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Recently, considerable attention has been paid to photochromic (PC) materials owing to its great potential application in various fields, such as rewritable copy papers, erasable optical memory media, smartwindows, sensors, photoswitches and so on. Up until now, most of PC materials come from organics. However, we report that a self-activated LAG phosphor possesses PC property based on non-doped Mg2SnO4 synthesized via a traditional solid-state reaction method. The photoluminescence and long afterglow (LAG) properties were investigated. Interestingly, the white surface color can be colored into brown by ultraviolet-light (UV) irradiation. After visible light irradiation or heat-treatment, the colored Mg2SnO4 can be bleached into white again. The reversible white-brown PC properties of Mg2SnO4 were characterized by diffuse reflectance spectra. Based on the observed phenomena and the obtained experimental results, a tentative model was constructed to illustrate the LAG and PC mechanism.
© 2017 Optical Society of America
1. Introduction
Inorganic luminescent materials, generally include photoluminescence phosphor and long-persistent materials, have been used in various fields for decades especially in solid-state lighting [1,2]. Owing to the advantages of saving-energy, environment friendly, long lifetime and reliability, considerable efforts have been devoted to explore some new phosphors with excellent luminescent proprieties or superior afterglow behavior [3,4]. Recently, a interesting phenomenon was observed in rare-earth or transition mental ions doped phosphors, such as BaMgSiO4: Eu2+ [5], Ba3MgSi2O8:Eu2+ [6], Sr2SnO4: Eu3+ [7], CaAl2O4: Eu2+, Nd3+ [8], Ba5(PO4)3Cl: Eu2+ [9], ZnGa2O4: Bi3+ [10] and LiGa5O8: Cr3+ [11]. MgGeO3: Yb3+ [12], Sr3YNa(PO4)3F: Eu2+ [13], Mg4Ga8Ge2O20: Cr3+ [14], Na0.5Bi2.5Nb2O9: Re3+ (Re = Sm, Pr, Er) [15–17], Na0.5Bi4.5Ti4O15: Re3+ (Re = Sm, Pr, Er) [18]. After irradiation by external light, the surface color of these substances mentioned above changed from their original color to other colors. Therefore, this kind of materials was defined as PC materials. Namely, PC denotes a reversible medium-color change upon exposure to light [6]. Since the discovery of a PC material that containing silver halide (AgX) glass [19], much attention had been paid in the mechanism and application of PC materials. But the main focus of research was on glasses and organic compounds [20,21]. At the same time, great progress and substantial achievement were obtained in the organic PC materials field. However, the research and application of inorganic PC materials are still immature and in their initial stage. And the detailed mechanism of inorganic PC materials has not been revealed clearly. As many reports, the PC mechanism of organic PC materials was ascribed to the changes in molecular conformation or configuration. While PC mechanism in inorganic materials is quite different from that in organic materials and usually considered to be closely related to electron or hole trapped defects generated by photostimulation [13,14]. Therefore, the exploration of any more inorganic PC materials is helpful to revealing or improving the PC mechanism and promotes a general technique for exploring or designing practical PC materials in the future application.
Although the luminescence and LAG properties of rare earth ions or transition mental ions doped Mg2SnO4 have been studied systematically in the past few years [22–24], the PC property of un-doped Mg2SnO4 has not been reported yet. Herein, we report a self-activated PC phosphor Mg2SnO4. The PL, LAG and PC properties were investigated briefly by excitation and emission spectra, diffuse reflectance spectra, LAG decay curve, and TL curve. Finally, a schematic was constructed to discuss the mechanism of LAG and PC.
2. Experimental procedures
2.1 Sample preparation
The blue emitting LAG phosphor Mg2SnO4 powders were synthesized successfully via conventional high temperature solid state method in the ambient atmosphere. The raw materials used in the preparation were as follows: MgO (Aladdin, A.R. (Analytical Reagent), ≥99.5%) and SnO2 (Aladdin, 99.95%). Stoichiometric amounts of reactant powders were weighed out accurately and then ground for 1 h to form a homogeneous fine powders. Firstly, the homogeneous powders were dried at 150 °C for 4 h. After that, the homogeneous powders were prefired at 900 °C in air for 6 h, and then calcined at 1450 °C for another 6 h with intermediate grindings. After naturally cooling down to room temperature, the white Mg2SnO4 sample was obtained and then ground to fine powder sample for the following characterizations.
2.2 Measurements
The phase and purity of the as prepared sample was identified by using a XD-2 X-ray diffractometer (Beijing PGENERAL) with Cu Kα irradiation. Diffuse reflectance spectra were obtained by a UV-visible spectrophotometer (Shimadzu UV-2450, Japan) using BaSO4 as a reference. Before testing for the diffuse reflectance spectra, the Mg2SnO4 sample was pre-fired up to 600 K. After being cooled down to room temperature naturally, we colored it by a UV-radiation (220 nm). All experimental data were collected at the same test conditions. The room temperature photoluminescence excitation (PLE), emission (PL) spectra, LAG spectra and LAG decay curves were recorded with an FLS-980 fluorescence spectrophotometer (Edinburgh Instruments). The thermoluminescence (TL) glow curves were measured by FJ427A1 thermo-luminescent dosimeter (CNNC Beijing Nuclear Instrument Factory). For TL measurement, the sample was first exposed to a UV-radiation (220 nm) for 10 min and then put in dark for different delay time.
3. Results and discussion
3.1 Phase identification
Figure 1 shows the XRD pattern of un-doped Mg2SnO4 powder sample together with Joint Committee for Powder Diffraction Standard (JCPDS) card of Mg2SnO4. All the diffraction peaks of the obtained XRD pattern matched well with JCPDS card No. 24–0723, indicating that the obtained powders are single phase.
3.2 Photoluminescence and LAG properties
As many reports have been investigated and proved, Mg2SnO4 is a self-activated LAG phosphor. Figure 2 (a) presents the PLE and PL spectra of Mg2SnO4. Obviously, a broad emission band range from 470 to 550 nm and centered at ~498 nm was observed, which may be caused by the defects of Mg2SnO4 host [23]. The excitation band centered at ~220 nm is probably caused by the transition from the valence band (VB) to the conduction band (CB) of the Mg2SnO4 host. The PL and PLE spectra of Mg2SnO4 are consistent with that of reported by Jiachi Zhang [23,24]. Besides the broad blue luminescence, the Mg2SnO4 phosphor also exhibited bright persistent phosphorescence. After irradiation by UV lamp, a bright blue afterglow was observed from non-doped Mg2SnO4. Figure 2 (b) shows the afterglow intensity as a function of time by monitoring 498 nm emission of Mg2SnO4. The afterglow intensity decreased quickly in the first several minutes and then decayed with slow speed. Obviously, the blue afterglow can last more than 1 h after removing 220 nm UV-irradiation.
Fig. 2 (a). Excitation (λem = 498 nm) and emission (λex = 220 nm) spectra of non-doped Mg2SnO4; (b) LAG decay curve of Mg2SnO4 (excited at 220 nm for 10 min, monitored at 498 nm).
3.3 PC properties analysis
Interestingly, after UV light irradiation for a few minutes, the body color of Mg2SnO4 powder changed from white to brown. And then the colored powders can be substantially bleached back to its original color (white) after exposure to visible light source for several minutes. To further investigate the PC properties of Mg2SnO4, the diffuse reflectance spectra were performed to qualitatively characterize the PC properties of the Mg2SnO4 sample before and after UV irradiation. Figure 3(a) shows the diffuse reflectance spectra of Mg2SnO4 before and after the irradiation by a UV lamp. Clearly, there are two broad absorption bands at 200-250 nm and 250-350 nm. The band centered at ~220 nm which is consistent with the PLE spectra of Mg2SnO4 can be also ascribed to the host absorption. While the absorption band at 250-350 nm is originated from Sn4+-O2- charge transfer band. Before the irradiation, the pure Mg2SnO4 sample with white body color almost without obvious absorption in the wavelength range from 350 to 800 nm.
Fig. 3 (a). Diffuse reflectance spectra of Mg2SnO4 before and after UV irradiation; (b) Reflectivity intensity (at 420 nm) dependence on different delay times (0–480 h); (c) Body color changing photographs of Mg2SnO4 under UV, sunlight or heat treatment.
After UV irradiation, the white Mg2SnO4 sample was colored into brown and reflected in the fallen diffuse reflectivity in the wavelength range of 350 to 800 nm. The colored Mg2SnO4 can be gradually bleached back to its original color (white) again after sunlight irradiation for a long time. Also, the colored Mg2SnO4 sample can be gradually bleached in the dark room with about 20 days (shown in Fig. 3(b)). Furthermore, the colored Mg2SnO4 can be also bleached into white through heating treatment. Clearly, the reversible coloration–decoloration (white–brown) process can be intuitively observed in Fig. 3(c).
To further investigate the influence of temperature on the reflectance of colored Mg2SnO4, the colored Mg2SnO4 sample bleached by different temperature treatment (50-350 °C) was also performed. Figure 4(a) shows the reflectance dependence on the different fading temperature. With the rise of heating temperature, it was clearly seen that the reflectance increase drastically, especially the drops centered at 420 nm. Obviously, there are a slow increasing process at temperature below 180 °C and a fast increasing process above 180 °C. Therefore, high temperature with better bleaching effect. Figure 4(b) shows reversible coloration–decoloration processes (alternatively colored by 220 nm UV light and then immediately bleached by 532 nm visible light irradiation or 350 °C heating treatment). The intensity of the reflectance at 420 nm can be recovered to the initial level after bleaching by 350 °C treatment or 532 nm irradiation. After several cycle experiments, almost no fatigue or thermal degradation in the coloration–decoloration processes. It demonstrates that Mg2SnO4 possesses superior PC thermo-stability and high fatigue resistance. So, the information recording and erasing can be repeated well in Mg2SnO4. Therefore, it is a promising PC material for potentially used in optical memory device.
Fig. 4 (a). Diffuse reflectance spectra of colored Mg2SnO4 sample after different temperatures (50–350 °C) heat treatment for 3 min; (b) Reflectance intensity changes of Mg2SnO4 at 420 nm induced by colored by 220 nm irradiation and bleached by 532 nm light irradiation (Green dots) or 350 °C heating treatment (Red dots).
3.4 Thermoluminescence analysis
It was well known that traps played an essential role in LAG materials. It was also considered that the PC is related to the charge carriers trapping according to recent reports [13,14]. Therefore, to further investigate the LAG and PC phenomenon, the nature of traps was probed by the TL glow curve. Figure 5(a) shows the TL curve of Mg2SnO4 with 10 min delay time. And the result was fitted by a GlowFit program based on the following function [25]:
Fig. 5 TL curves of Mg2SnO4: Excited at 220 nm for 10 min then (a) Fitted results of 10 min delay time; (b) with different delay time (5 min-480 h).
Where I(t) is the intensity of TL glow curve, n is the concentration of excited electrons, b is the kinetic order parameter, is defined as the attempt-to-escape frequency, E0 is the activation energy for releasing the captured electrons, k is the Boltzmann constant, T(t) = T0 + βt (K) is the linear heating profile with the heating rate β.
In most cases, the TL bands close to the ideal temperature (50 °C-100 °C) is suitable for releasing charge carriers slowly at room temperature. Thereby Band 1 is suitable for the occurrence of room temperature LAG phenomenon. While the contribution of high temperature band (>100 °C, Band 2 and Band 3) is mainly to store energy to ensure the long duration of LAG. Furthermore, as our previous work showed that the high temperature band with deeper traps is responsible for PC [9,13,14,26]. To fully understand the nature of the traps and obtain more information about the trap structure and gain insight into the effect of traps on the origin of LAG and PC in un-doped Mg2SnO4, a series of TL curves of Mg2SnO4 with different delay time were performed, as exhibits in Fig. 5(b). As anticipated, with the delay time prolonged, the intensity of the TL peak reduces and the position of TL bands shifts to higher temperature gradually. And the intensity of Peak 1 decreased with remarkable rapid speed than that of Peak 2. It’s caused by the trapped electrons/holes in shallower depth of traps are released easier and faster, while that in deep traps are still remained. Therefore, it leads to the TL bands shift to higher temperature and the trap depth increase. An obvious phenomenon can be found in the Fig. 5(b), with increasing the delay time, the relative intensity of TL glow curve between the two peaks have changed. A reasonable explanation is that the trapped electrons in deep traps under thermo stimulus can partially move to shallow traps. Also, under a light stimulus, the trapped electrons in deep traps can partially move back to shallow traps [23]. And they finally can be released to recombine with holes. Therefore, the light and heating can bleach the colored Mg2SnO4 sample.
3.5 Possible LAG and PC mechanism for Mg2SnO4
Furthermore, in order to fully understand the generation of LAG and PC phenomena, the following observed phenomenon may be reasonable for the explanation of the difference between LAG and PC:
- (i) The LAG emission after excited by UV lamp for 10 min can be observed ~1 h by the naked eyes in the dark. But, after the disappearance of LAG, the colored Mg2SnO4 in the dark can’t be bleached obviously. Therefore, we may conclude that the shallower traps are suitable for the room temperature LAG, while the PC is mainly caused by deeper traps. It can be seen in Fig. 5(b) that the deep traps can capture electrons for a long time. And Fig. 5 is also a good explanation for the colored Mg2SnO4 can’t be bleached obviously in the dark even the LAG disappeared.
- (ii) As shown in Fig. 4(a), it is clearly seen that the temperature above 180 °C with better bleaching effect. When the colored Mg2SnO4 powders were heated at the temperature below 180 °C, they start to be bleached but the bleaching effect is not obvious. While they can be bleached substantially when the temperature above 180 °C. Figure 5 shows that the electrons captured by deep traps cannot release below 180 °C. So the deeper traps play a decisive role in PC. The bleaching effect of above 180 °C treatment is much better because of the electrons in deep traps can be released rapidly under higher temperature.
- (iii) After irradiation by sunlight or other artificial visible-light source, the colored Mg2SnO4 can also be bleached. As shown in Fig. 4(b), the colored Mg2SnO4 can be bleached back to the original body color (white) by 532 nm irradiation.
Based on the aforementioned phenomenon and results, a model as shows in Fig. 6 was constructed for the possible explanation of LAG and PC mechanism. For un-doped Mg2SnO4, electrons can be excited directly from VB into CB under 220 nm UV excitation (step 1), while holes left in the VB can be captured by hole traps. Upon the UV irradiation, the excited electrons are abundant, therefore, majorities of the electrons relaxed from CB to defects levels and then recombined with holes directly (step 2). This recombination process would induce the defects level as luminescence centers and generates a consistent blue PL centered at ~498 nm. However, the electrons (excited in the CB) and holes (created in the VB) can be captured by electron traps and hole traps, respectively (step 3). Namely, this process of storing energy is like charging a battery. Therefore, it’s reasonable for LAG and PC phenomenon. After switched off the excitation, the captured electrons with suitable trap depth can be released gradually with the aid of thermal vibration and returning back to the defects level through the CB and then cause the blue LAG emission at room temperature (step 4). Whereas deep traps can store captured electrons for a long time due to it’s difficult to empty at room temperature. Therefore, the colored Mg2SnO4 seems cannot be bleached after the LAG disappeared. While, the captured electrons in deep traps can be released or move back to shallow traps under thermo or light stimulus. Afterward, the captured electrons in deep traps can be back to the CB via interaction with photons or thermal vibration and consequently reach the ground states to recombine with holes (step 5). Therefore, the thermo and light stimulus can bleach the colored sample. As a result, the colored Mg2SnO4 powders (brown) are bleached into its original body color (white).
4. Conclusion
The reversible white-brown photochromic material Mg2SnO4 was synthesized. The PL, LAG and PC properties of Mg2SnO4 were investigated. The pure Mg2SnO4 shows blue PL and LAG emission centered at 498 nm, and the LAG duration is more than 1 h. The surface color of the Mg2SnO4 changed from white to brown after UV irradiation and can be bleached into the original color by heating or visible light irradiation. According to the TL glow curves and diffuse reflectance spectra, deep traps were responsible for the PC. The bleaching effect of heating treatment was much better when temperature above 180 °C. Accordingly, the LAG and PC mechanism was also discussed in detail based on the obtained results.
Funding
National Natural Science Foundation of China (No. 21671045).
References and links
1. C. Feldmann, T. Jüstel, C. R. Ronda, and P. J. Schmidt, “Inorganic luminescent materials: 100 years of research and application,” Adv. Funct. Mater. 13(7), 511–516 (2003). [CrossRef]
2. T. Jüstel, H. Nikol, and C. Ronda, “New developments in the field of luminescent materials for lighting and displays,” Angew. Chem. Int. Ed. 37(22), 3084–3103 (1998). [CrossRef]
3. E. Danielson, J. H. Golden, E. W. McFarland, C. M. Reaves, W. H. Weinberg, and X. D. Wu, “A combinatorial approach to the discovery and optimization of luminescent materials,” Nature 389(6654), 944–948 (1997). [CrossRef]
4. P. Escribano, B. Julián-López, J. Planelles-Aragó, E. Cordoncilloa, B. Vianab, and C. Sanchez, “Photonic and nanobiophotonic properties of luminescent lanthanide-doped hybrid organic–inorganic materials,” J. Mater. Chem. 18(1), 23–40 (2008). [CrossRef]
5. Y. Li, Y. Wang, Y. Gong, X. Xu, and F. Zhang, “Photoionization behavior of Eu2+-doped BaMgSiO4 long-persisting phosphor upon UV irradiation,” Acta Mater. 59(8), 3174–3183 (2011). [CrossRef]
6. Y. Yonezaki and S. Takei, “Photochromism and emission-color change in Ba3MgSi2O8-based phosphors,” J. Lumin. 173, 237–242 (2016). [CrossRef]
7. S. Kamimura, H. Yamada, and C. N. Xu, “Purple photochromism in Sr2SnO4: Eu3+ with layered perovskite-related structure,” Appl. Phys. Lett. 102(3), 031110 (2013). [CrossRef]
8. J. Ueda, T. Shinoda, and S. Tanabe, “Photochromism and near-infrared persistent luminescence in Eu2+, Nd3+-co-doped CaAl2O4 ceramics,” Opt. Mater. Express 3(6), 787–793 (2013). [CrossRef]
9. G. F. Ju, Y. H. Hu, L. Chen, and X. J. Wang, “Photochromism of rare earth doped barium haloapatite,” J. Photochem. Photobiol. Chem. 251, 100–105 (2013). [CrossRef]
10. Y. X. Zhuang, J. Ueda, and S. Tanabe, “Photochromism and white long-lasting persistent luminescence in Bi3+-doped ZnGa2O4 ceramics,” Opt. Mater. Express 2(10), 1378–1383 (2012). [CrossRef]
11. F. Liu, W. Yan, Y. J. Chuang, Z. Zhen, J. Xie, and Z. Pan, “Photostimulated near-infrared persistent luminescence as a new optical read-out from Cr3+-doped LiGa5O8,” Sci. Rep. 3, 1554 (2013). [CrossRef] [PubMed]
12. Y. J. Liang, F. Liu, Y. F. Chen, X. J. Wang, K. N. Sun, and Z. W. Pan, “New function of the Yb3+ ion as an efficient emitter of persistent luminescence in the short-wave infrared,” Light Sci. Appl. 5(7), e16124 (2016). [CrossRef]
13. Y. H. Jin, Y. H. Hu, Y. R. Fu, L. Chen, G. F. Ju, and Z. F. Mu, “Reversible colorless-cyan photochromism in Eu2+-doped Sr3YNa(PO4)3F powders,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(36), 9435–9443 (2015). [CrossRef]
14. Y. H. Jin, Y. H. Hu, L. F. Yuan, L. Chen, H. Y. Wu, G. F. Ju, H. Duan, and Z. F. Mu, “Multifunctional Near-Infrared Emitting Cr3+-doped Mg4Ga8Ge2O20 Particles with Long Persistent, Photostimulated Persistent Luminescence and Photochromism Properties,” J. Mater. Chem. C Mater. Opt. Electron. Devices 4(27), 6614–6625 (2016). [CrossRef]
15. Q. Zhang, X. Zheng, H. Sun, W. Li, X. Wang, X. Hao, and S. An, “Dual-Mode Luminescence Modulation upon Visible-Light-Driven Photochromism with High Contrast for Inorganic Luminescence Ferroelectrics,” ACS Appl. Mater. Interfaces 8(7), 4789–4794 (2016). [CrossRef] [PubMed]
16. Q. Zhang, H. Sun, X. Wang, X. Hao, and S. An, “Reversible Luminescence Modulation upon Photochromic Reactions in Rare-Earth Doped Ferroelectric Oxides by in Situ Photoluminescence Spectroscopy,” ACS Appl. Mater. Interfaces 7(45), 25289–25297 (2015). [CrossRef] [PubMed]
17. Q. W. Zhang, Y. Zhang, H. Q. Sun, Q. Sun, X. S. Wang, X. H. Hao, and S. An, “Photoluminescence, photochromism, and reversible luminescence modulation behavior of Sm-doped Na0.5Bi2.5Nb2O9 ferroelectrics,” J. Eur. Ceram. Soc. 37(3), 955–966 (2017). [CrossRef]
18. Q. Zhang, Y. Zhang, H. Sun, W. Geng, X. Wang, X. Hao, and S. An, “Tunable luminescence contrast of Na0.5Bi4.5Ti4O15: Re3+ (Re= Sm, Pr, Er) photochromics by controlling the excitation energy of luminescent centers,” ACS. ACS Appl. Mater. Interfaces 8(50), 34581–34589 (2016). [CrossRef] [PubMed]
19. D. Caurant, D. Gourier, and M. Prassas, “Electron-paramagnetic-resonance study of silver halide photochromic glasses: Darkening mechanism,” J. Appl. Phys. 71(3), 1081–1090 (1992). [CrossRef]
20. P. X. Li, M. S. Wang, and G. C. Guo, “Two New Coordination Compounds with a Photoactive Pyridinium-Based Inner Salt: Influence of Coordination on Photochromism,” Cryst. Growth Des. 16(7), 3709–3715 (2016). [CrossRef]
21. J. Ren, X. Q. Xu, W. Shen, G. R. Chen, S. Baccaro, and A. Cemmi, “Gamma-ray induced reversible photochromism of Mn2+ activated borophosphate glasses,” Sol. Energy Mater. Sol. Cells 143, 635–639 (2015). [CrossRef]
22. B. F. Lei, B. Li, X. J. Wang, and W. L. Li, “Green emitting long lasting phosphorescence (LLP) properties of Mg2SnO4: Mn2+ phosphor,” J. Lumin. 118(2), 173–178 (2006). [CrossRef]
23. J. C. Zhang, M. H. Yu, Q. S. Qin, H. L. Zhou, M. J. Zhou, X. H. Xu, and Y. H. Wang, “The persistent luminescence and up conversion photostimulated luminescence properties of nondoped Mg2SnO4 material,” J. Appl. Phys. 108(12), 123518 (2010). [CrossRef]
24. J. C. Zhang, Q. S. Qin, M. H. Yu, M. J. Zhou, and Y. H. Wang, “The photoluminescence, afterglow and up conversion photostimulated luminescence of Eu3+ doped Mg2SnO4 phosphors,” J. Lumin. 132(1), 23–26 (2012). [CrossRef]
25. S. W. S. McKeever and R. Chen, “Luminescence models,” Radiat. Meas. 27(5-6), 625–661 (1997). [CrossRef]
26. Y. H. Jin, Y. H. Hu, Y. R. Fu, Z. F. Mu, and G. F. Ju, “Reversible white and light gray photochromism in europium doped Zn2GeO4,” Mater. Lett. 134, 187–189 (2014). [CrossRef]
