Aggregation Kinetics and Self-Assembly Mechanisms of Graphene Quantum Dots in Aqueous Solutions: Cooperative Effects of pH and Electrolytes

The cooperative effects of pH and electrolytes on the aggregation of GQDs and the aggregate morphologies are characterized. Because GQDs have an average size of 9 nm with abundant O-functionalized edges, their suspension was very stable even in a high electrolyte concentration and low pH solution. Divalent cations (Mg2+ and Ca2+) excelled at aggregating the GQD nanoplates, while monovalent cations (Na+ and K+ did not disturb the stability. For Na+ and K+, positive linear correlations were observed between the critical coagulation concentration (CCC) and pH levels. For Mg2+ and Ca2+, negative, but nonlinear, correlations between CCC and pH values could not be explained and predicted by the traditional DLVO theory. Three-step mechanisms are proposed for the first time to elucidate the complex aggregation of GQDs. The first step is the protonation/deprotonation of GQDs under different pH values and the self-assembly of GQDs into GQD-water-GQD. The second step is the self-assembly of small GQD pieces into large plates (graphene oxide-like) induced by the coexisting Ca2+ and then conversion into 3D structures via pi-pi stacking. The third step is the aggregation of the 3D-assembled GQDs into precipitates via the suppression of the electric double layer. The self-assembly of GQDs prior to aggregation was supported by SEM and HRTEM imaging. Understanding of the colloidal behavior of ultrasmall nanoparticles like GQDs is significantly important for the precise prediction of their environmental fate and risk.