The surface chemistry of graphene oxide: Key towards enhanced performance of ZnO-GO photocatalysts

Abstract

Graphene oxide (GO) is known to facilitate the enhancement of the photocatalytic performance of semiconducting metal oxides such as TiO2 or ZnO. However, the influence of its particular surface chemistry has not been clarified. In this work we employ GO with distinct and well-defined oxidation degree for the preparation of ZnO-GO hybrid materials. We demonstrate that the surface chemistry of GO is key for facilitating favorable interface interactions. This results in a lowering of the alignment of energy levels at the ZnO-GO interface and enables the transfer of photo-excited electrons from ZnO to GO. The remaining positive charge at the valence band of ZnO then easily can promote oxidation reactions with the analyte and/or the solvent. In this way, GO reduces the recombination rate of the photogenerated electron-hole pair and enhances the photocatalytic activity of ZnO. We show that by solely controlling the oxidation degree of GO and by optimizing the loading fraction highly efficient ZnO-GO photocatalysts can be prepared. Most favorable conditions are achieved for intermediate oxidation degrees of GO. Optimized ZnO-GO hybrids thus reveal photocatalytic degradation of methylene blue at conversion rates of up to 80%, within times as short as 70 minutes at concentrations as low as 0.045 mg/mL, accompanied by a good recyclability behavior. The results are of general character and extendable to semiconducting metal oxides used in photocatalysis [1].