Spray-flame synthesis of LaMO3 (M = Mn, Fe, Co) perovskite nanomaterials: Effect of spray droplet size and esterification on particle size distribution

Perovskite nanomaterials such as LaMnO3 , LaFeO3 , and LaCoO3 were synthesized in a spray flame from metal nitrates dissolved in combustible liquids. The addition of low-boiling solvents such as 2-ethylhexanoic acid (2-EHA) to the ethanol-based solutions supports the formation of phase-pure particles with unimodal particle-size distribution in the 10-nm range attributed to enhanced evaporation through micro-explosions. Nevertheless, in many cases, a second particle mode with sizes of a few hundred nanometers is formed. In this paper, we investigate two possible reasons for the appearance of large particles. Firstly, we analyze the effect of the oxygen dispersion gas flow applied in the two-fluid nozzle on the droplet size distributions of burning sprays using phase Doppler anemometry. We identified that an increase of the dispersion gas flow significantly decreases the number concentration of large droplets ( > 30 mu m), which causes a significant increase of the BET surface area of as-synthesized LaMnO3 and LaCoO3 with increasing dispersion gas flow from 60 m(2)/g (5 slm dispersion gas) to 100 m(2) /g (8 slm). Secondly, the esterification in the mixture of solvents towards ethyl-2-ethylhexanoate, which is associated with the release of water as a byproduct, was analyzed by GC/MS. The ester concentration in the iron-containing solution was found to be up to nine times higher than in cobalt or manganese precursor solutions. Simultaneously, the produced LaFeO3 materials show lower BET surface areas and the increasing dispersion gas flow has a minor effect on this material than on the cobalt and manganese perovskite cases. We attribute this to the fact that water formed during esterification forces the hydrolysis of iron nitrate and the formation of large particles within the droplets. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.