PNNL

Abstract

Functionalization massively improves graphene as a material for nanoscale separations. Yet, a molecular level understanding of how specific functional groups influence the competitive adsorption of solvated ions and water on functionalized graphene surfaces remains an open issue. Here, we present an ab initio molecular dynamics study, supported by experimental evidence, of the adsorption behavior of solvated Pb(II) ions and water ¬– an essential solution component – on graphene functionalized with different O-containing groups. Common O-functional groups like OH and COOH exert a stronger influence on the water density, water-water interactions, water-functional group interactions, molecular adsorption structures, and hydrogen bond networks than O and H functional groups. Graphene functionalized with O-containing groups shows higher adsorption affinity for Pb(II), reflected by the stronger Pb-O bonds and weaker interaction between Pb(II) and the surrounding water molecules. When COOH functional groups are present, proton dissociation is observed, and the COO- groups formed especially stabilize Pb(II) through strong electrostatic interactions, suggesting that among all of the O-functional groups under study, COOH offers the best Pb(II) adsorption capacity for graphene oxide. Inspired by the findings of the theoretical investigation, cyclic voltammetry and electrochemical impedance spectroscopy measurements were performed on graphene oxide (GO) prepared with two different coverages of O-containing functional groups. Electrochemical measurements suggest that GO with a low coverage of O functional groups (C=O, COOH) allows more Pb(II) ions to diffuse through the GO layers rather than adsorbing on the surface, which in agreement with the theoretical predictions. Our findings have substantial implications towards understanding the selective transport of water and ions through GO laminate membranes used in separations and desalination.