Evidence of Trivalent Europium Incorporated in Anatase TiO₂ Nanocrystals with Multiple Sites

Abstract

Trivalent europium ions have been successfully incorporated in anatase TiO2 nanocrystals via a sol−gel solvothermal synthesis, in spite of a large mismatch in ionic radius between Eu3+ and Ti4+. The photoluminescence intensity of Eu3+:TiO2 nanocrystals has been significantly improved, which is comparable to that of common red phosphors. Multiple sites of Eu3+ in TiO2 nanocrystals have been identified by the technique of site-selective spectroscopy. Eu3+ ions at two lattice sites exhibit sharp emission and excitation peaks with site symmetries descending from D2d to approximate C2v and D2 as a result of the lattice distortion, whereas Eu3+ ions at disordered site near the surface are analogous to Eu3+ ions located in a glasslike environment. Extended X-ray absorption fine structure is utilized for identifying the local structure and Eu3+ coordination. The luminescence dynamics and crystal-field levels of Eu3+ at different sites have been analyzed. A growth mechanism for the incorporation of Eu3+ in the anatase lattice is also suggested.