General Approach to Well-Defined Perovskite MTiO3 (M = Ba, Sr, Ca

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General Approach to Well-Defined Perovskite MTiO3 (M = Ba, Sr, Ca, and Mg) Nanostructures Wenjun Dong,*,†,‡ Bingjie Li,† Yang Li,† Xuebin Wang,† Lina An,† Chaorong Li,† Benyong Chen,† Ge Wang,‡ and Zhan Shi§ †

Center for Optoelectronics Materials and Devices, College of Science and Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou 310018 ‡ School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 § State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, 130012, P.R. China

bS Supporting Information ABSTRACT: Well-defined perovskite MTiO3 (M = Ba, Sr, Ca and Mg) nanostructures were successfully synthesized by a convenient hydrothermal method, employing MCl2 as M source and titanate nanowire as Ti precursor. Different perovskite nanostructures such as nanoring, nanobowl, nanotube, and nanodisk were successfully prepared. Two types of growth mechanisms were proposed by investigating the formation processes of MTiO3 nanostructures. The formation of MTiO3 (M = Ba and Sr) undergoes a “nanowire-tube-ring” growth mechanism with a self-sacrifice of titanate nanowire framework rout, whereas the formation of CaTiO3 and MgTiO3 structures follows a simultaneous epitaxial growth of TiO2 nanotube on titanate nanowire and self-nucleation of MTiO3 (M = Mg and Ca) in the solution initially, and then a conversion of the nanowire and nanotube to MgTiO3/CaTiO3 nanostructure process. The electrical properties of the perovskite MTiO3 nanostructure including polarization-field hysteresis and capacitance-voltage characteristics were also investigated.

1. INTRODUCTION Perovskites have shown great interest in science and engineering of the electronics industry due to their important properties and applications, such as insulating dielectrics, ferro-/antiferroelectrics, ferro-/antiferromagnets, superconductors, and thermoelectrics.1-3 Barium titanate, one of the most investigated perovskite materials, has a high dielectric constant and ferroelectric property that are essential for thin-film electronic components and electro-optical materials.4-6 Recently, low-dimensional BaTiO3 nanostructures have received increasing attention for two reasons.7 First, BaTiO3 can provide useful information to fabricate next generation, 3D ferroelectrie random access memory structures with required bit density.8 Second, detailed ab initio calculations have predicted that BaTiO3 would be a new kind of ferroelectric order (circular or toroidal ordering of dipoles) in nanorods and nanodisks, and it could open a new avenue for the ferroelectric medium in data storage.9-13 As a semiconductor with band gap of about 3.2 eV,14 SrTiO3 is a promising candidate of electrodes for efficient photocatalysts15,16 for water splitting.17,18 CaTiO3 has been well investigated for ferroeletctricity, red luminescence, and electro-mechano-optical r 2011 American Chemical Society

conversion properties.19-21 Moreover, CaTiO3 is getting attention recently for the biocompatibility and implant bone applications.22,23 As for MgTiO3, a microwave dielectric material, it has been applied in communication systems such as cellular phones, broadcasting satellites, and global positioning systems.24-26 The development of nanoscale device elements and interconnects has led researchers great interests and attempts to synthesize low-dimensional MTiO3 (M = Ba, Sr, Ca, and Mg) nanostructures.27-31 For example, 1D ferroelectric and nonferroelectric nanostructures have been successfully developed by a solid-state reaction.7 BaTiO3, SrTiO3, and CaTiO3 nanostructures (nanoparticle, nanowire, and nanotube) have been prepared based on hydrothermal or solvothermal treatment.32-37 Monodispered porous SrTiO3 and BaTiO3 nanostructures with large specific surface areas and high photocatalytic activities have been prepared by a Kirkendall effect38 or a evaporationinduced self-assembly approach.39 Recently, BaTiO3 nanostructure was also prepared by biological and bioinspired method at Received: November 8, 2010 Revised: January 18, 2011 Published: February 21, 2011 3918

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The Journal of Physical Chemistry C room temperature.40,41 However, the methods mentioned above can only be used to synthesize one or two kinds of perovskite, a general approach to synthesis the morphology and phase-controllable perovskite nanostructures still remains a challenge. The inherently chemical reactivity of titanate nanostructures is beneficial for designing complex titanate-based composites.15 In this article, phase- and morphology-controlled MTiO3 nanostructures from titanate nanowires precursor have been prepared by a facile one-pot hydrothermal synthesis approach. Different perovskite nanostructures such as heteronanostructure of BaTiO3 nanoparticle on titanate nanowire, BaTiO3 nanotube, BaTiO3 nanoring, SrTiO3 nanobowl, CaTiO3 microtube, and MgTiO3 nanodisk columm have been successfully prepared. On the basis of the formation processes of MTiO3 nanostructures, two types of growth mechanisms were proposed. The growth mechanism of MTiO3 (M = Ba and Sr) nanostructure undergoes a “nanowire-tube-ring” growth process with a self-sacrifice of the titanate nanowire framework route. The formation of CaTiO3 and MgTiO3 nanostructures follows a simultaneous epitaxial growth of TiO2 nanotube on titanate nanowire and MTiO3 self-nucleation in the solution initially, and then a conversion of titanate nanowire and TiO2 nanotube to MTiO3 nanostructure process. Most interestingly, the phase of BaTiO3 can be easily controlled from the cubic to tetragonal phase in this method, and the ferroelectric properties of the BaTiO3 can be well tuned.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MTiO3 Nanostructures. Titanate nanowires (on Ti foil) were synthesized according to the procedures described in a previous reference,42 and the perovskite nanostructure film can be prepared directly on the Ti substrate. For instance, the BaTiO3 nanostructure was synthesized as following: first, 10 mL of BaCl2 saturated solution was added into a Teflonlined vessel, an amount of NaOH was dissolved in the BaCl2 solution with stirring, and then a piece of nanowires-on-Ti (1.0 cm 3.0 cm) was placed in the Teflon-lined vessel. Subsequently, the vessel was sealed and heated for 24 h. The products were collected and washed for three times with deionized water, and then dried in air. Other perovskite nanostructures (SrTiO3, CaTiO3, and MgTiO3) were prepared with a similar recipe. 2.2. Characterization. The morphology and size of the asobtained samples were characterized on a Hitachi 4800 scanning electron microscopy (FESEM). High-resolution transmission electron microscopy (HRTEM) studies were carried out on a JEOL JEM 2010 transmission electron microscope with an accelerating voltage of 200 KV. The phase structure was determined by powder X-ray diffraction (XRD) experiments on a Bruker AXS D8-discover with Cu KR radiation (λ = 1.54056 Å) scanning from 15° to 80° at a speed of 1°/s. The polarizationfield (P-E) hysteresis was measured by a Sawyer-Tower circuit using a precision LC unit from Radiant Technologies in the electric field range of 20-80 kV/cm, and capacitance-voltage (C-V) characteristics were measured on a Keithley 4200 semiconductor characterization system at the signal swing with 0.5 V in the frequency of 10 kHz.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure. The evolution of the BaTiO3 nanostructure at 200 °C in 1.0 mol/L NaOH condition with different reaction time was studied, and the morphologies of

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the as-prepared samples were shown in Figure 1. Part a of Figure 1 depicts the SEM image of the samples obtained after a 1 h reaction, and the uniform nanoparticles with the diameter in the range of 50-80 nm were grown on the titanate nanowire. After 3 h reaction, the nanowire was completely covered by dense nanoparticles and formed nanowire@nanoparticle core-shell structures (part b of Figure 1). When the reaction time extends to 6 h, the tubular structures with open ends were obtained due to dissolving of titanate nanowire core (as shown in part c of Figure 1). HRSEM image (insert in part c of Figure 1) shows that the inner diameter and outer diameter of the nanotube were ∼180 and ∼400 nm, respectively. After a 24 h reaction, the uniform nanorings with ∼100 nm of inner diameter and ∼200 nm of outer diameter were obtained (as shown in part d of Figure 1). The nanoring structure can be observed obviously as shown in the insert of part d of Figure 1. It is well-known that the crystallographic structure of BaTiO3 determined the dielectric properties.43 Interestingly, the phase transformation of as-prepared BaTiO3 from the cubic to the tetragonal can be easily controlled by the hydrothermal method. Part e of Figure 1 shows the XRD patterns of the products shown in parts a-d of Figure 1, and all prepared products could be identified as BaTiO3 phase. The XRD patterns of the BaTiO3 nanostructure confirmed that the BaTiO3 was tetragonal perovskite structure at 200 °C after 24 h reaction, whereas the BaTiO3 obtained from 1 to 20 h reaction exhibited cubic perovskite phase as evident in the XRD analysis. Usually, the degree of ferroelectricity is defined as the relative ratio of lattice parameter of c axis to a axis. The c/a ratio of the BaTiO3 nanoring is quantified to be 1.0075 according to the Rietveld refinement of the XRD pattern, which confirm the asprepared BaTiO3 belongs to fine tetragonal structure (insert in part e of Figure 1).44 The phase transformation is also detected from the splitting of the [200] peak of the cubic at 45.4° into [002] and [200] of the tetragonal phase. HRTEM image of the nanoring shows that 0.40 nm of the d value is consistent to the [001] plane of the tetragonal BaTiO3 perovskite lattice (shown in part f of Figure 1). The influence of the concentration of NaOH on morphology and size of as-prepared nanostructure was investigated at 200 °C for 24 h. A previous study reported that OH- played a vital role in the generation of BaTiO3 seeds.8 Experimental results show that BaTiO3 cannot be obtained without NaOH being present in this reaction system. When the concentration of NaOH was 0.5 mol/ L, sphere-like porous particles with diameter about 350 nm were obtained (part a of Figure S1 of the Supporting Information). Increased the concentration of NaOH to 1.5 mol/L, nanobowl structures were formed, and the nanobowl showed smooth surface with the ∼80 nm of the caliber and ∼70 nm of the wall thickness (as seen in part b of Figure S1 of the Supporting Information). When the NaOH concentration was increased to 2.0 mol/L, round nanoparticles with ∼100 nm in diameter were obtained (part c of Figure S1 of the Supporting Information). The temperature effect on the BaTiO3 morphology and size in the nanosynthesis was likewise studied in 1.0 mol/L NaOH solution for 24 h. At 120 °C, the cubelike BaTiO3 nanoparticles with nanowire vestige could be obtained, and the size of the nanoparticle was about 250 nm (as shown in part a of Figure S2 of the Supporting Information). Increasing the temperature to 160 °C, irregular particles were obtained as shown in part b of Figure S2 of the Supporting Information. An even higher reaction temperature (200 °C) would result in the uniform nanoring structures (as shown in part c of Figure S2 of the 3919

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The Journal of Physical Chemistry C

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Figure 1. SEM images (a-d) of as-obtained BaTiO3 products at different intervals of reaction time, (a) 1 h; (b) 3 h; (c) 6 h; (d) 24 h at 200 °C in 1.0 mol/L NaOH. (e) XRD patterns of BaTiO3 products, and magnified XRD patterns in the 2θ range of 44-46° (insert); (f) HRTEM image of the asobtained BaTiO3 nanorings.

Supporting Information). When the temperature was further increased to 240 °C, the BaTiO3 nanoparticles about 150 nm were formed, a split gap about 10 nm in width and ∼35 nm in depth appeared on each one, which may be induced by the dissolving of the titanate nanowire (part d of Figure S2 of the Supporting Information). The phase transformation from cubic to tetragonal structure was observed in the as-prepared BaTiO3 nanostructures when the temperature is above 200 °C as well (part e of Figure S2 of the Supporting Information). The morphology evolution of SrTiO3 nanostructure with the reaction time varied from 1 to 24 h at 160 °C in 1.0 mol/L NaOH condition was shown in Figure 2. Part a of Figure 2 depicted the SEM image of the sample obtained after 1 h, and spherical SrTiO3 nanoparticles were grafted on titanate nanowires. When the reaction was carried out for 3 h, the SEM image of the sample showed the core-shell structure with the titanate nanowires crossing the nanocubes (part b of Figure 2). When the reaction time was extended to 6 h, the aligned SrTiO3 nanoparticle structures were obtained, and the titanate nanowires were dissolved completely (as shown in part c of Figure 2). When the reaction time lasts for 24 h, the uniform ringlike SrTiO3 nanostructures about 50 nm in diameter were prepared (part d of Figure 2). The XRD patterns showed entire titanate nanowire precursor converted to perovskite structure after 3 h reaction (as shown in part e of Figure 2), and the growing process from titanate to SrTiO3 clearly. HRTEM image of the as-prepared SrTiO3 sample shows

that d value of 0.39 nm is consists with the [100] plane of the cubic SrTiO3 perovskite lattice (shown in part f of Figure 2). Usually, SrTiO3 and SrCO3 are coexistence in the air-operating conditions with hydrothermal method, so a CO2-free reaction environment and precursor are acquired to avoid the formation of SrCO3. Interestingly, lower reaction temperature can avoid the formation of SrCO3 in this research. Pure and welldefined SrTiO3 nanoparticles can be prepared when the temperature is around 160 °C. The uniform SrTiO3 nanoparticles about 50 nm in diameter were prepared (part a of Figure S3 of the Supporting Information). XRD results suggest that the asobtained SrTiO3 is good crystalline and no other impurity is detected (part d of Figure S3 of the Supporting Information). However, a high-temperature reaction above 180 °C would result in the formation of SrTiO3 and SrCO3 particles simultaneously (parts b and c of Figure S3 of the Supporting Information). CaTiO3 and MgTiO3 perovskite nanostructures were synthesized by a similar recipe. First, the reaction time influence on morphology evolution of CaTiO3 nanostructure at 200 °C in 1.0 mol/L NaOH solution was studied. After the hydrothermal reaction for 3 h, the titanate nanowires grew into nanorods with a diameter about 150 nm (as shown in part a of Figure 3). When the reaction time is extended to 6 h, a core-shell structure appeared (part b of Figure 3). The cross section of the core-shell structure reveals that the nanotube is a multiwalled nanotube structure. Typically, the diameter of the inner, middle and outer 3920



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Figure 2. SEM images (a-d) of as-obtained SrTiO3 products at 160 °C in 1.0 mol/L NaOH in different reaction time, (a) 1 h; (b) 3 h; (c) 6 h; (d) 24 h; (e) XRD patterns of the SrTiO3 products; (f) HRTEM image of the SrTiO3 nanoparticles.

shell were ∼60, ∼120, and ∼160 nm, respectively (insert of part b of Figure 3). At the same time, rectangular CaTiO3 crystals could also be obtained in the solution (as shown in part b of Figure 3). When the reaction time was extended to 12 h, the nanotubes would dissolve gradually and transform into rectangular CaTiO3 structures (as shown in part c of Figure 3). After a 24 h reaction, rectangular CaTiO3 structures became the prevalent products with a good crystallinity (as shown in part d of Figure 3). Part e of Figure 3 shows the XRD patterns of the CaTiO3 products obtained at different reaction durations. XRD result confirms that the core-shell structure is titanate nanowire@anatase TiO2 nanotube structure in the 6 h reaction, and all of the titanate nanowires and TiO2 nanotubes are converted into pure CaTiO3 when the reaction time is over 24 h. HRTEM image of a CaTiO3 nanoparticle shows that the products are well-crystallized, and the lattice fringe with spacing of about 0.38 nm is corresponded to the interplanar distance of the [101] planes in the orthorhombic CaTiO3 (part f of Figure 3). The growth process of MgTiO3 nanostructure from titanate nanowire was also investigated. Part a of Figure 4 showed the SEM image of the sample obtained in a hydrothermal reaction at 200 °C for 3 h. Detailed structures revealed that the nanotube shell was epitaxial growth on the titanate nanowire, and the nanotube was tens of micrometers in length and about 50-100 nm in diameter (part a of Figure 4). The XRD patterns confirmed the anatase TiO2 nanotube shell was epitaxial growth on the titanate nanowire (shown in part a of Figure S4 of the Supporting

Information). When the reaction time was extended to 6 h, entire nanowires were dissolved and nanotube structures were obtained. The cross-section of the as-prepared samples shows the multilayer core-shell nanotube structure (part b of Figure 4), and several nanometer gaps between adjacent TiO2 tube shells can be observed. XRD patterns also confirm the nanotube is pure anatase TiO2 structure (as shown in part b of Figure S4 of the Supporting Information). When the reaction time was extended to 12 h, dispersive lamellar MgTiO3 nanodisks were formed (as shown in part c of Figure 4). After a 24 h reaction, selfassembled MgTiO3 lamellar nanodisk columns with ∼70 nm of inner diameter, ∼180 nm of outer diameter, and ∼130 nm in length were obtained (part d of Figure 4). 3.2. Electrochemical Properties. To study the electrical properties of the MTiO3 film, the Pt/MTiO3/Ti capacitor device was prepared by sputtering platinum electrode on the MTiO3 nanostructure layer (Figure S5 of the Supporting Information). The investigation of the bias voltage dependence of the capacitance (C-V) is one of the most useful methods to characterize ferroelectric property of the MTiO3 nanostructures.45 C-V curves were measured at room temperature in the frequency range of 1 kHz-10 kHz under the voltage of 100 V. Parts a-c of Figure 5 show the C-V characteristics of BaTiO3, SrTiO3, and CaTiO3 thin films devices. The capacitance of BaTiO3 changed from 9.0 to 9.1 PF, with the applied voltage in the -12 to þ12 V at frequency of 10 kHz. The butterfly-shaped C-V curves 3921



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Figure 3. SEM images (a-d) of as-obtained CaTiO3 products at 200 °C in 1.0 mol/L NaOH, in different reaction time, (a) 3 h; (b) 6 h; (c) 12 h; (d) 24 h; (e) XRD patterns of CaTiO3 products; (f) HRTEM images of as-obtained CaTiO3 products.

Figure 4. SEM images of as-obtained MgTiO3 products at 200 °C in 1.0 mol/L NaOH at different reaction time, (a) 3 h; (b) 6 h; (c) 12 h; (d) 24 h.

indicate that the BaTiO3 shows a ferroelectric nature45 (part a of Figure 5). The capacitance of SrTiO3 and CaTiO3 revealed that

both of them have high dielectric constant, but have no ferroelectric nature (parts b and c of Figure 5). 3922



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Figure 5. CV curve (a-c) of the as-prepared products, (a) BaTiO3; (b) SrTiO3; (c) CaTiO3; (d) P-E hysteresis loops of BaTiO3.

Figure 6. Schematic illustration of the formation mechanism of MTiO3.

Hysteresis measurements of the BaTiO3 film capacitor were obtained using a Sawyer-Tower circuit at room temperature and the P-E hysteresis loops were well saturated curves. The distinct ferroelectric hysteresis loops of BaTiO3 capacitor were obtained for the BaTiO3 capacitor with driving electric field of 20-80 KV/cm2 and the frequency of 100 Hz (part d of Figure 5). It is well-known that high frequency can reduce the dielectric loss and minimized the leakage current contribution. When the electric field is 80 KV/cm2, the values of remanent polarization (Pr), spontaneous polarization (Ps), and coercive field (Ec) were 2.88, 5.91 μc/cm2 and 35 KV/cm, respectively. The polarization values are lower than that reported for BaTiO3 ceramics. It may attribute to the smaller grain size, lower packing density, or the decreasing of the thickness of the films, which are accordant with other report previously.46-49

3.3. Formation Mechanism. On the basis of the morphological evolution of the time-dependent experiment result, the intrinsic growth mechanisms of MTiO3 nanostructures were explored (Figure 6). With the progress of hydrothermal reaction, the concentration of TiO32- or TiO2(OH)22- would increase in the solution.49 The dissolved species redeposited on the surface of larger nanowires with the Ostwald ripening process.15 The diameter of the nanowire was increased to 100 nm, which is significantly larger than that of the titanate nanowire precursor (about 50 nm in diamter). On the other hand, the TiO surface termination and dissolved species would react with M2þ to form the MTiO3 nucleation. In the reaction, BaTiO3 or SrTiO3 cannot be prepared without OH- being present, and previous research confirmed that OHplayed a vital role in the generation of BaTiO3 or SrTiO3 seeds. 3923



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Figure 7. TEM (a) and HRTEM (b) image of a heteronanostructure of titanatenanowire@BaTiO3 nanoparticle; TEM (c) and HRTEM (d) image of a titanate nanowire@TiO2 nanotube core-shell structure.

It is well-known that titanate nanowire has the TiO surface termination in base solution, the majority of nanowire surfaces are determined to have either [010] or [001] facets, and abundant OH- would promote the fast formation of HTiO3.50 When Ba(OH)2 or Sr(OH)2 was added to solution, Ba2þ or Sr2þ would interact with the lattice oxygen, and the surface atoms are reconstructed into BaTiO3/SrTiO3 nucleation. Further, the vacancy injection on the titanate nanowire surface induced by the Ostwald ripening growth drove the agglomeration of SrTiO3 and BaTiO3 nucleation on the nanowire.39 In addition, BaTiO3/ SrTiO3 nanoparticles in the solution had a strong tendency to aggregate on the nanorods, and then developed into titanate nanowires@perovskite nanoparticles core-shell structures. TEM image of heteronanostructure of BaTiO3 nanoparticles on titanate nanowire shows two distinctly morphology contrasts (shown in part a of Figure 7). The HRTEM image of the interface between BaTiO3 and titanate was shown in part b of Figure 7, and two d values of 0.78 and 0.34 nm are consistent to the [200] and [003] planes of titanate structure, respectively.49 While d values of 0.28 nm are consistent with the [110] plane of the BaTiO3. With the eroding of the titanate nanorod cores, the titanate nanowire@BaTiO3/SrTiO3 nanoparticles core-shell structures transformed into nanotube structures gradually, and then developed into nanoring structure. For the formation of CaTiO3 and MgTiO3 nanostructures, another growth mechanism was investigated (based on SEM images Figure 3 and 4). After reaction for 3 h, titanate nanowire@TiO2 nanotube structures were obtained. Prolonging the reaction time to 6 h, multilayer nanotubes can be obtained due to the dissolving of the titanate nanowire core. Part c of Figure 7 displays a TEM image of titanate nanowire@TiO2 nanotube core-shell structure at 200 °C for 1 h reaction. The boundary between the titanate nanowire core and the epitaxial TiO2 shell is clearly shown. The large lattice mismatch between titanate and CaTiO3/MgTiO3 prevent the CaTiO3/MgTiO3 shell formation on the titanate nanowire. The d values of 0.35 nm is consistent to the [101] plane

of anatase TiO2 in the lattice structure (part d of Figure 7). After a 24 h reaction, the core-shell structures were dissolved and CaTiO3 tubes or MgTiO3 nanodisk columns were obtained. Different from the BaTiO3 and SrTiO3 growth, CaTiO3 and MgTiO3 structures follow a simultaneous epitaxial growth of TiO2 nanotube on titanate nanowire and self-nucleation of MTiO3 in the solution initially, and then a conversion of the nanowire and nanotube to MTiO3 (M = Ca and Mg) nanostructure process.

4. CONCLUSIONS In summary, well-defined MTiO3 (M = Ba, Sr, Ca, and Mg) nanostructures have been successfully prepared by a general and convenient hydrothermal method. Different morphologies of perovskite nanostructures such as nanoring, nanobowle, nanodisk, and rectangular tube structure were successfully prepared. The details nucleation and transformation of the MTiO3 nanostructure were represented by the topochemical reaction based on the close correlation between the perovskite nanoparticles and titanate nanowires. The effects of reaction temperature, reaction time and alkaline concentration on the phase transition and morphology evolution were studied. Various characterization techniques such as, XRD, FESEM, and TEM were employed to characterize the final products. C-V and P-E character revealed that dielectric properties of the MTiO3 nanostructures can be well controlled. Further, two different growth mechanisms were proposed to clarify the formation of perovskite nanostructures. The general method can be extended to the preparation of other metal perovskites with controlled morphology and structure, such as CoTiO3 and MnTiO3. ’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images of the as-obtained BaTiO3 in different concentrations of NaOH, SEM images and XRD patterns of the as-obtained MTiO3 (M = Ba and Sr) products with different reaction temperature, XRD

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The Journal of Physical Chemistry C patters of the titanate nanowire and anatase TiO2 nanotube, and a device scheme of the setup used for the electrochemical measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: þ86-571-86843587; fax: þ86-571-86843587.

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 50972130, 20701033, 50772100, 10874153); Qianjiang Talent Program of Zhejiang Province (QJD1002001); China Postdoctoral Science Foundation (No. 201003048, 20090450292). Open Project Program of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University. The authors acknowledge Dr. B. Liu, Hebei University for the hysteresis measurements. ’ REFERENCES (1) Chandler, C. D.; Roger, C.; Hampden-Smith, M. J. Chem. Rev. 1993, 93, 1205. (2) Pena, M. A.; Fierro, J. L. G. Chem. Rev. 2001, 101, 1981. (3) Bhalla, A. S.; Guo, R.; Roy, R. Mater. Res. Innovations 2000, 4, 3. (4) Setter, N.; Waser, R. Acta. Mater. 2000, 48, 151. (5) Hennings, D.; Klee, M.; Waser, R. Adv. Mater. 1991, 3, 334. (6) Haertling, G. H. J. Am. Ceram. Soc. 1999, 82, 797. (7) Buscaglia, M. T.; Harnagea, C.; Dapiaggi, M.; Buscaglia, V.; Pignolet, A.; Nanni, P. Chem. Mater. 2009, 21, 5058. (8) Scott, J. F.; Morrison, F. D.; Miyake, M.; Zubko, P.; Lou, X.; Kugler, V. M.; Rios, S.; Zhang, M.; Tatsuta, T.; Tsuji, O.; Leedham, T. J. J. Am. Ceram. Soc. 2005, 88, 1691. (9) Naumov, I. I.; Bellaiche, L.; Fu, H. Nature 2004, 432, 737. (10) Fu, H; Bellaiche, L. Phys. Rev. Lett. 2003, 91, 257601. (11) Scott, J. F. Nat. Mater. 2005, 4, 13. (12) Her, Y. -S.; Matijevic, E.; Chon, M. C. J. Mater. Res. 1995, 10, 3106. (13) Takeuchi, T.; Tabuchi, M.; Ado, K.; Honjo, K.; Nakamura, O.; Kageyama, H.; Suyama, Y.; Ohtori, N.; Nagasawa, M. J. Mater. Sci. 1997, 32, 4053. (14) Cardona, M. Phys. Rev. 1965, 140, 651. (15) Li, Y.; Gao, X. P.; Li, G. R.; Pan, G. L.; Yan, T. Y.; Zhu, H. Y. J. Phys. Chem. C 2009, 113, 4386. (16) Domen, K.; Kudo, A.; Onishi, T.; Kosugi, N.; Kuroda, H. J. Phys. Chem. 1986, 90, 292. (17) Wrighton, M. S.; Ellis, A. B.; Wolczanski, P. T.; Morse, D. L.; Abrahamson, H. B.; Ginley, D. S. J. Am. Chem. Soc. 1976, 98, 2774. (18) Burnside, S.; Moser, J. E.; Brooks, K.; Gr€atzel, M. J. Phys. Chem. B 1999, 103, 9328. (19) Lemanov, V. V.; Sotnikov, A. V.; Smirnova, E. P.; Weihnacht, M. Appl. Phys. Lett. 2002, 81, 886. (20) Diallo, P. T.; Boutinaud, P.; Mahiou, R.; Cousseins, J. C. Phys. Status Solidi A 1997, 160, 255. (21) Wang, X. S.; Xu, C. N.; Yamada, H.; Nishikubo, K.; Zheng, X. G. Adv. Mater. 2005, 17, 1254. (22) Pecharromon, C.; Esteban-Betegon, F.; Bartolome, J. F.; Lopez-Esteban, S.; Moya, J. S. Adv. Mater. 2001, 13, 1541. (23) Ohtsu, N.; Sato, K.; Yanagawa, A.; Saito, K.; Imai, Y.; Kohgo, T.; Yokoyama, A.; Asami, K.; Hanawa, T. J. Biomed. Mater. Res., Part A 2007, 82, 304. (24) Lee, B. D.; Lee, R. H.; Yoon, K. H.; Cho, Y. S. Ceram. Int. 2005, 31, 143.

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General Approach to Well-Defined Perovskite MTiO3 (M = Ba, Sr, Ca

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