The novel formation of barium titanate nanodendrites.
Over the past few years, the unique ferroelectric, piezoelectric, and thermoelectric properties of barium titanate (BaTi[O.sub.3]) nanoparticles have become increasingly important in the electronic ceramics industry. The BaTi[O.sub.3] nanoparticles have been extensively applied in various fields such as multilayer ceramic capacitors (MLCCs), integral capacitors in printed circuit boards (PCB), dynamic random access memories (DRAM), resistors with positive temperature coefficient of resistivity (PTCR), temperature-humidity-gas sensors, electrooptic devices, piezoelectric transducers, actuators, and thermistors [1-9]. Among these applications, performance and characteristics are strongly influenced by size, shape, composition, morphology, spatial ordering, and impurities of the BaTi[O.sub.3] nanoparticles. Thus, effectively controlling their shape and size is of high importance and is a challenging task for researchers and the industry. In this work, we have developed BaTi[O.sub.3] with novel dendrite-like structures. Very recently, nanoparticles with dendrite-like structures have received much attention because of their potential application in device [10,11]. However, finely controlling the morphology of the BaTi[O.sub.3] nanoparticles is extremely dependent on preparation method and synthesis procedure.
Traditionally, the BaTi[O.sub.3] particle is prepared by the solid-state reaction method through heating BaC[O.sub.3] and Ti[O.sub.2] at high temperature as 1200[degrees]C 12,13]. The disadvantage of this method is that high calcinations temperature may strongly cause aggregation between the particles, and it takes a long time to produce submicrometer particles (1~2 [micro]m). Up to now, various new preparation methods have been developed and reported in fabricating BaTi[O.sub.3] nanoparticles with high quality, well-controlled shape, and small size, such as the sol-gel method [14, 15], the hydrothermal method [16, 17], the Pechini processing using a citric or oxalate complex as the precursor [18, 19], the ball-milling method [20, 21], the polymeric precursor method , the soft chemical process , the glycolthermal method , and the coprecipitation method . Among these, the coprecipitation method is superior to other methods in terms of the following characteristics: high growth rate, modest equipment, low processing temperature, ease of controlling the yield, low cost, large amount synthesized, and high quality .
In the coprecipitation method, the preparation of BaTi[O.sub.3] nanoparticles through the coprecipitation of barium and titanium hydroxides from aqueous solutions has been reported since the early Flaschen research work . Synthesis of BaTi[O.sub.3] nanoparticles as the decomposition product of barium titanyl oxalate or barium titanyl citrate is a multistage process, depending on the gaseous medium, the dispersion of the starting reagents and intermediate phase (the degree of branching of the interphase surface), the regime in which the reaction occurs (kinetic or diffusion), the growth temperature, and the heating rate [28-32]. Although these previous studies succeeded in fabricating BaTi[O.sub.3] nanoparticles, the procedure is quite complicated. Furthermore, these procedures also require special conditions, such as judicious choice of the stabilizer, heat treatment, and time duration. Therefore, it will be a significant challenge to simplify the procedure for the fabrication of BaTi[O.sub.3] nanoparticles.
In our laboratory, we developed a simple procedure by slightly modifying the multistage process so it could be applied to fabricate BaTi[O.sub.3] nanoparticles with well-controlled size. In this simple procedure, appropriate amount of stock solution of titanium tetrachloride (Ti[Cl.sub.4]), barium chloride (Ba[Cl.sub.2]), and oxalic acid was added in deionized water to form growth solution. The BaTi[O.sub.3] nanoparticle was formed by coprecipitation of both barium and titanium precursor. During the coprecipitation process, titanium acted as the seed in the growth solution so that the barium could nucleate and precipitate onto the surfaces of titanium via the heterogeneous nucleation process. More importantly, it is found that the amount of added Ba[Cl.sub.2] can be critical for shape and size of BaTi[O.sub.3] nanoparticles.
In this study, we first reported the fabrication of BaTi[O.sub.3] nanoparticles with novel dendrite-like structures through the coprecipitation method, the so-called BaTi[O.sub.3] nanodendrites (BTNDs). It can be observed that the various amounts of added Ba[Cl.sub.2] during nucleation and growth process caused the alteration of the BaTi[O.sub.3] nanoparticles shape, forming the branch-like structures. Until now, to our knowledge, there are no reports yet on the synthesis of the BTNDs by coprecipitation method. A good understanding of the microstructure properties is a very important issue for the potential application of the BTNDs. Thus, a detailed model for the newly observed novel BTNDs is also proposed to explain their possible formation mechanism.
2. Experimental Details
Barium chloride (Ba[Cl.sub.2] x 2[H.sub.2]O, 99%) and oxalic acid ([C.sub.2][H.sub.2][O.sub.4] x 2[H.sup.2]O, 99%) were obtained from Riedel-deHaen (Sigma-Aldrich, USA). Titanium tetrachloride solution (Ti[Cl.sub.4], 99%, 0.1 M) was purchased from Fluka (Sigma-Aldrich, USA). All chemicals and materials were used without further purification. The distilled water used throughout the experiments was purified by a Milli-Q system (Millipore resistivity 18.2 M[OMEGA] cm). The BTNDs were fabricated by first dissolving Ba[Cl.sub.2] in distilled water at 50-70[degrees] C. Separately, oxalic acid was dissolved in distilled water at 65[degrees]C in an ultrasonic tank with titanium tetrachloride slowly added. The two solutions were mixed in an ultrasonic bath at 65[degrees] C. Nanometer-sized BaTi[O.sub.3] particles were formed at this stage. Finally, the growth time was 20 min.
The size and shape of the BTNDs were measured and analyzed by transmission electron microscopy (TEM, JEOL JEM1230) at an accelerating voltage of 80 kV. The microstructure of the BTNDs was observed by high-resolution transmission electron microscopy (HRTeM, Philips Tecnai G2 F20) with an accelerating voltage of 200 kV. The HRTEM was equipped with selected area electron diffraction (SAED) and an energy-dispersive X-ray (EDX) spectrometric element analyzer. The samples for TEM, SAED, and EDX were prepared by drop coating onto a standard 200-mesh, 3 mm, carbon-coated copper grid (Agar Scientific, UK).
3. Results and Discussion
Figures 1(a)-1(d) show the TEM images of BaTi[O.sub.3] nanoparticles obtained by adding 3,4, 5, and 6 mL of Ba[Cl.sub.2]. The results clearly show that the shape of the BaTi[O.sub.3] nanoparticles can be changed by altering the amount of Ba[Cl.sub.2]. When the amount of Ba[Cl.sub.2] was 3 mL, the BaTi[O.sub.3] nanoparticles with large quantities were almost spherical in shape and were small in size, as shown in Figure 1(a). The inset of Figure 1(a) shows the TEM image of BaTi[O.sub.3] nanoparticles at higher magnification, indicating that the particle size is about 20 nm. When the amount of Ba[Cl.sub.2] was increased from 4 to 5 mL, the shape of BaTi[O.sub.3] nanoparticles began to change from spherical to dendrite-like, as shown in Figures 1(b) and 1(c). When the amount of Ba[Cl.sub.2] was 6 mL, the BaTi[O.sub.3] nanoparticles were almost dendrite-like in shape, as shown in Figure 1(d). Even after sonication for TEM sample preparation, the branches of the dendrites were intact, indicating strong bonding between the grains. Thus, there is not any isolated spherical BaTi[O.sub.3] particles in TEM image. However, the role of Ba[Cl.sub.2] may be to act as shape-modifier to change BaTi[O.sub.3] nanoparticles' shape from spherical to dendrite-like structure when the Ba[Cl.sub.2] with high amount was added to growth solution during coprecipitation process. Besides, these results also show that the size of BaTi[O.sub.3] nanoparticles increased as the amount of Ba[Cl.sub.2] increased, as revealed TEM analysis (Figure 1).
Figure 2 shows the low-magnification TEM images of single BTND prepared with 6mL of Ba[Cl.sub.2]. As can be seen in Figure 2(a), the BTND described as dendritic structures has a large area of several square micrometers. The thickness of the central stem of BTND was ~300 nm. Along the central stem (with length of ~20 [micro]m), branching was seen for every ~300nm. The lengths of the side branches were found to be different for the same BTND. Also the angle between the main stem and the branch was not constant for all the cases, as shown in Figure 2(b). The aggregated crystallites may form a BTND by oriented attachment of the crystallites. The inset of Figure 2(b) shows the SAED pattern of the individual grain from the BTND. The characteristic ring in the polycrystalline diffraction pattern confirmed that the BTNDs are polycrystalline structures. Figure 2(c) shows high-magnification TEM image of stem of single BTND, which clearly shows that the dendrite-like structure consisted of eleven large BaTi[O.sub.3] particles and many small BaTi[O.sub.3] compounds between the particles. Figure 2(d) schematically shows the formation mechanism of BTNDs. The BTNDs were formed by aggregation of many small BaTi[O.sub.3] compounds between the large BaTi[O.sub.3] particles during the growth process, indicating that small BaTi[O.sub.3] compounds linked the large BaTi[O.sub.3] particles to form the dendrite-like shape. However, the present study is to show that the amount of Ba[Cl.sub.2] is a key parameter in the formation of BaTi[O.sub.3] nanoparticles with various sizes and shapes.
The BaTi[O.sub.3] nanoparticles produced using the coprecipitation method were analyzed by using EDX for studying the composition of BaTi[O.sub.3] nanoparticles, as shown in Figure 3. The elements detected should be carbon, oxygen, titanium (Ti), and barium (Ba) in the present method. No other elements were detected, indicating that the sample is purely BaTi[O.sub.3]. The peaks of copper (Cu) and carbon in this chart correspond to the Cu grid coated with a thin carbon film as a carrier of the BaTi[O.sub.3] nanoparticles during the test. The above findings support the hypothesis that the formation of BTNDs process is as follows. The relationship between the formation of BTNDs and the amount of Ba[Cl.sub.2] can be easily explained through the chemical formation of BaTi[O.sub.3] particles during oxalate process , as shown in Table 1. The precipitation of monodisperse BaTi[O.sub.3] particles is generally formed with the synthesis of mixed oxalate (Step 1) and the thermal decomposition (Step 2). According to (1) of Step 1, Ti (IV) hydroxo complexes or Ti (IV) polyanions are produced by hydrolysis and condensation reactions. According to (2) of Step 1, starting materials Ti[Cl.sub.4] and Ba[Cl.sub.2] are reacted with water and oxalic acid ([H.sub.2][C.sub.2][O.sub.4]) to precipitate a double oxalate (BaTiO[([C.sub.2][O.sub.4]).sub.2] x 4[H.sub.2]O) precursor. This precursor was obtained by the reaction which proceeds in two steps: (i) initial rapid formation of a Ti-rich gel phase and (ii) slower reaction between the gel phase and the [Ba.sup.2+] left in solution. According to Step 2, this precursor during growth process then results in formation of small BaTi[O.sub.3] compounds (at atomic- or molecular-level compositional homogeneity) through thermal decomposition. Finally, the aggregation and the agglomeration of many small BaTi[O.sub.3] compounds lead to the formation of crystalline BaTi[O.sub.3] particle, and a white BaTi[O.sub.3] particle precipitate can be readily observed. According to (2), the amount of double oxalate precursor is increased as the amount of Ba[Cl.sub.2] increases when the Ti[Cl.sub.4] is enough amounts. In other words, the amount of small BaTi[O.sub.3] compounds is increased with the increase in amount of double oxalate precursor, as shown in Step 2 of Table 1. Thus, the aggregation of small BaTi[O.sub.3] compounds is enhanced when the amount of small BaTi[O.sub.3] compounds increases, resulting in the growth of BaTi[O.sub.3] nanoparticles being enhanced and causing the size of the BaTi[O.sub.3] nanoparticles to be increased. However, the size of BaTi[O.sub.3] nanoparticles is directly proportioned to amount of Ba[Cl.sub.2], with the results being consistent with TEM analysis of Figure 1.
In this study, we propose that the addition of Ba[Cl.sub.2] causes the possible mechanism of BTNDs formation. It is found that a high amount of Ba[Cl.sub.2] led to formation of large BaTi[O.sub.3] particles and small BaTi[O.sub.3] compounds during the coprecipitation growth that caused particle agglomeration to form BTNDs in the growth solution, as shown in Figure 2. The small BaTi[O.sub.3] compounds aggregated onto the surface of the large BaTi[O.sub.3] particles by the van der Waals attractions forces during growth process. It is considered to comprise mainly two processes: (i) the formation of small BaTi[O.sub.3] compounds at the growth process and (ii) the subsequent anisotropic coalescence of these small BaTi[O.sub.3] compounds leading to the BTNDs formation; that is to say, these small BaTi[O.sub.3] compounds with an unstable state show a tendency to undergo fusion into dendrite-like structures. Hence, the amount of Ba[Cl.sub.2] definitely has a critical role in the formation of the BTNDs. However, formation mechanism for BTNDs using the coprecipitation method via Ba[Cl.sub.2] addition is still under investigation.
In summary, this study prepares polycrystalline BTNDs by a simple coprecipitation method. It has been observed that the amount of Ba[Cl.sub.2] plays an important role in the formation of BTNDs. Change in the amount of Ba[Cl.sub.2] from 3 to 6 mL strongly affected the shape of particles from sphere to dendrite-like shape. The formation of BTNDs was induced by aggregation of many small BaTi[O.sub.3] compounds between the several large BaTi[O.sub.3] particles during growth, causing the small BaTi[O.sub.3] compounds to linkto the large BaTi[O.sub.3] particles forming dendrite-like structures. Further measurements are now necessary to get a better understanding of these BTNDs. This preparation of BTNDs is proven to be a simple and effective synthesis method.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was partially supported by the National Science Council of Taiwan (NSCT) under Contract no. NSC 1022221-E-390-019-MY2. The authors gratefully acknowledge the Southern Taiwan University of Technology (Taiwan) for the TEM measurement.
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Chien-Jung Huang, (1) Kan-Lin Chen, (2) Pin-Hsiang Chiu, (3) Po-Wen Sze, (4) and Yeong-Her Wang (3,5)
(1) Department of Applied Physics, National University of Kaohsiung, Nanzih, Kaohsiung, Taiwan
(2) Department of Electronic Engineering, Fortune Institute of Technology, Kaohsiung, Taiwan
(3) Institute of Electro-Optical Science and Engineering, Institute of Microelectronics, National Cheng-Kung University, Tainan, Taiwan
(4) Department of Electro-Optical Science and Engineering, Kao Yuan University, Kaohsiung, Taiwan
(5) Department of Electrical Engineering, Institute of Microelectronics, National Cheng-Kung University, Tainan, Taiwan
Correspondence should be addressed to Chien-Jung Huang; firstname.lastname@example.org and Kan-Lin Chen; email@example.com
Received 15 March 2014; Accepted 26 March 2014; Published 30 April 2014
Academic Editor: Fu-Ken Liu
TABLE 1: Preparation of BaTi[O.sub.3] particles using oxalate process. Step 1: synthesis of mixed oxalate Ti[Cl.sub.4] + [H.sub.2]O [right arrow] TiO[Cl.sub.2] + 2HCl (1) Ba[Cl.sub.2] + TiO[Cl.sub.2] + 2[H.sub.2][C.sub.2][O.sub.4] + 4[H.sub.2]O [right arrow] BaTiO([C.sub.2][O.sub.4])2 x 4[H.sub.2]O + 4HCl (2) Step 2: thermal decomposition BaTiO[([C.sub.2][O.sub.4]).sub.2] x 4[H.sub.2]O [right arrow] BaTiO[([C.sub.2][O.sub.4]).sub.2] + 4[H.sub.2]O (3) BaTiO[([C.sub.2][O.sub.4]).sub.2] [right arrow] 0.5Ba[Ti.sub.2][O.sub.5] + 0.5BaC[O.sub.3] + 2CO + 1.5C[O.sub.2] (4) 0.5Ba[Ti.sub.2][O.sub.5] + 0.5BaC[O.sub.3] [right arrow] BaTi[O.sub.3] [down arrow] + 0.5C[O.sub.2] (5)
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|Title Annotation:||Research Article|
|Author:||Huang, Chien-Jung; Chen, Kan-Lin; Chiu, Pin-Hsiang; Sze, Po-Wen; Wang, Yeong-Her|
|Publication:||Journal of Nanomaterials|
|Date:||Jan 1, 2014|
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