Eco-Friendly Synthesis of Zn-Doped CuO Nanoparticles Using Aloysia citrodora Extract for Highly Efficient Fenton-like Dye Degradation

Abstract

The development of efficient, sustainable, and low-cost catalysts for wastewater treatment remains a major environmental challenge. In this work, Zn-doped CuO nanostructures were successfully synthesized via a green route using Aloysia citrodora leaf extract as a natural reducing and stabilizing agent. The structural and morphological properties of the prepared catalysts were systematically characterized by XRD, Raman spectroscopy, FTIR, SEM, and EDX analyses. The results revealed the formation of highly crystalline monoclinic CuO nanoparticles, whose defect density and surface properties were significantly modified by Zn incorporation. The catalytic performance of the synthesized materials was evaluated through the heterogeneous Fenton-like degradation of Rhodamine B in aqueous solution under dark conditions. The Zn-doped CuO catalyst exhibited outstanding degradation efficiency (~99.97%) within only 30 min, using a low catalyst dosage of 15 mg and a minimal H₂O₂ amount of 25 μL. The enhanced catalytic activity is attributed to the synergistic interaction between Zn-induced lattice defects and the Cu²⁺/Cu⁺ redox cycle, which promotes efficient H₂O₂ activation and •OH radical generation. Radical scavenging experiments confirmed the dominant role of hydroxyl radicals in the degradation process. Compared with previously reported CuO-based catalysts, the present system demonstrates superior performance in terms of reaction rate, oxidant consumption, and energy efficiency. These findings highlight the potential of Zn-doped CuO synthesized via green chemistry as a promising and sustainable catalyst for advanced wastewater treatment applications.

1. Introduction

Copper is a vital element for humans, animals, and plants in various oxidation states (Cu⁺ and Cu²⁺). Researchers are interested in materials made of copper because they are used in a wide range of technologies [1], including the creation of supercapacitors, near-infrared filters, magnetic storage media, sensors, catalysis, semiconductors, and more [2,3,4]. These materials have attracted increasing attention due to their low cost, natural abundance, and favorable physicochemical properties [5,6]. Rapid advancements in nanotechnology have produced a variety of nanoparticles (NPs) [7]. These nanomaterials can be classified into several categories based on their size, shape, and chemical composition, including carbon-based nanomaterials, metal and metal oxide nanoparticles, polymeric nanoparticles, metal–organic framework (MOFs), and fullerenes [8,9,10]. In particular, hybrid and supported nanostructures [11] have attracted increasing attention due to their enhanced stability and catalytic performance [12]. Among them, metal oxides are crucial, drawing the attention of researchers from a wide range of disciplines, including energy storage, drug delivery [13], sensing [14,15], environmental applications such as bee-safe nano insecticides [16], biomedical applications, electronics, catalysis, and others [17,18,19,20,21]. Cupric oxide (CuO) is an inorganic transition metal oxide semiconductor with a low band gap (1.2–1.5 eV) which makes it considerably more stable than organic nanoparticles [22]. CuO NPs have been produced using a variety of chemical and physical methods. In the case of chemical routes, the sol–gel method [23], chemical reduction [24], precipitation [25], and polyol synthesis [26] are the most studied [27]. In contrast, physical synthesis approaches include laser ablation [28], pulsed laser deposition [29], microwave-assisted synthesis [30], etc. The rapid recombination of photogenerated electrons (e⁻) and holes (h⁺) limits its practical utilization. Therefore, efficiently separating photogenerated electrons and holes is essential to improving CuO’s efficiency [31,32,33,34,35]. This can be achieved by lowering their recombination via techniques including doping, composite modification, or the creation of heterogeneous junctions [36]. Additionally, elemental doping is believed to influence the shape of nanoparticles as well as their optical, physical, and chemical properties [5]. Several studies have focused on doping CuO with various metals, including Ni, Mn, Ag, Zn, Pd, and Fe [36]. Zn is a better doping material because the ionic radius of Zn²⁺ (0.74 Å) is closer to that of Cu²⁺ (0.72 Å) [37]. Although Zn²⁺ and Cu²⁺ have the same charge, the slightly larger Zn²⁺ can distort the lattice, promote oxygen vacancy formation and improve catalytic activity [38]. Therefore, it was suggested that Zn might be incorporated into CuO using a preparation procedure that does not require calcination [39].

Recent studies have demonstrated the effectiveness of Advanced Oxidation Processes (AOPs), such as photo-Fenton, ozonation, sonolysis, and ionizing radiation, for the removal of recalcitrant water pollutants. Among these approaches, metal oxide-based catalysts have gained increasing attention due to their ability to generate highly reactive oxygen species (ROS), including superoxide (•O₂⁻) and hydroxyl (•OH) radicals during oxidant activation [40]. Among AOPs, the Fenton reaction has attracted considerable attention due to its high efficiency, simple operation, and low cost [41]. In this context, copper has emerged as a promising alternative to iron for Fenton-like catalysis, owing to its abundance, low toxicity, and high redox activity. The reversible Cu⁺/Cu²⁺ redox cycle enables efficient hydroxyl radical generation from H₂O₂, with faster kinetics than the conventional Fe²⁺/Fe³⁺ system [42]. Heterogeneous Fenton-like oxidation has emerged as an efficient and sustainable approach for organic pollutant removal [43]. In such systems, charge transfer properties play a crucial role in oxidant activation. Therefore, strategies such as elemental doping, defect engineering, and heterojunction construction are widely employed to enhance charge separation efficiency and improve H₂O₂ activation, leading to improved degradation performance [44]. In this context, Rhodamine B (RhB) is frequently used as a model organic pollutant. As a synthetic xanthene dye, it is utilized in the dying of cotton, weed, and leather, as well as in the preparation of ball pen and stamp pad inks. RhB is highly stable and resistant to conventional biological treatments [45], making it an ideal candidate for evaluating the performance of new Fenton-like catalysts. In parallel, a biological approach to material synthesis has emerged, utilizing renewable resources and non-toxic phytochemicals as reducing and stabilizing agents [46]. Environmentally friendly, plant-mediated techniques have already been used to produce Zn-doped CuO for various therapeutic applications [47,48].

In this study, pure and Zn-doped CuO nanoparticles were synthesized via a simple and cost-effective co-precipitation route using Aloysia citrodora leaf extract. This medicinal plant, rich in polyphenols and other bioactive molecules, has not yet been reported for the synthesis of these materials. It was selected due to its abundant phytochemical content, rapid growth, and high productivity, making it a sustainable and renewable resource [49]. The extract contains diverse phenolic constituents, including phenylpropanoids and glycosylated flavones, which can act as efficient reducing and stabilizing agents during the synthesis process [50]. The present work further examines Zn incorporation and its influence on structural defects and surface reactivity, which may promote H₂O₂ activation into hydroxyl radicals (•OH). Although Zn doping has been widely reported to enhance photocatalytic performance, its contribution to heterogeneous Fenton-like systems remains insufficiently explored. Notably, heterogeneous Fenton-like processes can operate efficiently under dark conditions, thereby lowering energy demand and offering a sustainable pathway for wastewater treatment.

The novelty of this study resides in the development of a sustainable plant-mediated route for the synthesis of Zn-doped CuO nanoparticles using Aloysia citrodora extract as a reducing and stabilizing agent, which has not previously been reported for this material. This work further provides a comprehensive correlation between Zn-induced structural defects, morphological evolution, and the enhancement in heterogeneous Fenton-like catalytic activity toward Rhodamine B degradation under dark conditions.

2. Results and Discussion

2.1. XRD Pattern Interpretation

X-ray diffraction patterns of pure CuO and Zn-doped CuO samples (CuO-Zn0.5, CuO-Zn1, CuO-Zn2, CuO-Zn3, and CuO-Zn5) were recorded in the 2θ range of 20–90° (Figure 1a). All samples exhibited the characteristic monoclinic CuO structure in agreement with JCPDS card No. 01-080-1268, with well-defined diffraction peaks at 32.5°, 35.6°, 38.9°, 46.3°, 48.8°, 51.5°, 53.6°, 58.4°, 61.8°, 65.9°, 66.3°, 68.0°, 72.5°, 75.0°, and 75.3° corresponding to the crystallographic planes (110), (−111), (111), (−112), (−202), (112), (020), (202), (−113), (−311), (310), (220), (311), (004), (−222), (−114), (−313), (312), (400), and (−402), respectively. The preservation of the monoclinic CuO structure across all doping concentrations indicates successful incorporation of Zn²⁺ ions without phase transformation or secondary phase formation related to copper.

Minor additional peaks were observed in Zn-doped samples at approximately 22–25°, 31.8°, and 33.6°. The peak at 31.8° may correspond to the (100) plane of ZnO, though the absence of other characteristic ZnO reflections suggests either phase segregation below the detection threshold or trace impurities rather than significant ZnO formation. The peaks observed in the 22–25° region may be attributed to residual organic compounds originating from the plant extract used during the synthesis. The CuO diffraction peaks exhibited no significant 2θ shifts upon doping, consistent with previous studies that reported minimal peak displacement for Zn-doped CuO systems [51,52]. However, pronounced changes in peak broadening and intensities were observed, particularly for the (−111) and (111) reflections, indicating altered crystallographic texture or preferential orientation effects induced by doping (Figure 1b).

The absence of significant peak shifts coupled with changes in peak broadening and intensities strongly suggests iso valent substitution of Cu²⁺ by Zn²⁺, which have the same charge and similar ionic radii (Cu²⁺: 0.73 Å, Zn²⁺: 0.74 Å), without significantly altering the overall charge balance or crystal structure [53]. This behavior contrasts with studies involving dopant ions with substantially different ionic sizes, such as Ni²⁺ (r = 0.95 Å) or Mn²⁺ (r = 0.69 Å) in CuO, where higher peak shifts were reported due to more significant lattice expansion or contraction [54]. This comparison underscores the critical role of ionic size matching in determining doping mechanisms in metal oxide systems.

The lattice parameters for the monoclinic structure were calculated using standard indexing methods [55]. The results in

Table 1. Variation in lattice constants, cell volume, dislocation density, and full width at half maxima (FWHM) with dopant concentration.

Property	CuO	CuO-Zn0.5	CuO-Zn1	CuO-Zn2	CuO-Zn3	CuO-Zn5
x (Å)	4.6823	4.6844	4.6810	4.6812	4.6852	4.6838
y (Å)	3.4174	3.4192	3.4183	3.4157	3.4175	3.4139
z (Å)	5.1268	5.1242	5.1272	5.1262	5.1238	5.1251
b (°)	99.475	99.538	99.517	99.560	99.542	99.530
Cell volume (Å)	80.917	80.941	80.912	80.825	80.906	80.820
Dislocation density (nm−2) 10−3	0.750	1.147	2.473	2.061	1.298	1.808
FWHM (hlk = −111) (°)	0.239	0.327	0.378	0.323	0.335	0.299
FWHM (hlk = 111) (°)	0.302	0.421	0.494	0.415	0.437	0.359

show irregular variations, with Zn concentration deviating from Vegard’s law, which typically predicts linear relationships between lattice parameters and dopant concentration. a ranged from 4.6810 to 4.6852 Å, b from 3.4139 to 3.4192 Å, and c from 5.1238 to 5.1272 Å. These values are in agreement with values reported in the literature [56] and the earlier-mentioned JCPDS card. The monoclinic angle β remained relatively constant (~99.5°), and unit cell volume fluctuated between 80.82 and 80.94 ų without monotonic correlation with Zn concentration. This deviation from Vegard’s law suggests that the doping mechanism is more complex than simple substitutional incorporation, potentially arising from preferential occupation of specific Cu²⁺ sites by Zn²⁺, compensation effects involving point defects such as oxygen or vacancies, local distortions around Zn²⁺ dopants, or the clustering of dopant atoms at higher concentrations [57].

Crystallite size analysis using both Scherrer’s equation (Equation (1)) [58] and the Williamson–Hall (W-H) (Equation (2)) method [37] revealed intriguing non-monotonic trends with doping concentration. Scherrer analysis involved only the (−111) and (111) peaks. The results showed that crystallite sizes decreased from 31.6 nm for pure CuO to 18.8 nm for CuO-Zn1, and then increased to 25.6 nm for CuO-Zn5, as shown in Figure 1c,d. The W-H method, which accounts for strain effects, yielded different absolute values but similar trends, with sizes ranging from 55.5 nm for pure CuO to 63.1 nm for CuO-Zn2. The consistently larger crystallite sizes obtained from the W-H method compared to Scherrer’s equation highlight the importance of strain considerations in nanocrystalline materials, as Scherrer’s equation assumes strain-free conditions while the W-H method separates size and strain contributions to peak broadening.

Micro-strain analysis revealed a systematic increase from 1.30 × 10⁻³ for pure CuO to 3.2 × 10⁻³ for CuO-Zn5 using the W-H method (Figure 2), corroborated by rising dislocation density (Equation (4)) from 0.75 × 10¹⁴ m⁻² for pure CuO to 2.47 × 10¹⁴ m⁻² for CuO-Zn1. The full width at half maximum (FWHM) of the (−111) and (111) peaks broadened significantly from 0.239° to 0.378°, further evidencing enhanced structural disorder.

$$\mathrm{A}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e} \mathrm{c}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{e} \mathrm{s}\mathrm{i}\mathrm{z}\mathrm{e} (\mathrm{D})={\displaystyle \frac{K\times \lambda }{\beta \times Cos\theta }}$$

(1)

$$\beta \times Cos\left(\theta \right)={\displaystyle \frac{k\lambda }{D}}+4\mathsf{\epsilon }\times \mathrm{s}\mathrm{i}\mathrm{n}\theta$$

(2)

$$\mathrm{M}\mathrm{i}\mathrm{r}\mathrm{o}-\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}(\mathsf{\epsilon })={\displaystyle \frac{\beta }{4tan\theta }}$$

(3)

$$\mathrm{D}\mathrm{i}\mathrm{s}\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y} (\delta )={\displaystyle \frac{1}{D^2}}$$

(4)

where K is the Scherrer constant or shape factor = 0.9, θ is the diffraction angle in radians, λ signifies the X-ray wavelength (1.5406 Å for Cu Kα radiation), and β indicates line broadening at half the maximum intensity (FWHM), expressed in radians.

2.2. Raman Spectroscopy Analysis

Structural confirmation of both pure and doped samples was performed using Raman spectroscopy. Cupric oxide (CuO) has a monoclinic structure and belongs to the C6₂h (C2/c) space group. It exhibits three Raman-active modes: one A_g and two B_g modes [59]. Figure 3 presents the Raman spectra of the pure and doped samples, recorded using a 532 nm laser at 10 mW power. Three characteristic peaks are observed at 288 cm⁻¹ (A_g mode), 337.15 cm⁻¹ (B_1g mode), and 626.54 cm⁻¹ (B_2g mode) [60]. No peaks associated with impurities or the substrate are detected, consistent with the XRD results, confirming the high purity of the samples. After Zn incorporation, the B_1g mode disappears, indicating a reduction in phonon coherence due to lattice disorder and strain induced by Zn²⁺ incorporation in the fabricated CuO nanoparticles.

Additionally, the A_g mode exhibits a noticeable shift toward lower wavenumbers (≈270–287 cm⁻¹), suggesting lattice distortion and phonon confinement effects associated with crystallite size reduction. The B_2g mode remains nearly constant around ~626 cm⁻¹ for low Zn contents (0.5–3%), while a significant shift to 616.46 cm⁻¹ is observed at 5% Zn, reflecting stronger defect formation and lattice distortion at high doping. These Raman observations are consistent with XRD peak broadening and Williamson–Hall analysis, which evidence the evolution of crystallite size and micro-strain with Zn concentration (

Table 2. Raman peak shifts induced by Zn doping.

Sample	Ag (cm−1)	ΔAg (cm−1)	B2g (cm−1)	ΔB2g (cm−1)
CuO	288.00	0.00	626.54	0.00
CuO-Zn0.5	280.00	−8.00	626.00	−0.54
CuO-Zn1	287.19	−0.81	625.92	−0.62
CuO-Zn2	272.22	−15.78	626.56	+0.02
CuO-Zn3	270.58	−17.42	626.25	−0.29
CuO-Zn5	274.22	−13.78	616.46	−10.08

).

2.3. Fourier Transform Infrared (FTIR) Spectroscopy Analysis

FTIR spectroscopy is essential for identifying functional groups and chemical bonds in nanomaterials. Figure 4 illustrates the FTIR spectra of pure and Zn-doped CuO NPs at various concentrations. In the low-frequency region, the absorption bands observed between 522 cm⁻¹ and 580 cm⁻¹ are assigned to the metal–oxygen (Cu–O) stretching modes, confirming the formation of the CuO monoclinic structure [61]. For the pure sample, the primary Cu–O peak appears at 526 cm⁻¹, while for Zn-doped samples (notably CuO-Zn0.5 and CuO-Zn3), this band shifts to approximately 528–530 cm⁻¹. This shift toward higher wavenumbers suggests lattice distortion and micro-strain resulting from the substitution of Cu²⁺ ions by Zn²⁺ ions, which have slightly different ionic radii.

A prominent band at 1093–1112 cm⁻¹ is observed across all samples. This corresponds to the C–O stretching vibrations of alcohols, esters, and ethers [62]. These functional groups are indicative of flavonoid glycosides and phenolic acids from the Aloysia citrodora extract used during synthesis, consistent with previous studies [49]. FTIR spectral shifts indicate that these molecules may be either physically adsorbed or chemically coordinated to surface Cu²⁺/Zn²⁺ sites [63]. The band appearing near 1436 cm⁻¹ in the Zn-doped samples is attributed to the symmetric and asymmetric stretching of carboxylate groups, likely arising from residual acetate species from the chemical precursors [64,65]. Overall, these results confirm that Zn doping modifies both the Cu–O vibrations and the surface chemistry. These FTIR trends correlate with XRD observations: the shift and broadening of the Cu–O modes are explained by the increased dislocation density and lattice defects induced by the dopant. The stabilization of the Cu–O bond around 530 cm⁻¹ in doped samples supports the successful incorporation of Zn into the CuO framework, which is expected to enhance the material’s catalytic performance.

2.4. Morphological and Compositional Analysis (SEM-EDX)

2.4.1. SEM Results

SEM analysis was performed to examine the surface morphology and particle size distribution of pure CuO and Zn-doped CuO nanoparticles. The SEM images of the undoped CuO sample are shown in Figure 5, revealing agglomerated nanostructures made up of irregularly shaped grains with sizes ranging from roughly 50 nm to 160 nm; porous aggregates are also formed, a typical characteristic of CuO synthesized via a green route, typically arising from the high surface energy of the nanocrystals and the dual reducing–stabilizing role of plant biomolecules [66]. In contrast, Zn doping induces a clear modification in the morphology. At very low doping (0.5%), the particles remain agglomerated but tend to be smaller and more spherical, with an estimated size range of 30–80 nm. With further increases in Zn content (≥1%), the morphological modification becomes more pronounced and consistent across all compositions (1–5%), leading to nearly spherical to sub-spherical nanoparticles with a narrower size distribution of approximately 20–50 nm; this morphological refinement can be attributed to the incorporation of Zn²⁺ ions, which influence nucleation and growth processes during the formation of CuO crystallites. Several studies have reported that Zn incorporation significantly affects the morphology of CuO. It can induce a transformation from irregular structures to spherical shapes [67], convert initially octahedral particles into more diffuse spherical morphologies [52], and promote the transition from irregular plates to well-defined nanoparticles [39].

2.4.2. EDX Results

The EDX analysis (Figure 6) confirms the elemental composition of pure CuO and Zn-doped CuO. For the undoped sample, only copper and oxygen signals are detected, indicating the successful formation of CuO. Upon Zn doping, characteristic Zn peaks appear, and their intensity gradually increases with dopant concentration, demonstrating the effective incorporation of Zn into the CuO matrix. Additionally, slight variations in the atomic percentages of Cu and O are observed as the Zn content increases, reflecting compositional adjustments during doping. Overall, the results confirm the successful synthesis of Zn-doped CuO without the presence of undesirable impurities.

2.5. Catalytic Degradation of Rhodamine B

2.5.1. Effect of Zn Doping Amount on Rhodamine B Removal

The degradation efficiency of Rhodamine B using pure CuO and Zn-doped CuO was first evaluated under identical experimental conditions (RhB = 5 ppm, V_solution = 25 mL; natural pH, catalyst mass = 25 mg, H₂O₂ = 100 μL, ambient temperature). As shown in Figure 7a, the 0.5% Zn-doped CuO sample exhibits the highest catalytic performance, achieving approximately 80% degradation over a total reaction time of 60 min in the presence of H₂O₂. This peak efficiency is directly correlated with a sharp reduction in crystallite size (~23 nm via Scherrer and ~31 nm via W-H), which maximizes the active surface area. At this low doping level, the micro-strain remains minimal, ensuring efficient charge transfer, and may reduce the recombination of electron–hole pairs. Beyond 0.5%, a significant drop in activity is observed, with the 1% Zn sample even falling below the performance of pure CuO. This decline coincides with an increase in structural disorder (micro-strain) and the recovery of crystallite size, as shown by the W-H method. In this range, excessive doping likely creates structural defects that may act as deep traps for charge carriers, hindering the activation of H₂O₂. Interestingly, partial recovery of the degradation efficiency is observed for Zn contents ≥ 3%. This trend coincides with changes in the structural parameters derived from the W–H analysis, suggesting a modification of lattice distortion at higher doping levels. In particular, after reaching a maximum micro-strain, relative relaxation of the crystal lattice is observed at higher Zn contents. Such structural evolution may favor the formation of additional surface defects or catalytically active sites, which can enhance the interaction between the catalyst surface and H₂O₂, leading to increased generation of reactive oxygen species (ROS). Nevertheless, this recovered activity remains lower than the optimum performance obtained for the 0.5% Zn-doped CuO sample.

To further quantify these observations, a kinetic study (Figure 7b) was performed using the pseudo-first-order model. The calculated rate constant k and correlation coefficient R² provide clear numerical validation of the catalytic trends. The standard deviation of the rate constant was calculated for pure CuO and CuO-Zn0.5 and found to be very low (±0.0004 min⁻¹) and (±0.0002 min⁻¹), respectively, confirming excellent reproducibility for the undoped sample. However, the pseudo-first-order model does not fully describe all samples: CuO-Zn1 shows a low R² (0.28), suggesting contributions from adsorption or changes in the reaction mechanism. In contrast, CuO-Zn0.5 (R² = 0.92) shows a reasonably good fit; alternative models, such as intraparticle diffusion, could better describe these kinetics. The 0.5% Zn-doped CuO sample exhibited the highest rate constant k = 0.00873 min⁻¹, which is approximately 2.6 times higher than that of pure CuO, which has k = 0.00338 min⁻¹. This significant enhancement confirms that the reduction in crystallite size and the minimization of micro-strain at this specific doping level drastically accelerate the reaction kinetics. For the 1% Zn sample, the sharp decline in activity is reflected by a substantial drop in the rate constant, reinforcing the hypothesis that excessive structural defects hinder the activation of H₂O₂. Interestingly, the partial recovery observed for higher Zn contents (5% Zn) is characterized by a stabilized rate constant k = 0.00368 min⁻¹ with an excellent fit R² = 0.971. This suggests that while lattice relaxation at higher concentrations creates new active sites, the overall catalytic efficiency remains limited compared to the 0.5% Zn optimum, where the synergy between surface area and charge transfer is most favorable.

2.5.2. Effect of pH

The influence of pH on RhB degradation was investigated at pH 5, natural pH, and pH 9 using the optimal catalyst (0.5% Zn-doped CuO). The results indicate that the degradation efficiency is strongly pH-dependent (Figure 8). At acidic pH, complete degradation (nearly 100%) was achieved within only 30 min, followed by a plateau signifying the total removal of the dye. In contrast, at natural pH and basic pH, the process was significantly slower, reaching approximately 80% and 74%, respectively, after 60 min. The enhanced performance at acidic pH can be attributed to the superior activation of H₂O₂ and the accelerated generation of hydroxyl radicals (•OH), which are the primary oxidants in heterogeneous Fenton-like systems [68]. Under acidic pH~5, the scavenging of radicals is minimized, and the stability of H₂O₂ favors the catalytic cycle. These findings confirm a heterogeneous Fenton-like mechanism, where the Zn-doped CuO surface effectively mediates the decomposition of H₂O₂ into highly reactive species, and the rapid kinetics at slightly acidic pH, compared to the slower degradation at higher pH [69]. These findings confirm that the process remains effective within the pH range of 5–9.

2.5.3. Effect of Catalyst Dosage

The influence of catalyst dosage on the degradation of Rhodamine B was investigated by varying the amount of 0.5% Zn-doped CuO from 15 mg to 25 mg. As shown in Figure 9, a dosage of 15 mg was sufficient to achieve nearly complete degradation within 30 min. Interestingly, increasing the catalyst mass to 20 mg or 25 mg did not yield any significant improvement in the degradation rate or final efficiency, as all tested dosages reached the 99.95% threshold at the same 30-min mark. This behavior suggests that at 15 mg, the system already possesses a sufficient number of active sites to fully activate the H₂O₂ and generate the required reactive oxygen species (ROS) for the concentration of RhB present. The lack of enhancement at higher dosages (20–25 mg) can be attributed to several factors. Higher catalyst concentrations can lead to the formation of clusters, reducing the effective surface area that was previously gained through Zn doping (as seen in the reduced crystallite size). An excess of catalyst may increase the turbidity of the solution or lead to the scavenging of hydroxyl radicals •OH by the catalyst surface itself, preventing further kinetic gains. Consequently, 15 mg was selected as the optimal catalyst dosage for subsequent parametric studies to ensure both high efficiency and economic feasibility.

2.5.4. Effect of H₂O₂ Amount

The effect of H₂O₂ dosage on Rhodamine B degradation was evaluated by varying the volume from 25 to 100 μL. The experimental results, illustrated in Figure 10, indicate that nearly complete degradation was successfully achieved within 30 min for all investigated dosages. This behavior suggests that even the minimum dosage of 25 μL provides a sufficient quantity of H₂O₂ molecules to be activated by the 0.5% Zn-doped CuO surface for total dye removal. A total of 25 μL was identified as the optimal dosage, demonstrating a high catalytic efficiency of the Zn-doped CuO system and its ability to achieve total degradation with minimal oxidant consumption, which is a significant advantage for cost-effective wastewater treatment.

2.5.5. Effect of Initial Rhodamine B Concentration

The effect of the initial Rhodamine B (RhB) concentration on the degradation efficiency was investigated by varying the dye concentration from 5 to 20 ppm under dark conditions using the CuO-Zn0.5/H₂O₂ heterogeneous Fenton-like system. As shown in Figure 11, the degradation efficiency strongly depends on the initial pollutant concentration. At 5 ppm, complete degradation (~100%) was rapidly achieved within 30 min. However, increasing the initial RhB concentration to 10, 15, and 20 ppm resulted in a progressive decrease in degradation efficiency, reaching approximately 97%, 85%, and 68% after 60 min, respectively.

This inverse relationship is mainly attributed to the limited production of reactive oxygen species, particularly hydroxyl radicals (•OH), which remains constant at fixed catalyst loading (15 mg) and oxidant dosage (25 μL of H₂O₂). At higher RhB concentrations, the ratio of generated •OH radicals to dye molecules decreases, leading to insufficient oxidative species to mineralize the pollutant completely. Moreover, higher dye concentrations promote competition for the active catalytic sites on the Zn-doped CuO surface, resulting in partial surface saturation and reduced degradation kinetics [70].

2.5.6. Effect of Scavengers

To elucidate the degradation mechanism of Rhodamine B and identify the main reactive species involved in the Zn-doped CuO/H₂O₂ system, scavenger experiments were carried out under the optimized conditions as shown in Figure 12. Tert-butanol (TBA), EDTA, and ascorbic acid were used as a hydroxyl radical (•OH) scavenger, a metal ion chelating agent, and a non-selective reducing agent/ROS scavenger, respectively. The addition of TBA resulted in a significant decrease in degradation efficiency to approximately 55%, indicating that hydroxyl radicals (•OH) play a major role in the degradation process. However, since the reaction was not completely suppressed, other reactive species may also contribute to RhB degradation [71]. Upon the addition of EDTA, the degradation efficiency further decreased to about 41%. This strong inhibition highlights the crucial role of metal active sites in activating H₂O₂ and generating reactive species through a heterogeneous Fenton-like process.

In contrast, the presence of ascorbic acid did not inhibit degradation, and near-complete removal (~100%) was still achieved. This behavior can be attributed to the dual role of ascorbic acid, which may act as a reducing agent promoting the Cu²⁺/Cu⁺ redox cycle, thereby enhancing the generation of reactive species rather than suppressing them.

Overall, these results demonstrate that the degradation of Rhodamine B is mainly governed by a heterogeneous Fenton-like mechanism, where hydroxyl radicals generated via metal-assisted H₂O₂ activation are the dominant oxidizing species.

2.5.7. Suggested Heterogeneous Fenton-like Reaction Mechanism

The enhanced heterogeneous Fenton-like performance of Zn-doped CuO can be explained by the synergistic interplay between Cu redox cycling and Zn-induced lattice defects, as illustrated in Figure 13. The incorporation of Zn²⁺ into the CuO lattice generates oxygen vacancies and structural distortions, as confirmed in [52], which facilitate interfacial electron transfer and promote the activation of H₂O₂ molecules. These oxygen-deficient sites act as catalytic hotspots, accelerating interfacial electron transfer and radical generation. Similar improvements in electron transfer efficiency have been reported in biohybrid catalytic systems, where conductive nanomaterials facilitate charge transport between active sites and pollutants [72]. The catalytic cycle proceeds through the reversible Cu²⁺/Cu⁺ redox coupling. Similar strategies aimed at enhancing redox cycling have been reported in Fenton-based systems, where the Fe³⁺/Fe²⁺ cycle is promoted by suitable additives to improve catalytic efficiency and pollutant degradation [73]. This redox process leads to the generation of highly reactive species such as •OH radicals [74], according to the following reactions:

$$\begin{gathered} {Cu}^{2+}+{H_{2}O}_2\to {Cu}^{+}+H{O}_2•+H^{+} \\ {Cu}^{+}+{H_{2}O}_2\to {Cu}^{2+}+•OH+O{H}^{-} \\ 2H{O}_2•{{\to H}_{2}O}_2+O_2 \end{gathered}$$

•OH radicals then participate in the degradation of Rhodamine B:

$$•OH+RhB\to degraded products$$

The dominant role of •OH radicals was confirmed by scavenger experiments using tert-butanol, which significantly suppressed RhB degradation. Meanwhile, Zn doping can enhance charge transfer kinetics and stabilize surface Cu⁺ species, leading to faster H₂O₂ activation and improved catalytic efficiency.

2.5.8. Reusability of Catalyst

The reusability of the optimal CuO-Zn0.5 catalyst was evaluated using 15 mg of catalyst, 25 µL of H₂O₂, at slightly acidic pH, and 5 ppm of Rhodamine B solution. After each degradation cycle, the catalyst was recovered by washing and then dried at 150 °C before reuse. As shown in Figure 14, the catalyst exhibited excellent stability and reusability. In the first cycle, near-complete degradation of Rhodamine B was achieved, with a removal efficiency of 99.97%; in the second cycle, a slight drop in efficiency to 90% was observed, which may be due to temporary surface fouling by adsorbed intermediates, minor rearrangements of active sites, or small experimental variations. The subsequent drying at 150 °C restored the activity, leading to near-complete degradation in the following cycles, demonstrating the robustness and recyclability of the catalyst. These results indicate that the 0.5% Zn-doped CuO nanoparticles maintain high catalytic performance over multiple cycles, making them promising for practical applications in dye degradation.

2.5.9. Comparative Study

Table 3 presents a comparative analysis between the present work and previously reported CuO-based catalysts employed for heterogeneous Fenton-like degradation of organic dyes. The comparison clearly demonstrates that the Zn-doped CuO synthesized via a green route using Aloysia citrodora exhibits superior catalytic efficiency, achieving almost complete degradation (~99.97%) of Rhodamine B within only 30 min, under mild conditions, low catalyst dosage, and minimal H₂O₂ consumption.

In contrast, most reported CuO-based systems require longer reaction times (60–120 min), higher catalyst loading, and significantly larger amounts of oxidant. Notably, the present catalyst operates efficiently under dark conditions, which highlights its energy-saving advantage and practical applicability. These results confirm the synergistic effect of Zn doping and green synthesis in enhancing the heterogeneous Fenton-like catalytic performance of CuO, making the present material a promising candidate for wastewater remediation. These findings are consistent with previous studies on functional materials for water treatment, where structural characteristics and surface properties play a crucial role in determining removal efficiency [75].

Table 3. Comparison of previously reported CuO-based catalysts for heterogeneous Fenton-like degradation of organic dyes.

Catalyst	Synthesis Method	Degradation Process	Time (min)	Catalyst Loading	H2O2 Concentration	Degradation Efficiency (%)	Ref.
CuO NPs	Green synthesis (S. Tenacissima L.)	Heterogeneous Fenton-like	60	0.2 g/L	NR	99.88% (Tartrazine) 99.97% (Nile Blue)	[42]
CuO NPs	Green synthesis (Cimin grape)	Heterogeneous Fenton-like	120	25 mg/mL	0.8 µg/mL	97.8% (Methylene Blue)	[76]
ZnO–Co3O4–CuO	Mechano-chemical (grinding)	Fenton-like	60	5 g/L	175 µL/50 mL	MO, MB, RhB 100%	[77]
Mn-doped CuO	Co-precipitation	Heterogeneous Fenton-like	90	1 g/L	60 mmol/L	81% (Ciprofloxacin)	[41]
Zn-doped CuO	Green synthesis (Aloysia citrodora)	Heterogeneous Fenton-like (dark)	30	0.6 g/L	25 µL	~99.97% Rhodamine B	This work

Note: NR: not reported.

Table 3. Comparison of previously reported CuO-based catalysts for heterogeneous Fenton-like degradation of organic dyes.

Catalyst	Synthesis Method	Degradation Process	Time (min)	Catalyst Loading	H2O2 Concentration	Degradation Efficiency (%)	Ref.
CuO NPs	Green synthesis (S. Tenacissima L.)	Heterogeneous Fenton-like	60	0.2 g/L	NR	99.88% (Tartrazine) 99.97% (Nile Blue)	[42]
CuO NPs	Green synthesis (Cimin grape)	Heterogeneous Fenton-like	120	25 mg/mL	0.8 µg/mL	97.8% (Methylene Blue)	[76]
ZnO–Co3O4–CuO	Mechano-chemical (grinding)	Fenton-like	60	5 g/L	175 µL/50 mL	MO, MB, RhB 100%	[77]
Mn-doped CuO	Co-precipitation	Heterogeneous Fenton-like	90	1 g/L	60 mmol/L	81% (Ciprofloxacin)	[41]
Zn-doped CuO	Green synthesis (Aloysia citrodora)	Heterogeneous Fenton-like (dark)	30	0.6 g/L	25 µL	~99.97% Rhodamine B	This work

Note: NR: not reported.

3. Materials and Method

3.1. Chemicals and Reagents

Copper Sulfate pentahydrate (CuSO₄·5H₂O, ≥99%, BIOCHEM, Chemopharma, Cosne-Cours-sur-Loire, France, M.W. = 249.68 g/mol) and zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O, ≥99%, Analar Normapur, Prolabo, Paris, France, M.W. = 219.50 g/mol) were used as precursors for the preparation of pure and Zn-doped CuO nanostructures with 0.5, 1, 2, 3, 5% weight of Zn. Sodium hydroxide (NaOH, pellets, ≥99%, Merck, Darmstadt, Germany, M.W. = 40.00 g/mol), Rhodamine B (BIOCHEM, M.W. = 479.02 g/mol) and distillated water are the products and reagents utilized in this work. All of the chemical reagents used were of analytical quality and did not need to be further purified.

3.2. Synthesis of Pure CuO and Zn-Doped CuO

A 1% (w/v) aqueous extract of Aloysia citrodora “Lemon verbena” was prepared by treating dried leaf powder with boiling distilled water, following established green synthesis protocols mediated by plant extracts, as demonstrated in [78].

Aloysia citrodora leaf extract was used as a reducing agent in a green co-precipitation process to create both pure CuO and Zn-doped CuO nanoparticles, as shown in Figure 15. To manufacture pure CuO, 50 mL of lemon verbena leaf extract was added to 100 mL of 0.1 M copper sulfate solution. The same process was used to create Zn-doped CuO nanoparticles, using zinc acetate (Zn[OOCCH₃]₂·2H₂O) as the doping precursor. The Zn-doped CuO nanoparticles were named CuO-Zn0.5, CuO-Zn1, CuO-Zn2, CuO-Zn3, and CuO-Zn5. The necessary stoichiometric amounts of zinc acetate were added to the copper sulfate solution to produce Zn doping concentrations of 0.5%, 1%, 2%, 3%, and 5%. The mixture was heated with magnetic stirring for 1 h at around 100 °C, and the pH of the mixture was adjusted by dropwise addition of NaOH solution (0.1 M) to reach and maintain a pH of 11 for all the samples. The resulting solution had yet to produce a black precipitate of all synthesized, and the obtained precipitate was dried in a hot air oven at 120 °C for 24 h. Subsequently, it was calcined at 600 °C as reported in [79] for 2 h. XRD, RAMAN, FTIR, and SEM/EDX analyses were used to characterize the final black fine powder of pure and Zn-doped CuO.

3.3. Characterization

Room-temperature optical absorption spectra of the pure CuO and Zn-doped CuO samples were recorded over a wavelength range of 200–800 nm in the absorption mode using the UV-Vis Spectrophotometer (SHIMADZU 1900i, Kyoto, Japan); phase analysis was carried out using X-ray diffraction with a Bruker model D8 ADVANCE (Billerica, MA, USA), operating with Cu-Kα = 1.54184 Å radiation and Bragg–Brentano configuration, a scanning rate of 0.01 °/s, and a 2θ scanning range of 20° to 90°. Raman spectra were acquired using a Thermo Scientific DXR (Waltham, MA, USA) equipped with a 532 nm excitation and 10 mW laser power. At the same time, the materials’ functional group was verified using a Fourier Transformation Infrared Spectrometer (Bruker-INVENIO-R, Billerica, MA, USA), and the shape and elemental composition of the as-prepared content were measured using a Thermo Fisher Apreo 2 C (Waltham, MA, USA) with a FEG-Schottky-type electrical source.

3.4. Degradation of Rhodamine B by Pure and Zn-Doped CuO

The degradation of Rhodamine B (RhB) was investigated under laboratory conditions at ambient temperature using pure CuO and Zn-doped CuO as catalysts. An aqueous RhB solution with an initial concentration of 5 ppm and a total volume of 25 mL was prepared at natural pH. A catalyst mass of 25 mg was added to the solution, and the suspension was magnetically stirred in the dark for 15 min to establish adsorption–desorption equilibrium, while preliminary adsorption experiments confirmed that adsorption–desorption equilibrium was reached after 15 min in the dark. After this equilibration step, 100 μL of H₂O₂ was added to initiate the heterogeneous Fenton-like reaction. At given time intervals, 3 mL aliquots were withdrawn, centrifuged to remove the catalyst particles, and the residual RhB concentration was determined using a UV–Vis spectrophotometer by measuring the absorbance at 554 nm and 557 nm at acidic pH.

$$\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y} \left(\mathrm{D}\right)=\left[\right(C_0-C)/C_0]\times 100\%$$

(5)

where C₀ represents the initial concentration of dye and C is the residual concentration of dye.

Initially, the degradation efficiencies of pure CuO and Zn-doped CuO were compared in order to select the most effective catalyst based on degradation yield. Subsequently, a series of parametric studies were carried out using the selected catalyst to evaluate the influence of different operational parameters, including solution pH (5, natural pH, and 9), catalyst dosage (15, 20, and 25 mg), H₂O₂ amount (25, 50, 75, and 100 μL), and initial RhB concentration (5, 10, 15, and 20 ppm). These investigations were performed to determine the optimal conditions for Rhodamine B degradation by the CuO/H₂O₂ and Zn-CuO/H₂O₂ heterogeneous Fenton-like system.

4. Conclusions

In this study, pure and Zn-doped CuO nanoparticles were successfully synthesized via a sustainable, low-cost, and environmentally friendly co-precipitation method using Aloysia citrodora leaf extract as a green reducing and stabilizing agent. Structural and morphological characterizations confirmed the effective incorporation of Zn into the CuO lattice and revealed a clear evolution of the crystallite size and surface morphology as a function of Zn content. The introduction of Zn at low concentration (0.5%) resulted in a pronounced reduction in crystallite size and a transition toward finer, more granular and quasi-spherical nanoparticles, leading to an increased density of accessible surface-active sites. As a consequence, the 0.5% Zn-doped CuO sample exhibited the highest catalytic performance in the heterogeneous Fenton-like degradation of Rhodamine B under dark conditions. Under optimized reaction parameters (mildly acidic pH, low catalyst dosage, and moderate H₂O₂ concentration), rapid and efficient dye degradation was achieved within a short reaction time. At higher Zn contents, although further morphological refinement was observed, the catalytic activity did not surpass that of the optimal composition, indicating the existence of a balance between structural modification and catalytic efficiency. Overall, this work demonstrates that green-synthesized Zn-doped CuO nanoparticles represent an efficient and sustainable catalyst for heterogeneous Fenton-like wastewater treatment. The proposed strategy offers a promising approach for the removal of organic pollutants under mild conditions while minimizing chemical consumption and environmental impact.