Microwave assisted synthesis of zinc vanadate nanoparticles and photocatalytic application

In recent years, the nanotechnology has gained much attention since the nanoparticles (NPs) have applications in every field of life. The hetero-structured are of special interest due to their higher photocatalytic activity. In present investigation, Zinc vanadate NPs were synthesized from vanadium and zinc salts by microwave assisted precipitation method. The Zinc vanadate NPs were characterized by x-ray Powder Diffraction (XRD), Energy-dispersive x-ray spectroscopy (EDX), Scanning electron microscope (SEM) and UV-visible techniques. The photocatalytic activity (PCA) was evaluated by degrading the methylene blue (MB) dye and process variables were optimized. The Zinc vanadate size was <100 nm and particles were in aggregates form. The MB dye degradation was performed at different conditions of process variables and it was observed that MB dye was significantly degraded using Zinc vanadate NPs under UV light irradiation. The reaction time, catalyst dose and dye initial concentration showed variable effect on dye degradation. Based on results, it can be concluded that the microwave irradiation is viable for the synthesis of Zinc vanadate NPs for photocatalytic activity. In view of promising efficiency of Zinc vanadate NPs, it can be used for the dye degradation and for the remediation of textile effluents.


Introduction
Metal oxide play pivotal role in material science and can adopt various structural geometries with electronic distribution that exhibit metallic or semiconductor character. The semiconductor pure and doped metal oxide properties depend on their morphology, structure and grain size [1,2]. Metal oxide nanoparticles have been used in electroanalysis and detection of biomolecules. The nanoparticles are useful with respect to structural changes that allow the changes in symmetry and cell parameters. On the other hand, the changes in electrochemical characteristics and in surface properties increase the band gap that effects the chemical activity of the nanoparticles [3,4]. So far, these materials have the ability to improve the properties and have enormous applications in many fields such asmicroelectronics, batteries, sensing device, nano probes and nano medicine. Nanostructure of vanadium oxide (VO) have been studied [5][6][7][8] and a variety of nanostructures in the form of 1D to 3D of V 2 O 5 have been preferred as a model system for the depiction of nanostructured materials. One of the major applications of VO is lithium ion batteries. These NPs are usually prepared via hydrothermal heating of aqueous solution [9,10]. Nano wires, nano belts or nano-urchin shapes are synthesized by chemical parameters which depends on the arrangement of a VO network that act as a precursor in synthesized nanotubes and nanorods [11][12][13][14].
The VO act as the catalyst with band gap around 2.6 eV. These active band have an arrow gap makes in a wide region of UV light frequencies approaches are frequently implemented. Different parameters of nanomaterials application have been used for food safety and food packing such as silver NPs as potent antimicrobial agent, polymer/clay nano composites as high barrier packaging materials, nanomaterial, nano-sensors based assays for the detection of food-relevant analytes. With various compositions NPs have been preparedby physical and chemical methods. The physical methods may be evaporation, sputtering, laser ablation, ion ejection, and electron-beam lithography [15,16,21].
The chemical methods consist of a salt reduction method, micelles, electrochemical sol-gel method, gasliquid interface, thermolysis and decomposition on ultrasonic treatment. They improved the energy efficiency of photo generated supported various composition for appropriate co-catalysts. The atavistic has been improved at which the photocatalytic reaction occurs in which electrons moves to the holes from bulk to reaction active sites [22][23][24][25][26][27].
However, in view of the drawbacks faced by low efficiency, instability, semiconductor pure oxides in photocatalytic process, researchers are now focusing on the synthesis of nano hetero-structured photo catalysts [28][29][30][31]. Moreover, owing to their robust nature and versatility in morphology, the hetero-structured NPs show better performance as photo-catalysts and are gaining attention of scientific community [32,33]. Zinc vanadate and their derivatives have also been investigated for extensive chemical sensing, lubrication cathode materials catalysis and plating [18,20,34].
More recently, Zn 3 (VO 4 ) 2 nano rods with visible light-driven capacity were prepared by hydrothermal method. The degradation of MB and 4-nitrophenol over the Zn 3 (VO 4 ) 2 nano rods were reported, which was synthesized by hydrothermal route. As-synthesized 3D Zinc vanadate nanoflower was characterized by XRD, SEM, AFM, EDX, Particle size and UV-vis techniques. Moreover, PCA was evaluated for the degradation of cetirizine hydrochloride (C-HCl) using RSM under CCD [35,36].
The reports concerning the crystal growth and dimension control of Zinc vanadate without any template under microwave irradiation are rare. Therefore, the present study was conducted for the synthesis of Zinc vanadate under microwave irradiation. The synthesized Zinc vanadate was characterized by SEM, XRD, EDX, UV-vis techniques. Moreover, the PCA was evaluated for the degradation of methyl blue dye (figure 1).

Materials and methods
All the chemicals and reagents employed in this study were of analytical grade and were purchased from Sigma-Aldrich. Ultra-pure water (resistivity18.2 Mohm.com) was obtained from Milli-Q plant. Standard stock solution of each required reagents was prepared by dilution method. Desired concentrations were obtained from stock solution [37].

Synthesis procedure
For the synthesis, ammonium vanadate and ZnCl 2 solutions was prepared and mixed with each other at 85°C under continuous stirring. Then, solution was divided into two parts. One was heated on magnetic hot plate, other under microwave treatment and ammonium hydroxide was added for pH adjustment of the solution. The pH was adjusted 8.8 and then content in the beaker was left over night at 85°C to allow the precipitates to settle down. Solvent was decanted and precipitates was washed with ultra-pure water followed by ethanol washing (thrice) to ensure the removal of unreacted ions and then, calcined at 500°C for 5 h [38,39].

Characterization
In order to characterize the Zinc vanadate catalyst different characterization techniques were used. For shape and morphology (structure) of catalyst, scanning electron microscopy was performed using SEM (JSM-5910, JEOL). For elemental analysis, Energy Dispersive x-ray Spectroscopy (EDX) was used. For crystal size and phase, XRD (JDX-3532) was used [40].

Photocatalytic activity
The PCA was evaluated under UV light irradiation using low pressure mercury lamps. For the photolysis and photocatalysis of aqueous solution of MB dye, a photochemical apparatus fitted with a 4W low pressure Hg-UV lamp (PENARY USA) was used. The wavelength of the emitted light by Hg-UV lamp was 254 nm. A magnetic stirrer was used for stirring the sample to achieve homogenous flow of UV radiation throughout the solution [41].
The MB dye solutions of different concentrations were prepared in ultra-pure water and catalyst was added at specific ratios. In order to dissolve the catalyst, sonication was carried out for 10 min. To achieve the adsorption-desorption equilibrium, dark stirring was carried out for 30 min. Sample of dark stirring were collected and analyzed to get the spectra. The UV light irradiated samples were collected at different times and filtered through micro syringe filter. Then the remaining concentration of MB dye was determined by spectrophotometric analysis. All the experiments were performed in a glass beaker of 250 ml. Specific quantity of catalyst powder was mixed with 50 mL of MB dye solution. The pH of the solution was adjusted using acid base solution. The solution in the beaker was sonicated for 10 min and then continuously for 30 min in dark for the establishment of adsorption-desorption equilibrium. To quantify the degradation, 3 mL of sample was collected at specific time from the reactor, filtered through 0.22 mm micro syringe filters and then, analyzed through UV spectrophotometer (CE Cecil 7200 UK). The percent of MB dye degradation was measured using relation shown in equation (1).
Where, C 0 and C i are the initial concentration of MB dye and the concentration at different time intervals, respectively. All the samples were irradiated in triplicate and data was averaged and reported as mean ±standard deviation.

Results and discussion
In recent years, the nanotechnology has gained much attention since the nanoparticles (NPs) have huge applications in every field of life. The hetero-structured NPs are of special interest due their higher photocatalytic activity [42,43]. In present investigation, Zinc vanadate NPs were synthesized from vanadium and zinc salts by microwave assisted precipitation method. The Zinc vanadate NPs were characterized by XRD, EDX, SEM, UV-visible techniques. The particles were round in shape and size of synthesized material was 100 nm. The XRD pattern (0-80°) of synthesized zinc vanadate is shown in figure 2. XRD analysis exhibited the formation of pure form of Zinc vanadate and well indexed with JCPDS card . This study seizures the direct precipitation approach for the synthesis of 3-D spherical Zinc vanadate flower. The synthesized Zinc vanadate was in pure phase hexagonal form. Peaks obtained at sharp edges reflect the higher crystallinity of the compound without any impurity peak.
The morphology was determined with SEM and results are shown figure 3 and 4. The Zinc vanadate was first formed in small particle (nano range), then these were combined to form aggregates. The microwave treated effect was also significant since the morphology was changed as the samples were irradiated with microwave. The composition of Zinc vanadate was confirmed by EDX analysis and results are shown in figure 5. The EDX spectrum shows peaks related to Zinc vanadate and no additional peak was observed, which indicate the adopted method was used sucessfuly for the synthesis of Zinc vanadate. The EDX revealed the presence of V(21.22%), O(19.85%), Zn(58.93%) and the atomic percentages were 16.28%, 48.49% and 35.23%. These results are in line with reported study for Zinc vanadate , for example, Timmaji [44] synthesized BiVO 4 by solution combustion synthesis and electrodeposition. The first deposition of Bi film on a Pt substrate in an acidic BiCl 3 medium. This film anodically exposed those medium that contain hydrolyzed vanadium precursor to produce Bi 3+ and subsequent BiVO 4 formation by in situ precipitation. Glycine, citric acid, urea was used at the same time as the fuel. The probe of Methyl orange was used to test the combustion synthesized (CS) samples and photocatalytic attributes.The nanoparticles (AgBiW 2 O 8 ) were prepared for the first time by solution combustion analysis via bismuth nitrate sodium tungstate, silver nitrate as precursors for Bi, Ag and W correspondingly and urea used as the fuel. Similarly, Ibrahim, Khan [45] synthesized nanoflakes WO 3 /BiVO 4 hterojuncation prepared by facile hydrothermal method.   figure 6. The first order linear relationship was revealed by the plots of the (C/C 0 ) vs. irradiation time (t), where C was the concentration of MB dye at the irradiation time (t) and C 0 was the concentration before irradiation. When time interval is increased from 0 to 80 min the decolorization efficiency of dye also increases from 0 to 66% and then complete degradation occurs at 160 min. The interaction of catalyst with UV light increases which is indicative of the fact that active sites of the catalyst increased which further enhance the production of highly reactive species. This confirms that activity of catalyst increases with time. The data revealed that the degradation efficiency of the dye increased with extending irradiation time. Absorbance of the solution at 554 nm with time was determined to monitor the MB dye concentration. The decrease in the concentration of MB dye with time was due to its degradation by catalyst. It can be seen that in about 3 h, almost all the MB dye was degraded. So, time of 160 min was selected for further study. The same MB dye was degraded by Mahadik, Shinde [46] who studied the Fe 2 O 3 as a photocatalyst, but they obtained low degradation efficiency for longer reaction time. So the prepared catalyst was very active by degrading the MB dye up to 83% in 160 min, [47] prepared (m-BiVO 4 ) photocatalytic and antimicrobial activity was studied. Photocatalytic activity of m-BiVO 4 nanostructure was evaluated by degrading the MB dye, which is a waste of textiles industries.

Effect of catalyst dose
In order to evaluate the photocatalytic degradation of the MB dye under UV light, blank experiment was performed. The first order linear relationship was revealed by the plots of the (C/C 0 ) vs. catalyst dose (figure 7). The rate of degradation process is negligible when experiment was carried out in the absence of catalyst. It suggests that the catalyst must be used in combination with UV light for the degradation of dye. The degradation of the MB dye increased when ZnV 2 O 8 catalyst dose was increased. This indicates that the degradation of MB dye increases in the presence of photocatalyst. The increase in degradation rate may be attributed to the production of highly reactive oxidizing species and high generation of free radicals by catalyst in the presence of UV light which degraded the dye to a large extent [48][49][50][51][52][53]. Under these experimental conditions, and after 10 min of UV irradiation, the degradation reaches 83% of MB dye. The concentration of MB dye in the solution was also mentioned spectrophotometrically by measuring the absorbance of the solution at 665 nm. After ascertaining the activity shown by the as synthesized ZnV 2 O 8 the effective parameters were selected for further study. Also, [16] evaluated the photodegradation efficiency of BiVO4 and graphene oxide (RGO) composite. The photocatalysts RGO-BiVO4 show high degradation efficiency versus pure BiVO4 photocatalyst under visible light. Specially, composite photocatalyst of the 2 wt. % RGO-BiVO 4 exhibits the highest CIP degradation ratio (68.2%) in 60 min, which is over 3 times than that (22.7%) of the pure BiVO 4 particles. However, the catalysts which act as active site and play a role as proton donors in the photodegradation process of the organic compounds due to higher surface area.

Effect of dye concentration
Solutions of different dye concentrations (10, 20, 30, 40, 50 mg l −1 ) were prepared. The absorbance spectra of the solutions of different concentrations were carried out and their respective corresponding maximum absorbance were noted ( Figure. 7). All the absorbance data were recorded at 665 nm which is the maximum corresponding absorbance wavelength of MB dye. A graph of absorbance versus different concentrations was plotted. Graph shows that with the increase in dye concentration, the degradation was decreased. Also, [54] prepared BiOCl/BiVO 4 photocatalyst with heterojunction structure using a hydrothermal method. Under visible light irradiations, the heterojunction have been suggested for the degradation of methyl orange (MO).The photocatalytic efficiency for MO photodegradation was increased versus BiOCl, BiVO 4 and Degussa P 25 .This activity has been obtained usingheterojunction composite of 13 mol % BiOCl and 87 mol % BiVO 4 .The removal of MO has been mainly initiated by consuming the conduction band electrons valence-band holes that play important role in photocatalytic activity. Wang, Shao [55] also synthesized novel BiOCl-C 3 N 4 heterojunctions through-liquid-assisted solvent-thermal method. UV-visible light diffusion reflectance spectrometry was performed. These results show that they are dispersed to form heterojunction structures to absorb visible light. The methyl orange (MO) degradation indicated heterojunction proportion that increased the visible light absorption for photocatalytic activity.

Conclusions
In present investigation, Zinc vanadate NPs were synthesized from vanadium and zinc salts by microwave assisted precipitation method. The Zinc vanadate NPs were characterized by XRD, EDX, SEM, UV-vis. techniques. The Zinc vanadate size was <100 nm and particles were in aggregates form. The MB dye degradation was performed at different conditions of process variables and it was observed that MB dye was significantly degraded using Zinc vanadate NPs under UV light irradiation. The reaction time, catalyst dose and dye initial concentration showed variable effect on dye degradation. Based on results, it can be concluded that the microwave irradiation is viable for the synthesis of Zinc vanadate NPs for photocatalytic application. In view of promising efficiency of Zinc vanadate NPs, it can be used for the degradation of dye and for the remediation of other industrial pollutants present in wastewater.