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Structural and electrochemical characterization of carbon electrode modified by multi-walled carbon nanotubes and surfactant
Accepted Manuscript Title: Structural and Electerochemical Characterization of Carbon Electrode Modified by Multi-Walled Carbon Nanotubes and Surfactant Authors: Elmira Pajootan Mokhtar Arami PII: DOI: Reference:
S0013-4686(13)01734-9 http://dx.doi.org/doi:10.1016/j.electacta.2013.09.012 EA 21229
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-7-2013 3-9-2013 3-9-2013
Please cite this article as: E. Pajootan, M. Arami, Structural and Electerochemical Characterization of Carbon Electrode Modified by MultiWalled Carbon Nanotubes and Surfactant, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.09.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and Electerochemical Characterization of Carbon Electrode Modified by Multi-Walled Carbon Nanotubes and Surfactant
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Elmira Pajootan1, Mokhtar Arami2∗ Textile Engineering Department, Amirkabir University of Technology,424 Hafez Ave, Tehran, Iran, 15875-4413. Tel: +982164542614, Fax: +98 2166400245, E-mail: [email protected]
2
Textile Engineering Department, Amirkabir University of Technology,424 Hafez Ave, Tehran, Iran, 15875-4413. Tel: +982164542614, Fax: +98 2166400245, E-mail: [email protected]
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1
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Abstract
In this study, multi-walled carbon nanotubes were successfully deposited on the surface of
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the carbon electrode (CE) using a cationic surfactant (Cetyl Trimethyl Ammonium Bromide, CTAB) by simple electrodeposition method. FT-IR spectra, SEM images and the contact angle
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measurements were employed to characterize the structure of the modified electrode. The electrochemical impedance spectroscopy and cyclic voltammetry were performed to evaluate the
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electroanalytical behavior of electrodes through the modification process. The results indicated
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that the impedance of the modified electrode was reduced about 95% at pH 3 and 7.4 and 91% at pH 11 due to the deposition of the nanotubes on its surface. The equivalent circuit for EIS measurements was perfectly fitted at three pH values (3, 7.4 and 11) on the basis of the transmission line model that represents the impedance response of a diffusion process to characterize the properties of the bulk films. Also, the electrochemical behavior of methylene blue on the surface of the modified electrode was investigated. The obtained cyclic voltammograms showed three redox peaks, which can be related to the formation of semimethylene blue and leucomethylene blue.
Keywords: Multi-Walled Carbon Nanotubes, Electrodeposition, Electrochemical Impedance Spectroscopy, Cyclic Voltammetry, Transmission Line Model. ∗
Author to whom all correspondence should be addressed: 424 Hafez Ave, Tehran, Iran, 15875-4413. Tel: +98 //2164542614, Fax: +98 2166400245, Email: [email protected].
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1. Introduction Electrode material based on carbonaceous compounds has been widely applied in electrochemistry and electroanalysis as working electrode due to their special properties including low resistivity, chemical inertness, and unique surface chemistry, which make them a
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proper choice to determine a wide range of substances in electrocatalytic area [1-7]. Also carbon electrodes (CEs) have the advantage of having a larger hydrogen overvoltage rather than metals
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[8]. Various types of CEs such as carbon paste [9, 10], carbon fiber [11], glassy carbon [12], screen-printed carbon [13], carbon nanotubes (CNTs) [14, 15], etc. have been developed and/or
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modified to achieve specific properties for a variety of electrochemical purposes.
The nano-scale structure, large surface area, high mechanical strength and extraordinary
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electronic properties of CNTs have made them useful in several attractive applications including nanosized semiconductor devices, high performance nanocomposites, energy conversion devices, sensors, etc [14, 15]. The modification of electrodes using multi-walled carbon
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nanotubes (MWCNTs) to design new analytical sensors has been prevalently reported [16-20]. Homogenous and consistent coating of CNTs on the surface of the electrodes can be attained by
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electrically charged CNTs [21].
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simple, low cost and feasible electrodeposition method followed by the uniform dispersing of The previous investigations reported that the CNTs have been negatively or positively
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charged by carboxylic or amino functionalization which helps disperse individual CNTs in the suspension for electrodeposition process [22, 23]. Also several studies have surveyed the electrodeposition of the combination of CNTs/metal ions or CNTs/metal oxides on the surface of various electrode substrates [24-27].
Another appropriate and simple method to overcome the poor solubility of CNTs to attain the stable dispersion is the application of surfactants. In this procedure, the surfactant is adsorbed on the surface of CNTs, and subsequent ultrasonication of the solution, which takes several minutes, will cleave apart their aggregations and debundle nanotubes by steric or electrostatic repulsions resulted from the charge of surfactant hydrophilic groups [28-30]. In this paper, MWCNTs have been electrodeposited by applying direct current onto the CE under ambient conditions in mild solution with high efficiency. The electrodeposition solution contained MWCNTs and a cationic surfactant (Cetyl Trimethyl Ammonium Bromide, CTAB) that sonicated for 60 min. The literature reviews indicated that the dispersion of MWCNTs via
Page 2 of 30
cationic surfactant and further fabrication of CEs by electrodeposition has not yet been investigated. Surface morphology and structure of the prepared electrodes were studied using scanning electron microscope (SEM). The dynamic contact angle of the electrodes was also measured to determine the hydrophilicity of the surface. FT-IR analysis was employed to
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compare the surface groups of the electrode before and after the deposition. Furthermore, in the current study, the electrochemical properties of the electrodes during different steps of
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preparation have been illustrated. The electrochemical impedance spectroscopy of electrodes was carried out to understand the transport and reaction processes occurring in the film; and the
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transmission line model to describe the diffusive layer allowed for a satisfactory description of the data obtained. The cyclic voltammetry was also established to investigate the determination of the organic 3,7-bis(dimethylamino)phenothiazine-5-ium-chloride as the model dye
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(Methylene Blue, MB) in aqueous solutions by modified electrode. MB has been widely utilized in electrocatalysis and electrochromics; and there are many redox reactions available to MB [31].
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Finally, a mechanism was proposed for the reaction of MB on the surface of the modified
2.1. Chemicals and materials
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2. Materials and methods
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electrode.
In all experiments the reagents used were of analytical grade. Cetyl Trimethyl Ammonium
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Bromide ((C16H33)N(CH3)3Br, CTAB) and multi-walled carbon nanotubes (MWCNTs) (purity> 95%, length 10-20 μm and diameter 30-50 nm) were purchased from Merck and Neutrino, respectively. C.I. Basic Blue 9 (Methylene Blue, MB) was obtained from Ciba Co. Buffer solutions including pH 3 (citrate and hydrochloric acid), pH 7.4 (phosphate buffer solution) and pH 11 (boric acid, potassium chloride and sodium hydroxide), all with the conductivity of 6 mS/cm were also purchased from Merck. 2.2. Preparation of the modified electrode Prior to modification, carbon electrodes (graphite with the dimension of 3×20×80 mm3) were hand-polished with emery paper, then washed with distilled water and pretreated with NaOH (10%), HNO3: H2O (1:1, v/v) and acetone, each for 5 min, respectively. Finally the electrodes (pretreated-CE) were rinsed with distilled water. The electrode modification by electrodeposition
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method was carried out by applying DC voltage of 17.5 (V) to the solution containing 0.3 g/L MWCNTs and 0.2 g/L CTAB, which was sonicated for 60 min using Delta D68H Ultrasonic. The ultrasonication of MWCNTs via CTAB will lead to the dispersion of nanotubes, and fix the surfactants on the surface of MWCNTs (possible arrangements of CTAB on MWCNTs are
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illustrated in Fig. 1(a), (b) and (c) [28]). It can be described that the cationic surfactant will make the nanotubes positively charged; and through simple electrodeposition process, these charged
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CNTs are driven toward cathode to form a thin layer at the electrode surface.
Eventually, the coated electrodes (MWCNTs/CTAB-CE) were immersed in the bicarbonate
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solution (0.01 M) for 30 min in order to extract the residual surfactants from the surface of electrode. The modified electrodes were washed with distilled water and dried in room temperature (MWCNTs-CE). The modification steps are schematically shown in Fig. 1. In this
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study, the structural as well as the electrochemical characterization of the modified electrode
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(MWCNTs-CE) in the course of the preparation processes have been investigated. Figure 1.
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The
Fourier
transform
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2.3. Characterization of the modified electrode infrared
(FT-IR)
spectroscopy
of
CE,
pretreated
CE,
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MWCNTs/CTAB-CE and MWCNTs-CE were measured with a Thermo Nicolet Avatar 360 FTIR Spectrometer within the range of 500- 4000 cm-1. The surface morphology of CE at different steps of modification was observed using a field emission scanning electron microscope ((FESEM) JSM-6700F, JEOL, Japan) operated at the voltage of 20.0 kV.
Dynamic contact angle of the electrodes were measured using KRŰSS processor tensiometer K100 MK2/SF/C (Germany) to investigate the surface hydrophobicity to confirm the suggested modification mechanism. Impedance measurements were carried out in a three-electrode system made by Ivium Compactstat (Eindhoven, the Netherlands). A saturated calomel electrode (SCE) and graphite rod were used as a reference and auxiliary electrodes, respectively. The software used for determining the equivalent circuit and data analysis was Ivium Equivalent Circuit Evaluator
Page 4 of 30
(Eindhoven, the Netherlands). EIS was measured in solutions with pH of 3, 7.4 and 11 at 25 ˚C. The frequency was varied in the range of 100 Hz to 106 Hz. The cyclic voltammetry (CV) was performed in a three-electrode cell with Ag/AgCl as reference electrode and Pt as the counter electrode. CV was once made in solutions with pH of 3,
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7.4 and 11 to study the capacitive behavior of electrodes and once in solution containing MB and Fe3+ to determine MB in the solution, between the potentials of -0.6 V and +0.8 V (vs. Ag/AgCl)
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at scan rate of 50 mV/s.
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3. Results and discussions 3.1. Structural characterization of the electrodes
The contact angle is the angle where a liquid/vapor interface meets a solid surface, which
an
quantifies the wettability of a solid surface by a liquid via the Young’s equation [32, 33]. γsv = γsl.Cos θ + γsl
(1)
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where γsv, γsl and θ are the solid surface free energy, solid/liquid interfacial free energy and contact angle, respectively.
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In this study the dynamic contact angle of the electrode surface with water has been evaluated, when the electrode is entering the water (advancing contact angle). In this regard,
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samples of electrodes with the dimension of 1×10×20 mm3 were prepared and entered into the water container by a holder. As a result of the wetting process, advancing angles always simulate
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a fresh surface for the contact angle; this is formed immediately after the creation of the contact between the liquid and the surface. This type of measurement is the most reproducible way of measuring contact angles.
According to the obtained results, the surface of the CE and pretreated CE has the contact angle of 94.04˚ and 99.44˚, respectively. It is evident that the surface of carbon materials is hydrophobe and their contact angle is higher than 90˚. MWCNTs/CTAB-CE has the lowest contact angle (77.95˚) due to the existence of surfactants (CTAB) on the electrode surface. In other words, the surfactants have made the surface more hydrophile. After the extraction of surfactants from the surface, the contact angle increased again to reach the value of 92.16˚. Considering the above, the results of contact angle measurements confirmed the proposed mechanism for the preparation of the modified CE (Fig. 1), specially the extraction of the residual surfactants from the surface.
Page 5 of 30
FT-IR spectra of different electrodes are presented in Fig. 2. The addition of cationic surfactant (CTAB) for charging and dispersing the nanotubes affected the peaks that appeared in the FT-IR spectra. The broad peak at 3430 cm-1 is attributed to the O–H stretching bond which is
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due to the water absorption of the carbon material in the atmosphere. The weak peak observed in all samples at 1725 cm-1 is related to the C=O bond which is probably due to the impurities of
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carbon and MWCNTs. The corresponding peaks of C–H (alkene bending), C=C and C≡C are placed at 802, 1625 and 2028 cm-1, respectively, which appeared only on CE and Pretreated CE,
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meaning that either these bonds do not exist in MWCNTs or they are not numerous. The peaks at 2923 and 2853 cm-1 are related to the symmetric alkane stretching of C–H bond, the intensity of which is higher in the MWCNTs/CTAB-CE due to the presence of the surfactants. Also the
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peaks – appearing at 1465 and 1253 cm-1 attributed to the CH2 (alkane bending) and C–N amine bond – are exclusively observed in the MWCNTs/CTAB-CE sample due to the existence of the
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surfactants.
d
Figure 2.
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The variation of the surface morphology of CE during the modification process was studied by FESEM images. The surface of CE before any treatment (Fig. 3(a)) was smooth and even.
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Based on the performed experiments, the nanotubes did not deposit on the surface of the untreated CEs. According to Fig 3(b), the pretreated CE showed a rough and scaly surface structure which increased the surface area of CE and facilitated the electrodeposition of nanotubes. Fig 3(c) and (d) illustrated that the nanotubes had been successfully deposited on the surface of the pretreated electrodes to form a highly porous film, and the subsequent removal of surfactants from the surface did not affect the nonotubes arrangement on the surface. Figure 3. Also the FESEM images of the cross section of modified electrode are shown in Fig. 3(e) and (f). It can be seen that a thin layer of MWCNTs with the thickness of about 6 μm has been successfully deposited on the surface of the pretreated CE.
Page 6 of 30
3.2. Electrochemical characterization of the electrodes The electrochemical characteristics of the prepared electrodes were investigated by cyclic voltammetry. Fig. 4 represents the comparative cyclic voltammograms for different CEs measured in three pH values (3, 7.4 and 11) at a scan rate of 50 mV/s. According to Fig. 4, the
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area under the pulse gets larger in the order of: MWCNTs-CE > MWCNTs/CTAB-CE > Pretreated CE > CE for all pH values, which can be related to the increase of electrochemical
cr
surface area after the pretreatment steps with further electrodeposition of MWCNTs on the surface; this improves the charge transfer and confirms the formation of a thin layer on the
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pretreated CE, which significantly improves its capacitive properties by showing a fairly large capacitance-like currents. According to the Fig. 4 the CE sample does not show any redox peaks at all pH values. The Pretreated CE exhibits a weak redox peak at 0.21 V; while, the similar peak
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with significantly enhanced current has been observed for MWCNTs-CE sample at pH 7.4. At pH 3, the redox peaks have been observed at 0.45 and 0.6 V for Pretreated CE and MWCNTs-
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CE, respectively, the appearance of which could be related to water discharge. In addition, the redox peaks observed at the negative potential values for MWCNTs-CE and MWCNTs/CTAB-
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CE at pH 7.4 and for MWCNTs-CE at pH 3 are probably attributed to the reduction of the absorbed oxygen [14, 34]. The results of the redox peaks indicate that these responses are pH
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dependent and are due to the redox reaction by the functional groups existing in the carbonaceous material (as explained earlier in FT-IR analysis) like C=O as impurities. The
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obtained results are consonant with the previous studies [35, 36]. Figure 4.
Also Fig. 4 indicates that the pH 3 solution produces the largest area enclosed by the CV curves and the pseudo-capacitive behaviors are less significant for electrodes in the solution with higher pH values. Perhaps at lower pH values more hydrogen ions are available as a reagent for Faradaic reaction [37]. These acquired results are in agreement with previous reports and suggest that the prepared MWCNTs-CE could be used in energy storage applications in acidic electrolytes [21]. Fig. 5 exhibits the electrochemical impedance of the prepared electrodes. EIS is a sensitive and non-destructive technique, which is often applied to determine the characterization of
Page 7 of 30
electrode processes and complex interfaces [38]. Therefore, in this research the EIS has been applied to survey the electrochemical behavior and charge transfer ability across the prepared electrodes interface. According to Fig. 5, the electrode impedances at frequencies above 102 Hz do not differ
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much and the electrode conductivities are close to each other; this means that the deposition of nanotubes on the surface of the electrodes does not affect the impedance at high frequencies. But
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the impedances of electrodes at low frequencies became distant from each other. For example the impedance at 1 Hz was reduced up to 95% at both pH 3 and 7.4, and about 91% at pH 11 after
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the deposition of nanotubes and further removal of surfactants from the surface, which may be attributed to the large surface area and high electrical conductivity of the coated nanotubes on the
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surface of the electrode lowering the electrode resistance more effectively.
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Figure 5.
In order to explain the impedance behavior of different electrodes, the proposed equivalent circuit model estimated from the shape of the data curve in Nyquist plots at pH values of 3, 7.4
te
d
and 11 (Fig. 6) was used to fit the EIS measurement results using Zview software. Figure 6.
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In the present study, the measured data are fitted to the selected circuit to determine the circuit parameters. According to the circuit model (Fig. 7), Rs is the solution resistance, ZD is the diffusion impedance and Rct is the charge transfer resistance. ZCPE represents the double layer constant phase element (CPE), which is explained by equation (2):
where
(2)
, ω is the angular frequency (rad/s) = 2πf; f is frequency (Hz). The parameter q
indicates the value of the capacitance of the CPE as n approaches 1. The parameter n (0 < n < 1) determines the micro fractal and distribution of the phase-phase interface. When n = 0.5 the impedance equals the Warburg impedance and when n = 1, the CPE is identical to a capacitor [14, 39].
Page 8 of 30
The semi-circle-like complex plane plots at high frequencies were observed in all electrodes, while the low frequency line becomes considerably smaller after the pretreatment and deposition of CNTs on the surface. Since the sloped line similar to Warburg impedance was observed in the Nyquist plots at low frequencies, the diffusive impedance (Fig. 7) which is generally expressed
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with the transmission line model was employed to represent the impedance response of the diffusion process. This model has been used to analyze different systems where the charge
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transfer is controlled by a diffusion mechanism [40-42]. According to Fig. 6 the adoptive model has shown excellent fitting to the experimental data. In this model ZD is determined using the
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(3)
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following equations (3-5):
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where
(5)
te
d
and
(4)
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The parameter x1 and x2 refer to the impedance per unit length (Ω/cm) for liquid and solid phase, respectively, and ζ is the impedance length (Ω.cm) corresponding to the diffusion length L (cm).
Figure 7.
In the equivalent circuit, the interface and liquid phase is explained by q3 and r1, respectively, and the solid phase is represented by the parallel connection of r2 and q2. The corresponding fitting data are listed in Table 1. In all samples, L was chosen as 1 and Rs was 10 Ω. R1 represents the diffusive resistance in the pores of the film, Q3 is related to the trapping, motion or delay of local charges at the liquid/solid interface; while the combination of R2 and Q2 are correlated to the disordered solid phase [40, 43]. The obtained results in Table 1 indicate that the resistances in the diffusive model (R1 and R2) decrease after the deposition of
Page 9 of 30
nanotubes on electrode surface for all pH measurements. The reduction of the ion transfer resistance in pores and interface has significantly decreased the diffusive impedance of the porous structured MWCNTs -CE and MWCNTs/CTAB-CE. The maximum diffusion impedance (ZD-Max) obtained at the lowest frequencies (1 Hz) are also given in Table 1. The ZD-Max of the
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nanotubes deposited electrodes are 36.90, 47.85 and 134.48 Ω for MWCNTs-CE at pH 3, 7.4 and 11, respectively; which implies the lower energy barrier of charge transfer within the CNTs
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deposited films at pH 3 and confirms the data of cyclic voltammetery.
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Table 1
Therefore, the equivalent circuit model adopted to explain the shape of the EIS spectra,
an
suggested that a distributed transfer process was the reason for the disordered line observed at low frequencies. Also one can conclude that the overall effect of charge transfer in bulk,
M
electrode/liquid boundaries and the solid phase influence the electrochemical behavior of electrodes at various range of frequencies in different media.
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3.3. Determination of MB
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The electrochemical response of 3×10-2 mM MB at the surface of MWCNTs-CE was
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investigated in 0.1 mM Fe2(SO4)3 at three pH values (3 for acidic, 7 for neutral and 11 for alkali media). As it can be seen in Fig. 8, the CE and pretreated CE showed no peak current at pH 3 in this potential range indicating that there was no redox process on the surface of CE in the presence of MB. It can be seen that the MWCNTs/CTAB-CE has shown a weak peak at -0.09 V due to the oxidation of MB at the surface of the electrode. This may be due to the repulsion of cationic MB and positively charged CTAB on the surface of the electrode, which reduces the concentration of MB on the surface of the modified electrode. On the other hand, the MWCNTsCE has demonstrated different behavior, which means that the nanostructured MWCNTs-CE acts as an electrocatalyst for the reduction of MB. The huge specific surface area of CNTs can increase the effective area of the electrode; as a result, the peak current increases too. At pH 11 no peak was observed in the reverse scan suggesting that the oxidation of MB at MWCNTs-CE
Page 10 of 30
is irreversible and weak. Three sets of redox peaks could be discerned at pH 3 and 7. The sharper and higher current peak at pH 3 suggests that the redox reaction happens faster. According to the results, the determination of MB and its possible mechanism was investigated further at pH 3. There are many reports about the reduction of MB in electron
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transfer reaction showing one couple redox peaks or two oxidation peaks relating to either electropolymerization of methylene blue at positive potential or oxidation of MB immobilized on
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the surface of the electrode [44-46].
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Figure 8.
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To determine the mechanism of the oxidation of MB at the surface of the MWCNTs-CE, the cyclic voltammetry were performed 4 times, illustrated in Fig. 9(a). The voltammograms indicates that the reaction at the surface of the electrode is not the electropolymerization of MB
M
due to the increasing of the peaks heights by increasing the cycle numbers. If this was an electropolymerization process, the height of the first peak would decrease; whereas, the height of
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the second one (polymer formation) would increase [44, 47].
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Fig. 9(b) exhibits the corresponding voltammograms of MB at potential range of 0.25 to 0.8 V for 4 cycles. When the experiment is performed in this potential range, no redox peaks are
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observed. This means that the appearance of the second reduction peak is dependent on the existence of the first, and also the height of the second peak is higher than the first one.
Figure 9.
There are various studies that have investigated the redox mechanism of MB+ in different media (acidic, neutral and alkali media) on the surface of various electrodes. For example Bauldreay and Archer reported the oxidation of MB+ on SnO2 electrodes at pH 1 [48], Karyakin et al. also investigated the oxidation of MB+ at glassy carbon electrodes in aqueous solution at pH 9.1 [49], Pennarun et al. have discussed the reduction of MB+ at pH 8 at micro-optical ring electrodes [45], Žutić and Svetliĉić have also studied the reduction of MB+ at gold electrode at
Page 11 of 30
pH 7.9 [50, 51], etc. In the present study, the possible mechanism proposed by the obtained voltammograms could be described as below: According to the literatures [46, 52, 53], semi-methylene blue (HMB +•) could be produced in acid solution (pH < 6) by taking one electron and one proton, then the unstable semi-
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methylene blue takes another electron and forms leucomethylene blue (MBH32+). Then MBH32+/ HMB +• can be oxidized by Fe3+ and form HMB +•/MB+ and Fe2+. The HMB +• can also be
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reduced by Fe2+ in acidic media to form MBH32+ [52]. The possible reactions of MB+ occurring
an
Figure 10.
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at the surface of the MWCNTs-CE can be written as follows (Fig. 10):
At higher pH values, the formation of insoluble Fe(OH)3 and Fe(OH)2 in the solution could decrease the redox reactions by Fe3+/Fe2+; and also these iron hydroxides could be polymerized
M
at pH > 3.5 and there is a possibility that they can remove the dissolved MB+ by surface complexation or electrostatic attraction that decrease the concentration of MB+ at the surface of
d
the MWCNTs-CE to lower the current peaks [54].
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According to our results, a mechanism for the redox reaction of MB at the surface of the MWCNTs-CE has been proposed. The work herein did not aim to provide the definite and
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detailed mechanism for MB reactions, and our main interest was to compare the voltammograms resulted from the CE before and after the modification with nanotubes in the presence of MB in the aqueous media. However, the more precise, comprehensive and quantified survey is needed to satisfactorily justify the oxidation and reduction mechanism of MB+ at the surface of the MWCNTs-CE. 4. Conclusion
In this paper the construction and formation of the thin layer deposition of MWCNTs on the surface of the carbon electrode were performed. Detection of methylene blue as a hazardous organic compound in aqueous solution was described. The electrodeposition of the MWCNTs accompanied by cationic surfactants (CTAB) as the driving force toward the cathode has been demonstrated. The characterization and properties of the electrode through the stages of preparation have been studied by SEM images, FT-IR spectra and the advancing contact angle.
Page 12 of 30
The obtained data confirmed the structure variation within the modification process; and a mechanism for the deposition of nanotubes was proposed. The electrochemical impedance spectroscopy and cyclic voltammetry of CE, pretreated CE, MWCNTs/CTAB-CE and MWCNTs-CE were also investigated at three pH values (3, 7.4 and 11). The pseudo-capacitive
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behavior of the modified electrode was more significant at pH 3. The preparation of MWCNTsCE has served two purposes: first to lower the impedance and second, to enlarge the interacting
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surface area for the detection of MB. EIS data were fitted with the suggested equivalent circuit, and the CNTs deposited electrodes showed the lowest diffusive impedance. Due to the unique
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electronic structures of theirs, the nanotubes can enhance the electrochemical reaction; therefore, methylene blue dye was strongly adsorbed on the surface of the electrode and exhibited three redox peaks. The resulted voltammograms revealed that MB reduces on the surface of
an
MWCNTs-CE by two electrons in a consecutive two one-electron steps and forming semimethylene blue (HMB +•) and leucomethylene blue (MBH32+). The promising results obtained by
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this modified electrode might also be used for the detection of organic and colored hazardous
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Ac ce p
105.
te
organic dyes by electrochemical methods: A general review, Appl. Catal., B, 87 (2009)
Page 18 of 30
Q3(Fs β -1)
β
CE
175.4
164.2
7.44 ×10-8
1
9.62 ×10-5
0.87
Pretreated CE
185.3
173.4
4.98×10-8
1
3.25×10-4
MWCNTs/CTABCE
3.58
4.881
3.19×10-2
0.33
5.44×10-3
MWCNTs-CE
4.762
24.75
2.633×10-3
1
3.87×10-3
CE
24.99
5.5×106 1.47×10-5
1
4.11×10-4
Pretreated CE
14.12
60.24
3.14×10-4
0.87
MWCNTs/CTABCE
4.609
37.95
1.244×10-3
MWCNTs-CE
2.263
8.915
CE
107.6
Pretreated CE
ZD-Max (Ω)
ZCPE (μF)
n
1154.278
0.00185
1
cr
α
558.4493
0.001063
1
0.94
18.64538
5.47×10-8
1
1
36.90365
5.11×10-8
1
0.82
2421.503
6.59×10-3
1
8.36×10-4
0.85
193.8133
1.06×10-2
0.95
0.88
4.109×10-3
1
41.09485
1.23×10-2
0.97
1.302×10-3
1
2.294×10-3
1
47.85473
2.25×10-2
0.99
100.8
2.37×10-7
0.93
2.16×10-4
0.91
502.8436
0.001136
0.96
75.82
116.9
3.33×10-8
1
0.010228
1
94.74851
0.01382
1
MWCNTs/CTABCE
8.342
106.4
0.010282
1
2.599×10-3
0.90
41.34213
4.62×10-8
1
MWCNTs-CE
74.41
74.84
3.76×10-8
1
3.917×10-3
1
134.4833
0.013536
1
M
an
us
0.80
d
pH 11
Q2 (Fs α-1)
te
pH 7
R2 (Ω)
Ac ce p
pH 3
R1 (Ω)
ip t
Table 1 EIS fitting parameters for different electrodes.
Page 19 of 30
Figure Captions: Fig. 1. Schematic of the proposed modification steps. Fig. 2. FT-IR spectra of CE, Pretreated CE, MWCNTs/CTAB-CE and MWCNTs-CE. and (d): MWCNTs-CE and the cross section of MWCNTs-CE: (e) and (f).
ip t
Fig. 3. FESEM images of the surface of (a): CE, (b): Pretreated CE, (c): MWCNTs/CTAB-CE Fig. 4. Cyclic voltammograms of different CEs at pH 3, 7.4 and 11 at a scan rate of 50 mV/s.
cr
Fig. 5. Bode modulus plots of electrochemical impedance for different CEs at pH 3, 7.4 and 11. Fig. 6. Nyquist plots of electrochemical impedance for different CEs at pH 3, 7.4 and 11, the Fig. 7. Adoptive equivalent circuit model for electrodes.
us
fitting results are shown with the solid line.
Fig. 8. Cyclic voltammograms of 3×10-2 mM MB at different pH values in 0.1 mM Fe2(SO4)3
an
and scan rate of 50 mV/s. of 50 mV/s. Fig. 10. Mechanism of MB reduction.
te Ac ce p
d
M
Fig. 9. Four cyclic voltammograms of 3×10-2 mM MB at pH 3 in 0.1 mM Fe2(SO4)3 at scan rate
Page 20 of 30
Ac ce p
te
d
M
an
us
cr
ip t
Figure(s)
Page 21 of 30
Ac
ce
pt
ed
M
an
us
cr
i
Figure(s)
Page 22 of 30
Ac ce p
te
d
M
an
us
cr
ip t
Figure(s)
Page 23 of 30
Ac ce p
te
d
M
an
us
cr
ip t
Figure(s)
Page 24 of 30
Ac ce p
te
d
M
an
us
cr
ip t
Figure(s)
Page 25 of 30
Ac
ce
pt
ed
M
an
us
cr
i
Figure(s)
Page 26 of 30
Ac ce p
te
d
M
an
us
cr
ip t
Figure(s)
Page 27 of 30
Ac
ce
pt
ed
M
an
us
cr
i
Figure(s)
Page 28 of 30
Ac
ce
pt
ed
M
an
us
cr
i
Figure(s)
Page 29 of 30
Ac
ce
pt
ed
M
an
us
cr
i
Figure(s)
Page 30 of 30
S0013-4686(13)01734-9 http://dx.doi.org/doi:10.1016/j.electacta.2013.09.012 EA 21229
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-7-2013 3-9-2013 3-9-2013
Please cite this article as: E. Pajootan, M. Arami, Structural and Electerochemical Characterization of Carbon Electrode Modified by MultiWalled Carbon Nanotubes and Surfactant, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.09.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and Electerochemical Characterization of Carbon Electrode Modified by Multi-Walled Carbon Nanotubes and Surfactant
ip t
Elmira Pajootan1, Mokhtar Arami2∗ Textile Engineering Department, Amirkabir University of Technology,424 Hafez Ave, Tehran, Iran, 15875-4413. Tel: +982164542614, Fax: +98 2166400245, E-mail: [email protected]
2
Textile Engineering Department, Amirkabir University of Technology,424 Hafez Ave, Tehran, Iran, 15875-4413. Tel: +982164542614, Fax: +98 2166400245, E-mail: [email protected]
us
cr
1
an
Abstract
In this study, multi-walled carbon nanotubes were successfully deposited on the surface of
M
the carbon electrode (CE) using a cationic surfactant (Cetyl Trimethyl Ammonium Bromide, CTAB) by simple electrodeposition method. FT-IR spectra, SEM images and the contact angle
d
measurements were employed to characterize the structure of the modified electrode. The electrochemical impedance spectroscopy and cyclic voltammetry were performed to evaluate the
te
electroanalytical behavior of electrodes through the modification process. The results indicated
Ac ce p
that the impedance of the modified electrode was reduced about 95% at pH 3 and 7.4 and 91% at pH 11 due to the deposition of the nanotubes on its surface. The equivalent circuit for EIS measurements was perfectly fitted at three pH values (3, 7.4 and 11) on the basis of the transmission line model that represents the impedance response of a diffusion process to characterize the properties of the bulk films. Also, the electrochemical behavior of methylene blue on the surface of the modified electrode was investigated. The obtained cyclic voltammograms showed three redox peaks, which can be related to the formation of semimethylene blue and leucomethylene blue.
Keywords: Multi-Walled Carbon Nanotubes, Electrodeposition, Electrochemical Impedance Spectroscopy, Cyclic Voltammetry, Transmission Line Model. ∗
Author to whom all correspondence should be addressed: 424 Hafez Ave, Tehran, Iran, 15875-4413. Tel: +98 //2164542614, Fax: +98 2166400245, Email: [email protected].
Page 1 of 30
1. Introduction Electrode material based on carbonaceous compounds has been widely applied in electrochemistry and electroanalysis as working electrode due to their special properties including low resistivity, chemical inertness, and unique surface chemistry, which make them a
ip t
proper choice to determine a wide range of substances in electrocatalytic area [1-7]. Also carbon electrodes (CEs) have the advantage of having a larger hydrogen overvoltage rather than metals
cr
[8]. Various types of CEs such as carbon paste [9, 10], carbon fiber [11], glassy carbon [12], screen-printed carbon [13], carbon nanotubes (CNTs) [14, 15], etc. have been developed and/or
us
modified to achieve specific properties for a variety of electrochemical purposes.
The nano-scale structure, large surface area, high mechanical strength and extraordinary
an
electronic properties of CNTs have made them useful in several attractive applications including nanosized semiconductor devices, high performance nanocomposites, energy conversion devices, sensors, etc [14, 15]. The modification of electrodes using multi-walled carbon
M
nanotubes (MWCNTs) to design new analytical sensors has been prevalently reported [16-20]. Homogenous and consistent coating of CNTs on the surface of the electrodes can be attained by
te
electrically charged CNTs [21].
d
simple, low cost and feasible electrodeposition method followed by the uniform dispersing of The previous investigations reported that the CNTs have been negatively or positively
Ac ce p
charged by carboxylic or amino functionalization which helps disperse individual CNTs in the suspension for electrodeposition process [22, 23]. Also several studies have surveyed the electrodeposition of the combination of CNTs/metal ions or CNTs/metal oxides on the surface of various electrode substrates [24-27].
Another appropriate and simple method to overcome the poor solubility of CNTs to attain the stable dispersion is the application of surfactants. In this procedure, the surfactant is adsorbed on the surface of CNTs, and subsequent ultrasonication of the solution, which takes several minutes, will cleave apart their aggregations and debundle nanotubes by steric or electrostatic repulsions resulted from the charge of surfactant hydrophilic groups [28-30]. In this paper, MWCNTs have been electrodeposited by applying direct current onto the CE under ambient conditions in mild solution with high efficiency. The electrodeposition solution contained MWCNTs and a cationic surfactant (Cetyl Trimethyl Ammonium Bromide, CTAB) that sonicated for 60 min. The literature reviews indicated that the dispersion of MWCNTs via
Page 2 of 30
cationic surfactant and further fabrication of CEs by electrodeposition has not yet been investigated. Surface morphology and structure of the prepared electrodes were studied using scanning electron microscope (SEM). The dynamic contact angle of the electrodes was also measured to determine the hydrophilicity of the surface. FT-IR analysis was employed to
ip t
compare the surface groups of the electrode before and after the deposition. Furthermore, in the current study, the electrochemical properties of the electrodes during different steps of
cr
preparation have been illustrated. The electrochemical impedance spectroscopy of electrodes was carried out to understand the transport and reaction processes occurring in the film; and the
us
transmission line model to describe the diffusive layer allowed for a satisfactory description of the data obtained. The cyclic voltammetry was also established to investigate the determination of the organic 3,7-bis(dimethylamino)phenothiazine-5-ium-chloride as the model dye
an
(Methylene Blue, MB) in aqueous solutions by modified electrode. MB has been widely utilized in electrocatalysis and electrochromics; and there are many redox reactions available to MB [31].
M
Finally, a mechanism was proposed for the reaction of MB on the surface of the modified
2.1. Chemicals and materials
te
2. Materials and methods
d
electrode.
In all experiments the reagents used were of analytical grade. Cetyl Trimethyl Ammonium
Ac ce p
Bromide ((C16H33)N(CH3)3Br, CTAB) and multi-walled carbon nanotubes (MWCNTs) (purity> 95%, length 10-20 μm and diameter 30-50 nm) were purchased from Merck and Neutrino, respectively. C.I. Basic Blue 9 (Methylene Blue, MB) was obtained from Ciba Co. Buffer solutions including pH 3 (citrate and hydrochloric acid), pH 7.4 (phosphate buffer solution) and pH 11 (boric acid, potassium chloride and sodium hydroxide), all with the conductivity of 6 mS/cm were also purchased from Merck. 2.2. Preparation of the modified electrode Prior to modification, carbon electrodes (graphite with the dimension of 3×20×80 mm3) were hand-polished with emery paper, then washed with distilled water and pretreated with NaOH (10%), HNO3: H2O (1:1, v/v) and acetone, each for 5 min, respectively. Finally the electrodes (pretreated-CE) were rinsed with distilled water. The electrode modification by electrodeposition
Page 3 of 30
method was carried out by applying DC voltage of 17.5 (V) to the solution containing 0.3 g/L MWCNTs and 0.2 g/L CTAB, which was sonicated for 60 min using Delta D68H Ultrasonic. The ultrasonication of MWCNTs via CTAB will lead to the dispersion of nanotubes, and fix the surfactants on the surface of MWCNTs (possible arrangements of CTAB on MWCNTs are
ip t
illustrated in Fig. 1(a), (b) and (c) [28]). It can be described that the cationic surfactant will make the nanotubes positively charged; and through simple electrodeposition process, these charged
cr
CNTs are driven toward cathode to form a thin layer at the electrode surface.
Eventually, the coated electrodes (MWCNTs/CTAB-CE) were immersed in the bicarbonate
us
solution (0.01 M) for 30 min in order to extract the residual surfactants from the surface of electrode. The modified electrodes were washed with distilled water and dried in room temperature (MWCNTs-CE). The modification steps are schematically shown in Fig. 1. In this
an
study, the structural as well as the electrochemical characterization of the modified electrode
M
(MWCNTs-CE) in the course of the preparation processes have been investigated. Figure 1.
d
The
Fourier
transform
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2.3. Characterization of the modified electrode infrared
(FT-IR)
spectroscopy
of
CE,
pretreated
CE,
Ac ce p
MWCNTs/CTAB-CE and MWCNTs-CE were measured with a Thermo Nicolet Avatar 360 FTIR Spectrometer within the range of 500- 4000 cm-1. The surface morphology of CE at different steps of modification was observed using a field emission scanning electron microscope ((FESEM) JSM-6700F, JEOL, Japan) operated at the voltage of 20.0 kV.
Dynamic contact angle of the electrodes were measured using KRŰSS processor tensiometer K100 MK2/SF/C (Germany) to investigate the surface hydrophobicity to confirm the suggested modification mechanism. Impedance measurements were carried out in a three-electrode system made by Ivium Compactstat (Eindhoven, the Netherlands). A saturated calomel electrode (SCE) and graphite rod were used as a reference and auxiliary electrodes, respectively. The software used for determining the equivalent circuit and data analysis was Ivium Equivalent Circuit Evaluator
Page 4 of 30
(Eindhoven, the Netherlands). EIS was measured in solutions with pH of 3, 7.4 and 11 at 25 ˚C. The frequency was varied in the range of 100 Hz to 106 Hz. The cyclic voltammetry (CV) was performed in a three-electrode cell with Ag/AgCl as reference electrode and Pt as the counter electrode. CV was once made in solutions with pH of 3,
ip t
7.4 and 11 to study the capacitive behavior of electrodes and once in solution containing MB and Fe3+ to determine MB in the solution, between the potentials of -0.6 V and +0.8 V (vs. Ag/AgCl)
cr
at scan rate of 50 mV/s.
us
3. Results and discussions 3.1. Structural characterization of the electrodes
The contact angle is the angle where a liquid/vapor interface meets a solid surface, which
an
quantifies the wettability of a solid surface by a liquid via the Young’s equation [32, 33]. γsv = γsl.Cos θ + γsl
(1)
M
where γsv, γsl and θ are the solid surface free energy, solid/liquid interfacial free energy and contact angle, respectively.
d
In this study the dynamic contact angle of the electrode surface with water has been evaluated, when the electrode is entering the water (advancing contact angle). In this regard,
te
samples of electrodes with the dimension of 1×10×20 mm3 were prepared and entered into the water container by a holder. As a result of the wetting process, advancing angles always simulate
Ac ce p
a fresh surface for the contact angle; this is formed immediately after the creation of the contact between the liquid and the surface. This type of measurement is the most reproducible way of measuring contact angles.
According to the obtained results, the surface of the CE and pretreated CE has the contact angle of 94.04˚ and 99.44˚, respectively. It is evident that the surface of carbon materials is hydrophobe and their contact angle is higher than 90˚. MWCNTs/CTAB-CE has the lowest contact angle (77.95˚) due to the existence of surfactants (CTAB) on the electrode surface. In other words, the surfactants have made the surface more hydrophile. After the extraction of surfactants from the surface, the contact angle increased again to reach the value of 92.16˚. Considering the above, the results of contact angle measurements confirmed the proposed mechanism for the preparation of the modified CE (Fig. 1), specially the extraction of the residual surfactants from the surface.
Page 5 of 30
FT-IR spectra of different electrodes are presented in Fig. 2. The addition of cationic surfactant (CTAB) for charging and dispersing the nanotubes affected the peaks that appeared in the FT-IR spectra. The broad peak at 3430 cm-1 is attributed to the O–H stretching bond which is
ip t
due to the water absorption of the carbon material in the atmosphere. The weak peak observed in all samples at 1725 cm-1 is related to the C=O bond which is probably due to the impurities of
cr
carbon and MWCNTs. The corresponding peaks of C–H (alkene bending), C=C and C≡C are placed at 802, 1625 and 2028 cm-1, respectively, which appeared only on CE and Pretreated CE,
us
meaning that either these bonds do not exist in MWCNTs or they are not numerous. The peaks at 2923 and 2853 cm-1 are related to the symmetric alkane stretching of C–H bond, the intensity of which is higher in the MWCNTs/CTAB-CE due to the presence of the surfactants. Also the
an
peaks – appearing at 1465 and 1253 cm-1 attributed to the CH2 (alkane bending) and C–N amine bond – are exclusively observed in the MWCNTs/CTAB-CE sample due to the existence of the
M
surfactants.
d
Figure 2.
te
The variation of the surface morphology of CE during the modification process was studied by FESEM images. The surface of CE before any treatment (Fig. 3(a)) was smooth and even.
Ac ce p
Based on the performed experiments, the nanotubes did not deposit on the surface of the untreated CEs. According to Fig 3(b), the pretreated CE showed a rough and scaly surface structure which increased the surface area of CE and facilitated the electrodeposition of nanotubes. Fig 3(c) and (d) illustrated that the nanotubes had been successfully deposited on the surface of the pretreated electrodes to form a highly porous film, and the subsequent removal of surfactants from the surface did not affect the nonotubes arrangement on the surface. Figure 3. Also the FESEM images of the cross section of modified electrode are shown in Fig. 3(e) and (f). It can be seen that a thin layer of MWCNTs with the thickness of about 6 μm has been successfully deposited on the surface of the pretreated CE.
Page 6 of 30
3.2. Electrochemical characterization of the electrodes The electrochemical characteristics of the prepared electrodes were investigated by cyclic voltammetry. Fig. 4 represents the comparative cyclic voltammograms for different CEs measured in three pH values (3, 7.4 and 11) at a scan rate of 50 mV/s. According to Fig. 4, the
ip t
area under the pulse gets larger in the order of: MWCNTs-CE > MWCNTs/CTAB-CE > Pretreated CE > CE for all pH values, which can be related to the increase of electrochemical
cr
surface area after the pretreatment steps with further electrodeposition of MWCNTs on the surface; this improves the charge transfer and confirms the formation of a thin layer on the
us
pretreated CE, which significantly improves its capacitive properties by showing a fairly large capacitance-like currents. According to the Fig. 4 the CE sample does not show any redox peaks at all pH values. The Pretreated CE exhibits a weak redox peak at 0.21 V; while, the similar peak
an
with significantly enhanced current has been observed for MWCNTs-CE sample at pH 7.4. At pH 3, the redox peaks have been observed at 0.45 and 0.6 V for Pretreated CE and MWCNTs-
M
CE, respectively, the appearance of which could be related to water discharge. In addition, the redox peaks observed at the negative potential values for MWCNTs-CE and MWCNTs/CTAB-
d
CE at pH 7.4 and for MWCNTs-CE at pH 3 are probably attributed to the reduction of the absorbed oxygen [14, 34]. The results of the redox peaks indicate that these responses are pH
te
dependent and are due to the redox reaction by the functional groups existing in the carbonaceous material (as explained earlier in FT-IR analysis) like C=O as impurities. The
Ac ce p
obtained results are consonant with the previous studies [35, 36]. Figure 4.
Also Fig. 4 indicates that the pH 3 solution produces the largest area enclosed by the CV curves and the pseudo-capacitive behaviors are less significant for electrodes in the solution with higher pH values. Perhaps at lower pH values more hydrogen ions are available as a reagent for Faradaic reaction [37]. These acquired results are in agreement with previous reports and suggest that the prepared MWCNTs-CE could be used in energy storage applications in acidic electrolytes [21]. Fig. 5 exhibits the electrochemical impedance of the prepared electrodes. EIS is a sensitive and non-destructive technique, which is often applied to determine the characterization of
Page 7 of 30
electrode processes and complex interfaces [38]. Therefore, in this research the EIS has been applied to survey the electrochemical behavior and charge transfer ability across the prepared electrodes interface. According to Fig. 5, the electrode impedances at frequencies above 102 Hz do not differ
ip t
much and the electrode conductivities are close to each other; this means that the deposition of nanotubes on the surface of the electrodes does not affect the impedance at high frequencies. But
cr
the impedances of electrodes at low frequencies became distant from each other. For example the impedance at 1 Hz was reduced up to 95% at both pH 3 and 7.4, and about 91% at pH 11 after
us
the deposition of nanotubes and further removal of surfactants from the surface, which may be attributed to the large surface area and high electrical conductivity of the coated nanotubes on the
an
surface of the electrode lowering the electrode resistance more effectively.
M
Figure 5.
In order to explain the impedance behavior of different electrodes, the proposed equivalent circuit model estimated from the shape of the data curve in Nyquist plots at pH values of 3, 7.4
te
d
and 11 (Fig. 6) was used to fit the EIS measurement results using Zview software. Figure 6.
Ac ce p
In the present study, the measured data are fitted to the selected circuit to determine the circuit parameters. According to the circuit model (Fig. 7), Rs is the solution resistance, ZD is the diffusion impedance and Rct is the charge transfer resistance. ZCPE represents the double layer constant phase element (CPE), which is explained by equation (2):
where
(2)
, ω is the angular frequency (rad/s) = 2πf; f is frequency (Hz). The parameter q
indicates the value of the capacitance of the CPE as n approaches 1. The parameter n (0 < n < 1) determines the micro fractal and distribution of the phase-phase interface. When n = 0.5 the impedance equals the Warburg impedance and when n = 1, the CPE is identical to a capacitor [14, 39].
Page 8 of 30
The semi-circle-like complex plane plots at high frequencies were observed in all electrodes, while the low frequency line becomes considerably smaller after the pretreatment and deposition of CNTs on the surface. Since the sloped line similar to Warburg impedance was observed in the Nyquist plots at low frequencies, the diffusive impedance (Fig. 7) which is generally expressed
ip t
with the transmission line model was employed to represent the impedance response of the diffusion process. This model has been used to analyze different systems where the charge
cr
transfer is controlled by a diffusion mechanism [40-42]. According to Fig. 6 the adoptive model has shown excellent fitting to the experimental data. In this model ZD is determined using the
an
(3)
us
following equations (3-5):
M
where
(5)
te
d
and
(4)
Ac ce p
The parameter x1 and x2 refer to the impedance per unit length (Ω/cm) for liquid and solid phase, respectively, and ζ is the impedance length (Ω.cm) corresponding to the diffusion length L (cm).
Figure 7.
In the equivalent circuit, the interface and liquid phase is explained by q3 and r1, respectively, and the solid phase is represented by the parallel connection of r2 and q2. The corresponding fitting data are listed in Table 1. In all samples, L was chosen as 1 and Rs was 10 Ω. R1 represents the diffusive resistance in the pores of the film, Q3 is related to the trapping, motion or delay of local charges at the liquid/solid interface; while the combination of R2 and Q2 are correlated to the disordered solid phase [40, 43]. The obtained results in Table 1 indicate that the resistances in the diffusive model (R1 and R2) decrease after the deposition of
Page 9 of 30
nanotubes on electrode surface for all pH measurements. The reduction of the ion transfer resistance in pores and interface has significantly decreased the diffusive impedance of the porous structured MWCNTs -CE and MWCNTs/CTAB-CE. The maximum diffusion impedance (ZD-Max) obtained at the lowest frequencies (1 Hz) are also given in Table 1. The ZD-Max of the
ip t
nanotubes deposited electrodes are 36.90, 47.85 and 134.48 Ω for MWCNTs-CE at pH 3, 7.4 and 11, respectively; which implies the lower energy barrier of charge transfer within the CNTs
cr
deposited films at pH 3 and confirms the data of cyclic voltammetery.
us
Table 1
Therefore, the equivalent circuit model adopted to explain the shape of the EIS spectra,
an
suggested that a distributed transfer process was the reason for the disordered line observed at low frequencies. Also one can conclude that the overall effect of charge transfer in bulk,
M
electrode/liquid boundaries and the solid phase influence the electrochemical behavior of electrodes at various range of frequencies in different media.
te
3.3. Determination of MB
d
The electrochemical response of 3×10-2 mM MB at the surface of MWCNTs-CE was
Ac ce p
investigated in 0.1 mM Fe2(SO4)3 at three pH values (3 for acidic, 7 for neutral and 11 for alkali media). As it can be seen in Fig. 8, the CE and pretreated CE showed no peak current at pH 3 in this potential range indicating that there was no redox process on the surface of CE in the presence of MB. It can be seen that the MWCNTs/CTAB-CE has shown a weak peak at -0.09 V due to the oxidation of MB at the surface of the electrode. This may be due to the repulsion of cationic MB and positively charged CTAB on the surface of the electrode, which reduces the concentration of MB on the surface of the modified electrode. On the other hand, the MWCNTsCE has demonstrated different behavior, which means that the nanostructured MWCNTs-CE acts as an electrocatalyst for the reduction of MB. The huge specific surface area of CNTs can increase the effective area of the electrode; as a result, the peak current increases too. At pH 11 no peak was observed in the reverse scan suggesting that the oxidation of MB at MWCNTs-CE
Page 10 of 30
is irreversible and weak. Three sets of redox peaks could be discerned at pH 3 and 7. The sharper and higher current peak at pH 3 suggests that the redox reaction happens faster. According to the results, the determination of MB and its possible mechanism was investigated further at pH 3. There are many reports about the reduction of MB in electron
ip t
transfer reaction showing one couple redox peaks or two oxidation peaks relating to either electropolymerization of methylene blue at positive potential or oxidation of MB immobilized on
cr
the surface of the electrode [44-46].
us
Figure 8.
an
To determine the mechanism of the oxidation of MB at the surface of the MWCNTs-CE, the cyclic voltammetry were performed 4 times, illustrated in Fig. 9(a). The voltammograms indicates that the reaction at the surface of the electrode is not the electropolymerization of MB
M
due to the increasing of the peaks heights by increasing the cycle numbers. If this was an electropolymerization process, the height of the first peak would decrease; whereas, the height of
d
the second one (polymer formation) would increase [44, 47].
te
Fig. 9(b) exhibits the corresponding voltammograms of MB at potential range of 0.25 to 0.8 V for 4 cycles. When the experiment is performed in this potential range, no redox peaks are
Ac ce p
observed. This means that the appearance of the second reduction peak is dependent on the existence of the first, and also the height of the second peak is higher than the first one.
Figure 9.
There are various studies that have investigated the redox mechanism of MB+ in different media (acidic, neutral and alkali media) on the surface of various electrodes. For example Bauldreay and Archer reported the oxidation of MB+ on SnO2 electrodes at pH 1 [48], Karyakin et al. also investigated the oxidation of MB+ at glassy carbon electrodes in aqueous solution at pH 9.1 [49], Pennarun et al. have discussed the reduction of MB+ at pH 8 at micro-optical ring electrodes [45], Žutić and Svetliĉić have also studied the reduction of MB+ at gold electrode at
Page 11 of 30
pH 7.9 [50, 51], etc. In the present study, the possible mechanism proposed by the obtained voltammograms could be described as below: According to the literatures [46, 52, 53], semi-methylene blue (HMB +•) could be produced in acid solution (pH < 6) by taking one electron and one proton, then the unstable semi-
ip t
methylene blue takes another electron and forms leucomethylene blue (MBH32+). Then MBH32+/ HMB +• can be oxidized by Fe3+ and form HMB +•/MB+ and Fe2+. The HMB +• can also be
cr
reduced by Fe2+ in acidic media to form MBH32+ [52]. The possible reactions of MB+ occurring
an
Figure 10.
us
at the surface of the MWCNTs-CE can be written as follows (Fig. 10):
At higher pH values, the formation of insoluble Fe(OH)3 and Fe(OH)2 in the solution could decrease the redox reactions by Fe3+/Fe2+; and also these iron hydroxides could be polymerized
M
at pH > 3.5 and there is a possibility that they can remove the dissolved MB+ by surface complexation or electrostatic attraction that decrease the concentration of MB+ at the surface of
d
the MWCNTs-CE to lower the current peaks [54].
te
According to our results, a mechanism for the redox reaction of MB at the surface of the MWCNTs-CE has been proposed. The work herein did not aim to provide the definite and
Ac ce p
detailed mechanism for MB reactions, and our main interest was to compare the voltammograms resulted from the CE before and after the modification with nanotubes in the presence of MB in the aqueous media. However, the more precise, comprehensive and quantified survey is needed to satisfactorily justify the oxidation and reduction mechanism of MB+ at the surface of the MWCNTs-CE. 4. Conclusion
In this paper the construction and formation of the thin layer deposition of MWCNTs on the surface of the carbon electrode were performed. Detection of methylene blue as a hazardous organic compound in aqueous solution was described. The electrodeposition of the MWCNTs accompanied by cationic surfactants (CTAB) as the driving force toward the cathode has been demonstrated. The characterization and properties of the electrode through the stages of preparation have been studied by SEM images, FT-IR spectra and the advancing contact angle.
Page 12 of 30
The obtained data confirmed the structure variation within the modification process; and a mechanism for the deposition of nanotubes was proposed. The electrochemical impedance spectroscopy and cyclic voltammetry of CE, pretreated CE, MWCNTs/CTAB-CE and MWCNTs-CE were also investigated at three pH values (3, 7.4 and 11). The pseudo-capacitive
ip t
behavior of the modified electrode was more significant at pH 3. The preparation of MWCNTsCE has served two purposes: first to lower the impedance and second, to enlarge the interacting
cr
surface area for the detection of MB. EIS data were fitted with the suggested equivalent circuit, and the CNTs deposited electrodes showed the lowest diffusive impedance. Due to the unique
us
electronic structures of theirs, the nanotubes can enhance the electrochemical reaction; therefore, methylene blue dye was strongly adsorbed on the surface of the electrode and exhibited three redox peaks. The resulted voltammograms revealed that MB reduces on the surface of
an
MWCNTs-CE by two electrons in a consecutive two one-electron steps and forming semimethylene blue (HMB +•) and leucomethylene blue (MBH32+). The promising results obtained by
M
this modified electrode might also be used for the detection of organic and colored hazardous
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Page 18 of 30
Q3(Fs β -1)
β
CE
175.4
164.2
7.44 ×10-8
1
9.62 ×10-5
0.87
Pretreated CE
185.3
173.4
4.98×10-8
1
3.25×10-4
MWCNTs/CTABCE
3.58
4.881
3.19×10-2
0.33
5.44×10-3
MWCNTs-CE
4.762
24.75
2.633×10-3
1
3.87×10-3
CE
24.99
5.5×106 1.47×10-5
1
4.11×10-4
Pretreated CE
14.12
60.24
3.14×10-4
0.87
MWCNTs/CTABCE
4.609
37.95
1.244×10-3
MWCNTs-CE
2.263
8.915
CE
107.6
Pretreated CE
ZD-Max (Ω)
ZCPE (μF)
n
1154.278
0.00185
1
cr
α
558.4493
0.001063
1
0.94
18.64538
5.47×10-8
1
1
36.90365
5.11×10-8
1
0.82
2421.503
6.59×10-3
1
8.36×10-4
0.85
193.8133
1.06×10-2
0.95
0.88
4.109×10-3
1
41.09485
1.23×10-2
0.97
1.302×10-3
1
2.294×10-3
1
47.85473
2.25×10-2
0.99
100.8
2.37×10-7
0.93
2.16×10-4
0.91
502.8436
0.001136
0.96
75.82
116.9
3.33×10-8
1
0.010228
1
94.74851
0.01382
1
MWCNTs/CTABCE
8.342
106.4
0.010282
1
2.599×10-3
0.90
41.34213
4.62×10-8
1
MWCNTs-CE
74.41
74.84
3.76×10-8
1
3.917×10-3
1
134.4833
0.013536
1
M
an
us
0.80
d
pH 11
Q2 (Fs α-1)
te
pH 7
R2 (Ω)
Ac ce p
pH 3
R1 (Ω)
ip t
Table 1 EIS fitting parameters for different electrodes.
Page 19 of 30
Figure Captions: Fig. 1. Schematic of the proposed modification steps. Fig. 2. FT-IR spectra of CE, Pretreated CE, MWCNTs/CTAB-CE and MWCNTs-CE. and (d): MWCNTs-CE and the cross section of MWCNTs-CE: (e) and (f).
ip t
Fig. 3. FESEM images of the surface of (a): CE, (b): Pretreated CE, (c): MWCNTs/CTAB-CE Fig. 4. Cyclic voltammograms of different CEs at pH 3, 7.4 and 11 at a scan rate of 50 mV/s.
cr
Fig. 5. Bode modulus plots of electrochemical impedance for different CEs at pH 3, 7.4 and 11. Fig. 6. Nyquist plots of electrochemical impedance for different CEs at pH 3, 7.4 and 11, the Fig. 7. Adoptive equivalent circuit model for electrodes.
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fitting results are shown with the solid line.
Fig. 8. Cyclic voltammograms of 3×10-2 mM MB at different pH values in 0.1 mM Fe2(SO4)3
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and scan rate of 50 mV/s. of 50 mV/s. Fig. 10. Mechanism of MB reduction.
te Ac ce p
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M
Fig. 9. Four cyclic voltammograms of 3×10-2 mM MB at pH 3 in 0.1 mM Fe2(SO4)3 at scan rate
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