Duration-controlled microwave engineering of Bi-rich Bi₇O₉I₃ with tunable band structure for selective Cr(VI) reduction and organic oxidation under visible light

Bismuth oxyiodide (BiOI) is a visible-light-responsive photocatalyst; however, its performance is restricted by rapid charge recombination and limited redox capability. In this study, a duration-controlled microwave strategy (5, 10, 20, and 30 irradiation cycles at 640 W) was employed to engineer phase transformation from BiOI to Bi-rich Bi₇O₉I₃, enabling systematic tuning of band-edge positions. Notably, even within the Bi₇O₉I₃ phase obtained at different irradiation cycles, variations in conduction band (CB) and valence band (VB) positions are observed, indicating that microwave duration enables tuning of electronic structure within a single phase. The structural evolution from flower-like BiOI to plate-assembled and micro-rectangular Bi₇O₉I₃ was accompanied by progressive modulation of the CB and VB. Their photocatalytic activity was evaluated through reduction of Cr(VI) and degradation of bisphenol A (BPA) and ciprofloxacin (CIP) under visible light, with additional investigation of Cr(VI) concentration, catalyst dosage, and solution pH. All Bi-rich Bi₇O₉I₃ samples exhibited higher activity compared with BiOI. The Bi₇O₉I₃ photocatalyst synthesized at 30 cycles (Bi₇O₉I₃ (30 cycles)) achieved the highest Cr(VI) reduction efficiency (67% at 40 ppm after 360 min), which increased to 93% under acidic condition (pH 3). In contrast, the Bi₇O₉I₃ photocatalyst synthesized at 10 cycles (Bi₇O₉I₃ (10 cycles)) showed the highest oxidation performance, achieving 69% of BPA and 92% of CIP degradation after 300 min. Mott–Schottky analysis revealed that Bi enrichment shifted the CB toward more negative potentials while adjusting the VB position. The Bi₇O₉I₃ (30 cycles) photocatalyst, possessing the most negative CB, favored electron-driven Cr(VI) reduction, whereas the Bi₇O₉I₃ (10 cycles) photocatalyst provided more favorable VB alignment for hole-mediated oxidation of organic pollutants. In a Cr(VI)/CIP combined system, the presence of coexisting pollutant further improved the selective photocatalytic activities for both Bi₇O₉I₃ (10 cycles) and Bi₇O₉I₃ (30 cycles). These activity trends indicate that duration-controlled phase evolution enables selective regulation of reduction and oxidation pathways through band-edge modulation.