Pressure-enhanced surface interactions between nano-TiO2 and ionic liquid mixtures probed by high pressure IR spectroscopy

The pressure-dependent interactions between the ionic liquid mixture ([MPI][I-1.5]) and nano-TiO2 surfaces have been studied up to 2.5 GPa. The results of infrared spectroscopic profiles of [MPI][I-1.5] and [MPI][I-1.5]- nano-TiO2 indicated that no appreciable changes in the C-H stretching bands with the addition of nano-TiO2 were observed under ambient pressure. As the pressure was elevated to 0.7 GPa, the C-H stretching absorption of [MPI][I-1.5] underwent band-narrowing and red-shifts in frequency. In contrast to the results of [MPI][I-1.5], the spectra of [MPI][I-1.5]- nano- TiO2 do not show dramatic changes under high pressures. A possible explanation for this observation is the formation of certain pressure-enhanced C-H...nano-TiO2 interactions around the imidazolium C-H and alkyl C-H groups. As imidazolium C-H...I- is replaced by the weaker imidazolium C-H...polyiodide, the splitting of the imidazolium C-H stretching bands was observed. The experimental results indicate that both nano-TiO2 and polyiodides are capable of disturbing the self-assembly of ionic liquids. This study suggests the possibility to tune the efficiency of dye-sensitized solar cells via a high pressure method.