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Electrochemical in situ synthesis of polypyrrole nanowires
Electrochemistry Communications 102 (2019) 94–98
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Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrochemical in situ synthesis of polypyrrole nanowires a
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A.M.R. Ramírez , M.A. Gacitúa , E. Ortega , F.R. Díaz , M.A. del Valle a b c
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Universidad Mayor, Núcleo Química y Bioquímica, Facultad de Estudios Interdisciplinarios, Laboratorio de Electroquímica, Av. Alemania 0281, 4801043 Temuco, Chile Universidad de Santiago de Chile, Facultad de Química y Biología, Avenida Libertador Bernardo O'Higgins 3363, 7254758 Santiago, Chile Pontificia Universidad Católica de Chile, Laboratorio de Electroquímica de Polímeros, Av. V. Mackenna 4860, 7820436, Macul, Santiago, Chile
ARTICLE INFO
ABSTRACT
Keywords: Conducting polymer Electrochemical-polymerization Modified electrode Doping/undoping process Nanostructure
Modification with polypyrrole nanowires, PPy-nw, is accomplished directly upon the working electrode by electrochemical polymerization methods using mesoporous silica as a template. The silica template is prepared by a potentiostatic method, generating a homogeneous film over a previously deposited thin layer of PPy, so that PPy-nw grows within the nanochannels of the mesoporous silica and adheres firmly to the surface. Subsequently the template is removed to obtain intact Pt|PPy-nw with stable and reproducible electrochemical properties, and with an enhanced (about 360 times higher charge capacity) response after charge–discharge experiments compared to equivalent electrodes modified with polymer deposits in the bulk (PPy) form. SEM reveals the brush-type conformation of PPy-nw (30 nm in diameter). Thus, a cheap, simple, highly repeatable method is used in situ to prepare electrodes modified with nano-structured polymers, using electrochemical techniques alone. This could have a great impact on a wide range of applications of conducting polymers.
1. Introduction Conducting polymers (CPs) have attracted much attention due to their excellent properties [1,2] and have been used in the development of photovoltaic cells [3], sensors [4–7], capacitors [8,9], electrochromic devices [10], and in microextraction [11,12], among other applications. One of the most studied CPs is polypyrrole (PPy), since its characteristics make it suitable for energy/gas storage devices [13,14], electrochemical sensors [5,15], and supercapacitors [9]. In a comprehensive review, Huang et al. [9] summarizes different applications of nanostructured PPy in the study of supercapacitors, concluding that its high electrical conductivity and flexibility are very promising features for future miniaturized, flexible, wearable batteries. Nevertheless, in that review, most of the nanostructured PPy-based materials considered are actually in the micrometer domain (no dimension lower than 100 nm), and resulted from a combination of polypyrrole with other materials (NiCo2O4; graphene; WO3, etc.). Conducting polymer nanostructures such as nanowires have been studied over the last decade with the aim of enhancing properties such as the response time for electrontransfer/ion-transport processes and the surface area, properties that are key in the design of electronic applications [2,16–22]. For instance, Long et al. [19] reported that polypyrrole tubes achieved an 80-fold increase in electrical conductivity when their diameter was reduced from 500 to 150 nm. However, much of the literature is concerned with
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expensive techniques like electrospinning and nanoimprinting as well as forming composites with other nano-sized materials [9,13,15,19,21–24], resulting in discrete-to-good performance during electrochemical charge–discharge experiments (p-doping/undoping response). A reasonable approach to producing cost-effective, scalable, nanostructured, conducting polymer-based devices is to use simple methods like electrochemical polymerization. In this regard, our research group has made interesting contributions, being the first (2009) to prepare polythiophene nanowires directly on the electrode using electrochemical techniques alone [25]. We used the hard-template approach, employing an electrochemically-prepared mesoporous silica film developed by Walcarius et al. [26]. Other authors have since used this template to deposit polypyrrole inside the pores of the silica template [27], but have not proved nanowire morphology nor the enhancement of charge–discharge characteristics. On the other hand, our method has been perfected over time by experience gained with other polymers including polythiophene [17,25], polyterthiophene [28], poly (3,4-ethylenedioxythiophene [18,29–32] and poly(1-amino-9,10-anthraquinone) [33–35]; each of them successfully tested for different applications. Through this experience our group has identified and dealt with two major difficulties encountered when preparing CP nanowires by electrochemical methods. Firstly, the weak adherence of the nanowires to the electrode is solved by depositing a thin layer of
Corresponding author. E-mail address: [email protected] (M.A. del Valle).
https://doi.org/10.1016/j.elecom.2019.04.007 Received 8 February 2019; Received in revised form 10 April 2019; Accepted 11 April 2019 Available online 12 April 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 102 (2019) 94–98
A.M.R. Ramírez, et al.
polymer on the surface prior to deposition of the template. When the wires are grown, the nanostructures are covalently bonded to the base polymer layer [30]. Secondly, complete removal of the mesoporous silica used as a template by means of alkali solution washing steps, leaving the nanowire deposit perfectly attached to the electrode and exposed for further use and characterization [34]. In this way, the adherence and stability of the nanowires is ensured for subsequent use, even becoming independent of the electrode material. The current report presents outcomes for the electrochemical preparation of 30 nm wide polypyrrole nanowire deposits with highly enhanced charge–discharge capabilities compared to regular deposits. The approach is solely based on electrochemical techniques, ensuring repeatability and low cost, which are key aspects for scaling up to industrial applications. These methods are protected by patent through patent application [36]. 2. Experimental Electrochemical polymerization was conducted in an anchor-type glass cell with three compartments, using a polycrystalline platinum (Pt) disk working electrode of 0.07 cm2 geometric area, polished to a mirror finish with a cloth pad using a 0.3 μm alumina slurry. A platinum wire with an area 20 times larger was employed as the counter electrode. Ag|AgCl immersed in a tetrabutylammonium chloride solution, whose potential is matched to that of a saturated calomel electrode (SCE), was the reference electrode suitable for anhydrous conditions [37]. All the electrochemical studies were carried out on a CH Instruments 900B potentiostat/galvanostat at 20 °C and under a high-purity argon atmosphere. The deposition of the first layer of bulk polymer, to prepare the Pt|PPy modified electrode, is carried out by cyclic voltammetry (CV), scanning between −1.20 and 1.40 V at 0.100 V s−1 scan rate for three cycles. The working solution consists of 0.01 mol L−1 double distilled pyrrole and 0.1 mol L−1 tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile. The original method for producing a mesoporous silica template was intended for deposition over metallic and graphite electrodes [26]. Here we adapted the method of Walcarius et al. for deposition of a silica template on a Pt|PPy modified electrode, as described in previous studies for PEDOT deposits [29]. The template was prepared in an ethanol:milli-Q water 50:50 mixture containing 0.05 mol L−1 of KNO3, 0.0034 mol L−1 tetraethyl orthosilicate and 0.115 mol L−1 hexadecyltrimethylammonium bromide, by applying a fixed reducing potential for 5 s. The surfactant was then removed by washing the electrode with 0.1 mol L−1 HCl in the same ethanol:water mixture. To evaluate the permeability of the silica film, CV profiles were recorded in a 0.01 mol L−1 ferrocene and 0.1 mol L−1 TBAPF6 solution in CH3CN. This produces a mesoporous silica film over the original polypyrrole layer (Pt|PPy-silica). Next, in order to prepare nanowires, Pt|PPy-silica was used as the working electrode and cyclic voltammetry performed using the same conditions described above, but for only one cycle. Finally, the template was removed by spraying with 0.5 mol L−1 NaOH followed by 0.5 mol L−1 NaHCO3 and finally with plenty of water [33], leaving the polypyrrole nanowires (denoted Pt|PPy-nw) exposed. The pdoping/undoping process of these electrodes was studied (via charge–discharge experiments) in a supporting electrolyte solution and compared to the results obtained using equivalent electrodes with bulk PPy deposits (Pt|PPy). The morphology of the electrodes was characterized by SEM (Inspect F50 FEI Scanning Electron Microscope).
Fig. 1. Voltammetric profiles of Pt|PPy-silica prepared by applying different electroforming potentials vs SCE for 5 s. Interphase: 0.100 mol L−1 TBAPF6 + 0.010 mol L−1 ferrocene in CH3CN, scan rate 0.1 V s−1.
towards the ferrocene-couple response. Fig. 1 displays the ferrocene couple on the modified Pt|PPy-silica electrodes obtained by applying 5 s steps at different potential values. The ferrocene redox process, and its current density, is directly related to the electrode active area, which means that the deposit of silica on the polymer layer produces a decrease in current density proportional to the coating. When the silica template is prepared using −1.10 V vs. SCE electrode, the coating is incomplete to the naked eye, leaving a large part of the polymer surface exposed and explaining the large ferrocene current observed. On the other hand, when the template is deposited at −1.30 or −1.40 V, the silica film is so thick that it spontaneously cracks, exposing part of the PPy layer. This explains the high current for ferrocene at these potentials shown in Fig. 1. It was deduced that a more complete and homogeneous coating for PPy layers is achieved at −1.20 V. To establish the optimal conditions, the active area was calculated based on the potential scan rate and the RandlesSevcik equation; in the optimal case, i.e. when the silica is electrodeposited at −1.20 V, the area is 4.27 · 10−4 cm2 which implies a large coating of the surface. After the conditions for producing the silica template had been optimized, the Pt|PPy-silica electrodes were used for the electrochemical polymerization of polypyrrole, shown in Fig. 2A. Electrochemical polymerization of bulk compared to nanowire PPy deposits is presented in Fig. 2A. Important features are observed. First, the decrease in the current density during the growth of PPy-nw compared to bulk PPy is explained by the fact that the silica template decreases the electrode active area. Second, the monomer oxidation potential is 0.180 V lower for the nanowires compared to the regular deposit. This is due to the electrocatalysis effect observed inside confined spaces, which is the basis for the use of mesoporous electrodes in preconcentration electroanalysis [38]. The monomer oxidation potential shift due to the effect of the silica template has also been observed for polythiophene deposits [25,39]. Fig. 2B shows the cyclic voltammograms obtained during charge–discharge experiments with Pt|PPy (—) and Pt|PPy-nw (—) modified electrodes after synthesizing the nanowires and wholly removing the template. It is observed that, as expected, the charge for the p-doping/ undoping process is significantly greater (360 times more) for the electrode modified with PPy-nw than with bulk PPy. Considering that only electrochemical methods were employed and no other components (metal oxides, carbon nanotubes, graphene, etc.) were added, this charge–discharge outcome is very impressive. Nevertheless, there are previous reports of polypyrrole micro/nanostructures obtained using
3. Results and discussion Firstly, it is necessary to select the potential used to prepare the silica template on the base PPy layer. As noted in previous reports [25], this can be optimized effectively by preparing silica templates at different potential/times and studying the CV response of the deposits 95
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Fig. 2. (A) Voltammetric profiles (5th cycle) during the growth of PPy on: (—) Pt|PPy; (—) Pt|PPy-silica. Interphase: 0.01 mol L−1 pyrrole + 0.100 mol L−1 TBAPF6 in anhydrous CH3CN (inset: detail of first cycle to obtain monomer oxidation potential). (B) Charge–discharge experiments or voltammetric p-doping/undoping responses of (—) Pt|PPy and (—) Pt|PPynw (inset: enlargement of the Pt|PPy electrode response). Interphase: 0.100 mol L−1 TBAPF6, in anhydrous CH3CN. Scan rate 0.100 V s−1.
with respect to regular PPy. Wysocka-Zołopa and Winkler [41] employed anionic surfactants during the electrochemical-polymerization process, producing 10 μm conical polypyrrole deposits which displayed three times more current than regular PPy during electrochemical charge–discharge experiments in electrolyte solution. Finally, Qian et al. [42] made use of commercial aluminum oxide templates over electrodes to prepare highly oriented vertically aligned poly-(3,4-etethylenedioxythiophene) PEDOT/PPy composite nanowires with 200 nm diameter and 15 μm length, which displayed about two times more specific capacitance than the homopolymers, but no comparison is made with regular non-nano deposits. Therefore, the outcomes in terms of charge–discharge experiments with PPy nanowires in the present work surpass any past report made in the area. In addition, the
Table 1 Summary of charge–discharge experiments on bulk and nano-structured PPy. Electrode
Ep⁎/V
Qd/mC cm−2
Qud/mC cm−2
Qpd:Qpu
Pt|PPy Pt|PPy-nw
0.557 0.517
0.3513 127.80
0.3524 126.52
1.00 1.01
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p-Doping potential.
electrochemical polymerization by other authors. Turco et al. [40] produced PPy nanowires using the non-static solution-surface electrochemical polymerization method. They produced 30–400-nm-wide wires arranged in a disordered way over the electrode surface, but no electrochemical charge–discharge experiments outcomes are presented
Fig. 3. SEM micrographs of p-doped modified electrodes Pt |PPy-nw: (A–B) top views; (C–D) cross-sectional views. 96
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reversibility of the charge–discharge process at the PPy-nw is maintained (see Table 1). First, doping processes take place at a slightly lower potential (Ep) for PPy-nw than for PPy, which is consistent with the electrochemical polymerization outcomes. Furthermore, a huge difference between PPy and PPy-nw deposits in terms of charge can be seen when comparing the different p-doping (Qd) and undoping charge densities (Qud). In addition, and as many other authors have pointed out, the reversible character of the charge–discharge processes evidenced by the Qpd:Qpu ratio makes these materials suitable for applications in capacitors and batteries. Finally, complementing the impressive charge behavior of the deposits, which indirectly provides some hints about their structure, Fig. 3 shows the morphology of the PPy nanowires (micrographs obtained using SEM microscopy). The micrographs in Fig. 3 clearly show that nanoscale PPy wires have been produced. The top views (Fig. 3A and B) demonstrate the presence of nanowires with an average length and diameter of 1970 and 30 nm, respectively. Compared to work by other authors, the present report provides evidence for the smallest dimensions achieved for this material, considering the proposed methods. The cross-sections (Fig. 3C and D) show the presence of a bulk PPy layer ca. 1 μm thick to which nanowires adhere, in an arrangement that we have called “brush type”. The success of the proposed methodology is thus verified, and can be expanded to other depositing materials such as metals and metal oxides. If care is taken at each experimental stage, repeatable nanowire deposits with great stability may be obtained, features which are very difficult to achieve when the nanostructures are prepared elsewhere and then somehow placed over the electrode. Furthermore, it is verified that the Pt|PPy-nw modified electrodes have a reversible p-doping/ undoping charge which is much higher than that of regular PPy-modified electrodes. This suggests that these nanostructured electrodes may be of great utility in applications involving conducting polymers.
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Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Electrochemical in situ synthesis of polypyrrole nanowires a
b
c
c
A.M.R. Ramírez , M.A. Gacitúa , E. Ortega , F.R. Díaz , M.A. del Valle a b c
c,⁎
T
Universidad Mayor, Núcleo Química y Bioquímica, Facultad de Estudios Interdisciplinarios, Laboratorio de Electroquímica, Av. Alemania 0281, 4801043 Temuco, Chile Universidad de Santiago de Chile, Facultad de Química y Biología, Avenida Libertador Bernardo O'Higgins 3363, 7254758 Santiago, Chile Pontificia Universidad Católica de Chile, Laboratorio de Electroquímica de Polímeros, Av. V. Mackenna 4860, 7820436, Macul, Santiago, Chile
ARTICLE INFO
ABSTRACT
Keywords: Conducting polymer Electrochemical-polymerization Modified electrode Doping/undoping process Nanostructure
Modification with polypyrrole nanowires, PPy-nw, is accomplished directly upon the working electrode by electrochemical polymerization methods using mesoporous silica as a template. The silica template is prepared by a potentiostatic method, generating a homogeneous film over a previously deposited thin layer of PPy, so that PPy-nw grows within the nanochannels of the mesoporous silica and adheres firmly to the surface. Subsequently the template is removed to obtain intact Pt|PPy-nw with stable and reproducible electrochemical properties, and with an enhanced (about 360 times higher charge capacity) response after charge–discharge experiments compared to equivalent electrodes modified with polymer deposits in the bulk (PPy) form. SEM reveals the brush-type conformation of PPy-nw (30 nm in diameter). Thus, a cheap, simple, highly repeatable method is used in situ to prepare electrodes modified with nano-structured polymers, using electrochemical techniques alone. This could have a great impact on a wide range of applications of conducting polymers.
1. Introduction Conducting polymers (CPs) have attracted much attention due to their excellent properties [1,2] and have been used in the development of photovoltaic cells [3], sensors [4–7], capacitors [8,9], electrochromic devices [10], and in microextraction [11,12], among other applications. One of the most studied CPs is polypyrrole (PPy), since its characteristics make it suitable for energy/gas storage devices [13,14], electrochemical sensors [5,15], and supercapacitors [9]. In a comprehensive review, Huang et al. [9] summarizes different applications of nanostructured PPy in the study of supercapacitors, concluding that its high electrical conductivity and flexibility are very promising features for future miniaturized, flexible, wearable batteries. Nevertheless, in that review, most of the nanostructured PPy-based materials considered are actually in the micrometer domain (no dimension lower than 100 nm), and resulted from a combination of polypyrrole with other materials (NiCo2O4; graphene; WO3, etc.). Conducting polymer nanostructures such as nanowires have been studied over the last decade with the aim of enhancing properties such as the response time for electrontransfer/ion-transport processes and the surface area, properties that are key in the design of electronic applications [2,16–22]. For instance, Long et al. [19] reported that polypyrrole tubes achieved an 80-fold increase in electrical conductivity when their diameter was reduced from 500 to 150 nm. However, much of the literature is concerned with
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expensive techniques like electrospinning and nanoimprinting as well as forming composites with other nano-sized materials [9,13,15,19,21–24], resulting in discrete-to-good performance during electrochemical charge–discharge experiments (p-doping/undoping response). A reasonable approach to producing cost-effective, scalable, nanostructured, conducting polymer-based devices is to use simple methods like electrochemical polymerization. In this regard, our research group has made interesting contributions, being the first (2009) to prepare polythiophene nanowires directly on the electrode using electrochemical techniques alone [25]. We used the hard-template approach, employing an electrochemically-prepared mesoporous silica film developed by Walcarius et al. [26]. Other authors have since used this template to deposit polypyrrole inside the pores of the silica template [27], but have not proved nanowire morphology nor the enhancement of charge–discharge characteristics. On the other hand, our method has been perfected over time by experience gained with other polymers including polythiophene [17,25], polyterthiophene [28], poly (3,4-ethylenedioxythiophene [18,29–32] and poly(1-amino-9,10-anthraquinone) [33–35]; each of them successfully tested for different applications. Through this experience our group has identified and dealt with two major difficulties encountered when preparing CP nanowires by electrochemical methods. Firstly, the weak adherence of the nanowires to the electrode is solved by depositing a thin layer of
Corresponding author. E-mail address: [email protected] (M.A. del Valle).
https://doi.org/10.1016/j.elecom.2019.04.007 Received 8 February 2019; Received in revised form 10 April 2019; Accepted 11 April 2019 Available online 12 April 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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polymer on the surface prior to deposition of the template. When the wires are grown, the nanostructures are covalently bonded to the base polymer layer [30]. Secondly, complete removal of the mesoporous silica used as a template by means of alkali solution washing steps, leaving the nanowire deposit perfectly attached to the electrode and exposed for further use and characterization [34]. In this way, the adherence and stability of the nanowires is ensured for subsequent use, even becoming independent of the electrode material. The current report presents outcomes for the electrochemical preparation of 30 nm wide polypyrrole nanowire deposits with highly enhanced charge–discharge capabilities compared to regular deposits. The approach is solely based on electrochemical techniques, ensuring repeatability and low cost, which are key aspects for scaling up to industrial applications. These methods are protected by patent through patent application [36]. 2. Experimental Electrochemical polymerization was conducted in an anchor-type glass cell with three compartments, using a polycrystalline platinum (Pt) disk working electrode of 0.07 cm2 geometric area, polished to a mirror finish with a cloth pad using a 0.3 μm alumina slurry. A platinum wire with an area 20 times larger was employed as the counter electrode. Ag|AgCl immersed in a tetrabutylammonium chloride solution, whose potential is matched to that of a saturated calomel electrode (SCE), was the reference electrode suitable for anhydrous conditions [37]. All the electrochemical studies were carried out on a CH Instruments 900B potentiostat/galvanostat at 20 °C and under a high-purity argon atmosphere. The deposition of the first layer of bulk polymer, to prepare the Pt|PPy modified electrode, is carried out by cyclic voltammetry (CV), scanning between −1.20 and 1.40 V at 0.100 V s−1 scan rate for three cycles. The working solution consists of 0.01 mol L−1 double distilled pyrrole and 0.1 mol L−1 tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile. The original method for producing a mesoporous silica template was intended for deposition over metallic and graphite electrodes [26]. Here we adapted the method of Walcarius et al. for deposition of a silica template on a Pt|PPy modified electrode, as described in previous studies for PEDOT deposits [29]. The template was prepared in an ethanol:milli-Q water 50:50 mixture containing 0.05 mol L−1 of KNO3, 0.0034 mol L−1 tetraethyl orthosilicate and 0.115 mol L−1 hexadecyltrimethylammonium bromide, by applying a fixed reducing potential for 5 s. The surfactant was then removed by washing the electrode with 0.1 mol L−1 HCl in the same ethanol:water mixture. To evaluate the permeability of the silica film, CV profiles were recorded in a 0.01 mol L−1 ferrocene and 0.1 mol L−1 TBAPF6 solution in CH3CN. This produces a mesoporous silica film over the original polypyrrole layer (Pt|PPy-silica). Next, in order to prepare nanowires, Pt|PPy-silica was used as the working electrode and cyclic voltammetry performed using the same conditions described above, but for only one cycle. Finally, the template was removed by spraying with 0.5 mol L−1 NaOH followed by 0.5 mol L−1 NaHCO3 and finally with plenty of water [33], leaving the polypyrrole nanowires (denoted Pt|PPy-nw) exposed. The pdoping/undoping process of these electrodes was studied (via charge–discharge experiments) in a supporting electrolyte solution and compared to the results obtained using equivalent electrodes with bulk PPy deposits (Pt|PPy). The morphology of the electrodes was characterized by SEM (Inspect F50 FEI Scanning Electron Microscope).
Fig. 1. Voltammetric profiles of Pt|PPy-silica prepared by applying different electroforming potentials vs SCE for 5 s. Interphase: 0.100 mol L−1 TBAPF6 + 0.010 mol L−1 ferrocene in CH3CN, scan rate 0.1 V s−1.
towards the ferrocene-couple response. Fig. 1 displays the ferrocene couple on the modified Pt|PPy-silica electrodes obtained by applying 5 s steps at different potential values. The ferrocene redox process, and its current density, is directly related to the electrode active area, which means that the deposit of silica on the polymer layer produces a decrease in current density proportional to the coating. When the silica template is prepared using −1.10 V vs. SCE electrode, the coating is incomplete to the naked eye, leaving a large part of the polymer surface exposed and explaining the large ferrocene current observed. On the other hand, when the template is deposited at −1.30 or −1.40 V, the silica film is so thick that it spontaneously cracks, exposing part of the PPy layer. This explains the high current for ferrocene at these potentials shown in Fig. 1. It was deduced that a more complete and homogeneous coating for PPy layers is achieved at −1.20 V. To establish the optimal conditions, the active area was calculated based on the potential scan rate and the RandlesSevcik equation; in the optimal case, i.e. when the silica is electrodeposited at −1.20 V, the area is 4.27 · 10−4 cm2 which implies a large coating of the surface. After the conditions for producing the silica template had been optimized, the Pt|PPy-silica electrodes were used for the electrochemical polymerization of polypyrrole, shown in Fig. 2A. Electrochemical polymerization of bulk compared to nanowire PPy deposits is presented in Fig. 2A. Important features are observed. First, the decrease in the current density during the growth of PPy-nw compared to bulk PPy is explained by the fact that the silica template decreases the electrode active area. Second, the monomer oxidation potential is 0.180 V lower for the nanowires compared to the regular deposit. This is due to the electrocatalysis effect observed inside confined spaces, which is the basis for the use of mesoporous electrodes in preconcentration electroanalysis [38]. The monomer oxidation potential shift due to the effect of the silica template has also been observed for polythiophene deposits [25,39]. Fig. 2B shows the cyclic voltammograms obtained during charge–discharge experiments with Pt|PPy (—) and Pt|PPy-nw (—) modified electrodes after synthesizing the nanowires and wholly removing the template. It is observed that, as expected, the charge for the p-doping/ undoping process is significantly greater (360 times more) for the electrode modified with PPy-nw than with bulk PPy. Considering that only electrochemical methods were employed and no other components (metal oxides, carbon nanotubes, graphene, etc.) were added, this charge–discharge outcome is very impressive. Nevertheless, there are previous reports of polypyrrole micro/nanostructures obtained using
3. Results and discussion Firstly, it is necessary to select the potential used to prepare the silica template on the base PPy layer. As noted in previous reports [25], this can be optimized effectively by preparing silica templates at different potential/times and studying the CV response of the deposits 95
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Fig. 2. (A) Voltammetric profiles (5th cycle) during the growth of PPy on: (—) Pt|PPy; (—) Pt|PPy-silica. Interphase: 0.01 mol L−1 pyrrole + 0.100 mol L−1 TBAPF6 in anhydrous CH3CN (inset: detail of first cycle to obtain monomer oxidation potential). (B) Charge–discharge experiments or voltammetric p-doping/undoping responses of (—) Pt|PPy and (—) Pt|PPynw (inset: enlargement of the Pt|PPy electrode response). Interphase: 0.100 mol L−1 TBAPF6, in anhydrous CH3CN. Scan rate 0.100 V s−1.
with respect to regular PPy. Wysocka-Zołopa and Winkler [41] employed anionic surfactants during the electrochemical-polymerization process, producing 10 μm conical polypyrrole deposits which displayed three times more current than regular PPy during electrochemical charge–discharge experiments in electrolyte solution. Finally, Qian et al. [42] made use of commercial aluminum oxide templates over electrodes to prepare highly oriented vertically aligned poly-(3,4-etethylenedioxythiophene) PEDOT/PPy composite nanowires with 200 nm diameter and 15 μm length, which displayed about two times more specific capacitance than the homopolymers, but no comparison is made with regular non-nano deposits. Therefore, the outcomes in terms of charge–discharge experiments with PPy nanowires in the present work surpass any past report made in the area. In addition, the
Table 1 Summary of charge–discharge experiments on bulk and nano-structured PPy. Electrode
Ep⁎/V
Qd/mC cm−2
Qud/mC cm−2
Qpd:Qpu
Pt|PPy Pt|PPy-nw
0.557 0.517
0.3513 127.80
0.3524 126.52
1.00 1.01
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p-Doping potential.
electrochemical polymerization by other authors. Turco et al. [40] produced PPy nanowires using the non-static solution-surface electrochemical polymerization method. They produced 30–400-nm-wide wires arranged in a disordered way over the electrode surface, but no electrochemical charge–discharge experiments outcomes are presented
Fig. 3. SEM micrographs of p-doped modified electrodes Pt |PPy-nw: (A–B) top views; (C–D) cross-sectional views. 96
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reversibility of the charge–discharge process at the PPy-nw is maintained (see Table 1). First, doping processes take place at a slightly lower potential (Ep) for PPy-nw than for PPy, which is consistent with the electrochemical polymerization outcomes. Furthermore, a huge difference between PPy and PPy-nw deposits in terms of charge can be seen when comparing the different p-doping (Qd) and undoping charge densities (Qud). In addition, and as many other authors have pointed out, the reversible character of the charge–discharge processes evidenced by the Qpd:Qpu ratio makes these materials suitable for applications in capacitors and batteries. Finally, complementing the impressive charge behavior of the deposits, which indirectly provides some hints about their structure, Fig. 3 shows the morphology of the PPy nanowires (micrographs obtained using SEM microscopy). The micrographs in Fig. 3 clearly show that nanoscale PPy wires have been produced. The top views (Fig. 3A and B) demonstrate the presence of nanowires with an average length and diameter of 1970 and 30 nm, respectively. Compared to work by other authors, the present report provides evidence for the smallest dimensions achieved for this material, considering the proposed methods. The cross-sections (Fig. 3C and D) show the presence of a bulk PPy layer ca. 1 μm thick to which nanowires adhere, in an arrangement that we have called “brush type”. The success of the proposed methodology is thus verified, and can be expanded to other depositing materials such as metals and metal oxides. If care is taken at each experimental stage, repeatable nanowire deposits with great stability may be obtained, features which are very difficult to achieve when the nanostructures are prepared elsewhere and then somehow placed over the electrode. Furthermore, it is verified that the Pt|PPy-nw modified electrodes have a reversible p-doping/ undoping charge which is much higher than that of regular PPy-modified electrodes. This suggests that these nanostructured electrodes may be of great utility in applications involving conducting polymers.
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