일차원 나노구조를 가진 페롭스카이트 물질의 합성과 그 특성분석
- 일차원 나노구조를 가진 페롭스카이트 물질의 합성과 그 특성분석
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- Among TiO2-based perovskite materials, PbTiO3, BaTiO3, and SrTiO3 have been most widely studied because of ferroelectric, piezoelectric, dielectric, pyroelectric, electro-optic, photoelectric and catalytic properties. Also a SrTiO3 is an important n-type semiconductor, photacatalyst and photoelectrode. So they can be applied to various devices such as memory devices, thermistors, multilayer capacitors and electro-optical devices, sensors, actuators and photocatalyst.
And nanostructures of these materials are needed because of the recent trend of the highly integrated, high-performance and miniaturized devices. In the past several decades, the synthesis of these materials has been focused on nanoparticles, which can be molded into the desired shape. However, the molding method cannot be scaled down to nanosize. And powder type 1D nanostructures are difficult to fabricate on a flat conducting substrate with a good ordering because it is hard to control the direction of the nanostructure.
So we are synthesized 1-D nanostructures on substrate directly in these works. In fact, 1-dimensional (1-D) materials have many physical and chemical properties better than 0-dimensional nanoparticles such as superior charge transport, large surface area and so on. Also the well-aligned ferroelectric nanowire arrays on a flat substrate are suitable for three-dimensional device elements in miniaturized ferroelectric devices.
In our works, we synthesized 1-D perovskite nanostructures from titanium oxide as a starting materials by various method. First, BaTiO3 nanowires thin film could be synthesized on a conducting glass by following procedures. TiO2 particles as seeds is spray coated on the conducting glass in the first step, and the TiO2 particles were grown BaTiO3 nanowires under basic hydrothermal treatment. The crystal growth takes place via an Ostwald ripening process. And as-synthesized BaTiO3 nanowires have the single crystalline, cubic structure and 50~100 nm diameter. The BaTiO3 nanowires on FTO glass have also 4 μm film thickness with the same thickness of spray-coated TiO2. The product film thickness could be controlled as adjusting the thickness of TiO2 coating. The capacitance of BaTiO3 1-D nanowire array film was found to be 4.5 times higher than that of nanoparticle film by measuring the impedance. The piezoelectric property of individual BaTiO3 nanowire was also demonstrated by piezoforce microscopy.
We also synthesized a vertically aligned PbTiO3 nanowire array on Ti substrate from anodic titanium oxide nanotubes. High quality TiO2 nanotubes are first fabricated by anodic oxidation of Ti foil under organic solution. And then they treated hydrothermally in a lead acetate solution to obtain PbTiO3. As a result, we obtained single-crystalline PbTiO3 nanowires on Ti substrate. This nanotube-to-nanowire transformation on conducting substrate could be described by a swelling-and-rupture mechanism, in which individually well-developed nanotubes are swollen and broken along the friction planes because of difference in the directions of expansion force. The ferroelectric property of individual PbTiO3 nanowires and whole area of nanowire arrays has been demonstrated by piezo-response force microscopy. The as-prepared nanowires have a uniform diameter of ca. 40 nm and a length of 10 μm. And the density of nanowires becomes ultrahigh density corresponding to ca. 0.22 Tb inch-2. At the anodization conditions, the TiO2 tube thickness and film thickness were dependant to the voltages and time. And the nanowires diameter and PbTiO3 film thickness are determined by TiO2 tube thickness and TiO2 film thickness, respectively.
And highly-ordered nanoporous PbTiO3, BaTiO3, and SrTiO3 were synthesized by hydrothermal treatment of highly-organized nanoporous titanium oxide. The highly-organized hexagonal titanate was prepared by twice anodization of Ti foil. The morphology of these three perovskite nanoporous materials are highly-regular, densely-packed hexagonal nanoporous structures. And the nanopore thickness of as-prepared perovskite is thicker than the that of titanium oxide by entering the metal ions. Also desired film thickness could be controlled by anodization time. The XRD pattern revealed that the as-obtained samples are in good agreement with tetragonal phase for pure PbTiO3 and cubic phase for pure BaTiO3 and SrTiO3.
Also, we investigate the ferroelectric properties of PbTiO3 nanowires and PbTiO3 nanoporous structures by PFM. Both samples show well-defined hysteresis loops which are another evidence of their ferroelectricity. The saturated d33 value of nanowire sample is higher than that of nanoporous sample, while the coercive field of nanowire sample is smaller than that of nanoporous sample. This means the magnitude of polarization of nanowire sample is larger than that of nanoporous sample, but polarization of nanowire sample is easier to flip than that of nanoporous sample.
Furthermore, we can obtained desired morphologies of these structures like as diameter, thickness, length and so on by adjusting the various factors. And it is expected these methods of synthesizing the these 1-D nanomaterials on substrate could be generic synthesis route for other 1-D TiO2-based perovskite nanostructures.
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