Design, Synthesis, and Crystal Structure of Naturally Occurring Arsenic-Based Materials: A Sustainable Approach toward Photocatalytic Dye Degradation, Catalytic Solvent-Free Organic Transformations, Electrochemical and Theoretical Studies

A naturally occurring arsenic-based mineral, i.e., lammerite, Cu3As2O8, has been successfully synthesized in the laboratory by employing a hydrothermal technique, and its sustainable approach toward photocatalytic dye degradation and catalytic solvent-free organic transformations has been explored along with electrochemical studies. Single-crystal X-ray diffraction studies reveal that it crystallizes in monoclinic symmetry with a = 5.3938(6) Å, b = 11.6233(15) Å, c = 5.0804(7) Å, β = 111.74(6)°, V = 295.86(71) Å3, SG: P21/c, and Z = 2. The building unit comprises [AsO4] tetrahedral, [CuO5] trigonal bipyramidal, and [CuO4] square planar units. [AsO4] and [CuO5] are connected to each other through a bridging oxide linkage to form a [CuAsO8]n unit. Further, [CuO4] is connected to the [CuAsO8]n unit to form Cu3As2O8. The material served as an efficient photocatalyst for the complete degradation of the methylene blue (MB) dye, although the degradation efficiency was enhanced by approximately 2-fold upon addition of 0.1 mL of 6% H2O2. The SEM and TEM studies revealed multilayered, porous, nanosized particles with enhanced surface-active area. The radical scavenger experiment confirms that the hydroxyl radical is responsible for the dye degradation. Electrochemical studies confirm the quasi-reversible electron transfer process of the Cu2+/Cu+ and Cu+/Cu0 redox system. Further, the synthesized lammerite exhibited promising catalytic efficiency toward a solvent-free Knoevenagel condensation reaction by varying different electron-withdrawing and -donating substituents.