The Bi can be employed as a charge transfer medium between different semiconductors because it possesses good conductivity [
66]. When Bi is coupled with semiconductors, Bi can transfer photoinduced electrons from one semiconductor conduction band to another semiconductor conduction band as a charge mediator, as shown in
Fig. 2a. The presence of Bi accelerates the migration speed of photogenerated charges, thereby significantly increasing the removal efficiency of pollutants, as shown in the photocatalytic systems of CdS QDs/Bi/Bi
2WO
6 [
67], Bi/
α-Bi
2O
3/g-C
3N
4 [
68] and Bi
2MoO
6/Bi/TiO
2 [
69]. Moreover, the introduction of Bi can also constitute an all-solid-sate Z-Scheme heterojunction (
Fig. 2b). The Z-Scheme migration mechanism of carriers not only facilitates the spatial separation of photoexcited electron-hole pairs, but also retains strong redox capacity [
17]. Xu
et al. [
23] constructed a ternary Cu
2O/Bi/Bi
2MoO
6 Z-Scheme heterojunction system by combining Bi spheres with Cu
2O/Bi
2MoO
6 hollow spheres. The removal efficiency of sulfadiazine (SDZ) and Ni(Ⅱ) reached 98.6% and 93.2% after 100 min and 60 min over this composite under visible light irradiation, respectively. In a mixed system containing organic pollutants and heavy metal ions, the Cu
2O/Bi/Bi
2MoO
6 Zscheme heterojunction removed both contaminants at the same time, and exhibited superior photocatalytic performance compared to Bi
2MoO
6. The significant increase of photocatalytic activity was ascribed to the effective divorcement of photogenerated carriers in the ternary system. In detail, the photoinduced electrons on the conduction band of Bi
2MoO
6 easily migrated to Bi. The strengthened electric field around the Bi spheres facilitated the continuous migration of electrons to the valence band of Cu
2O, and then recombined with photoinduced holes. In the carrier transmission pathway of the ternary Cu
2O/Bi/Bi
2MoO
6 composite, Bi played a role of an electronic medium. Deng
et al. [
66] synthesized graphene-functionalized Zscheme heterojunction through utilizing Bi as a bridge to connect BiOCl and Bi
2O
3. This multi-component system showed the highest photocatalytic activity (99.7%) under visible light in terms of 2-nitrophenol removal. The synergistic effect of multi-channel charge transport (Bi-bridge and rGO) and efficient charge separation was the key for the increased activity. The photoexcited electrons generated by Bi
2O
3 were transferred to the valence band of BiOCl with oxygen vacancies through the Bi-bridge. The holes with strong oxidizing capability remaining on Bi
2O
3 can oxidize 2-nitrophenol. The removal efficiency of chemical oxygen demand (COD) in actual industrial wastewater reached 70.3% with the addition of 1 mL 30% H
2O
2 in BiOCl-Bi-Bi
2O
3/rGO system under visible light for 11 h. In the heterojunction structure containing Bi, Bi plays the part of charge transport bridges. The similar carrier migration mechanisms have also been reported in other systems, such as Bi-Bi
2MoO
6 nanosheet/CdS-diethylenetriamine [
70], Bi/BiPO
4/Bi
2WO
6 [
71] and Bi-BiOCl/AgCl [
72]. The Bi metal has the potential of replacing precious metals as a charge transport channel to enhance separation efficiency of electron-hole pairs.