Variable range hopping conductivity of hydrothermally reduced graphene oxide fibers

We report applying the autoclaved hydrothermal method for obtaining conductive reduced graphene oxide (rGO) fibers for potential flexible electronic applications, such as supercapacitors, transistors, or sensing applications. The reduction of GO was performed in the temperature range 120 to 180 °C under increased pressure of ca. 8 bar in a sealed Teflon lined up, stainless steel autoclave. The fiber’s diameter and length were defined by the glass tube used as the mold for reducing GO water suspension (diameter of 600 µm and length of 8 cm). After drying, in an ambient atmosphere, the hydrogel fiber shrinks to ca. 50 µm in diameter and 6 cm in length (collapsed pore structure). The drying process, in addition to enhancing electrical conductivity, also increases the mechanical strength of the fibers due to the stronger overlapping of the graphene flakes. The best performance was observed in the fiber reduced at the highest temperature studied, 180 °C, and a minimum temperature of 120 °C is necessary to obtain a fiber. Electrical conductivity was measured using the 4-probe method. The results were analyzed within the framework of variable range hopping and Arrhenius models to pinpoint the best model describing electrical conductivity in dry rGO fibers.