Microwave-assisted synthesis of nanocrystalline MWO4 (M: Ca, Ni) via water-based citrate complex precursor
Introduction
CaWO4 and NiWO4 are important inorganic materials of the metal tungstate families that have high application potential in various fields, such as in photoluminescence [1], microwave applications [2], optical fibers [3], scintillator materials [4], humidity sensors [5], magnetic properties [6] and catalysis [7]. Metal tungstate of relatively large bivalent cations (MWO4, ionic radius > 0.99 Å, M: Ca, Ba, Pb, Sr) exist in the so-called scheelite structure form (scheelite: CaWO4), where the tungsten atom adopts tetrahedral coordination. Tungstates of smaller bivalent cations (MWO4, ionic radius < 0.77 Å: M: Fe, Mn, Ni, Mg), however, belong to the wolframite structure (wolframite: (Fe, Mn)WO4), where the tungsten atom adopts an overall six-fold coordination [8].
Most previous approaches to the preparation of these families of compounds need high-temperature and harsh reaction conditions, such as the Czochralski method [9], reaction in aqueous medium followed by heating of the precipitates [10], the conventional solid-state method [11] and the hydro-thermal reaction over an extensive period [12]. However, metal tungstate particles prepared by these processes are relatively large with inhomogeneous morphology and composition. Inhomogeneous compounds of CaWO4 and NiWO4 might be easily formed because the WO3 has a tendency to vaporize at high temperatures [13], and the temperature for the solid-state reaction is relatively high, almost above 1000 °C for 24 h [11].
These problems could be solved by applying advanced wet chemical methods. Polymerized complex method as a modified Pechini method [14], where several metal ions in a solution could be first chelated to form metal complexes and then polymerized to form a gel, seems to be most suitable among chemical solution processes, because rigidly fixed cations are homogeneously dispersed in the polymer network and have few chances to segregate even during pyrolysis. This method has already been successfully used to prepare highly pure powders of various double oxides such as BaTiO3 [15], Y6WO12 [16], mixed-cation oxides [17] and even for various superconductors [18] with multiple cationic compositions. However, in spite of the many advantages of the polymeric complex method, the weakness of this method is the difficulty of the effective removal of the large amount of organic substances. Based on this consideration, the citrate complex method as another chemical solution process was tried in this work for the synthesis of nanocrystalline CaWO4 and NiWO4 powders. In this process, metal citrate complexes without a network structure are formed from water instead of ethylene glycol.
On the other hand, microwave irradiation as a heating source has been found and developed for a number of applications in chemical and ceramic processing [19], [20], [21]. Compared with the usual methods, microwave synthesis has the advantages of shortening the reaction time, giving products with small particle size, narrow particle size distribution and high purity. Jansen et al. [19] suggested that these advantages could be attributed to fast homogeneous nucleation and easy dissolution of the gel.
In this work, we report the synthesis of nanocrystalline CaWO4 and NiWO4 powders from water-based citrate complex precursor using microwave irradiation. The precursors and powders were evaluated through their crystallization process and thermal decomposition processes and by identifying particle morphology.
Section snippets
Experimental
Metal nitrates (Ca(NO3)2·4H2O, Ni(NO3)2·6H2O, Junsei Chemical Co. Ltd., Japan) and ammonium tungstate ((NH4)10W12O41·5H2O, Junsei Chemical Co. Ltd., Japan) were used as the metallic cations. De-ionized water (DW) and citric acid (HOC(CO2H)(CH2CO2H)2, CA, Yukiri Pure Chemical Co. Ltd., Japan) were used as the solvent and chealating agent for the process. Fig. 1 shows the schematic flow chart for the synthesis of nano-sized CaWO4 and NiWO4 powders by the modified citrate complex method using
Results and discussion
Fig. 2, Fig. 3 show the phase identification of the NiWO4 and CaWO4 particles heated for 3 h as a function of heating temperature in detail using XRD. In Fig. 2(a) and (b), the precursor and powders of NiWO4 at 300 °C were amorphous without any crystallized phases. Above 350 °C in Fig. 2(c) and (f), the particles could be identified as the NiWO4 phase. At 350 °C in Fig. 2(c), unreacted WO3 phase was observed and it seemed to be an additional phase to the NiWO4 phase. For a higher heating
Conclusion
Nano-sized CaWO4 and NiWO4 powders were successfully synthesized by the modified citrate complex method under microwave irradiation. Crystallization of CaWO4 and NiWO4 particles were detected at 300 and 350 °C, respectively, and completed entirely at a temperature of 400 °C. Most of the CaWO4 and NiWO4 nanocrystalline powders heat-treated between 350 and 450 °C showed primarily spherical and homogeneous morphology. The average crystallite sizes of CaWO4 were between 12 and 35 nm, and those for NiWO4
References (22)
- et al.
J. Lumine.
(1997) - et al.
Mater. Res. Bull.
(2004) - et al.
Physica B
(1997) - et al.
Sens. Actuators B
(1995) - et al.
Mater. Res. Bull.
(2003) - et al.
Phys. Rev.
(1962) - et al.
Phys. Rev. B
(1992) - et al.
Nucl. Instrum. Methods Phys. Res., A
(1997) - et al.
Adv. Funct. Mater.
(2003) - et al.
Phys. Status Solidi B
(1975)