Structure and morphology of synthesized lanthanum hydroxide La(OH)(3) nanocrystalline powders: study on fuel to oxidant ratio

摘要

An efficient process based on a solution combustion technique has been developed to produce high crystalline and nanometer-sized particles of lanthanum hydroxide [La(OH)(3)] using metal nitrate [La(NO3)(3).6H(2)O] as oxidant and citric acid [C6H8O7] as fuel. Three fuel/oxidant ratios were chosen to study the effect of the fuel content on the phase formation and powder properties. X-ray diffraction analysis reveals the hexagonal phase [space group: P6(3)/m (176)] of La(OH)(3) (JCPDS no. 36-1481) without any additional impurity peaks. Since F/O ratio has significant control over flame temperature and the amount of gas release, the prepared La(OH)(3) has F/O-dependent properties variation. SEM images of the prepared samples have different morphologies due to the change in the flame temperature, which in turn releases a lot of gases. The Raman spectral bands at 225, 278, 334, and 445 cm(-1) and a weak broader peak 593 cm(-1) confirmed the phase formation as reported for crystalline La(OH)(3). The room-temperature photoluminescence spectra of La(OH)(3) for the excitation at lambda(ex) = 325 nm exhibit emission bands at 417 and 493 cm(-1) corresponding to the typical blue band of La3+ ions. The recorded EDAX spectra indicate only the presence of La and O elements. It is observed that the sample prepared for F/O = 1.0 is almost stoichiometry than the other two samples, fuel lean and fuel rich. TEM images show typical degree of agglomeration and polydisperse with uniform size distribution. The dielectric constant of La(OH)(3) has higher value (401) in the lower-frequency (100 Hz) than the value (54) measured at higher frequency (1 kHz). The fuel-to-oxidant ratio and temperature have been found to significantly effect the dielectric constant of La(OH)(3). The physicochemical properties of [La(OH)(3)] nanoparticles are valuable for developing La-based applications such as catalysts and phosphors.