Synthesis RMn2O5 (R = Gd and Sm) nano- and microstructures by a simple hydrothermal method

Materials Chemistry and Physics 118 (2009) 467–472

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Synthesis RMn2 O5 (R = Gd and Sm) nano- and microstructures by a simple hydrothermal method Gangqiang Zhu a,∗ , Peng Liu a , Mirabbos Hojamberdiev b , Bao Ge a , Yun Liu c , Hongyan Miao c , Guoqiang Tan c a b c

School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, PR China Shaanxi Key Laboratory of Nano-materials and Technology, Xi’an University of Architecture and Technology, Xi’an 710055, PR China College of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, PR China

a r t i c l e

i n f o

Article history: Received 21 November 2008 Received in revised form 4 July 2009 Accepted 16 August 2009 Keywords: Nanostructures Chemical synthesis Transmission electron microscopy X-ray diffraction

a b s t r a c t Single-phase RMn2 O5 (R = Gd and Sm) nano- and microstructures have been successfully synthesized via a simple hydrothermal process at 250 ◦ C for 24 h using NaOH as mineralizer. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selective area electron diffraction patterns (SAED) were used to characterize the as-synthesized GdMn2 O5 and SmMn2 O5 samples. The effect of NaOH concentration and the molar ratio of Mn2+ /Mn7+ on the morphology and size of the final products was studied, and a possible formation mechanism of RMn2 O5 (R = Gd and Sm) nanoplates and nanorods under hydrothermal conditions was proposed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Synthesis of low-dimensional nanomaterials with wellcontrolled size, morphology, and chemical composition has attracted considerable research interest due to their distinctive geometries, novel physical and chemical properties, which make them to be potentially applied in modern electronics and photonics [1–3]. Recently, multiferroics materials, exhibiting simultaneous magnetic and ferroelectric order, have been receiving intensive attention [4–6]. Because of the spontaneous electric polarization (magnetization), which can be switched by an applied electric field (magnetic field) and intrinsic coupling of ferroelectric and magnetic ordering, they have a wide range of applications including data storage, transducers, actuators, nonvolatile memory [7] and gate ferroelectrics in field-effect transistors [8]. Till now, many studies have been directed at finding new multiferroic materials and investigate their properties in known oxides, such as BiMO3 (M = Mn, Fe, Sc, Co, etc.) [9–12], and rare-earth manganates [13–21]. RMn2 O5 (R = rare-earth, Y, and Bi) represents a family of compounds containing Mn3+ and Mn4+ ions in two crystallographic sites. The scientists developed single-crystal phases of RMn2 O5 from a Bi2 O3 flux for R = rare-earth, and gave the unit-cell parame-

∗ Corresponding author. Tel.: +86 29 85303823; fax: +86 29 85303823. E-mail address: [email protected] (G. Zhu). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.08.019

ters for the complete series in the early 1960s [22,23]. The crystal structure of RMn2 O5 is orthorhombic (space group Pbam) and consists of the octahedrally coordinated Mn4+ (4f) and square-planar pyramid Mn3+ (4h) [24]. Neutron diffraction studies showed that the magnetic structure is rather complex and magnetic moments of Mn3+ and Mn4+ ions form a helical magnetic ordering below TN [25]. In the past few years, much attention has been paid to the RMn2 O5 system, because these oxides exhibit exceptional properties, e.g., giant magnet-capacitance, giant magneto-elastic coupling and magnetoelectric [6,26,27]. Normally, the mixed-valence manganese oxides are prepared by traditional high-temperature solid-state reaction. However, high oxygen pressure is necessary for the synthesis of single-phased RMn2 O5 . Some alternative synthetic routes, namely, fused-salt electrolysis, SHS and citrate-gel and hydrothermal methods [28], have been successfully employed to prepare manganese oxides at relatively low temperature. Among these methods, the hydrothermal method is an attractive route to prepare the inorganic solids with narrow particle size distribution and well-controlled morphologies through one-step procedure at lower temperature. In this paper, we report a simple hydrothermal route to synthesis different shapes and sizes of RMn2 O5 (R = Gd and Sm) nano- and microstructures. The effect of NaOH concentration as a mineralizer and the molar ratio of Mn2+ /Mn7+ on the size and structure of the final products have been studied, and a dissolution–recrystallization mechanism of RMn2 O5 (R = Gd and Sm) was proposed. This method is single-step and can be applied as a very green chemistry route

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G. Zhu et al. / Materials Chemistry and Physics 118 (2009) 467–472 2. Experimental 2.1. Synthesis

Fig. 1. XRD patterns of GdMn2 O5 (a) and SmMn2 O5 (b) synthesized at 250 ◦ C for 24 h using the NaOH concentration of 5 M.

The starting materials used in the present work included gadolinium nitrate [Gd(NO3 )3 ·6H2 O], samarium nitrate [Sm(NO3 )3 ·6H2 O], potassium manganate (KMnO4 ) and manganese acetate [Mn(C2 H3 O2 )2 ]. All chemicals were analytical grade and used without further purification. Preparation of RMn2 O5 (R = Gd and Sm) nanostructures was as follows: firstly, 12.5 ml R(NO3 )3 (0.2 M) solution was prepared in a 50 ml beaker with distilled water and then 35 mmol Mn(C2 H3 O2 )2 and 15 mmol KMnO4 (the molar ratio of 7:3) were dissolved at the same time in the solution and constantly stirred for 30 min. 0.3 M (or 5 M) NaOH was slowly added into the homogenous solution and vigorously stirred for several minutes. The amorphous precursor was transferred into a 50 ml stainless steel Teflon-lined autoclave with a filling capacity of 60%. The hydrothermal treatment was performed by putting the autoclave in an oven at a temperature of 250 ◦ C for 24 h. After the system was cooled down to room temperature, the final products were washed with distilled water for several times, and dried at 80 ◦ C for 4 h. To prepare RMn2 O5 (R = Gd and Sm) microplates, the molar ratio of manganese sources was changed to 4:1, that is, 20 mmol Mn(C2 H3 O2 )2 and 5 mmol KMnO4 , and 5 NaOH was used as mineralizer. 2.2. Characterization

without any other organic dispersants and capping agents. The high-aspect ratio of the as-synthesized particles can play essential role in preparing well-oriented ceramic materials at a low sintering temperature.

The phase composition of as-prepared powder samples was detected before and after washing with diluted nitric acid (1 mol/l for 2 h) by X-ray powder diffraction (XRD; Model D/MAX2550, Rigaku Co., Tokyo, Japan) with Cu K␣ radiation ( = 1.5406 Å) at 40 kV and 50 mA. Scanning electron microscopic (SEM) observation was performed using a FEI Quanta 200 (FEI Ltd., Eindhoven, Netherlands).

Fig. 2. TEM images of the GdMn2 O5 (a) and SmMn2 O5 (c) nanorods synthesized at 250 ◦ C for 24 h using the NaOH concentration of 5 M. HRTEM images of the GdMn2 O5 (b) and SmMn2 O5 (d) nanorods.

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3. Results and discussion

Fig. 3. XRD patterns of the GdMn2 O5 obtained via the hydrothermal reaction at 250 ◦ C for 24 h using different NaOH concentrations of 0.3 M (a), 1 M (b), 3 M (c) and 5 M (d).

The transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) and selective area electron diffraction (SAED) images were taken with a JEM-2100F electron microscope (JEOL, Tokyo, Japan) with an accelerating voltage of 200 kV.

Fig. 1 shows the typical XRD patterns of GdMn2 O5 and SmMn2 O5 synthesized at 250 ◦ C for 24 using NaOH concentration of 5 M. All the peaks in Fig. 1a can be indexed to an orthorhombic phase of GdMn2 O5 . The calculated lattice parameters are a = 0.7365 nm, b = 0.8534 nm and c = 0.5694 nm which are compatible with the previous reported data (JCPDS Card No.52-0301, a = 0.7383 nm, b = 0.8517 nm and c = 0.5682 nm). All the peaks in Fig. 1b can be indexed to an orthorhombic phase of SmMn2 O5 . The calculated lattice parameters are a = 0.7436 nm, b = 0.8568 nm and c = 0.5692 nm which are compatible with the previous reported data (JCPDS Card No.52-1096, a = 0.7433 nm, b = 0.8587 nm and c = 0.5696 nm). No peaks of other impurity phases can be observed, which indicates a high purity of the obtained samples. The morphology and size of the as-synthesized samples was characterized by TEM. The morphology of GdMn2 O5 and SmMn2 O5 synthesized at 250 ◦ C for 24 h with the NaOH concentration of 5 M using KMnO4 and Mn(C2 H3 O2 )2 as the manganese sources is illustrated in Fig. 2. Fig. 2a gives a TEM image of the as-synthesized GdMn2 O5 powders, in which the sample compounded with largescale nanoplates with a width in the range of 100–200 nm and the length in the range of 500–1000 nm. Further insight into the nanoplate was gained by HRTEM observation on the edge of the plate as represented in Fig. 2b. The lattice fringe of 0.57 nm in

Fig. 4. TEM (a) and HRTEM (b) images of the GdMn2 O5 nanoplates synthesized at 250 ◦ C for 24 h using the NaOH concentration of 0.3 M. TEM (c) and HRTEM (d) images of the SmMn2 O5 nanoparticles synthesized at 250 ◦ C for 24 h using the NaOH concentration of 0.3 M.

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Fig. 5. XRD patterns of GdMn2 O5 (a) and SmMn2 O5 (b) synthesized at 250 ◦ C for 24 h using NaOH concentration of 5 M with Mn2+ /Mn7+ molar ratio of 4:1. XRD patterns of GdMn2 O5 (c) and SmMn2 O5 (d) after washing with diluted nitric acid.

the observed nano-crystallites agrees well the (0 0 1) lattice plane, which indicates the nanoplates with preferring growth of the caxis. Fig. 2c show a low-magnification TEM image of SmMn2 O5 nanorods. Each nanorod has a uniform diameter along its entire length and the nanorods with a diameter about 100 nm and their length range from 500 nm to 1000 nm. Fig. 2d shows a representative HRTEM image of the edge area of SmMn2 O5 nanorod. The image reveals that the nanorod is single crystalline with interplanar spacing of about 0.57 nm agreed well the (0 0 1) lattice plane of orthorhombic phase SmMn2 O5 , which indicates the nanorods also with preferring growth of the c-axis. In previous reports, the concentration of alkaline mineralizer has a great effect on the phase structure and size of metal oxides and perovskite oxides [29–31]. In order to study the influence of concentration of NaOH mineralizer on the morphology and phase structure of GdMn2 O5 and SmMn2 O5 , a set of experiments were carried out with variation NaOH concentration. Fig. 3 shows the XRD patterns of GdMn2 O5 synthesized with different NaOH concentration of 0.3, 1, 3 and 5 M, respectively. It can be seen that pure GdMn2 O5 can be obtained with the NaOH concentration from 0.3 to 5 M, and the relative intensity of the XRD patterns increased with increase of NaOH concentration, which indicates the high-crystalline powders can be synthesized with high alkaline concentration without any impurity phases. Fig. 4a depicts low-magnification TEM images of the as-synthesized GdMn2 O5 nanostructures after hydrothermal treatment at 250 ◦ C for 24 h with the NaOH concentration of 0.3 M. It can be seen that most crystallites with a diameter about 20–50 nm. The plate was further characterized using HRTEM, as shown in Fig. 4b. The lattice fringe of 0.57 nm in the observed nano-crystallite agrees well the (0 0 1) lattice planes, which with the same result to GdMn2 O5 nanoplates synthesized with the NaOH of 5 M. Fig. 4c shows the TEM image of the SmMn2 O5 nanostructures synthesized with the NaOH concentration of 0.3 M. Nanoparticles with a diameter about 50 nm and short rod-like particles with a length no more than 150 nm were obtained. To further verify the morphological characteristic of the SmMn2 O5 nanostructures, the HRTEM image of the particle is displayed in Fig. 4d. Fig. 4d shows the HRTEM image of a single nanoparticle, the lattice fringe of 0.34 nm and 0.57 nm in the observed nano-crystallite agrees well the (0 2 1) and (0 0 1) lattice planes, respectively. The molar ratio of Mn2+ /Mn7+ has a great effect on the shape and structural of the final products. Fig. 5 represents the XRD patterns of GdMn2 O5 and SmMn2 O5 synthesized at 250 ◦ C for 24 h using 5 M NaOH as mineralizer with Mn2+ /Mn7+ molar ratio of 4:1. The peaks

in the XRD pattern shown in Fig. 5a can be indexed to orthorhombic GdMn2 O5 and hexagonal Gd(OH)3 (JCPDS Card No. 38-1042). Fig. 5b shows the XRD patterns of the final product which consisted of orthorhombic SmMn2 O5 and hexagonal Sm(OH)3 (JCPDS Card No. 83-2036). Fig. 5c and d show the XRD patterns of the as-synthesized samples after washing with diluted nitric acid. The result shows that the impurity including Gd(OH)3 and Sm(OH)3 were dissolved in diluted nitric acid and all the peaks in Fig. 5c and d can be indexed to pure orthorhombic GdMn2 O5 and SmMn2 O5 , respectively. Fig. 6a and b show the SEM images of the GdMn2 O5 and SmMn2 O5 samples synthesized at 250 ◦ C for 24 h using 5 M NaOH as mineralizer with Mn2+ /Mn7+ molar ratio of 4:1 before washing with diluted nitric acid. Visually, the powders are mainly consisted of microrods and well-defined hexagonal shape microplates. Fig. 6c shows the SEM image of GdMn2 O5 after being washed with diluted nitric acid and the powders are mainly consisted of hexagonal shape microplates and flower-like microstructures, the hexagonal shape microplates with a width about 5 ␮m and the thickness about 600 nm as can be seen in Fig. 6d. The flowerlike microstructures are consisted of well-defined hexagon shape microplates (Fig. 6e). Fig. 6f shows the SEM image of SmMn2 O5 after being washed with diluted nitric acid and the powders are also consisted of hexagonal shape microplates and flower-like microstructures. The width and the thickness of the plates are about 6 ␮m and 500–1000 nm, respectively, as shown in Fig. 6g. The high-magnification of SmMn2 O5 flower-like microstructures are consisted of hexagonal shape microplates with the smaller size than those of GdMn2 O5 (Fig. 6h). Results of XRD patterns obtained under different synthesis conditions provide important information to prove the formation process of the final product although it may be complicated in the hydrothermal synthesis system. Under the high-pressure condition of the hydrothermal process, the redox reaction between KMnO4 and Mn(C2 H3 O2 )2 in an alkaline solution can be carried out completely, and the Mn valence in the product depends on the molar ratio of Mn2+ and Mn7+ ions in the reactants. Therefore, the molar ratio of Mn2+ and Mn7+ is also critical to the final product. The formation mechanism of GdMn2 O5 and SmMn2 O5 synthesized with different molar ratio of Mn2+ and Mn7+ can be expressed as follows: 5Re(NO3 )3 (Re = GdandSm) + 3KMnO4 + 7Mn(C2 H3 O2 )2 + 26NaOH → 5ReMn2 O5 + 3KNO3 + 12NaNO3 + 7H2 O + 14NaC2 H3 O2 (Mn2+ /Mn7+ = 7 : 3) 80Gd(NO3 )3 + 24KMnO4 + 96Mn(C2 H3 O2 )2 + 15O2 + 408NaOH → 60GdMn2 O5 + 20Gd(OH)3 + 216NaNO3 + 24KNO3 + 96NaC2 H3 O2 + 174H2 O(Mn2+ /Mn7+ = 4 : 1) 80Sm(NO3 )3 + 24KMnO4 + 96Mn(C2 H3 O2 )2 + 15O2 + 408NaOH → 60SmMn2 O5 + 20Sm(OH)3 + 216NaNO3 + 24KNO3 + 96NaC2 H3 O2 + 174H2 O(Mn2+ /Mn7+ = 4 : 1) In the hydrothermal process, the size and morphology of the final product depend on the competition between nucleation and crystal growth, which are determined by inherent crystal structure and the chemical potential in the precursor solution. The RMn2 O5 (R = rare-earth and Y) is of orthorhombic phase structure with high anisotropy structure, which leads to an anisotropic growth behavior of RMn2 O5 particles under a suitable synthesis condition. In the present research, the NaOH concentration in the synthesis system had a great effect on the shape and size of the final product.

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Fig. 6. Low-magnification SEM images of GdMn2 O5 synthesized at 250 ◦ C for 24 h using NaOH concentration of 5 M with Mn2+ /Mn7+ molar ratio of 4:1 before (a) and after (c) washing with diluted nitric acid, high-magnification SEM image of GdMn2 O5 after washing with diluted nitric acid (d), SEM image of GdMn2 O5 with flower-like microstructures (e), low-magnification SEM images of SmMn2 O5 synthesized at 250 ◦ C for 24 h using NaOH concentration of 5 M with Mn2+ /Mn7+ molar ratio of 4:1 before (b) and after (f) washing with diluted nitric acid, high-magnification SEM image of SmMn2 O5 after washing with diluted nitric acid (g), SEM image of SmMn2 O5 with flower-like microstructures (h).

When the hydrothermal treatment was performed in a relative low alkali condition, the amorphous precursors were hardly dissolved. Therefore, the main formation mechanism is in situ transformation process in which amorphous precursors crystallize into small particles by removal of structure water at the temperature 250 ◦ C. With increasing the alkali concentration in the hydrothermal treatment system, the dissolving rate of the amorphous precursors increased and accelerated the dissolution–recrystallization process. Once the

solution is supersaturated, nucleation and crystallization take place in the solution, and finally self-organized into high-aspect ratio of nanostructures. 4. Conclusions In this paper, we report a simple hydrothermal route to synthesis high-crystalline RMn2 O5 (R = Gd and Sm) nano- and

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microstructures at 250 ◦ C for 24 h using NaOH as mineralizer. The size and the morphology of final products were strongly depended on the NaOH concentration in the reaction system. High aspect RMn2 O5 (R = Gd and Sm) nanostructures were formed by a dissolution–recrystallization formation mechanism. The highaspect ratio of particles can be expected to prepare well-oriented ceramic materials at a low sintering temperature and the present conditions can be widely applied to fabricate other rare-earth manganates. References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] C.G. Hu, H. Liu, W.T. Dong, Y.Y. Zhang, G. Bao, C.S. Lao, Z.L. Wang, Adv. Mater. 19 (2007) 470. [3] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [4] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. [5] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [6] C.R. dela Cruz, B. Lorenz, Y.Y. Sun, Y. Wang, S. Park, S.-W. Cheong, M.M. Gospodinov, C.W. Chu, Phys. Rev. B 76 (2007) 174106. [7] N. Fujimura, T. Ishida, T. Yoshimura, T. Ito, Appl. Phys. Lett. 69 (1996) 1011. [8] D. Ito, N. Fujimura, T. Yoshimura, T. Ito, J. Appl. Phys. 93 (2003) 5563. [9] H. Yang, Z.H. Chi, J.L. Jiang, W.J. Feng, Z.E. Cao, T. Xian, C.Q. Jin, R.C. Yu, J. Alloy. Compd. 461 (2008) 1. [10] Y. Wang, Q.H. Jiang, H.C. He, C.W. Nan, Appl. Phys. Lett. 88 (2006) 142503. [11] A.A. Belik, S. Iikubo, K. Kodama, N. Igawa, S. Shamoto, M. Maie, T. Nagai, Y. Matsui, S.Y. Stefanovich, B.I. Lazoryak, E. Takayama-Muromachi, J. Am. Chem. Soc. 128 (2006) 706. [12] A.A. Belik, S. Iikubo, K. Kodama, N. Igawa, S. Shamoto, S. Niitaka, M. Azuma, Y. Shimakawa, M. Takano, F. Izumi, E. Takayama-Muromachi, Chem. Mater. 18 (2006) 798.

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