Impact of Tb4+ and morphology on the thermal evolution of Tb-doped TiO2 nanostructured hollow spheres and nanoparticles

Abstract Tb-doped TiO2 hollow spheres (HSs) in the range 0.0–2.0 at.% have been synthesized by the first time to the best of our knowledge. The HSs are compared with nanoparticles (NPs) to evaluate the impact of morphology on their physicochemical and photoluminescence (PL) behavior upon increasing calcination temperature. After calcination at 550 °C, the particles are anatase with a primary average size of 10.0 ± 0.2 nm for the NPs and 12.0 ± 0.2 nm for those that form the micron sized hollow spheres of 1.8 μm diameter and ca. 64 nm shell thickness. The temperature of the anatase–rutile transition is found to be strongly dependent on the presence of Tb as well as on morphology. Contrarily to the usual stabilization of anatase when doping with trivalent rare-earth ions, the transition temperature is reduced when doping with Tb. The rutile phase is further favored for the HSs compared to the NPs probably related to the low density of the HSs and/or a more efficient packing of the nanoparticles that form those spheres with respect to the packing of the NPs. A slight unit-cell volume decrease for the anatase structure is observed upon Tb doping, in both the NPs and in the HSs, contrary to the expected increment due to the larger ionic radius of Tb3+ compared to Ti4+. In addition, the intensity of the characteristic f-f Tb3+ emission bands is extremely weak both in the anatase and rutile phases. The transition is accompanied with the emergence of an infrared emission band centered at 810 nm related to the formation of defects during the structural transformation providing deep levels in the gap that partly quench the f-f emissions in the rutile phase. The results are consistent with the presence of Tb in both +3 and + 4 valence states. XPS measurements confirmed the presence of Tb3+ as well as of Tb4+ in both HSs and NPs with a Tb4+ fraction that increases with increasing Tb doping. The large fraction of Tb4+ present in the samples originates the weak f-f emission intensity, the slight decrease of the cell parameters and the destabilization of the anatase phase.