Electrocatalytic activity of nitroxyl mixed self-assembled monolayers: combined effects of the nanoscale organization and the composition

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Electrocatalytic activity of nitroxyl mixed self-assembled monolayers:

combined effects of the nanoscale organization and the composition

Olivier Al ev^ eque,* Tony Breton* and Eric Levillain

Received 20th December 2011, Accepted 20th January 2012 DOI: 10.1039/c2sm07423k

The aim of this article is to demonstrate that the composition and distribution of electroactive species immobilized on a gold surface can have a significant influence on the reactivity of modified surfaces. It will be shown that on mixed SAMs, where the electroactive species (TEMPO) are diluted with alkanethiols of different lengths, the contribution of the surface distribution on the electrocatalytic activity is as important as the composition. Without any information on the distribution of species within the monolayer, the interpretation of results cannot be reliable.

Introduction

Since the pioneering work¹by Nuzzo and Allara in 1983, self- assembled monolayers (SAMs) of alkanethiols on solid surfaces have gained much attention in interfacial electrochemistry.

During the last decade, increasing attention has been dedicated to the design and elaboration of redox-responsive SAMs for various applications like catalysis,²recognition,^3,4organic elec- tronic^5,6 and chemical analysis.⁷ These works have resulted in some elegant examples that incorporate sophisticated receptors on the electrode surfaces. A SAM is a molecular building spon- taneously formed between molecules and a substrate having a common affinity, to form a single layer with a high degree of organization.⁸ The molecules involved are composed of three different parts: the head group bonded to the solid surface, the spacer group which confers stability and organization due to the establishment of intermolecular interactions, and the functional group which modulate chemical properties of the initial substrate. When SAMs with specific functional groups are immobilized to tune surface properties, mixed SAMs involving a passive matrix (alkanethiols) can be used to lower the density of these active elements and maintain a high molecular order. Such a strategy allows overcoming spatial limitations, especially for interfacial reactions and can improve the transposition of the property of an active element from the solution to the surface- confined state.^9,10

The reactivity of a surface cannot be only linked to the surface coverage and a key factor in most applications is the control of surface distribution. The determination of structure/property relationships followed by structure/reactivity relationships thus becomes a rule to follow. Unfortunately, as mentioned by Whitesides et al.,⁸ ‘‘experimental data are missing to establish

detailed structure-property relationships for interfacial reactions on SAM, especially in the case of mixed SAMs’’.

Based on this citation, we recently reported the possibility of easily discriminating phase-segregated and homogeneously nitroxyl mixed SAMs using macroscopic characterizations by cyclic voltammetry,¹¹i.e.without using expensive and complex techniques.^12–14The determination of the surface distribution is based on the use of the generalized lateral interactions model,¹⁵ which can link the non-idealities of the obtained experimental voltammograms (apparent potential (Eapp), full width at half maximum (FWHM) and peak intensity (ip)) and the amount of lateral surface interactions between redox entities. This work also confirms the XPS results of Shaporenko et al.¹⁶ and Watch- arinyanon et al.¹² dealing with the role of the elaboration protocol on the organization of the mixed SAMs when the two binaries used have nearly the same length,i.e.coadsorption leads to phase segregation whereas successive adsorptions lead to randomly distributed species. We have also highlighted a difference of electrocatalytic activity between nitroxyl mixed SAMs presenting randomly and non-randomly distributed electroactive sites,¹⁷by showing that electrocatalytical oxidation of phenethyl alcohol is not only due to the amount of electroactive centers immobilized on the Au surface, but also to their distribution.

Many other works showed that several factors contribute to different distributions,e.g.surface topography of the substrate or temperature.^18,19Another factor is inherent to the binary used.

Several studies concur to show that the greater the difference in chain length, the more favoured the phase segregation.^18,20When the difference in the length of alkyl chain is greater than four methylene groups, phase separation is generally observed. This last factor is interesting because it permits us to modulate, a priori, the composition and the surface organization of SAMs at the same time.

In this current paper, we prepared electroactive mixed SAMs from a nitroxyl derivative and different alkanethiols. Electro- chemical and electrocatalytical behaviours of SAMs will be Laboratoire MOLTECH-Anjou, Universite d’Angers—CNRS, UMR

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E-mail: olivier.aleveque@univ-angers.fr; tony.breton@univ-angers.fr;

Fax: +33241735405; Tel: +33241735090

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compared for various backbones of the alkanethiols used (C6H13SH, C8H17SH, C10H21SH and C12H25SH) and for the two elaboration protocols (successive adsorptions and co-adsorption). Interpretation of the differences will be discussed in terms of composition and organization of the mixed self-assembled monolayers.

Experimental

Materials

All self-assembled mixed monolayers used in this study were formed from two constituents.

The electroactive one (C15T) was composed of a 2,2,6,6-tet- ramethylpiperidine-1-oxyl (TEMPO) unit linked to a 15-carbon alkyl tail. Synthesis and characterisations of C15T (Scheme 1) were described in ref. 21.

The second one was a commercial alkanethiol (CxH2x+1SH—

abbreviated CxSH) (Sigma-Aldrich) that acts like a passive matrix to prevent reactions directly on the gold substrate. Four alkanethiols were used alternatively: 1-hexanethiol (C₆H₁₃SH), 1-octanethiol (C₈H₁₇SH), 1-decanethiol (C₁₀H₂₁SH) and 1-dodecanethiol (C12H25SH). Alkyl chain lengths were chosen to modulate the nanoscale organization of SAMs and allow the positioning of electroactive sites (TEMPO) on the surface of the SAM.

Interfacial reactivity of these mixed SAMs was evaluated through the electrocatalytical oxidation ofsec-phenethyl alcohol (Sigma-Aldrich) by the oxidized form of the TEMPO radical.

Gold substrates elaboration

Au substrates were prepared by deposition ofca.5 nm of chro- mium followed by ca. 100 nm of gold onto a glass substrate through a shadow mask (MECACHIMIQUE/France) using a physical vapor deposition system (PVD ME300 PLASSYS/

France) and were made immediately before use. This protocol, commonly used in the literature,^22,23 provides reproducible Au(111) surfaces with high crystallographic quality, low roughness (R_aless than 2 nm) and with a defined geometry. They do not undergo post-treatment after completion and that is an advantage.

Mixed SAMs elaboration

Mixed SAMs were prepared on fresh Au substrates according to two protocols:

Protocol A: successive adsorptions.Successive adsorptions of C15T and alkanethiols were performed by immersing the Au

substrate for 15 min in a millimolar solution of C15T in methylene chloride and then in a millimolar solution of alkanethiols in methylene chloride. In order to obtain the expected C15T surface coverages, the time of immersion in alkanethiol solutions was adjusted according to the chosen alkanethiol.

Protocol B: co-adsorption.The co-adsorption was carried out by immersing the Au substrate for 15 min in a millimolar solution of C15T/C_xSH in methylene chloride. In order to obtain the expected C15T surface coverages, the proportions of the two components in solution were adjusted according to the chosen alkanethiol.

Electrochemical experiments

Electrochemical experiments were carried out with a Biologic SP-300 potentiostat in a glove box containing dry, oxygen-free (<1 ppm) argon. Cyclic voltammetry (CV) was performed in a three-electrode cell controlled at a temperature of 293 K. Working electrodes were functionalized Au substrates. Counter electrodes were platinum plates. Reference electrodes were Ag/AgNO₃(0.01 M CH₃CN). CVs were recorded in dry HPLC- grade methylene chloride (CH2Cl2) and the supporting electrolyte was tetrabutylammonium hexafluorophosphate (Bu4NPF6).

Based on repetitive measurements, absolute errors on poten- tials were found to be approximately 3 mV.

The normalized surface coverageqis defined by the ratio of the experimental surface coverage (G) to the maximum surface coverage (Gmax) according toq¼G/Gmax. Experimental surface coverages (G) were deduced by integration of the voltammetric signals of SAMs. At 293 K, the maximum surface coverage (Gmax) was estimated²⁴to be 5.00.1 10¹⁰mol cm²in CH₂Cl₂.

Electrocatalytical experiments

Electrocatalytical experiments were carried out by adding 40 mmol L¹ofsec-phenethyl alcohol (Sigma-Aldrich) and 80 mmol L¹of 2,6-lutidine (base necessary for the TEMPO regeneration) (Sigma-Aldrich) to the electrochemical media. No stirring was performed.

Experimental difficulties and precautions taken

Drastic experimental conditions were required to obtain accurate data. First, the PVD procedure to elaborate Au substrates must be reproducible in roughness and in area. Note that all attempts to obtain rational results with Au electrodes extensively polished to a smooth, mirror-like finish failed. Second, the elaboration and electrochemical characterizations of SAMs were performed under thermostatted conditions. Third, TEMPO derivatives were pure with only trace amounts (<5%) of disulfides in order not to impede the formation or alter the structure of the SAM.²⁵Note that all the compounds were purified using column chromatog- raphy to remove impurities and trace of disulfure. And then, the time of electrochemical experiments does not exceed 1 h, in order to avoid desorption of immobilized species (1% of desorption in 1 h²⁴). Without this drastic protocol, CVs of SAMs are not reproducible in charge and in shape.

Scheme 1 Derivative of TEMPO radical: C15T.

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Results and discussion

Electrochemical characterisations and nanoscale organisations In CH₂Cl₂, cyclic voltammograms of the mixed SAMs prepared exhibit a reversible one-electron process close to 0.49 V, according to the reaction:

C15T#C15T⁴+ e (R1)

The shape of voltammetric waves and the linear dependency between peak intensities and scan rates are characteristic of the surface-confined redox species.²⁶

For each mixed SAM, CV parameters (E_p,i_p, FWHM) are extracted from experimental data. For more clarity and simplicity, interpretations of the results are only based on the full width at half maximum (FWHM) parameter, i.e. the most

precise parameter and less sensitive to experimental errors. Fig. 1 (left part—1, 3) shows the evolution of the FWHMs recorded for the different mixed SAMs prepared using protocols A (top) and B (bottom). All full widths at half maximum are lower than the expected value (89/nmV at 293 K) of an ideal Nernstian system based on a Langmuir isotherm (i.e. all adsorption sites are equivalent and there are no interactions between immobilized electroactive centers). At high TEMPO surface coverage, the FWHM values are clearly lower (35 mV) than the theoretical expected one, this value is almost reached when the electroactive moieties are highly diluted in the passive matrix.

These unusual characteristics have been already presented and discussed in our previous studies.^11,15,21 According to these works, we assumed that the FWHM evolutions can be explained by the use of the generalized lateral interactions model (i.e.all adsorption sites are equivalent and lateral interactions exist

Fig. 1 Experimental anodic full widths at half maximum (FWHM) of mixed SAMs elaborated with (1) protocol A and (3) protocol B, extracted from CVs obtained in 0.1 MnBu4NPF6/CH2Cl2,v¼50 mV s¹, as a function of the surface coverage of C15T. Segregation factor (f) deduced (eqn (2)) from experimental FWHMs of mixed SAMs elaborated with (2) protocol A and (4) protocol B as a function of the surface coverage of C15T. Four diluents were used for these mixed SAMs: (:) 1-dodecanethiol (C12SH), (-) 1-decanethiol (C10SH), (C) 1-octanethiol (C8SH), and (;) 1-hexanethiol (C6SH).

The dashed lines are just trend curves. Errors: Standard Deviation (FWHM)¼5 mV and Relative Standard Deviation (q)¼3%.

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between immobilized electroactive centers during an electrochemical process), which can be formalized by this formula:

FWHMðqÞ ¼RT nF

"

2 ln

2 ffiffiffi p2

þ3

3 ffiffiffi

p2 2 GfðqÞ

# (1) where G is an interaction constant between immobilized electroactive molecules, and f(q) a dimensionless quantity named

‘‘segregation factor’’, representative of the average number of lateral interactions per electroactive site, and thus representative of the spatial distribution (2D). Iff(q)¼ q, this model can be reduced to the Laviron’s model,²⁷ providing evidence of a random distribution of electroactive centers on the surface under the assumption of the model, whereas, iff(q) >q, phase segregation occurs. In order to directly interpret the results in term of spatial distribution, it is possible to normalize the FWHM (FWHM^Norm), and thus determinef(q). This is formalized by the following equation:¹¹

FWHM^Norm¼fðqÞ ¼ FWHMðqÞ FWHMðq¼0Þ FWHMðq¼1Þ FWHMðq¼0Þ (2) with FWHM(q¼1)¼35 mV, and FWHM(q¼0)¼89 mV at 293 K.

Fig. 1 (right part—2, 4) shows the evolution of the calculated segregation factors for the mixed SAMs obtained with the two protocols. According to the protocol used, f(q)vs.normalized surface coverage (q) clearly differ for each binary used, as previously seen in the same way with FWHMs.

When protocol B (co-adsorption) is used to modulate the surface coverage, the trends off(q) for the four mixed SAMs are very close independent of the chain length of the diluent. The non-linearity indicates that phase segregation occurs, and in the same order of magnitude regardless of the alkanethiol used. We can assume that:

fCA,C₁₂SH¼fCA,C₁₀SH¼fCA,C₈SH¼fCA,C₆SH¼f^segregated (3) When protocol A (successive adsorptions) is used, the trends off(q) for the four mixed SAMs clearly differ and are linked to the chain length of the diluent. The linear dependence between f(q) and the normalized surface coverage is consistent with a random replacement of the immobilized C15T centers by the dodecanethiols (C₁₂SH), involving a random distribution of the two components in the mixed SAM.²¹Decreasing the length of the alkanethiols,f(q) increases gradually to reach the level of segregation obtained with the protocol B. We can assume that:

f^random¼fCA,C₁₂SH<fCA,C₁₀SH<fCA,C₈SH

<fCA,C₆SH¼f^segregated (4)

These results corroborate recent XPS and NEXAFS measurements showing that elaboration protocols and binaries used can influence the molecular organization of mixed SAMs.^12,16

Interfacial reactivity/catalytic effects

TEMPO and its derivatives have been extensively studied as a redox mediator in the field of organic synthesis, and mostly, for the oxidation of primary and secondary alcohols.^28,29 In this

context, the reactivity of the mixed SAMs prepared in this work were evaluated through the electrocatalytic oxidation of phenethyl alcohol. The process can be described by these three successive reactions:

C15T#C15T⁺+ e (R2-1)

ROH + C15T⁴#RO + C15TÔH (R2-2) C15TÔH+ C15T⁴#2 C15T (R2-3) Our preliminary study¹⁷ on mixed SAMs of C15T/C12SH showed, for rapid electron transfer (ks> 20 s¹) (R2-1), that the catalytic process was rate limited by two processes in competi- tion: (R2-2) the direct oxidation of the alcohol (ROH) by the oxoammonium cation (C15T⁺), giving a ketone (RO) and a hydroxylamine (C15TÔH) and (R2-3) the regeneration of the TEMPO by the comproportionation reaction of the hydroxylamine produced with an oxoammonium cation. The process (R2-2) is favoured for a random distribution because the steric hindrance, induced by the proximity of redox centers, is less important in this case and allows a better approach of alcohol, whereas the process (R2-3) is favoured for a segregated distribution, the regeneration of the TEMPO involving the direct proximity of an oxoammonium cation and a hydroxylamine.

Fig. 2 displays CVs of mixed C15T/C_xSH SAMs having normalized surface coverage near 0.2 in the presence of phenethyl alcohol. CVs are typical of a catalytic reaction occurring on an electroactive SAM.^30,31A coverage of 0.2 has been chosen to highlight the difference in the electrocatalytic behaviour between the different SAMs because it leads to the greatest gap betweenf^segregatedandf^random(see Fig. 1).

When protocol B (co-adsorption) is used, the catalytic current is initiated faster as the alkane chains are short, indicating a faster regeneration of the TEMPO on the surface. A gap of 50 mV is observed between the currents generated by the C15T/C₆SH and C15T/C12SH mixed monolayers. All these monolayers present the same degree of segregation (eqn (3)), so this phenomenon can be explained by a progressive increase in the number of degrees of freedom of TEMPO terminal functions when the alkanethiol chain length of the passive matrix is decreased: two non-adjacent TEMPO are therefore able to approach each other and make the regeneration process even more favourable.

When protocol A (successive adsorptions) is used, and as previously seen,¹⁷the catalytic current is initiated much slower on monolayers presenting a random distribution than on phase segregated monolayers. As observed for protocol B, the catalytic current is initiated faster as the alkane chains are short, indicating a faster regeneration of the TEMPO units on the surface. This time, the potential shifts are more pronounced, with a gap of 110 mV between the currents generated by the C15T/C₆SH and C15T/C₁₂SH mixed monolayers. This result can be explained by the combination of the effect of the distribution (fincreases from f^randomtof^segregated when the chain length decreases, according to eqn (4)) and the effect of the alkanethiol chain length. An important and particular result is that the catalytic behavior for monolayers of C15T/C₆SH is protocol independent (same catalytic curves), pointing out the crucial role of the distribution, onefvalue producing only one catalytic response for a specific binary used.

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Conclusions

Results presented here demonstrate that the composition and distribution of electroactive species immobilized on the gold surface have a significant influence on the reactivity of modified surfaces.

This work shows that on mixed self-assembled monolayers, where the electroactive species (TEMPO) are diluted with alkanethiols of different lengths, the contribution of the surface

distribution on the electrocatalytic activity is as important as the composition.

In this particular context, the catalytic effect is enhanced by a better regeneration of electroactive sites (TEMPO) for mixed SAMs having segregated distributions (proximity of redox centers) and for mixed SAMs exhibiting a significant difference between chain length of electroactive species and alkanethiols (by an increase in the number of degrees of freedom of TEMPO terminal functions).

Knowledge of both distribution and composition of SAMs helps avoid jumping to conclusions. Indeed, we could interpret the reactivity changes by a difference in composition and not by a combined effect of the composition and distribution.

This highlights a rule for studies dealing with the interfacial reactivity of mixed monolayers: the study of the reactivity must be preceded by a determination of the organization.

Future work will be dedicated to the analysis of the overall reaction kinetics as well as its numerical modelling.

Acknowledgements

This work was supported by the Centre National de la Recherche Scientifique (CNRS/France) and the Universite d’Angers (Angers-France). The authors thank the Contrats de Projets Etat Region (CPER 2007–2013) and the Plateforme d’Ingenierie et d’Analyse Moleculaire (PIAM/Angers-France) for the acquisi- tion of the electrochemical instrumentation. The authors express their gratitude to Flavy Alev^eque for her critical reading of the manuscript.

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Fig. 2 CVs of mixed SAMs obtained in 0.1 MnBu4NPF6/CH2Cl2,v¼ 50 mV s¹, in the presence of 40 mmol L¹ofsec-phenethyl alcohol. These SAMs were elaborated with (top) protocol A and (bottom) protocol B and prepared from C15T and (:) 1-dodecanethiol (C12SH), (-) 1-decanethiol (C10SH), (C) 1-octanethiol (C8SH), and (;) 1-hexanethiol (C6SH). Insets: segregation factor (f) deduced (eqn (2)) from experimental FWHMs of mixed SAMs used before catalytic experiments. The arrows show the shift of the CVs when the chain length of the alkanethiols is decreased.

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Electrocatalytic activity of nitroxyl mixed self-assembled monolayers: combined effects of the nanoscale organization and the composition