Synthesis of Ag/ZnO-AC composite photocatalyst: spectroscopic investigation, parameter optimization, synergistic effect and performance enhancement for cost-effective photocatalytic degradation of phenols and dyes

The pollution of natural water bodies due to the discharge of industrial wastewater is a major global concern. These pollutants are highly hazardous not only to the health of human beings but also to the entire ecosystem. There are various methods for the mitigation of harmful and recalcitrant organic, inorganic, and microbial pollutants including membrane processes (Owen et al. 1995), biological methods (Sonune & Ghate 2004), adsorption (Jusoh et al. 2007), advanced oxidation processes (AOPs) (Andreozzi et al. 1999) and electrochemical methods (Chen 2004). Among these AOPs using a metal oxide semiconductor is a promising technique for the complete destruction of all kinds of deleterious pollutants present in wastewater (Dewil et al. 2017). AOP was chosen for the remediation of pollutants due to their potential of conversion of recalcitrant organic pollutants to biodegradable compounds together with potential of disinfection, decolorization, and deodorization. AOP results in the formation of highly active hydroxyl radicals which can degrade harmful and deleterious components to harmless compounds providing an ultimate solution for wastewater treatment (Legrini et al. 1993).

Various metal oxide semiconductors have been reported to be good photocatalysts such as titanium dioxide (TiO₂) (Galindo et al. 2000), zinc oxide (ZnO) (Chiu et al. 2010), tungstate (WO₃) (Deng et al. 2019), and molybdate (MoO₄) (Bi et al. 2009). Among these ZnO stands out because of its low cost, wide band gap, high stability, eco-friendly, reusable and excellent photocatalytic properties for the degradation of pollutants such as phenols and dyes (Ye et al. 2015; Amornpitoksuk et al. 2018). Its efficiency has been reported to be particularly noticeable in the advanced oxidation of pulp mill bleaching wastewater, the photooxidation of 2-phenylphenol and photocatalyzed oxidation of phenol (Marcì et al. 2001; Amano et al. 2013). However, problems with the use of ZnO itself as a photocatalyst are well recognized; specifically, fast recombination rate of electron-hole pair, inefficient utilization of UV/visible light and poor adsorption capacity for recalcitrant organic pollutants (Zhou & Deng 2007). One strategy to overcome the limitations of ZnO and to improve its photocatalytic activity is decoration of different kinds of metals such as Pt, Pd and Ag etc. onto ZnO (Xia et al. 2009; Tian et al. 2011; Zhao et al. 2011) The decoration of metals onto ZnO can act as an effective electron acceptor and transporter, trapping the photo induced charge carriers, thereby improving the charge transfer processes and photocatalytic activity. Of the noble metals, silver is cited to be cost-effective (Tian et al. 2011; Zhang et al. 2014).

Another efficient strategy to overcome the limitations of ZnO is the use of photocatalyst support. Various methods of photocatalyst particle support have been investigated such as alumina, zeolite, silica gel, quartz, stainless steels, clays and activated carbon (AC) (Fernández et al. 1995; Chen & Mao 2007). Of these, AC attracts attention because of its highly porous structure, greater surface area, rich functional groups, easy availability and cost-effectiveness (Velasco et al. 2010; Chen et al. 2017). The use of AC as photocatalyst support also acts as a photosensitizer for bandgap narrowing, which is favorable for expanding the visible light absorption region of semiconductors (Han et al. 2017). Moreover, it also prevents the recombination of photogenerated electron-hole pair resulting in enhancement of photocatalytic activity (Li et al. 2016). Additionally, the use of AC as support to Ag/ZnO causes the synergistic effect leading to improved photocatalytic degradation of pollutants. When AC is used as a support on Ag decorated ZnO, there is a possibility of easier separation from the suspension and thus can be reused, increasing the life of the photocatalyst. Thus, the efficiency of photocatalytic degradation by ZnO is improved in order to meet the requirements of environmental protection (Chandraboss et al. 2015).

A facile, cost-effective and sustainable approach was explored to synthesize Ag/ZnO-AC composite photocatalyst. To the best of our knowledge, this is first time it has been reported that the degradation of PNP by Ag/ZnO-AC composite photocatalyst. PNP is one of the recalcitrant organic pollutants present in water bodies owing to its higher solubility and stability in water. It is listed as a hazardous organic compound by the Environmental Protection Agency (San et al. 2002). PNP used in the raw material for the manufacture of pesticides, herbicides, synthetic dyes, pharmaceuticals, and for leather treatment is a common constituent in the effluents from industries involved in manufacturing these chemicals. (Oturan et al. 2000). PNP is highly toxic to plants, animals and microorganisms due to its carcinogenic properties (Liu et al. 2017). Removal of PNP from natural water bodies and from contaminated sites is therefore of prime importance. For this reason, the photocatalytic degradation of PNP was used as a model pollutant to evaluate the photocatalytic activity of Ag/ZnO-AC composite photocatalyst. Ag/ZnO-AC composite photocatalyst also showed the degradation for ONP and MO which confirms its non-selective nature. Thus, Ag/ZnO-AC composite photocatalyst can be extended to study the degradation of other recalcitrant and deleterious organic pollutants present in wastewater.

Various types of materials have been developed and explored for the photocatalytic degradation of PNP. Recently Chakraborty et al. (2021) carried out degradation of 4-Nitrophenol by Ag₂O-ZnO composite nanocones (Chakraborty et al. 2021). Photocatalytic degradation of PNP over BiVO₄ in the presence of H₂O₂, under visible light, was studied by Umabala (2015). Wei et al. (2020) studied the degradation of 4-nitrophenol by Z-scheme Ag/In₂S₃/ZnO nanorods composite photocatalysts (Wei et al. 2020). Taha et al. (2020) observed 78.6% degradation of PNP after irradiating it for 120 minutes by Activated Carbon-Supported Ag and ZnO nanocomposite synthesized by the green method (Taha et al. 2020).

To the best of our knowledge, the photocatalytic performance for degradation of PNP by Ag/ZnO-AC has not been well-explored. Hence, this area of research needs more attention and scientific contributions. We have carried the extensive studies of photocatalytic degradation of PNP by Ag/ZnO-AC composite photocatalyst. Different operational parameters including an initial concentration of pollutant, composite dose, effect of calcination temperature and the effect of H₂O₂ and ethanol were studied and investigated. Our present work showed 99.9% degradation of PNP after irradiating it for 100 minutes under UV light in the presence of Ag/ZnO-AC composite photocatalyst by using the optimum amount of chemicals, making this study cost effective.

To overcome the limitations of bare ZnO as a photocatalyst, a decoration of Ag on ZnO with AC as a support were used to synthesize Ag/ZnO-AC composite for the improved photocatalytic degradation of organic pollutants. Thus, here we report the synthesis of ZnO, Ag/ZnO and Ag/ZnO-AC composites and their photocatalytic activities for the degradation of p-Nitrophenol (PNP), o- Nitrophenol (ONP), and dye methyl orange (MO) under UV irradiation. The calcinated ZnO (400 °C) and optimized loading of Ag (1%) on ZnO were used in synthesizing Ag/ZnO-AC composite. The combination effect of ZnO, Ag and AC improved the photocatalytic degradation of organic pollutants over bare ZnO. The synergistic effect in the composite is due to the combination of the enhanced photocatalytic activity of Ag decorated ZnO because of the prevention of recombination of generated electron-hole pairs and increased adsorption capacity due to AC as support. The % degradation of all the three pollutants under study, PNP, ONP and MO, were found to be in the order Ag/ZnO-AC > Ag/ZnO > ZnO. The plausible mechanism for enhanced photocatalytic degradation of pollutants using Ag/ZnO-AC composite is also discussed. Ag/ZnO-AC composite photocatalyst not only shows the degradation efficiency for the pollutant PNP but also for other pollutants such as ONP and MO. This confirms the non-selective nature of Ag/ZnO-AC composite photocatalyst.

Analytical Grade (A.R.) reagents were used throughout the present studies. The following chemicals were purchased from various companies and used without further purification. AC was procured from Lurgi Aktivkohle GmbH Germany, AgNO₃ (Spectrochem Pvt. Ltd India) ONP and PNP were purchased from Sisco Research Laboratory, Pvt. Ltd, India. MO, Oxalic acid (OA) were procured from Merck, Germany. Zinc acetate dihydrate was purchased from Ranbaxy chemicals and ethanol was procured from Changshu Yangyuan Chemical, China.

The sol-gel method has advantages such as low cost, easy to handle, and safe procedure to prepare ZnO nanoparticles over conventional synthesis methods such as magnetic sputtering, chemical vapor deposition, and hydrothermal reaction (Lü et al. 2010). ZnO nanoparticles were prepared by sol-gel method from zinc acetate in a solution of oxalic acid, using ethanol. Zinc acetate (3.66 g) was dissolved in 100 mL ethanol and refluxed at 75 °C under vigorous stirring for 30 minutes. OA (4.2 g) was mixed with 75 mL of ethanol and added to the previous solution dropwise. The final mixture was refluxed at 80 °C for 60 minutes and then left to cool down to room temperature. Finally, the gel was dried at 80 °C overnight (Xerogel) and the powder was calcined for 3 h at different temperatures, 400, 600 and 800 °C.

In the Photodeposition method ZnO decorated with Ag metal was prepared by photoreducing Ag⁺ ions to Ag metal on the ZnO surface according to the following steps. First, 500 mg ZnO was added to 50 mL deionized water. Then the required amount of AgNO₃ was added to the ZnO suspension, where the silver concentration was 0.5, 1.0 and 1.5% (mole ratio) versus ZnO for synthesizing Ag decorated ZnO. To use the minimum quantity of Ag for cost-effectiveness, the concentration of Ag for doping on ZnO was studied for 0.5, 1 and 1.5%. The mixtures were then irradiated with UV light (four 8 W low pressure mercury vapor lamps) for 3 h and then dried in an air oven at 110 °C for 2 h.

AC was available in granular form. It was washed with distilled water several times and then dried at 110 °C for 2 h in an oven. Dried granules of AC were crushed in a mortar into a fine powder and the obtained powder was calcinated at 200 °C for 2 h to remove moisture and was kept in desiccators until further use. AC was used for the preparation of composite. A definite amount of Ag/ZnO and AC was dispersed in distilled water and magnetically stirred for 3 h. The product was filtered and dried at room temperature to get Ag/ZnO-AC ternary composite.

All the prepared samples were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX), Transmission Electron Microscopy (TEM), X-Ray Photoelectron Spectroscopy (XPS), UV-visible absorption spectra and Photoluminescence spectra (PL). The analysis of XRD was used to determine the crystal structure of the composite. It was carried out by using the Powder XRD (Phillips PW1729) which was equipped with a Cu anode and graphite monochromator and the diffractograms were recorded by using CuKα radiation over a range of 20–80°. The morphology of ZnO, Ag/ZnO and Ag/ZnO-AC photocatalysts were observed using an SEM (Jeol JSM-6360) equipped with EDX. TEM images were obtained by a TEM (FEI-Technai G-2 ultra twin), XPS was carried out on Kα + , Thermo Fischer Scientific Instruments, UK, with Al Kα radiation to elucidate the element composition and surface chemistry of the composite. UV-visible absorbance spectra of solid materials were recorded between the range of 200–800 nm using a UV-visible spectrometer (JASCO V-670) equipped with DRS and BaSO₄ was employed as a reference substance. UV-visible absorbance spectra of solution were recorded using UV visible spectrometer (Schimadzu UV-1800) double beam spectrophotometer. The PL spectra were obtained using spectrofluorometer (JASCO FP-8300). Fourier Transform Infrared Spectra (FTIR) was recorded on Bruker Tensor 37 in the mid IR region (400–4000 cm⁻¹).

To investigate the photocatalytic degradation of pollutants by synthesized nanoparticles, the experiments were performed in batch process. A multilamp photoreactor constituting four 8 W low pressure mercury vapor lamps at 30 ± 1 °C was used as a source for UV irradiation. The light intensity near the sample was measured by a radiometer; the intensity was found to be 15 W/m². Forty mg of photocatalyst was dispersed into the photoreactor containing 100 mL of 0.1 mM/L PNP solution. Air was bubbled through the reaction solution using an aerator with constant speed. The temperature of the reaction solution was controlled at room temperature by circulating fans. The whole reactor assembly was mounted on a magnetic stirrer for mixing the aqueous solution during the reaction in order to ensure that the solution was well-mixed and the photocatalyst did not settle down inside the container as depicted in Figure 1. During the photocatalytic experiment, the slurry containing aqueous pollutant solution and photocatalyst was magnetically stirred for 10 minutes in the dark and then it was irradiated with UV light. Aliquots (1 mL) were withdrawn at specific time intervals and were immediately filtered to separate the photocatalyst through a 0.2 μm, 13 mm diameter millipore disc. Changes in absorption spectra were recorded at λ_max using Schimadzu (UV-1800) UV-visible spectrophotometer with a 1 cm path length of cuvette made of quartz cell.

From the calculations, the average crystallite size of the ZnO nanoparticles was found to be 32 nm.

The SEM studies were used to investigate the surface morphology of Ag/ZnO-AC ternary composite. The SEM micrograph obtained for Ag/ZnO-AC composite was found to be spherical (Figure 3(a) and 3(b)). The EDX pattern of as synthesized Ag/ZnO-AC ternary composite indicates that it is composed of Zn, O, Ag and AC, as shown in Figure 4(g). No evidence of other impurities was found. This observation was in good agreement with XRD results. Elemental mapping of Ag/ZnO-AC ternary composite depicts the good dispersion of Ag and AC on ZnO as shown in Figure 4(a)–4(f). Figure 4(b) shows the co-existence of Zn, O, Ag and AC in Ag/ZnO-AC composite. Figure 4(c)–4(f) depicts the corresponding elements of the composite.

TEM images low resolution TEM (LRTEM) and high-resolution TEM (HRTEM) are presented in Figure 3(c)–3(e) which clearly reveals ternary composite formation made up of Ag decorated ZnO with AC as support. The aggregation of Ag decorated ZnO over AC indicates the supporting role of AC as a center for deposition and growth of Ag/ZnO nanoparticles. The presence of transparent layers is seen in Figure 3(d), indicating the carbonaceous layers with a high degree of porosity interspersed among Ag decorated ZnO. The selective area electron diffraction pattern (SAED) is shown in Figure 3(f), indicating a set of rings with spots which shows that the as synthesized composite is polycrystalline in nature. The lattice spacing was measured as 0.26 and 0.235 nm for ZnO (100) and Ag (111) respectively which is in agreement with the literature value as shown in Figure 3(e) (Kim et al. 2012).

To further calculate quantitatively the actual content of Ag and investigate the chemical states of the as-prepared samples, XPS analysis was also carried out. The survey scan of as synthesized Ag/ZnO-AC composite depicts the presence of Zn, O, Ag and AC, shown in Figure 5(a), and no peaks of any other elements were observed. Figure 5(b)–5(e) shows the high resolution XPS spectra of Zn 2 p, O 1 s, C 1 s and Ag 3 d respectively. Figure 5(b) shows Zn 2 p peaks at binding energies of 1045.36 eV and 1022.32 eV corresponding to Zn 2 p_1/2 and 2 p_3/2 respectively, confirming that the Zn element exists mainly in the form of Zn²⁺ state (Muthulingam et al. 2015). XPS spectra of O1 s shown in Figure 5(c) are asymmetric, indicating the presence of multi-component oxygen species on the surface. The curve was deconvoluted by Gaussian fittings into two separate peaks located at 531.17 and 532.72 eV respectively. The 531.17 eV peak is attributed to the lattice oxygen whilst the higher energy peak located at 532.72 eV is associated with chemisorbed oxygen from the surface hydroxyl (Saravanan et al. 2013). Figure 5(d) shows C 1 s deconvoluted peak at binding energies of 287.78, 285.59 and 284.61 eV corresponding to the presence of sp³ hybridized carbon (Wang et al. 2016). Figure 5(e) shows Ag 3 d peaks at binding energies of 375.12 and 368.37 eV corresponding to Ag 3 d_3/2 and 3 d_5/2 respectively (Saravanan et al. 2013).

Figure 6(b) shows the room temperature PL spectra of the as prepared bare ZnO, Ag/ZnO and Ag/ZnO-AC photocatalyst. PL provides useful information about the efficiency of charge carrier trapping and recombination in semiconductor particles since the PL emission comes from the recombination of free charge carriers (Xin et al. 2005). The PL spectra of ZnO, Ag/ZnO, Ag/ZnO-AC were determined at an excitation wavelength of 370 nm and the corresponding maximum emission peak was found to be at 431 nm. The PL spectra of Ag/ZnO and Ag/ZnO-AC are similar to that of ZnO nanoparticles with a peak position at 431 nm. The order for the intensity of PL signal is ZnO>Ag/ZnO>Ag/ZnO-AC. A lower PL intensity means a lower electron-hole recombination rate and hence a longer life of photogenerated carriers (Khan et al. 2013). This indicates that Ag/ZnO and Ag/ZnO-AC composite has obtained the lowest recombination possibility of photoexcited electrons and holes under UV light irradiation as compared to bare ZnO.

The photocatalytic activity was checked under UV light irradiation using bare ZnO, Ag/ZnO and Ag/ZnO-AC catalysts for the degradation of PNP as a model organic pollutant. The degradation was found to be in the order Ag/ZnO-AC>Ag/ZnO>ZnO (Figure 7(c)). The degradation of PNP was found to be 65% using bare ZnO (calcinated at 400 °C) after 40 minutes of irradiation. To overcome the limitations and to improve the photocatalytic degradation of PNP, Ag decorated ZnO (Ag/ZnO) composite was synthesized. The percentage degradation of PNP using optimized 1.0% Ag/ZnO was found to be 79%. To further improve the rate and its photocatalytic activity and increase the adsorption of pollutants onto the surface of Ag/ZnO, AC was used as a support to prepare Ag/ZnO-AC composite. The degradation of PNP using Ag/ZnO-AC was observed to be 90% within 40 minutes of irradiation. This is ascribed to the increased charge separation of photogenerated electron-hole pair (Figure 6(b)) and a higher rate of adsorption of pollutant onto Ag/ZnO-AC catalyst. The degradation of PNP was studied in the dark using Ag/ZnO-AC, only 70% PNP was degraded. This degradation was achieved due to the adsorption phenomenon. The irradiation under UV light in the presence of Ag/ZnO-AC caused 99.9% degradation of PNP during the same period of irradiation. The results are depicted in Figure 7(b). From this data, it can be concluded that the improved degradation is due to adsorption followed by photocatalytic degradation.

The adsorption behaviour of Ag/ZnO-AC photocatalyst was studied and it was observed that only 70% PNP was degraded. The results are depicted in Figure 7(a). The adsorption capacity was found to be in the order Ag/ZnO-AC>Ag/ZnO>ZnO. The higher adsorption capacity of Ag/ZnO-AC composite photocatalyst can be attributed due to interactions between the graphitic carbon layer (sp² bonding) and the aromatic ring of the PNP (Hsu et al. 2009). The highly porous and greater surface area of AC is another factor for the increased adsorption behavior of Ag/ZnO-AC. In order to observe the effects of Ag/ZnO-AC catalyst and UV light, PNP was photochemically degraded for 100 minutes, in the absence of Ag/ZnO-AC under UV light (photolysis), in the presence of Ag/ZnO-AC in the dark (adsorption) and in the presence of Ag/ZnO-AC and UV light (photocatalysis). The results obtained are depicted in Figure 7(b). Direct photolysis did not cause any significant degradation under UV irradiation and the degradation of PNP was found to be 8% after irradiating for 100 minutes. After 10 minutes of irradiation using Ag/ZnO-AC as photocatalyst, 66% degradation was achieved. The degradation of PNP was found to be only 8% after irradiating for 10 minutes without catalysis, whereas in the dark but in the presence of Ag/ZnO-AC as catalyst 60% degradation was achieved. The degradation of PNP was found to be only 8% after irradiating with UV light for 100 minutes. Similar results are obtained by Assi et al. (2015) The irradiation under UV light in the presence of Ag/ZnO-AC as photocatalyst caused 99.9% degradation of PNP during 100 minutes of irradiation.

The irradiation under UV light in the presence of Ag/ZnO-AC caused 99.9% degradation of PNP during the same period of irradiation. The enhanced degradation efficiency of Ag/ZnO-AC is caused by the combination effect of AC as well as Ag. AC has a large specific surface and stronger adsorption capacity (Tsumura et al. 2002) whereas the role of Ag is to suppress the hole electron pair recombination (Peng et al. 2007).

The synergy factor has been estimated quantitatively using equation R = k_{(Ag/ZnO−AC)}/k_(ZnO) (Matos et al. 1998) for Ag/ZnO-AC photocatalyst. Similarly, the synergistic factor of Ag/ZnO and ZnO-AC can be determined. The synergistic factor of Ag/ZnO and ZnO-AC was found to be 1.75 and 1.6 respectively. However, on combination of Ag and AC, i.e for Ag/ZnO-AC photocatalyst, the synergistic factor was found to be 2.3. Thus, from this data it may be concluded that there is a synergistic effect between Ag and AC leading to improved degradation efficiency of PNP. Thus, the combination of Ag and AC caused a synergistic effect in the Ag/ZnO-AC composite leading to improved degradation of PNP.

The kinetic study was investigated for the degradation of PNP using the as prepared bare ZnO, Ag/ZnO and Ag/ZnO-AC photocatalysts (Figure 7(d)) and the reaction rate constants of the prepared photocatalyst is depicted in Table 1. The k value of the Ag/ZnO-AC is observed to be 2.23 times larger than that of the bare ZnO in degrading PNP. Thus, Ag/ZnO-AC causes significant improvement in photocatalytic degradation of PNP under UV irradiation.

The effect of calcination was studied at 400 °C and its photocatalytic degradation study was evaluated and compared with the uncalcined ZnO. The results are depicted in Figure 8(a) and the photocatalytic performance for the degradation of PNP was enhanced for ZnO calcined at 400 °C. This phenomenon may be explained by the following two reasons: (1) At 400 °C, the crystallinity of ZnO was improved (Figure 2). With increase in calcination temperature, intensity of diffraction peaks increases, which indicates strengthening of ZnO phase. (2) The second reason may be explained on the basis of crystallite size of the nanoparticles as a function of calcination temperature. With an increase in calcination temperature there is a decrease in crystallite size, leading to a greater surface area causing the greater photocatalytic performance for the degradation of PNP at higher temperature as compared to RT (Ashraf et al. 2015).

Figure 8(b) shows the effect of the concentration of Ag on ZnO, which shows that the photocatalytic activity of Ag/ZnO increases with an increase in the Ag loading up to optimum loading and then decreases. The optimized loading of Ag on ZnO was found to be 1.0%. The improved photocatalytic activity is due to suppression of the recombination of photogenerated electron-hole pair in the semiconductors. The increase in silver loading above the optimum level has a detrimental effect on the photocatalytic activity of the composite. It may be due to the deposition of Ag at the active sites on the ZnO surface for the desired photocatalytic reactions causing the ZnO to lose its activity. Besides this, negatively charged silver sites begin to attract holes and subsequently recombine them with electrons. So, the metal deposits become recombination centers (Coleman et al. 2005).

In order to avoid an excess of catalyst and to ensure a total absorption of efficient photons, the optimum dose of the Ag/ZnO-AC composite photocatalyst was determined. Experiments were carried out by taking different amounts of Ag/ZnO-AC, keeping the PNP concentration constant (0.1 mM/L). Figure 8(c) illustrates the effect of different amounts (0.2–0.6 gL⁻¹) of Ag/ZnO-AC composite on the photocatalytic degradation of PNP. It is clear from Figure 8(c) that the degradation of PNP increases with an increase in the dose of catalyst up to an amount of 0.4 g L⁻¹. An increase in photocatalytic degradation is due to an increase in the number of active sites on Ag/ZnO-AC available for the reaction, which in turn increases the rate of radical formation. The reduction in photocatalytic degradation above the specified amount is due to a reduction in the penetration of light into the solution. The addition of surplus catalyst also results in the deactivation of activated molecules by collision with ground state molecules (Subramani et al. 2007). From Figure 8(c), 0.4 g L⁻¹ dose of the composite catalyst Ag/ZnO-AC (1.0%) showed a better result for the degradation of PNP.

Using the optimized dose of composite (0.4 g L⁻¹), studies were carried out for the effect of initial PNP concentration. For this, the concentration of PNP was varied from 0.05 to 0.5 mM/L. The rate of degradation of PNP was found to be less for concentrations 0.05, 0.25 and 0.5 mM/L. The degradation rate decreases at a low initial PNP concentration of 0.05 mM/L. It may be attributed to the fact that the life time of ^·OH radical is very short (only a few nano seconds), and it can only react at or near the location where they are formed. The increased concentration enhances the probability of collision between the^·OH radicals and PNP molecules. However, when the initial concentration of PNP is increased further to 0.1 mM/L, the % reduction in the initial concentration of PNP decreases because at the higher concentration, the number of collisions between molecules increases whereas the number of collisions between molecules and ^·OH radical decreases. Consequently, the rate of reaction is retarded (Amine et al. 2001). The results obtained are depicted in Figure 8(d). Maximum degradation of PNP was obtained at 0.1 mM/L concentration (Augugliaro et al. 1991).

The addition of H₂O₂ to the heterogeneous system increases the concentration of ^·OH radicals (Sauer et al. 2002). The experimental results for UV + Ag/ZnO-AC + H₂O₂ system are depicted in Figure 8(e) for PNP, which showed a drastic increase in the rate of degradation of PNP due to the increased concentration of ^·OH radicals. As an electron acceptor, H₂O₂ does not only generate ^·OH radicals but inhibits the electron-hole recombination process (Shintre & Thakur 2016) at the same time, which is one of the most important practical problems in using ZnO as a photocatalyst (Chen et al. 1999). Hydrogen peroxide can also absorb light at 254 nm and decompose to produce ^·OH radicals, which would lead to an increased rate of PNP degradation (Shintre & Thakur 2012). Alcohols such as ethanol, are commonly used to quench hydroxyl radicals. It was observed that small amounts of ethanol inhibited the photocatalytic degradation of PNP. Thus, hydroxyl radicals play a major role in photocatalytic oxidation. It is unlikely that ethoxy radicals (C₂H₅O^·) are generated. Otherwise, it would have increased the degradation efficiency of PNP (Alahiane et al. 2014).

To investigate the robustness of Ag/ZnO-AC photocatalyst composite, reusability studies were assessed for the degradation of PNP. The results are depicted in Figure 9. The photocatalytic efficiency hardly reduced (less than 5%) after four cycles, demonstrating high photostability potentials for reuse under UV irradiation towards the removal of organic pollutants present in wastewater. In order to confirm that composite catalyst is stable for reuse, FT-IR spectra of PNP (Figure 10(a)), Ag/ZnO-AC before irradiation; fresh catalyst (Figure 10(b)) and after irradiation were determined (Figure 10(c)). FT-IR spectrum of catalyst before degradation of PNP and after degradation of PNP was found to be similar confirming the stability of Ag/ZnO-AC. The peak obtained at about 1375 cm⁻¹ in Figure 10(b) can be attributed to the presence of hydroxyl residue which may be due to the atmospheric moisture (Chaouch & Mustapha 2001). Dissolution of Ag and Zn after the degradation of degradation of PNP was not found. This may be explained on the basis of FT-IR data. FT-IR of Ag/ZnO-AC before irradiation (Figure 10(b)) and after irradiation were determined (Figure 10(c)). FT-IR spectrum of catalyst before degradation of PNP and after degradation of PNP was found to be similar, confirming the stability of Ag/ZnO-AC.

To study the non-selective nature of photocatalyst, the photocatalytic degradation of ONP and MO by ZnO, Ag/ZnO and Ag/ZnO-AC were studied and the results are depicted in Figure 11(a) and 11(b) respectively. The results obtained are similar to PNP degradation which implies the non-selective nature of composite.

The obtained results highlight the improved photocatalytic degradation of PNP using ternary Ag/ZnO-AC composite. The proposed mechanism of Ag/ZnO-AC composite is shown in Figure 12.

When ZnO is irradiated with UV light having energy greater than or equal to the band gap energy, the electrons from the valance band are excited to the conduction band leaving behind holes in the valance band. The photogenerated electrons are captured by dissolved oxygen to generate reactive radicals like superoxide radicals (^·O₂⁻) and the holes react with water molecules/hydroxide ions to generate hydroxyl radicals (^·OH). These ^·OH oxidize the pollutants into carbon dioxide and water moieties (Rakibuddin & Ananthakrishnan 2016). There is a possibility of recombination of photogenerated electron-hole pair thereby reducing the photocatalytic activity of ZnO. When Ag is decorated onto the surface of ZnO, Ag traps the photogenerated electrons leading to better separation of electron-hole pair (Online et al. 2015). This is evidenced by the lower PL intensity observed for Ag/ZnO as compared to bare ZnO (Figure 6(b)). Now, when AC is used as photocatalyst support, not only does it provide a greater surface area thereby increasing the concentration of PNP (Figure 7(a)), but it also causes greater light absorption (Figure 6(a)). Moreover, the use of AC has further increased the charge separation preventing the recombination of photogenerated electron hole pair (Figure 6(b)). In the absence of AC, pollutants must collide with the Ag/ZnO by chance, and remain in contact for the photocatalysis to proceed. When this is not achieved, the reactants or intermediate products will pass back into solution and can only react further when they collide with ZnO again.

The synergy factor has been estimated quantitatively using equation R = k_{(Ag/ZnO−AC)}/k_(ZnO) (Matos et al. 1998). The synergistic factor of Ag/ZnO-AC composite was found to be 2.3. Similar observations about the synergic effect of activated carbon as an additive to ZnO in the photodegradation of organic pollutants have been described in the literature (Lee et al. 2004; Ao et al. 2008).

Thus, the combination of Ag and AC on ZnO has resulted in the improved degradation of not only PNP but also other pollutants.

A new facile ternary Ag/ZnO-AC composite photocatalyst has been synthesized and confirmed by different spectroscopic techniques showing improved degradation of phenols and dyes using UV light. Decoration of Ag and use of AC as support on ZnO has led to the inhibition of photogenerated electron-hole pair recombination evident from the lower photoluminescence intensity, improving its photocatalytic performance. The rate of degradation for PNP, ONP and MO were found to be in the order Ag/ZnO-AC>Ag/ZnO>ZnO. Owing to the use of optimized concentration of Ag for decoration onto ZnO and the use of an optimized dose of photocatalyst throughout the experiment has led to the economic feasibility of photocatalyst. The reusability studies performed on Ag/ZnO-AC reveals the sustainability of photocatalyst composite towards the remediation of PNP present in wastewater. The present study highlights the modifications of ZnO onto its surface causing the synergistic effect leading to a remarkable improvement in the degradation of phenols as well as dyes. Synthesized Ag/ZnO-AC photocatalyst composite may be successfully used for the degradation of other recalcitrant pollutants present in wastewater for practical applications.

The authors would like to acknowledge the Board of Control and University Development (BCUD), UPE II, Savitribai Phule Pune University, Pune and UGC, New Delhi for financial support.

All relevant data are included in the paper or its Supplementary Information.