Rapid-gel combustion synthesis, structure and luminescence investigations of trivalent europium doped MGdAlO4 (M = Mg2+, Ca2+, Sr2+ and Ba2+) nanophosphors

Optik - International Journal for Light and Electron Optics 200 (2020) 163450

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Original research article

Rapid-gel combustion synthesis, structure and luminescence investigations of trivalent europium doped MGdAlO4 (M = Mg2+, Ca2+, Sr2+ and Ba2+) nanophosphors

T

Sonika Kadyana, Sitender Singha, Anura Simantillekeb, Bernabe Maric, ⁎ Devender Singha, a

Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, India Centro de Fisica, Universidade of Minho, Braga 4710057, Portugal c Departament de Física Aplicada, Universitat Politècnica de València, València 46022, Spain b

A R T IC LE I N F O

ABS TRA CT

Keywords: Rapid-gel combustion MGdAlO4:Eu3+ Luminescence Tetragonal Fluorescent panels

A series of novel red MGdAlO4:Eu3+ (M = Mg2+, Ca2+, Sr2+ and Ba2+) was synthesizedand investigated. The samples were synthesized via rapid-gel combustion route at a temperature of 600 °C and examined over large doping concentration of 0.01 to 0.05 mol. The optimal concentration of activator ion has been found to be 0.03 mols for MGdAlO4 lattice. Using the excitation wavelength of 393 nm, these phosphors exhibited strong emission in the red region owing to 5D0→7F2 (electric dipole allowed) transition located at 610–615 nm. The effect of reaction temperature on luminescence was also analyzed for these materials. The structural characteristics of phosphors were studied via joint approach of X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Diffraction patterns of prepared phosphor contain sharp peaks in 10°–80° region. The proposed materials have single phase tetragonal structure having I4/mmm space group. The degree of crystallization has been found to increase with temperature. The TEM micrographs exhibited the spherical shape of particles in 15–35 nm size. The FTIR spectra of CaGdAlO4 showed peaks at 699 and 461 cm−1 analogous to Al-O and Gd-O stretching vibrational modes. Due to excellent luminescent response the Eu3+ doped MGdAlO4 can be excellent materials for the growth of effective red phosphor in WLEDs and fluorescent panels.

1. Introduction The various applications of optical materials such as solid state lighting, biomedical imaging, safety signal, luminescent paint and in optoelectronic devices encourage the researcher to work continuously in the field of rare earth activated luminescent materials [1]. For the synthesis of these kinds of phosphors materials, the primary requisite is a suitable host lattice and an efficient activator [2]. Due to excellent chemical and thermal stability MGdAlO4 (M = Mg, Ca, Sr and Ba) materials proved themselves as apposite host for luminescent materials [3,4]. These days, the key condition for modern displays is emission in the red region. Large numbers of trivalent rare earth ions have been used as the activator ions in luminescence materials because the excited states of these ions can get populated easily and coupled with the prevalence of radiative decay to the ground state rather than non-radiative decays [5–7].

⁎

Corresponding author at: Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, Haryana, India. E-mail address: [email protected] (D. Singh).

https://doi.org/10.1016/j.ijleo.2019.163450 Received 20 June 2019; Accepted 18 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.

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Among trivalent rare earth ions, europium requires in small amount, exhibit intrinsic luminescence and strong red emission around 612 nm [8–10]. Therefore, over the past decade, europium containing aluminates have engrossed remarkable attention of the researchers to explore the optical properties of these materials [11–16]. The luminescence intensity of phosphor materials is also affected by local environment. Hence, matrices of aluminates, oxides, sulphates, silicates, sulfides etc. have been estimated as host lattice for optical materials of rare earth ions [17–19]. Presently researchers use various techniques for synthesis of nanoparticles of desired range like hydrothermal [20], solid state reaction [21], sol-gel [22], and rapid-gel combustion [23]. The main drawbacks of solid-state reaction is the high temperature synthesis, large grain size, more time consumption and frequent grinding which leads to loss of luminescence. The sluggish gelformation is major limitation of sol-gel method which makes it slower method than former techniques [24]. In present research work, we have utilized the urea stimulated rapid-gel combustion procedure to prepare a series of MGd1-xAlO4:xEu3+ (M = Mg2+, Ca2+, Sr2+, Ba2+, and x = 0.03) nanophosphors, because this technique has a number of advantages such as; fabrication of high purity homogenous materials at lower temperature with fine particle size in few minutes, low-cost raw materials and easy synthesis method for handling [25]. In particular, the present rare-earth activated aluminate phosphors find many applications in luminescence world like light emitting diodes (LED’s), luminous paints, dial plates of wrist watches, fluorescent bulbs, cathode ray tubes, scintillators, plasma display panels (PDP’s) and field emission displays. In the present article, the work is executed to prepare a series of MGd1-xAlO4:xEu3+ luminescent materials through urea-assisted rapid-gel combustion method and characterize by various studies comprising photoluminescence (PL), XRD, FTIR and TEM respectively. Structure as well as surface morphological properties of synthesized phosphor samples were studied broadly approving the excellent crystallinity of these materials which increases further with increase in temperature. The peaks in photoluminescence emission spectra of these nanophosphors ratify them as strong red light producing materials which support that these can be valuable components for various light emitting materials. 2. Experimental 2.1. Materials and synthesis MGd1-xAlO4:xEu3+ luminous solids were synthesized through rapid-gel combustion route and urea was used as a fuel to accelerate the reaction. The chemicals used in the reaction were in highly pure form and purchased from Sigma Aldrich. The required raw materials, nitrates of metals like Gadolinium, Aluminium, Magnesium, Calcium, Strontium, Barium and Europium were weighed according to their stoichiometric ratios as in the general formula of above mentioned materials on an electric balance. These chemicals were taken in silica crucible of 50 ml size and then deliquesced in marginal volume of double distilled water. Calculated quantity of urea (NH2CONH2) was used as fuel which was done by considering the overall oxidising and reducing valences of metal nitrates and fuel respectively. The crucible containing all prime components was then heated at a temperature of nearly 80 °C to get a gel type material. Thus attained gel was then introduced into the furnace upheld at 600 °C. Here, decomposition of the gel would occur followed by dehydration resulting in the evolution of gases like water vapours, carbondioxide, ammonia and various oxides of nitrogen. The evolved gases would develop it in spongy, frothy and soft solid [26]. The required energy for present redox reaction was compensated by explosion of evolved gases. After 15 min, the combustion would be completed and then crucible was taken out of furnace. Covered the crucible with an appropriate lid and then allow it, to cool at room temperature. After cooling, the white and soft material was obtained which was then converted to a homogeneous powder by crushing into a pestle mortar. These homogeneous powders were then seen under ultra-violet lamp to prelude test their red light emission. A portion of the obtained materials was further heated to illustrate the altered luminescence and crystallinity with temperature. Fig. 1 sums up the entire steps used in the synthesis technique. 2.2. Materials characterization The emission spectra and color-coordinates of synthesized phosphors were explained in detail using Xenon source based Horiba Jobin YVON Fluorolog spectrophotometer in solid state at normal temperature. The structure and phase of prepared luminous solids MGd1-x AlO4:xEu3+ (M = Mg2+, Ca2+, Sr2+, and Ba2+, x = 0.03) have been studied via performing the X-ray diffraction spectra on Rigaku Mini Flex 600 Diffractometer using CuKα irradiation (40 mA current and 40 kV voltage supply). These XRD pattern were recorded from 2θ = 10°–80° with 2°/min scanning speed. The particle sizes of synthesized nanophosphors were calculated by the Scherer’s equation. The FTIR technique was used to explain the chemical bonding of metal-oxygen bonds in particular phosphor materials and these spectra were noted in 400–4000 cm−1 spectral range using a Bruker spectrophotometer linked with OPUS software. Besides, transmission electron micrographs (TEM) were seen under TECNAI 200 kV transmission electron microscope (Fei, Electron Optics). 3. Results and discussions 3.1. Photoluminescence investigation The optical behaviour of Eu3+ activated MGdAlO4 (M = Mg2+, Ca2+, Sr2+ and Ba2+) type phosphors have been studied in detail using the PL spectroscopy. The characteristics red emission of the as-prepared samples under ultraviolet light in the lamp is shown by 2

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Fig. 1. Flowchart showing different steps used in the synthetic procedure.

Fig. 2. Images of MGdAlO4:Eu3+ (a–b) MgGdAlO4, (c–d) CaGdAlO4, (e–f) SrGdAlO4 and (g–h) BaGdAlO4 down conversion phosphors by stimulating them in UV light and in the absence of UV light. 3

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Fig. 3. Photo Luminescence Excitation (PLE) spectra for Eu3+ doped CaGdAlO4 nanophosphor.

the images in Fig. 2. The prime condition for the origin of luminescence in inorganic materials is the presence of suitable activator ions in the lattice [27,28]. The mechanism of luminescence involves absorption of energy by the host crystal from the external source and the transmission of this absorbed energy to the activator ion. The dopant ion having a large number of energy states undergoes excitation to higher energy states. Afterward, it relaxes to the ground energy level causing the production of the light of longer wavelength, which appears as a peak in optical emission spectra [29]. The absorption spectra were recorded at the 612 nm emission wavelength. Fig. 3 shows the photo luminescent excitation (PLE) spectra of CaGdAlO4 phosphors at 600 °C recorded using the PL emission wavelength of 612 nm. The excitation includes transitions between f-levels of europium(III) ions in the 350 to 550 nm region. The main peaks were detected at 361 (7F0→5D4), 381 (7F0→5L7), 393 (7F0→5L6), 413 (7F0→5D3), 463 (7F0→5D2) and 531 nm (7F0→5D1) in the PLE spectra. The most intense absorption transition was observed at 393 nm (7F0→5L6) [30]. The photoluminescence emissions of the synthesized materials were recorded at 393 nm as wavelength of excitation. Fig. 4 shows the typical photo-emissive spectra of CaGdAlO4 prepared using europium ion concentration of 0.01–0.05 mol. The emissive power of focused phosphor is found to increase without any change in the spectral shape upon the increase of the concentration of the Eu3+ ion. However, photoluminescent quenching starts as the concentration exceeds 0.03 mols. The increased amount of dopant in the lattice results in an enhanced non-radiative decay, consequently, radiative emission and luminous intensity decrease [31]. The most effective concentration of the Eu3+ ion in the present matrix is three-mole percent. Hence, the photoluminescence characteristics are profound to the composition of activator ion in the crystal matrix. Fig. 5(a–d) depicts the fluorescence spectra of MGdAlO4 materials recorded using 393 nm as an exciting wavelength. These emission exhibit peaks at 588–592, 610–615 and 651–655 nm. These peaks can be ascribed to 5D0 to 7F1, 7F2 and 7F3 transitions respectively. Out of these, 5D0 to 7F1, 7F2 are magnetic dipole and electric dipole transition correspondingly. The electrically allowed transition is more intense compared to the magnetic dipole transition [32,33]. The magnetic dipole transition (5D0→7F1) is allowed according to the Judd–Ofelt theory. However, the electric dipole transition (5D0→7F2) becomes unusually permitted when the local center of symmetry is absent around the trivalent europium ion. The greater intensity of 5D0→7F2 over the others also tells us about the non-centrosymmetric position of europium(III) site. The lack of inversion symmetry around europium(III) ions provides bright red and efficient phosphors. The lack of centre of inversion in for Eu3+ ion is verified by calculating the ratio of intensity of 5D0→7F2 to 5D0→7F1 transitions. Higher the value of intensity ratio greater will be asymmetry around the Eu3+ ions in the crystal structure. The effect of temperature was also investigated by recording the luminescence emission spectra at different temperatures viz. 600,

Fig. 4. Graph showing a variation of fluorescent intensity with the concentration of Eu3+ ions doped CaGdAlO4 materials at 600 °C. 4

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Fig. 5. Photoluminescence emission spectra for Eu3+ activated MGdAlO4 phosphors. (a) MgGdAlO4, (b) CaGdAlO4, (c) SrGdAlO4 and (d) BaGdAlO4.

Fig. 6. Relative PL emission spectra for Eu3+ activated fluorescent materials at 900 °C.

900 and 1050 °C (Fig. 6) and it was found that the luminescence intensity increases with temperature because of the increase in radiative phenomenon at a higher temperature [34,35]. From here, we conclude that, amongst these materials the highest luminescence intensity was observed for BaGdAlO4 lattice. Fig. 7 shows the different transitions undergone by europium(III) ion during excitation and emission phenomenon [36]. Table 1 presents the positions of spectral peaks, dominant transition and color of emitted light under UV excitation. Table 2 shows the color coordinates value calculated by CIE calculator for the respective phosphors at different temperatures, which are very helpful to confirm the emissive region in the proposed phosphors. Color coordinates are very significant parameters for the inorganic phosphors and these were calculated using the chromaticity calculator. The CIE triangle is demonstrating (Fig. 8) the strong red emission clearly [37]. Red emission is the prime requirement for advanced displays; hence the presently synthesized materials may prove exceptional phosphors in future display technologies. 3.2. Powder X-ray diffraction measurements The determination of host crystal lattice and phase purity of fabricated nanophosphors was done by PXRD analysis. Fig. 9(a–d) depicts the X-ray diffraction patterns of the Eu3+ doped MGdAlO4 aluminate phosphors synthesized via the combustion process. Due 5

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Fig. 7. Various energy levels transitions occurring in europium(III) ion present in host lattice. Table 1 Various transitions, Main peak and color corresponding to Eu3+ activated aluminate materials. D0→7F1 (nm)

D0→7F2 (nm)

D0→7F3 (nm)

Lattice type

5

5

5

MgGdAlO4 CaGdAlO4 SrGdAlO4 BaGdAlO4

588 590 592 592

611 615 610 611

651 655 651 651

Most Intense Transition 615 615 610 611

(5D0→7F2) (5D0→7F2) (5D0→7F2) (5D0→7F2)

Color Red Red Red Red

Table 2 CIE color coordinates values for Eu3+ activated aluminate nanophosphors at respective temperatures. Host Lattice

MgGdAlO4 xy

As prepared 900 °C 1050 °C

0.592 0.593 0.596

CaGdAlO4 xy 0.351 0.355 0.359

0.612 0.615 0.618

SrGdAlO4 xy 0.341 0.345 0.348

Fig. 8. Color triangle demonstrating CIE coordinates of focused phosphors. (a) MgGdAlO4, (b) CaGdAlO4, (c) SrGdAlO4 and (d) BaGdAlO4.

6

0.611 0.613 0.617

BaGdAlO4 xy 0.318 0.321 0.327

0.653 0.656 0.659

0.341 0.344 0.348

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Fig. 9. Powder XRD spectrum of MGdAlO4:Eu3+ samples at various temperatures. (a) MgGdAlO4, (b) CaGdAlO4, (c) SrGdAlO4 and (d) BaGdAlO4.

to the similar ionic size and same valence state, Eu3+ ions can substitute the Gd3+ metal ions present in undertaken crystal matrix. The variation of alkaline earth metals ions (Mg2+, Ca2+, Sr2+ and Ba2+) in MGdAlO4 materials have altered X-ray spectra of the resulting phosphors. Gd3+ and M2+ ions occupy the dissimilar co-ordination positions in the focused lattice. Diffraction peaks for CaGdAlO4 phosphor at 600 °C are very sharp from 10 to 70 degree. The presence of sharp XRD peaks indicates the higher degree of the crystallinity in the proposed phosphor. The diffraction peaks of CaGdAlO4 material are closely matched with JCPDS No. 24-0192 [38]. The observed diffraction peaks are in close agreement with tetragonal crystal structure and space group I4/mmm. Further heating of this material at 900 and 1050 °C shows the peaks with enhanced intensity. Powder XRD patterns of SrGdAlO4 are shown in Fig. 9(c) having diffraction peaks corresponding to tetragonal crystal structure, I4/mmm (space group) and 139 (group number). Xray diffraction spectra for MgGdAlO4 and BaGdAlO4 are nearly same having no available literature data. A significant enhancement in the intensity is observed with increased re-heating temperature. The crystallite sizes for these materials have been determined applying Scherer’s equation. According to this D = kλ/βcosθ where, D = crystallite size, k = 0.89 (constant value), β denotes full width at half maximum, λ is wavelength of X-ray and θ stands for incident Bragg’s angle. The crystallite size and crystallinity of the materials strongly influence the luminescence intensity of the phosphors. The Gd3+ ions present in the host lattice have been effectively replaced by Eu3+ ions due to comparable ionic radii and similar valence of two ions. Table 3 shows the various characteristics of the materials obtained from PXRD analysis such as main peak position, FWHM (Full Width at Half Maximum) and sizes of particles.

Table 3 Different characteristics of XRD analysis for MGdAlO4:Eu3+ materials. Type of phosphor

Area

2θ

Width

FWHM

Crystallite Size (nm)

MgGdAlO4:Eu3+ CaGdAlO4:Eu3+ SrGdAlO4:Eu3+ BaGdAlO4:Eu3+

41722.75 35947.41 30082.20 20274.03

28.58 33.98 28.52 28.44

03.80 10.36 13.23 08.17

0.05 0.09 0.03 0.07

30.25 18.29 39.43 19.60

7

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Fig. 10. FTIR plots of for Eu3+ doped MGdAlO4 phosphors. (a) MgGdAlO4, (b) CaGdAlO4, (c) SrGdAlO4 and (d) BaGdAlO4.

3.3. FTIR studies Vibrational peaks which explain the bonding characteristics were analyzed using their Fourier Transform Infrared (FTIR) spectra. Fig. 10(a–d) depicts the FTIR spectra recorded at room temperature in 4000–400 cm−1 range for the MGd1-xAlO4:xEu3+ (where M = Mg2+, Ca2+, Sr2+, Ba2+ and x = 0.03) calcined on 900 °C. This analysis was done to know about the entity which can behave as quencher for photoluminescence in phosphors. It was found that eOH group enhances the non-radiative phenomenon that may result in de-activation of Eu3+ ions from higher states. It is worthful to notice that eOH vibrations are due to moisture absorbed by alkaline earth metals. The peaks in the infrared spectra appear only for the vibrations which undergo the alteration in electric dipole moment of the species. These spectra are very useful for the study of various metal-oxygen bonds. For MgGdAlO4 lattice their IR peaks have been appeared at 431, 547, 675 cm−1 confirming bending and stretching vibrations of gadolinium-oxygen and aluminium-oxygen stretching vibrations respectively. IR peaks for CaGdAlO4 lattice include gadolinium-oxygen and aluminium-oxygen stretching modes have been obtained at 461 and 669 cm−1. The IR bands for SrGdAlO4 material were observed at 468, 760 and 871 cm−1 indicating gadolinium-oxygen bending and aluminium-oxygen bending and stretching vibrational modes. In case of BaGdAlO4 phosphor sharp peak obtained at 443 cm−1 is due to Ba2+ ions while peaks at 547, 645, 706 and 846 indicates the gadolinium-oxygen and aluminium-oxygen bending and stretching vibrations [39]. For all the present materials peaks corresponding to eOH stretching and bending modes have not been appeared in the IR spectra demonstrating the absence of moisture in the lattices.

3.4. Transmission Electron Microscopy (TEM) The morphological studies including shape and size of particles were investigated via transmission electron microscopic analysis. Fig. 11(a–d) displays the TEM images of the europium(III) doped MGdAlO4 (M = Mg2+, Ca2+, Sr2+ and Ba2+) phosphor materials. The particles are spindle shaped in magnesium and calcium while these are spherical in strontium and barium. There are two types of particles dark and light colored indicating two types of metal ions in the lattice, one being divalent and other is trivalent. The unbalanced distribution of the flow of mass and temperature during the combustion of material is responsible for the abnormality in the shape and size of particles [40]. The size of the particles lies between 15–35 nm. The particle size as estimated from these micrographs has been found to in accordance with the size calculated from XRD investigations. The nano-range particles have made them suitable for various advanced display technologies and lightening purposes. Table 4 shows the particle size of the samples 8

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Fig. 11. Transmission electron micrographs of MGdAlO4 samples re-heated at 900 °C. (a) MgGdAlO4, (b) CaGdAlO4, (c) SrGdAlO4 and (d) BaGdAlO4. Table 4 Particle size values investigated from diffraction data and TEM. Type of Analysis

XRD TEM

Particle size (nm) MgGdAlO4:Eu3+

CaGdAlO4:Eu3+

SrGdAlO4:Eu3+

BaGdAlO4:Eu3+

30.25 28.06

18.29 21.64

39.43 35.01

19.60 20.07

investigated via TEM analysis and Scherer’s equation. 4. Conclusion Novel red phosphors MGdAlO4:xEu3+ have been designed via easy, quick and cost-effectively feasible combustion process at temperature 600 °C with urea, using as a fuel. In addition, heating of each sample at higher temperature of 900 and 1050 °C for two hours show enhanced crystallinity and optical intensity. Excellent photoluminescence response was obtained at 0.03 mol percents concentration of europium(III) ions. Photoluminescence analysis has demonstrated that the proposed phosphors show strong emission in red region at 610–615 nm belongs to 5D0→7F2 (forced electric dipole) transition of Eu3+ in existing matrixes. The lack of 9

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inversion centre around europium ion in the lattice appears to be responsible for higher intensity of 5D0→7F2 transition (610–615 nm). The weak intensity peak due to 5D0→7F1 transition (magnetic dipole) was situated at 588–592 nm. Color coordinate values (x and y) for the emitted red light were explained by CIE triangle. The crystalline environment and single phase structure of the recommended lattices are explained by X-ray diffraction technique. SrGdAlO4:Eu3+ phosphor with tetragonal lattice takes I4/ mmm space group. The chemical bonding of metal-oxygen bonds in proposed materials have been explained using FTIR technique. The information of three dimensional arrangements and crystallite size is provided by TEM investigation. It is found that size of particle as derived from XRD and TEM respectively, are in decent settlement with each other. These studies recommend the reported phosphors to be used for different lighting purposes in future. 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