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

Abstract

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.