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Synthesis of sheaf-like CuO from aqueous solution and their application in lithium-ion batteries
Journal of Alloys and Compounds 484 (2009) 322–326
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis of sheaf-like CuO from aqueous solution and their application in lithium-ion batteries Qingtao Pan, Kai Huang, Shibing Ni, Feng Yang, Shumei Lin, Deyan He ∗ Department of Physics, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 19 March 2009 Received in revised form 16 April 2009 Accepted 17 April 2009 Available online 23 April 2009 Keywords: Oxides Nanostructures Chemical synthesis Energy storage
a b s t r a c t Sheaf-like CuO nanostructures have been synthesized by a simple hydrothermal process conducted at 120 ◦ C for 24 h. The crystalline structure and morphology of the as-synthesized powder have been characterized by using X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM). And the growth process of the sheaf-like nanostructures was also clarified. The electrochemical performance as anode material for lithium-ion batteries was further evaluated by charge–discharge measurements. It was found that the sheaf-like CuO electrode can exhibit a high initial discharge capacity of 965 mAh/g. After 41 cycles, the electrode can deliver a capacity of 580 mAh/g. And a high rate capability was also obtained. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the past several years, the large-scale self-assembly of inorganic nanocrystals with different sizes and shapes has been strongly motivated by the requirement to uncover and map their size- and shape-properties and to achieve their applications [1–3]. Until now, much excellent research on the shape-controlled synthesis of inorganic crystals has been reported, such as rods/wires [4,5], belts [6], flowers [7], and dendrites [8–10]. As a p-type semiconductor with a narrow band gap (1.4 eV), CuO has attracted a great deal of attention owing to their important applications, such as high temperature superconductors and giant magneto resistance materials [11]. Furthermore, it has also been shown to act as a rechargeable electrode material that reacts with two Li per formula unit, exhibiting higher capacity than the carbonaceous substances (e.g. maximum of 372 mAh/g for graphite) that are used currently in commercial lithium-ion batteries [12]. It has been considered that most of these functions not only depended strongly on the structure but also on the morphology of the electrode components [13]. It is evident that the purpose of preparing CuO in various sizes and shapes has been driven by the strong interest in their novel properties and potential applications [14,15]. To date, complex CuO architectures such as dandelions-like hollow microspheres [16], pricky-like microspheres [17] and dendrite-like nanostructures [18]
∗ Corresponding author. Fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.090
have been obtained by hydrothermal method and wet chemical method. However, most of the methods involved environmentally malignant chemicals and organic solvents, which were toxic and not easily degraded in the environment. Recently, the sheaf-like CuO nanostructures were prepared assisted by ionic-liquids [19], which might introduce impurities into the products. In this letter, we successfully prepared the sheaf-like CuO nanostructures by hydrothermal method, and electrochemical properties of their as anode materials for lithium-ion batteries were investigated. 2. Experimental All the chemical reagents were analytically pure and used without further purification. Sheaf-like CuO were synthesized as follows: in a typical experimental procedure, 5 mmol Cu(NO3 )2 ·H2 O and (CH2 )6 N4 were dissolved in 34 ml of distilled water. After stirring for several minutes, the obtained clear solution was transferred into a 45 ml Teflonlined autoclave. The autoclave was maintained at 120 ◦ C for 24 h. After the mixture cooled naturally to room temperature, the product was centrifugally separated and repeatedly washed with ethanol and distilled water to remove the impurities. Finally, the samples were dried at 60 ◦ C for 8 h. The as-prepared products were characterized by powder X-ray powder diffraction (XRD) on a Rigaku D/max-RA X-ray diffractometer with Cu K␣ radiation ( = 0.15406 nm). The morphologies and sizes of the products were determined by field-emission scanning electron microscopy (FESEM, S-4800, Hitachi) and transmission electron microscope (TEM, H-800, Hitachi). X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB MKII with Mg K␣ (h = 1253.6 eV) as the exciting source at a pressure of 1.0 × 10−4 Pa and a resolution of 1.00 eV. The data of the binding energies were corrected for specimen charging by reference the C1s to 284.8 eV. In the process of fabricating the Li-ion battery, electrodes were prepared by compressing a mixture of sheaf-like CuO (80 wt.%), acetylene black (10 wt.%), and polyvinylidene fluoride (PVDF, 10 wt.%) on copper foil. Coin-type cells (2032) of Li/1 M LiPF6 in ethylene carbonate, diethyl carbonate and ethyl methyl carbonate (EC/DMC/EMC, 1:1:1, v/v/v)/CuO were assembled in an argon-filled dry box. A Cel-
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323
Fig. 3. TEM images of a single complex nanostructure: (a) low and (b) high magnification. Fig. 1. XRD pattern of the sample prepared at 120 ◦ C for 24 h by the hydrothermal route. gard 2320 microporous polypropylene was used as the separator membrane. The weight of active material on each CuO electrode was between 1 and 2 mg. The cells thus fabricated were cycled galvanostatically in the voltage range between 0.02 and 3.0 V with a multichannel battery test system (Neware BTS-610).
3. Results and discussion Fig. 1 shows the XRD pattern of as-synthesized powder prepared using the hydrothermal method. As observed, the positions of the characteristic peaks of the products are consistent with the literature values (JCPDS 48-1548), suggesting that there are no other compounds present in the final products. The sharp peaks confirm the high crystallinity of the products. Fig. 2a shows the low magnification of SEM image of the asprepared CuO products. It can be seen that the complex CuO nanostructures are obtained on a large scale. Enlarged SEM image
displayed in Fig. 2b further conforms the complex configurations, which are made of two to four half-sheaves originating from the same core. Furthermore, the half-sheaf structure with a folding fanshape is also obtained. Fig. 2c shows a single complex structure, which are composed of well-aligned nanoplateletes with length up to ∼4 m, looking like a sheaf of straw tied in the middle. It can be seen that the nanoplateletes are ellipsoidal and the thickness of the CuO nanoplateletes is 30–60 nm. Fig. 2d shows a single half-sheaf structure with a folding fan-shape. As can be seen, the nanoplateletes are tied at one end, and the individual nanoplatelet has an average width of ∼250 nm and sheaves are ∼2 m in length. Further more, we can also see that the nanoplatelets may be assembly by the nanoparticles. Fig. 3a shows the typical TEM image of a single complex structure. As can be seen, a bundle of nanoplateletes crystals have been bandage in the middle, with the end fanning out, and the end is thinner than the center (Fig. 3b). The purity and the composition of the sheaf-like CuO were further investigated by X-ray photoelectron spectroscopy. Fig. 4a is the
Fig. 2. SEM images of the sample prepared at 120 ◦ C for 24 h: (a) low magnification, (b) high magnification, (c) a single complex nanostructure and (d) a single half-sheaf with folding fan-like shape.
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Fig. 4. XPS spectra of sheaf-like CuO: (a) typical XPS survey spectrum; (b) close-up surveys for Cu 2p core; (c) close-up survey for O 1s core, with a peak deconvoluted by Gaussian function.
Fig. 5. TEM images of samples obtained at 120 ◦ C for different times: (a and b) 4 h, (c and d) 8 h and (e and f) 16 h.
Q. Pan et al. / Journal of Alloys and Compounds 484 (2009) 322–326
typical survey spectrum, showing the presence of only Cu and O without any other obvious contaminant species, except for a small amount of adsorbed carbon was used to calibrate the acquired spectra, and Al from the aluminum substrate used to support the CuO samples. Fig. 4b and c shows the XPS spectra of binding energy of Cu 2p and O 1s, respectively. The Cu 2p peaks at 933.6 and 953.5 eV correspond to the binding energy of Cu 2p3/2 and Cu 2p1/2 , respectively, which is consistent with observed one in CuO [20,21]. Moreover, the Cu 2p core-level XPS spectrum of the as-synthesized CuO powders displays several strong satellite peaks. As shown in Fig. 4c, the O 1s core-level spectrum is broad, and two O 1s peaks (marked as i and ii) were resolved by using a curve-fitting procedure. The binding energies have been identified as O2− in CuO at 529.6 eV [peak (i)], and oxygen adsorbed onto the surface of CuO sheaves at 531.6 eV [peak (ii)]. Thus, the XPS results support the that the sample is composed only of CuO. In order to clarify the growth process of the sheaf-like CuO nanostructures, the samples formed at 120 ◦ C for different times were examined by TEM and XRD. Fig. 5 (see a–f) shows the change in morphology of as-prepared CuO as a function of time. When the hydrothermal reaction was processed for 4 h, it can be seen that the majority are nanoparticles and the minority are sheaf-like CuO with small sizes (Fig. 5a and b). With the reaction time prolonged to 8 h, sheaf-like CuO will grow and more are obtained (Fig. 5c and d). Fig. 5e and f are the images of the products after reacting for 16 h, revealing that only sheaf-like CuO (with larger sizes) existed. Fig. 6 shows the XRD pattern of the products processed for different times. Fig. 6a shows that the products processed for 4 h are mainly Cu2 (OH)3 NO3 , only three small peaks are indexed to CuO. When the reaction time was prolonged to 8 h, the XRD pattern is shown in Fig. 6b. As can be seen, more CuO peaks appear. With the reaction time increasing to 16 h, almost all of the peaks can be indexed to CuO, only some small peaks are indexed to
Fig. 6. XRD patterns of samples obtained at 120 ◦ C for different times: (a) 4 h, (b) 8 h and (c) 16 h.
325
Scheme 1. Formation process of sheaf-like copper oxide.
Cu2 (OH)3 NO3 . For obtaining the pure CuO phase, we prolonged the reaction time to 24 h. From the observed morphologies and XRD patterns of the samples processed for different times, it is possible to interpret the formation process of the sheaf-like structures as follows: (1) under the hydrothermal conditions, (CH2 )6 N4 will hydrolyze and release NH3 . In the alkaline solution, Cu2 (OH)3 NO3 nanoparticles are initially formed and the initially formed nanoparticles tend to directionally self-aggregate or self-organize slowly to form sheaf-like nanostructures through a possible oriented attachment process. (2) With the reaction continuing, sheaf-like Cu2 (OH)3 NO3 will grow in size, and part of the Cu2 (OH)3 NO3 will be decomposed to CuO. (3) When the reaction time continuing enough, all the Cu2 (OH)3 NO3 will be decomposed to CuO. The formation process is summarized in Scheme 1. The discharge–charge curves of the electrode made by the sheaflike CuO at the discharging and charging rate of C/2 at room temperature are shown in Fig. 7a. In the discharge curves of the first cycle, there are two obvious potential plateaus around 1.45 and 0.9 V, followed by a gradual decrease in the potential down to
Fig. 7. (a) Discharge–charge plots and (b) cycle performance of the sheaf-like CuO electrodes with the cut-off voltage of 0.02 and 3.0 V (versus Li). Rate: C/2.
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4. Conclusions In summary, sheaf-like CuO nanostructures were synthesized by hydrothermal method. The growth process of the sheaf-like CuO was clarified. Probably due to the short diffusion and large surface area, the material shows a high initial discharge capacity of 965 mAh/g at a rate of C/2. After 41 cycles, the electrode can deliver a capacity of 580 mAh/g. And the sheaf-like CuO electrodes show a high rate capability. Acknowledgments The authors appreciate the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education of China (20040730029) and Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China. Fig. 8. Discharge capacity of the sheaf-like electrode at different discharge–charge rates (0.5 C, 1.5 C and 2.5 C).
0.02 V. For the 40th and 41st cycles, the amplitude of the plateau is markedly reduced, so that only a discharge slope is observed. The initial discharge capacity of the electrode is 965 mAh/g, which is larger than the normal CuO powders from the decomposition of Cu(OH)2 of 540 mAh/g [22], and the initial Coulombic efficiency is 98%. Based on a maximum uptake of 2Li/CuO, the theoretical of 674 mAh/g is attained [22]. The large excesses observed in capacity may originate from the decomposition of the electrolyte and subsequent formation of an organic layer deposited on the surface of the particles [23]. Fig. 7b shows the charge–discharge capacities and charge–discharge efficiencies of the sheaf-like CuO electrode as a function of cycle number. Solid and open circles denote charge and discharge capacities, respectively, and solid squares denote charge–discharge efficiencies. The capacities gradually decrease with increasing the cycle number. After 41 cycles, the capacity is still as high as 580 mAh/g, which is larger than graphite of 372 mAh/g, indicating the good discharge properties. Another excellent property associated with this CuO electrode is its high rate capability. The discharge capacity as a function of the cycling number at different rates is illustrated in Fig. 8. As can be seen, the sheaf-like CuO electrodes can deliver 645 mAh/g after 21 cycles at a rate of 0.5 C; then this value decreases gradually to 341 mAh/g after another 21 cycles at a rate of 1.5 C; the electrode still have a capacity of 270 mAh/g after further 10 cycles at a rate of 2.5 C. The high capacity of the two electrodes mainly originates from their sheaf-like nanostructures, such as short diffusion paths both for electrons and lithium ions as well as large electrode–electrolyte contact area [24,25].
References [1] S. Sun, C.B. Murry, D. Weller, L. Folks, A. Moster, Science 287 (2000) 1989. [2] J.F. Wang, M.S. Gudiksen, X.F. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1455. [3] W.D. Yu, W.Y. Vivian, J. Am. Chem. Soc. 126 (2004) 13200. [4] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137. [5] Z.D. Xiao, L.D. Zhang, Z.Y. Wang, Q.F. Lu, X.K. Tian, H.B. Zeng, Mater. Lett. 61 (2007) 1718. [6] Z.P. Liu, S. Li, Y. Yang, S. Peng, Z.K. Hu, Y.T. Qian, Adv. Mater. 15 (2003) 1946. [7] X.D. Gao, X.M. Li, W.D. Yu, J. Phys. Chem. B 109 (2005) 1155. [8] M.H. Cao, T.F. Liu, S. Gao, G.B. Sun, X.L. Wu, C.W. Hu, Z.L. Wang, Angew. Chem. Int. Ed. 44 (2005) 2. [9] D.B. Kuang, A.W. Xu, Y.P. Fang, H.Q. Liu, C. Frommen, D. Fenske, Adv. Mater. 15 (2003) 1747. [10] Q.T. Pan, K. Huang, S.B. Ni, Q. Wang, F. Yang, D.Y. He, Mater. Lett. 61 (2007) 4773. [11] X.G. Zheng, C.N. Xu, Y. Tomokiyo, E. Tanaka, H. Yamada, Y. Soejima, Phys. Rev. Lett. 85 (2000) 5170. [12] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [13] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [14] D. Chen, G.Z. Shen, K.B. Tang, Y.T. Qian, J. Cryst. Growth 254 (2003) 225. [15] M. Vaseem, A. Umar, S.H. Kim, A. Al-Hajry, Y.B. Hahn, Mater. Lett. 62 (2008) 1659. [16] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 5124. [17] Y.J. He, Mater. Res. Bull. 42 (2007) 190. [18] H. Zhang, S.Z. Li, X.Y. Ma, D.R. Yang, Mater. Res. Bull. 43 (2008) 1291. [19] M. Zhang, X.D. Xu, M.L. Zhang, Mater. Lett. 62 (2008) 385. [20] W. Wang, Z. Liu, Y. Liu, C. Xu, C. Zeng, G. Wang, Appl. Phys. A Mater. Sci. Process. 76 (2003) 417. [21] Y.C. Zhang, J.Y. Tang, G.L. Wang, M. Zhang, X.Y. Hu, J. Cryst. Growth 294 (2006) 278. [22] P. Novak, B. Klapste, P. Podhajecky, J. Power Sources 15 (1985) 101. [23] D. Larcher, C. Masquecier, D. Bonnin, Y. Chabre, V. Masson, J.B. Leriche, J.M. Tarascona, J. Electrochem. Soc. 150 (2003) A133. [24] Q.M. Pan, H.Z. Jin, H.B. Wang, G.P. Yin, Electrochim. Acta 53 (2007) 951. [25] C.J. Curtis, J. Wang, D.C. Schulz, J. Electrochem. Soc. 151 (2004) A590.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis of sheaf-like CuO from aqueous solution and their application in lithium-ion batteries Qingtao Pan, Kai Huang, Shibing Ni, Feng Yang, Shumei Lin, Deyan He ∗ Department of Physics, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 19 March 2009 Received in revised form 16 April 2009 Accepted 17 April 2009 Available online 23 April 2009 Keywords: Oxides Nanostructures Chemical synthesis Energy storage
a b s t r a c t Sheaf-like CuO nanostructures have been synthesized by a simple hydrothermal process conducted at 120 ◦ C for 24 h. The crystalline structure and morphology of the as-synthesized powder have been characterized by using X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM). And the growth process of the sheaf-like nanostructures was also clarified. The electrochemical performance as anode material for lithium-ion batteries was further evaluated by charge–discharge measurements. It was found that the sheaf-like CuO electrode can exhibit a high initial discharge capacity of 965 mAh/g. After 41 cycles, the electrode can deliver a capacity of 580 mAh/g. And a high rate capability was also obtained. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the past several years, the large-scale self-assembly of inorganic nanocrystals with different sizes and shapes has been strongly motivated by the requirement to uncover and map their size- and shape-properties and to achieve their applications [1–3]. Until now, much excellent research on the shape-controlled synthesis of inorganic crystals has been reported, such as rods/wires [4,5], belts [6], flowers [7], and dendrites [8–10]. As a p-type semiconductor with a narrow band gap (1.4 eV), CuO has attracted a great deal of attention owing to their important applications, such as high temperature superconductors and giant magneto resistance materials [11]. Furthermore, it has also been shown to act as a rechargeable electrode material that reacts with two Li per formula unit, exhibiting higher capacity than the carbonaceous substances (e.g. maximum of 372 mAh/g for graphite) that are used currently in commercial lithium-ion batteries [12]. It has been considered that most of these functions not only depended strongly on the structure but also on the morphology of the electrode components [13]. It is evident that the purpose of preparing CuO in various sizes and shapes has been driven by the strong interest in their novel properties and potential applications [14,15]. To date, complex CuO architectures such as dandelions-like hollow microspheres [16], pricky-like microspheres [17] and dendrite-like nanostructures [18]
∗ Corresponding author. Fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.090
have been obtained by hydrothermal method and wet chemical method. However, most of the methods involved environmentally malignant chemicals and organic solvents, which were toxic and not easily degraded in the environment. Recently, the sheaf-like CuO nanostructures were prepared assisted by ionic-liquids [19], which might introduce impurities into the products. In this letter, we successfully prepared the sheaf-like CuO nanostructures by hydrothermal method, and electrochemical properties of their as anode materials for lithium-ion batteries were investigated. 2. Experimental All the chemical reagents were analytically pure and used without further purification. Sheaf-like CuO were synthesized as follows: in a typical experimental procedure, 5 mmol Cu(NO3 )2 ·H2 O and (CH2 )6 N4 were dissolved in 34 ml of distilled water. After stirring for several minutes, the obtained clear solution was transferred into a 45 ml Teflonlined autoclave. The autoclave was maintained at 120 ◦ C for 24 h. After the mixture cooled naturally to room temperature, the product was centrifugally separated and repeatedly washed with ethanol and distilled water to remove the impurities. Finally, the samples were dried at 60 ◦ C for 8 h. The as-prepared products were characterized by powder X-ray powder diffraction (XRD) on a Rigaku D/max-RA X-ray diffractometer with Cu K␣ radiation ( = 0.15406 nm). The morphologies and sizes of the products were determined by field-emission scanning electron microscopy (FESEM, S-4800, Hitachi) and transmission electron microscope (TEM, H-800, Hitachi). X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB MKII with Mg K␣ (h = 1253.6 eV) as the exciting source at a pressure of 1.0 × 10−4 Pa and a resolution of 1.00 eV. The data of the binding energies were corrected for specimen charging by reference the C1s to 284.8 eV. In the process of fabricating the Li-ion battery, electrodes were prepared by compressing a mixture of sheaf-like CuO (80 wt.%), acetylene black (10 wt.%), and polyvinylidene fluoride (PVDF, 10 wt.%) on copper foil. Coin-type cells (2032) of Li/1 M LiPF6 in ethylene carbonate, diethyl carbonate and ethyl methyl carbonate (EC/DMC/EMC, 1:1:1, v/v/v)/CuO were assembled in an argon-filled dry box. A Cel-
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323
Fig. 3. TEM images of a single complex nanostructure: (a) low and (b) high magnification. Fig. 1. XRD pattern of the sample prepared at 120 ◦ C for 24 h by the hydrothermal route. gard 2320 microporous polypropylene was used as the separator membrane. The weight of active material on each CuO electrode was between 1 and 2 mg. The cells thus fabricated were cycled galvanostatically in the voltage range between 0.02 and 3.0 V with a multichannel battery test system (Neware BTS-610).
3. Results and discussion Fig. 1 shows the XRD pattern of as-synthesized powder prepared using the hydrothermal method. As observed, the positions of the characteristic peaks of the products are consistent with the literature values (JCPDS 48-1548), suggesting that there are no other compounds present in the final products. The sharp peaks confirm the high crystallinity of the products. Fig. 2a shows the low magnification of SEM image of the asprepared CuO products. It can be seen that the complex CuO nanostructures are obtained on a large scale. Enlarged SEM image
displayed in Fig. 2b further conforms the complex configurations, which are made of two to four half-sheaves originating from the same core. Furthermore, the half-sheaf structure with a folding fanshape is also obtained. Fig. 2c shows a single complex structure, which are composed of well-aligned nanoplateletes with length up to ∼4 m, looking like a sheaf of straw tied in the middle. It can be seen that the nanoplateletes are ellipsoidal and the thickness of the CuO nanoplateletes is 30–60 nm. Fig. 2d shows a single half-sheaf structure with a folding fan-shape. As can be seen, the nanoplateletes are tied at one end, and the individual nanoplatelet has an average width of ∼250 nm and sheaves are ∼2 m in length. Further more, we can also see that the nanoplatelets may be assembly by the nanoparticles. Fig. 3a shows the typical TEM image of a single complex structure. As can be seen, a bundle of nanoplateletes crystals have been bandage in the middle, with the end fanning out, and the end is thinner than the center (Fig. 3b). The purity and the composition of the sheaf-like CuO were further investigated by X-ray photoelectron spectroscopy. Fig. 4a is the
Fig. 2. SEM images of the sample prepared at 120 ◦ C for 24 h: (a) low magnification, (b) high magnification, (c) a single complex nanostructure and (d) a single half-sheaf with folding fan-like shape.
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Fig. 4. XPS spectra of sheaf-like CuO: (a) typical XPS survey spectrum; (b) close-up surveys for Cu 2p core; (c) close-up survey for O 1s core, with a peak deconvoluted by Gaussian function.
Fig. 5. TEM images of samples obtained at 120 ◦ C for different times: (a and b) 4 h, (c and d) 8 h and (e and f) 16 h.
Q. Pan et al. / Journal of Alloys and Compounds 484 (2009) 322–326
typical survey spectrum, showing the presence of only Cu and O without any other obvious contaminant species, except for a small amount of adsorbed carbon was used to calibrate the acquired spectra, and Al from the aluminum substrate used to support the CuO samples. Fig. 4b and c shows the XPS spectra of binding energy of Cu 2p and O 1s, respectively. The Cu 2p peaks at 933.6 and 953.5 eV correspond to the binding energy of Cu 2p3/2 and Cu 2p1/2 , respectively, which is consistent with observed one in CuO [20,21]. Moreover, the Cu 2p core-level XPS spectrum of the as-synthesized CuO powders displays several strong satellite peaks. As shown in Fig. 4c, the O 1s core-level spectrum is broad, and two O 1s peaks (marked as i and ii) were resolved by using a curve-fitting procedure. The binding energies have been identified as O2− in CuO at 529.6 eV [peak (i)], and oxygen adsorbed onto the surface of CuO sheaves at 531.6 eV [peak (ii)]. Thus, the XPS results support the that the sample is composed only of CuO. In order to clarify the growth process of the sheaf-like CuO nanostructures, the samples formed at 120 ◦ C for different times were examined by TEM and XRD. Fig. 5 (see a–f) shows the change in morphology of as-prepared CuO as a function of time. When the hydrothermal reaction was processed for 4 h, it can be seen that the majority are nanoparticles and the minority are sheaf-like CuO with small sizes (Fig. 5a and b). With the reaction time prolonged to 8 h, sheaf-like CuO will grow and more are obtained (Fig. 5c and d). Fig. 5e and f are the images of the products after reacting for 16 h, revealing that only sheaf-like CuO (with larger sizes) existed. Fig. 6 shows the XRD pattern of the products processed for different times. Fig. 6a shows that the products processed for 4 h are mainly Cu2 (OH)3 NO3 , only three small peaks are indexed to CuO. When the reaction time was prolonged to 8 h, the XRD pattern is shown in Fig. 6b. As can be seen, more CuO peaks appear. With the reaction time increasing to 16 h, almost all of the peaks can be indexed to CuO, only some small peaks are indexed to
Fig. 6. XRD patterns of samples obtained at 120 ◦ C for different times: (a) 4 h, (b) 8 h and (c) 16 h.
325
Scheme 1. Formation process of sheaf-like copper oxide.
Cu2 (OH)3 NO3 . For obtaining the pure CuO phase, we prolonged the reaction time to 24 h. From the observed morphologies and XRD patterns of the samples processed for different times, it is possible to interpret the formation process of the sheaf-like structures as follows: (1) under the hydrothermal conditions, (CH2 )6 N4 will hydrolyze and release NH3 . In the alkaline solution, Cu2 (OH)3 NO3 nanoparticles are initially formed and the initially formed nanoparticles tend to directionally self-aggregate or self-organize slowly to form sheaf-like nanostructures through a possible oriented attachment process. (2) With the reaction continuing, sheaf-like Cu2 (OH)3 NO3 will grow in size, and part of the Cu2 (OH)3 NO3 will be decomposed to CuO. (3) When the reaction time continuing enough, all the Cu2 (OH)3 NO3 will be decomposed to CuO. The formation process is summarized in Scheme 1. The discharge–charge curves of the electrode made by the sheaflike CuO at the discharging and charging rate of C/2 at room temperature are shown in Fig. 7a. In the discharge curves of the first cycle, there are two obvious potential plateaus around 1.45 and 0.9 V, followed by a gradual decrease in the potential down to
Fig. 7. (a) Discharge–charge plots and (b) cycle performance of the sheaf-like CuO electrodes with the cut-off voltage of 0.02 and 3.0 V (versus Li). Rate: C/2.
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Q. Pan et al. / Journal of Alloys and Compounds 484 (2009) 322–326
4. Conclusions In summary, sheaf-like CuO nanostructures were synthesized by hydrothermal method. The growth process of the sheaf-like CuO was clarified. Probably due to the short diffusion and large surface area, the material shows a high initial discharge capacity of 965 mAh/g at a rate of C/2. After 41 cycles, the electrode can deliver a capacity of 580 mAh/g. And the sheaf-like CuO electrodes show a high rate capability. Acknowledgments The authors appreciate the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education of China (20040730029) and Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China. Fig. 8. Discharge capacity of the sheaf-like electrode at different discharge–charge rates (0.5 C, 1.5 C and 2.5 C).
0.02 V. For the 40th and 41st cycles, the amplitude of the plateau is markedly reduced, so that only a discharge slope is observed. The initial discharge capacity of the electrode is 965 mAh/g, which is larger than the normal CuO powders from the decomposition of Cu(OH)2 of 540 mAh/g [22], and the initial Coulombic efficiency is 98%. Based on a maximum uptake of 2Li/CuO, the theoretical of 674 mAh/g is attained [22]. The large excesses observed in capacity may originate from the decomposition of the electrolyte and subsequent formation of an organic layer deposited on the surface of the particles [23]. Fig. 7b shows the charge–discharge capacities and charge–discharge efficiencies of the sheaf-like CuO electrode as a function of cycle number. Solid and open circles denote charge and discharge capacities, respectively, and solid squares denote charge–discharge efficiencies. The capacities gradually decrease with increasing the cycle number. After 41 cycles, the capacity is still as high as 580 mAh/g, which is larger than graphite of 372 mAh/g, indicating the good discharge properties. Another excellent property associated with this CuO electrode is its high rate capability. The discharge capacity as a function of the cycling number at different rates is illustrated in Fig. 8. As can be seen, the sheaf-like CuO electrodes can deliver 645 mAh/g after 21 cycles at a rate of 0.5 C; then this value decreases gradually to 341 mAh/g after another 21 cycles at a rate of 1.5 C; the electrode still have a capacity of 270 mAh/g after further 10 cycles at a rate of 2.5 C. The high capacity of the two electrodes mainly originates from their sheaf-like nanostructures, such as short diffusion paths both for electrons and lithium ions as well as large electrode–electrolyte contact area [24,25].
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