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J. Phys. Chem. C 2009, 113, 3612–3616
Removal of Ferromagnetic Metals for the Large-Scale Purification of Single-Walled Carbon Nanotubes Chuxin Wu, Jiaxin Li, Guofa Dong, and Lunhui Guan* State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, YangQiao West Road 155#, Fuzhou, Fujian 350002, P.R. China ReceiVed: NoVember 19, 2008; ReVised Manuscript ReceiVed: December 26, 2008
An effective method has been developed to remove ferromagnetic metals for the large scale purification of single-walled carbon nanotubes (SWNTs) produced by arc-discharge, which are generally regarded as the most challenging soot to remove metallic catalyst residues. The efficiency of the method has been evaluated by electron microcopy, magnetic and thermogravimetric analysis. As a result, at least 200 mg SWNTs with 99% purity were obtained from 3.6 g raw soot. The metal percentage dropped from ∼24.0% to ∼1.0% and the magnetic metal percentage in the purified sample was as low as 0.2 wt %. Nearly all ferromagnetic metals were eliminated from the process. 1. Introduction
2. Experimental Section
Single-walled carbon nanotubes (SWNTs) have emerged as fascinating materials for their superior characteristics and promising wide applications.1-4 As-synthesized SWNTs prepared by current methods such as chemical vapor deposition (CVD),5 laser-ablation,6 or arc-discharge7 contain large amount of impurities, typically amorphous carbon, graphite nanoparticles and catalyst metals (Fe, Co, or Ni). The presence of impurities (especially ferromagnetic metal nanoparticles) has hampered the accurate characterization of the intrinsic bulk properties of SWNTs, such as thermal, magnetic and electrical properties.8,9 Purification is a bottleneck issue in the use of SWNTs, so various methods have been employed to purify SWNTs, for instance gas-phase oxidation, wet-chemical and heat treatment, centrifugation, and microfiltration.10-14 However, those reported methods were not effective to remove catalyst particles encapsulated inside multishelled graphite particles. To circumvent this problem, some researchers synthesized SWNTs by using nonferromagnetic metals.15,16 However, the existence of the catalyst particles still hampered the accurate measurements of the thermal and electrical properties of SWNTs. Recently researchers specially designed magnetic filtration to completely remove ferromagnetic metals.17-19 The ferromagnetic content was reduced by nearly 2 orders of magnitude. It was very attractive; meanwhile, the experimental setups were relatively complex, time-consuming, and can only deal with SWNTs on test tube scale (mg/h). So it is emergent to explore an easy and effective way to obtain a large quantity of SWNTs with ultrahigh purity. Here we report a simple method that nearly completely removed ferromagnetic metals from the raw arc-discharged SWNTs, which are generally regarded as the most challenging soot to remove metallic catalyst residue. Our method, which combined only air oxidation and chemical treatments, can easily deal with SWNT soot on large scale (over 3 g one time). After purification, the metal percentage dropped from ∼24.0% to ∼1.0% and the magnetic metal percentage in the purified sample was as low as 0.2 wt %.
Raw SWNT soot was produced by arc-discharge based on previous reports.20 In brief, the anode was a graphite rod (Φ6 × 150 mm) in which a hole (Φ4) was drilled and filled with a powder mixture of YNi2 alloy, graphite and FeS with weight ratio of 17.2:10:1. An arc was created by a current of 80 A. The gap between two electrodes was kept about 5 mm. Typical time for consuming an anode rod was decreased to 8 min. About 1.5-2.0 g of cotton-like soot and cloth-like soot were collected from the cathode and the chamber wall. The as-produced soot was ground and dried at 80 °C in air. A multiprocess was adopted to purify the SWNT soot. About 3.6 g of raw soot was heated in air at 300 °C for 1 h to remove a small part of the amorphous carbon. The remaining sample (about 2.7 g) was dispersed in 300 mL 15% H2O2 solution with the help of ultrasonic. The solution was refluxed for 3 h. When the suspension cooled, a black deposit appeared. After150 mL of the upper solution was decanted; 150 mL of 37% HCl solution was further added into the suspension followed by refluxing for 3 h. The products were denoted as HCl-treated SWNTs. The sample was filtered with a 0.8 µm microfilm, washed with deionized water, and directly annealed in air ranging from 400 to 450 °C for 1 h and then from 450 to 470 °C for 0.5 h. The so-obtained samples were denoted as thermaltreated SWNTs. The sample was refluxed in 2.6 M HNO3 for 6 h, filtered with 0.8 µm microfilm and washed with a large amount of deionized water. The microfilm with SWNTs was dried under IR lamp. Upon drying, the black deposit on the film was self-rolled into a SWNT buckypaper, which were denoted as purified SWNTs. Finally, we obtained ∼200 mg SWNTs with ultrahigh purity. In order to recover the structure of the purified SWNTs, the obtained sample was annealed at 1200 °C in vacuum for 10 h. A variety of methods were used to monitor the effect of the purification. Magnetometry which probes the permanent moment associated with residual magnetic catalyst particles was performed in a Quantum Design physical property measurement system (PPMS). The morphology and purity of the samples were characterized by tramsmission electron microscope (TEM, JEOL-2010) and scanning electron microscope (SEM, JSM6700F) equipped with electronic dispersive X-ray spectroscopy (EDX).
* To whom correspondence should be addressed. Telephone: 86-59183792835. Fax: 86-591-83792835. E-mail:
[email protected].
10.1021/jp810163u CCC: $40.75 2009 American Chemical Society Published on Web 02/11/2009
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Figure 1. (a) SEM image of the raw soot, (b) TEM image of the raw soot, (c) SEM image of the purified sample, and (d) TEM image of the purified sample.
The thermogravimetric analysis (TGA) measured the total metal content with a heating rate of 10 °C /min in air (NETZSCH STA-449C). Raman spectroscopy (Renishaw 2000, excited at 514.5 nm) was employed to check the structural changes from the raw soot to the purified SWNTs. 3. Results and Discussion Previous report indicated that in the raw soot produced by arc-discharge, the cotton-like soot contained much fewer multishelled graphite nanoparticle compared with the cloth-like soot, thus was much easier to be purified.20 To evaluate the efficiency of our method, we tentatively chose the cloth-like soot with relative bad quality (containing ∼10% SWNTs) as starting materials. Figure 1a shows a typical SEM image of the raw soot. Similar with the previous reports, the raw soot contained a lot of impurities, including metal particles ranging from 5 to 100 nm and a large amount of carbonaceous particles. A TEM image (figure 1b) indicated that some Ni particles were surrounded by amorphous carbon, while some Ni particles were encapsulated into the multishelled graphite nanoparticles. The SWNT content in the raw soot was estimated to be around 10 wt %. The purification removed nearly all impurities, while it did not change SWNTs to a great extent. Figure 1c and Figure 1d show SEM and TEM images of the purified product, which were of ultrahigh purity. No visible impurities, including other forms of carbon materials and metal particles, were detected. Figure 2 shows mass-normalized magnetic hysteresis loops of the raw soot and the purified samples measured by PPMS at
Figure 2. Mass-normalized magnetic moments of the raw soot (10.85 mg, black line) and the purified sample (20.88 mg, red line). The samples were measured under the magnetic field of 60000 Oe, shown here the maximal field was 10000 Oe. The data are obtained by subtracting the linear diamagnetic response from the SWNTs. The inset shows the saturated permanent moment of the remaining ferromagnetic particles in the purified sample.
300 K. The raw root was dominated by permanent moment contribution, the measured saturation magnetic moment (Ms) was 6.21 emu/g. The purification dramatically reduced the moment to 0.12 emu/g, which was rather smaller than that of the purified HiPco (around 2 emu/g) SWNTs with 98 wt % purity21 and also the sample (0.422 emu/g)19 purified by recent magnetophoretic method. Assuming that the ferromagnetic component arisen from Ni is of a saturation moment of 50 emu/g,
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Figure 3. TGA curves of (a) the raw soot, (b) the purified sample annealed in 1200 °C, and the EDX of their TGA residues.
a rough estimation of the ferromagnetic metal content was about 0.2%, accorded with ∼0.2% checked by TGA and EDX. Figure 3 shows TGA results of the raw soot and the purified sample annealed in 1200 °C, together with EDX results of the TGA residues. The solid lines show the weight loss whereas the dot lines show its derivative curves as a function of temperature from 30 to 1000 °C. The SWNTs and the carbonaceous impurities were completely burned off near 1000 °C and the residual materials were metal oxide. In the raw soot, as shown in Figure 3a, the initial burning temperature is 340 °C and there are two peaks at 400 and 520 °C, respectively. The first peak is attributed to the burning of amorphous carbon,22 as amorphous carbon can be more easily burned due to the faster oxidation rates.23 The second peak is assigned to the burning of the mixture of SWNTs and nanocrystalline graphite particles that are more stable and consequently burned at higher temperature. The derivative curves of the purified sample after annealing in 1200 °C (Figure 3b) show a single peak at 760 °C which is assigned to the thermal oxidative destruction of SWNTs. The initial burning temperature and rapid oxidation temperature all drastically increase due to much fewer metal catalytic particles which can catalyze the oxidation of the carbonaceous materials including amorphous carbon, SWNTs and nanocrystalline graphite particles, as demonstrated by Chiang et al. We confirm the removal of the nanocrystalline graphite particles and the metallic particles impurities, which is consistent with the results checked by TEM and SEM. We also monitored the TGA residues by EDX, assuming that Fe, Ni and Y were oxidized to Fe2O3, NiO and Y2O3. For the raw soot, EDS shows that the residue after heating in air to 1000 °C has a (Ni + Fe):Y atomic ratios of ∼8:1. It is surprising that the purified sample contains much lower (Ni + Fe):Y ratios of ∼1:4. The results implies that the ferromagnetic metal particles were selectively removed by the purification process, they only account for 0.2% of the purified SWNTs. Our method was a gentle way for purifying SWNTs, thus did not introduce prominent defects on SWNTs. Raman spectroscopy was employed to check the changes from the raw soot to the purified SWNTs. (Figure 4). The representative peaks of SWNTs are tangential band (G band), the disorder induced band (D band) and the radial breath mode (RBM) which is
Figure 4. Raman spectra of the raw soot and the purified SWNTs. Inserted are the HR-TEM images of the purified SWNTs with perfect tubular structures.
inversely proportional to the diameters of SWNTs.23 The peak intensity ratio of the G band to the D-band (G/D value) strongly reflects the purity and quality of SWNTs. The G/D value increased from 5.6 for the raw soot to 43.6 for the purified SWNTs, indicating that the purity of the SWNT was increased remarkably by removing amorphous carbon and metal particles. The high G/D value also indicated that the purification did not introduce many prominent defects on SWNTs, as also demonstrated by the inserted HR-TEM images of the purified SWNTs. The RBM appears at around 169 cm-1, indicating that the mean diameter of SWNTs is around 1.4 nm, accorded with HR-TEM observations. The purification combined heat treatment and acid washing, which was similar with conventional approach but much more efficient. We monitored the products of each step to reveal the reason. Figure 5a shows a TEM image of the sample after refluxing in H2O2 and HCl. Although most impurities existed, especially the nanocrystalline graphite with metal particles, the naked metal particles and a small quantity of amorphous carbon were removed. The corresponding HR-TEM image (Figure 5b) indicated that refluxing in H2O2 open gaps (indicated by arrows) on the multishelled graphite particles. We checked more than
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Figure 5. TEM and HRTEM images of (a) the HCl-refluxed products, (b) a typical multishelled graphite nanoparticles encapsulating Ni particle after HCl-refulxing, the arrows indicate gaps introduced by refluxing, (c) the thermal-treated products, (d) an example of sintering of metal catalyst particles due to the thermal treatment.
20 graphite particles, all of them had gaps on their surface. It was a very important procedure, which paved the way for removing the graphite particles by next process (thermal treatment). The problem of purifying SWNTs mainly resulted from the subtle reactivity between SWNTs and the graphite particles. However, if there were gaps on the graphite particles, the metal particles, which contacted with air during the thermal treatment, catalyzed the oxidation of the graphite particles, and then decreased their burning temperature. After the thermal treatment (from 400 to 450 °C for 1 h and then from 450 to 470 °C for 0.5 h), all the amorphous carbon and graphite particles were eliminated. Figure 5c shows the dramatic improvement of the SWNTs-to-impurity ratio after the thermal treatment. The HR-TEM image (Figure 5d) indicated that the crystal structure of the graphite particles encapsulating metal nanoparticles eventually collapsed. The regular spacing of the lattice plane is 0.24 nm, corresponding to the separation of (111) planes of cubic NiO. Most images of the nanoparticles after oxidizing in air exhibited these general features. We concluded that during oxidizing in air, the carbon coating on the metal particles was primarily destroyed. The metal (Ni) was converted to metal oxide, which were removed by the final step (refluxing in dilute HNO3). On the contrary, the structure of the SWNTs retained even after refluxing in HNO3 for 6 h. It should be noted that the burning temperature of SWNTs and graphitic nanoparticles was only slightly different. For comparison, the HCltreated sample was likewise oxidized in air at 480 °C for 0.5 h or at 450 °C for 1 h. In the sample oxidized at 480 °C, both nanocrystalline graphitic particles and SWNTs were terribly
damaged. Instead, the sample oxidized at 450 °C still contained many nanocrystalline graphitic particles with metal catalytic particles. 4. Conclusions We present an efficient way to purify SWNTs from the arcdischarge raw soot to ultrahigh purity on large scale. As a result, at least 200 mg SWNTs with 99 wt % purity was obtained from 3.6 g raw soot with bad quality. Taking it into account that the SWNTs only accounted for ∼10% of the raw soot, the yield of SWNTs in our purification was ∼56% (200 mg/ 360 mg). TEM, SEM, TGA and magnetic measurements demonstrated that the metal percentage dropped from ∼24.0% to ∼1.0% and the magnetic metal content in the purified sample was as low as 0.2 wt %. Nearly all ferromagnetic metal impurities were successfully eliminated from the process. The purification of SWNTs based on our methods can be easily scaled up, thus paved a way to the mass-production of SWNTs with ultrahigh purity. We thank Mr. Y. Li and Prof. R. Chen at Fujian Normal University for helping Raman measurement. Acknowledgment. Financial support for this study was provided by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences and the National Key Project on Basic Research (grant no. 2009CB939801).The authors thank Mrs. L. Zhou and Mr. F. Bao for helping with the SEM and TEM.
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