Abstract:

Objective The energy crisis and environmental pollution are the two major tests facing mankind. Nitrogen-containing dyes with chromophores have stable chemical properties, which can darken the color of water bodies and affect the light transmission and are hazardous in causing genetic mutations. BiOBr has a good visible light response, but its forbidden band width is slightly larger, the visible light utilization is low, and the photogenerated carriers are prone to compounding. The combination of CdS and BiOBr is believed to have the capacity for the effective separation of electrons and holes to increase the visible light utilization to improve the photocatalytic performance.

Method BiOBr nanosheets were prepared by the solvothermal method, followed by the preparation of the CdS/BiOBr composites by growing CdS particles directly on the layered BiOBr surface by the hydrothermal method, where the morphology of the composite catalysts was adjusted by adjusting the molar ratio of the two. The prepared photocatalysts were characterized by transmission electron microscope(TEM), X-ray diffraction(XRD), UV-Vis DRS, etc., and the degradation effect and stability of the catalysts on Rhodamine B (RhB) were investigated by visible light catalysis experiments. The main active substances in the degradation process were identified by active species capture experiments.

Results When the molar ratio of CdS to BiOBr was 1∶3, the catalyst CdS/BiOBr (1∶3) morphology showed a three-dimensional ping-pong chrysanthemum-like hierarchical structure, and the CdS nanoparticles were uniformly distributed on the surface of BiOBr petal-like flakes (Fig. 2(e) and (f)). Peaks associated with Bi, Br, O, Cd and S were observed in the EDS spectrum, confirming the presence of these six elements in the composite (Fig. 3(a)), the successful formation of heterojunction between CdS and BiOBr was confirmed by the analysis of TEM (Fig. 3(b) and (c)), and the XRD spectrum indicated that the CdS/BiOBr composite was successfully prepared with high purity (Fig. 4). The catalyst CdS/BiOBr(1∶3) showed a significant red shift compared with both pure BiOBr and pure CdS, and in addition, the absorption intensity of the catalyst was significantly enhanced at 550-800 nm (Fig. 5(a)). The forbidden band widths of BiOBr and CdS were 2.70 eV and 2.28 eV (Fig. 5(b) and (c)), respectively, and the CB and VB positions of 0.33 eV and 3.03 eV for BiOBr and -0.59 eV and 1.69 eV for CdS were calculated according to equations. The fluorescence spectrum intensity of the catalyst CdS/BiOBr (1∶3) was significantly weaker compared to that of the pure BiOBr (Fig. 6), and the specific surface area of the catalyst CdS/BiOBr (1∶3) was increased compared with that of the pure component BiOBr (Fig. 7). The results of the visible photocatalytic experiments demonstrated that the light degraded 99.5% of RhB for 120 min with a primary kinetic constant of 0.044, which was 3.67 times the rate constant of pure BiOBr (Fig. 8). After 7 cycles of photocatalytic degradation of RhB, the photocatalytic activity of the three-dimensional ping-pong chrysanthemum-like catalyst CdS/BiOBr(1∶3) still reached 90.1%, and no significant changes were found in the XRD diffraction peaks of the catalyst before and after the reaction (Fig. 9-11); and by adding EDTA and BQ to the photocatalytic system, the degradation activity of RhB was obviously inhibited and the degradation rate constant was significantly reduced (Fig. 12). The EPR results showed that a large number of h⁺ and ·O₂^- radicals were indeed generated in the photocatalytic degradation reaction, which confirmed the accuracy of the radical capture experiment (Fig. 13).

Conclusion In summary, a regularly combined three-dimensional ping-pong chrysanthemum-like CdS/BiOBr photocatalyst was synthesized for the first time, which exhibited excellent photocatalytic activity and cycling stability in the photocatalytic degradation of Rhodamine B. This was attributed to the fact that the prepared CdS/BiOBr catalyst showed higher visible light absorption efficiency, lower electron-hole pair complexation chance, and a larger specific surface area of the three-dimensional ping-pong chrysanthemum-like layered structure, which provided sufficient transport paths for reactant molecules and facilitates the transfer of interfacial photogenerated carriers, thus improving the catalytic efficiency of the photocatalyst.

Key words: photocatalysis, printing and dyeing waste water treatment, rhodamine B, hydrothermal method, degradation mechanism, BiOBr, CdS