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J. Phys. Chem. B 2006, 110, 22318-22322
Purification and Opening of Carbon Nanotubes Using Steam Gerard Tobias,* Lidong Shao, Christoph G. Salzmann, Yoon Huh,† and Malcolm L. H. Green Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QR, U.K. ReceiVed: May 24, 2006; In Final Form: August 10, 2006
Purification and opening of carbon nanotubes has been carried out by treatment of as-made single-wall carbon nanotubes (SWNTs) with pure steam at 1 atm pressure. Treated samples have been characterized by highresolution transmission electron microscopy and IR and Raman spectroscopy. Comparison between the steam purification and the standard nitric acid purification treatment shows that steam is less aggressive toward damage to the tubular nanotube wall structure and forms fewer functional groups.
Introduction The synthesis of single-wall carbon nanotubes (SWNTs) by both arc and chemical vapor deposition (CVD) methods provides samples which contain impurities such as catalyst particles and amorphous and related graphitic particles. Many attempts have been made to remove these impurities, for example, by using oxidation by air and hydrochloric acid treatment,1 nitric acid,2 or microfiltration methods3 among others. The nitric acid purification leads to partial reaction of SWNTs and consequential damage of the tubular structure.4,5 Cutting SWNTs using a mixture of sulfuric and nitric acids has also been used to obtain opened tubes.6 Gas adsorption studies have suggested that the functional groups created during this treatment can block the opened parts of the SWNTs.7,8 These functionalities can be removed by thermal annealing. Also, it has been shown that SWNTs reseal at high temperatures (800 °C); opened SWNTs can become closed.9,10 Clearly vacuum annealing of SWNTs should be avoided if opened tubes are required. In light of the above we have sought more selective methods for the opening and purification of carbon nanotubes. It is well-established that during the CVD synthesis of carbon nanotubes water enhances the activity and lifetime of the catalyst11 and reduces the formation of amorphous carbon materials. The amount of water present has to be low (ppm) during the synthesis of either SWNTs11 or multiwall carbon nanotubes (MWNTs)12 since larger amounts (∼17%) can result in the formation of highly defective nanotubes.13 There are a few experimental studies on the use of water (steam) to purify postsynthesized carbon nanotubes. Hauge et al. have used two different gas mixtures containing water: Cl2H2O-HCl14 and wet oxygen.15 While the gas mixture of Cl2H2O-HCl was found to remove unwanted carbon species, water alone did not have purification effects under the reported conditions.14 Dillon et al. reported the opening of SWNTs and removal of amorphous carbon using steam as inferred from hydrogen adsorption experiments.16 The degree to which SWNTs could be purified by steam was limited since treatment at 700 °C in 1.3 × 10-3 atm water pressure already resulted in the consumption of SWNTs.16 * To whom correspondence should be addressed. Telephone: +44 (0)1865 272641. Fax: +44 (0)1865 272690. E-mail:
[email protected]. ac.uk. † Present address: Division of Advanced Technology, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea.
Here we present further evidence of the use of steam for the purification and opening of single-walled and multiwalled carbon nanotubes, at 1 atm water pressure. Experimental Section Carbon nanotube samples were supplied by Thomas Swan & Co. Ltd. in a “wet cake” form (ca. 95% water by weight). The samples also contained amorphous carbon, graphitic particles (carbonaceous crystalline materials having few graphitic layers), and some metal particles. The samples were dried at 60 °C before use. Multiwalled carbon nanotube samples were provided by Thomas Swan as a dry powder. Distilled water was first flushed with argon to remove dissolved oxygen. To measure the gas evolution during the reaction of different forms of carbon with steam, 400 mg of the desired carbon compound (SWNTs, MWNTs, or graphite) was placed into a silica tube (25 mm diameter) and sonicated with 10 mL of water for 15 min. This dispersion was then frozen in liquid nitrogen and attached to a vacuum line through a condenser. After pumping and filling the whole system three times with argon, the sample was kept under argon and allowed to warm to room temperature. The furnace was then heated to the desired temperature, and the volume of gas evolved was measured. Alternatively, SWNTs or MWNTs were placed in a silica tube (9 mm diameter) and immobilized with silica wool at both sides. Steam was introduced by bubbling argon (190 mL/min) through hot water (98 °C). Both arrangements produced similar results. To monitor the opening of the SWNTs and MWNTs, typically 10 mg of steam-purified carbon nanotubes was selected and stirred at 70 °C for 2 days in a saturated solution of uranyl acetate {UO2(MeCO2)2(H2O)2} (BDH Chemicals). Uranyl acetate was chosen since the heavy element component uranium can be easily observed by transmission electron microscopy (TEM). The mixture was then filtered using a 0.2 µm polycarbonate membrane and dried overnight. The filtered samples were dried but not washed after the filtration. High-resolution transmission electron microscopy (HRTEM) images were taken in a JEOL JEM-4000EX highresolution electron microscope (point resolution, 0.16 nm). Samples for HRTEM observation were ground and dispersed in CHCl3 and placed dropwise onto a holey carbon support grid. FT-IR spectra were recorded in transmission mode on a Nicolet MAGNA-IR 560 spectrometer at 4 cm-1 resolution by
10.1021/jp0631883 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006
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Figure 1. Gas evolution during the annealing of (a) 400 mg of asmade SWNTs with 10 mL of water and (b) 10 mL of water. Heating was at 5 °C/min to 1075 °C (210 min), and the samples were kept at this temperature for 3 h. The dashed line correlates time and temperature (right axis). The vertical line separates the dynamic and isothermic annealing regimes.
co-adding 128 scans (MCT/B detector; CsI beam splitter; HappGenzel apodization). Thin films of the carbon nanotube samples were prepared on ZnSe disks (2 mm thickness) by dropping 2-propanol dispersions (∼10 mg/mL) onto the preheated substrates (∼70 °C).17 Film thicknesses were increased until the absorbance of the background was ∼1. Spectra were then scaled so that the lowest point of the background equals 1. Background corrections were performed by subtracting straight lines in the displayed spectral ranges. Raman spectra were recorded on a Jobin Yvon Labram spectrometer equipped with a microscope, through a 50-fold magnification objective (Olympus Co.), by adding four sets of spectra together. A 40 mW argon-ion laser (514 nm) was used with a reduced output power of 4 mW. The 1800 L/mm grating provides a resolution starting from 1.1 cm-1 at 150 cm-1 up to 0.6 cm-1 at 3600 cm-1. The abscissa was calibrated with a silicon standard, and the sharp Raman shifts are accurate to (2 cm-1. Raman spectra were directly taken from the thin films prepared for IR spectroscopy. To ensure homogeneity of the samples, three spectra were recorded from different spots on the sample (laser spot diameter ∼ 1 µm). Raman spectra were then normalized for the intensity of the G-band. Gas chromatography was carried out using a Perkin-Elmer Autosystem XL GC, Arnel. Results and Discussion Figure 1 shows the gas evolution during the treatment with steam both with and without the presence of the as-made SWNTs. The difference between the two reflects the reaction between the SWNTs and the steam. The reaction temperature was increased at 5 °C/min up to 1075 °C (210 min) and then kept at this temperature until essentially no more gas was evolved. The vertical line separates the dynamic and isothermal annealing regimes. Temperature is shown on the right axis by interpolation of the straight dashed lines. The two different areas both show that a larger amount of gas evolved from the system when SWNTs are present: at around 200 °C (up to 50 min) and above 750 °C (145 min). The gas evolved at the lower temperature can be attributed to the desorption of physisorbed material.16 The gas evolved above 750 °C was shown to be H2, CO, and CO2 by gas chromatography (sample taken at 900 °C). This is consistent with the known reactions:
C + H2O ) CO + H2
and
CO + H2O ) CO2 + H2
Figure 2. Typical HRTEM pictures of as-made SWNTs taken at (a) 10 000× and (b) 50 000× and steam-purified SWNTs at (c) 10 000× and (d) 50 000×.
Almost all of the sample, except for the metal particles, was consumed at the end of the experiment, indicating that steam had reacted with all the carbon materials present in the as-made SWNTs (carbon nanotubes, amorphous carbon, and graphitic particles). To better understand the reactivity of steam with the different components of the sample (carbon nanotubes, amorphous carbon, and graphitic particles), as-made SWNTs were isothermically treated at 750 and 900 °C. Almost no gas evolved after 2 h at 750 °C, indicating that the kinetics of the reaction was too slow to get useful data. Samples were characterized by HRTEM after different reaction periods at 900 °C. We observed that amorphous carbon was much more reactive with the steam than carbon nanotubes and graphitic particles. No amorphous carbon was present after 2 h treatment. Although graphitic particles cannot be removed from the sample without consuming SWNTs, we found that 4 h treatment was a good compromise for purifying the as-made SWNTs. Figure 2 shows HRTEM pictures at low and high magnification of as-made SWNTs and after steam treatment at 900 °C for 4 h. The purification of the sample with steam clearly removes amorphous carbon and some graphitic particles entangling the as-made SWNTs (Figure 2a,b), leaving behind cleaner SWNTs (Figure 2c,d), where metal particles (catalyst) can more easily be seen. We will refer to this sample as steam-purified SWNTs (see Figure 3 and Supporting Information for higher magnification pictures). Refluxing this steam-purified sample with concentrated HCl removes most of the metal particles, as determined by exhaustive HRTEM analysis (Supporting Information, Figure S1). The ends of the SWNTs are also removed during the steam treatment. Thus, a sample of steam-purified SWNTs was treated with an aqueous solution of uranyl acetate. Figure 3 shows a HRTEM image of the resulting filtered but unwashed SWNT materials, showing them to be filled with heavy atoms of uranium. HRTEM has been previously used to differentiate between the encapsulated and external material on SWNT samples.18 The successful room-temperature filling of steamtreated SWNTs by non-oxidazing aqueous solutions of heavy elements provides direct evidence that the steam-treated SWNTs
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Figure 3. HRTEM picture of an uranium compound encapsulated inside steam-purified SWNTs.
are open at the ends.10 The filling compounds can be easily distinguished from the catalytic particles used during the synthesis (Supporting Information, Figure S2). Recent studies showed that the ends of the tubes can be closed by heating in a vacuum at 800 °C.9,10 In contrast, the presence of steam during the cooling process (from 900 °C to room temperature) prevents the re-sealing of the tips. A more detailed study is currently in progress to determine the extent to which this re-sealing is diameter-dependent. Raman spectra (Figure 4) show a decrease in the ratio of D/G band intensity (ID/IG) from 8.2 ( 1.4% for the as-made SWNTs (Figure 4a) to 4.6 ( 0.4% after steam treatment at 900 °C for 4 h (Figure 4b). There is no significant change in ID/IG when a sample of as-made SWNTs is heated at 900 °C for 4 h under argon (ID/IG ) 7.7 ( 1.3%; spectrum not shown). We propose that two different factors can increase ID/IG: namely, the number of defects present in the tubular wall structure of SWNTs, as already reported,19 and also the interaction between carbonaceous fragments and SWNTs. A more detailed spectroscopic study on these interactions will be reported elsewhere.20 The observed decrease in ID/IG can therefore be attributed to the removal of carbonaceous impurities during the steam purification, as shown by HRTEM (Figure 2c,d) and, possibly, also to a more efficient oxidation of the highly defective SWNTs. The radial breathing modes (RBM) do not show significant changes
Tobias et al. before and after the steam purification, which indicates that the tubular structures of the SWNTs are preserved during the steam treatment. Functional groups on carbon nanotubes can be detected by infrared (IR) spectroscopy (Figure 5). Especially carbonylcontaining groups such as carboxylic acid functions can be identified readily by the appearance of CdO stretching transitions in the spectral range around 1740 cm-1. Figure 5a shows the IR spectrum of as-made SWNTs. The absence of peaks around 1740 cm-1 indicates a low degree of CdO-containing functional groups. This is also found after steam treatment in spectrum b, which shows that the steam purification does not cause the formation of CdO-containing functional groups. The effects of steam purification on the spectroscopic activity of SWNTs are next compared to those of the standard nitric acid purification treatment for the removal of amorphous carbon. A sample of as-made SWNTs was refluxed in 3 M nitric acid for 45 h6,21 and washed several times with distilled water to completely remove the nitric acid. The Raman spectrum (Figure 4c) shows significant changes: the intensities of the RBMs decrease, the spectral features of the G band become less resolved, and the D/G band ratio increases to 44.7 ( 0.5%, all of which indicates a large amount of defects, if compared to the as-made SWNTs (Figure 4a). This is consistent with recent studies which show that nitric acid purification affects the SWNT structure.4,5 The IR spectrum (Figure 5c) shows the appearance of broad bands at 1741, 1585, and ∼1200 cm-1, which have been previously assigned to CdO, CdC, and C-O stretching transitions, respectively.17 The prominent broad band in the 3100-3600 cm-1 range can be related to contributions from a variety of O-H stretching modes.17 We have observed changes in the intensities of the bands at ∼3470 and ∼3240 cm-1 for different acid-treated samples. This could arise from different amounts of adsorbed water onto the nanotube surface.22 The band at 3565 cm-1 can be assigned to non-hydrogen-bonded O-H corresponding to the acid functionality on the SWNT sample. The comparison of the Raman and IR spectra of steam and nitric acid purified SWNTs (spectra b and c in Figures 4 and 5) clearly shows that the steam treatment is less aggressive toward attacking the tubular nanotube structure and additionally avoids the formation of functional groups to a detectable level by IR. Since it is unlikely to have carbon dangling bonds at the end of the SWNTs (opened by steam), we suggest hydrogen termination of nanotube dangling bonds,16 although hydroxyl groups may also be present.15
Figure 4. Raman spectra of (a) as-made SWNTs, (b) steam-purified SWNTs at 900 °C for 4 h, (c) after refluxing as-made SWNTs with 3 M HNO3 for 45 h, (d) sample c annealed at 900 °C for 2 h under argon, and (e) steam-treated sample c at 900 °C for 2 h. Scale bars indicate the intensity differences between the left and the right panels. Spectra have been normalized for equal intensity of the G-band. Indicated D/G band intensity ratios are average values from three spectra recorded from different spots on the sample.
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Figure 5. FT-IR spectra of (a) as-made SWNTs, (b) steam-purified SWNTs at 900 °C for 4 h, (c) after refluxing as-made SWNTs with 3 M HNO3 for 45 h, (d) sample c annealed at 900 °C for 2 h under argon, (e) steam-treated sample c at 900 °C for 2 h. Spectral artifacts are indicated with asterisks.17 No bands were observed between 2800 and 2000 cm-1.
Figure 6. Gas evolution during the annealing of (a) 400 mg of asmade MWNTs with 10 mL of water and (b) 10 mL of water. Heating was at 5 °C/min to 1075 °C (210 min), and the sample was kept at this temperature. The dashed line correlates time and temperature (right axis). The vertical line separates the dynamic and isothermic annealing regimes.
To further investigate the effects of steam treatment, samples treated with nitric acid were subsequently then annealed at 900 °C for 2 h under flowing argon with and without the presence of steam. It is well-known that the functional groups created during the acid treatment can be removed by thermal decomposition.7,8 As expected, a loss of CdO and C-O band intensities is found in the IR spectra of both samples, which indicates the removal of functional groups (spectra d and e in Figure 5). Also, D band intensity in the Raman spectrum is decreased during argon annealing (D/G band ratio of 21.6 ( 1.6% (Figure 4d) compared to 44.7 ( 0.5% after nitric acid treatment (Figure 4c). This decrease is even more dramatic for the steam-treated sample, which shows a D/G band ratio of 3.3 ( 0.2% (Figure 4e). RBM intensity is also regained especially after steam treatment, as it can be seen in spectrum e. This confirms that steam is highly efficient in removing defective SWNTs and carbonaceous impurities, as already observed for the as-made steam-purified SWNTs. To complete this study, multiwalled carbon nanotubes were also sonicated in water and annealed to 1075 °C using the same conditions used for as-made SWNTs. The gas evolution during this treatment is given in Figure 6. Due to the experimental setup, the gas formed during the reaction of MWNTs with steam has to be determined by the difference between both curves (water-MWNTs and only water). The vertical line separates the dynamic and isothermal annealing regimes, and temperature can be read on the right axis by interpolation of the straight
dashed line. In contrast to as-made SWNTs, and within the experimental error, there is no release of gas due to the desorption of physisorbed species at low temperature (up to 50 min).16 The reaction between steam and as-made MWNTs starts at the same temperature as that for as-made SWNTs, but the sample is consumed in a shorter period of time. This is consistent with the fact that the amount of graphitic particles in as-made MWNTs is low. The reaction of pure graphite (1-2 µm average particle size, Sigma-Aldrich) with steam requires higher temperature and time compared to as-made SWNTs and MWNTs (Supporting Information, Figure S3). HRTEM pictures of as-made MWNT and steam-treated MWNTs at 900 °C for 1 h are presented in Figure 7. Extensive HRTEM observations confirm the opening or severe damage of most of the ends. A sample of steam-opened MWNTs was successfully filled by an aqueous solution of uranyl acetate (Supporting Information, Figure S4) following the same procedure described for the SWNTs. Solution filling of inorganic compounds containing heavy elements has long been used to provide direct evidence of the opening of MWNTs.23 Conclusions Large-scale purification and opening of carbon nanotubes has been carried out using steam at 1 atm pressure as a mild oxidizing agent. In contrast to previous report of the complete destruction of SWNTs by temperature programmed desorption at 700 °C (1.3 × 10-3 atm water pressure),16 we found a higher resistance of the carbon nanotubes toward steam. In contrast to purification treatment using nitric acid the steam purification does not modify the nanotube tubular structure and avoids the formation of defects and functional groups. It is clear from the demonstration of filling by aqueous solution of uranyl acetate that the SWNTs are opened during the steam purification. Since the ends of the tubes are more reactive than their wall structure, it is likely that SWNTs can be opened at temperatures lower than 900 °C. Further experiments are being carried out to establish a lower temperature limit for the opening. Possible candidates for the chemical nature of the end-offrame carbons after the steam treatment include C-H and C-OH. Acknowledgment. The authors thank Thomas Swan Co. Ltd. for supplying the samples of SWNTs and MWNTs used in this study. We thank J. Singh (Thomas Swan Co. Ltd.) and B. Ballesteros (University of Oxford) for discussion. This research was supported in part by Grant RTN HPRN-CT-2002-00192,
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Tobias et al. HRTEM pictures of MWNTs (as-made and steam-purified after stirring the sample with a solution of uranyl acetate). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
Figure 7. (a and b) HRTEM micrographs of as-made MWNTs; (ce) MWNTs after steam treatment at 900 °C for 1 h (damage of the end of the tubes can be clearly seen; the arrows point to an opening perforation).
Nanotemp (European Community’s Human Potential Program) and by a Marie Curie Intra-European Fellowship within the 6th European Community Framework Program MEIF-CT-2006024542) (G.T.). We also acknowledge the financial support provided through the Austrian Science Funds (FWF; Project J2446) (C.G.S.), and Leverhulme Trust (Y.H.). Supporting Information Available: HRTEM pictures of SWNTs(as-made, steam-purified, steam-purified refluxed in HCl, and steam-purified after stirring the sample with a solution of uranyl acetate) and reaction of pure graphite with steam.
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