Spatial charge separation between wurtzite CdS polar (0001) and (0001¯) facets and their enhanced visible-light photocatalytic activity

The films assembled from wurtzite CdS nanocrystals with average size of 8.0 nm and clean {0001} facets have been synthesized by heating Cd(NO3)2-thiourea aqueous solution on Si substrate at 300 °C for 20 min. The CdS films show superior photoreactivity, in comparison with CdS dendrites and flowerlike architectures assembled from nanoflakes with exposed {0001} facets. The superior photocatalytic activity is attributed to the active {0001} facets. Structure of the wurtzite CdS {0001} facets was studied using periodic density functional theory. The exposed CdS {0001} facets were found to be polar surfaces, thus a charge separation model between polar Cd-CdS (0001) and S-CdS (0001¯) surfaces was proposed. The internal electric field between the polar Cd-CdS (0001) and S-CdS (0001¯) surfaces provides an effective driving force for the separation of photogenerated electrons and holes. The reduction and oxidation reactions selectively occur on the positive Cd-CdS (0001) and negative S-CdS (0001¯) planes. The good charge separation results in high photocatalytic activity. The charge separation model may provide new insights into the understanding of charge transfer in the semiconductor nanomaterials with reactive facets and the design of advanced photocatalysts and photoelectronic devices.