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Facile synthesis and lithium storage performance of hollow CuO microspheres
Materials Letters 129 (2014) 5–7
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis and lithium storage performance of hollow CuO microspheres Simin He, Jinshan Li, Jun Wang, Guangcheng Yang, Zhiqiang Qiao n Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China
art ic l e i nf o Article history: Received 14 December 2013 Accepted 3 May 2014 Available online 14 May 2014 Keywords: CuO Microstructure Energy storage and conversion Hollow microsphere
a b s t r a c t Hierarchical CuO hollow microspheres (2–3 μm) have been successfully prepared via a facile hydrothermal process on a large scale by using cupric acetate as a reagent in the presence of polyvinylpyrrolidone. The hollow sphere composed by many nanorods with wall thickness of 400 nm. The surface area of the hollow CuO microspheres measured by a BET-nitrogen adsorption method is 25.7 m2/g. It is also found that such unique hierarchical architecture shows excellent electrochemical performance for lithium ion batteries. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Cupric oxide (CuO) is a popular p-type transition versatile semiconductor [1]. By controlling the morphologies which include nanospindles [2], nanowires [3,4], flower-like [5], as well as nano/ microspheres nanostructures [6–13], CuO has been widely utilized in variety of practical applications, such as gas-sensing [12,14], catalysis [9,11], solar cells, nanoenergetic materials (nEMs) [3,10] and lithium ion (Li-ion) batteries [4–7,15]. Among these morphologies, hollow spheres have recently been subjected to extensive research for their excellent characteristics [6–10,16]. The hollow spheres have less gas diffusion distance and larger specific surface area compared with bulk particles. Moreover, the hollow spheres with either mesoporous or microporous shells can provide effective transport channels and active sites which are crucial for high sensitivity and quick response [12]. It means that the ions have high mobility and reactivity to assemble and reassemble Li-ion batteries. Several methods have been developed to fabricate hollow nano/microspheres, including colloidal templating and a solvothermal method using organic solvent. However, the multistep process, the required toxic raw materials, and the contamination of the byproducts limit their exploitation. Therefore, more facile and simple synthesis strategies still should be extended. In this work, we demonstrate a facile hydrothermal process for controllable synthesization of hollow CuO microspheres. The hollow spherical shell is composed of small nanorods.
n
Corresponding author. Tel.: þ 86 816 2485072. E-mail address: [email protected] (Z. Qiao).
http://dx.doi.org/10.1016/j.matlet.2014.05.034 0167-577X/& 2014 Elsevier B.V. All rights reserved.
This interesting hierarchical structure significantly improves electrochemical performances.
2. Materials and methods Cupric acetate (CuAc, Cu(CH3COO)2 H2O) and Polyvinylpyrrolidone (PVP) were purchased from Alfa Aesar Company. Deionized water was obtained from a Millipore Milli-Q system. CuO microstructures were synthesized via a hydrothermal process. In a typical procedure, 0.8 g CuAc and 10 wt% PVP were added into 30 mL deionized water and dissolved after constantly stirring for 10 min. The solution was then transferred into a 50 mL stainless steel autoclave with a Teflon liner and heated in an oven at 120–180 1C for 12–24 h. After cooling to room temperature, the resulting products were separated via centrifugation and washed several times with deionized water and ethanol. Finally, the products were dried in air at 80 1C for 3 h. The products were characterized via X-ray diffraction (XRD, a Bruker D8 Advance diffractometer with Cu-Kα radiation (λ ¼1.5406 Å)). The particle size and morphology were visualized by field emission scanning electron microscopy (FESEM, Apollo 300) and high-resolution transmittal electronic microscopy (HRTEM, Libra 200). The surface area of the as-obtained sample was computed from the results of N2 physisorption at 77 K (model: BECKMAN SA3100 COULTER) by using Brunauer– Emmett–Teller (BET) formalism. Electrode material was prepared by dispersing 80 wt% CuO powders, 10 wt% carbon black (TIMCAL Super P), and 10 wt% polyvinyldiene fluoride in acetone. The slurry was then coated onto the etched copper-foil and dried in an air oven at 120 1C for
6
S. He et al. / Materials Letters 129 (2014) 5–7
12 h. Later the copper-foil was pressed between twin rollers and cut into circular discs with a diameter of 16 mm. The coin-type test cells (size 2016) were fabricated by assembling the composite electrode as the anode, 1 M LiPF6 dissolved in ethylene carbonate and dimethylcarbonate (1:1 by weight) was used as the electrolyte, whatman glass fiber paper (Aldrich) as the separator, and Li-foil as the counter. They were assembled by using a coin cell crimper (Hosen, Japan) in an Ar-filled glovebox. The cells were analyzed via galvanostatic cycling at 445 mA g 1 between 0.01 and 5 V versus Li þ/Li at room temperature (25 1C).
3. Results and discussions Structural analysis of the as-obtained sample was obtained by XRD. As shown in Fig. 1, all of the diffraction peaks are consistent with the monoclinic phase of CuO (JCPDS no. 48-1548). Fig. 2 represents the SEM and HRTEM images of the uniform hollow CuO microspheres synthesized at 140 1C for 16 h. The morphology of the product is in large quality and uniformity,
Fig. 1. Typical XRD pattern of the as-obtained CuO products.
and typical diameters of the spherical architectures are range 2–3 μm (Fig. 2a). The close view (Fig. 2b and c) displays that the surface of the spherical architecture is not smooth. The wall of hollow sphere is consists of many nanorods with around 400 nm in thickness. The close-up view of a crashed microsphere vividly demonstrates that the nanorods regularly connect with each other perpendicularly and align to the spherical wall, pointing toward a common center to form a hollow interior. Moreover, the BET surface area of this sample is 25.7 m2/g, which is larger than the results previously reported twinned-hemisphere [13] and Urchinlike CuO [17]. As a result of the large surface area, the as-prepared hierarchically porous CuO structures could promote the diffusion of molecular and provide numerous active sites for surface contact reactions [14]. A typical HRTEM image of a single nanowire and the selected area electron diffraction (SAED) pattern obtained from a particle-rich region (Fig. 2d). In the HRTEM image, the lattice spacing of 0.232 nm corresponds to that of the (111) crystal planes of monoclinic phase CuO. The first discharge–charge voltage profiles of CuO samples are shown in Fig. 3a. The onset of Li insertion into CuO can be observed at 2.3 V characterized by a sloping potential specific charge profile. However, once the discharge process starts, the voltage rapidly decreased to 1.2 V. When Li is inserted into CuO in the first discharge, two obvious voltage plateaus (1.2–1.0 V and 0.9–0.7 V versus Liþ/Li) are found for the reaction of Li and CuO sheets. However, for potentials below 0.5 V, the particle sizedependent growth of a solid electrolyte interphase (SEI) layer becomes evident. The behavior is similar to that described in the literatures [15]. The electrode synthesized at 140 1C for 16 h (sample c) shows a high initial specific charge beyond 881.7 mA h g 1 and first charge capacity of 586.1 mA h g 1, which is larger than that of reported CuO microspheres [6]. This result indicates that the construction of hollow microstructure is an effective way to improve the electrochemical performance. As a good controlling of mobile charge carriers, the whole sphere can be considered as a 3D current collector network and Li-ion buffering reservoirs, and it also provide negligible diffusion (short diffusion distance) to enhance electronic
Fig. 2. (a), (b) and (c) Displays typical SEM images of the hollow CuO microspheres obtained from the hydrothermal treatments at 140 1C for 16 h; (d) HRTEM image of one part of the CuO sphere and corresponding SAED spectra (inset).
S. He et al. / Materials Letters 129 (2014) 5–7
7
Fig. 3. (A) first galvanostatic discharge–charge cycling curves of the CuO samples; (B) discharge capacity versus cycle number for 50 cycles. CuO products made from (a) 160 1C for 16 h, (b) 140 1C for 18 h, and (c) 140 1C for 16 h.
conductivity. Ion-permeable porous shells contain mesopores and micropores that allow the ion diffusion toward the entire active surface are highly desired. The mesopores provide direct diffusion channels, facilitate Li-ion diffusion kinetics and fast uptake-release on the interface. The abundant quasi-micropores in the nanorods that compose the CuO shells endow the material a large quantity of active site for ion. The quasi-micropores are also considered one of the key roles for high initial discharge capacity. This feature is particularly useful when the battery is cycled at high currents, and more conductive pathways become available. With the reaction temperature increased from 140 1C to 160 1C or the reaction time increased from 16 h to 18 h, the initial charge and discharge capacity decreased and voltage plateau became lower in the charge/discharge curves. These phenomena could be described as following: with the reaction temperature increasing or the activating reaction time prolonging, the size of crystallite CuO increased gradually and CuO particles agglomerated easily. This change reduced the surface area of the anode materials and the contact area between the electrode and the electrolyte, thus the storage location for Li þ decreased and results in a weaker activity of CuO electrode. The cycling performance of the CuO products is shown in Fig. 3b. After the second cycle, a continuing increase in specific charge was observed from all of the three CuO samples. Due to the kinetically activated electrolyte degradation and the formation of an organic layer on the surface of the nanorods in the low potential region, a polymeric gel-like film grew reversibly and caused increase of the specific capacity. Accordingly, the electrode’s capacity started to decline after repeated charging and discharging. The exaggerated electrolyte degrade on the microsized particles may be caused by the increased metal-electrolyte contact area and the charge transfer resistance. Anyway, all of the above effects become more pronounced as the specific surface area and the number of grain boundaries increased with the smaller particles. As shown in Fig. 3b, the specific capacities of the three CuO particles still remain at 400 mA h g 1 after 50 cycles, which indicates excellent reversibility.
4. Conclusions In summary, hollow CuO microspheres have been synthesized via a simple hydrothermal process. The nanorods self-organized into a hierarchical hollow architecture, and the tiny holes within the spheres shells ensure good electrical contact of current collector and enhance the charge transfer or Li-ion transport. Results of electrochemical test show that the samples have a good electrochemical performance with a high initial discharge capacity and a specific capacity of 400 mA h g 1 up to 50 cycles at a rate of 445 mA g 1. It is believed that this uniform hollow architecture could be an advanced material for its potential applications in the area such as gas-sensing, optical materials and so on.
Acknowledgments This work is financially supported by National Natural Science Foundation of China (Nos. 11372288, 11202193 and 11272292). References [1] Rakhshani AE. Solid State Electron 1986;29:7–17. [2] Yuvaraj H, Jae-Jin S. Mater Lett 2014;116:5–8. [3] Kaili Z, Carole R, Marine P, Nicolas Mauran. J Microelectromech Syst 2008;17:832–6. [4] Yang Z, Wang D, Li F, et al. Mater Lett 2013;90:4–7. [5] Zhang YX, Huang M, Li F. Int J Electrochem Sci 2013;8:8645–61. [6] Gao S, Yang S, Shu J, et al. J Phys Chem C 2008;112:1932–8. [7] Ji H, Miao X, Wang L, Qian B, Yang G. Powder Technol 2013;241:43–8. [8] Deng C, Hu H, Zhu W, Han C, Shao G. Mater Lett 2011;65:575–8. [9] Liu R, Yin J, Du W, et al. Eur J Inorg Chem 2013;8:1358–62. [10] Jian G, Liu L, Zachariah MR. Adv Funct Mater 2013;23:1341–6. [11] Zhang Z, Che H, Wang Y, et al. Catal Sci Technol 2012;2:1953–60. [12] Qin Y, Zhang F, Chen Y, et al. J Phys Chem C 2012;116:11994–2000. [13] Wang C, Ye Y, Tao B, Geng B. CrystEngComm 2012;14:3677–83. [14] Sumanta Kumar M, G Ranga R. Nanoscale 2013;5:2089–99. [15] Reddy MV, Yu C, Jiahuan F, Loh KP. Appl Mater Interfaces 2013;5:4361–6. [16] Tian ZP, Zhou Y, Li ZD, Liu Q, Zou ZG. J Mater Chem A 2013;1:3575–9. [17] Xu L, Sithambaram S, Zhang Y, et al. Chem Mater 2009;21:1253–9.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile synthesis and lithium storage performance of hollow CuO microspheres Simin He, Jinshan Li, Jun Wang, Guangcheng Yang, Zhiqiang Qiao n Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China
art ic l e i nf o Article history: Received 14 December 2013 Accepted 3 May 2014 Available online 14 May 2014 Keywords: CuO Microstructure Energy storage and conversion Hollow microsphere
a b s t r a c t Hierarchical CuO hollow microspheres (2–3 μm) have been successfully prepared via a facile hydrothermal process on a large scale by using cupric acetate as a reagent in the presence of polyvinylpyrrolidone. The hollow sphere composed by many nanorods with wall thickness of 400 nm. The surface area of the hollow CuO microspheres measured by a BET-nitrogen adsorption method is 25.7 m2/g. It is also found that such unique hierarchical architecture shows excellent electrochemical performance for lithium ion batteries. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Cupric oxide (CuO) is a popular p-type transition versatile semiconductor [1]. By controlling the morphologies which include nanospindles [2], nanowires [3,4], flower-like [5], as well as nano/ microspheres nanostructures [6–13], CuO has been widely utilized in variety of practical applications, such as gas-sensing [12,14], catalysis [9,11], solar cells, nanoenergetic materials (nEMs) [3,10] and lithium ion (Li-ion) batteries [4–7,15]. Among these morphologies, hollow spheres have recently been subjected to extensive research for their excellent characteristics [6–10,16]. The hollow spheres have less gas diffusion distance and larger specific surface area compared with bulk particles. Moreover, the hollow spheres with either mesoporous or microporous shells can provide effective transport channels and active sites which are crucial for high sensitivity and quick response [12]. It means that the ions have high mobility and reactivity to assemble and reassemble Li-ion batteries. Several methods have been developed to fabricate hollow nano/microspheres, including colloidal templating and a solvothermal method using organic solvent. However, the multistep process, the required toxic raw materials, and the contamination of the byproducts limit their exploitation. Therefore, more facile and simple synthesis strategies still should be extended. In this work, we demonstrate a facile hydrothermal process for controllable synthesization of hollow CuO microspheres. The hollow spherical shell is composed of small nanorods.
n
Corresponding author. Tel.: þ 86 816 2485072. E-mail address: [email protected] (Z. Qiao).
http://dx.doi.org/10.1016/j.matlet.2014.05.034 0167-577X/& 2014 Elsevier B.V. All rights reserved.
This interesting hierarchical structure significantly improves electrochemical performances.
2. Materials and methods Cupric acetate (CuAc, Cu(CH3COO)2 H2O) and Polyvinylpyrrolidone (PVP) were purchased from Alfa Aesar Company. Deionized water was obtained from a Millipore Milli-Q system. CuO microstructures were synthesized via a hydrothermal process. In a typical procedure, 0.8 g CuAc and 10 wt% PVP were added into 30 mL deionized water and dissolved after constantly stirring for 10 min. The solution was then transferred into a 50 mL stainless steel autoclave with a Teflon liner and heated in an oven at 120–180 1C for 12–24 h. After cooling to room temperature, the resulting products were separated via centrifugation and washed several times with deionized water and ethanol. Finally, the products were dried in air at 80 1C for 3 h. The products were characterized via X-ray diffraction (XRD, a Bruker D8 Advance diffractometer with Cu-Kα radiation (λ ¼1.5406 Å)). The particle size and morphology were visualized by field emission scanning electron microscopy (FESEM, Apollo 300) and high-resolution transmittal electronic microscopy (HRTEM, Libra 200). The surface area of the as-obtained sample was computed from the results of N2 physisorption at 77 K (model: BECKMAN SA3100 COULTER) by using Brunauer– Emmett–Teller (BET) formalism. Electrode material was prepared by dispersing 80 wt% CuO powders, 10 wt% carbon black (TIMCAL Super P), and 10 wt% polyvinyldiene fluoride in acetone. The slurry was then coated onto the etched copper-foil and dried in an air oven at 120 1C for
6
S. He et al. / Materials Letters 129 (2014) 5–7
12 h. Later the copper-foil was pressed between twin rollers and cut into circular discs with a diameter of 16 mm. The coin-type test cells (size 2016) were fabricated by assembling the composite electrode as the anode, 1 M LiPF6 dissolved in ethylene carbonate and dimethylcarbonate (1:1 by weight) was used as the electrolyte, whatman glass fiber paper (Aldrich) as the separator, and Li-foil as the counter. They were assembled by using a coin cell crimper (Hosen, Japan) in an Ar-filled glovebox. The cells were analyzed via galvanostatic cycling at 445 mA g 1 between 0.01 and 5 V versus Li þ/Li at room temperature (25 1C).
3. Results and discussions Structural analysis of the as-obtained sample was obtained by XRD. As shown in Fig. 1, all of the diffraction peaks are consistent with the monoclinic phase of CuO (JCPDS no. 48-1548). Fig. 2 represents the SEM and HRTEM images of the uniform hollow CuO microspheres synthesized at 140 1C for 16 h. The morphology of the product is in large quality and uniformity,
Fig. 1. Typical XRD pattern of the as-obtained CuO products.
and typical diameters of the spherical architectures are range 2–3 μm (Fig. 2a). The close view (Fig. 2b and c) displays that the surface of the spherical architecture is not smooth. The wall of hollow sphere is consists of many nanorods with around 400 nm in thickness. The close-up view of a crashed microsphere vividly demonstrates that the nanorods regularly connect with each other perpendicularly and align to the spherical wall, pointing toward a common center to form a hollow interior. Moreover, the BET surface area of this sample is 25.7 m2/g, which is larger than the results previously reported twinned-hemisphere [13] and Urchinlike CuO [17]. As a result of the large surface area, the as-prepared hierarchically porous CuO structures could promote the diffusion of molecular and provide numerous active sites for surface contact reactions [14]. A typical HRTEM image of a single nanowire and the selected area electron diffraction (SAED) pattern obtained from a particle-rich region (Fig. 2d). In the HRTEM image, the lattice spacing of 0.232 nm corresponds to that of the (111) crystal planes of monoclinic phase CuO. The first discharge–charge voltage profiles of CuO samples are shown in Fig. 3a. The onset of Li insertion into CuO can be observed at 2.3 V characterized by a sloping potential specific charge profile. However, once the discharge process starts, the voltage rapidly decreased to 1.2 V. When Li is inserted into CuO in the first discharge, two obvious voltage plateaus (1.2–1.0 V and 0.9–0.7 V versus Liþ/Li) are found for the reaction of Li and CuO sheets. However, for potentials below 0.5 V, the particle sizedependent growth of a solid electrolyte interphase (SEI) layer becomes evident. The behavior is similar to that described in the literatures [15]. The electrode synthesized at 140 1C for 16 h (sample c) shows a high initial specific charge beyond 881.7 mA h g 1 and first charge capacity of 586.1 mA h g 1, which is larger than that of reported CuO microspheres [6]. This result indicates that the construction of hollow microstructure is an effective way to improve the electrochemical performance. As a good controlling of mobile charge carriers, the whole sphere can be considered as a 3D current collector network and Li-ion buffering reservoirs, and it also provide negligible diffusion (short diffusion distance) to enhance electronic
Fig. 2. (a), (b) and (c) Displays typical SEM images of the hollow CuO microspheres obtained from the hydrothermal treatments at 140 1C for 16 h; (d) HRTEM image of one part of the CuO sphere and corresponding SAED spectra (inset).
S. He et al. / Materials Letters 129 (2014) 5–7
7
Fig. 3. (A) first galvanostatic discharge–charge cycling curves of the CuO samples; (B) discharge capacity versus cycle number for 50 cycles. CuO products made from (a) 160 1C for 16 h, (b) 140 1C for 18 h, and (c) 140 1C for 16 h.
conductivity. Ion-permeable porous shells contain mesopores and micropores that allow the ion diffusion toward the entire active surface are highly desired. The mesopores provide direct diffusion channels, facilitate Li-ion diffusion kinetics and fast uptake-release on the interface. The abundant quasi-micropores in the nanorods that compose the CuO shells endow the material a large quantity of active site for ion. The quasi-micropores are also considered one of the key roles for high initial discharge capacity. This feature is particularly useful when the battery is cycled at high currents, and more conductive pathways become available. With the reaction temperature increased from 140 1C to 160 1C or the reaction time increased from 16 h to 18 h, the initial charge and discharge capacity decreased and voltage plateau became lower in the charge/discharge curves. These phenomena could be described as following: with the reaction temperature increasing or the activating reaction time prolonging, the size of crystallite CuO increased gradually and CuO particles agglomerated easily. This change reduced the surface area of the anode materials and the contact area between the electrode and the electrolyte, thus the storage location for Li þ decreased and results in a weaker activity of CuO electrode. The cycling performance of the CuO products is shown in Fig. 3b. After the second cycle, a continuing increase in specific charge was observed from all of the three CuO samples. Due to the kinetically activated electrolyte degradation and the formation of an organic layer on the surface of the nanorods in the low potential region, a polymeric gel-like film grew reversibly and caused increase of the specific capacity. Accordingly, the electrode’s capacity started to decline after repeated charging and discharging. The exaggerated electrolyte degrade on the microsized particles may be caused by the increased metal-electrolyte contact area and the charge transfer resistance. Anyway, all of the above effects become more pronounced as the specific surface area and the number of grain boundaries increased with the smaller particles. As shown in Fig. 3b, the specific capacities of the three CuO particles still remain at 400 mA h g 1 after 50 cycles, which indicates excellent reversibility.
4. Conclusions In summary, hollow CuO microspheres have been synthesized via a simple hydrothermal process. The nanorods self-organized into a hierarchical hollow architecture, and the tiny holes within the spheres shells ensure good electrical contact of current collector and enhance the charge transfer or Li-ion transport. Results of electrochemical test show that the samples have a good electrochemical performance with a high initial discharge capacity and a specific capacity of 400 mA h g 1 up to 50 cycles at a rate of 445 mA g 1. It is believed that this uniform hollow architecture could be an advanced material for its potential applications in the area such as gas-sensing, optical materials and so on.
Acknowledgments This work is financially supported by National Natural Science Foundation of China (Nos. 11372288, 11202193 and 11272292). References [1] Rakhshani AE. Solid State Electron 1986;29:7–17. [2] Yuvaraj H, Jae-Jin S. Mater Lett 2014;116:5–8. [3] Kaili Z, Carole R, Marine P, Nicolas Mauran. J Microelectromech Syst 2008;17:832–6. [4] Yang Z, Wang D, Li F, et al. Mater Lett 2013;90:4–7. [5] Zhang YX, Huang M, Li F. Int J Electrochem Sci 2013;8:8645–61. [6] Gao S, Yang S, Shu J, et al. J Phys Chem C 2008;112:1932–8. [7] Ji H, Miao X, Wang L, Qian B, Yang G. Powder Technol 2013;241:43–8. [8] Deng C, Hu H, Zhu W, Han C, Shao G. Mater Lett 2011;65:575–8. [9] Liu R, Yin J, Du W, et al. Eur J Inorg Chem 2013;8:1358–62. [10] Jian G, Liu L, Zachariah MR. Adv Funct Mater 2013;23:1341–6. [11] Zhang Z, Che H, Wang Y, et al. Catal Sci Technol 2012;2:1953–60. [12] Qin Y, Zhang F, Chen Y, et al. J Phys Chem C 2012;116:11994–2000. [13] Wang C, Ye Y, Tao B, Geng B. CrystEngComm 2012;14:3677–83. [14] Sumanta Kumar M, G Ranga R. Nanoscale 2013;5:2089–99. [15] Reddy MV, Yu C, Jiahuan F, Loh KP. Appl Mater Interfaces 2013;5:4361–6. [16] Tian ZP, Zhou Y, Li ZD, Liu Q, Zou ZG. J Mater Chem A 2013;1:3575–9. [17] Xu L, Sithambaram S, Zhang Y, et al. Chem Mater 2009;21:1253–9.