Density Functional Theory Study and Photocatalytic Activity of ZnO/N-Doped TiO2 Heterojunctions

Abstract

The beneficial effects on ZnO and TiO2 photocatalytic activity resulting from a suitable combination of doping and heterojunctions were proposed and investigated by a synergistic experimental and theoretical study. In detail, the ZnO/N-doped TiO2 heterojunction was synthesized and tested in the photocatalytic degradation of atrazine under UV and visible light. Wide-angle X-ray diffraction (WAXD) analysis reveals the presence of both ZnO and TiO2 crystalline phases in the heterojunction. UV–vis DRS analysis shows that the simultaneous presence of ZnO and N-doped TiO2 in the heterojunction results in a slight increase in the band gap value in the UV region , while the shoulder in the visible region typically of bare N-doped TiO2 is preserved. On the basis of these experimental observations, density functional theory (DFT) calculations were carried out to model both the N-doped TiO2 structure and to rationalize the experimental decrease in the band gap energy. The DFT modeling of band alignment of the ZnO/N-doped TiO2 heterojunction allowed us to define the “minimal band gap” (MBG), corresponding to the interface gap, which resulted in smaller band gap energy than the two separate semiconductor band gaps. Furthermore, the DFT modeling of the electronic structure of N-doped TiO2 predicted the existence of additional energy levels between the TiO2 valence and conduction bands, which allowed us to justify the activity of the heterojunction under visible light irradiation. Photocatalytic tests showed that the ZnO/N-doped TiO2 heterojunction performance was better than that of both N-doped TiO2 and ZnO alone, allowing us to achieve almost complete atrazine degradation under UV light irradiation, whereas 24% of atrazine degradation was achieved after 180 min of visible light irradiation. Finally, the photocatalytic results achieved by using scavenger molecules for reactive oxygen species showed that ·OH and ·O2– are both reactive species in atrazine photocatalytic degradation under UV irradiation, while ·OH is responsible for the photocatalytic processes under visible irradiation. DFT modeling, validated by these results, finally allowed us to define a model of the band alignment and photogenerated charge-transfer mechanism for the ZnO/N-doped TiO2 heterojunction.