Preparing and Studying Structural and Optical Properties of Pb1-xCdxS Nanoparticles of Solar Cells Applications

Nanoparticles of Pb1-xCdxS within the composition of 0≤x≤1 were prepared from the reaction of aqueous solution of cadmium acetate, lead acetate, thiourea, and NaOH by chemical co-precipitation. The prepared samples were characterized by UV-Vis spectroscopy(in the range 300-1100nm) to study the optical properties, AFM and SEM to check the surface morphology(Roughness average and shape) and the particle size. XRD technique was used to determine the crystalline structure, XRD technique was used to determine the purity of the phase and the crystalline structure, The crystalline size average of the nanoparticles have been found to be 20.7, 15.48, 11.9, 11.8, and 13.65 nm for PbS, Pb0.75Cd0.25S, Pb0.5Cd0.5S, Pb0.25Cd0.75S, and CdS respectively. The results indicate that crystalline structure of all prepared samples is cubic except CdS which shows hexagonal and cubic structure. The particle size was found within the range of (64.81 to 91.14) nm, with a high purity.


Introduction:
Processing by means of chemical coprecipitation of bi-and ternary semiconducting matter in different sizes and formats is one of the fastest areas of material growth research because this method of deposit is simple, cost-effective and produces high quality that is important for applications in different optical and electrical devices (1)(2)(3). Cadmium sulfide (CdS) and lead sulphide (PbS) are examples of semiconductor material which have received great attention and can be deposited by the method of precipitation. Both semiconductors, which have originated in the specific photovoltaic and luminescent applications, are good respondents to the visible spectrum light (4).
CdS has in simple 2.42eV band gap at room temperature within the visible spectrum (5). PbS with a small direct band gap of 0.41eV at 300 K (at room temperature) is one of the materials commonly studied as a result of their possible application on non-lineal optical devices by group IV-VI semiconstructors (6), it is also used in many other areas such Pb +2 ion detectors, photographs and absorbers (7). PbS appears to have strong effects below the exitonic Bohr radius of 18 nm on the quantum scale and can be changed from 0.41ev (bulk) to 4.00ev (8).
PbS nanoparticles were synthesized by the reaction between lead nitrate and sodium sulphide and suggests a method for detecting bacterial specimens based on the commemorative properties of quantum dote semiconductors (9).Its nanoparticles are also made from lead (II) nitrate and sodium sulfide using the chemical coprecipitation process (10). CdS nanoparticles are formed in the presence and absence of the methylene blue pigment, by the interaction of cadmium acetate with thiourea (11).Also, the green synthesis of CdS nanoparticles was considered the most promising technique for potential use in biological systems (12) The PbS and CdS and their mixture ( Pb 1-x Cd x S ) have a significant interest since they give the advantages of tuning optical and Opto-electroequipment properties pbs (13)(14)(15). CdS usually comes out as a hexagon or cubic, but at higher pressures, the CdS takes the form of a NaCl structure (16). PbS has a facially centered cubic crystal structure (8). Partly crystalline compatibility exists between the CdS and the PbS, which allows the two materials to form cubic structures according to FCC atoms with identical constants (17,18).
The objective of this research is to prepare PbS, CdS, and Pb 1-x Cd x S nanoparticles using chemical precipitation method and investigate their optical and structural properties for solar cell applications.

Materials and Methods: Chemicals
The starting materials for the preparation include cadmium acetate [Cd(CH 3 COO) 2 .2H 2 O] as a source of Cd +2 cation, lead acetate [Pb(CH 3 OO) 2 .2H 2 O ] as a source of Pb +2 cation, thiourea as a source of anion S -2 , and sodium hydroxide (NaOH) to ensure maximum growth of medium alkaline. The volumes and molarities of chemicals used in the preparation of nanoparticles are listed in Table 1.

Procedure
Lead solution or cadmium solution or together lead and cadmium acetate solutions were heated up by 45°C with magnetic stirring principle. The alkaline NaOH solution was then added drop by drop to reach10 pH.
The color of the solution of PbS was light gray in 30 minutes. Then its color became darker as the reaction time rose until it completely changed into dark gray. The color of the CdS solution in 30 minutes was light yellow. Then its color grew darker as the reaction time rose until it changed completely to dark yellow to orange. The color of Pb 0.75 Cd 0.25 S was first light yellow in 15 minutes and then medium gray in 20 minutes, after 30 min became dark gray color. The Pb 0.5 Cd 0.5 S color was bright yellow for the first time in 15 minutes, medium gray for 20 minutes and final became dark gray. Pb 0.25 Cd 0.75 S color was at least 15 minutes bright yellow and 20 minutes bright green, and at the end it became dark green (19,20).
The color precipitate was separated by filtration, washed with distilled water, and dried at 80 °C in an oven.

Characterization instruments
XRD technique was used to determine the crystalline structure, the crystalline size of the nanoparticles and the purity of the phase using Xray diffractometer of copper-filtered monochromatic CuKα radiation (λ=1.54 0Å), Shimadzu, Japan and samples were analyzed at scales . Ultraviolet-Vis spectrometer (Shimadzu, Japan ) has been used to measure the optical absorption obtained from solution. Atomic force microscopy (AFM), SPMAA 3000, Advanced Angestrum Inc., USA, and field emission scanning electron microscopy (FE-SEM),(SEM; Oxford instruments model SEM:S-3200N), analyzed the morphology and roughness of the surface.

Results and Discussion:
Nanomaterials can most easily be analyzed by diffracting X-rays. If crystallite size is less than 100 nm there may be substantial broadening of the X-ray diffraction line, as shown in Fig.1. This broadening is used to measure the medium crystalline scale, with the most precise results obtained from the small size range using this method. The crystallite size of all prepared samples was calculated using the Scherer eq.1 (21) D= 0.9λ /β cosӨ, ---1 Where λ is wavelength of x-ray (Å), β is FWHM (radian) is the intrinsic full width at Half Maximum of the Peak and θ is the Bragg's diffraction angle of the respective XRD Peak. Figure 1 shows the X-ray diffraction pattern obtained from the prepared nanoparticles. The  Table 2, which was indexed within a cubic structure and was confirmed using a standard card (JCPDS No. 96-901-3403). The absence of any other peaks reveals that the PbS is free of impurities. XRD peaks clearly indicate a significant broadening after Cd doping which may be due to the difference in the stable crystal structure of PbS (cub.) than CdS (hex.) (22). As a result, when Cd 2+ occupies more and more of the space originally occupied by Pb 2+ , the internal strain increases and the crystal structure of the PbCdS solid becomes unstable. This clearly shows that Cd 2+ ions are well dissolved in the PbS lattice, as the ionic radius of Cd 2+ is less than that of Pb 2+ .As the Cd concentration increases, the diffraction peaks becomes slightly broader due to a reduction in the grain size. Additionally, the intensity of the diffraction peaks decreases which can be attributed to doping-induced structural disorder (23). (311) as shown in  (24) and by Nada et al (25).  643 AFM technology provides digital images that quantitatively evaluate surface characteristics, including grain size (nm) and roughness average (nm). The 3D and 2D pictures, Fig. 2 demonstrates the spherical shapes for all samples studied. The images also show a non-compact surface which is not smooth. The grain size (average diameter) and roughness average (Ra) obtained from AFM measurements are listed in Table 3. The sizes of nanoparticles obtained from the AFM images appear bigger than the values that obtained from XRD measurements (26). Those results can be interpreted for several reasons; the first explanation is that the nanoparticles tend to form aggregates on the surface during deposition. The second explanation is related to the shape of the tip AFM that may cause misleading cross-sectional views of the sample. The results show that the grain size and Ra of PbS is larger than other samples which are consistent with X-Ray results except the roughness values in CdS.  The samples provided by the preceding have a broad optical absorption spectrum in the ultraviolet range. This could serve to suggest the development of smaller-sized Pb 1-x Cd x S when the pinnacle position changes into UV field.
The energy consumed in UV or visible areas actually changes the molecule's electronic excitation and thus changes their ability to absorb lights in the UV-visible area of the electromagnetic radiation. In this way, the electronic excitation of molecule was changed (9). Therefore, a relation was determined between the energy absorbed in the electronic transition and the frequency (υ) which is used in eq.2 (27) If E g is the energy band gap (eV), h is Planck constant, c is the light velocity. Energy gab for Pb 1-  The result of Table 4 shows that the values of band gab are 4.4, 4.36, 4.9, 4.34 and4.39 eV for PbS, Pb 0.75 Cd 0.25 S, Pb 0.5 Cd 0.5 S, Pb 0.25 Cd 0.75 S and CdS respectively. This means that the Pb 0.5 Cd 0.5 S is the best sample which may be used in the solar cells with high performance.

Conclusions:
Nanoparticals of CdS, PbS, and Pb 1-x Cd x S with 0, 0.25, 0.5, 0.75, and 1)at.wt% have been prepared by chemical co-precipitation method. The XRD analysis shows that PbS has a cubic structure while CdS has cubic and hexagonal structure and reveals that they are free from impurities. cubic structure. The 3D and 2D AFM pictures demonstrate the spherical shapes for all the samples studied and also show a non-compact surface which is not smooth and this increases the absorbance for the prepared samples. The diffuse absorption spectra of PbS, CdS, and Pb 1-x Cd x S nanoparticle showed a sharp increase in the energy gap due to the quantum confinement effect. From the measurement of the optical energy gap for Pb 1- x Cd x S nanoparticles, it has been found that: the optical transitions are direct transitions and the Pb 0.5 Cd 0.5 S is the best sample which may be used in the solar cells with high performance. SEM images of prepared samples display nanoparticle of various diameters with spherical shape and show uniformly homogeneous surface which confirms the results of XRD and AFM.