Effects of hydroxylation on the acidic and basic strengths of anatase TiO2 surfaces

Titanium dioxide (TiO2) is one of prototype, water-splitting catalysts, while its catalytic performance is strongly affected by the presence of water molecules or hydroxyl groups on the surface. The adsorption of water on the TiO2(101) and TiO2(001) surfaces at different water/hydroxyl coverages was firstly studied using density functional theory (DFT) calculations. It has been found that the surface hydroxylation state has a significant influence on water adsorption behaviour. On the basis of calculated Gibbs free surface energies as a function of temperature and hydroxylation extent, the fully dehydrated TiO2(101) and TiO2(001) surfaces will be reached at 500 and 1000 K, respectively. In the present work, BF3 and NH3 were used as probe molecules to study the Lewis acidic and Brønsted basic strengths of TiO2(101) and TiO2(001) surfaces under different hydroxylation conditions. As the hydroxyl coverage increases, the Brønsted basic strength becomes stronger while the Lewis acidic strength is weakened. Finally, it is noted that the TiO2 surface is slightly oxidised at low hydroxyl coverage but being reduced at high hydroxyl coverage.