Hole distribution in nitrogen-doped TiO2 anatase nanoparticles

Hole distributions introduced by a N doping atom in N-doped TiO 2 anatase nanoparticles are studied with first principles calculations. A nitrogen atom substitutes an oxygen atom in a TiO 2 nanoparticle and introduces a hole in the particle. The empty state may be on the top of the valence band or inside the band gap as an impurity band. If the empty state is on the top of the valence band, the hole mainly distributes on the nitrogen atom and the two dangling atoms. The empty states on the dangling oxygen atoms introduced by the nitrogen doping atom can activate the dangling oxygen atoms and increase their oxidation ability. For example, the dangling oxygen atoms may oxidize absorbed small toxic molecules. The summation of the empty states on the nitrogen atom and the two dangling oxygen atoms is close to a constant and which is about one half of the total empty state of the particle for most of the substitutional configurations. Nitrogen atom can also generate an impurity band inside the band gap. This impurity band can make the N-doped anatase particle absorb the visible lights. Eventually, the photo-catalytic properties of anatase particles may be changed by the N-doping processes. Most of the dopant formation energies in the particles are lower than that in the bulk. So, nitrogen doping process maybe easier in the small particle sample than in the bulk materials.