Fe-doping effects on the structural, vibrational, magnetic, and electronic properties of ceria nanoparticles

摘要

In this work, we report on a single-pot synthesis route based on a polymeric precursor method used for successfully producing undoped and iron-doped CeO_2 nanoparticles with iron contents up to 10.0 mol. %. The formation of high-crystalline nanoparticles with a cubic fluorite structure is determined for all the studied samples. Meanwhile, the magnetic measurements of the undoped ceria nanoparticles revealed the occurrence of ferromagnetism of bound magnetic polarons of a fraction of Ce~(3+) at room temperature, and only a paramagnetic behavior of Fe~(3+) ions was determined for Fe-doped ceria nanoparticles. A monotonous reduction of the effective magnetic moment of the Fe~(3+) ions was determined. It suggests a change from a high-spin to low-spin state of Fe ions as the Fe content is increased. The 3+ valence state of the iron ions has been confirmed by the Fe K-edge X-ray absorption near-edge structure (XANES) and Mössbauer spectroscopy measurements. X-ray photoelectron spectroscopy data analysis evidenced a coexistence of Ce~(3+) and Ce~(4+) ions and a decreasing tendency of the relative fraction of Ce~(3+) ions in the surface region of the particles as the iron content is increased. Although the coexistence of Ce~(3+) and Ce~(4+) is confirmed by results obtained via Ce L_3-edge XANES measurements, any clear dependence of the relative relation of Ce~(3+) ions on the iron content is determined, suggesting a homogeneous distribution of Ce~(3+) and Ce~(4+)-ions in the whole volume of the particles. Ce L_3-edge extended X-ray absorption fine structure revealed that the Ce-O bond distance shows a monotonous decrease as the Fe content is increased, which is in good agreement with the shrinking of the unit cell volume with the iron content determined from XRD data analysis, reinforcing the substitutional solution of Ce and Fe ions in the CeO_2 matrix.