NANO LETTERS
Existence and Kinetics of Graphitic Carbonaceous Impurities in Carbon Nanotube Forests to Assess the Absolute Purity
2009 Vol. 9, No. 2 769-773
Satoshi Yasuda,† Tatsuki Hiraoka,† Don N. Futaba,† Takeo Yamada,† Motoo Yumura,† and Kenji Hata*,†,‡ Nanotube Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan, and Japan Science and Technology Agency (JST), Kawaguchi, 332-0012, Japan Received November 9, 2008; Revised Manuscript Received December 26, 2008
ABSTRACT We present an approach that can identify and assess carbonaceous impurities in carbon nanotube (CNT) forests. First, the kinetics of the impurity accumulation was elucidated by investigating the time evolution of both the height and weight of the forests. Second, the kinetics was used to extract a power scaling law revealing the carbonaceous impurity level to solely depend on the total volume of carbon exposure. Third, the power scaling law allowed for a quantitative model describing both the growth of CNTs and accumulation of the carbonaceous impurities. Lastly, from the model the absolute purities of the SWNT forests were evaluated as above 95% with a high of 99.5%.
When carbon nanotubes (CNTs) are efficiently grown by chemical vapor deposition (CVD) from catalysts deposited on a substrate, the subsequently grown CNTs align vertically into a bulk material called a CNT forest.1-9 Numerous efforts have been carried out to understand their growth mechanism, to engineer their structure, and to develop applications.10,11 Until now, various CVD approaches have been proposed to synthesize forests spanning from plasma CVD,5 alcohol CVD,2 water-assisted CVD (denoted as “super-growth CVD”),4,6-9 and even “flying carpets”.12 These studies have made clear that a forest is a very universal form of CNTs; a highly efficient growth from a moderate density of catalysts located on a substrate inevitably results in a forest. Such progress in synthesis has accelerated the research of CNT forests, as envisioned by the rapidly increasing applications of the forest spanning from supercapacitors,13,14 sensing applications,15 field-emitters in flat-screen displays,1,16 thermal interface materials,17 electrical interconnects in nanoscale devices,18 and superhydrophobic surfaces for self-cleaning surfaces.19 This broad range of utility of the forests stems from the exceptional properties of the CNTs within the forests, such as high surface area, alignment, good electrical conductivity, and long length. * To whom correspondence should be addressed. E-mail: Kenji-hata@ aist.go.jp. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Japan Science and Technology Agency (JST). 10.1021/nl803389v CCC: $40.75 Published on Web 01/16/2009
2009 American Chemical Society
An additional important characteristic of the forest is high carbon purity. In general, CNT forests synthesized by currently known growth techniques contain metal catalysts and carbonaceous impurities. Recently, a carbon purity of 99.98% estimated from X-ray fluorescence spectrometry has been reported from a 2.5 mm height SWNT forest, representing one of the purest CNTs ever made.4 Only a negligible amount of metal impurities were present in the forest, and therefore in the following discussion we neglect these. The high carbon purity of the forest stems from the synthesis of a massive amount of SWNTs from a minimal amount of catalyst, and the easy separation of the forest from the substrate and the catalysts. Highly pure CNTs require no purification before use, in contrast to the low purity CNTs grown in the past where purification was indispensible. The purification processes are not only complicated and expensive but also degrade the CNTs.20-23 Therefore, highly pure CNTs are advantageous to extract the intrinsic properties of CNTs. For example, high carbon purity is crucial to realize high surface area, because the surface areas of impurities, such as metal, are low. Hence, high purity is the key for efficient energy and material storage important for gas-sensors and supercapacitors. Additionally, impurities are chemically reactive and limit the lifetime of such devices. Although high carbon purity represents a substantial progress, high carbon purity does not necessary mean that the forest is solely composed from CNTs, because other
Figure 1. Time evolution of SWNT forest growth. (a) Overlaid plots of forest height (red line) and weight per area (blue line) as a function of growth time. (b,c) The growth time dependence of the density and G/D ratio, respectively. (d) Raman spectra at each growth time.
carbonaceous structures might exist as impurities. In general, carbonaceous impurities are extremely difficult to discriminate quantitatively from CNTs. A procedure to assess the absolute purity using near-infrared spectroscopy on dispersed SWNT soot has been reported,24 but the accuracy was limited by the ability to disperse well the SWNTs. In this article, the carbon and purity is defined as the carbon weight percent of a CNT sample while the absolute purity is meant to be the carbon nanotube weight percent of a CNT sample. As such, despite its importance, we do not know much about carbonaceous impurities, for example, if CNT forests include carbonaceous impurities, and if so, how much, how do they accumulate, and what drives their accumulation? In this article, we addressed these critical issues by presenting direct evidence that CNT forests do in fact include graphitic carbonaceous impurities that are generated by the absorption of carbon species onto the forest and are difficult to remove or discriminate. We found that the kinetics of the impurity accumulation followed a power scaling law. This finding allowed for the quantitative modeling of the impurity accumulation to estimate the absolute purities of the SWNT forest as above 95% with a high of 99.5% for typical growth conditions. These results significantly deepen our understanding regarding the presence of carbonaceous impurities in CNT forests, and would serve as a guide in CNT metrology to characterize purity. First, we showed that highly pure forests were not solely composed from CNTs but indeed contained carbonaceous impurities. To demonstrate this aspect, we investigated the time evolutions of the height and weight of a SWNT forest. The SWNT forests were synthesized by supergrowth. Briefly, a 2 × 2 cm2 silicon wafer sputtered with a Al2O3 (30 nm)/ Fe (1 nm) thin catalytic film was loaded into furnace at 750 °C, and the forest was grown from ethylene (75 standard cubic centimeters per minute (sccm)) and helium (925 sccm) as the carrier gases with water (concentration 200 ppm).4,25 As known, the forest height growth rate (Figure 1a, red line) gradually decreased and terminated after 20 min.25 In contrast, the forest weight (Figure 1a, blue line) steadily increased particularly after the growth termination. Notably, 770
the density did not increase appreciably during (average 45 mg/cm3) and immediately after the growth termination as reported,25 but did so significantly at longer growth times (Figure 1b). Raman (532 nm wavelength) spectra (Figure 1d) revealed a gradual decrease in the graphitic (G-band) to disorder band (D-band) ratio. The radial breathing mode peaks, an indicator of SWNTs, remained constant throughout. These data are readily explainable by assuming an accumulation of carbonaceous impurities to the forest. Characterization of the carbonaceous impurities was implemented to classify their structures and properties. For a 5 min growth (Figure 2a), as reported, transmission electron microscopy (TEM) showed SWNTs clean and free from carbonaceous impurities. In contrast, for a 90 min growth (Figure 2b), the SWNTs were coated with layers of carbon and carbonaceous aggregates. With this coating, SWNTs resembled multiwalled CNTs (Figure 2c). Thermogravimetric analysis (TGA) profiles (1 °C/min with 10 sccm air) of clean (Figure 2d, black line) and dirty forests (Figure 2d, red line) were nearly identical with negligible weight reduction at low temperatures below 500 °C, followed by the main combustion in the range of 550-700 °C, and no measurable residue above 750 °C. These results strongly implied that the carbonaceous coating was not amorphous but graphitic since the burning temperature of amorphous carbon was much lower than 500 °C. Also, they implied that the removal of the impurities and assessment of the absolute purity of CNT forests by combustion were impossible. Brunauer-EmmettTeller (BET) surface areas were estimated from nitrogen adsorption isotherms at 77 K of 30 mg SWNT forests. Importantly, the dirty forests (176 m2/g) showed a 7-fold drop in the surface area compared to that of a clean forest (1230 m2/g). This meant that the SWNTs were coated by tightly stacked graphitic layers and particles that caused a degradation of the forest properties. Summarizing, the forest contained a non-negligible amount of graphitic carbonaceous impurities that were not only difficult to discriminate and remove but also made the SWNT forests resemble a lowquality MWNT forest. To elucidate the adsorption kinetics of carbonaceous impurities on forests, we designed an experiment that Nano Lett., Vol. 9, No. 2, 2009
Figure 3. Plots of the relative impurity level defined as the increase in weight relative to the initial forest weight as a function of (a) the time of ethylene exposure and (b) total volume of ethylene exposure for different ethylene concentration, respectively.
Figure 2. TEM images of SWNTs for (a) 5 min and (b) 90 min growth. (c) Schematic diagram illustrating how the SWNTs resembles multiwalled CNTs by adsorption of carbonaceous impurities. (d) TGA weight loss and these derivatives for 5 min (black curve) and 90 min (red curve) growth.
separated the growth of SWNTs from the accumulation of impurities. Specifically, a clean forest with a minimal amount of impurities was grown, removed from the substrate,4 and exposed to ethylene (impurity accumulation process). As evidenced by the lack of residual matter in the TGA data, the forest was catalyst free thus no further CNTs could grow; therefore any further weight increase must have resulted from the accumulation of carbonaceous impurities. The 1230 m2/g specific surface area for the clean SWNT forests (5 min growth) was close to the theoretical value of 1315 m2/g.26 Since the presence of carbonaceous impurities would decrease the surface area, the absolute purity of the clean SWNT forests was expected to be very high, as also supported by the clean SWNTs viewed by TEM (Figure 2a). These clean forests were exposed to ethylene at 750 °C with different times and concentrations. We defined the Nano Lett., Vol. 9, No. 2, 2009
“relative impurity level” as the increase in weight relative to the initial forest weight g(t) ) 100 ×
(
∆G mSWNT
)
(1)
where ∆G is the weight of impurities, and mSWNT is the initial weight of the SWNT forest, respectively. Generally, the relative impurity level rose nonlinearly with time (Figure 3a). Furthermore, higher ethylene concentrations exhibited faster increase. We found that this phenomenon fit best to a power law g(t) ) a × tb
(2)
where the fitting parameters, a and b, for differing ethylene concentrations were 0.42 ( 0.16 and 1.48 ( 0.06 (200 sccm), and 0.11 ( 0.02 and 1.41 ( 0.04 (75 sccm), respectively. Interestingly, the scaling exponent, b, was nearly identical for both cases, and the coefficient, a, was proportional to the ethylene concentration. This meant that a scaling curve (Figure 4b) existed between the relative impurity level and the total volume of exposed ethylene. The existence of such scaling curve indicated that the amount of carbonaceous impurities exclusively depended on the total amount of ethylene exposed to the forest regardless of the ethylene input level. The nonlinear increase of the impurity level with 771
Figure 4. Schematic model of the progression of CNT growth and the carbonaceous impurity formation during forest growth.
ethylene exposure demonstrated that the presence of an impurity initiates further impurity formation and adsorption. Therefore, once the forest began to get dirty, it became dirtier with increasing speed. We suspect that upon contact with the nanotube, reacted carbon species formed clusters and layers on the tubes. Over time, these clusters and layers acted as nucleation points for more accumulation of impurities. Consequently, as they grew in size and number, the crosssection of the intertube spacing reduced, which initiated a cascading process. These results point to the importance to choose the optimum growth conditions to balance the impurity level against SWNTs yield. We believe that substantial progress in synthesizing CNTs with higher efficiency and absolute purity can be achieved by elucidating and controlling the key factors that determine the scaling exponent, and we hope that this research would invoke such future efforts. From the impurity kinetics, a forest growth model was developed, which encompassed both CNT growth and impurity accumulation. Through this forest growth model, the absolute purity of the SWNT forest was estimated. As described, the relative impurity level (g(t)) increased nonlinearly (eq 2) with the time the CNTs were exposed to the growth ambient. As shown schematically in Figure 4, during CNT growth the earlier a CNT section was grown, the longer it would be exposed to the growth ambient and therefore increased its impurity level. Quantitatively, at growth time T, the impurity level (∆G) that has attached to a SWNT grown at time t becomes ∆G(t) ) ∆SWNT(t)g(T - t) ×
1 100
(3)
where ∆SWNT(t), and g(t) are the weight of the grown CNT and the relative impurity level at time t, respectively. As displayed in Figure 1a (red line), the growth of CNT heights agreed well with the radioactivity decay model H(t) ) βτ0{1 - e(-t⁄τ0)}
(4)
where β ) 54.3 µm and τ0 ) 7.4 min are the initial growth rate and catalyst lifetime, respectively. 772
By definition, the absolute purity of the CNT forest at growth time T is P(T) ) 100 ×
SWNT(T) SWNT(T) + G(T)
(5)
where SWNT(T) ) H(t) × area × density, and G(T) are the weights of the CNTs and impurities at growth time T. Substituting, eqs 3 and 4 to 5, we obtain P(T) ) 100 ×
1 - e(-T⁄τ0) a T (-t⁄τ0) 1 - e(-T⁄τ0) + e (T - t)bdt τ0 0
∫
(6)
Accordingly, the time evolution of the weights of the SWNTs (Figure 5a, red line), impurities (Figure 5a, black line), the forest (Figure 5a, blue line), and the absolute purity (Figure 5b, solid line) could be numerically calculated as shown in Figure 5. The good agreement (Figure 5a, blue line) with the experimental data (Figure 5a, blue squares) highlights the accuracy of our model. Over the same time duration the weight of the SWNTs initially increased rapidly, gradually slowed, and stopped, while the weight of the impurities initially increased slowly and rapidly increased with time. Consequently, the model (Figure 5a, blue line) predicted that the forest weight initially increases sharply, gradually slow, and continues to increase steadily, and that the absolute purity remains very high initially and drop with the rise in impurity level. On the basis of this model, the absolute purities of forests grown (Figure 2a) for 5 min with specific area of 1230 m2/g and grown for 20 min (until growth saturation) were estimated as 99.5 and 95.3%, respectively. It should be noted that our estimation of the absolute purity did not rely on the initial purity of the forest used in experiments described in Figure 3. This was because our absolute purity was estimated by a numerical model where the parameters were determined solely and unambiguously from experiments, and none of these parameters rely on the initial purity. For example, due to the scaling behavior of the accumulation process of carbonaceous impurities, the parameters are universal and do not depend on the initial purity of the forest. Such high absolute purity would have Nano Lett., Vol. 9, No. 2, 2009
carbonaceous impurities to achieve an absolutely pure CNT material in the future. Acknowledgment. Support from the Nanotechnology Program “Carbon Nanotube Capacitor Development Project” (2006-2010) by the New Energy and Industrial Technology Development Organization (“NEDO”) is acknowledged. References
Figure 5. (a) Time evolution of calculated weights of the SWNTs (red line), impurities (black line), the forest (blue line), and the experimental data (blue squares) shown in Figure 1a. (b) Time evolution of the absolute purity calculated from eq 6.
been difficult to estimate from other existing approaches.24 This high absolute purity for CNT forests stemmed from the opposing CNT and impurity trends. This trend provided a valuable opportunity to tune the growth time to achieve a forest with high yield and absolute purity. For example, terminating the growth process prior to growth saturation was important to achieve forests of high absolute purity, and such control is now easy to implement by an in situ telecentric height monitoring system.27 As shown, provided the kinetics of the SWNT growth and accumulation of impurities are known, modeling of the forest growth with the accumulation of impurities incorporated is straightforward. It would be interesting to see whether we can achieve growth with long lifetime, high initial growth rate, and low scaling exponent in the future. In conclusion, we have studied the accumulation process of carbonaceous impurities on CNT forests. This approach revealed the existence of carbonaceous impurities on CNT forests, elucidated a power law scaling kinetics of impurity accumulation, and established a quantitative model of the growth of CNTs incorporated with the accumulation of carbonaceous impurities. From these studies, the absolute purity of forests was quantitatively assessed for the first time. We hope that these results would serve as a key to control
Nano Lett., Vol. 9, No. 2, 2009
(1) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512–514. (2) Maruyama, S.; Kojima, R.; Miyauchi, Y.; Chiashi, S.; Kohno, M. Chem. Phys. Lett. 2002, 360, 229–234. (3) Jeong, H. J.; Kim, K. K.; Jeong, S. Y.; Park, M. H.; Yang, C. W.; Lee, Y. H. J. Phys. Chem. B 2004, 108, 17695–17698. (4) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362–1365. (5) Hofmann, S.; Ducati, C.; Kleinsorge, B.; Robertson, R. Appl. Phys. Lett. 2003, 83, 4661–4663. (6) Yamada, T.; Namai, T.; Hata, K.; Futaba, D. N.; Mizuno, K.; Fan, J.; Yudasaka, M.; Yumura, M.; Iijima, S. Nat. Nanotechnol. 2006, 1, 131– 136. (7) Chakrabarti, S.; Nagasaka, T.; Yoshikawa, Y.; Pan, L.; Nakayama, Y. Jpn. J. Appl. Phys. 2006, 45, L720–L722. (8) Noda, S.; Hasegawa, K.; Sugime, H.; Kakehi, K.; Zhang, Z.; Maruyama, S.; Yamaguchi, Y. J. Appl. Phys. 2007, 46, L399–L401. (9) Zhao, B.; Futaba, D. N.; Yasuda, S.; Akoshima, M.; Yamada, T.; Hata, K. ACS Nano 2009, ASAP. (10) Zhang, M.; Atkinson, K. R.; Baughman, R. H. Science 2004, 306, 1358–1361. (11) Cao, A. Y.; Dickrell, P. L.; Sawyer, W. G.; Ghasemi-Nejhad, M. N.; Ajayan, P. M. Science 2005, 310, 1307–1310. (12) Pint, C. L.; Pheasant, S. T.; Pasquali, M.; Coulter, K. E.; Schmidt, H. K.; Hauge, R. H. Nano Lett. 2008, 7, 1879–1883. (13) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987–994. (14) Niu, C. M.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl. Phys. Lett. 1997, 70, 1480–1482. (15) Zhang, H.; Cao, G. P.; Yang, Y. S.; Gu, Z. N. J. Electrochem. Soc. 2008, 155, K19–K22. (16) Hiraoka, T.; Yamada, T.; Hata, K.; Futaba, Don N.; Kurachi, H.; Uemura, S.; Yumura, M.; Iijima, S. J. Am. Chem. Soc. 2006, 128, 13338–13339. (17) Huang, H.; Liu, C. H.; Wu, Y.; Fan, S. S. AdV. Mater. 2005, 17, 1652– 1656. (18) Ye, J. Li. Q.; Cassell, A.; Ng, H. T.; Stevens, R.; Han, J.; Meyyappan, M. Appl. Phys. Lett. 2003, 82, 2491–2493. (19) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; Mckinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701–1705. (20) Rinzler, A. G.; Liu, J.; Dai, H.; Nilolaev, P.; Huffman, C. B.; Rodriguez-Macias, 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 1998, 67, 29–37. (21) Zhang, M.; Yudasaka, M.; Koshio, A.; Iijima, S. Chem. Phys. Lett. 2002, 364, 420–426. (22) Harutyunyan, A. R.; Pradhan, B. K.; Chang, J.; Chen, G.; Eklund, P. C. J. Phys. Chem. B 2002, 106, 8671–8675. (23) Dillon, A. C.; Gennett, T.; Jones, K. M.; Alleman, J. L.; Parilla, P. A.; Heben, M. J. AdV. Mater. 1999, 11, 1354–1358. (24) Itkis, M. E.; Perea, D. E.; Niyogi, S.; Rickard, M.; Hamon, M. A.; Hu, H.; Zhao, B.; Haddon, R. C. Nano Lett. 2003, 3, 309–314. (25) Futaba, D. N.; Hata, K.; Yamada, T.; Mizuno, K.; Yumura, M.; Iijima, S. Phys. ReV. Lett. 2005, 95, 056104-1056104-4. (26) Peigney, A.; Laurent, C.; Flahaut, E.; Bacsa, R. R.; Rousset, A. Carbon 2001, 39, 507–514. (27) Yasuda, S.; Futaba, D. N.; Yumura, M.; Iijima, S.; Hata, K. Appl. Phys. Lett. 2008, 93, 143115–143117.
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