Photocatalytic activity of the visible-light-driven spherical Ag2S modifying the CdS synthesized by the facile chemical methods for the degradation of methylene blue and rhodamine B

Abstract

Semiconductor photocatalyst is a simple, efficient, low-cost, and green solution for dealing with the environmental crisis. To suppress the recombination of electrons and holes and the photocorrosion of the CdS synthesized via a controllable chemical precipitation method without using any surfactants or templates, the CdS–Ag2S heterostructure was synthesized by a facile ion exchange technique. The CdS and CdS–Ag2S photocatalysts were characterized, and the results confirmed the formation of Ag2S particles on the surface of CdS spheres. Based on the optical properties of the CdS–Ag2S heterostructure compared with those of the pure CdS, investigated by ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS) and photoluminescence (PL), the greater absorption capacity, narrower band gap (2.02 eV), and more efficient transfer and separation of charge carriers were observed. The value of the specific surface area of CdS–Ag2S (415.93 m2/g) was studied via the nitrogen adsorption and desorption isotherms. Accordingly, it is higher than the specific surface area of the bare CdS (33.385 m2/g) due to having more active sites. The photocatalytic activities of CdS and the CdS–Ag2S heterostructure were evaluated by the degradation of the dyes of methylene blue (MB) and rhodamine B (RhB), and the uncolored organic contaminant of phenol. The photodegradation of MB, RhB, and phenol by CdS increased from 80%, 67%, and 61%–98%, 90%, and 81% by CdS–Ag2S, respectively, within 180 min under the visible light irradiation. Furthermore, the CdS–Ag2S heterostructure exhibited a suitable stability after being used in three cycles. The efficiencies of sunlight and visible light irradiation for photodegradation of MB and RhB were compared. The effects of the initial concentration of the dyes, concentration of the catalyst, pH, and the presence of Cl− and CO32− ions (available in the wastewater containing dyes) on the photodegradation of MB and RhB by the CdS–Ag2S heterostructure were also investigated. The electrochemical impedance spectroscopy (EIS) measurement revealed that the CdS–Ag2S heterostructure accelerated the photogenerated transfer of charge carriers. The present work has recommended a new approach using the chemical precipitation and ion exchange to synthesize 3-dimensional CdS–Ag2S spheres with an excellent photocatalytic performance. A proper mechanism was suggested to investigate the photocatalytic process of the CdS–Ag2S heterostructure. Also, the radical trapping experiment was carried out via various scavengers to distinguish the role of the active species of O2•-, OH•, h+, and e−, respectively. Finally, the kinetics of the photodegradation of MB and RhB could be studied by Langmuir-Hinshelwood model explained as pseudo-first-order.