Hairy Carbon Nanotube@Nano-Pd Heterostructures: Design

Sep 21, 2010 - Poly(glycidyl methacrylate), PGMA, was prepared via ATRP in bulk solution, and its epoxy groups were further acid-hydrolyzed in order t...
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Hairy Carbon Nanotube@Nano-Pd Heterostructures: Design, Characterization, and Application in Suzuki C-C Coupling Reaction Samia Mahouche Chergui,*,† Alexandre Ledebt,† Fayna Mammeri,† Frederic Herbst,† Benjamin Carbonnier,‡ Hatem Ben Romdhane,§ Michel Delamar,† and Mohamed M. Chehimi*,† ‡

† ITODYS, University Denis Diderot & CNRS (UMR 7086), 15 rue Jean de Baı¨f, 75013 Paris, France, Institut de Chimie et des Mat eriaux de Paris Est, CNRS (UMR 7182), 2-8 rue H. Dunant, 94320 Thiais, France, and §Laboratoire de Chimie Structurale et Organique, Facult e des Sciences de Tunis, Campus Universitaire, 2092 Manar II, Tunisia

Received July 13, 2010. Revised Manuscript Received September 6, 2010 Poly(glycidyl methacrylate), PGMA, was prepared via ATRP in bulk solution, and its epoxy groups were further acid-hydrolyzed in order to obtain a polymer with glycerol moieties (noted POH). The POH chain end C-Br bonds were subjected to a nucleophilic attack by NaN3, resulting in azide-terminated POH (POH-N3). The CNTs were modified by in-situ-generated alkynylated diazonium cations from the para-alkynylated aniline of the formulas H2N-C6H4CtC-H, yielding CNT-C6H4-CtC-H nanotubes. The azide-functionalized polymer POH-N3 was clicked to the alkynyl-modified CNTs giving CNT@POH hybrids, which were further subjected to an oxidation resulting in carboxylated polymer-modified CNTs (noted CNT@PCOOH). The as-designed hairy CNTs served as efficient platforms for the in-situ synthesis and massive loading of 3 nm sized palladium nanoparticles (NPs). The CNT@ PCOOH@Pd heterostructures prepared so far exhibited an efficient catalytic effect in the C-C Suzuki coupling reaction and were regenerated up to four times without any significant loss of catalytic activity.

Introduction Carbon nanotube-supported nanoparticles heterostructures are nanometer scale and high surface area objects that are the subject of numerous academic and applied technological studies1 within the domains of catalysis,2 energy conversion/fuel storage,3 electronic nanodevices,4 and sensors.5 In the catalysis domain, carbon nanotubes (CNTs) have been used as an alternative supports due to their unique tubular structure, high surface area, high thermal and electrical conductivity, and high corrosion resistance. For electronics devices, CNT-supported catalysts were employed to design catalysts arrays.4 As far as sensor devices are concerned, it was shown that CNTs have great potential in recognizing molecular species, with a very good detection limit, provided that they are decorated by polymers or by metallic nanoparticles (NPs).6 From these few examples, it is clear that an efficient support (e.g., CNTs) rests on its ability to strongly bind and disperse metallic nanoparticles. This can be achieved by e.g. laser-assisted physical treatment of CNTs7 which leads to an effective immobilization of Pd nanoparticles leading to stable CNT/Pd heterostructures. In the absence of such a treatment, no decoration of the MWCNTs was observed by simple mixing with nanoparticle suspensions. A dry method employing plasma surface treatment of CNTs *Corresponding authors: e-mail [email protected], Ph þ33157278854 (S.M.C.); e-mail [email protected], Ph þ33157276863 (M.M.C.). (1) Wildgoose, G. G.; Banks, C. E.; Compton, R. G. Small 2006, 2, 182. (2) Yang, S. D.; Zhang, X. G.; Mi, H. Y.; Ye, X. G. J. Power Sources 2008, 175, 26. (3) Georgakilas, V.; Gournis, D.; Tzitzios, V.; Pasquato, L.; Guldi, D. M.; Prato, M. J. Mater. Chem. 2007, 17, 2679. (4) Mubeen, S.; Zhang, T.; Yoo, B.; Deshusses, M. A.; Myung, N. V. J. Phys. Chem. C 2007, 111, 6321. (5) Wang, Y.; Yeow, J. T. W. J. Sens. 2009, Article ID 493904, doi: 10.1155/2009/ 493904. (6) Gao, G.; He, H.; Zhou, L.; Zheng, X.; Zhang, Y. Chem. Mater. 2009, 21, 360. (7) Henley, S. J.; Watts, P. C. P.; Mureau, N.; Silva, S. R. P. Appl. Phys. A: Mater. Sci. Process. 2008, 93, 875.

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induced a uniform dispersion of Pd clusters at the carbon nanosupport in comparison to the pristine nanotubes.8 Surface chemical modification can be achieved at the synthesis stage of the CNTs (in-situ modification)9 or by post-functionalization employing molecular modifiers such as aminophenyl groups.10 In order to control the CNT/NP interface and to favor a high NP loading, coordinating polymer grafts can act as macromolecular coupling agents as they carry multiple binding sites. In this regard, polypyrrole (PPy) has been used as an ultrathin layer on CNTs to bind metallic nanoparticles for electrocatalysis applications.11 As an alternative to conductive polypyrrole or poly(ethylene imine), the team led by Chao Gao grafted poly(meth)acrylates to oxidized CNTs via surface-initiated ATRP. They demonstrated that hairy nanomaterials could serve as support for the in-situ nucleation of Ag, Pt, and Au nanoparticles.12,13 They have also grafted CNTs with polymers bearing alkynyl (azide) for the immobilization of magnetic particles modified by azide (alkynyl) groups.14 Alternatively, it is possible to achieve covalent bonding of polymers to CNTs, in mild conditions, through grafted aryl layers derived from either isolated15,16 or in-situ-generated17-19 (8) Felten, A.; Ghijsen, J.; Pireaux, J.-J.; Drube, W.; Johnson, R. L.; Liang, D.; Hecq, M.; van Tendeloo, G.; Bittencourt, C. Micron 2009, 40, 74. (9) Jiang, H.; Zhuc, L.; Moon, K.-S.; Wong, C. P. Carbon 2007, 45, 655. (10) Guo, D. J.; Li, H. L. Carbon 2005, 43, 1259. (11) Selvaraj, V.; Alagar, M. Electrochem. Commun. 2007, 9, 1145. (12) Gao, C.; Vo, C. D.; Jin, Y. Z.; Li, W.; Armes, S. P. Macromolecules 2005, 38, 8634. (13) Gao, C.; Li, W.; Jin, Y. Z.; Kong, H. Nanotechnology 2006, 17, 2882. (14) He, H.; Zhang, Y.; Gao, C.; Wu J. Chem. Soc., Chem. Commun. 2009, 1655. (15) Price, B. K.; Hudson, J. L.; Tour, J. M. J. Am. Chem. Soc. 2005, 127, 14867. (16) Dyke, C. A.; Tour, J. M. Nano Lett. 2003, 3, 1215. (17) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (18) Hudson, J. L.; Casavant, M. J.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 11158. (19) Lawson, G.; Gonzaga, F.; Huang, J.; De Silveira, G.; Brooka, M. A.; Adronov, A. J. Mater. Chem. 2008, 18, 1694.

Published on Web 09/21/2010

DOI: 10.1021/la102801d

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aryldiazonium coumpounds. Indeed, it is well-known that aryl layers bind to graphene-type carbon by strong covalent bonds as judged from theoretical functional density calculations.20 This actually accounts for resistance to degrafting in aggressive conditions as shown by Pinson and co-workers.21,22 In this regard, there are basically two main options to prepare hairy nano-objects: via in-situ polymerization employing a pretreated CNT as a macroinitiator,23-25 termed grafting from, or via attachment of prepared polymers with reactive sites directly to pretreated CNTs, the so-called grafting onto route. The latter can be achieved via click chemistry, an efficient and versatile approach for strong covalent bonding of polymers to CNTs.26,27 Indeed, on the one hand, Adronov et al. clicked polymers to CNTs via aryl layers to improve their solubility,26,27 whereas on the other hand Gao and co-workers have grown chelatant polymers from carbon nanotubes using surface-initiated atom transfer radical polymerization (SI-ATRP) for immobilizing metallic nanoparticles.12 This paper bridges the gap between these two elegant approaches (aryl layers and click chemistry on the hand; grafted, chelatant polymers on the other hand) by gathering unique opportunities for preparing heterostructures based on the following sequential steps: (i) use of in-situ-generated diazonium compounds to modify CNTs by aryl layers; (ii) click chemistry to bind reactive ATRP polymers to CNTs; (iii) in-situ deposition of metallic NPs by chemical reduction of previously immobilized metal ions on the hairy nanotubes prepared in steps i and ii. These three steps enabled us to design CNT@polymer@nanoparticles. The target polymer is poly(glycidyl methacrylate), PGMA, hydrolyzed after synthesis by ATRP (POH) and further functionalized by azide groups (POH-N3). Note that ATR polymers are much more flexible than conductive polymers. The CNTs were functionalized by alkynyl groups using the diazonium salt BF4-,þN2-C6H4-CtCH. The azide-functionalized polymer POH-N3 was then clicked to the alkynylated carbon nanotubes (CNT-C6H4-CH2CtCH). From the above, it is therefore highly expected that the synergy of diazonium compounds with ATRP polymers and click chemistry holds promises concerning the design of robust reactive and functional polymer-modified carbon nanotubes. Toward this end, the hairy nanotubes CNT@POH were employed as nanometerscale platforms for the in-situ synthesis of palladium NPs from their metallic salt precursors using borohydride sodium as reductant.2 The resulting heterostructures CNT@PCOOH@NP and their precursors were characterized by IR, TGA, EDX, XPS, XRD, and TEM. The heterostructures were evaluated as catalysts for the well-known Suzuki C-C coupling reaction between bromobenzene and phenylboronic acid, and the reaction products were examined by 1H NMR and GC-MS.

Experimental Section Materials. Glycidyl methacrylate (GMA, Sigma) was passed through silica column in order to remove the polymerization (20) Jiang, D. E.; Sumpter, B. G.; Dai, S. J. Phys. Chem. B 2006, 110, 23628. (21) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (22) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429. (23) (a) Matrab, T.; Chancolon, J.; L’hermite, M. M.; Rouzaud, J. N.; Deniau, G.; Boudoue, J. P.; Chehimi, M. M.; Delamar, M. Colloids Surf., A 2006, 287, 217. (b) Wu, W.; Tsarevsky, N. V.; Hudson, J. L.; Tour, J. M.; Matyjaszewski, K.; Kowalewski, T. Small 2007, 3, 1803. (24) Wang, G.-J.; Huang, S.-Z.; Wang, Y.; Liu, L.; Qiu, J.; Li, Y. Polymer 2007, 48, 728. (25) Tour, J. M.; Hudson, J. L.; Krishnamoorti, R.; Yurelki, K.; Mitchell, C. A. WO 05030858A3, 2005. (26) Li, H.; Cheng, F.; Duft, A. M.; Adronov, A. J. Am. Chem. Soc. 2005, 127, 14518. (27) Mayo, G. D.; Behal, S.; Adronov, A. J. Polym. Sci., Part A: Polym. Chem. 2008, 47, 450.

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inhibitor and stored under argon at -4 °C. Multiwalled carbon nanotubes (MWCNTs, 1-10 nm diameter and 0.1-10 μm length, Aldrich), 2,2-bipyridine (bpy, Alfa-Aesar), copper bromide (CuBr, Aldrich), copper chloride (CuCl, Aldrich), ethyl 2-bromoisobutyrate (EBiB, Aldrich), sodium carbonate (Na2CO3, Acros), ethynylaniline (C8H7N, Aldrich), isoamyl nitrite (C5H11NO2, Alfa-Aesar), N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA, Aldrich), sodium azide (NaN3, Prolabo), potassium permanganate (KMnO4, Prolabo), palladium chloride (PdCl2, Acros), sodium hydroxide (NaOH, Acros), and sodium borohydride (NaBH4, Aldrich) were used as received. Chloroform (CHCl3), N,N-dimethylformamide (DMF), dichloromethane (DCM), acetonitrile (ACN), sulfuric acid (H2SO4), methanol (CH3OH), and chlorhydric acid (HCl) were all of analytical reagent grade. Milli-Q water was obtained by a Millipore purification system.

Synthesis and Functionalization of PGMA. ATRP of Glycidyl Methacrylate. CuBr (27 mg, 0.19 mmol), CuCl

(19 mg, 0.19 mmol), and PMDETA (39 μL, 0.19 mmol) were loaded into a 50 mL flask and deoxygenated for 5 min. A mixture of GMA (5 mL, 38 mmol) and diethyl ether (5 mL), prebubbled through argon flow for 10 min, was added to the salts. To ensure complete dissolution of copper, the mixture was sonicated for a few minutes. Finally, the argon-purged initiator EBriB (28 μL, 0.19 mmol) was injected into the reaction system via an argonpurged syringe. ATRP was stopped after 6 h by exposing the reactor to air. The resulting PGMA was dissolved in an excess of CHCl3, and the solution was filtered through a neutral alumina column to remove the ATRP catalyst. The solvent was then removed by rotary evaporation, and the PGMA was precipitated in cold ether and dried under vacuum. The weight-average molecular weight (Mw) and polydispersity index (PDI) were found to be 41 500 g/mol and 1.51, respectively. Hydrolysis of Oxirane Groups of PGMA28. Dry PGMA (4.45 g) was added to a mixture of CHCl3 (100 mL) and H2SO4 (50 mL, 0.1M). After 48 h of stirring at 60 °C, the solvent was reduced by evaporation, and the pH of the polymer solution was adjusted to 7 with a Na2CO3 saturated solution. The hydrolyzed PGMA (POH) was filtered, washed several times with water, and dried under vacuum.

Preparation of Azido-Terminated Hydrolyzed Poly(glycidyl methacrylate). 4 g of POH and 401 mg of NaN3 (6.17 mmol) were deoxygenated for 5 min in a dry flask, and then 50 mL of degassed DMF was added via a purged argon syringe. The solution was stirred under argon flow for 3 h, and the resulting azide-functionalized polymer was precipitated in 50/50 CH3OH/H2O mixture. The POH-N3 polymer was dried under vacuum.

Preparation of Hairy MWCNTs. In-Situ Generation of Diazonium Salt for Grafting Alkynylated Aryl Layers to MWCNTs. CNTs (100 mg, 8.33 mmol) and the commercially available ethynylaniline (1.95 g, 16.66 mmol) of were added to a 50 mL flask equipped with a stir bar; the mixture was then deoxygenated by bubbling with argon for at least 15 min. Isoamyl nitrite (2.23 mL, 16.66 mmol) was then added via an argon-purged syringe, and the solution was heated at 60 °C for 3 h. The product was then purified by filtration, washed with acetone, and then redispersed in DCM and then ACN with sonication for 5 min, prior to filtration and washing with water. Water washing was continued for two repeated filtration-redispersion cycles. The functionalized MWCNTs (in short CNT-C6H4-CtCH) were then dried overnight at 60 °C.

Click Coupling of Azide End-Functionalized POH and Alkyne-Functionalized MWCNTs. CNT-C6H4-CtCH (30 mg) were dispersed in a two-neck flask containing 40 mL of DMF and sonicated for few minutes. POH-N3 (250 mg) was added, and the mixture was purged for 10 min under argon flow. A solution of (28) Smigol, V.; Svec, F.; Frechet, J. M. J. Macromolecules 1993, 26, 5615.

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Scheme 1. Synthetic Route for the in-Situ Grafting of Alkylynated Aryl Groups and Click Reaction of the ATRP-Prepared PGMA on CNTs

CuBr (30 mg, 0.20 mmol) in DMF (10 mL) was sonicated for 2 min in order to ensure its complete dissolution and bubbled through argon flow for 5 min. It was added to the CNTs suspension, followed by addition of PMDETA (42 μL, 0.2 mmol) under an argon atmosphere. The mixture was stirred for 5 h at room temperature, filtered, and ultrasonically washed with THF. To remove the copper catalyst, the product was extracted with acetylacetone/ethanol solution, in a volume ratio of 1/5 for 1 h, followed by filtration and washing with excess C2H5OH, H2O, and THF. The CNT@POH were then dried under vacuum.

Oxidation of POH Grafts on the Surface of CNTs29. CNT@POH (450 mg) was dispersed in a 50 mL solution of H2SO4 (1 M) containing 49.9 mg of KMnO4, and stirred at room temperature for 3 h. The mixture was filtered and washed with H2O five times, and the resulting chelatant nanocomposite (CNT@PCOOH) was dried under vacuum. In-Situ Synthesis of Palladium Nanoparticles. Chelatant nanosupports CNT@PCOOH (50 mg) were ultrasonically dispersed in 10 mL of NaOH solution (1 M), and then 10 mL of an aqueous solution of PdCl2 (10-2 M) was added. After stirring for 1 h, a fresh aqueous solution of NaBH4 (0.1 M) was added drop by drop to the mixture. The solution was stirred for 1 h at room temperature. The resulting palladium nanoparticle-decorated CNTs were filtered, then washed several times under sonication with copious amounts of water, and dried under vacuum. Catalytic Suzuki Coupling Reaction. The test of the Suzuki coupling reaction30 was carried out by dissolving 4-bromobenzene (314 mg, 2 mmol), benzeneboronic acid (292 mg, 2.4 mmol), and Na2CO3 (640 mg, 6 mmol) in 10 mL of DMF. Then 10 mg of (29) Horaka, D.; Rittichb, B.; Spanova, A. J. Magn. Magn. Mater. 2007, 311, 249. (30) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.

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CNT@PCOOH@Pd was added; the solution was sonicated for 5 min and then stirred at 100 °C for 2 h. The catalyst was separated by filtration, washed several times with DMF and water, and then dried under vacuum. The same catalyst was used in four other cycles of Suzuki reaction. The resulting mixture was added to 50 mL of water and was then extracted with 50 mL of ethyl acetate three times. The organic products were dried over sodium sulfate, evaporated under reduced pressure, and analyzed by gas chromatography-mass spectrometry (GC-MS) and 1H NMR. Characterization. FTIR. FTIR spectra were recorded in a Nicolet Magna-860 FTIR (Thermo-Elctron) spectrometer. The samples were mixed with KBr, and compressed pellets were prepared for analysis in the range between 400 and 4000 cm-1, with a resolution of 4 cm-1. All spectra were baseline-corrected with Omnic 6.1 software. TGA. The thermal and decomposition characteristics of the materials were determined by thermal gravimetric analyses, conducted on a Netzsch STA 409C, in the temperature range of 10-800 °C with a heating rate of 10 °C/min under a flow of air at 80 mL/min. EDX. X-ray energy dispersion spectroscopy of the palladiumhybrid nanocomposite was conducted with an EDAX GENESIS analyzer installed on a scanning electron microscope (SEM 6100 JEOL). TEM. Transmission electron micrographs were obtained on a JEOL 2010 microscope equipped with a CCD camera. Samples were prepared by dispersing uncoated and coated carbon nanotubes in ethanol using an ultrasonic bath. Few drops from these suspensions were then gently deposited onto a lacey carbon grid and left to dry in air prior to TEM observation. XRD. The XRD pattern of the hybrid material was performed on X’pert Pro diffractometer. X-rays of 1.7902 A˚ wavelength were DOI: 10.1021/la102801d

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Figure 1. FTIR spectra over pressed KBr pellets of (a) PGMA, (b) POH, and (c) POH-N3.

Figure 2. FTIR spectra over pressed KBr pellets of (a) CNTC6H5-CtCH, (b) CNT@POH, and (c) CNT@PCOOH.

generated by a Co KR source. The angle of diffraction was varied from 5° to 40° at 40 kV and 40 mA. XPS. The spectra were recorded using a Thermo VG Scientific ESCALAB 250 system fitted with a microfocused monochromatic Al KR X-ray beam (1486.6 eV, 650 μm spot size). The samples were mounted on double-sided adhesive tape and pumped in the fast entry lock prior to analysis. A flood gun was used (filament current: 3 A; emission current: ∼3 mA; acceleration: 4 eV) to achieve a uniform charge over the sample surface. The Avantage software, version 3.51, was used for digital acquisition and data processing. Spectral calibration was determined by setting the aliphatic C-C/C-H C 1s component at 285 eV. The surface composition was determined using the integrated peak areas, and the corresponding Scofield sensitivity factors were corrected for the analyzer transmission function.

symmetric and asymmetric vibrations of the epoxy rings, respectively. This result is confirmed by the appearance of a broad peak centered at 3200-3600 cm-1 assigned to OH stretching vibrations (Figure 1b) formed after ring-opening of epoxy group of PGMA, resulting in vicinal diol. Figure 1c indicates an additional band characteristic of azide vibrations at 2110 cm-1, which account for bromine substitution by N3.35 The PGMA structure was also characterized, before and after hydrolysis by 1H NMR spectroscopy (using a Bruker 400 MHz Avance III instrument) at room temperature with CDCl3 as solvent. 1H NMR of PGMA detects the protons: δ 0.93 (COC(CH3)2), 1.09 (CH3-CH2), 1.42 (CH2C-CH3), 1.70 (CH3CH2-O), 1.89 (C(CH3)2-CH2), 2.63 and 2.83 (CH-CH2-O), 3.22 (CH-OCHH2), 3.80 and 4.30 (O-CH2-CHO). After treatment with acidic medium, the 1H NMR spectrum displays disappearance of epoxy protons and apparition of new peaks at δ 3.46 (CH2-CH-OH, CH-OH, and CH2-OH) and 4.76 (CH-OH and CH2-OH) confirmed complete opening of epoxy groups. Functionalization of Carbon Nanotubes by Diazonium Compounds and via Click Chemistry. The resulting azideterminated POH can readily be clicked to alkyne-functionalized carbon nanotubes via the very well-known 1,3-cycloaddition reaction as described elsewhere.26,36 In a second step, the grafted POH chains were further oxidized to PCOOH at the surface of the nanotubes. Figure 2 displays FTIR spectra of CNT-C6H5CtCH, CNT@POH, and CNT@PCOOH. The spectrum of CNT-C6H5-CtCH exhibits a narrow band at 2097 cm-1, attributed to CtC stretching vibration bands, and a broad peak at 3284 cm-1 assigned to the alkyne C-H vibrations. In addition, the aryl ring stretching vibrations are visible at 1168 and 1503 cm-1. These spectra indicate that the in-situgenerated diazonium salt was successfully reduced and the resulting aryl radical grafted to the CNT surface. Figure 2b shows a complete disappearance of the peak corresponding to the alkyne groups after click reaction, while two new strong peaks at 1730 and 1161 cm-1 attributed to CdO and C-O groups appear; they correspond to the PGMA ester functions. At this stage, it is noteworthy that the CNT@POH prepared so far were evaluated as supports for the immobilization of metallic nanoparticles. However, they did not retain any palladium NPs. For this reason, the hydroxyl groups of the grafted POH chains were reacted with a diluted solution of KMnO4 in order to

Results and Discussion General Strategy for the Preparation of Hairy Carbon Nanotubes via Tandem Aryl Layer Grafting and Click Chemistry. The ATRP polymers are halogen-terminated which makes them suitable for functionalization with various nucleophilic substitution reactions, for example, 1,3-dipolar cycloaddition click reaction.31,32 The synthesis route is illustrated in Scheme 1. Preparation and Functionalization of Poly(glycidyl methacrylate). Poly(glycidyl methacrylate) is an interesting polymer because it bears a highly reactive epoxy group in each repeat unit12,33,34 and simultaneously exhibits a high resistance to hydrolysis of the ester groups. Figure 1 shows the FTIR spectra of PGMA synthesized by ATRP before and after modification, leading to its azidefunctionalized form. Figure 1a exhibits, for the neat PGMA, the characteristic stretching vibrations of CdO and C-O from the ester groups, centered at 1734 and 1157 cm-1, respectively. The peak at 3060 cm-1 is assigned to the C-H vibration in the epoxy group, whereas the bands centered at 1247 and 907 cm-1 correspond to the C-O vibrations, in the same ring. One can also notice the presence of symmetric and antisymmetric CH2 stretching peaks at 2940 and 2993 cm-1, respectively. After hydrolysis of PGMA, The FTIR spectrum (Figure 1b) shows clearly complete disappearance of the peaks at 1247 and 907 cm-1, attributed to (31) Golas, P. L.; Matyjaszewski, K. QSAR Comb. Sci. 2007, 26, 1116. (32) Mamidyala, S. K.; Finn, M. G. Chem. Soc. Rev. 2010, 39, 1252. (33) Mahouche Chergui, S.; Abbas, N.; Matrab, T.; Turmine, M.; Bon Nguyen, E.; Losno, R.; Pinson, J.; Chehimi, M. M. Carbon 2010, 48, 2106. (34) Djouani, F.; Herbst, F.; Chehimi, M. M.; Benzarti, K. Constr. Build. Mater. 2010, in press.

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(35) Zhang, Y.; He, H.; Gao, C.; Wu, J. Langmuir 2009, 25, 5814. (36) Zhang, Y.; He, H.; Chao Gao, C. Macromolecules 2008, 41, 9581.

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Figure 3. Thermogravimetric analysis curves of PGMA, alkynylated CNT, CNT, and CNT-POH.

transform them into carboxylic acid functions which are known to have a strong chelating power.37 These reaction products (COOH groups) are highlighted by the significant increase of the width of the band at 3300 cm-1 and the appearance of the characteristic vibration peak of acid carbonyl (CdO) at 1631 cm-1. The conversion percentage of OH to COOH groups is difficult to determine by IR. Alternatively, one could use XPS to check the change in the C 1s(COOR)/C 1s(C-O) intensity ratio, which increases from 0.33 to ∼0.66 on going from CNT@POH to CNT@PCOOH. For a full conversion, the ratio is expected to increase up to 2 (see structures in Scheme 1), which implies that the conversion is partial. However, the underlying diazoniummodified CNT is detected by XPS and has a contribution of C-O bonds to the C 1s peak of CNT@PCOOH which thus induces an underestimation of the conversion of POH into PCOOH. Peakfitted C 1s regions from CNT-C6H5-CtCH, CNT@POH, and CNT@PCOOH are given in the Supporting Information to account for the conversion of OH into COOH. TGA was used to assess mass loading of alkynylated aryl groups and grafting POH chains to CNTs (Figure 3). The pristine CNTs remain stable until 650 °C, the onset for a mass loss of 91 ( 1%. The thermogram of alkynylated CNTs (CNT-C6H5CtCH) indicates a 30 wt % loss, which occurs in the 150-490 °C range and which is due to the decomposition of the grafted aryl layer. For pristine and alkynylated CNTs, the residual products above 750 °C are due to the metallic catalyst used in the synthesis of the carbon nanotubes. One can notice the decrease of the degradation temperature of the grafted CNTs as shown elsewhere for the aryl layers grafted onto CNTs systems.38 After POH grafting, the weight loss increased to 97%, showing substantial vinylic polymer mass loading of ∼67 wt % determined by difference between the TGA curves of the alkynylated CNT and CNT@POH. This reflects the efficiency of polymer grafting to CNTs through the click chemistry route. Unfortunately, it is difficult to estimate the yield of the “click” reaction between the alkynylated CNTs and POH-N3. Actually, the main problem lies in the difficulty to compute the azo bridges that exist within the aryl layer (see for example discussions in refs 22 and 39). Indeed, (37) Liu, P.; Liu, Y.; Su, Z. Ind. Eng. Chem. Res. 2006, 45, 2255. (38) Chen, X.; Wang, J.; Zhong, W.; Feng, T.; Yang, X.; Chen, J. Macromol. Chem. Phys. 2008, 209, 846. (39) (a) Gam-Derouich, S.; Carbonnier, B.; Turmine, M.; Lang, P.; Jouini, M.; Ben Hassen-Chehimi, D.; Chehimi, M. M. Langmuir 2010, 26, 11830. (b) Shewchuk, D. M.; McDermott, M. T. Langmuir 2009, 25, 4556. (c) Toupin, M.; Belanger, D. J. Phys. Chem. C 2007, 111, 5394. (d) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Chem. Mater. 2007, 19, 4570. (e) Hurley, B. L.; McCreery, R. L. J. Electrochem. Soc. 2004, 151, B252.

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Figure 4. Photographs of the solubility/dispersibility behavior of the (a) pristine CNTs, (b) CNT-CtCH, and (c) CNT@PCOOH in DMF.

once the click reaction has been performed, the end CNT-grafted polymer contains nitrogen atoms from azo bridges and others from the triazole cycle. It would perhaps be much more straightforward and elegant to analyze the CNT@POH nanomaterials prepared so far by solid-state 1H NMR, the utility of which was illustrated elsewhere in the study of CNT-grafted PMMA.40 A simple, qualitative test used to confirm successful CNTs functionalization is to check their solubility in common organic solvents. Figure 4 shows three vials containing equal volumes of DMF and equal masses of (a) pristine CNTs, (b) alkynylated CNTs, and (c) CNT@PCOOH. The unmodified CNTs are completely insoluble and sediment in DMF, whereas the alkynylated and CNT@PCOOH composites form a clear, dark solution that exhibits no discernible particulate materials, hence their high dispersibility in DMF. Preparation and Characterization of the Hairy Carbon Nanotube-Supported Palladium Nanoparticle Heterostructures. After in-situ synthesis of Pd nanoparticles in the presence of CNT@PCOOH, the end hairy nanotube-supported nanoparticles CNT@PCOOH@Pd were examined by EDX, XPS, XRD, and TEM. Figure 5 shows EDX (a), XPS (b), XRD (c), and TEM (d) analyses of the hairy nanotube-supported Pd heterostructures. EDX patterns show C and O originating from the underlying CNT@PCOOH and intense peaks of Pd, therefore confirming immobilization of palladium on the hairy CNTs platforms. Figure 5b shows the XPS survey spectrum of the CNT@ PCOOH@Pd heterostructures; the high-resolution Pd 3d doublet is shown in the inset. The Pd 3d5/2 binding energy (BE) was found at 335.7 eV, which is in agreement with the range of values compiled by Wagner et al.41 for palladium in the metallic state. To confirm such a chemical state, the Auger parameter was determined and found to be equal to 662.6 eV, a value that is in line with 662.9 ( 0.3 eV reported by Brun et al.42 It is to note that the Pd 3d/C 1s intensity ratio is much higher than that obtained elsewhere for the self-regulated reduction of palladium acetate on MWCNT with the aid of sodium dodecyl sulfate (SDS).43 XPS was further used to roughly estimate the extent of palladium mass loading per gram of CNT@PCOOH to be in the 176-247 mg/g range. (40) Cahill, L. S.; Yao, Z.; Adronov, A.; Penner, J.; Moonoosawmy, K. R.; Kruse, P.; Goward, G. R. J. Phys. Chem. B 2004, 108, 11412. (41) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble Jr., J. R. NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 3.5. http://srdata.nist.gov/xps/Default. aspx. (42) Brun, M.; Berthet, A.; Bertolini, J. C. J. Electron Spectrosc. Relat. Phenom. 1999, 104, 55. (43) Karousis, N.; Tsotsou, G.-E.; Evangelista, F.; Rudolf, P.; Ragoussis, N.; Tagmatarchis, N. J. Phys. Chem. C 2008, 112, 13463.

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Figure 5. Analysis of CNT@PCOOH@Pd nanoparticle heterostructures by (a) EDX and (b) XPS. The inset in (b) shows the high-resolution Pd 3d doublet (c) XRD pattern and (d) TEM image.

Scheme 2. Suzuki C-C Coupling Reaction Catalyzed by CNT@PCOOH@Pd Heterostructure

To do so, the polymer mass loading on CNTs (determined by TGA analysis of CNT@POH) was combined to the C 1s peak fittings of POH-N3 and CNT@PCOOH@Pd (see Supporting Information). The XRD pattern (Figure 5c) of Pd-supported CNT@ PCOOH reveals typical Pd nanocrystals peaks at 2θ = 46.28°, 53.23°, 79.51°, and 96.85° which correspond to respectively (111), (200), (311), and (400) planes characteristic of face-centered-cubic (fcc) structure of metallic Pd nanoparticles, according to the standard powder diffraction data (00-046-1043 for Pd, cubic). XRD analysis confirms thus the metallic state of the immobilized Pd NPs. Note that the peaks are broad which suggests a nanocrystallinity of the palladium particles. Their average size was calculated with measurements of the full width at half-maximum for the diffraction peaks of Pd using the Scherrer equation and found to be approximately equal to 3 nm. The morphology and distribution of the generated palladium NPs was explored by TEM (Figure 5d). We can clearly see that the immobilized Pd nanoparticles are well distributed and densely packed on the surface of the hairy chelatant carbon nanotubes as it was shown for Ag NPs immobilized on CNT@poly(acrylic acid).13 The NPs are very small and have a nearly uniform size of 3 ( 0.5 nm. The small difference noted with the value which that determined by XRD can be explained by either the 10% error of the Scherrer method and/or the aggregation of the nanoparticles 16120 DOI: 10.1021/la102801d

shown by TEM images. It is well-known that using of NaBH4 reducing agent permits very quick nucleation and growth of palladium nanoparticles, which led to the formation of small particles. The Pd nanoparticles are tightly bound to the densely packed PCOOH chains as they resisted ultrasonic shaking. Indeed, aryl layers bind strongly to the sp2 carbon materials9,22 and spontaneously act as efficient coupling agents for the polymers which in turn act as macromolecular coupling agent for nano-Pd through the chelatant bonding sites. Catalytic Characteristic of CNT@PCOOH@Pd. In order to evaluate the catalytic activity of the immobilized nanoPd, we have used the heterostructure CNT@PCOOH@Pd in Suzuki C-C coupling reaction44 between 4-bromobenzene and phenylboronic acid as shown in Scheme 2. First, we have carried out this reaction in the absence of the CNT@PCOOH@Pd and noticed no conversion of reactants after 16 h. Similarly, neither pristine CNTs nor alkynylated ones exhibited any measurable catalytic activity. In contrast, in the presence of the heterostructures, 77% of the reactants were converted to biphenyl after 2 h. This result demonstrates the catalytic role of the hairy CNT-supported nano-Pd. (44) Fujii, S.; Matsuzawa, S.; Nakamura, Y.; Ohtaka, A.; Teratani, T. Langmuir 2010, 26, 6230.

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H NMR spectra (not shown) displayed additional peaks assigned to the formation of biphenyl (7.6, 7.4, 7.3 ppm). GC-MS results have confirmed the satisfactory reaction yield, showing formation of pure biphenyl and absence of byproduct generally resulting from homocoupling of aryl bromides.45 The same catalyst was reused for four other Suzuki reaction cycles, and the results indicate that the catalyst can be effectively recycled and remain highly stabile without significant loss of its catalytic capacity. The conversion yield for the fifth cycle was 69%, which is only 10% relative loss compared to the first use. This slight decrease in the catalytic activity of the heterostructures could be ascribed to coalescence of Pd nanoparticles after each cycle. It is believed that desorption of the NPs is unlikely after each catalysis cycle since they resisted the ultrasound-assisted washing procedure prior to their use to catalyze the Suzuki C-C coupling reaction. Moreover, visually we have observed nothing that has leached in the reaction medium which remained transparent. Note that the activity effect of heterogeneous catalysts is highly dependent on its solubility in the reaction medium. In our case, the covalent bonding of chelatant polymers to CNTs improved clearly their solubility/dispersibility as is shown in Figure 4.

Conclusion We have developed a simple and efficient method for elaboration of palladium nanoparticles decorated-CNT@ polymer supports. The PGMA chains were prepared by bulk solution ATRP and hydrolyzed prior to end-functionalization by sodium azide. The resulting azide-funtionalized, hydrolyzed PGMA (POH-N3) were clicked to alkynylated CNTs which were prepared using (45) Pittelkow, M.; Moth-Poulsen, K.; Boas, U.; Christensen, J. B. Langmuir 2003, 19, 7682.

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in-situ-generated diazonium salts of the formulas BF4-,þN2C6H4-CtC-H. Sequential reactions leading to click chemistry nanosized products were monitored by FTIR which highlighted the successful modification of the nanotubes and functionalization of PGMA prior to click coupling. The hairy nanotubes CNT@POH contained up to 67 wt % polymer as judged from TGA measurements. They were further oxidized to CNT@ PCOOH and served for the in-situ synthesis of palladium nanoparticles. EDX, XPS, and TEM indicated an important attachment of Pd nanoparticles at the surface of the heterostructures CNT@PCOOH@Pd. The palladium NPs were found to be in the metallic state as judged by XPS and XRD and 3 nm sized as estimated by XRD. It is thus concluded that the novel strategy devised in this work for clicking macromolecules to grafted aryl layers is a versatile approach for decorating nanometer scale objects (e.g., carbon nanotubes). The hairy CNTs designed so far by tandem diazonium salt reduction and click chemistry efficiently served for tailoring novel heterostructures, namely CNT@polymethacrylate@ Pd with high loading of supported nanoparticles. This work highlights the essential role of functionalization of carbon nanotubes by polymer grafts, through the intermediate aryl layers, for the immobilization of monodisperse palladium NPs. The end heterostructures CNT@polymer@Pd exhibit very satisfactory activity and regeneration in Suzuki C-C coupling reaction. The catalyst stability is due to (i) efficient role of aryl groups as coupling agents for chelatant polymers and to (ii) high chelating power of the tethered polymer toward NPs. Supporting Information Available: Conversion of OH to COOH and Pd loading on hairy CNTs. This material is available free of charge via the Internet at http://pubs.acs.org.

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