Fabrication of quaternary Ag/BiOBr@Bi2MoO6/ZnO/g-C3N4 composite for direct Red 23 dye removal from water

This study presents the design, synthesis, and performance evaluation of a quaternary Ag/BiOBr@Bi 2 MoO 6 /ZnO/g-C 3 N 4 nanocomposite developed via a hydrothermal-photoassisted route for the near-visible-light-driven removal of Direct Red 23 (DR23) dye from aqueous solutions. Integrating plasmonic Ag nanoparticles with ZnO/g-C 3 N 4 and BiOBr@Bi 2 MoO 6 heterostructures produced a multi-junction system which is expected to enhanced photon absorption, charge carrier separation, and surface redox reactivity. Comprehensive characterization using FTIR, XRD, SEM, EDS, TEM, XPS, BET, UV-Vis DRS and PL analyses confirmed the formation of a coherent, a well-integrated heterostructured architecture with a narrowed optical band gap (2.64 eV) and significantly reduced recombination rates. The composite achieved 80.8% degradation of DR23 within 110 min under UV-visible (near-visible) irradiation, showing improved performance compared with the corresponding binary systems. Molecular dynamics (MD) simulations performed with Materials Studio and LAMMPS demonstrated strong dye-surface interactions and high thermodynamic stability (adsorption energy = -57.302 kcal.mol −1 .atom −1 ), corroborating experimental efficiency. The simulations also revealed an optimal adsorption temperature of 340 K and an 88.15% atomic removal rate of DR23 molecules. The proposed S-scheme charge transfer mechanism, facilitated by the plasmonic effect of Ag nanoparticles and favorable band alignment among the four semiconductors, enables efficient generation of reactive oxygen species ( . OH, . O 2 − and 1 O 2 ). These reactive oxygen species initiate DR23 degradation primarily via cleavage of the azo (-N=N-) bond, followed by sequential deamination, desulfonation, aromatic ring opening and further oxidation into low-molecular-weight intermediates, as confirmed by LC-ESI-MS/MS analysis. Overall, the enhanced photocatalytic performance of the Ag/BiOBr@Bi 2 MoO 6 /ZnO/g-C 3 N 4 composite originates from the synergistic effects of heterojunction-induced charge separation, plasmonic enhancement by Ag nanoparticles and efficient generation of reactive oxygen species.