Growth of Ni loaded CdS in nanorods structure for photocatalytic and dye degradation applications under solar irradiation

Due to the exponential increase in global energy consumption and the degradation of environmental conditions caused by fossil fuels, it is critical to improve inexhaustible and sustainable resources. Generally, solar energy is one of the clean and environmentally agreeable energy sources. By harvesting solar energy for photocatalysis and considering it as a promising solution for various energy generation applications such as hydrogen production. Herein we are using Cadmium Sulphide and Nickel-doped Cadmium Sulphide in 0.5, 1 and 5 weight percent which act as photocatalyst for water splitting which will eventually produce an enormous amount of Hydrogen (H2). Cadmium sulphide was prepared through the chemical precipitation method and Ni-CdS by hydrothermal technique. The purity and phase formation were examined by the X-ray diffraction (XRD) and validated via Rietveld refinement by using Full Prof software. The surface morphology and the structure of as-synthesized material were evaluated by Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscope (TEM) spectroscopic techniques. Following the results, the Ni-CdS nanocomposite having 1.0 wt% of Ni exhibits the highest H2 evolution rate of 9 mmolg−1 in 5 h with strong photo-stability, which is about 50 times higher than that of CdS. The material was tested to degrade organic dye for its photocatalytic operations. The newly prepared composite materials (CdS-Ni-NiO) were used for the photocatalytic degradation of the methylene blue (MB) dye. Ni(1.0 wt%)-CdS shows an optimal degradation percentage of 95.436 in the presence of artificial solar light in 90 min. Crystal growth mechanism shows the spherical structure of CdS agglomerate to form nanorods structure when doped with Ni metal which is also verified by the TEM images of CdS and Ni-doped CdS. The XPS peaks observed at 854.88 eV and 861.07 eV for Ni2+ with an energy separation of 6.18 eV confirmed the existence of NiO with Ni/CdS. The Raman bands of pure CdS and Ni (1.0 wt%)-CdS nanorods were observed at 300 cm-1 and 293 cm−1 for 1LO phonon and 601 cm−1 and 586 cm−1 for 2LO phonon corresponds. The Ni tuned the CdS band gap from 2.36 to 2.20 eV. The results pave the way for designing multi-component CdS-Ni nano-composites for highly efficient H2 evolution and other environmental applications.


Introduction
In recent years, the growing interest in design and development of semiconductor materials with specific surface morphology has gained thrust because of incomparable novel electronic and optical properties [1][2][3], and thus its wide range of potential applications, like bioimaging [4], chemical separations media, solar energy conversion [5], photocatalysis [6], etc.Several approaches have been developed to design both inorganic and organic materials in specific morphologies.The approaches include hydrothermal method [7][8][9], solvent-relief-selfseeding (SRSS) process [10], self-assembly method [11], and template methods [12][13][14][15], or CVD growth in a confined environment [16].The Cadmium sulphide (CdS, II-VI group) nanocrystals and its composites being an important class of materials among other semiconductors.Due to the reasonable band-gap, it attracts the researchers and the scientific community worldwide to design in control crystal phase, shapes, size and morphologies.CdS is a direct band gap semiconductor possess structural dependent band gap such as wurzite (Hexagonal) structure has 2.40 eV whereas the Zincblend (cubic) structure has 2.38 eV [17,18].The unique tuneable bandgap feature now made it one of the model materials for exploring the inimitable optical and electronic properties of quantum-confined semiconductors.It is reported that the surface defect states of CdS instantly trapped the optically excited electrons and holes, results successful quenching of their recombination [19].Thus, it is widely applied for the nonlinear optical materials, optoelectronics, biological labeling, light emitting diode for flat panel display, photoelectric conversion in solar cells and photocatalytic application for water-splitting hydrogen production and industrial dye mineralization [20][21][22][23].
Among several developed synthesis approaches of CdS nanocrystals, the production of good quality CdS using a one-pot synthetic method [22], synthesis of large quantity single-crystal CdS of micro belts morphology from the modified thermal evaporation method [23], and development of capping agent stabilized CdS nanoparticles using a simple and green route [20], have been reported.
In this specific context, the current research outlines a simple chemical technique for transforming CdS nanoparticles into CdS nanorods, utilizing a Ni catalyst.The process yielded a considerable amount of uniform CdS nanorods with integrated Ni.As anticipated, this procedure facilitated the presence of both Ni and Ni 2+ phases on the CdS rods.

Synthesis of CdS and Ni-loaded CdS
The production of CdS nanoparticles involved a straightforward chemical process.In this demonstration, 4.02 g of cadmium chloride (CdCl 2 .H 2 O) and 0.305 g of thiourea [(NH 2 )2CS] were separately dissolved in 50 ml of water in a beaker under magnetic stirring at room temperature.The Ammonium Chloride solution was gradually added to the cadmium chloride solution until the pH reached 10 to 11.Subsequently, the thiourea solution was combined with the above solution and stirred at 80 °C in a silicon oil bath until the entire solution turned yellow.The resulting yellow precipitates were washed multiple times with distilled water, followed by centrifugation to collect the powder sample.The sample was then dried in an oven at 50 °C for 24 h.This synthesized nanomaterial was designated as CdS nanoparticles.The same procedure was employed to synthesize Ni-doped CdS (referred to as Ni-CdS) nanoparticles.Different amounts of Ni (0.5, 1.0, 2.0, and 5.0 wt%) were loaded onto the CdS surface to create the nanocomposite photocatalysts.

Materials characterizations
X-ray diffraction is a standard technique used for identifying the incident source of the crystallographic phases of the sample using x-rays.As XRD experiment is multifaceted and non-destructive to samples, structure data dependent on the elastic dispersion of x-rays from individual atoms can be shown.As the substance has a unique lattice structure, the Bragg equation creates specific diffraction patterns.
Here d is the distance between the adjacent diffracting planes, θ is an incident angle of wave and planes, λ is an incident wave wavelength and n is the integer.The CdS and Ni-loaded CdS powders underwent characterization using x-ray diffraction (XRD) on a Bruker D8-Advance x-ray diffractometer with mono-chromatized CuKα radiation (λ = 1.5418Å).The XRD analysis involved an accelerating voltage of 40 kV and an applied current of 40 mA.The lattice parameters were determined by using the equation [24,25].
The formula is used to calculate the relative percentage variation in the interplanar distance 'd' [26].

JCPDS
Taking into account the first-order approximation n = 1 for computing the lattice parameters, we have The Calculation of the lattice constant 'a' is performed specifically for the {100} planes.
The lattice constant 'c' is determined for the {002} planes through the following calculations In the hexagonal arrangement of CdS, the Cd and S atoms are coordinated tetrahedrally.The distance between Cd and S atom in the Cd-S bond is denoted as 'L' and can be expressed as [27] ⎜ The position parameter 'u' in the wurtzite structure represents the cumulative displacement of each atom along the 'c' axis.It is defined as [28] ( ) The calculation of the volume of the hexagonal primitive cell is carried out using the following formula = V a c As the c/a ratio decreases, the value of 'u' increases, resulting in almost unchanged tetrahedral distances despite the distortion of tetrahedral angles.
The b q Cos hkl hkl values on the y-axis have been determined as a function of q 4Sin hkl on the x-axis and from linear fit, crystalline size D and micro strain ä have been calculated.
In many situations, homogeneity and isotropy are also not accepted.An anisotropic strategy is introduced in order to incorporate more practical conditions.Hence, Williamson-Hall is altered by an anisotropic strain ä.The lattice stress deformation is believed to be uniform in every crystallographic direction in the uniform stress deformation model (USDM) and to be present with a minimal micro strain in the fine particulates.In USDM, the Hooke's law applies to the strain and a linear ratio of the stress to strain is given by [27][28][29][30] Where σ is crystal stress, ä is anisotropic micro strain, this is dependent on the crystallographic orientation and Y hkl is an elasticity modulus or the Young modulus.This approximation holds true under the condition of a relatively small strain.However, as the strain increases, the particles deviate from this linear relationship.To address this, the equation of Williamson Hall is modified by substituting the value of ε from equation (12), resulting in the following expression [26-29] The following relationship gives Young's modulus for a hexagonal crystal [29-32] In this equation 'a' and 'c' represent the lattice parameters, while S 11 , S 13 , S 33 and S 44 correspond to the elastic compliances of CdS.These compliances have specific values: 2.069 × 10 −11 , −0.581 × 10 −11 , 1.697 × 10 −11 and 6.649 × 10 −11 m 2 N −1 , respectively [33].
There is a different method called the uniform deformation energy density model (UDEDM), which can be used to calculate a crystal's energy density.The energy density of u ed (energy per unit volume) is a function of strain for a system which follows the Hooke's law.
Thus, the energy-stress relationship from equation (16), equation (12) can be rewritten as By knowing the Y hkl value, the lattice strain can be determined.The W-H plots defined an isotropic extension of the graph.This highlights that owing to the contribution of micro strain, the diffracting domains is isotropic.When expanding the line of isotropic parameters, an average 'size-strain map' will help evaluate size-strain parameters (SSP).This methodology has an advantage because data from high angles reflection, where the accuracy is typically less significant, are given less priority [34].It is presumed that a Gaussian function explains 'strain profile,' and a Lorentzian function explains the 'crystallite size' profile and is supported by [28,35]  In this context, 'dhkl' represents the lattice distance between the planes of 〈h k l〉, 'Vs' denotes the apparent volume weighted average size, and 'äa' stands for an apparent strain measurement that is in alignment with the root mean square (RMS) strain [28] ⎛ ⎝ ⎞ ⎠

RMS a
The volume average true size for spherical crystallites is represented as D v , and it is calculated as D v = V s 4/3 The Wilson equation was used in order to determine the upper limit ä hkl , and the root mean square (RMS) ä RMS in the following direction using correlation RMS hkl The upper limit micros train is derived from Wilson's law in Bragg and a Gaussian micro strain distribution was supposed to extract the root mean square micro strain from the upper limit micro strains.The root-mean square (RMS) lattice strain is provided by the replacement of the ä hkl value in equation (21), we get The observed and optimal interplanar spacing values here are d and d 0 .
A nonlinear curve fitting function called Voigt estimates the full width half maximum FWHM) for all peaks.This approach yields the most accurate fit to the experimental data.Furthermore, the Voigt function is a combination of Gaussian and Lorentzian functions [25], and FWHM is computed with the following formula In this equation β 0 represents the observed Full Width at Half Maximum (FWHM), W G is the Gaussian width, and W L is the Lorentzian width.To eliminate the instrumental broadening (β i ) the following equation is utilized Quantitative information about the preferred orientation of crystals can be derived from the Texture Coefficient, TC [28].Where TC (hkl) is a coefficient of texture, I hkl be the XRD sample intensity and N is the number of reflections considered for diffraction and I 0 hkl is the XRD intensity from JCPDS card.If TC (hkl) ≈ 1 is taken into consideration for all á ñ hkl planes, the nanoparticles are identical to JCPDS with randomly orchestrating crystallites, while TC higher than 1 are suggested by the grain abundance in a certain á ñ hkl direction.Values 0 < TC (hkl) < 1 suggest the lack in that direction of the grain.By using the Rietveld refinement technique, all of the patterns for X-ray diffraction (XRD) were analyzed using the Fullprof program.The XRD pattern can be refined for all samples through the space group P 63 m c.A well-known technique for collecting structural details from powder diffraction data is the Rietveld Refinement process.The procedure contrasts the Bragg intensities with those dependent upon a possible structural model by using a lesser-quadratic method.The parameters such as background and scale factors were optimized in the first stage of refinement.Subsequently, the process involves the step-by-step refinement of various structural parameters, including lattice parameters, profile and width parameters, desired orientation, asymmetry, isothermal parameters, atomic coordinates, and site occupancies.To assess the fitting accuracy of the experimental results, parameters such as 'goodness of fit' χ2 and R factors (R P = Profile factor, R B = Bragg factor, and R F = Crystallographic factor) are calculated.When these parameters reach their maximum values, the crystal structure is deemed adequately fitted to the experimental diffraction data.
Where 'Y i ' is the point (experimental) observed and 'Y c,i ,' is the point measured and the number of data points is n.Weighted profile factor: Where is the observation variance.Expected Weight factor: Here (n-p) is the number of degrees of freedom.'n' is the cumulative number of points of the experiment and 'p' is the number of parameters refined.Reduced chi-square: Where, 'h' is the reflection vector of the Bragg.The I obs,h is the integrated intensities that are observed and I calc,h the intensities calculated.
Crystallographic factor: The x-ray density ẍ of the actual samples have been estimated by means of an estimated lattice constant according to relation Where NA is the Avogadro number ( ´mol 6.02 10 23  1 ), M corresponds to the CdS sample molecular weight (CdS = 144.46g mol −1 ), Z corresponds to the number of formula unit in the unit cell (Z = 2) and V indicates a Volume of unit cell ( = V 3 /2 a 2 c).Jeol Jem TEM is used to obtain TEM images and SAED Pattern.The values of interplanar spacing (d) pertaining to all diffraction rings were estimated using the following equations: Where R is the distance between the center bright spot and the matching rings, L is the camera length between the sample and the photographic film, and λ is the wavelength of the electron beam depending on the accelerating voltage: 200 kV equals 0.02736 Å.We have used a camera length of 50 cm.MB degradation experiment is done in the absence of light i.e. in the dark and in the presence of artificial solar light.Around 200 mg of a sample of pristine CdS and Ni (0.5-5.0 wt%)-CdS is used to degrade the Methylene blue which is dissolved in water in the molar ratio 10 −5 :1.The supernatant of the Methylene Blue solution is taken after periodic interval for UV-vis spectroscopy measurement.The Shimadzu UV-1900i spectrophotometer was used for UV-vis absorption spectra.
The chemical states of the species available in the samples were identified using X-ray photoelectron spectroscopy (XPS) analysis.It was performed using an Axis Ultra system and a spectrometer with a monochromatic Mg-Kα source (hv =1253.6 eV) and a hemispherical analyzer.The C1s line at 284.6 eV was kept as base to correct the all XPS spectra.The Raman spectra of the material were obtained with an iHR550 Raman spectrophotometer, Horiba Jobin Yvon, with HeNe laser (632.8 nm) as the excitation source.Diffuse reflectance UV-visible spectra (DRUVS) were obtained in the 200-700 nm range using BaSO 4 as a reference and using a Carry 500 Diffuse reflectance UV-visible spectrophotometer.X-ray peak analysis using the Williamson-Hall (W-H) method was employed to assess the lattice strain and crystallite size.The Williamson-Hall method is known for its ease of distinguishing between peak broadening caused by the size and strain effects when considering the peak width as a function of 2 θ hkl .The local lattice distortion induces strain, leading to the expansion of peaks.Equation (12) represents the W-H equation of the isotropic strain model, assuming uniform strain in all crystallographic directions.Sherrer plot of CdS and Ni(5wt%)-CdS represented in figure 1.The graph plotted between b q Cos hkl hkl and q 4 Sin hkl represents the W-H plot of the ISM, which is fitted straight and seen in figure 2 for the CdS and Ni (5 wt%)-CdS nanoparticles.From the intercept of the fitted line through the Y-axis, the crystallite size is determined and the strain is estimated from the fitting slope.

Result discussion
Anisotropic strain model (ASM) The strain is generally not fully isotropic in nature for any substance.Thus, in the second term of equation ( 14) anisotropic strain should be used.A graph is then drawn in a straight line between b q Cos hkl hkl and q .

Y 4Sin hkl hkl
Young's Modulus is calculated from the equation (15).The slope and y-interception of the graph respectively measure the uniform deformation stress and the crystallite size.Figure 3     The crystallite average size can also be calculated using the modified Scherrer equation.The importance of the modified Scherrer formulation is to minimize the error source and estimate the crystallite size.In order to mathematically suppress defects, the average crystallite size is computed by using the least square method taking account of all peaks.Figure 6 illustrates the standard modified Scherrer plot for CdS and Ni-CdS for ln(β hkl )   versus.ln(1/Cos θ hkl ).The intercept of a minimum square regression line passing by all available peaks achieves the value of d.Table 1 represents the crystalline size of Cds and Ni (5 wt%)-CdS using different W-H methods.
Table 2 represents the JCPDS and Observed 2θ and d spacing values for different hkl for the CdS nanoparticles.
Young's Modulus is calculated from the equation (15) shown in table 3 for different peak positions for CdS, Ni (1 wt%)-CdS and Ni (5 wt%)-CdS respectively.

Reitveld refinement of CdS and Ni (5 wt%)-CdS
XRD pattern Rietveld Refinement procedure was done for all the samples of pristine CdS and Ni-doped CdS in the hexagonal phase with space group P63mc using the Full Prof software for all of the structural parameters such as atoms fractional coordinates, lattice parameters, thermal parameters, site occupancy and microstructural parameters such as average crystallite size.The Rietveld refined data in addition to x-ray diffraction patterns for Pure CdS, Ni (1 wt%)-CdS and Ni (5 wt%)-CdS is shown in figure 7. It can be shown that the profiles for the measured and determined ones align and that for the P63 mc space group Bragg 2θ positions are permitted for all experimental peaks.The positioning of sulfur during the refinement was taken as free parameters.Cadmium has been fixed in an atomic fractional position.Isothermal parameters and occupancies are firmed for both zinc and oxygen.During fitting, free parameters were adopted in the parameters such as lattice constants, scale factors, and shape parameters.The first stage of refinement was to optimize global parameters such as background and scale factors.The following stage consisted of refining structural parameters such as lattice parameters, profile shape, width parameter and desired orientation, and atomic and asymmetric coordinates.The background was fitted with polynomials of the sixth order, while pseudo-voigt   In order to evaluate the fitting qualities of the experimental results, parameters such as 'goodness of fit' χ2 and different R-factors like Rp (profile factor), Rwp (weighted profile factor), Rexp (expected weighted profile factor), RB (Bragg factor) and RF (crystallographic factor).R p, R wp , R B , R F, and χ 2 are the reliability factors (R factors), and lattice parameters along with the crystalline size shown in table 7. The low values of different R factors and 'fitness of good' obtained justify the consistency between the refined models and the experimental data.
Texture Coefficient can be calculated from the equation (25).If TC (hkl) ≈ 1 is taken into consideration for all á ñ hkl planes, the nanoparticles are identical to JCPDS with randomly orchestrating crystallites, while TC higher than 1 are suggested by the grain abundance in a certain á ñ hkl direction.Values 0 < TC (hkl) < 1 suggest the lack in that direction of the grain.
The preferential growth of the crystallites perpendicular to the 〈 hkl 〉 plane is higher with the rise of TC (hkl) .The Texture coefficient of CdS, Ni (1 wt%)-CdS and Ni (5 wt%)-CdS values is shown in table 8.
Figure 8 illustrates the hexagonal CdS structure and Ni(5 wt%)-CdS structure in the ball and stick style image produced by the CIF-file of the VESTA software obtained from Full Prof structural refinement of experimental results.
Table 9 represents the bond angles and bond lengths between different atoms in CdS and Ni-CdS nanocomposites.

2-D and 3-D electron density mapping
The analysis of electron density mapping was conducted using the GFourier software in the FullProf kit to simulate the distribution of electron densities within a unit cell.In determining the location of the atoms inside the unit cell, the representation of electron density is important.The electron density refers to the geometrical structure element Fourier transform which occupies the whole cell unit.The scattering density of the electron is viewed as a two-dimensional or three-dimensional map of Fourier.Usually, the two-dimensional maps of Fourier are drawn as curves, showing the distribution of electron densities between individual atoms of the compound elements.If the curves of electron density are dense and thick, the location of a comparatively heavier element is indicated in the unit cell.In contrast, the Fourier 3-D maps encompass a network in the form of the chicken wire that shows a single degree of electron density.The black color in figure 9(a) applies to the density  The physical properties of the materials can clearly be influenced by a redistribution of the electron density between cations and anions.Among them are strongly suspended bond strength vibrational properties, including the phonon modes, based on electron density and lattice distribution.Raman spectroscopy will also provide useful knowledge that could be specifically connected to the creation of lattice defects and changes in the density of electrons in accordance with the doping process.

Crystal growth mechanism for CdS and Ni-CdS
The mechanism for growth involved in Ni-CdS nanorods heterostructure is as shown in proposed scheme 1.
When the synthesis is conducted as an aqueous solution at the initial phase, the thiourea acts as a bidentate ligand and forms a more stable Cd-thiourea complex.In the simple chemical process wherein stirring takes place in the reaction mixture.This leads to the slow release of the Cd 2+ ions and weakens the stable Cd-thiourea complex.

Photocatalytic methylene blue degradation (MB) activity
The photocatalytic activity performance of pure CdS and Ni@NiO-CdS photocatalysts are evaluated over MB dye in the absence of light (dark) and the presence of visible light irradiation with a 300 W xenon lamp equipped with UV cut-off filter, and presented in figures 14(a)-(e).It is found that with an increase of the reaction time, the methylene blue characteristic peak is gradually decreased, which indicates the concentration of methylene blue is gradually decreased.The aliquots of the solution are examined at an interval of fifteen minutes by UV spectrophotometer.A small change in the intensity of the MB absorption peak (∼666 nm) is observed in the dark condition due to adsorption-desorption of dye onto the photocatalysts surface and considerable change is observed in light due to photo-activity of photocatalysts.
All Ni dopedCdS nanocomposite photocatalysts are showed the photocatalytic activity for the MB degradation under light irradiation compared to pure CdS.A maximum MB degradation efficiency of ∼ 95% in 90 min is recorded with Ni 1.0 wt% in CdS (Ni1CdS) nano-photocatalysts (figure 14(c)); and ∼ 69% for Ni 0.5 wt% in CdS (Ni05CdS), ∼ 72% for Ni 2.0 wt% in CdS (Ni2CdS) and ∼69% for Ni 5.0 wt% in CdS (Ni5CdS) photocatalysts under light irradiation (figures 14(a), (b) and (d).The bare CdS only degrades MB of ∼29%.The improved performance of the CdS in the presence of Ni is attributed to the plasmonic effect and formation of the Schottky barrier at the surface interface.Furthermore, the catalytic activity Ni-CdS samples were recorded in the dark condition and shown in figure 14(a).In dark conditions, a maximum value of MB degradation efficiency of ∼40% is recorded for the Ni1CdS photocatalyst in comparison to ∼8% for bare CdS in 45 min.One of the deciding factors for photocatalysis activity is the adsorption performance of photocatalytic materials to the target pollutant molecules.The adsorption activity of the synthesized photo-catalysts in the dark is calculated and seen in figure 16 in order to investigate the adsorption properties.Experimental findings reveal that the composite Ni-CdS has progressively improved adsorption efficiency as the Ni content is increased.
In contrast with pure CdS particles, the more specific surface area of the Ni-CdS composites also makes it simpler to adsorb positive loaded MB molecules.The details comparison of the MB degradation in dark and light with developed photocatalysts is tabulated in table 10.
Being a photoactive phenothiazine dye, the MB interacts with Ni-CdS photocatalysts under light irradiation, the whole molecule possibly degrades through synergetic competitive approaches, such as demethylation of MB dye followed by decomposition of the aromatic rings in the dye molecule as presented in scheme 2. The inset of scheme 2 shows the color change of the methylene blue and scheme 3 depicts the mechanism of photocatalytic methylene blue degradation [36][37][38].The obtained values of k are equal to 0.0039, 0.0062, 0.0088, 0.0069, and 0.0062 min −1 for CdS, Ni05CdS, Ni1CdS, Ni2CdS and Ni5CdS, respectively as depicted in table 10.

Possible mechanism of methylene blue degradation
The efficient separation of charges plays a crucial role in determining the photocatalytic activity of a semiconducting photocatalyst.Thus, it was essential to ascertain the conduction band (CB) and valence band (VB) potentials of the Ni1CdS photocatalyst using the following empirical equations: Here, E VB represents the valence band (VB) edge potential, E CB denotes the conduction band (CB) edge potential, χ stands for the electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg represents the band gap energy of the semiconductor [39].
The calculated electronegativity value (χ) of Ni1CdS was found to be 2.026, while E CB and E VB were estimated to be −1.374 and +0.826 eV, respectively, relative to the normal hydrogen electrode (NHE).The conduction band of Ni1CdS was more negative than that of O 2   The photo-generated carriers are spatially separated according to the Z-scheme charge transfer process, which inhibits significantly the recombination of photo-generated electrons and holes.As a result, the oxidation or reduction potential of the photo carrier can be improved, thereby increasing the lifetime of photo-generated holes and electrons from Ni-CdS.

Surface elemental analysis
X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the chemical composition of various species within the architecture of CdS nanorods loaded with Ni (1.0 wt%).The XPS

Raman analysis
The Raman spectra depicting pure CdS and Ni-loaded CdS nanocomposites are illustrated in figure 17.In figure 4(a), the Raman bands of pure CdS at 300 cm −1 and 601 cm −1 , corresponding to the 1LO and 2LO phonons, are presented.A subtle shift, specifically at 293 cm −1 and 586 cm −1 , is observed in the 1LO and 2LO Raman bands of Ni-loaded CdS nanoparticles compared to pure CdS.This shift could be attributed to the smaller ionic radius of Ni 2+ ions (Ni2+ = 0.62 Å) compared to Cd 2+ ions (Cd 2+ = 0.97 Å), as confirmed by XPS, indicating the formation of Ni 2+ on the surface of Ni.
Furthermore, an enhancement in the intensity of CdS peaks is noted after Ni doping, with the maximum observed for 1.0 wt% Ni.This enhancement may be attributed to the plasmonic effect of Ni ultrafine nanoparticles.The intensity ratio of the 2LO to 1LO modes (I2LO/I1LO) provides additional support for the strength of the exciton-phonon coupling following Ni doping, compared to pure CdS.However, this intensity ratio slightly decreases as the concentration of Ni increases, as depicted in figures 17(b) and (e).

Charge carriers separation and transportation study
Figure 18 displays the diffuse reflectance UV-V is optical absorption characteristics of both pure CdS and Niloaded CdS nanocomposites within the 400-700 nm range.In the pure CdS sample, an absorption edge appears around 530 nm, with a corresponding estimated Eg of 2.36 eV, closely aligning with the intrinsic absorption band gap of CdS particles.Following Ni loading, there is not a substantial alteration in the absorption edge of CdS, suggesting that Ni is dispersed on the CdS surface rather than being lattice incorporated.
Nevertheless, an observed enhancement in the absorption spectra of CdS after Ni loading implies increased charge accumulation at the CdS surface.The maximum absorption is recorded for Ni (1.0 wt%) loaded CdS nanocomposites.The progressive improvement in absorption indicates a heightened charge density buildup at the CdS surface, potentially attributed to the formation of heterostructures such as CdS-Ni@NiO near the surface, as evident in XPS spectra.These varied properties facilitate the amalgamation of different electronic structures, leading to an expanded light response and improved charge separation and electron transfer.
It's important to observe that the presence of a heterostructure at the material surface not only results in a color change, causing a shift in peak position but also induces a change in the refractive index at the material surface.The introduction of Ni also alters the band gap energy of CdS, as illustrated in figure 19.Tauc's method was employed to estimate the band gap energy, revealing a decrease from 2.36 eV (for CdS) to 2.21 eV for 0.5 wt% Ni, 2.20 eV for 1.0 wt% Ni, and a subsequent increase to 2.29 eV for 5.0 wt% Ni-loaded CdS samples.
In addition to absorption studies, an analysis of the interface charge transfer strength can be conducted through photoluminescence (PL) emission spectra.Figure 20 presents a comparison of the PL emission spectra  to elucidate the charge separation and transfer process of photo-induced charge carriers in both pure CdS and Ni-loaded CdS nanocomposites.Previously reported results support the notion that emissions in CdS nanostructures arise from band-edge emission and surface-defect emission.Band-edge emission in CdS nanostructures is influenced by the size-sensitive quantum confinement effect, typically positioned in the 420-500 nm range, while peaks in the 530-680 nm range correspond to surface defect emission caused by sulfur vacancies or sulfur dangling bonds.Electrons and holes persist in the conduction band of CdS and the valence band of NiO.
Figure 20 illustrates two emission peaks in pure CdS at 523 nm and 540 nm under an excitation wavelength of 340 nm, attributed to surface defect states.The emission peak at 540 nm is triggered by the formation of sulfur vacancies in CdS, possibly due to the smaller ionic radius of S 2− compared to that of Cd 2+ .Conversely, the broader emission peak at 523 nm in CdS results from the recombination of holes from the CdS valence band with electrons trapped in sulfur vacancies.Upon Ni doping, the intensity of the CdS peaks at 523 nm significantly diminishes, indicating a weakening of the emission band intensity due to the transfer of electronhole pairs between various sections of the composites.
In this case, the reduction in PL band emission intensity in the CdS-Ni@NiO sample is attributed to the formation of a Z-scheme interface for electron-hole pair transfer.The electron-hole transfer process is specifically attributed to Ni nanoparticles at the interface of CdS-Ni@NiO, explaining the fluorescence quenching observed in CdS-Ni@NiO.

Photocatalytic hydrogen evolution
The hydrogen (H 2 ) production behavior of CdS and Ni-loaded CdS samples with varying amounts of Ni loading was assessed during solar irradiation in the visible region, as illustrated in figure 21.In the absence of a photocatalyst in a controlled reaction, no H 2 evolution was observed.However, a significant amount of H 2 evolution was detected when photo-catalysts were present, indicating that photo-catalytic activity occurred exclusively in the presence of these catalysts.Figure 21 illustrates that pure CdS exhibited minimal H 2 evolution, specifically 0.18 mmol.This limited evolution could be attributed to the rapid recombination rate of electrons and holes or the participation of only a small number of electrons and holes in the photo-catalytic reaction.Additionally, a substantial overpotential on the CdS surface resulted in a swift backward reaction.The introduction of 0.5 wt% Ni demonstrates a significant increase in H 2 evolution, specifically a 22.22-fold enhancement (4.0 mmol), compared to pure CdS.With a Ni loading of 1.0 wt%, the acceleration of H 2 production is even more pronounced, reaching 9.0 mmol, which is approximately 50 times greater than that observed with pure CdS (see figure 21).Consequently, the notable improvement in H 2 evolution following Ni loading underscores the impact of Ni, particularly with surface NiO, on the photocatalytic activity.The heightened H 2 evolution attributed to Ni@NiO on the CdS surface may be attributed to the presence of two competitive mechanisms: an outstanding capability to transport charge carriers (electrons and holes) and the effective hindrance of the recombination of photo-excited charge carriers (electrons and holes).

Conclusion
The pure CdS and Ni-CdS samples were successfully synthesized by the facile chemical process, affording an effective photocatalyst that can be used for hydrogen production and degradation of methylene blue.The estimation of crystallite size is done by using different Williamson-Hall methods.ISM, ASM, UDEDM, Size strain plot, Scherrer, and Modified Scherrer plot are obtained.Rietveld Refinement of CdS and Ni-CdS from the XRD data by using Full Prof software is done.FESEM and TEM data show the change in shape from spherical to nanorods with an increase in the doping concentration of Ni in CdS as depicted.FESEM, TEM, and Crystal growth mechanisms show the changing of CdS spherical structure to Ni-CdSnanorods.Under visible light, the loading of 1.0 wt percent Ni in CdS produced the highest H 2 evolution of 9 mmol at a rate of 1.8 mmol h −1 , which is 50 times higher than pure CdS (180 mol @ 36 mol h −1 ).Ni (1 wt%)-CdS shows the highest degradation percentage of 95% in just 90 min.It shows the highest environment mineralization efficiency and photocatalytic degradation in comparison to pure CdS.As a result of its high visible light photoactivity, the current Ni@NiO-CdS composite could be a promising candidate for energy renewal and conversion.

3. 1 .
Structural analysis and Rietveld refinement of CdS and Ni-CdS 3.1.1.Measurement of strain and crystallite size using Williamson-hall methods Isotropic Strain Model (ISM) reveals the 'CdS and Ni (5 wt%)-CdS' ASM W-H plot.Uniform deformation energy density model (UDEDM)A graph is then drawn and fitted into a straight line between b q Cos and slope of the fitted line, the crystallite size and uniform deformation energy density (U) is measured.The W-H plot of the UDEDM system is shown in figure4for CdS and Ni (5 wt%)-CdS.The average lattice strain is determined by ( ) = u Y2 hkl from the energy density of deformation and Y hkl value.Strain size plot of CdS and Ni (5 wt%)-CdS is represented by figure 5.Modified Sherrer Method

Figure 7 .
Figure 7. Rietveld refined of CdS, Ni(3 wt%)-CdS and Ni (5 wt%)-CdS sample XRD pattern.The dot is experimental while the solid line is a representation of refined data from Rietveld.The background shows how experimental and refined information varies.

Figure 9 (
b) shows the mapping of Cd and S elements of Fourier 3-dimensional electron density in the CdS unit cell at x = 0.After doping of Ni into the CdS the electron density has increased drastically and it depends upon the concentration of doping.Figure 10 shows the electron density pattern of Ni (1 wt%)-CdS which has increased in comparison with pure CdS electron density.Again, figure 11 reveals the Ni (5 wt%)-CdS electron density pattern which has the highest electron density with respect to pure CdS and Ni (1 wt%)-CdS.

Figure 8 .
Figure 8. Ball and Stick representation of hexagonal structure generated by VESTA program of sample (a) CdS (b) Ni (5 wt%)-CdS.

Figure 9 .
Figure 9. (a) 2D Electron Density Mapping in CdS unit cell.(b) 3D-Electron Density Mapping in CdS unit cell.The density of the electron is measured in e/Å 3 .
Then thiourea is invaded by the heavily nucleophilic O atoms of H 2 O, resulting in the weakening of the S--C double bonds, which are slowly shattered to release S 2− anions.S 2− anions will then be released in reaction with the pre-released Cd, which grows CdS nuclei that serve as seeds for further development into CdSplatelets.When Ni is doped into the CdS with concentration Ni (0.5 wt%).First, Cd 2+ and Ni 2+ can coordinate with the thiourea, which, as suggested in scheme 1, leads to the development of the metallic complex M 2+ -thiourea (M = Cd, Ni).Due to their relative stability, the thermolysis continues slowly and leads to the formation of certain Cd and Ni sulphide nuclei when heated.These newly formed nuclei are unstable in the solution and contain many unsafe bonds or defects or traps at the nuclei surface that can contribute to the formation of CdScrystalline and favor the introduction of Ni 2+ into the crystalline CdS.At constant temperature i.e. 180 °C for 5 h, due to the thermal and hydrolytic stability of the sulfur metal bond, Ni−S−Cd containing metal sulphide nanoparticles are formed which supply assembly centers for random moving metal sulphides nanoparticles.These nanoparticles are attached and aggregated on the liquid interface, forming compact shells.The ongoing aggregation and the ensuing growth process is suggested to lead to the formation of Ni-doped CdS spheres.When further Ni doping into the CdS with higher concentration i.e.Ni (1 wt%)-CdS and Ni (5 wt%)-CdS, Ni 2+ , Cd 2+, and thiourea complex formed.The bonds between randomly moving metal sulphide nanoparticles change their structure to form one-dimensional rod-like crystals following the first nucleation of Ni 2+ , Cd 2+, and thiourea.In this situation, the DLA model (The random walk of particles due to Brown's Motion Cluster forms aggregates of these particles together) includes a unified assembly to which the Ni 2+ , Cd 2+, and S 2- monomers spread from the solution and reach an area right next to a crystal surface.When the neighboring nanoparticles have reached the place adjacent to the edge of the movement, the further particles are attached to them in the same crystallographic direction.This leads to the formation of nanorods by an oriented growth process of the attachment.Reaction time also plays an unavoidable role in studying the CdS nanorods development process by maintaining the other reaction conditions.
Scheme 1. Crystal growth mechanism of CdS and Ni-doped CdS.

Figure 12 .
Figure 12.(a) FESEM image of pure CdS (b) Selected area magnification of FESEM image of pure CdS (c) TEM image of pure CdS nanoparticles (d) Selected Area Electron Diffraction Pattern (e) CdS nanoparticles fringes with d spacing (f) Particle Size Distribution.

Figure 15 .Figure 16 .
Figure 15.(a) C t /C o versus Time for Dark (b) C t /C o versus Time for light irradiation; (c) logarithmic of (Co/Ct) versus Time for dark (d) logarithmic of (Co/Ct) versus Time for light.

Figure 16 (
d) presents two resolved peaks of S2p at binding energies around 161.36 eV (S 2p3/2) and 162.67 eV (S 2p1/2) with a spin-orbital partition of 1.31 eV.The 2:1 area ratio confirms the S 2-chemical state of S, and a peak at 159.97 eV may indicate the presence of bridging disulfides S22−.Additionally, XPS spectra show a small peak of metallic Ni at 852.15 eV, and an intense NiO peak, suggesting surface transformation of Ni into NiO upon exposure to air.

Figure 16 (
c) displays peaks at 854.88 eV and 861.07 eV with a 6.18 eV energy separation, corresponding to Ni 2p3/2 for Ni2+ in Ni surface.Peaks at 872.54 eV and 878.14 eV are identified as satellite peaks of Ni2+ in NiO, confirming surface oxidation of Ni into NiO in ambient conditions.The presence of Ni metallic features at a photon energy of 1253.6 eV suggests that the NiO layer over the Ni surface is within the estimated inelastic mean free path of electrons (1.05 nm at 400 eV).Consequently, the observed phenomena support the formation of an ultra-thin layer of NiO over the Ni surface.

Table 2 .
The normal and measured d hkl interplanar spacing and relative percentage variance for some major XRD peaks for the {hkl} respective planes.

Table 1 .
Crystallite size was computed using different W-H method models, the Scherrer method, and the modified Scherrer method.

Table 4 .
Fractional atomic coordinates and isothermal parameters of various atoms derived from the Rietveld XRD sample study for CdS.

Table 5 .
Fractional atomic coordinates and isothermal parameters of various atoms derived from the Rietveld XRD sample study for Ni(1.0 wt%)-CdS.

Table 6 .
Fractional atomic coordinates and isothermal parameters of various atoms derived from the Rietveld XRD sample study for Ni(5.0 wt%)-CdS.

Table 7 .
R-factors, the goodness of fit, lattice parameters, crystalline size, X-ray density, and crystalline size.

Table 9 .
Bond angle and bond length of CdS and Ni-CdS.

Table 10 .
CdS and Ni-CdS photocatalytic activity.The plausible degradation mechanism of MB by Ni-CdS under light illumination is presented as follows