Visible light responding photocatalysts have been widely investigated for the degradation of various pollutants [1–3] water splitting [4,5] and photoelectrochemical (PEC) cells [6,7]. A number of these photocatalysts do perform adequately under visible light. This is due to an unfavorable balance between the enhanced visible light absorption and a negative impact on charge recombination . Recently, many studies have highlighted the way of coupling a wide band-gap semiconductor (with the proper band positions) to form a junction structure, which has been found to enhance the charge separation, leading to improvement of photocatalytic activity [9–13].
ZrO2 is an n-type semiconductor, with a wide bandgap and large negative value of the conduction band [14–16]. The photocatalytic activity of ZrO2 is usually investigated at around 250 nm, but studies were carried out for some samples in the range between 290–390 nm . It was also reported that ZrO2 showed photocatalytic activity at 320 nm, due to the absorption band emerging from an impurity level . ZrO2 was used in the oxidation of 2-propanoltoacetone , in the photocatalytic degradation of 4-chlorophenol, 4-nitrophenol, and methylene blue [20–22]. But, due to its limitation factor of the high percentage of electron-hole recombination and practical difficulties, various methods have been explored for ZrO2 material .
Tungstate nanomaterials with novel architectures and physical and chemical properties are very useful for many potential applications such as flashing materials, LED, magnetic and fluorescent materials, light-emitting materials, photocatalytic materials, scintillator, laser host, and nanoordered substrate materials, so they are considered as an important class of functional materials. As an important photocatalyst, ZnWO4 has been applied for photocatalytic hydrogen production from water and mineralization of organic pollutants under UV light irradiation.
Azo dyes, especially methyl orange, are mainly used in the textile, paper, synthetic leather, and food industry due to their high chemical stability. Methyl orange (or Sodium 4-[(4-dimethylamino) phenyldiazenyl] benzenesulfonate) is a common azo dye with features of both a dye and an acid-base indicator in aqueous solutions. Since methyl orange does not degrade easily in a conventional manner, the treatment of methyl orange in the effluent is a major challenge for the dyeing industry. In treating the printing and dyeing wastewater, methyl orange of a certain concentration is selected as a typical degradation compound.
Moreover, to reduce the electron-hole recombination and completely contribute to the redox process, that can couple with other visible light responding photocatalysts for further charge transfer. ZnWO4, a novel visible responding photocatalyst with a 3.40 eV band gap, has been extensively used to degrade organic compounds due to its highly efficient oxidation ability and changeable morphologies [24-26]. Herein, we demonstrate a convenient hydrothermal method at room temperature for the synthesis of novel ZnWO4/ZrO2 nanoparticles. The synthesized nanoparticles are carefully characterized using UV-vis DRS, FTIR, XRD, SEM, EDX, BET, and TEM techniques. Also, the photocatalytic activity of the ZnWO4/ZrO2 nanoparticles in the degradation of methyl orange (MO) is investigated.
Preparation of photocatalyst
Analytical grade Zinc acetate, Sodium Tungstate, Zirconium Oxy Chloride, Sodium Hydroxide, Ethanol were purchased from Merck (India) and used without purification. Double distilled water was used in the photocatalytic experiments.
Preparation of ZrO2
In a typical experimental procedure, 5 g of ZrOCl2.8H2O was dissolved in 90 ml distilled water. This solution was stirred for 2 h. The pH of the solution was maintained at pH 12 by adding 10% NaOH solution. The resulting suspension was refluxed and heated to 60 oC for 2 h. The solution was gradually cooled to room temperature. The resultant white precipitate was washed several times and dried at 60 oC for 1 h in a hot air oven. The above precipitate was calcinated in a muffle furnace at 600 oC for 2 h.
Preparation of ZnWO4
ZnWO4 was prepared by dissolving 5 g of Zinc acetate in 50 ml distilled water. 4.3 g of Sodium Tungstate dissolved in 50 ml distilled water was added to the above solution with constant stirring. Then the mixture was stirred for 2 h. The obtained precipitate was washed with distilled water several times and dried in a hot air oven at 120 oC for 1 h. The obtained ZnWO4 was calcinated in a muffle furnace at 500 oC for 1 h.
Preparation of ZnWO4/ZrO2 nanocomposite
2 g of ZrO2 and 0.5084 g of ZnWO4 were dissolved in 100 ml of ethanol. The whole mixture was sonicated for 30 minutes, refluxed for 2 h, and stirred for 2 h. Then the precipitate was washed with ethanol solution and dried at 60 oC for 1 h. It was calcinated in a muffle furnace at 500oC for 2 h to obtain ZnWO4 /ZrO2 nanocomposite .
Photocatalytic activity experiments
Photocatalytic experiments were carried out in an immersion-type photoreactor. 300 ml of MO with an initial concentration of 10 µm was taken in a cylindrical glass vessel which was surrounded by a circulating water jacket to cool the lamp. The air was bubbled continuously into the aliquot by an air pump in order to provide a constant source of dissolved oxygen. Before irradiation, the reaction mixture was stirred in dark for 15 minutes to achieve the adsorption-desorption equilibrium between the catalyst and dye molecules [28, 29]. 300 W Xe lamp with an ultraviolet cut-off filter was used as the visible light radiation, 5ml of aliquot was collected at regular time intervals. Then the sample was centrifuged to remove the photocatalyst and the filtrate was analyzed by UV-vis spectrophotometer at maximum absorption of 464 nm. The photodegradation of MO was calculated by the formula given below.
Where, CO is the concentration of MO before irradiation time, and C is the concentration of MO after a certain irradiation time.
Characterization of Photocatalyst
UV visible diffuse reflectance spectra were acquired using a Shimadzu UV-2450 spectrophotometer. BaSO4 was used as the reflectance standard. The surface structure was characterized by a Fourier-transform infrared (FT-IR) spectrophotometer (JASCO-FT-IR-460 plus). X-ray diffraction (XRD) patterns of the prepared photocatalytic samples were obtained using an X-ray diffractometer (XRD; XPERT PRO X-RAY) with monochromatic Cu Kα radiation. XRD scanning was performed under ambient conditions over a 2θ region of 20°−60° at a rate of 2 °/min (40 kV, 20 mA). The surface morphology was examined using scanning electron microscopy (SEM) (JSM 6701F—6701) in both secondary and backscattered electron modes and the elemental analysis was also detected. Transmission electron microscopy (TEM) mapping images were obtained using the TEM-TECNAI G2 model. pH was monitored using a EUTECH instrument pH meter.
RESULTS AND DISCUSSION
The optical properties of the nanocomposite influence their photocatalytic activity. The UV-vis-DRS of ZrO2, ZnWO4, and ZnWO4/ZrO2 were shown in Fig.1 (a). It was seen from the figure that the absorption peaks for ZnWO4/ZrO2 are highly redshifted when compared to ZrO2 and ZnWO4. The optical band gap Eg of the nanoparticles was determined by extrapolation of the linear portion of α2 versus hν plots using the following equation .
Where α is the absorption coefficient, hν is the photon energy, Eg is the optical band gap energy and c is the constant depending on the electron-hole mobility. Tauc plots of ZrO2, ZnWO4, and ZnWO4/ZrO2 were shown in Fig. 1(b), 1(c), and 1(d). The band gaps of ZrO2, ZnWO4, and ZnWO4/ZrO2 were found to be 3.6 eV, 3.2 eV, and 2.5 eV respectively. The bandgap of ZnWO4/ZrO2 is sharper and highly suitable for receiving visible light.
Fig. 2 (a), (b), and (c) shows the FT-IR spectra of ZrO2, ZnWO4, and ZnWO4/ZrO2 respectively. The absorption peaks at 761 and 890 cm-1 are originated from the bending and stretching vibration of Zn-O-W. The two peaks at 3473 and 1635 cm-1 implying that the basic hydroxyl group in nanocomposite  and are also attributed to stretching vibrations of the OH groups in W-OH. The Zr-O band was observed at 1064 cm-1. The band at 670 cm-1 was due to a Zr-O bending vibration. Peaks that occurred less than 610 cm-1 belong to Zr-O band vibrations. The surface OH group density increases with tungsten incorporation. The larger surface hydroxyl group density will lead to the enhancement of the photocatalytic activity because they can interact with photogenerated holes, which gives better charge transfer and inhibits the recombination of electron-hole pairs [32, 33].
The phase composition of ZrO2, ZnWO4, and ZnWO4/ZrO2 were shown in Fig.3. The observed peaks of the nanocomposite were in good consistency with the characteristic peaks of ZrO2 (based on a JCPDS card no 730958) with Rhombohedral phase and Rhombo-centered lattice) and ZnWO4 (based on a JCPDS card no 880251) with Monoclinic phase and Primitive lattice). The average crystallize size was calculated using the Debye-Scherrer equation 
Where β is the full width at half height maximum of the most intense 2θ peak, K is the shape factor (0.89). θ, λ are the incident angle and wavelength of X – rays respectively. The average particle size obtained for ZrO2, ZnWO4, and ZnWO4/ZrO2 nanoparticles were found to be 29.20 nm, 23.78 nm, and 20.14 nm respectively.
SEM, EDX, and TEM
The surface morphology of ZrO2, ZnWO4, and ZnWO4/ZrO2 was observed by SEM and shown in Fig. 4 (a), 4 (b), 4 (c) respectively. Plate-like morphology was observed for ZrO2 and spongy sphere for ZnWO4 . On doping with ZnWO4, ZrO2 shows agglomeration. The compositions of ZrO2, ZnWO4, and ZnWO4/ZrO2 nanocomposite were determined by EDX shown in Fig.5.(a),(b) and (c) respectively. The EDX result shows that the sample containing Zr, Zn, W, and O were observed at their energy levels without any impurities The EDX elemental analysis of ZrO2, ZnWO4, and ZnWO4/ZrO2 is given in Table. 1. TEM image of nanocomposite was shown in Fig.6. It was observed to be a plate with sponge spheres and the average crystallization was found to be 28-39 nm which is in good agreement with the XRD results.
Photocatalytic degradation of MO
Fig.7. shows the photocatalytic activity of ZrO2, ZnWO4, and ZnWO4/ZrO2 for the degradation of MO (Reaction conditions: MO 1.5 µM and catalyst concentration 0.125 g/L). The results revealed that the degradation of MO has only achieved 30% in the presence of ZnWO4 and 23% of MO is degraded by ZrO2. However, the highest photocatalytic activity is shown by ZnWO4/ZrO2 (95%). The photodegradation of MO was also carried out without light (only catalyst) for up to 180 min. The adsorption percentage of the MO at 180 min of contact time is negligible (16%) without nanocomposites.
The highest photocatalytic activity of ZnWO4/ZrO2 can be explained as follows: when the photocatalyst is irradiated with visible light, electrons and holes are formed in the conduction band and valence band of ZnWO4/ZrO2 (narrow bandgap semiconductor). Therefore photogenerated electrons of ZnWO4 will be easily transferred to the conduction band of ZrO2 (wide bandgap semiconductor). Therefore, efficient electron-hole separation is achieved on the photocatalyst surface. The electron reacts with surface adsorbed O2 to produce O2.- and holes react with H2O to create .OH. This results in more charge carriers forming reactive species, which promote the degradation of MO .
Effect of Catalyst dosage
In order to optimize the photocatalyst dosage on the degradation of MO, experiments were carried out with varying ZnWO4/ZrO2 dosage from 0.050 g/L to 0.150 g/L, at a constant MO concentration of 1.5 µM and the results are shown in Fig. 8. The photodegradation of MO is negligible (15.75%) without the addition of any photocatalyst during 180 min of irradiation. The photodegradation of MO increases with the increase of ZnWO4/ZrO2 dosage from 0.050 g/L to 0.125 g/L and then decreases. This is due to the increase of the total active surface area and the availability of the more active sites on the catalyst surface for photoreaction. However, the high catalyst dosage increases the turbidity of the suspension leading to a shielding effect on the penetration of light . Therefore, the absorption of light by the photocatalyst is limited, and fewer catalyst sites can be activated. The surface area was also reduced due to the agglomeration of the nanoparticles at high photocatalyst dosage.
Effect of the initial concentration of MO
The influence of initial MO concentration on its photodegradation was examined in the presence of ZnWO4/ZrO2 (0.125 g/L) and the results are displayed in Fig.9. The photodegradation of MO decreases with an increase in MO concentration from 1.5 µM to 2.5 µM. At high MO concentrations, more MO molecules are adsorbed on the surface of ZnWO4/ZrO2 and fewer photons are available to reach the catalyst surface and therefore the less number of. OH is formed, thus causing inhibition in degradation percentage. Also, an increase in MO concentration can lead to a decrease in the path length of the photon entering the aqueous solution of MO and reduce the catalytic efficiency .
Effect of pH
The degradation of MO was studied at different pH ranges from 2-6 with an initial MO concentration of 1.5 µM, catalyst concentration (ZnWO4/ZrO2) of 0.125 g/L, and irradiation time of 180 min. The degradation of MO as a function of pH is shown in Fig.10. The photodegradation of MO increases with an increase in pH from 2 to 4 and a further increase in pH leads to a decrease in photodegradation. The photocatalytic activity of ZnWO4/ZrO2 was found to be 95% at pH 4 [39, 40].
The kinetics of photodegradation of MO
The kinetics of photodegradation of MO was studied by conducting the experiments under optimum operation conditions (MO concentration 1.5 µM, ZnWO4/ZrO2 concentration 0.125 g/L, and irradiation time 90 min). In all experiments, the degradation followed first-order kinetics plots of –ln[C/C0] versus time showed a linear relationship, where C is the concentration of MO remaining in the solution at irradiation time of t, and C0 is the initial concentration at t = 15 min). First-order rate constants were evaluated from the slopes of the –ln [C/C0] versus time plots in Fig.11. . The observed rate constant for the photodegradation of MO in the presence of ZnWO4/ZrO2 is 1.45x10-2 s-1, which is significantly higher than that observed for ZrO2 (5.8x10-3 s-1), ZnWO4 (1.08x10-2 s-1) and without catalyst (1.8x10-3 s-1). Moreover, Fig. 12 shows the mechanism of ZnWO4/ZrO2 on MO.
From Fig. 13 the study of reusability of the catalyst composite. To evaluate changes in the efficiency and kinetic constants of reused photocatalyst, 0.125 g/L of the catalyst was tested for three cycles. The dye decolorization was mildly decreasing from 92 % (1st cycle) to 90 % in the second cycle and 85 % in the third cycle. Generally, the catalyst remained sufficiently efficient after repeated use. The loss of activity of the photocatalyst might be due to the adsorption of insensitive species on the photocatalyst surface and the supporting matrix. Moreover, the comparison of methyl orange degradation efficiency for different catalysts is shown in Table 2.
In this present work, ZnWO4/ZrO2 has been successfully synthesized and effectively utilized for visible-light-driven photocatalyst for the degradation of methyl orange. The prepared nanoparticles have been characterized using UV – vis DRS, FTIR, XRD, SEM, TEM, and EDX techniques. It was observed to be a plate with sponge spheres for ZnWO4/ZrO2 nanocomposite and the average crystallization was found to be 28-39 nm which is in good agreement with the XRD results. The photocatalyst was successfully applied for the degradation of MO under visible light and the reaction parameters were optimized. ZnWO4/ZrO2 (95%) shows higher photocatalytic activity than ZrO2 (60 %) and ZnWO4 (72%).
CONFLICTS OF INTEREST
There are no conflicts to declare.