Selective oxidation of excess amorphous carbon during single-walled carbon nanotubes synthesis by induction thermal plasma process

A. Shahverdi1, K. S. Kim1, Y. Alinejad1, G. Soucy1, J. Mostaghimi2

1Department of Chemical Engineering and Biotechnological Engineering, Université de Sherbrooke, Sherbrooke, Canada, J1K2R1

2Depatement of Mechanical and Industrial Engineering, Centre for Advanced Coatings Technologies, University of Toronto, Toronto, Canada, M5S1A4

Abstract: The induction thermal plasma process has been employed for the production of high quality and moderate pure single-walled carbon nanotubes, SWCNTs, (i.e. purity of ~ 40 wt. %). An effort has been made to improve the purity of SWCNTs materials during their production (i.e. an in situ purification process). Thermal oxidation process could help to remove most of amorphous carbon (a-C) impurities from the final SWCNT product resulting an increment of SWCNT purity from ~ 40 wt% to ~ 60 wt%.

Keywords: Induction thermal plasma, SWCNTs, In situ purification, Thermal oxidation

1. Introduction 2. Experimental set-up and procedure

Single-walled carbon nanotubes (SWCNTs) were The synthesis of SWCNT was carried out using the discovered by Ijima in 1993[1]. Since their discovery, set-up shown in Fig. 1. SWCNTs have become a new field of research and received a lot of interests due to their exceptional electrical, mechanical and thermal properties as well as nano-metric morphology. These newly synthetic materials are characterized by their almost perfect cylindrical structure of seamless graphite, as well as their remarkably very high aspect ratios [2].

Up to now, many synthesis methods have been proposed including arc-discharge, laser ablation, chemical vapor deposition (CVD) and induction thermal plasma [3]. Among all, induction thermal plasma process has been found a promising method for a large-scale production of SWCNT (i.e. ~ 120 g h-1) at relatively high quality and moderate purity (i.e. ~ 40 wt%) [4]. However, the presence of impurities can hinder obtaining the optimal performance of SWCNT in many promising applications in nano-electronics, nano-probes, nano- structural composites, field-emission displays, chemical sensors, etc [5]. Fig. 1 Schematic diagram of SWCNT synthesis set-up by In this work, the purity of SWCNT synthesized by induction thermal plasma process. induction thermal plasma system was improved in situ. The initial characterization techniques indicated that the A radio frequency (RF) inductively coupled plasma torch major impurity of the synthesized SWCNT soot could be (TEKNA PL-50) was used to efficiently evaporate the amorphous carbon (a-C). Therefore, a selective gas-phase solid feed stock materials containing carbon black as thermal oxidation process was selected to eliminate a-C carbon source and nickel (Ni), cobalt (Co) and yttria and improve the final purity of SWCNT materials. For this (Y2O3) as catalysts. The solid feedstock is injected by Ar reason, pre-heated oxygen was injected into the synthesis via a water cooled probe into the plasma torch. Ar and He system to burn out the excess of a- C during the synthesis as central and sheath gases, respectively, are used for of SWCNT. The purified samples were characterized by thermal plasma generation. The reactor section of the several techniques including high resolution scanning synthesis set-up provides a suitable environment for the electron microscopy (HRSEM), transmission electron initiation of SWCNT formation. This section is followed microscopy (TEM), Raman spectroscopy and thermo- by a quenching section, shown in Fig. 1, where the gravimetric analysis (TGA). formation and growth of SWCNT are terminated [4]. Solid products are separated from exhaust gases and collected separate weight-losses observed in TGA profile, reveals a via a filtration system constructed by three metallic porous dominant peak at ~ 320°C which can be assigned to the filters. The selective thermal oxidation of SWCNT soot is non-crystalline structure of carbonaceous materials (i.e. done in the filtration system. Therefore, this section is amorphous carbons) as their burning appears at the lowest equipped by a gas heater, as shown in Fig. 1. Pure pre- temperature. Therefore, to remove amorphous carbon from heated oxygen, at 17.22 SLPM, is injected into the the final product, pre-heated oxygen is injected into the filtration system so that the filtration chamber is occupied filtration system during the synthesis process. HRSEM by 10 vol% of oxygen. In order to identify the major results of both the raw and the purified SWCNT soot are impurities in the as-produced SWNT, and to estimate presented in Fig. 4. changes in the SWNT samples through the applied in situ purification process, the material properties of the samples are analyzed by x-ray diffraction (XRD), scanning electron TGA 320°C microscopy (SEM, Hitachi, S4700), transmission electron microscopy (TEM, Hitachi, H750), TGA (TA instruments, 370°C TGA 2050) and Raman spectroscopy. For the TGA measurements, approximately 10 mg of SWNT material is oxidized in flowing air (40 sccm), using a temperature DTG ramp of 10 ºC/min from room temperature to 900 ºC. Raman measurements are made, using the 632.8 nm excitation (He-Ne laser).

3. Results and discussion

XRD pattern of synthesized SWCNT soot, shown in Fig. 2, indicates the presence of graphitic materials, residual Fig. 3 TGA / DTG graphs of the raw SWCNT soot synthesized by induction thermal plasma process. catalysts (i.e. Ni, Co and Y2O3) and yttrium carbide (YC2). (a)

500 nm

(b) Fig. 2 X-ray diffraction pattern of the raw soot containing SWCNTs synthesized by induction thermal plasma process.

From Fig.2, it is evident that the major impurities, besides residual catalysts, are carbonaceous materials such as amorphous carbons and partially-graphitized particles. Identifying different type of carbonaceous materials from XRD is somehow impossible. In the thermal oxidation of 500 nm SWCNT soot, the temperatures in which these materials react with oxygen and burn are very important. Thermal Fig. 4 HRSEM images of (a) raw SWCNT soot, (b) in situ oxidation behaviors of synthesized SWCNT soot are purified SWCNT soot. studied by means of TGA and the result is shown in Fig. 3. TGA graph clearly indicates a multiple weight-losses Fig.4 (a) is a typical SEM image of the as-produced related to the burning of carbonaceous materials containing sample and shows the existence of the SWNT bundles SWCNTs. The residual mostly contains catalysts having entangled with many carbonaceous impurities. A 17 wt% of the raw SWCNT soot. The derivative weight comparison between Fig.4 (a) and (b) clearly indicates that loss (DTG) profile, simplifying the identification of the more tubes are present in the in situ thermally oxidized sample than as-produced one. To justify and confirm the All Raman spectra in Fig.6 indicate an intense radial result of HRSEM, TGA is performed on the purified breathing mode (RBM) feature in a range of 120- 200 cm-1 samples and the result is shown in Fig.5. as well as a high frequency feature, so-called G-band, in the range of 1500-1600 cm-1. Both of these features are characteristic of SWCNT in the Raman spectrum [6, 7] and TGA therefore, precisely indicate the presence of SWCNT in the 370°C sample. A large structure (1340 cm-1) assigned to ill-organized graphite, so-called D-band, [7] is present in both Raman spectra, as seen in Fig.6. G/D ratio in the Raman spectrum can be a good measure for the purity DTG 22 wt% evaluation of SWCNT sample [8]. As the spectra in Fig.6 are normalized with G-band intensity, the weaker D-band intensity of the purified SWCNT sample than that of raw SWCNT sample indicates its lower amount of amorphous carbon. Moreover, the full width at half maximum (FWHM) of RBMs and tangential G-bands provide a good measure of wall damage in the SWCNT [9]. As observed Fig. 5 TGA/DTG graphs of in situ purified SWCNT soot from Fig.6, both of these bands of in situ purified SWCNT synthesized by induction thermal plasma process. and raw SWCNT are comparable. This confirms that the selective thermal oxidation process did not induce As seen in Fig. 5, the amount of residual (i.e. fully noticeable wall defects [10]. As a complementary oxidized catalysts) is increased from 17 wt% to 22 wt% technique for Raman spectroscopy, TEM is applied to due to the preferential oxidation of amorphous carbon. The monitor morphological changes on the SWCNT sample peak at 320°C, observed in Fig.3, is totally removed from after the purification process. Fig. 7 (a) shows TEM image TGA graph of the purified SWCNT soot. This is consistent of the raw SWCNT soot. As seen in this figure, the tubes with the increase of catalysts proportion in the sample. To are present beside shape-less gray islands, indicated by estimate the purity of the tubes in the sample before and arrows, representing amorphous carbon materials. after purification process, DTG curves are fitted to a series of Lorentzian functions in order to calculate the under- (a) curve area of each peak. The results, not shown here, indicated ~20 wt% improvement from the initial purity (i.e. 40 wt%, [4]) of SWCNT. Although both TGA and HRSEM results indicate the improvement of SWCNT purity after thermal oxidation, the effect of such process on the SWCNT should be studied by using Raman spectroscopy and TEM. Harsh process conditions such as oxidative environment at high temperatures can cause defects on, or destroy, the SWCNTs [5]. Raman spectra of both purified and raw SWCNT are shown in Fig. 6. 100nm

λexc=632.8 nm (b)

100 nm Fig. 7 TEM images of (a) raw SWCNT (b) in situ purified SWCNT with selective thermal oxidation process. Fig. 6 Normalized Raman spectra of (a) raw SWCNT soot (black line) (b) in situ purified SWCNT soot (red line). Black dots in the gray islands can be assigned to dense [7] T. Belin, F. Epron, Characterization Methods of materials such as metallic catalysts and graphitic particles Carbon Nanotubes: a Review, Materials Science and in the sample. Fig. 7 (b) represents TEM image of the Engineering B, (Solid-State Materials for Advanced in situ purified SWCNT sample with selective thermal Technology), 119 (2005), 105-118. oxidation process. The tubes are clear in this image and the presence of gray islands is minimized. Thus, it is certain [8] Y. Lian, Y. Maeda, T. Wakahara, T. Akasaka, S. that most of amorphous carbon materials are effectively Kazaoui, N. Minami, T. Shimizu, N. Choi, and H. removed from raw SWCNT soot. Tokumoto, Nondestructive and High-Recovery-Yield Purification of Single-Walled Carbon Nanotubes by 4. Conclusion Chemical Functionalization, Journal of Physical Chemistry B, 108 (2004), 8848-8854. To conclude, a simple and effective procedure for in situ purification of SWCNT product of the induction thermal [9] D. Nepal, D. S. Kim, K. E. Geckeler, A Facile and plasma process has been presented. Pre-heated oxygen was Rapid Purification Method for Single-Walled Carbon injected into the filtration system to remove amorphous Nanotubes, Carbon, 43 (2005), 651–673. carbon impurities from the final products. The results clearly indicate that the selective thermal oxidation process [10] M. Monthioux, B. W. Smith, B. Burteaux, A. Claye, can be a suitable procedure for removal of amorphous J. E. Fischer, D. E. Luzzi, Carbon, 39 (2001), 1251– carbon materials during the synthesis of SWCNT and 1272. consequently increase the purity of the tubes from ~40 wt% to ~60 wt% without noticeable damages.

Acknowledgement

This work was supported by the Natural Science and Engineering Research Council (NSERC) of Canada.

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