A team of scientists from the Ulsan National Institute of Science and Technology and Case Western Reserve University has developed a new technique to mass-produce superior quality graphene nanosheets at a lower cost by adding a small amount of dry ice to a simple industrial process.
The research team was led by Jong-Beom Baek from the Ulsan National Institute of Science and Technology. During the process, the research team added frozen carbon dioxide or dry ice and graphite in a ball miller packed with stainless steel balls. After turning the miller for two days, graphite flakes were produced from the mechanical force. The flakes’ edges were carboxylated subsequent to the chemical interaction with carboxylic acid generated during the milling process.
The graphite with carboxylated edges now becomes soluble in polar aprotic solvents such as dimethyl sulfoxide, and in protic solvents such as methanol and water. When the flakes are dissolved in a solvent, they isolate into graphene naonsheets of 5 or less layers. The research team then compressed the samples of the material into pellets in order to study the material’s workability to create molded items for electronic applications. These pellets demonstrated 688 folds more electrical conductivity than the compressed acid-oxidation pellets.
When the pellets made of the ball-mill–derived graphene nanosheets were heated at 900°C, the edges of the graphene nanosheets get connected through a strong hydrogen bonding to adjoining sheets due to decarboxylation and stay cohesive. However, the compressed acid-oxidation pellets get destroyed when heated.
To fabricate large-area nanosheet films of graphene, a solution containing the edge-carboxylated graphene nanosheets and solvent was cast over 3.5x5 cm silicon wafers, and then heated at 900°C to decarboxylate the edges, which in turn bonded with edges of adjoining pieces. According to the team, the only limiting factor of the process is the wafer size. These large-area graphene nanosheet films demonstrated better electrical conductivity than materials produced by the acid oxidation, even at a high level of optical transmittance.
Baek explained that by utilizing different solvents and replacing dry lice with sulfur trioxide or ammonia, the edges of the graphene nanosheets can be customized for a variety of applications and can be arranged to make 2D and 3D structures.