Sunlight-driven fast photo-degradation of Eriochrome Black T dye using highly efficient La-doped Ag3PO4 decorated with ZnS QDs

Abstract

The untreated discharge of dye-contaminated effluents into aquatic environments poses serious risks to both environmental integrity and public health. Therefore, the development of efficient dye removal strategies is essential for pollution control and ecosystem protection. This study aims to investigate and optimize the photocatalytic degradation of Eriochrome Black T (EBT) in aqueous solutions using novel Ag₃PO₄-based composites, including lanthanum-doped Ag₃PO₄, and ZnS quantum dots under sunlight irradiation. Pure Ag₃PO₄, 2% and 6% La-doped Ag₃PO₄, and ZnS quantum dots were synthesized via a co-precipitation method, and their composite photocatalysts were fabricated using a hybrid mixing approach. The characterizations of the materials were carried out using X-ray diffraction, BET surface area analysis, UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS) to evaluate their structural, optical, and morphological properties. The synthesized photocatalysts, including pure Ag₃PO₄, 2% and 6% La-doped Ag₃PO₄, ZnS quantum dots, and their composite systems, exhibited well-defined crystalline structures, as confirmed by X-ray diffraction (XRD) analysis. UV-vis DRS analysis showed that pure Ag₃PO₄ had a band gap of 2.41 eV, which decreased to 2.39 eV for 2% La-doped Ag₃PO₄ and 2.36 eV for 6% La-doped Ag₃PO₄, while ZnS quantum dots exhibited a band gap of 3.6 eV, and the 6% La-doped Ag₃PO₄/ZnS QD composite showed a significantly reduced band gap of 1.5 eV. Upon the incorporation of ZnS QDs into the 6% La-Ag₃PO₄ particles, the surface area of the 6% La-Ag₃PO₄ heterojunction composite increased from 133.446 to 141.120 m² g⁻¹. The photocatalytic activity of the synthesized Ag₃PO₄-based materials was evaluated through the degradation of Eriochrome Black T (EBT) under light irradiation. The influence of key operational parameters, including the solution pH (3–10), dye concentration (10–20 ppm), photocatalyst dosage (0.05–0.15 g), and irradiation time (5–120 min), was systematically investigated to assess their effect on degradation efficiency. Among the examined photocatalysts, 6% La-doped Ag₃PO₄ exhibited the highest photocatalytic performance, demonstrating the beneficial role of lanthanum incorporation. The degradation efficiencies of 97.84% and 84.88% were achieved using 6% La-doped Ag₃PO₄ and the 6% La-doped Ag₃PO₄/ZnS QD composite, respectively, under the optimized conditions of pH 6 and an irradiation time of 120 min. Overall, these results indicate that La-modified Ag₃PO₄-based photocatalysts are promising and sustainable materials for the effective treatment of dye-contaminated wastewater, offering significant potential for environmental remediation applications.