DOI: 10.1002/chem.201304118
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& Li-ion Batteries
Using Simple Spray Pyrolysis to Prepare Yolk–Shell-Structured ZnO–Mn3O4 Systems with the Optimum Composition for Superior Electrochemical Properties Seung Ho Choi and Yun Chan Kang*[a] Abstract: A spray-pyrolysis process is introduced as an effective tool for the preparation of yolk–shell-structured materials with electrochemical properties suitable for anode materials in Li-ion batteries (LIBs). Yolk–shell-structured ZnO–Mn3O4 systems with various molar ratios of the Zn and Mn components are prepared. The yolk–shellstructured ZnO–Mn3O4 powders with a molar ratio of 1:1 of the Zn and Mn components are shown to have high capacities and good cycling performances.
Transition-metal oxides, such as ZnO, Mn3O4, Fe2O3, and NiO, are considered to be promising anodic materials for lithiumion batteries (LIBs) owing to their advantages in terms of high theoretical capacity, safety, and low cost.[1–26] Even though nanostructuring of transition-metal oxides is expected to improve their electrochemical properties, most metal oxides still suffer from poor cyclability, arising from the large specific volume change (above 200 %) during the repetitive charging and discharging processes.[1–10] Binary transition-metal oxides with single phases, such as ABO4 and AB2O4 (A = Zn, Ni, Cu; B = Mn, Co, Fe, Mo), have been studied to overcome the poor cycling performance of single element transition-metal oxides.[1, 11–22] Binary transition-metal oxides get converted into fine nanocomposites of each of the metal oxides and Li2O immediately after the first electrochemical conversion reaction.[1, 11–22] These separated metal oxides and Li2O offer a buffering effect, thereby accommodating the volume change during the charge/discharge process.[14, 15] That said, the ratio of the metal components comprising the binary transition-metal oxides strongly affects their electrochemical properties for lithium-ion storage. In addition, the optimum ratio of metal components necessary for realizing the best electrochemical performance is dependent on the system of binary transitionmetal oxides. However, the composition of binary transitionmetal oxides that have a single phase is strictly restricted. Mixed oxides of the form x AOy (1 x) BOz with various crystal structures could be considered to show better electrochemical [a] S. H. Choi, Prof. Y. C. Kang Department of Chemical Engineering, Konkuk University 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304118. Chem. Eur. J. 2014, 20, 3014 – 3018
properties than binary transition-metal oxides that have single phases, such as ABO3 and AB2O4. The realization of mixed oxides of x AOy (1 x) BOz with good electrochemical properties necessitates uniform mixing of A and B components. Thus far, electrochemical properties of binary transitionmetal oxides, such as Co–Fe–O, Sn–Cu–O, and Co–Mn–O systems, have been extensively studied as the anode materials for LIBs.[22–26] In addition, yolk–shell-structured powders with distinctive core@void@shell configurations have been widely studied as anode materials for LIBs as these structures provide good electrochemical properties as a result of shortened diffusion length. Also, the void spaces in the yolk–shell structure accommodates the volume expansion during charge/recharge cycles.[16, 27–30] According to previous reports, yolk–shell-structured powders were mainly prepared by using a multistep solution method.[31–33] To the best of our knowledge, the preparation and the electrochemical properties of yolk–shell-structured binary transition-metal oxides of Zn and Mn components have not been reported yet. Among the binary transitionmetal oxides, ZnMn2O4 powders have been studied as promising anode materials for LIBs because of their low potential plateaus, high abundance, and the fact they are environmental friendly.[15–19] Recently, a simple spray pyrolysis process was successfully applied to the preparation of yolk–shell-structured powders.[34–37] In previous studies, the electrochemical properties of the yolk–shell-structured materials with single or binary metal oxides prepared by spray pyrolysis were investigated.[34–37] The electrochemical properties of the yolk–shell-structured binary metal oxides were not compared to those of the yolk–shellstructured single metal oxide in the previous reports. Herein, we report the successful synthesis of yolk–shell-structured ZnO–Mn3O4 systems, with various molar ratios of the Zn and Mn components, by using a one-pot spray-pyrolysis process. The prepared yolk–shell particles had various crystal structures depending on the composition of Zn and Mn components. However, the uniform mixing of the Zn and Mn components could be achieved because of a microscale reaction inside the micron-sized droplets. Furthermore, the effect of composition on the electrochemical properties of the ZnO–Mn3O4 systems has also been investigated. For comparison, powders composed of single-component ZnO and Mn3O4 yolk–shell materials were also prepared. Figure 1 shows the morphology of the ZnO–Mn3O4 systems with various compositions that were prepared directly by spray pyrolysis of the spray solution containing metal salts and
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Figure 2. XRD patterns of the yolk–shell-structured ZnO–Mn3O4 powders with various molar ratios of the Zn and Mn components.
Zn/Mn molar ratios of 3:1 and 2:1 had mixed crystal structures of hexagonal ZnO and the cubic ZnMnO3 phases. On the other hand, the XRD patterns of the ZnO–Mn3O4 systems with Zn/Mn molar ratios of 1:1, 1:2, and 1:3 all had the same crystal structure, indicating the formation of solid solutions. The ZnO– Mn3O4 yolk–shell-structured powders with mixed crystal structures of hexagonal ZnO and the cubic ZnMnO3 phases had multishells, as shown in Figures 1 and 2. Figure 3 a shows the cyclic voltammograms (CVs) of the single-component ZnO and Mn3O4 powders with yolk–shell structures at a scan rate of 0.07 mV s 1 in the voltage range of
Figure 1. TEM and dot-mapping images of the yolk–shell-structured ZnO– Mn3O4 powders with various molar ratios of Zn and Mn components; a) ZnO, b) Zn/Mn(3:1), c) Zn/Mn(2:1), d) Zn/Mn(1:1), e) Zn/Mn(1:2), f) Zn/Mn(1:3), and g) Mn3O4.
sucrose. One yolk–shell particle with distinctive core@void@shell configuration was formed from each droplet, irrespective of the molar ratio of Zn and Mn components. However, the internal structure of the yolk–shell particles was affected by the compositions of the powder. In this study, pure ZnO and Mn3O4 powders prepared by spray pyrolysis of zinc nitrate and manganese nitrate solution, respectively, led to single-shell particles. However, zinc-rich ZnO–Mn3O4 systems, as shown in Figure 1 b and c, gave multishell particles. On the other hand, ZnO–Mn3O4 systems with Mn/Zn molar ratios of 1:1, 2:1, and 3:1, as shown in Figure 1 d–f, had single shells. Furthermore, the TEM images of the different samples, as shown in Figure 1 and Figure S2 (see the Supporting Information), indicated that the powders had a porous structure with very fine crystallite size, irrespective of its composition. The dot-mapping images shown in Figure 1 revealed that the Zn and Mn components were uniformly distributed all over the ZnO–Mn3O4 composite powders, irrespective of the molar ratios of Mn/Zn. The Brunauer–Emmett–Teller (BET) specific surface areas of the ZnO–Mn3O4 systems with Zn/Mn mole ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 were determined to be 16, 21, 21, 22, 19, 21, and 19 m2 g 1, respectively. Figure 2 shows the XRD patterns of the yolk–shell-structured ZnO–Mn3O4 systems with various molar ratios of the Zn and Mn components. The single-component powders, prepared by spray pyrolysis of zinc nitrate and manganese nitrate solutions, had pure crystal structures of hexagonal ZnO and tetragonal Mn3O4, as shown in Figure S4 (see the Supporting Information). The XRD patterns of the zinc-rich ZnO–Mn3O4 systems with Chem. Eur. J. 2014, 20, 3014 – 3018
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Figure 3. Cyclic voltammograms for the first and second cycles at a scan rate of 0.07 mV s 1; a) yolk–shell-structured ZnO and Mn3O4 powders, b) yolk–shell-structured ZnO–Mn3O4 powders with Zn/Mn molar ratios of 2:1, 1:1, and 1:2.
0.001–3 V. In the case of ZnO, there is only one strong reduction peak at 0.3 V in the cathodic scan. The first electrochemical process of the ZnO powder involves the reduction of ZnO to Zn and Li2O, formation of a Li–Zn alloy, and growth of the solid electrolyte interphase (SEI) layer.[5–7, 15] The oxidation peaks appearing at 0.3, 0.53, and 0.6 V in the first anodic scan indicate the multistep delithiation process of the Li–Zn
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Communication alloy.[5–7, 15] The oxidation peak at 1.3 V could be related to the oxidation of Zn upon reaction between Zn and Li2O.[6] Similarly, Mn3O4 yolk–shell-structured powders had two main peaks at 1.07 and 0.08 V in the first cathodic scan, which could be attributed to the reduction of Mn3O4 to MnO and reduction of MnO into Mn and Li2O, respectively.[8–10] The main peak appearing at 1.2 V in the first anodic scan could be assigned to the formation of Mn3O4.[8–10] The cathodic peaks of pure ZnO and Mn3O4 yolk–shell-structured powders shifted to higher voltage regions in the second cycles. The CVs of ZnO–Mn3O4 composites with various Zn/Mn molar ratios showed features similar to those of ZnO and Mn3O4, as shown in Figure 3 b. Intensive reduction peaks at 0.2–0.25 V were observed in the first cathodic scans, irrespective of compositions. This could be attributed to the irreversible reduction of ZnO–Mn3O4 composites to form metallic nanograins (Zn0, Mn0) dispersed in an amorphous Li2O matrix.[15–21] The two oxidation peaks at around 1.2 and 1.5 V can be associated with the oxidation of metallic Zn and Mn nanograins to ZnO (1.5 V) and Mn3O4 (1.2 V), respectively.[15–21] The cathodic peaks of the ZnO–Mn3O4 composite yolk–shell-structured powders also shifted to higher voltage regions in the second cycles. Figure 4 a shows the initial charge and discharge curves of the yolk–shell-structured ZnO–Mn3O4 systems with various molar ratios of the Zn and Mn components at a constant current density of 700 mA g 1. The initial discharge capacities of the ZnO–Mn3O4 systems with Zn/Mn molar ratios of 1:0, 2:1, 1:1, 1:2, and 0:1 were 1118, 1224, 1470, 1312, and 1192 mA h g 1, respectively, the charge capacities were 680, 724, 919, 802, and 699 mA h g 1, respectively, and the respective corresponding Coulombic efficiencies were 61, 59, 63, 61, and 59 %. Intriguingly, ZnO–Mn3O4 yolk–shell-structured powders with an equal molar content of Zn and Mn components had the highest initial charge and discharge capacities. Figure 4 b shows the cycling performances of the ZnO–Mn3O4 yolk–shell-structured powders at a constant current density of 700 mA g 1. The ZnO–Mn3O4 systems with Zn/Mn molar ratios of 1:0, 2:1, 1:1, 1:2, and 0:1 delivered discharge capacities of 202, 634, 912, 806, and 324 mA h g 1, respectively, after 100 cycles, and the corresponding capacity retentions measured after the first cycle were 29, 86, 100, 102, and 48 %. During cycling, the discharge capacities of the yolk–shell-structured ZnO–Mn3O4 composite powders increased gradually, primarily owing to the formation of a polymeric gel-like film on the active material.[15, 16, 27, 38] The capacity increasing during cycling by formation of this polymeric gel-like film on the yolk–shellstructured ZnO–Mn3O4 systems could be seen to occur irrespective of Zn/Mn molar ratios. However, the capacity fading of the pure ZnO and Mn3O4 yolk–shell-structured powders diminished the capacity increasing by formation of a polymeric gel-like film on the active material. Of all the different samples analyzed in this study, yolk–shell-structured powders with Zn/Mn molar ratio of 1:1 exhibited the best cycle performance at a high current density of 2000 mA g 1, as shown in the Figure S6 (see the Supporting Information). Also, the electrodes with Zn/Mn (3:1) and Zn/Mn (1:3) molar ratios have better electrochemical properties than the single-component metal Chem. Eur. J. 2014, 20, 3014 – 3018
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Figure 4. Electrochemical properties of the yolk–shell-structured ZnO–Mn3O4 powders; a) initial cycle profiles in the voltage range of 0.001–3 V at 700 mA g 1, b) cycle performances at 700 mA g 1, c) rate performance of powders with Zn/Mn molar ratio of 1:1.
oxide, as shown in Figure S7 (see the Supporting Information). Figure 4 c shows the rate performances of the ZnO–Mn3O4 yolk–shell-structured powders with Zn/Mn molar ratio of 1:1. The current density was increased from 500–3000 mA g 1 in a step-by-step manner and restored to 500 mA g 1. For each step, 10 cycles were measured to evaluate the rate performance. The stable reversible discharge capacities of the yolk–shell-structured powder decreased from 815 to 460 mA h g 1 as the current density was increased from 500 to 3000 mA g 1. The discharge capacity recovered to 790 mA h g 1 as the current density was restored to 500 mA g 1. The structural stabilities of the yolk–shell-structured pure ZnO and Mn3O4, and ZnO–Mn3O4 system with Zn/Mn molar ratio of 1:1 during cycling were proved by electrochemical impedance spectroscopy (EIS) measurements, as shown in Figure S10 (see the Supporting Information). Impedance measurements were carried out at room temperature on cells after 1 and 50 cycles, in the potential range 0.01–3.0 V at a current density of 700 mA g 1. After 50 cycles, the Nyquist plots of the electrodes were composed of a semicircle in the medium frequency region and an inclined line at low frequencies. The medium-frequency semicircle was assigned to the charge-
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Communication transfer resistance, and the line inclined at approximately 458 to the real axis corresponded to the lithium diffusion process within the electrodes.[35] The charge-transfer resistances of the yolk–shell-structured pure ZnO and Mn3O4 were much higher than that of the ZnO–Mn3O4 system with Zn/Mn molar of 1:1. However, the yolk–shell-structured pure ZnO and Mn3O4, and ZnO–Mn3O4 system had similar charge-transfer resistances after one cycle. The TEM images of the ZnO, Mn3O4, and ZnO– Mn3O4 system with Zn/Mn molar ratio of 1:1 after 50 cycles are shown in Figure S11 (see the Supporting Information). The ZnO and Mn3O4 powders lost their yolk–shell structures after cycling as shown in the TEM images. However, the yolk–shell structure of the ZnO–Mn3O4 system with Zn/Mn molar ratio of 1:1 was maintained even after cycling. The yolk–shell-structured ZnO–Mn3O4 system with Zn/Mn molar ratio of 1:1 with stable structure had the lowest resistance compared to the other samples after cycling. In summary, yolk–shell-structured ZnO–Mn3O4 systems with different molar ratios of Zn/Mn were directly prepared by a spray-pyrolysis process, and their electrochemical properties were analyzed. One yolk–shell particle with either a single shell or double shells was formed from each droplet, according to the molar ratio of the Zn and Mn components. The uniform mixing of the Zn and Mn components in the yolk–shell-structured ZnO–Mn3O4 systems was achieved because of the microscale reaction inside the micron-sized droplets. Electrochemical analysis of the different samples prepared in this study indicated that the yolk–shell-structured ZnO–Mn3O4 powders with the same molar content of Zn and Mn components had the highest initial charge and discharge capacities, and good cycling performances both at low and high current densities of 700 and 2000 mA g 1, respectively.
Experimental Section Synthesis Yolk–shell-structured ZnO, Mn3O4, and ZnxMn1-xOy powders with various molar ratios of the Zn and Mn components were directly prepared by one-pot spray pyrolysis. Figure S1 (see the Supporting Information) shows the schematic illustration of the ultrasonic spray-pyrolysis system used in this study. A quartz reactor of length 1200 mm and diameter 50 mm was used. The reactor temperature was maintained at 800 8C. The flow rate of the air used as the carrier gas was 10 L min 1. The spray solutions were obtained by dissolving zinc nitrate hexahydrate [Zn(NO3)2·6 H2O] and manganese nitrate tetrahydrate [Mn(NO3)2·4 H2O] in distilled water. The total concentration of Zn and Mn components was fixed at 0.2 m, whereas the concentration of sucrose, which was used as the carbon source material, was fixed at 0.7 m.
Characterizations The crystal structure of the as-prepared powders was investigated by using X-ray diffraction (XRD, X’pert PRO MPD) with CuKa radiation (l = 1.5418 ) at the Korea Basic Science Institute (Daegu). Morphological characteristics were investigated by using high-resolution transmission electron microscopy (TEM, JEOL-2100F) at 200 kV. The specific surface areas of the powders were calculated Chem. Eur. J. 2014, 20, 3014 – 3018
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from Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000).
Electrochemical measurements The capacity and cycling performance of the powders were determined by constructing a 2032-type coin cell. The anode was prepared by mixing 80 wt % active material, 10 wt % Super P, and 10 wt % sodium carboxymethyl cellulose (CMC) binder. Lithium metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was 1 m LiPF6 in a 1:1 mixture by volume of ethylene carbonate/dimethyl carbonate (EC/DMC) with 2 % vinylene carbonate. The charge/discharge characteristics of the samples were determined through cycling in the potential range of 0.001–3 V at diverse current densities. Cyclic voltammetry measurements were carried out at a scan rate of 0.07 mV s 1.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST; No. 2012R1A2A2A02046367). This study was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-50210). Keywords: anode materials · lithium-ion batteries · metal oxides · spray pyrolysis · yolk–shell materials [1] M. V. Reddy, G. V. S. Rao, B. V. R. Chowdari, Chem. Rev. 2013, 113, 5364 – 5457. [2] J. Jiang, Y. Y. Li, J. P. Liu, X. T. Huang, C. Z. Yuan, X. W. Lou, Adv. Mater. 2012, 24, 5166 – 5180. [3] X. Y. Lai, J. E. Halpert, D. Wang, Energy Environ. Sci. 2012, 5, 5604 – 5618. [4] L. Hu, P. Zhang, H. Zhong, X. R. Zheng, N. Yan, Q. W. Chen, Chem. Eur. J. 2012, 18, 15049 – 15056. [5] X. H. Huang, X. H. Xia, Y. F. Yuan, F. Zhou, Electrochim. Acta 2011, 56, 4960 – 4965. [6] X. Y. Shen, D. B. Mu, S. Chen, B. R. Wu, F. Wu, ACS Appl. Mater. Interfaces 2013, 5, 3118 – 3125. [7] M. S. Wu, H. W. Chang, J. Phys. Chem. C 2013, 117, 2590 – 2599. [8] H. L. Wang, L. F. Cui, Y. Yang, H. S. Casalongue, J. T. Robinson, Y. Y. Liang, Y. Cui, H. J. Dai, J. Am. Chem. Soc. 2010, 132, 13978 – 13980. [9] J. Gao, M. A. Lowe, H. D. AbruÇa, Chem. Mater. 2011, 23, 3223 – 3227. [10] Z. Q. Li, N. N. Liu, X. K. Wang, C. B. Wang, Y. X. Qi, L. W. Yin, J. Mater. Chem. 2012, 22, 16640 – 16648. [11] M. V. Reddy, C. Yu, F. Jiahuan, K. P. Loh, B. V. R. Chowdari, RSC Adv. 2012, 2, 9619 – 9625. [12] C. T. Cherian, M. V. Reddy, S. C. Haur, B. V. R. Chowdari, ACS Appl. Mater. Interfaces 2013, 5, 918 – 923. [13] Y. Sharma, N. Sharma, G. V. S. Rao, B. V. R. Chowdari, Adv. Funct. Mater. 2007, 17, 2855 – 2861. [14] B. Liu, J. Zhang, X. F. Wang, G. Chen, D. Chen, C. W. Zhou, G. Z. Shen, Nano Lett. 2012, 12, 3005 – 3011. [15] F. M. Courtel, H. Duncan, Y. Abu-Lebdeh, I. J. Davidson, J. Mater. Chem. 2011, 21, 10206 – 10218. [16] G. Q. Zhang, L. Yu, H. B. Wu, H. E. Hoster, X. W. Lou, Adv. Mater. 2012, 24, 4609 – 4613. [17] L. A. Zhou, H. B. Wu, T. Zhu, X. W. Lou, J. Mater. Chem. 2012, 22, 827 – 829. [18] S. W. Kim, H. W. Lee, P. Muralidharan, D. H. Seo, W. S. Yoon, D. K. Kim, K. S. Kang, Nano Res. 2011, 4, 505 – 510. [19] Y. F. Deng, S. D. Tang, Q. M. Zhang, Z. C. Shi, L. T. Zhang, S. Z. Zhan, G. H. Chen, J. Mater. Chem. 2011, 21, 11987 – 11995.
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Communication [20] L. F. Xiao, Y. Y. Yang, J. Yin, Q. Li, L. Z. Zhang, J. Power Sources 2009, 194, 1089 – 1093. [21] F. M. Courtel, Y. Abu-Lebdeh, I. J. Davidson, Electrochim. Acta 2012, 71, 123 – 127. [22] M. X. Li, Y. X. Yin, C. J. Li, F. Z. Zhang, L. J. Wan, S. L. Xu, D. G. Evans, Chem. Commun. 2012, 48, 410 – 412. [23] S. H. Choi, Y. C. Kang, Nanoscale 2013, 5, 4662 – 4668. [24] K. S. Park, S. D. Seo, H. W. Shim, D. W. Kim, Nanoscale Res. Lett. 2012, 7, 35 – 41. [25] P. F. Teh, S. S. Pramana, Y. Sharma, Y. W. Ko, S. Madhavi, ACS Appl. Mater. Interfaces 2013, 5, 5461 – 5467. [26] L. Yu, L. Zhang, H. B. Wu, G. Q. Zhang, X. W. Lou, Energy Environ. Sci. 2013, 6, 2664 – 2671. [27] L. Zhou, D. Y. Zhao, X. W. Lou, Adv. Mater. 2012, 24, 745 – 748. [28] J. Liu, S. Z. Qiao, J. S. Chen, X. W. Lou, X. R. Xing, G. Q. Lu, Chem. Commun. 2011, 47, 12578 – 12591. [29] N. Liu, H. Wu, M. T. McDowell, Y. Yao, C. M. Wang, Y. Cui, Nano Lett. 2012, 12, 3315 – 3321.
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[30] J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang, D. Wang, Angew. Chem. 2013, 125, 6545 – 6548; Angew. Chem. Int. Ed. 2013, 52, 6417 – 6420. [31] X. W. Lou, L. A. Archer, Z. C. Yang, Adv. Mater. 2008, 20, 3987 – 4019. [32] J. Liu, H. Q. Yang, F. Kleitz, Z. G. Chen, T. Y. Yang, E. Strounina, G. Q. Lu, S. Z. Qiao, Adv. Funct. Mater. 2012, 22, 591 – 599. [33] T. Y. Yang, J. Liu, Y. Zheng, M. J. Monteiro, S. Z. Qiao, Chem. Eur. J. 2013, 19, 6942 – 6945. [34] Y. J. Hong, M. Y. Son, Y. C. Kang, Adv. Mater. 2013, 25, 2279 – 2283. [35] S. H. Choi, Y. C. Kang, ChemSusChem 2013, 6, 2111 – 2116. [36] K. M. Yang, Y. J. Hong, Y. C. Kang, ChemSusChem 2013, 6, 2299 – 2303. [37] C. M. Sim, S. H. Choi, Y. C. Kang, Chem. Commun. 2013, 49, 5978 – 5980. [38] J. L. Cheng, B. Wang, C. M. Park, Y. P. Wu, H. Huang, F. Nie, Chem. Eur. J. 2013, 19, 9866 – 9874. Received: October 22, 2013 Revised: December 15, 2013 Published online on February 13, 2014
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Using Simple Spray Pyrolysis to Prepare Yolk–Shell-Structured ZnO–Mn3O4 Systems with the Optimum Composition for Superior Electrochemical Properties Seung Ho Choi and Yun Chan Kang*[a] Abstract: A spray-pyrolysis process is introduced as an effective tool for the preparation of yolk–shell-structured materials with electrochemical properties suitable for anode materials in Li-ion batteries (LIBs). Yolk–shell-structured ZnO–Mn3O4 systems with various molar ratios of the Zn and Mn components are prepared. The yolk–shellstructured ZnO–Mn3O4 powders with a molar ratio of 1:1 of the Zn and Mn components are shown to have high capacities and good cycling performances.
Transition-metal oxides, such as ZnO, Mn3O4, Fe2O3, and NiO, are considered to be promising anodic materials for lithiumion batteries (LIBs) owing to their advantages in terms of high theoretical capacity, safety, and low cost.[1–26] Even though nanostructuring of transition-metal oxides is expected to improve their electrochemical properties, most metal oxides still suffer from poor cyclability, arising from the large specific volume change (above 200 %) during the repetitive charging and discharging processes.[1–10] Binary transition-metal oxides with single phases, such as ABO4 and AB2O4 (A = Zn, Ni, Cu; B = Mn, Co, Fe, Mo), have been studied to overcome the poor cycling performance of single element transition-metal oxides.[1, 11–22] Binary transition-metal oxides get converted into fine nanocomposites of each of the metal oxides and Li2O immediately after the first electrochemical conversion reaction.[1, 11–22] These separated metal oxides and Li2O offer a buffering effect, thereby accommodating the volume change during the charge/discharge process.[14, 15] That said, the ratio of the metal components comprising the binary transition-metal oxides strongly affects their electrochemical properties for lithium-ion storage. In addition, the optimum ratio of metal components necessary for realizing the best electrochemical performance is dependent on the system of binary transitionmetal oxides. However, the composition of binary transitionmetal oxides that have a single phase is strictly restricted. Mixed oxides of the form x AOy (1 x) BOz with various crystal structures could be considered to show better electrochemical [a] S. H. Choi, Prof. Y. C. Kang Department of Chemical Engineering, Konkuk University 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304118. Chem. Eur. J. 2014, 20, 3014 – 3018
properties than binary transition-metal oxides that have single phases, such as ABO3 and AB2O4. The realization of mixed oxides of x AOy (1 x) BOz with good electrochemical properties necessitates uniform mixing of A and B components. Thus far, electrochemical properties of binary transitionmetal oxides, such as Co–Fe–O, Sn–Cu–O, and Co–Mn–O systems, have been extensively studied as the anode materials for LIBs.[22–26] In addition, yolk–shell-structured powders with distinctive core@void@shell configurations have been widely studied as anode materials for LIBs as these structures provide good electrochemical properties as a result of shortened diffusion length. Also, the void spaces in the yolk–shell structure accommodates the volume expansion during charge/recharge cycles.[16, 27–30] According to previous reports, yolk–shell-structured powders were mainly prepared by using a multistep solution method.[31–33] To the best of our knowledge, the preparation and the electrochemical properties of yolk–shell-structured binary transition-metal oxides of Zn and Mn components have not been reported yet. Among the binary transitionmetal oxides, ZnMn2O4 powders have been studied as promising anode materials for LIBs because of their low potential plateaus, high abundance, and the fact they are environmental friendly.[15–19] Recently, a simple spray pyrolysis process was successfully applied to the preparation of yolk–shell-structured powders.[34–37] In previous studies, the electrochemical properties of the yolk–shell-structured materials with single or binary metal oxides prepared by spray pyrolysis were investigated.[34–37] The electrochemical properties of the yolk–shell-structured binary metal oxides were not compared to those of the yolk–shellstructured single metal oxide in the previous reports. Herein, we report the successful synthesis of yolk–shell-structured ZnO–Mn3O4 systems, with various molar ratios of the Zn and Mn components, by using a one-pot spray-pyrolysis process. The prepared yolk–shell particles had various crystal structures depending on the composition of Zn and Mn components. However, the uniform mixing of the Zn and Mn components could be achieved because of a microscale reaction inside the micron-sized droplets. Furthermore, the effect of composition on the electrochemical properties of the ZnO–Mn3O4 systems has also been investigated. For comparison, powders composed of single-component ZnO and Mn3O4 yolk–shell materials were also prepared. Figure 1 shows the morphology of the ZnO–Mn3O4 systems with various compositions that were prepared directly by spray pyrolysis of the spray solution containing metal salts and
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Figure 2. XRD patterns of the yolk–shell-structured ZnO–Mn3O4 powders with various molar ratios of the Zn and Mn components.
Zn/Mn molar ratios of 3:1 and 2:1 had mixed crystal structures of hexagonal ZnO and the cubic ZnMnO3 phases. On the other hand, the XRD patterns of the ZnO–Mn3O4 systems with Zn/Mn molar ratios of 1:1, 1:2, and 1:3 all had the same crystal structure, indicating the formation of solid solutions. The ZnO– Mn3O4 yolk–shell-structured powders with mixed crystal structures of hexagonal ZnO and the cubic ZnMnO3 phases had multishells, as shown in Figures 1 and 2. Figure 3 a shows the cyclic voltammograms (CVs) of the single-component ZnO and Mn3O4 powders with yolk–shell structures at a scan rate of 0.07 mV s 1 in the voltage range of
Figure 1. TEM and dot-mapping images of the yolk–shell-structured ZnO– Mn3O4 powders with various molar ratios of Zn and Mn components; a) ZnO, b) Zn/Mn(3:1), c) Zn/Mn(2:1), d) Zn/Mn(1:1), e) Zn/Mn(1:2), f) Zn/Mn(1:3), and g) Mn3O4.
sucrose. One yolk–shell particle with distinctive core@void@shell configuration was formed from each droplet, irrespective of the molar ratio of Zn and Mn components. However, the internal structure of the yolk–shell particles was affected by the compositions of the powder. In this study, pure ZnO and Mn3O4 powders prepared by spray pyrolysis of zinc nitrate and manganese nitrate solution, respectively, led to single-shell particles. However, zinc-rich ZnO–Mn3O4 systems, as shown in Figure 1 b and c, gave multishell particles. On the other hand, ZnO–Mn3O4 systems with Mn/Zn molar ratios of 1:1, 2:1, and 3:1, as shown in Figure 1 d–f, had single shells. Furthermore, the TEM images of the different samples, as shown in Figure 1 and Figure S2 (see the Supporting Information), indicated that the powders had a porous structure with very fine crystallite size, irrespective of its composition. The dot-mapping images shown in Figure 1 revealed that the Zn and Mn components were uniformly distributed all over the ZnO–Mn3O4 composite powders, irrespective of the molar ratios of Mn/Zn. The Brunauer–Emmett–Teller (BET) specific surface areas of the ZnO–Mn3O4 systems with Zn/Mn mole ratios of 1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1 were determined to be 16, 21, 21, 22, 19, 21, and 19 m2 g 1, respectively. Figure 2 shows the XRD patterns of the yolk–shell-structured ZnO–Mn3O4 systems with various molar ratios of the Zn and Mn components. The single-component powders, prepared by spray pyrolysis of zinc nitrate and manganese nitrate solutions, had pure crystal structures of hexagonal ZnO and tetragonal Mn3O4, as shown in Figure S4 (see the Supporting Information). The XRD patterns of the zinc-rich ZnO–Mn3O4 systems with Chem. Eur. J. 2014, 20, 3014 – 3018
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Figure 3. Cyclic voltammograms for the first and second cycles at a scan rate of 0.07 mV s 1; a) yolk–shell-structured ZnO and Mn3O4 powders, b) yolk–shell-structured ZnO–Mn3O4 powders with Zn/Mn molar ratios of 2:1, 1:1, and 1:2.
0.001–3 V. In the case of ZnO, there is only one strong reduction peak at 0.3 V in the cathodic scan. The first electrochemical process of the ZnO powder involves the reduction of ZnO to Zn and Li2O, formation of a Li–Zn alloy, and growth of the solid electrolyte interphase (SEI) layer.[5–7, 15] The oxidation peaks appearing at 0.3, 0.53, and 0.6 V in the first anodic scan indicate the multistep delithiation process of the Li–Zn
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Communication alloy.[5–7, 15] The oxidation peak at 1.3 V could be related to the oxidation of Zn upon reaction between Zn and Li2O.[6] Similarly, Mn3O4 yolk–shell-structured powders had two main peaks at 1.07 and 0.08 V in the first cathodic scan, which could be attributed to the reduction of Mn3O4 to MnO and reduction of MnO into Mn and Li2O, respectively.[8–10] The main peak appearing at 1.2 V in the first anodic scan could be assigned to the formation of Mn3O4.[8–10] The cathodic peaks of pure ZnO and Mn3O4 yolk–shell-structured powders shifted to higher voltage regions in the second cycles. The CVs of ZnO–Mn3O4 composites with various Zn/Mn molar ratios showed features similar to those of ZnO and Mn3O4, as shown in Figure 3 b. Intensive reduction peaks at 0.2–0.25 V were observed in the first cathodic scans, irrespective of compositions. This could be attributed to the irreversible reduction of ZnO–Mn3O4 composites to form metallic nanograins (Zn0, Mn0) dispersed in an amorphous Li2O matrix.[15–21] The two oxidation peaks at around 1.2 and 1.5 V can be associated with the oxidation of metallic Zn and Mn nanograins to ZnO (1.5 V) and Mn3O4 (1.2 V), respectively.[15–21] The cathodic peaks of the ZnO–Mn3O4 composite yolk–shell-structured powders also shifted to higher voltage regions in the second cycles. Figure 4 a shows the initial charge and discharge curves of the yolk–shell-structured ZnO–Mn3O4 systems with various molar ratios of the Zn and Mn components at a constant current density of 700 mA g 1. The initial discharge capacities of the ZnO–Mn3O4 systems with Zn/Mn molar ratios of 1:0, 2:1, 1:1, 1:2, and 0:1 were 1118, 1224, 1470, 1312, and 1192 mA h g 1, respectively, the charge capacities were 680, 724, 919, 802, and 699 mA h g 1, respectively, and the respective corresponding Coulombic efficiencies were 61, 59, 63, 61, and 59 %. Intriguingly, ZnO–Mn3O4 yolk–shell-structured powders with an equal molar content of Zn and Mn components had the highest initial charge and discharge capacities. Figure 4 b shows the cycling performances of the ZnO–Mn3O4 yolk–shell-structured powders at a constant current density of 700 mA g 1. The ZnO–Mn3O4 systems with Zn/Mn molar ratios of 1:0, 2:1, 1:1, 1:2, and 0:1 delivered discharge capacities of 202, 634, 912, 806, and 324 mA h g 1, respectively, after 100 cycles, and the corresponding capacity retentions measured after the first cycle were 29, 86, 100, 102, and 48 %. During cycling, the discharge capacities of the yolk–shell-structured ZnO–Mn3O4 composite powders increased gradually, primarily owing to the formation of a polymeric gel-like film on the active material.[15, 16, 27, 38] The capacity increasing during cycling by formation of this polymeric gel-like film on the yolk–shellstructured ZnO–Mn3O4 systems could be seen to occur irrespective of Zn/Mn molar ratios. However, the capacity fading of the pure ZnO and Mn3O4 yolk–shell-structured powders diminished the capacity increasing by formation of a polymeric gel-like film on the active material. Of all the different samples analyzed in this study, yolk–shell-structured powders with Zn/Mn molar ratio of 1:1 exhibited the best cycle performance at a high current density of 2000 mA g 1, as shown in the Figure S6 (see the Supporting Information). Also, the electrodes with Zn/Mn (3:1) and Zn/Mn (1:3) molar ratios have better electrochemical properties than the single-component metal Chem. Eur. J. 2014, 20, 3014 – 3018
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Figure 4. Electrochemical properties of the yolk–shell-structured ZnO–Mn3O4 powders; a) initial cycle profiles in the voltage range of 0.001–3 V at 700 mA g 1, b) cycle performances at 700 mA g 1, c) rate performance of powders with Zn/Mn molar ratio of 1:1.
oxide, as shown in Figure S7 (see the Supporting Information). Figure 4 c shows the rate performances of the ZnO–Mn3O4 yolk–shell-structured powders with Zn/Mn molar ratio of 1:1. The current density was increased from 500–3000 mA g 1 in a step-by-step manner and restored to 500 mA g 1. For each step, 10 cycles were measured to evaluate the rate performance. The stable reversible discharge capacities of the yolk–shell-structured powder decreased from 815 to 460 mA h g 1 as the current density was increased from 500 to 3000 mA g 1. The discharge capacity recovered to 790 mA h g 1 as the current density was restored to 500 mA g 1. The structural stabilities of the yolk–shell-structured pure ZnO and Mn3O4, and ZnO–Mn3O4 system with Zn/Mn molar ratio of 1:1 during cycling were proved by electrochemical impedance spectroscopy (EIS) measurements, as shown in Figure S10 (see the Supporting Information). Impedance measurements were carried out at room temperature on cells after 1 and 50 cycles, in the potential range 0.01–3.0 V at a current density of 700 mA g 1. After 50 cycles, the Nyquist plots of the electrodes were composed of a semicircle in the medium frequency region and an inclined line at low frequencies. The medium-frequency semicircle was assigned to the charge-
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Communication transfer resistance, and the line inclined at approximately 458 to the real axis corresponded to the lithium diffusion process within the electrodes.[35] The charge-transfer resistances of the yolk–shell-structured pure ZnO and Mn3O4 were much higher than that of the ZnO–Mn3O4 system with Zn/Mn molar of 1:1. However, the yolk–shell-structured pure ZnO and Mn3O4, and ZnO–Mn3O4 system had similar charge-transfer resistances after one cycle. The TEM images of the ZnO, Mn3O4, and ZnO– Mn3O4 system with Zn/Mn molar ratio of 1:1 after 50 cycles are shown in Figure S11 (see the Supporting Information). The ZnO and Mn3O4 powders lost their yolk–shell structures after cycling as shown in the TEM images. However, the yolk–shell structure of the ZnO–Mn3O4 system with Zn/Mn molar ratio of 1:1 was maintained even after cycling. The yolk–shell-structured ZnO–Mn3O4 system with Zn/Mn molar ratio of 1:1 with stable structure had the lowest resistance compared to the other samples after cycling. In summary, yolk–shell-structured ZnO–Mn3O4 systems with different molar ratios of Zn/Mn were directly prepared by a spray-pyrolysis process, and their electrochemical properties were analyzed. One yolk–shell particle with either a single shell or double shells was formed from each droplet, according to the molar ratio of the Zn and Mn components. The uniform mixing of the Zn and Mn components in the yolk–shell-structured ZnO–Mn3O4 systems was achieved because of the microscale reaction inside the micron-sized droplets. Electrochemical analysis of the different samples prepared in this study indicated that the yolk–shell-structured ZnO–Mn3O4 powders with the same molar content of Zn and Mn components had the highest initial charge and discharge capacities, and good cycling performances both at low and high current densities of 700 and 2000 mA g 1, respectively.
Experimental Section Synthesis Yolk–shell-structured ZnO, Mn3O4, and ZnxMn1-xOy powders with various molar ratios of the Zn and Mn components were directly prepared by one-pot spray pyrolysis. Figure S1 (see the Supporting Information) shows the schematic illustration of the ultrasonic spray-pyrolysis system used in this study. A quartz reactor of length 1200 mm and diameter 50 mm was used. The reactor temperature was maintained at 800 8C. The flow rate of the air used as the carrier gas was 10 L min 1. The spray solutions were obtained by dissolving zinc nitrate hexahydrate [Zn(NO3)2·6 H2O] and manganese nitrate tetrahydrate [Mn(NO3)2·4 H2O] in distilled water. The total concentration of Zn and Mn components was fixed at 0.2 m, whereas the concentration of sucrose, which was used as the carbon source material, was fixed at 0.7 m.
Characterizations The crystal structure of the as-prepared powders was investigated by using X-ray diffraction (XRD, X’pert PRO MPD) with CuKa radiation (l = 1.5418 ) at the Korea Basic Science Institute (Daegu). Morphological characteristics were investigated by using high-resolution transmission electron microscopy (TEM, JEOL-2100F) at 200 kV. The specific surface areas of the powders were calculated Chem. Eur. J. 2014, 20, 3014 – 3018
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from Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000).
Electrochemical measurements The capacity and cycling performance of the powders were determined by constructing a 2032-type coin cell. The anode was prepared by mixing 80 wt % active material, 10 wt % Super P, and 10 wt % sodium carboxymethyl cellulose (CMC) binder. Lithium metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was 1 m LiPF6 in a 1:1 mixture by volume of ethylene carbonate/dimethyl carbonate (EC/DMC) with 2 % vinylene carbonate. The charge/discharge characteristics of the samples were determined through cycling in the potential range of 0.001–3 V at diverse current densities. Cyclic voltammetry measurements were carried out at a scan rate of 0.07 mV s 1.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST; No. 2012R1A2A2A02046367). This study was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-50210). Keywords: anode materials · lithium-ion batteries · metal oxides · spray pyrolysis · yolk–shell materials [1] M. V. Reddy, G. V. S. Rao, B. V. R. Chowdari, Chem. Rev. 2013, 113, 5364 – 5457. [2] J. Jiang, Y. Y. Li, J. P. Liu, X. T. Huang, C. Z. Yuan, X. W. Lou, Adv. Mater. 2012, 24, 5166 – 5180. [3] X. Y. Lai, J. E. Halpert, D. Wang, Energy Environ. Sci. 2012, 5, 5604 – 5618. [4] L. Hu, P. Zhang, H. Zhong, X. R. Zheng, N. Yan, Q. W. Chen, Chem. Eur. J. 2012, 18, 15049 – 15056. [5] X. H. Huang, X. H. Xia, Y. F. Yuan, F. Zhou, Electrochim. Acta 2011, 56, 4960 – 4965. [6] X. Y. Shen, D. B. Mu, S. Chen, B. R. Wu, F. Wu, ACS Appl. Mater. Interfaces 2013, 5, 3118 – 3125. [7] M. S. Wu, H. W. Chang, J. Phys. Chem. C 2013, 117, 2590 – 2599. [8] H. L. Wang, L. F. Cui, Y. Yang, H. S. Casalongue, J. T. Robinson, Y. Y. Liang, Y. Cui, H. J. Dai, J. Am. Chem. Soc. 2010, 132, 13978 – 13980. [9] J. Gao, M. A. Lowe, H. D. AbruÇa, Chem. Mater. 2011, 23, 3223 – 3227. [10] Z. Q. Li, N. N. Liu, X. K. Wang, C. B. Wang, Y. X. Qi, L. W. Yin, J. Mater. Chem. 2012, 22, 16640 – 16648. [11] M. V. Reddy, C. Yu, F. Jiahuan, K. P. Loh, B. V. R. Chowdari, RSC Adv. 2012, 2, 9619 – 9625. [12] C. T. Cherian, M. V. Reddy, S. C. Haur, B. V. R. Chowdari, ACS Appl. Mater. Interfaces 2013, 5, 918 – 923. [13] Y. Sharma, N. Sharma, G. V. S. Rao, B. V. R. Chowdari, Adv. Funct. Mater. 2007, 17, 2855 – 2861. [14] B. Liu, J. Zhang, X. F. Wang, G. Chen, D. Chen, C. W. Zhou, G. Z. Shen, Nano Lett. 2012, 12, 3005 – 3011. [15] F. M. Courtel, H. Duncan, Y. Abu-Lebdeh, I. J. Davidson, J. Mater. Chem. 2011, 21, 10206 – 10218. [16] G. Q. Zhang, L. Yu, H. B. Wu, H. E. Hoster, X. W. Lou, Adv. Mater. 2012, 24, 4609 – 4613. [17] L. A. Zhou, H. B. Wu, T. Zhu, X. W. Lou, J. Mater. Chem. 2012, 22, 827 – 829. [18] S. W. Kim, H. W. Lee, P. Muralidharan, D. H. Seo, W. S. Yoon, D. K. Kim, K. S. Kang, Nano Res. 2011, 4, 505 – 510. [19] Y. F. Deng, S. D. Tang, Q. M. Zhang, Z. C. Shi, L. T. Zhang, S. Z. Zhan, G. H. Chen, J. Mater. Chem. 2011, 21, 11987 – 11995.
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Communication [20] L. F. Xiao, Y. Y. Yang, J. Yin, Q. Li, L. Z. Zhang, J. Power Sources 2009, 194, 1089 – 1093. [21] F. M. Courtel, Y. Abu-Lebdeh, I. J. Davidson, Electrochim. Acta 2012, 71, 123 – 127. [22] M. X. Li, Y. X. Yin, C. J. Li, F. Z. Zhang, L. J. Wan, S. L. Xu, D. G. Evans, Chem. Commun. 2012, 48, 410 – 412. [23] S. H. Choi, Y. C. Kang, Nanoscale 2013, 5, 4662 – 4668. [24] K. S. Park, S. D. Seo, H. W. Shim, D. W. Kim, Nanoscale Res. Lett. 2012, 7, 35 – 41. [25] P. F. Teh, S. S. Pramana, Y. Sharma, Y. W. Ko, S. Madhavi, ACS Appl. Mater. Interfaces 2013, 5, 5461 – 5467. [26] L. Yu, L. Zhang, H. B. Wu, G. Q. Zhang, X. W. Lou, Energy Environ. Sci. 2013, 6, 2664 – 2671. [27] L. Zhou, D. Y. Zhao, X. W. Lou, Adv. Mater. 2012, 24, 745 – 748. [28] J. Liu, S. Z. Qiao, J. S. Chen, X. W. Lou, X. R. Xing, G. Q. Lu, Chem. Commun. 2011, 47, 12578 – 12591. [29] N. Liu, H. Wu, M. T. McDowell, Y. Yao, C. M. Wang, Y. Cui, Nano Lett. 2012, 12, 3315 – 3321.
Chem. Eur. J. 2014, 20, 3014 – 3018
www.chemeurj.org
[30] J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang, D. Wang, Angew. Chem. 2013, 125, 6545 – 6548; Angew. Chem. Int. Ed. 2013, 52, 6417 – 6420. [31] X. W. Lou, L. A. Archer, Z. C. Yang, Adv. Mater. 2008, 20, 3987 – 4019. [32] J. Liu, H. Q. Yang, F. Kleitz, Z. G. Chen, T. Y. Yang, E. Strounina, G. Q. Lu, S. Z. Qiao, Adv. Funct. Mater. 2012, 22, 591 – 599. [33] T. Y. Yang, J. Liu, Y. Zheng, M. J. Monteiro, S. Z. Qiao, Chem. Eur. J. 2013, 19, 6942 – 6945. [34] Y. J. Hong, M. Y. Son, Y. C. Kang, Adv. Mater. 2013, 25, 2279 – 2283. [35] S. H. Choi, Y. C. Kang, ChemSusChem 2013, 6, 2111 – 2116. [36] K. M. Yang, Y. J. Hong, Y. C. Kang, ChemSusChem 2013, 6, 2299 – 2303. [37] C. M. Sim, S. H. Choi, Y. C. Kang, Chem. Commun. 2013, 49, 5978 – 5980. [38] J. L. Cheng, B. Wang, C. M. Park, Y. P. Wu, H. Huang, F. Nie, Chem. Eur. J. 2013, 19, 9866 – 9874. Received: October 22, 2013 Revised: December 15, 2013 Published online on February 13, 2014
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