- Email: [email protected]
Combustion synthesis and photoluminescence of SrAl2O4:Eu,Dy phosphor nanoparticles
Materials Letters 58 (2004) 352 – 356 www.elsevier.com/locate/matlet
Combustion synthesis and photoluminescence of SrAl2O4:Eu,Dy phosphor nanoparticles Tianyou Peng a,*, Huanping Yang a, Xuli Pu a, Bin Hu a, Zucheng Jiang a, Chunhua Yan b a
b
Department of Chemistry, Wuhan University, Wuhan 430072, PR China State Key Laboratory of Rare Earth Materials Chemistry and Application, Peking University, Beijing, PR China Received 31 March 2003; accepted 5 June 2003
Abstract Eu2 +,Dy3 + co-doped strontium aluminate (SrAl2O4) phosphor nanoparticles with high brightness and long afterglow were prepared by glycine – nitrate solution combustion synthesis at 500 jC, followed by heating the resultant combustion ash at 1100 jC in a weak reductive atmosphere of active carbon. The average particle size of the SrAl2O4:Eu,Dy phosphor nanoparticles ranges from 15 to 45 nm as indicated by transmission electron microscopy (TEM). The broad-band UV-excited luminescence of the SrAl2O4:Eu,Dy phosphor nanoparticles was observed at kmax = 513 nm due to transitions from the 4f65d1 to the 4f7 configuration of the Eu2 + ion. The results indicated that the main peaks in the emission and excitation spectrum of phosphor nanoparticles shifted to the short wavelength compared with the phosphor obtained by the solid-state reaction synthesis method. The decay speed of the afterglow for phosphor nanoparticles was faster than that obtained by the solid-state reaction method. D 2003 Elsevier B.V. All rights reserved. Keywords: Long-afterglow phosphor; Combustion synthesis; Strontium aluminate; Hole trap; Optical properties
1. Introduction The photoluminescence at the blue-green visible region of Eu 2 + -doped alkaline earth aluminates MAl 2 O 4 :Eu 2 + (M = Ca, Sr, Ba) phosphors have been studied extensively [1 –5]. The afterglow lifetime and intensity can be enhanced by co-doping with some other rare earth ions [4 –6]. Those phosphors exhibit a rapid initial decay from the Eu2 + ion followed by a long persistence. This effect has been ascribed to the thermal activation of holes from traps followed by the emission of Eu2 + [7– 10]. Compared with sulfide phosphorescent phosphors, SrAl2O4:Eu,Dy phosphor possesses safer, chemically stable, very bright and long-lasting photoluminescence with no radiation [6 – 12], which results in an unexpectedly large field of applications, such as luminous paints in highway, airport, buildings and ceramics products, as well as in textile, the dial plate of glow watch, warning signs and escape routine, etc. [7,13]. The grain size of phosphor powders prepared through solid-state reaction method is in several tens of micrometers. Phosphors of small particles must be obtained by grinding the larger phosphor * Corresponding author. Fax: +86-278-764-7617. E-mail address: [email protected] (T. Peng). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-577X(03)00499-3
particles. Those processes easily introduce additional defects and greatly reduce luminescence efficiency [14]. With the development of scientific technologies on materials, several chemical synthesis techniques, such as co-precipitation [8], sol –gel [15] and combustion synthesis methods [16], have been applied to prepare SrAl2O4 and/or its phosphors. All of these methods were conducted in liquid phases so that each component can be accurately controlled and uniformly mixed. For sol – gel or co-precipitation techniques, processing routines to prepare the precursor powders are complicated and the duration is long. The combustion process to prepare the precursor powders, however, is very facile and only takes a few minutes, which has been extensively applied to the preparation of various nano-scale oxide materials. This synthesis technique makes use of the heat energy liberated by the redox exothermic reaction at a relative low igniting temperature between metal nitrates and urea or other fuels. Furthermore, the process is also safe, instantaneous and energy saving. Therefore, the combustion synthesis appears to hold promise for the preparation of nanosized aluminate phosphors. These phosphor nanoparticles are potential for construction materials in detecting damage in a bridge or high buildings, as well as in display and decorative materials in hotels, the plaza and instrument,
T. Peng et al. / Materials Letters 58 (2004) 352–356
etc. To the best of our knowledge, reports on the preparation of SrAl2O4:Eu,Dy nanoparticles is still rare [8], although ultrafine SrAl2O4 has been prepared by the combustion of corresponding metal nitrate – aluminium nitrate –urea mixture synthesis [16]. In view of the special properties concerning the nano-scale materials, it is interesting to know the differences in properties between the nanoparticles and micrometer-scale particles of SrAl2O4:Eu,Dy phosphors. In this paper, combustion synthesis is applied to prepare the SrAl2O4:Eu,Dy phosphor nanoparticles. The microstructure of the as-prepared SrAl2O4:Eu,Dy precursor powders are investigated by X-ray diffraction (XRD), scanning electron microscopy(SEM) and transmission electron microscopy (TEM). The photoluminescent phosphor nanoparticles with high brightness and long afterglow were obtained by heating the resultant combustion ash precursor powders at 1100 jC in a weak reductive atmosphere of active carbon.
2. Experimental Sr0.97Al2O4:Eu0.01,Dy0.02 phosphor nanoparticles were prepared by solution combustion synthesis method followed by heating the precursor combustion ash at 1100 jC in a reductive atmosphere of active carbon. In a cylindrical quartz container (80 mm diameter 200 mm height), stoichiometric composition of aluminum nitrate (Al(NO3)3), strontium nitrate (Sr(NO3)2), dysprosium nitrate (Dy(NO3)3) and europium nitrate (Eu(NO3)3) were dissolved in a minimum amount of distilled water together with 1.5 times excess amount of glycine. The precursor solution was introduced into a muffle furnace maintained at 500 jC. Initially, the solution boiled and underwent dehydration, followed by decomposition with the evolution of large amounts of gases (oxides of carbon, nitrogen and ammonia). Then, spontaneous ignition occurred and underwent smouldering combustion with enormous swelling, producing white foamy and voluminous SrAl2O4:Eu,Dy. The whole process is over within less than 3 min. The voluminous and foamy combustion ash can be easily milled to obtain the precursor powder of SrAl2O4:Eu,Dy. The well-milled precursor powder is subsequently annealed at 1100 jC for 2 h in an active carbon atmosphere, producing SrAl2O4:Eu,Dy phosphor nanoparticles. Sr0.97Al2O4:Eu0.01,Dy0.02 phosphors were also synthesized at 1300 jC by the usual solid-state reaction technique under a weak reductive atmosphere of active carbon for comparing, and boric acid was added to the mixture as flux [6]. All starting materials were of the analytical purity. The morphological studies are carried out on a H-510 scanning electron microscope (SEM) and a H-600 transmission electron microscope (TEM). Crystal structures of samples were checked by means of a Rigaku RINT-1400 ˚ . The photolumidiffractometer using Cu Ka k = 1.5406 A nescent properties of the obtained phosphor nanoparticles were measured at room temperature using a Perkin Elmer
353
LS-5 spectrometer. The excitation (kexc = 350 nm) source was a xenon lamp. The decay profiles were also recorded using the same instrument after the samples were sufficiently excited for about 20 min. Prior to the afterglow measurements, samples were exposed to irradiation from an 11-W conventional tricolor fluorescent lamp.
3. Results and discussion A typical X-ray diffraction pattern of the resultant SrAl2O4:Eu,Dy precursor powders are shown in Fig. 1a. As can be seen, pure monoclinic phase diffraction peaks of SrAl2O4 are predominant in the XRD patterns, which is similar with the earlier results [15]. No other product or starting material was observed, implying that the phase composition of the precursor powders are all low-temperature monoclinic phase (a-phase), and the little amount of doped rare earth ions have almost no effect on the SrAl2O4 phase composition [8]. Fig. 1b shows the diffraction pattern of phosphor nanoparticles obtained by heating the precursor powders of SrAl2O4:Eu,Dy at 1100 jC for 2 h. There is no observable differences between the two diffraction patterns in Fig. 1a and b, indicating that the pure monoclinic phase of SrAl2O4 has already formed in the first combustion steps. In conventional solid-state reactions, the pure SrAl2O4: Eu,Dy monoclinic phase appears at 1300 jC [6]. Although no flux is added, SrAl2O4:Eu,Dy phase with high purity can be obtained at 500 jC through the combustion process to the starting materials, whereas it is impossible to happen for solid-state reaction method due to impurities as M3Al2O6 are formed at lower temperatures. Furthermore, pure singlephase MAl2O4 (M = Mg, Ca, Sr, Ba and Zn) has already been prepared by combustion synthesis method [16]. Those metal aluminates were obtained by the combustion of corresponding metal nitrate – aluminium nitrate – urea mixture at 500 jC as described above. Furthermore, it has been also proved that metal aluminates with different structures such as CaAl2O4, Ca3Al2O6, CaAl12O19 and MgCeAl11O19 can be prepared by just changing the metal nitrate – alumi-
Fig. 1. X-ray diffraction patterns of the SrAl2O4:Eu,Dy: (a) as-prepared, (b) deoxidized at 1100 jC in active carbon atmosphere.
354
T. Peng et al. / Materials Letters 58 (2004) 352–356
ninm nitrate – urea ratio in the combustion reaction. The reason may be attributed to the above 1500 jC of flame temperature during the combustion synthesis processing [17]. Moreover, the uniform component in solution phase is also beneficial for the formation of the pure phase at relative low temperature. The line broadening feature in Fig. 1 can be seen due to their smaller particle size, indicting that the phosphor nanocrystallines can be prepared in a far low temperature (about 500 jC). With the mean crystalline size of precursor powders and phosphor nanoparticles obtained at 1100 jC deduced according to the Scherrer’s equation using full-width at the half-maximum (FWHM) data [18], the calculated crystalline sizes are 12 and 25 nm, respectively. The microstructure of the precursor powders and SrAl2O4:Eu,Dy phosphor nanoparticles deoxidized at 1100 jC are studied on their SEM and TEM micrographs (Fig. 2). The morphology of the precursor powders is unconsolidated, reflecting the inherent nature of the combustion process (Fig. 2a). The surfaces of the foams show a lot of cracks, voids and pores formed by the escaping gases during combustion reaction. In order to achieve accurate data of the grain size of SrAl2O4:Eu,Dy precursor powders, its TEM image is recorded in Fig. 2b. Most of the particles of SrAl2O4:Eu,Dy precursor powders appear to be irregularly elliptical with aggregation and an average particle size of 5 – 30 nm (Fig. 2b). The grain size of SrAl2O4:Eu,Dy phosphor deoxidized at 1100 jC is increased to 15 – 45 nm with some hard agglomeration accompanied with the formation of necked crystallites, as can be observed from Fig. 2c. Furthermore, the crystalline size according to Scherrer’s equation is very closed to the TEM observation, also implying that the obtained phosphor nanoparticles have good crystallinity, which is apparently different from that of SrAl2O4 resulted from solid-state reaction method. Figs. 3 and 4 show the excitation and emission spectra at room temperature of SrAl2O4:Eu,Dy phosphor prepared by solution combustion synthesis and solid-state reaction methods, respectively. It is observed that the main peaks of excitation spectra of phosphor nanoparticles shift to shorter wavelength (from 258 to 242 nm, and 323 to 313 nm) compared with the phosphor resulted from solid-state reaction method. This may be associated with the quantum size effect of the phosphor nanoparticles, which increased the kinetic energy of the electrons and resulted in a larger band gap, thus, needing higher energy to excite the luminescent powders [8]. The UV-excited SrAl2O4:Eu,Dy phosphor nanoparticles at room temperature yielded high bright green luminescence (kmax = 513 nm) with only one band (Fig. 4). The bandwidth was quite large, f 67 nm, but symmetric, indicating only one luminescent centre, which is Eu2 + luminescence and excitation result from transitions between the 4f65d1 and 4f7 electron configurations. The resultant SrAl2O4:Eu,Dy combustion ash precursor powders has not shown this emission spectra, indicating that the active carbon can effectively deoxidize the Eu3 + to Eu2 +. More-
Fig. 2. SEM and TEM images of SrAl2O4:Eu,Dy. (a) SEM of precursor powders, (b) TEM of precursor powders, (c) TEM of phosphors deoxidized at 1100 jC in active carbon atmosphere.
over, compared with the sample obtained by solid-state reaction method, the emission maximum peak of phosphor nanoparticles shifted to shorter wavelength (from 519 to 513 nm). A similar result has also been observed in ultrafine needle-like SrAl2O4:Eu,Dy phosphors reported by Lin et al. [8]. The slight blue shift in the emission band may be attributed to the changes of the crystal field around Eu2 +. Since the excited 4f65d1 configurations of Eu2 + ion is extremely sensitive to the change in the lattice environment in contrast to the shielded 4f7 ground configurations due to the shielding function of the electrons in the inner shell, the 5d electron may couple strongly to the lattice. Hence, the mixed states of 4f and 5d will be split by the crystal field, which may lead to the blue shift of its emission peak. The afterglow band (Fig. 4c) had the same position, shape and
T. Peng et al. / Materials Letters 58 (2004) 352–356
Fig. 3. Excitation spectra of SrAl2O4:Eu,Dy phosphors resulting from (a) solution combustion method, (b) solid state reaction method.
bandwidth as those of the UV-excited spectra, indicating that the emitting centre is still Eu2 + ion. In the present system, Eu2 + ions are the luminescent centers, the photoexcited luminescence is considered to be due the transition from 5d level to 4f level of Eu2 + and holes in the traps are responsible for the long afterglow [6]. Therefore, Dy3 + played an important role and worked as hole traps. The trap levels lie in between the excited state and the ground state of Eu2 +. This is because the existence of stable tetravalent state Dy4 + of this rare earth ion. Moreover, Dy3 + ion possesses relatively high transfer energies than any other trivalent rare earth ions. After excitement by the ultraviolet lights, electrons of Eu2 + ion in the 4f level transfer to 5d level, and holes were produced from 4f level to the valence band and Eu2 + ions change to Eu+. Some free holes created in the valence band of the host were transported in the conduction band and captured by the Dy3 + hole traps. Large amounts of Dy3 + is populated, having captured holes from the valence band, and stay as Dy4 +. When the excitation source was cut off, some holes captured by the Dy3 + hole traps were thermally released slowly to
Fig. 4. Emission spectra of SrAl2O4:Eu,Dy phosphors resulting from (a) solution combustion method, (b) solid state reaction method, (c) afterglow spectrum of SrAl2O4/Eu,Dy resulting from solution combustion method at room temperature determined at 2 min after removal of the illumination of conventional tricolor fluorescent lamp.
355
valence band, and the released holes travel back of Eu2 +, finally returning to the ground state of Eu2 + accompanied with emitting the lights. Furthermore, the hole mobility may also play an important role and need to be taken into account. Low rate of hole mobility and electron-hole recombination will increase retraping probability and further slow down the decay process [9]. The decay characteristics of SrAl2O4:Eu,Dy phosphor indicated that the decay process both of two kinds of phosphors contained the rapid-decaying process and the slow-decaying one (Fig. 5). The afterglow of phosphor nanoparticles, which allowed the time to be visually recognized ( z 0.32 mcd/m2), lasted for over 16 h after the excited source was cut off. The initial intensity of nanometer phosphor (Fig. 5a) is lower than that of the phosphor powders prepared by the solid-state reaction method after exciting for 20 min (see also Fig. 4). Moreover, the phosphor nanoparticles decayed more rapidly than that prepared by the solid-state reaction method. The reason may be that the nanoparticles made the phosphor crystallize easily and completely during heattreatment process as described above, which may lead to decreasing the amount of defects in the inner phosphor and fewer crystallographic distortions, as well as shallower trap level than that of phosphor obtained from solid-state reaction method, so that the relative intensity of afterglow is lowered and the decay of afterglow is hastened. In addition, a lot of defects are dispersed on the surface of phosphors because of the high surface area of the nanoparticles, and which may result in relatively less amount of luminescent center Eu2 + in the SrAl2O4 lattice available for the direct radiation. Therefore, it may result in the weaker initial intensity of phosphor nanoparticles. Furthermore, as described above, fast speed of hole mobility and electron-hole recombination in nanoparticles with good crystallinity will decrease retraping probability and further prompt the decay process. Of course, the
Fig. 5. Decay curve of SrAl2O4:Eu,Dy phosphors after exciting for 20 min: (a) solution combustion method, (b) solid state reaction method (room temperature 25 jC, excitation time 20 min, source conventional tricolor luminescent lamp).
356
T. Peng et al. / Materials Letters 58 (2004) 352–356
detailed reasons need to be further investigated, and relative research is under way.
4. Conclusion The SrAl2O4:Eu,Dy phosphor nanoparticles can be synthesized by combustion synthesis processing, along with heating the resultant combustion ash precursor powder at 1100 jC in a weak reductive atmosphere of active carbon. Analytical results showed that the nanometer phosphors have pure monoclinic SrAl2O4:Eu,Dy phase, with the average particles size in the range of 15 – 45 nm. Compared with the solid-state reaction method, the blue shift of main peaks of excitation and emission of the nanometer phosphors occur.
Acknowledgements Financial support from the State Key Laboratory of Rare Earth Materials Chemistry and Application of Peking University and Hubei Province Science and Technology Commission in China are gratefully acknowledged.
References [1] F.C. Palilla, A.K. Levine, M.R. Tomkus, J. Electrochem. Soc. 115 (1968) 642. [2] G. Blasse, A. Bril, Philips Res. Rep. 23 (1968) 201. [3] S.H.M. Poort, W.P. Blokpoel, G. Blasse, Chem. Mater. 7 (1995) 1547. [4] Y. Lin, Z. Tang, Z. Zhang, C. Nan, J. Eur. Ceram. Soc. 23 (2003) 175. [5] R. Sakai, T. Katsumata, S. Komuro, T. Morikawa, J. Lumin. 85 (1999) 149. [6] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [7] H. Yamamoto, T. Matsuzawa, J. Lumin. 72 – 74 (1997) 287. [8] Y. Lin, Z. Zhang, F. Zhang, Z. Tang, Q. Chen, Mater. Chem. Phys. 65 (2000) 103. [9] W. Jia, H. Yuan, L. Lu, H. Liu, W.M. Yen, J. Lumin. 76 – 77 (1998) 424. [10] E. Nakazawa, T. Mochida, J. Lumin. 72 – 74 (1997) 236. [11] W.Y. Jia, H.B. Yuan, W.M. Yen, J. Lumin. 76 (1998) 424. [12] G. Groppi, C. Cristiani, P. Forzatti, J. Mater. Sci. 29 (1994) 3441. [13] Y. Murayama, N. Takeuchi, Y. Aoki, T. Matsuzawa, Phosphorescent Phosphor, US Patent 5424006 (1995). [14] R.P. Rao, J. Electrochem. Soc. 143 (1996) 189. [15] L.K. Kurihara, S.L. Suib, Chem. Mater. 5 (1993) 609. [16] J.J. Kingsley, K. Suersh, K.C. Patil, J. Mater. Sci. 25 (1990) 1305. [17] Y. Tao, G. Zhao, W. Zhang, S. Xia, Mater. Res. Bull. 52 (1997) 501. [18] B.B. Cullity, Elements of X-ray Diffraction, 2nd ed., Addison-Wesley, Reading, MA, 1997, p. 102.
Combustion synthesis and photoluminescence of SrAl2O4:Eu,Dy phosphor nanoparticles Tianyou Peng a,*, Huanping Yang a, Xuli Pu a, Bin Hu a, Zucheng Jiang a, Chunhua Yan b a
b
Department of Chemistry, Wuhan University, Wuhan 430072, PR China State Key Laboratory of Rare Earth Materials Chemistry and Application, Peking University, Beijing, PR China Received 31 March 2003; accepted 5 June 2003
Abstract Eu2 +,Dy3 + co-doped strontium aluminate (SrAl2O4) phosphor nanoparticles with high brightness and long afterglow were prepared by glycine – nitrate solution combustion synthesis at 500 jC, followed by heating the resultant combustion ash at 1100 jC in a weak reductive atmosphere of active carbon. The average particle size of the SrAl2O4:Eu,Dy phosphor nanoparticles ranges from 15 to 45 nm as indicated by transmission electron microscopy (TEM). The broad-band UV-excited luminescence of the SrAl2O4:Eu,Dy phosphor nanoparticles was observed at kmax = 513 nm due to transitions from the 4f65d1 to the 4f7 configuration of the Eu2 + ion. The results indicated that the main peaks in the emission and excitation spectrum of phosphor nanoparticles shifted to the short wavelength compared with the phosphor obtained by the solid-state reaction synthesis method. The decay speed of the afterglow for phosphor nanoparticles was faster than that obtained by the solid-state reaction method. D 2003 Elsevier B.V. All rights reserved. Keywords: Long-afterglow phosphor; Combustion synthesis; Strontium aluminate; Hole trap; Optical properties
1. Introduction The photoluminescence at the blue-green visible region of Eu 2 + -doped alkaline earth aluminates MAl 2 O 4 :Eu 2 + (M = Ca, Sr, Ba) phosphors have been studied extensively [1 –5]. The afterglow lifetime and intensity can be enhanced by co-doping with some other rare earth ions [4 –6]. Those phosphors exhibit a rapid initial decay from the Eu2 + ion followed by a long persistence. This effect has been ascribed to the thermal activation of holes from traps followed by the emission of Eu2 + [7– 10]. Compared with sulfide phosphorescent phosphors, SrAl2O4:Eu,Dy phosphor possesses safer, chemically stable, very bright and long-lasting photoluminescence with no radiation [6 – 12], which results in an unexpectedly large field of applications, such as luminous paints in highway, airport, buildings and ceramics products, as well as in textile, the dial plate of glow watch, warning signs and escape routine, etc. [7,13]. The grain size of phosphor powders prepared through solid-state reaction method is in several tens of micrometers. Phosphors of small particles must be obtained by grinding the larger phosphor * Corresponding author. Fax: +86-278-764-7617. E-mail address: [email protected] (T. Peng). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-577X(03)00499-3
particles. Those processes easily introduce additional defects and greatly reduce luminescence efficiency [14]. With the development of scientific technologies on materials, several chemical synthesis techniques, such as co-precipitation [8], sol –gel [15] and combustion synthesis methods [16], have been applied to prepare SrAl2O4 and/or its phosphors. All of these methods were conducted in liquid phases so that each component can be accurately controlled and uniformly mixed. For sol – gel or co-precipitation techniques, processing routines to prepare the precursor powders are complicated and the duration is long. The combustion process to prepare the precursor powders, however, is very facile and only takes a few minutes, which has been extensively applied to the preparation of various nano-scale oxide materials. This synthesis technique makes use of the heat energy liberated by the redox exothermic reaction at a relative low igniting temperature between metal nitrates and urea or other fuels. Furthermore, the process is also safe, instantaneous and energy saving. Therefore, the combustion synthesis appears to hold promise for the preparation of nanosized aluminate phosphors. These phosphor nanoparticles are potential for construction materials in detecting damage in a bridge or high buildings, as well as in display and decorative materials in hotels, the plaza and instrument,
T. Peng et al. / Materials Letters 58 (2004) 352–356
etc. To the best of our knowledge, reports on the preparation of SrAl2O4:Eu,Dy nanoparticles is still rare [8], although ultrafine SrAl2O4 has been prepared by the combustion of corresponding metal nitrate – aluminium nitrate –urea mixture synthesis [16]. In view of the special properties concerning the nano-scale materials, it is interesting to know the differences in properties between the nanoparticles and micrometer-scale particles of SrAl2O4:Eu,Dy phosphors. In this paper, combustion synthesis is applied to prepare the SrAl2O4:Eu,Dy phosphor nanoparticles. The microstructure of the as-prepared SrAl2O4:Eu,Dy precursor powders are investigated by X-ray diffraction (XRD), scanning electron microscopy(SEM) and transmission electron microscopy (TEM). The photoluminescent phosphor nanoparticles with high brightness and long afterglow were obtained by heating the resultant combustion ash precursor powders at 1100 jC in a weak reductive atmosphere of active carbon.
2. Experimental Sr0.97Al2O4:Eu0.01,Dy0.02 phosphor nanoparticles were prepared by solution combustion synthesis method followed by heating the precursor combustion ash at 1100 jC in a reductive atmosphere of active carbon. In a cylindrical quartz container (80 mm diameter 200 mm height), stoichiometric composition of aluminum nitrate (Al(NO3)3), strontium nitrate (Sr(NO3)2), dysprosium nitrate (Dy(NO3)3) and europium nitrate (Eu(NO3)3) were dissolved in a minimum amount of distilled water together with 1.5 times excess amount of glycine. The precursor solution was introduced into a muffle furnace maintained at 500 jC. Initially, the solution boiled and underwent dehydration, followed by decomposition with the evolution of large amounts of gases (oxides of carbon, nitrogen and ammonia). Then, spontaneous ignition occurred and underwent smouldering combustion with enormous swelling, producing white foamy and voluminous SrAl2O4:Eu,Dy. The whole process is over within less than 3 min. The voluminous and foamy combustion ash can be easily milled to obtain the precursor powder of SrAl2O4:Eu,Dy. The well-milled precursor powder is subsequently annealed at 1100 jC for 2 h in an active carbon atmosphere, producing SrAl2O4:Eu,Dy phosphor nanoparticles. Sr0.97Al2O4:Eu0.01,Dy0.02 phosphors were also synthesized at 1300 jC by the usual solid-state reaction technique under a weak reductive atmosphere of active carbon for comparing, and boric acid was added to the mixture as flux [6]. All starting materials were of the analytical purity. The morphological studies are carried out on a H-510 scanning electron microscope (SEM) and a H-600 transmission electron microscope (TEM). Crystal structures of samples were checked by means of a Rigaku RINT-1400 ˚ . The photolumidiffractometer using Cu Ka k = 1.5406 A nescent properties of the obtained phosphor nanoparticles were measured at room temperature using a Perkin Elmer
353
LS-5 spectrometer. The excitation (kexc = 350 nm) source was a xenon lamp. The decay profiles were also recorded using the same instrument after the samples were sufficiently excited for about 20 min. Prior to the afterglow measurements, samples were exposed to irradiation from an 11-W conventional tricolor fluorescent lamp.
3. Results and discussion A typical X-ray diffraction pattern of the resultant SrAl2O4:Eu,Dy precursor powders are shown in Fig. 1a. As can be seen, pure monoclinic phase diffraction peaks of SrAl2O4 are predominant in the XRD patterns, which is similar with the earlier results [15]. No other product or starting material was observed, implying that the phase composition of the precursor powders are all low-temperature monoclinic phase (a-phase), and the little amount of doped rare earth ions have almost no effect on the SrAl2O4 phase composition [8]. Fig. 1b shows the diffraction pattern of phosphor nanoparticles obtained by heating the precursor powders of SrAl2O4:Eu,Dy at 1100 jC for 2 h. There is no observable differences between the two diffraction patterns in Fig. 1a and b, indicating that the pure monoclinic phase of SrAl2O4 has already formed in the first combustion steps. In conventional solid-state reactions, the pure SrAl2O4: Eu,Dy monoclinic phase appears at 1300 jC [6]. Although no flux is added, SrAl2O4:Eu,Dy phase with high purity can be obtained at 500 jC through the combustion process to the starting materials, whereas it is impossible to happen for solid-state reaction method due to impurities as M3Al2O6 are formed at lower temperatures. Furthermore, pure singlephase MAl2O4 (M = Mg, Ca, Sr, Ba and Zn) has already been prepared by combustion synthesis method [16]. Those metal aluminates were obtained by the combustion of corresponding metal nitrate – aluminium nitrate – urea mixture at 500 jC as described above. Furthermore, it has been also proved that metal aluminates with different structures such as CaAl2O4, Ca3Al2O6, CaAl12O19 and MgCeAl11O19 can be prepared by just changing the metal nitrate – alumi-
Fig. 1. X-ray diffraction patterns of the SrAl2O4:Eu,Dy: (a) as-prepared, (b) deoxidized at 1100 jC in active carbon atmosphere.
354
T. Peng et al. / Materials Letters 58 (2004) 352–356
ninm nitrate – urea ratio in the combustion reaction. The reason may be attributed to the above 1500 jC of flame temperature during the combustion synthesis processing [17]. Moreover, the uniform component in solution phase is also beneficial for the formation of the pure phase at relative low temperature. The line broadening feature in Fig. 1 can be seen due to their smaller particle size, indicting that the phosphor nanocrystallines can be prepared in a far low temperature (about 500 jC). With the mean crystalline size of precursor powders and phosphor nanoparticles obtained at 1100 jC deduced according to the Scherrer’s equation using full-width at the half-maximum (FWHM) data [18], the calculated crystalline sizes are 12 and 25 nm, respectively. The microstructure of the precursor powders and SrAl2O4:Eu,Dy phosphor nanoparticles deoxidized at 1100 jC are studied on their SEM and TEM micrographs (Fig. 2). The morphology of the precursor powders is unconsolidated, reflecting the inherent nature of the combustion process (Fig. 2a). The surfaces of the foams show a lot of cracks, voids and pores formed by the escaping gases during combustion reaction. In order to achieve accurate data of the grain size of SrAl2O4:Eu,Dy precursor powders, its TEM image is recorded in Fig. 2b. Most of the particles of SrAl2O4:Eu,Dy precursor powders appear to be irregularly elliptical with aggregation and an average particle size of 5 – 30 nm (Fig. 2b). The grain size of SrAl2O4:Eu,Dy phosphor deoxidized at 1100 jC is increased to 15 – 45 nm with some hard agglomeration accompanied with the formation of necked crystallites, as can be observed from Fig. 2c. Furthermore, the crystalline size according to Scherrer’s equation is very closed to the TEM observation, also implying that the obtained phosphor nanoparticles have good crystallinity, which is apparently different from that of SrAl2O4 resulted from solid-state reaction method. Figs. 3 and 4 show the excitation and emission spectra at room temperature of SrAl2O4:Eu,Dy phosphor prepared by solution combustion synthesis and solid-state reaction methods, respectively. It is observed that the main peaks of excitation spectra of phosphor nanoparticles shift to shorter wavelength (from 258 to 242 nm, and 323 to 313 nm) compared with the phosphor resulted from solid-state reaction method. This may be associated with the quantum size effect of the phosphor nanoparticles, which increased the kinetic energy of the electrons and resulted in a larger band gap, thus, needing higher energy to excite the luminescent powders [8]. The UV-excited SrAl2O4:Eu,Dy phosphor nanoparticles at room temperature yielded high bright green luminescence (kmax = 513 nm) with only one band (Fig. 4). The bandwidth was quite large, f 67 nm, but symmetric, indicating only one luminescent centre, which is Eu2 + luminescence and excitation result from transitions between the 4f65d1 and 4f7 electron configurations. The resultant SrAl2O4:Eu,Dy combustion ash precursor powders has not shown this emission spectra, indicating that the active carbon can effectively deoxidize the Eu3 + to Eu2 +. More-
Fig. 2. SEM and TEM images of SrAl2O4:Eu,Dy. (a) SEM of precursor powders, (b) TEM of precursor powders, (c) TEM of phosphors deoxidized at 1100 jC in active carbon atmosphere.
over, compared with the sample obtained by solid-state reaction method, the emission maximum peak of phosphor nanoparticles shifted to shorter wavelength (from 519 to 513 nm). A similar result has also been observed in ultrafine needle-like SrAl2O4:Eu,Dy phosphors reported by Lin et al. [8]. The slight blue shift in the emission band may be attributed to the changes of the crystal field around Eu2 +. Since the excited 4f65d1 configurations of Eu2 + ion is extremely sensitive to the change in the lattice environment in contrast to the shielded 4f7 ground configurations due to the shielding function of the electrons in the inner shell, the 5d electron may couple strongly to the lattice. Hence, the mixed states of 4f and 5d will be split by the crystal field, which may lead to the blue shift of its emission peak. The afterglow band (Fig. 4c) had the same position, shape and
T. Peng et al. / Materials Letters 58 (2004) 352–356
Fig. 3. Excitation spectra of SrAl2O4:Eu,Dy phosphors resulting from (a) solution combustion method, (b) solid state reaction method.
bandwidth as those of the UV-excited spectra, indicating that the emitting centre is still Eu2 + ion. In the present system, Eu2 + ions are the luminescent centers, the photoexcited luminescence is considered to be due the transition from 5d level to 4f level of Eu2 + and holes in the traps are responsible for the long afterglow [6]. Therefore, Dy3 + played an important role and worked as hole traps. The trap levels lie in between the excited state and the ground state of Eu2 +. This is because the existence of stable tetravalent state Dy4 + of this rare earth ion. Moreover, Dy3 + ion possesses relatively high transfer energies than any other trivalent rare earth ions. After excitement by the ultraviolet lights, electrons of Eu2 + ion in the 4f level transfer to 5d level, and holes were produced from 4f level to the valence band and Eu2 + ions change to Eu+. Some free holes created in the valence band of the host were transported in the conduction band and captured by the Dy3 + hole traps. Large amounts of Dy3 + is populated, having captured holes from the valence band, and stay as Dy4 +. When the excitation source was cut off, some holes captured by the Dy3 + hole traps were thermally released slowly to
Fig. 4. Emission spectra of SrAl2O4:Eu,Dy phosphors resulting from (a) solution combustion method, (b) solid state reaction method, (c) afterglow spectrum of SrAl2O4/Eu,Dy resulting from solution combustion method at room temperature determined at 2 min after removal of the illumination of conventional tricolor fluorescent lamp.
355
valence band, and the released holes travel back of Eu2 +, finally returning to the ground state of Eu2 + accompanied with emitting the lights. Furthermore, the hole mobility may also play an important role and need to be taken into account. Low rate of hole mobility and electron-hole recombination will increase retraping probability and further slow down the decay process [9]. The decay characteristics of SrAl2O4:Eu,Dy phosphor indicated that the decay process both of two kinds of phosphors contained the rapid-decaying process and the slow-decaying one (Fig. 5). The afterglow of phosphor nanoparticles, which allowed the time to be visually recognized ( z 0.32 mcd/m2), lasted for over 16 h after the excited source was cut off. The initial intensity of nanometer phosphor (Fig. 5a) is lower than that of the phosphor powders prepared by the solid-state reaction method after exciting for 20 min (see also Fig. 4). Moreover, the phosphor nanoparticles decayed more rapidly than that prepared by the solid-state reaction method. The reason may be that the nanoparticles made the phosphor crystallize easily and completely during heattreatment process as described above, which may lead to decreasing the amount of defects in the inner phosphor and fewer crystallographic distortions, as well as shallower trap level than that of phosphor obtained from solid-state reaction method, so that the relative intensity of afterglow is lowered and the decay of afterglow is hastened. In addition, a lot of defects are dispersed on the surface of phosphors because of the high surface area of the nanoparticles, and which may result in relatively less amount of luminescent center Eu2 + in the SrAl2O4 lattice available for the direct radiation. Therefore, it may result in the weaker initial intensity of phosphor nanoparticles. Furthermore, as described above, fast speed of hole mobility and electron-hole recombination in nanoparticles with good crystallinity will decrease retraping probability and further prompt the decay process. Of course, the
Fig. 5. Decay curve of SrAl2O4:Eu,Dy phosphors after exciting for 20 min: (a) solution combustion method, (b) solid state reaction method (room temperature 25 jC, excitation time 20 min, source conventional tricolor luminescent lamp).
356
T. Peng et al. / Materials Letters 58 (2004) 352–356
detailed reasons need to be further investigated, and relative research is under way.
4. Conclusion The SrAl2O4:Eu,Dy phosphor nanoparticles can be synthesized by combustion synthesis processing, along with heating the resultant combustion ash precursor powder at 1100 jC in a weak reductive atmosphere of active carbon. Analytical results showed that the nanometer phosphors have pure monoclinic SrAl2O4:Eu,Dy phase, with the average particles size in the range of 15 – 45 nm. Compared with the solid-state reaction method, the blue shift of main peaks of excitation and emission of the nanometer phosphors occur.
Acknowledgements Financial support from the State Key Laboratory of Rare Earth Materials Chemistry and Application of Peking University and Hubei Province Science and Technology Commission in China are gratefully acknowledged.
References [1] F.C. Palilla, A.K. Levine, M.R. Tomkus, J. Electrochem. Soc. 115 (1968) 642. [2] G. Blasse, A. Bril, Philips Res. Rep. 23 (1968) 201. [3] S.H.M. Poort, W.P. Blokpoel, G. Blasse, Chem. Mater. 7 (1995) 1547. [4] Y. Lin, Z. Tang, Z. Zhang, C. Nan, J. Eur. Ceram. Soc. 23 (2003) 175. [5] R. Sakai, T. Katsumata, S. Komuro, T. Morikawa, J. Lumin. 85 (1999) 149. [6] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [7] H. Yamamoto, T. Matsuzawa, J. Lumin. 72 – 74 (1997) 287. [8] Y. Lin, Z. Zhang, F. Zhang, Z. Tang, Q. Chen, Mater. Chem. Phys. 65 (2000) 103. [9] W. Jia, H. Yuan, L. Lu, H. Liu, W.M. Yen, J. Lumin. 76 – 77 (1998) 424. [10] E. Nakazawa, T. Mochida, J. Lumin. 72 – 74 (1997) 236. [11] W.Y. Jia, H.B. Yuan, W.M. Yen, J. Lumin. 76 (1998) 424. [12] G. Groppi, C. Cristiani, P. Forzatti, J. Mater. Sci. 29 (1994) 3441. [13] Y. Murayama, N. Takeuchi, Y. Aoki, T. Matsuzawa, Phosphorescent Phosphor, US Patent 5424006 (1995). [14] R.P. Rao, J. Electrochem. Soc. 143 (1996) 189. [15] L.K. Kurihara, S.L. Suib, Chem. Mater. 5 (1993) 609. [16] J.J. Kingsley, K. Suersh, K.C. Patil, J. Mater. Sci. 25 (1990) 1305. [17] Y. Tao, G. Zhao, W. Zhang, S. Xia, Mater. Res. Bull. 52 (1997) 501. [18] B.B. Cullity, Elements of X-ray Diffraction, 2nd ed., Addison-Wesley, Reading, MA, 1997, p. 102.