Praseodymium-Doped Cerium Oxide (PrxCe1–xO2−δ) Nanoparticles with High Water Dispersibility: The Nature of Pr-Related Optical Transitions Studied by MCD Spectroscopy

In this article, we report optical and magneto-optical properties of praseodymium-doped cerium oxide nanoparticles with oxygen deficiencies (referred to as PCO; Pr_xCe_1–xO_2−δ), which can be synthesized by a common solvothermal protocol. The as-obtained products have a confeito-like morphology with high water dispersibility, showing a pale yellow dispersion. The high water dispersibility remains unchanged after thermal annealing of the products at 600 °C, giving a reddish dispersion of PCO nanoparticles. On the basis of both optical absorption and magnetic circular dichroism (MCD) spectra of the annealed PCO dispersions, two different optical transitions responsible for the red coloration are, for the first time, resolved in the region of 350–650 nm. Notably, the transitions can distinctly be recognized by the MCD signals with opposite signs arising from two interacting Faraday B-terms and thus would be ascribed to those from the valence band involving the spin–orbit split band of occupied Pr⁴⁺ (4f) states to the unoccupied Pr⁴⁺ (4f) states. We believe that the methodology using MCD will be universal to effectively unveil the characteristic features of aliovalent-dopant-related optical transitions in various transition metal oxide nanostructures.