Preparation and Tunable Luminescence of Eu Doped KNN Ceramics

Abstract: Rare-earth doped inorganic ferroelectrics are considered as novel photochromic materials, with potential applications for optical switch and information storage (K0.5Na0.5)1–xEuxNbO3 (KNN:xEu) ceramics were prepared by high temperature calcination, with precursor powder obtained by hydrothermal method. Strong red emission at 615 nm was observed which corresponds to the 5D07F2 transition of Eu3+ under excitation of 465 nm. Under UV light irradiation for 3 min, the color of the ceramics turned from milky white to dark gray. The colored samples returned to the original color when heated at 200 ℃ for 10 min, showing strong photochromic behavior. Meanwhile, the luminescence intensity of Eu3+ can be tuned without obvious degradation by alternating UV light and heat stimulus. Upon UV light irradiation, large luminescence modulation ratio (ΔRt) up to 83.9% was achieved for KNN:0.06Eu, indicating good luminescence switching behavior. A possible mechanism for non-radiative energy transfer from the luminescent center to the color center was proposed according to their luminescent behavior.

Key words: K0.5Na0.5NbO3(KNN); luminescence; photochromism

Photochromic (PC) materials, changing color between the coloring state and bleaching state upon light irradiation, have aroused increasing attention with great potential applications in photo-switching and related optoelectronic devices[1-4]. Compared with traditional organic PC system, inorganic materials exhibit better thermal stability, higher chemical and fatigue resistance[5-7]. Some inorganic materials have been reported for their PC behaviors, such as WO3, TiO2, and V2O5[7-9]. However, the luminescence modulations of these materials are extremely poor, which limit their potential application for optical data storage. Recently, rare-earth element doped ferroelectric ceramics, including K0.5Na0.5NbO3 (KNN), Na0.5Bi2.5Nb2O9 (NBN) and Na0.5Bi4.5Ti4O15 (NBT), have been reported to show good luminescence switching behaviors[10-11]. Rare earth doped ferroelectrics are capable of combining ferroelectric energy storage and optical memories, which make them fascinating candidates as high density information storage media[12].  

K0.5Na0.5NbO3 (KNN) lead-free piezoelectric ceramics have been considered as the substitute for lead zirconate titanate (PZT) due to the large piezoelectric coeffiecient (d33) and high Curie temperature (TC)[13-14]. Now,KNN:Er3+ and KNN:Er3+/Yb3+ display the upconversion luminescence modulation performance[15-17]. With visible light irradiation, the luminescent switching contrast (ΔRt)up to about 60% is observed in KNN:Sm3+ ceramics[18]. Eu3+ doped inorganic materials have also been proved to possess excellent photochromic behavior under UV irradiation, like Sr2SnO4:Eu3+, BaMgSiO4:Eu3+, Sr3SnMO7: Eu3+[19-21]. The underlying mechanism may be various from the valence state transition of Eu3+ to volatilizationinduced photochromism[19]. Upon to now, photoluminescence and ferroelectric property of KNN:Eu3+ have been studied, while its luminescence based on PC reactions has not been reported[22-23].  

In this study, Eu3+ doped K0.5Na0.5NbO3 (KNN:xEu) powder with uniform morphology was fabricated by the hydrothermal method. KNN:xEu ceramics were obtained by calcining the precursor powder at high temperature. Hydrothermal method displays many merits in regulating the grains size, distribution and lower preparation temperature. Upon UV light irradiation and thermal stimulus, KNN:xEu ceramics display both photochromic reaction and tunable luminescence behavior with excellent reversibility. High value of ΔRt (up to 83.9%) is achieved for KNN:0.06Eu. Possible luminescence switching mechanism was also proposed, regarding the energy transfer between luminescence center and color center. 

1 Experimental

The Eu3+ doped K0.5Na0.5NbO3 (KNN:xEu) (x=0, 0.02, 0.04, 0.06, 0.08, 0.10) precursor powder was prepared by the hydrothermal method. Sodium hydroxide (NaOH), potassium hydroxide (KOH), niobium pentoxide (Nb2O5) and europia (III) oxide (Eu2O3) served as raw materials. First, KOH and NaOH were weighed according to K+/Na+ ratio of 3:1. They dissolved in distilled water to form a solution in which the concentration of OHchanged from 10 mol/L to 12 mol/L. Thereafter, Nb2O5 was added to the above solution and stirred for 30 min. The obtained suspension was placed in a 100-mL reaction kettle up to 70% of the total volume. Then the reaction kettle was sealed in a stainless-steel tank and heated at 200 ℃ for 12 h. The resulting products were obtained by centrifugation, washing and drying. The synthesized powders were granulated with 6wt% polyvinyl alcohol(PVA) binder, and then pressed into 12 mm disk-shaped pellets. KNN:xEu ceramics were prepared by calcination at 1140 ℃ for 4 h.  

The crystal structure was characterized by powder X-ray diffraction (D/max-2400, Rigaku, Japan) using a Cu Kα radiation. The microstructures of the ceramics were analyzed by scanning electron microscopy (S-4800,Hitachi, Japan). The UV-VIS-NIR spectrophotometer (Cary 5000, Agilent) were used to test the diffuse reflectance spectra. The coloration of the ceramics was carried out under the UV-light (λ<400 nm) output of a 300 W Xe lamp (PLS-SXE300, Beijing Zhongjiaojinyuan Technology Co. Ltd). The luminescence spectra and luminescence quenching behavior (at 300, 350, 390, 407 and 450 nm) were characterized by a fluorescence spectrophotometer (F-7000, Hitachi, Japan).  

2 Results and discussion

Fig. 1(a) shows the XRD patterns of the KNN precursor powder synthesized at 200 ℃ for 12 h (K+/Na+=3 : 1) with [OH] concentration from 10 mol/L to 12 mol/L. When [OH] was 10 mol/L, small amount of NaNbO3 was observed in the sample, except for the main perovskite phase of KNN (PDF #77-0038). With the increase of [OH-] from 11 mol/L to 12 mol/L, the NaNbO3 phase gradually disappeared. This result indicates that slight increase of [OH-] concentration can speed up the reaction process and facilitate the formation of pure KNN phase. Fig. 1(b) shows XRD patterns of the KNN:xEu (x=0.02, 0.04, 0.06, 0.08, 0.10) powders obtained by the hydrothermal method. Pure KNN phase has  been obtained with [OH] of 11 mol/L, indicating that Eu3+ is helpful for the formation of pure KNN phase. In Fig. 1(c), with increasing Eu3+ concentration, (110) diffraction peak moves slightly to a higher angle, suggesting that Eu3+ has partly replaced K+ and Na+ of the KNN matrix. The ion radius of Eu3+ (0.112 nm) is smaller than those of Na+ (0.139 nm) and K+ (0.164 nm), resulting in the shrink of the crystal lattice[22]

Fig. 1 XRD patterns of the precursor powders synthesized at 200 ℃ for 12 h (K+/Na+=3:1) with different [OH-] concentrations (a), XRD patterns of KNN:xEu (x=0.02, 0.04, 0.06, 0.08, 0.10) powders synthesized at 200 ℃ for 12 h (K+/Na+=3:1, [OH-]=11 mol/L) (b) and zoomed XRD patterns from (b) within 30°–33° (c) 

Fig. 2 displays the surface morphology of the KNN:0.06Eu ceramics calcined at 1140 ℃. Regular grains with cubic shape are well crystallized, with average size of about 0.3 μm. This grain size is very similar to those of hot-press sintered KNN ceramics and solidstate sintered KNN:Er ceramics[15, 24]. Rare earth (like Er3+ and Eu3+) may inhibit the grain growth of KNN ceramics[15]. According to the EDS analysis, the molar ratio of K+ and Na+ of the ceramics is almost 0.78, which is different from the initial composition. The radius of K+ (0.164 nm) is much bigger than that of Na+ (0.139 nm), so it is much more difficult for K+ to diffuse into the crystal lattice[25].

Fig. 2 SEM image of KNN:0.06Eu ceramics synthesized at 1140 ℃ for 4 h

Fig. 3(a) shows the photoluminescence excitation (PLE) spectra of the KNN:0.06 ceramic and photoluminescence (PL) spectra of KNN:xEu ceramics. The PLE spectrum monitored at 618 nm reveals three characteristic absorption peaks, wherein the band at 395 nm corresponding to the 7F05L6 transition, the 465 nm band corresponding to the 7F05D2 and the 526 nm band corresponding to the 7F05D1. The emission spectra excited at 465 nm of KNN:xEu ceramics consists of a group of intense and sharp characteristic lines ranging within 570–750 nm, which mainly result from the characteristic f-f transition from the excited states 5D0 to the ground state 7FJ (J=0–4) of Eu3+ ions[23]. The strongest red emission at 615 nm comes from the 5D07F2 of Eu3+. The positions of emission peak and line shape of the emission spectra maintain the same, but the emission intensity of Eu3+ is a function of its doping concentration. With the increase of Eu3+ concentration, the red emission intensity gradually increased and reached maximum when x=0.08. The intensity decreases gradually beyond this critical concentration. 

Fig. 3 Excitaion (λem=618 nm) spectra of the KNN:0.06Eu ceramics and emission (λex=465 nm) spectra of KNN:xEu ceramics at room temperature (a) and the dynamic decay curves on Eu3+ concentrations for KNN:Eu samples under 465 excitation (b)

The dynamic decay curves on Eu3+ concentrations for KNN:Eu ceramics under 465 nm excitation are shown in Fig. 3(b). The lifetimes are well fitted by using second-order exponential as follows:  

Where I0 is the emission intensity at t0, A1 and A2 are constant, τ1 and τ2 are the lifetimes of the exponential component. 

With Eu3+ concentration increasing, the average lifetime gradually increases and reaches maximum when x=0.08, which is mainly related to the radiative energy transfer between Eu3+ neighbor ions. With Eu3+ concentration further increasing, the ion distance between Eu3+-Eu3+ becomes shorter, possibly resulting in the decrease of average lifetime[26-27]

Fig. 4 shows the reflectance spectra and photochromic reaction of KNN:xEu ceramics under UV irradiation and thermal stimulus. Fig. 4(a) gives the reflection spectra of KNN:0.06Eu ceramic irradiated under UV light for different time (0 s to 3 min). The reflection intensity decreases significantly in the region 400–750 nm with increasing irradiation time, indicating that the irradiated sample absorbs part of the visible light. The absorption ratio can be evaluated by comparing the difference of the reflective intensity before (RE1) and after (RE2) irradiation using the formula: Δabs = RE1–RE2 (%)[28]. Fig. 4(b) shows that KNN:0.06Eu has a broad absorption from 400 nm to 750 nm, with the absorption maximum at about 566 nm. In the inset of Fig. 4(b), upon UV light irradiation for 3 min, the color of the sample turns from milky white to gray as a colored sample. The colored sample recovered its initial stage when heated at 200 ℃ for 10 min (marked as Δ). In Fig. 4(c), KNN:0.06Eu ceramic shows the best photochromic behavior, which is indicated by the change of Δabs (at 566 nm) with Eu3+ concentration. KNN:0.06Eu ceramic is treated repeatedly with UV light irradiation (3 min) and thermal stimulus (200 ℃ for 10 min). As seen in Fig. 4(d), the reflectance intensity at 566 nm shows no obvious degradation for 10 cycles, showing good fatigue resistance of the color switching. 

Fig. 4 Reflectance spectra for the KNN:0.06Eu by UV light irradiation (0 s, 30 s, 60 s, 2 and 3 min) (a), difference absorption (Δabs) spectra for the KNN:0.06Eu by UV irradiation 3 min with inset showing photographs of color changes of ceramic before and after UV irradiation (b), the Δabs vs Eu concentration (c) and reflectance intensity changes (d) of KNN:0.06Eu by alternating UV irradiation and heat treatment

The photoluminescence spectra of KNN:0.06Eu ceramics before and after UV irradiation for 3 min are shown in Fig. 5(a). Upon UV light irradiation, the luminescence intensity decreases significantly, showing strong luminescence quenching effect. The quenching behavior may be explained by the parameter ΔRt (luminescence modulation ratio) using the equation: ΔRt = (R0–Rt)/R0×100 (%), where R0 and Rt are the luminescence intensity before and after light irradiation, respectively[10,15]. Fig. 5(b) shows the change of ΔRt at 615 nm as a function of Eu concentration. Upon UV light irradiation for 3 min, the emission intensities of all samples decreased significantly. The calculated ΔRt values of the KNN:xEu samples are 34.2% (x=0.02), 47.6% (x=0.04), 83.9%(x=0.06), 46.4% (x=0.08) and 48.3% (x=0.10), respectively. The maximum ΔRt value (83.9%) is achieved for KNN:0.06Eu3+, which is comparably high among rare earth doped KNN ferroelectrics. 

Fig. 5 Changes of emission spectra (λex=465 nm) of KNN:0.06 Eu ceramics before and after UV irradiation for 3 min (a), luminescence switching ratio (ΔRt) at 615 nm as a function of Eu concentration (b) and ΔRt of KNN:0.06Eu ceramics by alternating UV light irradiation and thermal stimulus for 7 cycles (c)

Furthermore, KNN:0.06Eu ceramic was subjected to alternating UV light irradiation (3 min) and the thermal stimulus (200 ℃ for 10 min) for 7 cycles. For each cycle, the PL spectra were recorded immediately after UV light irradiation and the thermal stimulus. Accordingly, ΔRt at 615 nm was calculated and the result was shown in Fig. 5(c). The decreased emission intensity after UV irradiation is able to almost recover its initial state upon heating treatment. ΔRt has no obvious degradation for at least 7 cycles, illustrating good reversibility of the luminescence switching. 

In order to further investigate the effect of irradiation wavelength on the tunability of luminescence, the emission spectra of KNN:0.06Eu ceramic (λex=465 nm) under different wavelength irradiation (300, 350, 390, 407 and 450 nm) were measured, as shown in Fig. 6(a). For this experiment, a xenon lamp equipped with an F-7000 spectrometer acted as the irradiation light source. In Fig. 6(a), with irradiation time (20 s, 40 s, 1 min, 2 min,3 min and 4 min) increasing, ΔRt of the KNN:0.06Eu gradually increases. ΔRt decreases dramatically with increasing irradiation wavelength, and the largest ΔRt is obtained with irradiation wavelength of 300 nm. This result verifies that the luminescence of KNN:0.06Eu can be effectively tuned by UV light irradiation. There are some reports of Eu based photochromic materials which is also effectively excited by UV light, like Sr2SnO4:Eu3+, BaMgSiO4:Eu3+, Sr3SnMO7:Eu3+, although the origins of PC processes may be varied[19-21]

Fig. 6(b) illustrates the possible mechanism of luminescence switching based on PC reactions. It may be explained by the free or trapped charge carrier and resonance energy transfer (RET) mechanism, which bases on the overlap of the emission band of Eu3+ ions and the absorption bands of photochromic KNN ceramics[29-30]. Due to many vacancy-related defects in KNN host, additional defect energy levels form within the forbidden gap of KNN. Upon UV light illumination, the electrons from the valence band (Ev) are excited to higher energy level (defect level), and are trapped by the oxygen vacancies (Vo)[10,15]. The excited holes are also trapped by the K and Na vacancies (VA). Both of these defects are color centers which have a broad absorption band in the range from 400 to 700 nm (Fig. 4). The emission positions of Eu3+ overlaps with the absorption band range of these color centers (Fig. 3, 4 and 5). When Eu3+ is excited, the emission can be effectively absorbed by the color centers through the resonance energy transfer, resulting in the significant decrease of the Eu3+ luminescence intensity[19]. When the ceramic is heated to 200 ℃ for 10 min, the colored sample fade back into their original color state. Meanwhile, electrons can be released from the traps and the color centers disappears. Correspondingly, the emission intensities of Eu3+ are also recovered. 

Fig. 6 Luminescence modulation ratio (ΔRt) as a function of irradiation time under different irradiation wavelengths (a) and schematic diagram of luminescence modulation upon photochromic reactions for KNN:xEu ceramics (VO is oxygen vacancy, and VA is K and Na vacancy) (b)

The luminescence mechanism of KNN:xEu is similar with that of Sr3SnMO7:Eu which is also irradiated by UV (254 nm) light[19]. Other KNN based ceramics, like KNN:Er, KNN:Er/Yb, KNN:Ho/Yb and KNN:Sm, have been reported to show photochromic and luminescence switching behavior by visible light (407 nm) illumination[16-18]. Furthermore, the emission intensity reached maximum for KNN:0.08Eu, while the largest luminescence switching ratio is achieved for KNN:0.06Eu. The luminescence switching ratio mainly relates to the energy transfer between luminescence centers and color centers. And the color centers relates to the traps or defects within the host lattice. The unique feature of KNN:xEu is possibly caused by the varied trap depths or distributions of defects within this system, which will be systematically studied in the future. 

3 Conclusions

Eu doped K0.5Na0.5NbO3 (KNN:xEu) precursor powders were hydrothermally synthesized at 200 ℃ for 12 h (K+/Na+=3:1, [OH] =11 mol/L). The phase-pure perovskite KNN:xEu ceramics exhibit homogeneous morphology when calcined at 1140 ℃ for 4 h. KNN:xEu ceramics show remarkable photochromic and luminescence switching behavior. Upon UV light irradiation, the sample colors change from the initial milky white to dark gray. By alternating UV irradiation and thermal stimulus, the luminescence modulation ratio (ΔRt) of KNN:0.06Eu reach high values of 83.9%. These results indicate that KNN:xEu system is a potential candidate for novel photo-electronic device. 


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