Visible Light Photocatalytic Degradation of Methylene Blue and Malachite Green Dyes with CuWO4-GO Nano Composite

Copper Tungstate-Graphene Oxide nano composites have been successfully applied as excellent catalysts for the photocatalytic degradation with Methylene blue and Malachite green dyes under visible light irradiation. A facile solid state metathesis synthesis of copper tungstate (CuWO4) followed by ball milling and subsequent preparation of copper tungstate-graphene oxide (CuWO4-GO) nano composite using a colloidal blending process and its application as a visible light photocatalyst for the degradation of Malachite green and Methylene blue dyes. The morphology and composition of copper tungstate (CuWO4) nano composite have been characterized using X-Ray Diffraction (XRD), UV-Visible Diffuse Reflectance Spectra (UV-DRS), Raman Spectra, Field Emission Scanning Electron Microscopy (FESEM)-EDS and UV Visible Spectroscopy. It shows a band gap value of 2.13 eV, an increase in range and intensity of light absorption and the reduction of electron-hole pair recombination in CuWO4 with the introducing of GO on to it.


Introduction
Many efforts have been made in the past, and in recent years semiconductor Modern Research in Catalysis based photocatalysts with high photocatalytic activity for environmental protection have been developed. Procedures such as water disinfection, air purification, polluted waste water remediation, etc. are taken. It has been a promising technique owing to its strong oxidizing nature, chemical inertness, economic viability and non-toxicity [1] [2] [3]. The unique arrangement of electronic structure, light absorption properties and charge transport characteristics in most of the metal oxides and tungstates have proven them to be superior photocatalysts.
Transition metal tungstates are considerable inorganic materials that have a significant application in various fields. Some of the divalent transition metal tungstates have also gained commercial interest in fluorescent lamps and lasers lights, due to its excellent electrical conductivity. In addition to this, these are also used as humidity sensors and catalysts. Copper tungstate is a well-known semiconductor with potential technological applications in scintillates, detectors, photon-odes, laser hosts, optical fibers etc. [4] [5].
Solid-state metathetic approach has been successfully applied for the synthesis of many oxide materials. For example, Gopalakrishnan [11]. Coppertungstate (CuWO 4 ) crystals exhibit only wolframite-type monoclinic structure at high pressure [12]. Many metal tungstates like CuWO 4 are also used for water splitting and photocatalysis [13] [14]. Generally, the oxidized graphene sheets, namely, GO, acquire multiple defects and the degree of the defects is subject to the additive amount of oxidant and the oxidizing time [15]. Graphene oxide consists of water-dispersible, soft carbon sheets that can be easily converted to a conductive form and this 2D material continues to inspire many interesting applications and discoveries in a wide variety of fields including liquid-crystal display technology, bioscience, and materials science [16]- [21].
Graphene oxide (GO) is a two-dimensional material derived from graphite by introducing covalent C-O bonds [22]. A large number of oxygen-containing functional groups have been implanted on both sides of a single graphite sheet overcomes the inter sheet Vander Waals force and enlarges the interlayer spacing. The sheets in such an expanded structure are easily pulled up using an external force by ultrasonication. Now the copper tungstate nano particles are directly grown on graphene oxide which appears to exhibit strong interactions with the underlying graphene oxide sheets. Since ultrasonication would not lead to any dissociation of the sheets ,due to its strong coupling leading to an ad-  The reasons for these improvements are attributed to the fact that copper tungstate has a direct band gap positioned near the optical value of the solar spectrum and high energy of CB (Conduction Band), giving the photo electrons to a strong reducing ability and graphene oxide possesses a remarkable electrical transport property [23]. By this, the CuWO 4 -GO composite also possesses a remarkable electrical transport property. Now, in the CuWO 4 -GO composites, a possible reaction mechanism can be proposed based on the observed enhanced photo catalytic activity. Here, a portion of graphene oxide acts as a photo carrier in conducting electrons to the surface of the graphene oxide ,which improves the separation of the electron-hole pairs and photo catalytic efficiency. It plays a conventional role as an elector acceptor and transporter, which is reported in most of the graphene oxide-semiconductor composites.
In the present study, a facile metathesis synthesis of CuWO 4 and CuWO 4 -GO nano composite has been reported. The morphology and composition have been characterized using XRD, SEM and EDAX. The photo catalysis has been performed to study degradation of Methylene blue and Malachite green dyes by the prepared materials.

Materials and Methods
All the chemicals were purchased of analytical grade which can be used as directly without any further purification. All the reactions were carried out using deionized water. The dyes used for this study are Malachite green and Methylene blue whose chemical formulae are C 23  The resultant compound CuWO 4 obtained from the solid-state method was taken and grinded mechanically in low energy ball mill to obtain nano sized copper tungstate. The balls to powder weight ratio was maintained at 10:1 and the mixture is ground at 350 rpm for 4 hrs in methanol medium to obtain homogeneous compound. The residue after ball milling was dried in a hot air oven at 80˚C.

Synthesis of Graphene Oxide
Graphene oxide (GO) was prepared by the well-known modified Hummers method [15] from an expanded acid washed graphite flakes (Sigma Aldrich). In this, 5 g of graphite was added into a mixture of 108 mL of sulphuric acid (Molychem-AR) and 12 mL of orthophosphoric acid (Molychem-AR) and stirred for 10 min. To it, 2.5 g of sodium nitrate (Molychem-AR) was added. Subsequently, the beaker with reagents was put in an ice bath in order to keep it below 5˚C. Now, 15 g of potassium permanganate (Molychem-AR) was added in portions into the mixture, which was vigorously stirred. After addition of the oxidant, the beaker was heated and kept at 35˚C to 40˚C with continuous stirring. In the next step, 280 mL of deionized water was added to the beaker and heated to 95˚C and maintained the same conditions for about 60 min. To complete the reaction 5 ml of 30% hydrogen peroxide (Molychem-AR) is added, now the color of the solution changes to bright yellow .The mixture is then washed with 5% hydrochloric acid (Molychem-AR) solution to remove sulfate ions and with deionized water in order to remove the chloride ions to maintain neutral pH. After centrifugation the gel like substance is vacuum dried at 60˚C for more than 6 hrs to get GO as powder. Reflectance spectroscopy, Raman Spectroscopy.

PhotoCatalytic Experiments
Photo catalytic activity of the synthesized CuWO 4 -GO nano composite was eva- Percentage degradation of the dye was calculated using the following formula.
( ) where A o is absorbance of dye at initial stage A t is absorbance of dye at time t.

Instrumentation
Retsch® Planetary Ball Mill PM 100 bench top grinding station has been used for ball milling. The resulting powder was characterized using X-Ray Diffractometer (PANalytical-X' Pert PRO, Japan) at room temperature, using Nickel Filter Cu-Kα radiation (λ = 1.54059 Å), over a wide range of 10˚ ≤ 2θ ≤ 80˚ with a scanning speed of 2˚ min −1 . The morphology of the as-synthesized samples was investigated by field emission scanning electron microscopy (FESEM, LEO1550).
Band gaps were calculated using Single Monochromator UV-2600 (optional ISR-2600 Plus, λ up to 1400 nm). Figure 2 shows the X-Ray Diffraction plot of nanographene oxide and the peaks are in good agreement with the characteristic peaks of graphene oxide . Figure 3 shows

FT-IR
The FTIR spectrum of CuWO 4 is shown in Figure 4 The spectrum exhibits a broad band near 3446 cm −1 due to the OH-stretching vibrations of free and hydrogen-bonded hydroxyl groups .The band at 614 cm −1 and the peak at 476 cm −1 can be assigned to the Cu-O stretching vibration along the direction and the peak at 1028 cm −1 is due the presence of W-OH bond [26].

UV Diffuse Reflectance Spectral Studies
The optical properties of pure copper tungstate, nano copper tungstate and     the λ max is observed at 580 nm and the band gap is found to be 2.1 eV in Figure 8 and Figure 9.  Figure 10 shows Raman spectra of CuWO 4 -GO nano composite. In terms of group theoretical analysis, Wolframite structure belonging to P 2 /c (z = 2) monoclinic structure is expected to give 18 (8 A g + 10 B g ) Raman-active bands out of 36 possible lattice modes. As our interest is limited to show the phase formation of tungstates, the Raman modes CuWO 4 -GO is shows the vibrations at 910 cm −1 which is an evidence for wolframite structure.

The Photo Catalytic Activity
The photo catalytic activity of the nanoCuWO 4 Figure 13 shown in comparison with micro and nanoCuWO 4 . Aiming at eliminating the effect of adsorption on the dye degradation efficiency, before each photocatalytic process, the adsorption-desorption equilibria between the dye and the photocatalysts were first obtained at 30 min and it is sufficient time to reach equilibrium. After 30 min the reaction mixture is exposed to visible light under constant magnetic stirring, and aliquot of the reaction mixture is collected for every 10 min and subjected to UV-Vis analysis. It can be seen that the intensity of the absorption peaks decreased as the reaction progressed with CuWO 4 -GO as the catalyst. We observed that the MG dye solution is completely degraded in 60 min and MB dye degraded in 80 min.

Effect of Graphene Oxide % Composition on the Photocatalytic Activity
Malachite green and Methylene blue dye can be more easily adsorbed by GO (MG&MB degradation efficiency ~75%) rather than the CuWO 4 (MG & MB degradation efficiency ~3%) catalyst. Its adsorption capacity was enhanced when GO was introduced on CuWO 4 . The performances of the prepared samples, including pure CuWO 4 and modified CuWO 4 by graphene oxide with different percent compositions, were investigated with regard to the degradation of MG&MB under visible light irradiation. There is almost no photodegradation in the absence of the catalyst. This suggested that, negligible amount of degradation by light can be ignored. In the presence of nano CuWO 4 , Malachite green and Methylene blue dyes were degraded in 80 to100 min under visible light irradiation. The CuWO 4 -GO nano composite has enhanced the photocatalytic degradation of Malachite green and Methylene blue dyes. The efficiencies of dye degradation were significantly improved from 80% to 100% in a time interval of 60 to 80 min by improving the amount of graphene oxide from 5% to 10% in the CuWO 4 -GO composite. Even though by adding upto 20% of graphene oxide in the composite, degradation efficiency remained the same. This suggests that graphene oxide with an amount of 10% in the CuWO 4 -GO composite performs the best in removing the organic pollutants in wastewater under visible light irradiation ( Figure 14).

Effect of Amount of CuWO4-GO Nano Composite
The    of the catalysts dosage quantity from 4 to 6 g/L will slightly decrease the efficiency of the degradation of Malachite Green and Methylene blue. This phenomena, maybe because of the increase in the amount of catalysts dosage, which would increase the reactive sites that can correspondingly produce more reactive oxidative species. However, too much catalyst dispersed in the system will possibly increase light scattering and decrease light penetration [28], resulting in the reduction of degradation efficiency of MG & MB in a system with excessive photocatalysts. CuWO 4 -10% and CuWO 4 -20% GO nano are having similar percentage of degradation so the experiment was carriedout with CuWO 4 -10% GO composite

Effect of pH of Initial Dye Solution
Considering that the pH of wastewater is possibly different, its effect on the photocatalytically degrading dye (MG/MB) in the presence of CuWO 4 -GO under visible light irradiation was explored. It has been reported in Figure 16 that, with an increase of pH of the dye (MG/MB) solution, it may reduce the adsorption of dye (MG/MB) on the photocatalyst. This resulted in the improvement of degradation efficiencies when pH of the MG solution increased from 5 to 7 and for MB solution increased from 3 to 5 [27]. And the transformation of CuWO 4 damages the CuWO 4 -GO structure and eventually reduces the photocatalytic activity.

Effect of Temperature
As shown in Figure 17, temperature of the photocatalytic reacting system was also varied from 0˚C to 80˚C to explore its effect on the photocatalytic performances of the prepared samples under visible light irradiation. When the temperature of the reacting system is in the range of 20˚C -60˚C, the photocatalytic performances in degrading dye (MG/MB) were similar and only a slight increase is found with the increase in temperature. However, when the temperature was fixed at 0˚C, the photocatalytic activity was significantly reduced. This might be because the mass transfer of pollutants to the surface of photocatalysts was decreased and the generation rate of oxidative species was also reduced. When the temperature was as high as 80˚C, the photocatalytic activity was greatly decreased. High temperature favors the recombination of charge carriers and desorption of adsorbed organics on the photocatalysts. The results can be regarded as evidence of temperature controller needed for solar devices.

Reusability of the Catalyst
The reusability of the prepared sample was assessed by recycling CuWO 4 -GO composite three times and the profiles of MG/MB concentrations is shown in   difficult to be destroyed [28]. Despite of this slight reduction in degradation efficiency, the stability of the reused CuWO 4 -GO photocatalysts after degradation of dye (MG/MB) is still significant.

Plausible Photocatalytic Mechanism
The plausible mechanism for the photocatalytic activity of the catalyst can be attributed to the presence of π-conjugation and 2D planar structures of graphene oxide in the CuWO 4 -GO composite which can adsorb organic molecules easily Modern Research in Catalysis on its surface via strong π-π interactions [29]. Additionally, graphene oxide possesses high charge carrier mobility and can be regarded as an electron acceptor.
It can greatly decrease the recombination rate of photogenerated electrons and holes. Upon visible light excitation, the electron-hole pairs (h + , e − ) are generated on the copper tungstate-graphene oxide surface followed by the instant transfer of photogenerated electrons onto graphene oxide via a percolation mechanism and then the negatively charged graphene oxide can activate the dissolved oxygen to produce superoxide anion radical, while the holes can react with the adsorbed water to form hydroxyl radical. Finally, the active species, holes, superoxide anion radical and hydroxyl radical oxidize the Methylene blue and Malachite green dye molecules adsorbed on the active sites of the copper tungstate-graphene oxide system through the π-π stacking and electrostatic attraction.

Conclusion
In summary, we have demonstrated the synthesis of