Effects of pH and ascorbic acid on the synthesis of Bi-modified bismuth oxyiodide composite catalysts

•A series of Bi-modified bismuth oxyiodide composites were synthesized.•The pH strongly affects the composition, morphology and performance of composites.•Ascorbic acid can reduce Bi3+ to Bi, inhibit BiOI growth and help to form Bi4O5I2.•The ternary Bi/BiOI/Bi4O5I2 exhibits the highest photocatalytic activity.•An indirect Z-scheme ternary photocatalytic mechanism of BBI-AA-0.24 is proposed. Multi-heterojunction composites exhibit unparalleled superiority in light absorption and spatial separation of charges, but their synthesis remains a challenge. Herein, we report the one-step hydrothermal synthesis of Bi-modified bismuth oxyiodide composites by modulating the pH of the reaction system using ascorbic acid (AA) as reducing agent. The pH highly influences the composition, morphology and properties of the samples. AA can reduce Bi3+ to metallic Bi, inhibit grain growth of BiOI and help to form Bi4O5I2, thus realizing the one-step synthesis of ternary composites. The sample obtained at pH 6.5 and AA of 0.24 g is the BiOI/Bi/Bi4O5I2 composites with flower-like hierarchical structure, which has the highest degradation efficiency for bisphenol A and RhB. In addition, formation mechanism of the composites was systematically researched based on the composition of intermediates, and an indirect Z-scheme photocatalytic mechanism of BBI-AA-0.24 based on the electronic bridge of metal Bi was proposed. [Display omitted]