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graphite-based electrode
Materials Science and Engineering C 29 (2009) 187–192
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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Fabrication and electrochemical behavior of single-walled carbon nanotube/graphitebased electrode Abdolmajid Bayandori Moghaddam a,b, Mohammad Reza Ganjali a,⁎, Rassoul Dinarvand b, Taherehsadat Razavi a, Siavash Riahi c, Saeed Rezaei-Zarchi d, Parviz Norouzi a a
Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran P.O. Box 14155-6455, Tehran, Iran Medical Nanotechnology Research Centre, Medical Sciences/University of Tehran, P.O. Box 14155-6451, Tehran, Iran Institute of Petroleum Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran d Department of Biology, Payam-e-Noor University, Yazd, Iran b c
A R T I C L E
I N F O
Article history: Received 12 December 2007 Received in revised form 26 April 2008 Accepted 13 June 2008 Available online 26 June 2008 Keywords: Single-walled carbon nanotube Atomic force microscopy Atomic charges Cyclic voltammetry
A B S T R A C T An electrochemical method for determining the dihydroxybenzene derivatives on glassy carbon (GC) has been developed. In this method, the performance of a single-walled carbon nanotube (SWCNT)/graphitebased electrode, prepared by mixing SWCNTs and graphite powder, was described. The resulting electrode shows an excellent behavior for redox of 3,4-dihydroxybenzoic acid (DBA). SWCNT/graphite-based electrode presents a significant decrease in the overvoltage for DBA oxidation as well as a dramatic improvement in the reversibility of DBA redox behavior in comparison with graphite-based and glassy carbon (GC) electrodes. In addition, scanning electron microscopy (SEM) and atomic force microscopy (AFM) procedures performed for used SWCNTs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The idea of using nanostructures in electrochemical studies both as a scientific challenge and for practical reasons. In recent years, scientists have learned various techniques for nanofabrication. Electrodes made from various carbon materials (partially graphitized glassy carbon, activated carbon and graphite fibers) are widely used in important electrochemical applications [1–3]. In the work reported by Adams [4], carbon composite electrodes have received enormous attention. Among them, the composites made of graphite powder and mineral oil are the most widely known [5]. Iijima discovered microtubules of graphitic carbon, naming these tubes multiwalled carbon nanotubes (MWCNTs) [6]. Single-walled carbon nanotubes (SWCNTs) were first reported in 1993 [7,8]. The closed topological and tubular structures of carbon nanotubes [9] make them unique among different carbon forms and provide pathways for chemical studies. A number of investigations carried out to find applications of carbon nanotube in modified electrode could impart strong electrocatalytic activity to some important biomolecules [10–16]. O-dihydroxybenzenes are well known in biological systems, often as metabolites of simpler aromatic hydrocarbons such benzene or naphthalene. Due to the presence of two exchangeable hydrogen atoms, the aromatic diols tend to be biologically reactive molecules,
⁎ Corresponding author. Tel.: +98 21 61112788; fax: +98 21 66405141. E-mail address: [email protected] (M.R. Ganjali). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.06.016
capable of exhibiting both anti- and pro-oxidant behaviors [17]. It was realized that the oxidation mechanisms on the electrode and in body share similar principles [18]. On the other hand, the appearance of nanotechnology was opening a new entrance for new electrodes in the field of electrochemical applications [19]. Subsequently, it was obvious that the essential coordination of these scientific branches could lead to fruitful findings. The nano-scale effects in the catalytic properties of gold particles are well-known chemically inert. Gold turns catalytically active when the particle size is below 3–4 nm [20,21]. Electrodes with nanometer dimensions provide exciting new tools for electrochemical studies. The small dimensions lead to a high current density on the electrode surface, allowing the study of fast heterogeneous electron-transfer kinetics, molecular interactions and mass transport in nanometer regime [22]. We report here, 3,4-dihydroxybenzoic acid (DBA) electrochemical behavior at a SWCNT/graphite-based electrode by cyclic voltammetry (CV). 2. Experimental details 2.1. Chemicals and materials DBA and paraffin oil (high purity) were purchased from Aldrich. Graphite powder with a 1–2 μm particle size, acids and the used salts for preparation of buffer solutions, such as CH3COONa/CH3COOH (c = 0.15 M, pH = 4 and 5), K2HPO4/KH2PO4 (c = 0.15 M, pH = 6, 7 and 8) and K3Fe(CN)6 were reagent grade materials from Merck. Glassy
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Fig. 1. (a and b) SEM images of SWCNTs.
carbon rods were prepared from Azar Electrode Co. (Iran). Singlewalled carbon nanotubes (SWCNTs) were purchased from Research Institute of Petroleum Industry (Iran). The SWCNTs were prepared by chemical vapor deposition, with up to 80% yield of high quality and inner wall diameter distribution close to 1.0–1.2 nm were obtained. An economical production method for SWCNTs in large quantity using MgO nanoporous catalyst was developed by Rashidi et al. [23]. Co–Mo catalyst supported MgO nanoporous were prepared by sol–gel method. Methane decomposition at 900–1000 °C over catalyst surface produced carbon nanotubes with single wall structure using a special rotary reactor. Removing of metallic impurity from the product was performed by washing in HNO3.
2.2. Measurements All electrochemical experiments were performed by Autolab potentiostat PGSTAT 30, including a three-electrode cell; a glassy carbon (GC), graphite-based and SWCNT/graphite-based electrodes acted as working electrodes, a platinum wire as counter electrode and Ag/AgCl (KCl, 3 M) as reference electrode. Furthermore, scanning electron microscopic (SEM) studies were performed using ZEISS DSM 960. Atomic force microscopic (AFM) studies were performed with the help of a DME, controller: Dual Scope C-21, and scanner: DS 95-50. Gaussian 98, Revision A. 6, program has been used for the quantum mechanic calculations. The B3LYP/6-31G⁎ method is employed for the
Fig. 2. (a and b) AFM images of SWCNTs, (c) obtained image profile for selected direction by arrow from (a).
A.B. Moghaddam et al. / Materials Science and Engineering C 29 (2009) 187–192
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Fig. 3. (a) Electrochemical behavior of SWCNT-based electrode during continuous CVs (0.25 V/s), comparative CVs of 1 mM DBA on the surface of (b) SWCNT-based electrode, (c) SWCNT/graphite (1/2)-based electrode, (d) SWCNT/graphite (1/1)-based electrode and (e) SWCNT/graphite (2/1)-based electrode in phosphate buffer solution (pH = 6.0), scan rate: 0.1 V/s. (e) Variation of half sum of the anodic and cathodic peak potentials, for DBA/related o-quinone redox couple at SWCNT/graphite-based electrode.
gas-phase molecular geometry and HF/6-31G⁎ method is used for the obtained atomic charges (ESP fit) [24]. Note that the limitation of this approach is that the polarization effect associated with the condensed phase environment is not explicitly included, although the tendency for the HF/6-31G⁎ QM (Quantum Mechanics) level of theory to overestimate dipole moments has been suggested to account for this deficiency [25].
the ratio of 1:1 (w/w). Regarding the casting procedure, a small amount of melted paraffin (~ 20% by weight) was added to SWCNT/ graphite powder mixture. A portion of composite mixture was packed into the end of a polyamide tube. Electrical contact was made by forcing a glassy carbon rod (r = 2 mm) down into the tube and into the back of composite and, then, a thin film on the surface of glassy carbon electrode was prepared. This strategy increased the compactness and
2.3. Preparation of SWCNT/graphite-based electrode For electrochemical studies, SWCNT/graphite-based electrodes were prepared from a mixture of SWCNTs and graphite powder in Table 1 Electrochemical data Electrode
Epa (V)
Epc (V)
ΔEp
Glassy carbon Graphite-based SWCNT/graphite-based
0.543 0.494 0.350
0.104 0.145 0.277
0.439 0.349 0.073
Anodic and cathodic peak potentials of the cyclic voltammograms were obtained at 0.10 V/s. Other conditions as in Fig. 3.
Fig. 4. Redox mechanism of DBA.
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stability of composite on the surface of glassy carbon electrode. Here, it should be mentioned that only a very small amount of melted paraffin had to be used for attachment of composite, because in this way a hard composite creation was achieved, illustrating excellent electrochemical properties. In addition, employment of a greater melted paraffin amount raised the peak potentials. Pure SWCNT, SWCNT/graphite (2/1), SWCNT/graphite (1/2) and graphite-based electrodes were also made using the separate materials, dispersed in a small quantity of melted paraffin, in the same way as for composite electrodes described above. The GC rod was carefully polished with alumina (0.05 μm) on a polishing cloth. The electrode was placed in ethanol and sonicated to remove the adsorbed particles.
The microscopic areas of SWCNT/graphite-based electrode and the bare GC electrode were obtained by CV using 5 mM K3Fe(CN)6 as a probe at different scan rates. For a reversible process, the following equation exists: 3=2
Ipa ¼ 2:69 105 n
1=2 1=2
Ac0 DR
v
ð1Þ
Here, Ipa refers to the anodic peak current. For K3Fe(CN)6, n = 1, DR = 5.9 · 10− 5 cm/s [26], then from the slope of Ipa − v1/2 relation, microscopic areas can be calculated. SWCNT/graphite-based electrode increased nearly 38% in area in comparison to GC electrode.
Fig. 5. (a) CVs of 1 mM DBA in phosphate buffer solution (pH = 7.0) at various sweep rates. From outer to inner; 0.30, 0.25, 0.20, 0.175, 0.150, 0.125, 0.10, 0.075 and 0.05 V/s, at the SWCNT/ graphite-based electrode. The plots of anodic and cathodic peak currents vs. scan rate (b) and square root of scan rate (c). (d) and (e) similar to (b) and (c) for second redox peaks.
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3. Results and discussions 3.1. SEM and AFM characterization Fig. 1(a) and (b) illustrates the SEM images. SWCNTs were formed as boundless tubes because of the van der Waals forces. Fig. 2(a) and (b) depicts the AFM images of SWCNTs. Additionally, Fig. 2(c) demonstrates the related image profile for selected direction by arrow from related AFM image. This profile displays the diameters in selected points of profile with the values of 11.0 and 22.1 nm that these may be the diameter of bundles of SWCNTs. 3.2. Electrochemical behavior of DBA at SWCNT/graphite-based electrode In accordance with Fig. 3(a), SWCNT-based electrode has no electrochemical activity in used solutions, but the background current becomes larger, which attributed to the fact that SWCNTs can increase the surface activity noticeably. This is an advantage for SWCNT substrates, as they are sufficiently stable in a wide potential area. Oxidation or reduction of the substrate surface may lead to unfavorable process. Fig. 3(b) shows the cyclic voltammogram of 1.0 mM DBA on the surface of SWCNT-based electrode. The pure SWCNT-based electrode did not exhibit excellent electrochemical behavior for DBA, on the grounds that the attachment and compactness of pure SWCNTs by melted paraffin is feeble. However, attachment of composite increased with the increasing graphite ratio in the composite. Fig. 3(c) and (d) depicts comparative cyclic voltammograms (CVs) obtained at SWCNT/graphite (1/2)-based electrode and SWCNT/graphite (1/1)-based electrode, after the addition of 1.0 mM DBA in phosphate buffer solution (PBS, pH = 7.0), respectively. The best electrochemical behavior was obtained on SWCNT/graphite surface with the ratio of 1:1, named earlier as SWCNT/graphite-based electrode. The presence of well-defined anodic and cathodic peaks indicated improved electrochemical reactivity for DBA oxidation–reduction reaction on SWCNT/graphite-
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based electrode (Fig. 3(d)). Fig. 3(e) shows the CV of DBA at SWCNT/ graphite (2/1)-based electrode, the electrode with the greater SWCNT ratio did not exhibit excellent electrochemical behavior for DBA. Data in Table 1 demonstrated the characteristic anodic and cathodic peak potentials for electrodes as mentioned above. Thus, the reversibility of DBA was significantly improved at SWCNT/graphite-based electrode. The reason for the better performance of SWCNT/graphite-based electrode may be due to the nanometer dimensions of SWCNTs, electronic structure and presence of large numbers of topological defects (e.g., bond rotational defects or pairs of 5–7 rings; defects which would not create any visible change in the overall topology or curvature) has been suggested [9]. Hence, the surface of nanotubes could be inherently more reactive compared to their graphite counterparts [27]. Depending on their atomic structure, CNTs behave electrically as a metal or as a semiconductor [28]. The subtle electronic properties suggest that CNTs have the ability to promote electrontransfer reactions, when used as an electrode in chemical reactions [29]. In the meantime, SWCNTs augment the effective area of electrode. As DBA oxidation involves the transfer of two protons, we have analyzed the effect of pH on cyclic voltammogram at SWCNT/ graphite-based electrode. Fig. 3(f) consists of relation (Epa +Epc)/2 − pH at SWCNT/graphite-based electrode. This average can be considered approximately as the standard (formal) potential. At SWCNT/graphitebased electrode, standard potential (and the cathodic and anodic peak potentials) decreased by about 55 mV per pH unit, corresponding to a two-electron and two-proton overall redox process (Fig. 4). To further investigate the characteristics of DBA at SWCNT/graphite-based electrode, influence of the scan rate on redox behavior of DBA was studied by cyclic voltammetry (Fig. 5). This figure shows two anodic and corresponding cathodic peaks. The first redox peaks, one anodic peak (A1, at 0.133 V) and one corresponding cathodic peak (C1, at 0.096 V), corresponded to the adsorbed DBA electro-oxidation and vice versa within a two-electron and two-proton process (ΔEp = 0.037 V). Second redox peaks, one anodic peak (A2, at 0.350 V) and one corresponding cathodic peak (C2, at 0.277 V) corresponded to
Fig. 6. (a) CVs of 0.25 mM DBA in phosphate buffer solution (pH = 7.0) at various sweep rates. From outer to inner; 0.25, 0.15, 0.08, 0.04, 0.02 and 0.01 V/s, at graphite-based electrode. Relationship between peak currents (Ipa, Ipc) vs. square root of sweep rates (b) and sweep rates (c).
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4. Conclusions The attractive properties of this new composite material open a new entrance for new electrodes in the field of nanoelectrochemistry. Undoubtedly, nanotechnology in combination with electrochemistry can extremely influence the development rate of these scientific fields. However, a number of challenges remain to be faced, which are related to the processing of nanostructured materials arranged to modify electrodes in a main controlled way. Acknowledgements The authors would like to thank Mr. Hashemi for performance of SEM studies from Science College Electron Microscopy Laboratory, University of Tehran. Additionally, the authors wish to express their appreciation for useful comments of the referees. Moreover, financial support provided by University of Tehran Research Affairs is gratefully acknowledged.
Fig. 7. Optimized geometry (B3LYP/6-31G⁎) and atomic charges of DBA.
the diffusive DBA electro-oxidation and vice versa within a twoelectron and two-proton process (ΔEp = 0.073 V). There is a good linear relationship between the first redox peak currents and scan rates (v) (Fig. 5). This means that the electrode process is controlled by adsorption. In accordance with Fig. 5 there is a good linear relationship between the second redox peak currents and square root of scan rates (v) in the range of 0.05–0.30 V/s (Fig. 5), indicating that the electrode process is controlled by diffusion. On the other hand, with increase of scan rates, the oxidation peaks shift to more positive potentials, while the reduction peaks shift to more negative potentials, indicating that electron-transfer rate is not very fast and electrochemical reaction gradually becomes less reversible. According to Fig. 6, the redox peak currents of DBA increased linearly with the square root of scan rate. This means that DBA electrochemical behavior at graphite-based electrode is a diffusion-controlled process. 3.3. Theoretical characteristics of DBA Electrode process was controlled by adsorption and diffusion on the SWCNT/graphite-based electrode surface (Fig. 5) and it was controlled by diffusion on the surface of graphite-based electrode (Fig. 6). On the other hand, we applied positive potentials to electrode during the DBA electro-oxidation. Thus, DBA atoms with the more negative charges could be the adsorbed sites of DBA on SWCNTs surfaces. For this reason, calculation of DBA atomic charges might be very useful. According to DBA atomic charges (Fig. 7), oxygen atom presents the maximum value of atomic charges and the highest negative charge among the oxygen atoms is attributed to the oxygen atom of hydroxyl group in carboxylic acid group. In detail, the oxygen atom of hydroxyl in carboxylic acid group presents the highest negative charge (−0.688), in contrast to the other oxygen atoms, which exhibits negative charge values of −0.658, and −0.628. Therefore, carbon atom in the carboxylic acid group displays the highest positive charge (0.829). Carbon atoms in aromatic ring are negatively charged. Nevertheless, in the aromatic ring with electron-withdrawing groups, carbon atoms, which are connected to these kinds of groups, are positively charged.
References [1] K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, Wiley, New York, 1988. [2] G.G. Hu, W.L. Wang, K.J. Liao, J. Phys. Chem. Solids 65 (2004) 1731. [3] H. Shimotani, T. Kanbara, Y. Iwasa, Appl. Phys. Lett. 88 (2006) 73104. [4] R.N. Adams, Anal. Chem. 30 (1958) 1576. [5] K. Kalcher, J.M. Kauffmann, J. Wang, Electroanalysis 7 (1995) 5. [6] S. Iijima, Nature 354 (1991) 56. [7] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [8] D.S. Bethune, C.H. Kiang, M.D. de Vries, Nature 363 (1993) 605. [9] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996. [10] J. Huang, Z. Song, J. Li, Y. Yang, H. Shi, B. Wu, J.-I. Anzai, T. Osa, Q. Chen, Mat. Sci. Eng. C 27 (2007) 29. [11] J. Huang, Y. Yang, H. Shi, Z. Song, Z. Zhao, J.-I. Anzai, T. Osa, Q. Chen, Mat. Sci. Eng. C 26 (2006) 113. [12] K.-H. Xue, F.-F. Tao, S.-Y. Yin, Chem. Phys. Lett. 391 (2004) 243. [13] K.-H. Xue, J.-M. Liu, R.B. Wei, Chem. Phys. 327 (2006) 319. [14] X. He, L. Jiang, C. Fan, Chem. Phys. 334 (2007) 253. [15] J. Wang, M. Li, Z. Shi, N. Li, Z. Gu, Anal. Chem. 74 (2002) 1993. [16] M.M. Rahman, I.C. Jeon, J. Braz. Chem. Soc. 18 (2007) 1150. [17] A. Chichirau, M. Flueraru, L.L. Chepelev, J.S. Wright, W.G. Willmore, T. Durst, H.H. Hussain, M. Charron, Free Rad. Biol. Med. 38 (2005) 344. [18] S. Suzen, B.T. Dermircigil, S.A. Ozkan, N. J. Chem. 6 (2003) 1001. [19] A. Bayandori Moghaddam, M.R. Ganjali, R. Dinarvand, A.A. Saboury, T. Razavi, A.A. Moosavi-Movahedi, P. Norouzi, Biophys. Chem. 129 (2007) 259. [20] M. Haruta, Catal. Today 36 (1997) 153. [21] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [22] I. Heller, J. Kong, H.A. Heering, K.A. Williams, S.G. Lemay, C. Dekker, Nano Lett. 5 (2005) 137. [23] A.M. Rashidi, M.M. Akbarnejad, A. Ahmadpour, A.A. Khodadadi, Y. Mortazavi, M. Mahdyarfar, H.R. Ghafarian, The International Conference on Carbon, Oviedo, Spain, July 6–10 2003. [24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., GAUSSIAN 98, Revision A.7, Gaussian, Inc., Pittsburgh, PA, 1998. [25] O.M. Becker, A.D. McKerell Jr., B. Roux, M. Watanabe, Computational Biochemistry and Biophysics, Marcel Dekker Inc., New York, 2001. [26] X.-X. Yan, D.-W. Pang, Z.-X. Lu, J.-Q. Lu, H. Tong, J. Electroanal. Chem. 569 (2004) 47. [27] P.J. Britto, K.S.V. Santhanam, A. Rubio, Adv. Mater. 11 (1999) 154. [28] D. Tekleab, R. Czerw, D.L. Carroll, Appl. Phys. Lett. 76 (2000) 3594. [29] J.M. Nugent, K.S.V. Santhanam, A. Rubio, Nano Lett. 1 (2001) 87.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Fabrication and electrochemical behavior of single-walled carbon nanotube/graphitebased electrode Abdolmajid Bayandori Moghaddam a,b, Mohammad Reza Ganjali a,⁎, Rassoul Dinarvand b, Taherehsadat Razavi a, Siavash Riahi c, Saeed Rezaei-Zarchi d, Parviz Norouzi a a
Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran P.O. Box 14155-6455, Tehran, Iran Medical Nanotechnology Research Centre, Medical Sciences/University of Tehran, P.O. Box 14155-6451, Tehran, Iran Institute of Petroleum Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran d Department of Biology, Payam-e-Noor University, Yazd, Iran b c
A R T I C L E
I N F O
Article history: Received 12 December 2007 Received in revised form 26 April 2008 Accepted 13 June 2008 Available online 26 June 2008 Keywords: Single-walled carbon nanotube Atomic force microscopy Atomic charges Cyclic voltammetry
A B S T R A C T An electrochemical method for determining the dihydroxybenzene derivatives on glassy carbon (GC) has been developed. In this method, the performance of a single-walled carbon nanotube (SWCNT)/graphitebased electrode, prepared by mixing SWCNTs and graphite powder, was described. The resulting electrode shows an excellent behavior for redox of 3,4-dihydroxybenzoic acid (DBA). SWCNT/graphite-based electrode presents a significant decrease in the overvoltage for DBA oxidation as well as a dramatic improvement in the reversibility of DBA redox behavior in comparison with graphite-based and glassy carbon (GC) electrodes. In addition, scanning electron microscopy (SEM) and atomic force microscopy (AFM) procedures performed for used SWCNTs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The idea of using nanostructures in electrochemical studies both as a scientific challenge and for practical reasons. In recent years, scientists have learned various techniques for nanofabrication. Electrodes made from various carbon materials (partially graphitized glassy carbon, activated carbon and graphite fibers) are widely used in important electrochemical applications [1–3]. In the work reported by Adams [4], carbon composite electrodes have received enormous attention. Among them, the composites made of graphite powder and mineral oil are the most widely known [5]. Iijima discovered microtubules of graphitic carbon, naming these tubes multiwalled carbon nanotubes (MWCNTs) [6]. Single-walled carbon nanotubes (SWCNTs) were first reported in 1993 [7,8]. The closed topological and tubular structures of carbon nanotubes [9] make them unique among different carbon forms and provide pathways for chemical studies. A number of investigations carried out to find applications of carbon nanotube in modified electrode could impart strong electrocatalytic activity to some important biomolecules [10–16]. O-dihydroxybenzenes are well known in biological systems, often as metabolites of simpler aromatic hydrocarbons such benzene or naphthalene. Due to the presence of two exchangeable hydrogen atoms, the aromatic diols tend to be biologically reactive molecules,
⁎ Corresponding author. Tel.: +98 21 61112788; fax: +98 21 66405141. E-mail address: [email protected] (M.R. Ganjali). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.06.016
capable of exhibiting both anti- and pro-oxidant behaviors [17]. It was realized that the oxidation mechanisms on the electrode and in body share similar principles [18]. On the other hand, the appearance of nanotechnology was opening a new entrance for new electrodes in the field of electrochemical applications [19]. Subsequently, it was obvious that the essential coordination of these scientific branches could lead to fruitful findings. The nano-scale effects in the catalytic properties of gold particles are well-known chemically inert. Gold turns catalytically active when the particle size is below 3–4 nm [20,21]. Electrodes with nanometer dimensions provide exciting new tools for electrochemical studies. The small dimensions lead to a high current density on the electrode surface, allowing the study of fast heterogeneous electron-transfer kinetics, molecular interactions and mass transport in nanometer regime [22]. We report here, 3,4-dihydroxybenzoic acid (DBA) electrochemical behavior at a SWCNT/graphite-based electrode by cyclic voltammetry (CV). 2. Experimental details 2.1. Chemicals and materials DBA and paraffin oil (high purity) were purchased from Aldrich. Graphite powder with a 1–2 μm particle size, acids and the used salts for preparation of buffer solutions, such as CH3COONa/CH3COOH (c = 0.15 M, pH = 4 and 5), K2HPO4/KH2PO4 (c = 0.15 M, pH = 6, 7 and 8) and K3Fe(CN)6 were reagent grade materials from Merck. Glassy
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Fig. 1. (a and b) SEM images of SWCNTs.
carbon rods were prepared from Azar Electrode Co. (Iran). Singlewalled carbon nanotubes (SWCNTs) were purchased from Research Institute of Petroleum Industry (Iran). The SWCNTs were prepared by chemical vapor deposition, with up to 80% yield of high quality and inner wall diameter distribution close to 1.0–1.2 nm were obtained. An economical production method for SWCNTs in large quantity using MgO nanoporous catalyst was developed by Rashidi et al. [23]. Co–Mo catalyst supported MgO nanoporous were prepared by sol–gel method. Methane decomposition at 900–1000 °C over catalyst surface produced carbon nanotubes with single wall structure using a special rotary reactor. Removing of metallic impurity from the product was performed by washing in HNO3.
2.2. Measurements All electrochemical experiments were performed by Autolab potentiostat PGSTAT 30, including a three-electrode cell; a glassy carbon (GC), graphite-based and SWCNT/graphite-based electrodes acted as working electrodes, a platinum wire as counter electrode and Ag/AgCl (KCl, 3 M) as reference electrode. Furthermore, scanning electron microscopic (SEM) studies were performed using ZEISS DSM 960. Atomic force microscopic (AFM) studies were performed with the help of a DME, controller: Dual Scope C-21, and scanner: DS 95-50. Gaussian 98, Revision A. 6, program has been used for the quantum mechanic calculations. The B3LYP/6-31G⁎ method is employed for the
Fig. 2. (a and b) AFM images of SWCNTs, (c) obtained image profile for selected direction by arrow from (a).
A.B. Moghaddam et al. / Materials Science and Engineering C 29 (2009) 187–192
189
Fig. 3. (a) Electrochemical behavior of SWCNT-based electrode during continuous CVs (0.25 V/s), comparative CVs of 1 mM DBA on the surface of (b) SWCNT-based electrode, (c) SWCNT/graphite (1/2)-based electrode, (d) SWCNT/graphite (1/1)-based electrode and (e) SWCNT/graphite (2/1)-based electrode in phosphate buffer solution (pH = 6.0), scan rate: 0.1 V/s. (e) Variation of half sum of the anodic and cathodic peak potentials, for DBA/related o-quinone redox couple at SWCNT/graphite-based electrode.
gas-phase molecular geometry and HF/6-31G⁎ method is used for the obtained atomic charges (ESP fit) [24]. Note that the limitation of this approach is that the polarization effect associated with the condensed phase environment is not explicitly included, although the tendency for the HF/6-31G⁎ QM (Quantum Mechanics) level of theory to overestimate dipole moments has been suggested to account for this deficiency [25].
the ratio of 1:1 (w/w). Regarding the casting procedure, a small amount of melted paraffin (~ 20% by weight) was added to SWCNT/ graphite powder mixture. A portion of composite mixture was packed into the end of a polyamide tube. Electrical contact was made by forcing a glassy carbon rod (r = 2 mm) down into the tube and into the back of composite and, then, a thin film on the surface of glassy carbon electrode was prepared. This strategy increased the compactness and
2.3. Preparation of SWCNT/graphite-based electrode For electrochemical studies, SWCNT/graphite-based electrodes were prepared from a mixture of SWCNTs and graphite powder in Table 1 Electrochemical data Electrode
Epa (V)
Epc (V)
ΔEp
Glassy carbon Graphite-based SWCNT/graphite-based
0.543 0.494 0.350
0.104 0.145 0.277
0.439 0.349 0.073
Anodic and cathodic peak potentials of the cyclic voltammograms were obtained at 0.10 V/s. Other conditions as in Fig. 3.
Fig. 4. Redox mechanism of DBA.
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stability of composite on the surface of glassy carbon electrode. Here, it should be mentioned that only a very small amount of melted paraffin had to be used for attachment of composite, because in this way a hard composite creation was achieved, illustrating excellent electrochemical properties. In addition, employment of a greater melted paraffin amount raised the peak potentials. Pure SWCNT, SWCNT/graphite (2/1), SWCNT/graphite (1/2) and graphite-based electrodes were also made using the separate materials, dispersed in a small quantity of melted paraffin, in the same way as for composite electrodes described above. The GC rod was carefully polished with alumina (0.05 μm) on a polishing cloth. The electrode was placed in ethanol and sonicated to remove the adsorbed particles.
The microscopic areas of SWCNT/graphite-based electrode and the bare GC electrode were obtained by CV using 5 mM K3Fe(CN)6 as a probe at different scan rates. For a reversible process, the following equation exists: 3=2
Ipa ¼ 2:69 105 n
1=2 1=2
Ac0 DR
v
ð1Þ
Here, Ipa refers to the anodic peak current. For K3Fe(CN)6, n = 1, DR = 5.9 · 10− 5 cm/s [26], then from the slope of Ipa − v1/2 relation, microscopic areas can be calculated. SWCNT/graphite-based electrode increased nearly 38% in area in comparison to GC electrode.
Fig. 5. (a) CVs of 1 mM DBA in phosphate buffer solution (pH = 7.0) at various sweep rates. From outer to inner; 0.30, 0.25, 0.20, 0.175, 0.150, 0.125, 0.10, 0.075 and 0.05 V/s, at the SWCNT/ graphite-based electrode. The plots of anodic and cathodic peak currents vs. scan rate (b) and square root of scan rate (c). (d) and (e) similar to (b) and (c) for second redox peaks.
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3. Results and discussions 3.1. SEM and AFM characterization Fig. 1(a) and (b) illustrates the SEM images. SWCNTs were formed as boundless tubes because of the van der Waals forces. Fig. 2(a) and (b) depicts the AFM images of SWCNTs. Additionally, Fig. 2(c) demonstrates the related image profile for selected direction by arrow from related AFM image. This profile displays the diameters in selected points of profile with the values of 11.0 and 22.1 nm that these may be the diameter of bundles of SWCNTs. 3.2. Electrochemical behavior of DBA at SWCNT/graphite-based electrode In accordance with Fig. 3(a), SWCNT-based electrode has no electrochemical activity in used solutions, but the background current becomes larger, which attributed to the fact that SWCNTs can increase the surface activity noticeably. This is an advantage for SWCNT substrates, as they are sufficiently stable in a wide potential area. Oxidation or reduction of the substrate surface may lead to unfavorable process. Fig. 3(b) shows the cyclic voltammogram of 1.0 mM DBA on the surface of SWCNT-based electrode. The pure SWCNT-based electrode did not exhibit excellent electrochemical behavior for DBA, on the grounds that the attachment and compactness of pure SWCNTs by melted paraffin is feeble. However, attachment of composite increased with the increasing graphite ratio in the composite. Fig. 3(c) and (d) depicts comparative cyclic voltammograms (CVs) obtained at SWCNT/graphite (1/2)-based electrode and SWCNT/graphite (1/1)-based electrode, after the addition of 1.0 mM DBA in phosphate buffer solution (PBS, pH = 7.0), respectively. The best electrochemical behavior was obtained on SWCNT/graphite surface with the ratio of 1:1, named earlier as SWCNT/graphite-based electrode. The presence of well-defined anodic and cathodic peaks indicated improved electrochemical reactivity for DBA oxidation–reduction reaction on SWCNT/graphite-
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based electrode (Fig. 3(d)). Fig. 3(e) shows the CV of DBA at SWCNT/ graphite (2/1)-based electrode, the electrode with the greater SWCNT ratio did not exhibit excellent electrochemical behavior for DBA. Data in Table 1 demonstrated the characteristic anodic and cathodic peak potentials for electrodes as mentioned above. Thus, the reversibility of DBA was significantly improved at SWCNT/graphite-based electrode. The reason for the better performance of SWCNT/graphite-based electrode may be due to the nanometer dimensions of SWCNTs, electronic structure and presence of large numbers of topological defects (e.g., bond rotational defects or pairs of 5–7 rings; defects which would not create any visible change in the overall topology or curvature) has been suggested [9]. Hence, the surface of nanotubes could be inherently more reactive compared to their graphite counterparts [27]. Depending on their atomic structure, CNTs behave electrically as a metal or as a semiconductor [28]. The subtle electronic properties suggest that CNTs have the ability to promote electrontransfer reactions, when used as an electrode in chemical reactions [29]. In the meantime, SWCNTs augment the effective area of electrode. As DBA oxidation involves the transfer of two protons, we have analyzed the effect of pH on cyclic voltammogram at SWCNT/ graphite-based electrode. Fig. 3(f) consists of relation (Epa +Epc)/2 − pH at SWCNT/graphite-based electrode. This average can be considered approximately as the standard (formal) potential. At SWCNT/graphitebased electrode, standard potential (and the cathodic and anodic peak potentials) decreased by about 55 mV per pH unit, corresponding to a two-electron and two-proton overall redox process (Fig. 4). To further investigate the characteristics of DBA at SWCNT/graphite-based electrode, influence of the scan rate on redox behavior of DBA was studied by cyclic voltammetry (Fig. 5). This figure shows two anodic and corresponding cathodic peaks. The first redox peaks, one anodic peak (A1, at 0.133 V) and one corresponding cathodic peak (C1, at 0.096 V), corresponded to the adsorbed DBA electro-oxidation and vice versa within a two-electron and two-proton process (ΔEp = 0.037 V). Second redox peaks, one anodic peak (A2, at 0.350 V) and one corresponding cathodic peak (C2, at 0.277 V) corresponded to
Fig. 6. (a) CVs of 0.25 mM DBA in phosphate buffer solution (pH = 7.0) at various sweep rates. From outer to inner; 0.25, 0.15, 0.08, 0.04, 0.02 and 0.01 V/s, at graphite-based electrode. Relationship between peak currents (Ipa, Ipc) vs. square root of sweep rates (b) and sweep rates (c).
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4. Conclusions The attractive properties of this new composite material open a new entrance for new electrodes in the field of nanoelectrochemistry. Undoubtedly, nanotechnology in combination with electrochemistry can extremely influence the development rate of these scientific fields. However, a number of challenges remain to be faced, which are related to the processing of nanostructured materials arranged to modify electrodes in a main controlled way. Acknowledgements The authors would like to thank Mr. Hashemi for performance of SEM studies from Science College Electron Microscopy Laboratory, University of Tehran. Additionally, the authors wish to express their appreciation for useful comments of the referees. Moreover, financial support provided by University of Tehran Research Affairs is gratefully acknowledged.
Fig. 7. Optimized geometry (B3LYP/6-31G⁎) and atomic charges of DBA.
the diffusive DBA electro-oxidation and vice versa within a twoelectron and two-proton process (ΔEp = 0.073 V). There is a good linear relationship between the first redox peak currents and scan rates (v) (Fig. 5). This means that the electrode process is controlled by adsorption. In accordance with Fig. 5 there is a good linear relationship between the second redox peak currents and square root of scan rates (v) in the range of 0.05–0.30 V/s (Fig. 5), indicating that the electrode process is controlled by diffusion. On the other hand, with increase of scan rates, the oxidation peaks shift to more positive potentials, while the reduction peaks shift to more negative potentials, indicating that electron-transfer rate is not very fast and electrochemical reaction gradually becomes less reversible. According to Fig. 6, the redox peak currents of DBA increased linearly with the square root of scan rate. This means that DBA electrochemical behavior at graphite-based electrode is a diffusion-controlled process. 3.3. Theoretical characteristics of DBA Electrode process was controlled by adsorption and diffusion on the SWCNT/graphite-based electrode surface (Fig. 5) and it was controlled by diffusion on the surface of graphite-based electrode (Fig. 6). On the other hand, we applied positive potentials to electrode during the DBA electro-oxidation. Thus, DBA atoms with the more negative charges could be the adsorbed sites of DBA on SWCNTs surfaces. For this reason, calculation of DBA atomic charges might be very useful. According to DBA atomic charges (Fig. 7), oxygen atom presents the maximum value of atomic charges and the highest negative charge among the oxygen atoms is attributed to the oxygen atom of hydroxyl group in carboxylic acid group. In detail, the oxygen atom of hydroxyl in carboxylic acid group presents the highest negative charge (−0.688), in contrast to the other oxygen atoms, which exhibits negative charge values of −0.658, and −0.628. Therefore, carbon atom in the carboxylic acid group displays the highest positive charge (0.829). Carbon atoms in aromatic ring are negatively charged. Nevertheless, in the aromatic ring with electron-withdrawing groups, carbon atoms, which are connected to these kinds of groups, are positively charged.
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