Synthesis and characterization of nanocrystalline copper sulfide powders

Nanocrystalline copper sulphide (Cu2-xS) powders were synthesized by chemical precipitation from their aqueous solutions composed of different molar ratio of copper sulfate dehydrate (CuSO4.5H2O) and thiorea (NH2)2CS as source of Cu+2, S-2 ions respectively, and sodium ethylene diamine tetra acetic acid dehydrate (EDTA) as a complex agent. The compositions, morphological and structural properties of the nanopowders were characterized by energy dispersive spectroscopy (EDS), scanning electron microscope (SEM), and X-ray diffraction (XRD), respectively. The compositional results showed that the copper content was high and the Sulfur content was low for both CuS and Cu2S nanopowders. SEM images shows that all products consist of aggregate of fine nanospheres with uniform distribution and the size of the particles formed are in nanometer range. XRD results revealed that the obtained powders contains a mixture of copper sulfide phases specially the intermediate phases and the rough estimate of the average crystallite size using the Scherrer formula gives a range of values (4.1-36.9) nm.


Introduction
The development of nanostructured materials in the form of thin films has appropriate place in the research of solid state technology, it has high surface area to volume ratio therefore it have different structural, optical, electrical, magnetic and dielectric properties than bulk [1]. The nanomaterials especially metal chalcogenides, like selenides, sulfides, and telluride are being studied widely due to their quantum confinement effects that associated to their small crystalline size [2,3], which give them a large potential applications in the light emitting diodes, solar cells, fuel cell, drug delivery, catalysts for industrial transformation, gas and photo-sensing mechanism [2,4]. The transition sulfides metal are very promising semiconducting materials that have been used in many applications due to their excellent properties that includes a unique structure morphology, direct band gap, and high absorption coefficient of more than 10 5 cm -1 [5].
Cu 2-x S nanocrystals act as p-type semiconductor material mainly due to that of copper vacancies occurring within the lattice, which is the reason for their use in optoelectronic devices [6]. It is also used in photothermal conversion applications, solar control coatings, photovoltaic applications, electronic devices, optical filters as well as in low temperature gas sensor applications [7,8].
Copper sulfide (Cu 2-x S) has five stable phases that naturally occurs depends on the varying of Cu/S ratio, Covellite (CuS), anillite (Cu 1.75 S), degenite (Cu 1.8 S), djurelite (Cu 1.95 S), and chalcocite (Cu 2 S) [9], Other phases that exist include yarowite (Cu 1.12 S) and spionkopite (Cu 1.14 S) [7]. The synthesis of Copper sulfide nanoparticles by a simple method is still a challenge [10]. Many methods have been developed to synthesize copper sulphide nanoparticles such as microwave [11], solvothermal [12], electrosynthesis [13], organometallic precursor route [14], hydrothermal [15], and chemical precipitation [16]. The present paper deals with the synthesis Cu 2-x S nanocrystalline powders with different molar ratio of Cu/S, and study the compositional, The procedure for prepare CuS film involve, taking 15 ml of copper salt solution as Cu +2 ion source in 100ml beaker, 15 ml of complex agent is mixed drop wise , the solution is stirred constantly for few min for getting homogenous mixture. Then add NH 4 OH drop wise until the color of the solution changes from light blue to dark blue (pH=10). After this 15ml of thiorea add as S -2 ion source, stir the solution for 15 min, then the solution color become olive. The reaction mixture keep in water bath at 60 o C. After 90 min the reaction was completed and the particles settled down at the bottom of the beaker. During the precipitate formation a change in color from purple-silver to black was observed. The as synthesized product was filtered and rinsed several times with distilled water, then it was dried in oven at 100 o C for 1hr to obtain powder.
Five reaction baths were used trying to obtain different copper sulfide powders with different phases (CuS, Cu 1.75 S, Cu 1.8 S, Cu 1.95 S and Cu 2 S) by changing Cu/S molar ratio in the solution.
The compositional, structural, and morphological properties of the prepared powder were studied. Compositions of elements were recorded by EDS, type Bruker company-Germany, X-Flash 6l10model (Physics Department, College of Science, Al-Nahrain University), and x-ray diffraction was determined by (Miniflex II Rigaku company, Japan) diffractometer equipped with a Cu-Kα radiation. The morphological features for the prepared powders were studied by scanning electron microscopy (SEM), model (TOSHIBA 4160), with high resolution mode, accelerating voltage (200 eV and 30 keV), and SEM magnification from (3-1000000).

Results and discussion
The elemental composition of Cu 2-x S powders for different values of x (1, 0.25, 0) were investigated using EDX and the charts are shown in Fig. 1. Peaks of Cu and S exhibit the presence of these elements in these powders.
There are small peaks of Oxygen, Nitrogen and Carbon originated from the precursor material which are used in the chemical reaction or from the contaminated with the elements exist in the environment. The elemental analysis was carried out only for Cu and S and the mass percentage of Cu:S in these powders were calculated and listed in Table 1. : a-x=1, b-x=0.25, c-x=0. It is clear that the composition ratio of the elements deviation from the calculated ratio, where the copper ratio was high in the CuS and Cu 2 S powders and the sulfur was low content. This is may be occurs because of the interaction between copper and complex agent, which may effect the operation of released ions that can be high. In different way sulfur was bound in solution after the reaction, therefore the ratio of sulfur was low.

Fig. 1: EDS chart of Cu 2-x S powders at different ratios of Cu
Scanning electron microscopy is a convenient method to study the surface morphology of powder and thin films.
It gives as important information regarding growth, shape and size of the particles. Surface morphology of material plays an important role in solar energy conversion efficiency of the device. Fig. 2 shows the surface morphology of the Cu 2-x S powders of different molar ratio (x=1, 0.25, 0) with different magnification. It was observed from these images that all products consist of aggregate of fine nanospheres with uniform distribution, the size of the particles formed are in nanometer range. : (a) x=1,  (b)x=0.25, (c) Table 2. One can see that the obtained powders contains a mixture of copper sulfide phases, this result agrees with the result of Pop et al. [16]. The high copper content phases have problem, which represent by that the copper atoms instability towards the formation of copper vacancies, this leads to the formation of different crystal structures depending on Cu vacancy concentrations [17][18][19]. A rough estimate of the average crystallite size using the Scherrer formula gives a range of values (4.1-36.9) nm.

Conclusions
Copper sulfide (Cu 2-x S) nanoparticles with different molar ratios (Cu/S) were prepared by chemical precipitation. The composition ratio of the elements was slightly deviated from the calculated ratios. Scanning electron microscope images showed uniform surface with agglomerate nanoparticles for all samples. X-ray diffraction analysis of Cu 2-x S powders showed that the samples have crystalline structure, and the molar concentration ratio of the precursors was important parameters for synthesizing Cu 2-x S powder.