Metastable TiO<sub>2</sub> polymorphs are more promising
materials
than rutile for specific applications such as photocatalysis or catalysis
support. This was clearly demonstrated for the anatase phase but still
under consideration for brookite, which is difficult to obtain as
pure phase. Moreover, the surface doping of anatase with lanthanum
ions is known to both increase the thermal stability of the metastable
phase and improve its photocatalytic activity. In this study, TiO<sub>2</sub> nanoparticles of almost only the brookite structure were
prepared by a simple sol–gel procedure in aqueous solution.
The nanoparticles were then doped with lanthanum(III) ions. The thermal
stability of the nanoparticles was analyzed by X-ray diffraction and
kinetic models were successfully applied to quantify phases evolutions.
The presence of surface-sorbed lanthanum(III) ions increased the phase
stability of at least 200
°C and this temperature shift was attributed to the selective
phase stabilization of metastable TiO<sub>2</sub> polymorphs. Moreover,
the combination of the surface doping ions and the thermal treatment
induces the vanishing of the secondary anatase phase, and the photocatalytic
tests on the doped brookite nanoparticles demonstrated that the doping
increased photocatalytic activity and that the extent depended on
the duration of the sintering treatment
