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J. Phys. Chem. C 2008, 112, 19908–19915
Fabrication, Purification, and Characterization of Double-Wall Carbon Nanotubes via Pulsed Arc Discharge Hiromichi Yoshida, Toshiki Sugai, and Hisanori Shinohara* Department of Chemistry and Institute for AdVanced Research, Nagoya UniVersity, Nagoya 464-8602, Japan ReceiVed: July 23, 2008; ReVised Manuscript ReceiVed: September 13, 2008
Fabrication of high-purity double-wall carbon nanotubes (DWNTs) with purity more than 95% has been achieved by developing synthetic and purification methods. High-temperature pulsed arc discharge (HTPAD) is used to synthesize DWNTs with controlled combinations on catalysts of transition metals (Ni/Co/Fe) and rare earth metals (Y/La). The results show that both rare earth metals and cobalt concentrations are crucial to enhance production of DWNTs and to reduce that of residual single-wall carbon nanotubes (SWNTs). The as-produced DWNTs are purified by establishing a dispersion-assisted oxidation procedure. To optimize purification processes, stabilities against oxidation in terms of the number of layers (DWNTs and SWNTs) and of the diameters are investigated. The oxidation rate of DWNTs is twice as low as that of SWNTs with the same diameter of around of 1.6 nm at a suitable temperature. As the results of these optimizations, DWNTs with a purity of 95% have been obtained from as-produced 10% DWNT samples. To achieve this high purification with minimum loss of DWNTs, hybrid oxidation with hot air and refluxing in hydrogen peroxide have been also developed. 1. Introduction Structural control of carbon nanotubes (CNTs) is a key technology for nanoscience and nanotechnology as well as practical applications. In particular, the control of the number of layers is one of the most important techniques, which can provide the possibility to explore novel properties of CNTs originating from layer interaction. For example, double-wall carbon nanotubes (DWNTs),1-7 which have the thinnest CNTs with the layer interaction, exhibit ambipolar properties as channels of field effect transistors (FET).8 Furthermore, structural and electrical properties of DWNTs are studied increasingly with both experimental9-13 and theoretical14-17 approaches. DWNTs have been produced by the conventional arc discharge6,18 using sulfur and hydrogen, but they have never been produced by laser ablation.19 These results suggest that the number of layers is controlled by the vaporization procedures. Catalyst effects for production of DWNTs have been also reported with various chemical vapor deposition (CVD) methods in terms of Co and Fe concentration.2,3,20,21 For the control of the numbers of layers in CNTs, we have developed the hightemperature pulsed arc discharge (HTPAD)22 method. HTPAD can produce thin and high-graphitization single-wall carbon nanotubes (SWNTs) like laser ablation. Moreover, HTPAD can produce DWNTs using only Ni/Y as the catalyst.1 The DWNTs synthesized have diameters of 1.6-2.0 nm,1 which are still one of the most structurally uniform DWNTs with high graphitization.23-25 In spite of these advantages, HTPAD can only achieve a concentration of 10% of DWNTs among SWNTs and DWNTs,1 which is much lower than the best record of 80%.18,21 Purifying the DWNTs is, thus, the key subject for HTPAD as well as for any other production methods.1 The purification method of DWNTs has been developed by the present group utilizing the higher chemical durability of DWNTs.1 This method has been applied to various DWNTs * Corresponding author. Phone: +81-52-789-2482. Fax: +81-52-7476442. E-mail:
[email protected].
Figure 1. Apparatus of high-temperature pulsed arc discharge.
from CVD, thereby proving its universality and usefulness.4,20 The chemical durability of the DWNTs from CVD has been determined to be several hundred times as durable against oxidation as SWNTs.4 In these successful cases, the contaminant SWNTs are much narrower than the DWNTs and they are not bundles but quite dispersed. Since narrower SWNTs are reported to be unstable26 compared to thicker ones, we need to make a fair comparison between DWNTs and SWNTs having the same diameter to deduce the true effect of the layer interaction. In addition, DWNTs have been purified by heating in air;1,4 however, that hot air has been known to oxidize metallic CNTs selectively.27 By utilizing other oxidizers, we may eliminate contaminated SWNTs in much milder condition and thus get pure DWNTs much effectively. The bundle effect of CNTs, on the other hand, should also play a crucial role in the purification of DWNTs, suggesting that SWNTs in a thick bundle could be burned slower than DWNTs in a thin bundle or isolated DWNTs. In this case, purification must be performed in the dispersed conditions. To clarify these four key factors for pure DWNTs, catalyst optimization, the diameter, the bundle, and the oxidizer effects, here we present optimization of catalysts together with the development of new purification methods that utilize a disper-
10.1021/jp806529v CCC: $40.75 2008 American Chemical Society Published on Web 11/18/2008
Pulsed Arc Discharge Synthesis of DWNTs
Figure 2. Purification scheme of the dispersion technique. Each step corresponds to (a) as-produced CNTs, (b) dispersed CNTs with surfactant, (c) mixed CNTs with fumed silica, (d) CNTs without surfactant by oxidation, (e) DWNTs without SWNTs by oxidation, and (f) pure DWNTs fumed silica by rinsing with NaOH, respectively.
sion technique to ensure pure DWNTs with purity greater than 95%. We also report that the durability of DWNTs against oxidation is still twice as high as that of SWNTs with the same diameter (1.6 nm) under optimum conditions that reveal the real effect of the layer interaction. 2. Experimental Section 2.1. Effect of Metal Catalysts on HTPAD Synthesis of DWNTs. DWNTs together with SWNTs were produced by HTPAD with handmade metal composite graphite electrodes. The schematic of the apparatus is shown in Figure 1. The cathode was made from a mixture of graphite (99.99%, 23 µm, Toyotanso Co. Ltd.), nickel (3-7 µm, 99.9%, Nilaco), cobalt (10 µm, 99.9%, Nilaco), yttrium (40 mesh, 99.9%, Nilaco), and lanthanum oxide (99.9%, WAKO Chemical). The composition ratios were Ni/Co/Y ) x/(4 - x)/1 atom % (x ) 0-4) and Ni/M ) 4/y atom % (M ) Y, La; y ) 0-7.5). These composite electrodes were typically made from a mixture of 1.5 g of these metals/graphite, which were mixed for half a day, and it was molded into a rectangular rod with a size of 4 × 6 × 40 mm3 at a pressure of 160 MPa. This composite rod was attached to a Mo electrode and used as a cathode. Different from steady arc discharge, the cathode is consumed by the pulsed arc discharge.28 A graphite electrode was used as an anode. These electrodes were inserted into a quartz tube (φ, 28 mm), through which Ar buffer gas was passed at a flow rate of 500 sccm (1 atm). The electrodes were located at the center of the furnace, where the temperature was controlled at 1250 °C. The cathode was vaporized by pulsed arc discharges at 600 µs, 50 A, and 50 Hz generated by a handmade HV power supply. The sublimed vapors were converted into SWNTs and DWNTs, and they were collected on a water-cooled trap. 2.2. Purification with Dispersion Technique. The purification scheme is shown in Figure 2. The as-produced soot was heated at 360 °C to remove amorphous carbon (cf., Figure 2a). The CNTs were dispersed into 100 mL of 1% of sodium dodecyl sulfate (SDS) solution (Figure 2b). The solution was centrifuged at 4800g and decanted to remove the residue of metals and nanocapsules. The supernatant, which was the solution of CNTs wrapped with SDS, was mixed with 2 g of fumed silica (particle size, 7 nm; Aldrich). The fumed silica was used as a guard against bundling of CNTs in both liquid and solid phases even after losing the surfactant through the oxidation. This mixture was stirred and dried to powder (Figure 2c). The powder was heated at 360 °C, and the SDS was simultaneously decomposed.
J. Phys. Chem. C, Vol. 112, No. 50, 2008 19909
Figure 3. Typical TEM image of as-produced soot. Diameters of marked outer and inner DWNTs are 1.8 and 1.1 nm and that of SWNTs is 1.6 nm, respectively.
The powder was then rinsed with acetone and hydrochloric acid. Catalyst metals and some decomposed SDS were removed at this stage (Figure 2d). The powder was then dried again by heating and refluxed in hydrogen peroxide for a half-day. Some of the SWNTs, especially defective SWNTs, were removed at this stage. To evaluate the effect of dispersion, CNTs were also purified without the dispersion technique shown in Figure 2b-d. These CNTs were rinsed with hydrochloric acid to remove metals and heated at 500 °C (cf., section 3.3). The powder was divided into several patches for the investigation. To estimate the stability of DWNTs, these patches were oxidized in air at three different temperatures for four different time periods. The diameter distribution of each patch was estimated by transmission electron microscopy (TEM) observation. The stability of DWNTs can be estimated by the changing of the DWNTs/(SWNTs + DWNTs) ratio with diameters of 1.6 to ∼1.7 nm. Most of the SWNTs were removed at this heat treatment (Figure 2e) (cf., section 3.4). Finally, purified DWNTs were obtained by removing fumed silica by rinsing them with sodium hydroxide water solution (Figure 2f). To clarify the dependence of the oxidizer on purification of DWNTs, CNTs were also purified by one-step oxidation, i.e., refluxing in hydrogen peroxide or heating in air (cf., section 3.5). DWNTs produced with Ni/Y catalyst (4/1 and 4/0.5 atom %) were used to obtain high-purity DWNTs and to analyze the layer interaction, respectively. DWNTs produced with less Y catalyst are suitable for the studies of the interaction since Y has a significant catalytic effect in the oxidation processes (cf., section 3.2). The products were characterized by TEM (JEOL JEM-2100F) and Raman spectroscopy with excitation at 632.8 nm (Horiba Jobin Yvon HR-800) to evaluate the yield of DWNTs. The concentrations of DWNTs with margins of error were estimated by TEM counting of individual SWNTs and DWNTs with normal distribution analyses. The effect of catalyst metals on oxidation and the ratio of CNTs among the total weight of asproduced soot were analyzed by thermogravimetric analysis (TGA) (Shimadzu TA-60). 3. Results and Discussion 3.1. Effect of Transition Metal Catalysts on DWNT Synthesis. Figure 3 shows a typical TEM image of the as-produced DWNTs. The distributions of the diameters of the outer DWNTs and the inner DWNTs were determined to be 1.8 ( 0.2 nm
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Yoshida et al.
Figure 4. Catalyst dependence of the ratio of the DWNTs (bar graph) and the quantity of the DWNTs (line graph). The ratios are evaluated by TEM observation. The quantity of the DWNTs is estimated by TEM images and the corresponding weight loss of TGA. Details are described in the text.
and 1.0 ( 0.2 nm, respectively, whereas those of SWNTs were determined to be 1.5 ( 0.4 nm. SWNTs with the diameter less than 1.1 nm were not observed in as-produced CNTs. The dependence of the synthesis of the DWNTs on the Ni/Co ratio together with Y (1 atom %) is shown in Figure 4. The ratio of the DWNTs in CNTs estimated by TEM increases as the Co concentration increases. On the other hand, the total quantity of DWNTs decreased slightly as the concentration of Co increased because of the rapid decrease in the production of as-produced soot. The quantity of DWNTs was obtained by multiplying the weight of the soot, the ratio of the DWNTs among CNTs, and the concentration of CNTs in the soot. The last factor, i.e., the concentration of the CNTs (10-15 wt %), was determined by TGA and was found to be independent of the Ni/Co ratio. The catalytic dependence of the mean diameters of the DWNTs and SWNTs was also examined by Raman spectroscopy. Figure 5 shows the Raman spectra (radial breathing mode (RBM) region) of these different catalyst concentrations. Lightblue and light-red areas of Figure 5 correspond to RBM of inner and outer DWNTs, respectively, which are determined by TEM observations. When the Co concentration increases, the peak intensity ratio between that in the light-red area around 138 cm-1 and that in a colorless area around 165 cm-1 increased, which is consistent with the results in Figure 4. Together with the DWNTs concentration, the peak at 148 cm-1 became more prominent than that at 165 cm-1 with the increase of the Co concentration in Figure 5, where both peaks are arrowed. These peaks both in the colorless SWNTs area show that the mean diameters of SWNTs also increased. This correlation between the increase in mean diameter caused by the Co and the increase in DWNTs concentration was confirmed by the TEM observation. This Co effect in diameter has been also observed in the laser ablation method.29,30 The result shows that DWNTs tend to be produced under the condition where thick CNTs are produced, which suggests that the precursors of the thicker SWNTs could be converted into those of DWNTs. The precursor model is also supported by the presence of a threshold diameter for the DWNTs and the temperature dependence of DWNT production as reported previously by the present group.1 3.2. Effect of Rare Earth Metal Catalysts on Synthesis and Oxidation of DWNTs. Figure 6 shows the relative yield of the DWNTs of samples synthesized with pure Ni, Ni/Y, and Ni/La. The yields were estimated by considering the RBM intensity, as described in the previous section. The results show that La shows catalytic effect in the synthesis of DWNTs.
Figure 5. Catalyst dependence of RBM. Light-blue and light-red areas correspond to inner and outer layers of DWNTs, respectively. Intensities are normalized by the power of the exciting laser. The noncolored area corresponds to SWNTs, where some peaks are arrowed for analyses. Details are described in the text. Graduations at the top show diameters calculated by the formula d (nm) ) 248/ω (cm-1).
Figure 6. Y/La dependence of DWNT ratio defined as DWNTs/ (DWNTs + SWNTs) estimated by Raman spectra. Blue and red lines correspond to the concentration of Y and La, respectively. An open square and an open diamond at 5 atom % of La and 10 atom % of Y with dashed lines represent that no RBM peaks or no DWNTs and SWNTs were observed. The data were calibrated by the value of Ni/Y ) 4/1 atom %.
Production quantity of La was smaller than that of Y, which was evaluated to be 0.01 mg/h and was 4 times smaller than that of Y. This low efficiency can be explained by the actual lower catalytic effect of La, but this La was introduced into the composite rod not as metal like Y but as oxide. Since the catalytic effects strongly depend on these introduction forms,18 we have not concluded that the yield difference comes from the actual catalytic effects. The concentrations and yields of the DWNTs were almost proportional to the concentrations of the lanthanide catalysts in a low-concentration regions, which had lanthanide concentrations of less than 2.5 atom %. In contrast, the presence of lanthanide catalysts in a concentration of >5% suppressed the growth of DWNTs and caused an increase in the yield of amorphous carbon.31 In the absence of the lanthanide catalysts, only thin SWNTs were produced. Y was reported to produce thick CNTs for arc discharge.32 The relationship between the ratio of DWNTs and the concentration of Y also suggests that thick SWNTs tend to convert into DWNTs as discussed in section 3.1. We also found that the rates of production and the consumption of electrodes without lanthanide were an order of magnitude less than those of electrodes with
Pulsed Arc Discharge Synthesis of DWNTs
Figure 7. Catalyst dependence on oxidation of as-produced soot. Each peak in ascending order of oxidation temperature corresponds to burning of amorphous carbon, SWNTs, and DWNTs, respectively.
Y, suggesting that the presence of Y results in easier sublimation of the graphite electrode and increase in the production of CNTs. A similar catalytic effect was reported for the sublimation of electrodes during the conventional (steady) arc discharge.32 These catalytic metals play significant roles in purification as well as in high-yield synthesis. Figure 7 shows the dependence of the Y catalyst on the differential TGA curve of the as-produced soot. The peaks of the curve, which are in ascending order of temperature, correspond to the burning of amorphous carbon, SWNTs, and DWNTs, respectively. It is observed that the burning temperatures decreased with an increase in the concentration of Y. It was previously reported that the burning (oxidation) temperatures of SWNTs and amorphous carbon reduce significantly when the concentration of the catalyst metal increases.33 The results in this study show that DWNTs also have the same catalytic effect. In order to investigate the interaction between the inner and outer layer of the DWNTs due to oxidation, the concentration of yttrium was chosen to be 0.5 atom % since at this concentration the effect of metals is reduced and the interaction between the layers is enhanced. The high concentration of yttrium resulted in a high relative ratio of DWNTs against SWNTs. A yttrium concentration of 1 atom % is considered as optimum to obtain highly purified DWNTs. 3.3. Effect of New Dispersion Technique on Purification. Parts a-d of Figure 8 show low-magnification TEM images of the as-produced soot (10% ( 3%), DWNTs purified by the conventional method (90% ( 5%), intermediate DWNTs (cf., Figure 2b) without particles by the dispersion method (10% ( 5%), and final DWNTs (cf., Figure 2f) with purity of 95% ( 3% and without particles (cf., section 3.5), respectively. Impurities due to metal catalysts and amorphous carbons, which are from nearly 90% of the weight of the as-produced soot, can also be observed in Figure 8a. The ratio of DWNTs increased up to 90% ( 5% with the conventional hot air oxidation method, as shown in Figure 8b. However, the sample had many hollow and metal-filled nanocapsules even after the acid treatment (cf., Figure 8b). These nanocapsules produced from the metal particles cannot be removed by any oxidation method because of their thicker graphitic layers than those of SWNTs and DWNTs. In addition to the contamination, the final yield of DWNTs of this purification method was found to be quite low (
