Anatase-to-Rutile Phase Transition in TiO2 Nanoparticles Irradiated by Visible Light

The light-induced phase transition of TiO2
nanoparticles from anatase to rutile structure is reported
depending on the surrounding environment, the transition
being accomplished under oxygen-poor conditions. The
transition mechanism is interpreted in the framework of oxygen
adsorption and desorption phenomena with the involvement of
surface oxygen vacancies and F centers. It is shown that the
observed phase transition is not thermally driven because the
local temperature of the nanoparticles during irradiation is
about 370 K (estimated through the Stokes to anti-Stokes
Raman peaks ratio). On the contrary, the phase transition is
initiated by intragap irradiation (with the exception of the red
light one) that acts as TiO2 surface sensitizer, promoting the
activation of the surface and the nucleation of rutile crystallites starting from two activated anatase neighboring nanoparticles.