Graphene is an allotrope of carbon and was discovered in 2004 by scientists using graphite and scotch tape. It is composed of carbon atoms linked together by covalent bonds, where each carbon is bonded to three other carbon atoms, creating hexagonal shapes(Refer to top of figure 2). The covalent bonds between the carbon atoms are very strong and contribute to graphene’s high tensile strength, which is its ability to bend without breaking. When comparing graphene to carbon nanotubes, graphene has a flat structure while nanotubes are cylindrical. Due to the flat structure of graphene, every atom is on the surface and accessible from both sides therefore there is more interactions with other molecules. In addition, the sp2 hybridization, as well as very thin atomic thickness allows for strength, electricity and heat conduction. The covalent bonds that hold graphene together play such a key role in it’s properties. For instance, the 0.42 Nm-long carbon bonds between the atoms in graphene allow it to be one of the strongest materials discovered. Also, besides the fact that graphene is an allotrope of carbon, it behaves much like a metal. Due to the fact that each carbon atom in graphene is only bonded to three other carbons, there is one pi orbital, for each carbon, free for electron conduction and therefore graphene is usually referred to as a semimetal or semiconductor.
Figure 1: Animated image of graphene filter in use
Image credits: (http://news.mit.edu/2012/graphene-water-desalination-0702)
Structure of Carbon
In terms of the chemical structure of carbon, a carbon atom is composed of 6 electrons, 4 being in its valence energy level. Out of the 4 outer energy electrons, two are in the 2s energy level and one in each of the two 2p orbitals, with one empty 2p level. When bonding comes into play, promotion occurs where the 2s orbital promotes its electron to the third 2p orbital, creating 4 half filled orbitals. In graphene, carbon only bonds to three other carbons, therefore the hybridization would be sp2 with one unhybridized p orbital. The carbon’s sp2 atomic orbitals would overlap creating a sigma bond between each of the three carbons, and the unhybridized 2p orbital would result in delocalized electrons throughout the whole molecule. As for the 3D geometry of the carbon atoms, they would form the shape trigonal planar because there are 3 hybridized orbitals formed and the last 2p orbital does not affect the shape. The bond angles would be 120 degrees. All information in this section can be visually seen in figure 2.
Figure 2: Drawings of graphene sheet structure, orbital configuration and 3D geometry of Carbon
Graphene for Water Desalination
In order for efficient desalination to occur, water must easily flow through a membrane, while salt be held back, and that is exactly what graphene is capable of. For the process of water desalination using graphene, the water passes through artificially made holes, called pores, while salt, containing sodium and chlorine ions, is blocked from passing. One main reason for why this filter works so well is because of graphene’s hydrophobic property. Hydrophobic refers to something that is “water-hating” and does not wish to interact with water. The carbon atoms of graphene have no interest in interacting with water, therefore leading water smoothly through the pores, meanwhile salts stay back. The smooth flow of water through the pores is due to the polarity of the graphene sheets. Since completely composed of carbon atoms, the bonds between the carbons in graphene are nonpolar, meaning they have an electronegativity difference of 0 and therefore the overall molecule is nonpolar. When water, a polar molecule, approaches the graphene, it has no intention to interact with the carbon atoms due to the polarity difference and glides through. The ability for graphene to desalinate water is also due to the nanometer-scale pores created. The size of the pores is what allows for this filtration method to work, where the pore diameter is big enough for water molecules to pass through, but small enough for ions to be blocked. Graphene is impermeable, however, by creating pores and controlling their size and density, it creates a permeable membrane for water desalination. The pores are the key structure for this method of filtration. However, the pores are not naturally found in graphene, but have to be man made. After conducting several experiments at MIT, researchers have come up with a way of making the nanosized pores. The process involves two steps. First, the graphene is struck with gallium ions which disturb the carbon bonds amongst the graphene. Then, an oxidizing solution is coated across the graphene, which strongly reacts with the disturbed carbon bonds, producing a hole where the gallium ions were struck. The size of the pores can be controlled based on how long the graphene sheet is left in the oxidizing solution. “To better understand how small and dense these graphene pores are, if our graphene membrane were to be magnified about a million times, the pores would be less than 1 millimeter in size, spaced about 4 millimeters apart, and span over 38 square miles, an area roughly half the size of Boston,” says Sean O’Hern, leader of the experiment and a graduate student of MIT. Also, the extreme strength, caused by strong covalent bonds between carbons which requires high amount of energy to break, and thinness of graphene filters allows them to be able to sustain a high flow of water, therefore proving to be more effective compared to other filtration systems, such as the reverse-osmosis desalination plants.
Figure 3: Animated picture of graphene filter in use for water Desalination
Picture credit:(http://www.pitt.edu/~klm196/graphenefilter.jpg)
Environmental/Economic Impact
Availability of fresh water is a rising issue all around the world today, where the amount of available freshwater gets worse as population grows. One large source of water is seawater, however it requires desalination in order to be potable. In present day, there are a few water desalination plants/technologies that exists, such as the Reverse-Osmosis, however, present methods are proven to be very expensive and require huge amounts of water to manufacture. They also produce a lot of excess waste water. This is where graphene filters come into play and can save the day. For instance, graphene filters can be cheaply produced in labs, therefore prove to be a relatively inexpensive alternative. They also require a lot less energy to be built and require no energy to be used. So not only does the use of this nanotechnology vastly help in terms of environmental issues, but it also helps economic issues in terms of being a cheaper alternative compared to other desalination technologies. There is also no leading research or specific information on any negative impacts that may arise from the use of graphene filters, however, it is a fairly new concept so until it has been put into use, there are no known cons present to this nanotechnology. All in all, through the mass of information and research of graphene, we can see the potential of using graphene as a filter for water. The future holds many ideas and possibilities for this nanotechnology.
...(download the rest of the essay above)