Surface Characterization of TiO₂ Polymorphic Nanocrystals through ¹H-TD-NMR

Nanocrystals (NCs) surface characterization is a fundamental step for understanding the physical and chemical phenomena involved at the nanoscale. Surface energy and chemistry depend on particle size and composition, and, in turn, determine the interaction of NCs with the surrounding environment, their properties and stability, and the feasibility of nanocomposites. This work aims at extracting more information on the surface of different titanium dioxide polymorphs using ¹H-TD-NMR of water. Taking advantage of the interaction between water molecules and titanium dioxide NCs, it is possible to correlate the proton transverse relaxation times ( T₂) as the function of the concentration and the specific surface area (δ_p· C_m) and use it as an indicator of the crystal phase. Examples of three different crystals phase, rutile, anatase, and brookite, have been finely characterized and their behavior in water solution have been studied with TD-NMR. The results show a linear correlation between relaxivity ( R₂) and their concentration C_m. The resulting slopes, after normalization for the specific surface, represent the surface/water interaction and range from 1.28 g m^-2 s^-1 of 50 nm rutile nanocrystals to 0.52 for similar sized brookite. Even higher slopes (1.85) characterize smaller rutile NCs, in qualitative accordance with the trends of surface energy. Thanks to proton relaxation phenomena that occur at the NCs surface, it is possible to differentiate the crystal phase and the specific surface area of titanium dioxide polymorphs in water solution.