Hydrogen Chemical Configuration and Thermal Stability in Tungsten Disulfide Nanoparticles Exposed to Hydrogen Plasma

Abstract

The chemical configuration and interaction mechanism of hydrogen adsorbed in inorganic nanoparticles of WS are investigated. Our recent approaches of using hydrogen activated by either microwave or radiofrequency plasma dramatically increased the efficiency of its adsorption on the nanoparticles surface. In the current work we make an emphasis on elucidation of the chemical configuration of the adsorbed hydrogen. This configuration is of primary importance as it affects its adsorption stability and possibility of release. To get insight on the chemical configuration, we combined the experimental analysis methods with theoretical modeling based on the density functional theory (DFT). Micro-Raman spectroscopy was used as a primary tool to elucidate chemical bonding of hydrogen and to distinguish between chemi- and physisorption. Hydrogen adsorbed in molecular form (H) was clearly identified in all the plasma-hydrogenated WS nanoparticles samples. It was shown that the adsorbed hydrogen is generally stable under high vacuum conditions at room temperature, which implies its stability at the ambient atmosphere. A DFT model was developed to simulate the adsorption of hydrogen in the WS nanoparticles. This model considers various adsorption sites and identifies the preferential locations of the adsorbed hydrogen in several WS structures, demonstrating good concordance between theory and experiment and providing tools for optimizing of hydrogen exposure conditions and the type of substrate materials.

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

“…Recent experiments of plasma-hydrogenation of WS 2 materials did not reveal the existence of chemisorbed atomic hydrogen 26. This observation is now supported by the following calculations.…”

“…This also justifies that the intercalation energies per atom in Table 5 are close to the chemisorption energies (1.37 eV) of H atoms on the surface of a single WS 2 layer. However, the experimental Raman measurements did not detect H—S bonds (see below) 26. We conclude that the amount of atomic hydrogen on the surface or intercalated between layers of WS 2 materials is non-significant because H 2 has a high dissociation energy, and perhaps there was not enough atomic H in the RF plasma.…”

“…Tenne and co-workers were the first who reported that the TMDCs with layered structures, like WS 2 and MoS 2 , can form closed-cage nanoparticles with polyhedral or nanotubular shapes 25. An experimental investigation of the interaction of hydrogen with WS 2 nanotubes and fullerenic-like nanoparticles, and the capacity of these materials to store hydrogen, has been reported in two previous publications,10,26 and some preliminary ideas on the interaction have been proposed 26,27. Various hydrogenation methods were applied in those experiments: hydrogenation by high-pressure molecular hydrogen, as well as microwave and radiofrequency (RF) plasma-enhanced hydrogenation performed at low gas pressures.…”

“…In a previous work we reported a series of experimental micro-Raman spectra of WS 2 nanoparticles (NPs): a) non-hydrogenated NPs, b) RF plasma hydrogenated NPs, and c) RF plasma hydrogenated NPs vacuum annealed up to 450 °C 26. The three spectra correspond to the black-solid lines in Figure 13.…”

“…Of course those barriers are no relevant if chemisorbed H is produced in experiments already starting with atomic hydrogen. In fact, neutral H atoms and H + cations are generated in a hydrogen plasma, in addition to molecular hydrogen,10,26 although in a concentration usually too small compared to that of H 2 .…”