3.1. XRD study
The XRD pattern of (2.5, 5 and 10wt.%) Cu doped ZnS/Fe3O4 nanocomposites displayed in Fig. 1. As stated in our prior investigation, the XRD patterns of the synthesized samples exhibit discernible peaks corresponding to magnetite and ZnS, while no peaks indicative of impurity can be identified, and therefore the inclusion of dopants does not affect the phase structure of ZnS/Fe3O4, and the Cu ions are effectively integrated into the lattice structure of ZnS without forming CuS, as mentioned in the literatures [30, 31]. Substitution doping is deemed beneficial when the two elements possess equivalent ionic radii. It is worth noting that Cu2+ and Zn2+ have similar ionic radii, with values of 0.73 and 0.74 Å, respectively. So Cu2+ can easily substitute with Zn2+ in the ZnS lattices, compared to S2− [32]. The mean size of the crystallite (D) in all samples was determined using Scherrer's formula (Eq. (3)) [33, 34]:
$$\text{D}=\frac{0.9{\lambda }}{{\beta }\text{c}\text{o}\text{s}{\theta }}$$
3
Table 1 presents the average crystallite size of Fe3O4 and ZnS as determined through the implementation of the Scherrer technique on the peaks exhibiting considerable intensity. It is evident that the size of the crystallites in each nanocomposite varies from 12 to 58 nm, thereby confirming their nano-scale nature.
Table 1
The average Crystallite size (nm) ascertained through XRD.
Sample | Fe3O4 (311) | Fe3O4 (400) | ZnS (111) | ZnS (220) |
2.5wt.% Cu doped ZnS/Fe3O4 | 33 | 12 | 14 | 58 |
5wt.% Cu doped ZnS/Fe3O4 | 28 | 14 | 10 | 17 |
10wt.% Cu doped ZnS/Fe3O4 | 20 | 23 | 23 | 14 |
3.2. FTIR study
Figure 2 exhibits the FTIR spectra of (5 and 10 wt.%) Cu doped ZnS/Fe3O4 nanocomposites. As documented in various literary works, the absorption bands observed at 1622.75 cm− 1 and 3435.29 cm− 1 can be ascribed to the stretching vibrations of C = O and O-H, respectively [30]. In 5wt.% Cu doped ZnS/Fe3O4 nanocomposite, the Fe-O bond stretching vibrations are also associated with the notable peaks of absorption at 629.72 cm− 1 and 556.76 cm− 1, indicating a properly formed Fe3O4 structure [35]. Also, the stretching vibration of Zn-S is represented by the peak at 1013 cm− 1. The peaks observed at 1305.06 cm− 1 is ascribed to the presence of C-O bond [26]. Furthermore, the lack of any absorption band associated with CuS suggests that Cu is present in a pure state. Notably, there has been no occurrence of any form of chemical bonding between Cu and the nanocomposite. According to the current investigation, the substitution of transition metals in the crystal structure of ZnS nanoparticles has been adequately accomplished. The allocation of peaks for (5 and 10wt.%) Cu-doped ZnS/Fe3O4 is presented in Table 2.
Table 2
Peaks assignment of (5 and 10wt.%) Cu doped ZnS/Fe3O4.
Assignments | Cu (5%) | Cu (10%) |
Fe-O stretching vibrations | 629.72, 556.76 | 625.46, 557.08 |
Zn-S stretching vibrations | 1013 | 1120.28 |
3.3. Photoluminescence (PL) spectroscopy study
To investigate the optical characteristics of (5 and 10wt.%) Cu doped ZnS/Fe3O4 nanocomposites, photoluminescence spectroscopy, as illustrated in Fig. 3, was performed with an excitation wavelength of 200 nm and a range of 200 nm–800 nm. The PL emissions of samples are at approximately 423, 480, and 516 nm, which maximum emission was measured at 516 and 423 nm, although a faint emission peak was recorded at 479 nm. The recombination of electrons from a shallow state to the sulfur vacancies causes the emission peak at 423 nm. Zinc vacancies in the ZnS lattice are also responsible for the emission peak at 480 nm [36]. Fundamentally, the samples emit light at these specific wavelengths due to the presence of different types of vacancies in the material, with sulfur and zinc vacancies contributing to the observed emissions at 423 and 480 nm, respectively. The PL intensities of 10wt.% Cu doped photocatalysts were lower than those of 5wt.% Cu doped ZnS/Fe3O4. The intensity of this PL spectrum falls as the weight percent of dopant increases. Photo-generated e−/h+ pair recombination decreases as intensity decreases. This proves that photocatalytic efficiency is influenced by PL results. Higher photocatalytic activity is found in photocatalysts with decreased exciton recombination [37]. Table 3 shows the position of emission bands of (5 and 10% Cu doped ZnS/Fe3O4) nanocomposites with just a modest shift in PL spectra.
Table 3
PL bands position in (5 and 10wt.%) Cu doped ZnS/Fe3O4.
Sample | PL bands position (nm) |
5wt.% Cu doped ZnS/Fe3O4 | 421 | 483 | 513 |
10wt.% Cu doped ZnS/Fe3O4 | 424 | 481 | 517 |
3.4. UV visible (UV-Vis) spectroscopic study
Utilizing a UV-Vis spectrometer, the optical characteristics of the nanocomposites were examined. Figure 4 (a) and (b) show the absorbance spectra and their corresponding estimated band gaps. Because of their absorption, the synthesized samples are acceptable materials for the treatment of wastewater. For 2.5wt.% Cu, 5wt.%Cu and 10wt.%Cu doped ZnS/Fe3O4, the absorption band emerged at 212, 211, and 264 nm, respectively. The bandgap value alternates as the Cu concentrations change. For 2.5wt.%Cu, 5wt.%Cu and 10wt.%Cu, the computed band values are 4.77, 4.67, and 4.55 eV, respectively. The closing in the bandgap values is a result of fluctuating carrier concentrations, which arise from the introduction of defects, leading to the formation of new energy levels between the valence and conduction bands in the samples [29]. In comparison to other samples, the band gap energy is smaller in 10wt.%Cu doped ZnS/Fe3O4. This means that more e−/h+ couples may be generated by UV light energy and more separation rate of charge carriers, which increases photocatalytic performance [38].
3.5. Electronic structure:
Mott-Schottky analysis is commonly employed in electrochemical analysis to measure the charge-current density and flat band potential (Efb), and also to analyze the electronic behavior of various materials, including n-type/p-type/p-n type [39]. The Mott-Schottky (M-S) test, as depicted in Fig. 5, was conducted to determine the precise locations of the conduction band (CB) and valence band (VB) of synthesized samples. The positive slopes observed in the linear portions of the MS curves signifying that the 2.5, 5, and 10 are n-types semiconductors [40]. Additionally, the slope of the Mott-Schottky plot and its related intercept with the x-axis were calculated for each sample in order to determine the carrier concentration and flat band potential, respectively (as shown in Table 4) [41]. According to data analysis, 10wt.% Cu-doped ZnS/Fe3O4 has the highest and 2.5wt.% Cu doped ZnS/Fe3O4 has the lowest carrier concentration. The 10wt.% Cu doped ZnS/Fe3O4 nanocomposite's electrical resistance may be significantly reduced as a result of the enormous increase of charge carriers. This result is in excellent agreement with the outcomes of PL investigations and the successful separation of charges. Furthermore, the 10wt.% Cu doped ZnS/Fe3O4 shows the largest flat band potential (0.5 V), indicating that a smaller overpotential can be used to start the charge-transfer reaction [41]. The Ag/AgCl reference electrode was used to calculate the samples' flat band potentials. Based on the observation, the flat-band potential (EFB) of n-type semiconductors was generally near the CB position, the estimated CB of (2.5, 5, and 10wt.%) Cu doped ZnS/Fe3O4 were − 0.873, 0.397 and 0.597 eV (NHE), respectively. The calculated values of the VB were 3.89, 5.07, and 5.15 eV, respectively, based on the formula ECB = EVB – Eg [42].
Table 4
Carrier Concentrations (ND) and Flat Band Potentials of the (2.5, 5 and 10wt.%Cu) doped ZnS/Fe3O4.
Sample | Carrier concentration (cm− 3) | Flat band potential (V) |
2.5wt.% Cu doped ZnS/Fe3O4 | 6.97*1010 | -0.97 |
5wt.% Cu doped ZnS/Fe3O4 | 1.15*1011 | 0.3 |
10wt.% Cu doped ZnS/Fe3O4 | 2.47*1011 | 0.5 |
3.6. Morphological and compositional analysis (FE-SEM & EDX)
Structural and compositional examination of the synthesized samples has been carried out using the FE-SEM and EDS method, as depicted in Fig. 6 (a) and (b). The FE-SEM images of the (5 and 10wt.%) Cu-doped ZnS/Fe3O4 nanocomposites, reveal a uniform and symmetrical spherical morphology with some degree of agglomeration. No discernible variations in morphology were observed between the 5wt.% Cu doped ZnS/Fe3O4 and 10wt.% Cu doped ZnS/Fe3O4. The average size of the particles in the 5wt.% Cu doped ZnS/Fe3O4 and 10wt.% Cu doped ZnS/Fe3O4 nanocomposites is 42.61 and 44.04 nm, respectively. In addition, the elemental mapping of nanocomposites elucidates the dispersion of copper, iron, oxygen, zinc, and sulfur elements. The presence of Cu, Fe, O, Zn, and S elements is confirmed by the EDS spectra of the Cu doped ZnS/Fe3O4 samples, without any impurities, which is consistent with the XRD results. The synthesized materials are devoid of impurities and residual chemicals, thereby ensuring their safety for photocatalytic applications. The weight and atomic percentage of elements in synthesized materials showed in Table 5.
Table 5
EDS quantification results.
Element | Weight (%) | Atomic (%) |
5wt.% Cu doped ZnS-Fe3O4 | 10wt.% Cu doped ZnS-Fe3O4 | 5wt.% Cu doped ZnS-Fe3O4 | 10wt.% Cu doped ZnS-Fe3O4 |
O | 17.30 | 23.29 | 30.88 | 33.94 |
S | 45.32 | 40.38 | 45.60 | 38.01 |
Fe | 13.06 | 14.20 | 11.39 | 17.67 |
Cu | 7.58 | 11.92 | 3.86 | 5.66 |
Zn | 16.74 | 10.21 | 8.27 | 4.72 |
Total | 100 | 100 | 100 | 100 |
3.7. Adsorption ability of (2.5, 5 and 10wt.%) Cu doped ZnS/Fe3O4
To evaluate the adsorption abilities of (2.5, 5 and 10wt.%) Cu-doped ZnS/Fe3O4, RhB and MB have been selected as the organic dyes. The impact of time on the adsorption efficiency was examined through the utilization of UV-vis spectra (Fig. 7 (a)). As illustrated, the quantity of dye adsorbed onto the photocatalysts from aqueous solution grows rapidly over time. Furthermore, an investigation of adsorption efficiency in various samples reveals that, 10wt.% Cu doped ZnS/Fe3O4 has the maximum adsorption efficiency for MB and RhB, with 100% of MB adsorbed in 20 min. For RhB, the equilibrium adsorption capacity of (2.5, 5, and 10wt.%) Cu doped ZnS/Fe3O4 is 1.84 mg g− 1, 3.19 mg g− 1, and 4.615 mg g− 1, respectively and for MB the equilibrium adsorption capacity of (2.5 and 5wt.%) Cu doped ZnS/Fe3O4 is 1.99 mg g− 1 and 3.14 mg g− 1, respectively (Fig. 7 (b)). As shown in Fig. 7 (c), these results corresponded to the Ct/C0 plots of (2.5, 5, and 10wt.%) Cu doped ZnS/Fe3O4.
3.8. Photocatalytic mechanism and degradation study:
When photocatalysis process occurs, the dye molecules react with electron-hole pairs at the conduction and valence bands, resulting in superoxide radicals (•O2 –) and hydroxyl radicals (•OH), upon exposure to light of an energy level surpassing the prepared sample's bandgap. The degradation of dyes such as RhB, resulting in the formation of H2O and CO2, can be attributed to radicals such as superoxide, and hydroxyl. Another factor contributing to the enhancement of photocatalytic activity is the generation of a novel energy level situated beneath the conduction band, which occurs as the dopant concentration increases [29].
The assessment of the photocatalytic efficacy of fabricated samples is conducted through the utilization of rhodamine B (RhB). Figure 8 show the UV spectrum of RhB dye in relation to reaction time, effectively demonstrating the photocatalytic capabilities of the synthesized samples in degradation of RhB dye. Before irradiation, the dye solutions accompanied by a catalyst, were placed in a dark environment (30 min), to attain adsorption-desorption equilibrium. For 180 min, the RhB dye solution was exposed to UV light. After a uniform 30 min interval, the aqueous dye solution was removed and subjected to UV-Vis spectroscopy for characterization. Figure 8 illustrates the alteration of the absorbance peak at 554 nm with an increase in the concentration of dopant from 2.5 to 10wt.%. As absorption reduces with respect to illumination time, the degradation efficiency rises. At the same degradation situations applied to the all prepared samples, the 10wt.%Cu had the greatest reduction in the absorption peak.
After 60 min of irradiation, 10wt.% Cu doped ZnS/Fe3O4 had the most degradation efficiency (i.e. 98.8%). With the assistance of PL spectroscopy information, which demonstrates that the sample with the smallest intensity in PL spectra has stronger photocatalysis degradation, this highest efficiency and effectiveness of the 10wt.% Cu doped ZnS/Fe3O4 may be thoroughly substantiated. As can be observed, the sample containing 10wt.%Cu has larger degrading activity due to its lower PL spectrum intensity.
3.9. Reusability of recycled photocatalysts
The photocatalytic process is typically conducted within a suspension comprised of semiconductor nanostructures, necessitating a subsequent separation procedure to eliminate the catalyst from the aqueous medium. The cost incurred in eliminating nanoparticles, from substantial quantities of water, poses a significant impediment to the photocatalytic technique employed for wastewater treatment [43]. However, the nanocomposites generated in this investigation show superparamagnetic characteristics at room temperature, which simplifies the previously discussed problem. When creating photocatalysts for industrial applications, reusability is of the utmost importance [44]. In the present study, an investigation was conducted to assess the reusability of 10wt.% Cu doped ZnS/Fe3O4. The photocatalyst underwent a sequence of procedures involving continuous testing, recycling through the use of a magnet, washing, drying, and subsequent reuse for a total of five cycles. The outcomes of these experiments are illustrated in Fig. 9. The degradation efficiency gradually decreased from the first to the fifth cycles, exhibiting values of 97.7%, 96%, 92%, 91%, and 89%, respectively. Since 100% recovery of photocatalyst is not attainable, the decline may be the result of catalyst loss in the recovering operations or obstruction of some active sites by dye adsorption that cannot be totally eliminated in each cycle by ethanol or water [45]. The findings indicate that after five cycles, 89% of the photocatalytic activity of 10wt.% Cu doped ZnS/Fe3O4 may be sustained without a discernible decrease.