Exploring the Differences in the Photocatalytic Degradation Capacity and Carrier Migration Mechanism of Type-II Heterojunction g-C3N4/Ag2CO3 and Ag2CO3/Bi2WO6

Photocatalysis technology has demonstrated significant potential in addressing environmental pollution and clean energy challenges. The role of heterojunctions in enhancing the photocatalytic performance is crucial. This study successfully synthesizes two types of type-II heterojunction photocatalysts: g-C₃N₄/Ag₂CO₃ (CA) and Ag₂CO₃/Bi₂WO₆ (AB). Immediately after, according to the mass ratio of g-C₃N₄ in the g-C₃N₄/Ag₂CO₃ heterojunction, they are named CA-1, CA-3, CA-5, CA-7, CA-9, and CA-11. According to the mass ratio of Ag₂CO₃ in the Ag₂CO₃/Bi₂WO₆ heterojunction, they were named AB-1, AB-3, AB-5, AB-7, AB-9, and AB-11. The morphology, structure, and optical properties of the heterojunctions were characterized by SEM, TEM, XPS, and DRS. Experimental results indicate that both type-II Ag₂CO₃/Bi₂WO₆ and type-II g-C₃N₄/Ag₂CO₃ heterojunctions effectively degrade the antibiotic levofloxacin (LEV). Notably, during degradation experiments on various pollutants, the type-II CA-9 heterojunction exhibited superior degradation performance against quinolone antibiotics and azo dyes, whereas the type-II AB-9 heterojunction showed enhanced effectiveness in degrading quinolone antibiotics specifically. This behavior is explained through analyses based on the internal electric field and density functional theory (DFT). The conduction band and Fermi level of silver carbonate cannot be higher than those of Bi₂WO₆ at the same time. In contrast, g-C₃N₄ has a higher conduction band and Fermi level than silver carbonate. Based on the built-in electric field and conduction band distribution of the heterojunction, this paper innovatively names the situation where both the Fermi level and the conduction band can be higher than those of another material as type-II-I heterojunctions, such as g-C₃N₄/Ag₂CO₃, while naming the situation where both the Fermi level and the conduction band cannot be higher than those of another material at the same time as type-II-II heterojunctions, such as Ag₂CO₃/Bi₂WO₆. The results show that the type-II-I heterojunction g-C₃N₄/Ag₂CO₃ exhibits a strong photocatalytic degradation ability, while the type-II-II heterojunction Ag₂CO₃/Bi₂WO₆ has great potential for selective degradation due to its relatively weak photocatalytic performance. This research is expected to offer valuable insights into the carrier migration mechanisms of type-II heterojunctions and highlight the potential applications of type-II-II heterojunctions in selective degradation processes.