NANO LETTERS
Mesogenicity Drives Fractionation in Lyotropic Aqueous Suspensions of Multiwall Carbon Nanotubes
2006 Vol. 6, No. 3 568-572
Shanju Zhang, Ian A. Kinloch, and Alan H. Windle* Department of Materials Science and Metallurgy, UniVersity of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. Received October 28, 2005; Revised Manuscript Received January 1, 2006
ABSTRACT We describe a simple method for separating carbon nanotubes on the basis of their mesogenicity by fractionating biphasic aqueous suspensions within the Flory chimney of the lyotropic phase diagram. Macroscopic phase separation occurs on centrifuging the biphasic nanotube suspension or allowing it to stand. Long, straight nanotubes with higher mesogenicity (liquid crystalline forming ability) segregate preferentially to the liquid crystalline phase, whereas shorter nanotubes and impurities with lower mesogenicity segregate preferentially to the isotropic phase.
Carbon nanotubes (CNTs) have attracted great interest since their discovery because of their unusual chemical structure and intriguing physical properties.1 However, processing difficulties limit the realization of their full potential. In many cases, the nanotubes cannot be purified or dispersed in easily handled solvents at a significant concentration. The dispersion of CNTs in water is especially important due to potential applications in biology and medicine. Reports of the aqueous dispersion of CNTs through oxidation, grafting, wrapping, and surfactant addition show that, upon suitable treatment, nanotubes can be well dispersed in deionized water over a wide range of concentrations.2-6 (These dispersion methods may also separate nanotubes from graphitic impurities.7) However, these treated nanotubes typically possess a wide distribution of sizes and properties. The separation of a desired nanotube species from such polydisperse dispersions has been reported by single-strand DNA wrapping, electrophoresis, flocculation, and chromatography.3,8-14 The DNA method has opened up the possibilities of separating metallic from semiconducting single-walled nanotubes. While, the chromatography does separate by length, there can be difficulties in recovering the nanotubes from the separation medium used. It has been suggested that carbon nanotubes are simply exceptionally stiff examples of polymeric molecules15 and that nanotubes, like other rigid-chain polymers such as polyp-phenylenebenzobisoxazole (PPTA)16 and tobacco mosaic virus (TMV),17 should form a lyotropic liquid crystalline phase in a suspension (or solution) above a critical concen* Corresponding author:
[email protected]; tel, +44 1223 334321; fax, + 44 1223 334366. 10.1021/nl0521322 CCC: $33.50 Published on Web 02/07/2006
© 2006 American Chemical Society
tration.18 Recently, this group reported experimental evidence of liquid crystallinity of multiwall CNTs in an aqueous suspension,19 with a following paper from the Rice group reporting the liquid crystalline behavior of single-wall CNTs in a superacid suspension.20 It has been subsequently shown that the formation of this liquid crystalline phase opens a door to the development of lyotropic processing routes for carbon nanotubes analogous to those used for rigid-chain polymers.21 The ability of a rodlike object to form a liquid crystalline phase, its mesogenicity, depends predominantly on its straightness and its aspect (length/diameter) ratio. In the case of lyotropic liquid crystalline systems, the greater the mesogenicity of the rods, the smaller the volume fraction needed to form the liquid crystalline phase.22 Where the aspect ratio is very high, such as with liquid crystalline polymers and carbon nanotubes,23 then other, more subtle considerations come into play as entanglements may militate against ordering as this leads to elastic bending of the rodlike entities. Additionally, entanglements can greatly slow the kinetics in any approach to equilibrium. The polymer/solvent equilibrium diagram (Figure 1) shows that the isotropic and liquid crystalline phases in equilibrium are separated by a biphasic region (the so-called Flory chimney) extending over a fixed range of composition, the bounds of which are dependent on the mesogenicity and the strength of the polymer/polymer interaction divided by the temperature.24-26 Where there is wide polydispersivity, the polymeric molecules of molecular weights sufficiently low to lower their mesogenicity (i.e., lengths below their persistence length) will be expected to diffuse preferentially to the disordered isotropic phase. As a result, a measure of size fractionation
Figure 1. The phase diagram for a lyotropic liquid crystalline material in a solvent. The Flory chimney contains the biphasic region, with the critical concentrations for the lower and upper limits of the chimney marked as c1* and c2*, respectively.
will occur between the two phases of the Flory chimney in the lyotropic phase diagram. If the phases can be physically separated, the liquid crystalline phase can be removed and refractionated by the addition of further solvent. This process can then be repeated successively to give further enhancement of the mesogenicity of the species forming the liquid crystalline phase. As the length of carbon nanotubes is considered to be exceptionally high, fractionation is possible over a considerable nanotube length range within which the aspect ratios are greater than 100:1.27 This fractionation would then allow nanotubes to be purified on the basis of their affinity for the liquid crystalline phase, which is their
mesogenicity, to produce monodispersed samples, which would be of significant scientific and technological value. Furthermore, the use of nanotubes, which can be directly observed in the scanning electron microscope in dried samples, adds scientific understanding to lyotropic liquid crystals and their phase diagram. We prepared a nematic, liquid crystalline phase of nanotubes by oxidizing multiwalled carbon nanotubes and dispersing the treated nanotubes in deionized water.19 The multiwall carbon nanotubes were synthesized in our own laboratory by the pyrolysis of a solution of ferrocene in toluene.28 The nanotubes produced were reasonably straight with outer diameters in the range of 10-100 nm and lengths from 20 to 200 µm. To aqueously disperse the nanotubes, the pristine tubes were oxidized at 50 °C for 24 h in a 3:1 mixture of concentrated sulfuric acid and concentrated nitric acid under ultrasonication and then washing with deionized water.19 The resultant nanotubes were shortened to a length of ∼900 nm and a diameter of ∼30 nm. These tubes were then dispersed in deionized water to the required concentration. The phase transition from isotropic to nematic was observed upon increasing the concentration of nanotubes in the water above a critical value, which depended upon the length distribution of the nanotubes, as discussed later. A series of aqueous nanotube suspensions were produced that covered a range of concentrations. For those concentrations within the biphasic region of the phase diagram, the phases separated and coarsened over time such that after 1 month in a sealed container, the separated phases were clearly visible under a polarized light microscope. Parts a and b of
Figure 2. Micrographs of the nematic structures of 8 wt % aqueous suspension of shortened nanotubes. Cross-polar optical micrographs (a) before and (b) after 1 month at room temperature. In (b) the two phases are separated by a sharp interface with an isotropic drop of dark texture within a continuous nematic phase of Schlieren texture. (c) FEGSEM micrograph of the liquid crystalline phase in image after removal of water (b), significantly, the longer CNTs dominate the nematic phase. (d) FEGSEM micrograph of the isotropic drop in image (b), here, shorter CNTs and carbon impurities make up the isotropic phase. Nano Lett., Vol. 6, No. 3, 2006
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Figure 3. Length distributions for the nanotubes/nanoparticles from each phase of the sample in Figure 2: initial material before separation (a) and the separated nematic (b) and isotropic (c) phases.
Figure 4. Plots showing (a) the volume fraction of nanotubes in each of the phases coexisting in the Flory chimney as a function of overall concentration after centrifuging the sample, (b) the volume faction of the nematic phase over the same range of total concentration, and (c, d) FEGSEM images of the dried samples of the (c) isotropic and (d) nematic phases.
Figure 2 show typical reflected polarized optical micrographs of the textures obtained before and after the sample (8 wt % in water) had been held for 1 month at room temperature (Olympus BX-50). Considerable coursing of the two-phase structure can be observed in Figure 2b. Dark regions, which remain unchanged on cross-polar rotation and correspond to the isotropic phase, are set within the Schlieren texture typical of the nematic liquid crystalline phase. The phase structure, including the singularities of the Schlieren texture, is preserved as a thin film after the solvent is completely evaporated from the suspension. This preservation enables the microstructures to be observed with a field emission gun scanning electron microscope (FEGSEM, JEOL 6340FFF) at a resolution sufficient to observe the organization of the individual nanotubes.19 Parts c and d of Figure 2 show typical FEGSEM micrographs for the nanotube distributions and alignments in both liquid crystalline and isotropic phases after
removal of the solvent. The liquid crystalline phase consists of long and straight nanotubes that self-organize into ordered alignments (Figure 2c). In contrast to the liquid crystalline phase, shorter nanotubes and carbon impurities were found in the isotropic phase (Figure 2d). These nanotubes showed neither long range nor short range orientational order, while the impurities included nanosized carbon particles and highly distorted nanotubes. Figure 3 shows length distributions by number of CNTs in the starting material and in the two separated phases; the counts for each of the distributions being on ∼100 nanotubes, taken from different areas of the sample. These distributions confirm the microscopy observations in showing that nanotubes of higher mesogenicity (i.e., the longer and straighter ones) diffuse preferentially to the liquid crystalline phase while those of lower mesogenicity along with other nanosized particles of carbon impurity segregate to the isotropic region.
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Figure 5. Iterative fractionation of the nematic phase: (a) volume fraction of nanotubes in the nematic phase after each centrifugation cycle as a function of total suspension concentration; (b) critical concentrations of onset and upper limit of biphasic behavior versus centrifugation cycle.
To exploit the interphase segregation on the basis of mesogenicity as a purification method for carbon nanotubes, the nematic and isotropic phases were physically separated by centrifugation. Typically 1 mL of biphasic sample was centrifuged at 3500 rpm for 90 min in a MISTRAL 1000 centrifuge. The denser nematic phase collected at the bottom of the tube and the isotropic phase supernatant collected above it, with complete phase separation occurring within 90 min. Additionally, such macroscopic separation provided a good base for the measurement of concentrations within each of the phases, as well as their relative volumes. The weight concentrations in both isotropic and liquid crystalline phases were determined by collecting aliquots which were
then dried in an oven. Figure 4a shows the relative concentrations found in each phase for a range of overall concentrations. In accord with the prediction of the phase diagram, the concentrations within each phase are essentially constant across the two-phase field, the overall concentration affecting only the relative proportions of each phase, as shown in Figure 4b. The lower concentration limit of the biphasic region of the phase diagram, the “Flory chimney”, was estimated from the experimental data in Figure 3 by extrapolating the linear plot to zero value of volume fraction of the liquid crystalline phase. This critical concentration, defined as c1*, had a value of ∼1.6 wt %. According to the rigid rod theory,22,29,30 a liquid crystalline phase starts to form at a weight fraction of about 3.3Fd/l, where F, d, and l are, respectively, rod density, rod diameter, and rod length. For the shortened nanotubes used in this work, F ∼ 1.75,19 d ∼ 30 nm, and l ∼ 900 nm; suggesting a critical concentration at the start of phase transition of ∼1.9 wt %, in general agreement with the experimental data especially where the simplification inherent in the theory and are taken into account. Similarly, the upper limit of biphasic behavior was estimated by extrapolating the linear curve to the value of 1.0 for volume fraction of liquid crystalline phase, indicating a critical concentration, c2* of ∼14.8 wt %. The Flory chimney width defined as c2* - c1* is, therefore, ∼13 wt %. This value is much larger than that predicted by theory due to the polydispersity in the nanotube length.22,29 FEGSEM micrographs (parts c and d of Figure 4) of material drawn from each of the macroscopically separated phases showed the longer, straighter tubes dominating the nematic phases with shorter, deformed tubes and other impurities fractionated into the isotropic phases in agreement with observation of the two-phase microstructure (Figure 2). Interphase fractionation driven by mesogenicity has been extended by an iterative process. The ability to decant off the isotropic phase, replace with water to restore the same overall concentration, and then centrifuge again provided a basis for successive fractionation stages. At each stage, the process can be followed by measuring the concentration and volume fractions of each phase. Figure 5a shows the volume fraction of liquid crystalline phase against the total concen-
Figure 6. FEGSEM micrographs of a -1/2 disclination in the nematic phase: (a) an unpurified sample showing segregation of shorter nanotubes and impurities to the core; (b) a similar disclination in a sample purified by successive fractionation. Nano Lett., Vol. 6, No. 3, 2006
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tration for different cycles of centrifugation. For each cycle, the dependence of the volume fraction of liquid crystalline phase on the overall concentration becomes steeper, although the value of critical concentration for the appearance of the nematic phase (c1*) remains unchanged for each cycle, e.g., c1* ∼ 1.6 wt %. The data from Figure 5a thus show that the Flory chimney decreases in width with each successive cycle. Figure 5b is a plot of the critical concentrations for the limits of the two-phase region, (c1*) and (c2*), against reciprocal of the cycle number (N), which enables extrapolation to N f ∞. The ratio of c2*/c1* at the infinite value for cycle number of centrifugation is about 2.8, which is in agreement with the theoretic value of 2-3 for a system of rigid rods with the most probable distribution. It is interesting to note that segregation on the basis of tube’s mesogenicity also occurs around the disclination cores within the nematic phase. The aggregation of shorter nanotubes and carbon impurities within the disclination cores was frequently observed in the liquid crystalline phase. Before purification, the shorter nanotubes and carbon impurities spontaneously separate from longer nanotubes and preferentially enter the disclination cores with disordered alignments, as shown in Figure 6a. The role of low mesogenicity material in segregating to disclination cores, where it reduces the high elastic distortion energies in that region, has long been a speculation in liquid crystal polymer science. The additional imaging capability characteristic of nanotube-based liquid crystallinity means that such segregation can be observed. After purification, the disclination core shows little or no segregation of low-mesogenicity material (Figure 6b), attesting to the fact that the large scale segregation between isotropic and liquid crystalline phases is dominant and is not compromised by any tendency of dislocation cores to keep low-mesogenicity material in the nematic phase. In summary, we present a novel and simple method for the separation and purification of carbon nanotubes based their relative mesogenicity. This method offers the advantage of selecting nanotubes from other contaminant species and therefore acting as a purification process. We anticipate that this method will be useful for the analysis and purification of nanotubes in aqueous medium for their potential biological applications. References (1) Harris, P. J. F. Carbon Nanotubes and Related Structures: New Materials for the Twenty-first Century; Cambridge University Press: Cambridge, 1999. (2) Liu, J.; Rinzler, A. G.; Dai, H.; 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.
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