Synthesis, Characterization, and Photocatalytic Activity of Cu-Doped MgO Nanoparticles on Degradation of Methyl Orange (MO)

Abstract

The purpose of this study is to synthesize Cu-doped MgO nanoparticles and test the performance of photocatalytic degradation of methyl orange (MO). Mg(NO₃)₂, CuCl_2, NaOH, and fresh Calotropis procera leaf extract were used as precursors. The prepared nanoparticles were characterized by using FT-IR, XRD, SEM, and UV-Vis spectrometer to study the functional group, crystal structure, surface morphology, and absorption edge, respectively. The wide band above 3000 cm⁻¹ from the FT-IR spectrum corresponds to the stretching vibrations of flavonoids and phenolic compounds of Calotropis procera leaf extract. Furthermore, the Mg-O bonding of undoped MgO and Cu-doped MgO NPs is represented by new peaks which appeared at 831 and 835 cm⁻¹, respectively. The crystal size of undoped MgO and Cu-doped MgO nanoparticles is 13.04 nm and 12.08 nm, respectively. The SEM microstructure of pure MgO showed higher agglomeration than the Cu-doped MgO nanoparticles. The degradation efficiency of the Cu-doped MgO NPs was compared with that of the MgO NPs, and the photocatalytic activity of these NPs was evaluated using the photocatalytic degradation rates of MO dye. Cu-doped MgO NPs showed higher degradation efficiency than pure MgO NPs. The insertion of Cu in the MgO structure improved the photocatalytic efficiency of the MgO NPs under optimal conditions. Therefore, Cu-doped MgO exhibits high photocatalytic activity compared with undoped MgO nanoparticles under sunlight irradiation.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The primary causes of the global contamination of water resources are growing urbanization and industrialization [1]. Water contamination is mostly caused by a considerable amount of synthetic colors among these pollutants. Remaining dye pollutants are primarily found in the wastewater from the paper, pulp, and textile industries. These toxins are dark in color, chemically stable, and non-biodegradable. Furthermore, the chemical sector releases hazardous and dangerous substances into water bodies, including phenols, herbicides, and pesticides [2].

Most of the textile industries emit organic dyes (phenol red, methyl orange, methyl red, methyl blue, and methyl violet), acidic gases, toxic gases, and heavy metals (Cd, Hg, and Pb) as raw materials which cause serious pollution to land and water [3]. Organic dyes inhibit light penetration through water, slow down photosynthesis reactions, hinder sunlight penetration, lower photosynthesis activity, and cause oxygen deficit, all of which have an impact on water quality. Furthermore, certain chemical dyes like azo dyes can cause mutagenic and carcinogenic effects [4]. Synthetic dyes, which are used in large quantities in the textile industry, are chemically stable and harmful to the environment. Because of their stability, azo dyes, such as methyl orange, are commonly used in the textile, paper, and food industries [5]. That is why research has been intensified on the treatment and management of azo dyes in wastewater to reduce their negative effects.

In developed countries, organic dyes from water and soil can be treated with modern technology. Photocatalytic nanoparticles have received a lot of attention due to their unique properties compared to bulk materials. Metal oxide nanoparticles such as ZnO, Cu2O, TiO2, and MgO are suitable for the photodegradation of organic dyes. The degradation of methylene blue and Congo red was tested with ZnO and MgO nanoparticles [5, 6], respectively, at the laboratory scale. MgO nanoparticles show enhanced catalytic activity due to large surface area and high chemical stability [7]. MgO, on the other hand, absorbs only ultraviolet electromagnetic radiation due to its high bandgap energy. As a result, its photocatalytic activity under solar radiation is low, as only 5% of solar energy that reaches the earth is ultraviolet (UV) [8].

To increase photocatalytic activity, doped metals have recently been added to established photocatalysts such magnesium oxide (MgO). Transition metal doping changes the energy band gap, creates charge traps for photogenerated carriers, improves photocatalytic effectiveness by suppressing charge carrier recombination, and reduces kinetics [9]. It is commonly recognized that adding transition metals results in smaller nanoparticles with higher specific surface area and inhibits particle development. By increasing the surface area and surface defects of metal oxides through doping with copper, zinc, and silver, one can shift the optical absorbance towards the visible range [10]. It may also alter the optical characteristics, reduce the band gap, move the absorption band to a longer wavelength, and change the morphology of the MgO nanostructure; copper is the element of choice for doping MgO nanoparticles [11, 12].

Metal oxide nanoparticles can be synthesized by condensation, evaporation, sol-gel, precipitation, co-precipitation, and green methods [13]. Among theme, green synthesis has recently been of great interest to produce metal oxide nanoparticles such as ZnO, MnO₂, MgO, and TiO₂ [14]. In green synthesis, metal nitrate acts as an oxidizing agent and phytochemicals present in plant extracts act as a reducing and stabilizing agents for the synthesis of nanoparticles. Currently, plant-based synthesis is considered as the best alternative due to its stability and higher nanoparticle production rate [15].

Calotropis procera is a plant that contains various phytochemicals such as flavonoids, terpenoids, and phenols. Various biomolecules with functional groups such as C=C (alkenyl), C=N (amide), -O-H (phenolic and alcohol), N-H (amine), C-H, and COO- (carboxylic group) are primarily responsible for the reduction and stabilization of nanoparticles [16].

Therefore, this study including synthesis and characterization aimed to test Cu-doped MgO nanoparticles using leaf extract of Calotropis procera and photocatalytic degradation of methyl orange (MO) under solar radiation.

2. Materials and Methods

2.1. Chemicals

All the chemicals such as magnesium nitrate (99%), copper chloride dihydrate (98%), sodium hydroxide (99%), methyl orange (99.9%), and dimethyl sulfoxide (99%), deionized water, and fresh leaf of Calotropis procera were employed without further purification and were of analytical grade.

2.2. Plant Extraction and Synthesis of the Photocatalysts

2.2.1. Preparation of Calotropis procera Leaf Extract

Fresh leaf of Calotropis procera (Figure 1) was collected from Adwa District (located at 14°15′N and 38°52′E, with an elevation range from 1500 to 2700 m (a.s.l.)), Tigray, Northern Ethiopia [17]. The collected leaf was washed thoroughly using tap water, followed by deionized water until the dust particles and other undesired substances were removed. Then, the leaf was dried in open air to remove the residual moisture and changed into powder using a grinder. The powder was sieved using 250 micrometer sieve. About 4 g of the plant powder was boiled in 100 mL of deionized water for 15 min at 80°C. The leaf extract was cooled and filtered through Whatman No. 41 filter paper and centrifuged at 1500 rpm for 5 min to remove biomaterial residue. Finally, the filtrate of plant extract was stored in a refrigerator for further experiments [17].

[figure(s) omitted; refer to PDF]

2.2.2. Synthesis of MgO NPs Using Calotropis procera Leaf Extract

To prepare MgO nanoparticles, 0.1 M of Mg(NO₃)₂ was dissolved in 100 mL deionized water. The solution was mixed with 10 mL of leaf extract. Then, 5 mL of 1 M NaOH solution was added drop wise while continuously stirred for 3 h at 60°C. Upon the addition of NaOH solution a sharp change in color from light yellow to deep yellow was observed which confirmed the formation of Mg(OH)₂ nanoparticles. The precipitate was then centrifuged at 3000 rpm for 15 min, washed using deionized water several times, and dried in an oven for 12 h at 80°C. At this point, the yellow color product (mixture of MgO and Mg(OH)₂) did not show higher crystallinity. Then, it was calcined at 500°C for 3 h to remove organic impurities and obtain higher crystallinity MgO nanostructures [18].

2.2.3. Synthesis of Cu-Doped MgO NPs Using Calotropis procera Leaf Extract

To prepare Cu-doped MgO nanoparticles, 0.1 M of Mg(NO₃)₂ and 0.01 M of CuCl₂ were dissolved in 100 mL of deionized water. About 10 mL of leaf extract was added drop wise into the mixture with continuous stirring. Then, 5 mL of 1 M NaOH was also added drop wise with continuous stirring for 3 h at 60°C. The precipitate was centrifuged at 3000 rpm for 15 min and washed with deionized water several times to remove the impurities. After 12 h in an oven at 80°C, the precipitate changed into a dry powder. In addition, it was calcined at 500°C for 3 h in a muffle furnace [19].

2.3. Characterization

The crystal structures and average crystallite size of the samples were scanned using X-ray diffraction (XRD-7000, Shimadzu, Japan) equipped with a Cu-Kα radiation (λ = 1.5406 Å). The step scan mode is used to know crystal structures and average crystallite size in the range of 10° to 80°. A scanning electron microscope [20] (JSM-5610, JEOL, Japan) was used to examine the sample’s surface morphology. The chemical functionality of the samples was analyzed using Fourier transform infrared spectroscopy (FT-IR, PerkinElmer, USA) in the range 4000-400 cm⁻¹. The optical properties of the nanoparticles were investigated using a PerkinElmer Lambda 35 spectrometer with a wavelength range of 200-800 nm [11].

2.4. Photocatalytic Degradation Studies

Methyl orange (MO) was selected as a model pollutant for photocatalysis due to excessive presence in textile effluents. Photocatalytic experiments were carried out in 500 mL beakers. The mixture of 1 g/L of MgO, Cu-doped MgO, and 200 mL of 10 ppm MO dye was stirred in a dark for 30 min to allow adsorption/desorption equilibrium before irradiating the sunlight [11]. The irradiation was then conducted under sunlight on sunny days to minimize fluctuations in solar intensity. After 20 min of irradiation, 10 mL of the heterogeneous mixture was withdrawn, centrifuged for 5 min at 3000 rpm, and then filtered through filter paper. The concentration of MO was determined by measuring the absorption intensity at maximum absorbance wavelength. The percentage degradation of MO solution was calculated using the following formula [21]:$$\begin{array}{}\text{(1)}& \%\text{\hspace{0.17em}degradation}=\frac{{\mathrm{A}}_0-{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_0}\times 100,\end{array}$$where $A_0$ and $A_t$ represent absorbance of MO at initial and each time interval, respectively.

3. Results and Discussion

3.1. Fourier Transform Infrared (FT-IR) Spectroscopy

The FT-IR spectra of Calotropis procera leaf extract, undoped MgO NPs, and Cu-doped MgO NPs are shown in Figure 2. The broad bands appeared around 3260-3300 cm⁻¹ which correspond to N-H and O-H stretching vibrations from amino and phenolic compounds of flavonoids introduced from the Calotropis procera leaf extract. Minor peaks located at 2922 and 2852 cm⁻¹ were attributed to C-H stretching vibrations of –CH₃ and –CH₂ aliphatic groups. The weak absorption peak at 1732 cm⁻¹ is due to C=O stretching vibration of flavonoids. The strong peak at 1616 cm⁻¹ represents the stretching vibration of aromatic C=C bond. Other scholar also reported occurrence of a broad absorption band at 3247 cm⁻¹ from the investigation of Calotropis gigantea leaf extract which is peculiar characteristics of N-H and O-H starching vibration of amino and phenolic compounds of glycosides and flavonoids [22].

[figure(s) omitted; refer to PDF]

New peaks appeared at 831 cm⁻¹ and 835 cm⁻¹ which represent the stretching vibration of Mg-O bonding from MgO and Cu-doped Mg-O compounds, respectively. Cu-doped MgO showed slight shift to higher wave number which might be due to the introduction of Cu as a dopant. Other scholars also reported similar findings [23]. On the other hand, bands appearing at 3697 and 3699 cm⁻¹ belong to antisymmetric stretching vibration of Mg(OH)₂ crystal structure. These findings are in good agreement with the reported literature values [24].

3.2. XRD Analysis

The XRD diffractograms of undoped MgO and Cu-doped MgO NPs calcined at 500°C are shown in Figure 3. The XRD pattern of undoped MgO NPs appeared at 2θ = 36.89°, 42.88°, 62.24°, 74.59°, and 78.54° which correspond to (111), (200), (220), (311), and (222) plane of reflection, respectively. The diffraction peaks were well indexed to cubic crystal structure (JCPDS card No. 00-045-0946) with space group of Fm-3m. The sharp, clear line broadening of the diffraction peaks situated at 2θ showed that the as-synthesized MgO NPs have crystalline nature. This might be an indication of formation of pure phase of MgO NPs. These results are congruent with reported literature values [25]. The diffraction peaks of Cu-doped MgO at 2θ value of 36.86°, 42.87°, 62.22°, 74.59°, and 78.51° correspond to the Miller index of (111), (200), (220), (311), and (222). This shows that the XRD pattern of Cu-doped MgO changes towards the lower angle after doping with copper. These diffraction patterns of Cu-doped MgO nanoparticles are similar to the undoped MgO nanoparticles except for minor differences probably caused by the presence of trace impurities [21]. Moreover, smaller differences in ionic radii between Cu²⁺ (0.073 nm) and Mg²⁺ (0.066 nm) impacted the crystal lattice very little. This further enhanced creating the surface defects which might improve photocatalytic activity of MgO NPs [26].

[figure(s) omitted; refer to PDF]

The intensity of the strongest diffraction peak (2θ = 42.88°) decreased and the full width at half maximum (FWHM) increased when Cu is doped. From the Debye–Scherrer equation (equation (2)), the intense peak was used to calculate the average crystallite size of the nanoparticles [31].$$\begin{array}{}\text{(2)}& D=\frac{K\lambda }{\beta \cos \theta },\end{array}$$where D is the crystallite size, λ is the wavelength of X-ray source (Cu-Kα radiation = 1.5406 Å), K is the shape factor = 0.94, θ is the Bragg angle, and β is the full width at half maximum in radian.

The crystallite domain sizes of all undoped MgO and Cu-doped MgO were found to be 12.56 nm, 13.04 nm, 13.06 nm, 13.97 nm, and 13.7 nm and 13.035 nm, 12.08 nm, 11.5 nm, 12.65 nm, and 12.28 nm, respectively. The crystallite size decreased in Cu-doped MgO compared to the pure MgO which might be due to the insertion of copper into MgO crystal lattice. The insertion of copper into the MgO nanoparticles lattices can disrupt the particle growth process, which could explain the reduction in crystallite size in Cu-doped MgO nanoparticles [21].

3.3. Surface Morphology Analysis

Surface morphology of the undoped MgO and Cu-doped MgO nanoparticles was examined by SEM as shown in Figure 4. SEM microstructure shows that surface morphology of MgO nanoparticles changes significantly after doping with copper. Pure MgO nanoparticles (Figure 4(a)) are connected to each other to make large network systems which have irregular pore sizes, shapes, and many agglomerations. The SEM micrographs of Cu-doped MgO nanoparticles (Figure 4(b)) indicate that the morphology is relatively uniformly distributed of spherical particles and exhibit good crystalline quality where agglomerations of particles are comparably less than the undoped MgO nanoparticles. This might be due to the copper doping which reduces the crystalline size of the particle which is similar to the reported value [27, 28]. This result is in line with the findings of crystallite size estimated from XRD measurements (Table 1).

[figure(s) omitted; refer to PDF]

Table 1

Major crystalline size of as-synthesized nanoparticles.

Nanoparticles	2θ (deg)	FWHM (rad.)	Crystalline size (nm)	d-spacing
Undoped MgO	42.8756	0.65490	13.04	2.10758
Cu-doped MgO	42.8672	0.70710	12.08	2.10797

3.4. Optical Property Analysis

UV-Vis diffuse reflectance spectroscopy was used to investigate the produced nanoparticles’ electromagnetic radiation absorption region. The spectra for the absorption of light by MgO and Cu-doped MgO are given in Figure 5. MgO and Cu-doped MgO absorb at 336 nm and 400 nm, respectively. MgO absorption edge was in the UV region. Because of the copper, the Cu-doped MgO nanoparticle absorbed light in the visible region [29]. The absorption edges were increased to 400 nm by doping copper with MgO.

[figure(s) omitted; refer to PDF]

The band gap energy of nanoparticles is estimated by extrapolating the linear section of the graph between the modified Kubelka–Munk function ${\mathrm{F}\mathrm{R}\text{hv}}^2$ and photon energy (hν) [30] as shown in Figure 5(b). MgO band gap energy (3.7 eV) is reduced to 3.1 eV when it is doped with Cu. This clearly shows that the inclusion of Cu atoms contributes to the narrowing of MgO band gap, extending its absorption edge to the visible range and capturing more photons from sunlight [31, 35].

3.5. Effect of Photocatalytic Parameters on Degradation of Methyl Orange

3.5.1. Effect of Catalyst Loading

The photocatalytic degradation of methyl orange using the undoped MgO and Cu-doped MgO NPs under sunlight irradiation is shown in Figure 6. In order to study the effect of catalyst loading, a series of photocatalytic degradation experiments were carried out by varying the amounts of catalyst loading from 0.1 to 2 g/L solution with 10 ppm MO and 3 h irradiation time as optimized by Ashebir et al. [32]. After 3 h, the percentage of MO degradation increased from 72% to 85% as the amount of undoped MgO NPs loading increased from 0.1 to 1 g/L (Figure 6(a)). This might be due to an increase in the amount of catalyst which increases the availability of active sites on the catalyst surface, which in turn increases the number of hydroxyl and superoxide radicals. Similar findings have already been reported by other scholars [33]. However, further increase in catalyst dosage from 1 g/L to 2 g/L reduced the degradation of MO from 85% to 68% after 3 h. This might be due to blockage of light penetration by the existence of excessive amount of photocatalyst. Moreover, agglomeration might also occur. When the catalyst concentration is higher, it reduces the surface area and decreases the degradation of the catalyst efficiency [34].

[figure(s) omitted; refer to PDF]

Similarly, when the amount of Cu-doped MgO nanoparticles increased from 0.1 g/L to 1 g/L, the percent of degradation also increased from 80% to 95% after 3 h (Figure 6(b)) [35]. This might be caused by an increase in catalyst loading, which would increase the photocatalyst surface area and reaction sites. However, as the catalyst dosage was increased beyond 1 g/L, the degradation efficiency further decreased. This could be due to the reduction of catalyst surface area [35]. Therefore, the degradation efficiency of Cu-doped MgO nanoparticles was higher than that of the undoped MgO nanoparticles. The optimum amount of the catalyst dosage was 1 g/L.

3.5.2. Effect of Concentration of Methyl Orange

The photocatalytic degradation experiment was carried out by varying the initial concentration of MO (10 to 17.5 mg/L) with an increment of 2.5 mg/L while catalyst loading and irradiation time were kept constant at 1 g/L and 3 h, respectively. As shown in Figure 7(a), by increasing the dye concentration from 10 mg/L to 17.5 mg/L, the degradation efficiency reduced from 88% to 60% after 3 h. The decrease in degradation efficiency with increasing dye concentration is due to a reduction in the amount of hydroxyl radicals, which in turn reduces the catalyst’s degradation performance [36]. As indicated in Figure 7(b), when the initial concentration of MO increases from 10 mg/L to 17.5 mg/L, the percentage of degradation also decreased from 94% to 65% after 3 h and at 1 g/L of Cu-MgO nanoparticles. This might be due the existence of excess dye molecules with fixed amount of catalyst to compete for the active sites thus reducing the degradation rate. Moreover, high concentration of a dye also hinders light penetration [37].

[figure(s) omitted; refer to PDF]

The comparison of photocatalytic degradation efficiency of the undoped MgO and Cu-doped MgO nanoparticles for MO is shown in Figure 8. The photocatalytic degradation efficiency of the undoped MgO and Cu-doped MgO nanoparticles was compared in the degradation of MO using sunlight irradiation as source of energy. The degradation efficiency of the MO (10 ppm) after sunlight irradiation of aqueous solution using 1g/L undoped MgO and Cu-doped MgO photocatalyst were found to be 80% and 95%, respectively; after 3 h irradiation time. As a result of increased surface area from Cu-doping, the photocatalytic degradation efficiency was significantly increased. Despite the low MO concentration, it took about 180 minutes of irradiation to completely degrade MO in the presence of undoped MgO and Cu-doped MgO NPs. This is due to the photocatalyst being placed on a surface, which significantly reduces light absorption and dye interaction [28]. In general, the photocatalytic degradation efficiency of Cu-doped MgO nanoparticles is greater than that of undoped MgO nanoparticles. Even though there is no crystal size difference with the as-synthesized nanoparticles, Cu-doped has greater efficiency than undoped of MgO nanoparticles; this is possibly due to the narrow band gap energy [12].

[figure(s) omitted; refer to PDF]

The proposed degradation mechanism of MO by the undoped MgO and Cu-doped MgO photocatalytic nanoparticles is illustrated in Scheme 1. During the irradiation time, the undoped MgO photocatalysts could easily absorb the light energy to create holes (h_VB⁺) in the valance band and electrons ${\mathrm{e}}_{\text{CB}}^{-}$ in the conduction band (equations (3) and (4)) [7]. Then, the holes and electrons can react with H₂O and oxygen already adsorbed at the surface of MgO nanoparticles to generate hydroxyl (^.OH) and superoxide ${\mathrm{O}}_2^{-}$ radicals (equations (5) and (6)). Copper is actually used to trap the photogenerated electron by acting as an electron sink to prevent the charge-carrier recombination. Furthermore, the ${\text{Cu}}^{2+}$ ions trap electrons and convert into less stable ${\text{Cu}}^{1+}$ ions which are ready to react with oxygen molecules used as oxidizing agent for superoxide formation (equations (7) and (8)). Finally, the hydroxyl (^.OH) radicals and superoxide ${\mathrm{O}}_2^{-}$ radicals are readily available for further decomposition of MO (equation (9)) [12, 39]. The overall elementary reaction mechanisms are summarized from (3)–(12).$$\begin{array}{}\text{(3)}& \text{MgO}+\mathrm{h}\nu ⟶\text{MgO}{\mathrm{h}}_{\text{VB}}^{+}+\text{MgO}{\mathrm{e}}_{\text{CB}}^{‐}\text{(4)}& {\mathrm{H}}_2\mathrm{O}⟶{\mathrm{H}}^{+}+{\text{OH}}^{‐}\text{(5)}& \text{MgO\hspace{0.17em}}{\mathrm{h}}_{\text{VB}}^{+}+{\text{OH}}^{‐}⟶\text{OH}\text{(6)}& {\text{Cu}}^{2+}+{\mathrm{e}}_{\text{CB}}^{‐}⟶{\text{Cu}}^{+1}\text{electron\hspace{0.17em}trap}\text{(7)}& {\text{Cu}}^{+1}+{\mathrm{O}}_2\text{ads}⟶{\text{Cu}}^{2+}+{\mathrm{O}}_2^{–}\text{electron\hspace{0.17em}release}\text{(8)}& {\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}⟶{\text{HO}}_2\text{(9)}& {\text{HO}}_2+{\text{HO}}_2⟶{\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2\text{(10)}& {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2^{.–}⟶{\text{OH}}^{‐}+\text{OH}+{\mathrm{O}}_2\text{(11)}& \text{OH}+\text{Dye}⟶{\mathrm{H}}_2\mathrm{O}+{\text{CO}}_2\text{(12)}& \text{OH}+\text{Dye}⟶{\mathrm{H}}_2\mathrm{O}+{\text{CO}}_2\end{array}$$

[figure(s) omitted; refer to PDF]

3.6. Kinetic Study of the Photocatalytic Degradation of MO

The rate of reaction for the photocatalytic degradation of MO using the undoped MgO and Cu-doped MgO NPs is shown in Figure 9. The graph of first-order kinetics was drawn by plotting $\ln C_0/C_t$ versus irradiation time. The experimental results demonstrated that photocatalytic degradation of MO is a first-order kinetic reaction. A good correlation coefficient $R^2>0.97$ confirmed that the experimental data are nearly linearly fit [12, 40]. The photocatalytic degradation of MO using undoped MgO and Cu-doped MgO nanoparticles has rate constants of 0.00544 and 0.0096 min⁻¹, respectively. The rate of kinetics for photocatalytic degradation of MO is more efficient for Cu-doped MgO nanoparticles.

[figure(s) omitted; refer to PDF]

4. Conclusion

Pure MgO and Cu-doped MgO nanoparticles were successfully synthesized by using Calotropis procera leaf extract as a natural reducing and stabilizer agent. The as-synthesized pure MgO and Cu-doped MgO NPs were characterized by FT-IR, XRD, SEM, and UV-Vis diffuse reflectance spectrophotometer. The FT-IR spectrum shows the presence of stretching vibrations of the O-H, N-H, and C= O groups involved in the stabilization of the nanoparticles. The XRD results confirmed that the inclusion of copper as a doping agent reduced the crystal size of the undoped MgO NPs. The SEM micrographs also revealed that the pure MgO NPs have irregular pore size and high agglomeration compared with the Cu-doped MgO NPs. The inclusion of Cu as a dopant for the MgO nanoparticles reduced the band gap energy from 3.7 to 3.1 eV. The as-synthesized Cu-doped MgO nanoparticles have better photocatalytic degradation efficiency than the undoped MgO nanoparticles for MO. The experimental results of as-synthesized nanoparticles for photocatalytic degradation of MO indicate that rate of degradation is dependent on initial dye concentration and catalyst loading. The kinetic reaction of MO for photocatalytic degradation was consistent first-order reaction.

As a result, Cu-doped MgO nanoparticles are considered an efficient solar light active photocatalyst for the removal of organic dyes from wastewater.

Acknowledgments

The first author acknowledges the financial support from Mekelle University (NORAD Project) (Grant ID: PG/MSc/CNCS/MU-NMBU/60/2020).

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