Synthesis and Photocatalytic Properties of ZnWO4 Nanocrystals via a Fast Microwave-Assisted Method

High crystallinity of ZnWO4 nanoparticles has been successfully synthesized via a highly effective and environmentally friendly microwave route by controlling the reaction time and temperature. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Fourier infrared spectrum (FT-IR). The crystallinity was enhanced with the increase of the reaction temperature and time. The photocatalytic activities of ZnWO4 nanocrystals were evaluated by testing the photodegradation of rhodamine B (RhB) dye under ultraviolet (UV) light irradiation. The results indicated that as-prepared ZnWO4 was highly effective for the degradation of RhB. The degradation rate of RhB reached 98.01% after 6 h of UV illumination.

However, there are some shortcomings for the traditional synthetic methods: (1) the reaction temperature is too high; (2) the reaction cycle is too long; (3) the procedure is complex [23]. Obviously, the long reaction cycle will waste time. It is worse that the long reaction time may lead to the dendritic growth and increase the particle size. Chen and coworkers have synthesized ZnWO 4 powder by hydrothermal method and demonstrating that particle aggregation is stimulated by temperature and time increase [2]. Wu et al. have successfully produced ZnWO 4 photocatalyst via the sol-gel process in a temperature range of 450-800 ∘ C, but the grain size is difficult to be controlled [25]. Thus, the application of ZnWO 4 photocatalyst has been limited, and an energy saving and environmently friendly synthesis method of nanoscale tungstate has received extensive attention.
To date, as far as we know, the synthesis of ZnWO 4 photocatalyst by microwave-assisted method is rare. Wu et al. have reported a microwave solvothermal route to synthesize ZnWO 4 nanoparticles at 160 ∘ C. However, the route needed longer time (1-3 h) and organic solvent (ethylene glycol) [29]. Therefore, the development of fast and environmently friendly microwave-assisted methods for the synthesis of ZnWO 4 nanocrystals is of great importance for broadening and improving their industrial applications. In this paper, we first successfully obtained high crystallinity ZnWO 4 photocatalyst with a microwave-assisted process by several minutes and using water as solvent. Compared with the conventional methods, microwave irradiation is preferable due to its unique effects, such as volumetric heating, higher reaction rates, shorter reaction time, selectivity energy saving, and being environmentally friendly. During the degradation of rhodamine B (RhB) under UV light irradiation, the ZnWO 4  exhibited high photocatalytic activities. The measurements suggested that photocatalytic property of the sample is related to the crystallinity, surface area, and dimension of particles. Moreover, the catalyst is relatively stable and can be reused.

Experimental Section
2.1. Synthesis. All syntheses were performed using a commercial multimode microwave synthesizer coupled with an automation system (Initiator 8 Exp). The reactions were conducted in 20 mL vessels. In a typical synthesis of the ZnWO 4 powder, all the chemicals were of analytical grade and used without further purification. Sodium tungstate (Na 2 WO 4 ⋅2H 2 O) and zinc chloride (ZnCl 2 ) were the staring materials. Na 2 WO 4 ⋅2H 2 O (0.14 g), ZnCl 2 (0.13 g), H 2 O (10.0 mL), and poly(ethylene glycol) (0.12 g) were mixed. When the vessel was protected by a sealing cap, it was then transferred into the microwave cavity, and the temperature was increased rapidly to 180 ∘ C at a rate of 50 ∘ C/min. The reaction was performed at 180 ∘ C for several minutes, and the reaction vessel was quickly cooled to room temperature by a forced-N 2 flow. The as-synthesized ZnWO 4 nanocrystals were collected by centrifugation, washed with distilled water and absolute ethanol several times to remove impurities, and finally dried in vacuum at 80 ∘ C for 4 h.

Characterization.
The morphology and microstructure of the sample were observed with a JEM-2010 transmission electron microscope operated at 120 kV. Further structural characterization was performed on FEI Tecnai F20 highresolution field-emission transmission electron microscope (HRTEM) operated at 200 kV. XRD patterns of ZnWO 4 photocatalysts were recorded by a MiniFlex II X-ray diffractometer operated at 40 kV and 40 mA using Cu K radiation ( = 0.15406 nm). UV-Vis DRS was performed on a Hitachi U-3010, and BaSO 4 was used as a reference. Fourier transform infrared spectra (FT-IR) were recorded on a Perkin-Elmer 1600 FT-IR spectrometer with a KBr disk. BET surface area was determined by nitrogen adsorption isotherm measurements at 77 K on a Micrometrics ASAP 2010.

Measurements of Photocatalytic
Activity. The photocatalytic activity of ZnWO 4 catalysts was evaluated by degradation of RhB under a 300 W UV irradiation. ZnWO 4 photocatalyst (25 mg) was dispersed in RhB solution (10 mg/L). Before UV irradiation, an adsorption-desorption equilibrium was established by ultrasonic and mechanical stirring for 30 min. After that, the solution was exposed to UV light irradiation under magnetic stirring. A little amount of reaction solution for UV-Vis spectroscopy analysis was taken from the photoreactor at appropriate time interval.

Results and Discussion
The phase composition of the samples prepared at different temperatures was analyzed by X-ray powder diffraction (XRD). As shown in Figure  peaks is observed at 180 ∘ C, and all peaks can be indexed in reference to pure ZnWO 4 , space group P2/c, and unit cell parameters consistent with those reported in JCPDS 15-0774. Therefore, it is reasonable to deduce that the increase of the microwave temperature improves the crystalline of ZnWO 4 powder. Thus, the temperature increase stimulates effectively the crystallization of ZnWO 4 nanoparticles. The growth process ZnWO 4 nanocrystals was monitored by investigating the products obtained at different stages of the reaction using XRD techniques. Figure 2 shows the XRD patterns of ZnWO 4 samples irradiated at 180 ∘ C at different reaction stages. XRD patterns could be easily identified as a pure monoclinic wolframite structure ZnWO 4 based on a JCPDS Card (no. 15-0774). The results indicated that the intensity of the diffraction peaks strengthens with the increase of reaction time, especially in the (010), (100), (011), (110), (111), and (021) crystal planes of ZnWO 4 . Small intensity (010) peak can be found in XRD patterns with 5 min reaction time. When the reaction time extended to 15 min, the (010) peak appeared. These results show that reaction time has an influence on the phase formation of ZnWO 4 . It also suggests that microwave method can achieve crystallization of samples in higher kinetics, which economized the energy and shortened the time.
The FT-IR spectrum of ZnWO 4 powders is shown in Figure 3. of irradiation, the degradation rate of RhB can be nearly 98.01%. The kinetics curves of photodegradation of RhB are shown in Figure 5(d) where the curves 1, 2, and 3 are related to samples prepared at 140, 160, and 180 ∘ C, respectively. The linear relationship between and time demonstrates that the photocatalytic degradation of RhB follows a pseudofirst-order kinetics: where 0 / is normalized RhB concentration, is the reaction time, and is the reaction rate constant (min −1 ). The figure also shows that the degradation rate of samples prepared at 140 ∘ C is the best (curve 1), the reaction rate constant of which is calculated to be 0.6798 min −1 . The degradation rate of samples prepared at 160 ∘ C is center (curve 2), the reaction rate constant of which is calculated to be 0.6760 min −1 . The degradation rate of samples prepared at 180 ∘ C is the lowest (curve 3), the reaction rate constant of which is calculated to be 0.4133 min −1 . The sample prepared at as high temperature as 180 ∘ C exhibits the lower ability to transform RhB compared with those prepared at 140-160 ∘ C. This indicates that the crystallinity structure of photocatalyst is the vital factor that affects the photocatalytic activity of materials.
To obtain the information about the specific surface area of the as-prepared ZnWO 4 samples, BET N 2 adsorption analysis was performed. As shown in Figure 6, the nitrogen adsorption isotherm belongs to type II. The analysis showed that the specific surface areas of the samples prepared at 160 ∘ C and 140 ∘ C were 25.05 m 2 /g and 28.10 m 2 /g, respectively. The experimental results revealed that the specific surface area of samples prepared at 140 ∘ C is higher than the sample prepared at 160 ∘ C. Therefore, these demonstrations indicate that photocatalytic properties of ZnWO 4 can be significantly improved by specific surface areas [33]. Figure 6 inset shows the photocatalytic effect of ZnWO 4 prepared at 140 ∘ C for 5 min. We can see that the color of RhB gradually changes from violet red to shallow. This result coincided with the UV-Vis absorption spectra ( Figure 5(a)). To evaluate further the photocatalytic activity, the final RhB solution was tested for the carbonation rate. The carbon content of nondegradated RhB solution was 6.445 m 2 /g, and the presence of carbon was not detected in the final RhB solution, which suggested that the dye almost completely was mineralized into CO 2 and H 2 O. In this work, we also studied the photocatalytic degradation of ZnWO 4 prepared at 180 ∘ C for different reaction times. The UV-visible absorption spectrum and dynamics curve of the aqueous solution of RhB with ZnWO 4 under exposure to the ultraviolet light lamp for various durations are shown in Figure 7. The results show that the sample prepared at 180 ∘ C for 5 min has the highest photocatalytic activity, and after 6 hours of irradiation, the conversion of RhB can be nearly 92.12%. Figure 7 rate of samples with reaction time of 15 min is the lowest (curve 2), the reaction rate constant of which is calculated to be 0.1261 min −1 . Curve 4 shows almost no degradation of RhB after 6 hours of irradiation. When the reaction time is 5 min, samples have poor crystalline, small particles, and good dispersion, which lead to the higher photocatalytic activity. When the reaction time extended to 15 min, the sample has the best crystalline, large particle size, and large aspect ratio, but its photocatalytic activity is poorer than that of the sample prepared by 5 min [34]. This is consistent with previous conclusions. From the above analysis, we know that ZnWO 4 nanopowders prepared at 180 ∘ C for 5 min have better photocatalytic activity, and its photocatalytic effect is shown in Scheme 1. Therefore, the crystallinity and particle size play important roles in photocatalytic activity of as-prepared ZnWO 4 .  observed that the ZnWO 4 materials prepared at 140 ∘ C for 5 min have the maximal degradation rate. With higher reaction temperature, a decrease in photocatalytic activity is observed. These results can be explained by the poor crystallinity at low temperature, which leads to good catalytic activity. Also, with the extension of reaction time, the photocatalytic activity lets down which can be attributed to the large particle size.
On the basis of the above experimental observation, photocatalytic mechanism for ZnWO 4 photocatalytic degradation RhB is speculated (Scheme 2). ZnWO 4 energy band structure was composed by discrete, full-of-low-energy electron valence band (VB) and empty high-energy conduction band (CB); the valence band and conduction band are separated by the forbidden band. When the energy of absorption light is greater than the band gap, the valence electrons are stimulated to the conduction band, making the conduction band with charge, which has a reduction and the valence band resulting hole (h + ) with oxidation. Simultaneously, the poor crystallinity leads to abundant lattice defects acting as holes [35]. These free electrons and holes migrate to the catalyst surface and react with adsorbed water, dissolved oxygen, which generate the high activity of hydroxyl radical ( • OH) and superoxide anion (O 2 − ), further react with organics in the dyes, and ultimately degraded to CO 2 and H 2 O [36].
In addition, we repeated degradation of dye RhB three times with samples prepared at 180 ∘ C for 5 min to further study the stability of photocatalyst ZnWO 4 ; the degradation rate remained at 90%. The experimental results showed that the catalyst was relatively stable and can be reused.

Conclusions
In this paper, the ZnWO 4 nanoparticles were successfully synthesized by the microwave process. It is possible to control effectively the particle size and crystallization of ZnWO 4 by adjusting the reaction temperature and reaction time.
Comparing the photocatalytic property of ZnWO 4 prepared at a different reaction time and temperature, we conclude that photocatalytic property of the sample is related to the crystallinity, surface area, and dimension of particles. The photocatalytic activity of samples prepared at reaction temperature of 140 ∘ C and reaction time of 5 min is the best. Furthermore, the ZnWO 4 powder products showed stable photocatalytic activity for the degradation of RhB under UV light irradiation.