Nanocarbon aerogel synthesis

Nanocarbon aerogel synthesis

Nanocarbon aerogels are the products of the self-assembly of discrete carbon nanostructures, such as graphene, graphene oxide or carbon nanotubes, into continuous networks [31]. Various synthetic strategies to create carbon nanostructure aerogels have been investigated over the last 5-10 years. An important method of manufacturing nanocarbon aerogels is chemical vapor deposition. This manufacturing process is based on the decomposition of carbon-containing gaseous precursors on sacrificial porous foams (e.g. metal foams, metal oxide foams) to form porous graphitic foams and aerogels. More commonly however, nanocarbon aerogels are produced by wet-chemical assembly of individualised carbon nanostructures in solution. In this approach, a homogenous suspension of fully dispersed nanocarbons is produced and used for self-assembly. This method is usually carried out in aqueous solution, therefore the nanocarbons are oxidized before dispersing them in water. After a stable exfoliated suspension is formed, nanocarbons can form a 3D network in solution (nanocarbon hydrogel), where crosslinking of the carbon nanostructures is based on the intermolecular forces between the nanocarbons or aided by gelling agents, such as polymers.  Subsequently, a mild drying process is initiated (e.g. freeze drying or supercritical CO2 drying) to remove water, producing 3D nanocarbon network where the dispersed phase is air (nanocarbon aerogel) [32].

Assembly of the nanocarbons in the first step can be classified in two categories: (a) random assembly in concentrated dispersion, or (b) templating strategies (e.g. templating around ice crystals or emulsion droplets). This project will explore both strategies to synthesis new GNF aerogels.

Random self-assembly method

As shown in Figure6 [33], a typical procedure of random self-assembly method is illustrated. Initially, dilute nanoparticles suspension evaporates to semi-dilute suspension with a large volume compression. With the help of a gelling agent and the increasing concentration, the nanoparticles are cross-linked into a 3D network, forming a wet-gel. Subsequently, the liquid from the obtained wet-gel is removed, typically by critical point drying (CPD) [33]. In a typical supercritical dried process, ethanol is added into the obtained gel to remove water through solvent-exchange [34]. Supercritical CO2 (scCO₂) is fully miscible with ethanol [34] so that it can be entirely replaced by scCO₂ during the CPD process. The surface tension of supercritical CO₂ is practically zero. Therefore, scCO₂ imposes no capillary pressure during drying, avoiding capillary collapse and thereby keeping the aerogel microstructure intact [34].

The vital advantage is to keep porousity structure of nanocarbon aerogels intact, thus improving the surface area of aerogels, which is likely to be of crucial importance for utilizing them as support frameworks for catalytic nanoparticles.

When assembling aerogels from nanostructures, achieving individualization of the nanostructures in the initial dispersion medium and crosslinking of the continuous network are of crucial importance. For carbon nanostructures, individualization is most commonly achieved by chemical (covalent or non-covalent) functionalization of the nanocarbon to improve compatibility with the dispersing medium in order to facilitate exfoliation and formation of stable nanocarbon dispersions. The subsequent crosslinking of the carbon nanostructures can be achieved through either covalent chemistry or through promoting non-covalent interactions such as van-der-Waals interactions or hydrogen bonding. For graphene derived aerogels, most approaches are utilizing GO as a starting material, enabling efficient exfoliation into stable dispersions of individual nanosheets. For example, Wu et al. [35] prepared a GO-derived aerogel with a high C/O proportion. In this hydrothermal reaction, an aqueous GO suspension of relatively high concentration (5 mg/mL) was aged at 85°C to produce a GO hydrogel, followed by supercritical CO₂ dying to create the GO aerogel. The as-prepared GO aerogel exhibited a huge specific surface area (870 m²/g). Based on the structural characterization, the GO sheets within the aerogel were self-assembled via hydrogen-bonding interactions between oxygen-containing groups, see Figure7. After reduction under hydrogen atmosphere, the obtained reduced GO aerogel showed substantially enhanced electrical conductivity [35].

Worsley et al. reported for the first time using CPD approach prepared monolithic DWCNT aerogels which exhibited large surface areas (585 m²/g) [36]. As both graphene and CNT aerogels without additives are often fragile, they may be reinforced with additives such as polyvinyl alcohol (PVA) in the suspension. Bryning et al. used CPD approach to fabricate PVA enhanced CNT aerogels, which can support 8000 times their weight, but had a decrease of electrical conductivity [37].

Templating methods

Nanocarbon aerogels can be templated through a variety of substrates. Here, we will focus on the templating by ice-crystals and emulsion droplets as these are most relevant approaches for this PhD research. Liu et al. synthesized [38] highly compressible, low density (8.7 mg/cm³) reduced GO aerogels with anisotropic porosity, derived from directional freeze-drying Figure 8. Various concentrations of GO suspensions were mixed with vitamin C (1:1, w/w) and, then heated at 70°C to acquire GO hydrogels. Crosslinking of the aerogels is induced through reduction of the GO by Vitamin C, leading to a (partial) restoration of the graphitic properties and resulting in strong, cross-linking van-der-Waals interactions between the sheets [38]. As phase separation leads to the isolation of solid nanoparticles from the ice, thus solid nanoparticles will pile up in-between adjoining ice crystals to form anisotropic structures [39]. After freeze-drying, the rGO aerogel exhibits anisotropic porous structure presented in Figure8 [38]. This anisotropic pore structure induced by directional freeze-drying plays a vital role in enhancing the mechanical property of the rGO aerogel.

Ice-templating can be combined with emulsion-templating to create nanocarbon aerogels with more complex microstructures. The mechanism of the emulsion-templated method utilises GO sheets as surface-active amphilphiles stabilizing the interface between the oil phase and the water phase in emulsions [40]. The GO sheets have a high concentration of hydroxyl and carboxylic acid groups at their surface [41]. Therefore, the GO sheets interfacial activity is highly depended on pH. According to this property, the pH of GO suspensions is adjusted to a relatively low value to decrease the deprotonation of oxygen-contained functional groups on GO surface which reduces the hydrophilicity of GO, resulting in preferential distribution GO at the oil/water interfaces [40]. In a typical emulsion-templating synthesis as it shown in Figure9, aqueous GO suspensions are emulsified with toluene which acts as the oil phase, the GO emulsions are put into moulds and subsequently directionally frozen in liquid nitrogen [40]. In the process of unidirectional freezing of GO emulsions, large ice crystals are formed in the water phase and surrounded by the GO-coated, liquid oil droplets (as solidifying temperature of oil droplet is much lower than water). After removing the solvents during freeze-drying, cylindrical GO aerogel monoliths are obtained with cellular GO microstructure shaped by both the large ice crystals and the smaller emulsion droplets. After thermal reduction, reduced GO cellular network aerogel are obtained in Figure9 [42].