Ionic liquid ordering at an oxide surface: 1-butyl-3-methylimidazolium tetrafluoroborate, [C4C1Im][BF4], interaction with the anatase TiO2(101) surface

The interaction of the ionic liquid [C4C1Im][BF4] with anatase TiO2, a model photoanode material, has been studied using a combination of synchrotron radiation photoelectron spectroscopy and Near-Edge X-ray Absorption Fine Structure spectroscopy. The system is of interest as a model for fundamental electrolyte-electrode and dye-sensitized solar cells. The initial interaction involves degradation of the [BF4]- anion, resulting in incorporation of F into O-vacancies in the anatase surface. At low coverages, [C4C1Im][BF4] is found to order at the anatase(101) surface via electrostatic attraction, with the imidazolium ring oriented 32 ± 4 ° from the anatase TiO2 surface. As the coverage of ionic liquid increases, the influence of the oxide surface on the topmost layers is reduced and the ordering is lost.