Green Photocatalysis: A Comprehensive Review of Plant-Based Materials for Sustainable Water Purification

Abstract

Green synthesis represents a sustainable, reliable, and eco-friendly approach for producing various materials and nanomaterials, including metal and metal oxide nanoparticles. This environmentally conscious method has garnered significant attention from materials scientists. In recent years, interest in plant-mediated nanoparticle synthesis has grown markedly, owing to advantages such as enhanced product stability, low synthesis costs, and the use of non-toxic, renewable resources. This review specifically focuses on the green synthesis of metal oxide nanoparticles using plant extracts, highlighting five key oxides: TiO₂, ZnO, WO₃, CuO, and Fe₂O₃, which are prepared through various plant-based methods. The release of toxic effluents like synthetic dyes into the environment poses serious threats to aquatic ecosystems and human health. Therefore, the application of biosynthesized nanoparticles in removing such pollutants from industrial wastewater is critically examined. This paper discusses the synthesis routes, characterization techniques, green synthesis methodologies, and evaluates the photocatalytic performance and dye degradation mechanisms of these plant-derived nanoparticles.

1. Introduction

In recent years, nanotechnology has emerged as a transformative discipline in materials science, serving as a powerful tool that has significantly contributed to advancements in various scientific, industrial, and technological domains worldwide. Metal oxide nanoparticles (NPs) show exceptional promise for applications in photocatalysis, environmental remediation, and biomedical fields. The distinctive capability of this technology lies in its ability to enable the fabrication of materials with dimensions ranging from 1 to 100 nm [1,2,3,4]. Nanotechnology relies on two fundamental approaches for the fabrication of nanomaterials: the top-down and bottom-up strategies (Figure 1). The top-down approach involves breaking down bulk materials into nanoscale structures through physical or mechanical processes. Techniques such as scanning lithography, photolithography, nanosphere lithography, soft lithography, imaging probe lithography, dispersion lithography, optical machining, deposition, ion implantation, and diffusion are commonly employed. Despite its usefulness, the top-down method presents several limitations, including high processing costs, extended etching durations, and the potential introduction of defects or imperfections in the resulting nanostructures. In contrast, the bottom-up approach constructs nanoparticles starting from individual atoms, ions, or molecules, allowing for the formation of well-defined nanostructures through controlled nucleation and growth. This approach encompasses a variety of chemical and physicochemical methods such as wet chemical synthesis, the sol–gel process, metal–organic decomposition, atomic or molecular condensation, chemical vapor deposition, spray pyrolysis, laser pyrolysis, and aerosol-based techniques [5,6,7,8,9,10]. In the realm of nanotechnology, the synthesis of nanoparticles (NPs) generally falls into three primary categories: chemical, physical, and biological (commonly referred to as green synthesis). The chemical approach comprises a broad spectrum of techniques, including but not limited to chemical reduction, sol–gel processing, hydrothermal synthesis, ion-exchange processes, co-precipitation, solvothermal methods, template-assisted synthesis, spray pyrolysis, thermal decomposition, the reflux method, chemical vapor deposition, solution combustion, colloidal methods, and polymer-based techniques [11,12,13,14,15,16,17,18,19,20,21,22,23,24]. The physical approach involves methodologies such as vapor-phase synthesis, mechanical milling, sono-chemical synthesis, physical vapor deposition, sputtering, electrochemical methods, laser ablation, laser pyrolysis, microemulsion techniques, microwave-assisted synthesis, arc discharge, evaporation-condensation, the polyol process, and pulsed wire discharge [25,26,27,28,29,30,31,32,33,34,35,36,37,38]. The biological approach, or green synthesis, has gained substantial attention as a sustainable and environmentally friendly alternative to conventional methods. This strategy employs biological entities such as plant extracts [39], bacteria [40], fungi [41], and yeast [42] as natural reducing and stabilizing agents in the fabrication of nanoparticles. Owing to its biocompatibility, simplicity, and reduced ecological impact, green synthesis is increasingly viewed as a promising route for large-scale and low-toxicity nanomaterial production [43]. Plant-mediated synthesis is especially advantageous due to its cost-effectiveness, simplicity, and ability to utilize various plant parts such as leaves, roots, peels, and seeds, thereby enhancing versatility and resource efficiency. Additionally, phytochemicals in these extracts act as capping agents, improving nanoparticle stability and functional performance. A growing body of research demonstrates the application of green-synthesized nanoparticles (TiO₂, ZnO, WO₃, CuO, Fe₂O₃) in photocatalysis, particularly for the degradation of organic pollutants under visible or UV light. However, photocatalytic efficiencies vary based on synthesis routes and require systematic evaluation.

This review systematically examines the green synthesis, structural characterization, and photocatalytic performance of TiO₂, ZnO, WO₃, CuO, and Fe₂O₃ nanoparticles prepared using plant extracts via different methods, with the aim of highlighting their environmental remediation potential.

2. Mechanism of Photocatalysis

Reactive oxygen species (ROS) are fundamental in understanding the underlying mechanisms of photocatalytic degradation. At the onset of the process, highly reactive ROS such as superoxide anions (O₂⁻), hydroxyl radicals (${\mathrm{O}\mathrm{H}}^{\mathrm{⦁}}$), sulfate radicals (SO₄^•−), and singlet oxygen (¹O₂) are generated [44]. These species possess strong redox potentials, making them effective oxidants capable of breaking down and mineralizing organic pollutants into environmentally benign end-products like carbon dioxide (CO₂) and water (H₂O). According to Mallah et al. [45], the interaction between ROS and organic contaminants during photocatalysis occurs through three distinct phases: Photoexcitation, Radical formation and pollutant trapping, and Oxidative pollutant degradation. In the Photoexcitation stage, when a photon with energy equal to or greater than the bandgap of the semiconductor strikes the material, it excites an electron from the valence band (VB) to the conduction band (CB). This excitation leaves behind a hole (h⁺) in the VB while the electron (e⁻) now occupies the CB (Equation (1)). If the electron and hole remain weakly bound by Coulombic attraction, they are collectively referred to as an exciton. The excited electrons typically migrate toward the surface of the photocatalyst, enabling subsequent redox reactions. In the Radical formation and pollutant trapping stage, the photogenerated charge carriers initiate surface reactions leading to the generation of reactive oxygen species (ROS). Electrons in the CB reduce adsorbed oxygen molecules, producing superoxide radicals (${\mathrm{O}}_2^{\mathrm{⦁}-}$), which can subsequently yield hydrogen peroxide and additional hydroxyl radicals (${\mathrm{O}\mathrm{H}}^{\mathrm{⦁}}$) (Equation (2)). Concurrently, holes in the VB oxidize surface hydroxyl groups or water molecules, resulting in the formation of highly reactive hydroxyl radicals (${\mathrm{O}\mathrm{H}}^{\mathrm{⦁}}$) (Equations (3)–(5)). At the same time, pollutants become adsorbed onto the catalyst surface, a critical step that enhances the likelihood of direct interactions between the generated ROS and the contaminant molecules. Finally, in the Oxidative pollutant degradation stage, the ROS attack and progressively degrade the adsorbed pollutants. Hydroxyl radicals, with their high oxidation potential, indiscriminately cleave C–C, C–H, and C=C bonds, while superoxide radicals act both directly and indirectly by producing secondary oxidizing species. These processes convert complex organic molecules into smaller intermediates, such as alcohols, aldehydes, and carboxylic acids, which are ultimately mineralized into environmentally benign end products, primarily CO₂, H₂O, and inorganic ions (Equation (6)) [45,46]. Figure 2 schematically illustrates this three-step photocatalytic mechanism. It should be noted that the photocatalytic efficiency is strongly influenced by the catalytic conditions employed during nanoparticle synthesis and photocatalysis. Parameters such as calcination temperature, pH, irradiation source, nanoparticle dosage, and pollutant concentration strongly influence photocatalytic performance. In addition, the type of photocatalyst (e.g., doped photocatalysts, bio-nanocomposites, or clay-based photocatalysts) can alter the morphology, particle size, surface area, and crystallinity of the nanoparticles. These structural features in turn affect the bandgap energy and charge carrier dynamics, including the generation, separation, and lifetime of electron–hole pairs, which are directly linked to ROS formation. For example, smaller particles with higher surface area provide more active sites and facilitate charge separation, enhancing ROS production and pollutant degradation. By considering these experimental factors, the photocatalytic mechanism can be more accurately correlated with the structural and electronic properties of the nanoparticles, providing a comprehensive understanding of their performance [45].

$$\mathrm{P}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+\mathrm{h}\mathsf{\nu }\to {\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}+{\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}$$

(1)

$${\mathrm{O}}_2+{\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\to {\mathrm{O}}_2^{\mathrm{⦁}-}$$

(2)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\to {\mathrm{H}}^{+}+{\mathrm{O}\mathrm{H}}^{\mathrm{⦁}}$$

(3)

$$2{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}+2{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{H}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2$$

(4)

$${\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{O}\mathrm{H}}^{\mathrm{⦁}}$$

(5)

$$\mathrm{R}\mathrm{O}\mathrm{S}+\mathrm{P}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}\to \mathrm{S}\mathrm{i}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{r} \mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{d}+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(6)

In heterogeneous photocatalysis, a wide range of semiconductors have been utilized as photocatalysts to promote the degradation of dyes into smaller, harmless molecules such as CO₂ and H₂O. These semiconductor materials exhibit different band gap energies, which influence their light absorption capabilities. For example, TiO₂, ZnO, SnO₂, and CuO have been extensively investigated, with respective band gap energies of approximately 3.2 eV, 3.4 eV, 3.6 eV, and 1.9 eV [47,48,49,50].

3. Plant-Mediated Green Synthesis of Nanoparticles

Given the toxic and hazardous effects associated with conventional synthesis methods, green synthesis has emerged as a promising and sustainable alternative [51]. Biological approaches to nanoparticle synthesis typically offer advantages such as mild reaction conditions, the absence of toxic reagents, and the use of eco-friendly, cost-effective resources, as demonstrated in Table 1. A variety of biological systems, including plants, algae, yeast, and fungi, are employed in nanoparticle fabrication [52] (Figure 3). Among these, plant-based or phyto-mediated synthesis stands out due to its rapidity, simplicity, low production cost, and access to a vast and diverse range of natural resources. Notably, phyto-synthesis can utilize different parts of the plant, including fruits, roots, stems, and peels, thus enhancing its versatility and sustainability [53]. One of the notable advantages of green synthesis is its versatility in utilizing nearly all parts of a plant for nanoparticle production, including seeds and seed waste [54], arils [55], fruits [56], flowers [57], roots [58], and others.

The emergence of green technology, particularly through the biological or green synthesis of nanomaterials (NMs), marks the beginning of a new era in safe and sustainable nanotechnology. Consequently, this approach has gained significant attention in recent years due to its environmental and health-related benefits [59].

Table 1. Advantages and disadvantages of Plant-Mediated Photocatalysts [60].

Aspect	Advantages	Limitations
Eco-friendliness	Use non-toxic, biodegradable plant extracts; environmentally sustainable.	Some processes may still involve non-green conditions (e.g., high temperatures, long synthesis time).
Phytochemical content	Rich in natural compounds (e.g., flavonoids, phenolics, tannins) that serve as reducing and stabilizing agents.	Phytochemical composition varies by species, plant part, region, and season, affecting reproducibility.
Simplicity and cost-efficiency	Requires no sterile conditions, complex culturing, or expensive equipment.	Industrial scalability is limited; plant materials may require storage or preservation.
Safety	No use of hazardous chemicals; safe for researchers and the environment.	Lack of precise control over reaction conditions may result in inconsistent product quality.
Photocatalytic performance	Produces TiO2 NPs with small particle sizes, reduced bandgap, and high dye degradation efficiency (>90%).	Some materials show reduced reusability or stability after one photocatalytic cycle.
Versatility	Enables synthesis of various NP shapes (spherical, tubular, etc.) with good surface properties.	Irregular particle shapes and low crystallinity may occur, limiting some functional applications.
Mechanistic understanding	Offers natural and sustainable synthesis pathways.	The exact reaction mechanisms are still not fully understood, limiting optimization strategies.

Table 1. Advantages and disadvantages of Plant-Mediated Photocatalysts [60].

Aspect	Advantages	Limitations
Eco-friendliness	Use non-toxic, biodegradable plant extracts; environmentally sustainable.	Some processes may still involve non-green conditions (e.g., high temperatures, long synthesis time).
Phytochemical content	Rich in natural compounds (e.g., flavonoids, phenolics, tannins) that serve as reducing and stabilizing agents.	Phytochemical composition varies by species, plant part, region, and season, affecting reproducibility.
Simplicity and cost-efficiency	Requires no sterile conditions, complex culturing, or expensive equipment.	Industrial scalability is limited; plant materials may require storage or preservation.
Safety	No use of hazardous chemicals; safe for researchers and the environment.	Lack of precise control over reaction conditions may result in inconsistent product quality.
Photocatalytic performance	Produces TiO2 NPs with small particle sizes, reduced bandgap, and high dye degradation efficiency (>90%).	Some materials show reduced reusability or stability after one photocatalytic cycle.
Versatility	Enables synthesis of various NP shapes (spherical, tubular, etc.) with good surface properties.	Irregular particle shapes and low crystallinity may occur, limiting some functional applications.
Mechanistic understanding	Offers natural and sustainable synthesis pathways.	The exact reaction mechanisms are still not fully understood, limiting optimization strategies.

4. Green Synthesis of Oxide-Based Photocatalysts Using Plant Extracts

Nanomaterials are typically synthesized using two main approaches: top-down and bottom-up methods, each differing in terms of quality, processing speed, and cost-effectiveness. Plant-extract-mediated synthesis processes (PPS) fall under the bottom-up approach of nanomaterial fabrication [61]. In most chemical methods, nanoparticles (NPs) are produced through the interaction of three key components: a metal precursor, a reducing agent, and a capping or stabilizing agent. In the case of PPS synthesis, biomolecules present in plant extracts serve as eco-friendly alternatives to conventional, often toxic, reducing and capping agents such as sodium borohydride and polyvinyl alcohol [62]. The characteristics and properties of the resulting NPs are largely influenced by the chosen synthesis route. Moreover, even the final stages of synthesis, particularly the separation technique (typically centrifugation) and the purification method, play a crucial role in determining the nature and quality of the synthesized NPs [63]. Although the biological synthesis of PPS is chemically assisted, it is distinguished by the substitution of synthetic agents with plant-derived biomolecules. Several methods have been employed for PPS synthesis, and key approaches are outlined below.

In the field of nanoparticle synthesis, several wet-chemical methods have been developed to ensure controlled particle size, morphology, and surface characteristics. Among these, the hydrothermal route was used for the synthesis of tungsten oxide (WO₃) NPs using Tamarindus indica leaf extract [64]. The co-precipitation method is widely used, involving the simultaneous precipitation of metal ions, usually as hydroxides in a common aqueous medium. This process includes key steps such as nucleation, particle growth, and agglomeration. Aloe vera and Beetroot extract are used to synthesize TiO₂ [65] and ZnO [66], respectively. It offers significant advantages, including low operating temperatures, minimal energy requirements, exclusion of organic solvents, and enhanced control over nanoparticle morphology as demonstrated in biosynthesis of CuO from Euphorbia alata [67] et Bougainvillea [68]. Another frequently employed approach is the sol–gel method, which is based on the hydrolysis and polycondensation of metal alkoxides or salts to form a colloidal sol that gradually evolves into a gel-like network. The sol–gel process enables the synthesis of highly homogeneous and pure nanoparticles, often at relatively low temperatures such as synthesis of TiO₂ by Tinospora cordifolia leaf [69] and Mulberry extract [70]. In this review, we provide a comprehensive overview of the green synthesis, physicochemical characterization, and photocatalytic performance evaluation of five widely studied metal oxide nanoparticles: titanium dioxide (TiO₂), zinc oxide (ZnO), tungsten trioxide (WO₃), cupric oxide (CuO), and hematite (Fe₂O₃). These metal oxides have been synthesized using a variety of plant-derived extracts, which serve as natural reducing, stabilizing, and capping agents, offering an eco-friendly and sustainable alternative to conventional chemical routes. Different synthesis approaches including co-precipitation, sol–gel, and hydrothermal methods are examined in detail to highlight their influence on the morphological, structural, and optical properties of the resulting nanoparticles. Furthermore, the photocatalytic activity of these bio-synthesized nanomaterials is discussed in the context of organic dye degradation under visible or UV light irradiation, emphasizing their potential for environmental remediation applications.

4.1. Titanium Dioxyde NPs (TiO₂)

Titanium dioxide (TiO₂) is among the most efficient and widely studied photocatalysts due to its excellent stability, non-toxicity, and strong oxidative power. For this reason, it has been extensively synthesized and investigated, particularly through green synthesis approaches that utilize plant-based extracts as environmentally friendly alternatives to conventional chemical methods. TiO₂ nanoparticles were synthesized via a green route by mixing equal volumes of titanium isopropoxide (TTIP) and Tinospora cordifolia extract at 50 °C under stirring. A color change from white to pale yellow indicated the reduction of Ti⁴⁺ ions. The formed precipitate was filtered, washed with ethanol, dried, ground into powder, and calcined at 400 °C (Figure 4).

The XRD analysis confirms that the synthesized TiO₂ nanoparticles are highly crystalline and predominantly anatase phase, with no detectable impurities or secondary phases (Brookite, Rutil), such as the most intense peak at 2θ = 25.33°, corresponding to the (101) plane, which is a characteristic diffraction peak of anatase-phase TiO₂. Other identified anatase peaks are shown in

Table 2. XRD Peak positions and corresponding crystal planes of green TiO₂.

2θ	25.33°	37.90°	47.89°	53.90°	54.94°	62.74°	70.18°	74.96°
Crystal plane	(101)	(004)	(200)	(105)	(211)	(204)	(220)	(215)

. Anatase phase is known for its high photocatalytic activity [69].

FTIR analysis of Tinospora cordifolia extract and green-synthesized TiO₂ nanoparticles confirmed the presence of key functional groups involved in the synthesis process. Characteristic Ti–O stretching at 791 cm⁻¹ and broad bands between 1000–500 cm⁻¹ indicated the anatase phase. The nanoparticles displayed an average crystallite size of 15.02 nm, a slightly reduced band gap of 3.13 eV compared to commercial TiO₂ (3.20 eV), and an irregular morphology, suggesting enhanced optical properties through green synthesis Figure 5 [69].

Green-synthesized TiO₂ nanoparticles demonstrated excellent photocatalytic activity, achieving 94.43% degradation of Acid Blue 113 dye under UV light within 80 min (Figure 6) [69].

In another preparation by Vanda Wellia et al, using the rind of Aloe vera (L.) Burm extract as a green-capping agent, TiO₂ nanoparticles were synthesized by adding 5 mL of TiCl₄ (1 M) to deionized water, followed by the dropwise addition of Aloe vera rind extract (4% v/v). The pH was adjusted to 7 with NaOH, and the mixture was stirred for 24 h. The formed precipitate was collected by centrifugation, washed with ethanol and water, dried at 100 °C, and calcined at 500 °C. The product was labeled TOAv4. Similar procedures were followed using 10% and 20% extract concentrations (TOAv10 and TOAv20). A control sample without extract, labeled TO, was also prepared for comparison. Full sample details are provided in

Table 3. Variation in precursor and Aloe vera (L.) Burm. f. rind extracts volume ratio [65].

Sample Code	Percent Extact (v/v)	Volume (mL) Ratio Precursor/Extract
TO	0	5:0
TOAv4	4%	5:0.2
TOAv10	10%	5:0.5
TOAv20	20%	5:1

. [65].

The obtained nanoparticles exhibited the anatase phase with minor NaCl impurities and UV absorption between 326–330 nm. FTIR and EDS analyses confirmed the presence of Ti–O bonds and functional bioactive compounds from the extract. Thermal analysis validated anatase formation between 400–500 °C [71,72,73,74,75]. SEM revealed morphologies ranging from spherical to rod-like, with particle sizes between 14 and 23.4 nm. The TOAv10 sample had the highest surface area, while TOAv4 demonstrated the best photocatalytic efficiency, degrading 51.44% of methylene blue, outperforming other formulations as illustrated in Figure 7 [65]. However, it is important to note that key experimental parameters such as the duration of the photodegradation process, the initial concentration of pollutant, and the amount of TiO₂ nanoparticles used were not specified in the study. The absence of these critical details limits the ability to fully assess and compare the photocatalytic efficiency and reaction kinetics. This underlines the necessity of clearly reporting all relevant experimental conditions in photocatalysis research to ensure the reproducibility and scientific validity of the results.

Then, in the research of Ghulam Nabi et al, lemon peel extract was used, and TiO₂ NPs were obtained with spherical anatase crystals with the size 80–140 nm (Table 3). TiO₂ nanoparticles were prepared by mixing 1.25 g of bulk titania, 2.5 mL of distilled water, and 35 mL of lemon peel extract. A color shift from white to yellow confirmed the reaction. The mixture was stirred for 5 h at room temperature, followed by centrifugation, triple washing, air-drying, and calcination at 500 °C for 2 h. The absorption spectrum confirmed the formation of TiO₂ nanoparticles with a band gap of 3.08 eV (Table 3). These synthesized nanoparticles demonstrated over 70% photocatalytic efficiency, outperforming commercial TiO₂ [76]. However, the study did not specify the initial concentration of the pollutant or the amount of TiO₂ nanoparticles used during the photocatalytic tests. The lack of such critical experimental details hinders a comprehensive evaluation of the reported efficiency and limits the reproducibility and comparability of the results. Titanium dioxide nanoparticles with pure anatase phase were synthesized via a green, cost-effective method using Citrus limetta extract; the synthesis steps are illustrated in Figure 8. Characterization by XRD, SEM, EDS, and UV–Vis confirmed their successful formation with particle sizes between 80–100 nm and a band gap of 3.22 eV (

Table 4. Different plant extracts and precursors were used to synthesize TiO₂ NPs.

Plant Extract	Metal Precursor	Size (nm)	BG (eV)	Ref.
Aloe vera	TiO2	14–24	3.15–3.18	[65]
Tinospora cordifolia	TiO2	15.02	3.13	[69]
Mulberry	TiO2	24	3.16	[70]
Lemon peel	TiO2	80–140	3.08	[76]
Citrus limetta	TiO2	80–100	3.22	[77]
Syzygium cumini	TiO2	10	3.48	[78]
Piper betel	TiO2	6.6	-	[79]
Ocimum tenuiflorum	TiO2	7.0	-	[79]
Moringa oleifera	TiO2	6.6	-	[79]
Mentha spicata	TiO2	11	3.22–3.25	[80]
Terminalia catappa and carissa carandas	TiO2	10–21	3.21	[81]
Trigonella foenum-graecum	TiO2	40–60	-	[82]
Coriandrum sativum	TiO2	6.8	-	[79]
Monsonia burkeana	TiO2	8.93	3.53	[83]
Jatropha curcas	TiO2	13	3.28	[84]

). The nanoparticles demonstrated high photocatalytic efficiency, degrading over 90% of Rhodamine B within 80 min. This performance is attributed to their small size, clean surface, and high surface-to-volume ratio, highlighting their potential for eco-friendly wastewater treatment applications [77]. In a study conducted by Annin K. Shimi et al., TiO₂ nanoparticles were synthesized by using Mulberry plant extract as a natural reducing agent. A 1 M solution of titanium isopropoxide (TTIP) was mixed with 10 mL of Mulberry extract and stirred at 80 °C and 800 rpm. A milky white precipitate formed within 30 min, indicating the formation of nanophase materials. The precipitate was then centrifuged at 15,000 rpm for 10 min, with the process repeated three times. The resulting samples were filtered using Whatman No. 1 filter paper and subsequently dried at 100 °C for 1 h. The dried powders were stored in a desiccator for further characterization. This study highlights an effective and eco-friendly route for producing TiO₂ nanoparticles. In this study, TiO₂ nanoparticles were successfully synthesized using mulberry plant extract. The obtained nanoparticles exhibited a wide band gap and a pure anatase crystalline phase, confirmed by the presence of the dominant (101) peak at 25.38°, which is known for its superior photocatalytic activity, was observed by the XRD pattern exhibited in Figure 9. Under UV illumination, the TiO₂ nanoparticles achieved 96% degradation of methylene blue within 120 min. The photocatalytic process followed pseudo-first-order kinetics with a rate constant of 0.02398 min⁻¹, indicating strong catalytic efficiency Figure 10. Furthermore, the nanoparticles demonstrated good stability and reusability over five consecutive cycles Figure 11. Antibacterial tests showed that TiO₂ exhibited significant antibacterial activity, particularly against the Staphylococcus aureus strain [70].

TiO₂ nanoparticles were successfully synthesized using Syzygium cumini leaf extract, and their application as photocatalysts for industrial wastewater treatment resulted in significant removal of Pb²⁺ (82.53%) and COD (75.5%), both following first-order kinetics [78]. In a comparative study on the green synthesis of TiO₂ nanoparticles, four different plant leaf extracts, Piper betel, Ocimum tenuiflorum, Moringa oleifera, and Coriandrum sativum, were employed as natural reducing and stabilizing agents. The aim was to evaluate the influence of each extract on the physicochemical properties and photocatalytic performance of the synthesized TiO₂. In this green synthesis protocol, 5 mL of leaf extract (Piper betel, Ocimum tenuiflorum, Moringa oleifera, or Coriandrum sativum) was mixed with 5 mL of titanium tetraisopropoxide and 25 mL of double-distilled water. The reaction mixture was stirred continuously using a magnetic stirrer at 70 °C for 3 h. Upon cooling to room temperature (30 °C), a white precipitate was formed and subsequently separated by filtration using 0.5 mm Whatman filter paper. The obtained solid was then subjected to calcination at 400 °C for 3 h in a muffle furnace. The resulting TiO₂ nanoparticles were cooled and prepared for further physicochemical characterization. Among the four-leaf extracts tested Piper betel (PB), Ocimum tenuiflorum (OT), Moringa oleifera (MO), and Coriandrum sativum (CS), Moringa oleifera extract proved to be the most effective reducing agent and demonstrated superior photocatalytic performance in the degradation of Malachite Green dye (

Table 5. Comparative studies of green-TiO₂ on the photocatalytic activities.

Photocatalyst	Plant Used	Pollutant	Experimental Conditions	% Degradation	Ref.
TiO2	Aloe vera	MB	-Calcination temperature: T = 500 °C (1 h) -Irradiation source: UV light	48%, 51.44%, 47.82%, 49.29% (without plant extract, with 4%, 10%, 20%, respectively)	[65]
TiO2	Tinospora cordifolia	Acid Blue 113 dye	-Calcination temperature: T = 400 °C; -Irradiation source: UV light -Time of photocatalysis: 80 min; -Pollutant concentration: 50 mg/L -Amount of NPs: 2 g/L; -pH = 4	94.43%	[69]
TiO2	Mulberry	MB	-Irradiation source: UV light; -Time of photocatalysis: 120 min -Pollutant concentration: 10 ppm (10 mL); -Mass of NPs: 10 mg	96%	[70]
TiO2	Lemon peel	RhB	-Calcination temperature: T = 500 °C (2 h); -Irradiation source: UV light -Time of photocatalysis: 120 min	>70%	[76]
TiO2	Citrus limetta		-Calcination temperature: 550 °C (2 h); -Irradiation source: UV light; -Time of photocatalysis: 80 min; -Pollutant concentration: 10 mg/L (50 mL); -Mass of NPs: 0.7 g.	>90%	[77]
TiO2	Syzygium cumini	Pb	-Calcination temperature: 570 °C (3 h); -Irradiation source: UV light; -Time of photocatalysis: 17 h; -pollutant concentration: 8.6 ppm (500 mL); -Mass of NPs: 0.3 g.	75.5%	[78]
		COD	-Calcination temperature: 570 °C (3 h); -Irradiation source: UV light; -Time of photocatalysis: 17 h; -Pollutant concentration: 8450 mg/L (500 mL); -Mass of NPs: 0.3 g.	82.53%
TiO2	Piper betel	Malachite green dye.	-Calcination temperature: T = 400 °C (3 h); -Irradiation source: Solar light; -Time of photocatalysis: 30 min; -pollutant concentration: 100 ppm (50 mL); -Mass of NPs: 100 mg	~50%	[79]
	Ocimum tenuiflorum			~70%
	Moringa oleifera			~100%
	Coriandrum sativum			~60%
TiO2	Mentha spicata	Reactive Black 5	-Calcination temperature: T = 400 °C; -Irradiation source: UV light -Time of photocatalysis: 90 min; -Pollutant concentration: 5 ppm (75 mL) -Mass of NPs: 0.075 g	96%	[80]
TiO2	Monsonia burkeana	MB	-Calcination temperature: T = 500 °C (1 h); -Irradiation source: UV light -Time of photocatalysis: 120 min; -Pollutant concentration: 20 mg/L -Mass of NPs: 60 mg; -pH = 10	85.5%	[83]
TiO2	Jatropha curcas	COD	-Calcination temperature: T = 450 °C (3 h); -Irradiation source: Solar light. -Time of photocatalysis: 5 h; -Pollutant concentration: 1428 mg/L.	82.26%	[84]

) [79]. TiO₂ anatase nanoparticles have been prepared via extraction of Mentha spicata in water, a process that combined hydrothermal activation at two different temperatures: M0 (150 °C for 8 h) and M0-H (180 °C for 12 h), followed by heating at 400 °C. The extract of the plant served as a stabilizer and a capping agent that enabled the photocatalytic and antimicrobial performances of the material to be doubled. The sample M0-H showed the highest photocatalytic activity for RB5 dye degradation, which can be attributed to its larger specific surface area, higher pore volume, better dispersion, and smaller particle size (8–12 nm). Moreover, the plant-assisted TiO₂ possessed increased hydroxyl content, reduced lattice oxygen, and slightly higher band gap energies (3.22 eV for M0 and 3.25 eV for M0-H) in comparison to the reference. Such improvements in structure and surface properties cause more dye adsorption, better light absorption, as well as less charge recombination, which in turn results in higher photocatalytic efficiency (Figure 12) [80].

Table 5 presents a comparative study of TiO₂ nanoparticles synthesized via green methods using various plant extracts. In addition to reporting the photocatalytic efficiency of the synthesized nanoparticles, the table provides detailed experimental conditions used during synthesis, including the type of plant extract, calcination temperature, irradiation source, photocatalysis time, pollutant type and concentration, nanoparticle dosage, and solution pH. The systematic consideration of these parameters is crucial, as they strongly influence the structural, optical, and catalytic properties of the nanoparticles, thereby determining their efficiency and applicability in wastewater treatment. The use of different plant sources not only provides environmentally friendly synthesis routes but also affects key characteristics such as particle size, surface area, crystallinity, and dye degradation efficiency. This study underscores the role of plant-mediated synthesis in tuning the functional properties of TiO₂ nanoparticles for enhanced photocatalytic applications. Comparable analyses of ZnO, CuO, and Fe₂O₃ are also presented in this review, providing an integrated perspective for cross-material comparison and facilitating the identification of both common trends and material-specific behaviors in photocatalytic efficiency.

4.2. Zinc Oxide NPs (ZnO)

This section discusses the preparation of ZnO nanoparticles from various plants such as Beetroot, Myrtus communis, spinach leaves, and Camellia sinensis. ZnO nanoparticles were synthesized using Beetroot extract. The extract was heated to 60 °C, followed by the addition of zinc acetate. The pH was adjusted to 8, and the mixture was stirred at this temperature for 40 min until a pale-yellow solution formed. The resulting product was centrifuged, washed with double-distilled water, and dried at 80 °C. Finally, the dried powder was calcined at 450 °C for 15 min [66]. In the same study Darshan Singh synthesizes calcium-coated ZnO by the coprecipitation method Zn (CH₃COO)₂. 2H₂O and calcium chloride. Zinc acetate was dissolved in the extract at 60 °C and pH ~8, followed by the addition of calcium chloride. The mixture was stirred for one hour, then centrifuged to isolate the nanocomposite. After washing with double-distilled water to remove impurities, the product was dried and calcined at 450 °C for 15 min in a muffle furnace [85]. The structural and morphological characteristics of zinc oxide (ZnO) and calcium-coated zinc oxide (Ca-ZnO) nanoparticles were analyzed using SEM, TEM, EDX, and XRD techniques. SEM and TEM images confirmed a uniform distribution of ZnO nanoparticles with a hexagonal shape and particle sizes ranging from 20 to 100 nm. TEM analysis indicated that calcium coating did not alter the morphology of the ZnO nanoparticles, consistent with previous findings [86]. EDX analysis verified the elemental composition, revealing the presence of zinc and oxygen in ZnO, and calcium, zinc, and oxygen in Ca-ZnO. Chlorine traces were also detected due to calcium salt contamination. XRD patterns showed that the synthesized ZnO nanoparticles crystallized in the hexagonal wurtzite phase, characterized by strong and sharp diffraction peaks. Similar patterns were observed for Ca-ZnO nanocomposites with 1:1 and 1:2 ratios, as shown in Figure 13 [66].

This study demonstrates the photocatalytic efficiency of both uncoated (blank) and calcium-coated ZnO nanoparticles (ratios 1:1 and 1:2) in degrading organic dyes. The uncoated ZnO nanoparticles achieved 100% degradation of methylene blue within 40 min, while the calcium-coated variants showed 80–90% degradation after 60 min. However, calcium coating significantly reduced the photocatalytic performance, particularly in the case of rhodamine B, where the degradation dropped from 55.2% (blank ZnO) to 5.4% and 3.4% for the 1:1 and 1:2 Ca-ZnO ratios, respectively. These findings suggest that while blank ZnO nanoparticles exhibit high photocatalytic activity, especially with methylene blue, the calcium-coated versions result in slower or negligible degradation. Nonetheless, both forms show potential for application in wastewater treatment targeting toxic organic pollutants [66]. A similar analysis of the photocatalytic degradation of methylene blue (MB) using ZnO nanoparticles synthesized with Vitex negundo extract showed nearly 98.50% degradation within 60 min [87]. The same type of analysis on MB dye photocatalytic degradation with ZnO-Myrtus communis NPs also resulted in nearly 94% of ZnO-Acetate and 95% of ZnO-Nitrate degradation in 30 min. Fresh Myrtus communis leaves were dried, ground, and extracted using 70% ethanol at 100 °C for 1 h. The filtered extract was then used to synthesize zinc oxide nanoparticles (ZnO NPs) by adding zinc nitrate or zinc acetate, followed by calcination at different temperatures (400, 600, and 800 °C). The resulting ZnO NPs were characterized using field-emission scanning electron microscopy (FESEM-EDX), X-ray diffraction (XRD), UV-Vis spectroscopy, photoluminescence (PL), and Fourier-transform infrared spectroscopy (FTIR). Samples made with zinc acetate were labeled ZnO-M400, ZnO-M600, and ZnO-M800, while those from zinc nitrate were named NZnO-M400, NZnO-M600, and NZnO-M800. X-ray diffraction (XRD) analysis confirmed that all synthesized ZnO nanoparticles (NPs) exhibit a hexagonal wurtzite crystal structure with a polycrystalline nature and the presence of peaks (100), (002), (101), (102), (110), (103), (200), (112), and (201) in PXRD results in dicates that ZnO nanoparticles were synthesized with high purity [88]. Fourier-transform infrared spectroscopy (FTIR) revealed the presence of active vibrational modes corresponding to functional groups such as hydroxyl (OH), carboxylate (COO), carbon dioxide (CO₂), and water (H–OH), indicating the successful formation of ZnO. Ultraviolet-visible (UV–Vis) spectroscopy demonstrated that the optical bandgap values of all ZnO NPs are lower than that of bulk ZnO (Eg = 3.26), suggesting quantum size effects [89]. Photoluminescence (PL) measurements established a correlation between the morphology of the nanomaterials and their defect distribution. Field emission scanning electron microscopy (FESEM) analysis showed that the use of Myrtus communis plant extract enabled the green synthesis of ZnO NPs with well-defined hexagonal structures, including spherical, pyramidal, cylindrical, and rod-like shapes, as well as a variety of particle sizes. The morphology of the synthesized samples was significantly influenced by the choice of precursor and calcination temperature, leading to variations in the hexagonal forms. This study concludes that the size and morphology of green-synthesized ZnO powders are strongly dependent on both the precursor type and the calcination temperature. Notably, smaller particle sizes are associated with larger surface areas, which in turn contribute to enhanced photocatalytic performance, confirming that ZnO NPs preserved their high photocatalytic activity up to four cycles, as supported by numerous previous studies (Figure 14) [88].

ZnO nanoparticles were synthesized via another green method using Spinacia oleracea leaf extract by Velmurugan and al. The process involved mixing the plant extract with zinc acetate and ammonium solution, followed by heating at 90 °C for 3 h. After cooling and filtration, the precipitate was washed with ethanol and deionized water, then dried at 130 °C and calcined at 300 °C [90]. XRD analysis confirmed that the synthesized ZnO nanoparticles possess a hexagonal Wurtzite structure with an average crystallite size of 37.6 nm, which was further supported by SEM and TEM observations. UV–Vis spectroscopy revealed a surface plasmon resonance (SPR) range between 320 and 390 nm, and the Tauc plot indicated a direct bandgap energy of approximately 3.12 eV (

Table 6. Different plant extracts and precursors were used to synthesize ZnO NPs.

Plant Extract	Metal Precursor	Size (nm)	BG (eV)	Ref.
Beetroot	Ca-ZnO	20–50	-	[66]
Vitex negundo	ZnO	19	3.16	[87]
Myrtus communis	Acetate-ZnO	100	3.21	[88]
	Nitrate-ZnO	100	3.24
Spinacia oleracea	ZnO	35–40	3.12	[91]
Camellia sinensis	ZnO	12.2	3.15	[92]
Lawsonia inermis	ZnO	22	3.37	[93]
	ZnO/Fe2O3	39	2.8

).

FTIR spectra identified characteristic functional groups from biomolecules involved in the synthesis process, with a distinct absorption band at 505 cm⁻¹, verifying the formation of ZnO nanoparticles. Biosynthesized ZnO nanoparticles demonstrated effective photocatalytic activity under simulated sunlight, achieving significant degradation of organic pollutants in gasoline refinery effluent, particularly toluene (84.26%) and xylene (90.36%) at pH 8 after 240 min of exposure (

Table 7. Comparative studies of green-ZnO NPs on the photocatalytic activities.

Photocatalyst	Plant Used	Pollutant	Experimental Conditions	Degradation (%)	Ref.
ZnO	Beetroot	MB	-Calcination temperature: T = 450 °C (15 min); -Time of photocatalysis: 40 min; -Irradiation source: sunlight; -Pollutant concentration: 10 mg/L (100 mL); -Masse of NPs: 0.01 g	100%	[66]
Ca-ZnO			-Calcination temperature: T = 450 °C (15 min); -Time of photocatalysis: 60 min; -Irradiation source: sunlight; -Pollutant concentration: 10 mg/L (100 mL); -Masse of NPs: 0.01 g	90%
ZnO-acetate	Myrtus communis	MB	-Calcination temperature: T = 400 °C; -Irradiation source: UV light.; Time photocatalysis: 50 min; -Pollutant concentration: 10 ppm (100 mL); -Masse of NPs: 0.1 g.	98%	[88]
ZnO-Nitrate			-Calcination temperature: T = 400 °C; -Irradiation source: UV light.; -Time of photocatalysis: 60 min; -Pollutant concentration: 10 ppm (100 mL); -Masse of NPs: 0.1 g	99%
ZnO	Spinach leaves	Toluene	-Calcination temperature: T = 300 °C (120 min); -Irradiation source: sunlight; Time photocatalysis: 240 min; Pollutant concentration: 0.5 g/L (150 mL); -Masse of NPs: 0.05 g; pH = 8	84.26%	[91]
		Xylene	-Calcination temperature: T = 300 °C (120 min); -Irradiation source: sunlight; Time photocatalysis: 240 min; Pollutant concentration: 0.5 g/L (150 mL); -Masse of NPs: 0.05 g; pH = 8	90.36%
		COD	-Calcination temperature: T = 300 °C (120 min); -Irradiation source: sunlight; -Time photocatalysis: 240 min; Pollutant concentration: 0.5 g/L (150 mL); -Masse of NPs: 0.05 g; pH = 8	81.24%
ZnO	Camellia sinensis	chlortetracycline	-Calcination temperature: T 400 °C (3 h); -Irradiation source: UV-light; -Time photocatalysis: 60 min; -Pollutant concentration: 10 mg/L (50 mL); -Masse of NPs: 50 mg; pH = 3.5	94.70%	[92]
ZnO/H2O2	Lawsonia inermis	Effluent dye mixture	-Irradiation source: Solar light; -Time photocatalysis: 3 h; -Pollutant (50 mL); -Masse of NPs: 1 mg; pH = 7	70.50%	[93]
ZnO/Fe2O3/H2O2			-Irradiation source: Solar light; -Time photocatalysis: 3 h; -Pollutant (50 mL); -Masse of NPs: 1 mg; pH = 7	90.40%
ZnO	Solanum trilobatum	MB	-Irradiation source: Sunlight; -Time photocatalysis: 90 min; -Pollutant concentration: 10 μm; -Catalyst Amount: 0.6 g/L	94.07%	[94]
ZnO	Lepidagathis ananthapuramensis	MB	-Irradiation source: Sunlight; -Time photocatalysis: 120 min; -Pollutant concentration: 10 ppm; -Catalyst Amount: 5 mg	98.50%	[95]
ZnO	Euphorbia milii	MB	-Irradiation source: Sunlight; -Time photocatalysis: 50 min; -Pollutant concentration: 10 μm -Catalyst Amount: 0.6 g/L	98.17%	[96]
ZnO	Phoenix roebelenii	MB	-Irradiation source: UV lamp; -Time photocatalysis: 105 min; -Pollutant concentration: 10 ppm; -Masse of NPs: 0.2 g	98%	[97]

). In treating paper mill effluents, successive photocatalytic cycles led to a progressive decrease in COD levels, reaching 81.24%, 74.14%, 68.92%, 60.24%, and 53.82%, respectively (Figure 15). Moreover, radical scavenging assays confirmed the strong antioxidant potential of the ZnO NPs, especially under high-concentration conditions [91].

4.3. Tungsten Oxide (WO₃)

WO₃ nanoparticles were green-synthesized by Fadhila Anggraini et al. using Beetroot extract and a 0.1 M Na₂WO₄·2H₂O solution. The influence of pH (4, 7, and 11) was assessed using a phosphate buffer. The extract and precursor were mixed at a 5:1 volume ratio, stirred for 6 h, and aged overnight. The obtained precipitate was dried at 80 °C and sintered at 250 °C or 550 °C for 2 h in air, yielding WO₃ nanoparticles. WO₃ nanoparticles were successfully synthesized using Beetroot extract. The optimal conditions were found at pH 4 and a sintering temperature of 550 °C. Characterization by XRD, SEM-EDX, and XPS confirmed the formation of a single-phase WO₃ with a crystallite size of 26.4 nm and an average particle size of 24 nm. The reflectance spectrum (Figure 16a) depicts the characteristic edge wavelength at around 398 nm. The synthesized nanoparticles exhibited a band gap of 2.9 eV as shown in Figure 16b. Under both UV and visible light, the WO₃ NPs showed remarkable photocatalytic activity, achieving 99.67% degradation of Rhodamine B within 30 min of UV exposure. Additionally, the photocatalyst maintained high stability and reusability over five consecutive cycles [98].

In another study, WO₃ nanoparticles were successfully synthesized through a green chemistry approach using Hyphaene Thebaica extract as a natural reducing agent. Sodium tungstate was dissolved in the extract, stirred for 24 h, then centrifuged, washed, and dried at 80 °C. The dried product was partially characterized, while the rest was annealed at 400 °C for 4 h to complete the formation of WO₃ nanostructures.

The nanoflakes achieved removal efficiencies of 93% for Congo Red and over 98% for Methylene Blue, underscoring their strong potential for wastewater treatment applications (Figure 17). FTIR analysis confirmed the structural evolution of WO₃ nanostructures upon annealing [99]. The as-synthesized WO₃ exhibited characteristic bands for adsorbed water (3000–3700 cm⁻¹, 1610 cm⁻¹) and W=O stretching (938 cm⁻¹). A broad band between 400–1000 cm⁻¹ was attributed to W–O vibrations. After annealing at 400 °C, hydroxyl-related bands diminished due to dehydration, and the 938 cm⁻¹ peak merged into a broader 400–1200 cm⁻¹ band, indicating the formation of a well-structured O–W–O lattice in crystalline WO₃ [100]. The material exhibited excellent recyclability and structural stability over multiple cycles. Its notable adsorption capacity toward both dyes further supports its application in pollutant removal (Figure 18) [99].

In new research, Jerold Antony tungsten oxide (WO₃) nanoparticles were synthesized via a green hydrothermal route using Tamarindus indica extract as a natural reducing and stabilizing agent. The reaction mixture was treated at 180 °C for 3 h, and the product was purified and dried. This method provides an eco-friendly and efficient alternative for WO₃ nanoparticle synthesis [64,101]. The experimental procedure for the green synthesis of WO₃ nanoparticles is provided in Figure 19.

Hexagonal WO₃ nanoparticles synthesized via a green method using Tamarindus indica extract demonstrated high photocatalytic efficiency for degrading the antibiotic ofloxacin in wastewater, green WO₃ was tested for reusability over six photocatalytic cycles after thorough washing with deionised water, and the final degradation efficiency was recorded (Figure 20) Photodegradation followed pseudo-first-order kinetics, achieving significant removal within 150 min, with hydroxyl radicals playing a key role. The material exhibited a surface area of 19.112 m²·g⁻¹, a pore diameter of 0.09 nm, and a band gap of 2.41 eV. XPS analysis confirmed the presence of tungsten (W) and oxygen (O) without impurities. The O1s peak at 530.09 eV corresponds to W–O bonding (Figure 21). This eco-friendly approach offers a promising and sustainable alternative for pharmaceutical pollutant remediation in wastewater [102].

4.4. Copper Oxide NPs (CuO/Cu₂O)

Copper oxide (CuO) nanoparticles were synthesized using a green method based on Euphorbia alata extract. The aerial parts of the plant were dried, ground, and extracted by infusion using distilled water for one week. The resulting extract was heated to 90 °C, and a solution of CuSO₄·5H₂O was added. The mixture was stirred for 2 h, centrifuged, washed, and then dried at 40 °C. Finally, the product was calcined at 400 °C for 4 h to obtain CuO nanoparticles. Following a green synthesis method, a conventional chemical hydrothermal process was also used to prepare CuO nanoparticles. CuSO₄·5H₂O was dissolved in deionized water, and KOH was added dropwise until pH 10 was reached. The mixture was stirred at 90 °C for 2 h, forming a dark precipitate, which was then centrifuged, washed, dried at 80 °C, and finally calcined at 400 °C for 4 h to yield CuO nanoparticles [67]. CuO nanoparticles exhibited a monoclinic structure with sizes between 10–16 nm. FTIR confirmed the presence of phenolic and flavonoid groups on biosynthesized CuO-NPs (Figure 22). UV-Vis analysis showed band gap energies of 1.77 eV (chemical) and 1.92 eV (green). Morphological analysis revealed spherical shapes for chemically synthesized CuO-NPs and octahedral clusters for green ones. The green CuO-NPs showed enhanced photocatalytic, antibacterial, and antifungal activities, particularly against S. aureus, B. subtilis, C. albicans, and S. cerevisiae. The E. alata-based synthesis offers high purity, simplicity, low cost, and eco-friendliness, making it suitable for water remediation and biomedical use. The histogram (Figure 23) illustrates that the photocatalytic degradation efficiency of methylene blue (MB) diminishes as the dye concentration increases under sunlight irradiation. Moreover, the biosynthesized CuO-NPs exhibited higher degradation efficiency compared to the chemically synthesized ones [67].

In a continuation of green synthesis approaches, Al-Faouri et al. employed Bougainvillea plant extract as a natural reducing and stabilizing agent for the preparation of copper oxide nanoparticles (CuO-NPs). Bougainvillea leaves were thoroughly washed with tap and deionized water, then air-dried in a shaded, well-ventilated area at room temperature for 10 days. Once fully dried, the leaves were ground into a fine powder using a blender and sieved through a 100 μm mesh to eliminate coarse particles [68]. A total of 10 g of dried Bougainvillea leaf powder was added to 100 mL of deionized water and heated at 80 °C with constant stirring for 20 min, producing a greenish solution. After standing at room temperature for 24 h, the extract was filtered using Whatman No. 1 paper to remove solid residues, yielding a deep honey-colored aqueous extract. This extract was analyzed by FT-IR to identify functional groups and was stored at 277 K for use as a natural reducing and capping agent in nanoparticle synthesis. A 0.14 M solution of cupric acetate (50 g in 1000 mL deionized water) was prepared and stirred, then 100 mL of Bougainvillea extract was added dropwise under continuous stirring. The color transition from blue to emerald green indicated nanoparticle formation. After 4 h of stirring and 48 h of settling, the precipitate was separated via filtration, washed with deionized water and ethanol, and dried at 80 °C for 4 h. The final calcination at 400 °C for 2 h yielded CuO nanoparticles as a fine black powder, which was cooled, ground, and stored for further analysis. Copper oxide nanoparticles (CuO-NPs) synthesized via a green method using Bougainvillea leaf extract demonstrate effective reduction and stabilization through phytochemicals acting as capping agents. These nanoparticles show strong UV absorption, a band gap of 2.74 eV, and favorable optical properties, including notable refractive index and optical conductivity. Structural analyses (XRD) confirm a crystalline monoclinic phase (Figure 24), while SEM and TEM reveal a nearly spherical morphology with minimal agglomeration. FT-IR analysis further confirms the involvement of bioactive compounds in nanoparticle stabilization and formation [68].

The thorough analysis of the optical and structural characteristics of the synthesized CuO-NPs provides valuable insights into their light–matter interactions and supports their potential deployment across a wide spectrum of practical applications [68]. However, the photocatalytic performance of these nanoparticles remains to be validated through experimental degradation studies, which represents a notable limitation of the current work. In another study conducted by Rupashree et al., copper oxide nanoparticles (CuO-NPs) were synthesized via a green method using cupric nitrate trihydrate and green tea extract in a 1:1 ratio. The components were dissolved in distilled water, stirred magnetically for homogenization, and heated at 120 °C to form a gel. The gel was then calcined at 380 °C for 10 min in a muffle furnace. After cooling to room temperature, phase-pure CuO-NPs were obtained. Copper oxide nanoparticles (CuO-NPs) were successfully synthesized via an environmentally friendly green method using green tea extract as a fuel. Their structural, compositional, and morphological properties were characterized by XRD (Figure 25a), FTIR (Figure 25b), UV-Vis spectroscopy (Figure 25c), EDX, SEM, and TEM. The CuO-NPs demonstrated effective catalytic degradation of methylene blue dye in aqueous solution, achieving complete degradation of a 5 ppm dye concentration within 180 min using 200 mg of catalyst at pH 9. Photocatalytic degradation reached 92% under sunlight and 99% under UV light (Table 10). Figure 25d shows the degradation efficiency of methylene blue (MB) over time using 200 mg of CuO nanoparticles, which represents the optimal mass for maximum degradation. The photocatalytic mechanism was also thoroughly discussed. These features highlight the potential of the fabricated sensor for reliable industrial applications [103].

CuO nanopowders were synthesized via a green sol–gel method using copper (II) chloride dihydrate, sodium hydroxide, and Lantana camara leaf extract. A 0.1 M precursor solution was mixed, heated, and combined with plant extract, followed by the dropwise addition of NaOH, leading to a rapid formation of CuO precipitate. The product was washed, dried at 100 °C, and calcined at 500 °C. The synthesis was also carried out at higher precursor concentrations (0.2 M and 0.3 M) to study the effect on nanoparticle formation [104]. XRD pattern shows the two prominent peaks of 2θ and the values 35.5O and 38.7O are assigned to (−111) and (111), which is the evidence of CuO nanoparticles, and revealed that CuO nanoparticles synthesized at 0.2 M molarity exhibited the smallest crystallite size (17 nm) compared to other concentrations (Figure 26) [105]. Optical studies indicated a strong absorption edge and a blue shift at 343 nm, confirming the presence of a distinct optical band gap, is shown in Figure 27 [106]. SEM analysis showed morphological transformation to seed-like spherical particles. FTIR confirmed the presence of functional groups associated with CuO such as peaks at 597,507 and 431 cm⁻¹ corresponded to the CuO deformation vibration. The biosynthesized CuO-NPs demonstrated significant antibacterial activity against four bacterial strains and excellent photocatalytic efficiency, achieving 94.07% degradation of methylene blue within 90 min (Figure 28). The photocatalytic degradation mechanism of MB by CuO nanostructures begins with dye adsorption onto the nanoparticle surface. Upon sunlight exposure, CuO generates electron–hole (e⁻/h⁺) pairs, which interact with molecular oxygen and hydroxyl groups to form reactive oxygen species (^•O₂⁻ and ^•OH) [107]. These radicals are responsible for breaking down the dye molecules into smaller intermediates, leading to effective degradation [108]. Overall, CuO-NPs synthesized at 0.2 M via green synthesis showed superior properties in terms of size, morphology, and functional performance, particularly in antibacterial and photocatalytic applications. The work offers valuable insights into the synthesis and activity of CuO nanocomposites; however, the absence of reusability studies limits the assessment of its practical potential.

The biosynthesis of copper oxide nanoparticles (CuO NPs) involves using 1 M copper sulfate as a precursor. A 50 mL copper sulfate solution is mixed with 50 mL of Psidium. guajava leaf extract under continuous magnetic stirring at 100 °C. The pH is adjusted using 0.2 M sodium hydroxide (NaOH). A color change from dark green to brownish-black indicates the formation of monodispersed CuO NPs. The resulting precipitate is centrifuged, washed with distilled water to remove impurities, and dried at 90 °C to obtain fine CuO NPs powder for further use [109]. In the present study, CuO nanoparticles (CuO-NPs) were successfully synthesized via a green route using Psidium guajava leaf extract, marking the first reported use of this plant in the green synthesis of CuO-NPs. The eco-friendly approach leverages the phytochemicals present in the extract, which act as natural reducing, capping, and stabilizing agents, resulting in a less toxic and efficient synthesis process. The biosynthesized CuO-NPs were characterized using FT-IR and XRD, confirming the involvement of biomolecules in the formation of crystalline CuO structures, such as the characteristic peaks observed at 624 cm⁻¹, 701 cm⁻¹, and 777 cm⁻¹ in the FT-IR pattern, which are attributed to the formation of biosynthesized CuO metal. Similar findings in the literature have reported absorption peaks at 799 cm⁻¹ and 855 cm⁻¹, which correspond to the stretching and vibrational modes associated with CuO nanoparticles [110]. The XRD spectrum of CuO is characterized by intense, sharp peaks as shown in

Table 8. XRD Peak Positions and corresponding crystal Planes of green CuO.

2θ	25.01°	28.91°	32.21°	33.21°	33.85°	35.5°	37.7°	44.96°	65.11°
Crystal plane	(202)	(400)	(110)	(200)	(002)	(111)	(−142)	(112)	(113)

[111]. Morphological analysis by FE-SEM and HR-TEM revealed predominantly spherical particles with sizes ranging from 40 to 150 nm. These morphological features align with previous studies, such as the green synthesis using Tamarix gallica leaf extract, which also produced spherical particles of varying sizes [112], and the neem seed-mediated synthesis that yielded platelet-like structures [113], reported by Revathi and Thambidurai. Elemental composition was confirmed through EDS, while DLS analysis provided the average particle size distribution. Furthermore, photocatalytic studies demonstrated a high degradation efficiency of methylene blue (89%) compared to Congo red (81%) under sunlight irradiation (Figure 29), indicating the strong potential of these biosynthesized CuO-NPs for both biomedical applications and the treatment of industrial wastewater [109]. Consistent with previous studies, effective photodegradation of methylene blue was observed using Mg-doped ZnO nanoparticles under simulated sunlight irradiation [114].

In addition to copper (II) oxide (Cu₂O), another important copper oxide is cuprous oxide (Cu₂O). While Cu₂O contains copper in the +2-oxidation state, Cu₂O consists of copper in the +1 state. Both oxides exhibit distinct physical, chemical, and crystallographic properties, and their applications differ accordingly. Cu₂O, in particular, possesses a cubic crystal structure and a direct band gap of approximately 2.0 eV, making it suitable for applications in photocatalysis, photovoltaics, and sensing [115]. Cuprous oxide nanoparticles (Cu₂O NPs) were synthesized using an eco-friendly method with curcumin serving as both reducing and capping agent. Curcumin was first dissolved in NaOH to prepare a homogeneous solution, which was then mixed with a 0.1 M copper nitrate solution and stirred at 80–90 °C for 20 min. The reaction proceeded for 3 h, during which a color change from blue to reddish brown was observed. The nanoparticles were collected by centrifugation, washed with ethanol, and dried under vacuum. Characterization was performed using XRD, SEM, and UV–Vis analyses. The synthesis was reproducible under identical conditions, confirming the reliability of the method [116]. The six prominent diffraction peaks located at 29.33°, 36.23°, 42.01°, 61.10°, 73.39°, and 77.40° correspond to the crystallographic planes (110), (111), (200), (220), (311), and (222), respectively, which are characteristic of cubic Cu₂O, in agreement with the JCPDS reference file No. 78-2076 [117]. These results confirm the successful formation of Cu₂O nanoparticles under the applied synthesis conditions. The SEM image reveals that most of the particles exhibit a nearly spherical morphology, with sizes predominantly in the range of 30–40 nm. The corresponding particle size distribution curve is illustrated in Figure 30 [116].

The photocatalytic performance of cuprous oxide nanoparticles can be attributed to their strong UV light absorption capability and their subsequent generation of reactive oxygen species, including superoxide (O₂^−•) and hydroxyl (^•OH) radicals. Although no explicit degradation percentage was reported, the qualitative analysis strongly supports the high photocatalytic efficiency of the Cu₂O nanoparticles synthesized via curcumin mediation. The structural and electronic properties such as high surface area, optimal charge distribution, and strong interaction between curcumin and the Cu₂O surface are key factors known to enhance photocatalytic performance. Density Functional Theory (DFT) calculations further confirmed the electronic charge transfer from curcumin to the Cu₂O nanoclusters, facilitating the generation of reactive species under UV light. These qualitative insights suggest that the synthesized Cu₂O NPs possess a strong potential for efficient degradation of organic pollutants like methyl orange (

Table 9. Comparative studies of green-CuO NPs on the photocatalytic activities.

Photocatalyst	Plant Used	Pollutant	Experimental Conditions	Degradation Efficiency	Ref.
CuO	Ephedra Alata	MB	-Calcination temperature: T = 400 °C (4 h); -Irradiation source: Sunlight -Time of photocatalysis 3 h; -Pollutant concentration: 10 mg/L; -Mass of NPs: 20 mg	93.4%	[67]
CuO	Green tea extract	MB	-Calcination temperature: T = 380 °C (10 min) -Irradiation source: Sunlight (850 Wm−2); -Time of photocatalysis: 3 h -Pollutant concentration: (5–20 ppm); -Mass of NPs: 50–200 mg	92%	[103]
			-Calcination temperature: T = 380 °C (10 min) -Irradiation source: UV-light (125 Wm−2); -Time of photocatalysis: 3 h -Pollutant concentration: (5–20 ppm); -Mass of NPs: 50–200 mg	99%
CuO	Lantana camara	MB	-Calcination temperature: T = 500 °C (4 h); -Irradiation source: Sunlight; -Time of photocatalysis: 90 min; -Molarity of NPs: 0.2 M	94%	[104]
CuO	Psidium guajava	MB	-Irradiation source: sunlight; -Time of photocatalysis: 2 h; -Pollutant concentration: 30 mg/L; -Mass of NPs: 20 mg	89%	[109]
		Congo Red	-Irradiation source: sunlight; -Time of photocatalysis: 2 h; -Pollutant concentration: 30 mg/L; -Mass of NPs: 20 mg	81%
Cu2O	Curcumin	MO	-Irradiation source: UV-light; -Time of photocatalysis: 2 h -Pollutant concentration: 150 μm; -Mass of NPs: 50 mg; pH = 3	94.5%	[116]

), even though specific degradation rates were not numerically provided [116].

4.5. Iron Oxide NPs (Fe₂O₃)

To investigate the effect of Syzygium cumini (L.) Skeels extract concentration on the synthesis of α-Fe₂O₃ nanoparticles, powders were prepared by dissolving 0.025 mol of Fe(NO₃)₃·9H₂O in 100 mL of plant extract at two concentrations (17.5% and 22.5%). The mixtures were heated and stirred at 60 °C until gel formation, then dried at 100 °C for 24 h. The dried gels were ground and subsequently calcined at 500 °C and 650 °C for 2 h with a heating rate of 10 °C/min. The resulting samples were labeled A1 (17.5%, 500 °C) and A2 (22.5%, 650 °C) [118]. In this study, α-Fe₂O₃ nanoparticles were successfully synthesized using varying concentrations of Syzygium cumini extract. XRD analysis confirmed the exclusive formation of the α-Fe₂O₃ crystalline phase. Variations in extract concentration and calcination temperature significantly influenced the morphology of the nanoparticles, particularly in terms of particle size and aggregation. To better understand how the extract acts in the process of nucleation and formation of α-Fe₂O₃-NPs, it is first necessary to elucidate the phytocompounds present in the extract of the leaves of S. cumini (L.) Skeels [119]. The sample synthesized with a higher extract concentration (A1) exhibited enhanced photocatalytic activity, with a degradation rate of 2.868 × 10² ± 0.019 min⁻¹ toward norfloxacin (NORF), and achieved ~32% mineralization as shown by TOC analysis. On the other hand, the band gap energy obtained from the materials was 1.9 and 2.0 eV (Figure 31). The material also demonstrated excellent stability over three successive photocatalytic cycles (Figure 32) [118]. Scavenger tests indicated that photogenerated holes (h⁺) play a dominant role in the degradation mechanism, followed by hydroxyl radicals (^•OH) [120]. These findings highlight that the concentration of S. cumini extract not only facilitates green synthesis of α-Fe₂O₃ but also modulates surface defect formation, thereby enhancing photocatalytic efficiency. In this context, the structural, morphological, and photocatalytic analyses collectively confirm the effectiveness of S. cumini extract as both a reducing and capping agent. This finding is particularly noteworthy, as it suggests the potential of this extract to be employed in the green synthesis of other functionalized nanoparticles such as ZnO, NiO, CuO [121,122,123], and TiO₂ [78] for a wide range of applications.

In another eco-friendly and cost-effective study conducted by Awais Ahmad et al., for the synthesis of α-Fe₂O₃ nanoparticles from a medicinal plant extract of Trachyspermum ammi, thus eliminating the need for hazardous chemicals. The successful formation of α-Fe₂O₃ was confirmed by XRD and FTIR analyses. The hematite phase of α-Fe₂O₃ nanoparticles is confirmed by IR bands at 445 and 550 cm⁻¹, indicating metal–oxygen bonding and surface hydroxyl groups that enhance radical formation and charge separation [124]. These features contribute to improved photocatalytic and antibacterial performance. Additionally, the interaction between plant extracts and nitrate precursors favors the formation of highly crystalline, phase-pure α-Fe₂O₃ with enhanced optical properties (Figure 33). HRTEM and FESEM images revealed spherical nanoparticles with an average diameter of 26 nm, consistent with the crystallite size determined from XRD data [125,126]. The reduced particle size contributed to the improved photocatalytic and antibacterial performance of the material. The green-synthesized α-Fe₂O₃ demonstrated strong antibacterial activity against Staphylococcus aureus and Escherichia coli. Furthermore, it showed significant photocatalytic efficiency, reaching 91% degradation of methylene blue under visible light within 120 min. These results highlight the potential of T. ammi-derived α-Fe₂O₃ nanoparticles as promising candidates for environmental remediation and biomedical applications [127]. On the other hand, α-Fe₂O₃ nanoparticles were successfully synthesized using Carica papaya leaf extract and ferric chloride hexahydrate (FeCl₃·6H₂O) as the iron precursor, mixed in a 1:1 volume ratio (50 mL each) at room temperature. The pH of the solution was adjusted to 11 using 1 M NaOH, and the mixture was stirred for 30 min, leading to the formation of a dark-colored solution, confirming nanoparticle formation. The resulting nanoparticles were separated by centrifugation at 8000 rpm for 20 min, thoroughly washed with ethanol and distilled water, then dried in a hot-air oven at 80 °C for 3 h and stored for further use. The synthesized α-Fe₂O₃ nanoparticles exhibited notable photocatalytic activity, achieving approximately 77% degradation of Remazol Yellow RR dye under sunlight irradiation within 6 h at a dosage of 0.8 mg/L (Figure 34) [128].

In a related study, Carica papaya leaf extract was also employed for the green synthesis of silver-doped iron oxide (Ag–Fe₂O₃) nanoparticles. The presence of phytochemicals in the extract acted as both reducing and stabilizing agents, facilitating the formation of well-dispersed, doped nanoparticles. The incorporation of silver into the Fe₂O₃ matrix aimed to enhance the material’s photocatalytic and antimicrobial performance by improving charge separation and reducing electron–hole recombination.

Fe₂O₃ nanoparticles (NPs) and silver-doped iron oxide nanocomposites (Ag/Fe₂O₃-NCs) were successfully synthesized using a polyphenol-rich plant extract (PPE) through a green approach. Key reaction parameters, including temperature, pH, precursor concentration, and PPE volume, were systematically optimized. The resulting materials were characterized using UV–Vis spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The Ag/Fe₂O₃ nanocomposites exhibited excellent photocatalytic activity, achieving complete degradation of methylene blue (MB) within 90 min using only 20 mg of catalyst. Ag/Fe₂O₃ NCs’ photocatalytic degradation of MB through a potential schematic mechanism is illustrated in Figure 35. Furthermore, the material retained high photocatalytic performance upon reuse, with less than a 10% decrease in degradation efficiency after a second cycle (Figure 36). In addition to their photocatalytic potential, the synthesized Ag/Fe₂O₃ NPs demonstrated antioxidant properties through radical scavenging activity, and hold promise as multifunctional agents for wastewater disinfection, water purification, and remediation of dye-contaminated water [129].

Table 10. Photocatalytic performance of α-Fe₂O₃ nanoparticles against various dyes.

Sample	Plant Extract	Pollutant/Dye	Volume	Pollutant Dose	Catalyst Dose	Light Irradiation	Contact Time	BG (eV)	Crystallite Size (nm)	Removal (%)	Ref.
α-Fe2O3	Syzygium cumini	Norfloxacin	50 mL	50 mg/L	50 mg	UV	90 min	1.9–2.0	28.51–37.64	96	[118]
Fe2O3	Trachyspermum ammi	MB	100 mL	10 ppm	10 mg	Visible	120 min	-	26	91.0	[127]
α-Fe2O3	Carica papaya	Remazol Yellow	100 mL	50 ppm	0.8 g/L	Sunlight	250 min	-	21.59	75	[128]
Ag/Fe2O3	Carica papaya	MB	-	20 mg/L	0.2 g/L	Visible	90 min	2.91	29.17	92.97	[129]
Fe2O3	Pomegranate seeds	Reactive Blue	100 mL	20 mg/L	15 mg	UV	56 min	-	25–55	95.08	[130]
Fe2O3	Cynometra ramiflora	MB	50 mL	20 ppm	30 mg	Sunlight	4 h	-	68.17	94.0	[131]
Fe2O3	Cassia auriculata	Malachite Green	100 mL	-	20 mg	Visible	150 min	-	-	91.2	[132]
Fe2O3	Withania coagulans	Safranin	100 mL	10 ppm	0.5 mg	Sunlight	180 min	4.20	16 ± 2	68.8	[133]

provides a comparative overview of the photocatalytic performance of Fe₂O₃ nanoparticles tested against various organic dyes. It summarizes key experimental parameters such as dye type, solution volume, catalyst dosage, and observed degradation efficiency. The data highlight the effectiveness of Fe₂O₃ under different conditions and for different pollutants, with references to the corresponding studies.

5. Quantitative Assessment of Environmental Impact

While the green synthesis of metal oxide nanoparticles using plant extracts offers eco-friendly and cost-effective alternatives to conventional methods, it is crucial to quantitatively evaluate the environmental impact of these processes. Recent studies have employed life cycle analysis (LCA) and other assessment tools to measure parameters such as energy consumption, solvent usage, and waste generation during synthesis. For instance, a study on the green synthesis of zinc oxide nanoparticles from Sida acuta leaves demonstrated a significant reduction in energy consumption and chemical waste compared to traditional chemical methods [134]. Similarly, research on the synthesis of iron oxide nanoparticles highlighted a decrease in solvent usage and improved biodegradability of by-products [135].

These findings underscore the importance of integrating environmental impact assessments into the development of green synthesis protocols. By quantitatively evaluating the sustainability of these processes, researchers can optimize synthesis conditions to minimize environmental footprints, thereby advancing the field of sustainable nanotechnology.

6. Conclusions and Future Perspective

This review has highlighted the recent progress in the green synthesis of TiO₂, ZnO, WO₃, CuO, and Fe₂O₃ nanoparticles using various plant extracts. Plant-mediated approaches have been shown to provide an eco-friendly and cost-effective alternative to conventional methods, enabling the fabrication of nanomaterials with controlled morphology, crystallinity, and photocatalytic efficiency. Importantly, these green-synthesized photocatalysts demonstrate remarkable potential for degrading organic dyes and pollutants under both UV and visible light, underscoring their suitability for environmental remediation.

Nevertheless, several challenges remain. Most studies are still confined to laboratory-scale demonstrations, with limited exploration of photocatalyst stability, recyclability, and long-term performance under real environmental conditions. Moreover, the mechanistic role of phytochemicals in tailoring nanoparticle properties is not yet fully understood, restricting the rational design of optimized materials.

Future research should therefore address the following directions:

• Industrial-scale expansion: Develop scalable synthesis routes that ensure reproducibility, cost-efficiency, and quality control for large-scale production.
• Advanced material design: Engineer composite and heterostructured catalysts (e.g., doped systems, hybrid nanocomposites, or heterojunctions) to extend light absorption into the visible spectrum and suppress charge recombination.
• Real-world applicability: Test photocatalysts in actual wastewater systems containing complex pollutant mixtures, variable pH, and competing ions to evaluate their robustness and practical efficiency.
• Mechanistic insights: Elucidate the molecular role of phytochemicals in nucleation, growth, and surface functionalization to better tune particle size, band structure, and catalytic activity.
• Catalysis–membrane integration: Explore the development of photocatalytic membranes that combine degradation and separation processes, thereby reducing secondary contamination, improving water quality, and enhancing process sustainability.
• Multifunctional applications: Explore additional roles such as antimicrobial activity, energy harvesting, or pollutant sensing, thereby broadening the technological relevance of these nanomaterials.

In summary, plant-mediated synthesis of metal oxide nanoparticles represents a promising step toward sustainable nanotechnology. By combining green chemistry principles with innovative material design, catalytic membrane integration, and industrial scaling, future research can bridge the gap between laboratory-scale advances and real-world applications, ultimately contributing to clean water technologies and global environmental sustainability.