GO-enhanced photocatalytic breakdown of emamectin benzoate by an MgO–CuO nanocomposite under visible light illumination

Abstract

The persistent accumulation of recalcitrant pesticide residues poses a significant environmental challenge. Emamectin benzoate, a highly toxic and recalcitrant pesticide extensively used in modern agriculture, has emerged as a critical environmental contaminant due to its persistence, bioaccumulation potential and resistance to conventional degradation pathways. Conventional photocatalysts are limited by their inefficient interfacial charge transfer and rapid charge recombination. To address these limitations, an MgO–CuO/GO ternary heterostructure is rationally engineered, in which GO acts as a conductive carbon-based mediator to facilitate charge migration. The resulting Z-scheme pathway enables efficient charge separation while preserving strong redox potentials for enhanced reactive oxygen species generation. XRD reveals the improved crystallinity of the nanosheets upon the incorporation of GO. SEM indicates the uniform distribution of the MgO–CuO nanostructures over the GO sheets, indicating effective integration and reduced agglomeration, while TEM images confirm the intimate interfacial contact between MgO–CuO and GO, forming a well-dispersed heterostructure with enhanced structural coupling. 4.0 wt% MgO–CuO/GO nanosheets exhibit a significantly reduced band gap (1.72 eV) owing to the synergistic coupling of metal oxides with GO, thereby enhancing visible-light absorption. FTIR spectroscopy indicates distinct shifts in the C–O and C O vibrations, suggesting strong interfacial interaction and chemical coordination between the GO and metal oxides. The 4.0 wt% MgO–CuO/GO nanocomposite shows the highest degradation activity of 75% with a rate constant of 0.27 h⁻¹ for emamectin benzoate degradation. The combined effect of these components results in superior photocatalytic degradation of emamectin benzoate, affording highly efficient materials for environmental remediation.

The GO–MgO–CuO composite exhibits enhanced crystallinity, efficient charge separation, and improved stability, enabling effective EMB degradation and demonstrating strong potential for photocatalytic environmental remediation.

1. Introduction

The extensive use of synthetic pesticides in modern agricultural practices has led to the accumulation of these compounds in aquatic environments, raising serious concerns regarding ecological safety and human health.^1,2 Among these contaminants, the semisynthetic avermectin derivative emamectin benzoate (EMB) is particularly problematic due to its bioaccumulation potential, high chemical stability and resistance to biodegradation.³ Various treatment approaches, including adsorption, biological treatment and advanced oxidation processes, have been investigated for emamectin benzoate removal; however, these methods often suffer from incomplete degradation and secondary pollutant formation. Recently, CuO- and GO-based photocatalytic systems have attracted considerable interest owing to their visible-light responsiveness, enhanced interfacial charge transfer and improved suppression of electron–hole recombination.⁴ The removal of such contaminants is especially challenging for conventional purification techniques, which are often limited by incomplete oxidation, leading to the formation of toxic secondary intermediates. The development of sustainable and effective methods for pollution control utilizing sunlight-based strategies has become a priority in environmental research.⁵ Semiconductor-based photocatalysis has emerged as a promising approach for the degradation of organic pollutants owing to its ability to harness solar energy for the generation of reactive oxygen species (ROS).⁶ Nevertheless, the practical efficiency of single-component photocatalysts is fundamentally constrained by the rapid recombination of photogenerated charge carriers, limited visible-light absorption and an insufficient redox potential for driving multi-step oxidation reactions. These limitations are intrinsically governed by the electronic band structure and charge carrier dynamics, necessitating the rational design of advanced hetero-structured systems that can simultaneously enhance light utilization and promote efficient charge separation.^7–12

p-Type CuO has attracted considerable attention due to its narrow band gap and strong absorption in the visible region; however, its photocatalytic performance is significantly hindered by rapid electron–hole recombination and weak oxidation capability. In contrast, n-type MgO exhibits excellent chemical stability, high surface basicity and strong adsorption affinity toward organic pollutants, yet its wide band gap restricts its activity primarily to the ultraviolet region.^13–18 The integration of these two semiconductors can form a heterojunction interface; however, conventional type-II charge transfer pathways often lead to a reduced redox ability, thereby limiting the formation of highly reactive species such as hydroxyl radicals. This trade-off between charge separation and redox strength remains a critical bottleneck in designing efficient photocatalytic systems.

To address these challenges, the incorporation of carbon-based conductive materials has emerged as an effective strategy to regulate interfacial charge transfer and suppress recombination losses. In particular, graphene oxide (GO), a two-dimensional layered carbonaceous material enriched with oxygen-containing functional groups, offers a unique platform for enhancing photocatalytic performance due to its high electrical conductivity, large specific surface area and high electron mobility.^19–21 GO can function as an electron acceptor, a transport pathway and a temporary charge storage reservoir, thereby facilitating rapid interfacial electron migration. More importantly, its integration enables the construction of a direct Z-scheme heterojunction, in which photogenerated electrons in the conduction band of one semiconductor selectively recombine with the holes in the valence band of the other semiconductor, preserving the strong redox potentials of the remaining charge carriers.^22,23 Such a configuration not only enhances charge separation efficiency but also maximizes the generation of highly oxidative ˙OH and reductive ˙O₂⁻ radicals.

Furthermore, the incorporation of GO into metal oxides can allow the modulation of their interfacial electronic properties, reducing charge transfer resistance and providing additional active centers for adsorption and surface reactions. Such interactions can contribute greatly to accelerating the photocatalytic kinetics and increasing the quantum efficiency. Despite these advantages, the rational design of MgO–CuO/GO ternary heterostructures for the degradation of persistent pesticide molecules such as EMB remains largely unexplored, particularly in terms of elucidating the underlying charge transfer mechanism and interfacial synergy.^24,25

As toxic pesticide residues from agricultural wastewater continue to leach into drinking water systems and aquatic environments, pesticide contamination in water resources has become a major environmental and public health concern. Pesticide accumulation in water can harm aquatic life, upset ecological balance and, most importantly, pose significant health risks to humans, including causing neurological disorders, hormonal imbalance and other chronic diseases. Therefore, the main goal of the present research is to develop efficient visible-light-active MgO–CuO/GO ternary nanocomposites for the removal of hazardous pesticides. In this study, the MgO–CuO/GO ternary nanocomposite with enhanced photocatalytic activity is successfully prepared via a coprecipitation route for the degradation of emamectin benzoate (EMB). Unlike conventional heterojunction systems that often sacrifice redox strength for improved charge separation, this work is specifically directed toward constructing an interfacial, regulated Z-scheme architecture capable of retaining highly reactive charge carriers for the efficient oxidative degradation of recalcitrant emamectin benzoate molecules under visible-light irradiation. The engineered heterointerface promotes the efficient spatial separation of charge carriers, wherein GO acts as a solid-state electron mediator with a band gap (1.72 eV) that facilitates the recombination of low-energy charge carriers while preserving highly energetic electrons and holes for redox reactions. This unique charge transfer pathway enables the enhanced generation of reactive oxygen species, leading to superior photocatalytic performance compared to that of individual components and binary systems. The present study not only provides mechanistic insights into charge carrier dynamics in multicomponent systems but also establishes a viable strategy for designing next-generation photocatalysts for environmental remediation under solar irradiation.

2. Experimental section

2.1. Materials

Materials used in experiments are as follows: Mg(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, sodium hydroxide (NaOH), graphite powder (C), concentrated sulphuric acid (H₂SO₄, 98.0%), potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂), hydrochloric acid (HCl), absolute ethanol (C₂H₆O) and thiourea (CH₄N₂S). Magnesium nitrate, copper nitrate, sodium hydroxide, potassium permanganate, hydrochloric acid, ethanol and thiourea were purchased from Sigma-Aldrich, while the graphite powder and sulphuric acid were obtained from Merck. All materials utilized in the experiments were of analytical grade (AR) and employed without additional purification.

2.2. Synthesis of copper oxide (CuO) by the co-precipitation method

A 0.1 M solution of Cu(NO₃)·3H₂O and a 0.1 M solution of NaOH were prepared in water. Next, the second solution was added to the first solution dropwise. The mixture was stirred until the blue color changed to light blue, and dark green-blue precipitates were formed. The solution was stirred for 2 hours, and the pH of the mixture was maintained at 14. The mixture was centrifuged and washed four times with ethanol and distilled water. It was dried at 80 °C for 6 hours and then calcined at 500 °C for 4 hours. Black CuO nanoparticles were obtained.

2.3. Synthesis of magnesium oxide (MgO) by the co-precipitation method

A 0.2 M solution of Mg(NO₃)·6H₂O and a 0.5 M solution of NaOH were prepared in water. Then, the second solution was added to the first solution dropwise and stirred. After 1 hour of stirring, milky-white precipitates appeared. The pH of the solution was maintained at 11–12. The obtained precipitates were filtered, centrifuged and washed with ethanol and distilled water. They were dried at 60 °C for 6 hours. The obtained MgO powder was calcined at 300 °C for 4 hours.

2.4. Synthesis of graphene oxide (GO)

Graphene oxide was synthesized through the modified Hummer's method.²⁴ Expanded graphite was used as a precursor. To prepare expanded graphite, 15.0 mL of H₂SO₄ was added to 15.0 mL of HNO₃ dropwise to make a solution. In the next step, the graphite powder was added to the acid solution and kept for 3 days. After 3 days, the soaked graphite powder was washed several times with distilled water and ethanol. After that, the mixture was centrifuged and dried at 60 °C for 3 hours. Expanded graphite or graphite flakes were obtained.

To obtain GO, 40.0 mL of H₂SO₄ was added to a beaker, and 2.5 g of expanded graphite was added in small intervals and stirred at room temperature. After stirring, the beaker was placed in an ice bath, and 7.5 g of potassium permanganate (KMnO₄) was added to the beaker at intervals of 20 min and stirred slowly until a uniform liquid paste was formed. The ice bath was removed after the formation of this paste.

After the dark-green paste was achieved, stirring was stopped, and deionized water was added to dilute this solution. Then, the solution was placed in a water bath and subjected to uniform heating at 90 °C for 2 h, which brought back the dark green color to the solution again. After the removal of the water bath, 2.0 mL of H₂O₂ was added to the solution. Yellowish bubbles started to appear. Next, the prepared solution was centrifuged to separate the particles. Then, a 1.0 M solution of HCl was added and stirred slowly for 3–4 h. This solution was centrifuged again to obtain the particles. Deionized water was added to remove unwanted HCl and stirred for half an hour. Then, the above solution was centrifuged to separate the particles. The final product was dried at 60 °C on a heating plate, and the GO powder was obtained.

2.5. Synthesis of the ternary composite (MgO–CuO/GO)

First, MgO–CuO nanocomposites were synthesized. 0.01 g of CuO and 0.01 g of MgO were separately dispersed in 100 mL of distilled water and stirred for 60 min. Then, both solutions were mixed, sonicated for 1 h and left to settle for 20 min.

The obtained nanoparticles were repeatedly rinsed with distilled water and ethanol. Nanoparticles were collected by centrifugation and dried at 60 °C for 1 h. 0.02 g of MgO–CuO and 1 g of the GO nanocomposite were dispersed in 50 mL of a distilled water and ethanol mixture with a 1 : 1 ratio and stirred until dissolved. The mixture was washed with ethanol and distilled water and then dried at 60 °C. The ternary composite 2 wt% MgO–CuO/GO was obtained. Similarly, 4 and 6 wt% composites were prepared. Scheme 1 provides the diagram of the synthetic protocol of the ternary composite.

Scheme 1. Schematic of the ternary composite.

2.6. Experimental photocatalytic measurements

EMB was selected as the model pesticide to assess the photocatalytic degradation efficacy of different composite materials. A 10 ppm solution of EMB was prepared, and 1.0 g L⁻¹ of each sample, including the bare material and nanocomposites with different graphene oxide (GO) contents, was introduced into the dye solution at 50 mL. A 250 mL cylindrical quartz vessel served as the reactor, and the lamp was placed 10 cm away from the reaction mixture's surface. To make sure the adsorption and desorption processes were in equilibrium, the mixture was agitated for half an hour in the dark. To evaluate the performance in photocatalytic degradation, the suspension was then subjected to radiation from a 1000 W xenon lamp at predetermined intervals. To guarantee consistent suspension and irradiation throughout the experiment, a magnetic stirrer was used to continuously stir the suspension containing the MgO–CuO/GO catalyst and EMB solution at 400 rpm. A water-circulating bath was used to keep the reaction temperature at 25 °C ± 2 °C. All experimental conditions were kept consistent throughout the analysis. At designated intervals, small aliquots were withdrawn and analyzed using UV-Vis spectroscopy at 495.0 nm to monitor absorbance. To ensure reliability and minimize error, the degradation experiments for each synthesized photocatalyst were conducted twice.

2.7. Characterizations

X-ray diffraction (XRD) analysis was used to analyze the phase and structural properties of the composites using Cu Kα radiation (λ = 1.5 Å) at an operating voltage of 40 kV and a current of 100 mA. Energy-dispersive X-ray spectroscopy (EDAX) in combination with scanning electron microscopy (SEM, JEOL-JAD 2300 model) was used to evaluate the surface morphology and elemental distribution. The presence of nitrogen within the GO structure was verified through Fourier transform infrared (FTIR) spectroscopy, utilizing a Bruker FTIR ALPHA II instrument to investigate the chemical bonds present. A Shimadzu UV-Vis spectrophotometer (model: 2700) was used to capture optical absorption spectra. Additionally, using a JEOL JEM-2200FS system running at 200 kV, transmission electron microscopy (TEM) was performed to examine the two-dimensional morphology of both composite and bare samples. Current–voltage (I–V) characteristics were further analyzed using an ASMEC system for material electro-physical characterization.

3. Results and discussion

3.1. Crystal structure and morphology

Fig. 1A–D shows the X-ray diffraction patterns of CuO, MgO, GO and the MgO–CuO/GO ternary composites, respectively. Fig. 1A shows that CuO's monoclinic phase is confirmed by the diffraction peaks at 2θ ≈ 35° and 38°, as well as the additional reflections corresponding to the (110), (002), (−111), (202), (−113), (311) and (200) planes. According to the standard diffraction data, the MgO pattern shows distinctive reflections at 43°, 62°, and 74°, which are indexed to the (200), (220) and (222) planes, respectively (Fig. 1B). A noticeable peak at a scattering angle of 11.0°, which corresponds to the (001) plane, is visible in the diffraction pattern of graphene oxide (GO), as seen in Fig. 1C. This confirms the characteristic layered structure of graphene oxide. The effective production of graphene oxide (GO) is implied by the development of a clear diffraction peak at 10.2° (001).²⁵Fig. 1D presents the XRD spectra of MgO–CuO/GO ternary materials. These samples contain all relevant peaks of GO. As the concentration of MgO–CuO increases, new peaks at 35.01° (002), 39.31° (111) and 49.08° (−002) appear, indicating the presence of CuO, and the peak of (200) at 42.4° represents the presence of MgO in the GO structure.^26,27 This suggests that in the ternary heterojunction (Fig. 1D), the addition of MgO–CuO partially reduces GO by restoring the graphitic structure through the interaction of the CuO and MgO basal planes.

Fig. 1. XRD patterns of (A) CuO, (B) MgO, (C) GO and (D) the ternary composites of (d) 2.0 wt%, (e) 4.0 wt%, and (f) 6.0 wt% MgO–CuO/GO.

The diffraction angle increases and the intensity drops as the MgO–CuO content increases from 2.0 wt% to 6.0 wt%, as seen in Fig. 1D. The characteristic GO diffraction peak centered around ∼11° exhibits a slight shift towards higher diffraction angles as the MgO–CuO loading increases from 2.0 wt% to 6.0 wt%, indicating a reduction in the interlayer spacing due to the interfacial interaction and structural strain induced by the incorporation of metal oxides. In addition, a gradual variation in the peak intensity is observed, which may be associated with changes in the crystallinity, stacking behavior and dispersion of GO layers within the ternary heterostructure.²⁸ These results support the formation of effective heterostructured photocatalysts.

3.2. SEM analysis

To investigate the morphology of the samples, scanning electron microscopy (SEM) analysis was conducted. Fig. 2A presents the SEM images of CuO nanosheets captured at various magnifications. The material predominantly comprises numerous nanosheets, each formed by stacked, leaf-like nanostructures ranging from 80 to 120 nm in length and 10 to 15 nm in thickness. These nanosheets exhibit smooth surfaces, and their edges appear notably sharp and thick. The nanosheets are a result of the accumulation of numerous CuO nanosheets. The nanostructures are believed to form through a sequential process involving initial nucleation, followed by aggregation and self-assembly into larger formations. According to the literature, the Ostwald ripening mechanism plays a crucial role in the development of these CuO nanosheets.²⁹

Fig. 2. SEM images of CuO (A), MgO (B), GO (C), and 4.0 wt% (D) and 6.0 wt% MgO–CuO/GO (E).

The SEM images reveal the uniform distribution of CuO and MgO nanosheets on the GO surface, indicating successful doping. The GO sheets exhibit a wrinkled and folded structure with an average thickness of 1–2 nm. The CuO nanosheets are rectangular with an average length of 80–120 nm, while the MgO nanosheets have a spherical shape with an average diameter of 10–20 nm. The doped GO nanocomposite shows a rough and porous surface, which indicates the high adsorption of pollutants and hence more photodegradation.

3.3. Energy-dispersive X-ray analysis

In order to determine the elemental composition of 4.0 wt% MgO–CuO/GO, energy-dispersive X-ray spectroscopy and elemental mapping analysis were carried out, as depicted in Fig. 3. Fig. 3A reveals that C, O, Cu and Mg were unevenly present in the synthesized 4.0 wt% MgO–CuO/GO. The comparatively lower Mg signal intensity observed relative to that of Cu may be attributed to the lower atomic number and weaker X-ray emission efficiency of Mg, which generally results in reduced EDX sensitivity toward lighter elements. Additionally, the partial embedding and nonuniform distribution of MgO within the GO-supported heterostructure may further contribute to the comparatively lower detected Mg content. The elemental mapping (represented by different colors) of the synthesized 4.0 wt% MgO–CuO/GO provided strong evidence of the haphazard distribution of these elements (C, O, Cu and Mg), as shown in Fig. 3B. Table 1 presents the atomic and weight percentages of different elements found in the synthesized nanomaterials.

Fig. 3. (A) EDX analysis and (B) elemental mapping images of the 4.0 wt% MgO–CuO/GO ternary composite nanostructure.

Table 1. Weight and atomic percentages of elements in 4.0 wt% MgO–CuO/GO, as determined by elemental analysis.

Element	4.0 wt% MgO–CuO/GO
	Weight (%)	Atomic (%)
C	66.2	76.03
O	25.5	22.01
Cu	7.7	1.67
S	0.3	0.13
Mg	0.2	0.12
Al	0.1	0.05
Total	100	100

3.4. Transmission electron microscopy

Transmission electron microscopy (TEM) was employed to examine the detailed morphological characteristics of the samples. The TEM images of the ternary composite 4.0 wt% MgO–CuO/GO are shown in Fig. 4. The TEM image of the ternary composite (Fig. 4A) reveals a homogenous distribution of GO sheets with CuO and MgO nanoparticles. The uniform distribution of MgO–CuO over transparent GO sheets is shown in Fig. 4B. GO exhibits a more uniform distribution and fewer wrinkles. Some regions show the stacking of GO sheets, indicating potential restacking, which leads to a reduced interlayer spacing. Agglomerated MgO–CuO nanoparticles distributed over thin GO nanosheets are visible in Fig. 4C, which shows a TEM image of the 4.0 wt% MgO–CuO/GO nanocomposite. The dark contrast region corresponds to MgO–CuO nanoparticles uniformly distributed over GO sheets.

Fig. 4. TEM images of the 4.0 wt% MgO–CuO/GO ternary composite showing (A) GO sheets incorprated with MgO–CuO nanoparticles, (B) Distribution of MgO–CuO/GO on the surface of GO and (C) aggregation of MgO–CuO component on GO sheets at higher resolution.

3.5. Optical properties

Diffuse reflectance spectroscopy (DRS) was utilized to investigate the optical characteristics of pure CuO, MgO, and GO and their composites. All samples demonstrated significant absorption within the wavelength range of 200–800 nm. As shown in Fig. 5A(a), CuO exhibits an absorption maximum at around 715 nm. MgO (Fig. 5A(b)) shows a broad absorption peak between 230 and 360 nm. GO (Fig. 5A(c)) exhibits two absorption peaks: one at 250 nm, which is most likely caused by the C–C bonds' π → π* transition, and another shoulder at 320 nm, which is associated with the C O bonds' n → π* transition. Fig. 5B shows the absorption spectra of the ternary composites. According to Fig. 5B(d), 2.0 wt% MgO–CuO/GO (Fig. 5B(d)) shows a red shift in the absorption at 693 nm, and it lies in the visible region; 6.0 wt% MgO–CuO/GO (Fig. 5B(f)) shows an absorption shift at 491 nm, while 4.0 wt% MgO–CuO/GO (Fig. 5B(e)) shows the highest red shift and broadest absorption at 720 nm. Fig. 5C and D show the Tauc plots of the pure samples and ternary composites. The band gap for CuO, MgO, and GO is 1.8 eV, 5.2 eV and 2.1 eV respectively (Fig. 5C). The estimated band gap values of the composite materials show a significant decrease with increasing MgO–CuO content. The 4.0 wt% MgO–CuO/GO composite shows a 1.72 eV band gap (much lower than that of both pure CuO and MgO). The 2.0 wt% MgO–CuO/GO ternary composite shows a 1.79 eV band gap, and the 6.0 wt% MgO–CuO/GO ternary composite shows a band gap of 2.53 eV. The decrease in the band gap is the effect of doping MgO and CuO phases into the GO matrix, which converges electronic transitions.³⁰

Fig. 5. Diffuse reflectance spectra of (A) CuO (a), MgO (b), and GO (c) and (B) the ternary composites of 2.0 wt% (d), 4.0 wt% (e), and 6.0 wt% (f) MgO–CuO/GO. Tauc plots of (C) CuO (a), MgO (b), and GO (c) and (D) the ternary composites of 2.0 wt% (d), 4.0 wt% (e), and 6.0 wt% (f) MgO–CuO/GO.

After being excited by light, the 4.0 wt% MgO–CuO/GO composite's narrower band gap generates greater energy photo-carriers. During photocatalytic applications, materials with smaller band gaps can absorb more photons and promote the production of charge carriers.^31,32 The enhancement in absorption is because the presence of GO reduces light reflection, which plays a role in improving light absorption and electronic interactions within the composite. GO facilitates light absorption and promotes efficient charge transfer, which can contribute to improved photocatalytic performance.

In 6.0 wt% MgO–CuO/GO, the band gap increases to 2.53 eV, likely due to the excessive metal oxide loading causing nanoparticle agglomeration and blocking GO's active sites. This weakens electronic coupling and charge transfer while also introducing recombination centers. Unlike the 4.0 wt% composite, which optimally narrows the band gap, the higher loading disrupts the composite's structural and electronic balance, reducing photocatalytic efficiency. Table 2 shows a comparison of the optical properties, structural properties and degradation percentages of the synthesized photocatalysts. Table 2 indicates that the 4.0 wt% MgO–CuO/GO ternary composite with a narrow band gap and lower particle size exhibits the greatest photocatalytic activity.

Table 2. Optical and structural properties and photodegradation values.

Photocatalyst	Optical properties		Structural properties		Photodegradation (%)
	Absorption (nm)	Band gap (eV)	Theta (°)	Particle size (nm)
CuO	688	1.8	17.5	14.7	26
MgO	238	5.2	21.5	16.7	60
GO	564	2.2	11.1	20	33
2.0 wt% MgO–CuO/GO	720	1.79	11.2	14.5	40
4.0 wt% MgO–CuO/GO	693	1.72	11.3	11.5	75
6.0 wt% MgO–CuO/GO	491	2.53	11.4	9.5	68

3.6. Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is a potent method for examining a material's functional groups and chemical structure. The Fourier transform infrared (FTIR) spectra of CuO, MgO and GO are illustrated in Fig. 6A, displaying their distinctive functional groups and bonding characteristics. In the FTIR spectrum of CuO (a), the O–H stretching of adsorbed water is responsible for the broad band at 3499 cm⁻¹, whereas surface carbonate species are associated with the peaks at 1517 cm⁻¹ and 1388 cm⁻¹. CuO formation is confirmed by the peak at about 990 cm⁻¹, which corresponds to Cu–O lattice vibrations. For MgO (b), the broad peak at ∼3695 cm⁻¹ is associated with surface –OH groups, while the peak at ∼851 cm⁻¹ corresponds to Mg–O vibrations, confirming MgO formation. A broad O–H stretching band at ∼3350 cm⁻¹, a C O peak at ∼1713 cm⁻¹ and a C C band at approximately 1612 cm⁻¹ are all visible in the GO spectrum (c). The presence of oxygen-containing functional groups is confirmed by the peaks at 1165 and 1080 cm⁻¹, which correspond to C–O–C and C–O stretching vibrations, respectively.

Fig. 6. FTIR spectra of (A) CuO (a), MgO (b), and GO (c) and (B) the ternary composites of 2.0 wt% (d), 4.0 wt% (e) and 6.0 wt% (f) MgO–CuO/GO.

FTIR analysis can be used to determine the interaction between the constituents and offer insights into their structural characteristics for a ternary composite made of MgO, CuO, and GO with different MgO–CuO ratios (2.0 wt%, 4.0 wt%, and 6.0 wt%), as shown in Fig. 6B. The hydroxyl groups are responsible for the large peak at 3200–3600 cm⁻¹ (O–H stretching). The faint signal for C O stretching is located between 1700 and 1720 cm⁻¹ (carboxyl groups). C–O stretching is represented by the peaks (epoxy and alkoxy groups) at 1000–1200 cm⁻¹. C C stretching is responsible for the peak at about 1600 cm⁻¹ (unoxidized graphitic domains). Usually, the peaks linked to Mg–O stretching vibrations appear below 600 cm⁻¹. Cu–O stretching vibrations are found between 400 and 600 cm⁻¹. As the MgO–CuO levels increase, the intensity of the O–H and C O peaks may decrease, suggesting interactions between GO and MgO–CuO. The interaction between GO and metal oxides may cause shifts in the C–O and C O peaks, indicating chemical bonding or coordination. With increasing MgO–CuO content (2.0 wt%, 4.0 wt%, and 6.0 wt%), the intensity of GO-specific peaks decreases proportionally, reflecting the compositional impact. The decreasing intensity or disappearance of certain GO peaks (e.g., O–H and C O) indicates chemical reduction or bonding with MgO and CuO. The appearance of new peaks or shifts in existing ones demonstrates the interaction between GO and metal oxides. GO's FTIR spectrum reveals distinctive functional groups that contain oxygen, including –OH, C O, and C–O–C. There is also a slight signal that corresponds to species that contain nitrogen, which is probably caused by ammonium or nitric acid ions that were left behind after the oxidation process. This signal does not show intrinsic nitrogen in the GO structure because no deliberate nitrogen doping was done.

Strong interfacial interactions, rather than simple physical mixing, are confirmed by the observed shifts and the broadening of characteristic peaks in the composite's spectrum compared to the spectra of individual components.³⁸

3.7. Photocatalytic degradation of aqueous emamectin benzoate

This study used a ternary composite of GO, MgO, and CuO to evaluate the photocatalytic breakdown of the insecticide emamectin benzoate under visible light.³³ Kinetic analysis, recyclability testing, and UV-Vis absorption spectroscopy were used to evaluate the degradation efficiency. The results show that the MgO–CuO/GO composite has higher photocatalytic activity than individual metal oxides, indicating that it could be a useful photocatalyst for environmental applications.^34,35

The degradation efficiency of emamectin benzoate was compared for CuO, MgO, GO, and their composites with varying MgO–CuO contents under visible light irradiation (Fig. 7A).

Fig. 7. Standard degradation graphs for (A) CuO (a), MgO (b), GO (c), and 2.0 wt% (d), 4.0 wt% (e) and 6.0 wt% MgO–CuO/GO (f). (B): kinetic curves for CuO (a), MgO (b), GO (c), and 2.0 wt% (d), 4.0 wt% (e) and 6.0 wt% (f) MgO–CuO/GO ternary composites.

The adsorption–deposition equilibrium between EMB molecules and the catalyst surface was established by magnetically stirring the reaction mixture in the dark for 30 minutes prior to light irradiation. This experiment confirms that the surface adsorption is the main cause of the initial drop in the EMB concentration, whereas the photocatalytic degradation is responsible for the drop when illuminated under UV light. The contribution of adsorption is confirmed by the slight decrease in the EMB concentration during the dark phase. Although very little adsorption occurs in the dark phase, the higher degradation under UV light proves that photocatalytic degradation is the principal mechanism. The degradation percentage over time follows a decreasing trend, with 4.0 wt% MgO–CuO/GO showing the highest degradation rate. The pure metal oxides (CuO and MgO) exhibit relatively lower degradation efficiencies, likely due to the rapid recombination of photogenerated electron–hole pairs. The MgO–CuO loading in the MgO–CuO/GO ternary composite has a significant impact on the photocatalytic degradation of EMB. When incorporated into the ternary composite with GO, MgO–CuO enhances charge separation, facilitates efficient electron transfer and improves the generation of reactive oxygen species, due to the synergistic interaction between MgO, CuO and GO. The increased MgO–CuO content up to 4.0 wt% optimizes the light absorption and surface area, thereby promoting higher degradation efficiency. However, excessive MgO–CuO loadings slightly reduce the degradation of EMB. This reduction in degradation efficiency is associated with excessive surface coverage, catalyst particle agglomeration and the partial blockage of photoactive sites. This result confirms that an optimal proportion of MgO–CuO in the composite enhances the degradation process, making the 4.0 wt% MgO–CuO/GO composite the most effective catalyst for emamectin benzoate breakdown.

The kinetic behavior of the photocatalytic degradation process was analyzed using a pseudo-first-order reaction model based on the Langmuir–Hinshelwood mechanism.

In the equation ln C₀/C_t = kt, C₀ and C_t represent the concentrations of EMB at irradiation times 0 and t, respectively, and k is the apparent reaction rate constant. The plot of ln(C₀/C) vs. time demonstrates a linear relationship. The apparent rate constants were calculated using pseudo-first-order kinetic plots ((ln C₀/C) vs. irradiation time) (Fig. 7B). The rate constant (k) values for different catalysts indicate a significant variation in degradation rates. CuO (0.05 h⁻¹) and MgO (0.08 h⁻¹) show relatively lower rate constants due to higher electron–hole recombination. By contrast, GO (0.1 h⁻¹) exhibits a slightly higher rate due to its charge transport ability. Similarly, 2.0 wt% MgO–CuO/GO (0.18 h⁻¹) and 6.0 wt% MgO–CuO/GO (0.21 h⁻¹) show improved rates. 4.0 wt% MgO–CuO/GO (0.27 h⁻¹) achieves the highest rate constant, highlighting the optimal synergy between MgO, CuO, and GO.

This significant enhancement in the rate constant suggests that the 4.0 wt% MgO–CuO/GO composite provides the best combination of a high surface area, efficient charge separation, and photocatalytic activity. GO increases conductivity and electron transfer, while CuO and MgO act as photocatalytically active sites, promoting oxidation–reduction reactions. The rate constants (k) of different photocatalysts for EMB degradation are summarized in Table 3. Table 3 clearly shows the superior performance of the 4.0 wt% MgO–CuO/GO composite.

Table 3. Reaction rate constants (k) for the photocatalytic degradation of EMB using different photocatalysts.

Photocatalyst	Reaction constant k (h−1)
CuO	0.05
MgO	0.08
GO	0.1
2.0 wt% MgO–CuO/GO	0.18
4.0 wt% MgO–CuO/GO	0.21
6.0 wt% MgO–CuO/GO	0.27

UV-Vis absorption spectra (Fig. S1) provide insights into the degradation of EMB in the presence of the 4.0 wt% MgO–CuO/GO composite as a function of time. The absorbance of emamectin benzoate exhibits characteristic peaks in the range of 270–360 nm, and the gradual reduction in peak intensity over 420 min confirms the breakdown of pesticide molecules. This decline in absorbance suggests that the photocatalyst is effectively facilitating the degradation process. The rapid decrease in absorption intensity within the first few hours indicates a high degradation efficiency, attributed to the synergistic effect of GO, MgO, and CuO. GO plays a crucial role in charge carrier separation, while MgO and CuO provide active sites for photocatalytic reactions. The results indicate that the ternary composite significantly accelerates the degradation of EMB compared to non-composite materials (Fig. S1).

3.7.1. Efficiency of the composite

A photocatalyst's practical applicability is mainly determined by its structural stability and reusability during repeated photocatalytic cycles, in addition to its degradation efficiency. The recyclability of the 4.0 wt% MgO–CuO/GO composite was evaluated over three consecutive runs to determine its stability and long-term usability under visible light irradiation (Fig. 8). After three cycles, the photocatalyst shows excellent stability with only a minor decrease in degradation efficiency.

Fig. 8. Recyclability of the 4.0 wt% MgO–CuO/GO ternary composite for the photocatalytic degradation of EMB under visible light irradiation over three consecutive cycles.

Only a slight decline in performance is noted in the second and third cycles, suggesting that the composite maintains the majority of its catalytic activity despite prolonged irradiation and frequent use. The slight decline in efficiency is attributed to surface fouling or minor material loss. The degradation efficiency remains relatively high even after the third cycle, confirming the stability and reusability of the catalyst. The catalyst retains a significant portion of its initial activity. This sustainability makes 4.0 wt% MgO–CuO/GO a promising material for practical wastewater treatment applications, as it can be repeatedly used with minimal efficiency loss. The strong interfacial interaction between GO, CuO, and MgO effectively suppresses photocorrosion and improves charge separation during photocatalytic reactions, which accounts for the MgO–CuO/GO composite's excellent cyclic stability.

The photocatalytic degradation of EMB, a pesticide, using the ternary composite of CuO, MgO, and GO with varying MgO–CuO loadings (2.0 wt%, 4.0 wt%, 6.0 wt%) is an exciting application. As photocatalysts, CuO and MgO absorb light energy and produce electron–hole pairs. Because of its superior electron conductivity, GO improves charge separation and lowers electron–hole pair recombination, increasing photocatalytic efficiency. Superoxide radicals (O₂˙⁻) are created when electrons and oxygen interact. Hydroxyl radicals (˙OH) are created when holes interact with water or hydroxyl ions.³⁶ These reactive oxygen species (ROS) attack EMB molecules, breaking them down into smaller, less harmful compounds. Lower MgO–CuO contents, up to 2.0 wt%, may result in insufficient charge transfer but higher light absorption due to CuO and MgO. An optimal ratio up to 4.0 wt% enhances charge separation and light utilization. Excessive contents up to 6.0 wt% may reduce light absorption by GO, lowering activity. A higher intensity increases electron–hole pair generation. Excessive amounts of the catalyst may cause scattering, reducing light penetration. High concentrations may saturate the catalyst's active sites. The presence of GO increases light-harvesting efficiency, extending the response range into the visible spectrum. GO acts as an electron acceptor, minimizing the recombination of electron–hole pairs generated under light irradiation. CuO facilitates electron transfer and provides catalytically active sites, while MgO stabilizes the structure and prevents aggregation.^37–39 The composite structure provides a large surface area for effective interaction with pollutant molecules, thus improving degradation kinetics. Furthermore, a comparison of the photocatalytic efficiency of MgO–CuO/GO and other photocatalysts reported in the literature for the degradation of EMB is provided in Table 4. The MgO–CuO/GO nanocomposite shows competitive degradation efficiency under visible light irradiation compared to previously reported photocatalysts. The ternary composite's reduced band-gap energy and improved charge separation and the synergistic effect of GO incorporation could all be responsible for the improved performance.

Table 4. Comparison of the photocatalytic efficiency of MgO–CuO/GO and other reported photocatalysts for the degradation of EMB.

S. no.	Photocatalyst	Irradiation time	Temperature	Catalyst dosage (g)	Degradation efficiency of EMB (%)	Reference
1	g-C3N4@ZIF-8	60 min	Room temperature	2 mg	62	22
2	CoO nanoparticles	150 min	Room temperature	0.05 g	35	40
3	Ba-doped CoO	150 min	Room temperature	0.05 g	58	40
4	Fe in distilled water	24 h	Room temperature	1 mg mL−1 Fe	75.0	5
5	Fe in methanol	24 h	Room temperature	Not reported	59.3	5
6	MgO–CuO/GO	420 min	25 °C ± 2 °C	0.05 g	75.0	This study

In Fig. 9, the impact of the pH and catalyst dosage on EMB breakdown is depicted. Fig. 9A shows the effect of pH on the degradation of EMB using the 4.0 wt% MgO–CuO/GO ternary composite.

Fig. 9. (A) Effect of pH on EMB degradation. (B) Effect of catalyst dosage on EMB degradation using the 4.0 wt% MgO–CuO/GO ternary composite.

The impact of pH on the degradation of EMB is closely linked with the chemical structure and ionization behavior of the EMB molecules. Several oxygen-containing functional groups present in EMB, such as hydroxyl, ether, and benzoate moieties, are prone to protonation and deprotonation at various pH levels. In an acidic medium, excessive H⁺ ions can protonate both the EMB molecules and the MgO–CuO/GO photocatalyst's active surface sites. This decreases the adsorption interaction between the pesticide and photocatalyst surface and inhibits the production of reactive oxygen species (ROS), particularly hydroxyl radicals (˙OH). Therefore, a lower degradation efficiency is observed under strongly acidic conditions (pH 2.0–4.0). In an alkaline medium (pH 8.0–10.0), although OH⁻ ions can encourage the formation of ˙OH radicals, strong alkalinity can cause radical recombination reactions, agglomeration of the catalyst and increased electrostatic repulsion, all of which have a negative effect on the efficiency of the photocatalyst. The adsorption behavior of EMB on the surface of the catalyst may change if its ionic state changes under alkaline conditions. The maximum efficiency (75%) recorded at near-neutral pH (pH 6.0) indicates the existence of an optimal balance between the adsorption of EMB, catalyst surface charge and reactive oxygen species generation, which leads to improved photocatalytic performance.

It was discovered that the catalyst dose was a crucial experimental parameter for both validating the range of catalysts in industries and preventing their overuse. Thus, the dependence of the catalyst dosage on the breakdown of EMB was examined by varying the amount of the 4.0 wt% MgO–CuO/GO ternary composite between 0.2 g L⁻¹ and 1.2 g L⁻¹ while keeping the other parameters constant (Fig. 9B). As the catalyst amount increases from 0.2 g L⁻¹ to 1.0 g L⁻¹, the degradation efficiency increases from ∼39% to ∼75%, as shown in Fig. 9B. This increase in degradation efficiency is explained by the increase in active sites and the enhanced generation of reactive oxygen species (ROS) when exposed to visible light. At 1.0 g L⁻¹, the 4.0 wt% MgO–CuO/GO ternary composite shows the maximum degradation of 75%. This is because more photons are absorbed when the catalyst dose is increased, which will result in the production of more charge carriers. However, the degradation efficiency slightly decreases to 70% when the catalyst dosage increases to 1.2 g L⁻¹. An excessive catalyst loading and the radical reaction mechanism can both account for this decline. The excessive generation of superoxide radicals (˙O₂⁻) and hydroxyl radicals (˙OH) at higher catalyst concentrations may encourage radical–radical recombination reactions like ˙OH + ˙OH → H₂O₂, which lowers the amount of active radicals available for EMB oxidation. Furthermore, an increased catalyst loading limits photon absorption by active sites and reduces light penetration by increasing turbidity and particle agglomeration. As a result, beyond the ideal catalyst dosage, the efficient use of ROS and photogenerated charge carriers declines.^40–44

3.8. Kinetic analysis and reactive species identification for the MgO–CuO/GO composite

The apparent rate constants (k) calculated from the pseudo-first-order kinetic model (ln(C₀/C) versus time) are presented in h⁻¹ for all samples (Fig. 10A). The 4.0 wt% MgO–CuO/GO composite exhibits the highest rate constant (0.272 h⁻¹), which is approximately three to four times higher than that of pristine CuO and MgO. The significant enhancement confirms the synergistic interfacial coupling among GO, CuO and MgO, facilitating efficient electron transport and charge separation. Increasing the GO content causes a slight decrease in the rate constant, suggesting that excessive GO may partially shield active sites or limit light penetration. Therefore, 4.0 wt% MgO–CuO/GO is identified as the optimal composition, delivering superior photocatalytic efficiency due to balanced charge transfer and surface reactivity. To elucidate dominant reactive species involved in the photocatalytic degradation process, active species trapping experiments were conducted using specific scavengers, including BZQ (for ˙O₂⁻), TBA (for ˙OH), AO (for h⁺) and AgNO₃ (for e⁻), as shown in Fig. 10B. In the absence of a scavenger, the photocatalyst exhibits high decolorization efficiency (∼92%) after 5 h of irradiation. However, a significant suppression in degradation efficiency is observed upon the introduction of scavengers. Notably, the addition of AO (h⁺ scavenger) results in the most drastic decline in activity, followed by TBA (˙OH), indicating that photogenerated holes and hydroxyl radicals play dominant roles in the degradation pathway. The moderate decrease observed with BZQ suggests the involvement of superoxide radicals (˙O₂⁻), while the presence of AgNO₃ confirms the participation of electrons in the reaction mechanism. This finding is in line with recent research on photocatalytic and (photo)electrocatalytic systems, where the production of ROS is largely dependent on molecular oxygen activation. According to reports, the photocatalytic activation of O₂ produces superoxide radicals (˙O₂⁻), which can then contribute to the production of hydroxyl radicals (˙OH), both of which play a major role in the degradation of pollutants.⁴⁵ Furthermore, advanced photoelectro-Fenton systems have shown that ROS-driven synergistic oxidative and reductive pathways can efficiently break down organic pollutants, underscoring the significance of these species in environmental remediation.⁴⁵ Consequently, the scavenging experiment results from this investigation are consistent with recent research results, demonstrating that the main sources of EMB degradation are ˙O₂⁻ and ˙OH radicals.

Fig. 10. (A) Apparent pseudo-first-order rate constants (k, h⁻¹) of CuO, MgO, and the 4.0 wt% MgO–CuO/GO composites. (B) Active species trapping experimental results showing the effect of BZQ (˙O₂⁻), TBA (˙OH), AO (h⁺), and AgNO₃ (e⁻) on the photocatalytic degradation efficiency.

3.9. Band edge position analysis based on the Mulliken electronegativity theory

The conduction (CB) and valence band (VB) edge potentials of GO, CuO and MgO were estimated using the Mulliken electronegativity theory, which provides a theoretical approach to determine semiconductor band positions. The band edge potentials are calculated using the following equations:E_VB = E_CB + E_g,where χ is the absolute electronegativity, E_e is the energy of free electrons on the hydrogen scale (≈4.5 eV), E_g is the band gap energy, and E_CB and E_VB are the conduction and valence band edge potentials, respectively. Using the experimentally determined band gaps of CuO, MgO, and GO and the electronegativity values of GO, CuO, and MgO, the CB and VB positions were calculated.

The Mulliken electronegativity theory was used to estimate the band edge positions of MgO and CuO; the results are shown in Table 5. CuO was found to have conduction band (CB) and valence band (VB) potentials of +0.41 eV and +2.21 eV, respectively, whereas MgO showed CB and VB positions of −1.31 eV and +5.19 eV, respectively. The direct Z-scheme mechanism showed that CuO and MgO both produce electron–hole pairs when exposed to UV light. Through the conductive GO matrix, which serves as an effective electron mediator, the photogenerated electrons in the CB of CuO tend to recombine with the holes in the VB of MgO. The highly reductive electrons in the CB of MgO (−1.31 eV), which can reduce O₂ to ˙O₂⁻ radicals, are preserved by this recombination pathway. Simultaneously, the highly oxidative holes in CuO's VB (+2.21 eV) can oxidize H₂O or OH⁻ to produce ˙OH radicals. This Z-scheme charge transfer pathway's synergistic effect improves charge separation and sustains strong redox potentials, greatly increasing the photocatalytic degradation efficiency of EMB. Thus, the Mulliken-based band alignment confirms the feasibility of efficient charge separation in the MgO–CuO/GO composite, supporting enhanced photocatalytic performance through improved redox potential matching and suppressed electron–hole recombination.⁴⁶

Table 5. Band gap (E_g) and Mulliken-derived band edge potentials (E_CB and E_VB) of GO, CuO, and MgO.

Material	E g (eV)	χ (eV)	E CB (eV vs. NHE)	E VB (eV vs. NHE)
CuO	1.8	5.81	+0.41	+2.21
MgO	5.2	5.67	−1.31	+3.89
GO	2.2	5.50	−1.00	+1.20

3.10. Proposed photocatalytic mechanism

Scheme 2 shows the proposed degradation mechanism of EMB using the ternary nanocomposite (MgO–CuO/GO). Both n-type MgO and p-type CuO absorb photons and produce electron–hole pairs when exposed to light. In contrast to MgO, which has a larger band gap of roughly 5.2 eV, CuO excites electrons from the valence band (VB) to the conduction band (CB) across a band gap of 1.8 eV. The photogenerated electrons migrate toward the VB of MgO due to the favorable band alignment between CuO, MgO, and graphene oxide (GO), which serves as an electron acceptor and a transport mediator. The lifetime of the charge carriers is extended by this charge transfer, which inhibits direct electron–hole recombination. While the holes left in the VB of CuO oxidize H₂O/OH⁻ to produce hydroxyl radicals (˙OH), the accumulated electrons on MgO reduce dissolved O₂ to superoxide radicals (˙O₂⁻). Emamectin benzoate is then attacked and mineralized by these extremely reactive oxygen species, primarily ˙O₂⁻ and ˙OH, into innocuous byproducts like CO₂ and H₂O. The brief mechanism is given below:

MgO–CuO/GO + hν → MgO–CuO/GO (h⁺ + e⁻), 1

MgO–CuO/GO (e⁻) → MgO–CuO + GO/(e⁻), 2

GO/(e⁻) + O₂ → GO + O₂˙, 3

MgO–CuO (h⁺) + OH⁻ → MgO–CuO, 4

2O₂˙⁻ + 2H₂O → H₂O₂ + O₂ + OH˙, 5

MgO–CuO (h⁺) + H₂O → MgO–CuO + OH˙ + H⁺, 6

O₂˙⁻/OH˙ + EMB → H₂O + CO₂. 7

Scheme 2. Proposed degradation mechanism of emamectin benzoate via the MgO–CuO/GO photocatalyst under visible light.

4. Conclusions

An effective approach has been introduced to develop ternary composites of graphene oxide loaded on magnesium oxide and copper oxide. For the breakdown of EMB in aqueous solutions, the ternary composite of graphene oxide loaded with magnesium oxide and copper oxide shows exceptional photocatalytic performance. By expanding the response range into the visible spectrum, GO improves the light-harvesting efficiency. By acting as an electron acceptor, GO reduces the amount of electron–hole pairs that recombine when exposed to light. Varying the ratios of GO loaded on CuO and MgO allows the optimization of the composite's photocatalytic activity. The optimized composite (4.0 wt% MgO–CuO/GO) achieves high degradation efficiency under optimal reaction conditions, emphasizing the synergistic interaction between the components. CuO facilitates electron transfer and provides catalytically active sites, while MgO stabilizes the structure and prevents aggregation. The composite structure provides a large surface area for effective interaction with pollutant molecules, improving degradation kinetics. This study offers a promising path toward creating effective and long-lasting solutions for the cleanup of pesticide-contaminated water and emphasizes the significance of customized material design in improving photocatalytic characteristics. To enable their practical use in environmental protection, more investigation into the long-term stability and scalability of these composites is advised.

Conflicts of interest

The authors declare no conflicts of work in the manuscript.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra03436e.