Modelling of adsorption and intercalation of hydrogen on/into tungsten disulphide multilayers and multiwall nanotubes

Abstract

Understanding the interaction of hydrogen with layered materials is crucial in the fields of sensors, catalysis, fuel cells and hydrogen storage, among others. Density functional theory, improved by the introduction of van der Waals dispersion forces, provides an efficient and practical workbench to investigate the interaction of molecular and atomic hydrogen with WS2 multilayers and nanotubes. We find that H-2 physisorbs on the surface of those materials on top of W atoms, while atomic H chemisorbs on top of S atoms. In the case of nanotubes, the chemisorption strength is sensitive to the nanotube diameter. Diffusion of H-2 on the surface of WS2 encounters quite small activation barriers whose magnitude helps to explain previous and new experimental results for the observed dependence of the hydrogen concentration with temperature. Intercalation of H-2 between adjacent planar WS2 layers reveals an endothermic character. Intercalating H atoms is energetically favorable, but the intercalation energy does not compensate for the cost of dissociating the molecules. When H-2 molecules are intercalated between the walls of a double wall nanotube, the rigid confinement induces the dissociation of the confined molecules. A remarkable result is that the presence of a full H-2 monolayer adsorbed on top of the first WS2 layer of a WS2 multilayer system strongly facilitates the intercalation of H-2 between WS2 layers underneath. This opens up an additional gate to intercalation processes.