Enhancing Photocatalytic Activity with the Substantial Optical Absorption of Bi2S3-SiO2-TiO2/TiO2 Nanotube Arrays for Azo Dye Wastewater Treatment

One-dimensional TiO2 nanotube arrays (TNAs) were vertically aligned and obtained via the electrochemical anodization method. In this study, Bi2S3-TiO2-SiO2/TNA heterojunction photocatalysts were successfully prepared with different amounts of Bismuth(III) sulfide (Bi2S3) loading on the TNAs by the successive ionic layer adsorption and reaction (SILAR) method and characterized by X-ray diffraction (XRD) patterns, field-emission scanning electron microscope–energy-dispersive spectroscopy (FESEM-EDS), Fourier transform infrared (FTIR) spectra, ultraviolet-visible diffuse reflectance spectra (UV–Vis/DRS), and electrochemical impedance spectroscopy (EIS) techniques. The photocatalytic performances of the samples were investigated by degrading Basic Yellow 28 (BY 28) under visible-light irradiation. Optimization of the condition using the response surface methodology (RSM) and central composite rotatable design (CCRD) technique resulted in the degradation of BY 28 dye, showing that the catalyst with 9.6 mg/cm2 (designated as Bi2S3(9.6)-TiO2-SiO2/TNA) showed the maximum yield in the degradation process. The crystallite size of about 17.03 nm was estimated using the Williamson–Hall method. The band gap energies of TiO2-SiO2/TNA and Bi2S3(9.6)-TiO2-SiO2/TNA were determined at 3.27 and 1.87 eV for the direct electronic transitions, respectively. The EIS of the ternary system exhibited the smallest arc diameter, indicating an accelerated charge transfer rate that favors photocatalytic activity.