Cathodo-, Thermo-, and Photoluminescent Properties of Nano-Y2O3:Eu 3+ Fabricated by Controlled Combustion Synthesis

Y 2 O 3 :Eu nanophosphors were prepared through combustion reaction under controlled condition of the fuel ethylenediaminetetraacetic acid (EDTA-Na 2 ) and in the temperature range from 350 to 700C. The products were characterized by X-ray diffraction (XRD), field emission scattering electron microscopy (FESEM), and energy dispersive spectroscopy (EDS). The results showed that Y 2 O 3 :Eu nanoparticles were successfully synthesized by combustion method at low temperature and in short reaction time. The light-emitting ability of Y 2 O 3 :Eu 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 117C after UV exposure and at least two peaks at 125 and 336C with Gamma irradiation. Photoluminescent spectra of Y 2 O 3 :Eu 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 Y 2 O 3 :Eu nanoparticles on annealing temperature and concentration of Eu was also studied.


Introduction
Light emitting under electromagnetic fields exposure of the nanoparticles based on oxide yttrium (Y 2 O 3 ) 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 Y 2 O 3 doped with rare earth ions (Y 2 O 3 :Ln 3+ ) 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 Y 2 O 3 :Ln 3+ [5].Among them nano-Y 2 O 3 :Eu 3+ 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, Y 2 O 3 :Ln 3+ 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][21][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-Y 2 O 3 :Eu 3+ with controlled morphology and in particular the site-selectivity and relative behavior of the strongest 5 D 0 -7 F 2 (red) transition from Eu 3+ needs to be further investigated for strong red phosphor application in the different fields.
In the present work Y 2 O 3 :Eu 3+ nanoparticles were prepared by combustion method with EDTA-Na 2 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-Y 2 O 3 :Eu 3+ have been investigated to explore the application potential in display or dosimeter.The photoluminescent properties of the fabricated Y 2 O 3 :Eu 3+ have been characterized by photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra to search for a novel red stronger luminescent nanophosphor based on Y 2 O 3 :Eu 3+ to develop labeling or imaging tool in biomedicine.

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-Na 2 (99%, Sigma), and urea (99.5%, Merck) were used as starting materials and fuels to prepare Y 2 O 3 :Eu 3+ .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-Na 2 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 Y 2 O 3 :Eu 3+ 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 K, 40 kv, 20 mA.CL spectra, image, and EDS of Y 2 O 3 :Eu 3+ 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 Y 2 O 3 :Eu 3+ could be compared precisely.The TL glow curves of Y 2 O 3 :Eu 3+ were obtained in the temperature range of 50-450 ∘ C. The TL glow curves of Y 2 O 3 :Eu 3+ 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.

Results and Discussion
The powder XRD patterns of Y 2 O 3 :5 mol% Eu 3+ sample, which was prepared by combustion method with EDTA-Na 2 as fuel, annealed at 700 ∘ C, are shown in Figure 1.The appearance of all prominent diffraction peaks of the planes and [145] 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].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-Na 2 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].
These estimated particles sizes of Y 2 O 3 :5 mol% Eu 3+ prepared by combustion reaction in using EDTA-Na 2 as fuel were greater than those of Y 2 O 3 :5 mol% Eu 3+ synthesized with urea CO(NH 2 ) 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-Na 2 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 Y 2 O 3 :3 mol% Eu 3+ 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 Y 2 O 3 doped with Eu 3+ ions.Figure 4 presents EDS layered image of Y 2 O 3 :3 mol% Eu 3+ sample, in which the position of ions yttrium (green), oxygen (yellow), and europium (red dot) on yttrium oxide nanoparticles Y 2 O 3 can be recognized.Based on these EDS spectra it can be considered that Eu 3+ ions were distributed rather regularly on the particles of yttrium oxide doped europium ions.
3.1.Cathodoluminescence. Nanophosphors are efficient luminescent materials and irreplaceable components of lightemitting 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][28][29].Therefore we have studied the lightemitting ability of Y 2 O 3 :Eu 3+ 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 Y 2 O 3 :3 mol% Eu 3+ samples and it is indicated that our samples have strong CL intensity even at low doping concentration of 3 mol% Eu 3+ .These results have indicated that the obtained Y 2 O 3 :Eu 3+ can be applied for display technology.

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].RE 3+ doped Y 2 O 3 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 Y 2 O 3 :Eu 3+ is sensitive enough to measure background UV radiation such as sun light and bulb light [31].Therefore they have investigated the TL behavior of Y 2 O 3 :Eu 3+ after UV exposure.In this report the primary results of TL glow curves of Y 2 O 3 :5 mol% Eu 3+ after UV exposure and Gamma irradiation are presented for comparison.The Y 2 O 3 :Eu 3+ 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 Y 2 O 3 :5 mol% Eu 3+ 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 Y 2 O 3 :Eu 3+ 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 Y 2 O 3 :Eu 3+ 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.

Photoluminescence.
In order to study the influence of excitation energy, the sample Y 2 O 3 doped with 3 mol%, 5 mol% Eu 3+ synthesized in using EDTA-Na 2 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 TL intensity (a.u.) showed characteristic optical properties of Eu 3+ ions in the cubic Y 2 O 3 host [21].The emission spectra lines of Eu 3+ 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 Eu 3+ emission transition [1,3,4].Figure 9 shows the PL emission spectra of 5 mol% Eu 3+ doped Y 2 O 3 samples prepared by combustion method with different fuels: (1) urea and (2) EDTA-Na 2 .It can be seen in the combustion synthesis with EDTA-Na 2 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].
The peaks at 582, 588, 590, 600, 612, and 632 nm are corresponding to the 5 D 0 → 7 F j ( = 0, 1, 2, 3, and 4) transition of Eu 3+ ion in host matrix.The 5 D 0 → 7 F 0 , 5 D 0 → 7 F 2 , and 5 D 0 → 7 F 3 transitions originate from C 2 sites by electronic dipole transition and the 5 D 0 → 7 F 1 by magnetic dipole transition situated at both S 6 and C 2 sites [17,36].The magnetic dipole transition 5 D 0 → 7 F 1 is nearly independent of the host matrix and other electric dipoles allowed 5 D 0 → 7 F j ( = 2, 4, and 6) transitions are strongly influenced by the local structure and site asymmetry around the Eu 3+ ion [4,25,37].Table 1 showed the luminescence intensity ratios of Eu 3+ ion doped Y 2 O 3 matrix.Strong energy transfer from the Eu 3+ ions occupying S 6 site to C 2 site increased the luminescence efficiency at 612 nm.The photoluminescence intensity ratio of 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 (red to orange) provides valuable information about the site occupied by Eu 3+ 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.
Figure 10 shows photoluminescent intensity of Y 2 O 3 :5 mol% Eu 3+ 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 Eu 3+ rare earth ions were found to increase with the increasing of Eu 3+ 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 Eu 3+ is 5 mol% and optimal temperature is 350 ∘ C. Figure 11 shows the PL excitation spectra of the Y 2 O 3 :5 mol% Eu 3+ 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 Eu 3+ 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 (Eu 3+ ← O 2− ) [31].

Conclusions
Y 2 O 3 :Eu 3+ nanoparticles have been successfully synthesized by the EDTA-Na 2 fuel assisted combustion method at low temperature and in short reaction time.XRD studies confirmed the formation of pure cubic phase of Y 2 O 3 .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 5 D 0 -7 F j ( = 0, 1, 2, 3, and 4) transition of Eu 3+ ion.The highest of luminescent intensity at 612 nm under an excitation wavelength of 254 nm is from transition 5 D 0 -7 F 2 .CL intensity of Y 2 O 3 :3% mol Eu 3+ was measured independently on the excitation powers which were 5, 10, and 15 kV.The PL intensity was found to increase with increasing Eu 3+ doping concentration up to 5 mol%.The nanophosphor Y 2 O 3 :Eu 3+ has great potential to develop display and thermoemission dosimeter, especially for biomedical applications.