Green Synthesis of a CuO–ZnO Nanocomposite for Efficient Photodegradation of Methylene Blue and Reduction of 4-Nitrophenol

CuO–ZnO nanocomposites (NCs) were synthesized using an aqueous extract of Verbascum sinaiticum Benth. (GH) plant. X-ray diffraction (XRD), spectroscopic, and microscopic methods were used to explore the crystallinity, optical properties, morphology, and other features of the CuO–ZnO samples. Furthermore, catalytic performances were investigated for methylene blue (MB) degradation and 4-nitrophenol (4-NP) reduction. According to the results, CuO–ZnO NCs with 20 wt % CuO showed enhanced photocatalytic activity against MB dye with a 0.017 min–1 rate constant compared to 0.0027 min–1 for ZnO nanoparticles (NPs). Similarly, a ratio constant of 5.925 min–1 g–1 4-NP reductions was achieved with CuO–ZnO NCs. The results signified enhanced performance of CuO–ZnO NCs relative to ZnO NPs. The enhancement could be due to the synergy between ZnO and CuO, resulting in improved absorption of visible light and reduced electron–hole (e–/h+) recombination rate. In addition, variations in the CuO content affected the performance of the CuO–ZnO NCs. Thus, the CuO–ZnO NCs prepared using V. sinaiticum Benth. extract could make the material a desirable catalyst for the elimination of organic pollutants.


INTRODUCTION
Freshwater is one of the most vital resources needed for the well-being of mankind. Nowadays, water pollution has become one of the challenges our world faces, particularly due to toxic and carcinogenic 1,2 soluble organic pollutants released from industries such as textiles, 2,3 paper and pulp, 1,4 pharmaceuticals, and tanneries. 5 Thus, removing these pollutants before releasing effluents into the environment is essential. Conventional wastewater treatment techniques have limitations due to low efficiency, high cost, or generation of secondary contaminants. Therefore, developing eco-friendly, efficient, and low-cost materials and fabrication techniques are necessary. 6 Currently, researchers are exploring the use of nanostructured material-based catalysts as alternatives to conventional techniques. The unique characteristics of nanoscale materials have led to their use as catalysts for the degradation of pollutants into benign compounds. 7,8 Recently, nanostructured material-based photocatalysis has shown remarkable development in the applications of water splitting, 9,10 dye degradation, 11,12 and drug degradation. 13−16 Specifically, metal oxide-based nanostructure photocatalysis is a promising approach for efficiently removing organic contaminants from wastewater.
As a result of their unique properties and versatility, metal oxides (ZnO, TiO 2 , WO 3 , CuO, Cu 2 O, Fe 2 O 3 , etc.) play a key role in catalysis. 17,18 ZnO is among the metal oxides that have been explored most for wastewater treatment for its low cost, abundant availability, nontoxicity, and favorable exciton binding energy. 19,20 However, wide bandgap for visible light activation, fast e − /h + recombination, and photocorrosion in aqueous media are drawbacks limiting the practical application of pristine ZnO as photocatalyst. 7,19 Several properties have been demonstrated by coupling of ZnO with low-bandgap ptype CuO semiconductors: (i) a p−n heterojunction is formed, which reduces the e − /h + recombination rate; 21 (ii) narrow bandgaps are created, which improve visible light harvesting; 22,23 and (iii) improved reusability without significant performance loss. 24 Therefore, developing CuO−ZnO nanostructured material is necessary to achieve this goal.
The current work describes a green approach for CuO−ZnO NC synthesis utilizing an aqueous extract of GH with an efficient, low-cost, simple, and rapid MW-assisted method. Microscopy, spectroscopy, and other methods were used to study the resulting samples. The catalytic activity of CuO− ZnO NCs was also evaluated against MB degradation and 4-NP reduction. For comparison purposes, the degradation performance of ZnO was also evaluated. CuO−ZnO NCs prepared with the aid of GH extract are anticipated to have enhanced catalytic reactions due to improved response to visible light and reduced e − /h + recombination rates.  Figure S1) were collected, washed in distilled water, air-dried, and then ground to powder using a grinder. Then, 10 g of GH powder and 100 mL of DW were mixed in a 1 L beaker and then left to boil under magnetic stirring for 10 min. After cooling, filtration was used to separate the extract, which was then stored at 4°C for use in the subsequent experiments. While it is being stirred, 50 mL of extract was slowly added to the solution. NaOH (20 wt %) was used for adjusting the pH and then stirred for 20 min. Subsequently, the mixture was MW-irradiated using a household oven (1000 W, Comet) for 10 min at 50% power output. After purification with ample DW and ethanol, the brown precipitate obtained was dried at 80°C for 12 h. A pestle was used to hand-crush the resultant solid. Eventually, calcination of the powder was conducted at 500°C for 3 h using a muffle furnace. The resulting product was a 20 wt % CuO content sample.

Characterizations.
The crystallinity and crystal phase of the obtained powder was probed by an X-ray diffraction (XRD) instrument (XRD-7000S Shimadzu) with Cu Kα radiation. The elemental composition on the surface was collected using an X-ray photoelectron spectroscope (XPS, Thermo Scientific, Al Kα (hν = 1350 eV)). The surface morphology and elemental mapping were measured by a field emission scanning electron microscope (FESEM; HITACHI, S-4800) and a transmission electron microscope (TEM; FEI Tecnai G2 F30). UV−visible spectral characterization of the nanomaterials was performed using a UV−vis spectrophotometer (JASCO V-670). The emission spectra were measured using a fluorescence spectrophotometer (Agilent Cary Eclipse Fluorescence Spectrophotometer). The surface area analysis was conducted using a Micromeritics ASAP 2420 (V2.09 J) instrument by N 2 adsorption at 77 K. The progress of degradation and reduction reactions was observed by a UV− vis spectrophotometer (Azzota: SM-1600).

Photocatalytic Performance Test.
The photocatalytic efficiency of the CuO−ZnO NC sample was evaluated against the MB degradation (with a 150 W halogen lamp). The reaction suspension was prepared by mixing 120 mL of MB (10 ppm) and 20 mg of catalyst. To assure homogeneous dispersion of the catalyst and dye molecules, a 30-min sonication was used. Then, it was irradiated by visible light while being magnetically stirred continuously. Every 15 min, a 4 mL of sample was taken and centrifuged to separate the catalysts from the liquid before the absorbance measurement. After photocatalytic degradation of MB, the absorbance of each sample was measured. The percentage of photocatalytic degradation (D%) and rate of reaction were evaluated using eqs 1 and 2, respectively. 11,50 where k represents rate constant, and C 0 and C t are the dye (pollutant) concentrations before (or at time = 0 min) and after illumination (or at time = t min), respectively.

Catalytic Reduction Test.
To assess the catalytic capability of CuO−ZnO, 4-NP was used as a test nitroaromatic compound in the presence of NaBH 4 as a reductant. Typically, 40 mg of NaBH 4 and 40 mg of catalyst were dispersed in a 250 mL beaker containing 100 mL of 20 ppm 4-NP. The reduction progress was tracked by a UV−vis spectrophotometer. 51 Every 3 min, 3 mL sample of the mixture was withdrawn and its absorbance spectrum was then collected. Furthermore, the rate of reduction was evaluated using eq 2.

RESULTS AND DISCUSSION
The crystallinity and crystal phase of the calcined samples were investigated using the data from XRD measurements. XRD patterns of CuO, ZnO, and CuO−ZnO samples are shown in Figure 52 Following examination by highresolution TEM, the lattice fringes for ZnO−CuO were observed. Figure 1C,D displays the line intensity profile for the selected lines in the inverse fast Fourier transform (IFFT) image of the CuO−ZnO NC sample corresponding to the dspacing average of 0.26 nm ( Figure 1C) and 0.27 nm ( Figure  1D). According to the results, 0.26 nm is the interplanar spacing between the (002) plane of hexagonal ZnO (card no. 000-036-1451), while 0.27 nm is the spacing between the (110) plane of monoclinic CuO (card no. 00-048-1548) (Figure 1 B−D). This result is very consistent with the literature reports 29,53 and XRD analysis.
The average crystallite size was determined using Scherrer's formula (eq 3) for the first three intense peaks: (100), (002), and (101) for ZnO; (11−1), (111), and (20−2) for CuO; and (100), (002), and (101) for the CuO−ZnO NCs. 54 The calculated average crystallite sizes were 22, 14, and 18 nm for ZnO, CuO, and CuO−ZnO samples, respectively. The values obtained indicated that the synthesized samples have crystallites in the nanosize range. In view of the possibility that photocatalysts could be improved by reducing the crystallites size, the nanosize range crystallites of the synthesized CuO−ZnO samples make the materials an attractive photocatalyst candidate. 55 where λ stands for the wavelength of the incident X-ray, θ stands for the diffraction angle, and β represents the peak width at half-maximum of its height corresponding to the peak on the XRD pattern.
The absorption spectra of ZnO and CuO−ZnO in the UV− vis range are given in Figure 1E. Both ZnO and CuO−ZnO NC samples absorb strongly in the UV region (λ max = 374 nm), which is consistent with previous works. 56,57 Portions of the absorption spectrum fall in the visible range with the tail reaching 800 nm, which could be attributed to the light scattering in the colloidal suspension. 56 In comparison to pristine ZnO NPs, the CuO−ZnO sample showed a broader absorption spectrum over the range of 250−800 nm. The presence of CuO is responsible for a broader absorbance in the visible region, 58 indicating enhanced absorption capacity of the CuO−ZnO NCs in the visible light range. The bandgap calculated from Tauc's formula 12,33 was approximately 3.01 eV for ZnO and 2.74 eV for the CuO−ZnO NCs ( Figure S2). The result showed that the bandgap of the CuO−ZnO NCs is lower than ZnO NPs alone. The improved efficiency of visible light-harvesting capacity makes CuO−ZnO NCs suitable candidates for photocatalytic applications. 59 The photoluminescence (PL) properties of ZnO and CuO− ZnO suspension in methanol are displayed in Figure 1F. When the samples are excited at 340 nm, the PL spectra showed a strong emission band around 388 nm and relatively weaker emission band around 463 nm, including a shoulder band around 418 nm. The strong narrow emission band around 388 nm was attributed to the exciton recombination corresponding to the near band edge, 60 whereas the other peaks that appeared around 418 and 463 nm could be originated from the deeplevel defects. 61 Since PL emission is induced when the photogenerated e − /h + recombine, the relatively lower PL intensity of the CuO−ZnO NCs compared to ZnO NPs indicated the decreased radiative recombination rate in the composite, which could be attributed to the better e − /h + separation and prolonged exciton lifetime. 60 Thus, coupling CuO and ZnO revealed a decreased rate of exciton recombination with improved visible light absorption. 62 Figure 2A−C illustrates SEM images of the synthesized catalysts using GH extract. The micrograms indicated that ZnO and CuO were found in a cluster of spherical-like NPs, while CuO−ZnO NCs were found in a plate-like morphology. The morphology and crystal structure of the CuO−ZnO sample was further evaluated by TEM analysis, as displayed in Figures 2D and S3. The results illustrated that the prepared CuO−ZnO NCs possess plate-like nanostructures, supporting the SEM analysis. Furthermore, the distribution of CuO and ZnO in CuO−ZnO NCs was evaluated from EDS mapping images. The typical high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and corresponding EDS mapping of CuO−ZnO NCs ( Figure  2E,F) and Figure S4 demonstrated the even distribution of O, Cu, and Zn elements throughout the CuO−ZnO sample. 63 XPS study was utilized for the survey of oxidation states and compositions of the CuO−ZnO sample surface. The resulting XPS spectrum of the CuO−ZnO sample is presented in Figure  3. The peaks observed in the full survey spectrum displayed in Figure 3A indicate the presence of O, Cu, and Zn species. As shown in Figure 3B, XPS peaks at 1021.68 and 1044.77 eV were assigned to the Zn energy levels designated as 2p 3/2 and 2p 1/2 , respectively. 64 Similarly, the peaks centered at 933.1 and 953.01 eV were related to the 2p 3/2 and 2p 1/2 levels of Cu, respectively ( Figure 3C). 62 Satellite peaks appeared at the upper energy sides, 942.96 and 940.68 eV for Cu 2p 3/2 and 961.56 eV for Cu 2p 1/2 ( Figure 3C), confirming the existence of Cu in Cu 2+ state, i.e., CuO. 65 Furthermore, the peak located at 530.18 eV is attributed to the O 1s from the lattice oxygen (O 2− ) bound to metals, and the shoulder peak at 531.67 corresponds to the oxygen (O 2− ) in the oxygen-deficient region indicating the presence of oxygen vacancy (O V ). Hence, the XPS study revealed the presence of CuO and ZnO in the CuO−ZnO sample, which is consistent with the results from XRD, HRTEM, and EDS elemental mapping studies.
The adsorption−desorption isotherm of N 2 for the CuO− ZnO NC sample exhibited type IV isotherm having narrow adsorption−desorption of H3-type hysteresis loops ( Figure  S6), indicating the presence of a mesoporous structure. 66   The photocatalytic activities of CuO−ZnO NCs were tested against MB degradation using visible light, and the corresponding results are given in Figure 4. The UV−vis absorbance of MB showed a strong peak at 664 nm. 47 In the presence of catalysts, irradiating MB aqueous solution with visible light resulted in decreasing absorbance with time, indicating photodegradation of the MB. More specifically, the CuO−ZnO NCs showed improved photocatalytic performance compared to the pristine ZnO NPs with an efficiency of 24% for ZnO and 82% for CuO−ZnO NCs, as shown in Figure 4A−D. The rate constant (k) calculated from the linear fitted plots of ln(C t /C 0 ) vs time ( Figure 4D) was found to be 0.0027 and 0.017 min −1 for ZnO and CuO−ZnO NCs, respectively. The enhanced performance could be the result of coupling a p-type CuO with an n-type ZnO. Specifically, the improved visible light absorption capacity of CuO−ZnO (from UV−vis analysis), and the minimized rate of e − /h + recombination (from PL analysis) are responsible for the enhancement. 19 A study on how the CuO contents affect the activity of the prepared catalyst against MB was also conducted by varying CuO contents. The results depicted in Figure 5A−C show the performance of 24, 65, 82, 75, and 76% MB degradation for CuO wt % of 0, 10, 20, 30, and 50% samples of CuO−ZnO NCs, respectively. The calculated rate constants were 0.0027, 0.0112, 0.0170, 0.0139, and 0.0144 min −1 for ZnO (0%), CuO−ZnO (10%), CuO−ZnO (20%), CuO−ZnO (30%), and CuO−ZnO (50%) contents of CuO, respectively ( Figure   5D). While all CuO−ZnO samples showed higher performance than the ZnO NPs alone, CuO−ZnO NCs with 20 wt % CuO displayed the highest performance. This implies that there exists an optimum amount of CuO component in CuO− ZnO beyond which photocatalytic activity diminishes. 69 As reported in previous studies, the decreased performance of CuO−ZnO with increased CuO content beyond the optimum value could be explained by the increased agglomeration of CuO that masks surface photoactive sites or enhances the rate of e − /h + recombination. 70 In general, the processes involved in photocatalysis include absorption of light, e − /h + pairs generation and separation, transfer of e − /h + to the active sites for a redox reaction, recombination of e − /h + , and adsorption/desorption of reactants/products. Depending on the redox reaction between the charge carriers and the adsorbed species or surrounding species, reactive oxygen species (ROS) like hydroxyl radicals ( • OH), superoxide anion radicals (O 2 •− ), singlet oxygen species ( 1 O 2 ), and peroxide molecules (H 2 O 2 ) may form. 7,8 These species have a high tendency of oxidizing organic pollutants into less-toxic substances or mineralize them into CO 2 , H 2 O, and others.
In the case of CuO−ZnO NCs, light of appropriate energy illumination generates exciton, where e − accumulate in the conduction band (CB) and h + in the valance band (VB). Coupling CuO with ZnO semiconductor provides the band alignment in which the e − in the CB of CuO are pushed to the relatively less negative CB of ZnO. Thus, the h + in the VB oxidizes H 2 O/HO − (or organic pollutant RH) resulting in HO • (or R • ) depending on the potential of the hole in the VB   (eq 4). Similarly, the e − in the CB will be trapped by the dissolved oxygen and produce ROS that finally degrades the pollutant (eq 4). 47 Table 1 compares the activities of this work to the earlier reports using ZnO-based NCs against MB degradation under different experimental conditions. Despite the different experimental conditions, the CuO−ZnO NCs obtained using GH exhibited superior photocatalytic performance compared to the previously reported literature values based on their ratio constants (Table 1). It degraded 82% of the initial MB amount within 105 min with a rate constant of 0.017 min −1 or ratio constant of 0.85 min −1 g −1 (ratio of k to the mass of catalyst).
Furthermore, the catalytic reduction activity of the nanostructured catalysts was also tested for 4-NP along with the strong reducing agent NaBH 4 . Figure 6 shows the progress of reduction and corresponding kinetics in the presence of ZnO and CuO−ZnO catalysts. The aqueous solution of 4-NP displayed an absorbance peak at 320 nm ( Figure 6A). Upon NaBH 4 addition, the peak at 320 nm (from light yellow solution) disappeared, while a strong peak around 405 nm (intense yellowish-orange solution) was observed, indicating the formation of 4-nitrophenolate ion in alkaline media ( Figure  6A). The peak at 405 nm showed no change in intensity over an hour. However, when ZnO and CuO−ZnO were added, the peak intensity at 405 nm drops, while a new peak at 300 nm emerges, confirming the 4-AP formation. CuO−ZnO NCs showed strong catalytic efficiency with reaction completion in less than 12 min ( Figure 6B,C). On the other hand, ZnO showed insignificant change in the 4-nitrophenolate ion (in 12 min), suggesting the lower catalytic performance of ZnO than CuO−ZnO NCs against the reduction of 4-NP.
The reduction kinetics of 4-NP with NaBH 4 and catalysts is shown in Figure 6D. The rate constants of the reduction reaction were obtained from the linear fitted plot of ln(C t /C 0 ) vs time of reaction, as depicted in Figure 6D (inset) applying eq 2. In this regard, the ratio constants (K) of 0.125 and 5.925  min −1 g −1 were obtained for ZnO and CuO−ZnO nanocatalysts, respectively. 64 The reduction of 4-NP into 4-AP by NaBH 4 is a thermodynamically feasible reaction. 51 However, the rate of reduction reaction is slow due to the kinetic barrier in the reaction steps. 51 Catalysts allow the reaction to take routes with relatively lower energy barriers leading to a faster rate of reaction. Nanostructured catalysts were often used to accelerate the reduction of 4-NP into 4-AP by NaBH 4 . The proposed reaction mechanism for the conversion of 4-NP into 4-AP by NaBH 4 along with CuO−ZnO NCs catalysts is given in Scheme 2. Generally, the mechanisms involve processes such as adsorption/desorption and transfer of electrons and protons. From the aqueous solution, BH 4 − and 4-NP adsorb on the surface of the catalyst, where electron and proton transfers take place. Electrons transfer from BH 4 − to the adsorbed 4-NP through the CuO−ZnO catalyst leading to reduction. 75 Similarly, the protons transfer from the BH 4 − and aqueous solution to the 4-NP, completing the reduction reaction. 76 The adsorption process and transfer of charged species are facilitated by the built-in electric field of the p−n heterojunction of CuO−ZnO. 51 After completion of the redox reaction, the desorption step takes place, as shown in Scheme 2.
The prepared CuO−ZnO NCs have shown good recyclability up to four cycles against the MB using visible light and 4-NP reduction using NaBH 4 ( Figure S5).

CONCLUSIONS
CuO−ZnO NCs have been prepared successfully using the MW-assisted method with GH extract. The synthesized sample was characterized using various techniques. The analysis indicated that crystalline CuO−ZnO NCs with plate-like morphology were obtained. Similarly, the XRD patterns, EDS elemental mappings, and XPS spectra confirmed that the composite was composed of CuO and ZnO with uniform distribution. And also, the UV−vis absorbance and PL analysis of the CuO−ZnO NCs showed enhanced absorption in the visible range with a suppressed recombination rate of e − /h + compared to the ZnO NPs. Furthermore, the prepared sample showed improved photocatalytic degradation of MB and catalytic reduction of 4-NP with rate constants of 0.017 and 0.237 min −1 , respectively. Therefore, CuO−ZnO NCs obtained using abundantly available GH extract could be used for the remediation of water polluted by organic dyes.