Figure 1 (a-c) shows the XRD patterns of ZnS and Co2+: ZnS NPs. The observed XRD peaks corresponding to three indexed planes such as, (111), (220) and (311) with the other phase peaks were not observed due to impurities, to confirm the formation of cubic structure (JCPDS. NO. # 05-0566) . By using Debye-Scherer’s, the average crystalline sizes were found to be 3.7, 3.2 and 2.9 nm, respectively [14–15]. From XRD, the obtained diffraction peaks are decreases with the increase of dopant concentration as Co2+, it’s indicate a smaller crystallite size with low crystallinity due to lattice distortions [15–16]. Since, the ionic radius of Co2+ (0.65 nm) is slightly less than that of the Zn2+ (0.74 nm) ion and the lattice parameters (a = 5.386Å) of Co2+: ZnS sample are slightly less than the ZnS (a = 5.391 Å) [16–17]. Finally, the XRD peaks of Co2+: ZnS (3%) are broader than that of other samples, its showed smaller size of the particles.
Figure 2 (a-f) shows the SEM-EDX of ZnS and Co2+: ZnS NPs. The spherical-like morphology was observed with the compositional elements were presented, such as cobalt, sulfur and zinc. The cobalt ions are equally distributed in the ZnS lattice with the extra peaks was not present, its high purity [15–18]. Figure 3 (a-c) shows the TEM image of ZnS and Co2+: ZnS NPs. The particle sizes were found to be 3.7, 3.2 and 2.9 nm, respectively. Co2+: ZnS particles have a smaller size than the ZnS particles. These dots and rings are clearly indicating that the synthesized samples has cubic structure and there was no structural change, this was due to cobalt doping in the ZnS system, its indicates high purity [18–20].
Figure 4 (a-c) shows the UV-Visible of ZnS and Co2+: ZnS NPs. The synthesized samples were occurred in the UV region, its indicate blue shift compared to bulk ZnS due to the quantum confinement effect. By applying the value of n = ½, the observed band gap are calculated as 3.66, 3.89 and 4.10 eV, respectively. It is concluded, 3% of Co2+: ZnS particle have higher optical band gap compared to other samples due to smaller particle size. Consequently, the increased optical band gap materials suggest that the synthesized nanoparticles influence in the optical devices based applications, it can be tuned by adding the dopant as Co2+ [19–23]. Figure 5 (a-c) shows PL spectra of ZnS and Co2+: ZnS NPs with a excitation wavelength of 350 nm. The emissions were observed at 445 nm due to recombination of sulfur vacancy defects as internal vacancy of zinc (holes trapped) and sulfur (donor level) atom. Since, the emission spectra were observed in blue shift compared to bulk counterparts and it is useful for optoelectronic devices [22–24, 12].
Figure 6 (a-b) shows the photocatalytic activity of undoped and Co2+: ZnS. The characteristic absorption peak was observed at 645 nm at various time intervals upto 120 min. It’s corresponding to the absorption intensity slightly decreased as well as irradiation time increased with their new absorption bands is not present due to impurities. In pure ZnS, the photo generation rate of holes and electrons is much faster than separation rate; it’s producing low carrier separation efficiency. In Co2+: ZnS, the photo-electrons preferentially transfer to the doping level due to Co2+ atoms in the ZnS. It’s providing the electron trapping centers and recombination of holes and electrons, resulting in the increase of the carrier separation efficiency. Among them, Co2+: ZnS catalyst has a large surface area, small grain size and catalytic activity is higher than that of undoped ZnS catalyst. Figure 7 (a-b) shows the degradation efficiency of undoped and Co2+: ZnS catalyst. The degradation efficiency was found to be and 60 and 93.22 %, respectively. Among these values, Co2+: ZnS particles are enhanced to the higher photocatalytic activity due to enhancement of the energy transfer. During process, Co2+ doped ions inserts various localized energy states in the forbidden energy gap as recombination and charge carrier trapping on the surface charge transfer and it’s indicate that the Co2+ ions are provided extra energy level with reduced bandgap due to smaller size of particles in the ZnS lattice.
Table 1 gives the degradation of absence and presence of Co2+: ZnS catalyst using MeB under UV light irradiation. A blank experiment was found to be 0.31 x 10− 3. During synthesis, 2 % of Co2+: ZnS catalyst was found to be 1.87 x 10− 3 and 4.43 x 10− 3 and 3 % of Co2+: ZnS catalyst was found to be 2.33 x 10− 3 and 6.36 x 10− 3, respectively. It concluded that the 3 % of Co2+: ZnS catalyst has strong efficiency of degradation. Total Organic Carbon (TOC) analysis of the Co2+: ZnS catalyst in the presence of MeB dye was performed under UV light irradiation as shown in Fig. 11 (a-c). In TOC images, Co2+: ZnS catalyst of TOC removal was found to be 10.24, 72.18 and 88.23 % after 120 min of irradiation. Among these results, it’s indicates that the color disappearance of the MeB dye was faster than the degree of mineralization. During process, the quick disappearance was arising from the cleavage of the MeB dye bond and their minimal interaction with aliphatic chains. After 120 min, MeB dye molecules were converted to other intermediate forms, which exist in the solution irrespective of the dye de-colorization lead to complete mineralization beyond 120 min. Table 2 gives the COD values of initial and final treated of Co2+: ZnS catalyst under MeB. It is used to measure the amount of organic components in terms of the total amount of O2 required to oxidize it to CO2 and H2O via dichromate reflux method. In table 2, the synthesized catalyst has high potential for the removal of MeB due to decreased COD values. Consequently, TOC and COD results showed that the 3% of Co2+: ZnS catalyst has higher photocatalytic activities than the other samples. In addition, the detailed drgradation mechanism of undoped and Co2+: ZnS catalyst can be summerized below.
BET images of ZnS and Co2+: ZnS NPs with the N2 adsorption-desorption isotherms as shown in Fig. 10 (a-c). The surface area and pore volume of synthesized samples were calculated as 97, 112 and 148 m2g− 1 and 0.25, 0.34 and 0.63 cm3g− 1, respectively. Its corresponds to pore size distribution at ~(12–15), (9–13) and (5–9) nm, respectively. Since, the surface area of ZnS samples gradually increases with the increase of cobalt ions concentration due to smaller particle size. It is concluded that the synthesized samples was composed of many fine crystallites [27, 30].
Figure 11 shows the magnetic properties of 3% doped Co2+: ZnS with the applied magnetic field from − 20 to 20 K Oe at room temperature. The prepared ZnS nanoparticles favor the ferromagnetic behavior that reflect low magnetic moment and high coercivity value due to smaller particle size. Further, present case, the coercivity (Hc) of 223 Oe and the saturation magnetic moment of (Ms) 0.00025 (emu/g), respectively. As a result, a low value of saturation magnetic moment and higher value of coercivity were obtained for the 3% Co2+: ZnS sample compared with bulk ZnS . In ferromagnetism, the cubic structure was not changed due to substitution of Co2+ ions in the Zn2+ surface and it would be very useful for spintronic devices [29, 31].