Introduction

Paraoxon is one of the most important commercial organophosphate insecticides due to its high insecticidal efficiency, wide range of applications, and sufficient solubility in water. Nevertheless, due to its wide use in agriculture, it is commonly recognized as an environmental pollutant in surface water, soil, and groundwater [1]. In this regard, advanced technologies including microbial degradation, the use of nanomembrane, and photocatalytic decomposition have been developed for the removal of insecticides from contaminated environments [2-4]. Among the mentioned technologies, photocatalytic decomposition of the insecticides has some merits including [5, 6].  

ZnO is a widely used semiconductor photocatalyst, which has a wide application in the photocatalytic degradation of different pollutants. The major advantages of ZnO are its good chemical stability, non-toxicity, low production cost, and high eco-friendly [7, 8]. However, it has a high energy band-gap value, and low visible light photocatalytic activity, furthermore, the fast recombination of photogenerated electrons and holes remarkably reduces its photocatalytic performance [9]. Therefore, to solve these problems and to improve the visible light photocatalytic efficiency of  ZnO, different techniques have been developed in recent years such as doping [10], surface modification [11], morphology manipulation [12], defect engineering [13], and formation of hybrids and heterojunctions [9]. Among the above strategies, coupling of ZnO with other semiconductors in the heterojunction composite form has gained considerable interest, due to the potential capability of this method to enhance the photocatalytic activity via extending the visible-light harvesting and suppression of the recombination of the photogenerated electron–hole pairs. In this regard, various heterojunction photocatalysts of ZnO have been prepared such as ZnO/ZnBi₂O₄ [14], ZnO/CuO [15], ZnO/MoS₂[16], ZnO/g-C₃N₄ [17], ZnO/Bi₂MoO₆ [18], and Ag₂O/ZnO [19].

During the past decades, the p-type Copper (I) oxide (Cu₂O) semiconductor has gained remarkable consideration as a promising photocatalyst for the photocatalytic decomposition of various pollutants [20]. Nevertheless, it has a narrow band gap, and because of the fast recombination of the photoinduced electron–hole pairs on its surface, Cu₂O has low photocatalytic efficiency. In order to overcome this restriction, various techniques have been studied for the reduction of the charge carriers recombination on this semiconductor, one of the most promising strategies is the compositing of Cu₂O with other semiconductors in heterojunction form, such as TiO₂/Cu₂O [21], Ag₂O/Cu₂O [22], Cu₂O/Bi₂S₃ [23], Fe₂O₃/Cu₂O [24], Cu₂O/Cu₂V₂O₇ [25], and SrTiO₃/Cu₂O [26].

In the current study, ZnO-Cu₂O heterojunction photocatalyst was produced by an innovative hydrothermal method from ZnO nanorods, and Cu₂O nanoparticles and was applied as a visible light active photocatalyst for the degradation of Paraoxon insecticide. Based on the literature review, there is no report on the construction of ZnO-Cu₂O heterojunctions with nanorods and nanoparticles morphologies; in addition, there is no published article about the usage of ZnO-Cu₂O photocatalyst for the degradation of Paraoxon insecticide. The prepared photocatalysts were characterized by XRD, EDS, FESEM, Mott-Schottky, photocurrent, and DRS analysis.

EXPERIMENTAL

Materials

Zn (NO₃)₂.6H₂O, Ethylene glycol, Na₂CO₃, ammonia solution (25%), ethanol, Copper(II) nitrate trihydrate, Glycine, Potassium hydroxide were purchased in analytical grade from Merck, Germany, and were used as raw materials without any purification.

Synthesis of ZnO nanorods

To prepare ZnO nanorods, 1.5 g of zinc nitrate hexahydrate and 3 ml of were dissolved in 50 ml then 0.5 g of sodium carbonate was added and stirred for 2 hours. The resulting solution was transferred into a Teflon-lined stainless autoclave and subjected to a hydrothermal process at 120 ℃ for 12 hours. The final precipitates were immediately separated by centrifugation, washed with water and ethanol, and dried at 80°C. In this method, sodium carbonate was used as a weak base for the shape-controlled conversion of zinc cations to ZnO nanorods which were converted to CO₂ gas during the hydrothermal process.

Synthesis of Cu2O nanoparticles

To synthesize Cu₂O nanoparticles, 1.21 g of Copper (II) nitrate trihydrate was dissolved in 50 ml deionized water, and in another beaker, 0.75 g of Glycine amino acid was dissolved in 50 ml deionized water. These two solutions were mixed and under ultrasonication, 0.56 g Potassium hydroxide was added to the mixed solution. Later on, the solution was poured into a Teflon-lined stainless autoclave and was subjected to the hydrothermal process for 5 hours at a temperature of 200 ℃. The resulting nanoparticles were separated by centrifugation, washed with water and ethanol, and dried at 80°C.

Synthesis of ZnO-Cu₂O nanocomposite

In a typical procedure, for the preparation of ZnO-Cu₂O heterojunction photocatalyst with 10, 20, 30, 40, and 50% weight percentage of Cu₂O nanoparticles, which are labeled as ZnO-10Cu2O, ZnO-20Cu2O, ZnO-30Cu2O, ZnO-40Cu2O, and ZnO-50Cu2O, desired amounts of the prepared ZnO nanorods was fully dispersed by probe ultrasonication in the final solution of Cu₂O as described in the above section. At that point, the final suspension was poured into Teflon lined stainless autoclave and subjected to the hydrothermal process for 5 h at a temperature of 200 ℃. After completing the process, the prepared samples were extracted by centrifuge, and after washing with deionized water and ethanol, dried at 80 ℃.

Photocatalytic activity

The photocatalytic efficiencies of the synthesized samples were investigated by measuring the degradation of Paraoxon insecticide under visible light irradiation. A 570W Xenon lamp equipped with an L41 UV-cut-off filter (Kenko Co.) was used as a visible light source. Briefly, 50 mg of photocatalyst sample was fully dispersed in 100 mL of the aqueous solution of Paraoxon with the initial concentration of 30 mg/L. The resulting suspension was maintained under dark conditions and stirred for 4 h to reach an adsorption–desorption equilibrium, and afterward was irradiated at the pH level of 7 and temperature of 25 ℃. Every 15 min, 5 mL of aliquot was sampled and immediately centrifuged to deposit the remnant photocatalyst nanocomposites, and the remaining concentration of Paraoxon insecticide was measured via Cary 100 Bio UV–Vis spectrophotometer.

RESULTS AND DISCUSSION

The crystal structure of the synthesized samples was characterized by X-ray diffraction (XRD) using a Philips X’Pert MPD X-ray diffractometer (Netherlands) with Cu Kα radiation (λ = 1.54056 Å). In Fig. 1, for the ZnO nanorods, the major diffraction peaks positioned at 2θ = 31.9°,34.7°, 36.5°, 47.9, 56.8°, 62.9°, 68.1°, and 69.3° can be respectively assigned to (100), (002), (101), (102), (110). (103), (112), and (201) lattice planes of the hexagonal phase of ZnO (JCPDS # 80-0074) [27]. In the graph of Cu₂O nanoparticles, the peaks at 2θ = 29.9°,37.8°, 42.9°, 61.9, 74.1°, and 78.1° can be assigned to the (110), (111), (200), (220), (311) and (222) planes respectively, corresponding to the cubic Cu₂O structure (JCPDS 65-3288) [28]. In the XRD graph of the ZnO-Cu₂O heterojunction sample, the diffraction lines of ZnO and Cu₂O are seen, indicating the victorious synthesis of the heterojunction nanocomposite. Broadening of the XRD lines indicates the nanostructure nature of the prepared samples and the estimated crystallite sizes based on Debye-Scherrer’s formula [29] for the ZnO and Cu₂O samples are about 18 nm and 20 nm, respectively.

The FE-SEM experiment using Tescan MIRA 3 FESEM (Czech Republic), was used to investigate the size and morphology of the prepared nanostructures. The FE-SEM images of the synthesized ZnO-40Cu2O heterojunction are seen in Figs. 2A and B. In this image, the ZnO nanorods with an approximate diameter of 50-70 nm can be observed, and Cu₂O nanoparticles are also seen in this image. The difference between particle size approximated from Debye-Scherrer’s equation and that of the FE-SEM images is mainly related to the limitations of Debye-Scherrer’s equation [30].

To confirm the presence of ZnO and Cu₂O compounds in the ZnO-Cu₂O nanocomposite, the EDS analysis was carried out on the ZnO-40Cu₂O sample to identify the elemental composition and to authenticate the presence of Cu₂O and ZnO in this sample. The peaks of Zn, O, and Cu elements are seen in the EDS spectrum of the ZnO-40Cu₂O sample in Fig. 2C, indicating the victorious synthesis of the ZnO-Cu₂O heterojunction. Based on the result of this analysis, the weight percentage of the Zn, O, and Cu elements in the ZnO-40Cu₂O sample are about 54%, 20%, and 26%, respectively. Therefore, it could be concluded that the weight percentage of Cu₂O in this sample is lower than that of the expected 40%, which may be related to the incomplete reaction of the Cu₂O preparation method.

The optical characteristics of the synthesized photocatalysts were studied by UV-Vis diffuse reflectance spectroscopy (UV-DRS) on Shimadzu UV-2550 UV–vis spectrophotometer (Japan). As shown in Fig. 3, the absorption edge of the ZnO nanorods is located at about 400 nm and the absorption edge of the Cu₂O nanoparticles is about 630 nm. Furthermore, compared with ZnO, the absorption edge of the ZnO-Cu₂O nanocomposite has a redshift, therefore compositing of ZnO with Cu₂O could enhance the visible light harvesting capability of ZnO. The band gap energy (Eg) of the synthesized photocatalysts was estimated by the Kubelka–Munk function [24]. Based on Fig. 3b, the Eg values of ZnO and Cu₂O are about 3 eV and 1.3 eV, respectively. Whereas the ZnO-Cu₂O heterojunction photocatalyst has two Eg values at 2.8 and 1.3 eV, resulting from ZnO and Cu₂O semiconductors. Decreasing the Eg of ZnO confirms the considerable effect of the heterojunction compositing on the narrowing of the ZnO band gap [31], which could result in the enhancement of its visible light absorption capability and visible light photocatalytic efficiency.

The photocurrent analysis could be applied for investigating the charge carriers’ separation on the synthesized samples. Any increase in the photocurrent density could be related to the suppression of charge carriers’ recombination, which could result in the improvement of the photocatalytic efficiency [32]. As seen in Fig. 4, the photocurrent density of the ZnO-40Cu2O heterojunction nanocomposite is remarkably higher than that of the bare ZnO sample. Therefore, this increase in the photocurrent density could be related to the reduction of the recombination of charge carriers. Therefore, because of the decreasing recombination rate of charge carriers on the ZnO-Cu₂O nanocomposite, this sample could have improved photocatalytic activity. 

The potentials of the conduction band edge (E_C) and the valance band edge (E_V)  of the ZnO and Cu₂O samples was estimated by the Mott-Schottky experiment, as seen in Fig. 5. The positive slope of the Mott-Schottky plot of the ZnO photocatalyst demonstrates the n-type nature of this semiconductor. But, the negative slope of the Mott-Schottky plot of the Cu₂O sample, indicates that is the p-type nature of Cu₂O [33, 34]. As shown in Fig. 5a and Fig. 5b, the E_FB of Cu₂O and ZnO are around +0.1 V and -0.6 V (vs. Ag/AgCl) (+0.3 V and -0.4 V vs. NHE), respectively. As previously reported, for an n-type semiconductor, the E_FB value is 0.1 V lower than the E_C, and for a p-type semiconductor, E_FB is about ~0.1 eV higher than the E_V [35]. Therefore, the E_C of Cu₂O and ZnO samples are about +0.4 and -0.5 eV vs. NHE, respectively. E_V of Cu₂O and ZnO samples are estimated by E_V = E_C + Eg equation, hence, the E_V of these samples are 0.9 and 2.5 eV vs. NHE, respectively.

The photocatalytic efficiencies of the produced samples were examined by evaluating the photocatalytic degradation of Paraoxon insecticide on the prepared photocatalysts under the irradiation of visible light. As seen in Fig. 6(A), without the addition of any photocatalyst (Blank) sample, Paraoxon is not degraded, indicating its stability under visible light. On the other hand, considerable decomposition is seen in the existence of ZnO and ZnO-Cu₂O heterojunction nanocomposites, and about 100% of Paraoxon is decomposed after 180 min of irradiation, on ZnO-40Cu₂O heterojunction sample with 40% weight percentage of Cu₂O nanoparticles which has the best performance. Because of the enhancement of the visible light harvesting capability and due to the reduction of the electron-hole pairs recombination on the ZnO-Cu₂O samples, these samples have considerably enhanced photocatalytic activity. The kinetics of the photocatalytic degradation reactions of Paraoxon over the prepared photocatalysts were evaluated based on the Pseudo first-order equation (Eq. (1)) according to the Langmuir-Hinshelwood (L–H) model [36].

               (1)

Where  is apparent reaction constant, and  and  are the Paraoxon concentrations at illumination time (t) of 0 and t respectively. Fig. 6(B) shows the obtained plots for the photocatalytic decomposition of Paraoxon over the synthesized photocatalysts.  of the ZnO, and ZnO-40Cu₂O samples are measured as 0.0021, and 0.0201 min^-1, respectively. Therefore, the existence of Cu₂O in the ZnO-Cu₂O heterojunction considerably improves the visible light photocatalytic efficiency of ZnO. In table.1 the photocatalytic performance of the ZnO-40Cu₂O heterojunction with previously reported works, as seen in this table this sample has improved photocatalytic performance.

To study the function of hydroxyl radical (OH^•), superoxide radical (O₂^•−), and hole, on the photocatalytic degradation of Paraoxon over the ZnO-Cu₂O heterojunction, tert-Butyl alcohol (t-BUOH), benzoquinone (BQ), and Ethylenediaminetetraacetic acid (EDTA) were added into the reaction solution as scavengers of these species, respectively [42]. As seen in Fig. 7 the highest decrease in the photocatalytic performance is observed in presence of benzoquinone, indicating the main function of superoxide radicals for the photocatalytic decomposition of Paraoxon. Furthermore, the photocatalytic degradation is also decreased in presence of t-BUOH. Therefore, hydroxyl and superoxide radicals are the major oxidizing species responsible for the photocatalytic activity of ZnO-Cu₂O nanocomposite under visible light illumination.

Fig. 8 indicates a schematic view of the band energy diagram and type II charge transfer processes for the photocatalytic activity of the ZnO-Cu₂O heterojunction photocatalyst, under visible light illumination. With an incidence of visible light photons onto this photocatalyst, the electrons–hole pairs are produced in these semiconductors. The photo-generated electron on the CB of Cu₂O transfers to the CB of ZnO, and simultaneously, the hole on the VB of ZnO transfers to the VB of Cu₂O [43]. Therefore, the recombination of electron–hole pairs is considerably suppressed, and the life span of photoinduced holes and electrons is greatly increased. As a result, more O₂^•− and OH radicals are generated, and the oxidation power of the photoinduced electrons and holes is improved, resulting in the enhancement of the photocatalytic performance. According to the acquired results, the following degradation mechanism could be proposed for the degradation of Paraoxon over the ZnO-Cu₂O heterojunction nanocomposite:

Paraoxon (aq) + OH^• (aq) + O₂^•− (aq)   CO₂ (g)+ H₂O (l) + NO₃⁻ (aq) + PO₄³⁻ (aq) (2)

CONCLUSION

In the current study, a novel heterojunction nanocomposite was prepared via a hydrothermal route from ZnO nanorods and Cu₂O nanoparticles and was applied for the visible light photocatalytic degradation of Paraoxon insecticide. As the results indicated, the best photocatalytic efficiency was obtained for the ZnO-Cu₂O heterojunction nanocomposite with a 40% weight percentage of Cu₂O nanoparticles which has a photocatalytic activity of 0.0201 min^-1. Due to the suppression of the recombination of the photoinduced electron-hole pairs, and enhancement of the visible light harvesting ability, the ZnO-Cu₂O heterojunction has increased photocatalytic activity. Also, based on the Mott-Schottky experiments and the radical trapping tests, a type II charge transfer process was demonstrated for the decomposition of Paraoxon, and superoxide radical was proved as the main active species for the degradation reaction.

CONFLICT OF INTEREST

The authors declare no conflict of interest.