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

Abstract

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 m²/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. ~200 °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.