R.Vijayalakshmi and V. Rajendran
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1833-122X) Volume 6 December 2010
Results and Discussion
The single-crystalline perovskite barium titanate nanorods were successfully synthesized by a hydrothermal method. The synthesis was accomplished by using barium chloride and titanium tetrachloride as the starting materials and NaOH as minerlizer, respectively. The crystal structure, morphology and optical properties of the nanorods were characterized by X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Ultra-Violet (UV) and Photoluminescence (PL). The XRD results suggested that the hydrothermally synthesized BaTiO3 nanorods are cubic phase. Well-isolated single-crystalline cubic perovskite BaTiO3 nanorods with diameters ranging from 20 to 30 nm and lengths reaching up to >90 nm can be easily fabricated by this route. PL spectrum showed high UV emission and very low visible emission, indicating optical quality of particles with little surface defects. The mechanism of the formation of the single-crystalline cubic perovskite BaTiO3 nanorods was discussed.
Barium Titanate, Hydrothermal Method, Optical Properties, Photoluminescence, Nanorods
Ceramic materials based on perovskite-like oxides are of significant interest because of their applications in electrical and electronic devices. The extensive application of barium titanate (BaTiO3) in the ceramic industry is based on the properties of its polymorphs [1, 2]. The temperatures higher than about 130°C (Curie temperature), BaTiO3 exists in the cubic perovskite structure. In this crystal structure, the Ba+2 ions occupy the corners of the elementary cell, the Ti4+ ions are in the volume centre and the O2- ions in the surface centre. Because of the high symmetry of the cubic phase, barium titanate exhibits paraelectricity and an isotropic dielectricity. Below the Curie point, the crystal structure transforms from the cubic phase to the distorted tetragonal structure with a displacement of the centers of positive and negative charges within the sublattice. As a result, a dipole moment parallel to one of the cubic areas of the original phase arises. Such a generated spontaneous polarization in the tetragonal structure is the origin of its ferroelectric and piezoelectric behaviour. Further reduction of the temperature changes the structure of BaTiO3 into an orthorhombic structure at about 5°C and finally into the rhombohedral structure at 90°C. In fact, little study has been done for the latter two low-temperature modifications of BaTiO3 and no industrial application of them has been reported [3, 4].
Cubic BaTiO3 phase exhibit high dielectric constants . As a dielectric material, BaTiO3 is mainly used for capacitors such as multilayer ceramic capacitors (MLCCs)  and integral capacitors in printed circuit boards (PCB) . Its polarization below Curie point can be applied for the use in dynamic random access memories (DRAM) . Its piezo electric properties enable its use in transducers and actuators . Barium titanate nanorods have been synthesized by using different methods such as sol-gel processing , precipitation method , soft chemical process , sol-precipitation process , and a hydrothermal method . Among them, hydrothermal method shows advantages as a simple system, low cost, without higher temperature calcinations process, controllable grain size and easy preparation for multi component sample etc. In this paper, we report the results on preparing nanorods of BaTiO3 by the facile hydrothermal technique. The crystal structure, morphology and optical properties of BaTiO3 nanorods are discussed for the first time via hydrothermal method.
Analytical grade barium chloride, titanium tetrachloride were adopted as the source material for barium and titanium, and sodium hydroxide as minerlizer. An aqueous solution of barium and titanium was obtained by mixing one molar stoichiometric of BaCl2.2H2O with 0.6 mol of TiCl4 in 50 ml distilled water. A 1 mol concentration of NaOH was added to neutralize HCl formation from TiCl4 hydrolysis and to keep the pH value 5. After adding NaOH the solution was stirred for 30 min, and resulting in a white colloidal sol, the final volume was adjusted to 90ml using deionized water. Thereafter, 90 ml sol was transferred to a 100 ml Teflon-lined auto clave vessel. The sealed vessel was heated to 240°C for 20 h. Same procedure was followed for the different mol 2 & 3% concentration of NaOH and also maintains the pH value 7 & 9. After that, the resultant precipitate was centrifuged and washed with water for several times and finally dried at 80°C for 12 h in a vacuum oven.
The as-prepared BaTiO3 powder sample was characterized by X-ray powder diffraction on a D8 Advance X-ray diffractometer with CuKα irradiation at λ=1.5406 Å. The microstructure and grain size were analysed through a Scanning Electron Microscope Hitachi S-4500 and Transmission Electron Microscopy (TEM) using a JEM-2010 transmission electron microscope. The photoluminescence (PL) spectra were recorded at room temperature on a Hitachi model F-4500 fluorescence spectrophotometer. The optical absorption spectra were measured by a Cary 5E UV-Vis spectrophotometer (UV-2550).
Results and Discussion
Fig. 1 shows a XRD pattern of hydrothermally as-synthesized barium titanate nanorods with different (1-3%) concentration of NaOH. It is shown that all patterns fit well with the peak positions of standard cubic-phase BaTiO3. The inset Fig. 1 shows that only a single reflection is presented around a 2θ of 45°. Typically, the presence of the tetragonal form of barium titanate is inferred from the powder diffraction pattern containing two reflections due to (200) + (020) around a 2θ of 45°,whereas in the cubic form only a single reflection (200) is presented in this region. Therefore, the XRD pattern shown in Fig. 1 confirms that the BaTiO3 are composed of cubic form of barium titanate. The peaks in the XRD pattern are strong and sharp, which indicate relatively high crystallinity of the powders. The existence of chlorine ion is a possible reason for the high crystallinity because chlorine ion has been proven to assist in the selective formation of crystalline oxides rather than amorphous oxides .
The crystallinity of BaTiO3 (cubic) was estimated by measuring the XRD intensities of BaTiO3 (cubic) (100) peak at 2θ = 22.160° with few by-product of BaCO3 (marked with asterisks) which can be removed by a washing process. The unit cell parameter for the crystalline BaTiO3 is determined to be a=3.994 Å and c=4.035 Å which are consistent with the literature data of JCPDS (31-1741). The crystal size of BaTiO3 was calculated to be = 25-30 nm from the broadening of the corresponding XRD peaks using the Scherrer equation: L=Kλ/ (βcosθ), where L is the crystal size; λ is the wavelength of the X-ray radiation (λ=0.15406 nm) for Cu Kα; K is usually taken as 0.89; and β is the line width at half-maximum height.
SEM micrographs of BaTiO3 powders synthesized at three different concentration of NaOH (1, 2& 3%) are shown in Fig.2 (a, b & c). Particle synthesized in 1% NaOH appear to be very uniform spherical morphology. In the case of 2% NaOH, particles were modified spherical into rod-like morphology. However, the BaTiO3 particles synthesized in the 3% NaOH were completely changed into nanorods. With increasing the concentration of NaOH, the particles lose their acidity nature and then it converts ampoteric into basicity. By choosing the adequate concentration we could able to form nanorods.
Fig. 1. Powder XRD of the BaTiO3 powders with 1-3% concentration of NaOH. Inset shows that only a single reflection is present around a 2θ of 45°
Fig. 2. SEM image of the BaTiO3 powders with different concentration of NaOH. (a) 1% NaOH, (b) 2% NaOH, (c) 3% NaOH.
Fig. 3(a) shows a TEM image of the barium titanate nanorods obtained by the addition of 3% NaOH. The BaTiO3 nanorods are very straight and have a high regularity. TEM image shows that almost all of the nanoparticles in the reaction system were incorporated into the BaTiO3 nanorods by oriented attachment mechanism. The diameter of the BaTiO3 nanorods range from 20 to 30 nm and lengths reaching up to >90 nm. As shown in fig.3 (b) Selected Area Electron Diffraction (SAED) pattern exhibits a sharp diffraction pattern consistent with the cubic structure, which demonstrated that all of the BaTiO3 nanorods were single crystalline. The first four dots are assigned to the (100), (110), (111), and (200) reflections of the cubic phase and are in good agreement with the XRD measurements. Energy dispersive Spectrum (EDS) displayed in Fig.3(c) and inset table furnish the composition of various elements in the prepared sample.
Fig. 3.(a) TEM image of the BaTiO3 powders with 1-3% concentration of NaOH. 3(b) Inset showed the SAED pattern of BaTiO3 powders with 3% concentration of NaOH. 3(c) EDS of the BaTiO3 powders with 3% concentration of NaOH
Photoluminescence (PL) spectra of BaTiO3 nanorods excited with a 350 nm laser source at room temperature are presented in Fig 4. PL spectra of BaTiO3 nanorods consist of two bands: near band edge excitonic UV emission and defect related deep level emission in the visible range. The spectra display a strong UV bandgap emission around 369, 366 and 363 nm respectively. The emission in the UV region is attributed to the recombination between electrons in the conduction band and holes in the valence band. The visible emission is related to the intrinsic structural defects related to deep level emission, such as oxygen vacancies, surface states, OH- defects and noncentral symmetric Ti3+ in the nanophase barium titanate. All these structural defects can give rise to a change in the octahedron configuration. A configuration co-ordinate model can be employed to elucidate the luminescence process in nanorods BaTiO3. In such a model, configuration coordinates can be used to describe the relative position of the ions inside the TiO6 octahedron in perovskites BaTiO3. The electron band structure of BaTiO3 has low-lying narrow conduction bands from Ti3+-3d states and valence bands from O2—2p states . Such a recombination corresponding to a charge transfer from the central Ti3+ ion to a neighboring O2- ion inside the TiO68- octahedron via intrinsic defects leads to luminescence in the nanorods BaTiO3 system. Therefore a study of the PL property of BaTiO3 can provide valuable information on the quality and purity of this material.
Fig. 4. Photoluminescence of BaTiO3 powders for various concentrations (a) 3% NaOH (b) 2% NaOH (c) 1% NaOH.
The room temperature UV-Vis absorption spectrum of BaTiO3 nanorods is shown in Fig. 5. The sharp rise of the spectra (1, 2 & 3% NaOH) at the absorption edge demonstrates highly crystalline nanocrystals with less surface defects. The absorption band edges of the samples, showing obvious blue shift are estimated around 344,329 & 315 nm. The values of bandgap energies in all three cases (3.6, 3.7 & 3.9 eV) are larger than that of the reported value for bulk BaTiO3 (3.2 eV) which were also attributed to the formation of nanocrystalline BaTiO3 particles . Considering the blue shift of the absorption positions from the bulk BaTiO3, the absorption onsets of the present samples can be assigned to the direct transition of electron in the BaTiO3 nanorods. Therefore, the BaTiO3 nanorods would be a promising candidate for optoelectronic devices and UV Laser.
Fig. 5. UV-Vis Absorption spectrum of BaTiO3 powders for various concentration (a) 3% NaOH (b) 2% NaOH (c) 1% NaOH.
The formation of cubic phase BaTiO3 may be understood in the context of a dissolution-recrystallization mechanism described by Eckert et.al. . Compared other previous works used TiO2 solid as titanium source; TiCl4 could suppose more titanium ions for formation of BaTiO3. At room temperature, when the sodium hydroxide was added in mixture solution of BaCl2 and TiCl4, the TiO2 sol was formation. Increasing NaOH, the TiO2 sol solubility also increases, thereby increasing the dissolution rate of titanium into solution and increasing the rate of BaTiO3 formation.
From above results, it is proved that the importance of the hydroxide ions (OH-). They are not only vital in nucleation of BaTiO3 crystals under hydrothermal conditions, from the standpoint of thermodynamics, as Leneka et.al. reported, but they also seem to act as a catalyst by promoting the growth of BaTiO3. Because sodium hydroxide solubility is more than barium hydroxide, in this work there are more OH- ions in the solution than other previous works, then present work can prepared cubic BaTiO3 by hydrothermal reaction only for 20 h. Two proposed mechanisms involve a condensation reaction of Ti(OH)62- with Ba2+12b and migration of Ba2+l2a into the TiO2 structure with resulting breakage of Ti-O-Ti bands and incorporation of Ba2+. In the latter mechanism, the role of OH- could be to facilitate the hydrolysis of Ti-O-Ti bands. The dynamic nature of the interaction between TiO2, Ba2+, and OH- leads to a crystallization mechanism involving nucleation, growth, and crystal dissolution.
The role of the Cl- seems to be to help the nucleation of larger crystals with the smaller crystals acting as seed nuclei. It has been noted that in the presence of halide ions, the reactivity of TiO2 is decreased. Even though the exact reason for this is unclear, it is possible that decreased reactivity could be due to absorption of the Cl- on the TiO2 surface, thus impeding the diffusion of Ba2+. If, as mentioned above, the cubic BaTiO3 grows by a dissolution/recrystallization process and Cl- retards the crystal growth process, then it may be able to stabilize the formation of larger crystals . This suggested role of Cl- is speculative. However the data shown in this paper clearly suggest those additives that do not directly enter into the chemical reaction in the hydrothermal synthesis process can stabilize the formation of the cubic BaTiO3 nanorods.
We have successfully synthesized nanorods BaTiO3 by using the hydrothermal method. XRD results show that the nanorods are cubic phase. Well-isolated single-crystalline cubic form BaTiO3 nanorods with diameters ranging from 20 to 30 nm and lengths reaching up to >90 nm can be easily fabricated by this route. PL spectrum showed high UV emission and very low visible emission, indicating optical quality of particles with little surface defects. Blue shifts occurred in all the cases (344,329 & 315 nm) from that of bulk (390 nm), but a prominent shift was observed for 3% NaOH. Increasing the NaOH concentration was benefit for preparation of BaTiO3 nanorods. The barium titanate nanorods would be a promising candidate for optoelectronic devices and UV laser.
The authors are grateful to the University Grant Commission, for extending financial assistance to carry out this work.
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R.Vijayalakshmi and V. Rajendran
Department of Physics, Presidency College, Chennai, TamilNadu, India.
Email: [email protected]
This paper was also published in print form in "Advances in Technology
of Materials and Materials Processing", 12 (2010) 43-49.