A Novel, Controlled, and High-Yield Solvothermal Drying Route to

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A Novel, Controlled, and High-Yield Solvothermal Drying Route to Nanosized Barium Titanate Powders Yury V. Kolen’ko,*,† Kirill A. Kovnir,‡ Ine´ s S. Neira,† Takaaki Taniguchi,† Tadashi Ishigaki,† Tomoaki Watanabe,† Naonori Sakamoto,† and Masahiro Yoshimura† Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, and Max-Planck-Institut fu¨r Chemische Physik fester Stoffe, No¨thnitzer str. 40, 01187, Dresden, Germany ReceiVed: NoVember 24, 2006; In Final Form: March 8, 2007

In this report, we describe a novel solvothermal procedure for the synthesis of nanosized particles of barium titanate (BaTiO3). We have been able to synthesize large amounts of nearly uniform sized BaTiO3 nanocrystals in the size range of 5-37 nm. The advantages of our technique over other previously reported hydrothermal/ solvothermal approaches are the high yield and the simple but precise control of the size of the particles, which is very conveniently achieved by changing the water content of the reaction mixture in a measured way. The particles are systematically characterized by powder X-ray diffraction (XRD), Raman scattering, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED), high-resolution TEM (HRTEM), disc centrifugation, thermogravimetric and differential thermal analyses (TGADTA), infrared spectroscopy (IR), and inductively coupled plasma-optical emission spectrometer (ICP-OES). The as-synthesized BaTiO3 nanopowders contain BaCO3 byproduct as well as internal OH- groups and residual solvent species that can be removed by acid washing following heating. However, it is shown that this procedure results in the substantial change of the chemical composition and strong degradation of real microstructure of nanosized BaTiO3 particles.

Introduction Barium titanate is a key member of the family of technologically potential materials mainly because of its ferroelectric response and high dielectric constant1 that are regarded to be quite useful for the further development of the electronics industry. In particular, BaTiO3 can be a suitable material for electro-optical applications,2 thermistors,3 capacitors,4 microwave absorbers,5 photonic crystals,6 luminescent,7 high-capacity memory cells,8 ceramics,9 and so forth. It is well-known that barium titanate exists in several polymorphous modifications, of which the tetragonal (ferroelectric) and cubic (paraelectric) phases are most investigated. Tetragonal phase is the room-temperature stable form which transforms to cubic phase upon heating above the Curie temperature (Tc) of 130 °C (for bulk crystals).10 In the cubic perovskite structure of BaTiO3 (space group Pm3hm), TiO6 octahedra share all their corners forming a 3D framework with Ba atoms lying in the large cubooctahedral voids (see Supporting Information (SI) Figures S1A, S1B). This perfect perovskite structure, upon cooling below the Curie temperature, undergoes a structural deformation with the Ti atom displaced from the octahedron center toward one of the six O atoms (see SI Figures S1C, S1D), and this structural transformation leads to ferroelectric stabilization in tetragonal BaTiO3.11 Nanomaterials are often characterized by novel properties that can be considerably different from those of the bulk phase, and consistent with that observation, the polymorphous transformation in BaTiO3 nanoparticles has been found to be suppressed.12 For nano-BaTiO3, * To whom correspondence should be addressed. Phone: +81-(0)45924-5368; fax: +81-(0)45-924-5358; e-mail: [email protected]. † Tokyo Institute of Technology. ‡ Max-Planck-Institut fu ¨ r Chemische Physik fester Stoffe.

Tc becomes progressively lower with size, making the paraelectric cubic phase to be the room-temperature stable phase. The correlation between the size of the particles and the ferroelectric response is predicted in the frames of the Landau-GinsburgDevonshire thermodynamic theory.13 Together with it, various models for the explanation have been proposed, for example, critical crystallite size effect,14 lattice impurity effect,15 effect of oxygen vacancies,16 and the core (tetragonal)-shell (cubic) model.14a However, it is still an open question if the decreasing of the critical particle size of BaTiO3 can reduce ferroelectricity to zero. Nevertheless, Fu et al.17 suggested on the basis of microscopic first-principle calculations that the large ferroelectric off-center displacements can exist in barium titanate quantum dots with sizes as low as 5 nm. Therefore, both from the point of view of technology and pure physics, synthesis of phasepure barium titanate with nanoscaled particle size, uniform size distribution, distinct shape, and high crystallinity remains to be an extremely important task, and hence there is considerable interest in the development of methods that can produce BaTiO3 with different morphologies and dispersion. Over the past decades, several “soft chemistry” approaches18 have been developed for the preparation of nano/microsized barium titanate which cannot be obtained by classical solidstate routes. Various phase compositions and morphologies have been obtained by solution chemical,19 spray,20 glycothermal,21 catecholate,22 electrophoretic deposition,23 sol-gel,24 microwave,25 high-gravity reactive precipitation,26 gas evaporation,27 pyrolysis,28 room-temperature biosynthesis,29 microemulsionmediated process,30 and alkoxide-hydroxide sol precipitation31 as well as mainly by hydrothermal route32 including microwavehydrothermal,33 hydrothermal-electrochemical,34 and solvothermal syntheses.35 In addition, BaTiO3 in the form of lowdimensional nanostructures has also been successfully synthesized

10.1021/jp0678103 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007

Solvothermal Drying Route to Barium Titanate using soft chemistry routes.36 Although numerous methods have been described for the fabrication of nanomaterials on the basis of barium titanate, some problems are still unsettled, namely, there are just only a few reports available on BaTiO3 nanocrystals having a size smaller than 10 nm possessing nearly spherical morphology.19a,24c,24e,29,30b,31d,31e,32c,35 In this work, we focus on the preparation and investigation of BaTiO3 nanocrystals of sizes down to 5 nm. For the synthesis, we used direct reaction between barium hydroxide octahydrate and titanium(IV) tetraisopropoxide under solvothermal conditions. A similar approach was used in the literature to produce barium titanate nanoparticles,31d,31e however, without solvothermal treatment. This synthetic procedure takes advantage of the fact that when Ba(OH)2‚8H2O is dissolved, the in-situ released water provides the hydrolysis and condensation reaction of titanium(IV) tetraisopropoxide, resulting in the title compound. At the same time, this approach is not well developed in terms of high yield, feasibility of large-scale production, and homogeneity of microstructural characteristics (defined shape, narrower particle size range, etc.), as it is reasonably expected to be in a moderate-temperature solvothermal context. In this paper, we provide a synthetic route to nearly uniform sized BaTiO3 nanoparticles and describe their structural properties. The nanoparticles are produced by the solvothermal and solvothermal drying routes. The key features of a last one novel synthetic procedure is a high yield (up to 100%) which can be important for large-scale production in industries as well as the precise control of the size of the nanocrystals in the range of 5-37 nm by simple addition of a fixed amount of water during the solvothermal drying synthesis. Preliminary results on the chemical and physical degradation, in terms of the chemical composition and real microstructure, of nanosized BaTiO3 particles with acid washing and following heat treatment are also presented. Experimental Methods Starting Materials. Ti[OHC(CH3)2]4 (97.0% Kanto), Ba(OH)2‚8H2O (98.0% Kanto), absolute ethanol CH3CH2OHabs (99.5% Kanto), 2-methoxyethanol CH3O(CH2)2OH (99.0% Kanto), and acetic acid CH3COOH (99.7% Wako) were used as received. Distilled water was utilized as an additional agent during the solvothermal process as well as for preparation of the aqueous solutions and washing. Solvothermal Syntheses of Nanopowders. Nanosized BaTiO3 powders were prepared solvothermaly (shown schematically in Figure 1). First, the barium hydroxide octahydrate (10 mol % excess, 4.73 mmol) was dissolved in 2-methoxyethanol (20 mL) under continuous stirring (600 rpm) for 1 h. The Ti containing solution consisting of titanium(IV) tetraisopropoxide (4.3 mmol) diluted in absolute ethanol (10 mL) was added dropwise to the barium containing precursor at room temperature under continuous stirring (400 rpm). Finally, this mixture was placed in a polytetrafluoroethylene (PTFE) vessel (volume 40 cm3), and the vessel was capped by a PTFE cover and was placed inside a stainless steel autoclave (see SI Figure S2). The autoclave was sealed and kept at 240 °C for 6 h under autogenous pressure. The product of the solvothermal process was collected by centrifugation (at 4000 rpm for 1 h), was washed twice with distilled water and once with ethanol, and then was dried at 80 °C for 12 h on air. To remove most of the BaCO3 admixture from as-produced powder, a washing of grinded product (∼0.2 g) was carried out by ultrasonication for 5 min in 20 mL of 0.1% aqueous acetic acid solution. Throughout this work, a set of acronyms is used. The first three letters mean the desired BaTiO3 (BTO) and the number

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Figure 1. Solvothermal (solid lines) and solvothermal drying (dotted lines) syntheses flowcharts.

of the sample; the last letter indicates (if applicable) the applied further processing: w, washing by diluted acetic acid solution. Solvothermal Drying Syntheses of Nanopowders. The nanopowders of BaTiO3 were also prepared by a novel solvothermal drying technique (shown schematically in Figure 1). All reagents were mixed in a similar manner as in the case of the described above solvothermal synthesis. The main difference of the solvothermal drying technique is that a lowdensity (0.6 g/cm3) 1.5 mm thick HYPER-SHEET gasket (GORE-TEX) with porous structure of PTFE polymer was placed between the vessel and cover (see SI Figure S3A). Under the elevated temperatures and pressures, a slow shrinkage of the gasket takes place (see SI Figure S3B) that leads to the gradual evaporation of the solvents during the synthesis. On the basis of our observations, the solvothermal drying synthesis at 240 °C resulted in off-white powders (probably connected with the presence of organic species) already after 6 h. To remove most of the BaCO3 byproduct from as-produced powders, a washing of grinded powders (∼0.2 g) was carried out by ultrasonication for 5 min in 20 mL of 0.5% aqueous acetic acid solution. To investigate the possibility of particle size control in the course of solvothermal drying synthesis, the Ba(OH)2‚8H2O was dissolved in 2-methoxyethanol along with fixed amounts of distilled water (1, 2, 3, 4, and 6 mL). All other reagents were mixed in a similar manner as in the case of conventional solvothermal synthesis. Characterization. The crystalline products were characterized by powder X-ray diffraction (XRD) using a Rigaku RINT 2000 diffractometer with Ni-filtered Cu KR radiation (λ ) 1.54178 Å). Data were collected in the 2θ range of 10-80°, with a scan speed of 0.5°/min and a 0.02° step width. The size of the barium titanate crystallites was calculated by means of the Debye-Scherrer formula from the average broadening of the (100), (110), and (111) XRD reflections after KR2 correction. Full width at half-maximum (fwhm) values were obtained using the PeakFit software program. For the calculations of the unit cell parameters, the selected samples were characterized by means of XRD using a Huber G670 Image Plate Camera, Cu KR1 radiation, λ ) 1.540598 Å. The unit cell parameters were calculated from least-squares fits using LaB6 (cubic, a ) 4.15692 Å) as an internal standard utilizing the program package

7308 J. Phys. Chem. C, Vol. 111, No. 20, 2007 WinCSD.37 For all selected samples, the cubic symmetry was assumed, space group Pm3hm (No. 221). The room-temperature Raman scattering measurements were carried out on a Jobin Yvon T64000 spectrometer with visible laser light (wavelength 514.5 nm) as the excitation light. The slits were adjusted so that the resolution was 1 cm-1. The scattered light was collected in the back-scattering geometry using a liquid-nitrogen-cooled charge-coupled device (CCD) detector. All measurements were carried out under the microscope (the laser spot diameter was estimated to be between 1 and 2 µm). The system was operated with an output power of 50 mW. Prior to each measurement, the spectrometer was calibrated using a mercury lamp. The morphology was studied by scanning electron microscopy (SEM) using a Hitachi S-4500 microscope operating at 15 kV. Transmission electron microscopy (TEM), electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM) investigations were performed using a Hitachi H 9000 NAR microscope operating at 300 kV. For TEM measurements, one drop of the sample solution (sample dispersed in ethanol) as well as viscous reaction solution after centrifuging was deposited onto a holey carbon grid. Calculated electron diffraction patterns by fast Fourier transformation (FFT) were obtained using the ImageJ software program. Particle size distributions (PSD) of the selected samples were determined using a sedimentation analysis on a CPS Disc Centrifuge DC24000 equipped with an optically clear spinning disc and laser analyzer. The disc centrifuge was calibrated with polyvinyl chloride standard dispersed in distillated water with particle size 376.5 nm. For the measurement, typically 2-3 mg of the titanate powder was dispersed into 50 mL of distillated water using ultrasonic bath for 5 min. Thermogravimetric and differential thermal analyses (TGADTA) were performed using a Mac Science Type-2020 TGDTA thermoanalyzer, on air at a heating rate of 10 °C/min. The room-temperature diffuse reflectance infrared (IR) Fourier transform spectra were recorded on a Jeol JIR-7000 spectrometer with a resolution of 4 cm-1 and an accumulation of 100 scans. The crystalline products (4 mg) were thoroughly ground with (200 mg) potassium bromide powder (KBr for IR, Wako) in a mortar and pestle to have a fine mixture and were subjected to IR analysis. For the background spectrum, a finely grinded KBr powder was used. Chemical analysis for the Ba and Ti content was performed by inductively coupled plasma-optical emission spectrometer (ICP-OES) methods on a Varian Vista RL spectrometer. All values are the average of at least three replicates. Total specific surface area was calculated according to the BET method using N2 physisorption at 77 K (Micromeritics FlowSorb III). Results and Discussion Barium titanate nanopowders were prepared by direct reaction between solutions of barium hydroxide octahydrate and titanium(IV) tetraisopropoxide in 2-methoxyethanol and absolute ethanol, respectively. The chemical reaction under solvothermal condition conducted in current study likely occurs by eq 1, predominate in very basic solution.31b,31d In Figure 1 (solid lines), details of the synthetic procedures are summarized. According to eq 1, eight moles of water in the solid precursor of Ba(OH)2‚ 8H2O should be sufficient for the hydrolysis and condensation reaction of titanium(IV) tetraisopropoxide.31d Hence, after Ba(OH)2‚8H2O dissolution stage, the in-situ released H2O provides the formation of the title compound by eq 1.

Kolen’ko et al.

Ti[OHC(CH3)2]4 + H2O + Ba2+ + 2OH) BaTiO3 + 4(CH3)2CHOH (1) The crystal structure and average particles size of the solvothermally derived BaTiO3 are given in Table 1 (sample BTO-1). According to the powder XRD (Figure 2A), the asprepared sample BTO-1 is found to be a mixture of the BaTiO3 phase and barium carbonate (orthorhombic, JCPDS No. 5-378) byproduct. XRD reveals that the washing of BTO-1 in diluted acetic acid solution results in phase-pure barium titanate (sample BTO-1w) although it is difficult to assign the crystal symmetry of the nanoparticles to either cubic or tetragonal from this XRD pattern (sample BTO-1w, Figure 2A) because of the linebroadening effect and low intensity. Line-broadening analysis reveals that the crystallite size is about 5 nm in this case. Along with the XRD analysis, which gives average and static symmetry, the local and dynamic symmetry of solvothermally produced barium titanate is also monitored by Raman spectroscopy. According to the selection rules, all of the optic modes of BaTiO3 with perfect cubic symmetry should be Raman inactive while the same for the polar tetragonal and orthorhombic polymorphous forms should be Raman active,38 and therefore, Raman scattering becomes an essential study to be performed. Although the local structures of single crystalline and polycrystalline BaTiO3 as well as films have been characterized extensively by Raman scattering, the assignment of the vibrational frequencies to the Raman bands of BaTiO3 is still under discussion.39 This factor complicates the assignment of measured Raman bands. In current study, the observed Raman peaks have been assigned to more than one phonon mode, in accordance with the previous report,40 where in the Raman spectra of the bulk barium titanate, sharp bands around 175 cm-1 [A1(TO), E(LO)] and 305 cm-1 [B1, E(TO + LO)] and broad bands around 265 cm-1 [A1(TO)], 520 cm-1 [A1, E(TO)], and 720 cm-1 [A1, E(LO)] are characteristic of tetragonal phase. In Figure 2B, the Raman scattering spectra from powder samples BTO-1 and BTO-1w along with one representative spectrum from the bulk orthorhombic barium carbonate (99% Wako) are shown. All our products have typical spectrum for nanopowders with relatively broader peaks in comparison with the BaTiO3 single crystal39a mostly because of the random particle orientation in powder. The set of observed bands and spectrum features agrees fairly well with the reported Raman data for locally disordered cubic BaTiO3.40,41 The detected bands centered near 178, 529, and 715 cm-1 can be assigned to the [A1(TO), E(LO)], [A1, E(TO)], and [A1, E(LO)] phonon modes of the tetragonal BaTiO3 phase, respectively.40 However, the smearing/weakening of the band at about 265 cm-1 and the absence of a sharp peak at 305 cm-1 correspond to the [A1(TO)] and [B1, E(TO + LO)] modes, respectively, indicating an essentially symmetric local environment (i.e., the cubic symmetry) with small distortions.40,42 Raman scattering results also prove that BaCO3 byproduct can be completely removed from solvothermaly prepared sample by washing with dilute acetic acid (Figure 2B), which is in good agreement with the XRD analysis. Scanning electron microscopy was used to investigate the morphology of the obtained powders. Figure 3A shows a typical SEM image of the BTO-1 sample. Two different morphologies of the as-prepared powder are observed. In Figure 3A, relatively big aggregates ranging from a few to tens of micrometers without any texture are seen. Moreover, a large number of comparatively smaller aggregates (in the range of a few hundred nanometers to a few micrometers) are found to be present on

Solvothermal Drying Route to Barium Titanate


TABLE 1: Phase Compositions and Average Crystallite Sizes of the Nanosized Barium Titanate Powders Prepared by Solvothermal and Solvothermal Drying Routesa

sample

amounts of added water, mL

BaTiO3 modification detected by Raman scattering

average crystallite size, nm ((5%)b

BTO-1

SolVothermal Synthesis cubic

5

BTO-2 BTO-3c BTO-4 BTO-5 BTO-6 BTO-7 BTO-8

SolVothermal Drying Synthesis cubic cubic cubic cubic tetragonal tetragonal tetragonal

5 6 8 11 17 27 37

1 2 3 4 6

a Synthetic conditions: T ) 240 °C, t ) 6 h. According to XRD and Raman scattering, orthorhombic BaCO3 byproduct is present in all the as-produced samples. b Average crystallite sizes are estimated by means of the Debye-Scherrer formula. c Double amounts of the barium and titanium precursors were utilized for synthesis.

Figure 3. SEM microphotographs are shown for (A) solvothermally prepared BTO-1 sample and (B) the same sample after washing in diluted acetic acid (sample BTO-1w). (C) HRTEM image of the BTO-1 sample (inset: a representative HRTEM image of the lamellar single BaTiO3 particle) and (D) corresponding ED pattern from sample BTO1.

Figure 2. (A) Comparison of the X-ray powder diffraction patterns of the solvothermally prepared sample BTO-1 and washed BTO-1w sample. Tick marks below the patterns correspond to the positions of the Bragg reflections expected for the orthorhombic BaCO3 (JCPDS No. 5-378). Positions of the BaTiO3 diffraction peaks are shown by asterisks. (B) Raman spectra from samples BTO-1 and BTO-1w along with the data from bulk barium carbonate (orthorhombic). Dotted lines correspond to the observed set of bands for barium titanate.

top of the previously mentioned bigger aggregates (Figure 3A), which mostly disappear after washing with diluted acetic acid solution although the morphology of the bigger aggregates

remains nearly unaffected after washing (Figure 3B). From these observations, the smaller aggregates can be identified as BaCO3 byproduct. Transmission electron microscopy was applied to investigate the fine microstructure of the nanocrystals; the observed results are in agreement with the XRD and Raman spectroscopy data. Figure 3C shows typical HRTEM images from nanosized BTO-1 sample with two different magnifications. The powder consists of nanoscaled crystals with a mean particle size in the range from 5 to 7 nm, indicating their single-crystalline nature that is assessed by line-broadening analysis data. The edges of the particles are sharp faceted and free of any amorphous or secondary phase at the surface. Electron diffraction from the BTO-1 sample shows a distinct ring pattern, typical for an agglomerate of nanoparticles (Figure 3D). The rings of the ED pattern can be completely indexed in the barium titanate cubic Pm3hm space group, using the BaTiO3 unit cell parameter (JCPDS No. 31-174) further supporting the conclusions of Raman scattering measurements. Although centrifugation was applied at 4000 rpm for 1 h to collect the product after solvothermal synthesis, the results of TEM and ED investigation (see SI Figure S4) clearly illustrate that the nearly colorless and viscous reaction solution, along with the byproduct orthorhombic acicular BaCO3, still contains the target product. The cubic quite sharp faceted BaTiO3 nanoparticles are nearly uniform and seem to be well dispersed within viscous reaction solution (Figure S4). Although in the present study we found out that the cubic nanosized barium titanate can be successfully produced by conventional solvothermal method, the mass yield of BaTiO3 was only ∼25%, which is unsuitable for any large-scale production. Careful investigation of the full synthesis method indicates that the BaCO3 byproduct in sample BTO-1 is mainly formed while opening the PTFE Teflon vessel as well as during the product collection. Evidently, the complete reaction between the precursors did not take place in this case, which suggests that longer duration of solvothermal process would be required to obtain significant mass yield of BaTiO3, since the temperature of synthesis is limited by utilization of a pressure vessel

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Figure 5. A graph showing dependence of the average particle size of barium titanate over amount of added water during the solvothermal drying synthesis.

Figure 4. (A) X-ray powder diffraction patterns of the solvothermal drying syntheses products consist primarily of the BaTiO3, with a small amount of BaCO3 byproduct. The dotted line corresponds to the position of the most intense (111) peak for the orthorhombic BaCO3 (JCPDS No. 5-378). (B) The angular regions of the X-ray powder diffraction patterns from 36.0° < 2θ < 49.0° and 62.0° < 2θ < 68.0° where (111), (200), and (220) peaks of the cubic BaTiO3 are expected to be. Tick marks below the patterns correspond to the positions of the Bragg reflections expected for the byproduct orthorhombic BaCO3 (JCPDS No. 5-378).

fabricated with PTFE Teflon (Tmax e 250 °C). Moreover, the product collection by centrifugation was incomplete, as observed from the low-resolution TEM experiment (see SI Figure S4). The yield of the solvothermal process can be improved by the gradual evaporation of the solvents during the synthesis along with the product formation. Therefore, the nanopowders were also prepared by a solvothermal drying technique (see Experimental Methods). This process results in the formation of only solid products after reaction whereas in case of the conventional solvothermal synthesis both solid products and solution are formed. The advantage of this technique is that the products can be obtained with a high yield (up to 100%). In contrast to the conventional solvothermal process, the mass yield of barium titanate nanopowders, prepared by solvothermal drying method, in all the cases was higher than 95%. The powder XRD patterns from the set of samples prepared by solvothermal drying route is presented in Figure 4A. Similar to the case of BTO-1, all the patterns in these cases also reveal a mixture of BaTiO3 and BaCO3 as the products of solvothermal

drying synthesis; however, the amount of the admixture is significantly reduced compared to convenient solvothermal synthesis. Since the cubic and tetragonal structures of BaTiO3 are closely related (see SI Figure S1), their powder XRD patterns are similar and the quality of our X-ray data (very broad peaks and high background) does not allow us to distinguish between them. In particular, for all the series of solvothermally prepared samples and washed ones, no splitting of the peaks corresponding to cubic Bragg index (200) and (220) into tetragonal (200)/ (002) and (202)/(220), respectively, was detected by powder XRD (Figures 4B, S5). Moreover, the formation of both structures cannot be excluded which makes the interpretation of the XRD data even more complicated. It can be expected that the hydrolysis reaction of Ti[OHC(CH3)2]4 prior to the nanocrystal formation becomes kinetically more efficient in the presence of water31e,43 and consequently leads to the increase of size of the particles, thus providing a very useful handle to control the particle sizes by adding a suitable amount of water to the reaction mixture. Figure 5 demonstrates that a precise tuning of the barium titanate averaged particle size could be realized by simply adding fixed amounts of water to the reaction mixture prior to the solvothermal drying synthesis. According to the average half-width data of the (100), (110), and (111) XRD reflections, the particle size of as-prepared nanopowders increases from ∼5 nm (sample BTO-2) to ∼37 nm (sample BTO-8) with the increase of water addition (Table 1, Figure 5). Indeed, the same trend for the particles size was detected in washed samples. XRD analysis also reveals that the washing of all samples in diluted acetic acid solution results in phase-pure barium titanate which together with TEM investigation proves good thermal stability of the as-produced nanoparticles powders, since annealing of the washed samples at 600 °C for 10 h on air does not lead to any significant changes in phase composition and particles sizes. Notice that the sintering under heat treatment was not observed, at least in the case of nanoparticles with overall size of 37 nm. As an example, we show results for BTO-8 series of samples (see SI Figures S5, S6). The local crystal structure of solvothermal drying synthesis products was further monitored by Raman spectroscopy. In Figure 6, the Raman scattering spectra from washed samples, along with one representative spectrum from the bulk tetragonal BaTiO3 (t-BTO), are shown. A broad band centered at 265 cm-1, a sharp band at 305 cm-1, and an asymmetric and broad band


Figure 6. Raman spectra from the barium titanate samples prepared by solvothermal drying route along with the data from bulk tetragonal BaTiO3. Dotted blue lines correspond to the set of bands observed for the bulk tetragonal BaTiO3. Raman spectra collected from the BTO-2w and BTO-8w samples are almost identical to spectra from BTO-3w and BTO-7w samples, respectively, and are omitted for clarity.

at 517 cm-1 as well as a broad band at 716 cm-1 observed for the bulk t-BTO (black line in Figure 6) can be assigned to the [A1(TO)], [B1, E(TO + LO)], [A1, E(TO)], and [A1, E(LO)] phonon modes of the barium titanate phase, respectively.40 In addition, a spectral dip at 178 cm-1, which appears for tetragonal barium titanate single crystals and coarse ceramics owing to the anharmonic coupling among three [A1(TO)] phonons,40 was also observed for the bulk t-BTO. Practically no differences in the intensity and band positions for the BTO-2w and BTO-3w as well as for the BTO-7w and BTO-8w samples were detected by Raman scattering, indicating absence of any significant change in their local structures. The collected spectra from the samples BTO-3w-BTO-5w, shown in Figure 6, resemble well those reported previously for locally disordered cubic BaTiO340,41 as well as with the spectrum collected from the BTO1w sample (Figure 2B). Accordingly, the smearing/weakening of the band at about 265 cm-1 as well as the absence of a sharp peak around 305 cm-1 gives clear evidence for an essentially symmetric local environment with small distortions40,42 in asproduced BaTiO3 nanopowders with nanocrystal size lying in the range of 5-11 nm (Table 1). At the same time, a careful inspection of the collected spectra from the samples BTO-6w and BTO-7w (Figure 6) clearly shows that the appearance of a sharp peak around 305 cm-1 corresponds to the [B1, E(TO + LO)] mode of tetragonal BaTiO3 phase40 and together with the clear similarity of these set of spectra with the representative bulk tetragonal BaTiO3 spectrum confirms acentric local environment (i.e., the tetragonal symmetry) for the nanocrystals in the size range of 17-37 nm (Table 1). Our experimental findings are very much in accordance with recent Raman scattering investigations of Pithan et al.,30b wherein the transition from a locally distorted cubic structure (particles with size 15 nm) has been also observed. An intensive band at around 178 cm-1 is clearly observed in all synthesized samples (Figures 2B and 6). The appearance of similar bands is in fact well described in the literature and have been related with the local orthorhombic distortion in the nanosized BaTiO3.42,44 However, the presence of a Raman peak

J. Phys. Chem. C, Vol. 111, No. 20, 2007 7311 at about 178 cm-1 rather than a spectral dip is probably not conclusive proof of the orthorhombic local distortion of the average cubic lattice since the Raman scattering features in this spectral region are strongly dependent on the lattice polarization state, the polarization orientation, and the boundary conditions. In light of this, the most convincing picture of the crystal structure and symmetry of BaTiO3 nanocrystals with diameter of about 5-10 nm was suggested recently by Petkov45 et al. from the analysis of the atomic pair distribution functions. The local symmetry (distance range: 2-2.5 nm) of nanosized BaTiO3 is tetragonal but the slight tetragonal distortions seem to average out and the structure of the material is better described in terms of a cubic-like ordering at longer range distances. Thus, one can assume that our as-prepared BaTiO3 with nanocrystals size in the range of 5-11 nm consists mainly of a global cubic structure with local tetragonal clusters (Table 1, samples BTO-1-BTO-5). Additionally, the inspection of the two bands centered around 178 and 517 cm-1 (Figure 6), representing [A1(TO), E(LO)] and [A1, E(TO)] phonon modes, respectively, clearly reveals the existence of simultaneous band shifts toward the high-frequency region for the [A1(TO), E(LO)] mode and toward the low-frequency region for the [A1, E(TO)] mode with increasing particle size. This observation may be related to a global cubic structure with local tetragonal clusters to tetragonal global structure transition in barium titanate with the increasing particle size (Table 1).46 SEM image of the sample BTO-3 shows the typical morphology of the powders prepared by solvothermal drying technique (Figure 7A). The sample BTO-3 consists of small and relatively big aggregates without any texturing, the particle size ranging from a few hundred nanometers to several micrometers. Washing of the BTO-3 sample in diluted acetic acid does not lead to significant changes in the microstructure (Figure 7B), however, a slight increase in the average size of aggregates was observed. Moreover, the surface of big aggregates appears to be not so sharp, that is, slightly degraded as compared to those presented in Figure 7A. This may be related to a removing of a BaCO3 byproduct from the asprepared BTO-3 sample in agreement with XRD analysis. The HRTEM image of as-produced BTO-3 is presented in Figure 7C. In this figure, randomly oriented nanoparticles with well-developed lattice fringes are seen. The crystallite sizes lie within the range of 5-7 nm, which is almost the same compared with products of conventional solvothermal synthesis (sample BTO-1). The nanoparticles have good crystallinity and are defect-free. Corresponding electron diffraction from the BTO-3 sample shows a distinct ring pattern and can be completely indexed in the BaTiO3 cubic Pm3hm space group (Figure 7D). No rings corresponding to secondary phase are present. Figure 7E shows HRTEM image of single barium titanate nanoparticle. The corresponding fast Fourier transform (FFT) patterns of the single BaTiO3 nanoparticles are shown as an inset. This pattern could be indexed according to the cubic BaTiO3 structure. As a general feature for the barium titanate nanopowders obtained by solvothermal drying route, the syntheses in the presence of a fixed amount of water (6 mL) lead to the formation of mostly large aggregates (Figure 8A, sample BTO-8) consisting of nanocrystals (inset in Figure 8A). The general morphology of the washed samples is quite similar to that of as-produced ones, but increase of the average size of aggregates and shaping of their surface were observed (Figure 8B, sample BTO-8w). Moreover, acidic treatment does not change fine aggregate structure which is revealed by SEM (inset in Figure 8B). The size distributions for as-synthesized samples, which were


Kolen’ko et al.

Figure 7. SEM microphotographs are shown for (A) BTO-3 sample synthesized by solvothermal drying route and (B) the same sample after washing in diluted acetic acid (sample BTO-3w). TEM images of the BTO-3 sample: (C) HRTEM image, (D) corresponding ED pattern, and (E) a representative [111] HRTEM image (inset: FFT of the HRTEM image) of the lamellar single BaTiO3 particle.

estimated from SEM data, are rather broad. However, the average size of samples BTO-7w and BTO-8w of ∼25 and ∼35 nm, respectively, is in agreement with our line-broadening analysis data (Table 1) and TEM results. Accordingly, lowresolution TEM picture of the sample BTO-8 shows the typical fine microstructure of the prepared nanopowder (Figure 8C). The sample consists of well-shaped nearly rectangular nanocrystals with average particle sizes ∼35 nm. The line-broadening analysis of the crystallite size (Table 1) is in good agreement with the data obtained by TEM. The particles should therefore be considered as single domains. The electron diffraction pattern (inset in Figure 8C) from BTO-8 sample resembles well with the cubic/tetragonal BaTiO3 patterns but does not allow us to conclusively identify the structure to be a cubic or a tetragonal. However, the weakly resolved rings from d002 and d200 as well as d values tend to a tetragonal P4mm space group over the cubic one. Moreover, in our earlier report,47 we confirmed the existence of ferroelectricity in nanosized samples BTO-7w and BTO-8w by probing them using direct piezoresponse force microscopy technique, which confirms the presence of global tetragonal symmetry in them. Although our TEM investigations confirm that all solvothermally synthesized products are found to be nanosized, the SEM analyses clearly reveal heavy agglomeration of these BaTiO3

nanocrystals (Figures 3, 7, and 8). Surely, the ultimate preparation aim is to produce barium titanate nanocrystals with narrow size distribution and without large agglomerates after their redispersion in water or in another solvent. Overall, the synthesis of such nanoparticles in the form of a monodispersed assembly inevitably requires specific surface treatments (formation of core-shell structures or templates) which strongly suppress the surface of the ferroelectric materials and thus were not applied in the current study. However, we estimated the particle size distribution (PSD) for the as-prepared BTO-3 and BTO-8 samples after ultrasonic redispersion in water for 5 min using disc centrifuge analysis. According to the PSD analysis, sample BTO-3 had a relatively narrow peak at about 6 nm with an fwhm of 0.6 nm, while the sample BTO-8 had a rather broad peak centered approximately at 35 nm and an fwhm of 10 nm. Apparently, these peaks corresponded to the overall sizes of the nanocrystals in those nanopowders. Obtained values are in good agreement with the line-broadening analysis (Table 1) and TEM observation for these samples (Figures 7C and 8C). In addition, the disc centrifugation analysis also reveals the presence of a relatively broad peak centered approximately at 100 nm with an fwhm of 70 nm as well as a broad peak at about 120 nm with an fwhm of 100 nm for the BTO-3 and BTO-8 samples, respectively, which would correspond to the



Figure 8. Low (main panel) and high (inset) magnification SEM microphotographs are shown for (A) BTO-8 sample synthesized by solvothermal drying route and (B) the same sample after washing in diluted acetic acid (sample BTO-8w). (C) A low-magnification TEM image (inset: corresponding ED pattern) of the sample BTO-8.

overall sizes of the nanocrystal agglomerates. The agglomerates may form during suspension of the titanate nanopowders in water. Thus, we were able to synthesize nanoparticles with a quite narrow particle size distribution, and our PSD data correlate well with the line-broadening and TEM analyses. However, PSD results clearly suggest that those nanocrystals are not completely redispersed in water and, owing to the surface energy stabilization forces inevitably presented for the nanocrystals, exist in the suspensions in the form of agglomerates with sizes of up to ∼0.2 µm as well as in the form of separated crystals with nanometer sizes. TGA-DTA was performed to investigate the thermal behavior of the nanosized BaTiO3 particles produced by solvothermal drying technique, and representative results of TGA-DTA measurements from the samples BTO-3 and BTO-3w are shown in Figure 9A and 9B, respectively. The maximum value of weight loss between 25 and 1100 °C was recorded for the samples BTO-2 and BTO-3 (∼17 wt %) and decreases significantly with the increase of average particle size, for example, loss becomes ∼9 wt % for sample BTO-8. On the basis of our observations, for all nanopowders synthesized by solvothermal drying route, the total weight loss consists of four contributions (Figure 9A). The first step (up to 260 °C) is probably attributed to the endothermic desorption of physically adsorbed water as well as solvents. The second step with a strong weight loss between 260 and 410 °C is owing to the burning out of organic species absorbed by the nanoparticles during the synthesis. This was supported by the presence of two sharp exothermic peaks at around 320 and 390 °C in the DTA curve indicating the combustion of organic residues in at least two exothermic reactions.30b The third step (410-600 °C) can be mostly attributed to the loss of chemically bounded hydroxyl groups as well as partially to the loss of organic and

Figure 9. Thermogravimetric (TGA) and differential thermal analyses (DTA) results for the samples BTO-3 (A) and BTO-3w (B).


Figure 10. Infrared spectroscopy data in the region 400-3700 cm-1 taken from the BTO-3 and BTO-3w samples before and after heat treatment at 600 °C for 10 h on air. The positions of the observed characteristic bands of the CO32- group attributed to the BaCO3 byproduct are shown by asterisks.

inorganic carbonic residues.30b,48 Finally, the weight loss above 800 °C indicates CO2 release during the endothermic decomposition of BaCO3 byproduct. As expected, the first step weight loss under the same TGA-DTA experimental conditions of all washed samples is higher in comparison with solvothermally produced samples because of the higher amount of physically and probably chemically adsorbed water (vide infra) which comes from the washing procedure (Figure 9B). At the same time, the absence of the final step of weight loss above 800 °C for washed nanopowders shows the absence of barium carbonate, in good agreement with the above-mentioned XRD and Raman analyses. The results of TGA-DTA analyses were very much consistent with the infrared spectroscopy measurements. Figure 10 displays the representative IR spectra for the BTO-3 and BTO-3w samples before and after heat treatment at 600 °C for 10 h on air. The assignments of the observed bands were accomplished according to the literature data;42,49-52 the summary of the assignments and the IR wavenumber is presented in Supporting Information (see SI Table S1). For all samples, the two broad characteristic bands at 577 and 420 cm-1 are assigned to the Ti-O vibration modes.49 Another characteristic band of the crystalline barium titanate phase is observed at 1396 cm-1.50 The broad band in the region between 3000 and 3700 cm-1 and the band at 1614 cm-1 in the IR spectra of all samples are assigned to the asymmetric and symmetric stretching modes of O-H and the bending mode of H-O-H, respectively.51 These bands are contributed from the physically adsorbed water as well as internal hydroxyl groups OH-.42,49 For the sample BTO-3 prepared by solvothermal drying method, the bands in the regions 2900-3050 cm-1 and 800-1200 cm-1 are assigned to the remaining organic compounds from the synthesis procedure, namely, iPrOH and EtOH,52 which are almost fully oxidized after calcination at 600 °C for 10 h (Figure 10). In additional, the set of characteristic bands attributed to CO32groups from BaCO3 byproduct has been observed in IR spectra of BTO-3 sample both before and after calcination (labeled by asterisks in Figure 10).50,51

Kolen’ko et al. According to the infrared spectroscopy, washing of the sample BTO-3 in dilute acetic acid followed by calcination at 600 °C for 10 h leads to the liberation of the impurities from the barium titanate nanopowders. The bands representing CO32- groups and organic species are almost not seen in the IR spectrum of the annealed BTO-3w sample (Figure 10), moreover, the spectral features are very similar with the same reported for bulk barium titanate.50 With regard to the application of BaTiO3 whereby it should have the correct stoichiometry (Ba/Ti ) 1), it is also beneficial to understand the evolution of the chemical composition and the possibility of the point defects formation as related to the synthesis, acid washing, and post-washing calcination. Analogously to the preparation of barium titanate by various synthetic methods, to avoid the possibility of titanium dioxide admixture formation, we have used 10 mol % excess of Ba(OH)2‚8H2O precursor. So, it is not surprising that all our solvothermal drying synthesis products contain small amounts of BaCO3 byproduct, which are insoluble in water but can be completely removed by washing under acidic conditions, as was shown above. At the same time, BaTiO3 is well-known to have a high ability for the barium ion release from the surface during the washing with water, even at neutral pH.53 Moreover, the results of Neubrand et al.54 clearly illustrate that the leaching of Ba+2 is much faster in acidic conditions, according to eq 253a BaTiO3(s) + 2H+(aq) ) Ba2+(aq) + TiO2(s) + H2O(aq) (2)

resulting in a Ti enriched surface, and thus overall Ba/Ti ratio becomes less than unity. In the present study, the chemical compositions of the selected BaTiO3 products were quantified by inductively coupled plasma-optical emission spectrometry (ICP-OES) and are summarized in Table 2. According to ICP-OES data, the assynthesized samples have an excess of Ba associated with 10 mol % Ba excess established for initial precursor mixture, probably in the form of the BaCO3 admixture. The washing of the nanopowders in 0.5% acetic acid by ultrasonication for 5 min, together with BaCO3 byproduct dissolution, results in the substantial barium loss: Ba/Ti ratio decreases to 0.6 and 0.8 for the BTO-5w and BTO-6w samples, respectively. The chemical compositions with such a large Ba nonstoichiometry did not appear to be influenced by the post-washing thermal treatment (Table 2) while the total metal content is slightly increased indicating possible water and hydroxyl groups elimination. The observed disparity of overall Ba nonstoichiometry for BTO-5w (∼38 atomic percent (at. %)) and BTO-6w (∼20 at. %) may be a result of the high but different total specific surface areas of these samples. The BTO-5 powder with overall particle sizes of 11 nm has a high specific surface area (68 m2/g), which decreases to 29 m2/g for the BTO-6 powder with overall particle sizes of 17 nm. This agrees well with Neubrand et al.’s work,54 where they also reported that the barium loss increases with enhancing of a total specific surface area of the BaTiO3 powders, as incongruent dissolution of barium is a surface reaction where only barium from near-surface sites is leached. Additionally, owing to nanometer scale of as-produced BaTiO3 particles, the possibility of a partial incorporation of Ba vacancies into the core of nanocrystals during acidic treatment under ultrasonication cannot be excluded. This may give an additional contribution to the enhanced Ba nonstoichiometry. Overall, the developing of Ba vacancies in BaTiO3 nanocrystals with washing procedure results in an exothermic peak at ∼830 °C in DTA



TABLE 2: Unit Cell Parameters and Metal Atom Content for the Selected BaTiO3 Products Ba1-xTiO3-2x(OH)2x‚yH2Oc sample

unit cell parameter, Å

BTO-5 BTO-5w BTO-5wd (600 °C/10 h) BTO-6 BTO-6w BTO-6wd (600 °C/10 h)

4.0319(5) 4.0340(8) 4.0243(7) 4.0226(5) 4.0236(4) 4.0158(3)

a

Ba content, wt % 52.1(6) 43.8(2) 45.6(1) 56.9(5) 50.9(6) 51.4(1)

b

Ti content, wt % 17.5(2) 24.7(1) 25.8(1) 17.9(2) 22.1(2) 22.3(1)

b

1-x 1.04 0.62 0.62 1.11 0.80 0.80

y 0.67 0.24 0.55 0.43

a Least-squares fit of the position of nine diffraction maxima utilizing LaB6 as internal standard. For all samples, the cubic symmetry was assumed, space group Pm3hm (No. 221). b ICP-OES analysis results. c The formula Ba1-xTiO3-2x(OH)2x‚yH2O for the washed and annealed samples was assumed. The amount of “extra” water, y, was not calculated for the as-prepared samples because of the presence of barium excess and BaCO3 admixture. d Post-washing heat treatment at 600 °C for 10 h on air.

curve of BTO-3w sample (Figure 9B) corresponding to the formation of BaTi2O5 and Ba2Ti5O12 as was confirmed by XRD (JCPDS No. 34-133 and No. 17-661, respectively), which is expected to be in case of beneficial barium vacancies formation.48 In preparative chemistry of BaTiO3, there are several key features which are influenced on stoichiometry:48,55 (1) synthesis in the presence of water (hydrothermal, solvothermal, sol-gel, alkoxide, etc.) leads to the hydroxyl groups OH- incorporation into the regular oxygen sublattice of BaTiO3; (2) the proton defects in the oxygen sublattice are compensated by vacancies on both Ba and Ti sites; (3) it is believed that these point defects on the different lattice sites are combined and thus annihilate/ compensate each other to neutrality; (4) upon dehydration during the heat treatment (up to 800 °C), a vanishing of such compensated vacancies owing to the liberation of internal OHgroups results in the formation of the particles with fine intragranular nanoporous microstructure. The solvothermal reactions conducted in current study were performed in the presence of water originating from Ba(OH)2‚ 8H2O precursor. Also, water was utilized as an additional agent during the several solvothermal syntheses (Table 1). Indeed, the described above pathway of point defects formation may be realized in our case. The compensated vacancies seem to occur already after solvothermal drying synthesis. Notice that the presence of internal OH- groups was detected by IR spectroscopy in solvothermal drying synthesis products. In addition, our TGA-DTA data clearly indicates a significant weight loss in the range between 400 and 600 °C because of the liberation of chemically bound hydroxyl groups. Moreover, drying at 240 °C can also lead to evaporation of water leaving a significant amount of oxygen vacancies behind according to eq 3:32b,48

2OH•(O) f O(O) + V••(O) + H2O

(3)

Thus, even as-synthesized samples may contain some amount of Ba vacancies.48 ICP-OES analysis of washed samples shows the enhanced barium nonstoichiometry for the acidic treated samples. We have refined unit cell parameters of barium titanate samples from least-squares fits using LaB6 as an internal standard (Table 2). The cubic unit cell parameter for the washed samples slightly increases compared to as-synthesized ones, indicating that washing leads not only to the removal of Ba atoms but also to filling of oxygen vacancies with hydroxyl groups (inverse eq 3). Supporting evidence is that when BaCO3 byproduct is washed away, the chemical analysis shows a remarkably strong excess of oxygen in washed nanopowders, which tends to reduce with a heat treatment (Table 2). Further annealing of these samples on air may compensate for oxygen vacancies or may

annihilate the point defects by migration of cation and anion vacancies to the same point thereby making pore-like structures with smaller unit cell.10,48 Indeed, a distinct cell volume contraction is observed for the annealed barium titanate samples (Table 2). To understand a fine microstructure evolution with washing and post-washing heat treatment, the TEM, ED, and HRTEM measurements of the BTO-8 series of samples were carried out. Representative low-magnification TEM images are shown in Figure S7 of the Supporting Information, where in ED patterns from BTO-8 series of samples, no rings corresponding to a secondary phase were observed. Figure 11A shows HRTEM images of single BaTiO3 nanocrystals prepared by solvothermal drying synthesis (sample BTO-8). The individual nanoparticles are well defined and have nearly rectangular shape. In contrast, investigation of washed BTO-8w sample clearly revealed a presence of local variations of contrast (dot-bright zones) (Figure 11B), which was subsequently confirmed to be a common feature of almost all nanocrystals in this powder. In addition, the facets of BaTiO3 nanocrystals were altered after acidic washing, that is, nanocrystals become more “shapeless” owing to the smoothing of corners, consistent with the enhanced barium leaching from nearsurface sites. However, the surface of the particles was free of any amorphous or secondary phase. The latter observation suggests that the formation of core-shell structures because of a Ti enriched surface of washed nanocrystals can be excluded, at least in the case of nanoparticles with overall size of 37 nm. After further heat treatment of BTO-8w sample, as it can be seen in Figure 11C, the dot-bright zones become more sharp faceted and tend to have a rectangular shape. It is also seen in the representative low-magnification TEM image (Figure S7C). Although the nature of local variations of contrast in HRTEM image was not identified in current study, it is believed that these “inhomogeneities” might be related to thickness changes, that is, either voids in the core of the nanoparticles or thickness changes at the surface of the particles. Similar inhomogeneities, so-called intragranular nanopores, are observed quite regularly in TEM images of annealed BaTiO3 nanopowders produced in the presence of water,48,56 and they appear to be a result of the compensated vacancies vanishing because of the liberation of internal OH- groups.55 Thus, although our XRD, Raman scattering, and TEM data clearly show that washing of solvothermally dried products under acidic conditions as well as post-washing calcination does not lead to any significant changes in phase composition and particles sizes, it evidently appears that washed BaTiO3 samples have strong Ba nonstoichiometry (ICP-OES), depending upon their total specific surface area, as well as nanocrystallites that do not become physically perfect (HRTEM). In spite of the significant Ba nonstoichiometry, it can be speculated that such


Kolen’ko et al. Conclusions

Figure 11. HRTEM images (scale bar 10 nm) from the samples: (A) BTO-8, (B) BTO-8w, and (C) BTO-8w (600 °C/10 h).

crystal structure is stabilized by an additional incorporation of OH- groups into the regular oxygen sublattice during washing. Upon dehydration by annealing at 600 °C, those OH- groups were partially liberated, leading to the observed reduction in unit cell parameter (XRD) as well as leaving behind large quantities of O vacancies in addition to enhanced Ba ones (TGADTA). Finally, owing to the overall annihilation by physical migration of vacancies to close spatial proximity, the voids in the core of the nanoparticles or thickness changes at the surface of the particles were formed. It is reported in the literature that intragranular nanopores may not form during heat treatment of BaTiO3 with overall particle size of 10 nm produced in the presence of water, and it was explained in terms of thermodynamic and kinetic aspects.30b We do not know whether the acid washing of BaTiO3 nanocrystals followed by post-washing calcination is a major factor providing fine intragranular nanoporous microstructure in our case, but also the solvothermal drying synthesis in the presence of water likely plays an important role in this regard, and overall it appears that the observed phenomenon is rather intrinsic to barium titanate nanoparticles. We will continue the investigations to correlate stoichiometry, structure, and real microstructure of the title compound.

Summarizing the results of all used characterization techniques, we are able to make a conclusion concerning the proposed solvothermal technique. Nanosized (5 nm) BaTiO3 particles have been prepared by a conventional solvothermal route. It was shown that overall crystal structure of the as-produced barium titanate matched well with the cubic one, while Raman scattering experiments confirmed a global cubic structure with local tetragonal clusters. At the same time, mass yield of this synthesis procedure was established to be remarkably low ∼25%. This drawback was successfully overcome by the development of a novel solvothermal drying technique, demonstrating that BaTiO3 nanoparticles with average particle sizes of 5 nm can be easily produced in high yield (>95%). The results of our experiments display how the size of the barium titanate nanocrystals can be easily controlled in range from 5 to 37 nm by simply adding a fixed amount of water to the reaction mixture prior to the solvothermal drying synthesis. The as-synthesized BaTiO3 nanopowders have a regular shape and quite narrow particle size distribution. Detailed Raman studies of the as-synthesized barium titanate nanopowders confirm their acentric local environment for particles with size larger than 11 nm. XRD, Raman scattering, SEM, and TEM investigations did not reveal any significant differences when comparing the phase composition, local structure, morphology, particle size, and fine microstructure of the samples BTO-2 and BTO-3. However, the last one was synthesized using double amount of barium and titanium precursors. This important result clearly points out that the solvothermal drying route can open the scaling up possibility of the production of BaTiO3 nanocrystals in a single reaction. We have established that the solvothermal drying synthesis products contain BaCO3 byproduct as well as internal OHgroups and residual solvents species that can be removed by acid washing following calcination. However, this procedure leads to vanishing of the chemical compositions and fine microstructure of BaTiO3 nanocrystals. This finding is consistent with the literature, and although more data are needed to be certain, it appears to be rather intrinsic to barium titanate nanoparticles produced in the presence of water. Therefore, further research is currently underway to optimize our preparation procedure to reduce the amounts of BaCO3 byproduct in the final BaTiO3 nanopowders required for their industrial applications. Acknowledgment. We are grateful to Dr. G. Auffermann for the ICP-OES analysis, Dr. H. Borrmann and Dr. Yu. Prots for collecting XRD data, and Dr. S. Ray and Dr. A.V. Olenev for the fruitful discussion. Yu. V. K. thanks the FY2005 JSPS Postdoctoral Fellowships for Foreign Researchers. Supporting Information Available: Representation showing general and [100] views of the cubic and tetragonal barium titanate crystal structures (Figure S1). Images, giving general overview of conventional solvothermal setup (Figure S2) as well as gasket evolution after solvothermal drying process (Figure S3). TEM image and ED pattern showing the presence of cubic BaTiO3 nanoparticles in viscous solvothermal reaction solution even after centrifugation at 4000 rpm for 1 h (Figure S4). The angular regions of the XRD patterns from washed samples proving that no splitting of peaks corresponding to tetragonal symmetry was detected (Figure S5). XRD patterns indicating that the BaCO3 byproduct can be completely removed by washing from the solvothermally derived powders (Figure S6), as well as in conjunction with TEM proving the thermal stability

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A Novel, Controlled, and High-Yield Solvothermal Drying Route to

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