1. Introduction

The wastes discharged from various industries and plants are the major source of water pollution, ringing alarm bells for human society. Organic dyes are mainly used in industries related to textile, cosmetics, paper, pharmaceuticals, food, printing and dyeing. Around 20% of these dyes are lost during the process of coloring. Since these are lost into the water their removal from the water sources has become a major environmental issue ^[1,2]. For the treatment of hazardous wastes, several conventional methods like reverse osmosis, adsorption, ultra filtration, coagulation by chemical agents, enzymes, etc, are used ^[3,4,5,6,7]. But these nondestructive processes just transfer the pollutants from one phase to another, creating secondary pollutants. Hence for mineralization of the organic dyes with aromatic moieties, an eco-friendly cost effective method has to be implemented in large scale. Photocatalysis is one such process which has been looked into for a past couple of decades, but the high recombination rate of photo generated electrons and holes has prevented it from commercialization of the application ^[8,9,10].

Semiconducting materials with narrow band gap have been used as photocatalyst since the band gap energy corresponds to the UV or visible light region of the solar spectrum and also they help in charge separation. Materials like TiO₂, ZnO, Fe₂O₃, NiO, Co₃O₄, CeO₂, ZrO₂ have tunable physical and optical properties ^{[11,12,13,14,15,16]}. Among these, TiO₂ is a well-known photocatalyst with high chemical stability, non toxicity, photo corrosion resistivity and low cost. But it suffers from the disadvantage of having a wide band gap of 3.2 eV which leads to absorption of only a very small portion of the solar energy and has a higher recombination rate of electron-hole pairs. Multi metal oxides like SrTiO₃, LaFeO₃, NaTaO₃, BaFeO₃, K₄Nb₆O₁₇, CaIn₂O₄, CaBi₂O₄ and BiVO₄ have also been used for photodegradation of organic pollutants ^[15,17]. Of late M²⁺WO₄ type metal tungstates is under the research radar and nano/micro structured materials like CdWO₄, ZnWO₄ and NiWO₄ have been applied in variety of areas ^[1,15,18,19].

BaWO₄ which has a wide variety of applications in the field of lasers, catalysis, scintillators, fluorescent lamps, photoluminescence and optical fibers, have been studied very less in the field of photocatalysis ^[20]. Although several methods of preparation have been reported for this material such as Czochralski, precipitation, hydrothermal, solid state and pulsed laser deposition the material still suffered instability and slow electron transfer rate. Since visible region of the solar spectrum is more dominant, BaWO₄ which is a UV catalyst has to be transformed to visible light absorbing entity to make it more applicable ^[20]. For visible light absorption the band gap has to be small which means the recombination rate will be very high. By doping carbon containing species the transport property and the recombination rate can be altered desirably to improve the photocatalytic efficiency ^[1,18,19].

Graphene known to be mother of all 2D materials has excellent properties ^[21,22]. Reduced graphene oxide (RGO) can be prepared from graphene oxide (GO) via Hummer's and modified Hummer's method during which metal and metal oxides can be added to decorate the nanosheets ^[23,24]. RGO can act as a support system in the composite while also retaining the photogenerated electron thus reducing the recombination rate of electron-hole pair; it improves the adsorption of the dye molecules via π-π interaction. Several RGO based nanocomposites have been reported for photocatalytic applications but to the best of our knowledge RGO-BaWO₄ composites have not yet been studied for their photocatalytic performance ^[24].

Microwave approach of synthesis has an upper hand over the conventional thermal synthesis due to the factors like microwave irradiation directly heats the material rather than heating the reaction vessel and hence consumes less energy and time; uniform heating; selective heating; acceleration in reaction rate and high chemical yield. Recently, microwave approach has been used to synthesize RGO, metal tungstates and RGO hybrid composites for application in the field of photocatalysis, hydrogen generation and supercapacitors ^[21,25,26].

In view of the aforesaid facts, we thought it is worthwhile to synthesize RGO-BaWO₄ composites of varied compositions using microwave approach. The as synthesized materials were characterized using X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy to determine the phase structure, morphology and elemental composition. Photoluminescence spectroscopy (PL) and Diffuse reflectance spectroscopy (DRS) techniques were used to study their optical properties. The photocatalytic activity was studied through photodegradation of methylene blue (MB) and methyl orange (MO) dyes under visible light. The stability and reusability of the composites were also determined. A reaction mechanism was proposed based on active species trapping experiment to determine the species involved in photocatalytic degradation of dyes.

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2. Materials and Method

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2.1. Materials

All the reagents procured were of analytical grade and utilized without further purification. Deionized water was utilized for all the experiments.

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2.2. Synthesis of GO

GO was synthesized as reported earlier using modified Hummer's method ^[27,28]. The typical procedure is as follows: graphite flakes (about 1 g) was mixed well with KMnO₄ (about 4 g). To this conc. sulphuric acid (90 mL) was added slowly in about 30 minutes under magnetic stirring. The mixture was further stirred for 12 h after which deionized water (80 mL) and hydrogen peroxide (30%) were added till the color changed from greenish purple to bright yellow. This suspension was centrifuged and the supernatant was decanted. The precipitate was washed with water and hydrochloric acid (10%). It was vacuum dried at 60 ℃ for 12 h.

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2.3. Synthesis of RGO-BaWO₄ Composites

In a typical experiment, Ba (CH₃COO)₂ and Na₂WO₄·2H₂O each was dissolved in 20 mL mixture of ethylene glycol and water (1:1) under magnetic stirring for about 20 minutes at room temperature. 0.005 M solutions (20 mL each) of both were mixed to which calculated amounts of exfoliated GO was added and stirred for 30 minutes to prepare a series of xRGO-BaWO₄ composites where x = 0, 1, 2.5, 5 and 10. The exfoliation of GO was done by ultrasonicating the GO solution for 30 minutes. The solution containing GO, Ba (CH₃COO)₂ and Na₂WO₄·2H₂O was then taken into a 100 mL reactor and irradiated with microwave of 350 W for 2 minutes. After the solution cooled down naturally the precipitate was collected by centrifuging. It was washed with water and ethanol (10%) several times and vacuum dried at 60 ℃ for 12 h. The prepared samples were denoted as 1% RGO-BaWO₄, 2.5% RGO-BaWO₄, 5% RGO-BaWO₄ and 10% RGO-BaWO₄, respectively, with the number referring to the weight ratios of RGO to BaWO₄. A control sample of BaWO₄ was synthesized using a similar process without addition of GO.

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2.4. Characterization

The crystal structures of RGO-BaWO₄ composites were determined by XRD (X-6000, Shimadzu) with Cu K_α radiation (λ = 0.154 nm). The morphologies of the composites were examined with a SEM (JEOL JSE-6390) well equipped with EDX operated at a voltage of 15.0 kV. The elemental composition of the composites was determined using XPS (XPS, Kratos XSAM800) equipped with standard and monochromatic X-ray sources (Al-Kα). Raman spectra were obtained by Horiba Jobin Yvon, Japan at room temperature using a 532 nm laser as radiation source. DRS of the composites were recorded by utilizing UV-visible spectrometer (Analytik Jena). The room temperature PL spectra of the composites were obtained by using a Horiba Jobin Yvon PL instrument. The specific surface area of the samples was measured on SMART SORB 92/93 (Smart Instruments Company Private Limited, INDIA) using Brunauer-Emmett-Teller (BET) method.

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2.5. Photocatalytic Measurements

The photocatalytic performance of the RGO-BaWO₄ composites was determined in visible light region by the degradation of MB and MO solution. A pyrex glass photocatalytic reactor furnished with water cooled immersion tube, 250 W Hg lamp (emission range from 300 to 800 nm), cut-off filter of wavelength 400 nm and sample extracting facility was utilized to illuminate the suspension (100 mL) containing the dye (10 mg/L) and the photocatalyst (0.1 g). The reaction mixture was maintained at a pH of 3 using 0.1 N H₂SO₄. The suspension was allowed to achieve adsorption-desorption equilibrium by stirring it for 30 minutes in dark. Once the solution is irradiated at regular intervals, 4 ml of the suspension is withdrawn. It is centrifuged and the supernatant is subjected to UV-visible analysis. The photodegradation efficiency was estimated according to the equation, degradation % = [(C₀-C)/C₀] × 100, where C₀ represents the initial concentration of the dye and C represents the concentration of the dye after irradiation at time interval "t".

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3. Results and Discussion

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3.1. X-ray Diffraction Analyses

The XRD patterns of graphite, GO, RGO, BaWO₄ and RGO-BaWO₄ composites with various RGO weight proportions are shown in Figure 1. The XRD pattern of GO revealed only one sharp peak. The diffraction peak (002) of GO is situated at 2θ = 10.5° with d-spacing of about 0.842 nm. This value of d-spacing is larger than that of pristine graphite (0.339 nm) indicating the presence of intercalated oxygen containing functional groups ^[29]. The absence of peaks corresponding to graphite in GO pattern indicates that graphite has been completely oxidized to GO. The diffraction peaks corresponding to RGO-BaWO₄ composite can be indexed to scheelite type tetragonal crystal of BaWO₄ with space group Ⅰ41/a indicating that incorporation of BaWO₄ into RGO matrix does not change the crystal structure of the former. The diffraction peaks found at 2θ values 17.25°, 26.45°, 28.06°, 31.87°, 42.97°, 45.68°, 48.69°, 53.57°, 54.48°, 66.59°, 67.74°, 69.36°, 73.0°, 73.77°, 75.75° and 76.83° can be ascribed to (101), (112), (004), (200), (204), (220), (116), (312), (224), (400), (208), (411), (332), (404), (420) and (228) planes of BaWO₄ (JCPDS card no. 43-0646), respectively. The absence of impurity peaks indicated the high purity of the samples.

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3.2. SEM and EDX analyses

The morphologies of the BaWO₄ and RGO-BaWO₄ composites were studied using SEM (Figure 2). The SEM image reveals narrow elliptical BaWO₄ particles. In the composite we notice the BaWO₄ particles wrapped in RGO sheets. The elemental composition was found to be C, Ba, W and O by EDX analyses (Figure 3). Ba, W and O were present in the ratio 1:1:4 indicating the presence of BaWO₄. Elemental mapping of C, Ba, W and O using spot EDX revealed uniform distribution of BaWO₄ in RGO matrix in RGO-BaWO₄ composites (Figure 3a-d).

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3.3. XPS Analyses

XPS is a highly sensitive surface technique used to determine the elemental composition, chemical and electronic state of the element within a material. The XPS survey spectrum (Figure 4a) of RGO-BaWO₄ composite exhibits peaks corresponding to C 1s, O 1s, Ba 3d and W 4f. The high resolution deconvoluted C 1s XPS spectra for GO (Figure 4b), reveals four peaks located at 284.6 eV, 285.8 eV, 286.9 eV and 288.7 eV which corresponds to the C-C (sp² bonded carbon), C-OH (hydroxyls), C=O (carbonyls) and O-C=O (carboxyl) functional groups, respectively indicating high oxygen content on the surface ^[18]. High resolution deconvoluted C 1s spectrum of RGO-BaWO₄ (Figure 4c) reveals a peak around 284.6 eV, corresponding to C-C (sp² bonded carbon). The intensity of peaks corresponding to the oxygen functional groups decreased drastically indicating the reduction of GO into RGO successfully ^[19].

The high-resolution Ba 3d spectrum (Figure 4d) comprises of two peaks at 779.4 eV and 794.8 eV, corresponding to Ba 3d_5/2 and Ba 3d_3/2, respectively. The high-resolution W 4f spectrum (Figure 4e) also comprises of two peaks at 35.1 eV and 37.2 eV, which corresponds to the W 4f_7/2 and W 4f_5/2, respectively. The splitting energy between the 4f doublet of W is 2.1 eV indicates that W is in +6 state ^[19]. The high-resolution O 1s spectrum (Figure 4f) consists three peaks at 529.7 eV, 530.9 eV and 531.8 eV, which corresponds to the Ba-O, W-O-W and H-O-H (surface absorbed hydroxyl oxygen species), respectively ^[1,18].

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3.4. Raman Analyses

Raman spectroscopy is a fast and nondestructive technique to obtain vibrational information about the chemical bonds and symmetry of the molecule. The Raman spectra for GO, RGO, BaWO₄, and RGO-BaWO₄ composite are shown in Figure 5. The Raman spectra of GO reveals two prominent peaks at around 1346.4 cm⁻¹ and 1598.1 cm⁻¹ corresponding to the D band (the breathing mode of the symmetry A_1g) and the G band (the E_2g mode of sp² carbon atoms), respectively ^[18,19]. BaWO₄ on the other hand exhibits three peaks around 333.6 cm⁻¹, 794.5 cm⁻¹ and 927.1 cm⁻¹, which corresponds to the Raman active vibrations of A_g, B_g and E_g modes, where A and B are non-degenerate modes while E is doubly degenerate mode ^[30]. The subscript g refers to the centrosymmetric nature.

Two prominent D and G bands are exhibited around 1339.6 cm⁻¹ and1594.8 cm⁻¹ by RGO and around 1338.2 cm⁻¹ and 1592.5 cm⁻¹ by RGO-BaWO₄ composite, respectively. The 2D bands appear at 2653.2 cm⁻¹ and 2639.1 cm⁻¹ in the case of RGO and RGO-BaWO₄, respectively. The decrease in the wavenumbers of D and G bands from GO to RGO, indicates the conversion of GO to RGO ^[18,19]. The further decrease in the wavenumbers after the formation of the composite is due to the smaller size of BaWO₄ crystals.

The I_D/I_G intensity ratio of pure RGO and RGO in RGO-BaWO₄composite (1.07 and 1.10, respectively) is slightly greater than that of GO (0.75). This is may be due to the decrease in the sp² domain size as GO gets reduced to RGO nanosheets ^[19]. The domain size can be calculated by using equation (1),

${\text{L = 4}}{\text{.4/}}\left( {{{\text{I}}_{\text{D}}}{\text{/}}{{\text{I}}_{\text{G}}}} \right){\text{}}$

(1)

where, L = domain size and I_D/I_G is the intensity ratio of D and G bands. The calculated domain size of GO, RGO and RGO-BaWO₄ are 5.87 nm, 4.11 nm and 4.0 nm, respectively.

The number of RGO layers can be determined by the ratio I_2D/I_G, where I_2D is intensity of 2D band and I_G is intensity of G band. The ratio I_2D/I_G of > 2, 1 to 2, and < 1 corresponds to single, double and multi-layer of RGO sheets, respectively. In the present work I_2D/I_G was found to be 0.34 and 0.42 for RGO and RGO-BaWO₄ composite, respectively indicating the multilayered nature of RGO sheets.

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3.5. DRS Analyses

The optical properties of the as-synthesized composites were studied using DRS (Figure 6a). The results indicated that BaWO₄ absorbs in the UV region while the absorption edge of the RGO-BaWO₄ composites shifts towards the visible region with increase in the intensity of absorption. The DRS of pure RGO shows that although its absorption maximum occurs in the UV region it has sufficiently high absorbance in the visible region compared to that of pure BaWO₄. Thus, the observed red shift in the absorption edge of the composites may be due to the increasing amounts of RGO in the composites. The results also suggest that BaWO₄ is incapable of acting as a photocatalyst in the visible region while the composites can be active in the visible region as photocatalysts due to appropriate band gap.

The band gap energy was calculated employing the Kubelka-Munk equation (2),

$\text{ }\!\!\alpha\!\!\text{ }\left( \text{h }\!\!\nu\!\!\text{ } \right)\text{=A(h }\!\!\nu\!\!\text{ -}{{\text{E}}_{\text{g}}}{{\text{)}}^{\text{n/2}}}$

(2)

where α, h, ν, E_g and A are the absorption coefficient, Plank constant, frequency of light, band gap energy and constant, respectively. The "n" value of 1 corresponds to direct and 4 corresponds to indirect transition in the semiconductor material. The band gap is determined by a plot of (αhν)^n/2 versus energy of photon (hν). Direct band gap energy of 4.43 eV, 3.76 eV, 3.60 eV, 3.39 eV, 3.13 eV and indirect band gap energy of 3.30 eV, 2.47 eV, 2.19 eV, 1.99 eV, 1.22 eV corresponding to pure BaWO₄, 1% RGO-BaWO₄, 10% RGO-BaWO₄, 5% RGO-BaWO₄ and 2.5% RGO-BaWO₄ respectively (Figure 7). The values of direct and indirect band gaps indicate that the RGO-BaWO₄ composites respond to both UV as well as visible light.

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3.6. PL Analyses

PL spectra can be used to determine the effectiveness of the charge separation and recombination rate of the photogenerated electron hole pairs. If the intensity of PL peak is lower it means the recombination rate is lower ^[1]. The PL spectra of the as-synthesized BaWO₄ composites with various amount of RGO are shown in Figure 6b. The excitation wavelength corresponds to 400 nm while the emission peak is exhibited around 545 nm (green emission). BaWO₄ shows highly intense peak indicating high recombination rate. The composites exhibit lower intensity than pure BaWO₄. 2.5% RGO-BaWO₄ shows very less intensity indicating the low recombination rate of electron hole pairs generated by the visible light irradiation.

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3.7. Surface Area Analyses

The BET surface area analyses of the composite samples were carried out to study the effect of addition of RGO on the overall surface area of prepared nanocomposites. The measured BET surface areas of the samples were, 3.41 m²/g, 14.14 m²/g, 19.45 m²/g, 18.95 m²/g and 15.95 m²/g for pure BaWO₄, 1% RGO-BaWO₄, 2.5% RGO-BaWO₄, 5% RGO-BaWO₄ and 10% RGO-BaWO₄ respectively. The addition of RGO content initially increased the surface area of the sample up to RGO content of 2.5%. Further additions of RGO did not increase the surface area. This may be due to agglomeration of RGO and BaWO₄ in the formed composite.

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3.8. Photocatalytic Analyses

The photocatalytic degradation of MB and MO were studied in the presence of RGO-BaWO₄ in aqueous solution using visible light source. Degradation of dye (ⅰ) in presence of photocatalyst, in the absence of irradiation and (ⅱ) in the absence of photocatalyst with visible light irradiation, were considered as control (blank test). Under visible light irradiation the color of the solution changed with time indicating the photodegradation was in progress. The absorption intensity decreased with time as shown in Figure 8.

The absorbance peaks of MB and MO appearing at 664 nm and 464 nm decreased with time and disappeared completely after 120 minutes and 165 minutes, respectively. The photodegradation efficiencies of various catalyst compositions were compared (Figure 9a-b) and it was found that both in the case of MB and MO dye degradation 2.5% RGO-BaWO₄ composite showed maximum efficiency.

The kinetics of the photocatalytic degradation was studied by the first-order simplification ^[18] of Langmuir-Hinshelwood (L-H) kinetics, employing equation (3),

${\text{ln(}}{{\text{C}}_{\text{0}}}{\text{/C) = kt}}$

(3)

where C₀/C is the ratio between the concentration of the dye molecules at adsorption-desorption equilibrium and at irradiation time "t" and "k" is the first-order rate constant (min⁻¹). The "k" value is found from the plot of ln (C₀/C) versus irradiation time "t" (Figure 9c-d). The efficiency of photodegradation of the RGO-BaWO₄ composites increased with increasing the RGO contents from 1 to 10 wt% (Table 1). With increase in the RGO content, initially the efficiency of photocatalytic activity increases up to 2.5%. At 2.5% RGO content, the nanocomposite showed higher efficiency compared to other samples. This may be ascribed to the higher surface area available for charge separation which in turn enhances the catalytic efficiency. The decrease in the activity above 2.5% RGO content may be ascribed to the coverage of RGO on the semiconductor material and hence preventing the latter from effective absorption of incident light radiation and causing low activity ^[8,22]. This results of surface area analyses of the samples also support this statement.

As 2.5 % RGO-BaWO₄exhibited maximum performance, the photocatalytic stability and reusability of this composite are investigated for five consecutive cycles (Table 2). The photocatalytic activity of MB and MO degradation decreased slightly from 99.70 % to 97.68 % and 97.99 % to 89.59 % after five cycles, which indicates that 2.5 % RGO-BaWO₄nanocomposite has excellent reusability. The decreased efficiency may be due to loss of the catalyst during recovery process. Overall, the results indicate that the synthesized composites have excellent recyclability and photocatalytic efficiency.

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3.9. Mechanism of Photocatalytic Activity

The possible photocatalytic mechanisms involved in dye degradation was determined via trapping experiments using the chemicals, 5mM of tertiary butanol (TBA); 5 mM of disodium salt of ethylenediaminetetraaceticacid (EDTA), 5 mM of benzoquinone (BQ) and 10 mM of silver nitrate (AgNO₃) to act as scavengers for hydroxyl radicals, holes, superoxide radicals, and electrons respectively during the photocatalytic process ^[18]. The photodegradation reaction rate showed no much change when TBA was added indicating hydroxyl radicals are not the active species. However, the addition of EDTA, BQ and AgNO₃ to the system decreased the efficiency drastically showing that holes and super oxide anion radicals are mainly involved in the photodegradation process (Table 3). As indicated by the results below, a possible schematic scheme for the photocatalytic degradation is given in Figure 10. RGO-BaWO₄ composite absorbs the visible light and electron-hole pairs are created in BaWO₄. RGO due to its high charge carrier mobility can act as electron acceptor and transporter and delay the recombination of photogenerated electron hole pair. The negatively charged electrons react with dissolved oxygen to produce the superoxide anion radicals (O₂·^-). The holes react with the water molecules to form hydroxyl radicals (OH·).

These active species degrade the dye molecules into products like carbon dioxide and water. The reaction steps are shown in equation 4 to 9.

${\text{BaW}}{{\text{O}}_{\text{4}}}{\text{ + h}}\nu \to {\text{BaW}}{{\text{O}}_{\text{4}}}\left( {{{\text{e}}^{{\text{ - }}}}{\text{ + }}{{\text{h}}^{{\text{ + }}}}} \right)$

(4)

$\text{BaW}{{\text{O}}_{\text{4}}}\left( {{\text{e}}^{\text{-}}} \right)\text{ + RGO }\!\!~\!\!\text{ }\to \text{BaW}{{\text{O}}_{\text{4}}}\text{+ RGO }\left( {{\text{e}}^{\text{-}}} \right)\text{ }\!\!~\!\!\text{ }$

(5)

${{\text{e}}^{\text{-}}}\text{+ }{{\text{O}}_{\text{2 }\!\!~\!\!\text{ }\!\!~\!\!\text{ }}}\to {{\text{O}}_{\text{2}}}{{^{\text{.}}}^{\text{-}}}$

(6)

$\text{O}{{\text{H}}^{\text{-}}}\text{+ }{{\text{h}}^{\text{+}}}\text{ }\!\!~\!\!\text{ }\to \text{O}{{\text{H}}^{\text{.}}}\text{ }\!\!~\!\!\text{ }$

(7)

$\text{Dye + }{{\text{O}}_{\text{2}}}{{^{\text{.}}}^{\text{-}}}\text{+ O}{{\text{H}}^{\text{.}}}\text{+ }{{\text{h}}^{\text{+}}}\to \text{C}{{\text{O}}_{\text{2}}}\text{+ }{{\text{H}}_{\text{2}}}\text{O}$

(8)

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4. Conclusions

A series of RGO-BaWO₄ composites with various compositions have been synthesized via microwave irradiation method. The prepared materials have been characterized by diffraction, microscopic and spectroscopic techniques. The photocatalytic activities of the RGO-BaWO₄ composites have been studied using MB and MO dye. The dye degradation efficiency has been determined in the presence of visible light. We observed that the RGO-BaWO₄ composites with the optimal RGO content of 2.5 wt% shows the maximum photocatalytic performance of dye degradation in comparison to the other ratios studied. The improved photocatalytic performance of the BaWO₄ is due to the increase in visible light absorption range and the reduction in recombination rate of photogenerated electron-hole pair due to the composited RGO. The photocatalytic degradation mechanism involved superoxide anion radical and holes as the active species. The synthesized RGO-BaWO₄ had excellent stability and reusability. Hence, the above results indicate that RGO-BaWO₄ composite can be a promising eco-friendly photocatalytic material for the treatment of organic dyes.

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Acknowledgements

The author M.M.S.J. acknowledges the financial support from the National Institute of Technology Karnataka, Surathkal in the form of research fellowship.

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Conflicts of Interest

The authors declare no competing financial interest.

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