Luminescence properties of alkali metal ions sensitized CaFCl:Tb3+ nanophosphors

Luminescence properties of alkali metal ions sensitized CaFCl:Tb3+ nanophosphors

JOURNAL OF RARE EARTHS, Vol. 33, No. 10, Oct. 2015, P. 1026 Luminescence properties of alkali metal ions sensitized CaFCl:Tb3+ nanophosphors LIN Lin ...

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JOURNAL OF RARE EARTHS, Vol. 33, No. 10, Oct. 2015, P. 1026

Luminescence properties of alkali metal ions sensitized CaFCl:Tb3+ nanophosphors LIN Lin (林 林)1,2, LIN Hui (林 慧)1,2, WANG Zhezhe (王哲哲)1,2, ZHENG Biao (郑 标)1,2, CHEN Jixing (谌基兴)1,2, XU Senyuan (徐森元)1,2, FENG Zhuohong (冯卓宏)1,2, ZHENG Zhiqiang (郑志强)1,2,* (1. College of Physics and Energy, Fujian Normal University, Fuzhou 350007, China; 2. Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, Fuzhou 350007, China) Received 23 January 2015; revised 20 July 2015

Abstract: A series of CaFCl:Tb3+ and CaFCl:Tb3+,A+ (A=Li, Na and K) nanophosphors were synthesized by the one-step sol-gel method, which were reported for the first time. The sample consisted of monodisperse particles, the average size of which was 37 nm. The emissions of Tb3+ ions and oxygen defects OF‫ ׳‬were demonstrated in the CaFCl:Tb3+ samples. The former was made up of several peaks at 488, 545, 587 and 623 nm, ascribed to 5D4→7FJ (J=6–3) transitions of Tb3+ ions. The latter was shown as a broad band peaked at about 450 nm. Alkali metal ions A+ (A=Li, Na and K) were introduced as the charge compensators to improve the luminescence of samples. The influence of charge compensators on the emissions of Tb3+ ions and oxygen defects OF‫ ׳‬was investigated by the measurement of fluorescence spectra and luminescence decay curves. The results indicated that all the charge compensators weakened the defects emission. Furthermore, Li+ ion was the best charge compensator, because it not only reduced the defects emission but also increased the emission intensity of Tb3+ significantly. Our results suggested that this nanophosphor sensitized by the charge compensator might broaden potential applications of rare-earth doped CaFCl. Keywords: luminescence; CaFCl:Tb3+; alkali metal ions; charge compensator; rare earths

Alkaline-earth fluorochlorides MFCl (M=Ca, Sr, Ba) are very important and attractive from both fundamental and applications[1]. Besides, they are suitable hosts of rare-earth (RE) doped phosphors, in some sense, more suitable than oxides, which have been widely reported in recent years[2,3]. Because of the low phonon energy of MFCl (~300 cm–1, even lower than fluoride[4]), the competitive phonon-assisted non-radiative deactivation may be inhibited to the maximum. Consequently, high efficient radiative emissions of RE ions can be expected. RE3+ doped MFCl have been used in several technological applications, e.g., pressure calibrators and X-ray storage phosphors[5,6]. It can also be applied for new research fields, such as light-emitting diodes (LEDs), upconversion and quantum cutting. For example, monodisperse SrFCl:Yb3+,Er3+ nanocrystals show their stronger upconversion emissions than those of the SrF2:Yb3+,Er3+ [1]. Nevertheless, RE3+ doped MFCl have a charge imbalance which results in defect formation, blocking the efficient emission of RE ions[4,7]. Therefore, curbing the defects is important for achieving high efficient luminescence. Charge compensation is considered as an effective technique to curb the defects and improve the lumines-

cent efficiency. The utilization of alkali metal ions A+ (A=Li, Na and K) as charge compensation has been studied in different host lattices[8–10]. However, no attention has been paid to the effect of charge compensation on the luminescent properties of the MFCl compounds. In this paper, CaFCl:Tb3+ nanophosphors were synthesized by the one-step sol-gel method, which was reported for the first time. Trifluoroacetic acid and trichloroacetic acid were both added, acting as fluorine and chlorine source, respectively. This method is more convenient than the MF2 seed-based chlorination route[1]. The luminescent properties of CaFCl:Tb3+ were also investigated. Furthermore, alkali metal ions A+ were introduced in the CaFCl as charge compensators. And the effect of charge compensators on the luminescence properties of CaFCl:Tb3+ nanophosphors was studied in detail. For high luminous efficiency of Tb3+ is known to all, low phonon energy of CaFCl host and the nano-size of the samples, this nano-phosphor has potential application values as a fluorescent nano-thermometer. In addition, the Tb3+-doped nano-phosphor has been reported as a viable indicator for the optical sensing of temperature[11].

Foundation item: Project supported by the National Natural Science Foundation of China (11204039, 51202033), the Science Foundation of the Educational Department of Fujian Province of China (JA13084) and the Natural Science Foundation of Fujian Province of China (2015J01243) * Corresponding author: ZHENG Zhiqiang (E-mail: [email protected]; Tel.: +86-591-22868522) DOI: 10.1016/S1002-0721(14)60521-4

LIN Lin et al., Luminescence properties of alkali metal ions sensitized CaFCl:Tb3+ nanophosphors

1 Experimental The CaFCl:Tb3+ and CaFCl:Tb3+,A+ (A=Li, Na and K) samples were synthesized by the one-step sol-gel method which was reported for the first time. The reactants including Ca(CH3COO)2·2H2O (A.R.), CF3COOH (A.R.), CCl3COOH (A.R.), Tb4O7 (99.99%, A.R.), CH3COOA (A=Li, Na and K) (A.R.) were weighed by appropriate stoichiometric ratio. The series of the samples were CaFCl:x%Tb3+ (x=15, 20, 25, 28, 30) and CaFCl:10%Tb3+, 10%A+. All the atomic ratios of doped ions were relative to Ca2+ sites. The CaFCl:Tb3+,A+ samples were prepared as follows. First, Tb4O7 was dissolved in nitric acid (3:1) and dried out. 0.02 mol Ca(CH3COO)2·2H2O, 0.002 mol CH3COOA and 0.01 mol NH4Cl were dissolved in 12.5 mL water. Then, the dried nitrate was dissolved in the mixed solution, and trifluoroacetic acid, trichloroacetic acid, anhydrous ethanol, isopropanol were added. Second, the solution was stirred at 70 ºC for 3 h to form the sol. Then, the sol kept at 70 ºC for 36 h to form the gel. At last, the gel was heated at 600 ºC for 1 h in Ar atmosphere to obtain the samples. The X-ray diffraction patterns were examined using a Rigaku MiniFlex II X-Ray diffractometer with Cu Kα1 radiation (λ=0.154 nm). The transmission electron microscopy (TEM) observations were performed on a JEOL-2010 TEM with an acceleration voltage of 200 kV. The fluorescence spectra were carried out using a Fluorolog-3 spectrophotometer with a xenon lamp as the excitation source. The luminescence decay curves were measured on a FLS920 spectrofluorometer.

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CaF2 (JCPDS 35-0816). The results indicate that the one step sol-gel method is a feasible route to prepare CaFCl:Tb3+ nanophosphors. According to Fig. 1, the average crystallite size is 36.7 nm estimated via the Scherrer formula[12]. Fig. 2 shows the TEM micrograph of CaFCl:Tb3+,A+ nanophosphors. It can be seen from Fig. 2 that the samples consist of non-agglomerated, monodisperse particles. The average size of the particles was about 37 nm, according with the calculated result by the Scherrer formula. 2.2 Photoluminescence properties The excitation spectrum and the emission spectrum of CaFCl:10%Tb3+ are shown in Fig. 3. Either of the spectra consists of several peaks and a broad band. The former can be attributed to Tb3+ ions obviously. The major excitation peaks at 317, 341, 351, 369, 376 and 484 nm can be ascribed to the transitions from 7F6 to 5H7, 5L7, 5L9, 5 L10, 5D3 and 5D4 of Tb3+, respectively. Under 376 nm excitation, the samples exhibit emissions located at 488, 545, 587 and 623 nm, ascribed to transitions from the 5D4 level to the 7FJ (J=6–3) levels of Tb3+ ions[13], as seen from Fig. 3(b). The origin of the latter will be discussed in the next paragraph. In this paragraph, we will discuss the origin of the broad band, which can be seen from either of the fluorescence spectra (Fig. 3). For further discussion, this band was separated from the spectrum. The separating method is described as follows: First, since the fluores-

2 Results and discussion 2.1 Crystallization and morphology The XRD patterns of CaFCl:Tb3+,A+ samples are depicted in Fig. 1. As shown in Fig. 1, the diffraction peaks of each sample can be well indexed to the pure tetragonal CaFCl (JCPDS No. 24-0185), only except some weak peaks in the range of 27º–30°, attributed to

Fig. 1 X-ray diffraction pattern of CaFCl:Tb3+,A+ nanophosphors

Fig. 2 TEM images of CaFCl:Tb3+,A+ nanoparticles

Fig. 3 Excitation spectrum (λem=545 nm) (a) and emission spectrum (λex=376 nm) (b) of CaFCl:10%Tb3+ nanophosphors (Dash line: separated excitation/emission band of oxygen defects by Gaussian fitting)

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cence of Tb3+ is only made up of some peaks, and the start point and end point of each peak is known, each peak was roughly subtracted in the fluorescence spectrum, instead of a line from start point to end point. Second, the remaining was fitted by single Gaussian peak, represented in Fig. 3 as dash lines. It can be seen that the fitted Gaussian peak was in a good coincidence with corresponding curve. Third, the Gaussian peak was subtracted in the original fluorescence spectrum and the result represented the fluorescence of Tb3+ ions. The broad emission band (~450 nm) and the corresponding excitation band (~350 nm) were attributed to the oxygen defects OF‫[ ׳‬14]. The reasons are represented as follows: It has been reported that oxygen is often introduced into the host lattice of alkaline-earth fluorochlorides during the preparation[7]. Then the oxygen defects (O2–) substitute for halide anions to form electronegativity defect OF‫׳‬ and OX‫[ ׳‬15]. A broad band can also be observed in the emission spectrum of BaFCl:Eu3+, attributed to OF‫[ ׳‬4]. Therefore, here we draw the same conclusion. Doped Tb3+ ions occupy Ca2+ site to form electronpositivity defects TbCa·, which will facilitate the formation of oxygen defects OF‫׳‬. The oxygen defects OF‫ ׳‬in the MFCl have several absorption bands[7], located in the near ultraviolet region and visible region which partly overlap with the absorption of Tb3+. On the other hand, the main quenching mechanism of 5D4→7FJ emission of Tb3+ is the energy migration to killers[16]. Here these defects serve as killers to Tb3+ emission. Therefore, the oxygen defects surely impair luminescent intensity of Tb3+. 2.3

Effect of co-doped charge compensation on CaFCl:Tb3+ luminescence

To remove the emission of defects, alkali metal ions A+ (A=Li, Na and K) were introduced as the charge compensators. A+ ions occupy lattice sites of Ca2+ in CaFCl nanophosphors and form electronegativity defects ACa‫׳‬, which will promote the formation of electronpositivity defects TbCa· and curb the formation of electronegativity defects OF‫׳‬. Therefore, A+ doping is expected to remove the emission of defects. A+ are added into the material, the molar ratio of which is equal to Tb3+ to maintain the charge balance: 2Ca2+=TbCa·+ACa‫[ ׳‬17,18]. Fig. 4(a) shows the emission spectra of Tb3+-doped samples co-doped with and without each charge compensator. The separated emission band of oxygen defects of each spectrum is also depicted in Fig. 4(a). The emission intensity of Tb3+ and oxygen defects is represented in Fig. 4(b). Based on Fig. 4(b), each of the charge compensators decreases the emission intensity of oxygen defects, according with our suggestion. On the other hand, Li+ doping and Na+ doping strengthen Tb3+ emission, the former does significantly, the latter does weakly, but K+ does not. The possible reasons are given as follows.

Fig. 4 (a) Solid line: emission spectra of CaFCl:10%Tb3+,10%A+ and CaFCl:10%Tb3+ nanophosphors (λex=376 nm) (dash line: separated emission band of oxygen defects by Gaussian fitting) and (b) emission intensity of Tb3+ (545 nm, taking off the defect emission) and oxygen defects

The ionic radius size of Li+, Na+, K+ and Ca2+ is 0.059, 0.116, 0.133 and 0.118 nm, respectively[19]. The difference in the ionic radii brings about diversity in the crystal lattice structure and influences the luminescent properties. The radius of Li+ is much smaller than that of Ca2+, which leads to easy substitution without introducing other defects. Furthermore, the substitution may break the crystal field symmetry of Tb3+ and release spin-forbidden rule of 4f-4f transitions more thoroughly[20]. All of the above results in the remarkable enhancement of luminescence of Tb3+ [21]. On the contrary, Na+ and K+ are larger, so they are more difficult to be fully introduced into Ca2+ sites, and they bring in other cation vacancies[22] that play a role in concentration quenching centers[23,24]. In the case of doping K+, things get worse because K+ ion is larger than Ca2+. Therefore, while samples are co-doped with Na+ or K+, an obvious enhancement of Tb3+ cannot be observed due to the introduced quenching centers, although the emission of oxygen defects is surely weakened. To confirm this suggestion, the decay curves of Tb3+ emission of the four samples were measured (Fig. 5). The fast decay exponent derived from oxygen defects was removed. Generally speaking, the decay of Tb3+ emission follows the single exponential decay without any acceptor,

LIN Lin et al., Luminescence properties of alkali metal ions sensitized CaFCl:Tb3+ nanophosphors

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tential application values, due to high luminous efficiency of Tb3+, low phonon energy of CaFCl host and nano-size of samples.

References:

Fig. 5 Normalized decay curves of Tb3+ 545 nm emission (λex= 376 nm) for CaFCl:10%Tb3+,10%A+ and CaFCl:10%Tb3+ nanophosphors

but follows a non-exponential decay (Inokuti-Hirayama equation) with acceptors[16,25]. In the former case, the decay is described by I = I 0 exp( −t / τ 0 ) (1) where I0 is the emission intensity at time t=0, τ0 is the intrinsic lifetime of the donors in the absence of acceptors. In the latter case, the formula of the decay is given as: I = I 0 exp( −t / τ 0 − Ct

3/ n

)

(2)

where C is a parameter containing the concentration of donors (here is Tb3+) and the interaction strength between donors and acceptors, n≥6 depending on the nature of the multipolar interaction. From Eqs. (1) and (2), it can be seen that the emission of Tb3+ will decay faster with the defects serving as acceptors. According to Fig. 5, in Li-doping case, the decay is the slowest and close to single exponential decay, indicating that the defects are rare. In the case of doping Na+ or K+, decay of Tb3+ emission is faster than that of doping Li+, inferring that other quenching centers are imported. In conclusion, doped Li+ not only weakens the emission of defects but also strengthens the emission intensity of Tb3+ significantly. Therefore, Li+ ion is the best charge compensator.

3 Conclusions In summary, a series of CaFCl:Tb3+ and CaFCl:Tb3+,A+ (A=Li, Na and K) nanophosphors were prepared by the one-step sol-gel method. The luminescent properties of CaFCl:Tb3+ were investigated. The effect of charge compensator A+ (A=Li, Na and K) co-doped with Tb3+ in CaFCl was also studied. The oxygen defects OF‫ ׳‬were suitably restrained by the addition of A+ in the host. Moreover, Li+ ion was the best charge compensator, because it not only restrained the defects emission but also enhanced the emission intensity of Tb3+. This nanophosphor sensitized by the charge compensator had po-

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