended particles. Furthermore, particulate suspensions are not easily applicable in continuous flow systems. Several approaches were taken in order to overcome these problems. One of these approaches was to immobilize the powder on polymeric films either by TiO2 solution casting and sintering5 or by mounting them on a polymer nanofiber formed by electrospinning (ES).6 It is noteworthy that low adsorptivity of the contaminants on TiO2 might limit the process. This mass transport problem is crucial in particular because many of the chlorinated xenobiotic compounds are hydrophobic and, hence, hardly adsorb on the photocatalyst. It was shown that coupling the photocatalyst to an inert adsorbent may help in overcoming mass transport limitations by concentrating the contaminants at the active sites, either specifically7 or nonspecifically.8 Following these works, we report hereby on the fabrication of electrospun polymeric nanofibers containing TiO2 nanoparticles immobilized together with carbon nanotubes serving as the adsorptive sites. The unique photocatalytic properties of these nanofiber reactors will be reported elsewhere. * To whom correspondence should be addressed: e-mail,
[email protected]. (1) Blake, D. M. Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds from Water and Air; NREL: Golden, CO, 1999. (2) Holmann, M. M. Photodegradation of Water Pollutants; CRC Press: Boca Raton, FL, 1996. (3) Mills, A.; Le Hunte, S. J. Photochem. Photobiol., A 1997, 108, 1-35. (4) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis; BKC: Tokyo, 1999. (5) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1996, 98, 79-86. (6) He, C. H.; Gong, J. Polym. Degrad. Stab. 2003, 81, 117-124. (7) Ghosh-Mukerji, S.; Haick, H.; Paz, Y. J. J. Photochem. Photobiol., A 2003, 160, 77-85. (8) Hidaka, H.; Nohara, K.; Zhao, J. C.; Serpone, N.; Pelizzetti, E. J. Photochem. Photobiol., A 1992, 64, 247-254.
Carbon nanotubes (CNTs), discovered by Iijima in 1991,9 have been studied extensively due to their unique properties, such as high tensile modulus, good heat and electrical conduction, unique optical and electronic properties, etc.10 CNTs are used as reinforcing elements when dispersed in a polymer matrix. Some examples are extruded poly(methyl metacrylate) (PMMA) with CNT,11 thin polymer films containing CNTs,12,13 and melt fiber spinning of poly(carbonate) (PC) with CNTs.14 The ES technique15 is a simple method for fabricating ultrathin fibers, either oriented or laid in a random fashion as a fibrous mat. It is based on electrostatic surface charging of a polymer solution droplet, drawing a jet moving at a high speed toward a grounded stationary or rotating surface. The highly extensional flow, which ensues, results in ultrahigh draw ratios, which lead to the formation of a continuous nanofiber, whose diameter is typically in the range of tens to hundreds of nanometers. This technique has already been used to fabricate TiO2 nanofibers,16 ceramic hollow nanofibers,17 CNT-reinforced polymer nanofibers (either single or multiwalled),18-22 and as a substrate for synthesizing CNTs on polymer nanofibers containing the proper catalyst.23 In this paper we report on the fabrication, by the ES technique, of a composite nanofiber made of poly(acry(9) Iijima, S. Nature 1991, 354, 56-58. (10) Hilding, J.; Grulke, E. A.; Zhang, Z. G.; Lockwood, F. J. Dispersion Sci. Technol. 2003, 24, 1-41. (11) Cooper, C. A.; Ravich, D.; Lips, D.; Mayer, J.; Wagner, H. D. Compos. Sci. Technol. 2002, 62, 1105-1112. (12) Kim, S. H.; Min, B. G.; Lee, S. C.; Park, S. B.; Lee, T. D.; Park, M.; Kumar, S. Fibers Polym. 2004, 5, 198-203. (13) Pirlot, C.; Willems, I.; Fonseca, A.; Nagy, J. B.; Delhalle, J. Adv. Eng. Mater. 2002, 4, 109-114. (14) Sennett, M.; Welsh, E.; Wright, J. B.; Li, W. Z.; Wen, J. G.; Ren, Z. F. Appl. Phys. A: Mater. Sci. Process. 2003, 76, 111-113. (15) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531-4547. (16) Li, D.; Xia, Y. N. Nano Lett. 2003, 3, 555-560. (17) Li, D.; Xia, Y. N. Nano Lett. 2004, 4, 933-938. (18) Ye, H. H.; Lam, H.; Titchenal, N.; Gogotsi, Y.; Ko, F. Appl. Phys. Lett. 2004, 85, 1775-1777. (19) Dror, Y.; Salalha, W.; Khalfin, R. L.; Cohen, Y.; Yarin, A. L.; Zussman, E. Langmuir 2003, 19, 7012-7020. (20) Salalha, W.; Dror, Y.; Khalfin, R. L.; Cohen, Y.; Yarin, A. L.; Zussman, E. Langmuir 2004, 20, 9852-9855. (21) Sreekumar, T. V.; Liu, T.; Min, B. G.; Guo, H.; Kumar, S.; Hauge, R. H.; Smalley, R. E. Adv. Mater. 2004, 16, 58-61. (22) Ge, J. J.; Hou, H. Q.; Li, Q.; Graham, M. J.; Greiner, A.; Reneker, D. H.; Harris, F. W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2004, 126, 15754-15761. (23) Hou, H. Q.; Reneker, D. H. Adv. Mater. 2004, 16, 69-73.
10.1021/la0502443 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/11/2005
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Figure 1. Cryo-TEM images of vitrified PAN solutions in DMF containing different nanoparticles: (A) TiO2 dispersion; (B) MWCNT dispersion; (C, D) titanium dioxide and MWCNT in a combined dispersion. CF denotes parts of the holey carbon film. The vitrified dispersions cover the holes, revealing the MWCNT and TiO2 particles by mass and phase contrast. Bar ) 100 nm.
lonitrile) (PAN, [-CH2-C(CN)H-]n), in which axially oriented multiwalled CNTs (MWCNTs) as well as TiO2 particles are embedded. Such composite nanofibers may offer new potential advantages, such as enhanced durability and mechanical properties due to the CNT reinforcement18,22 and enhanced effectiveness due to the use of CNTs as adsorptive sites for the contaminants. Fabrication of polymer nanofibers from a joint dispersion of nanotubes and nanoparticles has not been studied before, although CNTs were used as templates for the synthesis of TiO2 nanoparticles.24,25 Experimental Section MWCNTs were purchased from NanoLab and used as received. The MWCNTs were added to dimethylformamide (DMF, AR, Gadot) in 3% (w/w) loading and were sonicated for 15 min in a 43 kHz Delta D2000 sonicator, to form a black slurry. PAN (Scientific Polymer Products, Inc., Mw ≈ 150 000) was dissolved in DMF (9% w/w) and the solution was added to the CNT slurry in a weight ratio of 1:1. The fluid was magnetically stirred for 15 min and then sonicated for 3 h. The resulting dispersions were homogeneous, exhibiting a dark inklike appearance, and were stable, exhibiting no particulate settling for several weeks. Titanium dioxide powder (Degussa P-25) was added to DMF in a 6% (w/w) load and was sonicated for 2 h to form a homogeneous milklike appearance. The MWCNT and/or TiO2 dispersions were added to the viscous PAN solution in order to achieve viscoelastic spinnable dispersions. The compositions of the dispersions used for electrospinning are shown in Table 1. The electrospinning process was conducted at room temperature using a voltage of 12 kV between a syringe needle (0.5 mm in diameter) situated 15 cm above a rotating grounded disk (∼1000 rpm), as described in detail elsewhere.26 (24) Huang, Q.; Gao, L. J. Mater. Chem. 2003, 13, 1517-1519. (25) Jing Sun, M. I. L. G. Q. Z. Carbon 2004, 42, 885-901.
Table 1. Composition of the Dispersions Used for Electrospinning and Cryo-TEM electrospinning (wt %)
cryo-TEM (wt %)
component PAN TiO2 MWCNT
4.5 2.0
4.5 1.5
4.5 2.0 0.5
1.5 0.67
1.5 0.5
1.5 0.67 0.17
The nanofiber specimens for imaging by transmission electron microscopy (TEM) were prepared by direct deposition onto a copper grid, coated by a holey carbon film. The dispersion structure was imaged by cryo-TEM. Samples were prepared by rapid vitrification of thin films using a controlled-environment vitrification system (CEVS).27 To form suitable thin liquid films in the CEVS, the dispersions used for electrospinning were diluted 3-fold (Table 1). Imaging was carried out using a low electron dose at acceleration voltage of 120 kV in a Philips CM120 TEM. Images were recorded with a Gatan MultiScan 791 CCD camera, using the Gatan DigitalMicrograph 3.1 software package. Samples for high-resolution scanning electron microscopy (HRSEM) were also prepared by direct deposition of the electrospun nanofibers onto a piece of silicon wafer. The micrographs were obtained by a secondary scattered electrons detector using a Leo Gemini 982 high-resolution SEM (HRSEM) at acceleration voltage of 2-4 kV and sample-detector distance of 1-3 mm.
Results and Discussion Before discussion of the composite nanofibers, it is important to evaluate the quality of the dispersions used for electrospinning, which controls the quality of the fabricated fibers. This is done by direct imaging using cryo-TEM. Thin films of the dispersions (compositions (26) Theron, A.; Zussman, E.; Yarin, A. L. Nanotechnology 2001, 12, 384-390. (27) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87-111.
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Figure 2. Images of electrospun PAN nanofibers: (A-C) HRSEM image, showing the wavy surface of a thick nanofiber (B) and smooth surface of a thin PAN nanofiber (C); (D) TEM image of nanofibers showing the varying contrast due to difference in thickness and surface features.
given in Table 1) were deposited onto TEM grids covered by a holey carbon film and rapidly vitrified in liquid nitrogen. The titanium dioxide nanoparticles dispersion in PAN solution proved to be homogeneous, containing mainly separated particles even though some aggregates are present, as can be seen in Figure 1A. Images of the MWCNT dispersion in the polymer solution (e.g., Figure 1B) show them to be well-separated, indicating that PAN is a good dispersing agent for these nanotubes. It should be noted that the nanotubes were not treated in any particular way, such as surface oxidation,22 yet a high degree of separation of the initially entangled nanotube bundles was achieved. An image of the tertiary dispersion containing TiO2, CNTs, and PAN can be seen in panels C and D of Figure 1. As seen in many cryo-TEM micrographs, the CNTs are well dispersed and separated, and moreover the TiO2 particles tend to attach themselves as aggregates upon the surface of the nanotubes. Continuous nanofibers of controlled diameter were obtained as oriented ropes by deposition on a rotating tapered wheel, on which HRSEM holders and TEM copper grids were placed. Figure 2 shows TEM and HRSEM images of PAN nanofibers woven from a 4.5% w/w PAN solution, at an electrospinning voltage of 12 kV. The fiber diameters ranged from 20 to 140 nm. HRSEM images show that thick fibers have a wavy surface (Figure 2B) whereas thin fibers exhibited a smooth texture (Figure 2C). This observation differs from the findings of Ge et al.,28 who attributed the wavy surface of PAN nanofibers to the presence of embedded CNTs. PAN nanofibers, containing 30% TiO2, were fabricated from solution containing 4.5% PAN and 2% TiO2 particles, at a voltage of 6 kV. The diameter distribution of the fibers increased and ranged between 50 and 300 nm. As can be seen in Figure 3A, the fibers’ surface seems to be heavily loaded with TiO2 particles which hinder formation of nanofibers (28) Ge, J. J.; Hou, H. Q.; Li, Q.; Graham, M. J.; Greiner, A.; Reneker, D. H.; Harris, F. W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2004, 126, 15754-15761
Figure 3. Electron microscopy images of PAN/TiO2 nanofibers: (A) HRSEM image of a few fibers showing TiO2 particles on the fibers’ surface; (B) TEM image of nanofibers showing the distribution of TiO2 particles. Bar ) 200 nm.
Figure 4. TEM images of an oriented MWCNT within a PAN nanofiber. Bar ) 100 nm.
with a uniform diameter, as was seen in the PAN fibers (Figure 2). TEM images corroborated the good dispersion of titanium dioxide particles in the nanofibers (Figure 3B), as could be seen at all the fibers examined. From observations of the electrospun nanofibers made of MWCNT dispersions in PAN solutions, it was evident that individual MWCNT were successfully embedded in the polymer matrix. In many regions of the nanofibers, such as shown in Figure 4, the individual MWCNTs appeared to be well oriented along the fiber axis and almost no indication was found for twisted, knotted, and entangled nanotubes. Some parts of the nanotubes were observed to protrude from the fiber surface, as has been reported previously.19 Composite PAN nanofibers, containing both 28% TiO2 nanoparticles and 7% MWCNTs, were made by ES at a
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Figure 5. TEM images of composite PAN nanofiber containing aligned MWCNT and TiO2 nanoparticles. Bar ) 50 nm.
voltage of 12 kV, yielding fibers of diameters ranging between 40 and 180 nm. Some parts of the nanofibers, which are heavily loaded with nanoparticles, show local thickening. Figure 5 shows TEM images of the composite fibers showing individual MWCNTs oriented along the fiber axis. Evidently, the heavy TiO2 loading (more than 4 times the CNT loading) did not hinder the nanotubes alignment. Figure 5A shows a very thick MWCNT, which fills a major part of the fiber diameter. The excellent alignment of the tube and fiber contour, shown in this image, strongly supports the alignment of the carbon nanotubes along the flow streamlines in the spinning process.19 As can be seen from the TEM images, the nanotubes and the TiO2 particles are in close proximity, often in direct contact with each other. Screening many TEM images did not lead to any substantial conclusion about the nature of the nanotube/particle interfaces. The observations varied from seemingly direct contact to separation by a polymer film several nanometers in thickness. These cases are exemplified by the TiO2 particles at the left and right sides of Figure 5B, respectively. The HRSEM image shown in Figure 6A demonstrates the heavy load of titanium dioxide particles on the fiber surface. Moreover, these images indicate close contact between the MWCNT and the TiO2 particles on the nanofiber surface (Figure 6B). The dispersions, which contain carbon nanotubes and titanium dioxide particles, are complex systems, which may interact in unique ways with the electrostatics and dynamics of the electrospinning process. Measurements were made in order to discern the relation between the applied voltage and the fiber diameter, determined by
TEM imaging. Figure 7a shows a typical diameter distribution of ternary nanofibers electrospun at 15 kV. The nanofibers diameter ranged between 10 and 120 nm, the number average diameter being 75 ( 4 nm. The narrow diameter distribution in this spinning process is evident. Figure 7b shows the average nanofibers diameter as a function of the electrospinning voltage, averaging at least 50 different fibers for each data point. It can be seen that the average diameter decreases with increasing spinning voltage, which creates a stronger electric force drawing the fibers toward the grounded plate, increasing the drawing ratio, and thus thinning the nanofibers diameter. The dependence is rather linear, as observed by the linear fit in Figure 7b. The ability of the electrospinning process to fabricate rather homogeneous nanofibers consisting of long-thin nanotubes and small spherical nanoparticles, as demonstrated in this work, is primarily attributed to the quality of the mixed dispersions of these very dissimilar objects. The issue of mixing together rods and spheres has been studied experimentally, theoretically, and by computer simulations.29 A wealth of complex behaviors and a variety of morphologies can be attained in such mixtures, as demonstrated with colloidal rodlike viruses and spherelike latex particles. Hence the creation of isotropic dispersions containing rodlike MWCNT particles with spherical-like TiO2 nanoparticles in the polymeric solution may pose a stability challenge. The observed dispersion homogeneity may reflect a miscible thermodynamic state, which is in (29) Adams, M.; Dogic, Z.; Keller, S. L.; Fraden, S. Nature 1998, 393, 349-352.
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Figure 6. HRSEM images of composite PAN nanofiber containing aligned MWCNT and dispersed TiO2 nanoparticles: (A) aligned nanofibers with TiO2 particles on the surface; (B) larger magnification showing MWCNT protruding from the fiber, between the nanoparticles. 29
accord with the phase diagram of Adams et al. (bearing in mind the differences between the systems). Alternatively, adsorption of nanoparticles to the nanotube surface, as shown in Figure 1, may add to the stability. In conclusion, we have demonstrated the simple fabrication, through the electrospinning technique, of continuous composite polymer nanofibers of controlled diameter, in which spherical TiO2 particles and axially aligned MWCNT are embedded. A fine dispersion of the nanoparticles and nanotubes was achieved at reasonably high loading. The structure of the composite nanofibers was characterized by electron microscopy, and the effect of electrospinning voltage on the nanofiber diameter was
Figure 7. (a) Histogram showing the diameter distribution of composite PAN/MWCNT/TiO2 nanofibers spun at 15 kV, as measured by TEM. (b) The dependence on spinning voltage of the average diameter of nanofibers as in (a). Solid line is a linear regression.
determined. The photocatalytic activity of these new composite nanofibers is currently under study. Acknowledgment. The work was supported by the Israel Science Foundation, Grant 26/03. The authors thank Professor Eyal Zussman for the use of the electrospinning apparatus and many helpful discussions. LA0502443
