DOI: 10.1021/cg1005727
Nearly Monodisperse Ferroelectric BaTiO3 Hollow Nanoparticles: Size-Related Solid Evacuation in Ostwald-Ripening-Induced Hollowing Process
2010, Vol. 10 3990–3995
Xuelin Tian,† Juan Li,‡ Kai Chen,§ Jian Han,† Shilie Pan,*,† Yongjiang Wang,† Xiaoyun Fan,† Feng Li,† and Zhongxiang Zhou† †
Xinjiang Key Laboratory of Electronic Information Materials and Devices, and ‡Key Laboratory of Plant Resources and Chemistry of Arid Zone, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urmuqi 830011, China, and §Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China Received April 29, 2010; Revised Manuscript Received June 25, 2010
ABSTRACT: Nearly monodisperse BaTiO3 hollow nanoparticles were prepared through a one-pot template-free route in molten hydrated salt medium. The composition and morphologies of the time-dependent products were investigated in detail, and Ostwald ripening is concluded to be responsible for the hollowing evolvement of the BaTiO3 particles. The size of the initial solid particles is proposed to play an important role in determining the evacuation manner of the solid in the ripening-induced hollowing process, that is, evacuating from the center region or underneath the surface layer of the solid aggregates, and could result in the morphology variation (i.e., hollow geometry or core-shell one) of the final products. Our study also points out that a highly active precursor is crucial to improve the uniformity of the products, and elevated reaction temperature is a prerequisite to activate the hollowing evolvement of solid aggregates. Second-harmonic-generation and Raman analyses revealed the ferroelectric tetragonal characteristic of the obtained BaTiO3 hollow nanoparticles, and the tetragonality (c/a) was quantified to be about 1.005 according to the Rietveld refinement of the X-ray diffraction pattern.
Introduction It is generally accepted that the precise control of size and morphology of nanomaterials provides an effective strategy for tuning their physical and chemical properties.1-8 Recently, the design and synthesis of hollow nanostructures with a narrow size distribution and well-defined morphology have attracted extensive attention, as their unique structures and related properties are highly desirable in a number of applications, including nanoscale reactors, drug-carrier, hydrogen storage, optical devices, and catalysis.2,7,9-22 The most prevalent strategy for synthesizing hollow nanostructures is using templates, hard or soft, to guide the formation of outer shells. Although the template methods have proven very effective and versatile for synthesizing various hollow structures, the templateremoval step significantly complicates the fabrication procedure and often unfavorably affects the quality (e.g., remnant impurity and inevitable shell collapse) of the products.23 Ideally, templatefree methods working under one-pot conditions are even more attractive for preparation of hollow nanostructures. BaTiO3 is noteworthy for its advantageous ferroelectric, piezoelectric, and dielectric properties with corresponding applications in nonvolatile memories, actuators and multilayer capacitors.24,25 Because the above properties are dependent on structure and finite size, the synthesis of BaTiO3 nanostructures with well-defined size and morphology is of high interest. Although many efforts have been devoted to the synthesis of various BaTiO3 nanostructures,24-31 very limited successes have been achieved on the preparation of BaTiO3 hollow nanoparticles. Lee et al. reported the fabrication of BaTiO3 hollow spheres using Ni particles as the template to *To whom correspondence should be addressed. Phone: (86)9913674558. Fax: (86)991-3838957. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 07/29/2010
direct the formation of the BaTiO3 shell layer.13 Buscaglia et al. also synthesized ferroelectric BaTiO3 hollow particles by a two-step process combining colloidal chemistry and solidstate reaction.32 Nevertheless, the above methods not only needed multistep operations but also employed a hightemperature calcination process (at 400 or 700 °C) to ensure the synthesis of BaTiO3. Very recently, a molten hydrated salt (MHS) method was put forward by our group for the synthesis of core-shell and hollow BaTiO3 particles through the reaction between Ba(OH)2 3 8H2O and homemade microscale rutile particles below 200 °C.33,34 Although the above research showed the attractive potential of the MHS method for the synthesis of complex-oxide functional materials in a straightforward, scalable, and low-temperature way, the obtained BaTiO3 particles showed poor monodispersity and had a very wide size distribution. In addition, it was also hard to separate the hollow BaTiO3 particles from the associated core-shell ones, which hindered the preparation of hollow products at high purity.34 In our follow-up research, we have made an endeavor to perform studies addressing the above issues. The nonuniformity in the size and morphology of the BaTiO3 products in the MHS synthesis is speculated to be related to the low reaction activity of the rutile precursor, which results in that the dissolution and transformation of rutile into BaTiO3 takes a longer period of time. Accordingly, the nucleation, growth, aggregation, and ripening process of BaTiO 3 particles34 would become much more complex and could not be separated effectively. As a result, BaTiO3 particles with nonuniform size and morphology become the dominant products. With the above considerations in mind, we deem that using more a highly active TiO2 precursor (e.g., thermodynamically metastable phase or nanoscale powder) should be a feasible r 2010 American Chemical Society
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Figure 1. (a) XRD, (b) SEM, (c) TEM, and (d) size distribution of the as-prepared BaTiO3 hollow particles.
route to improve the uniformity of the products. In this study, commercial anatase particles with a size in the range of 4085 nm were chosen as the precursor to react with Ba(OH)2 3 8H2O, and BaTiO3 hollow nanoparticles were successfully prepared by controlling the reaction duration and temperature. Simultaneously, the monodispersity of the BaTiO3 particles was dramatically improved. Another very interesting finding is that the Ostwald-ripening-induced hollowing evolvement initiates from the center of the solid aggregates herein, whereas in our previous report34 the solid evacuation starts underneath the surface layer of the solid particle. The difference and correlation of the hollowing mechanism in the two cases are analyzed, and the size of the initial solid aggregates is suggested to play an important role in determining the evacuation manner of the solid in the ripening-induced hollowing process, which could result in different morphologies of the final products (i.e., hollow particles or core-shell ones). The ferroelectric tetragonal characteristic of the BaTiO3 hollow particles was also revealed by second-harmonic-generation (SHG) and Raman investigations, and the tetragonality (c/a) was quantified to be about 1.005 according to the Rietveld refinement of the X-ray powder diffraction (XRD) pattern. Experimental Section Synthesis of BaTiO3 Hollow Particles. All the reagents were of analytical purity and were used as received. Ba(OH)2 3 8H2O was purchased from Tianjin third Chemicals, and anatase TiO2 particles was purchased from Beijing Huateng Chemical Co. Ltd. with a size in the range of 40-85 nm. The XRD pattern and transmission electron microscope (TEM) image of the anatase particles are shown in Figure S1 in the Supporting Information. The synthetic procedure is similar to a previous report34 except that microscale rutile powder was replaced with nanoscale anatase powder. In a typical synthesis, 3 g of Ba(OH)2 3 8H2O was melted at 90 °C, and 0.2 g of anatase particles was added into the molten hydrated salt. After thoroughly stirring, the mixture was immediately transferred into a Teflon-lined stainless steel autoclave of 23 mL and maintained at 180 °C for 3 h. When cooled to room temperature, the obtained solid was washed with dilute acetic acid (0.1 M), distilled water, and ethanol, respectively. Hot water at above 50 °C could be used as an alternative to the dilute acetic acid. The final product was dried at ambient conditions for further characterization, and the yield of the BaTiO3 hollow samples is more than 96% based on TiO2. Characterization. XRD patterns were recorded on a Bruker D8 Advance diffractometer with graphite monochromatized Cu KR radiation (λ = 1.5418 A˚). For the data used for cell refinement, the
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Figure 2. (a-c) Magnified images of the BaTiO3 hollow particles. (d) The typical electron diffraction pattern from a single particle. (e) HRTEM image taken from a single particle. XRD pattern was collected with a step interval of 0.00746° and a counting time of 2 s, in a 2θ range from 10 to 120° with 0.1 mm receiving slit. Crystal structure was identified and refined by the Rietveld method using the TOPAS ACADEMIC V4 software. The morphologies of the samples were characterized by a scanning electron microscope (SEM, JEOL JSM-6700F) and TEM (Hitachi H-600). High-resolution transmission electron microscope (HRTEM) investigations were performed on a JEOL JEM-2010 microscope. For the SHG investigation, the 1064 nm output of a Q-switched Nd: YAG laser (Lingyun LYPE10-SG-WL1064, Wuhan) with a beam diameter of about 1 mm was focused onto the BaTiO3 hollow samples (∼0.3 g) loaded into a glass vial. The double-frequency (DF) emission at 532 nm was recorded by a digital camera (Canon, IXUS 850IS). Raman spectra of various BaTiO3 samples were obtained on a Bruker Vertex 70 RAMII FT-Raman spectrometer, equipped with liquid nitrogen cooled Germanium detector and Nd: YAG laser excitation at 1064 nm in a spectral resolution of 4 cm-1. FT-IR measurement was conducted on a BIO-RAD FTS165 Fourier transform infrared spectrometer, using KBr as a binding material in the range of 400-4000 cm-1.
Results and Discussion Structural Characterization. The phase structure of the obtained products was investigated by XRD (Figure 1a), and all of the reflections can be readily indexed to tetragonal BaTiO3 (JCPDS No. 05-0626). Figure 1b shows the SEM image of the BaTiO3 products. Most of the particles have a size of about 110-140 nm, and it can be found that many particles present the exposed inner space. The TEM investigation further confirmed the hollow character of the products (Figure 1c). The shell thicknesses of the BaTiO3 hollow particles are estimated to be about 18-23 nm according to the TEM image. A low-magnification TEM image (see Figure S2 in the Supporting Information) presents the overall morphology and the uniform size characteristic of the products. Figure 1d shows the size distribution of the BaTiO3 hollow nanoparticles, which is obtained based on a count of 300 particles on TEM images. The detailed morphology of the BaTiO3 hollow particles was shown in Figure 2a-c. Interestingly, the hollow particles present nearly polygonal morphologies, rather than spherical ones. The shell of the quasi-polygon is not uniform and composed of many subunits which have almost straight lateral boundary. The unique morphology of the BaTiO3 particles may favor the exposure of certain low-energy faces,
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Figure 4. TEM images of the products after synthesizing for (a) 30 min, (b) 1 h, and (c) 2 h at 180 °C. Figure 3. XRD patterns of the intermediate products after synthesizing for 30 min, 1 h, and 2 h at 180 °C.
such as (100), (111), and (110) planes, on the subunits and thus reduce the total surface energy of the particle. The ED investigation indicated the polycrystalline characteristic of the hollow particles (Figure 2d), and the lattice distances measured from the diffraction rings are in perfect agreement with the perovskite BaTiO3. Figure 2e shows the HRTEM image of a hollow particle, which further verified the straight lateral boundary on the subunits mentioned above. It can also be found that the products are well-crystallized, and two sets of lattice fringe with spacing of about 0.28 nm were clearly distinguished which correspond to the interplanar distance of the {110} planes in the tetragonal BaTiO3. Evolution Process and Formation Mechanism of BaTiO3 Hollow Particles. The formation process of the BaTiO3 hollow particles was surveyed in detail by XRD and TEM investigations. Figure 3 shows the XRD patterns of the products obtained at different reaction durations. It can be found that the transformation of nanoscale anatase particles into BaTiO3 can be completed in no more than 30 min, whereas in our previous report34 the transformation of microscale rutile powder into BaTiO3 needs about 3 h under similar reaction conditions. This difference is evidently attributed to the higher chemical activity of nanoscale anatase powder. Corresponding to the above phase evolution, the timedependent morphologies of the products are demonstrated in Figure 4a-c. The products obtained at a reaction time of 30 min are almost solid BaTiO3 particles (Figure 4a). After synthesizing for 1 h, many hollow particles appear in the products, and they do not show apparent size change compared to the solid ones (Figure 4b). As the reaction time increased to 2 h, BaTiO3 hollow particles with a much thinner shell become the dominant products, and the size of the particles seems to increase slightly (Figure 4c). Note that the hollowing evolvement shown here is quite different from our previous report34 as well as many other cases,35-40 in which the solid evacuation starts underneath the surface layer of the initial particles, and thus homogeneous coreshell particles appear in the products, whereas in this study the solid evacuation initiates from around the center part of the particles, and no core-shell structured particle is observed in the products. The average diameter and shell thickness of the BaTiO3 particles at different reaction duration are calculated by counting about 150 particles on TEM images and are presented in Figure 5. When the reaction time increased from 30 min to 3 h, the average shell thickness of the BaTiO3 particles decreased from about 58 to 21 nm.
Figure 5. The relationship between the average diameter (blue), shell thickness (red) of the BaTiO3 products and the reaction duration.
Figure 6. Schematic illustration of the hollowing evolvement process. Shallow blue: smaller or loosely packed crystallites. Deep blue: larger or closely packed crystallites.
At the same time, the average size of the particles shows a slight increase from about 116 to 124 nm. On the basis of the above phase and morphology evolution investigations, the formation mechanism of the BaTiO3 hollow particles was proposed (Figure 6), and Ostwald ripening is believed to play a key role in the hollowing process.23,41 In our previous study, the microscale rutile particles can dissolve into soluble species such as [Ti(OH)x4-x] in the MHS synthesis, and then homogeneous nucleation and aggregation of BaTiO3 primary crystallites take place.34 The above process should also occur here for nanoscale anatase particles have higher activity and are more prone to be dissolved. And the aggregation of the primary crystallites results in the polycrystalline characteristic of the obtained BaTiO3 particle. Intrinsic density variations should exist in the BaTiO3 solid aggregates. As the reaction proceeds, some interior space would be generated within the solid aggregates through the Ostwald ripening process, as larger crystallites are essentially immobile while smaller or loosely packed crystallites ones are undergoing the mass transport through dissolving, diffusing outward, and recrystallizing. With the transportation of the mass, the void space would become larger and larger, and then hollow BaTiO3 products come into being. Although the Ostwald-ripening mechanism dominates in both cases, the difference in the evacuation manner of the
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Figure 7. HRTEM image of a BaTiO3 hollow particle. The white arrows indicate the mesopores on the shell of the particle.
solid is very significant. As mentioned above, the solid evacuation starts from around the center part herein rather than from underneath the surface layer of the particles. According to Liu and Zeng,35 and Lou et al.,23 where the evacuation process starts from may depend on the packing of primary nanoparticles and ripening characteristics. Here, we suggest that another important factor, that is, the size of the initial solid particles, may also play an important role in determining the hollowing process. When the initial particles have a larger size, it is very difficult for the solid evacuation to initiate at regions around the center of the solid particles, because the thick shell would inhibit the outward diffusion of the dissolved species. And thus the solid evacuation can only start underneath the surface layer of the particles, which results in the formation of homogeneous core-shell structures. On the contrary, smaller initial particles would favor the solid evacuation from the center region, and only hollow products come into being in the ripening process. In our previous report, the initial solid particles have a size in the range of 400-800 nm, and core-shell BaTiO3 particles become the dominant products.34 In this study, the initial particles have an average size of about 116 nm, and thus only hollow BaTiO3 particles appear in the products. These facts provide powerful evidence to our suggestion. The size difference in the two cases may be due to the different formation time of the initial solid particles, as a longer time would favor the collision and aggregation of more primary crystallites as well as the formation of larger solid aggregates. For the reported homogeneous core-shell particles obtained by the Ostwald ripening mechanism, it can also be found that they all have a relatively large size, such as ZnS (800-900 nm),35 MnS (500-800 nm),36 Fe3O4 (300-800 nm),37 γ-Fe2O3 (>1 μm),38 CdMoO4 (∼4 μm),39 and WO3 3 1/3H2O (2-15 μm),40 and no homogeneous core-shell particles with a size of less than 150 nm have been reported. All these facts further support our suggestion on the size effect in the ripening-induced hollowing process, and smaller particles (e.g.,
