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ZnO
Materials Science and Engineering A 430 (2006) 248–253
Sol–gel-template synthesis of ZnO nanotubes and its coaxial nanocomposites of LiMn2O4/ZnO Xiaohong Liu a,b , Jinqing Wang a , Junyan Zhang a,∗ , Shengrong Yang a,∗∗ a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received 17 March 2006; received in revised form 14 May 2006; accepted 18 May 2006
Abstract The coaxial nanostructure composite materials of LiMn2 O4 nanowires encapsulated in ZnO nanotubes are fabricated successfully via sol–gel method by using two-step template process. Transmission electron microscopy (TEM) and scanning electronic microscopy (SEM) results conformably show that the synthesized ZnO nanotubes possess the explicit end-opened tubular structure in uniform out diameter and wall thickness. Selected area electron diffraction (SAED) pattern, X-ray diffractometer (XRD), and X-ray photoelectron spectroscope (XPS) analysis jointly demonstrate that the main body of the fabricated coaxial composites is spinel structure LiMn2 O4 . It is expected that the two-step template process can be used to mass-produce coaxial LiMn2 O4 /ZnO nanocomposite materials as a novel cathode materials in lithium ion battery. © 2006 Elsevier B.V. All rights reserved. Keywords: Coaxial nanocomposites; ZnO; LiMn2 O4 ; Sol–gel; AAO template; Lithium ion battery
1. Introduction It has been made rapid progress in the development of one-dimensional nanostructure materials due to their superior electronic, magnetic, optical, and mechanical properties in comparison with the corresponding bulk counterparts [1–4]. Various methods have been developed to fabricate one-dimensional nanostructure materials, such as the arc discharge, laser ablation, catalytic CVD growth, and template synthesis, etc. [5], among which template method was extensively adopted due to its convenient and inexpensive characteristics [6–12]. Anodic aluminum oxide (AAO) template, characterized by regular and anisotropic porous structures in which the pores are generally parallel each other and perpendicular to the membrane surface [10], offers a promising route to synthesize ordered nanostructure materials with high surface area. Recently, considerable interests were focused on the fabrication of one-dimensional coaxial layered nanoscale composite materials because of their remarkable
∗
Corresponding author. Tel.: +86 931 4968295; fax: +86 931 8277088. Corresponding author. Tel.: +86 931 4968088; fax: +86 931 8277088. E-mail addresses: [email protected] (J. Zhang), [email protected] (S. Yang). ∗∗
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.059
properties different from those of individual nanomaterials [13], and which are expected to be potential applications in electronic transportation and photoelectronic nanodevices. Various mutilayer systems made from one-dimensional coaxial nanostructures have been prepared successfully by various techniques [14–16]. As an important functional material, ZnO is widely used in the fields of catalysis, optoelectronics, varistors, and gas sensors, etc., since it possesses wide direct bandgap, excellent chemical and thermal stability, and large exciton binding energy [17]. Onedimensional ZnO materials and its coaxial composite materials prepared by using different techniques, such as thermal reduction route and vapor phase transport technique, have been widely reported [18,19]. However, to our best of knowledge, sol–gel method used to fabricate the coaxial ZnO composite materials has not been developed yet. LiMn2 O4 , as a cathode material, has a lot of advantages such as low cost, small adverse impact on environment, and high capacity, etc. However, its capacity can fade slowly, which is generally dominated by the dissolution of Mn3+ , Jahn–Teller effect, and lattice instability at high oxidation levels. Moreover, the capacity fading become more severely due to the dissolution of Mn at elevated temperature (40–50 ◦ C) [20,21]. To overcome above mentioned problems, some strategies have been adopted including substituting portion of the
X. Liu et al. / Materials Science and Engineering A 430 (2006) 248–253
Mn3+ ions by mono-, di-, or tri-valent ions and surface treatment of electrode active materials [22–26]. It also has been proved that coating ZnO onto the cathode material surface is an effective method to decrease the capacity fading [27]. Accordingly, filling LiMn2 O4 into ZnO nanotubes is probably to be a good way in resolving the fading problem resulted from the Mn dissolution. In this work, by using sol–gel method, high-ordered ZnO nanotube arrays are firstly synthesized using AAO as the template, and then coaxial nanostructure composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes are fabricated using the synthesized ZnO nanotube arrays as the second-ordered template. It is expected to obtain a kind of composite nanostructure cathode material with good stability. 2. Experimental 2.1. Synthesis of ZnO nanotube arrays All chemicals used in this work are analytical grade without further purification. The ZnO nanotubes were prepared by using a method similar to what has been reported by Sakohara et al. [28]. In detail, 0.35 g zinc acetate was added to 20 ml ethanol, and the resulting mixture was stirred and boiled until a clear solution was obtained. The volume was increased to 20 ml with ethanol, and 0.08 g KOH was added in. The resulting solution was ultrasonically agitated until a white suspension was obtained (about 1 h). Then, the AAO membrane was immersed into the suspension for 1 min. After being taken out, it was allowed to dry in air at room temperature for 30 min. Finally, the membrane was undergone a heat-treatment at 600 ◦ C for 4 h in air. Thus, the ZnO nanotubes were synthesized inside the pores of the AAO template. 2.2. Fabrication of coaxial nanocomposites of LiMn2 O4 /ZnO Stoichiometric amounts of lithium acetate and manganese acetate (the molar ratio of Li:Mn = 1:2) were dissolved in deionized water, then citric acid and ethylene glycol (the molar ratio is 1:4) were added as the monomers for forming the polymeric matrix. The molar ratio of total metal ions to citric acid is 1:1. The pH value of the solution was adjusted to 8.0 with ammonium hydroxide. Then, with continuous stirring, the solution was heated at 80 ◦ C for 5 h until a transparent sol was obtained, which was very stable at room temperature and no precipitate was observed even for a long time. The pre-synthesized ZnO nanotubes/AAO membrane was placed into the LiMn2 O4 precursor sol, and then the whole systems was heated to 80 ◦ C and held for 2 h in oven for concentrating. After being taken out, the membrane was wiped off to eliminate the excess sol on the surface, and then dried in air environment at 80 ◦ C for 1 h and 140 ◦ C for another 0.5 h. Finally, the dried membrane was heat-treated at 600 ◦ C for 6 h in air environment. Thus, the coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes were fabricated successfully inside of the AAO pores.
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2.3. Characterization methods The morphology, crystallography, and composition of the synthesized ZnO nanotubes and its coaxial nanocomposites of LiMn2 O4 /ZnO were characterized. Scanning electronic microscopy (SEM) images were obtained with a JSM-5600LV microscope. For SEM sample preparation, the alumina membrane was attached on a Cu cylinder. Then, a few droplets of 3 M NaOH aqueous solution were dropped on the sample to dissolve membrane, and the samples were sputter-coated with gold prior to observation. The size and shape of the ZnO nanotubes and LiMn2 O4 /ZnO composite nanostructures were also confirmed by transmission electron microscopy (TEM) with JEM-1200EX instrument. For TEM sample preparation, the alumina membrane was etched via dissolving with 3 M NaOH aqueous solution, and then diluted with deionized water. Droplets of suspension containing ZnO nanotubes or LiMn2 O4 /ZnO composites were placed onto the copper grids for TEM observation. The phase structure of the coaxial ZnO/LiMn2 O4 composites with template membrane was determined on a D/max-RB Xray diffractometer (Rigaku Corp., Tokyo, Japan) with Cu K␣ radiation at a scanning step size of 2θ = 0.017◦ . The data were collected in the 2θ range from 10◦ to 80◦ . The chemical states of typical elements of the coaxial LiMn2 O4 /ZnO composites with the template membrane were analyzed on a PHI-5702 multifunctional X-ray photoelectron spectroscope (Physical Electronics, USA) operating with Al K␣ irradiation (hν = 1486.6 eV) at a pass energy of 29.35 eV. The binding energy of contaminated carbon (C 1s: 284.8 eV) was used as the reference and the resolution is about ±0.3 eV. 3. Results and discussion The typical SEM images of ZnO nanotubes grown within the AAO pores are shown in Fig. 1. It is obvious from Fig. 1a that the synthesized ZnO nanotubes are separated completely from the alumina membrane. These nanotubes pile together and are in a parallel state over a broad area even if they collapse. A large number of ZnO nanotubes cluster is displayed in Fig. 1b, from which it can be found obviously that the ZnO nanotubes round together and are aligned closely, which implies that the ZnO nanotubes can be mass-produced by this method. It also can be observed from Fig. 1c and d that the ZnO nanotubes possess characteristics of explicit end-opened tubular structure in uniform out diameter and wall thickness. In other words, the synthesized ZnO nanotubes are highly ordered and perfect arrays in uniform out diameter and wall thickness, while the length of the ZnO nanotubes is consistent with the thickness of the alumina membrane. The TEM image of a single ZnO nanobube after removal of the alumina membrane completely and the corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 2. From the TEM image (Fig. 2a), it can be obtained that the pore diameter and the wall thickness of the ZnO nanotubes are 175 and 25 nm, respectively. The corresponding SAED pattern indicates that the synthesized nanotubes are consisted of hexagonal single crystal ZnO and amorphous ZnO nanoparticles (Fig. 2b).
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Fig. 1. SEM images of ZnO nanotube arrays.
The important characteristic of end-opened tubular structure for the synthesized ZnO nanotubes (see Fig. 1c and d) makes it possible to fill other materials into the nanotubes, or to carry out chemical reactions inside of the nanotubes. In this work, the coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes are fabricated using the synthesized ZnO nanotube arrays as the second-ordered template. The TEM image and the corresponding SAED pattern of LiMn2 O4 nanowires encapsulated in ZnO nanotubes after removal of the alumina membrane are shown in Fig. 3. It is obvious that the ZnO nanotubes have been fully filled with LiMn2 O4 nanoparticles (Fig. 3a). The corresponding SEAD pattern (Fig. 3b) demonstrates that the fabricated LiMn2 O4 is polycrystal. The diffraction spots, from inner to outer, corre-
spond to the (1 1 1), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) diffraction planes of spinel LiMn2 O4 , respectively. And the diffraction spots of ZnO single crystal can also be observed. Thus, it can be concluded that the coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes has been successfully fabricated and the LiMn2 O4 nanowires possess well-defined spinel structure. The XRD pattern of the LiMn2 O4 /ZnO composites with the alumina template membrane is shown in Fig. 4. The diagnostic peaks of ZnO are not appeared because the amount of ZnO is relative lower comparing with that of LiMn2 O4 . Some diagnostic peaks appearing at 2θ = 18.8◦ , 36.3◦ , 44.2◦ , 48.5◦ , 58.6◦ , 64.3◦ , and 67.6◦ , are assigned to the (1 1 1), (3 1 1), (4 0 0), (3 3 1), (5 1 1), (4 4 0), and (5 3 1) planes of the spinel phase
Fig. 2. (a) TEM image of a ZnO nanobube after removal of the alumina membrane; (b) SAED pattern of the corresponding ZnO nanotube.
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Fig. 3. (a) TEM image of LiMn2 O4 nanowires encapsulated in ZnO nanotubes after removal of the alumina membrane; (b) SAED pattern of the corresponding composite nanostructure.
LiMn2 O4 , respectively. So it can be confirmed that the LiMn2 O4 nanowires encapsulated in ZnO nanotubes have well-defined spinel structure, which is greatly consistent with the result of SAED analysis. The chemical composition of the LiMn2 O4 /ZnO composites with the alumina template membrane is also characterized by XPS analysis. The survey scan XPS spectrum of the prepared sample is shown in Fig. 5. It is clear that the signals of Li, Mn, Al, O, and Zn are appeared on the spectrum. In addition, the signal of C is also detected at 284.8 eV, which is assigned to the ubiquitous adventitious carbon and cited as reference. Fig. 6a–d display the XPS spectra of Li 1s, Mn 2p, Zn 2p, Al 2p, and O 1s core levels, respectively. The peak of Li 1s is located at 54.2 eV (Fig. 6a) with a relatively low intensity, indicating that Li exists in the state of Li+ . In the literatures, the binding energy of Mn 2p3/2 was generally reported to be appeared at 642.6 eV for Mn4+ in LiMn2 3+,4+ O4 and Mn4+ O2 (pyrolusite), and 641.6 eV for Mn3+ in LiMn2 3+,4+ O4 and Mn2 3+ O3 (bixbyite) [29]. In our case, the two peaks of Mn 2p (Mn 2p3/2 and Mn 2p1/2 ) are located at 642.2 and 653.7 eV, respectively, with an energy separation of 11.5 eV. It is obvious that the binding energy of Mn 2p3/2 peak is intervenient between those of Mn4+ (642.6 eV) and
Mn3+ (641.6 eV), which is accordance with its spinel structure. To determine the average oxidation state of the Mn element, the relative ratio of Mn3+ to Mn4+ in the coaxial composite materials of LiMn2 O4 /ZnO is calculated by least-squares fitting analysis with two spectra of Mn2 O3 and MnO2 . The best-fitting result is compared with the experimental spectrum in Fig. 6b. In virtue of the relative concentration data of both Mn3+ (52.22%) and Mn4+ (47.78%) ions, the average oxidation state of Mn is calculated to be 3.48, which is in accordance with the Mn valence in stoichiometric LiMn2 O4 . Peak of Zn 2p (Zn 2p3/2 ) appeared at 1021.6 eV (Fig. 6c) is assigned to the low-spin of Zn2+ . Moreover, the relative intensity of Zn 2p signal is lower, which is attributed to its low content in the coaxial composite. Peaks of O 1s and Al 2p are appeared at 531.2 eV (Fig. 6d) and 74.5 eV (Fig. 6e), respectively, which indicate that the O element and Al element are existed in the state of O2− and Al3+ , respectively. In conclusion, the analytical results integrating all of obtained data demonstrates that the fabrication of coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes has been achieved successfully and the main body of the fabricated coaxial composites is spinel structure LiMn2 O4 .
Fig. 4. XRD pattern of LiMn2 O4 nanowires encapsulated in ZnO nanotubes within the alumina membrane.
Fig. 5. XPS survey spectrum of LiMn2 O4 nanowires encapsulated in ZnO nanotubes within the alumina membrane.
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Fig. 6. XPS spectra of (a) Li 1s; (b) Mn 2p; (c) Zn 2p; (d) O 1s; (e) Al 2p core levels for LiMn2 O4 nanowires encapsulated in ZnO nanotubes within the alumina membrane.
4. Conclusions In summary, the coaxial nanostructure composite materials of LiMn2 O4 nanowires encapsulated in ZnO nanotubes have been fabricated successfully by a two-step template process: (1) highly ordered ZnO nanotube arrays with explicit end-opened tubular structure by the sol–gel process was firstly synthesized; (2) precursor sol of LiMn2 O4 nanoparticles was then filled into the synthesized ZnO nanotubes. As a result, nanostructure composite materials of LiMn2 O4 nanowires encapsulated in ZnO nanotubes were obtained, which was confirmed by means of SEM, TEM/SEAD, XRD, and XPS analysis. It is expected that ZnO nanotubes could protect the LiMn2 O4 from the dissolution of Mn and then prevent its capacity fading as cathode materials in lithium ion battery. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (Grant No. 50375151, 50323007,
and 50572107), and “Hundreds Talent Program” of Chinese Academy of Sciences for financial support. References [1] Y. Cui, Q.Q. Wei, H.K. Park, C.M. Lieber, Science 293 (2001) 1289–1292. [2] Y. Huang, X. Duan, Y. Cui, L.J. Lauhon, K.H. Kim, C.M. Lieber, Science 294 (2001) 1313–1317. [3] P.G. Collins, M.S. Arnold, P. Avouris, Science 292 (2001) 706– 709. [4] A. Bachtold, P. Hadley, T. Nakanishi, C. Dekker, Science 294 (2001) 1317–1320. [5] W.B. Zhao, J.J. Zhu, H.Y. Chen, J. Cryst. Growth 258 (2003) 176–180. [6] C.R. Martin, Science 266 (1994) 1961–1966. [7] T.M. Whitney, J.S. Jiang, P.C. Searson, C.L. Chien, Science 261 (1993) 1316–1319. [8] P.P. Nguyen, D.H. Pearson, R.J. Tonucci, K. Babcock, J. Electrochem. Soc. 145 (1998) 247–251. [9] D. Routkevitch, A.A. Tager, J. Haruyama, D. Almawlawi, M. Moskovits, J.M. Xu, IEEE. Trans. Electron Devices 43 (1996) 1646–1658. [10] J.C. Hulteen, C.R. Martin, J. Mater. Chem. 7 (1997) 1075–1087.
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[20] J.C. Hunter, J. Solid State Chem. 39 (1981) 142–147. [21] A. Blyr, C. Sigala, G.G. Amatucci, D. Guyomard, Y. Chabres, J.M. Tarascon, J. Electrochem. Soc. 145 (1998) 194–209. [22] Y. Xia, N. Kumada, M. Yoshio, J. Power Sources 90 (2000) 135–138. [23] G. Amatucci, J.M. Tarascon, US Patent No. 5,674,645 (1997). [24] Y.K. Sun, G.S. Park, Y.S. Lee, M. Yoshio, K.S. Nahm, J. Electrochem. Soc. 148 (2001) A994–A998. [25] H.J. Kweon, D.G. Park, Electrochem. Solid-State Lett. 3 (2000) 128–130. [26] S.S. Kim, Y. Kadoma, H. Ikuta, Y. Uchimoto, M. Wakihara, Electrochem. Solid-State Lett. 4 (2001) A109–A112. [27] Y.K. Sun, K.J. Hong, Jai Prakash, J. Electrochem. Soc. 150 (2003) A970–A972. [28] S.J. Sakohara, L.D. Tickanen, M.A. Anderson, J. Phys. Chem. 96 (1992) 11086–11091. [29] N. Treuil, C. Labrugere, M. Menetrier, J. Portier, G. Gampet, A. Deshayes, J.C. Frison, S.J. Hwang, S.W. Song, J.H. Choy, J. Phys. Chem. B 103 (1999) 2100–2106.
Sol–gel-template synthesis of ZnO nanotubes and its coaxial nanocomposites of LiMn2O4/ZnO Xiaohong Liu a,b , Jinqing Wang a , Junyan Zhang a,∗ , Shengrong Yang a,∗∗ a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China Received 17 March 2006; received in revised form 14 May 2006; accepted 18 May 2006
Abstract The coaxial nanostructure composite materials of LiMn2 O4 nanowires encapsulated in ZnO nanotubes are fabricated successfully via sol–gel method by using two-step template process. Transmission electron microscopy (TEM) and scanning electronic microscopy (SEM) results conformably show that the synthesized ZnO nanotubes possess the explicit end-opened tubular structure in uniform out diameter and wall thickness. Selected area electron diffraction (SAED) pattern, X-ray diffractometer (XRD), and X-ray photoelectron spectroscope (XPS) analysis jointly demonstrate that the main body of the fabricated coaxial composites is spinel structure LiMn2 O4 . It is expected that the two-step template process can be used to mass-produce coaxial LiMn2 O4 /ZnO nanocomposite materials as a novel cathode materials in lithium ion battery. © 2006 Elsevier B.V. All rights reserved. Keywords: Coaxial nanocomposites; ZnO; LiMn2 O4 ; Sol–gel; AAO template; Lithium ion battery
1. Introduction It has been made rapid progress in the development of one-dimensional nanostructure materials due to their superior electronic, magnetic, optical, and mechanical properties in comparison with the corresponding bulk counterparts [1–4]. Various methods have been developed to fabricate one-dimensional nanostructure materials, such as the arc discharge, laser ablation, catalytic CVD growth, and template synthesis, etc. [5], among which template method was extensively adopted due to its convenient and inexpensive characteristics [6–12]. Anodic aluminum oxide (AAO) template, characterized by regular and anisotropic porous structures in which the pores are generally parallel each other and perpendicular to the membrane surface [10], offers a promising route to synthesize ordered nanostructure materials with high surface area. Recently, considerable interests were focused on the fabrication of one-dimensional coaxial layered nanoscale composite materials because of their remarkable
∗
Corresponding author. Tel.: +86 931 4968295; fax: +86 931 8277088. Corresponding author. Tel.: +86 931 4968088; fax: +86 931 8277088. E-mail addresses: [email protected] (J. Zhang), [email protected] (S. Yang). ∗∗
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.059
properties different from those of individual nanomaterials [13], and which are expected to be potential applications in electronic transportation and photoelectronic nanodevices. Various mutilayer systems made from one-dimensional coaxial nanostructures have been prepared successfully by various techniques [14–16]. As an important functional material, ZnO is widely used in the fields of catalysis, optoelectronics, varistors, and gas sensors, etc., since it possesses wide direct bandgap, excellent chemical and thermal stability, and large exciton binding energy [17]. Onedimensional ZnO materials and its coaxial composite materials prepared by using different techniques, such as thermal reduction route and vapor phase transport technique, have been widely reported [18,19]. However, to our best of knowledge, sol–gel method used to fabricate the coaxial ZnO composite materials has not been developed yet. LiMn2 O4 , as a cathode material, has a lot of advantages such as low cost, small adverse impact on environment, and high capacity, etc. However, its capacity can fade slowly, which is generally dominated by the dissolution of Mn3+ , Jahn–Teller effect, and lattice instability at high oxidation levels. Moreover, the capacity fading become more severely due to the dissolution of Mn at elevated temperature (40–50 ◦ C) [20,21]. To overcome above mentioned problems, some strategies have been adopted including substituting portion of the
X. Liu et al. / Materials Science and Engineering A 430 (2006) 248–253
Mn3+ ions by mono-, di-, or tri-valent ions and surface treatment of electrode active materials [22–26]. It also has been proved that coating ZnO onto the cathode material surface is an effective method to decrease the capacity fading [27]. Accordingly, filling LiMn2 O4 into ZnO nanotubes is probably to be a good way in resolving the fading problem resulted from the Mn dissolution. In this work, by using sol–gel method, high-ordered ZnO nanotube arrays are firstly synthesized using AAO as the template, and then coaxial nanostructure composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes are fabricated using the synthesized ZnO nanotube arrays as the second-ordered template. It is expected to obtain a kind of composite nanostructure cathode material with good stability. 2. Experimental 2.1. Synthesis of ZnO nanotube arrays All chemicals used in this work are analytical grade without further purification. The ZnO nanotubes were prepared by using a method similar to what has been reported by Sakohara et al. [28]. In detail, 0.35 g zinc acetate was added to 20 ml ethanol, and the resulting mixture was stirred and boiled until a clear solution was obtained. The volume was increased to 20 ml with ethanol, and 0.08 g KOH was added in. The resulting solution was ultrasonically agitated until a white suspension was obtained (about 1 h). Then, the AAO membrane was immersed into the suspension for 1 min. After being taken out, it was allowed to dry in air at room temperature for 30 min. Finally, the membrane was undergone a heat-treatment at 600 ◦ C for 4 h in air. Thus, the ZnO nanotubes were synthesized inside the pores of the AAO template. 2.2. Fabrication of coaxial nanocomposites of LiMn2 O4 /ZnO Stoichiometric amounts of lithium acetate and manganese acetate (the molar ratio of Li:Mn = 1:2) were dissolved in deionized water, then citric acid and ethylene glycol (the molar ratio is 1:4) were added as the monomers for forming the polymeric matrix. The molar ratio of total metal ions to citric acid is 1:1. The pH value of the solution was adjusted to 8.0 with ammonium hydroxide. Then, with continuous stirring, the solution was heated at 80 ◦ C for 5 h until a transparent sol was obtained, which was very stable at room temperature and no precipitate was observed even for a long time. The pre-synthesized ZnO nanotubes/AAO membrane was placed into the LiMn2 O4 precursor sol, and then the whole systems was heated to 80 ◦ C and held for 2 h in oven for concentrating. After being taken out, the membrane was wiped off to eliminate the excess sol on the surface, and then dried in air environment at 80 ◦ C for 1 h and 140 ◦ C for another 0.5 h. Finally, the dried membrane was heat-treated at 600 ◦ C for 6 h in air environment. Thus, the coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes were fabricated successfully inside of the AAO pores.
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2.3. Characterization methods The morphology, crystallography, and composition of the synthesized ZnO nanotubes and its coaxial nanocomposites of LiMn2 O4 /ZnO were characterized. Scanning electronic microscopy (SEM) images were obtained with a JSM-5600LV microscope. For SEM sample preparation, the alumina membrane was attached on a Cu cylinder. Then, a few droplets of 3 M NaOH aqueous solution were dropped on the sample to dissolve membrane, and the samples were sputter-coated with gold prior to observation. The size and shape of the ZnO nanotubes and LiMn2 O4 /ZnO composite nanostructures were also confirmed by transmission electron microscopy (TEM) with JEM-1200EX instrument. For TEM sample preparation, the alumina membrane was etched via dissolving with 3 M NaOH aqueous solution, and then diluted with deionized water. Droplets of suspension containing ZnO nanotubes or LiMn2 O4 /ZnO composites were placed onto the copper grids for TEM observation. The phase structure of the coaxial ZnO/LiMn2 O4 composites with template membrane was determined on a D/max-RB Xray diffractometer (Rigaku Corp., Tokyo, Japan) with Cu K␣ radiation at a scanning step size of 2θ = 0.017◦ . The data were collected in the 2θ range from 10◦ to 80◦ . The chemical states of typical elements of the coaxial LiMn2 O4 /ZnO composites with the template membrane were analyzed on a PHI-5702 multifunctional X-ray photoelectron spectroscope (Physical Electronics, USA) operating with Al K␣ irradiation (hν = 1486.6 eV) at a pass energy of 29.35 eV. The binding energy of contaminated carbon (C 1s: 284.8 eV) was used as the reference and the resolution is about ±0.3 eV. 3. Results and discussion The typical SEM images of ZnO nanotubes grown within the AAO pores are shown in Fig. 1. It is obvious from Fig. 1a that the synthesized ZnO nanotubes are separated completely from the alumina membrane. These nanotubes pile together and are in a parallel state over a broad area even if they collapse. A large number of ZnO nanotubes cluster is displayed in Fig. 1b, from which it can be found obviously that the ZnO nanotubes round together and are aligned closely, which implies that the ZnO nanotubes can be mass-produced by this method. It also can be observed from Fig. 1c and d that the ZnO nanotubes possess characteristics of explicit end-opened tubular structure in uniform out diameter and wall thickness. In other words, the synthesized ZnO nanotubes are highly ordered and perfect arrays in uniform out diameter and wall thickness, while the length of the ZnO nanotubes is consistent with the thickness of the alumina membrane. The TEM image of a single ZnO nanobube after removal of the alumina membrane completely and the corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 2. From the TEM image (Fig. 2a), it can be obtained that the pore diameter and the wall thickness of the ZnO nanotubes are 175 and 25 nm, respectively. The corresponding SAED pattern indicates that the synthesized nanotubes are consisted of hexagonal single crystal ZnO and amorphous ZnO nanoparticles (Fig. 2b).
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Fig. 1. SEM images of ZnO nanotube arrays.
The important characteristic of end-opened tubular structure for the synthesized ZnO nanotubes (see Fig. 1c and d) makes it possible to fill other materials into the nanotubes, or to carry out chemical reactions inside of the nanotubes. In this work, the coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes are fabricated using the synthesized ZnO nanotube arrays as the second-ordered template. The TEM image and the corresponding SAED pattern of LiMn2 O4 nanowires encapsulated in ZnO nanotubes after removal of the alumina membrane are shown in Fig. 3. It is obvious that the ZnO nanotubes have been fully filled with LiMn2 O4 nanoparticles (Fig. 3a). The corresponding SEAD pattern (Fig. 3b) demonstrates that the fabricated LiMn2 O4 is polycrystal. The diffraction spots, from inner to outer, corre-
spond to the (1 1 1), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) diffraction planes of spinel LiMn2 O4 , respectively. And the diffraction spots of ZnO single crystal can also be observed. Thus, it can be concluded that the coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes has been successfully fabricated and the LiMn2 O4 nanowires possess well-defined spinel structure. The XRD pattern of the LiMn2 O4 /ZnO composites with the alumina template membrane is shown in Fig. 4. The diagnostic peaks of ZnO are not appeared because the amount of ZnO is relative lower comparing with that of LiMn2 O4 . Some diagnostic peaks appearing at 2θ = 18.8◦ , 36.3◦ , 44.2◦ , 48.5◦ , 58.6◦ , 64.3◦ , and 67.6◦ , are assigned to the (1 1 1), (3 1 1), (4 0 0), (3 3 1), (5 1 1), (4 4 0), and (5 3 1) planes of the spinel phase
Fig. 2. (a) TEM image of a ZnO nanobube after removal of the alumina membrane; (b) SAED pattern of the corresponding ZnO nanotube.
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Fig. 3. (a) TEM image of LiMn2 O4 nanowires encapsulated in ZnO nanotubes after removal of the alumina membrane; (b) SAED pattern of the corresponding composite nanostructure.
LiMn2 O4 , respectively. So it can be confirmed that the LiMn2 O4 nanowires encapsulated in ZnO nanotubes have well-defined spinel structure, which is greatly consistent with the result of SAED analysis. The chemical composition of the LiMn2 O4 /ZnO composites with the alumina template membrane is also characterized by XPS analysis. The survey scan XPS spectrum of the prepared sample is shown in Fig. 5. It is clear that the signals of Li, Mn, Al, O, and Zn are appeared on the spectrum. In addition, the signal of C is also detected at 284.8 eV, which is assigned to the ubiquitous adventitious carbon and cited as reference. Fig. 6a–d display the XPS spectra of Li 1s, Mn 2p, Zn 2p, Al 2p, and O 1s core levels, respectively. The peak of Li 1s is located at 54.2 eV (Fig. 6a) with a relatively low intensity, indicating that Li exists in the state of Li+ . In the literatures, the binding energy of Mn 2p3/2 was generally reported to be appeared at 642.6 eV for Mn4+ in LiMn2 3+,4+ O4 and Mn4+ O2 (pyrolusite), and 641.6 eV for Mn3+ in LiMn2 3+,4+ O4 and Mn2 3+ O3 (bixbyite) [29]. In our case, the two peaks of Mn 2p (Mn 2p3/2 and Mn 2p1/2 ) are located at 642.2 and 653.7 eV, respectively, with an energy separation of 11.5 eV. It is obvious that the binding energy of Mn 2p3/2 peak is intervenient between those of Mn4+ (642.6 eV) and
Mn3+ (641.6 eV), which is accordance with its spinel structure. To determine the average oxidation state of the Mn element, the relative ratio of Mn3+ to Mn4+ in the coaxial composite materials of LiMn2 O4 /ZnO is calculated by least-squares fitting analysis with two spectra of Mn2 O3 and MnO2 . The best-fitting result is compared with the experimental spectrum in Fig. 6b. In virtue of the relative concentration data of both Mn3+ (52.22%) and Mn4+ (47.78%) ions, the average oxidation state of Mn is calculated to be 3.48, which is in accordance with the Mn valence in stoichiometric LiMn2 O4 . Peak of Zn 2p (Zn 2p3/2 ) appeared at 1021.6 eV (Fig. 6c) is assigned to the low-spin of Zn2+ . Moreover, the relative intensity of Zn 2p signal is lower, which is attributed to its low content in the coaxial composite. Peaks of O 1s and Al 2p are appeared at 531.2 eV (Fig. 6d) and 74.5 eV (Fig. 6e), respectively, which indicate that the O element and Al element are existed in the state of O2− and Al3+ , respectively. In conclusion, the analytical results integrating all of obtained data demonstrates that the fabrication of coaxial composites of LiMn2 O4 nanowires encapsulated in ZnO nanotubes has been achieved successfully and the main body of the fabricated coaxial composites is spinel structure LiMn2 O4 .
Fig. 4. XRD pattern of LiMn2 O4 nanowires encapsulated in ZnO nanotubes within the alumina membrane.
Fig. 5. XPS survey spectrum of LiMn2 O4 nanowires encapsulated in ZnO nanotubes within the alumina membrane.
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Fig. 6. XPS spectra of (a) Li 1s; (b) Mn 2p; (c) Zn 2p; (d) O 1s; (e) Al 2p core levels for LiMn2 O4 nanowires encapsulated in ZnO nanotubes within the alumina membrane.
4. Conclusions In summary, the coaxial nanostructure composite materials of LiMn2 O4 nanowires encapsulated in ZnO nanotubes have been fabricated successfully by a two-step template process: (1) highly ordered ZnO nanotube arrays with explicit end-opened tubular structure by the sol–gel process was firstly synthesized; (2) precursor sol of LiMn2 O4 nanoparticles was then filled into the synthesized ZnO nanotubes. As a result, nanostructure composite materials of LiMn2 O4 nanowires encapsulated in ZnO nanotubes were obtained, which was confirmed by means of SEM, TEM/SEAD, XRD, and XPS analysis. It is expected that ZnO nanotubes could protect the LiMn2 O4 from the dissolution of Mn and then prevent its capacity fading as cathode materials in lithium ion battery. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (Grant No. 50375151, 50323007,
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