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Effect of Post Production Processing on Dispersion of Carbon Nanofibers in Water Jian Zhao* The Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong ProVincial Key Laboratory of Rubber-Plastics (Qingdao UniVersity of Science and Technology), Qingdao, China, 266042, Key Laboratory of Molecular Engineering of Polymers (Fudan UniVersity), Ministry of Education, Shanghai, China, 200433 and Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, (Hangzhou Normal UniVersity), Hangzhou, China, 310012
Dispersion of carbon nanofibers subjected to post production processing, heat treatment (HT) and pyrolytical stripping (PS), is investigated in aqueous suspension using small-angle light scattering. Both samples exhibit a hierarchical morphology consisting of small-scale aggregates and large-scale fractal agglomerates that precipitate rapidly. Although small-scale objects are formed by side-by-side aggregation of the nanofibers in both cases, the determination of the size distribution by assuming a tube model indicates that the PS sample is much less aggregated because of the presence of chemical vapor deposition layer. For HT fibers, the size of large agglomerates remains nearly unchanged during the precipitation process. For PS fibers, large agglomerates precipitate with time, leaving small entities in suspension, which never agglomerate. The HT nanofibers show greater degree of aggregation and agglomeration due to their highly ordered graphite nature. The carbon nanofiber structure significantly influences the dispersion of carbon nanofibers. These findings have significant implications for potential applications of carbon nanofibers in the fields of nanocomposites and biomedical nanostructures. Introduction Carbon nanofibers are used for a range of applications such as reinforcing fillers, field emitters, and nanoelectronic devices, etc.1-4 Carbon nanofibers (CNF) are different from carbon nanotubes in that they have many more walls of crystalline carbon and usually have more structural defects than carbon nanotubes. The cost of preparing carbon nanofibers is significantly less than carbon nanotubes due to the synthesis techniques used, defects, and the remaining amorphous carbon. Chemical vapor deposition method has been widely used to make vapor grown carbon fibers because of the ease of being scaled up and the relative cheapness of the components.5,6 The commercial production of these nanofibers is preformed in a continuous reactor with floating catalysts.7,8 During production, the nanofibers get coated in the reactor with an outer layer consisting of nested graphene tubes that is similar to multiwalled carbon nanotubes, but turbostratic.9 This layer surrounding each fiber is called “CVD layer” (chemical vapor deposition layer). Some nongraphene aromatic groups are also present on the surface of the fibers and are usually removed before use. To optimize their structural features for different applications, the carbon nanofibers are also usually processed upon production. Such post production processing can render these nanocarbons to possess more desirable strength and electrical conductivity. For instance, heat treatment (HT) (up to 3000 °C) is performed to graphitize (CVD) carbon (far less ordered) present on the surface of as-grown carbon nanofibers (PR19). The outer layer of the heat-treated nanofibers is highly ordered after significant structural changes, thus possessing highly electrical conductivity.10 As-produced carbon nanofibers also can be pyrolytically stripped (PS) in carbon dioxide through a rapid heating process below 1000 °C.11 Typically, polyaromatic hydrocarbons (PAH) * To whom correspondence should be addressed. E-mail: zhaojian@ gmail.com.
are removed during this processing. PAHs are the byproduct of incomplete combustion of carbon-based materials and have proven highly toxic to humans and other animal species. The pyrolytically stripped carbon nanofibers retain a chemically vapor deposited (CVD) layer of carbon on the surface of the fiber over a graphitic tubular core fiber. The structure and surface properties of these carbon nanofibers were studied by Raman, ESEM, and XPS, etc.12-15 Dispersion of carbon nanofibers is a major challenge for manipulation of these nanospecies. While post production thermal processing changes the structure of nanofibers and alters surface properties of nanofibers, thus influencing their sensitivity to chemical treatment, little is known about their effect on the state dispersion of carbon nanofibers in media. Researchers recently demonstrated that scattering technique is a unique tool to quantify the dispersion of carbon nanofibers.16-20 We used light scattering to demonstrate that plasma treatment significantly improves the dispersion of carbon nanofibers. The carbon nanofibers we used are HT fibers.21 In this article, smallangle light scattering is performed to compare the evolution of the dispersion of the carbon nanofibers produced under different post production processing (PS and HT). Comparison of dispersion behavior of PS and HT nanofibers suspended in water shows that HT nanofibers are more difficult to disperse due to their ordered graphite surface nature. The presence of the CVD layer renders PS nanofibers less aggregated and agglomerated, making them easier to be manipulated. Applied Sciences, Inc. (ASI) made the samples used in this research using full-scale chemical vapor deposition (CVD). Pyrograf-III carbon nanofibers are commercially available and widely used as raw materials in literature for their potential applications such as nanocomposites, nanodevices, and biomedical nanostructures.12-14 Experimental Section Vapor-grown carbon nanofibers (VGCNF) were provided by Applied Sciences Inc., Cedarville, OH. The commercial produc-
10.1021/ie100295d 2011 American Chemical Society Published on Web 01/04/2011
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tion of these nanofibers is performed in a continuous reactor with floating catalysts. Pyrograf-III PR19HT (high heat treated) nanofibers are vapor grown and subsequently heated to temperatures up to 3000 °C. Pyrograf-III PR19PS nanofibers are pyrolytically stripped in carbon dioxide to remove polyaromatic hydrocarbons from surface. The Pyrograf-III VGCNF normally contain a few concentric cylinders but may also be nested truncated cones. These fibers have an outer diameter of 50-200 nm and a wall thickness of about 20 nm. TEM samples were prepared by allowing a drop of nanofiber suspension to dry onto Cu grids with holey 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 was 200 kV. The dispersion efficiency was determined using a low-angle light scattering photometer-a Micromeritics Saturn Digisizer (www.micromeritics.com). The data are reported in reciprocal space as intensity versus the magnitude of the scattering vector, q. Light scattering covers the regime 10-6 Å-1 < q < 10-3 Å-1, where q ) (4π sin θ)/λ, θ being one-half the scattering angle, and λ being the wavelength of the radiation in the medium. This q range corresponds to length-scales (∼q-1) from 100 µm at low-q to 1000 Å (0.1 µm) at high-q. In the light scattering experiment, deionized water was used as the background. Aqueous suspensions of carbon nanofibers were sonicated at 10 W for 5 min before the observations began. Initially, the concentration of HT or PS carbon nanofibers was 5.0 × 10-6 g/mL. No attempt was made to control the pH or the temperature of the sonicated suspensions, although it was noted that the temperature of suspensions rose to 65 °C during sonication. The light scattering measurements were taken in the batch mode. That is, the sample chamber was filled with the undiluted suspension and observed without stirring, agitation, or circulation. We also estimate the size distribution from the light scattering data using the maximum entropy method.22 We use the Irena code to get the maximum entropy solution. Results and Discussion Presented in Figure 1, bright-field and high-resolution TEM images of as-received carbon nanofibers PR19PS show that the PS nanofibers are wrapped with a smooth CVD layer. The ordered graphite structure is observed under the CVD layer. Defects are occasionally found on the surface. The CVD layer is not much ordered but highly graphitic with curving planes parallel to the fiber axis. As shown in Figure 2, the Pyrograf-III PR19HT nanofibers possess the graphite structure with the interlayer spacing d ) 0.34 nm. Heat treatment at high temperature changes the CVD layer into graphite.11 As compared to the PS nanofibers, the outer layer of the PR19HT nanofibers is highly ordered. Graphitization of turbostratic carbon leads to its transformation to well-ordered graphite and thus hydrophobic behavior of PR19HT. Pristine carbon nanofibers suspended in water are not very stable in unsonicated suspension at any significant concentration. Fibers precipitate rapidly. We use light scattering to monitor this process. As investigated previously, the time evolution of the light scattering profiles for the HT carbon nanofibers in water indicates that the overall intensity decreases gradually with time, consistent with precipitation.21 Here, we also show several scattering profiles at different time points (Figure 3) when the data were obtained in batch mode. As can be seen, two crossover
Figure 1. TEMs of carbon nanofibers PR19PS. CVD layers are visible at both magnifications. The bars are 20 and 5 nm.
regions, each followed by one quasi power-law regime, are found, indicating two structural levels length scales. The crossover region is referred to as Guinier scattering. The curvature in the Guinier regime defines a length scale (Guinier radius or radius of gyration, Rg, in the case of independent scatterers). Beaucage’s Unified Model was used to fit the curves to extract Rg, the power-law exponents, P, the Guinier prefactors, G, and the power-law prefactor, B, associated with each length scale.23,24 For the purpose of comparison, these parameters are displayed in Table 1 for the two structural levels. The slope near -2 (P ) 2) on a log-log plot around q ) 0.002 Å-1 could be attributed to a hollow tube because the wall of such an object is two-dimensional on scales larger than its wall thickness and shorter than the radius. The slope can also originate from more complex aggregated structures.19,20,25 The crossover length scale (q-1 = 1 µm) between the two power-law regimes corresponds to the maximum radius of the fiber aggregates. Minimal change in Rg and P is found for q > 10-4 Å-1, showing minimal change in morphology with time on length scales below ∼1 µm. These small-scale structures found at high-q form large-scale objects, which we call agglomerates. The prefactor, G, extracted from high-q level (level 1) decreases with time, indicating that the number and/or molecular weight of the small-scale entities is decreasing. All the carbon precipitates after 2 days. We also studied the dispersion behavior of pyrolytically stripped carbon nanofibers (PR19PS) suspended in water using light scattering. Figure 4 shows the light scattering profiles as a function of time for carbon nanofibers PR19PS in water at a concentration of 5.0 × 10-6 g/mL. Similar to the HT case in Figure 3, two structural levels are present. The associated
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Figure 3. Evolution of the light scattering profile of carbon nanofibers PR19HT for 1 day following dispersion by sonication. 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 experiment. The lines are two-level unified fits.
Figure 2. TEMs of carbon nanofibers PR19HT. Graphitic layers are visible. The low-resolution image shows a variety of tube shapes and morphologies including concentric cylinders and stacked cones. The bars are 20 and 5 nm.
parameters are listed in Table 2. In contrast to the time evolution of the scattering profile of the HT nanofibers, which show minor decreases over time, the scattered intensity at low-q for PS nanofibers drops sharply. A dramatic decrease in the intensity at low-q is observed after 5 h. In dilute solution, the intensity at q f 0 is proportional to molecular weight, so the data in Figure 4 imply a decrease of molecular weight by a factor of 10 between 5 min and 5 h. The Rgs extracted from low-q region correspond to the size of agglomerates. For HT nanofibers, the large-scale agglomerates appear immediately after sonication, and their sizes are almost unchanged during the precipitation process. For PS nanofibers, however, the Rgs extracted from low-q dramatically decrease with time. When suspended, larger agglomerates precipitate, leaving smaller entities in suspension. In the PS case, the agglomerates are easier to break and do not grow. Figure 5 compares the scattering curves for HT and PS carbon nanofibers at 24 h after sonication. As compared to the HT sample, the scattered intensity at low-q (G) for the PS sample is considerably smaller, indicating small entities in the suspension. The Guinier radius derived from low-q region is 4.06 µm for the PS case, as compared to 19.5 µm for the HT sample. These findings are in good agreement with enhanced dispersion because of the presence of the CVD layer on the surface of the nanofibers. Figure 6 compares the Guinier radius extracted from high-q as a function of time for HT and PS nanofibers (Figure 6). In any case, the Rgs (Figure 6) derived from these high-q data show
Table 1. Guinier Radii and Exponents as a Function of Time for As-Received Carbon Nanofibers PR19HT time
low-q
high-q
Rg (µm) P G 106B Rg (µm) P G 108B
5 min
1h
2h
5h
8h
24 h
21.3 1.48 160.2 0.59 0.86 2.01 1.53 4.53
20.9 1.44 110.1 4.23 0.84 2.07 1.24 2.93
20.8 1.43 74.2 4.14 0.88 2.15 1.46 1.67
19.4 1.22 51.5 29.72 0.83 2.14 0.92 1.77
18.6 1.23 44.9 23.24 0.87 2.08 1.00 2.56
19.5 1.32 65.2 10.67 0.83 2.00 0.67 3.90
little change with time, implying that the short-scale morphology remains unchanged during the precipitation process. The Rgs extracted from high-q for the PS sample are considerably smaller than those for HT sample (Figures 6), in agreement with size distribution analysis shown below. Pyrolytically stripped nanofibers show less degree of aggregation, although aggregated structure (bundles) still exists. Because of the presence of the CVD layer on the surface of carbon nanofibers, the bundles do not further aggregate with time To better understand the bundle morphology, the data were analyzed to determine size distributions using maximum-entropy method.22 Figures 1 and 2 show that carbon nanofibers are more tube-like. We therefore use a tube model to extract size distribution from high-q region. A simplified tube model used here was developed recently.25 Assuming a tube form factor, Figure 7 shows the volume distributions with a wall thickness of 200 Å for 24 h PS and HT samples. Using the tube model, we obtain the diameter distribution with a peak around 0.52 µm for the 24 h HT fibers. The diameter at the peak is much larger than the fiber diameters seen in TEM, implying that the scattering entities are side-by-side fiber
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Figure 4. Dispersion of carbon nanofibers PR19PS in water during 1 day suspension. The suspensions were sonicated at 10 W for 5 min before data were taken using light scattering in batch mode. The lines are two-level unified fits.
Figure 5. Comparison of the scattering profiles for PR19HT and PR19PS carbon nanofibers 24 h after sonication.
Table 2. Guinier Radii and Exponents as a Function of Time for As-Received Carbon Nanofibers PR19PS time
low-q
high-q
Rg (µm) P G 106B Rg (µm) P G 108B
5 min
1h
2h
5h
24 h
16.3 1.23 49.9 0.16 0.68 1.81 0.28 14.53
12.9 1.05 16.9 0.74 0.60 2.01 0.25 3.36
10.8 1.03 10.8 11.12 0.59 1.98 0.23 4.60
4.1 1.09 3.5 0.47 0.66 1.83 0.22 13.01
4..06 1.10 3.1 0.36 0.69 1.77 0.18 17.34
bundles, not individual fibers. Comparison of the size distributions for PS and HT samples (Figures 7) shows that the size distributions of PS nanofibers shift to smaller bundle sizes. In other words, it is easier to break the side-by-side aggregates in the PS case. Because the contrast is not known, the volume distributions (ordinate in Figure 7) are on an arbitrary scale. A peak in the diameter distribution for the PS sample is found at around 0.37 µm, which is considerably smaller than that for the HT sample, but still larger than the largest fibers found in TEM. As we know, heat treatment (HT) (up to 3000 °C) can graphitize chemical vapor deposited (CVD) carbon present on the surface of as-grown carbon nanofibers. Because of the extended π electron system, these systems are highly polarizable and thus subject to large attractive van der Waals forces. These forces are responsible for the secondary bonding that holds graphitic layers together. These forces lead to so-called “bundles”, extended structures formed by side-by-side aggregation of the nanofibers. The linear structure of carbon nanofibers leads to a cooperative effect that enhances the forces described above. Whereas spherical particles touch at a point, rods interact along
Figure 6. Radius of gyration (Rg) derived from high-q region as a function of time for PR19HT and PR19PS carbon nanofibers.
a line. As a result, the above forces are augmented by filler geometry. For the mutual attraction of single- and multiwalled carbon nanotubes, the structure of the outer graphene tubes have been identified as a primary contributor to the dispersion forces.26 It is likely that this behavior functions for carbon nanofibers as well. The disordered nature of the CVD layer over a tubular core fiber likely reduces the large attractive forces.
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matrices and nanofibers, thus resulting in enhanced performance of PS nanofiber-reinforced polymer composites. Conclusions We studied the effect of post production processing (heat treatment and pyrolytical stripping) on the dispersion behavior of carbon nanofibers suspended in water under quiescent conditions. Both samples show a hierarchical morphology consisting of small-scale aggregates and large-scale agglomerates. The aggregates are small side-by-side bundles of individual nanofibers, which form large-scale fractal clusters and precipitate rapidly. In the HT case, the agglomerates appear immediately after sonication, and their size remains almost unchanged during the precipitation process. In the PS case, the bundle size is substantially reduced. Small entities are observed in solution upon the precipitation process of large agglomerates. The presence of the CVD layer on the PS nanofibers leads to improved dispersion not only by suppressing agglomeration, but also by inhibiting side-by-side aggregation. Pyrolytical stripped carbon nanofibers are more suitable for biomedical and nanocomposite applications.
Figure 7. Comparison of volume distributions assuming tubes with a wall thickness of 200 Å for the 24 h PR19HT and PR19PS data.
More significantly, depending on the post production processing, each type of fiber shows a different surface structure, and thus different hydrophilicity. The HT fibers have graphitized surfaces without chemically actived angling bonds, leading to hydrophobic behavior.27 After CO2 treatment of as-grown carbon nanofibers, oxygen-terminated bonds form on the surface of the resulting pyrolytically stripped nanofibers. The presence of hydroxyl or carbonyl groups on the outer surface was confirmed, although their coverage is low.28,29 The outer disordered CVD layer (oxygen surface termination) and its turbostratic surface structure enables adsorption of water, leading to partially hydrophilic behavior and thus improved dispersion in aqueous solution. All the above-mentioned factors lead to less aggregated and agglomerated structure of the PS nanofibers suspended in water, which is favorable for their biomedical applications. For instance, the partly hydrophilic behavior and less aggregated structure of the PS nanofibers improve their capacity for biomolecule adsorption that is critical for the potential use of carbon nanofibers.14 These observations have significant implications for the selection of as-received carbon nanofibers as reinforcing fillers for polymer composites. Because of the generic entropic penalty, it is reasonable that carbon nanofibers still aggregate and agglomerate when dispersed in polymer matrices.30 The reinforcing elements are disordered fractal objects (agglomerates), not isolated tubes. If too large, they behave as large voids that are responsible for decreasing composite moduli. The presence of the CVD coating layer on the PS nanofibers leads to improved dispersion by lessening the degree of aggregation and agglomeration. Less energy is required to achieve dispersion (electrical conductivity is not the concern for polymer composites in this case), thus allowing greater retention of fiber length during processing. It is envisioned that the functional groups present on the CVD layer improve interfacial bonding between polymer resins and nanofibers, the smaller agglomerates mitigate their void behavior, and the reduced bundle size increases the interface area between polymer
Acknowledgment I thank Jie Lian at the University of Michigan for the TEM images and Micromeritics Corp. for use of the Saturn Digisizer 5200. I am grateful to Dr. Jan Ilavsky (Argonne National Laboratory), Professor Dale W. Schaefer, and Gregory Beaucage (University of Cincinnati) for their valuable help in the analysis of the scattering data. The work was funded in part by the National Natural Science Foundation of China (No. 50943026 and 51073082), MOE Key Laboratory of Molecular Engineering of Polymers (Fudan University), MOE Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education (Hangzhou Normal University), and the Key Laboratory of Rubber-Plastics (Qingdao University of Science and Technology), Ministry of Education, China. Literature Cited (1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties and Applications; Springer: Berlin, 2001. (2) Gong, Q.; Li, Z.; Zhou, X.; Wu, J.; Wang, Y.; Liang, J. Synthesis and characterization of in situ grown carbon nanofiber/nanotube reinforced carbon/carbon composites. Carbon 2005, 43, 2426–2429. (3) Li, P.; Zhao, T.; Zhou, J.; Sui, Z.; Dai, Y.; Yuan, W. Deuterated water as super solvent for short carbon nanotubes wrapped by DNA. Carbon 2005, 43, 2701–2703. (4) Safadi, B.; Andrews, R.; Grulke, E. Multiwalled carbon nanotube polymer composites: synthesis and characterization of thin films. J. Appl. Polym. Sci. 2002, 84, 2660–2669. (5) Ajayan, P. Nanotubes from carbon. Chem. ReV. 1999, 99, 1787– 1799. (6) Dai, H. Carbon nanotubes: opportunities and challenges. Surf. Sci. 2002, 500, 318–322. (7) Tibbetts, G. G.; Doll, G. L.; Gorkiewicz, D. W.; Moleski, J. J.; Perry, T. A.; Dasch, C. J.; Balogh, M. J. Physical properties of vapor-grown carbon fibers. Carbon 1993, 31, 1039–1047. (8) Tibbetts, G. G.; Gorkiewicz, D. W.; Alig, R. L. A new reactor for growing carbon fibers from liquid- and vapor-phase hydrocarbons. Carbon 1993, 31, 809–814. (9) Wang, Y.; Santiago-Aviles, J. J.; Furlan, R. R. Pyrolysis temperature and time dependence of electrical conductivity evolution for electrostatically generated carbon nanofibers. IEEE Trans. Nanotechnol. 2003, 2, 39–43. (10) Paredes, J. I.; Burghard, M.; Marti’nez-Alonso, A.; Tasco’n, J. M. D. Graphitization of carbon nanofibers: visualizing the structural evolution on the nanometer and atomic scales by scanning tunneling microscopy. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 675–682.
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ReceiVed for reView February 6, 2010 ReVised manuscript receiVed November 1, 2010 Accepted November 28, 2010 IE100295D
