Diameter-control of single-walled carbon nanotubes produced by magnetic field-assisted arc discharge

CARBON 50 (2012) 2556 2562 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon Diameter-control of single-walled carbon nanotubes produced by magnetic field-assisted arc discharge Yanjie Su, Yaozhong Zhang, Hao Wei, Zhi Yang, Eric Siu-Wai Kong, Yafei Zhang * Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Research Institute of Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China ARTICLE INFO ABSTRACT Article history: Received 2 December 2011 Accepted 7 February 2012 Available online 14 February 2012 We have demonstrated a scalable approach to synthesize single-walled carbon nanotubes (SWCNTs) with selected diameter distributions by applying a magnetic field perpendicular to the electric field in the arc plasma. It is found that the purity and orientation of SWCNTs can be controlled by the magnetic field. SWCNTs with different diameter distributions can be separated into two different regions by the applied magnetic field, and the diameterselection efficiency is improved by modifying the direction of the magnetic field. Our findings suggest that the motion of the catalyst particles with different sizes, positive carbon ions and electrons are significantly influenced by Lorentz forces, resulting in the difference in the growth processes of the SWCNTs due to the collective interactions between the arc plasma and the magnetic field. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Single-walled carbon nanotubes (SWCNTs) can be semiconducting or metallic depending on their diameter and chiral angle [1 2], which are identified as the chiral indices (n,m). Moreover, the bandgap energies of semiconducting SWCNTs are inversely proportional to their diameters. However, the as-grown SWCNTs always come as a mixture of nanotubes with different chiralities, resulting in the heterogeneous performance of SWCNT-based devices. This presents a major obstacle to many advanced applications of SWCNTs (such as field-effect transistors [3], solar cells [4,5], and optical sensors [6]). Although specific SWCNTs with controlled chiralities can be successfully achieved through selective oxidation [7], density gradient ultracentrifugation [8,9], and gel chromatography [10], these methods still suffer from setbacks such as low yield and high cost. In order to obtain SWCNT devices with homogeneous properties, one must be able to prepare SWCNTs with tailor-made chiral angle or diameter. Within the last decade, controllable synthesis of SWCNTs with narrow-chirality distribution has been widely investigated by chemical vapor deposition (CVD) with flexible control of reaction parameters (such as catalyst type and particle size, growth conditions, and carbon source feedstock/concentration) [11 14]. The as-synthesized SWCNTs, however, are still known to contain some structural defects. Direct current (DC) arc discharge method, generally used to synthesize defect-free SWCNTs, is having difficulty to be utilized for controllable synthesis of SWCNTs with a narrow-chirality distribution, due to the complicated nucleation and growth issues [15,16]. Our earlier investigation suggested that low-pressure CO can influence the catalyst formation and carbon precipitation, resulting in controllable synthesis of SWCNTs with narrow diameter distribution [17]. Recently, we have demonstrated that the arc plasma morphology responds strongly to transverse magnetic field due to electromagnetic interactions, i.e., arc plasma parameters can be easily controlled by the transverse magnetic field. Volotskova et al. [18] reported that the chirality distribution of SWCNTs * Corresponding author: Fax: +86 21 3420 5665. E-mail address: yfzhang@sjtu.edu.cn (Y. Zhang). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.02.013

CARBON 50 (2012) 2556 2562 2557 synthesized by DC arc discharge could be tailored by applying an axial magnetic field to the arc plasma in the inter-electrode gap. However, the weak magnetism of the catalyst (Ni/Y) and the violently tumbling behavior of SWCNTs soot in arc chamber will weaken the diameter-selectivity. On the other hand, the diameter distribution of SWCNTs synthesized using Ni/ Y as catalyst is narrower than those of SWCNTs synthesized using Fe/Mo or Fe/Co/Ni as catalysts. Here we report a diameter-selected synthesis of SWCNTs using the arc discharge process, in which Fe/Mo are used as catalysts and different transverse magnetic fields have been applied to the arc plasma. 2. Experimental The synthesis of SWCNTs was performed in a DC arc discharge furnace [19]. The anode was a sintered graphite rod (6 mm diameter, 100 mm length) containing the catalyst Fe/Mo, in which the mole ratio among C/Fe/Mo was 97.5:2.0:0.5. The cathode was a pure graphite rod with a diameter of 8-mm diameter and a length of 300-mm. A permanent magnet was used to generate a transverse magnetic field perpendicular to the electric field. The magnetic field strength in the inter-electrode gap was about 10 G (as measured by a Gaussmeter). SWCNTs were synthesized with a current of 90 A under Ar-H 2 atmosphere at 30 kpa, and the interelectrode gap was kept constant at 2 mm. To characterize the morphology, diameter distribution and SWCNT purity of the as-synthesized SWCNTs, we employed a combination of scanning electron microscopy (SEM), Raman spectroscopy, thermogravimetric analysis (TGA). SEM images were acquired on a Zeiss Ultra55 FE-SEM instrument operating at 5 kv. Raman spectroscopy was characterized using a Bruker Senterra dispersive Raman microscope with laser excitation at 633 nm and 785 nm. TGA measurements were performed on a PerkinElmer Pyris 1. Samples were analyzed at a heating rate of 10 C/min up to 900 C under air atmosphere. 3. Results and discussion As we know, the arc plasma in the inter-electrode gap exhibits almost-spherical shape during the arc discharge process for SWCNTs (see Supplementary Fig. S1). After applying a magnetic field perpendicular to the electric field in the arc plasma region, however, one can see in Fig. 1a that the morphology of the arc plasma can be easily altered by the applied magnetic field. After magnetic field assisted arc discharge, the macroscopic morphologies of SWCNT products are shown in Fig. 1b. It can be clearly observed that the SWCNTs were directionally deposited in the front region of oriented arc plasma, under the influence of a transverse magnetic field to the arc plasma. This directional deposition of SWCNTs can be used for large-scale synthesis of high-quality SWCNTs by changing the direction of the magnetic field. According to Lorentz force formula, one can infer that positive ions in the arc plasma play a dominant role in the formation of the oriented arc plasma, which will influence the growth process of SWCNTs. To understand the effect of magnetic field on the morphologies of SWCNTs in different regions, SEM was performed in order to evaluate the morphologies of SWCNT samples from the front and back side of oriented arc plasma, marked by F- SWCNT and B-SWCNT, respectively. It is clearly seen in Fig. 1c that the as-synthesized SWCNTs have a smooth and clean surface, only a few impurity particles attached. Low resolution SEM image of SWCNTs shows that the SWCNT bundles in F-SWCNT sample are closely packed in parallelaligned arrays by magnetic field (see Supplementary Fig. S2). It means that well-aligned SWCNT films can be acquired directly on a substrate when it was put in the deposition region. While SWCNTs in B-SWCNT sample (Fig. 1d) are covered by numerous impurities and exhibit low degree of structural order. From the above findings, we deduce that the motion of positive carbon ions in inter-electrode gap are accelerated by Lorentz forces, resulting in higher velocity than the thermal velocity of neutral carbon clusters [20]. Furthermore, low electron density caused by the transverse magnetic field will result in the SWCNTs with positive charges [21], which is helpful for selective deposition in the front region of oriented arc plasma. To further investigate the effect of magnetic field on the catalysts, the corresponding energy dispersive X-ray spectroscopy (EDS) spectra of F-SWCNT and B-SWCNT samples show that the catalyst content in F-SWCNT sample is smaller than that in B-SWCNT sample (see Supplementary Fig. S3), indicating that Fe/Mo catalysts had been controlled by weak magnetic field after vaporizing from the anode. Higher Fe content in B-SWCNT sample is attributed to higher magnetic field strength in the region. Fig. 1e and f show the TGA curves of SWCNT samples in two different regions. One can see clearly that F-SWCNT sample was completely burned off at about 750 C, while the weight loss of B-SWCNT sample stopped after about 880 C, indicating that the purity in F-SWCNT sample is higher than that in B-SWCNT sample. In addition, the derivative thermogravimetric (DTG) data exhibit four peaks at about 325370 C, 425450 C, 600611 C and 750 C, which correspond to the oxidization of Fe catalysts (positive peak) and mass loss by carbon oxidation (negative peaks). The first exothermic peak at 425 C can be assigned to the combustion of amorphous carbon, while the second peak corresponds to the mixture of SWCNTs. However, the third exothermic peak at 750 C only exists in B-SWCNT sample, indicating the content of graphitic particles in F-SWCNT sample is much lower than that in B-SWCNT sample. Hence, the purity of SWCNTs can be influenced by applying a transverse magnetic field in the arc plasma. To further investigate the effects of magnetic field on the diameter distribution of SWCNTs, we synthesized two SWCNT samples through changing the direction of transverse magnetic field. According to the orientations of arc plasma induced by the transverse magnetic field, the directions of transverse magnetic field are marked by X and X, respectively. Fig. 2 shows the Raman spectra of as-synthesized SWCNT samples using 633 and 785 nm as excitation wavelengths. The radial breathing mode (RBM) in Raman spectra depends strongly upon the diameter of SWCNTs, and has been widely used to determine the diameter of SWCNTs [22]. Fig. 2b and c show Raman spectra of SWCNT samples synthesized by arc discharge with X magnetic field. Compared with RBM peaks in B-SWCNT sample, the peak intensities at 132 cm 1 (1.90 nm), 186.5 cm 1 (1.30 nm), and 211 cm 1 (1.14 nm)

2558 CARBON 50 (2012) 2556 2562 Fig. 1 (a) A typical digital image of arc plasma with transverse magnetic field. (b) A typical digital photograph of as-synthesized SWCNT product. Two SWCNT samples were collected in the Front and Back regions, marked by F-SWCNT and B-SWCNT, respectively. SEM images of (c) F-SWCNT sample and (d) B-SWCNT sample. TGA and DTG thermographs of (e) F-SWCNT sample and (f) B-SWCNT sample. decrease (Fig. 2b), and two RBM peaks at 142 cm 1 (1.75 nm) and 162.5 cm 1 (1.51 nm) disappear in F-SWCNT sample, indicating that the average diameter of SWCNTs in F-SWCNT sample is smaller than that in B-SWCNT sample, The 785 nm excitation line has also been used to characterize the Raman RBM spectra of the SWCNTs in Fig. 2c. Compared with RBM peaks in B-SWCNT sample, the peak intensities at 202 cm 1 (1.19 nm) and 227 cm 1 (1.05 nm) in RBM spectrum of F-SWCNT sample increases while the peak intensity at 146.5 cm 1 (1.69 nm) decreases, indicating that the SWCNTs with smaller diameters are enriched in F-SWCNT sample and suggesting that SWCNTs with different diameter distributions can effectively be separated by a transverse magnetic field. These Raman spectral data therefore strongly provide further support for magnetic field induced diameterselectivity of SWCNTs. We further investigate Raman spectra of SWCNT samples synthesized by arc discharge with X magnetic field, as shown in Fig. 2e and f. Compared with B-SWCNT sample, two main RBM peaks at 133.5 cm 1 (1.87 nm) and 189 cm 1 (1.28 nm) (Fig. 2e) have been found in F-SWCNT sample, indicating that the diameter distribution of SWCNTs has been separated after adding X magnetic field to the arc plasma. Only one RBM peak at 146 cm 1 (1.70 nm) can be found in Fig. 2f when a 785 nm laser is used for excitation, further supporting an effective diameter-selective synthesis of SWCNTs during the arc discharge process.

CARBON 50 (2012) 2556 2562 2559 Fig. 2 Schematic of arc plasma morphologies under different magnetic field directions (a and d), marked as X and X, respectively. Raman spectra of SWCNT samples with 633 nm excitation (b and e) and 785 nm excitation (c and f). Front and Back samples were collected in corresponding regions in Fig. 1b and Fig. S4. The tangential mode vibration (G-band) is another important feature in the Raman spectra of SWCNTs, which can be used for diameter characterization and to distinguish between semiconducting and metallic SWCNTs [23]. The frequencies of G bands in our samples are at around 1585 cm 1, which can be attributed to collective effects between diameter-dependent Raman shifts [24] and high-frequency x + G of metallic SWCNTs in bundles [25]. In addition, the frequencies of G bands are also influenced by the resonance between scattered photons (E laser E + G ) and E ii electronic transition of SWCNTs [25]. Raman shifts of G bands are clearly observed in the Raman spectra of F-SWCNT and B-SWCNT samples (Fig. 3). Since the SWCNTs contain semiconducting and metallic SWCNTs in our samples, the Raman shifts are ascribed to the changes of SWCNT diameters. Especially for Raman shifts in Fig. 3c, one can see clearly that, compared with the SWCNTs in B-SWCNT sample, the enhancement of the SWCNTs with large diameter in F-SWCNT sample results in up-shift of G band because low-frequency peak x G increases with increasing SWCNT diameter [25]. These diameter-dependent Raman shifts further verifies that transverse magnetic field can be used to separate SWCNTs in situ during arc discharge processes. Second-order Raman spectra (2D band) of SWCNTs have also been investigated in our samples. One-peak Raman 2D-band spectra are clearly observed, indicating that only one E ii transition is occurring with the incident photon energy E laser [26]. Therefore, 2D-band spectra are also useful for verifying the (n,m) assignment given by the RBM mode. The 2D-band frequency in F-SWCNT sample is different from that in B-SWCNT, which can be attributed to the change in diameter distributions of SWCNTs due to the application of the transverse magnetic field. The aforementioned results show that the diameter distribution of SWCNTs can be separated efficiently into two different regions by applying a transverse magnetic field perpendicular to the electric field. Moreover, one can see from Fig. 2e and f that magnetic field strength in the SWCNT growth region is an important factor in improving the separation efficiency. Hence, one can conclude that the in situ separation process of SWCNTs with different diameter distributions is closely related to the changes of catalyst Fe particle motion and arc plasma parameters in the nonuniform magnetic field [18,27]. In our experiments, the arc morphology and the drag direction of oriented arc plasma have been altered significantly through applying magnetic field to the arc plasma and changing the direction of magnetic field, indicating that the density distributions of positive carbon ions and electrons can be significantly altered by Lorentz forces, as shown in Fig. 4. Meanwhile, the plasma density and electron temperature in the center of arc plasma are tailored through applying magnetic field to the arc plasma [28], suggesting that the ratio between ions and neutral density and the SWCNT-plasma interactions will be changed, which will also influence the SWCNT growth process. The motions of the charged carbon species and the size distribution of catalysts in non-uniform magnetic field will be altered and differentiated in different directions due to different motion (e.g., velocity and direction) in the arc plasma, which result

2560 CARBON 50 (2012) 2556 2562 Fig. 3 G- and 2D-band frequencies of SWCNT samples synthesized by X and X magnetic fields with 633 nm excitation (a and c) and 785 nm excitation (b and d). Fig. 4 Schematic of charged ion motion in arc plasma after applying a magnetic field. V therm, thermal velocity of positive ions; V, actual velocity of positive ions; and F E B, Lorentz force. in different SWCNTs growth rate. The SWCNTs with different diameter may be formed and remove from the arc plasma due to different nucleation process and catalysts Fe with different diameters in a magnetic field [18]. On the other hand, the geometry and plasma density of arc plasma are dramatically altered by transverse magnetic field, which lead to SWCNTs with positive charges due to the decreased electron flux to SWCNTs from arc plasma [21]. The growth processes of SWCNTs, therefore, are affected by the applied magnetic field. Therefore, we conclude that the collective interactions aforementioned between arc plasma and magnetic field would alter the nucleation process and growth process of the SWCNTs. The SWCNTs with different diameters could be differentiated from each other in accordance to their variable motion (e.g., velocity and direction) in the transverse magnetic field.

CARBON 50 (2012) 2556 2562 2561 4. Summary SWCNTs with selected diameter distributions have been synthesized by applying a magnetic field perpendicular to the electric field in the arc plasma. We have demonstrated a scalable method, by which magnetic field-induced separation strategy enables the control of the purity and orientation of SWCNTs. More importantly, SWCNTs with different diameter distributions can be separated into two different regions by applying a transverse magnetic field; while the diameterselection efficiency can be improved by modifying the direction of the magnetic field. Our findings support the conclusion that the motion of the catalyst particles with different sizes in the arc plasma is influenced by the applied magnetic field, leading to SWCNTs with different diameters. 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