Trap States in\nReduced Colloidal Titanium Dioxide\nNanoparticles Have Different Proton Stoichiometries

Added electrons and holes in semiconducting (nano)materials\ntypically\noccupy “trap states,” which often determine their photophysical\nproperties and chemical reactivity. However, trap states are usually\nill-defined, with few insights into their stoichiometry or structure.\nOur laboratory previously reported that aqueous colloidal TiO<sub>2</sub> nanoparticles prepared from TiCl<sub>4</sub> + H<sub>2</sub>O have two classes of electron trap states, termed <b>Blue</b> and <b>Red</b>. Herein, we show that the formation of <b>Red</b> from oxidized TiO<sub>2</sub> requires 1<i>e</i><sup>–</sup> + 1H<sup>+</sup>, while <b>Blue</b> requires\n1<i>e</i><sup>–</sup> + <u>2</u>H<sup>+</sup>. The two states are in a protic equilibrium, <b>Blue</b> ⇌ <b>Red</b> + H<sup>+</sup>, with <i>K</i><sub>eq</sub> = 2.65 mM. The <b>Blue</b> states in the TiO<sub>2</sub> NPs behave just like a soluble molecular acid with this <i>K</i><sub>eq</sub> as their <i>K</i><sub>a</sub>,\nas supported by solvent isotope studies. Because the trap states have\ndifferent compositions, their population and depopulation occur with\nthe making and breaking of chemical bonds and not (as commonly assumed)\njust by the movement of electrons. In addition, the direct observation\nof a <u>2</u>H<sup>+</sup>/1<i>e</i><sup>–</sup> trap state contradicts the emerging H atom transfer\n(1H<sup>+</sup>/1<i>e</i><sup>–</sup>) paradigm for\noxide/solution interfaces. Finally, this work emphasizes the importance\nof chemical stoichiometries, not just electronic energies, in understanding\nand directing the reactivity at solid/solution interfaces.