Origin of the increased photocatalytic performance of TiO₂ nanocrystal composed of pure core and heavily nitrogen-doped shell: a theoretical study.

We have carried out a theoretical study to explain the photocatalytic performance of the newly synthesized special core (pure TiO2)-shell (heavily nitrogen (N)-doped TiO2) structure of TiO2 nanocrystal using advanced first-principles calculations. The conventional N doping models by maximizing the mutual distances between dopants are found to only introduce localized gap states irrespective of doping concentrations, which is in agreement with previous theoretical results but cannot explain the experimental results. In comparison, the electronically coupled N doping of TiO2, which is almost as stable as the conventional doping models and generally overlooked in previous works, can not only narrow the overall band gap but also decrease the carrier recombination rate. In particular, in the special core-shell structure of TiO2 nanocrystal, perfect type-II-like homojunction is formed, which can further decrease the carrier recombination rate. The present study conclusively accounts for the recent experimental results and indicates that the final electronic structures of doping system are very sensitive to the models used to conduct calculations, which can rationalize the distinct conclusions about N-doped TiO2 in previous theoretical works.