Zinc silicates with tunable morphology by surfactant assisted sonochemical route suitable for NUV excitable white light emitting diodes

Accepted Manuscript Zinc silicates with tunable morphology by surfactant assisted sonochemical route suitable for NUV excitable white light emitting diodes R.B. Basavaraj, H. Nagabhushana, B. Daruka Prasad, G.R. Vijayakumar PII: DOI: Reference:

S1350-4177(16)30243-7 http://dx.doi.org/10.1016/j.ultsonch.2016.07.002 ULTSON 3300

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

17 April 2016 3 July 2016 4 July 2016

Please cite this article as: R.B. Basavaraj, H. Nagabhushana, B. Daruka Prasad, G.R. Vijayakumar, Zinc silicates with tunable morphology by surfactant assisted sonochemical route suitable for NUV excitable white light emitting diodes, Ultrasonics Sonochemistry (2016), doi: http://dx.doi.org/10.1016/j.ultsonch.2016.07.002

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Zinc silicates with tunable morphology by surfactant assisted sonochemical route suitable for NUV excitable white light emitting diodes R.B. Basavaraj1, H. Nagabhushana1,*, B. Daruka Prasad2, G.R. Vijayakumar3 1

Prof. C.N.R. Rao Centre for Advanced Materials, Tumkur University, Tumkur - 572 103, India 2 Department of Physics, B M S Institute of Technology, VTU Affiliated, Bangalore - 560064, India 3 Department of Chemistry, University College of Science, Tumkur University, Tumkur - 572103, India Abstract The cationic surfactants assisted ultrasound route was used to prepare Dy3+ doped Zn2SiO4 nanophosphors.

The final products were characterized by powder X-ray diffraction (PXRD),

ultraviolet visible spectroscopy, scanning electron microscopy, transmission electron microscopy and photoluminescence. Orthorhombic phase of Zn2SiO4:Dy3+ (JCPDS card no. 35-1485) was confirmed from PXRD. It was evident that the morphology of spherical and broom like structures were obtained with epigallocatechin gallate (EGCG) and cetyltrimethylammonium bromide (CTAB) surfactants respectively.

Further the size and agglomeration of the products were varied with

surfactants concentration, sonication time, pH and sonication power. The probable formation mechanisms to obtain various micro/nano superstructures were discussed. The characteristic PL peaks were observed at 484, 574 and 666 nm due to the electronic transitions 4F9/2 → 6Hj (j=15/2, 13/2, 11/2) of Dy3+ ions upon excited at NUV pumping wavelength of 350 nm [6H15/2 → 6P7/2 (4M15/2)]. The Judd – Ofelt intensity parameters and radiative properties were estimated by using PL emission data. The photometric studies indicated that the obtained phosphors could be promising materials in white light emitting diodes (wLED’s). The present synthesis route was rapid, environmentally benign, cost-effective and useful for industrial applications such as solid state lighting and display devices. Keywords: Sonochemical synthesis; Nanophosphor; Photoluminescence; Cationic surfactant; Solid state lighting * Corresponding author: +91- 9945954010, E-mail addresses: [email protected] (H. Nagabhushana).

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1. Introduction Over the last few years, rare earth ions doped inorganic nanophosphors were attractive due to their exceptional luminescent properties [1, 2]. It has been demonstrated by many researchers that the luminescent properties of nano scale materials were completely different from their micro/bulk materials [3]. Further, it was well known that the luminescent properties of inorganic materials with nano/micro superstructure were highly dependent on size, shape, morphology, compositions and crystallinity [4]. Currently, the research on efficient and low-cost nanophosphors preparation and their use in solid state lighting industries is essential. The dense particle phosphors with spherical morphology can increase the screen brightness and improve the resolution because of lower scattering of evolved light and higher packing densities than irregular particles obtained by other synthesis routes [5].

The ideal morphology of luminescent particles should include a perfect

spherical shape, narrow size distribution (0.5 – 2 µm) and non-agglomeration. As a result, the research on the synthesis methods for the massive production of phosphors of required morphology receives significant attention. WLEDs were considered to be the next generation lighting systems because of their excellent properties such as high luminous efficiency, low power consumption, environmental friendly features, reliable and long life [6, 7]. It was well known that the silicates were effective luminescent hosts due to stable crystal structure, excellent long term stability and low cost [8, 9]. Consequently, the spectroscopic properties of rare earth ions in silicate materials doped with rare earth ions has been used as commercial phosphors in tricolor fluorescent lamps, plasma display panel (PDP), scintillators. Further, they exhibit excellent water resistance and strong absorption in the near-UV region [10-12]. Ultrasound assisted route has been demonstrated to be a suitable technique for the synthesis of advanced materials with unusual properties. When liquids were irradiated with high-intensity ultrasound irradiation, acoustic cavitation provides the primary mechanism for sonochemical effects. During cavitation, bubble collapse creates intense local heating, high pressures and tremendously rapid cooling rates. These transient localized hot spots can encourage various chemical reactions

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namely oxidation, reduction and dissolution [13, 14]. Till date only a few studies were reported on the sonochemical synthesis of silicate hosts and to the best of our knowledge, no reports were available for preparation of Zn2SiO4: Dy3+ nanophosphors using cationic surfactant assisted ultrasound route. Dy3+ doped hosts were shown an encouraging applications as a white light emitting phosphors since it exhibit two intense photoluminescence (PL) peaks at bluish-green (at ~ 485 nm) and yellowish (at ~575 nm) colors. The blue-green and the yellowish emissions were attributed due to 4F9/2→ 6H15/2 and 4F9/2 → 6H13/2 ; f-f electronic transitions of Dy3+ [15].

The yellow emission

attributed to electric dipole transition was greatly sensitive to the crystal field around Dy3+ ions. On the other hand, the bluish emission was attributed to magnetic dipole transition and hardly varies with the crystal field around Dy3+ ions. The crystal field environment of Dy3+ has significant impact on the intensity of yellow emission, but has little effect on that of the blue emission. Consequently, it was possible to acquire white light emission from Dy3+ activated luminescent materials by varying the intensity ratio of yellow to blue emission (Y/B) via changing the matrix or adjusting the host composition. Therefore Dy3+ activated luminescent phosphors fascinated with much consideration because of their significant applications as potential single phase white phosphors [16]. It was evident from the literature, surfactant molecules such as EGCG / CTAB acts as a molecularly dissolved surface modifier or stabilizer acts as capping agent which were chemically absorbed onto the surface of nanoparticles. EGCG was the most rich and potent component in green tea which is biologically active due to B & D diol hydrophilic rings and A & C hydrophobic rings [17]. CTAB was a combination of ammonium head and hydrocarbon tails. The structures and important chemical properties of both EGCG and CTAB were represented in Table.S1. In this paper, Zn2SiO4 host was selected due to its distinct properties namely low cost, easy preparation, excellent thermal as well as chemical stability and strong absorption in the near-UV region.

Herein, reported the controlled synthesis of Zn2SiO4:Dy3+ with several hierarchical

nano/micro superstructures using EGCG/CTAB surfactant assisted ultrasound synthesis. Further,

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various self-oriented superstructures were also obtained by varying the experimental conditions such as sonochemical time, power and the pH of the precursor solution. The probable formation mechanism for different hierarchical nano/micro superstructures was put forward on the basis of time-dependent experiments. To evaluate the potential applications of the product, PL, CIE, CCT and color purity of Zn2SiO4:Dy3+ nano/micro superstructures were studied and interpreted in detail. 2. Synthesis Characterization Pure and Dy3+ (1-9 mol %) doped Zn2SiO4 nano/micro superstructures were synthesized by using two different surfactants EGCG & CTAB via ultrasound method. The Table.S2 shows the list of chemicals used for ultrasound synthesis. Initially the stoichiometric quantities of precursor solutions of zinc nitrate [(Zn (NO3)3; 14.8745 g, Sigma Aldrich], dysprosium nitrate [Dy (NO3)3: 0.0348 g for 1 mol %] and tetraethyl orthosilicate [C8 H2OO4Si: 10.41 ml, Sigma Aldrich] were dissolved in 50 ml distilled water. These aqueous solutions were thoroughly mixed in a magnetic stirrer to get a clear solution. The surfactant solutions were prepared by adding 5g of EGCG / CTAB in 100 ml double distilled water.

Before ultra-sonication, 5 ml of surfactant solutions were added to the clear

precursor solution. Further, the resultant solution was subjected to the ultra-sonication (ultrasonic frequency ~20 kHz, power ~300 W) at a fixed temperature of ~ 60 °C under different sonication time (1 - 6 h). Similar steps were repeated for the different surfactant quantities (10, 15, 20, 25 and 30 ml) and the dopant concentrations (1 - 9 mol %). The precipitate was filtered and washed several times by distilled water and ethanol to remove ammonium ions. The product was dried at 60 °C for 3 h and calcined at 950 ±5 oC. The flow chart for the synthesis of Dy3+ doped Zn2SiO4 nanophosphor was shown in Fig.S1. Further, in order to investigate the effect of pH in the synthesis of Dy3+ doped Zn2SiO4 with EGCG /CTAB surfactant, the initial precursors were exposed to ultrasound irradiation up to 6 h with different pH values (1 - 8). Phase purity and crystallinity of nanophosphors were measured using a powder X-ray diffractometer (XRD, Shimadzu 7000), Cukα (1.541Ǻ) radiation with nickel filter. Scanning electron microscopy (SEM) measurements were performed on a Hitachi, Model TM 3000 for studying the 4

morphology of the samples. Transmission electron microscopy (TEM) was performed on a Hitachi H-8100 accelerating voltage up to 200 KV, LaB6 filament equipped with EDS (Kevex sigma TM Quasar, USA). The prepared samples were dispersed on a sticky carbon pad. The thin layer of gold (Au) was deposited on the sample to get better image quality. The diffuse reflectance spectroscopy of the samples was recorded on spectrometer PerkinElmer (Lambda-35). The Jobin Yvon Spectroflourimeter Fluorolog-3 operational with 450W Xenon lamp as an excitation source was used for photoluminescence (PL) measurement. 3. Results and discussion 3.1 Morphological studies Morphological studies were carried out to optimize the time of ultrasonic irradiation and also EGCG concentration for the zinc silicate with 5 mol % of Dy3+ for which the PL was optimized as discussed in the subsequent sections. Fig.1 shows the consequence of period of ultrasonic irradiation to tune the morphology of the product with fixed EGCG quantity of 30 ml.

Microstructures produced at

sonication time of 1 h were found to be agglomerated spherical shape particles. With increase in sonication time, there is dispersion of the particles and more spherically uniform particles were observed. For the sonication time of 4 - 6 h, uniformity of the particles was improved and also much better dispersed.

Optimal time for the synthesis of fine and uniform spherical well dispersed

Zn2SiO4 nanoparticles was achieved for 6 h. Fig.2 shows the schematic representation of various stages involved in the formation of the spherical shaped particles. In the process of ultrasonication, TEOS and EGCG act as source of silicate and surfactant respectively. Initially there was a fast nucleation due to high energy input which slowly changes to self-assembly of particles. These particles changes to more spherical nature at the time of acoustic bubble formation then leads to well dispersed spherical shape particles of micro range due to cleavage of acoustic bubbles. Fig.3 shows the SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphors with different quantity of EGCG (5, 10, 15, 20, 25 and 30 ml) irradiated for 6 h. When the quantity of EGCG was 5 ml and 15 ml, non-uniform spherical agglomerated particles dominated (Figs. 4a, b & c). When the quantity of

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EGCG was increased to 25 ml, uniform spherical shaped particles were obtained (Figs.4 d & e). Further with increase in EGCG to 30 ml, uniform spherical shaped particles were completely retained with diameter of ~ 5µm (inset of Fig.3f).

This clearly evident that to tune the morphology

of the product the quantity of EGCG plays an important role. Fig.4. shows the SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with CTAB surfactant (5, 10, 15, 20, 25 and 30 ml) with 1h of ultrasonic irradiation time.

When the CTAB concentration was 5 - 10 ml, the micrographs

consist of particles along with uneven stiff fibers (Fig.4 a, b). However, when the CTAB concentration was ~ 15 ml, the particles were almost disappearing and fibers oriented in random directions (Fig.4 c, d). Further the concentration of CTAB was increased to 25- 30 ml, uniform broom-like structures were appeared (Fig.4 e, f). The diffusion induced branching growth mechanism may be responsible for the formation of broom-like morphology was explained schematically as shown in Fig.5. The hierarchical branching morphologies were formed through a self-organization process under non-equilibrium conditions. The crystallization arrangement was normally affected by the distance between the nucleation spots and the driving force for the crystallization. For low CTAB concentration, after the formation of nuclei, a concentric diffusion field forms around the crystal. This was due to the quick consumption of reactants present near the surface of the crystals. However at higher concentrations of CTAB, the apexes of crystals projected into the regions of diffusion layer make them grow much faster than the central parts of facets leads to the formation of multi armed structures. The hyper-branched micro fibers up to 50 µm in length were evident from Fig.4f. This was due to the fact that CTAB, acting as a ‘surfactant / capping agent /self-assembling agent’, to bound on some specific facets and provides preferred growth direction to nanocrystals. The aggregation of the multi branch like fibers implies that these will grow radially from central particles. To the best of our knowledge, so far such welloriented trunks and branches in the same plane have not been reported for Zn2SiO4 micro/nano superstructures. It was well documented that, when two particles of dissimilar sizes bump into each other, there will be a tendency for ions to dissolve from the surface of a smaller particle and

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precipitate on the surface of a larger one due to the transmission of electrons. Therefore, the particles having larger sizes will grow at the expense of the smaller ones. The incessant irreversible process of hitting and sticking along the preferred growth directions initiates the preliminary growth. This large, new and random supramolecular structure represents various forms of growth in which one particle after another was formed and then diffuses, sticks to the growing structure. Liu et al showed the formation of individual nanorods of Co3O4 where, Ostwald ripening processes along with dissolution and recrystallization lead to individual nanorods [18-20].

But in the present case, only

decomposition, dissolution and recrystallizations were observed without the Ostwald ripening. Hence there were no free-standing rods formed from the broom-like structure. Fig.6 shows the influence of pH on the morphological features of Zn2SiO4:Dy3+ (5 mol %). It was witnessed that the pH may greatly influence the morphology of the product. In the evolution stage, various tiny assembly units extend to grow into hierarchical superstructures to reduce the surface energy. Several factors may influence to obtain different superstructures, namely crystalface attraction, electrostatic and dipolar fields associated with the aggregate, Vander Waals forces, intrinsic structures and external factors. When compared with conventional routes, optimized sonication condition normally results in reduced particle sizes, highly uniform in particle distribution and reduction in synthesis time. Therefore, the effect of sonication power on structures was performed under different frequencies (20-28 kHz). When the frequency was set to 20 kHz, agglomerated particles were observed (Fig. 7a). However, with the increase of sonication power from 22 kHz to 26 kHz similar type of morphologies were observed (Fig.7 b-d) but when increased the frequency of sonication to 28 kHz, agglomerated particles were converted to urchin type with well-connected network morphology (Fig.7e) due to the supply of sufficient energy to break the agglomeration. SEM micrographs of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with sonication time of 1 h (Fig. 8a) and 5 h (Fig. 8b &c) without surfactant clearly evident that no definite shapes were observed by changing the sonication time.

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The step by step reaction mechanism for the formation of Zn2SiO4 phosphors was shown in Fig.9. During the ultrasound irradiation, homolytic cleavage of water takes place (step 1) lead to the formation of H2O2 which reacts with TEOS forms orthosilisic acid (H4SiO4). H4SiO4 reacts with four ethanol molecules and zinc nitrate to form zinc orthosilicate (Step 3). Zn2SiO4 spherical and broom-like rods structures were resulted in presence of cationic surfactants EGCG and CTAB respectively.

The different morphologies were obtained for various reaction conditions was

tabulated in Table.1. In the presence of EGCG a coordinate bond with zinc silicate to form micelles. The micelles undergo rearrangement and assisted the typical nucleation (Fig.10). Further it was expected that micelle coordinated zinc with rings B and D diol groups become polar hydrophilic exterior and A and C rings of EGCG hydrophobic and come to be interior. Similarly in the presence of CTAB which also been acts as cationic surfactant form a micelles with the preliminary formed Zn2SiO4 NPs. Herein, the long aliphatic cetyl chain was hydrophobic and become interior and polar trimethylammonium bromide become exterior and attached to the polar Zn2SiO4 superstructures. In the further step, rearrangement of micelles and typical nucleation will result the broom type of structures (Fig.11). It was clear that presence of EGCG/CTAB play a vital role in controlling the nucleation and growth rate during the reaction. Fig.12. shows TEM, HRTEM and EDAX images of Zn2SiO4 :Dy3+ (5 mol %) formed in presence of optimized concentrations of EGCG and CTAB sonicated for 6 h. It was evident from TEM images that the crystallites were almost spherical in shape and their size was found to be ~ 3035 nm. The obtained crystallite sizes were in good agreement with the values estimated from PXRD analysis. Fig.12e shows the EDAX which confirms the elements present in the product and also it confirms the purity of the product. The interplanar spacing was estimated to be 0.32 - 0.35 nm. To know the surface topography of the product, AFM images of Zn2SiO4 :Dy3+ (5 mol %) was showed in Fig. S2. The average particles size was found to be ~ 13 nm (Fig.S2 b). Also the 3-D phase diagram with axis scan (2 µm x 2 µm x 0.31 nm) along X, Y and Z- axis respectively was observed (Fig.S2 c).

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3.2 Characterization When the as-formed sample was calcined to 950 °C, orthorhombic phase of Zn2SiO4 :Dy3+ (JCPDS card no. 35-1485) was obtained for all the mol % of Dy3+ (Fig. 13 a) [21]. Compared to other methods of preparation to prepare silicates, in the present case there was a reduction of calcination temperature by ~ 250 °C [22]. The small impurity peak of Dy2O3 was observed around 2θ = 36° in PXRD profiles and which was readily indexed to cubic phase with JCPDS card no. 22-0612. Further, no other phase was detected indicating that Dy3+ homogeneously mixed and effectively doped in the host lattice in Zn2+ sites since, the ionic radii was close to each other (the ionic radii of RDy3+ = 0.91 nm and RZn2+ = 0.88 nm were very close to each other). However, at higher Dy3+ doping concentration, the PXRD patterns slightly shifted to lower angle side and also at higher concentration of Dy3+, the prominent peak of Dy2O3 was observed as shown in Fig. 13(b). In the present case estimated value of acceptable percentage difference (Dr) for Zn2SiO4:Dy3+ was found to be 28 % [23]. So, it was believed that the Zn2+ was replaced by Dy 3+ in the host. The detailed explanation of the estimation was presented in supplementary section.

The average

crystallite size (D) was calculated for different Dy3+ doped Zn2SiO4 nanophosphors were summarized in Table.2. It can be seen from the table that, the crystallite size increases with increase of dopant Dy3+ concentration. This was due to the increase in strain leading the replacement of Zn2+ ions by Dy3+ ions of smaller radii.

To calculate the crystallite size and strain present in the

nano/microstructures W–H (Williamson–Hall) fitting method was used (Fig.S4) [24]. The estimated values of size and strain were summarized in a Table.2. The diffuse reflectance spectra (DRS) of undoped and Dy3+ (1-9 mol %) doped Zn2SiO4 nanophosphor recorded in the range 200 - 1100 nm at RT were shown in Fig.S5. In the shorter wavelength region the spectrum exhibit a weak absorption band may be due to formation of metastable states between valence band and conduction band by the Dy3+ ions. The numerous electronic absorption bands were observed in the longer wavelength region which was assigned from ground state 7F0 to various excited states of Dy3+ ions [25]. In the region above 400 nm, each characteristic 9

absorption bands can be attributed to the transitions from the ground state to the excited states of the Dy3+ ion. The peaks at 471, 755, 809, 907 and 1100 nm were assigned to 6H15/2 → 4F9/2, 6H15/2 → 6

F3/2, 6H15/2 → 6F5/2, 6H15/2 → 6F7/2 and 6H15/2 → 6H7/2 + 6F9/2 respectively [26]. Due to variation in

particle size red shift of the bands were observed in the DRS with increase in Dy3+ concentration. Kubelka–Munk (K-M) function was applied to the DRS data and determined the energy band 1/ 2

gap (Eg) of the product [27]. The intercepts of the tangents to the plots of [F (R∞ ) hν ] versus photon energy hν was shown in Fig. 14. The calculated band gap energies were summarized in Table.2. The degree of structural order and disorder into the matrix lead to changes in Eg and also which will change the distribution of energy levels within in the band gap. Furthermore, the degree of structural order–disorder of the matrix was mainly depend on the preparation methods as well as experimental conditions which can favor or slow up the formation of structural defects leads to the variation in the Eg [28]. Photoluminescence studies (PL) Fig.15 shows the excitation spectrum of Zn2SiO4: Dy3+ (5 mol %) nanophosphor monitored at 484 nm emission which was attributed to 4F9/2 → 6H13/2 transition of Dy3+ ion. The excitation spectrum consist a sharp intense band at 350 nm corresponding to 6 H15/2 → 6P7/2 + 4M17/2 transition attributed to the direct excitation of Dy3+ ion. Fig. 16 shows the PL emission spectra of Zn2SiO4: Dy3+ (1-9 mol %) nanophosphor excited at 350 nm which consist of three main peaks in blue (484 nm), yellow (574 nm) and red (666 nm) region respectively. The positions of the emission peaks were not influenced by the Dy3+ concentration. These emission peaks can be assigned to the electronic transitions of 4F9/2 → 6H15/2, 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 respectively [29]. The blue emission corresponds to the magnetic dipole transition and the yellow emission belongs to the hypersensitive (forced electric dipole) transition with the selection rule ∆J=2 [30, 31]. Among these, transitions, 4F9/2 → 6 H13/2 were purely electric dipole (ED) transitions but the 4F9/2 → 6 H15/2 obey magnetic dipole (MD) transitions A MD transition does not change with the host environment significantly but the ED transition was sensitive to the crystal field [32]. The different energy levels 10

of Dy3+ ions were shown in Fig.S6. The dependences of 4F9/2 → 6H13/2 emission on the Dy3+ doping concentration in the Zn2SiO4 : Dy3+ (1-9 mol %) NPs excited under 350 nm and the variation of PL intensity of 480 nm centered peak for different Dy3+ mol concentration were shown in Fig. 17. As Dy3+ concentration was increased, the PL intensity of transition centered at 484, 574 & 666 nm peaks were increased up to 5 mol %; then diminished due to concentration quenching. It was observed that the maximum emission intensity of Zn2SiO4 : Dy3+ phosphor showed at 5 mol %. The concentration quenching occurs due to the energy transfer from one activator to the neighboring ion [33]. The critical distance for energy transfer (Rc) in Zn2SiO4 : Dy3+ NPs was estimated from the structural parameters as discussed in supplementary section and found to be ~ 13.56 Å [34]. The schematic representation of Blasse’s formula was shown in Fig.S7. When critical energy distance between Dy3+ ions and Zn2SiO4 was greater than 5 Å; the overlapping between the excitation and emission spectra decreases. The energy transfer between Dy3+ ion take places due to electric multipolar interaction which can be determined by the equation: −1 I = k 1 + β (X)Q 3    X

------------ (1)

Where X; Dy3+ ion concentration, k and β; constants, Q = 6, 8 and 10 for dipole – dipole, dipole – quadruple and quadruple – quadruple interactions and less than 6 was due to the energy transfer mechanism [35]. The value of Q was determined by plotting log (X) Vs log (I/X) (Fig. 18) which gives a linear graph having a slope = - 0.9807 and intercept = 7.3446. The Q value was ~ 3 indicates that the concentration quenching in Zn2SiO4 was due to energy transfer mechanism [36]. To measure the degree of distortion from the inversion symmetry of the local environment of the Eu3+ ions in the host matrix, the asymmetric ratio (A21) was an important parameter [37];

∫ I ( D → F )dλ = ∫ I ( D → F )dλ 5

A21

2

7

0

5

1

0

11

1

7

2

--------- (2)

where I1 and I2; intensity of magnetic dipole transition at 574 nm and electric dipole transition at 666 nm respectively. Therefore the value of A21 for Zn2SiO4:Dy3+ increases with the increase in Dy3+ concentration (Fig. 19). The variation of asymmetric ratio with doping concentration can influence the luminescent property of a material and also used to determine the optimum concentration of rareearth in the host for luminescence applications. Judd-Ofelt analysis The Judd-Ofelt intensity parameters (Ω2 & Ω 4) and various radiative properties such as radiative transition probability (AT), radiative (τrad) lifetime, branching ratio (βR) and asymmetric ratio (A21) were calculated for detailed investigation of site symmetry and luminescence dynamics of rare earth Dy3+ ion in the host Zn2SiO4 matrix. [38]. The relation between radiative emission rates and the integrated emission intensities was given by the equation [39]:

A0−2, 4 A0−1

=

I 0 −2 , 4 I 0−1

=

hυ 0−1 hυ0−2, 4

------------- (3)

where I0-J and hν0–J; integrated emission intensity and energies corresponding to transition 4F0→6HJ (J=1, 2, 4) respectively. The radiative transition rates (A0-J) of electric dipole can be expressed as 3

A( 0− J ) =

(

64π 4ϑ J n n 2 + 2 3h (2 J + 1) 9

)

2

∑ Ωλ λ

4

F0 U (λ ) 6 H J

2

----------------- (4)

= 2, 4

where A(0− J ) ; the coefficient of spontaneous emission, e ; the electronic charge, ϑ J ; the wave number of the corresponding transition, h ; the Planck's constant, Smd ; the strength of the magnetic dipole and n ; the RI of the prepared sample.

4

6

F0 U (λ ) H J

2

; squared reduced matrix element of

Dy3+ and were 0.0033 and 0.0023 for J = 2 and 4 respectively and these value was independent of the chemical environment [40]. Thus, by using Eqs. (12) & (13), the values of Ω2 and Ω4 were calculate and tabulated in a Table.3. The variation in the value of Ω2 with different Dy3+ concentration specifies that Ω2 was more sensitive to the ligand environment. The parameter Ω2 shows it dependence on the covalency between rare earth ions and ligand anions because parameter Ω2 12

replicate the asymmetry of the local environment at the Dy3+ ion site. The increase in the value of Ω2 with concentration shows decrease in the symmetric nature of Dy3+ in this host. The radiative properties such as transition probabilities (AT), radiative lifetimes (τrad) and branching ratios (βR) for the excited states of Dy3+ ions were calculated by using estimated value of J–O parameters with following equations [41, 42]. AT (ψJ ) = ∑ AJ − J ′

--------------- (5)

J′

τ rad (ψ J ) =

1

----------------- (6)

AT (ψ J )

A(ψJ ,ψ ′J ′) AT (ψJ )

β (ψJ ) =

----------------- (7)

The measured branching ratio for Dy3+ doped Zn2SiO4 nanophosphor to be 9.98 ≥ 0.50 suggests that present nanophosphor can emit laser radiation more effectively and was suitable for white color displaying devices. The various parameters obtained by Judd-Ofelt technique were tabulated in Table.4. The quantum efficiency (QE) of Zn2SiO4:Dy3+ (5 mol %) was estimated using the relation

QE =

EC − E a La − Lc

……(8)

Where EC; Integrated lumen of the phosphor caused by direct excitation, Ea; integrated lumen from the empty integer along sphere (without sample), La; excitation profile from the empty integrating sphere and Lc; integrated excitation profile when the phosphor was directly emitted by incident beam [43]. The QE of the specified phosphor was found to be ~ 85.14%. Color purities of the obtained sample were also checked using the relation.

Colour Purity =

(xs (xd

2

2

− xi ) + ( y s − y i ) 2

2

− xi ) + ( y d − y i )

×100 %

……..(9)

where (xs, ys) were the coordinates of a sample point, (xd, yd) were the coordinates of the dominant wavelength and (xi, yi) the coordinates corresponds to the highest illumination points [44]. The color factor values had been tabulated in Table.4. The color rending index (CRI) values varied very close 13

to 93.38% specifies that the prepared phosphor materials may be outstanding materials for WLED applications. To estimate the material performance on color luminescence emission, CIE (Commission International de I’Eclairage) chromaticity coordinates were used. The color of any light source can be denoted as an (x, y) coordinates in colors pace [45, 46]. The CIE diagram of Zn2SiO4: Dy3+ (1-9 mol %) nanophosphors were presented in Fig. 19(a), magnified view is shown in Fig. 19(b) and CIE coordinates in Table.4. As it can be seen from the CIE diagram, the product shows pure white emission. The correlated color temperature (CCT) was a specification of the color appearance of the light emitted by a light source, relating its color to the color of light with respect to a reference light source when heated up to a specific temperature, in kelvin (K). The CCT score for a lamp or a source was a preferred ‘‘warmth’’ or ‘‘coolness’’ degree of its look. but, contrary to the temperature scale, lamps with a CCT rating beneath 3200 K were generally considered ‘‘heat’’ resources, at the same time as those with a CCT above 4000 K was normally taken into consideration ‘‘cool’’ in look when heated up to a specific temperature, in kelvin. The CCT was calculated by converting the (x, y) coordinates of the light source to ( U ′ , V ′ ) by using the following relations, and by determining the temperature of the closest point of the Planckian locus to the light source on the ( U ′ , V ′ ) uniform chromaticity diagram (Fig. 20 a and b) [47]. U′ =

4x − 2 x + 12 y + 3

--------------- (10)

V′ =

9y − 2 x + 12 y + 3

------------- (11)

The quality of white light from CCT was calculated by McCamy empirical formula CCT = − 437 n 3 + 3601 n 2 − 6861 n + 5514 . 31 , where n = (x − x c ) ( y − y c ) ; the inverse slope

line and chromaticity epicenter was at xc = 0.3176 and yc = 0.3173 [48]. The values of both CIE and CCT were summarized in Table.4.

In the present study, the CCT value of Zn2SiO4: Dy3+

nanophosphor was found to be ~ 6151 K which was well within the range of vertical daylight. Thus it can be useful for artificial production of warm white light in illumination devices. 14

Conclusions

White light emitting Zn2SiO4 :Dy3+ (1 – 9 mol %) nanophosphors were prepared by using facile EGCG / CTAB surfactant assisted ultrasound method. It was evident that the hierarchical superstructures largely depend on the sonication time, pH and concentration of EGCG / CTAB. The growth mechanism for various hierarchical superstructures was proposed. The PXRD patterns of phosphors calcined at 950 °C confirm single orthorhombic Zn2SiO4. The energy gap (Eg) value varies from 4.25–5.12 eV. PL intensity increases up to 5 mol % and then diminishes due to selfquenching effect. The optimized white emitting phosphor exhibits high efficiency and color purity. Further, the phosphor showed excellent CIE chromaticity co-ordinates (x = 0.31769, y = 0.31731) and CCT value (6151 K) of the standard white LEDs. Hence, the phosphor obtained by ultrasound route was highly useful for display and solid state lightning applications.

15

Acknowledgement

The author Dr. H Nagabhushana thanks to DST-SERB (Project No. SR/FTP/PS-135/2010) New Delhi for the sanction of this Project.

16

References

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19

Figure Captions:

Fig.1. SEM images of Zn2SiO4: Dy3+ (5 mol %) nanophosphor with different sonication times (1, 2, 3, 4, 5 and 6 h) with EGCG (30 ml). Fig.2 Schematic representation of formation process of spherical shape morphology in Zn2SiO4 :Dy3+ (5 mol %) nanophosphor. Fig.3 SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with different concentration of EGCG (a) 5 ml (b) 10 ml (c) 15 ml (d) 20 ml (e) 25 ml and (f) 30 ml with 3h of ultrasonic irradiation time. Fig.4. SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with CTAB surfactant (a) 5 ml (b) 10 ml (c) 15 ml (d) 20 ml (e) 25 ml and (f) 30 ml with 1h of ultrasonic irradiation time. Fig.5. Schematic representation of formation process of broom-like morphology (in presence of CTAB) in Zn2SiO4: Dy3+ (5 mol %) nanophosphor. Fig.6. Formation micro superstructures of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with various pH values. Fig.7. SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with different sonication power (a) 20 kHz (b) 22 kHz (c) 24 kHz (d) 26 kHz (e) 28 kHz in presence of EGCG (30 ml). Fig.8. SEM micrographs of Zn2SiO4:Dy3+ (5 mol %) nanophosphor obtained without surfactant with a sonication time of (a) 1 h and (b) 5 h (c) the magnified image of (b). Fig.9. Step by step reaction mechanism for the formation of Zn2SiO4 phosphor. Fig.10. Schematic representation of Zn2SiO4 nanostructures in the presence of EGCG. Fig.11. Schematic representation of Zn2SiO4 nanostructures in the presence of CTAB. Fig.12. TEM (a, b), HRTEM (c and d) images & EDAX of Zn2SiO4:Dy3+ (5 mol %) nanophosphor. Fig.13. (a) PXRD patterns of Dy3+ (1-9 mol %) doped Zn2SiO4 nanophosphors and (b) shift in the prominent peak of PXRD patterns of Dy3+ (1-9 mol %) doped Zn2SiO4 nanophosphors. Fig.14. Energy band gap plot of pure & Dy3+ (1-9 mol %) Zn2SiO4 nanophosphor. Fig.15. PL excitation spectrum of Zn2SiO4:Dy3+ (5 mol %) nanophosphor at λemi = 574 nm. Fig.16. PL emission spectra of Zn2SiO4:Dy3+ (a) 1 mol %, (b) 3 mol % , (c) 5 mol % , (d) 7 mol % and (e) 9 mol % nanophosphor excited at λexi= 350 nm. Fig.17. Effect of concentration of Dy3+ on the 574 nm emission and the variation of asymmetric ratio in Zn2SiO4 nanophosphor. Fig.18. Relation between log(x) and log (I/x) in Zn2SiO4:Dy3+ (1-9 mol %) nanophosphor. Fig.19. (a) CIE (b) magnified view of CIE coordinates of Zn2SiO4:Dy3+ (1-9 mol %) nanophosphor. Fig.20. (a) CCT (b) magnified view of CCT coordinates of Zn2SiO4:Dy3+ (1-9 mol %) nanophosphor. 20

Table captions:

Table.1. Various morphologies obtained in the present study with respective to different reaction conditions. Table.2. Estimated crystallite size, strain & Eg values of Zn2SiO4: Dy3+ (1- 9 mol %) nanostructures. Table.3 Judd-Ofelt intensity parameters (Ω 2, Ω4), radiative transition probability (AT), calculated radiative (τrad) lifetime, branching ratio (βR) and asymmetric ratio (A21) of Zn2SiO4 : Dy3+ nanostructures. (λexi = 350 nm). Table.4 Photometric characteristics of Dy3+ (1-9 mol %) doped Zn2SiO4 nanophosphor.

21

Fig.1. SEM images of Zn2SiO4: Dy3+ (5 mol %) nanophosphor with different sonication times (1, 2, 3, 4, 5 and 6 h) with EGCG (30 ml).

22

Fig.2. Schematic representation of formation process of spherical shape morphology in Zn2SiO4:Dy3+ (5 mol %) nanophosphors.

23

Fig.3. SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with different concentration of EGCG (a) 5 ml (b) 10 ml (c) 15 ml (d) 20 ml (e) 25 ml and (f) 30 ml with 3h of ultrasonic irradiation time.

24

Fig.4. SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with CTAB surfactant (a) 5 ml (b) 10 ml (c) 15 ml (d) 20 ml (e) 25 ml and (f) 30 ml with 1h of ultrasonic irradiation time.

25

Fig.5.Schematic representation of formation process of broom-like morphology (in presence of CTAB) in Zn2SiO4: Dy3+ (5 mol %) nanophosphor.

26

20 µm

20 µm

Mixture of Zn2+ +TEOS+EGCG

pH 7

pH 2

20 µm

20 µm

20 µm

20 µm 20 µm

20 µm

Fig.6. Formation micro superstructures of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with various pH values.

27

a

b

c

e

d

Fig.7. SEM images of Zn2SiO4:Dy3+ (5 mol %) nanophosphor with different sonication power (a) 20 kHz (b) 22 kHz (c) 24 kHz (d) 26 kHz (e) 28 kHz in presence of EGCG (30 ml).

28

(a)

(b)

(c)

Fig.8. SEM micrographs of Zn2SiO4:Dy3+ (5 mol %) nanophosphor obtained without surfactant with a sonication time of (a) 1 h and (b) 5 h (c) the magnified image of (b).

29

. .

H2O

. . . . H + OH . . OH + OH

H + OH H2

H+ H

STEP:1

H2O H2O2 OH

OEt

H2O2

STEP:2

+ EtO

Si

HO

OEt

Si

OH

+

4C2H5OH

OH

OEt

Orthosilicic acid TEOS

OH

STEP:3

HO

Zn2+

Si

OH

+ 2Zn(NO )

-

O

3 2

Zinc nitrate

OH

OSi

O-

O-

+

4HNO3

Zn2+

Zinc Orthosilicate bulk Zn2+

STEP:4

-

O

OSi

O-

O-

+

EGCG

Zn2+

Zinc Silicate NPs

Fig.9. Step by step reaction mechanism for the formation of Zn2SiO4 phosphors.

30

OH

O-

OH

Zn2+

B HO

O OH

A

+

-

Si

O

OH

+

Non Polar

Zn2+

O

O-

OH

O

O-

Zn2SiO4 Primary NPs

D OH

OH

Zinc silicate

Self assembled EGCG

EGCG

Ultra sonication Non Polar

Non Polar

Micelle arrangement

Nucleation

20 µm

Non Polar Non Polar Non Polar

Zn2SiO4 Super structures

Non Polar

Non Polar

Micelle formation Fig.10. Schematic representation of Zn2SiO4 nanostructures in the presence of EGCG.

31

Fig.11. Schematic representation of Zn2SiO4 nanostructures in the presence of CTAB.

32

Fig.12.TEM (a, b), HRTEM (c, d) & (e) EDAX of Zn2SiO4:Dy3+ (5 mol %) nanophosphor.

33

(JCPDS. NO. 37-1485)

9 mol%

*

10

20

(113)

30

40

9mol%

7 mol%

7mol%

5 mol%

5mol%

3 mol%

3mol%

(333)

50

(220)

1 mol%

(713)

(410) (223)

(012) (211) (300) (220)

(110)

Intensity (a.u)

*

b

(226) (633) (413)

a

60

70

2 θ (degree)

80 30.5

31.0

31.5

1mol%

32.0

32.5

2θ (degree)

Fig.13. (a) PXRD patterns of Dy3+ (1-9 mol %) doped Zn2SiO4 nanophosphors and (b) shift in the prominent peak of PXRD patterns of Dy3+ (1-9 mol %) doped Zn2SiO4 nanophosphors.

34

3+

(F(R) hν )2

Zn2SiO4:Dy (1-9 mol%)

5.32 eV 5.48 eV 5.36 eV5.40 eV 5.43 eV

Pure 1 mol% 3 mol% 5 mol% 7 mol% 9 mol%

5.52 eV

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2

E=hν (eV) Fig.14. Energy band gap plot of pure & Dy3+ (1-9 mol %) Zn2SiO4 nanophosphor.

35

3+

300

λemi = 574 nm

363 nm 386 nm

324 nm

421 nm 452 nm

350 nm

PL Intensity(a.u)

Zn2SiO4:Dy (5 mol%)

325

350

375

400

425

450

Wavelength(nm) Fig.15. PL excitation spectrum of Zn2SiO4:Dy3+ (5 mol %) nanophosphor at λemi= 574 nm.

36

Fig.16. PL emission spectra of Zn2SiO4:Dy3+ (a) 1 mol %, (b) 3 mol % , (c) 5 mol % , (d) 7 mol % and (e) 9 mol % nanophosphor excited at λexi= 350 nm.

37

for 574 nmpeak

PL Intensity(a.u) 0

2

4 6 8 Dy concentration (mol%)

I574 nm/ I483 nm

3+

Asymmetric ratio vs Dy conc.

10

3+

Fig.17. Effect of concentration of Dy3+ on the 574 nm emission and the variation of asymmetric ratio in Zn2SiO4 nanophosphors.

38

9.4

3+

Zn2SiO 4:Dy (1-9 mol%) Linear Fit

Log (I/x)

9.2 9.0 8.8 Equation

8.6 8.4 8.2

y = a + b*x

No Weight Weight Residual 4.3886E-4 Sum of Squares -0.9996 Pearson's r 0.99895 Adj. R-Squ Value Intercept

B B

Slope

-2.0

Standard E

7.3447

0.02298

-0.9807

0.01593

-1.8

-1.6

-1.4

-1.2

-1.0

Log x Fig.18. Relation between log(x) and log (I/x) in Zn2SiO4:Dy3+ (1-9 mol %) nanophosphor.

39

0.9

(a)

Zn2SiO4:Dy3+(1-9 mol%)

λexi = 350 nm

(b)

0.6

CIE Y

9 mol % 7 mol %

3 mol % 0.3

1 mol %

5 mol %

0.0 0.0

0.2

0.4

0.6

0.8

CIEX Fig.19. (a) CIE (b) magnified view of CIE coordinates of Zn2SiO4:Dy3+ (1-9 mol %) nanophosphor.

40

0.6 0.468

0.5

(a)

~ 6151 K 0.4

0.466

9 mol %

(b) 7 mol %

0.3

Zn2SiO4:Dy (1-9 mol %)

V'

V'

3+

0.464

3 mol %

λexi = 350 nm

0.2

mol % 1 3 5 7

0.1

9

0.1

0.2

0.3

U' 0.2044 0.2069 0.2058 0.2071

V' 0.462 0.466 0.462 0.467

CCT 6439.9 5999.5 6303.5 5961.0

0.20635 0.4627 6267.3

0.4

0.5

0.6

0.462

5 mol % 1 mol %

0.460 0.205

0.206

0.207

U'

U’ U' Fig.20. (a) CCT (b) magnified view of CCT coordinates of Zn2SiO4:Dy3+ (1-9 mol %) nanophosphor.

41

Table.1.Various morphologies obtained in the present study with respective to different reaction conditions. Sample No

Surfactant

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

EGCG EGCG EGCG EGCG EGCG EGCG CTAB CTAB CTAB CTAB EGCG EGCG EGCG EGCG EGCG EGCG EGCG EGCG Nil

Sonication Time (min) 60 120 180 240 300 360 120 180 360 360 120 120 120 120 120 120 120 120 30

pH value

Tempera ture (0C) 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

Product

Morphology

8 8 8 8 8 8 8 8 8 8 1 2 3 4 5 6 7 8 8

Ultrasonic power (kHz) 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20

Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4 Zn2SiO4

Spherical Spherical Grapes-like Grapes-like Solid sphere Solid sphere fiber-like fiber-like Broom-like Broom-like Cylindrical Flower-bud Stack-like Cactus-like Cauliflower Flower-like Flower-like Flower-like No specific shape No specific shape Agglomerate Urchin type

20.

Nil

180

8

20

60

Zn2SiO4

21. 22.

EGCG EGCG

360 360

8 8

22 26

60 60

Zn2SiO4 Zn2SiO4

42

Table.2. Estimated crystallite size, strain & Eg values of Zn2SiO4: Dy3+ (1- 9 mol %) nano structures. Dy3+ (mol %)

Pure

Crystallite size (nm) Scherrer’s W-H approach approach 40 40

Strain ε x (10-3)

Energy gap (Eg) in eV

1.01

5.16

1

34

45

1.02

4.82

3

31

43

1.10

4.78

5

31

30

1.12

4.75

7

27

31

1.27

4.69

9

26

20

1.34

4.25

43

Table.3. Judd-Ofelt intensity parameters (Ω2, Ω4), radiative transition probability (AT), calculated radiative (τrad) lifetime, branching ratio (βR) and asymmetric ratio (A21) of Zn2SiO4 : Dy3+ nano structures. (λexi = 350 nm). Dy3+ J–O intensity Emission AT βR A21 τrad Conc. parameters peak (s-1) (ms) wavelength mol% (×10-20 cm2) λp (nm) Ω2 Ω4 1

2.35

1.89

574

332

30.07

9.983

1.08

3

2.74

2.32

574

284

35.16

9.985

1.05

5

2.76

2.05

574

282

35.42

9.988

1.04

7

2.66

1.92

574

292

34.16

9.974

1.05

9

2.47

1.92

575

315

31.73

9.994

1.06

44

Table.4. Photometric characteristics of Dy3+ (1-9 mol %) doped Zn2SiO4 nanophosphors. Dy3+ (mol %)

CIE

CCT

CCT (K)

CRI (%)

1

X 0.31541

Y 0.31691

U’ V’ 0.20441 0.46211 6439.90

95.12

3

0.32262

0.32340

0.20695 0.46677 5999.53

93.75

5

0.31769

0.31731

0.20588 0.46268 6303.50

94.18

7

0.32330

0.32410

0.20716 0.46726 5961.07

92.34

9

0.31832

0.31726

0.20635 0.46274 6267.33

91.52

45

Research highlights

1. Dy3+ doped Zn2SiO4: prepared using cationic surfactants ultrasound assisted route. 2. Nucleation and growth mechanisms were analyzed with various parameters. 3. Possible mechanisms for the morphological growth of superstructures were proposed. 4. Prepared samples find applications in white light emitting diodes (WLED’s).

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