Strontium titanates with perovskite structure as photo catalysts for reduction of CO2 by water: Influence of co-doping with N, S & Fe

Abstract

Strontium titanates, SrTiO3, Sr3Ti2O7 and Sr4Ti3O10, with perovskite structure, have been modified by co-doping with N, S and Fe and characterized by X-ray diffraction (XRD), Diffuse reflectance Spectroscopy (DRS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDXA), X-ray Photoelectron Spectroscopy (XPS) and Photo Luminescence Spectroscopy (PLS) techniques. XRD, EDXA and XPS results substantiate the incorporation of the three dopants into the titanate matrices. Based on the XPS binding energy data, it is inferred that doped nitrogen in SrTiO3 and Sr4Ti3O10 phases occupies O2− ion sites by substitution and interstitial sites in Sr3Ti2O7. Fe as Fe3+ and S as cationic S6+ occupy Ti4+ ion sites in the respective lattice in all three titanates. Additional energy levels created by the dopants within the band gap result in narrowing of the band gap, as revealed by DRS data and thus enable visible light absorption by the modified titanates. Significant reduction in the intensity of photo luminescence emission lines of modified titanates indicates that doping retards the recombination rates of charge carriers, enhancing their life time. Changes in the photo-physical properties brought out by modification with dopants improve the activity for photo catalytic reduction of CO2 vis-a-vis pristine titanates. Both Sr3Ti2O7 and Sr4Ti3O10, perovskites with layered structure, display higher activity when compared to simple perovskite, SrTiO3. Layered structure facilitates faster transport of charge carriers and the interlayer space could function as oxidation/reduction reaction sites, leading to the separation of charge carriers. Amongst the two layered perovskites, double layered Sr3Ti2O7 displays higher activity compared to triple layered Sr4Ti3O10 since the conduction band minimum of Sr3Ti2O7 is at higher negative energy level vis-à-vis that for Sr4Ti3O10 and the inter layer spacing is appropriate for CO2 reduction and splitting of water as well.