Eu3+-doped gadolinium oxyhydroxide Gd1-xEuxOOH crystals were synthesized by the flux method. The X-ray diffraction data for the crystals were well refined assuming a monoclinic structure with the P21/m space group. Gd1-xEuxOOH (x ≤ 0.2) crystals showed strong red emission, and the highest fluorescence quantum yield (Φf) was 0.27, obtained for x = 0.10. Φf decreased rapidly as the Eu3+ content x increased above 0.2, owing to concentration quenching. Analysis with a percolation model indicated three-dimensional energy transfer between the Eu3+ ions.
Eu3+-doped gadolinium oxyhydroxide Gd1−xEuxOOH crystals were synthesized by the flux method. The X-ray diffraction data for the crystals were well refined assuming a monoclinic structure with the P21/m space group. Gd1−xEuxOOH (x ≤ 0.2) crystals showed strong red emission, and the highest fluorescence quantum yield (Φf) was 0.27, obtained for x = 0.10. Φf decreased rapidly as the Eu3+ content x increased above 0.2, owing to concentration quenching. Analysis with a percolation model indicated three-dimensional energy transfer between the Eu3+ ions.
Keywords:Gadolinium Oxyhydroxide; Fluorescence Quantum Yield; Percolation Model
Some rare-earth compounds have excellent optical properties and hence are widely used in high-performance luminescence devices and as catalyst supports. Trivalent rare-earth ions such as Eu3+, Tb3+, and Tm3+-doped in a suitable host material show strong emission based on electron transition between the 4f orbitals. The luminescence properties depend strongly on the chemical composition and crystal structure of the host material. One of the Gd(III) compounds, gadolinium oxyhydroxide (GdOOH), which has a monoclinic structure with the P21/m space group, can be obtained as a stable phase by the thermal dehydration of hydroxide Gd(OH)3 [
In this study, we synthesized Gd1−xEuxOOH crystals and evaluated their luminescence properties, including Φf. In addition, we evaluated the critical Eu3+ concentration in the Gd1−xEuxOOH system by using a percolation model.
Crystals of Eu3+-doped gadolinium oxyhydroxide, Gd1−xEuxOOH (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1), were synthesized by the flux method. Appropriate amounts of Gd2O3 powder (99.9%) and Eu2O3 powder (99.9%) were mixed in an agate mortar for 1 h. Then, the Gd2O3/Eu2O3 mixture was placed in a 20-mL Zr crucible and covered by a layer of a NaOH (15 g)/KOH (5 g) mixture. The crucible was placed in an alumina container, covered with an alumina lid, and heated in an electric furnace at 330˚C for 72 h in ambient atmosphere. Subsequently, the flux was dissolved in large excess of water; the obtained crystals were washed thoroughly in distilled water to remove any residual flux and dried on a hot plate at 80˚C for 1 h.
The chemical composition of the crystals was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES; Seiko Instruments Inc., SPS1700HVR) and an energy-dispersive X-ray spectrometer (EDS; JEOL, EX-23000 BU). The crystal structures were identified by powder X-ray diffraction (XRD; Rigaku, RINT2000). The diffraction data were acquired using CuKα radiation generated at 40 kV and 20 mA, in the 2θ range 10~80˚ at room temperature, and refined by the Rietveld method [
Gd1−xEuxOOH crystallized at the bottom of the crucible. As shown in
The TG curve of the as-grown crystals indicated no apparent weight loss in the temperature range 30~400˚C, indicating that the as-grown crystals did not include Gd(OH)3, which normally decomposes at 270˚C [
the O2− to Eu3+ charge transfer band (CTB) and the band due to energy transfer from Gd3+ to Eu3+ [
The critical Eu3+ concentration was evaluated by using the percolation model, which has been used to explain energy transfer in the solid state [
where Pc, z, and d are the critical concentration, number of neighbor sites, and number of dimensions, respectively [
with three-dimensional energy transfer. This result indicates three-dimensional energy transfer occurs between the Eu3+ ions in this system.
To improve the luminescence properties of the crystals, it is necessary to increase the critical concentration of Eu3+. In general, compounds with a low-dimensional arrangement of rare-earth ions show a high critical concentration. On the basis of the results obtained using the percolation model in this study, we state that two-dimensional models have a higher critical concentration (0.50 and 0.33) than do three-dimensional models. When the increase in the distance between the rare-earth layers restrains interlayer energy transfer, the critical concentration of Eu3+ in this system can increase. Therefore, excellent luminescence properties can be obtained by co-doping larger ions in the rare-earth sublattice with a high level of Eu3+ doping. The high alkali durability of the Gd1−xEuxOOH crystals is confirmed by the fact that they are stable in NaOH/KOH solution at 330˚C.
Gd1−xEuxOOH crystals with a maximum length of 1.2 mm were synthesized by the flux method. The Gd1−xEuxOOH (x ≤ 0.2) crystals showed strong red emission, with the maximum Φf of 0.27 for x = 0.10. The critical Eu3+ content was 0.2, because Φf decreased rapidly beyond this level owing to concentration quenching. This critical content was the closest to that estimated by a percolation model for Eu3+-Eu3+ interactions within 0.404 nm, wherein Eu3+ had eight neighbors, indicating three-dimensional energy transfer between the Eu3+ ions in the rare-earth sublattice. The synthesized crystals showed excellent alkali durability and hence may be used as a high-efficiency host material in unique photomedical applications such as photochemical internalization and photodynamic therapy.
This study was supported by JSPS KAKENHI Grant Number 21560696, 24560827.