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Evaluation of Na2TiO3 formation for producing crystalline BaTiO3 nanoparticles by liquid–solid–solution process at low temperature
Journal of Alloys and Compounds 695 (2017) 2160e2164
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Evaluation of Na2TiO3 formation for producing crystalline BaTiO3 nanoparticles by liquidesolidesolution process at low temperature Wooje Han a, Hong-Sub Lee a, Byungwook Yoo b, Hyung-Ho Park a, * a b
Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, Republic of Korea Display Materials & Components Research Center, Korea Electronics Technology Institute, Seongnam-si, Gyeonggi-do, 13509, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2016 Received in revised form 3 November 2016 Accepted 4 November 2016 Available online 5 November 2016
Crystalline barium titanate (BaTiO3) nanoparticles were synthesized by refluxing in an air atmosphere at a low temperature using sodium hydroxide. The reaction mechanism of BaTiO3 nanoparticle formation by this new liquidesolidesolution (LSS) synthesis route with refluxing in an air atmosphere at a low temperature of 80 C was demonstrated to be a facile fabrication process for BaTiO3 nanoparticles with a high crystalline quality and minimized agglomeration rate. BaTiO3 nanoparticles were formed via sodium titanate (Na2TiO3), and the size of the BaTiO3 nanoparticles decreased with increasing sodium hydroxide concentration. The formation of a Na2TiO3 intermediate phase was confirmed by energy dispersive X-ray spectroscopy and X-ray diffraction analysis. The capping ligands of BaTiO3 suggest that hydroxyl and oleic acid are present. The dielectric constant of the crystalline BaTiO3 nanoparticles was higher than 150. © 2016 Elsevier B.V. All rights reserved.
Keywords: BaTiO3 nanoparticles Liquidesolidesolution process Na2TiO3 NaOH reaction mechanism
1. Introduction Barium titanate (BaTiO3) has a typical perovskite structure and has been one of the most actively promoted among ferroelectric materials because of its high dielectric constant. It is widely used in thermistors, sensors, electro-optical devices, and multi-layer ceramic capacitors (MLCCs) as a global electronic ceramic [1e9]. Owing to recent developments of these applications, the demand for thin layer materials with a high dielectric constant has increased for the miniaturization and performance enhancement of the devices [10]. In these applications, the device performance is strongly influenced by the size, shape, and composition of the BaTiO3 nanoparticles. Thus, the facile synthesis of nanosized BaTiO3 without impurities is of prime importance and is a challenging task for researchers and the industry. BaTiO3 nanoparticles are generally prepared by a solid-state reaction method by heating BaCO3 and TiO2 at temperatures as high of 1200 C, but calcination at such high temperatures results in agglomerations of particles [11]. Various other preparation methods have also been reported for the preparation of BaTiO3 nanoparticles with high quality, such as the solgel method [12e14], hydrothermal method [15,16], ball-milling method [17,18], glycolthermal method [19], liquidesolidesolution
* Corresponding author. E-mail address: [email protected] (H.-H. Park). http://dx.doi.org/10.1016/j.jallcom.2016.11.062 0925-8388/© 2016 Elsevier B.V. All rights reserved.
process (LSS) [20], and co-precipitation method [21]. Wet chemical methods are very useful for obtaining nanosized BaTiO3 because of the control over the agglomeration of particles and the low temperature involved in the synthesis. Among such methods, hydrothermal and sol-gel processes have been extensively studied for synthesis of BaTiO3 nanoparticles. The sol-gel process has received considerable attention because of the relative ease, reliability, and homogeneity of the synthesis and processing of nanomaterials with high purity. The use of metal alkoxides as a precursor in the sol-gel process provides more benefits compared to the solid-state reaction [12e14] and hydrothermal process because it enables control of the structural composition at the molecular level [4]. However, during the sol-gel process, non-crystalline BaTiO3 is also obtained at room temperature, and annealing treatment converts it to crystalline BaTiO3 [22]. The heat treatment leads to agglomeration of the particles, which increases the size of the particles. Although the conventional hydrothermal method enables direct synthesis of spherical BaTiO3 with small powder particles (<0.1 mm) at a low reaction temperature, it requires relatively harsh reaction conditions, and therefore, special and expensive precursors are needed that are capable of withstanding high pH levels, temperature, and pressures [15,16,23]. Wang et al. proposed the LSS process followed by the hydrothermal method in order to synthesize nanoparticles of various noble metals and metal oxides while minimizing particle agglomeration. However, because of the hydrothermal method involved, the LSS process has some disadvantages, such as thermal
W. Han et al. / Journal of Alloys and Compounds 695 (2017) 2160e2164
treatment temperature higher than 180 C, long processing time, and expensive precursors [24]. The sol-gel process can produce high-purity and uniform particle sized powders at a low synthesis cost. Therefore, in this work, BaTiO3 nanoparticles were synthesized using a new LSS synthetic process at a low temperature of 80 C with refluxing in an air atmosphere instead of applying a hydrothermal method. Furthermore, the new LSS synthetic process from hydrothermal to refluxing enabled investigation of the fabrication procedure of BaTiO3 nanoparticles, including intermediate phase formation. The detailed results suggest that this process is a facile fabrication process for BaTiO3 nanoparticles resulting in high crystalline quality and minimized agglomeration. 2. Experimental procedure Barium nitrate (Ba(NO3)2, 99.999% trace metals basis, SigmaAldrich, USA); titanium (IV) butoxide (Ti(OCH2CH2CH2CH3)4, reagent grade, 97%, Sigma-Aldrich); n-butanol (99.5%, Duksan, South Korea); deionized (D.I.) water; and sodium hydroxide (NaOH, reagent grade, 98%, pellets (anhydrous), Sigma-Aldrich) were used as the three starting materials, solvent, and reacting agent, respectively. Oleic acid (CH3(CH2)7CH]CH(CH2)7COOH, technical grade, 90%, Sigma-Aldrich) was used as a capping agent without any purification. The experiments generally followed a recently proposed LSS process [20]. The Ba:Ti molar ratio of the precursor was kept constant at 1:1, and the concentration of the precursor was 0.2 mol/L. The concentration of the capping agent was 1.5 mol/ L. Initially, barium nitrate as a Ba precursor was dissolved in water in a round-bottom flask. Then, titanium (IV) butoxide as a Ti precursor and oleic acid as a capping agent were dissolved separately in n-butanol. NaOH as a catalyst was then dissolved in water and mixed with the Ti precursor solution. Oleic acid and barium precursor solution were sequentially added into the solution. NaOH stock solution was made using anhydrate NaOH pellets, and a highconcentration NaOH stock solution was easily obtained by cooling the beaker with flowing water. The mixing of solution created three immiscible layers where the top and bottom layers were clear and the middle layer was cloudy. Fig. 1 shows the reaction mechanism with a photograph of the immiscible layers observed in the roundbottom flask. The top layer is the Ti precursor and capping agent with butanol, and the bottom layer is the Ba precursor with H2O. The formed BaTiO3 nanoparticles are located in the middle cloudy layer. The solution was refluxed at 80 C for 2 h in an air atmosphere. Then, the residue in the solution was separated by a centrifuge at 20,000 rpm for 10 min. The separated particles were washed several times with distilled water and n-butanol to remove the remaining organic materials and impurities. Finally, the BaTiO3 particles were dried at 50 C for 12 h in an oven under an air atmosphere. In the synthesis, the basicity of the solution is the prime control parameter during the synthesis of the crystalline BaTiO3
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nanoparticles. Therefore, the concentration of NaOH was varied, and the reaction mechanism of the formation of crystalline BaTiO3 nanoparticles was studied at a low temperature. Fourier transform-infrared spectroscopy (FT-IR, PerkinElmer, USA) was employed for the confirmation of the capping agent and BaTiO3 nanoparticles in the precipitate and final product. The crystalline phases and chemical composition were evaluated by Xray diffraction (XRD, Ultima model, Rigaku, Japan) and energy dispersive X-ray spectroscopy (EDX, AMETECK, USA), respectively. For the XRD analysis, Cu Ka radiation was used at a 2q range of 20 e80 . Scanning electron microscopy (SEM, AIS-2000C, SERON, South Korea) was used to observe the morphology and size of the nanoparticles. After pelletizing, the currentevoltage and leakage current (4294A, Agilent, USA) behaviors were measured by a dielectric test fixture (16451B, Agilent) to analyze the dielectric properties. 3. Results and discussions Barium titanate (BaTiO3) nanoparticles were synthesized using a new LSS synthetic process by a refluxing method in an air atmosphere. The concentration of NaOH is known to be a significant factor during the formation of crystalline BaTiO3 [25]. Therefore, the concentration of NaOH was varied from 2.5 to 15 M, and the detailed reaction mechanism was studied. The new LSS synthetic process involves phase separation, and a detailed photograph and the reaction mechanism are shown in Fig. 1. After final addition of Ba(NO3)2 to the solution mixture, three separate layers were formed in the solution. As the top layer contains butanol and the bottom layer contains water, these two layers form clear solutions. The middle layer becomes cloudy because of the capping agent ligand attached with Na/Ti. The capping agent contains a long hydrocarbon chain, which enhances the non-polar nature of the capping agent. This intermediate complex was suspended between the butanol and water layers. This phase separation in the solution is a main aspect of the LSS process, in which further refluxing of the solution enhances the reaction rate of the nanoparticle formation. However, in the new LSS synthetic process, the main mechanism of BaTiO3 formation is passing through Na2TiO3, and the size dependence of the BaTiO3 nanoparticles on the NaOH concentration can be explained by the following reaction equations. Ti(OC4H9)4 þ 4H2O / Ti(OH)4 þ 4C4H9OH
(1)
2NaOH þ Ti(OH)4 / Na2TiO3 þ 3H2O
(2)
Ba(NO3)2 þ 2H2O / Ba(OH)2 þ 2HNO3
(3)
Na2TiO3 þ Ba(OH)2 / BaTiO3Y þ 2NaOH
(4)
Fig. 1. Schematic of the BaTiO3 nanoparticles phase transfer synthesis strategy.
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Titanium (IV) butoxide converted into butanol forms Ti(OH)4 by hydrolysis, and NaOH combines with Ti(OH)4 to form Na2TiO3. The hydroxylation of Ba(NO3)2 proceeded, and the formation of BaTiO3 proceeded with Na2TiO3 as chemical intermediate. Increasing the concentration of NaOH promotes the Na2TiO3 generation, which promotes the BaTiO3 nucleation. The rate of formation of BaTiO3 through Na2TiO3 increases with increasing NaOH concentration. Thus, the formation of crystalline BaTiO3 in the middle layer immediately settles down in the bottom layer, as shown in Fig. 1. To validate the generation of Na2TiO3 and the proposed reaction mechanism of the formation of BaTiO3 through the Na2TiO3 intermediate, an experiment was performed without the barium precursor under the same conditions. Fig. 2(a) shows the EDX spectra of the samples prepared using the Ti precursor and NaOH without the Ba precursor. The existence of Na, Ti, and O peaks and the atomic concentration data reveal the presence of sodium titanate. The atomic ratio of Na:Ti was 2:1, as observed in the EDX data shown in the inset of Fig. 2(a), which confirms the formation of the Na2TiO3 phase. The Pt peak was observed owing to the conductive surface coating in the SEM sample preparation. Therefore, the generation of Na2TiO3 was confirmed, and the proposed reaction mechanism was validated. To support this, the XRD spectra of the samples prepared without the Ba precursor were obtained with and without annealing treatment, as shown in Fig. 2(b). It is clear that before annealing, the sample has an amorphous state, but after annealing at 500 C, the sample is crystalline [26]. Therefore, this result also confirmed the formation of the chemical intermediate Na2TiO3 phase during the synthesis of BaTiO3 nanoparticles, according to the proposed reaction mechanism given in Equation (2). FT-IR measurements were carried out for the BaTiO3 nanoparticles prepared at various NaOH concentrations (2.5e15 M), and the
Fig. 3. FT-IR spectra of the precipitates of BaTiO3 nanoparticles prepared at NaOH concentrations of 2.5, 5, 7.5, 10, and 15 M.
results are shown in Fig. 3. The absorption peak around 2800e2900 cm1 was attributed to the CeH bond from oleic acid [27]. The peaks observed at 1500 and 1400 cm1 were attributed to the CH2 and CH3 bending modes, respectively. The absorption peak of the hydroxyl group was observed at 3400 cm1 [28]. The formation of BaTiO3 was confirmed by a detection of the TieO octahedral bond absorption peak at 495 cm1 [29,30]. This peak was observed only in the samples prepared with NaOH concentration above 7.5 M. For the NaOH concentrations of 2.5 and 5 M, sodium mostly reacts with the capping agent and is consumed according to Equation (5). Because of the low pH of the solution, the TieO octahedral bond may not be formed with NaOH concentrations of 2.5 and 5 M. CH3(CH2)7CH]CH(CH2)7COOH þ NaOH / CH3(CH2)7CH] CH(CH2)7COONa þ H2O
(5)
The crystalline phase of the BaTiO3 nanoparticles was analyzed using XRD. The diffraction patterns were obtained for BaTiO3 nanoparticles prepared with different NaOH concentrations, and the results are shown in Fig. 4. The existence of peaks for the different concentrations of NaOH reveals the formation of crystalline BaTiO3 synthesized by refluxing in an air atmosphere at a temperature of 80 C. It is clear that NaOH plays an important role
Fig. 2. (a) EDX spectrum of the Na2TiO3 phase shown with the SEM image in the inset, and the (b) XRD spectra of the Na2TiO3 sample prepared without the Ba precursor before and after annealing at 500 C.
Fig. 4. XRD spectra of the precipitates of BaTiO3 nanoparticles prepared at NaOH concentrations of 2.5, 5, 7.5, 10, and 15 M.
W. Han et al. / Journal of Alloys and Compounds 695 (2017) 2160e2164
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the major diffraction peak (110) in the BaTiO3 XRD spectra. The Scherrer equation is shown below.
D¼
Fig. 5. SEM images of BaTiO3 nanoparticles synthesized at NaOH concentrations of (a) 7.5, (b) 10, and (c) 15 M.
in the formation of the nanosized crystalline BaTiO3 phase. The samples obtained with NaOH concentrations of 7.5, 10, and 15 M were crystalline in nature, as observed in the XRD spectra. All of the diffraction peaks are indexed by comparing with the standard reference XRD pattern (JCPDS # 31-0174). The crystal structure of all of the BaTiO3 nanoparticles was cubic. A mixed phase of BaTiO3 and BaCO3 was obtained for the sample synthesized at a NaOH concentration of 7.5 M BaCO3 was formed during refluxing in an air atmosphere. The carbon dioxide from the atmosphere may react with Ba to form BaCO3. The samples obtained at NaOH concentrations of 2.5 and 5 M were amorphous and did not reveal formation of BaTiO3, as observed in the XRD spectra and FT-IR results. Because a low concentration of NaOH inhibits formation of the Ti octahedral, there is no possibility of the formation of crystalline BaTiO3. However, a pure BaTiO3 crystalline phase was obtained at high NaOH concentrations of 10 and 15 M at a low temperature by refluxing in an air atmosphere. The crystallite sizes of the BaTiO3 nanoparticles were calculated using the Scherrer equation [31] for
Kl bcos q
(6)
Here, b is the width of the observed diffraction line at its half intensity maximum, K is the shape factor set to 0.9, and l is the wavelength of Cu Ka. The crystallite sizes of the BaTiO3 powders obtained with NaOH concentrations of 7.5, 10, and 15 M were 34.3, 23.9, and 23.2 nm, respectively. Through refluxing in an air atmosphere at a low temperature, pure crystalline phase BaTiO3 nanoparticles (~23 nm) could be synthesized. As a result, the FT-IR and XRD results showed that BaTiO3 nanoparticles were formed with NaOH concentrations of 7.5, 10, and 15 M. The size and surface morphology of the crystalline BaTiO3 nanoparticles prepared with NaOH concentrations of 7.5, 10, and 15 M were observed by using SEM, and the results are shown in Fig. 5(a)e(c), respectively. Grapeshaped small nanoparticles were formed owing to the low temperature and hydroxyl ligand effects [32]. The fine nanoparticle distribution with high concentration of NaOH was induced by many nuclei produced by the chemical intermediates. A large amount of Na2TiO3 participates in the formation of many small clusters, which acted as nuclei for BaTiO3 nanoparticle formation. As a result, smaller nanoparticles were observed at NaOH concentrations of 10 and 15 M. This new LSS synthetic process could thus obtain a fine distribution of nanoparticles below 100 nm because the Na2TiO3 chemical intermediate with the Ti octahedron participates in the formation of BaTiO3. The dielectric constant and dielectric loss of the BaTiO3 nanoparticles synthesized at a NaOH concentration of 10 M were measured from 1 kHz to 1000 kHz, and the results are shown in Fig. 6. The dielectric constants of the BaTiO3 nanoparticles were found to be greater than 165 and slightly decreased with increasing NaOH concentration. This decreasing dielectric constant with decreasing nanoparticle size could be explained by the increase in the thickness of the boundary and surface charge compensation layer [33]. However, this dielectric constant was almost 2 times larger than the value reported by Huang et al. [34]. Furthermore, the dielectric loss was less than 0.035. From these results, it can be said that pure crystalline BaTiO3 nanoparticles with a high dielectric constant and low dielectric loss were synthesized using the modified LSS process with refluxing in an air atmosphere at 80 C.
Fig. 6. Dielectric properties of BaTiO3 nanoparticles synthesized with varying NaOH concentrations.
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4. Conclusion Pure crystalline BaTiO3 nanoparticles were synthesized at a low temperature of 80 C using a modified LSS process with refluxing in an air atmosphere. At various molar concentrations of NaOH, BaTiO3 nanoparticles were formed through Na2TiO3 as a chemical intermediate, where the concentration of NaOH plays an important role in controlling the size of the nanoparticles and phase formation. The most important factor for controlling the size of BaTiO3 nanoparticles was determined to be the NaOH concentration. The new LSS process with refluxing in an air atmosphere is simple, reliable, and cost effective for the synthesis of crystalline BaTiO3 nanoparticles at a low temperature and can be used for mass producing BaTiO3 for application in MLCCs. Acknowledgements This study was supported by a grant (#10041220) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A15054541). And this work was supported by LG Display under LGD-Yonsei University Incubation Program. References [1] P. Fiorenza, R.L. Nigro, P. Delugas, V. Raineri, A.G. Mould, D.C. Sinclair, Direct imaging of the core-shell effect in positive temperature coefficient of resistance-BaTiO3 ceramics, Appl. Phys. Lett. 95 (2009) 142904. [2] M. Wegmann, R. Bronnimann, F. Clemens, T. Graule, Barium titanate-based PTCR thermistor fibers: processing and properties, Sens. Actuat. A Phys. 135 (2) (2007) 394e404. [3] J. Wang, Q. Lin, R. Zhou, B. Xu, Humidity sensors based on composite material of nano-BaTiO3 and polymer RMX, Sens. Actuat. B Chem. 81 (2e3) (2002) 248e253. [4] Y. He, T. Zhang, W. Zheng, R. Wang, X. Liu, Y. Xia, J. Zhao, Humidity sensing properties of BaTiO3 nanofiber prepared via electrospinning, Sens. Actuat. B Chem. 146 (1) (2010) 98e102. [5] X. Wang, X.-N. Xu, H. Yamada, K. Nishikubo, X.-G. Zheng, Electro-mechanooptical conversions in Pr3þ-Doped BaTiO3eCaTiO3 ceramics, Adv. Mater. 17 (10) (2005) 1254e1258. [6] P. Tang, D.J. Towner, A.L. Meier, B.W. Wessels, Low-voltage, Polarizationinsensitive, electro-optic modulator based on a polydomain barium titanate thin film, Appl. Phys. Lett. 85 (2004) 4615. [7] C. Pithan, D. Hennings, R. Waser, Progress in the synthesis of nanocrystalline BaTiO3 powders for MLCC, Inter. J. Appl. Ceram. Tech. 2 (1) (2005) 1e14. [8] D.-H. Yoon, B.I. Lee, Processing of barium titanate tapes with different binders for MLCC applicationsdPart I: optimization using design of experiments, J. Eur. Ceram. Soc. 24 (5) (2004) 739e752. [9] Z. Tian, X. Wang, L. Shu, T. Wang, T.-H. Song, Z. Gui, L. Li, Preparation of nano BaTiO3-based ceramics for multilayer ceramic capacitor application by chemical coating method, J. Am. Ceram. Soc. 92 (4) (2009) 830e833. [10] Z. Tian, X. Wang, H. Gon, T.-H. Song, K.H. Hur, L. Li, CoreeShell structure in nanocrystalline modified BaTiO3 dielectric ceramics prepared by different sintering methods, J. Am. Ceram. Soc. 94 (4) (2011) 973e977. [11] S. Shao, J. Zhang, Z. Zhang, P. Zheng, M. Zhao, J. Li, C. Wang, High piezoelectric properties and domain configuration in BaTiO3 ceramics obtained through the
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Evaluation of Na2TiO3 formation for producing crystalline BaTiO3 nanoparticles by liquidesolidesolution process at low temperature Wooje Han a, Hong-Sub Lee a, Byungwook Yoo b, Hyung-Ho Park a, * a b
Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, Republic of Korea Display Materials & Components Research Center, Korea Electronics Technology Institute, Seongnam-si, Gyeonggi-do, 13509, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2016 Received in revised form 3 November 2016 Accepted 4 November 2016 Available online 5 November 2016
Crystalline barium titanate (BaTiO3) nanoparticles were synthesized by refluxing in an air atmosphere at a low temperature using sodium hydroxide. The reaction mechanism of BaTiO3 nanoparticle formation by this new liquidesolidesolution (LSS) synthesis route with refluxing in an air atmosphere at a low temperature of 80 C was demonstrated to be a facile fabrication process for BaTiO3 nanoparticles with a high crystalline quality and minimized agglomeration rate. BaTiO3 nanoparticles were formed via sodium titanate (Na2TiO3), and the size of the BaTiO3 nanoparticles decreased with increasing sodium hydroxide concentration. The formation of a Na2TiO3 intermediate phase was confirmed by energy dispersive X-ray spectroscopy and X-ray diffraction analysis. The capping ligands of BaTiO3 suggest that hydroxyl and oleic acid are present. The dielectric constant of the crystalline BaTiO3 nanoparticles was higher than 150. © 2016 Elsevier B.V. All rights reserved.
Keywords: BaTiO3 nanoparticles Liquidesolidesolution process Na2TiO3 NaOH reaction mechanism
1. Introduction Barium titanate (BaTiO3) has a typical perovskite structure and has been one of the most actively promoted among ferroelectric materials because of its high dielectric constant. It is widely used in thermistors, sensors, electro-optical devices, and multi-layer ceramic capacitors (MLCCs) as a global electronic ceramic [1e9]. Owing to recent developments of these applications, the demand for thin layer materials with a high dielectric constant has increased for the miniaturization and performance enhancement of the devices [10]. In these applications, the device performance is strongly influenced by the size, shape, and composition of the BaTiO3 nanoparticles. Thus, the facile synthesis of nanosized BaTiO3 without impurities is of prime importance and is a challenging task for researchers and the industry. BaTiO3 nanoparticles are generally prepared by a solid-state reaction method by heating BaCO3 and TiO2 at temperatures as high of 1200 C, but calcination at such high temperatures results in agglomerations of particles [11]. Various other preparation methods have also been reported for the preparation of BaTiO3 nanoparticles with high quality, such as the solgel method [12e14], hydrothermal method [15,16], ball-milling method [17,18], glycolthermal method [19], liquidesolidesolution
* Corresponding author. E-mail address: [email protected] (H.-H. Park). http://dx.doi.org/10.1016/j.jallcom.2016.11.062 0925-8388/© 2016 Elsevier B.V. All rights reserved.
process (LSS) [20], and co-precipitation method [21]. Wet chemical methods are very useful for obtaining nanosized BaTiO3 because of the control over the agglomeration of particles and the low temperature involved in the synthesis. Among such methods, hydrothermal and sol-gel processes have been extensively studied for synthesis of BaTiO3 nanoparticles. The sol-gel process has received considerable attention because of the relative ease, reliability, and homogeneity of the synthesis and processing of nanomaterials with high purity. The use of metal alkoxides as a precursor in the sol-gel process provides more benefits compared to the solid-state reaction [12e14] and hydrothermal process because it enables control of the structural composition at the molecular level [4]. However, during the sol-gel process, non-crystalline BaTiO3 is also obtained at room temperature, and annealing treatment converts it to crystalline BaTiO3 [22]. The heat treatment leads to agglomeration of the particles, which increases the size of the particles. Although the conventional hydrothermal method enables direct synthesis of spherical BaTiO3 with small powder particles (<0.1 mm) at a low reaction temperature, it requires relatively harsh reaction conditions, and therefore, special and expensive precursors are needed that are capable of withstanding high pH levels, temperature, and pressures [15,16,23]. Wang et al. proposed the LSS process followed by the hydrothermal method in order to synthesize nanoparticles of various noble metals and metal oxides while minimizing particle agglomeration. However, because of the hydrothermal method involved, the LSS process has some disadvantages, such as thermal
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treatment temperature higher than 180 C, long processing time, and expensive precursors [24]. The sol-gel process can produce high-purity and uniform particle sized powders at a low synthesis cost. Therefore, in this work, BaTiO3 nanoparticles were synthesized using a new LSS synthetic process at a low temperature of 80 C with refluxing in an air atmosphere instead of applying a hydrothermal method. Furthermore, the new LSS synthetic process from hydrothermal to refluxing enabled investigation of the fabrication procedure of BaTiO3 nanoparticles, including intermediate phase formation. The detailed results suggest that this process is a facile fabrication process for BaTiO3 nanoparticles resulting in high crystalline quality and minimized agglomeration. 2. Experimental procedure Barium nitrate (Ba(NO3)2, 99.999% trace metals basis, SigmaAldrich, USA); titanium (IV) butoxide (Ti(OCH2CH2CH2CH3)4, reagent grade, 97%, Sigma-Aldrich); n-butanol (99.5%, Duksan, South Korea); deionized (D.I.) water; and sodium hydroxide (NaOH, reagent grade, 98%, pellets (anhydrous), Sigma-Aldrich) were used as the three starting materials, solvent, and reacting agent, respectively. Oleic acid (CH3(CH2)7CH]CH(CH2)7COOH, technical grade, 90%, Sigma-Aldrich) was used as a capping agent without any purification. The experiments generally followed a recently proposed LSS process [20]. The Ba:Ti molar ratio of the precursor was kept constant at 1:1, and the concentration of the precursor was 0.2 mol/L. The concentration of the capping agent was 1.5 mol/ L. Initially, barium nitrate as a Ba precursor was dissolved in water in a round-bottom flask. Then, titanium (IV) butoxide as a Ti precursor and oleic acid as a capping agent were dissolved separately in n-butanol. NaOH as a catalyst was then dissolved in water and mixed with the Ti precursor solution. Oleic acid and barium precursor solution were sequentially added into the solution. NaOH stock solution was made using anhydrate NaOH pellets, and a highconcentration NaOH stock solution was easily obtained by cooling the beaker with flowing water. The mixing of solution created three immiscible layers where the top and bottom layers were clear and the middle layer was cloudy. Fig. 1 shows the reaction mechanism with a photograph of the immiscible layers observed in the roundbottom flask. The top layer is the Ti precursor and capping agent with butanol, and the bottom layer is the Ba precursor with H2O. The formed BaTiO3 nanoparticles are located in the middle cloudy layer. The solution was refluxed at 80 C for 2 h in an air atmosphere. Then, the residue in the solution was separated by a centrifuge at 20,000 rpm for 10 min. The separated particles were washed several times with distilled water and n-butanol to remove the remaining organic materials and impurities. Finally, the BaTiO3 particles were dried at 50 C for 12 h in an oven under an air atmosphere. In the synthesis, the basicity of the solution is the prime control parameter during the synthesis of the crystalline BaTiO3
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nanoparticles. Therefore, the concentration of NaOH was varied, and the reaction mechanism of the formation of crystalline BaTiO3 nanoparticles was studied at a low temperature. Fourier transform-infrared spectroscopy (FT-IR, PerkinElmer, USA) was employed for the confirmation of the capping agent and BaTiO3 nanoparticles in the precipitate and final product. The crystalline phases and chemical composition were evaluated by Xray diffraction (XRD, Ultima model, Rigaku, Japan) and energy dispersive X-ray spectroscopy (EDX, AMETECK, USA), respectively. For the XRD analysis, Cu Ka radiation was used at a 2q range of 20 e80 . Scanning electron microscopy (SEM, AIS-2000C, SERON, South Korea) was used to observe the morphology and size of the nanoparticles. After pelletizing, the currentevoltage and leakage current (4294A, Agilent, USA) behaviors were measured by a dielectric test fixture (16451B, Agilent) to analyze the dielectric properties. 3. Results and discussions Barium titanate (BaTiO3) nanoparticles were synthesized using a new LSS synthetic process by a refluxing method in an air atmosphere. The concentration of NaOH is known to be a significant factor during the formation of crystalline BaTiO3 [25]. Therefore, the concentration of NaOH was varied from 2.5 to 15 M, and the detailed reaction mechanism was studied. The new LSS synthetic process involves phase separation, and a detailed photograph and the reaction mechanism are shown in Fig. 1. After final addition of Ba(NO3)2 to the solution mixture, three separate layers were formed in the solution. As the top layer contains butanol and the bottom layer contains water, these two layers form clear solutions. The middle layer becomes cloudy because of the capping agent ligand attached with Na/Ti. The capping agent contains a long hydrocarbon chain, which enhances the non-polar nature of the capping agent. This intermediate complex was suspended between the butanol and water layers. This phase separation in the solution is a main aspect of the LSS process, in which further refluxing of the solution enhances the reaction rate of the nanoparticle formation. However, in the new LSS synthetic process, the main mechanism of BaTiO3 formation is passing through Na2TiO3, and the size dependence of the BaTiO3 nanoparticles on the NaOH concentration can be explained by the following reaction equations. Ti(OC4H9)4 þ 4H2O / Ti(OH)4 þ 4C4H9OH
(1)
2NaOH þ Ti(OH)4 / Na2TiO3 þ 3H2O
(2)
Ba(NO3)2 þ 2H2O / Ba(OH)2 þ 2HNO3
(3)
Na2TiO3 þ Ba(OH)2 / BaTiO3Y þ 2NaOH
(4)
Fig. 1. Schematic of the BaTiO3 nanoparticles phase transfer synthesis strategy.
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Titanium (IV) butoxide converted into butanol forms Ti(OH)4 by hydrolysis, and NaOH combines with Ti(OH)4 to form Na2TiO3. The hydroxylation of Ba(NO3)2 proceeded, and the formation of BaTiO3 proceeded with Na2TiO3 as chemical intermediate. Increasing the concentration of NaOH promotes the Na2TiO3 generation, which promotes the BaTiO3 nucleation. The rate of formation of BaTiO3 through Na2TiO3 increases with increasing NaOH concentration. Thus, the formation of crystalline BaTiO3 in the middle layer immediately settles down in the bottom layer, as shown in Fig. 1. To validate the generation of Na2TiO3 and the proposed reaction mechanism of the formation of BaTiO3 through the Na2TiO3 intermediate, an experiment was performed without the barium precursor under the same conditions. Fig. 2(a) shows the EDX spectra of the samples prepared using the Ti precursor and NaOH without the Ba precursor. The existence of Na, Ti, and O peaks and the atomic concentration data reveal the presence of sodium titanate. The atomic ratio of Na:Ti was 2:1, as observed in the EDX data shown in the inset of Fig. 2(a), which confirms the formation of the Na2TiO3 phase. The Pt peak was observed owing to the conductive surface coating in the SEM sample preparation. Therefore, the generation of Na2TiO3 was confirmed, and the proposed reaction mechanism was validated. To support this, the XRD spectra of the samples prepared without the Ba precursor were obtained with and without annealing treatment, as shown in Fig. 2(b). It is clear that before annealing, the sample has an amorphous state, but after annealing at 500 C, the sample is crystalline [26]. Therefore, this result also confirmed the formation of the chemical intermediate Na2TiO3 phase during the synthesis of BaTiO3 nanoparticles, according to the proposed reaction mechanism given in Equation (2). FT-IR measurements were carried out for the BaTiO3 nanoparticles prepared at various NaOH concentrations (2.5e15 M), and the
Fig. 3. FT-IR spectra of the precipitates of BaTiO3 nanoparticles prepared at NaOH concentrations of 2.5, 5, 7.5, 10, and 15 M.
results are shown in Fig. 3. The absorption peak around 2800e2900 cm1 was attributed to the CeH bond from oleic acid [27]. The peaks observed at 1500 and 1400 cm1 were attributed to the CH2 and CH3 bending modes, respectively. The absorption peak of the hydroxyl group was observed at 3400 cm1 [28]. The formation of BaTiO3 was confirmed by a detection of the TieO octahedral bond absorption peak at 495 cm1 [29,30]. This peak was observed only in the samples prepared with NaOH concentration above 7.5 M. For the NaOH concentrations of 2.5 and 5 M, sodium mostly reacts with the capping agent and is consumed according to Equation (5). Because of the low pH of the solution, the TieO octahedral bond may not be formed with NaOH concentrations of 2.5 and 5 M. CH3(CH2)7CH]CH(CH2)7COOH þ NaOH / CH3(CH2)7CH] CH(CH2)7COONa þ H2O
(5)
The crystalline phase of the BaTiO3 nanoparticles was analyzed using XRD. The diffraction patterns were obtained for BaTiO3 nanoparticles prepared with different NaOH concentrations, and the results are shown in Fig. 4. The existence of peaks for the different concentrations of NaOH reveals the formation of crystalline BaTiO3 synthesized by refluxing in an air atmosphere at a temperature of 80 C. It is clear that NaOH plays an important role
Fig. 2. (a) EDX spectrum of the Na2TiO3 phase shown with the SEM image in the inset, and the (b) XRD spectra of the Na2TiO3 sample prepared without the Ba precursor before and after annealing at 500 C.
Fig. 4. XRD spectra of the precipitates of BaTiO3 nanoparticles prepared at NaOH concentrations of 2.5, 5, 7.5, 10, and 15 M.
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the major diffraction peak (110) in the BaTiO3 XRD spectra. The Scherrer equation is shown below.
D¼
Fig. 5. SEM images of BaTiO3 nanoparticles synthesized at NaOH concentrations of (a) 7.5, (b) 10, and (c) 15 M.
in the formation of the nanosized crystalline BaTiO3 phase. The samples obtained with NaOH concentrations of 7.5, 10, and 15 M were crystalline in nature, as observed in the XRD spectra. All of the diffraction peaks are indexed by comparing with the standard reference XRD pattern (JCPDS # 31-0174). The crystal structure of all of the BaTiO3 nanoparticles was cubic. A mixed phase of BaTiO3 and BaCO3 was obtained for the sample synthesized at a NaOH concentration of 7.5 M BaCO3 was formed during refluxing in an air atmosphere. The carbon dioxide from the atmosphere may react with Ba to form BaCO3. The samples obtained at NaOH concentrations of 2.5 and 5 M were amorphous and did not reveal formation of BaTiO3, as observed in the XRD spectra and FT-IR results. Because a low concentration of NaOH inhibits formation of the Ti octahedral, there is no possibility of the formation of crystalline BaTiO3. However, a pure BaTiO3 crystalline phase was obtained at high NaOH concentrations of 10 and 15 M at a low temperature by refluxing in an air atmosphere. The crystallite sizes of the BaTiO3 nanoparticles were calculated using the Scherrer equation [31] for
Kl bcos q
(6)
Here, b is the width of the observed diffraction line at its half intensity maximum, K is the shape factor set to 0.9, and l is the wavelength of Cu Ka. The crystallite sizes of the BaTiO3 powders obtained with NaOH concentrations of 7.5, 10, and 15 M were 34.3, 23.9, and 23.2 nm, respectively. Through refluxing in an air atmosphere at a low temperature, pure crystalline phase BaTiO3 nanoparticles (~23 nm) could be synthesized. As a result, the FT-IR and XRD results showed that BaTiO3 nanoparticles were formed with NaOH concentrations of 7.5, 10, and 15 M. The size and surface morphology of the crystalline BaTiO3 nanoparticles prepared with NaOH concentrations of 7.5, 10, and 15 M were observed by using SEM, and the results are shown in Fig. 5(a)e(c), respectively. Grapeshaped small nanoparticles were formed owing to the low temperature and hydroxyl ligand effects [32]. The fine nanoparticle distribution with high concentration of NaOH was induced by many nuclei produced by the chemical intermediates. A large amount of Na2TiO3 participates in the formation of many small clusters, which acted as nuclei for BaTiO3 nanoparticle formation. As a result, smaller nanoparticles were observed at NaOH concentrations of 10 and 15 M. This new LSS synthetic process could thus obtain a fine distribution of nanoparticles below 100 nm because the Na2TiO3 chemical intermediate with the Ti octahedron participates in the formation of BaTiO3. The dielectric constant and dielectric loss of the BaTiO3 nanoparticles synthesized at a NaOH concentration of 10 M were measured from 1 kHz to 1000 kHz, and the results are shown in Fig. 6. The dielectric constants of the BaTiO3 nanoparticles were found to be greater than 165 and slightly decreased with increasing NaOH concentration. This decreasing dielectric constant with decreasing nanoparticle size could be explained by the increase in the thickness of the boundary and surface charge compensation layer [33]. However, this dielectric constant was almost 2 times larger than the value reported by Huang et al. [34]. Furthermore, the dielectric loss was less than 0.035. From these results, it can be said that pure crystalline BaTiO3 nanoparticles with a high dielectric constant and low dielectric loss were synthesized using the modified LSS process with refluxing in an air atmosphere at 80 C.
Fig. 6. Dielectric properties of BaTiO3 nanoparticles synthesized with varying NaOH concentrations.
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4. Conclusion Pure crystalline BaTiO3 nanoparticles were synthesized at a low temperature of 80 C using a modified LSS process with refluxing in an air atmosphere. At various molar concentrations of NaOH, BaTiO3 nanoparticles were formed through Na2TiO3 as a chemical intermediate, where the concentration of NaOH plays an important role in controlling the size of the nanoparticles and phase formation. The most important factor for controlling the size of BaTiO3 nanoparticles was determined to be the NaOH concentration. The new LSS process with refluxing in an air atmosphere is simple, reliable, and cost effective for the synthesis of crystalline BaTiO3 nanoparticles at a low temperature and can be used for mass producing BaTiO3 for application in MLCCs. Acknowledgements This study was supported by a grant (#10041220) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A15054541). And this work was supported by LG Display under LGD-Yonsei University Incubation Program. References [1] P. Fiorenza, R.L. Nigro, P. Delugas, V. Raineri, A.G. Mould, D.C. Sinclair, Direct imaging of the core-shell effect in positive temperature coefficient of resistance-BaTiO3 ceramics, Appl. Phys. Lett. 95 (2009) 142904. [2] M. Wegmann, R. Bronnimann, F. Clemens, T. Graule, Barium titanate-based PTCR thermistor fibers: processing and properties, Sens. Actuat. A Phys. 135 (2) (2007) 394e404. [3] J. Wang, Q. Lin, R. Zhou, B. Xu, Humidity sensors based on composite material of nano-BaTiO3 and polymer RMX, Sens. Actuat. B Chem. 81 (2e3) (2002) 248e253. [4] Y. He, T. Zhang, W. Zheng, R. Wang, X. Liu, Y. Xia, J. Zhao, Humidity sensing properties of BaTiO3 nanofiber prepared via electrospinning, Sens. Actuat. B Chem. 146 (1) (2010) 98e102. [5] X. Wang, X.-N. Xu, H. Yamada, K. Nishikubo, X.-G. Zheng, Electro-mechanooptical conversions in Pr3þ-Doped BaTiO3eCaTiO3 ceramics, Adv. Mater. 17 (10) (2005) 1254e1258. [6] P. Tang, D.J. Towner, A.L. Meier, B.W. Wessels, Low-voltage, Polarizationinsensitive, electro-optic modulator based on a polydomain barium titanate thin film, Appl. Phys. Lett. 85 (2004) 4615. [7] C. Pithan, D. Hennings, R. Waser, Progress in the synthesis of nanocrystalline BaTiO3 powders for MLCC, Inter. J. Appl. Ceram. Tech. 2 (1) (2005) 1e14. [8] D.-H. Yoon, B.I. Lee, Processing of barium titanate tapes with different binders for MLCC applicationsdPart I: optimization using design of experiments, J. Eur. Ceram. Soc. 24 (5) (2004) 739e752. [9] Z. Tian, X. Wang, L. Shu, T. Wang, T.-H. Song, Z. Gui, L. Li, Preparation of nano BaTiO3-based ceramics for multilayer ceramic capacitor application by chemical coating method, J. Am. Ceram. Soc. 92 (4) (2009) 830e833. [10] Z. Tian, X. Wang, H. Gon, T.-H. Song, K.H. Hur, L. Li, CoreeShell structure in nanocrystalline modified BaTiO3 dielectric ceramics prepared by different sintering methods, J. Am. Ceram. Soc. 94 (4) (2011) 973e977. [11] S. Shao, J. Zhang, Z. Zhang, P. Zheng, M. Zhao, J. Li, C. Wang, High piezoelectric properties and domain configuration in BaTiO3 ceramics obtained through the
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