BET surface area analysis and nitrogen adsorption-desorption isotherms were used to evaluate the properties like surface area, pore volume, pore diameter of CdS/NiS nanocomposite. Surface characteristics of the sample were analyzed using quanta chrome Nova1000 BET instruments, prior to the analysis samples that were degassed at 180˚C for 4 hrs, the nitrogen adsorption-desorption isotherm was recorded by passing nitrogen gas to the sample under bath temperature 77K. Nitrogen adsorption-desorption isotherms, surface area and pore size distribution were obtained by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method [18] [19] [20]. The micro pore volume, micro pore area and external surface area were obtained by Volume-Thickness (v-t) method using Asiqwin data reduction software (version 3.0). As prepared CdS/NiS nanocomposite showing identical hysteresis loop of IV isotherms. BET surface Analysis data has been reported in Figure 2 and Table 1.

UV-Visible spectrum Figure 3(a) of CdS/NiS nanocomposite over the range 200 - 700 nm showed photoabsorption properties no longer than 320.70 nm, which suggest that the catalyst is photo active under UV light irradiation. Assuming the CdS/NiS solid as direct semiconductor and it is possible to calculate the band gap of CdS/NiS by constructing a Tauc plot [21]. The Tauc plot of CdS/NiS is displayed in Figure 3(b). The energy of the band gap of CdS/NiS could be thus estimated to be 3.26 eV. It is reported that Cd loading results in enlarge surface area of the CdS/NiS photocatalyst. However, the increase of surface area is likely not the main factor affecting the photocatalytic activity of CdS/NiS. Other factor

that could affect photocatalytic efficiency are such as availability of active sites, crystalline structure, pore size and number/nature of trapped sites [22] [23]. According to the literatures and the fact that cadmium acting as an electron trap [24], an enhanced photocatalytic activity of CdS/NiS nanocomposite found in our study was likely ascribed to a decrease of electron-hole pair recombination and thus promoting the photocatalytic activity [25].

The XRD patterns of CdS/NiS nanocomposite Figure 4(a) exhibit hexagonal structure with similar peaks. The broadening of XRD peaks confirms nanocrystalline nature of the as prepared samples. The diffracted peaks obtained at diffraction angles 2θ of 27.11, 37.21, 43.28, 63.01, 75.48 and 79.51 corresponds to the (111), (211), (300) (400) (332) and (422) planes of CdS/NiS peaks with hexagonal phase (JCPDS-80-0006) [23]. The crystallite size was calculated using Williamson-Hall (W-H) plot method Figure 4(b). W-H method reported that the XRD pattern broadening is attributed to both crystallite size and lattice strain. The XRD peak broadening due to micro strain is given by

β ε = 4 ε tan θ (1)

where β_ε is broadening due to strain, ε is the strain and θis the peak position in radians.

β cos θ = k λ / D + 4 ε sin θ (2)

From Equation (2) is Williamson-Hall equation and represents the uniform deformation model (UDM) by plotting 4sinθalong the x-axis and βcosθalong the y-axis and from the linear fit of the data, the crystalline size was estimated from the Y-intercept and it was found to be 23.22 nm, and the strain ε was estimated from the slope [26] [27] and it was found to be 6.6324 × 10⁻⁴. From the XRD data the cell parameters are calculated and it is found to be a= b≠ c (a=

4.176 Å, b= 4.176 Å and c= 13.412 Å) and α = β = 90˚, γ ≠ 90˚. Accordingily CdS/NiS nanocomposite belongs to hexagonal structure [28] [29] [30].

Particle size and surface area contributes to a larger extent on the photocatalytic activities of photocatalysts. The morphological studies of synthesized CdS/NiS nanocomposite from electrochemical method were analyzed by scanning electron microscopy are shown in Figure 5. Sample shows that it consists of nano rod shape of CdS/NiS nanocomposite. The elemental analysis of the CdS/NiS nanocomposite was carried out using EDAX (JOEL, JED-2300, Germany). The EDAX analysis spectrum Figure 6 reveals elemental composition of CdS/NiS nanocomposite. It is clear from the graph that the peaks corresponding to Cd, S and Ni are present in the prepared samples. The elements present and their relative proportions or quantitative results obtained by SEM-EDAX analysis and the corresponding data are given in Table 2.

The reaction was performed with different concentrations of MB with constant weight of CdS/NiS photocatalyst. The change in concentration of the MB was recorded by change in colour using Spectrophotometer. A plot of log%T (percent transmittance of light) versus time was liner up to 50% of the reaction indicating the disappearance of MB follows 1^st order kinetics as shown in Figure 7. The rate constant values are tabulated in Table 3. The reaction rate decreases with increase in concentration of MB. This is because with increase in the dye concentration, the solution becomes more intensely colour and the path length of the photons entering the solution is decreased thereby few photons reaches

the catalyst surface. Hence the production of hydroxyl radicals is reduced. Therefore the photodegradation efficiency is reduced. The pH and COD for MB solutions before and after degradation were measured and given in Table 3 and Figure 7.

The experiments were performed by taking different amount of CdS/NiS photocatalyst varying from 0.005 - 0.03 g in order to study the effect of catalyst loading. The study showed that increase in catalyst loading from 0.005 g - 0.02 g increased dye removal efficiency. Further increase in catalyst above 0.02 g decreased the photoactivity of the catalyst, which is due to the aggregation of CdS/NiS nanocomposite at high concentration causing a decrease in the number of surface active sites and increase in the opacity and light scattering of CdS/NiS nanocomposite at high concentration. This tends to decrease the passage of light through the sample. Further, in the present study indicated, from economic point of view, the optimized photocatalyst loading is 0.02 g/20ml. Effect of catalyst on photodegradation of MB is shown in Figure 8 and rate constant values are tabulated in Table 4. A result of COD effect is shown in Figure 8(b).

The solution pH is an important variable in the evaluation of aqueous phase mediated photocatalytic reactions. The pH of the solution was adjusted by adding 0.001M HNO₃ or 0.001M NaOH solution. The effect of pH was studied at pH 4.02, pH 6.0, pH 9.5 by keeping all other experimental conditions constant. The results are illustrated in Figure 9 and tabulated in Table 5. The rate of

degradation is observed to be slow at lower pH (<4.02) and higher (>9.5) pH. It was observed that the amount of material recovered after the experiment was lowered at lower and higher pH because of the dissolution of semiconductor sulfide at extreme pH values. Results of COD effects are illustrated in Figure 9(b). The optimum selected pH is 9.5 at which photodegradation is high.

Temperature is one of the essential factors which effect the rate of photodegradation. To examine the effect of temperature, the experiment was carried out at three different temperatures. It is confirmed that when temperature is increased the degradation efficiency of MB is increased, and observed that the rate of degradation is not very significant at lowtemperature. However, the reaction is more significantly influenced at high temperature since the diffusion rate increased with temperature. An increase of temperature could bring about an increase in the degradation rate [31]. The rate constant and COD values are reported in (Table 6 and Figure 10). The thermodynamic parameters were calculated and are reported in Table 7.

The rate of HER increases with the increase of CdS/NiS photocatalyst concentration from (0.001 g - 0.003 g). Rate constants were calculated by plotting log(V_t - V₀) v/s time in mins. The values of pseudo 1^st order rate constants are shown in Table 9 and Figure 12(a) & Figure 12(b). The rate of HER becomes slow after 50% of the reaction, hence the rate constants are obtained for approximately 50% reaction.

The rate constants for HER at different concentration of hydrazine (0.1M - 0.5M) are obtained. The values of rate constants are not affected by the concentration, since the experiments are carried out at excess concentration of hydrazine than photocatalyst. The comparison of the rate constants for Visible light and UV light are shown in Table 10, Figure 13(a) and Figure 13(b), which indicates the kinetics of rate of hydrogen evolution are carried out in visible light and UV light, the rate of HER is higher in UV light than Visible light. The current density (i) for HER has been calculated by using potential and resistance measured during the experiment has shown in Figure 14(a) and Figure 14(b).

Figure 15(a) and Figure 15(b) shows the plot of Current v/s Potential for HER, Figure 16(a) and Figure 16(b) shows the plot of current v/s time in mins, which indicates the increase of potential, increases the current with the increase of rate of H₂ evolution. The rate of HER is very high in the presence of CdS/NiS photocatalyst compared to blank. This indicates the photocatalyst played an important role in the evolution of H₂ from N₂H₄H₂O in UV light compared to visible light.

In view of the above facts, the rate constants for HER are in following order

k UVlight > k Visiblelight ≫ k Blank .

Hence the rate of HER by CdS/NiS nanocomposite is sensitive to UV light compared to sunlight, as the photocatalyst energy gap is 3.24 eV which comes under UV light. The rate constants for HER at different temperature are depicted in Table 11. Thermodynamic parameters for visible light and UV light are calculated and are depicted in Table 12. The increase of temperature slightly increases the rate of HER.

The different Tafel slopes for HER under UV light and visible light compared to blank are depicted in Figure 17(a) and Figure 17(b). The Tafel slope is higher

under UV light than visible light indicates the higher HER for UV light compared to visible light in presence of CdS/NiS photocatalyst. The values of Tafel slopes are shown in Table 13.

In view of the above results it is confirmed that initially HER rate in presence of the CdS/NiS photocatalyst is very high and after 50% of HER the reaction tends to slow. The results show that HER occur at very low cathodic current density. Considiring the decomposition of 3N 2 H 4 H 2 O ( I ) → 4 ( 1 − x ) NH 3 ( g ) + ( 1 + 2 x ) N 2 ( g ) + 6 x H 2 ( g ) + 3H 2 O ( g ) the

maximum gas evolution would be 91.1 mmol. 50 vol% hydrogen gas if all the

hydrazine hydrate was converted into nitrogen and hydrogen, i.e. x= 1. Thus the N₂H₄H₂O can be used as a hydrogen source which contains large amount of hydrogen undergo oxidation with OH radical in presence of the CdS/NiS photocatalyst and converted to hydrogen by following two ways

N 2 H 4 → N 2 ( g ) + 2H 2 ( g ) ,     Δ H = − 95.4   kJ ⋅ mol − 1

3N 2 H 4 → 4NH 3 + N 2 ( g ) ,       Δ H = − 157   kJ ⋅ mol − 1 .

Splitting of N₂H₄H₂O into H₂ and N₂ by photo-catalyzing HER may be due to interaction of OH radicals produced during the reaction with N₂H₄H₂O which leads to oxidation of N₂H₄H₂O. It is evidence that the formation of higher concentration of OH radical in presence of UV light compared to Visible light as the rate constant of HER was higher in UV light than Visible light by CdS/NiS photocatalyst.

The rate of HER is limited by either proton adsorption onto an active site or evolution of formed hydrogen from the surface. A high Tafel slope is indicative of proton adsorption as the rate limiting step, while a lower Tafel slope (20 - 45 mV) indicates that the evolution of molecules hydrogen from the catalyst is rate limiting. In the present case the Tafel slope for visible light is 23.5 mV and 42.5 mV for UV light. Blank experiments show poor activity for HER (10.1 - 13.5 mV).