Structural and optical characterization of metal tungstates (MWO4; M=Ni, Ba, Bi) synthesized by a sucrose-templated method
© M Zawawi et al.; licensee Chemistry Central Ltd. 2013
Received: 29 January 2013
Accepted: 16 April 2013
Published: 1 May 2013
Metal tungstates have attracted much attention due to their interesting structural and photoluminescence properties. Depending on the size of the bivalent cation present, the metal tungstates will adopt structures with different phases. In this work, three different phases of metal tungstates MWO4 (M= Ba, Ni and Bi) were synthesized via the sucrose templated method.
The powders of BaWO4 (tetragonal), NiWO4 (monoclinic) and Bi2WO6 (orthorhombic) formed after calcination temperatures of 750, 650 and 600°C for 4 h respectively are found to be crystalline and exist in their pure phase. Based on Scherrer estimation, their crystallite size are of nanosized. BET results showed NiWO4 has the highest surface area. BaWO4 exhibited less Raman vibrations than the NiWO4 because of the increased lattice symmetry but Bi2WO6 showed almost the same Raman vibrations as BaWO4. From the UV-vis spectra, the band gap transition of the metal tungstates are of the order of BaWO4 > Bi2WO6 > NiWO4. Broad blue-green emission peaks were detected in photoluminescence spectra and the results showed the great dependence on morphology, crystallinity and size of the metal tungstates.
Three different phases of metal tungstates of BaWO4 (scheelite), NiWO4 (wolframite) and Bi2WO6 (perovskite layer) in their pure phase were successfully prepared by the simple and economical sucrose-templated method. The highest surface area is exhibited by NiWO4 while largest band gap is shown by BaWO4. These materials showed promising optical properties.
Metal tungstates with formula MWO4 have attracted much attention due to their interesting structural and photoluminescence properties [1–5]. These materials have found applications in scintillation counters, lasers and optical fibers [6, 7]. Some of the divalent transition metal tungstates have also gained commercial interest in lasers and fluorescent lamps, while some are of special importance due to their electrical conductivity and magnetic properties. In addition, these materials also find applications as catalysts and humidity sensors [8, 9].
In the MWO4 compounds, if M2+ has small ionic radius < 0.77 Å (Ni = 0.69), it will belong to the wolframite-type monoclinic structure where the tungsten atom adopts an overall six-fold coordination . However, if larger bivalent cations with ionic radius > 0.99 Å (Ba=1.35), they exist in the so-called scheelite-type tetragonal structure where the tungsten atom adopts tetrahedral coordination. Bismuth tungsten oxide belongs to the orthorhombic system, space group Pca21, and crystallizes in a layered crystal structure including the corner-shared WO6. The Bi atom layers are sandwiched between WO6 octahedral layers . It is the simplest member of the Aurivillius family from Bi2An-1BnO3n+3 (A=Ca, Sr, Ba, Pb, Bi, Na, K and B=Ti, Nb, Ta, Mo, W, Fe) (when n=1) of layered perovskites, which structurally comprises of alternating perovskite-like slabs of WO6 and [Bi2O2]2+ layers. Recently, many studies have been reported on the preparation and characterization of metal tungstates using various preparation methods such as Czochralski , precipitation [13, 14], hydrothermal [11, 15], solid state , pulsed laser deposition . Meanwhile the nanostructures of metal tungstates in different crystal structures including nanorods, nanoparticles, hollow clusters and others have been prepared by chemical and physical methods. For Bi2WO6, its nanometer sheet shaped was obtained through hydrothermal treatment at pH=11, heated at 200°C for 24 hours and finally thermally treated at 400, 600 and 800°C for 3 hours . BaWO4 in the rhombic shape was prepared by a molten flux reaction using alkali metal nitrates as the reaction media . Nickel tungstate (NiWO4) nanoparticles were successfully synthesized at low temperatures by a molten salt method at a temperature as low as 270°C, where the mixture of NaNO3 and LiNO3 was used as the molten salt medium with 6:1 mass ratio of the salt to the NiWO4 precursor . Generally, these methods require expensive and sophisticated equipment, high temperatures with long processing times, expensive precursors and high consumption of electric energy.
Prabhakaran et al.  had used a cheaper and simpler method of using sucrose in order to synthesize yttria-stabilized zirconia (YSZ) nanoparticles in both acidic and basic solutions. The analyses consistently reported to have fairly uniform nanoparticles with small size, containing both tetragonal and monoclinic phases with crystallite size between 10 and 30 nm. Due to its simplicity, the sucrose-template method has great potential for manufacturing high quality ultrafine ceramic oxides economically  and this creates a new approach for synthesis of the other ceramic materials. In this method, the -OH and -COOH groups of the decomposed sucrose products help in binding the metal ions in the homogeneous solution, which reduces the chances of precipitation. During the decomposition process, a voluminous, organic-based, black, fluffy mass of carbonaceous material is formed which upon heating will decompose further into carbon dioxide and water and a large amount of heat is generated. The outgoing gases prevent agglomeration, and form pores and fine particles with high surface area in the final products. The aim of this paper is to synthesize the different crystal structures of BaWO4, NiWO4 and Bi2WO6 by a sucrose templated method and to characterize the materials for their structural and optical property by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Brunaer-Emmet-Teller (BET) and Raman spectroscopy while optical properties were investigated using UV-vis and photoluminescence spectroscopy.
Preparation of powders
The desired metal nitrates [Ba(NO3)2, Ni(NO3)2∙ 6H2O, Bi(NO3)2] of 2.6135, 2.9081 and 4.8511 g were individually dissolved in distilled water before being mixed into an aqueous solution of sucrose. This is followed by addition of an equal volume of 2.4633 g of ammonium metatungstate to maintain stoichiometric ratio (1:1) with continuous stirring. Sucrose acts as a template and the ratio of sucrose to metal used was 3:1. Towards the end of the evaporation, the precursor solution (after further heating) gave rise to a fluffy black organic mass. The carbon rich mass was easily crushed to form the precursor powders. Precursor powders are denoted as MWp (M= Ba, Ni, and Bi). Calcination treatment was applied in the next step because of the large amount of organic compounds present in the crunchy powders. The temperatures and durations for calcinations were derived from the results of the thermogravimetric analysis whereby processes such as dehydration and other volatilizations to go to completion before proceeding to higher temperatures.
The calcination treatment applied to the samples involved heating at the rate of 10°C/min and the temperature was held constant for 4 h for each thermal change as inferred from the thermal analysis to allow completion of each of the processes. The three powdered precursors, MWp (M= Ba, Ni, and Bi) were subsequently calcined at 750, 650 and 600°C respectively for 4 h and the samples were denoted as MWO4 (M= Ba, Ni, and Bi).
The formation of oxides was monitored by X-ray diffraction (XRD) measurements using Siemen D5000 with a copper Kα radiation tube and wavelength λ of 1.54 Å, operated at 40 kV and 40 mA. The X-ray powder diffraction patterns were obtained in the range 5-60°, with increments of 0.05°. The crystalline phases were identified by using the International Centre for Diffraction Data (ICDD). The full width at half maximum (FWHM) of the diffraction peaks obtained from the refinement have been used to calculate the crystallite size. Specific surface area (SBET) measurements were made with a Quantachrome AUTOSORB-1 model by nitrogen adsorption at -196°C using the BET isotherm. Samples were degassed under flowing argon at 250°C for 9 h before being adsorbed by nitrogen. The surface morphology of the samples was e analyzed using the Field Emission Scanning Electron Microscope, FESEM JSM-7500F/7500FA (JEOL) at magnification of 20,000 ×. This morphological analysis can provide information on the prevalent surface features. FESEM images allowed us to estimate the average particle size distribution of all three samples through the counting of approximately 150 particles using Image tool software. Diffuse reflectance spectra were obtained using a UV-Visible Spectrophotometer (Shimadzu). Raman spectra was collected by InVia Raman Microscope Renishaw spectrometer using UV lens set at λUV = 325 nm and equipped with 2,400 l/nm diffraction grating. The same equipment was also used for photoluminesence (PL) analysis by using a visible lens set and equipped with 1,200 l/nm diffraction grating.
Results and discussion
Summary of metal tungstates phase formation and calculated crystallite sizes
Body cubic tetragonal
FESEM and BET
Summary of metal tungstates specific surface area, pore volume and pore size distribution
Pore volume (cm3/g)
Pore size distribution, (nm)
Even though the NiWO4 sample has larger crystallite size (according XRD), its surface area is fivefold larger (20.06 m2g-1) than Bi2WO6 (3.58 m2g-1). This phenomenon is attributed to the higher pore distribution (Table 2) and less agglomeration of NiWO4 itself (Figure 2(b)). This finding shows that the prepared NiWO4 sample using sucrose solution evaporation has higher BET surface area compared to NiWO4 synthesized by combustion method (< 11 m2g-1) even though a spherical-like morphology was obtained in both cases . Bi2WO6 synthesized by using co-precipitation method also resulted in similar spherical particles (after calcinations at 600, 700 and 800°C) as reported by Alfaro and de la Cruz , but the size of particles were in microns; sizes (~1-2 μm) and BET values obtained were 0.3- 1.5 m2g-1, which was 10 times lower than the findings in this work shown in Table 2.
FESEM images can also allow the estimation of the average particle size distribution of samples by counting approximately 150 particles using an Image tool software. The particles are assumed spherical-like (Figure 2(a1-c1)). Figure 2(a1) shows the average particle size distribution (diameter) in the range from 0.80-0.94 μm for BaWO4. The figure shows that 59% of the particles with a spherical-like morphology presented an average area of 0.84-0.88 μm. Figure 2(b1) shows the average particle area distribution of Bi2WO6 is 20-55 nm and that 62% of the particles presented an average area of 25-35 nm, smaller than BaWO4 (in micron range). As for NiWO4 (Figure 2c1) with the plate-like morphology the average particle area distribution is 30-90 nm, which is in close relationship with the above grain size (FESEM image). These results show that the sucrose-templated method is able to influence the growth process into nano-range for samples NiWO4 and Bi2WO6, except for sample BaWO4 which is in micron size. However, the particle size distribution of BaWO4 synthesized using a sucrose-templated method shows smaller dimension (0.84-0.88 μm) compared to BaWO4 synthesized by co-precipitation followed by domestic microwave-hydrothermal at 413 K for different times which resulted in a large self-assembled microcrystal of height (0.30–11.85 μm) and width (0.25–2.30 μm) .
where all 13 vibrations Ag, Bg and Eg are Raman-active. As shown in Figure 4, the tetragonal BaWO4 has two strong vibrations at 924 and 330 cm-1 and four weak vibrations at 829, 797, 716 and 272 cm-1. It is predicted to have less Raman vibrations when compared to monoclinic NiWO4 because of the increased lattice symmetry. The two strong vibrations of 924 and 330 cm-1 and weak mode at 797 cm-1 can be assigned to the W-O stretching vibration of WO4 tetrahedra. The medium mode at 272 cm-1 is derived from symmetric stretching vibration of the BaO6 octahedra. All these modes are characteristic of the tetragonal scheelite structure as reported previously [32–36]. However, in our samples, the vibrations were slightly shifted and some vibration modes were not detected. These observations can be attributed to some differences in their geometries, particle sizes and nature of the products.
Here, 18 even (g) vibrations are Raman-active modes. As for monoclinic NiWO4, the corresponding spectrum in Figure 4 shows only three strong vibrations at 891, 778 and 698 cm-1 and five weak vibrations at 328, 374, 552, 616 and 1036 cm-1 corresponding to the normal W-O vibration of the WO6 octahedra. Unlike the ideal WO4 structure (scheelite) where four normal vibrational modes of the tetrahedral structure are Raman active, WO6 structure has six normal modes of vibration of which only three are Raman active. The isolated WO6 wolframite structure found in the bulk crystalline NiWO4 has 891 cm-1 which is associated with the WO6 symmetric stretching vibration and this agrees well with the results reported by Ross-Medgaarden and Wachs .
The factor group analysis predicts that there should be 105 optical modes for Pca21 structure of Bi2WO6 distributed among 26A1 + 27A2 + 26B1 + 26B2 irreducible representations. The A1, B1 and B2 modes are both Raman and IR active whereas the A2 modes are only Raman active. Bi2WO6 shows two strong peaks at 797 and 295 cm-1 and weak peaks at 410 and 716 cm-1. The strongest peak at 797 cm-1 can be assigned to the symmetric and asymmetric stretching modes of the WO6 octahedra involved in the motions of the apical oxygen atoms perpendicular to the layer . The weak Raman peak at 716 cm-1, is due to asymmetric stretching mode of the WO6 octahedra, involving mainly vibrations of the equatorial oxygen atoms within layers. The peak at 295 cm-1 region originates from the bending mode of the bismuth-oxygen polyhedral.
Diffuse reflectance UV-visible spectroscopy
A unique feature of UV-vis for the isolated WO4 reference compounds is that they only possess a single ligand to metal charge transfer (LMCT) band in the general region of 218-274 nm, with many of the band maxima occurring at 220-250 nm. The exact location of this band maximum depends on the extent of distortion of the isolated WO4 structure . Optical absorbances of samples BaWO4 and Bi2WO6 show only one absorption band, while NiWO4 shows four absorption bands. Worth noting to report that the absorption peak of BaWO4 from this work was found close to what has been reported .
For the NiWO4 sample, 100 nm shift to a lower wavelength was observed as compared to the same material synthesized by the molten salt method . Four bands observed from the NiWO4 sample at both UV and visible range (Figure 5(b)) are due to the oxidation state of the cations . Cimino et al.  had reported that absorption bands at 1.21, 1.65-1.74, 2.00-2.11, 2.83-2.88 and 3.35 eV from Ni2+O6 are due to the transition from 3A2g to the excited states 3T2g, 1Eg, 3T1g, 1T2g, and 3T1g, respectively. Similar data were also obtained by Lenglet et al.  who reported the same bands at about 1.08-1.13, 1.72-1.75, 1.77-1.95, 2.71-2.79 and 2.97-3.00 eV. In the present work, four absorbance bands at 299 nm (2.97 eV), 453 nm (2.71 eV), 738 nm (1.68 eV) and 842 nm (1.47 eV) are observed; the first and second bands with high intensity are in the ultraviolet range while the third and forth with low intensity is in the blue range. The first band at 2.97 eV may be attributed to the charge transfer transition in the WO6 matrix. Bands at 2.71 and 1.68 eV are assigned to the forbidden electronic transition from 3A2g to 1Eg and 1T2g, respectively. The band at 1.47 eV can be assigned to the presence of Ni2+O4 arising from Frenkel defects with dislocation of Ni2+ from the octahedral to tetrahedral sites. This result is in agreement with that of de Oliveira et al. .
For Bi2WO6, the band gap value obtained (3.05 eV) is higher than that found by Fu et al. , as the Eg value in d° perovskites was shown to depend upon the electro-negativity of the transition metal ion, the connectivity of the polyhedral and the deviation from linearity of the M-O-M bonds. In addition, the forms of the solid samples often have strong effect on the optical properties of the material .
Summary of wavelength at maximum peak and peak intensity of BaWO 4 , NiWO 4 and Bi 2 WO 6 samples
For Bi2WO6, smaller grain size also contributes considerably to high PL intensity. Similar observations were also observed by Dong Young et al. who synthesized Bi2WO6 hydrothermally and obtained higher PL intensity with the smaller crystallite size of 23 nm as calculated from XRD . These phenomenon closely agrees to that reported by Quintana-Melgoza et al.  in which the optical response of material is largely determined by its underlying electronic properties that are closely related to its chemical or ions, atomic arrangement and physical dimension for nanometer-sized materials. Low intensity of the PL curve has been shown to be due to the oxygen atoms playing the role of electron capturers, thereby depressing the recombination process. In addition, PL intensity also depends on whether the added tungsten metal acts as an electron capturer or not. The PL curve of NiWO4 powder tends to shift slightly to a higher wavelength as compared to Bi2WO6 and BaWO4. This blue shift is observed when the dimensions of nanocrystalline particles approach the exciton Bohr radius (ao) due to the quantum-size effect (quantum confinement phenomenon) which can be attributed to the wider band gap  thus agreeing with the finding on band gap calculation in Table 3. Lee et al.  on discussing the effective mass model has assumed that blue shift in the band gap energy occurs due to spatial confinement of an exciton. Hence to generate a free exciton, energy higher than the effective band gap energy must be available. In the absence of additional levels introduced by defects, radiative electron-hole recombination of this free exciton should result in photon emission with energy equivalent to the band gap energy. Although there are different opinions explaining the origin of the emission bands and the nature of the optical transition is unclear, the WO42− complex and the slight deviation from a perfect crystal structure are believed to be responsible for the emission bands.
Three different phases of metal tungstates of BaWO4 (scheelite), NiWO4 (wolframite) and Bi2WO6 (perovskite layer) were successfully prepared by the simple and economical sucrose-templated method. The highest surface area (20.06 m2g-1) contributed by of NiWO4 is believed to arise from higher pore distribution and less particle agglomeration due to the presence of sucrose. Raman spectra showed that the vibration modes of the products are in accordance to those of the tungstate compounds. Microstructure vibrations of three different phases of scheelite-type BaWO4 were shown to have less Raman active modes when compared to wolframite NiWO4, caused from the increased lattice symmetry while layered perovskite Bi2WO6 exhibited only four peaks involving oxygen motion, perpendicular and within the layer. Slight shifting of the detected vibration modes and that some vibration modes were not detected can be attributed to some differences in their geometries, particle sizes and nature of the products. The UV spectra revealed the highest band gap associated with BaWO4 followed by Bi2WO6 and NiWO4. Broad blue-green emission peaks in PL were detected at ~ 600 nm for all samples. The blue-shift in PL spectra is due to the quantum size effect as a result of the wider band gap. Results also showed great dependence of the PL intensity on smaller grain sizes (~ 50-80 nm) with homogenous spherical particle morphology. The materials showed promising PL results for fluorescence lamp application.
The authors gratefully acknowledged the financial support granted by the Ministry of Higher Education, Malaysia (FRGS: FP056/2008C) and University of Malaya (PPP: PV035/2011B).
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