Metal adsorbate interactions and the convergence of density functional calculations

TL;DR

This study investigates how system size affects metal atom adsorption energies on nanostructures, revealing size-dependent interactions and providing a scaling relation to accurately estimate binding strengths in computational models.

Contribution

It identifies the mechanisms of size-dependent interactions in DFT calculations of metal adsorption and proposes a scaling relation to improve accuracy in binding energy estimations.

Findings

System size significantly influences adsorption energies in metallic and semi-metallic systems.

Insulating substrates show negligible size dependence.

A scaling relation enables extrapolation to the isolated-impurity limit.

Abstract

The adsorption of metal atoms on nanostructures, such as graphene and nanotubes, plays an important role in catalysis, electronic doping, and tuning material properties. Quantum chemical calculations permit the investigation of this process to discover desirable interactions and obtain mechanistic insights into adsorbate behavior, of which the binding strength is a central quantity. However, binding strengths vary widely in the literature, even when using almost identical computational methods. To address this issue, we investigate the adsorption of a variety of metals onto graphene, carbon nanotubes, and boron nitride nanotubes. As is well-known, calculations on periodic structures require a sufficiently large system size to remove interactions between periodic images. Our results indicate that there are both direct and indirect mechanisms for this interaction, where the latter can require even larger system sizes than typically employed. The magnitude and distance of the effect depend on the electronic state of the substrate and the open- or closed-shell nature of the adsorbate. For instance, insulating substrates (e.g., boron nitride nanotubes) show essentially no dependence on system size, whereas metallic or semi-metallic systems can have a substantial effect due to the delocalized nature of the electronic states interacting with the adsorbate. We derive a scaling relation for the length dependence with a representative tight-binding model. These results demonstrate how to extrapolate the binding energies to the isolated-impurity limit.