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Electrochimica Acta 54 (2009) 6584–6593

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

lectrochemistry of surface grafted copper(II) benzoate complexes

veliina Repo a,1, Elisabet Ahlberg b, Lasse Murtomäki a,∗, Kyösti Kontturi a, David J. Schiffrin c

Helsinki University of Technology, Faculty of Chemistry and Materials Sciences, Department of Chemistry, PO Box 6100, FI-02015 TKK, FinlandDepartment of Chemistry, University of Gothenburg, S-412 96 Göteborg, SwedenChemistry Department, University of Liverpool, L69 7ZD, United Kingdom

r t i c l e i n f o

rticle history:eceived 12 March 2009eceived in revised form 2 June 2009ccepted 6 June 2009vailable online 16 June 2009

eywords:yclic voltammetry

a b s t r a c t

Glassy carbon electrodes were grafted with carboxyphenyl groups by reduction of 4-carboxyphenyldiazonium tetrafluoroborate and these modified electrodes were characterised bycyclic voltammetry and ac impedance measurements. Cu(II) was reacted with the carboxyphenyl groupsin the film to give a surface voltammetric response for the immobilised Cu(II)/Cu(I) couple. The resultsindicated an ECEC mechanism, in which the chemical steps correspond to the change of coordinationenvironment following the electron transfer steps. The relaxation half-life time for the Cu(I) speciesformed after electron transfer was estimated at (140 ± 11) s. The large value of the peak width of ∼200 mV

urface graftingelf-assemblyopper-benzoatecheme of square

was analysed by modelling the voltammograms and the large value of the full width at half maximum(FWHM) could be explained by dispersion in the formal potentials of Cu centres present in a variety ofenvironments in the films studied. An ECEC mechanism (scheme of square) is proposed for the electrontransfer reaction considering that the chemical step after reduction of the Cu(II) complex corresponds toconformational changes within the attached layer. Experimental data clearly show that the oxidation ofthe reduced film can take place from different Cu(I) complexes formed along the reduction to the fully

relaxed Cu(I) species.

. Introduction

The attachment of organic molecules on conducting surfacesas been intensively studied in recent years due to the wide rangef applications of the resulting structures, including molecularlectronics [1,2], electrocatalysis [3,4] and bioelectronics [5]. Inddition, immobilisation of redox active molecules on electrodesas provided a convenient way to study electron transfer acrossolecular bridges between a redox centre placed at a well-defined

istance from the electrode surface and in the absence of otherffects such as diffusion, convection and adsorption [6].

A common way of modifying conducting surfaces has been thedsorption of thiols on gold to form self-assembled monolayersSAMs). These layers are electrochemically stable in a potential spanf approximately 1.2 V [7]. Other modification techniques, such as

hemical grafting of carbon surfaces by reduction of aryl diazoniumalts [8,9], are of considerable interest. The latter relies on the elec-rochemical formation of aryl radicals by reduction of a diazoniumompound. These then bind to the surface through the formation

∗ Corresponding author. Tel.: +358 9 4512575; fax: +358 9 4512580.E-mail address: lasse.murtomaki@tkk.fi (L. Murtomäki).

1 Current address: Laboratory of Applied Environmental Chemistry, Departmentf Environmental Sciences, University of Kuopio, Patteristonkatu 1, FI-50100 Mikkeli,inland.

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.06.019

© 2009 Elsevier Ltd. All rights reserved.

of covalent C–C bonds [9] and therefore, the grafted layers are morestable than those in SAMs, with a very wide potential window,from −2 to 1.8 V vs. Ag/AgCl/sat. KCl [10]. In addition, grafting is notrestricted to carbon surfaces and for example gold [11] and iron [12]can be modified using this method. In the latter case, the graftedlayer provides protection against corrosion [12].

Electron transfer kinetics of surface bound redox moleculesat self-assembled monolayers have been extensively investigated[13] but electrochemical studies of grafted glassy carbon elec-trodes have mainly focused on the blocking effect induced by theattached surface groups [14,15]. The properties of redox centresin SAMs are, however, somewhat different from those directlyattached to an electrode surface, in particular in relation to thewidth of the voltammetric waves (FWHM). For instance, the hetero-geneous rate constants of ferrocene moieties attached to a glassycarbon electrode using grafted 4-carboxyphenyl groups as linkageshave been previously measured and analysed using the Marcustheory of electron transfer [16]. In this and other work, the val-ues of the FWHM were considerably larger than that expectedfor a reversible reaction, indicating a variety of environments inwhich the redox centre is present in a SAM [16–18]. In addi-

tion, grafted electroactive layers have been used in biologicalapplications to bind proteins [19]. An interesting application ofgrafted layers is to provide attachment sites for groups suitablefor the coordination of transition metal ions for example, throughamino groups attached to glassy carbon employing diazonium

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hemistry [19] onto which nanoparticles can be grown on surfaces20].

The aim of this work was to study the electrochemical prop-rties of surface bound Cu(II) benzoate complexes with a view tonvestigating the kinetics of the Cu(II)/Cu(I) couple when the lig-nd is a constituent of a film grafted to an electrode surface andherefore, when the ligand exhibits restricted mobility. Copper is aonvenient transition metal ion for these studies since its changesn coordination geometry on reduction, from octahedral to tetra-edral, are well documented. In addition, it was of interest to useryl carboxylic residues as ligands since they participate in manyeactions of supramolecular chemistry [17]. For this purpose, glassyarbon electrodes were grafted by reduction of a solution of 4-arboxyphenyldiazonium tetrafluoroborate salt [9–12]. The Cu(II)ons were subsequently coordinated to the free carboxyl groups andhe electrochemical properties of the electroactive layers formedere studied by cyclic voltammetry.

The paper is divided in four sections. We describe first therafting procedure and the electrochemical characterisation of theu(II)/Cu(I) couple incorporated within a carboxyphenyl film. Wehen analyse the voltammetric response of this couple and dis-uss the results in terms of the known inorganic chemistry of theopper-benzoate system. Third, we present simulations of the cyclicoltammograms to elucidate the origin of the large value of the fullidth at half maximum (FWHM) observed and finally, a plausible

eaction mechanism corresponding to the voltammograms mea-ured is discussed.

. Experimental

.1. Reagents

All reagents were of analytical grade unless otherwisetated. Tetrabutylammonium tetrafluoroborate (TBABF4, 95%)as obtained from Acros Organics and was recrystallised from

cetone. 4-carboxyphenyldiazonium tetrafluoroborate was syn-hesized from 4-aminobenzoic acid (Aldrich, 99%) followingell-established procedures [21]. Sodium perchlorate (Merck)as recrystallised from Milli-Q® water. Potassium hexacyano-

errate(III), KCl, NaCl (Merck), acetonitrile (HPLC grade, Merck),uSO4·5H2O (Riedel-de-Haën) and 4-mercaptobenzoic acid (4-BA, Aldrich, 97%) were used as received. Milli-Q® water was used

hroughout.

.2. Instrumentation and methodology

Electrochemical measurements were carried out using a three-lectrode cell. The counter electrode was a twisted platinum wire.lassy carbon (GC) and gold working electrodes were employed.he glassy carbon electrodes were 3 mm diameter rods sealed inlass or PTFE. Commercial electrodes purchased from Bioanalyticalystem Inc. (MF-2012) were also used for comparison purposes. Theold electrode was prepared by melting gold (99.95%) on a platinumire sealed in glass. After melting, the polycrystalline gold bead

ormed was cooled and then cleaned electrochemically by cyclingn 0.05 M H2SO4 between −0.3 and 1.5 V vs. the saturated calomellectrode (SCE) at a scan rate of 1 V s−1 until reproducible voltam-ograms were obtained. The surface area (0.46 cm2) of the gold

lectrode was determined from the oxide stripping peak in 0.05 M2SO4 [22].

A three-way glass stopcock was used to separate the referencelectrode from the electrolyte solution. Different reference elec-rodes were employed. The reference electrode in the non-aqueousolution was Ag/Ag+ (0.01 M AgNO3) in acetonitrile, E0 = 0.568 V23]. The ferricenium/ferrocene couple was used as an internal ref-

ta 54 (2009) 6584–6593 6585

erence with a half wave potential (E1/2) of 0.062 V with respect tothis reference electrode. For the measurements in aqueous solu-tions, a Ag/AgCl/0.1 M NaCl electrode was used (E = 0.045 V vs. SCE[24]). The potentials vs. Ag/AgCl in the figures refer to Ag/AgCl/0.1 MNaCl and those given vs. Ag/Ag+, to the Ag/Ag+ electrode in acetoni-trile.

The solutions were deareated by bubbling with N2 and a flowof N2 was kept over the space above the solution during the wholemeasurement time. All the experiments were carried out at roomtemperature (22 ± 2) ◦C. The potential was controlled with an Auto-lab PGSTAT100 potentiostat (Ecochemie, The Netherlands), and theac impedance measurements were carried out with a frequencyresponse analyzer SI1250 (Solartron Instruments) connected tothe measuring cell through an electrochemical interface (SI1286Solartron Instruments). Impedance measurements were performedat open circuit potential in the frequency range 65 kHz to 0.1 Hzusing a 20 mV sine wave amplitude. The impedance data wereanalysed using an equivalent circuit with constant phase elementcomponents. The CVs were recorded at different scan rates. Peakpotentials and currents were obtained from the cyclic voltammo-grams (CVs) corrected for background currents obtained beforeCu(II) attachment and using the peak function facility in the Originsoftware employed.

2.3. Electrode modification

Before surface modification, the glassy carbon electrodes werepolished with 1 and 0.3 �m alumina slurries and sonicated in waterfor 10 min followed by thorough rinsing with water and acetone.Grafting was carried out by cycling the potential ten times from0.2 to −0.4, −0.6 or −0.8 V vs. Ag/Ag+ (scan method) in 10 mM 4-carboxyphenyldiazonium tetrafluoroborate solution in acetonitrilecontaining 0.1 M TBABF4 as base electrolyte. In some cases (markedin figure legends) the potential was held at the switching potentialfor 10 min after the initial scans. For comparison, a potential stepmethod was also employed for grafting. Successive −0.2 V poten-tial steps were made and the electrode was kept at each potentialstep for 5 s. For example, the first step was from 0.2 to 0 V andthe electrode was held at 0 V for 5 s, after which the potential wasstepped to −0.2 V and held at that value for 5 s. Finally the poten-tial was stepped to −0.4 V and held at this value for 60, 120, 300or 600 s. This procedure was repeated with final potentials of −0.6and −0.8 V. The grafting solution was freshly prepared before eachmodification and bubbled with N2 before use.

After grafting, the electrodes were rinsed with acetone and thenwater and finally cycled in 0.1 M NaClO4 until the voltammogramswere reproducible in order to remove loosely bound molecules.Copper(II) was attached to this functionalised surface by immers-ing the electrode in a 5 mM aqueous CuSO4 solution for 15 min.The amount of Cu(II) attached was determined by cyclic voltam-metry and no changes in the peak currents were observed forlonger immersion times. Self-assembly of the 4-MBA monolayerwas carried out by immersing the gold electrode in 20 mM 4-MBAin ethanol for 24 h.

3. Results and discussion

3.1. Grafting

Fig. 1 shows cyclic voltammograms for the reduction of 4-

carboxyphenyldiazonium tetrafluoroborate in acetonitrile. Thepeak at ca. −0.5 V vs. Ag/Ag+ in the first scan (a) corresponds tothe reduction of the diazonium cation to the radical, which thenbinds covalently to the surface. In the second scan (b) a small reduc-tion current can still be seen and by the 10th scan the surface is

6586 E. Repo et al. / Electrochimica Acta 54 (2009) 6584–6593

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Table 1Charge transfer (RCT) resistance forgrafted glassy carbon electrode in5 mM K3/4[Fe(CN)6] at different valuesof pH. Electrode area = 0.0707 cm2.

pH RCT (k�)

1.5 1.0

ig. 1. Grafting of glassy carbon electrode in 10 mM 4-carboxyphenyldiazoniumetrafluoroborate salt in acetonitrile. 0.1 M TBABF4 was used as supporting elec-rolyte (a), (b) and (c) correspond to the first, second and tenth scan, respectively.can rate: 0.2 V s−1.

lmost completely blocked by the grafted carboxyphenyl groups.he reduction wave appearing in the first scan at ca. −0.1 V haseen previously observed; its origin is unclear [21,25].

.2. Electrochemical characterisation

.2.1. Glassy carbon electrodesFunctionalised electrodes were characterised using the hex-

cyanoferrate(III)/(II) couple as a redox probe. As previouslybserved by Saby et al. [14], electron transfer to this couple is com-letely blocked at pH 6.6 (Fig. 2) due to electrostatic repulsionetween the –COO− surface groups and the hexacyanoferrate(III)

on. The pKa of surface grafted benzoic acid is 2.8 [14], this groups ionised at pH 6.6, and hence, tunnelling distance effects stronglynhibit the rate of electron transfer [15,21]. In acid media, no suchnhibition is observed (Fig. S1).

To demonstrate functionalisation by carboxyphenyl groups, theH dependence of the ac impedance spectra of the grafted elec-rode in contact with solutions containing the ferri-/ferrocyanideouple (Fig. S2) was measured. The results are similar to previousork [14]. The charge transfer resistance (RCT) (Table 1) decreases

ith decreasing pH due to protonation of the –COO− electrode ter-ination, with the consequent decrease in electrostatic repulsion

etween the components of the redox couple and the electrodeurface.

ig. 2. Cyclic voltammograms for bare and carboxyphenyl grafted glassy carbonlectrode (GC) in aqueous 1 mM K3[Fe(CN)6]/0.1 M KCl at pH 6.6. The electrode wasrafted by keeping the potential at −0.8 V vs. Ag/Ag+ for 10 min.

2.9 64.4 306.6 90

The values of RCT differ, however, from previous work. For exam-ple, for the same electrode area, values of RCT of 10 k� [14] and46 k� [21] were reported at pH 3 in comparison with an RCT of6 k� at pH 2.9 obtained here. This variability is most likely dueto differences in grafting. A value of RCT of 10 k� was obtainedusing a shorter grafting time (4 min) but at a more negative graft-ing potential (−0.61 vs. Ag/AgCl/0.1 M NaCl). More negative graftingpotentials have been observed to produce denser layers [26] andaccordingly, a greater blocking effect, which explains the higher RCTvalue. The value of RCT of 46 k� [21] was obtained using an in situgrafting method in an acidic aqueous solution i.e., the preparationconditions were very different from those employed in the presentstudy. For 4-nitrophenyldiazonium salt it has been observed thatgrafting in aqueous solution leads to a lower surface coverage butstill to layers that are more blocking compared to those graftedin acetonitrile [25]. Baranton and Bélanger [21], however, reportedthe opposite effect. The reason for this discrepancy is not clear. Fur-thermore, different RCT values have been reported for unmodifiedglassy carbon electrodes even under very similar conditions [14,21].It should also be noticed that at pH 1.5 the charge transfer resis-tance of the grafted electrode is much larger than that for the bareglassy carbon electrode (1.0 k� compared with 93 �), indicatingthe importance of physical blocking.

The variability in the values of RCT observed for the differentgrafted surfaces points to the difficulty of comparing different sub-strates and film growth conditions. In spite of these uncertainties,the characterisation experiments clearly show that a carboxylatepH dependent termination is produced.

3.2.2. Self-assembled monolayers on goldThe possibility of using self-assembled monolayer of 4-

mercaptobenzoic acid (4-MBA) on gold for the surface complex-ation of Cu(II) was investigated. At pH 6.6, electron transfer tohexacyanoferrate(III) is not inhibited by the 4-MBA SAM (Fig. S3)in contrast with the behaviour of a thiophenyl monolayer or car-boxylic acid terminated alkanethiols on gold [13,15,27,28], and ofa carboxyphenyl layer grafted on glassy carbon. 4-MBA can attachto the gold surface through the SH and –COO− groups interactingwith the metal through the �-electron system. Therefore, a parallelorientation to the surface is favoured for adsorbed 4-MBA. Since itwas desirable to have a Cu(II) centre coordinated to a –COO− ter-mination without direct contact with the electrode surface theseSAMs are unsuitable for studying the electrochemical properties ofcoordinated Cu(II).

3.3. Electron transfer to copper(II) coordinated to graftedbenzoate groups

3.3.1. Preparation of the surface Cu(II) benzoate complexCoordination of Cu(II) to the benzoate complex was carried

out in situ by immersing the functionalised electrode in 5 mMCuSO4. Coordination was confirmed by comparing voltammogramsin 0.1 M NaClO4 before and after immersion in the Cu(II) solution(Fig. 3). The carboxyphenyl layer is not electroactive in the potentialrange studied but after immersion in the Cu2+ solution, the electron

E. Repo et al. / Electrochimica Acta 54 (2009) 6584–6593 6587

Fig. 3. Cyclic voltammograms of grafted glassy carbon electrode in 0.1 M NaClO4

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Table 2Surface concentration of copper(II) benzoate complex as a function of grafting timeand potential. Grafting was made by the potential step method and the electrolysistime at the grafting potential (Egr) was varied.

Electrolysis time (s) Surface concentration (×10−10 mol cm−2)

Egr = −0.4 V Egr = −0.6 V Egr = −0.8 V

60 4.6 5.6 6.7120 6.8 5.1 6.2300 6.5 6.2 6.3600 7.1 8.4 6.0

Fig. 5. Cyclic voltammograms for grafted copper(II) benzoate complexes in 0.1 M+

efore (a) and after (b) immersion in 5 mM CuSO4 for 15 min. Scan rate: 0.1 V s−1.he electrode was grafted by the potential scan method (10 scans between 0.2 and0.6 V vs. Ag/Ag+).

ransfer (ET) reaction corresponding to the Cu(II)/Cu(I) redox cou-le is observed, indicating the formation of a Cu(II) benzoate surfaceound complex.

The stability of the Cu(II) species formed was assessed by repeti-ive potential scanning. Fig. 4 shows that the peak current decreasesapidly at first due to the loss of weakly bound copper, but as cyclingroceeds, the changes in the CV become negligible. There is hardlyny difference in the currents between the 80th and 90th scanemonstrating the stability of the complex formed.

.3.2. Cu(II) surface concentrationThe surface concentration of copper complexes attached to the

rafted layer was evaluated from the area under the reduction peakf the background subtracted voltammograms. The influence ofxperimental conditions on the resulting surface concentration ofu(II) was studied for potential scans and steps to investigate the

nfluence of grafting potential and time on the surface concentra-ion of the complex. The potential is the most important parameteretermining coverage, which approaches a saturation value with

ncreasing reaction time [26]. The results for the potential stepethod are shown in Table 2. The average value for the Cu(II)

urface concentration for grafting times of 120 s or greater was

6.6 ± 0.5) × 10−10 mol cm−2 and little potential dependence wasbserved. The average reproducibility of the measured coverage forll potentials and times of reduction was 8%.

ig. 4. Cyclic voltammograms of a glassy carbon electrode grafted with copper(II)enzoate complexes after multiple scanning in 0.1 M NaClO4. 1st, 20th, 40th, 60th,0th and 90th scans are shown in the figure. Scan rate: 0.1 V s−1. The electrode wasrafted by keeping the potential at −0.8 V vs. Ag/Ag+ for 10 min.

NaClO4. The grafting potentials were (a) −0.4 V, (b) −0.6 V and (c) −0.8 V vs. Ag/Agin acetonitrile using the potential scan method (10 scans). Scan rate: 0.1 V s−1. Inset:surface concentration (� ) calculated from the peak area of reduction of graftedcopper(II) benzoate complex as a function of the grafting potential.

In contrast with these results, a linear increase in coverage withswitching potential was observed when the scan grafting methodwas used, as shown in the inset to Fig. 5. For long electrolysistimes, an apparent decrease in the –COOH groups available for Cu(II)attachment was observed, probably due to multilayer formation(see below).

3.3.3. Stoichiometry of the surface Cu(II) complexIn order to calculate the stoichiometry of the Cu(II) surface

coordination compound formed, the coverage by phenyl car-boxylate groups must be known. The charge transferred duringthe reductive grafting of the diazonium salt using the poten-tial step method corresponds to surface concentrations from (2.2to 5.4) × 10−8 mol cm−2 (potential ranges from 0.2 to −0.4, −0.6and −0.8 V). When comparing these results with the concen-tration of –COOH groups in the film available for coordination(6.6 × 10−10 mol cm−2), this implies that only a small fraction of theradicals generated are attached to the surface (∼1%) and the restdecays through other reactions, e.g. polymerization in solution. Asimilar high value for the surface concentration of radicals initiallyformed by the reduction of the diazonium cation using the scanmethod (2.1 × 10−8 mol cm−2) was calculated by integration of thefirst grafting scan (Fig. 1). The charges associated with the reduc-tion of the diazonium compound do not give an indication of theresulting surface coverage.

A surface coverage of 7.4 × 10−10 mol−1 cm−2 with car-boxyphenyl groups has been calculated by Liu et al. for graftingGC with the corresponding diazonium salt [16] whereas a value

of 5.5 × 10−10 mol−1 cm−2 [21] has been estimated from X-rayphotoelectron spectroscopy (XPS) for electrodes prepared usingin situ generated diazonium cations in an aqueous solution. Thisvalue is smaller by a factor of two than that predicted for a close

6 ica Acta 54 (2009) 6584–6593

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acked monolayer of carboxyphenyl groups (12 × 10−10 mol cm−2)29]. XPS may not be the best way to measure the coverage ofrafted groups containing only carbon and oxygen [10,14] but nev-rtheless, it can be concluded that a full single compact monolayers not formed by electrochemical grafting of glassy carbon. Similarbservations for the coverage of glassy carbon by nitrophenylroups have been made by McCreery et al. [29,30].

The similarity in the surface concentration of Cu(II) measured inhe present work, � Cu(II), of 6.6 × 10−10 mol cm−2, and the reportedCOOH surface concentration, indicates that each Cu(II) ion isound approximately to one carboxyphenyl group. Two differentopper-benzoate complexes are formed when considering the spe-iation of Cu(II) in benzoate aqueous solutions, CuL+ and CuL2,here L is the benzoate ligand and the remaining ligands are water

31]. The formation of the 1:2 complex in aqueous solution haseen found to be negligible [32], indicating that the proposed 1:1toichiometry of the surface complex is in agreement with the solu-ion thermodynamic data. Although the above are rough estimatesonsidering the uncertainty in the value of the coverage by car-oxyphenyl groups, the surface stoichiometry estimated requireshat the film electroneutrality is provided by electrolyte anions, inroad agreement with the known properties of the aqueous Cu(II)enzoate complexes [32].

The unknown structure of the grafted layer poses uncertaintiesor defining the coverage by Cu(II). In contrast to SAMs, grafting onC can give rise to multilayer growth [33]. In this case the Cu(II)

ons coordinated to the film will be present in a range of distancesrom the surface resulting in different degrees of inhibition tolectron transfer and hence, the reduction charge measured forhe bound Cu((II) may not fully represent the total amount ofarboxylate functionalities available for coordination. The X-raytructure of Cu(C6H5COO)2·3H2O shows that Cu(II) is bound towo benzoate molecules through one oxygen group and fourater molecules occupy the remaining coordination sites to formdistorted octahedron [34]. Thus, the 1:1 surface stoichiometry

stimated above requires that an electrolyte anion must provideharge neutralisation.

.4. Kinetics and mechanism of electron transfer to the grafted

enzoate Cu(II) complex

.4.1. Cyclic voltammetry for the Cu(II) benzoate film.Fig. 6 shows the sweep rate dependence of the cyclic voltamme-

ry of the Cu(II) benzoate surface. The linear dependence of peak

ig. 6. Cyclic voltammograms in 0.1 M ClO4 of an electrode grafted by the potentialcan method (10 scans between 0.2 and 0.6 V vs. Ag/Ag+) and then reacted withu(II). Scan rates: 0.02, 0.05, 0.1, 0.2, 0.5, 0.7 and 1 V s−1. Inset: dependence of theeak current on scan rate for the grafted Cu(II) benzoate complex. The slopes of theathodic and anodic linear fittings are 17.6 and 16.8 �A s V−1, respectively.

Fig. 7. Dependence of the cathodic (�) and anodic (�) peak potentials on sweep ratefor the results shown in Fig. 8.

current on scan rate is shown in the inset to the figure, confirmingthat the redox reaction is due to surface bound species. The slopesof the cathodic and anodic peak currents are very close to each other(−17.6 and 16.8 �A s V−1, respectively) and the lines pass throughthe origin. Fig. 7 shows the dependence of the peak potentials onsweep rate. Although the peak separation increases for faster sweeprates, these results do not correspond to a simple quasi-reversibleelectron transfer reaction to a surface confined species since thepeak separation reaches a constant value different from zero at lowsweep rates. The limiting value of the difference in peak potentials,�Ep, at low sweep rates shown in the trumpet plot in Fig. 7, is notzero as would be expected for a reversible reaction of an adsorbedcouple. This behaviour has been previously observed for surfaceattached quinones [35]. The difference in the peak potentials forthe reduction and oxidation processes indicates that the formalpotential of these two reactions is not the same and these two pro-cesses must be treated separately but closely related. The origin ofthis effect might be related to differences in charge compensationenvironments of the redox centres since peak separations at lowsweep rates would be expected for an electrochemical–chemical(EC) mechanism.

Fig. 7 also shows that the peak potentials shift as the sweeprate is increased to high values. The uncompensated resistancewas calculated from the AC impedance results and a value of(0.09 ± 0.02) k� was obtained. The maximum error in the peakpotentials (at 1 V s−1) is of the order of 1.5 mV. This is an averagefrom eight different experiments. Therefore, the IR drop in the filmdoes not affect the results of the rate constant estimated from thetrumpet plot (Fig. 7). The rate constants for the cathodic and anodicprocesses were estimated from these shifts using the method ofLaviron [36] and values of the standard surface electron transfer rateconstant, k0 [37], of approximately 17 and 28 s−1 were calculatedfor the cathodic and anodic reactions, respectively.

The full width at half maximum, FWHM, (ca. 200 mV and inde-pendent of sweep rate) is unexpectedly larger than that for areversible reaction, 90.6 mV/n in the absence of lateral interactions[22]. The origin of this effect is discussed in Section 3.4.6.

3.4.2. The structure of Cu(I) and Cu(II) benzoatesIn order to analyse the redox processes in the Cu(II) benzoate

film, it is useful to discuss first the structural information availablefor the Cu(I) and Cu(II) benzoates. In the solid state, the stable formof Cu(II) benzoate obtained from aqueous solutions is the trihy-

drate. Its structure consists of columns of Cu atoms, each one havingtwo carboxyl groups and four water molecules as nearest neigh-bours, giving rise to distorted octahedra [34]. Two water moleculesare shared between each octahedron and the resulting structure hasaligned rows of Cu atoms. This structure results in only one oxygen

ica Acta 54 (2009) 6584–6593 6589

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tom of each carboxylate group being associated with each Cu atom.he structure of anhydrous Cu benzoate has been characterised byowder diffraction from material obtained by dehydration of therihydrate [38]. In this case, the loss of water necessarily results inery large structural changes. However, considering that the sta-le phase obtained from aqueous solutions is the trihydrate [34], it

s most likely that the material formed by the reaction of aqueousu(II) with the attached benzoate functionality will contain waters a ligand. Since a surface film contains a termination that is absentn a bulk crystal, it would be expected that the Cu(II) ion will beresent in a wide variety of coordination environments.

The chemistry of Cu(I) carboxylates has attracted considerablenterest due to their catalytic properties in organic reactions suchs decarboxylations and ester formation by reaction with organicalides [39,40]. Edwards and Richards [41] demonstrated a robustnd simple synthetic procedure for this family of compounds byeacting Cu2O with the corresponding dry carboxylic acid. Cu(I)enzoate is easily hydrated but the anhydrous compound has beenrepared and its structure determined [42]. This compound hascomplex crystalline structure with two independent tetramers

resent in an asymmetric unit. This has four Cu atoms present in alane with the –COO− benzoate groups bridging the four Cu atomsnd alternating their orientation above and below the Cu atomslane. This structural motif has also been found for substituted Cuenzoates and importantly, the Cu–Cu distance is close to the sumf the van der Waals radii indicating metallophilic interactions asbserved for Au(I) compounds [43]; see also Ref. [44] and referencesited therein.

These Cu(I) complexes are stable in the solid state and they showsurprising stability in aqueous media [45]. Saphier et al. have

roposed that this results from the binding of the Cu ion to theromatic ring of the benzoate ligand and not to the oxygen atomsn the carboxylate groups. The suggestion that copper-benzoate is a→ � complex is in contrast with the X-ray structures determinedy Drew et al. [42] who showed that binding occurs through thearboxylate group. The differences between these structures coulde related to differences in the solid state and solution environmentut the reported stability observed for the aqueous Cu(I)-benzoateystem is unusual (see further discussion in Section 3.4.5). It isherefore, difficult to predict the structures that could be formedn the electrode surface on reduction of the Cu(II) benzoate filmsrepared.

The above discussion indicates that the structures of the Cu(II)nd Cu(I) benzoates show significant differences in the solid state.or the former, only one of the oxygen atoms is close to the Cuentre, whereas for the latter, a ring structure prevails in whichhe carboxylate acts as a bridging group. Although the structure ofhe Cu(II) films studied here is likely to differ from those of singlerystals due to the different coordination environments of a solid-D compared with a 3-D structure, it is clear that large structuralhanges would be expected to take place on reduction of the Cu(II)enzoate films investigated.

.4.3. Reaction mechanismFrom the above considerations, it is proposed that the reduc-

ion mechanism could be best described by the scheme of squareshown in Scheme 1 [46] with coupled electrochemical–chemicaleactions (ECEC). In this reaction sequence, the chemical steps arehe structural changes of the coordination environment aroundhe copper ion determined by the change in oxidation state of the

etal centre. In the forward sweep, the proposed electron trans-

er sequence corresponding to the voltammograms shown in Fig. 6tarts therefore with a first electron transfer to the Cu(II) speciesccording to:

u(II) + e− � Cu(I)∗ (I)

Scheme 1. Scheme of squares describing the ECEC mechanism of the electrochem-ical reduction of Cu(II) benzoate followed by its reoxidation.

For clarity, the ligands associated with the metal ion (the car-boxylate environment) are not indicated in what follows. Cu(I)* isthe Cu(I) species formed immediately after electron transfer i.e., ina ligand environment corresponding to Cu(II). Cu(I)* then relaxes tothe equilibrium ligand configuration of the Cu(I) ion in the benzoatefilm:

Cu(I)∗ → Cu(I) (II)

During the reverse sweep, the oxidation of the Cu(I) speciesfollows a similar sequence:

Cu(I) � Cu(II)∗ + e− (III)

Cu(II)* (a Cu(II) ion in a non-equilibrium environment) finallyrelaxes to the original Cu(II) compound according to:

Cu(II)∗ → Cu(II) (IV)

3.4.4. Relaxation kinetics of Cu(I)*

In order to analyse the mechanism described above, the rateof the first chemical step was investigated. These would involve areorganisation of the ligand environment after electron transfer i.e.,reaction (II). The kinetics of the relaxation reaction (II) were studiedby holding the potential at −0.3, −0.35 or −0.4 V for different timesat the switching potential and then carrying out an oxidation scan.Fig. 8a shows an example of the voltammograms obtained whenstopping the scan at −0.35 V for different times. The charge underthe oxidation peak at −0.14 V corresponds to the amount of theCu(I)* species remaining on the surface after reduction for differenttimes since the oxidation of the relaxed Cu(I) species occurs at morepositive potentials. The onset of this reaction can be observed inFig. 8a for potentials more positive than 0.15 V.

The reoxidation wave for the initial species formed, Cu(I)*,decreases with increasing holding time due to the decay of the reac-tion intermediate. The total amount of Cu(I)* remaining for differentholding times was calculated by integration of the first wave in thereoxidation scan. The inset in Fig. 8a shows that the decay of Cu(I)*(reaction (II)) follows first order kinetics with a rate constant k1 ofapproximately (5 ± 0.4) × 10−3 s−1, corresponding to a half-life ofapproximately (140 ± 11) s. Thus, the decay of Cu(I)* is very slow. Itis not surprising, therefore, that this chemical step is not observedin the results shown in Figs. 3–5 obtained at 0.1 V s−1 for whichthe whole anodic transient is measured within a time scale of 5 sand hence, the electrochemical properties measured relate only toreaction (I).

Fig. 8b shows the same experiment but at a more negative hold-ing potential. The reoxidation waves for the relaxed forms of Cu(I)

can be clearly observed. An unusual aspect of these results is thatthe oxidation potential of the Cu(I) species formed by decay of theCu(I)* intermediate shifts to more positive values for longer hold-ing times. It is important to note that no further reduction to Cu(0)was observed by extending the voltammetric scan up to −0.9 V

6 ica Ac

(rotvtCtwtss

diasctct

3

bsbcd

FeSAdpfi

590 E. Repo et al. / Electrochim

data not shown), demonstrating that the changes illustrated in theeoxidation scans in Fig. 8b depend only on the chemical processesccurring after the reduction of Cu(II). The standard potentials ofhe Cu(I)/Cu(0) and the Cu(II)/Cu(0) couples are 0.521 and 0.3419 Vs. SHE, respectively [47] and from the dissociation constant ofhe Cu(I)Bz complex in solution [45], the standard potential of theu(I)Bz/Cu(0) couple is 0.241 V vs. the SHE or 0.044 V with respecto the Ag/AgCl electrode employed in this work. A reduction featureas never observed at this potential, which is not surprising since

he reduction of Cu(I)Bz would have to take place from the solidtate in the case of the benzoate films studied, for which there is notabilisation mechanism for Cu(0).

The above considerations are in agreement with the known largeifference in the solid state structures of Cu(I) and Cu(II) discussed

n Section 3.4.2. The shift in peak potential with increasing timest the negative scan limit indicates a slow stabilisation of the Cu(I)pecies formed from Cu(I)*. It is proposed that the rate of the chemi-al step is slow due to the presence of the transition metal ion withinhe rigid structure of the film with little room for the structuralhanges required by reaction (II), which restricts the relaxation ofhe structures formed after electron transfer.

.4.5. The stability of the Cu(I) and Cu(II) benzoate complexesIt is of interest to compare the redox potential of the Cu(II)/Cu(I)

enzoate couple when present in the film and in solution. The

tandard potential of the latter is not available in the literatureut it can be calculated from the reported stability constants ofopper-benzoate. The stability constant of Cu(I) benzoate has beenetermined by fast scan cyclic voltammetry and high frequency

ig. 8. Cyclic voltammograms for grafted Cu(II) benzoate in 0.1 M NaClO4 for differ-nt times at the negative limit of the scan. (1) 30 s, (2) 60 s, (3) 120 s and (4) 300 s.can rate = 0.1 V s−1. The electrode was grafted by keeping the potential at −0.8 V vs.g/Ag+ for 10 min. Negative potential limits: (a) −300 mV and (b) −400 mV. Inset:ecay plot corresponding to first order kinetics for the reoxidation of the reductionroduct of Cu(II) benzoate in the film (Cu(I)*). Q(t) is the integrated current under therst reoxidation wave for different times of reduction at a cathodic limit of −0.35 V.

ta 54 (2009) 6584–6593

square wave voltammetry. The dominant complex is Cu–L, whereL = benzoic acid/benzoate (Eq. (V)) and log(K1) is 4.7 at pH 5.41[45]. For the Cu(II) benzoate complex Cu–L+, Eq. (VI), values oflog(K1) = 1.6 ± 0.2 [32] and 1.51 ± 0.03 [48] have been reported.

Cu(I)(aq) + L(aq) � Cu(I) − L(aq) (V)

Cu(II)(aq) + L(aq) � Cu(II) − L+(aq) (VI)

The much larger stability constant for the Cu(I) complex com-pared to that of Cu(II) has been ascribed to d → � interactionsbetween Cu(I) and the aromatic �-system and confirmed by 1HNMR [45]. From these results, a standard potential of 0.34 V vs.the standard hydrogen electrode (SHE) can be calculated for thereaction:

Cu(II) − L(aq) + e− � Cu(I) − L(aq) (L = benzoate) (VII)

The redox potential of the corresponding surface copper com-plexes,

Cu(II) − L(s) + e− � Cu(I) − L(s) (VIII)

was estimated at approximately 0.11 V vs. the SHE from the aver-age of the peak potentials for the reduction and oxidation processes(Fig. 7). Thus, for the surface bound complexes, the ratio of equilib-rium constants (K1) is only 5.8 compared with approximately 1500for the aqueous complexes (reactions (V) and (VI)).

A linear dependence of the stability constant of Cu(I)–L com-plexes for aromatic carboxylates on the pKa of the correspondingacids has been demonstrated (see Fig. 6, Ref. [45]) from which thestability constants for the surface bound copper-benzoate com-plexes can be estimated. The pKa of surface grafted benzoic acidis 2.8 [14] and this value leads to log(K1) = 2.8 for the surface boundCu(I) complex. This is more than two orders of magnitude lowerthan the equilibrium constant in solution. By contrast, the calcu-lated value of log(K1) for the surface bound Cu(II) complex is 2,estimated using the ratio of 5.8 between the stability constantsof Cu(I)–L/Cu(II)–L calculated above. Thus, Cu(II)–L(s) is a slightlystronger complex compared with the solution species. The lowervalue of the stability constant for the surface bound Cu(I) complex isprobably due to a decreased overlap between the d-orbitals of Cu(I)and the �-system of the aromatic ring resulting from geometricalconstraints within the film.

3.4.6. The full width at half maximum (FWHM)The very large value of the FWHM observed, of approximately

200 mV, is much greater than that expected for a reversible oneelectron reaction, 90.6 mV [13,49]. Large values have been previ-ously observed for redox groups present in SAMs [16,18,49]. Theresults in Fig. 5 show that the FWHM does not depend on coverageand therefore, a Langmuir isotherm should be applicable. Thus, amodel that considers interactions between adsorbed species is notapplicable to the present experimental conditions. The voltammet-ric response was analysed considering first a distribution of formalpotentials within the film and employing a Gaussian distributionof formal potentials [50–52]. Such a distribution can be attributedto differences in charge neutralisation within the film, site–siteinteractions or a distribution of coordination environments. It isvery likely that the grafted copper complex layer is heterogeneousbecause of the structural issues discussed in Section 3.4.2 and theheterogeneity of the polished glassy carbon surface [53]. A Gaus-sian distribution of formal potentials of redox species j within the

film of the form:

f (E0′j ) = 1√

2��exp

[−

(E0′j

− E0′)2

2�2

](1)

ica Ac

fp�c

�

ws

�

�

mctusfpr

3

resapbv[tfrowr

Fl�mw

E. Repo et al. / Electrochim

originally described by Albery et al. [51] was employed. E0′j

is the

ormal potential of species j, E0′is the average value of the formal

otential of the Cu(II)/Cu(I) benzoate redox species in the film andis the standard deviation of the distribution. The dimensionless

urrent, �(E), was calculated from [50]:

(E) =Ef∫Ei

f (E0′j

)�(E0′j

)[1 + �(E0′

j)]2

dE0′j (2)

ith Ei and Ef the initial and final potentials of the voltammetriccan and

(E0′j ) = exp

(nF

RT(E − E0′

j ))

(3)

The dimensionless current is given by

(E) =(

RT

A�T vF2

)I(E) (4)

Eq. (2) was computed using Mathcad 14® and Table S1 (supple-entary information) shows the dependence of the FWHM on �. A

omparison between a measured and a simulated curve is illus-rated in Fig. 9. The basic features of the CVs can be describedsing the above distribution of formal potentials and this figurehows a good match between predictions and the measurementsor � = 93 mV. The suitability of this model is in agreement with theroposal that the copper species will be present in the film in a wideange of chemical environments.

.4.7. The influence of coupled ion and electron transferThe influence of counter charge balance in the voltammetric

esponse was analysed. In order to preserve film electroneutrality,lectron transfer to the Cu(II)/Cu(I) redox couple must be compen-ated either by transfer of a cation into the film or elimination ofn anion from it. The analysis of this problem has been previouslyresented by Anson’s group for steady-state conditions [54–58],y Smith and White [59] and Myland and Oldham [60] for cyclicoltammetry, and by Scholz et al. for square wave voltammetry61]. The central issue is that although cation transfer does not limithe reaction rate, the potential drop across the film–solution inter-

ace changes as the film is reduced or oxidised and this in turn,educes the film–solution potential difference, i.e. the driving forcef the redox reaction. A full derivation of the equations employed asell as an analysis of the parameters determining the voltammet-

ic response for this case is given as supplementary information.

ig. 9. Comparison between a measured (solid line) and a simulated (dashedine) voltammogram considering a Gaussian distribution of formal potentials with

= 0.1 V and a reversible reaction; v = 0.1 V s−1 and n = 1. The experimental voltam-ogram was measured in 0.1 M NaClO4 with the scan rate of 0.1 V s−1. The electrodeas grafted by the potential scan method (10 scans between 0.2 and 0.6 V vs. Ag/Ag+.

ta 54 (2009) 6584–6593 6591

Fig. 10a shows a comparison of simulations for a reversible redoxreaction with and without considering counter-ion transfer. Forconvenience, the results are given in terms of the reduced current� given by Eq. (4).

A major difference in the simulated voltammograms is observedwhen ion transfer is included in the kinetic model, leading to lowerand broader current peaks. Fig. 10b shows the calculated depen-dence of the potential difference at the film–solution interface onapplied potential for a linear sweep voltammetry calculation. Therate of change of potential drop across the film/solution interfaceis significantly lower than that of the total applied potential, byas much as 50% at the switching potential. Therefore, the drivingforce for electron transfer is lower and changes more slowly thanthe total applied potential showing that charge compensation bycounter-ions can have an important influence in the value of theFWHM. In the example shown in Fig. 10a, the FWHA is increasedfrom 90.6 to 130 mV for a very fast reaction when charge compensa-tion is taken into account. Further analysis of this problem is givenin the supplementary information.

It is concluded that the potential drop across an immobilisedredox film and the electrolyte solution has a significant effect onthe cyclic voltammograms by lowering and broadening the currentpeaks. It is not possible at present to distinguish between the twopossible reasons of the observed peak broadening discussed above.This analysis has been presented to highlight that the properties ofthese films require a careful analysis and that more than a simpleexplanation for the broadening of the voltammetric peaks must beconsidered.

3.4.8. Scheme of squares simulation for surface attachedcomplexes

Although the above models are applicable for explaining theobserved broadening of the voltammetric peaks, the mechanismshown in Scheme 1 cannot account for the results shown in Fig. 8bsince it is clear that more than a single relaxed reduction product isformed. The reoxidation can take place from different Cu(I)* speciesformed in the transition from the initial reduction product to thefully relaxed Cu(I).

In the scheme of squares shown in Scheme 1, two well-definedCu(I) species are shown, corresponding to the unrelaxed andthe fully relaxed complexes. However, the experimental resultsclearly show that the reoxidation of Cu(I) depends on time i.e., theoxidation takes place from Cu(I) species present in different envi-ronments on the way from Cu(I)* to Cu(I). In the simulation of thereaction mechanism, the experimental values of the time at theswitching potential and of the redox potentials related to this timehave been chosen.

For simplicity, the relaxation of the Cu(II)* species has not beenconsidered since the experiments were started with a fully oxi-dised relaxed film, as shown by the presence of a single reductionpeak. The formal potential of the Cu(I)/Cu(II)* couple depends onthe rearrangement of the layer following electron transfer. As hasbeen discussed in Sections 3.3.1 and 3.4.2, large structural changesmust take place after electron transfer resulting in the stabilisationof the cuprous species. A consequence of this stabilisation is a shiftof different Cu(I)/Cu(II)* formal potentials to more positive values,as clearly observed by the shift in the peak potentials with time ofreduction at −400 mV shown in Fig. 8b.

In order to analyse the general features of these reactions, theexperiments holding the potential at the negative end of the scanwere simulated considering the above mechanism and a simplified

reaction sequence. Reactions (I) and (III) are the electrochemicalsteps for which it is assumed that the rate constants follow theButler-Volmer formalism [22]. The equations used for the sim-ulation are given as supplementary information. This simplifiedmodel did not consider the change in oxidation potential of the

6592 E. Repo et al. / Electrochimica Acta 54 (2009) 6584–6593

Fig. 10. (a) Simulations of reversible redox reactions without (dotted line) and with counsee supplementary information Eq. (S5). k0 = 1000 s−1, v = 1 mV s−1, = 100.

Fig. 11. Scheme of squares simulations showing the effect of the waiting time at thes(aK

ctfiiwcrrSdo

4

efptsbm

witching potential −200 mV, expressed as vt: (a) 3000 (solid), (b) 6000 (dashed),c) 12,000 (dotted) and (d) 30,000 mV (dash-dotted), corresponding to 30, 60, 120nd 300 s waiting time at the switching potential of the scan for v = 100 mV/s. K0

1 =02 = 7, K1 and K2 = 0.0013; E0′

2 − E0′1 = 100 mV.

uprous species present in different environments. The results ofhe simulation are shown in Fig. 11 where it can be seen that therst peak gradually disappears and the second peak increases with

ncreasing time at the switching potential. This general featureas also observed experimentally as demonstrated in Fig. 8b; it is

lear that the time spent at the switching potential influences theeoxidation sweep. These simulations show that the voltammetricesponse can be described by the scheme of squares indicated incheme 1. A full quantitative analysis including the involvement ofifferent unrelaxed Cu(I) species, however, was outside the scopef the present work.

. Conclusions

4-Carboxyphenyl groups were grafted on glassy carbonlectrodes and characterised electrochemically using hexacyano-errate(III) as a redox probe. As previously observed, depending on

H, both physical blocking and electrostatic repulsion determinehe rate of electron transfer across the attached layer. However, theame experiments with 4-MBA coated gold electrode showed poorlocking, which was attributed to the surface orientation of 4-MBAolecules.

ter-ion transport (solid line). (b) Potential drop across the film–solution interface,

The grafted 4-carboxyphenyl groups were used to coordinatecopper(II) ions to form stable surface bound complexes. The elec-trochemistry of the attached copper(II) benzoate complexes wasstudied by cyclic voltammetry. Simulations including dispersion inthe redox potentials within the film and ion and electron transfercoupling were used to model the voltammograms and both modelscould explain the observed peak broadening. The decay of the ini-tial Cu(II) reduction product was investigated and a first order decayconstant of (5 ± 0.4) × 10−3 s−1 was estimated. An ECEC mechanismis proposed with the chemical steps corresponding to the reor-ganisation of the ligand environment. Simulations based on thismechanism can predict the general features of the voltammetricresponse.

The importance of the present work is to demonstrate that thecarboxylate group is an appropriate functionality for coordinativeattachment of Cu(II) to a –COO− functionalised electrode and thatelectron transfer to this metal redox centre is strongly influencedby the rigidity of the ligand environment, in particular these resultin a change in the coordination geometry of the metal centre withinthe film.

Acknowledgments

The authors gratefully acknowledge the preparation of 4-carboxyphenyldiazonium tetrafluoroborate by Dr J. Paprotny,Chemistry Department, University of Liverpool and support fromthe European Union FP6 programme (DYNAMO project).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.electacta.2009.06.019.

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