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Synthesis, Characterization, and Exchange Bias Effect in Single Crystalline Li0.44MnO2 Nanoribbons Xianke Zhang,† Shaolong Tang,*,‡ and Youwei Du‡ † ‡
College of Physics and Electronics, Gannan Normal University, Ganzhou 341000, China Nanjing National Laboratory of Microstructures, Jiangsu Provincial Laboratory for NanoTechnology, Department of Physics, Nanjing University, Nanjing 210093, China ABSTRACT: Single crystalline Li0.44MnO2 nanoribbons are synthesized via the Na0.44MnO2 nanoribbons precursor template in LiNO3-LiCl eutectic molten salt. Li0.44MnO2 and Na0.44MnO2 nanoribbons are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy techniques. The influences of reaction temperature on the formation of Li0.44MnO2 nanoribbons have also been discussed. Magnetization measurements show that Li0.44MnO2 nanoribbons exhibit interesting exchange bias effects and the magnitude of exchange bias field can be tuned by the strength of the cooling field. The magnetic behaviors of Li0.44MnO2 nanoribbons can be interpreted based on a core-shell model.
’ INTRODUCTION The exchange anisotropy interaction at the interface between a ferromagnetic (FM) and an antiferromagnetic (AFM) component usually results in the exchange bias (EB) effect, which can manifest itself as a shift exchange bias field (HE) of the magnetization hysteresis loop along the field axis and has been extensively investigated in recent years due to the intriguing physics and its importance in technological applications.1 The EB phenomena has been observed in many different systems: particles where the cores couple to the shells,2-5 inhomogeneous spin glass (SG) where FM domains couple to AFM domains,6 thin films consisting of bilayer where a FM layer couples to an AFM layer,7-11 or double superlattices where an artificial FM superlattice couples to an artificial AFM superlattice.12 Recently, Ali et al. reported the observation of an EB effect in a FM/SG bilayer system and proposed that this is a phenomenon of greater generality.13 The EB has been observed in the intrinsically phase-separated spin and/ or cluster glass bulk manganites14 and cobalties15 when considering coupling between the FM clusters and the AFM matrix or SG regions. Furthermore, the EB effect has also been found in one-dimensional (1D) magnetic nanostructures, such as Co3O4 nanowires,16,17 SrMn3O6-x nanoribbons,18 CaMn2O4 nanowires,19 Li4Mn5O12 nanosticks,20 and La0.25Ca0.75MnO3 nanowires.21 In recent years, extensive efforts have been devoted to the investigation of lithium manganese oxides as positive electrodes for rechargeable lithium batteries due to their low cost, low toxicity, and superior safety. Li0.44MnO2 has been widely researched as a positive electrode material for secondary lithium batteries.22-27 Generally speaking, Li0.44MnO2 can be prepared via an ion r 2011 American Chemical Society
exchange method in molten salt using the corresponding sodium manganese oxide (Na0.44MnO2) as the parent compound.26,27 Furthermore, it is clearly confirmed that the Li0.44MnO2 material is interesting not only for its good cycle performance23 but also for its high voltage and high specific capacity.24,25 The single crystalline Li0.44MnO2 nanowires have also been synthesized by ion exchange reaction, and electrochemical measurements show Li0.44MnO2 nanowires possess both the large specific capacity and good high current density charge-discharge performance based on the nanostructure nature for a lithium ion battery.28 While studies on the magnetic nature of Li0.44MnO2 are very limited, in this paper we report the preparation and magnetic properties of single crystalline Li0.44MnO2 nanoribbons.
’ EXPERIMENTAL SECTION The single crystalline Li0.44MnO2 nanoribbons are synthesized by a facile reaction using long single crystalline Na0.44MnO2 nanoribbons as the self-template, which is prepared by a simple molten salt synthesis (MSS) process. In a typical procedure of preparing Na0.44MnO2 nanoribbons, 1 mmol of MnCO3 and 0.44 mmol of Na2CO3 were mixed with 5.0 g of NaCl, ground homogeneously in a mortar for 30 min. The mixture was then placed into an alumina crucible, and annealed at 850 °C for 5 h in a crucible furnace, and subsequently naturally cooled to room temperature. The resulting product was washed several times with distilled water and then dried at 100 °C in a drying oven. The Received: October 14, 2010 Revised: December 7, 2010 Published: January 27, 2011 2644
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Figure 1. XRD patterns of the as-synthesized products: (a) Na0.44MnO2 and (b) Li0.44MnO2.
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Figure 2. FE-SEM images of (a) Na0.44MnO2 and (b) Li0.44MnO2 nanoribbons.
synthesis of Li0.44MnO2 nanoribbons was achieved via Na0.44MnO2 nanoribbons followed by ion exchange to replace sodium by lithium. The sodium/lithium ion-exchange experiments were performed using a molten salt composed of a mixture of LiNO3 (88 mol %) and LiCl (12 mol %) at 400 °C for 1 h in air. The as-synthesized Na0.44MnO2 and Li0.44MnO2 nanoribbons were characterized using an X-ray diffractometer (XRD, Rigaku, D/ Max-RA) with Cu Ka radiation (λ = 1.54 Å). The morphology of these nanoribbons was observed on a field emission scanning electron microscope (FE-SEM, HITACHI X650) and a highresolution transmission electron microscope (HRTEM, JEM, JEOL 2010EX). The selected-area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS) patterns were obtained in TEM observation. Magnetic properties of Li0.44MnO2 nanoribbons were measured on a commercial superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum Design).
’ RESULTS AND DISCUSSION The purity and crystallinity of the precursor template Na0.44MnO2 nanoribbons were examined by powder XRD measurements (Figure 1a). It is evident that the observed pattern of the collected sample displayed all of the expected peaks emanating from pure orthorhombic Na4Mn9O18 (JCPDS No. 27-0750). Figure 1b shows the XRD pattern of the Li0.44MnO2 nanoribbons. Compared with the Na0.44MnO2, we find the Li0.44MnO2 peaks with the same indexes to Na0.44MnO2 move toward to higher angle since the diameter of lithium ion (Liþ) is smaller than that of a sodium ion (Naþ). Figure 2a presents the FE-SEM image of the Na0.44MnO2 nanoribbons. A large quantity of nanoribbons with diameters ranging from 100 nm to a few hundred nanometers and length up to tens of micrometers were obtained. The starting materials were mostly transformed into the nanoribbons products from SEM observations of the samples. The FE-SEM image revealing the morphologies of Li0.44MnO2 is shown in Figure 2b. The final product still preserves the 1D nanostructure, and the length and width of Li0.44MnO2 nanoribbons are almost same as that of Na0.44MnO2 nanoribbons, and the thickness of most Li0.44MnO2 and Na0.44MnO2 nanoribbons is less than 100 nm, which confirm
Figure 3. (a) Typical TEM image of a single Na0.44MnO2 nanoribbon. Inset of (a) is the SAED pattern recorded from the [010] zone axis. (b) HRTEM image of the nanoribbon. (c) EDS analysis of as-prepared Na0.44MnO2 nanoribbons.
the precursor template (Na0.44MnO2 nanoribbons) efect on the ultimate products (Li0.44MnO2 nanoribbons). This synthesis route is thought to be template techniques, the so-called self-sacrificing template process.29-32 A typical single Na0.44MnO2 nanoribbon is shown in Figure 3a, the width of the nanoribbon is about 150 nm. The SAED (inset of Figure 3a) pattern recorded along [010] zone axis indicates that the nanoribbon is single crystalline in nature. In Figure 3b, the HRTEM image obtained from part of the individual nanoribbon shows an interplanar spacing of 0.45 nm, which corresponds to the (200) plane distance of the orthorhombic Na0.44MnO2. On the basis of HRTEM and SAED, the nanoribbon grows along its [001] crystallographic direction, which is consistent with other related reports.33 In order to confirm the chemical compositions of the as-prepared 2645
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Figure 4. (a) TEM image of the Li0.44MnO2 nanoribbons. (b) An individual Li0.44MnO2 nanoribbon with a diameter of 200 nm. Inset of (b) is its corresponding SAED pattern. (c) HRTEM image of the Li0.44MnO2 nanoribbon lattice and (d) its EDS data.
structures, EDS spectra measurements were taken. EDS analysis (Figure 3c) shows that the chemical components of the nanoribbon are Na, Mn, and O. The Cu peak originates from TEM grids. The single crystalline Na0.44MnO2 nanoribbons then were converted into lithium manganese oxide nanoribbons in LiNO3LiCl eutectic molten salt. Figure 4a gives the TEM image with a few Li0.44MnO2 nanoribbons. From the image, we can see the diameters of these nanoribbons are between 100 nm and several hundred nanometers, which is consistent with SEM observation. A typical TEM image of single Li0.44MnO2 nanoribbon is shown in Figure 4b. The SAED pattern (inset of Figure 4b) taken from this individual nanoribbon shows the presence of sharp diffraction spots revealing the formation of well-developed, single crystalline Li0.44MnO2. The HRTEM image of the nanoribbon (Figure 4c) shows the fringe spacing is 0.43 nm, which corresponds to the (200) interplanar distance of the Li0.44MnO2. The growth direction is determined to its [001] crystallographic orientation according to HRTEM and SAED, which indicates Li0.44MnO2 nanoribbons have the same growth direction as Na0.44MnO2 nanoribbons. EDS data (Figure 4d) suggest that the Naþ in Na0.44MnO2 has been displaced with Liþ, which is from the composite molten salt. The Li element cannot be detected by EDS. These images of Li0.44MnO2 nanoribbons indicate that the morphology of single crystalline Na0.44MnO2 nanoribbons is maintained after the molten salt treatment. From a view of crystal structure, Na0.44MnO2 has two types of tunnels. Two sodium sites are situated in large S-shaped tunnels, while another site is found in smaller tunnels. According to this structure, the [001] direction is the main path for sodium diffusion.34 Therefore, the Liþ in molten salt diffuse into crystal lattice of Na0.44MnO2 mainly along the [001] orientation, which is the growth direction of Na0.44MnO2 nanoribbons. This further indicates the substitution of Liþ for Naþ should be along the axis of Na0.44MnO2 nanoribbons. The [001] growth direction of Li0.44MnO2 nanoribbons may virtually manifest this point. The formation reaction from Na0.44MnO2 to Li0.44MnO2 nanoribbons can be explained as 0:44Liþ þ Na0:44 MnO2 f Li0:44 MnO2 þ 0:44Naþ
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Figure 5. The XRD patterns of product after the ion exchange between Liþ and Naþ: (a) 350 °C for 1 h; (b) 450 °C for 1 h; (c) 465 °C for 1 h; (d) 500 °C for 1 h. The triangles and squares denote Li0.44MnO2 and Li2MnO3 diffraction peaks, respectively.
In our experiments, it was found that the temperature of eutectic molten salt has an important influence on the final product. Figure 5a presents the XRD pattern that the ion exchange reaction proceeds at 350 °C for 1 h in air, which can be identified as the Li0.44MnO2 phase. When the reaction temperature is raised to 450 °C, the two phases of Li0.44MnO2 and Li2MnO3 have both been confirmed in the XRD patterns as shown in Figure 5b. The main phase agrees with Li0.44MnO2, and the second phase should be Li2MnO3. Figure 5c shows the XRD pattern of product performed at 465 °C for 1 h in air, which can be indexed to Li2MnO3 according to JCPDS data No. 84-1634 with monoclinic symmetry. With further increase in the molten salt temperature (500 °C), the phase of the ultimate product is still Li2MnO3 as indicated in Figure 5d. It is worth noting that the morphology of the final product at different reaction temperature after the molten salt treatment is 1D nanostructure. In a very recent paper,35 Kikkawa et al. reported the atomic and electronic structures of the mixture of Li0.44MnO2 nanowires and byproduct Li2MnO3 nanoparticles formed after the conversion of Na0.44MnO2 nanowires into Li0.44MnO2 nanowires. Comparing our results with that of mentioned above, we can find the temperature of eutectic molten salt has really a critical effect on the final product. When the temperature is 450 °C, the XRD pattern confirmed that the main phase agrees with Li0.44MnO2, and the second phase is Li2MnO3 (Figure 3 in ref 35). The byproduct Li2MnO3 nanoparticles adhered to the faces of single crystalline Li0.44MnO2 nanowires with the specific relationship of crystallographic orientations between the Li2MnO3 nanoparticles and Li0.44MnO2 nanowires. However, the XRD patterns of our products performed at 465 and 500 °C also show a pure phase of Li2MnO3, and there is not the second phase. Some Li2MnO3 buds adhered to the surface of Li2MnO3 nanoribbons. This will be discussed in detail in our future papers. The magnetic characterization of Li0.44MnO2 nanoribbons was performed by using a SQUID magnetometer. Any effect on the magnetic properties caused by the shape anisotropy of the nanoribbons will 2646
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Figure 6. ZFC (squares) and FC (circles) magnetization curves measured in an applied magnetic field of 20 Oe as a function of temperature. Inset: M (T) curves for ZFC and FC under a 1 kOe field.
not be considered in our experiments because the measurements were done on powders with randomly oriented Li0.44MnO2 nanoribbons. Figure 6 shows the magnetizations of the Li0.44MnO2 nanoribbons as a function of temperature at an applied magnetic field strength of 20 Oe after zero field cooling (ZFC) and also with field cooling (FC). In ZFC measurement, the sample is cooled from room temperature to 3 K without applying a magnetic field, and then the magnetization is recorded on warming and in a weak external field of 20 Oe. For the FC, the sample is cooled from room temperature in an applied magnetic field of 20 Oe. Two characteristic features can be found in Figure 6. First, a bifurcation of the FC and ZFC magnetization below 300 K can be seen, which is a typical feature of SG, superparamagnetic (SPM), or diluted AFM in a field (DAFF) behavior. The separation between FC and ZFC curves indicates a nonequilibrium magnetization state below 300 K for the ZFC case. Second, a sharp peak in the ZFC curve is presented about at T = 10 K, which usually signifies SG, SPM, or simply AFM behavior. Below 50 K, both ZFC and FC magnetizations increase rapidly with decreasing temperature, which may reveal the presence of local FM ordering among clusters as well as weak FM tendency at low temperature. The inset of Figure 6 presents the ZFC and FC magnetization of our sample measured at H = 1000 Oe. In this case, the ZFC and FC magnetization curves show strong overlapping, while, the peak in ZFC curve is also visible at low temperature. So, the higher a magnetic field is, the less inconspicuous the difference between ZFC and FC curves becomes and then the lower the bifurcation temperature is. This behavior indicates that the high magnetic field suppresses the relaxation behavior of SG. The fitting to the data of 1000 Oe ZFC susceptibility in Li0.44MnO2 nanoribbons above 160 K by the Curie-Weiss law, χ = χ0 þ C/(T - ΘW) presents the parameters: the temperature-independent contribution χ0 = 5.7 10-4 emu/mol Oe, the Curie constant C = 2.45 emu K/ mol, and the Curie-Weiss temperature ΘW = -264 K. The negative sign and the high absolute value of ΘW indicate strong AFM exchange between the manganese ions. Figure 7 indicates the ZFC magnetizations measured at different applied magnetic fields (H = 20, 100, and 1000 Oe) and the solid lines are the peaks fitting with Lorentzian function via the Origin 7.0 software. Each ZFC susceptibility curve exhibits a peak at lower temperature; the peak position is denote as Tf. We can obtain that Tf is 10.00, 8.16, and 6.09 K at H = 20, 100, and 1000 Oe by Lorentzian fitting, individually. Therefore, Tf shifts to lower temperatures with increasing magnetic fields. The peak position
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Tf as a function of applied magnetic field is plotted in Figure 7d. As we know, in most AFM systems the ZFC peak position is not expected to show any significant shift with increasing field. So, the ZFC peak position Tf indicates the spin glass freezing in our sample. As a rule, higher magnetic fields suppress the energy barriers and thus reduce the freezing temperature, which is consistent with a SG transition. This virtually matches with the observation seen in Figure 7d. The origin of SG transition in the Li0.44MnO2 nanoribbons will be discussed. Kodama et al. and Martínez et al. have argued that freezing of surface spins in nanoparticles can lead to a SG phase.36,37 Kodama et al. have proposed a model in which the spins at the surface of a particle are disordered, leading to frustration and spin-glass-like (SG-like) freezing. Li0.44MnO2 exhibits a composite tunnel structure. The walls of tunnels consist of double chains of edge-sharing MnO6 octahedra, linked by corners to each other. The Mn-Mn network forms an isosceles triangular lattice within the double chains, which is expected to cause geometrical frustrations, which can give rise to SG behavior. So, the SG behavior formation may be ascribed to the surface spin disorder and/or geometrical frustration. The clarification as to whether any or all of these factors are operative behind our observation is beyond the scope of this present work and will form the subject of future studies. The magnetization dependence on the applied magnetic field (hysteresis loops) was measured at T = 3 K after cooling in zero field and after cooling in a field of 40 kOe; see Figure 8. The M (H) curve after ZFC is symmetrical and centered about the origin with a small coercivity of 290 Oe and a nearly linear shape in the field range used, which is usually thought to be from a regular AFM system. It is also observed in the hysteresis loop that the magnetization saturation did not show in fields up to 30 kOe like other conventional SG systems.14 A more careful examination of the FC hysteresis loops measured in 40 kOe reveals two notable features, namely, a hysteresis (coercivity) and a shift toward negative magnetic fields as well as to the positive magnetization axis of the M (H) curve from the origin, i.e., EB effect (inset of Figure 8). The two features clearly show that, superposed on the predominant AFM signal, there is a minor FM component. The origin of this weak ferromagnetism at low temperature may be due to the small dimensions of the naoribbons. A bulk antiferromagnet has zero net magnetic moment due to its two mutually compensating sublattices in a zero field. If the surface to volume becomes sufficiently large for an AFM particle, then the surface always leads to a breaking of the sublattice pairing and thus leads to net magnetic moment. Different models have been proposed to explain this weak ferromagnetism in small AFM nanoparticles.38,39 As generally accepted, the exchange bias field is defined as HE = |H1 þ H2|/2, where H1 and H2 are the negative field and the positive field where the magnetization equals to zero, respectively. In order to investigate the cooling field dependence of exchange bias field, we measured the hysteresis loop of Li0.44MnO2 nanoribbons at 3 K in the field range of -50 kOe e H e 50 kOe under cooling fields of 10, 20, and 60 kOe. Figure 9 shows the enlarged view of the low field region, which indicates the shift of the hysteresis curve due to exchange bias coupling. For the FC loops under the cooling field of 10, 20, and 60 kOe, the value of HE is about 176, 207, and 224 Oe, respectively, indicating the exchange bias is strongly dependent on the cooling field. We can observe that the HE increases with increasing cooling field from the inset of Figure 9. Let us discuss the origin of EB effect in our sample. Recently, a core-shell phenomenological model was proposed to describe the magnetic structure of manganite nanoparticles.40 In the core-shell 2647
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Figure 7. The ZFC magnetization curves measured at applied magnetic fields at low temperature: (a) H = 20 Oe, (b) H = 100 Oe, (c) H = 1000 Oe. The solid lines are the fitting of peak configuration with Lorentzian function. (d) The dependence of Tf on the applied magnetic field (H).
Figure 8. Magnetization as a function of external magnetic field at T = 3 K for Li0.44MnO2 nanoribbons after ZFC (open squares) and FC (open circles). Inset: the enlarged view of the central part, which indicates the shift of the hysteresis loop due to exchange bias coupling.
structure, the inner part of the particle (the core) has the same properties as the bulk materials, whereas the outer layer (the shell) contains most of the oxygen faults and vacancies in the crystallographic structure, which give rise to the relaxation of superexchange interaction on the surface of AFM nanowires or nanoparticles. Then, a FM or SG shell will form at the surface of these materials, resulting in a natural AFM/FM or AFM/SG interface.4,5 On the basis of the analysis of our experiment results, we think this model may be suitable in our case. We suggest that the core of the Li0.44MnO2 nanoribbons is AFM at low temperature, while the shell is FM due to the breaking of the sublattice pairing on the surface layer. If the FM shell is as thin as a few lattice units, its spin magnetization may behave as a SG-like layer.1,5 This agrees well with our results that the Li0.44MnO2 nanoribbons show SG-like behavior at low temperature. So, the EB effect in our sample can be attributed to the exchange interaction between the AFM core and the SG-like shell. Our experiments indicate the HE in the Li0.44MnO2 nanoribbons increases with increasing cooling field.
Figure 9. Central part of FC hysteresis loops measured at T = 3 K under the cooling field of 10, 20, and 60 kOe. Inset shows the influence of the cooling field on the magnitude of the exchange bias field.
It could be explained as follows: when applying a small cooling field, only a part of spins at the SG/AFM interface are pinned along the cooling field direction. By increasing the magnitude of the cooling field, more spins are aligned with the magnetic field and rotate, and the exchange interaction is enhanced. Thus, a tunable exchange bias field is obtained by changing the cooling field, which contributes to the development of multifunctional spin electronics devices.
’ CONCLUSIONS In summary, highly crystalline Li0.44MnO2 nanoribbons have been prepared via Na0.44MnO2 nanoribbons followed by ion exchange to replace the sodium by lithium in LiNO3-LiCl eutectic molten salt, which is thought to be a self-sacrificing template process. Both Li0.44MnO2 and Na0.44MnO2 nanoribbons were all characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy techniques. Magnetization measurements show that Li0.44MnO2 nanoribbons exhibit weak ferromagnetism at low temperature, and a significant EB effect is also observed. The magnitude 2648
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The Journal of Physical Chemistry C of exchange bias field can be tuned by the strength of the cooling field. The magnetic behaviors of Li0.44MnO2 nanoribbons can be interpreted by considering the exchange interaction between the AFM core and the SG-like shell.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Key Project of Fundamental Research of China (No. 2005CB623605 and No. 2010CB923404). ’ REFERENCES (1) Nogues, J.; Sort, J.; Langlais, V.; Skumryev, V.; Suri~ nach, S.; Mu~noz, J. S.; Baro, M. D. Phys. Rep. 2005, 422, 65. (2) Stamps, R. L. J. Phys. D 2000, 33, R247. (3) Niebieskikwiat, D.; Salamon, M. B. Phys. Rev. B 2005, 72, 174422. (4) Markovich, V.; Fita, I.; Wisniewski, A.; Puzniak, R.; Mogilyansky, D.; Titelman, L.; Vradman, L.; Herskowitz, M.; Gorodetsky, G. Phys. Rev. B 2008, 77, 054410. (5) Dong, S.; Gao, F.; Wang, Z. Q.; Liu, J.-M.; Ren, Z. F. Appl. Phys. Lett. 2007, 90, 082508. (6) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogues, J. Nature (London) 2003, 423, 850. (7) Nogues, J.; Schuller, I. K. J. Magn. Magn. Mater. 1999, 192, 203. (8) Suess, D.; Kirschner, M.; Schrefl, T.; Fidler, J.; Stamps, R. L.; Kim, J.-V. Phys. Rev. B 2003, 67, 054419. (9) Gruyters, M. Phys. Rev. B 2009, 79, 134415. (10) Hong, J.; Leo, T.; Smith, D. J.; Berkowitz, A. E. Phys. Rev. Lett. 2006, 96, 117204. (11) Ouyang, H.; Lin, K. W.; Liu, C. C.; Lo, S. C.; Tzeng, Y. M.; Guo, Z. Y.; Lierop, J. Phys. Rev. Lett. 2007, 98, 097204. (12) Velthuis, S. G. E.; Felcher, G. P.; Jiang, J. S.; Inomata, A.; Nelson, C. S.; Berger, A.; Bader, S. D. Appl. Phys. Lett. 1999, 75, 4174. (13) Ali, M.; Adie, P.; Marrows, C. H.; Greig, D.; Hickey, B. J.; Stamps, R. L. Nat. Mater. 2007, 6, 70. (14) Karmakar, S.; Taran, S.; Bose, E.; Chaudhuri, B. K.; Sun, C. P.; Huang, C. L.; Yang, H. D. Phys. Rev. B 2008, 77, 144409. (15) Tang, Y. K.; Sun, Y.; Cheng, Z. H. Phys. Rev. B 2006, 73, 174419. (16) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Sch€uth, F. Nano Lett. 2006, 6, 2977. (17) Benitez, M. J.; Petracic, O.; Salabas, E. L.; Radu, F.; T€uysu€uz, H.; Sch€uth, F.; Zabel, H. Phys. Rev. Lett. 2008, 101, 097206. (18) Yu, J. Y.; Tang, S. L.; Zhang, X. K.; Zhai, L.; Shi, Y. G.; Deng, Y.; Du, Y. W. Appl. Phys. Lett. 2009, 94, 182506. (19) Arnold, D. C.; Kazakova, O.; Audoit, G.; Tobin, J. M.; Kulkarni, J. S.; Nikitenko, S.; Morris, M. A.; Holmes, J. D. ChemPhysChem 2007, 8, 1694. (20) Xu, M. H.; Zhong, W.; Yu, J. Y.; Zang, W. C.; Au, C.; Yang, Z. X.; Lv, L. Y.; Du, Y. W. J. Phys. Chem. C 2010, 114, 16143. (21) Zhang, T.; Wang, X. P.; Fang, Q. F. J. Phys. Chem. C 2010, 114, 11796. (22) Jeong, Y. U.; Manthiram, A. Electrochem. Solid State Lett. 1999, 2, 421. (23) Doeff, M. M.; Richardson, T. J.; Hwang, K.-T. J. Power Sources 2004, 135, 240. (24) Akimoto, J.; Awaka, J.; Takahashi, Y.; Kijima, N.; Tabuchi, M.; Nakashima, A.; Sakaebe, H.; Tatsumi, K. Electrochem. Solid State Lett. 2005, 8, A554. (25) Awaka, J.; Akimoto, J.; Hayakawa, H.; Takahashi, Y.; Kijima, N.; Tabuchi, M.; Sakaebe, H.; Tatsumi, K. J. Power Sources 2007, 174, 1218.
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