Tunable Morphologies and Optical Properties of ZnS Nanostructures Doped with Ni, Co, and Mn: A Solvothermal Synthesis Approach

INTRODUCTION
Diluted magnetic semiconductors (DMSs) can be produced by substituting transition metal ions with doping of semiconductor nanostructures [1-4]. DMSs are estimate one of the most hopeful candidate materials for spintronic applications because of their nanostructure and crystalline nature, which manifest novel electro-optical and magnetic properties [5-7]. ZnS-based DMS is widely studied [8-11] due to the high solubility of magnetic atoms in the II– VI host lattice, and these materials can be ferromagnetic when transition magnetic ions are induced into the host ZnS [12]. The optical energy band gap of the ZnS compound is about 3.6 eV [13-15]; Therefore, it represents un optimum candidate for utilizing in electroluminescence, cathode luminescence [16], and sensors devices. Doping with this compound leads to other characteristics and this depends largely on the type of impurity ions. The impurity ions occupy the Zn lattice and act as electron–hole traps within the optical bandgap between the VB. and CB. In fact, nickel, manganese, and cobalt dopant elements possess magnetic properties that enhance crystalline growth when these dopants are introduced into the crystal structure of a material, as reported previously [17-20]. This enhancement is critical to the tuneable morphology of the nanoparticles, which significantly affects the physical properties of the crystal structure.  Therefore, herein, the authors report how the dopant elements Ni, Mn, and Co are synthesised with doped ZnS nanostructures, and then investigate how the structural, morphological, optical, and magnetic properties are affected by incorporating these dopants into the host ZnS. The main idea of improving the characteristics of the zinc sulfide compound is to change the morphology of nanoparticles from sphere-like to rod-like, flower-like, and sheet-like nanoparticles, which in turn provide new properties of the ZnS nanostructure. 

MATERIALS AND METHODS
The Ni, Mn, and Co dopant elements were doped with pure ZnS nanospheres and synthesized using the solvothermal method. All precursors were purchased from Merck. In general, 0.13632, 0.12959, 0.25844, and 0.12984g from ZnCl2, NiCl2, MnCl2, and CoCl2 precursors were synthesized to prepare doped and undoped ZnS nanoparticles. Using 0.32g of capping agent (polyvinylpyrrolidone), It was resolved in 40 ml of EG as solvent with regular stirring for 3 h at 80 °C. The produced resolution was carried into an autoclave and then het up at 250°C in an oven for 6 hours. The autoclave is then cooled to room temperature. Finally, by washing the sediments with distilled water, mixing them with absolute ethanol many times, and then drying them at 80 °C for 2 h in a vacuum oven, the resulting product is a nano-powder.

Characterization
Doped and undoped zinc sulphide nanostructures were characterised by XRD patterning with Cu-Kα radiation (λ=0.15406 nm) (D8 Advance Bruke). FESEM (Zeiss, PIJMA-VP) was performed to observe the morphology of the nanoparticles. UV-Vis spectroscopy (shimadzu UV-1800) was used to study the optical properties. Finally, a vibrating sample magnetometer (Lakeshore VSM-7410) was used to investigate the behaviour of the magnetic and the doped ZnS nanoparticles properties.

RESULTS AND DISCUSSION
XRD pattern study
Fig. 1 shows the XRD patterns of the synthesized pure ZnS and x-doped ZnS (x=Ni, Mn and Co) nanoparticles. Each sample exhibits a single phase (cubic zinc blend) structure without detectable secondary phase formation, confirming the successful incorporation of dopants into the ZnS lattice. It was observed that the dominant diffraction peaks, located at the 2θ positions 28.42°, 47.50°, and 56.35° indexed to the diffractions of (111), (200), (220), and (311) planes, and which matches the standard JCPDS card number 89-7385, as summarised in Table 1. It can be seen that doping with nickel, manganese and cobalt does not later the lattice structure of the pure ZnS, but affect the crystal structure parameters, and induces subtle modification in the lattice parameters. This effect can be observed in the tiny diffraction peaks in the nickel-doped ZnS. Moreover, the slight shifts of the dominant diffraction peaks of the pure ZnS toward lower angles (inset of Fig. 1), indicative of lattice expansion according to Bragg’s law.
This means that the dopant cations occupy the Zn cation position in the crystal lattice. The smaller ionic radius of Ni2+ (0.69Å) than that of the Zn2+ (0.74Å) would be expected to contract the lattice, shifting peaks to higher angles. However, the observed shift toward lower angles reveal that defect formation such as Zn or S vacancies along with local strain effects, counterbalances the anticipated contraction. This may be due to the magnetic property of nickel, which affects the redistribution of cations and then the lattice strain of the unit cell. The larger ionic radius of Mn2+ (0.80Å) inherently expands the lattice, while the ionic radius of Co2+ (0.745Å) nearly identical to Zn2+, induces minimal volumetric change, with strain related distortion demonstrating its structural impact [21]. The size of crystal D was evaluated from the dominant XRD peak using the Debye– Scherrer formula [22] D=0.94λ/βcosθ, where β is the full width at half maximum of the dominant XRD peak, and θ is the position of the diffraction peak. The Table 1 summarizes, the values of lattice parameter “a”, unit cell volume “V”, and crystallite size “D”. in the Ni2+ doped ZnS sample slight reduction in “a”, “V” and “D”, with increased strain. In the Mn2+ doped ZnS sample exhibits the highest strain and the smallest crystallite size, consistent with lattice expansion coupled with substantial micro-strain from ionic radius mismatch. While, Co2+ doped ZnS sample reveals minimal change in “a” and “V” but significant strain and reduced crystallite size indicating defect induced distortion as the dominant effect [22]. In all doped samples, the crystallite size decreases relative to pure ZnS, suggesting that dopant incorporation hinders crystal growth during nucleation. The concurrent increase in microstrain values reflects enhanced lattice distortion, which is expected to influence carrier transport, defect state density, and consequently, the optical performance of the materials.

Morphological study
The shape of nanoparticles of the undoped and doped ZnS nanoparticles was observed by FESEM, as shown in Fig. 2. For Pure ZnS, the nanoparticles show spherical morphology in a consistent and uniform shape and size with large agglomeration may be ascribed to the high surface energy and van der Waals forces. 
Upon doping process was incorporated into host ZnS, the nanoparticle morphology changed from spherical to rod-like, flower-like (hierarchical assemblies), and thin nanosheet-like morphologies in Ni, Co, and Mn-doped ZnS, respectively (Fig. 2a-d). 
From the XRD patterns and according to the relationship of nanoparticle size, it was observed that the particle size decreases with the incorporation of dopants, as summarized in Table 2. Decreasing the particle size was accompanied by an increase in dislocation density with the incorporation of dopants into the host’s ZnS, resulting in a raise in the lattice strain of the structure. This substitution leads to local distortions in the lattice, which act as nucleation centers, changing the crystal growth kinetics and thus the final shape of the nanoparticles.
This transformation in the nanoparticles shape was accompanied by an increase in defects with a decrease in crystal size. The altration in particle shape from spherical to rod, flower, and sheet is as a result of the decrease in pressure inside the unit cell, this in turn results in less cation disorder in the crystal structure because of the ionic radius of the doped ions compared to the ionic radius of Zn+2. Based on what was mentioned, the effect of doping on the ZnS by changing the shape of the nanoparticles may help enhance the optical properties of the ZnS particles. Morphological variation suggests that similitudes influence the anisotropic growth rates of zinc sulfide (ZnS) crystal planes, due to changes in surface energy and preferential adsorption of similitude ions on specific crystal faces. Controlling the shape of nanoparticles can have a profound impact on the physical and functional properties of ZnS nanostructures. For example, anisotropic structures, such as nanorods and nanosheets, typically exhibit enhanced charge carrier transport and increased surface-to-volume ratio, which improves photocatalytic and optoelectronic performance.

Optical study
UV– visible spectroscopy was utilized to investigate the optical properties of the pure ZnS and doped ZnS nanoparticles. It was noticed that the doping process leads to the insertion of intermediate states between the conduction and valence bands, which may enhance the optical properties of the material. As shown in Fig. 3, the absorbance of the pure ZnS is improved when the dopant elements are introduced into the host’s ZnS due to the induction of additional optical levels within the forbidden gap, which in turn leads to rise the optical transitions and improved absorbance. The morphology of the nanoparticles is another reason for improving the optical property of the zinc sulfide compound.  It was found that rod-like, flower-like, and sheet-like nanoparticles are preferred over spherical-like nanoparticles because of their hierarchical assembly, crystallinity, and tunable geometrical parameters, which in turn leads to an increase in the absorption coefficient of the material. 
Furthermore, the optical band gap contributes to the enhancement of the absorbance intensity and the broad band of the wavelengths. As shown in Fig. 3, when dopant elements are incorporated into the structure of the zinc sulphide, the optical band gap decreases from 3.78 eV to 3.72 3.59 eV and 3.14 eV for pure ZnS, Co-doped ZnS, Ni-doped ZnS, and Mn-doped ZnS, respectively. The energy band gap (Eg) was deduced according to the Tauk’s relation [22] αhv=A(hυ-Eg) where A is a constant, α is the absorption coefficient and h is plank’s constant. By plotting of (αhv)2 vs the photon energy (hυ) and taking the intercept on the hv-axis, the optical band gap can be estimated. Electronegativity affects the optical property of the material through hybridisation between cations and anions. The electronegativity of an atom is strongly correlated with the ionisation energy. Therefore, electrons with high ionisation energies have low electronegativity, as in the Zn element, because their nuclei do not exert a strong attractive force on electrons. Elements with low ionisation energy, such as the dopant elements Ni, Mn, and Co, have higher electronegativity, which resulted in stronger ionic interaction of Dopant-S compared with Zn-S.  

Magnetic study
The magnetic property of the Ni/Mn- and Co-doped zinc sulfide nanostructures were investigated by VSM, as shown in Fig. 4. It shows the magnetization vs the applied magnetic field measured at room temperature. The weak magnetic hysteresis loops indicate that all samples are ferromagnetic at room temperature. It was found that changing the morphology of nanoparticles results from incorporating magnetic elements into the ZnS host, which results in an increase in the magnetic property of the material through increasing saturation magnetization Ms. The crystal size or particle size of the Ni-doped ZnS sample represents the critical size at which coericivity Hc is high. Therefore, it can be said that the coericivity and remnant magnetization Mr increases with decreasing crystal size or particle size before reaching the critical size due to the strong spin interaction during nanoparticle alignment and hence transforms from a single domain region to a multi-domain region. 
There is a relationship between the coericivity Hc of the material and grain size. where increasing the value of Hc means that more energy is required for demagnetisation of the material, i.e., increasing the grain boundaries accompanied by decreasing the grain size. To investigate the state of nanoparticles in a single magnetic domain or multi-magnetic domain, the sureness ratio of the material was determined according to the relation SQR=Mr/Ms [23]. I It was found that all samples have an SQR> 0.5 at which the samples are in the multimagnetic domain. In addition, the Co-doped ZnS sample exhibited higher disordering than the other samples because of its smaller SQR value. In addition, the coericivity of the material has an important effect on the anisotropy of the properties in any crystal, as shown in relation K1=(Mr×Hc)/0.98 [23], where K1 is the magnitude of the anisotropy constant. It was found that the Mn-doped ZnS sample had a smaller anisotropy than the Co/Ni-doped ZnS samples. 

CONCLUSION
Pure ZnS, Ni/Mn, and Co-doped ZnS nanoparticles were successfully synthesized using the solvothermal method. The incorporation of the dopant elements Ni, Mn, and Co into the ZnS nanostructure plays an important factor in the crystal growth and morphology of nanoparticles. The nanoparticles of each sample exhibit a cubic zinc blende structure without any secondary phase formation. Crystalline parameters such as the lattice constant, unit cell volume, and crystalline size decreased with the incorporation of dopant elements into the host’s ZnS, accompanied by increased the lattice strain within the volume of unit cell. Morphology of the nanoparticles changed from spherical-like in the pure ZnS nanoparticles to rod-like, flower-like, and sheet-like in the Ni, Co, Mn doped ZnS nanoparticles, respectively, resulting in an increase in the optical transitions and an improvement in the absorption coefficient. In addition, the magnetic study showed that the as-synthesized Ni, Mn, and Co-doped ZnS nanoparticles exhibited ferromagnetism at room temperature. This result denoted that dopant elements significantly influence the ZnS compound and may improve its optical properties.

ACKNOWLEDGMENTS
The authors thank university of kufa, and Hilla university college, and the authors state that there is no competing interest.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.