Comparative Studies of the Optical Absorbance and Structural Properties of Ironcopper Sulphide(CuS-Fe) and Iron Lead Sulphide(PbS-Fe) Thin Films Deposited by Silar Method

The influence of iron on lead sulphide(PbS)  and Copper Sulphide (CuS) thin films deposited on glass substrates via successive ionic layer adsorption (SILAR) Technique using lead acetate, Pb(CH 3 COO) 2 , Cupric Acetate Cu(CH 3 COO) 2 , thioacetamide ( S 2 H 5 NS) , Iron (II) Chloride dehydrate(Fe Cl 2 . 2H 2 O), ethanol and ammonia by  in alkaline medium annealed between  283K and 500K was investigated.   The structural and morphological studies were performed by X-ray diffraction (XRD) Analysis and  scanning electron microscopy(SEM) respectively. The Uv-visible studies were done using spectrometer in the Technical University, Ibadan.   The XRD showed  films of cubic  crystalline   PbS thin films, cubic and face-centred crystalline  PbSFe thin films, cubic CuS thin film, hexagonal Cu 2 S thin films and cubic and hexagonal crystalline natured CuSFe thin films with the preferential (111),(002)(004) (311) orientations. Keywords :  Copper Sulphide thin Films: Lead Sulphide thin Films: Absorbance: structural properties DOI : 10.7176/APTA/82-04 Publication date: January 31 st 2020

involved six steps. After pre-treatment of the substrates, the synthesis were done using .05M lead acetate and thioacetamide solution. Ammonia was used to control the pH. It was done between pH between 8.5 and 11.5. The iron ions were got from iron(II) chloride dehydrate. The copper ions were got from cupric acetate. It was equally deposited in alkaline environment too.
For a SILAR growth of PbS thin film, only four steps are involved, namely: • The glass substrate was first immersed in lead acetate solution for 35minutes , where lead ions were adsorbed on the surface of the substrate. • The second step involves the rinsing of the substrate for 35 seconds in deionised water to remove loose and unadsorbed lead ions from the surface. • The substrate was then immersed in thioacetamide solution for 35seconds, where the sulphur ions react with the pre-adsorbed lead ions on the substrate surface to form lead sulphide layer, • Finally, the substrate was rinsed again with deionised water to remove unadsorbed and loose material from the substrate surface. • A SILAR growth cycle for PbS x Fe (1-x) thin films has six.steps, namely: • The glass substrate was first immersed in lead acetate solution for 35 seconds , where lead ions were adsorbed on the surface of the substrate. • The second step involves the rinsing of the substrate for 35 seconds in deionised water to remove loose and unadsorbed lead ions from the surface. • The substrate was then immersed in thioacetamide solution for 35seconds, where the sulphur ions react with the pre-adsorbed lead ions on the substrate surface to form lead sulphide layer, • Finally, the substrate was rinsed again with deionised water to remove unadsorbed and loose material from the substrate surface, • The substrate was immersed in iron(II) Chloride dehydrate solution to adsorb iron ions on the preadsorbed lead sulphide layer, • The unadsorbed iron ions were removed from the substrate by rinsing in deionised water for 35seconds. After repeating for sufficient number of cycles( 90 cycles), PbS x Fe (1-x) composite thin films were deposited. The number of deposition cycles for PbS and Fe were adjusted to obtain various compositions of PbS x Fe (1-x) thin films(see table 1 below)  The densities of the composite (PbS)x(Fe)(1−x) thin films were estimated by considering compositional parameter 'x'. The weights of the deposited films were determined by using an electronic microbalance. In the present investigation thickness of (PbS)x(Fe)(1−x) films measured using sensitive microbalance is listed in table 2 above. The site for the research work was the crystal growth laboratory, Physics and Astronomy Department, University of Nigeria, Nsukka, Nigeria. The structural properties of the (PbS)x(Fe)(1−x) composite thin films were studied by X-ray diffractometer with CuKα radiation of wavelength 0.154 nm. The surface morphological investigations were performed using scanning electron microscopy analysis and energy dispersive spectrometry (EDS) analysis at the Department of Industrial Chemistry , The Technical University, Ibadan Nigeria.
B. Copper Sulphide and Copper Sulphide Iron thin films. The substrates were pre-treated as in the case above. For the SILAR deposition of (CuS)(1−x) thin films, 0.05 M cupric acetate solutions were taken as cationic precursor and 0.05 M thioacetamide as anionic precursor. The pH of the anionic and cationic precursors was adjusted to 12 and 8 by ammonia addition. Substrate was Advances in Physics Theories and Applications www.iiste.org ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.82, 2020 immersed in the cupric acetate solution for 35 s to adsorb Cu 2+ ions. (a) The un-adsorbed Cu 2+ ions were removed from the substrate by rinsing it in deionised water for 35 s. (b) The substrate was then again immersed in thioacetamide solution for 35 s, where S 2− ions reacted with Cu 2+ to form a layer of CuS. After repeating a sufficient number of cycles. It was removed and dried in an oven to avoid dust and oxidation. . For the SILAR deposition of (CuS)(1−x) Fe (1-x) thin films, the pre-treated glass substrates were immersed into 0.05 M cupric acetate solutions taken as cationic precursor, then rinsed in deionised water for 35 seconds before immersing into 0.05 M thioacetamide , taken as anionic precursor for 35 seconds before rinsing in deionised water. This was repeated for several cycles before the substrate was immersed in iron(II) Chloride dehydrate solution to adsorb iron ions on the pre-adsorbed copper sulphide layer.
The unadsorbed iron ions were removed from the substrate by rinsing in deionised water for 35seconds. It is worthy to note that the substrate was again immersed in thioactetamide solution where S 2− ions react with Cu 2+ to form a layer of CuS. After repeating a sufficient number of cycles, (Fe)1−x(CuS)(x) composite thin films were deposited. The number of deposition cycles for CuS and Fe was adjusted to obtain various compositions of (Fe) 1−x(CuS)x .
where D is the average crystallite size, k is the particle shape factor that varies with the method of taking the breadth and shape of crystallites , λ is the X-ray wavelength used(0.1542 nm), β is the angular line width of halfmaximum intensity (FWHM) of the diffraction peak, and θ is the Bragg's angle in degrees. See tables 1 and 2 below.

Morphological Studies
The morphological characterisation of CuS, CuSFe, PbS and PbSFe thin films were done using the scanning electron microscopy analysis(SEM) and Energy Dispersive spectrometry analysis .

Scanning electron microscopy(SEM) analysis
The    Fig.7 shows the plots of absorbance against wavelength of PbSFe thin films while Figure 9 depicts the plots of transmittance against wavelength. Figure 8 shows the plot of absorbance against wavelength of CuSFe thin films while Figure 10 depicts the plots of transmittance against wavelength for CuSFe thin films. The absorbance spectra of PbSFe thin films (Fig.7) vary in a manner, decreasing continuously from 1.40 (a.u) at 400nm to about 0.50 at 900nm. This range of absorbance is in agreement with the reports of Uhugebu (2007) for FePbS thin films, Agbo and Nnabuchi (2011) for TiO2/PbO thin films, and Onah,Ugwu and Ekpe (2015) for TiO2/CuO thin films. The absorbance spectra of CuSFe thin films (Fig.8) shows that the thin films decreased with wavelength within 400-500nm and then remained fairly constant within the wavelength range 550-700nm. Thereafter, it increased within the wavelength range 750-900nm. The maximum absorbance is about 0.5 (a.u.) at 400nm. This value of absorbance is below the maximum of 1.50 (a.u) by Uhuegbu (2007) for FeCuS2 thin films. The high absorbance displayed by PbSFe films may be used as spectrally selective coating for solar thermal applications. Solar collectors for heating fluids require increasing the reception area of the solar radiation, and/or to increase the absorbance of the surface coating in order to improve thermal efficiency (Oliva, Maldonado, Diaz and Montolvo, 2013). These findings are in agreement with the report of Augustine and Nnabuchi (2018) for CuO/PbS thin films. SUMMARY:X-ray diffraction patterns of CuS-Fe and (PbS)x(Fe)1−x composite films were shown in Fig.1 and 2 above. The peaks of XRD patterns have been assigned from the x-ray diffraction files ref. numbers :  (511), which corresponds to 2θ angles ranging from10.098-85.846. The XRD of doped PbS and CuS annealed at about 650K has been included. These had thirteen and seven peaks ranging from angles 2θ ranging from 10.429-85.9645 and 18.012-80.012 respectively. The (0 0 2) and (0 0 4) orientations due to hexagonal lattice are prominent in CuSFe and (1 1 1) and (2 0 0) orientations due to cubic lattice are distinct in pure PbS and CuS thin films. The PbSFe thin films annealed at temperature less than 500K were crystals that was cubic and face-centred. However, at x = 0.5 i.e. for (PbS)0.5(Fe)0.5, and (CuS)0.5(Fe)0.5 strong orientations disappear showing the non-formation of crystals due to the sp-d orientation. The crystallite sizes of the deposited materials were calculated using Debye-Scherer's formula.
From literature, the lead Sulphide thin films have been reported as having thermal stability as observed in this study. The samples(doped and undoped) were annealed between temperatures of 293K and 493K and from the XRD, the intensity ratio some diffractions changed but no additional peaks were observed up to 475K; This showed that the PbS nanofilm was not oxidized. The change in the diffraction reflection intensities was attributed to the fact that the phase transition to cubic structure takes place in the PbS film at 375K (Qadri et al. 2003).
The presence of oxygen atoms as shown by the EDS studies showed that the proportion ofiron to lead sulphide and iron to copper sulphide were not in equal proportion and also oxidation must have taken place because of their large surface area (Qadri et al. 2003). Based on this finding, the lead sulphide thin films (doped and undoped ) can be used in devices as fire alarm sensors, flame sensors and heat source detection systems.

Conclusions
A simple, cheap and convenient SILAR method was be employed to deposit good quality CuSFe and (PbS)x (Fe)1−x composite thin films. The deposited films were uniform and adherent to the substrate. Their structural and morphological properties of those composite thin films were studied.The EDS Studies showed that in (PbS)x (Fe)1−x composite thin films, the composition of iron was 21.8wt% while in (CuS)x (Fe)1−x composite thin films, iron composition was 20.8wt%.. The XRD and morphological studies revealed that CuSFe and PbSx(Fe)(1−x) thin films were nanocrystalline in nature depending on film composition. The average crystallite size was found to vary for the CuSFe thin films between 35 and 17 nm and for PbSFe thin films 34 and16 depending on film composition. The variation in thickness, strain and dislocation densities were also composition dependent. Similar observation has been reported by Wang et al. The samples annealed at different temperatures (383K-500K) never showed any prominent peaks structurally and morphologically as confirmed by studies done by He et al., From literature, considerable changes can be seen for temperatures up to 700 0 K (Mote, 2012). The high absorbance displayed by PbSFe films may be used as spectrally selective coating for solar thermal applications. Solar collectors for heating fluids require increasing the reception area of the solar radiation, and/or to increase the absorbance of the surface coating in order to improve thermal efficiency. The relatively high transmittance of PbSFe and CuSFe thin films in the infrared region suggest that they may be used for coating the walls and roofs of poultry houses to facilitate the transmission of infrared radiation in order to generate the heat required for warming young chicken. These properties can be well used in solar energy conversion devices and optoelectronics.