Controlled loading of gold nanoparticles on carbon nanotubes by regenerative ion exchange

Materials Chemistry and Physics 116 (2009) 284–288

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Controlled loading of gold nanoparticles on carbon nanotubes by regenerative ion exchange Yusheng Gu a , Xianming Hou a,b , Haiyuan Hu a , Bo Yu(F) a , Lixia Wang b , Feng Zhou a,∗ a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Department of Chemistry, Taishan University, Taian 271021, PR China

a r t i c l e

i n f o

Article history: Received 8 October 2008 Received in revised form 17 February 2009 Accepted 26 March 2009 Keywords: Carbon nanotubes Nanoparticle

a b s t r a c t Highly dispersed gold nanoparticles supported on carbon nanotubes are generated using an easy twostep synthesis including ion exchange and reduction. Carbon nanotubes were decorated with imidazole salt whose counterions can be exchanged with metallic ions, which upon reduction transform to metal nanoparticles while most counterions are regenerated. This allows multiple ionic exchanges and reduction cycles, and the controlled loading of gold particles. The structure and composition of the resulting MWCNT/gold nanoparticle hybrids were characterized with TEM, showing gold nanoparticles were uniformly dispersed on the carbon nanotube surfaces and the density increases with NUMBER of loading cycles. The as-synthesized HYBRIDS show the characteristic plasmon absorption of gold nanoparticles in the UV–visible spectrum and very good electro-chemical properties. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Since their discovery [1], carbon nanotubes (CNTs) have been continuously attracting extensive interests in many areas of science and technology and have been the focus of current research for many years due to their unique structure-dependent optical, electrical, and mechanical properties. CNTs can be used as supports for preparing different CNT/metal nanoparticle (NP) composites, which have potential applications in catalysis, sensing, light harvesting, and electronic nanodevices [2] etc. Hybrid material may have collective properties that are dramatically different from individual component. Therefore, there is an ever-increasing interest in synthesizing CNT-metal NP composites [2,3]. In the past few years, various approaches and strategies, including spontaneous deposition [3,4], electrochemical deposition [5–7], substrate enhanced electroless deposition [8–10], NP decoration on chemically oxidized CNT side walls [11,12], and physical evaporation [13,14] etc, have been developed to assemble metal nanoclusters on the functionalized CNTs. To prepare well-defined NP-decorated CNTs, however, further development in the modification of CNTs is still required. Room-temperature ionic liquids (RTILs), as a new and environment-friendly solvent system, is increasingly arousing worldwide concern due to their high mobility, low melting points, negligible vapor pressure, thermal stability, low toxicity, large elec-

∗ Corresponding author. Tel.: +86 931 4968466; fax: +86 931 4968163. E-mail address: [email protected] (F. Zhou). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.03.029

trochemical window, non-flammability, and ability to dissolve a variety of chemicals [15–18]. In contrast to their successful applications in organic chemistry and inorganic synthesis, the use of RTIL as a linker to link two different species together is still in its infancy. Here we report on a wet-chemical method for the synthesis of CNT-metal NP assemblies with a wide range of NP coverage, which involves covalent coupling of the activated CNTs with ILs, subsequent anion exchange, and controllable assembly of gold NPs on the modified CNTs. Key to the synthesis lies in the IL-mediated loading of metal precursors on the functionalized CNTs. The procedure for preparing CNT-gold NP composites is shown in Fig. 1.

2. Experimental 2.1. Preparation of multiwalled carbon nanotube (MWCNT)/gold NP composites MWCNTs (from NTP Ltd., Shenzhen, China) were decorated with ionic liquids (ILs) via previously reported procedures [19]. Briefly, MWCNTs were activated in nitric acid solutions to give carboxyl group modified MWCNTs (MWCNT–COOH), followed by reaction with thionyl chloride and 1-hydroxyethyl-3-methyl imidazolium chloride (HEMImCl) in dry tetrahydrofuran (THF). This results in the IL-functionalized MWCNTs (MWCNT–ImCl). The prepared MWCNT–ImCl composites were mixed with moderate concentration chloroauric acid (HAuCl4 ) under violent stirring for 24 h to perform completely anion exchange, forming ImAuCl4 ion pair modified MWCNT (MWCNT–ImAuCl4 ). Reduction of MWCNT–ImAuCl4 with sodium borohydride (NaBH4 ) gives the final gold NP-loaded MWCNT (MWCNT/gold NP) hybrids. Since the resulting hybrids still contain the ImCl groups, it is possible to repeat this cycle multiple times, thereby controlling the number of particles attached to the nanotubes. The above-synthesized mixtures were fully washed with distilled water and absolute ethanol for several times. Finally, the products were collected by centrifugation and the precipitate was dried at 60 ◦ C in vacuum for 10 h.

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Fig. 1. (a) Schematic of modification of MWCNTs with ionic liquids. X = Cl; anionic exchange with AuCl4 − and subsequent reduction and regeneration of chloride counterion and repeated reduction. (b) Depiction of chemically reduced gold atoms collide to gold nanoparticles and recovery of ionic exchange capacity of the attached ionic liquids.

2.2. Sample characterization

3. Results and discussion

The morphology and dimension of the as-prepared MWCNT/gold NP composites were observed by transmission electron microscopy (TEM), which was taken on a Hitachi Model H-600 electron microscopy using an accelerating voltage of 100 kV. The crystalline structure of the product was characterized by X-ray diffractometer equipped with a monochromated Cu K␣ ( = 1.5418 Å) radiation. The powder X-ray diffraction (XRD) pattern is recorded from 15◦ to 90◦ in 2 with a scanning rate of 0.017 s−1 . The samples were studied with energy dispersive X-ray spectroscopy (EDS), which was performed on a JEOL-5600LV scanning electron microscope (SEM). The products were also analyzed by X-ray photoelectron spectroscopy (XPS), which was carried out on a PHI5702 multi-functional X-ray photoelectron spectrometer (Physical Electronics, USA), using Al K␣ X-ray as the excitation source. Ultraviolet (UV) absorption spectrum measurement was carried out with a Purkinje TU-1901 spectrophotometer by dispersing MWCNT/gold NP composite powder in anhydrous alcohol and using anhydrous alcohol as the reference. Electrochemical measurements were performed on CHI660B electrochemical working system. A gold electrode modified with MWCNT/gold NP composites was employed as working electrode. A platinum foil served as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The potentials were measured and reported with respect to the SCE.

3.1. TEM analysis Fig. 2 shows a number of representative TEM images and the corresponding selected area electron diffraction (SAED) patterns of acid activated MWCNTs and gold NP modified MWCNTs. Fig. 2a shows that the pristine CNTs have very smooth surface and the average diameter of 80 nm. Gold NPs are generated after a first round of ion exchange and reduction (Fig. 2b). The surface-deposited gold NPs are highly dispersed along MWCNTs with average sizes of 10–12 nm. The SAED pattern (insert in Fig. 2b) reveals that the gold NPs on MWCNTs have a single crystalline structure. The quantity of resultant gold NPs could not be substantially enhanced by increasing the AuCl4 − concentration beyond the saturated ionic exchange, indicating that the gold NPs are formed only from ionexchanged AuCl4 − monolayer. The consideration to carry out one more nanoparticle loading cycle lies in that as shown in Fig. 1b, the

Fig. 2. TEM images of (a) bare MWCNTs, (b) MWCNTs after a single cycle of ion exchange–reduction, and (c) after five cycles.

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Fig. 4. XPS spectra of the modified MWCNT composites in the Au (4f) level regions before (curve a) and after (curve b) reduction. Fig. 3. XRD pattern of exchange–reduction cycle.

MWCNT/gold

NP

composites

after

one

ionic

chemically reduced gold atoms cannot exist in the form of atom, but collide to larger gold clusters and nanoparticles. Therefore, the cationic imidazolium rings are freed from binding with gold species, but coordinate most possibly again with chlorine anions, making them have further ionic exchange capacity with AuCl4 − . Consequently, by reloading the MWCNTs (or more precisely the ImCl units) with AuCl4 − after the reduction process, more gold atoms can be introduced. So the particle loading process can be repeated. Fig. 2c shows the TEM image of CNT/gold NP composites after five cycles between of anion exchange and subsequent reduction. The number of gold NPs has increased dramatically, demonstrating successive attachment of gold NPs on modified CNTs, which cannot be achieved via the electrostatic absorption method [20,21]. The size enlargement of some NPs is caused by aggregation of freshly generated gold NPs on previously formed ones. We found that loading cycles can be repeated at least five times, resulting in stepwise increases in NP density. After that, loading becomes less efficient probably because the gold NPs then fully cover the surface, blocking ion exchange sites. 3.2. XRD analysis The XRD spectrum of MWCNT/gold NP composites after one ionic exchange–reduction cycle is shown in Fig. 3. The major diffraction peaks can be indexed as the gold face-centered cubic (fcc) phase based on the data of the JCPDS file [22]. The MWCNTs showed a typical peak of (0 0 2) phase of MWCNTs or graphite [23] and the peaks at 38.4◦ , 44.6◦ , 64.8◦ , 77.9◦ , and 81.9◦ can be assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystalline plane diffraction peaks of gold, respectively. The broad peaks in the XRD pattern indicate that in situ synthesized gold NPs are small. The average size of the gold NPs is around 10.6 nm calculated using the Scherrer formula, which is in good agreement with the result from the TEM image.

Au3+ in the form of a doublet at 87.3 (4f7/2 ) and 90.3 eV (4f5/2 ) [26]. After reduction, the disappearance of two peaks at 87.3 and 90.2 eV indicates that no Au3+ species exist in the MWCNT composites. The data support the in situ formation of gold NPs on the functionalized MWCNT surface. In order to further understand the ion exchange process, MWCNTs before and after reduction were also analyzed with EDS. The EDS images of both MWCNT–ImAuCl4 and MWCNT/gold NP composites are shown in Fig. 5a and b. The EDS spectrum of MWCNT–ImAuCl4 sample exhibits higher Cl to Au ratio, which is greatly reduced after the reduction reaction, but the Cl signal still remains detectable. This strongly suggests that concomitant with the reduction reaction, there is a regeneration of chloride counterions. It is worth noting that other counterions except AuCl4 − might be present, but apparently do not interfere with the ionic exchange process.

3.4. Optical absorption It is well known that the main feature of the absorption spectra for metal NPs is of the surface plasmon (SP) resonance band(s). Nanosized gold colloids show a very intense SP absorption band in the visible region. Therefore, the formation of gold NPs on the functionalized MWCNT surfaces was also verified with UV–visible spectroscopy (shown in Fig. 6). It is clearly seen that the MWCNT–COOH, MWCNT–ImCl and MWCNT–ImAuCl4 spectra have no absorption in the visible region. The weak absorption peak centered at about 532 nm in the spectrum of MWCNT/gold NP composites was in good agreement with the reported values for gold NPs [27], and attributable to the plasma excitation in gold NPs.

3.3. XPS and EDS analysis XPS measurements were made to further confirm the chemical reduction of AuCl4 − and subsequent formation of gold NPs on the IL-modified MWCNTs. As depicted in Fig. 4 (curve b), the XPS spectrum of as-prepared gold NP coated MWCNT composites shows the Au 4f7/2 and 4f5/2 doublet with the binding energies of 84.3 and 88.0 eV, respectively. These are typical values for Au0 [24], indicating the formation of gold NPs on the sidewalls of MWCNTs. Prior to reduction, the detected peaks corresponding to Au0 species may be attributable to the exposure of the sample to air [25]. Any AuCl4 − present in the MWCNT hybrids would give rise to XPS peaks for

Fig. 5. EDS images of MWCNT–ImAuCl4 (a) and MWCNT/gold NP (b) samples.

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Fig. 6. UV–vis absorption spectra taken from the MWCNT–COOH (a), MWCNT–ImCl (b), MWCNT–ImAuCl4 (c), and MWCNT/gold NP samples (d–f, corresponding to one, three and five deposition cycles, respectively) dispersed in ethanol.

4. Electrochemical properties The increased NP density greatly affects their electrochemical properties. To study these effects, the MWCNT/gold NP hybrids were precipitated onto 1,6-hexanedithiol modified gold electrodes. Fig. 7 shows the cyclic voltammetry (CV) response for different electrodes in 0.1 M K2 SO4 solution containing 10 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) at 50 mV/s. Redox reactions easily occurred on a bare gold substrate, evidenced by very large redox currents. After the gold substrate had been covered by a layer of dithiol, the CV curve dramatically changes from an electro-chemically reversible shape to a capacitive shape. Electrodes modified with one cycle deposited MWCNT/gold NP hybrids showed slightly increased electrochemical response compared to the monolayer-dithiol coated electrode. However, the electrodes coated with MWCNT/gold composites with three and five deposition cycles exhibited reversible voltammetry of Fe(CN)6 3−/4− with large cathodic/anodic peaks. Compared with the bare gold substrate, the bigger peak-topeak separation (Ep ) seems to indicate slow kinetics between the electrodes. But the fact that is responsible for the above phenomena may be explained by the strong electron-blocking effect of 1,6-hexanedithiol layer between gold NPs and the bare gold substrate. Fig. 8 shows that peak currents of the electrode coated with MWCNT/gold NP composites with five deposition cycles are proportional to the square root of scan rate. The roughly 100 mV

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Fig. 8. Scan rate dependence for CV curves of electrode coated with MWCNT/gold NP composites with five deposition cycles in 10 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) in 0.1 M K2 SO4 solutions, scan rate of (a–f) 10, 20, 50, 100, 200, 300 mV/s, respectively. (Inset) Graph of cathodic peak current (Ip ) vs square root of scan rate (v1/2 ).

peak-to-peak separation (Ep ) is obtained, which is almost consistent with the bare gold electrode. Therefore, we can conclude that these gold NPs on the modified MWCNTs not only provide the necessary conduction pathways, but also act like nanoscale electrodes in promoting the electron transfer between the analyte and the electrode surface. 5. Conclusions In conclusion, we reported on an easy and novel approach to the controlled loading of gold NPs on the IL-functionalized MWCNTs. Gold NPs were generated via reduction of the anionic gold precursor complex, which can be reloaded via regenerative ion exchange. The supported gold NPs disperse uniformly on the sidewalls of MWCNTs. The measurement results demonstrated the as-prepared composites have good optical absorption and excellent electrochemical properties. Potentially, the approach can be used to prepare a number of metal NPs (Au, Cu, Ag, Pt and Pd)–CNT composites, which hold promising applications in sensing and catalysis [12,28–30]. Acknowledgements The authors acknowledge the financial support of this work by “Top Hundred Talents” Program of CAS, and Young’s Scholarship in TaiShan University (Y07–2–05). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Fig. 7. Cyclic voltammetry of 10 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) in 0.1 M K2 SO4 solutions at 50 mV/s for (a) the bare gold substrate, (b) dithiol-modified gold substrate, (c–e) the electrodes coated with MWCNT/gold NP composites with one, three and five deposition cycles.

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