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Langmuir 2008, 24, 1910-1917
Spontaneous Functionalization of Carbon Black by Reaction with 4-Nitrophenyldiazonium Cations Mathieu Toupin and Daniel Be´langer* De´ partement de Chimie, UniVersite´ du Que´ bec a` Montre´ al, Case Postale 8888, succursale Centre-Ville, Montre´ al, Que´ bec H3C 3P8, Canada ReceiVed August 17, 2007. In Final Form: NoVember 21, 2007 The mechanism of the spontaneous chemical functionalization of Vulcan carbon black by reaction with 4-nitrophenyl diazonium cations was investigated by varying the reaction conditions. First, the carbon black was oxidized by nitric acid reflux to introduce oxygenated functionalities onto the surface prior to the functionalization step. Second, a reducing agent (H3PO2) was added to a solution containing 4-nitrobenzene diazonium tetrafluoroborate to generate 4-nitrophenyl radicals homogeneously in the bulk solution. The functionalized carbons were characterized by elemental analysis, X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption isotherms using the BET isotherm and DFT Monte Carlo simulations. These characterization methods were employed to determine the grafting yield as a function of the reaction conditions. Interestingly, the grafting yield was not affected by a change in the reaction conditions. An average nitrogen content of 1.4 ( 0.1 atom % was found by elemental analysis, and XPS showed a nitrogen surface concentration of about 3.5%. XPS also indicated an important decrease in the concentration of oxygenated functionalities upon grafting 4-nitrophenyl moieties onto the oxidized carbon black. Presumably, in this case the grafting involves either the coupling of carboxylate and 4-nitrophenyl radicals or, more likely, a concerted decarboxylation where the diazonium cation, acting as an electrophile, replaces the oxygenated groups and loss of CO2. The nitrogen adsorption isotherms of the functionalized carbon blacks suggested that the grafted groups were most probably localized at the entrance of the micropores.
Introduction The functionalization of carbon such as single-walled nanotubes (SWNTs), highly oriented pyrolytic graphite (HOPG), and carbon black relying on the in situ generation of a diazonium salt has been investigated in great detail in the past decade.1-3 This spontaneous modification methodology was first introduced by Belmont and was the subject of numerous patents issued to the Cabot Corporation.4 In situ-generated diazonium cations are easily obtained through a classic diazotation of a primary amine and can be realized in aqueous (with nitrous acid) or acetonitrile (with NOBF4) media.5 This modification procedure is very attractive because it avoids the use of oxidative conditions that * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
(1) (a) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3832. (b) Bahr, J. L.; Tour, J. M. J. Mater. Chem. 2002, 12, 1952. (c) Dyke, C. A.; Tour, J. M. Chem.s Eur. J. 2004, 10, 812. (d) Dyke, C. A.; Stewart, M. P. Tour J. Am. Chem. Soc. 2005, 127, 4497. (e) Dyke, C. A.; Tour, J. M. Nano Lett. 2003, 3, 1215. (f) Dyke, C. A.; Tour, J. M. J. Am. Chem. Soc. 2003, 125, 1156. (g) Dyke, C. A.; Stewart, M. P.; Maya, F. Tour J. M. Synlett 2004, 31, 155. (h) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H.; Kittrell, C.; Hauge. R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519. (i) Bahr, J.; Yang, J.; Kosynkin, K.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. L. Am. Chem. Soc. 2001, 123, 6536. (2) (a) Pandurangappa, M.; Lawrence, N. S.; Compton, R. G. Analyst 2002, 127, 1568. (b) Leventis, H. C.; Streeter, I.; Wildgoose, G. G.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Talanta 2004, 63, 1039. (c) Wildgoose, G. G.; Leventis, H. C.; Davies, I. J.; Crossley, A.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. J. Mater. Chem. 2005, 15, 2375. (3) Toupin, M.; Be´langer, D. J. Phys. Chem. C. 2007, 111, 5394. (4) (a) Belmont, J. A. U.S. Patent 5,554,739, September 10, 1996 (Cabot Corporation). (b) Belmont, J. A. U.S. Patent 5,851,280, December 22, 1998 (Cabot Corporation). (c) Belmont, J. A.; Adams, C. E. U.S. Patent 5,895,522, April 20, 1999 (Cabot Corporation). (d) Johnson, J. E.; Belmont, J. A. U.S. Patent 5,803,959, September 8, 1998. (e) Belmont, J. A.; Adams, C. E. U.S. Patent 5,713,988, February 3, 1998 (Cabot Corporation). (f) Adams, C. E.; Belmont, J. A. U.S. Patent 5,707,432, January 13, 1998 (Cabot Corporation). (g) Belmont, J. A. U.S. Patent 5,672,198, September 30, 1997 (Cabot Corporation). (h) Belmont, J. A.; Amici, R. M.; Galloway, C. P. U.S. Patent 6,042,643, March 28, 2000. (i) Belmont, J. A.; Amici, R. M.; Galloway, C. P. U.S. Patent 6,494,946, December 17, 2002. (j) Belmont, J. A.; Amici, R. M.; Galloway, C. P. U.S. Patent 6,740,151, May 25, 2004.
can lead to the detrimental oxidation of the carbon substrate6 and a wide variety of possible substituted aryl groups that can be grafted onto various carbon surfaces.4,7,8 The specific chemistry of the surface can be influenced by changing the nature of the functional groups of the grafted aryl moieties suitable for desired applications.9 For example, functionalities such as -COOH, -SO3H, and -N(C2H5)2 can be used to perform ion exchange with metal complexes that can be further chemically reduced to generate metallic nanoparticles.9 Furthermore, the wettability of the modified surface could be influenced by grafting hydrophobic (-CF3) or hydrophilic (-SO3H) functionalities.4 The latter materials were proposed for fuel cell applications, notably in membrane electrode assemblies (MEA), to improve transport phenomena occurring during cell operation.10 Other applications have been proposed in the literature where selected functionalities immobilized at electrode surfaces could be useful for sensing11 to immobilize enzyme and could be used as biosensors12 as an anchoring site at which to perform heterogeneous combinatorial chemistry.13 Previous reports have shown that a “one pot” modification of carbon can be achieved with an in situ-generated diazonium (5) March, J. AdVanced Organic Chemistry, 3rd ed.; John Wiley & Sons: New York, 1985. (6) (a) Fitzer, E.; Geigl, K. H.; Heitner, W.; Weiss, R. Carbon 1987, 18, 389. (b) Hoffman, W. P.; Curley, W. C.; Owens, T. W.; Phan, H. T. J. Mater. Sci. 1991, 26, 4545. (7) (a) Downard, A. J. Electroanalysis 2000, 12, 1085. (b) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429. (8) 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. (9) (a) Liu, J.; Cheng, L.; Li, B.; Dong, S. Langmuir 2000, 16, 7471. (b) Marwan, J.; Addou, T.; Be´langer, D. Chem. Mater. 2005, 17, 2395. (10) Saab, A. P.; Garzon, F. H.; Zawodzinski, T. A. J. Electrochem. Soc. 2003, 150, A214. (11) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303. (12) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (13) Coulon, E.; Pinson, J.; Bourzat, J. D.; Commercu¨on, A.; Pulicani, J. P. J. Org. Chem. 2002, 67, 8513.
10.1021/la702556n CCC: $40.75 © 2008 American Chemical Society Published on Web 01/23/2008
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Scheme 1. Summary of the Reaction Pathways for the Carbons
salt.1-4 This spontaneous grafting of organic groups at the surface of carbon powders occurs because of the injection of electrons of carbon into the diazonium cations, which leads to the formation of reactive aryl radicals.1,2 The spontaneous modification with diazonium salt was also demonstrated for metals14 and silicon.15,16 Simplicity, flexibility, and efficiency make this functionalization method very convenient for mass production. The drawback of this approach seems to be the limited number of moieties that can be grafted onto the surface compared to the electrochemical route that can achieve much thicker layers.17 Unfortunately, very little is known about the factors influencing the grafting procedure, and no procedure has been established to control the grafting yield other that using electrochemistry to drive the layer growth. Accordingly, the aim of this article is to gain some insight into the grafting mechanism and determine the factors that control the grafting yield. Two approaches were investigated in this work. First, the impact of the carbon black pretreatment prior to the modification was studied in an attempt to assess the importance of the carbon surface state. This was investigated using oxidative pretreatment or heat treatment prior to spontaneous grafting.18 Second, a reducing agent (H3PO2) was used to potentially increase the number of generated 4-nitrophenyl radicals in solution.2 The resulting modified carbons were characterized by X-ray photoelectron spectroscopy (XPS) and elemental analysis. Finally, DFT-Monte Carlo simulations of the adsorption isotherms of the modified carbon black were performed to determine the effect of the various treatments (e.g., oxidation, heat-treatment, grafting) on the carbon microporous structure and to gain some insight into the grafting sites. (14) Adenier, A.; Cabet-Deliry, E.; Chausse´, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491. (15) Hunger, R.; Jaegermann, W.; Merson, A.; Shapira, Y.; Pettenkofer, C.; Rappich, J. J. Phys. Chem. B 2006, 110, 15432. (16) (a) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370. (b) Wang, W.; Lee, T.; Kamdar, M.; Reed, M. A.; Stewart, M. P.; Hwang, J.-J.; Tour, J. M. Superlattices Microstruct. 2003, 33, 217. (17) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038. (18) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; John Wiley and Sons: New York, 1988.
Experimental Section Carbon Substrate Preparation. Vulcan XC72R was obtained from the Cabot Corporation and is named Vulc. A flow chart describing all of the subsequent modifications made to Vulc is shown in Scheme 1. Typically, Vulc (1 g) was oxidized by acid reflux for 30 min using 200 mL of concentrated nitric acid (Fisherbrand) and is hereafter referred to as VulcOx. After the mixture cooled, deionized water (Barnstead Nanopure II) was added to lower the acid concentration to about 1 M, and the dispersion was filtered on a nylon membrane having a pore size of 0.45 µm (OSMONICS Inc.). VulcOx was washed on the filter with 1 L of deionized water and dried under vacuum at 40 °C overnight. VulcOxHT results from the heat treatment of VulOx. The temperature program for the heat treatment was a ramp of 10 °C/min from room temperature to 1000 °C in argon (PRAXAIR). Subsequently, the sample was kept at the latter temperature for 15 min and then cooled. In addition, both Vulc and in situ-modified Vulc were heat treated by the same method to yield VulcHT and Vulc-ISHT, respectively. Carbon Modification. Two modification methods were employed to graft 4-nitrophenyl moieties onto the surface of the carbon substrates. The in situ-generated diazonium cation modification was conducted by dispersing 1 g of carbon in 50 mL of deionized water to which 8.3 mmol of 4-nitroaniline (ACS, Fisher) was added, followed by 8.3 mmol of sodium nitrite (ACS, Fisher) and finally 10 mL of concentrated hydrochloric acid (Fisher). The mixture was stirred overnight at ambient temperature, filtered, and washed successively with water, methanol, dimethylformamide, and acetone. The powder was dried overnight under vacuum and is named VulcIS (Scheme 1). The second method was performed by dispersing 1 g of carbon in 50 mL of water, followed by the addition of 0.1 equiv of 4-nitrophenyldiazonium tetrafluoroborate (fast red GC salt, Aldrich) and 0.1 equiv of hypophosphorous acid, H3PO2 (50 wt %, Aldrich). The solution was stirred overnight, washed, and filtered the next day.2a This modified powder was named Vulc-RD. The same procedures were used to modify VulcOx and VulcOxHT and to obtain VulcOx-RD and VulcOxHT-RD, respectively (Scheme 1). X-ray Photoelectron Spectroscopy (XPS). The VG Escalab 220iXL spectrometer using the Al KR monochromatic beam (1486.6 eV) and a hemispherical multichannel detector was used for all analyses. The survey spectra were recorded at a resolution of 1 meV, and the core-level spectra were recorded at a resolution of 50 meV. A complete set of survey spectra for almost all of the samples prepared
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Table 1. Chemical Composition of the Modified Carbon Determined by Elemental Analysis and XPS elemental analysis Vulc VulcOx VulcOxHT Vulc-IS Vulc-RD VulcOx-IS VulcOx-RD VulcOxHT-IS VulcOxHT-RD
XPS
%N
%C
%N
%O
%C
%NO2/Ca
%O/Cb
∆%O/Cc
na na na 1.50 1.48 1.26 1.43 1.57 1.28
na na na 91.14 92.50 92.96 86.81 92.91 91.65
0.0 0.0 0.0 3.1 4.0 3.5 3.5 2.7 3.7
1.8 13.3 1.9 7.6 8.9 7.6 10.8 5.3 7.7
98.2 86.7 98.1 89.3 87.1 88.9 85.7 92.0 88.6
0.0 0.0 0.0 2.5 3.0 2.9 2.3 2.2 3.0
1.9 15.4 1.9 8.5 10.2 8.6 12.6 5.7 8.7
6.6 8.3 -6.8 -2.8 3.8 6.8
a Value of the N 1s 406 eV signal divided by the C 1s total signal (in %). b Value of the O 1s signal divided by the total C 1s signal (in %). c Difference between the %O/C values of Vulc, VulcOx, and VulcOxHT and the corresponding carbons modified with 4-nitrophenyl groups.
in this work can be found as Supporting Information (Figures SI1-SI3). The pressure in the analytical chamber of the spectrometer was kept below 10-9 Torr. Every spectrum was corrected according to C 1s at 284.5 eV. Adsorption Isotherms and Porosity Analysis. Adsorption isotherms were realized using an Autosorb-1 instrument (Quantachrome Instrument). The adsorption isotherms were determined using nitrogen as an adsorbent at 77 K. The volume of N2 adsorbed for relative pressures ranging from 1 × 10-6 to 1 was recorded. The specific surface area was calculated by the BET isotherm using Quantachrome software (ASW1). Macroscopic theories such as the Dubinin-Radushkevich approach (DR), the BJH method, and the semiempirical treatments of Horvath and Kawazoe (HK) and Saito and Foley do not a give realistic description of the filling of micropores and narrow mesopores. To prevent an underestimation of pore sizes and achieve a more realistic description, microscopic theories that describe the sorption and phase behavior of fluids in narrow pores on a molecular level are necessary. Treatments such as density functional theory (DFT) or molecular simulation methods such as Monte Carlo simulation (MC) provide a much more accurate approach to pore size analysis. Methods such as the DFT of inhomogeneous fluids19,20 and Monte Carlo simulations21,22 bridge the gap between the molecular-level and macroscopic approaches. This is why the microporosity information was obtained through the simulation of the isotherm by DFT Monte Carlo calculations using the same software (ASW1).
Figure 1. XPS C 1s core-level spectra of Vulc (Vulc), acid-refluxed carbon (VulcOx), and VulcOx following reaction with in situgenerated 4-nitrophenyldiazonium cations (VulcOx-IS).
Results X-ray Photoelectron Spectroscopy. C 1s Region. The C 1s core-level spectrum of unmodified carbon black (Vulc) shows a strong graphite component at 284.5 eV, followed by the surface oxides components (e.g., C-OH, CdO, and COOH) at binding energies of 285.4, 286.4, and 288.7 eV, respectively.23 For the oxidized carbon (VulcOx), a much stronger component is observed for COOH at 288.7 eV. The formation of -COOH surface groups by nitric acid reflux has been demonstrated by XPS24 and also by FTIR.25 For VulcOx after heat treatment (VulcOxHT), the 288.7 eV component decreases back to the level of the unmodified carbon, showing the expected departure for the oxygenated functionalities as demonstrated by a substantial decrease in the O content reported in Table 1.18 Finally, no changes in the C 1s spectra (Figure 1) were observed following (19) Evans, R.; Marconi, U. M. B.; Tarzona, P. J. Chem. Soc., Faraday Trans. 2 1986, 82, 1763. (20) Ravikovitch, P. I.; Haller, G. L.; Neimark, A. V. AdV. Colloid Interface Sci. 1998, 76-77, 203. (21) Gubbins, K. E. Physical Adsorption: Experiment, Theory and Application; Fraissard, J., Ed.; Kluwer: Dordrecht, The Netherlands, 1997. (22) Gelb, L. D.; Gubbins, K. E.; Radhakrsihnan, R.; Sliwinska-Bartowiak, M. Rep. Prog. Phys. 1999, 62, 1573. (23) Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Roy, C. J. Anal. Appl. Pyrolysis 2003, 67, 55. (24) Shao, Y.; Yin, G.; Zhang, J.; Gao, Y. Electrochim. Acta 2006, 51, 5853. (25) Jia, N.; Martin, R. B.; Qi, Z.; Lefebvre, M. C.; Pickup, P. G. Electrochim. Acta 2001, 46, 2863.
Figure 2. XPS N 1s core-level spectra of Vulcan before (Vulc) and after modification by reaction with in situ-generated 4-nitrophenyldiazonium cations (Vulc-IS).
modification by reaction with diazonium cations, except for VulcOx for which the component at 288.7 eV disappeared after the grafting of 4-nitrophenyl onto the surface (VulcOx-IS) (Figure 1). A set of C 1s core-level spectra for all of the carbons prepared in this work can be found as Supporting Information (Figures SI4-SI6). N 1s Region. The unmodified Vulc, VulcOx, and VulcOxHT do not show any significant signals in the N 1s region of the XPS spectra as depicted in Figure 2 and SI 7. However, following modification by reaction with in situ-generated 4-nitrophenyldiazonium cations, the N 1s core-level spectra show two
Spontaneous Functionalization of Carbon Black
Figure 3. XPS O 1s core-level spectra of Vulcan before (Vulc) and after chemical modification by reaction with in situ-generated 4-nitrophenyldiazonium cations (Vulc-IS).
components at 406 and 400 eV (Figure 2 and SI 8). The major component at 406 eV is due to the nitro functionalities14,26 and confirms that 4-nitrophenyl groups were not degraded during the grafting step. The 400 eV component is always observed for 4-nitrophenyl-modified surfaces and was attributed to the reduction of NO2 inside the spectrometer by the X-ray beam14,26 and the coupling of the diazonium cations with carbon surfaces having phenolic-type oxygenated functionalities to form an azo bond.3,27 The N 1s spectra for the carbons modified with chemically reduced 4-nitrophenyldiazonium salts are similar to those recorded for the carbons modified with in situ-generated diazonium cations (Figure SI 9). Thus, the method to produce the 4-nitrophenyl radicals does not affect the nature of the grafted moieties. In addition, Table 1 shows for samples Vulc-IS and Vulc-RD that the nitrogen content, evaluated from the core-level spectra and elemental analysis, is not significantly influenced by the modification procedure (e.g., spontaneous or induced generation of the 4-nitrophenyl radical). O 1s Region. In Figure 3, the O 1s spectrum of the unmodified Vulc (Vulc) shows two components at 531.9 and 533.6 eV for the CdO and C-O bonds, respectively.28 Following modification by the in situ-generated diazonium cations (Vulc-IS), a new component at 532.7 eV now dominates the O 1s spectrum of Vulc-IS and is attributed to the N-O bond29 of the 4-nitrophenyl grafted groups. Figure 4 shows that the O 1s spectrum for VulcOx can also be fitted with two components for CdO and C-O at 531.9 and 533.6 eV, respectively. This observation is consistent with the introduction of oxygenated functionalities as a result of acid treatment. Second, once modified, this oxidized carbon (VulcOx-IS) shows, as in the case of Vulc-IS (Figure 3), a component at 532.7 eV (Figure 4) attributed to the NO2 functionalities. A complete set of O 1s core-level spectra can also be found as Supporting Information (Figures SI10-SI12) for various modified carbons. Interestingly, Figure 4 shows a (26) (a) Elliott, C. M.; Murray, R. W. Anal. Chem. 1976, 48, 1247. (b) Sharma, L.; Matsuoka, T.; Kimura, T.; Matsuda, H. Polym. AdV. Technol. 2002, 13, 481. (c) Allongue, P.; Delamar, M.; Desbat, M.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. (d) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 805. (27) (a) Saby, C.; Ortiz, B.; Champagne, G. Y. Be´langer, D. Langmuir 1997, 13, 6805. (b) Baranton, S.; Be´langer, D. J. Phys. Chem. B 2005, 109, 24401. (c) Lyskawa, J.; Be´langer, D. Chem. Mater. 2006, 18, 4755. (d) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Chem. Mater. 2007, 19, 4570. (28) Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Roy, C. Appl. Surf. Sci. 2003, 217, 181. (29) Sharma, J.; Garret, W. L.; Owens, F. J.; Vogel, V. L. J. Phys. Chem. 1982, 86, 1657.
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Figure 4. XPS O 1s core-level spectra of oxidized Vulcan before (VulcOx) and after reaction with in situ-generated 4-nitrophenyldiazonium cations (VulcOx-IS).
Figure 5. N2 adsorption isotherms of the carbon substrates before (Vulc) and after acid pretreatment (Oxidized ) VulcOx) and following thermal treatment of the oxidized carbon at 1000 °C in Ar (Thermal ) VulcOxHT).
decrease in the width at half-maximum following modification with in situ-generated diazonium cations that suggests a decrease in the contribution from the surface oxides (531.9 and 533.6 eV). This is confirmed by the lower oxygen content of VulcOx-IS relative to that of VulcOx reported in Table 1 and is consistent with the disappearance of the peak at 288.7 eV on the C 1s spectrum (Figure 1) for VulcOx following modification with in situ-generated diazonium salts. The same observation is made for VulcOx modified in the presence of the diazonium salt and H3PO2 (Table 1 for VulcOx-RD and Figure SI12). Gas Adsorption Isotherms. The gas adsorption isotherm for Vulc prior to modification is shown in Figure 5. According to IUPAC, Vulc displays a mixed isotherm of types I and II for the low and high relative pressures (P/P0), respectively.30 The high volume of nitrogen adsorbed on the surface at low relative pressure is characteristic of an extended microporous structure, but the sloped plateau in the median relative pressure range indicates a contribution from the outer surface that is generally related to adsorption in the mesopores.31 The structure of Vulc is composed of nanobeads (∼50 nm diameter) sintered in larger agglomerates.32 Hence, the voids between the nanobeads would be responsible for the important microporous structure. However, the nanobeads would also explain the important external surface that is probably responsible for the mesoporous-like behavior of the isotherm. The BET surface area of Vulc is 223 m2/g, and DFT Monte Carlo simulation yielded a specific surface of 187 m2/g (Table 2). (30) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (31) Ohkubo, T.; Miyawaki, J.; Kaneko, K.; Ryoo, R.; Seaton, N. J. Phys. Chem. B 2002, 106, 6523. (32) Zhang, X.; Wang, W.; Chen, J.; Shen, Z. Langmuir 2003, 19, 6088.
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Table 2. Specific Surface Areasa of the Carbons before and after Modification Vulc VulcHT VulcOx VulcOxHT Vulc-IS Vulc-ISHT
BET (m2/g)
DFT (m2/g)
223 237 76 191 121 186
187 220 52 163 78 162
a See Experimental Section for the determination of BET and DFT specific surface areas.
Figure 7. N2 adsorption isotherms of the Vulc carbon black before (Vulc) and after reaction with in situ-generated 4-nitrophenyl diazonium cations (Vulc-IS) and following thermal treatment of the modified carbon at 1000 °C under Ar (Vulc-ISHT).
Figure 6. Pore size distribution calculated by a DFT-Monte Carlo simulation of the carbon substrates before (Vulc) and after acid pretreatment (VulcOx) and following thermal treatment of the oxidized carbon at 1000 °C in Ar (VulcOxHT).
These values are in agreement with literature data for the surface area of Vulc.33 After the acid oxidation of Vulc, the adsorption isotherm displays a remarkable drop in the volume at low relative pressure (Figure 5, VulcOx). This translates to lower specific surface areas of 76 and 52 m2/g according to BET and DFT Monte Carlo, respectively (Table 2). Furthermore, according to the DFT Monte Carlo simulations presented in Figure 6, pores having a diameter smaller than 20 Å are the most influenced. This drop in the volume adsorbed by the powder is ascribed to the “constriction” phenomena of the surface oxides present at the pore entrance preventing gaseous molecules from filling the micropores.34 This is not surprising if one considers the average cross section of nitrogen molecules to be 16.2 Å.2 Hence, the presence of functionalities at the opening of the pore will restrain the entrance of gas. This is further demonstrated by the volume recovery at low relative pressure when the oxygenated functionalities generated by the acid treatment are removed following heat treatment at 1000 °C under Ar (Figure 5, VulcOxHT). Here, VulcOxHT recovers a BET specific surface close to its initial value of 191 m2/g (Table 2) and its microporous structure according to the DFT Monte Carlo simulation (Figure 6). The effect of grafted molecules on the nitrogen adsorption isotherm was also investigated. Figure 7 shows the adsorption isotherms for Vulc after its modification with in situ-generated diazonium cations (Vulc-IS) and the modified carbon that is thermally treated at 1000 °C in Ar (Vulc-ISHT). Similar to the case of oxidation, the grafting of 4-nitrophenyl groups on carbon causes a severe drop in the adsorbed volume at low relative pressure, which is recovered following heat treatment in Ar (Figure 7). This translates to a substantial decrease in the BET specific surface from 223 to 121 m2/g after modification and an increase to 186 m2/g after the heat treatment, as shown in Table (33) Raghuveer, V.; Manthiram, A. Electrochem. Solid-State Lett. 2004, 7, A336. (34) Moreno-Castilla, C.; Ferro-Garcia, M. A.; Joly, J. P.; Baustista-Toledo, I.; Carrasco-Marin, F.; Rivera-Utrilla, J. Langmuir 1995, 11, 4386.
Figure 8. Pore size distribution calculated by DFT-Monte Carlo simulation of the carbon substrate (Vulc) and 4-nitrophenyl-modified (Vulc-IS) and thermally treated modified carbon (Vulc-ISHT).
2. Following heat treatment, the nitrogen content of Vulc-IS decreases from 1.5 to 0.35 atom % N, suggesting that the 4-nitrophenyl functionalities were removed from the surface. The remaining 0.35% of nitrogen (of Vulc-ISHT) can be attributed to the formation of a small amount of graphitic pyrrolic and pyridinic functionalities when 4-nitrophenyl-modified carbon is heated under these conditions.3 Figure 8 shows that there is an important decrease in the microporous surface for pore sizes smaller than 20 Å when the carbon is modified. In agreement with the BET surface area data, this microporous surface is partially recovered when the modified carbon is heat treated (Figure 8). To verify whether the incomplete recovery of the initial surface area of Vulc following the heat treatment of the oxidized and modified carbon could be due to the thermal treatment affecting the carbon microporous structure itself, Vulc was subjected to the same heat treatment (VulcHT). In this case, no loss of surface area was found following the heat treatment (Table 2).
Discussion For the electrochemically induced modification of carbon, the in situ-generated diazonium cations are reduced by applying the appropriate potential to form the reactive radical species at the carbon surface.26c However, the spontaneous modification of carbon (e.g., powder and single-walled carbon nanotubes) requires that the carbon surface is able to reduce the diazonium cations in order to initiate the reaction.1 The decomposition process of the diazonium cations during the modification of carbon surfaces remains elusive and has not yet been directly addressed in the literature. Diazonium ions are known to decompose in acidic water in the presence of a nucleophile such as electron-donating π-electron systems.35 Here, the carbon π-electron system is (35) Zollinger, H. Chem. Res. 1973, 6, 335.
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Scheme 2. Dediazotation by Nucleophilic Attack of the Carbon Surface on the Benzenediazonium Cation Followed by the Heterolytic Formation of a Covalent C-C Bond Between the Surface and the Aryl Group
presumably the nucleophile, which transfers electrons needed for the reduction of the diazonium cations that leads to the formation of phenyl radicals.1i This is followed by the heterolytic formation of a covalent C-C bond between the surface and the phenyl radical as shown in Scheme 2. Analogous spontaneous reduction processes involving the reduction of metal complexes by carbon surfaces have recently been reported.36-38 It was demonstrated that single-walled nanotubes (SWNT) can reduce metallic cations such as PtCl42-and AuCl4- to the respective metal by electron transfer, leaving positive holes in the SWNT.36 According to the estimated SWNT work function (∼5 eV), the Fermi level is at about 0.5 V versus SHE, which is positive enough to allow for the electroless deposition of a metal onto the surface. Recently Chen et al. observed that the functionalization of carbon nanotubes (CNTs) by nitric acid oxidation improved the electroless deposition yield of Pt.37 It was claimed that the open circuit potential of the oxygenated functionalities introduced onto the surface by acid treatment was responsible for the galvanic process involved in the electroless deposition of Pt. However, this contradicts prior observations made for the electroless deposition of Pt on highly oriented pyrolytic graphite (HOPG).38 In fact, Zoval et al. demonstrated that the introduction of oxygenated functionalities by the anodic oxidation of HOPG prior to contact with the Pt complex solution inhibited the spontaneous reduction process.38 Here, we hypothesize that the oxidation of Vulc would lead to a lower grafting yield of the 4-nitrophenyl groups. Obviously this is not the case because both XPS and elementary analysis data (Table 1) show similar N contents for Vulc-IS and VulcOxIS. In addition to the role of oxygenated groups mentioned above, it is well known that oxygenated functionalities are present on edge and defect sites of graphite planes.18,39 Thus, the observation of similar N content is somewhat surprising considering that both 4-nitrophenyl moieties and oxygenated functionalities cannot simultaneously occupy the same sites. However, it is instructive to look at the variation of the amount of oxygen present before and after modification (Table 1). For example, the oxygen content of Vulc of 1.8% deduced from XPS (%O/C ) 1.9; Table 1) is comparable to literature data.24 Once modified by reaction with the in situ-generated diazonium cations, Vulc-IS shows an increase in the %O/C ratio to 8.5 (Table 1). If this number is corrected for the initial contribution of the oxygenated functionalities initially present on the surface, then a net gain in oxygen of 6.6% is obtained (∆%O/C; Table 1), which is in agreement with the introduction of oxygen atoms by the nitrophenyl functionalities considering the uncertainties in XPS measurements and curvefitting procedures. The same observation can be made for all modified samples with the exception of VulcOx-IS and VulcOxRD. For the latter, one can observe a significant loss of oxygen (∆%O/C; Table 1). Hence, one has to consider the possibility (36) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc. 2002, 124, 9058. (37) Chen, J.; Wang, M.; Liu, B.; Fan, Z.; Cui, K.; Kuang, Y. J. Phys. Chem. B 2006, 110, 11775. (38) Zoval, J. V.; Lee, J.; Gorer, S.; Penner, R. M. J. Phys. Chem. B 1998, 102, 1166. (39) Jiang, W.; Nadeau, G.; Zaghib, K.; Kinoshita, K. Thermochim. Acta 2000, 351, 85.
Scheme 3. Coupling between Carboxylate and the Phenyl Radicals
that the latter oxides have been replaced by 4-nitrophenyl groups. This can occur by a chemical transformation analogous to the well-known Kolbe reaction that involves the oxidation of a carboxylate40 and the coupling of a carboxylate radical (by delocalization of the positive charge on the carbon substrate, thus leaving the radical on the carboxylate group) with a 4-nitrophenyl radical (from the corresponding diazonium cation) (Scheme 3). However, this mechanism is unlikely, unless the carboxylate groups on the carbon surface are easier to oxidize than when they are part of anthracenecarboxylic acid. Alternately, we propose that the carbon π system still plays the role of the nucleophile for the decomposition of the diazonium ion whether oxygenated functionalities exist on its edges or not. But in the case where oxygenated functionalities are present, the reaction changes, where the oxygenated functionalities undergo a concerted decarboxylation by the arenium ion mechanism (Scheme 4). Hence, in this scenario, the diazonium cation would be the electrophile replacing the oxygenated functionality on the edge, and the latter would be the CO2 leaving group.5 In addition, the formation of an azo bond, to which the N 1s peak at 400 eV is associated, could be possible if nitrogen departure does not occur. However, it should be noted that a previous report has shown that this reaction, which is equivalent to the reaction of 9-anthroic acid with 4-nitrophenyl diazonium cations, did not led to the departure of the carboxylic group.41 This suggests that another mechanism is operative for a surface reaction. It is also important to point out that the decomposition of the diazonium ion can be caused by the presence of oxygen35 as well as by the photolytic dediazoniation42 depending on the reaction conditions. Hence, many factors can lead to the homolytic cleavage of the C-N bond between the diazonium ion and the phenyl radical required for the functionalization process. In addition, to confirm that the (40) Scha¨fer, H. J. Top. Curr. Chem. 1990, 152, 90. (41) Dickerman, S. C.; Levy, L. B.; Schwartz, A. W. Chem. Ind. 1958, 360. (42) Milanesi, S.; Fagnoni, M.; Albini A. Chem. Commun. 2003, 216.
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Toupin and Be´ langer
Scheme 4. Concerted Dediazoniation of the Diazonium Cations and Decarboxylation of the Carbon Black Edges When VulcOx Is Functionalized
decarboxylation was not caused by other reagents (e.g., NaNO2, HCl, NaNO2 + HCl, and H3PO2) present in the solution when performing the modification, VulcOx was left to react with these individual reagents (in the absence of the amine) for 4 h. XPS measurements performed on such treated Vulc powders showed no significant difference in the oxygen content (Figure SI13). It might be of interest at this point to estimate the surface coverage of the carbon black by 4-nitrophenyl moieties. It is generally believed that the aryl radicals will react predominantly with carbon atoms on the more reactive edge sites.14,39 To evaluate the surface coverage of the grafted functionalities, the number of potentially active sites on the surface was determined by estimating the fraction of exposed edge planes to be approximately 1% of the total number of carbon atoms. (See the Supporting Information for more details about this calculation.) In our case, elemental analysis (Table 1) of the 4-nitrophenyl-modified carbons suggests that an average of 1.4 ( 0.1% of the carbon surface atoms are linked to a nitrogen atom if one assumes, for simplicity, only the direct grafting of an aryl group without the formation of an azo linkage.27 This data indicates that the available active edge sites would be saturated by 4-nitrophenyl groups. In addition, there is some evidence that grafting could also occur on a basal plane,26c and more recently, Strano et al. explained the sidewall modification of SWNTs by diazonium salts by the conversion of sp2 atoms to sp3, allowing covalent bond formation.1h Thus, some grafting on the basal plane of the carbon powder is expected because carbon blacks have highly amorphous structures with presumably a fraction of the carbon atoms having sp3 hybridization.43 The spontaneous grafting of 4-nitrophenyl groups onto a glassy carbon electrode was also observed, and the electrochemical data suggested that the layer thickness of the grafted film varied from 1 to 4 monolayers for an immersion time of 1 to 60 min.14 These observations are consistent with the saturation of the reactive sites and also confirm that extensive layer growth is not observed without electrochemical induction. In contrast to the latter study, no difference in the grafting yield was observed for a carbon black modified for an immersion time of 1, 4, or 12 h or by a 10-fold increase in the concentration of the reagents. However, multilayer films were observed for the electroless modification of metallic substrates by diazoniums salts.14 It was demonstrated by atomic force microscopy (AFM) that layers up to roughly 10 nm could be grown on an iron surface when immersed in a solution of diazonium salts for 6 h. This growth was attributed to the initial grafting of a monolayer and then by the subsequent attack of another 4-nitrophenyl radical on the already grafted phenyl ring. The second aryl radical would be generated by tunneling of the electron from the metal trough of the conjugated π system of the phenyl layer to the diazonium (43) Ja¨ger, C.; Henning, Th.; Schlo¨gl, R.; Spillecke, O. J. Non-Cryst. Solids 1999, 285, 161.
ions of the solution. A nonuniform layer on top of the metal substrate showing humps indicates that the metallic surface had cathodic and anodic regions responsible of the galvanic cycle supplying electrons for the reaction.44 However, carbon supplies electrons for the reduction processes by hole injection in its π system, as demonstrated by conductivity measurements.36 This will increase the open circuit potential of the carbon to a point that, when the reaction proceeds, a phenomenon will self-limit the reduction reaction36 and will explain the difference in behavior between the carbon and metallic substrates. In an effort to increase the surface coverage and eventually achieve the growth of multilayers, Vulc was modified using the 4-nitrobenzene diazonium tetrafluoroborate salt and a reducing agent (hypophosphorous acid) in the reaction mixture. This would allow the complete transformation of the diazonium cations to the reactive radical species and favor the formation of multilayers as in the electrochemical modification method.2a Samples modified in this way were named Vulc-RD, VulcOx-RD, and VulcOxHT-RD. Surprisingly, Table 1 shows that the grafting yield is not significantly influenced by the modification method. The addition of H3PO2 to the diazonium salt solution leads to the homogeneous generation of aryl radicals in the solution. This process occurs in conjunction with the heterogeneous electron transfer at the carbon powder surface with diazonium cations, which also generates aryl radicals that can graft to the surface. The fact that the grafting yield is similar with or without H3PO2, under our experimental conditions, indicates that the heterogeneous process governs the grafting process. In our case, the data does not suggest the formation of a multilayered film but a monolayer localized at the available reactive sites. Tour and co-workers reported similar observations for the functionalization of SWNT, where electron microprobe analysis revealed that 1 out of 20-30 carbon atoms of the nanotube was attached to a moiety and humps of roughly 4-6 Å were observed by TEM.1i Obviously, the electrochemical driving force (e.g., external source or galvanic cycle) is fundamental for the growth of multilayer films, not only for the dediazoniation of the salt but also for the attack on the already grafted phenyl rings.45 However, it is obvious that other approaches need to be developed to increase the grafting yield at the carbon powder substrate. Creating surface oxygenated functionalities by acid reflux is known to increase the number of defect sites on carbon surfaces such as graphite, HOPG, and glassy carbon.18 Hence, the removal of surface oxides by thermal treatment up to 1000 °C led to the departure of CO2 and the formation of new reactive sites available for reaction with aryl radicals.39 This could allow for an increase in the number of 4-nitrophenyl moieties grafted onto the surface. Thus, VulcOxHT, which is characterized by a relatively low (44) Adenier, A.; Barre´, N.; Cabet-Deliry, E.; Chausse´, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Surf. Sci. 2006, 600, 4801. (45) Marcoux, P.; Hapiot, P.; Batail, P.; Pinson, J. New J. Chem. 2004, 28, 302.
Spontaneous Functionalization of Carbon Black Scheme 5. Idealized Models of a Micropore of a Graphite Structure: (a) Unmodified and (b) Modified by 4-Nitrophenyl Moieties.
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the departure of CO and CO2, which are associated with the oxygenated functionalities present at the carbon surface following chemical oxidation.46
Conclusions
oxygen content (Table 1), was subsequently modified by in situgenerated salts (VulcOxHT-IS), and again no significant change in the grafting yield was observed. This could be explained by the fact that carbon black is a highly amorphous carbon structure that cannot be significantly disorganized by oxidation in contrast to carbon having highly crystallized domains.32,46 Adsorption isotherms clearly demonstrated the effect of modification by oxidation, heat treatment, and the grafting of organic groups on the microporosity of the carbon. A simple model is proposed to explain the loss of microporosity and the decrease in BET surface area. Scheme 5 shows an idealized micropore of 10 Å in part a together with a representation of that micropore entrance once modified by 4-nitrophenyl moieties in part b. Thus, micropores with a diameter of 10 Å or less will be blocked or filled by the organic moieties. This model is consistent with the significant decrease in the adsorbed gas volume observed at low relative pressures and the loss of micropores upon the grafting of 4-nitrophenyl groups. Interestingly, the chemical oxidation of Vulc led to a similar blocking of the micropores. This is not surprising considering that both grafting and oxidation will most likely occur on the more reactive edge plane sites. Finally, the recovery of the BET surface area and microporosity following the heat treatment of Vulc-IS is consistent with TG-MS and XPS data, which demonstrated that heat treatment at 1000 °C in Ar is sufficient to decompose the grafted layer.3 Analogously, previous TG-MS data for oxidized carbon revealed (46) Kangasniemi, K. H.; Condit, D. A.; Jarvi, T. D. J. Electrochem. Soc. 2004, 151, E125.
This work demonstrates that the electroless functionalization of Vulc by 4-nitrophenyl groups is limited to about 1.4 atom %N despite the fact that the experimental conditions potentially limiting (oxidation of carbon) or presumably favoring surface grafting (adding a reducing agent) were used. The issue of the spontaneous reduction of the diazonium was partially addressed because this work established that, apparently, a galvanic process of the carbon would not be responsible for the reduction of the diazonium salts and the functionalization. Our results suggest that an electrochemical driving force is required for the formation of multilayered films or that spontaneous grafting should be performed under other experimental conditions or by using other approaches. This can be a positive aspect for some applications, where a self-limited monolayer instead of thick multilayers is desired. Also, the presence of oxygenated functionalities does not hinder the grafting yield, and apparently there is a concerted decarboxylation reaction where the diazonium cation would be the electrophile replacing the oxygenated functionality leaving as CO2. The fact that the microporous structure of carbon is affected by the grafting process might have important consequences for applications of such modified carbons. Nonetheless, the microporosity can be recovered following heat treatment. Acknowledgment. The financial support of the Natural Science and Engineering Research Council of Canada (NSERC) is acknowledged for a research grant to D.B. and for an equipment grant for the XPS spectrometer. Dimitre Karpusov is acknowledged for XPS measurements at the University of Alberta. M.T. also acknowledges the NSERC for a post-graduate fellowship. We also thank Jean Pinson for the mechanism described in Scheme 3, which arose from the discussion held during the Ph.D. defense of M.T.. Supporting Information Available: A complete set of survey spectra for almost all of the samples prepared in this work, a set of C 1s core-level spectra for all of the carbons prepared in this work, and a complete set of O 1s core-level spectra. Details of the calculation of the number of potentially active sites on the surface determined by estimating the fraction of exposed edge planes. This material is available free of charge via the Internet at http://pubs.acs.org. LA702556N
