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A simple and controllable hydrothermal route for the synthesis of monodispersed cube-like barium titanate nanocrystals
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 4514–4522 www.elsevier.com/locate/ceramint
A simple and controllable hydrothermal route for the synthesis of monodispersed cube-like barium titanate nanocrystals Wen Caia,b,d,n, Tingke Raob, Aiwu Wangb, Jie Huc, Junqing Wangd, Jiasong Zhonga, Weidong Xianga b
a School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region c Department of Electrical Engineering, KU Leuven, 3001 Leuven, Belgium d State Key Laboratory of Molecular Vaccinology and Molecular Diagnostics & Center for Molecular Imaging and Translational Medicine, School of Public Health, Xiamen University, Xiamen 361102, PR China
Received 8 November 2014; received in revised form 26 November 2014; accepted 27 November 2014 Available online 5 December 2014
Abstract Monodispersed BaTiO3 (barium titanate) nanocrystals with well-controlled size have been successfully prepared via a simple hydrothermal route. The phase structure, morphology and surface composition of the BaTiO3 nanocrystals are characterized by means of XRD, TEM, FTIR and TGA. Results demonstrate that the synthesized products consist of cube-like nanocrystals with a size of 5–15 nm, which can be well-controlled by tuning the Ba/Ti molar ratio in the starting materials, and that the amount of the organic capping agents on the surface of BaTiO3 nanocrystals is about 32 wt%. The present simple synthesis route would provide a general strategy for the preparation of other perovskite-type nanocrystals. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Surfaces; D. BaTiO3 and titanates; D. Perovskites; E. Capacitors
1. Introduction Over the past decades, the synthesis of perovskite-type metal oxide nanocrystals has attracted much attention due to their widespread applications in electronics, sensing, catalysis, and nonlinear optics [1–4]. BaTiO3, as one kind of perovskite-type metal oxides, shows a variety of potential applications in multilayered capacitors, random access memories, thermistors, pressure transducers, and waveguide modulators due to its excellent ferroelectric, pyroelectric, piezoelectric, electro-optic, dielectric, and elastic properties [5,6]. Currently, the BaTiO3 products applied in industry are commonly prepared by the catecholate process at a calcination temperature as high as 1000–1200 1C. To avoid the formation of severe agglomerates and obtain the final n Corresponding author at: School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China. E-mail address: [email protected] (W. Cai).
http://dx.doi.org/10.1016/j.ceramint.2014.11.146 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
product with high quality, the synthesis of re-dispersible BaTiO3 nanocrystals can make a great contribution. On the other hand, it is well-known that the structural and physical properties of perovskite nanocrystals are strongly dependent on their size, shape, crystallinity, and surface composition. Therefore synthesis of monodispersed BaTiO3 nanocrystals with well-defined shape, controllable size and high degree of compositional homogeneity is very important for the investigations of their chemical and physical properties. Although many efforts have been devoted to the fabrication of perovskites nanostructures with various morphologies, it is still a great challenge for researchers to prepare BaTiO3 nanostructured materials with well-controlled size and shape [7–9]. For instance, Hou et al. prepared nanoporous BaTiO3 crystallites [10]. Adireddy et al. [11], Zheng et al. [12] and Wang et al. [13] synthesized monodispersed cubelike BaTiO3 colloidal nanocrystals via a solution-based route. Hou et al. also prepared single-crystalline barium strontium titanate nanocubes by a solvothermal method [14]. Deng et al.
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obtained bowl-like single-crystalline BaTiO3 nanoparticles [15]. Chen et al. fabricated SrTiO3 and BaTiO3 nanotubes via a template-based route [16]. Urban et al. prepared single-crystalline perovskite nanorods composed of barium titanate and strontium titanate [17]. Urban et al. [18], Im et al. [19], Joshi et al. [20] and Yang et al. [21] synthesized barium titanate nanowire. Yang et al. synthesized single-crystal Ba1 xSrxTiO3 (x¼ 0–1) dendrites via a simple hydrothermal method [22]. Zhang et al. synthesized single-crystalline barium tetratitanate nanobelts via a selfsacrificing template process [23]. Yu et al. synthesized singlecrystalline barium dititanate nanobelts [24]. Jitputti et al. prepared SrTiO3 nanotube arrays by a hydrothermal route [25]. Among the above solution methods, hydrothermal route, as a relatively simple and cost-effective approach to control the size and shape of the products, has been widely used to fabricate metal oxide nanocrystals with high crystallinity and good dispersion properties at much lower temperatures. Here, we report a simple and well-controlled hydrothermal approach for the synthesis of freestanding and cube-like BaTiO3 nanocrystals with average length edges ranging from 5 to 15 nm by tuning Ba/Ti molar ratio in the starting materials. TEM images reveal that the synthesized BaTiO3 nanocrystals capped with oleic acid molecules are wellseparated and have the tendency to self-assemble into ordered planar structures. FTIR and TGA show that the prepared nanocrystals retain oleic acid molecules on their surfaces and can easily redisperse in organic solvents. The small size and organic capping agents offer great potential of using the BaTiO3 nanocrystals in high energy density nanocomposite capacitors. 2. Experimental procedures
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KOH in DI water + Ba(NO3)2 in DI water
Oleic acid in BuOH + Ti(O-Bu)4 in BuOH
Mixture Solution
Autoclave, 135 oC for 16 hrs
Washing and separation
BT nanocrystal
Fig. 1. Schematic flow-chart of hydrothermal synthesis of BaTiO3 nanocrystals.
components within the supernatant solution was performed by centrifugation. The precipitate was then collected, purified and finally dispersed in toluene yielding a stable milky colloidal solution. The yield of the products was about 90%. Fig. 1 shows the schematic flow-chart for the hydrothermal synthesis of BaTiO3 nanocrystals. The molar ratio of Ba/Ti in the starting materials was tuned from 1:0.70 to 1:1.06 to investigate its influence on the size of BaTiO3 nanocrystals. For the synthesis of (Ba, Sr)TiO3 nanocrystals, the molar ratio of Ba/Sr in the staring materials was set as 0.5 mmol/ 0.5 mmol. For the synthesis of SrTiO3 nanocrystals, the amount of Sr(NO3)2 in the starting materials was 1 mmol.
2.1. Chemicals Barium nitrate (Ba(NO3)2, 99%), strontium nitrate (Sr (NO3)2, 99%), potassium hydroxide (KOH, 90%), 1-butanol anhydrous (C4H10O (BuOH), 99.8%), titanium (IV) n-butoxide (C16H36O4Ti (Ti(O–Bu)4), 97%), oleic acid (C18H34O2, 99%), oleylamine (C18H37N, 70%) were all of analytical grade and purchased from Aldrich. All of them were used as received without further purification. 2.2. Synthesis of BaTiO3, SrTiO3 and (Ba, Sr)TiO3 nanocrystals The synthetic method is similar to that described in Ref. [11] with improvement by adjusting the molar ratio of Ba/Ti in the staring materials to control the size of the products. In a typical procedure for the preparation of BaTiO3 nanocrystals, different solutions containing 1 mmol of Ba(NO3)2 dissolved in 5 mL of deionized water, 11 mmol of KOH dissolved in 5 mL of deionized water, 0.80 mmol of Ti(O–Bu)4 dissolved in 5 mL of BuOH and 2.5 mL of oleic acid in 5 mL of BuOH were mixed together and the resulting solution was then transferred to a 23 mL of Teflon-lined stainless steel autoclave. The autoclave was sealed and then heated to 135 1C for 16 h. The reaction mixture was cooled down to room temperature after the completion of the reaction. The separation of
2.3. Characterization and measurements The crystal structure and phase composition of the asobtained white nanopowders were identified and analyzed by using X-ray powder diffraction technique. X-ray powder diffraction (XRD) was performed on a Panalytical X'Pert Pro diffractometer (40 kV, 40 mA) by using Cu Kα radiation (λ¼ 0.15405 nm) at a scanning rate of 0.021/s. The nanopowder samples for XRD characterization were prepared by drying the toluene dispersion of BaTiO3 nanocrystals at room temperature. The size, morphology and microstructure of the nanocrystals were investigated by transmission electron microscopy (TEM). TEM images were recorded on a Philips CM120 microscope (120 kV). For TEM observations, the toluene dispersion of BaTiO3 nanocrystals was dropped onto a carbon-coated copper-grid. Fourier transform infrared spectroscopy (FTIR, JASCO FT/IR-420) was used to analyze the attachment of capping agents onto the surface of BaTiO3 nanocrystals by preparing sample with the KBr-pellet method. All the above measurements were carried out at room temperature. Thermogravimetry analysis of BaTiO3 nanocrystals was conducted by Pyris Diamond TG/DTA (Perkin-Elmer) with temperature range of 25–800 1C at a heating rate of 10 1C/min in air-flow (30 ml/min).
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(200)
JCPDS NO 31-174
(110)
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Intensity(a.u.)
(220)
(211)
(210)
(200)
(111)
(100)
Intensity(a.u.)
(a) BT, Ti/Ba=0.7
(a) BT, Ti/Ba=0.7 (b) BT, Ti/Ba=0.8
(b) BT, Ti/Ba=0.8
(c) BT, Ti/Ba=0.85 (d) BST
(c) BT, Ti/Ba =0.85 (d) BST
(e) ST
(e) ST
10
20
30
40
50
60
70
40.7 41.8 42.9 44.0 45.1 46.2 47.3 48.4 49.5
2Theta(degree)
2Theta(degree)
O Ti Ba
Counts (a.u.)
Ti Ba
Ba Ti Ba Ba 0
1
2
3
4
5
6
7
8
9
10
Energy(Kev)
Fig. 2. (A) XRD patterns of the BaTiO3 (BT) (a–c), (Ba, Sr)TiO3 (BST) and SrTiO3 (ST) nanocrystals synthesized via a hydrothermal route at 130 1C for 16 h. (B) XRD patterns in the 2θ range of 43–481 of BT, BST and ST nanocrystals prepared by a hydrothermal method at 135 1C for 16 h. The Ba/Ti molar ratio used in the starting materials is 1:0.70 (a), 1:0.80 (b), and 1:0.85 (c). (C) EDS spectrum of the BaTiO3 nanocrystals.
3. Results and discussion Fig. 2A shows the XRD patterns of the as-prepared BaTiO3, SrTiO3 and (Ba, Sr)TiO3 nanocrystals. All the diffraction peaks in Fig. 2A-(a–c) correspond to the crystal planes of BaTiO3 (JCPDS no. 31-174). It can be seen from Fig. 2A-(b) that very little carbonate byproducts, which can be removed by an acid washing process, are present in the final products. The XRD patterns of SrTiO3 and (Ba, Sr)TiO3 nanocrystals are shown in Fig. 2A-(d and e). The strong and sharp peaks suggest that SrTiO3 and (Ba, Sr)TiO3 nanocrystals are highly crystalline. With the increase of the amount of Sr, the reflection positions shift to higher diffraction angles, owing to the smaller lattice constants caused by the substitution of the larger Ba ions by the smaller Sr ions. Furthermore, it can be seen from Fig. 2B that there is a peak with weak spilling at 451 corresponding to the (200) plane in the perovskite structure, indicating that the product shows tetragonal phase, which is consistent with the result described in Ref. [11]. Fig. 2C gives an EDS diagram of the BaTiO3 nanocrystals, it can be seen that there are only Ba, Ti, and O elements in the product without any other metal element, which indicates the highpurity of the product.
Fig. 3 illustrates the representative transmission electron micrographs (TEM) images of the barium titanate samples. Overview images (Fig. 3a, c, e, g, and i) at low magnification show that the sample entirely consists of nano-sized monodispersed barium titanate crystals without the presence of large crystals or agglomerates, and that the size of barium titanate nanocrystals is smaller than those reported in almost all the references. The high-magnification images (Fig. 3b, d, f, h, and j) reveal that the nanocrystals have cube-like morphology, and that the size of the nanocrystals is in the range of 5–15 nm. The influence of the molar ratio of Ba/Ti on the size of the BaTiO3 product was investigated by adjusting it from 1:0.70 to 1:1.06 in the starting materials, and it was found that the products with a small size of 5–8 nm (calculated from XRD by the Scherrer equation) had been successfully obtained with the ratio of 1:0.80, as shown in Fig. 3(c) and (d). However, the size of the products increased when this ratio was greater or less than 1:0.80. The effects of KOH, oleylamine, and reaction temperature and time on the size of products were also investigated when the amount of Ti(O–Bu)4 in the starting materials was kept as 0.8 mmol. It can be seen from Fig. 4(a and b) that the size of the products increased when the amount of KOH was more
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Fig. 3. TEM images of the BaTiO3 nanocrystals synthesized via a hydrothermal route at 130 1C for 16 h by using different Ba/Ti molar ratio in the starting materials: (a , b) Ba/Ti¼ 1:0.70 (BT 2); (c, d) Ba/Ti ¼1:0.80 (BT 3); (e, f) Ba/Ti ¼1:0.85 (BT 4); (g, h) Ba/Ti ¼1:1.0(BT 1); (i, j) Ba/Ti¼ 1:1.06 (BT 8).
than 11 mmol. However, the products appeared to be amorphous when the amount of KOH was less than 11 mmol, which had been explained in detail by Riman group [26]. From Fig. 4(c–f), it can be seen that the size of the products increased when the reaction temperature was elevated and the reaction time was elongated due to the higher crystal growth rate and longer crystal growth time. It was also found
that the addition of oleylamine facilitated the formation of products with a small size of 8 nm (Fig.4g and h). We also obtained the (Ba, Sr)TiO3 and SrTiO3 nanocrystals under the current hydrothermal conditions (130 1C for 16 h, and 0.8 mmol of Ti(O–Bu)4 was used). From their TEM images (Fig. 5), it can be seen that SrTiO3 nanocrystals take on nanosphere morphology with serious aggregation, and that the
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Fig. 3. (continued)
size of (Ba, Sr)TiO3 and SrTiO3 nanocrystals is about 15 nm and 4 nm, respectively. The influences of the above reaction conditions on the size of BaTiO3 product could be summarized in Table 1. Fourier transform infrared spectroscopy (FTIR) was performed to investigate the attachment of oleic acid molecules onto the surface of BaTiO3 nanocrystals. BaTiO3 nanocrystals capped with oleic acid molecules were washed repeatedly by using toluene to remove the physisorbed oleic acid molecules until a constant amount of oleic acid molecules was obtained before FTIR analysis. The FTIR spectra of oleic acid molecules and BaTiO3 sample capped with oleic acid were recorded as illustrated in Fig. 6A. The FTIR spectra of BaTiO3 sample capped with oleic acid are very similar to that of oleic acid molecules. The vibration bands centered at 2917 cm 1 and 2849 cm 1, corresponding to the CH2 asymmetric and CH2 symmetric stretches in the oleic acid molecules respectively, were observed in the BaTiO3 nanocrystals capped with oleic acid. The peaks at 1510 cm 1 and 1440 cm 1 are the characteristics of the asymmetric and symmetric stretches of COO groups in oleic acid molecules. Because frequency difference (Δυ) is 70 cm 1, it is reasonable that carboxyl groups absorbed onto the surface of BaTiO3 nanocrystals via a
bidentate chelation [27]. And so the oleic acid groups could be chemisorbed onto the surface BaTiO3 nanocrystals by coordinating the oxygen atoms in carboxyl groups with Ba atoms on the surface of BaTiO3 nanocrystals in the process of hydrothermal growth [11,27]. The broad band around 579 cm 1 is ascribed to the typical Ti–O vibrations in BaTiO3 [27,28], which is only observed in the FTIR spectra of nanocrystals capped with oleic acid. We also find that the CQO stretching vibration around 1714 cm 1 in oleic acid molecules disappears in the present FTIR spectrum of our BaTiO3 nanocrystals capped with oleic acid (Fig. 6A), which is consistent with the result in Ref. [27]. In order to determine the amount of oleic acid molecules on the surface of BaTiO3 nanocrystals and to investigate the thermal behavior of BaTiO3 nanocrystals capped with oleic acid, thermogravimetry analysis (TGA) was conducted in an atmosphere of air with samples heated from room temperature to 800 1C at a rate of 10 1C min 1. As shown in Fig. 6B, the initial weight losses before 250 1C mainly resulted from the release of the organic solvent and water. And a dramatic weight loss of about 32 wt% in the range of 250–600 1C is observed due to the decomposition and combustion of oleic acid molecules attached onto the surface of BaTiO3 nanocrystals [11,27].
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Fig. 4. TEM images of BaTiO3 nanocrystals (the size of nanocrystals changes with KOH, OA and OM, reaction temperature and time; the amount of Ti(O–Bu)4 was kept as 0.8 mmol): (a, b) KOH: 13 mmol (BT 5); (c, d) reaction time: 20 h (BT 11); (e, f) reaction temperature: 145 1C (BT 7); (g, h) OA and OM: 2.5 ml respectively (BT 6).
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Fig. 5. TEM images of (Ba, Sr)TiO3 (a) and SrTiO3 (b) nanocrystals prepared via a hydrothermal route at 130 1C for 16 h.
Table 1 The size of BaTiO3 nanocrystals prepared under different reaction conditions. Sample no.
Titanium (IV) n-butoxide (Ti(O–Bu)4, mmol)
Oleic acid (OA, ml); oleylamine (OM, ml)
Potassium hydroxide (KOH, mmol)
Reaction temperature (1C)
Reaction time (h)
Particle size (nm)
BT BT BT BT BT BT
1 2 3 4 5 6
1.0 0.7 0.8 0.85 0.8 0.8
11 11 11 11 13 11
135 135 135 135 135 135
16 16 16 16 16 16
7–9 6–10 5–8 6–11 12–15 8
BT BT BT BT BT
7 8 9 10 11
0.8 1.06 1.0 0.8 0.8
OA 2.5 OA 2.5 OA 2.5 OA 2.5 OA 2.5 OA 2.5 OM 2.5 OA 2.5 OA 2.5 OA 2.5 OM 2.5 OA 2.5
11 11 8.5/9.5 11 11
145 135 135 135 135
16 16 16 16 20
10–12 15 Amorphous 6–10 11–15
Fig. 6. (A) FTIR spectra of oleic acid molecules (A-a) and BaTiO3 nanocrystals capped with oleic acid (A-b), and TGA curve of the BaTiO3 nanocrystals capped with oleic acid (B).
The surfaces of BaTiO3 nanocrystals are capped by 32 wt% of oleic acid molecules, this surface modification not only plays an important role in the growth and morphology formation of BaTiO3 nanocrystals, but also ensures the BaTiO3 nanocrystals be perfectly dispersed in nonpolar solvent (such
as toluene) without any aggregation and further facilitates the formation of ordered planar array structures by creating a field with tetragonal symmetry [27]. It is common that small nanocrystals easily gather into larger crystals. However, based on the above FTIR analysis, when BaTiO3 nanocrystals are
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Fig. 7. Schematic diagram for the growth process of cube-like BaTiO3 nanocrystals under hydrothermal conditions with oleic acid assistance.
well capped by oleic acid, the oleic acid molecules will selectively and uniformly adsorb onto nanocrystal's surface with lower plane index by bidentate complexation with Ba2 þ or Ti4 þ ions on the surface of nanocrystals [27], which, on the one hand, promotes the nanocrystals to grow into a cube shape, and on the other hand, long alkyl chain of oleic acid molecules weakens the interactions between nanocrystals in non-polar solvent and controls the final morphology of BaTiO3 nanocrystals through the adsorption differences of oleic acid molecules on nanocrystal's different planes. This result is similar to that reported in Refs. [11,27] and the above formation and evolution process of cube-like BaTiO3 nanocrystals could be understood better with the help of schematic diagram in Fig. 7. The oleic acid capping agents also allow the BaTiO3 nanocrystals with small size to be well-dispersed in organic monomer for the preparation of high energy density nanocomposite capacitors [29,30]. 4. Conclusions The size-controllable and highly monodispersed cube-like BaTiO3 nanocrystals were synthesized by a simple hydrothermal route with the assistance of oleic acid molecules. TEM images show that the products consist of a large number of monodispersed nanocrystals with cubic-like morphology and the size is in the range of 5–15 nm, and that the products obtained at Ba/Ti molar ratio of 1:0.8 in the starting materials have a small size of 5–8 nm, which smaller than those reported in almost all the references. FTIR and TGA indicate that the amount of oleic acid molecules attached onto the surface of BaTiO3 nanocrystals is about 32 wt%. The small size and
organic capping agents offer great potential of using the BaTiO3 nanocrystals in high energy density nanocomposite capacitors, and the current simple synthesis route would provide a general strategy for the preparation of other perovskite-type nanocrystals. Acknowledgment The authors acknowledge the financial support from National Natural Sciences Foundations of China (Grants nos. 50972107 and 50772075). References [1] S.H. Shin, Y.H. Kim, M.H. Lee, J.Y. Jung, J. Nah, Hemispherically aggregated BaTiO3 nanoparticle composite thin film for highperformance flexible piezoelectric nanogenerator, ACS Nano 8 (3) (2014) 2766–2773. [2] K. Manoli, P. Oikonomou, E. Valamontes, I. Raptis, M. Sanopoulou, Polymer–BaTiO3 composites: dielectric constant and vapor sensing properties in chemocapacitor applications, J. Appl. Polym. Sci. 4 (125) (2012) 2577–2584. [3] Y.F. Cui, J. Briscoe, S. Dunn, Effect of ferroelectricity on solar-lightdriven photocatalytic activity of BaTiO3—influence on the carrier separation and stern layer formation, Chem. Mater. 25 (21) (2013) 4215–4223. [4] B.G. Yust, N. Razavi, F. Pedraza, Z. Elliott, A.T. Tsin, D.K. Sardar, Enhancement of nonlinear optical properties of BaTiO3 nanoparticles by the addition of silver seeds, Opt. Express 20 (24) (2012) 26511–26520. [5] J. Lott, C. Xia, L. Kosnovsky, C. Weder, J. Shan, Terahertz photonic crystals based on barium titanate/polymer nanocomposites, Adv. Mater. 20 (2008) 3649–3653. [6] I.I. Naumov, L. Bellaiche, H. Fu, Unusual phase transitions in ferroelectric nanodisks and nanorods, Nature 432 (2004) 737–740.
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W. Cai et al. / Ceramics International 41 (2015) 4514–4522
[7] X. Wang, J. Zhuang, Q. Peng, Y. Li, A general strategy for nanocrystal synthesis, Nature 437 (7055) (2005) 121–124. [8] S. O’Brien, L. Brus, C.B. Murray, Synthesis and characterization of nanocrystals of barium titanate, toward a generalized synthesis of oxide nanoparticles, J. Am. Chem. Soc. 123 (2001) 12085–12086. [9] V. Bansal, P. Poddar, A. Ahmad, M. Sastry, Room-temperature biosynthesis of ferroelectric barium titanate nanoparticles, J. Am. Chem. Soc. 128 (2006) 11958–11963. [10] R.Z. Hou, P. Ferreira, P.M. Vilarinho, Nanoporous BaTiO3 crystallites, Chem. Mater. 21 (2009) 3536–3541. [11] S. Adireddy, C.K. Lin, B.B. Cao, W.L. Zhou, G. Caruntu, Solution-based growth of monodisperse cube-like BaTiO3 colloidal nanocrystals, Chem. Mater. 22 (2010) 1946–1948. [12] H.J. Zheng, K.J. Zhu, Q.L. Wu, J.S. Liu, J.H. Qiu, Preparation and characterization of monodispersed BaTiO3 nanocrystal by sol–hydrothemal method, J. Cryst. Growth 363 (2013) 300–307. [13] W.W. Wang, L.X. Cao, W. Liu, G. Su, W.X. Zhang, Low-temperature synthesis of BaTiO3 powders by the sol–gel–hydrothermal method, Ceram. Int. 39 (2013) 7127–7134. [14] B. Hou, Y. Xu, D. Wu, Y.H. Sun, Preparation and characterization of single crystalline barium strontium titanate nanocubes via solvothermal method, Powder Technol. 170 (2006) 26–30. [15] Z. Deng, Y. Dai, W. Chen, X.M. Pei, J.H. Liao, Synthesis and characterization of bowl-like single-crystalline BaTiO3 nanoparticles, Nanoscale Res. Lett. 5 (2010) 1217–1221. [16] Y.Y. Chen, B.Y. Yu, J.H. Wang, R.E. Cochran, J.J. Shyue, Templatebased fabrication of SrTiO3 and BaTiO3 nanotubes, Inorg. Chem. 48 (2009) 681–686. [17] J.J. Urban, W.S. Yun, Q. Gu, H. Park, Synthesis of single-crystalline perovskite nanorods composed of barium titanate and strontium titanate, J. Am. Chem. Soc. 124 (2002) 1186–1187. [18] J.J. Urban, J.E. Spanier, L. Ouyang, W.S. Yun, H. Park, Singlecrystalline barium titanate nanowires, Adv. Mater. 15 (2003) 423–426. [19] B. Im, U.A. Joshi, K.H. Lee, Growth of single crystalline barium titanate nanowires from TiO2 seeds deposited on conducting glass, Nanotechnology 21 (2010) 425601-1–425601-5.
[20] U.A. Joshi, S. Yoon, S. Baik, J.S. Lee, Surfactant-free hydrothermal synthesis of highly tetragonal barium titanate nanowires: a structural investigation, J. Phys. Chem. B 110 (2006) 12249–12256. [21] J. Yang, J. Zhang, C.Y. Liang, M. Wang, P.F. Zhao, J.W. Liu, M.M. Liu, R.C. Che, Ultrathin BaTiO3 nanowires with high aspect ratio: a simple one-step hydrothermal synthesis and their strong microwave absorption, ACS Appl. Mater. Interfaces 5 (15) (2013) 7146–7151. [22] L.L. Yang, Y.G. Wang, Y.J. Wang, X.F. Wang, X.J. Guo, G.R. Han, Synthesis of single-crystal Ba1 xSrxTiO3 (x¼ 0–1) dendrites via a simple hydrothermal method, J. Alloys Compd. 500 (2010) L1–L5. [23] X.K. Zhang, S.L. Tang, J.Y. Yu, L. Zhai, Y.G. Shi, Y.W. Dou, Synthesis of single-crystalline barium tetratitanate nanobelts via self-sacrificing template process, Cryst. Growth Des. 9 (2009) 2971–2973. [24] J.Y. Yu, S.L. Tang, R.L. Wang, Y.G. Shi, B. Nie, L. Zhai, X.K. Zhang, Y.W. Dou, Synthesis of single-crystalline barium dititanate nanobelts, Cryst. Growth Des. 8 (2008) 1481–1483. [25] J. Jitputti, P. Charoensirithavorn, S. Yoshikawa, Hydrothermal production of SrTiO3 nanotube arrays, Chem. Lett. 36 (2007) 1508–1509. [26] M.M. Lencka, R.E. Riman, Thermodynamic modeling of hydrothermal synthesis of ceramic powders, Chem. Mater. 5 (1993) 61–70. [27] X.W. Yang, Y.W. Zeng, L.Q. Mo, L.X. Han, Oleic acid assisted glycothermal synthesis of cuboidal Ba0.6Sr0.4TiO3 nanocrystals and their ordered architectures via self-assembly, J. Colloid Interface Sci. 357 (2011) 308–316. [28] G. Busca, V. Buscagia, M. Leoni, P. Nanni, Solid-state and surface spectroscopic characterization of BaTiO3 fine powders, Chem. Mater. 6 (1994) 955–961. [29] P. Kim, N.M. Doss, J.P. Tillotson, P.J. Hotchkiss, M.J. Pan, S.R. Marder, J. Li, J.P. Calame, J.W. Perry, High energy density nanocomposites based on surface-modified BaTiO3 and a ferroelectric polymer, ACS Nano 3 (9) (2009) 2581–2592. [30] Y. Kim, K. Kook, S.K. Hwang, C. Park, J. Cho, Polymer/perovskite-type nanoparticle multilayers with multielectric properties prepared from ligand addition-induced layer-by-layer assembly, ACS Nano 8 (3) (2014) 2419–2430.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 4514–4522 www.elsevier.com/locate/ceramint
A simple and controllable hydrothermal route for the synthesis of monodispersed cube-like barium titanate nanocrystals Wen Caia,b,d,n, Tingke Raob, Aiwu Wangb, Jie Huc, Junqing Wangd, Jiasong Zhonga, Weidong Xianga b
a School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region c Department of Electrical Engineering, KU Leuven, 3001 Leuven, Belgium d State Key Laboratory of Molecular Vaccinology and Molecular Diagnostics & Center for Molecular Imaging and Translational Medicine, School of Public Health, Xiamen University, Xiamen 361102, PR China
Received 8 November 2014; received in revised form 26 November 2014; accepted 27 November 2014 Available online 5 December 2014
Abstract Monodispersed BaTiO3 (barium titanate) nanocrystals with well-controlled size have been successfully prepared via a simple hydrothermal route. The phase structure, morphology and surface composition of the BaTiO3 nanocrystals are characterized by means of XRD, TEM, FTIR and TGA. Results demonstrate that the synthesized products consist of cube-like nanocrystals with a size of 5–15 nm, which can be well-controlled by tuning the Ba/Ti molar ratio in the starting materials, and that the amount of the organic capping agents on the surface of BaTiO3 nanocrystals is about 32 wt%. The present simple synthesis route would provide a general strategy for the preparation of other perovskite-type nanocrystals. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Surfaces; D. BaTiO3 and titanates; D. Perovskites; E. Capacitors
1. Introduction Over the past decades, the synthesis of perovskite-type metal oxide nanocrystals has attracted much attention due to their widespread applications in electronics, sensing, catalysis, and nonlinear optics [1–4]. BaTiO3, as one kind of perovskite-type metal oxides, shows a variety of potential applications in multilayered capacitors, random access memories, thermistors, pressure transducers, and waveguide modulators due to its excellent ferroelectric, pyroelectric, piezoelectric, electro-optic, dielectric, and elastic properties [5,6]. Currently, the BaTiO3 products applied in industry are commonly prepared by the catecholate process at a calcination temperature as high as 1000–1200 1C. To avoid the formation of severe agglomerates and obtain the final n Corresponding author at: School of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China. E-mail address: [email protected] (W. Cai).
http://dx.doi.org/10.1016/j.ceramint.2014.11.146 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
product with high quality, the synthesis of re-dispersible BaTiO3 nanocrystals can make a great contribution. On the other hand, it is well-known that the structural and physical properties of perovskite nanocrystals are strongly dependent on their size, shape, crystallinity, and surface composition. Therefore synthesis of monodispersed BaTiO3 nanocrystals with well-defined shape, controllable size and high degree of compositional homogeneity is very important for the investigations of their chemical and physical properties. Although many efforts have been devoted to the fabrication of perovskites nanostructures with various morphologies, it is still a great challenge for researchers to prepare BaTiO3 nanostructured materials with well-controlled size and shape [7–9]. For instance, Hou et al. prepared nanoporous BaTiO3 crystallites [10]. Adireddy et al. [11], Zheng et al. [12] and Wang et al. [13] synthesized monodispersed cubelike BaTiO3 colloidal nanocrystals via a solution-based route. Hou et al. also prepared single-crystalline barium strontium titanate nanocubes by a solvothermal method [14]. Deng et al.
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obtained bowl-like single-crystalline BaTiO3 nanoparticles [15]. Chen et al. fabricated SrTiO3 and BaTiO3 nanotubes via a template-based route [16]. Urban et al. prepared single-crystalline perovskite nanorods composed of barium titanate and strontium titanate [17]. Urban et al. [18], Im et al. [19], Joshi et al. [20] and Yang et al. [21] synthesized barium titanate nanowire. Yang et al. synthesized single-crystal Ba1 xSrxTiO3 (x¼ 0–1) dendrites via a simple hydrothermal method [22]. Zhang et al. synthesized single-crystalline barium tetratitanate nanobelts via a selfsacrificing template process [23]. Yu et al. synthesized singlecrystalline barium dititanate nanobelts [24]. Jitputti et al. prepared SrTiO3 nanotube arrays by a hydrothermal route [25]. Among the above solution methods, hydrothermal route, as a relatively simple and cost-effective approach to control the size and shape of the products, has been widely used to fabricate metal oxide nanocrystals with high crystallinity and good dispersion properties at much lower temperatures. Here, we report a simple and well-controlled hydrothermal approach for the synthesis of freestanding and cube-like BaTiO3 nanocrystals with average length edges ranging from 5 to 15 nm by tuning Ba/Ti molar ratio in the starting materials. TEM images reveal that the synthesized BaTiO3 nanocrystals capped with oleic acid molecules are wellseparated and have the tendency to self-assemble into ordered planar structures. FTIR and TGA show that the prepared nanocrystals retain oleic acid molecules on their surfaces and can easily redisperse in organic solvents. The small size and organic capping agents offer great potential of using the BaTiO3 nanocrystals in high energy density nanocomposite capacitors. 2. Experimental procedures
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KOH in DI water + Ba(NO3)2 in DI water
Oleic acid in BuOH + Ti(O-Bu)4 in BuOH
Mixture Solution
Autoclave, 135 oC for 16 hrs
Washing and separation
BT nanocrystal
Fig. 1. Schematic flow-chart of hydrothermal synthesis of BaTiO3 nanocrystals.
components within the supernatant solution was performed by centrifugation. The precipitate was then collected, purified and finally dispersed in toluene yielding a stable milky colloidal solution. The yield of the products was about 90%. Fig. 1 shows the schematic flow-chart for the hydrothermal synthesis of BaTiO3 nanocrystals. The molar ratio of Ba/Ti in the starting materials was tuned from 1:0.70 to 1:1.06 to investigate its influence on the size of BaTiO3 nanocrystals. For the synthesis of (Ba, Sr)TiO3 nanocrystals, the molar ratio of Ba/Sr in the staring materials was set as 0.5 mmol/ 0.5 mmol. For the synthesis of SrTiO3 nanocrystals, the amount of Sr(NO3)2 in the starting materials was 1 mmol.
2.1. Chemicals Barium nitrate (Ba(NO3)2, 99%), strontium nitrate (Sr (NO3)2, 99%), potassium hydroxide (KOH, 90%), 1-butanol anhydrous (C4H10O (BuOH), 99.8%), titanium (IV) n-butoxide (C16H36O4Ti (Ti(O–Bu)4), 97%), oleic acid (C18H34O2, 99%), oleylamine (C18H37N, 70%) were all of analytical grade and purchased from Aldrich. All of them were used as received without further purification. 2.2. Synthesis of BaTiO3, SrTiO3 and (Ba, Sr)TiO3 nanocrystals The synthetic method is similar to that described in Ref. [11] with improvement by adjusting the molar ratio of Ba/Ti in the staring materials to control the size of the products. In a typical procedure for the preparation of BaTiO3 nanocrystals, different solutions containing 1 mmol of Ba(NO3)2 dissolved in 5 mL of deionized water, 11 mmol of KOH dissolved in 5 mL of deionized water, 0.80 mmol of Ti(O–Bu)4 dissolved in 5 mL of BuOH and 2.5 mL of oleic acid in 5 mL of BuOH were mixed together and the resulting solution was then transferred to a 23 mL of Teflon-lined stainless steel autoclave. The autoclave was sealed and then heated to 135 1C for 16 h. The reaction mixture was cooled down to room temperature after the completion of the reaction. The separation of
2.3. Characterization and measurements The crystal structure and phase composition of the asobtained white nanopowders were identified and analyzed by using X-ray powder diffraction technique. X-ray powder diffraction (XRD) was performed on a Panalytical X'Pert Pro diffractometer (40 kV, 40 mA) by using Cu Kα radiation (λ¼ 0.15405 nm) at a scanning rate of 0.021/s. The nanopowder samples for XRD characterization were prepared by drying the toluene dispersion of BaTiO3 nanocrystals at room temperature. The size, morphology and microstructure of the nanocrystals were investigated by transmission electron microscopy (TEM). TEM images were recorded on a Philips CM120 microscope (120 kV). For TEM observations, the toluene dispersion of BaTiO3 nanocrystals was dropped onto a carbon-coated copper-grid. Fourier transform infrared spectroscopy (FTIR, JASCO FT/IR-420) was used to analyze the attachment of capping agents onto the surface of BaTiO3 nanocrystals by preparing sample with the KBr-pellet method. All the above measurements were carried out at room temperature. Thermogravimetry analysis of BaTiO3 nanocrystals was conducted by Pyris Diamond TG/DTA (Perkin-Elmer) with temperature range of 25–800 1C at a heating rate of 10 1C/min in air-flow (30 ml/min).
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(200)
JCPDS NO 31-174
(110)
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Intensity(a.u.)
(220)
(211)
(210)
(200)
(111)
(100)
Intensity(a.u.)
(a) BT, Ti/Ba=0.7
(a) BT, Ti/Ba=0.7 (b) BT, Ti/Ba=0.8
(b) BT, Ti/Ba=0.8
(c) BT, Ti/Ba=0.85 (d) BST
(c) BT, Ti/Ba =0.85 (d) BST
(e) ST
(e) ST
10
20
30
40
50
60
70
40.7 41.8 42.9 44.0 45.1 46.2 47.3 48.4 49.5
2Theta(degree)
2Theta(degree)
O Ti Ba
Counts (a.u.)
Ti Ba
Ba Ti Ba Ba 0
1
2
3
4
5
6
7
8
9
10
Energy(Kev)
Fig. 2. (A) XRD patterns of the BaTiO3 (BT) (a–c), (Ba, Sr)TiO3 (BST) and SrTiO3 (ST) nanocrystals synthesized via a hydrothermal route at 130 1C for 16 h. (B) XRD patterns in the 2θ range of 43–481 of BT, BST and ST nanocrystals prepared by a hydrothermal method at 135 1C for 16 h. The Ba/Ti molar ratio used in the starting materials is 1:0.70 (a), 1:0.80 (b), and 1:0.85 (c). (C) EDS spectrum of the BaTiO3 nanocrystals.
3. Results and discussion Fig. 2A shows the XRD patterns of the as-prepared BaTiO3, SrTiO3 and (Ba, Sr)TiO3 nanocrystals. All the diffraction peaks in Fig. 2A-(a–c) correspond to the crystal planes of BaTiO3 (JCPDS no. 31-174). It can be seen from Fig. 2A-(b) that very little carbonate byproducts, which can be removed by an acid washing process, are present in the final products. The XRD patterns of SrTiO3 and (Ba, Sr)TiO3 nanocrystals are shown in Fig. 2A-(d and e). The strong and sharp peaks suggest that SrTiO3 and (Ba, Sr)TiO3 nanocrystals are highly crystalline. With the increase of the amount of Sr, the reflection positions shift to higher diffraction angles, owing to the smaller lattice constants caused by the substitution of the larger Ba ions by the smaller Sr ions. Furthermore, it can be seen from Fig. 2B that there is a peak with weak spilling at 451 corresponding to the (200) plane in the perovskite structure, indicating that the product shows tetragonal phase, which is consistent with the result described in Ref. [11]. Fig. 2C gives an EDS diagram of the BaTiO3 nanocrystals, it can be seen that there are only Ba, Ti, and O elements in the product without any other metal element, which indicates the highpurity of the product.
Fig. 3 illustrates the representative transmission electron micrographs (TEM) images of the barium titanate samples. Overview images (Fig. 3a, c, e, g, and i) at low magnification show that the sample entirely consists of nano-sized monodispersed barium titanate crystals without the presence of large crystals or agglomerates, and that the size of barium titanate nanocrystals is smaller than those reported in almost all the references. The high-magnification images (Fig. 3b, d, f, h, and j) reveal that the nanocrystals have cube-like morphology, and that the size of the nanocrystals is in the range of 5–15 nm. The influence of the molar ratio of Ba/Ti on the size of the BaTiO3 product was investigated by adjusting it from 1:0.70 to 1:1.06 in the starting materials, and it was found that the products with a small size of 5–8 nm (calculated from XRD by the Scherrer equation) had been successfully obtained with the ratio of 1:0.80, as shown in Fig. 3(c) and (d). However, the size of the products increased when this ratio was greater or less than 1:0.80. The effects of KOH, oleylamine, and reaction temperature and time on the size of products were also investigated when the amount of Ti(O–Bu)4 in the starting materials was kept as 0.8 mmol. It can be seen from Fig. 4(a and b) that the size of the products increased when the amount of KOH was more
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Fig. 3. TEM images of the BaTiO3 nanocrystals synthesized via a hydrothermal route at 130 1C for 16 h by using different Ba/Ti molar ratio in the starting materials: (a , b) Ba/Ti¼ 1:0.70 (BT 2); (c, d) Ba/Ti ¼1:0.80 (BT 3); (e, f) Ba/Ti ¼1:0.85 (BT 4); (g, h) Ba/Ti ¼1:1.0(BT 1); (i, j) Ba/Ti¼ 1:1.06 (BT 8).
than 11 mmol. However, the products appeared to be amorphous when the amount of KOH was less than 11 mmol, which had been explained in detail by Riman group [26]. From Fig. 4(c–f), it can be seen that the size of the products increased when the reaction temperature was elevated and the reaction time was elongated due to the higher crystal growth rate and longer crystal growth time. It was also found
that the addition of oleylamine facilitated the formation of products with a small size of 8 nm (Fig.4g and h). We also obtained the (Ba, Sr)TiO3 and SrTiO3 nanocrystals under the current hydrothermal conditions (130 1C for 16 h, and 0.8 mmol of Ti(O–Bu)4 was used). From their TEM images (Fig. 5), it can be seen that SrTiO3 nanocrystals take on nanosphere morphology with serious aggregation, and that the
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Fig. 3. (continued)
size of (Ba, Sr)TiO3 and SrTiO3 nanocrystals is about 15 nm and 4 nm, respectively. The influences of the above reaction conditions on the size of BaTiO3 product could be summarized in Table 1. Fourier transform infrared spectroscopy (FTIR) was performed to investigate the attachment of oleic acid molecules onto the surface of BaTiO3 nanocrystals. BaTiO3 nanocrystals capped with oleic acid molecules were washed repeatedly by using toluene to remove the physisorbed oleic acid molecules until a constant amount of oleic acid molecules was obtained before FTIR analysis. The FTIR spectra of oleic acid molecules and BaTiO3 sample capped with oleic acid were recorded as illustrated in Fig. 6A. The FTIR spectra of BaTiO3 sample capped with oleic acid are very similar to that of oleic acid molecules. The vibration bands centered at 2917 cm 1 and 2849 cm 1, corresponding to the CH2 asymmetric and CH2 symmetric stretches in the oleic acid molecules respectively, were observed in the BaTiO3 nanocrystals capped with oleic acid. The peaks at 1510 cm 1 and 1440 cm 1 are the characteristics of the asymmetric and symmetric stretches of COO groups in oleic acid molecules. Because frequency difference (Δυ) is 70 cm 1, it is reasonable that carboxyl groups absorbed onto the surface of BaTiO3 nanocrystals via a
bidentate chelation [27]. And so the oleic acid groups could be chemisorbed onto the surface BaTiO3 nanocrystals by coordinating the oxygen atoms in carboxyl groups with Ba atoms on the surface of BaTiO3 nanocrystals in the process of hydrothermal growth [11,27]. The broad band around 579 cm 1 is ascribed to the typical Ti–O vibrations in BaTiO3 [27,28], which is only observed in the FTIR spectra of nanocrystals capped with oleic acid. We also find that the CQO stretching vibration around 1714 cm 1 in oleic acid molecules disappears in the present FTIR spectrum of our BaTiO3 nanocrystals capped with oleic acid (Fig. 6A), which is consistent with the result in Ref. [27]. In order to determine the amount of oleic acid molecules on the surface of BaTiO3 nanocrystals and to investigate the thermal behavior of BaTiO3 nanocrystals capped with oleic acid, thermogravimetry analysis (TGA) was conducted in an atmosphere of air with samples heated from room temperature to 800 1C at a rate of 10 1C min 1. As shown in Fig. 6B, the initial weight losses before 250 1C mainly resulted from the release of the organic solvent and water. And a dramatic weight loss of about 32 wt% in the range of 250–600 1C is observed due to the decomposition and combustion of oleic acid molecules attached onto the surface of BaTiO3 nanocrystals [11,27].
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Fig. 4. TEM images of BaTiO3 nanocrystals (the size of nanocrystals changes with KOH, OA and OM, reaction temperature and time; the amount of Ti(O–Bu)4 was kept as 0.8 mmol): (a, b) KOH: 13 mmol (BT 5); (c, d) reaction time: 20 h (BT 11); (e, f) reaction temperature: 145 1C (BT 7); (g, h) OA and OM: 2.5 ml respectively (BT 6).
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Fig. 5. TEM images of (Ba, Sr)TiO3 (a) and SrTiO3 (b) nanocrystals prepared via a hydrothermal route at 130 1C for 16 h.
Table 1 The size of BaTiO3 nanocrystals prepared under different reaction conditions. Sample no.
Titanium (IV) n-butoxide (Ti(O–Bu)4, mmol)
Oleic acid (OA, ml); oleylamine (OM, ml)
Potassium hydroxide (KOH, mmol)
Reaction temperature (1C)
Reaction time (h)
Particle size (nm)
BT BT BT BT BT BT
1 2 3 4 5 6
1.0 0.7 0.8 0.85 0.8 0.8
11 11 11 11 13 11
135 135 135 135 135 135
16 16 16 16 16 16
7–9 6–10 5–8 6–11 12–15 8
BT BT BT BT BT
7 8 9 10 11
0.8 1.06 1.0 0.8 0.8
OA 2.5 OA 2.5 OA 2.5 OA 2.5 OA 2.5 OA 2.5 OM 2.5 OA 2.5 OA 2.5 OA 2.5 OM 2.5 OA 2.5
11 11 8.5/9.5 11 11
145 135 135 135 135
16 16 16 16 20
10–12 15 Amorphous 6–10 11–15
Fig. 6. (A) FTIR spectra of oleic acid molecules (A-a) and BaTiO3 nanocrystals capped with oleic acid (A-b), and TGA curve of the BaTiO3 nanocrystals capped with oleic acid (B).
The surfaces of BaTiO3 nanocrystals are capped by 32 wt% of oleic acid molecules, this surface modification not only plays an important role in the growth and morphology formation of BaTiO3 nanocrystals, but also ensures the BaTiO3 nanocrystals be perfectly dispersed in nonpolar solvent (such
as toluene) without any aggregation and further facilitates the formation of ordered planar array structures by creating a field with tetragonal symmetry [27]. It is common that small nanocrystals easily gather into larger crystals. However, based on the above FTIR analysis, when BaTiO3 nanocrystals are
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Fig. 7. Schematic diagram for the growth process of cube-like BaTiO3 nanocrystals under hydrothermal conditions with oleic acid assistance.
well capped by oleic acid, the oleic acid molecules will selectively and uniformly adsorb onto nanocrystal's surface with lower plane index by bidentate complexation with Ba2 þ or Ti4 þ ions on the surface of nanocrystals [27], which, on the one hand, promotes the nanocrystals to grow into a cube shape, and on the other hand, long alkyl chain of oleic acid molecules weakens the interactions between nanocrystals in non-polar solvent and controls the final morphology of BaTiO3 nanocrystals through the adsorption differences of oleic acid molecules on nanocrystal's different planes. This result is similar to that reported in Refs. [11,27] and the above formation and evolution process of cube-like BaTiO3 nanocrystals could be understood better with the help of schematic diagram in Fig. 7. The oleic acid capping agents also allow the BaTiO3 nanocrystals with small size to be well-dispersed in organic monomer for the preparation of high energy density nanocomposite capacitors [29,30]. 4. Conclusions The size-controllable and highly monodispersed cube-like BaTiO3 nanocrystals were synthesized by a simple hydrothermal route with the assistance of oleic acid molecules. TEM images show that the products consist of a large number of monodispersed nanocrystals with cubic-like morphology and the size is in the range of 5–15 nm, and that the products obtained at Ba/Ti molar ratio of 1:0.8 in the starting materials have a small size of 5–8 nm, which smaller than those reported in almost all the references. FTIR and TGA indicate that the amount of oleic acid molecules attached onto the surface of BaTiO3 nanocrystals is about 32 wt%. The small size and
organic capping agents offer great potential of using the BaTiO3 nanocrystals in high energy density nanocomposite capacitors, and the current simple synthesis route would provide a general strategy for the preparation of other perovskite-type nanocrystals. Acknowledgment The authors acknowledge the financial support from National Natural Sciences Foundations of China (Grants nos. 50972107 and 50772075). References [1] S.H. Shin, Y.H. Kim, M.H. Lee, J.Y. Jung, J. Nah, Hemispherically aggregated BaTiO3 nanoparticle composite thin film for highperformance flexible piezoelectric nanogenerator, ACS Nano 8 (3) (2014) 2766–2773. [2] K. Manoli, P. Oikonomou, E. Valamontes, I. Raptis, M. Sanopoulou, Polymer–BaTiO3 composites: dielectric constant and vapor sensing properties in chemocapacitor applications, J. Appl. Polym. Sci. 4 (125) (2012) 2577–2584. [3] Y.F. Cui, J. Briscoe, S. Dunn, Effect of ferroelectricity on solar-lightdriven photocatalytic activity of BaTiO3—influence on the carrier separation and stern layer formation, Chem. Mater. 25 (21) (2013) 4215–4223. [4] B.G. Yust, N. Razavi, F. Pedraza, Z. Elliott, A.T. Tsin, D.K. Sardar, Enhancement of nonlinear optical properties of BaTiO3 nanoparticles by the addition of silver seeds, Opt. Express 20 (24) (2012) 26511–26520. [5] J. Lott, C. Xia, L. Kosnovsky, C. Weder, J. Shan, Terahertz photonic crystals based on barium titanate/polymer nanocomposites, Adv. Mater. 20 (2008) 3649–3653. [6] I.I. Naumov, L. Bellaiche, H. Fu, Unusual phase transitions in ferroelectric nanodisks and nanorods, Nature 432 (2004) 737–740.
4522
W. Cai et al. / Ceramics International 41 (2015) 4514–4522
[7] X. Wang, J. Zhuang, Q. Peng, Y. Li, A general strategy for nanocrystal synthesis, Nature 437 (7055) (2005) 121–124. [8] S. O’Brien, L. Brus, C.B. Murray, Synthesis and characterization of nanocrystals of barium titanate, toward a generalized synthesis of oxide nanoparticles, J. Am. Chem. Soc. 123 (2001) 12085–12086. [9] V. Bansal, P. Poddar, A. Ahmad, M. Sastry, Room-temperature biosynthesis of ferroelectric barium titanate nanoparticles, J. Am. Chem. Soc. 128 (2006) 11958–11963. [10] R.Z. Hou, P. Ferreira, P.M. Vilarinho, Nanoporous BaTiO3 crystallites, Chem. Mater. 21 (2009) 3536–3541. [11] S. Adireddy, C.K. Lin, B.B. Cao, W.L. Zhou, G. Caruntu, Solution-based growth of monodisperse cube-like BaTiO3 colloidal nanocrystals, Chem. Mater. 22 (2010) 1946–1948. [12] H.J. Zheng, K.J. Zhu, Q.L. Wu, J.S. Liu, J.H. Qiu, Preparation and characterization of monodispersed BaTiO3 nanocrystal by sol–hydrothemal method, J. Cryst. Growth 363 (2013) 300–307. [13] W.W. Wang, L.X. Cao, W. Liu, G. Su, W.X. Zhang, Low-temperature synthesis of BaTiO3 powders by the sol–gel–hydrothermal method, Ceram. Int. 39 (2013) 7127–7134. [14] B. Hou, Y. Xu, D. Wu, Y.H. Sun, Preparation and characterization of single crystalline barium strontium titanate nanocubes via solvothermal method, Powder Technol. 170 (2006) 26–30. [15] Z. Deng, Y. Dai, W. Chen, X.M. Pei, J.H. Liao, Synthesis and characterization of bowl-like single-crystalline BaTiO3 nanoparticles, Nanoscale Res. Lett. 5 (2010) 1217–1221. [16] Y.Y. Chen, B.Y. Yu, J.H. Wang, R.E. Cochran, J.J. Shyue, Templatebased fabrication of SrTiO3 and BaTiO3 nanotubes, Inorg. Chem. 48 (2009) 681–686. [17] J.J. Urban, W.S. Yun, Q. Gu, H. Park, Synthesis of single-crystalline perovskite nanorods composed of barium titanate and strontium titanate, J. Am. Chem. Soc. 124 (2002) 1186–1187. [18] J.J. Urban, J.E. Spanier, L. Ouyang, W.S. Yun, H. Park, Singlecrystalline barium titanate nanowires, Adv. Mater. 15 (2003) 423–426. [19] B. Im, U.A. Joshi, K.H. Lee, Growth of single crystalline barium titanate nanowires from TiO2 seeds deposited on conducting glass, Nanotechnology 21 (2010) 425601-1–425601-5.
[20] U.A. Joshi, S. Yoon, S. Baik, J.S. Lee, Surfactant-free hydrothermal synthesis of highly tetragonal barium titanate nanowires: a structural investigation, J. Phys. Chem. B 110 (2006) 12249–12256. [21] J. Yang, J. Zhang, C.Y. Liang, M. Wang, P.F. Zhao, J.W. Liu, M.M. Liu, R.C. Che, Ultrathin BaTiO3 nanowires with high aspect ratio: a simple one-step hydrothermal synthesis and their strong microwave absorption, ACS Appl. Mater. Interfaces 5 (15) (2013) 7146–7151. [22] L.L. Yang, Y.G. Wang, Y.J. Wang, X.F. Wang, X.J. Guo, G.R. Han, Synthesis of single-crystal Ba1 xSrxTiO3 (x¼ 0–1) dendrites via a simple hydrothermal method, J. Alloys Compd. 500 (2010) L1–L5. [23] X.K. Zhang, S.L. Tang, J.Y. Yu, L. Zhai, Y.G. Shi, Y.W. Dou, Synthesis of single-crystalline barium tetratitanate nanobelts via self-sacrificing template process, Cryst. Growth Des. 9 (2009) 2971–2973. [24] J.Y. Yu, S.L. Tang, R.L. Wang, Y.G. Shi, B. Nie, L. Zhai, X.K. Zhang, Y.W. Dou, Synthesis of single-crystalline barium dititanate nanobelts, Cryst. Growth Des. 8 (2008) 1481–1483. [25] J. Jitputti, P. Charoensirithavorn, S. Yoshikawa, Hydrothermal production of SrTiO3 nanotube arrays, Chem. Lett. 36 (2007) 1508–1509. [26] M.M. Lencka, R.E. Riman, Thermodynamic modeling of hydrothermal synthesis of ceramic powders, Chem. Mater. 5 (1993) 61–70. [27] X.W. Yang, Y.W. Zeng, L.Q. Mo, L.X. Han, Oleic acid assisted glycothermal synthesis of cuboidal Ba0.6Sr0.4TiO3 nanocrystals and their ordered architectures via self-assembly, J. Colloid Interface Sci. 357 (2011) 308–316. [28] G. Busca, V. Buscagia, M. Leoni, P. Nanni, Solid-state and surface spectroscopic characterization of BaTiO3 fine powders, Chem. Mater. 6 (1994) 955–961. [29] P. Kim, N.M. Doss, J.P. Tillotson, P.J. Hotchkiss, M.J. Pan, S.R. Marder, J. Li, J.P. Calame, J.W. Perry, High energy density nanocomposites based on surface-modified BaTiO3 and a ferroelectric polymer, ACS Nano 3 (9) (2009) 2581–2592. [30] Y. Kim, K. Kook, S.K. Hwang, C. Park, J. Cho, Polymer/perovskite-type nanoparticle multilayers with multielectric properties prepared from ligand addition-induced layer-by-layer assembly, ACS Nano 8 (3) (2014) 2419–2430.