Introduction

In recent decades, water pollution has become one of the most significant environmental challenges worldwide. The release of various pollutants, including organic pigments from the textile, dyeing, and pharmaceutical industries, into water has raised serious concerns about human health and the environment, becoming a significant environmental challenge. Despite their chemical stability and high biodegradability, these dyes are difficult to remove from the environment and may have harmful effects on humans and the environment. For this reason, it is imperative to develop efficient methods for removing these pollutants. In this regard, various technologies have been developed to remove these pollutants, including physical, chemical, and biological processes1,2. However, many of these methods have limitations such as high cost, production of sludge and hazardous by-products, and inefficiency at low concentrations of pollutants3. Photocatalytic processes have attracted the attention of many researchers as a new and promising method for water purification. In this process, semiconductors such as titanium oxide (TiO2) and zinc oxide (ZnO) are used as catalysts, which, by absorbing ultraviolet or visible light, are capable of producing strong oxidizing free radicals that can decompose organic pigments and convert them into harmless substances4. ZnO is an attractive option among semiconductor materials due to its good photocatalytic properties, chemical stability, low cost, and availability, and has been widely studied5. Pure ZnO, however, has limitations, including a high band gap energy and a high electron–hole recombination rate, which reduce its photocatalytic efficiency6. To improve the photocatalytic properties of ZnO, the use of metal oxide nanocomposites has been considered. Combining ZnO with other metal oxides can lead to an increase in the specific surface area, a decrease in particle size, and the introduction of impurities that reduce the electron–hole recombination rate, thereby improving the photocatalytic efficiency7,8,9. For example, Bakre et al. (2020) showed that CuO-NiO-TiO2 nanocomposites have high catalytic efficiency in various applications10. Additionally, the method of synthesis of nanocomposites also affects their properties and effectiveness. Green synthesis methods have been proposed as a suitable alternative to traditional chemical methods due to the use of natural and non-toxic materials and agents11. Plant extracts contain organic compounds that can serve as reduction and stabilization agents in the manufacture of NPs12. Previous studies have shown that the usage of nanocomposites has been effective in the degradation of organic dyes13,14,15. For example, Kanmani and Ramachandran (2012) showed that TiO2/ZnO core–shell nanostructures have high efficiency in the degradation of MB16. Also, Sabouri et al. prepared NiO nanoparticles by a green method using Gum Arabic and investigated their cytotoxic, photocatalytic, and antibacterial effects17. In this study, a ternary metal oxide nanocomposite, ZnO-MgO-Gd2O3, was synthesized using a green synthesis method with the extract of Ocimum basilicum L. The physical and chemical properties of the produced nanocomposite were assessed using various methods such as XRD, FESEM, and UV–Vis. Also, the photocatalytic efficiency of this nanocomposite in the degradation of MB, MO, EBT, and RhB dyes was evaluated. Although numerous studies have been conducted on ZnO binary nanocomposites, reports on the green synthesis of ZnO-MgO-Gd₂O₃ (ZMG) ternary nanocomposites, particularly using basil seed extract, are very rare and lack sufficient detail. A comprehensive examination into the correlation between their structure, photonic properties, and performance is also lacking. Therefore, the primary objective of our research is to develop an environmentally friendly synthesis method and thoroughly examine the photocatalytic potential of this emerging nanocomposite.

Experimental process

Materials

Zn (NO3)2. 6H2O (98.5%), Mg (NO3)2. 6H2O (99%), Gd (NO3)3. 6H2O (99.9%), MB (C16H18ClN3S), RhB (C28H31ClN2O3), EBT (C20H12N3O7SNa), MO (C14H14N3NaO3S), HCl (98%), and NaOH were acquired from Merck Co. The solvent of choice was also distilled water. Ocimum basilicum L. seeds were commercially sourced from a supermarket in Dargaz City, Iran.

Production of Ocimum basilicum L. seed extract

A fresh aqueous Ocimum basilicum L. seed extract was prepared in our laboratory for the green synthesis of ZnO-MgO-Gd₂O₃ nanocomposite as reducing and capping agents. The seeds were commercially sourced from a supermarket in Dargaz City, Iran. Ocimum basilicum L. seed extract’s primary role was to serve as a multifunctional agent for the reduction of metal precursors and stabilization of the formed nanocomposite. After washing with distilled water and drying in the dark, 1 g of Ocimum basilicum L. seeds was mixed with 100 mL of deionized water and stirred for 2 h at 60 °C. Then, the prepared extract was concentrated using a rotary evaporator to increase the amount of active ingredient in the extract. Lastly, for the synthesis step, the filtered extract was maintained at 4 °C.

Preparation of nanocomposite

For the synthesis of the ZMG nanocomposite, aqueous solutions (0.5 M) of Zn (NO3)2·6H2O, Mg (NO3)2·6H2O, and Gd (NO3)3·6H2O were prepared. These solutions were combined in a single beaker and stirred continuously for 45 min. Following this mixing period, 40 mL of the Ocimum basilicum L. plant extract was added to the mixture solution of salts. The resulting solution was maintained at a constant temperature of 80 °C for 6 h for gel formation. The gel was subsequently dried in an oven at 80 °C for 8 h. Afterward, it was calcined in a muffle furnace at 500/600 °C for 2 h each to yield ZMG nanocomposite18. Figure 1 depicts the schematic diagram of the nanocomposite synthesis process.

Fig. 1
Fig. 1
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Schematic of ZnO-MgO-Gd2O3 Nanocomposite synthesis steps.

Characterization

To comprehensively investigate the properties of ZMG nanocomposites, detailed characterizations were performed on their structure, morphology, and optical properties. In this regard, the crystal structure of the sample was determined using XRD (D8-ADVANCE/ Bruker axs USA) in the range of 2θ = 10–80°, and the functional groups present in it were identified by FT-IR (AVATAR 370) in the range of 400–4000 cm⁻1. The surface morphology and particle dimensions were studied using FESEM (JEOL 7600F) and TEM (LeO910/Germany). The high-resolution images obtained were analyzed with Digimizer software to determine the particle size distribution. Simultaneously, elemental analysis was performed using EDS to confirm the weight and atomic percentage of the elements and their distribution in the nanocomposite structure. Finally, the optical characteristics and band gap energy of the material were evaluated using UV–Vis-DRS (SHIMADZU 3600/ Japan).

Experiments related to photodegradation

In order to evaluate the photocatalytic efficiency of ZMG nanocomposite, its performance in the degradation of four types of organic dyes, including MB, MO, EBT, and RhB, under ultraviolet UVA light irradiation was studied. For this purpose, first, 26 mg of photocatalyst was added to 100 mL of an aqueous solution of each dye with a concentration of 10−5 M. The resulting suspension was then kept in complete darkness for 45 min under continuous stirring to establish surface adsorption–desorption equilibrium between catalyst particles and dye molecules. After this stage, the photodegradation reaction was initiated by exposing the reactor to UVA light (UVA lamp power = 20 W, and λ = 365 nm). To monitor the reaction process, 2 mL samples were taken at regular intervals of 15 min. To determine the residual dye concentration, the samples were centrifuged for 10 min to completely separate the catalyst particles, and then the resulting clear solution was analyzed with a UV–Vis spectrophotometer. Measurements were made at the maximum absorption wavelength (λmax) of each dye, including 663 nm for MB, 464 nm for MO, 550 nm for EBT, and 555 nm for RhB19,20. Finally, the dye degradation efficiency was calculated using Eq. 1, which evaluates the concentration changes over time21,22.

$$\text{Degradation }(\text{\%})= [({\text{A}}_{0}-{\text{A}}_{\text{t}})/{\text{A}}_{0}] \times 100$$
(1)

where A0 represents the initial absorbance and At represents the absorbance at a given time t. Also, to predict the reaction path, the kinetics of the photocatalytic process were investigated using Eq. 2.

$$Ln \left(\frac{{C}_{t}}{{C}_{0}}\right)={K}_{Obs}t$$
(2)

In this equation, C0 refers to the initial concentration, while Ct denotes the concentration at a given time t, and Kobs is the observed rate constant.

Results and investigation

XRD

The XRD spectra of the ZMG nanocomposites were compared using X’pert High Score Plus software. X-ray diffraction patterns of the calcined nanocomposite at 500 and 600 °C are shown in Fig. 2. The diffraction peaks at angles of 31.76, 34.42, 36.25, 47.53, 56.59, 62.85, 67.94, and 69.08° correspond to crystal planes (100), (002), (101), (102), (110), (103), (112), and (201) for ZnO with hexagonal structure and space group P63mc (JCPDS No # 01–079-0206), respectively23. MgO crystallizes in a cubic structure characterized by the space group Fm-3 m. Remarkably, the XRD pattern of MgO reveals specific peak positions corresponding to crystal planes. The detected angles at 36.93, 42.91, 62.3, and 74.69°, which can be associated with (111), (200), (220), and (311) planes, respectively, for MgO, with the standard card (JCPDS No # 00–045-0946)24. On the other hand, peaks with angles of 20.10, 28.57, 33.11, 35.17, 42.59, 47.52, 52.1, 56.4, and 59.15° correspond to crystal planes (211), (222), (400), (411), (134), (440), (611), (622), and (444) of Gd2O3 with cubic structure and space group Ia3 (JCPDS No #00–043-1014)25,26. The crystallite size and strain due to lattice deformation were obtained using the Halder-Wagner method (Eq. 3). In this method, the peak broadening due to crystallite size and lattice strain is assumed to be Lorentzian and Gaussian, respectively27.

$${{\beta }^{2}}_{hkl}={\beta }_{l}{\beta }_{hkl}+{\beta }_{G}$$
(3)

where βl and βG are the broadenings of the Lorentzian and Gaussian distributions, respectively, the advantage of the Halder-Wagner method is that it assigns more weight to the diffraction peaks at low and intermediate angles, where the overlap of the diffraction peaks is minimal. In this method, the crystal size (DH-W) and strain are obtained from the following Eq. 4.

$${\left(\frac{{\beta }_{hkl}^{*}}{{d}_{hkl}^{*}}\right)}^{2}=\frac{1}{D} \frac{{\beta }_{hkl}^{*}}{\left({{d}_{hkl}^{*}}^{2}\right)}+\frac{{\varepsilon }^{2}}{4}$$
(4)

where ε and D represent the strain and crystal size, respectively, and \({\beta }_{hkl}^{*}=\frac{{\beta }_{hkl}\text{cos}\theta }{\lambda }\) and \({d}_{hkl}^{*}=\frac{{2d}_{hkl}\text{sin}\theta }{\lambda }\)

Fig. 2
Fig. 2
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XRD pattern of ZnO-MgO-Gd₂O₃ nanocomposites (500 and 600 °C).

In the Halder-Wagner method, first, the plot of \({\left(\frac{{\beta }_{hkl}^{*}}{{d}_{hkl}^{*}}\right)}^{2}\) is plotted against \(\frac{{\beta }_{hkl}^{*}}{\left({{d}_{hkl}^{*}}^{2}\right)}\) for the (hkl) peaks with higher intensity. Then the crystal size and strain can be obtained, respectively, by using the inverse of the slope and the width from the origin of the linear fit of the data. The Halder-Wagner curve of the ZMG nanocomposite calcined at two temperatures of 500 and 600 °C is shown in Fig. 3. According to the calculations, the crystallite size values of the ZMG nanocomposite calcined at temperatures of 500 and 600 °C were obtained as 12.06 and 12.34 nm, respectively.

Fig. 3
Fig. 3
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Curve of variation of \({\left(\frac{{\beta }_{hkl}^{*}}{{d}_{hkl}^{*}}\right)}^{2}\) in terms of \(\frac{{\upbeta }_{\text{hkl}}^{*}}{\left({{\text{d}}_{\text{hkl}}^{*}}^{2}\right)}\) For ZMG nanocomposite calcined at temperatures of 500 °C (a) and 600 °C (b).

FT-IR

FT-IR analysis of the synthesized ZMG nanocomposite is shown in Fig. 4. The functional groups present in this nanocomposite were investigated in the range of 400 to 4000 cm-1. Under 11,000 cm-1, the peak represents the stretching vibration of the M–O bond, indicating the metal oxide nanocomposite’s excellent dispersion28. In addition, the peak identified in the range of 1300 to 1600 cm-1 is usually related to the vibrations of the C-H bonds. Also, the absorption band at 3426 cm-1 indicates the presence of the stretching vibrations of the O–H group29. The data show that a temperature increase from 500 °C to 600 °C leads to a reduction in the intensity of the C-H and O–H peaks.

Fig. 4
Fig. 4
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FT-IR spectra of Ocimum basilicum L. seed extract and ZnO-MgO-Gd₂O₃ nanocomposites.

Morphological analysis

FE-SEM images of ZMG nanocomposites are presented in Fig. 5 (a, b). Increasing the synthesis temperature yields a nanocomposite with more uniform, highly regular spherical particles. To achieve precise measurements of particle size, the FE-SEM images were evaluated with DigiMizer software. Subsequently, the analysis was performed using IBM SPSS software, and the resulting particle size distribution curves are illustrated in Fig. 5 (c, d). The elements of the ZMG nanocomposite were identified and quantified using EDS analysis (Fig. 5 (e, f)), so that the presence of elements such as zinc, magnesium, gadolinium, and oxygen in the synthesized nanocomposite was confirmed. On the other hand, the elements’ atomic and weight percentages are inserted in a table in Fig. 5. Furthermore, the elemental map (Fig. 5 (g)) shows the nanocomposite’s elemental distribution, confirming the uniform dispersion of particles throughout the composite structure. The TEM image and particle size curve of the ZMG nanocomposite produced at 500 °C are shown in Fig. 6 (a, b). The results showed that the produced nanocomposite has a spherical morphology with a diameter of approximately 67 nm, with a homogeneous spatial distribution of particles.

Fig. 5
Fig. 5
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FE-SEM pictures of ZnO-MgO-Gd₂O₃ nanocomposites 500 °C (a) and 600 °C (b), particle size curve 500 °C (c) and 600 °C (d), EDX spectrum 500 °C (e) and 600 °C (f), and Mapping images (g).

Fig. 6
Fig. 6
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TEM image (a) and particle size curve (b) of ZnO-MgO-Gd₂O₃ nanocomposite at 500 °C.

UV–Vis-DRS analysis

The energy gap diagram and absorption spectrum of ZMG nanocomposite for two temperatures of 500 and 600 °C are shown in Fig. 7. The absorption edge in the absorption spectrum of the nanocomposite in the wavelength range of 380 nm indicates the presence of a band gap, which is an effective indicator in the analysis of the DRS spectrum. The energy gap values were calculated using Eqs. 5 and 6, and their values are reported in the diagram30. The results showed that the energy gap value decreases with increasing calcination temperature. The significant decrease in the band gap from 3.7 eV to 2.9 eV with increasing calcination temperature (from 500 °C to 600 °C) goes beyond being explained by a slight increase in crystallite size (12.06 nm to 12.34 nm). This phenomenon is strongly attributed to the dominance of lattice defects and impurities at 600°C due to quantum confinement. The higher temperature facilitates the formation and stabilization of oxygen vacancies (VO) and the creation of new energy levels within the band gap31. These defect levels shorten the electron–hole transfer paths and cause a strong red shift to 2.9 eV. This decrease in the band gap is highly desirable for enhancing the ability of the catalyst to absorb visible light and explains the improved photocatalytic performance.

Fig. 7
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Tauc diagrams of ZnO-MgO-Gd₂O₃ nanocomposite.

$$\alpha =F\left({R}_{\infty }\right)=\frac{K}{S}= \frac{{\left(1-{R}_{\infty }\right)}^{2}}{2{R}_{\infty }}$$
(5)
$${\left(\alpha .h\nu \right)}^\frac{1}{n}=A\left(h\nu -{E}_{g}\right)$$
(6)

Photocatalytic activity

The photocatalytic process is the absorption of a photon by a molecule or precursor, which causes electronic excitation and the formation of a high-energy excited state in the system. Photocatalysts are known as environmentally active materials that, under the irradiation of sunlight or fluorescent light, can separate pollutants from their surface, and this light irradiation induces chemical reactions without changing the original structure of the material32,33. The main mechanism of this process is based on electron transfer on the surface of the photocatalyst. When radiation energy equal to or greater than the energy gap of the semiconductor is applied to the photocatalyst, electrons are transferred from the valence band to the conduction band. The salient features of photocatalysts include the ability to absorb natural radiant energy, low energy consumption, and environmental compatibility. The fundamental difference between the photocatalytic and catalytic processes is in their activation method; In catalysts, the reaction rate is increased by the formation of catalytic intermediate species, while in photocatalytic processes, the formation of electron–hole pairs under light irradiation accelerates chemical reactions. The positive holes (h⁺) created in the valence band react with water molecules to produce hydroxyl radicals (OH∙). On the other hand, the excited electrons in the conduction band can react with oxygen molecules to form superoxide radicals (\({{O}^{\bullet }}_{2}^{-}\)). These radical species are considered the main factors in the degradation and oxidation processes of organic compounds and can convert organic molecules to simpler products through oxidative pathways34,35,36,37. The chemical reactions (1–5) governing the photocatalytic process are summarized below, and Fig. 8 shows a schematic diagram of this process.

Fig. 8
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The mechanism of the photocatalytic process.

$$h\nu \left(UV\right)+\left(Nanocomposite\right)\to {h}^{+}+{e}^{-}$$
(7)
$${H}_{2}O+{h}^{+}\to {H}^{+}+O$$
(8)
$${O}_{2}+ {e}^{-}\to {{O}^{\bullet }}_{2}^{-}$$
(9)
$$\text{Dye}+{\text{h}}^{+}\to \text{Oxidized products}$$
(10)
$$\text{Dye}+{\text{e}}^{-}\to \text{ Reduced products}$$
(11)

Considering the crucial role of the pH parameter in the efficiency of photocatalytic degradation processes, the effect of changes in pH on the degradation of four organic pollutants (MB, MO, EBT, and RhB) in the presence of the ternary ZMG nanocomposite was evaluated. The experiments were conducted in the concentration range of 10–5 M for dyes and 26 mg for the nanocomposite. In this regard, the effect of different pH levels on the degradation rate of dyes was investigated separately. For MB, MO, and EBT, pH values of 3, 7, and 10 were tested, while for RhB, values of 2, 7, and 10 were used. The pH of the dye solutions was adjusted with 2 M NaOH and 1 M HCl. The results are shown in Fig. 9. Based on the experimental results, the highest degradation efficiency was obtained for MB (98.5%), MO (91.5%), and EBT (99.5%) in an alkaline medium with pH = 10. In contrast, RhB in the acidic medium (pH = 2) showed the highest degradation percentage with a value of 72%. This difference in efficiency can be attributed to the different chemical structures of the dyes and their stability under different pH conditions. The graphs related to the investigation of the photocatalytic activity and kinetic analysis of the reactions for all dyes are presented in Fig. 10. Analysis of the kinetic data shows that the reactions adhere to a pseudo-first-order kinetic model, which indicates the dependence of the reaction rate on the initial concentration of the dyes38.

Fig. 9
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Effect of pH on the degradation of MB (a), EBT (b), MO (c), and RhB (d) dyes.

Fig. 10
Fig. 10
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Photocatalytic activity of ZnO-MgO-Gd₂O₃ nanocomposite at optimum pH of MB (a, pH = 10), EBT (b, pH = 10), MO (c, pH = 10), and RhB (d, pH = 2), pseudo-first-order kinetics of pollutants (e).

Conclusions and prospects

In this study, ZMG nanocomposite was synthesized using Ocimum Basilicum L. extract by the green chemistry method. The results of various analyses, such as XRD, FTIR, UV–Vis/DRS, FE-SEM/EDX/Mapping, indicated the successful synthesis of the aforementioned nanocomposite. The XRD pattern of the synthesized nanocomposite confirmed its crystalline nature. Increasing the calcination temperature resulted in the formation of clearer peaks and higher intensities, indicating an increase in the crystallinity of the nanocomposite. Also, increasing the temperature increased the crystallite size of the synthesized nanocomposite, which is due to the increase in atomic mobility, nucleation, and grain growth during the calcination process. The results of FT-IR examination confirmed the presence of different functional groups within the nanocomposite. Increasing the calcination temperature caused slight changes in specific peaks, which may be due to changes in the bonding environment, lattice vibrations, or molecular interactions. The UV–Vis/DRS spectra of the ZMG nanocomposite showed absorption bands in the range of 200 to 400 nm. Also, the energy gap values of the nanocomposite were calculated according to the DRS data, and it was found that increasing the temperature resulted in a decrease in the energy gap. The larger crystallite size and smaller energy gap in calcined samples are due to temperature differences, as higher temperatures cause thermal energy diffusion and recrystallization. EDS analysis also confirmed the presence of all elements in the nanocomposite. Finally, the photocatalytic activity of the nanocomposite in the degradation of MB, MO, EBT, and RhB dyes was investigated at the optimum pH. By examining the results, it was found that MB, MO, and EBT dyes had a significant degradation percentage in basic conditions, which were 98.5, 91.5, and 99.5, respectively. Also, RhB dye had 72% degradation in acidic conditions. While the green-synthesized nanocomposite shows promising performance, the scalability of its plant-based production method requires further development. Future efforts will therefore focus on scaling the synthesis for industrial applications, such as wastewater treatment, and conducting toxicity studies to ensure environmental safety.