Article pubs.acs.org/Langmuir
Covalent Modification of Glassy Carbon Surfaces by Electrochemical Grafting of Aryl Iodides Line Koefoed,† Steen U. Pedersen,*,† and Kim Daasbjerg*,†,‡ †
Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO) and ‡Carbon Dioxide Activation Center, Aarhus University, 8000 Aarhus C, Denmark S Supporting Information *
ABSTRACT: The reduction of an aryl iodide is generally believed to involve a clean-cut two-electron reduction to produce an aryl anion and iodide. This is in contradiction to what is observed if a highly efficient grafting agent, such as an aryldiazonium salt, is employed. The difference in behavior is explained by the much more extreme potentials required for reducing an aryl iodide, which facilitates the further reduction of the aryl radical formed as an intermediate. However, in this study we disclose that electrografting of aryl iodides is indeed possible upon extended voltammetric cycling. This implies that even if the number of aryl radicals left unreduced at the electrode surface is exceedingly small, a functionalization of the surface may still be promoted. In fact, the grafting efficiency is found to increase during the grafting process, which may be explained by the inhibiting effect the growing film exerts on the competing reduction of the aryl radical. The slow buildup of the organic film results in a well-ordered structure as shown by the well-defined electrochemical response from a grafted film containing ferrocenylmethyl groups. Hence, the reduction of aryl iodides allows a precisely controlled, albeit slow, growth of thin organic films.
■
step (pathway a)8 or a stepwise electron transfer fragmentation process (pathway b).9 In either case the highly efficient grafting agent, the aryl radical Ar•, is formed on or close to the electrode surface. In the aryldiazonium case, Ar• is formed at a potential insufficient for its further reduction to the aryl anion. The aryl radical will, therefore, react with whatever is close to it, i.e., the electrode surface, already grafted layers, another radical, or the solvent.7 If the aryl anion is formed, it will most likely be protonated to afford the corresponding arene (ArH). The electrochemical reduction of aryl iodides (as well as aryl bromides and chlorides) is well-established and proceeds via a two-electron process (a so-called ECE mechanism) to form ArH and iodide.4,7,10 Usually, the electrochemical reduction and dissociation of the carbon−halogen bond occur in a stepwise manner,10 although the concerted path may be feasible for a few specific iodobenzenes at low sweep rates.11 Noteworthy, if the dissociation is slow, Ar• is formed far from the electrode surface. Nonetheless, Ar• is still reduced, now by ArX•−, in a socalled DISP mechanism. Under such conditions no grafting can take place. Also, indirect reduction of 4-iodobenzonitrile via a mediator has been shown to afford the corresponding Ar• which, in the presence of high concentrations of activated olefins such as styrene or acrylonitrile, may add to these.12 In general, the
INTRODUCTION Electrochemically assisted modification of carbon surfaces is an important research area for the design of functional surfaces. This type of modification can be obtained either by oxidative grafting of e.g. amines1 and alcohols2 or by reductive grafting of aryldiazonium,3,4 diaryliodonium,5 and triarylsulfonium salts.6 Oxidative grafting is less useful since most metallic surfaces corrode at positive potentials. In particular, the electrochemical grafting of aryldiazonium salts has been widely explored due to the reproducible formation of covalently attached films at moderate potentials and the wide range of accessible salts.7 The electron-withdrawing nature of the diazonium functionality along with the driving force associated with the formation of the gaseous dinitrogen leaving group facilitates the reduction of aryldiazonium salts. Scheme 1 shows that aryldiazonium salts, ArN2+, or for that sake aryl iodides, ArI, are reduced by either a concerted oneScheme 1. Reaction Mechanism for the Reduction of ArX (X = N2+ or I), Where the Aryl Radical Intermediate May Graft the Electrode Surface Unless It Becomes Further Reduced to the Aryl Anion
Received: January 27, 2017 Revised: March 9, 2017 Published: March 23, 2017 © 2017 American Chemical Society
3217
DOI: 10.1021/acs.langmuir.7b00300 Langmuir 2017, 33, 3217−3222
Article
Langmuir
at ambient temperature, where Δ is the phase shift and tan(ψ) is the amplitude ratio upon reflection. The complex refractive index of the bare substrate was calculated from the measured Δs and ψs values. A three-layer optical model consisting of a substrate with a complex refractive index, the grafted layer with a refractive index and thickness, and the surrounding medium (air) was used to calculate the overall reflection coefficients for in-plane (Rp) and out-of-plane (Rs) polarized lights. The real and the imaginary parts of the refractive index of the bare substrate were obtained by measuring the clean plates prior to modification. Ellipsometric measurements were performed on the same area of the plates before and after each modification step. Because the measurements are carried out on a dried and, therefore collapsed film, the refractive index of the layer is fixed at a constant value (real = 1.55; imaginary = 0), independent of the thickness. The average values reported correspond to data points obtained from measuring the thickness at three spots on each plate. X-ray Photoelectron Spectroscopy. XPS analysis was done using a Kratos Axis Ultra-DLD spectrometer (Kratos Analytical Ltd., Manchester, UK) with a monochromatic Al Kα X-ray source at a power of 150 W with an analysis area of 300 × 700 μm2. Survey spectra were acquired by accumulating two sweeps in the 0−1350 eV range at a pass energy of 160 eV. Spectral processing was carried out using CasaXPS (Casa Software Ltd., Teignmouth, UK). A Shirley background subtraction was performed, and atomic surface concentrations were determined from the survey spectra using the manufacturer’s sensitivity factors. Binding energies of the components in the spectra were determined by calibrating against the C−C sp3 peak for C 1s at 285.0 eV. Analysis of Cyclic Voltammetric Data. The total charge (Q) associated with the redox process of surface bound ferrocenyl groups was obtained from background-subtracted cyclic voltammograms by integration of the electrochemical signal (see Figure S1, Supporting Information). The electroactive films were analyzed by means of cyclic voltammetry using sweep rates ranging from 0.05 to 10 V s−1. The sweep rates were applied in random order to deduce if changes in the chemical properties of the film arising over time had occurred due to the repetitive cycling. This was not the case. The surface coverage (Γ) was obtained from the expression Γ = Q/nFA, where n is the number of electrons in the heterogeneous electron transfer process (n = 1 for the ferrocenyl group), F is Faraday’s constant, and A is the geometrical surface area of the electrode. The first and last sweep were measured at a reference sweep rate (= 0.1 V s−1) to evaluate the stability of the film.
indirect approach offers better conditions for accomplishing radical chemistry, since Ar• is not formed directly at the strongly reducing electrode surface as in the direct approach. The crucial issue to consider for evaluating the likelihood of a radical-based grafting is the reduction potential of ArX relative to that of Ar•, noting that the aryl anion is a nongrafting species. In a recent study we determined the reduction potential of aryl radicals to be ∼−0.94 V vs SCE.13 With this knowledge at hand it is not surprising that diaryliodonium and, in particular, aryldiazonium salts with reduction potentials of typically −1.00 V vs SCE and −0.45 V vs SCE,5 respectively, are efficient grafting agents. This being said, surface derivatization is also possible with triarylsulfonium salts, in spite of their reductions potentials being around −1.7 V vs SCE in MeCN.6 This implies that even if the number of aryl radicals escaping reduction is exceedingly small, functionalization of the electrode surface may still take place. In this study, we go one step further by disclosing that even the more difficult reducible aryl iodides (like alkyl iodides14,15) may act as electrografting agents. This is surprising, considering that the electron transfer to aryl iodides usually is considered to be a nongrafting two-electron reduction leading to the formation of iodide and benzene (upon protonation).16 However, we present compelling evidence in favor of the grafting reaction from cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and ellipsometry. Cyclic voltammetry and XPS are further utilized to determine the surface coverage. The grafting of aryl iodides produces a stable and comparatively well-ordered molecular film structure that can withstand prolonged ultrasonication. With this new grafting agent at hand together with the triarylsulfonium,6 diaryliodonium,5 and, in particular, aryldiazonium salts,3 one possesses the tools required for forming a variety of aryl-based organic films.
■
EXPERIMENTAL SECTION
Chemicals. Acetonitrile (HPLC grade, ≥99.9%) was purchased from Sigma-Aldrich. Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was synthesized using standard procedures. Iodobenzene (≥99%) was purchased from Fluka, and 1-bromo-4-iodobenzene (98%), 1-iodo-4-nitrobenzene (98%), and 4-iodoanisole (98%) were from Sigma-Aldrich. Ferrocenylmethyl 4-iodobenzoate was synthesized as described in the Supporting Information. Electrochemical Setup. Glassy carbon (GC) rods (Sigradur G, HTW, diameter = 1 mm) imbedded in epoxy resin or GC plates (Sigradur G, HTW, 10 mm × 10 mm × 1 mm) were employed as working electrodes, while a Pt wire served as counter electrode. The reference electrode was a Ag/AgI pseudoreference electrode (i.e., a silver wire immersed in 0.1 M Bu4NBF4 + 0.01 M Bu4NI/MeCN). The measurements were performed in 1 or 2 mM solutions of the aryl iodide in 0.1 M Bu4NBF4/MeCN, which was deaerated with argon before use. The GC electrodes were carefully polished before each experiment by successive treatments with diamond suspensions (Struers, grain size: 9, 3, 1, and 0.25 μm) followed by thorough rinsing with water and ethanol and, finally, 10 min ultrasonication in HPLC grade acetone. GC plates were cleaned by sonication in Milli-Q water, HPLC grade acetone, and pentane (10 min in each solvent) prior to electrografting. At the end of each experiment the potential of the ferrocenium/ferrocene (Fc+/Fc) couple was measured, and all potentials were referenced against SCE using the previous determination of E0Fc+ = 0.41 vs SCE in MeCN.17 Consecutive voltammetric cycles were always recorded with 2 s stirring in between the cycles. Ellipsometry. Dry-film thicknesses were determined using a rotating analyzer ellipsometer (Dre, Germany). The GC plates were measured at 65° angle of incidence. The ellipsometric parameters of the bare (Δs, ψs) and grafted (Δg, ψg) substrates were measured in air
■
RESULTS AND DISCUSSION Chart 1 shows the structure of the three aryl iodides selected for this work. In addition to the unsubstituted and Chart 1. Chemical Structures of the Aryl Iodides Investigated
bromosubstituted aryl iodides, an electroactive ferrocenecontaining compound was synthesized to enable a direct electrochemical characterization of the organic films. Grafting of Iodobenzene. Figure 1a displays two consecutive cyclic voltammograms recorded at a glassy carbon (GC) electrode using a sweep rate, v, of 0.1 V s−1 on 2 mM iodobenzene in 0.1 M Bu4NBF4/MeCN. The two voltammograms with a reduction peak potential, Ep,red, ≈− 2.23 V vs SCE look similar, which seem to be fully consistent with the reported nongrafting two-electron reduction of iodobenzene to iodide and benzene (upon protonation).16 Yet, upon increasing the number of voltammetric cycles to 100, the electrode surface becomes passivated as evidenced from the decrease in wave 3218
DOI: 10.1021/acs.langmuir.7b00300 Langmuir 2017, 33, 3217−3222
Article
Langmuir
Figure 1. (a) First (black) and second (red) cyclic voltammograms (CV) along with (b) representative cyclic voltammograms recorded at a GC electrode using v = 0.1 V s−1 on 2 mM iodobenzene in 0.1 M Bu4NBF4/MeCN.
Table 1. Atomic Percentages Obtained by XPS for a GC Substrate Electrografted with 1-Bromo-4-iodobenzene along with Calculated Surface Concentrations, Γ, and Film Thicknesses, d, Measured by Ellipsometry C 1s (atom %) blank CV1 CV5b CV20 CV50b electrolysisd
97.7 98.9 98.7 97.4 93.3 83.0
O 1s (atom %)
± 0.2 ± 0.5 ± 0.0 ± 0.2
1.6 1.0 0.9 1.6 2.5 6.6
± 0.2 ± 0.3 ± 0.2 ± 0.4
Br 3p (atom %)
othera (atom %)
Γ (mol cm−2)
d (nm)
∼0 ∼0 1.5 × 10−11 7.7 × 10−11 2.7 × 10−10
