Hydrophobic signature on TiO2 nanoparticles in liquid water

Understanding of structural preferences and dynamics of water at the TiO2–H2O interface is fundamentally impor- tant in the perspective of broad range of TiO2 applications that extend from bio-adsorption for medical devices, photo- catalysis for energy production, to coatings with remarkable self-cleaning and anti-fogging properties. However, current knowledge on these systems under full hydration result almost entirely from modelling rather than direct experimental observations. Surface-sensitive techniques like electron microscopy (EM) or X-ray photoelectron spectroscopy (XPS) operate under high vacuum, therefore do not provide insights into phenomena underlying the real catalytic and bio-adsorption processes that take place under wet conditions. Herein, atomic-level characterization of TiO2 nanoparticles surface under full hydration was performed by a combination of 1H magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics density functional theory (MD-DFT) simulations. Thanks to partial proton to deuteron exchange and fast MAS, resolved NMR signals from OH/H2O species at the TiO2–H2O interface could be observed and interpreted with chemical shifts calculations performed on system evolution trajectory frames. Presence of the recently debated atmospheric carboxylic acids was confirmed, however, the amount of these was small compared to OH/H2O surface species. Surface protonation and evolution of hydrogen-bonded network are shown to govern the surface chemistry of TiO2 under wet conditions.