pubs.acs.org/Langmuir © 2010 American Chemical Society
SECM Investigations of Immobilized Porphyrins Films Yann Leroux,† Delphine Schaming,‡ Laurent Ruhlmann,*,‡ and Philippe Hapiot*,† †
Universit e de Rennes 1, Sciences Chimiques de Rennes (Equipe MaCSE), CNRS, UMR 6226, Campus de Beaulieu, Bat 10C, 35042 Rennes Cedex, France, and ‡Laboratoire de Chimie Physique (Groupe TEMiC), CNRS, UMR 8000, Universit e Paris-Sud, Bat 349, 91405 Orsay Cedex, France Received April 1, 2010. Revised Manuscript Received July 27, 2010
Electronic properties of electrogenerated Zn-porphyrin layers linked by an electroactive linker and immobilized on a semitransparent ITO electrode were investigated by steady-state SECM in unbiased conditions in view of the numerous possible applications of such surface. This SECM strategy took advantage of the variations of the charge transfer kinetics of the organic redox couple (the mediator used in SECM) on ITO surface with the standard potential of the mediator. After preliminary characterization of nonmodified ITO, analysis of the SECM approach curves recorded with a series of redox mediators allows the characterizations of both film permeability and charge transport inside the organic film in conditions close to a “real optoelectronic device”. Two types of porphyrin films were considered. In the first one, the film was produced by electropolymerization of a modified zinc-β-octaethylporphyrin in which the bipyridinium pendant substituent is first introduced. The second type of film was prepared directly from an in situ electropolymerization method in which the Zn porphyrin is simply oxidized in the presence of 4,40 -bipyridine. Experiments show the occurrence of efficient charge transport inside both films after initial reduction of the electroactive linker. However, the first preparation method leads to films with stronger blocking character versus organic molecules and higher charge injection rates.
Introduction Porphyrins, forming complexes with some metal ions, have found various important applications, as photosensitizer and/or catalysts in chemical and photochemical reactions or more recently in analytical chemistry.1 Many chemical models have been built in the past years aimed at mimicking natural biological systems; among these, polynuclear porphyrins have attracted considerable interest owing to the wide diversity of their electron transfer characteristics that could be modulated with their chemical structure.2 Many of the proposed applications require the immobilization of porphyrins. For this purpose, several methods of surface functionalization on different materials have been proposed as, for example, the formation of self-assembled monolayers,3,4 the attachment on silica,5 or, for what concerns the *Corresponding authors. E-mail:
[email protected],
[email protected]. (1) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (b) Lindsay Smith, J. R. in Metalloporphyrins in Catalytic Oxidation, Shelson, R. A., Ed.; Marcel Dekker: New York, 1994. (2) Kadish, K. M., Smith, K. M., Guilard, R., Eds. The Porphyrin Handbook; Academic Press: San Diego, CA, 2000; Vol 8-9. (3) (a) He, Y. F.; Ye, T.; Borguet, E. J. Am. Chem. Soc. 2002, 124, 11964. (b) Tsuda, A.; Sakamoto, S.; Yamaguchi, K.; Aida, T. J. Am. Chem. Soc. 2003, 125, 15722. (c) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (d) Li, Q.; Mathur, G.; Gowda, S.; Surthi, S.; Zhao, Q.; Yu, L.; Lindsey, J. S.; Bocian, D. F.; Misra, V. Adv. Mater. 2004, 16, 133. (e) Lu, X.; Yuan, H.; Zuo, G.; Yang, J. Thin Solid Films 2008, 516, 6476. (4) (a) Williams, M. E.; Hupp, J. T. J. Phys. Chem. B 2001, 105, 8944. (b) Lu, X.; Zhang, L.; Li, M.; Wang, X.; Zhang, Y.; Liu, X.; Zuo, G. ChemPhysChem 2006, 7, 854. (5) (a) Da Cruz, F.; Driaf, K.; Berthier, C.; Lameille, J.-M.; Armand, F. Thin Solid Films 1999, 349, 155. (b) Li, G.; Bhosale, S. V.; Wang, T.; Hackbarth, S.; Roeder, B.; Siggel, U.; Fuhrhop, J. H. J. Am. Chem. Soc. 2003, 125, 10694. (c) Zhang, M. Q.; Powell, H. V.; Mackenzie, S. R.; Unwin, P. R. Langmuir 2010, 26, 4004. (d) Dreas-Wlodarczak, A.; M€ullneritsch, M.; Juffmann, T.; Cioffi, C.; Arndt, M.; Mayor, M. Langmuir 2010, 26, 10822. (6) (a) Macor, K. A.; Spiro, T. G. J. Am. Chem. Soc. 1983, 105, 5601. (b) Bettelheim, A.; White, B. A.; Raybuck, S. A.; Murray, R. W. Inorg. Chem. 1987, 26, 1009. (c) Li, G.; Wang, T.; Schulz, A.; Bhosale, S.; Lauer, M.; Espindola, P.; Heinze, J.; Fuhrhop, J. H. Chem. Commun. 2004, 552. (d) Li, G.; Bhosale, S.; Tao, S.; Guo, R.; Bhosale, S.; Li, F.; Zhang, Y.; Wang, T.; Fuhrhop, J. H. Polymer 2005, 46, 5299. (e) Rault-Berthelot, J.; Paul-Roth, C.; Poriel, C.; Juillard, S.; Ballut, S.; Drouet, S.; Simonneaux, G. J. Electroanal. Chem. 2008, 623, 204.
Langmuir 2010, 26(18), 14983–14989
present study, polymerization of porphyrins to form electroactive films.6 Electrodes coated with such electroactive porphyrins are very attractive systems for fundamental studies, especially to probe the electronic coupling between or through monomeric porphyrins subunits, as well as for developments in the practical uses of the deposited materials.7 A common method to polymerize porphyrins relies on the use of bridging ligands to produce coordination polymers or oligomers8 but generally results in insoluble powders rather than film formation. Another common strategy that could be transposed to film formation6a consists of the electrochemical oxidative radical coupling of porphyrins via substituents attached on the ring periphery.6 If electropolymerization is considered as an easy process to obtain the formation of such electroactive film, this possibility is often counterbalanced by the somewhat complicated syntheses of the starting monomeric subunits. Following a different strategy based on nucleophilic attack onto porphyrin cation radicals,9,10 we have recently proposed a simple way for the electropolymerization of porphyrins based on the oxidation of the ring in the presence of the appropriate bridging ligand.11
(7) (a) This is a very active subject and numerous previous studies have been dedicated to the study of charge transfer between porphyrins connected by different types of linker, noticeably, in DAB systems. (See for example ref 7b and the references quoted in the introduction). (b) Regehly, M.; Wang, T.; Siggel, U.; Fuhrhop, J. H.; R€oder, B. J. Phys. Chem. B 2009, 113, 2526. (8) Collman, J. P.; McDevitt, J. T.; Leidner, C. R.; Yee, G. T.; Torrance, J. B.; Little, W. A. J. Am. Chem. Soc. 1987, 109, 4606. (b) Marvaud, V.; Launay, J.-P. Inorg. Chem. 1993, 32, 1376. (9) (a) Giraudeau, A.; Ruhlmann, L.; El Kahef, L.; Gross, M. J. Am. Chem. Soc. 1996, 118, 2969. (b) Ruhlmann, L.; Giraudeau, A. Eur. J. Inorg. Chem. 2001, 659. (c) Ruhlmann, L.; Schulz, A.; Giraudeau, A.; Messerschmidt, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1999, 121, 6664. (10) (a) Ruhlmann, L.; Hao, J.; Ping, Z.; Giraudeau, A. J. Electroanal. Chem. 2008, 621, 22. (b) Hao, J.; Giraudeau, A.; Ping, Z.; Ruhlmann, L. Langmuir 2008, 24, 1600. (11) (a) Giraudeau, A.; Schaming, D.; Hao, J.; Farha, R.; Goldmann, M.; Ruhlmann, L. J. Electroanal. Chem. 2010, 638, 70.(b) Schaming, D.; Allain, C.; Farha, R.; Goldmann, M.; Lobstein, S.; Giraudeau, A.; Hasenknopf, B.; Ruhlmann, L. Langmuir 2010, 26, 5101.
Published on Web 08/24/2010
DOI: 10.1021/la101294s
14983
Article
Leroux et al.
Among possible methods for investigating the electronic and permeation properties of the obtained film, scanning electrochemical microscopy (SECM) in feedback mode has been proven to be a powerful tool to analyze electronic transfer on various substrates.12 A few SECM studies have been reported in the literature on porphyrin-modified substrates but mainly concern the investigation of the blocking properties or the permeation of redox probe through the film.4 In the present work, we focus our study on the electronic properties of electrogenerated metal porphyrin films, and more precisely on the ability of porphyrin bridges to allow electronic transfer between redox active units. For this purpose, we used an approach in which the sample is not electrically connected (unbiased conditions) and a series of redox mediators which display different standard potentials.13 Steadystate SECM in unbiased mode has been used with success previously for the study of different modified surfaces, as it permits the investigation of the sample under conditions where the surface is not electrically connected.13,14 It is noticeable that this method is complementary to cyclic voltammetry, with the difference that SECM allows observations of the immobilized layer from the bulk solution and thus permits a different view of the same redox properties.14,15 Briefly, the SECM principle is based on the interaction of the substrate under investigation with a redox species (the mediator) that is electrogenerated at a microelectrode.12 This interaction is followed through the analysis of the current flowing at the microelectrode while it approaches the substrate. Depending on the nature of the redox couple, probing the permeation, the accessibility, or the redox reactivity of the functional group is possible by varying the size of redox couple or its standard potential. In this study, two types of film were considered. In the first one, the polymerization was performed by oxidation of a modified porphyrin subunit in which the bipyridinium linker was already introduced (poly-1).10 The second type of film (poly-2) was prepared directly from the in situ electropolymerization in which the ZnOEP porphyrin is oxidized in the presence of the 4,40 bipyridine.11 Both films were electrosynthesized using a semitransparent ITO electrode (indium tin oxide) that shows good adherence properties for both films and because of the wide variety of optoelectronic applications of this material. SECM investigations were made under these conditions in acetonitrile.
Experimental Section Chemicals. The zinc-β-octaethylporphyrin (ZnOEP) and the 4,40 -bipyridine were purchased from Sigma-Aldrich. The zinc-meso-bipyridinium-β-octaethylporphyrin (ZnOEP(bpy)þ, ClO4-) was synthesized and characterized as described previously (12) (a) Bard, A. J.; Mirkin, M. V.; Unwin, P. R.; Wipf, D. O. J. Phys. Chem. 1992, 96, 1861.(b) Bard, A. J., Mirkin, M. V., Eds. Scanning Electrochemical Microscopy; Marcel Dekker: New York, 2001;(c) Wei, C.; Bard, A. J.; Mirkin, M. V. J. Phys. Chem. 1995, 99, 16033. (d) Ufheil, J.; Hess, C.; Borgwarth, K.; Heinze, J. Phys. Chem. Chem. Phys. 2005, 7, 3185. (e) Wittstock, G.; Burchardt, M.; Pust, S. E.; Shen, Y.; Zhao, C. Angew. Chem., Int. Ed. 2007, 46, 1584. (f) Amemiya, S.; Bard, A. J.; Fan, F. R. F.; Mirkin, M. V.; Unwin, P. R. Annu. Rev. Anal. Chem. 2008, 1, 95. (13) (a) Ghilane, J.; Guilloux-Viry, M.; Lagrost, C.; Simonet, J.; Hapiot, P. J. Am. Chem. Soc. 2007, 129, 6654. (b) Hauquier, F.; Ghilane, J.; Fabre, B.; Hapiot, P. J. Am. Chem. Soc. 2008, 130, 2748. (c) Wang, A. F.; Ornelas, C.; Astruc, D.; Hapiot, P. J. Am. Chem. Soc. 2009, 131, 6652. (d) Zigah, D.; Noel, J.-M.; Lagrost, C.; Hapiot, P . J. Phys. Chem. C 2010, 114, 3075. (14) (a) Nicholson, P. G.; Ruiz, V.; Macpherson, J. V.; Unwin, P. R. Phys. Chem. Chem. Phys. 2006, 8, 5096. (b) Li, F.; Ciani, I.; Bertoncello, P.; Unwin, P. R.; Zhao, J.; Bradbury, C. R.; Fermin, D. J. J. Phys. Chem. C 2008, 112, 9686. (15) (a) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc. 2004, 126, 1485. (b) Ghilane, J.; Hauquier, F.; Fabre, B.; Hapiot, P. Anal. Chem. 2006, 78, 6019. (c) Wittstock, G.; Burchardt, M.; Pust, S. E.; Shen, Y.; Zhao, C. Angew. Chem., Int. Ed. 2007, 46, 1584.
14984 DOI: 10.1021/la101294s
by electrochemical oxidation of ZnOEP in the presence of an excess of 4,40 -bipyridine.9c Electropolymerizations. These were carried out with a threeelectrode system using a PARSTAT 2273 potentiostat. One-side indium tin oxide coated electrodes (ITO, Sigma-Aldrich, 8-12 Ω/ square) were used as working electrodes. A platinum wire was used as auxiliary electrode. The reference electrode was a saturated calomel electrode (SCE). It was electrically connected to the solution by a junction bridge filled with CH3CN containing 0.1 mol 3 L-1 of tetraethylammonium hexafluorophosphate (NEt4PF6). The polymers were prepared in similar electrochemical conditions. For both polymers, 25 iterative scans were carried out. After electropolymerization, the working electrodes were washed in CH3CN to remove traces of the conducting salt present on the deposited films. SECM experiments were all performed with freshly prepared films. Electropolymerization of poly-1 (Scheme 1) was carried out under an argon atmosphere in a 0.1 mol 3 L-1 solution of tetraethylammonium hexafluorophosphate in 1,2-C2H4Cl2 containing 2.5 10-4 mol L-1 of ZnOEP(bpy)þ, ClO4-. Cyclic scans (0.2 V s-1) of the working electrode were applied between -0.90 and þ1.60 V vs SCE. Electropolymerization of poly-2 (Scheme 2) was carried out as described above in the same medium and in the same conditions, but using 2.5 10-4 mol L-1 of ZnOEP and 2.5 10-4 mol L-1 of 4,40 -bipyridine instead of the substituted porphyrin. SECM Experiments. Measurements were performed using CHI900B from CH-Instruments equipped with an adjustable stage for the tilt angle correction. SECM experimentation consists of recording approach curves where the normalized current It = i/iinf is plotted versus the normalized distance L = d/a where i is the current at the tip electrode localized at a distance d from the substrate, iinf is the steady-state current when the tip is at an infinite distance from the substrate iinf = 4nFDCa, with n the number of electrons transferred per species, F the Faraday constant, D and C the diffusion coefficient and the initial concentration of the mediator, and a the radius of the UME.12,16 The applied potential at the UME is chosen as being sufficiently positive (or negative) to ensure a fast electron transfer at the UME (diffusion plateau of the mediator). The UME tip was a commercial (IJ Cambria) 5-μm-radius platinum disk with a typical RG = 10 (RG is the ratio of the total electrode radius including the glass insulator over UME radius). The UMEs were characterized by cyclic voltammetry and by typical approach curves recorded on conducting and insulating surfaces. We used a typical three-electrode configuration, with a platinum counter electrode and a Pt/Ppy reference electrode.17 The electrochemical cell was specifically designed to work under inert atmosphere (argon flowing) and is as represented in Scheme 3. For a defined sample, all approach curves are adjusted using the same zero origin. This was achieved by simply changing the solution containing the mediator between experiments without moving the UME. Absolute zero offset was estimated with the ferrocene curve that was first fitted to the insulating case and then slightly adjusted to take into account the low kel. All SECM experiments were performed at room temperature in unbiased conditions (the substrate is not electrically connected). All potentials are given versus the SCE electrode and were normalized as the E of ferrocene couple (0.405).
Determination of the Apparent Charge Transfer Rate Constants kel. To obtain a more quantitative analysis, the global charge transfer process was characterized by the apparent charge transfer rate constant kel that is the apparent constant for the reaction between the oxidized (or reduced) mediator and the (16) (a) Lefrou, C. J. Electroanal. Chem. 2007, 601, 94. (b) Cornut, R; Lefrou, C. J. Electroanal. Chem. 2007, 608, 59. (c) Cornut, R; Lefrou, C. J. Electroanal. Chem. 2008, 621, 178. (17) Ghilane, J.; Hapiot, P.; Bard, A. J. Anal. Chem. 2006, 78, 6868.
Langmuir 2010, 26(18), 14983–14989
Leroux et al.
Article Scheme 1. Electropolymerization of the Monosubstituted Porphyrin ZnOEP(bpy)þ to Form poly-1
Scheme 2. Electropolymerization of ZnOEP in the Presence of 4,4-Bipyridine to Form poly-2
surface under analysis.12a,b,13 This is an approximate procedure, as charge transport (or conductivity) inside the film are not taken into account. However, kel values could easily be derived from adjustments between the experimental approach curves (It versus L) and dimensionless theoretical curves assuming a irreversible electron transfer kinetics for which semiempirical solutions have been published.12b,16 Following the Bard-Mirkin formalism, Langmuir 2010, 26(18), 14983–14989
these fittings provide the dimensionless parameter κ = kela/D (where a is the electrode radius and D the diffusion coefficient of the redox mediator). D values were determined by classic electrochemical methods in the same experimental conditions as used for SECM experiments. Diffusion coefficients were taken equal for charge and neutral species of the mediators: Ferrocene (Fc) 1.7 10-5 cm2 s-1; tetrathiafulvalene (TTF) 1.5 10-5 cm2 s-1; DOI: 10.1021/la101294s
14985
Article
Leroux et al.
Scheme 3. SECM Cell Used to Work under Inert Atmosphere
Scheme 4. General Principle of Steady-State SECM in Unbiased Conditions Exemplified for a Reduction Process (Transposition to an Oxidation Process Is Straightforward)a
a
Charge transfer may occur through the substrate or the film (see text).
tetracyanoquinodimethane (TCNQ) 1.0 10-5 cm2 s-1; and 4-nitrobenzonitrile (4NB) 1.6 10-5 cm2 s-1.
Results and Discussion Our strategy for analyzing the redox properties of the porphyrins films is based on the determination on apparent electron transfer rate constants kel that are obtained with steady-state SECM in unbiased conditions. This global rate constant reflects the different steps of the charge injection process from the mediator to the film and transport by the sample. In steady-state SECM, the observation of charge transfer at the sample surface (positive feedback) implies the occurrence of several consecutive steps because the injected charges cannot accumulate in the sample (see Scheme 4). The first step is the charge transfer from the electrogenerated mediator to the sample (reduction or oxidation). This first transfer could occur at the film itself (direct electron tunneling) and by permeation of the mediator through the immobilized layer or pinholes or at the redox substrate depending of the nature of the film (mediated electron transfer). These steps are followed by charge transport outside the diffusion cone under the tip electrode, followed by the reverse reaction where the charge is returned to the mediator in solution. 14986 DOI: 10.1021/la101294s
The extension of the investigated surface depends on the competitions between these different steps but is at least a few times the diameter of the tip.12,18 Thus, depending on the nature of the redox couple, it is possible to probe the permeation, the accessibility of supporting substrate or the redox reactivity of the functional group by varying the size of redox couple or its standard potential, and the charge transport inside the film. The strategy is thus to consider different couples as mediators that are able or unable to exchange positive or negative charges with the porphyrins films. (See, for example, different investigations in ref 13.) Different approximate models have been developed to determine the apparent electron rate transfer with the relevant parameters. If the transport inside the organic film or by the substrate allows rapid evacuation of the charges, kel could be approximated as the sum of the different contributions from the mediated electron transfer and the permeation þ tunneling current: kel = kmedΓporph þ kdirect where Γporph is the surface concentration of active porphyrin units, kmed the rate constant between the reduced mediators in solution and the immobilized porphyrins, and kdirect the contribution from direct charge exchange with the substrate.15a Noticeably, when there is no limitation by the transport inside the film, kel is independent of the mediator concentration, and thus, similar curves should be obtained when the mediator concentration is changed.19,20 The case of a limitation by redox transport in the film is somewhat more complicated. The problem has been solved in a related situation where the charge transport inside the substrate is limited by ohmic drop.19 Moreover, an approximate analytical treatment has also been proposed for redox active films. In contrast to the preceding case, when transport becomes limiting, the apparent kel becomes dependent on the mediator concentration. Indeed, the layer is not able to efficiently evacuate the injected charge, and thus, the feedback is expected to be less “positive” when the concentration (or charge flow rate) increases.20 In this study, we chose four different reversible redox couples in acetonitrile: Ferrocene (Fc), tetrathiafulvalene (TTF), tetracyanoquinodimethane (TCNQ), and 4-nitrobenzonitrile (4NB). We could divide these mediators into two categories, Fc and TTF that work in oxidation and TCNQ and 4NB, which work in reduction. Figure 1 shows a typical voltammogram of poly-1 (similar patterns are observed with poly-2) that was recorded in a blank solution of acetonitrile containing only the supporting electrolyte. The reductive peaks have been previously ascribed to the reduction of the viologen moieties and the oxidation one to the oxidation of the ring.21 The reduction of the ring in the Zn porphyrin is also possible but only occurs at more negative potential (typically around -1.6 V). One may observe the negligible current at the starting potential showing that the viologen moieties are under their oxidized form (dication). For the first family of mediators (Fc and TTF), their oxidized forms (TTF. þ and Fcþ) cannot react with the porphyrin films regarding their oxidative redox potential (see the CV in Figure 1). However, they are good candidates to examine the permeation of the redox probe because they could rapidly exchange charge with the ITO substrate (see below). Concerning the second category, 4NB has a (18) (a) This general concept was presented by Amatore, C.; et al. in ref 18b, for a totally different chemical system. (b) Amatore, C.; Combellas, C.; Kanoufi, F.; Sella, C.; Thiebault, A.; Thouin, L. Chem.;Eur. J. 2000, 6, 820. (19) Whitworth, A. L.; Mandler, D.; Unwin, P. R. Phys. Chem. Chem. Phys. 2005, 7, 356. (20) Lie, L. H.; Mirkin, M. V.; Hakkarainen, S.; Houlton, A.; Horrocks, B. R. J. Electroanal. Chem. 2007, 603, 67. (21) Schaming, D.; Giraudeau, A.; Lobstein, S.; Farha, R.; Goldmann, M.; Gisselbrecht, J.-P.; Ruhlmann, L. J. Electroanal. Chem. 2009, 635, 20.
Langmuir 2010, 26(18), 14983–14989
Leroux et al.
Figure 1. Cyclic voltammetry of poly-1 on an ITO electrode in acetonitrile þ 0.1 mol L-1 NBu4PF6. Scan rate 0.2 V s-1.
standard potential sufficiently negative for its radical anion (4NB.-) being able to rapidly reduce the porphyrin film on the viologen moieties. On the contrary, TCNQ has a redox potential located at the foot of the reducing peak of the film. We could expect TCNQ.- to be able to reduce the films but with a less efficient electron transfer. SECM Experiments on Nonmodified ITO Substrate. Indium tin oxide (ITO) is a wide-band gap semiconductor material. ITO on glass substrate is one of the most common electrode materials for devices that require optical transparency in the visible range combined with quite good conductivity.22 Because of this high stability at positive potential, this electrode material was commonly used as transparent anode in many oxidative processes as, for example, in the electropolymerization of conducting polymers. However, it was shown that electrochemical performance of ITO surface in terms of electron transfer kinetics greatly depends on the processing of the material itself.23 The specificities of this material have permitted a lot of studies in organic electronic emerging fields, but are not innocent in terms of redox reactivity during SECM experiments. Notably, kinetics of the electron transfer on a semiconductor for SECM in unbiased conditions depends on the redox potential of the mediator and of different parameters as the flat band potential or the existence of surface states.15b,24 Recently, SECM approach curves of an ITO substrate were reported in water. The electron transfer kinetics at the surface was indeed found to depend on the potential of the redox couple.25 Thus, to be able to evaluate the contribution of the substrate in the analysis of the data, preliminary SECM studies are required and were performed with our ITO sample in our experimental conditions. Figure 2 represents the approach curves obtained, with the four considered mediators, on our naked ITO substrate. The approach curves exhibit an enhancement of the current when the tipsubstrate distance decreases for both Fc and TTF mediators. This behavior corresponds to a positive feedback and indicates the occurrence of a rapid regeneration of the mediator at the ITO surface. Both approach curves were similar suggesting that the process is mainly limited by the diffusion of the mediator from the (22) (a) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, R123. (b) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1. (23) Popovich, N. D.; Wong, S.-S.; Yen, B. K. H.; Yeom, H.-Y.; Paine, D. C. Anal. Chem. 2002, 74, 3127. (24) (a) Mandler, D.; Bard, A. J. Langmuir 1990, 6, 1489–1494. (b) Horrocks, B. R.; Mirkins, M. V.; Bard, A. J. J. Phys. Chem. 1994, 98, 9106–9114. (25) Neufeld, A. K.; O’Mullane, A. P. J. Solid State Electrochem. 2006, 10, 808.
Langmuir 2010, 26(18), 14983–14989
Article
Figure 2. SECM approach curves on an ITO electrode in acetonitrile (þ 0.1 mol L-1 NBu4PF6) with the different redox mediators: TTF (black O), Fc (magenta 4), TCNQ (green 0), 4NB (red 3). Lines are the theoretical curves for an irreversible electron transfer kinetics with κ = 6.6 (magenta), 0.15 (green).
Figure 3. SECM approach curves on an ITO electrode modified with the poly-1 film and using the different redox mediators in acetonitrile þ 0.1 mol L-1 NBu4PF6. Mediators: TTF (black O, ;), Fc (magenta 4, ;), TCNQ (green 0, ;), 4NB (red 3, ;). Lines are the fitting curves considering irreversible electron transfer kinetics and with the constants reported in Table 1.
tip to substrate.12 More precise fittings show a good adjustment with high values of the dimensionless parameter κ on the order of 6-7. In parallel, the same approach curves were recorded with the reductive mediators. They show a decrease of the current with the tip-substrate distance for both TCNQ and 4NB. This behavior corresponds to negative feedback, which points out a slow regeneration of mediators at the ITO surface. These negative feedback curves fit well with the theoretical curves calculated for rate constants on the order of κ = 0.15 indicating that, if electron transfer at the ITO surface remains possible, it is much slower for TCNQ/TCNQ.- and 4NB/4NB.- couples than for Fc/Fcþ and TTF/TTF.þ couples. From these preliminary analyses, we confirm that Fc and TTF are good mediators to probe the permeability of the film, because they should not react with the film but rapidly exchange charges with the ITO substrate. By contrast, TCNQ and 4NB are good candidates for probing the reactivity of the porphyrin film, as charge exchanges with the ITO substrate are negligible in our SECM conditions. SECM Analysis on Porphyrin Film poly-1. Figure 3 shows the SECM experiments made on porphyrin film poly-1 electrodeposited on ITO substrate, with all the mediators at a concentration of 10-3 mol L-1. For both reductive and oxidative mediators, the SECM approach curves exhibit shapes corresponding to intermediate kinetic situations between the totally DOI: 10.1021/la101294s
14987
Article
Leroux et al.
Table 1. Kinetic Rate Constant kel in cm s-1 Derived from the SECM Approach Curves mediator (E/SCE)/substrate
ITO
Poly-1 on ITO
Poly-2 on ITO
Fc/Fcþ (0.41 V/SCE) TTF/TTF.þ (0.35 V/SCE) TCNQ/TCNQ.- (0.27 V/SCE) 4NB/4NB.- (-0.60 V/SCE)
2.3 10-1 2.0 10-1 3.1 10-3 4.8 10-3
6.8 10-3 2.4 10-2 2.3 10-2 ¥
6.8 10-3 2.9 10-2 8.0 10-3 1.6 10-1
negative feedback and the positive feedback cases. Good fits are obtained with the theoretical curves suggesting that the global process is under the control of the charge transfer between the mediator and the sample. If we first examine the curves recorded with the reductive mediators TCNQ and 4NB, we could observe that kel values are much higher after modifications of the ITO by the porphyrins film than on the naked ITO itself. It is also noticeable that the highest rate constant is obtained with 4NB that displays the most negative reducing potential and for which the radical anion is the strongest reducing agent. In this case, the experimental curve could be fitted with the theoretical behavior expected for a system controlled by the diffusion of the mediator from the tip to the substrate. All these results fall in line with a reduction of poly-1 on the viologen moieties by the reduced form of the mediator. As we explained above, in steady-state SECM in unbiased conditions, such observations also imply that, after the first reduction of the viologen units, the negative charges are rapidly evacuated thanks to an efficient conduction process. In such case, it is also expected that the fastest global kinetics occur with the strongest reductive radical anion as is experimentally observed for 4NB as mediator. On the contrary, when using oxidative mediators TTF and Fc, approach curves display a higher “negative feedback character” after modification of the ITO by the porphyrins film (compare Figures 2 and 3). Because the oxidized forms of the mediators (TTF.þ and Fcþ) could not oxidize the film but rapidly exchange charges with the ITO substrate, kel are indicative of the permeation of the film toward these mediators. A partially blocked electrode behaves as the same electrode without the layer but with lower charge transfer kinetics.26 As seen in Table 1, the derived low kinetic rates kel show the good blocking character of poly-1 versus the organic probes that hardly reach the ITO. SECM Analysis on Porphyrin Film poly-2. Similar SECM experiments were performed with the ITO modified with poly-2. (See the approach curves in Figure 4.) As previously found for poly-1 on ITO, approach curves display good agreement with the behavior expected for an irreversible electron transfer kinetics allowing the extraction of the rate constants kel. By comparison with the poly-1 modified-ITO electrode, the approach curve recorded with TTF as mediator presents a slightly more “positive character” corresponding to higher values of kel. We could also notice that for both films the blocking character of the film appears slightly higher for Fcþ than for TTF.þ. It is not easy to provide a clear-cut answer to this difference that should likely reflect different interactions of the mediator with the film (that is positively charged). We could simply note that permeation seen in this type of experiment is often dependent on the considered organic probe. If we now examine the approach curves recorded with the two reductive mediators TCNQ and 4NB on poly-2 with those obtained for poly-1, one could observe that the positive feedback character is lower for poly-2 corresponding to smaller values of kel. Two hypotheses could be considered to explain this (26) Amatore, C; Saveant, J.-M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39.
14988 DOI: 10.1021/la101294s
Figure 4. SECM approach curves on an ITO electrode modified with the poly-2 film and using the different redox mediators in acetonitrile þ 0.1 mol 3 L-1 NBu4PF6. Mediators: TTF (black O, ;), Fc (magenta 4, ;), TCNQ (green 0, ;), 4NB (red 3, ;). Lines are the fitting curves considering irreversible electron transfer kinetics and with the constants reported in Table 1.
observation considering that, in that case, kel constants reflect the properties of the immobilized porphyrins films. First, this could be interpreted by a lower amount of redox active porphyrins, as the electron transfer at the film is directly proportional to the surface concentration of accessible electroactive species.15a Second, less efficient charge transfer transport inside the organic film could also lead to a decrease of the apparent kel.19,20 When the two films are examined by cyclic voltammetry, both films display similar patterns as shown in Figure 1 for poly-1, but lower currents are systematically obtained for poly-2 (around half) showing that the surface concentration of redox active sites is indeed smaller in that case. Even if the number of accessible sites visible by cyclic voltammetry (the transfer is initiated from the ITO side) and in SECM conditions (the film is observed from solution side) may be slightly different, this observation explains the SECM measurements and the lower performances of poly-2 versus poly-1. Concerning the second hypothesis, the presence of bipyridine during the electropolymerization could lead to an increase of the distance between redox active centers because of the formation of ligand-bridged zinc porphyrin (bipyridine axially ligated to zinc) and change in the film morphology resulting in a lower conductivity. Indeed, AFM experiments (see Figures S4 and S5 in the Supporting Information) indicate a more granular structure in the case of poly-2 compared to poly-1, which is generally associated with lower transport rates.27 To get more information about limitations by the lateral charge transport in the film, approach curves were recorded for different concentrations of mediators.19,20 The different curves are shown in Figure 5 for 4NB as mediator that corresponds to the fastest electron transfer, and derived kel values are shown in Table 2. In the case of poly-1, shapes of approach curves and the derived kel values are clearly independent of the mediator concentration. On the contrary, for poly-2, feedback becomes less positive when the mediator concentration increases, indicating limitations by the redox transport inside the porphyrin films as explained above.20 Similar experiments were performed with the other mediators (see Supporting Information). For TCNQ, small variations of kel are observed that could also be related to transport limitations. However, the situation is somewhat more complicated by the fact that TCNQ is not a sufficiently strong reducing agent for the (27) (a) See for example ref 26b and references therein. (b) Wang, Y.; Trand, H. D.; Liao, L.; Duan, X.; Kaner, R. B. J. Am. Chem. Soc. 2010, 132, 10365.
Langmuir 2010, 26(18), 14983–14989
Leroux et al.
Article
Figure 5. SECM approach curves on an ITO electrode modified with (a) the poly-1 film and (b) the poly-2 film, using the 4NB redox mediators in acetonitrile þ 0.1 mol L-1 NBu4PF6, at different concentrations: 10-3 mol L-1 (black 0, ;), 5 10-3 mol L-1 (red O, ;), 10-2 mol L-1 (blue Δ, ;). Lines are the fitting curves considering irreversible electron transfer kinetics and with the constants reported in Table 2. Table 2. Kinetic Rate Constant kel in cm s-1 Derived from the SECM Approach Curves of 4NB Redox Mediator at Different Concentrations
reduction of the porphyrins on the layer to be considered as reversible. For the two mediators working in the oxidation TTF and Fc in a range of potential where only the ITO substrate is responsible of the charge transport (see Supporting Information Figures S1 and S2), approach curves show very small variations of the apparent charge transfer rate (lower than 10% for 10-fold range of mediator concentration increase).
is immobilized on a semiconducting transparent electrode. The analysis took advantage of the variations of the charge transfer kinetics at the surface of a naked ITO substrate and of its conductivity with the standard potential of the redox mediator. Using a series of mediators allows probing both the permeability of the film and the charge communication between adjacent redox active units linked by metal porphyrins. In our case, where bipyridiniums are used to connect Zn porphyrins, the process involved first the reduction of the viologen subunit that appears as the limiting step and is followed by a rapid charge transport inside the film. By variation of the concentration, it appears that poly-1 (when the nucleophile bridge is included before the polymerization) presents high transport properties. In SECM, the global process appears limited by the number of accessible electroactive bipyridinium moieties.
Conclusions Electrochemistry is a convenient tool for producing electroactive porphyrin films on valuable substrates like ITO. SECM in feedback mode and in unbiased conditions allows the investigations of the redox and transport properties in conditions that are close to a “real optoelectronic device” where the redox active film
Supporting Information Available: SECM approach curves recorded at different concentrations (ferrocene (Fc), tetrathiafulvalene (TTF), tetracyanoquinodimethane (TCNQ)) are presented with the derived kel values. AFM imaging of poly-1 an poly-2 showing the structure of the porphyrins films on ITO are shown. This material is available free of charge via the Internet at http://pubs.acs.org.
concentration of 4NB in mol L-1
Poly-1 on ITO
Poly-2 on ITO
¥ ¥ ¥
1.6 10-1 4.2 10-2 3.5 10-2
-3
10 5 10-3 10-2
Langmuir 2010, 26(18), 14983–14989
DOI: 10.1021/la101294s
14989
