Role of Surface Coating in ZrO₂/Eu³⁺ Nanocrystals

Abstract

The sol−emulsion−gel method is used for the preparation of Eu³⁺ ion-doped and coated ZrO₂ nanocrystals. Here, we report the role of surface coating, dopant concentration, and temperature of heating in the modification of their crystal structure and photoluminescence properties. It is found that the volume fraction of the tetragonal phase increases from 28.08 to 91.56% because of surface coating. This is a significant modification of the crystal phase in ZrO₂ nanocrystals due to surface coating by Eu₂O₃. It is found that the photoluminescence properties are sensitive to the crystal structure, which is again controlled by surface coating, concentration, and heating temperature. It is found that the decay time (τ) of Eu-doped ZrO₂ nanocrystals increases with increasing the concentration of dopant and with increasing the temperature of heating because of changes in their crystal phase. The emission intensity of the peak at 611−617 nm (⁵D₀ → ⁷F₂) of the Eu³⁺ ion-activated ZrO₂ nanocrystals (doped and coated) is also found to be sensitive to the nanoenvironment. The average decay times are 770 and 488 μs for 1100 °C-heated 1.0 mol % Eu₂O₃-doped and coated ZrO₂ nanocrystals, respectively. Our analysis suggests that the site symmetry of the ions plays the most important role in the modifications of the radiative and nonradiative relaxation mechanisms as a result of the overall photoluminescence properties.