The Novel Formation of Barium Titanate Nanodendrites

1 Department of Applied Physics, National University of Kaohsiung, Nanzih, Kaohsiung, Taiwan 2Department 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 4Department of Electro-Optical Science and Engineering, Kao Yuan University, Kaohsiung, Taiwan 5Department of Electrical Engineering, Institute of Microelectronics, National Cheng-Kung University, Tainan, Taiwan


Introduction
Over the past few years, the unique ferroelectric, piezoelectric, and thermoelectric properties of barium titanate (BaTiO 3 ) nanoparticles have become increasingly important in the electronic ceramics industry.The BaTiO 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][2][3][4][5][6][7][8][9].Among these applications, performance and characteristics are strongly influenced by size, shape, composition, morphology, spatial ordering, and impurities of the BaTiO 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 BaTiO 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 BaTiO 3 nanoparticles is extremely dependent on preparation method and synthesis procedure.
Traditionally, the BaTiO 3 particle is prepared by the solidstate reaction method through heating BaCO 3 and TiO 2 at high temperature as 1200 ∘ 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 m).Up to now, various new preparation methods have been developed and reported in fabricating BaTiO 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 [22], the soft chemical process [23], the glycolthermal method [24], and the coprecipitation method [25].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 [26].
In the coprecipitation method, the preparation of BaTiO 3 nanoparticles through the coprecipitation of barium and titanium hydroxides from aqueous solutions has been reported since the early Flaschen research work [27].Synthesis of BaTiO 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][29][30][31][32].Although these previous studies succeeded in fabricating BaTiO 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 BaTiO 3 nanoparticles.
In our laboratory, we developed a simple procedure by slightly modifying the multistage process so it could be applied to fabricate BaTiO 3 nanoparticles with wellcontrolled size.In this simple procedure, appropriate amount of stock solution of titanium tetrachloride (TiCl 4 ), barium chloride (BaCl 2 ), and oxalic acid was added in deionized water to form growth solution.The BaTiO 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 BaCl 2 can be critical for shape and size of BaTiO 3 nanoparticles.
In this study, we first reported the fabrication of BaTiO 3 nanoparticles with novel dendrite-like structures through the coprecipitation method, the so-called BaTiO 3 nanodendrites (BTNDs).It can be observed that the various amounts of added BaCl 2 during nucleation and growth process caused the alteration of the BaTiO 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.

Experimental Details
Barium chloride (BaCl 2 ⋅2H 2 O, 99%) and oxalic acid (C 2 H 2 O 4 ⋅2H 2 O, 99%) were obtained from Riedel-deHa ë n (Sigma-Aldrich, USA).Titanium tetrachloride solution (TiCl 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 MΩ cm).The BTNDs were fabricated by first dissolving BaCl 2 in distilled water at 50-70 ∘ C.Separately, oxalic acid was dissolved in distilled water at 65 ∘ C in an ultrasonic tank with titanium tetrachloride slowly added.The two solutions were mixed in an ultrasonic bath at 65 ∘ C. Nanometer-sized BaTiO 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 JEM-1230) 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 energydispersive 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).

Results and Discussion
Figures 1(a)-1(d) show the TEM images of BaTiO 3 nanoparticles obtained by adding 3, 4, 5, and 6 mL of BaCl 2 .The results clearly show that the shape of the BaTiO 3 nanoparticles can be changed by altering the amount of BaCl 2 .When the amount of BaCl 2 was 3 mL, the BaTiO 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 BaTiO 3 nanoparticles at higher magnification, indicating that the particle size is about 20 nm.When the amount of BaCl 2 was increased from 4 to 5 mL, the shape of BaTiO 3 nanoparticles began to change from spherical to dendrite-like, as shown in Figures 1(b) and 1(c).When the amount of BaCl 2 was 6 mL, the BaTiO 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 BaTiO 3 particles in TEM image.However, the role of BaCl 2 may be to act as shape-modifier to change BaTiO 3 nanoparticles' shape from spherical to dendrite-like structure when the BaCl 2 with high amount was added to growth solution during coprecipitation process.Besides, these results also show that the size of BaTiO 3 nanoparticles increased as the amount of BaCl 2 increased, as revealed TEM analysis (Figure 1).
Figure 2 shows the low-magnification TEM images of single BTND prepared with 6 mL of BaCl 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 m), branching was seen for every ∼300 nm.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  shows the formation mechanism of BTNDs.The BTNDs were formed by aggregation of many small BaTiO 3 compounds between the large BaTiO 3 particles during the growth process, indicating that small BaTiO 3 compounds linked the large BaTiO 3 particles to form the dendrite-like shape.However, the present study is to show that the amount of BaCl 2 is a key parameter in the formation of BaTiO 3 nanoparticles with various sizes and shapes.The BaTiO 3 nanoparticles produced using the coprecipitation method were analyzed by using EDX for studying the composition of BaTiO 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 BaTiO 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 BaTiO 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 BaCl 2 can be easily explained through the chemical formation of BaTiO 3 particles during oxalate process [33], as shown in Table 1.Finally, the aggregation and the agglomeration of many small BaTiO 3 compounds lead to the formation of crystalline BaTiO 3 particle, and a white BaTiO 3 particle precipitate can be readily observed.According to (2), the amount of double oxalate precursor is increased as the amount of BaCl 2 increases when the TiCl 4 is enough amounts.In other words, the amount of small BaTiO 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 BaTiO 3 compounds is enhanced when the amount of small BaTiO 3 compounds increases, resulting in the growth of BaTiO 3 nanoparticles being enhanced and causing the size of the BaTiO 3 nanoparticles to be increased.However, the size of BaTiO 3 nanoparticles is directly proportioned to amount of BaCl 2 , with the results being consistent with TEM analysis of Figure 1.
In this study, we propose that the addition of BaCl 2 causes the possible mechanism of BTNDs formation.It is found that a high amount of BaCl 2 led to formation of large BaTiO 3 particles and small BaTiO 3 compounds during the coprecipitation growth that caused particle agglomeration to form BTNDs in the growth solution, as shown in Figure 2. The small BaTiO 3 compounds aggregated onto the surface of the large BaTiO 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 BaTiO 3 compounds at the growth process and (ii) the subsequent anisotropic coalescence of these small BaTiO 3 compounds leading to the BTNDs formation; that is to say, these small BaTiO 3 compounds with an unstable state show a tendency to undergo fusion into dendrite-like structures.Hence, the amount of BaCl 2 definitely has a critical role in the formation of the BTNDs.However, formation mechanism for BTNDs using the coprecipitation method via BaCl 2 addition is still under investigation.

Conclusions
In summary, this study prepares polycrystalline BTNDs by a simple coprecipitation method.It has been observed that the amount of BaCl 2 plays an important role in the formation of BTNDs.Change in the amount of BaCl 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 BaTiO 3 compounds between the several large BaTiO 3 particles during growth, causing the small BaTiO 3 compounds to link to the large BaTiO 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.

Figure 3 :
Figure 3: TEM image of single BTND and corresponding EDX spectra.