Indirect Reduction of Aryldiazonium Salts onto Cathodically Activated

May 24, 2005 - Centre National de la Recherche Scientifique-Universite´ de Rennes 1 Number 6510, and. Laboratoire de Chimie du Solide et Inorganique ...
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Indirect Reduction of Aryldiazonium Salts onto Cathodically Activated Platinum Surfaces: Formation of Metal-Organic Structures J. Ghilane,† M. Delamar,‡ M. Guilloux-Viry,§ C. Lagrost,† C. Mangeney,‡ and P. Hapiot*,† Laboratoire d’Electrochimie Mole´ culaire et Macromole´ culaire, SESO, Unite mixte de recherche Centre National de la Recherche Scientifique-Universite´ de Rennes 1 Number 6510, and Laboratoire de Chimie du Solide et Inorganique Mole´ culaire, Unite mixte de recherche Centre National de la Recherche Scientifique-Universite´ de Rennes 1 Number 6511, Institut de Chimie de Rennes, Campus de Beaulieu, F-35042 Rennes, France, and ITODYS, Unite mixte de recherche Centre National de la Recherche Scientifique-Universite´ Paris 7-Denis Diderot Number 7086, 1 rue Guy de la Brosse, F-75005, Paris, France Received February 14, 2005. In Final Form: April 2, 2005 Platinum phases of general formula [Ptn-, M+, MX] can be electrogenerated from cathodic polarization in dry dimethylformamide containing a supporting electrolyte, MX. The reaction of these electrogenerated Pt phases as reducing agent with aryldiazonium salts was investigated for preparing controlled metalorganic interfaces and characterizing the reactivity of the “reduced platinum phases”. In a two-step process, the “reduced platinum phase” locally reacts with aryldiazonium salts, leading to the attachment of aryl groups onto the metal surface in the previously modified areas. Detailed experiments using cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and in situ electrochemical atomic force microscopy (EC-AFM) were carried out to follow the reaction in solution with the example of NaI as supporting electrolyte (MX ) NaI). These studies demonstrate the irreversible attachment of aryl groups onto the platinum electrode. Comparison between the direct electroreduction of aryldiazonium compounds (4nitrophenyl- and 4-bromophenyldiazonium) on a platinum electrode and their reaction with [Pt2-, Na+, NaI] suggests that a similar general mechanism is responsible for the grafting. However in the second case, no applied potential is required to stimulate the binding thanks to the reductive properties of [Pt2-, Na+, NaI]. Competitive reduction of the organic layer and growth of the layer were observed and analyzed as a function of the injected charge used to initially produce [Pt2-, Na+, NaI]. Similar reactions are highly probable with other MX salts owing to the redox properties observed for this type of platinum phase ([Ptn-, M+, MX]).

Introduction The modifications of metallic surfaces by attaching (or including) organic molecules onto metal surfaces are important processes for providing new and special properties to the interfaces. For noble metals such as platinum, it was recently shown that new metal-organic interfaces could be obtained under cathodic polarization in dry aprotic solvents containing organic or inorganic salts.1 Under electrochemical conditions, the platinum surface is modified by the concomitant transfer of an electron with the insertion of the cation of the supporting electrolyte that leads to the formation of metal-organic phases of general formula [Ptn-, M+, MX], where M+ is the cation electrolyte, for example, Na+, K+, Cs+, or NAlk4+, and X- is the anion electrolyte (I-, BF4-, PF6-, ...).1 This unexpected process (for a noble metal that was considered during decades for preparing inert electrodes) formally corresponds to the reduction of the platinum metal. It occurs when the electrode is held at a negative potential in a dry organic solvent containing a supporting electrolyte (for example, * To whom correspondence should be addressed: e-mail [email protected]. † LEMM/SESO, UMR CNRS-University of Rennes 1 6510. ‡ ITODYS, UMR CNRS-University Denis Diderot (Paris 7) 7086. § LCSIM, UMR CNRS-University of Rennes 1 6511. (1) (a) Cougnon, C.; Simonet, J. Platinum Met. Rev. 2002, 46, 94. (b) Cougnon, C.; Simonet, J. Electrochem. Commun. 2002, 4, 266. (c) Cougnon, C.; Simonet, J. J. Electronal. Chem. 2002, 531, 179. (d) Simonet, J. Electrochem. Commun. 2003, 5, 439.

in very dry dimethylformamide + NaI).1 The reaction has been investigated by transient electrochemical methods and scanning electron microscopy (SEM), which show evidence of considerable modifications of the metal surfaces associated with formation of the new Pt phase.1 More recently, atomic force microscopy experiments performed simultaneously with electrochemical reduction (EC-AFM) showed that the process is completely reversible for thin layer modifications.2 The Pt surface is reduced to produce [Ptn-, M+, MX], and then upon reoxidation of the sample, the starting structure is recovered when the platinum metal is regenerated.1,2 It was shown that [Ptn-, M+, MX] phases exhibit strong reductive properties that were exemplified with the production of simple organic radical anion when the reduced sample is immersed in a solution containing a molecule with a reducible group.1c When these properties are taken into account, it can be envisaged to use [Ptn-, M+, MX] to trigger a chemical reaction involving first an electron transfer in the vicinity of the electrode. Concerning the immobilization of organic molecules onto the surface, there are several well-known reactions induced by the electron transfer to (or from) an organic molecule that lead to strong attachment of organic moieties onto the surface.3 A convenient reaction is certainly the (2) Bergamini, J.-F.; Ghilane, J.; Guilloux-Viry, M.; Hapiot, P. Electrochem. Commun. 2004, 6, 188. (3) For a general review on this subject, see for example Downard, A. J. Electroanalysis 2000, 12, 1085.

10.1021/la050401y CCC: $30.25 © 2005 American Chemical Society Published on Web 05/24/2005

Reduction of Aryldiazonium Salts onto Pt Surfaces Scheme 1. Schematic Illustration of the Direct Electrochemical Reduction of Aryldiazonium and Multilayer Formation via the Attachment of Aryl Radicals in Solution to the Pt Surface6

electrochemical grafting associated with the reduction of aryldiazonium salt, which was discovered in the beginning of the 1990s in the case of a carbon electrode (HOPG and graphite materials).4 The classical procedure involved the electrochemical reduction of an aryldiazonium salt to produce aryl radicals (and N2) in the vicinity of the electrode that react with the surface of the electrode by a proposed mechanism displayed in Scheme 1.5 Following these investigations performed on carbon surfaces,4,7 electrografting of aryl groups was reported on numerous materials such as silicon,8 iron,9 and several other metallic surfaces.10 Aryl groups layers immobilized on the electrode surface have been characterized by several different techniques such as Raman spectroscopy,11 infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS), and atomic force microscopy (AFM).12 However, nucleation and growth of multilayer films were often observed and illustrate the difficulties for controlling the amount of material deposited on the surface, especially for applica(4) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883. (5) Aryl radicals are very reactive species able to react rapidly with most organic molecules, including the solvent itself and other aromatic species. However, the reaction with the surface is favored because the generation of the radical occurs at a very short distance from the electrode surface (see ref 7b). (6) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. Langmuir, 2005, 21, 280. (7) (a) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. Electroanal. Chem. 1992, 336, 113. (b) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. (c) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant J.-M. Carbon. 1997, 35, 801. (8) (a) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B. 1997, 101, 2415. (b) Allongue, P.; Henry de Villeneuve, C.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791. (9) (a) Adenier, A.; Bernard, M. C.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 4541. (b) Adenier, A.; Cabet-Deliry, E.; Lalot, T.; Pinson, J.; Podvorica, F. Chem. Mater. 2002, 14, 4576. (c) Boukerma, K.; Chehimi, M. M.; Pinson, J.; Blomfield, C. Langmuir 2003, 19, 6333. (10) (a) Bernard, M. C.; Chausse´, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15, 3450. (b) Hurley, B. L.; McCreery, R. L. J. Electrochem. Soc. 2004, 15, B252. (11) (a) Liu, Y. C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254. (b) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958. (c) Itoh, T.; McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894.

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tions requiring the formation of a defined single monolayer.12b,13,14 As observed before, besides the attachment reaction of the aryl radicals onto the surface, a radical addition to the already grafted aromatic groups may occur that leads to the formation of a multilayer deposit as represented in Scheme 1.13 In these conditions, establishing the existence of the covalent bond between the aryl group layer and the supporting material is a difficult task that was clearly demonstrated only for carbon7b or iron surfaces.9a More generally, even if there is no clear-cut answer for many surface materials, the hypothesis of a covalent link is probable on the basis of the strong mechanical adherence of the layer that cannot be explained by weak interactions. Indeed, it is reported that only aggressive mechanical polishing, or vigorous corrosion of the surface, or scratching by an AFM tips, can remove the layer.7-11 Whatever the nature of the link, the existence of such a strong adherence remains a real advantage in applications where the stability of the grafting is basically required. In this work, we used the “reduced” platinum phases that were produced on an electrode surface as reducing agents.1c It is well-known that charged surfaces such as carbon can spontaneously reduce organic species present in solution. In this field, an interesting method based on the indirect grafting of phenyldiazonium salts onto reduced fluoropolymer has been reported.15c Even if the chemistry is completely different, the platinum surfaces can be electrochemically modified to achieve the grafting of diazonium salts, and in a process that is quite unexpected for a noble metal. We envisage exploiting the reduced platinum phases for a local induction of the reduction of the diazonium salt and to covalently bind aryl groups on noble metal surfaces in the absence of any externally applied potential. We can expect that the reaction only occurs in metal areas that have been previously modified, and its development is limited by the quantity of charge previously injected in the metal during the formation of [Ptn-, M+, MX] phases. This should lead to better control of the localization and of the thickness deposit because aryl radicals will be only produced on the sites where [Ptn-, M+, MX] phases were generated. Similarly, the final thickness of the deposit will be dependent on the charge injected in the material during the production of [Ptn-, M+, MX] phases. Besides the interest in potential applications for the development of such interfaces, our goal (12) (a) AFM techniques has been extensively used for the characterization of organic layers produced by reduction of phenyldiazonium salts, see for example refs 11b,c. (b) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. (c) Liu, S.; Tang, Z.; Wang, E.; Dong, S. Langmuir 1999, 15, 7268. (d) Brooksky, P. A.; Downard, A. J. Langmuir 2004, 20, 5038. (13) (a) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947. (b) Ortiz, B.; Saby, C.; Champagne, G. Y.; Be´langer, D. J. Electroanal. Chem. 1998, 455, 75. (c) Saby, C. Ortiz, B. Champagne, G. Y.; Be´langer, D. Langmuir 1997, 13, 6805. (d) Solak, A. O.; Eichorst, L. R.; Clark, W. J.; McCreery, R. L. Anal. Chem. 2003, 75, 296. (14) (a) This was also observed when the grafting occurs on carbon nanotubes.13b (b) Marcoux, P. R.; Hapiot, P.; Batail, P.; Pinson, J. New. J. Chem. 2004, 28, 302. (15) (a) It is noticeable that under exposure of the surface to a solution containing an aryldiazonium salt, the grafting of aryl groups was observed without electrochemical induction; see, for example, onto negatively charged carbon,15b,c carbon nanotubes15d,e and some other metallic surfaces.15f To explain this observation, a similar mechanism was proposed (see Scheme 1) in which the diazonium salt is reduced by thermal electron transfer at the open circuit potential of the substrate material in solution. Since the potential necessary to reduce a diazonium salt is quite positive (around 0 V/SCE), such nonstimulated reduction is unlikely to occur with noble metals such as Pt. (b) Belmont, J. A. U.S. Patent 5,554,739, 1996. (c) Combellas, C.; Kanoufi, F.; Mazouzi, D.; Thie´bault, A.; Bertrand, P.; Me´dard, N. Polymer 2003, 44, 19. (d) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (e) Dyke, C. A.; Tour, J. M. J. Am. Chem. Soc. 2003, 125, 1156. (f) Kosynkin, D. V.; Yang, J.; Tour, J. M. Mater. Res. Soc. Symp. Proc. 2001, 660, JJ3.5.1.

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was also to improve the basic knowledge about the reactivity of these unconventional “reduced” platinum phases. For comparison purposes, we also reexamined in more detail the direct electrochemical grafting under reductive electrochemical conditions on platinum, since this process has been characterized only from cyclic voltammetry studies.10 We performed a series of electrochemical atomic force microscopy (EC-AFM) experiments2 that allow in situ investigations of the electrochemical grafting. Finally, we present an other approach that leads to the immobilization of organic moieties using the reaction between the modified platinum (obtained by cathodic treatment of Pt surface in the presence of supporting electrolyte in dimethylformamide) and the diazonium salt without external applied electric potential, thanks to the reductive properties of the [Ptn-, M+, MX] phases.15 Experimental Section Chemicals. Sodium iodide (NaI) and tetrabutylammonium tetrafluoroborate (NBu4BF4) from Fluka (electrochemical grade) were used as supporting electrolytes without further purification. Dry N,N-dimethylformamide (DMF, puriss) was purchased from Fluka and stored over molecular sieves. The two diazonium salts (4-nitrophenyldiazonium and 4-bromophenyldiazonium tetrafluoroborate) were purchased from Acros and used as received. Electrochemical and EC-AFM Experiments and Procedures. The electrochemical EC-AFM cell was a three-electrode type and was previously described.2 The geometry of the cell was carefully designed to minimize the ohmic drop and improve the homogeneity of the modification on the surface sample.2 In situ investigations on surfaces with EC-AFM require well-defined and flat platinum substrates that can be used as electrode and sample for the AFM imaging. For this goal, we used (100) oriented Pt epitaxial thin films (thickness in the range of 50 nm), prepared by dc sputtering onto (100)MgO single-crystal substrate (5 × 5 mm2).16 We have previously shown that this type of structured sample allows clean observation and control of the surface electrode by AFM.2 The direct grafting of aryl compounds after cathodic modification of Pt was also performed with massive platinum disk electrodes (1 mm diameter) or platinum foils (5 × 5 mm2) purchased from Goodfellow. The shape of cyclic voltammograms (as in Figure 2) is distorted when an electrode with a too-large surface area is used. Curves with better resolution and higher reversibility are obtained when the same treatment is performed on a 1 mm diameter disk electrode. Platinum disk electrodes, platinum foils, and platinum deposits onto (100)MgO substrate were cleaned in an ultrasonic bath in DMF before any experiments to remove dust impurities. The topography of the modified Pt after grafting was characterized by contact-mode AFM on a PicoSPM II from Molecular Imaging (Tempe, AZ) with the integrated potentiostat. For classical cyclic voltammetry experiments, a conventional three-electrode cell was also used for electrochemical analyses and electrochemical modification of the platinum foils. The potentiostat was a PAR potentiostat/ galvanostat model M 273 (EG&G Princeton Applied Research). An Ag/AgNO3 electrode system and a platinum wire were used as reference electrode and counterelectrode. The potentials were checked versus the ferrocene/ferrocenium couple (taken as E° ) 0.400 V/SCE) and potentials were scaled versus the SCE electrode. All solutions were deoxygenated by bubbling argon gas for 15 min before the experiment. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo VG Escalab 250 instrument equipped with a monochromatic Al KR (hν ) 1486.6 eV) 200 W X-ray source. The X-ray spot size was 650 µm. The pass energy was set at 150 and 40 eV for the survey and the narrow scans, respectively. Additional high-resolution C1s regions were recorded with pass energy of 10 eV. For microstructured platinum substrates deposited on (100)MgO, charge compensation was achieved with a combination of electron and argon ion flood guns. The energy and emission current of the electrons were 4 eV and 0.35 mA, (16) Ducle`re, J. R.; Guilloux-Viry, M.; Perrin, A.; Cattan, E.; Soyer, C.; Re`miens, D. Appl. Phys. Lett. 2002, 81, 2067.

Ghilane et al. respectively. For the argon gun, the energy and the emission current were 0 eV and 0.1 mA, respectively. The partial pressure for the argon flood gun was 2 × 10-8 mbar. These standard conditions of charge compensation resulted in a negative but perfectly uniform static charge. Data acquisition and processing were achieved with Avantage software, version 1.85. Spectral calibration was determined by setting the main C1s component at 285 eV.17 The surface composition was determined by use of the manufacturer’s sensitivity factors. The fractional concentration of a particular element A (% A) was computed as

%A)

(IA/sA)

∑(I /s ) n

× 100

n

where In and sn are the integrated peak areas and the sensitivity factors, respectively.

Results and Discussion Direct Electrochemical Reduction of Diazonium Salts on Platinum Surfaces and EC-AFM Investigations. As explained in the Introduction, the grafting of aryl films to electrode surfaces through the electrochemical reduction of the corresponding aryldiazonium salts has been reported for a large variety of surface materials.4,7-14 Concerning the platinum, investigations are scarce, and to the best of our knowledge, the only report for this material is the cyclic voltammetry of a Pt electrode in a pure electrolyte solution showing the presence of an electroactive layer after the electrochemical reduction of 4-nitrophenyldiazonium.10 This experiment suggests that electrochemical grafting is possible on this metal, but for comparison purposes, it was useful to examine in more detail the direct electrochemical reduction of diazonium salts on Pt electrode. The evolution of the electrode surface during the reduction of 4-nitrophenyldiazonium salts was followed by in situ EC-AFM. A microstructured platinum substrate with low surface roughness was used as both a working electrode for the grafting and a sample for AFM analysis (see Experimental Section).2 Figure 1 shows some typical AFM topographic images of the Pt electrode, recorded under air and in solution, before and after the electrochemical grafting. The AFM image of the Pt sample (Figure 1a) shows the topography of the grown platinum electrode. One can observe dark areas representing holes and bright areas corresponding to the platinum terraces. This particular topography is due to the epitaxial growth of the Pt film onto the (100)MgO substrate and can be adjusted by careful control of the experimental conditions during the plasma deposition.16 There are two main interests in such a sample: first, the platinum areas are very flat (roughness lower than 1 nm), and second, their special patterns create useful marks to follow any surface changes. As seen in Figure 1b, after the sample was immersed in a dry DMF solution containing 0.01 mol‚L-1 4-nitrophenyldiazonium tetrafluoroborate and 0.1 mol‚L-1 NBu4BF4 for more than 1 h, AFM investigations did not evidence any modification of the surface topography. Comparison of Figure 1 panels a and b clearly indicates that the Pt layer is stable under these experimental conditions and that no spontaneous grafting reaction occurs. (This conclusion is confirmed by XPS analysis; see below.) Then a potential more negative than the reduction potential of 4-nitrophenyldiazonium was applied during 100 s to produce the aryl radicals. AFM images were immediately recorded in the solution. We checked on different surface areas that the same type of topography (17) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers, the Scienta ESCA300 Database; John Wiley & Sons: Chichester, U.K., 1992.

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Figure 2. Cyclic voltammetry in a DMF solution containing only a supporting electrolyte (0.1 mol‚L-1 NBu4BF4) of a (100)Pt sample (5 × 5 mm2) electrode after reduction of 4-nitrophenyldiazonium salts in DMF and cleaning of the electrode. Scan rate 0.1 V/s.

Figure 1. Direct electrochemical reduction of aryldiazonium on platinum surfaces: in situ EC-AFM images of a (100)Pt surface [Pt deposited on 100(MgO)]. (a) Pt substrate under air. (b, c) Pt substrate in a DMF solution containing 0.01 mol‚L-1 4-nitrophenyldiazonium tetrafluoroborate and 0.1 mol‚L-1 NBu4BF4 before any modification (b) and after electrolysis (c) at -0.8 V/SCE for 100 s. Charge passed for the modification Q ) 3.6 mC. (d) After exposure of the sample under air during one month. All image scans sizes are 3 × 3 µm2.

was visible, indicating homogeneous modification of the sample. As seen in Figure 1c, one can observe that small grains corresponding to the grafting of the aromatic molecules are formed only on the platinum terraces. Grains display diameter around 50 nm (measured by AFM on the lateral size) and height size around 5 nm, indicating that the electrochemical process results in films much thicker than one single monolayer. If we consider that the size of a nitrophenyl group is around 6.5 angstroms (calculated from a simple estimation of the equivalent sphere radius by AM1 semiempirical calculations), we can estimate that about 7-8 nitrophenyl moieties constitute the deposit layer. The same sample was then carefully rinsed in DMF in an ultrasonic bath for 20 min to ascertain that any organic material only deposited on the surface would be removed.4 A convenient way to detect the presence of the coating is examination of the modified electrode by cyclic voltammetry in a pure electrolyte solution.4 Due to the presence of electroactive nitro groups on the aromatic rings, the observation of an electrochemical signal is a signature that proves the presence of the organic layer.4 As seen in Figure 2, a reversible system characteristic of a nondiffusive species at a reduction potential very close to that of the reduction wave in nitrobenzene solution is detected, indicating that nitrophenyl groups are immobilized on the electrode. Another point that is worth outlining is the remarkable stability of the system after long time exposure to air. After the electrochemical characterization, the sample was carefully cleaned and kept in a simple plastic box under air. Reexamination of the sample by AFM shows that the small grains observed in Figure 1c were still visible (see Figure 1d) after 1 month of exposure under air. This sample was again put in an ultrasonic bath for 10 min in DMF and then reexamined by AFM, and we kept observing the same AFM images as those displayed in Figure 1c,d. All these results prove that the 4-nitrophenyl groups are strongly attached to the platinum surface and require mechanical abrasion to

be removed, suggesting a covalent bonding of the aryl group to the surface.4 The electrochemical studies were complemented by a similar series of experiments followed by XPS analysis of the sample. Comparing the XPS spectra of the platinum surface before and after electrochemical reduction of the 4-nitrophenyldiazonium salts in DMF evidences three major features: (i) The platinum signal is strongly attenuated after electrochemical reduction of the 4-nitrophenyldiazonium salt (from 31 at. % to 0.2 at. %), indicating the covering of the platinum surface. (ii) The carbon signal increases from 46 at. % to 69 at. %, indicating the adsorption of an organic layer. (The presence of carbon on the platinum surface before the electrochemical reduction of the diazonium salt is due to surface contamination.) (iii) A new peak appears at 406 eV, assigned to the N1s signal of the NO2 groups. Another component of the N1s signal is also observed at 400.3 eV. This may be due both to some reduced nitrogen (possibly from NO2 group reduction under the X-beam) and to the surface contamination of the platinum sample (this signal is visible on blank before any treatment).18 These results confirm the presence of the grafted organic layer on the platinum surface. As another example, the same series of experiments were carried out with the 4-bromophenyldiazonium salt. The same general trends as for 4-nitrophenyldiazonium salts were observed for this compound, except that the occurrence of the grafting is evidenced by the specific response of Br atoms. Indeed, the XPS spectrum of the platinum surface after reduction of the 4-bromophenyldiazonium salt exhibits the characteristic signal of Br3p19 at 183.6 eV (Br3p3/2) and 190.8 eV (Br3p1/2), showing that bromophenyl groups are immobilized onto the platinum surface. The Br3p signal has split into two components, with a shoulder (20% of the total Br3p signal) appearing on the low-energy side (at 182 eV) of the major peak. This last component may due to some bromide ions coming from the breaking of the C-Br bond after the electrochemical process or under the X-beam.18 For comparison, when the platinum electrode was first immersed in a solution of DMF + 0.1 mol‚L-1 Bu4NBF4 (18) (a) Mendes, P.; Belloni, M.; Ashworth, M.; Hardy, C.; Nikitin, K.; Fitzmaurice, D.; Critchley, K.; Evans, S.; Preece, J. Chem. Phys. Chem. 2003, 4, 884. (b) Adenier, A.; Cabet-Deliry, E.; Chausse´, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491. (19) The characteristic Br3d signal at 71 eV cannot be unambiguously ascribed since the Pt4f signal is located in the same binding energy range (70.7 eV).

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Figure 3. Cyclic voltammetry on a 1 mm diameter Pt disk electrode in a dry DMF solution containing 0.1 mol‚L-1 NaI. Scan rate 0.1 V/s.

containing 0.01 mol‚L-1 4-bromophenyldiazonium salt without applying any electrochemical treatment, the platinum surface did not show any Br signal. This experiment confirms our AFM studies in solution, showing that no spontaneous grafting reaction occurs on a nontreated platinum electrode, contrary to what is reported for easily oxidable materials.15b-f Reactivity of Cathodically Modified Pt Surface with Aryldiazonium. In this second part, we investigate the reductive properties of [Ptn-, M+, MX] phases for activating the grafting of diazonium salts in the absence of an externally applied potential. Two sets of experiments were performed, one with a massive platinum electrode and a second one with a microstructured Pt layer deposited on (100)MgO similar to the one used in Figure 1. Massive platinum electrodes allow the injection of large and different quantities of charges, as they are not limited by the thickness of the platinum layer. The second type of substrate has the advantage of being sufficiently flat to allow in situ investigation of the processes in solution by EC-AFM but impedes large charge injection to avoid the total reduction of the Pt layer and its destruction.2 As a supporting electrolyte, we chose NaI because the potential required for the formation of the corresponding reduced phase that depends on the nature of the supporting electrolyte is not too negative. It ensues that the reduced Pt phase is sufficiently stable for the modified sample to be easily handled. [The modified surface evolves only after a few hours under argon (or vacuum) or after a few minutes under ambient air.] Figure 3 displays a typical cyclic voltammogram recorded on a solid platinum disk electrode in a dry and deoxygenated DMF solution containing 0.1 mol‚L-1 sodium iodide (NaI). The voltammogram is similar to those previously reported1 and shows a quasi-reversible redox system. On the basis of detailed electrochemical investigations,1 the electrochemical reduction peak (for NaI, Ep(NaI) ) -1.7 V/SCE at a scan rate of 0.1 V‚s-1) was previously ascribed to the reversible formation of the metal-organic phase [Pt2-, Na+, NaI]. Previous electrochemical investigations and quartz microbalance have shown that the formula of the reduced phase corresponds to one electron per two platinum and that the charge is compensated by one sodium cation [Pt2-, Na+, NaI].1 Even if the peak potentials are not thermodynamic data, the value of Ep(NaI) indicates that [Pt2-, Na+, NaI] should react with reducible molecules such as aryldiazonium salts that display a reduction peak around 0.0 V/SCE.4 The procedure consists first of the electrogeneration of [Pt2-, Na+, NaI] and then of putting the modified sample in a solution containing the diazonium salt. For the first step on a solid platinum disk electrode, a typical sequence consisted of applying a potential step at -1.7 V/SCE during 100 s (the injected charge was followed by integration of the reductive current). Immediately after the Pt modifi-

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Figure 4. Cyclic voltammetry in a pure electrolyte solution (DMF + 0.1 mol‚L-1 NBu4BF4) of a 1 mm diameter Pt disk electrode after cathodic modification (0.18 C) followed by a 10 min immersion in a DMF solution containing 4-nitrophenyldiazonium salts. (a) Scan rate 0.1 V‚s-1. (b) Scan-rate dependency of the cathodic peak current for the 4-nitrophenyl grafted electrode.

cation, the electrode was quickly transferred into a deoxygenated DMF solution containing 0.1 mol‚L-1 4-nitrophenyldiazonium tetrafluoroborate without applying any external potential. The system was left to react for 10 min under argon to allow sufficient time for the reaction to be completed. (However, electrochemical examination performed a short time after exposure of the sample to the aryldiazonium solution suggests that the reaction is almost instantaneous.) Then the electrode was thoroughly rinsed several times with DMF and sonicated in DMF for 30 min, to remove the organic material that is only weakly bound on the platinum surface. Finally, the electrode was transferred to a pure electrolyte solution (DMF + 0.1 mol‚L-1 NBu4BF4) and examined by cyclic voltammetry as we did in the preceding part (Figure 4a). The same broad reversible system as the one displayed in Figure 2 is visible (Epc ) -1.0 V/SCE and Epa ) -0.91 V/SCE) at a potential very close to that of nitrobenzene itself.4 A linear dependence of the current versus the scan rate is also observed in the 0.05-1 V‚s-1 range (Figure 4b), which is indicative of a process where the electroactive species do not diffuse to and from the electrode20 and thus demonstrates that the nitroaryl species coming from the 4-nitrophenyldiazonium are grafted onto the platinum electrode surface. As we noticed for the direct electrochemical reduction experiments, the electrochemical signal of the nitrophenyl groups does not disappear after ultrasonic cleaning of the electrode in DMF, confirming that the attachment between the nitrophenyl groups and the platinum surface is strong enough to resist extensive rinsing in an ultrasonic bath. Taking into account that the reduction of the NO2 aryl group in DMF is monoelectronic, the coulometric integration of the cathodic peak provides an estimation of the aryl group surface concentration on the platinum surface (Γ in moles per square centimeter). This data treatment assumes that the NO2 groups remain available and active and also that the considered geometric surface is not too different from the real one. We plot in Figure 5 the surface coverage (derived as integration of the peak current of the grafted nitrophenyl) as a function of the quantity of the injected charge used to cathodically modify the platinum electrode (electrogeneration of [Pt2-, Na+, NaI]) in the experimental conditions of Figure 4a. Even if absolute values obtained by this method are relatively inaccurate because the geometric surface is certainly lower than the effective one, the dependency remains valid to follow the evolution of the coverage. We observed that the NO2 surface concentration first increases (20) Bard, A. J.; Faulkner, L. R. Electrochemical methods. Fundamentals and applications, 2nd ed.; John Wiley and Sons: New York, 2001.

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Figure 5. Surface concentration as a function of the charge injection used to modify the 1 mm diameter Pt disk electrode. Scheme 2. Schematic Mechanism Involved in Activation of Grafting of Phenyldiazonium Salts from Cathodically Modified Platinum Surface, Showing the Competition between Grafting and NO2 Reduction of the Already Immobilized Layers Figure 6. Reaction of [Pt2-, Na+, NaI] with 4-nitrophenyldiazonium salt: EC-AFM images of a (100)Pt sample [Pt deposited onto (100)MgO] in DMF. (a) Initial sample. (b) After cathodic modification (passage of 15 µC/cm2). (c) After addition of 0.01 mol‚L-1 4-nitrophenyldiazonium tetrafluoroborate. (d) Reexamination of the sample in a blank solution and after several reduction-oxidation cycles. Image scans size 3 × 3 µm2. Panels a-c were obtained in a deoxygenated DMF solution containing 0.1 mol‚L-1 NaI.

for injected charge up to 0.2 C‚cm-2 and then starts decreasing as the amount of charge injected to modify the platinum increases. The maximum of surface coverage of active NO2 is in the range of (4-5) × 10-9 mol‚cm-2. For charge injection larger than 0.32 C‚cm-2, the surface concentration of nitroaryl groups decreases to values about 2 × 10-9 mol‚cm-2. Similarly to what we concluded above for the direct electrochemical grafting, these Γ values seem too large when compared with expected value for a monolayer (Γ on the order of 10-10 mol‚cm-2), indicating that phenyl multilayers are formed onto the platinum surface. To explain the maximum, we should consider, in parallel with the grafting mechanism represented in the two first reactions of Scheme 2, the possibility of reduction of the NO2 groups by [Pt2-, Na+, NaI]. The reduction of NO2 will lead first to a radical anion that will evolve on a longer time scale to the irreversible formation of nonelectroactive products (in that range of potential).21 The observation of a maximum in Figure 5 suggests that the competition between the two processes varies during the grafting. At the beginning of the process, there is no nitrophenyl group on the Pt surface, and by comparison with NO2, the reduction of the diazonium moieties on a clean surface is favored by more than 0.8 eV (on the basis of the respective reduction potentials). Then when the aromatic layer appears, the reduction of the diazonium salt becomes more (21) The reduction of nitro compounds may lead to different groups (amine, hydroxylamine, ...) depending on the quantity of residual water and reaction time.

difficult as the electrons have to pass through the organic layer to reach the molecules in solution. This phenomenon that corresponds to a blocking effect of the electrode surface is well-known with aryldiazonium salts. It is commonly observed that the formation of the layer by direct electrochemical reduction of phenyldiazonium is accompanied by a rapid decrease of the electrochemical signal. (For example, during cyclic voltammetry experiments, the peak potential corresponding to the diazonium reduction rapidly vanishes after 1-3 cycles).4,7c Blocking effects on nitrophenyl-modified carbon electrodes have also been reported for the oxidation/reduction of several molecules present in solution for which the electrontransfer kinetics decrease significantly.13b-d The blocking effect will thus favor reduction of the NO2 group contained in the already grafted groups, allowing this process to compete with growth of the layer. The second series of experiments was performed on the microstructured platinum deposited on (100)MgO substrate (similar to the one used in the preceding part) in order to follow the cathodic modifications of platinum and its reaction with aryldiazonium by EC-AFM. As a preliminary experiment, immersion of the platinum electrode in a solution of DMF containing only the supporting electrolyte NaI shows no modification by comparison with the neat platinum sample (Figure 6a).2 When we started to apply cathodic polarization, small grains appeared on the platinum terrace (roughness of terrace around 10 nm; see Figure 6b). We limited the quantity of the injected charge to a low value to create only small modifications that remain compatible with good AFM imaging. This modification is indicative of the formation of metalorganic phases [Pt2-, Na+, NaI].2 As shown before, the amplitude of the modification depends directly on the injected charge quantity. If the charge injection is increased, areas without platinum are detected. This observation indicates that the adhesion between the new phase and the substrate (MgO) is certainly too low to maintain the deposit on the substrate and thus the

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platinum layer starts to disaggregate. In those conditions, when the applied potential is returned to a positive value, the initial structure of the sample is rapidly recovered and the sample can be reduced and reoxidized several times without noticeable changes in the recovered morphology, confirming that the formation of [Pt2-, Na+, NaI] is reversible. After exposure of the modified Pt sample under air, the structure of the starting platinum is recovered due to the reoxidation of [Pt2-, Na+, NaI] areas by O2. (See ref 2 for typical reduction/reoxidation experiments followed by EC-AFM.) After the formation of [Pt2-, Na+, NaI], a new solution containing 4-nitrophenyldiazonium in DMF is added in the cell (the electrode is disconnected) and the surface is examined by AFM. As seen in Figure 6c, smaller spheres are now visible and the calculated roughness of the platinum terraces decreases to 5 nm. It is noticeable that this topography and the one obtained after direct reduction of 4-nitrophenyldiazonium salt on the platinum electrode (compare Figures 1c,d and 6c,d) are very similar. Similarly, no modification of the new morphology occurs when a positive potential is applied (Figure 6d, roughness of platinum terrace 5 nm). This observation confirms that the deposit is now stable versus oxidation, contrary to [Pt2-, Na+, NaI], which is reversibly reoxidized under such conditions. Thus, the layers obtained before and after exposure of [Pt2-, Na+, NaI] to the 4-nitrophenyldiazonium salt solution present completely different chemical natures. At the end of the experiment, the sample was rinsed and examined in a solution containing only the supporting electrolyte. Cyclic voltammetry of the sample reveals the characteristic peaks of reduction of the immobilized nitroaryl groups (The general patterns of the cyclic voltammetry are the same as those observed after direct electrochemical grafting; see Figure 2.) Consequently, all these experiments clearly indicate that nitrophenyl groups have been attached on the Pt surface after reduction of the 4-nitrophenyldiazonium by [Pt2-, Na+, NaI]. At this point, it is interesting to notice that if the reduction power of [Pt2-, Na+, NaI] is negative enough to reduce the diazonium salt and the NO2 groups, it should certainly be able to reduce the generated aryl radical to its carbanion.22 Considering that the key reaction for formation of the first layer is reaction of the generated aryl radical with surface material, it is therefore surprising that the grafting occurs, as a reaction of a phenyl anion seems very unlikely in the case of Pt. This observation is not limited to our work. Several similar situations where the grafting is performed at very negative potential23 or indirectly with strong reducing species such as doped carbon materials have been reported in the literature.15c Considering possible formation of a phenyl anion, the reduction of the radical requires a second electron transfer from another [Pt2-, Na+, NaI] that is in competition with addition of the radical to the regenerated metallic Pt surface (the same is true in ref 15c with negatively doped carbon surfaces). Because we observed the grafting, we can simply conclude that the addition is faster. At this point and besides the experimental evidence, it remains difficult to get a clear-cut answer to this general and puzzling question that concerns the general reactivity of aromatic radicals with surface materials. The explanation for this phenomenon is not clear, but as was suggested (22) The standard redox potential of the radical/anion couple E°(Ph•/Ph-) was determined as 0.05 V/SCE: Andrieux, C. P.; Pinson, J. J. Am. Chem. Soc. 2003, 125, 14801. (23) Bureau, C.; Levy, E.; Viel, P. PCT Int. Appl. WO 03018212, 2003.

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Figure 7. XPS spectrum of platinum foils (5 × 5 mm2) after electrochemical formation of [Pt2-, Na+, NaI] followed by exposure to a solution of 4-bromophenyldiazonium and cleaning (see text). The spectrum displays the Br3p peaks of bromophenyl groups attached to a platinum surface.

before,23 a possible explanation may be very slow electrontransfer kinetics. In our case, we may also notice that the reducing agent [Pt2-, Na+, NaI] is not present in the close vicinity of the aryl radical after reduction of the diazonium because the metallic Pt is regenerated. This proximity of the radical with the metallic Pt should favor the addition of the radical instead of its reduction. Reaction of [Pt2-, Na+, NaI] with 4-Bromophenyldiazonium: XPS Analyses. The same route as described above was carried out with 4-bromophenyldiazonium on the platinum foils (5 × 5 mm2) (electrochemical formation of [Pt2-, Na+, NaI] and then addition of diazonium without external potential). After extensive rinsing and cleaning of the modified electrode in DMF for 20 min in an ultrasonic bath, the sample was examined by XPS spectroscopy. The spectrum shows signals corresponding to Br 3p3/2 and 3p1/2 at 183.6 and 190.8 eV, respectively (Figure 7). Furthermore, the curve of the Br3p signal intensity displays a maximum when plotted against the charge injected. This evolution is very similar to what was observed for NO2 surface concentration in the previous section, suggesting again a competition between reduction of the 4-bromophenyldiazonium and reduction of the bromophenyl substituent, leading to the breaking of some C-Br bonds. Conclusion As illustrated here in the case of [Pt2-, Na+, NaI], metalorganic platinum phases of general formula [Ptn-, M+, MX] display strong reductive properties that can be used to locally induce electrochemical grafting reactions such as the reduction of aryldiazonium salts. Cyclic voltammetry, X-ray photoelectron spectroscopy, and in situ ECAFM experiments show that the reaction results in the irreversible formation of strongly bound organic multilayers. Reduction of the aryldiazonium salts by [Pt2-, Na+, NaI] as reducing agent produces aryl radicals and concomitantly regenerates the platinum metal, allowing the reaction between aryl radicals and the metallic surface. Interestingly, the thickness of the layer can be adjusted through the quantity of charge injected in the platinum during its cathodic modification. The excess of charge can also create side reactions by reduction of the layer for which the incidence is also tunable through the initial quantity of injected charge. More generally, it is noticeable

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that similar phases (analogous to the Zintl phases24) have been reported for other metallic surfaces (Pd, Ni, ...)25 and that their reducing power can be adjusted by the choice of the supporting electrolyte (MX). Using the same procedure, we have already obtained the formation of similar organic layers with different supporting electrolytes MX. Consequently, it should be possible to extend the procedure presented here for the controlled preparation of several metal-organic and structured interface (24) (a) Corbett, J. D. Chem. Rev. 1985, 85, 383. (b) Scha¨fer, H.; Eisenmann, B. Rev. Inorg. Chem. 1981, 3, 29. (c) Kauzlarich, S. M. Chemistry, Structure and Bonding of Zintl phases and Ions; VCH Publishers: New York, 1996. (25) Cougnon, C.; Simonet, J. Electrochem. Commun. 2001, 507, 226.

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types based on the immobilization of aryl groups onto metallic surfaces. Acknowledgment. We gratefully acknowledge the community of Rennes Metropole for its financial support of the scanning probe microscopy. We also thank Dr. J. Simonet (UMR 6510, Universite´ de Rennes 1) for his valuable help and discussions about the electrochemical Pt modifications and Louis Pacheco and the companies Scientec and Molecular Imaging for their help in the setting up of the EC-AFM equipment in organic solvents. LA050401Y