Structural studies:
The structural properties of Au/Cd1 − xZnxS nanocomposites were analysed using XRD technique. All the patterns (Fig. 1) were exhibited broader peaks with high full width at half maximum (FWHM) indicating the small crystalline structured formed in all the samples via the conducted synthesis protocol. The pattern of pristine CdS nanoparticles exhibited three peaks at the 2θ positions 26.38°, 43.7° and 51.6° are representing the planes (111), (220) and (311), respectively in accordance to the JCPDS card # 75–0581 of cubic crystal system. For introducing the Zn2+ ions in the CdS system, the peak positions have shifted towards higher 2θ positions depend upon the concentration of Zn2+ [8, 9]. The pattern of pristine ZnS has exhibited the 3 peaks in the 2θ positions 28.80°, 47.83° and 56.62° representing the (111), (220), (311) planes, respectively in accordance to the JCPDS card # 77-2100 (cubic crystal system). The peak position shift for Zn2+ could be attributed to the relatively smaller ionic radii of Zn2+ (0.74A°) than the Cd2+ (0.97A°) [5]. The successive shift in the peak positions and absence of any other peaks corresponding to CdS or ZnS present in the CdZnS nanoparticles indicate the purity of the specimens and the avoidance of composite structures. This could be attributed to comparable electronegativity values of Zn (1.6) and Cd (1.7) atom [10]. The patterns of the Au/CdZnS indicated the crystalline peaks at 38° corresponding to (111) peak [11, 12].
We have calculated the crystallite size of the synthesized materials from the XRD data using Debye - Scherrer formula, D = Kλ/βcosθ, where, D is crystalline size, K is a constant related to particle morphology typically 0.9 for spherical particles, λ is the wavelength of X-ray (0.1542 nm), β full width of half maximum intensity (FWHM) of diffraction peak, θ is centre position of Bragg’s angle [13]. We have also calculated the lattice constant value (a) as a = d(h2 + k2 + l2)1/2, where d = nλ/2sinθ, n is the order of diffraction and (h k l) are the miller indices. Further we have also evaluated the lattice parameter values of the CdxZn1−xS and the graphs shown the linear decrease in the lattice constant values for the addition of Zn2+ ions.[14].
Morphological analysis:
The scanning electron micrographs (SEM) plays an effective role in the study of surface morphology of the nanomaterials. Figure 3(a-e) represent the SEM images of pristine and compound specimens (Cd1 − xZnxS). From the SEM images particle morphology of the materials retained in all the cases are identified. The images explicated the agglomerated nanoparticles. This agglomeration could be reasoned to many factors such as reaction rate, pH, impurity, surface energy, particle charges and product constant for solubility [15, 16, 17]. The SEM images of Au incorporated Cd1 − xZnxS nanoparticles are shown in Fig. 3(f-j). The SEM images indicated the unaltered morphology in the CdxZn1−xS specimens for Au incorporation.
Transmission electron microscopic images obtained for the Cd0.25Zn0.75S nanocompound (Fig. 4a&b) & Au/ Cd0.25Zn0.75S nanocomposites (Fig. 4c&d) have exhibited the particulate morphology of the obtained nanocomposites. The images indicated the size of the particles falls in between 3 nm to 5 nm and their average particle size was calculated to be Cd0.25Zn0.75S is 4 nm. The TEM images also reveal the particles possessing identical dimensions and are aggregated to each other. The concentration of Au on par to the CdZnS is quite low in the composites, since that TEM images of Au/ Cd0.25Zn0.75S are also mainly showcase the Cd0.25Zn0.75S nanoparticles rather the Au nanoparticles [18]. Figure 4(b&d) shows the lattice fringes with d-spacing of 0.32 nm & 0.33 nm could be assigned to the (111) lattice plane of zinc blend CdZnS & Au/CdZnS. The EDS spectrum is a tool to explore the chemical composition present in the nanocomposites. The EDS spectrum of Cd0.25Zn0.75S and Au/Cd0.25Zn0.75S were shown in Fig. 5., which confirm the existence of the elements Cd, Zn, S & Au in the corresponding specimens.
UV-visible absorption spectroscopy
We have obtained UV–vis absorption spectrum of the Au incorporated Cd1 − xZnxS nanoparticles to study the energy band positions and electronic transitions and are plotted as given in Fig. 6&7(a& b). The absorption edge of various CdZnS depends on the compositions of Cd2+ & Zn2+ ions, and lies within the values of ZnS (375 nm) and CdS (594 nm) band edge positions. The spectrum of pure ZnS shown the strong absorption of UV region photons and the absorption maximum at ~ 320 nm, whereas the absorption maximum of CdS falls in visible region with absorption maximum at ~ 500 nm. UV–vis absorption measurement is a convenient and effective method for investigating the band structures of photocatalyst [26]. The band gap of the Cd1 − xZnxS can be estimated by tauc plot of (αhυ)2 vs (hυ), where, α is the absorption coefficient and hυ is the photon energy. Thus, calculated bandgap value of pristine ZnS is 3.44 eV and CdS is 2.21 eV. The band gap value of bulk ZnS generally could be observed from 3.56 eV to 3.764 eV, and the bulk CdS exhibits at 2.4 eV [20]. Relatively reduced bandgap value of these nanomaterials compared to the bulk counterparts could be assigned to the materials formed at nanoscale. For the introduction of Zn2+ ions in CdS nanocompound resulted in significant blueshift in the band edge position have been observed and this further increased for increasing the concentration of Zn2+ ions. The calculated bandgap values of Cd0.75Zn0.25S, Cd0.5Zn0.5S, and Cd0.25Zn0.75S are 2.29 eV, 2.31 eV and 2.55 eV, respectively. The received high variation in bandgap value and less linearity in its change for high Zn2+ ions are well matching with the existing literature [21]. The spectrum obtained for the Au incorporated CdZnS nanocomposites have shown enhanced absorption in the visible region than their pristine counterparts.
Figure 8. The position of the conduction band minimum and valance band maximum can be calculated by electronegativity equations, ECB = χ − Ee – 0.5Eg and EVB= ECB+ Eg, where, ECB denotes conduction band potential, EVB is valence band potential, Eg band gap of the photocatalyst, χ absolute electronegativity of constituent atom Cd (4.3), Zn (4.45), S (6.62), Ee represents the energy of free electron in hydrogen scale (4.5eV) [27,28]. The position of conduction band and valence band of Cd1 − xZnxS were shifted more negative potential and more positive potential with increasing X value as shown in fig [22].
Sunlight photocatalytic activity
Photocatalytic activity of the attained nanocomposites was estimated by measuring degradation of MB in aqueous solution under sun light exposure. The degradation of MB dye molecules was estimated via the absorption intensity at 664 nm of the dye solution at serious of intervals. Based on the absorption intensity we have evaluated the C/C0 values for all the samples. The graph plotted for C/C0 versus time intervals illustrated the degradation rate of the dye molecules for various catalysts (Fig. 9a). Initially, pristine CdS nanoparticles have exhibited the photocatalytic percentage of degradation is about 49 % for 150 min. Further, the graph depicted the enhanced efficiency obtained for the CdZnS nanocomposites on par to the pristine CdS and ZnS. The percentage of degradation of MB dye molecules for the catalysts, Cd0.75Zn0.25S, Cd0.5Zn0.5S, Cd0.25Zn0.75S and ZnS are 62%, 70%, 77.6% and 69.5% respectively. Very low degradation efficiency of CdS over the other Zn2+ presented materials could be reasoned to the relatively lower conduction band position of Cd2+ leading to easy photocorrosion. The incorporation of Zn2+ in the CdS elevate the conduction band position to more negative values since the conduction position of ZnS is higher than the conduction band position of CdS. ZnS have possessed only 69.5% of degradation due to its wideband gap nature. We have attained the highest efficiency of 77.65 % of degradation for Cd0.25Zn0.75S nanocompound, in which 75 % of Zn2+ were utilized.
Immense increase in the efficiency of the Au incorporated nanomaterials were realized in the conducted photocatalytic reactions under sunlight. Addition of Au nanoparticles in CdZnS nanocompounds greatly improved the photocatalytic activity through LSPR properties [23]. Plasmonic properties of the Au nanoparticles intensifies the optical absorption of CdxZn1−xS directing to generation of more excitons. The Au/Cd0.25Zn0.75S photocatalyst exhibited highest photocatalytic activity towards the MB degradation. The photocatalytic degradation of Au incorporated Cd0.25Zn0.75S was achieved to 97% of degradation, which is 20% higher than the Cd0.25Zn0.75S. The percentage of degradation of other Au incorporated specimens of CdS, Cd0.75Zn0.25S, Cd0.5Zn0.5S and ZnS are respectively 93.75%, 91.5%, 93% and 94%. Thus, enhanced photocatalytic activity in CdZnS materials were achieved through the involvement of trivial loading of Au on Cd0.25Zn0.75S nanoparticles.
Charge transfer mechanism
Figure 9. To understand photoinduced charge transfer mechanism in the Au/Cd0.25Zn0.75S, we have plotted the energy band diagram based on the obtained results in the optical spectroscopic studies. The fermi level of Au was located at + 0.5 V versus NHE and conduction band of the Cd0.25Zn0.75S located at -0.45 V versus NHE. Upon the irradiation of the visible photons the excited electron from conduction band of Cd0.25Zn0.75S transferred to the Au due to this energy difference. Thus, Au nanoparticles trap the electrons from the conduction band of CdZnS by acting as electron sink and diminish the electron-hole recombination process. The transferred electrons in the Au nanoparticles reacts with oxygen molecules adsorbed on the surface of the catalyst to form superoxide radicals (O2·). Meanwhile the holes in valance band react with water and produce OH. radicals. These active radicals further conduct the efficient degradation process.