Synthesis and optical properties of Gd2O3: Pr3+ phosphor particles

J Sol-Gel Sci Technol (2012) 64:156–161 DOI 10.1007/s10971-012-2842-3

ORIGINAL PAPER

Synthesis and optical properties of Gd2O3:Pr3+ phosphor particles Hong Ha Thi Vu • Timur Sh. Atabaev • Yang-Do Kim • Jae-Ho Lee • Hyung-Kook Kim Yoon-Hwae Hwang

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Received: 25 May 2012 / Accepted: 7 July 2012 / Published online: 18 July 2012 Ó Springer Science+Business Media, LLC 2012

Abstract Spherical-shaped Gd2O3:Pr3? phosphor particles were prepared with different concentrations of Pr3? using the urea homogeneous precipitation method. The resulting Gd2O3:Pr3? phosphor particles were characterized by X-ray diffraction, field emission scanning electron microscope, and photoluminescence spectroscopy. The effects of the Pr3? doping concentration on the luminescent properties of Gd2O3:Pr3? phosphors were investigated. Photoluminescence measurements revealed the Gd2O3:1 % Pr3? phosphor particles to have the strongest emission. The luminescence properties of Gd2O3:Pr3? particles are strongly affected by the phosphor crystallinity and X-ray diffraction measurements confirmed that the crystallinity of Gd2O3 cubic structure could be enhanced by increasing the firing temperature. The luminescent Gd2O3:Pr3? phosphor particles have potential applications in areas, such as optical display systems, lamps and etc.

H. H. T. Vu T. Sh. Atabaev (&) H.-K. Kim (&) Y.-H. Hwang (&) BK 21 Nano Fusion Technology Division, Department of Nanomaterials Engineering, Pusan National University, Miryang 627-706, Republic of Korea e-mail: [email protected] H.-K. Kim e-mail: [email protected] Y.-H. Hwang e-mail: [email protected] Y.-D. Kim School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea J.-H. Lee Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Republic of Korea

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Keywords Gd2O3 Particles Pr3? doped Phosphor Luminescence

1 Introduction The synthesis and precise manipulation of phosphor materials on the submicron and nanoscale regime have attracted considerable attention for display and lighting technology. Phosphor particles with different luminescence emission spectra can be fabricated by the controlled doping of some specific lanthanide ions into a suitable host material. On the other hand, for each host material, there is an optimum concentration of dopant rare-earth (RE) ions. Moreover, the luminescent behavior of all RE-doped phosphors depends on the morphology, size and synthetic route. Phosphors comprised of small and ideally spherical particles are of interest because of their high packing densities and low light scattering, which can result in high brightness and a high resolution [1, 2]. Therefore, the ideal morphology of phosphor particles includes a perfect spherical shape, narrow size distribution and no agglomeration. Gadolinium oxide (Gd2O3) has been studied widely as the host matrix for downconversion [3–5] and upconversion [6, 7] luminescence processes because of its interesting physical properties, such as high melting point (*2320 °C), chemical durability, thermal stability and low phonon energy (*600 cm-1). Furthermore, the high density of Gd2O3 (q = 7.6 g/cm3) is also suitable for using RE-doped Gd2O3 as the perspective materials for X-ray detection and imaging purposes. The luminescence emission of RE ions is the result of unique electronic transitions, and different emission colors can be achieved by the controlled doping of specific lanthanide ions into the Gd2O3 host matrix. The Pr ion has a

J Sol-Gel Sci Technol (2012) 64:156–161

very complicated emission spectrum, and luminescence emission can be produced mainly by intra-4f transitions of Pr3?, when used as a dopant at trace amounts in host materials. The relative positions of the 1S0 state and lowest 4f5d state of Pr3? are environmentally dependent and differ according to the host material used [8]. The luminescence emission of Pr3? doped crystalline materials has been investigated for a range of applications, such as laser [9], field emission display devices [10] and fiber optical communication [11]. Nevertheless, few studies have reported the luminescent properties of host materials doped with Pr3? in submicron and nanoscale region, such as YAlO3 (sol–gel method) [8], CaTiO3 (solvothermal method) [12], and ZnO (coprecipitation method) [13]. Previous studies [2, 14] reported the large scale controlled synthesis of almost spherical RE doped Y2O3 phosphor particles using the urea homogeneous precipitation (UHP) method. The UHP method is an environmentally friendly, simple, fast and low cost method for the scalable preparation of spherical phosphor particles. In this study, spherical particles of Pr3? doped Gd2O3 phosphor material were prepared using the UHP method. To the best of the authors’ knowledge, the preparation of submicron, spherical-shaped Pr3? doped Gd2O3 phosphor particles has not been reported. Gd2O3 particles with different dopant concentrations were prepared and investigated by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy and photoluminescence (PL) measurements. The dopant concentration and crystallinity of the synthesized phosphor particles strongly affects their luminescent properties. This study suggests that Pr3? doped Gd2O3 phosphor particles are potential luminescence materials in light-emitting diodes (LEDs) and field emission displays owing to their visible red luminescence properties.

2 Experimental 2.1 Chemical synthesis Analytical grade gadolinium (III) oxide Gd2O3 (C99.9 %), praseodymium oxide Pr2O3 (C99.9 %), nitric acid HNO3 (70 %) and urea (99–100.5 %) were purchased from Sigma-Aldrich. All the chemicals were used as received. Spherical-shaped Pr3? doped Gd2O3 phosphor particles were prepared according to previous reports [2, 14]. In a typical synthesis, nitrates of gadolinium oxide and praseodymium oxide with a stoichiometric mol ratio (Gd/ Pr = 100 - x/x, where x = 1, 2, 3 and 5 mol % (a total of 0.001 mol for each sample)) were prepared by dissolving them in nitric acid. The solution was dried at 100 °C for one day to remove any excess nitric acid. Sealed beakers

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with freshly prepared aqueous urea solutions of nitrates (40 ml H2O, 0.5 g urea) were then placed into a water bath and heated to 90 °C for 1.5 h. Subsequently, the beakers were cooled immediately by placing them into a cold water bath to quench further particle growth. The precipitates were collected by centrifugation, thoroughly washed with deionized water and ethanol to remove the possible ions remaining in the products and dried in oven at 70 °C for 24 h. The collected dried precipitates were calcined in air at temperatures up to 1,000 °C for 1 h. 2.2 Physical characterization The structure of the prepared samples was examined by XRD (Bruker D8 Discover diffractometer) with Cu Ka radiation (k = 0.15405 nm) within a 2h scan range of 20°–60°. The structural properties were also investigated by FTIR (Jasco FT/IR6300) spectroscopy. The morphology of the particles was characterized by FESEM (Hitachi S-4700). Elemental analysis was carried out by energy dispersive X-ray spectroscopy (EDX; Horiba, 6853-H). The PL data were measured by placing the powders to standard powder holder on a Hitachi F-7000 spectrophotometer equipped with a 150 W Xenon lamp as the excitation source. All the measurements were taken at room temperature.

3 Results and discussion The UHP method is a well-known method for producing spherical phosphor particles of lanthanide oxides [2, 14]. On the other hand, the concentration of the urea source should be considered for the fabrication of uniform-sized spherical phosphor particles because the reaction rate increases with increasing temperature and urea concentration, and an excess of urea in solution leads to the formation of spherical particles with different sizes. Therefore, to examine the effect of the urea concentration on the morphology of phosphor particles, Gd2O3: 1 % Pr3? phosphor particles were synthesized under identical reaction conditions (T = 90 °C, 0.001 mol each sample) except for a range of urea concentrations in the solution. Figure 1 shows that almost uniformly-sized particles with sizes of approximately 100 ± 15 nm (a), 150 ± 20 nm (b) were synthesized when 0.3 and 0.5 g of urea was used for precursor precipitation, respectively. On the other hand, particles with different sizes were obtained when 1 g of urea was used for precipitation, and the final product consisted of polydispersed particles with sizes ranging from 200 nm up to several lm, as it shown in Fig. 1c. Such behavior can be explained by the fact that at high urea

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similar size distributions (not shown). Further, 0.5 g of urea was used for the preparation of Gd2O3:Pr3? phosphor particles. The intensity of luminescence emission depends strongly on the dopant concentration. For a comparative study of the optical properties, samples with different Pr3? concentrations were synthesized and characterized by PL. Y. Ji et al. [8] reported that YAlO3:Pr3? has a charge transfer band (CTB) centered at 288 nm, and strong blue (3P0?3H4, 487 nm) and red (1D2?3H4, 610 nm) peaks were observed upon excitation of the CTB. In contrast to YAlO3:Pr3? phosphor, J. W. Chung et al. [12] reported that the CTB in CaTiO3:Pr3? phosphor material was centered at 324 nm and direct excitation of the CTB produced only a single strong visible red emission (1D2?3H4, 612 nm). Figure 2 shows the photoluminescence excitation spectra (PLE) of Gd2O3:Pr3? phosphor particles with different dopant contents. In the excitation spectrum, a strong excitation band centered at 390 nm was observed due to CTB (monitoring the 1D2?3H4 transition at 614 nm). Weak signals in the region, 450–475 nm, appeared due to f–f transitions within Pr3? (3P1?3H4 transition). Figure 3 shows the normalized PL (unit peak intensity) emission spectra of Gd2O3:Pr3? phosphor particles measured in the range, 550–700 nm. The emission spectra of Gd2O3:Pr3? phosphor particles consist mainly of 1D2?3H4 centered at 614 nm. The integrated normalized PL intensity of 1 D2?3H4 transition observed at 614 nm decreased significantly with increasing Pr3? concentration from 1 to 5 mol % (Fig. 3 inset). Therefore, the dopant concentration plays an important role in the development of phosphor materials. The integrated luminescence intensity of the 1 D2?3H4 transition decreased gradually with increasing

Fig. 1 The FESEM images of Gd2O3:1 % Pr3? phosphor particles synthesized in the presence of a 0.3 g, b 0.5 g and c 1 g of urea

concentrations, more carbonate ions are generated and the pH increases more rapidly that at lower urea contents. The Oswald ripening process occurs then in solution, in which small particles tends to redeposit onto larger ones because larger particles are more energetically favored than smaller particles. Samples with higher dopant content also showed

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Fig. 2 The PL excitation of Gd2O3:Pr3? phosphor particles (kem. = 614 nm)


Fig. 3 The PL emission of Gd2O3:Pr3? phosphor particles (kexc. = 390 nm)

dopant content, which was attributed to the cross-relaxation process [8, 14–16]. The Gd2O3:1 % Pr3? sample was calcined at 600– 1000 °C for 1 h to further investigate the luminescence properties of Gd2O3:Pr3? phosphor particles. The luminescence emission of the Gd2O3:1 % Pr3? phosphor particles increased with increasing calcination temperature, as shown in Fig. 4. The integrated emission intensity of red emission (614 nm, 1D2?3H4) for Gd2O3:1 % Pr3? calcined at 1000 °C was *2.3 times higher than that calcined at 600 °C. The increased crystallinity and good dispersion of doping materials inside the host material lead to the observed luminescence enhancement [14]. A high calcination temperature leads to the formation of larger crystallites. The morphology and sizes of the Gd2O3:1 % Pr3? phosphor particles did not change dramatically with increasing calcination temperature (Fig. 5). On the other hand, the phosphor calcinated at higher temperatures showed larger crystallite sizes than that calcined at lower temperatures. EDX analysis of the Gd2O3:1 % Pr3? phosphor particles calcined at 1000 °C confirmed the presence of Pr3? in the Gd2O3 host (Fig. 5). Gd2O3:1 % Pr3? powders were calcined at different temperatures (600– 1000 °C) and examined by XRD (Fig. 6). These results matched the cubic phase of Gd2O3 (JCPDS card No. 882165). No additional diffraction peaks for the other phases were found, indicating the formation of a pure cubic phase. In addition, the diffraction peaks of the samples with a higher dopant content showed identical diffraction patterns (not shown). The XRD peaks of the Gd2O3: 1 % Pr3? powders calcined at 1000 °C were sharper and stronger than those calcined at 600 °C, suggesting higher

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Fig. 4 The PL emission of Gd2O3:1 % Pr3? phosphor particles (kexc. = 390 nm) calcinated at different temperatures ranging from 600 to 1000 °C

crystallinity. Moreover, the Gd2O3: 1 % Pr3? powders calcined at 1000 °C showed narrower peaks than those calcined at 600 °C, indicating crystallite growth inside the particles. The mean crystallite sizes (D) of the Gd2O3: 1 % Pr3? powders calcined at different temperatures were calculated based on the peak widths from the Debye–Scherrer’s equation: D¼

Kk b cos H

where K = 4/3 in the case of a spherical shape, D is the ˚ ), k is the wavelength of Cu Ka radicrystallite size (in A ation, and b is the corrected half-width diffraction peak. The estimated mean crystallite sizes of the Gd2O3: 1 % Pr3? powders calcined at 600, 800 and 1000 °C for 1 h were approximately 21.19, 27.95 and 36.97 nm, respectively ({222} plane). This shows that the crystallinity of the Gd2O3: 1 % Pr3? phosphor particles was improved and the crystallite sizes increased with increasing calcination temperature. On the other hand, it is known that the presence of –OH and –CO vibrational groups in phosphor materials can also quench or reduce the luminescence intensity [17]. Figure 7 presents the normalized FTIR spectra of the Gd2O3: 1 % Pr3? powders calcined at 600, 800 and 1000 °C for 1 h. The absorption band at *540 cm-1 was assigned to the characteristic metal-oxide (Gd–O) stretching vibrations of cubic Gd2O3 [18]. The weak broad absorption bands at 1500–1750 cm-1 and 3500–3800 cm-1 were assigned to OH groups, whereas the peak at approximately 1100 cm-1 were attributed to the presence of absorbed CO vibrational groups [2, 14]. The FTIR spectra of the Gd2O3: 1 % Pr3? phosphor particles

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Fig. 5 The FESEM images of Gd2O3:1 % Pr3? phosphor particles calcinated at a 600 °C, b 800 °C and c 1000 °C. The EDX spectra of Gd2O3:1 % Pr3? phosphor particles (Tcal. = 1000 °C)

Fig. 6 The XRD patterns of Gd2O3:1 % Pr3? phosphor particles calcinated at different temperatures ranging from 600 to 1000 ° C

Fig. 7 The FTIR patterns of Gd2O3:1 % Pr3? phosphor particles calcinated at different temperatures ranging from 600 to 1000 °C

were similar but the absorption of these two groups (CO and OH) decreased with increasing calcination temperature. Therefore, the observed luminescence enhancement can be explained by the high crystallinity, larger crystallite sizes and reduced amount of –OH and –CO vibrational groups in the phosphor materials.

4 Conclusions

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Spherical Gd2O3:Pr3? phosphor particles were synthesized using a facile, quick and inexpensive urea homogeneous precipitation method. The effects of the urea concentration on the particles sizes were examined. The CTB in the


Gd2O3:Pr3? phosphor particle was centered at 390 nm. Strong red emission due to the 1D2?3H4 transition in Pr3? (614 nm) was observed upon direct excitation of the CTB. The effects of the dopant concentration and calcination temperature on the luminescence performance of the Gd2O3:Pr3? phosphor particles were also evaluated. The Gd2O3:Pr3? phosphor is expected to find promising potential applications as a red light emitting source in areas of solid state illumination and cathodoluminescence. Acknowledgments This work was supported by the National Research Foundation of Korea (Grant No: 2010-0010575 and 2010-0027284). The authors would like to thank Prof. J. B. Lee for allowing the use of his equipment for the preparation and characterization of samples.

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