Porous, three-dimensional aerogels made from graphene are more appealing than flat versions of the material for battery electrodes and catalyst supports. But until now, graphene aerogels have had low-quality crystal structures and subsequently poor physical properties. Materials scientists now report a straightforward way to make highly crystalline graphene aerogels (ACS Nano 2014, DOI: 10.1021/nn505335u). The new materials can withstand temperatures 200 °C higher than graphene aerogels reported before and are up to 10 times as conductive and up to 14 times as stiff.
Other researchers have typically made aerogels by suspending graphene oxide nanoparticles in water, chemically reducing the suspensions, and then freeze-drying them to form 3-D networks of graphene. Those methods yield materials with very small crystal grains and numerous defects, says Marcus A. Worsley of Lawrence Livermore National Laboratory.
Worsley and his colleagues thought they could make crystals with larger grains and very few defects by annealing the material at a high temperature. They first added ammonium hydroxide to a graphene oxide suspension that they dried and heated at 1,050 °C in the presence of nitrogen for three hours. Then they heated the resulting aerogel at higher temperatures of 1,500 to 2,500 °C in the presence of helium for one hour. This high-temperature baking made the aerogel highly crystalline.
Raman spectroscopy showed that the aerogel had crystal grain sizes as large as 150 nm, about 10 times greater than previously reported graphene aerogels. Worsley adds that the individual flat graphene sheets in the aerogel come “about as close as you can get” to pristine sheets made by mechanical exfoliation. These high-quality building blocks form superior 3-D graphene aerogels, he says.