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Journal of Nanomaterials
Volume 2014, Article ID 585983, 9 pages
http://dx.doi.org/10.1155/2014/585983
Research Article

Effects of Two Purification Pretreatments on Electroless Copper Coating over Single-Walled Carbon Nanotubes

1Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, China
2School of Science, Hubei University of Technology, Wuhan 430068, China
3Capital Aerospace Machinery Corporation, Beijing 100076, China

Received 11 October 2014; Revised 14 December 2014; Accepted 14 December 2014; Published 29 December 2014

Academic Editor: Renyun Zhang

Copyright © 2014 Zhong Zheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

To achieve the reinforcement of copper matrix composite by single-walled carbon nanotubes, a three-step-refluxing purification of carbon nanotubes sample with HNO3-NaOH-HCl was proposed and demonstrated. A previously reported purification process using an electromagnetic stirring with H2O2/HCl mixture was also repeated. Then, the purified carbon nanotubes were coated with copper by the same electroless plating process. At the end, the effects of the method on carbon nanotubes themselves and on copper coating were determined by transmission electron microscope spectroscopy, scanning electron microscope spectroscopy, X-ray diffractometry, thermogravimetric analysis, Fourier transformed infrared spectroscopy, and energy dispersive spectrometry. It was clearly confirmed that both of the two processes could remove most of iron catalyst particles and carbonaceous impurities without significant damage to carbon nanotubes. The thermal stability of the sample purified by H2O2/HCl treatment was slightly higher than that purified by HNO3-NaOH-HCl treatment. Nevertheless, the purification by HNO3-NaOH-HCl treatment was more effective for carboxyl functionalization on nanotubes than that by H2O2/HCl treatment. The Cu-coating on carbon nanotubes purified by both purification processes was complete, homogenous, and continuous. However, the Cu-coating on carbon nanotubes purified by H2O2/HCl was oxidized more seriously than those on carbon nanotubes purified by HNO3-NaOH-HCl treatment.

1. Introduction

Single-walled carbon nanotubes (SWCNTs) have nearly perfect bonding structures and no adjacent walls. Compared with multi-walled carbon nanotubes (MWCNTs), whose adjacent walls are easy to bridge to compensate for unsaturated carbon and cause defects, these defects are much less. SWCNTs are of excellent properties in ballistic electric and thermal conductivity, thermal stability and strength, and so forth, which enable them to be applicant in many fields such as nanoelectronic devices, structural, and functional composites.

When carbon nanotubes (CNTs) are used as reinforcement for copper matrix composite, the wettability and bonding strength of interfaces between CNTs and copper matrix are very poor; it is because that the surface tension of pure copper is 1270 mN/m, far beyond the cut-off limit of surface tension (100~200 mN/m) to wet CNTs [1]. The physical and chemical properties of interfacial regions are discontinuous. It is difficult to achieve desirable macroeconomic performance of the CNTs reinforced copper matrix composite. At present, the most widely adopted method to solve the problem utilizes electroless plating process to prepare a layer of copper on the surface of CNTs as a transition layer. For instance, Daoush et al. [2] and Khaleghi et al. [3] successfully coated copper on the surface of CNTs by electroless plating and prepared copper matrix composite.

Nevertheless, commercial SWCNTs were synthesized with current techniques, such as chemical vapor deposition (CVD), high-pressure CO conversion (HiPCO), laser ablation and arc-discharge, and so forth [49] and usually contain several nanoimpurities including metal catalyst particles, amorphous carbon and flake graphite. Excellent properties of SWCNTs may be modified by these impurities, so that the full potential of SWCNTs is unable to be realized. Hence, it is essential to remove these impurities before SWCNTs are put into application.

Different purification methods have been developed and reported so far, which utilize one or morebasic techniques, like wet chemical oxidation with oxidants such as H2O2, nitric acid or KMnO4, gas phase thermal oxidation with O2, CO2, or H2O vapor at elevated temperature, microwave irradiation assisted acid digestion, microfiltration, and size-exclusion chromatography [1013]. Some aggressive processes usually damage SWCNTs severely and result in a low yield. There are still some problems in nondestructive processes such as microfiltration and size-exclusion chromatography, for example, long period, complex operation, and low yield. Other problems induced by the microwave process assisted acid digestion include large equipment investment, complex operation, and difficult control conditions.

Since the composition and the amount of impurities vary with synthesis methods of SWCNTs, the suitable purification methods are also different. The commercial grade SWCNTs synthesized by CVD were employed in this study. Thereupon, according to the composition and the amount of impurities in them, in comprehensive consideration of purification factors including cost, efficiency, operability and yield, purification methods with different oxidants were chosen to study. In fact, there are a few literatures describing these methods.

Fogden et al. [14] refluxed as-grown SWCNTs at 120°C either in concentrated HNO3 for 6 h or in a 3 : 1 mixture of concentrated sulphuric (98%) and nitric acid (65%) for 30 min. They found that the mixed acid was much more oxidative and destroyed more SWCNTs. Similarly, Monthioux et al. [15] investigated a series of purification methods as follows: as-grown SWCNTs were refluxed with 2-3 M nitric acid for 45 h, followed by NaOH washing, then purified in a 3 : 1 mixture of sulphuric (98%) and nitric (70%) acids at 70°C for 20–30 min, followed by NaOH washing, and finally oxidated with a 4 : 1 mixture of sulphuric acid (98%) and hydrogen peroxide (30%) at 70°C for 20–30 min, followed by NaOH washing. They found that strong acid was harmful to the SWCNTS structure, and a slight temperature increase could result in significant SWCNTS degradation, while mild to mid strength acid treatments were able to remove impurities and damaged SWCNTs with minimal effect on the sample. In addition, Ma and Wang [16] refluxed as-grown SWCNTs at 100°C for 4 h either in 12 M HCl or in 14 M HNO3. They found that impurities including iron nanoparticles and non-CNT carbon removed from the as-grown SWCNTs by 14 M HNO3 were much more than those by 12 M HCl. The nitric acid treatment in severe oxidation conditions, such as high concentration of nitric acid or long reaction time, could damage SWCNTs structure and dramatically decrease yield. Moreover, Mathur et al. [12] compared three purification methods of as-grown SWCNTs as follows: (a) treatment with 12 N HCl for 12 h at room temperature, (b) reflux with 2.8 N HNO3 for 6 h, and (c) reflux with 6 N HCl for 6 h and then with 2.8 N HNO3 for 6 h. They verified that great percentage of Ni and Co impurities could be removed from the as-grown SWCNTs by method (b) and (c). Wang et al. [17] heated the as-grown HiPco SWCNTs in an aqueous mixture of H2O2 and hydrochloric acid at 40–70°C for 4–8 h to remove carbon-coated iron catalyst nanoparticles. By lowering the pH to ~1–3, they found that the carbon-coated iron impurities were dissolved, leaving SWCNTs largely intact. This high selectivity was a result of the proximity effect of the iron-catalyzed Fenton chemistry. At pH ~1–3, iron was dissolved upon exposure and releases ferrous ions. The ferrous ions quickly diffused into acid solution, so the unwanted catalytic effect of iron and iron hydroxide precipitation on digestion of SWCNTs by H2O2 was confined.

Raymundo-Piñero et al. [18] investigated the mechanism of chemical activation by solid KOH and NaOH in MWCNTs. And they demonstrated that the effect of KOH and NaOH on the structural organization of pristine MWCNTs was different. NaOH only attacked disordered carbons, whereas KOH was found to interact with all forms of carbons. In addition, Chungchamroenkit et al. [19] combined NaOH solution leaching and froth flotation to purify the SWCNTs synthesized by disproportionation of CO over a Co-Mo/SiO2 catalyst. And they found that most of the SWCNTs in sample were recovered without damaging the physical and chemical structures of SWCNTs.

Most of the procedures concentrated on the reduction of metal and nonnanotube carbon impurity, the chemical modification (functionalization) of SWCNTs and the structural changes of SWCNTs. The influence of different purification techniques on the subsequent specific application has not been reported.

Here we carried out a three-step-refluxing procedure with HNO3-NaOH-HCl under mild conditions to purify SWCNTs sample. A previously reported purification process with H2O2/HCl was also repeated [17]. Then, the purified SWCNTs were coated with copper by the same electroless plating process. At the end, the effects of the two purification methods on the SWCNTs themselves and on electroless Cu-coating were determined by some modern analytical techniques and characteristic methods.

2. Experimental

SWCNTs synthesized by CVD were obtained from Chengdu Organic Chemicals co., LTD., Chinese Academy of Sciences. Analytical reagents such as hydrogen peroxide, nitric acid, sodium hydroxide, hydrochloric acid, and sodium dodecyl benzene sulfonate (SDBS) were commercially available. A CS-OY-CQ30 probe ultrasonic generator was employed to disperse SWCNTs before purification (Beijing Heng Odd Instrument Co., LTD.). A VOSHIN96-IIL bath ultrasonic generator was used to disperse SWCNTs during electroless plating (Wuxi Voshin Instrument Co., LTD.). Samples were filtered by 0.45 μm millipore filters.

The original, purified, and electroless coated SWCNTs were evaluated by a field-emission scanning electron microscope (FESEM, Quanta 450) equipped with an energy dispersive spectroscopy analyzer (EDS, SUPRA 55 SAPPHIRE), a transmission electron microscope (TEM, HITACHIH-7000FA), Chi600e electrochemical corrosion workshop, and an X-ray diffractometry (XRD, XD-2). Infrared transmittance spectra were obtained using a fourier transformed infrared spectroscopy (FT-IR, NEXUS). To evaluate the samples, thermogravimetric analysis was conducted in airflow with a heating rate of 10°C min−1 using a thermogravimetric analyzer (TGA, SDT Q600).

The detailed experimental procedures are presented as follows.

2.1. Ultrasonic Dispersion in SDBS Solution

A 0.025 w/v% of initial SWCNTs sample was dispersed in a 0.35 w/v% SDBS aqueous solution through ultrasonication for 30 min. The ultrasonic dispersion employs a probe ultrasonic generator with a frequency of 35 kHz and a power of 60 W; and it stands for 5 s after ultrasonic treatment for 5 s in each 10 s cycle.

2.2. Purification with HNO3-NaOH-HCl

More mild purification conditions were chosen to prevent damage or loss of SWCNTs. The complete purification procedure had three steps. The first one was a nitric acid treatment to remove metal catalyst particles; the second one was a hydroxide treatment to exfoliate disordered carbon enclosing metal particles and then form insoluble metal hydroxides from the reaction with the metal particles upon their exposure; and the third one was a hydrochloric acid treatment to dissolve the metal hydroxides and to further remove the residual metal particles, sodium ions and amorphous carbon from the sample.

The detailed procedures are shown as follows. A 0.035 w/v% of dispersed SWCNTs sample was refluxed in 6 M nitric acid at 118°C for 5 h, followed by deionized water washing. Then it was refluxed in 6 M Sodium hydroxide at 118°C for 3 h, followed by de-ionized water washing. Finally it was refluxed in 6 M hydrochloric acid at 118°C for 3 h, followed by deionized water washing. Finally, the sample was dried under vacuum for 3 h at 80°C.

2.3. Purification with H2O2/HCl Mixture

For comparison, a previously reported purification process heating dispersed sample in an aqueous mixture of 30% H2O2 and 1 N hydrochloric acid with an electromagnetic stirring at 60°C for 4 h was also carried out [17].

2.4. Electroless Copper Plating

A 0.025 w/v% of purified SWCNTs sample was activated in an aqueous solution of 0.1 M SnCl2/0.1 M HCl through ultrasonic dispersion for 10 min, followed by deionized water washing. Then it was sensitized in an aqueous solution of 0.014 M PdCl2/0.25 M HCl through ultrasonic dispersion for 8 min, followed by deionized water washing. Then it was dispersed in an electroless copper plating bath (composition as shown in Table 1, pH = 12.5) through ultrasonication at 45°C for 5 min, followed by deionized water washing. Finally, the sample was dried under vacuum for 3 h at room temperature.

Table 1: Composition of copper electroless plating bath.

3. Results and Discussion

3.1. Characterization of Purified SWCNTs

TEM, XRD, and TGA were employed to characterize original and purified SWCNTs samples.

3.1.1. TEM Analysis

The samples for TEM were prepared by dropping the ethanol suspension of SWCNTs onto a grid which was left to air-dry. TEM images before and after purification (see Figure 1) show clearly the effectiveness of the two purification processes.

Figure 1: Representative TEM images of (a) initial SWCNTs sample and purified sample with (b, c) HNO3-NaOH-HCl and (d) H2O2/HCl.

TEM image of initial sample (see Figure 1(a)) clearly shows entangled SWCNTs bundles in a mixture with metal nanoparticles (black dots), amorphous carbon encasing metal particles and free graphite particles flake graphite. The SWCNTs are typically ~6 nm in diameter and 5~30 um in length. The size of metal particles ranges from ten to several ten nm. There was no significant difference between the TEM micrographs of the sample purified by HNO3-NaOH-HCl treatment (see Figures 1(b) and 1(c)) and by H2O2/HCl treatment (see Figure 1(d)). Long SWCNTs with high structural quality were still observed. A comparison between Figures 1(c) and 1(d) (with a magnification of 150,000) reveals that both of the purification treatments can remove most of the metal particles and carbonaceous impurities without significant damage to SWNTs.

3.1.2. X-Ray Diffraction

Figure 2 shows that the XRD patterns of initial and purified SWCNTs samples over the range of angle shift values of 10° through 90°. In the diffraction pattern (2θ) of initial sample corresponding to Figure 2(a), two prominent peaks are observed at approximately 26.2 and 43.2, and two small peaks are observed at 56.3 and 80.0, respectively. The former two peaks are due to the (0 0 2) and (1 0 0) planes of CNTs, while the latter two peaks correspond to iron metal. It confirms that iron was the main metal impurity present. By contrast, in the diffraction pattern (2θ) of the samples purified by HNO3-NaOH-HCl treatment and H2O2/HCl treatment corresponding to Figure 2(b) and (c), only the two peaks corresponding to the (0 0 2) and (1 0 0) planes of CNTs can be clearly determined, and the two peaks corresponding to iron metal are too faint to be observed. The XRD results clearly confirm that most iron impurities were removed by the two purification treatments.

Figure 2: XRD patterns of (a) initial SWCNTs sample and purified sample with (b) HNO3-NaOH-HCl and (c) H2O2/HCl.
3.1.3. TG Analysis

The TGA curves of initial and purified SWCNTs samples (see Figure 3) show that the weight loss upon heating from 20 to 900°C. The final temperature (900°C) was sufficient for complete burning of CNTs and carbonaceous impurities, leaving reddish brown iron oxide on the sample holder, namely Fe2O3. Therefore the residual weights (%) after the heating process show the Fe2O3 contents. The TGA curves show that the corresponding ash content (Fe2O3) reduced from 13.3 wt% (initial sample) to 1.7 wt% (HNO3-NaOH-HCl purified), whereas it increased to 4.6 wt% after reacting with H2O2/HCl. And the corresponding iron contents (weight %) converted from Fe2O3 of initial sample, the samples purified by HNO3-NaOH-HCl treatment and H2O2/HCl treatment are 9.7, 1.2, and 3.3 wt%, respectively.

Figure 3: TGA curves of (a) initial SWCNTs sample and purified sample with (b) HNO3-NaOH-HCl and (c) H2O2/HCl.

The curve of initial sample corresponding to Figure 3(a) shows a slight increase in weight at low temperatures due to oxidation of the Fe metal impurity [20], while the weight of the purified samples corresponding to Figure 3(b) and (c) did not increase at the same temperature. The TGA curve of initial sample begins to show weight loss at around 235°C and drops to 13.3 wt% below 620°C. By contrast, purified samples show gentle degradation slopes above 620°C, indicating their higher thermal stability. It is because that decomposition temperature of amorphous carbon in initial sample is low, and iron catalyst particles accelerate the oxidation rate of amorphous carbon as well as SWCNTs at low temperatures. The elimination of iron particles and carbonaceous impurities by the two purification treatments increased the degradation temperatures of SWCNTs.

The TGA results clearly suggest that the two purification processes were effective in the removal of iron catalysts as well as carbonaceous impurities, as observed by TEM.

Furthermore, by comparison of degradation temperatures between Figure 3(b) (543°C~775°C) and (c) (580°C~797°C), it was found that the thermal stability of the sample purified by H2O2/HCl treatment was slightly higher than that purified by HNO3-NaOH-HCl treatment. It shows that the change or damage to the SWNTs purified by H2O2/HCl was less than those purified by HNO3-NaOH-HCl.

Carbon atoms in SWCNTs skeleton are linked together by sp2 hybrid C=C covalent bonds. Studies have shown that some sp2-C converted into sp3-C through refluxing SWCNTs in acid medium. Due to oxidizability of the metastable state sp3-C, defects could be caused in SWCNTs skeleton [21]. The stronger the acidity of refluxing solution is, the more susceptible to attacks the SWCNTs are [22]. In this study, the acidity of HNO3 solution (Ph = 0) was stronger than H2O2/HCl mixture (Ph = 3.5); thus, the change or damage to the SWCNTs purified by H2O2/HCl was less than those purified by HNO3-NaOH-HCl.

3.1.4. Infrared Spectroscopy Analysis

Figure 4 compares the IR spectra obtained from initial and purified SWCNTs samples. Infrared spectrum of initial sample corresponding to Figure 4(a) shows bands around 3435, 2927, and 1625 cm−1. The band at about 3435 cm−1 could be attributed to hydroxyl (νOH) group, suggesting that initial SWCNTs were likely to entrap trace moisture; the peak at about 1625 cm−1 could be due to the stretching vibrations of water δH=O or of aromatic C=C corresponding to the sp2-C structure in SWCNTs skeleton; and the peak at about 2927 cm−1 corresponding to aliphatic (νCH) group may be associated with debris from amorphous carbon in initial sample, as observed by TEM.

Figure 4: IR spectra of (a) initial SWCNTs sample and purified sample with (b) HNO3-NaOH-HCl and (c) H2O2/HCl.

By contrast, the band corresponding to aliphatic group was absent in IR spectra of the samples purified by HNO3-NaOH-HCl treatment and H2O2/HCl treatment corresponding to Figure 4(b) and (c), suggesting that amorphous carbon was not present. The results clearly confirm that most of the carbonaceous impurities were removed by the two purification treatments. Furthermore, both of the IR spectra from purified SWCNTs samples show bands around 3435, 1731, 1667 and 1462 cm−1. The band at about 3435 cm−1 could also be due to hydroxyl group, suggesting moisture; the peaks at about 1731 and 1462 cm−1 could corroborate the presence of the carbonyl (νC=O) group, supporting carboxyl-functionalization on the surface of SWCNTs; and the peak at about 1667 cm−1 could be attributed to the stretching vibrations of C=O in the carboxyl (νCOOH) group. The results suggest that the two purification processes were useful for SWCNTs functionalization. And it is positive for the electroless plating on the surface of SWCNTs.

Some oxygen-containing functional groups, such as hydroxyl groups and carboxyl groups, were introduced at defects or tips of SWCNTs skeleton through refluxing SWCNTs in acid medium. The degree of functionalization of SWCNTs depends upon the number of defects. The defects of the SWCNTs purified by H2O2/HCl were less than those purified by HNO3-NaOH-HCl, as described in previous section. Therefore, we could infer that the purification through HNO3-NaOH-HCl treatment may be more effective for carboxyl functionalization on SWCNTs than that through H2O2/HCl treatment according to the FTIR analysis.

3.2. Characterization of Electroless Copper Coating

SEM, EDS, XRD, and TG-DTA were employed to characterizethe electroless copper coating on the surface of SWCNTs.

3.2.1. SEM and EDS Analyses

Some representative SEM micrographs of Cu/SWCNTs samples purified by HNO3-NaOH-HCl treatment and H2O2/HCl treatment are shown in Figures 5(a) and 5(b). A comparison between Figures 5(a) and 5(b) indicates no significant differences in the morphology of Cu-coatings which shows the fine-grain appearance of coating with a complete, homogenous and continuous distribution. The Cu-deposited SWCNTs shown in Figure 5(a) are 110–210 nm in diameter, and those in Figure 5(b) are 140–360 nm in diameter. The diameter statistics of coated SWCNTs were collected about 30 diameters from each sample based on the measuring tool of SEM. These images indicate that the two purification treatments were all efficient to achieve Cu layer by an electroless deposition process on the surface of SWNTs.

Figure 5: Representative SEM images of Cu/SWCNTs samples purified (a) by HNO3-NaOH-HCl and (b) by H2O2/HCl.

Figures 6(a) and 6(b) show the EDX spectra for copper deposited SWCNTs corresponding to Figures 5(a) and 5(b). Both of the profiles show peaks for copper along with peaks for carbon and oxygen. Part of the observed oxygen might be the air trapped inside the SWCNTs or Cu-coating, the rest part might attribute to the oxidized coatings. Table 2 shows the tabulated element content values for Cu/SWCNTs samples purified by HNO3-NaOH-HCl and by H2O2/HCl. The copper-to-carbon fraction of Cu/SWCNTs purified by HNO3-NaOH-HCl was 8.8% in atom, while that of Cu/SWCNTs purified by H2O2/HCl was 10.8% in atom, indicating the latter Cu-coating was thicker, as observed by SEM. And the oxygen-to-copper fraction of Cu/SWCNTs purified by HNO3-NaOH-HCl was 59.2% in atom, while that of Cu/SWCNTs purified by H2O2/HCl was 145.8% in atom, indicating that the latter Cu-coating was oxidized more seriously.

Table 2: Element contents of EDX spectra for Cu-deposited SWCNTs.
Figure 6: EDX spectra of Cu/SWCNTs purified (a) by HNO3-NaOH-HCl and (b) by H2O2/HCl.

H2O2 is unstable under heat and easy to discompose to H2O and O2. The reactions are as follows: Some moisture and oxygen might be trapped inside the SWCNTs. They could cause the oxidation of Cu-coating. The density of copper is 8.96 g/cm3, and that of copper oxides is 6.0~6.3 g/cm3. Cu-coating produced a certain volume expansion after oxidation. The SWCNTs treated with H2O2/HCl oxidized more seriously and so they possess thicker Cu-coating than that treated with HNO3-NaOH-HCl.

3.2.2. X-Ray Diffraction

The XRD patterns of Cu/SWCNTs samples purified by HNO3-NaOH-HCl treatment and H2O2/HCl treatment over the range of angle shift values of 20° through 80° are shown in Figure 7. In the diffraction pattern (2θ) of Cu/SWCNTs purified sample by HNO3-NaOH-HCl corresponding to Figure 7(a), five peaks are observed at approximately 32.6, 41.2, 56.3, 62.21, and 73.26, respectively. The peaks at 41.2, 56.3 and 73.26 were assigned to the (111), (111), and (311) planes of the fcc phase of copper, and the rest two peaks at 32.6 and 62.21 correspond to CuO and Cu2O, respectively. By contrast, in the diffraction pattern of the Cu/SWCNTs purified sample by H2O2/HCl corresponding to Figure 7(b), ten peaks are observed at approximately 34.62, 38.6, 41.7, 47.6, 49.5, 57.2, 62.3, 67.8, 69.6, and 73.2, respectively. The three peaks at 38.6, 49.5, and 73.2 correspond to the (1 1 1), (2 0 0), and (2 2 0) planes of the fcc phase of copper, and the rest peaks are due to CuO or Cu2O.

Figure 7: XRD patterns of Cu/SWCNTs purified (a) by HNO3-NaOH-HCl and (b) by H2O2/HCl.

By comparing the diffraction peaks of purified SWCNTs samples in Figure 2, it was confirmed that layers of crystalline copper were newly introduced on surfaces of SWCNTs by electroless deposition. Furthermore, on comparison of the XRD results between Figure 7(a) and (b), it found that the Cu-coating on the SWCNTs purified by HNO3-NaOH-HCl were more compact than those on SWCNTs purified by H2O2/HCl, because they were composed of (111) and (311) fcc planes, whose atomic arrangements were more intensive than those in the Cu-coating on SWCNTs purified by H2O2/HCl. In addition, the XRD results also clearly indicate that the Cu-coating on SWCNTs purified by H2O2/HCl were oxidized more seriously, which was consistent with the above EDS results.

3.2.3. TG-DTA

Figure 8 shows the TGA and DTA curves of SWCNTs and Cu/SWCNTs samples purified by HNO3-NaOH-HCl and by H2O2/HCl at temperature from 20 to 900°C. The final temperature (900°C) was sufficient for complete burning of SWCNTs, leaving black CuO on the sample holder. Therefore the residual weights (%) after the heating process show the CuO contents. The TGA curves in Figure 8(a) show that the corresponding ash content (CuO) increased from 1.7 wt% (HNO3-NaOH-HCl purified sample) to 47.2 wt% (Cu/SWCNTs purified by HNO3-NaOH-HCl), whereas it increased from 4.6 wt% (H2O2/HCl purified sample) to 40.5 wt% (Cu/SWCNTs purified by H2O2/HCl). And the corresponding copper contents (weight %) converted from CuO of Cu/SWCNTs samples purified by HNO3-NaOH-HCl and by H2O2/HCl are 41.7 and 35.3 wt%, respectively.

Figure 8: (a) TGA and (b) DTA curves of Cu/SWCNTs purified (A) by HNO3-NaOH-HCl and (B) by H2O2/HCl, SWCNTs purified (C) by HNO3-NaOH-HCl and (D) by H2O2/HCl.

The TGA curve (A) in Figure 8(a) corresponding to the Cu/SWCNTs purified by HNO3-NaOH-HCl shows a slight increase in weight at low temperatures. It coincides with two exothermic peaks of DTA curve (A) in Figure 8(b) at approximately 150°C and 290°C, respectively.

The first exothermic peak corresponds to the oxide of metallic copper into Cu2O, and the second peak corresponds to the oxidization of Cu2O into CuO, simultaneously to the oxidization of metallic copper into Cu2O. The reactions are as follows [23]: While the weight of Cu/SWCNTs purified by H2O2/HCl has no significant increase (see TGA curve (B) in Figure 8(a)) at the same temperature and the two homologous peaks of DTA curve (B) in Figure 8(b) was much weaker at approximately 155°C and 320°C, respectively, it indicates that the latter Cu-coating was oxidized more seriously before TG-DTA testing, which was consistent with the EDS and XRD results.

The DTA curves (C and D) in Figure 8(b) corresponding to purified SWCNTs show exothermic peaks above 650°C was due to the SWCNTs oxidization, because it was accompanied by a sharp weight decrease in Figure 8(a). However, the degradation temperatures of Cu/SWCNTs samples were greatly reduced to 550°C (see DTA curve (A) in Figure 8(b)) and 600°C (see DTA curve (B) in Figure 8(b)), showing that the thermal stability of Cu/SWCNTs samples decreased. It is due the catalytic effect of copper on the oxidization of SWCNTs.

3.2.4. Electric and Thermal Conductivity

Cu/SWCNTs papers (25 mm × 8 mm × 0.225 mm) have been prepared through vacuum filtration, as black sheets shown in Figure 9. Both ends of each black sheet were clamped by two pieces of copper to be fixed on sheet glass. As a result of electrochemical corrosion workshop correction, curves of relationship between current and voltage (see Figure 10) were established to convert the original data value into resistivity. The resistivity of Cu/SWCNTs papers purified by HNO3-NaOH-HCl was 0.08111 Ω·mm, and that of Cu/SWCNTs papers purified by H2O2/HCl was 0.17805 Ω·mm. The results were affected by the dispersion, dispersant and the thickness of the sheets [24, 25]. Both of the papers were prepared and detected under the same conditions. It is because the SWCNTs treated with H2O2/HCl oxidized more seriously and their Cu-coating have lower electric conductivity than that treated with HNO3-NaOH-HCl.

Figure 9: Cu/SWCNTs papers (black sheets) purified (a) by HNO3-NaOH-HCl and (b) by H2O2/HCl.
Figure 10: - curves of Cu/SWCNTs papers purified (A) by HNO3-NaOH-HCl and (B) by H2O2/HCl.

The thermal conductivity of bulk Cu is 401 W·m−1·K−1 order in the literature, that of bulk CuO is 17 W·m−1·K−1. The intrinsic thermal conductivity of CuO film was lower than 4 W·m−1·K−1. Thus oxidation would cause the reduced thermal conductivity of Cu-coating.

4. Conclusions

In this paper, a three-step-refluxing procedure for the purification of SWCNTs was described. SWCNTs samples synthesized by CVD were treated in nitric acid, hydroxide, and hydrochloric acid successively. For comparison, a previously reported purification process heating SWCNTs sample in H2O2/HCl mixture was also carried out. After purifying using the two different methods, SWCNTs were all coated with copper by the same electroless plating process.

The results of TEM, XRD, TGA, and IR spectra clearly confirm that both of the purification treatments can remove most of the iron catalyst particles and carbonaceous impurities without significant damage to SWNTs, and the thermal stability of the sample purified by H2O2/HCl treatment was slightly higher than that purified by HNO3-NaOH-HCl treatment. However, the purification by HNO3-NaOH-HCl treatment was more effective for carboxyl functionalization on SWCNTs than that by H2O2/HCl treatment.

The results of SEM, EDS, XRD and TG-DTA reveal no significant differences in the morphology of Cu-coating which shows the fine-grain appearance of coating with a complete, homogenous and continuous distribution. Nevertheless, the Cu-coating on the SWCNTs purified by HNO3-NaOH-HCl was more compact, while the Cu-coating on SWCNTs purified by H2O2/HCl was oxidized more seriously.

When SWCNTs are used as reinforcement for copper matrix composite, the presence of copper oxide in Cu-coating on SWCNTs would weaken the performance of coating in electric and thermal conductivity and decrease the bonding strength of interfaces between SWCNTs and copper matrix. Therefore, the three-step-refluxing purification pretreatment was more suitable for the application of SWCNTs in copper matrix composite to achieve desirable macroeconomic performance of composite.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank the financial support from the National Natural Science Foundation of China (Grant no. 5120514) and Hubei Provincial Key Laboratory of Green Materials for Light Industry (Grant no. 20132-15).

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