Effect of Calcination Temperature on Structural, Photoluminescence

Article pubs.acs.org/JPCC

Effect of Calcination Temperature on Structural, Photoluminescence, and Thermoluminescence Properties of Y2O3:Eu3+ Nanophosphor R. Hari Krishna,†,‡ B. M. Nagabhushana,*,‡ H. Nagabhushana,§ N. Suriya Murthy,∥ S. C. Sharma,§ C. Shivakumara,⊥ and R. P. S. Chakradhar○ ‡

Department of Chemistry, M. S. Ramaiah Institute of Technology, Bangalore 560 054, India Visvesvaraya Technological University, Belgaum 590 018, India § Centre for Nanoscience Research (CNR), Tumkur University, Tumkur 572 103, India ∥ Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India ⊥ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India ○ CSIR-National Aerospace Laboratories, Bangalore 560017, India †

ABSTRACT: Red light emitting cubic Y1.95Eu0.05O3 nanophosphors have been synthesized by a low temperature solution combustion method using ethylene diamine tetra acetic acid (EDTA) as fuel. The systematic studies on the effect of calcination temperature on its structural, photoluminescence (PL), and thermoluminescence (TL) properties were reported. The crystallinity of the samples increases, and the strain is reduced with increasing calcination temperature. SEM micrographs reveal that samples lose their porous nature with an increase in calcination temperature. PL spectra show that the intensity of the red emission (611 nm) is highly dependent on the calcination temperature and is found to be 10 times higher when compared to as-formed samples. The optical band gap (Eg) was found to reduce with an increase of calcination temperature due to reduction of surface defects. The thermoluminescence (TL) intensity was found to be much enhanced in the 1000 °C calcined sample. The increase of PL and TL intensity with calcination temperature is attributed to the decrease of the nonradiative recombination probability, which occurs through the elimination of quenching defects. The trap parameters (E, b, s) were estimated from Chen’s glow peak shape method and are discussed in detail for their possible usage in dosimetry.

1. INTRODUCTION There has been a great demand for the development of new types of thermoluminescence dosimeter (TLD) phosphors for measuring high doses of ionizing radiation levels in personal and environmental fields. In this connection, significant advancements have been made in thermoluminescene (TL) experiments during the last couple of decades. However, the most important application of TL lies in radiation dosimetry1,2 which spans areas of health physics and other biological sciences, radiation protection, and personnel monitoring. TL experiments are equally helpful in defects and impurities related studies in solids. There are a number of commercially available thermoluminescent dosimeters, the most popular being LiF:Mg,Ti (TLD-100); CaSO4:Dy (TLD-900); LiF:Mg,Cu,P (TLD-00H); CaF2:Dy (TLD-200); and Al2O3 (TLD-500).3 However, efforts are still being made to improve the TL characteristics of these materials by preparing them using different techniques or by developing some new ones. Rare earth oxides are more stable than sulfur-containing phosphors, which undergo changes in surface chemistry when interacting with the electron beam, seriously degrading their PL, CL brightness, and releasing gases that can poison the field emitting tips.4,5 Y2O3:RE3+ nanoparticles are widely used as red © 2012 American Chemical Society

phosphor in display materials. In addition, they have been used in fluorescent lamps, projection televisions, and field emission displays5−9 due to their high chemical stability and good corrosion resistivity.10 The luminescence of Eu3+ is particularly interesting because its major emission is centered at 612 nm (red). Red emission is interesting, since it is one of the three primary colors (namely, red, blue, and green) from which a wide spectrum of colors can be generated by appropriate mixing. This strategy is in fact used for white light generation as well. For this reason, Eu3+ has been thoroughly investigated as a luminescent activator in many host lattices.11,12 Various chemical methods have been employed for preparing high-quality Y2O3:Eu3+ materials such as gas-phase condensation,13 coprecipitation method,14 electrochemical synthesis,15 sol−gel,16 pyrolysis,17 solid- to liquid-phase chemical route,18 combustion method,19 and hydrothermal/ solvothermal method.20,21 A considerable amount of work has been reported on PL and other physical properties of Received: September 29, 2012 Revised: November 30, 2012 Published: December 15, 2012 1915

dx.doi.org/10.1021/jp309684b | J. Phys. Chem. C 2013, 117, 1915−1924

The Journal of Physical Chemistry C

Article

Y 2 O 3 :Eu 3+ , whereas only a few reports exist on the thermoluminescence properties of this material. In the present work, Y2O3:Eu3+ nanopowders have been synthesized by the solution combustion technique using EDTA as fuel in a short time. The samples were calcined in an oxygen atmosphere at three different temperatures of 600, 800, and 1000 °C for 3 h and then characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV−vis spectroscopy, and Raman spectroscopy. Systematic measurements of crystallinity, crystallite size, and the effect of calcination temperature on its structural, photoluminescence (PL) intensity and thermoluminescence (TL) properties have been reported and analyzed in detail for their possible usage in dosimetric applications.

3. RESULTS AND DISCUSSION Figure 1 shows the powder X-ray diffraction patterns of Y1.95Eu0.05O3 of as-formed and calcined powders at 600, 800,

2. MATERIALS AND METHODS 2.1. Synthesis of Y2O3:Eu3+ Nanophosphor. The starting materials used for the combustion synthesis were analar grade yttrium oxide (Y2O3: 99.99%, CDH Ltd.), europium oxide (Eu2O3: 99.99%, Rolex Ltd.), nitric acid (HNO3: 99.99%, Merk Ltd.), and EDTA (C10H16N2O8: 99.99%, Merk Ltd.). For the preparation of Y1.95Eu0.05O3 nanophosphor, the oxide precursors were dissolved in 1:1 nitric acid to get corresponding nitrates, and the excess nitric acid was evaporated on a sand bath at 80 °C. EDTA was dissolved in deionized water, and the solution was subsequently added to the nitrate solution, while continuously stirring the mixture, to ensure homogeneous mixing. The Petri dish containing the homogeneous mixture of yttrium nitrate, europium nitrate, and EDTA was placed in a preheated muffle furnace maintained at 500 ± 10 °C. Initially, the solution boiled and underwent dehydration, followed by decomposition with the evolution of large amounts of gases (oxides of carbon, nitrogen). Then, spontaneous combustion with enormous swelling occurs, producing foamy and voluminous Y2O3:Eu3+ nanopowder. The theoretical equation of the combustion involving a redox mixture for the formation of Y2O3 nanoparticles using EDTA fuel can be represented by the following reaction:

Figure 1. PXRD patterns of Y2O3:Eu3+ calcined at different temperatures.

and 1000 °C for 3 h. The diffraction peak positions and relative intensities of all the samples are in good agreement with cubic Y2O3 (JCPDS No. 35-0734). It is observed that the width of the PXRD lines becomes narrower with increase in calcination due to improved crystallinity which in turn resulted in crystallite growth. The average crystallite size estimated from the Debye− Scherrer equation22 is found to be in the range 8−35 nm. Further, strain present in as-formed and calcined products was estimated using the W−H equation.23 0.9 β cos θ = + 4ε sin θ (2) λ where ε is the strain associated with the nanoparticles. Equation 2 represents a straight line between 4 sin θ (X-axis) and β cos θ (Y-axis). The slope of the line gives the strain (ε), and intercepts (0.9λ/D) of this line on the Y-axis give the grain size (D). Figure 2 shows the W−H plots of as-formed and calcined

8Y(NO3)3 + 3C10H16N2O8 → 4Y2O3 + 15N2 + 30CO2 + 24H 2O

(1)

The phase purity and the crystallinity of the nanophosphors calcined at different temperatures were measured using a powder X-ray diffractometer (PANalytical X‘Pert Pro) using Cu Kα (1.541 Å) radiation with a nickel filter. The FTIR studies have been performed on a Perkin-Elmer spectrometer (Spectrum 1000) using KBr pellets. The morphology and structure of the samples were inspected using a scanning electron microscopy (JEOL JSM 840A). Since the samples were insulating, electron micrography was aided by depositing a gold layer on the material using a sputtering process. The UV− vis absorption of the samples was recorded on a SL 159 ELICO UV−vis spectrophotometer. Raman studies were carried out on a Renishaw In-via Raman spectrometer with a 633 nm He−Cd laser and a Leica DMLM optical microscope equipped with a 50× objective, thus providing a laser spot of 2 μm in diameter. The photoluminescence (PL) measurements were performed on a Jobin Yvon spectrofluorimeter (Fluorolog-3) equipped with a 450 W xenon lamp as an excitation source. TL measurements were carried out at room temperature using a Nucleonix TL reader using UV as the excitation source.

Figure 2. W−H plots of Y2O3:Eu3+ calcined at different temperatures.

Y2O3:Eu3+ samples. It is observed that the strain present in asformed sample is more when compared to calcined samples, indicating the reduction in the number of surface atoms with increase in temperature and reduction of surface defects. The estimated crystallite size values and strain values using Scherer’s equation and W−H plots are given in Table 1. The cell 1916



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Table 1. Estimated Crystallite Size and Strain of As-Formed and Different Temperatures Calcined Y2O3:Eu3+ Nanophosphors crystallite size (nm) calcination temperature (oC)

Scherer’s formula

W−H plot

microstrain (×10−4)

as-formed 600 800 1000

8 14 23 35

13 21 32 43

36.79 14.750 9.600 8.470

parameters of 1000 °C calcined Y2O3:Eu3+ were evaluated using the Rietveld refinement method. The Rietveld refinement is a method in which various parameters of the XRD pattern (FWHM of peaks, asymmetry of peaks, peak shifts, etc.) can be used to estimate the crystal structure of the material under study. In our work, the Rietveld refinement was performed through the FULLPROF program.24 We utilize the pseudovoigt function in order to fit several parameters to the data point: one scale factor, one zero shifting, four background, three cell parameters, five shape and width of the peaks, one global thermal factor, and two asymmetric factors. A good agreement was obtained between the experimental relative intensity (observed XRD intensities) and stimulated intensity (calculated XRD intensities) from the model in Figure 3. The packing

Figure 4. Packing diagram of 1000 °C calcined Y2O3:Eu3+ nanophosphor.

Figure 5. Influence of sintering temperature on crystallite size and microstrain.

Figure 3. Reitveld analysis of 1000 °C calcined Y2O3:Eu phosphor.

3+

⎛ E ⎞ ⎟ D = C exp⎜ − ⎝ RT ⎠

nano-

(3)

where D is the particle size, C is a constant (=177.67), E is the activation energy for crystal growth, R is the ideal gas constant, and T is the absolute temperature. The calculated activation energy from the slope is 2.27 kJ/mol. Therefore, it is assumed that 2.27 kJ/mol of energy is necessary for the interfacial reaction to induce the growth of crystallite in cubic Y2O3. Figure 6 shows the Fourier transform infrared spectra of asformed and calcined Y2O3:Eu3+ samples. The absorption band centered at ∼3500 cm−1 was attributed to the O−H vibration mode of adsorbed water. The peaks centered around 1540 and 1460 cm−1 can be attributed to CO32− in the bond-stretching mode. The characteristic metal−oxygen stretching frequencies were observed at 440 and 560 cm−1. The peaks corresponding to the CO32− and adsorbed water present in as-formed samples were completely vanished in samples calcined at 800 and 1000 °C. These FTIR results are in agreement with the PXRD results shown in Figure 1.

diagram of Y2O3:Eu3+ after Rietveld refinement is shown in Figure 4. The refined parameters such as occupancy and atomic functional positions of the cubic phase Y2O3:Eu3+ are summarized in Table 2. The fitting parameters (Rp, Rwp, and χ2) show a good agreement between the refined and observed PXRD patterns. Figure 5 shows the variation of microstrain and the change in crystallite size of Y2O3:Eu3+ as a function of calcination temperature. Microstrain decreases and crystallite size increases with calcination temperature because of the decrease in surface defects and improved crystallinity resulting from higher calcination temperatures. Figure 5 (inset) shows the plot between ln(D) and 1/T, which is drawn on the basis of the Scott equation to describe the growth rate of nanoparticles during calcinations.25 1917



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Table 2. Rietveld Refined Structural Parameters for Y2O3:Eu3+ Nanophosphors Calcined at 1000 °Ca compound/crystal system

atoms

oxidation state

Wyckoff notation

X

Y

Z

B

occupancy

Y2O3:Eu3+cubic

Y1 Y2 O

+3 +3 −2

24d 8b 48e

−0.0319(2) 0.25000 0.3932(11)

0.0000 0.2500 0.1532(11)

0.2500 0.2500 0.3819(12)

0.05 0.05 0.05

1 1 1

Space group: Ia3̅ (No. 206). Lattice parameters: a = 10.6122(3) Å, cell volume = 1195.16(23) Å3, Rp = 10.0, Rwp = 15.5, Rexp = 13.46, χ2 = 1.33, RBragg = 4.56, RF = 3.58.

a

irregular size and shape. Further, the as-formed samples have a highly porous structure with free particles on the surface. With an increase in calcination temperature, the highly porous structure seen in the as-formed sample (Figure 7a) changes to flaky aggregates with pores in their structure (Figure 7b). Further increase in calcination temperature to 800 °C results in agglomeration of primary particles (Figure 7c), and when samples are calcined at 1000 °C, growth of particles due to the sintering effect can be clearly seen (Figure 7d). In this case, the particles fuse together, resulting in larger crystallite sizes, which is consistent with PXRD results (see Figure 1). The voids with pores are an inherent nature of combustion derived products due to the large amount of gases liberated during the combustion process. Figure 8 shows the transmission electron micrographs (parts a and b) and the corresponding selected area diffraction (SAED) pattern (part c) of the as-prepared Y2O3:Eu3+ sample. The TEM image of as-prepared nanopowder exhibits a crystallite-composed porous frame, a result which is consistent

Figure 6. FTIR spectra of Y2O3:Eu3+: (a) as-formed; (b) 600 °C; (c) 800 °C; (d) 1000 °C.

The scanning electron micrographs of as-formed and samples calcined at 600, 800, and 1000 °C for 3 h are shown in Figure 7a−d, respectively. The micrographs show that the particles are made up of agglomeration of many primary crystallites with

Figure 7. SEM micrographs of Y2O3:Eu3+: (a) as-formed; (b) 600 °C; (c) 800 °C; (d) 1000 °C. 1918



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Figure 9. UV−vis absorption spectra of Y2O3:Eu3+: (a) as-formed; (b) 600 °C; (c) 800 °C; (d) 1000 °C.

transition between the valence band and the conduction band. The absorption spectra also reveal that there is a slight red shift in prominent absorption bands with increase in calcination temperature. Lakshminarasappa et al. also reported two absorption bands in combustion derived Y2O3 in the wavelength region 206−360 nm.26 Transitions related to defects, surface traps, or impurities can cause absorption in the UV−vis region.27 It is a well-known fact that smaller particles have a high surface to volume ratio that results in the distribution of more surface defects. Thus, the lower the particle size, the nanomaterials exhibit strong and broad absorption bands.28 The optical band gap energy (Eg) of as-prepared and calcined phosphor was estimated by the Tauc relation29 given by α=A

(hv − Eg )1/2 hv

(4)

where “α” is the optical absorption coefficient, hν is the photon energy, “A” is a constant, and Eg is the band gap energy for direct transitions. The plots of (αhν)2 vs photon energy of asprepared and calcined samples are shown in Figure 10. It is observed from the figure that the Eg value is less in the asprepared sample when compared to calcined samples. The Eg values for as-prepared and calcined phosphor estimated from this relation are in good agreement with the literature26 and are in the range 5.1−5.25 eV. The variation in band gap values in

Figure 8. TEM images (a, b) and SAED pattern (c) of as-formed Y2O3:Eu3+ nanophosphor.

with the SEM results. As-prepared samples are agglomerates of sub-10 nm crystallites, which is in accordance with XRD-based particle size analysis. It should be pointed out that the nanocrystals are beam-sensitive because of their structure and composition, and high-resolution images could not be recorded. The SAED ring pattern for the as-prepared sample indicates significant disorder in the as-formed crystals. The UV−visible absorption spectra of as-prepared Y2O3:Eu3+ nanophosphor and calcined at different temperatures (600, 800, and 1000 °C) are shown in Figure 9. The spectra show a broad and prominent absorption band with a maximum absorption at ∼250−260 nm along with broad absorption bands centered at ∼480 nm. The absorption around ∼250 nm is attributed to the

Figure 10. Optical energy band gap of Y2O3:Eu3+calcined at different temperatures. 1919



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Table 3. Raman Mode Assignments for Y2O3:Eu3+ Nanophosphor Calcined at 1000 °C

as-formed and calcined samples might be due to the degree of structural order−disorder in the lattice that has a direct influence on the intermediate energy level distribution within the band gap. In the as-prepared sample, a high degree of structural defects may be present. When the sample is calcined at higher temperature, the host matrix becomes a more ordered structure with fewer defects. Consequently, intermediate energy levels (deep and shallow holes) are minimized within the optical band gap and Eg increases with calcination. Raman spectroscopy is a versatile tool that gives knowledge about the crystal structure, bonding, phase, and chemical environment. This spectroscopy probes molecular and crystal lattice vibrations, and is therefore very sensitive to small changes in crystal structure and bonding. PXRD results (Figure 1) show that Y2O3:Eu3+ samples calcined at 1000 °C are in the pure cubic phase without any additional phases or impurities. Cubic Y2O3 belongs to the Ia3̅ space group with C type structure that contains 32 cations and 48 anions. Among 32 cations, 8 Y3+ ions are on b sites and 24 Y3+ ions are on d sites, while all the 48 oxygens are in e sites. This structure is an analogue to the fluorite structure with each Y3+ ion located at the center of a cube from which two of the eight nearest neighbor oxygens of the fluorite structure have been removed.30 Because of its body-centered structure, the unit cell of Y2O3 contains the primitive structure twice, so that only eight unit formulas must be used to theoretically determine the number of vibrations.31 Therefore, the derived irreducible representations for the optical and acoustical modes are as follows: Γop = 4A g + 4Eg + 14Fg + 5A 2u + 5E u + 16Fu

Raman Shift (cm−1) experimental

ref 39

162 193 333 377 432 467 592

161 193 329 376 429 469 591

assignment of modes Fg Fg Fg Fg Fg Fg Fg

+ + + + + + +

Ag Eg Eg Ag Eg Ag Ag

Figure 12. Photoluminescence excitation spectra of Y2O3:Eu3+: (a) asformed; (b) 600 °C; (c) 800 °C; (d) 1000 °C. The vertical dashed lines are just to identify the peak positions.

(5)

where Ag, Eg, and Fg are Raman active, Fu is IR active, and A2u and Eu are inactive. On the whole, 22 Raman active lines are to be present for the pure Y2O3 compound.32,33 Figure 11 shows

of a broad band with a maximum at 240 nm due to the charge transfer (CT) between O2− and Eu3+.34 It is also worth noticing that the peak position shifts toward the higher wavelength side with calcination temperature. The CT energy position depends on the crystal field of ions surrounding O2−, the strength of anion binding, the size of the cation site, and the coordination number.35 Therefore, in this case, the shift in CTB might be attributed to a change in environment of O2− with calcination temperature. Figure 13 shows the photoluminescence emission spectra of the Y2O3:Eu3+ nanoparticles, as-prepared and calcined at 600−1000 °C for 3 h. The PL spectra were

Figure 11. Raman spectra of Y2O3:Eu3+ calcined at 1000 °C. The label * indicates the contribution of the Eu luminescence.

the observed Raman spectra of Y2O3:Eu3+ (calcined at 1000 °C) recorded at room temperature. Raman spectra of the Y2O3:Eu3+ compound show an intense peak at 426 cm−1 and weak peaks around 160, 194, 385, 586, and 603 cm−1. These results are in fair agreement with that of cubic Y2O3 powders. The assignment of modes of vibration for the Y2O3 compound is given in Table 3. Figure 12 shows the photoluminescence excitation spectra of (a) as-formed, (b) 600 °C, (c) 800 °C, and (d) 1000 °C (3 h calcined) Y1.95Eu0.05O3 phosphors. The excitation spectra were obtained by monitoring the 5D0 → 7F2 transition (611 nm) of Eu3+. It can be seen clearly that the excitation spectrum consists

Figure 13. Photoluminescence emission spectra of Y2O3:Eu3+: (a) asformed; (b) 600 °C; (c) 800 °C; (d) 1000 °C. (Inset: Variation of R/ O ratio and emission intensities as a function of different calcination temperatures.) 1920



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measured for all the samples under the same experimental conditions at room temperature. All the samples exhibited sharp and strong emission at 611 nm under excitation of 240 nm with variation in the emission intensity related to calcination temperature. The spectra show well resolved features, which can be noticed at 582, 588, 593, and 611 nm and assigned to the 5D0 → 7FJ (where J is 1, 2, and 3) transitions. The present study is dedicated to studying the effect of calcination temperature on luminescence behavior. Thus, it is important to correlate the luminescence behavior with the microstructural changes induced by thermal processing, and gain insight on the PL yield of the material. PL results show that the emission intensity of the as-prepared sample is very weak. We observe that the emission intensity of heat-treated phosphors increases with calcination temperature. It is also worth noticing that in as-prepared samples the magnetic dipole transitions (5D0 → 7F1) are not well pronounced but as the calcination temperature increases well resolved peaks at 583, 588, and 593 nm can be seen. The low PL yield in as-prepared samples can be ascribed to the high surface-to-volume ratio of nanoparticles together with the assumption of quenching defects on the surface. The continuous growth of crystallite size with temperature, as shown in Figure 5, results in a continuous decrease of the surface-to-volume ratio, which in turn eliminates the luminescence quenching defects. There is good correlation between the observed luminescence behavior and the degree of crystallinity extracted using PXRD. On the other hand, the reason for absence of 5D0 → 7F1 transitions in the as-prepared sample is ascribed to the nature of the Eu3+ environment in the host. The position and intensity of emission peaks tells about the nature of the local environment of the Eu3+ ion in cubic Y2O3 nanoparticles. The strongest emission at 611 nm is due to the 5D0 → 7F2 transition.36 It was reported that there are two Y3+ sites in cubic Y2O3; 75% of these sites are non-centrosymmetric with C2 symmetry, and the remaining 25% are centrosymmetric, having S6 symmetry.37,38 The most intense emission peak at 611 nm is due to the forced electric dipole transition arising from the low symmetry position of the Eu3+ ion with an inversion center. Therefore, the strongest emission is expected to come from Eu3+ ion on the C2 site. The other three 5D0 → 7F1 (583, 588, and 593 nm) transition lines are expected to arise from both Eu3+ C2 and S6 sites.39 Thus, it is clear that the Eu3+ environment changes from C2 symmetry to both C2 and S6 symmetry with calcination temperature. Generally, the color purity of Eu3+ luminescence is mainly determined by the ratio of the red emission transition (5D0 → 7 F2) to the orange emission transition (5D0 → 7F1) (asymmetry ratio, R/O), which is used as a sensitive parameter to understand the variation of the local symmetry around Eu3+ in the lattice.40 As shown in Figure 13 (inset), the R/O value shows variation with calcination temperature, indicating a change in symmetry of the crystal field around Eu3+. The structural rearrangement into a more ordered structure induced by calcination changes the local field of the Eu3+ ions, and is reflected in the emission spectra. The TL glow curves of as-formed and calcined Y2O3:Eu3+ (5 mol %) nanophosphors exposed to UV rays for 5 min are shown in Figure 14. The highest TL intensity was recorded for the 1000 °C calcined sample (inset of Figure 14). The asformed and calcined samples exhibit two satellite peaks at 90 and 162 °C along with a broad peak at 116 °C observed at a heating rate of 5 °C s−1. The nature of the glow curve remains

Figure 14. TL glow curves of Y2O3:Eu3+ calcined nanophosphor exposed to a UV source for 5 min: (a) as-formed; (b) 600 °C; (c) 800 °C; (d) 1000 °C. (Inset: Variation of TL intensity as a function of different calcination temperatures.)

the same, and a significant enhancement in TL intensity is observed for calcined samples. The effect of different UV exposure times on calcined samples (1000 °C) was studied and shown in Figure 15. A well resolved glow peak at 116 °C along

Figure 15. TL glow curves of Y2O3:Eu3+ nanophosphor calcined at 1000 °C exposed to UV radiations in the range 3−30 min.

with 90 and 162 °C glow peaks were recorded. These TL glow peaks indicate three different sets of traps are present. The TL intensity is found to be 25 times more in the 1000 °C calcined Y2O3:Eu3+ sample, when compared to the as-formed sample. This observation indicates an important correlation between decreased trap density and increased TL emission intensity in these materials. Figure 16 shows the variation of TL intensity with UV exposure time (3−30 min) for 1000 °C calcined Y2O3:Eu3+ samples. With an increase in the UV exposure time, the growth in the TL intensity of the 116 °C peak is more distinct than that of the lower temperature peak. Further, it is observed that the intensity of the glow peaks increases linearly up to an exposure time of ∼20 min and after that it decreases with an increase of UV exposure time. The density of the surface defects increases with an increase in UV exposure, leading to an 1921



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peak intensity and area under the peak decreases (see the inset in Figure 17), while the peak temperature shifts toward the higher temperature side (Figure 17). The glow peak (Tm) is shifted from 116 to 129 °C with an increase of heating rate from 5 to 10 °C s−1. The decrease in intensity with increase in heating rate is a phenomenon frequently observed in the practice of TL; it has been suggested to thermal quenching.43 The distinctions of traps in solids are useful to understand the luminescence process which can be obtained by TL studies. Further, the dosimetric characteristics of TL materials mainly depend on the kinetic parameters quantitatively describing the trapping emitting centers responsible for the TL emission.44 To obtain the parameters (E, b, and s), the glow curves are deconvoluted using commercially available ORIGIN 8.1 software (Figure 18) and then the individual deconvoluted Figure 16. Variation of TL intensity as a function of UV exposure time.

increase in peak intensity. The fall in TL intensity at higher exposure time is usually a consequence of competition between radiative and nonradiative centers or between different kinds of trapping centers. In the case of UV-exposed samples, the TL response mainly generates from the surface traps, since this radiation cannot penetrate deeper and hence will not induce lattice defects.41 The linear behavior over a wide range of exposure time may be explained on the basis of the track interaction model (TIM).42 According to this model, the number of traps generated by the high energy radiation in a track depends upon the cross section and the length of the track inside the matrix. In the case of nanomaterials, the length of the track generated by high energy radiation is of the order of a few tenths of nanomaterials. At low exposure time, there exist a few trap centers (TC)/luminescent centers (LC). As the exposure time increases, the TL intensity increases, since the number of particles irradiated increases linearly over time. However, as the irradiation time exceeds 20 min, new types of defects could be created, many of which are likely behaving as trap centers. This would explain the quenching of the observed thermoluminescence, beyond 20 min of irradiation. The influence of different heating rates between 5 and 10 °C s−1 on TL response has been investigated in 1000 °C calcined Y2O3:Eu3+ exposed to 5 min for UV rays and is shown in Figure 17. It is found that with the increase in heating rates the TL

Figure 18. Glow curve deconvolution of Y2O3:Eu3+ nanophosphor calcined at 1000 °C exposed to 7 min of UV radiation.

glow curves are analyzed using Chen’s peak shape method45 by the following equations. ⎛ KT 2 ⎞ Eα = Cα⎜ m ⎟ − bα(2KTm) ⎝ α ⎠

(6)

where α = τ, δ, and ω. δ = T2 − Tm

ω = T2 − T1

τ = Tm − T1

μg =

δ ω (7)

Cτ = 1.51 + 3.0(μg − 0.42) Cδ = 0.976 + 7.3(μg − 0.42)

(8)

Cω = 2.52 + 10.2(μg − 0.42) bτ = 1.58 + 4.2(μg − 0.42)

(9)

bδ = 0

bω = 1

(10)

The evaluated parameters E and b are then used as initial parameters in the kinetic equations. These parameters can be modified using the ORIGIN 8.1 software glow curve deconvolution (GCD) function, until the best fit is achieved. Similar studies have been reported by some authors for the calculation of trapping parameters using Chen’s peak shape method. Recently, similar studies have been reported by Manjunatha et al. for the calculation of trapping parameters.46 The isolated peaks were analyzed by Chen’s peak shape method to evaluate the peak parameters using the above equations. The calculated parameters were then used as initial

Figure 17. Effect of heating rates on TL glow curve for a sample exposed to 5 min of UV exposure: (a) 5 °C; (b) 7 °C; (c) 10 °C. (Inset: Variation of peak area as a function of different heating rates.) 1922



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Table 4. Estimated Trap Parameters of As-Formed and Different Temperatures Calcined Y2O3:Eu3+ Nanophosphors curve 1 Tm (°C)

calcination as-formed 600 °C 800 °C 1000 °C

curve 2

E (eV)

s (Hz)

1.29 1.06 0.93 1.13

× × × ×

93 74 93 80

1.2 8.2 1.8 2.4

Tm (°C) 22

10 1016 1014 1018

curve 3

E (eV)

s (Hz)

0.75 0.80 1.32 1.30

× × × ×

123 108 126 106

6.4 9.3 1.6 6.5

10

10 1011 1018 1018

Tm (°C)

E (eV)

146 147 151 142

0.94 0.70 0.92 0.84

s (Hz) 3.2 3.6 1.7 2.5

× × × ×

1013 109 1012 1011

Table 5. Estimated Trap Parameters of Y2O3:Eu3+ Nanophosphor Calcined at 1000 °C for Different UV Exposure Times activation energy (eV) UV exposure time (min)

peak

Tm (°C)

b (μg)

Eτ

Eδ

Eω

Eave

03

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

089 115 156 080 106 142 100 122 166 100 124 143 103 124 164 102 129 153 095 127 173 092 116 148

2(0.50) 2(0.50) 2(0.48) 2(0.48) 2(0.50) 2(0.50) 2(0.48) 2(0.52) 2(0.48) 2(0.50) 2(0.48) 2(0.50) 2(0.50) 2(0.49) 2(0.50) 2(0.50) 2(0.48) 2(0.48) 2(0.49) 2(0.49) 2(0.50) 2(0.48) 2(0.50) 2(0.50)

1.113 1.205 1.023 1.185 1.316 0.823 1.154 0.698 0.831 0.874 1.637 0.865 2.004 0.736 1.909 0.797 1.680 0.919 0.987 0.636 1.991 1.374 0.907 1.528

1.099 1.188 1.051 1.175 1.285 0.856 1.150 0.728 0.878 0.889 1.597 0.893 1.897 0.777 1.830 0.820 1.638 0.955 0.997 0.688 1.906 1.350 0.923 1.486

1.113 1.204 1.043 1.188 1.308 0.844 1.160 0.716 0.858 0.886 1.630 0.883 1.963 0.760 1.882 0.813 1.672 0.942 0.998 0.665 1.961 1.373 0.920 1.517

1.108 1.199 1.039 1.183 1.303 0.841 1.155 0.714 0.855 0.883 1.621 0.880 1.955 0.758 1.873 0.810 1.663 0.939 0.994 0.663 1.953 1.366 0.917 1.510

05

07

10

15

20

25

30

7.4 9.7 2.8 2.4 6.5 2.5 1.1 1.8 9.2 1.7 1.3 7.5 7.9 6.4 1.4 1.4 2.5 2.1 1.0 2.9 4.0 2.5 1.4 3.5

× × × × × × × × × × × × × × × × × × × × × × × ×

1016 1016 1013 1018 1018 1011 1017 1010 1010 1013 1022 1011 1025 1010 1023 1012 1022 1012 1015 109 1023 1020 1013 1019

formed to be at a comparably deeper site within the band gap of the host, suggesting that the hole traps are more prominent in the TL process. The simple glow curve structure and linear response over a wide range are some of the good characteristics of this cubic Y2O3:Eu3+ phosphor and may be useful for its application in high energy radiation.

parameters for the GCD basic function suggested by Kittis et al.47 for the second-order kinetics glow peaks given by ⎛ E T − Tm ⎞ I(t ) = 4Im exp⎜ ⎟ ⎝ kT Tm ⎠ ⎡ T2 ⎤−2 ⎧ E T − Tm ⎫ ⎬ + 1 + Δm ⎥ × ⎢ 2 (1 − Δ) exp⎨ ⎢⎣ Tm ⎥⎦ ⎩ kT Tm ⎭

4. CONCLUSIONS 5 mol % Eu3+ doped Y2O3 nanophosphors have been synthesized by the solution combustion method using ethylene diamine teta acetic acid (C10H16N2O8) as fuel. X-ray diffraction patterns confirm the cubic phase without impurity peaks, and we observed that crystallinity and crystallite size increase with calcination temperature. The average crystallite size calculated by the Debye−Scherer formula and the Williamson−Hall (W− H) plot are well comparable and were found to be in the range 8−43 nm which is also consistent with TEM results. Rietveld refinement confirmed the cubic phase with Ia3̅ (No. 206) space group, and the lattice parameter is found to be a = 10.6122(3) Å and cell volume (V) = 1195.16 (23) Å3. The electronic band gap of as-formed and calcined samples estimated from UV−vis spectroscopy is in the range 5.1−5.25 eV. Photoluminescence emission spectra excited at 240 nm show that there is a huge increase in emission intensity of calcined samples which is 10 times stronger than the as-synthesized product. The increase of

(11)

Here, Im is the main peak intensity, E is the activation energy (eV), and k is the Boltzmann constant. Once the activation energy (E) and order of kinetics (b) were determined, the frequency factor (s) was calculated from the equation ⎧ −E ⎫ βE ⎬[1 + (b − 1)Δm ] = s exp⎨ 2 2 kTm ⎩ kTm ⎭

frequency factor (Hz)

(12)

where Δ = 2kT/E, Δm = 2kTm/E, β is the linear heating rate, and k is the Boltzmann constant (8.6 × 10−5 eV K−1). The estimated trapping parameters (E, b, s) for 1000 °C cubic Y2O3:Eu3+ phosphor are shown in Tables 4 and 5. From the data, it is observed that a considerable amount of retrapping is taking place in all the TL second-order peaks of the phosphor. The activation energy (E) of all those peaks is 1923



Article

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PL intensity with annealing temperature is understood by the decrease of the probability of nonradiative recombination through the elimination of quenching defects. A thermoluminescence study of UV irradiated as-formed and calcined Eu3+ doped Y2O3 nanostructures has been studied in detail. We noted that the sample calcined at 1000 °C shows optimum TL intensity. TL intensity shows an increase for a wide range of UV exposure times. This has been explained on the basis of the track interaction model (TIM) and increased surface-to-volume ratio in the case of nanocrystalline Eu 3+ doped Y 2 O 3 nanophosphor. The samples exhibit a single broad TL glow curve at 116 οC which is due to overlapping of three closely lying peaks resolved by GCD functions. The kinetic parameters have also been evaluated using these functions. The studies reveal that crystallinity, crystallite size, and the elimination of structural disorder play a major role on the PL intensity and TL properties of these phosphors.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.M.N. gratefully acknowledges Visvesvaraya Technological University, Belgaum, for the financial support (VTU/Aca./ 2009-10/A-9/11714) to carry out this research work. R.H.K. is grateful to the Management, Principal and HOD Chemistry of M. S. Ramaiah Institute of Technology, Bangalore, for their constant support and encouragement. R.H.K. is also thankful to Dr. Tiju Thomas, Materials Research Center, IISc, Bangalore for his valuable suggestions and Dr. Anuradha Ashok and Mr. T. Vijayaraghavan, PSG Institute of Advanced Studies, Coimbatore, India for their help in recording TEM images.

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REFERENCES

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