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2009, 113, 2653–2656 Published on Web 01/27/2009
Quantitative Assessment of the Amount of Material Encapsulated in Filled Carbon Nanotubes Bele´n Ballesteros,* Gerard Tobias, Michael A. H. Ward, and Malcolm L. H. Green Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road OX1 3QR Oxford, United Kingdom ReceiVed: December 5, 2008; ReVised Manuscript ReceiVed: January 9, 2009
In this paper, we present the first methodology for the quantitative assessment of the amount of material encapsulated in filled carbon nanotubes. Samples of single-walled carbon nanotubes have been filled by moltenphase capillary wetting with CuI, FeCl2, and CuBr. A suitable solvent has been used in each case to remove the large amount of external material present after the filling step. Thermogravimetric analyses in air have been performed to the empty and filled nanotubes, and the data have been used to obtain the filling yield for each sample. By using different starting nanotube samples, we stress the importance of the correct analysis of the data, which becomes more crucial for samples with larger quantities of metal catalyst impurities. Highresolution transmission electron microscopy and X-ray electron dispersive spectroscopy experiments confirm the encapsulation of the desired compounds into the single-walled carbon nanotubes and the absence of external metal halides. The method proposed herein is accurate, simple, and can be easily implemented for a large variety of materials. Introduction Carbon nanotubes exhibit remarkable structural, mechanical, and electronic properties. Among their characteristics, their hollow cavity allows the encapsulation of a variety of materials which have been predicted and found to lead to novel structures and properties. Therefore, filled carbon nanotubes have been advocated for many applications such as nanoelectronic and nano-optoelectronic devices,1-3 diagnostic contrast agents,4 and biocompatible thermometers,5 and for the structural characterization of low dimensional materials 6,7 among others.8 Despite all the interest in materials encapsulated in singlewalled carbon nanotubes (SWCNTs), the determination of the filling yield has not been rigorously addressed and has usually been estimated by means of high-resolution transmission electron microscopy (HRTEM). Although extremely useful to determine the success of the filling experiment and the structure of the encapsulated materials, this technique is not suitable for quantitative sampling, and therefore, the determination of the filling yield can be subjective. This hinders the determination of the best conditions for optimization of filling experiments, which may well be desired for further applications of filled SWCNTs. Moreover, given that the amount of encapsulated material might affect the properties of the filled SWCNTs, an accurate assessment of the filling yield will be crucial for comparison of different filled samples and required for advances in this field. Thermogravimetric analysis (TGA) is a precise technique which requires a small amount of sample (on the order of a few milligrams). It has been used for CNT characterization to analyze both the metal content 9,10 and the functionalization * Tel: +44 (0) 1865 272600; Fax: +44 (0) 1865 272690, e-mail:
[email protected].
10.1021/jp810717b CCC: $40.75
degrees on CNT samples.11 In this work, we report on the use of TGA for the analysis of the filling yield of filled SWCNTs. Experimental Section Chemical vapor deposition (CVD) single-walled carbon nanotube samples were supplied by Thomas Swan & Co. Ltd. The samples also contained double-walled carbon nanotubes (DWCNTs), amorphous carbon, graphitic particles (carbonaceous crystalline materials having few graphitic layers), and some metal particles. DWCNTs have the same behavior under the reported experimental conditions. Thus, we will refer to the sample as SWCNTs. Purified SWCNTs were prepared by steam treatment for 4 h12 followed by an HCl wash13 to remove amorphous carbon and catalytic metal particles. As a general filling procedure, 50 mg of SWCNTs were ground with 500 mg of the desired filling material (namely, CuBr, FeCl2 and CuI) using an agate pestle and mortar, and then dried under vacuum at 100 °C for 12 h. The mixture was placed in a silica quartz ampule (under dinitrogen) and sealed under vacuum. The ampule was heated at 5 °C min-1 to 900 °C for 12 h, cooled to 50 °C below the melting point of the filling material at 1 °C min-1, and then allowed to cool to room temperature. The sample was removed from the quartz ampule and washed by sonication in a suitable solvent (a saturated aqueous solution of ammonia in the case of CuI@SWCNTs and CuBr@SWCNTs and 6 M HCl for FeCl2@SWCNTs) for 15 min to remove any material on the exterior of the filled nanotubes. Next, the samples were filtered on a polycarbonate membrane (Whatman Cyclopore, 1 µm pore size) and rinsed with deionized water. This protocol was repeated until a colorless filtrate was obtained. Finally, each sample was subjected to one further rigorous washing procedure by stirring them in the appropriate solution at 60 °C for 48 h, before 2009 American Chemical Society
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filtering them on a polycarbonate membrane, rinsing with deionized water, and then drying the samples in air at 60 °C. Thermogravimetric experiments were carried out on a PerkinElmer Pyris thermobalance by heating the sample up to 900 °C under flowing air at 5 °C min-1. This heating rate is expected to be slow relative to that of the oxidation reaction of carbon and the encapsulated materials. Typically, about 1-4 mg of sample were used. Filled SWCNT samples were also characterized by highresolution transmission electron microscopy (HRTEM) using a JEOL 4000EX microscope (operated at 200 kV) and a JEOL JEM-3000FX FEGTEM microscope (operated at 300 kV). X-ray electron dispersive spectroscopy (XEDS) analyses were carried out using a Oxford Instruments energy dispersive X-ray spectrometer with a super atmospheric thin window (SATW) detector running on an ISIS 300 software. For HRTEM and XEDS examination, the samples were ground and dispersed in ethanol and then placed dropwise onto a holey carbon support grid. XRD experiments were performed on a Philips PW1729 X-ray diffractometer using Cu KR radiation at a scanning rate of 0.04° s-1. Results and Discussion A sample of filled SWCNTs consists of carbon (from carbon nanotubes, graphitic particles, and amorphous carbon), metal catalyst impurities, and the desired encapsulated material. HRTEM studies together with XEDS analyses (Figure 1) on the samples showed that CuBr, FeCl2, and CuI were successfully encapsulated into SWCNTs and that no external material was present after the washing procedure. The complete absence of external material is required to get reliable data, since the TGA technique does not permit differentiation between internal and external material. Therefore, filling procedures that allow the complete removal of the external material without removing the encapsulated material are required. In this work, we filled the SWCNTs by high-temperature melting filling14 at 900 °C under vacuum. It is known that, when treating a sample of SWCNTs at temperatures above 800 °C under vacuum, the tubes reseal during the cooling process.15 Since the ends of the tubes were closed after the high-temperature filling process, the solvents used for the subsequent washing steps did not have access to the encapsulated material. Therefore, only the nonencapsulated material was dissolved during the washing procedure. The recently reported low-temperature fullerene-corked solution filling,16 which also allows the selective removal of the external material, introduces fullerenes in the sample that cannot be easily quantified, and hence, the filling yield assessment becomes more challenging. TGA analyses on both the as-made and purified SWCNTs samples reveal that the combustion starts at temperatures above 550 °C (Figure 2). For the filled SWCNTs, the onset temperature decreases considerably to around 300 °C, and different reaction steps can be observed in the curve due to the oxidation of the metal halides and the SWCNTs sample components. In all cases, the complete reaction is achieved at lower temperatures for the filled SWCNTs. This can be explained by the presence of the encapsulated metal halides, since it is well-known that the energy released in the reaction of a component in the sample will promote the reaction of the other components.17 The same phenomenon has been reported in samples of as-made SWCNTs with different amounts of metal catalyst impurities, where higher metal contents result in the decrease of the temperature required to oxidize the sample.18
Figure 1. HRTEM images and XEDS analyses of (a) CuBr, (b) FeCl2, and (c) CuI.
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Figure 3. XRD analysis of the sample resulting from FeCl2 oxidation in air.
On the other hand, if the filling material oxidizes to form a solid oxide, the residue of the TGA will be formed by the catalyst impurity residue and compound B (reaction (2)). air, ∆T
xA(s) 98 yB(s)
(2)
The value for RA can be then calculated taking into account the stoichiometry of this reaction by the expression:
RA )
Figure 2. TGA analyses in flowing air of SWCNTs before and after filling with (a) CuBr, (b) FeCl2, and (c) CuI.
During the TGA experiment on the filled samples in flowing air, the filling material may react with oxygen to form a solid residue, normally an oxide, or on the other hand may sublime or form a gaseous oxide. Also, both oxidation and sublimation may occur simultaneously. In any case, the filling yield (FY) can be accurately determined by the expression:
FY(wt %) )
100(R2 - R1) RA - R1
(1)
where R1, R2, and RA are the TGA residues in air for the empty carbon nanotubes, the filled carbon nanotubes, and the bulk filling material (A), respectively. The obtained FY is an average value and does not distinguish between the filling efficiency of SWCNTs of different diameters and lengths present in the sample. If the encapsulated material sublimes or reacts with oxygen to give a gaseous oxide during the TGA experiment, the residue will only consist of the catalyst impurity residue. The TGA residue for the filled SWCNTs (R2) will then be lower than that for the empty SWCNTs (R1), since a lower relative wt % of metal will be present in the filled tubes.
100 × x × MWB y × MWA
(3)
where x and y are the reaction stoichiometric constants and MWA and MWB the molecular weights of A and B, respectively. One should ensure that the product of the reaction (2) is the expected one, for instance, by XRD analysis (Figure 3). However, the calculation of RA shown above does not take into account that the oxidation of A to B might be occurring simultaneously to a sublimation process. To account for this, RA should preferably be obtained by TGA analysis at the same experimental conditions as those for the filled SWCNTs. Once the filling yield has been obtained, it is easy to calculate the amount of catalyst and carbon on the filled SWCNT sample and therefore know the exact composition of the sample. First, we should find out the content of catalyst impurities in the SWCNTs used for the filling experiments. The catalyst impurities in SWCNT samples can be in its metallic form, but also as metal oxides or carbides. In our case, the catalyst impurities consist of metallic iron, as proven by X-ray photoelectron spectroscopy analysis.13 When the catalyst is present in its metallic form, the TGA experiments carried out under flowing air will result in the oxidation of the carbon species and the metal (M) to carbon dioxide and a stable metal oxide (MmOn), respectively, as described by the reactions
C(s) + O2(g) f CO2(g) mM(s) + n ⁄ 2O2(g) f MmOn(s) The amount of catalyst in the carbon nanotube sample (M1) can be then accurately determined by the following equation:
M1(wt % ) )
m × R1 × MWM MWMmOn
(4)
where m is the stoichiometric constant and MWM and MWMmOn the molecular weights for the metal and the metal oxide, respectively.
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TABLE 1: Contents of the Samples (in wt %) Analyzed by TGA in Figure 1. sample
R1
R2
RA
FY
M2
C2
CuBr@SWCNTs 5.3% 15.0% 55.5% 19.3% 3.0% 77.7% FeCl2@SWCNTs 0.61% 12.1% 63.0% 18.4% 0.35% 81.3% CuI@SWCNTs 5.3% 14.1% 41.8% 24.1% 2.8% 73.1%
TABLE 2: Filling Yield Values Calculated by the Correct Expression and the Two Incorrect Approximations with Their Corresponding Associated Errors approx 1
approx 2
sample
FY
FY
error
FY
error
CuBr@SWCNTs FeCl2@SWCNTs CuI@SWCNTs
19.3% 18.4% 24.1%
9.7% 11.5% 8.8%
49.7% 37.5% 63.5%
17.5% 18.2% 21.1%
9.3% 1.1% 12.4%
The composition of the filled SWCNT sample will then be obtained by using the expressions below, where M2 refers to the metal catalyst and C2 to the carbon contents.
M2 ) 0.01 × M1 × (100 - FY)
(5)
C2 ) 0.01 × (100 - M1) × (100 - FY)
(6)
Also
FY + M2 + C2 ) 100
(7)
In Table 1, the contents in wt % of the SWCNTs filled with CuBr, FeCl2, and CuI are summarized, which allows comparison of the different samples. The filling yield in wt % ranges from 18.4% for FeCl2@SWCNTs to 24.1% for CuI@SWCNTs. In addition, the ratio in moles of filling material to moles of carbon present in the sample can be calculated. There is 1 mol of carbon for every 20.8, 21.4, and 20.8 mmol of CuBr, FeCl2, and CuI, respectively. Since the relative content of the different forms of carbon (namely, carbon nanotubes, graphitic particles, and amorphous carbon) varies within the different starting SWCNT sources, using the same source of starting SWCNTs would allow comparison of the encapsulation efficiency for different materials. This is the case for CuBr and CuI in our example. Although the filling yield is higher for CuI in terms of wt %, the content in moles encapsulated into carbon nanotubes is very similar for both. Finally, the volume of encapsulated material can be calculated assuming that the density of the material in the bulk is similar to that inside the SWCNT (previous reports6 have shown that this is not always the case). Every 100 mg of carbon, the encapsulated CuBr and CuI occupy a volume of 5.3 mm3 and 5.9 mm3, respectively. Therefore, a higher degree of filling for CuI@SWCNT would be assigned by HRTEM estimation. An obvious but wrong approximation to calculate the filling yield is to assume that the difference between the TGA residue from the starting nanotubes (R1) and the TGA residue of the filled material (R2) is due to the filling material. One could think that the filling yield corresponds to the direct substraction, namely, R2 - R1 (Approximation 1), or in a better approach, the substraction value would be used to calculate the filling yield taking into consideration the stoichiometry of reaction (2) (Approximation 2). Both approximations are incorrect, since they do not take into account that the relative metal content (in wt %) in the filled material is lower than in the empty carbon nanotubes. In Table 2, a summary of the filling yield values for the different reported samples using the correct expression and
both approximations is shown. The higher the catalyst metal content in the carbon nanotube sample, the bigger the error, and the more crucial it becomes to use the correct expression. For instance, the error is as high as 63.5% and 12.4% when using Approximations 1 and 2, respectively, for the CuI@SWCNTs sample, which has a 3.7 wt % of Fe in the starting SWCNT sample (calculated using R1 ) 5.3% in expression 4). Conclusions We report a simple and accurate method for quantitative determination of the filling yield in carbon nanotubes by means of TGA analysis. The filling yield calculation only requires the TGA residues of the filling material (RA) and the unfilled (R1) and filled (R2) carbon nanotube samples. The complete removal of the external material present after the filling step is essential. Filling yield determination has special interest for the optimization of the filling experiments and for comparison of different filled samples, since the differences in properties of filled nanotubes could be related to a different degree of material encapsulation. The higher the metal catalyst content in the starting carbon nanotube sample, the more important it becomes to use the correct expression to assess the filling yield. Acknowledgment. The authors thank Thomas Swan & Co. Ltd. for SWCNTs and funding (B.B. and G.T.) and the FP6 Marie Curie IE Fellowship MEIF-CT-2006-024542 (G.T.). References and Notes (1) Lee, J.; Kim, H.; Kahng, S. J.; Kim, G.; Son, Y. W.; Ihm, J.; Kato, H.; Wang, Z. W.; Okazaki, T.; Shinohara, H.; Kuk, Y. Nature 2002, 415, 1005. (2) Li, L.-J.; Khlobystov, A. N.; Wiltshire, J. G.; Briggs, G. A. D.; Nicholas, R. J. Nat. Mater. 2005, 4, 481. (3) Takenobu, T.; Takano, T.; Shiraishi, M.; Murakami, Y.; Ata, M.; Kataura, H.; Achiba, Y.; Iwasa, Y. Nat. Mater. 2003, 2, 683. (4) Prato, M.; Kostarelos, K.; Bianco, A. Acc. Chem. Res. 2008, 41, 60. (5) Vyalikh, A.; Klingeler, R.; Hampel, S.; Haase, D.; Ritschel, M.; Leonhardt, A.; Borowiak-Palen, E.; Rummeli, M.; Bachmatiuk, A.; Kalenczuk, R. J.; Grafe, H. J.; Buchner, B. Phys. Stat. Solidi B 2007, 244, 4092. (6) Carter, R.; Sloan, J.; Kirkland, A. I.; Meyer, R. R.; Lindan, P. J. D.; Lin, G.; Green, M. L. H.; Vlandas, A.; Hutchison, J. L.; Harding, J. Phys. ReV. Lett. 2006, 96, 215501. (7) Meyer, R. R.; Sloan, J.; Dunin-Borkowski, R. E.; Kirkland, A. I.; Novotny, M. C.; Bailey, S. R.; Hutchison, J. L.; Green, M. L. H. Science 2000, 289, 1324. (8) Monthioux, M. Carbon 2002, 40, 1809. (9) Dillon, A. C.; Gennett, T.; Jones, K. M.; Alleman, J. L.; Parilla, P. A.; Heben, M. J. AdV. Mater. 1999, 11, 1354. (10) Itkis, M. E.; Perea, D. E.; Jung, R.; Niyogi, S.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 3439. (11) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105. (12) Tobias, G.; Shao, L.; Salzmann, C. G.; Huh, Y.; Green, M. L. H. J. Phys. Chem. B 2006, 110, 22318. (13) Ballesteros, B.; Tobias, G.; Shao, L.; Pellicer, E.; Nogue´s, J.; Mendoza, E.; Green, M. L. H. Small 2008, 4, 1501. (14) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333. (15) Shao, L.; Tobias, G.; Huh, Y.; Green, M. L. H. Carbon 2006, 44, 2855. (16) Shao, L.; Lin, T. W.; Tobias, G.; Green, M. L. H. Chem. Commun. 2008, 2164. (17) Rinzler, A. G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C. B.; Rodrı´guez-Macı´as, F. J.; Boul, P. J.; Lu, A. H.; Heymann, D.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Appl. Phys. A: Mater. Sci. Proc. 1998, 67, 29. (18) Chiang, I. W.; Brinson, B. E.; Smalley, R. E.; Margrave, J. L.; Hauge, R. H. J. Phys. Chem. B 2001, 105, 1157.
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