Water Confined in Graphene Sheets: a Fast Field Cycling NMR Study

The production of high-quality graphene sheets is crucial for their practical application in electronics, optoelectronics, energy-storage devices, and composite materials. Among the preparation methods, electrochemical exfoliation of graphite in acidic electrolytes is particularly attractive because of its easy, fast, and environmentally friendly nature [1]. In this work, we aim at assessing the effect of applying different exfoliation voltages on the structure of graphene sheets by using 1H Fast Field Cycling (FFC) NMR relaxometry. The R1 dispersion curves of aqueous suspensions of graphene sheets obtained at different voltages are reported in Fig.1. At all voltages the curves exhibit similar features, including high-frequency plateau, dispersion centered at 10 MHz, and R1 dependence on the Larmor frequency below 1 MHz. The similarity of the features indicates that the voltage does not significantly affect the dynamics of water molecules interacting with graphene. As the voltage increases, R1 decreases, indicating that higher voltages cause a larger fraction of water not to interact with the graphene sheets, implying greater distances between sheets in a flake. EPR could help clarify the relaxation mechanism, aiding in selecting the best model for FFC NMR data analysis, from which structural parameters will be extracted by fitting the data.