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Electrochimica Acta 56 (2011) 7155– 7162

Contents lists available at ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

ynthesis of poly(phenylene oxide)-based fluoro-tin-oxide/ZrO2 nanoelectroderrays by hybrid organic/inorganic approach

adra Mosaa,∗, Olivier Fontainea, Paula Ferreirab, Rui P. Borgesc, Vincent Vivierd,avid Grossoa, Christel Laberty-Roberta, Clement Sancheza

Chimie de la Matière Condensée-UMR 7574 CNRS-Université Pierre et Marie CURIE, Collège de France, Bâtiment C, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, FranceCICECO – Centro de Investigac ão em Materiais Cerâmicos e Compósitos, Universidade de Aveiro Campus Universitário de Santiago, 3810-193 Aveiro, PortugalCentro de Física da Matéria Condensada, Universidade de Lisboa, Campo Grande, Ed. C8, 1749-016 Lisboa, PortugalLISE, UPR 15 du CNRS, Université Pierre et Marie CURIE, BP 133, 4 Place Jussieu, 75252 Paris Cedex 05, France

r t i c l e i n f o

rticle history:eceived 10 December 2010eceived in revised form 17 May 2011ccepted 19 May 2011

a b s t r a c t

This paper describes the synthesis and the characterization of poly(phenylene oxide)-based nanoelec-trode arrays. Using a hybrid organic/inorganic “sol–gel” solution, FTO (fluoro-tin-oxide) nanoelectrodearrays were synthesized with well-defined order, and controlled diameter of the nanoelectrode and the

vailable online 27 May 2011

eywords:TO/ZrO2 nanoelectrodesP-AFM

center-to-center distance. These FTO/ZrO2 nanoelectrodes were modified with ultrathin, polymer coat-ings based on the self-limiting electropolymerization of phenol. The polymer coated mainly the FTOnanoelectrodes. Impedance coupled with FTIR spectroscopies studies shows that the electrodepositedpolymer coating is uniform, conformal and pin-hole free. The CP-AFM (conductive probe-AFM) imagingstudies confirm that the ultrathin, uniform insulating poly(phenylene oxide) (PPO) layer homogeneously

des.

ybrid organic/inorganic “sol–gel” solution coats the FTO nanoelectro

. Introduction

Micro- or nano-electrode arrays have been used in a vari-ty of electroanalytical applications, i.e. end column detection inlectrophoresis [1,2], clinical chemistry [3,4] and environmentalonitoring [5–7]. Recently, there is a need for efficient elec-

rochemical sensor for environmental analysis. Accordingly, newlectrode materials and structures have been explored with aarticular emphasis on micro- and nanostructure on electrodeurfaces [8,9]. One emerging class of systems contains electri-ally conductive nanoelectrode array (NEA) that exhibits smallapacitive-charging currents, reduced iR drop and steady-state dif-usion limiting currents (for well dispersed electrodes) because of

combination of electrodes in the dimensions on the order of a fewens of nanometer [10–12].

These NEA can be considered as partially blocked electrodes13–15] or electrochemically heterogeneous electrodes [16]. Theesponse of such type of array strongly depends on the inter-lectrode distance, d, and the diameter of the electrode, r and onhe timescale of the experiment. A peak-shaped voltammogram is

btained for d < 12r while a sigmoidal shape is observed for d > 12r16]. Additionally, between these two limiting cases, the increasef the peak-to-peak separation with the inter-electrode distance

∗ Corresponding author.E-mail address: christel.laberty@upmc.fr (J. Mosa).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.05.074

© 2011 Elsevier Ltd. All rights reserved.

is also observed even for a fast electron transfer reaction occurs[13]. Accordingly, the electrochemical response of an array exhibit-ing large or small, d, lose its unique features and responds to amacroelectrode or an individual microelectrode, respectively. Amore complex behaviour is obtained when diffusion fields over-lap. For the latter, a single linear diffusion layer is reached if theexperimental time scale is sufficient.

Recently, we have shown using a bottom-up approach the fab-rication of nanoelectrode arrays with various metals, i.e. Pt, Au[18,21] which differs from most of the articles of the literaturethat refer to top-down approach [22]. Such nanoelectrodes aresurrounded by insulating ceramic layer such as TiO2, ZrO2, Al2O3.These heterogeneous surfaces associate both the conductivity ofthe underlying substrate (metal, FTO, etc.) and the physical andchemical properties of the porous network (temperature resis-tance, hardness, chemical inertia, reactivity, etc.). This hybridorganic–inorganic approach developed is simple, rapid, and flexibleas various Pt, Au, C nanoelectrode arrays with well-defined shapeand organisation can be synthesized. Compared to other nanoelec-trodes arrays (polymeric matrix), the ceramic coating provides highsensibility and selectivity and protects it from organic interference.Additionally, this protocol is very simple. A good control over thesize of the electrode and their center-to-center distance can be

easily achieved. Using impedance spectroscopy, the experimentalconditions where the NEA response is achieved, were defined. Tak-ing into account that d/r = 2 in our case, high scan rate (>2000 V s−1)are necessary to be in the behaviour of NEA.

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156 J. Mosa et al. / Electrochim

In this report, we extend our synthesis approach to the synthe-is of fluoro-tin-oxide (FTO) nanoelectrode arrays and we explorehe possibility of modifying these FTO nanoelectrode arrays with

polymer in order to fabricate hybrid organic–inorganic plat-orm. The main difficulty of this work is to obtain an ultrathinolymer layer only coated onto the FTO nanoelectrodes array inrder to keep the hybrid behaviour, i.e. ceramic–polymer. Thislectropolymerization occurs in confined media, the diameter ofhe electrode is about 65 ± 5 nm and the thickness of the ceramicayer is ∼10 nm. For three-dimensional architectures, confinementffects, diffusion of monomers within the pores, and surface rough-ess may alter the structure and the morphology of the polymerompared to an analogous electropolymerization process on a pla-ar substrate [23–26]. Additionally, the nature of the electrodeubstrate will determine the electrochemical stability of the elec-rodeposited polymer [27–29]. One observes that the long termtability is often critical specially when considering these polymersor energy storage or sensing applications. This confined environ-

ent may change the electropolymerization conditions and theontrolled designed of such new nano composite needs, at first,

perfect knowledge of their structure and electrochemical char-cteristic. In particular, we choose to electropolymerize a modelystem such as poly(phenylene oxide) (PPO) into the FTO/ZrO2anoelectrode arrays because (i) the polymerization of phenol haseen extensively studied, and (ii) their form insulating domainshat are easy to characterize. This hybrid-ceramic nanoelectrodes also interesting because, the presence of polymer componentnto FTO nanoelectrode can limit the access of molecules, sol-ent, or reactants to the nanoelectrode surface and can act as aemi-permeable membrane. This approach will then be extendedo other polymers such as for example poly-thiophene to constructriginal electrode. This study constitutes the first step of the workeveloping nanoarchitecture for sensing or energy conversionpplications.

. Experimental

.1. Synthesis and processing of FTO recessed nanoelectroderrays

All chemicals were obtained from Aldrich or Polymer Sourcend used as received. Transparent, conducting substrates ofTO films on glass slides (Solem, Re = 100 �, thickness: 80 nm)ere degreased in a 3 M HNO3 solution for 6 h and thenashed with water and stored in ethanol. Monolayers of hybrid

rganic–inorganic sol–gel ZrO2 solutions were made on FTO-oated glass slides as described previously and are noted heres FTO/ZrO2 nanoelectrode arrays [17–21]. Solution A con-isted of 10 g of polybutadiene-b-polyethilenoxide (PB-b-PEO)WPB = 320,00 g/mol and MWPB = 43,500 g/mol), 3 g of Ethanol

Ethyl alcohol absolute, Normapur) and 0.5 g of a concentrated 3 MCl aqueous solution was placed in an oven at 70 ◦C (for 2 h) untilopolymer PB-b-PEO was completely dissolved, and then it was leftt room temperature for equilibrium. Solution B, made of 140 mgf ZrCl4 and 1 g of EtOH, was added drop by drop to solution A.he final hybrid organic–inorganic solution C was stirred for 30 mint room temperature for homogenization. ZrO2-based films wereeposited by dip-dipping the glass-FTO substrates into the hybridrganic–inorganic solution C at a constant withdrawal speed of.3 mm/s under a constant air flow of about 10 L min−1, at a constantemperature of 70 ◦C and low relative humidity (<10% RH for all

he samples). The as-made films were directly transferred under-eath a curing IR lamp (450 ◦C at the sample surface for 5 min) toomplete the condensation of inorganic species of the film and toecompose the block copolymer template. The final system is com-

cta 56 (2011) 7155– 7162

posed of nanoholes with diameter of 65 ± 5 nm surrounded by ZrO2inorganic ceramic. The nanoholes defined the FTO nanoelectrodes.

The PPO films or dots were electrodeposited from an electrolyteconsisting of 50 mM phenol monomer, 0.1 M tetrabuthylammo-nium bromide (TBAB), and 50 mM tetramethylammonium hydrox-ide (TMAOH) in acetonitrile (all of them purchased from Aldrich)[23]. The working electrode, FTO/ZrO2, was placed in a standardthree-electrode cell consisting of a Pt wire as the counter elec-trode and a saturated calomel electrode (SCE-E = −0.241 V/NHE)or a silver/silver sulphate electrode (SSE-E = −0.680 V/NHE) as thereference electrode. For all the electrochemical experiments, thegeometrical surface area of the working electrode was 0.192 cm2.A potentiostat galvanostat (Princeton Applied Research – PAR262)was used for the electrosyntheses and voltammetric measure-ments.

The PPO films were grown on the FTO/ZrO2 working NEAs usingpotentiodynamic methods at a sweep rate of 50 mV s−1 by cyclingpotential between −0.6 V and 1 V/SCE for different cycles (1–30).The PPO-coated FTO/ZrO2 NEAs (PPO/FTO/ZrO2) were then rinsedwith deionised water and dried with argon.

2.2. Characterization

Scanning electron micrographs were obtained on a scanningelectron microscope equipped with a field emission gun (FEG-SEM, JEOL 6500F) using an accelerating voltage of 5 kV. As thesubstrate is an electron conductor, the uncoated and PPO-coatedFTO/ZrO2 NEAs electrodes were analyzed without any specific sur-face preparation. Grazing Incidence Small-Angle X-ray Scattering(GISAXS-Rigaku S-max 3000 equipped with a microfocus source� = 0.154 nm and a 2D Gabriel type detector place at 1480 mm fromthe sample) was used to assess the structure of the films and theperiod of the nanohole of the ZrO2 inorganic membrane at an angleof incidence of 0.21◦. The transmitted and specular reflected beamswere masked by a vertical beam-stop. Fig. 2a represents the GISAXSpatterns, i.e. the scattered intensity in the (qz and qy) plane. Diffrac-tion patterns were analyzed using Igor software.

To characterize the PPO films onto FTO nanoelectrodes,UV–Visible–NIR and FTIR spectra were performed. UV–Visible–NIRspectra were recorded between 2500 and 2000 nm with a PerkinElmer spectrophotometer (Lambda 950), with a resolution of 2 nm.FTIR spectra were recorded on a Perkin Elmer Spectrum 100 spec-trometer in the range 4000–450 cm−1 in transmittance mode witha resolution of 2 cm−1.

Current probe atomic force microscopy (CP-AFM) images ofFTO/ZrO2 bare nanoelectrodes and PPO-coated FTO/ZrO2 nano-electrodes were obtained using a Molecular Imaging picoLEmicroscope. CP-AFM operates in contact mode and simultaneouslyprobes the topography and the conductivity of the surface. Themeasured current (current sensing range of 10 nA) allowed for theconstruction of a spatially resolved map of conductivity. Pt-coatedsilicon cantilevers with spring constant k in the range 1–5 N/m anda tip radius <40 nm were used. All images were acquired with scanspeeds of 1500 nm/s while a bias ranging from +0.2 V to −0.2 V wasapplied between the sample and the grounded tip.

The FTO NEAs accessibility was evaluated by means of cyclicvoltammetry and electrochemical impedance spectroscopy in1 mM [Fe(CN)6]3−/[Fe(CN)6]4/0.1 M KCl solution before and afterelectrodeposition of the polymer. The uncoated and the PPO-coatedFTO/ZrO2 electrodes were scanned from 1 V to −1 V vs mercury ata scan rate ranging from 10 to 150 mV s−1 while the impedance

experiments were conducted with Solartron equipment coupledwith Princeton potentiostat in a frequency range of 1–105 Hz witha sinewave perturbation of 10 mV at the equilibrium potential. Notethat the error for the CV curves is ±5%. For the determination of the

ica Acta 56 (2011) 7155– 7162 7157

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J. Mosa et al. / Electrochim

overage fraction from the different methods, the errors changend we estimate an error ranging in between ±10 and 15%.

. Results and discussion

Before the deposition of PPO on these nanoelectrodes, the mor-hology and the microstructure of the FTO nanoelectrode arrayere studied as well as their electrochemical behaviour. Morpho-

ogical and structural characterizations of the FTO nanoelectrodesere carried out by FE-SEM, GISAXS and AFM techniques in order

o determine accurately morphological parameters such as the areaensity, the mean in-plane size, the mean center-to-center dis-ance, and the thickness of the ceramic layer.

.1. FTO/ZrO2 nanoelectrodes

.1.1. Structural characterizationScanning electron micrographs on FTO/ZrO2 NEAs show well-

ispersed in-plane circular shape FTO nanoelectrodes surroundedy ZrO2 inorganic matrices (Fig. 1). These nanoelectrodes are organ-

sed at short range distance into a hexagonal packing, proveny quantitative image analysis. The central part of the autocor-elation function (ACF) of the FE-SEM images displays a patternith hexagonal distance. Quantitative analysis by image process-

ng techniques allows the determination of the holes density, = 9 × 109 electrode cm−2, the mean diameter, <r> = 65 ± 5 nm and

center-to-center average distance between two electrodes ofd> = 110 nm. From this topographic studies, the accessible FTO sur-ace A is estimated to be 0.064 cm2 for a 0.192 cm2 macroelectrode.hickness of ZrO2 ceramic of 10 nm was measured from cross-ection SEM images (not shown), implying a ratio of <r>/thickness7. Note that there are some pin-holes and other defects in the

eramic layer. These imperfections are small and represent 2% ofhe surface of the layer (estimated from the SEM images analysis).

Advanced X-rays characterization methods such as Grazing Inci-ence Small-Angle X-Ray Scattering (GISAXS) (Fig. 2) and X-rayeflectivity (XRR) were used to assess the thickness homogeneity,he period of the array of holes and the roughness of the mem-ranes. Note that these results were obtained on Si-substrates andan be transposed to FTO substrates. XRR measurements allowed

s to highlight a surface film roughness close to 10% of the thicknessf the ceramic layer, in good agreement with AFM and TEM experi-ents. Compared to other assemblies of nanoelectrodes (polymericatrix), these FTO NEAs exhibit low surface roughness, and the

Fig. 2. (a) 2D GISAXS pattern of the ZrO2 nanostructured membranes on Si-substrate an

Fig. 1. SEM micrographs of the surface of the FTO/ZrO2 nanoelectrode.

ceramic layer is thin and dense. These characteristics are suitablefor limiting the mass transport phenomena of species inside theFTO nanoelectrodes, as the thickness of the ceramic layer is below10 nm.

3.1.2. Electrochemical characterizationCyclic voltammetry (CV) experiments were used to determine

the surface accessibility of the FTO nanoelectrodes and the dif-fusional regime (Fig. 3a) [18–21]. The faradic current obtainedin CV experiments in presence of a redox mediator confirms theaccessibility of the FTO substrates through the nanopores of theZrO2 inorganic matrix. The peak-shape response is described to alinear semi-infinite diffusion regime, which is also confirmed bythe linear relationship between the peak current and the square

root of the scan rate (Fig. 3b) [35]. This was also confirmed bythe estimation of dimensionless parameter, K�1/2 (with K�2 =(ra/(0.6r20 ))((D.R.T)/(F.v))1/2, ra represents the size of the activesite, � the scan rate (V s−1), F the faradic constant, D is the diffu-

d (b) X-ray reflectivity (XRR) of ZrO2 nanostructured membranes on Si-substrates.

7158 J. Mosa et al. / Electrochimica Acta 56 (2011) 7155– 7162

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ig. 3. (a) Cyclic voltammetry of FTO/ZrO2 nanoelectrode in 1 mM [Fe(CN)6]3−/[Fe(Current as a function of the square root of the scan rate.

ion coefficient (2.4 × 10−11 m s−1), 2R0, the half average distanceetween the active sites defined by Amatore et al. [13]. From ourxperimental data, K�1/2 is always higher than 10, which indicates

linear diffusion regime semi-infinite [13]. In the experimentalonditions used, these results show that the as-synthesized FTOanoelectrode arrays act as a macroelectrode, where the individ-al diffusion fields of the FTO nanoelectrode merge to form a lineariffusion regime. This result was expected as the d/r ratio is below2. Previous studies on comparable Pt and Au nanoelectrode arraysave shown that the scan rate necessary for observing a transitionetween linear diffusion at the macroelectrode to linear diffusion athe nanoelectrode is very important, i.e. >2000 V s−1. This scan ratealue is high and is not accessible with conventional equipment.hese results agree with our previous data on Gold and Platinumanoelectrode arrays and confirm the robustness of our protocol to

abricate NEAs on various substrates [21]. The FTO nanoelectrodesere stable in acidic conditions in the electrochemical stability

f water. SEM analysis was performed after cycling under theseonditions and no deterioration of the inorganic layer was found,ttesting the stability of the layer in these electrochemical condi-ions.

.2. PPO/FTO/zirconia nanoelectrodes

.2.1. Electropolymerization of phenolThe cyclic voltammograms of PPO on FTO/ZrO2 nanoelectrodes

s the same as that reported in the literature [22,30,37] (seeupplementary information). The positive current at E > 0.6 V/SCEn the potential sweep indicates the oxidation of the phenoxidenion to a phenoxy radical. A decrease of peak current upon theuccessive cycles is observed; indicating that further oxidation ofhe phenol monomer is limited by the formation of an insulatinglm. After 10 cycles, the voltammetric response is that of a capaci-ance envelope, confirming the formation of a self-limiting PPO filmn the FTO/ZrO2 nanoelectrodes.

.2.2. Electrochemical characterizationThe physical (e.g. film coverage and permeability) and

lectrochemical properties of polymer modified FTO/ZrO2 nano-lectrodes have been investigated by monitoring the electro-hemical response to solution-based redox probes such as

Fe(CN)6]3−/[Fe(CN)6]4 probe. Fig. 4 shows the CV curves of theTO NEA in 1 mM [Fe(CN)6]3−/[Fe(CN)6]4− in 0.1 M KCl at variousteps of the electropolymerization process. We notice an increasen the peak potential separation showing that the FTO electrodes

/0.1 M KCl for scan rates ranging from 10 to 150 mV s−1 at 298 K under Ar; (b) anodic

are blocked with the PPO layer. Before that the PPO layer covers allthe FTO nanoelectrodes, the system response is comparable to FTONEA. In this particular case, the NEA response is quasi a steady-statein the experimental conditions used.

Comparison of the voltammetric response on the ultrathin PPOmodified FTO/ZrO2 electrodes and native FTO/ZrO2 electrodes atneutral pH demonstrate that a part of the ferricyanide oxida-tion/reduction reactions were not completely suppressed. Thisindicates (i) a permeability of the PPO layer to the electrolytesolution or/and (ii) a partial coverage of the FTO nanoelectrodearrays with the PPO film. The fractional electrode coverage wasthen estimated from cyclo-voltammetry and impedance experi-ences in order to distinguish which phenomenon is predominant.The electrode coverage, �QCV , by the PPO film on the FTO nanoelec-trodes was first estimated by a comparison of the faradic current ofPPO/FTO/ZrO2 electrodes, called the modified electrode, obtainedfor different cycles to the bare FTO/ZrO2 electrode according to theequation [39,35]:

Fig. 4. Cyclic voltammograms of FTO/ZrO2 nanoelectrodes and PPO-coated FTO/ZrO2 nanoelectrodes for different cycles obtained in 1 mM[Fe(CN)6]3−/[Fe(CN)6]4/0.1 M KCl, Ar atmosphere, 298 K at a scan rate of 50 mV s−1

An experimental error of ∼10% has been estimated whatever the method used.

J. Mosa et al. / Electrochimica Ac

Fd(

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ig. 5. Evolution of the � (%) as function of the number of cycle determined fromifferent methods (i) the current of the cyclic voltammetry, (ii) the Warburg slope,iii) the transfer resistance, (iv) the apparent constant rate.

where imodified electrode is the peak current measured at the mod-fied electrode and ibare electrode is the peak current measured at theare electrode. The variation of �QCV as function of the number ofycles is reported in Fig. 5. After 10 cycles, a surface coverage of86% is estimated for PPO monolayer. This value is lower than the

eal one as the mass transfer and the degree of reversibility of theeaction were assumed to be the same for the different electrodestudied.

The surface coverage of the nanoholes was then calculatedrom the apparent rate constant [13]. In this method, the surfaceoverage is proportional to the peak distance. The nanoelectrodeoverage was estimated from Eq. (2) [35,40,41]:apS, modified electrode = kap

S, bare electrode(1 − �) (2)

The apparent rate constant, kapS, modified electrode is defined from

he Nicholson method [35,40,41] using Eq. (3) for a peak separation�Ep) higher than 60 mV.

0app = �

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RT

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Dox

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Dox and Dred represent the diffusion coefficient of the oxidantnd reductive species, respectively, � is the scan rate, is a dimen-ionless parameter tabulated [42] as function of the �Ep value.sing Eqs. (2) and (3), the fractional electrode coverage was esti-ated to 65% after 5 cycles and 97% after 10 cycles. This result

onfirms that the passivating film forms rapidly and that the FTOanoelectrodes were partially blocked after 10 cycles.

Impedance spectroscopy experiments were then used tostimate the surface coverage. Representative Nyquist of the elec-rochemical impedance spectroscopy of PPO/FTO/ZrO2 NEAs in

mM [Fe(CN)6]3−/[Fe(CN)6]4− in 0.1 M KCl at the open circuitotential are shown in Fig. 5 and are characterized by (i) a high-requency capacitive loop, ascribed to the transfer resistance inarallel with the double layer capacitance, (ii) a Warburg domain,hich occurs at midrange frequency associated with the diffusion

f ions within the PPO films. The high frequency limit correspondso the electrolyte resistance and the cell geometry, and its values then independent on the number of cycles. As the PPO depositshicken within the FTO nanoelectrode, the charge transfer resis-ance of the electrode increased from 4000 � after 1 cycle to5,000 � after 20 cycles. This increase of the charge transfer resis-ance indicates that the FTO nanoelectrodes were rapidly covered

y the polymer. The magnitude of the charge-transfer resistancean be related to the coverage of the electrode, �R

iS, assuming that

lectron-transfer reactions occurs only at the FTO nanoelectroden the electrode surface and that diffusion to these nanoelectrodes

ta 56 (2011) 7155– 7162 7159

was planar [39]. Such an assumption is consistent with the peak-shape of the CV curves presented in supplementary information.The following Eq. (4) for the apparent fractional coverage of theelectrode can be used:

�RiS = 1 −(

Rbare electrodect

Rmodified electrodect

)(4)

where Rbare electrodect is the charge-transfer resistance measured

at the bare electrode, and Rmodified electrodect the charge-transfer

resistance under the same conditions at the monolayer-coveredelectrode [38]. According to the data obtained from the Nyquistplots, a value of �R

iS= 99% is estimated after 30 cycles. For cover-

age approaching 100%, the slope of the diffusion part (w) of theNyquist plot can be used to calculate the surface coverage of theelectrode [38,42–44], �P

iS. Different behaviours are observed: (i) for

non-coated FTO nanoelectrodes, the slope is about 45◦, attestingto a complete merging of the individual nanoelectrodes [21]; (ii)for PPO-coated FTO nanoelectrodes, this linear domain exhibited aslope lower than 45◦, indicating that other phenomena occurred.This is related to diffusion of the species in the PPO layers. Thecoverage �P

iScan be estimated by the following equation [38,42–44]:

�PiS = 1 −(

wm − w

)(5)

where w is the Warburg coefficient calculated from the charac-terization of the bare electrode and m the slope of the linear intervalobserved in the high frequencies region of the Z′ vs ω−1/2 functionat the PPO-modified electrode. A value of 97% is estimated after10 cycles. These results compare well the ones obtained from theconstant rate.

This study indicates that the PPO layers cover the surface of theFTO nanoelectrodes and the reduction/oxidation peaks observedpreviously are mainly due to the permeabilities of the PPO nanodomains. Fig. 6 points out that the surface of the FTO nanoelec-trode is blocked after 10 cycles, which is fast compared to other3-D nanoarchitectures where 30 cycles are necessary with the sameconditions. This study demonstrates that after 10 cycles, the sur-faces of FTO nanoelectrodes are coated. However, the PPO filmshave certain permeability as some oxidation/reduction peaks areobservable on the CV curves.

3.2.3. Structural characterization of the PPO nanoelectrodesThe structure, chain length and thickness of the PPO film on the

FTO nanoelectrode array were studied using different techniquesas the resulting properties and performance of the PPO/ZrO2 hybridstructure will be directly related to its microstructure [34,36]. Scan-ning electron microscopy images of the PPO/FTO/ZrO2 NEA showthat the polymer does not completely coat the FTO nanoelectrodesafter 5 cycles (Fig. 7). Ten cycles are necessary to coat all theFTO nanoelectrodes. From these studies, the film thickness of PPOfilms is estimated to be around 20 nm. This value is higher thanthose obtained on Au substrates (<10 nm) [20] and comparable tothe one observed on carbon substrates [21]. Fig. 8 shows currentimages (400 nm × 400 nm) of a Pt/ZrO2 electrodes and PPO-coatedPt/ZrO2 nanoelectrodes using a bias voltage ranging from −2 V to2 V. Pt substrates were used because the roughness of the FTO sub-strates was too important to realize CS-AFM experiences. In thecurrent sensing image (Fig. 8b–d), regions of higher electrical con-ductivity are represented with brighter color and correspond tothe facilitated conduction of electrons between the tip and thesample. The uniformity in the current image suggests that theelectrical conductivity is homogenous. Only, small bright (yellow)

regions are observed, corresponding to defects in the ZrO2 mem-brane. Well-dispersed, bright regions are visible at the surface ofthe nanoelectrode, and these regions correspond to the Pt nano-electrodes and are at the same locations in the topographic images

7160 J. Mosa et al. / Electrochimica Acta 56 (2011) 7155– 7162

rent p

(trttimttUnFt

Fig. 6. Nyquist diagrams of PPO-coated FTO/ZrO2 nanoelectrodes with diffe

Fig. 8a). High currents are expected at such locations because ofhe much higher conductivity for Pt (resistivity of 8-12 � cm−1)elative to that for ZrO2 (∼10−6 S cm−1). It should be noted thathe conducting areas obtained from CP-analyses are always smallerhan the nanoelectrode size, due to the AFM tip (80 nm) which isn the same range as the nanoelectrode dimensions. The CP-AFM

easurements confirm the homogeneously conductive nature ofhe Pt/ZrO2 nanoelectrodes, which is critical for further modifica-ion of the Pt/ZrO2 nanoelectrodes via electrodeposition methods.ltrathin, passivating polymer coatings were achieved on Pt/ZrO2

anoelectrodes via the self-limiting electropolymerization of PPO.ig. 8c–d shows the current images of a PPO coated Pt/ZrO2 elec-rodes. Using a bias voltage of +2 V, immeasurable current levels

Fig. 7. FEG-SEM images of PPO-coated on FTO/ZrO2 nanoelectrodes (a) f

henol polymerization time. High frequency diagram is also shown (implot).

were seen for all regions where the PPO-Pt/ZrO2 nanoelectrodesare present, even when imaging was carried out with different biasvoltages (−2 V or +2 V). The absence of measurable current for thePPO-coated Pt/ZrO2 electrodes strongly suggests that the FTO nano-electrodes were completely passivated by the polymer coating (e.g.no pinholes or non-coated areas were detected).

The structure of the PPO film at the surface of the FTO nano-electrodes was studied by FTIR spectroscopy. Note that the signalto noise ratio is quite low because only a very thin layer of PPOwas deposited even after 10 cycles. Fig. 9 shows the FTIR spec-

tra recorded in transmittance mode in the range 1650–450 cm−1

of PPO-coated FTO/ZrO2 electrodes at different polymerizationtimes. Spectrum of the monomer was used as reference to assign

or 5 cycles and (b) for 20 cycles and (c) cross-section for 20 cycles.

J. Mosa et al. / Electrochimica Acta 56 (2011) 7155– 7162 7161

F urrenP coated

tP4oiObCreP[tPprb

Ft

ig. 8. (a) Topography images (400 nm × 400 nm) of a Pt/ZrO2 nanoelectrodes, (b) ct/ZrO2 nanoelectrodes, number of cycle: 5, Bias: +2 V, (d) current images of a PPO-

he FTIR bands for spectra [45,46]. The FTIR spectrum of thePO film on the FTO nanoelectrodes exhibit (i) bands in the50–600 cm−1 region that are associated with the in-plane and out-f-plane aromatic ring deformation vibrations [47], and (ii) bandsn the 1260–1400 cm−1 region, corresponding to the interaction of–H deformation and C–O stretching vibrations. Furthermore, theands in 900–1150 cm−1 region are also associated with the ether–O symmetric and asymmetric stretching vibration ( C–O–Cing). Their intensity increased with the number of cycling, morextremely for the band located at 1048 cm−1, indicating that thePO polymer was formed on the FTO nanoelectrodes surface48,49]. The presence of a wide band in this region demonstratedhe presence of quinine units in the structure of the as-depositedPO coating, indicating that PPO film arises from head-to-head cou-

ling of the phenol monomer [49]. The bands at 1490–1590 cm−1

efer to the symmetrical stretching C C vibration modes. Theseands indicate that the polymeric films maintain the aromatic char-

45060075090010501200135015001650

30 cycles

1 cycle

δ ring (C-C)

δ (O- H), υ (C -O)υs,υas(= C-O-C=)ring

υs(C=C)

Wavenumber /cm-1

Abs

orba

nce

/a.u

.

ig. 9. FTIR spectra of FTO/ZrO2 nanoelectrodes with different phenol polymeriza-ion times.

t images of a Pt/ZrO2 nanoelectrodes, Bias: +2 V, (c) current images of a PPO-coated Pt/ZrO2 nanoelectrodes, number of cycle: 20, Bias: +2 V.

acter. The spectra of polymeric films with a different number ofcycles are comparable, indicating that no secondary reactions wereinduced after a high number of cycles when complete polymeriza-tion was achieved.

4. Conclusions

Polymer-coated FTO nanoelectrode ensembles have been cre-ated by coating ultrathin PPO on FTO nanoelectrode ensemblessynthesized for the first time through a sol–gel derived approach.The electropolymerization of phenol was studied and used asmodel to understand the mechanism of the self-limiting elec-tropolymerization in these nano-domains (diameter ∼65 nm,thickness ∼10 nm and center-to-center distance = 110 nm). TheAFM topographic and the current data results show that the elec-trodeposition coats the FTO surface with polymer and that the PPOcoating is defect-free, robust, and insulating. Cyclic voltammetrycoupled with impedance spectroscopy of the non-coated and poly-mer coated FTO nanoelectrodes showed that the surface of the FTOwas completely blocked. A surface coverage of 97% was achievedafter 10 cycles.

By using electrodeposition method and hybrid organic/inorganic sol–gel approach, an original hybrid architecture wascreated exhibiting PPO domains into an insulating ZrO2 matrix.This design opens up many possibilities for electrochemical sens-ing or energy storage. The findings of this investigation on planarelectrode substrates will ultimately be exploited to design and opti-mize hybrid electrode nanoarchitectures comprising 3-D, porous,electrically conducting FTO/ZrO2 nanoelectrodes with ultrathin,conformal, electroactive polymer coatings.

Acknowledgements

The authors are grateful to FCT and Programa Opera-

cional Factores de Competitividade for financial support(PTDC/CTM/98130/2008). They also acknowledge the ExchangeCooperation Program CNRS/FCT. The authors acknowledge the useof the atomic force microscope at the Centro de Física da Matéria

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162 J. Mosa et al. / Electrochim

ondensada, Universidade de Lisboa. J.M. acknowledges the CNRSor financial support and L. Peláez for her assistance with thexperimental techniques. O.F. acknowledges the TERAGMASTORrogram. We acknowledge S. Borensztajn (L.I.S.E – UPR 15 CNRS)or assistance in SEM characterizations.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.electacta.2011.05.074.

eferences

[1] J. Liu, W.H. Zhou, T.Y. You, F.L. Li, E.K. Wang, S.J. Dong, Anal. Chem. 68 (1996)3350.

[2] T. Kappes, P.C. Hauser, Electroanalysis 12 (2000) 165.[3] S.Q. Zhang, H.J. Zhao, R. John, Anal. Chim. Acta 421 (2000) 175.[4] J. Albers, T. Grunwald, E. Nebling, G. Piechotta, R. Hintsche, Anal. Bioanal. Chem.

377 (2003) 521.[5] C. Belmont, M.L. Tercier, J. Buffle, G.C. Fiaccabrino, M. Koudelka-Hep, Anal. Chim.

Acta 329 (1996) 203.[6] R. Feeney, S.P. Kounaves, Anal. Chem. 72 (2000) 2222.[7] R. Feeney, S.P. Kounaves, Electroanalysis 12 (2000) 677.[8] R. Murray, Molecular Design of Electrode Surface, John Wiley, New York, NY,

1992.[9] C.R. Martin, in: A.J. Bard, I. Rubinstein (Eds.), Electroanalytical Chemistry, vol.

21, Marcel Dekker, New York, NY, 1999, p. 1.10] R.G. Compton, G.G. Wildgoose, N.V. Rees, I. Streeter, R. Baron, Chem. Phys. Lett.

459 (2008) 1.11] H.P. Wu, Anal. Chem. 65 (1993) 1643.12] X.-J. Huang, A.M. O’Mahony, R.G. Compton, Small 5 (2009) 776.13] C. Amatore, J.M. Savéant, D. Tessier, J. Electroanal. Chem. 147 (1983) 39.14] B.A. Brooks, T.J. Davies, A.C. Fisher, R.G. Evans, S.J. Wilkins, K. Yunus, J.D. Wad-

hawan, R.G. Compton, J. Phys. Chem. B 107 (2003) 1616.15] T.J. Davies, B.A. Brooks, A.C. Fisher, K. Yunus, S.J. Wilkins, P.R. Greene, J.D. Wad-

hawan, R.G. Compton, J. Phys. Chem. B 107 (2003) 6431.16] T.J. Davies, S.C.E. Bank, R.G. Compton, J. Solid State Electrochem. 9 (2005) 797.17] V.P. Menon, C.R. Martin, Anal. Chem. 67 (1995) 1920.

[

[[[

cta 56 (2011) 7155– 7162

18] C. Laberty-Robert, M. Kuemmel, J. Allouche, C. Boissière, L. Nicole, D. Grosso, C.Sanchez, J. Mater. Chem. 18 (2008) 1216.

19] M. Kuemmel, J. Allouche, L. Nicole, C. Boissière, C. Laberty-Robert, H. Amenistc-sch, C. Sanchez, D. Grosso, Chem. Mater. 19 (2007) 3717.

20] M. Kuemmel, C. Boissière, L. Nicole, C. Laberty-Robert, C. Sanchez, D. Grosso, J.Sol-gel Sci. Technol. 48 (2008) 102.

21] D. Lantiat, V. Vivier, D. Grosso, Ch. Laberty-Robert, C. Sanchez, Chem. Phys.Chem. 11 (9) (2010) 1971.

22] E.J. Menke, M.A. Thompson, C. Xiang, L.C. Yang, R.M. Penner, Nat. Mater. 5 (2006)914.

23] C.P. Rhodes, J.W. Long, M.S. Doescher, B.M. Dening, D.R. Rolinson, J. Non-Cryst.Solids 350 (2004) 73.

24] J.W. Long, D.R. Rolinson, Acc. Chem. Res. 40 (9) (2007) 854.25] J.W. Long, B.M. Dening, T.M. McEvoy, D.R. Rolinson, J. Non-Cryst. Solids 350

(2004) 97.26] T.M. McEvoy, J.W. Long, T.J. Smith, K.J. Stevenson, Langmuir 22 (2006) 4462.27] N. Oyama, T. Onsaka, Y. Ohnuki, T. Suzuki, J. Electrochem. Soc. 134 (1987) 3068.28] R.L. McCarley, E.A. Irene, R.W. Murray, J. Phys. Chem. 95 (1991) 2492.29] J.M. Bauldreay, M.D. Archer, Electrochim. Acta 28 (1983) 1515.30] S.E. Creager, G.T. Marks, D.A. Aikens, H.H. Richtol, J. Electroanal. Chem. 152

(1983) 197.34] Z. Ezerskis, G. Stalnionis, Z. Jusys, J. Appl. Electrochem. 32 (2002) 49.35] A.J. Bard, L.R. Faulkner, Electrochemical Methods, 2nd ed., J. Wiley & Sons, New

York, 2001.36] G.W. Muna, N. Tasheva, G.M. Swain, Environ. Sci. Technol. 38 (2004) 3674.37] F.A. Viva, E.M. Andrade, F.V. Molina, M.I. Florit, J. Electroanal. Chem. 471 (2)

(1999) 180.38] R.P. Janek, W.R. Fawcett, Langmuir 14 (1998) 3011.39] E. Sabatani, I. Rubinstein, R. Maoz, J. Savig, J. Electroanal. Chem. 219 (1987) 365.40] J.M. Savéant, Elements of Molecular and Biomolecular Electrochemistry, J.

Wiley & Sons, New York, 2006.41] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706.42] P. Dia, M. Guo, R. Tong, J. Electroanal. Chem. 495 (2001) 98.43] S. Campuzano, M. Pedro, C. Montemayor, E. Fatas, J.M. Pingarron, J. Electroanal.

Chem. 586 (2006) 112.44] L.V. Protsailo, W.R. Fawcett, Langmuir 18 (2002) 8933.45] Aldrich Condensed Phase Sample Library, Nicolet Instruments, 1988.46] G. Socrates, Infrared Characteristic Group Frequencies, John Wiley and Sons,

New York, 1994.47] R. Lapuente, J. Electroanal. Chem. 451 (1998) 163.48] M. Gratell, D.W. Kirk, J. Electrochem. Soc. 139 (1992) 2736.49] M. Ferreira, H. Varela, R.M. Torresi, G. Tremiliosi-Filho, Electrochim. Acta 52

(2006) 434.