Bi4O5Br2 nanosheets modified by carbon quantum dots: Efficient BPA degradation induced by enhanced absorption and Z-scheme charge transfer

Constructing semiconductor photocatalysts with enhanced absorption and rapid carrier dynamics is essential for applying semiconductor photocatalysis in environmental remediation and energy conversion. In this study, we modified carbon quantum dots (CQDs) onto the surface of Bi4O5Br2 using a one-step hydrothermal method. The modification enabled the excellent degradation of bisphenol A (BPA) due to the up-conversion luminescence of CQDs and the Z-scheme charge transfer path formed by the unique energy level of the two materials. The CQDs/Bi4O5Br2 composite added with 4 mg of CQDs completely degraded BPA (20 mg/L) within 100 min when the radiation wavelength exceeded 400 nm, and Bi4O5Br2 itself only degraded BPA by 1.44%. In sharp contrast, if a xenon lamp was directly irradiated without a 400 nm filter to sift the ultraviolet components, the degradation rate of the Bi4O5Br2 monomer could reach 40% under the same conditions. This indicates that introducing CQDs plays a vital role in visible-light-driven degradation. Furthermore, on the basis of the structural characterisation, the successful introduction of CQDs was confirmed in the CQDs/Bi4O5Br2 hybrid system. On this basis, the possibility of Z-scheme charge transfer was proved by X-ray photoelectron spectroscopy and ultraviolet electron spectroscopy measurements. The degradation mechanism of BPA under Z-type charge transfer was proposed through electron paramagnetic resonance, free radical capture and total organic carbon tests. This work is expected to provide a new idea for designing an energy band that matches semiconductor heterojunction photocatalysts, enhancing the unique narrow band gap structure and up-conversion properties of CQDs. In addition, it facilitates the construction of photocatalysts with better performance.