Hydrothermal Synthesis of Er-Doped Luminescent TiO2 Nanoparticles

Here we report the synthesis and characterization of a stable suspension of fluorescent erbium-doped titania nanoparticles and their assembly into thin films and photonic crystals. The nanoparticles were synthesized through a sol-gel process followed by peptization with tetramethylammonium hydroxide and hydrothermal treatment. As expected, following the hydrothermal treatment, X-ray diffraction shows the particles to be exclusively anatase. The nanoparticles form a stable suspension in water and were cast into 2and 20-μm-thick films and formed into an inverse opal structure through templating with a colloidal crystal. As synthesized, only a faint fluorescence at 1532 nm was observed from the particles when pumped with 795-nm laser radiation; however, heat treatment at temperatures as low as 100 °C resulted in a significant increase in the fluorescence. The fluorescence was observed to increase further with annealing up to 500 °C, perhaps due to removal of hydroxyl groups and other surface species. Through energy-dispersive X-ray spectroscopy and X-ray diffraction, we present evidence that the Er ions are doped into the TiO2 nanoparticles and that Er was not present as free Er2O3. This work demonstrates the potential of rare-earth doping of titania nanoparticles for photonic applications.