Unraveling the origin of the high photocatalytic properties of earth-abundant TiO/FeS heterojunctions: insights from first-principles density functional theory.

Herein, first-principles density functional theory calculations have been employed to unravel the interfacial geometries (composition and stability), electronic properties (density of states and differential charge densities), and charge carrier transfers (work function and energy band alignment) of a TiO(001)/FeS(100) heterojunction. Analyses of the structure and electronic properties reveal the formation of strong interfacial Fe-O and Ti-S ionic bonds, which stabilize the interface with an adhesion energy of -0.26 eV Å. The work function of the TiO(001)/FeS(100) heterojunction is predicted to be much smaller than those of the isolated FeS(100) and TiO(001) layers, indicating that less energy will be needed for electrons to transfer from the ground state to the surface to promote photochemical reactions. The difference in the work function between the FeS(100) and TiO(001) heterojunction components caused an electron density rearrangement at the heterojunction interface, which induces an electric field that separates the photo-generated electrons and holes. Consistently, a staggered band alignment is predicted at the interface with the conduction band edge and the valence-band edge of FeS lying 0.37 and 2.62 eV above those of anatase. These results point to efficient charge carrier separation in the TiO(001)/FeS(100) heterojunction, wherein photoinduced electrons would transfer from the FeS to the TiO layer. The atomistic insights into the mechanism of enhanced charge separation and transfer across the interface rationalize the observed high photocatalytic activity of the mixed TiO(001)/FeS(100) heterojunction over the individual components.