S-scheme Bi6O5(OH)3(NO3)5·3H2O/BiOBr0.8I0.2 Heterojunction Photocatalyst: Synthesis and Visible-Light Degradation Mechanism of Rhodamine B
A novel S-scheme heterojunction photocatalyst, Bi6O5(OH)3(NO3)5·3H2O/BiOBr0.8I0.2 (BON@BI), was synthesized via a one-step hydrothermal method using Bi6O5(OH)3(NO3)5·3H2O (BON), KBr, and KI as precursors. The mass ratio of BON to BiOBr0.8I0.2 (BI) was optimized, revealing that the 20% BON@BI composite (BON@BIOPT) exhibited the highest visible-light photocatalytic activity. Under 30 min of visible-light irradiation, BON@BIOPT achieved a 99.8% degradation efficiency of Rhodamine B (RhB), approximately twice that of pristine BI (52.2%). The composite displayed a rod-like morphology with uniform nanosheets, and its specific surface area increased from 32.54 m²·g⁻¹ (BI) to 44.7 m²·g⁻¹. The absorption edge red-shifted from 560 nm (BI) to 580 nm, narrowing the bandgap from 2.55 eV to 2.43 eV. The S-scheme heterojunction formed between BON and BI generates an internal electric field that effectively suppresses recombination of strongly reducing photogenerated electrons and strongly oxidizing holes, with superoxide radicals (O₂•⁻) and holes (h⁺) identified as the primary reactive species. BON@BIOPT exhibited excellent stability, retaining 88.6% degradation efficiency after seven consecutive cycles. It also demonstrated robust environmental adaptability, maintaining 85–98% degradation efficiency under various pH conditions and in the presence of interfering anions. The degradation pathway of RhB involves N-de-ethylation, cleavage of the conjugated chromophore, and deamination, ultimately mineralizing into low-molecular-weight organics, inorganic salts, CO₂, and H₂O. These results underscore the potential of BON@BIOPT for practical remediation of organic pollutants in water.