Ag-doped Bi2WO6/BiOI heterojunction used as photocatalyst for the enhanced degradation of tetracycline under visible-light and biodegradability improvement

A new Bi2WO6/BiOI/Ag heterojunction composite was fabricated by the deposition of Ag nanoparticles on Bi2WO6/BiOI via two-step hydrothermal process and characterized by X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM) analyses. The photoelectric properties of the as-prepared composites were investigated by UV–vis diffuse reflectance spectrum (DRS), transient photocurrent response, electrochemical impedance spectroscopy (EIS) and photoluminescence spectrum (PLS). It was found that the amount of Ag in Bi2WO6/BiOI/Ag exhibited an evident influence on photoelectric properties and photocatalytic activity. Compared with the other as-prepared photocatalysts, Bi2WO6/BiOI/Ag-8 presented the highest photocatalytic activity for tetracycline degradation ascribed to the heterojunction structure and the surface plasmon resonance (SPR) action, which evidently enhanced the visible-light absorbance, and promoted the separation/transfer of photo-generated electron/hole (e–/h+) pairs. The organic intermediates formed during tetracycline photocatalytic degradation were identified and the degradation pathway was proposed. For the recommended photocatalyst, the hydroxyl radical (∙OH) and superoxide anion radical (∙O2–) produced by the Z-scheme photocatalytic mechanism with Ag as co-catalyst were responsible for the degradation. In addition, the photocatalytic degradation of tetracycline evidently reduced the solution toxicity and enhanced the biodegradability.