Computational exploration of charge transfer dynamics in dye sensitized SnO2 and ZnS for photocatalytic applications

A computational investigation of some novel charge transfer complexes, prepared by the adsorption of dyes on SnO2 and ZnS, was carried out using semiempirical PM3 method. Natural dyes Chlorogenic acid (DTC) and Flavone (PHC) were adsorbed on 1  1  1 SnO2 and ZnS crystals to form SnO2-DTC, SnO2-DTC-PHC, and ZnS-DTC adsorption complexes. The electronic absorption spectra calculated using PM3 method showed a large bathochromic shift to 958 nm and 577 nm for SnO2-DTC and SnO2- DTC-PHC, respectively, suggesting the charge transfer complex formation. Moreover, the bandgap was reduced to 1.29 and 2.15 eV as compared to 3.4 eV of pristine SnO2. Subsequently, the adsorption of DTC on ZnS was accomplished and calculations were performed to determine the spectroscopic properties of the complex, where a considerable red shift was observed for ZnS-DTC as well. The semiempirical PM3 calculations pointed to stability of SnO2-DTC, SnO-DTC-PHC, and ZnS-DTC as predicted from negative adsorption energy values. The energies of Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) determined from single-point energy calculations were concomitant with the proposed photocatalytic mechanisms.