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

Abstract

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 ZrO2 NCs were dominantly of a tetragonal phase (t-ZrO2) with a small proportion of monoclinic ZrO2 (m-ZrO2). The high purity of t-ZrO2 NCs could be synthesized with more Eu3+ doping. It was found that the as-obtained ZrO2 NCs 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-ZrO2 NCs 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−1 at 230 K.

Epitaxial Core/Shell Nanocrystals of (Europium-Doped) Zirconia and Hafnia

“…49−52 While excitation of the ZrO 2 :Eu/ZrO 2 core/shells at 240− 260 nm yields only the 4f−4f transitions of Eu 3+ , excitation at 300 nm generates an additional broadband contribution ranging from 350 to 600 nm related to defect emission in ZrO 2 (Figure S14). 31,53−55 The coexistence of broadband and europium emission in ZrO 2 (with 0.4% Eu doping) was recently exploited for ratiometric fluorescence thermometry in the range from 130 to 230 K. 56 We did not observe this strong broadband emission for the ZrO 2 :Eu cores with high Eu doping (9%) (see Figure S14), implying that the presence of Eu ions suppresses the defect emission. This suggests that for the core/shell particles, the Eu ions remain located in the core during the shelling, since the presence of Eu ions in the shell would suppress the ZrO 2 defect emission (either by preventing the defect from forming or by short-range energy transfer to Eu).…”

“…While excitation of the ZrO 2 :Eu/ZrO 2 core/shells at 240–260 nm yields only the 4f–4f transitions of Eu 3+ , excitation at 300 nm generates an additional broadband contribution ranging from 350 to 600 nm related to defect emission in ZrO 2 (Figure S14). ,− The coexistence of broadband and europium emission in ZrO 2 (with 0.4% Eu doping) was recently exploited for ratiometric fluorescence thermometry in the range from 130 to 230 K . We did not observe this strong broadband emission for the ZrO 2 :Eu cores with high Eu doping (9%) (see Figure S14), implying that the presence of Eu ions suppresses the defect emission.…”