Institute of Physical Chemistry “Ilie Murgulescu” 202 Splaiul Independentei, 060021 Bucharest, RomaniaDepartment of Applied Physical Chemistry and Electrochemistry, University Politehnica of Bucharest, 132 Calea Grivitei, 010737 Bucharest, Romania
r t i c l e i n f o
rticle history:eceived 13 March 2012eceived in revised form 6 December 2012ccepted 9 December 2012vailable online 1 January 2013
eywords:anocomposite films
a b s t r a c t
Nanocomposite films based on polyaniline, functionalized single-walled carbon nanotubes and differentdopants were studied. These nanoporous composite films were grown electrochemically from aqueoussolutions such that constituents were deposited simultaneously onto substrate electrode.
The synthetic, morphological and electrical properties of the obtained nanocomposite films were com-pared. Scanning electron microscopy (SEM) revealed that the composite films consisted of nanoporousnetworks of SWCNTS (single-walled carbon nanotubes) coated with polymeric film. Cyclic voltammetry(CV) and electrochemical impedance spectroscopy (EIS) demonstrated that these composite films had
similar electrochemical response rates to pure polymeric films but a lower resistance and much improvedmechanical integrity. The negatively charged functionalized carbon nanotubes (CNTSF) served as anionicdopant during the electropolymerization to synthesize polymer/CNTSF composite films. The specific elec-trochemical capacitance of the composite films is a significantly greater value than that for pure polymerfilms prepared similarly. Using these composite films, the modified electrodes with improved properties
were obtained.
. Introduction
Without carbon nanotubes, the thin conducting polymer filmsrovide a high capacitance and reasonable response times, buthey suffer from mechanical and chemical instability in life cycleests, and low conductivity in the reduced or neutral states [1–8].
hen the thickness of the polymer film increases, charge trans-ort kinetics in the polymer becomes slow. These difficulties are allddressed by the addition of carbon nanotubes .For this reason, theesearchers have sought to combine carbon nanotubes (CNTs) andonducting polymers (CPs) for use as modified electrodes in differ-nt promising applications including conductive and high-strengthomposites, energy storage devices, biosensors and various otherevices.
Nanotubes themselves have provoked enormous interest overecent years, as a result of their unique properties and broad rangef potential applications. Their very high mechanical resilience,igh electrical conductivity and large surface areas are particularly
elevant to their applications in supercapacitors. Carbon nanotubesCNTs) and conductive polymers (CPs) are both interesting for theirnique electrochemical properties. Many efforts have focused on
the design and preparation of CNTs–CPs composites, in order toobtain a new material that would possess properties that would beuseful in particular applications [1–18]. Composite materials basedon the coupling of CPs and CNTs have been shown to possess prop-erties of the individual components with a synergistic effect [1]. TheCNTs doped CPs (conductive polymers) exhibit dramatically dif-ferent electronic properties [2–8] compared to CPs prepared withsmall anionic dopants. Such differences reflect to conductivity ofthe CNTs dopant compared to the common insulating dopants.The electron flow within the CPs/CNTs is apparently increased bythe entrapped CNTs due to an increased degree of delocalizationand CNTs bridging [9,10]. In fact, impedance studies indicated thatCPs/CNTs films are conducting even in their reduced state [9,10],i.e., a charge transfer occurs between the two constituents [7–15].CNTs are however difficult to process and insoluble in most sol-vents. CNTs can be divided into two main categories: single-walledcarbon nanotubes (SWCNTs) and multi-walled carbon nanotubes(MWCNTs). The first is formed by a single graphene sheet. The lat-ter are formed by additional graphene sheets wrapped around theSWCNT core. However, many of the potential applications of CNTsare hindered by the difficulties in their processing. The chemicalfunctionalization of CNTs will play a key role in the realization of
a material with much better properties. Thus, the solubilization ofCNTs by attachment of long-chain molecules to the open ends ofthe nanotubes was the first step in bringing CNTs into the realmof molecular chemistry [16]. Rapid progress in development of
Fig. 1. Cyclic voltammograms of PANI/FSWCNTs composite film in 0.25 M H2SO4
ethods for covalent attachment of various organic groups17–21], and biological molecules [22] has broadened the oppor-unities for applications of CNTs. With the advancement in thehemistry of CNTs it is now possible to tailor the electronic andhemical properties of CNTs [23,24]. Some of the important issuesddressed by the chemistry of CNTs involve dissolution of CNTsnd bundle exfoliation. The chemical functionalization of CNTs haseen shown to increase their solubility in organic solvents andacilitate their processing [11,25,26]. It has also been demonstratedhat the chemical functionalization affects the CNTs rope size andesults in exfoliation into smaller bundles and individual nano-ubes. These issues are especially important for the fabrication ofigh performance composite materials, controlled assembling ofolecular electronics and development of ultra-high sensitive sen-
or devises. Here, we outline chemical approaches to functionalizedingle-walled carbon nanotubes (FSWCNTs) with an emphasis onheir application for high-performance composites and biosensors.
In order to broaden their applications it was found necessaryo tailor their solubility properties. For this reason, single-walledarbon nanotubes (SWCNTs) were covalent functionalized with aater soluble conducting polymer (carboxylic acids and octade-
ylamine which was noted thus – FSWCNTs). The FSWCNTs graftopolymer has excellent solubilities in water and some organicolvents and it also exhibits an order of magnitude increase elec-rical conductivity over neat simple polymer [18]. The presence ofumerous functional groups in FSWCNTs means that there is poten-ial for covalent immobilization of various biomolecules or dopants.ased on the above we expect composite materials based on theoupling of CPs and FSWCNTs to possess properties of each of thendividual components with a synergistic effect.
In fact, composites of conducting polymers and carbon nano-ubes have been synthesized by either chemical or electrochemicalolymerization in the presence of carbon nanotubes (CNTs). In thehemical polymerization, an oxidant is needed but in this case theeduction product can affect the properties of the final productnanocomposite). The reaction product is always a powder which
eans a binder has to be used for the construction of an elec-rode [27,28]. The binder is often an insulator and hydrophobic andence, inevitably compromises the electrical and electrochemicalerformance. On the other hand, electrochemical polymerizationas a number of advantages. Particularly, there is no need for addedxidants and electrodeposited conducting polymers are naturallyntegrated as a continuous uniform film on electrode, saving these of a binder. Consequently, both the contact resistance within
he polymer and between the polymer and the current collectorre smaller than those of chemically prepared ones. Therefore,lectrodeposited films are ideal for the study of the electrochem-cal properties of these composites and for practical uses, such as
g solution (monomer free) at 25 ◦C with a scan rate of 50 mV/s and for 50 cycles.
sensors, electrocatalysts and supercapacitors as discussed before[29–32].
In this paper is described electrochemical synthesis ofnanocomposite films and electrochemical characterization ofthese nanocomposites by cyclic voltammetry, electrochemicalimpedance spectroscopy (EIS) and scanning electron microscopy(SEM).
2. Experimental
The electrochemical polymerizations were carried out using aconventional three electrodes system. A platinum electrode and asaturated calomel electrode (SCE) were used as counter and refer-ence electrode, respectively. The reference electrode was placed ina separate cell and was connected to the electrolytic cell via a saltbridge that ends as a Luggin capillary in the electrolytic cell. Thisarrangement helps in reducing the ohmic resistance of the electro-chemical system. The working electrode was made from a platinumdisk with surface area of 0.5 cm2. Functionalized single-walledcarbon nanotubes (FSWCNTs) were provided by CarbonSolutions,Inc. (www.carbonsolution.com, Riverside, CA), and Aniline (99.5%Fluka) was used as supplied. Bidistilled water was used for allsample preparations. All chemicals were of the highest quality com-mercially available and were used as received.
Cyclic voltammetry and electrochemical impedance spec-troscopy were used to investigate the electrochemical propertiesof the composite films. Electrochemical experiments were carriedout with an automated model VoltaLab 40 potentiostat/galvanostatwith EIS dynamic controlled by a personal computer. All the follow-ing potentials reported in this work are against the SCE (saturatedcalomel electrode). Scanning electron microscopy (SEM) was usedto compare the microstructures of the deposited films.
2.1. Preparation of modified electrodes
The Pt electrode was carefully polished with aqueous slurriesof fine alumina powder 0.05 �m on a polishing cloth until a mirrorfinish was obtained. After 20 min sonication, the electrodes wereimmersed in concentrated H2SO4, followed by thorough rinsingwith water and ethanol. The prepared electrodes were driedand used for modification immediately. Nanocomposite films ofCPs/FSWCNTs were prepared by electrochemical polymerizationfrom a solution containing both the functionalized carbon nano-tubes (FSWCNTs) and the corresponding monomer (aniline). Two
kinds of FSWCNTs were used in this work namely: single wallcarbon nanotubes (SWCNTs) functionalized with octadecylamineand single wall carbon nanotubes functionalized with carboxylicacids (see Schemes 1 and 2). In a first step, the FSWCNTs aqueous
634 F. Branzoi et al. / Progress in Organic Coatings 76 (2013) 632– 638
notub
sabecaei
aHsaaao
2
tpi0fie+wEwtca
So
Scheme 1. Chemical structure of single walled carbon na
uspension (usually 10 mg/L) was prepared via sonication (1 h)nd then the synthesis solution was prepared straightawayy dissolving the monomer and the corresponding supportinglectrolyte in the FSWCNTs aqueous suspension. The negativelyharged FSWCNTs in solution acted as sole supporting electrolytend dopant for the PANI depositions. For this reason FSWCNTs arenwrapped in polymers during the electropolymerization processn the form of counter ions or dopants.
Thus, PANI–FSWCNTs composite films were prepared, fromn aqueous solution containing 0.2 mol/L aniline + 0.25 mol/L2SO4 + 10 mg/L FSWCNTs by cyclic voltammetry in the potential
canning range of −250 to +900 mV at a scan rate of 10 mV/s and for cycles number of 10. The pure PANI films were prepared from anqueous solution of 0.2 mol/L aniline + 0.25 mol/L H2SO4, withoutdding FSWCNTs, also by cyclic voltammetry in the potential rangef −250 mV to +950 mV at a scan rate of 10 mV/s.
.2. Characterization of the modified electrodes
Electrochemical impedance spectroscopy (EIS) and cyclic vol-ammetry (CV) were used to investigate the electrochemicalroperties of the composite films. The electrochemical character-
zation of the PANI–FSWCNTs and PANI films was carried out in.25 mol/L H2SO4 and 0.25 mol/L Na2SO4 cycling aqueous solutionsor comparison and because the dopant anion of the polymeric filmss the same with the anion of the cycling solution. The workinglectrode potential was cycled in the potential range of −250 mV to900 mV with a scan rate of 50 mV/s. The impedance measurementsere performed using a VoltaLab 40 potentiostat/galvanostat with
IS dynamic in the frequency range of 100 kHz to 1 mHz with an AC
ave of 5 mV (peak-to-peak) overlaid on a DC bias potential and
he impedance data were obtained at a rate of 10 points per decadehange in frequency. All tests have been performed at 25 ◦C undertmospheric oxygen without agitation.
cheme 2. Chemical structure of single walled carbon nanotube functionalized withctadecylamine (SWCNTs–octadecylamine).
e functionalized with carboxylic acids (SWCNTs–COOH).
3. Results and discussion
In the first step, it was electrodeposited by cyclic voltammetrya polyaniline film onto a platinum substrate obtaining in thisway a modified electrode which was noted thus: PANI/platinumsubstrate. The cyclovoltammograms recorded during the elec-trodeposition of PANI film, have shown that at the cyclic potentialscanning on the range of −250 mV up to +900 mV, on the cyclicvoltammograms appear three redox peaks. This behavior can beexplained in the following mode: it is well known that polyanilinecan exist in three different oxidation states such as leucoemeral-dine (fully reduced form), emeraldine (partially oxidized form) andpernigraniline (fully oxidized form). These forms of polyanilineare dependent on the applied potential. At the increasing anodicscanning of the modified electrode potential the three oxidationstates of aniline take place at different potentials and on the anodicbranch of cyclovoltammograms appear three oxidation peaks. Atthe reverse scanning of modified electrode potential in the cathodicdirection appear three reduction states of aniline which corre-spond to oxidation states from the anodic scanning potential. Onecan say that, these three polyaniline oxidation forms correspondto the three anodic oxidation peaks from the anodic branch ofcyclovoltammograms while, the three polyaniline (PANI) reductionforms correspond to the three reduction peaks from the cathodicbranch of cyclovoltammograms. Hence, during the electrodeposi-tion process of PANI film take place three redox processes which canbe observed from cyclovoltammograms where appear three redoxpeaks (see Fig. 1 where the cyclovoltammograms have the sameshape with the cyclovoltammograms obtained during polyanilinefilm deposition). Further, we studied the electrochemical char-acteristics of obtained PANI films in a cycling solution of 0.25 MH2SO4 .The electrode potential was cycled on the potential rangefrom −250 up to +900 mV with a sweep rate of 50 mV/s and for acycles number of 50. In Fig. 1 is given the cyclic voltammogramsof polyaniline films in a cycling solution without monomer. Ana-lyzing the obtained results from cyclic voltammograms, it can beobserved that, at the anodic potential sweep on the anodic branchof the voltammograms appear three anodic oxidation peaks and atthe reverse potential sweep, on the cathodic branch of the voltam-mograms three reduction cathodic peaks appear. This fact pointsout the existence of three redox processes which take place in thePANI film on different potential ranges [33–35]. The first redoxpeak is commonly assumed to correspond to the electron trans-fer from/to the PANI film. In order to compensate the charge of the
PANI film, anion doping/dedoping of the PANI film occurs. The thirdredox peak corresponds to deprotonation and protonation process.Besides the proton/cation exchange, the anion is also expelled fromthe PANI film during deprotonation. The second small peak in the
F. Branzoi et al. / Progress in Organic Coatings 76 (2013) 632– 638 635
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Fig. 2. The Nyquist plots for: (a) pure PANI film, (b) PANI/SWCNTs–COOH nanocomposite film, (c) PANI/SWCNTs–octadecylamine nanocomposite film and the Bode plotsfor: (d) pure PANI film, (e) PANI/SWCNTs–COOH nanocomposite film and (f) PANI/SWCNTs–octadecylamine nanocomposite film.
636 F. Branzoi et al. / Progress in Organic Coatings 76 (2013) 632– 638
odifie
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electrons through the FSWCNTs and ions through the pore net-work or the direct interaction between the delocalised electronson polymer chains and the FSWCNTs.
Table 1Real impedance and capacitance values of pure PANI film and PANI/FSWCNTsnanocomposite film obtained by dripping method (DM) and by co-polymerizationusing the cyclic voltammetry (CV) at 0.01 Hz.
Polymeric film Slope valuesobtained fromgraph: −Zim = f(1/2�f)
Fig. 3. Capacitance evaluations for PANI/Pt and PANI/FSWCNTs/Pt m
otential range of (400–500) mV is probably due to a side reactionn the PANI film.
Further, were obtained PANI/FSWCNTs composite films on thelatinum substrate by cyclic voltammetry (co-polymerizare) using
synthesis solution of 0.25 M H2SO4 aqueous solution + 0.2 Mniline + 10 mg/L FSWCNTs (single-walled carbon nanotubes func-ionalizated with carboxylic acids or with octadecylamine).
Further, the obtained modified electrode typeANI/FSWCNTs/Pt was cycled in a cycling solution (monomerree) of 0.25 M H2SO4 at 25 ◦C with a scan rate of 50 mV/s andor 50 cycles. The obtained cyclovoltammograms are given inig. 1 and have the same shapes with those obtained for PANI/Ptodified electrode cycled in the same conditions, but in this
ase the anodic and cathodic peaks are much higher and largerhan those for PANI film. This fact can be explained taking intoccount that, FSWCNTs (in our case SWCNTs–COOH) are negativelyharged and they can act as doped anions and consequently, theonductivity of PANI/FSWCNTs composite film increases. Hence,he PANI/FSWCNTs composites exhibit higher currents than PANIlms, which can be translated into larger capacitance. This isn indication of faster kinetics in the composite, which can bettributed to the higher electronic conductivity of the FSWCNTsetwork. This means that the redox processes which take place
n nanocomposite film are more intense and more complex thanhose in the pure PANI film.
Analyzing in comparison the obtained results it can be observedhat, in all the cases the PANI/FCNTS composite film reveals anodicnd cathodic peaks much higher and much larger than PANIlm. This fact can be explained thus, in fully reduced form, theANI chains became neutral and the negative charge of immo-ile FCNTSF should be balanced by the cations with small sizerom the supporting electrolyte solution. Similar results haveeen obtained, in the same working conditions, for the modi-ed electrode type PANI/SWCNTs–octadecylamine as for modifiedlectrode type PANI/SWCNTs–COOH. Further, the composite andure polymer films were studied by EIS and for this reason theorking electrodes was placed in a cell in such a way that only
.5 cm2 area of the working electrode was exposed to the solu-
ion. EIS studies were performed at open circuit potential, in anqueous solution of 0.25 M H2SO4 and 25 ◦C. It is noted that EISeasurements were repeated at least three times to ensure repro-
ucibility. The resulting Nyquist plots and Bode plots for PANI,
d electrodes obtained by cyclic voltammetry or by dripping method.
PANI/SWCNTs–COOH and PANI/SWCNTs–octadecylamine systemsare shown in Fig. 2.
The Nyquist plots for both PANI and PANI/FSWCNTs(where FSWCNTs denote SWCNTs–COOH and/orSWCNTs–octadecylamine) composite films are featured by avertical trend at low frequencies, indicating a capacitive behavioraccording to the equivalent circuit theory [13,14]. Bode diagramspoint out also the capacitive behavior in concordance with Nyquistplots (see in comparison Fig. 2a–c and Fig. 2d–f).
The capacitances of the electrode materials were calculated,according to the equation [13,14]:
C = −12�fZim
f is the frequency; Zim is the imaginary impedance, from the slopeof the linear correlation between the imaginary impedance and thereciprocal of the frequency at low frequencies (see Figs. 2 and 3).From these figures and Table 1, one can observe higher capaci-tance the one order of magnitude value for PANI/FSWCNTs filmin respect with PANI pure polymeric films. Higher capacitanceof the composite films results obviously from the contribution ofthe embedded FSWCNTs that provide interconnected pathways for
PANI/CNT–octadecylamine –DM 0.5 mg
2 0.5 41.91
PANI/CNT–octadecylamine –CV 0.5 mg
1.78 0.56 36.83
F. Branzoi et al. / Progress in Organic Coatings 76 (2013) 632– 638 637
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Fig. 4. SEM images of: (A) PANI film, (B) PANI/SWCNTs–
Here, FSWCNTs is shorthand for SWCNTs–COOH (single-walledarbon nanotubes functionalized with carboxylic acids) and forWCNTs–octadecylamine (single-walled carbon nanotubes func-ionalized with octadecylamine).
The real impedance at low frequencies, where the capacitiveehavior dominates, is an indication of the combined resistance ofhe electrolyte and the film including both electronic and ionic con-ributions. The values of the real impedance at 0.01 Hz are given alson Table 1. It can be seen that the PANI/FSWCNTs films were signif-cantly lower in resistance than PANI films. It can also be seen thatANI/FSWCNTs offered much higher overall conductivity comparedith the PANI film. It has been already mentioned that, in general,
he real impedance of an electrode material also decreases as theaterial’s porosity increases due to improved ionic accessibility
9–15].This is in agreement with the SEM results presented below that
llustrated a smaller porosity in the PANI film than in the case ofhe composite film PANI/FSWCNTs. Analyzing in comparison theig. 4A–C, it can be observed that, the PANI and FSWCNTs filmseveal a morphological structure composed of nanofibers, but forhe pure polymeric ones they are slightly thicker than those inhe PANI/FSWCNTs composite film. This structural similarity mayxplain the relatively small difference in electrochemical behavioretween PANI and PANI/FSWCNTs as discussed above. However,he morphological structure of the PANI has been found to be dif-erent, the PANI/FSWCNTs composite film exhibiting a more porous
orphology. As shown in Fig. 4B and C the deposited PANI isostly wrapped around the FSWCNTs, this fact leads to a higher
onductivity and an improved electrochemical properties of theanocomposite films. The obtained results confirm findings madebove.
. Conclusions
Nanocomposite films were obtained by dripping method and byyclic voltammetry from a synthesis solution.
Electrochemically synthesized composite films of conductingolymers (PANI) and FSWCNTs have in common a porous structuret nano-meter scales. They have better mechanical integrity and aigher conductivity than the similarly prepared pure polymer.
The electrochemical activity of PANI/FSWCNTs/Pt modifiedlectrode in 0.25 M H2SO4 cycling solution is much more higher
han of PANI/Pt modified electrode in the same cycling solution.
The Nyquist plots for both PANI and composite films are fea-ured by a vertical trend at low frequencies, indicating a capacitiveehavior according to the equivalent circuit theory.
[
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cylamine composite film and (C) PANI/SWCNTs–COOH.
The higher capacitance of the composite films results obviouslyfrom the contribution of the embedded FSWCNTs that provideinterconnected pathways for electrons through the FSWCNTs andions through the pore network or the direct interaction betweenthe delocalised electrons on polymer chains and the FSWCNTs.
The PANI and FSWCNTs films reveal a morphological structurecomposed of nanofibers, but for the pure polymeric ones they areslightly thicker than those in the PANI/FSWCNTs composite film.
Microstructures of these composites suggest that PANI waswrapped around FSWCNTs. The obtained results can be employedto get the desired value of the capacitance by choosing the adequatepreparation method and so, by controlling the microstructure of thecomposites.
Acknowledgements
Financial support from PN-II-ID-PCE-2008-2 contract number596, code ID 716 (The National University Research Council) isgratefully acknowledged
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