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Superlattice of Octanethiol-Protected Copper Nanoparticles Teng-Yuan Dong,* Heng-Hsi Wu, and Ming-Cheng Lin Department of Chemistry, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen UniVersity, Kaohsiung, Taiwan ReceiVed December 19, 2005. In Final Form: April 25, 2006 The synthesis and spectroscopic characterizations of size-controlled Cu nanoparticles forming self-assembled 2D superlattices with hexagonal packing are described. The scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), and electron spectroscopy for chemical analysis (ESCA) techniques were used to characterize the octanethiol-protected copper nanoparticles.
In recent years, much attention has been paid to metal nanoparticles that give rise to unique electronic and optical properties that are useful in a variety of new technologies in optoelectronic devices, chemical sensors, molecular catalysts, and magnetic materials.1 We are currently interested in the application of copper nanoparticles as direct spray printing inks to form conductor lines. Copper is a widely used electronic material with a bulk resistivity of ∼1.7 µΩ cm. The chemistry of Cu(I) and Cu(II) complexes as precursors to Cu films has been researched extensively in the past 10 years.2 However, many direct-spray approaches to the formation of copper conductors are limited owing to the requirement of high concentration in organic solvents and processing in the presence of reducing agents at relatively high temperatures.2,3 In this letter, we present a study on octanethiol-protected copper nanoparticles. Recently, Chen et al. reported a study on hexanethiol-protected copper nanoparticles that had a quantized double-layer charging property.4 Furthermore, Chin et al. recently reported a study on using longer chains of octanethiol, decanethiol, and dodecanethiol as protecting groups to form more compact monolayers that are insoluble in most common solvents after drying.5 For the first time, we report the initial results for the preparation of octanethiolprotected copper MPCs that are soluble in most common organic solvents. Spin coating of octanethiol-protected Cu MPCs as an ink to produce Cu films will be reported in due course. The octanethiolated Cu MPCs were prepared in solution by using a modified literature method reported for Au MPCs.6-12 An aqueous solution (12.5 mL) of 0.125 M CuCl2 was added to * Corresponding author. E-mail:
[email protected]. Tel: +8867-5253907. Fax: +886-7-3908. (1) (a) Hayat, M. A., Ed. Colloidal Gold: Principles, Methods, and Applications; Academic Press: San Diego, CA, 1991. (b) Alivisatos, A. P. Science 1996, 271, 933-937. (c) Schmid, G. Chem. ReV. 1992, 92, 1709-1727. (d) Peschel, S.; Schmid, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1442-1443. (e) Duteil, A.; Schmid, G. J. Chem. Soc., Chem. Commun. 1995, 31-32. (f) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974-12983. (g) Pileni, M. P. Langmuir 1997, 13, 3266-3276. (h) Lizmarzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329-4335. (2) (a) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978-1981. (b) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876-9880. (c) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Yanagimoto, H. J. Am. Chem. Soc. 2004, 126, 10822-10823. (d) Schulz, D. L.; Curtis, C.; Ginley, D. S. Electrochem. Solid-State Lett. 2001, 4, C58-C61. (e) Temple, D.; Reisman, A. J. Electronchem. Soc. 1989, 136, 3525-3529. (f) Shin, H. K.; Chi, K. M.; Hampden-Smith, M. J.; Kodas, T. T.; Farr, J. D.; Paffett, M. Chem. Mater. 1992, 4, 788-795. (3) Kaloyeros, A. E.; Feng, A.; Garhart, J.; Brooks, K. C.; Gosh, S. K.; Saxena, A. N.; Luethrs, F. J. Electron. Mater. 1990, 19, 271-276. (4) Chen, S.; Sommers, J. M. J. Phys. Chem. B 2001, 105, 8816-8820. (5) Ang, T. P.; Wee, T. S. A.; Chin, W. S. J. Phys. Chem. B 2004, 108, 11001-11010. (6) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510-1514. (7) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548.
Figure 1. (a) SEM image of coralloid Cu MPCs. TEM images of a size-evolved Cu superlattice at a temperature of 150 ( 2 °C under N2 for (b) 2, (c) 20, and (d) 40 h.
10 mL of a deoxygenated toluene solution containing 3.40 g of TOAB (6.22 mmol, tetraoctylammonium bromide) under N2, and the resulting mixture was stirred vigorously for 8 h. Then 1.2 mL (6.915 mmol) of octanethiol was added to the solution, which turned from wine red to milk white after 5 min. Finally, a solution of NaBH4 (300 mg) in 25 mL of H2O was added dropwise. The change in the solution color from yellowish to brown indicated the formation of n-octanethiolated Cu MPCs. The organic layer was extracted with toluene and dried under reduced pressure. Free thiol and TOAB could be removed by rinsing the brown solids with methanol and methanol/ethyl acetate (4:1). Interestingly, this particular sample is initially soluble in common organic solvents (e.g., 30 wt % in toluene). However, these Cu MPCs precipitate out gradually after 7 days. As shown in Figure 1a, the SEM image of the Cu particles shows a coralloid structure. (8) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (9) Hostetler, M. J.; Wingate, J.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. W.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (10) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, W. P.; Murray, R. W. Langmuir 1998, 14, 5612-5619. (11) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (12) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655-1656.
10.1021/la053438r CCC: $33.50 © 2006 American Chemical Society Published on Web 07/07/2006
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Figure 3. Typical ESCA spectrum of Cu MPCs.
Figure 2. (a) 1H NMR spectrum of octanethiolated Cu nanoparticles, (b) 13C NMR spectrum of the free ligand, and (c) 13C NMR spectrum of octanethiolated Cu nanoparticles. 1H and 13C NMR were used to confirm that the Cu MPCs were
capped with octanethiols. The broad, high-field (10-40 ppm) peaks in the 13C NMR spectrum were assigned to the resonances coming from the -CH2- and -CH3 groups in octanethiol (Figure 2c). Figure 2b shows the 13C NMR spectrum of the free octanethiol. The 13C NMR resonance assignments of the free octanethiol were based on 2D 1H-1H, 13C-13C, and 1H-13C correlation experiments. The chemical shift assignments for the carbons on th octanethiol chain for the Cu MPCs (Figure 2c) can be reasonably inferred from those of the free octanethiol. It is readily apparent that the resonances narrow as the carbons are located further from the thiol functionality. The resonance from CR closest to the Cu core interface is broadened into the baseline. It is interesting that the octanethiol of Cu MPCs gives NMR signals at room temperature that are sharper than those of the octanethiolate Au MPCs.7 It is suggested that the smaller dipoledipole interactions of the discontinuity in the diamagnetic susceptibility at the Cu-octanethiol interface and dipolar interactions in the octanethiol monolayer decrease the efficiency
of the nuclear relaxation mechanism, lengthening T1 for the nucleus and causing the line to appear sharper.7 From Figure 2c, the absence of the CR resonance from free octanethiol and the resonances from TOAB suggests that the Cu nanoparticles are octanethiolated Cu MPCs. A thermal analysis mass spectrometric method was used to analyze the desorption behavior of octanethiolated Cu MPCs. No TOAB desorption has been detected. Alkyl sulfide (RS+), dialkyl disulfide (RSSR+), and dialkyl sulfide (R-S-R+) dominated the mass spectrum of the octanethiolated Cu MPCs. The data suggest that dimerization occurs as a result of the recombination of surface thiolates during desorption. The compositions of the Cu MPCs were also checked with ESCA (Figure 3) and TGA (Figure 4). The S2p3/2, S2p1/2, oxidized S2p3/2, oxidized S2p1/2, C1s, Cu2p3/2, and Cu2p1/2 compontents that appeared at 162.3, 163.8, 166.2, 167.7, ∼284.8, 932.4, and 952.2 eV, respectively, in the ESCA spectrum compare very well with the typical values of chemisorbed alkanethiolated MPCs.5,13,14 The C/S atomic ratio (7.69) was found to be slightly lower than that expected from the free octanethiol. Furthermore, the Cu/S atomic ratio (2.19) is consistent with that deduced from TGA. As shown in Figure 4, we noticed from the derivative thermograph that the weight loss is a three-step desorption (Tc at 183.0, 252.6, and 267.1 °C). We would hence suggest that the loss is due to the different binding sites of octanethiol fragments on coralloid Cu particles.15 The octanethiol coverage is estimated from the relative weight loss of organic to inorganic fragments. The Cu/S ratio (2.2) agrees well with the ESCA data. On the basis of the observations of the small Cu/S atomic ratio observed in the measurements of ESCA and TGA, high solubility in common organic solvents, and a lower melting point (130-140 °C) resulting in the difficulty of characterizing the Cu MPCs with the TEM technique, we suggest that the Cu MPCs have a subnanosize structure. Furthermore, the heat treatment to manipulate the size and shape of these coralloid Cu MPCs supports our suggestion. To bring about the shape and size evolution of the coralloid Cu MPCs, the solid was heat treated in molten TOAB (1:3 CuC8/ TOAB by weight) at a temperature of 150 ( 2 °C under N2 for 2, 20, and 40 h.16 Free thiol and TOAB were removed by rinsing with methanol and 4:1 methanol/ethyl acetate. Monodisperse Cu MPCs are also soluble in common organic solvents. The TEM images of monodisperse CuC8 MPCs after heat treatments are (13) Bensebaa, F.; Zhou, Y.; Deslandes, Y.; Kruus, E.; Ellis, T. H. Surf. Sci. 1998, 405, L472. (14) Bourg, M. C.; Badia, A.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 6562-6567. (15) Ang, T. P.; Chin, W. S. J. Phys. Chem. B 2005, 109, 22228-22236. (16) (a) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Langmuir 2000, 16, 490-497. (b) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719-2724.
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and a generation of various electronic devices. An expansion of this work is underway and will be reported in due course. Experimental Section
Figure 4. TGA and the first derivative thermographs of the Cu MPCs.
Figure 5. Particle size of controlled Cu MPCs with the heat treatment time of size evolution.
shown in Figure 1b-d, respectively. As shown in Figure 5, the size-controlled Cu MPCs clearly grew by increasing the heattreatment time, with the particle sizes being 1.13 ( 0.16, 2.89 ( 0.38, and 7.36 ( 0.59 nm for 2, 20, and 40 h, respectively. As shown in Figure 1b-d, the size-controlled Cu MPCs form 2D superlattices with hexagonal packing (each edge is 2.2, 4.3, and 10.1 nm for 2, 20, and 40 h, respectively) covering the carbon-coated copper grids during toluene evaporation. In summary, we have shown that Cu nanoparticles can be prepared and size evolved by heat treatment. The Cu MPCs that easily formed self-assembled 2D superlattices potentially have the capability to demonstrate the versatility in particle assembly
All chemicals were of analytical grade and were used as received without further purification. All manipulations involving air-sensitive materials were carried out by using standard Schlenk techniques under an atmosphere of nitrogen. The octanethiolated Cu MPCs were prepared in solution by using a modified literature method reported for Au MPCs.6-12 An aqueous solution (12.5 mL) of 0.125 M CuCl2 was added to 10 mL of deoxygenated toluene solution containing 3.40 g of TOAB (6.22 mmol, tetraoctylammonium bromide) under N2, and the resulting mixture was stirred vigorously for 8 h. Then, 1.2 mL (6.915 mmol) of octanethiol was added to the solution, which turned from wine red to milk white after 5 min. Finally, a solution of NaBH4 (300 mg) in 25 mL of H2O was added dropwise. The change in solution color from yellowish to brown indicated the formation of n-octanethiolated Cu MPCs. The organic layer was extracted with toluene and dried under reduced pressure. Free thiol and TOAB could be removed by rinsing the brown solids with methanol and 4:1 methanol/ethyl acetate. To evolve the shape and size of the coralloid Cu MPCs, the solid was heat treated in molten TOAB (1:3 CuC8/TOAB by weight) at a temperature of 150 ( 2 °C under N2 for 2, 20, and 40 h.16 Free thiol and TOAB were removed by rinsing with methanol and 4:1 methanol/ethyl acetate. The surface chemical composition of the copper nanoclusters was analyzed with a VG Scientific ESCALAB 250 operated at 400 W. A monochromatic Mg KR X-ray source at 1253.6 eV was used, and the system was calibrated with respect to the C 1s peak. 1H NMR spectra were run on a Varian Inova 500 MHz spectrometer. The size and morphology of the Cu nanoparticles were determined by transmission electron microscopy. A drop of nanoparticles dispersed in toluene was placed on a copper grid coated with a thin film of carbon and dried. Electron micrographs were taken with a JEOL JEM-3010 analytical scanning transmission electron microscope (TEM) and a JEOL JSM-6330 scanning electron microscope (SEM). The desorption temperature of the adsorbed octanethiols and the weight loss percentage of the nanoparticles were determined with a TA Instrument 5100 + Dynamic TGA 2950 instrument under nitrogen (flow rate ) 100 mL/min) at a heating rate of 10 °C/min.
Acknowledgment. This work was supported by the National Science Council (NSC93-2113-M-110-008), Taiwan, ROC, and Department of Chemistry and Center for Nanoscience and Nanotechnology at the National Sun Yat-Sen University. Supporting Information Available: Two-dimensional NMR spectra of free octanethiol. TA-MS of Cu MPCs. This material is available free of charge via the Internet at http://pubs.acs.org. LA053438R
