Enhanced internal electric field in S-scheme heterojunction for photothermal removal of antibiotics and hexavalent chromium

Abstract

The issue of insufficient internal electric field (IEF) strength in S-scheme heterojunction can be addressed by tuning the Fermi level of the materials. Herein, an S-scheme heterojunction of Bi₅O₇Br (BOB) and MnO₂ was constructed, which enhanced photocatalytic performance in pollutant removal through the IEF enhancement and photothermal effect. The composite material exhibited superior degradation performance over the individual materials in the removal of tetracycline (TC) and hexavalent chromium (Cr(VI)). When mixed solutions were treated, TC and Cr(VI) were removed 1.07 times and 1.27 times more efficiently by MnO₂/BOB-2.5% than in the single system. Owing to the photo-induced oxygen vacancies in BOB, the MnO₂/BOB composite sustained high activity through three successive cycles while exhibiting a widened Fermi-level gap. Additionally, the VB-XPS and DFT calculation results indicated that oxygen vacancies shortened the band structure and elevated the Fermi level, facilitating rapid electron transport. By monitoring changes in reaction temperature, it was found that the introduction of MnO₂ endows the composite material with a photothermal effect, raising the reaction system temperature and accelerating the reaction. Additionally, the toxicity of both intermediates and final products was evaluated, and the reaction mechanism underlying synergistic of MnO₂/BOB removal of TC and Cr(VI) was thoroughly examined. This work furnishes important perspectives on the development of high-efficiency S-scheme heterojunctions and their deployment in environmental cleanup.