1. Introduction

Many efforts have been done to make the solar energy more available in recent years. One of the proposals is to use the long-lasting phosphorescence (LLP) materials to absorb and store the sunlight energy in daytime and then emit slowly at night for illumination purposes [1]. At present, the best LLP materials are still Eu²⁺ doped alkaline earth aluminate phosphors, e.g. SrAl₂O₄: Eu²⁺, Dy³⁺ (green) and CaAl₂O₄: Eu²⁺, Nd³⁺ (blue) [2]. However, the aluminate-base materials have bad water resistance. Moreover, the synthetic process usually needs H₃BO₃ as a flux and the synthetic temperature of these materials is usually too high (>1400 °C) [2], which limits the application of LLP materials.

New applications for LLP materials have been presented lately, such as radiation detection [3], sensors for structural damage, fracture of materials [4–6] and temperature [7], which requires that the duration of LLP materials is long enough and the host materials of LLP are steadier than aluminate host.

Recently, more attention has been paid on the alkaline silicate phosphors for better chemical stability, heat stability, weather resistance and lower synthetic temperature compared with aluminate phosphors [8–10]. The best silicate-based long-lasting phosphors is still limit to Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺ [11] and Ca₂MgSi₂O₇: Eu²⁺,Dy³⁺ [12]. Unfortunately, the duration of the alkaline silicate phosphors is not comparable with the aluminate phosphors, which could not meet the demand of practical utilization. In our present work, a novel LLP Ba₄(Si₃O₈)₂: Eu²⁺, Dy³⁺ with a long lasting phosphorescence (>24 h) was synthesized by a solid state reaction. The fluorescence and phosphorescence properties were discussed. In addition, we have found that Eu²⁺ single doped Ba₄(Si₃O₈)₂ has a potential to be a promising candidate for storage phosphors.

2. Experimental

Ba₄(Si₃O₈)₂: Eu²⁺, Dy³⁺ were synthesized by a conventional solid–state reaction method. The raw materials were high purity BaCO₃,H₂SiO₃, Eu₂O₃ (99.99%), Dy₂O₃ (99.99%). Stoichiometric mixtures of starting materials were homogeneously mixed with ethanol in an agate mortar and ground. Then the samples were sintered in a tube furnace under a reducing atmosphere (N₂: H₂ = 95:5) for 6 h at 1250 °C. After annealing, the samples were cooled to room temperature in the furnace, and ground again into powder for subsequent use.

The phases of as-prepared phosphor samples were identified by powder X-ray diffraction (XRD) analysis (Rigaku D/max-2400/pc with Ni-filter Cu Ka radiation). The photoluminescence (PL) spectra and photoluminescence excitation spectra (PLE) were obtained using FLS920T spectrofluorometer with Xe 900 (450W xenon arc lamp) as the light source. The decay curves were performed by a PR305 Phosphorophotometer after the samples were irradiated by UV light (365nm) for 10 min. Thermoluminescence (TL) curves were measured on an FJ-427A TL meter (Beijing Nuclear Instrument Factory). The samples weights were kept constant (5 mg). Prior to the TL measurements, powder samples were first exposed for 10 min by UV light (365nm), and then heated from room temperature to 400°C with a heating rate of 1°C/s. All the measurements were performed at room temperature except TL spectra.

2. Results and discussion

Figure 1 shows the typical XRD patterns of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ and Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺. All the peaks can be indexed to the phase of Ba₄(Si₃O₈)₂ (JCPDS#83-1442). No impurity phase has been observed in any samples, clearly implying that the obtained samples are single phase and the doping of Eu²⁺ and Dy³⁺ does not cause any significant change in the host structure.

 

Fig. 1 XRD patterns of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺, Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ and JCPDS Card No.83-1442.

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Figure 2 displays the excitation and emission spectra of Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ and Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺. The emission spectra of Eu²⁺ single doped and Eu²⁺, Dy³⁺ co-doped samples are nearly the same, except for that the PL intensity of Eu²⁺, Dy³⁺ co-doped sample is a little higher than that of Eu²⁺ single doped sample. The above describe shows that Dy³⁺ plays a role of sensitize center. The phosphor reveals an asymmetric emission band centered at 506 nm which can be ascribed to the typical 4f⁶5d¹- 4f⁷ transition of Eu²⁺. Although the 4f electrons of Eu²⁺ are not sensitive to their surroundings, the 5d electrons are split by the crystal field. And when the crystal field is weak, the emission band of Eu²⁺ will exist in the short wavelength [13]. Two non-equivalent barium ions (Ba (I) and Ba (II)) which can be substituted by Eu²⁺ exist in Ba₄(Si₃O₈)₂. And either non-equivalent barium ion is coordinated by eight oxygen ions. However, the polyhedron around Ba (I) is more distorted than that around Ba (II) [14], leading to the crystal field of Eu (I) (substitute Ba (I) site) is much weaker than that of Eu (II) (substitute Ba (II) site) [15]. Based on the above analysis, the asymmetric emission band could be attributed to Eu²⁺ in different Ba²⁺ sites, and the emission bands centered at 500 nm and 550nm are due to the Eu (I) and Eu (II) who substitute Ba (I) and Ba (II) sites, respectively, as shown in Fig. 2 (the Gaussian profiles). The excitation spectrum monitored at 550 nm (the dash line) is different from the excitation spectrum excited by 500 nm (the dot line), which also means there could be two distinguishable cation sites in Ba_3.982(Si₃O₈)₂.

 

Fig. 2 The emission spectrum of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ and the excitation and the emission spectra of Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ (the dot line is the excitation spectrum monitored at 500nm and the dash line is the excitation spectrum monitored at 550 nm; the green lines are the Gaussian profiles).

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Figure 3 exhibits the afterglow decay curves of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ and Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺. The lasting time of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ is only 2 min which leads to the afterglow decay curve seems like a vertical line in Fig. 3. A very significant result from our present work is that by co-doping Dy³⁺, the lasting time increases largely. The duration of Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ is more than 24 h at a recognizable intensity level (≧0.32 mcd/m²), which is comparable for the commercial aluminate phosphors. The afterglow photographs of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ and Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ are also shown in the inset of Fig. 3. It is obvious that the afterglow of Eu²⁺, Dy³⁺ co-doped sample (inset of Fig. 3 A) is much stronger than that of Eu²⁺ single doped sample(inset of Fig. 3 B), indicating that Dy³⁺ plays an important role in the afterglow process.

 

Fig. 3 Afterglow decay curves of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ and Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺. Inset A: long afterglow photographs of Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺. Inset B: long afterglow photographs of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺. The photographs were taken in the darkroom for 1 min after the removal of the 365 nm ultraviolet lamp.

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Figure 4 reveals the phosphorescence spectra of Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ measured at different time after removing the excitation source. Only the emission peak of Eu²⁺ is found, which means that during the afterglow process Dy³⁺ may not serve as the luminescence center in Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺. As shown in Fig. 4, the emission peak of Eu²⁺ moves from 506 nm to 522 nm as the delay time increases, which suggests that both Eu (I) and Eu (II) have contribution to the afterglow process, however, the decay rate of Eu (I) (500 nm) is higher than that of Eu (II) (550nm).

 

Fig. 4 Phosphorescence spectra of Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ measure at different times after the removal of the excitation source (λ_ex = 367nm).

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The extraordinarily long afterglow could be attributed to the energy exchange processes between traps or traps and emission states [16]. The information with regard to the trap and trapping level can be obtained by the TL curve. As shown in Fig. 5 (dot line, we use the letter “T” to represent the trap), for Eu²⁺ single doped sample, at least three kinds of traps exist in the phosphor (T₁, T₂ and T₃) centered at 71, 124 and 250 °C, respectively. Ordinarily, the TL band at somewhere between 70 and 150 °C is suitable for LLP materials to free the trapped carriers slowly by thermal energy at room temperature [17,18]. If the TL temperature is above this range, the trap is too deep to free the carries trapped before and it is infaust to the LLP. Consequently, the traps of T₁ and T₂ are appropriate for LLP, on the contrary, the trap of T₃ is harmful for LLP. As discussed above, Dy³⁺ plays an important role in the afterglow process. By co-doping Dy³⁺ into the sample, the intensity of the TL bands of T₁ and T₂ increase largely, however, the TL band of T₃ disappears, as displayed in Fig. 5. It indicates that the codopant Dy³⁺ does not import new traps into the phosphor, however, it aggrandizes the traps which are benefit for the LLP materials and reduces the traps which are deleterious. Nevertheless, from the nowadays data, it is hard to identify whether the traps in the phosphor are the electron traps or the hole traps, which still needs further research.

 

Fig. 5 Thermoluminescence glow curves of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ (dot line) and Ba_3.982(Si₃O₈)₂: 0.008Eu²⁺, 0.01Dy³⁺ (solid line) after ceasing the UV irradiation for 1 min. Inset shows the thermoluminescence glow curves of Ba_3.992(Si₃O₈)₂: 0.008Eu²⁺ with different delay times after ceasing the UV irradiation.

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Another very interesting result from the work is that Eu²⁺ single doped Ba₄(Si₃O₈)₂ phosphor has a possibility to be a candidate for storage phosphor. The storage phosphor needs deep and stable traps which can immobilize the carriers permanently at room temperature and it has already found these phosphors have important applications as photostimulated materials [19,20]. As mentioned before, the duration of Ba₄(Si₃O₈)₂: Eu²⁺ is only 2 min (Fig. 3), which means there should not be any TL peak after ceasing the UV irradiation for 2 min. However, as shown in the inset of Fig. 5, the TL peaks of T₁ and T₂ still subsist, and the TL peak of T₃ does not change at all even after ceasing the UV irradiation for 40 h. The above description suggests that after ceasing the UV irradiation, some of the energy attributed to the recombination of electrons and holes released by T₁ and T₂ can be transferred to the luminescent center which causes 2 min afterglow, whereas most of the energy are released by non-radiative transition, such as thermal energy. The deep traps in the phosphor are stably existed and cannot release the carriers at room temperature, which are baneful for LLP materials but benefit for the storage phosphor.

4. Conclusion

A new long-lasting phosphor Ba₄(Si₃O₈)₂:Eu²⁺, Dy³⁺ was synthesized by a solid state reaction. The Eu substituted the Ba (I) and Ba (II) site shows an emission band centered at 500 and 550 nm, respectively. Both Eu (I) and Eu (II) have contribution to the afterglow process, however, the decay rate of Eu (I) is higher than that of Eu (II). The codopant Dy³⁺ can increase the traps which are benefit for LLP materials and decrease the traps which are deleterious compared with Eu²⁺ single doped sample. The duration of Eu²⁺, Dy³⁺ co-doped Ba₄(Si₃O₈)₂ is more than 24 h at a recognizable intensity level (≧0.32 mcd/m²). Eu²⁺ single doped Ba₄(Si₃O₈)₂ has a possibility to be a kind of storage phosphors due to their deep and stable traps which can immobilize the carriers permanently at room temperature.