Effect of dopant loading and calcination conditions on structural and optical properties of ZrO₂ nanopowders doped with copper and yttrium

Abstract

Undoped, Cu and/or Y doped ZrO2 nanopowders were synthesized with Zr, Y, and Cu nitrates using a co-precipitation approach. Their structural and optical properties were examined regarding dopant content (0.1-8.0 mol.% of CuO and 3-15 mol.% of Y2O3) and calcination conditions (400-1000 °C and, 1, 2 or 5 hours) through Raman scattering, XRD, TEM, EDS, AES, ESR, UV-Vis and FTIR diffused reflectance methods. The results showed that both Cu and Y dopants promoted the appearance of additional oxygen vacancies in ZrO2 host, while the formation of tetragonal and cubic ZrO2 phases was primarily influenced by the Y content, regardless of Cu loading. The bandgap of the most powders was observed within the 5.45-5.65 eV spectral range, while for those with high Y content it exceeded 5.8 eV. The (Cu,Y)-ZrO2 powders with 0.2 mol.% CuO and 3 mol.% Y2O3 calcined at 600°C for 2 hours demonstrated nanoscaled tetragonal grains (8-12 nm) and a significant surface area covered with dispersed CuxO species. For higher calcination temperatures, the formation of CuZr 2+ ESR centers, accompanied by tetragonal-to-monoclinic phase transformation. For fitting experimental FTIR reflection spectra, theoretical models with one, five, and seven oscillators were constructed for cubic, tetragonal, and monoclinic ZrO2 phases, respectively. Compared experimental and theoretical spectra, the parameters of various phonons were determined. It was found that the distinct position of the high-frequency FTIR reflection minimum is a unique feature for each crystalline phase. It centered at 700-720 cm-1, 790-800 cm-1, and 820-840 cm-1 for cubic, tetragonal, and monoclinic phases, respectively, showing minimal dependence on phonon damping coefficients. Based on the complementary nature of results obtained from structural and optical methods, an approach for monitoring powder properties and predicting catalytic activity can be proposed for ZrO2–based nanopowders.