Facet-dependent generation of superoxide radical anions by ZnO nanomaterials under simulated solar light

Abstract

The capability of semiconductor nanomaterials to generate reactive oxygen species (ROS) under solar irradiation, which is critical for their photocatalytic applications and may affect their environmental implications, can be substantially influenced by intrinsic nanomaterial properties, particularly exposed facets. However, the specific mechanisms controlling the facet-dependent ROS generation of widely used semiconductor nanomaterials, e.g. zinc oxide (ZnO), are not well understood. Here, we report that under identical irradiation conditions a ZnO nanoplate material with predominantly exposed {0001} facets generates nearly four times as much superoxide radical anions (O₂˙⁻) as a porous ZnO nanosheet material with predominantly exposed {100} facets, even though the former possesses much smaller specific surface area. The enhanced O₂˙⁻ generation of ZnO nanoplates with exposed {0001} facets can be ascribed to the following two aspects: (1) the {0001} facets, which contain more abundant coordinatively unsaturated zinc atoms, can adsorb a greater amount of molecular oxygen (O₂), and (2) the more negative conduction band potential of the {0001} facets can provide stronger thermodynamic driving force for the reduction of surface O₂ to O₂˙⁻. The enhanced O₂˙⁻ generation resulted in faster photocatalytic degradation of organic contaminants (using tetracycline as a model antibiotic contaminant) and greater bacterial inactivation (using Escherichia coli as a model microorganism). This study further identifies exposed facets as a critical nanomaterial property in dictating the photocatalytic activity and environmental implications of nanomaterials.