Hydrothermal Synthesis of Well-Defined Red-Emitting Eu-Doped GdPO4 Nanophosphors and Investigation of Their Morphology and Optical Properties

Abstract

Rare-earth-doped GdPO4 nanoparticles have recently attracted much scientific interest due to the simultaneous optical and magnetic properties of these materials and their possible application in bio-imaging. Herein, we report the hydrothermal synthesis of GdPO4:Eu3+ nanoparticles by varying different synthesis parameters: pH, <Gd>:<P> molar ratio, and Eu3+ concentration. It turned out that the Eu3+ content in the synthesized nanoparticles had little effect on particle shape and morphology. The synthesis media pH, however, has showed a pronounced impact on particle size and distribution, i.e., the nanoparticle length can be adjusted from hundreds to tens of nanometers by changing the pH from 2 to 11, respectively. Increasing the <Gd>:<P> molar ratio resulted in a decrease in nanoparticle length and an increase in its width. The temperature-dependent measurements in the 77–500 K range revealed that the GdPO4:50%Eu3+ sample maintains half of its emission intensity, even at room temperature (TQ1/2 = 291 ± 19 K).

Judd‐Ofelt analysis and optical properties of Eu3+‐doped GdPO4 phosphors synthesized by combustion method

“…Figure 3a shows that the characteristic absorptions at 1620 cm −1 and 3466 cm −1 were caused by the stretching and deformation vibrations of O─H groups in GdPO 4 ⋅H 2 O:Eu 3+ . 19 In Figure 3b, the absorptions at 1620 cm −1 and 3466 cm −1 are decreased by increasing the calcination temperature while the intensities of the other bands are almost unchanged. This proves that there is dehydration of hexagonal struc-ture (GdPO 4 ⋅H 2 O:Eu 3+ ) to form the high purity monoclinic phase GdPO 4 .…”

“…The band at 1075 cm −1 represents the asymmetric stretching vibration of the PO 4 3− group and the peaks observed at 620 cm −1 and 547 cm −1 are ascribed to the O─P─O bending vibration. Figure 3a shows that the characteristic absorptions at 1620 cm −1 and 3466 cm −1 were caused by the stretching and deformation vibrations of O─H groups in GdPO 4 ·H 2 O:Eu 3+ 19 . In Figure 3b, the absorptions at 1620 cm −1 and 3466 cm −1 are decreased by increasing the calcination temperature while the intensities of the other bands are almost unchanged.…”

“…The peak at 273 nm and the line group in the range from 300 to 320 nm are attributed to the 8 S 7/2 -6 I 11/2 and 8 S 7/2 -6 P J transition in Gd 3+ ions. 18,19 The appearance of the Gd 3+ excitation bands in the PLE spectrum of Eu 3+ ions shows the energy transfer from Gd 3+ to Eu 3+ in the GdPO 4 :Eu 3+ phosphor. In other words, for GdPO 4 :Eu 3+ , the Gd 3+ ions can cause the luminescence enhancement of Eu 3+ ions upon several excitation conditions (e.g., 273 and 310 nm).…”

“…Thus, its intensity depends strongly on the properties of the ligand field such as the asymmetry and the polarization whereas the intensity of the 5 D 0 → 7 F 1 transition is almost independent on those. 19,22 For this reason, the intensity ratio (R) of the 5 D 0 → 7 F 2 band to the 5 D 0 → 7 F 1 is used to estimate the ligand asymmetry as well as the covalence of the Eu 3+ -ligand bond. In our case, the R ratio varies slightly in the region of 0.725-0.765 when the Eu 3+ concentration is increased from 0.1 to 9.0 mol%.…”

“…According to this method, the 5 D 0 → 7 F 1 MD and 5 D 0 → 7 F 2,4,6 ED transitions are used for calculating the Ω λ parameters 33 . It is known that the transition probability of the 5 D 0 → 7 F 1 allowed MD transition is calculated by following expression: 19 $$$$\begin{equation}{A}_{M{\mathrm{D}}}({}^5{D}_0 \to {}^7{F}_1) = \frac{{64{\pi }^4{\nu }^3{n}^3{S}_{M{\mathrm{D}}}}}{{3h(2J + 1)}}\end{equation}$$$$where J is the total angular momentum of the 5 D 0 state ( J = 0), ν is the energy of the 5 D 0 → 7 F 1 transition, n is the refractive index of host matrix (refractive index of GdPO 4 was 1.97 33 ), and S MD is magnetic dipole line strength which depends only on rare earths and is almost independent of the compound 22 . Thus, the A MD ( 5 D 0 → 7 F 1 ) parameter of GdPO 4 :Eu 3+ can obtain through the following relationship: $$$$\begin{equation}\frac{{{A}_{M{\mathrm{D}}}}}{{{A}_{0M{\mathrm{D}}}}} = {\left( {\frac{n}{{{n}_0}}} \right)}^3\end{equation}$$$$where A 0MD and n 0 are the transition probability and refractive index of any published materials.…”