Letter pubs.acs.org/NanoLett
Solution−Solid−Solid Mechanism: Superionic Conductors Catalyze Nanowire Growth Junli Wang,*,†,‡,§ Kangmin Chen,† Ming Gong,∥ Bin Xu,⊥ and Qing Yang*,‡,§ †
Scientific Research Academy and School of Materials Science & Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, P. R. China ‡ Hefei National Laboratory of Physical Sciences at the Microscale & Department of Chemistry, University of Science and Technology of China (USTC), Hefei 230026, Anhui, P. R. China § CAS Key Laboratory of Materials for Energy Conversion, USTC, Hefei 230026, Anhui, P. R. China ∥ Experiment Center of Engineering and Materials Science, USTC, Hefei 230027, Anhui, P. R. China ⊥ Testing Center, Yangzhou University, Yangzhou 225002, Jiangsu, P. R. China S Supporting Information *
ABSTRACT: The catalytic mechanism offers an efficient tool to produce crystalline semiconductor nanowires, in which the choice, state, and structure of catalysts are active research issues of much interest. Here we report a novel solution− solid−solid (SSS) mechanism for nanowire growth catalyzed by solid-phase superionic conductor nanocrystals in lowtemperature solution. The preparation of Ag2Se-catalyzed ZnSe nanowires at 100−210 °C is exampled to elucidate the SSS model, which can be extendable to grow other II−VI semiconductor (e.g., CdSe, ZnS, and CdS) nanowires by the catalysis of nanoscale superionic-phase silver or copper(I) chalcogenides (Ag2Se, Ag2S, and Cu2S). The exceptional catalytic ability of these superionic conductors originates from their structure characteristics, known for high-density vacancies and fast mobility of silver or copper(I) cations in the rigid sublattice of Se2− or S2− ions. Insights into the SSS mechanism are provided based on the formation of solid solution and the solid-state ion diffusion/transport at solid−solid interface between catalyst and nanowire. KEYWORDS: Semiconductor nanowires, catalytic growth, superionic conductors, solid-phase catalyst, solution synthesis, nanoheterostructures
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and the associated catalytic kinetics and mass (ion or atom) transport processes during nanowire growth are debatable and full of many unclear problems. Here, we report that solid-state superionic (or fast ion) conductors of nanoscale silver or copper(I) chalcogenides (Ag2Se, Ag2S, and Cu2S)21−24 catalyze the growth of II−VI compound (ZnSe, CdSe, ZnS, and CdS) semiconductor nanowires in solution at low temperatures (typically at 100− 230 °C; see the Supporting Information for experimental details). At such temperatures, the catalyst particles of silver or copper(I) chalcogenides generated in the synthetic process are in solid state and proven to be crystallized in superionicconductor phase by means of variable-temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC), and in situ transmission electron microscopy (TEM), which is demonstrated in detail by superionic Ag2Se nanocrystals catalyzing the ZnSe nanowire growth. It is found that the catalytic capability of Ag2Se is closely related to the structure characteristics of its superionic phase,21−23 and accordingly a
emiconductor nanowires are a fascinating platform for fundamental researches in theoretic aspects and nanodevice applications in electronics and photonics.1,2 Up to now, the triple-phase metal-catalyzed mechanisms, such as vapor− liquid−solid (VLS),3,4 solution−liquid−solid (SLS),5,6 supercritical-fluid−liquid−solid (SFLS), 7,8 vapor−solid−solid (VSS),9−12 and supercritical-fluid−solid−solid (SFSS),13,14 have become the most successful strategies for growing various semiconductor crystalline nanowires. In these mechanisms, the state of catalysts is in dispute; they are in the liquid, solid, or other matters and states, including eutectics, bimetal alloys, and metastable phases. As noted, a liquid-phase catalyst is suggested for nanowire growth in the VLS, SLS, and SFLS regimes,3−8 whereas some solid-phase catalysts have recently been explored in the VSS and SFSS mechanisms,9−14 such as solid metal silicides/germanides, Al−Au and Ga−Au solid alloys preformed in the growth of highly covalent Si, Ge, and III−V nanowires. Interestingly, it is also found that the solid nanoparticles of Cu2−xS (Cu1.94S and Cu2S, etc.),15,16 Ag2S,17,18 and Ag2Se19,20 played catalyst roles in solution synthesis of highly ionic semiconductor nanowires/nanorods, such as In2S3, AgInZn7S9, and II−VI compounds. Unfortunately, the state and structure of these catalysts, the structure−catalytic capability relationship, © XXXX American Chemical Society
Received: February 20, 2013 Revised: May 8, 2013
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terminated by a darker Ag2Se particle at their tips, displaying the characteristic feature of a catalytic growth mechanism.3−20 The energy-dispersive spectroscopy (EDS) mapping (Figure 1C) reveals the elemental distributions of Ag, Zn, and Se in an Ag2Se-catalyzed ZnSe nanowire: Ag is limited to the tip and Zn to the wire part, while Se is distributed throughout. Room-temperature (RT) XRD analyses were performed on the ZnSe nanowires prepared at different amounts of silver precursor (AgNO3). As displayed in Figure 2A, at low Ag+/ Zn2+ molar ratio (3%) used for preparing ZnSe nanowires. The diffractions of Ag2Se catalyst are well indexed to tetragonal structure,20,25,26 consistent with the RT XRD results of pure Ag2Se nanocrystals prepared at the same conditions except no use of zinc source (Figure 2A, Figure S1 and Table S1). High-quality ZnSe nanowires can be produced by the Ag2Secatalyzed growth in a broad temperature range, typically at 100−210 °C (Figure S2). The presence of Ag2Se catalysts greatly reduces the crystalline temperature of ZnSe nanowires. To our knowledge, 100 °C is so far the lowest temperature reported for growing crystalline ZnSe nanowires. It is believed that the catalytic ability of a catalyst particle closely correlates with its matter state, composition, and crystal structure. In the temperature range of 100−210 °C for the growth of ZnSe nanowires, the catalyst of Ag2Se particle exists in solid phase according to the Ag2Se−ZnSe phase diagram (with a eutectic temperature at 850 °C, Figure S3).27 For the RT XRD results (Figure 2A) cannot reflect the intrinsic, in situ structure of catalyst particle during nanowire growth at elevated reaction temperature, we now use variable-temperature XRD, DSC, and in situ heating TEM techniques to reveal the intrinsic crystal structure of Ag2Se catalyst in the ZnSe nanowire growth at 100−210 °C and its structure changes with temperature variations, and thus we expect to make an insightful
deep, extendable understanding, based on the formation of Ag−Zn−Se solid solution inside Ag2Se catalyst particle and the solid-state ion diffusion/transport at Ag2Se/ZnSe solid−solid interface, is explored to elucidate the Ag2Se-catalyzed growth process of ZnSe nanowires. Similarly to the VLS, SLS, and VSS models mentioned above, the growth of a solid crystalline nanowire via a solid catalyst in a solution is here illustrated as the solution−solid−solid (SSS) mechanism (Figure 1A). This regime is typically demonstrated
Figure 1. (A) Schematic diagram of the SSS growth mechanism based on a solid-state catalyst. (B) TEM image of ZnSe nanowires catalyzed by Ag2Se particles, which are terminated at the end of nanowires. (C) STEM image and EDS elemental mapping of Ag, Zn, and Se for a typical Ag2Se-catalyzed ZnSe nanowire. Scale bars: 50 nm.
by the preparation of ZnSe nanowires using Ag2Se as catalyst in our study. Transmission electron microscopy (TEM) observations (Figure 1B) show that many of ZnSe nanowires are
Figure 2. (A) Room-temperature (RT) XRD patterns of Ag2Se catalyst particles from ZnSe nanowire prepared at 210 °C with different molar ratios of Ag+/Zn2+, compared with that of Ag2Se nanocrystals produced under the same conditions without zinc source. (B) Variable-temperature XRD data of Ag2Se catalyst particles from ZnSe nanowires prepared with Ag+/Zn2+ = 10% (molar ratio). The weak peak marked with an asterisk (∗) cannot be assigned. (C) DSC scans of Ag2Se catalyst particles from ZnSe nanowires prepared with Ag+/Zn2+ = 10% (molar ratio) at different temperatures (100−210 °C). B
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Figure 3. In situ HRTEM observation of phase transition in Ag2Se tips at (A) room temperature and (B) 115 °C. Insets are the fast Fourier transform (FFT) patterns of the corresponding HRTEM images of Ag2Se tips. The HRTEM images and their FFT patterns recorded at room temperature and at 115 °C are respectively well indexed to tetragonal Ag2Se along the [2̅32] orientation (the measured interplanar spacing of (202) is 2.23 Å and (221̅) is 2.03 Å, and the angle between these two planes is 89.34°) and cubic Ag2Se along the [111] orientation (the measured interplanar spacing of (101̅) is 3.52 Å and (011̅) is 3.52 Å, and the angle between these two planes is 60°). Scale bars: 5 nm.
Figure 4. TEM images of (A) Ag2Se-catalyzed CdSe nanowires, (B) Ag2S-catalyzed ZnS nanowires, (C) Ag2S-catalyzed CdS nanorods, (D) Cu2Scatalyzed ZnS nanorods, and (E) Ag2Se-catalyzed ZnSe nanorods. These short nanorods terminated by a catalyst tip in (C, D, and E) can be seen as matchstick-like nanoheterostructures. (F, G) TEM and STEM-EDS mapping images of Ag2Se−ZnSe Janus-like nanoheterostructures. Scale bars: 50 nm (A, B and E−G) and 20 nm (C, D).
an endothermic peak near 101−105 °C is detected, which suggests that the phase transition of tetragonal-cubic Ag2Se occurs at this temperature range (a slight difference with the synthetic temperature of samples from 100 to 210 °C). In comparison with the transition temperature (133−135 °C) reported for the orthorhombic-cubic transition of bulk or nanoscale Ag2Se,22,23,28 the tetragonal-cubic transition temperature declines significantly by 30 °C. For the nanowire sample prepared at 100 °C, the endothermic peak of tetragonal-cubic phase transition of Ag2Se catalyst has an onset of 96 °C in the
understanding of the Ag2Se-catalyzed SSS mechanism for nanowire growth. Figure 2B shows the variable-temperature XRD results in the range of 28°−44° (2θ) for Ag2Se catalyst tips from ZnSe nanowires, typically obtained at 10% molar ratio of Ag+/Zn2+. It clearly reveals that Ag2Se transforms from tetragonal to bodycentered cubic (bcc) phase at temperatures above 90 °C. DSC measurements provide detailed information on the phase transition behaviors of Ag2Se catalyst tips from the ZnSe nanowires. In the heating DSC scans (red lines in Figure 2C), C
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heating scan (Figure S4). It indicates that the Ag2Se catalyst particles will start to transform to cubic phase at ∼96 °C from the tetragonal form. This result is in good agreement with the variable-temperature XRD analyses (Figure 2B). Meanwhile, there are two exothermic peaks detected in the cooling DSC scans of Ag2Se catalyst tips (green lines in Figure 2C), which are probably attributed to the phase transitions of hightemperature cubic Ag2Se to two types of tetragonal Ag2Se with different lattice parameters (a = b = 0.706 nm and c = 0.498 or 0.476 nm;29 see Table S1). The pure tetragonal Ag2Se nanocrystals display similar phase change behaviors to the Ag2Se catalyst tips at ZnSe nanowires, proven by the variabletemperature XRD and DSC studies (Figures S1C and S5A). Interestingly, both the Ag2Se catalyst tips and the pure Ag2Se nanocrystals exhibit a reversible structure transformation between tetragonal and cubic phases (Figure S5). In addition, the tetragonal-cubic phase transformation of Ag 2Se is unambiguously observed by the in situ TEM technique, separately performed at room temperature and above 100 °C (Figure 3). The above investigations confirm that the in situ intrinsic crystal structure of Ag2Se catalyst is the bcc structure during the nanowire growth performed at temperatures ≥100 °C. As is known, the bcc-structured Ag2Se is a typical superionic conductor.21−23,28,30 Hereto, it can be concluded that it is the bbc superionic-phase Ag2Se that catalyzes the growth of ZnSe nanowires. The bcc superionic Ag2Se-catalyzed SSS regime can be efficiently extended to the synthesis of crystalline CdSe nanowires at the temperature above 100 °C, for example, at 100−210 °C (Figure 4A and Figure S6). Like Ag2Se, other chalcogenides of silver or copper(I) in superionic phase such as Ag2S and Cu2S, which are superionic in the bulk form at temperatures respectively above 179 °C21,23 and 105 °C,24 have also been used as catalysts, and accordingly II−VI semiconductor ZnS and CdS nanowires or nanorods (wires in low aspect ratio) are prepared (Figure 4B−D, Figures S6 and S7). These achievements show the generality of the SSS mechanism. Superionic conductor chalcogenides of silver or copper(I), Ag2Se, Ag2S, and Cu2S, have a rather high density of Ag+ or Cu+ vacancies (or deficiencies), a highly fast cation mobility, and a nearly rigid sublattice of Se2− or S2− anions.21−24,30 Such unique structure features enable the excellent catalytic ability of superionic Ag2Se for the growth of ZnSe nanowires at the low reaction temperature of 100−210 °C. Meanwhile, the small size of catalyst particles and the existence of ZnSe can both facilitate the formation and stabilization of superionic phase Ag2Se25,26,30−32 and thus will promote nanowire growth. Based on the structure features of superionic-phase Ag2Se, the growth process of Ag2Se-catalyzed ZnSe nanowires, i.e., the SSS mechanism, is outlined below, which should be applicable to other II−VI semiconductor nanowire growth by the catalysis of Ag2Se, Ag2S or Cu2S superionic conductors. (i) In superionic phase Ag2Se, the high-density cation Ag+ vacancies are favorable for foreign cation incorporation.15−17 The continual transport and incorporation of Zn/Se from solution into the preformed Ag2Se catalyst (Ksp: Ag2Se ≪ ZnSe20,31) will lead to the formation of Ag−Zn−Se solid solution27,32 in superionic phase Ag2Se catalyst particles; when it reaches supersaturation, ZnSe is precipitated from the solid solution and subsequently grows into crystalline nanowires. Meanwhile, the phase diagram of the Ag2Se−ZnSe system27 shows a low
solubility of Zn (ZnSe) in Ag2Se (as the same of Ag in ZnSe, Figure S3), which supports the precipitation and the subsequent growth of ZnSe nanowire from a solid catalyst particle of Ag2Se theoretically and experimentally. Similar phenomena have been previously reported in the metal-catalyzed growth of silicon, silicon− germanium, and II−VI and III−V semiconductor nanowires.6,11,12 (ii) The ion transport and diffusion at the Ag2Se/ZnSe solid−solid interface would also promote the growth of ZnSe anisotropically. Our experiments show that the Ag2Se catalyst tip of ZnSe nanowires undergoes a size reduction and even disappears as the reaction time increases (Figure S8). Meanwhile, short Ag2Se-catalyzed ZnSe nanorods (i.e., matchstick-like nanoheterostructures) and Ag2Se−ZnSe Janus-like nanoheterostructures are separately obtained by the reactions of Ag2Se nanocrystals with Zn/Se precursors and with only Zn precursor when excess Se precursor is used for the Ag2Se nanocrystal synthesis (Figure 4E−G and Figure S9). Such results indicate two diffusion processes: the thermodynamically dominated diffusion of Ag+ ions into ZnSe nanowires and the kinetically dominated diffusion of Zn2+ ions into Ag2Se nanocrystals. The two processes are competitive at the Ag2Se/ZnSe solid−solid interface to regulate the nanowire growth. The high mobility of Ag+ ions in superionic-phase Ag2Se21−23 and the high ionicity of both Ag2Se and ZnSe30,31 will favor the Ag+/Zn2+ ion diffusion and exchange. Particularly, Ag+ ion is highly mobile at the Ag2Se/ZnSe interface and can diffuse into ZnSe nanowires through the rigid Se2− ion sublattice. This process greatly improves the rate of ion diffusion/migration and thus the growth of ZnSe nanowires.16 The proposed SSS nanowire growth model displays the following characteristic features of a catalytic mechanism like the VLS, VSS, and SLS models, which are different from the seeded growth33−36 or other solution-phase growth regimes37,38 of semiconductor or noble metal nanowires/nanorods: (1) the semiconductor material has a low, limited solubility in the catalyst at the growth temperature,6,11,12,15,17 such as ZnSe and CdSe in Ag2Se27 (Figure S3); (2) the catalyst/semiconductor interface, such as the ZnSe/Ag2Se solid−solid interface, is the highly active site for nanowire growth,3−14 unlike the seeded growth mechanism where the growth site is the active facet at the end of nanorods or nanowires;33−36 (3) the catalyst particles greatly reduce the crystalline temperature of semiconductor nanowires and usually remain at the end of the resultant nanowires, and meanwhile the diameter of nanowires can be controlled by the size of catalyst particles;4,6 and (4) the low solubility of semiconductor material in the catalyst would provide the basis for the controlled growth of axial nanowire heterostructures composed of two distinct semiconductor materials.12 In conclusion, we have demonstrated a general catalytic growth model, the solution−solid−solid (SSS) mechanism, for the facile solution synthesis of II−VI semiconductor (ZnSe, CdSe, ZnS, and CdS) nanowires/nanorods, in which Ag2Se, Ag2S, and Cu2S crystallizing in superionic-conductor phase act as excellent catalysts. The intrinsic structures of catalysts, the associated catalytic kinetics, and ion transport processes are investigated and rationally discussed. Our work shows the wide D
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(23) Hoshino, S. Solid State Ionics 1991, 48, 179−201. (24) Wang, L. W. Phys. Rev. Lett. 2012, 108, 085703. (25) Sahu, A.; Qi, L.; Kang, M. S.; Deng, D.; Norris, D. J. J. Am. Chem. Soc. 2011, 133, 6509−6512. (26) Giinter, J. R.; Keusch, P. Ultramicroscopy 1993, 49, 293−307. (27) Trishchuk, L. I.; Oleinik, G. S.; Mizetskaya, I. B. Inorg. Mater. 1982, 18, 1540−1542. (28) Xiao, C.; Xu, J.; Li, K.; Feng, J.; Yang, J.; Xie, Y. J. Am. Chem. Soc. 2012, 134, 4287−4293. (29) De Ridder, R.; Amelinck, S. Phys. Status Solidi A 1973, 18, 99− 107. (30) Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P. Science 2004, 306, 1009−1012. (31) Wang, S.; Hu, B.; Liu, C.; Yu, S. H. J. Colloid Interface Sci. 2008, 325, 351−355. (32) Tadanaga, O.; Koide, Y.; Hashimoto, K.; Oku, T.; Teraguchi, N.; Tomomura, Y.; Suzuki, A.; Murakami, M. Jpn. J. Appl. Phys. 1996, 35, 1657−1663. (33) Talapin, D. V.; Koeppe, R.; Gotzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 1677−1681. (34) Carbone, L.; et al. Nano Lett. 2007, 7, 2942−2950. (35) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857− 13870. (36) Sun, Y. G.; Mayers, B.; Herricks, T.; Xia, Y. N. Nano Lett. 2003, 3, 955−960. (37) Mai, L. Q.; Xu, X.; Han, C. H.; Luo, Y. Z.; Xu, L.; Wu, Y. A.; Zhao, Y. L. Nano Lett. 2011, 11, 4992−4996. (38) Chen, P.; Xiao, T. Y.; Li, H. H.; Yang, J. J.; Wang, Z.; Yao, H. B.; Yu, S. H. ACS Nano 2012, 6, 712−719.
adaptability and high expansibility of the SSS model, which may be applied to more catalyst/semiconductor systems of high ionicity. Clearly, the SSS mechanism will complement or selectively replace conventional catalytic methods (e.g., VLS, VSS, SLS, SFLS, and SFSS) for semiconductor nanowire preparation.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, additional characterization data, and supporting figures (S1−S9). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (J.W.);
[email protected] (Q.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support for this research from the NSFC (21201086, 21071136, 51271173), the National Basic Research Program of China (2010CB934700, 2012CB932001), and the Research Foundation of Jiangsu University (11JDG071).
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REFERENCES
(1) Lieber, C. M. MRS Bull. 2011, 36, 1052−1063. (2) Yan, R.; Gargas, D.; Yang, P. Nat. Photonics 2009, 3, 569−576. (3) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89−90. (4) Duan, X.; Lieber, C. M. Adv. Mater. 2000, 12, 298−302. (5) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791−1794. (6) Wang, F. D.; Dong, A.; Sun, J.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45, 7511−7521. (7) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471−1473. (8) Davidson, F. M., III; Wiacek, R.; Korgel, B. A. Chem. Mater. 2005, 17, 230−233. (9) Persson, A. I.; Larsson, M. W.; Stenström, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Nat. Mater. 2004, 3, 677−681. (10) Kamins, T. I.; Williams, R. S.; Basile, D. P.; Hesjedal, T.; Harris, J. S. J. Appl. Phys. 2001, 89, 1008−1016. (11) Wang, Y.; Schmidt, V.; Senz, S.; Gösele, U. Nat. Nanotechnol. 2006, 1, 186−189. (12) Wen, C. Y.; Reuter, M. C.; Bruley, J.; Tersoff, J.; Kodambaka, S.; Stach, E. A.; Ross, F. M. Science 2009, 326, 1247−1250. (13) Tuan, H.-Y.; Lee, D. C.; Hanrath, T.; Korgel, B. A. Nano Lett. 2005, 5, 681−684. (14) Barth, S.; Koleśnik, M. M.; Donegan, K.; Krstić, V.; Holmes, J. D. Chem. Mater. 2011, 23, 3335−3340. (15) Han, W.; Yi, L.; Zhao, N.; Tang, A.; Gao, M.; Tang, Z. J. Am. Chem. Soc. 2008, 130, 13152−13161. (16) Connor, S. T.; Hsu, C.-M.; Weil, B. D.; Aloni, S.; Cui, Y. J. Am. Chem. Soc. 2009, 131, 4962−4966. (17) Zhu, G.; Xu, Z. J. Am. Chem. Soc. 2011, 133, 148−157. (18) Zou, C.; Li, M.; Zhang, L.; Yang, Y.; Li, Q.; Chen, X.; Xu, X.; Huang, S. CrystEngComm 2011, 13, 3515−3520. (19) Zhang, L.; Yang, H. Appl. Phys. A: Mater. Sci. Process. 2010, 98, 801−810. (20) Wang, J.; Yang, C.; Huang, Z.; Humphrey, M. G.; Jia, D.; You, T.; Chen, K.; Yang, Q.; Zhang, C. J. Mater. Chem. 2012, 22, 10009− 10014. (21) Kobayashi, M. Solid State Ionics 1990, 39, 121−149. (22) Boolchand, P.; Bresser, W. J. Nature 2001, 410, 1070−1073. E
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