Effect of ambient oxygen on the photoluminescence of sol-gel-derived nanocrystalline ZrO2:Eu, Nb

The development of inorganic nanophosphors is an active research field due to many applications, including optical gas sensing materials. We found a systematic dependence of the photoluminescence (PL) of europium (Eu3+) impurity ions in zirconia (ZrO2) nanocrystals on the ambient oxygen concentration in a O-2/N-2 mixture at normal pressure. Europium-doped ZrO2 powders were synthesized via a sol-gel route. Heat-treatment at 1200 degrees C resulted in a well-developed monoclinic phase (XRD crystallite size of similar to 50 nm) and an intense PL of Eu3+ ions residing in the dominant phase (Eu3+ was excited directly at 395 or 464 nm). Co-doping with niobium resulted in a narrowing of the PL emission lines. Only Nb5+ was detected by XPS and is believed to charge-compensate Eu3+ activators throughout the material leading to a more regular crystal lattice. At room temperature, the exposure to oxygen suppressed the Eu3+ fluorescence, whereas, at elevated temperatures (300 degrees C), the effect was reversed. At 300 degrees C and under a focused continuous laser beam, a substantial PL response (>50%) was achieved when switching 100% of N-2 for 100% of O-2. PL decay kinetics clearly showed that at 300 degrees C fluorescence quenching centers were induced within the material by oxygen desorption. The relatively fast (<5 min) and sub-linear PL response to the changes of oxygen concentration shows that ZrO2:Eu, Nb is a promising PL-based oxygen sensing material over a wide-range of oxygen pressures.