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.

Oxide Chemistry Below the Surface: Although metal oxides, such as titanium dioxide (TiO ₂ ), are used for catalytic oxidation reactions and photocatalysis, the O ₂ does not react directly with substrates. Vacancies in the surface region of the TiO ₂ rutile phase can transfer a negative charge to adsorbed O ₂ to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO ₂ particles—subsurface vacancies form. Setvin et al. (p. 988 ) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O ₂ ⁻ into a peroxo species and a bridging O ₂ dimer.

A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in ${\mathrm{TiO}}_2$ rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior.