Adsorption and electron injection of the N3 metal-organic dye on the TiO2 rutile (110) surface

Metal-organic ruthenium-based dyes are often used as a source of photogenerated electrons in dye-sensitised solar cells and photocatalysis. Here, we study the relationship between adsorption geometry and electron injection properties of one of the most successful metal-organic dyes, N3 (cis-bis(isothiocyanato)-bis(4,4'-dicarboxy-2,2'-bipyridyl)-ruthenium(II)), on the TiO2 rutile (110) surface. We systematically construct all possible adsorption configurations of the N3 molecule on this surface. By combining density-functional theory calculations and electron transfer calculations, we find that a large number of adsorption configurations are possible - more than ten structures, which differ in the number of carboxylic and thiocyanate groups adsorbed and in the adsorption mode of the carboxylic groups, have similar adsorption energies and similar electron injection times. Therefore, the observed fast electron injection from this dye may originate either from one adsorption configuration or from several co-existing configurations. Our results suggest that related substituted metal-organic dyes with fewer anchoring groups will also have good electron injection properties, even if only a small subset of adsorption configurations is available for them.