1. Introduction

Persistent phosphors show continuous luminescence for a long duration ranging from several minutes to a few hours even after the excitation light source is removed, as opposed to phosphors that show luminescence with a decay time on the nanosecond and millisecond scale because of intrinsic ion transitions. Persistent phosphors have found application in luminous paints in indoor and outdoor environments, for emergency signs, clock dials, and glow-in-the-dark road marks [1–4].

In 2014, we successfully developed new persistent ceramic phosphors with a new combination of lanthanide luminescent centers and transition metal trap centers in Y₃Al_5-xGa_xO₁₂(YAGG): Ce³⁺-Cr³⁺ (x = 2.5, 3, 3.5). The phosphors show Ce³⁺:5d-4f green persistent luminescence (λ_em = 510 nm) for several hours after blue-light excitation [5–8]. This phosphor can absorb blue light via Ce³⁺:4f-5d₁ transition with a large cross section and the excited electron can transfer to the conduction band (CB) easily due to the small energy gap between the 5d₁ level and the bottom of CB. In our previous work, we suggested that the Ce³⁺ ion loses an electron and the Cr³⁺ ion captures the electron under excitation, which is the main mechanism of persistent luminescence [5, 6]. However, the origin of the electron donors and acceptors in YAGG:Ce³⁺-Cr³⁺ persistent phosphors have never been directly demonstrated.

To elucidate whether the origin of the electron donors and acceptors related to the Ce and Cr ions, X-ray absorption spectroscopy (XAS) becomes a smart tool. So far, the nature of the electron donors and acceptors were investigated by using XAS in many persistent phosphors, such as aluminosilicate glasses doped with Eu²⁺ [9], (Ca,Sr)₂MgSi₂O₇:Eu²⁺-R³⁺ (R = Yb, Dy, Ce) [9], (Ca,Sr)₂MgSi₂O₇:Eu²⁺-R³⁺ (R = Yb, Dy, Ce) [10, 11], CaAl₂O₄:Eu²⁺-R³⁺(R = Yb, Sm, Nd, Ce) [12] and SrAl₂O₄:Eu²⁺-Dy³⁺ [13]. Carlson and Hölsä et al. reported that the valence state of Eu²⁺ changes into Eu³⁺ in Sr₂MgSi₂O₇:Eu²⁺ and the relative amount of Eu³⁺ increases with increasing the exposure time of X-ray in the X-ray absorption near-edge structure (XANES) spectra [11]. Also, for the most famous, brightest and longest persistent phosphor of SrAl₂O₄:Eu²⁺-Dy³⁺, Korthout and Smet et al. investigated the XANES spectra of the Eu L_III and Dy L_III edges after short and long X-ray charging. They concluded that a partial Eu²⁺ ion change into the Eu³⁺ ion by X-ray charging while the valence state of Dy³⁺ remains unchanged [13]. In this study, to investigate the valence change of the Y₃Al₂Ga₃O₁₂ persistent phosphors with new cation combination of Ce³⁺ and Cr³⁺, the XANES spectra were measured with and without UV charging during measurement.

2. Experiments

Polycrystalline ceramics of Ce³⁺(0.5%) singly-doped, Ce³⁺(0.5%)-Cr³⁺(0.05%)-codoped and Cr³⁺(0.2%) singly-doped Y₃Al₂Ga₃O₁₂ were synthesized by solid state reaction. The chemicals Y₂O₃ (99.99%), Al₂O₃ (99.99%), Ga₂O₃ (99.99%), CeO₂ (99.99%), Cr₂O₃ (99.9%) were used as starting materials. The powders were mixed by ball milling (Fritsch, Premium Line P-7) with ethanol and the obtained slurry was dried and pulverized. The dried powder was pressed into pellets sized 10 mm-ϕ × 2 mm thickness. The pellets were sintered at 1600 °C in air. The persistent luminescence spectra were measured by a radiance-meter (B&W Tek Glacier X) after ceasing 460 nm excitation for 5min. The X-ray absorption spectroscopy (XAS) was performed at the beamline BL9A of Photon Factory (KEK, Japan). The photon flux density of X-ray at 7 keV is 4×10¹² photons/sec/mm². The Ce L_III and Cr K X-ray absorption near edge structure (XANES) were recorded for all the samples in fluorescence mode with/without UV (250nm-400nm) illumination of a Xe lamp (Asahi Spectra, Max 302). The reason why the sample was illuminated by UV is to cause the ionization of Ce³⁺. As references, standard Ce(NO₃)₃, CeO₂, Cr₂O₃ samples were also measured. The Ce L_III and Cr K XANES for the reference samples were recorded in the transmission mode.

3. Results and discussion

Figure 1 (inset) shows the sample picture under w-LED illumination and after ceasing the w-LED illumination. Under the w-LED illumination, the body color of both Ce³⁺-singly doped and Ce³⁺-Cr³⁺ co-doped samples are yellow due to the strong Ce³⁺: 4f-5d absorption in the blue region. After ceasing the w-LED illumination, the only Ce³⁺-Cr³⁺ co-doped sample shows green persistent luminescence. The green persistent luminescence peaked at 510 nm is attributed to the transition from the lowest 5d₁ state to the 4f ground state of Ce³⁺ as shown in Fig. 1. The persistent luminescence spectrum matches with the sensitivity curve of the photopic vision of human eyes. Thus, the persistent luminescence of YAGG:Ce³⁺-Cr³⁺ is suitable for the luminous paint application such as emergency signs and road markers. The persistent intensity at 5 min after ceasing 460 nm excitation in YAGG:Ce³⁺-Cr³⁺ is 3900 times higher than that in YAGG:Ce³⁺ [3]. This result strongly indicates that the Cr³⁺ ion acts as an efficient carrier trapping center.

 

Fig. 1 Persistent luminescence spectra (μW/sr/m²/nm) of Y₃Al₂Ga₃O₁₂:Ce³⁺ and Y₃Al₂Ga₃O₁₂:Ce³⁺-Cr³⁺ 5 min after ceasing 460 nm excitation for 5 min, (inset) photograph of samples under white-LED and after ceasing white-LED illumination.

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To check the possibility that Ce³⁺ ions act as electron donors, the XANES spectra for the Ce L_III edge of the YAGG samples under different conditions are investigated as shown in Fig. 2(a). Figure 2(b) shows reference spectra of Ce(NO₃) and CeO₂ for Ce³⁺ and Ce⁴⁺, respectively. In Fig. 2(a), red and green solid lines show the XANES spectra for the YAGG:Ce³⁺-Cr³⁺ sample with and without UV illumination. In these XANES spectra, two main peaks were observed at 5725eV and 5735 eV. Compared with the reference XANES spectra in Fig. 2(b), the peaks at 5725 eV and 5735 eV are mainly contributed by the X-ray absorption of Ce³⁺ and Ce⁴⁺, respectively. The valence state of Ce³⁺ and Ce⁴⁺ can be well distinguished because the main XANES peak is different from each other. From the previous results of thermoluminescence excitation spectrum for YAGG:Ce³⁺-Cr³⁺, it was confirmed that this persistent phosphor can be charged by UV illumination [6]. It is expected that the Ce³⁺ and Ce⁴⁺ XANES peak intensities decrease and increase, respectively, by UV illumination. However, two XANES spectra with and without UV for Ce L_III edge in YAGG:Ce³⁺-Cr³⁺ are almost same to each other. This is probably because the strong X-ray irradiation also causes ionization of Ce³⁺ ion (charging process) without UV illumination. As an evidence of the efficient ionization, strong persistent luminescence was observed after one cycle of XANES measurement without UV illumination. In order to check the relationship between charging process and valence state change, the XANES spectrum of Ce L_III edge for YAGG:Ce³⁺ with UV illumination was measured as shown by a blue solid line of Fig. 2(a). In the XANES spectrum for YAGG:Ce³⁺, the XANES peak intensity of Ce³⁺ and Ce⁴⁺ becomes much stronger and weaker, respectively, compared with YAGG:Ce³⁺-Cr³⁺ as shown in Fig. 2(a). From the XANES spectrum, it is found that the valence state of Ce ion in YAGG:Ce³⁺ is almost trivalent even under UV illumination. Indeed, the XANES spectrum of YAGG:Ce³⁺ without UV illumination is completely same to that with UV illumination. These results indicate that Ce³⁺ does not change into Ce⁴⁺ in YAGG:Ce³⁺ by UV illumination although the photon energy of UV is large enough for the excitation of Ce³⁺:4f-electron to the conduction band based on the photoconductivity measurement and the vacuum referred binding energy (VRBE) for YAGG:Ce³⁺ [6, 14, 15]. Even though the 4f electron is ionized by UV illumination, the ionized electron could be bounded by Ce ion. However, in YAGG:Ce³⁺-Cr³⁺, a part of Ce³⁺ changes into Ce⁴⁺ from the XANES spectra with UV illumination as shown in Fig. 2(a). The valence state change from Ce³⁺ to Ce⁴⁺ can be caused by the existence of Cr³⁺ electron traps which attract the ionized electrons from Ce³⁺.

 

Fig. 2 (a) XANES spectra of Ce L_III edge in Y₃Al₂Ga₃O₁₂:Ce³⁺-Cr³⁺ illuminated with and without UV light and Y₃Al₂Ga₃O₁₂:Ce³⁺ illuminated with UV light, (b) reference XANES spectra of Ce(NO₃)₃ and CeO₂.

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In order to check the valence state changing of Cr ion, the XANES spectrum of Cr K edge for YAGG:Ce³⁺-Cr³⁺ and YAGG:Cr³⁺ with and without UV illumination were measured as shown in Fig. 3(a). Figure 3(b) shows the reference XANES spectra of Cr₂O₃ and CrCl₂ [16] for Cr³⁺ and Cr²⁺ ions. From the reference spectra, the energy of XANES edge related to the Cr²⁺-compound is much lower than that related to the Cr³⁺-compound. As with the XANES for Ce L_III, the XANES spectral shape for Cr K edge in YAGG:Ce³⁺-Cr³⁺ does not change significantly with and without UV illumination. This result also shows the charging process occur by X-ray irradiation during XANES measurement. On the other hand, the XANES spectrum for Cr K edge in YAGG:Cr³⁺ with UV illumination shows a different spectral shape compared with YAGG:Ce³⁺-Cr³⁺. When Ce³⁺ coexists in YAGG:Cr³⁺, the intensity of XANES edge in the range from 5985 eV to 6000 eV increases and the intensity at around 6007 eV decreases. From this result, the concentration ratio of Cr²⁺ respect to Cr³⁺ in YAGG:Ce³⁺-Cr³⁺ is much higher than that in YAGG:Cr³⁺. If there is the electron donor of Ce³⁺ which can be ionized by UV illumination, a number of Cr³⁺ ions can capture the ionized electrons and change into the Cr²⁺ ions. We, therefore, conclude that the combination of Ce³⁺ and Cr³⁺ causes the valence state change to Ce⁴⁺ and Cr²⁺ efficiently by UV illumination, which is the charging mechanism of persistent luminescence.

 

Fig. 3 (a) XANES spectra of Cr K edge in Y₃Al₂Ga₃O₁₂:Ce³⁺-Cr³⁺ illuminated with and without UV light and Y₃Al₂Ga₃O₁₂:Cr³⁺ illuminated with UV light, (b) reference XANES spectra of Cr₂O₃ and CrCl₂ [16].

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4. Conclusion

In summary, the valence states of Ce and Cr ions before and during charging were investigated by X-ray absorption near edge structure (XANES) spectra. To investigate the valence state change for Ce³⁺ and Cr³⁺ by UV illumination, the XANES spectra for Ce L_III and Cr K edges were measured with and without UV illumination. However, the XANES spectra do not change significantly by UV illumination in both Ce and Cr edges. This is probably because the strong X-ray irradiation also causes the release of the electron for Ce³⁺ and the capture of the electron for Cr³⁺ without UV illumination. On the other hands, in the XANES spectrum for Y₃Al₂Ga₃O₁₂:Ce³⁺, the XANES peak intensity of Ce³⁺ and Ce⁴⁺ becomes much stronger and weaker, respectively, compared with Y₃Al₂Ga₃O₁₂:Ce³⁺-Cr³⁺. This is because some part of Ce³⁺ can change into Ce⁴⁺ because the Cr³⁺ ions can capture electrons. In addition, when the Ce³⁺ ion co-exists in Y₃Al₂Ga₃O₁₂:Cr³⁺, the XANES peak of Cr²⁺ can be detected by UV illumination. These results show that the 4f electron of Ce³⁺ is excited by UV to the conduction band and act as electron donors. And then, the Cr³⁺ ion captures the ionized electron and changes into Cr²⁺. Therefore, we conclude that the Ce³⁺ is photo-ionized and Cr³⁺ captures the ionized electron for the charging process of Y₃Al₂Ga₃O₁₂:Ce³⁺-Cr³⁺ persistent phosphors.