Trap States in Reduced Colloidal Titanium Dioxide Nanoparticles Have Different Proton Stoichiometries.

Added electrons and holes in semiconducting (nano)materials typically occupy "trap states," which often determine their photophysical properties and chemical reactivity. However, trap states are usually ill-defined, with few insights into their stoichiometry or structure. Our laboratory previously reported that aqueous colloidal TiO nanoparticles prepared from TiCl + HO have two classes of electron trap states, termed and . Herein, we show that the formation of from oxidized TiO requires 1 + 1H, while requires 1 + 2H. The two states are in a protic equilibrium, ⇌ + H, with = 2.65 mM. The states in the TiO NPs behave just like a soluble molecular acid with this as their , as supported by solvent isotope studies. Because the trap states have different compositions, their population and depopulation occur with the making and breaking of chemical bonds and not (as commonly assumed) just by the movement of electrons. In addition, the direct observation of a 2H/1 trap state contradicts the emerging H atom transfer (1H/1 ) paradigm for oxide/solution interfaces. Finally, this work emphasizes the importance of chemical stoichiometries, not just electronic energies, in understanding and directing the reactivity at solid/solution interfaces.