Synthesis of Ce₂O₂S and Gd_{2(1–<i>y</i>)}Ce_2<i>y</i>O₂S Nanoparticles and Reactivity from in Situ X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy

Abstract

Lanthanide oxysulfide nanoparticles have recently attracted interest in view of their potential applications, such as lighting devices and MRI contrast agents, which requires a good stability in air and a controlled surface. In order to address these issues, in this work, air-sensitive Ce₂O₂S nanoparticles of hexagonal shape were successfully prepared and characterized under inert conditions. Bimetallic Gd_2(1-y)Ce_2yO₂S nanoparticles of similar shape and size were also synthesized for the whole composition range (y from 0 to 1). X-ray diffraction structural data are found to follow Vegard's law up to y = 0.4, which is attributed to the loss of stability in air of Ce-rich nanocrystals beyond this threshold. This picture is supported by X-ray absorption spectra taken at the S K-edge and Ce L₃-edge that show the partial oxidation of sulfide species and of Ce^III to Ce^IV in the presence of air or water. A complementary near-ambient-pressure X-ray photoelectron spectroscopy study shows that at least two types of oxidized sulfur species form on the nanoparticle surface. Even in Gd₂O₂S nanoparticles that are generally considered to be air-stable, we found that sulfide ions are partially oxidized to sulfate in air. These results unveil the physicochemical mechanisms responsible for the surface reactivity of lanthanide oxysulfides nanoparticles in air.