Shape-Controlled Monocrystalline Ferroelectric Barium Titanate

May 20, 2008 - or ordered architectures of coral-like nanostructures of assembled nanorods, starfish-like nanostructures, and sword-like nanostructure...
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J. Phys. Chem. C 2008, 112, 8634–8642

Shape-Controlled Monocrystalline Ferroelectric Barium Titanate Nanostructures: From Nanotubes and Nanowires to Ordered Nanostructures Ningzhong Bao,*,† Liming Shen,† Gopalan Srinivasan,‡ Kazumichi Yanagisawa,§ and Arunava Gupta*,† Center for Materials for Information Technology (MINT), The UniVersity of Alabama, Tuscaloosa, Alabama 35487, Physics Department, Oakland UniVersity, Rochester, Michigan 48309, and Research Laboratory of Hydrothermal Chemistry, Kochi UniVersity, Kochi 780-8520, Japan ReceiVed: March 8, 2008; ReVised Manuscript ReceiVed: April 23, 2008

We report for the first time on the controlled hydrothermal synthesis of barium titanate nanostructures using Na2Ti3O7 nanotubes and nanowires as synthetic precursors. A variety of nanostructured BaTiO3 have been prepared, exhibiting either simple shapes of nanowires, nanosheets, nanocubes, and hexagonal nanoparticles or ordered architectures of coral-like nanostructures of assembled nanorods, starfish-like nanostructures, and sword-like nanostructures. The shapes of the various BaTiO3 products are found to be dependent on the concentration of Ba(OH)2, the temperature, and the nature of the precursors. The synthesis route exploits the differences in the hydrothermal stability of the Na2Ti3O7 nanotubes and nanowires and the temperaturedependent crystal structure of barium titanate. Various nanoblocks, including nanosheets and nanorods formed from the Na2Ti3O7 nanotubes and nanowires, respectively, grow and assemble to form the ordered BaTiO3 nanostructures. This represents a new approach that is capable of assembling ordered perovskite nanostructures using relatively large nanoblocks formed from layered alkali-metal titanates. The process offers more flexibility than those using inorganic titanium salts or organometallic titanium compounds, which commonly leads to the formation of only BaTiO3 nanoparticles. 1. Introduction Nanomaterials are being widely studied because of their uniquesizeandshape-dependentpropertiesinreduceddimensions.1,2 In order to realize novel nanotechnology applications, the research work is now expanding toward the assembly of nanoparticles to form ordered nanostructures utilizing a variety of methods, including self-assembly of colloids, templating, etc., for which the monodispersity of the nanoparticles is important.3,4 However, nanostructures based on nonspherical nanoparticles are difficult to create directly because of their anisotropic structure, and thus solid templates, substrates, and patterned catalyst layers are often required.5–7 Thus, the development of simple and effective methods to directly create ordered architectures is of much interest. BaTiO3, SrTiO3, and mixed Ba1-xSrxTiO3 are important perovskite materials exhibiting a range of outstanding properties, such as ferroelectricity, piezoelectricity, high dielectric constant, and photorefractivity. They have therefore been exploited for a wide range of applications, including nonvolatile random access memory, transducers, gate dielectric, nonlinear optics, etc.8–11 The synthesis of BaTiO3 nanostructures has attracted much attention because of its novel shape- and size-dependent properties.12,13 For instance, the ferroelectric Curie temperature (Tc) of zero-dimensional (0D) BaTiO3 nanoparticles decreases progressively with size, resulting in room temperature stabilization of the paraelectric cubic phase.14,15 On the other hand, onedimensional (1D) BaTiO3 nanowires of 10 nm diameter still * To whom correspondence should be addressed. E-mail: nzhbao@ mint.ua.edu (N.B.), [email protected] (A.G.). † The University of Alabama. ‡ Oakland University. § Kochi University.

retain their ferroelectric properties, and nonvolatile polarization domains with dimensions as small as 100 nm2 can be induced in these nanowires. This opens up the possibility of fabricating BaTiO3 nanowire-based nonvolatile memory devices with an integration density approaching 1 terabit/cm2.16–23 In contrast to the simple 0D and 1D nanostructures, the synthesis of more complex nanostructures offers opportunities to explore other unique properties that can potentially lead to the fabrication of new types of nanoscale devices.24–26 However, most lowtemperature synthesis routes result in the formation of spherical and cubic BaTiO3 nanocrystals with cubic crystal structure. Complex nanostructures of assembled BaTiO3 nanocrystals are difficult to obtain because of the isotropy of the cubic crystal structure.8–17 Only the synthesis of aligned BaTiO3 nanotubes and nanowires prepared within confined spaces, such as porous alumina templates, has been reported.27 Thus, the development of a simple, effective, and economical process for directly creating BaTiO3 with desired architectures is of importance but remains a key research challenge. The nature of the precursor used for the syntheses often plays a key role in determining the shape and size of the nanomaterials. The commonly used titanium precursors are either inorganic salts, such as TiCl4 and TiOSO4, or organometallic compounds such as titanium acetylacetonate and titanium isopropoxide, which are usually quite expensive, corrosive, and sensitive to moisture. The nucleation and growth of BaTiO3 from the molecular or ionic titanium precursors in the presence of barium salts is often dictated by the nature of the cubic crystal structure, which results in the formation BaTiO3 nanocrystals with spherical or cubic shapes.12,13 On the other hand, the use of layered alkali-metal or layered protonic titanates precursors has been found to have a significant influence on both the phase formation and crystal growth of various TiO2 and titanate

10.1021/jp802055a CCC: $40.75  2008 American Chemical Society Published on Web 05/20/2008

Ferroelectric Barium Titanate Nanostructures

Figure 1. XRD patterns of (a1) Na2Ti3O7 nanotubes and (a2) Na2Ti3O7 nanowires and a standard XRD pattern of Na2Ti3O7 (JCPDS No. 311329). TEM images of (b1) Na2Ti3O7 nanotubes and (b2) Na2Ti3O7 nanowires.

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8635 for use in the assembly of more complex architectures or can dehydrate in situ and crystallize. As for the synthesis of BaTiO3, the shapes of large micrometer-sized precursors of platelike protonic titanates are maintained during their hydrothermal H+/ Ba2+ ion-exchange conversion to BaTiO3.33 Moreover, BaTiO3 of the same type of fibrous shape and size has been directly prepared from large micrometer-sized fibrous potassium titanates by hydrothermal K+/Ba2+ ion-exchange reactions without using the intermediates of protonic titanates as precursors.35,36 All of the above results indicate the possibility of opening up a new hydrothermal route that is capable of assembling novel ordered BaTiO3 architectures using relatively large BaTiO3 nanoblocks formed by self-splitting of the layered alkali-metal or layered protonic titanate precursors. This would be different from the growth of simple BaTiO3 nanoparticles from polymorphic growth units formed from Ti4+ or organometallic titanium compounds by hydrolysis. In the case of the niobates, perovskitetype nanosheets have been obtained by protonation of alkalimetal niobates followed by soft chemical exfoliation, and the chemical composition of the niobate fragments can be changed by reaction with different metal ions.37–39 Therefore, we believe it is possible to create and assemble novel BaTiO3 architectures using BaTiO3 nanoblocks formed from the layered alkali-metal and protonic titanates of small sizes by reactions with barium salts under hydrothermal conditions. Na2Ti3O7 nanotubes and nanowires are layered alkali-metal titanates that can be prepared by hydrothermal reaction. The protonic H2Ti3O7 can be prepared from Na2Ti3O7 by H+/Na+ ion-exchange reaction in a HCl solution. TiO2 nanomaterials with different shapes and sizes have been prepared from the protonic H2Ti3O7 nanotubes and nanowires.30,40–45 In contrast to the large-size titanates being used to prepare BaTiO3 with the same shape and size under hydrothermal conditions,33–36 Na2Ti3O7 nanotubes and nanowires exhibiting small size are able to split into smaller fragments for use in the assembly of more complex BaTiO3 nanostructures or can dehydrate in situ and crystallize to form nanowires and nanotubes. In the present study, we have developed the hydrothermal synthesis of shapecontrolled monocrystalline BaTiO3, SrTiO3, and mixed Ba1-xSrxTiO3 nanostructures using Na2Ti3O7 nanotubes and nanowires as synthetic precursors. Our synthesis route takes advantage of the differences in the hydrothermal stability of the Na2Ti3O7 nanotubes and nanowires and the temperaturedependent structural transformation of BaTiO3. 2. Experimental Section

Figure 2. Typical X-ray powder diffraction (XRD) patterns of asprepared samples of (a) BaTiO3, (b) Ba0.5Sr0.5TiO3, and (c) SrTiO3. Standard XRD patterns of JCPDS No. 31-0174 for BaTiO3, JCPDS No. 39-1395 for Ba0.5Sr0.5TiO3, and JCPDS No. 35-0734 for SrTiO3 are also shown.

products.28–34 The morphology and structure of the solid titanate precursors are either preserved using suitable conditions or rebuilt during the phase formation and transformation reactions.28–34 For example, TiO2 compounds with different morphologies and microstructures have been prepared from protonic titanate hydrates by either calcination or hydro/solvothermal reactions.28–31 This is due to the fact that protonic titanates are a group of solid TiO2 hydrates with layered crystal structure intercalated by protonated water molecules.32 Each layer in the crystal structure consists of side-by-side aligned TiO6 octahedra connected above and below by additionally shared apical edges of TiO6 octahedra at different levels.32 The layers within the titanate hydrates either can split into smaller fragments of TiO6 octahedra

Hydrothermal Synthesis of Na2Ti3O7 Nanotubes and Na2Ti3O7 Nanowires. A total of 1 g of TiO2 was dispersed in 20 mL of a 10 M NaOH aqueous solution in a Teflon-lined stainless steel autoclave by stirring for 1 h. After sealing, the autoclave was heated at different temperatures for 3 days and then cooled down to room temperature. The precipitate was collected, washed several times with distilled water and ethanol, and then dried in a vacuum oven at 70 °C for 12 h. The hydrothermal reaction temperature for the formation of Na2Ti3O7 nanotubes and nanowires is 130 and 200 °C, respectively. Hydrothermal Synthesis of Shape-Controlled Perovskite Nanostructures. Typically, a certain amount of either Na2Ti3O7 nanotubes or Na2Ti3O7 nanowires was dispersed in 15 mL of a Ba(OH)2 aqueous solution in a Teflon-lined stainless steel autoclave by stirring for 1 h. The solutions were saturated with N2 before the addition of Ba(OH)2. After adequate sealing, the autoclave was heated at different temperatures for 24 h and then cooled to room temperature. The precipitate was collected,

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Figure 3. TEM and HRTEM images of BaTiO3 nanostructures prepared from Na2Ti3O7 nanotubes: (a1-a3) hexagonal BaTiO3 nanoparticles [in 0.2 M Ba(OH)2 at 80 °C]; (b1-b3) coral-like nanostructures of assembled nanorods [in 0.1 M Ba(OH)2 at 120 °C]. TEM and HRTEM images of BaTiO3 nanostructures prepared from Na2Ti3O7 nanowires: (c1-c3) starfish-like nanoparticles [in 0.2 M Ba(OH)2 at 80 °C]; (d1-d3) sword-like nanostructures [in 0.1 M Ba(OH)2 at 120 °C].

washed several times with a 0.1 M aqueous solution of CH3COOH, distilled water and ethanol, and then dried in a vacuum oven at 70 °C for 12 h. BaTiO3 nanostructures with hexagonal, starfish-like, coral-like, and sword-like shapes were hydrothermally synthesized from 0.15 g of Na2Ti3O7 nanotubes and 0.2 M Ba(OH)2 at 80 °C, from Na2Ti3O7 nanowires and 0.2 M Ba(OH)2 at 80 °C, from 0.15 g of Na2Ti3O7 nanotubes and 0.1 M Ba(OH)2 at 120 °C, and from 0.15 g of Na2Ti3O7 nanowires and 0.1 M Ba(OH)2 at 120 °C, respectively. The reaction temperature for the formation of BaTiO3 has been determined to be >75 °C for 0.2 M Ba(OH)2. With an increase in the synthesis temperature to 140 °C, the concentration of Ba(OH)2 can be decreased to 0.1 M. SrTiO3 and mixedcomposition Ba1-xSrxTiO3 (where x is 0.77, 0.5, and 0.26) nanostructures were also prepared by reactions with 0.1-0.2 M Sr(OH)2 or mixed Ba(OH)2/Sr(OH)2 at 80-140 °C under

process conditions similar to those for studying the influence of the crystal structure. Characterization of Materials. The obtained samples were investigated using a combination of characterization technologies including transmission electron microscopy (TEM; coupled with high resolution (HR); Tecnai F-20), X-ray diffraction (XRD; Bruker D8 ADVANCE and Philips PW3830), and energydispersive spectroscopy (EDS) equipped on a scanning electron microscope (Philips X-30). The ferroelectric properties of individual BaTiO3 nanowires were investigated by dispersing them onto a Au-coated Si substrate and probing them using a scanned probe microscope (SPM). Topographic and phase images of the nanowires were obtained by operating the SPM in a noncontact atomic force microscopy (AFM) mode. Once a nanowire was located and its diameter measured, ferroelectric polarization was induced or “written” on the nanowire (per-

Ferroelectric Barium Titanate Nanostructures

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Figure 4. TEM images of BaTiO3 nanostructures prepared from Na2Ti3O7 nanotubes: (a1) nanosheet agglomerates [in 0.15 M Ba(OH)2 at 80 °C]; (a2) snowflake nanostructures [in 0.15 M Ba(OH)2 at 100 °C]; (a3) nanocubes [in 0.2 M Ba(OH)2 at 140 °C]. TEM images of BaTiO3 nanostructures prepared from the Na2Ti3O7 nanowires: (b1) nanowires [in 0.10 M Ba(OH)2 at 90 °C]; (b2) mixed starfish-like and sword-like nanostructures [in 0.15 M Ba(OH)2 at 100 °C]; (b3) nanocubes [0.2 M Ba(OH)2 at 140 °C].

Figure 5. Influence of the concentration of Ba(OH)2 and the hydrothermal reaction temperature on the morphology and structure of BaTiO3 products prepared from (a) Na2Ti3O7 nanotubes and (b) Na2Ti3O7 nanowires.

pendicular to the nanowire axis) by applying a large voltage (Vtip) to a conductive SPM tip. The stored polarization was then probed using electrostatic force microscopy (EFM). 3. Results and Discussion Figure 1 shows XRD patterns and TEM images of the Na2Ti3O7 nanotube and nanowire precursors used for the

assembly of BaTiO3 nanostructures. The XRD patterns (see parts a1 and a2 of Figure 1) of the Na2Ti3O7 nanotube and nanowire precursors are well indexed to the standard XRD pattern (JCPDS No. 31-1329), indicating a high purity for the obtained nanotubes and nanowires. The peak widths for the Na2Ti3O7 nanotubes (see Figure 1a1) are much larger than those for the Na2Ti3O7 nanowires (see Figure 1a2) because of the much smaller size

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Figure 6. TEM and HRTEM images of Ba0.5Sr0.5TiO3 nanostructures prepared from the Na2Ti3O7 nanotubes: (a1) nanoballs [in 0.1 M Ba(OH)2 and 0.1 M Sr(OH)2 at 80 °C], (a2) a mixture of nanoballs and nanocubes [in 0.1 M Ba(OH)2 and 0.1 M Sr(OH)2 at 120 °C]; (a3) typical HRTEM image showing clear crystal lattices. TEM and HRTEM images of Ba0.5Sr0.5TiO3 nanostructures prepared from the Na2Ti3O7 nanowires: (b1) a mixture of snowflake-like nanostructures and aligned nanorods [in 0.1 M Ba(OH)2 and 0.1 M Sr(OH)2 at 80 °C]; (b2) pure snowflake-like nanostructures [0.1 M Ba(OH)2 and 0.1 M Sr(OH)2 at 120 °C]; (b3) typical HRTEM image showing clear crystal lattice fringes.

of the former. As indicated by the TEM images, the Na2Ti3O7 nanotubes (see Figure 1b1) have a size of about 6 nm in diameter and up to 500 nm in length, which is much smaller than that of several hundreds of nanometers in diameter and length of micrometers for the Na2Ti3O7 nanowires (see Figure 1b2). All the samples, including BaTiO3, SrTiO3 and Ba0.5Sr0.5TiO3, were examined using powder XRD (see Figure 2) and exhibited high purity and very good crystallinity as established using standard XRD patterns for comparison. The shape and size of the synthesized BaTiO3 nanostructure have been investigated using TEM. Figure 3 shows representative images of four different nanostructures, which are of hexagonal (see Figure 3a1-3), coral-like (see Figure 3b1-3), starfish-like (see Figure 3c1-3), and sword-like (see Figure 3d1-3) configurations. The synthetic yield for each shape is over 90% (see Figure 1a1-d1). The BaTiO3 crystals (see Figure 3a1-2) exhibit an ordered hexagonal shape and uniform size of about 500 nm. Interestingly, by increasing the reaction temperature to 120 °C, and decreasing the concentration of Ba(OH)2 to 0.1 M, a coral-like architecture of BaTiO3 nanorods (see Figure 3b1-2) is formed, consisting both of short and relatively long nanorods. Several long nanorods are observed to grow out radially from the center, with relatively shorter nanorods growing out uniformly on them. Much shorter nanorods then grow on the short nanorods, leading to the formation of unusual coral-like nanostructures. The short and long nanorods have average diameters of around 100 and 200 nm, respectively. While the length of the short nanorods is up to several hundreds nanometers, those of the long nanorods is as much as a few micrometers. When the Na2Ti3O7 nanowires are used as precursors instead of the Na2Ti3O7 nanotubes, uniformly ordered starfish-like BaTiO3 nanostructures (see Figure 3c1), with sawtooth-like edges (see Figure 3c2), are obtained. The process conditions are identical to those used for

the synthesis of the hexagonal BaTiO3 particles (see Figure 3a1-2) using the Na2Ti3O7 nanotubes. A sword-like nanostructure (see Figure 3d1) develops by increasing the temperature to 120 °C and decreasing the concentration of Ba(OH)2 to 0.1 M, displaying double handles and bodies, both with sawtoothlike edges and sharp ends (see Figure 3d2). The observation of lattice fringes in the HRTEM images confirms the single crystalline nature of all the BaTiO3 products. The interlayer distances between adjacent lattice fringes in the HRTEM images of hexagonal (see Figure 3a3), coral-like (see Figure 3b3), starfish-like (see Figure 3c3), and sword-like (see Figure 3d3) nanostructures are measured to be 0.40 ( 0.005 nm, 0.28 ( 0.004 nm, 0.40 ( 0.004 nm, and 0.28 ( 0.005 nm, respectively. These are very close to the respective lattice spacing of the [100] planes at 0.405 nm and the [110] planes at 0.285 nm, for BaTiO3. The morphological evolution of the highly ordered BaTiO3 architectures as a function of process conditions is related to the hydrothermal stability of the Na2Ti3O7 nanotube and nanowire precursors. Layered titanates are good precursors for soft chemical synthesis because of their open structure and ionexchange properties.33,34 The morphologies of fibrous and platelike titanates, with sizes as large as several hundreds of micrometers, are maintained during their hydrothermal conversiontoBaTiO3 inaBa(OH)2 solutionforsuitableconcentrations.33,35,36 However, the Na2Ti3O7 nanotubes (about 6 nm in diameter and up to 500 nm in length) and Na2Ti3O7 nanowires (several hundreds of nanometers in diameter and length of micrometers), which are required for the creation of the various BaTiO3 nanostructures, may either collapse or partly dissolve and recrystallize through changes in the Ba(OH)2 concentration and the reaction temperature. The Na2Ti3O7 nanotubes and Na2Ti3O7 nanowires are formed by the rolling of nanosheets, which can

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Figure 7. TEM and HRTEM images of SrTiO3 nanostructures prepared from the Na2Ti3O7 nanotubes in 0.1 M Sr(OH)2 at 80-120 °C for 24 h: (a1) aggregates of nanoballs; (a2) a single aggregate of nanoballs; (a3) a single big nanoball; (a4) clear crystal lattices. TEM and HRTEM images of SrTiO3 nanostructures prepared from the Na2Ti3O7 nanowires in 0.1 M Sr(OH)2 at 80-120 °C for 24 h; (b1) assembled nanocubes showing the morphology of the original nanowire template; (b2) magnified TEM image of a local area of the assembled nanocubes; (b3) an invidual nanocube; (b4) clear crystal lattices.

Figure 8. Typical powder XRD patterns (a) in the 20-60° range for 2θ and (b) around the (100) peak for BaxSr1-xTiO3 (where x is 1, 0.77, 0.5, 0.26, and 0) prepared from either Na2Ti3O7 nanotubes or Na2Ti3O7 nanowires by reaction with 0.1-0.2 M Sr(OH)2, Ba(OH)2 or mixed Ba(OH)2/Sr(OH)2 at 80-140 °C for 24 h. The (100) peaks of standard XRD patterns of PDF No. 00-031-0174 for BaTiO3, PDF No. 00-0440093 for Ba0.77Sr0.23TiO3, PDF No. 00-039-1395 for Ba0.5Sr0.5TiO3, PDF No. 01-089-8211 for Ba0.26Sr0.74TiO3, and PDF No. 00-035-0734 for SrTiO3 are also shown.

be reversible for the nanotubes, but this is difficult for the nanowires because of the strong interlayer interaction and their relatively large size. Under very mild conditions, BaTiO3 nanosheets (see Figure 4a1) can be easily obtained from the Na2Ti3O7 nanotubes by exfoliation, and they further agglomerate or dissolve, recrystallize, to finally form ordered coral-like architectures (see parts b1 and b2 of Figure 3 and Figure 4a2) or hexagonal particles (see parts a1 and a2 of Figure 3) at higher Ba(OH)2 concentrations and/or reaction temperatures. In contrast, the Na2Ti3O7 nanowires tend to break and form short BaTiO3 nanowires (see Figure 4b1) under mild conditions and, for higher Ba(OH)2 concentrations and/or synthesis temperatures, convert to ordered starfish-like (see parts c1 and c2 of Figure 3), sword-like (see parts d1 and d2 of Figure 3), or some intermediate architectures (see Figure 4b2) via a dissolution and recrystallization of small nanowire fragments on larger nanowire fragments. At the highest reaction temperatures (g140 °C) and Ba(OH)2 concentrations (g0.2 M), both the Na2Ti3O7 nanotubes and Na2Ti3O7 nanowires dissolve and recrystallize, forming titanate clusters, from which cubic BaTiO3 particles are formed (see parts a3 and b3 of Figure 4). Figure 5 summarizes the

reaction temperature and Ba(OH)2 concentration-dependent morphological and structural changes of the BaTiO3 products, including simpler nanostructures of nanosheets, nanowires, nanocubes, and hexagonal nanoparticles or complex nanostructured architectures with shapes of starfish-like, coral-like, snowflake-like, and sword-like. The reaction yield for each type of the shape is over 90%. All of the synthesis conditions are in qualitative agreement with the thermodynamic predictions and previously observed experimental results.33,35,36,46 The morphological evolution of the highly ordered BaTiO3 architectures is also determined by the temperature-dependent structural transformation of BaTiO3. Two polymorphs of BaTiO3, namely, the tetragonal and cubic crystal forms, exist in the bulk under our synthesis temperature range of 70-140 °C. Tc of bulk BaTiO3 is 120 °C, which corresponds to the tetragonal-to-cubic transition temperature. In the tetragonal structure, the Ti4+ ion is distorted from the centrosymmetric position within the TiO6 octahedra. The asymmetry of tetragonal BaTiO3 can possibly contribute to the formation of the highly ordered architectures. We have performed additional experiments to test out this hypothesis. Both BaTiO3 and SrTiO3 are

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Figure 9. EDS spectra of typical BaxSr1-xTiO3 products, where x is (a) 1, (b) 0.77, (c) 0.5, (d) 0.26, and (e) 0. All of the BaxSr1-xTiO3 products were prepared from either Na2Ti3O7 nanotubes or Na2Ti3O7 nanowires by reaction with 0.1-0.2 M Sr(OH)2, Ba(OH)2, or mixed Ba(OH)2/Sr(OH)2 at 80-140 °C for 24 h. The Ba/Sr/Ti atomic ratio of each sample is also shown in the inset table.

ABO3-type perovskites, and there is a complete solution in the Ba1-xSrxTiO3 system, with the size of the unit cell decreasing

Bao et al. linearly with increasing Sr substitution. It should be noted that SrTiO3 exists only in the cubic structure, as does Ba0.5Sr0.5TiO3 over our experimental temperature range.8–11 Assuming that only the cubic polymorph, and no tetragonal polymorph, is formed during the synthesis of all of the products, they would be expected to exhibit similar morphology. However, unlike BaTiO3, we find that the Ba0.5Sr0.5TiO3 and SrTiO3 nanostructures are mostly cubic or spherical in shape (see Figures 6 and 7), with some complex structures formed only for Ba0.5Sr0.5TiO3 prepared using the Na2Ti3O7 nanowire precursor (see parts b1 and b2 of Figure 6) under mild conditions. Thus, the tetragonal crystal structure of BaTiO3 is likely to be an additional factor favoring the formation of the highly ordered architectures. Additionally, with an increase in the hydrothermal reaction temperature from 80 to 120 °C, the Ba0.5Sr0.5TiO3 nanostructures prepared from Na2Ti3O7 nanotubes change in shape from nanoballs (see Figure 6a1) to nanocubes (see Figure 6a2). Similarly, the main products prepared from the Na2Ti3O7 nanowires maintain the original morphology of the Na2Ti3O7 nanowires (see Figure 6b1) for low reaction temperatures, but the yield of snowflake-like nanostructures increases to 100% (see Figure 6b2) with an increase in the hydrothermal reaction temperature to 120 °C. For the SrTiO3 nanostructures prepared at 80-120 °C, the products prepared from the Na2Ti3O7 nanotubes (see Figure 7a1) consist of agglomerates of fine spherical nanoparticles (see Figure 7a2) and nanoballs (see Figure 7a3). In contrast, the products prepared from the Na2Ti3O7 nanowires (see parts b1-b3 of Figure 7) are composed

Figure 10. (a) TEM and HRTEM images of a typical BaTiO3 nanowire. (b) 3D AFM image of an individual BaTiO3 nanowire. (c) Topographic and (d) phase images of the individual BaTiO3 nanowire, showing the diameter of the nanowire and the antiparallel domains marked with arrows, respectively.

Ferroelectric Barium Titanate Nanostructures of nanocubes formed by the splitting of the Na2Ti3O7 nanowires. Again, these results confirm that the different hydrothermal stabilities of the Na2Ti3O7 nanotube and nanowire precursors play a key role in controlling the structures of different monocrystalline BaTiO3, SrTiO3 and mixed Ba1-xSrxTiO3 nanostructures. Stoichiometric compositions of Ba1-xSrxTiO3 can be prepared by changing the ratio of Ba(OH)2/Sr(OH)2 in a reaction solution. BaxSr1-xTiO3 (where x is 1, 0.77, 0.5, 0.26, and 0) were prepared from either Na2Ti3O7 nanotubes or Na2Ti3O7 nanowires by reactions with 0.1-0.2 M Sr(OH)2, Ba(OH)2, or mixed Ba(OH)2/ Sr(OH)2 at 80-140 °C for 24 h. Figure 8a shows typical powder XRD patterns of the prepared BaxSr1-xTiO3 samples with different compositions, along with BaTiO3 and SrTiO3 data for comparison. All of the samples exhibit single-phase diffraction patterns. The position of the d(100) diffraction peak (Figure 8b) of each stoichiometric Ba1-xSrxTiO3 sample agrees well with that of the corresponding standard XRD pattern, which shifts to higher angle (2θ) with increasing Sr concentration because of the decrease in the ionic radius from Ba2+ (1.35 Å) to Sr2+ (1.13 Å). Chemical compositions of typical BaTiO3, SrTiO3, and all of the stoichiometric Ba1-xSrxTiO3 samples were confirmed using EDS spectra, shown in Figure 9. The Ba/Sr/Ti atomic ratio of each sample, shown in the inset table, is about 0.967:0:1 for BaTiO3, 0.745:0.262:1 for Ba0.77Sr0.23TiO3, 0.516: 0.514:1 for Ba0.5Sr0.5TiO3, 0.264:0.781:1 for Ba0.26Sr0.74TiO3, and 0:1.04:1 for SrTiO3. Both the XRD and EDS analysis results indicate that the obtained products are stoichiometric Ba1-xSrxTiO3 with chemical compositions very close to the expected values. The ferroelectric properties of individual single-crystalline BaTiO3 nanowires (see Figure 10a) were investigated by dispersing them onto a Au-coated Si wafer and probing them using a SPM. As shown in Figure 10a, the investigated BaTiO3 nanowires are 300-400 nm in diameter and about 15 µm in length. Clear crystal lattices are observed in the HRTEM image (see the inset of Figure 10a) of the single-crystalline BaTiO3 nanowires. The crystal lattice spacing distance is 4.10 ( 0.04 Å, which corresponds to the distance between two adjacent [100] crystal planes of 4.04 Å. A three-dimensional (3D) topographic image (see Figure 10b) of an individual nanowire was obtained by operating the SPM in a noncontact AFM mode. Ferroelectric polarization was induced, perpendicular to the wire axis, on the nanowire by applying a large voltage to a conductive SPM tip. The polarized BaTiO3 nanowire was then probed using EFM with a small voltage applied at the tip. Antiparallel domains in the BaTiO3 nanowire are observed by applying a voltage perpendicular to the axis of the nanowire. The synthesis of barium titanate via a hydrothermal process is carried out in an excess water with very high OH concentration. Hydroxyl groups will not only adsorb on the surface of the barium titanate powder but also be incorporated into the barium titanate lattice.47 Such cationic vacancies and enlargement of the unit cell result in degradation of the dielectric properties of the final product. The adverse effects of the entrapped OH groups have been well documented.48,49 No method other than heating has been reported so far, which can effectively remove the lattice hydroxyls. It has been reported that the surface hydroxyl groups can be eliminated only after calcining the barium titanate powder for 1 h at 600-800 °C, and the incorporated lattice hydroxyl ion also diffuses to the surface of the barium titanate particles.50,51 The Ba1-xSrxTiO3 crystals reported in the present work were hydrothermally synthesized by the uptake of OH group in the lattice, which

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8641 definitely results in the formation of cation vacancies and, consequently, loss of dielectric properties of the final product. Therefore, a further annealing in oxygen is required for the removal of the hydroxyl groups, both on the surface and inside the lattices of the hydrothermally synthesized ferroelectric Ba1-xSrxTiO3 crystals. 4. Conclusion In summary, single-crystalline BaTiO3 nanostructures, exhibiting a wide variety of shapes and sizes, have been synthesized hydrothermally from Na2Ti3O7 nanotube and nanowire precursors by controlling the reaction temperature and the concentration of Ba(OH)2. The synthesis exploits the differences in the hydrothermal stability of Na2Ti3O7 nanotubes and nanowires as synthetic precursors and the temperature-dependent crystal structure transformation of BaTiO3. Under similar synthesis conditions, simpler nanostructures of monocrystalline SrTiO3 and stoichiometric Ba1-xSrxTiO3 (where x is 0.77, 0.5, and 0.26) are usually obtained by changing the Ba(OH)2/Sr(OH)2 ratio in reaction solutions. The reported process can potentially be expanded for the assembly of other complex perovskite [including ATiO3 (A ) Pb, Ca, Cd), Pb1-xSrxTiO3, etc.] nanostructures using relatively large nanoblocks formed from layered alkali-metal and protonic titanates. Acknowledgment. This work is supported by the National Science Foundation through Grant ECS-0621850. References and Notes (1) Lu, A. H.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (2) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (3) Limmer, S. J.; Gao, G. AdV. Mater. 2003, 15, 427. (4) Zeng, H.; Li, J.; Wang, Z.; Liu, J. P.; Sun, S. Nature 2002, 420, 395. (5) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (6) Wang, C.; Hou, Y.; Kim, J.; Sun, S. Angew. Chem., Int. Ed. 2007, 46, 6333. (7) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630. (8) Xu, Y. Ferroelectric Materials; North-Holland: New York, 1991. (9) Niederberger, M.; Garnweitner, G.; Pinna, N.; Antonietti, M. J. Am. Chem. Soc. 2004, 126, 9120. (10) Rupprecht, G.; Bell, R. Phys. ReV. 1964, 135, A748. (11) Fatuzzo, E.; Merz, W. Ferroelectricity; North-Holland: New York, 1967. (12) O’Brien, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085. (13) Bansal, V.; Poddar, P.; Ahmad, A.; Shastry, M. J. Am. Chem. Soc. 2006, 128, 11958. (14) Zhong, W. L.; Wang, Y. G.; Zhang, P. L.; Qu, B. D. Phys. ReV. B 1994, 50, 698. (15) Pertsev, N. A.; Zembilgotov, A. G.; Tagantsev, A. K. Phys. ReV. Lett. 1998, 80, 1988. (16) Urban, J. J.; Yun, W. S.; Park, H. J. Am. Chem. Soc. 2002, 124, 1186. (17) Mao, Y.; Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2003, 125, 15718. (18) Bansal, V.; Poddar, P.; Ahmad, A.; Shastry, M. J. Am. Chem. Soc. 2006, 128, 11958. (19) Mao, Y.; Banerjee, S.; Wong, S. S. Chem. Commun. 2003, 408. (20) Yun, W. S.; Urban, J. J.; Gu, Q.; Park, H. Nano Lett. 2002, 2, 447. (21) Wang, Z.; Hu, J.; Yu, M. F. Appl. Phys. Lett. 2006, 89, 263119. (22) Geneste, G.; Bousquet, E.; Junquera, J.; Ghosez, P. Appl. Phys. Lett. 2006, 88, 112906. (23) Naumov, I. I.; Fu, H. Phys. ReV. Lett. 2005, 95, 247602. (24) Lu, Q.; Gao, F.; Komarneni, S. J. Am. Chem. Soc. 2003, 126, 54. (25) Zheng, H.; Wang, J.; Lofland, S. E.; Mohaddes-Ardabili, L.; Zhao, T.; Salamanca-Riba, L.; Shinde, S. R.; Ogale, S. B.; Bai, F.; Viehland, D.; Jia, Y.; Schlom, D. G.; Wuttig, M.; Roytburd, A.; Ramesh, R. Science 2004, 303, 661.

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