Synthesis of Nickel Sul de Nanoparticles from Single Source Precursor (Metal Xanthate Complexes) Using Melting (Solvent-Less) Method


 Nickel sulfides are rich chemistry groups with discrete phases and stoichiometries, therefore they have various properties and applications. We herein prepare two single metal xanthate precursors [K(S2COBu)] Potassium butyl xanthate and [K(S2COPn)] Potassium pentyl xanthate using the melting method at two different temperatures 400 and 500°C to synthesize nickel sulfides nanoparticles. The nanoparticles were characterized using powder X-ray diffraction (p.XRD), energy-dispersive X-ray spectroscopy (EDX) and imaged using scanning electron microscopy (SEM). Two different nickel sulfides nanostructures obtained [Ni(S2COBu)3] and [Ni(S2COPn)3], the results from p.XRD show the sizes of the nanoparticles are (35.39±8.15 and 38.24±7.70 nm) at 400oC respectively, and (43.12±4.52 and 47.45±4.22 nm) respectively at 500oC. The result shows that the xanthate ligand affects the size of the nanoparticles as by increasing the alkyl chain length the size of the nanoparticles decrease. The (SEM) images show coral-like agglomerates, which are mainly assembled by spherical nanoparticles. EDX analysis shows stable and pure nickel sulfides with some variations in the elemental ratio of both nickel and sulfur according to the xanthate ligand used as a precursor to synthesis the nanoparticles. As a result, we introduce a simple, low cost and feasible synthesis of pure, stable, with definite size two nickel sulfides nanoparticles using metal xanthate ligands.


Introduction
An attractive target for many scientists due to the alterations in the optical [1], magnetic [2] properties and catalytic activities [3], which occur in nanometer-sized particles. Metal sul des have a great feature prominently among the phases investigated in nanocrystalline form especially nanocrystals of CdS [4,5] and the wideband gap semiconductor ZnS.[6-8] Interestingly, the magnetic properties of other metal sul de nanocrystals have been studied such as iron sul de [9], nickel sul de [10,11] phase systems, MnS, AgS [5], and numerous copper sul de phases. [5,12] Nickel II complexes are very important as they detect presences of pollutants, water vapour, and they have catalytic activates. They have unique optical, electrical properties and have different applications many studies report the high capacitance of the nickel sul des such as Shombe and co-workers studies the conversion e ciency and speci c capacitance of solvent-free synthesized different nickel sul de composites. Their results are based on X-ray diffraction, transmission, and scanning microscopy [13]. Sajjad and Khan used nickel sul de nanoparticles as electrode material for symmetric supercapacitor.
The synthesized nanoparticles showed a uniform shape and size. Nickel sul des electrodes showed enhanced electric properties such as high speci c capacitance of 2495 F g −1 at 1 A g −1 and excellent cycling stability based on the Ragone plot shows a high energy density of 52.4 W h kg −1 and an ultrahigh-power density of 13500.0 W kg −1 . [14] Marand et al. studied electric properties of nickel sul de reduced graphene oxide composite and reported a high speci c capacitance of 305 Fg −1 at a current density of 1.1 A g −1 and high-capacity retention of 91% after 3000 cycles. The composite was synthesized using a solution combustion method. [15] Nickel sul des are also used as electrodes in the battery to enhance their electrical properties. Li et al. developed nickel sul de nanoparticles with sulfur-doped reduced graphene oxide as dual-role anode materials for both lithium-ion battery (LIBs) and sodium-ion battery (SIBs) by increasing lithium and sodium storage e ciency. Their results showed good transition oxides/sul des in alkali metal-ion batteries [16]. Li and co-workers synthesized a composite of nickel sul de nanoparticles and reduced oxide nanosheets. The composites were synthesized via a simple one-step hydrothermal method using different temperatures. These composites avoided the agglomeration of the sodium ion batteries electrode materials and enhanced reversible capacity and better capability [17].
The photocatalytic activities of nickel sul des also attracted many researchers Kumari and co-workers synthesized nickel sul de nanostructures using the precipitation method. The surface of the nanostructure was functionalized and used for selective adsorption of anionic and cationic dyes and two different types of antibiotics. The adsorption of the dyes to the nanostructure was electrostatic, whereas the adsorption with the antibiotic was due to hydrogen bonding and metal coordination. The synthesized and functionalized nanostructure can be used as recyclable adsorbate for different organic pollutants[18]. Zhang and co-workers synthesized nickel sul de composite for e cient photocatalytic nitrogen xation upon sunlight irradiation. The composite system enhanced the electron transfer, increase the nickel sul de band potential with the synergetic internal electric eld and photogenerated electron-hole pairs. These results were con rmed by electrochemical impedance spectroscopy and photocurrent tests. The nickel sul de composite system showed a good photocatalytic nitrogen reduction and produced a high NH 3 rate [19]. Dev and Singh studied nickel sul de nanoparticles anchored graphene oxide among different metallic sul de nanoparticles. Moreover, they tested the photocatalytic activates through methylene blue reduction [20]. Lakshmanan et al., synthesized nickel sul des and nickel oxide nanoparticles using solvothermal and thermal decomposition, they characterized the synthesized nanoparticles by powder X-ray diffraction (pXRD), high resolution scanning electron microscopy (HRSEM), energy dispersive spectroscopy (EDS), and UV diffuse re ectance spectroscopy (UV-DRS). The photocatalytic activities of the nanoparticles were tested by detecting the degradation of methylene blue and rhodamine 6G upon UV irradiation. Their results showed that the nickel sul de nanoparticles are more photocatalytic active than nickel sul de nanoparticles [21].
As reported nickel II complexes are very important and have variable chemistry that let them applicable in different elds such as the detection of the presences of pollutants, water vapour, and they have catalytic activates. Nickle sul des have been used in different applications such as agriculture [22], solar cells [23,24] and as superconductors [25], pollutants degradation[26].
The properties of nanoparticles mainly depend on their shape and size. According to different synthesis methods nanoparticles, different nanoparticles crystalline phases and sizes can be obtained. The different applications of synthesized nanoparticles depend on their precise synthesis and characterization. Using a single-molecule precursor has many advantages if compared to multi-source synthetic protocols. Single molecules precursor synthesis resulted in constant and better composition nanoparticles and fewer crystal defects on its structure. Thus, a high-quality nanomaterial can be obtained. Synthesis of nickel sul des using multi-sources is commonly used and reported in many studies [27][28][29]. Using a long organic chain was also reposted in many studies to control the size and morphology of the synthesized nanomaterials but they limit their application as they caused ligand surface chemistry complexity [30,31].
Solventless thermolysis method is distinguished over other routes their ease of synthesis in which solidstate decomposition of a precursor is accomplished by thermal treatment under inert conditions. The solventless thermolysis method is considered an effective way to synthesize metal chalcogenide nanomaterials with a wide range of morphologies such as nanorods [39], nanowires [40], nanospheres [41], and nanodisks [42]. Interestingly, melt thermolysis can provide a simple and cost-effective way to scale up production. Another advantage of this approach is its ability to offer economic and environmental bene ts reduce the requirement for harsh materials, and typically, yields are frequently high. Melt reactions was used to synthesize a wide range of different nanoparticles materials including metal sul des such as Bi 2 S 3 [43], Cu 2 S [44], NiS [45], PbS[46], PdS [47] and CdS [48].
For single-source precursors, the origin of transition metals and non-metals materials of the target binary compound are associated with a single precursor species. Recently, this approach has been utilized widely in nanocrystal synthesis. Typically, it offers many great features such as ease of utilizing and high-quality products under relatively mild reaction conditions. [49][50][51] numerous transition metal sul de nanocrystals have been synthesized using single-source precursors e.g., ZnS [52,53], iron sul de [54,55], and nickel sul de nanocrystals [56,57].
Xanthates (alkyl dithiocarbonates or ROCS-2) are organic compounds that contain two groups a negatively charged group that react with metals and a hydrocarbon chain that react with non-polar are considered as a good choice to synthesis metal sul des as they decomposed easily and cleanly at low temperatures [59]. In addition, it can be used as a capping ligand for the synthesis of metal nanoparticles and self-assembly monolayer. Using xanthates for the synthesis of nanoparticles have many advantages, as the by-products generated due to xanthates decomposition are highly volatile and can easily be removed from the reaction leaving a pure and stable nanoparticle.
Here we used the melting (solvent-less) method at two different temperatures 400 and 500°C to synthesis two nickel sul de nanoparticles using two xanthate ligands [K(S 2 COBu)] Potassium butyl xanthate and [K(S 2 COPn)] Potassium Pentyl xanthate as a single-source precursor. Two nickel sul des nanoparticles were characterized using XRD and EDX. The crystallite size (D) is calculated using the Debye-Scherrer formula. The nanoparticles were imaged using SEM. We present a simple, low cost and feasible synthesis of pure, stable, with de nite size two nickel sul des nanoparticles using metal xanthate ligands.

Results And Discussion
Powder X-ray Diffraction (p.XRD) The p.XRD patterns are shown in Figure. As shown in gure 1, the intensity of the peaks at 35.66° and 45.61° decrease with the increase of the temperature this change in the intensity is related to the size changes at a different temperature, also the two peaks 52.53° and 53.34° at 400°C almost merge to form one peak at 500°C as the size of the complex changes. This is due to different phase structures resulting as the temperature changed as previously reported by Roffey and coworkers[60]. Figure 2 represents the p.XRD of second nickel sul de nanoparticles [Ni(S 2 COPn) 3 ] (Complex 2), peaks observed in the p.XRD pattern are the same as the one obtained for (complex 1). The peaks at 32.18° and 34.50° as well as the peaks at 52.53°, 53.34° at 400°C almost merged at 500°C, as discussed before due to different structure phases resulting from the temperature change. The peak at 34.50° at 400 disappears at 500 °C, which also con rm the structure phase changes at different temperature.
The crystallite size (D) is calculated using the Debye-Scherrer formula, the calculated crystallite size for Our results are in good agreement with the p.XRD pattern reported by Lakshman and coworkers for nickel sul des nanoparticles, they reported a crystallite size 62 nm, they used solvent and thermal deposition method to synthesis nickel sul de nanostructure [21]. Also, Almanqur et al. used xanthate single-source precursor to synthesis iron sul des nanostructures by using spin coating and deposition methods, they reported different crystallite sizes using p.XRD according to different methods they used [62].
Some studies used the electrochemical method to synthesis nickel sul de nanoparticles, they tested different experimental conditions and used maximum temperature at 60°C, their p.XRD showed α-phase nickel sul de (JCPDS card no. 75-0613) and 19 nm average particles size [63]. Others used aerosolassisted chemical vapour deposition at four different temperatures starting from 250 to 400°C to form a thin lm of nickel sul des, they used Bis(O-alkylxanthato) nickel (II), where the alkyls are hexyl and octyl.
Their p.XRD results showed a mixture of hexagonal Ni 17 S 18 and orthorhombic Ni 7 S 6 nickel sul de thin lms[64].
Salavati-Niasari and coworkers used microwave radiation to synthesis nickel sul de nanoparticles, and they tested different synthesis conditions as the effect of concentration of sulfur source, reaction time, and power of microwave irradiation. Their p.XRD results showed pure well-crystallized nickel sul de nanoparticles.
[65] Yu and Yoshimura used liquid-solid interaction between nickel and sulfur to synthesis nickel sul de thin lm and powder at a low temperature less than 250°C, there p.XRD results showed different nickel sul de phases produced as different solvents were used to synthesis nickel sul des[66].

Scanning electron microscopy (SEM)
The morphological features of the synthesized two nickel sul de nanoparticles [Ni(S 2 COBu) 3 ] (1) and [Ni(S 2 COPn) 3 ] (2) are studied by scanning electron microscopic technique. Fig. 3 illustrates SEM images of two nickel sul des nanoparticles obtained from two different xanthate ligands at two different temperatures. The SEM images of nickel sul des nanoparticles showed agglomerated small bead-like structures forming coral-like shapes. These agglomerates are assembled by different shapes of nanoparticles.
Energy Dispersive X-ray Spectroscopy (EDX) Further analysis of the synthesized nanostructures Energy-dispersive X-ray analysis was used to con rm the purity of the synthesized nanostructures through element analysis as shown in gure 4. The elemental composition obtained from EDX con rms a 1:1 ratio for the two nickel sul des nanoparticles, for [Ni(S 2 COBu) 3 ] the ratio Ni% was (49.22 and 49.17) at 400 and 500°C respectively. Therefore, the nickel percentage is relatively less than S% (50.78 and 50.83 3 ]. The results show that the size and phase structure of the synthesized nickel sul de nanoparticles are affected by changing the temperature, also as the alkali chain length increases the size of the nickel sul de nanoparticles decreases. The presented single source, solvent-less synthesis is a simple and low-cost method that produces pure, well crystallite nickel sul de nanoparticles.