using CTAB as soft template electrochemical sysntheis poly ANS

Electrochemical Preparation of a Nanostructured Poly(amino napthalene sulfonic acid) Electrode Using CTAB as a Soft Template and Its Electrocatalytic Application for the Reduction of Iodate A. Balamurugan,a Chia-Yu Lin,a Po-Chin Nien,a Kuo-Chuan Ho*a, b a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan tel: +886-2-2366-0739; fax: +886-2-2362-3040 b Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan *e-mail: kcho@ntu.edu.tw Received: August 29, 2011;& Accepted: November 1, 2011 Abstract A nanostructured poly(5-amino-2-naphthalene sulfonic acid) (nanostructured PANS) electrode has been prepared using cetyltrimethyl ammonium bromide (CTAB) as a soft template by potentiodynamic method. The effect of CTAB at various concentrations was analyzed during electrochemical synthesis of the PANS electrode. As the con- centration was near to cmc of CTAB, well growth and nanostructured PANS electrode was obtained. The formation mechanism and the reason for the increase of the peak current of the nanostructured PANS electrode are discussed. The surface morphology of PANS electrode was investigated using scanning electron microscopy (SEM) and a nanoribbon like structured PANS polymer was obtained near cmc of CTAB. The electrochemical properties of nanostructured PANS electrode were studied. The catalytic utility of nanostructured PANS electrode was investigat- ed and it exhibited electrocatalytic activity for the reduction of iodate with a low potential. The amperometric de- tection of iodate was tested at nanostructured PANS electrode. Linear range and detection limit were found to be 0.1 to 0.4 mM and 0.1 mM, respectively. The present work involves a simple, and one-step approach to fabricate a nanostructured PANS film with unique electrochemical properties, which can have great potential in various appli- cations such as sensors, and energy source systems. Keywords: CTAB, Electrocatalysis, Iodate reduction, Nanostructured PANS electrode DOI: 10.1002/elan.201100477 1 Introduction Conducting polymers (CPs) are a class of functional poly- mers that have extended conjugation throughout poly- meric chains [1]. Recently, the preparation of nanostruc- tured CPs (nCPs) have attracted great attention because of their potential applications in electronic circuits, chem- ical and electrochemical sensors, photovoltaic cells, elec- trochromic devices, and field-emission instruments [2]. Moreover, nCPs retain the properties of bulk CPs and have the characteristics of nanomaterials (e.g., large sur- face area, and quantum effect); these features further en- hance the merit of CPs for the above applications [3]. Several methods have been developed to synthesize nCPs, including chemical oxidative polymerization [4], template based electrochemical synthesis via hard tem- plates [5] and soft templates [6]. Although the hard tem- plate method allows the synthesis of nCPs, there exist drawbacks for obtaining pure nCPs, because the hard template materials have to be removed after synthesis, which often affects the properties of the nCPs. Moreover, these methods always require harmful organic solvents during an interfacial synthesis. Thus, soft template, such as surfactant, i.e. , CTAB, based electrochemical synthesis of nCPs stands out, not only because it is environmentally friendly, but also provides a simple and effective way for controlling surface morphology of deposited polymers via self-assembled micelles [7]. Meanwhile, there are numerous literature sources de- scribing the soft template based electrochemical synthesis of nCPs [8–12], even so, electrochemical synthesis of nCPs using CTAB remains unique. Very recently, Li et al. [13] synthesized poly(3,4-ethylenedioxythiophene) (PEDOT) micro/nanorods using CTAB as a soft template by electrochemical method. Raoof et al. [14] electrochem- ically prepared poly(m-toluidine)/NiO electrode using CTAB as soft template and observed enhanced electroca- talytic efficiency for methanol oxidation over poly(m-tol- uidine)/NiO electrode. In this work, nanostructured PANS, a dinuclear aro- matic polymer, modified electrode was prepared by an electrochemical deposition using CTAB as a soft tem- plate. PANS are a dinuclear aromatic polymer. In gener- al, poly(amino naphthalene) and its substituted deriva- Electroanalysis 2012, 24, No. 2, 325 – 331 � 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 325 Full Paper tives, poly(dinuclear aromatic amines), have more superi- or properties than other homonuclear conducting poly- mer, such as polyaniline, polypyrrole, etc.. These superior properties include the more delocalization of electron due to more extended p system, which lowers bandgap and modulates spectroscopy, electroactivity and charge transport properties in the polymer [15]. Earlier, poly (di- nuclear aromatic amines), such as poly(1,5-diaminonaph- thalene) [16], poly(1,8-diaminonaphthalene) [poly(1,8- DAN)] [17], substituted poly(naphthalenes) [18], and poly(5-amino-1-naphthol) [19], prepared by electropoly- merization and their electrochemical properties were studied. Recently, Atkinson et al. [20] reported the prepa- ration of PANS polymer by chemical oxidative polymeri- zation using ammonium persulfate as an oxidizing agent. Furthermore, Tagowska et al. [21] synthesized the nano- structured poly(1,8-diaminonaphthalene) by electrochem- ical and chemical methods. To the best of our knowledge, the preparation of nanostructured PANS electrode by either chemical or electrochemical method has not been reported earlier. In addition to the synthesis of PANS, the electrocatalyt- ic properties towards the reduction of iodate were also in- vestigated in the present work. Research topic for the de- termination of iodate in table salt has received considera- ble attention in order to eliminate iodine deficiency [22]; an excess of iodine or iodate can induce goiter as well as hyperthyroidism [23]. Recently, nanostructured electrodes such as PEDOT/Fe3+O nanospindles [24], nanocompo- sites of polyoxometallates [25], conducting polymer [26], and redox active molecule functionalized conducting polymer electrode [27] have been successfully used for electrocatalytic reduction of iodate. Thus, in the present work, one-step electrochemically prepared nanostruc- tured PANS electrode is studied for the electrocatalytic reduction of iodate and results are compared with those of other modified electrodes. 2 Experimental 2.1 Reagents and Apparatus ANS (95%) and CTAB (99%) were purchased from Al- drich. Potassium iodate (KIO3) was obtained from Wako Chemicals (Japan). All reagents were of analytical grade and used without any further purification. Solutions were prepared with doubly distilled water (DIW). High purity nitrogen was used for deaeration. The buffer and sample solutions were purged with highly purified nitrogen for at least 10 min prior to the experiments. Nitrogen atmos- phere was maintained over the solutions during the ex- periments. Electrochemical experiments were performed with CH Instruments (Model CHI-400) using CHI-750 potentio- stat. Glassy carbon electrode (GCE, geometric area= 0.071 cm2) obtained from BAS served as the working electrode. Pt wire was used as counter electrode and Ag/ AgCl with saturated KCl solution was used as reference electrode. All the potentials given in this paper were re- ferred to Ag/AgCl elelctrode (saturated KCl solution). The surface morphology of the modified electrode was characterized using Hitachi S-3000H SEM. The surface roughness of modified electrode was investigated using Atomic Force Microscopy under tapping mode (AFM, Digital Instruments, Dimension-3100 Multimode), and the AFM tip is a silicon-SPM sensor (tapping mode), thickness 4 mm, length 125 mm and width 30 mm. 2.2 Electrochemical Preparation of Nanostructured PANS Electrode Prior to modification, GCE was polished with 0.05 mm alumina on Buehler felt pads and then ultrasonically cleaned for 10 min in DI water. Finally, the electrode was washed thoroughly with DI Water. After being cleaned, the electrode was immersed into 0.1 M KCl solution con- taining 1 mM ANS and 1 mM CTAB for preparing the nanostructured PANS electrode. 3 Results and Discussion 3.1 Electrochemical Preparation of Nanostructured PANS Electrode Figure 1A shows cyclic voltammograms (CV) of the nanostructured PANS electrode in aqueous solution con- taining 1 mM ANS, 1 mM CTAB, and 0.1 M KCl at a scan rate of 100 mVs�1. It can be found that in the first cycle, an irreversible anodic peak was noticed at ~0.7 V, which is attributed to the oxidation of the amino group of ANS to a radical cation, and then to a dication. The ab- sence of a reversible cathodic peak indicates fast follow- up chemical reactions due to the consumption of the elec- trogenerated monocation radicals and dications to pro- duce polymeric products on the electrode surface [28]. In the subsequent cycle, redox peak at a formal potential (E8’)=0.20 V was observed. Upon continuous scanning, large redox peak current was observed which reflects the continuous formation of an electroactive polymeric film. It is worth mentioning here, that, after the first five cycles, the irreversible anodic peak at 0.7 V was found to increase as the number of cycle increases, indicating the formation of a conducting film, which mediates the oxida- tion of the reactive species [29]. Such a significant im- provement of monomeric oxidation in the course of film growth is the property of highly conductive polymers like polypyrrole, polyaniline, etc. However, we did not ob- serve such behavior when ANS was electropolymerized in absence of CTAB. Thus, we believe that CTAB plays crucial role in formation of conducting film. The mecha- nism of film formation will be discussed in the following section. 326 www.electroanalysis.wiley-vch.de � 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 2, 325 – 331 Full Paper A. Balamurugan et al. 3.1.1 Effect of CTAB Concentration on the Preparation of the Electrode We selected 1.0 mM CTAB (close to the value of 0.92 mM for cmc of CTAB), an optimum concentration for this research, unless otherwise specified, because the best results, in terms of the oxidative currents, were ach- ieved with this concentration as shown in Figure 1B. When the CTAB concentration was higher than 1.0 mM and lower than 0.75 mM, growth of peak currents was re- stricted within 2 cycles. The same trend was observed during the electropolymerization of ANS in the absence of CTAB. Therefore we proposed that the growth is greatly promoted by the formation of CTAB micelles. Without CTAB micelles, i.e. , at CTAB concentrations lower than 1.0 mM, the growth behavior of PANS is simi- lar to that prepared in absence of CTAB. However, as the concentration of CTAB is further increased, surfactant molecules self-assemble into aggregates with larger aver- age size and adsorb strongly on the electrode surface, in- hibiting further growth of PANS film. The CTAB concentration dependent morphology changes of the PANS polymer film were evaluated using SEM and AFM. Figure 2A–C display the SEM images of PANS polymer film prepared using CTAB with concen- trations of 0.5, 1.0, and 2.0 mM, respectively. It is ob- served that PANS film containing 1.0 mM CTAB formed a compact and ribbon like structure with the width rang- ing from 75–100 nm, as shown in Figure 2B. However, in other concentrations irregular structures of the polymer are obtained. Moreover, a PANS polymer film hardly forms on the electrode surface at 2 mM CTAB as shown in Figure 2C. The above observed results can be corrobo- rate with root mean square (RMS) of AFM images (see Figure 3) of PANS film. The observed RMS values of the PANS film are 3.45, 3.81, and 1.93, for CTAB concentra- tions of 0.5, 1.0 and 2.0 mM, respectively. Thus, the above results suggest that the optimum concentration of CTAB is 1 mM to obtain an orderly arranged and well-grown nanostructured PANS electrode. 3.1.2 Mechanism of Formation of Nanostructured PANS Electrode It has been seen that CTAB plays a very important role in the electropolymerization of ANS, by the ways such as specific orientation of molecules at the electrode surface and assistance for the formation of a well-grown PANS polymer film at 1.0 mM CTAB (close to cmc of CTAB). The reason for the growth of PANS polymer film is ex- plained below. The CTAB molecule is an amphiphilic surfactant (cat- ionic head group and hydrophobic tail), which exist as cationic CTA+ in solution at its cmc. It is well known that GCE contains hydrophilic ionic groups at the surface [31]. Meanwhile, Manne et al. [32] discovered via in situ AFM experiments that surfactants self assemble at the electrode surfaces as bilayers on hydrophilic surfaces and as monolayers on hydrophobic surfaces. On this basis, Kumar et al. [33] documented the way of formation of bi- layer and orientation of CTAB molecules on GCE sur- face; the cationic surfactant can adsorb onto the GCE surface preferentially with its polar head group forming surfactant self-assembly, followed by a second monolayer of CTA+ molecules organizing on top of the first layer forming a bilayer as a result of lateral hydrophobic inter- action amongst the neighboring tail groups, and exposing the second polar end of the surfactant to the aqueous so- lution. Therefore, during electrochemical polymerization of ANS in the presence of CTAB, negatively charged ANS is electrostatically attracted by the positively charged head group of CTAB. Thus, electrostatic attrac- tive force facilitates the formation of a well-grown nano- Fig. 1. (A) Cyclic voltammograms (CVs) of nanostructured PANS modified electrode in aqueous solution containing 1.0 mM ANS, 1.0 mM CTAB and 0.1 M KCl. Scan rate: 0.1 V/s. (B) Plot of anodic peak current (measured at a potential of 0.32 V) vs. number of cycles at different concentrations of CTAB. Other conditions are the same as used in (A). Electroanalysis 2012, 24, No. 2, 325 – 331 � 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.electroanalysis.wiley-vch.de 327 Nanostructured Poly(amino napthalene sulfonic acid) Electrode strucutred PANS polymer film. This electrostatic attrac- tive force may also act as driving force to the formation of nanoribbon like structure of PANS [34]. 3.2 Electrochemical Properties of Nanostructured PANS Electrode The effect of scan rate at nanostructured PANS modified electrode was investigated. As revealed in Figure 4, it is observed that anodic peak currents (at 0.27 V) are direct- ly proportional to the scan rate over the range of 10– 500 mV/s, indicating a surface confined redox process and facile charge transfer kinetics. The surface coverage (G) of the nanostructured PANS electrode was obtained by integrating the area under the anodic peak and was found to be 1.6�10�10 mol/cm2 (n=2) [21]. Electrochemical properties of the nanostructured PANS electrode were investigated in an aqueous solution at different pH values. As shown in Figure 5A, upon an increase of pH from 1 to 6, a shift of the formal potential for the redox reaction of PANS to more negative values, a decrease of the redox current, and an increase in the separation between anodic and cathodic peaks were no- ticed. This indicates that the redox properties of the nanostructured PANS electrode are pH dependent. The smaller redox peak currents, with increased pH, might be due to the reason that the amount of electroactive species is less [16]. The plot of the formal potential (estimated as (Epa+Epc)/2) versus pH (Figure 5B) is linear with the slope of �51 mV/pH (close to �59 mV), indicating that equal number of protons and electrons take part in the redox reaction of the polymer. 3.3 Electrocatalytic Reduction of Iodate at Nanostructured PANS Electrode Potassium iodate which is a well known oxidizing com- pound with a formal potential of 1.1 V should be reduced by the nanostructured PANS electrode via a 6-electron reduction (see Equation 1) [35]: IO�3 þ 6Hþ þ 6e� ! I� þ 3H2O ð1Þ This process requires protons, so an acidic electrolyte, i.e. , 0.2 M HNO3, is chosen for the study on electrocata- lytic behavior of iodate at nanostructured PANS elec- trode. As revealed in Figure 6, the electroreduction of iodate at GCE requires a large overpotential, and no ob- vious peak was observed in the range from +0.6 to �0.2 V in 0.2 M HNO3. However, with an addition of iodate, significant increase in reduction peak current along with a decrease in oxidation peak current was ob- Fig. 2. SEM images of PANS films prepared with CTAB concentrations of (A) 0.5, (B) 1.0, and (C) 2.0 mM. Scale bar: 400 nm. 328 www.electroanalysis.wiley-vch.de � 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 2, 325 – 331 Full Paper A. Balamurugan et al. served in the case of PANS modified GCE, indicating that the nanostructured PANS electrode shows remark- able electrocatalytic activities towards the reduction of iodate ions. On the basis of the voltammetric results described above, the nanostructured PANS electrode was further subjected to the amperometric detection of iodate at a relatively positive applied potential of 0.15 V in a magnet- ically stirred 0.2 M HNO3 solution. Figure 7 shows the typical current–time response at the nanostructured PANS electrode for iodate. The electrode response time was found to be less than 10 s after addition of iodate until steady-state values were obtained. The fast response time is attributed to an active film and short penetration depth of iodate. The Inset of Figure 7 shows the calibra- tion curve for iodate with a linear range of 0.1–0.4 mM, sensitivity of 4.22 mAmM�1 cm�2, and detection limit of 0.1 mM. Electroanalytical characteristics of nanostruc- tured PANS electrode and its comparison with other modified electrodes are shown in Table 1. Reproducibility of the nanostructured PANS electrode was estimated to- wards 0.2 mM iodate using amperometry method. The re- producible current response was observed with a relative standard deviation (RSD) of 5%. The stability of the nanostructured PANS electrode was tested by cyclic scanning in the potential range from 0.6 to �0.2 V at a scan rate of 100 mV/s for 100 cycles in 0.2 M HNO3 solution containing zero and 0.2 mM iodate. Fig. 3. AFM images of PANS films prepared with CTAB concentrations of (A) 0.5, (B) 1.0, and (C) 2.0 mM. Fig. 4. Plot of anodic peak current vs. scan rate. Electroanalysis 2012, 24, No. 2, 325 – 331 � 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.electroanalysis.wiley-vch.de 329 Nanostructured Poly(amino napthalene sulfonic acid) Electrode It was observed that the decay of current is 5%, and 15% without and with iodate, respectively. Furthermore, the selectivity of iodate reduction at nanostructured PANS electrode was verified in the presence of 200-fold greater amounts of foreign species such as nitrate, sulfate, and carbonate. We observed a selective response for iodate at nanostructured PANS electrode in the presence of the above mentioned species. 4 Conclusions We have successfully prepared a nanostrucutred PANS modified electrode using CTAB as a soft template. The optimal concentration of CTAB to obtain nanostrucutred PANS electrode with good electroactivity, in terms of peak current, was studied and found to be 1 mM. The for- mation mechanism of the nanostrucured PANS polymer film was discussed. Finally, electrochemical and electroca- talytic properties of the nanostrucured PANS polymer film were studied. The results show that nanostrucured PANS modified electrode can detect iodate ions at a rela- tively low potential with high sensitivity and stability. Fig. 5. (A) CVs of the PANS modified GCE prepared with CTAB concentration of 1 mM, at different pHs, and (B) plot of formal potential vs. solution p