Synthesis of La-doped ceria nanoparticles: impact of lanthanum depletion

The present work explores the possible issues involving the synthesis of La-doped ceria-based nanoparticles by co-precipitation; moreover demonstrates the consequence of such issues in the final system. The stoichiometric anomaly as revealed by the X-ray diffraction (XRD) peak shift studies confirmed a massive departure from the final target composition; indicating lanthanum depletion (~30 %) from the resultant oxide. This also revealed that a fraction of this unreacted lanthanum precursor stayed in the system alone. Transmission electron microscopy results revealed the presence of nanocrystalline La-oxycarbonate phase as a fine dispersion co-existing with the lanthanum cerate nanocrystals; XRD could not detect the presence of such a phase at 550 °C. Through Raman and Fourier transform infrared spectroscopy analyses, a possible pathway of this complex thermal transition for the free La-precursor was outlined (100–1200 °C). The Raman data unambiguously pointed out the transition zone for the decomposition sequence for its conversion from La-nitrate to oxycarbonate in the temperature range 400–550 °C. The overall study emphasises the need of special attention for synthesising La-based solid solution systems via colloidal routes; especially considering the solubility and mixing issues for the lanthanum precursor.