Pristine Graphene Aerogels by Room‐Temperature Freeze Gelation

Abstract

Aerogels can be fabricated from pristine graphene exfoliated nanosheets using freeze gelation with nonaqueous solvents and no heat treatment or reduction stage. Solvents are selected that disperse pristine graphene with a melting point above room temperature but with a high vapor pressure above the solid at room temperature, enabling sublimation (freeze drying) under ambient conditions. A single sheet of graphene naturally possesses an extremely large specific surface area.1 However, to enable the high surface area to be used in a practical device it must be packaged in a 3D volume. The highest surface area to volume ratio material based on chemically pure or pristine graphene (PG) is obtained using catalytic vapor deposition (CVD) to grow graphene on nanoporous templates, followed by template dissolution.2 More cost effective nanoporous structures can be fabricated using graphene oxide (GO). These methods are more scalable but compromise on the properties because GO, after appropriate heat treatment, forms the defective reduced graphene oxide (rGO). Fabrication of 3D nanoporous structures using chemically exfoliated GO has been achieved by various methods3 including aqueous freeze gelation4-6 and hydrothermal processing.7 Here we present a room temperature freeze gelation method to manufacture porous PG nanostructures from exfoliated material. The electrical properties of which are superior to conventional GO derived structures and approach those of CVD produced PG aerogels. Room-temperature freeze gelation (RTFG) was originally developed by Halloran and co-workers for the manufacture of ceramics.8 This processing route is similar to conventional aqueous freeze gelation (or freeze casting) but with the water replaced by an organic solvent selected to have a melting temperature greater than room temperature and a high vapor pressure. The material to be processed is mixed and dispersed in the solvent above its melting point, typically in the temperature range 50–120 °C and cooled to form a solid at room temperature. The solvent is selected to have a high vapor pressure above the solid at room temperature and thus rapidly sublimes at room temperature under ambient atmospheric conditions, leaving a porous solid of the original material.8, 9 The key advantages of this route over conventional aqueous freeze casting is that the broader range of solvents allow materials to be dispersed directly without surfactants or functionalization and, more significantly, the solid can be formed through conventional processing techniques prior to sublimation, whereas with aqueous routes one is restricted to purely freezing in a given shaped container. A schematic of how the RTFG process is used for graphene aerogel manufacture is illustrated in Figure 1a. Briefly, PG flakes are dispersed to the required concentration in an appropriate solvent at elevated temperature, using sonication to form a stable dispersion. This dispersion is then used with an appropriate forming method, e.g., molding, extrusion or printing, and cooled to promote solidification. The resulting waxy solid contains a uniform suspension of PG flakes. The solvent sublimes at room temperature leaving a nanoporous graphene aerogel. RTFG has two principal advantages over conventional routes for the manufacture of graphene aerogels by aqueous freeze gelation: i) the solvent has a high vapor pressure and thus the sublimation process can be carried out under ambient conditions at room temperature and standard atmospheric pressure, ii) the wide range of potential solvents allows the selection of a fluid in which PG flakes can be readily dispersed. Two organic solvents have been used here to demonstrate the process: i) phenol (C6H6O: melting temperature, TM = 40.5 °C, vapor pressure at room temperature, PT = 47 Pa)10 because of its prior use as a graphene exfoliant and solvent,11 and ii) camphene (C10H16: TM = 51.5 °C, PT = 400 Pa)8 because of its low toxicity (it is used in perfumes and