Altering the morphology of graphene aerogels through control of the gelation time and drying method

Abstract

Graphene aerogels are being increasingly investigated, however, a robust understanding of the effects of synthesis parameters on their morphology and performance remains underdeveloped. This work reports on the successful microstructural control of reduced graphene oxide (rGO) aerogels, prepared via low-temperature hydrothermal reduction of GO, through changes to their gelation time and drying method. Using ascorbic acid as the reducing agent, the hydrogels were allowed to set at three gelation times; until the ‘just-gelled’ state or onset of gelling, for 80 minutes, and for 720 minutes, the latter typically reported in the literature. The hydrogels were then subjected to either freeze drying, or CO2 supercritical drying, followed by a final pyrolysis step. The resulting aerogels were characterised by scanning electron microscopy and micro-Raman spectroscopy. The mechanical properties of the rGO aerogels were assessed under compression loading, whereas their ability to perform in water was assessed by a simple immersion test. The results showed that freeze drying of the just-gelled hydrogels produced aerogels with large and cellular pores, low compressive moduli, and rather poor water stability. Longer gelation times produced denser aerogels with smaller pores and improved water stability. Such results suggest that for the freeze dried aerogels, the gelation time influences the microstructure of the rGO gel – short gelation times lead to weaker, more pliable hydrogels, less resistant to ice crystal growth upon freeze drying. Supercritical drying produced aerogels with randomly oriented nanometrically sized pores, high compressive moduli, and good water stability, irrespective of the gelation time.