Novel green synthesis of S-doped TiO2 nanoparticles using Malva parviflora plant extract and their photocatalytic, antimicrobial and antioxidant activities under sunlight illumination.

Visible-light-responsible S-doped TiO2(GST) nanoparticles (NPs) are synthesized via sol-gel process, and an aqueous extract of Malva parviflora (MP) plant is used as the green and versatile medium with excellent reducing and capping properties to facilitate the S-doping and crystal growth of anatase. Compared with the counterpart (CST) derived from the conventional organic solvent. GST show a smaller average particle size (20.3 vs. 29.5 nm) with a larger surface area (135 vs 95 m2/g), together with the more significant red shift to longer wavelength in the visible light region. XPS analysis shows Ti4+ cations are substituted by S6+ ions into the lattice structure of TiO2 for both samples. The photodynamics of CST and GST catalysts are examined by electron paramagnetic resonance (EPR) spectroscopy, which shows the surface Ti3+ sites can be oxidized easily by the surface adsorbed O2 molecules, forming O2- radicals. Their photocatalytic activities (PCA) are evaluated by degrading methyl orange (MO) dye under the visible light irradiation. GST exhibit higher PCA in MO bleaching and chemical oxygen demand (COD) reduction. In addition, antimicrobial and antioxidant assays of CST and GST NPs also show that the irradiated NPs samples show higher antibacterial activities. GST NPs have a higher antibacterial activity than CST NPs against all tested bacteria and the minimum inhibitory concentration (MIC) is optimized to 25 μg/mL. The in-vitro antioxidant activity evaluated by the radical cation de-colorization test using 1,1-diphenyl-2-picrylhydrazyl (DPPH) further demonstrates that GST NPs give a better antioxidant activity.