UV radiation emitting LiAl5O8 doped with Gd3+ ceramic: Optical and EPR correlation study

Journal Pre-proof UV radiation emitting LiAl5 O8 doped with Gd3+ ceramic: optical and EPR correlation study Vijay Singh, S. Kokate, V. Natarajan

PII:

S0030-4026(19)31915-1

DOI:

https://doi.org/10.1016/j.ijleo.2019.164016

Reference:

IJLEO 164016

To appear in:

Optik

Received Date:

29 October 2019

Accepted Date:

6 December 2019

Please cite this article as: Singh V, Kokate S, Natarajan V, UV radiation emitting LiAl5 O8 doped with Gd3+ ceramic: optical and EPR correlation study, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164016

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UV radiation emitting LiAl5O8 doped with Gd3+ ceramic: optical and EPR correlation study Vijay Singh 1, *, S. Kokate 1, 2, V. Natarajan 3 1

Department of Chemical Engineering, Konkuk University, Seoul 05029, Korea 2 Department of Physics, SIRT, Bhopal, 462041, India 3 Institute of Advanced Research, Gandhinagar, Gujarat, 382007, India

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

Abstract

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By synthesizing gadolinium-doped LiAl5O8 sample via combustion method, electron paramagnetic

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resonance (EPR) and photoluminescence (PL) properties of gadolinium in the LiAl5O8 host were studied. From PL data, it was confirmed that gadolinium stabilized in the host system as Gd3+. The

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emission spectrum of the LiAl5O8 ceramic exhibited a sharp, narrow band associated with the transitions from the 6P7/2 excited state to the 8S7/2 ground state of Gd3+. EPR study revealed the

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presence of Gd3+ ions at distorted octahedral sites in the LiAl5O8 lattice. Keywords: Combustion; EPR; Gd3+; LiAl5O8; Ceramic; Luminescence

1. Introduction

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Aluminate spinels have gained much attention because of their wide range of technical uses, such as pigments, oxidation catalysts, high-alumina and cement refractories [1-3]. Lithium aluminate (LiAl5O8) has an inverse spinel structure. LiAl5O8 is reported to be found in a cubic-ordered form with a P4132/P4332 space group at low-temperature, and in a disordered form with an Fd3̅m space group at high-temperature [4]. Lithium aluminates are investigated widely as important host materials for transition metal and rare-earth ions. Because of their interesting chemical, mechanical, 1

magnetic, and luminescent properties, transition-metal ion doped aluminates such as LiAl5O8:Fe [5], LiAl5O8:Mn [6], LiAl5O8:Cr [7], and LiAl5O8:Co [8] have been reported extensively in the last decades. Recently, several research groups have studied the luminescent properties of the rare-earthions-doped LiAl5O8 phosphor. Nikhare et al. [9] reported photoluminescence (PL) and thermoluminescence (TL) in LiAl5O8:Ce. Singh and Rao [10] studied a Eu3+-doped LiAl5O8 redemitting phosphor in which they observed a strong emission peak at 613 nm caused by the 5D0–7F2

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transition of Eu3+. Pitale et al. [11] obtained intense luminescence in the green region from Tb3+-

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doped LiAl5O8phosphor caused by the magnetic dipole transition 5D4→7F5 of the Tb3+ ion at 543nm. Mu et al. [12] reported that Ce3+ and Dy3+co-doped LiAl5O8 can be a potential two-band (blue and

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yellow) phosphor. Singh et al. [13] reported visible up-conversion and NIR luminescence studies of

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LiAl5O8:Er phosphor co-doped with Yb3+ and Zn2+ ions. More recently, Mohapatra et al. [14] studied intense red-emitting LiAl5O8 doped with various concentrations of Eu3+ ions.

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Trivalent gadolinium (Gd3+) shows interesting magnetic and optical properties among the rare earth ions [15]. The Gd3+Activated host materials have become the primary choices to fabricate

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desirable ultraviolet (UV) light source [16, 17]. Gd3+ ion has seven unpaired electrons in its outermost 4f shell [18] and the energy gap of nearly 32000 cm-1 between 8S7/2 ground level and 6P7/2 excited level [19]. These characteristics make it a potential dopant into upconversion nanoparticles for magnetic and optical bimodal detecting [20]. Due to the ability of altering the relaxation time of

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surrounding water protons, Gd3+-doped nanoparticles are used enormously in MRI as contrast agents [21].

Gd3+ can transfer energy absorbed from UV radiations to other rare earth ions fully or partially

and therefore considered as one of the important sensitizers [22, 23]. Hence, activating a phosphor with Gd3+ co-doped with other rare-earth ions is a very promising route to obtain efficient luminescence [24]. In addition to these properties, Gd3+ ions, because of their 6P7/2 → 8S7/2 transition, 2

give the emission in a narrow-band (310-315nm) ultraviolet B region [25]. It is well known that the NB-UVB emission is very important for treating various skin diseases [26]. Commercial phosphor LaB3O6:Bi, Gd is basically used for fabricating phototherapy lamps [26]. Several reports show that the activating phosphates, borates, silicates, and aluminates host with gadolinium ions could be useful in phototherapy lamp for curing skin conditions [25, 27-29]. Due to the paramagnetic nature of Gd3+, its surrounding environment in a host lattice can be easily found using EPR (electron

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paramagnetic resonance) technique [30]. The gadolinium is very sensitive to electron spin resonance

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spectroscopy [31]; hence it helps to investigate different possible surroundings of Gd3+ ions in a selected host [28].

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In this work, we have chosen a Gd3 + ions doped spinel LiAl5O8 compound. LiAl5O8:Gd was

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prepared using an economical feasible solution-combustion method at 500 ± 10oC temperature within 5 min. In order to understand the role of Gd3+ in the LiAl5O8 host matrix, X-ray diffraction

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(XRD), photoluminescence (PL) spectroscopy, and electron paramagnetic resonance (EPR) techniques were used for the characterization of the prepared sample.

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2. Experimental setup

The LiAl5O8:0.01Gd ceramic was prepared using solution combustion method and urea was used as a combustion fuel. Starting materials, such as 0.0735 gm [LiNO3], 2gm [Al(NO3)3∙9H2O], 0.0048gm [Gd(NO3)3∙6H2O], and 0.8552gm [CO(NH2)2] were used for preparing the LiAl5O8:0.01Gd

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phosphor. The quantity of oxidizers (metal nitrates) and fuel (urea) were computed by keeping oxidisers to fuel ratio (O/F) =1 [32]. In a typical synthesis, stoichimetric amounts of the starting materials were taken in a 100-ml china dish and mixed thoroughly using a mortar and pestle. Then the china dish was kept into a muffle furnace preheated at 500 ± 10o C. The mixture boiled, underwent dehydration, and finally decomposed along with the evolution of various gases. The whole process completed in less than 5 min. After removing dish from the furnace, the resulting 3

fluffy product was milled gently using a mortar and pestle and the as prepared ceramic powder was utilized for further studies using different characterization method. An X’Pert Pro Diffractometer (Panalytical) was used to record XRD data at room temperature with Cu Kα radiation as an X-ray source in the 2 range of 10o to 80o. PL studies were conducted with the help of a Shimadzu RF-5301PC, spectrofluorophotometer equipped with a Xenon flash

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lamp. A JEOL FE1X ESR Spectrometer working in the X-band frequencies was used to record EPR spectra of the prepared powder.

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3. Results and discussion 3.1 X-ray diffraction

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The XRD pattern of the LiAl5O8:0.01Gd ceramic prepared using solution combustion is shown in

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Fig. 1(a). For comparison, a standard diffraction pattern for pure LiAl5O8 (JCPDS file no:-71-1736) is also presented in Fig. 1(b). The XRD measurements demonstrate the pure cubic phase of LiAl5O8

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which was synthesized successfully by combustion method at 500 ± 10oC. Absence of other secondary phases indicates that the Gd3+ ions fully occupied the LiAl5O8 lattice without affecting the

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host structure. The well-known Scherrer’s equation, D = 0.941λ/βcosθ was used to determine the crystallite size of the sample, where D is the average grain size, λ is the X-ray wavelength, θ is the diffraction angle, and β is the full-width at half-maximum of an observed peak. The strongest peak (311) was utilized to compute the average crystallite size (D) of the LiAl5O8:0.01Gd sample and was

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evaluated to be 30.33 nm.

Fig. 2 shows the crystal structure of LiAl5O8. At room temperature, LiAl5O8 crystallizes as pure

cubic spinel with space group P4332. Each LiAl5O8 unit cell contains four formula units and five inequivalent atoms that is, two Al ions (Al1 and Al2), two O ions (O1 and O2) and one Li ion (Li). There are two sites, i) 8c (tetrahedral) and ii) 4b, 12d (2 distorted octahedral) in the crystal structure

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of LiAl5O8. It has been reported that the Al3+ ions reside in both octahedral and tetrahedral sites, while Li+ ions occupy octahedral sites [14, 33]. 3.2 Photoluminescence Fig. 3 illustrates the PL spectra of the synthesized LiAl5O8:0.01Gd ceramic. Fig. 3a shows the excitation spectrum obtained for the LiAl5O8:0.01Gd ceramic under emission at 314 nm. The spectrum consists of six bands at 243, 246, 252, 273, 275, and 278 nm and can be assigned to the S7/2→6D5/2,3/2,8S7/2→6D7/2, 8S7/2→6D9/2, 8S7/2→6I11/2, 8S7/2→6I9/2, 6I17/2 and 8S7/2→6I7/2 transitions of

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Gd3+ ions, respectively. Alonso et al. [34] also reported the same peaks in Gd3+ -doped fluorozirconate glasses. Pathak et al. [35] observed six peaks at 244.4, 246.5, 252.9, 273.2, 276, and

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279.4 nm in Gd3+-doped Y3Ga5O12, when the emission wavelength was fixed at 312 nm. Fig. 3b

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shows the emission spectrum of LiAl5O8:0.01Gd measured at 273 nm excitation wavelength of the Gd3+ ion. The spectrum exhibits an intense peak and a weak peak at 314 (31847.13 cm-1) nm and 308

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nm (32467.53 cm-1), respectively. The strong peak at 314 nm has been attributed to the electricdipole 6P7/2 → 8S7/2 transition of Gd3+ions, whereas the weak peak at 308 nm has been attributed to

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the magnetic-dipole 6P5/2 → 8S7/2 transition of Gd3+ ions. The energy difference between these two levels is 620.4 cm-1, which matches well with those described by Shishonok et al. [36]. Li et al. [37] reported sharp peaks at 312 nm which were caused by the 4f → 4f intraconfiguration forbidden transitions of Gd3+ ions in the LaAlGe2O7 phosphor. Tian et al. [38] reported an intense peak at 313

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nm of Gd3+ ions in the NaY0.80Gd0.20FPO4 phosphor. Fig. 4 shows the energy level diagram of Gd3+ ions. Firstly, the ions get excited from the ground state 8S7/2 to the 6DJ, 6IJ level. Afterward, the ions non-radiatively decay to 6PJ state. Finally, the ions radiatively relax from 6PJ to 8S7/2 level and give emission at 314 nm. It is well known that the wavelength in the range of the narrow-band UVB (310-315 nm) region has great importance in the field of dermatology to cure skin diseases [39, 40].

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The prepared ceramic powder can be considered as a promising narrow-band UV light source for treating the various skin conditions. 3.3 Electron paramagnetic resonance As mentioned earlier, LiAl5O8 structure contains two distorted octahedral sites and one tetrahedral site for Al3+ ions. The Li+ ions occupy distorted octahedral sites, while the Al3+ ions occupy both tetrahedral as well as octahedral sites in the LiAl5O8 crystal lattice. When Gd3+ ion (ionic radius =

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0.94 Å for 6 coordination) is doped in LiAl5O8, it is not expected to replace Al3+ ions due to its

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larger ionic radius. However as reported earlier [14], it might substitute the alkali metal ion (along with charge compensating vacancies nearby or far off form the dopant site). It may be noted that the

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average Al-O bond distance is 1.90 Å in the octahedral site, while Li-O bond distance is 2.05 Å. The

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average bond distance of Gd-O in Gd2O3 (Metal nitrates in the starting material will decompose to oxides, before reacting with one another to form aluminates) is 2.2-2.5 Å. Hence the probability of

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doping Gd3+ at Li+ site is expected to be very high. The EPR spectrum of Gd3+ ion consists of seven fine structure transitions (∆Ms = ±1) for its 8S7/2 ground electronic state, in a cubic crystal field

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having the central line (-1/2 → =1/2) being the strongest. Fig. 5 shows the EPR spectrum of the as prepared sample. The strong signals around 900, 1500, 2300, 3400, 4400, 5300, 6000 (g ≈ 7.48, 4.5, 2.93, 1.98, 1.53, 1.27, 1.12) were observed, which is typical of Gd3+ ions occupying distorted

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octahedral Li+ sites in the host lattice.

4. Conclusions

A LiAl5O8:0.01Gd ceramic, emitting a narrow UV band was synthesized by a combustion reaction. The ceramics can be effectively excited at 273 nm and exhibit a narrow UV emission with a dominant peak at 314 nm. The results show that the ceramic LiAl5O8:0.01Gd could be a potential candidate for the UV component of phototherapy lamps. EPR spectra of the ceramic powder indicate 6

the presence of Gd3+ ions at distorted octahedral sites in the LiAl5O8 lattice, wherein doped Gd3+ ions are expected to substitute Li+ ions.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be

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considered as potential competing interests:

Acknowledgements

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Korea government (MSIT) (2018M2B2A9065656).

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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the

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Figure captions

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Fig. 1. (a) XRD pattern of LiAl5O8:0.01Gd ceramic, (b) JCPDS pattern of LiAl5O8 (File No-711736). Fig. 2. Crystal structure of LiAl5O8 ceramic Fig. 3. Photoluminescence spectra of the LiAl5O8:0.01Gd ceramic (a) Excitation spectrum (λem= 314 nm), (b) Emission spectrum (λexc= 273 nm)

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Fig. 4. Energy level diagram of Gd3+ ion

Fig. 5. EPR spectrum of LiAl5O8:0.01Gd ceramic

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(b) JCPDS File No:- 71-1736

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Fig. 1. (a) XRD pattern of LiAl5O8:0.01Gd ceramic, (b) JCPDS pattern of LiAl5O8 (File No-711736).

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Fig. 2. Crystal structure of LiAl5O8 ceramic

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(a) exc= 314nm

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(b) em= 273nm

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Fig. 3. Photoluminescence spectra of the LiAl5O8:0.01Gd ceramic (a) Excitation spectrum (λem= 314 nm), (b) Emission spectrum (λexc= 273 nm)

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Fig. 4. Energy level diagram of Gd3+ ion

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Fig. 5. EPR spectrum of LiAl5O8:0.01Gd ceramic

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