Bandgap Engineering and Plasmonically Enhanced Sun Light Photocatalyis in Au/Cd1-xZnxS Nanocomposites

Metal nanoparticles incorporated semiconductor nanomaterials generally holds a series of advantages especially enhanced electron-hole pair lifetime and thus exhibits superior solar energy conversions. In this study, we report a facile solution processing of Au incorporated Cd 1-x Zn x S, where x = 0, 0.25, 0.5, 0.75 & 1, nanocomposites and their enhanced photocatalytic applications. The Au/CdZnS nanocomposites were investigated for their structural, morphological, optical and photocatalytic properties. The XRD patterns indicated the crystalline sizes of CdZnS are found to fall with the range of 1-3 nm. The electron microscopic images publicized the average particle size of Cd 0.25 Zn 0.75 S is 4 nm. The bandgap values of pristine CdS, pristine ZnS, Cd 0.75 Zn 0.25 S, Cd 0.5 Zn 0.5 S and Cd 0.25 Zn 0.75 S are 2.21 eV, 3.4 eV, 2.29 eV, 2.31 eV and 2.53 eV, respectively. The optical band gap of the CdZnS nanomaterials have got reduced for Au incorporation due to the occurrence of red shift and the enhanced visible region absorption for the inclusion of Au. The visible light photocatalytic effect of the nanocomposites have been evaluated with methylene blue (MB) dye degradation reaction under sunlight light exposure. The Au incorporated Cd 0.25 Zn 0.75 S nanocompound had exhibited 97 % of photocatalytic degradation of MB dye molecules which is 20% higher than the bare Cd 0.25 Zn 0.75 S nanocompound. & 1 were synthesized by simple co-precipitation technique followed by SILAR. The obtained X-ray diffraction patterns indicated the phase purity of the obtained materials. The lattice parameters of CdZnS nanocomposites have got linear change for the linear change in the chemical compositions. The TEM images indicated the particle size of CdZnS falls in the range of 3–5 nm. The UV-vis absorption spectra indicated the Au incorporation enhanced visible region photons absorption in CdZnS nanocompounds. Further conducted photocatalytic investigations revealed the enhanced eciency in MB dye degradation for Au incorporation in CdZnS nanocomposite. Our studies suggest the plasmonic Au incorporated Cd 0.25 Zn 0.75 S photocatalyst possess the superior photocatalysis on par to the pristine ZnS and CdS nanomaterials under sunlight.


Introduction
The photo-assisted catalytic decomposition or fragmentation of organic pollutants employing visible light semiconductors have been recognized as promising methodology for e cient removal of pollutants. Amongst the semiconductors endeavoured for e cient photocatalysis of pollutant removals, metal sul des are considered to be the most preferable materials. CdS is one of the widely studied photocatalysts possess suitable band gap of 2.4 eV and appropriate energy band positions for photocatalytic reactions [1]. Nevertheless, low activity and stability of pure CdS and self photocorrosion strongly and toxicity issue limits from its practical application. In order to overcome this shortcoming, combining CdS with other metal sul des or incorporation of metal particles is extremely feasible fashion [2]. Meanwhile the band edge positions of CdS are comparatively lower than that of certain materials such as ZnS. Incorporating ZnS in CdS systems, resulting Cd 1 − x Zn x S nanocompounds could substantially elevate the energy level positions in which high e ciency could be achieved. Besides ZnS possess excellent charge transport properties, high electron mobility, better thermal stability, large exciton binding energy (40 meV) and as wide band gap on par to CdS which generally improve photoabsorption properties [3].
Making of Cd 1 − x Zn x S solid solution is a powerful strategy for improved photocatalytic activity compared to their pristine counterparts either CdS or ZnS [4,5], in which recombination of photogenerated charge carriers is the major drawback. The recombination can be further avoided abruptly via incorporating the plasmonic metal nanoparticles, such as Ag, Au, Pt, and Pd on the surface of the Cd 1 − x Zn x S nanomaterials [6,7]. These plasmonic metal nanoparticles inhibit the electron hole pair recombination by acting as electron trap and also enhance the incident photon absorbance by its localized surface plasmon resonance (LSPR) properties. Since, the studies on various methodologies on incorporation of various metal nanoparticles on CdZnS deserves more advantages.
Considering the above-mentioned parameters, we have synthesized Au incorporated Cd 1 − x Zn x S with X = 0 to 1. Band gap of the Cd 1 − x Zn x S tune by varying composition of Cd 2+ and Zn 2+ . The activity of Cd 1 − x Zn x S in photocatalytic dye degradation is much higher than bare CdS and ZnS, the most active photocatalyst in the Cd 1 − x Zn x S series is Cd 0.25 Zn 0.75 S and Au/ Cd 0.25 Zn 0.75 S.

Materials Synthesis of Au decorated Cd 1 − x Zn x S
We have followed co-precipitation methodology for the preparation of

Materials characterization
Crystal structure of the as-prepared photocatalyst was identi ed by PXRD (powdered X-ray diffractometer) by using PANalytical's X'pert pro with Cu Kα radiation (λ=1.5405 Α°) in the 2θ range from 20° to 80°. The optical absorption properties were studied using shimadzu (UV-3600) spectrometer with the absorption wavelength range 200 to 800 nm. The surface morphology and elemental composition of the photocatalyst were analysed by eld-emission scanning electron microscopy (FE-SEM) FEI Nova Nano-SEM operating at 15kv, attached with energy dispersive X-ray spectrometer. High-resolution transmission electron microscopic images (HR-TEM) were captured with a JEOL JEM-2010 (Japan) operated at 200 kv.

Sun light photocatalytic Studies
Photocatalytic activity of the obtained specimens was estimated by the degradation of methylene blue (MB) under the direct sunlight illumination. A typical test was carried out using 50 mg of as prepared photocatalyst nanomaterials and 50 ml of (10mg/L) MB dye solution. Firstly, the mixture was stirred magnetically in dark condition for 30 min in order to achieve absorption, adsorption and desorption equilibrium between nanomaterials and dye molecules. At every certain time interval, 3 ml of dye solution was collected after placed under bright sunlight and centrifuged to remove the catalysts. The concentration of the resultant dye in the solution was estimated by measuring the absorbance at the maximum absorption wavelength (λ = 664 nm) of MB using UV-vis spectrometer. The photodegradation percentage of the MB dye was calculated by the following equation:  [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 Zn 2+ could be attributed to the relatively smaller ionic radii of Zn 2+ (0.74A°) than the Cd 2+ (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(h 2 + k 2 + l 2 ) 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 Cd x Zn 1−x S and the graphs shown the linear decrease in the lattice constant values for the addition of Zn 2+ 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 (Cd 1 − x Zn x S). From the SEM images particle morphology of the materials retained in all the cases are identi ed. 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 Cd 1 − x Zn x S nanoparticles are shown in Fig. 3(fj). The SEM images indicated the unaltered morphology in the Cd x Zn 1−x S specimens for Au incorporation.

Conclusions
In this study, Au/Cd 1 − x Zn x S nanocomposite with X = 0, 0.25, 0.5, 0.75 & 1 were synthesized by simple coprecipitation technique followed by SILAR. The obtained X-ray diffraction patterns indicated the phase purity of the obtained materials. The lattice parameters of CdZnS nanocomposites have got linear change for the linear change in the chemical compositions. The TEM images indicated the particle size of CdZnS falls in the range of 3-5 nm. The UV-vis absorption spectra indicated the Au incorporation enhanced visible region photons absorption in CdZnS nanocompounds. Further conducted photocatalytic investigations revealed the enhanced e ciency in MB dye degradation for Au incorporation in CdZnS nanocomposite. Our studies suggest the plasmonic Au incorporated Cd 0.25 Zn 0.75 S photocatalyst possess the superior photocatalysis on par to the pristine ZnS and CdS nanomaterials under sunlight. Table   Table 1. Calculated crystalline parameters of Cd 1-x Zn x S nanocompounds and Au/Cd 1-x Zn x S nanocomposites