DFT Investigation of Substitutional and Interstitial Nitrogen-Doping Effects on a ZnO(100)–TiO2(101) Heterojunction

Density Functional Theory (DFT) calculations have been performed to investigate the structural and electronic properties of the ZnO(wurtzite)–ATiO2(anatase) heterojunction in the absence and presence of substitutional, interstitial nitrogen (N) doping and oxygen vacancies (OV). We report a detailed study of the interactions between the two nonpolar ZnO and TiO2 surfaces and on the role of N-doping and oxygen vacancies, which are decisive for improving the photocatalytic activity of the heterojunction. Our calculations show that substitutional N-doping is favored in the ATiO2 portion, whereas the interstitial one is favored in the ZnO region of the interface. Both substitutional and interstitial N-doped sites (i) induce gap states that act as deep electronic traps improving the charge separation and delaying electron–hole recombination, (ii) facilitate the OV formation causing a decrease in the formation energy (EFORM), and (iii) do not affect the band alignment when compared to the undoped analogue system. The presented results shed light on the N-doping effect on the electronic structure of the ZnO(100)–TiO2(101) heterojunction and how N-doping improves its photocatalytic properties.