Influence of Nitrogen Doping on the Defect Formation and Surface Properties of ${\mathrm{TiO}}_2$ Rutile and Anatase

Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of ${\mathrm{TiO}}_2$ rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N $2p$ states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti $3d$ band gap states at elevated temperatures. The increased O vacancy formation triggers the $1\times 2$ reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.

Figure 1

XPS of pristine (undoped) and N-doped and annealed rutile-${\mathrm{TiO}}_2$.

Figure 2

UP spectra of undoped (pristine) and N-doped ${\mathrm{TiO}}_2$ after room temperature N implantation and annealing for (a) rutile (110) and (b) anatase (101). The band gap region of anatase is magnified in (c), and the photon-energy dependence of two gap state features displayed in (d).

Figure 3

STM images of rutile, (a) pristine and (b) after N implantation and vacuum annealing, and anatase (101) (c),(e) pristine and (d),(f) N implanted/annealed. In (e), (f), and (g), the unit cell is indicated. In the high resolution STM images, the bright protrusions are assigned to the bridging twofold oxygen atoms. The model in (g) shows an anatase (101) surface with the potential position of a surface N dopant atom.