On the Chemistry and Diffusion of Hydrogen in the Interstitial Space of Layered Crystals h‐BN, MoS2, and Graphite

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 15; no. 43; pp. e1901722 - n/a
• Main Authors: An, Yun, Kuc, Agnieszka, Petkov, Petko, Lozada‐Hidalgo, Marcelo, Heine, Thomas
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.10.2019
• Subjects: 2D membranes; Boron nitride; diffusion; Diffusion coefficient; Diffusion layers; free‐energy surface; Hydrogen; Hydrogen isotopes; Isotope separation; Layered materials; Molybdenum disulfide; Nanotechnology; Organic chemistry; proton and atomic hydrogen diffusion; Protons; Transport; well‐tempered metadynamics simulations
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.201901722

Recent experiments have demonstrated transport and separation of hydrogen isotopes through the van der Waals gap in hexagonal boron nitride and molybdenum disulfide bulk layered materials. However, the experiments cannot distinguish if the transported particles are protons (H+) or protium (H) atoms. Here, reported are the theoretical studies, which indicate that protium atoms, rather than protons, are transported through the gap. First‐principles calculations combined with well‐tempered metadynamics simulations at finite temperature reveal that for h‐BN and MoS2, the diffusion mechanism of both protons and protium (H) atoms involves a hopping process between adjacent layers. This process is assisted by low‐energy phonon shear modes. The extracted diffusion coefficient of protium matches the experiment, while for protons it is several orders of magnitude smaller. This indicates that protium atoms are responsible for the experimental observations. These results allow for a comprehensive interpretation of experimental results on the transport of hydrogen isotopes through van der Waals gaps and can help identify other materials for hydrogen isotope separation applications. 2D layered materials h‐BN and MoS2 are demonstrated to separate hydrogen isotopes via nuclear quantum effects. The hydrogen diffusion through the interstitial space is explored for protium (H) atoms and protons (H+). In both cases, the diffusion is assisted by shear‐modes, with the free‐energy barrier of 0.08 eV for protium and 0.46 eV for protons in h‐BN.