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Enhancing the formation of tetragonal phase in perovskite nanocrystals using an ultrasound assisted wet chemical method
Ultrasonics Sonochemistry 33 (2016) 141–149
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Enhancing the formation of tetragonal phase in perovskite nanocrystals using an ultrasound assisted wet chemical method Abdolmajid Moghtada a, Rouholah Ashiri b,⇑ a b
Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran Department of Materials Science and Engineering, Dezful Branch, Islamic Azad University, P.O. Box 313, Dezful, Iran
a r t i c l e
i n f o
Article history: Received 3 April 2016 Received in revised form 2 May 2016 Accepted 2 May 2016 Available online 2 May 2016 Keywords: Titanate-based perovskites Nanoparticles Sonochemical method Tetragonal-phase Raman spectrum
a b s t r a c t Synthesis of highly-pure tetragonal perovskite nanocrystals is the key challenge facing the development of new electronic devices. Our results have indicated that ultrasonication is able in enhancing the formation of tetragonal phase in perovskite nanocrystals. In the current research, multicationic oxide perovskite (ATiO3; A: Ba, Sr, Ba0.6Sr0.4) nanopowders are synthesized successfully by a general methodology without a calcination step. The method is able to synthesize high-purity nanoscale ATiO3 (BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3) with tetragonal symmetry at a lower temperature and in a shorter time span in contrast to the literature. To reach an in-depth understanding of the scientific basis of the proposed methodology, in-detail analysis was carried out via XRD, FTIR, FT-Raman, FE-SEM and HR-TEM. The effects of the sonication time and sonication (bath) temperature on the tetragonality of nanoscale products were examined. Furthermore, Raman spectroscopy provides clear evidence for local tetragonal symmetries, in particular when a band is observed at 310 cm1. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Ternary Perovskite materials include a wide group of compounds used in various electroceramic device applications such as electronic, electro-optical and electromechanical devices. Perovskites have a cubic structure with general formula of ABO3, where A is usually an alkaline-earth or a large lanthanide, and B is usually a transition metal or a smaller lanthanide [1]. Perovskite structure has capability to host ions of different size. Focusing on parent compounds, in ABO3 series, only barium titanate (BaTiO3; BTO) is most studied and well explored compound in both bulk and thin film shapes. Although this compound shows a noteworthy range of interesting properties, however till now, only few studies have been reported on the sonochemical synthesis of perovskitetype materials. Sonochemistry uses the ultrasonic irradiation for inducing the formation of very fine powder products with high surface area in contrast to other synthesis methods [2]. It has been employed extensively for the synthesis of the nanostructured materials due to its rapid reaction rate, controllable reaction conditions, simplicity and safety. Moreover, powder particles synthesized through this method normally have uniform shape with narrow size distribution [1–4]. The crystal structure of BTO is typically observed by X-ray diffraction (XRD) and it appears to ⇑ Corresponding author. E-mail address: [email protected] (R. Ashiri). http://dx.doi.org/10.1016/j.ultsonch.2016.05.002 1350-4177/Ó 2016 Elsevier B.V. All rights reserved.
change from a tetragonal ferroelectric phase to a cubic paraelectric phase, which is inappropriate and is not very sensitive for transitions involving oxygen/titanium displacements [5]. However, vibrational spectroscopy is sensitive to this type of transformations, especially for Raman spectroscopy, which can detect local lattice distortions and crystallographic defects at the molecular level [6]. Chemical bonds vary widely in their sensitivity to scrutinizing by infrared techniques. Thus, the capability of infrared spectrophotometry (IR) is a function of the chemical bond, rather than being applicable as a general probe. FT-IR analysis was carried out for detecting the presence of the functional groups. Using this analysis, the reaction mechanisms in the sonochemical process can be detected. Raman and IR spectra are important in studying the ferroelectric materials, since ferroelectricity and lattice dynamics are closely related. Raman spectra give information on local symmetry and has been used by many researchers to study the relaxor behavior [7], the phase transitions [8], as well as in order to study other aspects like stress, strain [9] and grain size effect [10] of the ferroelectrics. In the present work, we have tried to develop an innovative method in order to synthesize a variety of ceramic nanoparticles with perovskite symmetry which has no by-products. This method is able to prepare the powder products at a low temperature of 333 K (60 °C) under the irradiation of the ultrasonic waves. Our approach provides a unified methodology for the synthesis of the perovskite materials which is a rapid one-step method with no
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need for calcination the products. On the other side, an elaborated explanation regarding mechanism of synthesis and phase development in sonochemical synthesis is not available in the literature. Few publications have studied the path of synthesis and characterized the phase formation, evolution and tetragonality of BTO powder. The goal of the present work is to study and characterize the structure and tetragonality dependences of the synthesized ATiO3 nanopowders using X-ray diffraction (XRD), Raman and IR spectroscopy which are able to identify the phase formation and evolution in the obtained nanopowders. 2. Experimental Synthesis temperature and purity of perovskite materials are key challenges facing the scientific community. This work addresses the challenges by developing a method for synthesizing the perovskite nanopowders. Our new approach is based on an ultrasound-assisted wet chemical processing method. To show that the developed method can be used as a general strategy for synthesizing carbonate-free perovskite nanocrystals, first BTO, strontium titanate (SrTiO3; STO) and barium strontium titanate (BaxSr1xTiO3; BSTO) nanocrystals were synthesized. Then, in order to study the tetragonality of the powder products, the second step of the study was focused on BTO as the most widely studied perovskite material. Titanium chloride (>99%), strontium chloride (>99%), barium chloride (>99%) anhydrous sodium hydroxide (>99%) and ethanol (99.8%) were obtained from Merck. The flowchart of the approach is shown in Fig. 1. The stoichiometric amounts of chloride salt and titanium chloride are dissolved in deionized water and ethanol, respectively. These solutions are added into a glass vessel containing NaOH solution. The concentration of the NaOH solution was required to guarantee a strong
alkaline environment (pH = 14) during reaction. The glass vessel containing precursor solution is subjected to an ultrasonic bath (Soner 220H, 53 kHz, 500 W, New Taipei City, Taiwan). The advantages of the sonochemical method include green synthesis, no waste product and ease of synthesis. The solution mixture was placed at the center of the ultrasonic bath and then was sonicated at 25, 50 and 60 °C for 5, 10 and 20 min to see the effect of sonication time and temperature. The sonication was conducted without cooling so that the temperature of the solution increased gradually up to 60 °C during synthesis. Our previous results [1] have shown that in order to synthesize BSTO (with general formula of BaxSr1xTiO3), first the corresponding molar ratios of the barium chloride (x) and strontium chloride (1 x) should be mixed together, then their mixture should be dissolved in deionized water. The successive steps of the synthesis are similar to those for BTO synthesis. After the reaction is finished and the mixture is cooled down to room temperature, then the powder product is separated, washed, and dried in an oven. The crystal structure and average crystallite size of the powder products are determined using an X-ray diffractometer (Philips PW3710). Functional groups in the product are detected using a FT-IR spectrophotometer (Hitachi 3140). FT-IR spectrum are recorded in the range of 400–4000 cm1 and measured on samples in KBr pellets. Raman spectrum was carried out with a FT-Raman 960 (Thermo Nicolet model) using a 5.5 mW laser with a wavelength of 636 nm. The morphological characteristics and microstructure of the nanoparticles were observed using field emission scanning electron microscopy (FESEM; Hitachi S4160, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM; ZEISS, LIBRA200, Oberkochen, Germany).
3. Results and discussion 3.1. XRD characterization
Fig. 1. Flowchart of the sonochemical method used in this work for the synthesis of the titanate-based perovskite nanocrystals.
It is known that advances in microelectronics and communication industries have led to substantial miniaturization of many electronic devices, while the performance requirements have increased [11]. As a result, smaller and more uniform particle sizes of ternary perovskite materials with tetragonal phase are required [12]. For instance, BTO powder with a narrow particle size distribution and high tetragonality is required for ceramic capacitors, selfcontrolled heaters, communication filters and non-volatile memories. Unfortunately, most of the methods used for the production of titanate-based (ATiO3; A: Ba, Sr, . . ..) powders, such as the conventional solid state reaction method, alkoxide-hydroxide route, solvothermal process, hydrothermal methods, etc. have not been fully successful in preparing perovskite nanopowders with a tetragonal phase. As a result, formation of tetragonal ATiO3 nanopowders remains as a challenging issue for scientific community. XRD method is a powerful nondestructive tool that can provide information regarding crystal structure, plane of orientation, strain relaxation, etc. The aim of the present work is phase identification, evolution and tetragonality of BTO powders. ATiO3-type ferroelectric (FE) compounds with perovskite structure often transform to a paraelectric (PE) phase when the temperature or pressure change [13,14]. For instance, PbTiO3 tetragonal perovskite (P4mm) transforms to cubic perovskite (PmN3m) at a high temperature of 763 K or at a high pressure of 11.2 GPa [15,16]. BaTiO3 also exhibits a tetragonal (P4mm) to cubic (PmN3m) phase transition at a high pressure of about 2 GPa or at 393 K [17]. XRD patterns of the sonochemically synthesized titanate-based (ATiO3) particles show strong diffraction peaks as it can be seen in Fig. 2. X-ray diffraction patterns are in good accordance with JCPDS No. 31-0174 (BTO), JCPDS No. 35-734 (SrTiO3; STO), JCPDS No. 34-0411 (Br0.6Sr0.4TiO3;
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Fig. 2. XRD patterns of (a) BTO (b) STO (c) BSTO nanocrystals synthesized at 333 K (60 °C) using our generalized methodology.
BSTO), respectively. Therefore, it can be said that titanate-based (ATiO3) perovskite powders have been synthesized successfully through our general methodology. Patterns of the synthesized products are consistent with other reports [18–21]. The titanatebased nanopowders were characterized by well-resolved peaks at 22.15, 31.49, 38.79, 45.23, 56.09, 65.85° corresponding to the (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 1), (2 2 0) planes. One difficulty in quantification of the tetragonality of BTO is the interpretation of the XRD measurement of (2 0 0) and (0 0 2) peaks. Theoretically,
100 pct tetragonal BTO has two separate peaks between 2h = 44 and 47° Fig. 3. Complete cubic barium titanate shows just one peak. A mixture of tetragonal and cubic barium titanate will show all intermediate forms between one and two peaks. The presence of clear splitting of (2 0 0) peak around 2h = 45° in X-ray diffraction patterns of BTO confirms its tetragonal symmetry at lower temperature. The extended scan of XRD around 2h = 45° is shown in Fig. 4. The XRD peak at 45.23° ((2 0 0) reflection), which is related to the c-axis of tetragonal phase of BaTiO3, shows much broader
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t = 1.022 shows tetragonal structures has formed at a low temperature of 333 K (60 °C). In this work the effect of two synthesis parameters including sonication time and sonication (bath) temperature on tetragonality of nanoparticles has been investigated. Among the obtained nanoparticles BTO is selected in order to study the effect of sonication time and synthesis temperature. Fig. 5 shows XRD patterns of the BTO powders synthesized at 333 K (60 °C) after different sonication times. It is obvious from XRD patterns that there is a minimum required sonication time to obtain a fully crystalline BTO powders. It took 20 min to obtain crystalline BTO particles using ultrasonic irradiation. Samples sonicated for shorter times contain both crystalline and amorphous phases. Fig. 6 clearly shows that by increasing in sonication time the amount of splitting of the mentioned peak increases. The tetragonality (c/a), calculated through the indexes of XRD patterns of BTO nanopowders at different sonication times is shown in Table 1. At sonication time lower than 5 min, the powder did not exhibit tetragonal structure. The tetragonality of powders increases with sonication time increasing, and after 20 min sonication, the tetragonality of sample was close to a fully tetragonal value of 1.022. Before 20 min sonication, the powder products show a single peak of (2 0 0) which is a characteristic of a cubic crystal structure. When the powder products are sonicated for 20 min, a splitting including (2 0 0) peak reacted at a higher region and (0 0 2) shoulder at the lower region are seen, which this are attributed to the characteristics of a tetragonal crystal structure. However, if the size of the synthesized crystallites decreases, the splitted peaks of tetragonal phase may overlap because of the broadening of the diffraction peaks induced by size of the crystallites. Fig. 7 shows the XRD of BTO synthesized at different sonication (bath) temperatures for 20 min sonication. The most intense reflection appearing in XRD pattern of sample sonicated for longer time indicates the formation of a BTO phase with tetragonal symmetry at sonication temperature of about 333 K (60 °C).The splitting of (2 0 0) peak is not visible in the case of lowest sonication (bath) temperature, but this peak is broader and asymmetric at higher sonication (bath) temperature. Hence, the splitting of the peak is due to the existence of two (0 0 2) and (2 0 0) peaks of tetragonal phase as shown in Fig. 8. At the lowest sonication (bath) temperature 298 K (25 °C) a very broad low intensity peak is seen which could be assigned to anatase phase listed in the JCPDS No. 84-1286.The tetragonality (c/a) calculated through the indexes of XRD patterns for BTO nanopowders at different sonication (bath) temperatures is shown in Table 1. The average crystallite size of ATiO3 perovskites calculated using Scherrer’s formula as follows:
Fig. 3. XRD curves of tetragonal and cubic barium titanate.
D¼
Fig. 4. XRD observation of the (2 0 0) peak splitting at 45.3° for BTO powders.
full-width half-maximum (FWHM) than those of the (1 0 0) and (1 1 0) reflections, which are related to a-axis of tetragonal phase of BaTiO3 [22]. Generally, it is known that the most stable structure of ABO3 is closely related to the tolerance factor as follows,
rA þ rO t ¼ pffiffiffi 2ðrB þ rO Þ
ð1Þ
where rA, rB and rO denote the ionic radii of A, B, and O ions, respectively. In general ferroelectric ABO3, the most stable structure is tetragonal for t J 1, cubic for t 1, and rhombohedral or orthorhombic for t [ 1 [23,24]. In our experiments, BTO with
0:9k bhkl cos h
ð2Þ
Where D is the average crystallite size, k = 1.541 Å (X-ray wavelength), and bhkl is the width of the diffraction peak at half maximum for the diffraction angle of 2h. The average crystallite size of BTO, STO and BSTO was calculated from the corresponding XRD patterns. Lattice parameters, unit cell volumes and crystallite sizes of the perovskite phases estimated from the XRD patterns are summarized in Table 2. These results clearly indicate that ultrasonication promotes the formation of tetragonal BTO phase in the final powder product. 3.2. FT-IR spectrum FT-IR spectrum of BTO synthesized nanopowders has been shown in Fig. 9. FT-IR spectrum interprets the existence of absorption bands at around 536, 1047, 1351, 1453, 1592 and 3400 cm1. It is known that the broad band in the range 3400–3097 cm1 is due to the stretching vibration of the hydroxyl (OAH) group and
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Fig. 5. XRD patterns of BTO powders synthesized at 333 K (60 °C) with different sonication times.
Fig. 6. XRD patterns magnification in 2h range of 40–50°.
Table 1 Tetragonality evaluation with respect to sonication time and sonication (bath) temperature. Sonication time and temperature
Tetragonality (c/a)
5 min at 333 K (60 °C) 10 min at 333 K (60 °C) 20 min at 333 K (60 °C) 20 min at 298 K (25 °C) 20 min at 323 K (50 °C) 20 min at 333 K (60 °C)
– 0.41 1.022 – 1.002 1.022
confirms the existence of water. Another band around 1592 cm1 is due to the bending vibration of the HAOAH bonds. The absorption bands at 1047, 1351 cm1 can be considered as the alcoholic bending vibrations (CAOH functional groups) [25]. The appearance of these absorption peaks is due to the washing stage of nanoparticles with alcohol in the final stage of synthesis. It is well known that the characteristic vibration bands corresponding to (MAO) metalAoxygen bonds are in the range of 400–800 cm1. Very strong bands related to TiAO and TiAOATi stretching vibrations appeared at 400–600 cm1 and 525–700 cm1, respectively [26].
Meanwhile the absorption at 536 cm1 is attributed to BaAO bond. Therefore, it can be said that the absorption at 536 cm1 corresponding to the (formation of the tetragonal BTO phase) is in agreement with XRD diffraction pattern [27].
3.3. FT-Raman spectrum Considering the structural results for cubic and tetragonal phases, in most of the cases the final product consists of a mixture of cubic and tetragonal phases. If their crystallite size is small enough to broaden the X-ray diffraction peaks, it is difficult to perform the qualitative phase analysis using X-ray powder diffraction data. Consequently, another approach is necessary to confirm the phase identification of BTO powder. Raman spectroscopy has been employed to determine the characteristic lattice vibration spectra of BTO powders. It is well known that symmetry-group analysis in cubic BTO exhibits no Raman active modes, while in tetragonal BTO (space group P4mm) there is eight optical Raman active modes: 4 modes of E symmetry, 3 modes of A1 symmetry and one mode of B1 symmetry [27]. Each of these modes splits into transverse (TO) and longitudinal (LO) optical components, due to
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Fig. 7. XRD patterns of BTO after 20 min sonication at different sonication (bath) temperatures.
Fig. 8. XRD patterns magnification in 2h range of 40–50°.
Table 2 Lattice parameters, unit cell volumes and crystallite sizes of BTO, STO, BSTO powders calculated from X-ray diffraction patterns. Synthesized powders
Dominant peak position
Lattice parameter, a, (Å)
Average crystallite size
Unit cell volume (Å)3
BaTiO3 SrTiO3 Ba0.6Sr0.4TiO3
31.45° 32.37° 32.00°
3.9440 3.92 3.9650
10.3 nm 4.8 nm 14 nm
62.75 60.24 62.33
long-range electrostatic forces associated with lattice ionicity [28]. In our investigation, five Raman peaks have been recorded and assigned to more than one phonon mode of tetragonal BTO, as presented in Fig. 10. According to literature data [29,30] Raman peaks are interpreted as follows: the sharp peak centered around 270 cm1 as a A1 (TO2) mode, a broad peak at 310 cm1 as B1 + E (LO2) + E (TO3), an asymmetric sharp peak at 515 cm1 as A1 (TO3) + E (TO4) and a broad weak peak at 742 cm1 as a sum of A1 (LO3) and E (LO4) modes. These are typical peaks in the Raman spectrum for the tetragonal BTO phase. Among them, the peaks around 310 and 742 cm1 disappear at above the Curie temperature, which is the stable region of the cubic phase. This means that it is possible to differentiate between the cubic and tetragonal phases by the presence of the two peaks in Raman spectrum. From the Raman spectrum, which is carried out on powder with crystallite size of 10.3 nm, it could be found that our sample contains the peaks corresponding to the tetragonal phase. Among all peaks, the
peak located at 310 cm1 is characteristic peak for the tetragonal BTO. This indicates that the results of the Raman spectroscopy measurements are in accordance with those obtained by X-ray analysis. The observed frequencies of Raman modes in BTO nanopowders as compared with those reported in previous works [31–33] are given in Table 3. 3.4. FE-SEM and HR-TEM analysis The morphology and particle size of ATiO3 (BTO, STO, BSTO) nanopowders synthesized in this work was studied by a field emission scanning electron microscopy (FE-SEM). FE-SEM micrographs of the ATiO3 are shown in Fig. 11. FE-SEM micrographs clearly show homogeneous morphology in shape and dimension, uniform crystal size and agglomerated nature of the nanopowders. The microstructure and size distribution of the powder products were studied using HRTEM technique. TEM micrographs of BTO and STO powder products synthesized in this work are shown in Fig. 12. Morphology of the powders is consistent with the FESEM results. Moreover, the crystallite size observed in TEM micrographs is in good agreement with the crystallite size calculated from XRD patterns. As can be seen, the powders are uniform in their size and shape. Our previous experiences in fabrication the nanoscale materials [34–45] critically indicate that a smart designing of the process and the use of the properly selected starting materials (or precursors) significantly influence the microstructure and performance of the final nanoscale product.
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Fig. 9. FT-IR spectrum of BTO nanocrystals synthesized at 333 K (60 °C).
Fig. 10. Raman spectrum of tetragonal BTO powders synthesized at 333 K (60 °C.
Table 3 Comparison of Raman mode frequencies (in cm1) observed from BTO with other literature. Mode
Ref. [31]
Ref. [32]
Ref. [33]
This work
A1(TO2) E(TO + LO) + B1 A1(LO2) A1(TO3) + E (TO4) A1(LO3) + E (LO4)
267 308 473 512 740
278 305 470 520 727
270 305 471 516 719
270 310 430 515 742
3.5. The role of the ultrasonication on the formation of tetragonal phase Functionalities such as ferroelectricity, piezoelectricity and piezoelectricity of the barium titanate are originated from domain structure of dipole which forms by slight asymmetry of the crystal structure [46–48]; this is affected by its processing. Therefore, the
synthesis methods resulting in formation of tetragonal phase are of significant importance. Most of the synthesis methods such as solgel processing [48–50] and solid-state synthesis [18,51–52] led to the formation of the cubic barium titanate as their final products. Our results have indicated that the synthesis method established here results in the formation of tetragonal phase at a low temperature in contrast to the literature [18,48–52]. It seems that ultrasonication is the reason behind this achievement. To discuss this, we should consider the ultrasonication effect and also the conditions required for the formation of the tetragonal phase simultaneously. The previous results have shown [46,48,52] that the formation of tetragonal barium titanate needs more energy in contrast to the cubic phase. Therefore, the formation of cubic phase is more preferred from the kinetic point of view. During sonication, ultrasonic waves radiate through the precursor solution. This causes alternating high and low pressure in the solution and also leads to the formation, growth, and implosive collapse of bubbles in the reaction mixture [2,53]. The collapse of bubbles with short
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Fig. 11. FE-SEM micrographs of (a) BTO, (b) STO and (c) BSTO nanocrystals synthesized in this work.
Fig. 12. HR-TEM micrographs of (a) BTO and (b) STO nanocrystals synthesized in this work.
lifetime produces intense local heating. According to the hot spot theory [2], very high temperatures (>5000 K) and pressures of roughly 1000 atm are obtained upon the collapse of a bubble. These critical conditions [54–55] provide the enough energy for the formation of the tetragonal phase instead of the cubic one. Therefore, it is expected to obtain better tetragonality with increasing the sonication time; a trend which is seen in our results (see Table 1 and Fig. 6). 4. Conclusions This work reported a simple and cost-effective general methodology for synthesizing ATiO3 (A: Ba, Sr, Ba0.6Sr0.4) nanocrystals. The methodology was a novel ultrasound-assisted wet chemical synthesis which was operated at a low temperature of 333 K (60 °C). The results indicated that ATiO3 nanocrystals with tailored morphology and narrow size distribution with the particles size in the range of 4–15 nm can be synthesized by using our developed method. X-ray diffraction pattern of BTO nanopowders suggested that in order to obtain tetragonal phase, a minimum temperature of 333 K (60 °C) is required. Raman spectrum of BTO showed a sharp peak at 310 cm1, which is characteristic of the formation of the tetragonal phase. The tetragonality of BTO nanopowders increased with increasing sonication time and sonication (bath) temperature. References [1] A. Moghtada, R. Ashiri, Ultrason. Sonochem. 26 (2015) 293–304. [2] T.J. Mason, J.P. Lolimer, Applied Sonochemistry, Wiley-VCH Verlag, U.K., 2002. [3] C.N.R. Rao, A. Müller, A.K. Cheetham, Wiley-VCH Verlag GmbH & Co. KGaA, Germany, 2005. [4] R. Ashiri, A. Moghtada, Metall. Mater. Trans. B 45 (2014) 1979–1986. [5] M. Yashima, K. Ohtake, M. Kakihana, H. Arashi, M. Yoshimura, J. Phys. Chem. Solids 57 (1996) 17–24. [6] P.S. Dobal, R.S. Katiyar, J. Raman Spectrosc. 33 (2002) 405–423.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Enhancing the formation of tetragonal phase in perovskite nanocrystals using an ultrasound assisted wet chemical method Abdolmajid Moghtada a, Rouholah Ashiri b,⇑ a b
Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran Department of Materials Science and Engineering, Dezful Branch, Islamic Azad University, P.O. Box 313, Dezful, Iran
a r t i c l e
i n f o
Article history: Received 3 April 2016 Received in revised form 2 May 2016 Accepted 2 May 2016 Available online 2 May 2016 Keywords: Titanate-based perovskites Nanoparticles Sonochemical method Tetragonal-phase Raman spectrum
a b s t r a c t Synthesis of highly-pure tetragonal perovskite nanocrystals is the key challenge facing the development of new electronic devices. Our results have indicated that ultrasonication is able in enhancing the formation of tetragonal phase in perovskite nanocrystals. In the current research, multicationic oxide perovskite (ATiO3; A: Ba, Sr, Ba0.6Sr0.4) nanopowders are synthesized successfully by a general methodology without a calcination step. The method is able to synthesize high-purity nanoscale ATiO3 (BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3) with tetragonal symmetry at a lower temperature and in a shorter time span in contrast to the literature. To reach an in-depth understanding of the scientific basis of the proposed methodology, in-detail analysis was carried out via XRD, FTIR, FT-Raman, FE-SEM and HR-TEM. The effects of the sonication time and sonication (bath) temperature on the tetragonality of nanoscale products were examined. Furthermore, Raman spectroscopy provides clear evidence for local tetragonal symmetries, in particular when a band is observed at 310 cm1. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Ternary Perovskite materials include a wide group of compounds used in various electroceramic device applications such as electronic, electro-optical and electromechanical devices. Perovskites have a cubic structure with general formula of ABO3, where A is usually an alkaline-earth or a large lanthanide, and B is usually a transition metal or a smaller lanthanide [1]. Perovskite structure has capability to host ions of different size. Focusing on parent compounds, in ABO3 series, only barium titanate (BaTiO3; BTO) is most studied and well explored compound in both bulk and thin film shapes. Although this compound shows a noteworthy range of interesting properties, however till now, only few studies have been reported on the sonochemical synthesis of perovskitetype materials. Sonochemistry uses the ultrasonic irradiation for inducing the formation of very fine powder products with high surface area in contrast to other synthesis methods [2]. It has been employed extensively for the synthesis of the nanostructured materials due to its rapid reaction rate, controllable reaction conditions, simplicity and safety. Moreover, powder particles synthesized through this method normally have uniform shape with narrow size distribution [1–4]. The crystal structure of BTO is typically observed by X-ray diffraction (XRD) and it appears to ⇑ Corresponding author. E-mail address: [email protected] (R. Ashiri). http://dx.doi.org/10.1016/j.ultsonch.2016.05.002 1350-4177/Ó 2016 Elsevier B.V. All rights reserved.
change from a tetragonal ferroelectric phase to a cubic paraelectric phase, which is inappropriate and is not very sensitive for transitions involving oxygen/titanium displacements [5]. However, vibrational spectroscopy is sensitive to this type of transformations, especially for Raman spectroscopy, which can detect local lattice distortions and crystallographic defects at the molecular level [6]. Chemical bonds vary widely in their sensitivity to scrutinizing by infrared techniques. Thus, the capability of infrared spectrophotometry (IR) is a function of the chemical bond, rather than being applicable as a general probe. FT-IR analysis was carried out for detecting the presence of the functional groups. Using this analysis, the reaction mechanisms in the sonochemical process can be detected. Raman and IR spectra are important in studying the ferroelectric materials, since ferroelectricity and lattice dynamics are closely related. Raman spectra give information on local symmetry and has been used by many researchers to study the relaxor behavior [7], the phase transitions [8], as well as in order to study other aspects like stress, strain [9] and grain size effect [10] of the ferroelectrics. In the present work, we have tried to develop an innovative method in order to synthesize a variety of ceramic nanoparticles with perovskite symmetry which has no by-products. This method is able to prepare the powder products at a low temperature of 333 K (60 °C) under the irradiation of the ultrasonic waves. Our approach provides a unified methodology for the synthesis of the perovskite materials which is a rapid one-step method with no
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need for calcination the products. On the other side, an elaborated explanation regarding mechanism of synthesis and phase development in sonochemical synthesis is not available in the literature. Few publications have studied the path of synthesis and characterized the phase formation, evolution and tetragonality of BTO powder. The goal of the present work is to study and characterize the structure and tetragonality dependences of the synthesized ATiO3 nanopowders using X-ray diffraction (XRD), Raman and IR spectroscopy which are able to identify the phase formation and evolution in the obtained nanopowders. 2. Experimental Synthesis temperature and purity of perovskite materials are key challenges facing the scientific community. This work addresses the challenges by developing a method for synthesizing the perovskite nanopowders. Our new approach is based on an ultrasound-assisted wet chemical processing method. To show that the developed method can be used as a general strategy for synthesizing carbonate-free perovskite nanocrystals, first BTO, strontium titanate (SrTiO3; STO) and barium strontium titanate (BaxSr1xTiO3; BSTO) nanocrystals were synthesized. Then, in order to study the tetragonality of the powder products, the second step of the study was focused on BTO as the most widely studied perovskite material. Titanium chloride (>99%), strontium chloride (>99%), barium chloride (>99%) anhydrous sodium hydroxide (>99%) and ethanol (99.8%) were obtained from Merck. The flowchart of the approach is shown in Fig. 1. The stoichiometric amounts of chloride salt and titanium chloride are dissolved in deionized water and ethanol, respectively. These solutions are added into a glass vessel containing NaOH solution. The concentration of the NaOH solution was required to guarantee a strong
alkaline environment (pH = 14) during reaction. The glass vessel containing precursor solution is subjected to an ultrasonic bath (Soner 220H, 53 kHz, 500 W, New Taipei City, Taiwan). The advantages of the sonochemical method include green synthesis, no waste product and ease of synthesis. The solution mixture was placed at the center of the ultrasonic bath and then was sonicated at 25, 50 and 60 °C for 5, 10 and 20 min to see the effect of sonication time and temperature. The sonication was conducted without cooling so that the temperature of the solution increased gradually up to 60 °C during synthesis. Our previous results [1] have shown that in order to synthesize BSTO (with general formula of BaxSr1xTiO3), first the corresponding molar ratios of the barium chloride (x) and strontium chloride (1 x) should be mixed together, then their mixture should be dissolved in deionized water. The successive steps of the synthesis are similar to those for BTO synthesis. After the reaction is finished and the mixture is cooled down to room temperature, then the powder product is separated, washed, and dried in an oven. The crystal structure and average crystallite size of the powder products are determined using an X-ray diffractometer (Philips PW3710). Functional groups in the product are detected using a FT-IR spectrophotometer (Hitachi 3140). FT-IR spectrum are recorded in the range of 400–4000 cm1 and measured on samples in KBr pellets. Raman spectrum was carried out with a FT-Raman 960 (Thermo Nicolet model) using a 5.5 mW laser with a wavelength of 636 nm. The morphological characteristics and microstructure of the nanoparticles were observed using field emission scanning electron microscopy (FESEM; Hitachi S4160, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM; ZEISS, LIBRA200, Oberkochen, Germany).
3. Results and discussion 3.1. XRD characterization
Fig. 1. Flowchart of the sonochemical method used in this work for the synthesis of the titanate-based perovskite nanocrystals.
It is known that advances in microelectronics and communication industries have led to substantial miniaturization of many electronic devices, while the performance requirements have increased [11]. As a result, smaller and more uniform particle sizes of ternary perovskite materials with tetragonal phase are required [12]. For instance, BTO powder with a narrow particle size distribution and high tetragonality is required for ceramic capacitors, selfcontrolled heaters, communication filters and non-volatile memories. Unfortunately, most of the methods used for the production of titanate-based (ATiO3; A: Ba, Sr, . . ..) powders, such as the conventional solid state reaction method, alkoxide-hydroxide route, solvothermal process, hydrothermal methods, etc. have not been fully successful in preparing perovskite nanopowders with a tetragonal phase. As a result, formation of tetragonal ATiO3 nanopowders remains as a challenging issue for scientific community. XRD method is a powerful nondestructive tool that can provide information regarding crystal structure, plane of orientation, strain relaxation, etc. The aim of the present work is phase identification, evolution and tetragonality of BTO powders. ATiO3-type ferroelectric (FE) compounds with perovskite structure often transform to a paraelectric (PE) phase when the temperature or pressure change [13,14]. For instance, PbTiO3 tetragonal perovskite (P4mm) transforms to cubic perovskite (PmN3m) at a high temperature of 763 K or at a high pressure of 11.2 GPa [15,16]. BaTiO3 also exhibits a tetragonal (P4mm) to cubic (PmN3m) phase transition at a high pressure of about 2 GPa or at 393 K [17]. XRD patterns of the sonochemically synthesized titanate-based (ATiO3) particles show strong diffraction peaks as it can be seen in Fig. 2. X-ray diffraction patterns are in good accordance with JCPDS No. 31-0174 (BTO), JCPDS No. 35-734 (SrTiO3; STO), JCPDS No. 34-0411 (Br0.6Sr0.4TiO3;
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Fig. 2. XRD patterns of (a) BTO (b) STO (c) BSTO nanocrystals synthesized at 333 K (60 °C) using our generalized methodology.
BSTO), respectively. Therefore, it can be said that titanate-based (ATiO3) perovskite powders have been synthesized successfully through our general methodology. Patterns of the synthesized products are consistent with other reports [18–21]. The titanatebased nanopowders were characterized by well-resolved peaks at 22.15, 31.49, 38.79, 45.23, 56.09, 65.85° corresponding to the (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 1), (2 2 0) planes. One difficulty in quantification of the tetragonality of BTO is the interpretation of the XRD measurement of (2 0 0) and (0 0 2) peaks. Theoretically,
100 pct tetragonal BTO has two separate peaks between 2h = 44 and 47° Fig. 3. Complete cubic barium titanate shows just one peak. A mixture of tetragonal and cubic barium titanate will show all intermediate forms between one and two peaks. The presence of clear splitting of (2 0 0) peak around 2h = 45° in X-ray diffraction patterns of BTO confirms its tetragonal symmetry at lower temperature. The extended scan of XRD around 2h = 45° is shown in Fig. 4. The XRD peak at 45.23° ((2 0 0) reflection), which is related to the c-axis of tetragonal phase of BaTiO3, shows much broader
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t = 1.022 shows tetragonal structures has formed at a low temperature of 333 K (60 °C). In this work the effect of two synthesis parameters including sonication time and sonication (bath) temperature on tetragonality of nanoparticles has been investigated. Among the obtained nanoparticles BTO is selected in order to study the effect of sonication time and synthesis temperature. Fig. 5 shows XRD patterns of the BTO powders synthesized at 333 K (60 °C) after different sonication times. It is obvious from XRD patterns that there is a minimum required sonication time to obtain a fully crystalline BTO powders. It took 20 min to obtain crystalline BTO particles using ultrasonic irradiation. Samples sonicated for shorter times contain both crystalline and amorphous phases. Fig. 6 clearly shows that by increasing in sonication time the amount of splitting of the mentioned peak increases. The tetragonality (c/a), calculated through the indexes of XRD patterns of BTO nanopowders at different sonication times is shown in Table 1. At sonication time lower than 5 min, the powder did not exhibit tetragonal structure. The tetragonality of powders increases with sonication time increasing, and after 20 min sonication, the tetragonality of sample was close to a fully tetragonal value of 1.022. Before 20 min sonication, the powder products show a single peak of (2 0 0) which is a characteristic of a cubic crystal structure. When the powder products are sonicated for 20 min, a splitting including (2 0 0) peak reacted at a higher region and (0 0 2) shoulder at the lower region are seen, which this are attributed to the characteristics of a tetragonal crystal structure. However, if the size of the synthesized crystallites decreases, the splitted peaks of tetragonal phase may overlap because of the broadening of the diffraction peaks induced by size of the crystallites. Fig. 7 shows the XRD of BTO synthesized at different sonication (bath) temperatures for 20 min sonication. The most intense reflection appearing in XRD pattern of sample sonicated for longer time indicates the formation of a BTO phase with tetragonal symmetry at sonication temperature of about 333 K (60 °C).The splitting of (2 0 0) peak is not visible in the case of lowest sonication (bath) temperature, but this peak is broader and asymmetric at higher sonication (bath) temperature. Hence, the splitting of the peak is due to the existence of two (0 0 2) and (2 0 0) peaks of tetragonal phase as shown in Fig. 8. At the lowest sonication (bath) temperature 298 K (25 °C) a very broad low intensity peak is seen which could be assigned to anatase phase listed in the JCPDS No. 84-1286.The tetragonality (c/a) calculated through the indexes of XRD patterns for BTO nanopowders at different sonication (bath) temperatures is shown in Table 1. The average crystallite size of ATiO3 perovskites calculated using Scherrer’s formula as follows:
Fig. 3. XRD curves of tetragonal and cubic barium titanate.
D¼
Fig. 4. XRD observation of the (2 0 0) peak splitting at 45.3° for BTO powders.
full-width half-maximum (FWHM) than those of the (1 0 0) and (1 1 0) reflections, which are related to a-axis of tetragonal phase of BaTiO3 [22]. Generally, it is known that the most stable structure of ABO3 is closely related to the tolerance factor as follows,
rA þ rO t ¼ pffiffiffi 2ðrB þ rO Þ
ð1Þ
where rA, rB and rO denote the ionic radii of A, B, and O ions, respectively. In general ferroelectric ABO3, the most stable structure is tetragonal for t J 1, cubic for t 1, and rhombohedral or orthorhombic for t [ 1 [23,24]. In our experiments, BTO with
0:9k bhkl cos h
ð2Þ
Where D is the average crystallite size, k = 1.541 Å (X-ray wavelength), and bhkl is the width of the diffraction peak at half maximum for the diffraction angle of 2h. The average crystallite size of BTO, STO and BSTO was calculated from the corresponding XRD patterns. Lattice parameters, unit cell volumes and crystallite sizes of the perovskite phases estimated from the XRD patterns are summarized in Table 2. These results clearly indicate that ultrasonication promotes the formation of tetragonal BTO phase in the final powder product. 3.2. FT-IR spectrum FT-IR spectrum of BTO synthesized nanopowders has been shown in Fig. 9. FT-IR spectrum interprets the existence of absorption bands at around 536, 1047, 1351, 1453, 1592 and 3400 cm1. It is known that the broad band in the range 3400–3097 cm1 is due to the stretching vibration of the hydroxyl (OAH) group and
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Fig. 5. XRD patterns of BTO powders synthesized at 333 K (60 °C) with different sonication times.
Fig. 6. XRD patterns magnification in 2h range of 40–50°.
Table 1 Tetragonality evaluation with respect to sonication time and sonication (bath) temperature. Sonication time and temperature
Tetragonality (c/a)
5 min at 333 K (60 °C) 10 min at 333 K (60 °C) 20 min at 333 K (60 °C) 20 min at 298 K (25 °C) 20 min at 323 K (50 °C) 20 min at 333 K (60 °C)
– 0.41 1.022 – 1.002 1.022
confirms the existence of water. Another band around 1592 cm1 is due to the bending vibration of the HAOAH bonds. The absorption bands at 1047, 1351 cm1 can be considered as the alcoholic bending vibrations (CAOH functional groups) [25]. The appearance of these absorption peaks is due to the washing stage of nanoparticles with alcohol in the final stage of synthesis. It is well known that the characteristic vibration bands corresponding to (MAO) metalAoxygen bonds are in the range of 400–800 cm1. Very strong bands related to TiAO and TiAOATi stretching vibrations appeared at 400–600 cm1 and 525–700 cm1, respectively [26].
Meanwhile the absorption at 536 cm1 is attributed to BaAO bond. Therefore, it can be said that the absorption at 536 cm1 corresponding to the (formation of the tetragonal BTO phase) is in agreement with XRD diffraction pattern [27].
3.3. FT-Raman spectrum Considering the structural results for cubic and tetragonal phases, in most of the cases the final product consists of a mixture of cubic and tetragonal phases. If their crystallite size is small enough to broaden the X-ray diffraction peaks, it is difficult to perform the qualitative phase analysis using X-ray powder diffraction data. Consequently, another approach is necessary to confirm the phase identification of BTO powder. Raman spectroscopy has been employed to determine the characteristic lattice vibration spectra of BTO powders. It is well known that symmetry-group analysis in cubic BTO exhibits no Raman active modes, while in tetragonal BTO (space group P4mm) there is eight optical Raman active modes: 4 modes of E symmetry, 3 modes of A1 symmetry and one mode of B1 symmetry [27]. Each of these modes splits into transverse (TO) and longitudinal (LO) optical components, due to
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Fig. 7. XRD patterns of BTO after 20 min sonication at different sonication (bath) temperatures.
Fig. 8. XRD patterns magnification in 2h range of 40–50°.
Table 2 Lattice parameters, unit cell volumes and crystallite sizes of BTO, STO, BSTO powders calculated from X-ray diffraction patterns. Synthesized powders
Dominant peak position
Lattice parameter, a, (Å)
Average crystallite size
Unit cell volume (Å)3
BaTiO3 SrTiO3 Ba0.6Sr0.4TiO3
31.45° 32.37° 32.00°
3.9440 3.92 3.9650
10.3 nm 4.8 nm 14 nm
62.75 60.24 62.33
long-range electrostatic forces associated with lattice ionicity [28]. In our investigation, five Raman peaks have been recorded and assigned to more than one phonon mode of tetragonal BTO, as presented in Fig. 10. According to literature data [29,30] Raman peaks are interpreted as follows: the sharp peak centered around 270 cm1 as a A1 (TO2) mode, a broad peak at 310 cm1 as B1 + E (LO2) + E (TO3), an asymmetric sharp peak at 515 cm1 as A1 (TO3) + E (TO4) and a broad weak peak at 742 cm1 as a sum of A1 (LO3) and E (LO4) modes. These are typical peaks in the Raman spectrum for the tetragonal BTO phase. Among them, the peaks around 310 and 742 cm1 disappear at above the Curie temperature, which is the stable region of the cubic phase. This means that it is possible to differentiate between the cubic and tetragonal phases by the presence of the two peaks in Raman spectrum. From the Raman spectrum, which is carried out on powder with crystallite size of 10.3 nm, it could be found that our sample contains the peaks corresponding to the tetragonal phase. Among all peaks, the
peak located at 310 cm1 is characteristic peak for the tetragonal BTO. This indicates that the results of the Raman spectroscopy measurements are in accordance with those obtained by X-ray analysis. The observed frequencies of Raman modes in BTO nanopowders as compared with those reported in previous works [31–33] are given in Table 3. 3.4. FE-SEM and HR-TEM analysis The morphology and particle size of ATiO3 (BTO, STO, BSTO) nanopowders synthesized in this work was studied by a field emission scanning electron microscopy (FE-SEM). FE-SEM micrographs of the ATiO3 are shown in Fig. 11. FE-SEM micrographs clearly show homogeneous morphology in shape and dimension, uniform crystal size and agglomerated nature of the nanopowders. The microstructure and size distribution of the powder products were studied using HRTEM technique. TEM micrographs of BTO and STO powder products synthesized in this work are shown in Fig. 12. Morphology of the powders is consistent with the FESEM results. Moreover, the crystallite size observed in TEM micrographs is in good agreement with the crystallite size calculated from XRD patterns. As can be seen, the powders are uniform in their size and shape. Our previous experiences in fabrication the nanoscale materials [34–45] critically indicate that a smart designing of the process and the use of the properly selected starting materials (or precursors) significantly influence the microstructure and performance of the final nanoscale product.
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Fig. 9. FT-IR spectrum of BTO nanocrystals synthesized at 333 K (60 °C).
Fig. 10. Raman spectrum of tetragonal BTO powders synthesized at 333 K (60 °C.
Table 3 Comparison of Raman mode frequencies (in cm1) observed from BTO with other literature. Mode
Ref. [31]
Ref. [32]
Ref. [33]
This work
A1(TO2) E(TO + LO) + B1 A1(LO2) A1(TO3) + E (TO4) A1(LO3) + E (LO4)
267 308 473 512 740
278 305 470 520 727
270 305 471 516 719
270 310 430 515 742
3.5. The role of the ultrasonication on the formation of tetragonal phase Functionalities such as ferroelectricity, piezoelectricity and piezoelectricity of the barium titanate are originated from domain structure of dipole which forms by slight asymmetry of the crystal structure [46–48]; this is affected by its processing. Therefore, the
synthesis methods resulting in formation of tetragonal phase are of significant importance. Most of the synthesis methods such as solgel processing [48–50] and solid-state synthesis [18,51–52] led to the formation of the cubic barium titanate as their final products. Our results have indicated that the synthesis method established here results in the formation of tetragonal phase at a low temperature in contrast to the literature [18,48–52]. It seems that ultrasonication is the reason behind this achievement. To discuss this, we should consider the ultrasonication effect and also the conditions required for the formation of the tetragonal phase simultaneously. The previous results have shown [46,48,52] that the formation of tetragonal barium titanate needs more energy in contrast to the cubic phase. Therefore, the formation of cubic phase is more preferred from the kinetic point of view. During sonication, ultrasonic waves radiate through the precursor solution. This causes alternating high and low pressure in the solution and also leads to the formation, growth, and implosive collapse of bubbles in the reaction mixture [2,53]. The collapse of bubbles with short
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Fig. 11. FE-SEM micrographs of (a) BTO, (b) STO and (c) BSTO nanocrystals synthesized in this work.
Fig. 12. HR-TEM micrographs of (a) BTO and (b) STO nanocrystals synthesized in this work.
lifetime produces intense local heating. According to the hot spot theory [2], very high temperatures (>5000 K) and pressures of roughly 1000 atm are obtained upon the collapse of a bubble. These critical conditions [54–55] provide the enough energy for the formation of the tetragonal phase instead of the cubic one. Therefore, it is expected to obtain better tetragonality with increasing the sonication time; a trend which is seen in our results (see Table 1 and Fig. 6). 4. Conclusions This work reported a simple and cost-effective general methodology for synthesizing ATiO3 (A: Ba, Sr, Ba0.6Sr0.4) nanocrystals. The methodology was a novel ultrasound-assisted wet chemical synthesis which was operated at a low temperature of 333 K (60 °C). The results indicated that ATiO3 nanocrystals with tailored morphology and narrow size distribution with the particles size in the range of 4–15 nm can be synthesized by using our developed method. X-ray diffraction pattern of BTO nanopowders suggested that in order to obtain tetragonal phase, a minimum temperature of 333 K (60 °C) is required. Raman spectrum of BTO showed a sharp peak at 310 cm1, which is characteristic of the formation of the tetragonal phase. The tetragonality of BTO nanopowders increased with increasing sonication time and sonication (bath) temperature. References [1] A. Moghtada, R. Ashiri, Ultrason. Sonochem. 26 (2015) 293–304. [2] T.J. Mason, J.P. Lolimer, Applied Sonochemistry, Wiley-VCH Verlag, U.K., 2002. [3] C.N.R. Rao, A. Müller, A.K. Cheetham, Wiley-VCH Verlag GmbH & Co. KGaA, Germany, 2005. [4] R. Ashiri, A. Moghtada, Metall. Mater. Trans. B 45 (2014) 1979–1986. [5] M. Yashima, K. Ohtake, M. Kakihana, H. Arashi, M. Yoshimura, J. Phys. Chem. Solids 57 (1996) 17–24. [6] P.S. Dobal, R.S. Katiyar, J. Raman Spectrosc. 33 (2002) 405–423.
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