Effects of Water and Formic Acid Adsorption on the Electronic Structure of Anatase TiO2(101)

Abstract

Water increases the photocatalytic oxidation (PCO) and decomposition (PCD) rates of formic acid on TiO2. To identify possible electronic origins for the rate increases, the effects of adsorption of combinations of key adsorbates (hydrogen, hydroxyl, water, formic acid, formate) on anatase TiO2(101) were investigated using density functional theory. The adsorption site and strength of bonds formed play key roles in altering the electronic structure to affect reactivity. Adsorption of hydrogen and water through the twofold-coordinated oxygen (2c-O) atom decreased the reducing power of the surface, and adsorption of hydroxyls through the fivefold-coordinated titanium (5c-Ti) atom increased the reducing power. The adsorption of water, formic acid, and monodentate and bidentate formate through the 2c-O and 5c-Ti atoms had varying effects on the positions of the valence and conduction band edges, depending on the strength of the bonds formed. Water coadsorption decreased the strength of the bonds between adsorbates and surface atoms, thus reducing the adsorbate’s effects. For monodentate formate, water coadsorption increased the reduction potential of the TiO2 surface, consistent with an increase in the photocatalytic reaction rate through a decrease in electron−hole recombination.