Graphene, the two-dimensional sheet of carbon atoms arranged in a honeycomb lattice, is one of the most promising new materials of the last decade. It was first isolated in 2003 by using ordinary adhesive tape to separate graphene flakes from graphite, which consists of many layers of graphene. Its structure, including a high electron density above and below each sheet, causes graphene to act as a semi-metal, with charges rapidly flowing along its surface. While individual graphene sheets are useful for certain high-tech applications (e.g transistors), it is necessary to layer graphene for other applications, such as electrodes and solar cells. Unfortunately, it is very difficult to process graphene using traditional solution-based methods because it is highly insoluble.
One relatively easy method to make graphene more soluble is by oxidizing it with strong acids and other oxidizing agents. This technique functionalizes the graphene with oxygen-containing groups, including alcohols and epoxides, creating what’s known graphene oxide (GO). Although these functional groups act as imperfections in the lattice, weakening the charge transport properties, this oxidized form can be dispersed in water, and the oxygen-containing groups allow it to more readily interact with other molecules. By alternating these GO sheets with a binder polymer, such as poly(diallyldimethylammonium chloride) (PDDA) or poly(acrylic acid) (PAA), it is possible to build up layers that are thick enough to use in a number of applications. After creating the layers, a simple heating is all that is required to remove these binders and the functional groups, restoring the transport properties of GO to near that of pristine graphene.
Solar cells and light-emitting diodes (LEDs) require transparent electrodes, as the devices must allow light to pass through them and conduct electricity. The abovementioned layer-by-layer (LBL) technique can be used to create transparent conducting films that are extremely thin, allowing for the transmission of light, but have good charge transport properties. Studies have also been done where the researchers used conducting functionalized carbon nanotubes instead of insulating polymer binders, further increasing the conductivity of these films.
Energy storage for everything from cellphones to electric cars is reliant on lithium-ion (Li-ion) batteries. The performance of these batteries is dependent on the electrode material; the electrodes require high surface area to store the charged ions. Layers of graphene, with their high electrical conductivity and maximized surface area, are ideal candidates for this application. Spacing between layers provided by binders, often poly(ethylene oxide) (PEO), in the LBL technique can readily accommodate lithium ions. Furthermore, defects in the honeycomb lattice are actually advantageous in this case, as they increase the overall surface area of the material. Another related technology, supercapacitors, would also benefit from layered electrodes. These devices transfer charge faradaically via of metal oxides, such as manganese oxide, instead of via lithium ion transport. By incorporating metal oxide nanoparticles into the binder between graphene layers in the LBL technique, high-performance supercapacitors can be assembled.
Similarly, integrating semiconductor nanoparticles, such as cadmium sulfur (CdS), between graphene sheets can be used to create quantum dot solar cells, with efficiencies three times higher than devices that lack high surface area graphene layers. The distribution of CdS nanoparticles combined with the fast charge transport of graphene allowed for a high absorbance of light and efficient charge separation, generating electricity.
Transistors, which act as switches, require a material that is semiconducting; it shouldn’t conduct when the switch is off. Because of this, there has also been research on making graphene less conducting. By cutting graphene into “ribbons,” functionalizing the edges, and then assembling it using the LBL technique, it is possible to turn graphene into a semiconductor for use in high-speed transistors. The solution-based processing is essential for the functionalization and deposition of the ribbons, and the layers create full connectivity along the devices.
Sensors function by monitoring the change in charge transport when an analyte is being detected. Owing to its high surface area and strong charge transport properties, graphene can be used to create highly responsive sensors. In order to sensitize graphene to the desired analyte, it can be functionalized from the oxygen-containing moieties of graphene oxide. This tailoring of the sensitivity allows the graphene to be very selective, and the responsiveness can be increased by stacking sheets using the LBL technique.
By controllably assembling graphene using an oxidized intermediate, there are a range of applications that can fully utilize its unique material properties; layer-by-layer techniques provide a huge surface area while maintaining high conductivity. The oxygen-containing functional groups provide both the ability to disperse graphene oxide in water and a capacity for interaction with other materials, and the controlled assembly of graphene sheets is invaluable for the creation of high-performance devices.
References
http://www.sciencedirect.com/science/article/pii/S1748013212000990