Facile Preparation of ZnO Nanoparticles and Ag/ZnO Nanocomposite and Their Photocatalytic Activities under Visible Light

Pham, Thi Anh Tuyet; Tran, Van Anh; Le, Van Duong; Nguyen, Minh Viet; Truong, Duc Duc; Do, Xuan Truong; Vu, Anh-Tuan

doi:https://doi.org/10.1155/2020/8897667

International Journal of Photoenergy

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

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Photocatalytic Nanomaterials for Energy Conversion and Storage

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Research Article | Open Access

Volume 2020 | Article ID 8897667 | https://doi.org/10.1155/2020/8897667

Facile Preparation of ZnO Nanoparticles and Ag/ZnO Nanocomposite and Their Photocatalytic Activities under Visible Light

Thi Anh Tuyet Pham,¹Van Anh Tran,¹Van Duong Le,¹Minh Viet Nguyen,²Duc Duc Truong,¹Xuan Truong Do,¹and Anh-Tuan Vu¹

Academic Editor: Hao Li

Received11 Aug 2020

Revised10 Sept 2020

Accepted07 Oct 2020

Published22 Oct 2020

Abstract

Zinc oxide (ZnO) has been known as an excellent photocatalyst for the degradation of a variety of organic pollutants under UV irradiation. This work describes a synthesis of ZnO nanoparticles via a facile precipitation method, and Ag was doped into Ag/ZnO nanocomposite to improve the photocatalytic degradation of BPA under visible light irradiation. The obtained ZnO nanoparticles were 20 nm in size and had a relatively high surface area and pore volume, 26.2 m²/g and 0.48 cm³/g, respectively. The deposition of Ag led to a decrease in the surface area, pore volume, and band gap energy () of ZnO nanoparticles. However, the photocatalytic activity of Ag/ZnO composite in the case increased. The performance of ZnO was compared with Ag/ZnO composites at the different molar ratios, and the kinetic reaction of BPA in these catalysts was investigated by the first-order kinetic model. The sample of Ag/ZnO-10 composite had the highest catalytic activity and showed the degradation efficiency, reaction rate, and degradation capacity of 100% in 120 min, 0.014 min^-1, and 40 mg/g, respectively. In comparison, the effects of Ag/ZnO molar ratio, catalyst dosage, solution pH, and concentration of BPA on photocatalytic degradation were investigated. Additionally, the photocatalytic performance of Ag/ZnO-10 composite was evaluated by the degradation of other persistent organic compounds such as phenol, tartrazine, and methylene blue and compared to other catalysts in literature.

1. Introduction

Today, significant development of inorganic nanoparticles has been their potential application in the environment, biology, electronics, optics, transport, and information technology [1]. That is reflected in the exponential increase in the number of scientific reports, patents, and companies related to science and nanotechnology, in which semiconductor technology from inorganic nanoparticles as the photocatalysts for environmental remediation has been widely studied. Zinc oxide (ZnO) is one of the commonly used semiconductors, with a wide band gap at room temperature (3.27 eV), the linear electronic displacement, and a large exciton binding energy (around 60 meV) [2]. In comparison to other oxide semiconductors, ZnO has many superior properties such as light sensitive, thermal and chemically stable, low cost, nontoxic, and sustainable with hydrogen environment [3]. The ZnO-based materials have many applications in industries, for example, rubbers, ceramics, pharmaceuticals, agricultural servants, paints, cosmetics, and photocatalysts [4–10]. Besides the advantages of this material due to the high electron aperture rate and low charge separation efficiency [11], the pure ZnO often exhibits relatively low photocatalytic activity under visible light irradiation, and therefore, ZnO does not meet the requirements of a photocatalyst when applied to practical processes [3, 12, 13]. That is why the enhanced catalytic activity of ZnO in visible light by doping other elements is a great concern to scientists.

To this moment, the ZnO nanomaterials with different shapes and sizes have been prepared by many methods such as hydrothermal, emulsion, chemical vaporization, and combustion methods [14–17]. Each method has its advantages and limitations. The hydrothermal method encounters a difficulty to adjust the shape of the material, and the obtained ZnO is often the rode and plate shapes; in the emulsion methods usually toxic solvents are used; the chemical evaporation method has a low efficiency leading to the high cost of materials; the combustion method faces difficulty in adjusting the particle sizes [18]. Therefore, a facile method for producing ZnO having high production and small particle sizes, at the same time an improved photocatalytic activity, is a crucial aspect for practical applications.

BPA (2,2-bis(4-hydroxyphenyl)propane) is chosen to be the starting material for many synthetic plastics, mainly some polycarbonates and epoxy resins, as well as certain polysulfones and certain suitable materials because BPA-based plastics are strong and tough. It is used in many popular consumer goods, such as plastic bottles, sports equipment, lining of water pipes, CDs, and DVDs [19]. About 4 million tons of BPA chemicals are produced every year to produce polycarbonates, which makes BPA one of the most produced chemicals worldwide. BPA is suspected of being contaminated with food (soluble in food) and has negative health effects such as changes in immune function, cardiovascular disorders, cancer, and infertility [20–23]. It can be found in natural water as a consequence of untreated industrial effluent, and it is very dangerous in the aquatic ecosystem due to its role as an estrogen receptor agonist [24]. According to the current U.S. Environmental Protection Agency standard evaluation procedures, Bisphenol A was moderately to slightly toxic to the fish and invertebrates tested, with LC50 or EC50 values from 1.1 to 10 mg/L. These data did not trigger freshwater or saltwater chronic tests [22]. Therefore, it is imperative and urgent to find a method to remove BPA from contaminated water.

Over the past decades, the treatment of BPA in the environment has been studied using many different approaches. Various technologies have been tested to remove BPA from wastewater like adsorption [25, 26], reverse osmosis [27], mechanical method [28], advanced oxidation processes by Fenton system [29], and photodegradation [30–32]. However, due to its eco-friendly operations, which do not create byproducts in the process, photodegradation has been preferred and extensively used. When a photocatalyst absorbs light irradiation for generating charge carriers and shifting electrons from the valence band (VB) to the conduction band (CB), photodegradation reaction occurs on its active surface through the formation of ^⋅OH and ^⋅O₂^-, which rapidly breakdown target pollutants into the end products of CO₂ and H₂O [33].

There have been many studies on synthesizing ZnO-based materials to decompose toxic organic substances in wastewater [9, 34–37], in which ZnO/TiO₂ and ZnO/rGO-rGH showed the effective materials of degrading BPA under UV irradiation [38, 39]. However, the use of UV light can affect the ecosystem as well as it is much more expensive than using visible light. Thereby, it is essential to improve the visible light absorption of ZnO by loading noble metal (Au, Ag, Pt, and Ru) [40]. On the other hand, modifying ZnO with noble metal nanoparticles also has attracted attention due to promoting the separation of photogenerated charges in photocatalysts through the formation of Schottky barrier and improving the photocatalytic stability [41, 42]. Among the various noble metal-semiconductor photocatalysts, Ag/ZnO can generate an effective Schottky barrier at the junction interface, supporting electron capture, thereby increasing the separation efficiency of electron-hole pairs and boost visible light harvesting capability.

In this study, a facile precipitation method was used to synthesize ZnO nanomaterials, and then, Ag was doped into Ag/ZnO composite with the different molar ratios. As-synthesized samples were characterized by the X-ray powder diffraction (XRD), the field emission scanning electron microscopy (FE-SEM), the transmission electron microscopy (TEM), the Fourier transform infrared spectroscopy (FTIR), and the UV-Vis diffuse reflectance spectra (DR-UV-Vis). The performance of the catalyst was evaluated by degrading BPA in water under visible light irradiation. The effects of Ag/ZnO molar ratio, a catalyst dosage, concentration of BPA, and solution pH on photocatalytic degradation of BPA were investigated. Ag was doped to enhance the photocatalytic performance of Ag/ZnO composite with the degradation of BPA. The effects of the Ag content, a dosage catalyst, a BPA concentration, and pH solution on degradation efficiency of BPA under visible light irradiation were investigated. The optimal reaction conditions and the photocatalytic performance of Ag/ZnO-10 composite were evaluated by degradation of other persistent organic compounds such as phenol, tartrazine (TA), and methylene blue (MB).

2. Materials and Methods

2.1. Materials

Bisphenol A (99%) was purchased from Sigma-Aldrich. AgNO₃ (99.8%), Zn(NO₃)₂·6H₂O (99%), hexamethylenetetramine (C₆H₁₂N₄, 99%), trisodium citrate (C₆H₅NaO₇S·2H₂O, 99%), and sodium borohydride (NaBH₄, 98%) were obtained from Merck. All reagents were used without any further purification. Distilled water was used throughout all experiments.

2.2. Preparation of ZnO

Nano ZnO modified from the precipitation method in the previous report [3]. Typically, 0.02 mol of Zn(NO₃)₂·6H₂O, 0.02 mol of hexamethylenetetramine, and 0.01 mol of trisodium citrate were dissolved in 100 mL distilled water; the solution was stirred and heated up to 90°C and then was kept at this temperature for 1 h without stirring. The precipitate was filtered and washed several times with distilled water. The powder was dried at 80°C overnight and calcined at 400°C for 2 h at a heating rate of 1°C/min to obtain nano ZnO.

2.3. Preparation of Nanocomposite Ag/ZnO

Typically, 1 g of ZnO and 0.1052 g of NaBH₄ were dissolved in 50 mL of distilled water. An amount of AgNO₃ corresponding to a desired molar ratio of Ag/ZnO was added into the solution. In this study, molar ratios of 2, 5, 10, and 10 were selected to investigate the effect of Ag content on photodegradation of BPA. The mixture was stirred for 1 h at 60°C and then filtered and washed several times with distilled water. The grey powder obtained after drying overnight at 60°C was denoted as Ag/ZnO-, in which is the molar ratio of Ag/ZnO.

2.4. Characterization

The crystalline phase of samples was investigated by the X-ray powder diffraction. XRD patterns were obtained by using the Bruker D8 Ax XRD diffractometer (Germany) with Cu Kα irradiation (40 kV, 40 mA). The ranging from 20 to 80° was selected for analyzing the crystal structure. The morphology and size of the samples were observed by transmission electron microscopy (TEM, JEM-2010). The morphology and size of the samples were observed by field emission scanning electron microscopy (FE-SEM, JEOL-7600F). The chemical composition of the composite was determined by energy dispersive spectrometry (EDS: JEOL-7600F). The textural properties were measured via N₂ adsorption/desorption isotherms using a Micromeritics (Gemini VII analyzer). The Fourier transform infrared spectroscopy (FTIR, Madison, WI, USA) measurement was carried out to explore the changes in functional groups of Ag/ZnO by Nicolet IS50. UV-Vis diffuse reflectance spectra of the as-synthesized samples were measured on a UV-Vis-NIR spectrometer (Cary 500).

2.5. Photocatalytic Experiment

The photodegradation of BPA was performed in a Pyrex beaker using the as-synthesized sample as a photocatalyst under visible light (250 W Hg lamp). BPA (20 mg/L) solution was prepared in 100 mL distilled water and mixed with 50 mg of catalyst. The mixture was stirred at a constant rate of 200 rpm. At regular time intervals for 30 min, analysis samples were taken from the reaction suspension and then filtered by a syringe filter (0.45 μm PTFE membrane) to remove the catalyst. The BPA concentration of the filtrate was analyzed by a UV-Vis spectrophotometer (Agilent 8453) at the maximum absorbance wavelength of 277 nm. The degradation efficiency and degradation capacity of BPA were calculated by the following equations:

The degradation rate of BPA was determined by fitting the degradation profile with the first-order kinetic model, as presented by the following equation:

where (min^-1) is the rate constant, is the initial concentration of dye, is the concentration of BPA in time (mg/L), is the volume of BPA solution (L), is the mass of the catalyst (g), and is the reaction time (min).

3. Results and Discussion

3.1. Characterization of the Catalyst

Figure 1 shows the SEM, EDS, and TEM images of the ZnO and Ag/ZnO-10 nanocomposite. ZnO showed an aggregated morphology (Figure 1(a)) consisting of many small particles with approximately 20 nm in size (Figure 1(b)). When Ag was doped into the composite, the morphology of Ag/ZnO-10 at low magnification (Figure 1(c)) was like ZnO, but at the higher magnification, it showed more aggregate than ZnO, and the Ag/ZnO sheets were observed in Figure 1(d). The elemental maps for Ag and Zn shown in Figures 1(e) and 1(f) indicated a good dispersion of Ag in the composite; nanoparticles were observed with high dispersion in the EDS layered image (Figure 1(g)). The EDS spectrum in Figure 1(h) showed the Ag, Zn, and O elements at the contents of 8.0, 68.0, and 15.8%, respectively. The TEM images (Figures 1(i) and 1(j)) and HR-TEM image (Figure 1(k)) gave the additional evidence of the dispersion of Ag in composite; Ag was crystal particles with the particle size of about 10-20 nm in Figures 1(i)–1(k). This result was consistent with the obtained results from SEM and element mapping.

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Figure 2 shows the XRD patterns of the ZnO and Ag/ZnO-10 nanocomposite. The diffraction peaks corresponding to (100), (002), (101), (102), (110), (103), (112), (201), and (202) planes were observed for pure ZnO; these could be indexed hexagonal wurtzite structure of ZnO (JCPDS 36–1451) [3]. These peaks were also seen in the Ag/ZnO-10 composite. The diffraction peaks corresponding to (111) and (200) of Ag were observed at 38.2 and 44.7°, respectively (JCPDS 04–0783) [43–45]. Moreover, there were no characteristic peaks of impurity phases such as Zn, Zn(OH)₂, and Zn(OH)₂CO₃ which were observed indicating the high purity of ZnO and Ag/ZnO-10 samples.

To study the effect of silver doping on the average crystallite size, the mean crystal size was estimated from the Debye–Scherrer equation as follows:

where is a dimension shape factor, a typical value of about 0.9, but varies with the actual shape of the crystallite; is the wavelength of X-ray used (1.5406 Å), is the full width at half maximum intensity (FWHM) in radians, and is Bragg’s diffraction angle. In order to obtain more detailed structural analyses, the Rietveld refinement method was applied to XRD data. The Rietveld refinement analyses were carried out using the FullProf program. The analytical results are shown in Table 1.

The change in the lattice parameters ( and ) from and for pure ZnO to and for Ag/ZnO-10 sample is observed; it clearly indicated the increase in the lattice size upon doping of Ag leading to the increase of the unit cell volume from 47.704 Å³ for ZnO to 47.790 Å³ for Ag/ZnO-10 because the radius of Ag⁺ (114 pm) is larger than Zn²⁺ (74 pm) and substitution of Ag⁺ ions in the lattice is larger than Zn²⁺ ions [46, 47]. The crystalline sizes of ZnO and Ag/ZnO-10 samples were estimated as 20.1 and 19.6 nm, respectively. These are consistent with the obtained results from SEM and TEM in Figure 1.

Figure 3 presents the N₂ adsorption/desorption isotherms and pore size distributions of the ZnO and Ag/ZnO-10 nanocomposite. According to the IUPAC classification, N₂ adsorption/desorption isotherms were type IV with H₃ hysteresis loop (Figure 3(a)), but the hysteresis loop of Ag/ZnO-10 was much lower than that of pure ZnO. Moreover, the hysteresis loops of both samples approached , indicating the presence of mesoporous and macropore together in both samples. Figure 3(b) shows the pore with distributions, which was determined by the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm. Both samples showed a mixture of pore size, but it could be seen that most of the pore possessed width in the range of 5-50 nm. The BET surface area, pore volume, and BJH average pore size were of composite slightly decreased with the doping of silver. BET surface areas were 26.2 and 24.2 m²/g, pore volumes were 0.48 and 0.21 cm³/g, and BJH average pore sizes were 26.8 and 21.1 cm³/g for ZnO and Ag/ZnO-10, respectively. Although samples were prepared by facile precipitation method, the surface areas of ZnO nanoparticles and Ag/ZnO nanocomposite were relatively high as compared to other samples in previous publications [48, 49]. In comparison, the pore size distributions of samples were analyzed by density functional theory (DFT) method. The DFT average pore size of ZnO was larger than that of Ag/ZnO, but the average pore size from the DFT method was larger than that of the BJH method as seen in Table 2.

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Figure 4 presents the FTIR spectra of the ZnO and Ag/ZnO-10 nanocomposite. The broad band at 3429 cm^-1 in ZnO could be assigned to the O–H stretching vibration of water. This peak became weaker and shifts to lower wavenumber at 3406 cm^-1 indicating that the addition of silver into composite led to decreased absorption of water on the surface of the sample. The peaks at 1636, 1509, and 1404 cm^-1 corresponded to the C=O stretching modes and O=C=O bonds due to the adsorption of CO₂ on the surface of the ZnO and Ag/ZnO nanocomposite. The peak location at 470 cm^-1 was assigned to the vibration of Zn–O in the lattice of the hexagonal wurtzite phase of ZnO.

Figure 5 shows the UV-Vis diffuse reflectance spectra and Tauc’s plot of ZnO and Ag/ZnO composites. The adsorption of Ag/ZnO composites was shifted to the longer wavelength from ZnO with the increase of the molar ratio of Ag/ZnO. The adsorption edges of Ag/ZnO-2 and Ag/ZnO-5 composites were steeper than those of ZnO, but it was similar to Ag/ZnO-10. When the molar ratio of Ag/ZnO increased to 15, the adsorption of the composite was shifted to a shorter wavelength from Ag/ZnO-10, showing a similarity to the shape of Ag/ZnO-5 (Figure 5(a)). The hν values were plotted against (αhν)² and extended to calculate the band gap energy of as-synthesized samples by Tauc’s method [50]; the results are presented in Figure 5(b). The band gap energy of ZnO was 3.220 eV. This was lower than that of Ag/ZnO-2 composite (3.230 eV) but larger than that of other composites; the band gap energies were 3.205, 3.125, and 3.181 eV for Ag/ZnO-5, Ag/ZnO-10, and Ag/ZnO-15, respectively. These results indicated that the doping of Ag into composite at the higher molar ratio than 2 reduced the band gap energy; it could increase the photocatalytic activity of composite under visible light.

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3.2. Photodegradation of BPA

3.2.1. Effect of Ag Doping on Photodegradation of BPA

Figure 6 presents the effect of silver doping on photocatalytic degradation of BPA under visible light irradiation. The degradation efficiency in 120 min and reaction rate of BPA in pure ZnO were 32.7% and 0.003 min^-1, respectively. The degradation efficiency of BPA increased when silver doped in the composite. The degradation efficiency and reaction rate increased to 49.9% and 0.005 min^-1, respectively, although the band gap energy of Ag/ZnO-2 composite was larger than that of ZnO. At the higher molar of Ag/ZnO than 2, the degradation efficiency and reaction rate increased when the molar ratio increased; the degradation efficiencies were 79.9 to 100% and the reaction rates were 0.008 to 0.014 min^-1 for Ag/ZnO-5 and Ag/ZnO-10 composites, respectively. With a further increase in the molar ratio of Ag/ZnO, the degradation efficiency was decreased.

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The photocatalytic mechanism of BPA on Ag/ZnO composite under visible light irradiation is presented in Figure 7. When visible light was irradiated to the surface of ZnO, the photons can excite electrons in the valence band (VB) to be moved up to the conduct band (CB); the electron and hole pair (e^- and h⁺) is generated. They can interact with water and oxygen on the surface of ZnO to generate oxidizing agents including H₂O₂, ^⋅O₂⁻, and ^⋅OH, in which ^⋅O₂⁻ and ^⋅OH are the strong oxidizing agents; they can decompose the organic compound into CO₂ and H₂O₂. However, the time of electrons (e^-) lying on the conduction band is extremely short; the electron (e^-) almost intermediately releases the energy to return to the ground state to recombine with hole (h⁺) originally. This is the significant limitation of pure ZnO nanoparticles.

In the presence of Ag metal, the Schottky barrier is formed between Ag and ZnO due to the energy level of CB of ZnO which is higher than the Fermi level of Ag/ZnO leading to free electrons in CB that can transfer from ZnO to Ag as described by the red arrow shown in Figure 7(b), thereby preventing the recombination between electrons and holes on the surface of ZnO. The electron on Ag nanoparticles can produce the ^⋅O₂⁻ free radical, while the holes in the VB can react with H₂O₂ to produce ^⋅OH radical. These free radicals can degrade organic compounds to CO₂ and H₂O. These can be explained by the following equations:

On the other hand, under visible light irradiation, the pure ZnO exhibited weak photocatalytic activity for degradation of BPA due to the fact that ZnO absorbs weakly in the visible light region, while the Ag/ZnO composites showed significantly enhanced photocatalytic activity, because under visible light irradiation electrons can be formed in the metallic Ag nanoparticles through the strong SPR effect, leading to strong absorption light in the visible region. Nevertheless, the Schottky barrier formed at the metal-semiconductor interface can hinder the transfer of electrons from Ag nanoparticles to ZnO. Up to now, many studies have proven that electrons can overcome the Schottky barrier at the interface because of its strong electron oscillating collectively on the SPR excitation [51–53]. Therefore, electrons can transfer from Ag to the CB of ZnO as illustrated by the green arrow in Figure 7(b). Then, they are scavenged by adsorbed oxygen molecules to yield superoxide radical anions (^⋅O₂^-) to degrade BPA molecules. Meanwhile, the photogenerated holes could transfer to the photocatalyst surface and directly oxidize the organic pollutants, resulting in an improved photocatalytic activity in the visible light region.

3.2.2. Effect of Catalyst Dosage on the Degradation of BPA

To investigate the influence of a catalyst mount, the catalytic oxidation experiments were carried out by employing the Ag/ZnO-10 composite at the different dosages (0.025, 0.050, 0.075, and 0.100 g) under constant reaction conditions: BPA concentration of 20 mg/L and . The catalytic efficiency as a function of catalyst amount is showed in Figure 8. It could be seen that the degradation efficiency and reaction rate of BPA increased with an increase in the dosage catalyst. The degradation efficiencies were 9.24, 100, 96.08, and 100% and reaction rates were 0.007, 0.014, 0.020, and 0.022 min^-1 at the catalyst dosage of 0.025, 0.050, 0.075, and 0.100 g, respectively. And the correlation coefficient () values were in the range of 0.92-0.98 (Figure 8(b)). However, the degradation capacity decreased from 47.4 to 20 mg/g when the dosage catalyst increased from 0.025 to 0.100 g (Figure 8(c)).

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Since the increase of catalyst dosage leads to an increase in the number of active sites on the surface of catalysts, the density of catalyst particles in the area of illumination is improved [54]. However, the degradation capacity of BPA per gram of catalyst was decreased with an increase in the catalyst dosage, as shown in Figure 8(c), due to increasing the suspended catalysts in a solution. The short wave tail photons are not able to enter the reaction mixture and a decrease in visible light penetration increasing in scattering effect [55, 56]. Also, as more catalyst was added, each catalyst has less chance to contact with BPA molecules because of fast reaction as shown in Figure 8(a). As a result, the reaction performance and rate can be improved with increasing catalyst dosage, but degradation capacity became smaller.

3.2.3. Effect of BPA Concentration on Photodegradation

The effect of the concentration of BPA on the photocatalytic activity of the catalyst is presented in Figure 9. The degradation efficiency and reaction rate were decreased with the concentration of BPA. The degradation efficiency was achieved 100% in 90 min for the concentration of 10 mg/L, showing the reaction rate of 0.018 min^-1. The degradation efficiency at 20 mg/L was also 100%, but at a lower reaction rate of 0.014 min^-1. The degradation efficiency and reaction rate were 38.1% and 0.004 min^-1 at a high concentration of 30 mg/L, respectively. values were in the range of 0.89-0.99.

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The negative effect of increased amount of BPA on reaction can be interpreted by the following reasons. (1) The number of BPA molecules, adsorbed on the active sites of the catalyst surface, increases with the initial concentration of BPA. Therefore, the generation rate of O₂^⋅- and OH^⋅ radicals on the same active sites became decreased. (2) Alternatively, increasing the dye concentration leads to generating many intermediates form dye molecules along with the reaction and may compete with BPA molecules in the constant total active sites. Despite lowering the degradation efficiency and reaction rate of BPA with increasing the initial concentration of BPA, the degradation capacity at 20 mg/L of 40 mg/g was higher than that of other concentrations (Figure 9(c)).

3.2.4. Effect of Solution pH

The pH value of wastewater is an important factor for well-designed reaction processes. It has a great influence not only on the functional groups and surface charges of adsorbents but also on the structure and ionization degree of pollutant molecules [57]. In this study, the effect of initial solution pH on the degradation of BPA by the as-synthesized Ag/ZnO-10 composite was investigated. The performance was evaluated at a wide range of pH values from 3.0 to 11.0 under the fixed other conditions (dosage catalyst of 0.05 g and BPA concentration of 20 mg/L).

Figure 10 presents the degradation of BPA with time at different values. The pH gave a critical impact on the degradation performance of BPA. When pH increased from 3.0 to 6.0, the degradation efficiency significantly increased from 16.3 to 100%, showing an improved reaction rate from 0.001 to 0.014 min^-1 and enhanced degradation capacity from 6.25 to 40 mg/g (Figures 10(b) and 10(c)). At pH value higher than 6.0, the reaction performance was decreased, the degradation efficiencies were 63.4 and 73%, and the reaction rates were 0.006 and 0.009 min^-1 at and 11.0, respectively. And values were in the range of 0.85-0.97.

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It was observed that the optimum pH for efficient degradation of BPA on Ag/ZnO-10 composite is 6.0. At lower pH than 6.0, the degradation efficiency is low; it is due to the dissolution of ZnO in an acid medium (equation (11)). At higher pH than 6, the efficient degradation is also low due to the dissolution of ZnO in an alkaline medium (equation (12)).

On the other hand, the surface charge property of nanoparticles, which depends on the pH point of zero charge (pH_PZC), is also the reason affecting the adsorption properties of dye molecules on the surface of photocatalyst. The pH_PZC of Ag/ZnO measured by the pH drift method reached 7.4, as shown in Figure 10(d). Hence, at the pH of 6.0, the catalytic surface will adsorb efficiently the organic anion (BPA donated protons) and lead to enhanced significant degradation of BPA on the Ag/ZnO composite.

3.2.5. Degradation of Other Organic Compounds and Comparison of Degradation of BPA with Other Catalysts

The degradation efficiency of organic compounds by photocatalysts depends not on only catalyst properties, such as surface area, pore volume and pore size distribution, and composition, but also on the characteristics of organic compounds. Therefore, the optimum condition for the degradation of the certain organic compounds depends on the characteristics of catalysts and solutions. In this study, the photocatalytic performance of Ag/ZnO-10 composite was evaluated by other persistent organic compounds such as phenol, tartrazine (TA), and methylene blue (MB) at the reaction conditions as follows: dosage catalyst of 0.05 g, BPA concentration of 20 mg, and pH of 6.0. As shown in Figure 11, the reaction efficiency and reaction rate ascended gradually from phenol to BPA, MB, and TA. The Ag/ZnO-10 composite showed a highly photocatalytic degradation of TA, showing the degradation efficiency and reaction rate of 100% in 60 min and 0.045 min^-1.

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Table 3 shows a brief comparison of BPA removal efficiency by different catalysts through photocatalytic activity. It could be observed that the degradation efficiency and reaction rate of the Ag/ZnO-10 composite under visible light irradiation were much higher than those of ZnO when adding C or Ce under UV light irradiation. In comparison with other catalysts such as H₃PW₁₂O₄₀/TiO₂, GO@BiOI/Bi₂WO₆, and Pd/mpg-C₃N₄, which remove BPA under simulated solar light irradiation by Xenon lamp, the degradation rate of BPA on Ag/ZnO-10 was also faster. Therefore, the Ag/ZnO-10 composite is expected as a good and eco-friendly catalyst in the removal of BPA from wastewater.

4. Conclusion

ZnO nanoparticles and Ag/ZnO nanocomposite were successfully prepared by the facile method. ZnO nanoparticles were approximately 20 nm in size and could degrade BPA under visible light irradiation showing the degradation efficiency and the reaction rate of 32.7% and 0.003 min^-1, respectively. When Ag was doped into composites at the molar ratio of Ag/ZnO larger than 2, the band gap energy of composite was decreased, and the degradation efficiency and the reaction rate increase. In comparison with the surface area and pore volume of the composite, the increase in degradation efficiency of BPA in composite mainly depended on the rate of electrons transferring from VB to CB and the recombination of electrons, in which the band gap energy and Ag content played a decisive role. At the Ag/ZnO ratio of 10, the composite approached the highest photocatalytic performance showing the degradation efficiency, reaction rate, and degradation capacity of 100%, 0.014 min^-1, and 40 mg/g, respectively. The optimal reaction conditions for the removal of BPA under visible light irradiation of Ag/ZnO-10 were pH of 6.0, catalyst dosage of 0.1 g, and BPA concentration of 10 mg/L for considering of degradation efficiency, which were , catalyst dosage of 0.025 g, and BPA concentration of 20 mg/g for considering the degradation capacity. Moreover, the catalytic performance of Ag/ZnO-10 composite was tested by degrading other persistent organic compounds. The reaction efficiency and the reaction rate increased gradually from phenol to BPA, MB, and TA, showing the degradation efficiency and reaction rate of 100% in 60 min and 0.045 min^-1 for TA. These results indicated the highly potential application of composite in the removal of toxic organic compounds in wastewater.

Data Availability

The research data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there is no conflict of interest.

Acknowledgments

The authors are grateful for the financial support of Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.05-2018.333.

References

E. Mohagheghpour, F. Moztarzadeh, M. Rabiee et al., “Micro-Emulsion Synthesis, Surface Modification, and Photophysical Properties of Zn_1−x Mn_x S Nanocrystals for Biomolecular Recognitio,” IEEE Transactions on Nanobioscience, vol. 11, no. 4, pp. 317–323, 2012.
View at: Publisher Site | Google Scholar
C. B. Ong, L. Y. Ng, and A. W. Mohammad, “A review of ZnO nanoparticles as solar photocatalysts: synthesis, mechanisms and applications,” Renewable and Sustainable Energy Reviews, vol. 81, pp. 536–551, 2018.
View at: Publisher Site | Google Scholar
V. A. Tu and V. A. Tuan, “A facile and fast solution chemistry synthesis of porous ZnO nanoparticles for high efficiency photodegradation of tartrazine,” Vietnam Journal of Chemistry, vol. 56, no. 2, pp. 214–219, 2018.
View at: Publisher Site | Google Scholar
R. Suntako, “Effect of synthesized ZnO nanograins using a precipitation method for the enhanced cushion rubber properties,” Materials Letters, vol. 158, pp. 399–402, 2015.
View at: Publisher Site | Google Scholar
P. Pascariu and M. Homocianu, “ZnO-based ceramic nanofibers: preparation, properties and applications,” Ceramics International, vol. 45, no. 9, pp. 11158–11173, 2019.
View at: Publisher Site | Google Scholar
X. Yang, C. Zhang, A. Li, J. Wang, and X. Cai, “Red fluorescent ZnO nanoparticle grafted with polyglycerol and conjugated RGD peptide as drug delivery vehicles for efficient target cancer therapy,” Materials Science and Engineering: C, vol. 95, pp. 104–113, 2019.
View at: Publisher Site | Google Scholar
C. García-Gómez, S. García-Gutiérrez, A. Obrador, and M. D. Fernández, “Study of Zn availability, uptake, and effects on earthworms of zinc oxide nanoparticle versus bulk applied to two agricultural soils: acidic and calcareous,” Chemosphere, vol. 239, article 124814, 2020.
View at: Publisher Site | Google Scholar
M. Baudys, J. Krýsa, M. Zlámal, and A. Mills, “Weathering tests of photocatalytic facade paints containing ZnO and TiO₂,” Chemical Engineering Journal, vol. 261, pp. 83–87, 2015.
View at: Publisher Site | Google Scholar
M. A. Bhatti, A. A. Shah, K. F. Almani et al., “Efficient photo catalysts based on silver doped ZnO nanorods for the photo degradation of methyl orange,” Ceramics International, vol. 45, no. 17, pp. 23289–23297, 2019.
View at: Publisher Site | Google Scholar
V. Sogne, F. Meier, T. Klein, and C. Contado, “Investigation of zinc oxide particles in cosmetic products by means of centrifugal and asymmetrical flow field-flow fractionation,” Journal of Chromatography A, vol. 1515, pp. 196–208, 2017.
View at: Publisher Site | Google Scholar
P. Sánchez-Cid, C. Jaramillo-Páez, J. A. Navío, A. N. Martín-Gómez, and M. C. Hidalgo, “Coupling of Ag₂CO₃ to an optimized ZnO photocatalyst: advantages vs. disadvantages,” Journal of Photochemistry and Photobiology A: Chemistry, vol. 369, pp. 119–132, 2019.
View at: Publisher Site | Google Scholar
M. R. D. Khaki, M. S. Shafeeyan, A. A. A. Raman, and W. M. A. W. Daud, “Application of doped photocatalysts for organic pollutant degradation - a review,” Journal of Environmental Management, vol. 198, Part 2, pp. 78–94, 2017.
View at: Publisher Site | Google Scholar
N. Yu, H. Peng, L. Qiu et al., “New pectin-induced green fabrication of Ag@AgCl/ZnO nanocomposites for visible-light triggered antibacterial activity,” International Journal of Biological Macromolecules, vol. 141, pp. 207–217, 2019.
View at: Publisher Site | Google Scholar
A. Stanković, S. Dimitrijević, and D. Uskoković, “Influence of size scale and morphology on antibacterial properties of ZnO powders hydrothemally synthesized using different surface stabilizing agents,” Colloids and Surfaces B: Biointerfaces, vol. 102, pp. 21–28, 2013.
View at: Publisher Site | Google Scholar
N. Neves, A. Lagoa, J. Calado et al., “Al-doped ZnO nanostructured powders by emulsion detonation synthesis – Improving materials for high quality sputtering targets manufacturing,” Journal of the European Ceramic Society, vol. 34, no. 10, pp. 2325–2338, 2014.
View at: Publisher Site | Google Scholar
B. P. Zhang, N. T. Binh, K. Wakatsuki et al., “Pressure-dependent ZnO nanocrsytal growth in a chemical vapor deposition process,” The Journal of Physical Chemistry B, vol. 108, no. 30, pp. 10899–10902, 2004.
View at: Publisher Site | Google Scholar
M. Park, S.-H. Lee, D. Kim, J. Kang, J.-Y. Lee, and S. M. Han, “Fabrication of a combustion-reacted high-performance ZnO electron transport layer with silver nanowire electrodes for organic solar cells,” ACS Applied Materials & Interfaces, vol. 10, no. 8, pp. 7214–7222, 2018.
View at: Publisher Site | Google Scholar
H. V. Vasei, S. M. Masoudpanah, M. Adeli, and M. R. Aboutalebi, “Solution combustion synthesis of ZnO powders using CTAB as fuel,” Ceramics International, vol. 44, no. 7, pp. 7741–7745, 2018.
View at: Publisher Site | Google Scholar
Y. Hu, Q. Zhu, X. Yan, C. Liao, and G. Jiang, “Occurrence, fate and risk assessment of BPA and its substituents in wastewater treatment plant: a review,” Environmental Research, vol. 178, article 108732, 2019.
View at: Publisher Site | Google Scholar
M. Clinic, “Nutrition and Healthy Eating,” in Healthy Lifestyle, B. A. Bauer, Ed., Mayo Clinic, 2019.
View at: Google Scholar
K. V. A. Kumar, B. Lakshminarayana, T. Vinodkumar, and C. Subrahmanyam, “Cu-ZnO for visible light induced mineralization of Bisphenol-A: impact of Cu ion doping,” Journal of Environmental Chemical Engineering, vol. 7, no. 3, article 103057, 2019.
View at: Publisher Site | Google Scholar
H. C. Alexander, D. C. Dill, L. W. Smith, P. D. Guiney, and P. Dorn, “Bisphenol a: acute aquatic toxicity,” Environmental Toxicology and Chemistry, vol. 7, no. 1, pp. 19–26, 1988.
View at: Publisher Site | Google Scholar
K. Pelch, J. A. Wignall, A. E. Goldstone et al., “A scoping review of the health and toxicological activity of bisphenol A (BPA) structural analogues and functional alternatives,” Toxicology, vol. 424, article 152235, 2019.
View at: Publisher Site | Google Scholar
M. F. Brugnera, K. Rajeshwar, J. C. Cardoso, and M. V. B. Zanoni, “Bisphenol A removal from wastewater using self-organized TIO₂ nanotubular array electrodes,” Chemosphere, vol. 78, no. 5, pp. 569–575, 2010.
View at: Publisher Site | Google Scholar
A. Bhatnagar and I. Anastopoulos, “Adsorptive removal of bisphenol A (BPA) from aqueous solution: a review,” Chemosphere, vol. 168, pp. 885–902, 2017.
View at: Publisher Site | Google Scholar
Y. Zhang, W. Cui, W. An, L. Liu, Y. Liang, and Y. Zhu, “Combination of photoelectrocatalysis and adsorption for removal of bisphenol A over TiO₂-graphene hydrogel with 3D network structure,” Applied Catalysis B: Environmental, vol. 221, pp. 36–46, 2018.
View at: Publisher Site | Google Scholar
G. Y. S. Chan, J. Chang, T. A. Kurniawan, C.-X. Fu, H. Jiang, and Y. Je, “Removal of non-biodegradable compounds from stabilized leachate using VSEPRO membrane filtration,” Desalination, vol. 202, no. 1-3, pp. 310–317, 2007.
View at: Publisher Site | Google Scholar
P. Wang, X. Zhou, Y. Zhang, L. Wang, K. Zhi, and Y. Jiang, “Synthesis and application of magnetic reduced graphene oxide composites for the removal of bisphenol A in aqueous solution—a mechanistic study,” RSC Advances, vol. 6, no. 104, pp. 102348–102358, 2016.
View at: Publisher Site | Google Scholar
M. Vilve, S. Vilhunen, M. Vepsäläinen et al., “Degradation of 1,2-dichloroethane from wash water of ion-exchange resin using Fenton’s oxidation,” Environmental Science and Pollution Research International, vol. 17, no. 4, pp. 875–884, 2010.
View at: Publisher Site | Google Scholar
Z. Mengting, T. Kurniawan, Y. Yanping, R. Avtar, and M. Othman, “2D graphene oxide (GO) doped p-n type BiOI/Bi₂WO₆ as a novel composite for photodegradation of bisphenol A (BPA) in aqueous solutions under UV-vis irradiation,” Materials Science and Engineering: C, vol. 108, article 110420, 2020.
View at: Publisher Site | Google Scholar
Q. Wang, C. Yang, G. Zhang, L. Hu, and P. Wang, “Photocatalytic Fe-doped TiO₂/PSF composite UF membranes: characterization and performance on BPA removal under visible-light irradiation,” Chemical Engineering Journal, vol. 319, pp. 39–47, 2017.
View at: Publisher Site | Google Scholar
H. Lan, G. Zhang, H. Zhang, H. Liu, R. Liu, and J. Qu, “Solvothermal synthesis of BiOI flower-like microspheres for efficient photocatalytic degradation of BPA under visible light irradiation,” Catalysis Communications, vol. 98, pp. 9–12, 2017.
View at: Publisher Site | Google Scholar
A. Sivakumar, B. Murugesan, A. Loganathan, and P. Sivakumar, “A review on decolourisation of dyes by photodegradation using various bismuth catalysts,” Journal of the Taiwan Institute of Chemical Engineers, vol. 45, no. 5, pp. 2300–2306, 2014.
View at: Publisher Site | Google Scholar
H. Xing, H. Ma, Y. Fu et al., “Preparation of g-C₃N₄/ZnO composites and their enhanced photocatalytic activity,” Materials Technology, vol. 30, no. 2, pp. 122–127, 2015.
View at: Publisher Site | Google Scholar
S. J. Yang, J. H. Im, T. Kim, K. Lee, and C. R. Park, “MOF-derived ZnO and ZnO@C composites with high photocatalytic activity and adsorption capacity,” Journal of Hazardous Materials, vol. 186, no. 1, pp. 376–382, 2011.
View at: Publisher Site | Google Scholar
T. Chang, Z. Li, G. Yun, Y. Jia, and H. Yang, “Enhanced photocatalytic activity of ZnO/CuO nanocomposites synthesized by hydrothermal method,” Nano-Micro Letters, vol. 5, no. 3, pp. 163–168, 2013.
View at: Publisher Site | Google Scholar
H. A. Kiwaan, T. M. Atwee, E. A. Azab, and A. A. El-Bindary, “Efficient photocatalytic degradation of Acid Red 57 using synthesized ZnO nanowires,” Journal of the Chinese Chemical Society, vol. 66, no. 1, pp. 89–98, 2019.
View at: Publisher Site | Google Scholar
S. Yang, P. Wu, M. Chen et al., “Enhanced photo-degradation of bisphenol a under simulated solar light irradiation by Zn–Ti mixed metal oxides loaded on graphene from aqueous media,” RSC Advances, vol. 6, no. 32, pp. 26495–26504, 2016.
View at: Publisher Site | Google Scholar
C. Liu, M. Yue, L. Liu, Y. Rui, and W. Cui, “A separation-free 3D network ZnO/rGO–rGH hydrogel: adsorption enriched photocatalysis for environmental applications,” RSC Advances, vol. 8, no. 40, pp. 22402–22410, 2018.
View at: Publisher Site | Google Scholar
S. S. Naik, S. J. Lee, T. Begildayeva, Y. Yu, H. Lee, and M. Y. Choi, “Pulsed laser synthesis of reduced graphene oxide supported ZnO/Au nanostructures in liquid with enhanced solar light photocatalytic activity,” Environmental Pollution, vol. 266, article 115247, 2020.
View at: Publisher Site | Google Scholar
S. Kuriakose, V. Choudhary, B. Satpati, and S. Mohapatra, “Enhanced photocatalytic activity of Ag-ZnO hybrid plasmonic nanostructures prepared by a facile wet chemical method,” Beilstein Journal of Nanotechnology, vol. 5, pp. 639–650, 2014.
View at: Publisher Site | Google Scholar
J. Lu, H. Wang, D. Peng, T. Chen, S. Dong, and Y. Chang, “Synthesis and properties of Au/ZnO nanorods as a plasmonic photocatalyst,” Physica E: Low-dimensional Systems and Nanostructures, vol. 78, pp. 41–48, 2016.
View at: Publisher Site | Google Scholar
E. B. Choi and J.-H. Lee, “Dewetting behavior of Ag in Ag-coated Cu particle with thick Ag shell,” Applied Surface Science, vol. 480, pp. 839–845, 2019.
View at: Publisher Site | Google Scholar
B. L. Martínez-Vargas, S. M. Durón-Torres, D. Bahena, J. L. Rodríguez-López, J. M. Peralta-Hernández, and A. Picos, “One-pot synthesis of ZnO–Ag and ZnO–Co nanohybrid materials for photocatalytic applications,” Journal of Physics and Chemistry of Solids, vol. 135, article 109120, 2019.
View at: Publisher Site | Google Scholar
R. Anugrahwidya, N. Yudasari, and D. Tahir, “Optical and structural investigation of synthesis ZnO/Ag nanoparticles prepared by laser ablation in liquid,” Materials Science in Semiconductor Processing, vol. 105, article 104712, 2020.
View at: Publisher Site | Google Scholar
P. Amornpitoksuk, S. Suwanboon, S. Sangkanu, A. Sukhoom, N. Muensit, and J. Baltrusaitis, “Synthesis, characterization, photocatalytic and antibacterial activities of Ag-doped ZnO powders modified with a diblock copolymer,” Powder Technology, vol. 219, pp. 158–164, 2012.
View at: Publisher Site | Google Scholar
Ö. A. Yıldırım, H. E. Unalan, and C. Durucan, “Highly efficient room temperature synthesis of silver-doped zinc oxide (ZnO:Ag) nanoparticles: structural, optical, and photocatalytic properties,” Journal of the American Ceramic Society, vol. 96, no. 3, pp. 766–773, 2013.
View at: Publisher Site | Google Scholar
A. C. Lucilha, M. R. . Silva, R. A. Ando, L. H. Dall'Antonia, and K. Takashima, “ZnO and Ag-ZnO crystals: synthesis, characterization, and application in heterogeneous photocatalysis,” Química Nova, vol. 39, no. 4, pp. 409–414, 2016.
View at: Publisher Site | Google Scholar
V. H. Tran Thi, T. H. Cao, T. N. Pham, T. T. Pham, and M. C. Le, “Synergistic adsorption and photocatalytic activity under visible irradiation using Ag-ZnO/GO nanoparticles derived at low temperature,” Journal of Chemistry, vol. 2019, Article ID 2979517, 13 pages, 2019.
View at: Publisher Site | Google Scholar
A. Kaur, G. Gupta, A. O. Ibhadon, D. B. Salunke, A. S. K. Sinha, and S. K. Kansal, “A facile synthesis of silver modified ZnO nanoplates for efficient removal of ofloxacin drug in aqueous phase under solar irradiation,” Journal of Environmental Chemical Engineering, vol. 6, no. 3, pp. 3621–3630, 2018.
View at: Publisher Site | Google Scholar
W. Li, F. Hua, J. Yue, and J. Li, “Ag@AgCl plasmon-induced sensitized ZnO particle for high-efficiency photocatalytic property under visible light,” Applied Surface Science, vol. 285, no. Part B, pp. 490–497, 2013.
View at: Publisher Site | Google Scholar
S.-i. Naya, T. Nikawa, K. Kimura, and H. Tada, “Rapid and complete removal of nonylphenol by gold nanoparticle/rutile titanium(IV) oxide plasmon photocatalyst,” ACS Catalysis, vol. 3, no. 5, pp. 903–907, 2013.
View at: Publisher Site | Google Scholar
H. Liu, Y. Hu, Z. Zhang, X. Liu, H. Jia, and B. Xu, “Synthesis of spherical Ag/ZnO heterostructural composites with excellent photocatalytic activity under visible light and UV irradiation,” Applied Surface Science, vol. 355, pp. 644–652, 2015.
View at: Publisher Site | Google Scholar
J. Y. C. Yang, Q. Li, and Y. Yu, “Facile synthesis of monodisperse porous ZnO nanospheres for organic pollutant degradation under simulated sunlight irradiation: the effect of operational parameters,” Materials Research Bulletin, vol. 87, pp. 72–83, 2017.
View at: Publisher Site | Google Scholar
M. Behnajady, N. Modirshahla, and R. Hamzavi, “Kinetic study on photocatalytic degradation of C.I. Acid Yellow 23 by ZnO photocatalyst,” Journal of Hazardous Materials, vol. 133, no. 1-3, pp. 226–232, 2006.
View at: Publisher Site | Google Scholar
A. Zyoud, A. Zu’bi, M. H. S. Helal, D. H. Park, G. Campet, and H. S. Hilal, “Optimizing photo-mineralization of aqueous methyl orange by nano-ZnO catalyst under simulated natural conditions,” Journal of Environmental Health Science & Engineering, vol. 13, no. 1, 2015.
View at: Publisher Site | Google Scholar
X. Yang, Y. Li, Q. Du et al., “Highly effective removal of basic fuchsin from aqueous solutions by anionic polyacrylamide/graphene oxide aerogels,” Journal of Colloid and Interface Science, vol. 453, pp. 107–114, 2015.
View at: Publisher Site | Google Scholar
N. Lu, Y. Lu, F. Liu et al., “H₃PW₁₂O₄₀/TiO₂ catalyst-induced photodegradation of bisphenol A (BPA): kinetics, toxicity and degradation pathways,” Chemosphere, vol. 91, no. 9, pp. 1266–1272, 2013.
View at: Publisher Site | Google Scholar
O. Bechambi, L. Jlaiel, W. Najjar, and S. Sayadi, “Photocatalytic degradation of bisphenol A in the presence of Ce–ZnO: evolution of kinetics, toxicity and photodegradation mechanism,” Materials Chemistry and Physics, vol. 173, pp. 95–105, 2016.
View at: Publisher Site | Google Scholar
O. Bechambi, S. Sayadi, and W. Najjar, “Photocatalytic degradation of bisphenol A in the presence of C-doped ZnO: effect of operational parameters and photodegradation mechanism,” Journal of Industrial and Engineering Chemistry, vol. 32, pp. 201–210, 2015.
View at: Publisher Site | Google Scholar
C. Chang, Y. Fu, M. Hu, C. Wang, G. Shan, and L. Zhu, “Photodegradation of bisphenol A by highly stable palladium-doped mesoporous graphite carbon nitride (Pd/mpg-C₃N₄) under simulated solar light irradiation,” Applied Catalysis B: Environmental, vol. 142-143, pp. 553–560, 2013.
View at: Publisher Site | Google Scholar

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