pubs.acs.org/Langmuir © 2010 American Chemical Society
Modification of Carbon Substrates by Aryl and Alkynyl Iodonium Salt Reduction Martin Weissmann, Steve Baranton,* and Christophe Coutanceau Laboratoire de Catalyse en Chimie Organique, Equipe Electrocatalyse, UMR 6503 CNRS Universit e de Poitiers, 40 Avenue du Recteur Pineau, F-86022 Poitiers Cedex, France Received June 15, 2010. Revised Manuscript Received July 31, 2010 Different carbon materials were modified using iodonium ion reduction creating radicals, which after reaction with carbon surfaces formed grafted layers of molecules. Several molecules (4-bromophenyl, 4-fluorophenyl, 6-chlorohexyne, and 4-bromobutyne) were grafted on glassy carbon and Vulcan XC72 carbon substrates. Carbon substrates were shown to be free of halogen atoms; therefore, the quantification of the grafted groups containing halogen atoms was facilitated. The grafting of the different molecules was first electrochemically studied on glassy carbon electrodes using cyclic voltammetry, in order to determine the reduction potential of the corresponding iodonium ions. Voltammetric study using Fe(CN)64- and Fe(CN)63- probe molecules and XPS characterization were also used to evidence the effectiveness of grafting from iodonium ion reduction reaction. Reduction potentials were found in the range from -0.9 V vs SCE to -1.0 V vs SCE, lower than those for corresponding diazonium ion reduction reaction on glassy carbon (close to -0.3 V vs SCE). Therefore, grafted layers from iodonium ions were carried out on carbon Vulcan XC72 powder using NaBH4 as reducing agent. Functionalized carbon powders were characterized by elemental analysis, thermogravimetric analysis, and X-ray photoelectron spectroscopy to evidence the presence of grafted molecules on the materials. However, low grafting yields were obtained. Then, several synthesis parameters were studied to optimize the grafting reactions, such as the control of the addition of reactants and their concentrations, leading to increase the surface concentration by a factor 2. At last, according to XPS measurements the grafting of alkinyliodonium ions led to very low surface concentrations (0.5 wt % for 6-chlorohexyne), whereas elemental analysis and TGA indicate ca. 2.4 wt % and ca. 5 wt %, respectively.
1. Introduction Carbon blacks are used in numerous applications such as printable ink, reinforcing agent, surface coating, sealing compounds, and formulation of plastics.1 Unfortunately, these materials are often not fully optimized for their final applications. Enhancement of their surface properties can be performed by different modification methods such as direct oxidation of the carbon support2,3 or grafting of molecules by reduction of a synthon.4-6 In the direct oxidation method, carbon black is exposed to an oxidizing agent (such as nitric acid or hydrogen peroxide) to increase the number of oxidized functions on the surface. These surface functions give interesting properties to carbon substrates for dispersion in aqueous media7 or for their use as catalyst supports2 in electrochemical devices. However, this method is destructive8 (carbon black structure is affected by the oxidizing treatment) and not selective (the precise control of the amount of oxidized functions is difficult to obtain). *To whom correspondence should be addressed. steve.baranton@ univ-poitiers.fr. (1) Kinoshita, K. in Carbon: Electrochemical and Physicochemical Properties; John Wiley and Sons, Wiley Interscience: New York, 1988. (2) Prado-Burgette, C.; Linares-Solano, A.; Rodriguez-Reinoso, F.; SalinasMartinez de Lecea, C. J. Catal. 1989, 115, 98–106. (3) Torres, G. C.; Jablonski, E. L.; Baronetti, G. T.; Castro, A. A.; de Miguel, S. R.; Scelza, O. A.; Blanco, M. D.; Pena Jimenez, M. A.; Fierro, J. L. G. Appl. Catal., A 1997, 161, 213–226. (4) Downard, A. J. Electroanalysis 2000, 12, 1085–1096. (5) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (6) Urchaga, P.; Weissmann, M.; Baranton, S.; Girardeau, T.; Coutanceau, C. Langmuir 2009, 25, 6543–6550. (7) Horita, K.; Nishibori, Y.; Ohshima, T. Carbon 1996, 34, 217–222. (8) Grolleau, C.; Coutanceau, C.; Pierre, F.; Leger, J.-M. Electrochim. Acta 2008, 53, 7157–7165.
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Surface modification by means of grafted molecules is an attractive technique to change the properties of the initial material. Delamar et al.4 demonstrated that carbon electrodes could be modified by electrochemical reduction of diazonium ions. This reduction reaction implies the formation of aryl radicals which are able to react with carbon surfaces forming grafted layers of molecules. The grafting of molecules on carbon surfaces seems a more attractive modification method, because it helps maintaining the support integrity and it can be realized for a wide variety of functional groups.9 This technique allows better control of the modification of the material properties by selecting the appropriate grafted groups. Moreover, carbon black modification can be performed by electroless diazonium ion reduction.10,11 The modifications initiated by diazonium ions are limited to the grafting of substituted phenyl groups;12 to enlarge the variety of grafted molecules, other precursors for grafting reactions were investigated such as halonium13-15 ions, sulfonium16 ions, or alkyl iodides.17 Among the different species, iodonium ions present particular interest: the surface modification initiated by electrochemical reduction of iodonium ions leads to the formation of (9) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201–207. (10) Toupin, M.; Belanger, D. J. Phys. Chem. C 2007, 111, 5394–5301. (11) Toupin, M.; Belanger, D. Langmuir 2008, 24, 1910–1917. (12) Vollhardt, P. K.; Schore, N. E. Organic Chemistry: Structure and Function, 4th ed.; W H Freeman & Co.: New York, 2003. (13) Datsenko, S.; Ignat’ev, N.; Barthen, P.; Frohn, H.-J.; Scholten, T.; Schroer, T.; Welting, D Z. Anorg. Allg. Chem. 1998, 624, 1669–1673. (14) Vase, K. H.; Holm, A. H.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2005, 21, 8085–8089. (15) Dirk, S. M.; Pylypenko, S.; Howell, S. W.; Fulghum, J. E.; Wheeler, D. R. Langmuir 2005, 21, 10899–10901. (16) Vase, K. H.; Holm, A. H.; Norrman, K.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2008, 24, 182–188.
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Article Scheme 1. General Mechanism of the Grafting Reaction of Iodonium Intermediates
homogeneous grafted layers on bulk conductive materials,14,18 and iodonium ions enable the grafting of a larger variety of molecules than diazonium ions, including alkynyl chains.14 Conversely to diazonium surface modifications, which can lead to the formation of azoic linkage,19 grafted layers obtained from iodonium precursors are expected to be linked to the carbon substrate through a C-C bond without incorporation of other moieties such as iodine. Furthermore, iodonium ion reduction potentials are lower than those of most of diazonium ions; this high stability of iodonium compounds implies that no spontaneous reaction will occur with carbon, reinforcing the control on the grafting reaction by a reducing agent. Aliphatic chains grafted on carbon black may present higher flexibility than aryl groups. Alkynyl chains carrying an ionic end group coupled with good flexibility should confer ionic conductive properties to carbon black materials. Ionic conductivity should be useful in a solid polymer electrolyte fuel cell application (SPEFC) where carbon black acts as a catalyst support. Such modified carbon black could improve qualitatively and quantitatively the triple phase boundaries (sites where electrochemical reactions take place) and further could increase platinum catalyst utilization efficiency.20 Surface modification by iodonium ion reduction was selected because of higher reduction potentials compared to sulfonium16 and other haloniums.13 Moreover, syntheses of iodonium ions are better referenced.21-23 The grafting reactions carried out via iodonium ion reduction follow the same principles as diazonium ion reduction: the reduction of iodonium ions produces radicals which are grafted on the material surface as described in Scheme 1. Grafted layers of 4-fluorophenyl and 4-bromophenyl were first formed on glassy carbon and Vulcan XC72 in order to compare with results in the literature concerning the grafting of diazonium and iodonium ions. Then, the developed method was transferred to grafting of 6-chlorohexyne and 4-bromobutyne on same substrates. The presence of halogen atoms is easily characterized by numerous techniques (XPS, elemental analysis): fluorine, chlorine, and bromine are not initially present in the different carbon materials. (17) Chehimi, M. M.; Hallais, G.; Matrab, T.; Pinson, J.; Podvorica, F. I. J. Phys. Chem. C 2008, 112, 18559–18565. (18) Holm, A. H.; Møller, R.; Vase, K. H.; Dong, M.; Norrman, K.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. New J. Chem. 2005, 29, 659–666. (19) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805–6813. (20) Srinivasan, S.; Velev, O. A.; Parthasarathy, A.; Manko, D. J.; Appleby, A. J. J. Power Sources 1991, 36, 299–320. (21) Beringer, F. M.; Drexler, M.; Gindler, E. M.; Lumpkin, C. C. J. Am. Chem. Soc. 1953, 75, 2705–2708. (22) Delwar Hossain, M.; Ikegami, Y.; Kitamura, T. J. Org. Chem. 2006, 71, 9903–9905. (23) Yoshida, M.; Nishimura, N.; Hara, S. Chem. Commun. 2002, 1014–1014.
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Modification of glassy carbon by the different halogenobenzene groups allowed determining the reduction potentials of iodonium ions on carbon surfaces and establishing the effectiveness of the grafting reaction (measurements in presence of probe molecules and reduction of grafted layers). The grafting reaction was then realized on carbon Vulcan XC72. The grafting protocol using iodonium ions is slightly different than that developed for diazonium ions:10 iodonium ions present a low reduction potential compared to that of diazonium ions and a reducing agent had to be added in order to perform the derivatization of carbon powders. Particular attention was given to the improvement of the grafting reaction protocol and several experimental parameters were studied (amount of reactants, addition of reactants, reaction time).
2. Experimental Section 2.1. Iodonium Compound Syntheses. Bis(halogenophenyl)iodonium compounds were synthesized according to the method described by Delwar-Hossain et al.22 All products were provided by Sigma Aldrich (ACS reagent grade, 98%). Iodine (5 mmol) and substituted benzene (fluorobenzene or bromophenyl, 50 mmol) were dissolved in a mixture of trifluoroacetic acid (100 mL) with 1,2-dichloroethane (50 mL). After complete dissolution of iodine, potassium peroxodisulfate (50 mmol) was added to the solution, which was then left to react 72 h at 40 °C. The organic phase was isolated and dried by filtration through MgSO4. The compounds were then retrieved by evaporation of the solvent under vacuum. In order to stabilize iodonium ions, trifluoroacetate counteranions (CF3COO-) were exchanged by trifluoromethanesulfonate ions (CF3SO3-). The exchange was carried out by dipping the iodonium salt in 100 mL of a concentrated aqueous solution of trifluoromethanesulfonate (1 mol L-1) for 12 h at room temperature; the compounds were then filtered and rinsed with water. This exchange procedure was repeated twice to ensure a complete replacement of anions. Alkynyliodonium ions were synthesized from iodosobenzene and the corresponding alkyne by using the method developed by Yoshida et al.23 Iodosobenzene (4.5 mmol) and mercuric oxide catalyst (0.02 mmol) were added to a mixture of 10 mL of dichloromethane and 2.2 mL of tetrafluoroboric acid solution. Finally, 3.75 mmol of 6-chlorohexyne or 4-bromobutyne were added to the solution, which was then stirred for two hours. The retrieval of the reaction products was made by separation of the aqueous phase from the organic one. The organic solvent was then evaporated under vacuum; the products were purified with ether and dried under vacuum. The iodonium compounds were then stored at room temperature, avoiding light exposure.22 2.2. Modification of Glassy Carbon Electrode. A glassy carbon electrode was polished with A1 (1 μm grain size), A3 (0.3 μm), and A5 grade (0.05 μm) alumina powders. After each polishing step, the electrode was washed with ultrapure water (Milli-Q, Millipore, 18.2 MΩ cm) and sonicated for 5 min. DOI: 10.1021/la1024313
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Article The electrochemical modification of the glassy carbon electrode was performed in a standard three electrode cell. The working electrode was a 3-mm-diameter glassy carbon disk, the counter electrode was a glassy carbon plate, and a SCE was used as reference electrode (Radiometer Analytical). The system was monitored by a Voltalab potentiostat (Radiometer Analytical, PGZ 100). Electrochemical modifications were carried out in deaerated acetonitrile containing tetrabutylammonium tetrafluorborate ([(C4H9)4Nþ BF4-] = 0.1 mol L-1) electrolyte, after total dissolution of iodonium ions (10 mmol L-1). The junction potential between the SCE and the acetonitrile solution was measured and was found to be close to 7.5 mV; it will then be neglected for this work. The grafted layer is generated by electrochemical reduction of iodonium ions on the glassy carbon working electrode under cyclic voltammetry conditions (scan rate was set at 50 mV s-1) between 0.4 V vs SCE and -1.3 V vs SCE (bis(4-fluorophenyl)iodonium ions) or 0.4 V vs SCE and -1.2 V vs SCE (bis(4-bromophenyl)iodonium ions). Five cycles are performed for the formation of a grafted layer on glassy carbon electrode. The presence of grafted layers on the electrode surface was confirmed by electrochemical measurements in 0.1 mol L-1 KCl aqueous solution (Acros Organics, 99%) containing ferricyanide and ferrocyanide probe molecules ([Fe(CN)64-] = [Fe(CN)63-] = 5 mmol L-1, Merck). The electrochemical reactivity of probe molecules is influenced by the presence of grafted groups on the electrode surface. For experiments related to diazonium ions, grafting reaction was carried out in aqueous medium using an in situ method of diazonium ion synthesis according to the protocol described by Baranton and Belanger.24 The grafted layer is generated by electrochemical reduction of 4-bromophenyldiazonium ions on the glassy carbon working electrode under cyclic voltammetry conditions (scan rate was set at 50 mV s-1) between 0.4 V vs SCE and -0.6 V vs SCE. 2.3. Modification of Carbon Powders. Carbon Vulcan XC72 (Cabot Corp) was dispersed in 50 mL of acetonitrile (VWR Prolabo, HiPerSolv CHROMANORM). Then, iodonium ions and sodium borohydride (Acros Organic, 99%, powder) were directly added to the solution. The reaction mixture was stirred for 24 h at room temperature. The mixture was then filtered under vacuum, and the carbon powder was washed by successive aliquots of water, methanol, and acetone. At last, the modified carbon powder was dried under vacuum at 75 °C for 12 h. The amounts of fluorine, chlorine, or bromine in the different samples were determined by elemental analysis (CNRS Central Analysis Department, Solaize, France). The amount of grafted groups is given in weight percent (wt%). 2.4. Thermogravimetric Analysis (TGA). TG analyses were performed on 10 mg carbon samples with a SDT Q600 (TA Instruments) setup. A temperature slope of 10 °C min-1 was applied to the different samples from 25 to 900 °C under air flow (U Quality, L’Air Liquide, 50 mL min-1).
2.5. X-ray Photoelectron Spectroscopy (XPS) Characterization. XPS measurements were performed with an Escalab MKII (VG Scientific) setup using the Magnesium monochromatic beam (1253.6 eV). The data were collected at room temperature, and the operating pressure in the analysis chamber was set below 8 10-9 Torr. The survey spectra were recorded between 0 and 1200 eV with a resolution of 1 eV. The core level spectra were recorded with a resolution of 50 meV.
3. Results 3.1. Electrochemical Reduction of Biphenyliodonium Ions. Bromophenyl and fluorophenyl groups were grafted on the surface of a glassy carbon electrode by electrochemical reduction of (24) Baranton, S.; Belanger, D. J. Phys. Chem. B 2005, 109, 24401–24410.
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bis(4-bromophenyl)iodonium ions and bis(4-fluorophenyl)iodonium ions, respectively. In order to compare with already developed derivatization methods, diazonium ions reduction on glassy carbon was also carried out. The concentrations of bis(4-bromophenyl)iodonium ions, bis(4-fluorophenyl)iodonium ions, and 4-bromophenyldiazonium ions were 10 mmol L-1, 10 mmol L-1, and 2 mmol L-1, respectively. Voltammograms recorded during the grafting reactions of iodonium ions are presented in Figure 1a,b. Two reduction peaks are present in the first cycle. In the following cycles, these two reduction peaks have completely disappeared, indicating the blocking of the surface, probably due to the formation of a grafted layer. The reduction peaks located at ca. -0.95 V vs SCE and -0.85 V vs SCE for the bis(4-fluorophenyl)iodonium ions and bis(4-bromophenyl)iodonium ions, respectively, correspond to the formation of radicals by reduction of the iodonium ions14 as described in Scheme 1. The reduction current peak observed at higher potential was already reported during the formation of grafted layers from diazonium ion reduction (Figure 1c).24 Benedetto et al. investigated in the case of diazonium ion reduction the origin of multipeaks on gold electrodes prepared on different substrates and exhibiting different surface structures.25 First, the necessity of a clean surface to observe multiple peaks during the diazonium ion reduction was pointed out as a reason for the low occurrence of this phenomenon in the literature.25 They also evidenced that the reduction potential of diazonium ions depended on the nature of the electrode surface, and linked it to the potential of zero charge (PZC) of the electrode surface. These observations can be transposed to the multiple reduction peaks observed during iodonium ion reduction on a glassy carbon electrode. However, the high complexity of the glassy carbon electrode surface makes it difficult to describe clearly the different active sites responsible for each reduction peak. Furthermore, the edges of carbon basal planes are the only sites which may initiate the formation of a grafted layer. Active sites for diazonium or iodonium ion reduction on carbon basal planes and carbon basal plane edges may not be blocked at the same step of the grafted layer formation process.26 Hence, the relation between the PZC of carbon electrode surface domains and the reduction potential of diazonium and iodonium ions is the only hypothesis which can be proposed, but it is difficult to establish whether it is applicable or not. Figure 1c indicates that the reduction of 4-bromophenyldiazonium ions occurs at a potential of ca. -0.3 V vs SCE, whereas the reduction of bis(4-bromophenyl)iodonium ions occurs at a significantly more negative potential (-0.85 V vs SCE). This behavior can be generalized to the reduction potential of most iodonium ions compared to the reduction potential of diazonium ions.27 Therefore, the spontaneous derivatization of a carbon powder (which is certainly not a strong enough reducing agent) could not be carried out without using a chemical reducing agent, conversely to the reaction mechanism described for the spontaneous reduction reaction of diazonium ions on carbon substrates.10 Measurements carried out in an aqueous medium in the presence of Fe(CN)64-/Fe(CN)63- probe molecules show similar effects on the charge transfer resistance for both iodoniummodified electrodes (Figure SI1, Supporting Information). The glassy carbon electrode modified with a bromophenyl layer (25) Benedetto, A.; Balog, M.; Viel, P.; Le Derf, F.; Salle, M.; Palacin, S. Electrochim. Acta 2008, 53, 7117–7122. (26) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534–6540. (27) Vase, K. H.; Holm, A. H.; Norrman, K.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2007, 23, 3786–3793.
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Figure 1. Cyclic voltammograms recorded in acetonitrile medium ([(C4H9)4NþBF4-] = 0.1 mol L-1) during the reduction reaction of 10 mmol L-1 iodonium ions for the grafting of (a) fluorophenyl, (b) bromophenyl groups, and (c) voltammograms recorded in aqueous medium ([HCl] = 0.5 mol L-1) during the reduction reaction of 2 mmol L-1 4-bromophenyldiazonium ions for the grafting of bromophenyl groups. (T = 20 °C; scan rate: 50 mV s-1.)
grafted by reduction of diazonium ions (dashed line in Figure SI1, Supporting Information) exhibits a slightly lower charge transfer resistance than a bromophenyl layer grafted by iodonium ion reduction. However, considering the complex relation between (28) Downard, A. J.; Prince, M. J. Langmuir 2001, 17, 5581–5586.
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Article
the grafted group surface coverage and the charge transfer resistance28 the current values measured at 0.6 V vs SCE indicate that the surface coverage is of the same order independently of the precursor ion used. These experiments demonstrate that derivatization by iodonium ion reduction can be used to functionalize carbon surfaces and can lead to surface coverages comparable to that obtained via the reduction of diazonium ions. The presence of bromophenyl groups on the electrode surface was quantified electrochemically using controlled reduction of bromine atoms (Figure SI2, Supporting Information).27 The charge involved in the reduction peak located at -2.3 V vs SCE (assuming that 2 electrons were exchanged during the process29) leads to a surface coverage of ca. 8 10-10 mol cm-2. This value is on the same order of magnitude as the surface coverage determined after diazonium grafting by electrochemical reduction of 4-bromophenyl groups9 or XPS measurements24 and is close to the surface coverage reported in the literature27 (5 10-10 mol cm-2 by electrochemical reduction of 4-bromophenyl groups and 8 10-10 mol cm-2 from XPS measurements). 3.2. Modification of Carbon Powders by Reduction of Diphenyliodonium Ions. Two samples of modified carbon powder were prepared by using different iodonium species (bis(4-fluorophenyl)iodonium ion and bis(4-bromophenyl)iodonium ion). The grafting reactions were performed following a simple protocol. Carbon Vulcan XC72 (0.2 g), iodonium compounds (0.4 g of bis(4-bromophenyl)iodonium ions or 0.5 g of bis(4-fluorophenyl)iodonium ions), and NaBH4 in excess (0.08 g) were dispersed in acetonitrile. Such amounts of reactants should theoretically lead to a 33 wt % loading in grafted groups, assuming a grafting reaction yield of 100%. Elemental analyses revealed the presence of fluorine (0.6 wt %) on the first sample and bromine (1.3 wt %) on the second sample. These elements were not detected in the unmodified carbon Vulcan XC72 sample; their presence on modified carbon powder is clearly linked to the achievement of the grafting reaction. These results give a loading of 3 wt % of fluorophenyl groups and 2.5 wt % of bromophenyl groups. The modification of carbon Vulcan XC72 (0.2 g) was also carried out by spontaneous reduction of 4-bromophenyldiazonium ions (1.3 10-3 mol L-1) in 50 mL of aqueous acidic medium (HCl 0.5 mol L-1). A loading of 10 wt % in bromophenyl groups was determined using elemental analysis. The lower yield of iodonium ion reduction reaction may be due to the necessity of adding a chemical reducing agent in the solution, whereas in the case of diazonium ions, carbon powder is assumed to be the reducing agent.10 The reduction reaction of iodonium ions can occur in bulk solution, i.e., far from the carbon substrate. The distance between the reduction sites and the grafting sites increases the possibility of side reactions between radicals formed by iodonium ion reduction. 3.3. X-ray Photoelectron Spectroscopy Characterization of Modified Carbon Powders. XPS survey spectrum of unmodified Vulcan XC72 carbon presents only traces of fluorine on its surface (not shown). XPS survey spectra realized on modified carbons reveal the presence of elements which are characteristic of the grafted groups (fluorine and bromine) on their surfaces (Figures 2a,b). Considering carbon powder functionalized with 4-fluorophenyl, the deconvolution of the F 1s core level spectrum (Figure 3a) shows two peaks located at 687.4 and 689.6 eV. These peaks correspond to fluorine atoms bound to a carbon atom. The peak located at 689.6 eV was also observed on unmodified carbon (29) Coulon, E.; Pinson, J. J. Org. Chem. 2002, 67, 8513–8518.
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Weissmann et al. Table 1. Loading of 4-Fluorophenyl Groups on the Carbon Powder Samples Measured by Elemental Analysis for Different Procedures of Reactant Addition
sample
interval between each iodonium addition (min)
interval between each NaBH4 addition (min)
loading of 4-fluorophenyl groups (wt%)
1 2 3 4 5 6 7 8 9
50 50 50 90 90 90 120 120 120
5 10 15 5 10 15 5 10 15
2.0 1.1 2.0 1.4 1.5 1.3 3.1 3.3 3.4
Table 2. Loading of 4-Fluorophenyl Groups on the Carbon Powder Samples Measured by Elemental Analysis for Different Amounts of Reactant Added time between each addition of NaBH4 loading of 4-fluorophenyl groups with 0.1 g of iodonium ions loading of 4-fluorophenyl groups with 0.5 g of iodonium ions
5 min 3.1
10 min 3.3
15 min 3.4
10.4
12.7
6.0
Figure 3. XPS spectra recorded for (a) core level F 1s and (b) core level Br 3d for 4-fluorophenyl and 4-bromophenyl modified carbon powder, respectively. Both carbon powder modifications were obtained as in Figure 2.
detected is 10 times higher for the modified carbon samples than for the unmodified one. This increase of fluorine concentration on the sample surface is related to the presence of grafted layers of 4-fluorophenyl groups. For carbon Vulcan XC72 modified with bromophenyl groups, the survey spectrum indicates the presence of bromine (Figure 2b). The surface of the unmodified carbon sample shows no presence of bromine. The Br 3d core level spectrum displays only one peak located at a binding energy of 71 eV (fitted with its Br 3d5/2 and Br 3d3/2 components in Figure 3b) attributed to bromophenyl grafted groups.9 The presence of these halogen atoms on the surfaces of the modified carbon samples is an evidence of the grafting reaction achievement. It is interesting to note that iodine atoms are not detected for all samples (modified and unmodified). XPS characterizations realized on modified carbon by diazonium cation reduction displayed a peak centered at 400 eV corresponding to the presence of nitrogen which was attributed to the formation of an azo bond between the grafted group and the carbon substrate.24 The absence of iodine on the samples prepared by reduction of iodonium ions indicates that all grafted groups are linked to the carbon substrate through a C-C bond. 3.4. Optimization of the Protocol of Carbon Powder Modification via Iodonium Ion Reduction. Previous experiments clearly demonstrated the presence of grafted layers of fluorophenyl and bromophenyl groups on the carbon surface. However, the loadings of grafted groups are significantly lower than those obtained with diazonium species. Although low loadings in grafted species may significantly modify the surface properties of carbon materials,30 it is important to improve and to better control the yield of the grafting reaction. So, the grafting reaction was carried out according to the following modified protocols. First, iodonium ions and reducing agent were added in several steps, and second, the effect of iodonium ion concentration was studied. The first series of experiments were performed by introducing a sufficient amount of reactants to reach a loading of 10 wt % in
Vulcan XC72 and is attributed to a small amount of impurities present in the initial Vulcan XC72 sample. The amount of fluorine
(30) Weissmann, M.; Baranton, S.; Clacens, J.-M.; Coutanceau, C. Carbon 2010, 48, 2755–2764.
Figure 2. XPS survey spectra recorded for (a) 4-fluorophenyl modified carbon powder, (b) bromophenyl modified carbon powder. Grafting reactions were carried out by reduction of 0.4 g bis(4bromophenyl)iodonium or 0.55 g bis(4-fluorophenyl)iodonium ions by NaBH4 in excess (0.08 g) on 0.2 g carbon Vulcan XC72.
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Scheme 2. Scheme Illustrating the Transfer of Hydroxyl Ions Induced by Flexible Alkyl Chains Grafted on a Carbon Surface
4-fluorophenyl groups (0.1 g of bis(4-fluorophenyl)iodonium ion, 0.016 g of NaBH4, 0.2 g of carbon vulcan XC72). Bis(4-fluorophenyl)iodonium ion was chosen because of its lower reduction potential: the control of its chemical grafting may be more difficult compared to bis(4-bromophenyl)iodonium ion. The reducing agent (20 mg of NaBH4) was first dissolved in 20 mL acetonitrile, and this solution was then used to reduce iodonium ions. The addition of iodonium ions was performed in four steps, regularly spaced over time (every 50, 90, or 120 min). Between each addition of iodonium ions, reducing agent solution was also added in four steps (1 mL each time) at intervals of 5, 10, or 15 min. Elemental analyses for the different samples are reported in Table 1. The results provide interesting information about the grafting method: the delay between additions of reducing agent has little or no influence on the yield in grafted groups. The longest time between two additions of iodonium ions leads to significant increase of the yield in grafted species. For lower intervals, results are very close. In the case of intervals of 120 min between additions of iodonium ions, most of the species from the previous addition step may already have reacted (by grafting or by side reactions). In contrast, for shorter addition intervals, side reactions could be enhanced by the increase of the concentration of iodonium ions or radicals in the solution. Then, the influence of iodonium ion concentration was studied to improve the loading of grafted groups. The series of experiments was carried out by adding reactants in order to obtain a maximum loading in 4-fluorophenyl groups of 33 wt % on carbon, assuming a grafting reaction yield of 100% (0.2 g of carbon, 0.5 g of bis(4-fluorophenyl)iodonium ions, and 0.08 g NaBH4). The optimum conditions determined previously (120 min between each addition of iodonium ions with a total of four additions) were used. Results are reported in Table 2. The increase of reactant amount results in a significant increase of the surface loading of 4-fluorophenyl groups. However, the formation of a grafted layer by reduction of iodonium ions remains difficult to control, as shown by the low loading reported with 0.5 g of iodonium and 15 min between each addition of NaBH4 solution. However, these experiments show that it is possible to increase the loading of grafted functional groups via the control of iodonium ion additions. In order to evaluate the progress in terms of loading in grafted groups, another sample of 4-bromobenzene modified carbon powder was prepared using iodonium ion reduction. The amounts of reactant were 0.2 g of carbon, 0.4 g of bis(4-bromophenyl)iodonium ions, and 0.08 g of NaBH4. These amounts correspond to those used in experiments performed in section 3.2, leading to a loading of 2.5 wt % measured by elemental analysis, without any improvement of the grafting protocol. The experiment was carried out with reactant additions according to protocol used for sample 8 in Table 1. An increase of the loading of grafted bromophenyl groups by a factor 2.5 was obtained (6.3 wt % loading on carbon powder). Langmuir 2010, 26(18), 15002–15009
3.5. Grafting of Alkynyl Groups. The modification of carbon materials by alkynyl groups is potentially interesting for electrochemical applications such as fuel cell systems where carbon blacks are used as catalyst supports in the electrodes. The presence of ionic grafted chains with a sufficient flexibility could ensure an ionic conduction all over the carbon material surface (Scheme 2),31 leading to an increase of the catalyst utilization efficiency.32 In order to evaluate the possibility of grafting alkynyl chains on carbon substrates, several experiments were performed with (1-bromobut-4-yne)phenyliodonium and (1-chlorohex-6-yne)phenyliodonium ions. Grafted layers of 6-chlorohexyne and 4-bromobutyne were realized using the same protocol as for substituted benzene groups. First, the grafting reactions were carried out on a glassy carbon electrode to determine the reduction potential of alkynyliodonium ion precursors. The grafting reaction of the different groups was performed by cyclic voltammetry with a linear potential variation of 50 mV s-1 from 0.4 V to -1.5 V vs SCE. The voltammograms obtained during the reduction reaction are given in Figure 4a,b. Their general shape is comparable to that of voltammograms recorded for the grafting reaction of phenyl groups. The reduction reaction occurs during the first cycle, and the carbon surface appears blocked for the following voltammetric cycles. Both alkynyliodonium species present a reduction current peak at a potential of ca. -0.95 V vs SCE. This reduction potential indicated the possibility of grafting the considered species on carbon Vulcan XC72 substrate using NaBH4 as the chemical reducing agent. However, this low reduction potential can lead to the reduction of newly formed radicals into anion species, almost simultaneously, which could decrease the yield in grafted groups. This mechanism was pointed out by Vase et al. in the case of the formation of grafted layers by reduction of iodonium ions14 and is also supported by investigations carried out on diazonium ion reduction reaction.33 Furthermore, the standard reduction potential of alkynyl radicals into alkynyl anions has been predicted by Fu et al. as being close to 1.35 V vs NHE (i.e., 1.1 V vs SCE).34 Hence, the alkynyl radicals produced by the reduction of iodonium ions can easily be reduced into alkynyl anions instead of being grafted on carbon surface. Experiments performed before and after modification of the electrode by alkynyl groups in the presence of probe molecules ([Fe(CN)6]4- and [Fe(CN)6]3-) are presented in Figure 5. Voltammograms recorded on modified surfaces display a total (31) Thompson, S. D.; Jordan, L. R.; Forsyth, M. Electrochim. Acta 2001, 46, 1657–1663. (32) Passalacqua, E.; Lufrano, L.; Squadrito, G.; Patti, A.; Giorgi, L. Electrochim. Acta 2001, 46, 799–805. (33) Andrieux, C. P.; Pinson, J. J. Am. Chem. Soc. 2003, 125, 14801–14806. (34) Fu, Y.; Liu, L.; Yu, H.-Z.; Wang, Y.-M.; Guo, Q.-X. J. Am. Chem. Soc. 2005, 127, 7227–7234.
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Figure 5. Cyclic voltammograms recorded in the presence of Fe(CN)64-and Fe(CN)63- (5/5 mmol L-1) in 0.1 mol L-1 KCl aqueous electrolyte on a bare glassy carbon electrode (solid line), on a glassy carbon electrode modified by 6-chlorohexyne molecules (dash-dotted line), and on a glassy carbon electrode modified by 4-bromobutyne molecules (dotted line) from iodonium ion reduction. (T = 20 °C; scan rate: 50 mV s-1.)
Figure 4. Cyclic voltammograms recorded in acetonitrile medium ([NBu4BF4] = 0.1 mol L-1) during the reduction reaction of 10 mmol L-1 iodonium ions for the grafting of (a) 6-chlorohexyne and (b) 4-bromobutyne groups. (T = 20 °C; scan rate: 50 mV s-1.)
inhibition of the redox behavior of probe molecules, which evidence the presence of grafted layers on the glassy carbon surface. Results obtained on glassy carbon electrodes led to investigations on the derivatization of Vulcan XC72 carbon powder by iodonium alkynyl chains. Layers of 4-bromobutyne and 6-chlorhexyne were grafted in acetonitrile (50 mL) medium using NaBH4 (0.08 g) as reducing agent. The amounts of reactants were calculated to reach a loading of grafted species of 33 wt %, considering a grafting reaction yield of 100% (320 mg of (1-bromobut-4-yne)phenyliodonium or 290 mg of (1-chlorohex-6-yne)phenyliodonium were introduced with 0.2 g of carbon powder). Loadings determined by elemental analyses are close to 0.5 wt % and 2.5 wt % for 4-bromobutyne and 6-chlorohexyne, respectively. These low loadings can have different explanations such as very low formation of multilayers with alkynyl groups due to a more difficult abstraction of H atom on the alkynyl chain than on a phenyl group, an important reduction of alkynyl radicals formed in solution into anions, or the use of a reducing agent that is too weak. Alternative reducing agents in acetonitrile medium such as hypophosphorous acid or hydrazine could lead to higher surface coverage in the case where the low loadings observed in the present study are due to the use of a reducing agent that is too weak. Thermogravimetric analyses carried out under air flow (50 mL min-1) with a temperature slope of 10 °C min-1 show important 15008 DOI: 10.1021/la1024313
mass losses between 150 and 400 °C (Figure 6a,b), corresponding to ca. 5% of the sample weights, which was not observed with a nonmodified carbon substrate. However, this important mass loss may not only be due to the alkynyl group degradation. Indeed, the departure of the grafted layer may lead to the presence of local defects on the carbon substrate which can locally facilitate substrate degradation. Hence, part of the mass loss recorded between 150 and 400 °C could include a part of the substrate. XPS survey spectrum recorded on 6-chlorohexyne modified carbon powder allows detection of traces of chlorine; bromine was not detected for the 4-bromobutyne modified sample. The Cl 2p core level spectrum obtained on the 6-chlorohexyne modified carbon powder sample presents two Cl 2p signals located at 197.8 and 200.1 eV, both fitted with their two components Cl 2p1/2 and Cl 2p3/2 (Figure 7). The Cl 2p signal at 200.1 eV confirms the presence of covalent chlorine coming from the 6-chlorohexyne grafted layer,35 with a very low at % (ca. 0.1 at %, which corresponds to ca. 0.5 wt %). The reason for this low surface concentration observed by XPS can be the elimination of chlorine or bromine atoms by reduction under the X-ray beam. This behavior was previously reported in the case of electroactive species36-38 and is consistent with the presence of the Cl 2p signal observed at 197.8 eV attributed to the presence of ionic chlorine on the surface. The discrepancies observed among alkynyl grafted group loadings measured by a different method show the necessity of performing parallel characterizations for these samples. However, elemental and TG analyses have confirmed the significant presence of grafted layers of alkynyl chains.
4. Conclusion The works presented in this article show the possibility of transferring the surface derivatization performed on a glassy (35) Du, J.; Wang, D.; Wilkie, C. A.; Wang, J. Polym. Degrad. Stab. 2003, 79, 319–324. (36) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491–501. (37) Elliott, C. M.; Murray, R. W. Anal. Chem. 1976, 48, 1247–1254. (38) Mendes, P.; Belloni, M.; Ashworth, M.; Hardy, C.; Nikitin, K.; Fitzmaurice, D.; Critchley, K.; Evans, S.; Preece, J. ChemPhysChem 2003, 4, 884–889.
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Figure 7. XPS spectra recorded for core level Cl 2p for a 6-chlorohexyne modified carbon Vulcan XC72 sample. The grafted layer was obtained by reduction 290 mg of (1-chlorohex-6-yne)phenyliodonium ions with an excess of NaBH4 (0.08 g) on 0.2 g carbon Vulcan XC72 powder.
Figure 6. TGA measurements of (a) 6-chlorohexyne and (b) 4-bromobutyne modified carbon Vulcan samples under air flow (50 mL min-1) with a temperature slope of 10 °C min-1.
carbon electrode by electrochemical reduction of iodonium ions to the derivatization of carbon powder surfaces. The modification of carbon powder surface is of great interest for fuel cell application in order to prepare high-performance and low-cost catalytic layers. The present study has shown that the use of iodonium ions allows grafting of alkynyl chains, conversely to
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diazonium ions. The nucleophilic substitution of halogen atoms by an aliphatic chain carrying an ionic end group (ammonium, phosphoric acid, sulfonic acid, carbonic acid, etc.) can easily be realized, and the presence of the ionic group coupled with the flexibility of such chains should confer ionic conduction properties to the catalyst support. Other specific properties can also be given to the carbon supports by choosing the appropriate end groups: hydrophilic or hydrophobic character, stabilization of metal nanoparticles,6 and so forth. The grafting conditions of alkynyl groups have to be improved in order to modulate the specific properties of the carbon substrate by controlling the grafting level. This could allow combination of different specific properties on the same substrate. These perspectives will be the focus of future work. Acknowledgment. We greatly acknowledge Suzie Poulin, Master in Chemical Science, and Responsible of the LASM (Ecole Polytechnique de Montreal - Canada), for XPS measurements. Supporting Information Available: Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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