Novel polyvinyl pyrrolidone capped pure, Ag (1–3%) and Cu doped (1–3%) zinc oxide (ZnO) nanoparticles (NPs) were successfully synthesized via the co-precipitation method. The synthesized NPs were characterized by UV-visible spectrophotometry, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and field emission scanning electron microscopy (FE-SEM). Compared to pure ZnO, the absorption bands of Ag and Cu doped ZnO NPs were shifted and, further, the band gap energy was also decreased which confirms the incorporation of Ag and Cu into the ZnO lattice. The XRD diffraction peak confirms that all the synthesized compounds are found to be of highly crystalline hexagonal wurtzite structure. In addition, the presence of Ag and Cu in the ZnO NPs was further evidenced from EDS analysis. FE-SEM images established the morphology of the doped ZnO NPs which was not affected by the addition of Ag and Cu. The photocatalytic activity of undoped, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs were tested with brilliant green dye under UV irradiation. Degradation study reveals that doping has a distinct effect on the photocatalytic behavior of ZnO NPs. In addition to that, kinetic, thermodynamic and reusability studies have been performed for the 2% Ag doped ZnO NPs.

INTRODUCTION

Wastewater generated from textile, paper, pulp and paint industries contains dyes and pigments which are highly toxic in nature and causes mutagenic and carcinogenic effects on humans, animals and aquatic systems (McKay et al. 1985; Bhattacharyya & Sarma 2003). Generally, dyes are classified based on the ionic charge of dye molecules such as non-ionic, cationic, anionic and zwitterionic (Hao et al. 2000). Brilliant green (BG) dye is a cationic dye which is more toxic than anionic dyes (Nandi et al. 2009). BG dye is extensively used in paper printing and dye industries (Mittal et al. 2008). Various types of treatment methods are available for the treatment of wastewater containing organic effluents such as physical, chemical and biological methods. Among them, heterogeneous photocatalytic degradation of organic effluents using semiconducting materials is proven as an effective technique because of its superiority over other methods, such as rapid oxidation, efficiency of degradation and also because there is no formation of polycyclic compounds (Pouretedal et al. 2009). In a photocatalytic reaction, the photocatalyst (semiconducting material) absorbs light having energy equal to or more than its band gap energy, which induces the hole-and-electron pairs for the formation of free-radicals to oxidize the organic dye molecules (Aarthi et al. 2007).

Currently, there are many semiconductor materials which are used as photocatalysts in the photocatalytic degradation of organic effluents such as TiO2, ZnS, ZnO, CdS, WO3, etc. Among the earlier catalysts, zinc oxide (ZnO) emerges as one of the most promising semiconducting materials due to its direct band gap of 3.2 eV and its high exciton binding energy (60 meV). ZnO is used for a variety of applications such as photocatalytic degradation (Velmurugan & Swaminathan 2011), field-effect transistors (Hong et al. 2008), lasers (Dorfman et al. 2006), photodiodes (Bao et al. 2006), and solar cells (Law et al. 2005). In any heterogeneous catalytic reaction, at least one reacting molecule should be adsorbed on the catalyst surface in order for the reaction to proceed. For this, the surface area of the catalyst and defects on the catalyst play an important role in the photocatalytic degradation of contaminants. Doping of a ZnO photocatalyst with transition metal or noble metal ions may produce crystal defects and influence the optical properties of ZnO, increasing the photocatalytic activity of prepared nanoparticles (NPs) (Gnanaprakasam et al. 2015). Further, theses doped metal ions can suppress the electron-hole pair recombinations which were induced during the light illumination (Wu et al. 2014a, 2014b). Recently, Ag and Cu modified metal oxides have been investigated for their prospective photocatalytic application. Lei et al. (2014) synthesized Ag doped TiO2 and studied its photocatalytic activity for the reduction of Cr(VI) under visible radiation. Gupta et al. (2013) investigated the photocatalytic performance of Ag doped TiO2 NPs prepared by the acid-catalyzed sol-gel route. Ganesh et al. (2014) studied photocatalytic activity of Cu doped TiO2 synthesized via the homogeneous co-precipitation method.

In the present article, polyvinyl pyrrolidone (PVP) capped pure, Ag and Cu doped ZnO NPs (flower-like microstructure with nano leaf) were synthesized via the co-precipitation method and their photocatalytic activity was tested with BG dye under UV-visible radiation. The synthesized NPs were characterized by UV-visible spectrophotometry, field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). To the best of our knowledge, no one has ever reported the synthesis of PVP capped Ag and Cu doped ZnO NPs and studied the effect of its photocatalytic behavior on the degradation of BG dye under UV-visible radiation. Further, the photocatalytic behavior of 2% Nd doped ZnO NPs has been extensively studied by varying catalyst loading, initial dye concentration and temperature, and reported. Thermodynamic study has been performed to confirm the feasibility of photocatalytic degradation of dye at the given condition. Also, the effectiveness of the prepared catalyst has been measured up to four cycles to check its reusability.

EXPERIMENTAL

Materials

ZnSO4·7H2O and PVP (surface stabilizer and growth modifier) were purchased from SD Fine-Chem Limited, India. CuSO4·5H2O was purchased from AD Lab, UK. AgNO3 was purchased from MP Biomedicals, India, and NaOH was purchased from Merck, India. All the chemicals used were analytical grade and used without any further purification. Double distilled water was used throughout the experiment.

Synthesis of NPs

Chemical co-precipitation methodology was used to synthesize pure, Ag and Cu doped ZnO NPs. The preparation of PVP capped 1% Ag doped ZnO NPs was as follows: in the first step, 0.25 M (7.19 g) ZnSO4.7H2O was dissolved in 100 mL of distilled water in which 1% (0.03237 g) of AgNO3 was added and the mixture was stirred for 30 min. After the stipulated time, 100 mL of 1 wt% PVP solution was added to the above precursor solution and the stirring was continued for 30 min. After stirring, 60 mL of 0.5 M of NaOH solution was added drop wise to the above mixture, which led to the formation of precipitate, and again stirred for 30 min. The synthesized NPs were separated by the centrifuge method and washed twice with ethanol and water and dried at 100 °C for 6 h. Finally, synthesized particles were calcined at 350 °C for 3 h to produce PVP capped 1% Ag doped ZnO NPs. A similar procedure was adopted for the synthesis of pure, 2%, and 3% Ag doped ZnO NPs and for (1–3%) Cu doped ZnO NPs.

Photocatalytic activity on BG dye

A volume of 100 mL of 10 ppm BG aqueous solution and 0.25 g of photocatalyst were put into a 250 mL beaker. The reaction mixture was stirred for 30 min under a dark condition to achieve adsorption–desorption equilibrium. Then, the UV light (medium pressure mercury vapor lamp 400 W, predominantly wavelength 366 nm) was switched on. At different time intervals (0 min, 30 min, 60 min, 90 min, 120 min, 150 min and 180 min) samples were taken out and centrifuged to separate the catalyst and then the supernatant was analyzed in a UV-visible spectrophotometer at λmax = 630 nm to study the photocatalytic degradation. The degradation percentage of dye was calculated using the following formula:
formula
1
where:
  • A0 is the initial absorption of dye supernatant when t = 0 (after 30 min stirring under dark condition).

  • A is the absorption of dye supernatant at time t.

Characterizations

The prepared undoped, Ag doped and Cu doped ZnO NPs were characterized by XRD using a Shimadzu XRD 6000 with Cu–Kα radiation at 40 kV (Japan). FE-SEM and EDS were studied using a SIGMA HV–Carl Zeiss with a Bruker Quantax 200–Z10 EDS Detector. Optical study and measurement of absorption of pure and degraded dye were carried out using a UV-visible spectrophotometer (Merck, USA). For the optical study, a small quantity of as-synthesized catalyst was dispersed in 3 mL distilled water with the help of an ultrasonicator to obtain transparent colloidal aqueous solutions and the absorbance vs. wavelength spectrum was generated using a UV-visible spectrophotometer (Merck, USA).

RESULTS AND DISCUSSION

Optical studies

The optical absorption spectra of undoped, Cu and Ag doped ZnO (1%, 2% and 3%) samples determined using a UV-visible spectrophotometer in the range of 200 to 800 nm are shown in Figure 1(a). It can be observed that all the samples exhibit good absorption of light in the UV and visible region. A strong excitonic absorption peak was observed around 373–382 nm for all the synthesized samples. The absorption peaks for prepared (1–3%) Ag doped and (1–3%) Cu doped ZnO NPs are presented in Table 1. The absorbed peaks in all the Ag doped and 2% Cu doped ZnO NPs exhibit red shift and 1% and 2% Cu doped ZnO particles exhibit blue shift with respect to pure ZnO NPs having absorption peak at 375 nm (Figure 1). The band gap (Eg) of synthesized compounds was calculated using Tauc's equation:
formula
2
where α is the absorption coefficient, is photon energy, A is constant, and the n value depends on transition type (1/2 allowed direct, 2 allowed indirect, 3/2 forbidden direct and 3 forbidden indirect). For ZnO, n is ½ since it is the direct band gap. The exact band gap values of prepared NPs were determined by Tauc's plot ((αhν)2 vs. ) shown in Figure 1(b). The band gap of pure ZnO is 3.09 eV and band gap values of Ag doped (1–3%) and Cu-doped (1–3%) ZnO are shown in Table 1. It has been observed that the optical band gaps of all prepared Ag and Cu doped ZnO NPs are lower than pure ZnO NPs. It can be observed that 2% Ag doped ZnO NPs exhibit lower optical band gap energy compared to all other prepared NPs. For both Ag and Cu doped NPs, the band gap value decreases up to 2% doping. The decrease in optical band gap energy in doped NPs may be due to the merging of the impurities band with the conduction band and p-d spin exchange interaction of band electrons and d electron of doped Ag and Cu particles (Wu et al. 2014a, 2014b). However, the band gap of 3% doped Ag and Cu begins to increase compared to 2% doped ZnO NPs which may be due to the defects (i.e. oxygen vacancies), carrier concentrations and structural parameters which may cause Burstein Moss shift (Manish et al. 2014b).
Table 1

Absorption peaks and band gap values for Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs

 Ag-doped
Cu-doped
Doping percentage (%)λmax (nm)Band gapλmax (nm)Band gap
377 3.00 373 3.07 
377 2.79 376 2.83 
382 2.84 374 2.85 
 Ag-doped
Cu-doped
Doping percentage (%)λmax (nm)Band gapλmax (nm)Band gap
377 3.00 373 3.07 
377 2.79 376 2.83 
382 2.84 374 2.85 
Figure 1

(a) UV-visible absorption spectra of pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs, (b) Touc's plot for pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs.

Figure 1

(a) UV-visible absorption spectra of pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs, (b) Touc's plot for pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs.

XRD studies

The XRD peaks of PVP capped pure, Ag (1–3%) and Cu (1–3%) doped ZnO NPs are shown in Figure 2. The strong diffraction peaks are assigned to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes and reveal that the prepared NPs are of hexagonal wurtzite phase (ICDD No. 36-1451). The diffraction peaks were narrower and highly intensive which confirms that both Cu and Ag doped ZnO NPs exhibit a good crystalline structure (Siva Vijayakumar et al. 2013). The crystallite sizes of prepared particles are determined by the Debye Scherer equation:
formula
3
where Dp is average crystallite size (nm), β is the full width at half maximum (radian), θ is Bragg angle (radian), λ is X-ray wavelength (nm). The average particles size is 68.01 nm for undoped, 65.503 nm for 1% Cu doped, 62.54 nm for 2% Cu doped, 58.383 nm for 3% Cu doped, 74.18 for 1% Ag doped, 55.28 for 2% Ag doped and 53.87 for 3% Ag doped ZnO NPs.
Figure 2

XRD pattern pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs.

Figure 2

XRD pattern pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs.

Incorporation of Cu and Ag ions in the ZnO lattice was confirmed by the shift in diffraction peaks and broadening of diffraction peaks with respect to undoped ZnO NPs (Saravanan et al. 2014). Further, the successful doping of Ag and Cu ions in ZnO NPs was confirmed by change in size of prepared Cu and Ag doped NPs compared with undoped ZnO NPs (Alkahlout et al. 2014). In the case of Cu doped ZnO, major peaks shifted towards the higher angle with respect to the pure ZnO and the size of the prepared Cu doped ZnO decreased with doping percentage. This phenomenon is due to the replacement of Zn2+ (0.074 nm) ions with Cu2+ (0.073 nm) ions with different ionic radius (Wu et al. 2014a, 2014b). Contrary to that, the size of the 1% Ag doped ZnO increased, because of the difference in the ionic radius of Zn2+ and Ag+ (1.15 nm). But, it was observed that the size of 2% and 3% Ag doped ZnO NPs were reduced. This is due to growth of the crystalline nature which might be hindered by the presence of higher (Ag) dopant concentration. All other secondary peaks correspond to zinc oxide sulfate (ICDD No. 712475) which was formed due to incomplete reaction.

FE-SEM and EDX studies

FE-SEM images of pure ZnO NPs, Ag (2%) and Cu (1%) doped ZnO NPs are shown in Figure 3. The images reveal that undoped, Ag (2%) and Cu (1%) doped ZnO NPs exhibit a flower-like microstructure with nano leaf and the morphology of the synthesized Ag and Cu doped ZnO NPs was not affected by the incorporation of Ag and Cu ions in the ZnO lattice. Figure 4 shows the EDS of Ag (1–3%) doped and Cu (1–3%) doped ZnO NPs. EDS analysis confirms the presence of the dopant ions (Ag and Cu) in the Ag and Cu doped ZnO NPs, respectively (Figure 4). A trace amount of sulfur and carbon are present in all the samples which may result from the incomplete conversion of ZnSO4 into Zn(OH)2 and capping agent PVP, respectively (Pung et al. 2012).
Figure 3

FE-SEM images of (a) pure ZnO NPs, (b) Ag (2%) doped ZnO NPs and (c) Cu (1%) doped ZnO NPs.

Figure 3

FE-SEM images of (a) pure ZnO NPs, (b) Ag (2%) doped ZnO NPs and (c) Cu (1%) doped ZnO NPs.

Figure 4

Energy dispersive spectra (counts per second (cps) vs. energy) of (a) Ag (1%) doped, (b) Ag (2%) doped, (c) Ag (3%) doped, (d) Cu (1%) doped, (e) Cu (2%) doped and (f) Cu (3%) doped ZnO NPs.

Figure 4

Energy dispersive spectra (counts per second (cps) vs. energy) of (a) Ag (1%) doped, (b) Ag (2%) doped, (c) Ag (3%) doped, (d) Cu (1%) doped, (e) Cu (2%) doped and (f) Cu (3%) doped ZnO NPs.

Photocatalytic activity

The photocatalytic degradation of undoped, Ag and Cu doped ZnO particles was tested with BG dye under UV-visible light. Figure 5(a) shows the percentage of removal of BG dye for pure, Ag and Cu doped ZnO NPs with respect to time. It was found that that 88.03% of BG dye was degraded under UV light in 3 h duration in the presence of pure ZnO NPs. This phenomenon may be due to the fact that when the undoped ZnO NPs are illuminated with light energy equal to or more than the band gap energy of ZnO, electron-hole pairs are induced. The photoinduced electrons could combine with oxygen to form super oxide radicals. In addition to that, the photogenerated holes may react with water molecules to generate hydroxyl radicals. Both hydroxyl and superoxide radicals are strong oxidizing agents which can directly oxidize the BG dye to produce CO2 and H2O (Pung et al. 2012). It was observed that degradation of BG dye was increased when ZnO NP was doped with Ag and Cu ions. This may be due to the dopant Ag and Cu ions trapping the photoinduced electrons to prevent electron-hole recombination which induces the active OH. radicals and oxygen vacancies defects (Manish et al. 2014a). In addition to that, formation of Fermi level and surface plasmon resonance may further increase the photocatalytic activity of prepared doped photocatalyst in the visible light region. The mechanism of photocatalytic activity of prepared Ag/Cu doped ZnO NPs is explained in Figure 6. It can be seen that maximum degradation of BG dye of 90.14% and 100% were achieved for 1% Cu doped ZnO NPs and 2% Ag doped ZnO NPs, respectively, in 3 h duration. From Figure 5(a), it can be seen that the photocatalytic degradation of the dye was not increased with doping percentage. This phenomenon may be due to a higher concentration of Ag and Cu doping promoting a cyclic process without inducing O. radicals (Sclafani et al. 1992). Actually, there is a need for an optimum concentration of dopant ions to match the light penetration with the thickness of the depletion region. In addition, a higher doping percentage may lead to increased recombination of photo induced electron-hole pairs which results in a reduction in the photocatalytic activity of the prepared photocatalyst (Yu et al. 2013). The photocatalytic degradation of BG dye on the surface of pure, Cu doped and Ag doped ZnO follows a pseudo-first-order kinetic law, which is expressed in the following equation:
formula
4
where C0 and C are the reactant concentration at time t = 0 and t = t, respectively, and k is the reaction rate constant. The relationship between ln(C0/C) and time is shown in Figure 5(b) and the reaction rate constant (k) and linear regression coefficient (R) are presented in Table 2. It was observed that maximum photocatalytic degradation rate and reaction rate constant of 0.031 min−1 were obtained for 2% Ag doped ZnO NPs under UV radiation compared to all other synthesized NPs. Figure 5(c) confirms the complete degradation of BG dye after the stipulated period of time.
Table 2

Reaction rate constant of BG dye degradation and regression coefficient for pure, Cu (1–3%) doped and Ag (1–3%) doped ZnO NPs

S. no.Catalystk (Reaction rate constant) (min1)R2 (Regression coefficient)
Pure ZnO 0.012 0.972 
1% Cu doped ZnO 0.013 0.959 
2% Cu doped ZnO 0.011 0.976 
3% Cu doped ZnO 0.09 0.977 
1% Ag doped ZnO 0.022 0.915 
2% Ag doped ZnO 0.031 0.915 
3% Ag doped ZnO 0.016 0.979 
S. no.Catalystk (Reaction rate constant) (min1)R2 (Regression coefficient)
Pure ZnO 0.012 0.972 
1% Cu doped ZnO 0.013 0.959 
2% Cu doped ZnO 0.011 0.976 
3% Cu doped ZnO 0.09 0.977 
1% Ag doped ZnO 0.022 0.915 
2% Ag doped ZnO 0.031 0.915 
3% Ag doped ZnO 0.016 0.979 
Figure 5

(a) Percentage removal of BG dye with respect to time for pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs, (b) kinetics of BG dye for pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs and (c) absorption spectra of BG dye aqueous solution for 3 h duration degraded by pure, Ag (2%) doped and Cu (1%) doped ZnO NPs.

Figure 5

(a) Percentage removal of BG dye with respect to time for pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs, (b) kinetics of BG dye for pure, Ag doped (1–3%) and Cu doped (1–3%) ZnO NPs and (c) absorption spectra of BG dye aqueous solution for 3 h duration degraded by pure, Ag (2%) doped and Cu (1%) doped ZnO NPs.

Figure 6

Photocatalytic degradation mechanism of Ag/Cu doped ZnO NPs.

Figure 6

Photocatalytic degradation mechanism of Ag/Cu doped ZnO NPs.

Effect of photocatalyst dosage

To study the effect of catalyst loading of 2% Ag doped ZnO NPs in the degradation of BG dye under UV-visible light, the concentration of catalyst was varied from 2 g/L to 4 g/L. Figure 7 shows the ln(C0/C) vs. irradiation time for the different photocatalyst loading. It was observed that the k value increases with catalyst loading up to 0.35 g/L; beyond that, the k value was reduced (Table 3). This is because when catalyst loading is increased, the number of available active sites for the adsorption of dye molecules increases and generation of oxidizing radicals increases which enhances the rate of degradation. But, when catalyst concentration is increased beyond the optimum level, the turbidity of the dye solution increases which reduces penetration of light through the slurry, i.e. producing a light screening effect. This results in the unavailability of photons for the degradation of adsorbed dye molecules on the surface of the catalyst. Therefore, there is a reduction in the photocatalytic degradation.
Table 3

Pseudo-first-order rate constant of photocatalytic degradation of BG dye and regression coefficient for the different catalyst dosage of 2% Ag doped ZnO NPs at 10 ppm initial dye concentration

S. no.Catalyst loading (g/L)k (Reaction rate constant) (min1)R2 (Regression coefficient)
2.0 0.014 0.940 
2.5 0.031 0.905 
3.0 0.031 0.928 
3.5 0.032 0.899 
4.0 0.021 0.923 
S. no.Catalyst loading (g/L)k (Reaction rate constant) (min1)R2 (Regression coefficient)
2.0 0.014 0.940 
2.5 0.031 0.905 
3.0 0.031 0.928 
3.5 0.032 0.899 
4.0 0.021 0.923 
Figure 7

Influence of catalyst loading on the photocatalytic degradation of BG dye using 2% Ag doped ZnO NPs.

Figure 7

Influence of catalyst loading on the photocatalytic degradation of BG dye using 2% Ag doped ZnO NPs.

Effect of initial dye concentration

The effect of initial dye concentration on the degradation of BG dye was studied by varying initial dye concentration from 10 ppm to 50 ppm. From the results, the k value decreases with increase in initial dye concentration (Table 4 and Figure 8). This phenomenon may be due to the increase of the initial dye concentration leading to enhanced adsorption of dye molecules on the surface of the photocatalyst. This reduces available active sites for the generation of hydroxyl radicals (Mai et al. 2008). Moreover, the higher concentration of dye may reduce the number of photons or path length of photons which travel to the surface of the photocatalyst (Beer-Lambert's law). Therefore, more light is absorbed by organic molecules and simultaneously the excitation of e- is prevented which results in the decrease in the photocatalytic activity of the prepared photocatalyst (Sobana & Swaminathan 2007; Sobana et al. 2008; Pouretedal et al. 2009; Sohrabnezhad et al. 2009).
Table 4

Pseudo-first-order rate constant of photocatalytic degradation of BG dye and regression coefficient for the different initial dye concentrations at 2.5 g/L of 2% Ag doped ZnO NPs

S. no.Initial dye concentration (ppm)k (Reaction rate constant) (min1)R2 (Regression coefficient)
10 0.031 0.915 
20 0.024 0.905 
30 0.007 0.923 
40 0.003 0.953 
50 0.001 0.940 
S. no.Initial dye concentration (ppm)k (Reaction rate constant) (min1)R2 (Regression coefficient)
10 0.031 0.915 
20 0.024 0.905 
30 0.007 0.923 
40 0.003 0.953 
50 0.001 0.940 
Figure 8

Effect of initial concentration of BG dye on the photocatalytic activity of 2% Ag doped ZnO NPs.

Figure 8

Effect of initial concentration of BG dye on the photocatalytic activity of 2% Ag doped ZnO NPs.

Effect of temperature

The influence of temperature on photocatalytic degradation was studied between 32 and 52 °C at 10 ppm initial dye concentration and 2.5 g/L catalyst loading. It was observed that rate of degradation increases with temperature. Activation energy, and change in entropy and enthalpy are calculated using Arrhenius and Eyring plots which are shown in Figure 9 and the change in the free energy is calculated using Equation (7).
formula
5
formula
6
formula
7
where k is the rate constant, k0 is the frequency factor, R is the gas constant, T is temperature, is change in enthalpy, is change in entropy, is change in free energy, kB is the Boltzmann constant, and h is Planck's constant. Activation energy is calculated as 15,389.21 kJ/kmol. The positive change in the entropy ( = 163.08 kJ/kmol) and negative change in the free energy change ( = −38,599.90 kJ/kmol) confirms the feasibility of reaction at the given condition. The positive change in the enthalpy (12,770.30 kJ/kmol) reveals the endothermic nature of the reaction. Also, the endothermic nature confirms that an increase in temperature results in an increase in the rate constants which is in accordance with the van 't Hoff equation.
Figure 9

(a) Arrhenius plot and (b) Eyring plot.

Figure 9

(a) Arrhenius plot and (b) Eyring plot.

Reusability of catalyst

Photocatalysis is a zero discharge technique, which eliminates solid waste disposal problems. The prepared Ag doped ZnO NPs can be recycled twice with only infinitesimal changes in their activity without any regeneration. Further use of used catalyst results in a significant drop in the photocatalytic activity. The reduction in the activity of reused catalyst may result from fouling of the photocatalyst due the deposition of hydroxide radicals on the surface of the catalyst.

CONCLUSION

In summary, PVP capped Ag and Cu doped ZnO photocatalyst was synthesized successfully via the co-precipitation method. The as-prepared catalyst exhibited a hexagonal wurtzite crystal structure with the size range of 55–69 nm. Meanwhile, Ag and Cu doping have significantly affected the optical property and narrowed the band gap energy level. Further, the 2% Ag doped and 1% Cu doped ZnO photocatalyst exhibited better photocatalytic activity for BG dye compared to pure ZnO NPs. The 2% Ag doped ZnO displayed the higher photocatalytic activity and optimum catalyst dosage was 3.5 g/L. Thermodynamic study demonstrated that temperature does not have a significant effect on the photocatalytic degradation of BG dye. Therefore, the as-prepared Ag and Cu doped ZnO catalyst are promising candidates for the photocatalytic degradation of toxic organic effluent at room temperature.

ACKNOWLEDGEMENTS

The authors convey sincere thanks to the Management and Principal of Coimbatore Institute of Technology through the Technical Education Quality Improvement Programme (TEQIP) fund, Coimbatore – 641014.

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