Biogenic ZnO/CuO/Fe₂O₃ Nanocomposite: A Groundbreaking Approach for Enhanced Degradation Capabilities and Reusability in Dye Removal Applications

Ocimum Basilicum L. were collected from east Algeria. Iron (III) chloride (FeCl₃, 98%), zinc chloride (ZnCl₂, 98%), copper (II) chloride hydrate (CuCl₂-2H₂O, 98%), sodium hydroxide (NaOH, 98%), were purchased from Sigma-Aldrich, Germany.

To synthesize the ZnO/CuO/Fe₂O₃ nanocomposites, a mixture containing 8.2 g of FeCl₃, 2.8 g of ZnCl₂, and 8.2 g of CuCl₂·2H₂O was combined with the prepared Ocimum Basilicum extract. The resulting mixture was subjected to continuous stirring and heating at 75 °C for approximately three hours. During this process, drops of NaOH (2 M) were added incrementally to adjust the solution's acidity until a noticeable change in color occurred, leading to the formation of a brown precipitate. To separate the precipitate, a centrifuge was employed, operating at a speed of 3000 rpm for a duration of 5 min. The obtained precipitate was then subjected to multiple washes using distilled water (DW) to eliminate impurities. Subsequently, the wet powder was dried by placing it in an oven set at 80 °C overnight. To obtain the final ZnO/CuO/Fe₂O₃ nanocomposite, the dried powder underwent annealing in an oven at a temperature of 500 °C for a duration of 3 h. This process facilitated the formation and stabilization of the desired nanocomposite structure. The experimental procedure for preparing the ZnO/CuO/Fe₂O₃ nanocomposite.

The UV–visible spectrophotometer (SECOMAM, model 9600, France) was employed to assess the optical properties within the wavelength range of 200–800 nm. To investigate the crystalline nature of the ZnO/CuO/Fe₂O₃ nanocomposite, XRD analysis was performed using the Proto Manufacturing Company's Benchtop model XRD machine (USA). The Fourier transform infrared spectroscopy (FTIR) technique was utilized (Thermo Fisher Scientific, Nicolet iS5 model, USA) to confirm the functional groups present in the nanocomposites. Additionally, a scanning electron microscope (TESCAN, VEGA3 model, USA) was utilized to examine the particle size and shape. The crystallite size was determined using the Scherrer formula (Eq. 1), where a prominent peak with the highest intensity was selected. The formula is as follows [52]:Here "k" represents the form factor (0.9), "D" represents the crystallite size (0.15418 nm, CuK), "β" denotes the Full Width at Half Maximum (FWHM), and "θ" represents the diffraction angle.

The degradation of Toluidine blue (TB), p-toluidine (PT) and m-Toluidine (MT) under sunlight at 29 °C, was studied using the nanocomposite as a photocatalyst. An aqueous solution containing the dyes at a concentration of 2.5 × 10^–5 M of TB, p-toluidine and m-Toluidine was used. 5 ml of each dye was placed separately in several beakers and 5 mg of ZnO/CuO/Fe₂O₃ was added. The resulting mixture was then irradiated under sunlight for 0, 15, 30, 45, 60, 75, 90 and 105 min. ZnO/CuO/Fe₂O₃ was separated from the solution using a photocatalyst powder removal centrifuge. The degradation activity was investigated in March 2023 and the average sunlight intensity was about 29 °C in El Oued, Algeria. The absorbance of dye was analyzed through UV–Vis spectrophotometry in the range of 200–900 nm. The percentage of hydrolysis of each dye was calculated using the following equation [53].where \({A}_{0}\) is the measured absorbance of dye solutions without the catalyst nanocomposite, \({A}_{(t)}\) is the measured absorbance when the catalyst is added after each time periods.

Figure 1 illustrates the UV–Vis absorption spectra of the ZnO/CuO/Fe₂O₃ nanocomposite, providing valuable insights into its optical properties.

In Fig. 1a, the absorption spectrum in the visible range reveals a peak at λ_max = 360.9 nm. This indicates that the nanocomposite is capable of absorbing light in the visible region, suggesting its potential for various optoelectronic applications. Furthermore, the band gap energy of the ZnO/CuO/Fe₂O₃ nanocomposite was determined using the (αhν)² versus energy function (eV) plot, as shown in Fig. 1b. By analyzing the data, the band gap energy was estimated to be 1.44 eV. The band gap energy is a crucial parameter in determining the semiconductor behavior and optical properties of materials. The obtained value of 1.44 eV indicates that the nanocomposite possesses a suitable band gap for absorbing light in the visible range. The successful synthesis of the ZnO/CuO/Fe₂O₃ nanocomposite is supported by the observed absorption in the visible spectrum and the determined band gap energy. These findings highlight the potential of the nanocomposite for applications such as photocatalysis, photovoltaics, and sensor technologies, where efficient light absorption and charge carrier generation are essential.

As shown in Fig. 2, the XRD pattern showed multiple peaks of the produced ZnO/CuO/Fe₂O₃ NC.

The broad peaks show that the material as it was produced comprises nanoscale-sized particles. The 2θ angles corresponding to the diffraction peaks are as follows: 31.779°, 34.430°, 36.265°, 47.554°, 56.615°, 62.876°, 66.400°, 67.971°, 69.112°, 72.587°, and 76.988°. These angles correspond to the crystallographic planes (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4), and (2 0 2), respectively. These observations indicate that the ZnO NPs possess a hexagonal crystal structure with a space group of P63mc (186) and lattice parameters of a = 3.2495 Å and c = 5.2069 Å, as specified in JCDPS Card No. 01-089-0510. Additionally, the presence of the CuO NPs was confirmed by the XRD spectrum, which included peaks other than the normal ZnO/CuO/Fe₂O₃ peaks at 2θ = 32,524°, 35,564°, 38,725°, 46,287°, 48,795°, 51,360°, 53,480°, 56,719°, 58,297°, 61,582°, 65,816°, 66,528°, 68,120°, 68,923°, 72,424°, 72,963°, 75,032°, corresponding to crystallographic reflection planes (1 1 0), (−1 1 1), (1 1 1), (−1 1 2), (−2 0 2), (1 1 2), (0 2 0), (0 2 1), (2 0 2), (−1 1 3), (0 2 2), (3 1 0), (2 2 0), (−2 2 1), (3 1 1), (2 2 1) and (0 0 4) respectively, where CuO NPs had a monoclinic crystal structure (Space group C2/c (15) and lattice parameters of a = 4,6830 Å b = 3,4240 Å and c = 5,1290 Å) JCDPS Card No. 01-089-5896. Furthermore, the XRD spectrum revealed peaks other than the typical Fe₂O₃ peaks, confirming the presence of the ZnO/CuO/Fe₂O₃ complex at 2θ = 23,851°, 32,856°, 35,080°, 39,076°, 40,321°, 42,866°, 48,823°, 53,511°, 55,254°, 56,544°, 57,194°, 61,527°, 62,931°, 65,114°, 68,891°, 71,636°, and 74,133°, corresponding to crystallographic reflection planes (0 1 2), (1 0 4), (1 1 0), (0 0 6), (1 1 3), (2 0 2), (0 2 4), (1 1 6), (2 1 1), (1 2 2), (0 1 8), (2 1 4), (3 0 0), (1 2 5), (2 0 8), (1 1 9), (2 2 0) respectively, where Fe₂O₃ NPs had a rhombohedral crystal structure (Space group R-3c (167) and lattice parameters of a = 5,1120 Å b = 5,1120 Å and c = 13,8200 Å) JCDPS Card No. 088-2359. The crystallite sizes of the ZnO/CuO/Fe₂O₃ nanocomposite sample were 29.1 nm, according to the Scherrer formula.

The FT-IR studies of Ocimum Basilicum leaf extract and synthesized ZnO/CuO/Fe₂O₃ nanocomposite are given in Fig. 3.

The FTIR analysis was performed to determine the purity and nature of the nanoparticles, as well as the existence of phytochemicals in the extract. The FTIR spectrum at around 3400–3800 cm⁻¹ for the Ocimum Basilicum extract could be attributed to the hydroxyl of the phenols stretching mode [54]. The O–H stretching vibrations could potentially account for the absorption peaks detected within the range of 3200–3500 cm⁻¹ in the spectrum of the ZnO/CuO/Fe₂O₃ nanocomposite [55]. The peaks around 2360 cm⁻¹ and 3073 cm⁻¹ are the H–O–H vibration of a cluster of crystallized water molecules and C–H stretching, respectively [56, 57]. The peak at 1508 cm⁻¹ was likely due to C=O [57]. The stretching vibration of C–O–C is responsible for the band observed at 1100 cm⁻¹ in both extract and ZnO/CuO/Fe₂O₃ nanocomposite spectrum. Furthermore, the peaks detected at 2878 cm⁻¹ were assigned to the stretching vibrations of the –CH₂ functional group [58]. According to related studies, the characteristic peaks of Fe–O in Fe₂O₃ were at 390–566 cm⁻¹, while the Cu–O stretching band in the monoclinic phase and the Zn–O stretching band were observed in 420–600 and 401–670 cm⁻¹ regions, respectively [59‐61].

The morphological dimensions of the produced ZnO/CuO/Fe₂O₃ nanocomposites were investigated using SEM images (Fig. 4a).

The average size distribution of the biosynthesized ZnO/CuO/Fe₂O₃ nanocomposite is around 65 nm, as shown by the particles size distribution histograms in Fig. 4b. As shown, ZnO/CuO/Fe₂O₃ nanocomposite particles come in a variety of sizes and irregular forms. The performance of photocatalysis can be significantly influenced by the geometry of a material's surface. Figure 4c illustrates the EDX analysis of the ternary nanocomposite ZnO/CuO/Fe₂O₃, displaying the mass and atomic percentages of different elements. The embedded table summarizes these value percentages for easy reference. The higher mass and atomic percentages of oxygen (O) and zinc (Zn) indicate their dominant presence in the composition. This could be attributed to the synthesis method, or the nature of the starting materials used. Conversely, iron (Fe), copper (Cu), and chlorine (Cl) exhibit lower percentages in comparison. These findings provide insights into the elemental compositions of the nanocomposite. Figure 4c enhances the understanding of the composition by visually representing the higher percentages of oxygen (O) and zinc (Zn), as well as the lower percentages of iron (Fe), copper (Cu), and chlorine (Cl). This graphical overview complements the table, facilitating comparisons between different elements and contributing to a comprehensive understanding of the nanocomposite's composition.

After the obtained results, this study confirmed the effectiveness of the nanocomposite in adsorption and photocatalysis in the decomposition of toluidine blue (TB), p-toluidine (PT) and m-Toluidine (MT) dyes under sunlight at normal atmospheric temperature. The intensity of the absorption peaks for all dyes used in this experiment decreased gradually with the increase in time without changing the maximum absorption wavelength. UV–Vis analysis revealed the absorption features at λ_max = 630.8 nm, 287.2 nm and 283 nm for TB, PT and MT, respectively. The results indicate that ZnO/CuO/Fe₂O₃ NPs have good disappearance efficacy TB, PT and MT dyes and showed a degradation rate of 99%, 99.1% and 99.7%, within 90 min, respectively (Fig. 5a, b, and e).

Kinetic studies, using a pseudo-first-order kinetic model, demonstrate the photocatalytic performance of the as-prepared photocatalysts (Eq. 3) [62] as shown in Fig. 5. Moreover, the rate constant of ZnO/CuO/Fe₂O₃ NC was estimated to be about 0.0314 min⁻¹, 0.0189 min ⁻¹ for TB, p-toluidine and m-Toluidine, respectively.where K is pseudo-first order rate constant, and t is irradiation time.

The photoremoval kinetics of the dyes was studied using pseudo-first-order kinetics. The degradation efficiency and rate constant for TB, p-toluidine and m-Toluidine were calculated from the reaction profiles and kinetic plots in Fig. 6 using the simple equations as follows:where R% is degradation efficiency of TB, p-toluidine and m-Toluidine, C₀ is the initial concentration and C_t is the concentration of days at different irradiation time.

The objective of this study was to investigate the stability and reusability of the ZnO/CuO/Fe₂O₃ nanocomposites, which was prepared for treating textile wastewater. This research also aimed to assess the applicability of the nanocomposite in industrial settings, considering sustainability and stability as important factors. In the initial cycle, a sample of the synthesized photocatalyst (20 mg) was added to a solution containing TB dye (2.5 × 10^–5 M) in a volume of 20 mL. Each solution was then exposed to sunlight for 105 min. Subsequently, the photocatalyst sample was separated from the solutions using filtration, washed multiple times with double distilled water, and dried in an oven at 60 ℃ for 4 h to remove any remaining moisture. The dried photocatalyst was reused in the subsequent cycles under the same experimental conditions as the first cycle. This process was repeated for five cycles, and the degradation efficiencies for each cycle, as well as the degradation percentages over time (C/C₀), are presented in Fig. 7a and b.

The degradation process of dyes in a solution is initiated by the catalyst's photoexcitation, which results in the generation of electron–hole pairs on the photocatalyst's surface (as shown in Eqs. 5–11) [63].

The positively charged holes (h +) in the valence band (hVB +) possess a high oxidative potential, enabling the direct oxidation of dyes into reactive oxidative intermediates (as illustrated in Eq. (6)). The degradation of TB, for instance, involves the participation of OH• radicals, which are formed either by the decomposition of water or through the reaction of h + with OH − molecules, OH• radicals, although non-selective, are potent oxidants that facilitate the partial or complete mineralization of organic compounds. When ZnO/CuO/Fe₂O₃ nanocomposites are irradiated with visible light, they undergo photoexcitation via surface absorption, leading to the injection of photoexcited electrons into O₂. As a result, the production of superoxide and hydroxyl radicals is accelerated, ultimately enhancing the rate of degradation for dye molecules. The process of photocatalytic degradation of dyes using ZnO/CuO/Fe₂O₃ nanocomposite under solar radiation is shown in the following equations [64]. Figure 8 illustrates the mechanism behind the enhanced photocatalytic activity of ZnO/CuO/Fe₂O₃ nanocomposites under solar irradiation.