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ScienceDirect Procedia Chemistry 19 (2016) 21 – 29
5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer (MAMIP), 4-6 August 2015
Development and characterization studies of Eu3+-doped Zn2SiO4 phosphors with waste silicate sources Nur Alia Sheh Omara, Yap Wing Fena,c†, Khamirul Amin Matorib,c, Sidek Hj Abdul Azizc, Zarifah Nadakkavil Alassanc, Nur Farhana Samsudinb b
a Functional Devices Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 Selangor, Malaysia Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 Selangor, Malaysia c Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Selangor, Malaysia
Abstract Structures, morphologies, and properties of europium doped zinc silicate were characterized using X-ray diffractometer, Field emission scanning electron microscope, Fourier transform infrared spectrometer, and UV-vis spectrophotometer. The density of doped zinc silicate shows the trend of increment when the sintering temperature increases. The XRD pattern shows that the material was highly crystalline, having sharp peaks, while the FESEM image reveals the presence of densely packed grains as sintering temperature increased 600 oC up to 1000 oC. The increase of transmission band intensities at 3443, 1630, 980, 650, 530 cm-1 confirmed the crystallization of Zn2SiO4 crystal in the glass matrix with increasing sintering temperature. Lastly, the increment of energy band gap after sintering temperature at 900 oC was related to the stabilization of α-Zn2SiO4 phase in material.
© Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2016 2016The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Keywords: Eu3+--doped zinc silicate; solid state method; sintering temperature; energy band gap
* Corresponding author. Tel.: +006-03-89466689; fax: +006-03-89454454. E-mail address:
[email protected]
1876-6196 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi:10.1016/j.proche.2016.03.006
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1. Introduction Among the inorganic silicate phosphors, zinc silicate or willemite (Zn2SiO4) has been identified as a very suitable host matrix for many rare earth and transition metal ions resulting in an excellent luminescence in the blue, green and red spectral zones1-2. Recently, rare earth ions-doped zinc silicates are receiving great interest because they are better candidates for luminescence activators in phosphor materials than transition metals3-5. Among rare earth ions, trivalent europium (Eu3+) is one of the vital luminescence activator ions due to its reddish emission of Eu3+ which are widely used in electroluminescent devices, optical amplifiers, and lasers6-7. To date the of preparation of rare earth ions doped zinc silicate, various techniques have been reported such as conventional solid state method, vapour deposition method, thermal evaporation method, hydrothermal method and sol-gel method8-9. Compared with these methods, the conventional solid states methods is regarded as a simple method due to the advantages of simplicity and to produce large amount of Zn2SiO4:Eu3+ 10. However, this method is usually used highly cost of SiO2 source to synthesize the zinc silicate. In order to overcome this drawback, it is needed to search a new silicate source from recycling of by-products for cost saving. In this work, Eu3+ doped Zn2SiO4 phosphors are fabricated by the waste soda lime silica glasses as SiO2 sources, thereby produce low cost production and reduce waste materials. The effects of 1 wt.% Eu2O3 by varying the sintering temperature of Eu3+ ions in Zn2SiO4 on the structural and optical properties were investigated.
Nomenclature D k Ȝ ß ș α hȞ k Egap
crystallite size scherrer‘s constant x-ray wavelength full width at half maximum (FWHM) in radiations angle of Bragg diffraction absorbance coefficient photon energy constant energy band gap
2. Materials and Methods Zn2SiO4 powder samples doped with 1 wt.% of europium ions were prepared using solid state method. 50 g of ZnO (99.99% Aldrich), soda lime silica waste glass, Eu2O3 (99.99% Aldrich) were used as the starting materials. The chemical compositions of the prepared samples, ((ZnO)0.5(SLS)0.5)1-y (Eu2O3)y where y = 0.01 were mixed together using milling process for 24 hours by using a ball milling jar. Then, the chemical batch was melted in an alumina crucible at 1400 °C in air with heating rate of 10 oCmin-1 for 2 hours in an electrically heated furnace. Later, the molten mixture was poured into the water to get a transparent glass fritz. The glass frit was cooled to room temperature and then crushed and sieved into fine powder about 63 µm. Next, the fine powders with an addition of 1.75 wt.% Polyvinyl Alcohol (PVA) as the binders, have been pressed at a pressure of 5 tons for 15 min to form the pellets. After that, all pellets were sintered at different temperatures from 600 °C to 1000 °C in an electrical furnace with duration of 2 h to form the glass ceramic compounds. Densities of the samples were calculated directly at room temperature by using an electronic densimeter MD300S that applied Archimedes‘ method with distilled water as the immersion liquid. The crystalline phase of the
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samples were investigated from X-ray diffraction spectrum recorded on a Phillips X‘pert X-ray Diffractometer and the data were collected over the βș range 10-80o at room temperature. The data then analysed by utilizing software X‘pert HighScore. The field emission scanning electron microscopy (FESEM) was carried out on the samples coated with gold (Au) to observe the surface morphology of the obtained glass ceramics at magnification of 5kx using Nova NanoSEM 30 Series under high vacuum. The infrared (IR) absorption measurements on the samples were made by employing the KBr pellet technique. The IR spectra were recorded at room temperature in the range 550-4000 cm-1 using VERTEX 70 instrument for identifying the chemical bonding of the samples. The UV-visible absorption spectra measurements in the wavelength range 250-800 nm were performed at room temperature by using Shimadzu UV-VIS-NIR spectrophotometer. In addition, the absorption edge for all samples was also observed to calculate the optical band gaps energy of the materials.
3. Results and Discussions The relationship of density with sintering temperatures is shown in Fig. 1. The results reveal increasing trend of the density with sintering temperature with the highest density being 3.353 g/cm3 at 1000 °C and the lowest density is 3.125 g/cm3 at 600 °C. This trend seems agreed with the normal expected phenomenon that increasing sintering temperature should increase the density which is attributed to the densification of the glass ceramics with progression of crystallization. These result might be explained through the sintering process where the organic solvent will evaporate and the oxides react to form crystallites, or grains of the required composition. Therefore, the grains will nucleate at discrete centers and grow outwards until the boundaries meet those of the neighboring crystallites 11.
Fig. 1. Density of Zn2SiO4:1 wt.% Eu3+ at different sintering temperature
Fig. 2 shows XRD spectra of 1 wt.% Eu3+ doped Zn2SiO4 glass ceramics sintered at different temperatures ranging from 600 to 1000 °C. At 600 °C, it is noticed that the characteristics peaks of prepared sample shows the lowest intensity that indicated the sample has poor crystallinity. It is found that the intensity of diffraction peaks became higher and sharper with the increase of sintering temperatures. Then, the XRD patterns revealed welldeveloped of major β-Zn2SiO4 (JCDPS file 14-0653) phase diffraction peaks with a small amount of ZnO (JCPDS
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Nur Alia Sheh Omar et al. / Procedia Chemistry 19 (2016) 21 – 29 30 No. 36-1451) as the prepared samples were sintered at 600 °C. Also some additional peaks corresponding to card those of AlNa(SiO4) (JCPDS card No. 02-0625) and K2(SO4)CrO4 (JCPDS card No. 25-1236) could be noticed at this temperature. When the sintering temperature was increased to 1000 °C, the β-Zn2SiO4 phase was completely
transformed to α-Zn2SiO4, which corresponded to JCDPS file 37-1485 and the minor phases of ZnO, AlNa(SiO4) and K2(SiO4)CrO4 disappeared12-13. From the sintering temperature of 700 °C, the samples displayed a good crystallization. The increase of sintering temperature has attributed to the improved crystallinity14-15. In XRD pattern, broader peaks were caused by smaller crystallite sizes and sharp peaks due to the formation of comparatively larger crystallites. In addition, the average crystallite sizes of the samples was calculated using full width at half maxima (FWHM) by using Debye-Scherrer’s formula16. D=
kO E cosT
where D was the crystallite size, k was Scherrer’s constant = 0.9, λ=1.5406 Å (X-ray wavelength), and ß was FWHM in radiations. As expected, with increase of sintering temperature the crystallite size also increases17. Tarafder et al. (2014) in their work also ascribed that the larger crystal sizes were formed due to high heat treatment temperature where the diffusion ions in the glasses increased, thereby accelerated the crystal growth rate in the glass matrix18-19. Therefore, the calculated average particle sizes (D) of 1 wt.% Eu3+ doped Zn2SiO4 were found to be increase in the range 20-53 nm over the investigated sintering temperature.
Fig. 2. X-ray diffraction pattern of Zn2SiO4:1 wt.% Eu3+ at different sintering temperature
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The surface morphology of the Zn2SiO4:1 wt.% Eu3+ glass ceramics are presented in Fig. 3. From the figure, it can be seen that all grains of the sintered samples are in melted-like form as the sintering temperature increased from 600 to 1000 °C. It is also clearly observed that the surface morphology of the particles was not uniform and the grains are irregular in shape20. As the sintering temperature increase, the particles tends to aggregate tightly with each other and was agglomerated into the larger of crystallite size21-22. It is found that the Fig. 3(d) and (e) displays the densely packed grains of sintered samples, thereby indicates the well crystallized of glass ceramics.
Fig. 3. FESEM images of Zn2SiO4:1 wt.% Eu3+ samples heat treatment at (a) 600 °C (b) 700 °C (c) 800 °C (d) 900 °C (e) 1000 °
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The quality and composition of different sintering temperature for Zn2SiO4:1 wt.% Eu3+ was characterized by FTIR spectroscopy at room temperature in the range 500-4000 cm-1 (Fig. 4). All FTIR spectra of samples are very similar regardless the heat treatment of temperature of the material. The FTIR spectrum of Zn2SiO4:1 wt.% Eu3+ contains clearly exhibits broad bands in the region (~3443.57 cm-1) of hydroxyl group show the stretching vibration of O-H groups23-24. Note that these broad peaks become weaker with increasing heat treatment temperature. Hence, the sintering process has proven to be suitable to remove all the organic molecules and OH groups which enable the samples to be more promising luminescent materials. The strong band was observed in the 980 cm-1 and below was due to SiO4 asymmetric stretching vibration. The peak in the ~650 cm-1 was ascribed to the bonding of Zn-O-Si symmetric stretching. The peak in the lower wavenumber at ~530 cm-1 was attributed to the ZnO symmetric stretching in ZnO4 group25-26. The absorption peaks of SiO4 (asymmetric) and ZnO4 become stronger due to increasing heat treatment temperature, suggesting crystallization of Zn 2SiO4 crystal in the glass matrix was formed. 30
Fig. 4. FTIR spectra of Zn2SiO4:1 wt.% Eu3+ at different sintering temperature. Dotted lines are drawn to guide the eye
UV-vis spectroscopy was used to characterize the optical absorbance of the Zn 2SiO4: 1 wt.% Eu3+. Fig. 5 shows the absorption spectrum of Eu3+ doped Zn2SiO4 in the spectral region from 200 to 800 nm at room temperature. The absorption profiles exhibit broad bands in the UV region (below 400nm). Additionally, the absorbance spectra of all heat treatment temperatures are very similar.
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Fig. 5. Absorption spectra of Zn2SiO4:1 wt.% Eu3+ at different sintering temperature
The absorption spectra of Eu3+ doped Zn2SiO4 were carried out to resolve the valence-conduction band transition, which allows us to calculate the band gap. UV-vis spectra indicate Zn2SiO4 single crystal is estimated has an optical band gap of 5.5 eV 23. The relation between the absorption coefficient (α) and incident photon energy (hν) for the case of indirect transition of Zn2SiO4 has a characteristic relation27:
DhQ 1 / 2
>
k hQ E gap
@
(1)
where k is constant, and Egap is the optical energy gap of the material. By extrapolating the linear portion of the optical absorption curve or its tail, it is possible to verify that indirect energy band gap value for 600, 700, 800, 900 and 1000 °C is 2.94 eV, 2.73 eV, 2.62 eV, 2.54 eV and 3.62 eV, respectively (Fig. 6). It can be observed that the values of the energy band gap decreased as the sintering temperature of Zn 2SiO4:1 wt% Eu3+ increases from 600-900 °C. This decreasing can be noted that the heat treatment at higher temperature induces a red shift of the electronic absorption edge towards larger wavelength indicating smaller energy band gap produced which is related to glass crystallization process28. Besides, the increase of particle sizes of the material also shows the decreasing in band gap value. However, the energy band gap was dramatically increased to 3.62 eV at 1000 °C. This might be due to the stabilized orthorhombic phase and improved crystallinity of the sample, thus, increased band gap by widening the absorption edge29.
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Fig. 6. Plot of (αhν)1/2 against hν for sample Zn2SiO4:1 wt.% Eu3+ at different sintering temperature.
4. Conclusions In this work, the crystallization of Zn2SiO4 compound doped 1 wt.% Eu3+ at different heat treatment was investigated by XRD, FESEM, FTIR, and UV-vis Spectroscopy. It was revealed that the density increasing when sintering temperature increasing. From XRD result, the progression of sintering temperature lead to an increase in diffraction peaks intensity and also crystallite sizes. Meanwhile, the FESEM analysis showed the samples aggregated tightly among each other due to high of sintering temperature. Lastly, the sintered samples exhibited decreasing energy band gap as sintering temperature increased up to 900 °C and then, increased dramatically to 3.62 eV when the heat treatment extended to 1000 °C. Acknowledgement The authors gratefully acknowledge the financial support for this study from the Malaysian Ministry of Education (MOE), Ministry of Science, Technology and Innovation (MOSTI) and Universiti Putra Malaysia (UPM) through Fundamental Research Grant Scheme (FRGS), Science Fund, and Putra Grant. The laboratory facilities provided by the Functional Devices Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, are also acknowledged. References 1. Yang RY, Peng YM, Lai HL, Chu CJ, Chiou B, Su YK. Effect of the different concentration of Eu 3+ ions on the microstructure and photoluminescent properties of Zn2SiO4:xEu3+ phosphors and synthesized with TEOS solution as silicate source. Opt Mater 2013; 35, 17191723. 2. Lojpur V, Nikolić MG, Jovanović D, Medić M, Antić Ž, Dramićanin MD. Luminescence thermometry with Zn2SiO4:Mn2+ powder. Appl Phys Lett 2013; 103, 1-3. 3. Tarafder A, Molla AR, Dey C, Karmakar B. Thermal, Structural, and Enhanced Photoluminescence Properties of Eu 3+-doped Transparent Willemite Glass-Ceramic Nanocomposites. J Am Ceram Soc 2013; 96, 2424-2431. 4. Dacanin Lj, Lukic SR, Petrovic DM, Nikolic M, Dramicanin MD. Judd-Ofelt analysis of luminescence emission from Zn 2SiO4:Eu3+ nanoparticles obtained by a polymer-assisted sol-gel method. Phys B 2011; 406, 2319-2322.
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