Effects of Surface Area of Titanium Dioxide Precursors on the

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Effects of Surface Area of Titanium Dioxide Precursors on the Hydrothermal Synthesis of Barium Titanate by Dissolution− Precipitation Ki Ho Ahn,† Young-Ho Lee,† Minsoo Kim,† Hong-shik Lee,‡ Yong-Suk Youn,† Jaehoon Kim,§ and Youn-Woo Lee*,† †

School of Chemical and Biological Engineering and Institute of Chemical Process, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea ‡ Supercritical Fluid Research Laboratory, Energy & Environment Research Division, Korea Institute of Science and Technology, 5 Wolsong-gil, Seongbuk-gu, Seoul 136-791, Korea § School of Mechanical Engineering, SKKU Advanced Institute of Nano Technology, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 440-746, Korea ABSTRACT: We have attempted to study the formation of BaTiO3 by hydrothermal synthesis and our experiments reveal the reaction by dissolution−precipitation as the dominant operating mechanism. Two major formation mechanisms of BaTiO3 by hydrothermal synthesis involving either in situ transformation or dissolution−precipitation have been previously proposed. However, the exact formation mechanism is yet to be clarified unambiguously. To investigate the mechanism in detail, we used Ba(OH)2 and three types of TiO2 nanoparticles as precursors for the synthesis. The morphology of the BaTiO3 obtained was cubic (with rounded edges), regardless of the morphology of the TiO2 used, which indicated that the synthesis followed the dissolution−precipitation mechanism. In this mechanism, the surface area of TiO2, which affects its dissolution rate, played a critical role in determining the crystal size of the BaTiO3 product. In this paper, we have also discussed the effects of the synthesis conditions on BaTiO3 particle formation.

1. INTRODUCTION Barium titanate, because of its outstanding dielectric properties, is one of the most widely used perovskite-structured materials.1−3 The applications of barium titanate in the electronics industry include its use in multilayer ceramic capacitors (MLCCs), thermistors, and gas sensors. Capacitors with small sizes and high capacitances are in demand with the view of miniaturizing advanced electronic devices.4 Consequently, to produce these capacitors, many research studies have pursued the synthesis of barium titanate nanoparticles with small sizes and narrow size distribution.5,6 Barium titanate has been synthesized by solid-state method,7 microwave synthesis,8 oxalate-route synthesis,9 sol−gel method,5,10−13 hydrothermal reaction,4,6,14−20 and spray pyrolysis.21,22 Among these synthesis methods, the hydrothermal method has many advantages. In the hydrothermal route, particles with small sizes, high crystallinity, and uniformity are achieved by a simple process. Hydrothermal synthesis uses titanium dioxide and barium hydroxide precursors, which react through two types of mechanisms.6,16 One is an in situ transformation mechanism, in which the surface of titanium dioxide reacts initially with barium ions. This reaction produces a barium titanate layer on the titanium dioxide surface, through which the barium ions diffuse and react with titanium dioxide gradually. The barium ions are supplied continuously until the titanium dioxide at the core is consumed and whole titanium dioxide precursor particle is transformed to barium titanate. Barium titanate particles obtained by this mechanism exhibit sizes and morphologies similar to that of the precursor titanium © 2013 American Chemical Society

dioxide, and both these morphologies are maintained during the transformation. The other mechanism of barium titanate formation is the dissolution−precipitation mechanism. By this mechanism, the Ti−O bonds are first dissolved in water to form [Ti(OH)4‑x]x+ ion. The hydroxyl titanium complex ion is sufficiently instable to react with barium ions to form barium titanate rapidly, following which the barium titanate particles are precipitated.23 Hydrothermal synthesis of metal oxide nanoparticles using supercritical fluids has been studied recently,24−31 which has been identified as a promising process to produce fine monodisperse particles rapidly and continuously. Atashfaraz et al.24 have suggested a mechanism for barium titanate formation in supercritical water and have reported on the effects of pH on titanium dioxide solubility in supercritical water. Masui et al.27 have synthesized barium titanate nanoparticles using a continuous system and have identified the structure of synthesized barium titanate nanoparticles. In the case of barium titanate, size is a significant factor affecting the crystal structure, which in turn determines the properties of the material.32−34 Many researchers have reported the size-tunable synthesis of barium titanate nanoparticles using various methods.35,36 Received: Revised: Accepted: Published: 13370

April 12, 2013 August 13, 2013 August 22, 2013 August 22, 2013 dx.doi.org/10.1021/ie401161x | Ind. Eng. Chem. Res. 2013, 52, 13370−13376

Industrial & Engineering Chemistry Research

Article

The objective of our study is to investigate the formation mechanism of barium titanate synthesized by the hydrothermal method using three types of titanium dioxide precursors with different sizes and shapes as starting materials.

microscopy (specifically, TEM), each type of titanium dioxide precursor used was also characterized by Brunauer, Emmett, and Teller (BET) analysis (ASAP2010, Micromeritics) to analyze the specific surface area characteristics.

2. EXPERIMENTAL SECTION 2.1. Materials. Barium hydroxide octahydrate ((Ba(OH)2· 8H2O, 97.0%, Junsei Chemicals Co., Ltd., Tokyo, Japan) was used as the barium precursor. Three different types of titanium dioxide precursors were used in the syntheses. These included (1) titanium dioxide powders (anatase, 99.8%, Aldrich Chemicals Co. Ltd., St. Louis, Missouri, USA) containing particles 110 nm in size, (2) P25 (Degussa, Frankfurt, Germany) with particles 25 nm in size, and (3) titanium dioxide nanorods prepared via hydrothermal synthesis.37 Each of these titanium dioxide precursors will henceforth be denoted as T110, T25, and TNR, respectively. Deionized water was prepared from the Milli-Q Advantage A10 ultrapure water purification system. 2.2. Experimental Procedure. Reactions were conducted in a batch type reactor (SUS 316, volume = 23 mL). Barium hydroxide, titanium dioxide, and water were added into the batch reactor. The molar concentration of titanium dioxide was fixed at 0.036 M and the molar ratio of barium hydroxide to titanium dioxide was varied in the range 1−3. The experimental conditions used are listed in Table 1. The batch reactor

3. RESULT AND DISCUSSION 3.1. Investigation of Formation Mechanism of Barium Titanate. Barium titanate nanoparticles synthesized using T110 and T25 (shown in Figure 1a, b) exhibited different sizes, as shown in the SEM and TEM images in Figure 1c−j. From T110 (Figure 1a), barium titanate nanoparticles of ∼110 nm (Figure 1c, d, g, and h) were synthesized. However, barium titanate nanoparticles with sizes of ∼30 nm (Figure 1e, f, i, j) were synthesized using T25 (Figure 1b). Despite the identical synthetic conditions (i.e., temperature and pressure of the reactions) used with each type of titanium dioxide particles (with different sizes) for the reaction, the sizes of the synthesized barium titanate were different. By controlling the reaction time, initial molar ratio of barium to titanium, and pH, without altering the sizes of the titanium dioxide precursors, barium titanate nanoparticles with similar sizes were obtained. Hence, from these results, it can be inferred that the size of the synthesized barium titanate particles was affected by the size of titanium dioxide precursor used. The sizes of the barium titanate nanoparticles synthesized with T110 and T25 were calculated from the XRD patterns in Figures 7 and 8 using Scherrer’s equation as 89 and 21 nm, respectively. These results also indicate that the sizes of the barium titanate nanoparticles synthesized depended on the size of the titanium dioxide nanoparticle precursors used. Because P25 was a mixture of anatase and rutile, it was necessary to determine whether the structure of the titanium dioxide precursor exerted any effect on the sizes of the barium titanate synthesized. Pure anatase powder containing particles ∼20 nm in size, prepared using supercritical water,38 were subsequently used to synthesize barium titanate. Images a and b in Figure 2 are the TEM and SEM images of the prepared pure anatase and barium titanate synthesized from the anatase powder, respectively. Although the structure of the precursor titanium dioxide was pure anatase, barium titanate particles with sizes similar to those synthesized using P25 were obtained. Hence, the effect of the crystal structure of titanium dioxide was negligible. It is noteworthy that Hu et al.5 reported a similar result for the synthesis of barium titanate by hydrothermal process. They indicated that because the size and morphology of the barium titanate synthesized were similar to that of the precursor titanium dioxide, barium titanate was formed via an in situ transformation mechanism. Habib et al.35 also reported the effect of the sizes of titanium dioxide precursors on the sizes of barium titanate synthesized and suggested that the synthesis occurred by in situ transformation. If barium titanate synthesis carried out in our study followed the in situ transformation mechanism, the morphology and size of barium titanate nanoparticles can be expected to be similar or identical to that of the titanium dioxide precursors. To examine the formation mechanism of barium titanate, we used TNR (Figure 3a) with average diameter of 11 nm (8−13 nm) and average length of 85 nm (77−150 nm) as the precursors for the barium titanate synthesis. Even though rod-shaped titanium dioxide was used as the precursor, cubic barium titanate nanoparticles with rounded edges were synthesized. This indicated that the barium titanate formation, under the experimental conditions we used, occurred by the dissolution−precipitation mechanism

Table 1. Experimental Conditions Used for the Synthesis of Barium Titanate run

T (°C)

pressure (bar)

1 2 3 4 5 6

400 400 400 400 400 400

300 300 300 300 300 300

7 8 9 10 11 12 13 14

400 400 400 400 400 400 400 400

300 300 300 300 300 300 300 300

type of TiO2 T110 T110 T25 T25 TNR T100:T25 = 1:1 T110 T110 T110 T110 T100 T100 T25 T25

time (min)

Ba/ Ti

10 20 10 20 10 20

2 2 2 2 2 2

10 20 20 20 20 20 10 10

1 1 1.5 2 3 2 1 1.5

KOH (mol) 0.2 0.2

0.05

containing the precursors in the reaction medium was placed in a molten salt bath. All the reactions were carried out at 400 °C and 300 bar. After the reaction, the synthesized particles were washed with water several times to remove any residual barium hydroxide reactant. The washed particles were then dried in a vacuum oven for 12 h at 60 °C. 2.3. Analytical Method. X-ray diffraction (XRD) patterns of the synthesized particles were obtained using a diffractometer (D-MAX-2500 PC, Rigaku Co., Tokyo, Japan) with Cu Kα radiation (generated at 50 kV and 200 mA). The detector moved stepwise at 10°/min covering 2θ values ranging from 10 to 90°. The particles obtained were characterized by transmission electron microscopy (TEM, JEM-3010, JEOL Co., Tokyo, Japan) and field-emission scanning electron microscopy (FE-SEM, Auriga, Carl Zeiss, Germany). In addition to electron 13371

dx.doi.org/10.1021/ie401161x | Ind. Eng. Chem. Res. 2013, 52, 13370−13376


Article

Figure 1. TEM images of (a) T110, (b) T25, (c) barium titanate nanoparticles synthesized using T110 with 10 min of reaction time, (d) barium titanate nanoparticles synthesized using T110 with 20 min of reaction time, (e) barium titanate nanoparticles synthesized using T25 with 10 min of reaction time, and (f) barium titanate nanoparticles synthesized using T25 with 20 min of reaction time. SEM images of barium titanate nanoparticles synthesized using (g) T110 with 20 min of reaction time, (h) T110 with 20 min of reaction time, (i) T25 with 10 min of reaction time, and (j) T25 with 20 min of reaction time.

Figure 2. (a) TEM image of the pure anatase nanopowders prepared by hydrothermal synthesis using supercritical water and (b) SEM image of the barium titanate nanaoparticles synthesized using these powders.

Figure 3. TEM images of (a) TNR and (b) barium titanate nanoparticles synthesized using TNR.

[Ti(OH)4 − x ]x + + Ba 2 + + 2OH− → BaTiO3(s) + 2H 2O (2)

and not by in situ transformation. Scheme 1 explains the formation mechanisms of barium titanate and the effects of the various types of titanium dioxide precursors on the barium titanate sizes. 3.2. Adjustment of Size of Barium Titanate during Hydrothermal Synthesis. In the case of dissolution− precipitation mechanism, barium titanate is formed by two reactions. TiO2 (s) + 2H 2O → [Ti(OH)4 − x ]x +

Following this mechanism, the differences in the sizes of barium titanate obtained from the different of titanium dioxide precursors can be explained. The nucleation rate of barium titanate is determined by the rate of reaction 1 or 2. Reaction 1 is the dissolution of titanium dioxide in water and reaction 2 is the precipitation of barium titanate. If the nucleation rate of barium titanate is determined by reaction 2, the size of the synthesized particles can be expected to be regular regardless of the types of titanium precursors. Thus, in our case, the nucleation rate of barium titanate was perhaps, governed by

(1) 13372



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Scheme 1. Representation of the Dissolution−Precipitation Mechanism and the Effect of the Specific Surface Area of Precursor Titanium Dioxide Particles on the Sizes of the Synthesized Barium Titanate Particles

the size of the barium titanate synthesized was affected by the specific surface area of the precursor titanium dioxide. The size of the barium titanate synthesized was specifically, inversely proportional to the specific surface area of titanium oxide, i.e., larger the specific surface area of titanium dioxide, faster was reaction 1. As shown in Scheme. 1, a higher number of barium titanate nuclei were formed from T25 than from T110 because of the higher initial dissolution rate of T25. Essentially, the dissolution rate of titanium dioxide determined the rate of supersaturation and nucleation, and hence, the number of nuclei. The number of nuclei dictated the sizes of the barium

reaction 1. Further, from the BET analysis of titanium dioxide (the results of which are shown in Table 2), it was assumed that Table 2. BET Analysis of Each Type of Titanium Oxide Precursor Used for the Synthesis of Barium Titanate sample

surface area (m2/g)

T110 T25 TNR*

8.1 43.0 34.2

Figure 4. XRD patterns of barium titanate nanoparticles synthesized using (a) T110 and (b) T25 at various temperatures. 13373



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Figure 5. XRD patters of synthesizes barium titanate using T110 with reaction time of 10 min and [Ba]/[Ti] of 1(Experiment 7 in Table 1).

Figure 6. (a) TEM image of barium titanate synthesizes barium titanate using T110 with reaction time of 10 min and [Ba]/[Ti] of 1(Experiment No. 7 in Table 1), and HR-TEM images of (b) small particle in a, (c) large particle in a.

residual titanium dioxide. XRD pattern of the synthesized particles is shown in Figure 5. Each of the peaks (2θ = 22.0, 31.5, 38.9, 45.1, 51.0, 56.2, 65.75, 70.5, 79.4, 83.5, and 87.7°) was matched to the barium titanate data available in the database (JCPDS card no.83−1879) and the sharp peaks show that particles with high crystallinity were produced. However, the peak corresponding to unreacted titanium dioxide was also observed at 2θ of ∼25.3°, which suggested that the reaction conditions were insufficient to synthesize pure barium titanate. Figure 6a shows TEM image of synthesized particles. The lattice of each large and small particle was examined. D-space of small particles in Figure 6a was 0.35 nm (Figure 6b), while that of large particles in Figure 6a was 0.28 and 0.4 nm (Figure 6c). From JCPDS data, each D-space value is matched to titanium dioxide (Card No. 84−1876) and barium titanate (Card No. 83−1879). From this analysis, it is inferred that large barium titanate particles with small segregated titanium dioxide particles are shown in Figure 6a. The presence of residual titanium dioxide indicated the low solubility of titanium dioxide. Hence, to increase the solubility of titanium dioxide, we adjusted several process parameters. An increase in the reaction time resulted in a decrease in the amount of unreacted titanium dioxide. The barium to titanium molar ratio in the reaction mixture was also modified (synthesis runs 5, 6, and 7 in Table 1). The XRD data revealed that the molar ratio of barium to titanium in the precursor mixture affected the titanium dioxide conversion. The amount of residual titanium dioxide decreased markedly with an increase in the molar ratio

titanate particles. Higher number of nuclei limited the crystal growth because of the lack of supply of precursors for crystal growth. From the Scheme 1 we propose, it can be concluded that the specific surface area of the precursor titanium dioxide particles determined the sizes of the barium titanate particles synthesized. The differences in the reaction rate of titanium oxide with different specific surface areas were demonstrated clearly at the low reaction temperatures used in the present study. Figure 4 shows the results of the XRD analysis of the synthesized particles at low temperature. Although all the synthesis conditions were identical, with the exception of the precursor particle sizes, the conversions of titanium dioxide were distinctly different. At 200, 250, and 300 °C, peak of unreacted titanium dioxide was rarely observed around 2θ of ∼25.3° in Figure 4b, whereas the peak of unreacted titanium dioxde was detected clearly in Figure 4a. Thus, it was clear that almost all of T25 was converted to barium titanate, whereas T110 was only partially converted to barium titanate under these conditions. 3.3. Optimization of Operation Conditions; Effect of Reaction Time, Initial Molar Ratio of Barium to Titanium, and pH. As discussed previously, the reaction rate of titanium dioxide depended on its specific surface area. Thus, it was necessary to study the optimal process conditions corresponding to the use of the different titanium dioxide precursors. Several experiments were carried out to investigate the optimal conditions to produce barium titanate without 13374



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of barium to titanium in the reaction mixture. Increasing the molar ratio of barium to titanium resulted in a higher pH of the reaction medium (i.e., water, in this case) than that obtained in synthesis run 4 (Table 1). Because for the solubility of titanium dioxide increases at higher pH, the molar ratio of barium to titanium in the precursor solution is critical to optimally synthesize barium titanate nanoparticles. However, solely increasing the molar ratio of barium to titanium in the reaction mixture still resulted in the synthesis of impure barium titanate particles. To synthesize pure barium titanate, KOH was added to the reaction medium to increase the pH of the solution (experiment runs 1, 9, and 10 in Table 1.). The solubility of titanium dioxide is higher in the presence of a strong base.24 The XRD patterns of the particles synthesized with KOH addition are shown in Figure 7a. Figure 8. XRD patterns of barium titanate particles synthesized using T25 by varying the barium to titanium ratios (synthesis runs 3, 13, and 14 in Table 1).

that the barium titanate nanoparticle synthesis occurred via the dissolution−precipitation mechanism. This mechanism consisted of two steps, which included the dissolution of titanium dioxide and the subsequent formation of barium titanate. The dissolution step determined the size of barium titanate, during which, the specific surface area of titanium dioxide played a key role in controlling the dissolution rate. Our results confirmed that the size of barium titanate was inversely proportional to the specific surface area of titanium dioxide. Thus, the specific surface area of titanium dioxide was a crucial factor in controlling the size of barium titanate synthesized by hydrothermal process. In addition, the synthesis conditions were also optimized. The reaction time, the molar ratio of barium to titanium in the initial reaction mixture, and the pH of the solution were adjusted by extensive experimentation. Optimum synthesis of pure barium titanate with T110 could be achieved by using reaction time of ∼20 min and molar ratio of barium to titanium in the initial reaction mixture of ∼2 along with addition of a suitable amount of KOH. Using T25, optimum preparation of pure barium titanate could be carried out with reaction mixtures containing barium and titanium in the molar ratio of ∼2 without KOH addition, using reaction time of ∼20 min.

Figure 7. XRD patterns of barium titanate particles synthesized with a (a) reaction time of 20 min, [Ba]/[Ti] = 2, and [KOH]/[Ti] = 2 (synthesis run 2, Table 1), (b) reaction time of 20 min, [Ba]/[Ti] = 2, and [KOH]/[Ti] = 0.5 (synthesis run 11, Table 1), and (c) reaction time of 20 min and [Ba]/[Ti] = 2, without the addition of KOH (synthesis run 12, Table 1).

Comparing Figure 7a to 7c, it can be observed that the conversion of titanium dioxide was different. The peak corresponding to the presence of unreacted titanium dioxide can be observed in Figure 7c and the synthesis of pure barium titanate nanoparticles found possible with the addition of KOH as shown in Figure 7a. Hence, the presence of KOH, a strong base (pKa = 0.5), was required to synthesize pure barium titanate using barium hydroxide, which is a weak base (pKb = −2.02). Adding KOH accelerated the solubility of titanium dioxide to result in pure barium titanate particles. The XRD results of barium titanate synthesized using T25 are shown in Figure 8 (synthesis run 2, Table 1). Unlike the samples obtained using T110, peaks of unreacted titanium dioxide were absent, even without KOH addition, indicating that the dissolution rate of T25 was different from that of T110. Consequently, by merely adjusting the molar ratio of barium to titanium in the initial reaction mixture, pure barium titanate nanoparticles were synthesized.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-880-1883. Fax: +82-2883-9124. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION In conclusion, barium titanate nanoparticles with different sizes were prepared by hydrothermal process by using various types of titanium dioxide precursors with different sizes and morphologies. From the results of our study, we observed 13375



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Effects of Surface Area of Titanium Dioxide Precursors on the

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