Template-Free and Scalable Synthesis of Core−Shell and Hollow

Synopsis. Molten hydrated salt was utilized, for the first time, as a new kind of solvent to explore the synthesis of complex oxide nanomaterials. Tet...
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DOI: 10.1021/cg900700u

Template-Free and Scalable Synthesis of Core-Shell and Hollow BaTiO3 Particles: Using Molten Hydrated Salt as a Solvent

2009, Vol. 9 4927–4932

Xuelin Tian,† Juan Li,‡ Kai Chen,§ Jian Han,† and Shilie Pan*,† †

Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, China, ‡Key Laboratory of Plant Resources and Chemistry of Arid Area, 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 June 23, 2009; Revised Manuscript Received July 29, 2009

ABSTRACT: Molten hydrated salt was used, for the first time, as a new kind of solvent to explore the synthesis of complex oxide nanomaterials. Employing Ba(OH)2 3 8H2O as the solvent, and rutile TiO2 as the reactant, core-shell and hollow nanoparticles of perovskite-type BaTiO3 were successfully fabricated under sealed conditions at 180 °C. No other additive was introduced into the reaction system. The effect of various reaction conditions on the morphology and size of the products was investigated. At high TiO2 dosage, core-shell BaTiO3 particles were the dominant products with their sizes between 400 and 800 nm, whereas relatively low TiO2 dosage favored the formation of hollow BaTiO3 particles less than 250 nm. The formation process of the products was investigated, and can be attributed to an Ostwald ripening induced hollowing mechanism. Because of its simplicity and scalability, the molten hydrated salt method may open up a promising route for the synthesis of complex oxide micro/nanomaterials for practical applications.

Introduction Hollow inorganic nanostructures have attracted extensive attention due to their unique chemicophysical properties as well as promising applications in energy, catalysis, sensing, and biomedical fields.1-14 A significant number of methods have been developed over the past few decades for the fabrication of simple hollow nanoparticles. More complex hollow structures, such as homogeneous core-shell particles, are receiving increasing attention owing to their unique structures and enhanced performances in various applications.15-23 For example, they could be used as more efficient nanoreactors compared with simple hollow spheres, for they possess higher surface area due to the simultaneously exposed core and inner shell surfaces. Moreover, if the core is entirely detached from the shell, the removable core can provide certain shaft work and favor better mixing of the inside reagents.16 The homogeneous core-shell spheres can also endow semiconductor materials with greatly enhanced photocatalytic activity attributed to multiple reflections of UV light within the interior voids.18 Because of the structure complexity and the same chemical composition of the core and shell parts, homogeneous core-shell structure is in general highly difficult to fabricate. Only very recently, a few materials with homogeneous core-shell structure, such as TiO2,15,18 ZnS,16 SiO2,21 and WO3 3 1/3H2O,22 have been successfully fabricated. Perovskite-type BaTiO3 is an important ferroelectric and piezoelectric material. It exhibits large dielectric constants and large nonlinear optical coefficients which are beneficial for widespread applications, such as multilayer capacitors, nonvolatile memories, thermistors, transducers, and electro-optical devices.24,25 Because of its novel shape- and sizedependent properties, the synthesis of BaTiO3 nanostructures has attracted considerable attention. In contrast to the *To whom correspondence should be addressed. Phone: (86)9913674558. Fax: (86)991-3838957. E-mail: [email protected]. r 2009 American Chemical Society

extensive studies on BaTiO3 nanospheres/cubes and nanorods/wires,25-32 very limited successes have been achieved on the fabrication of core-shell or hollow BaTiO3 nanoparticles. To the best of our knowledge, there are only two reports on the preparation of hollow BaTiO3 particles, with their methods suffering from complicated template removal7 or hightemperature calcination processes,33 respectively. In particular, no success on the synthesis of core-shell BaTiO3 particles has been reported. In this study, we report the fabrication of core-shell and hollow BaTiO3 particles through a novel molten hydrated salt (MHS) method, namely, using molten hydrated salt as the solvent in the synthesis. Molten salt (MS) synthesis, which uses melting salt as a solvent, is a well-established approach for the synthesis of various oxide nanomaterials.26,34-41 It possesses the merits of simplicity, versatility, and the ability for large-scale synthesis.37,42 However, because of the high melting point of common salts, the MS method has to operate at high temperature (usually higher than 800 °C for unitary salt and 400 °C for composite salts). Recently, Liu and co-workers reported the synthesis of nanostructures of complex oxides at 200 °C using mixed hydroxides as the solvent.27,43 Their method benefits from the low eutectic point of the NaOH/KOH mixture. To decrease the synthesis temperature, a desirable alternative is to use hydrated salts as solvents for they have a much lower melting point than common salts.44 The diverse hydrated salts existing in nature also provide abundant choices for the synthesis of various compounds. As hydrated salt will be dehydrated on heating under open circumstances, sealed reaction conditions can be employed to avoid the loss of water molecules from the solvent. Therefore, the primary concept of the MHS method is employing molten hydrated salts as solvents for materials synthesis under sealed conditions. Herein, we demonstrate the first implementation of MHS method, and BaTiO3 particles with unique structures are successfully obtained based on Ba(OH)2 3 8H2O hydrated salt. Published on Web 08/20/2009

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Figure 1. Powder XRD patterns of (a) initial rutile powder, and the products synthesized for (b) 30 min, (c) 1 h, (d) 3 h. The triangles denote the TiO2, and the rhombuses denote the BaTiO3. TiO2: 0.2 g.

Experimental Section Synthesis. For the preparation of core-shell BaTiO3 particles, 3 g of Ba(OH)2 3 8H2O (mp 78 °C) was melted at 90 °C. Then 0.2 g of rutile TiO2 powder (see Supporting Information) was added into the molten hydrated salt. The mixture was stirred by a Teflon bar to ensure the uniformity of the reactants. Then it was immediately transferred into a Teflon-lined stainless steel autoclave (TLSSA) of 23 mL and maintained at 180 °C for 10 h. After being cooled to room temperature, the obtained solid was washed with 0.1 M acetic acid, distilled water, and ethanol, respectively. The final product was dried at ambient condition for further characterization. For hollow BaTiO3 structures, a similar procedure was adopted except that a smaller amount of TiO2 (0.1 or 0.05 g) was used in the fabrication. In the MHS synthesis, BaTiO3 products of several hundred milligrams can be easily obtained at high yield (more than 95% based on TiO2), and the procedure can be scalable to multigram quantities when a larger TLSSA (e.g., 100 mL) is used instead. For the control experiment by hydrothermal synthesis, 0.72 g of Ba(OH)2 3 8H2O was dissolved into distilled water of about 9.7 mL. Then 0.048 g of TiO2 was added into the solution. After thoroughly mixing, the above mixture was transferred into a TLSSA of 23 mL and heated at 180 °C for 10 h. The obtained solid product was thoroughly washed with distilled water and ethanol. Characterization. The phase of the as-prepared samples was determined by X-ray powder diffraction (XRD, Bruker D8 Advance) with graphite monochromatized Cu KR radiation (λ = 1.5418 A˚). The morphology and structure of the samples were characterized by transmission electron microscope (TEM, Hitachi H-600). High-resolution transmission electron microscope (HRTEM) was performed on a JEOL JEM-2010 microscope. The second-harmonic generation (SHG) properties of the products were preliminarily investigated. About 0.3 g of BaTiO3 particles were loaded into a glass vial, and the 1064 nm output of a Q-switched Nd: YAG laser (Lingyun LYPE10-SG-WL1064, Wuhan) with beam diameter of about 1 mm was focused onto the powder samples. The double-frequency (DF) emission at 532 nm was recorded by a digital camera (Canon, IXUS 850IS).

Results and Discussion Evolution of Composition and Morphology, and Influencing Factor. In the MHS synthesis, Ba(OH)2 3 8H2O not only plays the role of the solvent, but also acts as a reactant, which simplifies the fabrication procedure to a large extent. Figure 1a shows the XRD pattern of the initial TiO2 precursor, which can be indexed as the rutile phase (JCPDS No. 21-1276). The transformation of TiO2 into BaTiO3 can be completed in 3 h (Figure 1b-d). The peaks shown in Figure 3d can be attributed to tetragonal BaTiO3 (JCPDS No. 05-0626), and prolonging the reaction time to 5 or 10 h did not result in any phase change of the products (Supporting Information).

Figure 2. TEM images of the BaTiO3 samples synthesized for (a, b) 3 h, (c) 5 h, and (d) magnified image of (c). Panel b shows a special particle with an unconspicuous inner void. The inset of (c) shows a fractured particle. The scale bars represent (a) 500 nm, (b) 200 nm, (c) 500 nm, and (d) 200 nm. TiO2: 0.2 g.

The evolution process of BaTiO3 particles during the MHS synthesis was elucidated by TEM. Figure 2a shows the morphology of the products obtained after MHS treatment for 3 h. The BaTiO3 particles exhibit solid structures and their sizes are almost in the range of 400-800 nm. A few particles with an unconspicuous inner void can also be found at this stage (Figure 2b). Core-shell structured BaTiO3 particles begin to appear after the MHS process for 5 h (Figure 2c,d). Compared to the aforementioned solid particles, these homogeneous core-shell particles do not show an apparent size change. It can be clearly seen from Figure 2d that the solid evacuation starts underneath the surface layer of the particles, and the core and shell parts are still connected to each other. Further evacuation of the BaTiO3 particles takes place with prolonged reaction time. The products obtained after MHS treatment for 10 h are shown in Figure 3. The coreshell BaTiO3 particles with a size in the range of 400-800 nm exhibit a much larger inner void relative to the products obtained after treatment for 5 h. It can be seen that the core is entirely detached from the shell (Figure 3a,b). The electron diffraction pattern indicates the particle is not single-crystal but is composed of many primary crystallites (Figure 3c). HRTEM images of the core-shell BaTiO3 particle are shown in Figure 3c,d, which indicate the products are wellcrystallized, and the lattice spacing is 0.28 nm which corresponds to the interplanar distance of the {101} planes in the tetragonal BaTiO3. As mentioned above, the obtained particles with detached core and shell parts may be used as more efficient nanoreactors for better mixing of the inside reagents. It should be noted very few hollow particles also appear in the products (indicated by the arrow in Figure 2b). To get a comprehensive picture of the evolution process, the products obtained after MHS treatment for 15 h were also investigated. Much more hollow particles with a size between 400 and 700 nm come forth, although core-shell particles were still the major products (Figure 4). Further

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Figure 3. (a, b) TEM images of the core-shell BaTiO3 particles synthesized for 10 h. (c) The typical electron diffraction pattern obtained from a single particle. (d) HRTEM image obtained from the shell, and (e) shows the clear lattice fringe image. The arrow in (b) indicates a hollow particle. TiO2: 0.2 g. The scale bar represent (a, b) 500 nm.

Figure 6. TEM images of the BaTiO3 particles synthesized for 10 h with the TiO2 dosage of (a-c) 0.1 g, and (d-f) 0.05 g. Panels (b, c) and (e, f) show the magnified images of nearly hollow or hollow particles of (a) and (d), respectively. The scale bar represents (a, d) 500 nm, and (b, c, e, f) 100 nm.

Figure 4. (a, b) TEM images of the BaTiO3 particles synthesized for 15 h. TiO2: 0.2 g. The scale bars represent (a) 500 and (b) 200 nm.

Figure 5. (a) Crystal structure of tetragonal BaTiO3. The red, blue, and green denote Ba, O, and Ti atoms, respectively. (b) Doublefrequency light emitting from the core-shell BaTiO3 particles synthesized for 10 h. The nongreen light spot is due to the chromatism of the digital camera used.

reaction to 20 h does not result in apparent morphology change of the products. To further verify the phase of the core-shell BaTiO3 particles, their SHG property was investigated. As is wellknown, the tetragonal distortion, that is, (c-a)/a (a, c are the cell parameters, Figure 5a), of BaTiO3 is usually no more than 1%, and therefore the angular resolution of peak-splitting is limited in particular for nanocrystalline materials, which results in the pseudocubic XRD pattern of the BaTiO3 particles.31 In this study, we explored the use of SHG as a simple and rapid way to identify the phase of BaTiO3. As a unique nonlinear optical effect, SHG can only be generated

in noncentrosymmetric crystals including ferroelectric materials.45,46 Tetragonal BaTiO3 is an important ferroelectric material and exhibits large nonlinear optical coefficients, whereas cubic BaTiO3 is centrosymmetric and has no SHG effect.24 Therefore, SHG investigation should be able to identify the phase of BaTiO3. As shown in Figure 5b, the core-shell BaTiO3 particles synthesized for 10 h showed very bright DF emission originating from SHG effect when the 1064 nm output of a Nd:YAG laser irradiated onto the powder sample. The products prepared for 15 h also have remarkable DF emission with similar brightness. This study not only confirms the tetragonal phase of as-synthesized core-shell BaTiO3 particles, but also indicates the SHG measurement might be a convenient and efficient route for identifying crystal structures of perovskite-type ferroelectric compounds. Interestingly, the BaTiO3 particles synthesized for 5 h exhibited much weaker DF emission and those particles synthesized for 3 h showed the weakest DF emission. These facts may suggest that the complete transformation of BaTiO3 into a tetragonal phase may need sufficient time in the MHS process. Detailed study on the synthesis-dependent SHG properties is in progress and will be published elsewhere. The influence of TiO2 dosage on the morphology and size of the products was investigated, and the formation of hollow BaTiO3 particles less than 250 nm was favored when TiO2 at lower dosage was used in the MHS synthesis. When the dosage of TiO2 decreased to 0.1 g, a large proportion of nearly hollow BaTiO3 particles with their size between 140 and 250 nm appeared, and larger particles in the range of 400-800 nm with nonhollow (i.e., core-shell or solid) structures were also mixed in the products (Figure 6a).

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Figure 7. (a) Schematic illustration of the formation process of core-shell and hollow BaTiO3 particles. Gray areas in the particle: loosely packed or smaller crystallites. Dark areas: closely packed or larger crystallites. (b) TEM images of a series of intermediates trapped during the synthesis of core-shell BaTiO3 particles. The scale bars in (b) represent 500 nm.

Figure 6b,c shows the magnified images of the nearly hollow particles. It can be seen that the particles have a very small core or are even totally hollow. When the dosage of TiO2 further decreased to 0.05 g, large amounts of smaller particles with their size between 120 and 240 nm came forth in the products, although nonhollow larger particles still existed (Figure 6d). The magnified images (Figure 6e,f) show the hollow inner structures of the particles. Formation Mechanism of Core-Shell and Hollow BaTiO3 Particles. It is interesting to understand the formation mechanism of the core-shell and hollow BaTiO3 particles since no hard or soft template was used in the MHS procedure. Up to now, there are two main mechanisms for the hollowing process in template-free synthesis. One is the Kirkendall effect which originates from the difference in diffusion rates of two interdiffusion species across an interface.1,47 The net flow of mass in one direction is balanced by an opposite flux of vacancies, which may then coalesce into inner voids preferably around the interface. The other one is Ostwald ripening mechanism.16,48 Because of the intrinsic density variations inside the starting solid particles, the inner space can be created when the less stable parts (smaller or loosely packed crystallites) in the particles are undergoing mass transport through dissolving and recrystallizing. In the MHS synthesis, the transformation of TiO2 into solid BaTiO3 particles can be completed in 3 h (Figure 1d), which means that the core-shell products obtained after a longer time treatment were not formed through the interdiffusion between two reacting species, such as TiO2 and barium oxysalt. In addition, the size of the core-shell BaTiO3 particles is in the range of 400-800 nm, which is quite different from that of the TiO2 precursor particles with their size between 0.8 and 2.5 μm (see Supporting Information). The above facts definitely exclude the possibility of Kirkendall mechanism in the MHS synthesis. Moreover, it can also be concluded that the transformation of TiO2 into BaTiO3 is not an in situ process by heterogeneous nucleation of BaTiO3 on the surface of TiO2, because an in situ transformation is likely to produce BaTiO3 particles with a similar size of the initial TiO2 particles. Thus, BaTiO3 should be formed through a dissolution-precipitation process. That is, the TiO2 is dissolved into soluble species such as [Ti(OH)x4-x] first, and homogeneous nucleation of BaTiO3 arises then

through the reaction between soluble barium and titanium sources.49 Ostwald ripening is believed to be responsible for the hollowing mechanism, and the formation process of coreshell and hollow products is illustrated in Figure 7a. At the early stage, the TiO2 is dissolved into soluble species and then homogeneous nucleation of BaTiO3 takes place, as described above. With continuous nucleation and growth of BaTiO3, larger BaTiO3 particles would form due to the aggregation of primary crystallites. The aggregation characteristic of the BaTiO3 particles can be revealed by the ED patterns of the early products obtained at 3 h (not shown), and the final core-shell particles (Figure 3c). Using the Scherrer-Debye formula, the size of the primary crystallites in the BaTiO3 particles obtained at 3 h (Figure 2a) is estimated to be 27 nm based on (101) diffraction peak, which also verifies the aggregation characteristic of the BaTiO3 particles. Further MHS treatment would result in the inside-out ripening of the particles.4,16 The smaller or loosely packed crystallites inside the particles would dissolve and diffuse outward. With the transportation of the mass, the void space between the core and the shell was generated, and became larger and larger after a longer process time. Figure 7b shows the morphology evolution of a series of intermediates trapped during the synthesis of core-shell BaTiO3 particles. For the final core-shell particles after MHS treatment for 10 h, the core is totally detached from the shell (Figure 3). The hollowing process will continue until the primary crystallites become highly closely packed and crystallize in a sufficiently stable phase. After reaction for 15 h, many hollow particles appear in the products (Figure 4), which further supports that the Ostwald ripening is the underlying mechanism for the hollowing evolvement in the MHS synthesis. The hollowing phenomena for crystalline particles through Ostwald ripening have been observed in diverse cases, such as for ZnS16 and TiO2,50 and the surface layer of the particle may be stabilized by inorganic ions in the solution.4 As mentioned above, longer MHS treatments favor the formation of thermodynamically stable tetragonal BaTiO3 with a strong SHG effect; therefore, the phase transformation from metastable (maybe cubic or amorphous) BaTiO3 into tetragonal BaTiO3 may provide the additional driving force for the Ostwald ripening.

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ensures the formation of dispersive micro/nanostructures without using an organic surface-capping materials.27 Therefore, the MHS strategy provides a desirable alternative for the synthesis of micro/nanostructures of complex functionaloxides. Conclusions

Figure 8. XRD pattern of the products synthesized for 10 h when saturated Ba(OH)2 solution was used as the solvent. The ratio between Ba(OH)2 and TiO2 is identical to that used for the synthesis of core-shell BaTiO3 particles. Ba(OH)2: 0.39 g, TiO2: 0.048 g, and H2O: 10 mL.

At low TiO2 dosage, relatively small BaTiO3 particles tend to appear due to the low concentration of primary crystallites. Then the inner mass of the particle is inclined to dissolve completely and recrystallize onto the shell. And therefore the hollow particles with size less than the core-shell particles come into being. The nonuniformity of the products may be attributed to the relatively large size of the TiO2 precursor particles. For larger TiO2 particles, the dissolution and transformation of TiO2 into BaTiO3 should be a longer period of time. Accordingly, the nucleation, growth, and aggregation process of BaTiO3 particles would become much more complex, and the above stages should not be separated effectively. Thus, BaTiO3 particles with nonuniform size and morphology become the dominant products. With these considerations in mind, a possible route to improve the uniformity of the products may be the use of smaller rutile particles as the precursor. Using anatase precursor particles might be another solution, which have higher activity than rutile particles and their transformation into BaTiO3 should be completed in a shorter period of time. Control Experiment by Solvothermal/Hydrothermal Synthesis. As the MHS synthesis was conducted under sealed conditions at elevated temperature, it can be viewed as a methodological extending of solvothermal/hydrothermal synthesis. To compare the MHS strategy with the conventional solvothermal/hydrothermal method, the control experiment was performed in which saturated Ba(OH)2 solution was used as the solvent for the synthesis of BaTiO3. The ratio between Ba(OH)2 and TiO2 is identical to that used for preparation of core-shell BaTiO3 in the MHS synthesis. As shown in Figure 8, in the hydrothermal synthesis the transformation of TiO2 into BaTiO3 could not be completed even after reaction for 10 h, whereas in MHS synthesis this transformation can be achieved in only 3 h (Figure 1d). In MHS synthesis, completed reaction can be achieved in a much shorter time, which may be due to the high polarity and reactivity of the molten hydrated salt.37,44 Moreover, because of the high viscosity of molten hydrated salts and short diffusion distances of species in them, reactants at high concentration can be introduced into the system for large-scale synthesis of micro/nanosized materials. The high viscosity can also restrain the agglomerate of the product particles, which

The MHS strategy was put forward for the synthesis of complex oxide nanomaterials. In our first attempt, tetragonal BaTiO3 was successfully prepared employing Ba(OH)2 3 8H2O as the solvent. The synthesis procedure is quite simple, and no additive is introduced. The novel core-shell or hollow structures of the products may bring out applications in various fields, such as drug delivery,4 ceramic capacitors,7 and catalyst supporting.51 The simplicity and scalability of the MHS method might provide a promising route for the synthesis of functional oxide nanomaterials for practical applications. Studies on improving the uniformity of the products and extending the strategy to other perovskite-type compounds are under way. Acknowledgment. The authors thank Ms. Jianmin Luo for her help in the TEM observations. This work is supported by the National Natural Science Foundation of China (Grant No. 50802110), the “One Hundred Talents” Program, the “Western Light” Program of the Chinese Academe of Sciences, the Natural Science Foundation of Xinjiang Uygur Autonomous Region of China (Grant No. 200821159) and the High Technology Research and Development Program of Xinjiang Uygur Autonomous Region of China (Grant No. 200816120). Supporting Information Available: Preparation and TEM images of the rutile precursor particles, and the XRD patterns of the BaTiO3 particles after MHS treatment for 5 and 10 h. This information is available free of charge via the Internet at http:// pubs.acs.org/.

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