Cu-Related Paramagnetic Centers in Cu- and (Cu,Y)-Doped ZrO₂ Nanopowders

In this work, we studied Cu-doped and (Cu,Y)-codoped ZrO₂ nanopowders produced through a coprecipitation approach to identify the nature of Cu-related bulk and surface paramagnetic centers. We conducted EPR, NMR, and Raman scattering studies on Cu- and (Cu,Y)-doped ZrO₂ powders calcined at different temperatures. At low calcination temperatures (400 °C) and low Cu loading (0.1-1.0 mol.% of CuO), the EPR signal was found to be attributed to surface-related Cu-H₂O complexes. For powders with higher Cu content (up to 8.0 mol.% of CuO), the superparamagnetic signal associated with the formation of copper clusters was observed. At higher calcination temperatures, the destruction of Cu-related surface complexes promotes the incorporation of Cu²⁺ ions into the bulk of ZrO₂ nanocrystals at Zr positions. Co-doping ZrO₂ with Cu and Y was observed to facilitate the incorporation of Cu²⁺ ions into cation sites at lower calcination temperatures when compared with Cu-doped ZrO₂.