A. Infrared analysis

To determine the chemical evolution of the sol–gel process for the synthesis of Eu³⁺, Bi³⁺ codoped Lu₂O₃ powders were obtained, as were the Fourier transformed infrared spectra at different annealing temperatures (100–900 °C) of 2.5 Eu³⁺ at.%, X Bi³⁺ at.% (X = 1.0, 2.5) [Figs. 1(a) and 1(b), respectively]. As can be observed, for the xerogel dried at 100 °C, there was a strong presence of the OH⁻ group, which is due to the presence of water and alcohol groups, characterized for the bands at 3400 cm⁻¹ (ν), 1650 cm⁻¹ (δ), and 750 cm⁻¹ (δ), ascribed to the O–H stretching (ν) and deformation (δ) vibrations. Indeed, since its occurrence remained until 900 °C, it is possible to assume that this group is trapped structurally in the host. However, as can be observed, this OH⁻ up to 100 and 200 °C had strong absorption bands at 1090 and 850 cm⁻¹, which can be attributed to a symmetrical stretching of C–O and deformation vibrations of C–O in CO₃²⁻ respectively. This stretching and vibration is brought about by the thermal decomposition of the carbon groups of acetic acid and DEG. Additionally, a small peak at 820–880, which is a characteristic of the C–C bond, can also be attributed particularly to the DEG decomposition.^{Reference Ksapabutr, Gulari and Wongkasemjit29} However, over 300 °C, all of these carbon-based bands were almost absent, thus eliminating the possibility of carbonates formation. On the other hand, the absorption band localized at 1380 cm⁻¹ is ascribed to a N–O stretching vibration^{Reference Chi and Chuang30} of NO₃⁻, which remained until 500 °C, whereas at 600 °C it was completely eliminated, indicating that beyond this temperature, only oxidized groups can be expected. Finally, the bands occurring at around 580 and 490 cm⁻¹, observed from 600 °C, are attributed to the Lu–O stretching vibrations of cubic Lu₂O₃ (an Lu₂O₃ host lattice vibration).^{Reference McDevitt and Baun31} This indicates that its crystallization was just beginning at this annealing temperature as confirmed by XRD and HSTEM observations.

FIG. 1.

Infrared spectra of Lu₂O₃: 2.5 at.% Eu³⁺, X at.% Bi³⁺ nanopowders at different annealing temperatures: (a) X = 1.0, (b) X = 2.5.

B. Structural and morphological studies

Figures 2(a) and 2(b) show the crystalline evolution of the Eu³⁺, Bi³⁺ codoped Lu₂O₃ powders at different annealing temperatures (600–900 °C) of 2.5 Eu³⁺ at.%, and two different Bi³⁺ at.% contents (1.0, 2.5), respectively. As observed for both samples, at 600 °C, the crystallization process occurred; however, a highly amorphous behavior remained. As the annealing temperature reached 700 °C, the nanopowders presented the cubic Lu₂O₃ structure (JCPDS 431021) with a spatial group Iā3 (lattice parameter 10.391 Å). It is important to note that the crystallization temperature was considerably lower than those reported (≈1100 °C) for solid-state reactions for Lu₂O₃ powders.^{Reference Zhang, Yang, Lu and Shi32,Reference Wang, Zhang and Wang33} Along with the increase of the annealing temperature up to 900 °C, the diffraction peaks became sharper, indicating that the crystallite size of the sample increases. There was no difference between both Bi³⁺ contents, since no other possible phases could be detected. The effect of the deformation of the lattice parameter as a function of the dopant ions, Eu³⁺ (2.5 at.%) and Bi³⁺ (0–3.0 at.%) is shown in Fig. 3. As observed, there was a slight increase for the monodoped sample, which increased from 10.402 to 10.481 Å at 2.5 at.% and resulted from the effect of the Bi³⁺ content. Table I shows the calculated crystallite sizes according to Scherer’s formula D = 0.9λ/β cosθ,^{Reference Cullity34} taking into account the broadening line of the diffracted peak, due to the effect of crystal size, where D is the crystal size of the powder, λ (0.15406 nm) is the wave length of the diffracted x-ray, β is the full-width radiation at half-maximum of the peak, and θ is the Bragg angle of the diffracted x-ray. The crystallite sizes varied from 6.2 ± 1.3 and 5.7 ± 2.1 nm at 600 °C to 20.2 ± 1.8 and 21.1 ± 1.9 nm at 900 °C for both Bi³⁺ contents. In light of this, there is no evidence of possible changes due to the composition in the crystal structure.

FIG. 2.

Structural evolution of Lu₂O₃: 2.5 at.% Eu³⁺, X at.% Bi³⁺ nanopowders as a function of the thermal treatment: (a) X = 1.0, (b) X = 2.5.

FIG. 3.

Lattice parameter of Lu₂O₃: 2.5 at.% Eu³⁺, X at.% Bi³⁺ (X = 0.0–3.0) nanopowders as a function of Bi³⁺ content.

TABLE I.

Crystallite size as a function of heat treatment temperature.

A TEM bright field micrograph of Lu₂O₃:Eu³⁺ 2.5 at.%, Bi³⁺ 1.0 at.% nanopowders annealed at 800 °C is shown in Fig. 4(a). As can be easily observed, the nanocrystallites presented a rounded morphology. The average size determined by the TEM observations was ≈20.2 nm, which is slightly higher than the XRD results. Figure 4(b) shows the HRTEM micrograph of a selected particle. The interplanar planes visible in the Eu³⁺, Bi³⁺ codoped Lu₂O₃ nanopowders were indexed by means of Fourier transform [Fig. 4(c)] to determine the interplanar distances, which were calculated as 2.94 and 2.56 Å, corresponding to the (222) and (400) planes, respectively [Fig. 3(d)], of the cubic Lu₂O₃ structure.

FIG. 4.

(a) TEM bright-field micrograph of Lu₂O₃:2.5 at.% Eu³⁺, 1.0 at.% Bi³⁺ nanopowders annealed at 800 °C, (b) HRTEM micrograph of a Lu₂O₃:2.5 at.% Eu³⁺, 1 at.% Bi³⁺ particle, (c) Fourier transform, and (d) (222) and (400) planes.

C. Luminescent properties

The evaluation of the luminescent properties was conducted on the Eu³⁺, Bi³⁺ codoped Lu₂O₃ nanopowders at a fixed Eu³⁺ level (2.5 at.%) and different Bi³⁺ levels (from 0.1 to 3.0 at.%) at different annealing temperatures (700–900 °C). Figure 5 shows the excitation spectra (611 nm) of the samples annealed at 900 °C with different Bi³⁺ doping levels. As can be observed, the common band was present from 230 to 270 nm, corresponding to the Eu³⁺–O²⁻ charge-transfer transition, as well as the ⁷F₀ → ⁵L₆ (395 nm), ⁷F₀ → ⁵D₂ (410 nm),^{Reference Zhang, Chen and Guo6,Reference Liu, Li, Xie, Hirosaki, Xu and Huang35,Reference Wang, Zhang, Lin, Baogui, Yibing and Min36} and ⁷F₀ → ⁵D₃ (412 nm)^{Reference Hreniak, Zych, Kepinsky and Strek27,Reference Takeshita, Isoha, Sawayama and Nikura37} transitions of Eu³⁺ in a Lu₂O₃ matrix. In addition to this, a strong broad band was observed in the range of 300–380 nm, which can be attributed to the ¹S₀ → ³P₁ transitions of the Bi³⁺,^{Reference Real, Vallet, Flament and Schamps38} at the C₂ site.^{Reference Jacobsohn, Blair, Tornga, Brown, Bennett and Muenchansen39} It can be observed that this absorption band increased with the increase of Bi³⁺ concentration, showing maximum efficiency at the 1.0 at.% doping level.

FIG. 5.

Excitation spectra (λ_em = 611 nm) of Lu₂O₃: 2.5 at.% Eu³⁺, X at.% Bi³⁺ (X = 0.1–3.0) nanopowders.

The emission spectra of Eu³⁺, Bi³⁺ codoped Lu₂O₃ nanopowders excited at 323 nm are shown in Fig. 6(a) for a monodoped sample (Eu³⁺ 2.5 at.%) and 1.0 Bi³⁺ at.% codoped sample, both annealed at 900 °C. As can be observed, the results presented the typical luminescence spectrum of Eu³⁺ ions in the cubic Lu₂O₃ host lattice, consisting of a group of five sharp peaks from 570 to 715 nm, ascribed to the ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) transitions of the Eu³⁺ ion. The most intense line, around 611 nm, corresponded to the transition of the ⁵D₀ → ⁷F₂ level of the Eu³⁺ ion, occupying the C₂ sites in the Lu₂O₃ host cubic lattice. Additionally, there were minor emission lines corresponding to the ⁵D₀ → ⁷F₀ (≈585 nm), ⁵D₀ → ⁷F₁ (≈589, ≈595, and ≈601 nm), ⁵D₀ → ⁷F₃ (≈651 and ≈664 nm), and ⁵D₀ → ⁷F₄ (≈690 and ≈695 nm) transitions. This predominant 611 nm Eu³⁺ emission, could be apparently caused by the population of split terms ⁷F_2,3,4 of the Eu³⁺ ion, which results from the change in its crystallographic surrounding upon penetration of Bi³⁺ ions into the matrix lattice; this has already been seen in a Y₂O₃ codoped matrix.^{Reference Strel’tsov, Dmitrienko, Akmaeva, Kudryavstev, Dmitrienko and Razumov20} It is important to note that the intensity of the codoped sample was nearly 400% higher than for the monodoped sample, thus indicating that a significant energy transfer process occurred between Bi³⁺ and Eu³⁺. This means that Bi³⁺ presents a sensitizer character of Eu³⁺.

FIG. 6.

(a) Emission spectra (λ_exc = 323 nm) of Lu₂O₃: 2.5 at.% Eu³⁺, 1.0 at.% Bi³⁺. (b) Emission spectra (λ_exc = 323 nm) of Lu₂O₃: 2.5 at.%Eu³⁺, X at.% Bi³⁺ (X = 0.0–3.0) nanopowders annealed at 900 °C, as a function of the Bi³⁺ at.% content.

Figure 6(b) shows the influence of Bi³⁺ concentration in the luminescence spectra, monitoring the 611 nm Eu³⁺ emission. As observed, all the samples presented the same characteristics of the previous analysis, while it is stated that the intensity is highly dependent on the Bi³⁺ doping level. With the increase of the Bi³⁺ content, the emission intensity of the 611 nm peak increased rapidly until 1.0 at.%. Above this concentration, the intensity diminished. The same results were observed for the excitation spectra shown in Fig. 5. On the other hand, Fig. 7 presents the emission spectra of the same 1.0 at.% Bi³⁺ sample, in a broader range (300–800 nm) thus making it possible to observe the bluish emission lines (³P₁→¹S₀) of Bi³⁺ in the range from 400 to 600 nm. The intensity of this emission is insignificant compared with the Eu³⁺ reddish emission; the inset shows an enlarged view of the Bi³⁺ emission, as well as the dependence of this light yield function on the Bi³⁺ content. However, this emission range overlapped the ⁷F₀ → ⁵L₆ (395 nm), ⁷F₀ → ⁵D₂ (466 nm) Eu³⁺, and ⁷F₀ → ⁵D₃ excitation bands, and therefore an energy transfer process occurred from Bi³⁺ to Eu³⁺ in the Lu₂O₃ codoped nanopowders, with a similar mechanism stated for Y₂O₃^{Reference Wu, Liang, Chen, Liu and Li12} and Gd₂O₃.^{Reference Dhananjay, Nagabhushana, Nagabhushana, Sharma, Rudraswamy, Suriyamurthy, Shivakumara and Chakradhar40} The proposed energy transfer processes and luminescence mechanism between Bi³⁺ and Eu³⁺ in Bi³⁺, Eu³⁺ codoped Lu₂O₃ are shown in Fig. 8. First, the phosphors were excited at 330 nm, where the Bi³⁺ ion absorbed the energy according to the ¹S₀→³P₁ Bi³⁺ transition. Later, the Bi³⁺ ions released the energy radiatively to the Eu³⁺ ions. Indeed, according to ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₂, and ⁷F₀ → ⁵D₃, Eu³⁺ emits light by the predominant ⁵D₀ → ⁷F₂ transition.

FIG. 7.

(a) Enlarged view of the emission spectra (λ_exc = 323 nm) of Lu₂O₃: 2.5 at.% Eu³⁺, 1.0 at.% Bi³⁺ nanopowders, at the Bi³⁺ bluish emission, (b) Light yield of the Bi³⁺ emission as a function of the Bi³⁺ at.% content.

FIG. 8.

The luminescence and energy transfer processes of Eu³⁺ and Bi³⁺ in Lu₂O₃: Eu³⁺, Bi³⁺ nanopowders.

Finally, Fig. 9 shows the relative intensity of the ⁵D₀ → ⁷F₂ emission at 611 nm as a function of the doping content and the annealing temperature. As observed, since at 700 °C the nanopowders presented a complete crystallization of the cubic structure, the increase in the emission intensity can be explained in terms of a better crystal refinement as a consequence of the thermal process. It may have also been due to the high temperature of the organic molecules present in samples, which are remnants of the solvents used during the sol–gel process. In addition, the specific removal of hydroxyl groups OH⁻ [Figs. 1(a) and 1(b)] increases the emission intensity since it acts like a quencher group which promotes nonradiative process in comparison to the radiative one.^{Reference Yulia, Yermolayeva, Tolmachev, Dobrotvorskaya and Vovk41,Reference Phaomei, Singh, Singh and Ningthoujam42} The same conclusions can be obtained by analyzing the effect of the Bi³⁺ content, since at the three annealing temperatures the same behavior was observed for 900 °C with the maximum peak at 1.0 Bi³⁺ at.%. However, the differences between the different annealing temperatures in the light yield temperatures were not significant.

FIG. 9.

Emission intensity (λ_exc = 323 nm, λ_em = 611 nm) of Lu₂O₃: 2.5 at.% Eu³⁺, 1.0 at.% Bi³⁺ powders as a function of the Bi³⁺ content and the annealing temperature.