Nitroxyl radical self-assembled monolayers on gold: Experimental data vs. Laviron’s interaction model

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Nitroxyl radical self-assembled monolayers on gold: Experimental data vs. Laviron’s interaction model

Olivier Alévêque, Pierre-Yves Blanchard, Tony Breton

, Marylène Dias, Christelle Gautier, Eric Levillain

, Fawzia Seladji

Laboratoire CIMA, Université d’Angers – CNRS, 2 Boulevard Lavoisier 49045, Angers cedex, France

a r t i c l e i n f o

Article history:

Received 24 June 2009 Accepted 7 July 2009 Available online 9 July 2009

Keywords:

Tempo

Self assembled monolayers Cyclic voltammetry Interaction model

a b s t r a c t

Mixed SAMs of nitroxyl radical derivative and alkanethiol have been studied by cyclic voltammetry in both aqueous and non-aqueous solvents. Cyclic voltammograms exhibit shapes as a function of surface coverage deviating from an ‘‘ideal system”. Confronted to Laviron’s interaction model, the agreement observed between theory and experiments provides evidence of a random distribution of electroactive centers on surface and indicates that the local ionic environment of charged redox centers plays a major role in the electrochemical behaviour of SAMs.

Ó2009 Elsevier B.V. All rights reserved.

1. Introduction

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and its deriva- tives have been extensively studied in the search for organic syn- thesis as a redox mediator, mostly for the oxidation of primary alcohols[1–4]. The electrochemical oxidation of TEMPO is known to be a stable and reversible one-electron process in both aqueous [5–7] and non-aqueous electrolytes [8,9]. Despite the wide electrochemical applications of nitroxyl radical, rare works have been devoted to design and elaborate redox-responsive TEMPO self-assembled monolayers (SAMs) [10–12]. In 1997, Fuchigami et al.[10], then Kashiwagi et al.[11]in 1999, have reported the first electrocatalysis attempts of alcohols and amines in acetonitrile. In 2008, Finklea et. al.[12]have worked on Au–S(CH2)10C(O)N(H)–

TEMPO SAM to the first estimation of the standard rate constant and the reorganization energies of TEMPO/TEMPO⁺in 1 M H2SO4. Recently, we have shown that nitroxyl radical SAMs afford a noteworthy electrocatalytic activity in both aqueous and organic media[13].

Here, we focused our attention on the shape of cyclic voltam- mograms (CVs) of mixed SAMs of nitroxyl radical derivative and alkanethiol in order to confront the experimental data with the Laviron’s interaction model.

2. Experimental

The synthesis and characterisations of nitroxyl radical deriva- tives1a,1band1c(Scheme 1) were described in Ref.[13].

Electrochemical experiments were carried out with a Biologic SP-150 potentiostat at 293 K. Cyclic voltammetry was performed in a three-electrode cell equipped with a platinum-plate counter electrode. Reference electrodes were Ag/AgNO3(0.01 M CH3CN) or Ag/AgCl/KClsat. CVs were recorded in dry HPLC-grade methylene chloride (CH2Cl2), HPLC-grade acetonitrile (CH3CN) or H2O (18 MX). Supporting electrolytes were tetrabutylammonium hexa- fluorophosphate (nBu4NPF6), sodium hexafluorophosphate (NaPF6), tetrabutylammonium perchlorate (nBu4NClO4) or sodium perchlo- rate (NaClO4).

The Au substrates were described in Ref.[14].

3. Results and discussion 3.1. Experimental data

In solution, cyclic voltammograms of 1a, 1b or 1c exhibit a reversible one-electron process close to 0.56 V in CH2Cl2 and 0.41 V in CH₃CN (vs. Ag/AgNO₃ in 0.1 M Bu₄NPF₆), respectively [13].

Electrochemical behaviour of mixed SAMs was studied in CH2Cl2, in CH3CN and H2O. In aqueous and non-aqueous media, the shape of voltammetric waves and the linear dependency be- tween peak intensities and scan rates were characteristic of surface-confined redox species [13]. CVs parameters are quasi 1388-2481/$ - see front matterÓ2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2009.07.015

* Corresponding authors. Address: Laboratoire CIMA, Université d’Angers – CNRS, UMR 6501 du CNRS, 2 Boulevard Lavoisier 49045, Angers cedex, France (E.

Levillain). Tel.: +33 241735095; fax: +33 241735405.

E-mail addresses: tony.breton@univ-angers.fr (T. Breton),eric.levillain@univ- angers.fr(E. Levillain).

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j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e l e c o m

chain-length (n) independent. Interestingly, shapes of voltammet- ric waves are solvent and supporting electrolyte dependent (Table 1). In CH2Cl2and CH3CN, the full width at half maximum (FWHM) deviate from the expected value (i.e.89 mV at 293 K) of an ‘‘ideal system”, corresponding e.g.to a Langmuir model.

The interfacial electron transfer process and the interactions be- tween the immobilized functional moieties have been studied

widely to clarify the ‘‘non-ideality” of experimental data[15–24]

but it still remains an open-ended question.

An approach to understand this striking behaviour is to dilute the electroactive species in the monolayer by mixing alkanethiols [25]. Indeed, CVs of mixed SAMs, prepared from 1a, 1b or 1c:alkanethiol mixtures, exhibit a noteworthy surface coverage dependence (Fig. 1).

3.2. Laviron’s interaction model

In experiments, the criteria for ideality are rarely met and inter- action models have to be developed in order to extract information about the adsorbed redox system of interest. In the last 30–

35 years, several models were developed [26–30]. Among these models, only two satisfy the diagnostic criteria[30]to predict cur- rent–voltage (i–E) wave shape in CV experiments on electroactive SAM: Laviron’s model and Smith and White’s approaches. In the Seventies, Laviron developed an interaction model, focused exclu- sively on non-idealities caused by lateral interactions[26]. This ap- proach, refined by Anson [27], relied on finding values of interaction coefficients which produced good fits with experimen- tal data. In 1992, the elegant electrostatic model of Smith and White [28], then refined by Yoneyama et al. [29], provided an analytical expression for the interfacial potential distribution.

However, because electrostatic approaches require intricate math- ematical treatments, we are focused on Laviron’s model.

According to the specific hypothesis developed by Laviron (Frumkin model),

The electroactive centers are random distributed on substrate, despite interactions,

aOO,aRRand aOR are the interaction constants between mole- cules of O, molecules of R and molecules of O and R, respectively (aiis positive for an attraction and negative for a repulsion; thea values are assumed to be independent of the potential), the i–E characteristic (IUPAC convention) can then be written [26a]:

iðtÞ ¼nFAksCmax½hOðtÞ

g

g

iðtÞ ¼ nFACmax dhRðtÞ dt

h i

¼nFACmax dhOðtÞ dt

h i

8>

><

>>

:

Table 1

CV parameters and interaction parameters of SAMs prepared from1c.

Media At 0.1 V s¹ CH2Cl2a

MeCN^a H2O^b 0.1 M nBu4NPF6 C(mol cm²) 4.710¹⁰ 5.310¹⁰ –

FWHM (mV) 36 113

Ep(V) 0.48^a 0.51^a

rOc +1.14 0.64

rRc

0 0

0.1 M nBu4NClO4 C(mol cm²) 4.710¹⁰ 5.010¹⁰ –

FWHM (mV) 32 71

Ep(V) 0.48 0.45

rOc +1.25 +0.30

rRc

0 0

0.1 M NaPF6 C(mol cm²) – 4.810¹⁰ 5.610¹⁰

FWHM (mV) 104 84

Ep(V) 0.45 0.69

rOc

0.27 +0.12

rRc

0 0

0.1 M NaClO4 C(mol cm²) – 5.710¹⁰ 5.010¹⁰

FWHM (mV) 71 94

Ep(V) 0.44^a 0.71^b

rOc

+0.34 0

rRc

0 0

avs. Ag/AgNO3(0.01 M).

bvs. Ag/AgCl/KClsat.

c vs. Standard deviation0.08.

Scheme 1.Nitroxyl radical derivatives1a,1band1c.

Fig. 1.(A) Experimental CVs of SAM in 0.1 M nBu4NPF6/CH2Cl2, prepared from different1c:decanethiol ratios, leading to 4.7, 3.7, 2.8, 2.1, 1.4 and 0.810¹⁰mol cm². (B) Experimental CVs of SAM in 0.1 M nBu4NPF6/CH3CN, prepared from different1c:decanethiol ratios, leading to 4.6, 3.3, 2.7, 2.3, 1.8 and 1.410¹⁰mol cm². (C) Experimental CVs of SAM in 0.1 M NaPF6/H2O, prepared from different1c:decanethiol ratios, leading to 5.3, 2.7, 2.2, 1.9, 1.3 and 1.010¹⁰mol cm². All CVs were performed at 0.1 V s¹.

with

n;F;A;ks;R;T;E0;t;hO;hRandhThave their usual meanings

g

and E⁰₀¼E0^RT_nFln ^b_b^O

R

8<

:

ð1Þ For a full reversible reaction (ks! þ1), two parameters,Gand S, play a primordial role.

G¼rRþrO

S¼rRrO

with rO¼ ðaOO aORÞ and rR¼ ðaRRaORÞ ð2Þ

rOandrRcan be defined as ‘‘interaction” parameters of O and R, respectively[27].

The parameterGdefines the shape of the peak (FWHM) and the peak intensity (ip) and the parameterS, the apparent formal peak

(Ep) as a function of hT. Final equations, derived from Laviron [26a]then Anson[27], are:

EpðhTÞ ¼E⁰_O=RþRT

nFShT ð3Þ

ipðhTÞ ¼n²F²vACmax

2RT

hT

ð2GhTÞ ð4Þ

FWHMðhTÞ ¼2R T

nF ln 1þ ffiffiffiffiffiffiffiffiffiffi

2GhT 4GhT

q 1 ffiffiffiffiffiffiffiffiffiffi

2GhT 4GhT

q 0 B@

1 CAG

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2GhT

4GhT

s

hT

2 64

3

75 ð5Þ

We pointed out that, for |GhT| < 1, FWHM varies quasi linearly withGhTand can be determined from Eq. (6) (via a Taylor serie atGhT= 0):

Fig. 2.i–Echaracteristics, calculated from Eqs. (1) and (2), as a function ofhT= {0.2, 0.4, 0.6, 0.8, 1.0} for given values of {rO,rR}. Others parameters:ks! þ1(full reversible reaction),v= 0.1 V s¹andT= 293 K.

FWHMðhTÞ R T

nF 2 ln 2 ffiffiffi p2

þ3

3 ffiffiffi p2 2 GhT

" #

ð6Þ

By mixing the electroactive species with alkanethiols, the parameterhTis modulated. In this outlook,i–Echaracteristics can be calculated, from Eqs. (1) and (2), as a function of {rO,rR} and h_T(Fig. 2) in order to display several abacus of CV shape. In order to exhibit a dimensionless current, i was normalized to ip ideal

(see Eq. (7)).

ip idealðhTÞ ¼n²F²vACmax

4RT hT ð7Þ

3.3. Experimental data vs. theory

To confront theory and experimental data,E_p, FWHM and ip parameters were extracted from experiment CVs for each electro- lyte medium. According to Eqs. (3), (4) and (6), linear regressions lead to estimateG andSparameters (Fig. 3) and then, calculate rOandrRinteraction parameters from Eq. (2) (Table 1).

Electrochemical behaviours of mixed SAMs, prepared from1a, 1bor1c:alkanethiol mixtures, fit with the interaction model devel- oped by Laviron. FromrOandrRparameters deduced from linear regressions, simulated CVs are in qualitative and quantitative agreement with CVs of mixed SAMs (Fig. 4). The excellent and rare [31]agreement observed between theory and experiments high- lights a random distribution of electroactive centers on gold.

These results show that the lateral interactions are governed by the interplay of coulombic repulsion, pairing ions and solvatation.

As suggested by Constentin and Saveant [32], the solvatation is essential to counteract coulombic repulsion because strong attrac- tive interactions between immobilized oxidized centers, leading to sharp shapes, are only observed in solvent with low dielectric con- stant such CH2Cl2. Conversely, the size of the counter anion (PF₆or ClO₄) plays a central role in attractive or repulsive interactions when the solvent has a high dielectric constant such CH3CN or H2O.

4. Conclusion

The non-ideality of CV shapes of nitroxyl radical mixed self- assembled monolayers on gold was modelized using lateral inter- Fig. 3.(A) Anodic apparent potential (Epa) as a function of the surface coverage. (B) Anodic peak intensity (ipa) as a function of the surface coverage. (C) Anodic full width at half maximum (FWHMa) as a function of the surface coverage. Mixed SAMs were prepared from different1c:decanethiol ratios in 0.1 M nBu4NPF6/CH2Cl2, leading to 4.7, 3.7, 2.8, 2.1, 1.4 and 0.810¹⁰mol cm².

Fig. 4. CVs calculated from Laviron model based on a Frumkin type isotherm. Only one variable changes at a time, the surface coverage. (A) Simulation of experimental CVs of SAM in 0.1 M nBu4NPF6/CH2Cl2(seeFig. 1A). Calculations were performed withrO= 1.14,rR= 0,ks= 90 s¹andh= {1.00, 0.79, 0.59, 0.44, 0.30, 0.17}. (B) Simulation of experimental CVs of SAM in 0.1 M nBu4NPF6/CH3CN (seeFig. 1B). Calculations were performed withrO=0.64,rR= 0,ks= 60 s¹andh= {1.00, 0.71, 0.58, 0.50, 0.39 and 0.29}.

(C) Simulation of experimental CVs of SAM in 0.1 M NaPF6/H2O (seeFig. 1C). Calculations were performed withrO= 0.12,rR= 0,ks= 60 s¹andh= {1.00, 0.50, 0.41, 0.33, 0.24, 0.19}.

actions between redox sites. Leading to excellent fits with experi- mental data, the Laviron’s model provides a simple way to estimate the interaction coefficients between redox centers and predict the randomness of the distribution of electroactive sites on SAMs. Re- cently, this latter property was illustrated by the catalytic activity of oxoammonium cation in aqueous and non-aqueous media[13].

Acknowledgments

This work was supported by the Centre National de la Recher- che Scientifique (CNRS - France), the ‘‘Agence Nationale de la Recherche” (ANR - France), and the ‘‘Région des Pays de la Loire”

(France).

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