Insight into the Transfer Mechanism of Photogenerated Carriers for WO₃/TiO₂ Heterojunction Photocatalysts: Is It the Transfer of Band–Band or Z-Scheme? Why?

The transfer mechanism of photoexcited charge carriers is always a hot topic in the photocatalysis research field. Coupling a photocatalyst with other photocatalysts is one of the most widely used strategies to realize effective transfer of the photogenerated carriers. In the study, a series of WO₃/TiO₂ composites with different weight ratios were prepared. The WO₃/TiO₂ composites were characterized in detail. The result showed that regardless of whether the primary part of WO₃/TiO₂ composites is TiO₂ or WO₃, the photocatalytic activities of WO₃/TiO₂ are much higher than those of pure TiO₂ or WO₃. The reason may be the generation of a relative p–n heterojunction between WO₃ and TiO₂. Under the effect of the built-in electric field, the transfer directions of the photogenerated charge carriers in the heterojunctions are opposite to the migration directions of the photogenerated charge carriers in the conduction band (CB) and valence band (VB) of WO₃ and TiO₂. Thus, the transfer of the photogenerated charge carriers adopts a Z-scheme system in the WO₃/TiO₂ heterojunctions. The accumulated photogenerated electrons in the CB of TiO₂ with more negative potential can reduce O₂ to a superoxide radical (•O₂–), and the photogenerated holes in the VB of WO₃ with more positive potential may oxidate H₂O (or OH–) into a hydroxyl radical (•OH). The photocatalytic activities of the WO₃/TiO₂ heterojunctions are significantly promoted. The transfer mechanisms and natural law for the WO₃/TiO₂ heterojunction photocatalysts were proved by physical and chemical methods. This work not only reveals the transfer mechanisms of photogenerated carriers and internal natural behavior of heterojunction photocatalysts, but also guides the design and constructing of composite photocatalysts, and thus has theoretical and practical significance.