“Self-sensitized” TiO2 photocatalysts sensitized by TiO2 nanocrystals with quantum structures were successfully prepared via a solvothermal process and a facile subsequent treatment process. We calculated an optimized structure by using a pseudopotential Hartree–Fock method and investigated the structure using X-ray diffraction, scanning, and transmission electron microscopy, and UV–Vis spectrophotometer. The self-sensitized products exhibit an extremely high photocatalytic activity due to their unique optical properties. To discuss the optical properties of the 3-D nanoparticles sensitized by the TiO2 nanocrystals, we proposed a model of the electron band structure of the as-prepared products to give a reasonable explanation for such a high photocatalytic activity.