Magnetic separation of Fe catalyst from single-walled carbon nanotubes in an aqueous surfactant solution

Transcription

1 Carbon 43 (2005) Magnetic separation of Fe catalyst from single-walled carbon nanotubes in an aqueous surfactant solution J.G. Wiltshire a, L.J. Li a, A.N. Khlobystov b, C.J. Padbury a, G.A.D. Briggs b, R.J. Nicholas a, * a Clarendon Laboratory, Department of Physics, Oxford University, Parks Road, Oxford, OX1 3PU, UK b Department of Materials, Oxford University, Parks Road, Oxford, OX1 3PH, UK Received 24 September 2004; accepted 7 December 2004 Abstract We report an efficient technique to separate ferromagnetic catalyst particles from an aqueous surfactant solution of single-walled carbon nanotubes (SWNTs) by the use of a 1.3 T permanent magnet. High resolution transmission electron microscopy (HRTEM) demonstrates that SWNTs are coated with a surfactant layer that stabilises the aqueous dispersions of SWNTs. The residual quantities of Fe catalyst (3%) can be effectively removed from a colloid solution of SWNTs in a magnetic field while absorbance spectra of the initial and purified solutions show that the nanotube diameter distribution remains unchanged. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Catalyst; Magnetic properties 1. Introduction * Corresponding author. Tel.: address: r.nicholas@physics.oxford.ac.uk (R.J. Nicholas). Current methods to synthesize single walled carbon nanotubes require a catalyst material, typically Fe(CO) 5 in the case of high pressure CO conversion (HiPCO) [1] or Y Ni mixtures for arc-discharge production [2]. However this metal catalyst becomes a contaminant in the final product along with amorphous carbon and graphitic shells. These contaminants hinder our ability to measure and control the properties of SWNTs, in particular the magnetic properties of bulk nanotube material are affected by the presence of impurities such as residual Fe catalyst. During the past decade, techniques to purify nanotubes have been developed, such as thermal [3,4] or chemical [5] oxidation which remove amorphous carbon and break open graphitic shells, followed by acid washing to dissolve the metal catalyst. Unfortunately both phases of this treatment are damaging, either introducing sidewall defects or shortening the nanotubes [6]. Moreover, diminishing returns means that repeated purification leads to ever reducing gains and increased loss of the actual SWNTs. A novel approach to removing catalyst containing graphitic shells was reported more recently [7], where the shells were opened and the catalyst particles were extracted by utilising their magnetic properties. This method used N,N-dimethyl formamide (DMF) as the solvent and introduced insoluble inorganic nano-powders such as ZrO 2 to act as a mechanical agent during ultrasonication. Unfortunately DMF has been shown to be very damaging to SWNTs [5] and the acidic treatment to dissolve the powder may introduce further defects. In this work we present a magnetic process for the removal of catalyst particles from raw nanotubes using magnetic field gradients that cause no additional damage and which should be relatively easily scalable /$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 1152 J.G. Wiltshire et al. / Carbon 43 (2005) Experimental The initial SWNT product was purchased from Carbon Nanotechnologies. It was synthesized by the HiP- CO method and consists of tubes with a broad diameter distribution of nm as determined by Raman spectroscopy [6]. This material has already undergone purification by the vendor and is quoted as being 92 94% SWNTs, with the remainder being approximately 3% residual Fe catalyst and 3 5% residual amorphous carbon 1. An aqueous suspension was prepared using 50 mg of the purchased nanotubes. These were dispersed and stirred in 100 ml of D 2 O with 1 wt% sodium dodecyl sulfate (SDS) surfactant. The dispersion was then treated by ultrasonication at a power of 250 W for 30 min. The colloidal solution formed was ultra-centrifuged at rpm for 4 h in order to separate individual nanotubes from nanotube bundles. An enriched solution of individual SWNTs was then collected from the upper supernatant. These colloidal solutions have been reported widely in other studies [9,11] and are extremely stable. For magnetic separation 1 ml of the aqueous surfactant solution was placed in a 5 mm diameter glass tube and positioned such that the top of the tube lay between the poles of a 1.3 T permanent magnet. After a few hours deposits of black material were observed accumulating adjacent to both poles of the magnet and further build up was discernable over a period of 3 days. After 4 days enough of the D 2 O had evaporated to leave the accumulated material in situ at the top of the glass tube, from where it could be collected for characterisation by high resolution transmission electron microscopy (HRTEM, JEOL JEM-4000EX, LaB 6, information limit <0.12 nm). In order to increase the yield and purification efficiency the system was modified so that the tube could be tracked through the poles of the magnet in a method similar to the idea of zone refining. The solution was scanned through the magnetic field region from the base of the tube at a rate of 10 mm per day, as shown schematically in Fig. 1. This ensured that the whole volume of solution had been passed through the field centre. The whole process was repeated several times to increase the total volume of treated solution. The untreated and treated colloidal solutions were then studied by UV vis-nir absorption, using a Perkin Elmer Lambda 9 Spectrophotometer. Both the solutions and the contaminant material were also studied by HRTEM. Non-surfactant based dispersions for HRTEM analysis were prepared by dispersing mg of a dry SWNT sample in 2 ml of MeOH and ultrasonicating 1 As determined by thermal gravimetric analysis (TGA) by Carbon Nanotechnologies. Fig. 1. Schematic diagram illustrating magnetic purification. (a) The initial setup of the experiment and (b) The final position ready for collecting the material at the poles of the magnet and the remaining solution. this for 15 min. This dispersion was then deposited onto copper TEM grids coated with a carbon film (Agar). When examining the aqueous surfactant solutions this was directly dropped onto a TEM grid without ultrasonication or the further use of solvents. 3. Results and discussion The basic observation is that a dark deposit accumulates in the region of highest magnetic field, opposite the pole pieces of the magnet. This suggests that the magnetic field gradient attracts paramagnetic impurities such as Fe or Ni and may offer a route to further purification of carbon nanotube material. In order to test this assertion we studied the solutions and the starting material by HRTEM and optical absorption HRTEM We first studied the raw HiPCO nanotubes (Fig. 2a and b). These images demonstrate the high SWNT purity, with the nanotubes formed into long (>10 lm) tightly bound bundles. The presence of catalyst appears to be low and consistent with the purity claimed by the manufacturer. In Fig. 2b a spherical catalyst particle can be seen, these are typically 2 10 nm in diameter and are spread uniformly on the surface of the nanotube bundles and do not form clusters. In most cases the SWNT sidewalls can be clearly seen to be free of amorphous materials. HRTEM characterisation of SWNT dispersions from D 2 O-SDS before magnetic treatment shows that the ultrasonication and ultracentrifugation procedure effectively breaks up the mats of nanotubes and increases the concentration of individual nanotubes and small bundles. In the dry state the SDS matrix appears to form web like structures with the nanotubes presumably pro-

3 J.G. Wiltshire et al. / Carbon 43 (2005) Fig. 2. HRTEM micrographs of as-received HiPCO nanotubes. Fig. 3. HRTEM micrographs of SWNT-SDS dispersions deposited from D 2 O. (a) Surfactant coated nanotubes laid across a hole in the carbon support grid. (b) Two particles of catalyst indicated by arrows. viding the backbone of support (Fig. 3a). Some residual nanotube bundles can be seen containing spherical particles of catalyst on their surface (Fig. 3b). The catalyst concentration in SWNT-D 2 O-SDS dispersions is low and comparable with the content of the catalyst in the as-received SWNTs (Fig. 2). High magnification imaging of isolated nanotubes suspended across the TEM grid reveals that the nanotube sidewalls are coated with amorphous material unevenly distributed along the nanotube surface (Fig. 4a and b). Such amorphous coating is not observed before the treatment with D 2 O-SDS, and therefore it can be attributed to the surfactant molecules adsorbed on SWNTs sidewalls. The thickness of the coating layer varies in the rage of 1 10 nm. Interestingly in a dry state the layer of surfactant seems to be thicker on one side of the nanotube than on the other. Richard et al. [8] have demonstrated that SDS and other surfactants form regular arrays of micelles on the surface of multi-walled carbon nanotubes (MWNTs). This requires a spacing of approximately 5 nm between neighbouring micelles [8]. MWNTs of large diameters can accommodate such uniform periodic structures on their surface. However, SWNTs with diameters of only nm have too small a surface area and too large a curvature to allow for the formation of ordered micelles. Instead the surfactant molecules are disordered and randomly distributed around the nanotubes. Fig. 5a and b show TEM images of the contaminant collected at the poles of the magnet. Fig. 5a shows a large area of the sample in which heavily contaminated nanotube ropes were found. A magnification of one of these contaminated areas is shown in Fig. 5b. The spherical particles of catalyst can often be found aggregated in large clusters incorporating a few nanotube bundles. The concentration of metallic impurities in the material magnetically separated from aqueous surfactant solution is many times greater than in the solution before exposure to the magnetic field. This demonstrates that particles of the ferromagnetic catalyst can be effectively removed from SWNTs in the magnetic field using the aqueous surfactant solutions. It was not feasible to use HRTEM to quantify the level of the residual Fe in the treated solutions due to their already high purity. Accurate absolute measurements of the impurity content

4 1154 J.G. Wiltshire et al. / Carbon 43 (2005) would require TGA (thermal gravimetric analysis) which is planned for the future once sufficiently large volumes of purified material have been produced Absorbance spectra Optical absorption was studied in order to determine whether the magnetic purification technique had any effect on the distribution of nanotube sizes in the purified Fig. 4. HRTEM micrographs showing the distribution of SDS surfactant on the surface of SWNTs, arrows indicate the sidewalls of nanotubes. Fig. 5. HRTEM micrographs of material separated in magnetic field from SWNT-SDS solution. Fig. 6. Absorbance spectra of (a) the E 11 band gap transition and (b) the E 22 band gap transition for (i) the initial solution, (ii) solution (i) after being stored for 18 days, (iii) solution (i) after being open to the atmosphere for 3 h, (iv) magnetically treated solution.

5 J.G. Wiltshire et al. / Carbon 43 (2005) product. Fig. 6 shows absorbance for the E 11 and E 22 modes of the nanotubes in solution at various stages of the procedure, (i) is the initial solution where the series of absorption peaks from 7000 to cm 1 corresponds to the individual diameter species. These are sharper than those observed for non-isolated nanotube samples [6], indicating that the solution indeed contains well isolated nanotubes [9]. The E 22 region shows a similar series of absorption peaks which also measure the size distribution. (ii) shows the same sample as in (i) but after storage for 18 days before being measured. The sharper features of the E 11 mode have broadened, which is attributed to oxygen absorption onto the nanotube sidewalls. (iii) is the same solution as measured in (i) after having been left open to the atmosphere for 3 h, the development of peaks at 6000 cm 1 and 7100 cm 1 are attributed to the absorption of H 2 O from the atmosphere into the solution [10]. Fig. 6 demonstrates that measurements of the E 11 mode are highly sensitive to the solution history (Fig. 6a) while the E 22 is largely insensitive (Fig. 6b), making it more reliable for the comparison of solutions before and after the magnetic field treatment. Spectrum (iv) was recorded for the magnetically treated solution. The large absorption below 7000 cm 1 is due to the presence of absorbed H 2 O due to the prolonged exposure to the atmosphere. By contrast the E 22 mode shows almost no changes in the absorbance spectra when compared to the untreated solutions. In particular the distribution of sharp E 22 peaks seems unchanged, showing that the purification process does not alter the nanotube diameter distribution. 4. Conclusions Recent developments in the production of aqueous surfactant solutions have opened the way for further progress in the production of well isolated SWNT systems, however we have shown that they also open new routes for the purification of the nanotubes. We have demonstrated that aqueous dispersions of SWNTs stabilised by surfactant can be effectively used for removing magnetic catalyst in the magnetic field. This route is not open in the solid state and not as easily achieved in the liquid state when using inorganic solvents. This technique is simple and non-destructive which will allow purification of large quantities of high quality SWNTs required for many applications. We have also performed high resolution TEM imaging of SWNT-SDS structures revealing for the first time a non-uniform distribution of the surfactant on a nanotube, however this was only observed in a dry state. The structure of the surfactant layer coating SWNTs differs significantly from the surfactant adsorbed on MWNTs which can be attributed to the smaller diameter of SWNTs. Absorbance spectra of the aqueous solution demonstrated that the magnetic purification does not alter the diameter distribution present in the solution. Acknowledgments Parts of this work were funded by EPSRC. ANK thanks DTI, EPSRC (GR/R66029/01) and Hitachi Europe Ltd for funding. We would also like to thank the Swire Group for providing a scholarship for L.J. Li. References [1] Nikolaev P, Bronikowski MJ, Bradley RK, Rohmund F, Colbert DT, Smith KA, et al. Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide. Chem Phys Lett 1999;91: [2] Journet C, Maser WK, Bernier P, Loiseau A, Chapelle ML, Lefrant S. Large-scale production of single-walled carbon nanotubes by the electric-arc technique. Nature 1997;388: [3] Moon JM, An KH, Lee YH, Park YS, Bae DJ, Park GS. 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