Carbon Quantum Dots‐Decorated Mn 0.5 Cd 0.5 S/BiOBr S‐Scheme Heterojunction With Interfacial Bi─S Bonding as an Efficient Piezocatalyst for Aqueous Cr(VI) Reduction

Hexavalent chromium [Cr(VI)] is a serious environmental problem that needs to be removed efficiently from water without the need for reagents. A two‐step hydrothermal/solvothermal method was employed to synthesize a ternary piezocatalyst of carbon quantum dots (CDs) coupled with Mn 0.5 Cd 0.5 S (MCS) and BiOBr (BOB), denoted MBC, with an interfacial Bi─S bond and tested for the piezocatalytic reduction of Cr(VI) under dark and ultrasound‐assisted conditions. BOB microsphere architecture which was also uniformly decorated with cauliflower‐like MCS aggregates and had a systematic increase in carbon content from 12.5 to 25.9 wt.% as the nominal loading of CDs in the BOB was increased from MBC‐1 to MBC‐3. In the case of Cr(VI), all three ternary MBC composites showed significant removal efficiency of the target ion compared to the respective BOB and MCS, with MBC‐2 showing the best removal efficiency (89%), whereas the control without the catalyst removed 5.2% of the Cr(VI), which indicates that the removal of Cr(VI) is not a sonolytic effect but a true piezo‐driven effect. Based on these results, an S‐scheme charge‐transfer mechanism that involves the interface between chemically bonded Bi─S and anchored CDs is proposed.