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

Authors: Thi Anh Tuyet Pham; Van Anh Tran; Van Duong Le; Minh Viet Nguyen; Duc Duc Truong; Xuan Truong Do; Anh-Tuan Vu
Journal: International Journal of Photoenergy
 (2020)

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## 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 (Eg) 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.


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## Body

## 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-x, in which x 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 2θ 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 (0.45μm 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:(1)Degradationefficiency%=C0−CtC0×100,(2)Degradationcapacitymgg=C0−Ct×Vm.The degradation rate of BPA was determined by fitting the degradation profile with the first-order kinetic model, as presented by the following equation:(3)lnC0Ct=kap×t,wherekap (min-1) is the rate constant, C0 is the initial concentration of dye, Ct is the concentration of BPA in time (mg/L), V is the volume of BPA solution (L), m is the mass of the catalyst (g), and t is the reaction time (min).


## 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-x, in which x 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 2θ 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 (0.45μm 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:(1)Degradationefficiency%=C0−CtC0×100,(2)Degradationcapacitymgg=C0−Ct×Vm.The degradation rate of BPA was determined by fitting the degradation profile with the first-order kinetic model, as presented by the following equation:(3)lnC0Ct=kap×t,wherekap (min-1) is the rate constant, C0 is the initial concentration of dye, Ct is the concentration of BPA in time (mg/L), V is the volume of BPA solution (L), m is the mass of the catalyst (g), and t is the reaction time (min).


## 3. Results and Discussion

### 3.1. Characterization of the Catalyst

Figure1 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.Figure 1
(a, b) SEM images of nano ZnO with the different scale bars; (c, d) SEM images with the different scale bars of the Ag/ZnO-10 nanocomposite; (e, f) element mapping images of Zn and Ag, respectively; (g, h) EDS layered image and EDS spectrum, respectively; (i, j) TEM images with the different scale bars; (k) HR-TEM image of the Ag/ZnO-10 nanocomposite.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)Figure2 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.Figure 2
XRD patterns of ZnO and Ag/ZnO-10 nanocomposite.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:(4)D=Kλβcosθ,whereK is a dimension shape factor, a typical value of about 0.9, but varies with the actual shape of the crystallite; λ is the wavelength of X-ray used (1.5406 Å), β is the full width at half maximum intensity (FWHM) in radians, and θ 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.Table 1
Lattice parameters and crystallite sizes of ZnO and Ag/ZnO-10.
SampleCrystal systemSpace groupCrystallite size (nm)Lattice parameter,a (Å)Lattice parameter,c (Å)Cell volume (Å3)ZnOHexagonalP63 mc20.13.251±0.0055.212±0.00247.704Ag/ZnO-10HexagonalP63 mc19.63.253±0.0075.215±0.00347.790The change in the lattice parameters (a and c) from 3.251±0.005Å and 5.212±0.002Å for pure ZnO to 3.253±0.007Å and 5.215±0.003Å 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.Figure3 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 P/Po=1, 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.Figure 3
(a) N2 adsorption/desorption isotherms and (b) pore size distributions of ZnO and Ag/ZnO-10 nanocomposite.
(a)(b)Table 2
The textural properties of ZnO and Ag/ZnO-10 composite.
SamplesSBET (m2/g)BJH pore volume (cm3/g)BJH average pore size (nm)DFT average pore size (nm)ZnO26.20.4826.930.1Ag/ZnO-1024.20.2121.125.2Figure4 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 4
FTIR spectra of the ZnO and Ag/ZnO-10 nanocomposite.Figure5 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.Figure 5
(a) UV-Vis diffuse reflectance spectra and (b) Tauc’s plot of the as-synthesized samples.
(a)(b)


### 3.2. Photodegradation of BPA

#### 3.2.1. Effect of Ag Doping on Photodegradation of BPA

Figure6 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.Figure 6
(a) Photodegradation of BPA in ZnO and Ag/ZnO and (b) the kinetic curves (the reaction conditions: dosage catalyst 0.5 g/L, BPA concentration of 20 mg/L, andpH=6.0).
(a)(b)The photocatalytic mechanism of BPA on Ag/ZnO composite under visible light irradiation is presented in Figure7. 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.Figure 7
Photocatalytic mechanism of BPA in Ag/ZnO nanocomposite under visible light.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 Figure7(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:(5)ZnO+hν→ZnOe−+h+(6)ZnOe−+Ag→ZnO+Age−(7)Age−+O2→⋅O2−+Ag(8)ZnOh++H2O→ZnO+H++⋅OH(9)⋅O2−+⋅OH+BPA→Intermediateproducts(10)⋅O2−+⋅OH+Intermediateproducts→CO2+H2OOn 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 andpH=6.0. 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 (R2) 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)).Figure 8
(a) Effect of dosage catalyst on photodegradation of BPA, (b) the kinetic curves, and (c) degradation capacity versus dosage catalyst. The reaction conditions are as follows: BPA concentration of 20 mg/L andpH=6.0.
(a)(b)(c)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 Figure9. 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. R2 values were in the range of 0.89-0.99.Figure 9
(a) Effect of concentration of BPA on photocatalytic activity, (b) the kinetic curves, and (c) degradation capacity versus concentration of BPA. The reaction conditions are as follows: dosage catalyst of 0.05 g andpH=6.0.
(a)(b)(c)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).Figure10 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 pH=9.0 and 11.0, respectively. And R2 values were in the range of 0.85-0.97.Figure 10
(a) Effect of pH solution on the degradation of BPA, (b) the kinetic curves, (c) degradation capacity versus pH solution, and (d) the pHPZC determination of Ag/ZnO. The reaction conditions are as follows: dosage catalyst of 0.05 g and BPA concentration of 20 mg/L.
(a)(b)(c)(d)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)).(11)ZnOs+2H+aq→Zn2++H2O(12)ZnOs+2OH−aq→ZnO2−+H2OOn the other hand, the surface charge property of nanoparticles, which depends on the pH point of zero charge (pHPZC), is also the reason affecting the adsorption properties of dye molecules on the surface of photocatalyst. The pHPZC 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 Figure11, 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.Figure 11
Degradation of the different persistent organic compounds in Ag/ZnO-10 composite. The reaction conditions are as follows: dosage catalyst of 0.05 g, BPA concentration of 20 mg/L, andpH=6.0.
(a)(b)Table3 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.Table 3
Comparison of photocatalytic degradation of BPA by various catalysts.
CatalystReaction conditionsObservationReference6.3% H3PW12O40/TiO2 composite filmBPA=5mg/L, pH=8.2, under light irradiation of Xenon lamp 300 W100% of BPA was removed in 240 min.[58]2% Ce-ZnOCat.=1g/L, BPA=50mg/L, pH=7.3, H2O2=5.10−3M, under UV light irradiation67% of BPA was removed in 180 min.[59]GO@BiOI/Bi2WO6Cat.=0.5g/L, BPA=10mg/L, pH=7, under Xenon illumination (500 W)81% of BPA was removed in 5 h.[30]4% C–ZnOCat.=1g/L, BPA=50mg/L, pH=8, H2O2=5.10−3M, under UV lamp 30 W100% of BPA was removed in 24 h.[60]1.5% Pd/mpg-C3N4 compositeCat.=0.5g/L, BPA=20mg/L, under Xenon 350 W, in a wide range of pH (3.1-11.0)Almost 100% of BPA was removed in 360 min.[61]ZnO nanoparticlesCat.=0.5g/L, BPA=20mg/L, pH=6.0, under visible light irradiation32.7% of BPA was removed in 120 min, degradation capacity of 13.08 mg/g.This workAg/ZnO-10 compositeCat.=0.5g/L, BPA=20mg/L, pH=6.0, under visible light irradiation100% of BPA was removed in 120 min, degradation capacity of 40 mg/g.This work


## 3.1. Characterization of the Catalyst

Figure1 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.Figure 1
(a, b) SEM images of nano ZnO with the different scale bars; (c, d) SEM images with the different scale bars of the Ag/ZnO-10 nanocomposite; (e, f) element mapping images of Zn and Ag, respectively; (g, h) EDS layered image and EDS spectrum, respectively; (i, j) TEM images with the different scale bars; (k) HR-TEM image of the Ag/ZnO-10 nanocomposite.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)Figure2 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.Figure 2
XRD patterns of ZnO and Ag/ZnO-10 nanocomposite.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:(4)D=Kλβcosθ,whereK is a dimension shape factor, a typical value of about 0.9, but varies with the actual shape of the crystallite; λ is the wavelength of X-ray used (1.5406 Å), β is the full width at half maximum intensity (FWHM) in radians, and θ 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.Table 1
Lattice parameters and crystallite sizes of ZnO and Ag/ZnO-10.
SampleCrystal systemSpace groupCrystallite size (nm)Lattice parameter,a (Å)Lattice parameter,c (Å)Cell volume (Å3)ZnOHexagonalP63 mc20.13.251±0.0055.212±0.00247.704Ag/ZnO-10HexagonalP63 mc19.63.253±0.0075.215±0.00347.790The change in the lattice parameters (a and c) from 3.251±0.005Å and 5.212±0.002Å for pure ZnO to 3.253±0.007Å and 5.215±0.003Å 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.Figure3 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 P/Po=1, 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.Figure 3
(a) N2 adsorption/desorption isotherms and (b) pore size distributions of ZnO and Ag/ZnO-10 nanocomposite.
(a)(b)Table 2
The textural properties of ZnO and Ag/ZnO-10 composite.
SamplesSBET (m2/g)BJH pore volume (cm3/g)BJH average pore size (nm)DFT average pore size (nm)ZnO26.20.4826.930.1Ag/ZnO-1024.20.2121.125.2Figure4 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 4
FTIR spectra of the ZnO and Ag/ZnO-10 nanocomposite.Figure5 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.Figure 5
(a) UV-Vis diffuse reflectance spectra and (b) Tauc’s plot of the as-synthesized samples.
(a)(b)


## 3.2. Photodegradation of BPA

### 3.2.1. Effect of Ag Doping on Photodegradation of BPA

Figure6 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.Figure 6
(a) Photodegradation of BPA in ZnO and Ag/ZnO and (b) the kinetic curves (the reaction conditions: dosage catalyst 0.5 g/L, BPA concentration of 20 mg/L, andpH=6.0).
(a)(b)The photocatalytic mechanism of BPA on Ag/ZnO composite under visible light irradiation is presented in Figure7. 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.Figure 7
Photocatalytic mechanism of BPA in Ag/ZnO nanocomposite under visible light.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 Figure7(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:(5)ZnO+hν→ZnOe−+h+(6)ZnOe−+Ag→ZnO+Age−(7)Age−+O2→⋅O2−+Ag(8)ZnOh++H2O→ZnO+H++⋅OH(9)⋅O2−+⋅OH+BPA→Intermediateproducts(10)⋅O2−+⋅OH+Intermediateproducts→CO2+H2OOn 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 andpH=6.0. 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 (R2) 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)).Figure 8
(a) Effect of dosage catalyst on photodegradation of BPA, (b) the kinetic curves, and (c) degradation capacity versus dosage catalyst. The reaction conditions are as follows: BPA concentration of 20 mg/L andpH=6.0.
(a)(b)(c)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 Figure9. 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. R2 values were in the range of 0.89-0.99.Figure 9
(a) Effect of concentration of BPA on photocatalytic activity, (b) the kinetic curves, and (c) degradation capacity versus concentration of BPA. The reaction conditions are as follows: dosage catalyst of 0.05 g andpH=6.0.
(a)(b)(c)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).Figure10 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 pH=9.0 and 11.0, respectively. And R2 values were in the range of 0.85-0.97.Figure 10
(a) Effect of pH solution on the degradation of BPA, (b) the kinetic curves, (c) degradation capacity versus pH solution, and (d) the pHPZC determination of Ag/ZnO. The reaction conditions are as follows: dosage catalyst of 0.05 g and BPA concentration of 20 mg/L.
(a)(b)(c)(d)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)).(11)ZnOs+2H+aq→Zn2++H2O(12)ZnOs+2OH−aq→ZnO2−+H2OOn the other hand, the surface charge property of nanoparticles, which depends on the pH point of zero charge (pHPZC), is also the reason affecting the adsorption properties of dye molecules on the surface of photocatalyst. The pHPZC 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 Figure11, 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.Figure 11
Degradation of the different persistent organic compounds in Ag/ZnO-10 composite. The reaction conditions are as follows: dosage catalyst of 0.05 g, BPA concentration of 20 mg/L, andpH=6.0.
(a)(b)Table3 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.Table 3
Comparison of photocatalytic degradation of BPA by various catalysts.
CatalystReaction conditionsObservationReference6.3% H3PW12O40/TiO2 composite filmBPA=5mg/L, pH=8.2, under light irradiation of Xenon lamp 300 W100% of BPA was removed in 240 min.[58]2% Ce-ZnOCat.=1g/L, BPA=50mg/L, pH=7.3, H2O2=5.10−3M, under UV light irradiation67% of BPA was removed in 180 min.[59]GO@BiOI/Bi2WO6Cat.=0.5g/L, BPA=10mg/L, pH=7, under Xenon illumination (500 W)81% of BPA was removed in 5 h.[30]4% C–ZnOCat.=1g/L, BPA=50mg/L, pH=8, H2O2=5.10−3M, under UV lamp 30 W100% of BPA was removed in 24 h.[60]1.5% Pd/mpg-C3N4 compositeCat.=0.5g/L, BPA=20mg/L, under Xenon 350 W, in a wide range of pH (3.1-11.0)Almost 100% of BPA was removed in 360 min.[61]ZnO nanoparticlesCat.=0.5g/L, BPA=20mg/L, pH=6.0, under visible light irradiation32.7% of BPA was removed in 120 min, degradation capacity of 13.08 mg/g.This workAg/ZnO-10 compositeCat.=0.5g/L, BPA=20mg/L, pH=6.0, under visible light irradiation100% of BPA was removed in 120 min, degradation capacity of 40 mg/g.This work


## 3.2.1. Effect of Ag Doping on Photodegradation of BPA

Figure6 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.Figure 6
(a) Photodegradation of BPA in ZnO and Ag/ZnO and (b) the kinetic curves (the reaction conditions: dosage catalyst 0.5 g/L, BPA concentration of 20 mg/L, andpH=6.0).
(a)(b)The photocatalytic mechanism of BPA on Ag/ZnO composite under visible light irradiation is presented in Figure7. 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.Figure 7
Photocatalytic mechanism of BPA in Ag/ZnO nanocomposite under visible light.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 Figure7(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:(5)ZnO+hν→ZnOe−+h+(6)ZnOe−+Ag→ZnO+Age−(7)Age−+O2→⋅O2−+Ag(8)ZnOh++H2O→ZnO+H++⋅OH(9)⋅O2−+⋅OH+BPA→Intermediateproducts(10)⋅O2−+⋅OH+Intermediateproducts→CO2+H2OOn 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 andpH=6.0. 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 (R2) 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)).Figure 8
(a) Effect of dosage catalyst on photodegradation of BPA, (b) the kinetic curves, and (c) degradation capacity versus dosage catalyst. The reaction conditions are as follows: BPA concentration of 20 mg/L andpH=6.0.
(a)(b)(c)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 Figure9. 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. R2 values were in the range of 0.89-0.99.Figure 9
(a) Effect of concentration of BPA on photocatalytic activity, (b) the kinetic curves, and (c) degradation capacity versus concentration of BPA. The reaction conditions are as follows: dosage catalyst of 0.05 g andpH=6.0.
(a)(b)(c)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).Figure10 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 pH=9.0 and 11.0, respectively. And R2 values were in the range of 0.85-0.97.Figure 10
(a) Effect of pH solution on the degradation of BPA, (b) the kinetic curves, (c) degradation capacity versus pH solution, and (d) the pHPZC determination of Ag/ZnO. The reaction conditions are as follows: dosage catalyst of 0.05 g and BPA concentration of 20 mg/L.
(a)(b)(c)(d)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)).(11)ZnOs+2H+aq→Zn2++H2O(12)ZnOs+2OH−aq→ZnO2−+H2OOn the other hand, the surface charge property of nanoparticles, which depends on the pH point of zero charge (pHPZC), is also the reason affecting the adsorption properties of dye molecules on the surface of photocatalyst. The pHPZC 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 Figure11, 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.Figure 11
Degradation of the different persistent organic compounds in Ag/ZnO-10 composite. The reaction conditions are as follows: dosage catalyst of 0.05 g, BPA concentration of 20 mg/L, andpH=6.0.
(a)(b)Table3 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.Table 3
Comparison of photocatalytic degradation of BPA by various catalysts.
CatalystReaction conditionsObservationReference6.3% H3PW12O40/TiO2 composite filmBPA=5mg/L, pH=8.2, under light irradiation of Xenon lamp 300 W100% of BPA was removed in 240 min.[58]2% Ce-ZnOCat.=1g/L, BPA=50mg/L, pH=7.3, H2O2=5.10−3M, under UV light irradiation67% of BPA was removed in 180 min.[59]GO@BiOI/Bi2WO6Cat.=0.5g/L, BPA=10mg/L, pH=7, under Xenon illumination (500 W)81% of BPA was removed in 5 h.[30]4% C–ZnOCat.=1g/L, BPA=50mg/L, pH=8, H2O2=5.10−3M, under UV lamp 30 W100% of BPA was removed in 24 h.[60]1.5% Pd/mpg-C3N4 compositeCat.=0.5g/L, BPA=20mg/L, under Xenon 350 W, in a wide range of pH (3.1-11.0)Almost 100% of BPA was removed in 360 min.[61]ZnO nanoparticlesCat.=0.5g/L, BPA=20mg/L, pH=6.0, under visible light irradiation32.7% of BPA was removed in 120 min, degradation capacity of 13.08 mg/g.This workAg/ZnO-10 compositeCat.=0.5g/L, BPA=20mg/L, pH=6.0, under visible light irradiation100% of BPA was removed in 120 min, degradation capacity of 40 mg/g.This work


## 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 pH=6.0, 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.



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