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Synthesis of SrAl2O4:Eu, Dy phosphor nanometer powders by sol–gel processes and its optical properties
Materials Chemistry and Physics 85 (2004) 68–72
Synthesis of SrAl2 O4 :Eu, Dy phosphor nanometer powders by sol–gel processes and its optical properties Tianyou Peng a,∗ , Liu Huajun a , Huanping Yang a , Chunhua Yan b b
a 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; received in revised form 10 October 2003; accepted 5 December 2003
Abstract Eu2+ , Dy3+ co-doped strontium aluminate (SrAl2 O4 ) phosphor nanometer powders with high brightness and long afterglow were prepared by heating the precursor gel (resulted from sol–gel method) at 900 ◦ C and a reductive atmosphere of active carbon. The average particle size of the SrAl2 O4 :Eu, Dy phosphor powders was 59 ± 7 nm and its optical properties have been studied systematically. The broad band UV excited luminescence of the SrAl2 O4 :Eu, Dy was observed at λmax = 506 nm due to transitions from the 4f6 5d1 to the 4f7 configuration of the Eu2+ ion. The results indicated that the main peaks in the emission and excitation spectrum shifted to the short wavelength compared with phosphor obtained by the solid-state reaction synthesis method. The decay speed of the afterglow for nanometer phosphors was faster than that obtained by the solid-state reaction method. © 2004 Elsevier B.V. All rights reserved. Keywords: Long afterglow phosphor; Sol–gel method; Strontium aluminate; Hole trap; Optical properties
1. Introduction Eu2+ doped alkaline earth aluminates MAl2 O4 :Eu2+ (M: Ca, Sr, Ba) phosphors with strong photoluminescence at the blue-green visible region have been studied extensively by many researchers [1–5]. The persistent luminescence lifetime and the intensity can be enhanced by co-doping with some other RE ions [4–6]. Those phosphors exhibit a rapid initial decay from the Eu2+ ion followed by a long persistence. This effect has been explained based on the thermal activation of holes from traps followed by the emission of Eu2+ [7–10]. However, many aspects of the mechanism for the long afterglow still remain unclear. Compared with sulfide phosphorescent phosphors, SrAl2 O4 :Eu, Dy phosphor possesses better safe, chemically stable, excellent photoresistance, very bright and long-lasting photoluminescence with no radiation [2,11,12], which resulted in an unexpectedly large field of applications, such as luminous paints in highway, airport, buildings and ceramics products [13]. In addition, it can also be applied in textile, the dial plates of glow watch, warning signs, escape routine and some instru∗ Corresponding author. Tel.: +86-2787-2187-44; fax: +86-2787-6476-17. E-mail addresses: [email protected], [email protected] (T. Peng).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.12.001
ments, etc. [7]. The sol–gel process is an efficient technique for the syntheses of phosphors due to the good mixing of starting materials and relatively low reaction temperature resulting in more homogeneous products than those obtained by solid-state reaction synthesis method. The direct solid-state preparation of monoclinic MAl2 O4 is usually carried out at around 1300 ◦ C due to impurities as M3 Al2 O6 are formed at lower temperatures. A lower temperature (900 ◦ C) has been reported for the successful preparation of MAl2 O4 (M: Mg, Co, Ca, Sr and Ba, etc.) powders by sol–gel technique [14]. On the other hand, a metastable orthorhombic form of Eu2+ doped CaAl2 O4 with long afterglow has also been prepared with a sol–gel method by heating the obtained gel at 850 ◦ C [15]. The SrAl2 O4 :Eu, Dy phosphor nanometer powders prepared by sol–gel method are deficient, although ultrafine needle-like SrAl2 O4 :Eu, Dy has been prepared from SrCO3 –Al2 O3 –RE(OH)3 (RE: Eu, Dy) precursor powders obtained by a soft chemical method [8]. These nanometer phosphors are very potential in the constructed materials to detect the damage of bridge or the high buildings, as well as the display and decorative materials for the hotel, the plaza, etc. In the present work, SrAl2 O4 is applied as the host materials with Eu2+ , Dy3+ doped as the activator and co-activator, respectively. The photoluminescent phosphor nanometer powders with high brightness and long afterglow were obtained by heating the
T. Peng et al. / Materials Chemistry and Physics 85 (2004) 68–72
3. Results and discussion About <1 wt.% weight loss was observed in the TG curve of the precursor gels in the temperature range from 25 to 140 ◦ C (Fig. 1), implying that the heterogeneous azeotropic distillation processing effectively removed water in gel, which is necessary for obtaining mono-dispersion nanoparticles [16]. A 62 wt.% weight loss was also observed in the temperature range between 140 and 600 ◦ C due to the vaporization of organic solvent and the decomposition of
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Sr0.97 Al2 O4 :Eu0.01 , Dy0.02 nanometer phosphors were prepared by sol–gel method. In a round-bottom flask, aluminum nitrate (Al(NO3 )3 ), strontium nitrate (Sr(NO3 )2 ), dysprosium nitrate (Dy(NO3 )3 ) and europium nitrate (Eu(NO3 )3 ) was dissolved in distilled water, using PEG 2000 as dispersers, and ammonium bicarbonate (NH4 HCO3 ) solution (0.1 M) containing 1 wt.% PEG 600 was added dropwise under stirring until pH arrive to 5.1. The resultant transparent sol was further refluxed with stirring for 3 h at 80 ◦ C. And then the sol was slowly concentrated until a gel formed. The obtained gel was first heated for 10 h at 100 ◦ C to remove water and then refluxed for 2 h in n-butanol at 114 ◦ C by a heterogeneous azeotropic distillation processing for further removal of water. The resulted precursor gel powder was heated for 16 h at different temperature in an active carbon atmosphere. Sr0.97 Al2 O4 :Eu0.01 , Dy0.02 phosphors were also synthesized at 1300 ◦ C by 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 morphology and particle size of the prepared nanometer phosphors were observed by transmission electron microscopy (TEM). Crystal structures of samples were checked by means of a Rigaku RINT-1400 diffractometer using Cu K␣ λ = 1.5406 Å. Thermoanalysis was carried out with a DT-40 simultaneous DTA–TGA thermoanalyzer. The sol–gel reaction was studied between 25 and 1000 ◦ C. The heating rate was 10 ◦ C min−1 and the gas (N2 –8% H2 ) with flow-rate of 120 cm3 min−1 . The luminescent prosperities of the obtained samples were measured at room temperature using a Perkin-Elmer LS-5 spectrometer. The excitation (λexc = 350 nm) source was a xenon lamp. The decay profiles were also recorded using the same instrument after the samples were sufficient excited for about 20 min. Prior to the afterglow measurements samples were exposed to irradiation from 11 W conventional tricolor fluorescent lamp.
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Temperature / C Fig. 1. TG–DTA curves of precursor gel resulted from sol–gel method (heating rate: 10 ◦ C min−1 ; gas flow rate: 120 cm3 min−1 , N2 –8%H2 ; sample weight: 32.1 mg).
nitrate in precursor gel. As can be seen from the DTA curves, there is an endotherm at 390 ◦ C, this endotherm is presumably the results of the pseudo-boehmit to ␥-Al2 O3 transformation. This transformation can occur between 350 and 450 ◦ C [14]. Exothermic peaks appeared at 490 and 850 ◦ C. The first one can be ascribed to the combustion of organic material while the second one is correlated with the phase transition of the crystal structure of SrAl2 O4 [8]. Fig. 2 indicates the XRD patterns of samples heated at different temperature. As can be seen, the patterns of samples heated at 600 and 700 ◦ C show mainly the ␥-Al2 O3 , which is consistent with the results of the thermal analyses. And when the temperature was enhanced to 800 ◦ C, the ␥-Al2 O3 transformed to a new transition phase, which cannot completely attributed to any known phase of aluminates except showing partly monoclinic SrAl2 O4 phase. And a pure monoclinic SrAl2 O4 phase was formed when heated at 900 ◦ C, no other products or starting materials were observed, which is similar with the earlier data in sol–gel method [14], implying the little amount of doped rare earth ions have almost no effect on the SrAl2 O4 phase composition. Furthermore, the
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precursor gel at 900 ◦ C and at reductive atmosphere of active carbon.
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Fig. 2. X-ray diffraction patterns of the SrAl2 O4 :Eu, Dy phosphors obtained by sol–gel method heated at: (a) 600 ◦ C; (b) 700 ◦ C; (c) 800 ◦ C; (d) 900 ◦ C; (e) solid-state reaction method.
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Wavelength / nm Fig. 4. Excitation spectra of SrAl2 O4 :Eu, Dy phosphors resulted from: (a) sol–gel method and (b) solid-state reaction method.
XRD pattern (Fig. 2d) of the nanometer phosphor also similar with that (Fig. 2e) of phosphor prepared by solid-state synthesis method except the difference in crystallinity [14], indicting that the phosphor can be prepared in a far low temperature (about 900 ◦ C). The mean crystalline size of sample can be deduced according to the Scherrers’ equation using FWHM data, the calculated crystalline sizes of samples obtained at 600, 700, 800 and 900 ◦ C are 16, 25, 29 and 43 nm, respectively. Fig. 3 shows the TEM images of phosphor nanometer powders heated at 900 and 1200 ◦ C. The electron micrograph (Fig. 3a) of the sample prepared at 900 ◦ C has reflected the basic particle morphology, where the smallest particle could be identified with the crystallite and/or their aggregates. The micrograph show a sharp distributions of particles with an average particle size of 59 ± 7 nm, which is closed to the values (43 nm) of crystalline size calculated from the Scherrers’ equation. The morphology is apparently different from the micrometer-scale particle and/or agglomerations of SrAl2 O4 resulted from solid-state reaction method, indicating that heterogeneous azeotropic distillation processing can effectively prevent the formation of larger agglomeration [16]. With the increase of temperature to 1200 ◦ C, the average diameter of SrAl2 O4 crystal is increased to 81 ± 9 nm (Fig. 3b). Compared with the phos-
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Fig. 3. TEM images of SrAl2 O4 :Eu, Dy nanometer phosphors heating at: (a) 900 ◦ C and (b) 1200 ◦ C under weak reductive atmosphere of active carbon.
phors prepared by the solid-state reaction method, the sintered temperature of the nanometer phosphors decreased by about 400 ◦ C due to the high activity of nanometer powder and the uniform components. The excitation and emission spectra at room temperature of SrAl2 O4 :Eu, Dy phosphor prepared by the sol–gel and solid-state reaction methods are shown in Figs. 4 and 5, respectively. It is observed that the main peaks of excitation spectra of nanometer phosphor shift to shorter wavelength (from 256 to 248 and 325 to 316 nm) as the phosphor particle size decreases compared with the phosphor resulted from solid-state reaction method. This may be associated with the quantum size effect of the nanometer phosphors, which increased the kinetic energy of the electrons and resulted in a larger band gap, and thus need higher energy to excite the luminescent powders [8]. The UV-excited SrAl2 O4 :Eu, Dy nanometer phosphor at room temperature yielded high bright green luminescence (λmax = 506 nm) with only one band (Fig. 5). The bandwidth was quite large, ∼64 nm, but symmetric, indicating only one luminescent
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Wavelength / nm Fig. 5. Emission spectra of SrAl2 O4 :Eu, Dy phosphors resulted from: (a) sol–gel method; (b) solid-state reaction method and (c) afterglow spectrum of SrAl2 O4 :Eu, Dy resulted from sol–gel method at room temperature, determined at 2 min after removal of the illumination of conventional tricolor fluorescent lamp.
centre, which is Eu2+ luminescence and excitation result from transitions between the 4f6 5d1 and 4f7 electron configurations. Moreover, compared with the sample obtained by solid-state reaction method, the emission maximum peak of nanometer phosphor shifted to shorter wavelength (from 519 to 506 nm). A similar results have also been observed in ultrafine needle-like SrAl2 O4 :Eu, Dy phosphors reported by Lin et al. [8]. Furthermore, a SrAl2 O4 :Eu, Dy phosphor nanoparticles with particle size of 15–45 nm previously produced by combustion synthesis in our group also exhibited an obvious shift of the emission maximum peak [17]. The slight blue shift in the emission band may be attributed to the changes of the crystal field around Eu2+ arising from the prepared technology. Since the excited 4f6 5d1 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. 5c) had the same position, shape and bandwidth as those of the UV-excited spectra indicating that the emitting centre is still Eu2+ ion. The same afterglow of SrAl2 O4 :Eu, Dy can already be observed with illumination of a conventional tricolor fluorescent lamp. It is well-known that in SrAl2 O4 :Eu2+ , Dy3+ phosphor, Eu2+ ions are the luminescent centers [6], the photo-excited 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. In the present system, it is the hole trapped–transported–detrapped process that resulted in the properties of long afterglow of SrAl2 O4 :Eu2+ , Dy3+ phosphor. Dy3+ played an important role and worked as hole traps, the trap levels lied in between the excited state and the ground state of Eu2+ . This is because the existence of stable tetravalent state of Dy4+ the rare earth ion. Moreover, Dy3+ ion possesses relatively high transfer energies than any other trivalent rare earth ions. Therefore, Dy3+ acts as hole trap. After excited 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+ , and some free holes created in the valence band of the host transported in the conduction band and captured by the Dy3+ hole traps, large amount 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 valence band, and the released holes travel back of Eu2+ , finally, returned 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 retrapping probability and further slow down of the decay process [9].
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T. Peng et al. / Materials Chemistry and Physics 85 (2004) 68–72
b a
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Fig. 6. Decay curve of SrAl2 O4 :Eu, Dy phosphors after exciting for 20 min: (a) sol–gel method; (b) solid-state reaction method (room temperature: 25 ◦ C; excitation time: 20 min; source: conventional tricolor luminescent lamp).
Fig. 6 shows the decay characteristics of SrAl2 O4 :Eu, Dy phosphors, the results indicated that the decay process both of two kinds of phosphors contained the rapid-decaying process and the slow-decaying one. The afterglow of nanometer phosphor, which allowed the time to be visually recognized (≥0.32 mcd m−2 ), lasted over 14 h after the excited source was cut-off. And the initial intensity of nanometer phosphor is lower than that of the phosphor powders prepared by the solid-state reaction method after exciting for 20 min (also refer Fig. 5a and b), moreover, the nanometer phosphor decayed more rapidly than that prepared by the solid-state reaction method. The reason may be that the nanometer particles made the phosphor crystallized easily and completely during heat-treatment process as described above, which maybe lead to the amount of defects in the inner phosphor decreasing 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 lowered and the decay of afterglow is faster. In addition, a lot of defects are dispersed on the surface of phosphors because of the high surface area of the nanometer powders, and which may result in the relative less amount of luminescent center Eu2+ in the SrAl2 O4 lattice available for direct radiation, therefore, it may result in the weaker initial intensity of nanometer phosphors. Furthermore, as described above, fast speed of hole mobility and electron–hole recombination in nanometer particles with good crystallinity will decrease retrapping probability and further prompt the decay process. Of course, the detailed reasons need to be further investigated, and the relative research is under processing. 4. Conclusion The SrAl2 O4 :Eu, Dy phosphor nanometer powders with homogeneous dispersion can be synthesized by sol–gel processing along with a heterogeneous azeotropic distillation processing, followed by heating the resulted precursor gel
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powders at 900 ◦ C and a weak reductive atmosphere of active carbon. Analytical results showed that the nanometer phosphors have pure monoclinic SrAl2 O4 :Eu, Dy phase with the average particles size of about 60 nm. Compared with the solid-state reaction method, the blue shift of main peaks of excitation and emission of the nanometer phosphors occur. Acknowledgements This work was financially supported by State Key Laboratory of Rare Earth Materials Chemistry and Application, Peking University, Beijing, PR China. 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] L.K. Kurihara, S.L. Suib, Chem. Mater. 5 (1993) 609. [15] T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, J. Niittkyoski, J. Alloys Compd. 341 (2002) 76. [16] T.Y. Peng, P.W. Du, B. Hu, Z.C. Jiang, J. Inorg. Mater. 15 (2000) 1087. [17] T.Y. Peng, H.P. Yang, X.L. Pu, B. Hu, Z.C. Jiang, C.H. Yan, Mater. Lett. 58 (2004) 352.
Synthesis of SrAl2 O4 :Eu, Dy phosphor nanometer powders by sol–gel processes and its optical properties Tianyou Peng a,∗ , Liu Huajun a , Huanping Yang a , Chunhua Yan b b
a 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; received in revised form 10 October 2003; accepted 5 December 2003
Abstract Eu2+ , Dy3+ co-doped strontium aluminate (SrAl2 O4 ) phosphor nanometer powders with high brightness and long afterglow were prepared by heating the precursor gel (resulted from sol–gel method) at 900 ◦ C and a reductive atmosphere of active carbon. The average particle size of the SrAl2 O4 :Eu, Dy phosphor powders was 59 ± 7 nm and its optical properties have been studied systematically. The broad band UV excited luminescence of the SrAl2 O4 :Eu, Dy was observed at λmax = 506 nm due to transitions from the 4f6 5d1 to the 4f7 configuration of the Eu2+ ion. The results indicated that the main peaks in the emission and excitation spectrum shifted to the short wavelength compared with phosphor obtained by the solid-state reaction synthesis method. The decay speed of the afterglow for nanometer phosphors was faster than that obtained by the solid-state reaction method. © 2004 Elsevier B.V. All rights reserved. Keywords: Long afterglow phosphor; Sol–gel method; Strontium aluminate; Hole trap; Optical properties
1. Introduction Eu2+ doped alkaline earth aluminates MAl2 O4 :Eu2+ (M: Ca, Sr, Ba) phosphors with strong photoluminescence at the blue-green visible region have been studied extensively by many researchers [1–5]. The persistent luminescence lifetime and the intensity can be enhanced by co-doping with some other RE ions [4–6]. Those phosphors exhibit a rapid initial decay from the Eu2+ ion followed by a long persistence. This effect has been explained based on the thermal activation of holes from traps followed by the emission of Eu2+ [7–10]. However, many aspects of the mechanism for the long afterglow still remain unclear. Compared with sulfide phosphorescent phosphors, SrAl2 O4 :Eu, Dy phosphor possesses better safe, chemically stable, excellent photoresistance, very bright and long-lasting photoluminescence with no radiation [2,11,12], which resulted in an unexpectedly large field of applications, such as luminous paints in highway, airport, buildings and ceramics products [13]. In addition, it can also be applied in textile, the dial plates of glow watch, warning signs, escape routine and some instru∗ Corresponding author. Tel.: +86-2787-2187-44; fax: +86-2787-6476-17. E-mail addresses: [email protected], [email protected] (T. Peng).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.12.001
ments, etc. [7]. The sol–gel process is an efficient technique for the syntheses of phosphors due to the good mixing of starting materials and relatively low reaction temperature resulting in more homogeneous products than those obtained by solid-state reaction synthesis method. The direct solid-state preparation of monoclinic MAl2 O4 is usually carried out at around 1300 ◦ C due to impurities as M3 Al2 O6 are formed at lower temperatures. A lower temperature (900 ◦ C) has been reported for the successful preparation of MAl2 O4 (M: Mg, Co, Ca, Sr and Ba, etc.) powders by sol–gel technique [14]. On the other hand, a metastable orthorhombic form of Eu2+ doped CaAl2 O4 with long afterglow has also been prepared with a sol–gel method by heating the obtained gel at 850 ◦ C [15]. The SrAl2 O4 :Eu, Dy phosphor nanometer powders prepared by sol–gel method are deficient, although ultrafine needle-like SrAl2 O4 :Eu, Dy has been prepared from SrCO3 –Al2 O3 –RE(OH)3 (RE: Eu, Dy) precursor powders obtained by a soft chemical method [8]. These nanometer phosphors are very potential in the constructed materials to detect the damage of bridge or the high buildings, as well as the display and decorative materials for the hotel, the plaza, etc. In the present work, SrAl2 O4 is applied as the host materials with Eu2+ , Dy3+ doped as the activator and co-activator, respectively. The photoluminescent phosphor nanometer powders with high brightness and long afterglow were obtained by heating the
T. Peng et al. / Materials Chemistry and Physics 85 (2004) 68–72
3. Results and discussion About <1 wt.% weight loss was observed in the TG curve of the precursor gels in the temperature range from 25 to 140 ◦ C (Fig. 1), implying that the heterogeneous azeotropic distillation processing effectively removed water in gel, which is necessary for obtaining mono-dispersion nanoparticles [16]. A 62 wt.% weight loss was also observed in the temperature range between 140 and 600 ◦ C due to the vaporization of organic solvent and the decomposition of
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Weight/%
Sr0.97 Al2 O4 :Eu0.01 , Dy0.02 nanometer phosphors were prepared by sol–gel method. In a round-bottom flask, aluminum nitrate (Al(NO3 )3 ), strontium nitrate (Sr(NO3 )2 ), dysprosium nitrate (Dy(NO3 )3 ) and europium nitrate (Eu(NO3 )3 ) was dissolved in distilled water, using PEG 2000 as dispersers, and ammonium bicarbonate (NH4 HCO3 ) solution (0.1 M) containing 1 wt.% PEG 600 was added dropwise under stirring until pH arrive to 5.1. The resultant transparent sol was further refluxed with stirring for 3 h at 80 ◦ C. And then the sol was slowly concentrated until a gel formed. The obtained gel was first heated for 10 h at 100 ◦ C to remove water and then refluxed for 2 h in n-butanol at 114 ◦ C by a heterogeneous azeotropic distillation processing for further removal of water. The resulted precursor gel powder was heated for 16 h at different temperature in an active carbon atmosphere. Sr0.97 Al2 O4 :Eu0.01 , Dy0.02 phosphors were also synthesized at 1300 ◦ C by 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 morphology and particle size of the prepared nanometer phosphors were observed by transmission electron microscopy (TEM). Crystal structures of samples were checked by means of a Rigaku RINT-1400 diffractometer using Cu K␣ λ = 1.5406 Å. Thermoanalysis was carried out with a DT-40 simultaneous DTA–TGA thermoanalyzer. The sol–gel reaction was studied between 25 and 1000 ◦ C. The heating rate was 10 ◦ C min−1 and the gas (N2 –8% H2 ) with flow-rate of 120 cm3 min−1 . The luminescent prosperities of the obtained samples were measured at room temperature using a Perkin-Elmer LS-5 spectrometer. The excitation (λexc = 350 nm) source was a xenon lamp. The decay profiles were also recorded using the same instrument after the samples were sufficient excited for about 20 min. Prior to the afterglow measurements samples were exposed to irradiation from 11 W conventional tricolor fluorescent lamp.
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Temperature / C Fig. 1. TG–DTA curves of precursor gel resulted from sol–gel method (heating rate: 10 ◦ C min−1 ; gas flow rate: 120 cm3 min−1 , N2 –8%H2 ; sample weight: 32.1 mg).
nitrate in precursor gel. As can be seen from the DTA curves, there is an endotherm at 390 ◦ C, this endotherm is presumably the results of the pseudo-boehmit to ␥-Al2 O3 transformation. This transformation can occur between 350 and 450 ◦ C [14]. Exothermic peaks appeared at 490 and 850 ◦ C. The first one can be ascribed to the combustion of organic material while the second one is correlated with the phase transition of the crystal structure of SrAl2 O4 [8]. Fig. 2 indicates the XRD patterns of samples heated at different temperature. As can be seen, the patterns of samples heated at 600 and 700 ◦ C show mainly the ␥-Al2 O3 , which is consistent with the results of the thermal analyses. And when the temperature was enhanced to 800 ◦ C, the ␥-Al2 O3 transformed to a new transition phase, which cannot completely attributed to any known phase of aluminates except showing partly monoclinic SrAl2 O4 phase. And a pure monoclinic SrAl2 O4 phase was formed when heated at 900 ◦ C, no other products or starting materials were observed, which is similar with the earlier data in sol–gel method [14], implying the little amount of doped rare earth ions have almost no effect on the SrAl2 O4 phase composition. Furthermore, the
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precursor gel at 900 ◦ C and at reductive atmosphere of active carbon.
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Fig. 2. X-ray diffraction patterns of the SrAl2 O4 :Eu, Dy phosphors obtained by sol–gel method heated at: (a) 600 ◦ C; (b) 700 ◦ C; (c) 800 ◦ C; (d) 900 ◦ C; (e) solid-state reaction method.
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Wavelength / nm Fig. 4. Excitation spectra of SrAl2 O4 :Eu, Dy phosphors resulted from: (a) sol–gel method and (b) solid-state reaction method.
XRD pattern (Fig. 2d) of the nanometer phosphor also similar with that (Fig. 2e) of phosphor prepared by solid-state synthesis method except the difference in crystallinity [14], indicting that the phosphor can be prepared in a far low temperature (about 900 ◦ C). The mean crystalline size of sample can be deduced according to the Scherrers’ equation using FWHM data, the calculated crystalline sizes of samples obtained at 600, 700, 800 and 900 ◦ C are 16, 25, 29 and 43 nm, respectively. Fig. 3 shows the TEM images of phosphor nanometer powders heated at 900 and 1200 ◦ C. The electron micrograph (Fig. 3a) of the sample prepared at 900 ◦ C has reflected the basic particle morphology, where the smallest particle could be identified with the crystallite and/or their aggregates. The micrograph show a sharp distributions of particles with an average particle size of 59 ± 7 nm, which is closed to the values (43 nm) of crystalline size calculated from the Scherrers’ equation. The morphology is apparently different from the micrometer-scale particle and/or agglomerations of SrAl2 O4 resulted from solid-state reaction method, indicating that heterogeneous azeotropic distillation processing can effectively prevent the formation of larger agglomeration [16]. With the increase of temperature to 1200 ◦ C, the average diameter of SrAl2 O4 crystal is increased to 81 ± 9 nm (Fig. 3b). Compared with the phos-
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Fig. 3. TEM images of SrAl2 O4 :Eu, Dy nanometer phosphors heating at: (a) 900 ◦ C and (b) 1200 ◦ C under weak reductive atmosphere of active carbon.
phors prepared by the solid-state reaction method, the sintered temperature of the nanometer phosphors decreased by about 400 ◦ C due to the high activity of nanometer powder and the uniform components. The excitation and emission spectra at room temperature of SrAl2 O4 :Eu, Dy phosphor prepared by the sol–gel and solid-state reaction methods are shown in Figs. 4 and 5, respectively. It is observed that the main peaks of excitation spectra of nanometer phosphor shift to shorter wavelength (from 256 to 248 and 325 to 316 nm) as the phosphor particle size decreases compared with the phosphor resulted from solid-state reaction method. This may be associated with the quantum size effect of the nanometer phosphors, which increased the kinetic energy of the electrons and resulted in a larger band gap, and thus need higher energy to excite the luminescent powders [8]. The UV-excited SrAl2 O4 :Eu, Dy nanometer phosphor at room temperature yielded high bright green luminescence (λmax = 506 nm) with only one band (Fig. 5). The bandwidth was quite large, ∼64 nm, but symmetric, indicating only one luminescent
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Wavelength / nm Fig. 5. Emission spectra of SrAl2 O4 :Eu, Dy phosphors resulted from: (a) sol–gel method; (b) solid-state reaction method and (c) afterglow spectrum of SrAl2 O4 :Eu, Dy resulted from sol–gel method at room temperature, determined at 2 min after removal of the illumination of conventional tricolor fluorescent lamp.
centre, which is Eu2+ luminescence and excitation result from transitions between the 4f6 5d1 and 4f7 electron configurations. Moreover, compared with the sample obtained by solid-state reaction method, the emission maximum peak of nanometer phosphor shifted to shorter wavelength (from 519 to 506 nm). A similar results have also been observed in ultrafine needle-like SrAl2 O4 :Eu, Dy phosphors reported by Lin et al. [8]. Furthermore, a SrAl2 O4 :Eu, Dy phosphor nanoparticles with particle size of 15–45 nm previously produced by combustion synthesis in our group also exhibited an obvious shift of the emission maximum peak [17]. The slight blue shift in the emission band may be attributed to the changes of the crystal field around Eu2+ arising from the prepared technology. Since the excited 4f6 5d1 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. 5c) had the same position, shape and bandwidth as those of the UV-excited spectra indicating that the emitting centre is still Eu2+ ion. The same afterglow of SrAl2 O4 :Eu, Dy can already be observed with illumination of a conventional tricolor fluorescent lamp. It is well-known that in SrAl2 O4 :Eu2+ , Dy3+ phosphor, Eu2+ ions are the luminescent centers [6], the photo-excited 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. In the present system, it is the hole trapped–transported–detrapped process that resulted in the properties of long afterglow of SrAl2 O4 :Eu2+ , Dy3+ phosphor. Dy3+ played an important role and worked as hole traps, the trap levels lied in between the excited state and the ground state of Eu2+ . This is because the existence of stable tetravalent state of Dy4+ the rare earth ion. Moreover, Dy3+ ion possesses relatively high transfer energies than any other trivalent rare earth ions. Therefore, Dy3+ acts as hole trap. After excited 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+ , and some free holes created in the valence band of the host transported in the conduction band and captured by the Dy3+ hole traps, large amount 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 valence band, and the released holes travel back of Eu2+ , finally, returned 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 retrapping probability and further slow down of the decay process [9].
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b a
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Fig. 6. Decay curve of SrAl2 O4 :Eu, Dy phosphors after exciting for 20 min: (a) sol–gel method; (b) solid-state reaction method (room temperature: 25 ◦ C; excitation time: 20 min; source: conventional tricolor luminescent lamp).
Fig. 6 shows the decay characteristics of SrAl2 O4 :Eu, Dy phosphors, the results indicated that the decay process both of two kinds of phosphors contained the rapid-decaying process and the slow-decaying one. The afterglow of nanometer phosphor, which allowed the time to be visually recognized (≥0.32 mcd m−2 ), lasted over 14 h after the excited source was cut-off. And the initial intensity of nanometer phosphor is lower than that of the phosphor powders prepared by the solid-state reaction method after exciting for 20 min (also refer Fig. 5a and b), moreover, the nanometer phosphor decayed more rapidly than that prepared by the solid-state reaction method. The reason may be that the nanometer particles made the phosphor crystallized easily and completely during heat-treatment process as described above, which maybe lead to the amount of defects in the inner phosphor decreasing 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 lowered and the decay of afterglow is faster. In addition, a lot of defects are dispersed on the surface of phosphors because of the high surface area of the nanometer powders, and which may result in the relative less amount of luminescent center Eu2+ in the SrAl2 O4 lattice available for direct radiation, therefore, it may result in the weaker initial intensity of nanometer phosphors. Furthermore, as described above, fast speed of hole mobility and electron–hole recombination in nanometer particles with good crystallinity will decrease retrapping probability and further prompt the decay process. Of course, the detailed reasons need to be further investigated, and the relative research is under processing. 4. Conclusion The SrAl2 O4 :Eu, Dy phosphor nanometer powders with homogeneous dispersion can be synthesized by sol–gel processing along with a heterogeneous azeotropic distillation processing, followed by heating the resulted precursor gel
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powders at 900 ◦ C and a weak reductive atmosphere of active carbon. Analytical results showed that the nanometer phosphors have pure monoclinic SrAl2 O4 :Eu, Dy phase with the average particles size of about 60 nm. Compared with the solid-state reaction method, the blue shift of main peaks of excitation and emission of the nanometer phosphors occur. Acknowledgements This work was financially supported by State Key Laboratory of Rare Earth Materials Chemistry and Application, Peking University, Beijing, PR China. 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] L.K. Kurihara, S.L. Suib, Chem. Mater. 5 (1993) 609. [15] T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, J. Niittkyoski, J. Alloys Compd. 341 (2002) 76. [16] T.Y. Peng, P.W. Du, B. Hu, Z.C. Jiang, J. Inorg. Mater. 15 (2000) 1087. [17] T.Y. Peng, H.P. Yang, X.L. Pu, B. Hu, Z.C. Jiang, C.H. Yan, Mater. Lett. 58 (2004) 352.