Highly anisotropic graphene aerogels fabricated by calcium ion-assisted unidirectional freezing for highly sensitive sensors and efficient cleanup of crude oil spills

Anisotropic graphene aerogels (GAs) are usually fabricated by unidirectional-freezing with graphene oxide (GO) dispersion as the precursor. However, polymer additives or pre-gelation are usually required to enhance the anisotropic architectures, which often cause attenuations in electrical conductivity and shape control accuracy. Herein, we demonstrate a calcium ion-assisted unidirectional-freezing approach for fabricating highly anisotropic GAs by unidirectionally freezing aqueous suspensions of GO with a trace amount of calcium ions, followed by freeze-drying and thermal reduction. The calcium ions could slightly crosslink GO sheets by coordination, and hence reduce the interaction between GO sheets and in situ grown ice pillars, benefiting vertical alignment of the GO sheets. After freeze-drying to sublime these ice pillars, high-temperature annealing is adopted to thermally reduce the anisotropic GO aerogels by removing their oxygen-containing groups, resulting in highly anisotropic GAs. The lightweight and anisotropic GA exhibits excellent sensitivity and durable reversibility as a piezoresistive sensor because of its highly aligned porous architecture. Furthermore, GA exhibits prominent adsorption capacity for many organic pollutants from wastewaters. Interestingly, the vertically aligned porous structure of GA enhances its solar-light absorption and solar-thermal energy conversion, making GA efficient in decreasing the viscosity of crude oil and thus enhancing its adsorption capacity.