Luminescent Properties of Zn_xCa_1–xTiO₃:yPr³⁺ Long-Lasting Phosphors

Introduction

Long-lasting phosphors (LLPs) are a kind of storage phosphor that can absorb and store energy when it is exposed to high-energy radiation by capturing charge carriers (electrons or holes) in traps (lattice defects or impurities) [1–3]. The stored energy can then be released for several seconds to hours in the form of visible light even after the stoppage of the excitation source at room temperature by thermal, optical or other physical stimulation [2–5]. In order to improve the luminous properties of LLP, many measures have been taken by varying the concentration of activator ion, hosts, different charge compensation agent and different synthesis methods [6]. Ping Huang et al. [7] researched the effects of Dy³⁺ concentration on the luminescent properties of Y₂O₂S: Dy³⁺, Mg²⁺, Si⁴⁺. All the emissions were ascribed to the 4f–4f transitions of Dy³⁺ and the optimal concentration of Dy³⁺ doped was 1 %. Wei Xie et al. [8] discussed the influence of Ba²⁺ and Ca²⁺ co-doping on the luminescent properties of SrAl₂O₄：Eu²⁺, Dy³⁺. Luyao Hou et al. [9, 10] demonstrated that various adding manner of host ZnO had different effects on the luminescent properties of ZnO–B₂O₃–SiO₂:Mn²⁺ optical storage glass–ceramics.

Since it was reported that CaTiO₃:Pr³⁺ had the red long afterglow by Diallo et al. [11], studies on alkaline earth titanate red LLP doped with rare earth ions have been hyperactive. In order to further optimize its luminescent performances, our research group studied the influence of host Zn²⁺ and activator ion Pr³⁺ on the luminescent properties of Zn_xCa_1–xTiO₃:yPr³⁺ (ZCTP) phosphors.

Experimental details

A series of ZCTP red LLPs were prepared with the sol–gel method. Preparation process was described below. Pr₆O₁₁ was dissolved in HNO₃ (30 wt %) to obtain Pr(NO₃)₃ crystal ultimately. Ca(NO₃)₂·4H₂O and Zn(NO₃)₂·6H₂O were separately dissolved in glacial acetic acid. (C₄H₉O)₄Ti and Pr(NO₃)₃ were dissolved in ethanol. These solutions were mixed together and stirred into a homogeneous precursor which was aged at 60 C and dried at 80 C. Samples were obtained after sintered at 900 C for 5 h in air.

X-ray diffraction (XRD) patterns were recorded by X-ray diffractometry (Rigaku D/Max-2500) with Cu-Kα radiation (λ=0.15406 nm). Excitation and emission spectra were measured using a fluorescence spectrophotometer (Hitachi F-7000) in the UV–visible region (250–550 nm). Long-lasting decay curves were detected at the same conditions. Optical storage property was investigated by a thermoluminescence (TL) dosimeter (FJ-427A, China).

Results and discussion

The XRD patterns of samples prepared with different values (x=0.1–0.4) and (y=0.1–0.8 %) are displayed in Figure 1, in which we can detect CaTiO₃, Zn₂TiO₄ and Ca₂Zn₄Ti₁₅O₁₆ phases. As shown in Figure 1(a), when x value is 0.1, the main crystal phase is CaTiO₃. When x=0.2 and 0.3, the diffraction intensity of CaTiO₃ was relatively strong. In addition, along with the content of Zn²⁺ increasing, the characteristic diffraction peaks of Zn₂TiO₄ can be found from the spectrum, which indicated that the content of Zn²⁺ has influenced the crystal composition and 20 mol % Zn²⁺ has reached the solid solubility limit.

Figure 1:

XRD patterns of Zn_xCa_1–xTiO₃:0.1 %Pr³⁺ (a) and Zn_0.2Ca_0.8TiO₃:yPr³⁺ (b).

The phases in Figure 1(b) are identical to those in Figure 1(a). On account of Pr³⁺ radius similar to that of Ca²⁺ shown in Figure 2, we confirm that Pr³⁺ accessed to perovskite structure and superseded Ca²⁺ position. Along with the increment of Pr³⁺ concentration, the characteristic diffraction peaks of CaTiO₃ receded little by little, mainly because Pr³⁺ distorted the perovskite structure.

Figure 2:

Radius of ions.

Figures 3 and 4 exhibit the excitation (a) and emission (b) spectra of ZCTP samples. As shown in Figure 3(a), the excitation bounds of Zn_xCa_1–xTiO₃:0.1 %Pr³⁺ situated from 325 to 335 nm which was corresponded to O (2p) → Ti (3d) interband transition. Compared with x=0.1, others’ excitation bounds emerged blue shift. The reason may be ascribed to the following. One reason is that Zn²⁺ radius is smaller than Ca²⁺ (shown in Figure 2), Zn²⁺ possibly occupies the position of Ca²⁺, resulting in the distortion of perovskite structure. Another reason is that the electronegativity of Zn (1.65) [12] is bigger than that of Ca (1), so the attraction of Zn²⁺ to electrons in O^2– is stronger, thus O (2p) → Ti (3d) interband transition becomes more difficult. When x=0.2, the excitation intensity was the strongest.

Figure 3:

Excitation and emission spectra of Zn_xCa_1–xTiO₃:0.1 %Pr³⁺.

Figure 4:

Excitation and emission spectra of Zn_0.2Ca_0.8TiO₃:yPr³⁺.

It can be seen in Figure 3(b) that Zn²⁺ content barely has effects on the position and shape of the emission peaks of Zn_xCa_1–xTiO₃:0.1 %Pr³⁺. Emission peak at 614 nm can be observed from the spectra of all the samples, which is due to the intra-4f transition from the excited ¹D₂ to the ground state ³H₄ of Pr³⁺. On the contrary, Zn²⁺ content profoundly influences luminescence intensity of samples which is consistent with the tendency of excitation spectral. When x is 20 mol %, the intensity reaches the maximum value. Lian Shixun et al. [13] discovered that CaTiO₃ and Ca₂Zn₄Ti₁₅O₃₆ can perform luminescence properties, but Zn₂TiO₄ does not. It has been noted that Zn²⁺ in CaTiO₃:Pr³⁺ can effectively improve the luminous intensity and prolong the afterglow time. But with the increment of Zn²⁺ content, the obtained Zn₂TiO₄ increases. These contradictory factors lead to the above result.

Compared with Figure 3, Pr³⁺ has the similar change trend to excitation and emission spectral of Zn_0.2Ca_0.8TiO₃:yPr³⁺ in Figure 4. When Pr³⁺ content reaches 0.2 and 0.3 mol %, the intensity of excitation and emission peaks presents the summit. And when Pr³⁺ content is 0.8 mol %, concentration quenching effect is obvious, probably due to an energy transfer to some unknown defect acting as a trap or to strong cross-relation effects [12].

Figure 5(a) shows the LLP decay curves with different Zn²⁺ content. Afterglow time refers to the time of the luminescence decaying to 10 % of the initial brightness. Depending on this, the Zn_0.2Ca_0.8TiO₃:0.1 %Pr³⁺ sample has the highest initial brightness and its afterglow time is the longest, which confirms that Zn²⁺ improves the initial brightness of GZTP LLP. In addition, all the curves exhibit an initial rapid decay followed by slow decay. When x=0.2, the decay advantage of this sample is more obvious in rapid decay process. Figure 5(b) exhibits the LLP decay curves with different Pr³⁺ molar concentration. These decay curves also present first rapid decay followed by slow decay. When Pr³⁺ concentration reached 0.2 mol %, its brightness throughout is higher than other samples. And its decay time is the longest which is above 50 s. Even though the luminous intensity of samples has little difference between Zn_0.2Ca_0.8TiO₃:0.2 %Pr³⁺ and Zn_0.2Ca_0.8TiO₃:0.3 %Pr³⁺ samples, the afterglow time of latter is shortened greatly.

Figure 5:

LLP decay curves with different Zn²⁺ content (a) and Pr³⁺ molar concentration (b).

Afterglow time of a sample is directly related to its energy-level depth. Figure 6(a) presents the TL curves of samples with different Zn²⁺ content. The traps whose TL temperature ranges located between 50 and 110 C is more suitable to generate long afterglow phenomenon [13]. As shown in Figure 6(a), the TL temperature of Zn_xCa_1–xTiO₃:0.1 %Pr³⁺ ranges from 40 to 45 °C and when x=0.2, the intensity is the strongest which is in accordance with the decay curves.

Figure 6:

TL curves of samples with different Zn²⁺ content (a) and Pr³⁺ molar concentration (b).

Many studies indicated that the existence of traps is associated with trapping, which they further related to Pr⁴⁺ and oxygen vacancies existing in samples. Along with Pr³⁺ concentration increasing, Pr³⁺ is easily oxidated to Pr⁴⁺, but when Pr³⁺ concentration exceeds a certain value, concentration quenching effect is obvious. The TL spectra of Zn_0.2Ca_0.8TiO₃:0.3 %Pr³⁺ samples can illustrate this viewpoint well. The TL temperature and intensity reached the maximum while Pr³⁺ concentration was 0.2 mol %. Unquestionably, y=0.2 % is the optimal value. Adversely the intensity reduced rapidly when y=0.6 %.

Conclusions

ZCTP red LLPs have been successfully prepared by a sol–gel method. Zn²⁺ has contradictory effects on the luminescent properties of ZCTP LLPs, so when Zn²⁺ content is 20 mol %, the luminescent properties of samples are the best. Pr³⁺ acting as active ion, when its concentration is 0.2 mol %, the luminescent properties of ZCTP LLPs are optimal.