Thermal transport in monolayer graphene oxide: Atomistic insights into phonon engineering through surface chemistry

Thermal transport in monolayer graphene oxide (GO) with randomized surface epoxy and hydroxyl groups, at various degrees of oxidation (O:C ratio), is investigated using non-equilibrium molecular dynamics simulations. We find that the in-plane thermal conductivity of finite sized pristine graphene or GO (5-50 nm in simulation) increases with length due to reduced phonon-boundary scattering. The intrinsic in-plane thermal conductivity and phonon mean free path of infinite pristine graphene or GO, are estimated based on the kinetic theory of phonon transport. We find that the thermal conductivity drops sharply to 17% of the pristine graphene value for a 1% O:C ratio, and to 1.5% for a typical GO with 20% O:C ratio, suggesting that typical GO is not a very good heat conductor compared to pristine graphene. Surface oxidation suppresses the density of state of the phonon mode due to C-C bonds (the G peak), reducing the phonon specific heat of this mode and hence, overall thermal conductivity. Phonon-defect scattering at the surface oxidized groups reduces the intrinsic mean free path of GO, also contributing to the reduction. Our results characterizes thermal transport in GO and offer insights into surface chemistry-mediated thermal transport in other 2D materials.