Received Oct. 16, 2006.

Abstract Long afterglow photoluminescent materials Sr_2-xCa_xMgSi₂O₇ doped with Eu²⁺,Dy³⁺ were prepared by gel-combustion method．The synthesized samples were characterized by X-ray diffraction and SEM. The excitation spectrum, emission spectrum and long decay curve were measured and analyzed. XRD patterns indicate that the phosphors have tetragonal crystal structure. SEM photographs show that the initial particles are very small with diameter below 100nm. Both the emission and excitation spectra are broad-band continuous spectrums. The main excitation peak is at about 400nm. With the increasing of Ca(x=0,0.5,1,1.5,2) doped in Sr_2-xCa_xMgSi₂O₇, the main emission peak shifts from 468nm to 483nm, 500nm, 512nm, 520nm respectively, the photoluminescent color ranges from blue to blue-green, green, yellow-green, yellow. The decay curves indicate that all of these phosphors have long afterglow features. Moreover，with the increasing of Ca doped in the host, the long afterglow brightness and duration time decreases. It results from the slight change of microstructure.
Keywords Sr_2-xCa_xMgSi₂O₇:Eu²⁺,Dy³⁺; gel-combustion; long afterglow; luminescence

1. INTRODUCTION
Long-lasting phosphors are materials with long decay time, ranging from a few minutes to tens of hours. These materials have received increasing interest in recent years due to their extensive applications in many fields such as road signs, billboards, graphic arts, interior decoration, etc^[1,2]. Long lasting aluminate-based phosphors attracted more research because of their excellent properties, such as no radiation, high brightness and long afterglow. But the properties of these phosphors may be decreased greatly while soaked in the water after several hours, thus limited their application^[3,4]. Alkaline earth silicates are useful luminescent hosts with high physical and chemical stability. Poort et al. prepared Sr₂MgSi₂O₇:Eu and Ca₂MgSi₂O₇:Eu phosphors by firing at 800^oC in a reducing atmosphere, and found that the main emission peaks located at 470 and 535 nm respectively^[5]. But the long afterglow was not observed in these phosphors. Recently, Lin^[6] and Sabbagh Alvani^[7] et al. prepared Eu²⁺, Dy³⁺ co-doped Sr₂MgSi₂O₇ phosphors through solid state reaction method, and found these phosphors would emit a blue-green light peaking at 476 nm upon UV illumination and show long afterglow. The blue-green emission is believed to be caused by the 4f 5d transitions. The 4f electrons of Eu²⁺ are not sensitive to the changes of the crystals field strength due to the shielding function of outer shell, the 5d electrons can split easily by these changes. Thus, the emission of 5d-4f from Eu²⁺ is strongly dependent on the host lattice.
    In our present work, a series of alkaline earth silicates-based phosphors Sr_2-xCa_xMgSi₂O₇: Eu²⁺, Dy³⁺ with long afterglow were prepared by gel-combustion method to obtain homogeneous products at relatively low reaction temperature. The effect of doped Ca on the crystal structure, photoluminescence and afterglow characteristics were examined and studied in detail.

2. EXPERIMENTAL
2.1. Experimental process
The long afterglow phosphors were synthesized by gel-combustion method. SrNO₃ (A.R), Mg(NO₃)₂·6H₂O (A.R), Ca(NO₃)₂.4H₂O (A.R), tetraethoxysilane(TEOS, A.R), anhydrous ethanol, Eu₂O₃(99.9%) and Dy₂O₃(99.9%) were employed as raw materials. Small quantities of H₃BO₃ (about 15mol% per mole of product) were added as flux. Urea (AR) was used as fuel.
    The procedure used to prepare Sr_2-xCa_xMgSi₂O₇:Eu²⁺_0.02,Dy³⁺_0.04 is as follows. Firstly, Eu₂O₃ and Dy₂O₃ were dissolved in appropriate concentration of nitric acid to form nitrate solution, then the accurate concentration was determined by EDTA complexing titration to ensure a desired stoichiometry. According to the nominal composition of Sr_2-xCa_xMgSi₂O₇:Eu²⁺_0.02,Dy³⁺_0.04 (x=0,0.5,1,1.5,2), stoichiometric TEOS, SrNO₃, Ca(NO₃)₂.4H₂O, Mg(NO₃)₂·6H₂O, Eu(NO₃)₃ and Dy(NO₃)₃ solutions were mixed in a 100ml crucible. Appropriate amounts of anhydrous ethanol, H₃BO₃, urea(1.5 times excess amounts of metal nitrates) and distilled water were added into it. The mixture was stirred to obtain a homogenous and transparent solution. Subsequently, a small amount of HNO₃(1mol.L^-1) was added as a catalyst and pH value of the solution was adjusted to 2-3. Next, the resulting solution was heated by water bath at 70-75^oC for several hours to evaporate superfluous water, the volume of the solution decreased and the viscosity increased continuously because of gradual polymerization. When the volume of the solution could not decrease, the solution became a transparent gel. Then the gel was put into oven and dried at 85^oC for 24h. The dry gel was ignited at 800^oC in a muffle furnace under air atmosphere. This combustion process only required 1 min. The dry white sponge sample (which was called precursor) was formed. Due to the evolved gases, the apparent volume expanded after combustion and the precursor became so loose that it could be broken into fine pieces easily. Finally, the precursor was calcined in muffle furnace under an active carbon atmosphere at 1000^oC for 75min to obtain the sample.
2.2 Characterization
Phases and crystallization of the samples were identified by X-ray diffraction analysis (XRD) with a Y2000 diffractometer using Cu-Ka radiation. The size and morphology of the samples were investigated with a KYKY 2800B scanning electron microscope (SEM). The excitation and emission spectrums of the samples were recorded as pellets on a RF-540 fluorescence spectrometer. The emission spectrum was measured at the excitation wavelength of 400nm. The decay curve of afterglow was measured by the ST-900PM brightness meter, and the samples were irradiated by the standard lamp for 15 min. All of the measurements were carried out at room temperature.

Fig. 1 XRD patterns of Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04

3. RESULTS AND DISCUSSION
3.1 XRD analysis of the obtained phosphors         
XRD patterns of the samples are shown in Fig.1. When x=0.0, nearly all of the peaks can be indexed to the Sr₂MgSi₂O₇, which are in excellent accord with powder data in JCPDS card( No.75-1736). According to that, the sample is tetragonal crystalline Sr₂MgSi₂O₇. With the increasing of Ca (x=0,0.5,1,1.5,2), the peaks move slightly to the direction of big angle. The reason is that the radius of Ca²⁺ (r_Ca) is a little smaller than Sr²⁺(r_Sr), with the increase of concentration of Ca²⁺ in Sr_2-xCa_xMgSi₂O₇ (x=0,0.5,1,1.5,2)，the cell parameters of Sr_2-xCa_xMgSi₂O₇ decrease, and interplanar distance decreases too. According to the Bragg equation, dsinq=l, where d is interplanar distance, q is Bragg angle, and l is radiation wavelength (0.154178nm for Cu Ka), Bragg angle increases with the decrease of d. When x=2.0, nearly all of the peaks are consistent with powder data in JCPDS card(No.35-592). According to that, the sample is tetragonal crystalline Ca₂MgSi₂O₇. Therefore, it can be seen that the 2q at each peak in the XRD patterns for Sr_2-xCa_xMgSi₂O₇ (x=0,0.5,1,1.5,2) samples are between that of Sr₂MgSi₂O₇ and Ca₂MgSi₂O₇, and the tetragonal structure of the host can be kept although slight change occurs in cell parameters.
    In these patterns, the peaks of compounds of Eu and Dy can not be found, which proves that Eu and Dy has entered the Sr_2-xCa_xMgSi₂O₇ crystal lattice, and have little effect on the crystal structure of the host.
3.2 SEM analysis of the obtained phosphors

Fig.2 SEM photograghs of Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 phosphors
(a) x=0 ; (b) x=2

The morphologies of the Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 (x=0，2) phosphors were observed by SEM and shown in Fig.2. It can be seen that the initial particles are very small with the diameter below 100nm, and nearly spherical in shape. It is because that the evolved gases ( N₂, NH₃, CO₂ ) can inhibit the particles agglomerating effectively. Meanwhile, some big particles with irregular lamellar structure can be observed, which may result from large surface energy of the initial small particles during the post calcination process.
    Crystallite size of powders was calculated from the peak broadening of X-ray diffraction peak using the Scherrer formula:
D = 0.89 l/bcosq
  Where D is the crystallite size in nm, the radiation wavelength l is 0.154178nm for Cu Ka, b is the corrected halfwidth, and q is the diffraction peak angle. The calculated crystallite size is about 60nm and 50nm respectively for Sr₂MgSi₂O₇ and Ca₂MgSi₂O₇. So, the crystallite size of as-synthesized powders is below 100nm.

3.3 Excitation and emission spectrum of the obtained phosphors
The excitation spectrum (monitored at 470nm) and the emission spectrum (excited by 400nm) of as-synthesized phosphors are shown in Fig 3 and Fig.4. It can be seen that both the emission and excitation spectra are broad-band continuous spectrum. The main excitation peak is at about 400nm. From Fig.4, it can be seen that with the increasing of Ca(x=0,0.5,1,1.5,2) doped in Sr_2-xCa_xMgSi₂O₇:Eu²⁺_0.02,Dy³⁺_0.04, the main emission peak of the phosphors moves to the longer wave-length. When x=0,0.5,1,1.5,2, the main peak value is located at 468nm, 483nm, 500nm, 512nm, 520nm, respectively. These emission peaks results from the 4f⁶5d→4f⁷(⁸S_7/2) transition of Eu²⁺ instead of the 4f⁷(⁶P_7/2)→4f⁷(⁸S_7/2) transition of Eu²⁺. Because 5d electron is an external naked state without being shielded and the division of its energy level is subject to strong influence of crystal field. So the 4f⁶5d→4f⁷(⁸S_7/2) transition of Eu²⁺ ion exhibits an emission peak with an obvious broad band. In addition, as previously reported ^[8-9], different host structures and crystallographic distortions will influence the crystal field environment of rare earth ions in the host structure, so the emission peaks will vary greatly. In the akermanite structure (Sr_2-xCa_xMgSi₂O₇), the alkaline earth ions form chains along the c axis. With the increasing of Ca(x=0,0.5,1,1.5,2), the length of the c axis decreases (r_Ca < r_Sr ), the orientation of an orbital will energetically become more favorable in this sequence, which implies that the emission is expected to shift to lower energy(longer wavelength)^[5]. This is clearly observed in the Sr_2-xCa_xMgSi₂O₇:Eu²⁺_0.02,Dy³⁺_0.04 phosphors, for Sr₂MgSi₂O₇:Eu,Dy(here x=0) phosphor, which has the longest c-axis, the shortest wavelength emission can be observed. For Ca₂MgSi₂O₇:Eu,Dy( here x=2) phosphor, which has the shortest c-axis, the longest wavelength emission can be observed. That is to say, with the increasing of Ca(x=0,0.5,1,1.5,2) in Sr_2-xCa_xMgSi₂O₇:Eu²⁺_0.02,Dy³⁺_0.04, the Eu²⁺ emission shifts to a long wavelength gradually. From these results, it follows that the influence of the orientation of one d orbital on the emission maximum is large. In addition, it can be seen from Fig.3 and Fig.4, the intensity of the excitation and emission spectrums decrease with the increase of Ca doped in Sr_2-xCa_xMgSi₂O₇. A further investigation is needed to explain this phenomenon.
    Under UV radiation at 254nm, no red-emitting of Eu³⁺ can be observed, which indicates that it has been reduced to Eu²⁺ completely. The special Dy³⁺ emission peak can not be observed in Fig.4, which shows that Dy³⁺ does not act as the luminescent centers in the Sr_2-xCa_xMgSi₂O₇ host crystal lattice, may be served as the traps of electrons or holes, or transfer the light energy to Eu²⁺.

Fig.5 Decay curves of Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 (x=0,0.5,1,1.5,2)

3.4 Long afterglow characteristic of the obtained phosphors
The luminescent decay curves of Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 (x=0,0.5,1,1.5,2) are shown in Fig.5. The results show that the decay trend of the samples consists of three parts. Initial rapid decay, middle transient decay and followed by very long slow decay. The initial rapid decay is due to short survival time of electron in Eu²⁺, the middle transient decay is due to the captures of Eu²⁺ ion by shallow trap energy level center, and the long slow afterglow decay is caused by the electrons captured in deep trap energy level resulted from Dy³⁺. It is obvious that the materials have different long persistent properties. Both the initial intensity after excitation and the afterglow intensity of the Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 (x=0) phosphors were higher than the other phosphors, it can be seen that with the increase of concentration of Ca²⁺(x=0,0.5,1,1.5,2), the decay time and intensity are decreasing, Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 (x=2) is the weakest one, which indicating that the difference in host material probably has great effect on the afterglow property of the obtained phosphors.
    Based on the work of Jia et al^[10], the possible configuration coordinate model for Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 long-lasting phosphorescence is given in Fig.6.

Fig.6 The configuration coordinate model for the Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 phosphor.

    The possible long-lasting phosphorescence mechanism of Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 phosphor is as the following: initially, visible light or UV light exposure excites the electrons transition from the ground state (A) of the Eu²⁺ to an excited state (B)(transition 1) and then one part of the excited electrons transit back to lower energy levels of Eu²⁺(transition 2) and the luminescence is observed. Meanwhile, some excited electrons can be stored in the traps energy level(C) through the relaxation process(transition 3). The long-lasting phosphorescence occurs when the electrons located in the trap energy level absorb energy and are re-excited back to the excited state(B) and then cascade to the ground state. The lasting time of phosphorescence depends on both the electrons number located in the trap energy level(C) and the energy that the electrons absorbed should be larger than the trap energy (E_T) to ensure the electrons to be re-excited back to the excited state(B). In this model, the trap may be created by the Dy³⁺ ions embedded in the Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04 matrix, or created by the oxygen vacancy formed in the reductive atmosphere. It has been known that the trap levels located at a suitable depth dominating the thermal release rate at room temperature have a pronounced effect on the afterglow^[11]. As for these Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04(x=0,0.5,1,1.5,2) phosphors, the duration time of Sr₂MgSi₂O₇:Eu, Dy is much greater than that of the others. The main reasons are ascribed to the more suitable depth of Dy³⁺ traps and the good nature of the Eu²⁺ surroundings in the Sr₂MgSi₂O₇ host.

4. CONCLUSIONS
Long afterglow luminescence materials Sr_2-xCa_xMgSi₂O₇:Eu²⁺_0.02,Dy³⁺_0.04 (x=0,0.5,1,1.5,2) was synthesized by gel-combustion method. This method requires relative shorter reaction time and lower calcination temperature, it is safe, instantaneous and energy saving. Both of the excitation spectrum and emission spectrum of long afterglow luminescent materials Sr_2-xCa_xMgSi₂O₇:Eu²⁺_0.02,Dy³⁺_0.04(x=0,0.5,1,1.5,2) are broad bands. The main excitation peak is at about 400nm. The main emission peak shifts from 468nm to 483nm, 500nm, 512nm, 520nm with the increasing of Ca(x=0,0.5,1,1.5,2) doped in host. As for these Sr_2-xCa_xMgSi₂O₇: Eu²⁺_0.02,Dy³⁺_0.04(x=0,0.5,1,1.5,2) phosphors, the dacay time and intensity decrease with the increase of concentration of Ca²⁺(x=0,0.5,1,1.5,2). Among them, the duration time of Sr₂MgSi₂O₇:Eu,Dy is much greater than the others. The main reasons are ascribed to the more suitable depth of Dy³⁺ traps and the good nature of the Eu²⁺ surroundings in the Sr₂MgSi₂O₇ host.

Acknowledgements  This work is supported by the Doctoral Foundation of Education Agency of Hebei province (No.B2004205) and Hebei University Research Foundation(y2004024).

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