Facile sol-gel combustion synthesis and photoluminescence enhancement of CaZrO3:Sm3+ nanophosphors via Gd3+ doping

JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012, P. 1000

Facile sol-gel combustion synthesis and photoluminescence enhancement of CaZrO3:Sm3+ nanophosphors via Gd3+ doping DU Qingqing (ᴰ䴦䴦)1, ZHOU Guangjun (਼ᑓ‫)ݯ‬1, ZHOU Juan (਼࿳)2, ZHOU Haifeng (਼⍋ዄ)1, ZHAN Jie (ሩᵄ)1, YANG Zhongsen (ᴼᖴỂ)1 (1. State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China; 2. Center for Disease Prevention and Control of Jinan Military Command, Jinan 250014, China) Received 1 April 2012; revised 26 April 2012

Abstract: CaZrO3:Sm and CaZrO3:Sm,Gd nanophosphors were synthesized by a facile and efficient sol-gel combustion method. Their structure and optical properties were studied. The photoluminesce (PL) results showed that the phosphor could be efficiently excited by irradiation at wavelengths in the visible light region (350–430 nm). The CaZrO3:Sm nanophosphor exhibited strong yellow-green, orange and red emissions with peak wavelength centered at 565 nm (4G5/26H5/2), 601 nm (4G5/26H7/2) and 645 nm (4G5/26H9/2), respectively. The incorporation of Gd3+ ions could greatly improve the luminescence intensity. The highest emission intensity was observed with 2 mol.% Gd3+ doped CaZrO3:3 mol.% Sm powder. The material had potential application in the development of materials for LED’s and other optical devices in the visible region. Keywords: optical properties; luminescence; sol-gel materials; rare earths

Inorganic phosphor materials based on lanthanide-doped oxides have received great attention for potential application in solid state lighting and display technology[1–3]. Among the various dopant species, europium in divalent or trivalent oxidation state is traditionally occupying a dominant role[4,5]. However, there is little research which has been focused on luminescence properties of Sm3+ ions that can also play the role of orange emission centers. And only a few attempts have been made to explore the possibility of using radiative properties of orange luminescence of Sm3+ in the development of materials for LED in the visible special region[6,7]. On the other hand, Gd3+ ions are often chosen to be used as sensitizers because of its special energy levels[8,9]. In addition, more and more attention was focused on perovskite-type oxide-base luminescent materials. As reported in previous lectures, A2+B4+O3 oxides phosphors are very stable and can steadily work in various environments. However, to our knowledge, little work has been performed on Sm3+, Gd3+ co-doped CaZrO3[10–12]. In our work, we chose CaZrO3 as host and Sm3+ ion as the activator ion to study the luminescence properties and energy transfer from Gd3+ to Sm3+ in the nanometer samples. PL properties of CaZrO3:Sm3+ and CaZrO3:Sm3+,Gd3+ samples were investigated with a variety of doping concentrations.

1 Experimental A facile sol-gel combustion method was used to prepare

the CaZrO3 samples: CaZrO3:xSm3+(x=0.5%, 1%, 2%, 3%, 4%, 5%) and CaZrO3:3%Sm3+,yGd3+(y=0.5%, 1%, 2%, 3%, 5%). The process is expressed briefly as below: Sm2O3 and Gd2O3 of analytical grade were used for preparing doping ions. Calcium nitrate (Ca(NO3)2·4H2O) was dissolved in deionized water and stoichiometric acetic acid oxygen zirconium (ZrO(AC)2) was added into this aqueous solution. Then 0.025 mol ammonium nitrate was added in the resultant solutions. After even mixing, citric acid was quickly injected into the solution to get white sol under vigorous stirring. The molar ratio of metal ions to ammonium nitrate and citric acid were 1:4 and 1:1.25, respectively. The whole process was under magnetic stirring at room temperature. The resulting sol was heated in a drying oven at 100 ºC for 12 h to prepare the xerogel. Then the precursor solutions were removed into crucibles and introduced into a muffle furnace maintained at 700 ºC for 30 min. The samarium nitrate (Sm(NO3)3, 0.05 mol/L) and gadolinium nitrate (Gd(NO3)3, 0.05 mol/L) were gained by dissolving samarium oxide (Sm2O3) and gadolinium oxide (Gd2O3) into diluted HNO3, respectively. The Sm3+-doped and Sm3+,Gd3+ co-doped CaZrO3 nanophosphors were synthesized just as the mentioned above method except that the Sm(NO3)3 and Gd(NO3)3 were introduced. The phase structure was characterized by powder XRD patterns (Germany Bruker axs D8-Advance X-ray diffract meter with graphite monochromatized Cu K irradiation (= 0.1542 nm)). Data were collected over the 2 range of

Foundation item: Project supported by National Natural Science Foundation of China (50972081, 51072098), the Chinese PLA Medical Science and Technique Foundation (CWS11J243) and Independent Innovation Foundation of Shandong University (2011JC024) Corresponding author: ZHOU Guangjun (E-mail: [email protected]; Tel.: +86-531-88361206) DOI: 10.1016/S1002-0721(12)60168-9

DU Qingqing et al., Facile sol-gel combustion synthesis and photoluminescence enhancement of CaZrO3:Sm3+…

20°–70°, with a step width of 0.02° and a count time of 0.2 s/step. Thermogravimetric analysis and differential scanning calorimeter were carried out simultaneously on a SDT Q600 TGA-DSC equipment. The analyses were carried out in air with a heating rate of 10 ºC/min from room temperature up to 800 ºC. The morphology and stoichiometry data were characterized by scanning electron microscopy (type JEOL JSM-6700f) and EDS (Horiba EMAX Energy, EX- 350), respectively. The PL properties were recorded using a fluorescence spectrophotometer (Edinburgh, FL920). A fluorescence microscope (type Olympus IX81) was used for the intuitionistic observation of the color of the samples. All the measurements were performed at room temperature.

2 Results and discussion 2.1 Characterization The results of the XRD analysis of the calcined powders as a function of the temperatures in the range of 600–800 ºC are shown in Fig. 1(a). The XRD patterns indicate that a direct combustion reaction at 600 ºC leads to the formation of crystallized CaZrO3. In comparison, much higher temperature (above 1400 ºC)[13] is needed for the preparation of well-crystallized CaZrO3 by conventional solid-state reaction. Higher temperature leads to improved crystalline. The diffraction peaks in Fig. 1(a) and (b) index well to the struc-

Fig. 1 XRD patterns of CaZrO3 pure samples with different sintering temperatures (a) and pure, 3% Sm3+ǃand 3% Sm3+, 2% Gd3+-doped CaZrO3 samples (b)

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ture of CaZrO3 (JCPDS 35-0645). In addition, the wellagreeing XRD results in Fig. 1(b) indicate that the limited rare-earth co-doping ions do not cause significant change in the host structure and phase purity. According to the Scherrer formula Dhkl=K/(cos), where K is a constant (0.89),  is the X-ray wavelength (0.15418 nm),  is the full width at half-maximum (FWHM),  is the diffraction angle, and Dhkl is the size along the (hkl) direction, the nanocrystallite size can be estimated. In our work, we chose diffraction peak at (101), (121) and (202) to calculate the nanoparticle sizes. The estimated average sizes of pure CaZrO3, CaZrO3:Sm and CaZrO3:Sm,Gd are 41, 40 and 37 nm, respectively. Fig. 2 shows the TGA-DSC curves of the precursor gel of CaZrO3:Sm,Gd dried at 100 °C for two days. The DSC curve shows two endothermic peaks around 127 and 246 °C because of the evaporation of water and the combustion of organic components. The mass loss assigned to the processes is about 58.82%, as seen on the TGA curve. The two exothermic peaks in the DSC curve are caused by the important combustion process due to the further combustion of the remaining organic groups and the citrate between 263 and 520 °C, with 13.64% and 11.27% mass loss, respectively. After the combustion process reached an end, the sample underwent no more transformations. Fig. 3(a) and (b) shows the typical SEM micrographs of pure CaZrO3 and 3 mol.% Sm3+, 2 mol.% Gd3+ co-doped CaZrO3 samples, respectively. The powder shows a relatively spherical and homogeneous morphology with an average size below 100 nm. Compared with the calculated results, the samples show a tendency to form agglomerates and the doped ions do not have significant effect on the size of the products. It is demonstrated that the sol-gel-derived nanophosphors with nano-size and spherical shape can effectively form a good phosphor layer in fabricating LEDs with high package density and low surface scattering[14]. In addition, the EDS analysis was utilized to determine the chemical composition of the doped samples. The EDS spectrum (Fig. 3(c)) reveals that the sample consists of calcium, zirconium, oxygen, samarium and gadolinium elements. To the CaZrO3:3%Sm3+,2Gd3+ sample, the actual Sm3+ and Gd3+ concentrations estimated by the EDS spectrum were 2.4 mol.% and 1.3 mol.%, respectively, indicating the presence

Fig. 2 TGA-DSC curves of the precursor gel of CaZrO3:Sm,Gd

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JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012

sition exhibits the strongest orange emission. In addition, the inset of Fig. 4(b) describes the variation of the PL intensity of 601 nm (4G5/26H7/2) with the doping ion ratio. The concentrations of Sm3+ as dopant were varied between 0.5 mol.% and 5 mol.% against Ca2+. It can be seen clearly that the optimal doping concentration of Sm3+ is 3 mol.%. If the doping concentration of Sm3+ is higher than the critical value, the intensity decreases due to the concentration quenching effect, which is considered to be associated with different contributions of the interactions among doped ions, energy migration through the lattice, and the cross-relaxation between neighboring Sm3+ ions[18,19]. In most matrix, such as Y3GaO6 [20], Y2O3 [21], SrAl2O4 [22], YVO4 [23], Y2SiO5 [24], CaTiO3 [25] and ZnAl2O4 [26], the red emission of Sm3+ is stronger than the orange emission. However, the result in our experiment is opposite. The emission intensity of the transitions between energy levels of different J value is related to the point group symmetry of the environment of Sm3+. According to Judd-Ofelt theory[27], when |J|=0 and 1, the magnetic dipole transition is allowed, which is insensitive to the environment. The 4G5/26H5/2 transition (|J|=0) is an allowable magnetic dipole transition, which shows orange emission and the intensity does not change with the matrix. On the other hand, 4G5/26H9/2 transition (|J|=2) is the typical electric dipole transition, which shows red emission and the intensity increases with the re-

Fig. 3 SEM micrographs of pure CaZrO3 (a), CaZrO3:3%Sm3+, 2%Gd3+ (b) and EDS spectra of the CaZrO3:3%Sm3+, 2%Gd3+ (c)

of appropriate Sm3+ and Gd3+ ions incorporated into the phosphors. 2.2 Luminescent properties Fig. 4(a) shows the excitation spectra of CaZrO3:Sm under the emission wavelength of 601 nm. These spectra are characterized by peaks 348, 366, 379 and 409 nm. These excitation peaks belong to the f-f high energy level transitions of Sm3+. Especially, the strongest peak centered at 409 nm is due to the 6H5/24L13/2 transition. Since the peak at 409 nm is the maximum, it is fixed as excitation wavelength for recording the emission spectra. The emission spectra of CaZrO3:Sm is shown in Fig. 4(b). Three obvious emission peaks centered at 565, 601 and 645 nm were observed in the PL spectrum of the samples with different doping concentrations of Sm3+ ions, which can be ascribed to the transitions from the excited 4G5/2 state to the 6H5/2, 6H7/2, and 6H9/2 states, respectively[15–17]. The emission bands for 4G5/26H11/2 transitions are too weak to be checked and the 4G5/26H7/2 tran-

Fig. 4 Excitation spectra of Sm3+-doped CaZrO3 sample (em= 601 nm) (a) and emission spectra of Sm3+-doped CaZrO3 samples (ex=409 nm) (b) (the inset is the evolution of the 601 nm emission intensity as a function of Sm3+ doping concentration)

DU Qingqing et al., Facile sol-gel combustion synthesis and photoluminescence enhancement of CaZrO3:Sm3+…

ducing of environment symmetry of Sm3+ ions. The 4G5/2 6 H7/2 transition partially belongs to the magnetic dipole transition. Usually, the electric dipole transition/ magnetic dipole transition emission ratio can be used as a measurement of the site symmetry of RE3+. When the ratio is bigger, the symmetry is lower. It is clearly shown from Fig. 4(b) that the emission intensity centered at 565 nm is much intenser than that of 645 nm, indicating that the magnetic dipole transition is dominant and Sm3+ ions prefer to hold a higher symmetry site in the host lattice. After the optimum content of Sm3+ was determined (3 mol.%), the effect of Gd3+ doping was studied. Fig. 5 shows the emission spectra of Gd3+ ions co-doped CaZrO3:Sm3+ sample. To obtain the optimal PL parameters of CaZrO3:3%Sm3+,Gd3+, we prepare the samples with different doping concentrations of Gd3+ ions. The incorporation of Gd3+ increases the average distance of metal and oxygen atoms. And the non-radiation of Sm3+ is decreased due to the decrease of the vibration of lattice, leading to the enhanced emission intensity of CaZrO3:Sm3+ by incorporation of Gd3+ ions[28–30]. The doping concentration dependence of the PL intensity is displayed in the inset of Fig. 5, which indicates that the optimal doping concentration of Gd3+ ions is 2 mol.%. When the doping concentration of Gd3+ ions exceeds 2 mol.%, the photoluminescence of CaZrO3:Sm3+,Gd3+ quenches quickly. The non-linear variation in the emission intensity with the increase of the concentration of Gd3+ ions indicates the formation of doped solid solution (CaZrO3:Sm). The emission spectrum of CaZrO3:Sm and CaZrO3:Sm,Gd calcined at different temperatures is shown in Fig. 6(a) and (b), respectively. With the increase of calcined temperature from 600–800 ºC, the PL intensity of the sample enhances gradually, which may be caused by the improvement of crystallinity and the decrease of fluorescence quenching centers[31]. In addition, calcining temperature has no effect on the emission peak position. In view of practicality, it is necessary to study how promising the PL and color-uniform levels of the phosphors are. CIE XY chromaticity diagram in Fig. 7 is plotted for further

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Fig. 6 Emission spectra of CaZrO3:3%Sm3+ (a) and CaZrO3:3%Sm3+, 2%Gd3+ (b) samples with different sintering temperatures

Fig. 7 CIE chromaticity diagram for CaZrO3:Sm,Gd (the inset is the microscope fluorescence image of CaZrO3:3%Sm3+,2%Gd3+ sample)

Fig. 5 Emission spectra of Sm3+, Gd3+ co-doped CaZrO3 samples (ex=409 nm) (the inset is the evolution of the emission intensities (601 nm) as a function of the Gd3+ doping concentration)

analysis of the phosphors. The CIE coordinates for CaZrO3:3%Sm3+,2%Gd3+ nanoparticle were found to be x=0.532, y=0.465, which was located in orange-yellow region. The microscope fluorescence image (the inset of Fig. 7) shows that the CaZrO3:3%Sm3+,2%Gd3+ phosphor could be used as orange-yellow components in the development of materials for LED and other optical devices in the visible region.

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3 Conclusions In summary, good CaZrO3:Sm,Gd nanophosphors were prepared by a facile and efficient sol-gel method. Their structural and luminescent properties were investigated. SEM results showed that the as-synthesized nanophosphors exhibited a relatively spherical and homogeneous morphology. On excitation with visible light, the CaZrO3:Sm nanophosphors exhibited strong yellow-green, orange and red emissions with peak wavelength centered at 565, 601 and 645 nm, respectively. The luminescent intensity was enhanced with the co-doping of Gd3+ ions into CaZrO3:Sm. Due to the chemical stability and good luminescence, CaZrO3:Sm,Gd nanophosphors were considered to be promising candidates for application in phosphor-converted LED and other optical devices in the visible region.

References: [1] Gong H L, Shi S K, Zhou J. Enhanced red luminescence of Eu3+ and R3+-doped La2Mo2O9 phosphors under blue light excitation. Curr. Appl. Phys., 2011, 11: 551. [2] Qian L W, Du W M, Gong Q, Qian X F. Controlled synthesis of light rare earth phosphate nanowires via a simple solution route. Mater. Chem. Phys., 2009, 114: 479. [3] Rambabu U, Munirathnam N R, Prakash T L, Buddhudu S. Emission spectra of LnPO4:RE3+ (Ln=La, Gd; RE=Eu, Tb and Ce) powder phosphors. Mater. Chem. Phys., 2003, 78: 160. [4] Mahalingam V, Vetrone F, Naccache R, Speghini A, Capobianco A. Colloidal Tm3+/Yb3+-doped LiYF4 nanocrystals: multiple luminescence spanning the UV to NIR regions via low-energy excitation. Adv. Mater., 2009, 21: 4025. [5] Liu Y S, Tu D T, Zhu H M, Li R F, Luo W Q, Chen X Y. A strategy to achieve efficient dual-mode luminescence of Eu3+ in lanthanides doped multifunctional NaGdF4 nanocrystals. Adv. Mater., 2010, 22: 3266. [6] Sharma Y K, Surana S S L, Singh R K. Spectroscopic investigations and luminescence spectra of Sm3+ doped soda lime silicate glasses. J. Rare Earths, 2009, 27: 773. [7] Zhu C F, Liang X L, Yang Y X, Chen G R. Luminescence properties of Tb doped and Tm/Tb/Sm co-doped glasses for LED applications. J. Lumin., 2010, 130: 74. [8] Wang H L, Tian L H. Luminescence properties of SrIn2O4: Eu3+ incorporated with Gd3+ or Sm3+ ions luminescence properties of SrIn2O4:Eu3+ incorporated with Gd3+ or Sm3+ ions. J. Alloys Compd., 2011, 509: 2659. [9] Chen L, Jiang Y, Chen S F, Zhang G B, Wang C, Li G H. The intermediate role of Gd3+ in energy transfer to produce light under VUV excitation. J. Lumin., 2008, 128: 2048. [10] Gonenli I E, Tas A C. Chemical synthesis of pure and Gddoped CaZrO3 powders. J. Eur. Ceram. Soc., 1999, 19: 2563. [11] Yuhei S, Sho S, Katsuhiko G, Yutaka N, Kazushige U. Tricolor luminescence in rare earth doped CaZrO3 perovskite oxides. Mater. Sci. Eng. B, 2009, 161: 100. [12] Zhang H W, Fu X V, Niu S Y, Xin Q. Blue luminescence of nanocrystalline CaZrO3:Tm phosphors synthesized by a modified Pechini sol-gel method.J. Lumin., 2008, 128: 1348. [13] Yajima T, Kazeoka H, Yogo T, Iwahara H. Proton conduction in sintered oxides based on CaZrO3. Solid State Ionics, 1991, 47: 271.

JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012 [14] Chang C K, Chen T M. White light generation under violetblue excitation from tunable green-to-red emitting Ca2MgSi2O7:Eu,Mn through energy transfer. Appl. Phys. Lett., 2007, 90: 161901. [15] Zhao J, Duan H, Ma Z, Liu L, Xie E. Effect of temperature on the photoluminescence of Eu3+ doped TiO2 nanofibers prepared by electrospinning. J. Optoelectron. Adv. Mater., 2008, 10: 3029. [16] Meng F G, Zhang X M, Seo H J. Optical properties of Sm3+ and Dy3+ ions in Gd2MoB2O9 host lattice. Opt. Laser Technol., 2012, 44: 185. [17] Zhang J, Wang Y, Wen Y H, Zhang F, Liu B T. Luminescence properties of Ca10K(PO4)7:RE3+ (RE=Ce, Tb, Dy, Tm and Sm) under vacuum ultraviolet excitation. J. Alloys Compd., 2011, 509: 4649. [18] Jamalaiah B C, Vijaya Kumar M V, Rama Gopal K. Fluorescence properties and energy transfer mechanism of Sm3+ ion in lead telluroborate glasses. Opt. Mater., 2011, 33: 1643. [19] Zhu W L, Ma Y Q, Zhai C, Yang K, Zhang X, Wu D D, Li G, Zheng G H. Photoluminescence properties of Sr3(VO4)2, Sr2Y2/3yEuy(VO4)2 and Sr2Y2/3zSmz(VO4)2. Opt. Mater., 2011, 33: 1162. [20] Liu F S, Liu Q L, Liang J K, Luo J, Yang L T, Song G B, Zhang Y, Wang L X, Yao J N, Rao G H. Optical spectra of Ln3+(Nd3+, Sm3+, Dy3+, Ho3+, Er3+)-doped Y3GaO6. J. Lumin., 2005, 111: 61. [21] Kodaira C A, Stefani R, Maia A S, Felinto M C F C, Brito H F. Optical investigation of Y2O3:Sm3+ nanophosphor prepared by combustion and Pechini methods. J. Lumin., 2007, 127: 616. [22] Tang T, Lee C, Yen F. The photoluminescence of SrAl2O4:Sm phosphors. Ceram. Int., 2006, 32: 665. [23] Zhang H W, Fu X Y, Niu S Y, Sun G Q, Xin Q. Photoluminescence of YVO4:Tm phosphor prepared by a polymerizable complex method. Solid State Commun., 2004, 132: 527. [24] Hirai T, Kondo Y. Preparation of Y2SiO5:Ln3+ (Ln=Eu, Tb, Sm) and Gd9.33(SiO4)6O2:Ln3+ (Ln=Eu, Tb) phosphor fine particles using an emulsion liquid membrane system. J. Phys. Chem. C, 2007, 111: 168. [25] Figueiredo A T D, Lazaro S D, Longo E, Paris E C, Varela J A, Joya M R, Pizani P S. Correlation among order-disorder, electronic levels, and photoluminescence in amorphous CT:Sm. Chem. Mater., 2006, 18: 2904. [26] Martinez-Sanchez E, Garcia-Hipolito M, Guzman J, RamosBrito F, Santoyo-Salazar J, Martinez-Martinez R, AlvarezFregoso O, Ramos-Cortes M I, Mendez-Delgado J J, Falcony C. Cathodoluminescent characteristics of Sm-doped ZnAl2O4 nanostructured powders. Phys. Stat. Sol.(a), 2005, 202: 102. [27] Ofelt G S. Intensities of crystal spectra of rare-earth ions. J. Chem. Phys., 1962, 37: 511. [28] Yuan J H, Yuan H H, Zhang Z H, Cheng Y M, Wang X J. Effects of doping trace Sm3+ and Gd3+ on luminescent character of Y2O2S:Eu3+. J. Chin. Rare Earths Soc. (in Chin.), 2005, 23: 421. [29] Liu X R. Recent development of cathodoluminescent materials. Chin. J. Lumin., 1989, 10: 243. [30] Wang H L, Tian L H. Luminescence properties of SrIn2O4:Eu3+ incorporated with Gd3+ or Sm3+ ions. J. Alloys Compd., 2011, 509: 2659. [31] Shea L E, Mckittrick J, Lopez O A. Synthesis of red-emitting, small particle size luminescent oxides using an optimized combustion process. J. Am. Ceram. Soc., 1996, 79: 3257.