Phase transfer of TiO₂ nanoparticles from water to ionic liquid triggered by phosphonic acid grafting

Controlling the interface between TiO₂ nanocrystals and ionic liquids is of high fundamental and applied interest for energy storage and conversion devices. Phase transfer of nanoparticles from a synthesis medium to a processing or an application medium plays a significant role in nanotechnology. Here we demonstrate that surface modification with phosphonic acids bearing cationic end-groups can trigger the phase transfer of TiO₂ nanoparticles from an aqueous sol to a typical water-immiscible ionic liquid, [Emim][NTf₂]. The transfer involves both the grafting of the phosphonic acid moiety and the exchange of the counter ion of the cationic end-group by NTf₂ anions, as demonstrated by solid-state NMR, elemental analysis and independent grafting and ion exchange experiments. Furthermore, the colloidal stability of the TiO₂ sols in [Emim][NTf₂] strongly depends on the hydrophobic character of the cationic end-groups.