One-dimensional cadmium sulphide nanotubes for photocatalytic water splitting

Recently, a two-dimensional (2D) stable CdS monolayer with remarkable visible-light photocatalytic activity for water splitting has been proposed via theoretical calculations. It is also interesting to explore the photocatalytic activity of one-dimensional (1D) CdS nanotubes (CSNTs). In the present study, we have designed atomic models for CSNTs in the 5.2-19.2 Å outer diameter range and investigated the basic properties using density functional theory. It is shown that the CSNTs have negative strain energies when the diameter is beyond 6.4 Å; this indicates that they can possibly be folded from 2D CdS nanosheets. Moreover, all the CSNTs are direct-energy-gap semiconductors, and their energy gaps monotonically increase as the outer diameter increases. Tunable band gaps provide strong evidence for the effectiveness of nanostructuring on the electronic properties of CSNTs. Due to their perfect band gaps and band energies for photocatalytic activity, CSNTs exhibit potential application in visible light photocatalysis for water splitting. Because of their better photooxidation capabilities and smaller effective masses of holes, CSNTs are more promising photocatalytic materials for the oxygen evolution reaction in Z-scheme systems than monolayer CdS. In addition, the designed CdS nanotube-planar (CSNTP) hetero-dimension junction can be used as a direct Z-scheme photocatalyst. Electron transfer is observed in the interface region, which induces an internal electric field that effectively promotes the separation of charge carriers at the interface and reduces the probability of recombination. This study helps to further understand the advanced strategies to design and improve potentially efficient photocatalysts for the overall water splitting reaction.