New insights into visible light active SnS semiconductor for the elimination of methylene blue: RSM and DFT approaches

Abstract

In this study, a response surface methodology (RSM) based on a central composite design (CCD) was performed by varying five pivotal variables: concentration of pollutant (mg/L), mass of catalyst (g), time (min), temperature (°C), and pH to evaluate the degradation efficiency of Methylene Blue (MB) using nanostructured SnS under visible light irradiation. The quadratic model displayed a notable level of reliability and fitness, as evidenced by its R2 value of 0.925. The optimized conditions obtained (Concentration of pollutant: 13.38 mg/L, mass of catalyst: 0.09 g, irradiation time: 25.26 min, temperature: 47.34 °C and pH: 8.36) yielded a degradation efficiency of 99 %, which is highly remarkable. Furthermore, based on density functional theory (DFT), the adsorption energy (Eads), charge transfer, and density of states (DOS) are investigated to study the application of tin sulfide as an adsorbent for Methylene Blue adsorption. When the MB molecule was positioned in a horizontal configuration at a distance of 3.136 Å from SnS, the significant adsorption energy of −3.22 eV was obtained. Due to the bond formation between tin atoms of SnS and carbon atoms of MB, it can be stated that there is a strong chemical interaction between SnS and MB molecule in this configuration. Regarding vertical orientation, adsorption energy was obtained to be −0.45 eV and no sign of any bond formation or breaking was observed, indicating that the adsorption process may be physisorption. The computational and experimental results proved that SnS is a promising candidate for MB adsorption and degradation.