Cyclic microwave-assisted synthesis and characterization of nano-crystalline alkaline earth metal tungstates

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1 Paper Cyclic microwave-assisted synthesis and characterization of nano-crystalline alkaline earth metal tungstates Anukorn PHURUANGRAT, * Titipun THONGTEM **, and Somchai THONGTEM * * Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand ** Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Nano-crystalline MWO 4 (M = Ca, Sr and Ba) were successfully produced using cyclic microwave radiation in propylene glycol. XRD, TEM, SAED and EDX analyses revealed the presence of nano-crystalline MWO 4 containing the corresponding alkaline earth metals, W and O. Their lattice parameters and crystallite sizes were determined, and enlarged with the increase in the ionic radii of the corresponding divalent metals. Six different vibrations were detected using Raman spectroscopy and very strong W O stretching of [WO 4] 2 tetrahedrons using FTIR The Ceramic Society of Japan. All rights reserved. Key-words : Alkaline earth metal tungstates, Cyclic microwave-assisted synthesis, Scheelite structure [Received October 1, 2007; Accepted March 21, 2008] 1. Introduction Alkaline earth metal tungstates (MWO 4, M = Ca, Sr and Ba) are very interesting materials due to their structural properties, and great potential and promising applications. 1) 3) CaWO 4 can be used for luminescence, thermoluminescence and stimulated Raman scattering behavior due to its superior luminescence properties as blue phosphor (433 nm). It can be used in oscilloscopes and as scintillating for detecting X-rays and γ-rays in medical applications. 1) CaWO 4 and SrWO 4 have attracted attention as laser host materials. 4) BaWO 4 is a potential material for stimulated Raman scattering 3) and for emission of blue luminescence. 5) There are a variety of methods used to produce alkaline earth metal tungstates, such as chemical solution reaction, 2) microemulsion-mediated hydrothermal synthesis 3) and spray pyrolysis. 4) Nano-materials have attracted interest for researchers due to their wide range of optical and electrical properties, 6) which are controlled by uniform shape and narrow size distribution. 7) Softchemistry route has very high possibility to mix the starting materials at atomic level. It can lead to chemical homogeneity compared with the conventional solid state reaction. 8) Solvents are very important to control particle morphologies in solutionbased synthesis. 9),10) For example, ethylene glycol has been widely used for synthesis of dispersion nano-materials due to its high efficiency in absorbing and stabilizing on particle surfaces. 11) Recently, microwave radiation is the candidate for synthesis of nano-materials. 9),11),12) It is able to utilize uniform growth environment. The radiation can promote formation of the products with fast and homogenous heating rate. 11) 13) The dissolving components are able to couple with microwave radiation leading to higher heating rate than that achieved by conventional method. It can solve the problems of temperature and concentration gradients. By focusing large amount of radiation into the solutions, vibrating electric field applied a force on charged particles which Corresponding author: T. Thongtem; ttpthongtem@yahoo. com; ttpthongtem@hotmail.com 2008 The Ceramic Society of Japan vibrated accordingly. Vibrations of the reactants have the influence on the reaction to proceed with shorter time and lower power. Finally, pure products were produced. 14) Previously, metal tungstates were produced via a citrate complex route assisted by microwave irradiation with the subsequent calcination at high temperature. 15),16) For the present research, alkaline earth metal tungstates were produced using cyclic microwave-assisted synthesis without any further calcination at high temperature. 2. Experiment The mol each of M(NO 3) 2 (M = Ca, Sr and Ba) and Na 2WO 4 were separately dissolved in 15 cm 3 propylene glycol (boiling point = 187 C 17) ), correspondingly mixed and stirred for s (30 min). Dielectric constant (relative permittivity) and dielectric loss for 2.45 GHz of propylene glycol at 25 C are 30.2 and 6.3, respectively; 18) therefore, it is highly susceptible to microwave radiation. 13) The mixtures were heated at 600 W cyclic microwave power for s. To avoid over heating, the microwave was on for 15 s and off for 85 s. During heating, test temperature (Fig. 1) was monotonically increased with the increase in the prolonged time. It tended to be constant at 132 C as 1,700 s passed. At the conclusion of the process, white pre- Fig. 1. Test temperatures at 600 W cyclic microwave power for different prolonged times. 605

2 JCS-Japan Phuruangrat et al.: Cyclic microwave-assisted synthesis and characterization of nano-crystalline alkaline earth metal tungstates cipitates were produced. They were washed with distilled water and absolute ethanol, and dried in air at 80 C for s (24 h). The final products were characterized using X-ray diffractometer (XRD) with the scanning angle (2θ) ranging from 10 to 60 deg, Fourier transform infrared spectrometer (FTIR) with KBr as a diluting agent, Raman spectroscopy with 50 mw Ar laser (λ = nm), transmission electron microscope (TEM) as well as selected area electron diffraction (SAED) technique operated at 200 kv and energy dispersive X-ray (EDX) analyzer operated at 20 kv. 3. Results and discussion Figure 2 shows XRD spectra of MWO 4 (M = Ca, Sr and Ba) which were specified the phase by comparing with the JCPDS standards, PDF numbers: (CaWO 4), (SrWO 4) and (BaWO 4). 19) They are scheelite structure which has body-centered tetragonal primitive cell and I4 1/a spacegroup. 2),4),7),19),20) It composes of two formula units. Each tungsten atom is surrounded by four equivalent O atoms in tetrahedral symmetry, and each divalent metal shares corners with eight adjacent [WO 4] 2 tetrahedrons. 7),20) Divalent metals (Ca 2+, Sr 2+ and Ba 2+ ) are surrounded by eight oxygen ions. 2) The spectra are Fig. 3. Raman spectra of CaWO 4, SrWO 4 and BaWO 4 produced at 600 Table 2. Raman Wavenumbers of MWO 4 Lattice vibration Wavenumber (cm 1 ) CaWO 4 SrWO 4 BaWO 4 ν 1(A g) ν 3(B g) ν 3(E g) ν 4(B g) ν 2(A g) ν f.r.(a g) Fig. 2. XRD spectra of CaWO 4, SrWO 4 and BaWO 4 produced at 600 Table 1. Lattice Parameters and Crystallite Sizes of MWO 4 Product Lattice parameter (nm) Crystallite size (nm) a c Scherrer formula TEM CaWO ± 2.97 SrWO ± 3.50 BaWO ± 4.28 Fig. 4. FTIR spectra of CaWO 4, SrWO 4 and BaWO 4 produced at

3 JCS-Japan very sharp with no detection of any other characteristic peaks, showing that the products are pure crystalline phase. Their lattice parameters, calculated from the relation between plane spacing equation for tetragonal structure and Bragg s law for diffraction, 21) are summarized in Table 1. Calculated lattice parameters of MWO 4 are very close to those of the corresponding JCPDS standards. 19) They were increased with the increase in the ionic radii of alkaline earth metals (Ca 2+ = nm, Sr 2+ = nm, Ba 2+ = nm). 22) The parameters are influenced by the electronegativities of divalent metals as well. To produce MWO 4, M(NO 3) 2 and Na 2WO 4 were used in order to retain the same structural blocks in the products by doubleexchange reaction between ions. 23) Possible reaction proceeded in propylene glycol as follows : M(NO 3) 2 + Na 2WO 4 MWO 4 + 2NaNO 3 Scheelite structure has molecular ionic group of [WO 4] 2 with strong covalent W O bonding. M 2+ cation has a weak coupling Fig. 5. SAED patterns and TEM images of (a) CaWO 4, (b) SrWO 4 and (c) BaWO 4 produced at 600 W cyclic microwave power for s. 607

4 JCS-Japan Phuruangrat et al.: Cyclic microwave-assisted synthesis and characterization of nano-crystalline alkaline earth metal tungstates with the ionic group. Raman spectra of MWO 4 can be divided into two groups, the internal and external modes. 13) The internal mode is the cause of oscillation inside [WO 4] 2 tetrahedrons. The external mode is called lattice phonon. It is caused by the motion of M 2+ cations relative to the rigid molecular units. In free space, the units have T d-symmetry. Vibrations composed of four ν 1(A 1), ν 2(E), ν 3(F 2) and ν 4(F 2) internal modes, one free rotation (f.r.) mode (F 1) and one translation mode (F 2). In lattice space, group theory calculation shows 26 different vibrations (3A g, 5A u, 5B g, 3B u, 5E g and 5E u). The 3A g, 5B g and 5E g vibrations are Raman active. The 4A u and 4E u vibrations are infrared active, but their remains are acoustic vibrations. The 3B u vibrations are silent modes. 22) Raman spectra of metal tungstates were characterized over the range cm 1 (Fig. 3). Six vibrations are Raman active and are summarized in Table 2. FTIR spectra of MWO 4 are showed in Fig. 4. The [WO 4] 2 tetrahedrons have ν 3(F 2) and ν 4(F 2) vibrations corresponding to the stretching and bending modes, respectively. 24),25) They showed very strong W O stretching at cm 1, and weak W O bending at 461 and 434 cm 1 for CaWO 4 and SrWO 4, respectively. 25),26) No bending mode was detected for BaWO 4. SAED patterns of MWO 4 (Fig. 5) were analyzed 13) and were specified as the metal tungstates. 19) Their diffraction rings are diffuse and hollow, showing that the products are nano-crystalline. TEM images of MWO 4 (Fig. 5) show that the products composed of a number of round nano-sized particles. Their sizes were measured from 500 particles on TEM images. Their distribution curves (Fig. 6) are very close to bell shapes which are the normal distribution. The averages and standard deviations are summarized in Table 1. Their sizes were increased with the increase in the ionic radii of the divalent metals. Crystallite sizes of the tungstates were calculated using (112) peaks of XRD spectra and Scherrer formula. 21) B = λk/lcosθ Where λ, θ, L, k and B are the wavelength of Cu Kα radiation ( nm), Bragg s angle, average crystallite size, a constant (0.89) and the full width at half maximum (FWHM) in radian unit, respectively. Their sizes (Table 1) were increased with the increase in the ionic radii of the divalent metals. There are some differences between the two values determined from the above methods due to the selection of k value in Scherrer formula, the measurement of FWHM and others. Previously, metal tungstates were prepared by the modified citrate complex method in water under microwave irradiation, and followed by the C calcination for s (3 h). 15),16) The precursor was produced using microwave irradiation, specified as amorphous phase of citrate complex. 15) At 300 C calcination, the product was amorphous without any crystalline phases. 15) Above 350 C, crystalline phase was detected. Their crystallite sizes are between 15 and 35 nm. 15) The degree of crystallinity, crystallite sizes and purity were increased with the increase in the calcination temperature. At C calcination, W O stretching vibrations of FTIR spectra were clearly Fig. 6. Particle size distributions of (a) CaWO 4, (b) SrWO 4 and (c) BaWO 4 produced at 600 Fig. 7. EDX spectra of CaWO 4, SrWO 4 and BaWO 4 produced at

5 JCS-Japan detected at 710 cm 1. 16) For present research, no calcination was required. The experimental time was shortened and the energy consumption was decreased. The products were pure crystalline phases. The crystallite sizes are a little smaller than those produced by the previous method. W O stretching vibrations were also detected at the same range as the previous explanation. Figure 7 shows EDX spectra of the tungstates. They revealed the presence of alkaline earth metals (Ca, Sr and Ba), W and O. Elemental analysis shows that M : W : O atomic ratios are very close to the corresponding stoichiometric tungstates. Cu of copper grids and C layers were also detected. 4. Conclusions Nano-crystalline MWO 4 (M = Ca, Sr and Ba) were successfully produced from M(NO 3) 2 and Na 2WO 4 in propylene glycol by the assistance of cyclic microwave radiation without any further calcination. XRD, SAED and EDX analyses revealed the presence of nano-crystalline MWO 4 containing the corresponding alkaline earth metals, W and O. The products composed of a number of round nano-sized particles characterized using TEM images. Their size distributions are very close to a bell shape. Their lattice parameters and crystallite sizes were enlarged with the increase in the ionic radii of divalent metals. Six different vibrations were detected using Raman spectroscopy, and very strong W O stretching of [WO 4] 2 tetrahedrons using FTIR. Acknowledgements We are extremely grateful to the Thailand Research Fund and the National Research Council of Thailand for supporting the research, and the Graduate School of Chiang Mai University for general funding. References 1) L. Sun, M. Cao, Y. Wang, G. Sun and C. Hu, J. Cryst. Growth, 289, (2006). 2) M. A. M. A. Maurera, A. G. Souza, L. E. B. Soledade, F. M. Pontes, E. Longo, E. R. Leite and J. A. Varela, Mater. Lett., 58, (2004). 3) Y. Liu and Y. Chu, Mater. Chem. Phys., 92, (2005). 4) Z. Lou and M. Cocivera, Mater. Res. Bull., 37, (2002). 5) X. Zhang, Y. Xie, F. Xu and X. Tian, J. Colloid Interf. Sci., 274, (2004). 6) C. An, K. Tang, G. Shen, C. Wang and Y. Qian, Mater. Lett., 57, (2002). 7) D. Chen, G. Shen, K. Tang, H. Zheng and Y. Qian, Mater. Res. Bull., 38, (2003). 8) P. Kalyani, N. Kalaiselvi and N. Muniyandi, Mater. Chem. Phys., 77, (2002). 9) T. Thongtem, A. Phuruangrat and S. Thongtem, J. Mater. Sci., 42, (2007). 10) T. Thongtem, A. Phuruangrat and S. Thongtem, Mater. Lett., 61, (2007). 11) D. Chen, K. Tang, G. Shen, J. Sheng, Z. Fang, X. Liu, H. Zheng and Y. Qian, Mater. Chem. Phys., 82, (2003). 12) Y. Zhao, X. H. Liao, J. M. Hong and J. J. Zhu, Mater. Chem. Phys., 87, (2004). 13) T. Thongtem, A. Phuruangrat and S. Thongtem, Current Appl. Phys., 8, (2008). 14) C. Gabriel, S. Gabriel, E. H. Grant, B. S. J. Halstead and D. M. P. Mingos, Chem. Soc. Rev., 27, (1998). 15) J. H. Ryu, J. W. Yoon, C. S. Lim, W. C. Oh and K. B. Shim, Ceram. Int., 31, (2005). 16) J. H. Ryu, C. S. Lim, W. C. Oh and K. B. Shim, J. Ceram. Process. Res., 5, (2004). 17) D. Andreescu, E. Matijević and D. V. Goia, Colloid. Surf. A, 291, (2006). 18) R. J. Sengwa, J. Molec. Liq., 108, (2003). 19) Powder Diffract. File, JCPDS Int. Centre for Diffract Data, U.S.A. (2001). 20) D. Errandonea, J. Pellicer-Porres, F. J. Manjón, A. Segura, Ch. Ferrer-Roca, R. S. Kumar, O. Tschauner, P. Rodríguez- Hernández, J. López Solano, S. Radescu, A. Mujica, A. Muñoz and G. Aquilanti, Phys. Rev. B, 72, (1) (14) (2005). 21) C. Suryanarayana and M. G. Norton, X-ray Diffraction, A Practical Approach, Plenum Press, New York (1998). 22) T. T. Basiev, A. A. Sobol. Y. K. Voronko and P. G. Zverev, Opt. Mater., 15, (2000). 23) V. Thangadurai, C. Knittlmayer and W. Weppner, Mater. Sci. Eng. B, 106, (2004). 24) G. M. Clark and W. P. Doyle, Spectrochim. Acta., 22, (1966). 25) A. P. de Azevedo Marques, D. M. A. de Melo, C. A. Paskocimas, P. S. Pizani, M. R. Joya, E. R. Leite and E. Longo, J. Solid State Chem., 179, (2006). 26) J. A. Gadsden, Infrared Spectra of Minerals and Related Inorganic Compounds, Butterworths (1975). 609