Negative contribution from defects responsible for low Young's modulus of graphene oxide at small defect densities

Graphene oxide has been extensively employed as an additive in several nanocomposites to enhance their mechanical stability even though its Young's modulus is significantly smaller than that of pristine graphene. In the past decade, various chemical functionalizations have been attempted to enhance the mechanical strength of graphene oxide. In this work, we analyze the atomic contributions to the Young's modulus (YM) of graphene oxide with relevant models to decouple the role of the defects and the oxygen functionalities. Based on our analysis we show that (1) the presence of defects is more important than the oxygen groups for reducing the YM and (2) the defect atoms provide negative contribution to the YM at low defect densities. The latter novel observation can be exploited, in principle, to perform selective substitution of the defect atoms to increase the YM of graphene oxide while keeping other functional groups in the non-defect region intact for further functionalization, if required. In the proof of concept example, 75% of the enhancement of the YM obtained upon substitution of the oxygen functionalities with two hydrogen atoms, in silico, can be accomplished by displacing just 10% of the oxygen atoms that are exclusive to the defects.