Abstract

It is known that when an n-type semiconductor and a p-type semiconductor (i.e., a p–n junction) are connected, an intrinsic electric field is formed due to the diffusion motion of majority carriers. The direction of this intrinsic electric field in the p–n junction runs from the n-type to the p-type semiconductor (n→p). If the migration directions of photogenerated charge carriers in the conduction band (CB) and valence band (VB) of the two contacting semiconductors align with the direction of the intrinsic electric field in the heterojunction, band-to-band transfer occurs. In experiments using TiO2-based composite photocatalysts, the heterojunction catalyst forms a structure analogous to a p–n junction relative to CdS/TiO2; however, due to differing carrier concentrations, TiO2 exhibits a p-type character while CdS shows an n-type character. Under the influence of the intrinsic electric field, photogenerated electrons migrate to the p-type TiO2 surface, while holes migrate to the n-type CdS surface. The migration directions of photogenerated electrons and holes in the CB and VB of both CdS and TiO2 match those observed in a typical p–n junction, confirming that the photocarrier migration mechanism in TiO2-dominated CdS/TiO2 systems follows a band-to-band transfer mechanism. When CdS serves as the dominant component, rapid recombination occurs between electrons in TiO2’s CB and holes in CdS’s VB, resulting in significant electron accumulation in TiO2’s CB and substantial hole generation in CdS’s VB. Electrons in the CB of TiO2, which carries a higher negative potential, reduce O2 to •O2−, while holes in the VB of CdS, possessing a higher positive potential, generate •OH−, thereby enhancing photocatalytic activity; thus, the photoexcited carrier transfer mechanism follows Scheme Z.

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