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 WO 3 /TiO 2 composites with different weight ratios were prepared. The WO 3 /TiO 2 composites were characterized in detail. The result showed that regardless of whether the primary part of WO 3 /TiO 2 composites is TiO 2 or WO 3, the photocatalytic activities of WO 3 /TiO 2 are much higher than those of pure TiO 2 or WO 3 . The reason may be the generation of a relative p–n heterojunction between WO 3 and TiO 2 . 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 WO 3 and TiO 2 . Thus, the transfer of the photogenerated charge carriers adopts a Z-scheme system in the WO 3 /TiO 2 heterojunctions. The accumulated photogenerated electrons in the CB of TiO 2 with more negative potential can reduce O 2 to a superoxide radical ( • O 2 – ), and the photogenerated holes in the VB of WO 3 with more positive potential may oxidate H 2 O (or OH – ) into a hydroxyl radical ( • OH). The photocatalytic activities of the WO 3 /TiO 2 heterojunctions are significantly promoted. The transfer mechanisms and natural law for the WO 3 /TiO 2 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.