Understanding the mechanism of the heterojunction is an important step towards controllable and tunable interfaces for photocatalytic and photovoltaic based devices. To this aim, we propose a thorough study of a double heterostructure system consisting of two semiconductors with large band gap, namely, wurtzite ZnO and anatase TiO2. We demonstrate via first-principle calculations two stable configurations of ZnO/TiO2 interfaces. Our structural analysis provides a key information on the nature of the complex interface and lattice distortions occurring when combining these materials. The study of the electronic properties of the sandwich nanostructure TiO2/ZnO/TiO2 reveals that conduction band arises mainly from Ti3d orbitals, while valence band is maintained by O2p of ZnO, and that the trapped states within the gap region frequent in single heterostructure are substantially reduced in the double interface system. Moreover, our work explains the origin of certain optical transitions observed in the experimental studies. Unexpectedly, as a consequence of different bond distortions, the results on the band alignments show electron accumulation in the left shell of TiO2 rather than the right one. Such behavior provides more choice for the sensitization and functionalization of TiO2 surfaces.* Our theoretical deductions are in agreement with the HRTEM image of the work done by Matt Law et al. [11]. ¶ It should be known that the appropriate choice of U depends on several parameters namely: the DFT code used, the choice of basis sets, pseudopotentials, and K-points mesh [42]. + It should be noted here that we also made relaxations for the two interfaces separately, i.e., for TiO2/ZnO (left) and ZnO/TiO2 (right). # Even if we can have an idea about band alignments around valence and conduction bands from results obtained with GGA+U functional, it is necessary to confirm or refute it by using another method namely double-macroscopic average technique [53,54,25].
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