Synthesis, Characterization, and Luminescent Properties of Pr3+-

Mar 25, 2008 - The corresponding bulk BaTiO3:Pr3+ was synthesized by a high-temperature solid-state reaction. All the products were systematically cha...
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J. Phys. Chem. C 2008, 112, 5724-5728

Synthesis, Characterization, and Luminescent Properties of Pr3+-Doped Bulk and Nanocrystalline BaTiO3 Phosphors Zuoling Fu, Byung Kee Moon, Hyun Kyoung Yang, and Jung Hyun Jeong* Department of Physics, Pukyong National UniVersity, Busan 608-737, South Korea ReceiVed: October 26, 2007; In Final Form: December 20, 2007

In this paper, a facile synthetic route for the preparation of nanocrystalline BaTiO3:Pr3+ by a solvothermal method is reported; the as-grown powders were found to be amorphous, which crystallized to the tetragonal phase without BaCO3 byproduct after calcination at 700 °C in air for 3 h. The corresponding bulk BaTiO3: Pr3+ was synthesized by a high-temperature solid-state reaction. All the products were systematically characterized by powder X-ray diffraction (XRD), infrared spectroscopy (IR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL), and photoluminescent excitation spectra (PLE). The luminescence mechanism and the size dependence of their fluorescence properties are also discussed.

1. Introduction Perovskites represent one of the most important class of functional ceramic used for electronic components. Among them, BaTiO3 is a typical representative perovskite material, not only for bulk crystals1 but also for nanocrystals.2,3 The effect of grain size on the properties of the BaTiO3 phase is strong and manifests itself in changes of all properties: phase transition temperatures, crystal structure, and optical properties. Recently, most research is focused on the dielectric property of BaTiO3,4-6 but few are concerned with the luminescent properties of rare earth doped BaTiO3.7-9 It is well-known that SrTiO3:Pr3+ has been developed as a red phosphor for field emission displays (FEDs). BaTiO3 is similar to SrTiO3 in structure and optical characteristics because BaTiO3 and SrTiO3 have perovskitetype crystal structure and a similar fundamental absorption edge (∼3.2 eV). Therefore, it is expected that BaTiO3:Pr3+ can be used as an FED phosphor for practical application. So it is very important to synthesize BaTiO3:Pr3+ and understand the luminescent mechanism. At present, wet chemical routes such as sol-gel,10 coprecipitation,11 hydrothermal,12 and organic precursor methods13 replaced the classical solid-state reaction for synthesis of BaTiO3. The advantage of the wet chemical preparative techniques is the quasiatomic dispersion of Ba and Ti in liquid precursors, which facilitates synthesis of the crystallized powder with submicrometer particle sizes at low temperatures. The properties of the powder may vary as different preparation methods are used. The selection of the preparation method is usually based upon the desired properties of the final material. The synthesis of mixed oxides from organic precursors has the distinct advantage over most other methods, namely, that very pure mixed oxides can be prepared.14 In this paper, nanocrystalline BaTiO3:Pr3+ was prepared by a solvothermal method, using barium acetylacetonate hydrate (Ba(C5H8O2)2‚ xH2O), praseodymium acetylacetonate hydrate (Pr(C5H8O2)3‚ xH2O), and titanium(IV) butoxide (Ti(OCH2CH2CH2CH3)4) as precursors. The use of the metal acetylacetonate and titanium butoxide in 2-proponal is a particularly attractive reaction system for several more reasons: (1) phase-pure BaTiO3:Pr3+ is obtained without other byproducts such as barium carbonate, * Corresponding author. E-mail: [email protected].

and therefore, subsequent separation steps can be avoided; (2) all precursors are commercially available; (3) the reaction is a simple, low-temperature process. 2. Experimental Section 2.1. Preparation of BaTiO3:Pr3+ Nanocrystals. 2.1.1. Materials. Titanium(IV) butoxide (Ti(OCH2CH2CH2CH3)4), barium acetylacetonate hydrate (Ba(C5H8O2)2‚xH2O), and praseodymium acetylacetonate hydrate (Pr(C5H8O2)3‚xH2O) were obtained from Aldrich. All of the chemicals were used without further purification. For the solvothermal treatment, we used 80 mL Teflon cups. 2.1.2. Synthesis. Barium acetylacetonate hydrate (Ba(C5H8O2)2‚ xH2O), and praseodymium acetylacetonate hydrate (Pr(C5H8O2)3‚ xH2O) was dissolved in 25 mL of 2-proponal, and the mixture was stirred for 1 h. Titanium(IV) butoxide (Ti(OCH2CH2CH2CH3)4) was added to the previous solution, and it was stirred again for 2 h. Finally, the mixture was placed in a poly(tetrafluoroethylene) (PTFE) vessel, and the vessel was capped by a PTFE cover and placed inside a stainless steel autoclave. The autoclave was sealed and kept at 250 °C for 10 h with stirring under autogenous pressure. The precipitates taken from the autoclave were dried at 50 °C in air. The precursor powders were calcined at 500, 600, and 700 °C, respectively. Finally, the pure phase BaTiO3:Pr3+ nanocrystals were obtained. For comparison, bulk BaTiO3:Pr3+ powders were obtained by a direct solid-state reaction method. The starting materials were BaCO3, TiO2, and PrF3. According to the nominal compositions of compounds BaTiO3:x% Pr3+ (x ) 0.05% and 0.2%), the appropriate amounts of starting materials were thoroughly mixed and ground, then heated at 800 °C for 4 h. After being reground, they were calcined at 1300 °C for 2 h. In addition, it should be mentioned that the activator content (Pr) was maintained at 0.2 mol % for all the prepared samples except one sample of bulk BaTiO3:0.05% Pr3+. 2.2. Characterization. The structural characteristics of the bulk and nanocrystalline BaTiO3:Pr3+ were measured from the X-ray diffraction (XRD) patterns using a Philips XPert/MPD diffraction system with Cu KR (λ ) 0.15405 nm) radiation and the Fourier transform near-infrared spectrometer (FT-IR, Perkin-

10.1021/jp7103424 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/25/2008

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Figure 1. XRD powder patterns of bulk and nanocrystalline BaTiO3: Pr3+ with different calcination temperatures: (a) [Ba] ) 0.05 mol/L, 500 °C, 3 h (BTO-1); (b) [Ba] ) 0.05 mol/L, 600 °C, 3 h (BTO-2); (c) [Ba] ) 0.05 mol/L, 700 °C, 3 h (BTO-3); (d) [Ba] ) 0.025 mol/L, 600 °C, 3 h (BTO-4); (e) [Ba] ) 0.025 mol/L, 700 °C, 3 h (BTO-5); (f) bulk, 1300 °C, 2 h (BTO-6b). The dotted line corresponds to the position of the most intense (111) peak for the orthorhombic BaCO3 (JCPDS no. 05-0378).

Elmer (U.S.A.) Spectrum GX). The morphology and the size of the obtained samples were observed with field emission scanning electron microscopy (FE-SEM, JSM-6700F, JEOL) and transmission electron microscopy (TEM, JEM-2010 JEOL). For the optical investigation, the photoluminescence (PL) and photoluminescence excitation (PLE) measurements were obtained at room temperature by using a luminescence spectrometer (Perkin-Elmer, LS-50B) with a 150 W Xe lamp as an excitation source. 3. Results and Discussion 3.1. Synthesis and Morphology of BaTiO3:Pr3+. BaTiO3 crystallizes in the tetragonal perovskite structure (space group P4/mmm). The barium atoms share the corners of the unit cell, and the titanium is at the center, surrounded by six oxygens that occupy the middle of the faces, in an elongated octahedral configuration. Figure 1 shows XRD powder patterns of bulk and nanocrystalline BaTiO3:Pr3+ with different concentrations and ignition temperatures (iT). When the concentration of Ba2+ is higher ([Ba] ) 0.05 mol/L), the BaTiO3 products always contain BaCO3 byproduct even though the iT is at 700 °C (BTO3). It is possible that the higher concentration can result in composition inhomogeneities and finally the second phase is formed. In a series of preliminary experiments, the mixing conditions were found to have a significant influence on the particle size distribution and on the formation of secondary phases. In particular, if the reactants are not efficiently mixed, composition inhomogeneities are produced and the crystallization of BaTiO3 is slowed down. With the decrease of Ba2+ concentration ([Ba] ) 0.025 mol/L), the pure phase BaTiO3:Pr was obtained at 700 °C (BTO-5), which can be related to the homogeneous solution. In addition, from Figure 1, it can be seen that, whereas at 500 °C (BTO-1) only BaCO3 is formed, with the decrease of Ba2+ concentration and the increase of iT, the pure BaTiO3 nanocrystals are obtained with the concentration [Ba] ) 0.025 mol/L at 700 °C (BTO-5). In addition, the XRD of bulk BaTiO3:Pr3+ is also shown in Figure 1f; the diffraction peaks are much sharper and more intense compared with those of the nanocrystalline BaTiO3:Pr3+. All of the peaks in bulk

Figure 2. (A) FT-IR spectra of TiO2, BaCO3, and as-prepared sample. (B) FT-IR spectra of nanocrystalline BaTiO3:Pr3+ with different calcination temperatures: (a) [Ba] ) 0.05 mol/L, 500 °C, 3 h (BTO1); (b) [Ba] ) 0.05 mol/L, 600 °C, 3 h (BTO-2); (c) [Ba] ) 0.05 mol/ L, 700 °C, 3 h (BTO-3); (d) [Ba] ) 0.025 mol/L, 600 °C, 3 h (BTO4); (e) [Ba] ) 0.025 mol/L, 700 °C, 3 h (BTO-5).

(BTO-6b) and nanocrystalline BaTiO3:Pr3+ (BTO-5) could be indexed to the tetragonal phase of BaTiO3, which were matched with BaTiO3 standard values given in JCPDS (no. 05-0626). In general, the nanocrystallite size can be estimated from the Scherrer equation, D ) 0.89λ/β cos θ, where D is the average crystallite size, λ is the X-ray wavelength (0.15405 nm), and θ and β are the diffraction angle and full width at half-maximum (in radians) of an observed peak, respectively. The estimated average crystallite sizes are about 24 and 43 nm for BTO-4 and BTO-5, respectively. For quality control and determination of structural characteristics, the infrared spectrophotometer was very useful. FTIR spectra of the BaTiO3:Pr3+ phosphors and precursors are shown in Figure 2, parts A and B. For the as-grown sample which is not calcined at high temperature, the typical absorption bands are found: a broad band near 3396 cm-1 can be attributed to the -OH stretching mode. Two sharp peaks near 1557 and 1423 cm-1 are assigned to stretching or bending of CO32-. A strong and broad band near 650 cm-1 is attributed to the bending mode of Ti-O. When compared with the IR spectra of TiO2 and BaCO3, all the assignments are in good agreement with the characterized peaks of BaCO3 and TiO2. In addition, it is

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Figure 3. FE-SEM image of BaTiO3:Pr3+ powders sintered at 600 °C (A) BTO-4 and 700 °C (B) BTO-5, respectively.

observed that differences of FT-IR spectra of nanocrystalline BaTiO3:Pr3+ start to evolve at temperatures above 600 °C from Figure 2B. The IR spectra of nanocrystals show a strongly decreasing absorption peak of carbonate ions. The heat treatment at 600 °C for 3 h results in formation of BaTiO3 as a major phase, but absorption characteristic for BaCO3 still persist as minor components (Figure 2B, spectra b-d). At 700 °C and the lower concentration of Ba2+, the peak has completely vanished. This means that nanocrystalline BaTiO3 is completely crystalline at T ) 700 °C (Figure 2B, spectrum e). The size of the particles prepared from solution depends, in general, on the relative rates of nuclei formation and crystallite growth. Both nucleation and growth are very sensitive to temperature, concentration, and mixing conditions; crystal agglomeration can also contribute substantially to the particle growth rate. A careful control of the above kinetic parameters should allow the synthesis of particles with desired size, from micrometer scale to nanometer scale. Parts A and B of Figure 3 are the FE-SEM images of BaTiO3:Pr3+ powders sintered at 600 and 700 °C, respectively. Figure 3A shows that the powders sintered at 600 °C are agglomerated in a lump. But when the temperature was increased to 700 °C, the surface morphology of the powders became granular and appears to have a homogeneous and isotropic distribution, as shown in Figure 3B. These results indicate that the BaTiO3:Pr3+ powders are

Fu et al.

Figure 4. TEM micrograph of BaTiO3:Pr3+ powders sintered at 600 °C (A) BTO-4 and 700 °C (B) BTO-5, respectively.

amorphous at 600 °C but have crystallized well at 700 °C. The fact supports the conclusion drawn from XRD and IR analysis. In addition, the typical TEM micrographs of the above corresponding BaTiO3:Pr3+ powders sintered at 600 and 700 °C are displayed in Figure 4, parts A and B. From Figure 4, it can be seen that the particle size in lower iT is smaller than that in the higher iT. The particle sizes of BTO-4 and BTO-5 are estimated about 30 and 50 nm, respectively, which are consistent with the calculated sizes by the Scherrer equation. 3.2. Spectral Analysis of Bulk and Nanocrystalline BaTiO3:Pr3+. Photoluminescent excitation spectra (PLE) were obtained for bulk and nanocrystalline BaTiO3:Pr3+ at different monitoring wavelengths of 616 and 600 nm (Figure 5). The strong broad bands peaking at about 300 and 360 nm are observed, and another sharp peak at 450 nm (3H4-3P2) and a shoulder at 470 nm (3H4-3P1) due to intra-4f transition are also detected. When the concentration of Pr is 0.05 mol %, only one broad band at about 300 nm is observed (Figure 5a), whose corresponding emission spectra (Figure 5, inset) show the strong sharp peak at 616 nm, which is attributed to the intra-4f transition 3P1 f 3F2 emission, which is an allowed magnetic dipole transition in BaTiO3 because Ba occupies a site with D4h symmetry. Upon 300 nm excitation, Pr3+-doped BaTiO3 exhibits the bright red Pr3+ emission, which indicates that the excitation band should be in relation to the 4f-5d transition of Pr3+. In

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Figure 5. PLE spectra of bulk (BTO-6b) and nanocrystalline (BTO-5) BaTiO3:Pr3+ for the emission at 616 and 600 nm, respectively. Curve a is the PLE spectrum of bulk BaTiO3:0.05% Pr3+ powder for the emission at 616 nm. In the inset the PLE and PL spectra of bulk BaTiO3:0.05% Pr3+ powders are shown.

Figure 6. PLE spectra of bulk BaTiO3:Ce3+ by high-temperature solidstate reaction for the emission at 585 nm in room temperature.

order to confirm the assignment of the excitation band, BaTiO3: 0.2% Ce3+ was prepared by the high-temperature solid-state method. As demonstrated by Dorenbos in an extensive review on the position of 5d transitions of lanthanides, it is possible to use the position of 5d levels of Ce3+ to predict those of all other lanthanides.16,17 This is true for Ce3+ and Pr3+ in BaTiO3, since, if put in the same compound, the first fd transition of Pr3+ always occurs at about 12200 ( 600 cm-1 higher in energy than that of Ce3+.16 Figure 6 shows the excitation spectra in BaTiO3:Ce3+ for the emission at 585 nm. From the observed energy of the lowest 5d state for Ce3+ in BaTiO3 (22222 cm-1 (450 nm)), the lowest 5d level of Pr3+ in this host lattice can be predicted to be about 296 nm. This result is in line with the band (about 300 nm) we observed in BaTiO3:Pr3+, which corroborates the attribution of this band. Figure 7 shows PL spectra of bulk (BTO-6b) and nanocrystalline (BTO-5) BaTiO3:Pr3+ for different excitations at 300 and

Figure 7. Emission spectra of bulk (BTO-6b) and nanocrystalline (BTO-5) BaTiO3:Pr3+ for the excitation at 300 and 360 nm. Curve a is the PL spectrum of bulk BaTiO3:0.05% Pr3+ powder for the excitation at 300 nm.

360 nm. From Figure 7, it can be seen that the PL spectrum has a strong peak at 616 nm for the excitation at 300 nm, whereas under a 360 nm excitation wavelength, a very weak peak at 600 nm can be observed, which is assigned to 1D2 f 3H emission.18,19 For the excitation band at 360 nm, it could 4 be due to exciton levels or a defect center which was caused by nonequivalent substitution.9,20,21 Further research is under way. In addition, the PL intensity in bulk materials is stronger compared with the nanocrystalline BaTiO3:Pr3+, which can be related to the surface defects on nanocrystal powders. 4. Conclusions BaTiO3:Pr3+ nanocrystals were synthesized by a solvothermal method. The as-grown powders were calcined at 700 °C in air for 3 h and crystallized to pure tetragonal phase. The estimated

5728 J. Phys. Chem. C, Vol. 112, No. 15, 2008 average crystallite size is about 43 nm from XRD, which is consistent with the size as indicated by TEM. The assignment of the excitation band at about 300 nm is clarified by PLE spectra of BaTiO3:0.2% Ce3+ in the first time. The PL intensity in bulk material is stronger compared with the nanocrystalline BaTiO3:Pr3+, which can be related to the surface defects on nanocrystal powders. Acknowledgment. This work was supported by grant-inaid for the National Core Research Center Program from MOST/ KOSEF (No. R15-2006-022-03001-0). References and Notes (1) Megaw, H. D. Nature 1945, 155, 484. (2) Amami, J.; Hreniak, D.; Guyot, Y.; Pazik, R.; Strek, W.; Goutaudier, C.; Boulon, G. J. Phys.: Condens. Matter 2007, 19, 096204. (3) Zhang, M. S.; Yu, J.; Chu, J. H.; Chen, Q.; Chen, W. C. J. Mater. Process. Technol. 2003, 137, 78. (4) Zhong, W. L.; Wang, Y. G.; Zang, P. L. Phys. ReV. B 1994, 50, 698. (5) Yoshikazu, O.; Hisamitsu, S.; Shinya, K.; Hiroshi, K. Jpn. J. Appl. Phys. 1994, 33, 5393.

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