Low-temperature synthesis of CuO-interlaced nanodiscs for lithium ion battery electrodes
© Seo et al; licensee Springer. 2011
Received: 15 February 2011
Accepted: 26 May 2011
Published: 26 May 2011
In this study, we report the high-yield synthesis of 2-dimensional cupric oxide (CuO) nanodiscs through dehydrogenation of 1-dimensional Cu(OH)2 nanowires at 60°C. Most of the nanodiscs had a diameter of approximately 500 nm and a thickness of approximately 50 nm. After further prolonged reaction times, secondary irregular nanodiscs gradually grew vertically into regular nanodiscs. These CuO nanostructures were characterized using X-ray diffraction, transmission electron microscopy, and Brunauer-Emmett-Teller measurements. The possible growth mechanism of the interlaced disc CuO nanostructures is systematically discussed. The electrochemical performances of the CuO nanodisc electrodes were evaluated in detail using cyclic voltammetry and galvanostatic cycling. Furthermore, we demonstrate that the incorporation of multiwalled carbon nanotubes enables the enhanced reversible capacities and capacity retention of CuO nanodisc electrodes on cycling by offering more efficient electron transport paths.
Inexpensive, environmentally innocuous, and easily producible cupric oxide (CuO) is an important p-type semiconductor with a bandgap of 1.2 eV that is widely studied in applications, including catalysts, gas sensors, photoconductive/photochemical cells, and other electronic devices [1–5]. Additionally, a great effort has recently been applied to the nanostructuring of CuO as it can deliver much higher reversible capacities than commercial graphite-based electrodes through the conversion reaction with Li (CuO + 2e- + 2Li+ ↔ Cu0 + Li2O). Thus, various CuO nanostructures (nanoparticles, nanowires, nanorods, nanotubes) have been shown to be good candidates as electrodes for lithium ion batteries [6–8]. Zhang et al. reported the size dependency of the electrochemical properties in zero-dimensional CuO nanoparticles synthesized by thermal decomposition of CuC2O4 precursor at 400°C . One-dimensional (1-D) CuO nanorod and nanowire CuO electrodes have also been produced via hydrothermal and wet chemical methods for enhanced reversible capacity [10, 11]. Recently, two-dimensional (2-D) CuO nanoribbons and other three-dimensional hierarchical nanostructures such as dendrites and spheres, assembled with nanoneedles, have been reported as high-performance anodes for Li ion batteries [12–14].
Herein, we demonstrate a low-temperature and large-scale conversion of initially prepared 1-D Cu(OH)2 nanowires into 2-D CuO nanodiscs and further vertically interlaced nanodisc structures. The detailed morphological evolution during the growth of the nanostructured CuO was examined by controlling the reaction conditions, such as synthesis time and temperature. The electrochemical reaction of Li with the obtained CuO nanodiscs was investigated by cyclic voltammetry (CV) and galvanostatic cycling. Furthermore, the enhanced reversible capacities and capacity retention in the CuO nanodisc composite electrodes, by the incorporation of multiwalled carbon nanotubes (MWCNTs), are reported by offering better efficient electron transport paths.
Cu(OH)2 nanowire precursors were prepared by a simple chemical solution route at room temperature . First, 30 mL of 0.15 M NH4OH (28-30% as ammonia, NH3, Dae-Jung Chemical, Shiheung, South Korea) was added to 100 mL of 0.04 M copper (II) sulfate pentahydrate (CuSO4·5H2O, 99.5%, JUNSEI Chemical, Tokyo, Japan), followed by drop-wise addition of 6.0 mL of 1.2 M NaOH (98%, Dae-Jung Chemical, Shiheung, South Korea) under magnetic stirring. The Cu(OH)2 precipitate appeared in the blue solution. The as-prepared solution containing the Cu(OH)2 precursor was stored at room temperature for 1 h and heat-treated at 60°C for 3 h in a convection oven to produce CuO nanostructures. The black powders were centrifuged and washed with deionized water and ethanol several times and were dried overnight at 70°C in a vacuum oven.
For preparation of the multiwalled carbon nanotube (MWCNT)/CuO composites, a calculated amount (60 mg) of synthetic multiwalled carbon nanotubes (CNT Co., Ltd., Incheon, South Korea) was first dispersed and sonicated for 3 h in 100 mL deionized water in the presence of cetyltrimethylammonium bromide (CTAB, 99%, 0.2 mg, Sigma-Aldrich, Saint Louis, MO, USA) . After complete dispersion of the MWCNTs, the same steps as those for the CuO nanopowders were followed.
The crystal structures and morphologies of each powder were investigated using X-ray powder diffraction (XRD; model D/MAX-2500V/PC, Rigaku, Tokyo, Japan), field emission scanning electron microscopy (FESEM; model JSM-6330F, JEOL, Tokyo, Japan), and high-resolution transmission electron microscopy (HRTEM; model JEM-3000F, JEOL, Tokyo, Japan). Additionally, the specific surface areas were examined using the Brunauer-Emmett-Teller (BET; Belsorp-mini, BEL Japan Inc., Osaka, Japan) method with a nitrogen adsorption/desorption process.
The electrochemical performance of each powder was evaluated by assembling Swagelok-type half cells, using a Li metal foil as the negative electrode. Positive electrodes were cast on Cu foil by mixing prepared powders (1.0-2.0 mg) with Super P carbon black (MMM Carbon, Brussels, Belgium) and the Kynar 2801 binder (PVdF-HFP) at a mass ratio of 70:15:15 in 1-methyl-2-pyrrolidinone (NMP; Sigma-Aldrich, St. Louis. MO, USA). A separator film of Celgard 2400 and liquid electrolyte (ethylene carbonate and dimethyl carbonate (1:1 by volume) with 1.0 M LiPF6, Techno Semichem Co., Ltd., Seongnam, South Korea) was also used. The assembled cells were galvanostatically cycled between 3.0 and 0.01 V using an automatic battery cycler (WBCS 3000, WonaTech, Seoul, South Korea). All cyclic voltammetry measurements were carried out at a scanning rate of 0.1 mV s-1.
Results and discussions
Figure 1b shows the low magnification FESEM image of CuO powders. It can be clearly observed that uniform 2-D disc-like morphologies with an average diameter of 500-700 nm and a thickness of 30-50 nm were obtained on a large scale. More interestingly, more than one standing disc was inserted into the central part of the lying discs, indicating CuO-interlaced nanodisc structures. This characteristic nanostructure was also confirmed by local contrast differences in a representative transmission electron microscopy (TEM) image of an individual disc (Figure 1c). The inset in Figure 1c depicts a typical CuO-interlaced nanodisc based on the FESEM and TEM observations. Figure 1d shows the magnified HRTEM image of the surface region in the nanodisc. The measured lattice spacings obtained from the HRTEM image were 2.76 and 2.30 Å, in accordance with the (110) and (200) planes of the monoclinic CuO structure, respectively.
After achieving a temperature of 60°C, most morphology changed suddenly to a disc shape by the acceleration of the oriented attachment (Figure 2e) because this 2-D compact nanostructure would be energetically favorable by reducing the interfacial energy of the 1-D nanowires [18, 21]. In addition, Cu(OH)2 almost completely transformed into CuO. However, a small amount of the Cu(OH)2 phase remained, supported by the presence of nanowires reminiscent of the Cu(OH)2 precurso.r With a reaction time extended to 3 h, complete conversion to CuO was observed using XRD (Figure 1a).
Another feature in this CuO nanostructure was the interlaced nanodisc morphologies, namely the vertically interconnected structure with standing nanodiscs in the center part of the lying nanodiscs (Figure 2f). The morphological evolution of each intermediate phase is schematically illustrated in Figure 2g. As a detailed transformation process from Cu(OH)2 to CuO suggested by Cudennec et al. , the possible formation mechanism of the interlaced disc nanostructures can be suggested via a different dissolution and recrystallization pathway, which can be supported by the coexistence of CuO nanodiscs and Cu(OH)2 nanowires (Figure 2e) . As the reaction time was prolonged, a Cu(OH)2 with a different dissolution rate, resulting in a different nucleation rate and secondary nucleation, may occur at high-energy sites on the surface of the primary nanodiscs . Finally, one or more secondary standing nanodiscs gradually evolved into the larger lying flat nanodiscs, finally forming interlaced disc nanostructures, as reported in similar CuO nanostructures, by hydrothermal conversion from Cu(OH)2 at 100-130°C [23, 24]. Therefore, the formation mechanism of the CuO-interlaced nanostructures during the phase conversion from Cu(OH)2 can be given via combined effects of the oriented attachment and subsequent dissolution-precipitation processes.
Figure 4c,d shows typical FESEM images of the CuO/MWCNT composite. MWCNTs were spatially dispersed in the composites without any appreciable agglomeration. In addition, the morphology of CuO in the composites was found to be mostly primary nanodiscs, not the interlaced disc nanostructures. It is believed that incorporation of MWCNT mitigated secondary nucleation and growth on the surface of the primary nanodiscs.
In summary, the successful low-temperature synthesis of phase-pure 2-D CuO-interlaced nanodiscs was demonstrated using simple dehydrogenation of 1-D Cu(OH)2 nanowires at 60°C in solution. The details of the growth aspects of the CuO-interlaced nanodiscs were suggested by the combined effects of the oriented attachment and subsequent dissolution-precipitation processes based on systematic temperature- and time-dependent morphology evolutions. These CuO nanostructures had a large surface area, approximately 60 m2 g-1, and the effects of their enhanced active sites by nanostructuring on the electrochemical performance of CuO could be further realized by the incorporation of MWCNTs.
This research was supported by Future-based Technology Development Program (Nano Fields) and the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0019116 and 2010-0029617).
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