Origin of Photocatalytic Activity in Ti⁴⁺/Ti³⁺ Core-Shell Titanium Oxide Nanocrystals

Colored titanium dioxide (TiO₂) nanocrystals have been studied intensively in recent years due to their substantial solar-driven photocatalytic activities. Because of the scarcity of information correlated among atomic structure, electronic structure, and photocatalytic property from isolated nanoparticles, the origin of photocatalytic activity of colored TiO₂ from microscopic and electronic level remains intensely debated. Here we demonstrated the origin of photocatalytic activity in Ti⁴⁺/Ti³⁺ core-shell TiO₂ nanocrystals mainly by a combination of high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and photo-Kelvin probe force microscopy (KPFM). The thickness of the Ti³⁺ shell layer increases with the annealing temperature, meanwhile the O/Ti atomic ratio of the outermost layer decrease. The differences in interplanar spacing between the surface and the subsurface generate strain effect, which increase carrier mobility and ultimately improve catalytic performance. Quantum size effect and specific surface area also have a big impact on catalytic performance. A prerequisite is to keep a good balance between the Ti⁴⁺/Ti³⁺ core-shell structure and the particle size to optimize its photocatalytic properties. By employing Au as a reference, we were able to estimate the Fermi levels of TiO₂ before and after illumination. The amplitude of the Fermi level upshift unambiguously relates with the photocatalytic activity. The largest Fermi level upshift under illumination means generating the most efficient photoinduced carries to perform a highly catalytic reaction, carried out by optimizing the electronic structure with appropriate oxygen vacancies. But too many oxygen vacancies increase the recombination center and reduce the concentration of photoinduced electrons, which can also be observed from the photo-KPFM result.