Lotus-Shaped CuO/ZnO Nanocomposites with Tunable Band Gap for UVA-Induced Photocatalytic Degradation of Organic Dyes

Abstract

Industrial discharge of organic pollutants poses severe risks to aquatic ecosystems and human health, underscoring the urgent need for efficient, reusable photocatalysts for their degradative removal. In this study, binary CuO/ZnO nanocomposites with varying CuO contents (5, 10, and 15 wt %, denoted as CZ-5, CZ-10, and CZ-15, respectively) were synthesized via a hydrothermal route at 100 °C for 24 h from their corresponding salt precursors. Structural and surface characterizations of the prepared composites were performed using state-of-the-art techniques, confirming the formation of highly crystalline wurtzite ZnO and monoclinic CuO phases with abundant oxygen vacancies. In addition, the CZ-10 composite exhibited a distinctive Lotus-like morphology, a reduced bandgap energy (3.13 eV), and suppressed electron–hole recombination via the S-scheme mechanism by reserving high potential electrons and holes by combining the lower potential of electrons and holes. The Point of Zero Charge (PZC) of CZ-10 was found to be 6.9. Furthermore, complete degradation of methylene blue (MB) and 96% of orange G (OG) could be achieved within 90 min under UVA irradiation using CZ-10 with rate constants of 0.06 and 0.03 min^–1, respectively. Optimization studies revealed the significant influence of pH, adsorbent dose, and dye concentration on photocatalytic efficiency. CZ-10 retained 87 and 84% of its activity for MB and OG removal from aqueous solution, respectively, after five successive cycles, demonstrating excellent stability and reusability. Lotus-shaped CuO/ZnO nanocomposites have significant potential as robust, efficient photocatalysts for the remediation of dye-contaminated wastewater under UVA light.

Introduction

Water is one of the most vital components of life. The availability of clean and safe surface water remains limited. Rapid population growth has driven global industrialization and urbanization, leading to the continuous discharge of persistent and hazardous pollutants into natural water bodies, such as rivers, lakes, and oceans, thereby reducing water quality. As a result, surface water becomes toxic for human consumption. The consumption of contaminated water can lead to severe health issues, such as dysentery, diarrhea, typhoid, cholera, and polio. Therefore, the key responsibilities of researchers are to develop efficient methods to remove such pollutants from wastewater before they are released into the natural water resources. Conventional treatments are often costly and inefficient in removing these persistent organic pollutants (POPs) from wastewater. , Chemical treatment methods, although effective in some cases, are also hazardous and expensive.

To overcome these limitations, advanced oxidation processes (AOPs) have emerged as promising, cost-effective, and environmentally friendly alternatives, , that generate and utilize the highly reactive hydroxyl radicals (·OH), which possess an oxidation potential of 2.80 V, significantly higher than that of hydrogen peroxide (1.78 V), allowing them to degrade even the most persistent organic compounds into harmless CO₂ and H₂O. − These radicals attack organic molecules nonselectively through electron transfer, hydrogen abstraction, and radical combination mechanisms. , Among the various AOPs, photocatalysis has attracted significant attention as a sustainable and efficient technique for removing POPs from wastewater. Photocatalysis uses optically active semiconductor materials that generate ·OH radicals upon exposure to light, which degrade organic contaminants. ,

ZnO has been widely used as a photocatalyst due to its strong photocatalytic activity, excellent chemical stability, abundance, nontoxicity, and low cost. , However, ZnO has some limitations, such as rapid electron–hole recombination, particle aggregation during synthesis, and weak absorption in the visible-light region. To enhance the photocatalytic performance of ZnO under sunlight, its band gap can be effectively reduced by coupling it with a narrow-band gap semiconductor, such as CuO. CuO, a p-type semiconductor with visible-light activity, forms a p–n heterojunction with ZnO that facilitates charge separation and diminishes the recombination of charge carriers via the S-scheme mechanism. Therefore, the generation of reactive oxygen species (ROS) increases. For instance, spheroid-shaped CuO/ZnO was reported as an effective photocatalyst for the degradation of MB. Similarly, CuO/ZnO aerogels were synthesized via the sol–gel method and utilized as a photocatalyst for the degradation of Rhodamine B (RhB). Recently, a spherical CuO-ZnO nanocomposite was synthesized through an environmentally friendly method employing Tragia involucrata L. leaf extract for the degradation of RhB.

In this study, CuO, ZnO, and CuO/ZnO binary nanocomposites with optimized weight percentage were synthesized via a hydrothermal method. The materials were characterized using state-of-the-art techniques. To represent real wastewater, which contains a mixture of different types of dyes and categorized as cationic, anionic, and nonionic based on ionic nature, methylene blue (MB), a cationic nonazo dye, and orange G (OG), an anionic azo dye were used to determine the photocatalytic efficiency of the prepared catalyst for the remediation of dye-contaminated wastewater under UVA light.

Experimental Section

Materials

Zinc nitrate hexahydrate (Merck, Germany), copper­(II) acetate monohydrate (Scharlau, Spain), sodium hydroxide pellets (Merck, Germany), glacial acetic acid (RCl Labscan), hydrochloric acid (Merck, Germany), methylene blue (Janssen Chimica, Belgium), and orange G (Merck, Germany) were used without further purification. Deionized water (conductivity: 0.055 μS cm^–1 at 25.0 °C) from an HPLC-grade water purification system (BOECO, Germany) was used.

Preparation of CuO/ZnO Composites

The hydrothermal method was employed to prepare the nanoscaled materials (Figure ). To an aqueous solution containing 33.17 mmol of Zn­(NO₃)₂·6H₂O, NaOH was added dropwise with magnetic stirring until the reaction was complete. A 3.77 mmol of Cu­(CH₃COO)₂·H₂O was dissolved, followed by the addition of 0.5 mL glacial acetic acid. This solution was then heated to 100 °C, and NaOH solution was added dropwise until the pH reached 12. The solution was then transferred to a Teflon-lined hydrothermal autoclave reactor and heated to 100 °C in an oven for 24 h. A gray residue of CuO/ZnO was formed, which was filtered, washed with water, then with ethanol, and finally dried in an oven at 60 °C overnight. The dried 10% CuO/ZnO sample was calcined at 300 °C for 4 h in a muffle furnace with a heating rate of 10 °C/min. After that, the calcined sample was stored in a container and denoted as CZ-10. The nanomaterials were prepared using the procedures above with different weight percentages of CuO (0, 5, 10, 15, and 100%), denoted as ZnO, CZ-5, CZ-10, CZ-15, and CuO, respectively.

$$\mathrm{Cu}({\mathrm{CH}}_3\mathrm{COO}{)}_2(\mathit{aq})+\mathrm{Zn}({\mathrm{NO}}_3{)}_2(\mathit{aq})\stackrel{\mathrm{NaOH}(\mathit{aq})}{\to }\mathrm{Cu}(\mathrm{OH}{)}_2/\mathrm{Zn}(\mathrm{OH}{)}_2+{\mathrm{CH}}_3\mathrm{COONa}(\mathit{aq})+\mathrm{NaN}{\mathrm{O}}_3(\mathit{aq})$$ 1

$$\mathrm{Zn}{(\mathrm{OH})}_2(s)+\mathrm{Cu}{(\mathrm{OH})}_2(s)\stackrel{\Delta }{\to }\mathrm{CuO}/\mathrm{ZnO}(s)+{\mathrm{H}}_2\mathrm{O}(l)$$ 2

1.

Scheme for the synthesis of CuO/ZnO composites.

Characterizations

The surface morphology and surface composition of the prepared nanomaterials were evaluated using field-emission scanning electron microscopy (FESEM) configured with an acceleration voltage of 5.0 keV of electron gun to ensure optimal imaging resolution for nanoscale features and energy-dispersive X-ray spectroscopy (EDX) (ZEISS, Germany), equipped with a 10.0 keV electron gun. For compositional analysis, Fourier Transform Infrared Spectroscopy (FTIR) (Frontier by PerkinElmer) was used. Spectra were collected with 32 scans per sample to ensure a high signal-to-noise ratio across the wavenumber range of 400–4000 cm^–1 at a resolution of 4.0 cm^–1. The crystallinity of the prepared materials was evaluated using an X-ray diffractometer (XRD) equipped with an XDC-700 Guinier-Hägg focusing camera and a Philips PW 1724 diffractometer, both equipped with strictly monochromatized Cu Kα1 radiation (λ = 0.1540598 nm). The formation of CuO/ZnO, the oxidation state of metal ions, and the electronic environment were confirmed by X-ray Photoelectron spectroscopy (XPS) and High-resolution X-ray photoelectron spectroscopy. The XPS spectrum was deconvoluted using XPS Peak41 software. Thermal behavior was analyzed using a Hitachi TG/DTA 7200 in a N₂ atmosphere, with a flow rate of 100 mL/min and a temperature increment of 10 °C/min from 30 to 1000 °C, using alumina pans as sample and reference holders. The optical properties of the synthesized materials were evaluated by a UV–visible absorption spectrophotometer (UVD-3500, Spectro UV–vis double beam, Labomed Inc.), where the ranges of 900–190 nm were measured with the accuracy of ±0.3 nm, and a UV–visible diffuse reflectance spectrophotometer (UV-DRS), Lambda 35 by PerkinElmer. The electronic and optical properties of the synthesized photocatalysts were analyzed using a Photoluminescence (PL) spectrophotometer (Hitachi F-7000) fitted with a 150-W xenon lamp as the radiant source.

Photocatalytic Test

Thirty mg catalyst was added to a 10 ppm, 100 mL dye solution, and the mixture was stirred continuously in the dark for 30 min to achieve adsorption–desorption equilibria. After that, the solution was transferred to the UVA chamber, where it was irradiated with a Philips 8W UVA light. Three mL of the solution was taken at 10 min intervals during irradiation, and the concentration was determined by measuring the UV–vis absorption (Figure S2). The concentrations of MB and OG were measured at their respective absorption maxima (λ_max) of 664 and 478 nm (Figure S3). The efficiency of photocatalytic degradation was calculated using-.

$$\mathrm{Degradation}\mathrm{efficiency}=\left(\frac{C_0-C}{C_0}\right)\times 100$$ 3

where C ₀ and C denote the respective concentrations of dyes at zero and t min. Eq was used to determine the kinetics of dyes’ photodegradation.

$$\ln \frac{C_0}{C}=kt$$ 4

where k is the rate constant of the pseudo-first order reaction in min^–1.

Results and Discussion

Composition and Crystal Structure Analysis

FTIR, XRD, TGA, and EDX analyses of the prepared materials were performed to evaluate the formation and determine the crystal structure of the metal oxide semiconductors. Figure shows the (a) FTIR, (b) XRD, (c) TGA, and (d) EDX of CuO, ZnO, and CZ-10.

2.

(a) FTIR spectra, (b) XRD patterns, (c) TGA patterns of the prepared materials, and (d) EDX spectrum and elemental mapping images of the prepared materials.

A band at 505 cm^–1 in the FTIR spectrum of CuO, associated with Cu–O vibration, indicates the presence of CuO particles. Similarly, a band at 415 cm^–1 in the spectrum of ZnO, associated with Zn–O vibration, indicates the presence of ZnO particles (Table S1). These bands are also observed in the spectrum of the CZ-10 composite, confirming the presence of both CuO and ZnO in the composite. When comparing the pure oxides to the composite, the CZ-10 exhibits minor shifting and broadening in these metal–oxygen bands. This broadening in the 415–505 cm^–1 range suggests the coupling between ZnO and CuO lattices at the interface in the heterojunction. The Zn–O and Cu–O vibrational modes merge in the composite spectrum, as expected when two oxide phases form a heterojunction. Such spectral merging, characterized by broader and less distinct bands, reflects interfacial bonding and confirms the formation of a heterojunction.

The XRD pattern of the prepared CuO was matched with previously recorded XRD pattern in the ICDD card number 00-045-0937, where peaks at 2θ values of 35.6, 38.8, 48.8, 58.3, 61.6, 66.2, and 68.1° are associated with (−111), (111), (−202), (202), (−113), (022), and (−311) planes, respectively (Figure b), indicating the formation of the monoclinic structure of CuO. The XRD pattern of the prepared ZnO was matched with previously recorded XRD patterns registered as ICDD card number 01-075-0576, indicating the formation of the hexagonal wurtzite structure of ZnO. The peaks at 2θ values of 31.7, 34.4, 36.2, 47.5, and 56.6° corresponded to the (100), (002), (101), (102), and (110) crystallographic planes, respectively (Figure b). These peaks were also observed in the XRD pattern of the CZ-10 composite, indicating the formation of two distinct crystalline phases: the monoclinic CuO and the hexagonal ZnO. Average crystallite sizes of the prepared catalyst were determined by the Debye–Scherrer equation,

$$D=\frac{k\lambda }{\beta \cos \theta }$$ 5

where Scherrer’s constant k = 0.98, wavelength of X-ray radiation λ = 0.154, β is the full width at half-maximum (FWHM) of the most intense peak, and θ is the Bragg angle with respective fwhm position. The average crystallite sizes were 20, 27, and 34 nm for CuO, ZnO, and CZ-10, respectively.

Figure c depicts the thermogram of the prepared catalyst. All of the semiconductor materials exhibited thermal stability up to 850 °C. A weight loss is observed in pure CuO nanoparticles at high temperatures. Between approximately 850 and 1000 °C, CuO decomposes into Cu₂O with the release of oxygen in an inert atmosphere:

$$4\mathrm{CuO}\stackrel{\Delta ,{\mathrm{N}}_2\mathrm{atmosphere}}{\to }2{\mathrm{Cu}}_2\mathrm{O}+{\mathrm{O}}_2$$ 6

This step releases about 13.5 wt % O₂, matching the observed mass loss. At higher temperatures, Cu₂O can further reduce to Cu metal, causing a minor second mass loss. ZnO remains stable up to 1000 °C in an inert atmosphere. ZnO remains unchanged at least up to its melting point, so any minor early loss (<1%) is likely due to desorbed water or surface hydroxyls. In the CZ-10 composite, the CuO weight percentage is only 10%. Therefore, CZ-10 exhibits minimal weight loss between 850 and 1000 °C because of the presence of 10% CuO in the CZ-10 catalyst.

Peaks at 0.2, 0.4, 8.0, and 8.2 keV in the EDX spectrum of CZ-10 correspond to the presence of Zn, Cu, and O (Figure d). Two distinct peaks were observed at 0.4 and 8.2 keV, confirming the presence of Zn in the sample. Additionally, the peaks at 0.4 and 8.0 keV are due to Cu, and the peak at 0.2 keV is due to oxygen in the sample. The peaks at 0.4 keV for copper and zinc were merged, making it the most intense peak. A distinct peak near 2 keV corresponds to the characteristic gold signal, consistently observed in the spectra of all samples. Gold was sputtered onto the sample to make it conductive. The elemental mapping images also confirm the presence of Cu, Zn, and O in the samples. EDX data, along with elemental mapping, indicated that the CZ-10 composite consists of Zn, Cu, and O.

Oxidation State and Chemical Environment of Elements

The XPS spectrum was obtained to evaluate the surface composition, chemical environment, and oxidation states of the elements in the CZ-10 composite. Figure depicts the XPS survey spectrum of CZ-10, along with the deconvolution of high-resolution XPS spectra for Cu, Zn, and O. The XPS survey spectrum of the CZ-10 nanocomposite confirms the presence of Cu, Zn, and O signals (Figure a).

3.

(a) XPS spectrum of CZ-10 composite and (b–d) deconvolution spectra of Zn, Cu, and O.

In wide-scan data, the O 1s peak (529 eV) and characteristic Cu 2p and Zn 2p photoemission peaks are clearly visible, indicating that both copper and zinc are present as oxides. In addition, FTIR signals corresponding to the vibrational modes of Cu–O and Zn–O further support the presence of copper and zinc in oxide form. This survey provides the elemental composition for detailed core-level analysis. High-resolution XPS of the Zn 2p region shows a well-defined doublet at 1021 and 1044 eV corresponding to Zn 2p_3/2 and Zn 2p_1/2 peaks, respectively (Figure b). These binding energies match expected values for Zn²⁺ in ZnO, indicating zinc is in the +2 oxidation state. The spin–orbit splitting of the Zn 2p doublet also corresponds to the known ZnO value (Δ ≈ 23 eV). No additional Zn 2p features or significant shifts are observed, suggesting a single chemical environment (Zn²⁺) with no detectable metallic Zn impurities. The Cu 2p region reveals two prominent peaks at 933 and 953 eV, corresponding to Cu 2p_3/2 and Cu 2p_1/2, respectively. These binding energies are typical of Cu²⁺ in CuO rather than Cu⁺ or metallic Cu, which additionally supports the TGA data of the CuO. Prominent shakeup satellite peaks appear at approximately 941 and 943 eV, corresponding to the 2p_3/2 satellite, and at 962 eV, corresponding to the 2p_1/2 satellite (Figure c). These satellites clearly indicate a Cu²⁺ (d⁹) electronic configuration. The spin–orbit splitting (ΔE) between Cu 2p_3/2 and Cu 2p_1/2 measures around 20.0 eV. The O 1s spectrum deconvolutes into multiple components. The peak at 529 eV is assigned to lattice O^2– bound in metal–oxygen frameworks, such as O in Zn–O and Cu–O bonds (Figure d). A peak at 530 eV is observed, which is typically attributed to oxygen-deficient or defect sites, such as oxygen vacancies (O-vac). A higher binding energy peak near 532 eV corresponds to surface-bound hydroxyl groups or adsorbed H₂O molecules. The prominent O-vac peak indicates that the composite has a significant number of O-vac, as observed in the CZ-10 heterostructure. These vacancies increase surface reactivity by offering electron-trapping sites and adsorption centers, which directly inform the enhanced photocatalytic behavior of the CZ-10 composite. These vacancies can capture photogenerated electrons, feeding them into the ZnO conduction band and thereby decreasing recombination. They also improve the adsorption of reactant molecules, facilitating redox reactions on the catalyst surface. The presence of Cu²⁺ (CuO) and Zn²⁺ (ZnO) phases indicates that the material forms a p-n heterojunction, which enhances charge separation. In CZ-10 composites, photoexcited electrons in the CuO conduction band tend to transfer to the lower-lying ZnO conduction band, while holes in ZnO migrate into CuO. This band alignment creates an internal electric field at the interface, driving the spatial separation of electrons and holes. The result is a significant reduction in recombination, as shown by longer carrier lifetimes in photoluminescence studies.

Morphology Analysis

FE-SEM images were taken to determine the morphology, including particle size, shape, and porosity of CuO, ZnO, and CZ-10 composite, as shown in Figure . It is evident that pure CuO and ZnO particles formed at the nanoscale. In contrast, the composite nanoparticles were larger than the pure materials. CuO particles exhibited a spherical structure in an aggregated form. Based on the histogram data (Figure S1), the particle diameter of CuO ranges from 25 to 30 nm. The ZnO particles form a double conical shape and also appear in aggregated forms. The diameter of the ZnO particles was found to be between 30 and 55 nm (Figure S1), slightly larger than the CuO particles. These data clearly indicate that the hydrothermal method is highly effective for preparing CuO and ZnO nanoparticles without the use of templates. CuO content is only 10% in the CZ-10 composites, which demonstrates the synergistic effect of the particle shape. Excellent lotus-shaped nanoparticles of the CZ-10 composite were observed, which were completely different in shape than the pure CuO and ZnO. ZnO and CuO particles stack together to form a lotus-shaped particle. To the ZnO particles, Cu²⁺ was added dropwise. From the literature survey, the zeta potential of the ZnO nanoparticles is negative. For that reason, Cu²⁺ ion adsorbed and bound to the oxygen atom on the surface of the ZnO. After that, NaOH solution was added to form Cu­(OH)₂ on the surface of the ZnO nanoparticles. The mixed solution was heated in the hydrothermal reactor. At high temperatures and pressures, with the synergistic effect of an optimal Cu²⁺/ZnO ratio, CuO/ZnO composite particles formed in a lotus-shaped morphology exhibited reduced aggregation. Petal in the lotus ranges in size from 20 to 300 nm (Figure S1), which is larger than the diameters of the pure ZnO and CuO particles. Lower aggregation behaviors are observed in these lotus-shaped particles due to the petals of the flowers.

4.

FE-SEM images of CuO, ZnO, and CZ-10 composite.

Lotus-shaped particles exhibit lower electron–hole recombination and better photocatalytic performance than spherical or double-conical particles, facilitating the generation of hydroxyl radicals upon light irradiation and enhancing the degradation of organic dyes. In the CZ-10 composite, nanochannels were also observed on the surface, facilitating electron transport. The average particle diameter was 34, 40, and 125 nm (corresponding to the petal-like structure of the lotus-shaped particles) for CuO, ZnO, and the CZ-10 composite, respectively.

Optical and Electronic Properties

UV-DRS was used to determine the optical band gaps of the prepared photocatalysts. The diffuse reflectance spectra (200–1100 nm) were transformed into absorption values using the Kubelka–Munk function, and the band gaps were estimated from Tauc plots. As shown in Figure , the calculated band gap energies were 1.64 and 3.23 eV for CuO and ZnO, respectively.

5.

UV-DRS spectra of (a) CuO, (b) ZnO, and (c) CZ-10 composite, and (d) photoluminescence spectra of ZnO and CZ-10.

ZnO is widely reported to have a band gap in the range of 3.3–3.4 eV, while CuO typically exhibits an indirect band gap of 1.2–1.7 eV. For the CZ-10 composite, a noticeable reduction in the band gap energy was observed compared to pure ZnO. The band gap energy was found to be 3.13 eV for the CZ-10 composite. This band gap narrowing arises from interfacial interactions between the narrow-bandgap CuO and the wide-bandgap ZnO, forming a heterojunction. The coupling of CuO and ZnO induces electronic band alignment, allowing CuO to act as a UVA light sensitizer and effectively shifting the composite’s absorption edge toward longer wavelengths. The narrowed band gap of the CZ-10 composite extends its optical response well into the UVA light range, which is nearly within the visible range. The PL spectra of the materials are shown in Figure d. Pure ZnO shows a strong, intense emission, whereas the CZ-10 composite exhibits a dramatically quenched PL intensity at 505 nm. This pronounced PL quenching in CZ-10 indicates that radiative recombination of photoexcited electrons and holes is strongly suppressed. The intimate junction between p-type CuO and n-type ZnO creates a built-in electric field that drives electrons into the ZnO and holes into the CuO. This spatial charge separation markedly inhibits electron–hole recombination. Photogenerated electrons flow from the conduction band of CuO into the lower conduction band of ZnO. In comparison, holes move from the valence band of ZnO into the higher valence band of CuO. This staggered band alignment thus partitions electrons and holes into different materials, prolonging their lifetimes.

Photocatalytic Degradation of Dyes

Two different types of dyes were selected for consideration in real wastewater: cationic (MB) and anionic dyes (OG). The degradation efficiency of the prepared catalyst was measured by maintaining constant experimental parameters, including 30 mg of catalyst dispersed in a 100 mL aqueous solution containing 10 ppm of dyes. Each catalyst adsorbed different amounts of dyes over a 30 min period when the solution was kept in the dark. Upon introduction of UVA light into the system, the catalyst was activated, initiating the photodegradation of the dye. For the kinetics study, 3 mL of the solution was taken at 10 min intervals during light irradiation to measure UV–vis absorption of the solution. Neutral pH was used in the photocatalytic experiment because the Point of Zero Charge (PZC) was 6.9 (Figure S11). Figure depicts the absorption spectra of dyes and the color change of an aqueous solution of dyes with UVA irradiation of 30 mg CZ-10 catalyst in a pH 7 and 10 ppm dye solution. The gradual decrease of the absorbance and color of the aqueous solution of both MB (Figure ) and OG dyes (Figure S4) with 10 min time intervals indicates that the CZ-10 catalyst was effective in removing both cationic and anionic dyes from aqueous solution, which means the practical application to treat real wastewater from textile industries.

6.

Absorption spectra for the photocatalytic degradation and the color change of MB with CZ-10 composite.

The color change during photocatalytic degradation of dyes indicated complete removal of the dyes from aqueous solution. MB and OG are the types of nonazo and azo dyes, respectively. The degradation profile also shows that the prepared binary heterogeneous catalyst is capable of degrading both azo and nonazo dyes from the aqueous solution.

The dynamic and kinetic plots of the photocatalytic degradation of MB and OG using different catalysts are shown in Figure . A blank run was performed without catalysts to investigate the effect of radiation on the aqueous dye solution (Figure S5). However, no reduction in dye concentration was observed, indicating that radiation alone cannot reduce it. The photocatalytic degradation of MB and OG was 62 and 40%, respectively, using pure ZnO within 90 min of irradiation. CuO shows only adsorption of both dyes, not degradation. The degradation efficiencies of MB and OG were 100 and 96%, respectively (Figure a,c), using the CZ-10 binary heterogeneous catalyst, indicating that the CZ-10 catalyst is more than twice as effective as the pure ZnO catalyst. The CZ-10 catalyst even showed similar degradation efficiency under natural sunlight irradiation (Figures S6 and S7), indicating the catalyst’s effectiveness across a wide range of radiation. Moreover, sunlight irradiation degrades the catalyst, making it a cost-effective solution for wastewater treatment.

7.

(a, c) Dynamic plot and (b, d) kinetic plot of the degradation of MB and OG.

Other binary catalysts, such as CZ-5 and CZ-15, yield results similar to those of ZnO, suggesting that CuO does not provide a synergistic effect in the composite materials, which would lead to improved photocatalytic degradation performance. In the CZ-10 composites, CuO exhibits synergism in the band structure and electronic properties, which enhances the light-harvesting properties and effectiveness in separating electron and hole pairs. This phenomenon stabilizes the electron and hole in their respective positions, allowing the production of hydroxyl radicals through the oxidation and reduction reactions of water and oxygen molecules, thereby enhancing the effectiveness of the CZ-10 catalyst in achieving a faster, more robust degradation profile. Approximately 40% of the initial MB solution concentration decreased within 10 min, and the half-life of this reaction was 14 min under UVA irradiation. The CZ-10 composites exhibited faster degradation of MB and OG than pure ZnO and other binary composites, such as CZ-5 and CZ-15. The results were fitted to zero-, first-, and second-order kinetics (Figure S8), yielding R ² values of 0.89, 0.99, and 0.07, respectively (Table S2). Thus, photocatalytic degradation of MB and OG by CZ-10 followed pseudo-first-order kinetics with rate constants of 0.06 and 0.03 min^–1 for MB and OG, respectively. For comparison, the rate constant of CZ-10 is more than five-times greater than that of pure ZnO. The rate constants for pure ZnO were 0.01 and 0.006 min^–1 for MB and OG, respectively. The comparison of the morphology-based performance of CuO/ZnO heterogeneous composite with reference to the same organic pollutants is given in Table .

1. Comparison of Morphology and Performance of CuO/ZnO Heterogeneous Composite.

Method	Morphology	Organic pollutant	rate constant (min –1 )	Reference
Hydrothermal	nanoplates	MB	0.0295
Hydrothermal	nanowires	MB	0.03543
Green synthesis	plate-like nanostructures	MB	0.017
Hydrothermal	lotus-shaped	MB	0.06	this work

In comparison with a study, the CuO/ZnO nanocomposite showed 96.1% efficiency in degrading 10 ppm RhB over 105 min at a pH of 9 with a catalyst dosage of 1 mg mL^–1. Likewise, spheroid-shaped CuO/ZnO exhibited 91% removal of MB under sunlight irradiation within 7 h, which is a prolonged period. In another study, CuO/ZnO nanocomposite degraded 82% of the 10 ppm MB amount within 105 min under visible light. In our work, CuO/ZnO demonstrated significantly better performance with both cationic and anionic dyes. CZ-10 exhibited 100 and 96% efficiencies in degrading MB and OG, respectively. Figure represents the MB dye removal efficiency per gram of catalysts (mg g^–1). ZnO showed the lowest dye removal efficiency of 20 mg g^–1. The degradation efficiency of MB with the addition of CuO up to the optimum loading ratio in the composites. However, further increasing the loading of CuO in the composite decreases the degradation efficiency.

8.

Removal of methylene blue per g of ZnO from aqueous solution.

Effect of pH

To investigate the effect of pH on the removal of MB and OG from the aqueous solution, the CZ-10 catalyst was dispersed in a range of pH (3, 5, 7, 9, 11, and 13). The pH of the solution was adjusted by adding 1.0 M NaOH and 1.0 M HCl solution. The catalyst dose was fixed at 0.3 g L-1, and the dye concentration was maintained at 10 ppm. Figure a illustrates the effect of pH on dye removal. The best photocatalytic performance was observed at pH 7, consistent with the PZC of CZ-10 (6.9), as reported in the SI (Figure S11). Moreover, at a basic pH (7–9), the highest removal of both MB and OG dyes was observed. This range of pH exhibits the highest photocatalytic degradation due to the availability of OH^–, which increases the production of HO^·, resulting in enhanced degradation of dyes. Above pH 9, the catalyst surface, ZnO, converts to Zn­(OH)₂, thus degradation efficiency was reduced. At acidic pH, the heterojunction breaks down, separating ZnO and CuO, and ultimately dissolving the catalyst in a highly corrosive environment, resulting in a drastic decrease in degradation efficiency. Such a pH of the solution was unfavorable for the degradation of organic dyes by the ZnO-based catalyst.

9.

(a) Degradation profile of MB and OG with varying (a) pH, (b) initial dye concentration, (c) catalyst dose, and (d) reusability of the CZ-10 catalyst.

Effect of Dye Concentration

A range of dye concentrations (5, 10, 15, 20, 25, 30 ppm) was used to investigate the effect of initial dye concentration on photocatalytic degradation. At lower initial dye concentrations (5, 10 ppm), complete photocatalytic degradation of MB and OG was observed. Increasing the initial dye concentration decreased the photocatalytic degradation performance. The reduction in high initial dye concentration was due to the intense color of the dyes, which inhibits the penetration of the radiation to the catalyst surface in the aqueous solution, reducing the light intensity on the surface of the catalyst, which decreases the production of hydroxyl radicals, resulting in a lower amount of dye degradation.

Effect of Catalyst Dose

The effect of catalyst dose was investigated by varying the amount of catalyst added to a 10 ppm, 100 mL aqueous solution of dyes. At a constant pH, lower catalyst doses produce fewer hydroxyl radicals, resulting in lower degradation efficiency. Increasing the catalyst dose to 0.3 g L^–1 or higher removed the catalysts, resulting in 100 and 96% degradation of MB and OG, respectively, within 90 min of irradiation. When catalyst doses exceed 0.7 g L^–1, degradation efficiency decreases due to the catalyst concentration, which blocks radiation. The optimal catalyst dose was found to be 0.2–0.7 g L^–1 in this degradation system.

Reusability and Recyclability of the Composite

The long-term stability and reusability of a photocatalyst are critical parameters for evaluating its potential in practical wastewater treatment applications. For this purpose, the photocatalytic degradation of MB and OG was performed over five consecutive cycles using the CZ-10 catalyst. After each cycle, the spent catalyst was carefully recovered by centrifugation at 9000 rpm for 10 min, followed by repeated washing with water and ethanol to remove any residual dye molecules adsorbed on the surface. This washing step is essential, as adsorbed organic residues can block active sites and photocatalytic efficiency in subsequent runs. The dye-free catalyst was then dried in an oven at 60 °C for 4 h before the next degradation experiment. Figure d shows the photocatalytic removal of MB and OG from aqueous solution for five consecutive runs with CZ-10. In the first run, the CZ-10 photocatalyst demonstrated excellent catalytic activity, achieving 100% removal of MB and 96% of OG after 90 min of irradiation. In the second cycle, efficiencies dropped to 94% for MB and 92% for OG, representing a decline of 6 and 4%, respectively. This negligible reduction highlights the catalyst’s structural stability and resistance to deactivation during reuse. As the number of cycles increased, a gradual decline in photocatalytic performance was observed. After five consecutive runs, the catalyst still maintained high degradation efficiencies of 87% for MB and 84% for OG. This corresponds to a total decrease of only 13% for MB and 9% for OG relative to the first run. This retention of activity demonstrates that the CZ-10 photocatalyst has excellent durability, making it suitable for repeated use without regeneration or replacement after each cycle. The observed decline in efficiency after multiple runs can be attributed to several factors, including minor structural changes, surface fouling by strongly adsorbed dye byproducts, and slight agglomeration of nanoparticles during the drying process, all of which reduce the available surface area and active sites. Despite these limitations, the overall performance indicates that CZ-10 retains the majority of its photocatalytic activity over successive cycles, reinforcing its potential as a cost-effective and sustainable photocatalyst for dye degradation in wastewater treatment. The correlation between catalytic performance and material properties is shown in Table .

2. Correlation between the Catalytic Performances and Material Properties.

Characteristics	CuO	ZnO	CZ-10
crystal structure	monoclinic	hexagonal wurtzite	mixture
crystallite Size (nm)	19	27	34
band gap (eV)	1.64	3.23	3.13
PL intensity		36.7	12.4
recombination rate (comparative)		3X	X
particle shape	spherical	double cone	lotus
particle size (nm)	34	40	125 (petals of flowers)
MB removal (%)	16	62	100
OG removal (%)	17	40	96
MB removal per g of ZnO (mg g–1)		20	67
rate constant (MB) (min–1)	0.002	0.012	0.06
rate constant (OG) (min–1)	0.001	0.006	0.03

Proposed Mechanism of Photocatalysis by CZ-10 Heterojunction

The CZ-10 composite produces hydroxyl radicals from water and oxygen via oxidation and reduction reactions, respectively, as illustrated in Figure . The valence band (VB) position of CuO and ZnO is reported to be 0.5 and 2.57 eV, while their conduction band positions are −0.71 and −0.62 eV, respectively. The conduction band (CB) of ZnO lies lower than that of CuO, whereas CuO possesses a lower VB energy compared to ZnO. The band gap of pure ZnO and pure CuO was 3.23 and 1.64 eV, respectively. After composite formation, the band gap of CZ-10 was found to be 3.13 eV, which is close to that of pure ZnO. Despite the similar band gaps, CZ-10 exhibits a higher degradation efficiency than pure ZnO nanoparticles. The electron–hole pair is better separated in the composite, slowing their recombination. Charge transfer between two semiconductors makes it feasible.

10.

Schematic representation of the proposed mechanism for the degradation of MB and OG via CZ-10 composite.

Two semiconductors can exchange charges in several ways. The Type II, Z-scheme, and S-scheme mechanisms are acceptable explanations for charge migration. From both dynamic and thermodynamic perspectives, the conventional Type II heterojunction is regarded as undesirable. In this heterojunction, holes migrate from a lower valence band VB to a higher one, and electrons transfer from a higher CB to a lower one. This charge transfer decreases the redox efficiency of the charge carriers. Based on the band alignment and experimental observations, neither the Type II nor the Z-scheme mechanism adequately describes the actual charge-transfer process. The S-scheme heterojunction has recently emerged as a more advanced model for predicting charge transfer between two distinct semiconductors under light irradiation. In this system, two semiconductors with different redox potentials are coupled so that one photocatalyst has a higher Fermi level (E _f) and a more negative CB potential than the other. The work function of the ZnO must also be greater than that of the CuO to facilitate efficient charge migration and band bending at the interface. It was previously considered that a thermal equilibrium state would be established; the E _f of CuO and ZnO would shift upward and downward, respectively, upon contact, until they reached the same level. However, the unequal distribution of charges makes this idea implausible. For the S-scheme heterojunction, Zhang et al. proposed a mechanism in which the charge-transfer process varies and the band locations are staggered. According to this theory, E _f of CuO gradually decreases while that of ZnO increases, resulting in band bending at their interface until thermal equilibrium is reached at the contacting point (Figure ). A CuO-to-ZnO-directed internal electric field (IEF) is created. A potential barrier is created by band bending and IEF. Following the creation of the S-scheme heterojunction and light illumination, the photoexcited ZnO electrons move to the interface region and recombine with the CuO photogenerated holes. Charge recombination within CuO and ZnO itself is also inhibited. Furthermore, high-energy electrons and holes at the CB of CuO and VB of ZnO, respectively, are preserved since the E _f in the bulk regions of ZnO and CuO remain unchanged as before contact. Redox and oxidizing processes at the surfaces of CuO and ZnO are facilitated by these electrons and holes, which have significant reduction and oxidation potentials, respectively. The mechanism of the production of hydroxyl radicals is given in eqs –.

$$\mathrm{CuO}+h\nu \to \mathrm{CuO}({\mathrm{e}}_{\mathrm{CB}}^{-})+\mathrm{CuO}({\mathrm{h}}_{\mathrm{VB}}^{+})$$ 7

$$\mathrm{ZnO}+h\nu \to \mathrm{ZnO}({\mathrm{e}}_{\mathrm{CB}}^{-})+\mathrm{ZnO}({\mathrm{h}}_{\mathrm{VB}}^{+})$$ 8

$$\mathrm{CuO}({\mathrm{h}}_{\mathrm{VB}}^{+})+\mathrm{ZnO}({\mathrm{e}}_{\mathrm{CB}}^{-})\to h\nu$$ 9

$$\mathrm{ZnO}({\mathrm{h}}_{\mathrm{VB}}^{+})+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HO}}^{·}+{\mathrm{H}}^{+}$$ 10

$$\mathrm{CuO}({\mathrm{e}}_{\mathrm{CB}}^{-})+{\mathrm{O}}_2\to {\mathrm{O}}_2^{-·}$$ 11

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{HO}}^{·}+{\mathrm{HO}}^{-}$$ 12

$${\mathrm{O}}_2^{-·}+2{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ 13

$$\mathrm{Dye}+{\mathrm{HO}}^{·}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$$ 14

A hole is left in the VB when light strikes and irradiates the CuO catalyst, since the electrons in the VB absorb the energy of the radiation and move into the CB. During irradiation, the ZnO catalyst experienced a similar situation. Due to the upward bending of the VB and CB in CuO semiconductors at the CuO-ZnO contact point, electrons in the CuO CB did not leap into the ZnO CB because their energy was too high. This is accomplished by electrons moving to the CuO surface, where they react with oxygen molecules to form superoxide radicals, which, in turn, generate hydroxyl radicals. Conversely, a hole in the VB of CuO may go to the depletion layer and recombine with an electron in the conduction band of ZnO. The CuO-ZnO contact point is where the downward bending in the ZnO semiconductor takes place. Because of its reduced energy in the descending direction and a hole in the upward direction, the electron can move downward. In the reaction with oxygen and water, respectively, these lower-energy electrons in the ZnO CB and the holes in the CuO VB can recombine to preserve the higher-energy electron in the CuO CB and the hole in the ZnO VB, which can efficiently generate the hydroxyl radicals. Hydroxyl radicals are generated when the hole reduces water molecules in the VB of ZnO, which subsequently oxidize the dyes, converting them into harmless products, such as CO₂ and H₂O.

Conclusions

CuO/ZnO could be synthesized at varying weight percentages using the hydrothermal method. The synthesized materials exhibited characteristic functional groups, along with Cu and Zn in the 2+ oxidation state in the nanocomposites. An oxygen vacancy was created in the composites. The composite showed high thermal stability up to 750 °C. CuO had a monoclinic structure, while ZnO was in the hexagonal wurtzite form in the composites. The optical band gaps were found to be 1.64, 3.23, and 3.13 eV for CuO, ZnO, and CZ-10, respectively, which indicated a reduction of band gap energy of the CZ-10 composite after the addition of CuO into ZnO that allows the absorption of the UVA region to produce hydroxyl radicals in the photocatalytic dye degradation. A reduction in electron–hole pair recombination in the CZ-10 composites was observed, thereby enhancing the degradation efficiency. The removal of MB and OG from aqueous solution was 100 and 96%, respectively, using CZ-10 material within 90 min of irradiation, which is more than twice as effective as the pure ZnO catalyst. The enhanced performance results from reduced electron–hole recombination and improved charge-carrier separation in the CZ-10 catalyst. The photocatalytic degradation of MB and OG by CZ-10 followed pseudo-first-order kinetics with rate constants of 0.06 and 0.03 min^–1 for MB and OG, respectively. The CZ-10 catalyst is recyclable and reusable, making it potentially suitable for practical applications.

Data will be made available on request.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c12559.

• Assignments of bands from FTIR spectra, histogram of particle size distribution determined from FE-SEM images, EDS spectra, and elemental mapping of the prepared catalysts; FESEM images of CZ-5 and CZ-15; peak positions at 2θ of XRD patterns for pristine ZnO, and CuO/ZnO nanocomposites; Point of Zero Charge (PZC) of CZ-10 catalyst; setup for UVA-induced photodegradation and spectral distribution of the UVA lamp; calibration curve of aqueous solution of MB and OG dye; UV–visible spectra and the color change photograph of OG dye during photocatalytic degradation; investigation of the change in concentration of MB dye by the irradiation of UVA light, and spectral monitoring of concentration changes of MB with time by the CZ-10 catalysts in the absence of radiation sources; dynamic and kinetic plot of the photocatalytic degradation of MB by metal oxide semiconductor with the irradiation of sunlight and UVA light; and linear fit of the data derived from the degradation reaction of MB by CZ-10: zero order, first order, and second order kinetics and their corresponding R ² values (PDF)

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University of Florida, Department of Chemistry, 165 Buckman Dr, Gainesville, Florida 32603, United States

M.S.: Methodology, Investigation, Visualization, Formal Analysis, Writing - Original Draft. A.J.: Investigation, Methodology, Writing - Original Draft. M.F.E.: Methodology, Investigation, Visualization, Formal Analysis. M.M.I.: Validation, Review and Editing. M.A.B.H.S.: Resources, Funding acquisition, Writing - Review and Editing. Muhammed Shah Miran: Conceptualization, Resources, Writing - Review and Editing, Funding acquisition, Visualization, Supervision.

The authors declare no competing financial interest.