Electronic Properties of Catechol Adsorbed on Rutile TiO2 and SnO2 (110) Surfaces: A Density Functional Theory Study

Abstract

The incorporation of organic molecules such as catechol onto TiO2 substrate to enhance TiO2 photocatalytic activity has led to improved Dye Sensitized Solar Cells (DSSCs) efficiency. Nonetheless, it still remains low for most practical applications hence more detailed description of the electronic structure of catechol-TiO2 rutile surface, could provide insight for further improvement. In this work, adsorption of catechol on rutile TiO2 and SnO2 (110) surfaces has been studied using first principle methods. The study investigated the role played by catechol in varying the electronic structure of TiO2 and SnO2 (110) surfaces. Results obtained showed that both the clean and catechol-terminated stoichiometric (110) TiO2 four layer surface had a band gap of 2.1 eV. The energy gap increased by 0.32 eV which represents an 18 % increment from 1.7 eV for clean stoichiometric TiO2 to 2.02 eV following adsorption of catechol molecule on the TiO2 (110) rutile 5-layer surface. The highest occupied molecular orbital (HOMO) in the four and five layered catechol terminated TiO2 (110) surfaces was found to be about 1 eV, above the valence band maximum edge but in SnO2 it nearly overlapped with bottom of conduction band. The lowest unoccupied molecular orbital (LUMO) in both TiO2 and SnO2 surfaces was located about 3 eV above the conduction band minimum, while the band gap of the molecule was in the range of 4.0 eV. The presence of catechol related C-2p orbitals within the energy gap and conduction band suggests that the energy level alignment of catechol adsorbed onto TiO2 suits the electron transfer processes that occur in DSSCs. The overlap of fermi level and closeness of catechol’s HOMO to conduction band minimum in catechol bound (110) rutile SnO2 surface shows that the surface may become conductive and hence, inappropriate for photocatalytic applications.