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Dispersing carbon nanotubes using surfactants
Current Opinion in Colloid & Interface Science 14 (2009) 364–371
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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s
Dispersing carbon nanotubes using surfactants Howard Wang Department of Mechanical Engineering and Institute for Materials Research, State University of New York, Binghamton, NY 13902, United States
a r t i c l e
i n f o
Article history: Received 14 June 2009 Received in revised form 17 June 2009 Accepted 23 June 2009 Available online 1 July 2009
a b s t r a c t We review the recent advances in dispersing single-wall carbon nanotubes (SWNTs) using amphiphilic surfactants in aqueous solutions. Three aspects are discussed. (1) On the organization of surfactant molecules with SWNTs, new insights at the microscopic level arise from electron microscopy and detailed computer simulation studies. (2) Quantitative measurements, such as molecular interactions between functional groups and SWNTs, the coverage of surfactant on SWNTs in solution, the charge state of the SWNT/surfactant complex, and the degree of dispersion are critical for better understanding dispersion mechanisms and for the further development of dispersion strategies. (3) The thermodynamic driving forces and the role of metastability in the structure of surfactant dispersed SWNT suspensions are analyzed. An outlook on practical and fundamental issues is also presented. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Single-wall carbon nanotubes (SWNTs) are interesting molecules initially illustrated by Iijima [1]. They are structurally unique materials that exhibit excellent mechanical, electrical, thermal, and optical properties [2], and offer promises for a number of novel applications [3–5]. Certain applications require pure and well isolated individual SWNTs, hence rather extensive effort has been devoted to achieving good dispersion of SWNTs through chemical functionalization [6–8] and physical interactions [9–25]. The former has been found effective but deteriorates the intrinsic properties of SWNTs [26,27]. A recent review provides a rather comprehensive account on dispersing carbon nanotubes with surface functionalization [28]. Physical approaches using amphiphilic surfactants have been proven capable of debundling SWNTs and stabilizing individual tubes while maintaining SWNT integrity and intrinsic properties [12,17]. Surfactant dispersion of SWNTs has been used since the onset of interests in these materials. The early demonstration of SWNT shortening and surfactant-assisted dispersion [9] has opened the door to reliable processes for building SWNT-based electronic devices [29] and exploring their biological applications [30], as well as incorporating SWNTs in matrix materials to form nanocomposites [5]. The needs of various applications demand more effective SWNT dispersion, which depends on interactions between SWNTs and surrounding molecules. In the initial rush toward picking up the best surfactant and best dispersion parameters, dozens of common and uncommon surfactants have been tested, and various processing conditions have been attempted. A notable achievement in this effort has been the use of the ionic surfactant sodium dodecylbenzene
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sulfonate (NaDDBS) by the Yodh group [10], followed up by further detailed studies [11,12,14]. Recent and future developments involve selective interactions with hence dissolution of semiconducting and metallic tubes [31,32] and SWNTs of different diameters [33–35]. The significance of such capability has been elegantly demonstrated recently in the self-sorting fabrication of SWNT transistor arrays [36]. In achieving better and more effective dispersion of SWNTs using surfactant, several fundamental questions have emerged. (1) What is the microscopic picture of surfactant interactions with SWNT? (2) How can we quantitatively assess the important characteristics in SWNT suspensions? (3) What are the thermodynamic and kinetic principles governing SWNT/surfactant systems? As advances in SWNT dispersion have reached a plateau in recent years, it is the goal of this article to review recent progresses in understanding fundamental aspects in dispersing SWNTs. As a recent review summarizes studies on surfactant dispersion of carbon nanotubes before 2006 [37], we focus on advances reported in the past 4 years. 2. Microscopic views of surfactant adsorption Surfactants disperse SWNTs in aqueous solutions mainly through hydrophobic/hydrophilic interactions, in which the hydrophobic tail of the surfactant molecule adsorbs on the surface of SWNT bundles while the hydrophilic head associates with water for dissolution [9– 23]. The microscopic pictures of how the adsorbed amphiphilic molecules organize on SWNTs, however, remain a topic of debate. Three most probable configurations are shown in Fig. 1 [38]. SWNTs can be encapsulated within cylindrical micelles [Fig. 1(a)] [15], or covered with either hemispherical micelles [Fig. 1(b)] [10] or randomly adsorbed molecules [Fig. 1(c)] [21]. Experimentally, microscopic pictures of surfactant adsorption on SWNTs can be revealed directly by cryo-TEM imaging or indirectly through neutron scattering.
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interactions and equilibrium thermodynamics dominate. We expect that a consensus on microscopic pictures of surfactant adsorptions will be reached for this concentration regime in the near future. At high concentrations, however, the interplay among short and long range interactions can result in non-equilibrium states that require quantitative assessments of the structure and dynamics of SWNT suspensions from microscopic to mesoscopic to macroscopic length scales. 3. Toward quantitative measurements
Fig. 1. Schematic illustration of various surfactant assembly structures on a SWNT, including (a) cylindrical micelles, side and cross-section views, (b) hemimicelle, and (c) random adsorption. Reprinted with permission from [38].
As the cylindrical inclusion picture represents a somewhat idealized scenario for dissolution, hemispherical micelle adsorption has captured researchers' imagination through a series of cryo-TEM micrographs, as shown in Fig. 2(a) [22]. Thus far, there has no direct evidence of spherical-micelle-forming surfactant packing densely on cylindrical nanotubes as shown in Fig. 1a. However, for cylinder-forming micelles, it was demonstrated that a simple shear flow could align SWNTs within worm-like-micelles (WLMs) to form well ordered local structures, as shown in Fig. 2(b), implying the inclusion of SWNTs in WLMs as illustrated in Fig. 2(c) [39]. Experimental evidence supporting the random adsorption is also limited. Small angle neutron scattering (SANS) studies suggest that the lack of cylindrical micelle features is due to the random absorption of surfactant on individual SWNTs [21]. New insights from detailed computer simulations represent most of our recent progress in understanding SWNT dispersion at the microscopic scale [34,38,40–46]. Simulations at the molecular level can reveal localized interactions among surfactant molecules and with SWNTs. In equilibrium, a dynamic balance is established, in which a surfactant molecule can exist in one of the three equal-energy states: as an isolated individual molecule, in micelles in the solution, or adsorbed to SWNT surfaces. This balance is sensitive to the surfactant concentration, particularly when the concentration is low. The concentration dependence of the microstructure of the surfactant adsorption on SWNTs is shown in Fig. 3 [46]. In dilute solution, the adsorption of surfactant on the tube appears to be random [Fig. 3(c)], while when concentration is high, most surfactant is in hemispheres on the tube [Fig. 3(d)], whereas at the intermediate concentration, both situations coexist [Fig. 3(a and b)]. Another interesting observation is that tube junctions are always favored for surfactant adsorption apparently due to the reduced energy penalty for more flexible tail conformations [46]. Different surfactant and lipids could behave differently because of varying interaction strengths and critical micelle concentrations, while the general trend of concentration dependence in the microstructure should remain [38]. If lipids with long saturated hydrocarbon tails are used, they would adsorb on a SWNT predominantly aligned along the tube axis [41], implying the onset of templated crystallization as observed for longer polymer chains on SWNTs [24,25]. On the other hand, a specific surfactant could have different interaction energies with SWNTs of different diameters, a subtle effect that could be potentially useful for sorting tubes by their diameters [34]. Although simulations suggest that structures of adsorbed surfactant be conveniently controlled via the bulk surfactant concentration, they apply only to very low surfactant and SWNT concentrations, where local
As the achievement of concentrated stable dispersions of SWNTs in surfactant solutions is still limited, good progresses have been documented in quantitative measurements of SWNT/surfactant systems, such as molecular interactions between functional groups with SWNTs, the coverage of surfactant on SWNTs in solution, the charge states of SWNT/surfactant complexes, and the degree of dispersion. These quantities are critical for better understanding and further development of the SWNT dispersion. This section reviews the recent advances in quantitative measurements. The knowledge of the strength of interactions between SWNTs and organic molecules remains scarce. Previous effort includes qualitative assessment of preferred interactions by testing dispersion with various surfactants [47], and direct adhesion measurement with modified AFM tips [48,49]. A comprehensive study combining experiments and simulations has recently been carried out to measure the interaction between modified AFM tips and SWNT bucky paper [50]. Some results are summarized in Table 1. Between the two types of endgroups, it is clear that the Aryl-thiols are more attractive to SWNTs than Alkyl ones. Among various Aryl endgroups, 4nitrilebenzene shows the largest favorable interaction, with a pulling-off force of 56.9 ± 15.5 pN per molecule, which is possibly due to strong donor–acceptor interactions. It is believed that SWNTs might behave as both donors and acceptors. In the presence of strong donors such as benzene rings, SWNTs behave as acceptors; and for the strong acceptor nitrile groups, they act as donors. So the attraction is maximal when both are present [50].
Fig. 2. (a) Cryo-TEM of SDS surfactant on SWNTs. Reprinted with permission from [22], (b) Cryo-TEM micrograph and (c) schematics of CTAT surfactant with SWNTs. Reprinted with permission from [39].
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Fig. 3. Simulation of surfactant on SWNT showing semi-micelles and preferential attraction to crosses. Concentration in (a) and (b) 1.59 × 10− 5. (c) 1.37 × 10− 5, and (d) 2.02 × 10− 5. (a) The hydrophilic head groups of surfactant in green and the hydrophobic tails in purple; and (b–d) showing only the hydrophobic tails. Reprinted with permission from [46].
The coverage of surfactant on SWNT surfaces essentially depends on the strength of attractive interactions. Surface coverage could be obtained through thermogravimetric analysis (TGA) to quantify the surfactant adsorbed on CNTs [51–53], zeta potential measurement to assess the surface charge states [54–61], or neutron scattering to measure the surfactant in free micelles [23]. The saturation adsorption ratio of surfactant vs. SWNT is ca. 0.004 mol/g for a non-ionic surfactant [23], which is close to the reported values of other surfactants [14,53]. Although values in terms of the surfactant molecule number per nm2 of SWNTs were reported, the comparison was made in the unit of molar surfactant per gram of SWNTs since the exact accessible surface area of SWNT in suspension is not known. It is most desirable to have a single parameter to quantify the degree of SWNT dispersion, which however, does not exist. Many different methods have been used to measure SWNT dispersion. Direct microscopic imaging over many length scales is recommended by a NIST-NASA work group [62]. Another popular method is UV-Vis absorption spectroscopy for examining the purity and dispersion of SWNTs because only isolated defect-free tubes would yield sharp optical spectra [54, 62–67]. However, this method applies only to very low concentrations, at odds with the push toward dispersing SWNTs at high loadings. To make suspensions measurable, they have to be greatly diluted. But dilution could change both the surfactant aggregates, as discussed above, and SWNT network structures, as will be illustrated below. The state of dispersion in suspensions prior to dilution may be lost, thus precluding the technique from true quantification of the degree of dispersion of interested suspensions. Other qualitative methods for assessing the SWNT dispersion include viscosity [68], dynamic light scattering [69], zeta potential [61], TGA measurements [51–53], size exclusion chromatography [70,71], etc.
Analytical ultracentrifugation is emerging as a potentially powerful tool for both purification and dispersion of SWNTs [31,33,47, 52,61,68, 72–92]. DNA-wrapped [33] or covalently functionalized [92] SWNTs could be sorted by their diameters in aqueous density gradients through ultracentrifugation; sodium deoxycholate (DOC) dispersed SWNTs were separated by their length [89,90], or when competitive surfactants were used, semiconducting and metallic tubes could be separated [31]. Ultracentrifugation can also be used to semi-quantitatively characterize the SWNT dispersion in surfactant
Table 1 Interaction strength between various functional groups and SWNT paper [50]. Experiment force/molecule (pN) Alkyl-thiol endgroup –OH –periluoro –SH –CHfCH2 –CH3 –QOOH –NH2
9.6 ± 2 8.7 ± 3 9.2 ± 3 8.1 ± 2 7.6 ± 2 12.2 ± 3 23.4 ± 4
Aryl-thiol endgroup 4-methylbenzene 4-nitrobenzene 4-aminebenzene 4-bromobenzene 4-hydroxybenzene 4-fluorobenzene 4-methoxybenzene H-benzene 4-nitrilebenzene
18.9 ± 5.7 21.8 ± 5.3 22.6 ± 4.7 26.9 ± 3.6 32.0 ± 8.4 39.5 ± 8.8 41.5 ± 10.9 46.8 ± 11.8 56.9 ± 15.5
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Fig. 4. Ultracentrifugation characterization of SWNT dispersion in surfactant solutions. Apparent and average diffusion coefficients determined with the preparative ultracentrifuge method (squares) and dynamic light scattering (circles). The inset shows the experimental and theoretical sedimentation functions of the corresponding precentrifuged dispersions. Reprinted with permission from [79].
solutions through measuring the sedimentation function. Fig. 4 shows the measurement of apparent and average diffusion coefficients of SWNTs in aqueous NaDDBS suspensions as determined with the preparative ultracentrifuge method (squares) and dynamic light scattering (circles), respectively, as suspensions undergo various times of pre-centrifugation followed by 4 h ultrasonication. The inset shows the experimentally measured and theoretical fitting of sedimentation functions of various pre-centrifuged dispersions [79]. The discrepancy in diffusion coefficients between the two methods is mainly due to the polydispersity in the suspended SWNT bundles. As pre-centrifugation time increases, the polydispersity decreases, the apparent diffusion coefficients from ultracentrifugation approach the bulk-averaged diffusion coefficient from dynamic light scattering. Hence the sedimentation measurement of SWNT dispersion applies most meaningfully to dilute and relatively monodisperse bundles in suspensions. Scattering offers unique insights into SWNT dispersion. Particularly, SANS has been used to assess the ensemble-averaged dispersion qualities in relatively high concentration SWNT suspensions [11,23,70,76,93–99]. As quantitative analyses of the scattering function would require relatively uniform tube structures, scaling arguments could yield characteristic signatures of SWNT dispersion. Fig. 5 shows a typical SANS spectrum on surfactant/SWNT dispersion showing dominant Q− 1 scaling behavior for isolated rods [11]. The SWNT scattering is characterized by power-law behavior. Individual SWNTs
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Fig. 6. The power exponents for both the 0.1% and 0.01% SWNT suspensions. It is evident that the power exponent has a minimum around 0.5% to 1% surfactant concentration, indicating the optimal condition for dispersing SWNTs. Reprinted with permission from [23].
behave like rigid rods, displaying Q− 1 scattering at the intermediate Q-range, whereas power exponents of 2–3 at lower Q suggest branching in SWNT bundles and tube network structures [94,95]. The power-law exponent, α, at intermediate Q-range could be used for semi-quantitatively assessing the degree of dispersion; the closer to 1, the better the dispersion. Fig. 6 shows α as a function of the surfactant concentration for both the 0.1% and 0.01% SWNT suspensions. The data suggest that dispersion is not sensitive to the SWNT concentration, and a minimum a around 0.5% to 1% surfactant concentration corresponds to the optimal condition for dispersing SWNTs. The observation that the overall surfactant concentration, rather than the ratio between surfactant and SWNTs, controls the effective dispersion is consistent with a mechanism of competing maximization of surfactant adsorption onto SWNT surfaces and a depletion interaction between SWNT bundles mediated by surfactant micelles [23]. SANS is becoming a useful tool not only for assessing the degree of SWNT dispersion in the relatively concentrated suspensions of interest, but also for contributing to new insights into the structure and thermodynamics of SWNT suspensions and networks. However, neutron scattering is not easily accessible to the broader research community, limiting the extent of its applications to studying SWNT dispersion. A fundamental question about the assessment of SWNT dispersion is whether structure measurement is sufficient while none of SWNT suspensions appear to be thermodynamically stable. 4. Thermodynamics, kinetics and network structures
Fig. 5. SANS spectrum on SWNT/NaDDBS dispersion showing dominant Q− 1 scaling behavior for isolated rods. Reprinted with permission from [11].
The effort toward better SWNT dispersion has been driven, consciously or subconsciously, by the quest for a fully-exfoliated SWNT suspension as a thermodynamically stable phase, which possesses a global free energy minimum. Most often, a somewhat metastable state is obtained. Careful examination of the thermodynamic and kinetic aspects of surfactant-dispersed SWNT suspension systems is much needed. In this section, the thermodynamics driving forces and the role of metastability in structure and stability of SWNT suspensions are discussed. The free energy density of the suspension system is composed of enthalpy terms such as the large attractive van der Waals interactions among tubes, surfactant mediated hydrophilic–hydrophobic interactions between surfactant and SWNTs and among surfactant molecules, and entropy terms related to spatial packing of SWNTs and surfactants. An accurate thermodynamic description of the system could offer to predict its stability, metastability or instability. However, given the heterogeneous nature of most SWNT suspensions, a practical and
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meaningful construction of the free energy is difficult, if not impossible. We therefore qualitatively examine several distinct states, namely the isolated-tube suspension, loosely aggregated network, densely interconnected gels, and large aggregates/precipitates. Isolated-tube suspensions apparently pose as thermodynamically stable phase. Nevertheless they fall to one of two metastable classes, namely thermodynamic metastability. The other is kinetic metastability as in networks, which will be discussed below. In a thermodynamic metastable state, an energy barrier hinders the relaxation of the system to the equilibrium state. In the case of surfactantassisted isolated tube solution, the electrostatic (for ionic surfactants) or steric (for nonionic surfactants) repulsion prevents SWNTs from getting close to each other and reaching a lower energy state of segregated tube bundles and surfactant micelles. Ultrasonication pumps the suspension system to a metastable state through shortening SWNTs and allowing them to be covered by surfactant molecules when transiently isolated in the suspension. As complete monolayer coverage of surfactant on already separated SWNTs would prevent the sticking of SWNTs, adsorption at junctions of SWNT crossing could stabilize the junction [46], making dispersion to individual tubes more difficult. Since SWNTs are born not uniform, the latter situation could dominate the population, resulting in SWNT network formation in suspensions. A SWNT network is characterized by short-ranged frictions or attractions at the “cross-linking” sites, which defy Brownian motion and thermal fluctuations and maintain the separation of the center of mass of tubes in the long range, resulting in long-lasting dispersions. This is the case of kinetic metastability, applicable to a wide range of SWNT systems from loose flocculation to strong gel networks. High medium viscosity and/or long range structures as in ionic surfactant solutions could further strengthen the kinetic metastability, resulting in apparently better dispersion. There has been no direct measurement of the continuous transition from isolated rods to percolated networks, however, studies point to the sensitivity of network structure to suspension parameters as exemplified by SANS data shown in Fig. 7. With reducing surfactant concentration [Fig. 7(a)] [95] or increasing pH value [Fig. 7(b)] [98], the quality of SWNT dispersion deteriorates, as evidenced by the shrinkage of Q− 1 regime and the expansion/ appearance of higher power-law scattering. On the other hand, the evidence of network-stabilized dispersion is clearly shown in Fig. 7(c) for covalently functionalized SWNTs in deuterated toluene. Upon the addition of solvent to the suspension, the percolated network with well-dispersed SWNTs collapses to form aggregates [99]. The transition occurs at a SWNT concentration of ca. 0.04% by mass, which corresponds to the percolation threshold in well-dispersed systems [5]. Below the percolation threshold, SWNTs do not exist as isolated individual rods in suspensions, they instead aggregate. This observation could be general for different SWNT dispersions. Many applications ultimately require an optimally percolated network, the nature of the percolated state depends on the strength of interactions [100], as shown in Fig. 8(a)–(d). As the depth of the effective potential well between contacted nanotubes (ε) increases, the network evolves toward a more isotropic, disordered, and metastable configuration with a lower percolation threshold, a scenario that is relevant to the engineering of conductive nanotube composites, for example. A challenge that remains is how to use surfactants, block copolymers and chemical functionalization, perhaps in combination, to ‘tune’ the effective interaction from short-range repulsive to strongly attractive in the same material by simply changing an external parameter. This tactic, of course, has to be carried out after the dispersion of nanotubes and formations of networks. As noted earlier, entropic depletion forces may be well-suited for this. Monte Carlo simulations demonstrate depletion induced isotropic–isotropic phase separation in suspensions of rod-like colloids [101], where short range reversible attractive interactions induced by spherical free surfactant micelles give rise to liquid–liquid phase
Fig. 7. SANS studies of the effect of (a) surfactant concentration [95], (b) pH value [98], and (c) dilution [99] on dispersion and aggregation of SWNTs in suspensions.
separation into rod-rich domains, as shown in Fig. 8(e). This might offer a way to self-assemble and control percolated structures. Simulations also demonstrate that depletion forces can significantly lower the percolation threshold of rigid rod suspensions, as shown in Fig. 8(f) [102], despite the anisotropic nature of the rod-rod depletion interaction and a global free energy minimum that favors parallel rod alignment. This tendency is more pronounced at higher aspect ratios, on which delocalized interactions and thermodynamic functions strongly depend. As many previous advances were achieved through delicate molecular chemistries, i.e., through engineering local chemical and
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Fig. 8. (a–d) Continuum percolation of carbon nanotubes with varying “stickiness”. Reprinted with permission from [100], (e) depletion-induced isotropic–isotropic phase separation in SWNT suspensions. Reprinted with permission from [101]. (f) Depletion-induced percolation in SWNT networks. Reprinted with permission from [102].
physical forces, further development toward real applications of carbon nanotubes requires “systems level” engineering through thermodynamics and statistical mechanics, in which transient, non-equilibrium, and metastable states play a crucial role. By revealing the energy landscape and controlling kinetic pathways, useful SWNT dispersions may be developed. 5. Summary and outlook In summary, despite the tremendous effort in trying to disperse SWNTs in aqueous solution using amphiphilic surfactants, achievements in stable suspensions with high concentration individual SWNTs have been limited. Somewhat better understanding of the task has accumulated over the past few years from visualizing microscopic structures, measuring molecular interactions, quantifying the degree of dispersion, and generalizing thermodynamic and kinetic theories. It is conceivable that this trend will continue and a principled approach with a combination of the chemistry of molecular designs and microscopic interactions and the physics of thermodynamics and kinetics of SWNT/surfactant suspensions will emerge. In this endeavor, quantitative measurements play an essential role for better assessment of materials systems and critical validation of theories and models. Significant progresses in surfactant dispersion of SWNTs will only be made through synergistic advances in materials and processing innovation, quantitative measurements, and computational and theoretical development. Acknowledgements HW acknowledges the support of NSF under DMR-0711013, and discussions with Prof. Christopher Li (Drexel), Dr. Cheol Park (NIA), and Dr. Erik Hobbie (NIST).
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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s
Dispersing carbon nanotubes using surfactants Howard Wang Department of Mechanical Engineering and Institute for Materials Research, State University of New York, Binghamton, NY 13902, United States
a r t i c l e
i n f o
Article history: Received 14 June 2009 Received in revised form 17 June 2009 Accepted 23 June 2009 Available online 1 July 2009
a b s t r a c t We review the recent advances in dispersing single-wall carbon nanotubes (SWNTs) using amphiphilic surfactants in aqueous solutions. Three aspects are discussed. (1) On the organization of surfactant molecules with SWNTs, new insights at the microscopic level arise from electron microscopy and detailed computer simulation studies. (2) Quantitative measurements, such as molecular interactions between functional groups and SWNTs, the coverage of surfactant on SWNTs in solution, the charge state of the SWNT/surfactant complex, and the degree of dispersion are critical for better understanding dispersion mechanisms and for the further development of dispersion strategies. (3) The thermodynamic driving forces and the role of metastability in the structure of surfactant dispersed SWNT suspensions are analyzed. An outlook on practical and fundamental issues is also presented. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Single-wall carbon nanotubes (SWNTs) are interesting molecules initially illustrated by Iijima [1]. They are structurally unique materials that exhibit excellent mechanical, electrical, thermal, and optical properties [2], and offer promises for a number of novel applications [3–5]. Certain applications require pure and well isolated individual SWNTs, hence rather extensive effort has been devoted to achieving good dispersion of SWNTs through chemical functionalization [6–8] and physical interactions [9–25]. The former has been found effective but deteriorates the intrinsic properties of SWNTs [26,27]. A recent review provides a rather comprehensive account on dispersing carbon nanotubes with surface functionalization [28]. Physical approaches using amphiphilic surfactants have been proven capable of debundling SWNTs and stabilizing individual tubes while maintaining SWNT integrity and intrinsic properties [12,17]. Surfactant dispersion of SWNTs has been used since the onset of interests in these materials. The early demonstration of SWNT shortening and surfactant-assisted dispersion [9] has opened the door to reliable processes for building SWNT-based electronic devices [29] and exploring their biological applications [30], as well as incorporating SWNTs in matrix materials to form nanocomposites [5]. The needs of various applications demand more effective SWNT dispersion, which depends on interactions between SWNTs and surrounding molecules. In the initial rush toward picking up the best surfactant and best dispersion parameters, dozens of common and uncommon surfactants have been tested, and various processing conditions have been attempted. A notable achievement in this effort has been the use of the ionic surfactant sodium dodecylbenzene
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sulfonate (NaDDBS) by the Yodh group [10], followed up by further detailed studies [11,12,14]. Recent and future developments involve selective interactions with hence dissolution of semiconducting and metallic tubes [31,32] and SWNTs of different diameters [33–35]. The significance of such capability has been elegantly demonstrated recently in the self-sorting fabrication of SWNT transistor arrays [36]. In achieving better and more effective dispersion of SWNTs using surfactant, several fundamental questions have emerged. (1) What is the microscopic picture of surfactant interactions with SWNT? (2) How can we quantitatively assess the important characteristics in SWNT suspensions? (3) What are the thermodynamic and kinetic principles governing SWNT/surfactant systems? As advances in SWNT dispersion have reached a plateau in recent years, it is the goal of this article to review recent progresses in understanding fundamental aspects in dispersing SWNTs. As a recent review summarizes studies on surfactant dispersion of carbon nanotubes before 2006 [37], we focus on advances reported in the past 4 years. 2. Microscopic views of surfactant adsorption Surfactants disperse SWNTs in aqueous solutions mainly through hydrophobic/hydrophilic interactions, in which the hydrophobic tail of the surfactant molecule adsorbs on the surface of SWNT bundles while the hydrophilic head associates with water for dissolution [9– 23]. The microscopic pictures of how the adsorbed amphiphilic molecules organize on SWNTs, however, remain a topic of debate. Three most probable configurations are shown in Fig. 1 [38]. SWNTs can be encapsulated within cylindrical micelles [Fig. 1(a)] [15], or covered with either hemispherical micelles [Fig. 1(b)] [10] or randomly adsorbed molecules [Fig. 1(c)] [21]. Experimentally, microscopic pictures of surfactant adsorption on SWNTs can be revealed directly by cryo-TEM imaging or indirectly through neutron scattering.
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interactions and equilibrium thermodynamics dominate. We expect that a consensus on microscopic pictures of surfactant adsorptions will be reached for this concentration regime in the near future. At high concentrations, however, the interplay among short and long range interactions can result in non-equilibrium states that require quantitative assessments of the structure and dynamics of SWNT suspensions from microscopic to mesoscopic to macroscopic length scales. 3. Toward quantitative measurements
Fig. 1. Schematic illustration of various surfactant assembly structures on a SWNT, including (a) cylindrical micelles, side and cross-section views, (b) hemimicelle, and (c) random adsorption. Reprinted with permission from [38].
As the cylindrical inclusion picture represents a somewhat idealized scenario for dissolution, hemispherical micelle adsorption has captured researchers' imagination through a series of cryo-TEM micrographs, as shown in Fig. 2(a) [22]. Thus far, there has no direct evidence of spherical-micelle-forming surfactant packing densely on cylindrical nanotubes as shown in Fig. 1a. However, for cylinder-forming micelles, it was demonstrated that a simple shear flow could align SWNTs within worm-like-micelles (WLMs) to form well ordered local structures, as shown in Fig. 2(b), implying the inclusion of SWNTs in WLMs as illustrated in Fig. 2(c) [39]. Experimental evidence supporting the random adsorption is also limited. Small angle neutron scattering (SANS) studies suggest that the lack of cylindrical micelle features is due to the random absorption of surfactant on individual SWNTs [21]. New insights from detailed computer simulations represent most of our recent progress in understanding SWNT dispersion at the microscopic scale [34,38,40–46]. Simulations at the molecular level can reveal localized interactions among surfactant molecules and with SWNTs. In equilibrium, a dynamic balance is established, in which a surfactant molecule can exist in one of the three equal-energy states: as an isolated individual molecule, in micelles in the solution, or adsorbed to SWNT surfaces. This balance is sensitive to the surfactant concentration, particularly when the concentration is low. The concentration dependence of the microstructure of the surfactant adsorption on SWNTs is shown in Fig. 3 [46]. In dilute solution, the adsorption of surfactant on the tube appears to be random [Fig. 3(c)], while when concentration is high, most surfactant is in hemispheres on the tube [Fig. 3(d)], whereas at the intermediate concentration, both situations coexist [Fig. 3(a and b)]. Another interesting observation is that tube junctions are always favored for surfactant adsorption apparently due to the reduced energy penalty for more flexible tail conformations [46]. Different surfactant and lipids could behave differently because of varying interaction strengths and critical micelle concentrations, while the general trend of concentration dependence in the microstructure should remain [38]. If lipids with long saturated hydrocarbon tails are used, they would adsorb on a SWNT predominantly aligned along the tube axis [41], implying the onset of templated crystallization as observed for longer polymer chains on SWNTs [24,25]. On the other hand, a specific surfactant could have different interaction energies with SWNTs of different diameters, a subtle effect that could be potentially useful for sorting tubes by their diameters [34]. Although simulations suggest that structures of adsorbed surfactant be conveniently controlled via the bulk surfactant concentration, they apply only to very low surfactant and SWNT concentrations, where local
As the achievement of concentrated stable dispersions of SWNTs in surfactant solutions is still limited, good progresses have been documented in quantitative measurements of SWNT/surfactant systems, such as molecular interactions between functional groups with SWNTs, the coverage of surfactant on SWNTs in solution, the charge states of SWNT/surfactant complexes, and the degree of dispersion. These quantities are critical for better understanding and further development of the SWNT dispersion. This section reviews the recent advances in quantitative measurements. The knowledge of the strength of interactions between SWNTs and organic molecules remains scarce. Previous effort includes qualitative assessment of preferred interactions by testing dispersion with various surfactants [47], and direct adhesion measurement with modified AFM tips [48,49]. A comprehensive study combining experiments and simulations has recently been carried out to measure the interaction between modified AFM tips and SWNT bucky paper [50]. Some results are summarized in Table 1. Between the two types of endgroups, it is clear that the Aryl-thiols are more attractive to SWNTs than Alkyl ones. Among various Aryl endgroups, 4nitrilebenzene shows the largest favorable interaction, with a pulling-off force of 56.9 ± 15.5 pN per molecule, which is possibly due to strong donor–acceptor interactions. It is believed that SWNTs might behave as both donors and acceptors. In the presence of strong donors such as benzene rings, SWNTs behave as acceptors; and for the strong acceptor nitrile groups, they act as donors. So the attraction is maximal when both are present [50].
Fig. 2. (a) Cryo-TEM of SDS surfactant on SWNTs. Reprinted with permission from [22], (b) Cryo-TEM micrograph and (c) schematics of CTAT surfactant with SWNTs. Reprinted with permission from [39].
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Fig. 3. Simulation of surfactant on SWNT showing semi-micelles and preferential attraction to crosses. Concentration in (a) and (b) 1.59 × 10− 5. (c) 1.37 × 10− 5, and (d) 2.02 × 10− 5. (a) The hydrophilic head groups of surfactant in green and the hydrophobic tails in purple; and (b–d) showing only the hydrophobic tails. Reprinted with permission from [46].
The coverage of surfactant on SWNT surfaces essentially depends on the strength of attractive interactions. Surface coverage could be obtained through thermogravimetric analysis (TGA) to quantify the surfactant adsorbed on CNTs [51–53], zeta potential measurement to assess the surface charge states [54–61], or neutron scattering to measure the surfactant in free micelles [23]. The saturation adsorption ratio of surfactant vs. SWNT is ca. 0.004 mol/g for a non-ionic surfactant [23], which is close to the reported values of other surfactants [14,53]. Although values in terms of the surfactant molecule number per nm2 of SWNTs were reported, the comparison was made in the unit of molar surfactant per gram of SWNTs since the exact accessible surface area of SWNT in suspension is not known. It is most desirable to have a single parameter to quantify the degree of SWNT dispersion, which however, does not exist. Many different methods have been used to measure SWNT dispersion. Direct microscopic imaging over many length scales is recommended by a NIST-NASA work group [62]. Another popular method is UV-Vis absorption spectroscopy for examining the purity and dispersion of SWNTs because only isolated defect-free tubes would yield sharp optical spectra [54, 62–67]. However, this method applies only to very low concentrations, at odds with the push toward dispersing SWNTs at high loadings. To make suspensions measurable, they have to be greatly diluted. But dilution could change both the surfactant aggregates, as discussed above, and SWNT network structures, as will be illustrated below. The state of dispersion in suspensions prior to dilution may be lost, thus precluding the technique from true quantification of the degree of dispersion of interested suspensions. Other qualitative methods for assessing the SWNT dispersion include viscosity [68], dynamic light scattering [69], zeta potential [61], TGA measurements [51–53], size exclusion chromatography [70,71], etc.
Analytical ultracentrifugation is emerging as a potentially powerful tool for both purification and dispersion of SWNTs [31,33,47, 52,61,68, 72–92]. DNA-wrapped [33] or covalently functionalized [92] SWNTs could be sorted by their diameters in aqueous density gradients through ultracentrifugation; sodium deoxycholate (DOC) dispersed SWNTs were separated by their length [89,90], or when competitive surfactants were used, semiconducting and metallic tubes could be separated [31]. Ultracentrifugation can also be used to semi-quantitatively characterize the SWNT dispersion in surfactant
Table 1 Interaction strength between various functional groups and SWNT paper [50]. Experiment force/molecule (pN) Alkyl-thiol endgroup –OH –periluoro –SH –CHfCH2 –CH3 –QOOH –NH2
9.6 ± 2 8.7 ± 3 9.2 ± 3 8.1 ± 2 7.6 ± 2 12.2 ± 3 23.4 ± 4
Aryl-thiol endgroup 4-methylbenzene 4-nitrobenzene 4-aminebenzene 4-bromobenzene 4-hydroxybenzene 4-fluorobenzene 4-methoxybenzene H-benzene 4-nitrilebenzene
18.9 ± 5.7 21.8 ± 5.3 22.6 ± 4.7 26.9 ± 3.6 32.0 ± 8.4 39.5 ± 8.8 41.5 ± 10.9 46.8 ± 11.8 56.9 ± 15.5
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Fig. 4. Ultracentrifugation characterization of SWNT dispersion in surfactant solutions. Apparent and average diffusion coefficients determined with the preparative ultracentrifuge method (squares) and dynamic light scattering (circles). The inset shows the experimental and theoretical sedimentation functions of the corresponding precentrifuged dispersions. Reprinted with permission from [79].
solutions through measuring the sedimentation function. Fig. 4 shows the measurement of apparent and average diffusion coefficients of SWNTs in aqueous NaDDBS suspensions as determined with the preparative ultracentrifuge method (squares) and dynamic light scattering (circles), respectively, as suspensions undergo various times of pre-centrifugation followed by 4 h ultrasonication. The inset shows the experimentally measured and theoretical fitting of sedimentation functions of various pre-centrifuged dispersions [79]. The discrepancy in diffusion coefficients between the two methods is mainly due to the polydispersity in the suspended SWNT bundles. As pre-centrifugation time increases, the polydispersity decreases, the apparent diffusion coefficients from ultracentrifugation approach the bulk-averaged diffusion coefficient from dynamic light scattering. Hence the sedimentation measurement of SWNT dispersion applies most meaningfully to dilute and relatively monodisperse bundles in suspensions. Scattering offers unique insights into SWNT dispersion. Particularly, SANS has been used to assess the ensemble-averaged dispersion qualities in relatively high concentration SWNT suspensions [11,23,70,76,93–99]. As quantitative analyses of the scattering function would require relatively uniform tube structures, scaling arguments could yield characteristic signatures of SWNT dispersion. Fig. 5 shows a typical SANS spectrum on surfactant/SWNT dispersion showing dominant Q− 1 scaling behavior for isolated rods [11]. The SWNT scattering is characterized by power-law behavior. Individual SWNTs
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Fig. 6. The power exponents for both the 0.1% and 0.01% SWNT suspensions. It is evident that the power exponent has a minimum around 0.5% to 1% surfactant concentration, indicating the optimal condition for dispersing SWNTs. Reprinted with permission from [23].
behave like rigid rods, displaying Q− 1 scattering at the intermediate Q-range, whereas power exponents of 2–3 at lower Q suggest branching in SWNT bundles and tube network structures [94,95]. The power-law exponent, α, at intermediate Q-range could be used for semi-quantitatively assessing the degree of dispersion; the closer to 1, the better the dispersion. Fig. 6 shows α as a function of the surfactant concentration for both the 0.1% and 0.01% SWNT suspensions. The data suggest that dispersion is not sensitive to the SWNT concentration, and a minimum a around 0.5% to 1% surfactant concentration corresponds to the optimal condition for dispersing SWNTs. The observation that the overall surfactant concentration, rather than the ratio between surfactant and SWNTs, controls the effective dispersion is consistent with a mechanism of competing maximization of surfactant adsorption onto SWNT surfaces and a depletion interaction between SWNT bundles mediated by surfactant micelles [23]. SANS is becoming a useful tool not only for assessing the degree of SWNT dispersion in the relatively concentrated suspensions of interest, but also for contributing to new insights into the structure and thermodynamics of SWNT suspensions and networks. However, neutron scattering is not easily accessible to the broader research community, limiting the extent of its applications to studying SWNT dispersion. A fundamental question about the assessment of SWNT dispersion is whether structure measurement is sufficient while none of SWNT suspensions appear to be thermodynamically stable. 4. Thermodynamics, kinetics and network structures
Fig. 5. SANS spectrum on SWNT/NaDDBS dispersion showing dominant Q− 1 scaling behavior for isolated rods. Reprinted with permission from [11].
The effort toward better SWNT dispersion has been driven, consciously or subconsciously, by the quest for a fully-exfoliated SWNT suspension as a thermodynamically stable phase, which possesses a global free energy minimum. Most often, a somewhat metastable state is obtained. Careful examination of the thermodynamic and kinetic aspects of surfactant-dispersed SWNT suspension systems is much needed. In this section, the thermodynamics driving forces and the role of metastability in structure and stability of SWNT suspensions are discussed. The free energy density of the suspension system is composed of enthalpy terms such as the large attractive van der Waals interactions among tubes, surfactant mediated hydrophilic–hydrophobic interactions between surfactant and SWNTs and among surfactant molecules, and entropy terms related to spatial packing of SWNTs and surfactants. An accurate thermodynamic description of the system could offer to predict its stability, metastability or instability. However, given the heterogeneous nature of most SWNT suspensions, a practical and
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meaningful construction of the free energy is difficult, if not impossible. We therefore qualitatively examine several distinct states, namely the isolated-tube suspension, loosely aggregated network, densely interconnected gels, and large aggregates/precipitates. Isolated-tube suspensions apparently pose as thermodynamically stable phase. Nevertheless they fall to one of two metastable classes, namely thermodynamic metastability. The other is kinetic metastability as in networks, which will be discussed below. In a thermodynamic metastable state, an energy barrier hinders the relaxation of the system to the equilibrium state. In the case of surfactantassisted isolated tube solution, the electrostatic (for ionic surfactants) or steric (for nonionic surfactants) repulsion prevents SWNTs from getting close to each other and reaching a lower energy state of segregated tube bundles and surfactant micelles. Ultrasonication pumps the suspension system to a metastable state through shortening SWNTs and allowing them to be covered by surfactant molecules when transiently isolated in the suspension. As complete monolayer coverage of surfactant on already separated SWNTs would prevent the sticking of SWNTs, adsorption at junctions of SWNT crossing could stabilize the junction [46], making dispersion to individual tubes more difficult. Since SWNTs are born not uniform, the latter situation could dominate the population, resulting in SWNT network formation in suspensions. A SWNT network is characterized by short-ranged frictions or attractions at the “cross-linking” sites, which defy Brownian motion and thermal fluctuations and maintain the separation of the center of mass of tubes in the long range, resulting in long-lasting dispersions. This is the case of kinetic metastability, applicable to a wide range of SWNT systems from loose flocculation to strong gel networks. High medium viscosity and/or long range structures as in ionic surfactant solutions could further strengthen the kinetic metastability, resulting in apparently better dispersion. There has been no direct measurement of the continuous transition from isolated rods to percolated networks, however, studies point to the sensitivity of network structure to suspension parameters as exemplified by SANS data shown in Fig. 7. With reducing surfactant concentration [Fig. 7(a)] [95] or increasing pH value [Fig. 7(b)] [98], the quality of SWNT dispersion deteriorates, as evidenced by the shrinkage of Q− 1 regime and the expansion/ appearance of higher power-law scattering. On the other hand, the evidence of network-stabilized dispersion is clearly shown in Fig. 7(c) for covalently functionalized SWNTs in deuterated toluene. Upon the addition of solvent to the suspension, the percolated network with well-dispersed SWNTs collapses to form aggregates [99]. The transition occurs at a SWNT concentration of ca. 0.04% by mass, which corresponds to the percolation threshold in well-dispersed systems [5]. Below the percolation threshold, SWNTs do not exist as isolated individual rods in suspensions, they instead aggregate. This observation could be general for different SWNT dispersions. Many applications ultimately require an optimally percolated network, the nature of the percolated state depends on the strength of interactions [100], as shown in Fig. 8(a)–(d). As the depth of the effective potential well between contacted nanotubes (ε) increases, the network evolves toward a more isotropic, disordered, and metastable configuration with a lower percolation threshold, a scenario that is relevant to the engineering of conductive nanotube composites, for example. A challenge that remains is how to use surfactants, block copolymers and chemical functionalization, perhaps in combination, to ‘tune’ the effective interaction from short-range repulsive to strongly attractive in the same material by simply changing an external parameter. This tactic, of course, has to be carried out after the dispersion of nanotubes and formations of networks. As noted earlier, entropic depletion forces may be well-suited for this. Monte Carlo simulations demonstrate depletion induced isotropic–isotropic phase separation in suspensions of rod-like colloids [101], where short range reversible attractive interactions induced by spherical free surfactant micelles give rise to liquid–liquid phase
Fig. 7. SANS studies of the effect of (a) surfactant concentration [95], (b) pH value [98], and (c) dilution [99] on dispersion and aggregation of SWNTs in suspensions.
separation into rod-rich domains, as shown in Fig. 8(e). This might offer a way to self-assemble and control percolated structures. Simulations also demonstrate that depletion forces can significantly lower the percolation threshold of rigid rod suspensions, as shown in Fig. 8(f) [102], despite the anisotropic nature of the rod-rod depletion interaction and a global free energy minimum that favors parallel rod alignment. This tendency is more pronounced at higher aspect ratios, on which delocalized interactions and thermodynamic functions strongly depend. As many previous advances were achieved through delicate molecular chemistries, i.e., through engineering local chemical and
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Fig. 8. (a–d) Continuum percolation of carbon nanotubes with varying “stickiness”. Reprinted with permission from [100], (e) depletion-induced isotropic–isotropic phase separation in SWNT suspensions. Reprinted with permission from [101]. (f) Depletion-induced percolation in SWNT networks. Reprinted with permission from [102].
physical forces, further development toward real applications of carbon nanotubes requires “systems level” engineering through thermodynamics and statistical mechanics, in which transient, non-equilibrium, and metastable states play a crucial role. By revealing the energy landscape and controlling kinetic pathways, useful SWNT dispersions may be developed. 5. Summary and outlook In summary, despite the tremendous effort in trying to disperse SWNTs in aqueous solution using amphiphilic surfactants, achievements in stable suspensions with high concentration individual SWNTs have been limited. Somewhat better understanding of the task has accumulated over the past few years from visualizing microscopic structures, measuring molecular interactions, quantifying the degree of dispersion, and generalizing thermodynamic and kinetic theories. It is conceivable that this trend will continue and a principled approach with a combination of the chemistry of molecular designs and microscopic interactions and the physics of thermodynamics and kinetics of SWNT/surfactant suspensions will emerge. In this endeavor, quantitative measurements play an essential role for better assessment of materials systems and critical validation of theories and models. Significant progresses in surfactant dispersion of SWNTs will only be made through synergistic advances in materials and processing innovation, quantitative measurements, and computational and theoretical development. Acknowledgements HW acknowledges the support of NSF under DMR-0711013, and discussions with Prof. Christopher Li (Drexel), Dr. Cheol Park (NIA), and Dr. Erik Hobbie (NIST).
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