Synthesis, Characterization and Optical Activity of RE-doped ZnWO4 Nanorods and Nanospheres by Hydrothermal Method

This work has investigated the effect of different dopants on structure, morphology and optical property of ZnWO4. Rare-earth doped ZnWO4 (ZnWO4:RE, with 0.5, 1, and 2 mol% of Eu3+ and Pr3+) were successfully synthesized by coprecipitation method followed by microwave-assisted hydrothermal system at 140 °C for 1 h. XRD indicated that the crystals have a wolframitetype monoclinic structure and with the addition of dopants the crystallite size decreased. HR-TEM images revealed interesting homogenous nanorods for pure ZnWO4 crystals with grow along (021) direction. For ZnWO4: RE we have found nanospheres morphologies, in which the decreasing crystal size were dependent on the RE doping concentration. IR spectra confirm the crystals structure. Ultraviolet–Visible diffuse reflectance spectra indicated that the optical band gap varies with increasing replacement of Zn2+ by RE ions. Egap was characteristic of semiconductor materials.

Doping strategy via chemistry solution using coprecipitation followed by microwave hydrothermal method (MWH) may provide wellcontrolled ways to modify the structures, morphologies, particle sizes, and surface features by means of adaptation on the compositions of the crystals [2,8,14,15]. Therefore, the incorporation of the rare earth ions on the pure ZnWO4 such as Eu 3+ and Pr 3+ ions replacing the Zn 2+ site can result in changes on sizes and shapes of pure ZnWO4 particles, as well as can to improve their property [14][15][16][17].

Results and Discussion
Fig. 1a-b shows the XRD pattern obtained for ZnWO4: RE (RE= Eu 3+ and Pr 3+ with 0.5, 1.0 and 2.0% mol). All diffraction peaks were indexed to the wolframite-type monoclinic structure according to the standard card (ICDD Card number: 00-015-0774).
The use MWH has provided the production of crystalline materials under moderate heating conditions, in aqueous and alkaline medium, with temperatures and times significantly lower than those reported in the literature [14,18]. Consequently, these conditions and the addition of dopants contribute to the reduction of particle size. By use the Scherrer's formula from the predominant plan (111) we have obtained the mean crystallite size of ZnWO4: RE 3+ . The found values were 9.60 nm for pure ZnWO4 and 7.32, 5.89 and 3.62 nm for 0.5, 1.0 and 2.0 mol% of Eu 3+ , respectively. For ZnWO4 doped with 0.5, 1.0 and 2.0 mol% of Pr 3+ the mean crystalline size were 9.50, 3.84 and 3.10 nm, respectively.
The decrease in crystallite size occurs through defects generated by the substitution of the network modifier (Zn 2+ ) by the dopant (Eu 3+ or Pr 3+ ), generating distortions in the crystalline lattice [19], which consequently contributes to technological applications. Optical band gap energy (Egap) values were calculated using the Kubelka-Munk equation [20]. From the extrapolation of the linear part of the curve in the Kulbelka-Munk function it is possible to establish Egap (Fig. 2). From this, Egap values decrease with increasing dopant concentration. This is due to the presence of RE in the lattice, resulting in intermediate energy levels between valence band (VB) and conduction band (CB) and electronic energy levels related to the additional 4f orbitals of the dopant [20]. This behavior contributes to applications in photoluminescence and photocatalysis area. FT-IR spectra of the RE 3+ -doped ZnWO4 (Fig. 3) show the main absorption bands at 400-1050 cm -1 . The bands at 464 and 668 cm −1 are assigned to bending vibrations of the W-O bonds. The peaks at 705 and 815 cm − 1 correspond to stretching vibration of the W-O bonds. The Zn-O-W bond vibrations result in a peak at 880 cm −1 owing to bending and stretching deformations. For the ZnO6 and WO6 octahedra, the symmetric and asymmetric deformation modes of the W-O and Zn-O bonds are observed at 560 and 432 cm −1 , respectively [3].
Figures 4a-f shows TEM images of Eu 3+doped ZnWO4. This technique was used to verify the particle size, homogeneity and shape of the crystals.
For pure ZnWO4 the HR-TEM images revealed homogenous nanorods (20-

Synthesis
Rare-earth doped ZnWO4 crystals (with 0.5, 1.0, and 2.0 mol% of Eu 3+ and Pr 3+ ) were synthesized by the coprecipitation method at room temperature followed by microwave hydrothermal method (MWH). First, 5 × 10 − 3 mol of Na2WO4 ·2H2O was dissolved in distilled water (50 mL). Separately, 5 × 10 −3 mol of Zn(NO3)2·6H2O was dissolved in distilled water (50 mL) at pH 6, and the dopant of Eu(NO3)3·5H2O or Pr(NO3)3·6H2O was added to the aqueous solutions containing Zn 2+ ions. Subsequently this solution was added to the WO4 2− solution at room temperature. Finally, the white suspension formed was placed in a Teflon autoclave. The Teflon autoclave was placed inside a microwave system (2.45 GHz, power of 800 W, pressure of 245 kPa) at 140 °C for 1 h at 25 °C min − 1 . The suspension was washed with distilled water until pH 7. The ZnWO4: Re 3+ were collected and dried at 60 °C for 6 h.

Conclusions
The insertion and variation of the rare earth dopants concentration has possibilited the formation of new structures as well as the change on the morphology from nanorods (for pure ZnWO4 crystals) to nanospheres (for RE doped ZnWO4 crystals). Consequently, the effect of the decrease in crystallite size and the creation of intermediate electronic levels enabled the decrease of Egap. These results suggest that our crystals have great potential for applications such as in phosphors, fluorescent lamps, display panels as well as photocatalysis applications and others.