Oxygen-Sensitive Photoluminescence of Rare Earth Ions in TiO2 Thin Films

Some nanocrystalline TiO2 materials are notorious for their photoluminescence (PL), which exhibits a pronounced sensitivity to ambient oxygen and has a potential for optical gas sensing. Here, we utilized pulsed laser deposition to obtain thin (70 nm) anatase TiO2 films containing 1-2 atom % of either samarium (Sm) or neodymium (Nd) impurity ions. The highly porous nanostructure consisted of quite regular, interconnected nanopillars with a wall thickness of 15-20 nm, fully exposing the material to ambient environment. The PL intensity. of both ions (induced by 355 nm pulsed laser) responded in a reversible manner to changes of oxygen volume fraction in dry O-2/N-2 flow. Sm3+:TiO2 worked effectively at room temperature, whereas Nd3+:TiO2 showed an improved relative response at 100-150 degrees C. Switching between pure nitrogen and oxygen atmospheres affected the PL intensities up to 6 times, with response time of 1 min to 02 and recovery time of 10 min in N-2. The PL decay kinetics of Sm3+:TiO2 showed unambiguously that both decreasing 02 concentration and increasing temperature reduced the fluorescence quantum yield of excited Sm3+ ions. Such behavior is compatible with the proposed model of energy acceptors created by (reversible) electron transfer from desorbed charged 02 species to certain lattice defects. In contrast, decreasing O-2 concentration increased the PL intensity of Nd3+:TiO2. In this case, PL decay kinetics showed an increased excitation efficiency of Nd3+ ions, suggesting that O-2 adsorption favors a concurrent relaxation path for the photoexcited charge carriers.