Abstract:
Nano titanium dioxide (TiO2) has been found to possess enhanced antibacterial properties and has thus been employed in various photocatalytic applications. Studies have confirmed higher photocatalytic activity when the TiO2 photocatalyst is doped with silver (Ag). The reported enhancement is due to valence band electrons being excited at longer wavelengths, hence extending absorbance further into the visible light region. For nano catalysts that interact in such a region, they are termed visible light active (VLA) photocatalysts. It is favourable to produce such catalysts since visible light accounts for about 50% of the solar spectrum whereas ultraviolet (UV) light accounts for just around 5%.
Pristine TiO2 and Ag doped TiO2 nanoparticles were synthesized by the sol-gel method. The nanoparticles were characterized by X-Ray diffraction (XRD), Brunauer Emmett Teller (BET), Thermogravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). COMSOL Multiphysics software was used to model the optical properties of undoped and Ag doped TiO2.
Bacteria inactivation in light conditions was mainly due to the generation of reactive oxygen species (ROS). Photocatalytic activity in the visible light region was successfully demonstrated by the Ag doped TiO2 nanoparticles for the degradation of Escherichia coli (E. coli) bacteria. The results for Ag doped TiO2 indicate 100% inactivation of E. coli after 2 hours under UV-A irradiation and 99% bacteria inactivation 4 hours when exposed to visible light. Photocatalytic experiments with bare TiO2 nanoparticles rendered E. coli colonies inactive at time of 4 hours in UV-A light condition. The inactivation rate was 100%. Under visible light, the undoped titania showed significant inactivation but it was clear that the photocatalytic effect could not improve due to insufficient ROS production. In dark condition, both Ag doped titania and bare titania had an effect on the E. coli population. This was attributed to direct contact with Ag ions resulting in the formation of toxic Ag species and effect of adsorption stresses respectfully. COMSOL simulations confirmed the surface plasmonic resonance effect of the Ag nanoparticles. This gave an insightful explanation to the experimental photocatalytic bacteria studies conducted.