Fabrication and photocatalytic degradation of reactive blue 19 by CuWO4, Ag3PO4 and CuWO4-Ag3PO4 composites under visible light irradiation
Mohammad javad Asadollahzadeh
1
(
Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood, Iran
)
Aliakbar Arabameri
2
(
Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood, Iran
)
Hamed Rashidi
3
(
Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood, Iran
)
Javad seyfi
4
(
Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood, Iran
)
Keywords:
Abstract :
Fabrication and photocatalytic degradation of reactive blue 19 by CuWO4, Ag3PO4, and CuWO4-Ag3PO4 composites under visible light irradiation
Abstract
CuWO4 nano-powder was synthesized via a hydrothermal reaction using CuCl2.2H2O and Na2WO4.2H2O in the stoichiometric 1:1 Cu:W molar ratio and sodium citrate as raw materials. Also, Ag3PO4 was synthesized by a precipitation method using Na2HPO4 and AgNO3. Finally, CuWO4-Ag3PO4 nanocomposite was synthesized in a precipitation route using the as-synthesized CuWO4 and Ag3PO4 as raw materials. The synthesized materials were characterized by the powder X-ray diffraction (PXRD) technique. In order to investigate the effect of concentration of the Basic solutions on the obtained materials morphology, the synthesized materials' morphologies were studied by field emission scanning electron microscopy (FESEM) technique. As shown by the FESEM images, the CuWO4 material morphology was spherical particles. Besides, the photocatalytic performance of the as-synthesized nanocomposites was studied for the degradation of Reactive Blue 19 (RB19) under direct visible light irradiation. For this purpose, several reaction parameters that affect the degradation yield, such as catalyst amount, pH value, and absence/presence of light, were investigated. The results indicate the higher photocatalytic yield at the presence of light irradiation when the solution's pH value was in the acidic range and the weight percent of silver phosphate in the composite mixture and the catalyst amount was more. The information revealed that a 0.05 g nanocomposite containing 0.24 mmole of Ag3PO4, with an initial pH of 3 and 100 mL of RB 19 with 30 mg/L concentration, approximately after 30 min, could remove completely under visible light illumination.
Keywords: Photocatalyst, Reactive Blue 19, Anthraquinone reactive dye, CuWO4-Ag3PO4, Nanocomposite.
1. Introduction
Heterogeneous semiconductor photocatalysis has been actively studied for environmental remediation. Copper tungstate (CuWO4) is belonging to the structurally divalent transition metal tungstates family. This compound contains 3d orbital corresponding to the transition metal [1]. Copper tungstate is a well-known n-type semiconductor with a wide range of applications as scintillation detectors, photoanodes, laser host, electrode material for photoelectrolysis, optical fibers, solar-assisted water splitting, and another usage. Copper tungstate can absorb light at wavelengths up to 540 nm, so it has a narrow bandgap (Eg) of 2.3–2.4 eV. The electronic properties related to CuWO4 crystals have been reported in different fields [2-4]. At room temperature and low pressure, the CuWO4 crystal is an important member of transition metal tungstates, which exhibits a triclinic structure [5].
CuWO4 composites have been widely used for applications such as electrical contacts, arc-resistant electrodes, microwave packages, heavy-duty electrical contacts, microelectronics devices, electro-discharge machining, and heat sinks materials for high-density integrated circuits. Due to their enhanced mechanical and physical properties, good arc erosion resistance, remarkable thermal and electrical conductivity, and high microwave absorption ability, they can serve in various applications [6,7].
Recently, Ag3PO4 has attracted considerable attention as a potentially visible light photocatalyst with a bandgap of 2.45 eV. So, the role of Ag3PO4 is a sensitizer absorbing visible light [8]. Silver phosphate (Ag3PO4) crystallizes in a body-centered cubic structure (bcc) with space group P-43n. This material has received considerable attention because of its photooxidative applications [9]. Ag3PO4 exhibits extremely high photooxidative capabilities for O2 evolution from water splitting and organic dye decomposition under visible light irradiation. Ag3PO4 has a direct bandgap of about 2.43 eV that can absorb energies with wavelengths shorter than about 530 nm. They can achieve a quantum efficiency of about 90% at the near 420 nm wavelengths in water oxidation with AgNO3 as a scavenger [10]. Ag3PO4 is the only compound that incorporates nonmetallic p-block specie into Ag2O [11]. In recent years, silver phosphate (Ag3PO4) is a new type of photocatalyst that is highly effective in visible light [12].
One approach to enhance the photocatalytic activity and promote the charge separation efficiency of Ag3PO4 is the coupling it with other semiconductors or noble metals. Some coupled systems such as Ag3PO4/TiO2, Ag3PO4/AgX (X=Cl, Br, I), Ag3PO4/SnO2, Fe3O4/Ag3PO4, Ag3PO4/Ag, Ag3PO4/BiOCl, Ag3PO4/reduced graphite oxide sheets, and carbon quantum dots/Ag3PO4 composites have recently been developed to improve the photocatalytic activity of Ag3PO4 [13]. Besides, some other composite materials of CuWO4 and Ag3PO4 nanomaterials, including Ag3PO4–ZnO [9], Ag3PO4-GO [14], ZnO/CuWO4 [15], CdS-CuWO4-TiO2 [16], Fe3O4/ZnO-CuWO4 [17], Ag3PO4/ZnFe2O4 [18] was reported.
Several methods have been reported for the synthesis of CuWO4 nanomaterials, including hydrothermal method using Na2WO4 and Cu(NO3)2.3H2O at 170 °C for 20 h followed by annealing at 500 °C for 3 h [1], precipitation method [2], ultrasonic method [3,4], chemical precipitation method [5], hydrolysis method [6], hydrothermal method for synthesizing CuWO4 film at 180 °C for 8 h followed by annealing at 500 °C for 2 h using H2N10O41W12·xH2O and CuCl2·2H2O [19], microwave method assisted solid state method [20], hydrothermal method at 100 °C for 10 h followed by annealing at 500 °C for 2 h using Cu(NO3)2 and Na2WO4 [21], co-precipitation method [7], hydrothermal method at 110 °C for 2 h followed by annealing at 500 °C using Cu(O2CCH3)2.H2O and Na2WO4.2H2O as raw materials [15], chemical impregnation method [16], refluxing method [17], hydrothermal method at 180 °C for 18 h followed by annealing at 500 °C using CuCl2 and Na2WO4 as raw materials [22], thermochemical method [23], co-precipitation method [24], hydrothermal method at 180 °C for 28 h followed by annealing at 400 - 700 °C for 2h using Cu(NO3)2·3H2O and Na2WO4·2H2O as raw materials [25], electrochemical method [26, 27]. Besides, several methods have been reported for the synthesis of Ag3PO4 materials, including precipitation method using sodium stearate, AgNO3 and Na2HPO4.12H2O agitated for 1h [18], co-precipitation method using AgNO3 and Na2HPO4.12H2O agitated for 1h [8], precipitation method AgNO3 and Na2HPO4.12H2O agitated for 1h [9], precipitation method using AgNO3 and Na2HPO4.12H2O [10], precipitation method using NaH2PO4, Na2HPO4, Na3PO4 and AgNO3 [12], precipitation method using oleic acid, AgNO3 and H3PO4 [13], precipitation method using AgNO3 and Na2HPO4 [14], precipitation method using Ag2CO3 and Na2HPO4 [28], precipitation method using oleic acid, and AgNO3, Na2HPO4 [29].
Anthraquinone-based dyes are more resistant to biodegradation because of their fused aromatic structures compared to azo-based ones [30]. Also, they may cause acute toxicity or even mutagenic effects on aquatic organisms [31]. Therefore, anthraquinone reactive dyes have gradually attracted significant attention from the toxicological and environmental perspectives, particularly considering the current increase in their applications.
Reactive Blue 19 (RB-19) is a commercially representative anthraquinone reactive dye. Few past studies concerning ozonation of RB-19 solution were focused on decolorization efficiency and color removal kinetics [32]. Our previous work has investigated the transformation of RB-19 under different ozonation conditions in 10 min of reaction time and found that partial oxidation was obtained [33]. Several catalysts have been reported for the photodegradation of RB 19, including K2S2O8 [34], TiO2 [35], ozonation [36], and bacterial flora [37].
In the present work, the synthesis of CuWO4 via hydrothermal method at 180 °C for 18h followed by calcination at 500 °C for 2h using CuCl2.2H2O and Na2WO4.2H2O in the stoichiometric 1:1 Cu:W molar ratio as raw materials is reported. Besides, Ag3PO4 is prepared by a simple precipitation method at 80 °C for 1 h using Na2HPO4 and AgNO3 as raw materials. Also, several CuWO4-Ag3PO4 composites with different molar ratios are prepared in the present work. The obtained materials are characterized by the PXRD technique. Besides, the morphology of the obtained materials is studied by FESEM images. Also, the photocatalytic degradation of RB 19 was investigated under direct visible light irradiation without using H2O2 (Scheme 1). Failure to utilize oxidants such as ozone (O3) and Hydrogen peroxides (H2O2) in the photocatalytic degradation process will reduce the risks of using these materials due to high corrosion, reduce costs due to high prices of these materials and respect for all living things on the earth. Reducing the cost of initial preparation of the effluent to remove the paint and eliminating the cost of neutralizing it are another advantages. Effects of reaction parameters such as pH value of dye solution and catalyst amount on the photocatalytic efficiency are also studied.
Scheme 1. Photocatalytic degradation of RB 19.
2. Experimental
2.1. Materials and methods
All chemicals were of analytical grade, obtained from commercial sources, and used without further purification. Phase identifications were performed on a powder X-ray diffractometer D5000 (Siemens AG, Munich, Germany) using CuKα radiation. The morphology of the obtained materials was examined with a Philips XL30 scanning electron microscope (Philips, Amsterdam, Netherlands). Absorption spectra were recorded on an Analytik Jena Specord 40 (Analytik Jena AG Analytical Instrumentation, Jena, Germany). The concentration of reactive blue 19 was determined at 600 nm using a Shimadzu UV-visible1650 PC spectrophotometer. A BEL PHS-3BW pH-meter with a combined glass-Ag/AgCl electrode was used to adjust the solution pH.
2.2. Synthesis of CuWO4 nanomaterial
In a typical experiment, 2.045 g (0.02 mol) of CuCl2.2H2O was added into 25 mL deionized water in a 100 mL beaker under stirring until the salt was solvated. When the salt was dissolved, sodium citrate (0.29 g or 1.0 mmol) was added to the resultant solution under stirring. The obtained mixture was stirred continuously for 30 min. Then 10 mL deionized water was added to the obtained solution. Na2WO4.2H2O (6.788 g or 0.02 mol) was added to the solution, and 5 mL of deionized water was combined with the mixture. The mixture was stirred for 30 min and was transferred to a Teflon-lined autoclave. Then the autoclave was sealed and treated thermally at 180 ℃ for 18 h. After the desired reaction time, the autoclave was cooled generally to room temperature. Then the precipitate was filtered and washed with deionized water and ethanol, respectively. The final powder was dried at 90 ℃ for 3h. Finally, produced dried powder was annealed at 500 ℃ for 2h. The synthesis yield was 86% for CuWO4 nanomaterial ( S1 ).
2.3. Synthesis of Ag3PO4 nanomaterial
Na2HPO4 (0.3549 g or 2.5 mmol) and AgNO3 (0.84935g or 5 mmol) were dissolved in 50 mL of deionized water, separately in two beakers. The obtained solution was stirred for 10 min. Then Na2HPO4 solution was transferred into a burette. The AgNO3 was titrated with the Na2HPO4 solution (25 drops per min). The titrated solution was then heated at 80 ℃ for 1h. The obtained precipitate was filtered and washed with deionized water. The final powder was dried at 100 ℃ for 1h. The synthesis yield was 75% for Ag3PO4 nanomaterial (S2).
2.3. Synthesis of CuWO4-Ag3PO4 nanocomposites
A certain amount of CuWO4 ( 0.97 g or 3.10 mmol) (S3), (0.95 g or 3.05 mmol) (S4), and (0.9 g or 2.89 mmol) (S5) was added in an 80 mL of deionized water existent in the beaker and was stirred for 90 min. In another beaker containing 5 mL deionized water, a specific amount of Ag3PO4 ( 0.03 g or 0.07 mmol), (0.05 g or 0.12 mmol), and (0.1 g or 0.24 mmol) was added and stirred for 30 min. Afterward, the two beakers were mixed in each other, and the obtained solution was heated until it was boiled. Then the solution was refluxed for 90 min. The produced precipitate was filtered and washed three times with deionized water. The obtained powder was dried at 90 ℃ for 3 h.
3. Results and discussion
3.1. Characterization
The X-ray diffraction patterns of the CuWO4 (S1) (JCPDS No 88-0269) and Ag3PO4 (S2) (JCPDS No. 06-0505) are reported in Figure 1. According to the structural analysis of Ag3PO4 nanoparticles using x-ray diffraction, five strong peaks at 33.38 °, 36.74 °, 52.73 °, 55.064 °, and 57.3080 ° are observed, which are in accordance with the standard (JCPDS No. 06-0505) and Indicates the formation of Ag3PO4 structure. Similarly, for CuWO4 nanoparticles, five strong peaks of 27.72 °, 30.11 °, 31.58 °, 48.65 °, and 53.1940 ° are observed, which are in accordance with the standard (JCPDS No. 88-0269) and indicate the formation of CuWO4. In addition, the XRD patterns of the prepared composites (S3, S4, and S5) are reported in Figure 2. The results show that the patterns for S1 and S2 have a main CuWO4 crystal structure with space group P-1. Lattice parameters were found as a = 4.87 Å, b = 5.82 Å and c = 4.68 Å with 
[1-5]. Besides, the PXRD pattern of S3 shows that Ag3PO4 crystallizes in the cubic crystal system with space group P-43n. The lattice parameters were found as a = b = c = 5.99 Å [8-13].
Figure 1. PXRD patterns of a) S1 and b) S2.
Figure 2 shows the PXRD patterns of S3 to S5. It was found that the crystal phase growth decreased with increasing the Ag3PO4 ratio to CuWO4 in the composite material, and Figure 2 confirms the conclusion. Besides, it shows that when the weight percent of Ag3PO4 was increased from 3% (S3) to 5% (S4), the peak shifts toward the lower 2θ value. It shows an expansion in the crystal system, but when increasing the Ag3PO4 value to 10% (S5), the peak shifts toward the higher 2θ value. So there is a contraction in the unit cell.
Figure 2. PXRD patterns of a) S3, b) S4, and c) S5.
Figure 3 shows the FESEM images of S1–S5 nanomaterials. The image of CuWO4 nanomaterial showed that the morphology of the obtained material was a spherical particle with a highly homogeneous morphology (a). The FESEM image of the synthesized Ag3PO4 nanomaterial showed that the morphology of the target was particles with homogeneous morphology (b). Figure 3d-f shows the FESEM images of S3 – S5. The images show that the targets' morphology was changed with increasing the weight percent of Ag3PO4 in the composite mixture. It was found that the morphology of the target was highly homogeneous spherical particles when the weight percents were 3 and 5 %. However, the morphology of the target was changed to particles in which the targets' morphology was heterogeneous.
Figure 3. FESEM images of a) S1, b) S2, c) S3, d) S4, and e) S5.
Figure 4a – f shows the particle size distribution profiles of S1–S5, respectively. The data for the size distribution of CuWO4 showed that the maximum particle diameter is 80–90 nm. For Ag3PO4, it can be seen the high homogeneity of the particle size, and the maximum size range was 60–70 nm. Figure 4c – e shows the data for the composite nanomaterials. The data indicate that the maximum diameter sizes were about 80–90, 60–70, and 60–70 nm for S3, S4, and S5, respectively. The average particle sizes were decreased with increasing Ag3PO4 weight percent in the composite material.
Figure 4. Particle size distribution profiles of a) S1, b) S2, c) S3, d) S4, and e) S5.
we calculated the mean size of crystalline domains with the Scherrer equation (
) and compared with this parameter from FE-SEM. The results show that the mean-size particles of larger crystals form nanometer-sized particles with larger sizes and the results are consistent with each other. According to XRD patterns data belong to CuWO4 and Ag3PO4, maximum FWHM occurs in 2θ=96.72, and 0.72 intensity and 2θ=65.72 and 0.7085 results are related to the average size of its crystals have been achieved 17.33 nm and 13.89 respectively. Mean-size by FE-SEM for CuWO4 and Ag3PO4 nanomaterials 80-90 nm and 60-70 nm have been identified, respectively.
3.2. Photocatalytic activity
The synthesized samples' photocatalytic activity was investigated for the degradation of RB 19 (Anthraquinone dye) under direct visible light irradiation. A light filter was used to prevent wavelengths below 400 nm. In order to prepare a 30 ppm RB 19 dye solution, 0.03 g of RB 19 powder was dissolved in 1000 mL of deionized water. The pH value of the dye solution was adjusted to the desired amount using 1M of HCl or NaOH solutions. In a typical photocatalytic process under the visible light irradiation, a certain amount (g) of the synthesized sample was added into 100 mL of the as-prepared solution and sonicated for 10 min in a dark room to establish an adsorption/desorption equilibrium between RB 19 molecules and the surface of the photocatalyst followed by further magnetic stirring (250 rpm) under direct visible light irradiation. The solution was drawn out at a certain time, and the photocatalyst was separated by centrifugation to measure the absorption spectra of RB 19 and calculate the RB 19 concentration using UV-Vis spectrophotometry. The following formula calculated the photodegradation yield (%) of RB 19:
That in this equation, A0 and At represent the initial absorbance of RB 19 at 602 nm and the absorbance at time t, respectively.
In order to study the degradation of RB 19 in a dark room under the typical catalytic degradation process, some tests were done. Figures 5a and 5b indicate the degradation of RB 19 using the various synthesized samples in different pH values and under dark conditions, respectively. The data in figure 5a indicates that there was no considerable degradation in the dark-room. Also, Figure 5b reveals that by changing the pH value from acidic to basic condition, the degradation of RB 19 decreased. However, the degradation percent was still low than under the dark condition. So, it can be concluded the degradation yield under dark-room for the samples in any solution pH was not considerable.
Figure 5. Plots of the photodegradation of RB 19 at the dark-room (a) the degradation efficiency for different nanocomposites (Reaction condition: pH= 6, 100 mL of 30 ppm dye solution and 0.05 g of catalyst) and (b) the effect of solution pH value on the degradation yield for the reference composite s5 (Reaction condition: 100 mL of 30 ppm dye solution and 0.05 g of catalyst).
The results for photocatalytic degradation of RB 19 under direct visible light irradiation shows that the degradation yield of the as-prepared nanomaterials was increased by increasing Ag3PO4 weight percent in the nanocomposite mixture (Figure 6a). It is found that under direct visible light irradiation when the catalyst weight in the reaction mixture is increased, the considerably higher degradation yield of RB 19 can be achieved. It can be concluded that the surface area that exists for the adsorb dye onto the catalyst surface is increased, and so the degradation in a certain time is done faster. However, it is clear that 0.03 g of catalyst in the reaction mixture can be the optimum value.
Figure 6. Plots of the photodegradation of RB 19 under direct visible light irradiation(a)degradation yield for the synthesized nanocomposites (Reaction condition: 0.03 g of catalyst, pH=6, 100 mL of 30 ppm dye solution) and b) effect of catalyst amount on the degradation yield for the reference composite s5 (Reaction condition: pH=3, 100 mL of 30 ppm dye solution).
Figure 7a and 7b presents the effect of the solution pH value on the degradation yield under the visible light irradiation. Figure 7a indicates that when the pH value was in the acidic range, the degradation yield increased for the samples in which the weight percent of Ag3PO4 was more. The observation indicates that existing H+ in the solution activated the oxidizing agents, so the degradation was done faster. Figure 7b shows that when the solution pH value was in the basic range, the degradation yield of all of the samples decreased considerably. Besides, we had mentioned in Figure 6a that when the pH value of the solution was in the neutral range, the degradation yield is low. The acidic medium increases the performance of the photo-degradation reaction because of change in the surface charge of the nanoparticles. Indeed, this surface charge leads to better dispersion of the nanoparticles in the suspension. As a result, the reaction between the dissolved oxygen and the organic compound will improve and the rate of free radical generation boosts accordingly. However, it is clear that the yield is more for the sample in which the weight percent of Ag3PO4 is more (S4). So, we can conclude that the pH=3 is the optimum value.
Figure 7. The efficiency of degradation yield of the as-synthesized nanomaterials when (a) the pH value is 3 and (b) the pH value is 10 (Reaction condition: 0.03 g of catalyst, 100 mL of 30 ppm dye solution).
Figure 8a and 8b presents the UV-Vis spectra and calibration diagram of the as-prepared RB 19 dye solution. The data reveal that there is no sensitive error in the preparation process of the dye solutions.
Figure 8. Plots of a) UV-Vis spectra of RB 19 solutions and b) calibration curve of the as-prepared solutions.
4. Conclusion
This study described the hydrothermal and precipitation syntheses of highly crystalline CuWO4, Ag3PO4, and CuWO4-Ag3PO4 nanomaterials. Considering the importance of removing rhodamine B dye from effluents and the problems caused by their presence in water sources, it is necessary to create methods to remove this dye type. The PXRD patterns indicated that the targets were crystallized well in triclinic (CuWO4) and cubic (Ag3PO4) crystal systems. The patterns showed that the unit cells were expanded by increasing the Ag3PO4 weight ratio in the composite mixture. The particle size distribution data showed that the diameter size ranges were decreased considerably by increasing the Ag3PO4 weight percent in the synthesized nanocomposite. FESEM images showed that Ag3PO4 weight percent for the synthesis of composite materials has the main effect on synthesizing CuWO4-Ag3PO4 nanocomposites. The RB 19 photocatalytic degradation data revealed that a 0.05 g nanocomposite containing 0.24 mmole of Ag3PO4, with an initial pH of 3 and Rhodamine B volume and concentration of 100 mL and 30 mg/L, respectively, could achieve removal of 100 % after 30 min under visible light illumination without using H2O2. One of the advantages of this work is performing a photocatalytic process without H2O2, which reduces the cost of initial preparation of the effluent to remove the dye and, after removal, does not include the cost of neutralizing it and this work could lead to further extensive research on the subject.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest regarding the publication of this manuscript.
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