Simultaneous fluorination and purification of natural block coaly graphite into fluorinated graphene with tunable fluorination degree

Abstract

Fluorinated graphene (FG) holds great promise for various applications, but the cost-effective production of FG is still challenging because of the high cost of the starting material such as fluorinated graphite, graphene and graphene oxide. Herein, a novel approach to prepare FG through electrochemical exfoliation of the starting natural block coaly graphite (NBCG) mineral, followed by a hydrothermal reaction is reported. This method uses NH4F solution in the whole process as fluorinating agent to provide fluoride source and purification agent to remove impurity minerals in natural graphite. The morphology, chemical composition and microstructure of the starting NBCG, a series of fluorinated graphene oxide (FGO) samples after electrochemical exfoliation and FG samples after hydrothermal process were characterized in detail by a number of analytical techniques such as TEM, SEM, XPS, TGA, XRD and Raman. The thick and compact graphitic platelets of NBCG have a considerable decrease in the thickness and lateral size during the process, and the prepared FG consists of thin separated nanosheet with a low ash content. Moreover, this method is effective in tuning the fluorination degree of FG with tunable electrical conductivity through the control of the electrolyte concentration in the first step of electrochemical exfoliation. The formation mechanism of FG from NBCG is proposed. It reveals that the electrochemical process results in the oxidation of NBCG and the adsorption of fluorine ions, and those oxygen-containing groups are essential for the further fluorination in subsequent hydrothermal process. Overall, this method paves a way to prepare FG with a low cost due to the avoid of prerequisite of crushing, grinding and purifying procedures for NBCG.