Preparation and Optical Properties of La₃PO₇:Eu³⁺ Nanophosphors Synthesized by Combustion Method

Abstract

Eu³⁺ doped La₃PO₇ nanoparticles were successfully synthesized under facial combustion method. The nano-phosphors then were characterized by some techniques including X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), photoluminescence spectrum (PL), and lifetime decay. SEM and TEM micrographs reveal exhibit of the La₃PO₇: Eu³⁺ nanopowders in spherical morphology with an average size of about 20 nm, matching with Scherrer’s formula calculation. Otherwise, the influence of annealing temperature and concentration of Eu³⁺ ions on optical characteristics of Eu³⁺ was doped La₃PO₇, and optical properties were investigated too. The photoluminescence shows the strong red emission originating from the ⁵D₀ → ⁷F_J (J = 1, 2, 3, and 4) transitions of Eu3+ and the 614 nm emission from the ⁵D₀ → ⁷F₂ electronic dipole transition is dominant.

1. Introduction

Phosphates of rare-earth ions have a wide range of potential applications because of their unique properties such as excellent luminescence and potential applications in luminescent devices. Primarily, applications focus on display equipment including lighting, field emission display (FED), cathode ray tubes (CRT), plasma display panels (PDP), biochemical probes, and catalysts.^1–6) In recent studies about these materials, many factors affect to the luminescence including particular size, morphology, crystallization, local symmetry, growth mechanism, doped proportion, and synthesis methods. In the La₂O₃–P₂O₅ system, there are seven intermediate compounds were suggested, i.e., La₅PO₁₀, La₃PO₇, La₇P₃O₁₈, LaPO₄, La₂P₄O₁₃, LaP₃O₉, and LaP₅O₁₄.⁷⁾ Among them, nano LaPO₄ materials are of great interest.^8,9) In 2004, LiXinYu and HongweiSong incorporated Eu³⁺ on LaPO₄ by the hydrothermal method.⁸⁾ On the other way, Ivica Vujcic and Tarama Gavrilovic prepared this material by the solid-state way in 2018.⁹⁾ As an activator, the Eu³⁺ ion has been investigated frequently due to its excellent fluorescent properties. In practical applications, the characteristic emission from LaPO₄: Eu³⁺ is composed of almost equal contributions from the ⁵D₀ → ⁷F₁ and the ⁵D₀ → ⁷F₂ transitions.^8,9) Therefore, LaPO₄: Eu³⁺ is an orange-red emission material instead of red colour purity. This changing depends on the crystal symmetry of the matrix. If the Eu³⁺ ions occupy the sites together inversion centers such as LaPO₄: Eu³⁺, orange emission transition from ⁵D₀ → ⁷F₁ of Eu³⁺ is dominant. As a result, pure red luminescence of ⁵D₀ → ⁷F₂ can obtain when Eu³⁺ occupies a site without an inversion center. Simillary, S. Lu and Y. Jin has demonstrated these special properties of La₃PO₇: Eu³⁺ nanomaterial.^10,11) Compound La₃PO₇ is a monoclinic structure, and the Eu³⁺ ion in this host lattice has no inversion symmetry. This phosphor warrants strong red fluorescence derived from ⁵D₀ → ⁷F₂ transition of Eu³⁺ upon UV excitation 266 nm.

However, there has been lacking the number of research on La₃PO₇ material until now. For a deeper understanding of this compound, we did an overcome of study in La₃PO₇: Eu³⁺ nanoparticles. In that, there are some methods for making the materials such as solid phase and co-precipitation and combustion method also were completion. In the study, red phosphor La₃PO₇: Eu³⁺ has been prepared by the combustion method. The photoluminescence (PL) emission intensity of Eu³⁺ ions depended on annealing temperature and concentration of the Eu³⁺ ions. All the results reveal that La₃PO₇: Eu³⁺ is a promising red-emitting phosphor for potential applications.

2. Experimental

La₂O₃ (Aldrich 99.99%), Eu₂O₃ (Aldrich 99.99%), urea (Merck 99.99%), HNO₃ (Merck, PA), (NH₄)₂HPO₄ (Merck, PA) were used as starting materials. La(NO₃)₃ and Eu(NO₃)₃ were prepared by dissolving La₂O₃ and Eu₂O₃ in nitric acid. La_1−xEu_x(NO₃)₃ was obtained from La(NO₃)₃ and Eu(NO₃)₃ with an appropriate ratio. Meanwhile, (NH₄)₂HPO₄ is a phosphorus source. The ratio of La(NO₃)₃ to (NH₄)₂HPO₄ was undoubtedly controlled to prevent the formation of other phases during the combustion process. Urea has been used as an organic fuel for combustion. Firstly, the mixing of La(NO₃)₃ and Eu(NO₃)₃ in a suitable ratio was heated at 80°C until drained condition. Then, add 5 mL distilled water and the appropriate amount urea, stirring for 30 min. Secondly, (NH₄)₂HPO₄ was added, vigorously stirring for 60 min at 50°C. Finally, the solution was heated until a lot of white gel. This precursor was calcined at 500, 600, 700, 800, and 900°C for 1 h in the air. In this process, the urea was oxidized by nitrate ions.

The crystalline structure was obtained on an Advance-Bruker D8 X-ray diffractometer using Cu K_α radiation resource (λ = 1.5406 Å), and the scan range was set from 20° to 80° (2θ) with a step of 0.03°s⁻¹. The thermal decomposition of precursor samples was investigated by simultaneous thermogravimetry and differential thermal analysis (TG-DTA) on a TG-DTA/DSC (France) instruments in the air with a heating rate of 10°C min⁻¹ from room temperature to 900°C in Vietnam Academy of Science and Technology. The morphology of the final product was examined using a scanning electron microscope (SEM) (HITACHI S-4800). Excitation and emission spectra were determined by a Cary Eclipse fluorescence spectrophotometer equipped with an 80 Hz xenon lamp as the excitation source.

3. Results and Discussion

For understanding the chemical reaction occurring during the calcination, thermal decomposition of the precursor samples was recorded by TG-DTA technique. Figure 1 demonstrated four weight-loss stages in the TG curve from 20 to 1000°C, indicated degradation of the precursor samples and the formation of La₃PO₇. In details, in range of 20–130°C, the first weight loss of 10.8 mass% due to the evaporation of water, similar to endothermal peak, recorded at 122.3°C on the DTA curve. Otherwise, the second step of weight loss (9.4 mass%) with an exothermal peak at 150°C revealed excessive urea burning with oxygen. Finally, the third step of weight loss (52.5 mass%) with two endothermal peaks at 240 and 321.7°C can be contributed by the decomposition of ammonium dihydrogen phosphate, ammonium nitrate, respectively. Beside, an exothermal peak at 340°C was observed as a result of the burning reaction of the nitrate, (NH₂)₂CO, and oxygen.

Fig. 1

TG-DTA curves of the as-synthesized precursor.

The crystal phase of La₃PO₇:Eu³⁺ nanoparticles was investigated by X-ray diffraction (XRD) using a Cu target radiation resource (λ = 1.5406 Å). Figure 2 shows the XRD pattern of La₃PO₇: Eu³⁺ phosphors particles using various annealing temperatures at 500, 600, 700, 800, and 900°C. The results pointed out that nanopowders showing in good crystallinities with a pure monoclinic phase, according to the results of JCPDS card 49-1023. Their lattice parameters are a = 13.085 Å, b = 13.585 Å, and c = 12.429 Å, respectively. When annealing temperature increased, the diffraction peaks became little by little narrow, sharp, and increased. There were no unwanted diffraction peaks in the XRD patterns. This proved that Eu³⁺ ions incorporated into La₃PO₇ completely. This result was perfectly agreement with reports of Bing Yan and Jianfeng Gu (2009).¹²⁾ The grains size (D) of La₃PO₇: Scherrer’s formula calculated Eu³⁺ nanoparticles:   

\begin{equation*} D = \frac{0.89\lambda}{\beta\,\mathit{cos}\,\theta} \end{equation*}

Fig. 2

XRD patterns of La₃PO₇ samples annealed at different temperatures.

Where β is the full width in radians at half-maximum (FWHM) of the peak at 2θ = 29.04°, and θ is the Bragg angle of the XRD peak.

The grain size of La₃PO₇: Eu³⁺ phosphors particles slight increased when annealing temperatures increased. These sizes were 7, 9, 11, 18, and 23 nm for annealing temperatures of 500, 600, 700, 800, and 900°C, respectively.

The surface morphology of La₃PO₇:5 mol%Eu³⁺ nanocrystals annealing at 500, 800, and 900°C were recorded by the SEM images (Fig. 3). It can be seen the La₃PO₇:5 mol%Eu³⁺ particles prepared by the combustion method have an almost spherical shape, smooth surface, well-order in the range of 10–20 nm. The morphology of La₃PO₇:5 mol%Eu³⁺ with monoclinic phase is present similar characteristics to the La₃PO₇: Eu³⁺ particles like works of Ye Jin and co-worker.¹¹⁾

Fig. 3

SEM images of La₃PO₇:5 mol%Eu³⁺ phosphor particle synthesized by combustion method at different temperatures.

Figure 4 showed the representative HR-TEM images of La₃PO₇:5 mol%Eu³⁺ nanoparticles. In that, Fig. 4(a) showed an aggregate composed of many nanoparticles. The sample was composed of relatively well-order monoclinic particles while the size of the average nanoparticles is about 18–20 nm. This matches the average crystallite with the XRD line profile analysis. The crystal planes of the nanoparticle found in Fig. 4(b). The interplanar distance is determined to be 0.306 nm, which corresponds to the $(\bar{4}11)$ crystal plane of monoclinic La₃PO₇. This result confirms that the samples are of high purity, and the material obtained in nanometer size.

Fig. 4

High-resolution TEM image of La₃PO₇:5 mol%Eu³⁺ for annealing temperature of 800°C.

The photoluminescence excitation spectrum (PLE) by monitoring the emission at 614 nm was performed in Fig. 5. All of the curves behave very similar characters. The PLE spectra revealed broadband and some sharp peaks. A band extended from 200 to 300 comes from the transition induced by charge transfer. On the other hand, the broad group is assigned to electron delocalization from an oxygen 2p orbital to an empty 4f orbital of Europium ions. The sharp lines are attributed to the f-f transitions of Eu³⁺ ion corresponding to ⁷F₀ → ⁵H₃ at 319 nm, ⁷F₀ → ⁵D₄ at 362 nm, ⁷F₀ → ⁵G₄ at 382 nm, ⁷F₀ → ⁵L₆ at 393 nm, ⁷F₀ → ⁵D₃ at 413 nm, ⁷F₀ → ⁵D₂ at 465 nm and ⁷F₀ → ⁵D₁ at 532 nm.¹³⁾

Fig. 5

Room-temperature photoluminescence excitation spectrum (PLE) (λ_em = 614 nm) of La₃PO₇:Eu³⁺.

Annealing temperature dependence of La₃PO₇ phosphor emission

Under 266 nm laser excitation, emission spectra of the annealed samples were recorded at different temperatures 500, 600, 700, 800, 900°C (see in Fig. 6). Collected spectra are composed of several sharp lines, ascribed to the ⁵D₀ → ⁷F₀ 578 nm, ⁵D₀ → ⁷F₁ 591 nm, ⁵D₀ → ⁷F₂ 614 nm, ⁵D₀ → ⁷F₃ 653 nm and ⁵D₀ → ⁷F₄ 702 nm. Although the main peak positions are identical to each other, the intensity peaks are different. In these samples, the intensity of emission transition ⁵D₀ → ⁷F₂ of Eu³⁺ is stronger than ⁵D₀ → ⁷F₁. It was well known that Eu³⁺ is sensitive to local symmetry. According to the rules of selection, the magnetic dipole transition is permitted, and the electric dipole transition is forbidden. The red emission at 614 nm from ⁵D₀ → ⁷F₂ transition is a typical electronic dipole transition, while the orange emission at 591 nm from ⁵D₀ → ⁷F₁ transition indicates an ideal magnetic dipole transition. When Eu³⁺ ion occupies the site with the inversion center, the ⁵D₀ → ⁷F₁ transition would be relatively strong, whereas the ⁵D₀ → ⁷F₂ transition is forbidden and very weak. As shown in Fig. 6, the ⁵D₀ → ⁷F₁ transition is prominent in all change, that means Eu³⁺ ion occupied at the site without inversion symmetry. As a consequently, the study all matches to previous publications (Wei-Ning Wang¹⁴⁾ and Bing Yan¹²⁾).

Fig. 6

Emission spectra (λ_ex = 266 nm) of La₃PO₇:Eu³⁺ samples annealed at different temperatures (500 (a), 600 (b), 700 (c), 800 (d), 900°C (e)). Inset: the dependence of the ratio of ⁵D₀ → ⁷F₂ (614 nm) to ⁵D₀ → ⁷F₁ (591 nm) on temperatures.

Figure 6 shows the emission spectra of La₃PO₇: 5 mol%Eu³⁺ samples, which annealed at different temperatures. It is noticeable that the intensities of the emission bands increase with increasing annealing temperature. It means the samples prepared at higher annealing temperature improved luminescence properties. The dependence of the ratio of the relative intensity of ⁵D₀ → ⁷F₂ (614 nm) to ⁵D₀ → ⁷F₁ (591 nm) on the annealing temperature was shown inset Fig. 6. It was found that the increase of annealing temperature from 500 to 900°C causes significantly promote emission intensities. Moreover, the emission intensity is not much different since the temperature exceeds over 800°C.

Concentration dependence of La₃PO₇ phosphor emission

Figure 7 indicated the emission intensities’ dependence on different Eu³⁺ concentrations in La₃PO₇: Eu³⁺ samples annealed at 800°C. The red emission intensity increases with increasing Eu³⁺ level from 1 to 7 mol% and then begins to decrease with the Eu³⁺ increasing concentration to 9 mol%. In the initial stage, an increase in doped concentration, the number of luminescent centers increases. However, when the Eu³⁺ concentration continues increasing, the internuclear distance of Eu³⁺–Eu³⁺ is reduced. In other words, the distance of luminescence centres decreases, that causes the non-radiative energy migration. Since the non-radiative transition occurred, the concentration of Eu³⁺ ions has reached a value of 9 mol%. That changing is called the concentration quenching. The dependence of the ratio of the relative intensity of ⁵D₀ → ⁷F₂ (614 nm) to ⁵D₀ → ⁷F₁ (591 nm) on Eu³⁺ concentrations points out inset of Fig. 7. This ratio rises with increasing Eu³⁺ concentration from 1 to 5 mol% and then begins to decrease with the Eu³⁺ concentration approaching 7 mol%.

Fig. 7

Emission spectra (λ_ex = 266 nm) of La₃PO₇:x mol%Eu³⁺ (x = 1 (a), 3 (b), 5 (c), 7 (d), 9 (e)) samples. Inset: the dependence of the ratio of ⁵D₀ → ⁷F₂ (614 nm) to ⁵D₀ → ⁷F₁ (591 nm) on Eu³⁺ concentrations.

The normalized luminescence decay curves for ⁵D₀ → ⁷F₂ of Eu³⁺ in La₃PO₇:5 mol%Eu³⁺ excited at 266 nm and monitored at 614 nm are shown in Fig. 8. It can be well fitted into double-exponential function with the equation:   

\begin{equation*} I = A_{1}\mathrm{epx}(-t/\tau_{1}) + A_{2}\mathrm{epx}(-t/\tau_{2}).^{13)} \end{equation*}

Where, I present to the emission intensity at time t, τ₁ and τ₂ are respectively the fast and slow constituent of luminescence lifetimes, A₁ and A₂ are the fitting parameters. The fitting results are listed in Table 1.

Fig. 8

Luminescence decay curves of La₃PO₇:5 mol%Eu³⁺ sample. The inset shows the relationship of the decay times of samples with different concentrations of Eu³⁺ ions.

Table 1 Decay lifetime of La₃PO₇:x mol%Eu³⁺ nanopowders (x = 1, 3, 5, 7, 9).

The equation $\tau = \frac{A_{1}\tau _{1}^{2} + A_{2}\tau _{2}^{2}}{A_{1}\tau _{1} + A_{2}\tau _{2}}$ was applied to calculate the average lifetime of Eu³⁺ for ⁵D₀ → ⁷F₂ emission.¹³⁾ It can also see that there is a decrease in the emission lifetime value from 0.826 ms to 0.760 ms when the Eu³⁺ ions concentration increases from 1 to 9 mol%. This is explained that, if the Eu³⁺ ions concentration increases in the lattice, the distances of Eu³⁺–Eu³⁺ internuclear will be reduced; leading to the extent of Eu³⁺–Eu³⁺ interaction would be increased. Thus, in the excited state, the energy transfer among different Eu³⁺ ions follows by non-radiative decay. It causes a decrease in the excited state lifetime corresponding to ⁵D₀ level of Eu³⁺ ions.¹⁵⁾

4. Conclusion

In summary, La₃PO₇: Eu³⁺ nanoparticles were successfully obtained via sample synthesis by a facial combustion method. Size and morphology characteristics of the La₃PO₇: Eu³⁺ nanoparticles were studied using X-ray, TEM, HR-TEM, and luminescence spectroscopy. The diffraction peaks of XRD are in good agreement with the results of JCPDS card No 49-1023 analysis with the monoclinic phase. The average size of nanoparticles, which is investigated by XRD, SEM, HR-TEM, is about 20 nm. Under 266 nm wavelength excitation, the emission intensities considerably improved with increase the annealing temperature of samples. The samples gained under 800°C exhibit a strong red emission at 614 nm, attributing to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The red emission intensity of samples rose when the concentration of Eu³⁺ ions increased from 1 to 7 mol% then decreased for the higher concentrations. The intensity ratio of ⁵D₀ → ⁷F₂ transition and ⁵D₀ → ⁷F₂ transition reached the maximum for 5% mol Eu³⁺-doped La₃PO₇ sample. Therefore, the best doping concentration of Eu³⁺ in La₃PO₇ is determined to be 5 mol%. The lifetime curve of La₃PO₇: Eu³⁺ nanophosphors can be well-fitted into a single exponential function. The lifetime of ⁵D₀ → ⁷F₂ transition decreased gradually with the increase of Eu³⁺ ions concentration.

Acknowledgments

This study was supported by Institute of Materials Science, Vietnamese Academy of Science and Technology (Grant No. CSCL1.01.19). A part of the work was done in the National Key Laboratory Electronic Materials and Devices.

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