Bi-isonicotinic acid on anatase (101): Insights from theory

The adsorption, on the TiO2 anatase (101) surface of the bi-isonicotinic acid (BINA) molecule has been investigated using density functional theory and a periodic approach. This molecule is of particular interest since it is the ligand used in coordination complexes for photovoltaic applications such as the so-called N3. Three different interaction modes (two bidentate dissociative modes and a monodentate molecular one) have been fully characterized at the geometric, energetic, and electronic levels. A fourth mode, corresponding to a chelating coordination, has been found to be unstable. Electronic properties have been rationalized by analyzing crystalline orbitals, and the effect of coverage and lateral interactions between adsorbates has been also investigated. Our results suggest a bridging bidentate, mode as the most stable one, the computed interaction energies being almost additive and directly related to the number of oxygen atoms involved in the interaction between the surface and the BINA molecule. Furthermore, the high flexibility of the latter has been evidenced, with rather high twists between its two pyridine rings without significant effect on the computed interaction energies, confirming the suitability of this ligand as an anchor group in such systems. Densities of states calculations revealed that, while the electronic coupling between the molecule and the substrate is strong, the upper part of the valence band is free of substrate states, and that no molecular states lie in the bottom of the conduction band for all adsorption modes. This suggests a favored electron promotion from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), in line with the classical chemical understanding of dye-sensitized solar cells, where optical excitation of the dye with visible light leads to excitation of the dye and injection of electrons into the conduction band of the semiconductor. Finally, the study of the reduced systems, by addition of an electron, revealed that spin densities are mainly located on the substrate, thus confirming that electron injection is favored in this system.