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 TiO₂ nanoparticles prepared from TiCl₄ + H₂O have two classes of electron trap states, termed Blue and Red. Herein, we show that the formation of Red from oxidized TiO₂ requires 1e ^- + 1H⁺, while Blue requires 1e ^- + 2H⁺. The two states are in a protic equilibrium, Blue ⇌ Red + H⁺, with K _eq = 2.65 mM. The Blue states in the TiO₂ NPs behave just like a soluble molecular acid with this K _eq as their K _a, 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⁺/1e ^- trap state contradicts the emerging H atom transfer (1H⁺/1e ^-) 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.