Electron transfer reactions at conductive diamond electrodes

Electrochimica Acta 47 (2002) 1641–1649

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Electron transfer reactions at conductive diamond electrodes

Sergio Ferro1, Achille De Battisti 1*Department of Chemistry, Uni�ersity of Ferrara, Via L. Borsari 46, I-44100 Ferrara, Italy

Received 30 October 2001; received in revised form 12 December 2001; accepted 19 December 2001

Abstract

The electrochemical behavior of the iron(III)/iron(II) and the ferri/ferro–cyanide redox couples in aqueous media has beeninvestigated at conductive diamond, to obtain information on the properties of this electrode material. The investigation has beencarried out at as prepared and oxidized diamond surfaces, under conditions of different concentration and temperature. Theelectron transfer kinetics has been followed by cyclic voltammetry, quasi-steady polarization (low-field conditions) and ACimpedance, and the results have been interpreted on the basis of the Hush model, attempting its extension to the ferri/ferro–cy-anide redox couple as well. © 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Electron transfer; Highly boron-doped diamond electrodes; Iron(III)/iron(II); Ferri/ferro–cyanide; Redox couples

1. Introduction

The electrochemical response of a redox couple insolution depends on the specific properties of the ion ormolecule under investigation, and on the electrode ma-terial. The first aspect is related with changes in thehydration shell (if simple electron transfer is involved)or with molecule reorganization, when a more complexbehavior can be expected. On the other hand, theelectrode material can be only a source/sink of elec-trons, or can interact with the redox couple, whencontact of electroactive species takes place.

Dealing with well-known redox couples, their electro-chemical behavior can be used to characterize a newelectrode material, with the aim of elucidating its prop-erties [1–23]; vice versa, a new redox couple should bestudied at an electrode material, whose characteristicsare established and, more important, well reproducible[24–42].

The main redox couples of iron, in aqueous solution,are the iron(III)/iron(II) and the ferri/ferro–cyanideredox equilibria, both studied at different electrodematerials and in different conditions of reactant con-centrations, supporting electrolyte and temperature.Both couples have been deeply investigated in the past,

but recent works have underlined some details that castdoubt on previous findings. In particular, the ferri/ferro–cyanide couple has been for long considered anouter-sphere reaction [28], while, on the contrary, it issignificantly dependent on the presence of chemicalfunctionalities on the electrode surface [6,9,32,34–36,40,43–45]. Also the kinetics for Fe3+/Fe2+ is highlysensitive to the electrode surface [2,5,31,41]; in particu-lar, at carbon electrodes, it seems stimulated by thepresence of oxides [7,43–45].

Taking into account these indications and also thoserecently reported on conducting diamond electrodes[10–13,17–19,21,22], in this paper the results of investi-gations are described on both iron couples, carried outat highly boron-doped diamond (BDD) thin film elec-trodes, with the aim of elucidating the electrochemicalbehavior of the films and to obtain information relatedwith the physical properties of this material.

2. Experimental

2.1. Electrodes

Highly BDD thin film electrodes were prepared onp-silicon (100) wafers (1–3 m� cm, 1 mm thick, 100mm diameter), as described elsewhere [46]; the p-dia-mond coating was conformal, pinhole-free and had athickness of 1 �m (�5%) and a resistivity of 15 (�

* Corresponding author. Fax: +39-0532-240709.E-mail address: [email protected] (A. De Battisti).1 ISE member.

0013-4686/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.PII: S 0 0 13 -4686 (01 )00898 -2

mailto:[email protected]

S. Ferro, A. De Battisti / Electrochimica Acta 47 (2002) 1641–16491642

30%) m� cm, consistent with a boron concentrationbetween 3500 and 5000 ppm. Moreover, elsewhere re-ported Raman investigation has shown that the non-di-amond to diamond ratio is less than 1%, indicatingfilms of good quality and with a significant diamondcharacter [47]. BDD electrodes, with a diameter slightlysmaller than 15 mm, were placed at the bottom of theelectrochemical cell and the exposed area had a nomi-nal value of 0.785 cm2 (a disk with a diameter of 1 cm,see Ref. [48]); a cylindrical platinum grid and a double-walled saturated calomel electrode (SCE), with an inter-mediate saturated NaNO3 solution, were the counterand the reference electrode, respectively. Electrodeswere washed by sonication in isopropanol, followed byan analogous treatment in MilliQ water, prior to thearrangement into the cell.

A ‘fully oxidized’ BDD electrode was obtained polar-izing at +3 V (vs. SCE) in 1 M HClO4, i.e. underextreme oxygen evolution conditions, for 20 min. XPSmeasurements have confirmed the obtainment of anoxygenated surface, the measured O/C ratio beingequal to 0.20 (which agrees well with the value of 0.18reported by Fujishima and coworkers [21]).

2.2. Chemicals

All solutions were prepared with MilliQ water (��18 M� cm), using analytical grade, hydrate reagents.Fe(ClO4)2, Fe(ClO4)3 (chloride�0.005%), K4Fe(CN)6

and K3Fe(CN)6 were supplied by Aldrich and usedwithout further purification. One molar HClO4 (Fluka)was adopted as background electrolyte for the iron(III)/iron(II) redox system, while 0.5 M H2SO4 (Riedel-deHaen) was adopted for the ferri/ferro–cyanide couple.Different concentrations (down to 1 mM) were pre-pared by diluting a freshly prepared 10 mM initialsolution, which contained both the reduced and theoxidized form of the ions under investigation.

2.3. Electrochemical measurements

Cyclic voltammetries were carried out by an AutolabPGSTAT20. Initial CVs were performed at room tem-perature (r.t.) and at different scan rates, between 0.01and 1 V s−1; the chosen potential range was cycledusing a step potential of 2 mV and repeating themeasure at least three times. In every case, the last cyclewas recorded, although reproducible signals were ob-tained just after the first measure.

Quasi-steady polarization curves were carried out ata scan rate of 0.1 mV s−1 and with a step potential of0.15 mV; curves were recorded in a range of �30 mVwith respect to the open circuit potential (OCP), start-ing from the higher value and conditioning the elec-trode at the initial potential for 120 s.

Electrochemical impedance spectroscopy was carriedout by an Autolab frequency response analyzer (FRA),performing measurements at the OCP, in the 10−1–103

Hz frequency range and with a measuring signal ampli-tude of 5 mV.

Measurements carried out at r.t. have been after-wards performed at different temperatures, from 3.5 to60 °C, changing the temperature of the measuring solu-tion by means of a thin-walled glass spiral tube inwhich cold or warm water was circulated; the tempera-ture inside the cell was thus controlled within �0.2 °C.The temperature variation involved only the workingand the counter electrodes, the reference electrode beingconnected by a thin glass tube [49].

The stability of the electrode materials was testedrecording a CV curve (as previously discussed) beforeand after every DC or AC polarization. All DC datahave been corrected for the ohmic drop on the basis ofthe solution resistance determined by the impedanceanalysis.

3. Results and discussion

3.1. Fe3+/Fe2+ redox couple

A blank cyclic voltammogram was recorded in 1 MHClO4 prior to the beginning of the measurements. Asknown, high quality BDD films have a wide potentialwindow of about 3 V without significant water decom-position [11,13]. In our case, for the as prepared film, acurrent density less than 2 �A cm−2 was recorded inthe potential range from −0.4 to +1.6 V versus SCE.As shown in Fig. 1, the background current is about120 times lower than those recorded in the presence ofthe less concentrated solution of the redox couple, atthe same scan rate (0.1 V s−1). The kinetics is not thatof a fully reversible system and �Ep progressively in-creases with increasing the scan rate, lying in the rangefrom 530 to 1040 mV. Since �Ep is larger than e.g. 200mV, the Nicholson’s approach [50] for the calculationof the apparent rate constant (kapp

0 ) cannot be applied.For this reason, the investigation has been carried

out by quasi-steady polarization curves, under low-fieldapproximation (data obtained at the oxidized BDDelectrode are reported in Fig. 2) and by impedanceanalyzes at the OCP. Both analyses provide kapp

0 valuesof (1.0�0.1)×10−5 and of (0.6�0.2)×10−5 cm s−1,for the as prepared and the oxidized BDD electrode,respectively. As discussed by McCreery and coworkers,in the case of sp2 carbon electrode [43–45], the behav-ior of the redox system under discussion seems to behighly sensitive to the presence of oxides, the effectbeing an enhancement of the reversibility. The samebehavior has been reported by Fujishima and cowork-ers [21] for BDD surfaces treated by oxygen plasma,

S. Ferro, A. De Battisti / Electrochimica Acta 47 (2002) 1641–1649 1643

which is also thought to allow insertion of carbonyls atthe diamond surface. In our case, the influence seems tobe the opposite, and anyway only a relatively smalleffect can be observed. Considering the conclusionsdrawn by McCreery et al., the most probable reasoncan be the absence of carbonyls at the diamond surface,hydroxyls being a reasonable alternative, also for thepreservation of the sp3 geometry for the lattice [23].

Fig. 3. CV curves for 5 mM Fe3+/2+ in 1 M HClO4 at differenttemperature (from 3.5 to 60 °C), at the as prepared BDD electrode.Scan rate: 0.1 V s−1.

Fig. 1. CV curves for Fe3+/2+ (1, 2.5, 5, 7.5 and 10 mM) in 1 MHClO4 at as prepared and oxidized BDD electrodes. Scan rate: 0.1 Vs−1. Room temperature.

Fig. 4. Apparent electron-transfer rate constant for the Fe3+/2+

redox couple.

Apart from these considerations, the rate constant val-ues agree quite well with the estimate recently given bySwain and coworkers [22] for a nominally similar elec-trode material, basing on a digital simulation of thecyclic voltammogram. However, Swain and coworkersused iron sulfate to prepare the solution (in perchloricacid) and it is well known that sulfates (as well chlorideions) have a role in increasing the reaction rate [31,41].

In order to better elucidate the process, the role ofthe temperature has been considered, changing its valuein the range from 3.5 to 60 °C, for the 5 mM redoxcouple concentration. Results, in terms of cyclicvoltammetry are given in Fig. 3 (for the as preparedBDD electrode), while the kinetic data are summarizedin Fig. 4. Again, the process is slightly slower at theoxidized BDD surface. The elaboration of data in termsof ‘Arrhenius plot’ allows the evaluation of the appar-ent electrochemical heat of activation (�H°�) for thereaction, while from the intercept of the plot of ln i0versus 1/T at 1/T�0 the pre-exponential factor can be

Fig. 2. Polarization curves (low-field approximation) recorded at theoxidized BDD electrode.


obtained, containing the entropy of activation (�S°�),within a constant [51]. Data for both the as preparedand the oxidized BDD electrode are reported in Fig. 5.The activation enthalpy results to be 8.53�0.27 kcalmol−1 for the first case, and 6.86�0.16 kcal mol−1 forthe reaction carried out at the oxidized BDD surface.These values seem at variance with results obtained interms of rate constants or exchange current densities:the reaction is slower at the oxidized BDD electrode,but the heat of activation at this material is smaller. Itis useful, at this point, to focus on the values of theother important thermodynamic quantity: the entropyof activation; from the ‘Arrhenius plot’, intercepts of−0.705�0.196 and of 0.662�0.116 are found at thetwo materials, respectively. From the fundamental rateequation:

lni0=K−�G°�

RT=K−

�H°�

RT+

�S°�

R

or else

−2.303× logi0=K �+�G°�

RT=K �+

�H°�

RT−

�S°�

R

it follows that the intercept is not directly a measure ofthe entropic contribution; on the other hand, it may beassumed that the constant contribution K remains un-changed in the two cases under discussion and thusdifference in entropy. The electrochemical reaction(which is tacitly assumed to be the oxidation of Fe2+)is thus accompanied by a more negative variation of theactivation entropy, when the reaction takes place at theoxidized BDD electrode. In other words, the hydroxylspresent at the oxidized BDD surface should interactwith the hydration shell of the reacting ion, and possi-bly also with solvent reorganization, allowing a largerdecrease of the activation entropy (loss of degrees offreedom).

The behavior of simple redox couples, at differentelectrode materials and solutions, has been described in

several papers, but only a limited number of themreports on the thermodynamics of the process. In par-ticular, Bockris et al. [2] and Trasatti and coworkers [5]considered different metals (Pt, Rh, Ir, Pd and Au inthe former paper, Ru and Ta in the latter), reporting�H°� values between 5.0 and 5.5 kcal mol−1, com-parable with those obtained in the present work, inspite of the different supporting electrolyte (1 M H2SO4

in Refs. [2,5]). Their faster kinetics can be examined inthe same view above-exposed, the adsorption of sulfatepossibly playing a role similar to those of the polargroups at the oxidized BDD surface [41]. Moreover,certain molecules can act as a bridge in allowing theelectron transfer between the electrode surface and theion in solution. Randles and Somerton [24] have founda value of 9 kcal mol−1 for the heat of activation at Ptin 1 M HClO4, together with an apparent rate constantof 5×10−3 cm s−1 (at 20 °C). The latter value isapparently very high and no information is given con-cerning the nature of the salts used for the solutionpreparation; however, 26 years later, the same setupwas reconsidered [31] and the reported rate constantwas 9×10−6 cm s−1. The authors explicitly examinedthe role of sulfates, showing how the kinetics is in-creased by adding little amounts of H2SO4 (see Table1).

3.2. [Fe(CN)6]3−/[Fe(CN)6]4− redox couple

In order to obtain data experimentally comparablewith those previously described, the redox couple solu-tions were prepared dissolving the hydrated salts in 0.5M H2SO4, in spite of the relative instability of com-plexes (ferricyanide in particular) in acidic media. How-ever, the freshly prepared solutions have been protectedfrom light during measurements. CV data, recorded at0.1 V s−1, for the different concentrations and at bothelectrode materials, are reported in Fig. 6. The charac-ter of the electrode surface is even more deciding in thiscase, the effect being measurable in terms of peakseparation, as well as peak current density. The formerparameter has been used for the calculation of theheterogeneous rate constant, following the Nicholson’sprocedure; �Ep are now between 60 and 275 mV, givingaverage kapp

0 values around 0.04 cm s−1 at the asprepared BDD, and values 10 times lower at the oxi-dized one.

At variance with the iron(III)/iron(II) redox couple,the ferri/ferro–cyanide kinetics (and, possibly, the con-comitant presence of both the oxidized and the reducedform of the couple) does not allow a reliable measureof the exchange current density, through the low-field polarization approximation. However, validatingevaluation of kapp

0 has also been performed byimpedance analysis (frequencies from 30 kHz to 3 Hz);as an example, Fig. 7 shows the impedance dataFig. 5. ‘Arrhenius plot’ for the Fe3+/2+ redox couple.


Table 1Non-exhaustive review of the pertaining literature on the iron(III)/iron(II) redox system

Reference Media Electrode�Ep (mV) Noteskapp0 (cm s−1)

[22] 1 M HClO4581 Diamond4.50E−05 High quality837�42 1.00E−05 1 M HClO4 Diamond

[21] 0.1 M NaClO4637 Diamond

0.1 M NaClO4466 Diamond ox O2 plasma treatment3.70E−06 [17] 0.01 M HClO4 Au (110)

0.1 M HClO4 Au (110)1.10E−060.5 M HClO48.00E−07 Au (110)

3.00E−04 0.1 M HClO4 Au (110) 0.18 �M NaCl1.20E−03 0.1 M HClO4 Au (110) 0.36 �M NaCl

0.1 M HClO42.80E−03 Au (110) 0.72 �M NaCl1.00E−05 [14] 0.1 M HClO4 Au (111)

Au (100)1.30E−06Au (110)6.40E−07

165�22 [44] 0.2 M HClO4 Polished GC FeNH4(SO4)2

(2.5�0.3) E−03 [43] 0.2 M HClO4 Polished GC FeNH4(SO4)2

[38] 0.5 M HClO46.00E−05 Au 25 °C, 100 atm4.00E−03 0.5 M HClO4 Au 125 °C, 100 atm4.00E−01 0.5 M HClO4 Au 225 °C, 100 atm

0.5 M HClO42.00E+00 Au 300 °C, 100 atm7.80E−03 [8] 0.1 M HClO4 Au Traces of Cl−

0.1 M HClO44.80E−03 Pt Traces of Cl−

9.00E−06 [31] 0.5 M HClO4 Pt0.5 M HClO4 Au5.00E−050.5 M HClO41.30E−04 Pt 30 �M H2SO4

5.60E−04 0.5 M HClO4 Pt 100 �M H2SO4

3.00E−03 0.5 M H2SO4 Pt0.5 M H2SO41.00E−02 Au

9.00E−03 [25] 1 M HClO4 Pt7.00E−03 0.5 M H2SO4 Pt

0.01 M H2SO45.00E−03 Pt 0.5 M K2SO4

0.5 M H2SO4 Au1.00E−02

This work673 1 M HClO41.06E−05 BDD Low-field1.26E−05 1 M HClO4 BDD FRA

1 M HClO4716 BDD ox6.78E−06 Low-field1 M HClO4 BDD ox1.19E−05 FRA

recorded at the oxidized BDD electrode, with different[Fe(CN)6]3−/4− concentrations and at room temperature.

Since the electrochemical behavior of the ferri/ferro–cyanide redox couple is influenced by a number ofparameters, like the supporting electrolyte concentra-tion [26,27,40] and the specific nature of ions present insolution [27,28,32], both complexation equilibria andthe inner-sphere character of the electron transfershould be taken into consideration. The literature on theargument is prone to consider effects of ions adsorptionat the electrode surface, as well as mechanisms in whichthe cations (potassium in particular [30]) play a role inthe electron transfer mechanism itself. In our case, suchhypotheses cannot have a significant application: noadsorption has been reported yet on diamond elec-trodes, neither K+ might play a role, the solution beingprepared in 0.5 M sulfuric acid. Conversely, the redoxcouple behavior is pH-dependent, the standard reduc-tion potential being equal to +0.69 V in 1 M H2SO4

and to +0.46 V in 0.01 M NaOH: this evidencesuggests that the electrochemical properties of the com-plex can be modified by interaction through the cyanidegroups. As discussed for the iron(III)/iron(II) redoxsystem, further indications have been obtained carryingout measurements at different temperature; kinetic dataare reported in Fig. 8, while the elaboration in terms of‘Arrhenius plot’ is shown in Fig. 9.

An activation enthalpy (�H°�) of 1.28�0.14 kcalmol−1, in the case of the as prepared BDD electrode,and of 3.99�0.18 kcal mol−1 for the oxidized one, havebeen found, respectively. Results agree well with thefaster kinetics of this redox couple, a larger exchangecurrent density being recorded where the activationenthalpy was lower. Moreover, it seems that the en-tropic contribution does not have a role particularlyimportant; for the intercepts of lines in Fig. 9, we have:1.147�0.103 (as prepared BDD) and 0.516�0.133 (ox-idized BDD). Remembering that �S°� and the in-


tercept have opposite signs, a smaller decrease of acti-vation entropy is associated with the reaction at theoxidized electrode.

The literature on the subject shows a number ofchemical and electrochemical investigations, at differentsolution composition. As above discussed, an importantrole is played by the nature of the cation; the rate of theelectrode reaction increases in the order Li+�Na+�

Fig. 8. Apparent electron-transfer rate constant for the ferri/ferro–cy-anide redox couple.

Fig. 6. CV curves for ferri/ferro–cyanide (1, 2.5, 5, 7.5 and 10 mM)in 0.5 M H2SO4 at as prepared and oxidized BDD electrodes. Scanrate: 0.1 V s−1. Room temperature.

Fig. 9. ‘Arrhenius plot’ for the ferri/ferro–cyanide redox couple.

K+�Rb+�Cs+ and Mg2+ �Sr2+ �Ba2+. As far asthe role of anions is concerned, the rate constant in-creases in the series F−�CNS−�SO4

2− �CH3COO−

�ClO4−�PO4

3− �NO3−�Cl−�Br−; however, the

former effect has been studied at a gold electrode (atwhich the reaction did not depend much on the anionused [27,28]), while the latter has been investigated atplatinum (using sodium salts [32]). Both cases should belargely different from diamond and, especially, from asprepared diamond. Focusing on diamond electrodes,unfortunately almost the totality of available data havebeen obtained in neutral solution of KCl [10–13,17,19,22] or Na2SO4 [21,23], the pH of the solutionbeing possibly important in determining the reactionrate. The paper by Swain and Ramesham [10] is theonly one that report measurements in acidic media (0.1N HClO4), but in that case the film resistivity wasabout 10 � cm and the kinetics of the ferri/ferro–cya-nide system was obviously influenced (�Ep was 104 mV,at 0.1 V s−1, which should be compared with a value of66 mV recorded by us, in conditions differing only inthe acid concentration).

Fig. 7. Impedance spectra of ferri/ferro–cyanide at the oxidized BDDelectrode. Room temperature.


3.3. Kinetic considerations

The results of the experimental investigation allowedthe definition of quantitative parameters of the kineticsof Fe3+/Fe2+ and [Fe(CN)6]3−/[Fe(CN)6]4− at con-ductive diamond electrodes. The above-exposed datacan be usefully collected as shown in Table 2, where acomparison has been tentatively done also between thetwo couples, and not only inside a single set of data.Different model approaches could be followed for try-ing a theoretical interpretation [52]; unfortunately, thelimited knowledge on the electrode material does notallow a discussion in terms, for example, of quantummechanics (the Gurney’s model [53] requires the densityof states and the Fermi distribution of electrons in theelectrode to be known). On the other hand, the tradi-tional continuum dielectric theory, historically due toBorn [54] and successively modified by Marcus [55], canbe used only for outer-sphere reactions, i.e. for reactingcomplexes that retain their own full coordination shellduring the electron transfer process. In inner-spherereactions, on the contrary, two complexes form anintermediate in which at least one ligand is sharedduring the electron transfer process.

A tentative, unitary approach to consider both theredox couples investigated in this work, may be basedon the molecular orbital/ligand field theory. Accord-ingly, differences should derive from the change ofligands around the central metal ion, the ligands beingH2O and CN−. The water molecule can interact withthe central iron ion, acting as a simple �-donor ligand;as a result, both hydrated (hexaaquo) Fe3+ and Fe2+

are high spin complexes and the electronic configura-tion of the metal can be written as t2g

4 eg2 or t2g

3 eg2,

respectively. Considering the ferri/ferro–cyanide redoxcouple, the cyanide ion is a �-donor, �-acceptor ligand;due to such interaction, [Fe(CN)6]3− and [Fe(CN)6]4−

are low spin complexes, the electronic configuration ofthe metal being t2g

6 and t2g5 , respectively. Interestingly,

the ligand �* orbitals can interact (overlap) also withthe electrode surface, acting as a molecular wire for theelectron transfer. This could be the reason for thedifferent mechanism of the two redox couples investi-gated, as well as for the ferri/ferro–cyanide faster kinet-ics. Moreover, this simple approach can give some

indications also on the structural differences betweenthe aquated FeIII and FeII ions, and on the lack ofstructural differences between ferri- and ferro–cyanideanions. In fact, a symmetrical decrease [56] in theFe�OH2 bond distance is observed after oxidation(from 2.10 to 1.97 A� ), while C�N bond distances arethe same [57], within experimental error, for the twooxidation states: 1.138(19) A� for [Fe(CN)6]4− and1.148(5) A� for [Fe(CN)6]3−.

The above-discussed model allows a qualitative inter-pretation of the differences between the kinetic parame-ters of the two redox couples; however, focusing oneach system, a quantitative approach would be prefer-able, especially for rationalizing the different behaviorof the same redox couple at the two electrode surfaces.Speaking again in terms of molecular orbital, and con-sidering that a higher interaction arises from similaratomic orbitals, it seems obvious that a less efficientoverlap can be obtained between an oxidized surfaceinstead of a hydrogenated one (the difference comes upas a consequence of electro-negativity). Thus, a slowerreaction rate would be expected for the electron trans-fer at the oxidized electrode surface.

A detailed analysis of the different parameters inTable 2 is not possible on the basis of the molecularorbital model. While the explanation of the thermody-namics for the ferri/ferro–cyanide couple requires con-siderations chiefly based on chemical flair, inconsidering the iron(III)/iron(II) redox couple it is pos-sible to apply the above-mentioned theories of Born[54] or, better, of Marcus [55]; the approach can beeven simpler, following the indications given by Hush[58]. For the activated state of the electron transferprocess, we can write:

�G*=(�ze0)2

8ai

�1−

1�w

�, �S*= −

(�ze0)2

8ai

1�w

2

��w

�T

and

�H*=(�ze0)2

8ai

�1−

1�w

−T�w

2

��w

�T�

where ai is the solvated ion radius, �w is the dielectricconstant of the solvent (water), and �z is the ion chargechange due to electron transfer.

Table 2Schematic summary of data for both the investigated redox systems; data obtained at as prepared and oxidized BDD electrodes

[Fe(CN)6]4− � [Fe(CN)6]3−+e[Fe(H2O)6]2+ � [Fe(H2O)6]3++e

as prepared BDD oxidized BDDoxidized BDDas prepared BDD

+++++++ +�H°� ++−�S°� −−−−−−−−−

�G°� �CN−+�S+++ � �S++++ � �CN−++


Considering that the solvent in the vicinity of acharged ion exists as a first organized layer [59], dielec-trically saturated [60], at variance with the Born theory,�w is not the bulk dielectric constant value. If �s1 and �s2

are the dielectric constant values of the hydration shellof the ion, in proximity of the hydrogenated and theoxidized electrode surface, respectively (with �s1��s2, asa consequence of the disordering interactions due to theelectrode surface), it follows that the required trends ofthe free energy of activation and of the activationentropy are obtained, provided a positive value is cho-sen for the derivative ��w/�T, with ��s1/�T��s2/�T.This is not a strange requirement, on the basis of theorienting interactions exerted by the charged ion andthe hydrophilic electrode surface (oxidized electrode),and on the disorienting action due to temperatureincrease.

Although strictly not applicable, the Hush simplifiedapproach seems to work well also in the case of theferri/ferro–cyanide redox couple; in fact, it is possibleagain to define a dielectric constant for the ‘hydrationshell’ of the central iron ion, the shell being constitutedof cyanide anions. The different behavior of this redoxcouple could be rationalized if an opposite dependenceof the derivative ��w/�T, with respect to the interactionswith the electrode surfaces, could be accepted (i.e.��CN,1/�T��CN,2/�T). However, this is again an ad-missible requirement, in view of the different nature ofthe ligand and considering that adsorption should takeplace at the electrode.

4. Conclusions

The electrochemical properties of diamond electrodesseem to be very dependent on the state of the surface(hydrogenated�hydrophobic or oxidized/oxy-genated�hydrophilic), but also the way the modifica-tion is obtained could have an important role. Asdiscussed by Fujishima et al., oxygen plasma treatmentinvolves the formation of oxygen-containing functional-ities, possibly identifiable as carbonyl groups [21]; onthe other hand, hydroxyl groups have been suggested asresulting from strong electrochemical oxidation [23].Also, a discriminating functionalization seems to de-pend on crystallographic faces [23], this aspect beingspecifically related to sample preparation.

Noteworthy is the electrochemical behavior of asprepared BDD electrodes with respect to well-knownredox couples: kinetic responses are comparable withthose recorded at (oxide-free) noble-metal electrodes,provided care is taken in solution preparation. Thisresult is probably the more intriguing, in considerationof the well-accepted relation between exchange currentdensities, density of states and Fermi distribution ofelectrons, in the case of metal electrodes. Accordingly,

it is difficult to explain how an even highly dopedsemi-conducting material (boron concentration �1020

atoms cm−3) can be compared with a metal (�1023

electrons in the conducting band), without showingotherwise obvious limitations. The detailed investiga-tion of different redox couples, having standard redoxpotentials spread all over the wide window of polariz-ability of BDD, is probably the only way for theunderstanding of the electrochemical and physicalproperties of this material, and will be the topic offuture works in our laboratory.

As far as the application of model approach toelectron transfer is concerned, the unique properties ofdiamond (e.g. the lack of any adsorptive interactionwith species in solution) has suggested the extension oftheories, referring to outer-sphere kinetics, to the caseof the ferri/ferro–cyanide redox couple, thus allowingthe discussion of a typical inner-sphere reaction.

Acknowledgements

The authors wish to express appreciation to Dr P.Barricelli for his help in experimental setup and discus-sion on part of the results. The authors are also gratefulto Professor B.E. Conway and to Professor C.A. Big-nozzi for revising the manuscript and for their advices.

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Electron transfer reactions at conductive diamond electrodes

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