Synthesis of Single-Crystalline Alkaline-Earth Metal Manganites

ARTICLE pubs.acs.org/crystal

Synthesis of Single-Crystalline Alkaline-Earth Metal Manganites Nanoribbons via Cation Exchange Xianke Zhang,†,‡ Zhibing Xu,† Shaolong Tang,*,† Yu Deng,§ and Youwei Du† †

Nanjing National Laboratory of Microstructures, Jiangsu Provincial Laboratory for NanoTechnology, Department of Physics, and Center for Materials Analysis, Nanjing University, Nanjing 210093, China ‡ College of Physics and Electronics, Gannan Normal University, Ganzhou 341000, China §

ABSTRACT: Single-crystalline alkaline-earth metal manganites CaMn3O6, SrMn3O6
x and Ba6Mn24O48 nanoribbons have been successfully synthesized by a facile ion-exchange approach based on molten-salt reaction between Na0.44MnO2 nanoribbons and CaCl2, SrCl2, and BaCl2. All of these as-prepared nanoribbons are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Taking the synthesis of single-crystalline SrMn3O6
x nanoribbons as an example, we researched in detail the conversion process from Na0.44MnO2 nanoribbons to SrMn3O6
x nanoribbons. Our results indicate that Na0.44MnO2 nanoribbons serve as the precursor template and the final products are obtained via ion-exchange reaction. Although there is a complete cation exchange, a change in stoichiometry, and a change in crystal structure, the morphology is nearly perfectly maintained. The synthetic method is thought to be a self-sacrificing templating route.

’ INTRODUCTION One-dimensional (1D) nanostructures of ternary complex oxides, such as nanowires, nanotubes, and nanobelts, have attracted a wide range of interests because of their fascinating size-dependent optical, electronic, magnetic, thermal, mechanical, and chemical properties. Many synthetic methods, such as hydrothermal reaction, template technique, molten-salt synthesis (MSS), and composite-hydroxide-mediated synthesis, have been developed to synthesize different kinds of 1D nanostructured ternary transition-metal oxides. However, only limited kinds of single-crystalline 1D ternary nanostructures have been obtained until now, for example, titanates,1
6 vanadates,7
9 niobates,10
13 chromates, and tungstates.14
17The synthesis of new ternary 1D nanostructures remains challenging to materials scientists. To the best of our knowledge, comparatively little work has been performed on the fabrication of ternary 1D manganites nanostructures. Colossal magnetoresistance (CMR) in mixed valence (Mn3þ/ Mn4þ) perovskite manganites with general formula R1
xAxMnO3 (R = rare earth ion; A = alkaline earth ion), resulting from competing magnetic properties, charge, and orbital ordering, has been a fascinating subject that attracted considerable research efforts in the past 20 years.18,19 On the basis of the crystal structural view, the mixed valence oxides constructed as the MnO6 octahedron can be classified into vertex-shared type compounds such as perovskite and edge-shared type compounds such as rutile or hollandite. Most physical studies have concentrated on the former owing to their possible technological applications in the field of magnetic devices and the possibility r 2011 American Chemical Society

of exploring the delicate interaction between Mn ions in basic physics. On the other hand, very little is known about the physical properties of edge-shared type manganese oxides. In most cases, these and other related materials, such as BaxMn8O16,20 Ba6Mn24O48,21 CaMn3O6,22 and SrMn3O6
x,23form complex tunnel structures in which the cations that occupy the tunnels are often subjected to displacements and agitations that result in modulated and sometimes incommensurate structures. Sato et al. have revealed interesting electronic and magnetic phase transitions in hollandite type manganese oxides24 and strong coupling between electric conduction and magnetism in rutile type manganese oxides.25 Methods such as imprinting, templating, and molding are usually used in the way of morphology transfer from one material to another, but they are strongly limited with respect to the available geometries and sizes. Techniques for morphology transfer on the nanoscale have so far remained a challenge to researchers. Employing pre-existing nanowires, nanorods, or nanotubes as templates could be a reasonable way to fabricate 1D nanostructures, which offers a very powerful means to increase the compositional and morphological diversity of materials. In this paper, we have utilized a facile ion-exchange approach to prepare mixed valence manganites CaMn3O6, SrMn3O6
x, and Ba6Mn24O48 nanoribbons based on molten-salt Received: December 17, 2010 Revised: April 4, 2011 Published: May 12, 2011 2852

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Figure 1. XRD patterns of as-prepared nanoribbons: (a) Na0.44MnO2; (b) CaMn3O6; (c) SrMn3O6
x; (d) Ba6Mn24O48.

reaction between Na0.44MnO2 nanoribbons and CaCl2, SrCl2, and BaCl2. The proposed synthetic route is considered to be a selfsacrificing templating process, which is similar to our previous report.26 Ion-exchange reaction may be a promising route for tuning the material compositions and properties of nanostructures.27
29 The MSS method is one of the simplest and most versatile and cost-effective approaches for obtaining crystalline, chemically purified, single-phase powders at lower temperatures and often in overall shorter reaction time with little residual impurities as compared with conventional solid-state reactions.

’ EXPERIMENTAL SECTION The single-crystalline CaMn3O6, SrMn3O6
x, and Ba6Mn24O48 nanoribbons were synthesized by an extended MSS method that consists of two processes. The first procedure is to synthesize Na0.44MnO2 nanoribbons. The second process is the ion-exchange reaction between Na0.44MnO2 nanoribbons and molten alkaline-earth metal chlorates. In a typical procedure of fabricating CaMn3O6 nanoribbons, first, 1 mmol MnCO3 and 0.22 mmol Na2CO3 were mixed with 5.0 g NaCl and ground homogeneously in a mortar for 30 min. The mixture was then placed into an alumina crucible and annealed at 870 °C for 5 h in a crucible furnace. Second, another mixture of 1 g CaCl2 and 1.5 g NaCl was added into the above-mentioned crucible at the end of the first process. This reaction proceeded at 870 °C for 4 h, and then the crucible was subsequently naturally cooled to room temperature. The resulting powders were washed for several times with distilled water to remove the residual NaCl and CaCl2 and then dried at 90 °C in a drying oven. The procedure of synthesizing SrMn3O6
x and Ba6Mn24O48 nanoribbons was almost the same as for CaMn3O6 nanoribbons. The reaction temperatures for the preparation of SrMn3O6
x and Ba6Mn24O48 nanoribbons were 850 and 910 °C, respectively. The amount of SrCl2 3 6H2O and BaCl2 3 2H2O in the second process was 0.72 and 0.66 g, respectively. The synthesized Na0.44MnO2, CaMn3O6, SrMn3O6
x, and Ba6Mn24O48 nanoribbons were characterized using an X-ray diffractometer (XRD, Rigaku, D/Max-RA) with Cu KR radiation (λ = 1.54 Å). The morphology of these nanoribbons was observed on a field emission scanning electron microscope (FE-SEM, HITACHI X650) or on a S-3400N II instrument, operating at 20 kV, and a high resolution transmission electron microscope (HRTEM, JEM, JEOL 2010EX).

Figure 2. (a) SEM image of Na0.44MnO2 nanoribbons. (b) Typical TEM image of an individual Na0.44MnO2 nanoribbon. Inset is the SAED pattern recorded from the [010] zone axis. (c) HRTEM image of the nanoribbon. (d) Energy-dispersive X-ray spectroscopy (EDS) analysis. The selected-area electron diffraction (SAED) pattern was obtained in TEM observation.

’ RESULTS AND DISCUSSION The XRD pattern of the precursor Na0.44MnO2 nanoribbons is shown in Figure 1a. All of the peaks can be easily indexed to that of the pure orthorhombic Na4Mn9O18 (JCPDS No. 270750). No additional impurity peaks are detected. Figure 2a presents the low-magnification FE-SEM image of the Na0.44MnO2. A large quantity of nanoribbons with diameters ranging from 100 nm to a few hundred nanometers and length up to tens of micrometers is obtained. The starting materials were mostly transformed into the nanoribbons products from SEM observations of the samples. A typical Na0.44MnO2 nanoribbon is shown in Figure 2b. The SAED pattern viewed along [010] zone axis (inset in Figure 2b) indicates that the nanoribbon is singlecrystalline. The HRTEM image of this nanoribbon (Figure 2c) shows that the fringe spacing is 0.45 nm, which corresponds to the (200) interplanar distance of the orthorhombic Na0.44MnO2. According to HRTEM and SAED, the nanoribbon grows along its [001] crystallographic direction, which is consistent with another related report.30 EDS analysis (Figure 2d) shows that the chemical components of the nanoribbon are the elements Na, Mn, and O. The Cu peak originates from TEM grids. Figure 1b presents the XRD pattern of CaMn3O6 nanoribbons obtained via ion-exchange reaction between Na0.44MnO2 nanoribbons and molten CaCl2 salt, which is agreement with the results of the previous reports.22,31 Figure 3a is the TEM micrograph of an individual CaMn3O6 nanoribbon. The SAED pattern (inset of Figure 3a) taken from a nanoribbon indicates that it is single-crystalline in nature. The HRTEM image of the single nanoribbon (Figure 3b) shows that the fringe spacing is 0.71 nm, which corresponds to the (001) interplanar distance of the monoclinic CaMn3O6. The EDS analysis (Figure 3c) 2853

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Figure 3. (a) TEM image of single CaMn3O6 nanoribbon. Inset is the SAED pattern recorded from the [100] zone axis. (b) HRTEM image of the nanoribbon. (c) EDS analysis of the nanoribbon. The C and Cu peaks originate from the TEM grids. (d) SEM image of CaMn3O6 nanoribbons.

suggests the chemical components of the nanoribbon are the elements Ca, Mn, and O. The Cu and C peaks originate from TEM grids. On the basis of the analysis of HRTEM and SAED, the growth direction of the nanoribbon is determined to be its [010] crystallographic direction. The morphology of the synthesized CaMn3O6 nanoribbons is further examined by FE-SEM, as shown in Figure 3d. The diameters of these nanoribbons are between 100 nm and a few hundred nanometers and length up to tens of micrometers, which is almost the same as that of the precursor. Figure 1c shows the XRD pattern of SrMn3O6
x nanoribbons prepared via ion-exchange reaction between Na0.44MnO2 nanoribbons and molten SrCl2. All of the diffraction peaks can be assigned to SrMn3O6
x (JCPDS Card No. 28-1233). Figure 4a gives the TEM image of SrMn3O6
x nanoribbons with different diameters. A typical single nanoribbon is shown in Figure 4b. SAED pattern (inset of Figure 4b) recorded from the single nanoribbon indicates that it is single-crystalline. Figure 4c shows the corresponding HRTEM image of the nanoribbon with the fringe spacing of 0.4 nm, which corresponds to the (003) interplanar distance of the SrMn3O6
x. The FE-SEM image of the SrMn3O6
x nanoribbons is presented in Figure 4d. A large quantity of nanoribbons with diameters ranging from one hundred nanometers to a few hundred nanometers are observed. The synthesis and magnetism of Ba6Mn24O48 nanoribbons have been reported in our previous work.32 Figure 1d gives the XRD pattern of Ba6Mn24O48 nanoribbons, which can be index to a mixed valence barium manganese oxide Ba6Mn24O48 first reported by Boullay et al.21 Figure 5a shows the typical FESEM image of the Ba6Mn24O48 nanoribbons. The TEM image of an individual nanoribbon with the diameter about 150 nm is shown in Figure 5b. The SAED pattern (the inset) viewed along [010] zone axis indicates that the nanoribbon is single-crystalline,

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Figure 4. (a) TEM image of SrMn3O6
x nanoribbons. (b) TEM image of an individual SrMn3O6
x nanoribbon. Inset is the SAED pattern of the nanoribbon. (c) HRTEM image of the nanoribbon. (d) SEM image of SrMn3O6
x nanoribbons.

Figure 5. (a) SEM image of the Ba6Mn24O48 nanoribbons. (b) TEM image of an individual Ba6Mn24O48 nanoribbon. Inset is the corresponding SAED pattern. (c) EDS data. (d) HRTEM image of the nanoribbon in panel b.

which is further proved by HRTEM image of the same nanoribbon (Figure 5d). The fringe spacing is 0.45 nm, which corresponds to the (400) interplanar distance of the Ba6Mn24O48. The nanoribbon grows along its [001] crystallographic direction. The EDS analysis (Figure 5c) shows that the chemical components of the nanoribbon are the elements Ba, Mn, and O, which indicates the complete conversion from Na0.44MnO2 nanoribbons to Ba6Mn24O48 nanoribbons. 2854

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Figure 6. XRD patterns of different reaction time in the process of preparing SrMn3O6
x nanoribbons: (a) ion-exchange reaction for 0 min; (b) ion-exchange reaction for 10 min; (c) ion-exchange reaction for 30 min; (d) ion-exchange reaction for 2 h.

Figure 7. (a) TEM image of the nanoribbon from ion-exchange reaction between Naþ and Sr2þ continued for10 min. (b) EDS analysis of this nanoribbon.

In order to investigate the chemical conversion process from single-crystalline Na0.44MnO2 nanoribbons to single-crystalline CaMn3O6, SrMn3O6
x, and Ba6Mn24O48 nanoribbons, further experiments were done. Taking the synthesis of 1D singlecrystalline SrMn3O6
x nanoribbons as an example, we researched the interesting formation process in detail. Figure 6 shows the XRD patterns of products attained for different reaction time. Figure 6a indicates the XRD pattern of the precursor, which means that the ion-exchange reaction between Na0.44MnO2 nanoribbons and molten SrCl2 did not arise. Figure 6b
d presents the XRD patterns that ion-exchange reaction between Na0.44MnO2 and SrCl2 proceeds for 10 min, 30 min and 2 h, respectively. Figure 6b and c reveal clearly that the two phases of Na0.44MnO2 and SrMn3O6
x coexist. By comparing Figure 6a and b, we can find that the strongest peak of Na0.44MnO2 moves toward a lower angle, which indicates that the corresponding crystalline plane distance increases. This may be caused by strontium ions (Sr2þ) entering the crystal lattice of Na0.44MnO2 via replacement with sodium ions (Naþ). Figure 6d shows Sr2þ have entirely substituted for Naþ, and then the SrMn3O6
x phase formed. From these images, we can determine

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Figure 8. Low-magnification SEM images of different reaction time of ion-exchange reaction between Naþ and Sr2þ. Panels a
d show the images when the ion-exchange reaction proceeded for 10 min, 30 min, 1 h, and 2 h, respectively.

that the peaks of Na0.44MnO2 become weak and the peaks of SrMn3O6
x reinforce when the reaction time is increased. Figure 7a gives the TEM image of single nanoribbon that ionexchange reaction between Na0.44MnO2 nanoribbons and molten SrCl2 produces after 10 min. The EDS analysis (Figure 7b) shows the chemical components of this nanoribbon are the elements Sr, Na, Mn, and O, which further reveals that Sr2þ really have gone into the crystal lattice of Na0.44MnO2 nanoribbons via replacement with Naþ and the 1D morphology of precursor is preserved. Figure 8 presents the low-magnification SEM images for different reaction time of ion-exchange between Na0.44 MnO2 nanoribbons and molten SrCl2 in the preparation of SrMn3O6
x nanoribbons. Panels a
d in Figure 8 show the ion-exchange reaction proceeding through 10 min, 30 min, 1 h, and 2 h, respectively. In all images, a large quantity of 1D nanostructures are observed and there are almost no nanoparticles. This may exclude the possibility that Na0.44MnO2 nanoribbons and molten SrCl2 react to form SrMn3O6
x nanoparticles, and then these nanoparticles grow into 1D SrMn3O6
x nanostructures. Figure 9a shows the high-magnification SEM image of the precursor Na0.44MnO2 nanoribbons. As shown, most nanoribbons tend to form bundles. The high-magnification SEM images of as-synthesized CaMn3O6, SrMn3O6
x, and Ba6Mn24O48 nanoribbons are presented in panels b
d in Figure 9, respectively. These ultimate nanoribbons also possess bundle-like morphology, which is the characteristic of the Na0.44MnO2 nanoribbons. Furthermore, the diameters of the initial Na0.44MnO2 nanoribbons are almost the same as that of the final products, which further validates the influence of the precursor template. On the basis of the above analysis, we deduce that the 1D Na0.44MnO2 nanoribbons serve as the precursor template and molten NaCl provides an apt solvent environment for 2855

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precursor Na0.44MnO2 nanoribbons by washing the reacting products and then mixing the Na0.44MnO2 nanoribbons with alkaline-earth metal chlorates. If the amount of alkaline-earth metal chlorates in the second process of our experiments is large enough, the alkaline-earth metal manganite nanoribbons would be heavily broken as a result of drastic ion-exchange reaction. So, the method used in our experiments is good at protecting the final 1D nanostructures from breaking.

’ CONCLUSIONS In summary, we have reported a facile ion-exchange approach to prepare different chemical compositions of 1D mixed valence alkaline-earth metal manganite nanostructures based on moltensalt reaction. CaMn3O6, SrMn3O6
x, and Ba6Mn24O48 nanoribbons can be synthesized via simple ion-exchange reaction from Na0.44MnO2 nanoribbons. The conversion mechanism from Na0.44MnO2 nanoribbons to alkaline-earth metal manganite nanoribbons was researched, and the synthesis method is suggested to be a self-sacrificing templating route. The synthetic strategy has been presented to fabricate 1D titanates26 and niobates12 nanostructures and may be extended to the synthesis of other ternary oxide 1D nanostructures. Figure 9. (a) High-magnification SEM image of the precursor Na0.44MnO2 nanoribbons. Panels b
d are the high-magnification SEM images of as-synthesized CaMn3O6, SrMn3O6
x, and Ba6Mn24O48 nanoribbons, respectively.

ion-exchange between Na0.44MnO2 and molten SrCl2 in the process of preparing SrMn3O6
x nanoribbons. The ultimate product retained the 1D nanostructures of the precursor, which confirmed the effect of the Na0.44MnO2 precursor template on SrMn3O6
x nanostructures. This synthesis route is thought to be a templating technique, a so-called self-sacrificing templating process, which has been extensively studied.12,33
39 However, this method is usually used to synthesize binary compounds. TiC, NbC, Fe3C, SiC, and GaN have been successfully prepared by reacting carbon nanotubes (CNTs) with the vapors of metal oxides or metal halides. SiO2, ZnO, CuO, Bi2O3, Co3O4, and SnO2 nanotubes could be obtained through the simple oxidation of corresponding elementary nanowires. Similar to CNTs, 1D Se and Te nanostructures have been used as chemical templates to produce various selenide and telluride nanostructures with similar morphology, such as Ag2Se, CdSe, RuSe2, Ag2Te, CoTe, PbTe, and Bi2Te3. To the best of our knowledge, only limited types of singlecrystalline ternary 1D nanostrcutures have been obtained via this method until now. The single-crystalline ZnAl2O4 and Fe2(MoO4)3 nanotubes can also be formed through a Kirkendalltype interfacial diffusion reaction.40
42 Xu and co-workers have developed a facile ion-exchange approach for the synthesis of NaNbO3 and CaNb2O6 nanorods based on molten-salt reaction between K2Nb8O21 nanowires and molten sodium and calcium salts.12Yang et al. reported the synthesis of single-crystalline LiMn2O4 nanotubes and nanowires via a low temperature molten-salt synthesis method, using prepared β-MnO2 nanotubes and R-MnO2 nanowires as the precursors and self-sacrificing template.43 We have attained a series of alkaline-earth metal manganites nanoribbons (CaMn3O6, SrMn3O6
x, and Ba6Mn24O48) via the ternary 1D nanostrcutures (Na0.44MnO2 nanoribbons) as the precursor template. In contrast to the method of Xu and co-workers,12 we did not directly obtain the

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86 25 8359 3817. Fax: þ86 25 8359 5535. E-mail: [email protected].

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