The unusual variations of photoluminescence and afterglow properties in monoclinic ZrO2 by annealing

The raw ZrO2 is annealed at 600–1550 °C for 6 h. It is found that the emission at 492 nm increases greatly when the annealing temperature is higher than 1200 °C and its afterglow shows a small improvement at 1200–1450 °C and a large enhancement after annealing at 1550 °C. The results that are obtained indicate that the impurity Ti4+ in ZrO2 is efficiently reduced to Ti3+ when the temperature is higher than 1200 °C, and the increase of Ti3+ centers contributes to the large improvement of emission at 492 nm. The thermoluminescence shows that at least two types of traps with different depths (0.65 eV and 1.46 eV) corresponding to oxygen vacancies exist in monoclinic ZrO2. After annealing at 1200–1450 °C, some new trap clusters related to oxygen vacancies and Ti3+ form and causes the small improvement of afterglow at 1200–1450 °C. The large improvement of afterglow after annealing at 1550 °C originates from the sharp increase of proper shallow traps (0.65 eV) in ZrO2. Accordingly, we present the feasible interpretations and luminescence mechanisms of monoclinic ZrO2 for our observations.