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Deposition of Gold Nanoparticles onto Thiol-Functionalized Multiwalled Carbon Nanotubes Rodolfo Zanella,*,† Elena V. Basiuk,† Patricia Santiago,‡ Vladimir A. Basiuk,§ Edgar Mireles,† Iva´ n Puente-Lee,⊥ and Jose´ M. Saniger† Centro de Ciencias Aplicadas y Desarrollo Tecnolo´ gico, UniVersidad Nacional Auto´ noma de Me´ xico (UNAM), Circuito Exterior S/N Ciudad UniVersitaria, A. P. 70-186, C.P. 04510 Me´ xico D.F., Mexico, Instituto de Fı´sica, UNAM, Circuito de la InVestigacio´ n Cientı´fica, C.U., 04510 Me´ xico D.F., Mexico, Instituto de Ciencias Nucleares, UNAM, Circuito Exterior, C.U., 04510 Me´ xico D.F., Mexico, and Laboratorio de Microscopı´a Electro´ nica, Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, UNAM, 04510 Me´ xico D.F., Mexico ReceiVed: April 25, 2005; In Final Form: June 30, 2005

Gold nanoparticles were deposited on the surface of multiwalled carbon nanotubes (MWNTs) functionalized with aliphatic bifunctional thiols (1,4-butanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol, and 2-aminoethanethiol) through a direct solvent-free procedure. Small gold particles, with a narrow particle size distribution around 1.7 nm, were obtained on 1,6-hexanedithiol-functionalized MWNTs. For MWNTs functionalized with the aminothiol, the average Au particle size was larger, 5.5 nm, apparently due to a coalescence phenomenon. Gatan image filter (GIF) observations show that sulfur is at the nanotube surface with a non-homogeneous distribution. A higher sulfur concentration was observed around the gold nanoparticles’ location.

1. Introduction The attachment of metal nanoparticles to carbon nanotubes (CNTs) is a way to obtain new hybrid materials with useful properties for gas sensor and catalytic application. Excellent molecular hydrogen sensors can be fabricated through decorating single-walled carbon nanotubes (SWNTs) with palladium nanoparticles.1 Gold supported on multiwalled carbon nanotubes (MWNTs) was proposed as a basic element for glucose biosensors.2 Other studies showed that metal nanoparticle-CNT systems can act as efficient catalysts. For example, SWNTsupported Pt nanoparticles with an average size of 1-2 nm exhibit high activity in the selective partial hydrogenation of R,β-unsaturated aldehydes.3 Nickel supported on CNTs catalyzes ethane decomposition.4 Pt, Ru, and Pt/Ru supported on CNT membranes can be used for O2 electrocatalytic reduction and methanol oxidation, as well as for the gas-phase catalysis of hydrocarbon decomposition.5 Ru nanoparticles deposited onto raw CNTs were efficient in liquid-phase hydrogenation of cinnamaldehyde.6 In most of these cases, the catalyst activity strongly depends on the nanoparticle size. It has been recently claimed that attaching SWNTs to gold tips allows atomic force and scanning tunneling microscopy observations7,8 and the potential of MWNTs as a solid phase for adsorption and concentration of trace metal ions.9 Several methods have been developed to attach nanoparticles to CNTs. Yu et al.10 showed that deposition of well-dispersed Pt as small metal clusters (10-20 nm) onto functionalized carbon nanotubes can be achieved when the nanotubes are previously oxidized by HNO3 or H2SO4-HNO3 mixture; Ye et al.11,12 reported an approach to synthesize Pt, Pd, Ni, and Cu nanowires, nanorods, or nanoparticles using MWNTs as tem* Corresponding author. E-mail: [email protected]. Tel: +(52) 5556228602, ext. 1115. Fax: +(52) 5556228651. † Centro de Ciencias Aplicadas y Desarrollo Tecnolo ´ gico. ‡ Instituto de Fı´sica. § Instituto de Ciencias Nucleares. ⊥ Laboratorio de Microscopı´a Electro ´ nica.

plates and supercritical CO2 as the reaction medium. Banerjee and Wong13 reported the reaction of both oxidized and pristine nanotubes with Vaska’s compound, trans-IrCl(CO)(PPh3)2, to form covalent CNT-metal complexes. The unique capability of supported gold nanoparticles to catalyze oxidation and hydrogenation reactions at low temperatures has been an active field of research during the last years.14-17 For catalytic applications, the optimum size of Au particles is smaller than 5 nm. The gold nanoparticles deposited onto Al2O3, SiO2, TiO2, MgO, Fe2O3, and activated carbon supports has been thoroughly studied, and the dependence of catalytic activity on the Au particle size and the nature of the support has been demonstrated.14-16,18-20 Fasi et al.21 deposited gold on MWNTs by employing gold colloid stabilized by THPC [tetrakis(hydroxymethyl)phosphonium chloride] and ultrasonic irradiation during preparation. Satishkumar et al.22 reported that an improved dispersion of Au nanoparticles could be obtained when CNTs are exposed to mild sonication during the deposition process; an intense sonication resulted in some of the nanoparticles entering into the nanotube. Gold nanoparticles were selectively attached to chemically functionalized surface sites on nitrogen-doped MWNTs by the adsorption of a cationic polyelectrolyte on the nanotube surface and then by reacting with negatively charged 10 nm Au nanoparticles.23 Smaller Au nanoparticles (1-3 nm) were also attached to functionalized MWNTs, preliminary treated with NH3 to neutralize acidic oxygenated functional groups,24 as well as to SWNTs via thioamides obtained by derivatizing carboxyl groups on oxidized SWNTs.25 Ellis et al.26 attached Au nanoparticles (1-3 nm) to CNT sidewalls through the hydrophobic interactions between octanethiol molecules capping the gold nanoclusters and acetoneactivated CNT surfaces. Carrillo et al.27 attached Au nanoparticles to graphite, SWNTs, and MWNTs by functionalizing their surfaces with multilayer polymeric films. Han et al.28 assembled alkanethiolate-capped Au nanoparticles of 2-5 nm core size onto CNTs in a nonpolar and hydrophobic solvent.

10.1021/jp0521454 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/05/2005

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From the above citations, it is clear that a great deal of attention and effort must be dedicated to get an adequate preliminary treatment of CNT supports. Since carbon nanotubes are chemically inert, activating their surface is an essential prerequisite for linking the metal nanoclusters to them. Chemical functionalization is the most common and widely used way to introduce the linkers, as well as to improve solubility or dispersibility of CNTs, which is also important for the efficient and uniform deposition of nanoparticles (see recent reviews29-33 and references therein). Sulfur-containing (in particular, thiol -SH) covalent and noncovalent linkers gained special attention.7,8,26,34-42 Their high affinity to gold, platinum, and other transition metals was successfully employed for anchoring metal nanoparticles to CNTs,26,34,38-40,42 attaching SWNTs to gold tips for atomic force and scanning tunneling microscopy,7,8 coating MWNTs with zinc sulfide,41 etc. While some applications can rely upon noncovalent (hydrophobic26,39,41,42 or ionic8,36) interactions of CNTs with sulfur-containing linkers, significant effort has been undertaken to develop covalent linking techniques.7,34-37 The latter, as a rule, include the following steps: (1) oxidation of CNTs with strong acids, to introduce reactive carboxylic groups at the tips and sidewall defect sites; (2) chemical activation of COOH groups with thionyl chloride (SOCl2) or carbodiimides; and finally (3) amide coupling of the activated COOH groups with amino substituted thiols.7,32,34-37 The whole reaction sequence is always performed in an organic solvent medium, often employs corrosive SOCl2, and is quite tedious due to such auxiliary steps as washing, centrifugation, drying, etc. A key element for the exploration of nanoparticle-CNT composites as sensor or catalyst materials is the possibility of effective and controllable assembly of nanoparticles on the surface of CNTs. Studying the attachment of metal nanoparticles onto CNTs could also serve to detect the presence of certain functional groups on their surfaces, and thus to prove that the derivatization was successful. With these considerations in mind, in the present work we developed a direct solvent-free functionalization of pristine MWNTs with aliphatic bifunctional thiols, aimed at the preparation of chemically modified nanotube surfaces capable of binding gold or other noble metal nanoparticles. An additional goal was to avoid the use of organic solvents and thionyl chloride activation, which are ecologically unfriendly. 2. Experimental Section The aliphatic bifunctional thiols (all from Aldrich) were represented by 1,4-butanedithiol, 1,6-hexanedithiol, and 1,8octanedithiol, as well as by 2-aminoethanethiol (as its hydrochloride). We expected thiol groups to react with (pentagonal) defect sites of pristine MWNTs, as was observed in the case of aliphatic amines.43,44 As prepared MWNTs from the CVD process (ILJIN Nanotech Co., Inc., Korea; 97%+ purity, diameter of 10-20 nm and length of 10-50 µm) were used. To perform the thiol functionalization, we employed the gasphase solvent-free procedure.43-45 MWNTs (100 mg) and dithiol or aminothiol (ca. 20 mg) were placed together into the reactor, and the reaction was performed at 130-150 °C for 2 h. During

this procedure, thiol vapors reacted with MWNTs; excess of the reagent condensed a few centimeters above the heated zone. To avoid the contamination of the derivatized MWNTs, before extracting them, the upper reactor part was wiped with cotton wool wetted with ethanol. In principle, the reaction can be equally performed by baking the reactant mixture in a sealed vial (which is essentially a solvent-free process as well);46 however the excess of dithiol or aminothiol should afterward be removed in some way (by washing or by evacuating with simultaneous heating). Of four thiol-derivatized products, in the further discussion we focus on the 1,6-dithiol derivative (referred to as 16DT-MWNTs) and 2-aminothiol derivative (ATMWNTs). Gold nanoparticles were prepared via the reaction of HAuCl4 (Aldrich) and citric acid (Baker) as reducing agent. In a typical experiment, 0.05 g of derivatized MWNTs was dispersed in 2-propanol (30 mL) and ultrasonicated for 3-5 min. Then 0.016 g of HAuCl4 and 0.017 g of citric acid, both dissolved separately in 10 mL of 2-propanol, were simultaneously added dropwise to the dispersion of MWNTs. The final volume was 50 mL. Ultrasonication was maintained during HAuCl4 and citric acid addition for about 3 min, then the dispersion was vigorously stirred at room temperature for 2 h. After the procedure, the solid phase was separated by centrifugation (5000 rpm for 10 min), washed in 20 mL of 2-propanol under stirring for 10 min, and then centrifuged again. The resulting MWNTs were dried under vacuum at room temperature for 2 h. The dry samples were stored away from light in a vacuum desiccator. As a control test, the same procedure of preparation was performed with pristine MWNTs. Transmission electron microscopy (TEM) observations of the samples were performed on a JEM 2010 FasTem analytical microscope equipped with GIF (Gatan image filter) and Zcontrast annular detectors. High-resolution TEM (HRTEM) images were obtained at the optimum focus condition (Sherzer condition). The chemical composition of MWNT samples was analyzed by using the GIF detector. The image filtering was performed using the S L-edge and C K-edge at 165 and 284 eV, respectively. The samples were mounted on a microgrid carbon-polymer supported on a copper grid by direct immersion of the grid into MWNT powder, without the use of any solvent. The histograms for the metal particle size distribution were established by measuring more than 800 particles. Scanning electron microscopy (SEM) observations were performed on a JEOL JSM-5900-LV instrument operating at 20 kV with an Oxford-ISIS energy-dispersive X-ray spectroscopic (EDS) detector, which was used to analyze the chemical composition of the samples. 3. Results and Discussion TEM images of 16DT-MWNTs show highly dispersed gold nanoparticles on the surface of MWNTs, as is evidenced in Figure 1. In this sample, the average Au particle size was very small (1.7 nm), with a narrow particle size distribution function (Figure 1b and Table 1). SEM studies (Figure 2a) show a carbon nanotube network of several micrometers in length. No Au particles can be distinguished under the present resolution,

TABLE 1: Au and S Content in the Functionalized MWNTs with Deposited Au Nanoparticles and the Average Au Particle Size sample

Au loading wt %

S loading wt %

Au (mmol/g)

S (mmol/g)

mole S/ mole Au

average particle size (nm)

standard deviation (nm)

Au@16DT-MWNTs Au@AT-MWNTs

5.3 2.9

5.3 3.2

0.26 0.15

1.65 0.99

6.4 6.7

1.7 5.5

0.8 2.6

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Figure 1. (a) TEM image of Au nanoparticles on 16DT-MWNTs; (b) HRTEM of gold nanoparticles of about 2 nm size; (c) size histogram of Au nanoparticles on 16DT-MWNTs.

Figure 3. TEM image of Au nanoparticles on pristine MWNTs.

Figure 2. (a) SEM image of 16DT-MWNTs after Au deposition, and their EDS spectrum (b).

indicating that gold was deposited only as small particles. The chemical analysis by energy dispersive spectroscopy (EDS) (Figure 2b) confirms the presence of gold and sulfur in the samples. Table 1 shows the gold and sulfur loading determined by EDS. A test experiment with pristine MWNTs was realized using the same procedure as for functionalized MWNTs, very few gold nanoparticles were found on the nanotube surface by TEM

(Figure 3). Most of the gold particles were bigger than 10 nm, but some of them were bigger than 100 nm. The gold loading in pristine MWNTs was less than 2 wt %, and as observed in Figure 3, the range of coverage of gold nanoparticles on MWNTs not functionalized was much lower than for MWNTs functionalized with thiol. Conventional HRTEM images are shown in Figure 4. The images were obtained at the optimum defocus condition to observe Au nanoparticles. Phase-contrast imaging in HRTEM relies on lattice fringes, which arise from the periodic structure of the crystal lattice; however, the images are highly dependent on the focus changes. For this reason, the spatial distribution of the nanoparticles in the nanotube is difficult to generalize, although it is possible to see the trend of the nanoparticles to follow the nanotube sidewall morphology. A closer view with

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Figure 4. (a) HRTEM image of Au nanoparticles on 16DT-MWNTs; (b) at higher magnification with the corresponding FFT inset showing the (111) reflection.

Figure 5. (a) HRTEM image of Au nanoparticles on AT-MWNTs showing their overlapping and coalescence phenomena; (b) size histogram of Au particles on AT-MWNTs.

the corresponding fast Fourier transform for gold is shown in Figure 4b (inset). The lattice parameter measured from the apparently truncated octahedral nanoparticle (white arrow) corresponds to (111) gold reflection.47 Different nanoparticle morphologies can be distinguished. For example, the particle marked with the dashed arrow exhibits a pattern quite similar to the one observed in icosahedral particles,48 whereas the black arrow points to a cubo-octahedral gold nanoparticle. The same procedure was used for the deposition of gold on AT-MWNTs. The average Au particle size obtained is larger (5.5 nm) than that obtained for gold on 16DT-MWNTs, and the particle size distribution is wider (Table 1 and Figure 5). Their characteristics from HRTEM can be appreciated in Figure 5. In this micrograph, an overlapping of the nanoparticles is evident (white solid arrow), which in some cases gives rise to their coalescence (dashed arrow). The coalescence phenomena could be the result of nonuniform capping of the aminothiol over the gold particle surface, avoiding the passivated metal nanoparticle; under these conditions the contact of the nanoparticles and the initial fusion with the orientational alignment of the particle planes generates a reorganizing particle interface, which produces the grown particle phenomenon. This process has been recently overviewed by Yacaman et al.49 This phenomenon can explain the larger Au nanoparticle size in the AT-MWNT samples as compared to the one typical for 16DTMWNTs and is apparently associated with the presence of amino groups in 2-aminoethanethiol used for the preparation of AT-

MWNTs. The experimental results have been summarized in Table 1. Another explanation for the larger gold nanoparticles obtainable with aminoethanethiol could be that Au prefers thiols to amines as ligands; if aminoethanethiol went down on the CNT with its sulfur end, then gold nanoparticles have less affinity for the N site and so might be more labile. Additional information on the spatial distribution of Au nanoparticles over the nanotubes was obtained from the Zcontrast imaging of AT-MWNTs (Figure 6). In this case, the depth of focus can be maximized through the optical configuration of the microscope, using a small condenser aperture to minimize the convergence angle.50 High-angle annular dark field technique (HAADF) and Z-contrast mode can provide highly detailed images of nanocrystal surfaces, 3D information, and mass contrast simultaneously. Figure 6 shows the gold nanoparticles surrounding the nanotubes surface. Here it is also possible to observe aggregation of the nanoparticles in some zones. HAADF-Z contrast technique generates images with high angle scattered electrons; therefore, the contrast in this type of images depends on the atomic number (Z). In Figure 6, the brighter zones correspond to Au nanoparticles; at the same time, it gives no direct information on the location of the sulfurcontaining functional groups. To clarify the latter point, a Gatan image filter attached to the microscope was employed to obtain the filtered images around the sulfur L-edge (165 eV) and carbon K-edge (284 eV). Figure 7a shows a bent nanotube with a gold nanoparticle of

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Figure 6. Z-contrast images of Au nanoparticles on AT-MWNTs: (a) lower and (b) higher magnification views.

Figure 7. (a) TEM image of a bent MWNT with a gold nanoparticle of about 6 nm size; (b) the corresponding image filtered around the carbon K-edge (284 eV) and (c) filtered image around the sulfur L-edge (165 eV).

about 6 nm size. The corresponding filtered image at the carbon K-edge peak is shown in Figure 7b, and the filtered image around the sulfur L-edge is presented in Figure 7c. From the latter image, it is evident that sulfur is present at the nanotube surface; however its distribution is not homogeneous, and the contrast differences on the sulfur mapping are evident from the bright contrast differences over the CNT surface: a notably higher sulfur concentration is observed around the gold nanoparticle location. In both types of deposited samples (AT-MWNTs and 16DTMWNTs), the interaction between the Au nanoparticles and the

nanotubes is strong enough to avoid the nanoparticle loss even when the samples were thoroughly washed. One should emphasize that the adequate choice of solvent to disperse carbon nanotubes is very important. Our attempts to attach Au nanoparticles to the derivatized MWNTs using water as a solvent medium were not particularly successful, giving rise to considerable agglomeration of gold over the nanotube bundles, obviously due to the known poor CNT dispersibility in water. The MWNTs nicely decorated with well-dispersed Au nanoparticles can be prepared only when water is substituted by 2-propanol.

Gold Nanoparticles on Thiol-Functionalized MWNT As shown in Table 1, 16DT-MWNTs have a higher gold loading (5-6 wt %) as compared to AT-MWNTs (3 wt %), in agreement with the sulfur content for each sample, which is 5% for dithiol-derivatized and 3% for aminothiol-derivatized MWNTs. Taking into account that the dithiol molecules contain two sulfur atoms whereas the aminothiol molecules contain only one S atom, the latter values also imply that the molar content of aminothiol moieties in AT-MWNTs is roughly equal to the molar content of dithiol moieties in 16DT-MWNTs. Thus the question arises what groups of aminothiol, SH or NH2, react with MWNTs. Supposing that Au nanoparticles bind mainly to thiol groups and not to amino groups, the difference of gold content for each kind of sample (Table 1) makes us believe that there is no obvious preference, and the SH or NH2 groups have approximately the same reactivity toward the reactive sites of MWNTs. 4. Conclusion Gold nanoparticles were anchored to the surface of MWNTs functionalized with aliphatic dithiols and aminothiols by a simple direct solvent-free procedure. Small gold particles with narrow particle size distribution were obtained for MWNTs derivatized with 1,6-hexanedithiol. For MWNTs derivatized with aminothiol the average particle size was greater. This difference in particle size is explained by a coalescence phenomenon of gold particles in aminothiol-derivatized samples that could be the result of nonuniform capping of the aminothiol over the gold particle surface, avoiding the passivated metal nanoparticle. Gatan image filter observations show that sulfur is at the nanotube surface with a higher concentration around the gold nanoparticles’ location. Taken together with the HAADF-Z contrast and filtering imaging techniques, it can also be used to visualize the sites of functional group attachment onto the carbon nanotube sidewalls. We expect this approach to be efficient to attach other nanostructures to CNTs through the dithiol interlinkers. This technique can also be useful for attaching CNTs to gold tips for atomic force and scanning tunneling microscopy and potentially for adsorption and concentration of trace metal ions. Acknowledgment. Financial support from the National Council of Science and Technology of Mexico (grant CONACYT-40399-Y) and from the National Autonomous University of Mexico (grants DGAPA IX101604, IN100402-3, and IN100303) is greatly appreciated. R.Z. and J.M.S. are also indebted to UNAM Nanoscience and Nanotechnology project for financial support. We are thankful to L. Rendon and J. Angel Flores for HRTEM and sample preparation assistance, respectively. The authors are also thankful to Central Microscopy Facilities of the Institute of Physics, UNAM, for providing its microscope tools used in this work. References and Notes (1) Kong, J.; Chapline, M. G.; Dai, H. AdV. Mater. 2001, 13, 1384. (2) Wang, S. G.; Zhang, Q.; Wang, R.; Yoon, S. F.; Ahn, J.; Yang, D. J.; Tian, J. Z.; Li, J. Q.; Zhou, Q. Electrochem. Commun. 2003, 5, 800. (3) Lordi, V.; Yao, N.; Wei, J. Chem. Mater. 2001, 13, 733. (4) Huu, C. P.; Keller, N.; Roddatis, V. V.; Mestl, G.; Schlo¨gl, R.; Ledoux, M. J. Phys. Chem. Chem. Phys. 2002, 4, 514. (5) Che, B.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Langmuir 1999, 15, 750. (6) Planeix, J. M.; Cousel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem. Soc. 1994, 116, 7935. (7) Yang, Y.; Zhang, J.; Nan, X.; Liu, Z. J. Phys Chem. B 2002, 106, 4139. (8) Nishino, T.; Ito, T.; Umezawa, Y. Anal. Chem. 2002, 74, 4275.

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