Ion-specific regulation of epoxy conversion and hydroxyl migration on graphene oxide

The dynamic behavior of oxygen-containing functional groups on the surface of graphene oxide (GO) is crucial to its various applications. Previous studies have confirmed that water molecules adsorbed on the GO surface can regulate the dynamic evolution of oxygen-containing functional groups, mainly involving two core reactions: (i) the conversion of epoxy groups to dangling oxygen groups and (ii) hydroxyl migration. However, the influence of widely existing ions in aqueous solutions on the behavior of these oxidizing groups remains elusive. This study employs density functional theory (DFT) to elucidate the distinct roles of several cations (Li⁺, Na⁺ and K⁺) and anions (Cl^- and SO₄^2-) in regulating these fundamental reactions of the oxidizing groups. Our results reveal that these reactions are mechanistically controlled by the coupled effects of the GO-ion interaction energy increment and GO internal energy increment during the reaction. Specifically, in the model system of this study, cations facilitate epoxy conversion by enhancing GO-ion interaction energy in the reaction process, while anions hinder this conversion by elevating GO internal energy increment relative to that of pristine GO. In addition, Na⁺, K⁺ and SO₄^2- promote hydroxyl migration by reducing GO internal energy increment compared with pristine GO during the reaction, whereas Li⁺ exhibits minimal impact and Cl^- suppresses migration due to weakened GO-ion interaction in the reaction. This work provides new atomic-level insights into the ion-specific regulation of surface functional group evolution, offering significant potential for advanced membrane technologies and surface functionalization applications.