Preparation of TiO₂ nanoparticles coated with ionic liquids: a supramolecular approach

Coated TiO2 nanoparticles by dicationic imidazolium-based ionic liquids (ILs) were prepared and studied by differential scanning calorimetry (DSC), dynamic light scattering (DLS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). Three ILs with different hydrophobicity degrees and structural characteristics were used (IL-1, IL-2, and IL-3). The interaction between IL molecules and the TiO2 surface was analyzed in both solid state and in solution. The physical and chemical properties of coated nanoparticles (TiO2 + IL-1, TiO2 + IL-2, and TiO2 + IL-3) were compared to pure materials (TiO2, IL-1, IL-2, and IL-3) in order to evaluate the interaction between both components. Thermal behavior, diffraction pattern, and morphologic characteristics were evaluated in the solid state. It was observed that all mixtures (TiO2 + IL) showed different behavior from that detected for pure substances, which is an evidence of film formation. DLS experiments were conducted to determine film thickness on the TiO2 surface comparing the size (hydrodynamic radius, Rh) of pure TiO2 with coated nanoparticles (TiO2 + IL). Results showed the thickness of the film increased with hydrophobicity of the IL compound. TEM images support this observation. Finally, X-ray diffraction patterns showed that, in coated samples, no structural changes in TiO2 diffraction peaks were observed, which is related to the maintenance of the crystalline structure. On the contrary, ILs showed different diffraction patterns, which confirms the hypothesis of interactions happening between IL and the TiO2 nanoparticles surface.