Insight into the Transfer Mechanisms of Photogenerated Carriers for Heterojunction Photocatalysts with the Analogous Positions of Valence Band and Conduction Band: A Case Study of ZnO/TiO2

In the paper, the synthesis of ZnO/TiO2 nanocomposites with different main parts (TiO2 or ZnO) is studied. When TiO2 is the main part of the ZnO/TiO2 heterojunction photocatalysts (ZnO/TiO2), the photocatalytic activity is decreased rapidly with the increase of the amount of ZnO. The reason may be attributed to the relative p–n junction (p-ZnO/n-TiO2) produced between ZnO and TiO2. The migration directions of the electrons and holes in the relative p–n junction are opposite to the transfer directions of the photogenerated electrons and holes in the valence band (VB) and conduction band (CB). However, when ZnO is the primary part of the heterojunction photocatalysts (TiO2/ZnO), the photocatalytic activity of the samples increases with the increase of the TiO2 amount up to 5% (95% ZnO/TiO2). The reason may be that the migration directions of the electrons and holes in the relative p–n junction (p-TiO2/n-ZnO) are the same as the transfer directions of the photoexcited electrons and holes in VB and CB between the two semiconductors. It is proposed that the conductivity of the heterojunction photocatalyst will be changed with the difference of content for the two semiconductors, which in turn affects the migration directions of the electrons and holes in the heterojunctions and their photocatalytic activity.