Synthesis and characterization of CaAl x O y :Eu 2+ phosphors prepared using solution-combustion method

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1 JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010, p. 272 Synthesis and characterization of CaAl x O y :Eu 2+ phosphors prepared using solution-combustion method F.B. Dejene 1, D.B. Bem 1, H.C. Swart 2 (1. Department of Physics, University of the Free State (Qwaqwa Campus), Private Bag X13, Phuthaditjhaba, 9866, South Africa; 2. Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, 9300, South Africa) Received 31 August 2010; revised 3 December 2010 Abstract: Europium-doped calcium aluminate (CaAl x O y :Eu 2+ ) phosphors were obtained at low temperatures (500 ºC) by the solution - combustion of corresponding metal nitrate urea solution mixtures. The particle size and morphology and the structural and luminescent properties of the as-synthesized phosphors were examined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), Auger spectroscopy, transmission electron spectroscopy (TEM) and photoluminescence (PL). It was found that the Ca:Al molar ratios showed greatly influence not only on the particle size and morphology, but also on their PL spectra and structure. With the Ca: Al molar ratios increasing from 6:100 to 26:100, the structure of as-synthesized phosphor changed from CaAl 12 O 19 to monoclinic CaAl 2 O 4 and the dominant emitting light from red to blue, implying that the oxidation state of doped europium ions changed from trivalent to divalent due to the structure variation of host lattice. A blue phosphor with almost pure phase can be easily prepared by solution combustion method with suitable Ca:Al molar ratio. Keywords: CaAl x O y :Eu 2+ ; solution-combustion; molar ratio; structure; luminescence; rare earths Aluminates of Ca, Sr and Ba doped with Eu 2+ activator ion possess safer, chemically stable and intense photoluminescence (PL) in visible [1,2], compared to the conventional sulfide-based phosphors. These properties makes them useful in many applications, such as luminous paints in highway, airport, buildings and ceramics products, in textiles, dial plate of glow watch, warning signs and the escape routes. But most of the literature deals with the strong green emission (530 nm) of Eu 2+ in stoichiometric SrAl 2 O 4 with cubic spinel structure [3,4]. The emission of Eu 2+ is strongly dependent on the host lattice and can occur from the ultraviolet to the red region of the electro-magnetic spectrum. This is because the 5d 4f transition is associated with the change in electric dipole and the 5d excited state is affected by crystal field effects. In the CaO-Al 2 O 3 system, several wellknown phosphor hosts exist, namely CaAl 2 O 4, CaAl 12 O 19, Ca 2 A l6 O 11, Ca 3 Al 2 O 6, CaAl 4 O 7 and Ca 4 Al 14 O [5 7] 25. CaAl 2 O 4 : Eu 2+, Nd 3+ and their solid solution in different Ba/Ca ratios is reported earlier [8,9]. To achieve various emission colours a change in the host lattice is regularly used by researchers all over the world [10,11]. This in turn opens up the involvement of various other aspects such as chemical nature, structure, reactivity, stability, etc, that are related to the host lattice. For example, europium gives blue (448 nm) luminescence in a CaAl 2 O 4 lattice and red (651 nm) in a CaS lattice. But tuning of emission colors in a single host lattice is a novel idea, which unfortunately has not been given serious thought. During our studies we found that without varying the conditions of preparation but slightly modifying the physico-chemical parameters of Eu doped CaAl x O y phosphors one can achieve a variety of emission wavelength and colors ranging from blue to red when excited by UV light. The endothermic processes during the solution-combustion process are determined by decomposition and removal of nitric oxides and significantly vary depending on the precursor composition and ratio of metal nitrate to urea [12,13]. This study is an attempt to investigate the effects on Ca:Al ratio on the morphological and structural properties of phosphors and the corresponding different possibilities of emissions with a single host lattice. 1 Experimental CaAl x O y :Eu phosphor powders with Ca:Al molar ratios of 6:100, 13:100, 19:100 and 26:100 were prepared by solutions combustion without flux material and post treatment. The doping concentration of Eu was fixed at 1 at.% of Ca component. The solutions were prepared by dissolving various amounts of the nitrate precursors of each component into distilled water. The resulting solution was transferred into a china crucible, which was then introduced into a muffle furnace maintained at 500 ºC for 5 6 min. The voluminous and foamy combustion ash were milled to obtain the final CaAl x O y :Eu 2+ phosphor. The crystal structures of the particles were studied by an X-ray diffraction (Bruker D8 diffractometer, Bruker Corporation of Germany, operating at 40 kv and 40 ma, and using Cu K = nm). The morphologies of particles were investigated using Shimadzu model ZU SSX-550 Superscan scanning electron microscope (SEM), coupled with an energy dispersive X-ray spec- Corresponding author: F. B. Dejene ( dejenebf@ufs.ac.za; Tel.: ) DOI: /S (10)

2 F.B. DEJENE et al., Synthesis and characterization of CaAl x O y :Eu 2+ phosphors prepared using solution-combustion method 273 trometer (EDS). Photoluminescence measurement was performed at room temperature on a Cary Eclipse fluorescence spectrophotometer (Model: LS 55) with a built-in 150 W xenon lamp as the excitation source and a grating to select a suitable wavelength for excitation. 2 Results 2.1 Morphological and structural properties It is well known that the final surface morphology and structure of a specific sample is significantly influenced by the overall bulk composition of the phosphor powder. To observe variations in morphology and structural features of the phosphor powders dependence on bulk composition, various samples are prepared with different Ca:Al molar ratios. The as-prepared powder reflects the foamy and agglomerate particle nature of the powder. The foamy structure of monoclinic CaAl 2 O 4 :Eu reflects the inherent nature of the reaction. The surface of the powder shows voids and pores, which may be formed by the evolved gases during combustion. Figs. 1 (a) and (b) depict SEM micrographs of the surface morphologies of the ground phosphors synthesized by solution-combustion process without fluxing agent for 6:100 and 26:100 molar ratio. Obviously, the particle size of phosphor synthesized with Sr/Al ratio of 6:100, are smaller, depicting highly uniform nanorods (Fig. 1 (a)) with diameters of about 30 nm and length from 1 2 micrometer. Typically phosphor morphology of samples with Sr/Al ratio of 26:100, was dominated by the presence of hexagonal platelets with well developed faces (Fig. 1 (b)), with sizes that varied from 1.0 to 2.0 m. Quasi-liquid interfaces are produced among crystal grains, and then the crystal grains reunite and result in the bigger particle size with well developed edges. Fig. 2 shows the EDS spectra of the nano-phosphors. The existence of little amount of doped rare earth active ions Eu 2+ in the samples is clear in their corresponding EDS spectra. There appeared no other emissions apart from Ca, Eu, O, Al, and C in the EDS spectra of the samples. The spectra depict the variation of Ca and Al elemental compositions. The elements appear in ratios similar with the proportions mixed in the starting materials, which shows the advantage of the liquid combustion process in controlling chemical components. Fig. 3 shows the XRD patterns for the CaAl x O y :Eu 2+ compositions with varying Ca:Al concentrations. The results of x-ray phase analysis of as prepared powders at 500 C demonstrated that several crystalline phases are formed depending on the ratio of the main components (Fig. 3). The XRD patterns show diffraction peaks of CaAl 12 O 19 phase for 6:100 molar ratios. With increasing Ca ions, the monoclinic CaAl 2 O 4 phase began to appear and the its diffraction peaks became sharper and stronger, indicating that the proportion of the monoclinic form was increased and led to CaAl 2 O 4 monoclinic as a single phase for 26:100 molar ratio (Contrast Joint Committee on Powder Diffraction Standards (JCPDS) data file No ). No other products or starting materials were observed for 26:100 molar ratios, implying that the small amounts of doped rare earth ions had almost no effect on the SrAl 2 O 4 phase composition [14]. It is apparent that a good crystalline CaAl 2 O 4 powder could be obtained at lower temperature by liquid combustion process, which is lower than that required for the sol-gel method [15] and the solid-state reaction method [16]. In fact, the volatile combustible gases ignite and burn with a flame and thus provide conditions for formation of phosphor lattice with dopant. Representative XRD pattern for 13:100 and 19: 100 molar ratios are also given for the comparison. Diffraction peaks for both these solid solution, show the mixed phase of both end members and presence of other phase such as Ca 3 Al 2 O 6 and CaAl 4 O 7. The predominant phase formed for most of these samples were CaAl 2 O 4 as the XRD pattern has shown minimum presence of other phases (Fig. 3 (b) and (c)). The structural transformation from CaAl 12 O 19 to monoclinic SrAl 2 O 4 can easily be seen from the XRD patterns. The average crystallite sizes were calculated using powder x-ray diffraction data by means of the Debye-Scherrer Eq. (1), D=K / cos (1) where D is the average diameter of the particles, K is the Scherrer constant equal to 0.9, is the wavelength of the Cu K radiation= nm, is the full width at half-maximum (FWHM), and is the Bragg angle. The calculated average crystallite sizes of the phosphor powder vary between 30 and 40 nm, which are consistent with the primary crystallite sizes from the SEM results. Fig. 1 SEM images of 6:100 (a) and 26:100 (b) molar ratios of Ca:Al with magnification (a) and magnification (b)

3 274 JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec Fig. 2 Point EDX results of the CaAl x O y :Eu 2+ phosphor powders for different Ca:Al molar ratios Fig. 3 XRD patterns of the CaAl x O y :Eu 2+ phosphor powders of the different molar ratios of Ca/Al 2.2 Photoluminescence spectra Fig. 4 (a) shows typical spectra of CaAl x O y :Eu 2+ phosphor powders with various Ca/Al molar ratio prepared by solution-combustion method. Under 325 nm excitation, the phosphor with high Ca/Al ratio emits predominantly in the blue domain, while that with lower Ca:Al ratio emits in the red domain i.e. 590 nm ( 5 D 0 7 F 1 ), 614 nm ( 5 D 0 7 F 2 ) and 680 nm ( 5 D 0 7 F 4 ) [17]. The broad blue emissions consist mainly of symmetrical bands having maxima at nm originate from the energy transitions between the ground state (4f 7 ) and the excited state (4f 6 5d 1 ) of Eu 2+ ions while the narrow emissions in red region arise from the f-f transitions of the remnant unreduced Eu 3+ ions. In case of doping with Eu 2+, the lowest excited state of 4f levels is located higher than the 4f 6 5d 1 level in most crystals, so that Eu 2+ usually gives broad-band emission due to f-d transitions. The wavelength positions of the emission bands depend very much on host, changing from near-uv to red. This dependence is interpreted as due to the crystal field splitting of the 5d level. With increasing crystal field strength, the emission bands shifts to longer wavelength. The emission in the vicinity of 600 nm is due to the magnetic dipole transition 5 D 0 7 F 1, which is insensitive to site symmetry. The emission around nm is due to the electric dipole transition of 5 D 0 7 F 2, induced by the lack of inversion symmetry at the Eu 3+ site, and is much stronger than that of the transition to the 7 F 1 state. The strong emission due to the electric dipole transition is utilized for commercial red phosphor applications, where the luminescence Eu 3+ ions occupy sites that have no inversion symmetry. If the Eu 3+ site has inversion symmetry, the electric dipole emissions weak and the dipole transition become relatively strong and dominate. As can be seen from Fig. 4 (b), PL intensities of broad emission of phosphors with high Ca:Al molar ratio are much higher than those of lower ratios. Also, it is found that that the peak wavelengths of the broad emission spectra drift slight in the direction of short wave, with an increase in Ca:Al ratio due to reduction in crystal field strength. The difference in PL intensity and position would be due to the difference in crystalline structure. It is known [16,18 20] that in contrast to ions Eu 3+, which yield red radiation in any matrix, the position of the radiation spectrum for Eu 2+ ions depends on the matrix, into which these ions are introduced. The mechanism is considered to include the photo-ionization of the Eu 2+ ions and a subsequent transfer of the electrons to traps formed by the lattice defects as oxide ion vacancies inside the band gap. When enough thermal energy is available, the captured electrons can escape again into the conduction band and recombine in a luminescent center leading to the Eu 2+ emission [21]. Fig. 4 (c) shows comparative curves of CaAl x O 4 :Eu 2+ phosphor with different Ca:Al molar ratios. The results indicate that the initial luminescence intensity of phosphors is enhanced when Ca:Al molar ratios are high (19:100 1nd 26:100). The sample with Ca:Al molar ratio of 19:100 significantly quenches the initial luminescence property of the phosphor. All the samples showed some decay property when the powders were efficiently excited by UV light. The decay behavior can be analyzed by a curve fitting procedure relying on the following triple exponential equation: Fig. 4 Emission spectra (a), Variation of maximum intensity and peak position of blue and red emission (b) and decay curves (c) of CaAl x O y :Eu 2+ phosphors of the different molar ratios of Ca/Al

4 F.B. DEJENE et al., Synthesis and characterization of CaAl x O y :Eu 2+ phosphors prepared using solution-combustion method 275 I=A 1 exp( t/ 1 )+A 2 exp( t/ 2 )+A 3 exp( t/ 3 ) (2) where I represents the phosphorescent intensity; A 1, A 2 and A 3 are constants; t is time; 1, 2 and 3 are the decay constants deciding the decay rate for the rapid, medium and slow exponential decay components, respectively. The fitting results of parameters of 1 and 2 are listed in Table 1. The decay constants have higher values for sample with Ca/Al ratio of 13:100 and lower value for sample with Ca:Al of 19:100. It is known that trap depth greatly influences the recombination between the excited electrons and trapped holes. Consequently, the decay components are determined by the carriers, which are trapped at different trapping levels. The long afterglow property of the phosphor samples can be described in terms of these carriers, whose nature has been shown to be holes [22,23]. Upon exposure of phosphors to the excitation radiation, the direct excitation of 4f 5d transition of Eu 2+ happens and plenty of holes are created; some of the free holes are released thermally to the valence band, meantime, one part of the released holes travel through the valence band, and are trapped by defects. When the excitation source is removed, the excited Eu + is relaxed to the metastable state (Eu + ) *. The trapped holes are detached thermally to the valence band and move to recombine with the excessive loosely bound electrons in the metastable state (Eu + ) * sites leading to the long afterglow [24]. Table 1 Results for fitted decay curves of the phosphor powders with different Ca:Al molar ratios Ca:Al molar ratio 6:100 13:100 19:100 26:100 Component Decay constants/s Fast ( ) Medium ( ) Slow ( ) Conclusions The CaAl x O y :Eu 2+ phosphors were successfully synthesized by solution combustion method and the effect of Ca/Al molar ratio on their structures and PL properties were examined. It was found that the phosphor with Ca/Al molar ratio of 26:100 was in favor of formation of CaAl 2 O 4 pure phase, which enhanced the intensity of blue emission of the phosphor while Ca/Al of 6:100 favors the CaAl 12 O 19 structure, enhances the red emission. The XRD results showed that the Ca/Al molar ratio of 26:100 retained the basic low-temperature monoclinic phase. The intermediate products formed at different Ca:Al molar ratio were identified as CaAl 4 O 7 and Ca 3 Al 2 O 6. The particle size of the phosphor became nano sized when the phosphor was synthesized with Ca/Al molar ratio of 6:100. 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