The enhancement of CdS ultrathin nanosheets photocatalytic activity for water splitting via activating the (001) polar facet by hydrogenation and its charge separation mechanism†

Abstract

The establishment of a general charge separation model is crucial for the design of high-performance photocatalytic materials. Based on charge separation between polar surfaces, herein, we synthesized atomically thick CdS nanosheets with exposed {001} polar facets though a reflux method. The photocatalytic H₂ evolution performance of ultrathin CdS nanosheets was remarkably enhanced by getting rid of O–H groups and Cl⁻ and (NH₂CH₂CH₂)₂N⁻ ions on the Cd-(001) polar surface by hydrogenation. A CdS-H170 ultrathin nanosheet shows an excellent photocatalytic H₂ production rate (26.5 mmol g⁻¹ h⁻¹), which is significantly higher than that of all pure CdS photocatalysts reported so far. Based on the CdS {001} polar structure, it was found that the exposed {001} surfaces of the CdS nanosheets are polar facets, and a spontaneous electric field (Es) exists between {001} polar planes. The main reason for the excellent photocatalytic H₂ evolution activity of hydrogenated ultrathin CdS nanoplates is an increase in the Es in CdS after the surface adsorbed ions or hydroxyl groups are removed by hydrogenation. Thus, the Es between Cd-(001) and S-(001̄) polar surfaces play an important role in the separation of photogenerated electrons and holes. Based on the concept of the Es in a polar structure, the mechanism of photocatalytic hydrogen production was proposed. This mechanism of improving photocatalytic performance by Es-driven charge separation should be a universal mechanism of photocatalytic hydrogen production. Thus, charge separation will be helpful in understanding the influence of crystal morphology and surface on photocatalytic activity and in designing and directing the synthesis of other high-performance photocatalysts.