Green Synthesis and Photocatalytic Dye Degradation Activity of CuO Nanoparticles

Abstract

The degradation of dyes is a difficult task due to their persistent and stable nature; therefore, developing materials with desirable properties to degrade dyes is an important area of research. In the present study, we propose a simple, one-pot mechanochemical approach to synthesize CuO nanoparticles (NPs) using the leaf extract of Seriphidium oliverianum, as a reducing and stabilizing agent. The CuO NPs were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) and Fourier-transform infrared spectroscopy (FTIR). The photocatalytic activity of CuO NPs was monitored using ultraviolet-visible (UV-Vis) spectroscopy. The CuO NPs exhibited high potential for the degradation of water-soluble industrial dyes. The degradation rates for methyl green (MG) and methyl orange (MO) were 65.231% ± 0.242 and 65.078% ± 0.392, respectively. Bio-mechanochemically synthesized CuO NPs proved to be good candidates for efficiently removing dyes from water.

1. Introduction

The development of industrialization has increased the risk of environmental pollution [1,2]. Several types of waste can seriously threaten water bodies [3,4]. Organic dyes are significant pollutants produced by different industries, such as food, pharmaceutical, leather, textile, inks, cosmetics, etc. [5,6,7]. Every year, tons of complex dyes are formed and discharged into water bodies, which cause harmful effects on aquatic life [8,9,10]. Developing methods for degrading dyes is a challenge for researchers [11,12,13,14,15]. Some of these methods are ion exchange, chemical oxidation, ozonolysis, photocatalytic degradation, and coagulation-based techniques [16,17,18,19,20,21]. Adsorption is often used to remove several water pollutants [22,23,24,25,26,27,28,29,30,31,32,33,34]. However, photocatalytic degradation using a semiconductor photocatalyst is considered a cost-effective and green approach to successfully degrade dyes [35,36,37,38,39].

Light active materials with large surface areas are needed to degrade organic dyes [40,41]. Several metal oxides or semiconductors are active photocatalysts under sunlight, due to their small band gap [42,43]. CuO is a p-type semiconductor having a narrow band gap of 2.1 to 2.71 eV [44]. CuO nanoparticles (NPs) show a proper response towards optical, mechanical, and photolytic applications [42,45,46,47]. Different methods are used to synthesize CuO NPs, including sol-gel, solvothermal, microwave irradiation, hydrothermal, arc discharge, etc. [48,49].

The synthesis of CuO NPs using plant extracts is considered the most practical and green approach [50,51]. Different biological methods for synthesizing CuO NPs involve the use of bacteria, fungi, algae, and plants as bioactive materials by using leaves, flowers, or stem extracts, including Solanum americanum, Solanum nigram, Camellia japonica, Pterospermum acerifolium, Gum karaya, Soya bean, etc. [38,51,52,53].

Several efforts were made to understand the biological mechanism and phytochemicals involved in the green synthesis of CuO NPs and their characteristics [54]. Different biomolecules responsible for stabilizing and reducing nanoparticles can be amino acids, proteins/enzymes, alkaloids, polysaccharides, vitamins, and alcoholic compounds [55,56]. The prepared CuO NPs depend on the reduction power of ions and the reduction capability of plants having biochemicals such as polyphenols, enzymes, and other chelating agents [57,58].

Seriphidium oliverianum belongs to the family Asteraceae, widely used in folk medicines [59,60]. It has many active biomolecules, such as cardenolides, anthraquinones, tannins, alkaloids, flavonoids, terpenoids, phenolic acids, and carbohydrates [61]. The biological CuO NPs can be effectively reduced by biomolecules [62].

Catalysis-based processes play a crucial role in producing high-value goods such as fuel, chemicals, pharmaceuticals, etc., from cheap raw materials. Catalysts are considered the engines behind these processes [63]. It is projected that the catalysis-based sector may produce commodities worth several trillion euros annually, with a total sales value of catalytic materials of about 20 billion euros, highlighting the importance of catalysis to our community [64]. The chemical industry mainly uses heterogeneous catalysis for several reasons, such as simple catalyst separation, durability, and suitability for continuous operation. However, the designing of catalyst materials is not a straightforward process due to their complex architecture and the poor understanding of active centers [65]. This idea becomes exceptionally crucial for the consistent mass production of solid catalysts. Despite the advanced level of technology in this sector, catalyst synthesis is often seen as more of an art than a science. Therefore, a significant amount of research focuses on the creation of catalysts, which is an indication of the tremendous effort put into understanding the rational synthesis of active, selective, and stable catalysts. This idea is crucial for the consistent mass production of solid catalysts. Consequently, despite the advanced level of technology in this sector, catalyst synthesis is frequently seen as more of an art than a science. Therefore, it is not surprising that there is much research focusing on the creation of catalysts, as they attest to the tremendous effort put into understanding the logical synthesis of active, selective, and stable catalysts [66,67].

Precipitation, deposition–precipitation, the hydrothermal approach, and impregnation are the main pathways for synthesizing industrial-scale catalysts that are currently developed to ensure a reasonable level of control over the catalyst properties and performance [68]. Other methods such as solid-state reactions and fusing can also be used. However, solution-based procedures always generate a significant amount of solvent waste due to their inherent nature. In addition, nitrate or chloride metal salts are commonly used as precursors, which may result in the production of poisonous gases during subsequent calcination stages. Necessary measures may be required to prevent these gases from escaping into the atmosphere [69]. Wet chemistry procedures are also frequently viewed as tedious and challenging to scale up for a particular formulation of the catalytic material. Furthermore, due to their energy requirements and possible role as producers of hazardous waste, solution processes and additional treatment stages, carried out at high temperatures, frequently fail to fulfill current environmental standards. Thus, there is a large interest in creating alternative synthetic procedures that are less harmful to the environment, easier, more economical, more productive, and scalable.

Superior features of the generated materials and more advantageous economic or environmental factors of the processes can stimulate the creation of innovative synthetic techniques. Due to the growing urgency of environmental issues and energy depletion resources, environmentally friendly production techniques of catalyst synthesis are particularly advantageous [70]. Reactive extrusion and ball milling are two prominent, quick, and efficient mechanochemical processes to create catalytic materials.

In the last decades, the mechanochemical approach was developed as a sustainable method for the large-scale production of various nanomaterials [71]. This procedure can generate well-dispersed metal oxide nanoparticles to be used in wide-ranging applications, including environmental monitoring, energy storage, conversion, or biomedical uses. The mechanochemical synthesis is relatively simple, easy to scale-up and create a uniform reaction [71]. The motivation of work is that chemical reactions can proceed in the absence of excess solvents or heating, making this a key reason for the recent interest in green chemistry.

To the best of our knowledge, the mechanical synthesis of CuO NPs using Seriphidium oliverianum extract was not yet reported in the literature. In this study, we develop a straightforward bio-mechanochemical approach using an electric mortar grinder mill to synthesize CuO NPs in the presence of Seriphidium oliverianum leaf extract. We also evaluated the degradation efficiency of water-soluble dyes, namely methyl green (MG) and methyl orange (MO).

2. Results and Discussion

2.1. X-ray Diffraction (XRD)

XRD was employed to study and explore the crystalline nature of the CuO nanostructured material. The average grain size of the material was obtained using the Debye–Scherrer’s formula:

(1)$\mathrm{D}=\mathrm{k}\mathsf{\lambda }/\mathsf{\beta }\cos \mathsf{\theta }$

where “D” is the crystallite size (nm), “k” is Scherrer’s constant, equal to 0.98, “β” is full width at half maximum (FWHM), and “θ” is the angle of diffraction.

The calculated average crystallite size of the NPs is 12.44 nm. The PXRD diffractogram (Figure 1) displays several characteristics peaks of the monoclinic structure for CuO NPs (standard JCPDS data card no. 00-001-1117 [27]).

2.2. Scanning Electron Microscopy (SEM)

The accumulation of fine CuO nanoparticles originated aggregates. High surface area to volume ratio of nanoparticles provides very high surface energy. To minimize its surface energy, the nanoparticles tend to agglomerate. Uncontrolled agglomeration may occur due to attractive van der Waals forces between particles. The average grain size obtained for CuO NPs was 1.48 µm. Figure 2 shows that the green synthesis of CuO NPs produces small, aggregated particles.

2.3. Fourier Transform Infrared (FTIR)

FTIR also allowed us to examine the composition and functional groups of bio-mechanochemically synthesized CuO NPs, from 400 to 4000 cm⁻¹. The strong vibrational bands found in the FTIR spectrum of CuO NPs (Figure 3) may be due to the biochemicals found in Seriphidium oliverianum extract (Figure S1), which capped the CuO NPs. Figure 3 shows a broad band at 3358 cm⁻¹_, which matches the hydroxyl functional group of alcoholic or phenolic compounds found on the NPs surface. Another FTIR band at 1616 cm⁻¹ corresponds to the aromatic bending vibrational frequency of the alkene group (C=C). It may be due to the bio components of leaves, which play a role in the reduction and stabilization of NPs. The sharp band at 1352 cm⁻¹ can be ascribed to frequencies of the C–H group of alkanes. The influential stretching band of C–O of the plant extract bio element alcoholic group is found at 1085 cm⁻¹. The bending vibration band of the aromatic group appears at 834 cm⁻¹. FTIR vibrational frequency ranges from 400 to 600 cm⁻¹, being attributed to Cu–O linkage, which confirms the formation of CuO NPs [72]. The functional groups associated with phytochemicals of Seriphidium oliverianum include glycoalkaloid, tropane alkaloid, and atropine, assigned to hydroxyl, aromatic, phenolic, and amino groups, which confirm the role of the plant extract as a reducing agent in the CuO NP synthesis.

2.4. UV-Visible Spectroscopy

Figure 4 shows the UV-Vis spectrum of CuO NPs, from 300 nm to 550 nm. It displays the expected absorption band at 324 nm for CuO NPs, and the band at 334 nm, due to the interband transition of Cu metal core electrons [73]. Bio components found in plant extracts play a role in the synthesis of stable CuO NPs [74]. Some aspects such as reaction time, temperature, concentration of precursor salt and aqueous leaf extract, and morphology of nanoparticles have an impact on the location of the absorption band in the UV-Vis spectrum. The sharp band also shows a high concentration of nanoparticles.

2.5. Photoluminescence Spectroscopy (PL)

PL allows us to reveal more details about exterior interstices, oxygen vacancies, surface flaws, optical emission facts, and photochemical characteristics of photocatalytic CuO NPs. The fluorescent process can also study the separation and transportation of electrons, the recombination process, and their effects on photocatalysis. The suggested mechanism of photoluminescence includes the movement of an electron from the valence to the conduction band after energy absorption via the generation of a hole. The recombination process occurs by shifting back the electron to the valence band with a simultaneous emission of energy. Furthermore, the small-sized particles are attributed to excellent facet fault and oxygen vacancies, resulting in a sharp luminescent peak [75].

The PL spectra of CuO NPs carried at 300 nm and 350 nm are shown in Figure 5. Two well-defined peaks are situated at 421 nm and 597 nm, for a wavelength of 300 nm (Figure 5a). The band at 421 nm is assigned to band edge-free excitons, and the band at 597 nm is attributed to bound excitons. The strong peak of PL spectra may be allocated to the small particle size and exterior defects. The intense band is also related to a high recombination rate. In Figure 5b, two distinct bands located at 450 nm and 699 nm are obtained by using a wavelength of 350 nm. These bands differ from those obtained at 300 nm (Figure 5a) because the excitation takes place at different wavelengths. Electron transformation occurs on different energy levels by absorbing different radiant energies and recombining them back to the valence band, with different conditions. So, the PL spectra covers different ranges. The intensity of band edge-free and bound excitons was found to be higher at a wavelength of 350 nm than at 300 nm. This confirms the existence of a UV adsorption band in the range of 324–334 nm, as previously mentioned in Section 3.4.

2.6. A Plausible Mechanism for Biogenic CuO NPs

The bio reduction of the precursor salt starts instantly, and the formation of CuO NPs is demonstrated by the solution color changing from blue to dark brown. The biochemicals found in the plant extracts play a main role in the stabilization of CuO NPs [38].

It is expected that several functional groups, found in flavonoids, can be used as reductants and contribute to NP formation [76]. Moreover, the release of H atoms during the conversion of enol flavonoids into keto flavonoids reduces Cu ions into metal Cu NPs. Nevertheless, the precise mechanism for the synthesis of CuO NPs mediated by plant extracts is still unknown. It is believed that the depth of the nanoparticles’ color, in an open environment, after one hour, can be attributed to oxidation, which is responsible for the formation of CuO NPs. Many factors might be involved, for e.g., it is possible that oxidation occurs due to environmental oxygen or biochemicals binding reduced metal ions before stabilization. Given electrostatic attraction, the ions of the metal oxide bind together forming NPs that are stabilized to prevent cluster formation. Despite the lack of a clear understanding of the mechanism, the use of plant extracts to synthesize nanoparticles is a promising approach due to its safe, environmentally friendly, and cost-effective nature. Figure 6 shows the proposed mechanism for CuO NP synthesis.

2.7. Evaluation of Photocatalytic Activity

The photocatalytic degradation of MG and MO, in the presence of CuO NPs, is depicted in Figure 7. All parameters, namely irradiation time, light source, concentration of dyes, and catalyst were identical for all reactions. The degradation of dyes was evaluated using natural sunlight as the light source. The confined bandgap and high surface area significantly influenced the degradation activity. Absorption spectra were measured at regular intervals using a UV-Vis spectrometer for all experiments. It was found that the intensity of the absorption band decreased as the illumination time under sunlight increased.

The main factors responsible for the decolorization of dyes were hydroxyl and oxy radicals, which degrade toxic contaminants formed when a hole–electron pair was created. Furthermore, the color of dyes simultaneously became lighter with time. Degradation efficiency for MG and MO was 65% and 65%, respectively, after 60 min of exposure to sunlight in the presence of a photocatalyst, as shown in Figure 8, Table 1. The rate constant is different for the two dyes, as they have different compositions and react differently. The higher the rate constant, the faster the reaction rate and vice versa.

2.7.1. Kinetic Studies

In order to determine the degradation rate of organic compounds under optimal conditions, kinetic studies were conducted using the following relationship:

(2)$\ln {\mathrm{A}}_{\mathrm{o}}/{\mathrm{A}}_{\mathrm{t}}=\mathrm{kt}$

In the equation, “A_o” is the absorbance of dyes at the time t = 0, “A_t” is the absorbance of dyes at time t, and “k” is the rate constant. Figure 9a shows the plot of A_o/A_t versus time, and Figure 9b shows ln A_o/A_t versus time. The slope of the graph represents the order of reaction (pseudo-first-order kinetics, which are attributable to the degradation of dyes) [77].

2.7.2. Mechanism

The primary species involved in detoxifying dyes under sunlight irradiation were identified via an analysis of the process. Figure 10 depicts the proposed mechanism for the photocatalytic degradation of dyes. Nanomaterials with a small bandgap promote the creation of hole–electron pairs, as low absorption energy is necessary for electrons to move between the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). When a catalyst absorbs, the absorption intensity is equal to the bad gap energy. The electron movement occurs from the ground state to the excited state, creating a gap, valence band hole (${\mathrm{h}}_{\mathrm{VB}}^{+}$), a free electron, and a conduction band electron (${\mathrm{e}}_{\mathrm{CB}}^{-}$). The positive hole is a promising candidate to accept an electron from the pollutants in order to degrade them. Highly oxidizing species convert the water molecules into hydroxyl radicals (OH˙), degrading the organic contaminants. Molecular oxygen combines with an electron and converts into a superoxide radical (O₂˙) [78]. The process involved in the reduction of pollutants is given as follows:

(3)$\mathrm{Photocatalyst}+\mathrm{h}\mathsf{\nu } \to { \mathrm{e}}_{\left(\mathrm{CB}\right)}^{-}+{ \mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}$

It is believed that hollow spaces and hydroxyl radicals are the main reactive species during photocatalytic degradation of water-soluble dyes.

2.7.3. Parameters Affecting Photocatalytic Degradation

In order to achieve a remarkable photocatalytic activity, some features must be considered, such as the concentration of dyes and photocatalyst, the nature and structural morphology of the catalyst, irradiation time, and light source. The effect of the concentration of the dyes and catalyst loading on the degradation are given in Figures S2 and S3 of Supporting Information. The thermodynamic parameters such as ΔG°, ΔS°, and ΔH° are given in Figure S4 and Tables S1 and S2. The standard calculation deviation and R² value are also given in Tables S3 and S4.

An inhibited reaction is observed when the concentration of dyes is increased, as there is no interaction with the active sites of the catalyst. This is due to the lack of absorption of light intensity by the dyes and the difficulty of electrons being able to migrate to the photocatalyst, resulting in insufficient hydroxyl radicals for dye degradation, and thus poor results are obtained [79]. MG and MO dye solution (with a 10-ppm concentration) showed much better results, compared to solutions with 15 or 20 ppm.

Upon increasing the photocatalyst amount, more interaction sites become available, leading to an enhanced production of hole–electron couples and hydroxyl radicals for the efficient detoxification of organic pollutants. The design, morphology, and particle size of the catalyst are important aspects in photocatalytic degradation. Smaller-sized particles provide more active sites for the absorption of dyes, increasing the efficiency of the process [80]. In our case, a good result was obtained using 10 mg CuO NPs. The removal rate of 65% was achieved after 60 min of illumination. 100% efficiency could be obtained if the exposure time was increased.

Although there is no report in the literature on the degradation of mechanochemical synthesized CuO NPs (used for the first time in the present work), other processes were used by several authors. Table 2 provides a detailed comparison between our synthesized photocatalyst and other CuO-based materials reported in the literature.

Concerning MO, UV light is more effective, as degradation rates varying from 45.23% [85] up to 96.4% [82] are obtained, using CuO NPs prepared via green synthesis. Using sunlight, the efficiencies vary from 31.95% [85] to 95% [86] for CuO NPs prepared via green synthesis, but are much smaller for CuO nanorods prepared via a hydrothermal method (22%) [87]. Our materials were the first prepared through mechanochemical synthesis and achieved a 65% degradation of MO using sunlight. This value is within the values reported in the literature for other preparation methods, but our method is simpler.

Concerning MG, no studies were found in the literature using CuO materials. A comparison is given for CuO-based materials used for the degradation of other dyes, showing the potential of this metal oxide.

2.7.4. Recyclability of Photocatalyst

The reusability of CuO NPs for the degradation of dyes was also analyzed. After complete degradation, the photocatalyst was removed and washed with deionized water. Then the photocatalyst was sonicated for a half an hour in 50 mL of deionized water and dried in air for 24 h. The CuO photocatalyst was then used in several consecutive degradation reactions. Excellent results were obtained for up to five runs, revealing the stability of CuO NPs (Figure S5). After the fifth run, the photocatalytic activity of the material decreased, possibly due to the formation of intermediates during the degradation of dyes, as suggested by other authors [100]. Additionally, the particle surface might decay during the degradation process, leading to a reduction in the overall activity after repeated use. However, the reusability of photocatalysts is a crucial factor in practical applications, as it allows for the effective and sustainable removal of dyes from wastewater.

3. Material and Methods

All chemical and precursor materials were purchased from Sigma-Aldrich (99.99%) and handled as acquired without any further treatment. Cu(NO₃)₂.3H₂O was used as precursor material. All other chemicals were of analytical grade and used without any additional purification. Deionized water was utilized for the preparation of standard solutions.

3.1. Preparation of Seriphidium Oliverianum Leaf Extract

The leaves of Seriphidium oliverianum were washed with distilled water and dried in air for a few days. Dry leaves were mashed to form a powder, using a mortar and a pestle. 5 g of powder was dispersed in 50 mL of deionized water. After that, the dispersion was kept for 24 h and then heated at 70 °C for 30 min under continuous stirring, followed by filtration with filter paper (Whatman No. 1) twice to remove the suspended particles altogether. The obtained leaf extract was stored for further experiments. We used dry leaves because morphology, size, and shape may vary for fresh leaves [101]. Additionally, the amounts and types of flavonoid groups change, depending on thermal stability during leaf drying and extract preparation. Moreover, when fresh leaves are used, the UV spectra may not give a clear absorption band, compared to dry leaves.

3.2. Synthesis of CuO NPs

The required amount of copper nitrate was crushed into a fine powder, and 40 mL of plant extract was added ([Cu] = 0.1 M), followed by continuous grinding for 3 h in an electric mortar grinder mill (Model 911MPEMG100) at 70 rpm speed. The formation of CuO NPs was noticed by a color change of the solution (from blue to dull, dark brown). After that, the solution was placed in an open-air atmosphere for one hour, intensifying the color from light to dark brown. The mixture was centrifuged at room temperature for 30 min at 4000 rpm and washed with deionized water to eliminate the excesses of leaf extract or precursor salt. The obtained CuO NPs were collected in a Petri dish and air-dried. The complete process is depicted in Figure 11.

3.3. Photocatalytic Experiments

The photocatalytic activity of photocatalytic CuO NPs was tested in the degradation of different dyes, namely, methyl green (MG) and methyl orange (MO), used as reference models (Table S5). The used sunlight came from a visible light source (average solar flux = 500 km h⁻¹m⁻²).

The reaction was initiated by adding 10 mg of CuO NPs to a 10-ppm solution (10 mgL⁻¹) of each dye. The mixture was stirred for 30 min in the dark in order to establish the adsorption–desorption equilibrium. A well-established spectrum of UV-vis absorption was seen in all experiments. Different bands were analyzed in the UV-Vis spectra for MG and MO, at 632 nm and 462 nm, respectively. The solution was stirred under sunlight irradiation, and 2 mL of suspension was withdrawn every 10 min, up to 60 min, to observe the absorption peak, which was considered the absorption of dyes at the time “t”, and analyzed with UV-Vis.

The following equation was used to measure the dye degradation [29].

(4)$\mathrm{Degradation} (\%)=\left(\frac{{\mathrm{A}}_0-{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_0}\right)\times 100$

where A₀ is the absorbance at time = 0 and A_t is the absorbance at time = t.

3.4. Characterization

The optical characteristics of synthesized CuO NPs were analyzed via UV-vis spectroscopy (Cecil 7500 UV-Vis Spectrometer), from 295 to 550 nm. The structural and chemical composition of CuO NPs were characterized using a Fourier-transform infrared spectrophotometer (FTIR, Tensor 27) that had a vibrational frequency ranging from 400 to 4000 cm⁻¹. A powder X-ray Diffractometer (PXRD) (Bruker D8 Advance PXRD) with Cu-Kα radiation source and wavelength λ equal to 1.540598 Å was employed to determine the crystallite size, nature, and phase description of CuO NPs [27]. In addition, CuO NPs morphology was investigated with scanning electron microscopy (SEM) using a MIRA-III TESCON apparatus. Surface deformity, photochemical, optical, and structural analysis of the obtained products were characterized using a Photoluminescence (PL) spectrometer at wavelengths ranging from 300 nm to 350 nm (Cary Eclipse Agilent technology) [28].

4. Conclusions

This study successfully reported an unprecedented environment-friendly bio mechanochemical approach for the synthesis of CuO NPs, using an aqueous extract from the Seriphidium oliverianum leaves. The bio components present in leaves were used as stabilizing and reducing agents. PXRD analysis identified a monoclinic CuO phase with a crystallite size of 12.44 nm. PL spectra identified two separate bands at 421 nm and 597 nm, indicating the presence of oxygen vacancies within the CuO NPs, which improved the photocatalytic activity. The synthesized nanosized material effectively demonstrated catalytic activity under sunlight illumination to degrade MG and MO dyes. The photocatalytic reduction followed a pseudo-first-order kinetics, with a removal rate of 65% for both MG and MO dyes. These promising results offered a new means for researchers to produce cost-effective and environmentally friendly photocatalysts to efficiently remove dyes from water.

Author Contributions

Conceptualization: M.B.T.; investigation: W.A., Mika Sillanpaa, S.A.C.C.; software: R.Y.; formal analysis: A.R. and T.J.; writing—original draft: S.A.; writing—review and editing: F.V., M.S., K.K., S.A.-F. and S.A.C.C., supervision: M.B.T., S.A.C.C. and I.B.; resources: I.B.; project administration: M.B.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are present within the manuscript body.

Conflicts of Interest

There are no conflict of interest between the authors.

Footnotes

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Figures and Tables

Figure 1. XRD diffractogram of biogenic CuO NPs (monoclinic CuO phase structure).

Figure 2. SEM image (a) and size distribution (b) of biogenic CuO NPs.

Figure 3. FTIR spectrum of biogenic CuO NPs.

Figure 4. UV-vis spectrum of biogenic CuO NPs.

Figure 5. PL spectra of biogenic CuO NPs at: (a) 300 nm, (b) 350 nm.

Figure 6. Mechanism of biogenic synthesis of CuO NPs.

Figure 7. Absorption spectra of the dyes at different time intervals: (a) MG, (b) MO.

Figure 8. Degradation rate of (a) MG and (b) MO dyes.

Figure 9. Kinetic results for dye degradation: (a) A/Ao vs. time; (b) ln (A/Ao) vs. time.

Figure 10. Schematic representation of photocatalytic degradation of dyes.

Figure 11. Mechanochemical synthesis of biogenic CuO NPs.

Table 1

Degradation rates (%) and rate constants for photocatalytic degradation of dyes using biogenic CuO NPs.

Dyes	Degradation Rate%	Rate Constant (min−1)
Methyl green (MG)	65.231	0.0175285
Methyl orange (MO)	65.078	0.0175348

Table 2

Comparative study of photocatalytic degradation of dyes using CuO based materials.

Photocatalyst	Synthesis Method	Time (min)	Light Source	Dye	Degradation Rate	Ref.
CuO NPs	Green Synthesis	24	UV light	MO	96%	[81]
CuO NPs	Green Synthesis	24	UV light	MO	96.4%	[82]
CuO NPs	Chemical Precipitation Method	120	UV light	MO	90%	[83]
CuO Microspheres	Reflux Condensation Method	93130	UV light	MOMB	89.39%92%	[84]
CuO NPs	Green Synthesis	6060	UV lightSunlight	MOMO	45.23%31.95%	[85]
CuO NPs	Green Synthesis	60	Sunlight	MO	95%	[86]
CuO Nanorod	Hydrothermal Method	90	Sunlight	MO	22%	[87]
CuO NPs	Co-precipitation Method	120	Xenon lamp	MO	39%	[88]
CuO NPs	Green Synthesis	41244	Visible light	MOMBMREY	80%91%89%97%	[89]
CuO NPs	Green Synthesis	120	Solar light	NBRY 160	93%81%	[90]
CuOnano leaves	Hydrothermal Synthesis	180	UV light	MBMV	89%96%	[91]
CuO NPs	Selective Method	60120	UV lightSunlight	MB	81%63%	[92]
CuO NPs	Green Synthesis	200	Sunlight	EYRh123MB	75.69%34.12%71.06%	[93]
CuO NPs	Precipitation Method	15	Visible source	MB	74%	[94]
CuO Nanosheets	RT Synthesis	6	Sunlight	AR	96.99%	[95]
CuO NPs	Green Synthesis	90	UV light source	RB	98%	[96]
CuO NPs	Microwave-assisted method	90	Sunlight	MB	99%	[97]
CuO NPs	Green Synthesis	150	Visible light	RB	84%	[98]
CuO NPs	Electrochemical Method	120	Sunlight	MBMRCR	93%90%85%	[99]
CuO NPs	Mechanochemical synthesis	60	Sunlight	MO	65%	This work
CuO NPs	Mechanochemical synthesis	60	Sunlight	MG	65%	This work

Abbreviations: NB—Nile blue; RY160—Reactive yellow; CBB—Coomassie brilliant blue; EY—Eosin yellow; Rh123—rhodamine 123; MR—Methyl red; CR—Congo red; AR—Allura red; AB—Acid black 210; RT—room temperature.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13030502/s1, Figure S1. FTIR spectrum of Seriphidium oliverianum; Figure S2: Effect of different concentrations of dyes on the degradation process; Figure S3. Effect of different concentrations of photocatalyst on the degradation process; Figure S4. Thermodynamic study of dyes degradation; Figure S5. Dyes degradation upon recycling of the photocatalyst; Table S1. Thermodynamic parameters for MG degradation on CuO NPs; Table S2. Thermodynamic parameters for MO degradation on CuO NPs; Table S3. RMSE calculations for degradation of MG; Table S4. RMSE calculations for degradation of MO. Table S5. Dyes used in this work. Refs. [102,103,104,105] are cited in supplementary materials.

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