Preparation and Photocatalytic Performance of BiOCl/CdS Composites

With the rapid development of industry and textile industry, problems, such as energy shortage and environmental pollution, are becoming increasingly serious. Semiconductor photocatalytic technology is regarded as one of the ideal means to solve the problems, due to its advantages of utilizing sunlight to degrade pollutants, decompose water to produce hydrogen and reduce CO₂. In bismuth oxyhalide materials, bismuth oxychloride (BiOCl) has attracted much attention, due to its unique layered structure, suitable band structure (~3.4 eV) and high chemical stability. The alternating stacking structure of [Bi₂O₂]²⁺ layer and Cl^- layer results in an internal electric field and effectively promote the separation of photo generated electron hole pairs, endowing it with excellent photo-oxidation ability. However, the absorption of visible light by BiOCl is limited by its wider bandgap (only 4%–5% of the solar spectrum), so that its practical applications are severely limited. Cadmium sulfide (CdS), a typical narrow-band semiconductor (~2.4 eV) complementary to BiOCl, is characterized by a wide visible light response range (absorption edge ~520 nm), a relatively negative reduction potential (−0.5 eV) and other advantages. CdS has high reactivity in hydrogen production and pollutant degradation. In response to the above issues, a hydrothermal thermal assisted liquid-phase self-assembly strategy was proposed to produce high interface coupling BiOCl/CdS heterojunction materials, where BiOCl was synthesized by using hydrothermal method and CdS was synthesized by using water bath method. Finally, BiOCl/CdS heterojunction was obtained by thermally assisted liquid-phase self-assembly method.

To synthesize BiOCl/CdS composite photocatalyst, 300 mg BiOCl catalyst was weighed and placed in a conical flask containing 60 mL H₂O. C₂H₅NS was added and the mixture was allowed to adsorb in the dark for 30 min. CdCl₂ was then added and the mixture was heated in water bath at 90℃ for 2 h. The precipitate was centrifuged alternately with deionized water and anhydrous ethanol four times. The treated precipitate was placed in a constant-temperature drying oven and dried at 60℃ to obtain target product.

The synthesized sample is indicated by XRD and SEM results to be a flower-shaped BiOCl powder with high crystallinity. The presence of Bi, O, Cl, S and Cd elements in the material was confirmed by using EDS and XPS. In TC degradation results, it was shown that, with increased content of CdS, the degradation performance of the BiOCl/CdS composite material exhibited a trend of initial increase and then decrease. Among these, the highest catalytic activity was exhibited by BiOCl/CdS-2 (66.69%), representing an approximately 15.17% improvement over the 51.52% observed for flower-shaped BiOCl. This enhancement is closely related to heterojunction construction and improved photogenerated carrier separation efficiency.

A BiOCl/CdS heterojunction photocatalyst was successfully constructed, whose degradation performance and mechanism of action on tetracycline hydrochloride were systematically studied. It is experimentally indicated that the visible light response ability is limited by the wide bandgap (~3.2 eV) of single flower-shaped BiOCl, whereas pure CdS despite possessing narrow bandgap advantages is characterized by weak photocatalytic degradation ability due to high photogenerated carrier recombination rates and photocorrosion issues. Optimized photocatalytic performance was achieved through regulation of CdS loading in the BiOCl/CdS heterojunction composite. Among them, BiOCl/CdS-2 showed the highest activity (degradation rate of 66.69%), which was 15.17% higher than that of pure BiOCl (51.52%). The performance enhancement is attributed to two collaborative mechanisms, i.e., band regulation and interface effects. With band regulation, the light response range is broadened and visible light capture capability is enhanced through the introduction of narrow bandgap CdS. IniInterface effect, photo-generated electrons are driven by the heterojunction to migrate from the CdS conduction band to the BiOCl conduction band, while holes are transferred to the CdS valence band, thus effectively suppressing carrier recombination and extending charge lifetime. It should be noted that a threshold exists for CdS loading. Excessive CdS (e.g., BiOCl/CdS-3) would cause particle aggregation on BiOCl surface, resulting in active sites being shielded, light absorption being hindered and degradation efficiency being decreased.