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

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.