UV emission from Gd3+ ions in LaAl11O18 phosphors

Optik 157 (2018) 1391–1396

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UV emission from Gd3+ ions in LaAl11 O18 phosphors Vijay Singh a,∗ , N. Singh a , M.S. Pathak a , S. Watanabe b , T.K. Gundu Rao b , Pramod K. Singh c , Vikas Dubey d a b c d

Department of Chemical Engineering, Konkuk University, Seoul, 05029, South Korea Institute of Physics, University of Sao Paulo, SP, 05508-090, Brazil Materials Research Laboratory, Sharda University, Greater Noida, 201310, India Department of Physics, Bhilai Institute of Technology, Raipur, Kendri, 493661, India

a r t i c l e

i n f o

Article history: Received 2 November 2017 Accepted 13 December 2017 Keywords: Combustion XRD ESR Gd3+ ions LaAl11 O18 Photoluminescence

a b s t r a c t Phosphors with the composition La1-x Gdx Al11 O18 (x = 0.01–0.10) were synthesized by combustion at a furnace temperature of 500 ◦ C. The formation of the as-prepared combustion products was confirmed by X-ray diffraction analysis. On excitation with 273 nm, an emission band for the Gd3+ ion was observed at 314 nm. This band progressively enhanced with increasing Gd3+ concentration. The phosphor with low concentrations of Gd3+ ions exhibits prominent ESR lines with geff ∼ 1.9, 2.0, 4.7, and 5.9. With high dopant concentrations, the spectrum shows features similar to those of a U-spectrum with dominant lines with geff ∼ 2.0, 2.9, 4.6, and 6.1. Low field lines arise from distortions in the immediate environment of the Gd3+ ion. © 2017 Elsevier GmbH. All rights reserved.

1. Introduction Lanthanum hexaluminates have become a very widely used host material for transition-metal and rare-earth ions [1,2]. Several reports have shown that lanthanum hexaluminates are outstanding materials because of their excellent low thermal conductivity, high corrosion resistance, and high thermal stability [1–5]. In recent years, hexaluminates doped with impurities have been successfully synthesized and characterized [1,2,6]. However, LaAl11 O18 has not received much attention as a luminescent host. Recently Singh et al. [7] reported Mn2+ , Eu2+ , and Eu3+ co-doped LaAl11 O18 phosphors. Mendhe et al. [2] investigated tunable luminescence and energy transfer in LaAl11 O18 :Eu, Tb phosphors. In this study, we have investigated the ultraviolet (UV) emission of gadolinium-activated LaAl11 O18 . Ultraviolet-emitting materials have been widely used in many fields, such as cosmetics, photochemistry, photolithography, photocopying, phototherapy, and photobiology [8–12]. Ultraviolet radiation is mainly divided into three parts, such as UVA, UVB, and UVC. The ultraviolet-emitting material of interest in this study is UVB, which has become the phototherapy treatment for diseases like psoriasis, vitiligo, atopic dermatitis, and other photo-responsive skin disorders [11,12]. Several investigations have shown that the most favorable range for the effective UVB treatment of psoriasis is in the long-wave part of the UVB spectrum, i.e., between 290 and 320 nm [13,14]. A narrow UVB source emitting at around 311 nm was made available around 1988. The radiation emitted by Gd3+ from the 6 P7/2 → 8 S7/2 transition at around 311–315 nm is ideally suited to phototherapy lamps. Recently, significant efforts have been devoted by several research groups toward the synthesis and characterization of UVB-emitting phosphors [15–17]. Singh et al. [18] studied UVB-emitting Gd3+ -doped ZrO2 phosphor.

∗ Corresponding author. E-mail address: [email protected] (V. Singh). https://doi.org/10.1016/j.ijleo.2017.12.034 0030-4026/© 2017 Elsevier GmbH. All rights reserved.

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Table 1 Detailed information on sample composition, sample codes, and starting materials. Sample composition

Sample code

Starting materials

La0.99 Al11 O18 :Gd0.01 La0.98 Al11 O18 :Gd0.02 La0.97 Al11 O18 :Gd0.03 La0.96 Al11 O18 :Gd0.04 La0.95 Al11 O18 :Gd0.05 La0.94 Al11 O18 :Gd0.06 La0.92 Al11 O18 :Gd0.08 La0.90 Al11 O18 :Gd0.10

LA1 LA2 LA3 LA4 LA5 LA6 LA7 LA8

La = 0.2597 g La = 0.2570 g La = 0.2544 g La = 0.2518 g La = 0.2492 g La = 0.2466 g La = 0.2413 g La = 0.2361 g

Al = 2.5 g Al = 2.5 g Al = 2.5 g Al = 2.5 g Al = 2.5 g Al = 2.5 g Al = 2.5 g Al = 2.5 g

U = 1.0916 g U = 1.0916 g U = 1.0916 g U = 1.0916 g U = 1.0916 g U = 1.0916 g U = 1.0916 g U = 1.0916 g

Gd = 0.0027 g Gd = 0.0054 g Gd = 0.0082 g Gd = 0.0109 g Gd = 0.0136 g Gd = 0.0164 g Gd = 0.0218 g Gd = 0.0273 g

(La = La(NO3 )3 ·6H2 O, Al = Al(NO3 )3 ·9H2 O, U = Urea, Gd = Gd(NO3 )3 ·6H2 O).

Chauhan et al. [19] reported a UVB-emitting LaPO4 :Gd phosphor for phototherapy lamps. Gawande et al. [20] reported narrow-band UVB-emitting LaB3 O6 :Bi,Gd and YBaB9 O16 :Bi,Gd phosphors for phototherapy applications. Singh et al. [21,22] investigated ultraviolet-emitting Gd-doped MgAl2 O4 and Y4 Zr3 O12 phosphors. Borate phosphors doped with rare earth and their UV emitting properties were investigated by Sonekar et al. [23]. Electron spin resonance (ESR) spectroscopy has become a powerful tool for investigating the rare-earth-doped inorganic phosphors. ESR spectroscopy of the Gd3+ ion, with 4f7 electronic configuration (8 S7/2 ), can be observed even at room temperature. To the best of our’ knowledge, no details about the ESR and luminescence properties of Gd3+ -doped LaAl11 O18 phosphors have been reported in the literature. Taking this point into consideration, we have undertaken detailed investigation of the combustion-derived Gd3+ -doped LaAl11 O18 phosphors using spectroscopic methods, such as X-ray diffraction (XRD), photoluminescence, and ESR techniques. 2. Experimental setup A series of samples with the general formula La1-x Gdx Al11 O18 (x = 0.01–0.10) was prepared by solution combustion. Lanthanum nitrate (La(NO3 )3 ·6H2 O), aluminium nitrate (Al(NO3 )3 ·9H2 O), urea (CH4 N2 O), and gadolinium nitrate (Gd(NO3 )3 ·6H2 O) were used as starting materials. The details of sample composition, sample codes, and starting materials are given in Table 1. The starting materials were weighed in stoichiometric proportion and dissolved in a minimal quantity of deionized water in a 100 ml china dish and kept in an oven at 80 ◦ C for 20 min to obtain a homogeneous solution. The solution was then placed into a furnace preheated to 500 ◦ C. At the beginning, the solution started burning and released much heat; the solution vaporized instantly with liberation of gaseous byproducts, such as oxides of carbon and nitrogen. The whole process was over within 5 min. After the combustion, the resulting voluminous foamy masses were gently ground and crushed into a fine powder. This powder was used for further characterization. The powder X-ray diffraction (XRD) patterns of the samples were recorded using a RIGAKU Miniflex-II diffractometer operating in the Bragg-Brentano focusing geometry. CuK␣ radiation (␭ = 1.5406 Å) was used as the X-ray source. The instrument was operated at 40 kV and 30 mA. The XRD patterns were taken with a scan rate of 5◦ /min in the 2␪ range of 10–80◦ . The photoluminescence emission and excitation spectra of the samples were recorded by using a spectrofluorophotometer (RF-5301PC SHIMADZU) equipped with Xenon lamp. The EPR spectra of the samples were recorded on a JEOL FE1X ESR Spectrometer, operating in the X-band frequencies, with a field modulation of 100 kHz. 3. Results and discussion 3.1. Crystal structure analyses The XRD was employed to explore the phase composition and crystal structure of the synthesized products. XRD patterns of La1-x Gdx Al11 O18 (x = 0.01–0.10) are depicted in Fig. 1. All the major diffraction peaks observed for the as-prepared samples matched the standard data for LaAl11 O18 (JCPDS No. 33-0699), confirming the formation of the hexagonal phase of LaAl11 O18 . Apart from the hexagonal LaAl11 O18 peaks, two additional weak reflexes (indicated by * ) were observed; these may belong to intermediate phases. Throughout the series, no additional XRD peaks that were caused by doping of gadolinium ions appeared. Considering the radii of La3+ (1.03 Å), Al3+ (0.53 Å). and Gd3+ (0.93 Å), we have substituted the Gd3+ for La3+ ions. It is worth mentioning that the hexagonal phase of LaAl11 O18 was obtained even at a 500 ◦ C furnace temperature. 3.2. Photoluminescence studies Fig. 2a shows the excitation spectra of La1-x Gdx Al11 O18 (x = 0.01–0.10) measured by monitoring the UV emission of the Gd3+ ion at 314 nm. As can be seen from Fig 2a, the excitation spectra exhibited bands in the UV region that correspond to 8 S7/2 → 6 D5/2 (∼244 nm), 8 S7/2 → 6 D7/2 (∼246 nm), 8 S7/2 → 6 D9/2 (∼252 nm), 8 S7/2 → 6 I15/2 ,6 I13/2 (∼273 nm), 8 S7/2 → 6 I11/2 (∼275 nm), and 8 S7/2 → 6 I7/2 (∼278 nm). Among all the excitations, the intense and strong excitation band corresponds to

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LA8 LA7 LA6 LA5 LA4 LA3

137

220

0214

127 0211

029

026

*

121

*

023 0010 025

110 112 017 114

LA1

014

011 012 006

LA2

JCPDS File No:- 33-0699

20

30

40

50

60

70

Fig. 1. Powder XRD pattern of LaAl11 O18 :Gd3+ (LA1–LA8) phosphors.

273 nm

314 nm

λemi=314nm (a)

225

240

255

LA1 LA2 LA3 LA4 LA5 LA6 LA7 LA8

270

308 nm

278 nm

252 nm

246 nm

244 nm

275 nm

λexi=273nm (b)

285

300

315

Fig. 2. Photoluminescence spectra of LaAl11 O18 :Gd3+ (LA1–LA8) phosphors: (a) excitation spectrum for emission at ␭emi = 314 nm, and (b) emission spectrum for excitation at ␭exi = 273 nm.

the 8 S7/2 → 6 I15/2 , 6 I13/2 (∼273 nm) transitions of the Gd3+ ion. Carnall et al. [24] shows that it is very difficult to distinguish 6I 6 –1 15/2 and I13/2 transitions, because the difference between them is only ∼15 cm . Fig. 2b shows the emission spectra of La1-x Gdx Al11 O18 (x = 0.01–0.10) measured by monitoring the UV excitation of the Gd3+ ion at 273 nm in the wavelength range from 290 to 330 nm. The spectra show the presence of an intense band at 314 nm and a weak band at 308 nm; these correspond to 6 P7/2 → 8 S7/2 and 6 P5/2 → 8 S7/2 transitions, respectively. According to Ref. [25], these transitions arise because of electric-dipole and magnetic-dipole transitions. Fig. 3 shows the variation in the emission transition (6 P7/2 → 8 S7/2 ) at 314 nm as a function of Gd3+ concentration. The figure shows that the luminescence efficiency of Gd3+ increases as the content of gadolinium increases. Hence, it can be mentioned that no luminescence quenching was observed for the prepared series of La1-x Gdx Al11 O18 (x = 0.01–0.10). According to previous reports [26,27], the observed emission of Gd3+ at 314 nm is quite beneficial in medical science. According to Refs. [28–30], the dose of narrowband UVB in treatment of early-stage psoriasis, mycosis fungoides, and vitiligo eliminates the possibility of side effects, such as erythema, and causes less irritation than broadband UVB does. Based on the PL results, the synthesized phosphor can be used as a source of narrow-band UV light (UVB) for these treatments. 3.3. Electron spin resonance studies Fig. 4 shows the ESR spectrum observed in the phosphor. For doping with low concentrations of Gd3+ ion (Fig. 4a), the phosphor LaAl11 O18 :Gd displays an ESR spectrum at room temperature with prominent lines at geff ∼ 1.9, 2.0, 4.7, and 5.9.

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LA1

LA2

LA3

LA4

LA5

LA6

LA7

LA8

Fig. 3. The differences in PL emission intensity of LaAl11 O18 :Gd3+ (LA1–LA8) phosphors.

4.7

(a) LA1 5.9

2.8 2.0 1.9 4.6 2.9

(b) LA8

6.1

2.0 1.9 0

200

400

600

800

Fig. 4. ESR spectra of LaAl11 O18 :Gd3+ (a) LA1 and (b) LA8 phosphors.

Low-intensity lines are observed at geff ∼ 2.1, 2.3, and 2.8. With increased Gd3+ ion concentration and at the maximum dopant concentration used in this study, ESR lines at geff ∼ 2.0, 2.9, 4.6, and 6.1 dominate the spectrum, which looks more like a U-spectrum normally observed in disordered systems like glasses and zeolites. This spectrum is shown in Fig. 4b. In ordered systems and in an environment with low distortions, S-state ions Eu2+ and Gd3+ display a spectrum that reflects a relatively weak crystal field experienced by the ions, a spectrum different from the ones observed in glassy or disordered systems like zeolites [31]. In disordered materials like glass, a distinct spectrum is observed with three characteristic lines with effective g-values 2.0, 2.8, and 5.9. This distinct spectrum is called a U-spectrum of Gd3+ and Eu2+ ions [32,33] (Fig. 4). There are cases where a spectrum similar to the one seen in disordered systems may be observed in polycrystalline materials [34,35]. In this case, there will be a distribution of crystal field at the Gd3+ sites that results in a spectrum similar to the one observed in structurally disordered materials. In the present system, when the Gd3+ concentration is high, such a spectrum is observed. The reason for the appearance of the U-spectrum has been discussed in detail by Brodbeck and Iton [36]. ESR lines confined to g ∼ 2.0 regions are observed in the spectrum when the ion happens to be in undistorted surroundings and sees a weak crystal field. Distortions lead to relatively stronger crystal fields and observation of lines in the low field region. A consideration of distribution of such fields among Gd3+ ions gives rise to a U-spectrum. The structural analysis of LaAl11 O18 by Iyi et al. [37] showed a structure of a magnetoplumbite type. Its crystal structure is similar to that of the mineral magnetoplumbite (PbM12 O19 ). LaAl11 O18 has the space group P63 /mmc with two formula units per unit cell. A substitution of divalent Sr ions by trivalent La ions in the structure of SaAl12 O19 [38] leads to the structure

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of LaAl11 O18 . This substitution results in a compound with a highly defective magnetoplumbite structure. The Sr2+ ion can be progressively removed from the lattice and replaced by the trivalent ion. The charge balance is achieved by removal of oxygens and creating oxygen vacancies [39]. If these oxygen vacancies are located near substituted Gd3+ sites in the phosphor LaAl11 O18 :Gd3+ , they may lead to distortions in the immediate environment of Gd3+ ions. The ESR spectrum of the Gd3+ ion is sensitive to structural changes in the vicinity of the ion. The crystal field experienced by the ion will change because of environmental changes, and the distortions make the ion see a stronger crystal field. As remarked by Brodbeck and Iton [36], ESR lines will now appear in the low field region of the spectrum. As oxygen vacancy locations can vary with respect to Gd3+ ion, there will be distribution in the crystal field seen by the ion. Because the Gd3+ ions are more concentrated in the lattice in the LA8 phosphor, the distribution will be more pronounced in this phosphor and will result in an ESR spectrum that looks like a U-spectrum. On the other hand, in the phosphor LA1, there is less distribution of the crystal field because of the low concentration of Gd3+ ion, and lines will appear in the low field region without the appearance of a U-spectrum. An ESR signal is observed in all the samples at geff ∼ 1.9 indicating a Gd3+ ion whose immediate environment is not disturbed by the oxygen vacancy. The ion experiences a relatively weak crystal field, and the oxygen vacancy is likely to be remote from this ion. Symmetry of the undisturbed lattice will prevail, and the line is observed near free-electron resonance. 4. Conclusion A series of La1-x Gdx Al11 O18 (x = 0.01–0.10) were prepared by solution combustion. XRD results confirmed the hexagonal phase of LaAl11 O18 , and doping Gd3+ ions did not alter the phase structure of the host LaAl11 O18 . UVB emission in a narrow band centered at around 314 from Gd-doped LaAl11 O18 can be a promising candidate for phototherapy lamps. Some lines have been observed in the ESR spectrum of the phosphor near free-electron resonance as well as in the low field region. A weak crystal field is experienced by some of the Gd ions, and surroundings of many of the ions are affected by oxygen vacancies, giving rise to low field lines as a consequence of the stronger crystal field seen by these ions. With a higher concentration of dopant ions, the spectrum looks like a U-spectrum because of a wider distribution of the crystal field. Acknowledgement This paper was supported by the KU Research Professor Program of Konkuk University. References [1] V. Singh, R.P.S. Chakradhar, J.L. Rao, Y.-D. 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