Growth Mechanism of Shape-Controlled Barium Titanate

CRYSTAL GROWTH & DESIGN

Growth Mechanism of Shape-Controlled Barium Titanate Nanostructures through Soft Chemical Reaction Sung-Oong Kang,*,† Bae Ho Park,† and Yong-Il Kim‡ Department of Physics, Konkuk UniVersity, Seoul 143-701, South Korea, and Korea Research Institute of Standards and Science, P.O. Box 102, Daejeon 305-600, South Korea

2008 VOL. 8, NO. 9 3180–3186

ReceiVed August 22, 2007; ReVised Manuscript ReceiVed NoVember 27, 2007

ABSTRACT: Morphology-controlled barium titanate (BaTiO3) nanostructures, from single-crystal nanorods to nanoparticles, were synthesized by tuning the reaction time of a soft chemical process. Based on the morphology change of the nanostructures, the growth mechanism of the shape-controlled BaTiO3 nanostructures by the soft chemical reaction method was suggested. With the reaction time of 42 h, single-crystal BaTiO3 nanorods with flat surface of facets and defined crystal axis were obtained. As the reaction time was extended to 50 h, a certain volume of nanorods converted to nanoparticles generated by a redissolution-growth reaction. When the reaction time was further elongated to 72 h, the volume of nanoparticles in the product notably increased with single grains of particles that have diameters up to ∼200 nm. It was determined that an ion exchange and the structural transformation from the layer structure of the precursor into the perovskite structure prevailed at an early stage of the soft chemical reaction, and then the nanorods changed to nanoparticles due to the redissolution-growth reaction for reaction times over 42 h.

1. Introduction Understanding the growth mechanism of perovskite nanostructures is important for designing shapes and dimensions of nanosized building blocks with perovskite structure, which have wide applications to microelectronics such as multilayer capacitors, electromechanics, high-k dielectrics, etc.1–3 In addition to interest about selective optical, magnetic, and electronic properties depending on the size and dimensionality of nanostructures,4–6 one-dimensional (1-D) ferroelectric perovskite oxides possess significant potential to address requirements for reducing the scale of nonvolatile memory devices and logic circuits. Also, the 1-D ferroelectric nanostructures, including nanorods or wires, nanotubes, and nanobelts, have been expected to contribute to improving performance of electronic devices with ultrahigh density of data bits, lower manufacturing cost, high operation speed, and less power consumption.7–11 Over the past decade, there has been much research into preparing perovskite nanoparticles using various synthetic methods.12 However, only a few efforts have resulted in controlled morphologies and dimensions of perovskite nanostructures. Urban et al. reported for the first time the synthesis of single-crystalline perovskite nanowires by solution-phase decomposition of bimetallic alkoxide precursors in the assistance of coordinating ligands.13 A one-step solid state reaction using nonionic surfactant also produced single-crystalline barium titanate (BaTiO3) nanorods and strontium titanate (SrTiO3) nanoparticles.14 Recently, a template-free hydrothermal process studied the growth of single-crystalline perovskite nanowires and suggested the mechanism of the anisotropic growth of 1-D perovskite nanostructures depending on a role of TiO2 nanoparticle seeds and reacting conditions.15 In our previous research, a soft chemical process for preparing single-crystal BaTiO3 nanorods was introduced by employing the structural transformation from the layer and tunnel structures of precursors. This synthesis approach is considered to be effective for obtaining the particular morphologies of BaTiO3 nanorods featuring * Corresponding author. Tel: +82-42-868-5741. Fax: +82-42-868-5635. E-mail address: [email protected]. † Konkuk University. ‡ Korea Research Institute of Standards and Science.

designed surfaces and defined crystal axis.16 As compared with the well-established mechanisms for the growth behavior of shape-controlled metal nanostructures utilizing the capping reagents,17 an explanation for the growth of perovskite nanostructures produced with different morphologies, especially through a specific synthesis process, has not been fully provided. In this work, we present shape control of BaTiO3 nanostructures, from single-crystal nanorods to nanoparticles, which is feasible in the soft chemical reaction system without using capping reagents. The growth model of morphology-controlled BaTiO3 nanostructures is also proposed by observing timedependent synthesis results of products. Microscopic analysis of nanostructures reveals the defined direction of crystal axis in BaTiO3 nanorods that was structurally transformed from the precursor and the size of single grains in the nanoparticles produced by the redissolution-growth reaction.

2. Experimental Section The starting precursor, 50 mmol of potassium methoxide (CH3OK), was dispersed in 10 mL of ethanol (99.9%), and 50 mmol of titanium ethoxide (Ti(OC2H5)4) was injected into the dispersion of CH3OK. The reactions of hydrolysis and condensation were controlled with 20 mL of deionized water and about 3.5 mL of hydrochloric acid (35%), which adjusted the pH of the solution to 7. The “sol” solution was prepared by stirring at 40 °C for 2 h and kept over 100 h under ambient conditions for aging. The obtained “wet gel” was placed in a drying oven and dried over 48 h at 100 °C. After the drying step, the “xerogel” was heated at 850 °C for 4 h to crystallize potassium tetratitanate (K2Ti4O9) nanostructures with an increasing and decreasing rate of temperature fixed at 300 °C per hour. The precrystallized K2O phase was rinsed out with hot deionized water several times, and the surfacerinsed K2Ti4O9 nanostructures were dried overnight in the drying oven at 80 °C. The precursor K2Ti4O9 nanostructures were dispersed with 40 mL of deionized water and mixed with the solution of barium precursor, 0.3 M barium hydroxide octahydrate (Ba(OH)2 · 8H2O) dissolved in 50 mL of deionized water. The soft chemical reaction was carried out at 90 °C without stirring for 42, 50, 72, and 90 h. After the soft chemical reaction in hot water, the autoclave was quenched with flowing water, and the final product was rinsed with deionized water several times at 90 °C. Phase identification and morphologic observations were performed by powder X-ray diffractometry (Rigaku Dmax2200V) and field emission scanning electron microscopy (FE-SEM, Sirion XI/FEG/ SFEG) and transmission electron microscopy (TEM), respectively. The

10.1021/cg700795q CCC: $40.75  2008 American Chemical Society Published on Web 07/25/2008

Growth of Shape-Controlled BaTiO3 Nanostructures

Crystal Growth & Design, Vol. 8, No. 9, 2008 3181

Figure 1. (a) FE-SEM image of K2Ti4O9 nanostructures; (b) XRD pattern of layer-structured K2Ti4O9 nanostructures; (c) FE-SEM image presenting BaTiO3 nanorods transformed from the precursor of K2Ti4O9 nanostructures; (d) XRD pattern of BaTiO3 nanorods converted from the structure of precursor; (e) representative morphology of BaTiO3 nanorods that have the flat and smooth surfaces of facets and the rectangular cross-section. lattice spacing, selected area diffraction pattern (SAED), energydispersive X-ray spectroscopy (EDS), and crystal axes of nanostructures were studied using a high-resolution transmission electron microscope (HRTEM, Jeol JEM-3000F).

3. Results and Discussion 3.1. Structural Transformation into Single-Crystal BaTiO3 Nanorods. The precursor layer-structured potassium tetratitanate (K2Ti4O9) nanostructures were used to be structurally transformed into BaTiO3 nanorods. The morphologies and structures of the K2Ti4O9 precursor and BaTiO3 nanorods observed by FE-SEM and XRD are presented in Figure 1a-d. In comparison of the morphology and structure of product with those of precursor, it is noticed that the morphology of precursor was retained; however, the structure of the product converted during the soft chemical reaction processed for 42 h at 90 °C.

Both structures of K2Ti4O9 nanostructures and BaTiO3 nanorods index on all Bragg positions of reported literature (K2Ti4O9, ICDD no. 32-0861; BaTiO3, ICDD no. 31-0174). Based on the identical morphologies between the precursor and the product and the converted structure from the precursor to the product, it may be evidenced that BaTiO3 nanorods synthesized in this work were directly transformed from the layer structure of the precursor without morphology modification. Additionally, the representative morphology of BaTiO3 nanorods in Figure 1e shows that BaTiO3 nanorods have flat and smooth surfaces of facets with different diameters of edges from several to ∼200 nm and a rectangular cross-section of nanorod. The structural transformation through the soft chemical reaction originates from the structural properties of K2Ti4O9. The layer structure of K2Ti4O9 has been reported to have a strong ion-exchanging property due to its open structure, as drawn in

3182 Crystal Growth & Design, Vol. 8, No. 9, 2008

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Figure 2. Schematic illustrations of structural transformation from the layer structure of K2Ti4O9 into the perovskite structure of BaTiO3: (a) the crystal structure of layer-structured K2Ti4O9 viewed along the b-axissthe open structure causes the strong ion-exchanging properties of precursor to be converted to the target structure of perovskite; (b) the schematic drawing of intermediate sublattices, in which the K+ sites were replaced with the Ba2+ ions; (c) the crystal structure of perovskite that was structurally transformed through the soft chemical reaction.

Figure 2a. A structural unit of four TiO6 octahedra forms a line of framework by edge-sharing and further constructs staggered sheets with a zigzag string by corner-sharing along the a-axis. Stacking the staggered sheets along the a-axis provides two types of interlayer spaces, narrow and wide interlayer spaces. K+ ions in the layer structure of K2Ti4O9 occupy only the positions in the wide interlayer spaces. The structure of K2Ti4O9 and the ion-exchanging reactions for the derivatives of hydrous titanium dioxide (H2Ti4O9 · nH2O) using an acid treatment have been reported.18–20 Based on the open structure and the ionexchanging properties of the precursor, it is first expected in our synthetic system that the K+ ions in the interlayer spaces are readily ion-exchanged with Ba2+ ions at an earlier stage of structural transformation steps. Then, intermediate sublattices sited by the Ba2+ ions, which are supposed to be structurally

Figure 3. (a) TEM image of BaTiO3 nanorods reacted for 42 h at 90 °C. It was observed that BaTiO3 nanorods have a high yield over 90% in the total volume of product. (b) TEM image of BaTiO3 nanorod. The inset shows the SAED pattern representing the singlecrystal characteristics of BaTiO3 nanorod. (c) HRTEM image of nanorod indicates that the crystal axis of BaTiO3 is vertically aligned to the growth axis of nanorod. (d) EDS analysis of the same BaTiO3 nanorod.



Figure 4. (a) Low-magnification TEM image of product reacted for 50 h at 90 °C. It was observed that a certain volume of nanorods converted to nanoparticle aggregates due to the redissolution-growth reaction. (b) XRD pattern of product reacted for 50 h demonstrates that there was only shape change of product without phase change during the reaction for 50 h.

Figure 5. (a) TEM image of a nanorod prepared for 50 h. The inset displays the SAED pattern of the nanorod showing the single-crystal characteristics of the BaTiO3 nanorod. (b) HRTEM image of the nanorod indicates identical crystal structure in comparison with that of BaTiO3 nanorod prepared for 42 h. (c) TEM image of nanoparticle aggregate prepared by the reaction time of 50 h. The inset shows the SAED pattern of the nanoparticle aggregate presenting the polycrystalline characteristics of nanoparticle aggregates. (d) EDS analysis of nanoparticle aggregate.

transformed into the perovskite structure, are formed as indicated by a dashed line in Figure 2b. The intermediate sublattices further transform into the perovskite structure (Figure 2c) by the shift of the Ba2+ ions and the TiO6 octahedra to the positions of perovskite at a subnanometer scale. During the structural transformation steps, a volume contraction from the intermediate

sublattice to the lattice of perovskite was calculated to be from 0.1404 nm3 of sublattice to 0.0649 nm3 of BaTiO3. When the reaction time was tuned for 42 h, BaTiO3 nanorods were observed to have a high yield of nanorods over 90% in the total volume of product. The low-magnification transmission electron microscopy (TEM) image of BaTiO3 nanorods (Figures


Figure 6. Schematic model of the morphology change to nanoparticle aggregates from nanorods due to the redissolution-growth reaction: (a) the drawing describes that the TiO2 layers in BaTiO3 nanorods dissolve into TiO32- ions, which are supposed to react with the Ba2+ ions, and then BaTiO3 nanoparicles are generated by the nucleation; (b) the schematic for the growth of nanoparticles up to BaTiO3 nanoparticle aggregates by way of the Oswald ripening.

3a) illustrates that the soft chemical reaction for 42 h at 90 °C induced the effective structural transformation into BaTiO3 nanorods from K2Ti4O9 nanostructures. BaTiO3 nanorods deposited on the TEM grid were collected from the same product as XRD pattern was measured and shown in Figure 1c. The selected area electron diffraction (SAED) pattern of the nanorod (the inset of Figure 3b) projected along the [111] direction exhibits clear diffracted spots and represents single-crystal characteristics of BaTiO3 nanorod. The high-resolution TEM (HRTEM) image of nanorod (Figure 3c) displays regularly arranged lattice fringes spaced at ∼4.02 Å. Calculation of lattice spacing and its orientation indicate that the crystal axis of the BaTiO3 nanorod is perpendicular to the growth axis of the nanorod. Figure 3d shows the energy dispersive X-ray spectroscopy (EDS) analysis of the nanorod that is essentially composed of Ba, Ti, and O. The element ratio between Ba and Ti for the BaTiO3 nanorod was measured to be of 53.7 wt % and of 26.4 atom % for Ba and of 30.9 wt % and of 43.6 atom % for Ti. Note that C and Cu peaks are due to the TEM grid. As a result, the designed morphology and the defined crystal axis of BaTiO3 nanorods were obtained by tuning the reaction time for 42 h, in which the ion-exchanging reaction and the structural transformation were dominantly processed at the first stage of soft chemical reaction.

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Figure 7. (a) Low-magnification TEM image of BaTiO3 nanoparticle aggregates prepared for 72 h and (b) XRD pattern of product reacted for 72 h.

3.2. Growth of Nanoparticles by Redissolution-Growth Reaction. In addition to the structural transformation, the growth of BaTiO3 nanoparticles was also expected, as reported the formation of fine BaTiO3 particles on the surfaces of plate-like BaTiO3 microparticles through the soft chemical reaction owing to the redissolution-growth reaction.18 The research about the formation of fine BaTiO3 particles revealed that the fine BaTiO3 particles have a nonpreferred orientation of the crystal axis and the formation of particles significantly depends on the concentration of Ba2+ ions. In our work, the formation of nanoparticle aggregates in the product reacted for 42 h was also observed with a volume of nanoparticles below 10% in the product in comparison with that of nanorods. As the reaction time was prolonged to 50 h, the volume of nanoparticle aggregates in the product increased, as recognized by the TEM image in Figure 4a. The XRD pattern of BaTiO3 nanostructures prepared for 50 h is shown in Figure 4b and indicates that there was no phase change between the precursor and the product despite the morphology change of the product. The TEM image and SAED pattern in Figure 5a display the single-crystal characteristics of the nanorods reacted for 50 h, taken with the electron beam along the [100] zone axis. The corresponding HRTEM in Figure 5b shows the interplanar spacing of lattices with ca. ∼4.02 Å, which is consistent with the separation of lattice planes perpendicularly aligned to the crystal axis of BaTiO3. The crystal structure of BaTiO3 nanorods in the product reacted for 50 h is identical to that of the nanorods reacted for 42 h. Hence, the increased volume of nanoparticles and the identical structure of BaTiO3 nanorods prepared for 50 h reasonably suggest that the nanoparticle aggregates in the



Figure 8. (a) TEM image of nanoparticle aggregate, (b) SAED pattern of the aggregate presents the polycrystalline characteristics, (c) TEM image of nanoparticle with the diameter of ∼200 nm, (d) SAED result of nanoparicle shows the single-crystal characteristics and thus demonstrates that the nanoparticle aggregates are composed of single grains with sizes up to ∼200 nm.

product for 50 h were directly generated from the source of nanorods prepared for 42 h, due to the redissolution-growth reaction. The TEM and SAED results of BaTiO3 nanoparicle aggregates in the product reacted for 50 h (Figure 5c and the inset) illustrate that the as-grown nanoparticle aggregates are polycrystalline. The EDS pattern (Figure 5d) also shows that the chemical components of nanoparticle aggregate consisted of Ba, Ti, and O. The element ratio between Ba and Ti for BaTiO3 nanoparticle aggregate was analyzed to be of 58.9 wt % and of 34.9 atom % for Ba, and of 29.3 wt % and of 49.8 atom % for Ti, respectively. The redissolution-growth reaction converting the morphology of nanorods to the nanoparticle aggregates is believed to originate from dissolution of the titanium oxide (TiO2) layer in the BaTiO3 nanorods into titanium trioxide (TiO32-) ions under a strong base condition. The redissolution of these weak acidic oxides into the metal trioxide ions has been reported in studies of the shape-controlling hydrothermal process and its mechanism of circular transformation on 1-D tellurium nanostructures.21,22 In the strong base solution used in our work (pH of hydrothermal solution was measured to be over 13), the as-dissolved TiO32ions react with the Ba2+ ions and then BaTiO3 nanopartcles with bimodal sizes are generated by way of homogeneous and inhomogeneous nucleation. In the following step, BaTiO3 nanoparticles of larger sizes continuously grow at the expense of smaller-sized nanoparticles through the well-known Oswald ripening process.23,24 The nucleation and growth steps through the redissolution-growth reaction are schematically presented in Figure 6a. The nucleated and grown BaTiO3 nanoparticles further undergo aggregation up to the polycrystalline nanoparticle aggregates, as the growth model is described in Figure

6b. The equations of structural transformation and redissolutiongrowth reaction in the soft chemical reaction may be expressed as follows

K2Ti4O9 + 4Ba2+ + 8OH- f 4BaTiO3 + 2KOH + 3H2O (1) BaTiO3 + 2OH- f TiO32- + Ba2+ + H2O f BaTiO3 + H2O

(2)

The low-magnification TEM image in Figure 7a shows that the volume and diameters of nanoparticle aggregates notably increased when the reaction time was extended to 72 h. The resulting XRD pattern of BaTiO3 nanostructures (Figure 7b) produced for 72 h also demonstrates that there was only morphology change without phase change of the product, identically as observed in the product for 50 h. The morphologic and structural investigations of product prepared for 72 h support that the redissolution-growth reaction further progressed as the reaction time was prolonged after the structural transformation steps. The TEM image and the SAED pattern of nanoparticle aggregates are displayed in Figure 8a,b. The ring pattern of the nanoparticle aggregates exhibits the polycrystalline characteristics of BaTiO3 nanoparticle aggregate composed of single grains of nanoparticles. From the SAED pattern of an individual nanoparticle showing the clear spots (Figure 8c,d), it is believed that the sizes of single grains composing the nanoparticle aggregates range up to ∼200 nm. With the reaction time over 90 h, the morphology of BaTiO3 nanostructures finally converted to the predominant volume of nanoparticle aggregates over 90% in the product, as shown in Figure 9a. The XRD pattern of the BaTiO3 nanoparticle


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4. Conclusion On a basis of microscopic and structural observations for the shape change of BaTiO3 nanostructures prepared by tuning the periods of reaction time, the mechanism on the growth of morphology-controlled BaTiO3 nanostructures through the soft chemical reaction system was proposed. In the first step of soft chemical reaction, the ion exchange and structural transformation were determined to prevail over the redissolution-growth reaction, and thus the production of single-crystal BaTiO3 nanorods was attainable. However, after the structural transformation steps, the product of BaTiO3 nanoparticle aggregates consisted of single grains with sizes up to ∼200 nm. In summary, a feasible synthesis approach to collectively produce the nanostructures with the different morphologies and crystallinity was proposed by controlling an experimental parameter, the reaction time of the soft chemical process. Acknowledgment. This work was supported by the KOSEF NRL Program grant funded by the Korea government (MEST; No. R0A-2008-000-20052-0) and Ministry of Knowledge Economy under the Program of Energy Technology Innovation Program (No. 2007-M-CC23-P-09-1-000).

References

Figure 9. (a) FE-SEM image of BaTiO3 nanoparticle aggregates produced by the reaction time for 90 h. The volume of nanoparticle aggregates was observed to be over 90% in the product. (b) XRD data of the product prepared for 90 h.

aggregates reacted for 90 h (Figure 9b) indicates that the phase of product was maintained while the shape of the nanostructures changed through the redissolution-growth reaction. As a result, polycrystalline BaTiO3 nanoparticle aggreagates were collectively prepared by tuning the reaction time for 90 h. In order to investigate the effects of factors dominating the crystallization of the BaTiO3 phase from K2Ti4O9 phase, such factors as reaction temperatures and concentration of the Ba2+ ions were selectively varied. As for the reaction temperatures, an optimized reaction temperature for the transformation to BaTiO3 nanorods ranges from 85 to 95 °C. However, when the reaction was carried out below 85 °C with concentration of Ba2+ ions fixed at 0.3 M, the exchange rate from the precursor to the product turned out to be low. For instance, the product soft chemically reacted at 80 °C for 72 had a relatively high portion of unreacted K2Ti4O9 nanostructure, over 20% determined by the intensities of XRD results. In contrast, reaction at >95 °C with the concentration of 0.3 M led to a high exchange rate that was difficult to control for tuning the morphologies of product to the nanorods. In the case of reaction at 100 °C for 42 h, the volume of nanoparticles increased over about 40% in the product and the morphology of nanorods changed from that of precursor. Additionally, the exchange rate also depends on the concentration of Ba2+ ions; an optimal concentration for synthesizing the nanorods is 0.3 M. At the concentration of 0.25 M, the exchange rate was low and produced a relatively high proportion of unreacted K2Ti4O9 precursor in the final product after reaction for 42 h at 90 °C. Alternatively, the reaction with the concentration of 0.35 M for 42 h at 85 °C produced a high volume of nanoparticle aggregates in the product owing to the high exchanging rate.

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Growth Mechanism of Shape-Controlled Barium Titanate

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