SrTiO3 Nanocomposites with

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Ferroelectric Mesocrystalline BaTiO3/SrTiO3 Nanocomposites with Enhanced Dielectric and Piezoelectric Responses Dengwei Hu, Hao Ma, Yasuhiro Tanaka, Lifang Zhao, and Qi Feng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01368 • Publication Date (Web): 06 Jul 2015 Downloaded from http://pubs.acs.org on July 12, 2015

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Chemistry of Materials

Ferroelectric Mesocrystalline BaTiO3/SrTiO3 Nanocomposites with Enhanced Dielectric and Piezoelectric Responses

Dengwei Hu,†,‡ Hao Ma, ‡ Yasuhiro Tanaka,‡ Lifang Zhao,† and Qi Feng∗,‡

†

College of Chemistry and Chemical Engineering, Baoji University of Arts and Science, 1 Hi-

Tech Avenue, Baoji, Shaanxi, 721013 PR China ‡

Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, 2217-

20 Hayashi-cho, Takamatsu-shi, 761-0396 Japan

∗ Corresponding author. E-mail: [email protected]; Fax: +81 (0)87-864-2402; Tel: +81 (0)87-864-2438

1

ACS Paragon Plus Environment


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ABSTRACT A platelike mesocrystalline BaTiO3/SrTiO3 (BT/ST) nanocomposite is prepared via a clever two-step solvothermal soft chemical process. Firstly a protonated titanate H1.07Ti1.73O4·nH2O (HT) crystal with a layered structure and platelike morphology is solvothermally treated in a Ba(OH)2 solution to generate a homogeneous platelike BaTiO3/HT (BT/HT) nanocomposite. Secondly the generated BT/HT nanocomposite is solvothermally treated in a Sr(OH)2 solution to generate the mesocrystalline BT/ST nanocomposite with platelike particle morphology. The transformation reactions from HT precursor to the mesocrystalline BT/ST nanocomposite are topochemical conversion reactions, and the formed BT/ST nanocomposite is constructed from wellaligned BT and ST nanocrystals in the same crystal-axis orientation. The BT/ST nanocomposite annealed at 900 oC shows a ferroelectric behavior and drastically enhanced piezoelectric and dielectric responses owing to the introduction of a lattice strain at a three dimensional heteroepitaxial interface between the BT and ST nanocrystals in the mesocrystal. The nanostructure of the BT/ST mesocrystal is suitable for simultaneous application of the strain engineering and the orientation engineering to develop high performance piezoelectric and dielectric materials.

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■ INTRODUCTION The study on ferroelectric metal oxide perovskites with an ABO3 formula is of great scientific and technological interest for their outstanding ferroelectric, piezoelectric, dielectric, pyroelectric, photoelectric, and catalytic responses. 1

,2,3

The lattice strain

engineering is one of the efficient approaches to further improve the electrical performances of the metal oxide perovskites.4 The solid solution near a morphotropic phase boundary (MPB) separating two crystal symmetries with different orientations of spontaneous polarization can usually exhibit an anomalously high piezoelectric and ,6,

dielectric responses.5 7 The enhancements of the piezoelectric and dielectric responses are associated with a lattice strain derived from the lattice mismatch between the two ,8,

morphotropic phases with slightly different lattice constants at their interface.5,7 9 The lattice strain induces an unstable spontaneous polarization at the interface, and then its polarization rotation becomes sensitive when a bias is applied, namely the energy for the polarization rotation becomes very low at MPB.4,6,10,11 The PbZrxTi1-xO3 (PZT) is a successful example of application of the MPB, which is widely used in the piezoelectric devices. Since PZT materials contain high content of toxic Pb, recently the studies on searching its alternative materials have become a hot topic. Some applications of the MPB to improve piezoelectric performances of Pb-free ferroelectric materials have been reported. However, the composition range of the MPB is very narrow, and also it is hard to get a temperature-independent MPB, 12 which limited the applications of the MPB. An artificial superlattice constructed from two kinds of perovskites can be applied also to enhance piezoelectric and dielectric responses. A BaTiO3/SrTiO3 (BT/ST) superlattice with a heteroepitaxial interface between ferroelectric BT phase and paraelectric ST phase, as shown in Figure 1a, has been developed and attracted 3



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considerable attentions. 13

14 , 15

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The BT/ST superlattices are reported occurring in

significant enhancements of dielectric constants and remanent polarization due to a horizontal strain within the two dimensional (2D) BT/ST heteroepitaxial interface.4, 16

, 17 , 18

This horizontal strain is derived from a ～2.2% lattice mismatch

between BT and ST phases.16,19

,20,21

Such artificially induced interfacial lattice strain

can also enhance the dielectric and piezoelectric responses, and it is called strain engineering. 22

, 23 , 24

The enhancements in the Curie temperature and remanent

polarization of ferroelectric materials can be achieved also by the interfacial lattice strain. 6,19,20 Since such lattice strain is derived from the lattice mismatch, it would show a less temperature-dependence than that derived from the MPB. The fabrication of the BT/ST superlattice has been achieved by a molecular beam epitaxy (MBE) method. However, such method is complicated and depends on the very sensitive substrate and expensive stacking configuration. 25

, 26 27

28

Furthermore,

although the 2D heteroepitaxial interface can be produced by MBE method, a three dimensional (3D) high-density heteroepitaxial interface is difficult to be produced by this method. For the practical applications, it is very expectative for the development of a facile process that can introduce 3D high-density heteroepitaxial interface, as shown in Figure 1b, to further improve the electrical properties of the BT/ST nanocomposites. Mesocrystal is an interesting polycrystal constructed from well-aligned nanocrystals with uniform orientation;29,30 it not only has some potential properties based on the individual nanocrystals, but also exhibits unique collective properties of nanocrystal ensembles.31 Therefore, it is a new class of material and has become an attractive research area in the past decade. The mesocrystal can offer some new

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opportunities for material designs, and can be applied to catalysis, sensing, and energy storage and conversion. 32

, 33 , 34 , 35

We have developed some perovskite titanate 3738394041

mesocrystals in our previous works.36

Recently some studies on BT and ST mesocrystals have been reported also by other authors. 42

, 43 , 44

A highly strained mesostructured BT/ST composite film has been

fabricated by a surfactant-templated sol–gel method.25 The fabricated BT/ST composite film shows larger dielectric and piezoelectric responses than the individual BT or ST film, because of an abundant compressive stress created by the lattice mismatch between the BT and ST nanocrystals. In addition, orderly assemblies of BT/ST mixture nanocube single crystals have been fabricated by capillary force assisted solution self-assembly method, and it has been found that the heteroepitaxial interface between BT and ST nanocubes could enhance piezoelectric and dielectric responses. 45,46 Herein, we report a clever solvothermal soft chemistry process for development of a platelike mesocrystalline BT/ST nanocomposite. Such platelike mesocrystalline nanocomposite is very difficult to be prepared via the conventional approaches due to their high nanocrystal-axis-orientation, chemical composition of two components, and high aspect ratio. The obtained mesocrystalline BT/ST nanocomposites are constructed from the well-aligned BT and ST nanocrystals in same crystal-axis orientation. The generated mesocrystalline BT/ST nanocomposite shows drastically enhanced piezoelectric and dielectric responses because the 3D high-density heteroepitaxial interface is successfully introduced into the mesocrystalline BT/ST nanocomposite (Figure 1b).

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■ EXPERIMENTAL SECTION Sample Preparations. Platelike H1.07Ti1.73O4·nH2O (HT) particles were synthesized as reported in our previous study.

47

Mesocrystalline BT/ST

nanocomposite and BaxSr1-xTiO3 (BST) solid solution were prepared by a two-step solvothermal process. Firstly a BT/HT nanocomposite was prepared by solvothermal treatment of the platelike HT crystals (0.096 g) and Ba(OH)2 (molar ratio of Ba/Ti = 0.5) in 30 mL water/ethanol mixed solvent at 200 oC under stirring conditions for 12 h. The obtained sample was washed with a 0.1 mol·L-1 acetic acid solution and distillated water. In the second step, the BT/HT nanocomposite (0.094 g) was solvothermally treated with Sr(OH)2 (molar ratio of Sr/Ti = 1) in 30 mL water/ethanol mixed solvent at 200 oC under stirring condition for 12 h. After the solvothermal treatment, the obtained sample was washed with the 0.1 mol·L-1 acetic acid solution and distillated water. Finally, the obtained sample was dried at room temperature. The as-prepared nanocomposite sample was annealed at a desired temperature for 3 h to obtain its annealing treatment sample. Material Characterizations. The structures of crystalline samples were investigated using a powder X-ray diffractometer (Shimadzu, XRD-6100) with Cu Kα (λ = 0.15418 nm) radiation. The particle size and morphology of samples were observed using scanning electron microscopy (SEM) (JEOL, JSM-5500S) or field emission scanning electron microscopy (FESEM) (Hitachi, S-900). TEM and high resolution TEM (HRTEM) observations, and also selected area electron diffraction (SAED) were carried out on a JEOL Model JEM-3010 system at 300 kV, where the sample was put on a Cu microgrid. Energy-dispersive spectroscopy (EDS) was measured on the TEM system (JEOL JED-2300T). Energy dispersive spectroscopy mapping (EDS-mapping) was observed using a HRTEM-EDS system (JEOL Model

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JEM-2100F HRTEM / EX-24063JGT EDX). Raman spectra were recorded by a JASCO NRS-3100 Raman spectrometer with a scanning step of 1 cm-1 at an excitation wavelength of 532 nm. Electrical Characterizations. The powder sample was pressed into a pelletshaped sample using a pellet press mold. A commercial BaTiO3 powder sample of BT-05 with a particle size of 50 nm was provided by Sakai Chemical Industry Co., Ltd. In the dielectric measurement, the pellet sample was mounted on an dielectric material test fixture with guarded electrode (Agilent 16451B), and then measured using an LCR meter (Agilent E4980A) at an applied bias of 1.0 V in a frequency range of 1 to 2000 kHz. The polarization-electric field (P-E) loop of the pellet sample was measured using a ferroelectric testing system (Toyo Corporation, FCE3-4KVSYS) at room temperature and 50 Hz. Piezoelectric response of the platelike particle sample was measured using a scanning probe microscopy system (SPM, SII, NanoTechnology Inc., Nano Navi Station) with an SPA-400 probe, combining atomic force microscopy (AFM) and piezoresponse force microscopy (PRM). A conductive Rh-coated Si cantilever (SIDF3-R) and Si substrate were coated with an Au thin film of 20 nm thickness by ion sputtering. The samples were dispersed on the Au-coated substrate by drop casting. The location of an individual platelike particle was detected by the AFM probe tip with the conductive Au-coated Si cantilever in the contact mode. After that a DC bias from -10 to 10 V was applied to the surface of the platelike particle, and the generated strain of the particle was detected simultaneously using the AFM system. The effective converse piezoelectric coefficient d*33 value was calculated by the formula d*33 = S / Va where S is the strain, and Va is the applied bias. Details of the measurement procedure 7



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and parameters can be found elsewhere.48,49

■ RESULTS AND DISCUSSION Syntheses of BaTiO3/HT nanocomposites For the synthesis of the platelike mesocrystalline BaTiO3/SrTiO3 (BT/ST) nanocomposite, a clever two-step solvothermal soft chemistry process was designed to inhibit the destruction of platelike morphology and prevent generating a BaxSr1xTiO3

(BST) solid solution. In the first step, a lepidocrocite-like layered titanate

H1.07Ti1.73O4·nH2O (HT) precursor with platelike morphology is solvothermally reacted partially with Ba(OH)2 to generate a BaTiO3/HT (BT/HT) nanocomposite with platelike morphology. In the second step, the obtained BT/HT nanocomposite is solvothermally reacted with Sr(OH)2 to transform unreacted HT in the nanocomposite to SrTiO3 (ST) to generate the platelike mesocrystalline BT/ST nanocomposite. The BT/HT nanocomposite samples obtained in the first step are named as BH-x/y, where x and y are volumes of water and ethanol added into the reactions solvent, respectively. The XRD and FESEM results indicate that after solvothermal reaction at 200 oC, HT is transformed into the BT phase partially, but remain platelike particle morphology (Figures S1 and S2). The BT phase formed in the water solvent exhibits a higher crystallinity than that in water/ethanol mixed solvent. In order to clear formation mechanism of the BT phase from the HT single crystal in detail, TEM/HRTEM observations and SAED were performed on the HT precursor and solvothermally treated samples (Figure 2). The platelike HT single crystal shows smooth surface, and clear and ordered diffraction spots in SAED pattern which can be

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assigned to orthorhombic HT symmetry located along a [010] zone axis (Figure 2b). The [010] direction (b-axis) is perpendicular to the basal-plane of the platelike HT particle. In BH-30/0 sample, the platelike crystals still maintain smooth surface except some nanoparticles on the surface of particles (Figure 2c). The nanocrystals with the sizes of about 10 nm at the comb-like edge of the platelike particle are observed. Two sets of SAED spots, corresponding the HT phase and the BT perovskite phase respectively, are simultaneously observed in one platelike crystal of BH-30/0 sample (Figure 2d), agreeing with XRD result in Figure S1b, which implies that BT and HT phases coexist in one platelike particle. The [1-10] and [001] directions of the generated BT phase correspond to the [001] and [100] directions of the matrix HT phase, respectively, namely [110] zone axis of the well-oriented BT nanocrystal corresponds to the [010] zone axis of the HT matrix, forming a BT/HT nanocomposite mesocrystal. In BH-10/20 and BH-5/25 samples synthesized by the solvothermal treatments, many BT nanocrystals are observed on the surfaces of the platelike crystals, and nanocrystal size of BT decreases with decreasing ethanol content in the reaction solvent (Figures 2e and g). Also two sets of SAED spots, corresponding to HT and BT phases, are observed simultaneously in one platelike polycrystal for these two kinds of samples (Figures 2f and h), suggesting the formation of mesocrystalline BT/HT nanocomposites. The [110] zone axis of BT nanocrystals also corresponds to the [010] zone axis of HT crystal, and the crystal orientations between the BT and HT phases are same as BH-30/0 sample. Many fringes with a width of about 3 nm are observed in the BT nanocrystals with a size of about 40 nm on platelike BH-5/25 particle (Figure 2g). The clear lattice fringes of 0.397 and 0.279–0.282 nm corresponding to the (001) and (1-10) planes of

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the BT phase, respectively, are observed in the HRTEM image (Figure 2i). A Fast Fourier Transform (FFT) pattern (Figure 2j) with reciprocal lattice spots of BT crystalline phase is consistent with the SAED result (Figure 2h). These results suggest that the nanocrystals on the surface of the platelike particle are BT nanocrystals. The fringes with width of 3 nm may be due to Moire effect derived from the interference between the diffractions of BT and HT phases. The above results suggest that in the water solvent, the formed BT nanoparticles are distributed uniformly in the platelike particle bulk, namely the formation of uniform platelike BT/HT nanocomposite; while in water/ethanol mixed solvent, the formed BT nanoparticles are preferentially distributed near the platelike particle surface. This difference can be explained by the different reactivities of Ba(OH)2 and HT in the solvents with different polarity. The solubilities of Ba(OH)2 and HT constructed by ionic bonds enhance with increasing the solvent polarity. Therefore, the solvothermal reactivity enhances with increasing the solvent polarity, namely, water/ethanol ratio. In the water solvent with high polarity, Ba(OH)2 shows a high solubility, and then Ba2+ ions can easily migrate into the HT crystal bulk from the interlayer space, and reaction with HT in the crystal bulk, resulting in the formation of the uniform BT/HT nanocomposite. On the other hand, in the ethanol solvent with low polarity, Ba(OH)2 shows a low solubility, and then the Ba2+ ions are difficult to migrate into the HT crystal bulk, resulting in that the formation reaction of BT preferentially occurs around the surface of the platelike HT particle. Therefore, the nanostructure of BT/HT nanocomposite can be effectively controlled by changing the properties of solvothermal reaction solvent. Solvothermal syntheses of BT/ST nanocomposites.

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In order to obtain the BT/ST nanocomposite, the platelike BT/HT nanocomposites are solvothermally treated in Sr(OH)2 solutions. The generated samples are named as BT/ST-x/y-m/n, where x and m are volumes of water, and y and n are volumes of ethanol in the reaction solvents of the first and second steps, respectively. Figure 3 shows the XRD patterns of the samples obtained by the solvothermal treatments of the BT/HT nanocomposites in Sr(OH)2 solutions at 200 oC. For BT/ST-30/0-10/20 sample obtained using BH-30/0 as the precursor, two sets of XRD patterns with a typical metal oxide perovskite structure are observed distinctly (Figure 3a). The XRD pattern with larger lattice constant can be easily indexed to the BT phase (JCPDS File No. 74-1964, cubic symmetry), and that with smaller lattice constant to ST phase (JCPDS File No. 74-1296, cubic symmetry), respectively. This observation reveals that a mixture of BT and ST phases is formed in BT/ST-30/0-10/20 sample. BT/ST5/25-10/20 sample obtained using BH-5/25 as the precursor also shows diffraction peaks of BT and ST phases, but the peaks are broad and overlapped together (Figure 3b), implying their low crystallinities. The result suggest that the ST phase is formed by the solvothermal reaction of the HT phase in the BT/HT nanocomposite with Sr(OH)2. The low crystallinity of BT/ST-5/25-10/20 is due to the low crystallinity of BT phase in the BH-5/25 precursor. When BH-30/0 and BH-5/25 samples are reacted with Sr(OH)2 in water solvent, Ba0.5Sr0.5TiO3 solid solution phase that can be identified by JCPDS File No. 39-1395 (cubic symmetry) is obtained (Figure 3c and d). The BST solid solution phase is formed owing to the high reactivity in the water solvent, where the formed ST nanoparticles can further react with the BT nanoparticles in the nanocomposite, which results the formation of BST solid solution. The XRD results reveal that the

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water/ethanol mixed solvent and BH-30/0 precursor are suitable for the preparation of the crystalline BT/ST nanocomposite. Figure 4 shows the TEM images and SAED patterns of the samples prepared by the solvothermal treatments of the BT/HT nanocomposites in the Sr(OH)2 solutions at 200 oC. All the products show the similar platelike morphology and polycrystalline nanostructures constructed from oriented nanocrystals, which suggests that HT phase in the BT/HT nanocomposite is transformed to BT or BST phase by topochemical conversion reaction. BT/ST-30/0-30/0 product has a larger crystal size of about 80 nm than other products, BT/ST-30/0-10/20, BT/ST-5/25-10/20, and BT/ST-5/25-30/0, those have a crystal size of about 40 nm. This result is consistent with the FESEM result in Figure S4. It is very interesting that two sets of single-crystal-like perovskite SAED spot patterns are observed simultaneously in one platelike particle for BT/ST-30/0-10/20 and BT/ST-5/25-10/20 products synthesized in the water/ethanol mixed solvent. BT/ST-30/0-10/20 polycrystalline platelike particle exhibits two sets of clear SAED spot patterns corresponding to the single-crystal-like BT and ST phases with [110] zone axis located on the basal-plane, respectively (Figure 4b), although it is a polycrystalline particle, which reveals that the BT and ST phases coexist in one polycrystalline platelike particle. Furthermore, it is worth pointing out that the SAED spot pattern of BT phase almost overlaps with that of the ST phase, except a little larger lattice constant of the BT phase than that of the ST phase, namely the crystal orientation of [110] zone axis of the BT phase entirely fits with the ST phase. This result reveals that the platelike particle is a mesocrystalline BT/ST nanocomposite, in which all the BT and ST nanocrystals in one platelike particle align in the same crystal-axis-orientation, and also suggests the formation of a heteroepitaxial interface

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between the BT and ST nanocrystals. In BT/ST-5/25-10/20 polycrystalline platelike particle, a less obvious BT and ST phases can be simultaneously observed also (Figure 4d), suggesting the formation of mesocrystalline BT/ST nanocomposite with low crystallinity. The SAED results of the formation of the BT and ST phases in these samples are consistent with the XRD results illustrated in Figures 3a and b. BT/ST-30/0-30/0 and BT/ST-5/25-30/0 samples synthesized in water solvent exhibit single-crystal-like SAED spot patterns of the Ba0.5Sr0.5TiO3 phase with [110] zone axis located on the basal-plane (Figures 4f and h). The result reveals that all the BST nanocrystals in a platelike particle align in the same crystal-axis-orientation, namely the formation of platelike BST mesocrystal. The results of the formation of the single Ba0.5Sr0.5TiO3 phase in BT/ST-30/0-30/0 and BT/ST-5/25-30/0 samples agree also with XRD patterns (Figure 3c, d). To investigate the solvent influence on the formation of BT/ST nanocomposite in detail, the BH-30/0 precursor is solvothermally treated in the Sr(OH)2 solutions of different solvents. Except reactions in pure ethanol solvent and pure water, the BT/ST nanocomposite can be obtained in the water/ethanol mixed solvents (Figure S5). These products show also the platelike morphology constructed from nanoparticles (Figure S6). The BH-30/0 precursor exhibits lower reactivity with Sr(OH)2 in the pure ethanol solvent, where the formation of the ST or BST phase is not observed. On the other hand, BH-30/0 precursor exhibits high reactivity with Sr(OH)2 in the high water content solvent, and then the formed ST nanoparticles can react further with the BT nanoparticles causing the formation of BST solid solution phase. Namely, the mesocrystalline BT/ST nanocomposite will disappear under high reactivity conditions. Therefore, the water/ethanol mixed solvent and BH-30/0 precursor with uniformly

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distributed BT nanocrystals are suitable for the synthesis of the platelike mesocrystalline BT/ST nanocomposite.

Nanostructural and nanocompositional analyses of mesocrystalline BT/ST nanocomposites. A chemical composition analysis is carried on the mesocrystalline BT/ST nanocomposite platelike particles using EDS (Figure S7). The Ti/Ba/Sr mole ratios in BT/ST-30/0-5/25,

BT/ST-30/0-10/20,

and

BT/ST-30/0-15/15

samples

are

1.00/0.51/0.39, 1.00/0.48/0.51, and 1.00/0.40/0.67, respectively (Table S1). The Sr content decreases with decreasing water content in the reaction solvent of the second step solvothermal reaction, and Ba and Sr contents are almost same in BT/ST-30/010/20 sample. The result suggests that the BT/ST ratio can be controlled by changing water and ethanol contents in the solvothermal reaction solvent. To investigate distribution of BT and ST nanocrystals in the BT/ST nanocomposites, EDS-mapping TEM image analysis is carried out on the nanocomposite samples. However, the BT and ST nanocrystals cannot be distinguished individually by the EDS-mapping TEM image (Figure S8). It may be due to the stacking nanostructure of the BT and ST nanocrystals or BT-ST core-shell nanostructure in the BT/ST nanocomposite. To confirm it, TEM and HRTEM observations, and EDS analysis are carried out on the nanoparticles obtained by grind and ultrasound dispersion treatments of the platelike BT/ST nanocomposite. The dispersed individual nanocrystals show clear lattice fringes of 0.282 and 0.273 nm in HRTEM images, which can be well assigned to the (110) planes of the BT and ST phases, respectively (Figure 5A). Figure 5B presents the corresponding EDS spectra detected at the (a, b, c, d) white pane areas in the TEM image of Figure 5A,

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respectively. The result suggests the each nanocrystal has a simple chemical composition of BT or ST, which agree with the HRTEM result. Furthermore, the heteroepitaxial interfaces between the BT and ST nanocrystals can be confirmed also by the HRTEM images (Figures 5C and S9), where the blue lines indicate the boundary regions between two individual nanocrystals. The above results reveal that the

platelike

mesocrystalline

BT/ST

nanocomposite

is

constructed

by

heteroepitaxially stacking individual BT and ST nanocrystals.

Topochemical conversion mechanism to platelike mesocrystalline BT/ST nanocomposite. The mesocrystalline conversion reactions from the HT single crystal to the mesocrystalline BT/ST nanocomposite and their topological correspondences can be illustrated as a schematic representation in Figure 6a. Platelike mesocrystalline BT/ST nanocomposite is formed by the two-step solvothermal topochemical process. In the first step, Ba2+ are intercalated into the interlayer space of HT crystal by an H+/Ba2+ exchange reaction, and then the intercalated Ba2+ react with the TiO6 octahedra of HT layered structure to form the BT nanocrystals in the crystal bulk. 50 In this solvothermal topochemical reaction, half of the HT crystal is transformed to the BT nanocrystals because the Ba/Ti mole ratio in the reaction system is 0.5/1. A dissolution-deposition reaction can occurs also on the platelike particle surface. In the present case, the dominant reaction is the topochemical conversion reaction due to the low concentration of Ba(OH)2. The platelike BT/HT nanocomposite with uniform distribution of BT nanoparticle is formed by the solvothermal reaction in water solvent. However, the BT nanoparticles are formed preferentially near the surface of the platelike particle in water/ethanol solvent, owing to low polarity of the solvent and

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low solubility of Ba(OH)2, in which the Ba2+ ions are difficult to be migrated in the HT crystal bulk. There is a definitely corresponding relationship between the crystalaxis directions of HT and BT nanocrystals because the conversion reaction from HT to BT is a topochemical reaction (Figures 2d, f and h). Namely, all the BT nanocrystals in one platelike crystal of the BT/HT nanocomposite present the same crystal-axis orientation, and [001], [1-10], and [110] directions of BT nanocrystals correspond to the [100], [001], and [010] directions of HT matrix crystal, respectively. In the second step, the solvothermal reaction of the BT/HT nanocomposite and Sr(OH)2 is also a topochemical conversion reaction similarly to reaction in the first step. When BT/HT nanocomposite with an uniform distribution of BT nanoparticles is used as a precursor, firstly Sr2+ are intercalated into the interlayer of residual HT crystal bulk by an H+/Sr2+ exchange reaction, and then Sr2+ react with the residual HT crystal to form ST nanoparticles until consuming all the HT phase. The mesocrystalline BT/ST nanocomposite is formed at this stage. However, BT and ST nanoparticles can react together further by the topochemical reaction, and then the platelike mesocrystal of BST solid solution is formed under the high reactivity conditions, such as in water solvent at 200 oC. When the BT/HT nanocomposite prepared in the water/ethanol mixed solvent is used as the precursor, the situation is different because the platelike particle surface of the BT/HT precursor is covered by BT nanoparticles. In this case, firstly the Sr2+ ions react with the BT nanoparticles locating on the platelike particle surface before migrating into the crystal bulk, which causes the formation of Ba0.5Sr0.5TiO3 solid solution nanoparticles from the platelike particle surface.

Therefore,

the

mesocrystalline BT/ST nanocomposite is difficult to be obtained by using such BT/HT precursor.

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In the second step reaction, dissolution-deposition reaction can occur also on the platelike particle surface, but topochemical conversion reaction is dominant, which keeps the platelike morphology of mesocrystalline BT/ST nanocomposite or Ba0.5Sr0.5TiO3 solid solution. Therefore, there is a corresponding relation between the crystal orientations of formed ST nanocrystals and HT precursor. Namely, [001], [110], and [110] directions of ST or BT nanocrystals correspond to the [100], [001], and [010] directions of HT matrix crystal, respectively, same as the BT nanocrystals. The topochemical reaction causes the formation of the BT/ST heteroepitaxial interface (grain boundary) in the mesocrystalline BT/ST nanocomposite (see Figure 5C-e and f). Figure 6b illustrates an in situ topochemical mesocrystal conversion reaction from the lepidocrocite-like layered structure of HT matrix crystal to the perovskite nanostructure of the mesocrystalline BT/ST nanocomposite. In the HT layered structure, the TiO6 octahedra are combined with each other via corner- and edgesharing to form a 2D TiO6 octahedral sheet. In the conversion reaction to the BT structure, the TiO6 octahedra in the layered structure are shifted to the positions of the perovskite structure with corner-sharing TiO6 octahedra, and this process has to satisfy the principle of minimum energy and space requirements to make the stable perovskite BT structure. In the conversion reaction to the ST structure, a similar reaction occurs also. At the heteroepitaxial interface of mesocrystalline BT/ST nanocomposite, the formation of a small amount of BST solid solution nano-layers is possible. Based on the formation mechanism described above, firstly the BT/HT nanocomposite precursor with uniform distribution and enough large crystal size of BT nanoparticles is necessary for the formation of the BT/ST nanocomposite. In the conversion of BT/HT to BT/ST nanocomposite, keeping appropriate reactivity in the

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solvothermal reaction system is very significant. The formed BT/ST nanocomposite can be transformed into BST solid solution under high reactive conditions. The appropriate conditions can be achieved by changing the polarity of solvothermal reaction solvent.

Annealing treatment of mesocrystalline BT/ST nanocomposites. The mesocrystalline BT/ST nanocomposite sample of BT/ST-30/0-10/20 is employed for the annealing investigation. The annealed products are named as BT/ST-x, where x is annealing temperature. Nanostructure of the mesocrystalline BT/ST nanocomposite does not change below 900 oC of the annealing temperature and a main phase of the BST solid solution is formed when the annealing temperature is elevated to 1300 oC (Figure S10). The platelike morphology of the nanocomposite can withstand up to 1100 oC, and starts to destroy at 1200 oC (Figure S11). Figure 7 presents the TEM images and SAED patterns of BT/ST-600 and BT/ST900 samples obtained by annealing treatments. BT/ST-600 particles show the platelike morphology constructed from the nanocrystals (Figure 7a). The nanocrystal size is not obvious change after the annealing treatment at 600 oC (Figure 4a). It still exhibits the SAED pattern of mesocrystalline BT/ST nanocomposite structure (Figure 7b), which agrees with the XRD result shown in Figure S10b. At the elevated annealing temperature of 900 oC, a sintered platelike particle was formed, in which the nanoparticle boundary became unclear (Figure 7c). Two sets of clear SAED spot patterns can be easily identified and assigned to BT and ST with the same [110] zone axis, respectively (Figure 7d). The results imply that dense platelike BT/ST nanocomposite particles can be formed after annealing at 900 oC.

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The transformation of pseudo-cubic BT phase to tetragonal BT phase by annealing treatment of the as-prepared BT/ST nanocomposite sample (BT/ST-RT) can be confirmed by Raman spectra, as illustrated in Figure 8. The as-prepared BT-RT mesocrystal and ST-RT mesocrystal show similar Raman spectra, in which except some unexpected bands at around 390, 450, and 890 cm-1, the other bands correspond to a pseudo-cubic BT phase and a cubic ST phase, respectively.51,52 These unexpected bands at around 390, 450, and 890 cm-1 of BT-RT may be assigned to the grainboundary regions of the BT nanocrystals in the mesocrystalline structure. 53 Asprepared ST-RT mesocrystal also displays similar unexpected bands at around 390, 450, and 890 cm-1 because it has similar mesocrystal nanostructure. In ST-RT mesocrystal spectrum, the four strong bands at 294, 557, 645, and 731 cm-1 can be assigned to the second-order Raman bands of ST structure, where its first-order Raman bands are not observed (Figure 8b), suggesting the cubic perovskite structure of ST before annealing. 54 , 55 The similar bands are observed in BT-RT spectrum (Figure 8a), due to its pseudo-cubic perovskite structure. The Raman spectrum of BT/ST-RT nanocomposite shows the vibration bands corresponding to BT-RT and ST-RT mesocrystals, revealing that BT/ST-RT nanocomposite is constructed from the pseudo-cubic BT phase and the cubic ST phase. The Raman spectrum of BT/ST-900 is different from that of BT/ST-RT. In BT/ST-900 spectrum, the vibration bands at 268, 303, 526, and 726 cm-1 can be assigned to characteristic bands of the tetragonal BT phase.51,56 The bands at 303, 362, 634, and 726 cm-1 correspond to the second-order Raman bands of ST structure, and the bands at 237, 482, and 813 cm-1 correspond to the first-order Raman bands of ST structure.55 The appearance of the first-order Raman bands suggests lattice distortion in the cubic structure of ST after the annealing. The vibration bands at 216, 864, and

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906 cm-1 cannot be assigned to the BT and ST structures. Therefore, we think these bands can be assigned to distorted lattice at BT/ST interface induced by the lattice mismatch of the BT and ST.28 The result implies the formation of BT/ST heteroepitaxial interface in the nanocomposite, and the in-plane compressive strain of lattice is induced by the heteroepitaxial interface. Such lattice strain at BT/ST heteroepitaxial interface is promising for the enhancements of the piezoelectric and dielectric performances.25

Electrical behaviors of mesocrystalline BT/ST nanocomposites. To confirm the BT/ST heteroepitaxial interface effect on the piezoelectric and dielectric properties, we study the electrical properties of the mesocrystalline BT/ST nanocomposites. The hysteresis loops of polarization-electric field (P-E) for BT/ST nanocomposites after annealing treatments at different temperatures are shown in Figure 9. With the increase of the annealing temperatures, the nonlinear P-E hysteresis behavior becomes obvious. Although saturated P-E hysteresis loops were not obtained due to the low density of the pellet samples prepared at low temperature, the enhancement of ferroelectricity after the annealing can be judged from the results. The remanent polarization Pr and coercive electric field Ec values evaluated from the P-E loop are 0.6 µC/cm2 and 5 kV/cm for BT/ST-900 at an applied electric field of 17 kV/cm. This result implies that the as-prepared BT/ST-RT is a paraelectric phase, and can be transformed into a ferroelectric phase after annealing at 900 oC due to the transformation of the pseudo-cubic BT phase to tetragonal BT phase, which agrees with the Raman spectrum result (Figure 8). The further larger Pr and Ec values can are expected for a high density sample.

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To evaluate the contribution of BT/ST interface to the dielectric behavior, relative permittivities (εr) of the BT and BT/ST mesocrystals pellet samples were measured in the frequency range of 1 kHz – 2 MHz, and the results are presented in Figure 10. These two samples have same platelike particle morphology and nanostructure but different nanocrystals interfaces, therefore, most of other effect, except the different nanocrystals interfaces, could be eliminated by comparison of their results.

As-

prepared BT-RT and BT/ST-RT mesocrystals show the similarly small εr values and small frequency dependences because they are paraelectric phases. BT-900 and BT/ST-900 samples exhibit larger εr values and larger frequency dependences in the low frequency range due to the transformations of the paraelectric phases to the ferroelectric phases after annealing at 900 oC. The permittivity of BT/ST-900 sample robustly enhances and is four times larger than that of BT-900 sample at 1 kHz. This result illustrates that BT/ST-900 sample exhibits an advantage on the enhancement of dielectric response. Although the permittivity of BT/ST-900 sample is not much higher than the normal high density BT ceramics, a further higher permittivity value is expected if a high density BT/ST mesocrystal sample can be prepared. The large frequency dependences in the low frequency range for BT-900 and BT/ST-900 samples suggest possibility of the effect of Maxwell-Wagner polarization or other dielectric relaxation processes from the nanocrystals interfaces. The variations of εr with the annealing temperature for BT/ST, BT mesocrystals, and also a commercial BaTiO3 sample of BT-05 are shown in Figure 11. With the increase of the annealing temperature, the εr values of the BT mesocrystal and commercial BT-05 sample increase continuously from 600 to 1300 oC due to transformation from the paraelectric phase to the ferroelectric phase and sintering of pellet samples. However, for the BT/ST mesocrystal, the εr value increases firstly to a

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maximum value of 2060 at 900 oC and then decreases, where the maximum value is much larger than the maximum values of BT mesocrystal and BT-05 samples annealed at 1300 oC. The permittivity decreasing above 900 oC is due to a transformation from BT/ST nanocomposite to BST solid solution phase that has a lower permittivity than BT/ST nanocomposite and BT phase. The higher permittivity of BT/ST-900 than BT-1300 is due to the lattice strain at the BT/ST heteroepitaxial interface that causes a specific enhancement in the permittivity.25 This result suggests that the high permittivity can be achieved by the introduction of the heteroepitaxial interfaces in the BT/ST nanocomposite. The piezoelectric responses of individual platelike particles of the BT/ST nanocomposite and BT mesocrystal are investigated by a scanning probe microscopy (SPM) technique. Figure 12 shows the strain-applied voltage (S-V) loops and d*33applied voltage (d*33-V) loops for BT/ST-900 and BT-900, where d*33 = S/V = strain (%)/Ea (V/m) is calculated from the S-V loop. BT/ST-900 exhibits a typical S-V loop of ferroelectric materials and an effective converse piezoelectric coefficient (d*33) of 306 pm V-1 at 10 V of applied voltage. However, BT-900 shows a much lower d*33 value of 103 pm V-1 than BT/ST-900. The annealing temperature dependence of d*33 for BT/ST nanocomposite is shown in Figure 11 and Figure S13. The d*33 value increases firstly to the maximum value at 900 oC and then decreases with increasing the annealing temperature. Similar to its permittivity behavior, the d*33 increasing from room temperature to 900 oC is also due to the transformation from paraelectric phase to ferroelectric phase, and the d*33 decreasing above 900 oC is due to the transformation from BT/ST nanocomposite to BST solid solution phase that has a lower d*33 than BT/ST nanocomposite.

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The d*33 value of BT/ST-900 nanocomposite (306 pm V-1) is about one order of magnitude higher than those of nanostructured BaTiO3 (28 pm V-1)

57

and the

mesostructured SrTiO3/BaTiO3 composite film fabricated by surfactant-templated sol–gel method25, and also higher than those of BT/ST nanocomposite (59 pm V-1)45 constructed from BT and ST nanocubes by solution self-assembly and the BT-900 mesocrystal (103 pm V-1) prepared in this study. This is due to the perfect orientation matching between the BT and ST nanocrystals in the mesocrystalline BT/ST nanocomposite prepared in this study. These results reveal that the introduction of the 3D high-density BT/ST heteroepitaxial interface in the BT/ST nanocomposite mesocrystal can give excellent piezoelectric response. Except of the lattice strain effect, the crystal-axis orientation effect also can contribute to the enhancement of piezoelectric response. It has been reported that [110]-oriented BT ceramic exhibits a much larger piezoelectric response than that of non-oriented BT ceramic.

58

Such orientation engineering, also called texture

engineering, is also an advanced technology to improve piezoelectric and dielectric responses for the ferroelectric materials. 59,60 Therefore, the [110]-orientation of the mesocrystalline BT/ST nanocomposite prepared in this study is also beneficial to enhancement of its piezoelectric response. The results of the electrical study described above imply that the mesocrystalline BT/ST nanocomposite is promising for the improvement of ferroelectric performances, including dielectric and piezoelectric responses, because except the strain engineering, the orientation engineering can be applied simultaneously in the mesocrystal nanomaterials. 34,61

■ CONCLUSIONS

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The two-step solvothermal topochemical process is an effective approach for the synthesis of the platelike mesocrystalline BT/ST nanocomposite. In the first step reaction, homogeneous BT/HT nanocomposite is formed by partial transformation of HT to BT. The nanostructure and BT nanoparticle size in the BT/HT nanocomposite are dependent strongly on polarity of reaction solvent. For preparation of the mesocrystalline BT/ST nanocomposite, the BT/HT nanocomposite with uniform BT nanoparticle distribution and enough large BT particle size obtained at 200 oC in water solvent is an optimal candidate as a precursor. In the second step reaction, mesocrystalline BT/ST nanocomposite is formed by reacting the BT/HT nanocomposite with Sr(OH)2. Keeping appropriate reactivity in the solvothermal reaction system is very significant to form the mesocrystalline BT/ST nanocomposite. Under high reactivity conditions, the formed BT/ST nanocomposite will be transformed further into BaxSr1-xTiO3 solid solution. The optimum conditions can be achieved by changing polarity of solvothermal reaction solvent. The BT nanocrystals and ST nanocrystals epitaxially connect together in the mesocrystalline BT/ST nanocomposite, which can provide a 3D high-density BT/ST heteroepitaxial interface. The as-prepared BT/ST nanocomposite shows a behavior similar to paraelectric material. However, after annealing treatment at 900 oC it shows ferroelectric behavior and significantly enhanced piezoelectric and dielectric responses owing to successful introduction of the lattice strain at ferroelectric BT/ST heteroepitaxial interface. The strain engineering and the orientation engineering can be applied simultaneously into the nanostructure of mesocrystalline nanocomposite for the drastic enhancement of piezoelectric and dielectric performances.

■ ASSOCIATED CONTENT 24


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Supporting Information Supplemental Experiments, Figures S1−S13, and Tables S1. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was financially supported in part by Grants-in-Aid for Scientific Research (B) (No. 26289240) from Japan Society for the Promotion of Science. This work was also supported in part by the National Natural Science Foundation of China (No.21005003) and the Doctoral Scientific Research Starting Foundation of Baoji University of Arts and Science (No. ZK15050).

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(41) Hu, D. W.; Kong, X. G.; Mori, K.; Tanaka, Y.; Shinagawa, K.; Feng, Q. Ferroelectric Mesocrystals of Bismuth Sodium Titanate: Formation Mechanism, Nanostructure, and Application to Piezoelectric Materials. Inorg. Chem. 2013, 52, 10542–10551. (42) Kalyani, V.; Vasile, B. S.; Ianculescu, A.; Buscaglia, M. T.; Buscaglia, V.; NanniCryst, P. Hydrothermal Synthesis of SrTiO3 Mesocrystals: Single Crystal to Mesocrystal Transformation Induced by Topochemical Reactions. Cryst. Growth Des. 2012, 12, 4450– 4456. (43) Park, N. H.; Wang, Y. F.; Seo, W.S.; Dang, F.; Wan, C.L.; Koumoto, K. Solution Synthesis and Growth Mechanism of SrTiO3 Mesocrystals. CrystEngComm 2013, 15, 679–685. (44) Dang, F.; Kato, K.; Imai, H.; Wada, S.; Hanedad, H.; Kuwabarae, M. Oriented Aggregation of BaTiO3 Nanocrystals and Large Particles in the Ultrasonic-assistant Synthesis. CrystEngComm 2010,12, 3441–3444. ( 45 ) Mimura, K.; Kato, K.; Imai, H.; Wada, S.; Haneda, H.; Kuwabara. M. Piezoresponse Properties of Orderly Assemblies of BaTiO3 and SrTiO3 Nanocube Single Crystals. Appl. Phys. Lett. 2012, 101, 012901. (46) Kato, K.; Dang, F.; Mimura, K.-I.; Kinemuchi, Y.; Imai, H.; Wada, S.; Osada, M.; Haneda, H.; Kuwabara, M. Nano-sized Cube-shaped Single Crystalline Oxides and Their Potentials; Composition, Assembly and Functions. Adv. Powder Technol. 2014, 25, 1401–1414. (47) Wen, P. H.; Itoh, H.; Tang W. P.; Feng, Q. Single Nanocrystals of Anatase-Type TiO2 Prepared from Layered Titanate Nanosheets: Formation Mechanism and Characterization of Surface Properties. Langmuir 2007, 23, 11782–11790. (48) Neuhausen, J; Evstaf'iev, V. K.; Block, T; Finckh, E. W.; Tremel, W; Augustin, L; Fuchs, H; Voss, D; Kruger, P; Mazur, A; Pollmann, J. Scanning Probe Microscopy Study of the MetalRich Layered Chalcogenides TaM2Te2 (M = Co, Ni). Chem. Mater. 1998, 10, 3870–3878.

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(49) Cheng, L.-Q.; Wang, K.; Li, J.-F. Synthesis of Highly Piezoelectric Lead-free (K, Na)NbO3 One-dimensional Perovskite Nanostructures. Chem. Commun. 2013, 49, 4003−4005. (50) Feng, Q.; Ishikawa, Y.; Makita, Y.; Yamamoto, Y. Solvothermal Soft Chemical Synthesis and Characterization of Plate-like Particles Constructed from Oriented BaTiO3 Nanocrystals. J. Ceram. Soc. Jpn. 2010, 118, 141–146. ( 51 ) Shiratori, Y.; Pithan, C.; Dornseiffer J.; Waser, R. Raman Scattering Studies on Nanocrystalline BaTiO3 Part I – isolated Particles and Aggregates. J. Raman Spectrosc. 2007, 38, 1288–1299. (52) Jang, H. W.; Kumar, A.; Denev, S.; Biegalski, M. D.; Maksymovych, P.; Bark, C. W.; Nelson, C. T.; Folkman, C. M.; Baek, S. H.; Balke, N.; Brooks, C. M.; Tenne, D. A.; Schlom, D. G.; Chen, L. Q.; Pan, X. Q.; Kalinin, S. V.; Gopalan, V.; Eom, C. B. Ferroelectricity in Strainfree SrTiO3 Thin Films. Phys. Rev. Lett. 2010, 104, 197601. (53) Venkateswaran, U. D.; Naik, V. M.; Naik, R. High-pressure Raman Studies of Polycrystalline BaTiO3. Phys. Rev. B 1998, 58, 14256–14260. (54) Petzelt, J.; Ostapchuk, T.; Gregora, I.; Rychetsky´, I.; Hoffmann-Eifert, S.; Pronin, A. V.; Yuzyuk, Y.; Gorshunov, B. P.; Kamba, S.; Bovtun, V.; Pokorny´, J.; Savinov, M.; Porokhonskyy, V.; Rafaja, D.; Vanek, P.; Almeida, A.; Chaves, M. R.; Volkov, A. A.; Dressel, M.; Waser, R. Dielectric, infrared, and Raman Response of Undoped SrTiO3 Ceramics: Evidence of Polar Grain Boundaries. Rev. B 2001, 64, 184111. ( 55 ) Rabuffetti, F. A.; Kim, H. S; Enterkin, J. A.; Wang, Y; Lanier, C. H.; Marks, L. D.; Poeppelmeier, K. R.; Stair, P. C. Synthesis-Dependent First-Order Raman Scattering in SrTiO3 Nanocubes at Room Temperature. Chem. Mater. 2008, 20, 5628–5635. (56) Hayashi, H.; Nakamura, T.; Ebina, T. In-situ Raman Spectroscopy of BaTiO3 Particles for Tetragonal-cubic Transformation. J. Phys. Chem. Sol. 2013, 74957–962. (57) Deng, Z.; Dai, Y.; Chen, W.; Pei, X. M.; Liao, J. H. Synthesis and Characterization of BowlLike Single-Crystalline BaTiO3 Nanoparticles. Nanoscale Res. Lett. 2010, 5, 1217−1221. (58) Wada, S.; Takeda, K.; Muraishi, T.; Kakemoto, H.; Tsurumi, T.; Kimura, T. Preparation of [1 1 0] Grain Oriented Barium Titanate Ceramics by Templatedgrain Growth Method and Their Piezoelectric Properties. Jpn. J. Appl. Phys. 2007, 46, 7039–7043. (59) Kimura, T. Application of texture Engineering to Piezoelectric Ceramics. J. Ceram. Soc. Jpn. 2006, 114, 15–25. (60) Hu, D. W.; Mori, K.; Kong, X. G.; Shinagawa, K.; Wada, S.; Feng, Q. Fabrication of [1 0 0]Oriented Bismuth Sodium Titanate Ceramics with Small Grain Size and High Density for Piezoelectric Materials. J. Eur. Ceram. Soc. 2014, 34, 1169−1180. (61) Cölfen, H.; Mann, S. Mesocrystals and Nonclassical Crystallization, John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England, 2008.

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Figure Captions Figure 1. Schematic illustrations of local environments for (a) 2D heteroepitaxial interface in BT/ST superlattice nanocomposite obtained by MBE approach and (b) possible 3D heteroepitaxial interface in mesocrystalline BT/ST nanocomposite of present study.

Figure 2. (a, c, e, g) TEM images and (b, d, f, h) SAED patterns of (a, b) HT precursor, (c, d) BH30/0, (e, f) BH-10/20, and (g, h) BH-5/25 samples obtained by solvothermal treatments of HT in Ba(OH)2 solutions at 200 oC for 12 h, respectively. (i) HRTEM image is an enlarged image derived from the (i) white pane in (g) TEM image, and (j) FFT pattern obtained from the whole region of (i) HRTEM image.

Figure 3. XRD patterns of (a) BT/ST-30/0-10/20, (b) BT/ST-5/25-10/20, (c) BT/ST-30/0-30/0, and (d) BT/ST-5/25-30/0 samples obtained by solvothermal treatments of BT/HT nanocomposites in Sr(OH)2 solutions at 200 oC for 12 h, respectively.

Figure 4. (a, c, e, g) TEM images and (b, d, f, h) SAED patterns of (a, b) BT/ST-30/0-10/20, (c, d) BT/ST-5/25-10/20, (e, f) BT/ST-30/0-30/0, and (g, h) BT/ST-5/25-30/0 samples obtained by solvothermal treatments of BT/HT nanocomposites in Sr(OH)2 solutions at 200 oC for 12 h, respectively.

Figure 5. (A, C) TEM images and (b, d, e, f) HRTEM images of individual nanocrystals derived from platelike mesocrystalline BT/ST nanocomposite (BT/ST-30/0-10/20 sample) after grind and ultrasound dispersion treatments. (b, d) and (e, f) HRTEM images are magnified from the (b, d) and (e, f) white panes in (A) and (C) TEM images, respectively. (B) EDS spectra of samples derived from (a, b, c, d) white panes of (A) TEM image, respectively. The electron beam size is approximately 50 nm.

Figure 6. (a) Schematic representation of formation mechanism of platelike mesocrystalline BT/ST nanocomposite via two-step topochemical conversion process from layered HT single crystal. (b) Schematic representation of crystallographic variation of crystal structure from lepidocrocite-like layered structure of HT matrix crystal to possible mesocrystalline perovskite

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nanocomposite with heteroepitaxial interface via in situ topochemical mesocrystalline conversion reaction.

Figure 7. (a, c) TEM images and (b, d) SAED patters of samples obtained by annealing platelike mesocrystalline BT/ST nanocomposite of BT/ST-30/0-10/20 at (a, b) 600 oC and (c, d) 900 oC for 3h, respectively.

Figure 8. Raman spectrum of (a) as-prepared BT mesocrystal, (b) as-prepared ST mesocrystal, (c) as-prepared mesocrystalline BT/ST nanocomposite of BT/ST-30/0-10/20, and samples obtained by annealing BT/ST-30/0-10/20 at (d) 600 oC and (e) 900 oC for 3 h, respectively.

Figure 9. P-E hysteresis loops of BT/ST nanocomposites obtained by annealing BT/ST-30/010/20 at different temperatures for 3 h, respectively.

Figure 10. Variations of relative permittivities (εr) for as-prepared BT mesocrystal, as-prepared BT/ST-30/0-10/20 nanocomposite, and samples obtained by annealing at 900oC in the frequency range of 1 kHz –2 MHz at 1.0 V bias.

Figure 11. Variations of relative permittivities (εr) at 1 Hz of frequency with annealing temperature for BT mesocrystal, BT/ST-30/0-10/20 nanocomposite, and commercial BT sample of BT-05, and variations of effective converse piezoelectric coefficient (d*33) with annealing temperature for BT/ST-30/0-10/20 nanocomposite and BT mesocrystal.

Figure 12 Strain-applied voltage loops and d33*-applied voltage loops for (a) BT/ST-900 nanocomposite and (b) BT-900 mesocrystal.

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Figure 1. Schematic illustrations of local environments for (a) 2D heteroepitaxial interface in BT/ST superlattice nanocomposite obtained by MBE approach and (b) possible 3D heteroepitaxial interface in mesocrystalline BT/ST nanocomposite of present study. 170x108mm (300 x 300 DPI)



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Figure 2. (a, c, e, g) TEM images and (b, d, f, h) SAED patterns of (a, b) HT precursor, (c, d) BH-30/0, (e, f) BH-10/20, and (g, h) BH-5/25 samples obtained by solvothermal treatments of HT in Ba(OH)2 solutions at 200 oC for 12 h, respectively. (i) HRTEM image is an enlarged image derived from the (i) white pane in (g) TEM image, and (j) FFT pattern obtained from the whole region of (i) HRTEM image. 82x182mm (300 x 300 DPI)


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Figure 3. XRD patterns of (a) BT/ST-30/0-10/20, (b) BT/ST-5/25-10/20, (c) BT/ST-30/0-30/0, and (d) BT/ST-5/25-30/0 samples obtained by solvothermal treatments of BT/HT nanocomposites in Sr(OH)2 solutions at 200 oC for 12 h, respectively 177x139mm (300 x 300 DPI)



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Figure 4. (a, c, e, g) TEM images and (b, d, f, h) SAED patterns of (a, b) BT/ST-30/0-10/20, (c, d) BT/ST5/25-10/20, (e, f) BT/ST-30/0-30/0, and (g, h) BT/ST-5/25-30/0 samples obtained by solvothermal treatments of BT/HT nanocomposites in Sr(OH)2 solutions at 200 oC for 12 h, respectively. 80x36mm (300 x 300 DPI)


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Figure 5. (A, C) TEM images and (b, d, e, f) HRTEM images of individual nanocrystals derived from platelike mesocrystalline BT/ST nanocomposite (BT/ST-30/0-10/20 sample) after grind and ultrasound dispersion treatments. (b, d) and (e, f) HRTEM images are magnified from the (b, d) and (e, f) white panes in (A) and (C) TEM images, respectively. (B) EDS spectra of samples derived from (a, b, c, d) white panes of (A) TEM image, respectively. The electron beam size is approximately 50 nm. 152x194mm (300 x 300 DPI)



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170x39mm (300 x 300 DPI)


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Figure 6. (a) Schematic representation of formation mechanism of platelike mesocrystalline BT/ST nanocomposite via two-step topochemical conversion process from layered HT single crystal. (b) Schematic representation of crystallographic variation of crystal structure from lepidocrocite-like layered structure of HT matrix crystal to possible mesocrystalline perovskite nanocomposite with heteroepitaxial interface via in situ topochemical mesocrystalline conversion reaction. 170x108mm (300 x 300 DPI)



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Figure 7. (a, c) TEM images and (b, d) SAED patters of samples obtained by annealing platelike mesocrystalline BT/ST nanocomposite of BT/ST-30/0-10/20 at (a, b) 600 oC and (c, d) 900 oC for 3h, respectively. 79x76mm (300 x 300 DPI)


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Figure 8. Raman spectrum of (a) as-prepared BT mesocrystal, (b) as-prepared ST mesocrystal, (c) asprepared mesocrystalline BT/ST nanocomposite of BT/ST-30/0-10/20, and samples obtained by annealing BT/ST-30/0-10/20 at (d) 600 oC and (e) 900 oC for 3 h, respectively. 80x113mm (300 x 300 DPI)



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Figure 9. P-E hysteresis loops of BT/ST nanocomposites obtained by annealing BT/ST-30/0-10/20 at different temperatures for 3 h, respectively. 82x71mm (300 x 300 DPI)


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Figure 10. Variations of relative permittivities (εr) for as-prepared BT mesocrystal, as-prepared BT/ST-30/010/20 nanocomposite, and samples obtained by annealing at 900oC in the frequency range of 1 kHz –2 MHz at 1.0 V bias. 82x84mm (300 x 300 DPI)



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Figure 11. Variations of relative permittivities (εr) at 1 Hz of frequency with annealing temperature for BT mesocrystal, BT/ST-30/0-10/20 nanocomposite, and commercial BT sample of BT-05, and variations of effective converse piezoelectric coefficient (d*33) with annealing temperature for BT/ST-30/0-10/20 nanocomposite and BT mesocrystal. 77x50mm (300 x 300 DPI)


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Figure 12 Strain-applied voltage loops and d33*-applied voltage loops for (a) BT/ST-900 nanocomposite and (b) BT-900 mesocrystal. 82x107mm (300 x 300 DPI)



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Figure For Table of Contents Use Only 170x108mm (300 x 300 DPI)


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SrTiO3 Nanocomposites with

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