Reversible Luminescence Modulation upon Photochromic Reactions

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Reversible luminescence modulation upon photochromic reactions in rareearth doped ferroelectric oxides by in-situ photoluminescence spectroscopy Qiwei Zhang, Haiqin Sun, Xusheng Wang, Xihong Hao, and Shengli An ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07345 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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ACS Applied Materials & Interfaces

Reversible luminescence modulation upon photochromic reactions in rare-earth doped ferroelectric oxides by in-situ photoluminescence spectroscopy Qiwei Zhang, *† Haiqin Sun, † Xusheng Wang, ‡ Xihong Hao, *† Shengli An† † School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, 7# Arerding Street, Kun District, Baotou 014010, China ‡ Functional Materials Research Laboratory, School of Materials Science and Engineering Tongji University, 4800 Caoyang Road, Shanghai 201804, China

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ABSTRACT: Reversible luminescence photo-switching upon photochromic reactions with excellent reproducibility is achieved in a new inorganic luminescence material: Na0.5Bi2.5Nb2O9: Pr3+ (NBN:Pr) ferroelectric oxides. Upon blue light (452 nm) or sunlight irradiation, the material exhibits a reversible photochromism (PC) performance between dark grey and green color by alternating visible light and thermal stimulus without inducing any structure changes, and accompanying by a red emission at 613 nm. The coloration and de-coloration process can be quantitatively evaluated by in-situ photoluminescence spectroscopy. Meanwhile,

the

luminescence emission intensity based on PC reactions is effectively tuned by changing irradiation time and excitation wavelength, and the degree of luminescence modulation has no significant degradation after several periods, showing very excellent reproducibility. Based on the luminescence modulation behavior, a double-exponential relaxation model is proposed, and a combined equation is adopted to well describe the luminescence response to light irradiation, being in agreement with the experimental data.

KEYWORDS: sunlight irradiation; photochromism; ferroelectric oxides; luminescence modulation; reproducibility;


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INTRODUCTION Photochromism (PC) is generally termed as a reversible photo-induced transformation of a chemical species between two forms (A and B), whose absorption spectra are distinguishably different.1,2 Each form represents “0” or “1” digital code, corresponding to “on” and “off” states, which can be repeatedly switched on and off with alternating ultraviolet (UV) and visible light (Vis) or thermal stimulation (∆), as follows:

(1) According to this reaction scheme, both the absorption and other physicochemical properties exhibit a change in response to light under UV or Vis light illumination, such as the refractive index, dielectric constant or oxidation/reduction potential etc..3-5 These features make it promising for a number of technological applications in the fields of high density optical memories, rewritable copy papers, sensors, smart windows and photo-switches. In order to satisfy practical applications above, nondestructive readout as an important parameter is frequently considered except for highly fatigue resistance, thermal stability and fast response time. Photo-induced switching of luminescent properties is one of the most attractive concepts for the realization of a nondestructive optical read-out system among several methods or ideas, such as readout using the changes of the refractive index or IR absorption, the molecular polarity, and so on.6 An alternative approach to modulate luminescence upon photochromic reactions is to attach covalently a luminescent group to a photochromic compound. For example, some novel photochromic compounds combined with some fluorescent moieties are designed to achieve non-destructive read out, such as organic π-conjugated molecules,7 metal complexes and nanocrystals.8,9 These luminescent photochromic materials show reversible changes in luminescence intensity with photochromic reaction, whose control



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mechanism of luminescence emission is generally based on the photochromic modulation of electron or energy transfer in the excited states.10,11 If successfully developed, these materials would have a potential value for future 3-D ultrahigh density optical recording and ultra high resolution bio-imaging technology. To date, most of the reported materials on reversible luminescence photo-switching are mainly organic molecule compounds (diarylethenes, fulgides, spiropyrans/oxazines, or their derivatives etc.),12-14 whereas the luminescence photo-switching phenomenon is seldom reported in inorganic compounds. Although switching of colors is frequently reported in inorganic compounds,15 the luminescence modulation upon photochromic reactions is rarely examined, thus their related mechanisms remain unclear, owing to their notably different molecule structures between inorganic and organic compounds. In contrast, inorganic PC materials have some remarkable advantages over organic compounds, for example, excellent thermal stability, mechanical strength and chemical resistance. These materials can be expected as compact high density optical memory materials with very small laser diodes. Since 1968, the PC performance in the transition metal (Fe/Mo)doped SrTiO3 was discovered by Faughnan et al.,16 more and more efforts have been made to investigate inorganic PC materials, such as: amorphous MoO3 films,17 V2O5,18 WO3 films,19 TiO2 films,20 or polyoxometalates etc..21 However, most metal oxides with PC performance show a strong sensitivity to UV light region, only a few sensitive to visible light, and they usually exhibit poor reversibility (e.g., difficultly photo-colored once for thermal bleached WO3 or MoO3), low fatigability and slow response time. Therefore, developing new inorganic PC materials sensitive to visible light with fast response time and good reproducibility is of great importance for efficiently utilizing solar energy and laser sources.


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Recently,

although

some

new

inorganic

materials

display

PC

(Sr2SnO4:Eu3+,

BaMgSiO4:Eu3+),22,23 the ferro-/piezoelectric materials may be specially attractive due to their intrinsic ferro-/piezoelectric performance. In our previous works, authors found that rare-earth doped ferroelectrics, such as BaTiO3, Bi0.5Na0.5TiO3 and K0.5Na0.5NbO3-based oxides etc.,24-27 exhibit visible light excited luminescent emission due to the characteristic 4f-4f transitions of rare-earth ions. These features may provide one possibility for realization of luminescence modulation upon PC materials sensitive to visible light, provided that RE-doped ferroelectrics have PC performance. Bismuth layer structure ferroelectrics (BLSFs) are well-known for potential applications in high-temperature piezoelectric devices, actuators and nonvolatile ferroelectric random access memories (FeRAMs), whose general formula is (Bi2O2)2+(An2 1BnO3n+1) ,

where A is Ca2+, Sr2+, Ba2+, Pb2+, Bi3+, K+, Na or some RE ions or a mixture of these,

and B is some highly charged cations of Nb5+, Ta5+, Ti4+ etc.. n is the number of octahedral layers in perovskite layers, its values change from 1 to 6, for example, the intensively investigated Na0.5Bi2.5Nb2O9 (NBN) (n=2),28 Bi4Ti3O12 (n=3),29 SrBiTi4O15 (n=4).30 Here, we report a novel PC luminescence material of Pr3+ doped Na0.5Bi2.5Nb2O9 (NBN:Pr) with bismuth layer structure sensitive to visible light or sunlight, showing a red emission at 613 nm. The green and dark grey colors can revert each other upon alternating visible light irradiation and thermal stimulus. Interestingly, the emission intensity of red emission can be qualitatively or quantitatively tuned by changing irradiation time during the ex-situ and in-situ photoluminescence spectroscopy.



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EXPERIMENTAL SECTION Powder synthesis: Pr doped Na0.5Bi2.5Nb2O9 ceramics were prepared by conventional solid-

state reaction method according to the formula: (Na0.5Bi0.5)1-xPrxBi2Nb2O9 (x=0, 0.001, 0.005, 0.015, 0.02, 0.03 and 0.04), abbreviated as NBN:xPr. The Na2CO3 (Alfa Aesar, 99.5%), Bi2O3 (Alfa Aesar, 99.975%), Nb2O5 (Alfa Aesar, 99.5%), Pr6O11 (Alfa Aesar, 99.9%) powders as starting materials were weighted. These powders were mixed with alcohol. After drying, they were calcined in an alumina crucible at 900 oC for 4h in air. The calcined powders were grinded, and pressed into disk-shaped pellets of 10 mm diameter by adding 8 wt% polyvinyl alcohol (PVA) binder. After that, these pellets were sintered at 1100 oC for 2h in air. Materials and properties characterization: Powder X-ray diffraction (XRD, D8 Advanced, Bruker, Germany) is used to characterize the phase structure at room temperature. Surface morphologies are examined by a field emission scanning electron microscopy (FESEM, JSM EMP-800, JEOL, Japan). The inductively coupled plasma atomic emission spectroscopy (ICPAES) (PROFILE SPEC, Leeman, AMERICA) was used to quantitatively investigate the composition deviation behavior of Na and Bi in NBN host. X-ray photoelectron spectroscopy (XPS) measurements were performed with a multifunctional imaging electron spectrometer (VG, ESCALAB 250XI, Thermo Scientific, Surrey, U.K.) with Al Ka (hν=1486.6 eV) radiation. The sunlight coloration was carried out by the visible-light output of a 300 W Xenon lamp (PLXSXE300UV, Beijing Perfect Light Technology Co. Ltd. China) with a AM1.5G filter. The diffuse reflectance spectra were tested by a UV/Vis spectrophotometer (U-4900, HITACHI, Japan). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured using a spectrofluorometer (F-4600, HITACHI, Japan) at room temperature. The absolute quantum yield (QY) and decay curves of the samples before and after irradiation were measured


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by a combined steady state & time resolved fluorescence spectrometer (FLS920, Edinburgh Instruments, United Kingdom). Reproducibility of luminescence modulation upon PC reactions is evaluated by measuring PL intensity changes with irradiation cycles, wherein a xenon lamp equipped with F-4600 was taken into account of not only as photo-irradiation sources but also as the exciting light sources. The measured wavelength range is from 500 nm to 700 nm. The scanning rate is 240 nm/min. Finishing a complete PL curve was defined as a cycle (n), namely, irradiation time of 50 s. RESULTS AND DISCUSSIONS Phase characterization and microstructure: Figure 1 shows the representative XRD patterns of

the NBN: xPr (x=0, 0.001) samples. The diffraction peak positions can be indexed based on the standard diffraction data of NBN (JCPDS#42-0397) with the A21am space group, no additional peaks were found except for a little mount of secondary phase of BiNbO4, similar results are observed in other compositions, which means that a main phase of bismuth layered structure with n=2 was achieved and Pr3+ ions were incorporated into NBN matrix to form solid solutions as desired. In addition, the diffraction peaks shift slightly to a higher angle because of an decrease in cell size, which can be ascribed to the formation of oxygen vacancies due to A-site Pr substitution to (Na0.5Bi0.5)2+ and the deficiency in A-site cations, without considering the Bi3+ site of the (Bi2O2)2+ layer and the small differences in ion radius of Pr3+ (CN=6, 0.99Å), Na+ (CN=6, 1.02Å), Bi3+ (CN=6, 1.03Å).31 These results were well consistent with some reports in some references related to NBN systems.32,33 The crystal structure viewed along [010] direction is shown in the inset of Figure 1.34 From SEM images of NBN and NBN:xPr samples shown in Figure S1 (Supporting information), plate-like morphologies are obviously observed, showing



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the typical Aurivillius type structure caused by the anisotropic nature of the crystal structure. No second phases are detected, which is consistent with the XRD result.

Figure 1. XRD patterns of NBN:xPr (x=0, 0.001). The inset is the crystal structure of NBN viewed along [010] direction.34 Photochromic and luminescent properties: The schematics and photographs of the NBN:xPr (x=0, 0.001) ceramic samples under sunlight irradiation and heat treatment are shown in Figure 2. The photographs clearly indicate the color change. The fresh samples sintered at 1100 oC for 2h show a green color before irradiation, whereas they instantly become dark grey after sunlight irradiation. When the sample is placed at heating stage with a temperature of 200 o

C, the dark grey quickly returns to its initial color (green). The precise coloration and de-

coloration time is difficultly obtained in present experiments. But, it is believed that the degree of coloration or de-coloration is directly proportional to the irradiation intensity and time, as well as heat treatment temperature and time. Besides, the green-dark grey coloration is completely reverted by alternating sunlight irradiation and thermal stimulus. The photochromic phenomenon observed in Figure 2 is clearly related to light irradiation, which can be reflected by measuring their reflection or absorption spectra. Figure 3 shows the 8 Environment ACS Paragon Plus

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changes in the reflection spectra of the NBN:xPr (x=0, 0.001) samples after exposure to a sunlight for 0s, 5s, 20s, respectively. Before sunlight irradiation, no obvious absorption bands are observed, except for that in the UV region corresponding to the optical band gap excitation (3.16 eV). After irradiation time of 5s, a broad absorption wave band appears in visible light range from 400 nm to 700 nm. With increasing irradiation time of 20s or more, the reflectance spectra only exhibit very a weak change. To investigate the absorption of the coloration sample, the absorption ratio (∆Abs.) between before and after sunlight irradiation of 5s is defined as: ∆Abs.=[R(0s)-R(5s)]/R(0s)×100%, where R(0s) and R(5s) are the reflectance intensity under 0s and 5s irradiation, respectively. The curves of ∆Abs. vs. wavelength are shown in the inset of Figure 3. There exists a broad absorption peak from 400 nm to 700 nm, and the strongest absorption is peaked at approximately 450 nm. Just as the results of Figure 2, the intensity of the absorption peak can be well repeatedly recovered by sunlight irradiation and heat treatment, and showing no obvious loss for several cycles. With further increasing Pr concentrations, the ∆Abs. values slightly decrease, as shown in Figure S2 (Supporting information).

Figure 2. Schematics and photographs of the NBN:Pr by alternating sunlight irradiation and thermal stimulus.



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Figure 3. Reflection spectra for the NBN:xPr (x=0, 0.001) by sunlight irradiation (0 s, 5 s, 20 s), the inset is the absorption difference (∆Abs.) between irradiation time of 0 s and 5 s. Although Pr3+ doping has no obvious influence on visible light absorption (Figure 3), Pr3+activated NBN gives rise to a red emission.35 PL and PLE spectra of the NBN:0.001Pr are shown in Figure 4. Under excitation at 452 nm, the material exhibits a sharp red emission from 1

D2→3H4 transition peaking at 613 nm, induced by the lack of inversion symmetry at the Pr3+

sites,36 and a much weak red emission located at 682 nm due to NBN host, whose excitation and emission spectra shown in Figure S3 (Supporting information). By monitoring the emission at 613 nm, the excited spectrum consists of three strong and sharp absorption peaks centered at 452 nm, 476 nm, and 491 nm, showing a typical f-f transition of Pr3+ from the 3H4 ground state absorption to the 3P0,1,2 excited states, specifically, originating from 3H4→3P2, 3H4→3P1, and 3

H4→3P0 transitions, respectively.25 The strongest absorption peak is located at the blue region

(452 nm), these emission bands can well match with all commercial blue LED chips (420-470 nm), which will be useful in the design of compact storage devices consisting a small LED.37 Furthermore, a weak broad band is observed at ～361 nm. According to the position of the


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absorption edge in the reflectance spectra, the weak band is assigned to the valence-to conduction band transition [O(2p)-Bi(6p)]. In Figure 4, we also depict the solar radiation spectrum received at the Earth’s surface,38 it is found that the NBN:0.001Pr material’s excitation spectrum strongly overlaps with the visible light portion of the solar radiation. This overlapping suggests that PL performance of the NBN:0.001Pr material may be influenced by visible-light radiation or solar radiation due to PC reactions (Figure 2). The excitation band at 452 nm strongly coinciding with the absorption peak (Figure 3) once confirms the statement above.

Figure 4. Excitation (λem=613 nm) and emission (λex=452 nm) spectra of the NBN:0.001Pr sample at room temperature. The green line is the solar spectrum received at the Earth’ surface at the air mass 1.5 global (AM 1.5G) condition.38 Luminescence modulation upon PC reactions: Figure 5a shows the emission spectral changes of NBN:0.001Pr excited by 465 nm excitation before and after sunlight irradiation of 5 s. After irradiation, the sample turn dark grey color from green color, the photoluminescence emission intensity is remarkably decreased. The emission spectral changes can be observed by the naked eye. The decreased degree (∆R1) before and after irradiation is obtained by the formula: ∆R1=(R0-R1)/R1×100%, wherein R0 and R1 are the relative intensity before and after



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irradiation time of 5s from the 1D2→3H4 transition at 613 nm. The ∆R1 value of the sample reaches up to 60.72%. Correspondingly, the absolute quantum yield (QY) obviously decreased to η=0.04 after irradiation, compared to η=0.09 before irradiation. Clearly, the red emission intensity can be modulated by PC reactions. The ∆R1 values exhibit a decreased trend with increasing Pr contents, especially for high concentrations of Pr ions, as shown in Figure S4 (Supporting information). Meanwhile, the decay curves for NBN:0.001Pr sample before and after irradiation are shown in Figure 5b and 5c, which can be well fitted by a double exponential fit, and the average decay lifetimes (τ) can be obtained by the formula as follows: I ( t ) = I 0 + A1 exp( − t / τ 1 ) + A 2 exp( − t / τ 2 )

(2)

A1τ 12 + A2τ 22 τ= A1τ 1 + A2τ 2

(3)

where I(t) and I0 are the emission intensity at time t and 0, respectively, AJ (J=1,2) is a constant, and τ1 and τ2 represent the decay times of the exponential components. The average decay lifetimes (τ) can be obtained by the Eq. (3). The average decay lifetimes of NBN:0.001 sample are 44.709 µs and 45.629 µs before and after irradiation, respectively.


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Figure 5. (a) PL spectral and quantum yield (η) changes of NBN:0.001Pr before and after sunlight irradiation of 5s. (b) and (c) The decay curves of NBN:0.001Pr before and after irradiation. However, the luminescence modulation upon PC reactions above (Figure 5) is only qualitatively investigated by PL measurements, due to very short response time to sunlight irradiation, especially, the light source irradiation and PL measurement is independent with each other, schematically illustrated in Figure 6a. In most literatures, the method is frequently adopted, and gradually recognized as one of the most attractive concepts for the realization of a non-destructive optical read-out system.39 How to quantitatively investigate the process of luminescence modulation is very critical and also helpful for precise evaluation of coloration and de-coloration process. A critical point for the design is to achieve PL measurements without interrupting irradiation, namely in-situ luminescence modulation. According to the aforementioned results, the overlapping of the absorption bands (Figure 3) with the excitation wavelength (Figure 4) signifies that the excitation light simultaneously acts as irradiation light



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source emission, then this feature can realize in-situ PL measurement without interrupting irradiation, as shown in Figure 6b.

Figure 6. Schematics of the ex-situ (a) and in-situ (b) photoluminescence measurements. Figure 7 shows the PL spectral changes of the NBN:0.001Pr sample with irradiation cycles under UV light (λex=361 nm) and visible light (λex=452 nm, 476 nm, 491 nm) by in-situ measurements (Figure 6b). Under different excitation wavelength, PL spectral patterns and center wavelength have no changes with different excitation light. And, the much weak emission at 682 nm only occurs at excitation wavelength of 452 nm, this is consistent with the results of Figure S3. Interestingly, the emission intensity exhibits strong dependence on irradiation cycles and excitation wavelength. The emission intensity decreases quickly with increasing irradiation cycles, especially for that excited by visible light range, as shown in Figure 7b, 7c and 7d, showing strong PL quenching behavior. Meanwhile, the in-situ irradiated area shown in Figure S5 gradually turns dark grey color (Supporting information). Compared to that under the visible light excitation, the emission intensity exhibits very weak change with irradiation cycles under the ultraviolet light excitation of 361 nm, similar behavior can be seen in other compositions (Figure S6, Supporting information), suggesting that the NBN:xPr is very sensitive to visible light absorption during the PC reactions and luminescence modulation.


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The degree of luminescence modulation behavior can be described by a parameter ∆Rn, as determined from the relative intensity ratio of the 1D2→3H4 (613 nm) transitions at every cycle against its initial emission intensity, as follows: ∆Rn=(R0-Rn)/R0×100%

(4)

where R0 and Rn are the initial relative intensity and the relative intensity under different irradiation cycles (n), respectively. The dependence of the ∆Rn on irradiation cycles is illustrated in Figure 8. The red emission intensity almost decreases to 49.74%, 48.60% and 50.16% of its initial intensity under 452 nm, 476 nm and 491 nm irradiation with 17 cycles, respectively, whereas only 3.67% under 361 nm irradiation. Such an intense dependence is certainly correlated to light absorption. As observed in the inset of Figure 3, the absorption ratios (∆Abs.) at 452 nm, 476 nm and 491 nm reach up to 21.33%, 21.32%, 20.68%, respectively, the ∆Abs. value at 361 nm is only 2.36%. The detailed results are shown in Table 1. Therefore, the light absorption plays an important role in determining the degree of luminescence modulation upon PC reactions.

Figure 7. PL spectral changes of the NBN:0.001Pr sample excited at 361 nm (a), 452 nm (b), 476 nm (c) and 491 nm (d) under different irradiation cycles at room temperature. 15 Environment ACS Paragon Plus


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From these results above, we found two important features for Pr doped NBN sample. First, the luminescence spectral change (∆Rn) is much significant under visible light irradiation than that under near UV light irradiation (∆Rn=3.67% at 361 nm). The high luminescence contrast above is desired for signal readout, which is essential for nondestructive photochromic readout application.40 Second, the photochromism behavior can be quantitatively evaluated by it-situ luminescence spectral measurements (Figure 7). In addition, we also investigate the reproducibility of the luminescence modulation upon photochromic reactions by alternating visible light irradiation and thermal stimulus, and have not observed significant luminescence intensity decay (∆Rn) while undergoing 5 periods, wherein every period includes 24 cycles.

Figure 8. The ∆Rn dependence on different irradiation cycles for NBN:0.001Pr at different excitation wavelength. Figure 9 illustrates the reproducibility of luminescence modulation for NBN:0.001Pr sample. When the sample undergoes in-situ irradiation of 24 cycles at room temperature, the luminescence emission intensity of the 1D2→3H4 can well recover its initial state as the sample is heated to 200 oC for 10 min. Most importantly, the change of luminescence modulation is


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reversible, and the emission intensity has no significant degradation after 5 periods. The degree of reproducibility behavior can be evaluated by standard deviation (SD), defined as i =5

SD =

(∆R ni ∑ i

− ∆R n )2

=1

(5)

5

where ∆Rni is the ∆Rn (n=24 cycle) value in every period (i=1, 2, 3, 4, 5), respectively, ∆R n is the average value of ∆Rni. The ∆Rni values are 53.35%, 52.65%, 51.61%, 53.11%, and 52.6%, respectively. The calculated value of SD is about SD=0.6413, indicating the excellent reproducibility for NBN:0.001Pr sample.

Figure 9. Reproducibility of the luminescence modulation (∆Rn) upon alternating 452 nm visible light and thermal stimulus under 5 periods.



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Table 1. Luminescence modulation properties of NBN:0.001Pr sample. 1

D2→3H4

∆Rn (17)

∆Abs.

(%)

(%)

Relative intensity

λex=361 nm

3.67

2.36

347

13(n=17)

λex=452 nm

49.74

21.33

972

-512(n=24)

λex=476 nm

48.60

21.32

899

-437(n=17)

λex=491 nm

50.16

20.68

969

-486(n=17)

∆REID

Wavelength

SD

0.6413

Luminescence modulation mechanism: So far, several theoretical models have been proposed to interpret the photochromic behavior. For amorphous materials, the models are based on the presence of color center or hydrogen bronze under irradiation, the obvious change in the absorption optical spectra is thought to be caused by the color center,41 intervanlence-charge transfer (IVCT),42 or small-polaron transition.43 While for some polycrystalline or nano crystals materials, the light absorption is mainly attributed to the free or trapped charge carriers.44 In present study, the PC behavior and luminescence modulation are simultaneously observed in ferroelectric oxides, the relevant reaction mechanisms between them are difficultly well described in terms of the existing theoretical models. Based on our experimental results and the trapped charge carriers model mentioned above, we propose the following luminescence modulation mechanism upon PC reactions, as shown in Figure 10.


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Figure 10. Luminescence modulation processes upon photochromic reactions for NBN:xPr samples. a) Carrier trapping. b) Luminescence modulation. c) De-coloration. (1) Photochromism-decoloration process: Generally, it is difficult to avoid the volatilization of alkali elements during the high temperature sintering, especially for Na and Bi elements in NBN host.45,46 Then, the volatilization results in compositional fluctuation, namely, the deviation of experimental results from theoretical values, Nb/Na or Nb/Bi mole ratio. The stoichiometric deviation behavior is carried out by the inductively coupled plasma atomic emission (ICP-AES). For pure Na0.5Bi2.5Nb2O9, the measured weight percentage of Na, Bi, and Nb are 1.23%, 20.39%, and 50.63% , respectively. The calculated Nb/Na and Nb/Bi mole ratios are 4.10 and 0.91, respectively. In comparison to theoretical values of Nb/Na and Nb/Bi mole ratio (Nb/Na=4, Nb/Bi=0.8), the measured mole ratio is higher than theoretical values, suggesting the volatilization of Na and Bi elements. Therefore, some vacancy-related defects caused by the volatilization of alkali elements would appear in NBN host, accordingly, forming some oxygen vacancies as charge compensation for the Bi or Na vacancies (VBi, VNa). The volatilization of Bi3+ gives rise to more oxygen vacancies because of higher valence state (3+). Meanwhile, Pr3+ substitutes A-site (Na0.5Bi0.5)2+, one positive charge center in A-site of the perovskite layers and one electron are created, which neutralize the positive holes, oxygen vacancies and A vacancies, and the positive-charge defects (PrNa0.5Bi0.5)+ can be effectively compensated for the formation of the negative-charge defects (Nb vacancy, VNb). The vacancy-related defects can be written as follows: (Bi ,Na )(highttemperature ) → V Bi′′′ + V Na′ + 2V&&O

(6)

5 Pr 3 +(−Bi0.5 Na 0.5 ) → 5 Pr&( Bi0.5Na0.5 ) + V Nb′′′′′

(7)



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So, these formed vacancy-related defects and conduction electrons, which attributed to form the F- centers, are responsible for green color.15 From Figure 3, NBN:xPr (x=0, 0.001) is a wide-band gap material, the absorption edge is located at about 330 nm-430 nm. The band gap energies (Eg) corresponding to the absorption edge can be obtained by extrapolating the absorption edge onto the energy axis by the conversion of the reflectance to absorbance data using Kubelka-Munk function (K-M):47 K /S =

(1 − R∞ ) 2 2 R∞

(8)

where K and S are the absorption coefficient and the scatting coefficient, respectively. R∞ is the reflectance ratio. The band gap energies of NBN:xPr (x=0, 0.001) are 3.16 eV and 3.12 eV, respectively. After irradiation, the Eg value only has slight changes. In the absorption spectra shown in Figure S7 (Supporting information), it can be decomposed into two parts, the first one is the steep absorption edge corresponding to the intrinsic absorption from the band-to band transitions, the second one, the extrinsic absorption, is a broad band at about 700 nm (1.77 eV). This absorption band is mainly due to populated vacancy-related defect centers mentioned above. Upon sunlight or visible light (452 nm, 2.75 eV) irradiation, the photo-generated electrons are excited from O-2p orbital to defect levels and Pr-4f orbital, and trapped at donor level (oxygen vacancies V¨O) within the forbidden gap of NBN, whose excitation energy is lower than the band gap of NBN), the photo-generated holes will shift to the A-site vacancies, as shown in Figure 10a. These trapped electrons are metastable, which can be optically excited to a higher energy level, resulting in a broad absorption in visible light region, the material turns to dark grey (Figure 10b). Accordingly, the broad absorption band occurs in the range from 400 nm to 700 nm. Besides, Pr-doped NBN sample does not show better absorption than un-doped one, thus,


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the color has no changes between them. This colorated process above could lead to a breakage of neighboring oxygen bonds: Nb-O and A(Bi, Na)-O bonds for the irradiated sample by capturing electron and modify the binding energy of them. For this purpose, the XPS spectra and fitting O 1s spectra before and after irradiation are illustrated in Figure 11, the fitting data are listed in Table S1 (Supporting information). It is found that the lattice oxygen peak from Nb-O and A-O bonds at 529.35 eV is accompanied by a shoulder peak at 530.69 eV. The shoulder peak is generally assigned to the absorbed oxygen due to the presence of oxygen vacancies.48,49 After irradiation, O 1s peaks slightly shift towards higher binding energy (about 0.49 eV), indicating the decrease of electronegativity of oxygen ions from A-O and Nb-O, the origin of lower electronegativity is obviously coming from the electrons trapped by oxygen vacancies.50 Correspondingly, the absorbed oxygen content would decrease, which can be well determined by the ratio of the fitting areas between lattice O and the absorbed O. From Table S1, The ratio (Vo/O2-) clearly decreases from 0.48 to 0.29. These results once prove our statements of photochromism mechanism discussed above in NBN:xPr.

Figure 11. (a) XPS spectra of NBN sample and (b) O1s spectra before and after sunlight irradiation, (c) and (d) fitting O1s spectra before and after sunlight irradiation.



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When the material with dark grey color is heated an appropriate temperature, the coloration fades gradually, and returns to its initial color because the trapped holes at A vacancies are released, as shown in Figure 10c. In consideration of the thermodynamic behavior of oxygen vacancies, the oxygen in air may re-enter the sample during thermal treatment, but we find that the samples annealed in air at 600 oC for 2h still exhibit reversible PC performance with excellent reproducibility, as like observed in Figure 9, which means that the F-color centers related to vacancies is very stable. (2) Luminescence modulation process: A common characteristic is observed in Pr doped NBN: the electrons from the ground state (valence band or Pr 4f) can be excited to higher levels (defect levels or Pr levels) under visible light irradiation, and accompanying by the photochromism behavior and the suppressed red emission with increasing irradiation cycles. Although the excitation energy (452 nm, 2.75 eV) is lower than the band gap of NBN (Eg=3.16 eV), the electron transitions to higher levels from the ground state still happen due to the presence of defect levels. According to the experimental results above, we can conclude that the luminescence quenching is correlated to the photochromism reactions, and the degree of luminescence quenching is almost proportional to the visible light absorption. The inherent reaction mechanisms among the photo-generated electrons, vacancies, emission and photochromism can be described by the following rules. Upon 452 nm excitation, the electrons with Pr3+ ground state (7F0) are excited to higher levels (5L6 or 5D2), among those excited electrons, only one portion of the excited state electrons take part in red emission by electron transitions from high levels to low levels, as observed in Figure 4, another portion of electrons are captured by vacancy-related defects, forming color centers (photochromism), which suppresses the red emission intensity.


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Through the above analysis, we only know that the PC deteriorates luminescence emission, why the curves of emission intensity vs. irradiation cycles (or irradiation time) show nonlinear behavior, rather than linear changes or others, what is the interplay between them? In order to answer these questions, first we need discuss the absorption behavior of materials to light. In Figure 3 or S2, we only observe the spectral changes between the initial state and the fully colored state with highest absorption, actually, the absorption difference ∆A(t) as a function of irradiation time (t) at a chosen irradiation light intensity and wavelength satisfies the exponential relaxation model,51 defined as: ∆A(t ) = A(t + t 0 ) − A(t 0 )

(9)

∆A(t ) = ∆A(∞ )[1 − exp(t / τ )]

(10)

where ∆A(∞) is the absorption difference at the fully colored state, t0 is the initial time, τ the relaxation time constant. From the Eq. (10), the ∆A(t) values exhibit the exponential decay trends with increasing irradiation time (t). Owing to the luminescence quenching degree strongly depends on the photochromism behavior, the photo-generated electrons from the ground state would be partly captured by vacancies of NBN host, contributing to

PC, the absorption

difference ∆A(t) also follow the Eq. (10). Therefore, we can infer that the luminescence emission intensity difference ∆REID (t) caused by PC should follow the rule, being proportional to the ∆A(t), as follows: ∆REID ∝ ∆A(t)

(11)

∆R EID (t ) = R[1 − exp(t / τ )]

(12)

∆REID (t) = R(t+t0)-R(t0) = Rn -R0

(13)

t=50 n

(14)



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where R is the emission intensity difference at the fully colored state, R0 and Rn are the initial relative intensity and the relative intensity under different irradiation cycles (n), respectively. According to the Eq. (12), a nonlinear relation of the ∆REID vs. t should be also obtained, and the relation can be fitted by the exponential function. In order to make clear the relation, the fitted ∆REID vs. irradiation cycles (n) curves at 452 nm excitation are shown in the Figure 12. Clearly, the ∆REID values exhibit a nonlinear change trend with increasing irradiation cycles, and these data can be fitted to the Eq. (12), but there is a distinct discrepancy between experimental data and fitting curve in a certain area, where the ∆REID almost becomes saturated. The distinct discrepancy may be related to the light absorption behavior from Pr ions. Namely, there is another relaxation mechanism caused by the light absorption of Pr ions, except for the relaxation from the light absorption of NBN host. A better fitting is obtained according to the modified exponential function: ∆R EID (t ) = B 0 + R 1 × [1 − exp(t / τ 1 )] + R 2 × [1 − exp(t / τ 2 )]

(15)

where the B0 is a constant, R1 and R2 are the luminescence emission intensity differencedependent coefficients, τ1 and τ2 are the relaxation time constants from the NBN host and Pr ion light absorption behavior, respectively.


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Figure 12. Irradiation cycles dependence of the luminescence emission intensity difference (∆REID) under 452 nm irradiation for NBN:0.001Pr. The symbol indicates the experimental data, and the solid line (blue) represents fitting to the Eq. (15), the dotted line (red) represents fitting to the Eq. (12). In the literature, the Eq. (10) was used by Dachraoui et al. in only dealing with the absorption behavior from host, whereas taking no consideration of other factors, just as rare-earth ions as activators. Thus, the fitted R value (absolute value, 489.64) is significantly lower than the experimental data at fully colored state (Figure 5). According to our experimental results of luminescence modulation, Pr doped NBN material should include two parts of absorption in response to light irradiation, one is coming from NBN host itself, the other is coming from the rare-earth ions. Upon the visible light irradiation, they have different relaxation time distribution, which is well consistent with the modified exponential function of Eq. (15) we proposed. Therefore, the first term represents the absorption behavior to light from NBN host, τ1 is the laxation time constant from the response of NBN host to light irradiation, the latter represents the response behavior of rare-earth ions to light irradiation, τ2 is the relaxation time. Therefore, the luminescence emission intensity difference is a combined result of R1 and R2, which is confirmed by the fitted data. From Eq. (15), the ∆REID should be zero if t is closer to 0, but the fitted B0 value is about 0.7046, so mall constant may be caused by the fitting error, no physical meanings. The detailed fitting parameters and errors are listed in Table S2 (Supporting information), Similar behavior is observed in other excitation wavelengths, as shown in Figure S5 (Supporting information). However, more investigations are needed to clarify this relation. CONCLUSIONS



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Based on high temperature piezoelectric material of Na0.5Bi2.5Nb2O9 with bismuth layer structure, the coexistence of photoluminescence emission and intrinsic photochromism was obtained by introducing rare-earth Pr3+ ions as activators. Upon exposure to visible light or sunlight, the material exhibited a reversible color change between green to dark grey by alternating visible light irradiation and thermal stimulus. The coloration-decoloration process can be quantitatively evaluated by in-situ PL measurements. Most importantly, the relative intensity of the photoluminescence emission can be effectively tuned by modifying irradiation cycles, and the emission intensity has no significant degradation after several periods, showing very excellent reproducibility. The results show that the new visible-light sensitive PC material has potential applications in the reusebale information storage media, data display, opical signal processing, chemical switch for computer and smart window (control of radiation intensity). Especially, the non-destructive readout capability in rare-doped ferroelectric oxides would have some benefits in the optical memory fields. ASSOCIATED CONTENT Supporting Information. SEM images of NBN:xPr samples, PL and PLE spectra of pure NBN, photographs before and after in-situ irradiation, absorption spectra of samples obtained by K-M functions, and the fitted curves to Eq. (12) and (15). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. cn,


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E-mail: [email protected] ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (No. 51562030, 51462028), the Natural Science Foundation of Inner Mongolia (No. 2015BS0503, 2014MS0522) and the Research Fund for Higher Education of Inner Mongolia (No. NJZZ14158). REFERENCES (1) Brown, G. H. Photochromism; Techniques of Chemistry. John Wiley & Sons, New York, 1971. (2) Crano, J. C.; Guglielmetti, R. Organic Photochromic and Thermochromic Compounds, 2nd ed; Plenum Press: New York, 1999. (3) Irie, M. Diarylethenes for Memories and Switches. Chem. Rev. 2000, 100, 1685-1716; (4) Tian H.; Yang, S. Recent Progresses on Diarylethene Based Photochromic Switches. Chem. Soc. Rev. 2004, 33, 85-87. (5) Taguchi, M.; Nakagawa, T.; Nakashima T., Kawai, T. Photochromic and Fluorescence Switching Properties of Oxidized Triangle Terarylenes in Solution and in Amorphous Solid States. J. Mater. Chem. 2011, 21, 17425-17432; (6) Hasegawa, Y.; Nakagawa T., Kawai, T. Recent Progress of Luminescent Metal Complexes with Photochromic Units. Coord. Chem. Rev. 2010, 254, 2643-2651. (7) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai N.; Kawai, T. Organic Chemistry: A Digital Fluorescent Molecular Photoswitch. Nature 2002, 420, 759-760. (8) Myles A. J.; Branda, N. R. 1, 2-Dithienylethene Photochromes and Non-destructive Erasable Memory. Adv. Funct. Mater. 2002, 12, 167-173.



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(51) Dachraoui, H.; Rupp, R. A.; Lengyel, K.; Ellabban, M. A.; Fally, M.; Corradi, G.; Kovács, L.; Ackermann, L. Photochromism of Doped Terbium Gallium Garnet. Phys. Rev. B 2006, 74, 144104-11. TOC graphics

Reversible luminescence modulation upon photochromic reactions with excellent reproducibility was achieved from Pr3+ doped Bi2.5Na0.5Nb2O9 ferroelectric oxides. The photochromic behavior and luminescence modulation exhibit very strong sensitivity to visible light or sunlight without inducing any structure changes.


Reversible Luminescence Modulation upon Photochromic Reactions

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