Morphology, phase and chemical composition of the nanostructures formed in the systems containing lanthanum, cerium, and silver

Abstract

X-ray phase and thermogravimetric analysis, scanning electron microscopy and energy-dispersion spectroscopy were used to study the products of phase formation during the precipitation of lanthanum and cerium salts in the presence of silver nitrate and recipients of precipitators, nucleating agents and hydrolysis regulators. Thermogravimetric analysis shows the completion of the La(OH)3 lattice dehydroxylation process at a temperature of ~ 300 °С and probable destruction of sulfates at a temperature of ~ 340 °С. The phase interaction of lanthanum oxide(III) with silver ends at T ~ 400 °C. The DTG curve shows a two-stage weight loss, which characterizes the destruction of lanthanum and silver hydroxides (250 °C) and the removal of sulfates (~ 340 °C), respectively. According to the TG, the total weight loss is 21.6 %. For the cerium-containing system the only endothermic effect of dehydroxylation of cerium hydroxide at T = 250 °C with its conversion into cerium dioxide is observed. The destruction of nitrates (anionic component of solutions) takes place at the temperature of 400 °C. Weight loss takes place at T = 150 °C and is 53.9 %. Thus, on the basis of TG-DTA data, it can be assumed that the formation of composites particles based on lanthanum and cerium oxides, modified with silver, ends at the temperature of 400 °C. The X-ray diffraction data shows that at the initial stage the system undergoes the formation of cerium and lanthanum hydroxides, and during lyophilization of the precipitate (T = 160 °C) the crystal lattice of hydroxides partial dehydroxylation takes place with the formation of trigonal oxides La2O3 and Ce2O3. It has been found that the presence of silver cations in the solution can affect the phase composition of lyophilized structures and the formation of the CeO2 phase. It is shown that the hydroxylamine chloride injection into the system can initiate the silver restoration on the lanthanum oxide surface and also partially restore it to the LaO phase. Temperature treatment of the samples (T = 400 °С) promotes homogenization of the precipitate composition: formation of 30 nm cerium dioxide particles with silver clusters evenly distributed on its surface, and hexagonal lanthanum oxide plates with individual silver particles as the second phase. In three-component systems, two modifications of lanthanum oxides (trigonal and cubic), cerium dioxide and metallic silver are formed. It is found that the chemical composition of the precipitates contains the main elements – La, Ce, O, Ag and impurity – S or Cl, as the anionic component of the initial solutions, N and K in the composition of the initial suspension. It is shown that the morphology of the samples is represented by hexagonal structures of lanthanum hydroxide and oxide, spherical and pseudocubic particles of cerium dioxide and lanthanum oxide, spherical clusters of silver.