Photocatalytic nanocomposites for increased optical activity

Abstract

To combat environmental pollution, photocatalytic decomposition provides degradation of organic and inorganic contaminants near the surface of the photocatalyst nanoparticles by converting optical energy of the absorbed light into chemical energy for the redox reactions. However, photocatalytic activities of such semiconductor metal-oxide nanoparticles are limited with their bandgap energy that allows for optical absorption typically in the ultraviolet spectral range. Yet another limitation is that the photocatalytic activity of these semiconductor nanoparticles is substantially reduced when they are immobilized in solid thin films, resulting from their effectively decreased active surface area. But such immobilized nanoparticles are much more desired in industrial applications, e.g., for mass environmental decontamination and outdoors/indoors self-cleaning on large surfaces. To address these issues, in this thesis, we investigated and demonstrated the spectral behavior and time evolution of optical activity curves of immobilized TiO2 and ZnO nanoparticles. We studied the nanoparticle size effect for the optical activity and demonstrated significant increase in the resulting photocatalysis with decreasing the size of such immobilized nanoparticles for the first time. We obtained optimal excitation conditions for TiO2 and ZnO nanocomposite films separately. We achieved maximum optical recovery levels of 93% for TiO2 nanoparticles and 55% for ZnO nanoparticles at the excitation wavelengths of 310 nm and 290 nm, respectively, after optical irradiation with an excitation density of 7.3 J/cm2 , where we observed no optical recovery for their respective negative control groups (with no nanoparticles). In these comparative spectral studies, we showed strong correlation between the differential optical recovery and the photocatalytic activity. For further substantial enhancement in the near ultraviolet and visible spectral ranges, we also proposed and demonstrated the use of a unique combination of TiO2-ZnO nanoparticles integrated together into the same resin. In this novel approach, we observed higher levels of photocatalytic activity under optical irradiation at and above 380 nm compared to the cases of only TiO2 or only ZnO nanocomposite films with the same total metal-oxide nanoparticle density. At 400 nm in the visible, we accomplished an optical recovery level of ~30% with the combination of TiO2-ZnO nanoparticles together while this level was only ~14% for the TiO2 nanoparticles alone and ~3% for the ZnO nanoparticles alone under identical conditions. Even at 440 nm, we obtained ~20% optical recovery using the TiO2-ZnO photocatalytic synergy, despite the optical activity of the single type of nanoparticles alone close to the zero base-line of their control group. These proof-of-concept experimental demonstrations indicate that such TiO2-ZnO combined nanocomposite films hold great promise for efficient environmental decontamination in daylight.