pH-Driven Hydrothermal Synthesis of Leiteite ZnAs ₂ O ₄ : Structure, Characterization, and Multifunctional Applications including Photocatalytic Dye Degradation, Catalytic Organic Transformation, and Antibacterial and Antioxidant Properties

The leiteite ZnAs2O4 mineral has been successfully synthesized in the laboratory by a pH-assisted hydrothermal technique, as discovered previously from the Tsumeb mine, Oshikoto, Namibia. The as-synthesized material has been developed as an efficient heterogeneous photocatalyst toward methylene blue dye degradation and catalyst toward solvent-free Knoevenagel condensation reaction and also is potent toward important antimicrobial and antioxidant activities. SCXRD studies reveal that no phase transition was observed upon lowering the temperature up to 100 K, as the lattice parameters were found to be a = 4.5318(3) Å, b = 5.0085(3) Å, c = 17.5707(11) Å, β = 90.914(4)°, V = 398.76(4) Å3, and SG: P21/c, with a slight contraction to the lattice parameters at room temperature. Solid-state DFT was employed to ascertain the nature of the semiconducting material. The chemical characteristics of the material toward MB degradation, radical scavenging, and antibacterial activity are explained by its band characteristics and partial density of states. Photocatalytic methylene blue dye degradation was demonstrated, and the rate was enhanced by 2-fold with the addition of 0.1 mL of 6% H2O2. The reusability of the catalyst has also been studied up to the 10th cycle. The catalytic activity was confirmed for the solvent-free Knoevenagel condensation reaction at moderate temperature, and the activities were also compared by varying the numbers of electron-donating and electron-withdrawing groups. The antibacterial activity on several Gram-negative and Gram-positive pathogens, i.e., Escherichia coli, Enterobacter sp., Bacillus sp., Klebsiella sp., and Pseudomonas sp., was studied. Furthermore, the antioxidant activity was also confirmed experimentally through the DPPH assays.