The current study is focused on fabrication of a ternary metal oxide nanocomposite (ZnO/CuO/Ag2O) as an efficient and superior photocatalyst by step-wise implanting of p-type CuO and Ag2O semiconductors onto an n-type semiconductor (ZnO) via a chemical method. The structural and textural characteristics of the manufactured samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy combined with electron dispersive spectroscopy (FESEM-EDS) and UV–visible spectroscopy. The photocatalytic performance of the fabricated ternary nanocomposite was tested against the photocatalytic degradation of crystal violet (CV) and rhodamine B (RhB) organic dyes under solar light irradiation. The ternary nanocomposite demonstrated about 99.05% and 97.38% degradation efficiency toward CV and RhB dyes under solar light irradiation in a time period of 105 min. The calculated rate constants (k, min−1) for degradation under solar light over the ZnO/CuO/Ag2O nanocomposite were 4.26 and 3.61 times higher than the k value obtained over ZnO nanoparticles for CV and RhB dyes, respectively. The main reactive species taking part in the photodegradation processes were •OH and O2 over ZnO/CuO/Ag2O photocatalysts under solar light illumination. Furthermore, the recycle experiments confirmed good reusability and photo-stability of the ZnO/CuO/Ag2O ternary nanocomposite.

  • Ternary metal oxide (TMO) nanocomposite (ZnO/CuO/Ag2O) was prepared by step-wise deposition of p-type CuO and Ag2O semiconductors onto an n-type ZnO semiconductor.

  • As-prepared TMO has displayed enhanced photocatalytic activities against the CV (97.38%) and RhB (99.05%) dyes compared with binary ZnO/CuO metal oxide and ZnO nanoparticles under the solar light irradiation.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Fast-growing populations, rapid rate of urbanization and industrialization have exploited natural resources extensively and large amounts of unchecked and unprocessed effluents are being discharged into the environment which pose grave threats to both biotic and abiotic elements of the ecosystem that have produced environmental disparities globally (Wayland et al. 2016). The created ecological inequities have materialized assorted environmental dilemmas on the planet including water pollution and crisis. In the recent decade, water pollution has emerged as a serious concern across the globe. Heavy metal ions, biomedical waste, agricultural discharge, dyes etc. are the main classes of pollutants by which water resource are being fatally contaminated. Among these, organic dyes are highly noxious pollutants that cause severe health hazards to human life. Thus, it is very much indispensable to eradicate organic dyes from water resources (Sundaram et al. 2019).

Currently, several practices are in application to wipe out water pollutants, such as electrochemical oxidation, filtration, adsorption, advance oxidation processes, and sedimentation processes, coagulation, etc. (Sankar et al. 2016) However, among these, conventional techniques like filtration, adsorption, coagulation etc. are expensive, less effective and environmentally not viable, while the advance oxidation process (AOP) seems to be the most appropriate method as it is an inexpensive, highly stable, and eco-friendly process (Upadhyay et al. 2020; Zarrin & Heshmatpour 2020). Furthermore, AOP is able to degrade organic pollutants into nonhazardous inorganic products without the creation of any other type of secondary pollutants. At this time, researchers across the globe have committed their endeavors to develop smarter and superior semiconducting nanostructures for proficient removal of pollutants from contaminated waters. In recent times, semiconducting metal oxide nanostructured materials have gained significant importance owing to their novel distinctiveness and potential applications in various fields (Shafi et al. 2019).

Particularly, transition metal oxides, for instance zinc oxide (ZnO), copper oxide (CuO), silver oxide (Ag2O), titanium oxide (TiO2) etc., are judged astonishing semiconducting metal oxides that effectively disintegrate organic pollutants including dyes from polluted water by absorbing light photons (Ghulam et al. 2013; Sundaram et al. 2019). ZnO is n-type semiconductor with a wide band gap (∼3.37 eV) and is considered a propitious photocatalyst due to its high chemical stability and excellent photocatalytic activity. The impressive photocatalytic properties of ZnO are due to its exciting physical and chemical characteristics such as its high electro-chemical stability, superior oxidative capability, and low toxicity (Naseem & Durrani 2021). ZnO nanoparticles (NPs) are typically utilized in many fields such as adsorption, photocatalysis, food preservation, and as pollutant sensors (Abebe et al. 2021).

However, the assorted intrinsic shortcomings of ZnO, such as wide band gap (∼3.37 eV), confine its practical application in the field of water purification and restrict its optical absorption in the UV region of the solar spectrum (Tian et al. 2012). Furthermore, in aqueous solution ZnO exhibits reduced photo-stability and poor photocatalytic efficiency due to the fast rate of electrons–holes recombinations of ZnO (Lee et al. 2016). In order to overcome these limitations and to enhance the photocatalytic potential of the ZnO photocatalyst, various researchers have dedicated their significant efforts (Tian et al. 2012; Wu et al. 2016; Xu et al. 2017; Naseem & Durrani 2021). In this regard mainly, attention has been paid to doping of metal ions and formation of heterostructures composed of narrow band gap semiconductors with wide band gap ZnO (Qian et al. 2017).

Copper oxide (CuO) is a p-type metal oxide with good characteristics like non-toxicity, availability and a narrow band gap (1.3–2.4 eV). CuO has attracted significant interest due to its unique and outstanding optical, electrical and thermal properties. It is widely used in magnetic storage media, photovoltaics, photocatalysis, gas sensing, and battery applications (Sundaram et al. 2019; Vivek et al. 2019). Assembling ZnO and CuO in a single nanosystem presents a unique p–n-type binary hybrid nanocomposite material that is able to demonstrate effective photocatalytic performance (Huang et al. 2018; Prabhu et al. 2019; Sakib et al. 2019) due to the shift of optical absorption into the visible range, improving charge-separation efficiency and shrinking the rate of electrons–holes recombinations (Pan & Zhao 2015; Tantubay et al. 2020).

Besides this, to bring further advancement in the photocatalytic performance and visible light response of binary metal oxide nanocomposite, designing ternary nanocomposites materials based on metal oxides has been considered an outstanding approach, since the ternary nanocomposites display advanced light absorption, improved stability, and superior catalytic activities. Additionally, the ternary metal oxide (TMO) nanocomposites are effectively utilized in various fields such as in photocatalysis, electrochemical sensing, and as energy storage devices. (Xu et al. 2017; Anjaneyulu et al. 2018).

Silver oxide (Ag2O) is a brown colored powder with and a narrow band gap semiconductor (1.2 eV). It is widely employed in several industrial activities, as colorants, cleaning agents, preservatives, electrode materials, and catalysts for alkane activation and olefin epoxidation (Zhou et al. 2010; Wang et al. 2011; Zhao et al. 2017). However, owing to its photosensitivity and unstable properties under light irradiation, Ag2O is seldom used as the primary photocatalytic material except as a co-catalyst (Wang et al. 2011). However, it is able to turn the UV light response into the visible region due to sensitization and efficient electron absorption under UV light irradiation (Liu 2015; Zhao et al. 2017). Thus, creating novel TMO nanocomposite materials based on transition metal oxide would be a significant initiative for decay of organic pollutants present in contaminated waters under solar light illumination.

Although many ternary nanocomposite photocatalysts have been reported, to the best of our knowledge, except for MOF- templated synthesis of ternary ZnO/CuO/Ag2O nanoheterostructures (Salari & Sadeghinia 2019) and their potential application in water treatment, no other study is available. This has motivated us to rationally design ternary ZnO/CuO/Ag2O nanocomposite photocatalysts with improved photocatalytic efficiency using a simple approach. Therefore, in the present work, for first time we have fabricated novel ternary ZnO/CuO/Ag2O nanocomposites by step-wise grafting of p-type semiconductors (CuO and Ag2O) onto an n-type semiconductor (ZnO) using a facile, inexpensive and reliable low temperature chemical method. Moreover, as-prepared ternary ZnO/CuO/Ag2O nanocomposite were found to exhibit improved photocatalytic activities toward the decomposition of rhodamine B (99.05%) and crystal violet (97.38%) dyes under exposure to visible light. In addition, the results showed that the ternary ZnO/CuO/Ag2O nanocomposites demonstrated significantly improved photocatalytic activities against the RhB and CV dyes, compared to ZnO nanopolygons and ZnO/CuO nanocomposites. The enhanced visible light photocatalytic efficiency, effective charge separation, and simultaneously advanced electronic properties render the synthesized nanocomposite as a potential photocatalyst in the fields of environmental remediation.

Materials used

Zinc acetate dehydrate [Zn(CH3COO)2·2H2O)] as zinc source, copper acetate monohydrate [Cu(CH3COO)2·H2O] as copper source, silver nitrate (AgNO3) as silver source, potassium hydroxide (KOH), sodium hydroxide (NaOH), acetic acid (C2H4O2), hydrochloric acid (HCl), isopropyl alcohol (IPA, C3H8O), benzoquinone (BQ, C6H4O2), ethylenediaminetetra-acetic acid (EDTA, C10H14N2O8Na2), dimethylsulfoxide (DMSO, C2H6SO), and cetyltrimethylammonium bromide (CTAB, C19H42BrN) were utilized in the present study. All chemical reagents utilized were purchased from Loba chemicals. Rhodamine B (RhB, C28H31ClN2O3) and crystal violet (CV, C25H30ClN3) dyes were used as the model pollutant. All chemicals were of analytical grade and used without any further purification.

Synthesis of photocatalysts

Synthesis of ZnO nanopolygons: In a typical synthesis, 10 g of zinc acetate was dissolved in 100 mL of double-distilled (DW) water with 1 mL of acetic acid and 1 g of CTAB and magnetically stirred for 60 min at room temperature. Then 8 g of KOH was dissolved in 100 ml of water separately and added drop-wise to the above solution under continuous stirring until the pH of solution reached 12. A white colored precipitate obtained was continuously stirred at 80 °C for 2 hours, and then further refluxed for the next 60 min at 100 °C. The obtained white colored precipitate was allowed to cool down at room temperature, filtered, and washed several times with DD water and ethanol. The obtained precipitate was then dried in a hot air oven at 85 °C for 5 hours.

Synthesis of ZnO/CuO binary nanocomposite: In this step CuO nanosheets were decorated onto ZnO nanopolygons by adsorption of Cu+2 ions and then further precipitated into CuO nanosheets in basic medium. For this, 1 g of the as-prepared ZnO nanopolygons were dispersed in 100 mL of DD water and stirred for 30 min magnetically to homogenize. Then, 1 g of copper acetate was mixed and stirred for 2 hours at room temperature. The white suspension was turned into a bluish white suspension due to the adsorption of Cu2+ ions on the surface of ZnO nanopolygons. After that, 2 M of KOH solution was added drop-wise until pH reached 11, a blue colored precipitate that appeared changed to a grey black color on heating the solution at 80 °C for 2 hours under constant stirring. The obtained precipitate was cooled down, filtered, washed several times with DD water and ethanol, and finally it was dried in a hot air oven at 85 °C for 5 hours.

Fabrication of ZnO/CuO/Ag2O ternary nanocomposite: In the last step of the synthesis process, Ag2O nanoparticles were deposited onto the ZnO/CuO binary nanocomposite (nanopolygon/nanosheets nanocomposite) by adsorption of Ag+ ions and their further precipitation in basic medium into Ag2O nanoparticles. In this step, 1 g of silver nitrate was dissolved in 100 mL of DD water. Then, 1 g of the as-fabricated binary metal oxide ZnO/CuO nanocomposite was dispersed in the silver nitrate solution and the mixture was stirred for 2 hours at room temperature. After that, 20 mL of 2 M KOH solution was added drop-wise under constant stirring and further stirred for the next 60 min at 80 °C. The Ag2O nanoparticles decorated ZnO/CuO nanocomposite were filtered, washed several times with DD water and ethanol, and dried in a hot air oven at 85 °C for 5 hours. The synthesis procedure for ZnO/CuO/Ag2O ternary nanocomposite is represented schematically in Figure 1.

Figure 1

Schematic illustration of the synthesis protocol of the ZnO/CuO/Ag2O ternary nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 1

Schematic illustration of the synthesis protocol of the ZnO/CuO/Ag2O ternary nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

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Characterization

The morphology and size of the as-synthesized ZnO nanoparticles, binary ZnO/CuO nanocomposite, and ternary ZnO/CuO/Ag2O nanocomposite were observed under a field-emission scanning electron microscope (FESEM) (Nova Nano FESEM-450 (FEI)) equipped with energy-dispersive X-ray spectroscopy (EDS). From the EDS, the chemical compositions of the as-prepared nanomaterials were investigated. The crystalline phases were observed by XRD patterns using a (PANalytical X'Pert PRO) diffraction meter equipped with a CuKα radiation source and with a 2θ range of 10°–80° with a step size of 0.05° and scan speed of 0.05°/min. UV-vis absorption spectra were created on a UV-vis spectrophotometer (LABBDA-750 Perkin Elmer) used for assessing optical properties. Fourier-transform infrared (FTIR) spectra (Perkin Elmer) were recorded to analyze the type of bonding and functional groups in the range 400–4,000 cm−1 with a resolution of 2 cm−1.

Photocatalytic degradation experiments

CV and RhB dyes were chosen as model pollutants to evaluate the photocatalytic activity of the as-prepared ZnO/CuO/Ag2O nanocomposite and other ZnO-based photocatalysts. In a typical experiment, 100 mL aqueous solutions of CV and RhB dye (20 mg/L) and 20 mg of photocatalysts were dispersed in a 250 mL capacity beaker. Prior to solar light irradiation, the suspension were magnetically stirred in the dark for 30 min to achieve adsorption–desorption equilibrium between the dye and the surface of the catalyst. Then, the mixtures were exposed to solar radiation for photocatalysis. At given irradiation time intervals, 5 mL of the dye solutions were collected and centrifuged to remove the catalyst particulates for quantification of degraded dyes. The residual CV and RhB dye concentrations were detected using a UV-vis spectrophotometer. Ultimately, photocatalytic degradation percentage was measured using the following equation:
formula
(1)
where C0 and Ct are, respectively, the initial concentration and concentration at certain times (Anjaneyulu et al. 2018; Zarrin & Heshmatpour 2020).

Characterization of synthesized samples

XRD analyses: The crystal structures and purity of the synthesized samples were analyzed using an X-ray diffraction (XRD) method. XRD patterns of ZnO nanopolygons, ZnO/CuO binary nanocomposite and ZnO/CuO/Ag2O ternary nanocomposite are shown in Figure 2. In the XRD pattern as illustrated in Figure 2(a), ZnO diffraction peaks were obtained at 2θ ≈ 31.80°, 34.54°, 36.30°, 47.57°, 56.66°, 62.94°, 66.44°, 67.92° and 69.20° which correspond to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes, respectively, and indexed to the hexagonal wurtzite crystal geometry of ZnO (JCPDS no. 36-1451) (Chen et al. 2020).

Figure 2

XRD patterns of (a) ZnO nanoparticle, (b) ZnO/CuO and (c) ZnO/CuO/Ag2O nanocomposites. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 2

XRD patterns of (a) ZnO nanoparticle, (b) ZnO/CuO and (c) ZnO/CuO/Ag2O nanocomposites. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

No other characteristic peaks were observed in the samples which designate the high degree of purity of crystal. In the XRD pattern of CuO/ZnO binary nanocomposite as represented in Figure 2(b), four diffraction peaks of CuO at 2θ ≈ 35.8°, 38.5°, 49.0° and 61.5° along with the other characteristic peaks of hexagonal wurtzite phases of ZnO observed, which was attributed to the (002), (111), (−202) and (−113) (JCPDS no. 48-1548) (Tantubay et al. 2020) crystal planes of CuO. The XRD pattern of the ZnO/CuO/Ag2O ternary nanocomposite is exhibited in Figure 2(c). In addition to the crystalline wurtzite ZnO diffraction peaks, four diffraction peaks (002) (111), (202) and (113) (JCPDS no. 48-1548) corresponding to CuO and four diffraction peaks at 2θ ≈ 32.6°, 37.9°, 55.0° and 65.0° corresponding to (111), (200), (220) and (311) crystal planes of Ag2O (JSPDS no. 41-1104) (Shi et al. 2014; Ma et al. 2015) and indexed to hexagonal crystal geometry of Ag2O were observed. The XRD patterns exhibited no other type of peak revealing that the fabricated ternary ZnO/CuO/Ag2O nanocomposite had excellent crystalline structure and high purity.

The average crystallite size was estimated using the Debye–Scherer relation as given below (2):
formula
(2)
where D is the crystallite size of the particle, K represents the Scherrer constant, which is equal to 0.9, λ is the wavelength of light used for diffraction, λ = 1.54 A°, β is the full width at half maximum (FWHM) of the diffraction peak and θ is the angle of reflection. The average crystallite sizes of the ZnO nanoparticles, ZnO/CuO binary and ZnO/CuO/Ag2O ternary nanocomposites were calculated to be 30, 28 and 25 nm respectively.

FESEM images and EDS analysis: The morphologies and size of the as-prepared ZnO nanoparticles, ZnO/CuO binary and ZnO/CuO/Ag2O ternary nanocomposites were investigated by FESEM observation. Figure 3(a)–3(c) show typical scanning electron microscopy (SEM) images of the ZnO nanoparticles at different magnifications, which have diameters of 18–50 nm. The pure ZnO nanoparticles are large in size due to agglomeration of primary particles and demonstrated disc-like and spherical-shaped morphology with a relatively smooth surface. EDS analysis as shown in Figure 4(d) represents the chemical composition of the ZnO nanoparticles and it is revealing that the nanoparticles are composed of Zn and O elements only. The SEM images of the ZnO/CuO nanocomposite are shown in Figure 4(a)–4(c) at different magnifications, which having spherical and sheet-shaped nanoheterostructures with smooth surfaces. The width of the nanosheets varied from 24.80 to 134.4 nm. The EDS analysis as represented in Figure 4(d) and the inset of Figure 4(d) reveals that the ZnO/CuO heterostructures are composed of Zn, O and Cu elements. Figure 5(a)–5(c) show the SEM micrographs of the ZnO/CuO/Ag2O nanocomposite particles that exhibit spherical and sheet-shaped morphology with rough surfaces. The rough surfaces are due to decoration of Ag2O nanoparticles on the surface of the ZnO/CuO particles. The EDS analysis as illustrated in Figure 5(d) and inset of Figure 5(d), displayed the chemical composition of the as-prepared ZnO/CuO/Ag2O nanocomposite, and it unveils that the ternary nanocomposite heterostructures are composed of Zn, O, Cu and Ag elements. The morphology and the EDS analysis supported the XRD results, which confirmed the purity of the fabricated ternary nanocomposite structures.

Figure 3

(a–c) SEM images at different magnification and (d) EDS spectra of ZnO, in inset of (d) elemental composition of ZnO nanoparticle. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 3

(a–c) SEM images at different magnification and (d) EDS spectra of ZnO, in inset of (d) elemental composition of ZnO nanoparticle. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

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Figure 4

(a–c) SEM images at different magnification and (d) EDS spectra of ZnO/CuO, in inset of (d) elemental composition of ZnO/CuO nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 4

(a–c) SEM images at different magnification and (d) EDS spectra of ZnO/CuO, in inset of (d) elemental composition of ZnO/CuO nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

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Figure 5

(a–c) SEM images at different magnification and (d) EDS spectra of ZnO/CuO/Ag2O, in inset of (d) elemental composition of ZnO/CuO/Ag2O nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 5

(a–c) SEM images at different magnification and (d) EDS spectra of ZnO/CuO/Ag2O, in inset of (d) elemental composition of ZnO/CuO/Ag2O nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

FTIR spectra: From the FTIR spectra the presence of functional groups and type of bonding in the synthesized samples can be discovered. Figure 6(a) displays the FTIR spectra of the as-prepared samples in the range 400–1,000 cm−1 at higher resolution in which FTIR bands contributed to metal oxide bonds (Zn–O, Cu–O and Ag–O bonds) properly labeled. Figure 6(b) represents the FTIR spectra related to the pure ZnO, ZnO/CuO and ZnO/CuO/Ag2O nanocomposites in the range 400–4,000 cm−1. The FTIR broad bands observed at 3,400–3,500 cm−1 are allocated to the stretching vibrations of the O–H bond on the surface of samples (Srinivasana & Punithavelan 2017) while the bands at ≈1,650 cm−1 have been attributed to the O–H bending mode of adsorbed water molecules (Figure 6(b)). Additionally, the sharp FTIR bands observed at 442 cm−1 correspond to the Zn–O bond vibration and confirm the formation ZnO nanoparticles (Tan et al. 2019). For the ZnO/CuO nanocomposite the bands located at 488 cm−1 and 444 cm−1 correspond to the Cu–O and Zn–O stretching vibration modes (Siva et al. 2020; Sivasakthi & Gurunathan. 2020). In the TMO nanocomposite ZnO/CuO/Ag2O the characteristic absorption bands reported at 446 cm−1, 490 cm−1 were ascribed to the Zn–O and Cu–O stretching vibrations, while bands noticed at 698 cm−1 and 882 cm−1 corresponded to the bending and stretching vibrations of the Ag–O bond (Wankhede et al. 2013; Kumar & Manisha 2018). Occurrence of vibration bands in the range of 400–1,000 cm−1 in Figure 6 is confirming the formation of metal oxygen bonds.

Figure 6

FTIR spectra of ZnO nanoparticle, ZnO/CuO and ZnO/CuO/Ag2O nanocomposites in the range of (a) 400–1,000 cm−1 and (b) 400–4000 cm−1. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 6

FTIR spectra of ZnO nanoparticle, ZnO/CuO and ZnO/CuO/Ag2O nanocomposites in the range of (a) 400–1,000 cm−1 and (b) 400–4000 cm−1. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

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Optical properties

The optical properties of the as-prepared photocatalysts were characterized to know their band gap energy. The UV–vis spectra of the as-prepared photocatalysts are shown in Figure 7(a) that shows sharp absorption bands at 368, 372 and 374 nm corresponding to pure ZnO nanoparticles, binary ZnO/CuO and ternary ZnO/CuO/Ag2O nanocomposites respectively. The decoration of the narrow band gap semiconductor CuO onto ZnO reflected the red shift in absorption and shows optical absorbance at 372 nm, it is further shifted towards higher wavelengths for ternary the ZnO/CuO/Ag2O nanocomposite due to deposition of narrow band gap semiconductor Ag2O onto the ZnO/CuO surface at 374 nm. The binary ZnO/CuO nanocomposites exhibited continuous light absorption in the range 400–800 nm, which may be related to efficient harvesting of visible light by the CuO due to its narrow band gap and provides the possibility of enhancing visible light absorption for the CuO/ZnO composite (Salari & Sadeghinia 2019). Coupling of CuO with ZnO can effectively realize the optical response shifting of ZnO-based nano photocatalysts from UV to the visible spectral range, which shows the potential of the CuO/ZnO p–n junction heterostructure for visible light-driven photocatalysis. Thus, the coupling of CuO and Ag2O with ZnO and the ZnO/CuO nanostructure respectively is showing a shift in optical response of ZnO and ZnO/CuO significantly from UV to the visible range and displaying the significance of the ternary ZnO/CuO/Ag2O nanocomposite in visible light-induced, driven photocatalysis. The band gap energy values for the as-fabricated ZnO-based nanomaterials were calculated using Equation (3):
formula
(3)
where α is the absorption coefficient, represents the energy of the photon, K is the proportionality constant and varies with the material, and n represents the index or optical transition of semiconductor, n = 1/2 or 2 for direct or indirect band gap semiconductors.
Figure 7

(a) UV-vis.absorption spectra and (b) Tauc plots of ZnO nanoparticle, ZnO/CuO and ZnO/CuO/Ag2O nanocomposites. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 7

(a) UV-vis.absorption spectra and (b) Tauc plots of ZnO nanoparticle, ZnO/CuO and ZnO/CuO/Ag2O nanocomposites. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

Band gaps of the as-prepared samples were calculated by extrapolation the linear portion of Tauc plots drawn between (αhν)2 vs. to cut the energy axis at α = 0, as illustrated in Figure 7(b). Calculated band gap energy values and maximum absorbance for all samples are summarized in Table 1. Figure 7(b) shows a slight red shift in band gap and more visible light absorption compared with CuO/ZnO (Salari & Sadeghinia 2019). The clear deviations in band gap values substantiate the interaction among ZnO, CuO and Ag₂O and validate the formation of hybrid nanocomposites.

Table 1

Optical parameters for prepared samples

NanomaterialsMaximum absorbance (nm)Calculated band gap energy (eV)
ZnO 368 3.1 
ZnO/CuO 372 2.75 
ZnO/CuO/Ag₂O 374 2.68 
NanomaterialsMaximum absorbance (nm)Calculated band gap energy (eV)
ZnO 368 3.1 
ZnO/CuO 372 2.75 
ZnO/CuO/Ag₂O 374 2.68 

Photocatalytic capability

The photocatalytic competencies of the ZnO-based mono, binary and ternary oxide nanomaterials were appraised towards the photodegradation of hazardous RhB and CV dyes, which in general are applied in the textile industries. Before studying the activity of prepared TMO hybrid nanocomposite catalyst, the photocatalytic activities of pure ZnO nanopolygons, binary ZnO/CuO catalysts for degradation of CV and RhB dyes, were evaluated under solar light irradiation. The obtained results of catalytic performances are displayed in Figures 8 and 9. It can be seen from the results that pure ZnO nanopolygons demonstrated a lower degradation efficiency compared to the ZnO/CuO binary nanocomposite. This might be because of the poor absorption of the visible light radiation due to the wide band gap and low charge-separation efficiency of pure ZnO. However, the deposition of CuO nanosheets onto the surface of ZnO nanopolygons has improved the photocatalytic efficiency of the ZnO/CuO binary nanocomposite compare to pure ZnO, which is mainly due to the shift in absorption at higher wavelengths in the visible range, increasing the generation of charge carriers and improving the charge-separation efficiency.

Figure 8

Time-dependent UV-vis absorption spectra for photodegradation of CV in the presence of (a) ZnO nanoparticles, (b) ZnO/CuO and (c) ZnO/CuO/Ag2O nanocomposites, and (d) % degradation of CV under solar light illumination.Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 8

Time-dependent UV-vis absorption spectra for photodegradation of CV in the presence of (a) ZnO nanoparticles, (b) ZnO/CuO and (c) ZnO/CuO/Ag2O nanocomposites, and (d) % degradation of CV under solar light illumination.Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

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Figure 9

Time-dependent UV-vis absorption spectra of RhB in the presence of (a) ZnO nanoparticles, (b) ZnO/CuO and (c) ZnO/CuO/Ag2O nanocomposites, and (d) % degradation of RhB under solar light illumination.Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 9

Time-dependent UV-vis absorption spectra of RhB in the presence of (a) ZnO nanoparticles, (b) ZnO/CuO and (c) ZnO/CuO/Ag2O nanocomposites, and (d) % degradation of RhB under solar light illumination.Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

Particularly, Figures 8(a)–8(c) and 9(a)–9(c) show the UV-vis absorption spectra of CV and RhB dyes, respectively, at different time intervals when illuminated with ZnO nanoparticles, ZnO/CuO binary and ZnO/CuO/Ag2O ternary nanocomposites. From the absorption spectra, it can be clearly observed that the intensities of characteristic adsorption peaks at 595 and 554 nm of CV and RhB dyes have gradually decreased with increasing illumination time. After 105 min of solar light illumination, the absorption spectral lines for both CV and RhB dyes become almost flat and the intensities of the peaks have been roughly observed to be zero for the ternary nanocomposite catalyst, indicating almost complete photodegradation of CV and RhB dyes over the ZnO/CuO/Ag2O ternary nanocomposites. However, it is noticed that the decrease in absorption intensities of UV-vis spectra for reactions catalyzed by pure ZnO photocatalysts are very less compared to ternary nanocomposites for both CV and RhB dyes. But, the decrease in absorption intensities for both CV and RhB dyes, photocatalyzed by ZnO/CuO binary nanocomposites were reported to be higher than for pure ZnO nanoparticles. The time-dependent % photodegradation of CV and RhB dyes in the absence of photocatalysts and in the presence of the photocatalysts (ZnO-based photocatalysts) are depicted in Figures 8(d) and 9(d).

In the case of binary the ZnO/CuO metal oxide nanocomposite, the optical response has shifted from the 368 to 372 nm UV region to the visible region because the band gap has decreased from 3.1 eV to 2.75 eV (Table 1) and the charge separation efficiency of the nanocomposite also improved, which has increased the degradation efficiency of the binary composite compare to pure ZnO. As shown in Figures 8(d) and 9(d), the ternary ZnO/CuO/Ag2O nanocomposites have displayed the greatest photocatalytic performance under solar light irradiation compared to pure ZnO and ZnO/CuO photocatalysts. This is due to the decoration of p-type semiconductor Ag2O onto the ZnO/CuO binary metal oxide nanocomposites surface that has extended the optical absorption of ternary ZnO/CuO/Ag2O nanocomposite toward the higher wavelengths in the visible range and reduced the band gap from 3.1 eV (ZnO) to 2.68 eV (ZnO/CuO/Ag2O). Moreover, the rate of generation of holes–electrons increased along with the charge-separation efficiency and the photocatalytic performance of the TMO ZnO/CuO/Ag2O nanocomposite was improved (Xu et al. 2017; Salari & Sadeghinia 2019). As shown in Figures 8(d) and 9(d) 97.38% of CV and 99.05% of RhB dyes were photodegraded in the presence of TMO ZnO/CuO/Ag2O nanocomposite under solar light irradiation for 105 min at optimum reaction conditions.

Kinetics of photocatalysis

The pseudo-first-order kinetics of the ZnO-based photocatalysts was investigated using the pseudo-first-order model as follows:
formula
(4)
where C0 and Ct are the initial concentration at (0) time and the concentration at different time intervals (t), respectively, and k is the reaction rate constant (min−1). Figure 10(a) and 10(b) exhibit the kinetic behaviors of CV and RhB dyes photodegradation, respectively, over the ZnO, ZnO/CuO and ZnO/CuO/Ag2O nanocomposites. The values of rate constants obtained from the slopes of the graphs drawn as function of time (t) indicated that rates of degradation of CV and RhB dyes by ternary ZnO/CuO/Ag2O are maximum, followed by binary ZnO/CuO and ZnO. The calculated rate constants (k, min−1) are represented by the bar graph in Figure 11. The ZnO/CuO/Ag2O nanocomposite shows that the maximum rate constant and obtained rate constant values for the photodegradation of CV and RhB dyes are 0.0452 min−1 and 0.0346 min−1 respectively, which are around three-fold and four-fold greater than that of the ZnO/CuO nanocomposite. This confirmed the beneficial presence of the Ag2O onto the ZnO/CuO nanocomposite. Furthermore, the influence of pH on kinetics of the photodegradation reactions for CV and RhB dyes was also studied and the results are illustrated in Figure 10(c) and 10(d). The calculated rate constants and R2 values at different pH values are summarized in Table 2. At lower pH values, the surface of the catalyst acquires a positive charge that electrostatically repels the cationic CV and RhB dye molecules, consequently the adsorption of these cationic dyes is reduced on the positively charged surface of the photocatalyst, and as result the photocatalytic degradation activity is lowered. Additionally, in the acidic medium the presence of H+ ions decreases the concentration of OH ions therefore generation of (OH·) hydroxyl radicals is lowered.
Table 2

Pseudo-first-order kinetics parameters for photodegradation of CV and RhB dyes at different pH values over the ZnO/CuO/Ag2O nanocomposite

RhB
CV
pHRate constant (min−1)Regression coefficient (R2)EfficiencyRate constant (min−1)Regression coefficient (R2)Efficiency
5.0 0.009576 0.953 61.40% 0.010659 0.952 65.43% 
7.0 0.015925 0.944 79.39% 0.018445 0.958 84.98% 
10.0 0.03459 0.948 97.05% 0.045174 0.953 99.05% 
RhB
CV
pHRate constant (min−1)Regression coefficient (R2)EfficiencyRate constant (min−1)Regression coefficient (R2)Efficiency
5.0 0.009576 0.953 61.40% 0.010659 0.952 65.43% 
7.0 0.015925 0.944 79.39% 0.018445 0.958 84.98% 
10.0 0.03459 0.948 97.05% 0.045174 0.953 99.05% 
Figure 10

Pseudo-first-order kinetics of photodegradation of (a) CV and (b) RhB, and effect of pH on kinetics of (c) CV and (d) RhB over the ZnO/CuO/Ag2O nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 10

Pseudo-first-order kinetics of photodegradation of (a) CV and (b) RhB, and effect of pH on kinetics of (c) CV and (d) RhB over the ZnO/CuO/Ag2O nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal
Figure 11

Bar graph showing pseudo-first-order rate constants for photodegradation of CV and RhB dyes over the ZnO/CuO/Ag2O nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 11

Bar graph showing pseudo-first-order rate constants for photodegradation of CV and RhB dyes over the ZnO/CuO/Ag2O nanocomposite. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

On increasing the pH of the solution photocatalytic degradation activity increased, because at higher pH values the photocatalyst possesses a negative charge on the surface that attracts the cationic dye molecules and facilitates the adsorption of cationic CV and RhB dyes on the negatively charged surface of the photocatalyst. Additionally, in basic medium, conversion of hydroxide ions into hydroxyl radicals by the catalyst increased (Kumari et al. 2020; Zarrin & Heshmatpour 2020). Therefore, the photodegradation efficiency increases as the pH increases up to 10. Thus, the significant photocatalytic degradation of CV and RhB dyes in basic pH can be credited to the increased adsorption and greater generation of hydroxyl radicals.

Role of active species in photodegradation

During the photodegradation process, different active species such as hydroxyl radicals (OH), holes (h+), electrons (e) and superoxide radical anions (O2) are mainly generated and take part in the degradation of harmful organic pollutants. To detect the main reactive species taking part in the photodegradation process, we have performed photodegradation experiments of CV and RhB dyes using different trapper solutions with ZnO/CuO/Ag2O nanocomposite catalysts under solar light irradiation. The influences of different scavengers were determined in terms of % decrease of photodegradation of CV and RhB dyes and results of the scavenger studies obtained with the ZnO/CuO/Ag2O nanocomposite photocatalysts are presented in Figure 12.

Figure 12

Effect of different scavengers on the photodegradation of CV and RhB dyes over the ZnO/CuO/Ag2O nanocomposite catalyst. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 12

Effect of different scavengers on the photodegradation of CV and RhB dyes over the ZnO/CuO/Ag2O nanocomposite catalyst. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

Four different scavengers such as EDTA, benzoquinone (BQ), isopropyl alcohol (IPA) and dimethyl sulphoxide (DMSO) were used as , , OH, and scavengers, respectively, in the reactive species trapping experiments. It can be seen from Figure 12 that when DMSO is added as e scavenger, a very small decrease in % degradation compare to without using scavenger for both CV and RhB dyes was observed, which signified that electrons (e) are not the main reactive species that are taking part in the photodegradation process for CV and RhB dyes under solar light illumination.

In contrast, a slight reduction in the photocatalytic degradation efficiency for both CV and RhB dyes was evidently observed when BQ and EDTA were used as O2 and h+ quenchers, indicating that O2 and h+ have taken part in the photodegradation process of CV and RhB dyes. However, the efficiencies for both dyes was depressed slightly more in the presence of BQ compared to EDTA, which reflected that the role of O2 is more significant compared to h+ in the degradation of both dyes. Furthermore, when IPA was added to the reaction solutions of CV and RhB dyes as OH scavenger, a drastic decrease in the degradation efficiencies was reported, which indicated that OH are the main reactive species that are involved in the photodegradation processes of CV and RhB dyes over ZnO/CuO/Ag2O under solar light illumination.

Mechanisms of photodegradation

The migration direction of the photoinduced holes and electrons basically depends on the band edge positions of the semiconductors. The conduction band (CB) edge position of ZnO is more negative than those of CuO and Ag2O CB edge positions before their contact or before the creation of heterojunctions, as shown in Figure 13. When they were combined together to create heterojunctions between p-type (CuO and Ag2O) and n-type (ZnO) semiconductors, the Fermi level (EF) of ZnO shifted downwards while the Fermi levels (EF) of (CuO and Ag2O) shifted upwards until the equilibration in the Fermi levels (EF) of all three constituents is achieved. Along with the Fermi levels (EF) shift, the band positions of ZnO, CuO and Ag2O also moved. Consequently, the CB and valence (VB) potentials of CuO and Ag2O become more negative compared with ZnO CB and VB potentials as shown in Figure 13 (Liu 2015; Li et al. 2019; Sang et al. 2020).

Figure 13

Schematic diagram for the photocatalytic mechanism of the ZnO/CuO/Ag2O nanocomposite catalyst under solar light irradiation. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 13

Schematic diagram for the photocatalytic mechanism of the ZnO/CuO/Ag2O nanocomposite catalyst under solar light irradiation. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

When the ZnO/CuO/Ag2O nanocomposite is irradiated with the solar light radiation, the light photons arriving at the photocatalyst surface would be absorbed by ZnO, CuO, and Ag2O. Consequently, the generation of electron–hole pairs from the corresponding VB would take place and move to the respective CBs. But, due to the wide band gap of ZnO (Eg = 3.1 eV), the photoinduced electrons in ZnO could not be directly transferred from the VB of ZnO to its CB under solar light illumination. However, these photoexcited electrons in ZnO might be transferred from the VB of ZnO to the VB of CuO and Ag2O. Subsequently, these electrons from the VBs of CuO and Ag2O can be further transferred to the CBs of CuO and Ag2O. Furthermore, photoexcited electrons accumulated in CBs of CuO and Ag2O will be finally assembled in the CB of ZnO while the holes would be assembled in the VBs of CuO and Ag2O. The accumulated electrons in the CB of ZnO interact with O2 molecules and produce O2 species, while the holes accumulated in the VB of CuO and Ag2O interact with H2O and oxidize into OH radicals, which can take part in photodegradation of CV and RhB dyes under solar light illumination.

Based upon the experimental results, we have proposed a mechanism for the photodegradation activity of the as-prepared ternary ZnO/CuO/Ag2O nanocomposite photocatalyst that is schematically illustrated in Figure 13. The possible reactions that are involved in the photodegradation process over ZnO/CuO/Ag2O nanocomposites can be given by Equations (5)–(13):
formula
(5)
formula
(6)
formula
(7)
formula
(8)
formula
(9)
formula
(10)
formula
(11)
formula
(12)
formula
(13)

Reusability and photo-stability of catalyst

To reveal the photo-stability and reliability of the as-fabricated ZnO/CuO/Ag2O photocatalyst for the potential practical applications, the cyclic photodegradation experiments for CV and RhB dyes were performed. The catalyst was successively used in five runs of cycles for the photodegradation of CV and RhB dyes in aqueous medium as shown in Figure 14(a) and 14(b). After completion of each cyclic experiment, the photocatalyst was recovered by centrifugation and then washed with DD water and ethanol repeatedly, dried for 3 hours at 85 °C in hot air oven and reused for photodegradation of CV and RhB dyes in the next cycle. In each run of the cyclic experiment 100 mL aqueous solutions of CV and RhB dyes (20 mg/L) with recovered ZnO/CuO/Ag2O photocatalyst were exposed to solar light radiation for 105 min for photocatalysis. The concentrations of degraded dyes were measured using a UV-vis spectrophotometer from 5 mL of the dye solutions extracted in the prescribed time intervals.

Figure 14

Recycling experiments of the ZnO/CuO/Ag2O nanocomposite catalyst for the photodegradation of (a) CV and (b) RhB under solar light irradiation. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 14

Recycling experiments of the ZnO/CuO/Ag2O nanocomposite catalyst for the photodegradation of (a) CV and (b) RhB under solar light irradiation. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

It is obvious from Figure 15 that after five runs of cyclic experiments the photodegradation efficiency of catalyst was decreased and approximately reduced by 10% and this was maintained at 88 and 86% for CV and RhB dyes, respectively. The small decline in the degradation potential of the catalyst designates its high stability and reusability in the photocatalysis process of CV and RhB dyes. The slight decrease in the effectiveness of the photocatalyst may be due to adsorption of intermediate species produced during the photodegradation of dyes onto active sites at the surface and may also be due to the photocorrosion of the catalyst. Table 3 shows the comparison of photocatalytic activities of CV and RhB dyes over some earlier reported binary and ternary nanocomposites photocatalysts.

Table 3

Comparison of photocatalytic performance of CV and RhB dyes over previously reported binary/ternary photocatalysts

PhotocatalystSynthesis methodologyDyeEfficiencyTime (min)Light sourceReference
ZnO/CuO/Ag2O ternary nanocomposite Stepwise grafting via chemical method RhB 97.38% 105 Solar light Present work 
ZnO/CuO hollow microspheres Self-assembly method RhB 97.8% 100 Solar light Chen et al. (2020)  
ZnO/CuO nanostructures Hydrothermal method RhB 96.84% 100 UV light Kumari et al. (2020)  
Ag2O/TiO2 NTA SILAR deposition RhB 59.19% 180 Solar light Hou et al. (2020)  
ZnO/CuO nanocomposite Hydrothermal method RhB 63.8% 120 Xe lamp (300 W) Cao et al. (2018)  
(rGO)-ZnO-TiO2 nanocomposite Sonochemical method CV 87.06% 20 UV light Potle et al. (2020)  
Ag3PO4/GO/g-C3N4 Precipitation method RhB 94.80% 50 Visible light Yan et al. (2018)  
ZnO/CuO)/rGO nanocomposites Solid-state method RhB 99% 20 Visible light Kumaresan et al. (2020)  
CoFe2O4 spinel nanoparticles Combustion method RhB 90% 270 min Visible light Nguyen et al. (2019)  
Nd3+ substituted ZnFe2O4 Urea co-precipitation method RhB 98% 210 min Visible light radiation Nguyen et al. (2021)  
(rGO)-ZnO-TiO2 nanocomposite Conventional method CV 72.10% 20 UV light Potle et al. (2020)  
Graphene-Ce-TiO2 & Graphene-Fe-TiO2 nanocomposites One-step in-situ ultrasound assisted method CV 78% 35 Ultrasonic irradiation Shende et al. (2017)  
rGO-TiO2/Co3O4 nanocomposite Co-precipitation method CV 80% 120 Visible light Ranjith et al. (2019)  
(rGO-ZnS-TiO2) ternary nanocomposite Conventional and sonochemical method CV 97% 50 UV light Kale et al. (2020)  
Bi–Fe- selenide Solvothermal method CV 99.22% 150 Solar light Ahmad et al. (2021)  
ZnO/CuO/Ag2O ternary nanocomposite Stepwise grafting via chemical method CV 99.05% 105 Solar light Present work 
PhotocatalystSynthesis methodologyDyeEfficiencyTime (min)Light sourceReference
ZnO/CuO/Ag2O ternary nanocomposite Stepwise grafting via chemical method RhB 97.38% 105 Solar light Present work 
ZnO/CuO hollow microspheres Self-assembly method RhB 97.8% 100 Solar light Chen et al. (2020)  
ZnO/CuO nanostructures Hydrothermal method RhB 96.84% 100 UV light Kumari et al. (2020)  
Ag2O/TiO2 NTA SILAR deposition RhB 59.19% 180 Solar light Hou et al. (2020)  
ZnO/CuO nanocomposite Hydrothermal method RhB 63.8% 120 Xe lamp (300 W) Cao et al. (2018)  
(rGO)-ZnO-TiO2 nanocomposite Sonochemical method CV 87.06% 20 UV light Potle et al. (2020)  
Ag3PO4/GO/g-C3N4 Precipitation method RhB 94.80% 50 Visible light Yan et al. (2018)  
ZnO/CuO)/rGO nanocomposites Solid-state method RhB 99% 20 Visible light Kumaresan et al. (2020)  
CoFe2O4 spinel nanoparticles Combustion method RhB 90% 270 min Visible light Nguyen et al. (2019)  
Nd3+ substituted ZnFe2O4 Urea co-precipitation method RhB 98% 210 min Visible light radiation Nguyen et al. (2021)  
(rGO)-ZnO-TiO2 nanocomposite Conventional method CV 72.10% 20 UV light Potle et al. (2020)  
Graphene-Ce-TiO2 & Graphene-Fe-TiO2 nanocomposites One-step in-situ ultrasound assisted method CV 78% 35 Ultrasonic irradiation Shende et al. (2017)  
rGO-TiO2/Co3O4 nanocomposite Co-precipitation method CV 80% 120 Visible light Ranjith et al. (2019)  
(rGO-ZnS-TiO2) ternary nanocomposite Conventional and sonochemical method CV 97% 50 UV light Kale et al. (2020)  
Bi–Fe- selenide Solvothermal method CV 99.22% 150 Solar light Ahmad et al. (2021)  
ZnO/CuO/Ag2O ternary nanocomposite Stepwise grafting via chemical method CV 99.05% 105 Solar light Present work 
Figure 15

Bar graph showing % degradation efficiency of the CV and RhB dyes photocatalyzed by the ZnO/CuO/Ag2O nanocomposite catalyst within five runs of cycles. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Figure 15

Bar graph showing % degradation efficiency of the CV and RhB dyes photocatalyzed by the ZnO/CuO/Ag2O nanocomposite catalyst within five runs of cycles. Please refer to the online version of this paper to see this figure in colour: http://dx.doi.org/10.2166/wst.2021.431.

Close modal

The sunlight active ZnO/CuO/Ag2O TMO nanocomposite photocatalyst was successfully prepared by step-wise grafting of p-type CuO and Ag2O semiconductors onto n-type ZnO nanopolygons via a chemical method. FESEM, FTIR, EDS, XRD and UV techniques were employed for the characterization of synthesized samples. The photocatalytic performance of the newly designed nanocomposite heterostructure was investigated against photodegradation of CV and RhB dyes under solar light illumination. The optical band gap of ZnO nanopolygons changed from 3.1 eV to 2.68 eV for the ZnO/CuOAg2O ternary nanocomposite. The ternary nanocomposite exhibited enhanced visible light absorption capacity and charge-separation efficiency, and also a reduced rate of electron–hole pair recombination rates due to the coupling of wide band gap n-type ZnO with narrow band gap p-type CuO and Ag2O semiconductors. The ternary nanocomposite demonstrated about 99.05% and 97.38% degradation efficiencies for CV and RhB dyes, respectively, under solar light irradiation in a time period of 105 min. The photocatalytic activity was shown to increase with increase in pH values of the dye solutions. The calculated rate constants (k) for the photodegradation of CV and RhB dyes under solar light over the ZnO/CuO/Ag2O nanocomposites were calculated to be 4.26 and 3.61 times higher than the k values obtained for the photodegradation over ZnO nanoparticles. The reactive species trapping experiment studies revealed that the main reactive species participating in the photodegradation processes were OH and O2 over the ZnO/CuO/Ag2O photocatalyst under solar light illumination. Furthermore, the recycle experiments authenticated that the ZnO/CuO/Ag2O ternary nanocomposite showed 88.60% and 86.73% of the degradation efficiency in the fifth run of the cycle for CV and RhB dyes, respectively, which substantiates good reusability and photo-stability. Thus, the current work serves as a novel design for metal oxide-based nanocomposites, which can be promising photocatalysts for photocatalytic applications under solar light irradiation and, furthermore, we believe that outcomes of the current research work may be prove supportive in the field of environmental remediation.

The authors are thankful to MRC, MINT, Jaipur and Department of Chemistry, University of Rajasthan, Jaipur (India) for providing research facilities and supporting.

All authors have contributed to this work and read and revised the manuscript.

Parmeshwar Lal Meena: Conceptualization, methodology, validation, investigation, formal analysis, and supervision. Krishna Poswal: experimental, original draft writing, resources, review and editing. Ajay Kumar Surela: methodology, validation, editing and correction. Jitendra Kumar Saini: experimental, resources and analysis.

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

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