Effects of H-, N-, and (H, N)-Doping on the Photocatalytic Activity of TiO2

We present a first-principles study on the phase-dependent doping effects on the photocatalytic activity of TiO2. Three TiO2 polymorphs-anatase, rutile, and brookite-doped with H, N, or both of them were systematically investigated to understand the mechanism for the improvement of photocatalytic performance induced by the doping. Our calculations revealed that the doping effect; on the electronic properties of TiO2 are phase-dependent. The n-type TiO2 was formed by the introduction of hydrogen interstitial or substitution, although interstitial state is stable for hydrogen in all of the TiO2 structures, revealing that hydrogen is one of important sources for the n-type character of TiO2. All of the bandgaps of TiO2 polymorphs were slightly narrowed by interstitial hydrogen, while only the bandgaps of brookite and rutile TiO2 were narrowed by substitutional hydrogen. Nitrogen also prefers the interstitial state in all of the TiO2 polymorphs. However, the effects of interstitial nitrogen on the electronic properties of TiO2 are phase-dependent: anatase and brookite TiO2 with interstitial nitrogen keep intrinsic and their bandgaps are unchanged, while rutile TiO2 with interstitial nitrogen shows p-type conductivity and bandgap narrowing. Our calculation further predicted that nitrogen substitution is enhanced in the presence of hydrogen interstitial bonded to nitrogen due to the reduced formation energy. The narrowing of the bandgaps, the improved mobility, intrinsic semiconducting characters, and reduced trapping centers in anatase and brookite TiO2 by the (H, N)-codoping result in the improvement of their photocatalytic performance because of the enhancement of absorption in visible light and reduction of recombination centers, while the performance of rutile TiO2 has not been improved because its bandgap is almost unaffected by the codoping.