Thermal evolution of impurities and hydroxyl groups in ZnO nanocrystals.

Abstract Impurities and surface hydroxyl groups play vital roles in tailoring the physicochemical properties of metal oxides. Herein, commercial ZnO nanocrystals (Sigma-Aldrich) were selected to trace the impurity and hydroxyl group evolution by temperature dependent photoluminescence spectrum, infrared spectrum and electron spin resonance spectrum. It is found that the structured Cu impurity emission in ZnO appeared when the temperature is down to 100 K. As thermal annealing reduced the donors significantly, the corresponding 5780 cm−1 and 5820 cm−1 IR peaks can be observed when the annealing temperature is above 600 °C. Further annealing at 800 °C increases the Cu2+ concentration as well as activates the diffusion of lithium impurity, both of which contribute to the visible luminescence. Besides, a group of IR peaks in the range of 3400–3700 cm−1 were found to be vanished after 800 °C thermal annealing, demonstrating their hydroxyl features rather than the previously proposed defect origin. [ABSTRACT FROM AUTHOR]