Efficient Anchoring of Silver Nanoparticles on N ... - Wiley Online Library

communications Decorated nanotubes DOI: 10.1002/smll.200500348

Efficient Anchoring of Silver Nanoparticles on N-Doped Carbon Nanotubes** Adalberto Zamudio, Ana L. Elas, Julio A. RodrguezManzo, Florentino Lpez-Uras, Geonel RodrguezGattorno, Fabio Lupo, Manfred R!hle, David J. Smith, Humberto Terrones, David Daz, and Mauricio Terrones* Carbon nanotubes exhibit fascinating electronic and mechanical properties. Soon after their identification by Iijima,[1] carbon nanoscience developed rapidly and various experimental researchers started to confirm these unique properties.[2] In addition, carbon nanotubes (CNTs) can be used as catalytic supports, in which metal particles are anchored to the surfaces of the tubes. These coated tubes have also be employed as fast responsive sensors or templates for generating metallic nanowires.[3] In order to establish a uniform coverage of metal particles on the surfaces of pure carbon nanotubes, it is necessary to activate the graphitic surface of the tubes, which is extremely inert. Therefore, nanotube surfaces need to be functionalized using different acid treatments to create carboxylic ( COOH), carbonyl ( C=O) and hydroxyl ( OH) groups, that are able to link different metal clusters.[4] Unfortunately, these acid treatments reduce considerably the mechanical and electronic performance of the tubes due to the introduction of large numbers of defects. However, nitrogen-doped multi-walled

[*] A. Zamudio, A. L. Elas, J. A. Rodrguez-Manzo, Dr. F. Lpez-Uras, Prof. H. Terrones, Prof. M. Terrones Advanced Materials Department, IPICYT Camino a la Presa San Jos, 2055, Col. Lomas 4a Seccin San Luis Potos 78216 (Mexico) Fax:(+444) 834-2040 E-mail: [email protected] Dr. G. Rodrguez-Gattorno, Dr. D. Daz Facultad de Qumica, Universidad Nacional Autnoma de M,xico Coyoac?n, M,xico DF, 04510 (Mexico) Prof. D. J. Smith Department of Physics and Astronomy and Center for Solid State Science Arizona State University, Tempe, AZ 85287-1504 (USA) Dr. F. Lupo, Prof. M. RAhle Max-Planck-Institut fAr Metallforschung Heisenbergstrasse 3, 70569 Stuttgart (Germany) [**] This work was also sponsored by CONACYT-M,xico grants: E-43662-F (D.D.), 45762 (H.T.), 45772 (M.T.), 41464-Inter American Collaboration (M.T.), 42428-Inter American Collaboration (H.T.), 2004-01-013/SALUD-CONACYT (M.T.), PUE-2004-CO2-9 Fondo Mixto de Puebla and PhD Scholarships (A.Z., J.A.R.M., A.L.E.). We also thank the Max-Planck-Gesellschaft, DFG Grant No. Ru342/112, Nanocomp HPRN-CT-2000-00037 (F.L.) and NSF DMR-030342 (DJS) for financial support. The authors are also grateful to K. Hahn, D. Ramrez Gonz?lez, Grisel Ramrez Manzanares, and Lisette Noyola for technical assistance.

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carbon nanotubes (CNxMWNTs) contain nitrogenated sites (substitutional and pyridinic nitrogen) that are chemically active. Therefore, it should be possible to avoid functionalization processes that use strong acid treatments, and attach metal particles onto the CNxMWNTs. In this account, we describe three different methods to anchor Ag nanoparticles (2–10 nm in diameter) on CNxMWNTs using a chemical route that does not require any acid treatment. High-resolution transmission electron microscopy (HR-TEM) and scanning electron microscopy (SEM) images reveal uniform coverage of Ag nanoparticles (NPs) on the surfaces of the CNxMWNTs. These small particles could be attached covalently via the nitrogenated groups located on the nanotube surfaces. It is important to mention that the Ag NPs attached to the CNx nanotubes are able to withstand ultrasonic treatments. The new Ag– CNx material could be used as bactericides, novel catalytic supports, sensors, and as nanoelectronic systems with enhanced electrical conductivity. When nanotubes are coated with metals, wetting becomes an important issue. In this context, Dujardin, et al.[5] reported that it is possible to wet narrow-diameter multiwalled nanotubes (e.g., < 20 nm outer diameter) when the surface tension of the metals is < 200 mN m 1. Recently, it has been demonstrated that the performance of nanotubebased sensors could be improved if metal particles such as Pd and Al were anchored to the tubes.[6–8] Zhao and coworkers[8] calculated the effects of Al13 clusters deposited on the surface of nanotubes, and found a considerable increase of electronic states around the Fermi level. The latter could be related to an enhancement of the chemical reactivity on the tube surface. Therefore, these systems could be used to detect low concentrations of NH3 molecules efficiently.[7] Jiang et al.[9] activated the surface of CNxMWNTs with a mixture of nitric and sulfuric acid. Subsequently, polyelectrolyte was added and adsorbed on the surfaces of the nanotubes by electrostatic interaction with the carboxylic sites. Gold particles were then successfully attached to the surfaces of the CNxMWNTs. These reactive sites allowed a uniform deposition of Au clusters along the nanotubes. This account reports a novel approach that allows Ag clusters to be anchored on the surfaces of CNx nanotubes. The method does not require any acid treatment and promotes an attachment of Ag particles during the reduction of the metallic salts. We synthesized Ag NPs with different average diameters (2–10 nm) by the reduction of a silver/2ethylhexanoate complex ([Ag-ethex]) or AgNO3, in two different solvents (DMSO and DMF) using, in some cases, sodium citrate tribasic dihydrate (Na3Cit·2 H2O) as a stabilizer. Subsequently, CNxMWNTs were added to the Ag NPs suspensions, which resulted in the generation of a uniform silver particle coating on the tube surfaces. It is important to note that when undoped nanotubes were mixed with these Ag suspensions, a poor coating of tubes was observed. N-doped multi-walled carbon nanotubes were obtained by aerosol thermolysis of solutions of ferrocene in benzylamine (PhCH2NH2–FeCp2) at 850 8C in an Ar atmosphere.[10] Pure carbon MWNTs were produced in a similar way but by using solutions of ferrocene in toluene (C7H8–

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FeCp2) during thermolysis.[11] Silver NPs were generated using methods previously reported in the literature.[12, 13] In this work, we describe three techniques that have been used to decorate carbon nanotubes with metal NPs (see Experimental Section). Figure 1 reveals TEM images of Ag NPs anchored on the surface of N-doped and undoped carbon nanotubes. Figure 1 a shows CNx nanotubes coated uniformly with numerous Ag NPs exhibiting diameters ranging from 2 to 5 nm.

Figure 1. HRTEM images of Ag NPs deposited on N-doped and undoped carbon nanotubes: a) Ag NPs (2–5 nm in diameter) deposited on CNxMWNTs (method 1; see Experimental Section). The image reveals a nanotube bundle that is uniformly coated with Ag NPs; b) Ag NPs (10–20 nm in diameter) poorly coating MWNTs (undoped); the latter sample was produced by the reduction of AgNO3 in DMF in the presence of MWCNTs (method 3). Note the clear absence of Ag NPs covering the undoped material.

These samples were prepared by mixing ready-made Ag particles with suspensions of CNx nanotubes (method 1). When MWNTs were coated with Ag particles using method 3 (see Experimental Section for details; Figure 1 b), we observed larger-diameter Ag particles (10–20 nm) covering the undoped tubes in a scattered fashion. In addition, these particles were poorly dispersed on the tubes surfaces (see Figure 1 b). From our observations, it was concluded that it is difficult to control the size of the Ag NPs and their interaction with the nanotubes when using method 3. This finding is due to the lack of a stabilizer in the reaction. Therefore, small Ag particles tend to coalesce heterogeneously into larger aggregates so as to form aggregates deposited on undoped tubes (Figure 2 a). In the case of synthesis method 3, Ag NPs of different sizes (e.g., from a few nanometers to hundreds of nanometers) are obtained due to the presence of DMF, which acts as a solvent and as a reducing agent. DMF is a stronger reducing agent than DMSO, and as a consequence the silver reducing reaction takes place more rapidly in this solvent. Additionally, in these reaction mixtures we do not incorporate any nanoparticle stabilizer such as citrate or 2-ethylhexanoate ions. However, it is possible to uniformly coat CNx nanotubes with Ag NPs of small diameter ( 2 nm) in the absence of a stabilizer but using DMSO instead of DMF (method 2, see small 2006, 2, No. 3, 346 – 350

Figure 2. HRTEM images of carbon nanotubes decorated with silver NPs: a) MWCNTs coated with Ag NPs. Here the sample was obtained by reducing AgNO3 in DMF suspensions containing carbon nanotubes (method 3; see Experimental Section). Note that the MWCNTs have encapsulated Fe due to their synthesis process. On the surface of these undoped tubes, we observed relatively large Ag NPs exhibiting different morphologies; b–d) CNxMWNTs decorated with Ag NPs. Here, the synthesis was carried out by reducing AgNO3 in DMSO using [Na-ethex] (method 2). It is important to emphasize that the Ag clusters (2 nm in diameter) are firmly attached to the tube surface, possibly due to the presence of DMSO (see text); e, f) sample obtained by mixing CNxMWNTs suspensions with pre-synthesized Ag NPs (method 1). In this case, one can observe that Ag particles are uniformly attached to the surface of the doped tubes. However a closer inspection of (f) reveals the presence of a crystalline core, which corresponds to Ag covered with an amorphous material, possibly due to the presence of DMSO, 2-ethylhexanoate, and citrate molecules (see also the EDX spectrum in Figure 3); g) schematic representation the particle imaged in (f) showing the crystalline Ag particle coated with amorphous material.

Experimental Section; Figure 2 b–d). Here, it is possible to observe the metal particles firmly attached to the outer surface of the N-doped tubes. When the CNxMWNTs were mixed with colloidal Ag NPs (method 1), the tubes were uniformly covered with particles with diameters < 10 nm (Figures 2 e). For large-diameter particles (6–10 nm), we could clearly observe a “wetting effect”. Higher-magnification imaging of one of these large particles deposited on the surface of a CNx tube depicts this phenomenon (Figure 2 f). However, a closer inspection of


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communications this particle shows the presence of an inner crystalline Ag particle ( 6 nm in diameter) coated with an amorphous material (Figure 2 g), possibly DMSO, 2-ethylhexanoate, and citrate agglomerates that stabilize the Ag clusters. This is consistent with EDX studies on these particles, which reveals the presence of S and traces of O (Figure 3). In order to explain the efficient anchoring of the Ag clusters on the CNx tube surface using methods 1 and 2, we believe that O groups from DMSO are reacting directly

Figure 3. a) TEM image of a CNxMWNT decorated with Ag NPs using method 1 (see Experimental Section); b) EDX spectrum of the region shown in (a). Note the presence of Ag, C, O, and S. It is believed that S atoms arise from DMSO surrounding larger Ag particles. The Cu signal arises from the TEM grid, whereas Si and Fe could come from the silica support and ferrocene used to grow our CNx tubes. Note the small traces of O possibly caused by DMSO, citrate, and 2-ethylhexanoate molecules.

with the nitrogenated sites (substitutional N embedded in the carbon network), and S atoms (from DMSO) interact strongly with the Ag cluster surface (see Figure 2 e–g). When DMF is used, it is possible that the pyridine-like (two-coordinated N) or substitutional N sites from the CNx tube react directly with O and some N atoms from DMF molecules that are bound to the Ag clusters. However, in order to confirm these mechanisms, first-principle theoretical calculations need to be performed. It should be emphasized that the presence of pyridinic or substitutional N sites in CNxMWNTs tenders the carbon nanotubes more reactive. These sites result in sharp peaks close to the Fermi energy when looking at the electronic density of states.[14] Therefore, the surface reactivity of CNxMWNTs modifies the p-molecular orbitals so that more chemical affinity with the Ag clusters is experienced. Figure 4 a and d show SEM images of two different samples prepared by mixing Ag NPs with N-doped and undoped MWCNTs, respectively (method 1). It is clear that CNx nanotubes exhibit a more uniform coverage with Ag particles when compared to undoped nanotubes. Brightand dark-field images are also depicted in Figure 4. From these images, it is possible to conclude that CNx tubes could interact more efficiently with Ag NPs of relatively small diameters (5–10 nm; Figure 4 b and c), as compared to undoped MWNTs. In addition, undoped tubes could only be covered with large agglomerates of Ag clusters (10–20 nm in diameter) that are inhomogeneously distributed on the

Figure 4. SEM and STEM images (bright- and dark-field) of carbon nanotubes decorated with Ag-NPs. The first row (a–c) corresponds to CNxMWNTs and the second row (d–f) depicts MWCNTs. Images (b) and (e) correspond to the dark-field images of (a) and (d), respectively. Bright-field (using the transmission mode) images are depicted in (c) and (f). When comparing the coverage between Ag-CNxMWNTs and AgMWNTs samples, it is clear that the N-doped tubes exhibit a more uniform coating with more monodisperse Ag NPs (5–10 nm in diameter). Samples using undoped MWNTs resulted in the inhomogeneous coverage of Ag clusters of larger size (10–20 nm in diameter) that tend to agglomerate with other Ag particles and do not cover the tube due to the lack of reactivity. Arrows in (f) indicate that various undoped MWNTs do not show any Ag NPs on their surface. This also demonstrates that MWNTs are not capable of uniformly anchoring Ag NPs. Both samples were synthesized using method 1 (see Experimental Section).

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tube surface (Figure 4 e and f). It is noteworthy that Ag particles could not cover other tubes uniformly due to the lack of surface reactivity of the MWNTs. We would like to point out that all samples studied by TEM and STEM were sonicated for 5 min in acetone or ethanol, and we clearly observed that the CNx nanotubes always retained their Ag-NP coating after sonication, whereas pure, undoped MWNTs do not exhibit a uniform coverage of particles among all of the tubes; some tubes were completely clean and no Ag particles were observed (Figure 4 d–f). The latter confirms that Ag NPs are attached more efficiently to N-doped nanotubes. Figure 5 shows X-ray diffraction patterns of N-doped and undoped carbon nanotubes decorated with Ag NPs. It is possible to observe reflections arising from the Ag NPs,

on the nanotubes, the results show that method 1 yields the best products. We also found that when using CNx tubes, it is not necessary to perform any acid treatment to activate the surface of the tubes. We suggest that the modified p-molecular orbitals of CNxMWNTs (resulting in sharp peaks close to the Fermi energy) are responsible to explain an enhanced chemical affinity for Ag NPs. The latter does not occur for pure, undoped CNTs. However, further experimental and theoretical studies now need to be carried out, involving such chemical systems, in order to clarify the Ag-anchoring mechanism. Ag NPs/CNxMWNT nanocomposites are excellent candidates for developing novel and more efficient catalyst supports and bactericides. In this context, preliminary experiments dealing with the reduction of NO (a pollutant and oxidizing agent) using Ag NPs have proved successful, and it is possible to reduce more than 587 NO molecules per Ag atom.[16] A careful study using CNx nanotubes coated with Ag NPs is currently underway. In addition, the effects of CNx nanotubes coated with Ag NPs in conjunction with amoebae are being studying in our group, in order to study the biocompatibility of these tubes with living organisms. Finally, we envisage that by anchoring CNx nanotubes with Ag NPs, one could enhance the electrical conductivity of the tubes.

Experimental Section Figure 5. X-ray diffraction patterns of Ag-decorated carbon nanotube samples (N-doped and undoped). The patterns come from the products of reactions with a) CNxMWNTs and b) MWCNTs following method 3 (see Experimental Section). In both cases, the upper curves correspond to the raw nanotube samples. Note the presence of graphite-like reflections corresponding to the (002) and (100) planes. The CNx tubes display lower-intensity (002) reflections, which is due to the presence of corrugated cylinders containing nitrogen atoms. The lower curves of (a) and (b) correspond to Ag-coated tubes. Here, it is possible to identify strong reflections arising from Ag, corresponding to the (111), (200), (220), and (311) planes.

exhibiting four main crystallographic planes, namely, (111), (200), (220), and (311). The raw nanotubes reveal reflections that correspond to the (002) and (101) planes of crystalline graphite-like materials. The low intensity observed for the (002) plane in the CNx tubes is common, and this is caused by the corrugated hexagonal networks within the doped tubes. Theoretical calculations performed by Ciraci et al.[15] demonstrate that the binding energy between Ag atoms and pure carbon nanotubes (undoped) is very weak. In our experiments, we believe that Ag clusters could be efficiently attached on the surface of CNx tubes due to their enhanced reactivity. In conclusion, we have developed a single-step method to efficiently attach Ag NPs onto the surface of N-doped carbon nanotubes. We found that the best approach to attach Ag clusters on nanotubes is when CNx nanotube suspensions in DMSO are mixed with colloidal pre-synthesized Ag NPs using [Ag-ethex] as the starting reagent (method 1). Although, the other methods are also able to coat particles small 2006, 2, No. 3, 346 – 350

Method 1: Carbon nanotubes were decorated by mixing presynthesized Ag NPs with carbon nanotube suspensions. The silver NPs were synthesized by dissolving [Ag-ethex] (1.25– 5.00 mg; Strem, 99 %) in DMSO (25 mL; Sigma), acting as the reducing agent. The nanoparticle size and the colloid stability were controlled by adding a capping agent (Na3Cit·2 H2O, Aldrich, 99 %) to the reaction mixture, using the same molar concentration as the metallic salt. This reaction mixture was stirred and heated at 60 8C for 30 min, to guarantee dispersion homogeneity and increase the silver-salt reduction rate. Subsequently, the silver colloidal dispersion (5 mL) was mixed with DMSO (5 mL) containing carbon nanotubes (0.1–0.3 mg; N-doped or undoped). This nanotube–DMSO suspension had been previously sonicated for 30 min in order to obtain a uniform dispersion. Method 2: Carbon nanotubes were decorated via an in situ synthesis of Ag NPs in carbon nanotube suspensions. Carbon nanotubes (0.1–0.3 mg; doped or undoped) were suspended in DMSO (25 mL) and ultrasonically dispersed for 30 min. Subsequently, AgNO3 (0.85–3.40 mg) was added to the CNT suspensions. In order to ensure the formation of silver NPs,[12] a capping agent (sodium 2-ethylhexanoate [Na-ethex]; Aldrich, 97 %) was added to the reaction mixtures, using the same AgNO3 concentrations. In this method, the temperature was fixed at 60 8C, and the suspension was stirred for 20 min. Method 3: Carbon nanotubes were also decorated by a similar method to the previous one, but N,N-dimethylformamide (DMF; Baker) was used instead of DMSO. This approach did not involve any capping agent such as [Na-ethex]. The carbon nanotubes (0.1–0.3 mg) were dispersed ultrasonically in DMF for 15 min, and subsequently a AgNO3 solution was added. The re-



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communications sulting suspension was heatedp to 60 8C under continuous stirring for 20 min. In all cases, the solvents (DMSO and DMF) are the reducing species; DMF is a better reducing agent than DMSO. Ethylhexanoate and citrate ions are capping agents that stabilize small silver NPs. The suspensions containing tubes decorated with Ag NPs were centrifuged four times (at 4000 rpm). In each step, the supernatant (clear part of the suspension) was removed and replaced with acetone. Finally, the acetone was evaporated to dryness at room temperature and the precipitates were collected and stored in vials. The samples were characterized by scanning electron microscopy (SEM) using a FEI XL30 SFEG (operating at 10–15 kV). Highresolution transmission electron microscopy (HR-TEM) was carried out in a JEOL JEM-4000 EX (operated at 400 kV). Energy-dispersive X-ray (EDX) analysis was carried out using a Zeiss EM 912 Omega operated at 120 kV. X-ray powder diffraction studies were performed in a Siemens D5000 and a Bruker D8 using CuKa radiation (l = 0.15406 nm). TEM samples were prepared by dispersing the products ultrasonically for 5 min in ethanol or acetone, and depositing a few drops of these suspensions onto holey carbon TEM grids.

Keywords:

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carbon nanotubes · doping · nanoparticles · nitrogen · silver

Received: September 17, 2005 Revised: October 18, 2005 Published online on December 28, 2005

[1] S. Iijima, Nature 1991, 354, 56 – 58.

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