Pb2+ doped CaY2Al4SiO12 garnet phosphor

Pb2+ doped CaY2Al4SiO12 garnet phosphor

Optik - International Journal for Light and Electron Optics 202 (2020) 163541 Contents lists available at ScienceDirect Optik journal homepage: www...

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Optik - International Journal for Light and Electron Optics 202 (2020) 163541

Contents lists available at ScienceDirect

Optik journal homepage: www.elsevier.com/locate/ijleo

Pb2+ doped CaY2Al4SiO12 garnet phosphor Vijay Singha,*, Manoj K. Tiwaria,b a b

T

Department of Chemical Engineering, Konkuk University, Seoul, 05029, Republic of Korea Department of Physics, MIET, Shahapur, Bhandara, 441906, India

A R T IC LE I N F O

ABS TRA CT

Keywords: Sol-gel XRD Pb2+ CaY2Al4SiO12 Photoluminescence UV

CaY2Al4SiO12 garnet phosphor doped with different Pb2+ concentrations were synthesized using sol-gel method. X-ray diffraction analysis confirmed that all samples crystallized in the cubic structure of garnet with la3d space group. Emission and excitation bands of Pb2+ doped CaY2Al4SiO12 were observed at 362 and 258 nm, respectively. The maximum emission was obtained for the sample doped with 0.07 mol of Pb2+. Stokes shift of CaY2Al4SiO12:Pb2+ phosphor was calculated to be 11,135 cm−1. The luminescence behavior of the Pb2+ ion in CaY2Al4SiO12 garnet is discussed.

1. Introduction Among inorganic host materials, garnet is one of the most important host materials for activator ions due to its excellent physical stability, high luminescent efficiency, good chemical stability, and high thermal properties [1–4]. Garnet-type structures have been used for different application due to their structural flexibility. Garnet structure has a general formula of A3B2C3O12, where A, B, and C are eight-, six-, and four coordinated with the surrounding O, forming dodecahedron, octahedron, and tetrahedron, respectively [3,5,6]. Garnets possess a cubic crystal structure with a complex arrangement of different cations in the unit cell. Garnet phosphors are unique in their tunability of the luminescence properties through variations in the {A}, [B] and (C) cation sub lattice [3]. Due to the flexibility of garnet structure, it allows replacing ions at dodecahedral, octahedral and tetrahedral sites [7]. The dodecahedral A site can be occupied by trivalent or divalent ions such as Y3+, Lu3+, Gd3+, La3+, Ca2+, and Mg2+ ions [7,8]. The octahedral B site can be occupied by Al3+, Ga3+, Sc3+, Sb3+, and In3+ while the tetrahedral C site contains differently charged ions such as Ga3+, Al3+, Si4+, and Ge4+ ions. In recent years, silicate-based MY2Al4SiO12 (where M = Ca, Ba, Mg) garnet compounds have been investigated by some research groups [8–12]. Katelnikovas et al. [13] have reported optical properties of yellow-emitting CaY2Al4SiO12:Ce3+ garnet. CaY2Al4SiO12:Ce3+ garnet phosphors showed a broad band emission in the range of 460–750 nm. Pan et al. [8] have studied red emission from Ce3+/Mn2+ co-doping suited garnet MgY2Al4SiO12 host and found that color-tunable emission of MgY2Al4SiO12:Ce3+,Mn2+ makes it a promising single-phased phosphor for warm WLEDs. Park et al. [14] have determined photoluminescence properties of yellow-emitting Y2BaAl4SiO12:Ce3+ phosphors and reported that Y2BaAl4SiO12:Ce3+ phosphors are promising yellow phosphors for the fabrication of WLEDs. Photoemission properties of a microcrystalline-powder Tb3+-doped CaY2Al4SiO12 garnet have been reported elsewhere [1]. The Tb3+-doped CaY2Al4SiO12 garnet showed a green-emission characteristic at around 544 nm with an excitation at 271 nm. The CaY2Al4SiO12 garnet contains {CaY2} sites that are dodecahedrons, [Al3+] sites that are octahedrons, and (Al2Si) sites that are tetrahedrons [1,3]. It is well known that photoluminescence properties of activator ions depend on the crystal structure and uniform distribution in the host lattice. Heavy metals ions such as Ti+, Pb2+, and Bi3+ are interesting due to their diverse optical properties. Recently, these



Corresponding author. E-mail address: [email protected] (V. Singh).

https://doi.org/10.1016/j.ijleo.2019.163541 Received 5 July 2019; Accepted 3 October 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.

Optik - International Journal for Light and Electron Optics 202 (2020) 163541

V. Singh and M.K. Tiwari

Table 1 Detailed information of sample composition and starting materials. Starting materials CaY2Al4SiO12:0.01Pb CaY2Al4SiO12:0.03Pb CaY2Al4SiO12:0.05Pb CaY2Al4SiO12:0.07Pb CaY2Al4SiO12:0.09Pb CaY2Al4SiO12:0.11Pb

Ca = 0.2361 g Ca = 0.2361 g Ca = 0.2361 g Ca = 0.2361 g Ca = 0.2361 g Ca = 0.2361 g

Y = 0.7660 g Y = 0.7660 g Y = 0.7660 g Y = 0.7660 g Y = 0.7660 g Y = 0.7660 g

Al = 1.5 g Al = 1.5 g Al = 1.5 g Al = 1.5 g Al = 1.5 g Al = 1.5 g

Si = 0.0600 g Si = 0.0600 g Si = 0.0600 g Si = 0.0600 g Si = 0.0600 g Si = 0.0600 g

C.A = 3.0736 g C.A = 3.0736 g C.A = 3.0736 g C.A = 3.0736 g C.A = 3.0736 g C.A = 3.0736 g

Pb = 0.0033 g Pb = 0.0099 g Pb = 0.0165 g Pb = 0.0231 g Pb = 0.0298 g Pb = 0.0364 g

Ca = Ca(NO3)2·4H2O, Y = Y(NO3)3·6H2O, Si = SiO2, Al = Al(NO3)3·9H2O, C.A = Citric acid, Pb = Pb(NO3)2.

heavy metal ions have been studied in a considerable number of host materials [15–17]. In particular, the luminescence of Pb2+ is quite diverse. It depends strongly on the host lattice. Pb2+ has attracted great scientific, medical, and industrial interest. For example, Pb2+ doped BaSi2O5 shows a broad band emission around 350 nm under UV excitation that is one of the earliest known phosphors for photocopying lamps [18]. (Ba, Zn, Mg)3Si2O7:Pb2+ phosphor has been used for fluorescent sun lamps [19]. In recent years, luminescence properties of lead (Pb2+) ions have been investigated in different hosts such as borate, silicate, aluminate, and phosphate [19–22]. In the present study, we investigated photoluminescence properties of Pb2+ ion in CaY2Al4SiO12 garnet. In the past decade, several synthesis methods have been developed and successfully used to synthesize garnet phosphors [23–26]. Among them, sol-gel method is considered as one of the most important techniques for the synthesis of various phosphor materials. Sol-gel method offers many advantages over conventional method such as higher purity, narrower particle size distribution, and lower energy consumption. To the best of our knowledge, no study has reported the preparation of divalent lead ion doped CaY2Al4SiO12 garnet by sol-gel process. Thus, the objective of this study was to prepare CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphors by sol-gel process and determine their photoluminescence (PL) properties. 2. Experimental A series of CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphors were prepared by using sol-gel method. In a typical synthesis, all starting materials of high purity were used without further purification. Details of sample composition and starting materials are given in Table 1. In a typical synthesis, stoichiometric quantity of starting materials such as Ca(NO3)2∙4H2O, Y(NO3)3∙6H2O, Al (NO3)3∙9H2O, SiO2, Pb(NO3)2, and citric acid (citric acid/metal ion 2:1, molar ratio) were firstly dissolved in 10 mL of deionized water under stirring at 500 rpm. A transparent aqueous solution was obtained after stirring for 1 h. The resultant transparent solution was kept at 110 °C in oven until homogeneous dried gel formed. The dried gel obtained was ground and sintered at 400 °C for 2 h. Finally, the resultant brown residual sample was fully ground and annealed at 1100 °C for 3 h in air to obtain the final sample. XRD patterns of samples were recorded using a RIGAKU Miniflex-II diffractometer. Cu-Kα radiation (λ = 1.5406 Å) was used an X-ray source. XRD patterns were taken at scan rate of 5°/minute in 2θ range of 10° to 80°. Photoluminescence measurements were carried out at room temperature on a Shimadzu RF-5301PC spectrofluorophotometer equipped with a Xenon flash lamp. 3. Results and discussion 3.1. XRD X-ray powder diffraction patterns of CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphors are shown in Fig. 1. Sharp diffraction

Fig. 1. Powder XRD pattern of CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphors. 2

Optik - International Journal for Light and Electron Optics 202 (2020) 163541

V. Singh and M.K. Tiwari

Fig. 2. Photoluminescence spectra of the CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphor (a) Excitation spectrum (λem = 362 nm) and (b) Emission spectrum of (λexc = 258 nm).

peaks showed that good crystallization of CaY2Al4SiO12 host lattice was constructed. X-ray diffraction patterns of samples were found to be in a good agreement with standard diffraction peaks of Y3Al5O12 (JCPDF card No. 33–0040). All peaks revealed the cubic structure of garnet with la3d space group. These XRD patterns also revealed a few minor impurity peaks indicated by (*). According to Katelnikovas et al. [13], these peaks might belong to Ca2Al2SiO7. No impurity peaks related to Pb2+ ions were detected for CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphor, indicating that Pb2+ ions had been effectively incorporated into the garnet host. As known that ionic radii of Y3+ and Ca2+ ions in eight coordination are 1.019 Å and 1.120 Å, respectively. Pb2+ (r =1.29 Å) ions are most likely to substitute Ca2+ site owing to their similar radii charge balance. The crystallite size was estimated using the Scherrer equation, D = kλ/ β cos θ, where D was the average grain size, k was the shape factor, λ was the X-ray wavelength (1.5406 Å), β was the full width at half maximum of the strongest diffraction peak, and θ was the diffraction angle. The crystallite size value of CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphor was found to be in the range of 28–31 nm. 3.2. Photoluminescence PL excitation and emission spectra of CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) are presented in Fig. 2. PL excitation spectra (see Fig. 2a) were monitored at 362 nm, corresponding to the 3P1→1S0 transition of Pb2+ ions. As shown in the figure, there was a strong broad excitation band in this spectrum within the range of 220–330 nm due to 6s2–6s16p2 interconfigurational transitions of Pb2+ ions [27]. The broad excitation band centred at 258 nm was assigned to 1S0→3P1 transitions of Pb2+ ions. Emission spectra of CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphor are shown in Fig. 2b. The emission spectra also consisted of broad band in the range of 280–510 nm. The emission band centred at 362 nm corresponded to 3P1 excited state level to 1S0 ground state transitions of Pb2+ ions. These emission bands of all the samples were in the UV region of the electromagnetic spectrum. As seen in Fig. 2, with different Pb2+ doping concentrations, shapes and positions of the excitation and emission peaks exhibited no obvious changes. The variation in the emission intensity of strong emission (362 nm) as a function of Pb2+ concentration is shown in Fig. 3. The emission intensity increased with increasing Pb2+ doping concentration up to 0.07 mol and decreased as Pb2+ ion concentration increased further due to concentration quenching. The concentration quenching was caused by an energy transfer process among Pb2+ ions. The energy transfer process usually takes place from exchange interaction or multipole-multipole interaction. When the distance between two lead ions is less than 5 Å, then exchange interaction is dominating. However, when the distance between two lead ions is more than 5 Å, then multipole–multipole interaction is responsible for concentration quenching mechanism. To better understand the 3

Optik - International Journal for Light and Electron Optics 202 (2020) 163541

V. Singh and M.K. Tiwari

Fig. 3. Variation in the emission intensity of strong emission (362 nm) as a function of Pb2+ concentration.

exact mechanism responsible for quenching, the critical distance (Rc) needs to be evaluated using the following equation:

R c = 2(

1 3V )3 4πXC N

where Xc is the critical concentration, N is the number of cations in the unit cell, and V is the volume of the unit cell. For CaY2Al4SiO12:Pb2+, Xc = 0.07, N = 8, V = 1710.3Å3, respectively [28,29]. The calculated Rc value was about 18.00 Å. This value showed that concentration quenching in CaY2Al4SiO12:xPb2+ (x = 0.01 ≤ x ≤ 0.11) phosphor occurred due to multipole-multipole interaction. The Stokes shift of the synthesized phosphor CaY2Al4SiO12:Pb2+ was calculated to be 11,135 cm−1 using the excitation band at 258 nm and the emission band at 362 nm. The stoke shift value was close to those reported for SrAl2B2O7:Pb2+ (12,292 cm−1) and Li2SrSiO4:Pb2+ (10,653 cm−1) phosphor [30,31]. Energy levels of ns2 cations such as Pb2+, Sn2+, and Bi3+ are interesting. Transitions from ground state 1S0 to excited states of 3 P0, 3P1, 3P2, and 1P1 are spin forbidden [17,30–32]. The 1S0 ↔ 3P1 transition is frequently observed in photoluminescence measurements. Energy levels and possible electronic transitions of Pb2+ ions are shown in Fig. 4. When the sample was excited with 258 nm, the electrons in the ground state got excited to 3P1 state. Afterward, electron in excited state radiatively decayed to ground state 1S0, giving the emission at 362 nm. 4. Conclusion Inorganic garnet phosphors were prepared by a sol-gel method. The dependence of emission intensity on Pb2+ concentration for CaY2Al4SiO12 was studied in detail. The phosphor CaY2Al4SiO12:Pb2+ showed emission at 362 nm upon excitation with 246 nm

Fig. 4. Energy level diagram of Pb2+ ions. 4

Optik - International Journal for Light and Electron Optics 202 (2020) 163541

V. Singh and M.K. Tiwari

wavelength. It was observed that the optimum concentration of Pb2+ in CaY2Al4SiO12 was 0.07 mol. At this concentration, the critical transfer distance (Rc) was calculated to be 18 Å. The emission band of the prepared garnet samples is in the UV region. It may be relevant for application in UV lamps. Acknowledgements This paper was supported by the KU Research Professor Program of Konkuk University. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018M2B2A9065656). References [1] M.S. Bhagat, K.N. Shinde, N. Singh, M.S. Pathak, P.K. Singh, S.U. Pawar, V. Singh, Photoluminescence properties of green emitting CaY2Al4SiO12:Tb3+ garnet phosphor, Optik 161 (2018) 111–117. [2] C.A. Geiger, E. Alekseev, B. Lazic, M. Fisch, T. Armbruster, R. Langner, M. Fechtelkord, N. Kim, T. Pettke, W. Weppner, Crystal chemistry and stability of “Li7La3Zr2O12” garnet: A fast lithium-ion conductor, Inorg. Chem. 50 (2011) 1089–1097. 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