Tailoring the Shape, Size, Crystal Structure, and Preferential Growth

Article pubs.acs.org/crystal

Tailoring the Shape, Size, Crystal Structure, and Preferential Growth Orientation of BaTiO3 Plates Synthesized through a Topochemical Conversion Process Marjeta Maček Kržmanc,*,# Boštjan Jančar,# Hana Uršič,§ Melita Tramšek,† and Danilo Suvorov# #

Advanced Materials Department, §Electronic Ceramics Department, and †Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova cesta 39, Ljubljana 1000, Slovenia S Supporting Information *

ABSTRACT: The conditions for the topochemical transformation of variously sized Bi4Ti3O12 template plates into BaTiO3 particles were studied in order to control their morphology, crystal structure, and preferential orientation. In the transformation from sub-200 nm- and μm-sized template plates in the presence of surplus BaCO3 (Bi4Ti3O12/BaCO3 = 1:10), the final BaTiO3 particles retained a memory of the precursor size when the conversion reaction in the molten salt (NaCl/KCl) occurred at 660 and 900 °C, respectively. In both cases the side length of the template was well preserved, while the thicknesses of the final BaTiO3 plates were larger compared to those of the templates. The morphology of the BaTiO3 particles formed from micrometer-sized Bi4Ti3O12 plates at 660 °C did not closely resemble the template shape because of the exfoliation and disintegration processes. Through the transformation of sub-200 nm Bi4Ti3O12 plates at 900 °C the formed BaTiO3 particles grew by Ostwald ripening, and thus also the shape of the final perovskite particles did not retain a memory of the template. We confirmed by Raman spectroscopy and X-ray diffraction that the BaTiO3 plates formed at 900 °C exhibited a higher tetragonality than those prepared at 660 °C. Ferroelectric hysteresis and piezoelectric butterfly curves, as obtained using a piezo-force microscope, indicated the significant ferroelectric response of [001] preferentially oriented micrometer-sized and sub-micrometer-sized BaTiO3 plates.

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special crystal planes of BaTiO3.12 In contrast, Bis(ammonium) lactate titanium dihydroxide (TALH) was recognized as an excellent precursor for the formation of BaTiO3 and SrTiO3 nanocubes with a very uniform size distribution.12,13 An ordered arrangement of these nanocubes in thin films and in heterostructure BaTiO3/SrTiO3 superlattices was also already demonstrated.13−15 In addition to nanocubes, anisotropic platelet or rod-shaped BaTiO3 micro- and nanocrystallites represent an advantage in achieving the optimum properties, on condition that they are properly preferentially aligned. Because of the high symmetry of the crystal structure, BaTiO3 and other ABO3-type perovskites do not show a tendency for the anisotropic growth of crystals.16 The most successful approach to the synthesis of plate-like perovskite particles is based on the topochemical conversion of two-dimensional structures, which already contain perovskite units. Typical examples of such structures are the Ruddlesden−Popper-type layered perovskite Sr3Ti2O7 and layered perovskite Aurivillius phases (Bi4Ti3O12, MBi 4 Ti 4 O 15 , M = Ba, Sr, Pb)). The preparation of approximately 10-μm-large, (001)-oriented MTiO3 plates

INTRODUCTION Among the many functional materials, ferroelectrics have been found to be one of the most versatile for applications in electronics, from piezoelectric sensors, energy harvesters, nonvolatile ferroelectric random-access memories (NVFeRAMs)1 to microwave tunable devices.2 Ferroelectric perovskite particles with a well-defined anisotropic shape are attracting increasing attention because of their unique shape- and sizedependent properties at low dimensions. These particles, with well-defined structures and surfaces and uniform size distributions, have the potential to be used as building blocks for the fabrication of functional nanodevices. The proper crystal orientation and alignment of the particles are an additional requirement for providing the desired functionality. BaTiO3 is, due to its outstanding ferroelectric, high dielectric constant, and optical properties, one of the most studied and also commercially used perovskite materials.3−6 Numerous literature reports about BaTiO3 formation under conventional hydrothermal and solvothermal conditions revealed the difficulties in preparing defined-shaped, nonaggregated nanoparticles when TiO2, Na2Ti3O7, K2Ti6O13, TiCl4, and Ti-alkoxide were used as the Ti-precursors.7−11 The reason most probably lies in the dissolution−precipitation mechanism and in the similar surface energies of the crystal planes, which hinders the growth of the © XXXX American Chemical Society

Received: February 1, 2017 Revised: April 26, 2017 Published: May 5, 2017 A

DOI: 10.1021/acs.cgd.7b00164 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Raman spectroscopy. The local piezoelectric properties of the BaTiO3 plates were examined using a PFM.

from Aurivillius MBi4 Ti 4 O 15 phases has already been reported.17−19 These 10-μm-sized plates were primarily meant for the preparation of highly oriented ceramics using a template grain growth (TGG) process, where the large plates serve as templates to seed oriented growth in a perovskite ceramic matrix.18,20 Oriented ferroelectric perovskite ceramics, thin and thick films are known to exhibit enhanced ferroelectric and piezoelectric properties.21,22 Nevertheless, for the realization of novel nanotechnology applications and the miniaturization of electronic devices, ferroelectric perovskite plates with dimensions considerably smaller than 10 μm are of great interest. The topochemical conversion from layered perovskite to ABO3-type perovskite seems to be one of the most promising approaches for the preparation of smaller micrometer- and nanosized, plate-like, ABO3 perovskite particles. In this work we studied strategies for controlling the shape, size, crystal structure, and preferential growth orientation of BaTiO3 plate-like particles transformed from Bi4Ti3O12 plates. We were focused on the preparation of considerably smaller BaTiO3 plates (side length 100 nm to 1.5 μm) compared to that already reported in the literature (side length: 10−20 μm).23 By varying the synthesis conditions (temperature, Bi4Ti3O12 template size, initial BaCO3/Bi4Ti3O12), we aimed to determine the boundary conditions for the preservation of the template morphology during the conversion. The already reported research of Su et al.23 was confined to one large Bi4Ti3O12 template size (10−20 μm), which was transformed to BaTiO3 only under certain conditions (1000−1040 °C, BaCO3/Bi4Ti3O12 = 1:10). Those large BaTiO3 plates were meant for the preparation of textured ceramics. However, in this research we looked at smaller dimensions to follow the demands of the miniaturization of electronic devices and novel applications, which require significantly smaller particle sizes than that so-far prepared using the molten-salt method. Taking into account that the preparation of smaller BaTiO3 plates through the topochemical conversion of Bi4Ti3O12 accordingly requires a smaller template size and lower conversion temperatures, we verified the dominance of various reaction and growth mechanisms for the transformation from different initial templates (side length: 200 nm to 1.5 μm) in the temperature range from 660 to 900 °C. The former temperature was only 10 °C above the eutectic temperature of the KCl/NaCl = 1:1 salt mixture. On the basis of this we aimed to examine the suitability of the molten-salt method for the preparation of nanosized, defined-shape perovskite particles. To the best of our knowledge the formation of perovskite particles by topochemical conversion in the molten salt at such a low temperature has not yet been described in the literature. Additionally, we examined the ferroelectric and piezoelectric characteristics of these particles using a piezo-force microscope (PFM), which has also not yet been reported. The other main objective of the work was to develop a better understanding of the topochemical conversion and thus be able to tailor the size, shape, and preferential orientation of the BaTiO3 plates. In particular, we wanted to fill the gap in the literature in terms of the formation of side products and determine the most effective methods for their elimination, which do not have a harmful influence on the target BaTiO3 plates. The development of the morphology, the compositional, and structural evolution during the topochemical transformation from Bi4Ti3O12 to BaTiO3 were studied by means of scanning (SEM) and transmission (TEM) electron microscopy, energy-

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EXPERIMENTAL SECTION

Preparation of Bi4Ti3O12 Particles. Bi4Ti3O12 plate-like template particles were prepared with the molten-salt method from Bi2O3 nanopowder (Sigma-Aldrich 99.8%) and TiO2 P25 (Degussa). The molar ratio of NaCl/KCl/Bi4Ti3O12 = 50:50:1 was selected based on the study of He et al.,24 who found that this amount of binary salt was optimal for the preparation of the thinnest plates. The typical procedure was as follows: first, KCl, NaCl, were mixed together in the molar ratio 1:1, and then Bi2O3 and TiO2 in the stoichiometric ratio of Bi4Ti3O12 were homogeneously mixed with the salts. The resulting mixture was placed in an Al2O3 crucible and covered in order to prevent contamination of the furnace with the salt and Bi2O3. For the preparation of micrometer-sized Bi4Ti3O12 plates the mixture was heated to 800 °C at 10 °C/min, isothermally heat-treated at this temperature for 2 h, and then cooled to room temperature at 10 °C/ min. Larger, 3-μm-sized Bi4Ti3O12 plates were prepared with the same heating and cooling regime, while the isothermal annealing was performed at 900 °C for 3 h. In the case of the synthesis of smaller sub-200 nm-sized Bi4Ti3O12 plates, the Al2O3 crucible with the reaction mixture was put directly into the furnace, preheated to 800 °C, and left to react at this temperature for 8 min, after which the furnace was cooled to room temperature by natural cooling. After the thermal treatment the salt was removed from the reaction product by washing with deionized water. In the final step the powders were washed with ethanol and dried at 60 °C. In order to remove the secondary phases both types of reaction products were washed with 3 M HNO3 (soaking time 15 min). Further washing with deionized water and ethanol was needed to completely remove the remains of the HNO3 and accelerate the drying, respectively. In terms of the morphology of the Bi4Ti3O12 plates, the synthesis procedure was found to be very reproducible, especially for longer annealing times. Preparation of BaTiO3 Plate- and Block-like Particles. For the study of the topochemical transformation from Bi4Ti3O12 template plates into BaTiO3, the reagents KCl, NaCl, and BaCO3 (Alfa Aesar, 99.8%) were mixed and ground together, and then Bi4Ti3O12 plates were added. The mixture was gently homogenized in the ethanol to avoid the destruction of the template plates. Prior to the heat treatment the ethanol was evaporated. The amounts of the salt (2.0 g KCl and 1.56 g NaCl) and Bi4Ti3O12 (0.8 g) were constant, while the amount of BaCO3 varied from the stoichiometric (Bi4Ti3O12/BaCO3 = 1:3) to more than three times in excess. The mixtures of the reagents with the following molar ratios: KCl/NaCl/Bi4Ti3O12/ BaCO3 = 39:39:1:3, KCl/NaCl/Bi4Ti3O12/BaCO3 = 39:39:1:5, KCl/ NaCl/Bi4Ti3O12/BaCO3 = 39:39:1:10, and KCl/NaCl/Bi4Ti3O12/ BaCO3 = 39:39:1:17 were put in the round Al2O3 crucible (d = 5 cm) and covered and heated to 600 °C at 10 °C/min and then to 900 °C at 0.5 °C/min, annealed at this temperature for 2 h and then cooled to room temperature with a cooling rate of 1 °C/min. For comparison the topochemical reaction was also performed with a faster heating rate of 10 °C/min to 900 °C, isothermal annealing for 2 h at this temperature and then cooled to room temperature at 10 °C/min. The transformation at a temperature slightly above the eutectic temperature of the salt was made with a heating rate of 10 °C/min to 600 °C and then with 0.1 °C/min to 660 °C, annealed at this temperature for 10 h and then cooled to room temperature with a cooling rate of 10 °C/min. After the reaction the salt was removed by washing with deionized water, and then the water-insoluble byproducts were eliminated from the BaTiO3 particles by sedimentation and/or by dissolution with 2 M and/or 3 M HNO3 (soaking time 5−15 min). A lower HNO3 concentration than reported in the literature was used18,23 in order to minimize the leaching of Ba from the BaTiO3 plates. Only large and high-density byproducts, which formed at a high temperature of 900 °C, were rather effectively separated from the BaTiO3 particles by sedimentation, whereas this was not possible for the smaller particles of superfluous phases formed at the lower B



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temperature of 660 °C. For the last, several washings with 2 M HNO3 or even with 3 M HNO3 were needed for their complete elimination. The soaking time in HNO3 solution never exceeded 15 min. The remains of the HNO3 were washed off with water, ethanol, and dried at 60 °C. The washing procedure was repeated in case the side products were not completely removed. In the case of the 900 °C reaction mixture, from which the majority of the large and high-density byproducts were eliminated by sedimentation, only one washing with 2 M HNO3 was usually sufficient to dissolve the eventual remains of superfluous phases and unreacted BaCO3, and thus single-phase BaTiO3 plates were obtained.

the BaTiO3 plates sticking to the PFM tip, the plates were fixed by means of epoxy resin. This involved the following procedure: the drop of suspension with the BaTiO3 plates in isopropanol was deposited on the heated (60 °C) flat and polished metal substrate, which was put on the bottom of a mold. The mixture of the resin (Epo Fix Resin M.W. ≤ 700), Struers, Ballerup, Denmark) and hardener (Epo Fix Hardener) was poured into the mold over the substrate with the plates. After the hardening (48 h), a piece of the resin was taken out of the mold and separated from the metal substrate. In this way the plates were imprinted in the hardened epoxy. The thin film of the epoxy that eventually covered the BaTiO3 plates and the loose BaTiO3 plates were removed by short and gentle polishing with fabric clothes. The above-described approach to the sample preparation for PFM measurements was very successful for the large, micrometer-sized BaTiO3 plates. Nevertheless, the problems appeared with the smaller, submicrometer particles, which were either covered with resin or not fixed and therefore immediately removed, even by extremely gentle polishing. For these reasons we decided to fix the 660 °C-annealed BaTiO3 plate-like particles on the SrTiO3 substrate using a thermal treatment (heating to 700 °C). We selected this nonconductive substrate in order to avoid electrical breakdown through the air, which could be a problem when measuring the small, sub-micrometer-sized plates on a large conductive substrate. Additionally, in this configuration the measurement was performed in the same floating-ground regime as in the case of fixation with epoxy resin. The topography (height image, contact mode) and the PFM images were recorded with an atomic force microscope (AFM; Asylum Research, Molecular Force Probe 3D, Santa Barbara, CA) equipped with a Dual AC Resonance Tracking (DART) switching spectroscopy (SS) mode. A Ti/Ir-coated Si tip with a radius of curvature ∼20 nm (Asyelec, AtomicForce F&E GmbH, Mannheim, Germany) was used, and the electric field was applied to the BaTiO3 plates/epoxy composite in the floating-ground regime. The out-of-plane amplitude and phase PFM images were measured in the DART mode (at 20 V and frequency 290 kHz), and the local hysteresis was measured in the SS mode with the waveform parameters: increasing step signal with maximum amplitude of 70 V and frequency 0.1 or 0.2 Hz; overlapping sinusoidal signal of amplitude 5 V and frequency 20 Hz, off-loop mode.

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CHARACTERIZATION The crystal structure of the samples was analyzed using a PANalytical X’Pert PRO MPD (Almelo, The Netherlands) and a D4 Endeavor (Bruker AXS, Karlsruhe, Germany) X-ray diffractometer. Both diffractometers were operated with Cu− Kα radiation (1.5406 Å). The search-match analyses of the Xray diffraction patterns were performed using the PANalytical HighScore Plus version 3.0e (3.0.5) software and the Web PDF-4+ 2014 database. For the estimation of the preferential growth orientation of the BaTiO3 plates, a few drops of the suspension of the BaTiO3 plates in isopropanol were deposited on a Si-single crystal. The majority of the plates deposited using this method were lying flat, with their larger surface on the Si-single crystal. In order to examine the influence of heating above the Curie temperature (Tc) on the orientation of the domains, the XRD pattern was measured for the as-deposited BaTiO3 plates and after heating at 150 °C. The particles were examined using a field-emission scanning electron microscope (FE-SEM, JSM-7600 F, JEOL) equipped with an Oxford Instruments Inca energy-dispersive X-ray spectrometer (EDS) and a transmission electron microscope (TEM, JEM 2100, JEOL, Tokyo, Japan) equipped with a Gatan ORIUS SC1000 CCD camera. Selected-area electron diffraction (SAED) was used for the examination of the crystal and domain structures of the individual particles. For the TEM investigation particles with thicknesses greater than 100 nm were embedded in epoxy resin, and the thus obtained composite was mechanically thinned, dimple ground, and ion milled to achieve a suitable electron transparency. The particle sizes (side length (l) and thickness (d)) were evaluated from the SEM images with the help of Smile View software (JEOL, Tokyo, Japan). For the statistics of the side length distribution 200−300 particles were assessed. Because the plate shape of the majority of the particles, fewer plates were lying perpendicular to the substrate, and thus the average thicknesses (d) were merely estimated from 10−80 particles. For the EDS analysis the BaTiO3 particles were deposited on the polished graphite substrate and coated with carbon (12 nm) to prevent charging. At least seven measurements on different BaTiO 3 particles were performed for each examined composition. EDS mapping of the individual particles was further performed by scanning transmission electron microscopy (STEM) using a probe Cs-corrected Jeol ARM 200 CF STEM. The Raman spectra were recorded at room temperature with a Horiba Jobin Yvon LabRam-HR spectrometer equipped with an Olympus BXFM-ILHS microscope and a CCD detector. The samples were excited by the 632.8 nm emission line of a He−Ne laser. The piezoelectric and ferroelectric characteristics of the BaTiO3 plates were examined with a PFM. In order to avoid

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RESULTS AND DISCUSSION Morphology Control of the Bi4Ti3O12 Plates. The formation of Bi4Ti3O12 plates in molten (KCl/NaCl) salt has been attributed to dissolution−precipitation because both the Bi2O3 and TiO2 dissolve in the molten salt and the Bi4Ti3O12 precipitates under a high degree of supersaturation. Further growth of the Bi4Ti3O12 plates occurred by Ostwaldripening.24,25 Bi4Ti3O12 started to form already at the eutectic temperature of the NaCl/KCl (650 °C). However, in our study the formation and growth of the plates were examined under various conditions at 800 °C and for NaCl/KCl/Bi4Ti3O12 = 50:50:1. This molar ratio was proved to be optimal for the preparation of the thin Bi4Ti3O12 plates.24 Discrete, welloriented Bi4Ti3O12 plates with an average side length of around 1−2 μm and a thickness of around 50 nm formed after 2 h at 800 °C (Figure 1a). The XRD analysis revealed that in addition to the prevailing monoclinic Bi4Ti3O12 phase (PDF: 04-0163435) a small amount of Na0.5Bi1.5ClO2 phase (PDF: 04-009C



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The structure of Bi4Ti3O12 is characterized by pseudoperovskite (Bi2Ti3O10)2− units, which are sandwiched between the (Bi2O2)2+ layers along the c-axis.24 According to the ab initio calculations of Noguchi et al.,26 the Bi vacancies are preferentially formed in the perovskite layers rather than in the (Bi2O2)2+ layers. Taking into account that Bi is removed in the course of the topotactic transformation from Bi4Ti3O12 to BaTiO3, the Bi vacancies are not expected to detrimentally influence this transformation. The increase in the aggregation of the Bi4Ti3O12 plates with the shortening of the synthesis time was clearly evident from the SEM micrographs (Figure 1), as well as from the decrease of the relative intensities of the diffraction peaks of the (00l)/ (117) planes (l = even number) (Figure S1, Supporting Information). The Bi4Ti3O12 plates synthesized at 800 °C/2 h showed a high (00l) preferential orientation, whereas the sample prepared directly at 800 °C for 8 min showed relative (00l)/(117) intensities that approach those in the PDF card (04-016-3435). Topochemical Conversion of Bi4Ti3O12 into BaTiO3 Plates. There is a need to develop a better understanding of the topochemical process in order to effectively control the shape, size, and crystal-growth orientation of micrometer and sub-micrometer BaTiO3 plates formed from Bi4Ti3O12 template plates. According to the study of Sue et al.23 the Bi4Ti3O12 plate-like particles with a mean side length of 10−20 μm transformed in the molten (KCl/NaCl) salt at 1000−1040 °C into BaTiO3 plates with a retained morphology. Actually, the authors reported on the preserved side length, whereas they did not mention how the thickness of BaTiO3 plates was related to that of the template. The reported topochemical conversion took place in the presence of excessive BaCO3 with the molar ratio Bi4Ti3O12/BaCO3 = 1:10. The restraining of the hydrolysis reaction of BaTiO3 (BaTiO3 + H2O ⇔ Ba2+ + TiO2 + 2OH−) during washing with a nitric acid solution was stated as one of the main reasons for the use of surplus BaCO3. Namely, the high concentration of Ba2+ in the solution is expected to shift the above-mentioned reaction to the left and therefore to prevent the leaching of Ba2+ from the BaTiO3 plates. Nevertheless, it seems that the necessity for using excessive BaCO3 in the topochemical conversion process cannot be entirely elucidated from this explanation. Since BaCO3 exhibited a higher dissolution rate than Bi4Ti3O12 in the molten KCl/NaCl salt, the formation of BaTiO3 during the topochemical conversion process is expected to occur via a dissolution−diffusion process in which the BaTiO3 product layer started to nucleate and grow on the surface of the Bi4Ti3O12.25 The reaction is then proposed to proceed by the diffusion of Ba2+ ions inward of the plate, where they replace Bi3+ ions in the pseudoperovskite layer. The diffusion of Bi3+ is expected to occur in the reverse direction, and consequently (Bi2O2)2+ layers are removed during the transformation. Highly reactive bismuth oxide, which is formed in the course of this process (eq 1, Supporting Information), is expected to react with other elements in the molten salt, such as Na, K, Ba, and/ or Cl. The reaction of bismuth-oxide-based compounds with the Ba are assumed to increase the required total amount of Ba. The higher concentration of Ba2+ in the salt is also expected to increase the diffusion rate of Ba2+ into the pseudoperovskite layer to replace the Bi3+. An insight into the reactions taking place at 660 and 900 °C in the salt mixture with Bi4Ti3O12/ BaCO3 = 1:10 was gained from the X-ray analyses of the waterwashed reaction products (Figure S2, Supporting Information).

Figure 1. SEM micrographs of Bi4Ti3O12 template plates obtained after annealing at 800 °C for 2 h (a) and 8 min (b).

0635) was also present (Figure S1, diffractogram a, Supporting Information). For the preparation of smaller sub-200 nm-sized Bi4Ti3O12 plates the annealing time and heating rate were shortened and increased, respectively. Because of the shorter synthesis time (8 min), the reaction did not proceed into the particle growth stage, and the Bi4Ti3O12 plates were smaller as well as highly aggregated (Figure 1b). On the basis of the XRD analysis it could be inferred that the amount of Na0.5Bi1.5ClO2 phase increased with the shortening of the reaction time (Figure S1, diffractogram b, Supporting Information). These results imply consecutive reactions where Bi2O3 and NaCl first react to form Na0.5Bi1.5ClO2, which is further consumed for the formation of Bi4Ti3O12. Since our interest was to prepare single-phase Bi4Ti3O12 template plates, we attempted to remove the Na0.5Bi1.5ClO2 phase by washing with dilute nitric acid (3 M HNO3). The XRD analysis of the washed products revealed the disappearance of the diffraction lines belonging to the Na0.5Bi1.5ClO2 phase. The appearance of the additional diffraction lines with very weak intensities indicated that a small amount of the new unknown phase formed during the washing process (Figure S1, diffractograms c and d, Supporting Information). Because of the formation of Na0.5Bi1.5ClO2 as the only secondary phase, the Bi4Ti3O12 plates were expected to be deficient in Bi when Bi2O3 and TiO2 were initially weighed in the stoichiometric ratio. This implies that smaller Bi4Ti3O12 plates, which were formed in a shorter time (8 min at 800 °C), contain more defects compared to the larger plates, obtained after a longer annealing time (2 h at 800 °C), since a larger amount of Na0.5Bi1.5ClO2 phase is formed in the former case (Figure S1, diffractograms a and b, Supporting Information). D



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Additionally, a knowledge of the byproducts is also important for a selection of an effective method for their separation from the target BaTiO3 particles. The color difference between the water-washed reaction products obtained at 660 °C (paleyellowish) and 900 °C (mixture of white and black particles) clearly indicated the different phase compositions and particle sizes. In the XRD pattern of the 660 °C-annealed sample the diffraction lines with the highest intensity could be ascribed to the BaBiO2Cl phase (PDF: 01-083-0442), while the rest of the diffraction lines corresponded to the unreacted BaCO3 (PDF: 04-074-2663) and BaTiO3 phases (Figure S2, diffractogram a, Supporting Information). All three phases were also detected in the XRD pattern of the 900 °C-annealed sample, in which the additional diffraction lines could be assigned to the black BaBiO3 phase (PDF: 04-008-8600) (Figure S2, diffractogram b, Supporting Information). The EDS analyses confirmed that the black crystals contained Ba and Bi in the ratio of 1:1, while no Cl, Ti or Na, K elements were detected. The high ratio between the intensities of the (100)/(110), (200)/(110), and (300)/ (110) diffraction lines implied the high [h00] preferential orientation of the BaBiO3 crystals. Similarly, the higher relative intensities of the diffraction peaks of the (020)/(131), (040)/ (131), (060)/(131) planes observed for the BaBiO2Cl phase which formed during the reaction at 900 °C, compared to that obtained at 660 °C, also indicated the [0k0] preferential orientation of the BaBiO2Cl crystallites obtained at the higher temperature. BaBiO2Cl and BaBiO3 were assumed to result from the reaction of Bi2O3 with the elements dissolved in the molten salt (eqs 2 and 3, Supporting Information). From the 660 °C-annealed sample the unwanted BaBiO2Cl and BaCO3 phases were eliminated from the BaTiO3 plates by washing with dilute nitric acid solution. The dissolution of the BaBiO2Cl led to the formation of a new BiOCl phase (PDF: 006-0249) (Figure S2, diffractogram c, eq 4, Supporting Information), which was further removed by the repeated washing with HNO3 solution (eq 5, Supporting Information). Because of the high solubility of BaCO3 in the HNO3 solution, BaCO3 was completely eliminated during the washing procedure. This was proven by thermogravimetric analyses. BaCO3 was not observed, either in the XRD pattern or in the Raman spectra of the washed powders. On the basis of their large size and high density, the majority of the BaBiO3 and BaBiO2Cl crystals, formed at 900 °C, were separated relatively well from the BaTiO3 particles by sedimentation. Afterward, only a single washing with 2 M HNO3 was enough to remove the remains of these byproducts and the unreacted BaCO3, after which the single-phase BaTiO3 particles were obtained (Figure 2, diffractograms b, c, d). After a similar washing procedure for the 900 °C-reacted mixture with a stoichiometric initial Ba/Ti ratio (Bi4Ti3O12/BaCO3 = 1:3), BaTiO3 with a cubic crystal structure and a large amount of unknown phase, which could not be removed using the above-described methods, were obtained (Figure S2, diffractogram d, Supporting Information). The XRD analysis confirmed that the complete conversion from layered Bi4Ti3O12 to BaTiO3 in the reaction mixture containing Bi4Ti3O12/BaCO3 in the ratio 1:10 is already possible at 660 °C (Figure 2, diffractogram a), which is slightly above the eutectic temperature of the salt. No clear splitting of the (100) diffraction lines indicated that these BaTiO3 particles exhibited a lower tetragonality compared to those formed at the higher temperature of 900 °C (Figure 2, diffractogram c). A

Figure 2. Powder XRD pattern of the BaTiO3 particles obtained by annealing the salt mixture with Bi4Ti3O12/BaCO3 = 1:10 at 660 °C for 10 h (a) and at 900 °C for 2 h (c, d) and the salt mixture with Bi4Ti3O12/BaCO3 = 1:5 at 900 °C for 2 h (b). The transformation was performed from 1.4-μm-sized (a, b, c) and 190 nm-sized (d) Bi4Ti3O12 template plates.

further analysis using Raman spectroscopy revealed that the structure of both powders was closer to tetragonal than cubic, for which no first-order Raman activity was expected.8,27−29 However, the Raman spectra also showed that the peak at 305 cm−1, which is assigned to the B1 mode and indicates asymmetry within the TiO6 octahedra of BaTiO3 on a local scale, was weaker for the 660 °C-annealed sample than for the highly tetragonal BaTiO3 plates, obtained at 900 °C (Figure 3).

Figure 3. Raman spectra of the BaTiO3 powders obtained from the 1.4-μm-sized (b, c) and 190 nm-sized (a, d) Bi4Ti3O12 template plates by annealing the salt mixture with Bi4Ti3O12/BaCO3 = 1:10 at 660 °C (a, b) and at 900 °C (c, d).

Namely, it is known that the greater sharpness of the peak at 305 cm−1 is related to a higher tetragonality.9,27,28 The spectral difference also appeared at around 180 cm−1, where a positive peak is observed in the Raman spectrum of the BaTiO3, formed at the lower temperature (Figure 3, curves a and b). In the literature, the origin of this peak is ascribed to the decoupling of the A1 (TO) phonons, induced by phonon damping due to the stress and lattice defects.28,29 The possibilities of tailoring the dimensions of the Bi4Ti3O12 plates raise expectations about controlling the size of BaTiO3 plates prepared by topochemical conversion. A knowledge of the conditions that provide the best retention of the shape and E



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exfoliation and disintegration of the initial template plates. Smaller plate-like particles with variable thickness and holes are assumed to be the result of these two processes. It can be inferred based on the greater thickness of the formed BaTiO3 particles compared to the parent Bi4Ti3O12 plates that the disintegration of the plates that were perpendicular to the larger surface prevailed over the exfoliation along the (Bi2O2)2+ layers. When the reaction continued to a higher temperature (800− 900 °C) the BaTiO3 crystallites aligned and sintered.17,18 These BaTiO3 plates exhibited a high tetragonality (Figures 2 and 3), and they were single crystalline in the area where the ED was taken (Figure 5). The BaTiO3 particles that formed from the nanosized Bi4Ti3O12 template (l = 190 nm, d ≈ 30 nm) at 660 °C exhibited a far more regular shape than that obtained from the 1.4-μm-sized template. The sizes of the BaTiO3 block-like particles are comparable to the template, but the thickness was higher (Figure 4b). Regarding the tetragonality, as observed by XRD and Raman, these well-defined-shape BaTiO3 nanoparticles exhibited a similar crystal structure to the BaTiO3 particles prepared from the larger, 1.4-μm-sized template plates at the same temperature of 660 °C (Figure 2, diffractogram a, Figure 3 curves a and b). It is interesting to note that although the 190 nm-sized Bi4Ti3O12 template plates were highly aggregated, the formed BaTiO3 particles, which showed a similar side length, were well separated and not grown together. This is another advantage of this method for the preparation of defined-shaped BaTiO3 particles. There was a question as to whether it is possible to achieve high tetragonality for the BaTiO3 particles transformed from the nanosized Bi4Ti3O12 plates and at the same time to preserve the small dimensions of the template. For this reason the topochemical transformation of the Bi4Ti3O12 plates with l = 190 nm and d ≈ 30 nm was performed using a typical annealing procedure, including slow heating to 900 °C, isothermal annealing for 2 h at this temperature, followed by a slow cooling rate. The result of the reaction was tetragonal (Figure 2. diffractogram d), μm-sized, plate- and block-like BaTiO3 particles with a broad size distribution (Figures S3 and S4, Supporting Information). The BaTiO3 particles, which already started to form close to the salt eutectic temperature (650 °C), were assumed to be initially small, but they grew to larger particles with an increase of the temperature to 900 °C. The growth occurred by the Ostwald ripening process, the rate of which depends on the diffusion coefficient, the solubility and the atomic structure of the particle surfaces.25 Taking into

enable control of the crystal structure and orientation is needed in order to benefit from this transformation reaction. First we examined which morphological changes accompanied the transformation at 660 °C in the case of differently sized template plates. The SEM examination of the BaTiO3 particles, obtained from the 1.4-μm-sized Bi4Ti3O12 at 660 °C, revealed that their size and shape differed significantly from the initial plates. Compared to the initial Bi4Ti3O12 template, the BaTiO3 particles had a considerably smaller side length (Figure 4 a), but a greater thickness and exhibited a plate-like shape and

Figure 4. SEM micrographs of the BaTiO3 particles obtained by annealing the salt mixture with Bi4Ti3O12/BaCO3 = 1:10 at 660 °C from the 1.4-μm-sized (a) and 190 nm-sized (b) Bi4Ti3O12 template plates.

also irregular shapes. The uneven surface, variable thickness, and the holes are rather characteristic for these BaTiO3 particles. We believe that such a morphology is a reflection of the processes occurring during the topochemical transformation at the low temperature of 660 °C. On the basis of the similarity between the Bi4Ti3O12 and MBi4Ti4O15 (M = Sr, Ba, Pb) template plates the growth of BaTiO3 on the Bi4Ti3O12 is expected to resemble the formation of MTiO3 on the MBi4Ti4O15.17,18 The first BaTiO3 crystallites that started to grow on the template are most probably misaligned from the parent Bi4Ti3O12 structure. The mismatch between the structures of the BaTiO3 and the pseudoperovskite units of the template is most probably one of the reasons for the partial

Figure 5. HR (a) and SAED pattern (b) of BaTiO3 plate prepared by annealing at 900 °C for 2 h viewed along the [1,̅ 1, 0] zone axis. Amorphous part at the edge of the particle formed during preparation of the sample. F



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Figure 6. Histograms of the side length distributions of the Bi4Ti3O12 template plates (a) obtained at 800 °C/2 h and BaTiO3 plates transformed from these Bi4Ti3O12 plates in the molten salt at 900 °C/2 h, whereas the initial mixture contained Bi4Ti3O12/BaCO3 = 1:5 (b, d) and Bi4Ti3O12/ BaCO3 = 1:10 (c, e) and the corresponding SEM micrographs (d, e).

a well-defined tetragonal crystal structure (Figure S2, diffractogram d, Supporting Information). On the contrary, some excess of BaCO3 (Bi4Ti3O12/BaCO3 = 1:5) led to single-phase and tetragonal BaTiO3 (Figure 2, diffractogram b) block-like particles with a shape that does not resemble that of the Bi4Ti3O12 template plates. This is also clearly evident from the particle size distribution analysis (Figure 6a,b). The BaTiO3 blocks with an average side length of 3.3 μm and thickness of around 1.4 μm are considerably bigger than the initial Bi4Ti3O12 template plates (Figure 6). We believe that due to the low concentration of Ba2+ the diffusion is slower and the formation of the first product BaTiO3 layer on the template plate is not fast enough to prevent the dissolution of the smaller and average-sized Bi4Ti3O12 plates. BaTiO3 then nucleates and grows on the larger template plates, which then become considerably thicker than the initial template plates. It could be inferred based on this morphology that in the case of a small excess of BaCO 3 , BaTiO 3 formed through both the dissolution−precipitation and dissolution−diffusion processes.25 A greater excess of BaCO3 (Bi4Ti3O12/BaCO3 = 1:10) led to BaTiO3 plates with an average side length of 1.4 (Figure 6c), which is similar to that of the template, while their average thickness of 380 nm was considerably greater than that of Bi4Ti3O12. Some, especially the small Bi4Ti3O12 plates, most

account that the reaction exposure time between 660 and 900 °C was about 14 h, Ostwald ripening is the most plausible mechanism for this large increase in the BaTiO3 particle size (Figures S3 and S4, Supporting Information). It is obvious from the above results that the transformation conditions, which include the small, 190 nm-sized Bi4Ti3O12 template, the slow heating rate to a high temperature of 900 °C, the isothermal (2h) annealing at this temperature, followed by slow cooling, led to BaTiO3 with a high tetragonality, but did not enable the preservation of the small size and the plate shape of the initial template. For this reason we decided to study systematically the influence of the various experimental conditions for the transformation from the micrometer-sized Bi4Ti3O12 template into BaTiO3 in the temperature range, where BaTiO3 with high tetragonality formed. First, we studied the importance of the excess BaCO3 in the preparation of the tetragonal plate-shaped BaTiO3 particles. The significant difference between the morphologies of the BaTiO3 particles obtained from the same Bi4Ti3O12 template plates and under the same thermal conditions but with different excess of BaCO3 revealed that the concentration of Ba2+ in the salt is very important. Single-phase BaTiO3 plates were not obtained in the case of the stoichiometric initial Ba/Ti ratio (Bi4Ti3O12/ BaCO3 = 1:3). Additionally, the formed BaTiO3 did not exhibit G



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only a- or those with only c-domains. Because of the higher average thickness there are greater possibilities for the BaTiO3 blocks to be also oriented perpendicular to the substrate, and thus the relative intensity of the (001) and (100) diffraction lines does not represent correctly their average crystal orientation. The poly domain nature of the individual plate was confirmed by the PFM measurements. The domains were mapped using PFM imaging, where topography, out-of-plane amplitude and phase signals were analyzed (Figure 8a−c). The

probably dissolved prior the formation of the BaTiO3 product layer. The dissolved Ti4+ ions are then consumed for the growth of BaTiO3 on the larger template plates. Nevertheless, this could only partially explain the greater thickness of the formed BaTiO3 compared to the initial Bi4Ti3O12 template plates. No significant improvement in the plate’s aspect ratio (side length/thickness) was noticed, either with increasing the BaCO3 concentration in the salt (Bi4Ti3O12/BaCO3 = 1:17) or with shortening the reaction time by increasing the heating and cooling rates. It was assumed that the matching of the 1.4-μm side length of the initial Bi4Ti3O12 and that of the BaTiO3 plates prepared at 900 °C is due to the preserved shape of the template. The good agreement between the 3.5-μm side length of the larger Bi4Ti3O12 template plates (obtained at 900 °C/3 h) and that of the BaTiO3 plates (3.2 μm), topochemically transformed from this template at 900 °C, additionally confirmed this hypothesis (Figure S5, Supporting Information). It seems that the conversion temperature of 900 °C in contrast to 660 °C was high enough such that the destructive consequences of the exfoliation and disintegration processes, which occurred during the initial stage of the transformation, are healed by sintering at high temperature and thus the BaTiO3 plate maintains the memory of the template shape. The intensities of the diffraction lines in the XRD patterns of the BaTiO3 blocks (prepared from Bi4Ti3O12/BaCO3 = 1:5) and plates (prepared from Bi4Ti3O12/BaCO3 = 1:10) cast on the Si-single crystal reflect the orientation of the particle on the substrate and the preferential crystal orientation of the BaTiO3 particles. The first depend a great deal on the particle shape. The average crystal preferential orientation could best be inferred from the XRD pattern of the BaTiO3 plates, because the majority of the plates are expected to lie flat with their largest surface on the Si-single crystal substrate. In the XRD pattern of the BaTiO3 plates the diffraction lines with the highest intensities were those produced by the (001), (100), (002), (200) planes, while the intensities of (101), (110), and (111) diffractions were very weak (Figure 7). The typical (001)/(100) peak splitting implied the presence of a- ((100) orientation (in-plane)) and c- ((001) orientation (out-of-plane)) domains in the individual plates, rather than that the sample consisted of two kinds of plates, i.e., those with

Figure 8. (a) Topography (height image), PFM out-of-plane (b) amplitude and (c) phase images of the BaTiO3 plates obtained by the molten salt synthesis at 900 °C/2 h from the initial salt with Bi4Ti3O12/BaCO3 = 1:10. A few examples of irregular domains are marked by arrows. (d) Local hysteresis loops; amplitude (below) and phase (above) measured in the spot marked by a cross in panel (a).

surface microstructure consists of BaTiO3 plates inserted in epoxy resin. The polished sample surface is flat, with only nanometer-sized variations in the height being observed (Figure 8a). By comparing the topography and amplitude PFM images, we can conclude that the enhancements in the PFM signal (bright regions) are observed at the positions of the BaTiO3 plates, providing evidence of their piezoelectric behavior. Even more, relatively large (≥200 nm) irregularly shaped ferroelectric domains are observed (Figure 8b,c). Note that the domain boundaries can be identified in the amplitude image as dark, nonactive boundaries inside the plates. Irregular domains were previously observed in ferroelectric Pb(Sc0.5Nb0.5)O3 and BiFeO3 bulk materials.30,31 After the scanning, the PFM switching spectroscopy experiment was performed. The local hysteresis loops typical for ferroelectric materials were measured (Figure 8d). No splitting of the reflections characteristic for a domain structure was noticed in the SAED (Figure 5). On the basis of the large size of the domains this is possible when the ED was taken from the single-domain regions of BaTiO3 plates. We noticed that the dominance of c-domains ((001)orientation) over a-domains ((100)-orientation) could be to some extent controlled by the experimental factors such as using an excess BaCO3 (3 ≤ Ba/Ti < 5), clean and complete

Figure 7. XRD pattern of the BaTiO3 plates (prepared from the Bi4Ti3O12/BaCO3 = 1:10 at 900 °C/2 h from the micrometer-sized Bi4Ti3O12) cast on the Si-single crystal (a-initial) and after heating at 150 °C (b). H



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Bi4Ti3O12 template plates without small scratches, high reaction temperatures (800 °C < T < 1000 °C), and slow heating and cooling rates. An XRD examination of the BaTiO3 plates after heating above the Curie temperature indicated that this caused an increase of a-domains at the expense of c-domains (Figure 7, curve b). The irregularly shaped ferroelectric domains were also observed in the 660 °C-annealed BaTiO3 plates deposited on the SrTiO3 substrate. The topography, PFM out-of-plane amplitude and the phase images are shown in Figure 9a−c. After the scanning, the PFM switching spectroscopy experiment was performed. The local hysteresis loop, typical for ferroelectric materials, was observed (Figure 9d).

Table 1. Atomic Compositions Calculated from the EDS Data for the Perovskite BaTiO3 Particles, Obtained from the Different Initial Bi4Ti3O12:BaCO3 ratio at 900 °C perovskite composition initial Bi4Ti3O12/ BaCO3 ratio 1:10 (μm-sized template) 1:10 (190 nm-sized template) 1:5 (μm-sized template)

Ba (atom %)

Bi (atom %)

Ti (atom %)

Ba/Bi/Ti ratio

20.07

0.35

19.67

1.02:0.02:1

19.73

0.49

19.77

1.00:0.03:1

19.59

0.44

19.98

0.98:0.022:1

determined for the SrTiO3 plates, which were obtained with optimized processing conditions for the topochemical microcrystal conversion from SrBi4Ti4O15 to SrTiO3.17 The low content of Bi3+ in our BaTiO3 plates implies that, regarding the elimination of Bi3+, there is no need for a two-step transformation from Bi4Ti3O12 via BaBi4Ti4O15 to BaTiO3 plates.18 In the EDS analyses results of the BaTiO3 plates, prepared from the Bi4Ti3O12/BaCO3 = 1:10 (Table 1), the ratio between the sum of Ba2+ and Bi3+ on the A site and Ti4+ on the B-site is slightly larger than 1. These results imply an ionic compensation mechanism via the formation of titanium vacancies, which can be represented by the following formula: Ba1−xBixTi1−x/4O3.35 Scanning TEM of the BaTiO3 plates revealed that like Ti4+ and Ba2+, the remains of Bi3+ were also homogeneously distributed in the plates, rather than being segregated at the defects (Figure 10). A low content of Bi3+ (Table 1) was also determined for the BaTiO3 plates prepared using a smaller excess of BaCO3 (Bi4Ti3O12/BaCO3 = 1:5).

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CONCLUSIONS The molten-salt method was found to be suitable for the preparation of nonaggregated, well-defined-shape, micrometersized and sub-micrometer-sized BaTiO3 particles through the

Figure 9. (a) Topography (height image), PFM out-of-plane (b) amplitude and (c) phase images of the BaTiO3 plates obtained by molten-salt synthesis at 660 °C and deposited on the SrTiO3 substrate. In the height image, it is clear that this is an agglomerate of at least four particles. For better clarity, two particles are highlighted by dotted lines. (d) Local hysteresis loops (frequency 0.2 Hz); amplitude (below) and phase (above) measured in the spot marked with a cross in Figure (a).

The remains of the bismuth in the BaTiO3 plates are expected to influence their dielectric, ferroelectric, and piezoelectric characteristics. Bismuth is a common dopant in commercial, BaTiO3-based, multilayer capacitors because it enables a lower sintering temperature. The solubility limit of bismuth in the sintered BaTiO3 ceramics was determined to be 3−5 atom %.32 It was shown by several authors that a low content of bismuth does not cause a deterioration of the functional properties. In the study of Wang et al.33 the best dielectric properties were obtained for the Ba0.97Bi0.02TiO3 composition. Similarly, Maurya et al.34 showed that a low content of bismuth in BaTiO3 ceramics (Ba0.975Bi0.025TiO3) enhanced its piezoelectric characteristics. On the basis of these studies it could be inferred that some small remains of bismuth in the BaTiO3 plates are allowed. Because of its large ionic radius, Bi3+ can only replace Ba2+ on the A-site, and not Ti4+ on the B-site in the perovskite structure. The remains of Bi3+ in the micrometer-sized BaTiO3 plates, as determined by the EDS, were less than 0.5 atom % (Table 1). According to the abovementioned literature reports this content is in the concentration range where an improvement in the properties is expected. The determined content of bismuth was less than 0.9 atom %, as

Figure 10. Scanning TEM of the BaTiO3 plates obtained by moltensalt synthesis at 900 °C/2 h from the initial salt mixture with Bi4Ti3O12/BaCO3 = 1:10. I



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Notes

topochemical transformation of Bi4Ti3O12 plates. Two types of Bi4Ti3O12 template plate-like particles with sub-200 nm- and μm-size side lengths were prepared in the molten salt (KCl/ NaCl) at 800 °C by varying the reaction time from 8 min to 2 h, respectively. In the next step the conditions for their transformation into BaTiO3 under molten-salt (NaCl/KCl) synthesis conditions were studied in order to be able to control their morphology, crystal structure, and preferential orientation. It was found that the shape of the formed BaTiO3 particles retained the memory of the template when dissolution− diffusion dominated over the dissolution−precipitation process. This was achieved with an excess amount of BaCO 3 (Bi4Ti3O12/BaCO3 = 1:10), which is believed to increase the diffusion rate of Ba. In the transformation of the sub-200 nmsized Bi4Ti3O12 plates, the formed BaTiO3 particles preserved the small size of the template when the reaction took place at a low temperature of 660 °C, wherein the side length of the BaTiO3 block-like particle was similar to that of the template, whereas the thickness was greater. When the conversion reaction for this type of template was performed at a high temperature of 900 °C, the dimensions of the large block-like BaTiO3 particles with high tetragonality did not resemble those of the initial template. The small BaTiO3 particles, which are believed to form initially from the sub-200 nm-sized template plates, most probably grew by Ostwald ripening during the heat treatment at a high temperature of 900 °C. In contrast to the transformation from the sub-200 nm-sized Bi4Ti3O12, the side length dimension of the micrometer-sized template plates was well preserved when the conversion to BaTiO3 took place at 900 °C, but not at 660 °C. Because of the exfoliation and disintegration processes during heat treatment at the final temperature, the formed BaTiO3 particles did not retain well the morphology of the initial template. Raman and XRD investigations showed that the BaTiO3 particles that formed at 900 °C exhibited greater tetragonality than those obtained at 660 °C, regardless of the size of the initial Bi4Ti3O12 template. Preferentially [001]-oriented micrometer-sized and submicrometer-sized BaTiO3 plates, obtained under optimized synthesis conditions, exhibited good ferroelectric and piezoelectric properties, as determined by PFM. Irrespective of the transformation temperature, the template size and its aggregation, the reported method enables the preparation of nonaggregated, well-defined shaped and preferentially oriented BaTiO3 particles. These advantages promise that other ABO3type perovskite particles (i.e., Ba1−xCaxTi1−yZryO3) with good functional properties could be prepared using a similar approach.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the project (Engineering of structural and microstructural characteristics in contemporary dielectrics and ferroelectrics with perovskite and perovskite-like crystal structures, J2-6753) and the M-era.Net project (Innovative nanomaterials and architectures for integrated piezoelectric energy harvesting applications, 3184 HarvEnPiez), which were financially supported by the Slovenian Research Agency and the Ministry of Higher Education Science and Technology, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00164. XRD patterns, histograms of the side-length distributions of the Bi4Ti3O12 template plates, SEM micrographs (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +384 1 4777 3292. ORCID

Marjeta Maček Kržmanc: 0000-0003-3436-5692 J



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