Green Synthesis of Fe₂O₃@Ag/AgCl Core–Shell Nanoparticles with Different AgNO₃ Concentrations for Enhanced Photocatalytic Degradation of Methylene Blue Under UV Irradiation

Nanotechnology has gained considerable attention in recent years due to its potential applications in medicine, catalysis, and environmental remediation (this includes the photodegradation of organic pollutants) [1]. Among these pollutants, methylene blue is a widely used dye in the textile and chemical industries that has caused water and environmental pollution due to its resistance to conventional biodegradation [2]. Photocatalysis using nanomaterials is therefore one of the most effective solutions for removing these dyes because of the high light absorption capacity of these materials and their ability to produce reactive oxygen species (ROS), which contribute to the breakdown of the dye molecules [3]. Among the various types of nanomaterials, iron oxide nanoparticles (Fe₂O₃ NPs) are of particular interest because of their excellent biocompatibility, magnetic properties, and chemical stability [4]. Specifically, α-Fe₂O₃ (hematite), the most thermodynamically stable phase of iron oxide under ambient conditions, it is an n-type semiconductor with a narrow band gap (~ 2.1 eV), enabling efficient light absorption in the visible region [5, 6]. Moreover, α-Fe₂O₃ possesses weak ferromagnetic properties at room temperature and excellent corrosion resistance, which make it highly suitable for catalytic, magnetic, and biomedical applications [7]. Hybrid nanostructures such as Fe₂O₃@Ag/AgCl core–shell nanoparticles have been developed, combining the unique physicochemical and biological features of iron oxide with silver and silver chloride [8]. The synergistic interaction between the core and shell components enhances charge separation, surface reactivity, and biological efficacy [9]. Traditional synthesis methods of such nanoparticles often involve toxic chemicals, high temperatures, and complex procedures, which can limit their biomedical applications due to potential toxicity and environmental hazards [10]. Therefore, green synthesis approaches have emerged as eco-friendly and sustainable alternatives, utilizing plant extracts as natural reducing and stabilizing agents [11]. Plant-based synthesis is cost-effective, simple, and environmentally benign, with phytochemicals such as polyphenols, flavonoids, and alkaloids playing a vital role in reducing metal ions and capping the resulting nanoparticle [12]. In this study, Fe₂O₃@Ag/AgCl core–shell nanoparticles were synthesized using mint leaf extract at two different concentrations of silver nitrate, the concentration of AgNO₃ is a crucial factor in green synthesis because it determines the amount of silver ions available during the reaction, which controls the formation of the Ag/AgCl shell around the Fe₂O₃ NPs in terms of thickness, regularity, and surface coverage. This, in turn, affects the optical properties of the material by enhancing light absorption and the surface plasmon effect of silver. It also helps improve charge separation and reduce electron and hole recombination, ultimately leading to enhanced photocatalytic efficiency in the decomposition of methylene blue dye.

3.1 Crystallographic analysis (XRD)

XRD patterns of Fe₂O₃@Ag/AgCl NPs prepared with two different silver nitrate concentrations (0.02 and 0.03 M) indicate a clear effect of concentration on the structure and crystallinity of the material. Fe₂O₃@Ag/AgCl (Figure 2(a)) with a lower silver nitrate concentration shows less sharp and lower intensity peaks compared to Fe₂O₃@Ag/AgCl (Figure 2(b)) with a higher concentration after annealing at 600℃, as shown in Figure 2. These peaks are due to α-Fe₂O₃, AgCl, and Ag with a hexagonal structure and a cubic structure corresponding to JCPDS card 96-900-0140, JCPDS card No. (01-085-1355) and (98-005-3759), respectively [15, 16]. The appearance of silver chloride (AgCl) in the samples, especially since it was not directly used in the reaction, is attributed to the use of mint leaf extract, which contains organic compounds and natural salts including chloride ions (Cl⁻), AgCl forms initially as a surface-associated phase, while Fe₂O₃ crystallizes during calcination to form the core, followed by stabilization of Ag/AgCl as the shell [17].

Figure 2. XRD pattern of Fe₂O₃@Ag/AgCl (a) with a lower concentration of silver nitrate (0.02 M) and Fe₂O₃@Ag/AgCl (b) with a higher concentration of silver nitrate (0.03 M)

The average crystal size of Fe₂O₃, Ag, and AgCl NPs for Fe₂O₃@Ag/AgCl NPs (0.02 M) was calculated using Eq. (2) [18] and found to be 27.43, 51.98, and 27.67 nm, respectively. In contrast, for Fe₂O₃@Ag/AgCl NPs with 0.03 M, the corresponding crystallite sizes decreased to 17.82, 23.69, and 19.46 nm for Fe₂O₃, Ag, and AgCl, respectively. This is in agreement with V. Madhubala and T. Kalaivani with increasing molar concentration of the shell, the crystal size decreases, and crystallinity increases [19]. The reduction in crystallite size with increasing shell concentration can be explained by enhanced nucleation and early surface passivation, which suppress further crystal growth and improve crystallinity [20]. Table 1(a and b) shows the structural parameters of Fe₂O₃@Ag/AgCl NPs with AgNO₃ (0.02 and 0.03 M).

D = K λ / β Cos θ            (2)

where,

D: Mean crystallite size

K: Scherrer constant (typically 0.9)

λ: Wavelength of the X-ray (1.5418 Å)

β: Full width at half maximum (FWHM)

θ: Bragg diffraction angle (in degrees)

Table 1a. The structural parameters of Fe₂O₃@Ag/AgCl NPs with 0.02 M of AgNO₃

Table 1b. The structural parameters of Fe₂O₃@Ag/AgCl NPs with 0.03 M of AgNO₃

3.2 Morphological and elemental analysis (TEM, EDX)

The TEM image, Figure 3(A) for Fe₂O₃@Ag/AgCl NPs, with 0.02M of AgNO₃, shows that the particles form a regular spherical shape, with a thin Ag/AgCl coating around the iron oxide [13]. In the TEM image (Figure 3(B)), with 0.03 M of AgNO₃, a thicker Ag/AgCl coating forms around the particles. This excess coating causes an increased deposition density and slight irregularity in the spherical shape of the particles, although the overall shape remains spherical, making the particles in image B less regular than in image A [21].

Figure 3. TEM images of Fe₂O₃@Ag/AgCl nanoparticles (NPs) prepared with AgNO₃ concentrations of (A) 0.02 M and (B) 0.03 M

Also, EDX analysis reveals, as shown in Figure 4(A and B), that the differences between the Fe₂O₃@Ag/AgCl NPs are primarily due to the effect of silver nitrate concentration on the reaction, as well as the use of mint leaf extract in the preparation. The weight and atomic percentages of the elements are shown in Table 2(a and b), where the silver nitrate concentration is low (0.02 M), silver and chloride are present in lower ratios, along with iron and oxygen, resulting in a thin Ag/AgCl coating around the iron oxide. However, when a higher silver nitrate concentration (0.03 M) is used, silver and chloride precipitation increase, as well as the presence of iron and oxygen, leading to a thicker Ag/AgCl coating [22]. Additionally, the presence of other elements from the plant extract, such as Si, Al, P, and S, was observed, indicating traces of organic compounds present in the extract [23]. 

Figure 4. EDX spectra of Fe₂O₃@Ag/AgCl nanoparticles (NPs) prepared with silver nitrate (AgNO₃) concentrations of (A) 0.02 M and (B) 0.03 M

Table 2a. The weight and atomic ratios of the elements for Fe₂O₃@Ag/AgCl NPs with silver nitrate (0.02 M(

Table 2b. The weight and atomic ratios of the elements for Fe₂O₃@Ag/AgCl NPs with silver nitrate) 0.03 M)

3.3 Surface functional group analysis (FTIR)

The two FTIR spectra presented in Figure 5 reflect the effect of different silver nitrate concentrations in the preparation of Fe₂O₃@AgAgCl using mint leaf extract. The peaks at 3436.34 cm⁻¹ and 3435.79 cm⁻¹ appear in the Fe₂O₃@AgAgCl (Figure 5(a and b)) spectra, respectively, indicating the presence of hydrogen bonds or adsorbed water on the surface of the particles [24]. While the peaks at 2957.11 cm⁻¹, 2924.59 cm⁻¹,2853.87 cm⁻¹ and 2957.02 cm⁻¹, 2924.31 cm⁻¹,2853.72 cm⁻¹ reflect the presence of extended C–H bonds, indicating the presence of hydrocarbon groups in the plant extract, and contribute to this enhanced organic interaction [25]. The peaks at 1740.97 cm⁻¹ and 1741.16 cm⁻¹ also appear, indicating the presence of C=O bonds in the organic compounds derived from the extract [26]. The peaks at 1632.01 cm⁻¹ and 1632.02 cm⁻¹ represent unsaturated C=C bonds, reflecting the organic effect of the extract [27]. The peaks at (1465.56 cm⁻¹ and 1384.88 cm⁻¹), (1465.83 cm⁻¹ and 1384.85 cm⁻¹) indicate C–H bending, reflecting the presence of methyl or ethyl groups in the organic compounds, enhancing the effect of the extract on reacting with metals [28]. The peaks at 1270.64 cm⁻¹ and 1269.88 cm⁻¹ represent C–O bonds, indicating the presence of oxide compounds [29]. The peaks at 1111.38 cm⁻¹ and 1112.70 cm⁻¹ reflect the presence of Si–O or Si–C bonds [30]. Furthermore, the presence of surface groups resulting from the plant extract components may be due to the decomposition of organic components after annealing [31].

Figure 5. FTIR spectra for Fe₂O₃@Ag/AgCl (a) with (0.02 M) of silver nitrate and Fe₂O₃@Ag/AgCl (b) with (0.03 M) of silver nitrate

Finally, the peaks at 551.46 cm⁻¹, 472.03 cm⁻¹, 618.25 cm⁻¹, and 463.43 cm⁻¹ reflect metallic bonds for Fe–O [32]. The spectrum for Fe₂O₃@Ag/AgCl (Figure 5(b)) indicates higher permeability, indicating a higher concentration of silver and silver chloride on the surface of the iron oxide; encapsulating the core isolates active surface sites in the Fe₂O₃ nucleus, such as surface defects and unsaturated bonds, which typically contribute to strong infrared absorption [33]. The absence of characteristic peaks for silver chloride is due to its ionic nature, which is not usually seen in FTIR [34].

3.4 Topographical analysis (AFM)

The AFM image in Figure 6(A) represents Fe₂O₃@AgAgCl NPs using 0.02 M of silver nitrate in the reaction. It shows an irregular surface with lower elevations and varying details, appearing to have rough, uneven textures, and the average particle size is 59.07 nm with roughness 23.16 nm. The AFM image in Figure 6(B), where a higher concentration of silver nitrate (0.03 M) was used, shows a more regular surface with sharp peaks and distinct shapes, indicating a more stable surface. The average particle size is 66.10 nm, and the roughness is 3.77 nm, reflecting a smoother, denser surface. This is an agreement with the study [35]. The decrease in surface roughness with increasing AgNO₃ concentration is attributed to the formation of an Ag/AgCl shell around the Fe₂O₃ nucleus. At higher concentrations, increased shell deposition covers surface protrusions and fills voids, resulting in a smoother surface. This behavior is consistent with TEM results showing improved shell regularity and continuity, as well as with XRD results indicating a decrease in crystal size with increasing shell concentration. This behavior is consistent with Graziani et al. [36], where increased coating deposition leads to improved surface homogeneity and a decrease in crystal size.

Figure 6. AFM images of Fe₂O₃@Ag/AgCl prepared with silver nitrate (AgNO₃) concentrations of (A) 0.02 M and (B) 0.03 M

3.5 Optical properties (UV-Vis DRS and band gap)

In this analysis, the effect of silver nitrate concentration on the properties of Fe₂O₃@Ag/AgCl was investigated using optical absorption spectroscopy and Tauc plot analysis using Eq. (3) [37]. Fe₂O₃@Ag/AgCl with a low concentration of silver nitrate (0.02 M) showed lower light absorption in the ultraviolet range, with an energy gap of 3.33 eV, as shown in Figure 7(a and b). In contrast, Fe₂O₃@Ag/AgCl with a high concentration of silver nitrate (0.03 M) showed increased light absorption in the ultraviolet range and a larger energy gap of 3.5 eV [38], as shown in Figure 7(c and d). The increase in the optical band gap with increasing silver nitrate concentration is attributed to structural and electronic changes resulting from the formation of Ag/AgCl shell around the Fe₂O₃ nucleus; therefore, higher AgNO₃ concentration limits crystal growth and reduces crystal size, thus widening the band gap due to the quantum confinement effect [39]. Furthermore, the presence of Ag and AgCl phases enhances absorption characteristics in the ultraviolet range. Ag promotes absorption via a localized surface plasmon effect, while AgCl modifies the electronic structure of the composite system [40].

$(a\ h v)^r=B(h v-E g)$             (3)

3.6 Photocatalytic performance of Fe₂O₃@Ag/AgCl core shell NPs

The optical absorption spectrum of methylene blue dye before and after treatment with Fe₂O₃@Ag/AgCl (0.02 and 0.03 M) and exposure to ultraviolet radiation at different times (15, 30, 45, and 60 min), as shown in the Figure 8(A and B), reveals a shift in the dye's wavelengths towards blue with increasing exposure time, particularly at 60 minutes, this may indicate to a degradation of methylene blue dye [41]. The results, as shown in Table 3a, indicate a faster and more efficient degradation (D%) of methylene blue dye when treated with Fe₂O₃@Ag/AgCl (0.03 M). The degradation rate starts at 74% after 15 minutes and reaches 92% after 60 minutes. This contrasts with the results in Table 3b for Fe₂O₃@Ag/AgCl (0.02 M), where the degradation rate (D%) starts at 70% after 15 minutes and continues to increase to 86% after 60 minutes, as shown in Figure 9(A and B), this may indicate that the increased proportion of reactants at the surface led to increased decomposition efficiency for Fe₂O₃@Ag/AgCl (0.03 M), and this is attributed to improved (e⁻/h⁺) separation and reduced reconnection [42].

To determine the concentration of dyes or the effect of reactants on the reaction rate in heterogeneous photo-oxidation systems under ultraviolet light irradiation, the Langmuir-Hinshelwood (L-H) equation is used [43].

$\operatorname{Ln}\left(\frac{C_t}{C_0}\right)=-K_{a p p} t$         (4)

where, t is the time in minutes, C_t is the concentration of methylene dye at time t, C₀ is the concentration before irradiation, and Kapp is the apparent reaction rate constant.

Figure 10. The concentration of methylene blue dye after UV exposure and treatment with Fe₂O₃@Ag/AgCl (0.02 M) and Fe₂O₃@Ag/AgCl (0.03 M), relative to its concentration before exposure and treatment

Figure 11. The slope of a straight line from the relationship between ln C_t/C₀ and time for Fe₂O₃@Ag/AgCl (0.02 and 0.03 M)

The C_t/C₀ for Fe₂O₃@Ag/AgCl )0.03 M), as shown in the Figure 10, decreases more significantly, starting at 1 (at time 0) and gradually decreasing to 0.13406 after 60 minutes, compared to Fe₂O₃@Ag/AgCl (0.02 M), where the ratio decreases from 1 to 0.079538 after 60 minutes, this indicates the Fe₂O₃@Ag/AgCl (0.03 M), was more effective at reducing the dye concentration. Therefore, the slope is calculated from the relationship between ln C_t/C₀ and time, as shown in Figure 11. The K_app value of Fe₂O₃@Ag/AgCl at 0.03 M was approximately 0.02445 min⁻¹ with an R² value of 0.8698, which is higher than the K_app of Fe₂O₃@Ag/AgCl at 0.02 M, which was 0.01937 min⁻¹ with an R² value of 0.9097. These results indicate that increasing the silver nitrate concentration from 0.02 to 0.03 M accelerates the reaction and thus proves more efficient in photocatalysis. The R² value of 0.8698 for a 0.03 M indicates that the first-order kinetic behavior is not perfectly aligned, and the graph shows slight scattering of some points. Perhaps this is due to the increased Ag/AgCl loading, which may contribute to effects such as adsorption, light blocking/scattering, or partial aggregation, resulting in slight nonlinearity.

3.7 Photocatalytic mechanism

The structural and electronic synergy between the Fe₂O₃ core and the Ag/AgCl shell. Upon light irradiation, the Fe₂O₃ core absorbs photons and generates electron–hole pairs through the transition of electrons from the valence band to the conduction band. Owing to the core–shell architecture, the photogenerated electrons readily migrate toward the Ag shell, where metallic Ag acts as an electron sink, thereby suppressing electron–hole recombination and prolonging the lifetime of the charge carriers [44].

The electrons accumulated on the Ag surface subsequently react with adsorbed oxygen molecules to form superoxide radicals (•O₂⁻), while the remaining holes in the valence band of the Fe₂O₃ core react with water molecules or hydroxyl ions to generate hydroxyl radicals (•OH) [45]. The presence of AgCl further contributes to modifying the electronic structure of the composite system and promoting efficient charge separation, in addition to enhancing the generation of reactive oxidizing species [46]. These •OH and •O₂⁻ radicals attack the methylene blue molecules adsorbed on the catalyst surface, leading to the cleavage of complex organic bonds and the gradual degradation of the dye into simpler, environmentally benign products such as CO₂ and H₂O [47]. Figure 12 illustrates the photocatalytic mechanism for Fe₂O₃@Ag/AgCl NPs of methylene dye degradation.

Table 3a. The main criteria used to determine the efficiency of Fe₂O₃@Ag/AgCl (0.03 M) for photocatalysis

Table 3b. The main criteria used to determine the efficiency of Fe₂O₃@Ag/AgCl (0.02 M) for photocatalysis

Figure 12. The photocatalytic mechanism for Fe₂O₃@Ag/AgCl NPs of methylene dye degradation