Green Synthesis of Cobalt Oxide Nanoparticles Using Hyphaene thebaica Fruit Extract and Their Photocatalytic Application

Abstract

Cobalt oxide, a multifunctional, anti-ferromagnetic p-type semiconductor with an optical bandgap of ~2.00 eV, exhibits remarkable catalytic, chemical, optical, magnetic, and electrical properties. In our study, cobalt oxide nanoparticles (Co₃O₄ NPs) were prepared by the green synthesis method using dried fruit extracts of Hyphaene thebaica (doum palm) as a cost-effective reducing and stabilizing agent. Scanning electron microscopy (SEM) depicts stable hollow spherical entities which, consist of interconnected Co₃O₄ NPs, while energy-dispersive X-ray spectroscopy (EDS) indicates the presence of Co and O. The obtained product was identified by X-ray diffraction (XRD) that showed a sharp peak at (220), (311), (222), (400), (511) indicating the high crystallinity of the product. The Raman peaks indicate the Co₃O₄ spinel structure with an average shift of Δν~9 cm⁻¹ (191~470~510~608~675 cm⁻¹). In the Fourier transform infrared spectroscopy (FT-IR) spectrum, the major bands at 3128 cm⁻¹, 1624 cm⁻¹, 1399 cm⁻¹, 667 cm⁻¹, and 577 cm⁻¹ can be attributed to the carbonyl functional groups, amides, and Co₃O₄ NPs, respectively. The photocatalytic activity of the synthesized NPs was evaluated by degrading methylene blue dye under visible light. Approximately 93% degradation was accomplished in the reaction time of 175 min at a catalyst loading of 1 g/L under neutral pH. This study has shown that Co₃O₄ is a promising material for photocatalytic degradation.

1. Introduction

As the world’s population continues to grow, the demand for various raw materials and finished products is increasing rapidly. This trend has been driven by global economic growth and industrial revolutions, which have led to a phase of rapid industrialization. Due to this rapid pace of industrialization and urbanization, water pollution is a top priority concern globally [1]. Approximately 2.3 million individuals lose their lives every year due to water pollution-related diseases, including typhoid, diarrhea, cholera, hepatitis, and cancer, as reported by the World Health Organization (WHO) [2]. Per annum, around 100,000 various dyes are used, and approximately 10–15% of these dyes are discharged directly or indirectly into the environment, resulting in detrimental consequences for ecosystems and living organisms [3]. The presence of nitrogenous chemicals in the environment can have detrimental effects on human health, including an increased risk of cancer and the potential development of conditions such as blue baby syndrome [4]. Additionally, exposure to water contaminants during pregnancy can lead to elevated rates of low birth weight, negatively impacting fetal health [5]. Water pollution poses significant threats to marine life, including seaweed, mollusks, birds, fish, and crustaceans, all of which serve as crucial food sources for humans. Moreover, the accumulation of insecticides such as dichloro-diphenyl-trichloroethane (DDT) in the food chain poses hazards to human health [6]. To keep up with the latest technological advances, synthetic dyes are widely used in various industries due to their significant structural diversity [7,8]. The textile industry annually uses a substantial quantity of carcinogenic organic dyes such as azo dyes, volatile organic chemicals (formaldehyde), lead, and chromium, that ultimately find their way into aquatic ecosystems [9]. Therefore, it is crucial to eliminate these particles before discharging them into rivers and lakes to prevent health risks. Most of these dyes are toxic to the plants, microbes, and protozoa in the water and are resistant to discoloration and degradation, making it necessary to take action [10,11].

The presence of certain dyes in water, such as methylene blue, malachite green, direct black 38, rhodamine blue, and orange II, can impede the passage of light, which can negatively impact the development and viability of aquatic plants and algae [12,13]. This, in turn, can disrupt the oxygen levels in the water and cause harm to aquatic animals [14]. Furthermore, dyes have the potential to influence the pH and temperature of water, which may elevate concentrations of suspended solids, chlorides, nitrates, and various metals, including manganese, sodium, lead, copper, chromium, and iron. Moreover, they contribute to higher levels of biological oxygen demand (BOD) and chemical oxygen demand (COD), ultimately affecting water quality and exerting effects on the behavior, reproductive success, and survival of aquatic organisms [15]. In some cases, dyes can accumulate in the tissues of these organisms, resulting in the buildup of harmful levels of the dye in the food chain and ultimately causing the death of affected organisms [16]. Humans who consume contaminated marine life can also be adversely affected by these toxic substances [17,18,19]. A sustainability plan based on green chemistry is being scrutinized for its potential to protect and preserve the environment and its inhabitants.

One of the dyes that is known to be toxic to aquatic organisms is methylene blue (MB). It is also known as [7-(dimethylamino)phenothiazin-3-ylidene]dimethylazanium chloride, an organic dye with a positive charge that finds widespread use in dyeing various items such as paper, clothes, office supplies, and leather [20]. Most of the MB produced, around 10–15%, is typically discharged into the environment through industrial wastewater [21]. The presence of dye in industrial wastewater, even at low concentrations, can visibly hinder sunlight from penetrating the water, causing a reduction in photosynthesis and imposing strict regulations on the organic content of polluted water. Additionally, long-term exposure to MB can be detrimental to human health, leading to gastrointestinal issues, central nervous system dysfunction, and kidney damage [22]. It is worth noting that MB exhibits high resistance to degradation from environmental factors such as light, temperature, and biological activity [23]. Hence, it is of utmost importance to identify the most efficient method for eliminating MB from water to safeguard both the environment and human well-being.

Wastewater treatment techniques for dye removal are typically grouped into three categories: physical, chemical, and biological. Physical methods such as adsorption, ion exchange, oxidation, and irradiation are commonly used but can be labor-intensive, consume a lot of power, and require multistage processing [24]. However, the photocatalytic degradation process is becoming increasingly popular as an effective dye removal method [25]. It is an effective and environmentally friendly method for the degradation of dyes in wastewater. Its selectivity, efficiency, versatility, and cost-effectiveness make it a promising option for the treatment of colored effluents from different industries. This process oxidizes the large molecules of dyes into smaller by-products such as water, carbon dioxide, and minerals. It is a contemporary technique extensively utilized for the breakdown or whitening of dyes [26]. This method entails the transfer of electrons from the valence band to the conduction band on a semiconductor surface when illuminated with light of a suitable wavelength. The excitons produced then interact with water or oxygen, generating hydroxide radicals and superoxide anions, which have potent oxidizing properties for breaking down various molecules, including dyes [27]. These decontamination procedures utilizing reactive oxygen species and other reactive species are collectively known as Advanced Oxidation Processes (AOPs) [28].

Conventional methods have been effective in removing dyes from water, but they often have limitations in terms of efficiency, selectivity, and cost [29]. Among the existing chemical, physical, and biological methods for the degradation of harmful dyes, nanotechnology reductive degradation has demonstrated significant qualities [30]. Nanotechnology-based approaches for the removal of dyes from water include the use of nanomaterials such as nanoparticles, nanotubes, and nanofibers. These materials offer a high surface area-to-volume ratio, which enhances their reactivity and makes them more effective in removing dyes from water. They can also be designed to be selective towards specific dyes, which increases their efficiency and specificity [31]. Recent studies have found that transition metal oxides have drawn research interest because of their compact size and large reactive surface area, which paved their way in various fields, including catalytic, chemical, optical, magnetic, and electrical [32,33,34]. In metal oxide NPs, the cobalt oxide (Co₃O₄) NPs and their composites are well known to be suitable catalysts for water oxidation and as light absorbers with wide band gaps because of their dual activity [35,36]. The non-stoichiometric polymorph Co₃O₄ has drawn significant interest among the other numerous cobalt oxidation states that are possible, including cobaltous oxide (CoO), cobaltic oxide (Co₂O₃), and cobaltosic oxide (Co₃O₄), because of its structural, redox and highest stability characteristics [37]. Cobalt oxide (Co₃O₄) is a significant p-type semiconductor with a band gap of 3.95–2.13 eV that comprises a spinel structure with tetrahedral sites (8a) occupied by Co²⁺ ions and octahedral sites (16d) by Co³⁺ ions [38], which makes it highly stable and anti-ferromagnetic [39]. Therefore, Co₃O₄-NPs have vast applications, including lithium-ion batteries, electrochromic devices, magneto-resistive devices, sensors for different organic and inorganic substrates, pigments and dyes, supercapacitors, and selective solar absorbers [40,41]. Recently, Sonkusare et al. reported a facile approach to prepare mesoporous octahedral Co₃O₄ NPs with a very narrow size distribution as an eco-friendly catalyst for the photo-degradation of toxic dyes under visible light [42]. Immobilization of the metal oxide particles on the surface of a natural polymer, such as chitosan (CS), is one of the ways to prevent aggregation, increase their catalytic efficiency, and improve their biocompatibility properties. The hydroxyl and amino groups on the surface of CS can adsorb the dye molecules, which facilitates the photocatalytic degradation reaction [43,44].

Utilizing plants and microorganisms to synthesize NPs is a promising alternative method for producing functional materials, thanks to their diverse structures and energy storage capacity. Specifically, the use of fruit extracts that are abundant in reducing agents has recently garnered attention due to their functional versatility and accessibility [28]. Phytochemicals, including primary and secondary metabolites such as antioxidants, flavonoids, catechins, flavones, isoflavones, anthocyanidins, isothiocyanates, carotenoids, and polyphenols, play a crucial role in reduction [45]. These compounds possess strong reducing properties that enable them to facilitate the conversion of metal ions into nanoparticles [46,47,48]. Additionally, these metabolites serve as capping agents to thwart potential NPs aggregation throughout the reaction process [49]. Various chemical and physical methods have been described for the synthesis of metal oxide NPs [50]. Therefore, the process of making Co₃O₄ NPs involves several common chemical and physical techniques, including the sol–gel method, sonochemical techniques, co-precipitation, wet polymerization, thermal decomposition, micro-emulsion, hydrothermal method, and spray pyrolysis [51]. However, these methods necessitate toxic and expensive chemicals, high temperatures, and pricey and complicated equipment; they also produce hazardous by-products that adversely affect the environment and are energy-intensive [52,53]. Therefore, they demand the development of clean, sustainable, and environmentally friendly synthesis methods [54,55]. The synthesis of NPs using plants and microorganisms is a promising alternative for making Co₃O₄ NPs because of their structural diversity and capacity for energy storage [56]. Due to diverse chemical quantities and mixtures in different plant extracts, the features of the produced NPs differed depending on the source of the plant [57,58]. Successful green synthesis of Co₃O₄ NPs has been reported using a variety of plant components, including leaves, stems, roots, seeds, latex, fruits, interior plant parts, peels, and shells. Rajeswari et al. synthesized eco-friendly Co₃O₄ NPs using the aqueous extract of P. dactylifera L seeds to evaluate the antimicrobial and photocatalytic activity [59]. According to Dubey et al., Calotropis Procera latex was used to manufacture Co₃O₄ NPs [60]. Punica granatum peel extract was utilized by Bibi et al. to create Co₃O₄ NPs from cobalt nitrate hexahydrate [61].

In an effort to produce Co₃O₄ NPs without the use of acid or base, Hyphaene thebaica L. fruit extracts were used as a bio reductant, taking into account the different physicochemical nature of Co₃O₄ NPs biosynthesized utilizing plant extracts [62]. Doum is the popular name for a type of palm tree, Hyphaene thebaica, a mint family member with edible oval fruit (Arecaceae) [45,63]. Common names include gingerbread palm, doum palm, zembaba, doom palm, mkoma, kam bash, and arkobkobai [64]. The northern portion of Africa is the doom palm’s natural habitat. Hyphaene thebaica is a plant recognized for its antioxidant, anticancer, anti-inflammatory, and antimicrobial properties due to its phenolic and flavonoid content [63]. Numerous preparations of H. thebaica have been used to treat hypertension, bleeding, dyslipidemia, haematuria, and decreasing blood pressure [65,66]. The fruit of H. thebaica was chosen for green synthesis because of its therapeutic applications and ethnopharmacological significance [67]. In the current study, Co₃O₄ NPs were produced using the plant extract of H. thebaica as a natural source for the synthesis, which addresses the environmentally friendly procedure. The physical properties of the synthesized nanoparticles were studied using diverse characterization techniques such as XRD, FTIR, UV–vis, EDS, and TEM/SEM. Furthermore, their photocatalytic application was measured under visible light irradiation.

2. Materials and Methods

2.1. Materials

Cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O) was obtained from Sigma-Aldrich (Cape Town, South Africa). Distilled water was used to prepare aqueous solutions.

2.2. Preparation of Hyphaene thebaica Fruit Extract

The fruits of H. thebaica were obtained from (Aswan) Egypt, from its native habitat. The collected fruit was first washed with tap water to remove any pollutants on the fruit. After that, they were repeatedly rinsed to remove the remaining moisture or unwanted impurities such as dust and other particulates. The dry material was finely milled into a powder. The plant extract was prepared as previously reported by Mohamed et al. [62]. Five grams of fruit powder was added to a 100 mL beaker of distilled water after being washed and dried. After that, the solution was heated to 80 °C on a hot plate while constantly stirring for two hours. A pale-yellow fruit extract was filtered and cooled at room temperature. The cooled fruit extract was filtered three times with Whatman filter paper to separate the macromolecules or residual material and stored in amber glass bottles to bio-synthesize Co₃O₄ NPs.

2.3. Synthesis of Cobalt Oxide Nanoparticles (Co₃O₄ NPs)

Co₃O₄ NPs were synthesized using the green synthesis method, which used Hyphaene thebaica aqueous extract as a reducing and capping agent. Accordingly, 0.2 M solution of Co(NO₃)2·6H₂O was prepared with 100 mL fruit extract of Hyphaene thebaica in a flask. Upon mixing, the solution turned from yellow to a red-brown color. After overnight aging, the solution was transferred into an oven at 130 °C for 5 h to obtain the dried precipitate. The residue was further crushed and annealed at 500 °C for 2 h. Finally, a dark brown colored powder was obtained, and the particles were stored in an air-tight container for further characterization and photocatalytic analysis. The step-by-step process is shown in Figure 1.

2.4. Characterization of H. thebaica Co₃O₄ NPs

After the synthesis of Co₃O₄ NPs, the nanoparticles were characterized using various techniques. The crystalline structure of the synthesized nanoparticles was analyzed using X-ray diffraction (XRD). XRD patterns were recorded in the 2θ range of 20–70°. The Debye–Scherrer equation was used to estimate crystallite sizes based on the width at half maxima. In order to find the functional groups, present in the samples, an FTIR analysis was carried out within the region of 4000–400 cm⁻¹. The surface morphology and shape of the Co₃O₄ NPs were confirmed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX). The internal morphology, size distribution, and particle size analysis of the nanoparticles were carried out using transmission electron microscopy (TEM). Image J software (1.53v) was used to perform TEM diameter measurements. Finally, the optical properties were characterized using the Raman spectroscopy technique by a Thermo Fisher DXR with a laser wavelength of 780 nm and a spot size of 0.5 nm.

2.5. Photocatalytic Tests

The photocatalytic activity of the synthesized Co₃O₄ NPs was evaluated by degrading methylene blue dye. One hundred milliliters of 10 ppm methylene blue (M.B.) aqueous solution was used for the photocatalytic experiment. A total of 1 g/L of the Co₃O₄ NPs was added to M.B. aqueous solution and stirred with a magnetic stirrer in the dark for 30 min to establish adsorption–desorption equilibrium between the solution and catalyst before irradiation from the 400 W high-pressure mercury lamp (λ ≥ 420 nm). The lamp was turned on, and after particular time intervals (0, 25, 50, 75, 100, 125, 150, and 175 min), the samples were filtered using centrifugation to settle down the suspended catalyst and then examined by a UV–visible spectrophotometer in the range of 400–800 nm. The absorbance measurements estimated the degradation degree of M.B. in the aqueous solution at about 665 nm. All the aqueous samples were at neutral pH, and all experiments were carried out at room temperature.

3. Results and Discussions

3.1. XRD Analysis

X-ray diffraction patterns (XRD) for samples of H. thebaica Co₃O₄ NPs were carried out using a Bruker Advanced D8 diffractometer with monochromatic Cu Kα radiation of wavelength 1.5406 Å operating at a current of 40 mA and a voltage of 40 kV in the Bragg–Brentano geometry. The observed intensity peaks at 31.50°, 37.05°, 38.78°, 44.98°, and 59.53° represent the corresponding planes at (220), (311), (222), (400), (511), respectively. These sharp peaks confirm that the synthesized Co₃O₄ NPs are crystalline in structure as shown in Figure 2. The fabricated Co₃O₄ NPs diffraction 2θ values correspond to the face-centered cubic crystalline phase of cobalt oxide (JCPDS card 073–1701) [68]. The average crystallite size of Co₃O₄ NPs was calculated by using Debye–Scherrer’s formula.

$$D=\frac{0.9\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$$

(1)

where λ is the wavelength of Cu-kα radiation, β is the full-width half-maximum (FWHM) in radians, and θ is the angle of diffraction (in radians). The average particle size and the lattice parameter (a = b = c) were found to be 11.87 nm and 8.082 Ǻ, respectively. No peaks of contaminants were observed, indicating the pure crystal structure of Co₃O₄. Thus, XRD confirms the formation of Co₃O₄ NPs with desired crystalline structure.

3.2. Microscopic Analysis

Scanning electron microscopy (SEM)—electron dispersive X-ray spectroscopy (EDS) was carried out on a Tescan MIRA3 GMU Univac SEM with a secondary electron detector and Drawbeam advanced/offline software to examine the Co₃O₄ NPs’ morphology and structural configuration. Figure 3a,b depicts stable hollow spherical entities which consist of interconnected nanoparticles. It was observed that the Co₃O₄ NPs spontaneously arranged themselves in a sieve-like pattern and adhered to the surface, creating interior cavities and loose structures. Agglomeration could also be observed because of the reduced size, greater surface area, and the presence of biomolecules [50]. Image J software (1.53v) was also used to identify the particle size by the histogram, as shown in Figure 3c, with an average diameter of about 42.7 nm.

Moreover, in order to further confirm the pores’ structure, TEM observations were carried out using an FEI Tecnai 20 transmission electron microscope, operating at 200 kV (Lab6 emitter). The high-magnification TEM images (Figure 4a–g) indicate that the pores structure consists of interconnected nanoparticles with an average diameter of about 42.7 nm. In the large area TEM image, it can be seen that a few nanoparticles are aggregated together in the form of chains.

3.3. Chemical Composition of Co₃O₄ NPs

Furthermore, EDS was performed parallel to SEM for the elemental analysis of Co₃O₄ NPs. The EDS range was between 0 and 8 keV. Observed peaks, as depicted in Figure 5, indicate the presence of Co and O. The spectra also reveal the presence of residual chloride ion impurities. Moreover, the spectra depicted the potassium signal due to the presence of bioactive compounds in the extracts. The elemental composition of the nanoparticles shows 72.07 weight percent cobalt and 25.85 weight percent oxygen corresponding to Co₃O₄. The sharp peaks between 0 and 2 KeV and between 6 and 8 KeV correlate to crystalline Co₃O₄ NPs. All these characterizations confirm the biosynthesized nanopowder to be Co₃O₄ NPs, with a porous surface, which enhances the surface area of the biogenic Co₃O₄ NPs.

3.4. Raman Spectroscopy

Raman scattering spectrometer was used to obtain the Raman spectra, which were recorded between 130 and 700 cm⁻¹ (Figure 6). Six Raman bands are identified at 189.3, 345.2, 478.3, 522.6, 615.6, and 655.8 cm⁻¹. According to the research on Co₃O₄ NPs characterization [69,70], these peaks indicate the Co₃O₄ spinel structure with an average shift of Δν∼9 cm⁻¹ (191~470~510~608~675 cm⁻¹). This shift may be attributed to surface strain or size effects. The notable Raman peaks correspond to the F2g (189.3 cm⁻¹) Eg (345.2 and 478.3 cm⁻¹), F2g (522.6 and 615 cm⁻¹), A1g (655.8 cm⁻¹) modes of the Co₃O₄ NPs crystalline phase. The Raman mode at 655.8 cm⁻¹ is attributed to characteristics of the octahedral sites (CoO₆) associated with the A1g symmetry, whereas the Eg and F2g modes are likely related to the combined vibrations of the tetrahedral site (CoO₄) and octahedral oxygen motions.

3.5. FTIR Analysis

Figure 7 reports the FTIR spectrum of the Co₃O₄ NPs in the spectral range of 400–4000 cm⁻¹. The major bands at 3128 cm⁻¹, 1624 cm⁻¹, and 1399 cm⁻¹ can be attributed to the hydroxyl, carbonyl, and functional groups of C–N, respectively [71]. The I.R. spectrum displays two strong bands due to vibration υ(Co–O) modes at ~667 cm⁻¹ and ~577 cm⁻¹, which originate from the fingerprint stretching vibrations of the cobalt–oxygen bond of the Co₃O₄ spinel oxide [59]. More precisely, the first band at 577 cm⁻¹ is a characteristic of OB₃ vibration in the spinel lattice, with B signifying the Co³⁺ in the octahedral site (O–Co), while the 667 cm⁻¹ band is attributed to the ABO₃ vibration where A denotes the Co²⁺ in a tetrahedral hole (Co²⁺ Co³⁺O₃). In addition, some representative peaks of aromatic rings centered at about 1624 cm⁻¹ and 1399 cm⁻¹ could be attributed to the presence of hydroxyl group (O–H) or C–O stretching of alcohols and carboxylic acids due to the absorption of moisture by the nanoparticles [72]. The peak around 3128 cm⁻¹ is the distinctive peak of the hydroxyl group of polyphenolic compounds. These results were consistent with other reports in which the FTIR spectra revealed the same characteristic peaks of Co₃O₄ NPs [61].

3.6. Photocatalytic Activity

Under visible light irradiation at ambient temperature, the photocatalytic activity of the as-synthesized Co₃O₄ NPs has been tested for the oxidation of organic dyes such as methylene blue (M.B.). Figure 8 displays the UV–vis spectra of M.B. aqueous solution under visible light irradiation (>420 nm) in the presence of Co₃O₄ NPs as a photocatalyst over various time intervals. The UV–visible spectrum of M.B. dye showed a maximum absorption peak at 662 nm. However, in the presence of Co₃O₄ NPs, the absorption peak of the M.B. dye solution decreased with the increase in time due to the continuous photocatalytic degradation of M.B. molecules in the system. The dye color started to fade considerably after 25 min of visible light irradiation, and after 175 min of exposure, the color reduction reached 93.45%. Therefore, the photocatalytic experiments show that the as-prepared Co₃O₄ NPs have high visible-light photocatalytic activity and can treat dyes in textile wastewater.

The photocatalytic degradation process of Co₃O₄ NPs involves the use of light energy to initiate a chemical reaction that breaks down organic compounds into smaller, harmless molecules. This is associated with the breakdown of the chromophoric group and the transformation of dye into low-molecular-weight by-products [73]. The dye degradation is mainly due to generating electrons and holes (e⁻ & h⁺) on the catalyst surface under irradiation [74]. Under visible light, Co₃O₄ NPs undergo charge separation, and electrons in the valence band of Co₃O₄ can be excited to its conduction band (e⁻ C.B.), causing the generation of holes in the valance band (h⁺ V.B.) simultaneously (Equation (2)). Thus, the OH· generated from the reaction (Equation (3)) is the main factor for the photodegradation of the dye because it is a strong oxidizing species, known to be an oxidation process. These ROSs can then react with organic molecules, such as pollutants or contaminants, breaking them down into smaller, less harmful compounds such as carbon dioxide and water.

Co₃O₄ + hv → e⁻ CB + h⁺ VB

(2)

H₂O + h⁺ VB → H⁺ + OH

(3)

MB + OH·→ degradation products (CO₂, H₂O, NO₃, etc.)

(4)

The average C.R. degradation percentage was calculated using the following equation.

Degradation efficiency (%) = (Co − Ct)/Co × 100

(5)

where Co was the initial concentration of the M.B. dye and Ct was the concentration of the dye after photocatalytic degradation. To optimize the photocatalytic degradation process of Co₃O₄ NPs, several factors need to be considered, including the type of organic compound being degraded, the intensity and wavelength of the light source, the size and shape of the nanoparticles, and the presence of other compounds in the environment.

4. Conclusions

In conclusion, Co₃O₄ NPs with an average particle size of 42.7 nm have been successfully prepared by a cost-effective and eco-friendly green synthesis method using Hyphaene thebaica L. fruit extracts. The prepared nanoparticles were analyzed by various techniques such as FTIR, XRD, Raman, TEM, and SEM. These techniques revealed the successful synthesis of Co₃O₄ NPs. The photocatalytic activity of synthesized Co₃O₄ NPs was analyzed. The as-prepared Co₃O₄ NPs exhibit excellent photocatalytic activity for the degradation of M.B. dye under visible light irradiation. Hence, it can be successfully employed in the treatment of textile and pharmaceutical industrial wastewater to remove the dye.