engleski

BaTiO3 nanorods: new insight by Raman spectroscopy

In our work we report on the synthesis of BaTiO3 nanorods by sol-gel electrophoretic deposition into template membranes. As a template we used track-etched hydrophilic polycarbonate (PC) membranes with pore diameters of 200 nm and thickness of 10-25 μ m. The template membrane was attached to aluminum working electrode while Pt mesh electrode was used as a counter electrode. For electrophoretic deposition of the sol into porous templates the potential of 30 V was applied between both electrodes for 30 min. After the deposition, the samples were annealed at elevated temperatures. This heating procedure was done in order to burn off the polycarbonate membrane and to make the nanorods dense and crystalline. Obtained nanorods were characterized by Raman spectroscopy (RS) and X-ray powder diffraction (XRD), while their sizes and the morphology were observed by scanning and transmission electron microscopy (SEM, TEM). RS experimets were made using micro-Raman with 50x LWD objective and 1 mW or 10 mW laser powers at the surface of the sample. XRD analysis (Fig. 1) indicated that nanorods consist of cubic (space group m-3m) BaTiO3. However, in the case of tetragonal 4mm simetry with c/a axis ratio close to one, it is hard or impossible to distinguish between cubic and tetragonal structure by XRD [1], so RS was applied. Raman spectra recorded using 1 mW at the sample showed bands of tetragonal structure in samples annealed at 700 and 800 °C (Fig. 2), although in the sample annealed at 700 °C the band corresponding to hexagonal structure [1] was also observed. Since the laser-induced thermal effects on the samples during the recording of the Raman spectra can induce structural phase transitions of the sample, the influence of the laser power to the observed structures was also studied. Thus, in BaTiO3 anealed at 800 °C the phase transition to hexagonal structure was observed using 10 mW laser power (Fig.1). We observed by SEM that the perovskite nanorods grown in a PC membrane have a uniform diameter throughout their entire length, while EDXS analysis confirmed that the composition of the observed nanorods corresponded to stoichiometric BaTiO3. By using TEM and selected area electron diffraction (SAED), we found that these nanorods are dense and polycrystalline, with the grain size ranging between 25 up to 50 nm. The length of nanorods is approximately 10  m with a diameter of an individual nanorod in the range of 100 to 180 nm. Since RS results suggests tetragonal structure with low a/c ratio, as well as possible coexistence of more phases in the samples [1, 2], these results will be discussed by comparison with HRTEM and SAED results observed at the individual crystallites as building blocks of nanorods. Moreover, the phase transitions induced by heating of the saples in the laser beam will be discussed in terms of the particle sizes by comparison with ? results that tetragonal BaTiO3 transforms to hexagonal form at temperatures higher then… °C. References: [1] W. Satoshi et.al., Jpn. J. Appl. Phys. 42 (2003) 6188-6195. [2] B.A. Hernandez et.al., Chem. Mater. 14 2002 480-482.

BaTiO3; nanorods; Raman spectroscopy; tetragonal structure

nije evidentirano