Novel bowl-like single-crystalline BaTiO3 nanoparticles were synthesized by a simple hydrothermal method using Ba(OH)2·8H2O and TiO2 as precursors. The as-prepared products were characterized by XRD, Raman spectroscopy, SEM and TEM. The results show that the bowl-like BaTiO3 nanoparticles are single-crystalline and have a size about 100–200 nm in diameter. Local piezoresponse force measurements indicate that the BaTiO3 nanoparticles have switchable polarization at room temperature. The local effective piezoelectric coefficient is approximately 28 pm/V.
KeywordsBaTiO3 Bowl-like Single-crystalline Piezoelectric properties
In recent years, increasing attention has been devoted to the synthesis of various nanostructured BaTiO3, such as nanoparticles, nanocubes, nanorods, nanowires, and nanotubes, because of the dependence of their ferroelectric and piezoelectric properties on the dimension and size, which is essential for realizing nanoscale devices for a wide range of applications, including memory, transducers, sensors, energy-harvesting devices,[1–6]. For instance, BaTiO3 nanoparticles transformed from tetragonal to cubic phase at room temperature with the critical crystallite size of approximately 40 nm due to the size effect . One-dimensional, stable ferroelectric monodomain was found in an 80 nm diameter single-crystalline BaTiO3 nanowire . And periodic voltage was generated from the BaTiO3 nanowire by applying periodically varying tensile mechanical strain . Furthermore, Fu and Bellaiche reported that the local dipoles in small BaTiO3 dots prefer to form a vortex-like pattern rather than the traditional viewed structures as arrayed in straight lines in neat rectangular rows and columns based on a first-principles approach [10, 11]. Following their research, Wang et al.  and Prosandeev et al.  also reported that, in sufficient small nanoscale ferroelectric objects (nanoparticles or nanodots), conventional domain structures should be replaced by vortex domain states based on atomistic simulation modeling. On the other side, Scott et al. [14–16] tried to experimentally verifying the existence of vortex domains, they pointed out that magnetic vortex domains can be stabilized through the physical removal of the vortex ‘core’, ferroelectrics with hollow structure should offer the greatest opportunity for experimentally creating polarization vortices in ferroelectrics. Periodic arrays of PZT nanorings were fabricated using a self-assembly technique namely the nanosphere lithography (NSL) method . PZT nanorings were synthesized through solution deposition inside of an anodized aluminum oxide nanopores thin film . However, these kinds of polarization vortices have not yet been experimentally observed in these PZT nanorings. Based on these points, the synthesis of hollow single-crystalline BaTiO3 will also offer the opportunity for experimental observation of such vortices domain. But comparatively little work has been performed on the fabrication of BaTiO3 with hollow structure. Only, Nakano et al.  and Buscaglia et al.  reported the synthesis of hollow poly-crystalline BaTiO3 structures by a layer-by-layer colloidal templating method and a two-step process combining colloidal chemistry and solid-state reaction, respectively. But the ferroelectric properties of the above hollow BaTiO3 structures were not yet investigated. All these features make the synthesis of hollow single-crystalline BaTiO3 of great significance.
Hydrothermal method has been considered as one of the most promising routes to synthesize oxide powders with controlled morphology, high crystallinity in a one-step process [19, 20]. This method has been widely used for the synthesis of BaTiO3 powders, nanotabulars and nanowires [21–23]. But there is no literature available about the synthesis of bowl-like BaTiO3 particles based on hydrothermal method. In this study, we report the novel single-crystalline BaTiO3 nanoparticles with bowl-like structure via hydrothermal method. Piezoresponse force microscope measurement has been employed to characterize the properties of the BaTiO3 nanoparticles.
Typical synthesis procedure: 0.0102 mol of Ba(OH)2·8H2O and 0.006 mol of TiO2 were mixed in 80 ml distilled water. The mixture was transferred into a Teflon-lined stainless steel autoclave (inner volume of 100 ml) and heated at 180°C for 72 h under autogenous pressure. The resulting BaTiO3 powders were filtered, washed with 0.1 M formic acid and deionized water several times, and finally dried at 80°C for 12 h in an oven. In our synthesis, after Titania was added with Ba(OH)2 in solution, TiO2 would interact with the OH− and H2O to form titanium hydroxyl species (probably HTiO3 −). Then, HTiO3 − reacted with Ba2+ ions to form BaTiO3 particles .
X-ray powder diffraction patterns (XRD) of the products were obtained on an X-ray diffractometer (Philips PW3050/60, MPSS) using Cu Kα radiation. Raman spectroscopy was performed at room temperature in a Raman spectrometer (Renishaw RM-1000), employing an Ar+ laser for excitation (λ = 514 nm). Scanning electron microscope (SEM) images were obtained by a scanning electron microscope (JEOL, JSM-5610LV). Transmission electron microscope (TEM) images and high-resolution electron microscope (HREM) images were recorded on a transmission electron microscope (JEOL, JEM-2100F). For TEM observations, the nanoparticles were ultrasonically dispersed in ethanol and then dropped onto carbon-coated copper grids.
The local polarization switching behaviors of the nanoparticles were characterized using high sensitivity piezoresponse force microscopy (PFM) [25, 26]. The characterization was conducted on scanning probe microscope (SEIKO, SPI4000N). A silicon tip coated with Ru (Micro cantilever, SI-DF3-R) was used. The spring constant of the cantilever was 1.6 N/m, and the free resonance frequency was 27 kHz. For sample preparation, BaTiO3 nanoparticles dispersed in water were drop-coated directly onto a highly oriented pyrolytic graphite (HOPG) substrate.
Results and Discussion
Ferroelectrics with hollow structure should offer the greatest opportunity for experimentally creating polarization vortices domain. But till now, similar with the forerunners’ research about the PZT nanorings [15, 16], detailed resolution of polarization behavior in hollow BaTiO3 particles has not been found to be achievable due to the lack of characterization technique, the vortices domains also cannot be measured. But the research of ferroelectric domain state in the unusual way should be a fertile area for further theory and experiment. The unique bowl-like single-crystalline BaTiO3 nanoparticles shown in the present study will provide an ideal candidate to investigate the polarization vortices in ferroelectric nanostructures. And this is clearly an objective for our future research.
In summary, bowl-like single-crystalline BaTiO3 nanoparticles have been synthesized through a hydrothermal method using Ba(OH)2·8H2O and TiO2 as starting materials. The bowl-like nanoparticles are about 100–200 nm in diameter. HRTEM characterization reveals that the whole nanoparticle is single-crystalline. Piezoresponse force microscope measurements indicate that the local polarization of the BaTiO3 nanoparticle is switchable at room temperature. The local effective piezoelectric coefficient is approximately 28 pm/V, which is comparable to the reported value (~22 pm/V) of the BaTiO3 ceramics. These unique BaTiO3 nanoparticles will provide an ideal candidate for fundamental studies of the ferroelectricity and piezoelectricity, which may prove useful in fabricating a variety of nanoscale functional devices.
This work was supported by the National Natural Science Foundation of China (No. 50672072, 50972115, 50932004, A3 Foresight Program-50821140308) and the Ph.D. Programs Foundation of Ministry of Education of China (No. 20090143110002).
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