Facile Synthesis of Copper Nanoparticles by Ionic Liquids and Its

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RESEARCH NOTES Facile Synthesis of Copper Nanoparticles by Ionic Liquids and Its Application to Facilitated Olefin Transport Membranes Jongho Kim,†,‡ Sang Wook Kang,‡,§ Sung Hyun Mun,† and Yong Soo Kang*,† Department of Chemical Engineering, Hanyang UniVersity, Seoul 133-791, South Korea, and School of Chemical & Biological Engineering, Seoul National UniVersity, Seoul 151-744, South Korea

We suggest a facile synthesis method of copper nanoparticles, and this process is facilitated by 1-butyl-3methylimidazolium tetrafluoroborate (BMIM+BF4-) which was employed to dissociate the copper metal into nanosized copper particles. The formation of copper nanoparticles was confirmed by UV-vis spectrum analysis and transmission electron microscope (TEM) imaging, and the interaction of BMIM+BF4- with the copper metal was confirmed by Fourier transform (FT)-Raman spectroscopy. Moreover, the fabricated copper nanoparticles could be used as an olefin carrier for propylene/propane separation. While BMIM+BF4-/Cu (stirred for 0 h) nanocomposite showed a selectivity of 1.1 and a permeance of 4.7 GPU, the separation performance of the BMIM+BF4-/Cu (stirred for 24 h) nanocomposite membrane was significantly improved; the selectivity was found to be 5.2 with a mixed gas permeance of 4.0 GPU. 1. Introduction In recent years, nanometer scale devices have attracted considerable attention because of their greater miniaturization and the fact that they exhibit unique properties which differ from the bulk properties.1 These unique properties are related to their size: high surface area and exceptional surface activity result in size-dependent properties due to the dramatic changes in the ratio of surface area to total volume.2-6 Many studies on the quantum size effect on photochemistry,7,8 nonlinear optical properties,9,10 and the emergence of metallic properties with the size of particles11-13 have been reported. Metal nanoparticles in particular have been vigorously investigated due to their specific properties. A wide spectrum of research has been conducted to control the size and shape of nanosized metal particles for the utilization and optimization of chemical/physical properties.14-17 A number of methods have been suggested for the preparation of metal nanoparticles, such as chemical reduction,18 gas condensation,19 laser irradiation,20 and sonochemical deposition.21 Recently, the versatility of metal nanoparticles has been reported. For example, nanobiotechnology,22,23 a substrate for surface-enhanced Raman spectroscopy (SERS),24 multiphoton adsorption and optical data storage,25,26 and surface-plasmonenhanced light adsorption for photovoltaic materials27,28 have been described as possible applications for metal nanoparticles. Among nanoparticles, noble metal nanoparticles, such as those composed of Au and Ag, have been the most common subjects of research due to their properties and potential applications.29-33 Due to its properties and wide variety of applications, silver is also one of the most interesting metals for nanofabrication. Because of the various characteristics of silver, it has played an important role in antibacterial applications,34 catalysis,35 and electronics,36 as the substrate for SERS13 and as an olefin * To whom correspondence should be addressed. Tel.: +82-2-22202336. Fax: +82-2-2296-2969. E-mail: [email protected]. † Hanyang University. ‡ Equally contributed as the first authors. § Seoul National University.

carrier.37,38 A number of preparation methods for silver nanoparticles have been developed. For example, the encapsulation of silver nanoparticles into a polymer shell via emulsion polymerization,39 in situ fabrication leading to free-standing polymer films incorporating silver nanoparticles,40 etc.41-43 However, noble metals such as gold and silver are relatively expensive materials to use for industrial devices. In this regard, copper is an inexpensive alternative candidate to gold and silver. A conventional study suggested that homologous elements have similar regular trends in physical and chemical properties. Therefore, it could be expected that the copper, a homologous element for gold and silver, can be utilized as a new alternative material for a variety of applications, including electronic,44 antifungal,45 and catalytic uses.46 In previous studies, copper nanoparticles were prepared from bulk copper metal using methods such as the discharge of bulk copper rods,47 photoconversion of copper flakes,48 and the reaction of iodobenzene with copper.46 For the application of metal nanoparticles, a simpler preparation process is needed. In the past, even though noble metal nanoparticles had been successfully synthesized, the fabricated procedures were relatively complicated.49,50 Thus, a simple and easy-handling procedure for synthesis is required for practical application. In this paper, we suggest a facile synthesis of copper nanoparticles employing ionic liquids; this is the first description of the fabrication of copper nanoparticles by stirring ionic liquids with microsized copper particles. The fabrication method we suggest is a relatively simple process and has many practical and economic advantages. Moreover, our group reported recently that the positively polarized Ag nanoparticles could be reversibly interacted with olefin molecules, resulting in the facilitated olefin transport.37,38 Since the characteristic of Ag is similar with Cu, it could be expected that polarized Cu nanoparticles also play role in interacting reversibly with olefin molecules.

10.1021/ie900150c CCC: $40.75  2009 American Chemical Society Published on Web 07/06/2009

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experiment, particular attention was paid to avoiding any potential contact of the membrane with water because water may deteriorate the separation performance. The upstream pressure is 40 psig. Gas permeance is expressed in units of GPU, where 1 GPU ) 1 × 10-6 cm3 STP/(cm2 s cmHg). The effectiveness of the IL/Cu nanocomposite membranes in separating a mixed gas (50:50 vol % propylene/propane) was evaluated using a gas chromatograph (Hewlett-Packard G1530A, MA) equipped with a thermal conductivity detector (TCD) and a unibead 2S 60/80 packed column. 3. Results and Discussion

Figure 1. Pictures of (a) flakes of Cu powder (b) neat BMIM+BF4- and (c) BMIM+BF4-/Cu powder composite solution.

2. Experimental Section Microsized copper particles (1-5 µm, 99%, purchased from Aldrich Chemical) were introduced into ionic liquids of 1-butyl3-methylimidazolium tetrafluoroborate (BMIM+BF4-, purchased from C-TRI). Transmission electron microscope (TEM) images were obtained by JEOL JEM-3000, operating at 300 kV. Samples for TEM imaging were prepared from a copper grid where the solution was dropped. The UV-vis absorption spectrum was obtained through Mecasys Optizen 2120 UV-vis spectrometer. Fourier transform (FT)-Raman spectra were detected in room temperature using JASCO NRS-3100 at a resolution of 1 cm-1. Separation membranes were prepared by coating BMIM+BF4-/ Cu nanocomposite dispersions onto a polyester microporous membrane support (Osmonics Inc., average pore size of 0.1 µM) using an RK Control Coater (Model 101, Control Coater RK Print-Coat instruments LTD, UK). The flow rates of mixed gas and sweep gas (helium) were controlled using mass flow controllers. The all gas flow rates represented by gas permeance were determined using a mass flow meter at the steady-state. Gas flow rates or gas permeances were measured with a mass flow meter at an upstream pressure in pounds per square inch (psig) and atmospheric downstream pressure. During the whole

After the solution of ionic liquid and microsized copper metal composites was stirred for 24 h, a color change from bronze to blue was observed, indicating the formation of copper nanoparticles.51 Figure 1 shows the copper flakes, neat BMIM+BF4- and BMIM+BF4-/Cu powder composite. The addition of metal copper into BMIM+BF4- at room temperatures with stirring led to the dissolution of the metal. Blue colors were observed in BMIM+BF4-/Cu powder composites after 24 h, indicating that nanoparticles had formed. TEM images were observed to investigate the size of the copper nanoparticles and the effect of ionic liquids on the size of the copper nanoparticles in Figure 2. In the TEM image, 20-200 nm particles were observed, while the size of copper powder particles without ionic liquid was 1-5 µM. In other words, pristine microsized copper powders existed as clusters and were dissociated into the copper nanoparticles by the introduction of ionic liquids. To confirm the size of the dissociated copper nanoparticles, UV-vis absorption was traced as shown in Figure 3. The plasmon resonance peak at about 580 nm is characteristic of nanosized copper particles. However, in the case of BMIM+BF4-/ Cu metal, the peak maximum was observed at 645 nm, which suggests that copper nanoparticles formed by BMIM+BF4- are 20-200 nm and that the dominant size of the particles is about 200 nm. This asymmetric peak means that various sizes of copper nanoparticles were present, and these results were consistent with TEM images. As determined from the TEM images and UV-vis spectrum, the size of the copper nanoparticles fabricated with BMIM+BF4- was 20-200 nm, which means considerably smaller particles were formed. It was thought that this dissociation was attributable to the favorable interaction between the surface of the entangled Cu and the ionic liquid.

Figure 2. Transmission electron micrograph of (a) copper powder and (b) dissociated-copper nanoparticles by ionic liquids. Weight ratio of BMIM+BF4-/ Cu powder ) 1/0.003.

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Table 1. Mixed Gas Selectivity and Permeance of Neat BMIM BF4and BMIM+BF4- /Cu Powder Composite Membrane

Neat BMIM+BF4BMIM+BF4-/Cu (stirring for 0 h) BMIM+BF4-/Cu (stirring for 24 h)

Figure 3. UV-visible absorption spectra of dissociated-copper particles.

Figure 4. FT-Raman spectra: neat BMIM+BF4- and BMIM+BF4-/Cu powder composite in the BF4- stretching region.

Interactions between Cu nanoparticles and BMIM+BF4- in BMIM+BF4-/Cu powder composites were investigated by FTRaman spectroscopy. The Raman spectra in the regions of the BF4- stretching bands are shown in Figure 4 for the BMIM+BF4-/ Cu powder composite. Note that the peaks for free ions, ion pairs, and higher order ion aggregates of BF4- appeared at 765, 770, and 774 cm-1, respectively. The peak intensity at 765 cm-1, corresponding to free BF4anions, was found to increase upon the addition of the Cu nanoparticles with a concomitant decrease in the peak intensity at 774 cm-1, corresponding to ion aggregates. The BF4-/Cu metal interaction results in the reduction of the interaction between BF4- and BMIM+, and consequently, the increase in the free anion concentration. This increase could be explained by the fact that the concentration of free anions increases by introducing inorganic nanoparticles in polymer electrolytes.52,53 It was therefore thought that the favored interactions between the Cu metal and BF4- caused the Cu metal powders to be disentangled to form nanoparticles.

selectivity (propylene/propane)

permeance (GPU)

0.9 1.1 5.2

0.5 4.7 4.0

From these data, the structures of BMIM+BF4-/Cu metal composites could be illustrated as shown in Scheme 1. The pristine copper powder existed as clusters, but the addition of ionic liquids caused the metal clusters to dissociate, resulting in the formation of Cu nanoparticles. The Cu nanoparticles formed in this manner were applied to an olefin separation membrane, and the separation performances for a propylene/propane gas mixture are shown in Table 1. Neat BMIM+BF4-, BMIM+BF4-/Cu (stirred for 0 h), and BMIM+BF4-/Cu (stirred for 24 h) composite membranes were investigated to confirm the facilitated transport as a new olefin carrier. The selectivity of propylene over propane was only about 0.9, and the mixed gas permeance was about 0.5 GPU51 for the neat BMIM+BF4- membrane. The BMIM+BF4-/Cu (stirred for 0 h) nanocomposite showed a selectivity of 1.1 and a permeance of 4.7 GPU. Compared with neat BMIM+BF4-, the selectivity barely changed; however, the permeance increased significantly. This result originated from the interfacial defects between BMIM+BF4- and Cu. However, the separation performance of the BMIM+BF4-/Cu (stirred for 24 h) nanocomposite membrane was significantly improved; the selectivity was found to be 5.2 with a mixed gas permeance of 4.0 GPU. These data indicated that copper nanoparticles can be utilized as a new olefin carrier for propylene/propane separation. In a previous study, silver nanoparticles activated by counteranions from ionic liquids showed the separation performance for propylene/propane.54 Likewise, the surface of the copper nanoparticles was activated by counteranions, resulting in the facilitated olefin transport. 4. Conclusions In summary, we succeeded in fabricating copper nanoparticles, and this process is facilitated by ionic liquids. The ionic liquid was employed to dissociate the copper metal into nanosized copper particles, and the interaction between the copper and ionic liquids, which was caused by various chemical forces, effected the dissolution of the microsized copper particles to nanosized particles. Moreover, the fabricated copper nanoparticles can be used as an olefin carrier for propylene/propane separation. The formation of copper nanoparticles was confirmed by UV-vis spectrum analysis and TEM imaging, and the interaction of BMIM+BF4- with the copper metal was confirmed by FT-Raman spectroscopy. This method is a valuable option for the fabrication of copper nanoparticles, since it is an

Scheme 1. Microsized Copper Dissociation by Ionic Liquids to Nanosized Copper Nanoparticles

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ReceiVed for reView January 28, 2009 ReVised manuscript receiVed June 16, 2009 Accepted June 22, 2009 IE900150C