Atomic transport at charged graphene: why hydrogen and oxygen are so different

TL;DR

This study uses density-functional calculations to reveal how charge doping in graphene distinctly influences the diffusion of hydrogen and oxygen atoms, with significant implications for controlling atomic transport.

Contribution

It demonstrates the opposite effects of electron and hole doping on H and O diffusion barriers and quantifies the impact on diffusion rates at room temperature.

Findings

Electron doping decreases O diffusion barrier but increases H barrier.

Hole doping increases O diffusion barrier but decreases H barrier.

Diffusion rates can vary by several orders of magnitude with doping levels.

Abstract

Using density-functional calculations, we show that electron or hole doped graphene can strongly change the mobility of adsorbed atoms H and O. Interestingly, charge doping affects the diffusion of H and O in the opposite way, namely, electron doping increases/reduces while hole doping reduces/increases the diffusion barrier of H/O, respectively. Specifically, on neutral graphene the diffusion barriers of O and H are 0.74 and 1.01 eV, which are, upon a hole doping of $+5.9\times10^{13}$ cm$^{-2}$, 0.90 and 0.77 eV, and upon an electron doping of $-5.9\times10^{13}$ cm$^{-2}$, 0.38 and 1.36 eV, respectively. This means, within the harmonic transition state theory, at room temperature, the diffusion rate of O can be decreased or increased by 470 or 2.2$\times 10^7$ times, and that of H can be increased or decreased by $10^5$ or $7\times 10^7$ times, by that hole or electron doping level. The difference between the H and O cases is interpreted in terms of the difference in geometric and bonding changes upon charge doping.