Langmuir 2003, 19, 6333-6335
X-ray Photoelectron Spectroscopy Evidence for the Covalent Bond between an Iron Surface and Aryl Groups Attached by the Electrochemical Reduction of Diazonium Salts Kada Boukerma,† Mohamed M. Chehimi,*,† Jean Pinson,‡ and Chris Blomfield§ Interfaces, Traitements, Organisation et Dynamique des Syste` mes (ITODYS), Universite´ Paris 7-Denis Diderot, associe´ au CNRS (UMR 7086), 1 rue Guy de la Brosse, F-75005 Paris, France, Laboratoire d’Electrochimie Mole´ culaire, Universite´ Paris 7-Denis Diderot, Unite´ Mixte Universite´ Paris 7-CNRS 7591, 2 Place Jussieu, 75251, Paris Cedex 05, France, and Kratos Analytical Limited, Wharfside, Trafford Wharf Road, Manchester M17 1GP, United Kingdom Received February 3, 2003. In Final Form: May 16, 2003
Introduction The electrochemical reduction of diazonium salts either in acetonitrile (ACN) or in acidic aqueous medium leads to the attachment of aryl groups to the surface of the electrode. The species responsible for the attachment to the surface is the aryl radical obtained upon one electron reduction of the diazonium salt. Because the synthesis of stable diazonium tetrafluoroborates from commercial amines is straightforward, and, moreover, the reduction occurs at low potentials within seconds to minutes, this reaction offers a very simple and versatile way of modifying the surface of conducting materials. The reduction of the diazonium salts has been achieved on carbon,1-4 silicon,5 and iron6 surfaces. In cyclic voltammetry, the reduction in ACN is characterized by a broad irreversible wave at a potential close to 0 V/saturated calomel electrode (SCE), which decreases and disappears upon successive scans. This blocking of the electrode corresponds to the grafting of organic groups to the surface. In every case, the layer of aryl groups is strongly attached to the surface and can only be removed by mechanical polishing or vigorous corrosion of the surface (silicon or * Author to whom correspondence should be addressed. † ITODYS, Universite ´ Paris 7-Denis Diderot. ‡ Laboratoire d’Electrochimie Mole ´ culaire, Universite´ Paris 7-Denis Diderot. § Kratos Analytical Limited. (1) (a) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (b) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (c) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J. M. J. Am. Chem. Soc. 1997, 119, 201. (d) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J. M. Carbon 1997, 36, 801. (2) Downard, A. J. Electroanalysis 2000, 12, 1085. (3) (a) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. (b) Liu, S.; Tang, Z.; Shi, Z.; Wang, E.; Dong, S. Langmuir 1999, 15, 7268. (4) (a) Saby, C.; Ortiz, B.; Champagne, G. Y.; Be´langer, D. Langmuir 1997, 13, 6805. (b) Ortiz, B.; Saby, C.; Champagne, G. Y.; Be´langer, D. J. Electroanal. Chem. 1998, 455, 75. (5) (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. (6) (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) Chausse´, A.; Chehimi, M. M.; Karsi, N.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2002, 14, 392. (c) Adenier, A.; Cabet-Deliry, E.; Lalot, T.; Pinson, J.; Podvorica, F. Chem. Mater. 2002, 14, 4576.
6333 Scheme 1
iron). The presence of aryl groups on the surface has been characterized by a variety of techniques, such as cyclic voltammetry, vibrational spectroscopy,7 X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy, and capacity measurements. XPS has permitted the characterization of the substituents of the aryl groups: -NO2, -I, -COOH,6a -CF3, and C6F13.6b Given the difficulty to remove the aryl layer (this can only be accomplished by mechanical polishing or corrosion of the metal buried underneath the organic layer), we believe that a carbon-electrode bond is formed. Therefore, a carbon-carbon bond would be formed on a glassy carbon or Highly Oriented Pyrolytic Graphite (HOPG) electrode; aryl radicals are known as very reactive species in organic chemistry,8 and it is not surprising to observe the formation of such bonds, although on HOPG a change of hybridization from sp2 to sp3 is required. On hydrogenated silicium, the reaction of alkyl radicals9 has been previously used for surface modification. However, on iron, the formation of a strong covalent bond with an aryl group is much less common although not unprecedented.10 We have shown previously by XPS that the C(1s) signal of a phenyl carboxylic group attached to an iron surface showed a slight shoulder centered at a binding energy position corresponding to a carbide species.6a The observation of this shoulder gave an indication of the existence of a C-Fe bond but could not be taken as compelling evidence. Because the characterization of this aryl-iron bond is of fundamental importance and crucial for potential future technological applications of this method of surface treatment, we envisaged to explore the possibilities offered by a modern XPS machine equipped with a monochromated X-ray source and an electron spectrometer that permits obtaining a full width at half-maximum as low as 0.65 eV for the ester C(1s) peak from poly(ethylene terephthalate). In this work, we investigated the surface composition of an iron surface after treatment by 4-carboxybenzene diazonium tetrafluoroborate (BF4- +N2C6H4COOH), as is shown in Scheme 1. Angle-resolved XPS was performed to probe the interfacial species formed between the diazonium salt and the iron surface. Experimental Section An iron disk was modified with -COOH groups by reduction of the tetrafluoroborate salt of the diazonium of 4-aminobenzoic acid in ACN and 0.1 M NBu4BF4 at -0.2 V/SCE, that is, at the potential of the voltammetric peak (-0.23 V/SCE) for 5 min. The disk was then thoroughly rinsed under ultrasonication in acetone and alcohol for 5 min each to remove any species loosely attached (7) (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) Ray, K., III; McCreery, R. L. Anal. Chem. 1997, 69, 4680. (8) Smith, M. B.; March, J. Advanced Organic Chemistry; John Wiley, Inc.: New York, 2001; p 904. (9) (a) Lindford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (b) Lindford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (10) Bowden, F. L.; Wood, L. H Compounds with iron-carbon σ-bonds. In The Organic Chemistry of Iron; Koerner Von Gustorf, E. A., Grevels, F. W., Fischler, I., Eds.; Academic Press: New York, 1978; Vol. 1, p 345.
10.1021/la030046g CCC: $25.00 © 2003 American Chemical Society Published on Web 06/26/2003
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Langmuir, Vol. 19, No. 15, 2003
Notes
to its surface. The Fourier transform infrared spectrum of the surface6c presents a broad OH stretching band at 3336 cm-1, which indicates the attachment of the aryl group to the iron surface. A Kratos Axis Ultra photoelectron spectrometer (Manchester, U.K.) was used to examine the treated iron substrate. This instrument is equipped with a monochromated Al KR X-ray source operating at 375 W and a fully automatic coaxial low-energy electron source that ensures a uniform charge compensation. Large areas of 700 × 300 µm from the sample surface were probed. The pass energy was set at 160 and 10 eV for the survey and the narrows scans, respectively. The takeoff angles (TOAs) were 0, 45, and 75° relative to the surface normal. Spectra were charge-referenced to the hydrocarbon component set at a 285-eV binding energy. The surface composition was determined using the manufacturer’s sensitivity factors. The fractional concentration of a particular element A (% A) was computed using
%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 Figure 1 shows the survey scans of an iron plate modified with 4-carboxyphenyl groups (referred to as Fe-COOH) at the indicated TOAs. The major core hole signals are C(1s), O(1s), and Fe(2p3/2) centered at 285, 531, and 710 eV, respectively. The iron substrate is clearly still visible even at 75°, an indication that the organic overlayer is (of the order of 2-3 nm) thinner than the sampling depth of the technique. Nevertheless, iron undergoes a slight decrease in the Fe(2p) peak intensity as the TOA is increased. This is expected because iron is the underlying substrate. In contrast, there is a distinct increase in the C(1s) intensity with increasing TOA, meaning that one goes from a bulk-sensitive analysis to an outermost-layers analysis. One can note the presence of a nitrogen N(1s) peak centered at 400 eV. This feature has already been reported and discussed by our group6a in a study of the modification of iron by the electrochemical reduction of aryldiazonium salts. It has also been reported in the case of carbon by Be´langer and co-workers.4a They concluded that the origin of the peak was unclear but suggested the reasonable possibility that it could be assigned to a hydrazine group obtained by the reaction of the diazonium function with surface phenol groups to give a hydrazine. However, on iron there is no such phenol groups on the surface but some remaining oxides (despite careful polishing immediately prior to the grafting) that could perhaps play a role similar to that of phenols on carbon surfaces. Figure 2 depicts the C(1s) region at angles of 0, 45, and 75°. The peaks are fitted with four components assigned to CC/CH, C-O, and COOH groups. The first minor component, centered at 283.3 ( 0.1 eV, is undoubtedly assigned to a carbide type of carbon11 that originates from the formation of a covalent iron-aryl bond. Figure S1 (Supporting Information) shows Fe(2p) regions at the indicated TOAs. Fe(2p3/2) has two main components centered at 706.6 and 710.1 eV that are assigned to iron in the metallic and oxide states, respectively. Clearly, there is an increase in the oxide/metal ratio as the TOA is increased toward the analysis of the outermost layers of the treated metal plate. Figure S2 (11) Shabanova, I. N.; Trapeznikov, V. A. J. Electron Spectrosc. Relat. Phenom. 1975, 6, 297.
Figure 1. Angle-resolved survey spectra of Fe-COOH at 0, 45, and 75°.
(Supporting Information) depicts the O(1s) region at the indicated TOAs. The two apparent maxima centered at 531 and 532 eV are assigned to the oxides and organic species, respectively. The latter arises mainly from the carboxylic acid CdO and CsOH groups. Angular effects are also observed for the O(1s) region because the organic/ inorganic ratio increases with the TOA. Figure 3 shows the surface composition in atomic percent versus the TOA. Whereas the carbon content increases monotonically, the total surface concentration of iron (in the metallic and oxide forms) decreases. The variation of the oxygen content is not really monotonic because this element belongs to both the inorganic substrate as oxide forms and the organic overlayer in the
Notes
Langmuir, Vol. 19, No. 15, 2003 6335
Figure 3. Surface composition of Fe-COOH at the indicated TOAs. In each bar, from top to bottom, the elements are C, N, O, Fe, and Na. Table 1. Peak Fitting Parameters for the C(1s) Regions at the Indicated TOAs CsO
OdCsO
TOA Angle of 0° BE (eV) 283.3 285.0 concentration (atom %) 2.1 64.3
carbide
CsC/CsH
286.3 11.2
288.6 22.4
BE (eV) concentration (atom %)
TOA Angle of 45° 283.4 285.0 1.07 68.1
286.1 13.3
288.5 17.5
BE (eV) concentration (atom %)
TOA Angle of 75° 283.4 285.1 0.90 72.6
286.2 12.5
288.5 13.9
carbide/COOH ratio (calculated from the results presented in Table 1) decreases versus the analysis angle, indicating that the functional groups of interest coexist in two close but different layers. This means that a COOH-rich layer is located on top of the carbide-rich diazonium-iron interface. Conclusion
Figure 2. Angle-resolved C(1s) spectra of Fe-COOH at 0, 45, and 75°. Note the C(1s) component centered at about 283.3 eV due to C-Fe bonds.
form of COOH groups and superficial adventitious contamination (which can hardly be avoided in the XPS analysis of materials surfaces). Table 1 reports the peak fitting parameters for the C(1s) region at the indicated TOAs. From the results presented, it would appear that the carboxylic acid group is not exactly at the outermost surface of the organic layer because the relative intensity of the peak at 288.5 eV slightly decreases on going from 0 to 75° TOA, which is when the sample is rotated to a more surface-sensitive analysis. This is in agreement with the existence of a polymeric layer of a substituted polyphenylene type, as was already observed on carbon by Kariuki and McDermott.3a Nevertheless, the
In summary, this note brings for the very first time direct evidence for the existence of a covalent bond between an aryl group and metallic substrate (in this case iron) using XPS as a sensitive surface analytical tool. Angularresolved XPS explicitly showed that the carbide bond formed at the diazonium-iron interface undergoes a monotonic decrease in relative concentration as a result of increasing TOA (relative to the surface normal) from 0 to 75°, meaning a gradual change from a bulk- to a surface-sensitive analysis. The existence of such a covalent bond is most probably the driving force for the excellent adhesion of aryl diazonium adlayers to metallic and carbon surfaces. Acknowledgment. The authors would like to thank the French Minstry of Research for financial support through the ACI Surfaces & Interfaces (Action Inte´gre´e Incitative Surfaces & Interfaces) Project No. S43-01. Supporting Information Available: Angle-resolved Fe(2p) and O(1s) spectra of Fe-COOH at 0, 45, and 75°. This material is available free of charge via the Internet at http://pubs.acs.org. LA030046G
