Effect of citric acid quantity on LaAlO3 formation in sol-gel synthesis with combustion

Abstract

Lanthanum aluminate (LaAlO₃) was synthesized using the nitrate-citrate sol-gel method followed by the combustion process. The synthesis was conducted using fuel-lean at pH 7, with metal precursor to citric acid mass ratios (Me:C₆H₈O₇) of 1:0.25, 1:0.37, and 1:0.48. The resultant products were analyzed using X-ray diffraction and scanning electron microscopy (SEM). After post-annealing at 900 °C, the formation of crystalline LaAlO₃ was confirmed, whereas the as-pyrolized product was found to be X-ray amorphous. It was established through thermal analysis that nitrate-citrate complexes decompose in several stages, and LaAlO₃ crystallizes at around 840 °C. An increase in citric acid concentration correlated with a reduction in microstress levels and a decrease in the size of coherent scattering regions, in agreement with the particle size measurement results obtained from SEM images. The synthesized nanoscale particles exhibited an average size in the range of 50–60 nm and a specific surface area greater than 15 m²/g. The synthesized powders were characterized by fragile porous aggregates composed of thin porous walls. Notably, an increase in citric acid concentration promoted the enlargement of pores within the aggregates while simultaneously reducing the pore size within the thin walls. Furthermore, the materials synthesized from the solutions with pH 1 demonstrated a larger average particle size (~75 nm) and smaller pore sizes, with combustion occurring at lower temperatures compared to the samples obtained from the solutions at pH 7.