**4. Conclusions**

In summary, we have developed a facile solvothermal approach to prepare n-p Bi2O2CO3/α-Bi2O3 heterojunction microtubes. Plasmonic Ag NPs were loaded onto the Bi2O2CO3/α-Bi2O3 microtubes using a simple photo-deposition process, to construct an Ag/Bi2O2CO3/α-Bi2O3 ternary photocatalytic system. This Ag/Bi2O2CO3/α-Bi2O3 ternary system showed much higher photocatalytic activity than α-Bi2O3 and Bi2O2CO3/<sup>α</sup>-Bi2O3. Under visible light irradiation, the well-defined interfaces between Bi2O2CO3 and α-Bi2O3 in the heterojunctions due to the in situ carbonation of α-Bi2O3 on the surface into Bi2O2CO3, facilitate the transfer of photoinduced electrons from the CB of α-Bi2O3 to that of Bi2O2CO3. Meanwhile, the superior electron conductivity of Ag NPs, the heterojunction at the interface between two semiconductors and Ag NPs, and the local electromagnetic field induced by the surface plasmon resonance effect of Ag NPs, further promote the transfer of photoinduced electrons and suppress the recombination of hole/electron pairs, leaving more holes in the VB of α-Bi2O3 to produce more ·OH for photodegradation of MO. After the photoinduced electrons in the CB of α-Bi2O3 and Bi2O2CO3 that cannot form ·O2<sup>−</sup> are transferred to Ag NPs, they combine with O2 to form ·O2<sup>−</sup>, which means that loading of Ag NPs onto Bi2O2CO3/α-Bi2O3 creates new reaction active sites and a new reactive species ·O2<sup>−</sup> for photocatalysis, compared with Bi2O2CO3/α-Bi2O3.

**Author Contributions:** Conceptualization, H.L. and L.Z.; formal analysis and data curation, X.L., Z.L., and G.H.; writing—original draft preparation, H.L., X.L., and G.H.; writing—review and editing, all authors; funding acquisition, L.Z. and H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The Talent Project of Yulin Normal University (No. G2020ZK14).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** Authors acknowledge the financial support from the Talent Project of Yulin Normal University (No. G2020ZK14).

**Conflicts of Interest:** The authors declare no conflict of interest.
