Direct quantification of oxygen vacancy on the TiO2 surface by 31P solid-state NMR

The surface oxygen vacancy of metal oxide generally plays a vital role in heterogeneous catalysis. However, because of the complicated structure of the metal oxide surface, it is not easy to distinguish surface oxygen vacancy quantitatively. Here, we utilized 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy with trimethylphosphine (TMP) as the probe molecule to study the anatase TiO2 treated at different temperatures under reductive, neutral, and oxidative atmospheres. By combining this with density functional theory (DFT) calculations, we successfully identified the surface oxygen vacancy site (surface Tivo) from the surface unsaturated metal site (surface Tinc, where nc is the number of coordination) and the surface with a weak electrostatic interaction (Sδ+). In addition, we determined the intrinsic photocatalytic efficiencies (H2 yield) of the above surface sites as surface Tivo ≥ Sδ+ > Ti5c in a water-splitting half-reaction. This study could provide a simple way to quantify the surface species of metal oxides and inspire the design of metal oxides with surface oxygen vacancies.