Tuning the charge carrier density and exciton pair separation in electrospun 1D ZnO-C composite nanofibers and its effect on photodegradation of emerging contaminants.

Maximizing anion (carbon) doping is thought to increase the charge carrier density in ZnO and other semiconductor metal oxide photocatalysts. It also enhances the photocatalytic activity of ZnO nanostructures by imparting visible light responsiveness. However, the effect of the carbon source on the doping efficiency, and in turn on the photocatalytic activity of ZnO nanostructures has been overlooked thus far. In this study one dimensional (1D) ZnO-Carbon composite nanofibers were prepared from different polymer (polyacrylonitrile, polystyrene, polyvinylpyrrolidone) precursor solutions and the C-doping efficiency and its effect on the photocatalytic activity were studied. The prepared nanofiber photocatalysts were characterized by XRD, XPS, FE-SEM, BET, TGA, FT-IR, photoelectrochemical and optical analyses techniques. Based on the thermal degradation profile of the polymer sources, the C-doping efficiencies varied among the samples prepared and so does their photocatalytic activity. Caffeine molecule was selected as a model emerging contaminant and its photodegradation was analyzed in the presence of the as-prepared photocatalysts. Upon the C-doping, new energy level was introduced within the bandgap of ZnO that lowers its bandgap energy by 0.35 eV. Additionally, the charge carrier density of ZnO increased and the flat band potential showed positive shift. These, together with the 1D nature of the photocatalysts, enhanced the photocatalytic activity of pristine ZnO by ~58% and 2.8 folds faster kinetics. Mechanistic study showed that hydroxyl radicals were the most active reactive species responsible for the caffeine molecule degradation. This study underscores that the photocatalytic activity of ZnO for the degradation of environmental pollutants can be maximized by C-doping through careful selection of the carbon source.