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J. Phys. Chem. C 2008, 112, 4176-4179
Facile Electrochemical Synthesis of Single-Crystalline Copper Nanospheres, Pyramids, and Truncated Pyramidal Nanoparticles from Lithia/Cuprous Oxide Composite Thin Films Yan Yu,†,‡ Yi Shi,† Chun-Hua Chen,*,† and Chunlei Wang*,‡ Laboratory of AdVanced Functional Materials and DeVices, Department of Materials Science and Engineering, UniVersity of Science and Technology of China, Hefei 230026, P. R. China, and Department of Mechanical and Materials Engineering, Florida International UniVersity, Miami, Florida 33174 ReceiVed: January 4, 2008
We developed a facile electrochemical charge-discharge method to synthesize single-crystalline Cu nanoparticles with different morphologies by using a Li-driven electrochemical reduction process. Threedimensional reticular Li2O/Cu2O composite thin films were fabricated on both Ni foam substrates and Cu foil substrates using the electrostatic spray deposition (ESD) technique. Cu nanospheres (on Ni foam substrate) and pyramid-like Cu particles (on Cu foil substrate) can be successfully produced with a high yield by fully discharging the assembly of three-dimensional reticular Li2O-Cu2O /Li electrochemical cells. It was demonstrated that the shape and size of copper nanoparticles could be controlled systematically by adjusting cycle numbers and the electrode substrate materials. In addition, a mechanistic consideration of the size control and shape formation is briefly discussed.
Introduction During the past few decades, synthesis of metal nanostructures has been a research focus due to their widespread applications in catalysis, photography, photonics, electronics, optoelectronics, plasmonics, information storage, optical sensing, biological labeling, imaging, and surface-enhanced Raman scattering (SERS).1-6 The properties of metal nanostructures are determined by parameters like size, shape, composition, crystallinity, and structure (e.g., solid versus hollow).7 Thus, rational control over the size, shape, and phase of nanocrystal particles has become an important research issue in recent years.8-10 Among all metals, copper is one of the most commonly used interconnect materials due to its high electrical conductivity and low price. In the past two decades, considerable attention has been paid to copper nanomaterials because of their unusual properties in nanodesigns, thermal conducting, lubrication, nanofluids, and catalysts.11,12 Various methods, including microemulsion, reverse micelles, irradiation method, polyol process, template-assisted synthesis, electrochemical deposition, chemical solution process, and chemical vapor deposition (CVD), have been applied to synthesize copper with different shapes.13-22 However, there are few reports on the morphology control of single-crystalline Cu nanoparticles. In this study, we developed an electrochemical milling method to fabricate Cu nanoparticles with controllable size and structure. During the exploration of new anode materials for lithiumion batteries, it has been found that nanoscale metallic clusters dispersed in an amorphous Li2O matrix could be obtained from the electrochemical reaction between some transition metal oxides (such as CoO, CuO, NiO, Co3O4, and MnO) and lithium.23-24 We have recently reported that Cu nanospheres and dentritic nanofibers could be synthesized by a one-step * To whom correspondence should be addressed. E-mail: cchchen@ ustc.edu.cn;
[email protected]. † University of Science and Technology of China. ‡ Florida International University.
discharge reaction between a micrometer-sized CuO working electrode and a lithium reference electrode.25 Herein, the facile method we developed is based on repeated charge-discharge treatment of porous lithia/cuprous oxide composite thin-film electrodes. It was found that Cu nanospheres could be fabricated on Ni foam substrates, and the pyramid-like Cu particles could be obtained on Cu foil substrates. In addition, it has been demonstrated that the morphology and size of Cu products could be modulated by controlling the charge-discharge cycle numbers. A possible formation mechanism was also discussed. Furthermore, the electrochemical technique can be potentially extended to the fabrication of other metal nanoparticles with various morphologies. Experimental Section Highly porous lithia/cuprous oxide composite thin-films were prepared by electrostatic spray deposition (ESD) technique.26-29 The precursor solution was prepared by mixing a 1.5 mmol copper acetate, 1.5 mmol lithium acetate in 50 mL of butyl carbitol (CH3(CH2)3OCH2OCH2CH2OH). The hybrid oxide thin films of copper oxide and lithium oxide were deposited for 3h on Cu foil and Ni foam disk substrates at 250 °C. A distance of 4∼5 cm was kept between the needle and the substrate. The applied voltage was 10∼18 kV. The feeding rate of the precursor solution was 4 mL/h. The as-deposited samples were examined by X-ray diffraction (XRD) (Philips X’Pert Pro Super, Cu KR radiation), X-ray photoelectron spectroscopy (XPS) (ESCALAB MK II), and field-emission scanning electron microscopy (FESEM) (JEOL JSM-6700F). The electrochemical discharge process was carried out on a multichannel battery test system (NEWARE BTS-610) using two-electrode coin cells (CR2032) of the Li/1M LiPF6 (EC/ DEC, 1:1)/Li2O-Cu2O, where EC and DEC stand for ethylene carbonate and diethyl carbonate, respectively. After different cycles of charge/discharge at a current density of 0.1 mA/cm2 between 3.0 and 0.01 V at room temperature, these cells were finally discharged to 0.0 V and disassembled in an argon-filled
10.1021/jp800071h CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008
Synthesis of Copper Nanoparticles
Figure 1. SEM images of Cu2O-Li2O thin films on different substrate: (a, b) porous nickel-foam substrate and (c, d) copper foil substrate. These thin films were deposited at 250 °C for 3 h in air.
glove box. Then the discharged Li2O-Cu2O electrodes were washed in turns with pure DEC, a dilute hydrochloric acid, and ethanol to remove the electrolyte residue, Li2O, and the residual copper oxide. The resulting thin film electrodes were separated into two parts. One was directly used for field-emission scanning electron microscopy (FESEM) test. The other was further washed in an ultrasonic bath. By ultrasonic cleaning process, limited weak contact between pyramidal (or truncated pyramidal) nanoparticles and substrate can be broken. The obtained powders were finally separated in a centrifuge. The obtained powder products were further analyzed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM; Hitachi 800), and high-resolution transmission electron microscopy (HRTEM; JEOL-2010). Results and Discussions The as-deposited films on Cu foils or on nickel foams are confirmed to be Cu2O and Li2O by XRD method (ref 29). FESEM was adopted to characterize the morphology of the asprepared copper sample. Figure 1 shows the SEM images of the two as-deposited films on Ni foam (Figure 1a) and on Cu foil (Figure 1c). Both of them are reticular structures with 3-D cross-linked pores of mean pore size of about 5 µm, showing a typical characteristic of ESD films.26-29 Panels b and d of Figure 1 display the higher-magnification SEM images of the 3D reticular thin-films on Ni foam and Cu foil, respectively. It can be clearly observed that the wall of three-dimensionally ordered macroporous structure is composed of nearly monodisperse spheres with an average size of ∼250 nm (Figure 1d shows average particles with average sizes of 150 nm). When the ESD-derived porous Li2O-Cu2O composite thinfilms were used as the working electrode in the Li2O-CuO/Li electrochemical cell, we found that the overall electrochemical reaction can be written as follows:29 discharge
Cu2O + 2Li+ + 2e- y\ z 2Cu + Li2O (initial cycles) charge discharge
z Cu + Li2O (subsequent cycles) CuO + 2Li+ + 2e- y\ charge From the above reactions, it is clearly seen that metal Cu can be fabricated during the electrochemical charge-discharge process. After discharge at different cycle numbers, the batteries were dissembled and soaked into EC electrolyte before SEM investigation. Figure 2 shows SEM images of Cu nanoparticles obtained from the fully discharged Li2O-Cu2O electrodes on Ni foam substrate cycled at a current density of 0.1 mA/cm2 at different cycle numbers. It is hard to recognize the boundary between each particle probably because of the incompletely washing the electrolyte and SEI (Solid Electrolyte Interface) film. It is found that the cycle number has great influence on the morphology and size of the Cu nanoparticles. After the first discharge and above-mentioned washing treatment, the as-
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Figure 2. FESEM images of the discharged products (metallic copper) of Li2O-Cu2O (Li:Cu ) 1:1) thin-film electrodes on Ni foam obtained after different charge-discharge cycles. (a) 1, (b) 50, and (c) 400 cycles. The current density was 0.1 mA/cm2.
prepared samples are mainly Cu nanoparticles with a size of about 200 nm (Figure 2a). When the working electrode is charged-discharged for 50 cycles, spherical Cu nanoparticles with a diameter of about 100 nm are observed (Figure 2b). When the charge-discharge cycle numbers are increased to 400 cycles, the obtained samples are copper nanospheres with a diameter of ca. 20 nm (Figure 2c). Therefore, the diameter of Cu nanoparticles decreases monotonously with increasing cycle numbers. After deep washing and dissolving the electrolyte by ultrasonic deep cleaning, the crystal structure, phase composition and morphology of the obtained Cu nanoparticles were characterized using XRD and TEM. Figure 3a displays a representative XRD pattern of the Cu nanoparticles prepared by the electrochemical charge-discharge for 50 cycles. The sharp and strong diffraction peaks (111), (220), and (311) can be readily indexed to the cubic phase of copper (JCPDS 85-0554). No impurities can be detected in this pattern, which indicates that the pure Cu can be obtained under current synthesis condition. Figure 3, panels b and c, shows the TEM image of Cu nanoparticles with a diameter of about 100 nm. The inset of Figure 3c proves a typical selected area electron diffraction (SAED) pattern of one Cu nanosphere. The appearance of regular spots in the SAED pattern indicates that the obtained Cu particles are single-crystalline. Through extensive investigations on individual spherical nanoparticles from the Cu products with SAED, we found that this single-crystalline character was maintained in all of the deep washed products. Note that here the round shapes of nanospheres look different from the samples shown in Figure 2b due to the existence of organic electrolytes or related products in SEM samples. Interestingly, Cu pyramid and truncated pyramidal nanoparticles can be successfully obtained by using Cu foils to replace Ni foam foils as the deposition substrates while other conditions remain unchanged. When the Cu2O-Li2O/Cu electrode was discharged in first cycle, the as-prepared samples were pyramidlike Cu microcrystals with an edge length of about 1.2 µm, as shown in Figure 4a. With the increasing charge-discharge cycles, the sample sizes decreased and the top of pyramid-like Cu particles were slowly razed out at the same time (Figure 4b,c). After discharging 50 times, the pyramid-like products became truncated pyramidal Cu nanoplates with a length of ca. 400 nm. This confirms that the cycle number has exerted a noticeable influence on the morphology and size of the Cu nanoparticles. In addition, the samples’ shapes are different from that spherical Cu nanoparticles (Figure 2), suggesting that Cu nanocrystals with different morphologies can be fabricated through choosing appropriate deposition substrate under current charge-discharge methods. Further TEM, SAED, and HRTEM show that the truncated pyramid-like Cu nanoplates are singlecrystalline (Figure 5). Although the exact mechanism for the charge-discharge preparation of Cu nanoparticles with modulated size and morphology is still under investigation, the influences of the
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Figure 3. (a) Typical XRD pattern, (b) overview TEM image, and (c) high-magnification TEM image of Cu nanospheres prepared after chargedischarge for 50 cycles. The insert in panel c is the SAED pattern of Cu nanospheres.
Figure 4. FESEM images of the discharged products (metallic copper) of Li2O-Cu2O (Li:Cu ) 1:1) thin-film electrodes on Cu foil obtained after different charge-discharge cycles. (a) 1, (b) 5, and (c) 50 cycles. The current density was 0.1mA/cm2. The inset in panel b is a side-view SEM image.
Figure 5. (a) TEM image, (b) the associated SAED pattern, and (c) HRTEM image of one truncated pyramid-like Cu nanoplates, suggesting the single-crystalline nature.
SCHEME 1: Possible Formation Mechanism of Cu Nanospheres and Pyramids
cycle numbers and the kinds of substrates on the morphologies of Cu nanoparticles are undoubtedly significant. During the discharging process, copper oxide can be reduced to Cu particles. When the electrode is charged, Cu particles were partially oxidized to Cu2O or CuO. When the electrode continues to be discharged, the newly produced Cu2O or CuO particles will be converted into Cu nanoparticles. In the following discharge process, some Cu particles will be deposited on either the surface of previously produced Cu nanoparticles or directly on the substrate. The particle size decreases with the increasing number of charge-discharge cycles, as we observed from above SEM
images. In addition, the density of Cu truncated pyramidal nanoparticles on the substrate is obviously higher than that of pyramid-like Cu particles, as displayed in Figure 4, confirming our explanation mentioned above. On the basis of experimental results and discussion, a possible mechanism for the formation of Cu spherical nanoparticles and truncated pyramidal nanoparticles is displayed in Scheme 1. As for the obvious difference in particles morphology when Cu foil and Ni foam were used as deposition substrate, we preliminarily believe that the degree of crystal lattice’ match between Cu and substrate may be the key reason, as reported in the preparation of nanostructures
Synthesis of Copper Nanoparticles using chemical vapor deposition (CVD) and solution-phase deposition.30-31 It is thought that the formation of the unique truncated pyramidal Cu on the Cu foil substrate could be attributed to some epitaxial effect of the copper growth. By an ultrasonic cleaning process, limited weak contact between pyramidal or truncated pyramidal nanoparticles and the substrate can be broken. Note that more systematic work needs to be done to further understand the formation mechanism of Cu nanomaterials under electrochemical charge-discharge method, the correlation between the substrate roughness and the epitaxial growth of Cu, which are now underway in our group. Conclusions In summary, a simple electrochemical method based on the charge-discharge process of as-prepared Li2O-Cu2O (Li:Cu ) 1:1) films on Ni foam substrates has been developed to synthesize Cu nanospheres. We found that different substrates have a noticeable influence on the morphologies of Cu products. By choosing Cu foil as the deposition substrate, pyramidal and single-crystalline truncated pyramidal Cu nanoparticles have been successfully fabricated. In addition, the sizes and morphologies of Cu particles have been modulated by controlling the charge-discharge cycles of the Li2O-Cu2O film electrode. A possible formation mechanism was also discussed. Importantly, the electrochemical technique can be potentially extended to the fabrication of other metal nanoparticles with different morphologies by choosing the proper substrate. These Cu nanoparticles are expected to find wide applications in catalysts, nanofluids, and surface-Raman scattering. Furthermore, the sizecontrolled synthesis of metal nanomaterials with controllable morphologies will offer great opportunities to explore the dependence of novel properties of nanomaterials on their morphology and size and also be essential for the manufacture of potential optoelectronic devices. Acknowledgment. This work is financially supported by National Science Foundation of China (No. 50372064) and the start-up funds from Florida International University. References and Notes (1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27.
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