Journal of Nanomaterials
Volume 2015 (2015), Article ID 637124, 7 pages
http://dx.doi.org/10.1155/2015/637124

Research Article

Cathodo-, Thermo-, and Photoluminescent Properties of Nano-Y₂O₃:Eu³⁺ Fabricated by Controlled Combustion Synthesis

Tran Kim Anh,¹ Pham Thi Minh Chau,¹ Nguyen Thi Quy Hai,¹ and Le Quoc Minh1,2

¹Institute for Research and Development of High Technology, Duy Tan University, K7/25 Quang Trung, Hai Chau, Da Nang, Vietnam
²Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam

Received 26 December 2014; Revised 19 March 2015; Accepted 20 March 2015

Academic Editor: Shifeng Zhou

Copyright © 2015 Tran Kim Anh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Y₂O₃:Eu³⁺ nanophosphors were prepared through combustion reaction under controlled condition of the fuel ethylenediaminetetraacetic acid (EDTA-Na₂) and in the temperature range from 350 to 700°C. The products were characterized by X-ray diffraction (XRD), field emission scattering electron microscopy (FESEM), and energy dispersive spectroscopy (EDS). The results showed that Y₂O₃:Eu³⁺ nanoparticles were successfully synthesized by combustion method at low temperature and in short reaction time. The light-emitting ability of Y₂O₃:Eu³⁺ nanoparticles upon the electron excitation has been studied at the potentials 5, 10, and 15 kV. The thermoluminescent glow curves have elucidated an intense peak at 117°C after UV exposure and at least two peaks at 125 and 336°C with Gamma irradiation. Photoluminescent spectra of Y₂O₃:Eu³⁺ nanoparticles exhibited strong red luminescent color with highest sharp band at 612 nm under excitation in ultraviolet at 254, 394 and in visible at 465 nm. The dependence of photoluminescent properties of Y₂O₃:Eu³⁺ nanoparticles on annealing temperature and concentration of Eu³⁺ was also studied.

1. Introduction

Light emitting under electromagnetic fields exposure of the nanoparticles based on oxide yttrium (Y₂O₃) doped lanthanide (Ln) ions are become the promising way that open new opportunities for the application in various fields of electronics, optics and also biomedicine [1]. Yttrium oxide is a well-known host for transition metals and various lanthanide ions due to large band gap (5.8 eV), high dielectric constant, high thermal stability, and low cut-off phonon energy (380 cm⁻¹). The light emission efficiency of the nanomaterials based on Y₂O₃ doped with rare earth ions (Y₂O₃:Ln³⁺) depends on the physical properties such as surface morphology [2], crystalline structures [3], and distribution of activator in matrix [4, 5]. High luminescence efficiency, long term thermal stability, high mechanical resistance, very good refractory properties, and chemical stability are interesting properties of Y₂O₃:Ln³⁺ [5]. Among them nano-Y₂O₃:Eu³⁺ has a great interest for application not only in the field emission display [6] and security printing [7] for electroluminescence [8], but also in biomedicine [9]. Nanoparticles synthesized by different methods show variation in their size, shape, and optical properties. To enhance the brightness and resolution of display of present technology, Y₂O₃:Ln³⁺ nanostructures of different morphologies have been synthesized by using sol-gel process [7, 10, 11], solvothermal [3, 12], hydrothermal [13, 14], colloidal and coprecipitation [15, 16], thermal and laser decomposition [17, 18], microwave [19], and combustion synthesis [7, 20–22]. Particularly, the combustion method has advantages due to its simplicity in nature and very short synthesis time. However, it is very important to develop phosphors nano-Y₂O₃:Eu³⁺ with controlled morphology and in particular the site-selectivity and relative behavior of the strongest ⁵D₀-⁷F₂ (red) transition from Eu³⁺ needs to be further investigated for strong red phosphor application in the different fields.

In the present work Y₂O₃:Eu³⁺ nanoparticles were prepared by combustion method with EDTA-Na₂ and with urea for comparison. The structural and morphological properties were studied by using XRD, FESEM, and EDS. The cathodoluminescent (CL) and thermoluminescent (TL) properties of nano-Y₂O₃:Eu³⁺ have been investigated to explore the application potential in display or dosimeter. The photoluminescent properties of the fabricated Y₂O₃:Eu³⁺ have been characterized by photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra to search for a novel red stronger luminescent nanophosphor based on Y₂O₃:Eu³⁺ to develop labeling or imaging tool in biomedicine.

2. Experiment

The sample was prepared by combustion method reported in [23]. All chemicals are of analytical grade. Yttrium oxide (99,99%, ALFA), europium oxide (99,95%, CERAC), nitric acid (99%, Sigma-Aldrich), EDTA-Na₂ (99%, Sigma), and urea (99.5%, Merck) were used as starting materials and fuels to prepare Y₂O₃:Eu³⁺. Stoichiometric amounts of yttrium and europium oxides were dissolved in aqueous nitric acid 1 : 1 ratio. The solution was heated to 100°C on hot plate while being vigorously stirred until the solution became transparent. The reaction solution was dried at 70°C for 3 hours to remove the excess nitric acid and water. After cooling down to room temperature, the dried rare earth salts were mixed with small amount of EDTA-Na₂ in deionized water and vigorously stirred for 1 hour to form clear solution. Sealed containers with freshly prepared solution were then placed into an electrical oven heated up to 105°C for 1 hour. After that the containers were moved to Muffle furnace whose temperature was maintained at 350°C for 2 min and raised to 600°C for 3 min. The combustion process was completed in about 6–10 min. After completion of heating process the furnace cooled down to the room temperature. The fabricated compound was collected and washed with deionized water and ethanol and filtered to the ultimate white product, which was dried in vacuum at 70°C for 15 h to obtain Y₂O₃:Eu³⁺ fine powder. The obtained samples were foamy, fluffy, and porous.

The samples were checked by the X-ray diffractometer (XRD) Model Bruckner D8-Advance using Cu Kα, 40 kv, 20 mA. CL spectra, image, and EDS of Y₂O₃:Eu³⁺ were measured by field emission scattering electron microscope (FESEM) F 6400 JEOL. The PLE spectra and PL spectra of as-prepared and annealed samples were recorded using Spectrophotometer Model FL3-22 HORIBA with double monochromator, photomultiplier, appropriate lens and filters, and equipment NanoLog iHR 320 HORIBA Jobin Yvon. The samples preparation technique and measurement condition for luminescent intensities have been maintained throughout the entire experiment work. Therefore PL intensity results of the as-prepared or the annealing samples of Y₂O₃:Eu³⁺ could be compared precisely. The TL glow curves of Y₂O₃:Eu³⁺ were obtained in the temperature range of 50–450°C. The TL glow curves of Y₂O₃:Eu³⁺ were recorded with a homemade TL setup consisting of a small metallic plate heating strip, temperature programmer, photomultiplier tube, and multimeter recorder at heating rate of 2°Cs⁻¹. The TL glow curves obtained above were figured on using Origin software.

3. Results and Discussion

The powder XRD patterns of Y₂O₃:5 mol% Eu³⁺ sample, which was prepared by combustion method with EDTA-Na₂ as fuel, annealed at 700°C, are shown in Figure 1. The appearance of all prominent diffraction peaks of the planes , , , , , , , and corresponding to cubic form of yttrium oxide is in agreement with the JCPD41-1105. The particle sizes of these materials were estimated and found to be in the range of 24 nm for ~350°C and 26 nm for 700°C annealed sample. The obtained data were calculated by using the Scherrer equation [23].

Figure 1: XRD spectra of Y₂O₃:5 mol% Eu³⁺, annealed at 700°C.

These calculated particles sizes were in good agreement with those from FESEM images shown in Figure 2. This indicates the spatial structure, fluffy nature with pores, and voids with loosely agglomerated particles. The morphology of the synthesized samples depends on the nature and concentration of EDTA-Na₂ or urea as organic fuel. During combustion, yttrium nitrate impregnates into the amorphous product and gets ignited. Heat dissipates by the evolution of gaseous products in minimization and thus it leads to localization of heat due to amorphous nature of the fuel. Further increasing to about 800°C for 2 hours did not much affect the morphology with particles still having a large degree of porosity and agglomeration [24].

(a)

(b)

Figure 2: (a) FESEM image of Y₂O₃:5 mol% Eu³⁺ annealed at 350°C. (b) FESEM image of Y₂O₃:5 mol% Eu³⁺ annealed at 700°C.

These estimated particles sizes of Y₂O₃:5 mol% Eu³⁺ prepared by combustion reaction in using EDTA-Na₂ as fuel were greater than those of Y₂O₃:5 mol% Eu³⁺ synthesized with urea CO(NH₂)₂ [7]. During the reaction process the reaction mixture underwent thermal dehydration and ignited at one spot with liberation of gaseous products such as oxides of nitrogen and carbon. The gaseous products during combustion reaction were liberated which increased surface area of powder products [25]. For the reaction mixture of EDTA-Na₂ need without further of any external heating. The heat of reaction is sufficient for decomposition of the mixture and resulted in the formation of particles. The combustion with urea can be controlled rate of the decomposition that would result in formation of particles with a small size and uniform distribution [23].

Figure 3 shows EDS spectrum of Y₂O₃:3 mol% Eu³⁺ sample, which shows that the product consists of the elements Y, O, and Eu. The EDS result in Figure 2 confirms the fabricated products were Y₂O₃ doped with Eu³⁺ ions. Figure 4 presents EDS layered image of Y₂O₃:3 mol% Eu³⁺ sample, in which the position of ions yttrium (green), oxygen (yellow), and europium (red dot) on yttrium oxide nanoparticles Y₂O₃ can be recognized. Based on these EDS spectra it can be considered that Eu³⁺ ions were distributed rather regularly on the particles of yttrium oxide doped europium ions.

Figure 3: EDS of Y₂O₃:3% mol Eu³⁺.

Figure 4: EDS layered image of Y₂O₃:Eu³⁺.

3.1. Cathodoluminescence

Nanophosphors are efficient luminescent materials and irreplaceable components of light-emitting devices like cathode ray tubes, plasma display panels, and field emission display [26]. The last ones have been recognized as one of the most promising technologies due to their most important features like great brightness, wide horizontal and vertical view angels, good contrast ration, and wide work temperature range. One of the most promising solutions is lanthanide doped oxides, garnets, and some perovskites [27–29]. Therefore we have studied the light-emitting ability of Y₂O₃:Eu³⁺ nanoparticles upon the electron excitation. CL spectra were measured by F 6400 JEOL system, which were observed upon the different potentials of 5, 10, and 15 kV in high vacuum of 10−7 mmHg. In Figure 5 are presented the CL spectra of Y₂O₃:3 mol% Eu³⁺ samples and it is indicated that our samples have strong CL intensity even at low doping concentration of 3 mol% Eu³⁺. These results have indicated that the obtained Y₂O₃:Eu³⁺ can be applied for display technology.

Figure 5: CL spectra of Y₂O₃:3 mol% Eu³⁺. Black curve: 5 kV, red curve: 10 kV, and blue curve: 15 kV.

3.2. Thermoluminescence

Recent studies on different luminescent nanomaterials have shown a potential application in dosimeter of ionizing radiation for measurements of high doses using the TL technique [21, 30]. RE³⁺ doped Y₂O₃ material is one of such promising materials and some attention has been paid to study its TL characteristics. In 1996 Yeh and Su have reported the possible use of rare earth doped oxide phosphor in UV light dosimeter and found that Y₂O₃:Eu³⁺ is sensitive enough to measure background UV radiation such as sun light and bulb light [31]. Therefore they have investigated the TL behavior of Y₂O₃:Eu³⁺ after UV exposure. In this report the primary results of TL glow curves of Y₂O₃:5 mol% Eu³⁺ after UV exposure and Gamma irradiation are presented for comparison.

The Y₂O₃:Eu³⁺ samples were irradiated with D2 Lampe for 1 min and with a Gamma source for 75 h, and then their TL glow curves were recorded. The TL glow curve of Y₂O₃:5 mol% Eu³⁺ after UV irradiation was measured from 50°C to 400°C with the heating rate 2°C/sec. These TL glow curves were figured on using Origin software and one intense peak that appeared at 117°C was well resolved as can be seen in Figure 6. In the case of Gamma ray irradiation, the TL glow curves of Y₂O₃:Eu³⁺ sample for a dose of 9 mSv and exposure time 75 h, at least one prominent TL glow peak at 336°C was found (Figure 7). Tamrakar indicated that the TL glow curve of UV irradiated sample RE doped nanophosphor shows the formation of shallow trap (surface trapping) and the Gamma irradiated sample shows the formation of deep trapping [32, 33]. The estimation of trap formation was evaluated by knowledge of trapping parameters. The trapping parameters such as activation energy, order of kinetics, and frequency factor were calculated by peak shape method. Here most of the peak shows second order of kinetics. It is possible in UV irradiated Y₂O₃:Eu³⁺ phosphors that the TL response mainly generates from the traps on surface. Since these radiations cannot penetrate deeper and hence will not induce lattice defects may be caused by the trapped carriers which are produced during the sample processing.

Figure 6: TL glow curve upon UV exposing.

Figure 7: TL glow curve upon Gamma irradiation.

3.3. Photoluminescence

In order to study the influence of excitation energy, the sample Y₂O₃ doped with 3 mol%, 5 mol% Eu³⁺ synthesized in using EDTA-Na₂ was excited by different wavelengths of 254, 465, and 394 nm from HORIBA system. All the samples showed strong PL emission with peak maximum at 612 nm under an excitation wavelength of 254 nm (Figure 8). We can note that the emission intensity when excited by the 254 nm wavelength was 1.6 times stronger than excitation by 394 nm. The spectra structure showed characteristic optical properties of Eu³⁺ ions in the cubic Y₂O₃ host [21]. The emission spectra lines of Eu³⁺ ion are shaped which is due to the screening of 4f orbital by 5s and 5d orbital from crystal field of the host lattice. The spectra have series of peaks well agreeing with the report values of Eu³⁺ emission transition [1, 3, 4]. Figure 9 shows the PL emission spectra of 5 mol% Eu³⁺ doped Y₂O₃ samples prepared by combustion method with different fuels: (1) urea and (2) EDTA-Na₂. It can be seen in the combustion synthesis with EDTA-Na₂ at temperature 350°C that the luminescent intensity of samples increased by near three orders in comparison with that of samples prepared by using urea. This may be due to the formation of well-crystallized cubic yttrium oxide phase [15, 34, 35].

Figure 8: Photoluminescent spectra of Y₂O₃:5 mol% Eu⁺³. : curve 1: 254 nm (I : 10), curve 2: 465, and curve 3: 394 nm.

Figure 9: Photoluminescent spectra of Y₂O₃:5 mol% Eu³. Curve 1: urea; curve 2: EDTA-Na₂.

The peaks at 582, 588, 590, 600, 612, and 632 nm are corresponding to the ⁵D₀⁷Fj ( = 0, 1, 2, 3, and 4) transition of Eu³⁺ ion in host matrix. The ⁵D₀⁷F₀, ⁵D₀⁷F₂, and ⁵D₀⁷F₃ transitions originate from C₂ sites by electronic dipole transition and the ⁵D₀⁷F₁ by magnetic dipole transition situated at both S₆ and C₂ sites [17, 36]. The magnetic dipole transition ⁵D₀⁷F₁ is nearly independent of the host matrix and other electric dipoles allowed ⁵D₀⁷Fj ( = 2, 4, and 6) transitions are strongly influenced by the local structure and site asymmetry around the Eu³⁺ ion [4, 25, 37].

Table 1 showed the luminescence intensity ratios of Eu³⁺ ion doped Y₂O₃ matrix. Strong energy transfer from the Eu³⁺ ions occupying S₆ site to C₂ site increased the luminescence efficiency at 612 nm. The photoluminescence intensity ratio of ⁵D₀⁷F₂ to ⁵D₀⁷F₁ (red to orange) provides valuable information about the site occupied by Eu³⁺ ions and covalent nature of the host matrix. The red/orange ratio of the sample heat treated at different temperatures from 350 up to 900°C only changed very little (10.6–11.3) as shown in Table 1. We suggested that further heat treatment at the higher temperatures will be not necessary.

Table 1: Photoluminescence intensity ratios of  ⁵D₀ → ⁷F₂ to ⁵D₀ of Y₂O₃:5 mol% Eu³⁺ synthesized in using EDTA-Na₂.

Sample name
Sample composition of Eu³⁺ ion
Temperature treatment (°C)
Used fuel
Red/orange ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁

Y₂O₃:5 mol% Eu³⁺
5 mol%
350
EDTA-Na₂
10.6

Y₂O₃:5 mol% Eu³⁺
5 mol%
700
EDTA-Na₂
10.2

Y₂O₃:5 mol% Eu³⁺
5 mol%
900
EDTA-Na₂
11.3

Figure 10 shows photoluminescent intensity of Y₂O₃:5 mol% Eu³⁺ prepared with EDTA fuel, annealed at 900°C (curve 1), 700°C (curve 2), and 350°C (curve 3). It was indicated that the sample prepared at 350°C exhibited the highest luminescence intensity. The emission intensities from Eu³⁺ rare earth ions were found to increase with the increasing of Eu³⁺ doping from 3 mol% to 5 mol% and to decrease with higher doping content to 7 mol%. It is found that the optimal concentration of Eu³⁺ is 5 mol% and optimal temperature is 350°C.

Figure 10: Photoluminescence spectra of Y₂O₃:5 mol% Eu³⁺. Curve 1: 900, curve 2: 700, and curve 3: 350°C.

Figure 11 shows the PL excitation spectra of the Y₂O₃:5 mol% Eu³⁺ nanoparticles. The excitation spectra show a broad peak at 250 nm along with higher side peaks at 395 nm and 465 nm. On increasing the annealing temperature, intensities of higher side peaks are reduced in comparison to 250 nm peak. These characteristic features of Eu³⁺ excitation indicate that at higher annealing temperature the energy required for excitation of nanophosphor is due to the 250 nm peak, which is a charge transfer transition (Eu³⁺ O²⁻) [31].

Figure 11: Photoluminescent excitation spectra of Y₂O₃:5 mol% Eu³⁺. Curve 1: 900, curve 2: 700, and curve 3: 350°C.

4. Conclusions

Y₂O₃:Eu³⁺ nanoparticles have been successfully synthesized by the EDTA-Na₂ fuel assisted combustion method at low temperature and in short reaction time. XRD studies confirmed the formation of pure cubic phase of Y₂O₃. The crystalline sizes of the nanoparticles were found to be 24 nm for the samples annealed at ~350°C and 26 nm for the samples annealed at 700°C. The intense peak at 117°C upon UV exposure and at least two peaks at 125 and 336°C upon Gamma irradiation were found in TL process. All the obtained samples showed bright emission under CL or PL exciting condition. The PL spectra presented a series of emission peaks between 570 nm and 650 nm corresponding to the ⁵D₀-⁷Fj (  =  0, 1, 2, 3, and 4) transition of Eu³⁺ ion. The highest of luminescent intensity at 612 nm under an excitation wavelength of 254 nm is from transition ⁵D₀-⁷F₂. CL intensity of Y₂O₃:3% mol Eu³⁺ was measured independently on the excitation powers which were 5, 10, and 15 kV. The PL intensity was found to increase with increasing Eu³⁺ doping concentration up to 5 mol%. The nanophosphor Y₂O₃:Eu³⁺ has great potential to develop display and thermoemission dosimeter, especially for biomedical applications.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors are thankful to the colleagues in the Institute for Research and development High Technology of Duy Tan University; Dr. Vu Thi Thai Ha, Mr. Anh Tuan, and Dr. Nguyen Duc Van in the Institute of Materials Science; and Dr. Do Quang Trung and Ph.D. degree student Nguyen Tu in the Advanced Institute of Science and Technology for CL and PL measurements.

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