Enhanced photocatalytic degradation of methylene blue using zinc vanadate nanomaterials with structural and electrochemical properties

Abstract

The development of Zn₃(VO₄)₂ nanomaterials was successfully achieved via a green chemistry method utilizing Moringa Oleifera extract. The photocatalytic performance of the synthesized nanomaterials was tested for the degradation of methylene blue (MB) under visible light irradiation. The optical properties, crystalline structure, and composition of the nanomaterials were analysed using photoluminescence (PL), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared spectroscopy (FTIR), and high-resolution transmission electron microscopy (HRTEM). XRD and HRTEM data revealed that the nanomaterials prepared at 500 °C and 700 °C exhibited high crystallinity and were quasi-spherical with a range of particle sizes and irregular shapes. The electrochemical properties were evaluated using cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS). The CV response showed broad redox peaks and peak separations indicative of pseudo-capacitive behaviour arising from faradaic reactions, which are pseudo-reversible. EIS results indicated that the electrochemical behaviour of the electrode material was influenced by both reaction kinetics and diffusion processes. Furthermore, the photocatalytic degradation of MB using Zn₃(VO₄)₂ nanomaterials was evaluated under visible light irradiation. The experiments considered various parameters, including MB concentration, catalyst loading, and pH. The results demonstrated an impressive degradation efficiency, reaching 87% removal of MB at pH 5.0 after 120 min of exposure to visible light. Kinetic analysis showed that the degradation followed a pseudo-first-order model (R² > 0.98), with high R² values and observed rate constants, highlighting the potential for optimizing catalyst use in environmental applications, particularly in the removal of organic pollutants like MB from wastewater.

Introduction

In recent years, the rapid growth of industrial operations particularly in sectors such as textiles, wood, paper, leather, cosmetics, and food has led to the significant discharge of pollutants, especially organic dyes, into the environment¹. These dyes pose serious health, environmental, and aesthetic concerns, as they can be visually detected in water even at low concentrations (10–50 mg/L). The textile industry is especially notable for its extensive dye usage, consuming large volumes of water (200–500 m³ per ton of product) and generating wastewater with dye concentrations ranging from 10 to 200 ppm. The release of contaminants like methylene blue (MB), a cationic thiazine dye widely used in textiles, paper, and pharmaceuticals, has raised significant environmental concerns. Its toxic and non-biodegradable nature threatens both human health and aquatic life^2,3.

Water pollution, arising from both synthetic and natural sources, has become a critical issue affecting freshwater quality and is difficult to address using conventional biological treatment methods. Consequently, there is an urgent need for effective technologies to remove these organic pollutants from wastewater, along with ongoing research to improve efficiency and reduce costs^{4, 5–6}. Photocatalysis, an advanced oxidation process (AOP) with considerable potential, employs semiconductor materials to facilitate the degradation of organic pollutants under light exposure^7,8. This technology offers numerous advantages, including high efficiency, environmental sustainability, cost-effectiveness, and compatibility with ambient conditions. Various semiconductor materials such as ZnO, SnO₂, TiO₂, WO₃, and V₂O₅ and their composites have been extensively studied for photocatalytic applications^{8, 9–10}. This catalytic process is vital for removing organic contaminants from the environment. However, these materials often face challenges, including wide band gaps that limit their activity under visible light and rapid electron-hole recombination rates, which reduce their photocatalytic efficiency^{9, 10–11}.

Recently, vanadate-based materials have gained significant interest due to their luminescent properties, functional capabilities, low cost, and potential applications in organic cathodes and photocatalysis^{8, 9–10}. Vanadate is an important transition metal oxide used for environmental and energy applications across various fields such as catalysis, batteries, antimicrobial activity, sensors, water splitting, solar cells, and photocatalysis. Recently, Zn₃(VO₄)₂ has gained recognition in the field of photocatalysis due to its favourable properties and versatility in environmental applications^{8, 9–10}. Zinc vanadate (Zn₃(VO₄)₂), a ternary metal oxide, has emerged as a potential photocatalytic material because of its unique electronic structure, excellent chemical stability, and relatively low toxicity^11,12. Typically, Zn₃(VO₄)₂ exhibits a narrow to moderate band gap within the visible spectrum, approximately 2.0–2.5 eV, allowing it to effectively absorb visible light. This absorption promotes electron transitions from the valence band to the conduction band, resulting in the formation of electron-hole pairs that are essential for photocatalytic activity. The ability to utilize visible light directly enhances its efficiency under solar illumination, making it suitable for environmental remediation. The nanostructured form of Zn₃(VO₄)₂ (such as nanoparticles, nanorods, and nanosheets) increases the surface area-to-volume ratio, providing more active sites for dye molecules to adsorb and react. This morphological feature promotes greater interaction between the photocatalyst and pollutants, thereby accelerating degradation rates^{12, 13, 14–15}. Visible light irradiation is crucial for the photocatalytic performance of Zn₃(VO₄)₂, as it supplies the energy needed to activate its electronic structure. Due to its appropriate band gap, the material efficiently absorbs visible light, facilitating electron excitation from the valence band to the conduction band. This process generates electron-hole pairs that participate in redox reactions, leading to the formation of reactive oxygen species such as hydroxyl radicals and superoxide anions. These reactive species play a key role in degrading dye molecules, including methylene blue. Our research demonstrates that the material not only harnesses visible light for direct excitation but also produces reactive species, thereby promoting efficient photocatalytic degradation of dyes under ambient light conditions^14,15. Recent advances in vanadium-based oxides have revealed their considerable promise as photocatalysts for dye degradation under visible and natural sunlight. Ahmad et al. (2025) synthesized PEG/ PEG/Eudragit-Zn₄V₂O₉ nanostructures via a low-temperature co-precipitation method, involving dissolving precursors in water, adjusting pH carefully, and drying. These nanostructures showed high catalytic efficiency, achieving 97.85% degradation of methyl orange in the absence of light¹⁶. Their findings highlight that incorporating polymers enhances active sites and charge transfer pathways. Similarly, Elizabeth et al. (2025) employed hydrothermal synthesis to produce Zn₃V₂O₈ nanorods supported on solanum betaceum-mediated reduced graphene oxide (SBRGO). This method involved hydrolyzing metal precursors in aqueous solutions at high temperature and pressure to form nanorods, followed by reduction of graphene oxide using natural plant extracts. This process facilitated heterostructure formation and bandgap tuning, resulting in a nanocomposite that achieved 92% degradation of methyl violet under visible light¹⁷. Further improvements were achieved by doping vanadium oxides with metals such as silver. Raman et al. (2025) used hydrothermal synthesis to incorporate Ag ions into α-Zn₂V₂O₇ nanoparticles at doping levels of 2–6 wt%. This involved combining precursor solutions with silver salts and treating them hydrothermally at high temperature. The doped nanoparticles exhibited reduced band gaps (down to 1.4 eV at 6 wt% Ag) and demonstrated nearly complete degradation of methylene blue (99.77%) under sunlight, along with enhanced antibacterial properties—likely due to increased surface area and improved charge separation. Moreover, the development of multifunctional heterostructures like V₄O₇/V₂O₅/Zn₂V₂O₇ microflowers was achieved through microwave-assisted synthesis, which involved rapid heating of precursor solutions containing vanadium and zinc salts. Ashish et al. (2025) reported that this method produced highly effective photocatalysts capable of degrading around 95% of methylene blue in just 45 min under natural sunlight, largely due to efficient charge separation within the composite¹⁹. These recent studies underscore the importance of synthesis techniques such as co-precipitation, hydrothermal, and microwave-assisted methods in controlling the morphology, doping levels, and heterostructure formation of vanadium oxides. These structural modifications significantly enhance their photocatalytic and antimicrobial properties, positioning vanadium-based nanomaterials as promising candidates for environmental remediation^{16, 17, 18–19}.

Researchers are also exploring various eco-friendly and sustainable synthetic approaches for fabricating Zn₃(VO₄)₂ nanostructures. Traditional chemical and physical methods often involve high temperatures, toxic reagents, waste generation, and high costs. In response, green synthesis techniques particularly those utilizing plant extracts have emerged as safe, cost-effective alternatives. These methods eliminate the need for harmful chemicals and leverage natural capping agents from plants, which can improve the properties of the resulting nanomaterials. For example, employing Moringa oleifera extract as a bio-template or reducing agent removes the necessity for harmful chemicals typically used in conventional synthesis. This eco-friendly approach reduces environmental impact, supports sustainable development, and minimizes toxic waste generation. The bioactive compounds in Moringa oleifera such as phenolics, flavonoids, and proteins act as natural capping and stabilizing agents^{20, 21–22}. They enable controlled nucleation and growth of nanostructures, producing uniform, well-defined nanoparticles with optimal sizes and shapes essential for effective photocatalytic activity.

The functional groups in plant extracts can modify the surface chemistry of nanomaterials, enhance surface hydrophilicity and increase active sites for photocatalytic reactions. This improves interactions with organic pollutants like methylene blue. Biomolecules can also serve as electron mediators or dopants, facilitating charge transfer and reducing electron-hole recombination, thus boosting photocatalytic efficiency under visible light. Additionally, natural extracts reduce dependence on costly, energy-intensive synthesis methods and reagents^{20, 21–22}.

Green synthesis often yields nanomaterials with enhanced stability due to natural capping agents that protect against agglomeration and degradation during photocatalytic cycles. These environmentally friendly methods induce structural and electronic modifications such as improved morphology, defect states, and surface chemistry that greatly enhance electrical conductivity, charge separation, and overall photocatalytic performance. Consequently, zinc vanadate nanomaterials produced via green methods are highly effective for environmental remediation while maintaining their eco-friendly advantages^{20, 21–22}. Moringa oleifera was selected primarily for its wide availability, cost-effectiveness, and rich phytochemical profile, including polyphenols, flavonoids, and antioxidants, all of which support green synthesis processes. Its proven ability to generate stable, well-dispersed nanoparticles, combined with sustainability and accessibility, makes it an attractive choice. While this study did not directly compare Moringa oleifera with other plant extracts, such comparisons could improve synthesis optimization and clarify the influence of various phytochemicals on nanoparticle properties. Future research should involve systematic evaluations of different plant extracts to determine their efficacy in nanoparticle synthesis, stability, and functional performance^{20, 21–22}. Green synthesis methods utilizing plant extracts are increasingly appealing due to their environmental benefits, cost savings, and simplicity. In theory, these techniques can be scaled up effectively if process parameters are carefully controlled, managed, and optimized. However, scaling up green synthesis requires addressing challenges such as reproducibility, standardization, resource availability, and process management. Overcoming these hurdles necessitates thorough process optimization, quality assurance, and potentially the development of standardized extract formulations. With proper planning and engineering, large-scale, sustainable, and economically viable production is feasible^{20, 21–22}.

This research focuses on the environmentally friendly synthesis of Zn₃(VO₄)₂ nanomaterials using Moringa oleifera extract as a natural reducing and capping agent. The choice of Moringa oleifera is motivated by its rich bioactive compounds, which facilitate eco-friendly synthesis processes. The study explores the mechanism behind the formation of these nanomaterials and evaluates their structural, optical, and electrochemical characteristics. Furthermore, the photocatalytic efficiency of the produced Zn₃(VO₄)₂ nanomaterials in degrading methylene blue (MB) under visible light exposure is evaluated. The results objective to advance sustainable approaches for wastewater treatment and environmental remediation. This project is closely aligned with the United Nations Sustainable Development Goals (UNSDGs), particularly Goal 6 (Clean Water and Sanitation), Goal 12 (Responsible Consumption and Production), and Goal 13 (Climate Action). The use of Moringa oleifera extract illustrates an environmentally friendly approach to synthesizing nanomaterials, promoting the development of sustainable, chemical-free technologies for water purification that enhance access to safe drinking water. The biodegradable and plant-derived properties of Moringa Oleifera support responsible resource utilization by reducing reliance on harmful chemicals, thereby minimizing environmental impact contributing to responsible consumption and production. Furthermore, employing renewable natural resources in green synthesis methods helps reduce greenhouse gas emissions associated with conventional manufacturing processes, supporting efforts to moderate climate change. This research contributes a novel, green synthesis pathway for zinc vanadate nanomaterials with enhanced photocatalytic activity, providing a promising path for sustainable environmental remediation technologies.

Experimental methods

Material

All chemicals were analytical grade and purchased from Merck.

Zinc nitrate (Zn(NO₃)₂) and vanadium (III) chloride, VCl₃, polyvinylidene fluoride and dimethyl formamide were purchased from Merck, distilled water was in our laboratory.

Dried leaves of Moringa oleifera were obtained from Burkina Faso, supplied by Zeba Abinatou from the Forestry Department of the Research Institute of Agriculture and Environment.

Preparation of a plant extract

Sample preparation: Dried Moringa oleifera leaves were accurately weighed (approximately 30 g) using an analytical balance. Place leaves into a clean container. Add 300 mL of freshly boiled deionized water (DI-H₂O). The mixture was maintained at 50 °C for 1 h and 45 min with continuous stirring to ensure thorough extraction. Allow the mixture to cool to room temperature. The extract was filtered through Whatman No. 1 filter paper to remove leaf remains. The resulting filtrate was stored in a clean, airtight container at 4 °C until further use.

Green synthesis of zinc vanadate {Zn₃(VO₄)₂}

Preparation of Precursors: About 3 g of each precursor vanadium III chloride (0.012 moles) and zinc nitrate (0.016 moles) were dissolved in 50 ml of the natural extract. Stir gently at room temperature until fully dissolved (~ 30 min). They were completely dissolved without any additional of toxic chemical and heat.

The solution was covered with aluminium foil to prevent contamination. Keep the solution at room temperature for 18 h. After 18 h from the observation there were no precipitation formed, therefore the formation of zinc vanadate complex was in the form of suspension. The oven was used to dried (100 °C) the solution. The powder was collected by centrifugation, then wash several times with distilled water to remove impurities. Dry the washed powder in the oven at 100 °C. The sample was divided into 3 equal parts. Sample, no further heat treatment (as synthesised). The two samples were annealed different in air for 2 h at 500 and 700 °C. This experimental method was extracted from the previous work²⁰.

Photocatalytic activity testing of Zn₃(VO₄)₂ nanomaterial

Preparation of Methylene Blue (MB) solution: Dissolve 10 mg of the dye in 1000 mL of distilled water, resulting in a concentration of 10 mg/L stock solution. The pH of this MB solution was then adjusted to 5.0 using HCl and NaOH. Following this, 100 mL of the prepared MB solution was combined with 10 mg of Zn₃(VO₄)₂ nanomaterial in a separate container to promote interaction between the dye and the nanomaterial. To enhance the adsorption of the dye onto the nanomaterial, the mixture was stirred for 30 min in the dark, allowing adequate time for equilibration before beginning the photocatalytic degradation process. Then exposed the mixture to a 270 W xenon arc lamp set to a wavelength of 460 nm to provide the necessary visible light for the photocatalytic reaction. At 30-minute interval, withdraw small aliquots and record UV-Vis absorbance spectra from 200 to 1000 nm, over a period of 2 h. Calculate the percentage degradation of MB using the absorbance at 664 nm. Repeat the experiment with initial MB concentration of 5, 10 and 15 ppm, while keeping all other reaction conditions unchanged. Further studies investigated the effect of different dosages of Zn₃(VO₄)₂ nanomaterial, specifically at levels of 2.5, 5, and 10 mg, while maintaining a constant initial concentration of methylene blue and pH of the solution. Additionally, the impact of pH on photodegradation rates was evaluated by adjusting the pH of the methylene blue solution to 3.0, 5.0, 7.5, and 10. The required pH for this experiment was adjusted using HCl and NaOH solutions before adding the Zn₃(VO₄)₂ photocatalyst.

Characterization

High resolution transmission electron microscopy (HRTEM) was used for their physical characterization as particle size, size distribution and morphology, on a Philips Technai TEM at120 kV. The elemental analysis spectra were obtained using energy dispersive X-ray spectroscopy (EDS) on an EDS Oxford instrument at 20 keV. A Fourier transform infrared (FT-IR) absorption spectrometer (Shimadzu 8400s spectrophotometer, 400–4000 cm^-1) was employed to verify the surface coating and the chemical bonding. X-ray powder diffraction (XRD) for crystallinity was performed using a Bruker AXS D8 Focus with CuKα radiation (λ = 0.15406 nm).

Electrochemical Performances: Conduct cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge- discharge measurements on an auto-lab potentiostat (CH Instruments, USA) electrochemical workstation with a three-electrode setup:

Working electrode: Glassy carbon working electrode coated with Zn₃(VO₄)₂.

Counter electrode: Platinum wire.

Reference electrode: Ag/AgCl (3 M NaCl).

Electrolyte: 0.1 M NaOH.

Deoxygenate solution with high- purity argon gas.

Polish working electrode with alumina slurries (1.0, 0.3, 0.05 μm) before and after measurements.

Voltametric Performance: Potential window from − 0.6 to 0.6 V with scan rates of 50 mV/s and various scan rates of 20–100 mV/s.

Impedance Performance: was measured at a of within a frequency region of 100 kHz–100 mHz at 10 mV perturbation amplitude.

Fabrication of a GCE/{Zn₃(VO₄)₂} electrode for the electrochemical studies

Preparation of catalyst ink: Mix 3 mg of Zn₃(VO₄)₂ nanomaterial with polyvinylidene fluoride and dimethyl formamide (DMF)as a binder.

Sonicate the solution for 15 min to obtain a uniform slurry.

Electrode coating: Drop-cast 3 µl of the slurry onto the surface area of the cleaned glassy carbon electrode.

Dry the coated electrode in the oven at 35 °C for 1 h.

Then rinse it gently with distilled water before electrochemical measurements.

HRTEM analysis

Figure 1 shows the HRTEM images of non-annealed and annealed (500 °C and 700 °C) Zn₃(VO₄)₂ nanomaterials. HRTEM is capable of visualizing atomic structures, allowing us to determine whether the material is crystalline, amorphous, or polycrystalline^{22, 23–24}. It can directly capture images of lattice fringes regular, periodic features that correspond to crystallographic planes within the crystal lattice. By measuring the distance between these lattice fringes, HRTEM provides accurate interplanar spacings, which can be used to identify specific crystal planes and phases^{22, 23–24}.

The images reveal that all samples are of nanometer size, with some variation in shape and particle irregularity. The non-annealed (RT) sample exhibits a combination of spherical and nanorod shapes with irregular particle sizes. Some particles notably exhibit a quasi-spherical shape, indicating they are nearly round but not perfectly spherical. These quasi-spherical particles appear almost round with minor deviations, which is common in nanomaterials due to growth dynamics and surface energy considerations. As the annealing temperature increases from RT to 500 °C and then to 700 °C, the particles tend to grow larger and predominantly adopt spherical and nanosheet morphologies. The presence of quasi-spherical particles at higher temperatures suggests a trend toward more isotropic growth, which can enhance surface properties by increasing surface area compared to irregular shapes^{22, 23–24}. The shape and size of these particles are crucial, as they directly influence the surface area available for catalytic or other surface-dependent activities. Quasi-spherical particles, being nearly round, possess a higher surface-to-volume ratio than larger, irregularly shaped particles. This increased surface area can improve performance in areas like catalysis, sensing, or energy storage^24,25.

At 700 °C, particle growth and agglomeration become more pronounced, which may lead to a decrease in accessible surface area despite the overall increase in particle size^25,26. Understanding particle morphology, particularly the existence of quasi-spherical particles, is vital for adjusting synthesis conditions to optimize surface characteristics and activity. At 500 °C, calcination facilitates the removal of residual organic materials from the green synthesis process and enhances crystallinity while maintaining nanoscale particle dimensions. This temperature is sufficient to improve structural ordering and stability without significant particle agglomeration^25,26. Conversely, at 700 °C, the increased thermal energy further enhances crystallinity and promotes particle growth and agglomeration, which can improve structural integrity but may reduce surface area. These temperature conditions were selected to study the balance between increased crystallinity and the preservation of nanoscale features, which are essential for photocatalytic performance. Lower temperatures may not fully achieve crystallization, while higher temperatures could cause excessive sintering and loss of nanostructure^22,26. Furthermore, EDX analysis confirmed the elemental composition of the samples, revealing the presence of zinc, vanadium, oxygen, and carbon (the latter likely originating from the carbon grid). The spectra confirm the successful formation of pure Zn₃(VO₄)₂ nanomaterials.

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Fig. 1

HRTEM images and EDS elements of GCE/ Zn₃(VO₄)₂ at R.T., 500 °C and 700 °C.

Optical properties of Zn₃(VO₄)₂

Figure 2(a) represents photoluminescence (PL) spectra of the green synthesized Zn₃(VO₄)₂ nanomaterials before annealing, after annealing at 500 °C and 700 °C. The measurement was done with 250 nm excitation wavelength LED light. As indicated in Fig. 2(a) the Zn₃(VO₄)₂ before annealing showed the emission wavelength maxima at 500 nm corresponding to 2.48 eV, however, after annealing at 500 °C and 700 °C the pure Zn₃(VO₄)₂ was obtained as a result an evident red shift to 554 nm corresponding to 2.24 eV is observed after annealing, corresponding to the literature^20,21. This is said to be due to the recombination of the hole generated as an electron is transferred from Oxygen 2p orbital to the 3d Orbital of vanadium forming the tetrahedral geometry (VO₄)²⁷. It is also reported that the observed sharp, strong peak might be due to those vacancies creates by the charge transition in the VO₄²⁸. However, the intensity is below 250 a. u which suggests that a larger fraction of the photogenerated electrons and holes are available for photocatalytic reactions, resulting in higher degradation rates of pollutants like dyes²⁹. it is, therefore, likely that this material absorbs visible light effectively AND efficiently converts it to reactive species. The PL spectrum provides direct evidence of visible light absorption, while the intensity variations predict the photocatalytic efficiency^{30, 31, 32–33}. Figure 2(b) shows the UV-Vis diffuse reflectance of the Zinc Vanadium Oxide measured in 200 to 1000 nm range with the corresponding Tauc plot. As expected from the oxides there is a reflection in the visible region. After annealing in 2 different temperatures the sample gave a reflection of up ~ 90%. Subramanian and co-authors states that the Zn₃(VO₄)₂ light absorption occurs in the UV region in the wavelength shorter than 380 nm, which agrees with what is observed in Fig. 1(b)³⁴. From the linear extrapolation of the absorption edge using Tauc plot ((αhv)² vs. energy), energy band gap was estimated to be 2.46 eV, 2.64 eV, and 2.61 eV for Zn₃(VO₄)₂ room temperature (RT), annealed at 500 °C, and 700 °C respectively. This corresponds with the Zn₃(VO₄)₂ band gap found in the literature which varies between 2.5 and 3.4 eV^{35, 36–37}. This, therefore, make this material suitable for visible light absorption as its excitation by visible light will generate electron-hole pairs that drive redox reactions. This explains why the Zn₃(VO₄)₂ material shows photocatalytic activity when exposed to the Visible light³⁷.

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Fig. 2

Shows the (a) photoluminescence and (b) UV-Vis diffuse reflectance and Tauc plot of the Zinc Vanadium Oxide (Zn₃(VO₄)₂).

Structural properties of the zinc vanadium oxide (Zn₃(VO₄)₂)

Figure 3 presents (a) the powder X-ray diffraction (XRD) pattern and (b) the FTIR spectrum of the synthesized Zinc Vanadium Oxide (Zn₃(VO₄)₂). The XRD pattern of the sample after annealing at 700 °C indicates that the material has a base-centered orthorhombic lattice with space group Abam and lattice parameters a = 8.2990 Å, b = 11.5284 Å, and c = 6.1116 Å, consistent with the standard card No.: 00-034-0378. The observed diffraction peaks correspond well with the standard positions associated with the orthorhombic Zn₃(VO₄)₂ phase, specifically at the 2θ values corresponding to the (0 2 0), (1 2 0), (2 0 0), (2 2 0), (2 1 1), (1 3 1), (1 4 0), (0 2 2), (3 2 0), (3 1 1), (0 4 2), (0 6 0), (3 1 3), (1 6 2), and (0 0 4) planes. The close match between the experimental peaks and the standard pattern confirms the formation of the Zn₃(VO₄)₂ crystalline phase. Importantly, no additional peaks indicative of impurity phases such as V₂O₅, ZnO, or other vanadates were observed within the detection limits of the XRD analysis, suggesting the phase purity of the synthesized material. The absence of extraneous peaks supports the conclusion that the sample is predominantly composed of pure Zn₃(VO₄)₂. The crystallite size was estimated using the Scherrer equation applied to the most prominent diffraction peak, yielding an average size of approximately 38.0 nm. The sharp and well-defined peaks observed in the pattern after annealing indicate high crystallinity of the Zn₃(VO₄)₂ nanoparticles, further corroborating the successful synthesis of a well-ordered crystalline phase^38,39.

FTIR analysis

Figure 3(b) shows the FTIR spectra recorded from 4000 cm^− 1 to 500 cm^− 1 for the three samples. The FTIR spectra indicated the existence of hydroxyl groups (broad band 3700–3000 cm⁻¹) in the unannealed sample, likely resulting from water adsorption. Following annealing at 700 °C and 500 °C, the spectra exhibit a division of this broad band and the appearance of bands in the range of 3300–3500 cm⁻¹ related to N–H stretching, which suggests the presence of residual organic materials from the plant extract. However, the absence of C = O vibrations (~ 1600 cm⁻¹) implies that organic compounds have mostly decomposed during the heat treatment, leaving only minimal organic residues. The spectra mainly reveal peaks associated with metal-oxygen bonds, such as V–O (~ 900 cm⁻¹) and Zn–O (~ 600–500 cm⁻¹), which indicate the development of metal-oxygen frameworks that provide stability to the synthesized materials^39,40. The initial organic residues from the synthesis are mostly removed after annealing, with the spectra primary showing metal-oxygen bonds that are crucial for the structural stability of the samples.

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Fig. 3

Shows the (a) XRD, and (b) FTIR spectrums of the bioengineered Zinc Vanadium Oxide {Zn₃(VO₄)₂} nanomaterials at R.T., 500 °C and 700 °C.

Photocatalytic activity

This research investigated the photocatalytic efficiency of zinc vanadate (Zn₃(VO₄)₂) nanomaterials in the degradation of methylene blue (MB) when subjected to UV light. To ensure that the degradation of MB was attributable to the photocatalytic properties of Zn₃(VO₄)₂, control experiments were performed without the catalyst. Methylene blue was selected as the model dye primarily because of its extensive industrial applications in textiles, printing, and dyeing, which make it a significant environmental contaminant. Its molecular structure as a cationic aromatic dye enables effective interaction with photocatalysts, and its strong absorption peak at 664 nm allows for easy, rapid, and sensitive monitoring of degradation through spectrophotometry. These characteristics collectively render MB an optimal and practical choice for evaluating photocatalytic activity^23,24.

Initially, the Zn₃(VO₄)₂ catalyst was allowed to interact with the MB solution in the absence of light during a pre-irradiation phase. This procedure established an adsorption equilibrium, ensuring effective adherence of MB molecules to the catalyst’s surface prior to the commencement of the photocatalytic reaction. Subsequently, the solution was exposed to a xenon arc lamp emitting 270 watts of visible light at a wavelength of 664 nm for a duration of two hours. The absorbance spectrum was examined over a range of 200 to 1000 nm, revealing two peaks at approximately 614 nm and 664 nm, which are associated with methylene blue, as depicted in Fig. 4(a)^41,42. Importantly, the intensity of the peak at 664 nm gradually diminished, with no new peaks appearing. This progressive decrease in MB absorbance over time indicates an effective photocatalytic process, highlighting the ability of Zn₃(VO₄)₂ to transform MB into non-toxic byproducts^41,42. Prior research has reported similar patterns, demonstrating the effectiveness of various photocatalysts in reducing MB concentrations under comparable experimental conditions^43,44. The percentage of degradation was calculated using the formula presented in Eq. (1) below^43,44:

1

In this equation, C₀ represents the initial concentration of methyl blue dye and C represents the final concentration. The results for degradation percentage, as depicted in Fig. 4A, reveal that the Zn₃(VO₄)₂ catalyst achieved approximately 87% degradation of methylene blue (MB) after 120 min of exposure to visible light. Similar trends were observed in the literature where the study reported by Jyothi and Royo, where various photocatalysts have exhibited degradation rates exceeding 85%^43,44. It is important to highlight that Zn₃(VO₄)₂ is distinguished by its cost-effectiveness, environmental safety, and ease of synthesis, which are critical factors in the development of sustainable photocatalytic materials.

The photocatalytic mechanism responsible for the degradation of methylene blue (MB) utilizing Zn₃(VO₄)₂ involves a sequence of reactions that facilitate the breakdown of the dye when exposed to visible light. This process can be outlined as follows:^43,44.

Step 1: Photoexcitation of Zn₃(VO₄)₂.

Absorption of visible light: When Zn₃(VO₄)₂ is exposed to visible light, photons with energy equal to or greater than its bandgap are absorbed. This process excites electrons from the valence band (VB) to the conduction band (CB).

Generation of electron-hole pairs denotes a photoexcited electron in the conduction band, and represents a positive hole in the valence band (Eq. 2)

2

Step 2: Charge Carrier Dynamics.

Separation and migration: Effective separation of electrons and holes is essential to avoid recombination. This spatial separation enables the charge carriers to engage in surface redox reactions (Eq. 3).

3

Step 3 & 4: Formation of Reactive Oxygen Species (ROS).

Reduction of molecular oxygen: The generated charge carriers (electrons and holes) move to the surface of Zn3(VO4)2, where they interact with adsorbed oxygen and water, leading to the formation of reactive oxygen species (ROS) like hydroxyl radicals (Eq. 4). The rate of this migration can be affected by the properties of the catalyst and the presence of trapping sites.

Oxidation of water or hydroxide ions: The holes oxidize water or hydroxide ions to produce hydroxyl radicals (Eq. 5).

4

5

.

Step 5.

Upon light activation, Zn₃(VO₄)₂ can produce reactive oxygen species (ROS) like superoxide anions (O2−), which are highly effective in degrading organic contaminants.

(Eq. 6).

6

Valence Band (VB): Primary responsible for oxidation reactions. Vacancies in the VB can oxidize water or hydroxide ions, leading to the production of hydroxyl radicals (•OH).

Conduction Band (CB): Primary responsible for reduction reactions. Electrons present in the CB can reduce oxygen molecules, resulting in the formation of superoxide radicals (•O₂⁻).

These reactive radicals, •OH and •O₂⁻, are extremely oxidative and are essential in the degradation of pollutants during photocatalytic processes.

Step 6: Final Mineralization:

Reactive oxygen species can subsequently interact with methylene blue, facilitating its oxidation or degradation. This mechanism generally leads to the disintegration of the MB molecule into smaller, less pigmented fragments or may result in its total mineralization into oxygen and water (Eqs. 7&8). Additionally, the electrons in the conduction band play a role in generating hydroxyl radicals, which are recognized as the key agents responsible for the degradation of organic compounds.

7

8

Effect of catalyst dose

The investigation into the impact of varying catalyst dosages (2.5, 5, and 10 mg) on methyl blue (MB) removal efficiency reveals a clear trend: increasing the catalyst mass enhances degradation performance up to a certain point. As depicted in Fig. 4B, the removal efficiency improves from lower catalyst amounts to a maximum of approximately 87% at 10 mg. This can be attributed to several factors, such as increasing the catalyst dose, which effectively enlarges the total surface area available for photocatalytic reactions, providing more active sites for adsorption and subsequent degradation of MB molecules. The greater number of active sites facilitates enhanced interaction with dye molecules and promotes the generation of reactive species, such as hydroxyl radicals, which are essential for breakdown processes. Higher catalyst loading also enhances photon absorption capacity, assuming the catalyst remains well-dispersed and non-aggregated. This leads to increased generation of electron-hole pairs vital for initiating photocatalytic reactions^{42, 43–44}. However, it is important to note that there is a saturation threshold beyond which additional increases in dosage may yield diminishing returns in removal efficiency, potentially due to the overlap of active sites or the aggregation of nanoparticles, thus contributing to surface saturation effects^{42, 43–44}. The effectiveness of photocatalytic reactions under sunlight exposure is influenced by various factors, including the catalyst’s properties, environmental conditions, and the nature of the incoming light. Both the spectral quality and intensity of sunlight, as well as temperature, are vital factors impacting photocatalytic reactions. It is critical to optimize these elements in conjunction with catalyst properties and reaction conditions to enhance efficiency in real-world applications⁴³.

Effect of concentration of methylene blue (MB)

The experiments tested different concentrations of MB while keeping other reaction parameters constant, as shown in Fig. 4C. The concentration of methylene blue is crucial for its photocatalytic degradation when using Zn₃(VO₄)₂ nanoparticles, with removal percentages observed over time at concentrations of 5, 10, and 15 ppm. The findings indicated that higher MB concentrations lead to improved removal efficiency. The increased ionic strength from more concentrated MB solutions enhances electrostatic interactions and creates more favorable conditions for adsorption. Previous studies, referenced in^{42, 43–44}, support this notion, suggesting that higher concentrations of ionic pollutants can initiate nucleation on the adsorbent’s surface, thereby facilitating more effective adsorption. Generally, higher methylene blue concentrations may result in greater generation of hydroxyl radicals and other reactive species due to the increased availability of dye molecules. However, this effect is usually effective only up to a certain concentration limit, beyond which light absorption may become a constraint. At extremely high concentrations, methylene blue may reach a saturation point, resulting in insufficient photocatalytic sites for all dye molecules, which could ultimately reduce the degradation rate^{42, 43–44}. The factors affecting the rates of photocatalytic degradation, such as recombination losses, surface saturation, and light intensity, interact with the concentration of MB in various ways: at low MB concentrations, the degradation rate generally increases with concentration due to greater substrate availability. At moderate to high concentrations, the rate may stabilize or decline due to surface saturation and recombination effects. Light intensity can improve some of these effects to a certain extent but cannot completely counteract the limitations caused by surface saturation and recombination at extremely high dye concentrations^43,44.

Kinetics of photodegradation of methylene blue (MB)

However, at varying concentrations, the order of the reaction may change, influencing the degradation rate. Indeed, both the catalyst’s surface charge and the dye’s ionization state play a crucial role in their interaction. Pseudo-first-order kinetics is applicable since either the catalyst or the oxidant is present in a large excess, which simplifies the rate law. This often suggests a low concentration of dye compared to the catalyst or oxidant, resulting in an apparent first-order behaviour^43,44.

The dye degradation reaction kinetics by the Zn₃(VO₄)₂ catalyst during the photocatalytic reaction were explained according to the pseudo-first-order kinetic model, in Eq. 9.

9

Where C_t and C₀ are the initial concentration of the dye and the concentration at the photocatalytic degradation time t (min), respectively. t is the time interval after each exposure of the suspension to the visible light, and K_a is the apparent rate constant (min^− 1)^43,44.

Figure 4 (D) illustrates the correlation between the plot of − ln (CO/Ct) and irradiation time. The photodegradation of Methyl Blue (MB) dye adheres to pseudo-first-order kinetics across different concentrations (5 ppm, 10 ppm, and 15 ppm), as evidenced by the high correlation coefficients (R² > 0.98) obtained from the kinetic analysis. The rate constants for the degradation process increase with higher MB concentrations, indicating a direct relationship between dye concentration and degradation rate (0.00919 min⁻¹ at 5 ppm, 0.01045 min⁻¹ at 10 ppm, and 0.01726 min⁻¹ at 15 ppm). This improvement in photodegradation efficiency at higher concentrations implies a greater number of MB molecules available for reaction, thereby increasing the overall degradation rate. These results highlight the potential of photodegradation techniques as an effective method for treating wastewater containing dyes, emphasizing the significance of understanding reaction kinetics to optimize degradation processes in environmental applications^43,44. The R² value reflects the degree to which the rate data aligns with a linear model of first-order kinetics, with values exceeding 0.98 indicating a strong correlation, thereby confirming that the degradation of Methylene Blue can be modelled as a first-order reaction under the experimental conditions applied.

An R² value exceeding 98% implies that the kinetic model (pseudo-first-order) aligns exceptionally well with the experimental data. This suggests that more than 98% of the variation in the decrease of dye concentration is accounted for by the model, indicating that the reaction kinetics adhere closely to the presumed order under the experimental conditions. The achievement of 87% degradation within 120 min demonstrates that the photocatalytic process is effective, but not rapid. Various factors influence this rate, including kinetic limitations, mass transfer resistance, catalyst surface saturation, and reaction equilibrium^43,44.

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Fig. 4

(a) Decolorization efficiencies for photocatalytic degradation of MB solutions, (b) (Zn₃(VO₄)₂) load, (c) MB different concentration (d) kinetics plot − ln (C_O/Ct) and irradiation time.

Influence of pH on the photodegradation efficiency of MB

The photocatalytic breakdown of organic dyes such as Methylene Blue (MB) is significantly affected by the solution’s pH, which influences various critical factors including the catalyst’s surface charge, dye speciation, and the production of reactive oxygen species (ROS)^43,44. This study systematically examined the impact of pH on the photocatalytic performance of Zn₃(VO₄)₂ across pH levels of 3.0, 5.0, 7.5, and 10.0, as shown in Fig. 5(a). The pH was adjusted using hydrochloric acid (HCl) and sodium hydroxide (NaOH).

The surface charge of Zn₃(VO₄)₂ varies with pH, which in turn affects the electrostatic interactions between the catalyst and dye molecules. In acidic conditions (pH 3.0), the catalyst surface tends to develop a positive charge, which can repel the cationic form of MB, leading to reduced adsorption and lower degradation efficiency⁴¹. As pH increases toward neutral and slightly alkaline conditions, the surface charge becomes increasingly negative, enhancing electrostatic attraction to the cationic MB molecules and facilitating adsorption, an essential step before photocatalytic degradation^41,42.

Experimental results revealed that the removal efficiency of MB increased from pH 3.0 to 5.0, reaching a maximum of approximately 87% at pH 5.0. This indicates that slightly acidic to neutral pH conditions are optimal for both adsorption and photocatalytic activity. Beyond pH 5.0, the degradation efficiency decreased at pH 7.5 and 10.0. This decline may be attributed to several factors, including changes in surface charge that reduce dye adsorption, nanoparticle agglomeration at higher pH levels which limits available active surface area, and potential alterations in the stability of reactive oxygen species or dye molecules in alkaline environments^{42, 43–44}. At pH 5, the combination of favorable surface charge, optimal generation of reactive oxygen species (ROS), and stability of MB molecules results in the highest photocatalytic degradation efficiency. Conversely, at neutral or alkaline pH levels, the surface charge and the formation of reactive species may be less effective, or the pollutants might be more resistant to degradation. From a molecular perspective, this process involves improved adsorption of MB onto the catalyst, increased formation of hydroxyl radicals, and effective attack on the dye’s aromatic structures, all influenced by pH-dependent surface chemistry^{42, 43–44}.

Additionally, at higher pH levels, the ionization state of MB affects its interaction with the catalyst surface. Protonation of dye molecules increases their susceptibility to oxidative degradation, thereby enhancing photocatalytic efficiency in mildly acidic conditions. Conversely, in highly alkaline environments, deprotonation and ionization of MB reduce its attraction to the negatively charged catalyst surface, leading to decreased adsorption and lower degradation rates^{42, 43–44}. MB was selected as the model dye primarily due to its extensive industrial application in textiles, printing, and dyeing, making it a significant environmental contaminant. Its molecular structure as a cationic aromatic dye enables effective interaction with photocatalysts. Additionally, MB’s strong absorption peak at 664 nm allows for easy, rapid, and sensitive monitoring of its degradation via spectrophotometry. These characteristics collectively make MB an ideal and practical choice for evaluating photocatalytic activity. Regarding the effect of pH on the photocatalytic efficiency of Zn₃(VO₄)₂, pH plays a crucial role by modifying the catalyst’s surface charge, thereby influencing dye adsorption. High pH (alkaline conditions) enhances the adsorption of cationic MB due to electrostatic attraction with negatively charged surfaces. Moreover, alkaline pH promotes the generation of reactive hydroxyl radicals (•OH), which are vital for dye degradation. Variations in pH also impact the stability and charge state of MB, consequently affecting its interaction with the photocatalyst^{41, 42–43}.

The stability test

The stability and reusability of Zn₃(VO₄)₂ nanomaterial as a photocatalyst were evaluated through a series of photodegradation experiments aimed at degrading methyl blue (MB) under visible light exposure. In these experiments, 10 mg of Zn₃(VO₄)₂ was used in each of five cycles to degrade MB at pH 5.0, as illustrated in Fig. 5(b). Initially, the photocatalytic performance was evaluated by exposing MB to visible light in the presence of the Zn₃(VO₄)₂ catalyst. After each cycle, the catalyst was recovered via centrifugation, thoroughly washed with water and ethanol to remove any residual dyes and reaction by-products and then dried for future reuse. This process was repeated for up to five cycles.

The results showed that Zn₃(VO₄)₂ maintained a high level of photocatalytic efficiency across the cycles, with only a slight decrease observed by the fifth cycle. This minor decline indicates that the catalyst exhibits good stability and reusability under the tested conditions. The slight performance drop could be attributed to factors such as surface fouling, minor structural changes, or incomplete removal of residues during recovery. In summary, these findings confirm that Zn₃(VO₄)₂ nanomaterial is a promising and durable photocatalyst for repeated dye degradation applications under visible light exposure^{40, 41, 42–43}.

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Fig. 5

(a) Effect of pH MB, (b) Recycling test for photocatalytic degradation of MB for five cycles.

Electrochemical properties

Cyclic voltammetry (CV) is a key electrochemical method used to explore the redox behavior, electron transfer kinetics, and electrochemical properties of electrode materials and analytes. It involves varying the potential of an electrode within a defined voltage range and measuring the resulting current, thus providing insights into the oxidation and reduction processes occurring at the electrode surface.

In this research focusing on the electrochemical properties of Zn₃(VO₄)₂ nanomaterials, CV has been employed to evaluate their electrochemical performance in an alkaline electrolyte (0.1 M NaOH) at a scan rate of 50 mV/s, as illustrated in Fig. 6(a). The unmodified glassy carbon electrode (GCE) showed no significant redox peaks, indicating low electrochemical activity. Modified GCEs with Zn₃(VO₄)₂ synthesized at room temperature (RT), 500 °C, and 700 °C exhibited clear redox behavior. Notably, the sample annealed at 500 °C demonstrated enhanced electrochemical activity, evidenced by increased peak currents and distinct oxidation and reduction peaks. The voltammograms displayed two sets of redox peaks. Anodic peaks were observed around 0.01 V and 0.46 V vs. SCE, associated with oxidation processes. Cathodic peaks appeared near 0.32 V and − 0.14 V vs. SCE, corresponding to reduction processes.

These peaks indicate Faradaic charge transfer reactions involving the Zn₃(VO₄)₂ nanomaterial, highlighting its electrochemical activity. The peak separation (ΔE = Epa – Epc) provides insights into the reversibility and kinetics of the redox process. Reversible systems typically display small peak separations (close to the theoretical value, e.g., ~ 59 mV/n for a one-electron process at 25 °C^{45, 46–47}. Larger peak separations suggest quasi-reversible or irreversible behavior, indicating slower electron transfer kinetics. In this case, the observed potential difference between the oxidation and reduction peaks, ΔE ≈ 0.52 V, is notably large. Larger ΔE values imply that the redox processes are either quasi-reversible or partially irreversible, suggesting slower electron transfer kinetics or potential diffusion limitations. However, the presence of distinct peaks indicates that some electron transfer occurs, making the material suitable for electrochemical applications, including high-performance energy storage devices and sensors^48,49. The CV analysis shows that Zn₃(VO₄)₂ nanomaterials, especially when prepared under optimal conditions (e.g., at 500 °C), demonstrate better electrochemical performance and higher peak currents compared to other samples. This suggests that the electrode material possesses greater electrical capacity⁴⁵.

The presence of clear redox peaks and pseudo-capacitive characteristics makes these materials promising candidates for high-performance energy storage devices, sensors, and catalytic applications. Broader Cyclic voltametric peaks typically indicate sluggish electron transfer, kinetic limitations, or irreversibility. Analysing ΔEp helps differentiate between reversible and irreversible behaviours, evaluate electrode kinetics, and understand the nature of the electrochemical process^48,50,51.

Figure 6(b) presents the square wave voltammetry (SWV) results for the three samples and bare GCE in 0.1 M NaOH. These SWV results confirm the findings observed in the cyclic voltammetry shown in Fig. 6(a). To further investigate the electrochemical kinetics of the modified GCE (GCE/Zn₃(VO₄)₂) annealed at 500 °C, CV measurements were performed at different scan rates (20, 40, 60, 80, and 100 mV/s), as displayed in Fig. 6(c). As the scan rate increases, all redox peaks remain observable and maintain similar shapes. The peak currents slightly increase with increasing scan rate. Additionally, there is a slight shift of the peaks toward lower and higher potentials during oxidation and reduction, respectively. This behavior suggests that the excellent performance of the modified electrode is controlled by pseudo-capacitance and diffusion processes^48,51. A plot of peak current (ip) versus the square root of the scan rate (ν^1/2) is shown in Fig. 6(d) and is used to characterize the electrochemically quasi-reversible redox system. The peak currents increase linearly with the square root of the scan rate, with correlation coefficients of 0.9538 and 0.9429, indicating good linearity consistent with quasi-.

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Fig. 6

(a). Cyclic voltammetry, (b) Square Wave voltammetry of GCE/ Zn₃(VO₄)₂ at R.T. (black line), GCE/ Zn₃(VO₄)₂ at 500 °C (blue line), GCE/ Zn₃(VO₄)₂ 700 °C (red line) at 50mV/s and (c) GCE/ Zn₃(VO₄)₂ at 500 °C at different scan rates (20–100 mV/s) in 0.1 M NaOH electrolyte. (d) Plot of i_p vs. ν^1/2 for redox peaks.

Further electrochemical characterization of the modified electrode was performed using electrochemical impedance spectroscopy (EIS). EIS measures the system’s response to a small AC potential perturbation over a range of frequencies, offering significant insights into the mechanisms of charge storage, charge transfer processes, and diffusion phenomena within the nanomaterial. The Nyquist plots for the GCE/ZnVO electrodes at room temperature (RT), 500 °C, and 700 °C are illustrated in Fig. 7. These plots show the real part of impedance plotted against the imaginary part over a range of frequencies. The Nyquist plots exhibit a distinctive combination of features: a semicircular region at high frequencies and a linear segment at low frequencies. The semicircle at high frequencies is associated with the charge transfer resistance (Rct), indicating the ease of electron transfer at the electrode/electrolyte interface, which may involve both physical double-layer charging and redox (faradaic) reactions^48,49. The linear segment at lower frequencies, referred to as Warburg impedance, signifies diffusion-controlled processes within the nanomaterial⁴⁶. The shape and extent of these features provide insights into the fundamental charge transport and diffusion behaviors.

In our analysis, the Nyquist plots reveal clear semicircular and linear regions, suggesting that charge storage involves both capacitive and resistive components, along with indications of diffusion processes. The presence of a semicircle implies that redox reactions contribute to charge storage, indicating that the material stores charge via double-layer capacitance and reversible redox reactions⁴⁷. The charge transfer resistances (Rct) for the electrodes at room temperature, 500 °C, and 700 °C were found to be 170, 125, and 151 ohms, respectively. The high Rct values observed in the EIS spectra suggest limited charge transport within the samples (RT and 700 °C). The relatively lower Rct for GCE/Zn₃(VO₄)₂ calcined at 500 °C may be attributed to improved crystallinity and enhanced electrical connectivity resulting from calcination at this temperature, which can facilitate charge transfer pathways⁴⁸.

Typically, a smaller semicircle indicates lower charge transfer resistance, reflecting better electrical conductivity or more effective charge transfer at the interface. Sometimes, a small semicircle may result from various factors, including high capacitance or fast kinetics; however, this does not necessarily imply high intrinsic conductivity of the bulk material. Instead, it primarily suggests improved charge transfer at the interface. Larger semicircles signify higher charge transfer resistance, indicating more obstructed electron transfer^{47, 48, 49, 50–51}. These EIS results are in good agreement with the cyclic voltammetry data. The linear segment observed at low frequencies confirms that ion diffusion is a crucial process, and the shape of the Nyquist plots indicates that both capacitive (double-layer and pseudocapacitive) and faradaic processes participate in overall charge storage.

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Fig. 7

Impedance plot: a. Nyquist plot: GCE/ Zn₃(VO₄)₂ at 700 °C (blue line), GCE/ Zn₃(VO₄)₂ 500 °C (red line) and GCE/ Zn₃(VO₄)₂ at R.T. (black line) electrode materials 0.1 M NaOH electrolyte.

Conclusion

This study demonstrates the successful synthesis of Zn₃(VO₄)₂ nanomaterials via an eco-friendly green chemistry approach utilizing Moringa Oleifera extract. Structural analyses, the FTIR results demonstrate the removal of organic phytochemicals, evidenced by the disappearance of C = O peaks, and the formation of metal-oxygen bonds (V–O and Zn–O), indicating the synthesis of metal oxide and vanadate phases through thermal decomposition and oxidation. XRD confirms the formation of phase-pure Zn₃(VO₄)₂ with high crystallinity, reflecting controlled nucleation facilitated by bio-organic compounds. HRTEM images reveal morphologies consistent with bio-mediated nucleation and growth, supporting the role of plant phytochemicals in directing the reduction process^{52, 53–54}.

Chemical reaction: Zn²⁺+ V⁵⁺ + plant phytochemicals → Zn₃(VO₄)₂ + oxidized organics.

This illustrates that phytochemicals act as reducing agents, transforming metal precursors into Zn₃(VO₄)₂ while themselves undergoing oxidation, confirming the green synthesis pathway [53–54].

The Zn₃(VO₄)₂ nanomaterials exhibit significant photocatalytic capabilities, achieving up to 87% degradation of methylene blue (MB) dye under visible light after 120 min under optimal conditions, particularly near pH 5.0. This high efficiency, aligned with pseudo-first-order kinetics, underscores their potential for environmental remediation of organic pollutants in wastewater. The photocatalytic process is sensitive to pH, with deviations leading to reduced performance due to decreased dye adsorption and increased nanoparticle aggregation. In addition to photocatalysis, the nanomaterials demonstrate promising electrochemical properties as electrode materials for supercapacitors. Cyclic voltammetry reveals broad redox peaks and high electroactivity, indicative of pseudo-capacitive behaviour driven by faradaic reactions. Impedance analysis shows charge transfer resistance and Warburg impedance consistent with diffusion-controlled processes, further supporting their suitability for energy storage applications. The nanomaterials also exhibit good stability and recyclability under repeated use, suggesting practical potential. However, further long-term stability assessments and evaluations under real environmental conditions are necessary to confirm their durability and applicability in complex wastewater matrices, which may contain various interfering substances. While Zn₃(VO₄)₂ nanomaterials are multifunctional, their performance suggests a slight preference for photocatalytic applications due to their semiconductor nature and proven effectiveness under light irradiation.

In overall, Zn₃(VO₄)₂ nanomaterials synthesized using an eco-friendly method exhibit strong potential for environmental remediation and energy storage. Their high photocatalytic efficiency, coupled with favourable electrochemical properties, makes them versatile candidates for multifunctional applications. Scaling up the green synthesis method utilizing plant extracts is possible through effective process optimization. Important factors to consider involve standardizing the preparation of extracts to guarantee reproducibility, regulating reaction conditions, and managing resource supply. By addressing these factors, it will be possible to achieve consistent, economical, and sustainable large-scale production of Zn₃(VO₄)₂ nanomaterials for environmental uses. Future research should focus on optimizing synthesis parameters, the scalability of the method, long-term stability, and performance valuation in real-world wastewater condition to maximize their potential.

Acknowledgements

This research was generously supported by the UNESCO-UNISA Africa Chair in Nanoscience and Nanotechnology, to whom we are all grateful.

Author contributions

N. Matinise: Conceptualization, Methodology, Investigation, Data curation, Analysis of figures, Writing – original draft. N. Botha: Resources, Analysis of figures. A. Fall: methodology Analysis of Figures. M. Maaza: Supervision. All authors approved the manuscript and this submission.

Data availability

All data generated or analysed during this study are included within the article.

Declarations

Competing interests

The authors declare no competing interests.

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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