Theoretical study on transport-scheme conversion of g-C3N4/TiO2 heterojunctions by oxygen vacancies

The energy band alignments, charge transfers and distributions of square of electronic wavefunctions of g-C3N4/(1 0 1)-TiO2 and g-C3N4/(0 0 1)-TiO2 heterojunctions with oxygen vacancies are investigated with first principles calculations. Our theoretical results show that TiO2 and g-C3N4 can contact with each other to form a van der Waals heterojunction. Three heterostructures for g-C3N4/(1 0 1)-TiO2, g-C3N4/VO-(1 0 1)-TiO2, and g-C3N4/(0 0 1)-TiO2 are Z-scheme heterojunctions while g-C3N4/VO-(0 0 1)-TiO2 is type-II heterojunction. The energy levels of (0 0 1) surface is higher than that of (1 0 1) surface in titanium dioxide, and the donor level formed by oxygen vacancy further increases its Fermi level. So in the process of forming heterojunction, the electrons will transfer from titanium dioxide to carbon nitride instead of from carbon nitride to titanium dioxide to form type-II heterojunction for g-C3N4/VO-(0 0 1)-TiO2. There exist several localized and delocalized shallow defect levels in energy gap of the g-C3N4/VO-(1 0 1)-TiO2 heterojunction. Its wavefunction of delocalized shallow defect level can span both sides of the heterojunction and play an important role in promoting photogenerated electrons migration from titanium dioxide to carbon nitride. However, there are partially localized deep defect levels in the energy gap of g-C3N4/VO-(0 0 1)-TiO2 heterojunction, whose wavefunctions only distribute over titania. They only act as the recombination centers of electron-hole pairs and makes the photocatalytic effect worse. Our calculations reveal the effects of delocalized wavefunction of shallow defect level covering both sides of heterojunction on photocatalytic activity in g-C3N4/TiO2 heterojunctions and provide a way to convert the transfer scheme of heterojunctions by introducing impurities.