Effect of the Structure of Small Anatase Nanoparticles on the Localization of Photogenerated Charge Carriers

Abstract

The structure of nanoparticles and the spatial arrangement of photogenerated thermalized charge carriers are studied for a series of isomers of small anatase nanoparticles (TiO₂)₂₉(H₂O)₄, (TiO₂)₇₀(H₂O)₄, and (TiO₂)₇₀ with faces (001) and (101) on the surface. It is shown that the location of surface hydroxyl groups and their replacement by surface oxygen atoms affect the nature and degree of deformation of the nanoparticle structure. The location of the boundary orbitals depends both on the size of the nanoparticles and on the location of the hydroxyl groups, as well as on the degree of dehydroxylation, which leads to the replacement of the hydroxyl groups by the surface oxygen atoms. In the case of a certain arrangement of hydroxyl groups or surface oxygen atoms, uncharged small stoichiometric anatase nanoparticles begin to absorb light in the visible region of the spectrum (the band gap width E_g decreasing to 2.25 eV). This is associated with the energy levels at the edge of the band gap near the valence band and the conduction band.