Organic Layers Bonded to Industrial, Coinage, and Noble Metals

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Chem. Mater. 2003, 15, 3450-3462

Organic Layers Bonded to Industrial, Coinage, and Noble Metals through Electrochemical Reduction of Aryldiazonium Salts Marie-Claude Bernard,| Annie Chausse´,§ Eva Cabet-Deliry,† Mohamed M. Chehimi,‡ Jean Pinson,*,† Fetah Podvorica,† and Christine Vautrin-Ul§ Laboratoire d’Electrochimie Mole´ culaire, Universite´ Paris 7-Denis Diderot, Unite´ Mixte de Recherche Universite´ Paris 7-CNRS 7591, 2 Place Jussieu, 75251 Paris Cedex 05, France, ITODYS, Universite´ Paris 7-Denis Diderot, Unite´ Mixte de Recherche Universite´ Paris 7-CNRS 7086, 1 rue Guy de la Brosse, F-75005 Paris, France, Laboratoire Analyse et Environnement de l’Universite´ Evry-Val d’Essonne, Unite´ Mixte de Recherche Universite´ d’Evry-CNRS-CEA 8587, Universite´ d’Evry, rue du Pe` re Jarland, 91025 Evry, France, and Laboratoire de Physique des Liquides et Electrochimie, Unite´ Propre de Recherche CNRS 15, Conventionne´ avec l’Universite´ Pierre et Marie Curie, 4 Place Jussieu, F-75004 Paris, France Received March 17, 2003. Revised Manuscript Received June 17, 2003

The reduction of diazonium salts in an aprotic medium permits the attachment of substituted aryl groups to a variety of metals: Co, Ni, Cu, Zn, Pt, and Au. These aryl groups are strongly bonded to the metal as they resist sustained rinsing under sonication in organic solvents. The organic layers have been characterized by cyclic voltammetry, infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, Rutherford backscattering, electrochemical impedance spectroscopy, and atomic force microscopy. From these data it is possible to propose a structure for these grafted layers.

Introduction Attachment of organic layers to metals is an important process which permits protection of the metal from the environment, but also provides particular properties to the surface. Therefore, metal coating with paints, polymers, and so forth is an industrial process of wide application and there exists a large variety of methods for this purpose.1-3 In these cases, only weak bonds are formed between the coating and the organic layer. The formation of stronger bonds between the metal and the organic layer has been achieved either from the metal itself or from surface oxides. For example, alkanesilanes form very stable monolayers on aluminum oxides. The headgroup of the molecule, a trichloro- or trialkoxysilane, forms a covalent bond with surface OH.4-7 This * To whom correspondence should be addressed. † Unite ´ Mixte de Recherche Universite´ Paris 7-CNRS 7591. ‡ Unite ´ Mixte de Recherche Universite´ Paris 7-CNRS 7086. § Unite ´ Mixte de Recherche Universite´ d’Evry-CNRS-CEA 8587. | Unite ´ Propre de Recherche CNRS 15. (1) Plankaert, R. Surface Coating. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Weinheim, 1994; Vol. A25, p 170. (2) Biethan, U.; Funke, W.; Hoppe, L.; Hasselkus, J.; Curtis, L. G.; Hoehne, K.; Zech, H.-J.; Heiling, P.; Yamabe, M.; Do¨ren, K.; Schupp, H.; Ku¨chenmeister, R.; Schmitthenner, M.; Kremer, W.; Wieczorrek, W.; Gempeler, H.; White, J. W.; Short, A. G.; Blank, W. J.; Calbo, L. J.; Plath, D.; Wagner, F.; Haller, W.; Ro¨dder, K.-M.; Streitberger, H.J.; Urbano, E.; Laible, R.; Meyer, B. D.; Bagda, E.; Waite, F. A.; Philips, M.; Ko¨hler, K.; Simmendinger, P.; Roelle, W.; Scholz, W.; Kortmann, W.; Valet, A.; Slongo, M.; Molz, T.; Hiller, R.; Thomer, K. W.; Vogel, K.; Schernau, U.; Hu¨ser, B.; Brandt, A.; Milne, A.; Weyers, H.; Plehn, W.; Lentze, H.-A. Paints and Coatings. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Weinheim, 1991; Vol. A18, p 359. (3) Hochberg, S. Industrial Coatings. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley: New York, 1979; p 427. (4) Polymeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978, 69, 1836. (5) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 2.

process has found industrial applications as a possible substitute for chromate treatments.6,8,9 Concerning the direct attachment to the metal itself, self-assembled monolayers (SAMs) of thiols on gold10,11 are probably the most popular examples. The success of this process is due to the ease of preparation (dipping the gold surface in a dilute solution of thiol) and to the formation of self-assembled monolayers. However, the details of the interaction between the thiol headgroup and the gold surface remain uncertain to some extent and the bond is not very strong. The heat of adsorption of dimethyl sulfide was measured in absence of any solvent; ∆Hads ) -117 kJ/mol12 and ∆Gads ) - 21 kJ/ mol were measured for octanethiol13 in hexane. It is of the order of liquid alcohol hydrogen bonds, therefore not very strong. Self-assembled monolayers of thiols have also been observed on copper,14 silver,15 and mercury.16 (6) Ro¨sch, L.; Peter, J.; Reitmeier, R. Silicon Compounds. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Weinheim, 1994; Vol. A24, p 21. (7) Ogarev, V. A.; Selector, S. L. Prog. Org. Coat. 1992, 31, 135. (8) Kayser, W. Surf. Eng. 2001, 17, 305. (9) Schubach, P. Galvanoteknik 2001, 92, 1825. (10) Self-Assembled Monolayers of Thiols, Thin Films; Ulman, A., Ed.; Academic Press: San Diego, 1998; Vol. 24. (11) Finklea H. O. J. Electroanal. Chem. 1996, 19, 109. (12) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 793. (13) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (14) (a) Vondrak, T.; Wang, H.; Winget, P.; Cramer, C. J.; Zhu X.Y. J. Am Chem Soc. 2000, 122, 4700. (b) Epple, M.; Bittner, A. M.; Kuhnke, K.; Kern, K.; Zheng, W. Q.; Tadjeddine, A. Langmuir 2002, 18, 773. (c) Kondo, H.; Saito, N.; Matsui, F.; Yokoyama, T.; Kuroda, H. J. Phys. Chem. B 2001, 105, 12810. (15) (a)Compagnini, G.; Galati, C.; Pignataro, S. Phys. Chem. Chem. Phys. 1999, 1, 2351. (b) Burleigh, T. D.; Shi, C.; Killic, S.; Kovacic, S.; Thompson, T.; Enick, R. M. Corrosion 2002, 58, 49.

10.1021/cm034167d CCC: $25.00 © 2003 American Chemical Society Published on Web 08/09/2003

Electrochemical Reduction of Aryldiazonium Salts

But their affinity to engineering metals such as steel is very limited.17 An interesting and very efficient way of attaching polymers to iron surfaces has been described by Van Alsten;18 self-assembled monolayers of alkyl-R,ωbisphosphonic acids on common engineering metals could be prepared. It was then possible to establish ionic bonds between the free phosphonate end, Zn2+, and a carboxylic-terminated tetrafloroethylene. This method has permitted the construction of metal/SAM/polymer assemblies of surprising durability, which provided an efficient protection of iron against corrosion. The attachment of polymers directly onto metallic surfaces is possible through electrochemical methods. The electrochemical reduction of an activated vinylic monomer on a metal (nickel, iron, etc.) leads to the covalent attachment of a polymer to an iron surface.19-30 This process developed by the CEA (Commisariat a` l’Energie Atomique) involves a radical anion obtained under anhydrous conditions; it is the species responsible for the reaction with the metal and for the propagation of the polymerization. A thin layer (2-5 µm)26 of polymer is covalently attached to the surface. This film as well as its mechanism of formation have been thoroughly investigated.31-34 Polymeric layers bonded to platinum electrodes can also be obtained by electrochemical oxidation of N-vinyl2-pyrrolidone.35,36 Electrooxidation of diamines such as ethylenediamine on a metallic (platinum, gold, or aluminum) electrode furnishes a polyethyleneimine coating. In this case, a carbon-metal bond may be formed (16) (a) Haag, R.; Rampi, M. A.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (b) Holmlin, R. E.; Haag, R.; Chabiync, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (17) Nozawa, K.; Nishihara, H.; Aramaki, K. Corros. Sci. 1997, 39, 1625. (18) Van Alsten, J. G. Langmuir 1999, 15, 7605. (19) Le´cayon, G.; Bouizem, Y.; Le Gressus, C.; Reynaud, C.; Boiziau, C.; Juret, C. Chem Phys. Lett. 1982, 91, 506-510. (20) Deniau, G.; Le´cayon, G.; Viel, P.; Hennico, G.; Delhalle, J. Langmuir 1992, 8, 267. (21) Viel, P.; de Cayeux, S.; Le´cayon, G. Surf. Interface Anal. 1993, 20, 468. (22) Bureau, C.; Defranscheschi, M.; Delhalle, J.; Deniau, G.; Tanguy, J.; Le´cayon, G. Surf. Sci. 1994, 311, 349. (23) Tanguy, J.; Viel, P.; Deniau, G.; Le´cayon, G. Electrochim. Acta 1993, 38, 175. (24) Tanguy, J.; Deniau, G.; Auge´, C.; Zalczer, G.; Le´cayon, G. J. Electroanal. Chem. 1994, 377, 115. (25) Tanguy, J.; Deniau, G.; Zalczer, G.; Le´cayon, G. J. Electroanal. Chem. 1996, 417, 175. (26) Deniau, G.; Le´cayon, G.; Bureau, C.; Tanguy J. In Protective Coatings and Thin Films; Pauleau, Y., Barna, P. B., Eds.; Kluwer Academic: Amsterdam, 1997; pp 265-278. (27) Bureau, C.; Deniau, G.; Viel, P.; Le´cayon, G. Macromolecules 1997, 30, 333. (28) Deniau, G.; Thome, T.; Gaudin, D.; Bureau, C.; Le´cayon, G. J. Electroanal. Chem. 1998, 451, 145. (29) Charlier, J.; Bureau, C.; Le´cayon, G. J. Electroanal. Chem. 1999, 465, 200. (30) Viel, P.; Bureau, C.; Deniau, G.; Zalczer, G.; Le´cayon, G. J. Electroanal. Chem. 1999, 470, 14. (31) Je´roˆme, C.; Geskin, V.; Lazzaroni, R.; Bredas, J. L.; Thibault, A.; Calberg, C.; Bodart, I.; Mertens, M.; Martinot, L.; Rodrigue, D.; Riga, J.; Je´roˆme, R. Chem. Mater. 2001, 13, 1656. (32) Baute, N.; Martinot, L.; Je´roˆme, R. J. Electroanal. Chem. 1999, 472, 83 and references therein. (33) Mertens, M.; Calberg, C.; Baute, N.; Jeroˆme, R.; Martinot, L. J. Electroanal. Chem. 1998, 441, 237. (34) Calberg, C.; Mertens, M.; Je´roˆme, R.; Arys, X.; Jonas, A. M.; Legras, R. Thin Solid Films 1997, 310, 148. (35) Doneux, C.; Caudano, R.; Delhalle, J.; Leonard-Stibbe, E.; Charlier, J.; Bureau, C.; Tanguy, G.; Le´cayon, G. Langmuir 1997, 13, 4898. (36) Calberg, C.; Kroonen, D.; Mertyens, M.; Je´roˆme, R.; Martinot, L. Polymer 1998, 39, 23.

Chem. Mater., Vol. 15, No. 18, 2003 3451 Scheme 1

but no proof has been obtained so far.37,38 We have shown previously that the electrochemical reduction of diazonium salts on carbon surfaces permits creation of a covalent bond between the carbon substrate (glassy carbon, carbon fibers, carbon felts, or HOPG) and a substituted aryl group.39,40 If the reduction is performed on an iron surface, it is also possible to observe the attachment of the aryl group to the substrate.41 This was demonstrated by cyclic voltammetry (CV), IR spectroscopy (IRRAS and PMIRRAS), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), and capacitance measurements (Scheme 1). The attachment of aryl groups can also take place on hydrogenated Si(111),42 the aryl group binding to a Si atom in place of an hydrogen. On carbon, iron, and hydrogenated silicium, the attachment of the organic layer is assigned to the very reactive aryl radical. In this paper we want to show that the reaction of aryldiazonium with conductive surfaces is not restricted to carbon, iron, or silicium but can be extended to a large variety of industrial, coinage, or noble metals. In this paper we have tried to prepare rather thick (several layers) films for further applications. An interesting alternative is the preparation of possibly organized monolayers, which is currently under investigation. The diazonium salts shown in Scheme 2 have been used and the modified surfaces will be termed: CuNO2 and NiCF3, by the symbol of the grafted metal and the substituent of the aryl ring or the aryl ring itself DAQ. The following metals have been investigated: Co, Ni, Cu, Zn, Pt, and Au. Results Cyclic Voltammetry. The voltammograms were recorded on carefully polished electrodes rinsed in degassed acetone and maintained under argon. The (37) Herlem, G.; Goux, C.; Fahys, B.; Dominati, F.; Gonc¸ alves, A.M.; Mathieu, C.; Sutter, E.; Trokourey, A.; Penneau, J. F. J. Electroanal. Chem. 1997, 435, 259. (38) Herlem, G.; Reybier, K.; Trokourey, A.; Fahys, B. J. Electrochem. Soc. 2000, 147, 597. (39) (a) Delamar, M.; Hitmi, R.; Pinson J.; Save´ant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (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) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux J., Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (d) Delamar, M.; De´sarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson J.; Save´ant, J. M. Carbon 1997, 35, 801. (e) Coulon, E.; Pinson, J.; Bourzat, J.-D.; Commerc¸ on, A.; Pulicani, J.-P. Langmuir 2001, 17, 7102. (40) (a) Downard, A. J. Electroanalysis 2000, 12, 1085. (b) Downnard, A. J. Langmuir 2000, 16, 9860.(b) Downnard, A. J.; Prince, M. J. Langmuir 2000, 16, 5581. (41) (a) Adenier, A.; Bernard, M. C.; Chehimi, M. M.; Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 4541. (b) Chausse´, A.; Chehimi, M. M.; Karsi, N.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2002, 14, 392. (42) (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.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 5791. (c) Allongue, P.; Henry de Villeneuve, C.; Pinson, J. Electrochim. Acta 2000, 45, 3241.

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Scheme 2

Table 1. Peak Potentials in Cyclic Voltammetry of Diazonium Salts on Nickel and Gold and Glassy Carbon diazonium salts

Ep V/SCE on a GCa electrode

Ep V/SCE on a nickel electrode

Ep V/SCE on a gold electrode

DNO2 DI DCF3 DC6F13 DC12H25

+0.20 -0.39 -0.05 -0.23 -0.35

0.00 -0.30 0.00 -0.15 -0.35

0.20 -0.10 -0.10 -0.30 -0.40

a

Glassy carbon.

Figure 1. Cyclic voltammograms of DNO2: (a) first scan, (b) second scan, and (c) in absence of diazonium on (A) gold (d ) 1 mm), (B) platinum (d ) 1 mm), and (C) nickel (d ) 3 mm) in ACN + 0.1 M NBu4BF4. c(DNO2) ) 3.0 mM. Reference SCE. Scan rate v ) 0.2 V/s.

voltammograms of DNO2 on the different metals investigated are shown in Figure 1. Similar voltammograms are obtained with other diazonium salts. (Table 1 shows the peak potentials on glassy carbon GCsas a references

Ni and Au.) In every case, the reduction takes place in the range of potentials 0 to -0.5 V/SCE along a broad one electron wave. On the second scan, the wave disappears or becomes very small (as shown in Figure 1). This phenomenon is also observed on carbon39 and iron;41 it provides a good indication that the grafting reaction is taking place. As on carbon,40c reversible systems such as nitrobenzene in solution can still be observed through the layer, albeit with a slower electron transfer than on a bare electrode. With more easily oxidized metals the reduction peak of the diazonium cannot be observed before the oxidation of the electrode. (This is the reason Ni and Au only are shown in Table 1.) When the reduction peak of the diazonium salt can be observed, it does not depend very much on the metal of the electrode as can be observed in Figure 1. After maintaining, with every metal, the potential of the electrode 300 mV after the peak potential measured on glassy carbon for 5 min41b (to increase the thickness of the organic layer as explained in the Introduction) followed by careful rinsing of the electrode in acetonitrile under sonication, the electrodes were transferred in a solution containing only acetonitrile and the supporting electrolyte (ACN + 0.1 M NBu4BF4). The curves which are obtained are shown in Figure 2. They show that electroactive groups have been transferred with the electrode and that their attachment has resisted sustained ultrasonic rinsing, and we have checked that the signals are located at the same potential as nitrobenzene (ZnNO2, NiNO2, CuNO2, CoNO2, AuNO2, PtNO2), anthracene (ZnAn), or anthraquinone (CuAQ) as shown in the Supporting Information (Figure S1, CuAQ, and Figure S2, CuNO2). All the voltammograms are reversible like nitrobenzene itself and one can clearly observe (ZnNO2, CuNO2, NiNO2, and CoNO2, for example) that the anodic and cathodic peaks are located at the same potential, indicating that the electroactive groups do not diffuse and are therefore attached to the surface. The lower reversibility of ZnAn is likely due to the higher basicity of the radical anion of anthracene. Infrared Spectroscopy. The modified surfaces were also examined by reflection FTIR spectroscopy by comparison with the transmission spectra (KBr pellet) of the parent diazonium salt. The FTIR spectra of the modified surfaces, although represented on the same scale for convenience, are much weaker than the transmission spectra and therefore affected by a larger signal-to-noise ratio. Figure 3 shows the spectra of DC6F13, ZnC6F13, and PtC6F13. The C-F stretching vibrations (strongly coupled with C-C stretching43) are clearly observed on the spectrum of DC6F13 as a series of strong bands between 1100 and 1300 cm-1. The spectra of perfluorooctanoic acid44a in a KBr pellet or adsorbed as a monolayer on silver and the spectrum of CF3(CF2)7(CH2)2SH44b adsorbed on Au(111) have been carefully analyzed by Porter.44 It is then possible, by comparison, to assign the bands of Figure 3. Table 2 summarizes the positions and assignment44 of these bands. The close similarity of the positions permits confident assignment of the bands of our spectra to the vibration (43) Socrates, G. Infrared Characteristics Group Frequency; Wiley: New York, 1994. (44) a) Chau, L.-K.; Porter, M. D. Chem. Phys. Lett. 1990, 167, 198. (b) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507.

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Figure 2. Cyclic voltammograms of modified metallic surfaces: (a) anthryl groups bonded to zinc, ZnAn; (b) nitrophenyl groups bonded to zinc, ZnNO2; (c) anthraquinone groups bonded to copper, CuAQ; (d) nitrophenyl groups bonded to copper, CuNO2; (e) nitrophenyl groups bonded to gold, AuNO2; (f) nitrophenyl groups bonded to nickel, NiNO2; (g) nitrophenyl groups bonded to platinum: PtNO2; and (h) nitrophenyl groups bonded to cobalt, CoNO2.

modes. The direction of the transition dipole νa(CF2, E1) νs(CF2, E1) is perpendicular to the chain axis; based on the IR selection rules of a reflection spectrum and the lower measured absorbance of the corresponding band relative to the calculated one, Porter44 concluded that

the chains of perfluoroalkanethiols were tilted by ∼20° relative to the surface normal. In the spectra of ZnC6F13 and PtC6F13, the strongest bands are located at 1249, 1258 and 1149, 1150 cm-1 for ZnC6F13 and PtC6F13, respectively. They correspond to the same νa(CF2, E1)

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Table 2. FTIR Spectra of DC6F13, ZnC6F13, and PtC6F13a PFOb 44 1366 1330 1295 1237 1204 1146 a

cm-1. b

DC6F13 KBr pellet 1364 1288 1235 1199 1145

PFOc 44

PFTd 44

ZnC6F13

PtC6F13

assignment44

1365 1321 1293 1251 1216 1151

1372 1335 1295 1246

1359

1360

1283 1249 1214 1149

1286 1258 1215 1150

νs (CF2, A1) νs (CF2, E2) ν (CC, E1) νs (CF3, E1) νa (CF2, A2)+ν(CF3) νs (CF2, E1)

1150 c

In Perfluorooctanoic acid, KBr pellet. Perfluorooctanoic acid, adsorbed as a monolayer on silver. monolayer on Au(111).

d

CF3(CF2)7(CH2)2SH as a

Figure 4. FTIR reflection spectra of (a) CuNO2, (b) NiNO2, and transmission spectrum of (c) 4-nitrobenzendiazonium (KBr pellet). Table 3. FTIR Spectra of DNO2, NiNO2, and CuNO2a NO2 stretching

DNO2

NiNO2

CuNO2

ν asymmetric ν symmetric

1358 1542

1351 1524

1354 1528

Figure 3. FTIR reflection spectra of (a) ZnC6F13, (b) PtC6F13, and (c) KBr pellet of DC6F13. a

νs(CF2, E1) transition dipoles, suggesting that the perfluoroalkyl chains are much further away from the normal to the surface than the 20° measured by Porter for perfluoroalkanethiols on Au(111). The vibrations of the aromatic ring are weak in the spectrum of the diazonium and absent in the reflection spectra of the modified surfaces. The in-plane CH vibrations43 of the aromatic ring appear as a strong band in the spectrum of DC6F13 at 1044 cm-1. This band appears much weaker in the spectra of ZnC6F13 and PtC6F13; this is also related to the IR selection rules, the CH vibrations being parallel to the surface as the phenyl ring stands up on the surface. The unsymmetrical 1,4-disubstitution of DC6F13 translates in the out-of-plane CH vibrations by two bands at 840 and 672 cm-1. It is however impossible to deduce the substitution pattern of ZnC6F13 and PtC6F13 from this part of the IR spectrum as the bands are very weak. The band at 734 cm-1 in the spectrum of PtC6F13 can be assigned to a CF deformation of the CF2CF3 group.43 The spectra of 4-nitrobenzenediazonium DNO2 as a reference and those of NiNO2 and CuNO2 are presented in Figure 4. As in the spectra of Figure 3, the vibrations of the aromatic ring (1613 cm-1 in DNO2) are very weak in the spectra of the modified surface, but NO2 stretching

In cm-1.

vibrations are very clearly observed; they are presented in Table 3. The difference ν asymmetric - ν symmetric ) 173174 cm-1 for the modified surfaces is what could be expected for a solid (159-177 cm-1).45 In this case also, the substitution pattern of the modified surfaces cannot be confidently deduced from the weak bands in the out-of-plane CH deformation region. The spectra of Figure 5 permit observation of the vibrations of the aromatic ring at 1418 and 1617 cm-1 for AuCF3 (by comparison: 1456, 1610 cm-1 for C6H5CF3). The strong band of the CF3 stretching appears at 1325 and 1321 cm-1 for AuCF3 and DCF3, respectively (by comparison, 1321 cm-1 for C6H5CF3). The 1,4disubstitution is clearly observed on DCF3 through the band at 850 cm-1, but also on AuCF3 (843 cm-1), suggesting that, in this case, there is no further attack on the first-grafted phenyl groups. In all grafted metallic plates, the absence of any significant band in the 2300-2130-cm-1 range where the NtN stretching could be expected indicates that the reduction of the diazonium group has occurred. As a (45) Nyquist, R. A. Interpreting Infrared, Raman and Nuclear Magnetic Spectra; Academic Press: New York, 2001; Vol. 2, p 173.

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Figure 5. FTIR reflection spectrum of (a) AuCF3 and transmission spectrum of (b) DCF3 (KBr pellet).

whole, these spectra leave no doubt about the attachment of aryl groups to the surfaces, but it is somewhat deceiving that the substitution patterns cannot be deduced due to the weakness of the bands in the CH out-of-plane deformation region. XPS Spectroscopy. To confirm the attachment of the aryl groups, the surface of metals were examined by XPS. Figure 6 displays the survey scans obtained at 90° for AuI, CuNO2, ZnNO2, and NiNO2. For the latter specimen, Figure 6 displays also a survey scan acquired with a ToA (take-off angle) of 30°. The characteristic peaks of the metal substrates are Pt 4f, Au 4f, Cu 2p3/2, Zn 2p3/2, and Ni 2p, centered at 70, 84, 932, 1022, and 856 eV, respectively. All survey spectra exhibit C 1s and O 1s peaks, which are partly due to adventitious contamination, and N 1s (except AuI) peaks (at ≈ 406 eV), which reflect the attachment of nitroaryl groups to the metallic surfaces. In the case of the AuI, an I 3d doublet located at 622-633 eV is clearly visible in the spectrum. It is worth noting that there is no contamination by unreacted diazonium tetrafluoroborate as absolutely no F 1s peak has been detected around 685 eV, indicating the efficiency of the rinsing process. One can see that coating on nickel is close to the analysis depth in XPS. Indeed, while the Ni 2p recorded at 90° ToA (Figure 6d) is very small and noisy, it almost completely disappears at 30° ToA, which is at an angle for which practically only core electrons from the aryl layers are collected. However, still we are left with a huge background due to inelastically scattered Ni 2p photoelectrons at low kinetic energy (high apparent binding energy). For all other metals, the main features from the substrates remain well-detected at 30°. Figure 7 depicts some specific regions from the copper substrate and the aryl layer. A typical N 1s spectrum is shown for the case of the CuNO2 surface (Figure 7a). For all other MetalNO2 specimens, the N 1s peak from -NO2 is at ca. 406 eV, in agreement with the value of 405.9 eV reported by Lindberg et al. for nitrobenzene.46

Figure 6. XPS spectra of (a) AuI, (b) CuNO2, (c) Zn NO2, and (d) NiNO2 at 90° ToA and (e) NiNO2 at 30° ToA.

It is important to note that the nitro N 1s feature has no shake-up satellite that has frequently been observed

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Figure 7. XPS spectra of (a) CuNO2 (N 1s peaks), (b) AuI (I peaks), (c) ZnNO2 (O peaks), and (d) CuNO2 (Cu peak).

in nitro-containing donor-acceptor-type molecules,47 such as p-nitroaniline. The second and minor peak in the N 1s region is located at 400.2 eV and due perhaps to a reduced form of the nitro group. This peak has already been observed on carbon during the grafting of DNO2.48 Figure 7b depicts a I 3d5/2-3d3/2 doublet from the AuI specimen. The main one is centered at 620.6 eV, matching the value reported by Zhou et al.,49 for a (46) Lindberg, B. J.; Hedman, J. Chem. Scr. 1975, 7, 155. (47) Chehimi, M. M.; Delamar, M. J. Electron Spectrosc. Relat. Phenom. 1988, 46, 427. (48) Saby, C.; Ortiz, B.; Champagne, G. Y.; Be´langer, D. Langmuir 1997, 13, 6805. (49) Zhou, X.-L.; Solymosi, F. P.; Blass, M.; Cannon, K. C.; White, J. M. Surf. Sci. 1989, 219, 294.

Bernard et al.

C-I bond in iodomethane adsorbed onto silver (620.8 eV) and attached as a monolayer (620.55 eV). Figure 7c displays a typical O 1s peak from a nitrophenyl group attached to a metallic substrate ZnNO2. The peak is fitted with three components centered at 530.6, 532.2, and 533.2 eV and assigned to a zinc oxide (in the present case), a nitro group from the attached nitrophenyl group, and surface contamination, respectively. The presence of ZnO has been checked by recording the Auger region of Zn, showing two peaks, a narrow and a wide one centered at 992 and 986.8 eV on the kinetic energy scale, and assigned to Zn0 and ZnO, respectively, indicating that some oxidation takes place on the surface, likely before the grafting reaction. All metals, except zinc (see above), exhibit practically the metallic states only as judged from the respective binding energies and the shape of the spectra. For example, Cu 2p3/2 has a single symmetric peak located at 932.6 eV (without any high binding energy shake-up satellite characteristic of CuII) that is assigned to the metallic state (Figure 7d). For all substrates, spectra were recorded at two ToA to determine the average thickness of the organic layer (see Experimental Section). Table 4 reports the surface chemical composition of the various plates at 30 and 90° takeoff angles and the estimated thickness of the aryl layer. Attachment of the nitrophenyl groups (by reduction of DNO2) is apparently more effective than that of the iodophenyl groups (from DI) since the N406/metal ratio is in the range 1-22 whereas the I/Au atomic ratio is about 0.33. However, DI yields the highest average thickness on gold, most probably reflecting a much more continuous coating and compact film. The thickness of the films obtained from XPS measurements will be compared below to that obtained from AFM measurements. Rutherford Backscattering. The modified surfaces CuI, NiI, and ZnI were also examined by Rutherford backscattering (RBS). This methods permits characterization of the atoms of the surface provided they are heavier than the substrate, but also permits measurement of the number of such atoms. Figure 8 shows the spectrum of CuI; one can clearly observe the peak corresponding to the iodine atoms. The surface coverage Γ varied from 10-8 mol cm-2 for CuI and 5 × 10-10 mol cm-2 for NiI. These results will be discussed below by comparison with the AFM images. Atomic Force Microscopy. We used atomic force microscopy (AFM) to examine the surface of metals before and after modification with organic layers. For this purpose, the surfaces were modified with dodecylphenyl groups in hope that the interactions between the long alkyl chains would lead to compact layers as for SAMs of thiols.11 It has been shown50 that on HOPG, nitrophenyl groups form an organized monolayer on the surface, likely due to the interaction, before the grafting reaction. Figures 9a, 10a, and 11a show the surface of untreated copper, nickel, and zinc disks carefully polished with 1-µm diamond paste and rinsed in acetone. One observes the grooves made by the diamond grains; these grooves are approximately 5-nm deep, some larger (50) Liu, S.; Tang, Z.; Shi, Z.; Niu, L.; Wang, E.; Dong, S. Langmuir 1999, 15, 7628.

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Figure 8. RBS spectrum of CuI. Table 4. Surface Chemical Composition (in at. %) as Determined by XPS for the Various Specimens at 30 and 90° Take-off Angles materials

C

O

N400

AuI (30°) AuI (90°) PtNO2 (