Facile synthesis of AgI/BiOI-Bi₂O₃ multi-heterojunctions with high visible light activity for Cr(VI) reduction

Graphical abstract: Highly visible-light-active AgI/BiOI-Bi₂O₃ with multi-heterojunctions was developed. - Highlights: • Visible-light-active AgI/BiOI-Bi₂O₃ with multi-heterojunctions was prepared. • Highly enhanced photocatalytic reduction of Cr(VI) was observed. • k_Cr(VI) on AgI/BiOI-Bi₂O₃ increased by ca.16 times relative to Bi₂O₃. • Decreased E_g, shifted E_fb and reduced charge transfer resistance were observed. • Simultaneous reduction of Cr(VI) and degradation of organics were achieved. - Abstract: AgI sensitized BiOI-Bi₂O₃ composite (AgI/BiOI-Bi₂O₃) with multi-heterojunctions was prepared using simple etching-deposition process. Different characterization techniques were performed to investigate the structural, optical and electrical properties of the as-prepared photocatalysts. It was found that the ternary AgI/BiOI-Bi₂O₃ composite exhibited: (1) improved photocurrent response, (2) smaller band gap, (3) greatly reduced charge transfer resistance and (4) negative shift of flat band potential, which finally led to easier generation and more efficient separation of photo-generated electron-hole pairs at the hetero-interfaces. Thus, for the reduction of Cr(VI), AgI/BiOI-Bi₂O₃ exhibited excellent photocatalytic activity under visible light irradiation at near neutral pH. AgI/BiOI-Bi₂O₃ was optimized when the initial molar ratio of KI to Bi₂O₃ and AgNO₃ to Bi₂O₃ was 1:1 and 10%, respectively. The estimated k_Cr(VI) on optimized AgI/BiOI-Bi₂O₃ was about 16 times that on pure Bi₂O₃. Good stability was also observed in cyclic runs, indicating that the current multi-heterostructured photocatalyst is highly desirable for the remediation of Cr(VI)-containing wastewater.