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Synthesis and luminescence properties of Ho3+ doped Y2O3 submicron particles
Journal of Physics and Chemistry of Solids 73 (2012) 176–181
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis and luminescence properties of Ho3 þ doped Y2O3 submicron particles Timur Sh. Atabaev a, Hong-Ha Thi Vu a, Yang-Do Kim b, Jae-Ho Lee c, Hyung-Kook Kim a,n, Yoon-Hwae Hwang a,n a
Department of Nanomaterials Engineering and BK 21 Nano Fusion Technology Division, Pusan National University, Miryang 627-706, Republic of Korea School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea c Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2011 Received in revised form 14 October 2011 Accepted 9 November 2011 Available online 18 November 2011
This paper presents comprehensive results of the eco-friendly, large scale fabrication of nearly spherical Ho3 þ -doped Y2O3 submicron particles, synthesized using the urea homogeneous precipitation method. The dependence of the photoluminescence emission on the doping concentration was examined to determine the optimum Ho3 þ concentration in the samples. X-ray diffraction data of the Y2O3:Ho3 þ particles revealed a cubic Y2O3 structure. Field emission scanning electron microscopy confirmed the formation of nearly spherical shape particles with a mean diameter of 200 7 50 nm. The luminescence emission intensity significantly increased with increasing calcination temperature due to the improved crystallinity of the synthesized particles. Strong visible green-yellowish emission due to 5F4; 5S2–5I8 Stokes transitions was observed under constant 450 nm excitation. Simple large scale fabrication along with the strong visible green-yellowish emission might give these particles wide area of applications. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials A. Oxides D. Luminescence
1. Introduction In recent years, there has been increasing interest in the area of rare-earth (RE)-ion-doped nanoparticles (NPs) for potential applications in photonic and biophotonic areas [1–4]. In contrast to semiconductor quantum dots, the emission wavelength of lanthanide-doped NPs is independent of the particle size. Moreover, nanoparticles with different emission wavelengths can be easily obtained by the controlled doping of lanthanide ions into an appropriate host material. Preliminary results showed that yttrium oxide (Y2O3) is a promising host material for labels in biotechnology owing to its stability and biocompatibility [5]. Moreover, Y2O3 has low phonon energy and excellent physical properties (phase stability, low thermal expansion and high melting point) [6]. The non-radiative relaxation rate appears to be relatively small for RE ions in the Y2O3 host, which can greatly enhance the luminescence emission. Up to now, the synthesis of irregular shaped RE doped Y2O3 NPs and bulk phosphor materials have been extensively studied using a co-precipitation method or even using some toxic organic solvents [6–8]. In contrast, only a few works addressed their
n
Corresponding authors. Tel.: þ 82 55 350 5845; fax: þ 82 55 353 5844. E-mail addresses: [email protected] (H.-K. Kim), [email protected] (Y.-H. Hwang). 0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.11.010
morphological characteristics. However, it is well known that uniform spherical-shaped particles can improve the optical performance due to the high packing density and reduction of light scattering [9]. The urea homogeneous precipitation has been proved to be a green, simple, quick and low-cost method that can be used for the scalable mass production of Y2O3 spherical particles. A broad luminescent spectrum ranging from blue to IR can be produced by the intra-4f transitions of Ho3 þ ions, where strong green luminescence normally dominates over the blue and red regions. Strong green luminescence due to 5F4; 5S2–5I8 transitions within the Ho3 þ ion was recently observed in LaAlO3 [10,11] and bismuth telluride hosts [12]. Strong green luminescence emission can be used in the lamp industry, cathode radiation tube (CRT), field emission displays (FED), security printing or even biologically oriented applications [10–13]. The efficiency of RE-doped phosphors for frequency conversion is often influenced by the dopant concentration or calcination temperature [14]. Therefore, the physical understanding of the frequency conversion processes of RE-doped phosphors and their dependence on size, calcination temperature and activators concentration is of fundamental importance. This paper reports the large scale controlled synthesis of nearly spherical Ho3 þ doped Y2O3 submicron particles prepared using the urea homogeneous precipitation method. The optical properties of synthesized submicron particles were investigated
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in detail. Strong visible green-yellowish luminescence was observed due to 5F4; 5S2–5I8 Stokes transitions within the Ho3 þ ions under constant 450 nm excitation. The luminescent properties of these particles were found to be strongly dependent on the Ho-ion concentration in the host material and calcination temperature, the parameters, which can be easily controlled during the fabrication.
2. Experimental 2.1. Chemical synthesis Analytical grade yttrium oxide Y2O3 (99.9%), holmium oxide Ho2O3 (99.9%), nitric acid HNO3 (70%) and urea (99–100.5%) were purchased from Sigma-Aldrich and used without further purification. Yttrium oxide and holmium oxide with a stoichiometric mol ratio (Y/Ho ¼100 x/x, where x ¼0.5, 1, 2 and 3 (a total of 0.001 mol for each sample)) were diluted with nitric acid and stirred vigorously until the solution became colorless. The solutions were then dried at 70 1C for one day to remove the excess nitric acid. Each sample of dried rare-earth salts was mixed with 40 ml of deionized water, 0.5 g of urea and then stirred vigorously for 30 min to form a clear solution. Sealed beakers with freshly prepared solutions were then placed in an electrical furnace and heated to 90 1C for 2 h, after which the beakers were immediately cooled in cold water bath to prevent the further growth of particles. After cooling to room temperature, the precipitates were centrifuged and thoroughly washed with deionized water and ethanol, and finally dried in an oven at 70 1C for 24 h. Dried precipitates were calcined in air at different temperatures up to 1000 1C for 1 h. 2.2. Physical characterization The structure of the prepared powders was examined by X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer with Cu Ka radiation (l ¼0.15405 nm) and a 2y scan range of 20–601. The structural properties were also analyzed using Fourier transform infrared spectroscopy (FTIR Jasco FT/IR6300). The morphologies of the particles were characterized by field-emission scanning-electron microscopy (FESEM, Hitachi S-4700) and field-emission transmission electron microscopy (FETEM, JEOL JEM-2100F). Elemental analysis was carried out by energy dispersive X-ray spectroscopy (EDX; Horiba, 6853-H). The PL measurements were taken with a Hitachi F-7000 spectrophotometer equipped with a 150 W Xenon lamp as the excitation source. The samples (50 mg) were placed into a cylindrical sample holder, 10 mm in diameter and pressed into pellets for the XRD and PL measurements. All the measurements were performed at room temperature.
3. Results and discussion Fig. 1 shows the XRD patterns taken in the scan range of 20–601 2y for four samples with different concentrations. All samples exhibited identical diffraction patterns, which were well consistent with the pure cubic Y2O3 structure (JCPDS no. 86-1107) with the space group of Ia3 (206) [15]. Since the concentration of Ho dopant is relatively low, no additional peaks of other phases were found, indicating the formation of a pure cubic Y2O3 phase. ˚ The similar ionic radius of these elements (Yir ¼0.90 A, ˚ Hoir ¼0.901 A) is also suitable for a successful replacement of the Y-ions with Ho-ions. Since a crystal symmetry depends on the concentration of the doping ions, the overall peaks intensity of the
Fig. 1. The X-ray diffraction patterns of Y2O3 particles doped with different concentrations of Ho3 þ (Tcal. ¼600 1C).
Y2O3 cubic structure decreased with increasing Ho3 þ concentration in particles. The mean crystallite size can be calculated by using Debye– Scherrer’s equation D¼
Kl b cos Y
where K ¼4/3 in the case of a spherical shape, D is the crystallite ˚ l is the wavelength of Cu Ka radiation, and b is the size (in A), corrected half-width diffraction peak. The strongest Y2O3:1% Ho3 þ peak at 29.121 (222) was selected and yielded a mean size of 41.7 nm, which means that the synthesized particles consists of smaller crystallites. Fig. 2 shows the FESEM images of Y2O3 particles doped with different concentrations of Ho3 þ ions. All images of the samples showed a spherical morphology. High magnification FESEM images in Fig. 2 clearly show that Y2O3 particles actually consist of smaller crystallites with size of approximately 50 nm, which is consistent with that estimated from XRD patterns using Debye– Scherrer’s equation. Some aggregated particles may be due to the preparation procedure and the effect of the high temperature during the calcination process being of most concern. The doping concentration of Ho-ions almost did not alter the final product and all particles showed sizes within the range of 200750 nm. FETEM of a single, randomly selected particle (Y2O3:1% Ho3 þ ) confirmed that particles consist of smaller crystallites as shown in Fig. 3. HRFETEM showed that the distance between crystal fringes equals 0.306 nm, which can be assigned to the {222} crystal plane of the Y2O3 bcc phase [15]. Selected area composition analysis of Y2O3:0.5% Ho3 þ particles, which was performed by energy-dispersive X-ray spectroscopy (EDX), revealed the presence of elemental Ho, Y and O, as shown in Fig. 3. Particles with higher dopant concentrations also showed the presence of Ho-ions in the host material (not shown). Fig. 4 presents the FTIR spectrum of Y2O3 particles doped with different concentrations of Ho-ions. Vibrational quanta of higher wavenumbers on the particles surface compared to the intrinsic phonons of yttria results in increasing multiphonon relaxations from all excited levels, which can finally quench the luminescence. The absorption band around the region of 570 cm 1 is due to the characteristic metal-oxide (Y–O) stretching vibrations of cubic Y2O3 [16]. Weak broad bands in the range of 1500– 1750 cm 1 and 3500–3800 cm 1 correspond to OH groups [17]. Another peak at approximately 1100 cm 1 can be attributed to the presence of carbon dioxide. Carbon dioxide and OH groups
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Fig. 2. The FESEM images of Y2O3:Ho3 þ particles doped with different concentrations of Ho3 þ (Tcal. ¼ 1000 1C).
were mostly absorbed from air because their signals are relatively low. The luminescence efficiency of RE-doped phosphors for a frequency conversion is often affected by the dopant concentration [14,18,19]. Therefore, it is very important to confirm the luminescence intensity dependence on the concentration of dopant ions. The authors of several previously published reports [10,12] have observed the strong absorption in the region of 440–460 nm in different host materials, which is related to 5 I8-5G6 absorption. Fig. 5 shows the luminescence emission spectra of Y2O3:xHo3 þ samples, where x¼0.5%, 1%, 2% and 3% in mol equivalent, taken in the scan range from 480 to 700 nm under 450 nm excitation (resonantly to 5G6 level). All samples showed
strong green-yellowish emission with some weak blue and far-red emission. Since the samples were prepared in the same way and were measured under identical conditions, their emission ratios could be compared. To select the sample with high emission intensity, the green emission peak intensity ( 549 nm) was measured as a function of the Ho3 þ concentration under constant 450 nm excitation (Fig. 5, inset). The green emission increases with increasing Ho3 þ concentration up to 1% in mol equivalent and then decreased when the concentration in the Y2O3 particles was larger than 1% mol equivalent. This clearly suggests a dependence of the luminescence intensity on the concentration of dopant ions. The most reasonable explanation for this phenomenon is cross-relaxation within Ho3 þ ions. The mean distance (R)
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Fig. 3. The FETEM images and EDX spectra of Y2O3:Ho3 þ particles calcinated at 1000 1C.
Fig. 4. The FT/IR spectra of Y2O3:Ho3 þ particles (Tcal. ¼ 1000 1C).
Fig. 5. The luminescence spectra of Y2O3:Ho3 þ particles (Tcal. ¼ 1000 1C). Inset is green emission peak intensity (around 549 nm) as a function of the holmium concentration under the constant 450 nm excitation.
between the Ho3 þ ions can be estimated by R¼ 0.62/(N)1/3 (where N is the concentration of ions) [20]. At low concentrations, dopant ions were rarely distributed in the crystal lattice. In other words, the amounts of the emission centers were quite small. At higher dopant concentrations the mean distance between dopant ions is much shorter; therefore ions can interact by an electric multipolar process leading to energy migration. The dipole–dipole quenching process is inversely proportional to the sixth power of ion–ion separation and thus to the square of the Ho3 þ concentration [20]. The cross relaxation process occurs followed by non-radiative decay of the two ions to the ground state. Therefore, we can insist that an interaction between the dopant ions leads to energy migration and non-radiative cross-relaxation processes to the ground state, which finally results in quenching of the luminescence intensity [14,19,20].
The optimum concentration of Y2O3:1% Ho3 þ particles was investigated further. Fig. 6(a) shows the excitation spectra of Y2O3:1% Ho3 þ particles (lem. ¼549 nm) calcined for 1 h at 1000 1C and Fig. 6(b) the emission spectra of Y2O3:1% Ho3 þ particles (lexc. ¼450 nm) calcined for 1 h at temperatures ranging from 600 to 1000 1C. One can observe that the emission intensity significantly increased with increase in calcination temperature. Fig. 7 shows XRD patterns of Y2O3:1% Ho3 þ particles calcined for 1 h at different temperatures. The XRD peak of the Y2O3:1% Ho3 þ particles calcinated at 1000 1C was quite sharp and strong compare to particles calcinated at 600 1C, indicating that the final product with high crystallinity can be synthesized at high calcination temperatures. The increased crystallinity and good dispersion of doping materials inside the host material lead to the observed
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Fig. 7. The X-ray diffraction patterns of Y2O3:1% Ho3 þ particles calcinated at different temperatures.
Fig. 6. (a) Excitation spectrum of Y2O3:1% Ho3 þ particles (Tcal. ¼1000 1C, lem. ¼549 nm); (b) emission spectrum of Y2O3:1% Ho3 þ particles (lexc. ¼450 nm) calcinated at different temperatures up to 1000 1C. Inset is eye visible greenyellowish emission of Y2O3:1% Ho3 þ particles (Tcal. ¼1000 1C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
luminescence enhancement. The integrated green emission intensity of 5F4; 5S2–5I8 Stokes transitions of particles calcined at 1000 1C is approximately 1.32 times higher than that calcined at 600 1C. This suggests that the luminescence intensity strongly depends on calcination temperatures and dopant concentration in the host material. Four emission bands in blue, green-yellow and far-red regions were observed under the constant 450 nm excitation due to Stokes transitions within Ho3 þ ions (Fig. 8). The blue luminescence emission was observed between 480–500 nm corresponding to the 5F2; 5F3; 3 K8-5I8 transitions. The green-yellowish emission in the region of 525–570 nm was attributed to the transition from 5S2; 5F4 levels to the ground 5I8 state. Feeble band in the region of 570–590 nm could be attributed to the 5F1; 5G6-5I7 transitions [10]. The far-red emission observed between 670 and 690 nm corresponds to the 5 F5-5I8 transition. The integrated intensity of green–yellowish
Fig. 8. Schematic diagram illustrating the energy band structure of Ho3 þ ion and Stokes transitions between levels (lexc. ¼450 nm).
emission (5S2; 5F4-5I8 transitions) of the particles calcined at 1000 1C was approximately 5 times more intense than those of the blue (5F2; 5F3; 3K8-5I8 transitions) and far-red (5F5-5I8 transition) emissions. Therefore, the total luminescence emission appears like a mixed green-yellowish emission by the naked eye (Fig.6(b), inset picture). These results indicate that uniform Y2O3:1% Ho3 þ particles show strong luminescence properties with narrow emission bands and can be used as a promising material for applications in optoelectronic devices, security printing, lamps for illumination purposes, biolabels technology, etc. [10–13].
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4. Conclusions Uniform-shaped nearly spherical shaped Y2O3 particles doped with different concentrations of Ho3 þ were synthesized using the urea homogeneous precipitation method. The synthesized uniformshaped spherical particles have a mean diameter of approximately 200750 nm. We showed that the luminescence properties of synthesized particles strongly depend on the dopant concentration and calcination temperature. Luminescence quenching was observed above 1% in mol equivalent of Ho3 þ -doped Y2O3, which indicated that 1 mol % Ho3 þ to be the optimal dopant concentration in samples produced by this synthesis route. A strong visible greenyellowish emission due to effective 5S2; 5F4-5I8 transitions is expected to find a wide range of applications in industries, especially for solid state illumination purposes.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology (No. 2010-0010575 and No. 2010-0027284). The authors would like to thank Professor J. B. Lee for allowing the use of his equipment for the preparation and characterization of samples. References [1] G. Sinha, A. Patra, Chem. Phys. Lett. 473 (2009) 151–154.
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[2] D.K. Chatterjee, A.J. Rufaihah, Y. Zhang, Biomaterials 29 (2008) 937–943. [3] F. Zhang, S.S. Wong, ACS Nano 4 (2010) 99. [4] Y.B. Mao, J.Y. Huang, R. Ostroumov, K.L. Wang, J.P. Chang, J. Phys. Chem. C 112 (2008) 2278. [5] Tamar Andelman, Simon Gordonov, Gabrielle Busto, Prabhas V. Moghe, Richard E. Riman, Nanoscale Res. Lett. 5 (2010) 263–273. [6] J.K. Han, G.A. Hirata, J.B. Talbot, J.McKittrick, Mater. Sci. Eng. B 176 (2011) 436-441. [7] A. Martinez, J. Morales, L.A. Diaz-Torres, P Salas, E. De la Rosa, J. Oliva, H. Desirena, Mater. Sci. Eng. B 174 (2010) 164–168. [8] Sreerenjini Chandra, Francis Leonard Deepak, John B. Gruber, Dhiraj K. Sardar, J. Phys. Chem. C 114 (2010) 874–880. [9] Y.C. Kang, I.W. Lenggoro, S.B. Park, K. Okuyama, J. Phys. Chem. Solids 60 (1999) 1855–1858. [10] P.J. Deren, J.-C. Krupa, J. Alloys Compd. 380 (2004) 362–367. [11] Vijay Singh, D.T. Naidu, R.P.S. Chakradhar, Y.C. Ratnakaram, Jun-Jie Zhu, Manish Soni, Physica B 403 (2008) 3781–3785. [12] X.Y. Wang, H.Y. Lin, D.N. Yang, L Lin, E.Y.B. Pun, J. Appl. Phys. 101 (2007) 113535. [13] Y. Li, D. Zhu, J. Shen, M. Tu, H. Sun, J. Rare Earths 24 (2006) 44. [14] T.S. Atabaev, Z. Piao, H.K. Kim, Y.H. Hwang, B.K. Kim, J. Nanosci. Nanotechnol. 11 (2011) 5892–5897. [15] Fan Zhang, Gary B. Braun, Yifeng Shi, Yichi Zhang, Xiaohong Sun, Norbert O. Reich, Dongyuan Zhao, Galen Stucky, J. Am. Chem. Soc. 132 (2010) 2850–2851. [16] X. Li, Q. Li, Z. Xia, L. Wang, W. Yan, J. Wang, R.R. Boughton, Crys. Growth Design 6 (2006) 2193–2196. [17] G. Yao, L. Su, J. Xu, X. Xu, L. Zheng, Y. Cheng, J. Cryst. Growth 310 (2008) 404–409. [18] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Chem. Mater. 15 (2003) 3650. [19] Franc- ois Auzel, Chem. Rev. 104 (2004) 139–173. [20] Amitava Patra, Pushpal Ghosh, Paramita Saha Chowdhury, Marcio A.R.C. Alencar, B. Whualkuer Lozano, Nikifor Rakov, Glauco S. Maciel, J. Phys. Chem. B 109 (2005) 10142–10146.
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis and luminescence properties of Ho3 þ doped Y2O3 submicron particles Timur Sh. Atabaev a, Hong-Ha Thi Vu a, Yang-Do Kim b, Jae-Ho Lee c, Hyung-Kook Kim a,n, Yoon-Hwae Hwang a,n a
Department of Nanomaterials Engineering and BK 21 Nano Fusion Technology Division, Pusan National University, Miryang 627-706, Republic of Korea School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea c Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2011 Received in revised form 14 October 2011 Accepted 9 November 2011 Available online 18 November 2011
This paper presents comprehensive results of the eco-friendly, large scale fabrication of nearly spherical Ho3 þ -doped Y2O3 submicron particles, synthesized using the urea homogeneous precipitation method. The dependence of the photoluminescence emission on the doping concentration was examined to determine the optimum Ho3 þ concentration in the samples. X-ray diffraction data of the Y2O3:Ho3 þ particles revealed a cubic Y2O3 structure. Field emission scanning electron microscopy confirmed the formation of nearly spherical shape particles with a mean diameter of 200 7 50 nm. The luminescence emission intensity significantly increased with increasing calcination temperature due to the improved crystallinity of the synthesized particles. Strong visible green-yellowish emission due to 5F4; 5S2–5I8 Stokes transitions was observed under constant 450 nm excitation. Simple large scale fabrication along with the strong visible green-yellowish emission might give these particles wide area of applications. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials A. Oxides D. Luminescence
1. Introduction In recent years, there has been increasing interest in the area of rare-earth (RE)-ion-doped nanoparticles (NPs) for potential applications in photonic and biophotonic areas [1–4]. In contrast to semiconductor quantum dots, the emission wavelength of lanthanide-doped NPs is independent of the particle size. Moreover, nanoparticles with different emission wavelengths can be easily obtained by the controlled doping of lanthanide ions into an appropriate host material. Preliminary results showed that yttrium oxide (Y2O3) is a promising host material for labels in biotechnology owing to its stability and biocompatibility [5]. Moreover, Y2O3 has low phonon energy and excellent physical properties (phase stability, low thermal expansion and high melting point) [6]. The non-radiative relaxation rate appears to be relatively small for RE ions in the Y2O3 host, which can greatly enhance the luminescence emission. Up to now, the synthesis of irregular shaped RE doped Y2O3 NPs and bulk phosphor materials have been extensively studied using a co-precipitation method or even using some toxic organic solvents [6–8]. In contrast, only a few works addressed their
n
Corresponding authors. Tel.: þ 82 55 350 5845; fax: þ 82 55 353 5844. E-mail addresses: [email protected] (H.-K. Kim), [email protected] (Y.-H. Hwang). 0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.11.010
morphological characteristics. However, it is well known that uniform spherical-shaped particles can improve the optical performance due to the high packing density and reduction of light scattering [9]. The urea homogeneous precipitation has been proved to be a green, simple, quick and low-cost method that can be used for the scalable mass production of Y2O3 spherical particles. A broad luminescent spectrum ranging from blue to IR can be produced by the intra-4f transitions of Ho3 þ ions, where strong green luminescence normally dominates over the blue and red regions. Strong green luminescence due to 5F4; 5S2–5I8 transitions within the Ho3 þ ion was recently observed in LaAlO3 [10,11] and bismuth telluride hosts [12]. Strong green luminescence emission can be used in the lamp industry, cathode radiation tube (CRT), field emission displays (FED), security printing or even biologically oriented applications [10–13]. The efficiency of RE-doped phosphors for frequency conversion is often influenced by the dopant concentration or calcination temperature [14]. Therefore, the physical understanding of the frequency conversion processes of RE-doped phosphors and their dependence on size, calcination temperature and activators concentration is of fundamental importance. This paper reports the large scale controlled synthesis of nearly spherical Ho3 þ doped Y2O3 submicron particles prepared using the urea homogeneous precipitation method. The optical properties of synthesized submicron particles were investigated
T.Sh. Atabaev et al. / Journal of Physics and Chemistry of Solids 73 (2012) 176–181
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in detail. Strong visible green-yellowish luminescence was observed due to 5F4; 5S2–5I8 Stokes transitions within the Ho3 þ ions under constant 450 nm excitation. The luminescent properties of these particles were found to be strongly dependent on the Ho-ion concentration in the host material and calcination temperature, the parameters, which can be easily controlled during the fabrication.
2. Experimental 2.1. Chemical synthesis Analytical grade yttrium oxide Y2O3 (99.9%), holmium oxide Ho2O3 (99.9%), nitric acid HNO3 (70%) and urea (99–100.5%) were purchased from Sigma-Aldrich and used without further purification. Yttrium oxide and holmium oxide with a stoichiometric mol ratio (Y/Ho ¼100 x/x, where x ¼0.5, 1, 2 and 3 (a total of 0.001 mol for each sample)) were diluted with nitric acid and stirred vigorously until the solution became colorless. The solutions were then dried at 70 1C for one day to remove the excess nitric acid. Each sample of dried rare-earth salts was mixed with 40 ml of deionized water, 0.5 g of urea and then stirred vigorously for 30 min to form a clear solution. Sealed beakers with freshly prepared solutions were then placed in an electrical furnace and heated to 90 1C for 2 h, after which the beakers were immediately cooled in cold water bath to prevent the further growth of particles. After cooling to room temperature, the precipitates were centrifuged and thoroughly washed with deionized water and ethanol, and finally dried in an oven at 70 1C for 24 h. Dried precipitates were calcined in air at different temperatures up to 1000 1C for 1 h. 2.2. Physical characterization The structure of the prepared powders was examined by X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer with Cu Ka radiation (l ¼0.15405 nm) and a 2y scan range of 20–601. The structural properties were also analyzed using Fourier transform infrared spectroscopy (FTIR Jasco FT/IR6300). The morphologies of the particles were characterized by field-emission scanning-electron microscopy (FESEM, Hitachi S-4700) and field-emission transmission electron microscopy (FETEM, JEOL JEM-2100F). Elemental analysis was carried out by energy dispersive X-ray spectroscopy (EDX; Horiba, 6853-H). The PL measurements were taken with a Hitachi F-7000 spectrophotometer equipped with a 150 W Xenon lamp as the excitation source. The samples (50 mg) were placed into a cylindrical sample holder, 10 mm in diameter and pressed into pellets for the XRD and PL measurements. All the measurements were performed at room temperature.
3. Results and discussion Fig. 1 shows the XRD patterns taken in the scan range of 20–601 2y for four samples with different concentrations. All samples exhibited identical diffraction patterns, which were well consistent with the pure cubic Y2O3 structure (JCPDS no. 86-1107) with the space group of Ia3 (206) [15]. Since the concentration of Ho dopant is relatively low, no additional peaks of other phases were found, indicating the formation of a pure cubic Y2O3 phase. ˚ The similar ionic radius of these elements (Yir ¼0.90 A, ˚ Hoir ¼0.901 A) is also suitable for a successful replacement of the Y-ions with Ho-ions. Since a crystal symmetry depends on the concentration of the doping ions, the overall peaks intensity of the
Fig. 1. The X-ray diffraction patterns of Y2O3 particles doped with different concentrations of Ho3 þ (Tcal. ¼600 1C).
Y2O3 cubic structure decreased with increasing Ho3 þ concentration in particles. The mean crystallite size can be calculated by using Debye– Scherrer’s equation D¼
Kl b cos Y
where K ¼4/3 in the case of a spherical shape, D is the crystallite ˚ l is the wavelength of Cu Ka radiation, and b is the size (in A), corrected half-width diffraction peak. The strongest Y2O3:1% Ho3 þ peak at 29.121 (222) was selected and yielded a mean size of 41.7 nm, which means that the synthesized particles consists of smaller crystallites. Fig. 2 shows the FESEM images of Y2O3 particles doped with different concentrations of Ho3 þ ions. All images of the samples showed a spherical morphology. High magnification FESEM images in Fig. 2 clearly show that Y2O3 particles actually consist of smaller crystallites with size of approximately 50 nm, which is consistent with that estimated from XRD patterns using Debye– Scherrer’s equation. Some aggregated particles may be due to the preparation procedure and the effect of the high temperature during the calcination process being of most concern. The doping concentration of Ho-ions almost did not alter the final product and all particles showed sizes within the range of 200750 nm. FETEM of a single, randomly selected particle (Y2O3:1% Ho3 þ ) confirmed that particles consist of smaller crystallites as shown in Fig. 3. HRFETEM showed that the distance between crystal fringes equals 0.306 nm, which can be assigned to the {222} crystal plane of the Y2O3 bcc phase [15]. Selected area composition analysis of Y2O3:0.5% Ho3 þ particles, which was performed by energy-dispersive X-ray spectroscopy (EDX), revealed the presence of elemental Ho, Y and O, as shown in Fig. 3. Particles with higher dopant concentrations also showed the presence of Ho-ions in the host material (not shown). Fig. 4 presents the FTIR spectrum of Y2O3 particles doped with different concentrations of Ho-ions. Vibrational quanta of higher wavenumbers on the particles surface compared to the intrinsic phonons of yttria results in increasing multiphonon relaxations from all excited levels, which can finally quench the luminescence. The absorption band around the region of 570 cm 1 is due to the characteristic metal-oxide (Y–O) stretching vibrations of cubic Y2O3 [16]. Weak broad bands in the range of 1500– 1750 cm 1 and 3500–3800 cm 1 correspond to OH groups [17]. Another peak at approximately 1100 cm 1 can be attributed to the presence of carbon dioxide. Carbon dioxide and OH groups
178
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Fig. 2. The FESEM images of Y2O3:Ho3 þ particles doped with different concentrations of Ho3 þ (Tcal. ¼ 1000 1C).
were mostly absorbed from air because their signals are relatively low. The luminescence efficiency of RE-doped phosphors for a frequency conversion is often affected by the dopant concentration [14,18,19]. Therefore, it is very important to confirm the luminescence intensity dependence on the concentration of dopant ions. The authors of several previously published reports [10,12] have observed the strong absorption in the region of 440–460 nm in different host materials, which is related to 5 I8-5G6 absorption. Fig. 5 shows the luminescence emission spectra of Y2O3:xHo3 þ samples, where x¼0.5%, 1%, 2% and 3% in mol equivalent, taken in the scan range from 480 to 700 nm under 450 nm excitation (resonantly to 5G6 level). All samples showed
strong green-yellowish emission with some weak blue and far-red emission. Since the samples were prepared in the same way and were measured under identical conditions, their emission ratios could be compared. To select the sample with high emission intensity, the green emission peak intensity ( 549 nm) was measured as a function of the Ho3 þ concentration under constant 450 nm excitation (Fig. 5, inset). The green emission increases with increasing Ho3 þ concentration up to 1% in mol equivalent and then decreased when the concentration in the Y2O3 particles was larger than 1% mol equivalent. This clearly suggests a dependence of the luminescence intensity on the concentration of dopant ions. The most reasonable explanation for this phenomenon is cross-relaxation within Ho3 þ ions. The mean distance (R)
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Fig. 3. The FETEM images and EDX spectra of Y2O3:Ho3 þ particles calcinated at 1000 1C.
Fig. 4. The FT/IR spectra of Y2O3:Ho3 þ particles (Tcal. ¼ 1000 1C).
Fig. 5. The luminescence spectra of Y2O3:Ho3 þ particles (Tcal. ¼ 1000 1C). Inset is green emission peak intensity (around 549 nm) as a function of the holmium concentration under the constant 450 nm excitation.
between the Ho3 þ ions can be estimated by R¼ 0.62/(N)1/3 (where N is the concentration of ions) [20]. At low concentrations, dopant ions were rarely distributed in the crystal lattice. In other words, the amounts of the emission centers were quite small. At higher dopant concentrations the mean distance between dopant ions is much shorter; therefore ions can interact by an electric multipolar process leading to energy migration. The dipole–dipole quenching process is inversely proportional to the sixth power of ion–ion separation and thus to the square of the Ho3 þ concentration [20]. The cross relaxation process occurs followed by non-radiative decay of the two ions to the ground state. Therefore, we can insist that an interaction between the dopant ions leads to energy migration and non-radiative cross-relaxation processes to the ground state, which finally results in quenching of the luminescence intensity [14,19,20].
The optimum concentration of Y2O3:1% Ho3 þ particles was investigated further. Fig. 6(a) shows the excitation spectra of Y2O3:1% Ho3 þ particles (lem. ¼549 nm) calcined for 1 h at 1000 1C and Fig. 6(b) the emission spectra of Y2O3:1% Ho3 þ particles (lexc. ¼450 nm) calcined for 1 h at temperatures ranging from 600 to 1000 1C. One can observe that the emission intensity significantly increased with increase in calcination temperature. Fig. 7 shows XRD patterns of Y2O3:1% Ho3 þ particles calcined for 1 h at different temperatures. The XRD peak of the Y2O3:1% Ho3 þ particles calcinated at 1000 1C was quite sharp and strong compare to particles calcinated at 600 1C, indicating that the final product with high crystallinity can be synthesized at high calcination temperatures. The increased crystallinity and good dispersion of doping materials inside the host material lead to the observed
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Fig. 7. The X-ray diffraction patterns of Y2O3:1% Ho3 þ particles calcinated at different temperatures.
Fig. 6. (a) Excitation spectrum of Y2O3:1% Ho3 þ particles (Tcal. ¼1000 1C, lem. ¼549 nm); (b) emission spectrum of Y2O3:1% Ho3 þ particles (lexc. ¼450 nm) calcinated at different temperatures up to 1000 1C. Inset is eye visible greenyellowish emission of Y2O3:1% Ho3 þ particles (Tcal. ¼1000 1C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
luminescence enhancement. The integrated green emission intensity of 5F4; 5S2–5I8 Stokes transitions of particles calcined at 1000 1C is approximately 1.32 times higher than that calcined at 600 1C. This suggests that the luminescence intensity strongly depends on calcination temperatures and dopant concentration in the host material. Four emission bands in blue, green-yellow and far-red regions were observed under the constant 450 nm excitation due to Stokes transitions within Ho3 þ ions (Fig. 8). The blue luminescence emission was observed between 480–500 nm corresponding to the 5F2; 5F3; 3 K8-5I8 transitions. The green-yellowish emission in the region of 525–570 nm was attributed to the transition from 5S2; 5F4 levels to the ground 5I8 state. Feeble band in the region of 570–590 nm could be attributed to the 5F1; 5G6-5I7 transitions [10]. The far-red emission observed between 670 and 690 nm corresponds to the 5 F5-5I8 transition. The integrated intensity of green–yellowish
Fig. 8. Schematic diagram illustrating the energy band structure of Ho3 þ ion and Stokes transitions between levels (lexc. ¼450 nm).
emission (5S2; 5F4-5I8 transitions) of the particles calcined at 1000 1C was approximately 5 times more intense than those of the blue (5F2; 5F3; 3K8-5I8 transitions) and far-red (5F5-5I8 transition) emissions. Therefore, the total luminescence emission appears like a mixed green-yellowish emission by the naked eye (Fig.6(b), inset picture). These results indicate that uniform Y2O3:1% Ho3 þ particles show strong luminescence properties with narrow emission bands and can be used as a promising material for applications in optoelectronic devices, security printing, lamps for illumination purposes, biolabels technology, etc. [10–13].
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4. Conclusions Uniform-shaped nearly spherical shaped Y2O3 particles doped with different concentrations of Ho3 þ were synthesized using the urea homogeneous precipitation method. The synthesized uniformshaped spherical particles have a mean diameter of approximately 200750 nm. We showed that the luminescence properties of synthesized particles strongly depend on the dopant concentration and calcination temperature. Luminescence quenching was observed above 1% in mol equivalent of Ho3 þ -doped Y2O3, which indicated that 1 mol % Ho3 þ to be the optimal dopant concentration in samples produced by this synthesis route. A strong visible greenyellowish emission due to effective 5S2; 5F4-5I8 transitions is expected to find a wide range of applications in industries, especially for solid state illumination purposes.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology (No. 2010-0010575 and No. 2010-0027284). The authors would like to thank Professor J. B. Lee for allowing the use of his equipment for the preparation and characterization of samples. References [1] G. Sinha, A. Patra, Chem. Phys. Lett. 473 (2009) 151–154.
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