Cerium oxide nanoparticles: Synthesis, characterization and tentative mechanism of particle formation

Given the scientific and technological interest in cerium oxide (ceria), in this paper nanoparticles of ceria (ceria-NPs) were synthesized using a modified polymer complex process (modified Pechini), while varying the pH of the system. This methodology made it possible to obtain, in a reproducible and controlled way, nanoparticles of ceria (< 100 nm) of a high chemical purity at low temperatures. The precalcined cerium solid obtained at 350 degrees C was characterized using differential thermal analysis (DTA), thermogravimetric analysis (TG) and IR spectroscopy. Very little organic phase was found in the respective spectra, indicating that the inorganic phase, cerium oxide, is predominant. Carbonaceous residues still present in the solids were removed by heating at temperatures above 500 degrees C and the samples obtained were characterized using X-ray diffraction (XRD), IR, UV-vis absorption and diffuse reflectance spectroscopies, and Transmission Electron Microscopy (TEM). The diffractograms of the samples showed that the only crystalline phase present was CeO2. From the results of UV-vis absorption and diffuse reflectance spectroscopy, two important energy values were obtained, 3.8 eV and 3.4 eV. These could be attributed to the energy gap value (3.8 eV) and to a possible "mid-gap" (3.4 eV). Furthermore, on increasing the synthesis pH, a reduction in particle size results, the particle being between 10 and 20 nm, with a spheroidal shape. By looking at the different stages of the synthesis process, a mechanism is proposed to explain how nanoparticles of ceria are formed.