Effect of Er3+ doping on the thermal stability of TiO2 nanoparticulate xerogels

The effect of Er3+ doping on the thermal stability of TiO2 nanoparticulate xerogels prepared by a colloidal sol-gel route was investigated. It was found that the as-synthesized xerogels crystallize as anatase phase with crystallite sizes in the low nanoscale range (<7 nm) and high-specific surface areas (>100 +/- 5 m(2)/g). Nevertheless, it was also found that the Er3+ cations are deposited on the surface of TiO2 nanocrystallites thus resulting in xerogels with smaller and more uniform nanoaggregates. Most importantly, detailed analyses using X-ray thermo-diffractometry together with selective analyses by transmission electron microscopy, selected area electron diffractometry, and X-ray energy-dispersive spectrometry showed that the thermal stability of these TiO2 nanoparticulate xerogels increases with increasing Er3+ doping. Specifically, the nanocrystallite growth is slowed down, the onset temperature of the anatase-to-rutile phase transformation is delayed, and the anatase phase is retained up to greater temperatures as the Er3+ content increases, which are all phenomena attributable to the formation at low temperatures (i.e. similar to 200 degrees C) of solid solutions with Er3+ solutes within the TiO2 host. Moreover, these solid solutions also have lower thermal expansion coefficient than the undoped crystal structure. Finally, Er3+ doping increasingly promotes the precipitation at high temperatures of Er2Ti2O7 from the rutile TiO2 solid solution, with the precipitation temperature decreasing with increasing Er3+ doping content.