Chirality-enriched semiconducting carbon nanotubes synthesized on high surface area MgO-supported catalyst

Materials Letters 65 (2011) 1878–1881

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Chirality-enriched semiconducting carbon nanotubes synthesized on high surface area MgO-supported catalyst Yang Xu a,⁎, Enkeleda Dervishi a, Alexandru R. Biris b, Alexandru S. Biris a,⁎ a b

Nanotechnology Center, University of Arkansas at Little Rock, AR 72204, United States National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj Napoca, RO-3400, Romania

a r t i c l e

i n f o

Article history: Received 9 December 2010 Accepted 12 March 2011 Available online 17 March 2011 Keywords: Carbon nanotubes Chirality CCVD

a b s t r a c t Single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution were synthesized by radio frequency-Catalytic Chemical Vapor Deposition (RF-CCVD) through the pyrolysis of CH4. Fe–Co bimetallic catalytic nanoclusters were supported on high-surface area MgO nanopowders and used in the nanotube synthesis process. Nanolog absorption fluorescence analysis was used to characterize the chiralities of the asproduced SWCNTs over this nanostructural catalyst. In the final SWCNT sample, the (7,5) semiconducting carbon nanotube species were found to be dominant, with a low chirality variation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Single-walled carbon nanotubes (SWCNTs) have shown unique electronic, spectroscopic, and optical properties because of their variable structures [1–3]. One of the biggest challenges is the synthesis of SWCNTs with narrow diameter distribution and specific chiralities. There are two major approaches in order to obtain nanotubes with such properties. One is by controlled synthesis of the SWCNTs [4,5], and the other is by separation of the as-synthesized or purified SWCNTs to obtain the species that are desired [6,7]. Several methods have been developed and reported to obtain SWCNTs with specific chiralities or diameters, but the limitations include the need to wrap nanotubes with polymeric chains (difficult to remove during the postsynthesis process) and the possibility that impurities may be introduced during the separation process [8]. Consequently, direct selective synthesis of SWCNTs with specific chirality is still a challenge. One of the most advanced approaches that were reported to generate nanotubes of high quality is the HiPCO process [9]. It generates large numbers of distinct SWCNT structures, which grow on unsupported iron catalyst clusters formed in situ by phase decomposition of iron pentacarbonyl in a continuous gas-phase process by using CO as a carbon source. The reported optimal synthesis conditions are of 1200 °C and 10 atm, respectively [9]. Another process that was reported to generate SWCNTs of highly controlled chirality and quality uses the CoMoCAT catalyst, which results in the formation of mostly (6,5) and (7,5) nanotube enriched species [4]. The synthesis process involves a silica-supported catalyst with bimetallic

⁎ Corresponding authors. Tel.: + 1 501 682 5166, + 1 501 683 7458. E-mail addresses: [email protected] (Y. Xu), [email protected] (A.S. Biris). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.040

catalytically active metallic clusters prepared from cobalt nitrate and ammonium heptamolybdate precursors. Generally, silica is less stable than MgO when exposed to high temperatures [10]. Additionally, the supported SiO2 material requires removal by HF, which is highly corrosive and has strong toxic effects with fast and deep tissue penetration on contact. Recently, the most intensively studied catalysts are those supported on porous MgO [11,12]. The porous support can produce a controlled number of metal particles with uniform dimensions due to their high surface area and porosity, which also could be reflected in a narrow diameter distribution synthesis of SWCNTs. Such approaches could lead to high specificity towards the desired nanotube type; furthermore, with a simple one-step HCl washing, most of the catalyst system can be removed, resulting in nanotubes of high quality and purity without structural defects. The Radio Frequency (RF)-CCVD method has previously been employed for the catalytic growth of carbon nanotubes [13,14]. The utilization of inductive heating in the synthesis of carbon nanotubes by CCVD can significantly reduce the energy consumption, enhance the crystallinity of carbon nanotubes, and reduce the forming of amorphous carbon [12–16]. In this study, high surface area nanopowder of MgO was used as the support to synthesize SWCNTs with narrow chirality distribution, and the (7,5) semiconducting SWCNT was the main resulting nanotube type. 2. Experimental The Fe–Co/MgO catalyst (molar ratio of Fe:Co:MgO was 2:1:50) was prepared by wet impregnation using the following procedure: Fe (NO3)3·9H2O and Co (NO3)2·6H2O were dissolved in ethanol mixed with the MgO powder (NanoScale Corporation, BET average surface

Y. Xu et al. / Materials Letters 65 (2011) 1878–1881

3. Results and discussions The contour plot in Fig. 1 (upper panel) shows the main peaks, which are labeled with the corresponding (n,m) indices. Clearly, only a few chiralities dominate the semiconducting nanotube distribution in the SA600 sample. A comparison with the fluorescence data obtained from a standard HiPCO sample (Fig. 1, bottom panel), indicates that the SA600 product exhibits a slightly narrower structure distribution. The SA600 product is composed of a high percentage of (7,5) nanotubes with the total relative fluorescence intensity of 38% (Table 1). CoMoCAT nanotubes have been reported to contain (7,5) and (6,5) nanotube species with an almost identical fractional intensity (~28%) from the total samples [4]. Table 1 lists the relative fluorescence intensity for the major structures detected in the SA600 sample and the corresponding ones in the CoMoCAT and HiPCO nanotube samples. Besides the (7,5) sample, two other species – (8,4) (25%) and (6,5) (14%), respectively – were found to be predominant in our SA600 carbon nanotube product. In total, these three species represent about 77% of all of the semiconducting tubes and 52% of all of the SWCNTs composing the SA600 sample (if approximately 2/3 of all the SWCNTs are considered semiconducting and 1/3 metallic [19]). Fig. 2 (left) shows the possible structures of our semiconducting SA600 SWCNTs with the chiral numbers (n,m) as formed based on the rolling of a graphene sheet map to form a cylinder. However, there are various other ways to roll the graphene layer, each way corresponding

1

720

Excitation Wavelength (nm)

700

0.85

(7,5)

680

0.70

660

0.55

640 0.41 620 0.26

600

0.11

580

(8,4)

560 540

0.039

(6,5)

900

1000

0 1100

1200

1300

1400

Emission Wavelength (nm)

Excitation Wavelength(nm)

area is around 600 m2/g− 1, particle size: 5 nm). SWCNTs were synthesized over the Fe–Co/MgO catalyst using the RF-CCVD method [12] and methane as the carbon sources. The RF inductive heating was performed using a RF generator operated at radio frequency of 365 kHz. 1 g of catalyst was placed in a graphite boat, and the boat was inserted into a quartz reactor (supporting information Fig. S1.). The final catalyst was placed in a graphite boat and annealed at 700 °C for 1 h in a N2 with a flow rate of 400 mL/min. All of the synthesis reactions were performed at 800–900 °C and a methane flow rate of 80 mL/min, while keeping the nitrogen flow constant. After 30 min of reaction time, the methane was turned off, and the resulting product was allowed to cool in nitrogen flow. To eliminate the MgO from this mixture, the solid product was suspended in a concentrated HCl solution and stirred overnight. The suspension was then filtered through a PTFE 0.2 μm membrane and washed to neutral pH with deionized water. The final nanotube product was named SA600. Next, the solid product was added to an aqueous solution containing the surfactant sodium cholate (SC, 1 wt.%) and dispersed by sonication for 1 h. The suspension was centrifuged for 2 h at 15,000 g using a high revolution centrifuge (Galaxy 16 Microcentrifuge, VWR International). The top solution was found to be stable and formed of individually dispersed SWCNTs that were further used for spectroscopy analysis. Fluorescence analysis of isolated SWCNTs offers a reliable technique to determine the chirality distribution of semiconducting SWCNTs [17,18]. In our work, a Spectrofluorometer, (Nanolog purchased from HORIBA Jobin Yvon, double-grating excitation monochromator, imaging emission spectrograph with a selectable grating turret, and multichannel liquid-N2-cooled InGaAs-array detector) was used to analyze the diameter and chirality distributions of SA600 and HiPCO nanotubes. The optical absorption spectra at UV– Vis-NIR range were recorded in 1 mm path-length quartz cells using a Shimadzu double-beam spectrophotometer UV-3600 with three detectors. X-ray diffraction technique was used for phase identification, quantitative phase. The X-ray power diffraction profiles of the catalyst system were recorded on a Bruker AXS D8 advanced diffractometer. The monochromatic Cu Kα radiation line and general area detector diffraction system were used as excitation source and detector, respectively. The experiments were carried out in Bragg– Brentano geometry. Quantitative analysis was performed with whole pattern fitting and Rietveld refinement using EVA software.

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700

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680

0.70

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640

0.41

620 0.26 600 0.11

580 560

0.039

540

0

900

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1100

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1300

1400

Emission Wavelength (nm) Fig. 1. Contour plots of normalized fluorescence intensities for the SA600 SWCNT sample (upper frame) and the HiPco sample (bottom frame).

to a particular tube diameter and helicity [20]. A helical vector represented by a pair of integers (n,m) has also been defined. The different colors of (n,m) gave the relative fluorescence intensity of each chiral angle SWCNT. The average diameter of SWCNTs in our sample is about 0.79 nm which is very close to CoMoCAT's 0.81 nm [4], but it is far smaller than HiPco's 0.93 nm [9]. The minimum capped tube diameter that has been reported is 0.71 nm [21]. A preferential chiral angle ranging from 15–27° for the semiconducting SWCNT structures corresponds to mostly armchair-types, a fact also confirmed in Fig. 2 (right). The UV–Vis-NIR optical absorbance spectra (Fig. 3) of our SA600 SWCNTs show the dominant peaks belonging to the first van Hove E11 range (900–1400 nm) corresponding to SWCNTs in the d–1 nm

Table 1 Relative fluorescence intensities of (n,m) segments in SA600, CoMoCAT, and HiPco. Chiral integer values (n,m)

Diameter (nm)

Chiral angle (deg)

Segmental intensity (%) SA600

CoMoCAT [4]

HiPco

(7,5) (8,4) (6,5) (8,3) (7,6) (9,2) (6,4) (8,6)

0.829 0.84 0.757 0.782 0.895 0.806 0.692 0.966

24.504 19.107 26.996 15.295 27.457 9.826 23.413 25.285

38 25 14 8 8 1.8 0.5 0.5

28 14 28 11 8.5 1.7 2.8 0.8

4.9 4.2 3.7 2.9 7.1 0.4 0.3 8.3

1880

Y. Xu et al. / Materials Letters 65 (2011) 1878–1881

a1

1.2

a2

(n,0) zigzag SWNT diameter (nm)

9,2 8,3 8,4

6,4

1.0

6,5 7,5

"9,5" "11,1" "9,2"

0.8

"8,4" "8,3"

"8,7" "8,6" "7,6" "7,5" "6,5" "6,4"

0.6 0.4 0.2

7,6 8,6 0.0

(n,n) armchair

0

5

10

15

20

25

30

35

Helical angle (deg) Fig. 2. (n,m)-intensity map (left), the thickness and color of each hexagonal cell in the graphene sheet is proportional to the observed intensity for that structure (red, brown, green, purple, and yellow demonstrated high to low intensity) and the diameter-chirality distribution (right) for the SA600 sample.

region. The spectra indicate that SA600 is composed of tubes with smaller diameters than those of the HiPCO SWNTs. The first van Hove transitions (S11) of the two dominant peaks 1030 nm and 1122 nm in the spectrum of SA600CNT can be assigned to specific semiconducting carbon nanotubes from the (7,5) and (8,4) chiral families [22]. Different dispersion processes, such as using sodium cholate (SC) surfactant, will induce a red shift compared to the sodium dodecyl sulfate (SDS)-based one. Among these species, (7,5) chiral SWCNT dominated when the reaction temperature reached 900 °C. Decreasing the temperature to 800 °C, there were more species formed, such as (9,2), due to the possible morphological temperature-driven changes in the catalyst morphology and structure [12]. Also, the dominant nanotube species shifted from (7,5) to (8,4) in the final product (Supporting information Fig. S2.). At a higher reaction temperature (900 °C), the Fe–Co catalytic species disperse uniformly over the high surface area of the MgO support in smaller metallic nanoclusters [12], and as a result, nanotubes with a small variation in diameter will be generated. In the CoMoCAT SWCNT synthesis process, SiO2 was found to [26] sinter when the temperature reached over 850 °C and induced lower SWCNT synthesis yields. On the contrary, MgO has a much better thermal stability even at temperatures as high as 1100 °C. Further increasing the reaction temperature to1350 °C, it was found that the surface properties of MgO started to change, surface area decreases, “neck growth” between particles and densification start to be observed [10] resulting in multiple-chiralities CNT species. Before the SWCNT growth, bimetal phase CoFe2O4 clusters were found uniformly formed and dispersed over the MgO surface during the high temperature calcination processes [23] as indicated by the SA600 HiPCO (7,5)

Abs

0.3

(8,4)

0.2

0.1

XRD analysis (Supporting information Fig.S3). Based upon the Rietveld refinement of the XRD data, the percentage of the crystalline phase CoFe2O4 component in the catalyst was 1.5%. The high surface area MgO shows high dispersion ability for the catalytic nanoparticles. The index peaks can be assigned to (220), (400), (422), (511), and (440) planes of a cubic structure CoFe2O4, which match well with the standard data of CoFe2O4 (JCPDS no. 79–1744) [24]. The bimetallic Fe/Co alloys are responsible for a more efficient nanotube growth process, due to a more energetically intense dissociation of the CH4 molecules and more favorable atomic interaction between the carbon atoms and the transitional metal atoms [21]. On the other hand, transition metals play a catalytic role at the atomic level [25], and previous research results have indicated that the bimetallic catalysts, compared to single metal ones, are more prone to produce narrower diameter distribution CNTs [25]. The nucleation bimetallic sites will form and anchor over the high surface area support nanomaterials and allow the carbon nanotube formation. The low dimensionality of the MgO support particles, are believed to hinder the migration of the metallic clusters and their coalescence at high temperatures resulting in uniform sized Fe–Co catalytic nanoparticles with excellent impact on the overall diameters of the resulting SWCNTs. Therefore, the nano-MgO's high surface area provides a better environment to protect the metallic nanoparticle from sintering [26]. Consequently, narrow diameter and chirality carbon nanotubes will start to form over catalysts supported on nanosized MgO particles [27]. In conclusion, high surface area of the support materials and the dimensional and compositional tuning of the catalytic nanoparticles are all critical parameters for the precise control of the SWCNT growth. However, the roles played by the metallic nanoparticle and support for certain chirality growth is still under investigation. The clearly highlighted different distributions between the SA600 and other nanotube species synthesized over various inorganic supports and/or reaction conditions indicate that the (n,m) chirality variation is a result of differences in the growth kinetics [26]. Further work is still required to fully understand the mechanism of growth of nanotubes species with narrow distribution in their chiral morphologies.

4. Conclusion 800

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Wavelength (nm) Fig. 3. UV–Vis-NIR spectra of SA600 and HiPco in sodium cholate (SC) 1 wt.% suspensions.

In summary, we synthesized a narrow chiral angle distribution of SWCNTs by using the RF-CCVD method and a catalytic system supported over nanoscale MgO particles. The semiconducting (7,5) was found to be dominant among the SWCNT species.

Y. Xu et al. / Materials Letters 65 (2011) 1878–1881

Acknowledgment This research was partially supported by the DOE (Grant No. DE-FG 36–06 GO 86072). Also financial support from the Arkansas Science and Technology Authority (ASTA) grant # 08-CAT-03 is greatly appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.matlet.2011.03.040. References [1] Dresselhaus MS, Dresselhaus G, Eklund PC. Academic press. CA: San Diego; 1996. [2] Wang C, Zhang J, Ryu K, Badmaev A, Zhou C. Nano Lett 2009;9:4285–92. [3] Mahmood M, Karmakar A, Fejleh A, Mocan T, Iancu C, Mocan L, et al. Nanomedicine 2009;4(8):883–93. [4] Bachilo SM, Balzano L, Herrera JE, Pompeo F, Resasco DE, Weisman RB. J Am Chem Soc 2003;125:11186–7. [5] Li X, Tu X, Zaric S, Welsher K, Seo W, Zhao W, et al. J Am Chem Soc 2007;129: 15770–1. [6] Zheng M, Jagota A, Semke ED, Diner BA, Mclean RS, Lustig SR, et al. Nat Mater 2003;2:338–42. [7] Tu X, Manohar S, Jagota A, Zheng M. Nature 2009;460:250–3. [8] Xu Y, Pehrsson PE, Chen L, Zhao W. J Am Chem Soc 2008;130(31):10054–5.

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