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Photo and cathodoluminescence characteristics of dysprosium doped yttrium oxide nanoparticles prepared by Polyol method
Journal of Luminescence 146 (2014) 497–501
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photo and cathodoluminescence characteristics of dysprosium doped yttrium oxide nanoparticles prepared by Polyol method R. Balderas-Xicohténcatl a,n, R. Martínez-Martínez b, Z. Rivera-Alvarez a, J. Santoyo-Salazar a, C. Falcony a a b
Centro de Investigación y de Estudios Avanzados-IPN, Departamento de Física, Apdo, Postal 14-470, Del. Gustavo A. Madero, C.P. 07360, México, D.F., Mexico Instituto de Física y Matemáticas, Universidad Tecnológica de la Mixteca, Carretera a Acatlima Km. 2.5, Huajuapan de León, Oaxaca 69000, Mexico
art ic l e i nf o
a b s t r a c t
Article history: Received 13 July 2013 Received in revised form 11 October 2013 Accepted 16 October 2013 Available online 25 October 2013
The luminescent characteristics of Dy3 þ -doped Y2O3 nanopowders synthesized using the polyol method are reported. The Y2O3 nanoparticles presented a cubic phase crystalline structure of Y2O3 after an annealing treatment in oxygen ambient at temperatures above 600 1C. The averaged crystallite size determined from the X-ray diffraction peaks width was in the 20–32 nm range depending on the annealing temperature. Scanning and transmission electron microscopy studies indicate the formation of nanoparticle aggregates up to 175 nm in diameter. Photoluminescence and cathodoluminescence measurements show a predominant emission at 573 nm, which is attributed to the 4F9/2-6H13/2 of the Dy3 þ ion. The luminescence emission dependence with the dopant concentration and postannealing temperatures is discussed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Polyol Yttria Dy3 þ Luminescence Y2O3
1. Introduction Over the last few years there has been a great interest in the synthesis and characterization of nanostructured materials because of their unique properties, which are different from those exhibited by microscopic structured or “bulk” materials. Optical properties of rare-earth ions in nanostructured materials have also been extensively studied, the lifetime emission, the quantumefficiency and the concentration quenching are some of the crystallite size-dependent properties in these materials [1–5]. The luminescent properties of these nano-materials make them attractive for many technological applications like display devices, up-conversion solar cells, white-light generation and detectors in medical diagnosis equipment among others [6,7]. Considerable efforts have been dedicated for the synthesis of rare-earth nanophosphors with uniform size and shapes, by chemical means with techniques such as co-precipitation and polyol. In the coprecipitation case, the urea homogenous precipitation method and the co-precipitation method using (NH4)2CO3 have been used to synthesize rare-earth doped Y2O3 nanopowders through a relatively clean and simple procedure [8,9]. In the case of the polyol method, initially developed for the preparation of metal
n
Corresponding author. Tel.: þ 52 55 56187398. E-mail addresses: rbalderas@fis.cinvestav.mx, [email protected] (R. Balderas-Xicohténcatl). 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.041
nanoparticles [10], it has been used for the synthesis of nanopowders of inorganic compounds such as oxides, phosphates or sulfides with very successful results [11–15]. This technique is also simple and clean as the co-precipitation techniques mentioned above, although it allows for a better control on the nanoparticle size below 50 nm as shown in the present work. Yttrium oxide in its nano-crystalline form has been produced by many techniques and has been doped with several rare-earth ions; in its cubic phase, it has a large bandgap of 5.8 eV, a high dielectric constant of 14–18 and is also optically isotropic, with a refractive index of 1.91. It also has a high thermal stability and it has a dominant phonon energy of 380 cm 1, which is one of the smallest phonon energies among metallic oxides; this low vibration energy is desirable for a host material as it improves the probability of radiative transitions among electronic energy levels of the rare earth ions [3,16]. Most of the studies reported for polyol-synthesized Y2O3 nanoparticles have been concentrated on Eu3 þ and Tb3 þ ions, making aside several interesting rare-earth ions [16–21], such as Dy3 þ ion, which has been studied in materials synthesized by other techniques [4,22–24]. In the present work, the synthesis of Y2O3 nano-powder doped with the yellow emitting ion Dy3 þ by the polyol method and its characterization are reported. In particular, the crystallinity, grain size and the photo and cathodoluminescent properties of the samples are discussed as a function of Dy3 þ ion concentration and annealing temperature of the powders.
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R. Balderas-Xicohténcatl et al. / Journal of Luminescence 146 (2014) 497–501
2. Experimental details The appropriate amounts of the precursor salts, yttrium (III) nitrate hexahydrate (Y N3O9 6H2O, Alfa Aesar, 99.9%), and dysprosium chloride hexahydrate (DyCl3 6H2O, MERCK, 99.99%) were dissolved in 100 ml of diethylene–glycol (Sigma-Aldrich, 99%) to form a solution with a total concentration of 0.078 M for the Dy3 þ doped nano-sized powders synthesis by the polyol method. The Dy concentration was studied in the 0–4 at% range with respect to the Y content. The synthesis process consisted of three stages in which the solution was subjected to a continuous mechanical agitation and forced hydrolysis of the dissolved precursor was obtained by adding 2 ml of de-ionized water (18 MΩ cm) to the starting solution. The temperature of the solution in the first stage was elevated up to 60 1C and maintained for 1 h, to propitiate a good dilution of the precursor salts; in the second stage, the temperature was raised up to 120 1C and maintained for 1 h. The process of formation of the nano-sized powders occurs in the third stage, in which the temperature was increased up to 175 1C and the polyol method related evaporation/condensation cycles started, this stage lasts 2 h. The resulting powder was separated from the solvent by sedimentation and it was washed three times with methyl-alcohol. After each wash, the product was filtered in order to recover the powders. Then, the powders were dried at 300 1C for 12 h in air. The samples with different dopant concentrations were annealed at different temperatures in the 300– 900 1C range in an oxygen atmosphere for 2 h. A 0.5 cm diameter pellet of these powders was used to facilitate the characterization. X-ray diffraction patterns of the powders were recorded using a SIEMENS, D5000 X-Ray diffractometer at a 1.540 Å wavelength. The morphology of the obtained powders was studied with both scanning and transmission electron microscopy (SEM and TEM). The SEM analysis was carried out using a JEOL, JSM-7401 F, field emission scanning electron microscope with an EDS attachment. TEM images were recorded using a JEOL, JEM-2010 transmission electron microscope. The photoluminescent (PL) characteristics of the powders were measured at room temperature with a spectrofluorometer HORIBA, FlouroMax-P operating on the phosphorescence mode with a 20 ms sampling window, a 0.01 ms delay time and a 50 ms time per flash. Cathodoluminescence (CL) measurements were carried out using a Relion CL system of electron beam in a vacuum chamber and it was spectrally resolved with the same spectrofluorometer on continuous (fluorescence) mode coupled with an optical fiber to the CL chamber. The CL measurements were carried out with an e-beam accelerating voltage of 5 kV and a current of 0.1 mA at room temperature.
3. Results and discussion Fig. 1 shows the XRD patterns of Y2O3:Dy3 þ (0.3 mol%) annealed at 300, 500, 600, 700 and 900 1C. Powders with different doping concentrations annealed at 900 1C were also measured (not shown) but no evident variation in the diffraction patterns was observed in this case. The powders were amorphous up to 600 1C, at this temperature the formation of crystalline peaks associated with Dy2O2(CO3) and Y2O3 starts to be observed. The presence of the Dy2O2(CO3) is apparently associated with an incomplete dissociation of the organic components in the starting solution. At 700 1C and above, the diffraction pattern corresponding to a body-centered cubic phase of yttrium oxide with a lattice parameter a¼ 10.5957 Å is clearly defined (ASTM database card no. 01-071-0049). The crystallinity of the samples is enhanced as the annealing temperature increases, this is suggested by the increment on the diffraction peaks intensity [15,16,19,20,24].
Fig. 1. XRD pattern of the Y2O3:Dy3 þ (0.3 at%) for different annealing temperatures under an oxygen atmosphere.
Fig. 2. Influence of the post-annealing temperature in the average grain size of the Y2O3:Dy3 þ (0.3 at%).
The average nano-crystallite size was calculated from the [222] diffraction peak (in Fig. 1, at 2θ ¼29.173) using Sherrer's equation. Fig. 2 shows the averaged crystallite size as a function of the annealing temperature, the average crystallite size grew from 20 nm at 600 1C to 32 nm at 900 1C. The SEM micrograph shown in Fig. 3(a) and (b) illustrates the typical morphology of the powders and in this case, corresponds to Y2O3:Dy3 þ (0.3 mol%) powder annealed at 900 1C. It is observed that the powder consists of agglomerated particles with quasispherical shape with an average diameter of 175 nm. The TEM micrographs shown in Fig. 3(c) and (d) for a similar sample at two different magnifications confirm that nano-crystallites of up to 50 nm cluster together in larger aggregates. The size of the observed nano-crystallites is of the order of that determined by Sherrer´s formula from the X-ray diffraction data ( 32 nm). Table 1 lists the results of EDS analysis for Y, O and Dy relative content in the powders annealed at 900 1C, these powders had an excess of oxygen content in relation to the 40–60 at% stoichiometry expected for Y2O3. The Dy content is found to increase proportionally to the content used in the starting solution. It is possible that the excess of oxygen in these powders is related to a
R. Balderas-Xicohténcatl et al. / Journal of Luminescence 146 (2014) 497–501
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Fig. 3. Micrographs of Y2O3:Dy3 þ (0.3 at%) sintered at 900 1C (a) and (b) SEM, and (c) and (d) TEM.
Table 1 Relative chemical composition concentration of the studied samples. Sample
B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8
(0%) (0.05%) (0.1%) (0.3%) (0.5%) (0.7%) (1%) (2%)
Concentration (at%) Y
O
Dy
33.67 0.5 34.17 0.1 32.7 7 3.0 37.3 7 0.9 34.7 7 0.4 34.0 7 0.2 32.6 7 0.7 35.27 0.9
66.47 0.5 65.9 7 0.2 67.2 7 3.0 62.6 7 0.9 64.97 0.7 65.6 7 0.3 66.97 0.8 63.6 7 1.0
0 0.02 70.01 0.077 0.02 0.187 0.02 0.247 0.01 0.32 70.13 0.517 0.16 1.29 7 0.06
not completely densified material, even at the highest annealing temperature (900 1C). Fig. 4 illustrates the room temperature photoluminescence excitation and emission spectral characteristic of the synthesized nanopowders, the spectra shown correspond to the Y2O3:Dy3 þ (0.3 mol%) powder annealed at 900 1C. The excitation spectrum was measured in the 200–450 nm range for the 573 nm emission peak and shows several peaks located at 209, 230, 353, 400 and 413 nm. The peak centered at 209 nm is associated with the yttrium oxide band-to-band absorption and charge transfer to the Dy3 þ ion. The peak centered at 230 nm is attributed to the Dy3 þ to O2 charge-transfer band (CTB) [24]. The peak centered at 353 nm corresponds to the transition 6H15/2-6P7/2. The localized peaks at 400 and 413 nm are associated with transitions within the electronic energy levels of the Dy3 þ dopant ion (6H15/2-4F7/2 þ 4 I13/2 and 6H15/2-4G11/2, respectively). The PL emission spectrum was obtained at an excitation wavelength of 209 nm, it exhibits four distinct emission peaks located at 573 nm, associated with the hypersensitive transition 4F9/2-6H13/2; at 485 nm corresponding to the less sensitive transition 4F9/2-6H15/2; at 667 nm corresponding to the 4F9/2-6H11/2 transition and at 766 nm corresponding to the transition 4F9/2-6H9/2. The overall luminescence
Fig. 4. PL excitation for the 573 nm emission (200–450 nm) and emission excited with 209 nm (450–800 nm) spectra of Y2O3:Dy3 þ (0.3 at%) powders.
intensity dependence with annealing temperature and doping density was monitored by plotting the 573 peak intensity as a function of each of these parameters (Fig. 5). The PL intensity was found to increase monotonically with increasing annealing temperature up to the highest temperature considered in this work (900 1C). This behavior is correlated with the crystallinity improvement with increasing annealing temperature shown in Fig. 1, therefore it is likely that the increase in luminescence emission is due to a stronger crystalline field on the Dy3 þ ion within the Y2O3 host. The inset in Fig. 5 shows the photoluminescent emission of the powders annealed at 900 1C for different dopant concentrations. It is possible to observe that the best emission efficiency is associated with a dopant concentration of 0.3 mol%. When the Dy concentration is more than 0.3 mol% the PL
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Fig. 5. PL emission intensity of the Dy doped Y2O3 (0.3 at%) nanoparticles taking the peak centered at 573 nm vs. the dopant concentration. Inset shows the PL emission intensity of the Dy doped Y2O3 (0.3 at%) nanoparticles taking the peak centered at 573 nm vs. the post-annealing temperature.
Fig. 6. Comparison of photoluminescence decays and lifetimes of the Dy doped Y2O3 (0.3 at%) nanoparticles for different excitation wavelengths.
emission decreases. At higher dopant concentrations the mean distance between dopant ions is much shorter; therefore these ions can interact by an electric multipolar process leading to energy migration and to an increase of the probability of a nonradiative recombination [25,26]. The optimum concentration for luminescent emission (0.3 mol%) of the Y2O3:Dy3 þ nanopowders was used in further experiments. Fig. 6 shows on a semi-logarithmic scale, the time decay of the luminescence intensity at the 573 nm emission peak when excited by either 209 or 413 nm light. In both cases the data is best fitted by a double exponential decay, although the short time exponential is clearly dominant. In the case of the 209 nm excitation, associated with a charge transfer from the host matrix, the decay times are 0.853 70.002 and 9.33 70.33 ms, while the decay times associated with an excitation within electronic states of the Dy3 þ ion (413 nm) are 0.709 70.001 and 5.79 7 0.01 ms. The total average decay time (τmean ¼ 2.257 0.14 ms for 209 nm and τmean ¼1.62 70.04 ms for 413 nm) is shorter in the excitation within electronic states case (413 nm) as expected from a less complex excitation-radiation mechanism as compared with the charge-transfer mediated process. The average decay time was
Fig. 7. CL emission spectra of the Y2O3:Dy3 þ (0.3 at%) nanoparticles for an acceleration voltage of 5 kV and a beam current of 0.1 mA. Inset shows a digital photograph of eye-visible catholuminescence emission from the nanoparticles.
Fig. 8. CL emission intensity vs. the dopant concentration of the Dy doped Y2O3 (0.3 at%) nanoparticles. Inset shows the CL emission intensity of the peak centered at 573 nm vs. the post-annealing temperature of the Dy doped Y2O3 (0.3 at%) nanoparticles.
calculated using the following equation: t mean ¼
At 21 þ Bt 22 At 1 þ Bt 2
ð1Þ
A typical cathodoluminescence emission spectrum is presented in Fig. 7 for the sample with 0.3 mol% dopant and 900 1C anneal. It was carried out with an e-beam accelerating voltage of 5 kV and with a current of 0.1 mA at room temperature. The inset shows a photograph of the sample within the cathodoluminescence chamber when the e-beam is on. The emission peaks observed are associated to the transitions within the electronic energy levels of the Dy3 þ ion as indicated for the photoluminescent emission (Fig. 4) although in this case, the relative intensity of the 573 nm peak is much more dominant with respect to the rest of the emission peaks. The origin of this phenomenon is not clear at present, although the excitation process differences seem to be responsible for it. Fig. 8 presents the CL emission intensity for the 573 nm peak as a function of doping density and annealing temperature. The inset of Fig. 8 shows that the peak of this
R. Balderas-Xicohténcatl et al. / Journal of Luminescence 146 (2014) 497–501
emission is observed at a doping concentration of 0.3 mol% as in the case of PL (inset Fig. 5), in fact the overall behavior of this emission with doping concentration is similar for both CL and PL. The CL intensity dependence with annealing temperature on the other hand exhibits a different behavior from that observed with PL. The CL emission is observed only above 600 1C and increases as the annealing temperature is increased. The optimal emission is obtained at 900 1C, as in the case of PL. It is possible that higher annealing temperatures further improve the emission intensity of the luminescence.
4. Conclusions
Acknowledgments The authors are grateful to the CONACyT and Physics Department of CINVESTAV-IPN for the financial support. We are also grateful to Marcela Guerrero and Ana Soto for the technical support provided. References [1] [2] [3] [4] [5] [6]
Nanoparticles of Dy3 þ doped Y2O3 have been synthesized by the polyol method with different dopant concentrations and different annealing temperatures and their photo and cathodoluminescent characteristics are reported for the first time. The Y2O3 nano-crystals present at annealing temperatures above 600 1C have a body-centered cubic pure phase and an average diameter of the order of 20–32 nm. SEM and TEM micrographs indicate the formation agglomerates of 175 nm of these nano-crystallites. The luminescence emissions show the characteristic peaks associated with the Dy3 þ ion inter-electronic energy levels transitions. It has been determined that the best luminescence emission efficiency is for 0.3 mol% dopant concentration, above this concentration a quenching effect resulting from the Dy–Dy interactions is observed. In the case of cathodoluminescent emission, the 573 nm peak is much more dominant with respect to the rest of the Dy3 þ ion related emission peaks present. The luminescence intensity was found to increase with increasing annealing temperature in the 300–900 1C range. This result is probably due to an improvement in the crystallographic arrangement of the Y2O3 host, since higher annealing temperatures induce a growth of the nano-crystallites and a reduction of crystalline defects. The synthesized-powders have good overall luminescent characteristics that open a possible option for technological applications due the simplicity, industrial scalability and low cost of the synthesis method used.
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P. Psuja, D. Hreniak, W. Strek, J. Nanomater. 2007 (2007). (Article ID 81350). T.-W. Kuo, T.-M. Chen, J. Lumin. 130 (2010) 483. C.X. Li, J. Lin, J. Mater. Chem. 20 (2010) 6831. Y. Wen, Y. Wang, B. Liu, F. Zhang, Opt. Mater. 34 (2012) 889. N. Niu, P. Yang, Y. Wang, W. Wang, F. He, S. Gai, D. Wang, J. Alloys Compd. 509 (2011) 3096. C. Feldmann, T. Justel, C.R. Ronda, P.J. Schmidt, Adv. Funct. Mater. 13 (2003) 511. H.A. Hoppe, Angew. Chem., Int. Ed. 48 (2009) 3572. T. Atabaev, H.K. Kim, Y.H. Hwang, J Colloid Interface Sci. 373 (2012) 14. D. Tu, Y.J. Liang, R. Liu, D.Y. Li, J. Lumin. 131 (2011) 2569. F. Fievet, J.P. Lagier, B. Blin, B. Beaudoin, M. Figlarz, Solid State Ionics 32-3 (1989) 198. C. Feldmann, H.O. Jungk, Angew. Chem. Int. Ed. 40 (2001) 359. O. Palchik, R. Kerner, A. Gedanken, A.M. Weiss, M.A. Slifkin, V. Palchik, J. Mater. Chem. 11 (2001) 874. C. Feldmann, Solid State Sci. 7 (2005) 868. Z.W. Quan, C.X. Li, X.M. Zhang, J. Yang, P.P. Yang, C.M. Zhang, J. Lin, Cryst. Growth Des. 8 (2008) 2384. M.A. Flores-Gonzalez, G. Ledoux, S. Roux, K. Lebbou, P. Perriat, O. Tillement, J. Solid State Chem. 178 (2005) 989. L.R. Singh, R.S. Ningthoujam, V. Sudarsan, I. Srivastava, S.D. Singh, G.K.Dey and S.K. Kulshreshtha, Nanotechnology 19, 2008, 055201. C. Feldmann, J. Merikhi, J. Mater. Sci. 38 (2003) 1731. M. Serantoni, E. Mercadelli, A.L. Costa, M. Blosi, L. Esposito, A. Sanson, Ceram. Int. 36 (2010) 103. T. Anh, P. Benalloul, C. Barthou, L.T. Giang, N. Vu, L. Minh, J. Nanomater. (2007) 48247. P. Psuja, D. Hreniak, W. Strek, J. Rare Earths 27 (2009) 574. R. Srinivasan, N.R. Yogamalar, J. Elanchezhiyan, R.J. Joseyphus, A.C. Bose, J. Alloys Compd. 496 (2010) 472. M. Diaz, C. Pecharroman, F. del Monte, J. Sanz, J.E. Iglesias, J.S. Moya, C. Yamagata, S. Mello-Castanho, Chem. Mater. 17 (2005) 1774. J. Zhang, Y. Wang, Y. Wen, F. Zhang, B. Liu, J. Alloys Compd. 509 (2011) 4649. M. Jayasimhadri, B.V. Ratnam, K. Jang, H.S. Lee, B.J. Chen, S.S. Yi, J.H. Jeong, L. R. Moorthy, J. Am. Ceram. Soc. 93 (2010) 494. T.S. Atabaev, H.H.T. Vu, Y.D. Kim, J.H. Lee, H.K. Kim, Y.H. Hwang, J. Phys. Chem. Solids 73 (2012) 176. T.S. Atabaev, H.T.V. Hong, H.K. Kim, Y.H. Hwang, J. Korean Phys Soc. 60 (2012) 244.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photo and cathodoluminescence characteristics of dysprosium doped yttrium oxide nanoparticles prepared by Polyol method R. Balderas-Xicohténcatl a,n, R. Martínez-Martínez b, Z. Rivera-Alvarez a, J. Santoyo-Salazar a, C. Falcony a a b
Centro de Investigación y de Estudios Avanzados-IPN, Departamento de Física, Apdo, Postal 14-470, Del. Gustavo A. Madero, C.P. 07360, México, D.F., Mexico Instituto de Física y Matemáticas, Universidad Tecnológica de la Mixteca, Carretera a Acatlima Km. 2.5, Huajuapan de León, Oaxaca 69000, Mexico
art ic l e i nf o
a b s t r a c t
Article history: Received 13 July 2013 Received in revised form 11 October 2013 Accepted 16 October 2013 Available online 25 October 2013
The luminescent characteristics of Dy3 þ -doped Y2O3 nanopowders synthesized using the polyol method are reported. The Y2O3 nanoparticles presented a cubic phase crystalline structure of Y2O3 after an annealing treatment in oxygen ambient at temperatures above 600 1C. The averaged crystallite size determined from the X-ray diffraction peaks width was in the 20–32 nm range depending on the annealing temperature. Scanning and transmission electron microscopy studies indicate the formation of nanoparticle aggregates up to 175 nm in diameter. Photoluminescence and cathodoluminescence measurements show a predominant emission at 573 nm, which is attributed to the 4F9/2-6H13/2 of the Dy3 þ ion. The luminescence emission dependence with the dopant concentration and postannealing temperatures is discussed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Polyol Yttria Dy3 þ Luminescence Y2O3
1. Introduction Over the last few years there has been a great interest in the synthesis and characterization of nanostructured materials because of their unique properties, which are different from those exhibited by microscopic structured or “bulk” materials. Optical properties of rare-earth ions in nanostructured materials have also been extensively studied, the lifetime emission, the quantumefficiency and the concentration quenching are some of the crystallite size-dependent properties in these materials [1–5]. The luminescent properties of these nano-materials make them attractive for many technological applications like display devices, up-conversion solar cells, white-light generation and detectors in medical diagnosis equipment among others [6,7]. Considerable efforts have been dedicated for the synthesis of rare-earth nanophosphors with uniform size and shapes, by chemical means with techniques such as co-precipitation and polyol. In the coprecipitation case, the urea homogenous precipitation method and the co-precipitation method using (NH4)2CO3 have been used to synthesize rare-earth doped Y2O3 nanopowders through a relatively clean and simple procedure [8,9]. In the case of the polyol method, initially developed for the preparation of metal
n
Corresponding author. Tel.: þ 52 55 56187398. E-mail addresses: rbalderas@fis.cinvestav.mx, [email protected] (R. Balderas-Xicohténcatl). 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.10.041
nanoparticles [10], it has been used for the synthesis of nanopowders of inorganic compounds such as oxides, phosphates or sulfides with very successful results [11–15]. This technique is also simple and clean as the co-precipitation techniques mentioned above, although it allows for a better control on the nanoparticle size below 50 nm as shown in the present work. Yttrium oxide in its nano-crystalline form has been produced by many techniques and has been doped with several rare-earth ions; in its cubic phase, it has a large bandgap of 5.8 eV, a high dielectric constant of 14–18 and is also optically isotropic, with a refractive index of 1.91. It also has a high thermal stability and it has a dominant phonon energy of 380 cm 1, which is one of the smallest phonon energies among metallic oxides; this low vibration energy is desirable for a host material as it improves the probability of radiative transitions among electronic energy levels of the rare earth ions [3,16]. Most of the studies reported for polyol-synthesized Y2O3 nanoparticles have been concentrated on Eu3 þ and Tb3 þ ions, making aside several interesting rare-earth ions [16–21], such as Dy3 þ ion, which has been studied in materials synthesized by other techniques [4,22–24]. In the present work, the synthesis of Y2O3 nano-powder doped with the yellow emitting ion Dy3 þ by the polyol method and its characterization are reported. In particular, the crystallinity, grain size and the photo and cathodoluminescent properties of the samples are discussed as a function of Dy3 þ ion concentration and annealing temperature of the powders.
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R. Balderas-Xicohténcatl et al. / Journal of Luminescence 146 (2014) 497–501
2. Experimental details The appropriate amounts of the precursor salts, yttrium (III) nitrate hexahydrate (Y N3O9 6H2O, Alfa Aesar, 99.9%), and dysprosium chloride hexahydrate (DyCl3 6H2O, MERCK, 99.99%) were dissolved in 100 ml of diethylene–glycol (Sigma-Aldrich, 99%) to form a solution with a total concentration of 0.078 M for the Dy3 þ doped nano-sized powders synthesis by the polyol method. The Dy concentration was studied in the 0–4 at% range with respect to the Y content. The synthesis process consisted of three stages in which the solution was subjected to a continuous mechanical agitation and forced hydrolysis of the dissolved precursor was obtained by adding 2 ml of de-ionized water (18 MΩ cm) to the starting solution. The temperature of the solution in the first stage was elevated up to 60 1C and maintained for 1 h, to propitiate a good dilution of the precursor salts; in the second stage, the temperature was raised up to 120 1C and maintained for 1 h. The process of formation of the nano-sized powders occurs in the third stage, in which the temperature was increased up to 175 1C and the polyol method related evaporation/condensation cycles started, this stage lasts 2 h. The resulting powder was separated from the solvent by sedimentation and it was washed three times with methyl-alcohol. After each wash, the product was filtered in order to recover the powders. Then, the powders were dried at 300 1C for 12 h in air. The samples with different dopant concentrations were annealed at different temperatures in the 300– 900 1C range in an oxygen atmosphere for 2 h. A 0.5 cm diameter pellet of these powders was used to facilitate the characterization. X-ray diffraction patterns of the powders were recorded using a SIEMENS, D5000 X-Ray diffractometer at a 1.540 Å wavelength. The morphology of the obtained powders was studied with both scanning and transmission electron microscopy (SEM and TEM). The SEM analysis was carried out using a JEOL, JSM-7401 F, field emission scanning electron microscope with an EDS attachment. TEM images were recorded using a JEOL, JEM-2010 transmission electron microscope. The photoluminescent (PL) characteristics of the powders were measured at room temperature with a spectrofluorometer HORIBA, FlouroMax-P operating on the phosphorescence mode with a 20 ms sampling window, a 0.01 ms delay time and a 50 ms time per flash. Cathodoluminescence (CL) measurements were carried out using a Relion CL system of electron beam in a vacuum chamber and it was spectrally resolved with the same spectrofluorometer on continuous (fluorescence) mode coupled with an optical fiber to the CL chamber. The CL measurements were carried out with an e-beam accelerating voltage of 5 kV and a current of 0.1 mA at room temperature.
3. Results and discussion Fig. 1 shows the XRD patterns of Y2O3:Dy3 þ (0.3 mol%) annealed at 300, 500, 600, 700 and 900 1C. Powders with different doping concentrations annealed at 900 1C were also measured (not shown) but no evident variation in the diffraction patterns was observed in this case. The powders were amorphous up to 600 1C, at this temperature the formation of crystalline peaks associated with Dy2O2(CO3) and Y2O3 starts to be observed. The presence of the Dy2O2(CO3) is apparently associated with an incomplete dissociation of the organic components in the starting solution. At 700 1C and above, the diffraction pattern corresponding to a body-centered cubic phase of yttrium oxide with a lattice parameter a¼ 10.5957 Å is clearly defined (ASTM database card no. 01-071-0049). The crystallinity of the samples is enhanced as the annealing temperature increases, this is suggested by the increment on the diffraction peaks intensity [15,16,19,20,24].
Fig. 1. XRD pattern of the Y2O3:Dy3 þ (0.3 at%) for different annealing temperatures under an oxygen atmosphere.
Fig. 2. Influence of the post-annealing temperature in the average grain size of the Y2O3:Dy3 þ (0.3 at%).
The average nano-crystallite size was calculated from the [222] diffraction peak (in Fig. 1, at 2θ ¼29.173) using Sherrer's equation. Fig. 2 shows the averaged crystallite size as a function of the annealing temperature, the average crystallite size grew from 20 nm at 600 1C to 32 nm at 900 1C. The SEM micrograph shown in Fig. 3(a) and (b) illustrates the typical morphology of the powders and in this case, corresponds to Y2O3:Dy3 þ (0.3 mol%) powder annealed at 900 1C. It is observed that the powder consists of agglomerated particles with quasispherical shape with an average diameter of 175 nm. The TEM micrographs shown in Fig. 3(c) and (d) for a similar sample at two different magnifications confirm that nano-crystallites of up to 50 nm cluster together in larger aggregates. The size of the observed nano-crystallites is of the order of that determined by Sherrer´s formula from the X-ray diffraction data ( 32 nm). Table 1 lists the results of EDS analysis for Y, O and Dy relative content in the powders annealed at 900 1C, these powders had an excess of oxygen content in relation to the 40–60 at% stoichiometry expected for Y2O3. The Dy content is found to increase proportionally to the content used in the starting solution. It is possible that the excess of oxygen in these powders is related to a
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Fig. 3. Micrographs of Y2O3:Dy3 þ (0.3 at%) sintered at 900 1C (a) and (b) SEM, and (c) and (d) TEM.
Table 1 Relative chemical composition concentration of the studied samples. Sample
B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8
(0%) (0.05%) (0.1%) (0.3%) (0.5%) (0.7%) (1%) (2%)
Concentration (at%) Y
O
Dy
33.67 0.5 34.17 0.1 32.7 7 3.0 37.3 7 0.9 34.7 7 0.4 34.0 7 0.2 32.6 7 0.7 35.27 0.9
66.47 0.5 65.9 7 0.2 67.2 7 3.0 62.6 7 0.9 64.97 0.7 65.6 7 0.3 66.97 0.8 63.6 7 1.0
0 0.02 70.01 0.077 0.02 0.187 0.02 0.247 0.01 0.32 70.13 0.517 0.16 1.29 7 0.06
not completely densified material, even at the highest annealing temperature (900 1C). Fig. 4 illustrates the room temperature photoluminescence excitation and emission spectral characteristic of the synthesized nanopowders, the spectra shown correspond to the Y2O3:Dy3 þ (0.3 mol%) powder annealed at 900 1C. The excitation spectrum was measured in the 200–450 nm range for the 573 nm emission peak and shows several peaks located at 209, 230, 353, 400 and 413 nm. The peak centered at 209 nm is associated with the yttrium oxide band-to-band absorption and charge transfer to the Dy3 þ ion. The peak centered at 230 nm is attributed to the Dy3 þ to O2 charge-transfer band (CTB) [24]. The peak centered at 353 nm corresponds to the transition 6H15/2-6P7/2. The localized peaks at 400 and 413 nm are associated with transitions within the electronic energy levels of the Dy3 þ dopant ion (6H15/2-4F7/2 þ 4 I13/2 and 6H15/2-4G11/2, respectively). The PL emission spectrum was obtained at an excitation wavelength of 209 nm, it exhibits four distinct emission peaks located at 573 nm, associated with the hypersensitive transition 4F9/2-6H13/2; at 485 nm corresponding to the less sensitive transition 4F9/2-6H15/2; at 667 nm corresponding to the 4F9/2-6H11/2 transition and at 766 nm corresponding to the transition 4F9/2-6H9/2. The overall luminescence
Fig. 4. PL excitation for the 573 nm emission (200–450 nm) and emission excited with 209 nm (450–800 nm) spectra of Y2O3:Dy3 þ (0.3 at%) powders.
intensity dependence with annealing temperature and doping density was monitored by plotting the 573 peak intensity as a function of each of these parameters (Fig. 5). The PL intensity was found to increase monotonically with increasing annealing temperature up to the highest temperature considered in this work (900 1C). This behavior is correlated with the crystallinity improvement with increasing annealing temperature shown in Fig. 1, therefore it is likely that the increase in luminescence emission is due to a stronger crystalline field on the Dy3 þ ion within the Y2O3 host. The inset in Fig. 5 shows the photoluminescent emission of the powders annealed at 900 1C for different dopant concentrations. It is possible to observe that the best emission efficiency is associated with a dopant concentration of 0.3 mol%. When the Dy concentration is more than 0.3 mol% the PL
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Fig. 5. PL emission intensity of the Dy doped Y2O3 (0.3 at%) nanoparticles taking the peak centered at 573 nm vs. the dopant concentration. Inset shows the PL emission intensity of the Dy doped Y2O3 (0.3 at%) nanoparticles taking the peak centered at 573 nm vs. the post-annealing temperature.
Fig. 6. Comparison of photoluminescence decays and lifetimes of the Dy doped Y2O3 (0.3 at%) nanoparticles for different excitation wavelengths.
emission decreases. At higher dopant concentrations the mean distance between dopant ions is much shorter; therefore these ions can interact by an electric multipolar process leading to energy migration and to an increase of the probability of a nonradiative recombination [25,26]. The optimum concentration for luminescent emission (0.3 mol%) of the Y2O3:Dy3 þ nanopowders was used in further experiments. Fig. 6 shows on a semi-logarithmic scale, the time decay of the luminescence intensity at the 573 nm emission peak when excited by either 209 or 413 nm light. In both cases the data is best fitted by a double exponential decay, although the short time exponential is clearly dominant. In the case of the 209 nm excitation, associated with a charge transfer from the host matrix, the decay times are 0.853 70.002 and 9.33 70.33 ms, while the decay times associated with an excitation within electronic states of the Dy3 þ ion (413 nm) are 0.709 70.001 and 5.79 7 0.01 ms. The total average decay time (τmean ¼ 2.257 0.14 ms for 209 nm and τmean ¼1.62 70.04 ms for 413 nm) is shorter in the excitation within electronic states case (413 nm) as expected from a less complex excitation-radiation mechanism as compared with the charge-transfer mediated process. The average decay time was
Fig. 7. CL emission spectra of the Y2O3:Dy3 þ (0.3 at%) nanoparticles for an acceleration voltage of 5 kV and a beam current of 0.1 mA. Inset shows a digital photograph of eye-visible catholuminescence emission from the nanoparticles.
Fig. 8. CL emission intensity vs. the dopant concentration of the Dy doped Y2O3 (0.3 at%) nanoparticles. Inset shows the CL emission intensity of the peak centered at 573 nm vs. the post-annealing temperature of the Dy doped Y2O3 (0.3 at%) nanoparticles.
calculated using the following equation: t mean ¼
At 21 þ Bt 22 At 1 þ Bt 2
ð1Þ
A typical cathodoluminescence emission spectrum is presented in Fig. 7 for the sample with 0.3 mol% dopant and 900 1C anneal. It was carried out with an e-beam accelerating voltage of 5 kV and with a current of 0.1 mA at room temperature. The inset shows a photograph of the sample within the cathodoluminescence chamber when the e-beam is on. The emission peaks observed are associated to the transitions within the electronic energy levels of the Dy3 þ ion as indicated for the photoluminescent emission (Fig. 4) although in this case, the relative intensity of the 573 nm peak is much more dominant with respect to the rest of the emission peaks. The origin of this phenomenon is not clear at present, although the excitation process differences seem to be responsible for it. Fig. 8 presents the CL emission intensity for the 573 nm peak as a function of doping density and annealing temperature. The inset of Fig. 8 shows that the peak of this
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emission is observed at a doping concentration of 0.3 mol% as in the case of PL (inset Fig. 5), in fact the overall behavior of this emission with doping concentration is similar for both CL and PL. The CL intensity dependence with annealing temperature on the other hand exhibits a different behavior from that observed with PL. The CL emission is observed only above 600 1C and increases as the annealing temperature is increased. The optimal emission is obtained at 900 1C, as in the case of PL. It is possible that higher annealing temperatures further improve the emission intensity of the luminescence.
4. Conclusions
Acknowledgments The authors are grateful to the CONACyT and Physics Department of CINVESTAV-IPN for the financial support. We are also grateful to Marcela Guerrero and Ana Soto for the technical support provided. References [1] [2] [3] [4] [5] [6]
Nanoparticles of Dy3 þ doped Y2O3 have been synthesized by the polyol method with different dopant concentrations and different annealing temperatures and their photo and cathodoluminescent characteristics are reported for the first time. The Y2O3 nano-crystals present at annealing temperatures above 600 1C have a body-centered cubic pure phase and an average diameter of the order of 20–32 nm. SEM and TEM micrographs indicate the formation agglomerates of 175 nm of these nano-crystallites. The luminescence emissions show the characteristic peaks associated with the Dy3 þ ion inter-electronic energy levels transitions. It has been determined that the best luminescence emission efficiency is for 0.3 mol% dopant concentration, above this concentration a quenching effect resulting from the Dy–Dy interactions is observed. In the case of cathodoluminescent emission, the 573 nm peak is much more dominant with respect to the rest of the Dy3 þ ion related emission peaks present. The luminescence intensity was found to increase with increasing annealing temperature in the 300–900 1C range. This result is probably due to an improvement in the crystallographic arrangement of the Y2O3 host, since higher annealing temperatures induce a growth of the nano-crystallites and a reduction of crystalline defects. The synthesized-powders have good overall luminescent characteristics that open a possible option for technological applications due the simplicity, industrial scalability and low cost of the synthesis method used.
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