Covalent modification of iron surfaces by electrochemical reduction of aryldiazonium salts

Article abstract of DOI:10.1021/ja003276f

Electrochemical reduction of aryldiazonium salts (in acetonitrile or acidic aqueous medium) on an iron or mild steel surface permits the strong bonding (which resists an ultrasonic cleaning) of aryl groups on these surfaces. Attachment of aryl groups was

Full text of DOI:10.1021/ja003276f

J. Am. Chem. Soc. 2001, 123, 4541-4549
4541
Covalent Modification of Iron Surfaces by Electrochemical Reduction
of Aryldiazonium Salts
†
‡
†
§
Alain Adenier, Marie-Claude Bernard, Mohamed M. Chehimi, Eva Cabet-Deliry,
⊥
§
,§
§
Bernard Desbat, Olivier Fagebaume, Jean Pinson,* and Fetah Podvorica
Contribution from the Laboratoire d’Electrochimie Mol e´ culaire de l’UniVersit e´ Paris 7, Unit e´ Mixte de
Recherche UniVersit e´ -CNRS No. 7591, UniVersit e´ Paris7-Denis Diderot, 2 Place Jussieu, F-75251,
Paris Cedex 05, France, ITODYS, UniVersit e´ Paris 7-Denis Diderot, associ e´ au CNRS (UPRESA 7086),
1
rue Guy de la Brosse, F-75005, Paris, France, the Laboratoire de Physico-Chimie Mol e´ culaire
UniVersit e´ Bordeaux 1, Unit e´ Mixte de Recherche UniVersit e´ -CNRS No. 5803, UniVersit e´ Bordeaux 1, 351
cours de la Lib e´ ration, F-33405, Talence Cedex Cedex, France, and the Laboratoire de Physique des
Liquides et Electrochimie, UPR 15CNRS, ConVentionn e´ aVec l’UniVersit e´ Pierre et Marie Curie,
4
Place Jussieu, F-75004, Paris, France
ReceiVed September 5, 2000. ReVised Manuscript ReceiVed NoVember 27, 2000
Abstract: Electrochemical reduction of aryldiazonium salts (in acetonitrile or acidic aqueous medium) on an
iron or mild steel surface permits the strong bonding (which resists an ultrasonic cleaning) of aryl groups on
these surfaces. Attachment of aryl groups was demonstrated by the combined used of electrochemistry, infrared
spectroscopy and polarization modulation infrared reflection spectroscopy (PMIRRAS), Rutherford backscat-
tering, X-ray photoelectron spectroscopy, and capacity measurements. The substituents of aryl groups, which
can be widely varied, include NO2, I, COOH, and long alkyl chains. It is shown that the attachment of the aryl
groups is to an iron and not to an oxygen atom and that the bond is covalent.
Introduction
by AFM4a and STM4b and it was concluded4a that the layer can
be thicker than a monolayer. The electrochemical blocking
properties of the films produced by this method have been
We have shown previously that the electrochemical reduction
of aryl diazonium salts on a carbon electrode permits the
covalent attachment of the aryl groups to the carbon surface.
The above electrochemical reaction can be performed in ACN
but also in acidic aqueous medium. The monolayers obtained
with a variety of starting diazonium salts have been characterized
by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS),
polarization modulation infrared spectroscopy (PMIRRAS),
5
1
investigated. These organic monolayers have been used to
1
b,6
7a
attach redox enzymes,
to reduce protein adsorption, to
differentiate the electrochemical signals of dopamine and
7
b
ascorbic acid, and to control the selectivity of glassy carbon
7
c
flow detectors. The mere reaction of aryl diazonium salts
without electrochemistry) has permitted the attachment of aryl
(
2
groups to carbon black to produce materials which may be useful
Raman spectroscopy, and Rutherford back scattering (RBS).
The grafting of aryl groups takes place on glassy carbon with
surface pending bonds but also on the basal plane of highly
8
in the plastic, rubber, and textile industries. It is also possible
9
to obtain closed packed aryl layers on silicon by electrochemical
1,2
reduction of diazonium salts on H-terminated Si (111).
It was therefore interesting to find out whether this process
could be extended to other substrates; in this paper we shall
describe the modification of iron and mild steel surfaces.10
Besides adhesion and lubrication, the modification of biomedical
surfaces, an obvious application of such a process would be
the protection of iron and steel against corrosion and we shall
oriented pyrolytic graphite (HOPG). On the basis of the
thoroughly investigated electrochemical reduction of aryl ha-
3
lides, the grafting was assigned to the reaction of the very
reactive aryl radical produced upon one-electron reduction of
the diazonium cation. The structure of the layer was examined
1
*
Address correspondence to this author.
ITODYS.
Laboratoire de Physique des Liquides et Electrochimie.
Laboratoire d’Electrochimie Mol e´ culaire.
Laboratoire de Spectroscopie Cristalline et Mol e´ culaire.
†
‡
§
⊥
(4) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534-6540.
(b) Liu, S.; Tang, Z.; Shi, Z.; Wang, E.; Dong, S. Langmuir 1999, 15, 7268-
7275.
(
1) (a) Delamar, M.; Hitmi, R.; Pinson, J.; Sav e´ ant, J. M. J. Am. Chem.
(5) (a) Saby, C.; Ortiz, B.; Champagne, G. Y.; B e´ langer, D. Langmuir
1997, 13, 6805-6813. (b) Ortiz, B.; Saby, C.; Champagne, G. Y.; B e´ langer,
D. J. Electroanal. Chem. 1998, 455, 75-81.
Soc. 1992, 114, 5883-5884. (b) Bourdillon, C.; Delamar, M.; Demaille,
C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336,
1
13-123. (c) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi,
(6) Dequaire, M.; Degrand, C.; Limoges, B. J. Am. Chem. Soc. 1999,
121, 6946-6947.
R.; Pinson, J; Sav e´ ant, J. M. J. Am. Chem. Soc. 1997, 119, 201-207. (d)
Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Sav e´ ant,
J. M. Carbon 1997, 36, 801-807.
(7) (a) Downard, A. J.; Roddick, A. D. Electroanalysis 1995, 7, 376-
378. (b) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta
1995, 317, 303-310. (c) Downard, A. J.; Roddick, A. D. Electroanalysis
1997, 9, 693-698.
(8) Belmont, J. A.; Amici, R. M.; Galloway C. P. (Cabot Corp.) PCT
WO 18,688, 1996 [Chem. Abstr. 1996, 125, 144212t].
(9) (a) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue,
P. J. Phys. Chem. B 1997, 101, 2415-2420. (b) Allongue, P.; Henry de
Villeneuve, C.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X.
Electrochim. Acta 1998, 43, 2791-2798.
(
2) (a) Liu, Y. C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254-
1
1259. (b) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958-3965.
Ray, K., III; McCreery, R. L. Anal. Chem. 1997, 69, 4680-4687.
3) (a) Hawley M. D. In Encyclopedia of Electrochemistry of the
(
Elements, Organic Section; Bard, A. J., Lund H., Eds.; Marcel Dekker:
New York, 1980; Vol. XIV, pp 179-280. (b) Pinson, J.; Sav e´ ant, J. M. In
Electroorganic Synthesis, Festschrift for Manuel Baizer; Little, R. D.,
Weinberg, N. L., Eds.; Marcel Dekker: New York, 1991; pp 29-44.
1
0.1021/ja003276f CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/21/2001

4542 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Adenier et al.
Scheme 1
discuss this point in a forthcoming paper. The experiments have
been performed both on chemically pure iron and mild (the
practical material which should be protected against corrosion)
and stainless steel. In the same way both acetonitrile and acidic
aqueous solutions have been utilized, the use of the first solvent
is, as we shall see later, much more convenient but of little
practical relevance. The different diazonium salts which have
been investigated are shown in Scheme 1. Compounds 1, 3,
1
1, and 12 were investigated as their substituents are easily
characterized by cyclic voltammetry (NO2, quinone, ben-
zoylphenyl), XPS or RBS (NO2, I, COOH), and IR (NO2,
benzoylphenyl); the other diazoniums were selected because they
could provide an alternative to thiols on gold but also for their
possible hydrophobic properties from which one could expect
some anticorrosion efficiency.
Results
Electrochemical Evidence for the Surface Modification.
The possibility of grafting aryl groups on iron surfaces has been
investigated in both aprotic [acetonitrile (ACN) + 0.1 M NBu4-
BF4] and acidic aqueous (diazonium salts are not stable in
aqueous solution above pH 2) media. In ACN + 0.1 M NBu4-
BF4 an iron electrode presents an electroactivity domain that is
restricted from -0.1 to -2.3 V/SCE (Figure 1a). In dilute (0.1
N) sulfuric acid this domain shrinks to nothing (Figure 1b). The
Figure 1. Cyclic voltammogram on a 1 mm diameter iron electrode
in (a) ACN + 0.1 M NBu
4 4
BF , scan rate V ) 0.2 V/s, and (b) 0.1 N
H
2
SO , scan rate V ) 0.02 V/s, with SCE reference.
4
11,12
open circuit potential (corrosion potential)
is located at
-
0.58 V/SCE, corresponds to a mixed potential since the
oxidation and reduction reactions are different, and is reached
when the anodic and cathodic currents are equal. The electro-
activity domains are therefore completely different from that
of a carbon electrode and it is not possible to observe as
1
described earlier the reduction wave of diazonium salts which
are located at potentials positive to the oxidation of the metal.
If the diazonium is substituted by an electron donating group
as in the case of 5, the reduction potential becomes negative
enough to permit the observation of the reduction wave by cyclic
voltammetry (Figure 2a). However, in the case of 1 bearing a
nitro electron withdrawing group, the wave becomes too positive
to be observed before the oxidation of iron. Therefore, in ACN,
the only general way to achieve the grafting of aryl groups on
the iron surface is to set the potential more negative than the
reduction of the diazonium salt, whether or not the cyclic
voltammetry wave can be observed. Coming back to the
voltammogram of 5, one observes the decrease of the wave on
Figure 2. Cyclic voltammetry of 5 (c ) 2 mM) in ACN + 0.1 M
NBu BF , 1 mm diameter iron electrode, scan rate V ) 0.2 V/s, SCF
reference: (a) first cycle, (b) second cycle, and (c) after 30 s of
electrolysis.
4
4
(
10) Fagebaume, O.; Pinson, J.; Podvorica, F. Pat. PCT Appl. FRO1
0388.
11) Brett, C.; Brett, A. Electrochemistry; Oxford University Press:
Oxford, 1993.
0
the second scan (Figure 2b) and its total disappearance after 30
s of electrolysis (Figure 2c); this is similar to what has been
previously observed on carbon. In sulfuric acid one should set
the potential negative to the corrosion potential (we will see
(
(12) O’M. Bockris, J.; Yang, B. J. Electrochem. Soc. 1991, 138, 2237-
2
252.

CoValent Modification of Iron Surfaces
J. Am. Chem. Soc., Vol. 123, No. 19, 2001 4543
later that it is possible to determine an optimal potential for
this grafting reaction at -0.75 V/SCE in 0.1 N H2SO4). With
such a procedure one assumes that diazonium salts are reduced
at similar potentials on carbon and iron. The reduction potential
of diazonium salts and the oxidation potential of iron in aqueous
media should make the direct reduction of diazonium salts by
iron thermodynamically possible. However, we shall see later
through XPS measurements that this spontaneous reaction is
unimportant for the grafting of the surface.
We have investigated the reduction of 4-nitrobenzenediazo-
nium tetrafluoroborate 1, which on carbon leads to the attach-
ment of 4-nitrophenyl groups. These groups are easily charac-
terized by their reversible reduction wave in aprotic medium
located at the same potential as nitrobenzene itself. We first
recorded the cyclic voltammogram of nitrobenzene in ACN +
0
.1 M NBu4BF4 on an iron electrode; one observes two waves
at -1.22 and -1.75 V/SCE (not shown), the height of the
second one being about double that of the first. The first wave
is reversible as an anodic wave is observed at -1.00 V/SCE
on the return scan. The standard potential of nitrobenzene is
therefore E° ) -1.11 V/SCE. This voltammogram is similar
to that observed on carbon electrodes (Figure 3). Then, the
potential of an iron electrode was maintained during 5 min at
-
0.8 V/SCE in an ACN + 0.1 M NBu4BF4 solution containing
1
(c ) 2mM). Thereafter the electrode was carefully rinsed in
acetone in an ultrasonic bath for 5 min in the absence of oxygen
and transferred to a new solution containing only the solvent
and the supporting electrolyte. Under such conditions, one can
observe a very broad reversible wave (Epc ) -1.17 V/SCE and
Epa ) -1.16 V/SCE) at a potential very close to that of
nitrobenzene itself (Figure 3c). This experiment clearly indicates
that nitrophenyl groups are transferred with the electrode and
that their bonding is strong enough to resist rinsing in an
ultrasonic bath. Figure 4 presents the result obtained with Fast
Red Al salt 11, which is a commercial diazonium salt of
anthraquinone. The iron electrode, which had been derivatized
and carefully rinsed under the same conditions as above, presents
a reversible system with a cathodic peak at Epc ) -0.93 V/SCE
and an anodic peak at Epa ) -0.88 V/SCE, which corresponds
to a standard potential E° ) -0.90 V/SCE. By comparison,
anthraquinone exhibits on an iron electrode a slow redox wave
(Epc ) -1.08 V/SCE and Epa ) -0.75 V/SCE) with E° ) -0.91
V/SCE, i.e., at the same potential as the modified electrode.
This experiment confirms the observations made with 1. In
addition, the symmetrical shape of the voltammogram is
indicative of a system that does not diffuse to and from the
electrode as could be expected for molecules bonded to the
electrode. It was also possible to observe the remarkable stability
of the system on repetitive scanning. Similar results were also
obtained with anthracenyl groups bonded to the surface through
reduction of 10 (Figure S1 in the Supporting Information).
An iron electrode modified by reduction of 11 at E ) -0.5
V/SCE in 0.1 N H2SO4 carefully rinsed and transferred to an
ACN + 0.1 M NBu4BF4 solution presents a voltammogram with
a low reversibility (Epc ) - 0.9 V/SCE and Epa ≈ -0.6 V /SCE)
at E° ≈ -0.7 V/SCE. This voltammogram (Figure 5) is very
similar to that of anthraquinone in ACN + 0.1 M NBu4BF4 on
an iron electrode previously maintained in a 0.1 N H2SO4
solution for 5 min. These experiments in acidic aqueous solution
indicate that it is possible to obtain the attachment of aryl groups
to the iron surface but the voltammogram of these aryl groups
is modified by reference to that obtained after grafting in ACN.
This appears through a loss of reversibility and a slower
electronic transfer. The loss of reversibility can be related to
4 4
Figure 3. Cyclic voltammetry in ACN + 0.1 M NBu BF of
nitrobenzene (c ) 2 mM) on (a) a glassy carbon electrode and (b) on
an iron electrode and (c) of an iron electrode derivatized with
4-nitrophenyl groups (see text), with V ) 0.2 V/s and SCE reference.
the protonation of the radical anions by acidic species transferred
with the electrode despite vigorous rinsing.
The modification of the iron surface is therefore possible in
ACN and acidic aqueous solution, but is more easily observed
after grafting in ACN. These results called for a characterization
of the surface by independent methods.
Infrared Spectra of the Organic Layer. As in the case of
monolayers on carbon it is difficult to obtain significant

4544 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Adenier et al.
4 4
Figure 5. Cyclic voltammograms in an ACN + 0.1 M NBu BF
Figure 4. Cyclic voltammetry of (a) an iron electrode modified with
solution of a 3 mm iron electrode (a) which has been derivatized with
anthraquinone groups (see text) and (b) in a 2 mM anthraquinone
solution (the electrode has been previously immersed for 5 min in 0.1
anthraquinone groups and transferred to an ACN + 0.1 M NBu
4
BF
+
4
solution and (b) an iron electrode in an ACN + 0.1 M NBu BF
4
4
3
mM anthraquinone solution, with V ) 0.2 V/s and SCE reference.
N H SO
2 4
and then rinsed).
reflection infrared spectra of monolayers. However, this can be
achieved with polarization modulation infrared absorption
2
By using a 5 cm glass plate covered by a thin layer of iron
deposited by sputtering, it was possible to obtain a specular
reflectance infrared spectrum of the organic layer obtained by
reduction of the corresponding diazonium salt 12. Figure 7
presents the IRTF spectra of the grafted benzoylphenyl groups
and that of a reference spectrum of melted benzophenone.15 Both
spectra present very similar absorptions at respectively (i) 1659
13
spectroscopy. The spectra were obtained with mild steel plates
for which a protection against corrosion would be of interest.
Before grafting, the steel plates were carefully polished and
rinsed in the absence of oxygen in an ultrasonic cleaner. An
-
1
ungrafted plate presents absorptions at 1050-1100 cm
characteristic of oxides as well as absorptions at 1400 and
-
1
16
-
1
and 1658 cm (assigned to the carbonyl group), (ii) 1317,
278 and 1311, 1275 cm (aromatic in-plane CH vibrations),
and (iii) 1200-1000 cm (rocking of the aromatic hydrogens).
There are, however, differences between the two spectra: (i)
the CdC stretching vibrations of the grafted benzoylphenyl
groups are very weak and (ii) the spectrum of benzophenone
1
(
600-1700 cm related to carbonates and hydrogenocarbonates
-
1
1
most likely stemming from reaction with atmospheric carbon
-1
dioxide after grafting) (Figure 6a). After grafting by 1 in an
ACN + 0.1 M NBu4BF4 solution the symmetrical and anti-
14
symmetrical vibrations of the NO2 group are clearly visible
-1
at 1350 and 1522 cm as well as the absorptions of the aromatic
-1
-
1
shows out-of-plane CH vibrations at 762 cm and an associated
group at 1263 and 1600 cm (Figure 6b). The peaks of oxides
are still visible, but those of carbonates seem to have disap-
peared. If the grafting is performed in 0.1 N sulfuric acid a
-
1
band at 698 cm which are characteristic of monosubstituted
benzenes. This last band disappears in the spectrum of the
grafted groups while two new absorptions appear at 805 and
-
1
very strong band is observed at 1180 cm that can be due to
either an increased oxide layer or the presence of sulfates;
however, the signal of the NO2 group can still be observed
-1
8
59 cm which can be confidently assigned to CH out-of-plane
(15) The Aldrich/ACD Library of FT-NMR Spectra; Aldrich Chemical
(
Figure 6c).
Co., Inc.: Milwaukee.
16) (a) Silverstein, R. M.; Basler, G. C.; Morill, T. C. Identification
Spectrom e´ trique de Compos e´ s Organiques; De Boeck Universit e´ : Paris,
1998. (b) Socrates, G. Infrared Characteristic Group Frequencies; Chicester,
England, 1994. (c) Nuttall, R. H.; Roberts, E. R.; Sharp, D. W. A.
Spectrochim. Acta, 1961, 17, 947-952.
(
(13) (a) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc.1991,
4
5, 380-389. (b) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.;
Escafre, N.; Pezolat, M.; Turlet M. Appl. Spectrosc. 1993, 47, 869-874.
(14) Varsanyi, G.; Holly, S.; Imre L. Spectrochim. Acta 1967, 23A, 1205.

CoValent Modification of Iron Surfaces
J. Am. Chem. Soc., Vol. 123, No. 19, 2001 4545
into competition with the surface oxidation, and with the
reduction of protons at more negative potential. The surface
concentration will be discussed later.
X-ray Photoelectron Spectroscopy (XPS). It is worth noting
that XPS and RBS are complementary surface analytical tools
since the sampling depth of the former is about 2 orders of
magnitude smaller than that of the latter. Figure 9 shows the
wide XPS scans of a mild clean steel plate (Figure 9a) and
4
4
-nitrophenyl- (Figure 9b), 4-carboxyphenyl- (Figure 9c), and
-iodophenyl-treated mild steel (Figure 9d). All wide scans show
C[1s], O[1s], and Fe[2p] peaks centered at ca. 285, 530, and 710
eV. Figure 9b shows a significant attachment of 1 following
the electrochemical treatment of the mild steel plate. This is
evidenced by a sharp increase in the relative intensity of C[1s]
(see below) on one hand and a small peak around 400 eV.
Enlargement of this region shows two peaks centered at 400
and 406 eV, the latter being due to the nitro group. The former
may be due to some reduction products deriving from the
5
a
diazonium salt. Note that the O[1s] peak was fitted with a
component centered at 533 eV, a binding energy that is
characteristic of the oxygen atom from a nitro group,17 together
with a second component at lower binding energy due to iron
oxides. After reduction of 4-carboxybenzenediazonium tet-
rafluroborate 3 the spectrum of the steel plate presents a C[1s]
peak at 289 eV (see below), a O[1s] signal fitted with two
components at 531 and 532 eV assigned to the carbon and the
CdO and OH oxygens of the carboxylic group, respectively,
and a F[1s] peak around 690 eV due to some surface contamina-
-
tion by the BF4 ions of the electrolyte (Figure 9c). When the
steel plate is modified with 4-iodophenyl groups the signal of
the iodine atom can be observed at 621 eV (4.2%) (Figure 9d).
The C/I ratio was 6.25, close to what is expected for a
iodophenyl group. To ascertain that the presence of the attached
group is not due to a mere adsorption or to a chemical reaction,
the same steel plate, polished and cleaned as before, was dipped
in a solution of 2 in ACN + 0.1 M NBu4BF4; under such
conditions only a small signal (0.45%) of iodine was observed.
This small signal can be due to a slow spontaneous reduction
of 2 by iron as indicated at the beginning of this paper. It is
worth noting that the attenuation of the Fe[2p] peak intensity by
the overlayer for 1, 2, and 3 treated mild steel plates. There is
also a noticeable increase in the noise level especially in the
case of the attachment of 2. The latter molecule seems to have
a fairly complete coverage as the Fe[2p] doublet is no longer
observed on the wide scan. The 700-1000 eV region charac-
teristic of Fe Auger lines shows instead a noisy background
due to increased inelastic energy loss.
As far as the C[1s] signals are concerned, we examined in
detail those corresponding to nitrophenyl and carboxyphenyl
groups attached to the surface by reduction of 1 and 3,
respectively (Figure 10), in order to determine if any part of
the signal could be assigned to the carbon-metal bond. For
nitrophenyl groups (obtained from 1), C[1s] can be fitted with
four components centered at 284.0, 285.0, 286.3, and 288.5 eV
assigned to the metal-bonded carbon, aromatic carbons, C-NO2
and/or C-O, and carboxylic carbons. The component assigned
to the carboxylic groups is small and the origin of these groups
is not clear. However, one must be aware that the main
component due to the C-C/CdC has a relative intensity which
exceeds that of the metal-bonded carbon. This, of course, makes
peak fitting of the C[1s] very complex especially when one has
Figure 6. PMIRRAS spectra of mild steel plates (a) polished, (b)
derivatized in ACN with 4-nitrophenyl groups, and (c) derivatized in
0
2 4
.1 N H SO .
vibrations of 1,4-disubsituted benzenes. In addition, one can
ascertain the loss of the diazonium group through the absence
of any strong absorption in the 2000-2130 cm-1 region.16b,c
Therefore this IR spectrum shows that during the electrochemi-
cal reduction the diazonium group has been lost and that one
of the benzene rings remains 1,4-disubtituted in good agreement
with the model of a benzoylphenyl group attached to the iron
surface by the position para to the carbonyl group.
Rutherford Backscattering Spectra (RBS). This method
+
measures the number of He ions backscattered by the surface
at a given energy, which permits the characterization of the
elements and their surface concentration. The escape depth of
the backscattered ions is of the order of the micron. It is
necessary that the atomic number of the atom to be identified
be about twice as large as that of iron to be differentiated. To
fulfill this condition we derivatized carefully polished mild steel
plates with 4-iodobenzene diazonium tetrafluoroborate. Figure
8
shows the RBS spectrum of such a plate after grafting in ACN.
Table 1 shows the results of different experiments as a function
of the solvent and potential. The only peak observed besides
that of iron corresponds to the iodine atoms, which confirms
the occurrence of the derivatization. Besides, analyses show that
the grafting is homogeneous throughout the sample and stable
under the ionic beam. In dilute sulfuric acid the optimum value
of the potential is -0.75 V/SCE leading to the highest surface
concentration; at more positive potentials the grafting comes
(
17) (a) Chehimi, M. M.; Delamar, M. J. Electron Spectrosc. Relat.
Phenom. 1988, 46, C1-C4. (b) Linndberg, B. J.; Hamrin, K.; Johanson,
G.; Gelius, U.; Fahlman, A.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970,
1, 286-298.

4546 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Adenier et al.
Figure 7. Reflection IRTF spectra of benzoylphenyl groups attached to the iron surface (top) and transmission IRTF spectrum of melted benzophenone
bottom).
(
increase of the oxygen peak from about 20% to about 40%),
one of them was then dipped in a solution of 2 in ACN + 0.1
M NBu4BF4, while the other one was grafted as previously
described in the same solution (a cathodic current is observed
which shows that the oxide layer is not thick enough to block
the electrode). In both cases only a small iodine signal was
observed amounting respectively to 0.3 and 0.4%. This signal
is much smaller than the signal observed after grafting on a
polished and cleaned plate indicating that the aryl groups are
bonded to iron and not to surface oxides.
Capacity of the Electrode. Attachment of an organic layer
to the surface of the electrode should decrease the capacity of
this electrode by adding a supplementary insulating layer in
series with the electrochemical double layer. The capacity can
be measured by an impedance method; we have recorded the
Nyquist diagrams (ZIm vs ZRe) (Figure S2 in the Supporting
Information). From the frequency of the maximum, it is possible
Figure 8. RBS spectrum of a mild steel plate modified (in ACN +
.1 M NBu BF ) with 4-iodophenyl groups.
0
4
4
to implement a low-intensity component at low-binding energy
due to the C-metal bond. Nevertheless, the rationale for
implementing such a component lies in the fact that the low-
binding energy side of the C[1s] peak exhibits indeed a slight
shoulder around a position corresponding to a carbide species.
In the case of carboxyphenyl groups attached through the
reduction of 3, it is worth noting that the carboxylic group leads,
as could be expected, to a much higher intensity of the apparent
maximum around 288-289 eV. The C[1s] is fitted with five
components as for nitrophenyl groups but with an additional
component (288.6 eV) due to -COOH. We also tentitatively
fitted the whole C[1s] spectrum with a C-metal component at
low binding energy (283.8 eV) that has an intensity comparable
to that of -COOH. The other components are assigned as
indicated above for nitrophenyl groups.
1
8
to obtain Cd. The capacity of a 3 mm diameter pure iron
2
electrode in 0.1 N H2SO4 was found to be 334 µF/cm . After
modification of the electrode surface with 4-hexadecyloxy
groups (by reduction of diazonium 8 in 0.1 N H2SO4), the
2
capacity falls to 192 µF/cm .
Surface Concentration of the Attached Groups. Two
methods can be used to measure the surface concentration of
aryl groups. The first one involves the measurement of the
charge used to reduce by one electron per molecule the nitro
groups bonded to the surface (after thorough rinsing of the
electrode and transfer to an ACN + 0.1 M NBu4BF4 solution).
This amounts to the integration of the voltammogram recorded
1
from an electrode modified with nitrophenyl groups. One of
the drawbacks of such a method is that it necessitates the
estimation of a baseline and of the end point of the voltammo-
gram. The second method relies on the integration of the RBS
spectra; in this case one must be certain that only the grafted
area of the electrode is included in the beam and that the surface
XPS spectra were also utilized to find out whether the grafting
was really taking place on iron or if the aryl group was bonded
to the oxide which can always be present on that surface. Two
steel plates were passivated in 1 N H2SO4 by sweeping the
potential to +1 V/SCE (a potential located in the passive range
of iron; passivation by the oxide layer can be observed by an
(18) Bard, A. J.; Faulkner, L. J. Electrochemical Methods; J. Wiley: New
York, 1993; p 345.

CoValent Modification of Iron Surfaces
J. Am. Chem. Soc., Vol. 123, No. 19, 2001 4547
Figure 10. XPS spectrum of the C[1s] peak of 4-nitrophenyl groups
Figure 9. XPS spectra of (a) a clean mild steel plate and (b) of a mild
steel plate derivatized with 4-nitrophenyl groups, (c) with 4-carboxy-
phenyl groups, and (d) with 4-iodophenyl groups.
(upper) and 4-carboxyphenyl groups (lower) attached to an iron surface.
Table 1. Surface Concentration of Aryl Groupsa
electrolysis
potential,
V/SCE
is stable under the ion beam. Both methods provide values
method of surface concn,
2 a
2
related to the geometrical surface (i.e. 1 cm for a 1 × 1 cm
sampleb
solventc
measurement
mol/cm
plate) of the electrode previously polished with 1 µm diamond
paste. The values in Table 1 provide results which, for similar
conditions, are in fair agreement. For example, grafting of
-
10
iron +1
ACN
-0.85
-0.85
-0.85
-0.85
-0.75
-0.75
VC
VC
VC
RBS
RBS
RBS
45 × 10
12 × 10
-10
iron + 11 ACN
iron + 12 ACN
steel +2 ACN
steel +2 0.1 N H
-10
8 × 10
48 × 10
32 × 10
0
-
-
10
10
4
-nitrophenyl or 4-iodophenyl groups of similar size led to
similar surface concentrations and increasing the size of the
attached group from 4-nitrophenyl to anthracenyl or anthracenyl-
2
2
SO
SO
4
steel
0.1 N H
4
9
,10-dione decreases the surface concentration. Surface con-
a
Surface concentration per geometrical surface.c Material of the
b
centrations obtained in dilute sulfuric acid reach at most 67%
of that obtained in ACN. This may be due to the oxidation of
the surface in sulfuric acid which decreases the surface available
for bonding aryl groups or to hydrogen formed on the surface
or adsorbed in the metal,19 which would change the properties
of the surface and prevent the electrochemical reaction. The
above surface concentrations can be compared with those
previously measured on carbon and particularly on HOPG (the
real surface of which is very close to the geometrical surface).
The surface concentrations of 4-nitrophenyl groups which have
electrode + diazonium salt used for derivatization. Used for deriva-
tization.
current). For the grafting of 5 the first transition time corre-
sponds to the reduction of the diazonium function. Nearly
identical transition times were obtained for iron and carbon
polished in the same way. This indicates that the same amount
of charge is needed to achieve complete coverage of the
electrode up to the time where the diazonium is no longer
reduced on the electrode. On HOPG such surface concentrations
have been shown to correspond to a closed-packed monolayer
of aryl rings standing up on the surface (a stereochemical
situation where 4-iodo- and 4-nitrophenyl groups occupy the
same surface and anthracenyl groups a larger one).
Atomic Force Microscopy (AFM) Imaging of the Surface.
Figure S3 (Supporting Information) shows the AFM images of
a mild steel surface that has been (a) carefully polished with 1
µm diamond paste and ultrasonically rinsed for 20 min in
acetone, (b) maintained in ACN + 0.1 M NBu4BF4 at -0.85
V/SCE for 30s, and (c) grafted by 4-n-butylphenyl groups
1
,2
-10
2
been measured lie in the range (12-18) × 10
mol/cm ;
1
however, on glassy carbon, polished under the same conditions
as the iron electrodes, or steel plates used in this paper, a value
-
10
2
of 40 × 10
mol/cm was obtained after reduction of 1.
Therefore, there is good agreement between the values obtained
on carbon and that on iron or steel. This was confirmed by
recording the chronopotentiometric curves (E ) f(t) at constant
(
19) O’M. Bockris, J. Surface Electrochemistry; Plenum: New York,
1
993; pp 745-852.

4548 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Adenier et al.
graft
(
through reduction of 4) in the same medium, at the same
I.E. ) 100(1 - icorr /icorr) reaches 82% for 4-butylphenyl
groups (by reduction of 5) in 5% KCl and 77% for dodecyloxy
groups (by reduction of 7) in 0.1 N H SO . These inhibition
potential for the same time. The polished surface (Figure S3a)
appears rather smooth with parallel grooves made by the
diamond grains as well as some small particles typically 20 nm
high which can be either iron or diamond particles that are not
removed from the surface by ultrasonic rinsing. Figure S3b does
not show significant changes by comparison with Figure S3a,
indicating, as could be expected from the i ) f(E) curves, that
no corrosion is taking place. Except for the particles initially
present on the surface, Figure S3c shows a homogeneous surface
where the grooves are still visible. The roughness of the surface
was measured: Ra varies from about 2 nm for Figure S3 (a and
b) and increases to only 3 nm upon grafting. The fact that no
significant change can be observed by AFM rules out the
existence of a nanometers thick polymeric layer but does not
exclude the formation of a multilayer (molecular models show
that a bilayer, obtained by substitution ortho to the butyl group,
of n-butylphenyl groups would not be thicker than a monolayer,
2
4
efficiencies correspond to the values which can be observed
when inhibitors such as alkynols and triazoles are added in
1
7,21,22
solution.
But starting from mild steel plates derivatized
as described in this paper it was possible to attach micrometer
thick polymeric layers which completely block the surface.
The formation of an organic layer covalently bonded to an
iron surface is not entirely unprecedented; the only similar proc-
ess of which we are aware has been described by G. Lecayon
and co-workers. Metallic surfaces such as Fe, Ni, Pt, and Au
have been covalently modified by electrochemical reduction of
vinylic monomers such as acrylonitrile, methacrylonitrile, and
butenenitrile. Thin polymeric layers (≈10-50 nm) covalently
bonded to the metal are obtained, the structure of which have
2
3-25
been thoroughly investigated.
corrosion by this process has proved to be quite efficient. The
Protection of metal from
25
2
6
∼
9.7 Å).
older investigations of Elofson and Gadallah should also be
quoted; they had investigated the reduction of diazonium salts
on mercury electrodes and they had used the radicals obtained
in this way for further cyclization reactions, but they also found
the formation of aryl mercury compounds which are analogues
of the aryl groups attached to the iron surface.
Many possible applications can be imagined for this process,
including the protection of iron against corrosion, the improve-
ment of adhesion between organic or polymeric layers and iron,
or the formation of iron surfaces with specific properties in the
biomedical field; it is indeed one of the characteristics of this
process that the organic groups which can be attached to the
iron surface can be varied at will and can be used as a further
a starting point for chemical reactions.
Discussion and Conclusion
We have shown that the electrochemical reduction of diazo-
nium salts on an iron or steel electrode leads to strongly bonded
aryl groups on the iron surface. Although this reaction should
be thermodynamically possible without electrochemical activa-
tion, it is too slow to occur to a significant extent as was
demonstrated by XPS experiments. The existence of attached
aryl groups was evidenced by cyclic voltammetry, XPS,
PMIRRAS, RBS, and capacity measurements. These different
methods rely on different physical principles; their combined
use and the convergent results obtained give confidence in the
reality of the grafting. XPS, FTIR, and PMIRRAS permit
qualitative characterization of either the atoms or the functional
groups; cyclic voltammetry and RBS provide quantitative data
on the surface concentration. Comparison with the data obtained
on carbon points to the formation of a monolayer, which is also
supported by AFM measurements that show no evidence for
the formation of a nanometer thick polymer layer. As iron is
very easily oxidized in contact with atmosphere or in aqueous
acidic solution, one could wonder whether an O-aryl bond is
formed instead of an Fe-aryl bond, but we have seen above
that XPS measurements indicate that the grafting on a purposely
oxidized surface is negligible. Another point that should be
discussed is the nature of the Fe-aryl bond. The grafting resists
ultrasonic rinsing in a variety of solvents and for long periods
of time but also several days of rinsing in boiling toluene. This
Further experiments are now in progress which show that
the process can be extended to other engineering (Zn, Cu, Ni,
Co) and coinage metals (Au, Pt).
Experimental Section
Chemicals, Electrodes. ACN was obtained from Merck (Uvasol),
H SO from Prolabo (Tritrinorm), and NBu BF from Fluka (puriss.).
2 4 4 4
Diazonium salts 1 and 11 are of commercial origin (Aldrich) and the
synthesis of 3 has been previously described;1c 2, 3-6, 9, 10, and 12
were obtained from the corresponding commercial amines by standard
27
methods. The diazonium salts were kept in a freezer before use.
28
4
-Iodobenzenze diazonium tetrafluroborate 2: mp 126 °C (lit.
1
mp 123-124 °C);
matics).
H NMR (200 MHz, DMSO) δ 8.4 (q, 4H, aro-
(
21) Epelboin, I.; Keddam M.; Takenouti, H. J. Appl. Electrochem. 1972,
, 71.
(22) Mernari, B.; El Attari, H.; Traisnel, M.; Bentiss, F.; Lagrenee, M.
Corros. Sci. 1998, 40, 391-399.
23) Lecayon, G.; Bouizem, Y.; Le Gressus, C.; Reynaud, C.; Boiziau,
2+
makes a mere adsorption unlikely. An ionic bond between Fe
2
3
+
or Fe and an aryl carbanion is also highly unlikely, as aryl
carbanions are very strong bases which would be readily
protonated even in ACN. Therefore one must conclude the
existence of a covalent Fe-aryl bond; such bonds are known
in organometallic chemistry,20 for example, the compound
FeArCp(CO)2 (Ar ) C6H4CH3 and Cp ) cyclopentadienyl) has
been prepared and fully characterized. This is also supported
(
C.; Juret, C. Chem Phys. 1982, 91, 506-510.
(24) Charlier, J.; Bureau, C.; Lecayon G. J. Electroanal. Chem. 1999,
465, 200-208 and references therein.
(
25) Deniau, G.; Lecayon, G.; Bureau, C.; Tanguy J. In ProtectiVe
Coatings and Thin Films; Pauleau, Y., Barna, P. B., Eds.; Kluwer
Academic: Amsterdam, 1997; pp 265-278.
(
i) by the observation of a shoulder at low binding energy on
(26) (a) Elofson, R. M.; Eldsberg, R. L.; Mecherly, P. A. J. Electrochem.
Soc. 1950, 97, 166-177. (b) Elofson, R. M.; Gadallah, F. F. J. Org. Chem.
the C[1s] peak which was assigned to a component corresponding
to the carbon bonded to the metal and (ii) by the observation
of a 1,4-substitution pattern on the FTIR spectrum of attached
benzoylphenyl groups.
The grafted organic layer provides some protection of the
mild steel surface against corrosion; the inhibition efficiency
1
3
969, 34, 854-857. (c) Gadallah F. F.; Elofson, R. M. J. Org. Chem. 1969,
4, 3335-3337. (d) Elofson, R. M.; Gadallah, F. F.; Schulz, K. F. J. Org.
Chem. 1971, 36, 1526-1531. (e) Elofson, R. M.; Gadallah, F. F. J. Org.
Chem. 1971, 36, 1769-1771. (f) Elofson, R. M.; Cantu, A. A.; Gadallah,
F. F. J. Org. Chem. 1973, 38, 2386-2393.
(
27) Furniss, S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R.
Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman:
London, 1989; p 920.
(
20) Bowden, F. L.; Wood, L. H. Compounds with iron-carbon σ-bonds.
(28) Korzeniowski, S. H.; Leopold, A.; Beadle, J. R.; Ahern, M. F.;
Sheppard, W. A.; Khanna, R. K.; Gokel, G. W. J. Org. Chem. 1981, 46,
2153-2159.
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.

CoValent Modification of Iron Surfaces
-Methylbenzenze diazonium tetrafluoroborate 4: mp 110 °C
J. Am. Chem. Soc., Vol. 123, No. 19, 2001 4549
3
0
4
currents were obtained from the Tafel plots with the EGG corrosion
software without ohmic drop compensation.
1
dec (lit. mp 110 °C dec); H NMR (200 MHz, DMSO) δ 2.5 (q, 3H,
CH ), 7.8-8.5 (two d, 4H, aromatics).
-n-Butylbenzenediazonium tetrafluoroborate 5: In an Erlen-
meyer flask placed in an ice bath, 4-n-butylaniline (149 mg, 1 mmol)
is dissolved in HBF (34%, d ) 1.23, 3 mmol, 30 mL) and stirred for
5 min, then NaNO (1.5 mmol, 103 mg) is added. The mixture is
stirred for 20 min. CH Cl (30 mL) is then added and the organic and
aqueous layers are separated in a separatory funnel. The organic phase
is dried over MgSO and evaporated to give a pale yellow solid. No
melting point could be measured as described in the literature.
3
Surface Characterization. XPS, PMIRRAS, and RBS equipment
2
8
1c
4
and conditions have been described previously. Elemental percentages
x
i
were obtained from x
i
i i i i i i i
) (A /S /∑ (A /S ), where A and S are
4
respectively the peak area of the detected element and the sensitivity
coefficient deduced from the study of the stoichiometric coefficient.
AFM images were obtained with a Nanoscope III. A series of
reproducible images were obtained from which we have selected that
of Figure S2. The average surface roughness expressed in nm, was
calculated by using the manufacturer’s software according to:
1
2
2
2
4
28 1
H
3
NMR (200 MHz, DMSO) δ 0.9 (t, J ) 6 Hz, 3H, CH
3
), 0.8-2.8 (m,
3
3
1
LxLy
Lx
Ly
5
H, CH
H, aromatics).
-n-Dodecylbenzenediazonium tetrafluoroborate 6: mp 70 °C
2
), 2.8 (t, J ) 8 Hz, 2H, CH benzylic), 7.6-8.5 (q, J ) 8 Hz,
2
Ra )
|f(x,y)| dx dy
∫ ∫
0
0
4
3
0
4
1
3
L and L are the dimensions of the surface and f(x,y) is the surface
dec; H NMR (200 MHz, DMSO) δ 0.85 (t, J ) 6 Hz, 3H, CH
3
),
x
y
3
relative to a calculated flat plane based upon the surface data that has
equal volume above and below the plane.
1
(
.1-2.8 (m, 20 H, CH
2
), 2.5 (t, J ) 8 Hz, 2H, CH
q, J ) 8 Hz, 4H, aromatics).
was prepared from 4-dodecyloxyaniline as described before:
2
benzylic), 7.8-8.5
3
3
1
7
1
Acknowledgment. We acknowledge the help of Mrs. Carole
Bilhem and Pascal Bargiela (ITODYS) for recording the XPS
spectra and of Dr. Mohammad Bousm e` ne for permission to use
his electrochemical corrosion equipment. We are grateful to M.
Gruselle (Laboratoire de Synth e` se des Compos e´ s Organom e´ t-
alliques, Universit e´ Pierre et Marie Curie) and to Dr. O. T.
Yurevna (INEOS Institute, Academy of Sciences, Moscow) for
the gift of the product FeArCp(CO)2. The C[1s] spectra were
fitted with the Winspec software kindly provided by Dr. Pierre
Lovette (Lise, Namur, Belgium).
white solid, H NMR (200 MHz, DMSO) δ 0.8-1.8 (m, 25H, aliphatic
3
protons), 4.3 (t, 2H, O-CH
2
), 7.5-8.5 (q, J ) 8 Hz, 4H, aromatics).
was prepared in a similar way but the diazonium salt still contained
8
1
some amine: white solid, H NMR (200 MHz, DMSO) δ 0.8-1.8 (m,
3H, aliphatic protons), 4.2 (t, 2H, O-CH ), 7.2-8.5 (q, J ) 8 Hz,
3
3
2
4
H, aromatics).
Electrodes were prepared either from 1 mm diameter iron wire
Johnson-Matthey 99.99%) embedded in epoxy resin or from 3 mm
(
diameter mild steel buttons maintained in a Teflon holder. Mild steel
plates (containing Fe 95.68%; C 0.31%; Mn 2.03%; P 0.05%; S 0.13%;
N 0.56%; Si 0.10%; Cu 0.07%; Ni 0.18%; Cr 0.30%; Sn 0.01%; Al
0
.58%) were kindly given by the Soci e´ t e´ Sollac.
Electrochemical Equipment. Electrochemical curves were obtained
Supporting Information Available: Figure S1 showing the
cyclic voltammogram of anthracene on an iron electrode and
of an iron electrode modified with anthracenyl groups; Figure
2 showing the impedance diagram in 0.1 N H2SO4 of an iron
electrode before and after modification by hexadecyloxyphenyl
groups; and Figure S3 showing AFM images of mild steel plates
before or after derivatization (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
either with a homemade potentiostat or a Versastat II from EGG
Priceton Applied Research). Impedance measurements were obtained
(
with an EGG potentiostat 263A and a lock-in amplifier 5210. The
frequencies investigated ranged from 0.001 Hz to 20 kHz and corrosion
(
29) Balz, G.; Schiemann, G. Berichte 1927, 60, 1188-1190.
(30) Bartulin, J.; Cardenas, G.; Maturana, H.; Ramirez, A.; Zunza, H.
Bol. Soc. Quim. Chil. Quim. 1981, 26, 1-5.
31) Zinsou, A.; Veber, M.; Strzelecka, H.; Jallabert, C.; Fourr e´ P. New
J. Chem. 1993, 17, 309-313.
(
JA003276F