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 O-2 to a superoxide radical (O-center dot(2)-), and the photogenerated holes in the VB of WO3 with more positive potential may oxidate H2O (or OH-) into a hydroxyl radical ((OH)-O-center dot). 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.