| 1. |
- Tiseanu, C., et al.
(författare)
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In situ Raman and time-resolved luminescence investigation of the local structure of ZrO 2 in the amorphous to crystalline phase transition
- 2012
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:31, s. 16776-16783
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Tidskriftsartikel (refereegranskat)abstract
- The local structure of europium doped and impregnated ZrO 2 in the amorphous state and during crystallization is investigated by in situ X-ray diffraction and in situ Raman, high-resolution transmission electron microscopy (HRTEM) and time-resolved photoluminescence spectroscopy. From Raman spectra excited at three different wavelengths (λ ex= 488, 514, and 633 nm), both phonon modes of ZrO 2 and photoluminescence (PL) corresponding to europium electronic transitions were investigated. In the as-synthetized state, samples were X-ray and Raman amorphous with few tetragonal (also monoclinic) crystallites being observed under HRTEM microscopy. In situ XRD patterns show that all samples crystallize in the tetragonal phase around 450 °C. The time-resolved PL spectra of europium doped and impregnated ZrO 2 show spectral dynamics with time delay after lamp/laser pulse which is assigned to the coexistence of the different amorphous and crystalline components or unreacted europium precursor. From in situ Raman spectra, crystallization was detected at 300-350 °C, monitoring for the characteristic tetragonal-like 5D 0- 7F 2 emission of europium at 606 nm. The ratio of tetragonal to amorphous emission increased abruptly from ca. 2-4% at 300-400 °C to almost 25% at 400-450 °C, whereas at 500 °C the emission is mostly tetragonal. A similar trend was found with the ex situ calcined samples, but relative strong tetragonal emission was observed at lower temperature in the range of 350 to 400 °C.
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| 2. |
- Tiseanu, C., et al.
(författare)
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Order and disorder effects in nano-ZrO 2 investigated by micro-Raman and spectrally and temporarily resolved photoluminescence
- 2012
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 14:37, s. 12970-12981
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Tidskriftsartikel (refereegranskat)abstract
- Pure and europium (Eu 3+) doped ZrO 2 synthesized by an oil-in-water microemulsion reaction method were investigated by in situ and ex situ X-ray diffraction (XRD), ex situ Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), steady state and time-resolved photoluminescence (PL) spectroscopies. Based on the Raman spectra excited at three different wavelengths i.e. 488, 514 and 633 nm and measured in the spectral range of 150-4000 cm -1 the correlation between the phonon spectra of ZrO 2 and luminescence of europium is clearly evidenced. The PL investigations span a variety of steady-state and time resolved measurements recorded either after direct excitation of the Eu 3+ f-f transitions or indirect excitation into UV charge-transfer bands. After annealing at 500°C, the overall Eu 3+ emission is dominated by Eu 3+ located in tetragonal symmetry lattice sites with a crystal-field splitting of the 5D 0- 7F 1 emission of 20 cm -1. Annealing of ZrO 2 at 1000°C leads to a superposition of Eu 3+ emissions from tetragonal and monoclinic lattice sites with monoclinic crystal-field splitting of 200 cm -1 for the 5D 0- 7F 1 transition. At all temperatures, a non-negligible amorphous/disordered content is also measured and determined to be of monoclinic nature. It was found that the evolutions with calcination temperature of the average PL lifetimes corresponding to europium emission in the tetragonal and monoclinic sites and the monoclinic phase content of the Eu 3+ doped ZrO 2 samples follow a similar trend. By use of specific excitation conditions, the distribution of europium on the amorphous/disordered surface or ordered/crystalline sites can be identified and related to the phase content of zirconia. The role of zirconia host as a sensitizer for the europium PL is also discussed in both tetragonal and monoclinic phases.
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| 3. |
- Tiseanu, C., et al.
(författare)
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Temperature induced conversion from surface to bulk sites in Eu 3-impregnated CeO 2 nanocrystals
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 112:1, s. 013521-
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Tidskriftsartikel (refereegranskat)abstract
- Evolution with calcination temperature of Eu 3 sites in CeO 2 nanocrystals is investigated by time-resolved photoluminescence spectroscopy. In the as-synthesized Eu 3 impregnated CeO 2, most of Eu 3 ions reside on surface (S) sites. The Eu 3emission in S sites is broad and short-lived (τ 240 Όs) being dominated by the electric dipole (ED) 5D 0- 7F 2 emission with little evidence for clustering. After calcination (between 500 and 1300 °C), Eu 3 is distributed on surface, cubic and up to three additional crystalline sites. Surface type emission could be detected until 1100 °C. In cubic sites, Eu 3 substitute for the lattice Ce 4 with O h symmetry (O sites). The emission of Eu 3 in O sites is characterized by relative long-lived (τ 1.8-2 ms) and ultra-narrow (FWHM 7 cm -1) magnetic dipole (MD) 5D 0- 7F 1 emission centered at ∌591 nm. Three more crystalline sites are attributed to the oxygen vacancy charge-compensated defects: trigonal with C 3v symmetry (C1 sites) and C2 and C3 sites with C 2v or lower symmetry. Eu 3 in C1 sites exhibits predominant ED 5D 0- 7F 2 emission centered at ∌610 and 632 nm with lifetime of 0.85-1 ms. The C1 sites are assigned to Eu 3-(oxygen vacancy)-associated cubic sites. The O 2- to Ce 4 electronic charge-transfer band sensitizes preferentially the Eu 3 emission in O and, to a less extent, C1 sites but not the S sites. Overall, the results show that the oxygen vacancies are distributed around both Eu 3 and Ce 4 and the Eu 3-oxygen vacancy interaction mode as nearest-neighbour or next-nearest-neighbour depends on the calcination temperature.
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