Growth and Organization of an Organic Molecular Mono layer on TiO2: Catechol on Anatase (101)

Anatase TiO2 is a widely used photocatalytic material, and catechol (1,2-benzendiol) is a model organic sensitizer for dye-sensitized solar cells. The growth and the organization of a catecholate monolayer on the anatase (101) surface were investigated with scanning tunneling microscopy and density functional theory calculations. Isolated molecules adsorb preferentially at steps. On anatase terraces, monodentate ('D1') and bidentate ('D2') conformations are both present in the dilute limit, and frequent interconversions can take place between these two species. A D1 catechol is mobile at room temperature and can explore the most favorable surface adsorption sites, whereas D2 is essentially immobile. When a D1 molecule arrives in proximity of another adsorbed catechol in an adjacent row, it is energetically convenient for them to pair up in nearest-neighbor positions taking a D2-D2 or D2-D1 configuration. This intermolecular interaction, which is largely substrate mediated, causes the formation of one-dimensional catecholate islands that can change in shape but are stable to break-up. The change between D1 and D2 conformations drives both the dynamics and the energetics of this model system and is possibly of importance in the functionalization of dye-sensitized solar cells.