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Morphology of PEG-Stabilized Carbon Nanofibers in Water Jian Zhao*,†,‡ and Dale W. Schaefer‡ The Key Laboratory of Rubber-plastics (Qingdao UniVersity of Science and Technology), Ministry of Education, Qingdao, China, and Department of Chemical and Materials Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0012 ReceiVed: June 04, 2008; ReVised Manuscript ReceiVed: July 16, 2008
Small-angle light scattering is used to assess the dispersion of poly(ethylene glycol) (PEG)-functionalized carbon nanofibers suspended in water. Analysis of these data elucidates the mechanism by which the functionalized nanofibers are solubilized in water. Linear, tube-like morphology is observed for the PEGfunctionalized nanofibers dispersed in water. However, dispersion is not down to the individual tube level as determined by analysis of the light scattering data in conjunction with transmission electron micrographs. Rather, scattering entities are polydisperse side-by-side fiber aggregates (bundles). Because of the presence of water-soluble PEG oligomers on the surfaces of the nanofibers these small-scale aggregates do not agglomerate to form the large-scale clusters that are observed for untreated and acid-treated nanofibers. Acidtreated nanofibers, by contrast, do agglomerate, but in an unusual fashion, showing a 10-h induction period of followed by linear growth of large-scale agglomerates. PEG-functionalization of the acid-treated fibers leads to stabilization by inhibiting formation of the large-scale agglomerates, not by disrupting the side-byside bundles. SCHEME 1
Introduction ago,1
carbon nanotubes Since their observation over a decade have been of great interest because of extraordinary mechanical and electronic properties.2,3 Unfortunately, these materials are insoluble in any solvent, which limits their potential applications.4 To date, the most successful approaches to solubilization involve functionalization of the sidewalls and tube tips.5-8 Although many groups have achieved solubilization of carbon nanotubes, measurement of the degree of dispersion remains challenging. Here we use light scattering to quantify dispersion of solubilized carbon nanofibers. The presence of oxygen-containing groups on carbon nanotubes after acid treatment provides rich chemistry for the attachment of moieties to the surface of nanotubes. The simplest route to dissolution of the carbon nanotubes is to directly react an amine with the carboxylated nanotubes. A zwitterion is produced through a simple acid-base reaction (Scheme 1). This method produces nanotubes that are soluble in water or organic solvents, depending on the chosen functional molecules. Research demonstrates that full-length nanotubes can be solubilized (>0.5 mg/mL) in tetrahydrofuran and 1,2-dichlorobenzene by the functionalization of sidewall defects after treatment by HNO3.7 Sun et al. reported that polymer-bound carbon nanotubes could be formed by attaching nanotubes to water-soluble linear polymers such as poly(ethylene glycol) (PEG) via acid-base zwitterionic interaction.9 Dispersions of functionalized carbon nanotubes have been characterized by atomic force microscopy (AFM), UV/visible and Raman spectra, etc.10,11 We12-17 and others18-23 have recently used scattering methods to determine the morphology of carbon nanotube suspensions and composites. Fagan et al.21 for example show that scattering is the most direct tool to assess * Corresponding author. E-mail:
[email protected]. † Qingdao University of Science and Technology. ‡ University of Cincinnati.
nanotube dispersion. Here, we use scattering to quantify the dispersion of PEG-functionalized carbon nanofibers and make comparisons with acid-treated and untreated nanofibers. Light scattering data are analyzed to show that the PEGfunctionalized fibers do not exist as individually suspended tubes, but as side-by-side aggregates or bundles that are stable for weeks. Acid-treated fibers also exist as bundles, but in this case the bundles further agglomerate and precipitate over a period of days. Experimental Section Multiwalled nanofibers (product number PR19-LHT) were provided by Pyrograf Products Inc. (www.apsci.com). PyrografIII PR19-HT nanofibers are vapor-grown and subsequently heated to temperatures up to 3000 °C. The multiwalled nanofibers (MWNFs) normally contain a few concentric cylinders but may also be nested truncated cones. Typically the cores are open. The carbon nanofibers were treated to attach carboxylic acid groups to their surfaces. The acid treatment procedure was as follows: A 100-mg amount of nanofibers was added to 400 mL of a mixture 98% H2SO4-70% HNO3 (3:1). The mixture was subject to ultrasonication for 4 h. The resulting mixture was diluted with deionized water to 2000 mL and then filtered through a PTFE membrane disk filter (Gelman, 0.2 µm pore size) followed by washing several times with deionized water until no residual acid was present. The sample was dried in a vacuum oven. Water-soluble, PEG-functionalized nanofibers were prepared as follows: a dried acid-treated nanofiber sample (50 mg) was mixed with diamine-terminated oligomeric poly(ethylene gly-
10.1021/jp804940n CCC: $40.75 2008 American Chemical Society Published on Web 09/09/2008
Morphology of PEG-Stabilized Carbon Nanofibers
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col), O,O′-bis(3-aminopropyl) poly(ethylene glycol)1500 (PEG1500N) (Aldrich, 500 mg), and the mixture was vigorously stirred at 100 °C for 5 days under nitrogen protection. Deionized water was added to the mixture, and the resulting dispersion was placed in membrane tubing (cutoff molecular weight ∼12 000) for dialysis against deionized water for 3 days. The water was changed every 8 h. A homogeneous dark-colored solution of PEG-functionalized nanofibers was obtained after the solid residue was separated from the dispersion by centrifuging. TEM samples were prepared by allowing a drop of nanofiber suspensions to dry onto carbon film. The high-resolution transmission electron microscopy (HRTEM) experiments were performed using a JEOL JEM 2010F electron microscope with a field emission source. The accelerating voltage used was 200 kV. The dispersion efficiency was determined using a low-angle light scattering photometer, a Micromeritics Saturn Digisizer 5200 (www.micromeritics.com). Light scattering covers the regime 10-6 Å-1 < q < 10-3 Å-1, where q is the magnitude of the scattering vector, q ) (4π sin θ)/λ, where θ is half the scattering angle, and λ is the wavelength of the radiation in the medium. The q-range corresponds to length-scales (∼q-1) from 100 µm at low q to 1000 Å (0.1 µm) at high q. Results and Discussion The raw Pyrograf-III PR19HT powder consists of loosely aggregated nanofibers. Figure 1 shows a HRTEM image of the powder. Defects on the sidewalls of nanofibers are occasionally observed. The graphite structure with the interlayer spacing d ) 0.34 nm is clearly observed. No iron catalyst particles are found by TEM. The bright field and high-resolution TEM images of the acidtreated carbon nanofibers are shown in Figure 2. Many defects formed on the sidewall and serious damage appears after oxidation. Disruption of outer graphitic layers is observed in the TEM image. The stripping of the altered outer graphite layers after strong oxidation can thin nanofibers. These observations are in agreement with literature.10,24 PEG-functionalized nanofibers look similar to acid-treated nanofibers in high resolution TEM. There is considerable experimental evidence for the presence of carboxylic acid groups bound to carbon nanotubes after acid treatment.6,25 Exploiting acid-base zwitterionic interactions, we reacted diamine-terminated oligomeric PEG with the carboxylic acids anchored on the surface of carbon nanofibers. This process yields a dark-colored homogeneous solution that is stable in water for months. This dispersion was characterized using light scattering. Figure 3 shows the light scattering profiles as a function of time for PEG-functionalized carbon nanofibers in water at a concentration of 5.0 × 10-6 g/mL. The data were obtained in the batch mode with no circulation or sonication. That is, the sample chamber was filled with the undiluted suspension and observed without stirring, agitation, or circulation. Deionized water was used as background. The scattering profiles are almost identical over 96 h, indicating minimal change in morphology. The scattering curve consists of two approximate power-law regimes separated by one intermediate Guinier crossover. The q value at the crossover gives the approximate size (radius-of-gyration, Rg) of the scattering entity as Rg = 2π/q. The slope near -1 at low q implies a linear object such as rod or tube on large scales. Electron microscopy shows that carbon nanofibers are more tube-like.
Figure 1. TEMs of unmodified carbon nanofibers at two magnifications. Graphitic layers are visible at both magnifications. The lowresolution image (top) shows a variety of tube shapes and morphologies including concentric cylinders and stacked cones. No metallic catalyst particles are observed. The bars are 20 and 2 nm.
Rg is the root-mean-square distance of the mass from the center of gravity. For a rod of length L and radius R, Rg2 ) R2/2 + L2/12. Thus, for a rod with a high aspect ratio, Rg is a measure of the rod length. However, no Rg can be extracted at low q because of the absence of a Guinier crossover in that region. It is possible that a Guinier crossover exists at lower q, but the intensity is not measurably above background. The slope near -2 on a log-log plot around q ) 0.003 Å-1 may arise from a hollow tube since the wall of such an object is a two-dimensional on scales larger than its wall thickness and shorter than the radius. Such a slope, however, can also arise from more complex structures. The crossover length scale at high q between the two power-law regimes corresponds to the largest radius of the individual tubes or tube aggregates. We argue below that the scattering objects are side-by-side tube aggregates or bundles.
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Figure 3. Evolution of the light scattering profile of PEG-functionalized nanofibers for four days. The suspensions were sonicated at 10 W for 5 min before the observations began. The measurements were taken in the batch mode, so the sample was undisturbed during the course of the observations.
Figure 2. TEMs of acid-treated and subsequently PEG-functionalized carbon nanofibers at two magnifications. Defects and serious damage on the walls are evident as well as breakage of graphite layers. The bars are 20 and 2 nm.
The data do not fit with a rod form factor although they do follow a tube-like form factor reasonably well. We fit the scattering curve of PEG-functionalized nanofibers at 72 h using a simplified tube form factor (Figure 4). The simplified tube form factor approximates an exact tube model but suppress the oscillations that are found for exact models.26 The simplified tube model fits the data reasonably well except at intermediate q, where it seems to underestimate the size of the relevant size scale (tube outer radius). On the basis of the fit to the simplified tube form factor, one might naively conclude that dispersion is down to the level of individual tubes. However, the outer diameter of 0.47 µm (calculated from the radius of 2350 Å) derived from the simplified tube model is substantially larger than the largest individual fibers seen in TEM. That is, scattering entities are not individual tubes, but side-by-side fiber bundles. A bundled structure also accounts for the deviation from the simplified tube form factor. Nevertheless, fitting with the simplified tube model implies that these tube aggregates are rigid and isolated. Timeevolution of dispersion behavior of PEG-nanofibers shows that
Figure 4. Light scattering data for PEG-functionalized nanofibers compared with the simplified tube model.
tube bundles remain well dispersed and do not form large-scale agglomerates over weeks. Figure 5 compares the scattering profiles of PEG-functionalized, acid-treated, and as-received carbon nanofibers two days after dispersion by sonication. The high-q features are similar. In contrast with the PEG-functionalized sample, a Guinier region at low q is present for untreated and acid-treated nanofibers, indicating the presence of large-length-scale objects that are likely formed by agglomeration of the smaller-scale bundles. No region with power-law scattering with a slope of -1 is visible at low q. In the untreated state a suspension of nanofibers is unstable in quiescent (unsonicated) suspension at any significant concentration. Tubes precipitate rapidly. Our previous investigation indicates that, for untreated nanofibers, the large-scale agglomerates appear immediately after sonication. The overall scattered intensity shows a nearly monotonic decrease in intensity with time, consistent with precipitation (not shown here).27 The Rgs extracted from low-q region correspond to the size of agglomerates and are virtually unchanged during the precipitation process.
Morphology of PEG-Stabilized Carbon Nanofibers
Figure 5. Comparison of the scattering profiles for PEG-functionalized, untreated and acid-treated carbon nanofibers at 44 h. The suspensions were sonicated at 10 W for 5 min before data were taken using light scattering in batch mode. The PEG sample does not show any agglomeration.
Figure 6. Evolution of the light scattering profile of acid-treated nanofibers in water over two days. The suspensions were sonicated at 10 W for 5 min before data were taken using light scattering in batch mode. Minimal change is observed at large q indicating minimal change in morphology below 1 µm. The micron-size entities originally present aggregate into larger structures in a hierarchical fashion. The lines are two-level unified fits. Low-q data are absent in regions where the signal is not significantly above background.
In contrast to the PEG and untreated samples, the scattering profile of acid-treated nanofibers shows a dramatic change over time. As displayed in Figure 6, the scattered intensity at low q initially decreases up to 8 h because of settling with minimal agglomeration. After ∼10 h, however, the intensity increases substantially at low-q, consistent with the fact that agglomeration dominates precipitation.27 Compared to the acid-treated and untreated samples, PEGfunctionalized nanofibers do not show any agglomeration with time. The homogeneous solution is stable in water for weeks. Attachment of water-soluble PEG oligomers to nanofibers alters the surface properties of nanofibers and renders the tubes hydrophilic, thus controlling agglomeration. As stated previously, the scattering curves of the acid-treated carbon nanofibers in Figure 6 consist of two power-law regimes
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Figure 7. Time dependence of the radii-of-gyration of the bundles (Level 1) and agglomerates (Level 2). The bundle size is stable, but the bundles agglomerate into large-scale clusters after a 10 h induction period.
and two Guinier regimes that define two “length scales.” Each Guinier regime is followed by a quasi power-law regime. One structural level is described by a Guinier regime and an associated power-law regime. To quantify the evolution of the acid-treated suspension in Figure 6, these data were analyzed using a two-level unified model28 to extract the size of the bundles (Level 1) and agglomerates of bundles (Level 2). The unified analysis allows one to extract Guinier radii and powerlaw exponents without commitment to any particular morphological model. In the case of Figure 6 there are two Guinier crossovers, each followed by a power-law regime. The results of the unified analysis are shown in Figure 7 where the Rg of each level is plotted vs time after initial suspension by sonication. Substantial agglomeration is observed after an induction period of 10 h. The Rgs derived from low q region (Level 2) increase linearly with time, consistent with agglomeration. All the carbon eventually precipitates.27 Acid treatment does not stop agglomeration and precipitation, but simply retards these processes. During the growth of the agglomerates, the Rg of the fiber bundles is unchanged. Conclusion We have studied dispersion behavior of PEG-functionalized nanofibers suspended in water under quiescent conditions. Although linear morphology is found, the dispersed objects are not individual tubes, but side-by-side bundles (aggregates) of tubes. Functionalization has minimal effect on the small-scale bundle morphology. Compared to untreated and acid-treated carbon nanofibers, which agglomerate to form large-scale agglomerates, PEG-functionalized samples do not form agglomerates and remain stable for weeks. The presence of the PEG oligomers leads to solubilization of nanofibers by suppressing agglomeration, not by disrupting side-by-side aggregates. Acknowledgment. We thank Jie Lian at University of Michigan for the TEM images and Micromeritics Corporation for use of the Saturn Digisizer 5200. The University of Dayton Research Institute supported work at the University of Cincinnati. The work was funded in part by Doctoral Fund of Qingdao University of Science and Technology and the Key Laboratory of Rubber-Plastics (Qingdao University of Science and Technology), Ministry of Education, China.
15310 J. Phys. Chem. C, Vol. 112, No. 39, 2008 References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Dai, L. M.; Mau, A. W. H. AdV. Mater. 2001, 13, 899. (3) Collins, P. G.; Zettl, A.; Bando, H.; Thess, A.; Smalley, R. E. Science 1997, 278, 100. (4) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y. S.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (5) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave, J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367. (6) Liu, J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; RodriguezMacias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (7) Chen, J.; Rao, A. M.; Lyuksyutov, S.; Itkis, M. E.; Hamon, M. A.; Hu, H.; Cohn, R. W.; Eklund, P. C.; Colbert, D. T.; Smalley, R. E.; Haddon, R. C. J. Phys. Chem. B 2001, 105, 2525. (8) Mickelson, E. T.; Chiang, I. W.; Zimmerman, J. L.; Boul, P. J.; Lozano, J.; Liu, J.; Smalley, R. E.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. B 1999, 103, 4318. (9) Huang, W. J.; Fernando, S.; Allard, L. F.; Sun, Y. P. Nano Lett. 2003, 3, 565. (10) Shaffer, M. S. P.; Fan, X.; Windle, A. H. Carbon 1998, 36, 1603. (11) Ausman, K. D.; Piner, R.; Lourie, O.; Ruoff, R. S.; Korobov, M. J. Phys. Chem. B 2000, 104, 8911. (12) Schaefer, D. W.; Brown, J. M.; Anderson, D. P.; Zhao, J.; Chokalingam, K.; Tomlin, D.; Ilavsky, J. J. Appl. Crystallogr. 2003, 36, 553. (13) Schaefer, D. W.; Zhao, J.; Brown, J. M.; Anderson, D. P.; Tomlin, D. W. Chem. Phys. Lett. 2003, 375, 369.
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