FTIR Analysis
Fig. 1 shows FTIR spectra pristine PPy, WS2 and WS2/PPy composite. Generally, in the WS2/PPy composite spectrum, most of peaks that presents in PPy and WS2 were observed with a slight shift in the wavenumber. The peaks or band that presents as C=C stretching of Py ring had been observed at 1528.95 cm-1 in PPy and at 1539.03 cm-1 in composite [22-23]. Besides, the C-N stretching was observed at 1468.72 cm-1 in PPy and at 1460.79 cm-1 in the composite which is slightly red-shifted from the original position. These shifts are due to the interaction between WS2 with PPy that causes the partial π-electron transfers from PPy to the WS2 [24]. The peaks at 782.88 cm-1 indicates W-S bond in composite spectrum, while for S-S bond of tungsten disulfide shows a peak at 889.92 cm-1 in WS2 and 889.37 cm-1 in nanocomposite [25]. Thus, it was proven that WS2 has successfully integrated with PPy and all the characteristic peaks were present in composite spectrum.
XRD Analysis
The phase purity and crystal structure of the composite have been identified by using X-ray diffraction. Fig. 2 (a) shows the X-ray diffractogram of PPy while Fig. 2 (b), (c) shows the WS2 and WS2/PPy diffractograms. Diffractogram of PPy shows an amorphous nature, and 2θ= 26.34°. WS2 displays crystalline peaks at 2θ=14.36°, 25.98°, 33.58°, 50.01° and 58.95° which can be indexed to the (002), (004), (101), (105) and (110) crystal planes of the WS2 structure, respectively (Pandey et al., 2005). The XRD pattern peaks (002), shows the presence of multi-layered sheets that were vertically stacked along the c-axis. However, XRD patterns of WS2/PPy does not show significant peaks of WS2 and the amorphous nature predominates [26]. It due to the agglomeration of crystalline phase during synthesis and has caused the diffraction peaks of WS2 to disappear.
FESEM Analysis
The micrographs of WS2 and WS2/PPy composite were observed using FESEM. The FESEM image of WS2 with magnification of 100kx is presented in Fig. 3(a), where plate-like entities exhibiting irregular morphology and the shape of hexagonal does not fully form that causes the overgrowth of fine particle [27] Meanwhile, the FESEM image of WS2/Ppy composite with magnification 150kx as shown in Fig. 3(b) shows the discrete spherical shape have been observed. This is because the PPy have been integrated to the surface of WS2. The elemental distributions of C, N, W and S at the surface were determined by EDX (energy dispersive X-ray spectrometry) mapping and corresponding results are shown in Fig. 4 (a), (b), (c) and (d) respectively. From EDX mapping, it was proved that there are element W, S, C and N inside the composite since in the spectrum of composite shows that there are presences of W-S bond and C-N stretching band and the elements were uniformly distributed.
Sunlight-assisted photocatalytic degradation
Comparison of Photocatalytic Activity of WS2 and WS2/PPy Composite
Photocatalytic properties of WS2 and WS2/PPy were observed under sunlight with MB as the target analyte. Fig. 5 shows the difference in absorbance intensity for the degradation MB with different catalyst. WS2/PPy evidently achieved stronger degradation of MB by diminishing the absorbance intensity of MB to max. This can be proven that the binary composites able to enhance photodegradation percentage of MB due to its excellent in optical absorption in the visible regions [28-29]. Furthermore, incorporation of PPy with WS2 results in photogenerated electrons to be transferred from excited state to the conduction band of WS2 particles efficiently [30]. Therefore, WS2/PPy was selected for the optimization analysis.
Effect of pH
Fig. 6 reveals the effect of pH versus percentage of MB degradation. Fig. 7, shows that at pH 3 (92.32%), the percentage of degradation was the highest compared to pH 7 (85.31%) and pH 9 (80.72%). This finding proved that photocatalytic activity able to occur in acidic condition compared to neutral and basic condition [31]. As we can observe in Fig. 7, the increase of pH will decrease the absorption intensity which makes the percentage degradation decrease too. The condition at pH 7 and pH 9 able to make photodegradation occur but in less efficiently. This is because the higher the pH, the formation of free radical •OH become slows which will affect the photocatalytic activity [32]. Thus, the best condition pH for photocatalytic activity of MB is pH 3.
Effect of Dosage
Based on Fig. 7, the different of percentage degradation has been observed due to the different of composite dosage been put in the MB. When 100 mg of composite was placed into MB and been exposed to the sunlight within 2 hours at pH 3, it has highest percentage of degradation (92.32 %) compared to when 20 mg (69.75 %) and 50 mg (78.27 %) were placed in MB. The result reveals the higher the dosage of composite, the higher the potential of composite to degrade dyes of MB [33]. This is also attributed to the fact that as the photocatalyst amount is increase, active surface sites of photocatalyst been exposed to the sunlight also increase [34]. Hence, more photon energy can be absorbed from the sunlight and activates the photodegradation of MB efficiently.
Effect of Contact Time
Fig. 8 shows that the degradation of different contact time exposure by using 100 mg of composite in 5 mg/L of MB at pH 3 within the range 0-180 minutes. Figure 8 depict that the degradation increased with increasing of exposure time to the sunlight. This is because more photon energy was absorbed by photocatalyst which provided by the sunlight. Therefore, more hydroxyl radical will be formed because more photon energy been absorbed, and electron-hole pairs was formed which is significant for photocatalytic activity [35].
Effect of Initial Concentration
Fig. 9, presents the degradation at different initial concentration of MB (MB). Fig. 9a and 9b reveals the degradation peaks of MB before and after the sunlight exposure at different range of initial concentrations. The significant degradation was observed after the exposure of sunlight. Meanwhile Fig.9b discloses percentage degradation of MB with MB initial concentration range 0-50 mg/L. When initial concentration of MB was 10 mg/L, the percentage degradation is 96.15%. However, when concentration of MB was increased, the percentage of degradation decreased. The lowest degradation is when initial concentration was 50 ppm (60.10%). This might be explained due to the increase of collisions between dyes molecules when concentration of dyes increases. Thus, the interaction between photocatalyst and MB decreases. Other than that, when the initial concentration of MB decreases, there are more ability for the light to penetrate the solution and irradiate the composite so that more photon energy can be absorbed and forming more electron-hole pairs and hydroxyl radical [35].