Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 24–33

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Template free synthesis of ZnO/Ag2O nanocomposites as a highly efficient visible active photocatalyst for detoxification of methyl orange Abhijit Kadam a, Rohant Dhabbe a, Anna Gophane b, Tukaram Sathe b, Kalyanrao Garadkar a,⁎ a b

Nanomaterials Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India Department of Zoology, Shivaji University, Kolhapur 416004, India

a r t i c l e

i n f o

Article history: Received 24 April 2015 Received in revised form 22 November 2015 Accepted 24 November 2015 Available online 02 December 2015 Keywords: Nanocomposites Optical properties Visible photocatalysis Comet assay

a b s t r a c t A simple and effective route for the synthesis of ZnO/Ag2O nanocomposites with different weight ratios (4:1 to 4:4) have been successfully obtained by combination of thermal decomposition and precipitation technique. The structure, composition, morphology and optical properties of the as-prepared ZnO/Ag2O composites were characterized by XRD, FT-IR, EDS, SEM, TEM, UV–Vis DRS and PL, respectively. The photocatalytic performance of the photocatalysts was evaluated towards the degradation of a methyl orange (MO) under UV and visible light. More specifically, the results showed that the photocatalytic activity with highest rate constant of MO degradation over ZnO/Ag2O (4:2) nanocomposites is more than 22 and 4 times than those of pure ZnO and Ag2O under visible light irradiation, respectively. An improved photocatalytic activity was attributed to the formation of heterostructure between Ag2O and ZnO, the strong visible light absorption and more separation efficiency of photoinduced electron–hole pairs. Moreover, the ZnO/Ag2O (4:2) nanocomposite showed excellent stability towards the photodegradation of MO under visible light. Finally, a possible mechanism for enhanced charge separation and photodegrdation is proposed. Genotoxicity of MO before and after photodegradation was also evaluated by simple comet assay technique. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Among the organic dyes, azo dyes are considered to be one of the largest group of pollutants that can be discharged into wastewater from textile and other industrial processes [1–2]. Industrial wastewater containing azo dyes causes serious problems to an aquatic environment due to their high toxicity, low biodegradability and potential carcinogenicity [3–4]. Nowadays, nanostructured semiconductor materials have been attracted much interest for the photodegradation of pollutants, solar energy conversion and gas sensing applications due to their unique properties [5]. In recent years, semiconductor-based photocatalysis is a kind of “green technology”, that has been received considerable attention since it represents an easy way to solve current energy and environmental crisis through the use of solar light [6]. Apart from the most commonly used TiO2, in particular, ZnO based nanocomposites have gained extreme importance due to their low cost. ZnO shows distinguishing advantages over TiO2 such as, tailoring of morphology, ease of crystallization, anisotropic growth, low cost, higher exciton binding energy (60 meV) and higher electron mobility (200 cm2 V−1 s−1). These are favorable for the enhancement of photocatalytic activity. It also absorb large fraction of solar spectrum than TiO2 [7–8]. However, in practical ⁎ Corresponding author. E-mail address: [email protected] (K. Garadkar).

http://dx.doi.org/10.1016/j.jphotobiol.2015.11.007 1011-1344/© 2015 Elsevier B.V. All rights reserved.

applications, the performance of ZnO is limited only in the UV region due to its large band gap [9]. Only about 4 to 5% of UV region comes under solar spectrum. A fast recombination rate of the photogenerated electron/hole pairs hinders the commercialization of ZnO nanomaterials [10–11]. Therefore, an effective use of visible light-responsive catalyst still remains a challenge for environmental cleaning applications. Hence, it is necessary to tune the photocatalytic activity of ZnO from UV to visible region with large charge carrier separation. In order to tune the optical absorption of ZnO into the visible region and an improvement of charge carrier separation, it is coupled with narrow band gap semiconductors such as Nb2O5/ZnO [12], Bi2O3/ZnO [13], Ag2O/ZnO [14] and CuO/ZnO [15]. Thus the coupled semiconductors have been actively studied to boost their photocatalytic activities into visible light [16]. Ag2O is a brown color p-type semiconductor that possesses a simple cubic structure has a band gap reported to be 1.2 eV with the energy level of the CB edge +0.2 eV (vs. SHE) [17]. It is also widely used in many industrial applications as cleaning agents, preservatives, colorants, electrode materials, photocatalysts for the environmental remediation and the organic transformation [18–19]. Ag2O is also found to be a stable and highly efficient photocatalyst under visible light [20–21]. The band-gap energy (1.2 eV) of Ag2O is close to an ideal value which is suitable for photocatalytic applications in the visible region [22]. All these properties of Ag2O play an important role for the formation of composite with ZnO nanorods for a better photocatalytic performance under visible light. Hence, the present investigation of coupling ZnO

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with Ag2O is undertaken. Here, ZnO nanorods are chosen as the host material due to one-dimensional nanostructure, which possesses a several advantages over the nanoparticles. The enhanced visible-light scattering and absorption, rapid diffusion-free electron transport along the long direction and the low number of grain boundaries are the advantages of such composites [12,23]. As-synthesized nanomaterials will be tested for the degradation of dyes. The toxicity assessment of photodegraded product is essential to apply photocatalysis in real life. Rapid and sensitive tools are needed for screening hazardous properties of dyes prior to intensive toxicological investigation and risk assessment [24]. Relative to other genotoxicity test, comet assay has several advantages which include sensitivity to detect low levels of DNA damage, the small number of cells per sample is enough, flexibility, low cost, ease application and the short time to complete the assessment [25]. The comet assay has been extensively used as a non-specific measure of genotoxic DNA damage in fish [26]. In most of the genotoxicity studies, fish has been considered as an ideal test organism to examine genotoxicity. It is likely that the low concentration of pollutant may not result in fish mortality, while it may be toxic to them. The comet assay is a simple method for measuring the damage of deoxyribonucleic acid (DNA) strand in eukaryotic cells. Therefore, the comet assay was used in the fish for the assessment of genotoxicity of the pollutant with greater accuracy. To the best of our knowledge we are the first to report on the genotoxicity of MO before and after degradation using the comet assay. This work will provide a simple strategy to design photocatalyst with higher efficiency and stability. The photocatalytic activity of ZnO/ Ag2O nanocomposites under UV and visible light is investigated towards the degradation of MO. It is envisaged that the present work will provide a template free ZnO/Ag2O nanocomposites for photocatalytic applications. The relationship between loading of Ag2O and the photocatalytic activities are discussed. The aim of the present research work is to develop highly efficient, visible active photocatalyst based on nanotechnology for detoxification of MO.

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2.3. Characterization of ZnO/Ag2O Nanocomposites XRD of as-synthesized ZnO, Ag2O, ZnO/Ag2O was recorded on a Panalytical diffractometer with CuKα radiation (λ = 1.5406 Å) in the range of 2θ, 10 to 80°. The surface morphology and composition of nanostructures were scanned by a scanning electron microscope (SEM, JEOL, JSM-6360) at various magnifications equipped with EDS. The shape and size of the particles were investigated using transmission electron microscopy with model TEM, JEOL 3010 (SAIF, Shilong). Fourier transform infrared spectra (FT-IR) of ZnO, Ag2O, ZnO/Ag2O were recorded on the Spectrum-one (Perkin Elmer) in the range of 4000 to 400 cm − 1 using KBr pellet. The UV–Vis. Diffuse reflectance spectra were recorded on a spectrophotometer (Varian Cary-5000 UV–Vis-NIR). Photoluminescence spectra of the samples were recorded on a spectrofluorometer (JASCO, Model F.P.750 Japan).

2.4. Evaluation of Photocatalytic Performance The photocatalytic activity of ZnO, Ag2O, and ZnO/Ag2O nanocomposites was evaluated for the degradation of MO under UV (365 nm) as well as visible light (520 nm) (Multi Lamp photoreactor: MLR-8, 9 W, along with power supply is purchased from Scientific Aids and Instruments Corporation (SAIC), Chennai, India.). Typically, in this experiment 100 mg photocatalyst was added in a photoreactor containing 100 mL of MO (20 ppm). Before irradiation, the dye solution was stirred for 30 min. in dark to ensure an adsorption-desorption equilibrium, then exposed to light. All photodegradation experiments were performed at ambient temperature. At a given time interval, aliquots were collected from photoreactor and then centrifuged to separate the particles of photocatalyst. The centrifuged solution was used to monitor the concentration of MO by recording the absorbance at λmax 463 nm by using a UV–Vis-NIR spectrophotometer.

2. Experimental Details 2.1. Materials and Methods

2.5. Genotoxicity of MO by Comet Assay

Zinc acetate dihydrate (Zn(AC)2) (99.5% SD Fine-Chemical Ltd.) used in the present study was analytical grade and used as received. AgNO3 and MO were purchased from Aldrich chemicals. Zn(AC)2 was used as the starting material for the synthesis of ZnO nanorods. In a typical synthesis, 6.0 g of Zn(AC)2 was taken in the agate pestle mortar, then grounded well for a period of 1 h, finally this powder was calcined in a silica crucible at the 300 °C for 3 h and then allowed to cool [27].

Genotoxicity of MO was tested on freshwater common carp, Cyprinus carpio (obtained from Govt. Fishery Farm, Dhom, Satara, MS, India). The 10 fingerlings per treatment group used for testing were of 5 ± 1 g weight and 5 ± 1 × 2.5 ± 1 cm size. For the experimental set up, glass tanks of 10 dm3 capacities for each treatment group were used. The tank was filled with tap water (pH 6.7) with temperature 26 ± 1 °C. For the negative control, set contains only tap water. The positive control contains 2 mg/dm3 methyl methane sulfonate (MMS). The two treatment groups were set for MO dye before and after photodegradation. The photoperiod of 12 h was maintained throughout the experiment. The blood samples were obtained from the fish with a sterilized insulin syringe by puncturing the heart. Then blood was diluted with an equal volume of Hank's balanced salt solution (HBSS). The alkaline (pH N 13) comet assay was performed for the assessment of MO genotoxicity according to the literature [25,28]. The 10 μL of each blood sample from different experimental groups were mixed with 180 μL of molten agarose (0.5%). These samples were spread over the slide which is precoated with agarose (1.5%). The slide was kept in lysis buffer for 1 h at 4 °C. For the separation of DNA, the Gel-electrophoresis was performed at 25 V for 20 min and 300 mA followed by neutralization, fixing and staining with Ethidium Bromide. Then the cells were examined under the fluorescence microscope (Nikon TS100-F) at 400 ×, using a 420–490 nm excitation and a 520 nm emission filter. The photographs of observed cells were taken for the DNA damage analysis. In order to determine percentage of DNA in the comet tail, images were analyzed with software OpenComet (v1.3) [29].

2.2. Preparation of ZnO/Ag2O Nanocomposites The typical preparation of the ZnO/Ag2O composite photocatalyst is as follows: as-synthesized 0.2 g of ZnO nanorods was added in 100 mL of distilled water and sonicated for 30 min. ZnO/Ag2O nanocomposites with different weight ratios ranging from 4:1 to 4:4 were prepared by the precipitation method. In particular, synthesis of ZnO/Ag2O (4:4) nanocomposite is as follows: 0.2 g of ZnO nanorods was dispersed in 100 mL of distilled water and in the same suspension 0.2 g of AgNO3 was added. Then the mixture was stirred continuously on a magnetic stirrer for 1 h, in the absence of light to reach complete adsorption– desorption equilibrium of Ag+ ions on the surface of ZnO nanorods. Then sufficient amount of NaOH solution (0.05 M) was added drop by drop under constant stirring to the above mixture of AgNO3 and ZnO. The obtained precipitate was collected by centrifugation and washed with distilled water and ethanol for several times. Finally, the powder was dried overnight at 60 °C. For the sake of comparison, the pure Ag2O was also prepared as a reference under the same conditions without adding ZnO nanorods.

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significantly reduced up to 22 nm. Further increase in concentration of Ag+ in composites the size of Ag2O increases up to 27 nm. 3.2. SEM and EDS Analysis

Fig. 1. XRD patterns of the as-synthesized products obtained. (a) ZnO, (b) 4:1 ZnO/Ag2O, (c) 4:2 ZnO/Ag2O, (d) 4:3 ZnO/Ag2O and (e) 4:4 ZnO/Ag2O nanocomposites and (f) Ag2O.

3. Results and Discussion

Fig. 2(a and b) shows a typical SEM images of Ag2O and ZnO/Ag2O (4:2) nanocomposites. The size of Ag2O nanoparticles is about 100 nm, which is much bigger than that of Ag2O nanoparticles coated on ZnO nanorods (shown in Fig. 2(b)). It is likely to be that surface of ZnO nanorods will be provided numerous nucleation sites for the growth of Ag2O nanoparticles, leading to the homogeneous dispersion of Ag2O on the ZnO nanorods with a smaller size. This is important for the improvement of the charge separation efficiency and the photocatalytic activity [20]. The SEM image shows spherical Ag2O NPs coated on the surface of ZnO nanorods and also supports that the Ag2O NPs are on the surface of ZnO rods. The EDS analysis was used to investigate the elements present in the material. Fig. 3 shows that the energy dispersive X-ray spectrum (EDS) of representative sample ZnO/Ag2O was recorded in the binding energy region of 0–10 keV. The spectrum consists of peaks corresponding to elemental zinc, silver and oxygen. No other additional peaks were observed from EDS, which confirms that the assynthesized ZnO/Ag2O nanocomposite is free from impurity.

3.1. X-ray Diffraction (XRD) 3.3. TEM Images Fig. 1(a) shows the XRD pattern of ZnO. The diffraction peaks in the pattern at 2θ = 31.83°, 34.49°, 36.34°, 47.63°, 56.69°, 62.93°, 66.48°, 67.91°, 69.20°, 72.65° and 77.0° are indexed for the planes of ZnO hexagonal wurtzite [100, 002, 101, 102, 110, 103, 200, 112, 201, 004, 202] respectively, which is in accordance to JCPDs Card No. 36-1451 [30]. The pattern of the Ag2O sample shows the peaks at 2θ of 33.1°, 38.3° and 55.3° are attributed to the [111, 200, 220] diffraction planes of cubic phase of Ag2O respectively (JCPDs Card No. 41-1104) [31] which is shown in Fig. 1(f). As the composition of Ag2O varies from 4:1 to 4:4 there is no shifting of XRD peaks of ZnO indicate that as-synthesized composites (b–e) were comprised of two phases hexagonal wurtzite (ZnO) and cubic (Ag2O) in the ZnO/Ag2O. The XRD peaks belonging to ZnO pattern in the ZnO/Ag2O composites did not show any shift in peak position as compared with the pure ZnO NRs, which can be implied that the Ag did not affect the ZnO lattices. These results indicate that Ag2O was deposited on the surface of ZnO. The XRD peaks of Ag2O gradually increase with increasing concentration of Ag+. No other additional impurity peaks were observed in the XRD pattern that indicates the phase purity of the product. The average crystallite size of ZnO, Ag2O and composites was calculated by using Scherrer's equation [32–33].and found to be 22 and 32 for ZnO and Ag2O, respectively. In case of composite materials (4:2 ratio) size of the ZnO is more or less constant, but the size of Ag2O was

Fig. 4(a) shows the TEM image of the ZnO nanorods having the diameter is in the range of 20 to 25 nm. As it can be seen from Fig. 4(b) that the nanoparticles of Ag2O with a diameter of 10–15 nm were decorated on the surface of ZnO nanorods. It is worth to note that Ag2O nanoparticles are strongly adsorbed towards the surface of ZnO. This is confirmed by ultrasonic dispersion in ethanol for 20 min. 3.4. FT-IR Analysis The existence of ZnO, Ag2O and ZnO/Ag2O nanocomposite was further confirmed by FT-IR analysis. Fig. 5 shows the FT-IR spectra of pure ZnO (a), pure Ag2O (b), and a representative of ZnO/Ag2O (4:2) composite(c) photocatalysts, respectively. For ZnO three main absorption regions can be observed clearly. The broad peaks at 3000–3500 and 1661 cm−1 are ascribed to the stretching and bending vibration of O–H of physically adsorbed water respectively [34]. The peaks in the range of 2923 to 2825 cm−1 corresponds to the stretching vibration of C–H bonds. The vibrational band observed at 489 cm−1 is the characteristic of Zn–O [35]. For pure Ag2O, the observed broad peak around 3500 cm−1 and peak at 1661 cm−1 belong to the O–H stretching vibration, while the peak at 1386 cm−1 belongs to the H–O–H bending vibration of the adsorbed water molecules on the surface [34].The vibrational

Fig. 2. SEM images of (a) Ag2O and (b) 4:2 ZnO/Ag2O nanocomposites.

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Fig. 3. Energy dispersive X-ray spectroscopic (EDS) of 4:2 ZnO/Ag2O.

band observed about 500 cm− 1 is characteristic of Ag–O [36]. The comparison of ZnO/Ag2O nanocomposites with pure ZnO nanorods and Ag2O nanoparticles shows the high intensity band at 3471 to 3200 cm−1, which is attributed to OH stretching and band at 1644 cm−1 is due to the OH bending mode of adsorbed H2O while the peak at 1386 cm−1 attributed H–O–H bending vibration on the surface of the nanocomposite which may have a crucial role in photocatalytic activity.

3.5. UV–Visible Diffuse Reflectance Spectra The photocatalytic behavior is mainly responsible on absorption coefficient and the band gap energy of the materials. Fig. 6. shows the optical properties of as-prepared ZnO, Ag2O, and the ZnO/Ag2O nanocomposites with various ratios measured by UV–Vis diffuse reflectance spectroscopy. ZnO nanorods shows a clear absorption edge around 370 nm and no significant absorbance in the visible region can be seen because of its wide band gap of 3.35 eV. As Ag2O mass ratio increases on the surface of ZnO nanorods, the optical absorption of the composites can be greatly tune into the visible region from 370 to 800 nm which relates to their excellent photocatalytic activity [21]. The results obtained from the UV–Vis. DRS suggests that the fabrication of heterostructure ZnO/Ag2O nanocompositesutilizes the entire solar light. The nanocomposite photocatalytic performance is better than individuals [37].

3.6. PL Spectra Photoluminescence (PL) spectra were used to investigate the separation efficiency of the photogenerated electron–hole pairs [38]. A signal of PL emission resulted from the recombination of photogenerated charge carries. A decrease in PL intensity of ZnO/Ag2O nanocomposites as compared to pristine ZnO indicates large separation efficiency of charge carrier [39–40]. PL spectra of ZnO and ZnO/Ag2O nanocomposites with different Ag2O contents were measured at excitation wavelength of 325 nm which is shown in Fig. 7. From the PL spectra it is seen that PL intensity around 400 nm gradually quenched with the increase of Ag2O content in the ZnO/Ag2O composites. This is an important feature for improved photocatalytic activity of ZnO with the modification of Ag2O. This implies that the heterostructure can deeply suppress the recombination of electron–hole pairs and leads to lower intensity of PL. [41]. In particular, the PL intensity is lowest, for ZnO and Ag2O at 4:2 ratio, which is well consistent with the result of the photocatalytic activity. 4. Photodegradation of Methyl Orange The photocatalytic activities of the ZnO/Ag2O nanocomposites with different Ag2O contents were evaluated towards photodegradation of MO under UV light (Fig. 8.). For comparison, the activities of pure ZnO and Ag2O were also tested under the similar experimental conditions.

Fig. 4. TEM images of (a) ZnO nanorods and (b) 4:2 ZnO/Ag2O nanocomposites.

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Fig. 5. FT-IR spectra of (a) ZnO, (b) Ag2O and 4:2 ZnO/Ag2O nanocomposites.

The degradation efficiency of MO was found to be 48 and 39% by using pure ZnO nanorods and Ag2O nanoparticles, respectively. It was observed that the photocatalytic activity increases with increase in Ag2O content from 4:1 to 4:2 as-prepared composite. ZnO/Ag2O (4:2) exhibited 99% photodegradation efficiency of MO within 25 min which is much higher than that of pure ZnO and Ag2O. The decrease in intensity of PL also supports the photocatalytic activity. The enhanced photocatalytic activity could be attributed to the formation of the heterostructure between Ag2O and ZnO, strong visible-light absorption, and higher charge separation efficiency. [42]. A further decrease in mass ratio of ZnO to Ag2O decreases the photocatalytic activity up to 65% which may be due to the excess loading of Ag2O that may occupy the active sites of ZnO nanorods. The similar results were reported by Chen et al. [43]. The another reason to decrease the photocatalytic activity with increase in the Ag2O loading is coverage of Ag2O on the surface of ZnO that decreased the surface contact between the ZnO and solution thereby resulting in low production of active radicals and charge carrier dissipation by trapping above a certain level of Ag2O loading. As the content of Ag2O loading increases, the size of the Ag2O increases this may be responsible for decrease in the photocatalytic activity [20]. 4.1. Photodegradation and Kinetic Study of Methyl Orange Under UV Light Fig. 9 shows the UV–Visible absorption spectra of MO degradation under UV light using optimal ZnO/Ag 2O nanocomposite (4:2). The

Fig. 6. UV–Vis diffuse reflectance spectra of (a) ZnO, (b) 4:1, (c) 4:2, (d) 4:3, (e) 4:4 ZnO/ Ag2O nanocomposites and (f) Ag2O.

Fig. 7. PL spectra of (a) ZnO, (b) 4:1, (c) 4:2, (d) 4:3 and (e) 4:4 ZnO/Ag2O nanocomposites.

intensity of the peak at 463 nm, which is related to the characteristic UV–Vis absorption of MO, decreases stepwise without shifting in the maximum absorption wavelength and almost disappears after 25 min. The change in MO concentration as a function of UV irradiation time is presented in Fig. 10(A). It is seen that the concentration of MO does not changes without the photocatalyst. The kinetic studies of an aqueous MO dye degradation process plays an important role in assessing the efficiency and feasibility of treating dye from contaminated water. Therefore, the kinetic studies of MO degradation under UV light have been discussed in the absence of a catalyst, presence of pure ZnO nanorods, Ag2O nanoparticles and ZnO/Ag2O nanocomposites. According to Langmuir–Hinshelwood (L–H) model [44], the rate expression at low initial concentration is given by. ln ðC0 =Ct Þ ¼ kt

ð1Þ

where C0 and Ct are the initial and concentration of the dye at time t respectively. A plot of lnC0/Ct versus time for MO photodegradation is shown in Fig. 10. The linear fit between lnC0/Ct and irradiation time supports the conclusion that the degradation follows first-order kinetics [45]. The values of regression coefficient (R2) of the experimental runs were more than 0.93 indicating that the degradation of MO satisfactorily followed an apparent-first-order kinetics [46–47].

Fig. 8. Photodegradation efficiency of different ratio of ZnO/Ag2O nanocomposites under UV light.

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Fig. 9. UV–Visible absorption spectra of degradation of MO under UV light.

Fig. 11. UV- Visible absorption spectra of degradation of MO under visible light.

The calculated k value and R2 for the ZnO nanorods and Ag2O nanoparticles are 0.0269 (0.979) and 0.01834 min−1 (0.963), respectively. For ZnO/Ag2O nanocomposite the calculated k value is 0.176 min− 1, (0.971) which is 7 and 10 times than k value of pure ZnO nanorods and Ag2O nanoparticles, respectively. This result reveals that the photocatalytic activity of the composite is improved greatly because of the formation of heterostructure between Ag2O and ZnO.

However, a ZnO/Ag2O nanocomposite shows 96% photocatalytic activity, which is much better than that of the ZnO and pure Ag2O nanoparticles within same irradiation time. From Fig. 12(B), the calculated k and corresponding R2 value of as-prepared ZnO/Ag2O nanocomposites is 0.0338 min− 1, (0.9970) which is 22 times and 4 times higher than ZnO nanorods (0.00156 min−1)(0.985) and pure Ag2O nanoparticles (0.00 808 min−1) (0.97), respectively.

4.2. Photodegradation and Kinetic Study of Methyl Orange Under Visible Light

4.3. Reusability of the Catalyst Under UV and Visible Light

Fig. 11 shows the intensity of the peak at 463 nm, which is related to the characteristic UV–Vis absorption of MO, a step wise decrease without shifting in the maximum absorption wavelength and almost disappears after 90 min. Fig. 12 (A) and (B) shows the photocatalytic activity and kinetics of the ZnO/Ag2O nanocomposite, ZnO nanorods, Ag2O nanoparticles and without catalyst for the degradation of MO under visible light irradiation. The change in MO concentration as a function of irradiation time shows that without catalyst the concentration of MO does not change with time. A ZnO has large band gap energy (3.35 eV) owing to this, photocatalysis proceeds only under UV light. Therefore ZnO nanorods have shown low photocatalytic activity under visible light, and the degradation of MO is found to be only 13.46% in 90 min. The pure Ag2O nanoparticles show good visible-light photocatalytic activity and the degradation was found to be 51.4% in 90 min.

From the viewpoint of industrial applications, the ZnO/Ag2O nanocomposite with the mass ratio of 4:2 as the optimized sample was selected to evaluate the reusability of composite under UV and visible light illumination. The photocatalytic activity of ZnO/Ag2O nanocomposites continuously decreases and the degradation efficiency for MO is found to be only 80% after three runs under UV light which is shown in Fig. 13(A). To find out the reason of the decrease in photocatalytic activity, the composition of the recyclable composite was characterized by XRD and is presented in Fig. 13(B). It is seen that some new diffraction peaks appeared, which can be ascribed to metallic Ag (JCPDS 04-0783) [48]. From this we can conclude that AgNPs were formed from Ag2O during the photocatalytic process, i.e. ZnO/Ag2O nanocomposite is unstable for repeated use under UV irradiation [49]. Moreover, the photocatalytic activity of ZnO/Ag2O nanocomposite under visible light was found to be stable after three runs. The photocatalytic

Fig. 10. Photocatalytic activity(A) and kinetics (B) of MO degradation under UV light irradiation (a) photolysis, (b) Ag2O (c) ZnO nanorods, and (d) 4:2 ZnO/Ag2O nanocomposites.

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Fig. 12. Photocatalytic activity(A) and kinetics (B) of MO degradation under visible light irradiation (a) photolysis, (b) ZnO nanorods, (c) Ag2O and (d) 4:2 ZnO/Ag2O nanocomposites.

degradation efficiency was more or less constant after three cycles and the photocatalytic degradation of MO is found to be 91% after three runs. To find reason for this observation, we examined the XRD patterns of the ZnO/Ag2O nanocomposite at the end of the third repeated experiment. The results are shown in Fig. 14(A). After repeating the third cycle of photocatalytic degradation of MO under visible light illumination, the peaks corresponding to Ag metallic are not detected in the XRD pattern, which is shown in Fig. 14(B) indicating that the ZnO/Ag2O is intact and photostable.

4.4. Genotoxicity Study In the present investigation, toxicity of methyl orange before degradation (MOBD) and methyl orange after degradation (MOAD) has been tested in the laboratory for screening the real toxic effects to the fish. The genotoxicity study of C. carpio blood showed that MO is a weak genotoxic. The % DNA damage is shown in Fig. 15. The fish exposed to MO before degradation shows 16.07 ± 1.21% of DNA damage. The exposure of fish to photodegraded product of MO showed very low DNA damage 2.11 ± 0.80%. Therefore, fish exposed to photodegraded products of MO showed DNA damage similar to negative control (1.24 ± 0.21% of DNA damage) which is shown in Fig. 16. Based on the results of the comet assay, the photodegraded product of MO seems to be less genotoxic.

4.5. Photocatalytic Degradation Mechanism 4.5.1. A] Under UV light Under UV irradiation both Ag2O and ZnO can be simultaneously excited to form electron–hole pairs and the photogenerated electrons from the lower conduction band (CB) of ZnO are transferred to the higher CB of Ag2O [14,50]. Therefore, the recombination of electron hole pairs is reduced. Thus, the Ag2O nanoparticle plays an important role to improve the photocatalytic activity of ZnO/Ag2O heterostructure. The obtained photogenerated electron can reduce the Ag2O into metallic silver nanoparticles by electron reducing action of ZnO nanorods and that can be confirmed by XRD results obtained by repeated photodegradation experiment under UV light irradiation. The better separation of electrons and holes in the ZnO/Ag2O heterostructure is confirmed by PL emission spectra; it is found that ZnO/Ag2O sample exhibits much lower fluorescence emission intensity than ZnO indicating that the recombination of photogenerated electron and hole is inhibited greatly.

4.5.2. B] Under visible light Under visible light irradiation (Fig. 17), only Ag2O can be excited and the photogenerated electrons on Ag2O can be migrated to the conduction band of ZnO [51]. An enhancement in photocatalytic degradation is mostly attributed to the process of charge carrier transfer, which is

Fig. 13. Three photocatalytic degradation cycles of MO using 4:2 ZnO/Ag2O nanocomposites under UV, (A) and XRD patterns of the as-prepared ZnO/Ag2O nanocomposites after the repeated photocatalytic degradation experiments for three times under UV light irradiation (B).

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Fig. 14. Three photocatalytic degradation cycles of MO using 4:2 ZnO/Ag2O nanocomposites under UV(A), and the XRD patterns of the as-prepared ZnO/Ag2O nanocomposites after the repeated photocatalytic degradation experiments for three times under visible light irradiation(B).

thermodynamically favorable because of less barrier exists between ZnO/Ag2O heterostructure. The conduction band and valence band of Ag2O (CB −1.3 eV and VB −0.2 eV, vs NHE at 0) lie above that of ZnO (CB, −0.86 and VB −2.42 eV, Vs NHE at 0) [52–53] which is shown in Fig. 17. The migration of photogenerated charge carriers is promoted because of less barrier between ZnO/Ag2O heterostructure. Therefore, the probability of electron and hole pairs recombination is reduced, a larger number of electrons concentrate on ZnO nanorods and holes locate on the Ag2O nanoparticles, which participated in a photochemical reaction and produced powerful superoxide radical as well as oxidizing − agent (·O− 2 , ·OH, ·OOH and OH ) that can degrade the organic dyes into CO2, H2O and some other minerals [54].

5. Conclusions A simple and effective route for the synthesis of ZnO/Ag2O nanocomposites by combining thermal decomposition and precipitation technique is proposed. As a Ag2O mass ratios increase on the surface of ZnO nanorods, the optical absorption of the composites can greatly tune in the visible region which is related to its excellent photocatalytic activity. The as-prepared ZnO/Ag2O nanocomposites showed significantly enhanced photocatalytic activity towards MO degradation under UV

and visible light irradiation than that of pristine ZnO nanorods and Ag2O nanoparticles. It could be observed that the photocatalytic activities increased with increasing Ag2O content from 4:1 to 4:2. The composite 4:2 ZnO/Ag2O exhibited 99% degradation within 25 min which is a much higher photocatalytic activity than that of pure ZnO or Ag2O. The PL peak reveals the lowest intensity, which is in consistent with the result of the photocatalytic activity. That is to say, in the case of 4:2 ZnO/ Ag2O nanocomposite, the photogenerated electron–hole pairs can efficiently transfer at the interface of heterostructure that results in the highest photocatalytic activity. 4:2 ZnO/Ag2O nanocomposites show 96% photodegradation efficiency under visible light within 90 min. The calculated k for MO degradation under visible light over the ZnO/Ag2O nanocomposites is 22 and 4 times higher than the k value obtained over ZnO nanorods and Ag2O nanoparticles, respectively. The composite showed excellent recyclability towards the photodegradation of MO under visible light. The comet assay study illustrated the photodegraded products of MO are less genotoxic. Thus, we believe that research outcome of the present work will be helpful to different environmental remediation.

Acknowledgments One of the authors (KMG) acknowledges DST for providing financial assistance under the Major Research Project (SR/S1/PC/0041/2010).

Fig. 15. OpenComet images for DNA damage, I) positive control, II) negative control, III) before degradation of MO, IV) AFTER degradation of MO under visible light.

Fig. 16. Bar graph of % DNA in tail.

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Fig. 17. Schematic view for the electron–hole pair separations and energy band matching of ZnO/Ag2O heterostructure under (A) UV- and (B) visible-light irradiation.

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Ag2O nanocomposites as a highly efficient visible active photocatalyst for detoxification of methyl orange.

A simple and effective route for the synthesis of ZnO/Ag2O nanocomposites with different weight ratios (4:1 to 4:4) have been successfully obtained by...
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