Structural and Surface Study of Praseodymium-Doped SnO2 Nanoparticles Prepared by the Polymeric Precursor Method

In the present study, we report on the successful synthesis of Pr-doped SnO2 (SnO2:Pr) nanoparticles using a polymeric precursor method while setting the Pr-content in the range from 0 to 10.0 mol %. The as-prepared samples were characterized in regard to their structural, morphological and surface properties. X-ray diffraction (XRD) patterns recorded from all samples revealed the tetragonal rutile-type structure with a systematic average size reduction (in the range from 11 to 4 nm) while enhancing the residual strain (in the range of 0.186 to 0.480%) as the Pr-content was increased. From the Rietveld refinement analysis we found that the lattice parameters (a, c, u, and V) showed a linear behavior, indicating a solid solution regimen for the Pr-doping. Transmission electron micrographs provided mean particle sizes of 8.7 +/- 0.5 nm, for 2.5 mol % Pr-content, and 5.2 +/- 0.5 nm,for 10.0 mol % Pr-content, which are in very good agreement with values obtained from the XRD data analysis: 7.4 +/- 1.0 nm and 4.0 +/- 1.0 nm, respectively. From X-ray photoelectron spectroscopy (XPS) measurements [O]/[Sn] = 1.44 ratio has been estimated at the surface of the undoped SnO2 nanoparticles, which is below the expected value for bulk compound ([O]/[Sn] = 2), suggesting that the system is strongly nonstoichiometric at the nanoparticle surface. Actually, we found the [O]/[Sn] ratio value increasing monotonically as the Pr-content was increased, which was interpreted as due to the elimination of the surface chemisorbed oxygen and/or oxygen-related vacancies. Moreover, a redshift of the Sn(3d) XPS peaks has been determined as the Pr-content was increased, evidencing the change of the oxidation state of tin ions from Sn4+ to Sn2+. Our analyzes of the Pr(3d) XPS peaks indicated the preference of the Pr-ions for the Pr3+ oxidation state, although small amounts of the Pr4+-ions could not be completely ruled out, particularly for the lower Pr-doping samples.