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Spectroscopic and Scanning Probe Studies of a Nondestructive Purification Method for SWNT Suspensions Jihye Shim,† Pornnipa Vichchulada, Qinghui Zhang, and Marcus D. Lay* Department of Chemistry and Nanoscale Science and Engineering Center (NanoSEC), UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: September 8, 2009; ReVised Manuscript ReceiVed: NoVember 16, 2009
A mild method for the bulk enrichment of high-aspect-ratio single-walled carbon nanotubes (SWNTs), while avoiding the damaging effects of acid purification methods, has been developed in this group. Various analytical techniques (atomic force microscopy, Raman microscopy, near IR, and UV-vis spectroscopy) have been used in order to attain a better understanding of the correlation between low-G centrifugation cycles and the quality of the SWNT suspension. Atomic force microscopy data of low-density SWNT networks formed from these suspensions indicate that the size and occurrence of globular impurities is lessened, while small bundles of high-aspect-ratio SWNTs remain. Raman spectroscopy is used to verify that this method produces suspensions of purified SWNTs without significantly damaging them. Near-infrared and UV-vis spectroscopy of these suspensions show prominent semiconducting and metallic interband transitions in the SWNTs and removal of impurities. Introduction Because of their unique physical, chemical, and mechanical properties,1,2 single-walled carbon nanotubes (SWNTs) have applications in a wide variety of scientific disciplines, including field emission devices,3 electronic devices,4 chemical sensors,5 batteries,6 hydrogen storage,7 and polymeric composites.7,8 However, most commercial synthesis methods for SWNT growth produce a large percentage (up to 60%) of carbonaceous impurities, such as amorphous carbons, fullerenes, nanocrystalline graphite, and transition-metal nanoparticles that are used as a catalyst during the synthesis.9 These impurities have a deleterious effect on the performance of various device structures, such as resistors, transistors, and sensors.10 Therefore, there is a great need for methods for purification of SWNT soot that do not damage the enhanced electrical properties of the SWNTs. Reported methods for purification, such as gas- or liquidphase oxidation, significantly damage pristine as-produced (AP) SWNTs. For example, gas-phase oxidation, used for the removal of carbonaceous impurities, usually occurs in an atmosphere containing a mixture of some of the following gases at elevated temperatures: air, oxygen, ozone, hydrogen chloride, chlorine, CCl4, or CO2.11,12 While any single one of these gases was not effective at purifying SWNT soot, of the dual-gas mixtures used, HCl(g) and H2O(g) were most effective at etching carbonaceous impurities. However, because the SWNTs are also composed entirely of carbon, these gases also etched the SWNTs, leaving either hydroxyl or chloride functional groups along the ends and sidewalls. Further, after purification, it is difficult to remove some of these gases from the sidewalls of the SWNTs. Therefore, in addition to creating defects in the SWNTs, the incomplete removal of these gases presents another problem, as electron-donating gases reduce the conductivity of SWNTs, which act as p-type semiconductors.13,14 Additionally, gas-phase * To whom correspondence should be addressed. E-mail: mlay@ chem.uga.edu. † Current address: Department of Applied Chemistry, Kyung Hee University, Gyeonggi-do, 446-701, Korea.
methods have not proven effective for bulk purification of SWNT soot. Liquid-phase oxidation processes, in strong acids, such as HNO3, HCl, HClO4, and H2SO4, are an alternative commonly used for bulk oxidative purification of SWNT soot. Although effective at reduction in carbonaceous impurities and metal nanocatalyst particles, acid treatments also attack the sp2 hybridized structure of the SWNTs, thus changing their intrinsic electronic transport properties.15,16 Therefore, the development of nondamaging methods for the purification of SWNTs is of great interest. The most common mild purification methods involve, first, creating an aqueous suspension of SWNTs. A surfactant is used to increase the solubility of the SWNTs without the need for chemical modification. Next, SWNTs are separated from impurities via filtration or centrifugation.17,18 Filtration methods work well for the production of threedimensional networks of transparent SWNTs. This material exhibits near metallic conduction, so it has applications similar to indium tin oxide (ITO). For example, when a dilute suspension of single-walled carbon nanotubes (SWNTs) in surfactant was vacuum-filtered onto a filtration membrane, followed by dissolving the membrane in solvent, the result was an SWNT film with a thickness of 50-150 nm.19 Although these films were composed of SWNTs that were bundled together, these films exhibited a high transmittance in the visible and nearIR. While this method was effective for formation of SWNT thin films with metallic conduction, the advantage of the purification and deposition method presented herein is that lowdensity, two-dimensional networks behave as transparent semiconductive materials.20,21 Recently, a combination of high-speed centrifugation and membrane filtration has been used for purification of SWNTs pretreated by oxidation with HNO3 or K2S2O8.22,23 The oxidation process was used to chemically modify the SWNTs in order to increase their solubility in H2O. Although bulk separation of carbonaceous impurities from SWNTs was accomplished, chemical modification of the SWNTs was crucial to the success of this work.
10.1021/jp9086738 2010 American Chemical Society Published on Web 12/14/2009
Nondestructive Purification Method for SWNT Suspensions An effective purification method that does not involve chemical modification of SWNTs is ultracentrifugation, which uses centrifugal forces of at least 100 000 G. This is an effective way of removing carbonaceous and metallic impurities from aqueous suspensions of SWNTs, as impurities usually have higher densities than short, unbundled SWNTs.24 However, in this technique, long unbundled SWNTs, which are useful for numerous electronic applications, are also removed due to their higher mass; the average length of SWNTs observed was only 200 nm. Therefore, it is important to develop an effective method for purification that removes the various impurities but leaves unbundled, high-aspect-ratio SWNTs in suspension. This manuscript reports the use of several analytical methods to study the properties of SWNTs that were purified by multiple cycles of centrifugation at low centrifugal force. This work shows that this purification method is an effective way to obtain bulk samples of purified SWNTs without removal of unbundled, high-aspect-ratio SWNTs; the average length observed by AFM exceeds 1 µm. Raman microscopy and near-IR transmission studies of SWNT suspensions confirmed that impurities were removed by this method. SWNTs deposited onto Si/SiOx substrates via laminar flow deposition (LDF) demonstrated that the suspensions were enriched in individual, high-aspect-ratio SWNTs.24,25 This deposition method prevents the formation of SWNT bundles during the deposition process. This provides a distinct advantage over other liquid deposition methods, which result in formation of large bundles of SWNTs,22 and allows determination of the effectiveness of the purification method presented herein. Experimental Methods Preparation and Purification of SWNT Solutions. Asproduced AP-grade CarboLex Inc. arc-discharge soot, with a guaranteed purity of 50-70 wt % SWNTs, was used for all experiments. Solutions of 1 mg/mL of SWNT soot were prepared in 1% (w/v) sodium dodecyl sulfate (SDS) by dispersion of SWNTs via probe sonication (model 500, Fisher) at 12 W for 30 min, followed by multiple centrifugation (Beckman Microfuge) cycles of 30 min each at 18 000 G. Each centrifugation cycle was followed by carefully collecting the upper half of the supernatant. Spectroscopic and scanning probe data were obtained after each centrifugation cycle in order to quantitate the effectiveness of this method for removal of impurities and enrichment in high-aspect-ratio SWNTs. Formation of SWNT Deposits through Laminar Flow Deposition. Si/SiO2 substrates were cut into fragments and then cleaned with compressed CO2. The substrates were then functionalized with a self-assembled monolayer to produce an amine-terminated surface by immersion in a fresh solution of 10 mM 3-aminopropyl triethoxysilane in 99.5% ethanol for 45 min. Ten deposition cycles were then used to form a low-density SWNT network. The deposition cycle has been described elsewhere.26 Briefly, each cycle is defined as depositing the SWNT solution onto the silane-coated Si/SiO2 substrate by drying in a stream of N2. The substrate was then rinsed with nanopure-H2O and dried under a stream of N2. Characterization of Purified SWNT Suspensions by AFM and Absorption Spectroscopy. AFM images were obtained in air using intermittent contact mode (Molecular Imaging PicoPlus). To determine the extent of purification, image analysis software (SPIP) was used to create a histogram of the height measurements for each pixel. This is an effective analysis method because the various impurities were consistently observed to have higher height than the average SWNT (∼1.2
J. Phys. Chem. C, Vol. 114, No. 1, 2010 653 nm). For each purification cycle, transmission near-infrared (NIR) spectroscopy (Thermo 6700 Nicolet FT-IR spectroscope) was performed on aqueous suspensions using a quartz cell with a path length of 1 cm. Ultraviolet-visible (UV-vis) absorption spectroscopy was performed on a Varian Cary 300 spectrophotometer in a similar quartz cell. Aqueous suspensions, as well as SWNT networks formed on Si wafer fragments (Si/SiOx), were characterized with Raman spectroscopy. Raman spectra were recorded on a Renishaw InVia Raman microscope, with a charge-coupled device (CCD) detector, using a diode laser with an excitation wavelength of 785 nm and 7 mW laser power. For aqueous suspensions, a capillary tube (1.5 × 90 mm) was used as a sample container. For Raman analysis of twodimensional networks, SWNTs deposited on Si/SiOx were examined without further modification. Analysis of the heights of bands in the Raman spectra was performed with WiRe 2.0 software. Results and Discussion Atomic Force Microscopy Studies. Although liquid deposition processes have the major advantage of decoupling the SWNT growth process (which occurs at ∼800 °C) from the deposition process, a major concern with liquid deposition processes is bundle formation during the deposition process. Laminar flow deposition is an effective way to prevent SWNT bundle formation during deposition of SWNT networks.25 Therefore, AFM analysis of deposits formed with LFD allows quantitation of the change in the density and size of impurities as a function of purification cycles, when the number of deposition cycles is held constant. The results indicate that an increase in centrifugation cycles decreases the size and density of impurities. Figure 1 shows the effect of centrifugation cycles at low G on the quality of SWNT networks deposited onto Si/SiOx substrates. Figure 1a shows a deposit formed from an unpurified SWNT suspension. An average height of 150 nm was observed, and the surface is largely covered by globular impurities. This type of deposit is typical of untreated suspensions. However, a dramatic improvement in the quality of deposit was observed after just one centrifugation cycle at low G (Figure 1b). The average height decreased to 55 nm due to removal of a significant portion of the larger impurities. SWNTs are clearly visible in this deposit. As the number of centrifugation cycles increases, the average height observed continues to decrease. After two centrifugation cycles (Figure 1c), a histogram of the heights in the z direction showed an average height of 12.5 nm, with a significant contribution from heights around 10 nm. Additionally, significant contributions from heights of less than 2 nm are present. There is a dramatic shift in the average height of the histograms between two and three centrifugation cycles; despite the fact that there is evidence of a bimodal distribution, for the first time, the average height is below 5 nm (Figure 1d). Four centrifugation cycles result in a small shift of the average of the height histogram, but with a larger representation from heights below 5 nm, as indicated by the asymmetric Gaussian curve observed in Figure 1e. After five centrifugation cycles, the average height approaches the range within one may expect to find SWNTs (∼2 nm), with only a small contribution from slightly larger heights (Figure 1f). As the average height range for arc-discharge SWNTs is 1.2-1.6 nm, this would indicate that, after five centrifugation cycles were performed on the deposition solution, the deposit is composed largely of individual SWNTs, with the presence
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Figure 1. AFM micrographs (8 × 8 µm) for deposits formed from solutions at various stages of purification. (a) AFM image and histogram of a deposit formed from an unpurified SWNT suspension with a concentration of 1 mg/mL of SWNT soot. The average height observed on this deposit is 150 nm. (b) The average height decreases to 55 nm for a deposit formed from a suspension that underwent one centrifugation cycle. (c) After two purification cycles, the average height observed is 12.5 nm. The shoulder on the left side of the peak in the histogram is indicative of the presence of a bimodal distribution, with heights of significantly less than 10 nm strongly represented. (d) After three centrifugation cycles, the average height is less than 5 nm, with a significant presence of particles up to 20 nm. (e) After four centrifugation cycles, the average height is 5 nm, with the large contribution of the height less than 5 nm. (f) Five low-G centrifugation cycles result in an average height of 2.0 nm, indicating the continued removal of various impurities and bundles, without the use of oxidizing treatments. Additionally, the deposit demonstrates enrichment in high-aspect-ratio SWNTs.
of very small diameter globular impurities. This indicates that multiple centrifugation cycles are an efficient, nonoxidizing way to remove impurities as well as large bundles of SWNTs. Raman Spectroscopy of SWNT Deposits and Suspensions. Raman Spectroscopy of SWNT Suspensions. Raman scattering has been used for many years as a probe of disorder in the carbon skeleton of sp2 and sp3 carbon materials.27 There are two Raman bands that are typically observed for sp2 carbon;28 the “G band,” which is called a “tangential mode,” is seen in the range of 1580-1600 cm-1. Its presence indicates a pristine, symmetrical graphene lattice.29 The second band observed is the “D band,” which is called a “dispersive” band. It is a relatively broad, disorder-induced band in the range of 1300-1370 cm-1.28,29 The D band, which is indicative of the presence of disordered sp2 hybridized carbon atoms, is caused by C-atom vacancies, impurities in the C lattice, or any other imperfections in the graphene lattice.30 Figure 2 shows the Raman spectra of SWNT suspensions treated with zero (0C, i.e. without centrifugation) to five low-G centrifugation cycles (5C). The G and D bands are observed at 1591-1595 and 1290-1310 cm-1, respectively. The magnitude of the IG/ID is an indicator of the purity of SWNTs.31 There is a consistent increase in the IG/ID ratio, as the G band was
Figure 2. Raman spectra showing the D band (1290-1310 cm-1) and the G band (1591-1595 cm-1) of the SWNT suspensions before centrifugation (0C) and after centrifugation from one to five cycles. (1C-5C). As the number of centrifugation cycles increased, IG/ID ratios increased.
observed to increase with each low-G centrifugation cycle (Table 1). Indeed, the IG/ID ratio increased from 4.34 to 17.05 over the course of five centrifugation cycles. This increase indicates enrichment in well-ordered sp2 bonds (such as those observed in defect-free SWNTs) and a corresponding reduction in defectcontaining carbonaceous impurities.32
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TABLE 1: IG/ID Ratios of the SWNTs in Raman Spectroscopy SWNT suspensions SWNT deposits
OC
1C
2C
3C
4C
5C
4.34 4.92
10.11 10.23
11.29 12.46
12.93 13.97
14.47 14.54
17.05 17.21
Raman Spectroscopy of 2-D SWNT Networks Deposited on Si/SiOx. Confocal Raman microscopy of 2-D SWNT networks deposited on Si wafer fragments also evidenced a consistent reduction in carbonaceous impurities with increasing low-G centrifugation cycles (Figure 3). This trend indicates that the quality of the deposition solution has a direct correlation on the quality of thin film formed using laminar flow deposition (Table 1); the IG/ID ratio increased from 4.92 to 17.21. Like the SWNT suspensions, the deposits show the highest IG/ID ratio after five centrifugation cycles. Therefore, Raman spectroscopy data concur with AFM data with regard to the effectiveness of low-G centrifugation for purification of SWNT soot. Near-Infrared Spectroscopy of SWNT Suspensions. The effectiveness of low-G centrifugation cycles for bulk purification of SWNT soot was also evaluated by near-infrared (NIR) spectroscopy in transmission mode. NIR spectroscopy has been reported as an important tool for characterizing the electronic band structure of SWNTs.33 When unbundled SWNTs are present in aqueous suspensions, a series of characteristic interband electronic transitions are observed; semiconducting SWNTs have first and second transitions starting with S11 ) 2Rβ/d and S22 ) 4Rβ/d, whereas the first transitions of the metallic SWNTs appear at M11 ) 6Rβ/d, where R is the C-C bond length (0.1424 nm); β ) ∼2.9 eV, the transfer integral between π orbitals (an interaction energy between neighboring C atoms); and d is the SWNT’s diameter (nm).33,34 These interband transitions produce prominent features in the NIR spectral range, provided the electronic transitions are not quenched by inter-SWNT interactions that occur in bundles, so they can be a basis for the evaluation of the enrichment of the suspension in unbundled SWNTs. Figure 4 shows NIR spectra of SWNT suspensions at varying degrees of processing. The S11, S22, and M11 transitions were observed at 1150, 980, and 750 nm, respectively. All of these absorption bands are suppressed in the untreated SWNT suspension. However, the intensities of these bands increase with increasing low-G centrifugation cycles.
Figure 3. Raman spectra, showing the D band (1290-1310 cm-1) and G band (1591-1595 cm-1) for SWNT deposits formed from suspensions treated with no centrifugation (0C) up to five (1C-5C). As the number of centrifugation cycles increased, IG/ID ratios increased.
Figure 4. NIR spectra showing metallic (M11), second semiconducting (S22), and first semiconducting (S11) transitions at 750-800, 980, and 1150 nm, respectively, of the SWNT suspensions before and after centrifugation.
TABLE 2: The Ratios, A(S)/A(T), of the SWNT Suspensions in NIR Spectroscopy sample
OC
1C
2C
3C
4C
A(S)/A(T) × 100% 96.52% 97.22% 97.43% 99.10% 99.96%
Studies by Haddon et al., have shown that, for acid-treated SWNT soot, the second semiconducting interband transition, S22, can be used for purity evaluation because it is less affected by doping during chemical processing.9,34 This method was used in this study for the S22 transition, which was used to quantify the extent of enrichment in unbundled SWNTs via the ratio A(S)/ A(T), where A(S) is the area of the S22 absorption feature after baseline subtraction and A(T) is the total area under the spectral curve. In Table 2, it is shown that multiple centrifugation cycles increase the purity ratio. The one-cycle centrifugation had a purity of 96.52%. The purity of the SWNT suspensions after two and three cycles of centrifugation increased to 97.22% and 97.43%, respectively. Four cycles showed a higher increase in purity compared with those that were subjected to fewer cycles. In five centrifugation cycles, the purity was remarkably high at 99.96%. UV-vis Absorption Spectroscopy of SWNT Suspensions. The absorbance of SWNT suspensions, depending on centrifugation cycles, was monitored by UV-vis absorption spectroscopy. The spectra showed that more centrifugation cycles decreased the absorbance of SWNTs. Current synthetic methods produce the mixtures of metallic and semiconducting SWNTs.33 In the UV-vis range, metallic SWNT showed first transition, M11, in 700-800 nm and the semiconducting transitions were shown in the NIR range.33,34 In Figure 5, the change of absorbance at 750 nm was plotted in the SWNT from zero (i.e., without centrifugation) to five centrifugation cycles. Figure 5 shows an exponential decay and leveling off with the decrease of the absorbances from 2.576 to 0.175, as centrifugation cycles increased. The uncentrifuged SWNT suspension had the highest absorbance due to bundles and other impurities. This indicates that most of the larger contaminants are removed in the first centrifugation cycle. This corroborates the observations made with AFM, where much of the larger amorphous C contaminants are removed after the first centrifugation cycles. Subsequent centrifugation cycles then remove bundles of SWNTs, as well as smaller globular contaminants. Conclusions Multiple cycles of centrifugation at low G are an efficient and nonoxidizing method for the purification of SWNT soot.
Nondestructive Purification Method for SWNT Suspensions
Figure 5. Plot of absorbance at 750 nm vs number of centrifugation cycles for an AP-grade SWNT soot suspension with a starting concentration of 1 mg/mL of SWNT soot. The absorbances decrease from 2.576 to 0.175 as the number of centrifugation cycles increases.
Experimental results showed that most of the heavier/larger contaminants are removed in the first centrifugation cycle, whereas subsequent cycles remove bundles of SWNTs and smaller globular contaminants. This results in suspensions enriched in long unbundled/undamaged SWNTs. The largest bundles of SWNTs observed by AFM after five centrifugation cycles were likely composed of about two SWNTs, as the total height was 2.6 nm. NIR study showed more prominent metallic and semiconducting transition interbands with a higher purity ratio, A(S)/A(T), according to the increasing number of centrifugation cycles. Also, both deposits and suspension-phase Raman spectroscopy provided higher ratios of IG/ID as the number of centrifugation cycles increased, indicating a reduced presence of defect sites due to C vacancies. Therefore, this mild purification technique can be applied for efficient and nondestructive purification of SWNT soot. Acknowledgment. The authors gratefully acknowledge financial support from the National Science Foundation through NSF Grant No. DMR-0906564. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Snow, E. S.; Perkins, F. K.; Houser, E. J.; Badescu, S. C.; Reinecke, T. L. Science 2005, 307, 1942. (3) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512. (4) Collins, P. G.; Zettl, A.; Bando, H.; Thess, A.; Smalley, R. E. Science 1997, 278, 100. (5) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894.
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