Langmuir 2007, 23, 6453-6458
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Carbon Nanotubes Contain Residual Metal Catalyst Nanoparticles even after Washing with Nitric Acid at Elevated Temperature Because These Metal Nanoparticles Are Sheathed by Several Graphene Sheets Martin Pumera* ICYS, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, Japan ReceiVed January 11, 2007. In Final Form: March 12, 2007 It is demonstrated that multiwalled (MWCNT) and single-walled (SWCNT) carbon nanotube materials contain residual metal impurities (Fe, Ni, Co, Mo) even after prolonged periods of “washing” with concentrated nitric acid at temperature of 80 °C. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) reveals that this is because such metal impurities are intercalated in the nanotube channel (in the case of MWCNT) or in the “bamboo” segment of the nanotube (in the case of “bamboo”-like MWCNT), or they create graphene sheet protected metal core/shell nanoparticles (in the case of SWCNT). TEM/energy-dispersive X-ray spectroscopy (TEM/EDS) elucidate that residual metal impurities presented in “washed” carbon nanotube materials are in some cases in the form of metal alloys or that there can be several different pure metal nanoparticles presented in one CNT material. It is shown by thermogravimetric analysis that “washing” with concentrated nitric acid removes up to 88% (w/w) of metal catalyst nanoparticles from as-received carbon nanotubes and that such removal has in some cases a significant effect on the electrochemical reduction of hydrogen peroxide.
1. Introduction
* Phone: +81-29-851-3354-8816. Fax: +81-29-860-4706. E-mail:
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of catalyst nanoparticles where it forms nanotubes. The metal catalyst particle remains at the base of the nanotube, on its tip, or it is incorporated within the nanotube.14 It is clear that, when such raw material is used, the electrocatalytic effect might originate from CNT itself or from catalyst nanoparticles. At the beginning of CNT electrochemistry research, carbon nanotube raw materials were simply treated in strong mineral acid (so-called “washing” procedure), and for a long time, it had been believed that the metal catalyst impurities were dissolved and removed from CNT materials (see, i.e., refs 15-20; however, most of the articles published in CNT-electrochemistry field could be cited here). Later, it was noted by Lawrence et al. using energy dispersive X-ray spectroscopy (EDS) that, even after the “washing” procedure, the CNT contain iron and copper impurities.21 This issue was investigated in much greater detail by Compton’s group, which evaluated the purity of MWCNT before and after the “washing” step using a scanning electron microscope (SEM) equipped with wavelength-dispersive X-ray spectroscopy (WDS) and independently by X-ray photoelectron spectroscopy (XPS).22 The authors demonstrated that even after treating CNT materials with strong mineral acid there is some residual amount of metal (namely iron) in the form of catalyst metal nanoparticles in the CNT sample22,23 and that that these metal nanoparticles are responsible for “electrocatalysis” of hydrazine22 and hydrogen peroxide23 on MWCNT-modified electrodes. The authors sug-
(1) Iijima, S. Nature (London) 1991, 354, 56. (2) Oberlin, A.; Endo, M.; Koyama, T. J. Cryst. Growth 1976, 32, 335. (3) Iijima, S.; Ichihashi, T. Nature (London) 1993, 363, 603. (4) Bethune, D. S.; Kiang, C. H.; De Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature (London) 1993, 363, 605. (5) Ajayan, P. M. Chem. ReV. 1999, 99, 1787. (6) Wang, J. Electroanalysis 2005, 17, 7. (7) Gooding, J. J. Electrochim. Acta 2005, 50, 3049. (8) Pumera, M.; Sa´nchez, S.; Ichinose, I.; Tang, J. Sens. Actuators, B, in press, doi: 10.1016/j.snb.2006.11.016. (9) Merkoc¸ i, A.; Pumera, M.; Llopis, X.; Perez, B.; del Valle, M.; Alegret, S. Trends Anal. Chem. 2005, 24, 826. (10) Banks, C. E.; Compton, R. G. Analyst 2006, 131, 15. (11) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 829. (12) Harris, P. J. F. Carbon 2007, 45, 229. (13) Takagi, D.; Homma, Y.; Hibino, H.; Suzuki, S.; Kobayashi, Y. Nano Lett. 2006, 6, 2642.
(14) Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; DuninBorkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. Nano Lett. 2007, 7, 602. (15) Musameh, M.; Wang, J.; Merkoci, A.; Lin, Y. Electrochem. Commun. 2002, 4, 743. (16) Valentini, F.; Amine, A.; Orlanducci, S.; Terranova, M. L.; Palleschi, G. Anal. Chem. 2003, 75, 5413. (17) Gong, K.; Dong, Y.; Xiong, S.; Chen, Y.; Mao, L. Biosens. Bioelectron. 2004, 20, 253. (18) Lawrence, N. S.; Deo, R. P.; Wang, J. Anal. Chim. Acta 2004, 517, 131. (19) Salimi, A.; Compton, R. G.; Hallaj, R. Anal. Biochem. 2004, 333, 49. (20) Pumera, M.; Merkoci, A.; Alegret, S. Sens. Actuators, B 2006, 113, 617. (21) Lawrence, N. S.; Deo, R. P.; Wang, J. Electroanalysis 2005, 17, 65. (22) Banks, C. E.; Crossley, A.; Salter, C.; Wilkins, S. J.; Compton, R. G. Angew. Chem., Int. Ed. 2006, 45, 2533. (23) Sˇ ljukic´, B.; Banks, C. E.; Compton, R. G. Nano Lett. 2006, 6, 1556.
Since the discovery of multiwalled carbon nanotubes (MWCNT)1,2 and their single-walled (SWCNT) counterparts,3,4 these nanoscale materials have attracted vast interest because of their unique chemical, mechanical, and electronic properties.5 Their electrochemical properties were intensively studied in the recent years,6-11 and it has been suggested that carbon nanotube materials-based electrodes can lead to lowering of overpotential (so-called “electrocatalytic effect”) of many electroactive substances; however, its magnitude is not always coherent across the literature. This is because researchers actually do not fully characterize their carbon nanotube materials, and therefore, they do have full control and understanding of their electrochemical properties. The main source of problems with carbon nanotube materials is their synthesis. CNT are usually grown by chemical vapor deposition (CVD) or arch-evaporation synthesis where metal catalyst nanoparticles are used (most common is nickel, cobalt, iron, or their combination).12,13 The actual mechanism of CVD growth and arch-evaporation is still under discussion, but it is suggested that the metal catalyst dissolves carbon from the carboncontaining gas and the carbon is later transported to the edges
10.1021/la070088v CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007
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gested that the metal impurities are likely trapped in graphitelike planes along the tube axis, or at the ends of the nanotubes, or contained within the carbon nanotubes; however, they did not provide any proof of how the nanoparticles are protected,22 even when they used transmission electron microscopy.24 The aim of this article is to show the proof that carbon nanotubes contain metal catalyst nanoparticles even after a prolonged period of “washing” in concentrated nitric acid at elevated temperature, because they are sheathed by several graphene sheets, either inside the nanotubes (in the case of MWCNT) or creating coreshell M@C nanoparticles (in the case of SWCNT). This manuscript also evaluates the effect of the “washing” procedure at elevated temperature on removal of the metal catalyst nanoparticles from CNT materials by thermogravimetric analysis and the effect of removal of metal catalyst nanoparticles on the electrochemical response of hydrogen peroxide on carbon nanotube-based electrodes. 2. Experimental Section 2.1. Apparatus. JEM 2100F field emission transmission electron microscope (JEOL, Tokyo, Japan) working at 200 kV was used to acquire TEM and HR-TEM images in a scanning TEM mode (spot size, 0.4 nm). TEM/EDS spectra were collected using the abovedescribed JEM 2100F equipped with an energy-dispersive X-ray spectrometer with an ultrathin window (JEOL, Tokyo, Japan). Thermogravimetric analysis was carried out using Exstar TG/DTA 6200 equipment (SII NanoTechnology, Japan) by placing the sample in the furnace with air atmosphere, heating to 120 °C with heating rate of 10 °C min-1, holding at 120 °C for 30 min to remove any moisture, and then again heating to 1000 °C with heating rate of 10 °C min-1. All voltammetric experiments were performed using an electrochemical analyzer µAutolabIII (Ecochemie, Utrecht, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems v 4.9 software (Ecochemie). Electrochemical experiments were carried out in a 5 mL voltammetric cell at room temperature (25 °C), using three-electrode configuration. A platinum electrode served as an auxiliary electrode and a Ag/AgCl as a reference electrode. All electrochemical potential in this paper are stated vs Ag/AgCl. 2.2. Materials. Multiwalled carbon nanotubes (MWCNT) were received from Sigma-Aldrich, Japan (MWCNT-A, o.d. 10-30 nm, i.d. 5-10 nm, length 0.5-50 µm; MWCNT-B, o.d. 3-10 nm, i.d. 1-3 nm, length 0.1-10 µm) and from NTP Nanotech Port, China (MWCNT-C, o.d. 10-20 nm; MWCNT-D, o.d.
