(317e) Visible-Light-Responsive Materials for Separation of Oil-Water Mixtures

Photocatalytic materials such as TiO₂ have attracted significant attention in altering the wettability of surfaces. These materials degrade any organic deposits and induce superhydrophilic properties to the surface upon irradiation with UV light. The excitation of TiO₂ by light with energy greater than the bandgap is the primary process underlying its vast applications in self-cleaning, antifogging, treatment of wastewater and most recently separation of oil-water mixtures. However, the large bandgap of TiO₂ (~ 3.2 eV for anatase and ~ 3.0 eV for rutile) limits its ability to absorb visible light or sunlight effectively. Given that only ~ 5 % of solar flux incident at the earth’s surface lies in this spectral regime (λ < 390 nm), utilization of a large fraction of sunlight for excitation process can thus be enhanced by tuning the bandgap of TiO₂ to the visible spectral regime. In this work, we have developed novel approaches to tune the bandgap of TiO₂ to be responsive in the visible spectral regime by surface modification. Under visible light irradiation, we show that these modified TiO₂ surfaces undergo hydrophobic-to-hydrophilic conversion in both air and oil environments. We explain the mechanism behind the change in wettability in terms of the surface regeneration processes. Such unique visible light-induced hydrophobic-to-hydrophilic conversion of our surfaces enables on-demand separation of oil-water mixtures. We anticipate that the materials that have such tunable light-responsive wettability can find broad applications in oil-water separation, wastewater treatment, self-cleaning, and phase transitions.