Z-Scheme versus type-II junction in g-C3N4/TiO2 and g-C3N4/SrTiO3/TiO2 heterostructures

Abstract

Composites formed by titania and graphitic C₃N₄, such as C₃N₄/TiO₂ and C₃N₄/SrTiO₃/TiO₂ heterojunctions, have attracted a considerable interest in recent years as efficient photocatalysts. Experimental evidence shows that the charge carriers' favorable migration path in g-C₃N₄/TiO₂ is opposite to what predicted by looking at the band edges alignment of the two units, and follows a direct Z-scheme. On the other hand, the typical migration scheme predicted by the type II band edges alignment is found in the g-C₃N₄/SrTiO₃/TiO₂ heterojunction, where a “buffer” layer of SrTiO₃ is present between the C₃N₄ and the TiO₂ phases. In this paper we provide a rationalization of the different behavior of the two heterojunctions based on density functional theory calculations using a hybrid functional approach. The analysis of the nature, stability, interface dipole, and band edges alignment of both g-C₃N₄/TiO₂ and g-C₃N₄/SrTiO₃/TiO₂ interfaces shows that the formation of the ternary system has the effect to reverse the sign of the interface dipole, thus reversing the junction nature in the two systems.