Synthesis of Monodisperse Copper Nanoparticles by Utilizing 1-Butyl

Dec 5, 2012 - Aiqin Mao , Mengling Ding , Xia Jin , Xiaolong Gu , Chun Cai , Chen ... Shuang-Shuang Zhang , Rong-Ji Liu , Guang-Jin Zhang , Zhan-Jun G...
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Synthesis of Monodisperse Copper Nanoparticles by Utilizing 1‑Butyl-3-methylimidazolium Nitrate and Its Role as Counteranion in Ionic Liquid in the Formation of Nanoparticles Gil Hwan Hong and Sang Wook Kang* Department of Chemistry, Sangmyung University, Seoul 110-743, Republic of Korea ABSTRACT: We successfully fabricated Cu nanoparticles from Cu flakes by utilizing the ionic liquid BMIM+NO3−. When BMIM+NO3− existing mostly in the free ion state was used for dissociation of Cu flakes, Cu nanoparticles were rapidly prepared in a relatively short amount of time compared to other ionic liquids. The favorable interaction between free NO3− of BMIM+NO3− and the Cu metal surface caused the flakes to dissociate in the ionic liquid, forming nanoparticles. The formation of Cu nanoparticles was confirmed by UV−vis spectral analysis and transmission electron microscopy (TEM) imaging, and the interaction of BMIM+NO3− with the Cu metal was confirmed by X-ray photoelectron spectroscopy (XPS). and thermal decomposition, have been utilized.15−20 Very recently, a simple process for the fabrication of copper nanoparticles from copper flakes utilizing ionic liquids (ILs) has been introduced.21,22 These studies indicate that copper flakes dissociate into copper nanoparticles when reacted with ionic liquid owing to strong interaction between the copper surface and the anion of the ionic liquid.21,22 Unfortunately, when 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4−), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+PF6−), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM+BF4−) were utilized, it took a relatively long time, i.e., 24 h, to fabricate the copper nanoparticles.21,22 Thus, it is desirable to develop another method for simple, fast fabrication of copper nanoparticles. In this report, we suggest a fast fabrication method for monodisperse copper nanoparticles utilizing 1-butyl-3-methylimidazolium nitrate (BMIM+NO3−) existing mostly as free ions and anion pairs. This method is expected to be widely utilized for fabrication of metal nanoparticles along with those reported previously because ILs have recently attracted attention for the synthesis and stabilization of nanoparticles.23−29

1. INTRODUCTION Nanoparticles have attracted much interest over the past decade because they show various properties that differ from the bulk state, such as the quantum size effect in photochemistry, nonlinear optical properties, and high reactivity.1−5 Recently, nanoparticles have been utilized for various applications such as surface-enhanced Raman spectroscopy (SERS), optical data storage, surface-plasmon-enhanced light adsorption, and antibacterial agents.6−9 Among metal particles, Ag nanoparticles have recently been of interest because they reversibly interact with olefins such as propylene and ethylene, in addition to having uses in antibacterial applications, catalysis, electronics, and SERS.10−12 It has been reported that electron acceptors such as p-benzoquinone (p-BQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) react with silver nanoparticles, resulting in activation of the metal surface, which becomes positively charged.10,12 Because of these properties, silver nanoparticles have been utilized as olefin carriers for olefin/paraffin separation membranes. For example, when p-BQ was introduced into poly(ethylene-co-propylene) (EPR)/Ag metal composite membranes, they showed separation performance comparable to the propylene/propane selectivity of 11 and a total mixed gas permeance of 0.5 GPU (1 GPU = 1 × 10−6 cm3 (STP)/(cm2 s cm Hg).10 Furthermore, when silver nanoparticles tuned by strong electron acceptor TCNQ were utilized for olefin/paraffin separation, the membrane showed significant improvement in separation performance with mixed gas selectivity of up to 50.12 However, even though silver nanoparticles can be utilized as olefin carriers, they are relatively expensive materials for use in industrial applications. Thus, copper, an element homologous to silver, has been an alternative candidate for electronic, antifungal, catalytic, and olefin carrier applications because homologous elements show similar physical and chemical properties.13−15 For the fabrication of copper nanoparticles, methods using bulk copper metals, such as discharge of bulk copper rods, photoconversion of copper flakes, the reaction of iodobenzene, © 2012 American Chemical Society

2. EXPERIMENTAL SECTION Microsized copper particles (1−5 μm, 99%, Aldrich Chemical) were introduced into ionic liquids of 1-butyl-3-methylimidazolium nitrate (BMIM+NO3−, C-TRI Co., Ltd.). Transmission electron microscope (TEM) images were obtained using a JEOL JEM-3000 operating at 300 kV. Samples for TEM imaging were prepared from a copper grid on which the solution was dropped. The UV−vis absorption spectrum was obtained using a Mecasys OPTIZEN 2120 UV−vis spectrophotometer for BMIM+NO3−/Cu composite solution. X-ray photoelectron spectroscopy (XPS) data were acquired using a PerkinElmer PHI 5400 X-ray photoelectron spectrometer for Received: Revised: Accepted: Published: 794

October 16, 2012 November 14, 2012 December 5, 2012 December 5, 2012 dx.doi.org/10.1021/ie302821g | Ind. Eng. Chem. Res. 2013, 52, 794−797

Industrial & Engineering Chemistry Research

Article

BMIM+NO3−/Cu composite solution. This system was equipped with a Mg X-ray source operated at 300 W (15 kV, 20 mA). The carbon (C 1s) line at 285.0 eV was used as the reference in our determinations of the binding energies of copper.

3. RESULTS AND DISCUSSION Scheme 1 shows the molecular structure of BMIM+NO3−. A previous study indicated that NO3− peaks for free ions, ion Scheme 1. 1-Butyl-3-methylimidazolium Nitrate

Figure 3. UV−visible absorption spectra of dissociated copper particles in BMIM+NO3−.

Figure 1. A photograph of (left) BMIM+BF4−/Cu flakes, (middle) BMIM+NO3−/Cu composite solution, and (right) BMIM+PF6−/Cu flakes.

Table 1. Viscosity of BMIM+NO3− and BMIM+NO3−/Cu viscosity (cP)

neat BMIM+NO3−

BMIM+NO3−/Cu composite solution

24.7

31.3

Figure 4. XPS for copper binding energy in the BMIM+NO3−/Cu composite.

liquid/copper flake weight ratios of 1:0.02) were stirred for 5 h. A color change from bronze to dark yellow was observed only for the BMIM+NO3−/Cu solution as shown in Figure 1, indicating the formation of copper nanoparticles. On the other hand, BMIM+BF4−/Cu and BMIM+PF 6−/Cu composite solutions did not change and the copper flakes remained constant, indicating that copper nanoparticles were hardly generated. When the composite solution was prepared with a weight ratio of BMIM+NO3−/Cu of 1:0.003, a color change from bronze to dark yellow was observed within 5 min while BMIM+BF4−/Cu and BMIM+PF6−/Cu composite solutions required 1440 min. From these results, it was thought that Cu flakes in the ionic liquid BMIM+NO3− easily dissociated to form nanoparticles. Table 1 shows the change of the viscosity of BMIM+NO3− after the copper nanoparticles were generated. When the copper nanoparticles were formed in BMIM+NO3−, the viscosity became 31.3 cP, while that of neat BMIM+NO3− is 24.7 cP, indicating that the generated nanoparticles were well dispersed in BMIM+NO3−. TEM images were observed to investigate the effect of BMIM+NO3− on the size of the copper nanoparticles (Figure 2). In the TEM images, monodisperse 10 nm particles were observed, while the average size of the copper flakes without ionic liquid was known to be 1−5 μm.21,22 These images indicate that monodisperse copper nanoparticles could be generated without specific agents such as surfactants and polymers.

Figure 2. Transmission electron microscopy of (a) dissociated-copper nanoparticles by ionic liquids and (b) the enlarged image. Weight ratio of BMIM+NO3−:Cu flakes = 1:0.02.

pairs, and higher order ion aggregates are at 1034, 1040, and 1045 cm−1, respectively, in FT-Raman spectra.30 In the case of BMIM+NO3−, the major Raman peak was observed at 1034 cm−1, indicating that BMIM+NO3− existed as mostly free ions.30 Thus, it could be expected that relatively free anions of BMIM+NO3− would strongly interact with the surfaces of copper metal flakes, resulting in the acceleration of dissociation to nanoparticles. Solutions of microsized copper metal composites in BMIM+NO3−, BMIM+BF4−, and BMIM+PF6− (with ionic 795

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Scheme 2. Formation of Nanosized Copper Nanoparticles by BMIM+NO3−

nanocomposite, as shown in Figure 4, was observed at 934.69 eV, while that of microsized Cu was known to be 934.05 eV. This increased binding energy was attributable to the strong interactions between the surface of the copper nanoparticles and the anion of BMIM+NO3−. Therefore, the partially positive-charged surface of the copper nanoparticles was generated as shown in Scheme 2. Because positively polarized copper nanoparticles could be used as CO2 carriers for facilitated transport membrane, it is expected that the small, monodisperse copper nanoparticles generated by BMIM+NO3− will be good carriers for separation of CO2/CH4 or CO2/N2 mixtures.

UV−vis absorption was observed to confirm the size of the dissociated copper nanoparticles (Figure 3). It is known that the plasmon resonance peak of nanosized copper particles is observed at approximately 580 nm.21 In the case of the BMIM+NO3−/Cu nanocomposite, the peak maximum was observed at 418 nm while that of BMIM+BF4−/Cu was observed at 645 nm, indicating that much smaller nanoparticles were generated. Furthermore, the spectrum shows a symmetric peak shape, meaning that monodisperse copper nanoparticles were formed in BMIM+NO3−. These spectra are consistent with TEM images shown in Figure 2. On the other hand, in the case of BMIM+BF4−/Cu and BMIM+PF6−/Cu nanocomposites, asymmetric peak shapes were observed with peak maxima at 645 and 660 cm−1, respectively, indicating that the particles were polydisperse and relatively large.21,22 It could be thought that the interaction between the surface of the copper metal and counteranions of BMIM+NO3− were much stronger than that of BMIM+BF4− and BMIM+PF6−, resulting in the fast and monodisperse dissociation of copper nanoparticles. To confirm the interaction between the surface of the copper particles and counteranions of BMIM+NO3−, XPS was conducted for the BMIM+NO3−/Cu nanocomposite. It was expected that the charge density of copper would change when the particles interacted because the negatively charged counteranions reacted with the metal surface. As a result, the binding energy of the copper in the BMIM+NO3−/Cu

4. CONCLUSIONS We successfully prepared monodisperse copper nanoparticles with average sizes of 10 nm by utilizing BMIM+NO3−. The size and distribution was confirmed by UV−vis analysis and TEM imaging. Because BMIM+NO3− existed as mostly free ions, it strongly interacted with the copper surface, as confirmed by XPS. As a result, the strong interaction between counteranions of BMIM+NO3− and the copper surface caused the flake state to dissociate rapidly to nanoparticles. This facile and fast method to fabricate small, monodisperse copper nanoparticles will be applied to various fields such as catalysis, antibacterial 796

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agents, conducting materials, and gas carriers as a substitute for costly Ag and Au in the near future.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82 2 2287 5362. Fax: +82 2 2287 5362. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a 2011 Research Grant from Sangmyung University.

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REFERENCES

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