Er³⁺ co-doped Sr_1.956MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺ phosphors and enhancement of luminescent properties

Abstract

Long afterglow phosphors of Sr_1.956MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺ and Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ were successfully synthesized via the sol–gel method. The phase compositions and luminescent properties of the phosphors were analyzed by X-ray diffraction (XRD), fluorescence spectra and decay curves. Compared with the undoped phosphor, the crystallinity of Er³⁺ co-doping phosphors is decreased. Luminescence spectra show the main peak of excitation was 274, 356 nm and the main emission peak was 467 nm. Furthermore, Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ show excellent luminescent properties, and its initial luminous intensity increased by 1.4 times than Sr_1.956MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺. The mechanism of Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ enhancement has been discussed.

1 Introduction

For more than a decade now, Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ has become a concern for long afterglow phosphors, because of its advantages of luminescent properties [1,2,3,4,5,6,7,8]. In 2005, Alvani et al. [9] successfully prepared Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ via high temperature solid phase method, and discussed the performance and structure of the phosphor in detail. Subsequently, Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ phosphors have been research to preparation methods and doping rare earth elements by many researchers [10,11,12,13,14]. There are two important research aspects: doping rare earth elements and preparation methods.

An important factor of the luminescent to long afterglow phosphors is the doping type and doping concentration of rare earth elements. The luminescence properties of the Sr₂MgSi₂O₇ phosphors can be created by doping rare earth ions. The doping type and concentration of rare earth elements are both important factors to phosphors of luminescent properties. Sahu et al. [15] successfully prepared Eu²⁺ and Ce²⁺ co-doping Sr₂MgSi₂O₇ phosphors via high temperature solid-state reaction. They focus on the effect of the Eu²⁺ and Ce³⁺ ion radius to decay curve. Wu et al. [16] synthesized the Eu²⁺ and Er³⁺ co-doping Sr₂MgSi₂O₇ phosphors with high temperature solid-state reaction. The results show that the afterglow time in Sr₂MgSi₂O₇: Eu²⁺, Er³⁺ is than Sr₂MgSi₂O₇: Eu²⁺, Er³⁺ is 1.2 times stronger than that in Sr₂MgSi₂O₇: Eu²⁺. In addition to the two rare earth ions co-doping Sr₂MgSi₂O₇ phosphors, Song et al. [17] also attempted that three rare earth ions of Eu²⁺, Dy³⁺ and Nd³⁺ co-doping Sr₂MgSi₂O₇ phosphors.

Although the advantages of high temperature solid-state reaction possesses are less synthesis steps and simple operation, it restricts development to high sintering temperature and unstable luminescent properties. Furusho et al. [18] researched the effect of different sintering temperatures on the luminescent properties. The result indicated the high temperature will influence the structure and phase purity of the Sr₂MgSi₂O₇ phosphors. Then, the researchers tried to reduce the synthetic temperature for improving the crystal structure and luminescent properties of the Sr₂MgSi₂O₇ phosphors by different preparation methods. Pan et al. [19] successfully synthesized Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ phosphors via co-precipitation method. The solid phase reaction can be completed at 1000 °C, indicating the particle size was uniform and the average particle size was about 1 µm. The Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ phosphors [17] was synthesized via combustion method with average particle size of about 20 nm. Zhang et al. [20] successfully prepared Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ phosphors via sol–gel method. This method has obvious advantages for preparing phosphors, such as a lower synthetic temperature and a fine powder.

In this paper, Sr_1.956MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺ and Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ phosphors were successfully prepared via sol–gel method, and the luminescent properties were improved. Er³⁺ ions are doped into Sr_1.956MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺ lattice improve the persistent luminescence due to the formation of exceedingly dense trapping levels situated at appropriate depth. Furthermore, the effects of and Er³⁺ on the phase compositions and luminescent properties of Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ phosphor were also discussed. The mechanism of Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ enhancement has been discussed.

2 Materials and methods

2.1 Preparation of the Sr_1.956MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺ (SMSED) and Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ (SMSED) phosphors

The SMSED and SMSEDE samples were prepared via sol–gel method. The rawmaterials Sr(NO₃)₂, Mg(NO₃)₂·6H₂O and H₂BO₃ were dissolved in H₂O to form solution A ,B and C, and the H₃BO₃ content was 7.5% of Sr₂MgSi₂O₇. The Dy₂O₃, Eu₂O₃ and Er₂O₃ was dissolved in 3.6M HNO₃ to form solution D,E and F. The Si(OC₂H₅)₄ was dissolved in absolute ethylalcohol to form solution G. All solutions were fully dissolved under mechanical stirring, respectively. The solutions of B, C, D, E, F were added to solution A under mechanical stirring, affording a mixture solutions. Then, the solution G was slowly added to the mixture and stirred at 70 °C for 2 h. The mixed solution became wet gel after 2 h and the resulting wet gel was dried in an oven at 70 °C for 72 h to obtain a dry gel. Finally, the dry gel was calcined in a muffle furnace at 1100 °C for 3 h. SMSED was prepared when F was not added.

2.2 Characterization

The X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer (Rigaku D/Max-2500) with Cu Ka radiation (λ = 0.15406 nm), and diffraction angles ranging from 10° to 80°. The luminescence spectra and decay curves were obtained via a Hitachi F-7000 fluorescence spectrophotometer (Japan) equipped with a 450W Xe lamp as the excitation light-source.

3 Results and discussion

Figure 1 shows the X-ray diffraction (XRD) patterns recorded for SMSED and SESEDE phosphors. It can be seen that all the phosphors are indexed to tetragonal Sr₂MgSi₂O₇ phase with the space group P-421m (No.113) according to JCPDF card no.75-1736. The three peaks (201), (211) and (212) are no offset. Er³⁺ co-doping has no significant influence on the structure of the phosphors. However, phosphors of Er³⁺ co-doping reduce the intensity of the peak resulting in a decrease in crystallinity. Eu²⁺,Dy³⁺ and Er³⁺ ions are regarded to occupy the Sr²⁺ sites in the cell because of the similar covalent radius (R_Sr = 1.95 Å, R_Eu = 1.98 Å, R_Dy = 1.92 Å, R_Er = 1.89 Å) [21, 22].

Fig. 1

XRD patterns of SMSED and SESEDE

Figure 2 shows the three-dimensional fluorescence spectra of SMSEDE phosphor. The center of strong emission region appeared at 390 and 467 nm, but the emission region at 390 nm is obviously weaker than 467 nm. This indicates that the best emission wavelength for the Eu²⁺ in the host crystal was 467 nm. These two strong emission peaks attributed to the 4f⁶5d → 4f⁷ transitions of Eu²⁺. Sr²⁺ at two different positions in the host lattice can be replaced by Eu²⁺ to form two emission centers Eu₁ and Eu₂ corresponding to emission at 390 and 467 nm [23]. This because that the R_Sr = 1.95 Å is close to R_Eu = 1.98 Å. The main excitation peaks are 274 and 356 nm, but due to the impact of instrument peak and peak intensity, the experiment will choose the monitoring wavelengths are 356 and 467 nm for emission spectra and excitation spectra, respectively.

Fig. 2

Three-dimensional fluorescence spectra of the SMSEDE

Figures 3 and 4 shows that the excitation spectra and emission spectra of the phosphors. This indicates that emission spectra and excitation spectra show that the fluorescence intensity is significantly increased after Er³⁺ co-doping. As shown in Fig. 3, under the monitoring wavelength of 467 nm, both of the two phosphors showed two different excitation ranges of 250–330 and 335–450 nm, and the main excitation peak were 274 and 356 nm, indicating that Er³⁺ had almost no influence to the excitation peak. The reason why the excitation spectra of SMSEDE is enhanced is that Er³⁺ as a co-doping ion that is Eu²⁺ ions makes the distribution of the Eu²⁺ ions becomes random and deep throughout the lattice. This result in enhancement of the two excitation peaks at 274 and 356 nm due to the increase of electrons excited by the two characteristic excitation wavelengths. The excitation peak at 274 nm can be attributed to the charge transition of Eu²⁺–O²⁻; the excitation peak at 365 nm can be attributed to the typical 4f–5d emission of Eu²⁺ in Sr₂MgSi₂O₇ [24]. As shown in Fig. 4 under the 356 nm excitation wavelength, emission spectra show that the fluorescence intensity is significantly increased after Er³⁺ co-doping. The reason why lattice defects are increased after the introduction of Er³⁺ and it acts as an trap to capture and release for electrons or holes [25]. The process affects the recombination of electron–hole pairs to make the intensities of the fluorescence spectra stronger. The emission of 467 nm was assigned to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺. The emission peak presented broad-featured, which could be attributed to the stronger crystal field strength of the host material. The characteristic emission peaks of Dy³⁺ and Er³⁺ are not detected, indicating the Eu²⁺ acted an activator, but Dy³⁺ and Er³⁺ were as co-activators in Sr₂MgSi₂O₇ phosphors. The maximum brightness is achieved when Er³⁺ and Dy³⁺ auxiliary luminescent particle are incorporated into the host lattice. The Commission Internationale de l’Eclairage (CIE) chromaticity diagrams [26] of the prepared phosphors are presented in Fig. 5. For all the prepared samples, the CIE chromaticity diagrams fall in the bright blue region.

Fig. 3

Excitation spectra of the SMSED and SMSEDE

Fig. 4

Emission spectra of the SMSED and SMSEDE

Fig. 5

Chromaticity diagram of SMSED and SESEDE

Figure 6 is shows the decay curve of SMSED and SMSEDE phosphors. Under the 467 nm excitation wavelength and the 356 nm emcitation wavelength, The afterglow decay process usually consists of three sub-decay processes, which can be described by a three-exponential function. The form of the equation is as follow [27]:

$${\text{I}}={{\text{I}}_0}+{{\text{I}}_1}\exp \left( { - {\text{t}}/{\tau _1}} \right)+{{\text{I}}_2}\exp \left( { - {\text{t}}/{\tau _2}} \right)+{{\text{I}}_3}\exp \left( { - {\text{t}}/{\tau _3}} \right)$$

(1)

where I is phosphorescent intensity, I₀, I₁, I₂, I₃ are the constants, t is the time, τ₁, τ₂ and τ₃ are time decay constants, respectively. The fitting results of the parameters of I₀, I₁, I₂, I_3, τ₁, τ₂ and τ₃ are listed in Table 1.

Fig. 6

Decay curves of SMSED and SMSEDE. (Color figure online)

Table 1 Constants of decay curves

The intensity of initial fluorescence (I₀) of SMSEDE is 1.4 times stronger than that in SMSED, because Er³⁺ co-doping could deepen the trap level allowing more free electrons to be trapped. As shown in Fig. 7, the ground state of Eu²⁺ transitions to the excited state (4f⁷ → 4f⁶5d¹) and then some of the electronic enter into the trap during excitation through the lattice. During the afterglow process, electrons are released from the trap to the excited state through thermal excitation and returned to the ground state of the Eu²⁺ from the excited state. In other words, the electron transition can be attributed to the 4f⁶5d¹ → 4f⁷. Compared with the undoped phosphor, the trap depth of Er³⁺ co-doping phosphors is increased, so that the fluorescence intensity and afterglow performance will be enhanced.

Fig. 7

The schematic diagram of long-afterglow phosphorescence mechanism of SMSEDE

4 Conclusions

In this paper, the Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ phosphor were successfully prepared via sol–gel method. The results show that phosphors of Er³⁺ co-doping reduce the intensity of the peak resulting in a decrease in crystallinity. Moreover the luminescent intensity and afterglow performance of Er³⁺ co-doping Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺ phosphors is improved and the Sr_1.94MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺, 0.016Er³⁺ phosphor initial luminous intensity increased by 1.4 times than Sr_1.956MgSi₂O₇: 0.004Eu²⁺, 0.04Dy³⁺ phosphor.