Citation

Shu-Wei Lin, Wen-Hu Qian, Hong-Jing Huo, Bao-Zong Li, Yi Li, Yong-Gang Yang. Preparation of optical active single-handed helical barium titanate nanotubes and characterization of dielectric properties[J]. Chin. Chem. Lett., 2017, 28(5): 1111-1113  

Shu-Wei Lin, Wen-Hu Qian, Hong-Jing Huo, Bao-Zong Li, Yi Li, Yong-Gang Yang     

BaTiO₃ has been widely used for its ferroelectricity at room temperature, high dielectric constant and low-loss. During the last decades, much effort has been carried out on controlling BaTiO₃ in nanoscale. Nanopowders can be prepared through ball-milling, molten salts, sol-gel and hydrothermal processes. Among them, the last method has been well studied. For example, nanoparticles [1], nanowires [2, 3] and nanotubes [4] are prepared using TiO₂ and Ba(OH)₂·8H₂O through this method. Ferroelectric and dielectric properties of nanosized BaTiO₃ have also been studied. It was found that Curie temperature was significant increased [5]. The ferroelectric phases were observed in the shells due to the complicated surface structure. Moreover, the dielectric constant decreased with decreasing particle size [6].

Single-handed helical nanofibers and nanotubes have been prepared through both self-templating and external templating approaches using chiral low-molecular-weight gelators (LMWGs) [7, 8]. The TiO₂ nanotubes and tubular nanoribbons prepared through the external templating approach exhibit interested optical activity and asymmetric catalysis property [9–12]. The optical activity is proposed to originate from the chiral defect on the inner surfaces of the tubular structures. A post-preparation approach has been developed by us for the preparation of LiTaO₃ tubular nanoribbons with optical activity using Ta₂O₅ tubular nanoribbons as starting materials [13]. Herein, single-handed helical BaTiO₃ nanotubes were prepared using TiO₂ nanotubes as starting materials. The BaTiO₃ nanotubes exhibited optical activity and dielectric properties.

The single-handed helical TiO₂ nanotubes were prepared through a supramolecular templating approach. The formation of them was shown as follows. First, the LMWGs self-assembled into single-handed twisted nanoribbons in the reaction mixture. Second, TiO₂ nanoparticles adsorbed and polycondensed on the surfaces of the organic twisted nanoribbons. Finally, after removing the templates with ethanol, the TiO₂ nanotubes were obtained. Single-handed helical BaTiO₃ nanotubes were prepared through the hydrothermal process using the TiO₂ nanotubes and Ba(OH)₂·8H₂O. Their FE-SEM and TEM images are shown in Fig. 1. The morphologies were almost kept after the impregnation of Ba²⁺ ions. The outer diameter and wall thickness are about 200–500 nm and 50 nm, respectively. These nanotubes are constructed by fine nanoparticles.

Figure 1

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Figure 1. (a) FE-SEM and (c) TEM images of P-BaTiO3. (b) FE-SEM and (d) TEM images of M-BaTiO3.

The WAXRD pattern of P-BaTiO₃ is shown in Fig. 2a. The WAXRD pattern shows the pervoskite BaTiO₃ with cubic/tetragonal symmetries. A broad peak is identified at 2θ=22.7°, indicating an amorphous structure. Therefore, P-BaTiO₃ should be constructed by partially crystalline nanoparticles. Raman spectroscopy is used to identify the tetragonal or cubic phase structure of BaTiO₃ (Fig. 2b) [14]. The band at about 715 cm^-1 is related to the highest frequency longitudinal optical mode A₁(LO). The bands at 185, 251, and 514 cm^-1 are assigned to the fundamental TO mode of the A₁ symmetry. The sharp band at 307 cm^-1 indicates the tetragonal phase. Therefore, P-BaTiO₃ should exhibits ferroelectric properties [5].

Figure 2

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Figure 2. (a) WAXRD pattern and (b) Raman spectrum of P-BaTiO3.

The DRCD and DRUV-vis spectra of M-BaTiO₃ and P-BaTiO₃ are shown in Fig. 3. Broad UV absorption bands at 200–383 nm were identified. The BaTiO₃ nanotubes exhibited DRCD signals at approximately 330 nm, indicating that the chirality originated from the intrinsic electronic structure. It was reported that asprepared titania tubular nanoribbons exhibited DRCD signals at 320 nm [12]. The DRCD signals shifted to long wavelength with the molecular structural transition and crystallization. Because the tubular structures were formed through an external templating approach, chiral defects on the inner surfaces were proposed to be the origin of the optical activity of the BaTiO₃ nanotubes.

Figure 3

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Figure 3. DRCD and DRUV-vis spectra of M-BaTiO3 and P-BaTiO3.

The dielectric constant and tanδ for P-BaTiO₃ are plotted vs. temperature in Fig. 4. The data were collected under nitrogen as temperature increased from low to high. At 10 and 100 Hz, one dielectric constant peak at 9.6 ℃ and one tanδ peak at 5.0 ℃ are observed at -120 ℃ to 180 ℃. The values of the dielectric constants are significantly lower than the reported bulk BaTiO₃ ceramics value [15]. The tanδ is lower than 0.3 at 10 Hz, and the tanδ is lower than 0.03 at 10, 000 Hz. Both the dielectric constant and tanδ decrease with increasing the frequency. The contribution of dipolar, electronic and ionic polarizations are proposed to drive the higher dielectric constant at lower frequencies.

Figure 4

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Figure 4. Plots of the dielectric constant and tanδ vs. temperature for P-BaTiO3.

Single-handed helical BaTiO₃ nanotubes with optical activity were prepared using TiO₂ through an impregnation approach. Chiral defects on the inner surfaces were proposed to be the origin of the optical activity of the BaTiO₃ nanotubes. Both the dielectric constant and tanδ decrease with increasing the frequency. At 10 and 100 Hz, one dielectric constant peak at 9.6 ℃ and one tanδ peak at 5.0 ℃ are observed at -120 ℃ to 180 ℃.

Characterization: Wide-angle X-ray diffraction (WAXRD) patterns were taken on an X'Pert-Pro MPD X-ray diffractometer using Cu Ka radiation with a Ni filter (1.542 Å). Raman spectra were obtained using a Renishaw Invia Raman microscope. The excitation wavelength was 532 nm. Field emission scanning electron microscopy (FE-SEM) images were obtained using a Hitachi 4800 instrument at 3.0 kV. Transmission electron microscopy (TEM) was performed using an FEI TecnaiG220 at 200 kV. The diffuse reflectance circular dichroism (DRCD) and diffuse reflectance ultraviolet-visible (DRUV-vis) spectra were obtained using a JASCO J-815 spectropolarimeter fitted with DRCD apparatus, with 10.0-nm bandwidth, 1.0-nm data pitch and 100-nm/min scan speed. Pellet of diameter 10 mm and thickness 1.18 mm are prepared from the BaTiO₃ powder. The relative dielectric constant (ε_r) and the dielectric loss (tanδ) are measured using Novocontrol Concept 80 Broadband Dielectric Spectrometer.

Chemicals: The synthesis of the as-prepared TiO₂ nanotubes has been reported previously [10]. Ba(OH)₂·8H₂O, ethanol and nitric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used in all the experiments.

Synthesis of single-handed twisted BaTiO₃ nanotubes: The asprepared TiO₂ nanotubes were put in a autoclave containing aqueous Ba(OH)₂·8H₂O solution. The sealed autoclave was then placed in an oven at 160 ℃ for 8 h. After cooling to room temperature, the solids were washed with dilute nitric acid, deionized water and ethanol and then dried overnight under vacuum at 40 ℃. The left-and right-handed helical BaTiO₃ nanotubes were named as M-BaTiO₃ and P-BaTiO₃, respectively.

This work was supported by the Priority Academic Program Development of Jiangsu High Education Institutions (PAPD).