Water-Hydroxyl Interactions on Small Anatase Nanoparticles Prepared by the Hydrothermal Route

Modification of metal oxides' characteristics by decreasing the nanoparticles' crystal size is usually interpreted in terms of increasing quantum size effects and/or oxygen vacancy concentration. However, some properties of TiO2 nanoparticles, such as water adsorption strength on anatase or photoactivity for toluene mineralization in the gas phase, are optimized when the mean anatase crystal size is close to 6 nm, indicating the action of two opposed effects. Here, we show that these effects are originated by the increasing acidity of bridging hydroxyls with decreasing crystal size. Increasing acidity favors, first, water hydrogen bonding to bridging hydroxyls and, then, simultaneously to bridging and terminal hydroxyls of adjacent particles, favoring nanoparticle agglomeration and interfacial hydroxyls and water stability. A too strong acidic character of bridging hydroxyls favors proton exchange of stabilized hydronium ions with bridging O2- and terminal hydroxyls, facilitating hydroxyl recombination and crystal growth.