Effect of the Concentration of Copper on the Properties of Copper Sulfide Nanostructure

Nanoparticles of copper sulfide have been prepared by simple reaction between using copper nitrate with different concentrations ratio 0.1, 0.3, and 0.5 mM, thiourea by a simple chemical route. The prepared Nano powders have been deposited onto glass substrates by casting method at 60°C. The structure of the product Nanofilms has been studied by x-ray diffraction, where the patterns showed that all the samples have a hexagonal structure of covellite copper sulfide with the average crystalline sizes 14.0716.51 nm. The morphology has been examined by atomic force microscopy, and field emission scan electron microscopy. The AFM images showed particles with almost spherical and rod shapes with average diameter sizes of 49.1190.64 nm. From the UV-Vis absorption studies, it has been noted that the increased absorption edge for all thin films leads to decreases in the energy gap of 3.5 to 3.0 eV for 1mM and 5mM respectively.


Introduction:
Because of its small size, the nanostructure has attracted a significant concern for its unique properties that cannot be obtained from the bulk structure.
The transition metal sulfides nanostructure such as PbS, HgS, CdS, and CuS shows remarkable chemical and physical characterizations in comparison with their bulks 1 . Copper sulfide, as a member of the chalcogenide Nano crystallized semiconductor, is an important element with multifunctional utilization in industry through the solar cells 2 , optical filters, sensors, photo catalysts, and lithium ion batteries 3 , and other devices are inexpensive due to the high absorption coefficient (10 4 cm -1 ) and small band gap (1.2 eV). NPs CuS show interesting physical and chemical properties according to their size and shape. Optical band gap energy of the CuS NPs varies from 1.2 to 2.5 eV depending on the stoichiometry. Copper sulfide NPs with different crystalline phases (such as chalcocite (Cu 2 S), djurleite (Cu 1⋅9 S), digenite (Cu 1⋅8 S), anilite (Cu 1⋅75 S), and covellite (CuS)) from the copper-rich (Cu 2 S) side to the copper-deficient side (CuS 2 ) have been reported extensively 4 . Nanostructure copper sulfide shows interesting structural and optical properties depending upon various growth conditions such as molar concentration, temperature, capping agents, surfactants, precursor solution composition etc. In order to synthesize copper sulfide nanostructure different methods have been adopted such as hydrothermal route 5, 6 , chemical bath deposition 1 , microwave irradiation 7 , green method 8 , mechanochemical synthesis 9 , a single step sonochemical method 10 , one-step solid-state reaction 11 , Spray pyrolysis deposition 12,13 , sol-gel method 14 , and chemical precipitation 15,4 and etc. Among these preparatory methods above, chemical precipitation technology is a simple, effective, and relatively low-cost method that uses the lowest possible number of chemicals, including those water-soluble minerals, and is also considered one of the most used methods for preparing nanomaterial 16 . In this research, the effect of concentration was investigated for Cu on properties of CuS nanostructures deposition by casting process. The optical, structural, and morphological properties of the prepared Nano films with different concentrations were investigated using XRD, AFM, FE-SEM, and UV-Vis spectrophotometer.

Preparation of CuO Nanoparticles
In a typical synthesis, CuS nanoparticles were prepared by reacting copper nitrate solution with different concentrations 0.1, 0.3, and 0.5 mM and thiourea in the presence of polyvinyl alcohol PVA as capping and stabilizing agents. The resulting solution was greenish in color. The synthesized powders were separated from the solution with a centrifugation at 4000 rpm per minute, and washed three times with distilled water. Finally; the products were dried at 80 o C for 1hr.

Deposition of Thin film
The prepared nanomaterial was precipitated on glass substrates by casting method, at temperature 60 o C. The glass substrates were ultrasonically cleaned in acetone, rinsed in distilled water, then in ethanol, and rinsed again.
The phase and the structure characterization of the as-prepared thin films were examined by XRD using X'pert Philips diffract meter with Cu Kα radiation beam λ=1.5406 ‫ە‬A for 2 values between 20 o -60 o . UV-Vis spectrophotometer SP-3000 plus, OPTIMA INC. Japan was used to measure the optical absorption measurements of samples over the range 300 nm to 1100 nm. The particle size and morphology of as-prepared thin films were studied through atomic force spectroscopy using SPM (Model AA30000), tip NSC35l//AIBS from Angstrom Advanced Inc (USA), and field emission scan electron microscopy by a MIRA3 TESCAN Mashhad (MUMS) model (Fill Emission Scanning Electron Microscope).

Structural analysis
To characterize the crystal structure, crystalline size, and orientation factor, XRD can be used, which is powerful non-destructive means of the samples. The X-ray diffraction patterns of CuS Nano crystalline at different concentrations 0.1, 0.3 and 0.5 mM are shown in Fig. 1 a, b, c respectively.

Figure. 1 XRD Pattern of CuS nano crystalline with different concentrations
From these patterns, it can be shown that the prepared thin films are of pure phase. For all the samples at different concentrations, the formation of the pure CuS was hexagonal phase. It has been affirmed by the standard peaks with the JCPDS card number 06-0464. of X-ray diffraction patterns, the diffraction peaks of the Nano crystalline were clear, indicating the formation of crystallites with Nano scale 17 , these results are consistent with M. Saranya et al. and S. Riyaz et al, 4,13 .
As shown from the Figures the number of peaks increases by increasing concentration, in addition to being stronger and sharper for larger concentrations indicating that the as-prepared thin films are well crystalline but larger in size. The increasing of the crystallization and crystal size may be due to the increasing of Cu concentration. The increase in the size of the crystals with increasing concentration is mainly attributable to the increase in the rate of reaction as well as on growth conditions, as the concentration increases; the crystalline nature of the film also increases. Crystallite size (D) of CuS Nano crystalline is founded using Scherrer equation (D = 0.9 λ/β cos θ). The result values of D illustrate that the size lies within the Nano scales range as shown in Table 1.  Fig. 2 a, b, c illustrates three dimensional (3D) profile and the granular distribution of AFM micrographs for CuS films deposited at different concentrations. Results of AFM measurements are listed in Table 2; From these results it is noted that the mean diameter value increases with increasing concentration as well as surface roughness, root mean square, and peak to peak. These results are consistent with another work, that of M.A. Sangamesha et al. 1 . Another observation is that the grain shape was mostly spherical.  Figure 3 shows FE-SEM images of the CuS nanostructure synthesized at various concentrations. From the images it can be remarked seen that the surface of the particles appears almost in spherical shape with the presence of agglomeration in concentration 0.5 mM, this leads to an increase in particle size.   Fig. 6 that the band gap of (CuS) nanostructure was direct for all samples. The values of energy gap were calculated from the intersection of the straight-line portion of the (αhν) 2 against the (hν). From the linear part it was noticed that the type of transition in these films has a direct nature. The determined energy-gap values of the samples are (3.5, 3, and 2.9) eV for (0.1, 0.3, and 0.5) mM respectively. The values of optical energy-gap for prepared nanostructures are higher compared with the bulk value of CuS (1.2 eV) due to the quantum confinement. The spectra of absorption display that the absorption peaks of exciton for the Nano crystal are blue-shifted in comparison to the bulk band gap and clearly demonstrate a strong quantum size effect 19 . Also, it has been observed that the energy gap of the films deceases when the concentration increase, this is attribute to the increase of crystalline size of the films. The results of the energy gaps are in agreement with a previous report by M.A. Sangamesha et.al 1 . Figure 7 illustrates the refractive index (n) with wavelength for different concentrations while Fig. 8 relates the extinction coefficient (k). The refractive index of materials is an important factor for the design of the device. The calculation of the refractive index is of great importance for applications in integrated optics. 3 While the extinction coefficient (k) can be measurement by k = α λ/4π 4 The values of thickness, energy gap, refractive index and extinction coefficient for the films are a tablet in Table 3.

Conclusions:
CuS nanostructures has been synthesized by employing chemical precipitation method at different concentrations, and then deposited on glass substrate using casting method. XRD patterns show that the structural nature of the CuS thin films is hexagonal phase for the all as-deposited. The average grain size is estimated by XRD, and AFM, where that it is concluded that the size increases with increasing concentration. The shape of grains is mostly spherical that's noted from AFM, and FE-SEM images. All the films show high transmission (~ >80%) and low absorbance in the UV-VIS region. The energy gaps values decrease with increasing concentrations. The optical characterizations for as-prepared CuS nanostructures indicate that the films are useful for optoelectronic devices such as photovoltaic cell as a window layer. These nanostructures can be used as a transparent dielectric material due to their high transparency in the visible region.