Open-Ended Aligned Carbon Nanotube Arrays Produced Using CO2

ARTICLE pubs.acs.org/JPCC

Open-Ended Aligned Carbon Nanotube Arrays Produced Using CO2-Assisted Floating-Ferrocene Chemical Vapor Deposition Xiaoshuang Yang,†,‡ Lixiang Yuan,*,† Vanessa K. Peterson,‡ Yongbai Yin,§ Andrew I. Minett,† and Andrew T. Harris† †

Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, Building J01, University of Sydney, NSW 2006, Australia ‡ The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia § Applied and Plasma Physics Group, School of Physics, University of Sydney, NSW 2006, Australia ABSTRACT:

The influence of CO2, in concentrations of up to 7600 ppm, on the preparation of aligned carbon nanotube (CNT) arrays from ethylene using floating-ferrocene chemical vapor deposition (CVD) at 750 °C was investigated. The CO2-assisted floating-ferrocene CVD method facilitates the well-controlled growth of aligned CNT arrays; the quality of the aligned CNT arrays was significantly improved in the presence of CO2, as demonstrated by improved alignment and crystallinity. Under the assistance of CO2, CNT arrays were linear and vertical. CNTs grown with CO2 were also higher than those grown in the absence of CO2, with the optimum CO2 concentration of 760 ppm producing a 50% enhancement in CNT height. Varying the concentration of CO2 also controlled the diameter and wall numbers of the aligned CNTs. CNTs synthesized in the presence of 7600 ppm CO2 had a diameter of 8.0 ( 1.6 nm, and an average wall number of 4 ( 1. In particular, both open-ended and triple-walled CNTs were clearly observed.

’ INTRODUCTION Carbon nanotubes (CNTs) have attracted interest due to their unique mechanical, electronic, and thermal properties.1,2 Aligned CNT arrays are composed of near-parallel CNTs that are selforiented, perpendicular to the substrate surface,3 and their ordered structure makes them promising candidate materials for use in field-emission devices,3 sensors,4 and membranes.5 Moreover, they are readily incorporated into these devices.6 Recently, highly efficient processes for CNT array growth have been examined.7,8 Chemical vapor deposition (CVD) is the most common method of preparing aligned CNT arrays. Various CVD techniques have been developed for the synthesis of wellaligned CNT arrays, including thermal CVD,3 plasma-enhanced CVD,9 and floating-catalyst CVD.10 In thermal and plasmaenhanced CVD, the catalyst is predeposited by physical vapor deposition, which remains difficult and expensive. In floatingcatalyst CVD, however, the catalyst is produced in situ through the simultaneous introduction of both the catalyst precursor and carbon source, leading to the continuous production of CNT r 2011 American Chemical Society

arrays.10 Thus, compared to other CVD techniques, floatingcatalyst CVD is attractive for the bulk-scale production of aligned CNTs due to its low cost, simplicity, and scalability. The properties of aligned CNT arrays, such as their conductivity, field-emission characteristics, and biocompatibility, depend strongly on the diameter and length of their component CNTs.11 13 The diameter and wall number of CNTs formed during CVD depend upon the morphology of the substratesupported catalyst and on the synthesis parameters and have been modified by adjusting the thickness of the metallic catalyst layer,14 by the application of chemical treatment,15 and by tuning the H2 pretreatment of the catalyst.16 In floating-catalyst CVD systems, the diameter of the CNTs can be modulated by changing the catalyst-precursor feeding rate.17,18

Received: April 13, 2011 Revised: June 20, 2011 Published: June 23, 2011 14093

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The Journal of Physical Chemistry C Several weak oxidizers, including H2O and CO2, have been used to enhance the growth of CNT arrays produced using CVD. For example, adding H2O selectively removes amorphous carbon without damaging the CNTs, and simultaneously promotes and preserves catalytic activity.7 The growth rate and morphology of a CNT array can be tuned using CO2, which etches away amorphous carbon and the outer walls of CNTs.19 Notably, CNT arrays that were modified by CO2 postsynthesis displayed enhanced field-emission properties due to the formation of defects on the CNT walls.20 Although adding CO2 to a CVD system enables the activity of a catalyst to be maintained, resulting in longer growth of CNT arrays, the effect of CO2 on CNT structure is not fully understood. Previous studies investigated the effect of CO2 concentration on the height of CNT arrays in a thermal CVD system,8,19,21 and the properties of CNT arrays that were treated in CO2 postsynthesis.20 Few reports have focused on the influence of the CO2 on the detailed CNT morphology, particularly under low CO2 concentrations in a floating-catalyst CVD system. In the current study, we have investigated the quality and morphology of CNT arrays synthesized in the presence of CO2 at various concentrations using a floating-ferrocene CVD method, where ferrocene was used as the catalyst precursor.

’ EXPERIMENTAL SECTION CNT array growth was conducted in a dual-zone furnace (OTF-1200  2-II, MTI) fitted with a quartz tube (inside diameter = 44 mm, length = 1000 mm). The flow rates of Ar, H2, C2H4, and CO2 were controlled by separate mass flow controllers (Alicat Scientific). Ferrocene (100 mg, g98%, Sigma Aldrich) was placed in a ceramic boat in the first zone, and the CNT arrays were formed in the second zone. A silicon wafer (5 mm  5 mm) with a 10-nm thick Al2O3 coating was used as a substrate for the CNT array. Under a flow of 475 sccm Ar and 25 sccm H2, the first zone was heated to 250 °C over 75 min to evaporate ferrocene while the second zone was heated to 750 °C. Ferrocene had completely evaporated from the first zone of the furnace by the time the second reached 750 °C. The zones were then maintained separately at these temperatures while a 400/ 140/115 sccm H2/Ar/C2H4 gas mixture was introduced to feed CNT growth. During this growth period, CO2 ([CO2] = 0, 760, 1500, 3000, or 7600 ppm) was introduced. After 30 min, the furnace was cooled to ambient temperature under 500 sccm Ar. The morphologies of the CNT arrays were measured using field-emission scanning electron microscopy (FESEM, Zeiss Ultra plus). The CNT diameters and wall numbers were analyzed from measurements of 50 nanotubes using high-resolution transmission electron microscopy (HRTEM, Philips CM120 Biofilter). Raman spectroscopy was performed using a Renishaw Raman with an Ar+ laser at 514 nm excitation. ’ RESULTS AND DISCUSSION A CNT array synthesized without CO2 grew to a height of ∼410 μm (Figure 1). In an atmosphere containing 760 ppm CO2, the minimum concentration that could be accurately controlled, CNT array growth was significantly promoted, producing an array 630 μm high. A further increase in CO2 concentration, to 1500 ppm, decreased the array height to 510 μm, and higher CO2 concentrations produced CNT arrays with a constant height of 480 μm. CNT arrays grown in the presence of relatively high CO2 concentrations (g3000 ppm) grew ca. 20% higher than

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Figure 1. CNT array height as a function of CO2 concentration during synthesis.

those without CO2, indicating that CO2 can be used to promote the growth of CNT arrays. Generally, CNT array growth terminates for two primary reasons: (a) the formation of amorphous carbon, from ethylene decomposition, coats the catalyst surface and deactivates it, and (b) Ostwald ripening of catalyst particles causes them to coalesce into larger, less-active particles.22 CO2 is a weak oxidant (E° = 0.52 V) that has been shown to selectively oxidize amorphous carbon. We postulate that CO2 removes the amorphous carbon coating from the catalyst, prolonging the catalyst lifetime and facilitating the growth of longer CNTs (higher yield). Additionally, a small amount of water is likely produced from the reaction between CO2 and H2 and may react with the catalyst support to produce hydroxyl groups that inhibit catalyst ripening, which would also extend CNT array growth.21 A side-view SEM image reveals that the CNT array grown with 760 ppm CO2 has a flat structure (Figure 2a), suggesting that the CNTs grew at uniform rates. High-magnification SEM images of the as-grown arrays were used to compare their morphologies. In all cases, a side view of the middle part of the array was examined. The CNT array synthesized without CO2 was composed of nonlinear, poorly aligned CNTs (Figure 2b). When CO2 was introduced as a promoter at low concentrations (760 3000 ppm), not only was the CNT array growth facilitated but alignment was improved (parts c e of Figure 2), though the latter effect was weak for CO2 concentrations above 1500 ppm. The production of wavy CNTs is induced by the uniform distribution of two types of catalysts, having different activities, on the substrate.23 This suggests that the wavy CNTs (Figure 2b) grown from catalysts derived from floating ferrocene were produced by catalyst particles with varying activity. In the floating-ferrocene system, it is possible for catalyst particles of various sizes to be produced as a result of the complex, autocatalytic decomposition of ferrocene. It is more likely, however, that large catalyst particles are formed when smaller particles agglomerate spontaneously under Ostwald ripening, leading to an inhomogeneous catalyst distribution and, consequently, relatively nonlinear CNT arrays. The production of hydroxyl groups on the substrate surface, caused indirectly by the presence of CO2, inhibits particulate agglomeration, thereby improving the catalyst distribution. Furthermore, the addition of CO2 at an appropriate concentration may improve the crystallinity of the CNTs during their growth process by cleaning the catalyst surface, and diminish the interference by removing the 14094

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Figure 2. Side-view SEM images of CNT arrays produced in the presence of CO2 at (a) 760 ppm and detailed structures at various concentrations: (b) no addition of CO2, (c) 760 ppm CO2, (d) 1500 ppm CO2, (e) 3000 ppm CO2, (f) 7600 ppm CO2.

Figure 3. (a) Raman spectra and (b) IG/ID of CNTs produced as a function of CO2 concentration.

amorphous carbon on the wall, thus facilitating straight, vertical growth of the CNTs. We propose that this causes well-aligned, vertically oriented CNT arrays to form in the presence of moderate concentrations of CO2. At higher CO2 concentrations the array alignment was only slightly improved (1500 or 3000 ppm CO2, parts d and e of Figure 2) or even worsened (7600 ppm CO2, Figure 2f). The aforementioned promotion of array height also diminished for CO2 concentrations over 1500 ppm (Figure 1). Similarly, Futaba et al. found that the enhancement of CNT array growth by CO2, H2O, and other oxidants was limited to low oxidant concentrations.8 At relatively high concentrations, CO2 may not only react with amorphous carbon but may also oxidize the Fe catalyst, deactivating it and disturbing the ethylene decomposition. The crystallinity of the CNTs was evaluated using Raman spectroscopy (Figure 3a). The Raman spectrum of a typical CNT sample contains a disordered carbon band (D band at ∼1327 cm 1), attributed to non-nanotube carbonaceous impurities (mainly amorphous carbon), and a graphitic carbon band (G band at ∼1580 cm 1), attributed to crystalline carbon. The intensity ratio IG/ID indicates the degree of crystallinity of a

CNT sample. Although a low concentration (760 ppm) of CO2 facilitated CNT growth, it did not significantly increase IG/ID (Figure 3b), suggesting that the amorphous carbon on the CNT walls was not significantly removed (Figure 4b). This was corroborated by TEM images, which show little difference in the amount of amorphous carbon on CNTs grown in the absence and presence of 760 ppm CO2 (parts a and b of Figure 4). When 1500 ppm of CO2 was introduced, IG/ID rose dramatically, indicating significant removal of the amorphous carbon covering the CNT walls. At higher CO2 concentrations (3000 and 7600 ppm), IG/ID increased only slightly, suggesting that the carbonaceous impurities on the CNT walls were almost completely removed in the presence of CO2 concentrations higher than 1500 ppm (Figure 4c). CNT outer diameters and wall numbers were calculated from TEM measurements (Figure 5). Both the outer diameters and wall numbers of the as-synthesized CNTs decreased with increasing CO2 concentration. A change in CO2 concentration from 0 to 7600 ppm resulted in a change to the outer diameters from 13.9 ( 2.9 to 8.0 ( 1.6 nm, respectively, and a change in the wall numbers from 9 ( 3 to 4 ( 1, respectively. We attribute this 14095

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Figure 4. Typical TEM images showing the amorphous carbon on CNTs grown (a) without CO2, (b) with 760 ppm CO2, (c and d) 7600 ppm CO2. (c) A typical triple-walled CNT and (d) an open-ended CNT prepared at 7600 ppm CO2.

Figure 5. CNT outer diameters and wall numbers as a function of CO2 concentration during synthesis.

trend to two main effects. The first is the suppression of catalyst aggregation by the CO2 during CNT growth as a result of inhibited catalyst particle agglomeration, as discussed earlier. This causes smaller catalyst particles to exist at higher CO2 concentrations, resulting in thinner CNTs.24 Moreover, Ostwald ripening causes inhomogenous catalyst particles to be formed as small particles agglomerate to form larger ones, leading to CNTs

with inhomogenous diameters. Thus when the Ostwald ripening was inhibited by the introduction of CO2, the catalyst particles and consequently the CNTs were more homogeneous. Therefore, CNTs grown in the presence of CO2 had less polydisperse diameters and lower wall numbers compared to the CNTs grown without CO2 (Figure 5). Second, CO2 can react with the CNT end caps and walls. CO2 was found to etch both CNT caps and walls when closed-ended CNTs were oxidized in CO2 at 850 °C for 5 h,25 and CNT walls can be etched by treatment with 6.8 36.8 mol % CO2 at 750 °C.19 Consistent with CNT walls being etched, we observed a few defects and the declining alignment of CNT arrays grown under high CO2 concentrations (Figure 2). Despite the mild conditions employed in the current study, open-ended CNTs (Figure 4d) were obtained by the in situ introduction of CO2 to the floating-ferrocene CVD during CNT growth. In particular, triple-walled CNTs were observed in arrays produced in the presence of 7600 ppm CO2. A typical TEM image of a triple-walled CNT is shown in Figure 4c.

’ CONCLUSIONS The growth of CNT arrays produced in a floating-ferrocene CVD system was promoted by the addition of CO2 to the Ar/ H2/C2H4 feed. We attribute this enhancement to the oxidation of amorphous carbon and the prevention of catalyst ripening, 14096

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The Journal of Physical Chemistry C both induced by the CO2. The vertical, straight growth of CNT arrays was also facilitated by CO2 due to the diminishing interference from amorphous carbon on the catalyst surface and CNT wall as a result of its oxidation by the CO2. CNT height was increased by up to 50% and reached up to 630 μm in 30 min, when synthesized in the presence of 760 ppm CO2. Arrays grown in the presence of low CO2 concentrations (760 3000 ppm) were well-aligned. Raman spectra reveal that, at relatively high CO2 concentrations (3000 ppm), CO2 removed carbonaceous impurities from the CNTs. Overall, CO2 addition is an effective approach to etching CNT walls, is able to modulate the wall numbers of the as-synthesized CNTs, and may represent a costeffective method to prepare aligned open-ended CNT arrays.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the Richard Claude Mankin postdoctoral scholarship and the Chinese Scholarship Council and the Australian Nuclear Science and Technology Organisation for postgraduate research support, Mr. V. Lo for his assistance with the HRTEM measurements, and Dr. T. L. Church for helpful discussions. ’ REFERENCES (1) Ajayan, P. M. Chem. Rev. 1999, 99, 1787. (2) Baughman, R. H.; Zakhidov, A. A.; De Heer, W. A. Science 2002, 297, 787. (3) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (4) Ding, D.; Chen, Z.; Rajaputra, S.; Singh, V. Sens. Actuators, B 2007, 124, 12. (5) Hinds, B. J.; Chopra, N.; Rantell, T.; Andrews, R.; Gavalas, V.; Bachas, L. G. Science 2004, 303, 62. (6) Constantopoulos, K. T.; Shearer, C. J.; Ellis, A. V.; Voelcker, N. H.; Shapter, J. G. Adv. Mater. 2010, 22, 557. (7) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362. (8) Futaba, D. N.; Goto, J.; Yasuda, S.; Yamada, T.; Yumura, M.; Hata, K. Adv. Mater. 2009, 21, 4811. (9) Chhowalla, M.; Teo, K. B. K.; Ducati, C.; Rupesinghe, N. L.; Amaratunga, G. A. J.; Ferrari, A. C.; Roy, D.; Robertson, J.; Milne, W. I. J. Appl. Phys. 2001, 90, 5308. (10) Andrews, R.; Jacques, D.; Rao, A. M.; Derbyshire, F.; Qian, D.; Fan, X.; Dickey, E. C.; Chen, J. Chem. Phys. Lett. 1999, 303, 467. (11) Li, H. J.; Lu, W. G.; Li, J. J.; Bai, X. D.; Gu, C. Z. Phys. Rev. Lett. 2005, 95, 086601. (12) Chhowalla, M.; Ducati, C.; Rupesinghe, N. L.; Teo, K. B. K.; Amaratunga, G. A. J. Appl. Phys. Lett. 2001, 79, 2079. (13) Correa-Duarte, M. A.; Wagner, N.; Rojas-Chapana, J.; Morsczeck, C.; Thie, M.; Giersig, M. Nano Lett. 2004, 4, 2233. (14) Zhao, B.; Futaba, D. N.; Yasuda, S.; Akoshima, M.; Yamada, T.; Hata, K. ACS Nano 2009, 3, 108. (15) Cantoro, M.; Hofmann, S.; Pisana, S.; Scardaci, V.; Parvez, A.; Ducati, C.; Ferrari, A. C.; Blackburn, A. M.; Wang, K. Y.; Robertson, J. Nano Lett. 2006, 6, 1107. (16) Nessim, G. D.; Hart, A. J.; Kim, J. S.; Acquaviva, D.; Oh, J.; Morgan, C. D.; Seita, M.; Leib, J. S.; Thompson, C. V. Nano Lett. 2008, 8, 3587. (17) Zhang, Q.; Huang, J. Q.; Zhao, M. Q.; Qian, W. Z.; Wei, F. Appl. Phys. A: Mater. Sci. Process 2009, 94, 853. 14097

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