Structure and stability of small H clusters on graphene

TL;DR

This study uses DFT calculations to analyze the structure, stability, and hydrogen desorption mechanisms of small H clusters on graphene, revealing stable configurations and diffusion-dependent desorption pathways.

Contribution

It systematically investigates cis- and trans-H clusters on graphene, identifying stable configurations and desorption pathways, which were not previously characterized in detail.

Findings

Stable cis- and trans-clusters identified with specific H arrangements.

H2 desorption pathways involve para-cis-dimer states and diffusion processes.

Diffusion energy barriers depend on reaction energies and cluster size parity.

Abstract

The structure and stability of small hydrogen clusters adsorbed on graphene is studied by means of Density Functional Theory (DFT) calculations. Clusters containing up to six H atoms are investigated systematically -- the clusters having either all H atoms on one side of the graphene sheet (\textit{cis}-clusters) or having the H atoms on both sides in an alternating manner (\textit{trans}-cluster). The most stable cis-clusters found have H atoms in ortho- and para-positions with respect to each other (two H's on neighboring or diagonally opposite carbon positions within one carbon hexagon) while the most stable trans-clusters found have H atoms in ortho-trans-positions with respect to each other (two H's on neighboring carbon positions, but on opposite sides of the graphene). Very stable trans-clusters with 13-22 H atoms were identified by optimizing the number of H atoms in ortho-trans-positions and thereby the number of closed, H-covered carbon hexagons. For the cis-clusters, the associative H$_2$ desorption was investigated. Generally, the desorption with the lowest activation energy proceeds via para-cis-dimer states, i.e.\ involving somewhere in the H clusters two H atoms that are positioned on opposite sites within one carbon hexagon. H$_2$ desorption from clusters lacking such H pairs is calculated to occur via hydrogen diffusion causing the formation of para-cis-dimer states. Studying the diffusion events showed a strong dependence of the diffusion energy barriers on the reaction energies and a general odd-even dependence on the number of H atoms in the cis-clusters.