Mint leaves purchased from local markets, Zinc nitrate Zn (No₃)₂·6H₂O (HiMedia Laboratories Pvt. Ltd., India, 99–100%), Sodium sulfide Na₂S·xH₂O (Daryaganj, Delhi, India), Silver nitrate (CDH, India, 99.9%), Deionized water (DW) and ethanol (96%).

Zinc nitrate Zn (NO₃)₂·6H₂O with (0.2 M) was dissolved in 50 mL of deionized water, then 30 mL of mint leaf extract was prepared by taking 10 g of dried leaves and adding 250 mL of DW to them was added dropwise to the solution. Then, sodium sulfide Na₂S·XH₂O at (0.2 M) dissolved in 50 mL of deionized water is added to the mixture with continuous stirring for one hour at room temperature. The resulting precipitate was separated by centrifugation at 4000 rpm for 30 min and then washed three times, twice with DW and once with ethanol for 30 min, and left to air dry overnight at room temperature.

0.3 g of the previously prepared ZnS NPs were weighed and 20 mL of DW was added, followed by mixing using a magnetic stirrer for approximately 15 min. After that, 30 mL of mint leaf extract was added dropwise and then concentrations of silver nitrate (0.005, 0.01, 0.02) M dissolved in 50 mL of DW were added. The reaction was left with continuous stirring for an hour. The precipitate was separated and washed twice with DW and once with ethanol for 30 min. The precipitate was left to dry overnight at room temperature and the resulting precipitate was black. Fig. 1 shows the steps for preparing ZnS@Ag₂S.

XRD pattern of ZnS NPs and ZnS@Ag₂S NPs were recorded using an Aeris Research Edition X-ray diffractometer (Malvern Panalytical) equipped with Cu Kα (λ = 0.154 nm) radiation. The instrument provides a high angular resolution of less than 0.04° (2θ), enabling accurate phase identification and crystallite size estimation. The particles morphology was examined by (TEM) PHILIPS, CM120, The Netherland. EDX analysis was performed using a Thermo Scientific™ Axia™ ChemiSEM (Thermo Fisher Scientific) to examine the elemental composition of the synthesized nanoparticles. The properties related to the functional groups on the surface in addition to the chemical bonds of the basic elements have been examined by FTIR analysis (Perkin Elmer TWO, USA), the spectra have been scanned in range (4000–400 cm⁻¹) at resolution (0.5 cm⁻¹). The optical absorbance spectrum was measured using a Shimadzu UV-1800 spectrophotometer (the wavelength ranging from 190–1100 nm). In addition, the particle size distribution was determined from TEM micrographs by measuring the diameter of 20 individual nanoparticles using ImageJ software.

XRD analysis was performed to analyze the crystal structure and phase purity of ZnS NPs and ZnS@Ag₂S NPs which were prepared using mint leaf extract with different concentrations of silver nitrate (0.005, 0.01, and 0.02 M), as shown in Fig. 2. The XRD results showed the ZnS NPs exhibited a cubic phase and polycrystalline nature with peaks at 2θ = 29.16° (hkl = 111), 2θ = 48.02° (hkl = 220), and 2θ = 56.92° (hkl = 311), correspond to JCPDS card (5-566) [17]. While XRD results for ZnS@Ag₂S NPs showed that the cubic phase of ZnS NPs appeared at 2θ = 29.3014° (hkl = 111), 2θ = 48.3094° (hkl = 220), and 2θ = 57.0874° (hkl = 311) at a concentration of 0.005 M, and at 2θ = 29.2794° (hkl = 111), 2θ = 48.3094° (hkl = 220), and 2θ = 57.0874° (hkl = 311), at a concentration of 0.01 M, and at 2θ = 28.5314° (hkl = 111), 2θ = 47.3194° (hkl = 220), and 2θ = 56.2294° (hkl = 311), at a concentration of 0.02 M, while peaks appeared at 2θ = 34.7684° (hkl = 022) and 2θ = 37.4524° (hkl = 103), reverts to Ag₂S in the monoclinic phase, but with low intensity at a concentration of 0.005 M [18]. As the silver nitrate concentration increased to 0.01 M and 0.02 M, an increase in the intensity of the monoclinic peaks was observed, along with the appearance of new peaks for Ag₂S at 2θ = 40.4664° (hkl = 031), and 2θ = 44.0634° (hkl = 103) for 0.01 M and 2θ = 40.2244° (hkl = 031), and 2θ = 43.0954° (hkl = 122) for 0.02 M, correspond to JCPDS card (14-0072) [19]. These changes indicate that increasing the silver nitrate concentration contributed to improved crystallization and enhanced silver sulfide formation in the monoclinic phase, which is in agreement with [20].

Also, the average crystalline size was calculated using Scherrer’s equation Eq. (1) [21,22], where it was found for ZnS NPs 2.01 nm. However, after the formation of Ag₂S NPs at a shell, an increase in the crystal size of the ZnS was observed, reaching to (2.78, 2.65, and 2.13 nm) for the concentration of silver nitrate (0.005, 0.01, and 0.02 M), respectively. The change in ZnS nanocrystal size during the formation of ZnS@Ag₂S NPs is attributed to the structural interaction between the two phases and the differential crystal structures of the two phases, leading to lattice mismatch and the generation of crystallographic stress at the interface [23]. As for the Ag₂S, it was observed that the average crystal size decreases with increasing concentration of silver nitrate, where it was (7.47, 5.89, and 3.83 nm) for (0.005, 0.01 and 0.02 M), respectively, this decrease is may be due to the rapid reaction between silver ions (Ag⁺) and sulfur ions (S²⁻) which leads to rapid saturation of the ZnS surface with Ag₂S, thus reducing the opportunity for crystal growth in the shell [24]. Furthermore, the change in the values of the interfacial distances (d_hkl) indicates the occurrence of internal network distortions resulting from the strong interaction between the two phases [25]. Table 1 shows the structural parameters of ZnS NPs and ZnS@Ag₂S core–shell NPs.

Fig. 3 shows TEM images for ZnS NPs and ZnS@Ag₂S NPs, with different concentrations of silver nitrate (0.005, 0.01, and 0.02 M), the image (A) shows clusters of ZnS NPs, lacking any silver sulfide layer [26]. However, when different concentrations of silver nitrate were added to ZnS NPs, a core and shell were observed around it, as shown in the images (B, C, and D). The image B (at 0.005 M) shows a combination of the core and shell, but it is less regular and homogeneous compared to image C (at 0.01 M), which shows a more regular and homogeneous arrangement of spherical particles. While, the image D (at 0.02 M) shows clusters of the core that are less homogeneous and surrounded by a regular shell, this is an agreement with [27]. Also, Fig. 4 shows the particle size distribution histograms, revealing that the homogeneity of the nanoparticles is clearly dependent on the silver nitrate concentration. ZnS NPs (A) exhibited a relatively narrow distribution within 10–20 nm, with the highest frequency in the 10–12 nm range. Upon incorporation of Ag₂S as shell (0.005 M), (B) the distribution shifted toward larger sizes (10–35 nm) with a dominant frequency around 15–20 nm, indicating enhanced growth. At higher concentrations (0.01 M and 0.02 M, C and D), the distributions became comparatively narrower (10–22 nm and 6–22 nm, respectively), with the highest frequency centered near 16–18 nm and 12–14 nm. These results demonstrate a concentration-dependent modulation of particle size and frequency, confirming that Ag₂S content significantly influences the nucleation–growth mechanism of the nanostructures.

In EDX analysis of ZnS NPs and ZnS@Ag₂S NPs with AgNO₃ (0.005, 0.01, and 0.02 M), as shown in Fig. 5, the primary elements in ZnS NPs (A) were found to be (Zn) and (S) [28]. Also, carbon (C), and oxygen (O) were present which attributed to the components of the mint leaf extract used in the preparation, in addition to the apparent Na content originates from the overlap between Na Kα and Zn L lines or may be indicate the actual presence of Na, this is in agreement with Senapati et al. [29]. While, in ZnS@Ag₂S NPs (B–D) at different concentrations of silver nitrate, the results showed that zinc (Zn) and sulfur (S) remained balanced within the core/shell structure, in addition to the continued presence of oxygen and carbon in the composition, this indicates the ongoing influence of the organic extract [30]. As the concentration of silver nitrate increased from 0.005 to 0.02 M, a significant increase in the silver (Ag) content was observed, reaching 35.1% by weight and 12.6% by atomic ratio. These changes reflect the formation of silver sulfide (Ag₂S) around the zinc sulfide (ZnS) [31]. The weight and atomic ratios of the elements for ZnS and ZnS@Ag₂S NPs are shown in Table 2.

The FTIR results of ZnS NPs and ZnS@Ag₂S NPs, prepared using mint leaf extract at different concentrations of silver nitrate (0.005, 0.01, and 0.02 M) were analyzed, are shown in Fig. 6. The vibrational frequencies at the peaks 3413.94 cm⁻¹, 3426.12 cm⁻¹, 3435.98 cm⁻¹, and 3426.26 cm⁻¹ for ZnS NPs and ZnS@Ag₂S NPs, respectively indicate to stretching vibrations of the hydroxyl (O-H) groups present in menthol and other polyphenolic compounds from the mint extract, which acting as reducing and stabilizing agents for the nanoparticles, as well as contributing to controlling their growth and preventing agglomeration [32]. Vibrational frequencies at (2924.47 cm⁻¹, 2924.97 cm⁻¹, 2925.13 cm⁻¹ and 2924.70 cm⁻¹), (2853.95 cm⁻¹, 2854.28 cm⁻¹, 2854.42 cm⁻¹, and 2854.03 cm⁻¹), for ZnS and ZnS@Ag₂S NPs, respectively indicate to stretching vibrations of C-H bonds in hydrocarbon compounds, reflecting the chemical composition of organic compounds in the extract [33]. Also, the frequencies at (1384.73 cm⁻¹, 1384.90 cm⁻¹, 1384.94 cm⁻¹, and 1384.93 cm⁻¹) for ZnS and ZnS@Ag₂S NPs, respectively and 1464.92, 1465.48, and 1465.82 cm⁻¹ for ZnS@Ag₂S NPs indicate the bending vibrations of C–H groups in saturated hydrocarbons, particularly the methyl (CH₃) and methylene (CH₂) groups found in alkanes, the presence of these frequencies is indicative of the presence of organic compounds such as menthol and other terpenes found in mint leaf extract [34]. The frequencies at 1607.26 cm⁻¹, 1630.99 cm⁻¹, 1631.47 cm⁻¹ and 1631.18 cm⁻¹ for ZnS and ZnS@Ag₂S NPs, respectively indicate C=C stretching vibrations in aromatic compounds such as phenols or menthol [17]. The frequencies at 1263.83 cm⁻¹, 1263.27 cm⁻¹, 1269.36 cm⁻¹, and 1269.05 cm⁻¹ for ZnS and ZnS@Ag₂S NPs, respectively refer to C-O stretching vibrations of alcohol and phenolic groups originating from menthol in the mint extract [35]. Also, the frequencies at 1114.75 cm⁻¹, 1113.21 cm⁻¹, 1112.53 cm⁻¹, and 1114.12 cm⁻¹ for ZnS and ZnS@Ag₂S NPs, respectively indicate to C-O or C-N bonds [36]. The frequencies at 622.62 cm⁻¹, 619.03 cm⁻¹, 619.55 cm⁻¹, and 618.33 cm⁻¹ for ZnS and ZnS@Ag₂S NPs, respectively indicate S-Zn bonds in zinc sulfide nanoparticles [37]. FTIR results also show significant changes in the spectrum of ZnS@Ag₂S NPs compared to ZnS NPs, the transmittance (T%) increasing with increasing silver nitrate concentration, this suggests the formation of Ag₂S as a coating around ZnS, furthermore, small peaks were observed between 450 and 900 cm⁻¹ in the spectrum of ZnS@Ag₂S NPs, possibly indicating vibrations associated with S-Ag bonds [38]. FTIR bonds for ZnS and ZnS @Ag₂S NPs synthesized by different concentrations of silver nitrate are listed in Table 3.

The optical absorption spectra and Tauc plot analysis of ZnS NPs and ZnS@Ag₂S NPs synthesized with different AgNO₃ concentrations (0.005, 0.01, and 0.02 M), as shown in Fig. 7, demonstrate a pronounced absorbance in the ultraviolet region [39]. The estimated optical band gap energies were found to be higher than that of bulk ZnS, which can be attributed to the quantum confinement effect arising from the nanoscale dimensions of the particles [40]. The band gap values were calculated to be 3.77, 3.61, 3.72, and 3.74 eV for ZnS NPs and ZnS@Ag₂S NPs with AgNO₃ of 0.005, 0.01, and 0.02 M, respectively [41], using the Tauc relation (Eq. (2)) [42,43]. Furthermore, a slight reduction in the band gap was observed after coating ZnS NPs with Ag₂S NPs, which can be ascribed to the formation of interfacial energy levels and electronic coupling between ZnS and Ag₂S [44].