Shifting and enhanced photoluminescence performance of the Sr1-xEuxMgAl10O17 phosphor

Accepted Manuscript Shifting and enhanced photoluminescence performance of the Sr1-xEuxMgAl10O17 phosphor Akshkumar Verma, Ashish Verma, G.V. Bramhe PII:

S0925-8388(18)33406-6

DOI:

10.1016/j.jallcom.2018.09.166

Reference:

JALCOM 47585

To appear in:

Journal of Alloys and Compounds

Received Date: 9 August 2018 Revised Date:

12 September 2018

Accepted Date: 15 September 2018

Please cite this article as: A. Verma, A. Verma, G.V. Bramhe, Shifting and enhanced photoluminescence performance of the Sr1-xEuxMgAl10O17 phosphor, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.09.166. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Shifting and enhanced photoluminescence performance of the Sr1-xEuxMgAl10O17 phosphor Akshkumar Verma1*, Ashish Verma1, G.V. Bramhe2 2

Department of Physics, Dr. Harisingh Gour Vishwavidyalaya Sagar- 470003, M.P., India. Department of Physics, Govt. Autonomous Postgraduate College Balaghat-481001, M.P., India.

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1*, 1

Abstract

In the present work, the SrMgAl10O17:Eu2+, efficient nanophosphors were prepared using combustion route by employing urea as a fuel. The structural and PL (Photoluminescence)

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properties were measured. The crystallite sizes of the synthesized phosphor were found in the order of ~15 nm to 90 nm by the Hall-Williamson equation. The surface area of the phosphor was found ~35.9 m2/g and the band gap was calculated ~5.29 eV. The Photoluminescence

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properties were studied for monitoring excitation (λem.= 460 nm) and emission spectra (λex.= 330nm). The phosphor shows strong blue luminescence in the range of ~460 nm to 470 nm due to the Eu (II) ions originating from the transitions 4f65d1→4f7. The photoluminescence intensity and quantum efficiency is upgraded by using an important parameter of the combustion process. The efficient SrMgAl10O17:Eu phosphors exhibits strong blue

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luminescence and 93.2% quantum efficiency for 8 mol% Eu. Long persistence phenomena of the phosphor was discussed using PL decay properties. The CIE color coordinate of the efficient phosphor was found (X= 0.16, Y= 0.18) suitable as a blue light emitting phosphor.

Corresponding Author

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1*

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Keywords: SrMgAl10O17:Eu phosphors, Combustion route, Structural and Photo-luminescence properties.

Mr. Akshkumar Verma

Email: [email protected] Department of Physics, Dr. Harisingh Gour Vishwavidyalaya Sagar- 470003, (M.P.), India.

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ACCEPTED MANUSCRIPT 1. Introduction Phosphor materials are found in wide application range from a fluorescent lamps to luminescence of modern lighting systems [1]. Classical inorganic phosphors usually consists of a host lattice within activators ions doped into it in small concentration. Some useful inorganic hosts such as sulphates, borates, phosphates, fluorides are known to be good photo-

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luminescence materials. After sulfide materials, alkaline earth aluminate host has gained recent importance and a large number of long- persistent phosphors were developed with aluminate as the matrix. Although aluminates are more chemically stable than sulfides while others are moisture sensitive. However aluminates exhibit long-lasting phosphorescence

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property due to the formation of a defect caused by charge compensation and cation disorders [2-3]. The rare earth doped alkali aluminates have been of considerable interest for the need of novel phosphors for lighting, it is one of the most important and urgent challenges to

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develop fluorescent material based solid state lighting (SSL) [4]. Various aluminates are used as hosts with doping rare earth ions for luminescent application [5]. Luminescence of strontium aluminate hosts (e.g. SrAl2O4, SrAl2B2O7, SrAl4O7, SrAl12O19, Sr3Al2O6, and Sr4Al14O25) and co-doped with other rare earth ions have attracted much attention due to their special long afterglow phenomenon [6]. Eu2+ ions used as an activator for aluminate host

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materials are typical blue emission centers which in terms of practical application are utilized in various fields in the emissive product. The europium and other rare earth activated phosphors such as BaMgAl10O17:Eu2+, CeMgAl10O17:Tb3+ as lamp phosphors, RAlO3:Ce3+ (Y and Gd) as a scintillator material, SrAl12O19:Mn is a green emitting phosphor used for

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plasma display panel, Pr and Nd doped SrAl12O19 crystal show good laser properties, SrAl2O4:Eu2+ Dy3+, Sr4Al14O25:Eu2+Dy3+ exhibit high brightness and long lasting afterglow

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and BaMgAl10O17:Eu2+ blue phosphor for display and lighting device [7-8]. The Eu2+- doped barium and strontium magnesium aluminates are recent commercially exploited luminescent materials. Strontium magnesium aluminate and other aluminate phosphors were produced by a traditional high-temperature solid-state reaction route [9], where oxides and carbonates are used as a raw material which demands the high annealing temperature and long durations, 1300 0C and 5-10 hour respectively [5]. The alkaline earth aluminate, SAM Eu2+, SAM:Eu2+, Mn2+ phosphor developed via solid-state method as well as combustion synthesis [10-11]. The PL and other properties greatly depend on the synthesis approaches, but sol-gel, hydrothermal, spray pyrolysis microwave irradiation, co-precipitation synthesis are wet chemical methods and solid-state method have some disadvantages [12]. To achieve a phase pure phosphor with high luminescence efficiency and uniform distribution of luminescence 2

ACCEPTED MANUSCRIPT centers urea fuel combustion synthesis was proposed [12-16]. Therefore in this work, we have used urea as a fuel material, strontium magnesium aluminate (SAM) as a host for aluminate phosphor material synthesized via combustion route. The reaction mechanism of the combustion is very complex. There are several parameters influencing the reaction such as the type of fuel-oxidizer, use of excess oxidizer, ignition temperature and amount of water

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contained in the precursor mixture, type of flame, generated gases, air, fuel-oxidant ratio and chemical composition of the precursor reagents. During the combustion synthesis reaction, there are four important temperatures that can affect the reaction process and the properties of the final product [17]. The use of an oxidizing agent could improve the reaction completion

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as well to tailor phosphorescent properties. The higher urea (oxidizing agent) contended mixture with HNO3 in combustion gives much combustion energy during the burning process of solution inside the furnace which increases crystallite sizes of the material. Therefore

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increases the efficiency of material or increases the intensity of material in the respective region of the visible region. Thus the photoluminescence intensity and crystallite size of composed material both are proportional to each other.

In another hand, the emission

intensity is also improved by after thermal treatment of synthesized powders proceeded without oxidizing agent [10]. S. Ekambaram, and his team have synthesized Eu2+ doped barium-based compound by rapidly heating an aqueous concentrated solution containing the

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stoichiometric amounts of metal nitrates and a fuel (urea) at 400/500 0C (carbohydrazide or diformyl hydrazine) [1]. Already some authors have reported of temperature 400-550 0C that is useful temperature for synthesis of Eu2+ emission of the SrMgAl10O17 phosphor. [18]

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Therefore in this work, we have described an alternative low and high temperature (400-700 0C) chemical combustion synthesis route for the preparation of SrMgAl10O17:Eu2+

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efficient phosphor material as shown in Fig. 1. Another way, chemical composition or proper mixing is a more important parameter since, nitrate materials have the presence of large crystallization water in aluminum nitrate, magnesium nitrate, strontium nitrate, and europium nitrate. In this present work, we have used the different timing for mixing before combustion reaction for the raw material and also used the different timing for grinding after combustion reaction for the foamy product. In addition, we have described the PL luminescence intensity and quantum efficiency of efficient SAM: Eu2+ phosphor as compared to the previous work of synthesized efficient strontium magnesium aluminate nanophosphor for their possible useful application. 2. Experimental Detail

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ACCEPTED MANUSCRIPT SrMgAl10O17:Eu nanopowder was prepared by solution combustion route. In this process, starting materials were taken Aluminum nitrate nona hydrate [Al(NO3)3.9H2O], Strontium nitrate hexa hydrate [Sr(NO3)2. 6H2O], Magnesium nitrate hexa hydrate [Mg(NO3)2. 6H2O] and Europium nitrate hexa hydrate [Eu(NO3)3.6H20)]. Then weighed quantities of each nitrate and urea mixed together and crushed with the help of mortar and

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pestle for a different time to form a thick paste. The resulting paste is transferred to a crucible and introduced into a vertical cylindrical muffle furnace (Muffle furnaces (0 0C o 10000C) MICRONIX MIT 961) maintained at an efficient temperature according to experiment. Initially, the mixture boils and undergoes dehydration with the evolution of a large number of

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gases (oxides of carbon, nitrogen, and ammonia). The process is highly exothermic continuous and the spontaneous ignition occurs. The solution underwent smoldering combustion with enormous swelling producing white foamy and voluminous ash. The flame

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temperature as high as 1600 -1800 0C converts [19], the vapor phase oxides into mixed aluminates. The flame persists for ~50 – 60 seconds. The crucible is then taken out of the furnace and allowed to cool to the room temperature, then the foamy product can easily be milled by mortar and pestle, and obtain the final powder Sr1-xEuxMgAl10O17. The detailed description of the method can be found in the original work of Patil and co-worker [19-23]. In this process, we have used some important parameter as shown below:

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1. The SrMgAl10O17:Eu0.01 phosphor synthesized by the 400 0C-700 0C heating temperature of combustion route, as tabulated in table 1.

2. When the doping concentration of europium has varied inside the host, then we have used

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40 min timing for mixing before combustion reaction for precursor material and also used 10 min timing for grinding after combustion reaction.

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2. Here we have used the heating temperature 400 0C for synthesis of product and also used grinding time 10 min for the synthesized foamy material while the material mixing time before combustion reaction (burning process) has varied for all samples. 3. Here we have used constantly material mixing time 2 hour before combustion and also has used constant temperature for synthesis while the grinding time has varied for synthesized material for all samples. Synthesized samples and used parameters are summarized in table 2 & table 3. The chemical reaction is shown in equation (1) for SAM:Eu phosphor synthesized by combustion route as shown in Fig. 1.

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ACCEPTED MANUSCRIPT 10Al NO ∙ 9  O + 1 − x Sr NO  ∙ 6H O + Mg NO  ∙ 6  O + x [Eu O  !" #

+ HNO ] + NH CONH $%%%%%%%%%%& Sr'() Eu) MgAl' O '# + H + N + NH + O + CO + H O 1 2.1 Phosphors Measurement Techniques

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The x-ray diffraction patterns of as-prepared samples were determined by powder Xray diffractometer (D8 ADVANCED BRUKER) by using Cu-Kα radiation. The morphology, porosity, particle size and composition of the products were examined by scanning electron microscopy (FESEM–FEI NOVA NANO SEMTM 450, 30kV) and transmitting electron

SORP MR6 surface area analyzer.

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microscopy (TECNAI G2 T30 (S-TWIN). The BET surface area was measured by BEL Infrared (IR) spectra were recorded on a Fourier

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Transform Infrared (FT-IR) spectrometer (SHIMADZU, model no. 8400S) with a resolution of 2 cm-1 and in the range 500-3800 cm-1. To measure IR spectra, the samples were mixed with KBr (Sigma Aldrich, 99.99%) in 1:5 ratio and then spectra was recorded. The Raman spectra of the sample were recorded by RENISHAW Micro Raman Spectrometer attached with a He-Ne laser excitation source of 633 nm. UV-VIS spectra was recorded using SYSTRONICS UV-2201 Double beam spectrophotometer in the absorbance mode. The

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sample was dispersed in the methanol and then UV-Vis spectra were recorded. The photoluminescence spectra measurement were carried out under ultraviolet excitation using 350 nm radiation from Nd-YAG laser and detected by CCD detector (RF- 5301 PC Shimadzu

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Model: QE 65000, Ocean Optics, USA) attached with the fiber. The lifetime decay was recorded with a fluorescence spectrometer (Hitachi F-4500) with a 100W flash Xenon lamp as the excitation source. The PL emission quantum efficiency under ultra-violet excitation

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was measured using an integrating sphere. The excitation source was a 500 W Xenon lamp. 3. Results and discussion 3.1. X-ray diffraction analysis The overall phase structure of the combustible product characterized by X-ray

diffractograms. The small number of doped europium ions have virtually no effect on phase structures. The Fig. 2 & Fig. 3 shows that the products are of high purity, β-Al2O3-type structure, perfect crystallite and well crystalline hexagonal lattice phase for Sr1-xMgAl10O17: Eux with the P63/mmc space group of phosphor sample [11, 24]. The structure is built of two spinel blocks of MgAl10O16 separated by one mirror plane (SrO). In such a unit cell, there are large numbers of octahedral coordinated [AlO6] sites, in which great Eu ions can be 5

ACCEPTED MANUSCRIPT accommodated and stabilized. Furthermore, the [AlO6] layer is separated by the [AlO4] and [SrO8] layer in the direction along the c-axis, which allows various substitutions, such as SAM:Eu2+, SAM:Mn2+, and SAM:Eu2+, Mn2+ etc. [5, 11, 25]. The predicated 1 mol% doped europium strontium magnesium aluminate compound has been synthesized by combustion route according to 400, 450,500, 550, 600, 650, and 7000C as shown in PXRD Fig. 2(a). The

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synthesized product has perfectly matched to the JCPDS file (26-0879) of PXRD pattern. The diffraction peak is corresponding to the (004), (110), (101), (102), (104), (105), (008), (107), (114), (200), (201), (202), (204), (205), (1010), (217), (1112), (308) and (2014) lattice planes of hexagonal phase parameter( a = 5.60Å, c = 22.46Å) by comparing the JCPDS card no.

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26-0879. The crystallite sizes (D) of the sample is estimated by the Scherrer equation (2) and the Hall-Williamson equation (3). The crystallite sizes and particle sizes of synthesized compound are summarised in table 4. Increased combustion heating temperature indicates

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that crystallite sizes (D) are increased and microstrain is decreased. The minimum particle size (~15nm-40nm) is observed at the 400 °C combustion temperature. The maximum particle size and crystallite sizes of product are found at 700 °C. Because in hightemperature, nanoparticles of phosphor get bulk nanostructure. According to observation bulk particle sizes are ~150-380nm and ~90nm crystallite sizes of phosphor has been observed, as

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tabulated in table 4. The raise of the combustion temperature results in crystallite size (D) increase and microstrain decrease [26]. *=

, 2

./012 ,+ 7 sin2 3

*

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. cos2 =

Thus, the slope of βcosθ/λ verses sinθ/λ plot has given the microstrain (η) as shown in Fig. 4,

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we have got a straight line with slope η and intercept kλ/ D. The value of η is the microstrain in the crystallites. The constant k is typically close to unity and ranges from 0.8 to1.39 [2728]. The typical microstrain values of samples were calculated by equation (3) using the Williamson-Hall (W-H) fitting method [29]. D is the crystallite sizes of the material, λ is the wavelength (0.154060 nm) of X-ray, β is the full-width half maxima of the diffraction peak, θ is the angle of diffraction, and k is the shape factor (k = 0.9). The microstrains of phosphor sample are indicated that the presence of defect and stress inside crystal as well as which may be due to crystal planes are not identical. The small nanoparticle has more microstrain than bulk nanoparticle which shows the compressive strain nature of the crystal due to doping of Eu and lattice distortion in the crystal. 6

ACCEPTED MANUSCRIPT The X-ray diffraction pattern of strontium magnesium aluminate doped with different concentration of europium reveals that the activators are incorporated into the lattice, and the doping of Eu does not change any significant in the host structure as shown in Fig. 2(b). The radius of Eu2+ is 0.112 Å and radius of Sr2+ is 0.126 Å, both are very close to each other, therefore Eu2+ is believed to perfectly reside on strontium ion site.

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The crystalline size is 30 nm, microstrain is 0.0085 and particle size ~15-40 nm, calculated of the Sr0.92Eu0.08MgAl10O17 (E5) compound while taking the diffraction angle 2θ=19.9033 degree. According to the PL result (Fig. 13), this sample (E5) gives more intensive result than other composition.

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While using the combustion reaction temperature 400 0C, there takes 10min timing for grinding for the synthesized foamy material when the material mixing time before combustion reaction (burning process) has varied for all samples. We observed, according to

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PL result (Fig. 14), sample E50 gives more efficient result than another sample. While taking diffraction angle 2θ=19.9033degree, the crystallites sizes have found to be in the range of ~29nm, microstrain ~0.0081, and particle size 4~15 to 25nm range for the more intense peak of E50 sample. Similarly when we have used constantly material mixing time 2 hour before combustion and also has used constant temperature for synthesis while the grinding time has

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varied for foamy product for all samples. We observed, according to the PL result (Fig. 15), the SAM:Eu sample (E509) shows efficient result of PXRD pattern as shown in Fig. 5. The crystallites sizes have found to be in the range of ~14nm, microstrain ~0.00751, and particle size was found

~14 to 20nm range for the more intense peak while taking the diffraction

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angle 2θ=19.9033degree, as tabulated in, table 5. This synthesis route led to a phase pure SAM:Eu0.08 with uniform, well-crystallized

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nanostructure where taking mixing time 2h before combustion reaction and 90 min grinding time for foamy product after combustion reaction.

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ACCEPTED MANUSCRIPT 3.2. High-resolution Transmission electron microscopy, Scanning electron microscopy & BET surface area analysis Here in, two instruments TEM and XRD are used for particle size determination. TEM is certainly the most direct method, providing real images and impression of homogeneity of the particles. Although, the elaboration of size distribution curves is limited to the consideration of typically several hundreds of particles. The Fig. 6 shows the TEM

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images of E1, E2, E3, E4, & E5 of SAM:Eu powder samples. As it can be seen, raising the heating temperature from 400 °C to 700 °C led to particles size growth. According to TEM images as shown in Fig. 6, domain particle size of samples heated at 400, 500, 600, and 700 °C range from 18 to 30 nm, 35 to 50 nm, 40 to 65 nm, and 70 to 120 nm, where their mean

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size is 25, 40, 50, and 90 nm, respectively. These results are in good agreement with the calculated results from XRD patterns.

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Again for sample E509, it is observed from the SEM and TEM micrographs, that the structures are about ~15 to 40nm in diameter compatible with the calculated values of the XRD data by the Debye Scherrer formula for the optimized concentration of Eu (8 mol%). Several micro/nano spherical particles are found within the grain can be easily observed from the high-resolution micrographs. The scanning electron micrographs also show the presence of pore, voids and regular homogeneity as shown in Fig. 7(a, b & c). The Fig. 7(d, e & f)

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shows the typical TEM images of one of the samples E509 under study. The non-uniformity of particle shapes & sizes are attributed to the combustion synthesis method by varying the fuel and fuel-to-oxidant ratio, the crystal phase or surface area or particle size or porosity could be suitably tailored [during which a large volume of volatiles release due to very high

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non-uniform temperature (~1600-1800 0C) distribution of the flame] [30]. The redox reactions with the nitrates become more exothermic & make them undergo combustion

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violently. Because of the uncontrolled dynamics of the process, the powder contains pores, irregular shape of particles. This is an inherent feature of this process. The SrMgAl10O17: Eu0.08 (E509) sample appeared to consist of spherical, hexagonal, regular and irregular particle distribution, were slightly agglomerated of micro and nanoparticles. Because the mortar and pestle used for mixing and grinding while timing for mixing and grinding of material was varied as summarized in table 4. The SrMgAl10O17:Eu powder composed of tiny particles have a high surface area (according to BET result), many vacancies in the crystal structure (corresponding to SEM result). It is well known that nanophosphor has many peculiar properties [31]. The BET surface area of the sample was found to be ~35.9m2/g. With the help of the urea combustion route, synthesized phosphors are well-crystallized and 8

ACCEPTED MANUSCRIPT nanostructure phosphors for the present communication are highly advantageous for showing the phenomena of blue luminescence and their related applications. 3.3. FT-IR spectra & Raman spectra analysis The FTIR spectra of SrMgAl10O17: Eu0.08 (E509) phosphor is shown in Fig. 8 (a) in the range from 400cm-1 to 3800cm-1. The SAM samples are exhibiting bands centered at

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377.5, 680, 1021.2, 2023 and 3552 cm-1. The sharp band at 680cm-1 are attributed to metal oxygen vibrations. The main sharp band at 680 cm-1 is due to the hexagonal structure of SAM:Eu. The weak band centered around 3552 cm-1 can be assigned to vibration mode of chemically bonded hydroxyl groups due to potassium bromide absorbed some water

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molecule. The band at 2054cm-1 to 1982cm-1 provides an evidence of molecular water in the structure which are assigned to H-O structure vibrations [32]. It is significant that there is no signature at 1387

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cm-1 indicative of the presence of nitrates. Nitrates were reported to be present in powders produced by urea fuel combustion method and further treatment at 700 0C eliminated the nitrate, organic impurity, and evaporated water molecule completely [33]. In our process, the furnace maintained at 550 0C and combustion occurred to produce an exothermic reaction and rise the reaction temperature in excess of 1800 0C, due to this high-temperature nitrates and organic products are removed from the final powder, no post thermal treatment was

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required in our process. Some sharp peaks between ~675 cm-1 to 150 cm-1 are due to the stretching or rotating, mode of Sr-Al-O in hexagonal lattice coordinate and in agreement with X-ray diffraction results for crystalline SrMgAl10O17:Eu phosphor material.

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In order to enhance the understanding of the doping effect from the structural point of view, Raman scattering study is a very useful tool for investigation. The lattice vibrational

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modes which provides details of lattice vibrational change [34], due to hexagonal symmetry of the compounds no material exclusion of vibrational modes occurs such that IR spectroscopy would afford the same result and was thus left out. To a first approximation, which attributes modes at a frequency higher than 1009 cm-1 to Al-O stretching vibration, and the narrow low-frequency peaks below 840 cm-1 to hexagonal vibration or tilts, the most intense band at 1292 cm-1 to the bending vibration of O-Al-O group within the hexagonal structure due to electron-phonon interactions as shown in Fig. 8(b). 3.4. VIS- UV analysis The study of optical absorption is important to understand the behavior of nanocrystals. A fundamental property is the band gap, the energy separation between the filled valence band and the empty conduction band. Optical excitation of electrons across the 9

ACCEPTED MANUSCRIPT band gap is strongly allowed producing an abrupt increase in absorption at the wavelength corresponding to the bandgap energy. This feature of the optical spectrum is known as the optical absorption edge. The optical absorption spectra recorded of Sr1-xMgAl10O17: Eux (E1 to E9) in the range of 190nm to 800nm is shown in Fig. 9(a). It can be seen that the spectra are featureless and no absorption occurs for wavelength λ > (visible) 390nm [35]. The

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absorption edge was found at λ= 220nm. In order to calculate the optical band gap by using the Wood and Tauc relation (equation 3), a graph is plotted between hυ versus (hυα)2 (photon energy) as shown in Fig. 9 (b).

;ℎ= = ,ℎ= − >? @ 3

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where α is the absorbance, h is the Planck constant, υ is the frequency Eg is the band gap and n is an exponent having values ½, 2, 3/2 or 3 depending on the type of transition such as direct allowed, indirect allowed and directly forbidden respectively [36]. The energy band

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gap was determined by extrapolating straight line portion of the curve to the intercept of the photon energy axes and only a slight decrease of the band gap is observed from 5.29 eV. 3.5. Photo-luminescence spectra analysis

The synthesized SrMgAl10O17:Eu phosphor is naturally white in appearance. The emission spectrum displays a symmetrically broad band centered at 460nm under the excitation of 330nm. The excitation spectra are recorded in the range of 200 to 400 nm and

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emission spectra are recorded in the range of 400 nm to 700 nm. The PL excitation spectra of three sample SAM:A, SAM:D, and SAM:F recorded while monitoring emission wavelength 460 nm. Similarly, the emission spectra recorded while monitoring excitation wavelength 330

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nm. The excitation spectra of SAM:Eu exhibited broadband emission at a lower temperature but at higher temperature exhibited three sharp peaks in different wavelengths due to different Eu positions.

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The PL emission spectra of SAM:A, SAM:B, SAM:C, SAM:D, SAM:E, & SAM:F

of SAM:Eu phosphor samples exhibited the raising temperature of combustion reaction from 400 °C up to 700 °C led to emission spectra of samples shifted toward the higher wavelength from lower wavelength. The higher temperature PL emission spectra exhibited red emission, but at low temperature, the PL emission spectra exhibited blue emission. The increased temperature of combustion reaction from 400 °C to 700 °C results in the PL emission spectra of phosphor exhibited first higher peak of SAM:F sample observed at 445 nm and the second peak of SAM:F sample observed at 601 nm. The first peak of SAM:F sample is more intensive and broad with respect to the second peak. Similarly, for SAM:E sample, the first peak observed is more intensive than the second peak but the second peak of SAM:E sample 10

ACCEPTED MANUSCRIPT is more intensive than the second peak of SAM:E sample. The similar order followed next sample just like SAM:F and SAM:E sample, as shown in Fig. 10&11. The PL intensity of the first peak continuously decreases and the intensity of the second peak is continuously increased if the combustion temperature of the furnace is increased. The first peak of samples shifts toward the second peak of the sample and vice

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versa, corresponding to the heating temperature. The first peak observed in the range of 440 nm to 490 nm due to Eu2+ ion, but the second peak observed in the range of ~560 nm to ~600 nm due to Eu3+ ions. The Blasse et al studied Eu3+ emission extensively in various hosts and they came up with the following conclusions. Eu3+ emission usually occurs from 5D0 → 7Fj transitions. There are three transitions, which are of prime importance 5D0→ 7F0 (around 570

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nm), 5D0→ 7F1 (around 595 nm) and 5D0 → 7F2 (around 610 nm). The first one is strongly forbidden transition, and yet observed with appreciable intensity in some hosts. The second

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one is the electric dipole forbidden but is allowed as a magnetic dipole. This is the only transition when Eu3+ occupies a site coinciding with a center of symmetry. When the Eu3+ ion is situated at a site that lacks the inversion symmetry, the transition corresponding to even values of j (except 0) are electric dipole allowed and red emission can be observed [37-40]. The 5D0 → 7F1 transition can also be observed as a magnetic dipole allowed transition as

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shown in Fig. 12. Further, all the lines corresponding to these transitions split in two number of components decided by the local symmetry [41]. The spectra are also sensitive to sizes of cations [42] and chemical bonding [43]. The emission band could be separated into three peaks, which are ascribed to Eu ions occupying BR, a-BR, and mO sites [44-46]. The d-f

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excitation level of the Eu2+ ions in solid state compounds depends strongly on the strength of crystal field around the Eu2+ ions, [47- 49]. Eu2+ ions occupy the different [Eu (1), Eu(2), and

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Eu (3)] sites emit at different peaks of 441, 461, and 497nm. Whereas the luminescence efficiency of the prepared phosphors which is greatly

dependent on the crystallite sizes and crystal strength of the powder [12]. Fig. 11 indicating that the PL luminescence intensity is increased because of crystal strength increases in the high-temperature combustion reaction while in the low-temperature of combustion reaction crystal strength decreases [47-48]. Therefore, if there is decrease in temperature of combustion reaction, second peak moves toward the first peak. Thus nanocrystal of SAM:Eu exhibits blue emission spectra in the range of ~430 nm to ~490 nm as shown in Fig 11. It is well known that surface morphology and particle size show impact role for luminescence properties of the material of nanophosphor. [12, 22, 32].

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ACCEPTED MANUSCRIPT The excitation and emission spectra of SAM:Eu2+ phosphors is shown in Fig. 13(a). The PL emission spectra is recorded of the Sr1-xEuxMgAl10O17 phosphor prepared via urea fuel combustion route, when the variation of concentration of activator and taken 40 min timing for material mixing before combustion reaction, and 10 min timing for grinding after combustion reaction for each sample. The monitored excitation spectrum wavelength 450 nm

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and monitored emission spectrum wavelength 330 nm, which corresponds to the 4f65d1→ 4f7 electric dipole transition of Eu2+ ions, which implies this blue phosphor is suitable for use with the near UV-LED chips. The maximum intensity was observed for 8 mol% of Eu, and minimum intensity was observed for 0.1% of Eu as shown in Fig. 13(b). The Eu2+ ions are

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particularly unique because its broad and intense band luminescence 4f65d→4f7 strongly depends on host (SrMgAl10O17) material with emission wavelength extending from UV to red wavelength of the electromagnetic spectrum. It is observed that the luminescence quenches at

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8 mol% of Eu2+ due to the more photons are involved in the luminescence process. The concentration quenching occurred due to the energy transfer from one activator to another if the activator was introduced solely to Z ion sites according to the Blasse et al. [50]. The probability of energy transfer is approximately proportional to the critical distance (Rc) that

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can be estimated by the equation (4).

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ACCEPTED MANUSCRIPT 3V RB = 2 C J 4πXH Z

'K

4

Where Z is the number of cations in the unit cell, V is the volume of the unit cell, and Xc is the critical concentration for quenching, if taking the values of Xc = 0.08, Z = 2, and V = 614.04 Å3, the critical distance Rc is then calculated to be about 19.42Å, according to the

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Dexter theory, a multipolar interaction between the magnetic dipole may cause the concentration quenching of the ion, when the ions are separated by a certain distance.

Again for the optimized concentration 8 mol% of Eu, the SAM:Eu0.08 phosphor prepared by modified combustion route, used by one of the important parameter tabulated in,

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table 3. The excitation and emission spectra of SAM:Eu2+ phosphors is shown in Fig. 14(a). The PL emission spectra is recorded for E51, E52, E53, E54, E55, E56, E57 and E58 (Sr0.92Eu0.08MgAl10O17). The maximum PL intensity was observed for E50 sample, and

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minimum PL intensity was observed for E51 as shown in Fig. 14(b). The great enhancements of PL intensity are about to ~199% when compared to that of the phosphor prepared by the traditional combustion route [16] due to the decreased particles size and reduced surface defects.

Again when used different timing for grinding for the foamy product after

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combustion reaction for the synthesized foamy product of SAM:Eu0.08, and then taken 2h timing for material mixing before combustion reaction, because of surface morphology and particle size show impact role for luminescence properties of material [12]. The PL emission spectra recorded for E501, E502, E503, E504, E505, E506, E507, E508 and E509

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(Sr0.92Eu0.08MgAl10O17) as shown in Fig. 15(a). The maximum PL intensity was observed for E509 sample and minimum PL intensity was observed for E501 sample as shown in Fig.

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15(b). The great enhancement of PL intensity ~72% when compared to PL intensity of E58 sample.

The chemical composition of raw material (urea and nitrates) is necessary for

homogenous thick paste to synthesize nanophosphor material since nitrates material have the presence of large crystalline water. Therefore, the total enhancement of PL intensity ~272% has been observed as compared to that of the traditional combustion route since the scattering effect is minimized due to the uniform particle size, regular morphology, minimum surface defects, and well-crystallite nanoparticle [16]. 3.6. Decay Analysis The decay curve is shown in Fig.16 (a) of the phosphor which is measured by the emission wavelength 350 nm and excitation wavelength 470 nm of Sr0.92MgAl10O17:Eu0.08 13

ACCEPTED MANUSCRIPT phosphor sample which shows an exponential nature. The decay behavior is well fitted by an empirical equation (5) [51]. (!

(!

(!

L = M' NOP Q T + M NOP Q T + M NOP Q T 5

RS

RU

RV

Where A is the emission intensity, after switching off the excitation source A1, A2, and A3 are weighting constant parameters τ1, τ2, and τ3 are the average lifetime of excited electron

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deciding the rate of rapid and exponential decay component respectively. The obtained decay curve which shows three different values of τ was found by simulating the decay curves as shown in table 6. This indicates that there are three different types of traps and hence three kinds of decay. The largest value of τ3 is related to the deepest trap center and slowest in the

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decay process. The mechanism of the long persistence is due to the holes trapped – transported de-trapped process.

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The International Commission on Illumination (CIE-1931) chromaticity diagram of Eu3+ doped in SrMgAl10O17 phosphor is shown in Fig. 16(b). The CIE chromaticity coordinates (X =0.16, Y=0.18) were calculated under 330 nm excitation. The PL excitation intensity and corresponding emission peak shows that the Sr0.92MgAl10O17:Eu0.08 phosphor found color coordinate in the blue region of the emission spectra. The color purity of color doped with Eu2+ is calculated 69.69% by the equation (6) [52]. [O − O\  + ] − ]\ 

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Color purity =

[O^ − O\  + ]^ − ]\ 

× 100% 6

Where (x, y) is (0.16, 0.18) for blue color are co-ordinate of sample point (xd, yd) is (0.14, 0.08) are coordinate of illumination wave-length by NTSC and (xi, yi) is (0.3101, 0.3162) are

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coordinate of white light in CIE diagram. Therefore this sample (E509) may be highly suitable as a blue phosphor for UV excited LED and may be used as a phosphor conversion

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blue optical LEDs etc.

3.10. Quantum Efficiency Analysis The quantum efficiency is defined as the ratio of the energy (quanta) required to excite

a phosphor to the energy (quanta) emitted from the sample. The quantum efficiency (Q) can be obtained from the following equation (7). Q =

number of photons emitted number of photons absorbed

Q = p

ijklmno

qoroqosto (pjklmno

14

 7

ACCEPTED MANUSCRIPT Here Isample is the emission intensity, Ereference and Esample are the intensities of the excitation light not absorbed by the sample and reference sample respectively [53]. The quantum efficiency of SAM has been measured by conventional methods using a standard material of well-known quantum efficiency. The incident beam is perpendicular to the surface of the sample, thus the observation angle is at 45° to the excitation source. It has been already found

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that the quantum efficiency is ~65% for SAM containing 2 mol% Eu (270 nm excitation) [54]. The BaMgAl10O17:Eu2+ (BAM) is a commercial blue emitting phosphor for PDP [55] and tri-color lamp [56] because its higher quantum efficiency is about 90% [55-58]. G. Ju et. al have compared quantum efficiency among BaMgAl10O17:Eu and SrMgAl10O17:Eu and they have predicted the quantum efficiency of SrMgAl10O17 is about 90% [11]. It was found that

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the quantum efficiency has been slightly dependent on the Eu2+ concentration. In this study, we measured the absolute value of the PL quantum efficiency (Q.E.) of several SAMs at

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room temperature. This is a more accurate and practical method than the indirect method using a standard material [59]. The SAM containing 0.1 mol% Eu converts only 56% of the absorbed photons to luminescence. As with the luminescence intensity, as the concentration of Eu ions increases, Q increases and reaches a maximum of QE ~86% for SAM containing 8mol % Eu. As the concentration increases further, Q decreases and is only 61% at 30 mol%

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Eu as shown in Fig 17. The concentration dependence of the luminescence intensity is also illustrated in Fig.13, 14 &15. The concentration dependence of both the quantum efficiency and the luminescence intensity indicate that the Eu concentration resulting in the maximum luminescence is 8 mol%. The maximum quantum efficiency (93.2%) has measured for

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sample E509. The quantum efficiency ~7% more than the sample of E5 is due to the properties of the nanoparticle of SAM:Eu0.08.

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4. Conclusion-

The PXRD pattern, TEM and SEM images, FT-IR spectra, the Raman spectra, and

the PL spectra of SAM:Eu is in good agreement with the calculated results. The luminescence efficiency of the prepared phosphors is greatly dependent on the crystallite sizes of the material. Low crystalline nanomaterials are synthesized at low temperature but highly crystalline nano bulk compounds are synthesized via high-temperature combustion process. Low-temperature combustion is process useful for blue spectra but high-temperature combustion is process useful for highly crystalline material for highly efficient red phosphor compound. This synthesis route led to a phase pure SAM:Eu0.08 with uniform, wellcrystallized nanostructure where taking mixing time 2h before combustion reaction and 90

15

ACCEPTED MANUSCRIPT min grinding time for foamy product after combustion reaction. The photoluminescence intensity enhanced ~272%, as compared to that of the phosphor prepared by the traditional combustion route (mixing time 1h before combustion and grinding time 25 min after combustion). The PL emission spectra of the Sr0.92MgAl10O17:Eu0.08 phosphor showed the characteristic Eu2+ emission peaks ~460 nm (blue color) originating from the transitions

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4f65d1→4f7. The CIE (International Commission on Illumination) color coordinate (X= 0.16, Y= 0.18) of the prepared phosphor was found suitable as a blue light emitting phosphor. The mechanism of the PL decay indicates a long persistence of the phosphor. The phosphor exhibits strong blue luminescence and ~93.2% quantum efficiency for 8 mol% of Eu.

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Therefore SrMgAl10O17:Eu (E509) nanophosphor may widely be used in plasma display panel (PDP) as a blue light emitting phosphor, mercury excited lamp, and cheap blue LED phosphor materials etc.

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ACCEPTED MANUSCRIPT Tables captions: Table 1. The various samples of SAM:Eu prepared via the urea fuel combustion process with the corresponding names. Table 2. The various samples of SAM:Eu prepared with the nominal composition of Sr1xEuxMgAl10O17.

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Table 3. The efficient samples of SAM:Eu (Sr0.92MgAl10O17:Eu0.08) prepared via using the timing for mixing before combustion reaction, and timing for grinding after combustion reaction. Table 4.

Typically calculated PXRD results via the Hall-Williamson and the Scherrer

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equation.

Table 5. Typically calculated PXRD results, crystallite size, microstrain, particle size, and

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quantum efficiency of SAM:Eu phosphor samples.

Table 6. Typical PL decay time data for the E1 phosphor.

Figure captions

Fig. 1. The typical schematic illustration of UFC route for the preparation of the SAM:Eu. Fig. 2. The typical diagram for the PXRD diffraction pattern of SAM:Eu powder samples.

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Fig. 3. Typical diagram for crystal structure of the SrMgAl10O17 compound. Fig. 4. The typical plot for microstrain measurement via the Hall-Williamson method. Fig. 5. The PXRD diffraction pattern and micro-strain calculation for efficient SAM:Eu samples.

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Fig. 6. The typical images [from (a) to (e)] for the particle size of the E1, E2, E3, E4, & E5 powder samples of SAM:Eu.

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Fig. 7. (a) The typical SEM images for the SrMgAl10O17: Eu phosphor sample. (b & c) the typical two SEM images for Sr0.92MgAl10O17:Eu0.08 phosphor sample in higher resolution, indicating the distribution of nanoparticles & clusters of the phosphor. (d) The SAED pattern for E509 sample, labeled with lattice planes. (e) HRTEM image of a sample exhibiting the lattice fringes. (f) The typical TEM image for E509 sample indicating the distribution of nanoparticles. Fig. 8. (a) The FT-IR spectra for Sr0.92MgAl10O17:Eu0.08 phosphor at room temperature. (b) The Raman (633nm by He- Ne source) spectra for Sr MgAl10O17:Eu0.08 phosphor material. Fig. 9. (a & b) The UV-Visible spectra and the band gap determination for the various samples of SrMgAl10O17:Eu phosphor. 21

ACCEPTED MANUSCRIPT Fig. 10. The typical PL excitation and emission spectra for Sr0.99MgAl10O17:Eu0.01 phosphor. Fig. 11. The typical plot for sample code vs PL intensity of two different peaks of samples. Fig. 12. The typical diagram for energy levels of Eu2+, and Eu3+ ions. Fig. 13. (a) The typical PL excitation and emission spectra for Sr1-xMgAl10O17:Eux with a variation of doping concentrations of Eu. (b) The plot for effect of contents of doped Eu ion

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on the PL intensity of SAM:Eu phosphor. Fig. 14. (a) The typical PL excitation and emission spectra for Sr0.92MgAl10O17:Eu0.08 with a different timing for mixing before combustion reaction of the sample. (b) Effect of timing for mixing before combustion reaction on the PL intensity.

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Fig.15. (a) The PL excitation and emission spectra for Sr0.92MgAl10O17:Eu0.08 with different timing for grinding after combustion reaction. (b) Effect of grinding time after combustion on the PL intensity.

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Fig. 16. (a) The typical decay curve for the SAM:Eu phosphor sample, (b) The CIE-1931 color- chromacity coordinates diagram for SrMgAl10O17: Eu phosphor.

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Fig. 17. The typical plot of efficient samples vs quantum efficiency of SAM:Eu phosphor.

22

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Table 1. The various samples of SAM:Eu prepared via the urea fuel combustion process with the corresponding names. Samples

Temperature(0C)

Sample code

1

SrMgAl10O17:Eu0.01

400

SAM:A

2

SrMgAl10O17:Eu0.01

500

SAM:B

3

SrMgAl10O17:Eu0.01

550

SAM:C

4

SrMgAl10O17:Eu0.01

600

5

SrMgAl10O17:Eu0.01

650

6

SrMgAl10O17:Eu0.01

700

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S.N.

SAM:D SAM:E

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SAM:F

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Table 2. The various samples of SAM:Eu prepared with the nominal composition of Sr1-xEuxMgAl10O17. Samples

1

Sr0.999MgAl1017:Eu0.001

E1

2

Sr0.99MgAl1017:Eu0.01

E2

3

Sr0.97MgAl1017:Eu0.03

E3

4

Sr0.95MgAl1017:Eu0.05

E4

5

Sr0.92MgAl1017:Eu0.08

E5

6

Sr0.9MgAl1017:Eu0.1

E6

Sr0.84MgAl1017:Eu0.16

E7

Sr0.80MgAl1017:Eu0.2

E8

Sr0.75MgAl1017:Eu0.25

E9

Sr0.7MgAl1017:Eu0.3

E10

7 8 9

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10

Sample code

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S.N.

23

ACCEPTED MANUSCRIPT Table 3. The efficient samples of SAM:Eu (Sr0.92MgAl10O17:Eu0.08) prepared via using the timing for mixing before combustion reaction, and timing for grinding after combustion reaction. Constant parameters

Timing for Grinding (minute)

Sample code

Constant parameters

E51

Tem.

10

E501

Tem. 4000C

2

50

E52

4000C,

20

E502

2hour timing

3

60

E53

10min

30

E503

for mixing

4

70

E54

Timing for

40

E504

before

5

80

E55

grinding

50

E505

combustion

6

90

E56

after

60

E506

reaction

7

100

E57

combustion

70

reaction

80

110

E58

9

120

E50

90

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8

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Sample code

1

Timing for Mixing (minute) 40

E507 E508

E509

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S.N.

Table 4. Typically calculated PXRD results via the Hall-Williamson and the Scherrer equation. Firing them. (0C)

D (nm)

Sr0.99MgAl10O17:

400

~21nm

Eu0.01

500

~32nm

550 600 700

Peak angle (θ)

Particle size (nm)

Microstrain

Scherrer equation (D114)

0.0446

~19nm

17.905

~15-40nm

0.0428

~23nm

17.88

~60-150nm

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Samples

Williamson Hall

~41nm

0.0422

~27nm

17.87

~130-300nm

~53nm

0.0391

~38nm

17.86

~140-350nm

~90nm

0.0348

~60nm

17.85

~150-380nm

E50 E509

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Table 5. Typically calculated PXRD results, crystallite size, microstrain, particle size of SAM:Eu phosphor sample. Efficient Lattice parameters(a =b≠ c) Crystallite size Microstrain samples a (Å) b (Å) Cell volume(Å3) (2θ=19.9033) 5.58 22.39 603.7267 30nm 0.0085 E5

and quantum efficiency Particle size (nm) ~15-40nm

Q.E.% 86

5.57

22.41

602.1020

29nm

0.0081

~15-25nm

89.8

5.63

22.37

614.0456

14nm

0.00751

~14-20nm

93.2

Table 6. Typical PL decay time data for the E1 phosphor. Phosphor

Time τ1

time τ2

time τ3

A1

A2

A3

Sr0.92MgAl10O17:Eu0.08

6min.

6.4min.

6.8 min.

3.4

4.9

5.9

24

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Figures

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Fig. 1. The typical schematic illustration of UFC route for the preparation of the SAM:Eu.

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Fig. 2. The typical diagram for the PXRD diffraction pattern of SAM:Eu powder samples.

Fig. 3. Typical diagram for crystal structure of the SrMgAl10O17 compound. 25

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Fig. 4. The typical plot for microstrain measurement via the Hall-Williamson method.

Fig. 5. The PXRD diffraction pattern and microstrain calculation for efficient SAM:Eu samples.

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Fig. 6. The typical images [from (a) to (e)] for the particle size of the E1, E2, E3, E4, & E5 powder samples of SAM:Eu.

Fig. 7. (a) The typical SEM images for theSrMgAl10O17: Eu phosphor sample. (b & c) the typical two SEM images for Sr0.92MgAl10O17:Eu0.08 phosphor sample in higher resolution, indicating the distribution of nanoparticles & clusters of the phosphor. (d) The SAED pattern for E509 sample, labeled with lattice planes. (e) HRTEM image of a sample exhibiting the lattice fringes. (f) The typical TEM image for E509 sample indicating the distribution of nanoparticles.

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Fig. 8. (a) The FT-IR spectra for Sr0.92MgAl10O17:Eu0.08 phosphor at room temperature. (b) The Raman (633nm by He- Ne source) spectra for Sr MgAl10O17:Eu0.08 phosphor material.

Fig. 9. (a & b) The UV-Visible spectra and the band gap determination for the various samples of SrMgAl10O17:Eu phosphor.

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Fig. 10. The typical PL excitation and emission spectra for Sr0.99MgAl10O17:Eu0.01 phosphor.

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Fig. 11. The typical plot for sample code vs PL intensity of two different peaks.

Fig. 12. The typical diagram for energy levels of Eu2+, and Eu3+ ions.

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Fig. 13. (a) The typical PL excitation and emission spectra for Sr1-xMgAl10O17:Eux with a variation of doping concentrations of Eu. (b) The plot for effect of contents of doped Eu ion on the PL intensity of SAM:Eu phosphor.

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Fig. 14. (a) The typical PL excitation and emission spectra for Sr0.92MgAl10O17:Eu0.08 (E5) with a different timing for mixing before combustion reaction of the sample. (b) Effect of timing for mixing before combustion reaction on the PL intensity.

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Fig.15. (a) The PL excitation and emission spectra for Sr0.92MgAl10O17:Eu0.08 with different timing for grinding after combustion reaction. (b) Effect of grinding time after combustion on the PL intensity.

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Fig. 16 (a) The typical decay curve for the SAM:Eu phosphor sample, (b) The CIE-1931, Color Chromacity coordinates diagram for SrMgAl10O17: Eu (E509) phosphor.

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Fig. 17. The typical plot of efficient samples vs quantum efficiency of SAM:Eu phosphor.

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Highlights-


The Crystalline nanoparticles have synthesized by UFC route.
Low-temperature UFC route is effective for blue emission spectra.
Upgraded quantum efficiency by using UFC route parameters.

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The SrMgAl10O17:Eu (E509) phosphor exhibits strong blue emission spectra.

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ACCEPTED MANUSCRIPT Novelty statement The SrMgAl10O17:Eu (strontium magnesium aluminate) phosphor already has been synthesized and reported via traditional urea fuel combustion route.

In this work, we

observed efficient temperature for combustion reaction for the efficient blue phosphor, the efficient concentration of europium as an activator, suitable timing for mixing before

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combustion reaction and has also measured suitable timing for grinding after combustion reaction. Using these parameters also recorded the shifting and enhancement of the photoluminescence performance. During this work, the PL intensity enhanced about to ~272% and quantum efficiency shift from 86% to 93.2% that compared with earlier reported traditional

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combustion route is more efficient. Additionally, the PXRD, TEM, SEM, FT-IR, Raman, and optical band gap have analysed with the help of various spectroscopic technique. Therefore this modification may be used as a synthesis method for blue phosphor material, blue LEDs material,

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PDP phosphor etc.