Optical and Photoluminescence Properties of Rare Earth Ions-Doped Anatase/Brookite Dual-Phase TiO2

For the first time, optical absorption and luminescent properties of nanostructured dual-phase anatase-brookite (A/B) TiO2 doped with Sm3+, Gd3+, Tb3+, Tm3+ and Yb3+ rear earth (RE) ions are reported. The RE-doped TiO2 particles had a spherical shape with a typical size of 10–25 μm in diameter and consisted of grown together nanocrystallite aggregates of 5–8 nm. SEM/EDS analysis confirmed successful incorporation of RE-dopants onto the surface of the TiO2 particles. With FTIR spectroscopy, a formation of RE–Ti–O and RE–OH bonds near the surface defect states was found. UV-Vis-NIR diffuse reflectance spectra exhibited a blue shift of the TiO2 main absorption band maximum for Gd3+ dopant and a red shift of the absorption edge for Yb3+. Several characteristic NIR absorption bands of RE ions were also detected for Sm3+ -, Tm3+ - and Yb3+ -doped TiO2, assigned to their 4f -4f transitions, while Gd3+ - and Tb3+ -doped samples show no absorption in this region. The calculated band gap energies (Eg) of RE-doped TiO2 were 3.02, 2.97, 2.93, 2.92, 2.88 and 2.91 eV, for undoped and doped with Sm3+, Tm3+, Tb3+, Yb3+, and Gd3+ samples, respectively. It was shown that under excitation of A/B TiO2 nanocrystals above the TiO2 band gap energy, the particles exhibited characteristic luminescence corresponding to 4f –4f transitions of RE3+ ions. It was explained that in RE-doped A/B dual-phase TiO2 nanostructures, the regions at A/B junctions or grain boundaries of TiO2 nanocrystals can easily accommodate RE3+ ions and provide for efficient energy transfer from TiO2 to RE3+ ions under appropriate alignment of excited state energy levels of the dopant in TiO2 band gap. Such energy transfer was observed only in samples doped with Sm3+, Tb3+, and Tm3+, while no emission was observed for Yb3+ - and Gd3+ -doped TiO2 due to the higher energy of their excited states. RE-doped A/B dual-phase TiO2 nanostructures may find potential applications in various optical and engineering fields, such as LED, optic systems, communication devices, and heterogeneous photocatalysis.