## 3 Results and Discussion  
### Characterization of SnIn\({}_{4}\)S\({}_{8}\)  
The X-ray diffraction pattern of the as-synthesized SnIn\({}_{4}\)S\({}_{8}\) powder is presented in Figure 1. According to the literature, the peaks were located at 20 = 14.0\({}^{\circ}\), 18.6\({}^{\circ}\), 23.3\({}^{\circ}\), 27.4\({}^{\circ}\), 28.7\({}^{\circ}\), 33.2\({}^{\circ}\), 41.0\({}^{\circ}\), 43.6\({}^{\circ}\), 47.8\({}^{\circ}\), 50.1\({}^{\circ}\), 55.9\({}^{\circ}\), 59.4\({}^{\circ}\), and 66.7\({}^{\circ}\). These peaks correspond to (111), (202), (220), (311), (222), (400), (422), (333), (440), (531), (533), (444), and (553) crystalline planes of cubic phase of SnIn\({}_{4}\)S\({}_{8}\) (JCPDS card #42-1305) [32,33]. The prepared materials show high purity since the spectrum does not show any impurity peak. The morphology of the prepared SnIn\({}_{4}\)S\({}_{8}\) powder is shown in the field-emission SEM image in Figure 2. Network-like microspheres were observed with an average diameter of 1-2 \(\upmu\)m. Close observation shows that the microspheres comprise numerous interleaving flakes with an approximate thickness equal to 10 nm.  
The main factor in assessment of the photocatalytic activity of a semiconductor is its optical absorption properties, which depend on its band structure [34, 35, 36]. Thus, the UV-vis diffuse reflectance spectrum of SnIn\({}_{4}\)S\({}_{8}\) was recorded (Figure 3a). The as-prepared SnIn\({}_{4}\)S\({}_{8}\) absorbs ultraviolet and visible light with wavelengths shorter than 550 nm. An onset of the absorption edge is observed starting around 550 nm in the visible light region with steep increase of absorption moving into the ultraviolet region. This absorption of visible light is attributed to the band-gap transition of the catalyst [24]. The equation \((\alpha\
u)^{n}=k(\hbar\
u\,-\,\text{Eg})\) can be used to estimate the band gap energy. In this equation, \(\alpha\) represents the absorption coefficient, hv refers to the photons' energy, \(k\) is the Coulomb constant, Eg denotes the absorption band gap energy, and value of \(n\) is 2 for a direct band gap semiconductor and 1/2 for an indirect one [26]. Figure 3b shows the plots of \((\alpha\
u)^{2}\) as a function of hv of the sample. The band gap (Eg) of SnIn\({}_{4}\)S\({}_{8}\) was calculated to be  
Figure 1: XRD pattern of the as-prepared SnIn\({}_{4}\)S\({}_{8}\) photocatalyst.  
Figure 2: SEM image of the as-prepared SnIn\({}_{4}\)S\({}_{8}\) photocatalyst.  
2.23 eV, which is suitable for photocatalytic decomposition of organic compounds under visible-light irradiation.  
XPS analysis was used to confirm the composition and valence state of elements of the SnIn\({}_{4}\)S\({}_{8}\) material (Figure 4). Two intense peaks in the Sn 3d region can be observed in Figure 4a at 486.0 eV (Sn 3d\({}_{5/2}\)) and 494.4 eV (Sn 3d\({}_{3/2}\)), which is relevant to Sn\({}^{4+}\) oxidation state on the surface of the sample [37]. Considering indium in Figure 4b, the characteristic peaks were located at 444.2 eV and 451.8 eV, which are associated to In 3d\({}_{5/2}\) and In 3d\({}_{3/2}\), respectively. These XPS peaks confirm the presence of In\({}^{3+}\) in the synthesized materials [27]. The S 2p spectrum exhibits a strong peak at 160.8 eV, indicating the presence of the S\({}^{2-}\) species in the sample. Peak deconvolution of S 2p spectrum (Figure 4c) has been performed. The binding energies of 160.8 eV and 161.9 eV correspond to S 2p\({}_{3/2}\) and S 2p\({}_{1/2}\), respectively. All these results are in agreement with those previously reported for SnIn\({}_{4}\)S\({}_{8}\) crystal [33].  
Figure 4: XPS spectra of the as-prepared SnIn\({}_{4}\)S\({}_{8}\) photocatalyst: (**a**) Sn 3d; (**b**) In 3d; (**c**) S 2p.  
Figure 3: (**a**) UV–vis diffuse reflectance spectrum of the as-prepared SnIn\({}_{4}\)S\({}_{8}\) photocatalyst; (**b**) Plots of (\(\alpha\)hv)\({}^{2}\) versus energy (hv) for calculating the band gap energy.