Molecular dynamics study of oxygen functionalization effects on graphene thermal conductivity : roles of C/O ratio and functional-group chemistry

Abstract

Graphene has been widely explored as a thermally conductive filler in nanocomposites and interface materials, yet its performance is highly sensitive to surface functionalization introduced during processing. Oxygen functional groups including epoxy (-O-), hydroxyl (OH), and carboxyl (COOH) are beneficial to composite matrices formation but simultaneously disrupt phonon transport by breaking the sp2 carbon network. A systematic non-equilibrium molecular dynamics study was conducted to elucidate how oxygen content and functional-group chemistry influence the thermal properties of graphene. Pristine graphene (2-12 nm lateral size) was first simulated to validate the methodology. Thereafter, the overall C/O ratio was varied from 300 to 4 while keeping the relative proportions of COOH, -O-, and OH groups constant; a steady decline in the relative thermal conductivity from 0.76 to 0.10 was observed as oxygen coverage increased. Finally, at a fixed C/O ratio of 7, the loading of each functional group was independently increased from 12 % to 65 %, and their respective impacts on thermal conductivity were quantified. COOH was found to yield the mildest thermal conductivity reduction due to the edge functionalization, whereas -O- functionalization caused the steepest decline (up to ≈0.15 at 65 % loading) because of the stronger interruption on the sp2 domain. The OH groups produced an intermediate, nonmonotonic response. These findings provide molecular-level design rules for balancing dispersion stability against thermal performance in graphene-based composites and thermal interface materials.