Carbon Nanotubes

Electrochemistry Communications 8 (2006) 1099–1105 www.elsevier.com/locate/elecom

Electrochemical properties of bamboo-shaped multiwalled carbon nanotubes generated by solid state pyrolysis Sangaraju Shanmugam, Aharon Gedanken

*

Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel Received 26 March 2006; received in revised form 2 May 2006; accepted 2 May 2006 Available online 9 June 2006

Abstract We report herein on an electrochemical study of bamboo-shaped multiwalled carbon nanotubes (BMWCNTs) that were synthesized by a simple and efficient solid state pyrolysis method. Cyclic voltammetry was used to evaluate the electrochemical behavior of BWCNTs in 0.1 M KCl containing 5 mM of K4Fe(CN)6. The electron transfer rates of as-synthesized bamboo-shaped multiwalled carbon nanotubes and commercial hollow multiwalled CNTs were evaluated and compared. Among the studied electrodes, BMWCNTs showed a faster electron transfer rate, as compared with the other electrodes. The higher electron transfer kinetics can be attributed to the surface oxide functional groups. IR, Raman, XPS, and electrochemical studies showed that the as-synthesized carbon nanotubes exhibit some functional groups on BMWCNTs. This phenomenon can be attributed to the larger amount of exposed edge planes in the as-prepared BMWCNTs, as compared with the ordinary multiwalled carbon nanotubes. The capacitive properties of BMWCNTs electrodes were studied in 1 M KNO3 using cyclic voltammetry with the scan rates ranging from 25 to 500 mV/s. The specific capacitance of the BMWCNTs electrodes were 11.5 and 6.7 F/g with scan rates of 25 and 500 mV/s, respectively. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Bamboo-shaped MWCNTs; Hydrophilic; Electrochemical; Solid state synthesis of CNTs; Capacitance

1. Introduction Carbon nanotubes (CNTs) have attracted much attention in recent years because of their unique structural, electronic, magnetic and mechanical properties [1,2]. CNTs have been proposed as a material for the storage of hydrogen [3] and as electrochemical super capacitors [4]. Various synthetic procedures for the preparation of carbon nanotubes have been developed, for example, arc-discharge [5– 7], laser ablation [8], and chemical vapour deposition (CVD) [9] of various molecules decomposed over metallic, metal supported catalysts. Various transition metal catalysts have been used for the synthesis of CNTs, such as iron [9], cobalt [10], nickel [11], copper [12], molybdenum [13], rhodium [14], palladium [15], gold [16], and CoAMo [17]. Carbon nanotubes have been received much attention as *

Corresponding author. E-mail address: [email protected] (A. Gedanken).

1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.05.001

potential electrode material for electrochemical capacitors [18–20]. Carbon nanotubes have been employed as catalyst supports for applications such as fuel cell and liquid phase reactions [21–23]. To support the active entities over the carbon nanostructures, they should possess some functional groups to anchor the active entities onto the CNTs. Unfortunately, however, the carbon nanotubes generated by the chemical vapour deposition method require some additional pretreatments in order to make them suitable as catalyst supports. Thus, introducing the functional groups required special methods [24–28]. The treatment of carbon nanotubes with existing oxidizing agents in the gas or liquid phase results in the formation of oxidic groups such as carboxyl, carboxylic anhydride, lactone, phenolic, carbonyl, and quinone groups, on the skeleton of the carbon nanotubes. In order to use carbon nanotube materials for electrochemical applications involving aqueous electrolytes, the carbon nanotube-based electrode needs to be pretreated so that it is easily wetted when immersed in the

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solution [29]. Various electrochemical oxidation methods have been proposed to introduce effectively oxygen functionalities [30]. By adopting harsh electrochemical methods, the longer tubes either broke down into smaller tubes, or a fracture of the carbon nanostructures was observed [31]. Chou et al. demonstrated the importance of oxygenated species at the ends of the CNTs for obtaining favorable electrochemical properties [32]. Li et al. studied electrochemical properties of three types of 3-D carbon nanotubes: single walled nanotube paper (SWNTP), as-produced multiwalled nanotubes towers (as-produced MWNTT), and heat-treated multiwalled nanotube towers (heat-treated MWNTT). The electron transfer rate is as follows: SWNTP > as-produced MWNTT > heat-treated MWNTT. This electron transfer rate was correlated with the extent of side-wall exposure and the density of graphite edge-planelike defects sites [33]. Compton et al. in their electrochemical studies of various carbon materials have suggested that bamboo-shaped carbon nanotubes exhibit a high percentage of graphitic layers that terminate at the surface of the tube, giving rise to a large number of edge-plane sites along the surface of tube, thus exhibiting a better electrochemical activity than the smooth, more basal plane-like regions in hollow-tube CNTs [34–37]. Very recently, Heng et al. demonstrated the advantageous of bamboo-like nanotubes over single walled carbon nanotubes for the biosensing of guanine and adenine [38]. The bamboo-CNTs showed better electrochemical performance, which was rationalized by the presence of end-caps located at regular intervals along the walls of bamboo-CNTs, which introduce edge planes of graphene all along the walls, when compared to hollow carbon tubes. In this communication, we present the electrochemical properties of as-synthesized BMWCNTs, which have been synthesized by a simple and efficient solid state method. The carbon nanotubes were characterized with SEM, TEM, XPS, and CV. Electron transfer kinetics was studied using K4Fe(CN)6 as a redox probe and also compared with commercial hollow multiwalled carbon nanotubes. Among the studied electrodes, BMWCNTs showed a faster electron transfer rate than other electrodes. The as-synthesized bamboo-shaped MWCNTs exhibit oxygen functional groups on the surface, which was revealed from IR, XPS, and electrochemical studies, that are responsible for higher electron transfer rates. The capacitive behavior of as-synthesized BMWCNTs was studied in 1 M KNO3 with cyclic voltammetry. 2. Experimental section The ruthenium (III) acetylacetonate was obtained from Aldrich and used as received. All other chemicals were of reagent grade and used without any purification. The LET-LOK union cells were procured from HAM-LET, Israel. A typical synthesis of bamboo-shaped carbon nanotubes is as follows: a 3/4 in. LET-LOK union part was

plugged from both sides by standard caps. Ruthenium acetylacetonate (0.5 g) was introduced into the cell at room temperature under atmospheric conditions. The filled cell was closed tightly with the other plug and then placed inside an iron pipe at the center of the furnace. The temperature was raised at a heating rate of 20 °C per min. The closed vessel cell was heated at 1000 °C for 10 h. The reaction took place under the autogenic pressure of the precursor. The closed vessel cell (LET-LOK) heated to 1000 °C was gradually cooled to room temperature, and opened with the release of a little pressure. The yield of the carbon nanotube is about 65% with the precursor. Commercial multiwalled CNTs (>90% purity, 140 ± 30 nm diameters, 7 ± 2 lm length) were obtained from MER Corporation (USA). Commercial MWCNTs were treated with HNO3– H2SO4 mixture for overnight. Surface morphology was studied using scanning electron microscopy (SEM), a JEOL-JSM 840 instrument operating at 10 kV, transmission electron microscopy (TEM) on a JEOL-JEM 100 SX microscope, working at an 80 kV accelerating voltage, and a JEOL-2010 HRTEM instrument with an accelerating voltage of 200 kV. Samples for TEM and HRTEM were prepared by ultrasonically dispersing the products into absolute ethanol, then placing a drop of this suspension onto a copper grid coated with an amorphous carbon film, and then drying under air. The elemental analysis of the sample was carried out by an Eager C, H, N, S analyzer. XPS measurements were performed using a Kratos in ultrahigh vacuum (UHV) with an axis HS monochromatized Al Ka cathode source at 75–150 W, using a low energy electron plod gun for charge neutralization. A survey, and high resolution individual metal emissions, was taken at medium resolution, with a pass energy of 80 eV, and step of 50 meV. For electrochemical studies, a conventional three-electrode single glass compartment was employed. The electrochemical studies were carried our with a Potentiostat/ Galvanostat Model 273 A. Pt wire and saturated calomel electrodes (SCE) were used as counter and reference electrodes, respectively. A 0.076 cm2 area glass carbon (GC) served as the working electrode. 10 mg of bamboo-shaped carbon nanotubes were dispersed in 0.5 ml of water for 20 min in an ultrasonicator. The dispersed carbon nanotubes (10 lL) were placed on a GC and dried in an oven at 90 °C for 2 min. 5 lL of 5% Nafion was dropped on a GC and dried at room temperature. The electrochemical behavior of BMWCNT electrodes was characterized by using a cyclic voltammetry technique. 5 mM K4Fe(CN)6 in 0.1 KCl was used to evaluate the electron transfer properties of BMWCNTs. Capacitance studies were measured in 1 M KNO3. The electrolyte was degassed with nitrogen gas before the electrochemical measurements. 3. Results and discussion Fig. 1(a) shows the SEM image of carbon nanotubes prepared by the solid state pyrolysis of ruthenium

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remove the Ru metallic nanoparticles from the as-synthesized BMWCNTs, we treated them by stirring overnight with an acid (HNO3 with KClO4). However, we failed to remove the metallic nanoparticles, as was observed from the XRD analysis (not shown). This may be due the ruthenium metal nanoparticles are not accessible for the acid because it is encapsulated in 30–45 graphitic layers of carbon nanotube, which is evidenced from HRTEM studies (not shown). The electrochemical properties of bamboo-shaped carbon nanotubes were evaluated in an acidic aqueous solution (1 M H2SO4). The results obtained are presented in Fig. 2. It is clear from the Fig. 2 that a well-defined anodic and cathodic peak were observed at 447 and 395 mV, respectively, for the as-synthesized CNTs. These peaks originate from the protonation/deprotonation of functional groups such as quinone/hydroquinone. In order to understand the phenomena, we carried out CV measurements at a series of scan rates from 10 to 40 mV/s, the anodic peak is shifted to a more positive potential and cathodic

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acetylacetonate. It is seen from the image that the carbon nanotubes are not straight. Fig. 1(b) shows a representative TEM image of the obtained carbon nanotubes, most of which have a bamboo structure with a few normal, twisted carbon nanotubes. The widths of the bamboo-shaped MWCNTs are in the range of 50–80 nm diameters, with an approximate length of 10 lm. The bamboo-shaped carbon nanotubes are comprised of an outer diameter of around 50–80 nm and an inner diameter of about 25– 35 nm. The compartments of the bamboo-shaped carbon nanotubes are almost uniform in size. The size of the compartments is about 200 nm in most of the carbon nanotubes [39]. Most of the compartments were empty; however a few tubes encapsulated with metallic ruthenium at their extreme ends. The compartments of the bamboo structure also possessed graphitic layers. The distance between two graphitic layers is 0.342 nm, which is in agreement with the graphitic carbon (0 0 2) d spacing plane. It is important to point out that the graphitic layers are not arranged parallel to the tube axis, and the wall has a herringbone-type graphitic arrangement. It is clear from the TEM studies that metal nanoparticles are embedded in the carbon nanotubes. The presence of metallic ruthenium nanoparticles is also evidenced from XRD studies. In order

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Fig. 1. (a) SEM image of the as-synthesized product and (b) TEM image showing the bamboo-shaped carbon nanotubes. Inset shows a HRTEM image of a bamboo-shaped CNT.

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Fig. 2. (a) Cyclic voltammetric response of BMWCNT/GC in 1 M H2SO4 at 25 mV/s between 0 and +1.0 V vs. SCE and (b) at different scan rates 10, 20, 30, 40 mV/s.

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peak is shifted to a negative one (Fig. 2(b)). The intensity of the anodic and cathodic peak currents is linearly dependent on the scan rate, indicating a surface confined process. To corroborate the origin of peaks in CV, we have supported the ruthenium on TiO2 and studied the electrochemical behavior under similar experimental conditions. We did not observe any peaks in the same potential region. This observation suggests that the redox peaks observed in BMWCNT electrode is originating from the oxidation/ reduction of surface functional groups. Ye et al. observed similar redox peaks for electrochemically treated (in various acid conditions) MWCNTs but did not detect redox peaks for untreated MWCNTs. Electrochemical treatment of MWCNTs introduce the quinoidal functional groups on the surface of nanotubes [40]. Generally, the carbon nanotubes synthesized by chemical vapor deposition are hydrophobic in nature and some harsh chemical methods have to be adopted in order to introduce some functional groups [24–28]. The current response for BMWCNTs is larger when compared to GC electrode, indicating a higher electrochemical surface and greater exposure of edge plane sites. In addition, the outer and inner surface of the nanotubes is also accessible for the electrolyte. The large background current is due to the capacitance and reversible redox behavior of the quinone/hydroquinone groups. Barisci et al. showed the resulting capacitance of acid-treated SWNT paper to consist of a fast, and slow processes [41]. The former mostly originates from the charging of the double layer, while the slow process appears to be a pseudocapacitance related to the surface Faradic reaction of surface functional groups. The capacitance could originate from the double layer at the electrode/electrolyte interface, which is proportional to the effective surface area and also from the Faradic reaction of the surface oxide functional groups that are present on the surface of the carbon nanotubes. We have derived the capacitance from CV studies at a potential of 0.7 V vs. SCE, found to be 9 F/g, which is consistent with the literature [41–43]. The electron transfer kinetics of as-synthesized BMWCNTs was examined using a well known redox probe (5 mM K4Fe(CN)6 in 0.1 M KCl). The peak splitting (DEp) is a function of the rate of the electron transfer of the electrode material. The CVs were carried out in the presence of a redox probe in order to evaluate their electron transfer properties. A typical response of a BMWCNT electrode for 3=4 a FeðCNÞ6 couple is shown in Fig. 3. The well-defined peaks obtained at +0.268 and +0.179 V in the forward and reverse scans are due to the Fe3+/Fe2+ redox couple. The peak splitting of 77 mV was observed for BMWCNTs, 146 mV for commercial MWCNTs (Fig. 3(a)), and 167 for GCE. We varied the scan rate from 10 to 100 mV (Fig. 3(b)), as the scan rate increases the anodic and cathodic peak potentials and the peak splitting increases from 77 mV (20 mV/s) to 89 mV (for 100 mV/s), indicating a quasireversible process and slow electron transfer kinetics. When comparing the electron transfer of three electrodes, BMWCNTs showed higher rate than commercial

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Fig. 3. (a) Cyclic voltammetric response of BMWCNT/GC and commercial CNT/GC electrodes in 5 mM K4Fe(CN)6 0.1 M KCl at 20 mV/s, and (b) cyclic voltammograms of BMWCNT/GC in 5 mM K4Fe(CN)6 0.1 M KCl at various scan rates.

MWCNTs and GCE. Chou et al. [32] recently observed a similar peak splitting for modified SWNTs. The experiments were conducted by modifying the electrode with SWNTs, which were randomly dispersed or well aligned. They observed a peak splitting of 105 mV for randomly dispersed SWCNTs, and 72 mV for the aligned SWCNTs on electrode. They showed that oxygenated species are of importance for favorable electron transfer kinetics. It is reported that the bamboo-shaped carbon nanotubes exhibit a high percentage of graphitic layers that terminate at the surface of the tube, giving rise to a large number of edge plane sites along the surface of tube, thus exhibiting a better electrochemical activity than the smooth, more basal plane-like regions in hollow-tube CNTs [34–37]. We believe that the bamboo-shaped CNTs exhibit large surface ends, and thus, more edge-plane sites are accessible for the electrolyte, enhancing the electrochemical behavior. The ends of a CNT show different electrochemical properties when compared to the sides. The end of CNT tube could show a reversible electrochemical and low redox potential, which is attributed to the oxygenated species. In case of bamboo CNTs, at the outer tubes, where compartments end, it is a

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possible to have more surface edge sites, so that each compartment exposes its surface to the electrolyte. However, in the case of hollow tubes, the graphitic layer is parallel to the tube axis, and thus only the ends of the tube are exposed to the electrolyte. To corroborate the electron transfer behavior of bamboo-shaped carbon nanotubes, we carried out X-ray photoelectron spectroscopy (XPS) to see the surface of as-synthesized BMWCNTs. The measured spectrum of BMWCNTs indicates the presence of C, Ru, and oxygen (Fig. 4(a)). The inset of Fig. 4(a) shows the high resolution XPS spectrum of O 1s, which exhibits a peak maximum at 532.5 eV that was deconvoluted into three peaks at 530.0, 531.6, and 533.1 eV, suggesting the presence of several different bonding structures of O. The C1s spectrum of as-synthesized BMWCNTs exhibits a peak centered at 285 ± 0.2 eV. The deconvoluted spectrum (inset of Fig. 4(b)) of C1s gave three peaks centered at 285 ± 0.2 eV, 285.6 ± 0.2 and 286.4 ± 0.2 eV, which can be attrib-

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uted to sp2 graphitic (CAC, CAH), sp3(fflCAO) and sp3 (C@O), respectively [44]. IR analysis of as-synthesized BMWCNTs shows the presence of a band at 1715 cm1, associated with C@O in carboxyls or carboxylic anhydrides, and a broad band centered at 1221 cm1, which can be attributed to the ethers, phenols, carboxyls, and carboxylic anhydrides [45]. We also carried out Raman spectra of bamboo-shaped carbon nanotubes to evaluate the defect sites present in the as-synthesized BMWCNTS. It is observed from the Raman spectra of carbon nanotubes, the presence of two strong peaks at 1346 cm1 and 1575 cm1, commonly known as D and G bands of carbon nanotubes. The peak at 1575 cm1 corresponds to an E2g mode of graphite, which is due to the sp2-bonded carbon atoms in a two-dimensional hexagonal graphitic layer. The D band at around 1346 cm1 is associated with the presence of defects in the hexagonal graphitic layers. The ratio of the intensities of the D and G bands is known to be correlated to the quality of the carbon nanotubes, which

Fig. 4. (a) XPS survey of a spectral region of 0–1200 eV. Inset in (a) is the HRXPS spectrum of oxygen 1s, and (b) the carbon 1s and Ru 3d region. Inset in (b) also shows the deconvoluted spectrum of C1s.

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is to estimate the degree of disorder in the graphitic carbon. The ID/IG ratio for the bamboo-shaped CNTs was found to be 0.520, suggesting that the CNTs are disordered and the existence of significant edge-plane sites. We also studied the capacitive nature of as-synthesized BMWCNTs in 1 M KNO3 solution at 25 °C cycled under a potential in the range from 0 to 0.8 V with a scan rate of 100 mV/s (Fig. 5(a)). The CV curve is rectangular in shape, which is a characteristic of the electrical double layer capacitor behavior of the as-synthesized carbon nanotubes. The electrochemical capacitive charging causes electrical energy to be stored in the electric double layer between the electrolyte and the carbon nanotube. The capacitance was calculated using the equation of I = (A)/(dV/dt), where I is the capacitive current and (dV/dt) denotes the scan rate. The specific capacitance of as-synthesized BMWCNT measured at a CV scan rate of 100 mV/s in 1 M KNO3 is 8.3 F/g. To understand the capacitive behavior of as-synthesized BMWCNT, the scan rate variation study was carried out at 25, 50, 100, 200, 300, 400 and 500 mV/s, the results are presented in Fig. 5(b). All the CVs curves showed almost rectangular and show a -3

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capacitive behavior. The mean specific capacitances of BMWCNT with scan rates of 25, 50, 100, 200, 300, 400 and 500 mV/s are 11.5, 10.3, 8.3, 7.7, 7.2, 6.9 and 6.6 F/g, respectively. The capacitance values obtained for as-synthesized bamboo-shaped carbon nanotubes is in agreement with the wide range of capacitance values reported for carbon nanotubes [41–43]. The electrochemical stability of the BMWCNT electrode was investigated by repetitive cycling in the potential range of 0 to 0.8 V vs. SCE for 300 cycles, the specific capacitance is fairly stable, only a 10% of initial capacitance is decreased after 300 cycles, suggesting that good electrochemical stability of the electrode in aqueous electrolyte. It is reported that the capacitance of carbon nanotubes can be improved by introducing various metal oxides such as MnO2, RuO2 [46–48]. These systems were fabricated by introducing the active oxides into/onto the carbon surface by various methods. In our case the Ru is encapsulated in bamboo-shaped CNTs, so it is possible to improve the capactitance of Ru-encapsulated bambooCNTs. The metallic Ru is electrochemically oxidized by applying 0.75 V vs. SCE in 1 M sulphuric acid for 3 h [49]. We did not observe any further improvement in capacitance even after electrochemical oxidation, indicating that the encapsulated Ru was not oxidized and also not accessible for electrolytes. This observation was further corroborated by XRD analysis. As we pointed out earlier, after acid treatment, BMWCNTs also exhibit the presence of metallic Ru, because the graphitic layers wrapped around the metal prevent its oxidation.

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The electrochemical characteristics of as-synthesized bamboo-shaped carbon nanotubes showed a different behavior from the hollow carbon nanotubes. The electron transfer rate of bamboo-shaped carbon nanotubes is faster when compared to commercial, multiwalled carbon nanotubes and glassy carbon electrodes. The observed behavior could be attributed to various factors such as better wettability, more exposure of edge-plane-like sites, and oxygen functional groups. Various studies revealed that as-synthesized bamboo-shaped CNTs exhibit some surface oxide functional groups, which could be responsible for this behavior. The specific capacitance of as-synthesized BMWCNT showed the specific capacitance of 11.5 F/g in 1 M KNO3. The encapsulated Ru nanoparticles were not accessible for the electrolytes, thus the carbon shell is protecting the electrochemical oxidation of metallic Ru to RuO2. Thus the obtained capacitance is attributed to the as-synthesized bamboo-shaped carbon nanotube.

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Fig. 5. (a) Cyclic voltammetric response of a BMWCNT/GC electrode in at 100 mV/s. (b) Effect of scan rate on the cyclic voltammetric behavior of BMWCNT/GC in 1 M KNO3, at 25 °C.

Acknowledgements S.S. is thankful for the Research Authority, Bar-Ilan University for Samuel and Helene Soref Young Scientist fellowship.

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