Article pubs.acs.org/Langmuir

One-Step Formation of Bifunctionnal Aryl/Alkyl Grafted Films on Conducting Surfaces by the Reduction of Diazonium Salts in the Presence of Alkyl Iodides Dardan Hetemi,†,‡ Hassan Hazimeh,§ Philippe Decorse,† Anouk Galtayries,∥ Catherine Combellas,†,§ Frédéric Kanoufi,†,§ Jean Pinson,*,† and Fetah I. Podvorica*,†,‡,§ †

Sorbonne Paris Cité, Univ Paris Diderot, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75013 Paris, France Chemistry Department of Natural Sciences Faculty, University of Prishtina, rr. “Nëna Tereze” nr. 5, 10000 Prishtina, Kosovo § ESPCI ParisTech, 10 rue Vauquelin, 75231 Paris Cedex 05, France ∥ Institut de Recherche de Chimie Paris, UMR 8247 CNRS, 11 rue Pierre et Marie Curie, 75005 Paris, France ‡

S Supporting Information *

ABSTRACT: The formation of partial perfluoroalkyl or alkyl radicals from partial perfluoroalkyl or alkyl iodides (ICH2CH2C6F13 and IC6H13) and their reaction with surfaces takes place at low driving force (∼−0.5 V/SCE) when the electrochemical reaction is performed in acetonitrile in the presence of diazonium salts (ArN2+), at a potential where the latter is reduced. By comparison to the direct grafting of ICH2CH2C6F13, this corresponds to a gain of ∼2.1 V in the case of 4-nitrobenzenediazonium. Such electrochemical reaction permits the modification of gold surfaces (and also carbon, iron, and copper) with mixed aryl−alkyl groups (Ar = 3-CH3−C6H4, 4-NO2−C6H4, and 4-Br−C6H4, R = C6H13 or (CH2)2−C6F13). These strongly bonded mixed layers are characterized by IRRAS, XPS, ToF-SIMS, ellipsometry, water contact angles, and cyclic voltammetry. The relative proportions of grafted aryl and alkyl groups can be varied along with the relative concentrations of diazonium and iodide components in the grafting solution. The formation of the films is assigned to the reaction of aryl and alkyl radicals on the surface and on the first grafted layer. The former is obtained from the electrochemical reduction of the diazonium salt; the latter results from the abstraction of an iodine atom by the aryl radical. The mechanism involved in the growth of the film provides an example of complex surface radical chemistry.



INTRODUCTION

sonication and mechanical tests. The electrochemical oxidation of alkyl carboxylates also permits the binding of alkyl chains to carbon (GC−R).7 For these two methods, only a limited number of substrates can withstand the oxidation potentials necessary for the reaction; as a consequence, mostly carbon has been investigated. The electrografting of alkyl chains by the oxidation of Grignard reagents was achieved on hydrogenated SiH (Si-R) under ultradry and oxygen-free conditions that limit the use of this method.8 Finally, alkyl iodides, R−I, were reduced on various metals (Au, Cu, Fe, and TiN) at quite negative potentials (∼−2.1 V/SCE on Au) to give metal−R films owing to the attack of R· radicals on the surface.9 On gold, the spontaneous grafting of both alkyl groups and iodine from alkyl iodides has been described.10 Electrochemical or spontaneous reduction of aryldiazonium salts permits the bonding of aryl groups onto various surfaces (carbon, metal, semiconductor, and polymer), leading to rich surface chemistry owing to the large range of substituents on

Surface modification by the attachment of organic groups is a subject of current interest both in the academic world1 and in industry.2 Several methods are available to bind organic molecules to various surface oxides, metals, carbon, or polymers. Long-chain alkyl thiol self-assembled monolayers (SAMs) can be obtained spontaneously on Au (and in fewer examples on silver, nickel, and copper), leading to Au−SR structures.3 The formation of the Au−S bond is reversible, which permits the motion of the −S−R chains on the surface to produce compact films, but at the same time, this limits the stability of the layer on the surface. As a consequence, thiol SAMs can be exchanged with other thiols; they are thermally poorly stable and present limited electrochemical stability.4,5 An alternative to thiols SAMs is provided by electrografting methods that allow bonding multilayers of alkyl chains by reduction or oxidation.1,6 Such reactions rely on the formation of alkyl radicals that strongly bind to the surface. Various examples of electrografting procedures have been developed on glassy carbon (GC). For example, the electrochemical oxidation of aliphatic amines provides GC−NH−R layers by the reaction of an aminyl radical, leading to films that withstand ultra© XXXX American Chemical Society

Received: February 27, 2015 Revised: April 16, 2015

A

DOI: 10.1021/acs.langmuir.5b00754 Langmuir XXXX, XXX, XXX−XXX

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Langmuir the aromatic ring.11−15 The attached organic film, the thickness of which varies from a monolayer16,17 to several hundreds of nanometers,18,19 is strongly bonded to the surface.20−23 The formation of these polyaryl films has been assigned to the attack of aryl radicals on the surface and on the first attached aryl group, leading to the growth of the layer.19,24 Regarding mixed layers, mixed-thiol SAMs have been obtained from thiols of different lengths (i.e., C8 and C16),25 and mixed alkyl/aryl thiols SAMs have been investigated for the immobilization of Cu complexes.26 Mixed nitroxyl/CH3terminated thiol SAMs27 have been prepared on Au with segregated phases or a random distribution, and their electrocatalytic reactivity was shown to depend on their distribution. Mixed aryl layers have also been obtained from diazonium salts (i.e., by the reduction of a mixture of two diazonium salts).28−30 A binary mixture of aryl diazonium salts bearing oppositely charged para substituents (−SO3− and −N(CH3)2+) has been electrografted;31 interestingly, (i) in the mixture solution, the two aryl diazonium salts undergo reductive adsorption at the same potential, which is distinctively less negative than the potential required for the reduction of either of the two aryldiazonium salts alone, and (ii) the surface ratio of the two phenyl derivatives is consistently 1:1 regardless of the ratio of the two aryl diazonium salts in the modification solutions. The grafting of the 1-(bithien-2-yl)-4-benzenediazonium (BTB) salt complexed by β-cyclodextrin (CD) provides a layer of grafted BTB oligomers complexed by CD. When CD is removed from the surface, pinholes are created and the nitrobenzenediazonium salt is reduced to graft a second component.32 Following the same strategy, after electrografting of the sterically hindered 4-((triisopropylsilyl)ethynyl)phenyl groups, the cleavage of the isopropyl groups and further grafting of nitrophenyl groups also permits to obtain mixed aryl layers.33 The formation of mixed aryl−alkyl SAMs has been described by the exchange of one thiol for another,34,35 but mixed aryl− alkyl films based on electrografting have never been reported. Here, we describe the preparation of mixed aryl−alkyl films by the electrografting of solutions containing a diazonium salt and an alkyl iodide. The electrografting is performed on various electrode surfaces: gold (Au), glassy carbon, (GC), iron, and copper. It gives mixed aryl−alkyl films of variable compositions and thicknesses. The mechanism involves a huge positive potential shift based on an iodine atom abstraction, larger than that previously reported with 2,6-dimethylbenzenediazonium that led to the exclusive grafting of alkyl groups on GC.36 The compounds electrografted are 3-methyl-, 4-nitro-, and 4bromo-benzenediazonium salts (3-MBz, 4-NBz) and, 1-iodo1H,1H,2H,2H-perfluoro-octane (1), and 1-iodohexane (2). The modified surfaces will be designated as Au(4-NBz + 1) or GC(3-MBz + 2). In the following text, the partial perfluoroalkyl group of 1 will be referred to as perfluoroalkyl for the sake of simplicity.



were used without further purification. 3-Methylbenzenediazonium tetrafluoroborate was synthesized in an aqueous solution in the presence of sodium nitrite by a standard procedure.37 Substrates. Gold-coated silicon wafers (1 × 1 cm2, Aldrich, 100 nm coating) were cleaned with concentrated H2SO4 at room temperature, and rinsed under sonication for 10 min in Milli-Q water. Before modification, the plates were dried in a stream of nitrogen. The polished glassy carbon (GC) plates were obtained from Sigradur (Germany). Before modification, they were rinsed in pure ethanol and dried under a stream of nitrogen. Vulcan XC-72 carbon powder was a gift from Cabot Corporation (USA), and mild steel and copper were industrial samples obtained from Weber Métaux (Paris). Electrodes for cyclic voltammetry were Au wires (d = 1 mm) and GC carbon rods (Tokai,̈ Japan, d = 2 mm) imbedded in epoxy resin. They were polished with different grades of polishing paper and finally with a 0.04 μm alumina slurry on a polishing cloth (DP-Nap, Struers, Denmark) using a Presi Mecatech 234 polishing machine. After being polished, the electrodes were rinsed with Milli-Q water and sonicated for 10 min in acetone to avoid organic contaminants. Functionalization of Electrodes and Plates. Electrografting of Au, GC electrodes, and plates was performed by chronoamperometry in ACN + 0.1 M NBu4PF6 solutions with different concentrations of aryl diazonium salt and alkyl iodide. The reference electrode was a saturated calomel electrode (SCE), and the counter electrode a platinum foil. The grafting potential was −0.5 V/SCE for all experiments; it was maintained for 10 min unless otherwise stated. Spontaneous grafting was performed on Vulcan XC 72, Fe and Cu by dipping the substrate into the solution containing the diazonium salt and the alkyl iodide for different duration times. In all experiments, after modification the plates were rinsed under ultrasonication for 10 min in ACN. IRRAS and IR-ATR. Spectra of the modified plates or carbon powder were recorded using a purged (low CO2, dry air) Jasco FT/IR6100 Fourier transform infrared spectrometer equipped with an MCT (mercury−cadmium−telluride) detector. For each spectrum, 1000 scans were accumulated with a spectral resolution of 4 cm−1. The background recorded before each spectrum was that of a clean substrate. IRATR spectra were recorded using a Ge crystal. XPS. X-ray photoelectron spectra were recorded using a Thermo VG Scientific ESCALAB 250 system fitted with a microfocused, monochromatic Al Kα X-ray source (1486.6 eV) and a magnetic lens, which increases the electron acceptance angle and hence the sensitivity. The pass energy was set at 150 and 40 eV for the survey and the narrow regions, respectively. The Avantage software, version 4.67, was used for digital acquisition and data processing. The spectra were calibrated against C 1s set at 285 eV. ToF-SIMS. Time-of-flight secondary-ion mass spectrometry data were acquired using a TOF-SIMS V spectrometer (ION-TOF GmbH). The analysis chamber was maintained at less than 5 × 10−7 Pa under the operating conditions. The total primary ion flux was below 1012 ions cm2 to ensure static conditions. The analysis beam was a pulsed 25 keV Bi+ primary ion source (liquid metal ion gun, LMIG) at a current of about 1 pA (high current bunched mode), rastered over a scan area of 500 × 500 μm2. Ellipsometry. The thicknesses (th) of films on Au and GC were measured with a Sentech SE400 monowavelength ellipsometer. The following values were taken as refractive indexes, ns, and dispersion coefficients, ks, for Au (ns = 0.185, ks = 3.399) and for GC (ns = 1.926 and ks = 0.743). These values were measured on clean surfaces before grafting, and the film thicknesses were deduced from the same plates after modification, taking ns = 1.46 and ks = 0 for the organic layer. Electrochemical Measurements. Electrochemical experiments were performed with an EG&G 263A potentiostat/galvanostat and Echem 4.30 version software. All experiments were carried out in ACN solutions deoxygenated with nitrogen. All potentials were measured versus the SCE electrode. Water Contact Angles. They were measured with a Kruss DSA3 instrument. A drop (3 μL) of Milli-Q water was automatically deposited on the top of the test sample placed in a horizontal position on the instrument stage. At least five measurements were made for

EXPERIMENTAL SECTION

Materials. 1-Iodo-1H,1H,2H,2H-perfluoro-octane (ICH2CH2C6F13, 1, 97%) was purchased from Alfa-Aesar; 1iodohexane (2, 98%), 4-nitro-, and 4-bromobenzenediazonium tetrafluoroborates (97 and 96%, respectively), and tetrabutylammonium hexafluorophosphate were purchased from Sigma-Aldrich; potassium chloride (99.5%) was purchased from Prolabo; and potassium ferricyanide (99%) was purchased from Merck. They B

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Table 1. Thicknesses and Water Contact Angles of Mixed Aryl and Perfluoroalkyl or Alkyl Films Electrografted on Au and GC Surfaces

a

1 or 2, mMa

entry

diazonium salt, mM

1 2 3 4 5 6 7

4-NBz/2 4-NBz/50 4-NBz/2 4-NBz/2 4-NBz/2 4-NBz/50 4-NBz/2

1/200 1/100 1/10 1/10 2/200 2/10

8 9 10

4-NBz/50 4-BrBz/50 4-BrBz/50

1/200 1/200 1/200

E V/SCE Au −0.5 −0.5 −0.5 −0.5 spontaneous −0.5 −0.5 GC −0.5 −0.5 spontaneous

reaction time/min

water contact angle θ/deg

thickness nmb

10 10 10 0.5 10 10 10

80 ± 1

7.4 ± 0.4 >100 33.0 ± 0.5 1.8 ± 0.7 from 0.8 to 1.6c 28.4 ± 1.1 8.5 ± 1.5

10 10 10

121 ± 2.9 84 ± 2.5 89 ± 1 87 ± 1 110 ± 2 111 ± 2

8.7 ± 0.3 8.2 ± 0.3 1.1 ± 0.2

1 = IC2H4C6F13; 2 = IC6H13. bMeasured by ellipsometry. cDepending on the location of the measurement.

each sample. The values of the contact angles were calculated by the tangent method using drop shape analysis software.

6%) and F 1s (689 eV, 29%). The N 1s peak can be deconvoluted into a first peak at 406 eV (2.3%) that pertains to the NO2 group and a second one at 400 eV (3.7%) that is also observed on a bare GC sample (2.2%). The deconvolution of the C 1s peak shows the terminal CF3 (293.7 eV, 2.2%); CF2 (−CF2)4−CF3 at 291.6 eV, 8.5%; −CH2−CF2 at 290.6 eV, 0.85%); and −CH2−CF2 (286.4 eV, 3.15%) and GC−CH2 (285.0 eV, 42.3%, which cannot be distinguished from the substrate itself or the aromatic carbons). This deconvolution is in agreement with the XPS spectrum of Au−S−(CH2)2−C6F13 (CF3, 293.8 eV; −(CF2)4−CF3, 291.8 eV; −CH2−CF2, 290.8 eV; CH2−CF2, 286.1 eV; and Au−CH2, 285.0 eV).38,39 The somewhat smaller relative intensity of −CH 2−CF2 by comparison with CF3 has been rationalized in ref 39. The N 1s (NO2, 406 eV) and F 1s intensities indicate equivalent numbers of nitrophenyl rings and perfluoroalkyl chains. By changing the take-off angle from 90 to 60 and 30°, there is no significant change of the N 1s or F 1s %, indicating a homogeneous film. On Au, a similar spectrum is obtained, but it also contains I 3d5/2 (619 eV, 2%) due to the spontaneous decomposition of 1 on the Au surface.10 Tof-SIMS for GC Plates. To obtain additional information on the structure of the films, ToF-SIMS spectra of GC surfaces modified with 4-bromobenzenediazonium (4-BrBz) were examined. 4-BrBz was used because the mass peaks will be identified with more certainty due to the two isotopes of bromine. Two films of different thicknesses were prepared (entries 10 and 9 in Table 1) from 4-BrBz and 1. The first one, GC(4-BrBz + 1)thin, that is 1.1 nm thick should allow the identification of the species at the beginning of the formation of the layer while the second one, GC(4-BrBz + 1)thick, that is 8.2 nm thick should allow the identification of the outer surface of a thicker film in static mode. The most significant fragments are gathered in Table 2. Both spectra indicate the presence of F−, CF2−, CF3−, Br−, C6H4-Br+ that reflect both the perfluoroalkyl chain and the aromatic group in the film. GC(4-BrBz + 1)thin also presents (CH2)2−CF2+ and (CH2)2−(CF2)2+ fragments that are related to the perfluoroalkyl chain on the surface whereas GC(4-BrBz + 1)thick shows the signature of mixed perfluoroalkyl-aryl fragments such as [C6H3(Br)(CH2)2−(CF2)2 + H]+ that indicate a substitution of the aromatic group by a perfluoroalkyl chain. We have also checked the absence of fragments such as C6H3BrCF+ that would be related to the formation of −·CF− radicals. To interpret the spectra, it must be remembered that



RESULTS AND DISCUSSION Infrared Spectroscopy, Ellipsometry, and Water Contact Angles for Au and GC Plates. Au(4-NBz + 1). When an Au plate has been modified by chronoamperometry (10 min at E = −0.5 V/SCE) in the presence of 4-NBz (2 mM) and 1 (100 mM), a 33 nm film is obtained on the Au surface (entry 3, Table 1). The IRRAS spectrum (Figure 1A) shows the signature of the perfluoroalkyl groups (1248 and 1147 cm−1) and the aryl groups (1600 cm−1) as well as that of the nitro group (1530 and 1350 cm−1). When the reaction has been performed without electrochemical activation (Au(4-NBz + 1)spont from 4-NBz (2 mM) + 1 (10 mM) for 10 min, (entry 5, Table 1, Figure 1B), the grafting of a mixed layer including both the nitrophenyl and the perfluoroalkyl groups is also observed (similar absorbances for both groups). The film is close to a monolayer but inhomogeneous: out of 18 ellipsometric measurements, 8 indicated a mean value of th = 0.8 ± 0.2 nm while 10 provided th = 1.6 ± 0.5 nm; 0.8 nm likely corresponds to a nitrophenyl group (∼0.7 nm from molecular models) whereas 1.6 nm corresponds to a C6H4(NO2)− (CH2)2C6F13 group. A similar Au(4-NBz + 1) thin film (th =1.8 ± 0.7 nm) was obtained by chronoamperometry at E = −0.5 V/SCE for 30 s from 2 mM 4-NBz and 10 mM 1 (entry 4, Table 1). On GC, the IR-ATR spectrum presents similar nitrophenyl and perfluoroalkyl bands (entry 8, Table 1, Figure 1D). However, the reaction is less efficient than on Au (compare entries 2 and 8). Au(4-NBz + 2). Two films of different thicknesses (28.4 and 8.5 nm) were prepared from 4-NBz and 2 (entry 6, Figure 1C; entry 7, Table 1). The IR bands corresponding to the alkyl (2956, 2933, and 2859 cm−1), aryl (1600 cm−1), and nitro (1530, 1350 cm−1) groups are clearly visible, and the signal of the nitro group is the most intense (Figure 1C). The results for the electrografting of 3-MBz in the presence of 1 or 2 are commented in the Supporting Information (Table S1 and Figure S1). The spontaneous reaction on carbon black (Vulcan XC72), mild steel, and copper from 4-NBz and 1 also leads to the grafting of the latter (Figure S2). XPS for GC Plates. The survey spectrum of GC(4-NBz + 1) (entry 8, Table 1, Figure 2) shows the signature of C 1s (282−295 eV, 57%), O 1s (533 eV, 8%), N 1s (400−406 eV, C

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Figure 2. XPS spectrum of GC(4-NBz + 1). (A) Survey spectrum and N 1s and F 1s (insets). (B) Deconvoluted C 1s spectrum.

SCE, the voltammetric behavior of 4-NBz and 1 was examined. The cyclic voltammetry at a carbon electrode of 1 (1 mM, Figure 3Aa) and 4-NBz (1 mM, Figure 3Ab) recorded separately shows peak potentials at respectively −1.92 and +0.15 V/SCE, that is, with a potential difference of ΔEp = 2.07 V. The reduction of 4-NBz involves a one-electron transfer at low concentrations (0.6 mM) and a relatively fast scan rate (1 V s−1), but at higher concentration, the height of the wave reaches a limiting value.40 The reduction of 1 involves a two-electron transfer.41 In Figure 3A, the height of the peaks of 4-NBz and 1 are not in the 1/2 ratio as could be expected; this is due to the modification of the electrode while the voltammogram of 4-NBz is recorded. When both 4-NBz and 1 are present in the same solution (Figure 3B), on the third scan the voltammetric peak of 4-NBz decreases to nearly zero, which is usually observed during the electrografting of diazonium salts. Figure 3C presents the voltammogram of a GC(4-NBz +1) electrode modified by chronoamperometry at E = −0.5 V/SCE for 5 min, after rinsing and transfer in an ACN + 0.1 M NBu4PF6 solution; the close to reversible signal of the nitrophenyl group is observed, indicating the presence of the latter groups in the film. Figures S3 and S4 (Supporting Information) present the electrografting under similar conditions by cyclic voltammetry and chronoamperometry on Au, using higher concentrations of both 4-NBz (4 mM) and 1 (20 mM). Similar but stronger trends are observed: the voltammetric peak of 4-NBz disappears on the second scan. Altogether, these voltammograms indicate the grafting of the

Figure 1. IR spectra of modified plates (1 cm2). (A) Au(4-NBz + 1), (B) Au(4-NBz + 1)spont, (C) Au(4-NBz +2), and (D) GC(4-NBz + 1). (A, C, and D) Obtained by chronoamperometry at E = −0.5 V/SCE for 10 min in ACN + 0.1 M NBu4PF6 containing (A) 4-NBz (2 mM) + 1 (100 mM), (C) 4-NBz (50 mM) + 2 (200 mM), and (D) 4-NBz (50 mM) + 1 (200 mM). (B) Obtained by spontaneous grafting for 10 min in ACN + 0.1 M NBu4PF6 containing 4-NBz (2 mM) + 1 (10 mM). (A−C) IRRAS and (D) IR-ATR.

static ToF-SIMS is concerned only with the outermost layer of the film. Therefore, the spectra indicate the presence of both the aromatic groups and the perfluoroalkyl chains at the beginning of growth and also the presence of aryl groups substituted by the perfluoroalkyl chains at the outer layer of a thicker film. Electrochemistry for GC Electrodes. To explain the formation of aryl-perfluoroalkyl layers at a potential of −0.5 V/ D

DOI: 10.1021/acs.langmuir.5b00754 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Table 2. ToF-SIMS Spectra for GC(4-BrBz + 1)thin and GC(4-BrBz + 1)thick GC(4-BrBz + 1)thin ions −

F CF2− CF3− [(CH2)2−CF2 + 2H]+ Br− [(CH2)2−(CF2)2 + 2H]+ C6H4Br+ [C6H3(Br)CH2+ 2H]+ [C6H3(Br)(CH2)2 + H]+ [C6H3(Br)(CH2)2−(CF2) + H]+ [C6H3(Br)(CH2)2−(CF2)2 + H]+

GC(4-BrBz + 1)thick

m/z calculated

m/z experimental

intensity

m/z experimental

intensity

18.998 49.996 68.994 80.020 78.918 80.913 130.024 154.950 156.948 169.974 171.972 182.982 184.980 232.978 234.974 282.974 284.972

18.995 49.980. 68.957 80.021 78.909 80.916 130.040 154.903 156.973

321 425 27 253 7775 12 490 433 660 402 114 5512 5899 5790

18.993 49.976 68.955

1 999 203 49 954 153 937

78.903 80.909

1 997 133 1 934 075

154.890 154.961 169.984 171.997 182.883 184.972 233.013 235.006 282.005 284.992

80 469 87 223 24 960 97 469 128 973 10 023 15 091 23 420 15 338

a function of time in Figure 5A. The ratio of the absorbances of the nitrophenyl (I1350) and perfluoroalkyl (I1248) groups, measured by IRRAS, is reported as a function of the thickness in Figure 5B. [Note that the IR beam of micrometer wavelength probes the whole thickness of thin films (0 < th

alkyl grafted films on conducting surfaces by the reduction of diazonium salts in the presence of alkyl iodides.

The formation of partial perfluoroalkyl or alkyl radicals from partial perfluoroalkyl or alkyl iodides (ICH2CH2C6F13 and IC6H13) and their reaction wi...
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