Molecular Mechanisms of Photoinduced Oxygen Evolution, PL Emission, and Surface Roughening at Atomically Smooth (110) and (100) n-TiO2 (Rutile) Surfaces in Aqueous Acidic Solutions

The success in preparing atomically smooth and stable (110) and (100) TiO2 (rutile) surfaces, combined with in situ photoluminescence (PL) and photocurrent measurements as well as atomic force microscopic (AFM) inspection, has enabled us to make systematic studies on molecular mechanisms of oxygen photoevolution and related processes on TiO2 (rutile), which are important for solar water splitting and photocatalytic environmental cleaning. The studies have revealed that various surface processes and properties, such as the flat-band potential (U fb), the spectrum and intensity of the PL from a precursor of the oxygen photoevolution reaction, and photoinduced surface roughening, have all strong dependences on the atomic-level structure of the TiO2 surface. Importantly, all the results have been explained on the basis of our recently proposed new mechanism that the oxygen photoevolution reaction is initiated by a nucleophilic attack of an H2O molecule to a surface-trapped hole, thus giving confirmative evidence to it. The molecular mechanisms for photoinduced primary processes at the TiO2 surface, clarified in the present work, will provide a typical model for photoreactions on metal oxides in contact with aqueous solutions.