Results The powders of BaWO 4 (tetragonal), NiWO 4 (monoclinic) and Bi 2WO 6 (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 NiWO 4 has the highest surface area. BaWO 4 exhibited less Raman vibrations than the NiWO 4 because of the increased lattice symmetry but Bi 2WO 6 showed almost the same Raman vibrations as BaWO 4. From the UV-vis spectra, the band gap transition of the metal tungstates are of the order of BaWO 4 Bi 2WO 6 NiWO 4. 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.

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XRD pattern can reveal the phase purity and crystallinity of the powder sample. Figure shows sharp diffraction peaks indicating that the oxide products are well crystallized and no peaks attributable to other impurities were observed. The pattern agrees well with the JCPDS file of NiWO 4, BaWO 4 and Bi 2WO 6 (PDF card 72-0480, 72-0746 and 79-2381). The NiWO 4 indexed in wolframite monoclinic structure (space group: P2/c, with Z = 2) is characterized by alternating layers of transition-metal and tungsten atoms parallel to the (100) plane. The oxygen atoms are hexagonally closely packed and the metal ions occupy a quarter of all the octahedral sites. For BaWO 4, the peaks from diffraction patterns are consistent with a body-center primitive tetragonal scheelite, space group I4 1/a and has C 4 h 6 point group with two formula units per primitive cell.

In an ideal scheelite type of ABO 4, larger A (Ba 2+) cation shows eight-fold coordination and smaller B (W 6+) cation shows four-fold coordination. The tungstates reported have strong covalent bonds of W-O in WO 4 2- molecular ionic units and weak coupling between WO 4 2- anions and Ba 2+ cations. All the peaks of Bi 2WO 6 are recognized with the crystal structure of orthorhombic symmetry crystal phase with space group Pca2 1 and crystallized in a layered crystal structure including the corner-shared WO 6. The Bi atom layers are sandwiched between WO 6 octahedral layers.

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Table shows that crystallite sizes of all the samples calculated from Scherer’s equation are in nano-size range: NiWO 4 at d 100, d 110 and d 011 are 19.3, 19.3 and 17.2 nm, while those of BaWO 4 at d 112 and d 004 are 18.9 and 17.4 nm. Smaller crystallite sizes of 15.5 and 14.9 nm are shown by Bi 2WO 6 at d 131 and d 002, respectively. Dong Young et al. have also synthesized similar compounds by commercial hydrothermal methods and obtained the crystallite size of 17-24 nm in same plane of d 131. NiWO 4 synthesized by reacting ammonium metatungstate and nickel nitrate as a function of temperature from 673 to 1073 K of 1 h reaction time has been reported by Quintana-Melgoza et al. and average crystallite size as determined by Scherrer analysis obtained was from 55 to 112 nm, which is three times bigger than that reported in this work.

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In the case of BaWO 4, calculated crystallite size synthesized by room temperature the metathetic reaction method has been reported to grow twofold in crystallite size (51 nm) along d 112. The existence of sucrose in the solution of the metal cations will form a matrix in which the metal cations are distributed through the sucrose structure. The sucrose molecule is hydrolyzed into glucose and fructose and in this way sugar recrystallization is prevented. The complex mass is obtained by complexation via gel formation and the final particles are obtained upon decomposition in the calcination process. During heating, the metal ion complex is decomposed into CO 2 and H 2O and a large amount of heat is generated. All these products are gaseous, preventing agglomeration and thus giving rise to pores and fine powders of smaller crystallite size (Table ).

The FESEM results demonstrated that the morphology of BaWO 4, NiWO 4 and Bi 2WO 6 samples strongly depend on size of particles while BET results showed the dependency of their surface areas on pore volume and pore distribution. All three samples show different morphologies: BaWO 4 particles (Figure (a)) grow in large spherical grain sizes between 0.8-0.9 μm. Samples NiWO 4 and Bi 2WO 6 in Figure (b & c) show smaller inter-connected grain sizes of 30-90 and 20-60 nm, respectively. From Figure, BaWO 4 shows mesoporous characteristics obtained from adsorption-desorption isothermal of type IV and the H3 and the hysteresis loop observed in the range of 0.70 – 0.95 P/P o (according to the IUPAC classification) agrees reasonably well with the small pore volume (0.05 cm 3g -1) and low surface area (2.30 m 2g -1), as shown in Table.

Both samples of NiWO 4 and Bi 2WO 6 show adsorption-desorption isotherms of a macroporous characteristic (type III) with absence of any hysteresis loop. Sample S BET(m 2g -1) Pore volume (cm 3/g) Pore size distribution, (nm) Bi 2WO 6 3.58 0.62 1.54 NiWO 4 20.06 0.54 2.46 BaWO 4 2.30 0.05 1.92 Even though the NiWO 4 sample has larger crystallite size (according XRD), its surface area is fivefold larger (20.06 m 2g -1) than Bi 2WO 6 (3.58 m 2g -1). This phenomenon is attributed to the higher pore distribution (Table ) and less agglomeration of NiWO 4 itself (Figure (b)). This finding shows that the prepared NiWO 4 sample using sucrose solution evaporation has higher BET surface area compared to NiWO 4 synthesized by combustion method (.

(1) where all 13 vibrations A g, B g and E g are Raman-active. As shown in Figure, the tetragonal BaWO 4 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 NiWO 4 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 WO 4 tetrahedra. The medium mode at 272 cm -1 is derived from symmetric stretching vibration of the BaO 6 octahedra. All these modes are characteristic of the tetragonal scheelite structure as reported previously –. 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. (2) Here, 18 even (g) vibrations are Raman-active modes. As for monoclinic NiWO 4, the corresponding spectrum in Figure 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 WO 6 octahedra. Unlike the ideal WO 4 structure (scheelite) where four normal vibrational modes of the tetrahedral structure are Raman active, WO 6 structure has six normal modes of vibration of which only three are Raman active.

The isolated WO 6 wolframite structure found in the bulk crystalline NiWO 4 has 891 cm -1 which is associated with the WO 6 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 P ca2 1 structure of Bi 2WO 6 distributed among 26A 1 + 27A 2 + 26B 1 + 26B 2 irreducible representations. The A 1, B 1 and B 2 modes are both Raman and IR active whereas the A 2 modes are only Raman active.

Bi 2WO 6 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 WO 6 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 WO 6 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. Figure shows the optical absorption spectra of BaWO 4, NiWO 4 and Bi 2WO 6 nanoparticles with an absorption edge in 200–900 nm region.

All samples have excellent optical transmission spectra as the maximum absorption edges appeared in the ultraviolet region: 223.0 nm for BaWO 4, 320.6 nm for Bi 2WO 6 and 299.0 nm for NiWO 4. The excitation from O 2p to Wt 2g in the (WO 4 2-) group absorbs ultraviolet irradiation in MWO 4. In the excited state of the (WO 4 2-) groups, the hole (on the oxygen) and the electron (on the tungsten) remain together as an exciton because of their strong interactions. Further absorption peaks in the visible region are exhibited by NiWO 4 which could be due to a charge transfer transition in which an oxygen 2 p electron goes into one of the empty tungsten 5 d orbital. Figure 5 Optical absorbance spectra of the metal tungstates. ( a) BaWO 4, ( b) NiWO 4 and ( c) Bi 2WO 6.

A unique feature of UV-vis for the isolated WO 4 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 WO 4 structure.

Optical absorbances of samples BaWO 4 and Bi 2WO 6 show only one absorption band, while NiWO 4 shows four absorption bands. Worth noting to report that the absorption peak of BaWO 4 from this work was found close to what has been reported. For the NiWO 4 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 NiWO 4 sample at both UV and visible range (Figure (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 Ni 2+O 6 are due to the transition from 3A 2g to the excited states 3T 2g, 1E g, 3T 1g, 1T 2g, and 3T 1g, 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 WO 6 matrix. Bands at 2.71 and 1.68 eV are assigned to the forbidden electronic transition from 3A 2g to 1E g and 1T 2g, respectively. The band at 1.47 eV can be assigned to the presence of Ni 2+O 4 arising from Frenkel defects with dislocation of Ni 2+ from the octahedral to tetrahedral sites. This result is in agreement with that of de Oliveira et al.

Quantification of the band gap (E g) was carried out for all three metal tungstate samples. The band gap transition is determined from the steep shape of the spectra and the equation αhν = A(hν – E g) m was employed where the absorption coefficient (α) is related to the incident photon energy (hν), A is constant, m is the index indicating the type of transition. The nature of the electropositive ions (Ba 2+, Ni 2+ and Bi 3+) seems to have small influence on the E g values. It is found that E g decreases according to the following sequence: BaWO 4 Bi 2WO 6 NiWO 4 (Table ). The band gap of BaWO 4 (4.60 eV) agrees well with the values reported , , while the value of the prepared NiWO 4 is significantly higher (3.05 eV) ,. Ross-Medgaarden and Wachs also reported the E g value of wolframite NiWO 4 as 4.5 eV, which is higher than this finding with ligand-to-metal charge transfer (LCMT) band maximum between 247-252 and 342-344 nm, due to the distortion nature in isolated WO 6 units.