3.1 XRD Analysis
The deposited pure CuS thin film of pure and Cd doped Cus is investigated by X-ray diffractometer. They are shown in Figure. The following values can be derived by using the data obtained from XRD analysis. The interatomic spacing (d), full width half maximum (β), and average particle size (D) values of pure CuS thin film are calculated and tabulated in Table [12].
2d Sin θ = nλ (Bragg’s Law) | (2) |
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\(\text{D}=\frac{0.94{\lambda }}{{\beta }\text{C}\text{o}\text{s}{\theta }}\) | (3) |
where λ is the wavelength of X-rays (0.1546 nm), n = 1 integer, d is the interplanar spacing between the atoms, β is FWHM (full width at half maximum), ‘θ’ is the diffraction angle and ‘D’ is crystalline size [12].
The formula, which is given as the length of dislocation lines per unit volume of the crystal, is used to calculate the dislocation density (ρ) of the deposited film using the diffractogram as well (4)
$$\rho = \frac{1}{{D}^{2}}$$
4
The microstrain value (ε) is obtained using Eq. (5). For the lattice parameter "a" use Eq. (6), which appears as cubic geometry above, where hkl stands for the lattice planes.
$$\epsilon = \frac{\beta cos \theta }{4}$$
5
$$\frac{1}{{a}^{2}}= \frac{({h}^{2}+{k}^{2}+{l}^{2})}{{a}^{2}}$$
6
The XRD peaks are observed from 20 ° to 80 °. Figure 2 shows the XRD pattern of a CuS thin film, indicating a sharp diffraction peak at 2 = 20.28 °. The deposited film at room temperature has only one sharp diffraction peak, which illustrates the amorphous nature of the films. The deposited pure CuS thin films are polycrystalline. Figure 2.1 of the CuS thin film shows the peaks corresponding to the (111), (200), (211), and (311) directions. The deposited film (200) has a high intensity. The diffraction peak (2 = 15.99 °) is due to the reflection in the cubic phase. The indexing is well matched with the JCPDS file (96-101-0921) [13]. This indicates that the CuS thin film has a crystalline structure with a preferential orientation along the (200) direction. The well-matched indexing with the JCPDS file confirms the formation of a high-quality CuS thin film.
The XRD peaks are observed from 20° to 80°. Figure 2.2 shows the deposited CuSe: Cd thin film at room temperature. It indicates a sharp diffraction peak at 2 = 20.29 ° and minor diffraction peaks at 16.54 ° and 24.54 °. Figure 2.2 shows the XRD pattern of CuS: Cd thin film broad diffraction at 2 = 24.71 °and expresses the amorphous nature of the film. The XRD peaks observed in Fig. 2.2 indicate that the deposited CuSe: Cd thin film has a crystalline structure, while the CuS: Cd thin film is amorphous in nature due to the broad diffraction peak at 2 = 24.71 °. These results suggest that the deposition process and conditions have a significant impact on the crystal structure of the thin films. The crystalline nature and the crystallite sizes are enhanced with the deposited CuS: Cd thin film.
Table 1
Structural parameters Crystallite size, d -Spacing, Dislocation Density (ρ), Microstrain (ε), and Lattice constants (a) of pure and Cd-doped CuS thin films.
Sample | Crystallite size ‘D’ (nm) | d -Spacing (nm) | Dislocation Density (ρ) | Microstrain (ε) | Lattice constants (a) |
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CuS | 57 | 0.6258 | 1.9822 | 0.00465 | 4.2245 |
CuS: Cd | 52 | 0.6446 | 1.9750 | 0.00458 | 4.2656 |
3.2 SEM Analysis
SEM micrographs of CuS thin films coated on a glass substrate are magnified for a 5-m square area. SEM images of thin films are given in Fig. 3. CuS film is coated uniformly and homogeneously. From Fig. 3, it is observed that the particles are in the spherical shape of an average grain or aggregate size (d) varying from 30 to 36 nm. The SEM micrographs also reveal that the films exhibit a rough to their face morphology with some visible cracks and voids. These features could be attributed to the deposition conditions and post-treatment processes used in the CuS, reaction of the films. Such an embedded surface looks like a germinating sprout of a seed. Due to high CuS, the concentration, small pits are present in the developed film [14]. The thickness of the layer may increase sulfur of the increased particle size. For CuS thin film resembles some porous surface, An average grain size (d) is varying from 30 to 40 nm. For CuS within film particle size is increased from 30–35 nm. The increase in size may be due to the increase in Sulphur content. Also, it reveals that small flakes like structure spread over the surface and the particles are clustered together in a spherical shape of an average grain size (d) varying from 30 to 90 nm. Maximum-sized particles spread throughout the film irregularly. The morphology of CuS thin film is influenced by the increase in Sulphur content, resulting in a larger particle size and irregular distribution of maximum-sized particles throughout the film. These findings suggest that controlling the Sulphur content can be a useful approach to tailor the properties of CuS thin films for various applications [15].
3.3 FTIR Analysis
In Fig. 4, the spectra of undoped CuS and Cd-doped CuS are shown (a, b). The wide curve between 3000 and 3500 cm-1 is formed by the bending of FTIR spectra at 1392 cm-1 and 1135 cm-1; a doubling is also seen; the peaks are caused by Cd bending vibration. The presence of Cd in the doped CuS sample causes a shift in the stretching vibration peak to a lower wavenumber, as observed in the spectra. This indicates that Cd has been successfully incorporated into the CuS lattice. Cu-O vibrations cause a faint absorbent band to be seen between 2306 and 2437 cm1. The results that were found agree well with an earlier report. For Cd2+-doped CuS, almost similar outcomes are observed [16]. Sulfate’s intensity has been reduced significantly, and FTIR analysis confirms that CuS vibration leads to a broad band at the metal peaks of 728, 742, 606, 623, and 594 cm1. This suggests that the doping of Cd2+ ions into CuS has an effect on the Cu-O vibrations and alters the sulfate's intensity. Further studies can be conducted to explore the potential applications of Cd2+doped CuS in various fields.
3.4 UV-Visible Analysis
Figure 5 shows the absorbance spectrum of wavelengths from 200 to 900 nm.
The next peak was obtained at 297 nm, which is the peak of the glass substrate, SiO2. The minimum peak of absorbance is observed at 379 nm, which is the peak of CuS. The absorbance increases with decreasing wavelength from 600 to 300 nm. For the increasing molar concentration, absorbance is higher in the UV-Vis regions but is lower in the NIR region [17]. This suggests that the synthesized CuS nanoparticles are well dispersed on the SiO2 substrate and exhibit strong absorbance in the UV-Vis region. The decrease in absorbance with increasing wavelength beyond 379 nm indicates that the synthesized CuS nanoparticles have limited absorption in the NIR region.
For all the concentrations of CuS film, the transmittance increases from 3 to 91 percent in the UV-Vis and NIR regions. In the UV-Vis range (300 to 600 nm), the film's transmittance goes up as the wavelength goes up, but it goes down in the NIR range (above 600 nm). For all the concentrations of CuS film, the average transmittance is greater than 70% throughout the UV, Vis, and NIR regions.
When the molarity of the precursor solution goes up, it is seen that the transmittance goes down. The Cd-doped CuS thin film is less transparent than the film-deposited CuS thin film. This property of high transmittance makes it a better material for optical coatings. From the spectra, it is revealed that the CuS films have low absorbance in the visible region, which is characteristic of the transmittance edge. It is shifted slightly towards a higher wavelength as the molar concentration is increased. This shift indicates a decrease in the band gap, which can be attributed to an increase in the molar concentration. In Figures a and b, peaks are attributed to the formation of excitons in CuS thin films, which increase with an increase in molar concentration. The formation of excitons in CuS thin films can be attributed to the presence of defects or impurities, which act as trapping centres for electrons and holes. The increase in peak intensity with an increase in molar concentration suggests that the defect density increases with increasing CuS concentration.
The band gap energies are calculated from the maximum transmittance of wavelength using the formula Eg = 1240/𝜆max eV. The band gap energy of CuS thin films decreases from 2.26 to 1.86 eV for the Cd doped CuS thin films, respectively, at room temperature [18].
3.5 Photocatalytic analysis
For the photodegradation of RhB dye under sunlight irradiation, the photocatalytic performance of the CuS: Cd thin films was evaluated in Fig. 6 [19]. Investigative scientists used the largest absorption peak at 622 nm in the visible spectra to track the photodegradation of RhB dye [20].
The change in the UV-Vis absorption spectra of RhB dye using Cd-doped CuS thin film as photocatalysts is depicted in Fig. 6. It was seen that, in the presence of CuS: Cd catalysts, the intensity of the major absorption peak of the RhB dye decreased gradually, almost averaging zero, and that the dye's color turned colorless in 180 minutes of sunlight irradiation, indicating that the RhB dye molecules were degraded [21]. The results indicate that CuS-Cd catalysts were more active under light irradiation, making them more suitable for use as photocatalyst materials [22]. Additionally, for all of the prepared CuS catalysts, the photodegradation efficiency of RhB dye was examined under the same experimental conditions [23]. The study found that the Cd-doped CuS thin film was effective in degrading RhB dye molecules under sunlight irradiation, with the major absorption peak decreasing gradually and the dye turning colorless in 180 minutes. This suggests that CuS: Cd catalysts are highly active in the presence of light and could be a promising material for photocatalysis [24].