DOI: 10.1002/cphc.201000929
Impact of the Nanoscale Organization of Nitroxyl Mixed Self-Assembled Monolayers on their Electrocatalytic Behaviour
Fawzia Seladji, Olivier AlvÞque, Christelle Gautier, Marylne Dias, Tony Breton,* and Eric Levillain*[a]
Self-assembled monolayers (SAMs) of thiolate derivatives have been widely studied and are known to form well-organized structures on various substrates.[1] Most of the work reported deal with the use of an alkane chain as spacer, linked to the desired functional moiety. The self-organization of those monolayers leads to packed structures and allows high surface coverage.[2] However, two main limitations arise owing to the architecture of SAMs. The first is the low structural quality ex- hibited by the monolayers when the structure-building forces are disturbed by the use of bulky active elements. In such cases, the strong interactions between the head groups result in poorly organized SAMs and possible ungrafted zones.[3]The second disadvantage of highly packed structure is the low per- formance of the functional groups because of their mutual in- teractions in confined spaces. Indeed, the transposition of the properties of an active moiety from the solution to the surface can only be successful if the available space around the immo- bilized active center is sufficient.[4] Mixed SAMs with a passive constituent are developed in order to overcome those limita- tions by lowering the density of functional groups on the sur- faces. This method allows, in some cases, to enhance the redox activity of a SAM.[5]Thus, knowing that the reactivity of a surface cannot be only linked to the surface coverage, we tried to establish a relationship between the nanoscale distri- bution of a redox-responsive SAM and its reactivity toward an interfacial reaction. More precisely, the aim of this work was to evidence the difference of electrocatalytic oxidation activity observed for nitroxyl mixed SAMs presenting randomly and nonrandomly distributed electroactive sites. The electroactive nitroxyl group was chosen because of its well-known ability to oxidize primary alcohol functions. Moreover, nitroxyl SAMs are electrochemically stable, presenting a fully reversible one-elec- tron oxidation in aqueous and usual nonaqueous solvents.[5]
Based on the elegant work of Shaporenko et al.[6] and Watcharinyanon et al.[7] dealing with the effect of the mixed SAM elaboration on the phase segregation, nitroxyl mixed SAMs were prepared using successive adsorptions (route1) and coadsorption (route2) in order to estimate the molecular
distribution by means of electrochemical parameters.[8] The binary was composed of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) units linked to a 15-carbon alkyl tail (C15T) and do- decanethiol (C12) as passive matrix. The alkyl chain lengths were chosen equal to 15- and 12-methylene groups for two reasons. First, it provides a high-quality packing as reported before.[2]Second, the adsorption/desorption equilibrium of the two constituents make the tuning of the TEMPO surface con- centration experimentally possible. For route1 the C15T sur- face coverage was varied by immersing a densely packed C15T SAM in a millimolar solution of C12 for different time periods.
For route2, the adsorption was carried out by immersing the gold substrate in a mixture of C15T/C12. The surface concen- tration was varied using different proportions of the two com- ponents.
To probe the intermolecular interactions on SAMs, we ex- tracted the full width at half maximum (FWHM) from experi- mental CVs of SAMs, prepared from routes1and2. As shown on Figure 1, FWHM vs normalized surface coverage (q) are de-
pendent of elaboration protocol, leading to a maximum FWHM gap between the two ways for approximatively 1 10 10mol cm 2. (q~20 %).
The linear dependence between FWHM and surface cover- age is consistent with a stochastic replacement of the C15T moieties by dodecanetthiol, involving a random distribution of the two components of the mixed SAM.[8]The nonlinearity of FWHM vs qagrees with a phase segregation of C15T and C12 Figure 1.FWHM vs normalized surface coverage (q)[8]for C15T/C12 mixed assembled monolayers. SAMs were elaborated following route 1 (~) and ela- borated following route 2 (!).
[a]F. Seladji,+O. AlvÞque, Dr. C. Gautier, Dr. M. Dias, Dr. T. Breton, Dr. E. Levillain
Laboratoire MOLTECH-Anjou, Universit d’Angers-CNRS UMR 6200 du CNRS, 2 Boulevard Lavoisier 49045 Angers Cedex (France)
Fax: (+33) 241 735 405
E-mail: [email protected] [email protected] [+] Current address:
Laboratoire de chimie et d’lectrochimie des complexes organomtalliques Universit des Sciences et de la Technologie Mohamed Boudiaf
BP 1505 - EL’M’naouer ORAN (Algrie)
ChemPhysChem2011, 12, 769 – 771 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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on Au surface, due to strong interaction existence between redox centers.[9] These results provide clear evidence of the elaboration influence of mixed SAMs on their molecular organ- ization and corroborate recent XPS and NEXAFS measurements showing variations on the chemical environment depending of the preparation protocol.[6, 7]
To evaluate the dynamic of the electron transfer in mixed SAMs, alternative current voltammetry (ac voltammetry) meas- urements[10] were performed on SAMs prepared from routes1 and 2. For instance, the electron-transfer rate constant (ks) at 1 10 10mol cm 2 (q=20 %) were estimated[11] as 50.5 s 1 and 100.8 s 1 for routes1and2 respectively (Figure 2). The
random replacement of C15T by C12 units leads to a weak de- crease of the electron transfer rate, compared to a coadsorp- tion of C15T and C12. It is also striking that theksvalues from mixed SAMs are lower than that of C15T unmixed SAM (ks= 201.5 s 1). The lowering of the electron-transfer rate con- stant has already been observed by Chidsey et al. on ferroce- nylalkanethiolate SAMs using the successive adsorptions method.[12]
Interfacial reactivity of mixed SAMs was evaluated through the electrocatalytical oxidation of phenethyl alcohol, which is known to undergo efficient oxidation in the presence of TEMPO+.[13] Figure 3 displays CVs of mixed C15T/C12 SAMs (from routes 1 and 2) having a surface coverage of 1 10 10mol cm 2in the presence of phenethyl alcohol.
Mixed SAMs prepared by coadsorption (route2) exhibit a well-defined electrocatalytic response, characterized by an oxi- dation peak followed by a current plateau. During the reverse
scan, a reduction peak is observed. The intensity of this peak was found to be proportional to the C15T surface coverage (not shown). At the opposite, the shape of CVs of mixed SAM prepared from route 1 (randomly mixed) is strongly different.
The oxidation process slowly reaches a current plateau, 1.3 fold higher, and does not show a reduction peak during the re- verse scan.
In order to estimate the catalytic component, it is necessary to subtract the electrochemical response of mixed SAMs re- corded in the absence of alcohol from the total response ob- tained in its presence (Figure 3bottom).[14]Two limit areas can be considered on the subtraction curves: a first one, below 0.45 V vs Ag/AgNO3, and a second one, up to 0.55 V vs Ag/
AgNO3. The comparison of the catalytic components for routes 1 and 2 highlights a strong catalytic activity on segre- gated mixed SAM in the first area, whereas this activity is en- hanced for randomly mixed SAM in the second area.
Considering all mass transport equal for our experimental conditions,[14]we can try to explain the difference of electroca- talytic behaviour on mixed nitroxyl SAMs. In the first area, the strong catalytic activity in phase segregation compared to random distribution involves that the proximity of redox cen- ters plays a decisive role. We suggest that this proximity allows the coproportionation reaction of hydroxylamine with oxoam- monium cation, which is a key step for the regeneration of the mediator.[15] For randomly mixed SAMs, the regeneration can only be possible at higher potential (i.e. for higher surface con- centrations of oxoammonium cations).
For a given surface coverage in the pseudo-diffusion step, the rate of oxidation of phenethyl alcohol by TEMPO+ limits the catalytic process.[14] The weaker catalytic activity observed for the phase segregation in this second area suggests that Figure 2.Representative ACV data plots for C15T (~) and C15T/C12 mixed
SAMs obtained from route 1 (&) and 2 (*). Straight line represent fitting of the three experiments using equivalent circuit models for electroactive monolayers. The standard electron transfer rates (ks) were estimated from the equivalent circuit model introduced by Creager et al.[10]
Figure 3.Top: Cyclic voltammogram of C15T/C12 mixed SAMs at
G=1 1010mol cm2obtained via routes 1 and 2 in 0.1mnBu4NPF6/CH2Cl2
under twelve repetitive cycles withoutsec-phenethyl alcohol (a) and with 40 mmolL1ofsec-phenethyl alcohol (c). The concentration of 2,6-luti- dine is 80 mmolL1in both cases. Bottom : Calculated CVs obtained by sub- tracting first vertex of experimental CVs in the absence of alcohol from first vertex of experimental CVs in its presence, for routes 1 and 2.
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www.chemphyschem.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem2011, 12, 769 – 771the steric hindrance, induced by the proximity of redox cen- ters, slows down the kinetic of this chemical oxidation.
Between the two limit areas, the comproportionation reac- tion and the mediator accessibility are in competition, leading to a progressive inversion of the rate determining process.
In summary, this work confirms that the elaboration protocol of mixed SAMs can drive the segregation rate of the SAM’s components and demonstrates that electrocatalytic activity of mixed nitroxyl SAMs is not only due to the amount of electro- active centers immobilized on Au surface, but also to their dis- tribution.
Experimental Section
Electrochemical experiments were carried out with a Biologic SP- 150 potentiostat, at 293 K. Cyclic voltammetry (CV) was performed in a three-electrode cell equipped with a platinum plate counter electrode. Reference electrode was Ag/AgNO3 (0.01mCH3CN). All CVs were recorded at 100 mV s 1 in dry HPLC-grade dichlorome- thane (CH2Cl2). The supporting electrolyte was tetrabutylammoni- um hexafluorophosphate (Bu4NPF6). Based on repeat measure- ments, absolute errors on potentials were found to be approxi- mately 5 mV. ACV experiments were carried out using the follow- ing parameters: scan rate of 5 mV s 1, AC amplitude of 25 mV with one point being acquired every 0.1/fs 1at a frequency of f Hz. Au substrates were prepared by deposition of ca. 5 nm of chromium followed by ca. 50 nm of gold onto a glass substrate using physical vapor deposition technique and were made immediately before use. SAMs were prepared on Au substrates according two routes.
Route 1: Successive adsorptions of C15T and dodecanethiol (C12) were performed by immersing the Au/glass substrate for 15 min in a millimolar solution of C15T in dichloromethane and then in a mil- limolar solution of C12 in dichloromethane. The immersion time in the C12 solution varied from 1 min to 48 h to obtain the expected C15T surface coverage. Route 2: Coadsorptions of C15T and C12 were performed by immersing the Au/glass substrate in a millimo- lar solution of C15T/C12 mixture in dichloromethane. The propor- tions in the mixture were respectively varied from 50/50 to 98/2 to obtain the expected C15T surface coverage.
Acknowledgements
This work was supported by the Centre National de la Recherche Scientifique (CNRS France), the ’’Agence Nationale de la Recher- che’’ (ANR France), and the ’’Rgion des Pays de la Loire’’
(France). The authors thank Flavy AlvÞque and Isabelle An- drouain for the critical reading of the manuscript and, Stphane Chesne for the design of electrochemical glass cells.
Keywords: electrocatalysis· monolayers·phase segregation · self-assembly·thiolates
[1] J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M. Whitesides,Chem.
Rev.2005,105, 1103 – 1169.
[2] F. Schreiber,Prog. Surf. Sci.2000,65, 151 – 256.
[3] S. Frey, A. Shaporenko, M. Zharnikov, P. Harder, D. L. Allara, J. Phys.
Chem. B2003,107, 7716 – 7725.
[4] L. G. Jensen, K. A. Nielsen, T. Breton, J. L. Sessler, J. O. Jeppesen, E. Levil- lain, L. Sanguinet,Chem. Eur. J.2009,15, 8128 – 8133.
[5] O. AlvÞque, F. Seladji, C. Gautier, M. Dias, T. Breton, E. Levillain,Chem- PhysChem2009,10, 2401 – 2404.
[6] A. Shaporenko, K. Rçssler, H. Lang, M. Zharnikov,J. Phys. Chem. B2006, 110, 24621 – 24628.
[7] S. Watcharinyanon, E. Moons, L. S. O. Johansson,J. Phys. Chem. C2009, 113, 1972 – 1979.
[8] O. AlvÞque, P-Y. Blanchard, T. Breton, M. Dias, C. Gautier, E. Levillain, F.
Seladji,Electrochem. Commun.2009,11, 1776 – 1780.
[9] O. AlvÞque, C. Gautier, M. Dias, T. Breton, E. Levillain, Phys. Chem.
Chem. Phys.2010,12, 12 584 – 12 590.
[10] S. E. Creager, T. T. Wooster,Anal. Chem.1998,70, 4257 – 4263.
[11] J. Li, K. Schuler, S. E. Creager,J. Electrochem. Soc.2000,147, 4584 – 4588.
[12] C. E. D. Chidsey, C. R. Bertozzi, T. M. Putvinski, A. M. Mujsce,J. Am. Chem.
Soc.1990,112, 4301 – 4306.
[13] Z. Ma, J. M. Bobbitt,J. Org. Chem.1991,56, 6110 – 6114.
[14] Y. Xie, F. C. Anson,J. Electroanal. Chem.1995,384, 145 – 153.
[15] T. Breton, D. Liaigre, E. M. Belgsir, Tetrahedron Lett. 2005, 46, 2487 – 2490.
Received: November 8, 2010
Published online on December 23, 2010
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