Investigation of the photocatalytic activity of a pentanary (AgxCu1−x)2ZnSnS4 compound

Toward the development of efficient and eco-friendly Cu<inf>2</inf>ZnSnS<inf>4</inf> (CZTS) photoactive materials, substitution strategies have emerged as an effective method to control the band gap energy and improve the properties of this material, thereby enhancing its performance. In this work, (Ag<inf>x</inf>Cu<inf>1−x</inf>)<inf>2</inf>ZnSnS<inf>4</inf> compounds (involving Cu–Ag substitution) with 0 ≤ x ≤ 1 were synthesized by a solid-state reaction of pure elements. X-ray diffraction and Raman spectroscopy indicated that while pure Cu<inf>2</inf>ZnSnS<inf>4</inf> adopted the kesterite-type structure, the fully substituted AgZnSnS<inf>4</inf> crystallized in the stannite-type structure, revealing a gradual phase transformation from the kesterite phase to the stannite phase with the increasing Ag content together with lattice expansion. Energy-dispersive X-ray spectroscopy measurements showed that the deviation between the experimental Ag concentrations and the theoretically calculated values decreased as the Ag content increased. Diffuse reflectance spectroscopy of the obtained samples revealed the tunability of the optical properties through cation (Cu–Ag) substitution. The band gap energy increased from 1.33 eV to 1.89 eV with the Ag substitution ratio, showing considerable potential in photovoltaic photocatalytic applications. Additionally, photocatalytic tests were carried out using these bulk materials in powder form in the presence of blue methylene dye, and their photodegradation was investigated under visible light. The efficiency for the photocatalytic degradation of blue methylene dye in the unsubstituted (free-Ag) powder material, which has the most developed crystal structure and the narrowest band gap, was the highest, reaching up to 94.6% with a degradation rate constant (k) of 0.0156 min⁻¹ after 180 minutes at pH 10 under non-scavenger conditions.