Novel Photoluminescence and Optical Thermometry of Solvothermally Derived Tetragonal ZrO2:Ti4+,Eu3+ Nanocrystals

In this paper, we report on the solvothermal preparation and detailed characterization of pristine and intentionally doped zirconium dioxide (ZrO2) nanocrystals (NCs, ~5 nm) with Eu3+or Ti4+/Eu3+ions using alkoxide precursors. The results indicated that the ZrO2NCs were dominantly of a tetragonal phase (t-ZrO2) with a small proportion of monoclinic ZrO2(m-ZrO2). The high purity of t-ZrO2NCs could be synthesized with more Eu3+doping. It was found that the as-obtained ZrO2NCs contain some naturally present Ti4+ions originating from precursors, but were being overlooked commonly, and some carbon impurities produced during synthesis. These species showed distinct photoluminescence (PL) properties. At least two types of Eu3+, located at low- and high-symmetry sites (probably sevenfold and eightfold oxygen coordination), respectively, were demonstrated to build into the lattice structure of t-ZrO2NCs together. The cationic dopants were illustrated to be distributed non-randomly over the sites normally occupied by Zr, while Ti impurities preferentially occupied the sites near the low-symmetry site of Eu3+, yielding efficient energy transfer from the titanate groups to the neighboring Eu3+. Luminescence nanothermometry could measure temperature in a non-contact and remote way and could find great potentials in micro/nano-electronics, integrated photonics, and biomedicine. On the basis of the dual-emitting combination strategy involving the white broadband CT (Ti3+→O−) emissions of the titanate groups and red sharp Eu3+emissions, t-ZrO2:Eu3+nanophosphors were demonstrated to be ratiometric self-referencing optical thermometric materials, with a working range of 130–230 K and a maxima of relative sensitivity of ~1.9% K−1at 230 K.