Abstract
The traditional freeze-casting route for processing graphene-based aerogels is generally restricted to aqueously dispersed flakes of graphene oxide (GO) and post-processing reduction treatments, which brings restrictions to the aerogels electrical properties. In this work, we report a versatile aqueous processing route that uses the ability of GO todisperse graphene nanoplatelets (GNP) to produce rGO-GNP lamellar aerogels via unidirectional freeze-casting. In order to optimise the properties of the aerogel, GO-GNP dispersions were partially reduced by L-ascorbic acid prior to freeze-casting to tune the carbon and oxygen (C/O) ratio. The aerogels were then heat treated after casting to fully reduce the GO. The chemical reduction time was found to control the microstructure of the resulting aeorgels and thus to tune their electrical and mechanical properties. An rGO-GNP lamellar aerogel with density of 20.8 ± 0.8 mg cm−3 reducing using a reduction of 60 min achieved an electrical conductivity of 42.3 S m−1. On the other hand, an optimal reduction time of 35 min led to an aerogel with compressive modulus of 0.51 ±0.06 MPa at a density of 23.2 ± 0.7 mg cm−3, revealing a compromise between the tuning of electrical and mechanical properties. We show the present processing route can also be easily applied to produce lamellar aerogels on other graphene-based materials such as electrochemically exfoliated graphene.
Highlights
The outstanding properties of graphene, including its high mechanical strength, high electrical conductivity, low density, and high surface area, grant it potential in a variety of application in composites, nano-electronics, energy storage, sensors, catalyst, and biomedicine [1,2,3]
We report a versatile aqueous processing route that uses the ability of graphene oxide (GO) todisperse graphene nanoplatelets (GNP) to produce rGO-GNP lamellar aerogels via unidirectional freeze-casting
A black-coloured aqueous suspension with a solid concentration of 25 mg ml−1 GO-GNP was formed. 50 mg of L-ascorbic acid was added to the suspension (1:1 mass ratio of GO to L-ascorbic acid), homogenised by shear mixing for 10 min in ice bath and placed into a water bath at 50 °C for a given time, t min
Summary
The outstanding properties of graphene, including its high mechanical strength, high electrical conductivity, low density, and high surface area, grant it potential in a variety of application in composites, nano-electronics, energy storage, sensors, catalyst, and biomedicine [1,2,3]. Wang et al [17] developed mechanically strong, super elastic rGO aerogel by using bidirectional freeze-casting method Several parameters, such as flake size, concentration, cooling rate, and mould shape have been found to influence the final microstructure and properties of the formed aerogels [18,19,20]. The modification via chemical route can disrupt the electronic paths in graphene and deteriorate the electrical and other quantum effect properties of the stuctures [25] To address this issue, some studies have adopted a non-covalent approach by using surfactant, including charged and uncharged polymers for dispering graphene materials [26, 27], though the stabilizing effect is still limted. It would be very useful to combine this approach with freeze casting to create high quality graphenebased aerogels
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