J. Phys. Chem. C 2008, 112, 19193–19202
19193
Preparative Ultracentrifuge Method for Characterization of Carbon Nanotube Dispersions Tao Liu,* Sida Luo, Zhiwei Xiao, Chuck Zhang, and Ben Wang High-Performance Materials Institute, FAMU-FSU College of Engineering, Florida State UniVersity, 2525 Pottsdamer Street, Tallahassee, Florida 32310-6046 ReceiVed: May 28, 2008; ReVised Manuscript ReceiVed: September 21, 2008
Quantitative characterization of carbon nanotube (CNT) dispersions is critical for developing various CNTbased applications, e.g., transparent conducting thin films, CNT thin film transistors, CNT-reinforced composites, etc. Using both of the preparative ultracentrifuge and absorption spectrum measurements, this paper presents a simple, easy-to-use, and reproducible method to experimentally determine the sedimentation function of carbon nanotube dispersions. This information can then be used for the semiquantitative characterization of the dispersion of CNT in various liquid media. Theoretical analysis of the sedimentation function based upon the conventional ultracentrifugation model allows for the determination of the apparent sedimentation and diffusion coefficients for a specific SWNT dispersion. As demonstrated in the examples, this method, along with dynamic light scattering, was used for studying the processing-structure relationship of SWNT/H2O dispersions. In brief: (1) size reduction of SWNT bundles due to exfoliation (diameter reduction) and shortening/cutting (length reduction) occurs simultaneously during the sonication process; power-law dependence of the diameter and length reduction on the sonication duration were observed; (2) size reduction of SWNT bundles reflected by the sedimentation function strongly depends upon the amount of dispersion being processed; for the same sonication conditions, the greater the amount of the dispersion, the less efficient the exfoliation of the SWNT bundles is; (3) under the same sonication conditions, the sedimentation coefficient of sodium dodecylbenzenesulfonate (SDBS) assisted SWNT/H2O is significantly smaller than that of Triton X-100, which indicates the superior dispersing capability of the former surfactant. 1. Introduction Single-walled carbon nanotubes (SWNTs) exhibit extraordinary physical properties. SWNTs are strong, stiff, and tough,1 possessing high thermal and electrical conductivity.2–5 Depending upon the diameter and chiral angle of an individual SWNT, it can be either metallic or semiconductive with different-sized energy gaps.6,7 Semiconductive SWNTs can be electrically switched on and off as field-effect transistors (FETs) for electronic applications,8,9 whereas metallic SWNTs can carry an extremely large current density5,10 for novel transparentconducting material applications. The broad range of various physical properties of SWNTs suggests that if they could successfully be incorporated into existing bulk materials, such as polymers, they could usher in a new wave of highperformance SWNT-enabled multifunctional materials and systems. One approach for developing SWNT-based devices and functional materials is through SWNT dispersion. SWNTs are first dispersed in various media, e.g., water, organic solvents, and polymer matrix. Afterward, post processes are applied for device fabrication. A wide range of CNT devices prepared by this approach can be found in the literature, e.g., CNT film strain sensors,11 high mobility CNT thin film transistors,12 SWNT thin film field effect electron sources,13 and various CNT film-based transparent electronics.14 SWNTs are rarely found as isolated individual tubes. Instead, through van der Waals interaction, SWNTs assemble into ropes or bundles,15 which possess a 2-D hexagonal-ordered lattice structure. The ultimate goal to achieve high-quality SWNT dispersion is to exfoliate the SWNT bundles to individual tubes. * Corresponding author. Tel.: +1 850 410 6606. Fax: +1 850 410 6342. E-mail address:
[email protected].
Many different potential applications of SWNTs rely on the fulfillment of this goal. Individual semiconductive SWNTs are visualized as the next-generation building blocks for highperformance electronics. Unfortunately, mixed metallic and semiconductive tubes are typical in the as-grown SWNTs. To overcome this difficulty, various schemes have been explored to separate the metallic SWNTs from the semiconductive ones.16–18 No matter which technique(s) could eventually succeed in the electronic separation of SWNTs, one necessary condition for high-efficiency separation is to have an exfoliated SWNT dispersion due to the coexistence of mixed types of SWNTs in one SWNT bundle.19 The importance of the exfoliation of SWNTs in developing high-performance SWNT-reinforced nanocomposites is well recognized.20,21 In carbon nanotube research, development of a repeatable process or method for producing high-quality exfoliated SWNT dispersions is important. However, it is equally important to possess quantitative characterization methods to determine the various properties of SWNT dispersions, e.g., degree of exfoliation, states of aggregation, dynamic properties, geometric dimension, number density of the dispersed particles, and the contents of the metallic versus semiconductive SWNTs. Such information is critical for quality control and optimization of a particular process for SWNT dispersions, as well as for screening various types of dispersing agents. Currently, the preferred methods in different laboratories to disperse and exfoliate SWNTs, i.e., ultrasonication assisted by surfactants,22,23 copolymers,24 polyelectrolytes,25 DNA molecules,26 and organic solvents,27,28 typically result in dispersion with mixed SWNT speciessSWNT bundles and individual tubes with varied diameter, length, and electronic types and entangled and aggregated structures of these long aspect ratio objects. For these reasons, having a simple and
10.1021/jp804720j CCC: $40.75 2008 American Chemical Society Published on Web 11/13/2008
19194 J. Phys. Chem. C, Vol. 112, No. 49, 2008 repeatable characterization technique that offers the ensembleaveraged quantitative information as previously mentioned for bulk SWNT dispersions is essential. Despite significant progress made toward quantitative understanding of SWNT dispersion based on various spectroscopic,29–33 scattering,34–37 and microscopic imaging techniques,38–40 a simple and easy-to-apply method for the quantitative characterization of bulk SWNT dispersions is still lacking.41 On the basis of specific viscosity measurements, Agarwal and co-workers24,41 recently developed a simple method that may serve as a valuable tool for achieving the quality control of carbon nanotube dispersions. This technique allows for qualitatively distinguishing the dispersion effects of high-power horn sonication from the mild bath sonication. In addition, by tracking the specific viscosity changes of carbon nanotube dispersion, an optimal sonication time can be identified.24 Analytical ultracentrifugation is a powerful and well-known technique in the areas of biochemistry, molecular biology, and macromolecular science for characterizing sedimentation, diffusion behaviors, and the molecular weights of both synthetic and natural macromolecules.42–47 The preparative ultracentrifuge also found applications on the characterization of proteins48,49 and macromolecules.50 Ultracentrifugation has been widely used in SWNT dispersion preparation in terms of eliminating aggregates and large bundles,11–14,16,17 in addition to separating metallic SWNTs from semiconductive ones.18 However, its usage for the characterization of SWNTs is limited to density measurements.51 This paper presents a method based on the use of a preparative ultracentrifuge for the semiquantitative characterization of carbon nanotube dispersions. This method uses simple, inexpensive, and easily operated instruments, i.e., a preparative ultracentrifuge and a UV-vis-NIR spectrometer, to experimentally determine the sedimentation function of a SWNT dispersion. The sedimentation function is uniquely determined by the distributed sedimentation and diffusion coefficients and, therefore, the distributed lengths and diameters of SWNT particles in a specific dispersion. Without further theoretical analysis, the experimentally determined sedimentation function can simply be used as a practical indicator for the quality comparison of the SWNT dispersions prepared under different processing conditions. Moreover, using the conventional analytical model of ultracentrifugation,42,48,54,55 we performed numerical fittings to the experimentally determined sedimentation functions to quantify the apparent sedimentation and diffusion coefficients of the dispersed SWNT nanoparticles prepared under different conditions. This information, along with the dynamic light scattering measurement, can be used to infer the average length and diameter of SWNTs in a bulk dispersion. Ongoing research is in progress to achieve the distributed lengths and diameters for a SWNT bulk dispersion. We expect that the preparative ultracentrifuge method presented in this paper will establish an easy-to-applyplatformforsystematicallystudyingtheprocessingstructure-property-performance relationship of SWNT dispersion-based functional materials. 2. Experimental Section Purified high-pressure catalytic decomposition of carbon monoxide (HiPCO) SWNTs were purchased from Carbon Nanotechnologies Inc. (batch # 91). Sodium dodecylbenzenesulfonate (SDBS, CAS number - 25155-30-10) and Triton X-100 (CAS number - 9002-93-1) were supplied by Sigma-Aldrich and used as received. By varying the sonication duration time (30 min, 2 h, 4 h, and 12 h), four different SWNT dispersions
Liu et al. were prepared by sonicating 16 mg of SWNTs in 100 mL of 0.7 wt % SDBS/deionized H2O solution by a horn sonicator (Misonix sonicator 3000, Frequency 20 kHz) in an ice bath. The sonication was operated at a pulse operation mode (on 10 s, off 30 s) with the power level set at 45 W for all of the dispersions investigated in this study. A similar procedure was used for preparing the 100 mL SWNT/Triton X-100/H2O and 200 mL SWNT/SDBS/ H2O dispersions. Additionally, a few replicated SWNT/SDBS/H2O dispersions that were sonicated for 30 min and 4 h were prepared to investigate the process variations. As one critical step for determining the sedimentation function of a given SWNT dispersion, each of the above prepared master batch dispersions (concentration of C0) and the corresponding series of diluted samples with concentrations of C0/2, C0/4, C0/16, C0/32, and C0/64 were subject to ultracentrifugation for varied periods of time (0.5, 1, 2, 5, 10, 30, and 60 min). The Eppendorf MiniSpin Plus Centrifuge was used at a rotation speed of 13 000 rpm with a nominal g-force value of 11 250g. The MiniSpin Plus is equipped with a fixed angle rotor (45°, Part# - F-45-12-11), and the maximum radial position is Rmax ) 6 cm. Standard 1.5 mL two-section (cylindrical and conical) centrifuge tubes were used for loading the dispersions in the centrifugation process. At the end of each specified centrifugation period, a fixed amount of supernatant (1.4 mL) was carefully extracted with a precision pipet and immediately subjected to absorption spectrum measurements with a Varian Cary 5000 UV-vis-NIR spectrometer. The absorbance of a supernatant obtained by the centrifugation of a diluted or undiluted sample with an initial concentration of Cd (Cd ) C0, C0/2, C0/4, C0/16, C0/32, or C0/ 64) for time t was marked as Ap (Cd, t). The subscript p denotes that the absorbance of the supernatant was closely associated with the processing conditions that were used for preparing the master batch dispersions, e.g., varying sonication times and changing the dispersing agents. Unless otherwise specified, the values of Ap (Cd, t) at 633 nm were used for various SWNT dispersions for determining the sedimentation function of a specific SWNT dispersion. In brief, the experimental method for determining the sedimentation function of a specific SWNT dispersion is summarized as follows: (1) dilution of a given CNT dispersion to a series of different concentrations; (2) ultracentrifugation of the original dispersion and the diluted samples for varied periods of times; (3) sampling a fixed amount of supernatants from each of the dispersions obtained in step (2); (4) UV-vis spectrum measurements of the supernatants obtained from step (3). On the basis of the UV-vis spectra of the supernatants and their corresponding dilution and ultracentrifugation information, the sedimentation function (eqs 1, 2, 3, and 10) was determined; details are discussed in section 3.1. Dynamic light scattering (DLS) measurements (Delsa Nano C Particle Size Instrumentation, Beckman Coulter, Inc.) were used to examine the diffusion coefficients of SWNT particles for selected SWNT/SDBS/H2O dispersions. The scattered light intensity autocorrelation function was measured at a 165° backscattering geometry with an incident laser of 658 nm in wavelength and was analyzed using the CONTIN method. The possible sources of errors on the determination of the diffusion coefficients of SWNT dispersions using the DLS method, which are induced by convection, thermal gradients, and the thermal lensing effect due to light absorption,56 were not taken into consideration.
Ultracentrifuge Method for CNT Characterization
Figure 1. UV-vis-NIR spectra for the 12 h sonicated SWNT/SDBS/ H2O dispersions. The group of Cd ) C0 represents the as-prepared and corresponding supernatants centrifuged for various times. Others represent the series-diluted dispersions and corresponding centrifuged supernatants. The inset plot shows the absorbance vs concentration of the series dispersions without centrifugation.
To verify the structural information of SWNTs determined by the preparative ultracentrifuge method, the tapping mode AFM (Veeco Instruments, Inc. Multimode) was used to acquire images of SWNTs on a silicon wafer substrate under ambient conditions. The sample preparation for AFM imaging follows a similar procedure given by Islam, et al.22 with some modifications. In brief, the selected SWNT dispersions were diluted to various concentrations and then subjected to spin casting at 2000 rpm on a silicon wafer (size
