Synthesis of graphene
Graphene is exciting material now a days because of highly electrical conductivity, highly optical transparency and more tensile strength than steel and flexibility, so it found application in transparent conductive electrodes.
Graphene is a basic building block of Graphite. Graphene is a 2D crystal made up of single atomic layer of carbon atoms arranged in honey comb structure making bonds with three neighbor atoms by sp2 hybridization. 10, 11
Lattice Structure of Graphite (left) and top view of Graphene Sheet showing bonding.
As shown in Fig Graphene consists of sp2 bonded carbon atoms arranged in hexagonal manner having bond length of 1.421 Å resulting in ultra-high strength.
In case of Electrical conductivity, there are varieties of results obtained by many researchers. This type of contradiction is mainly because of different routes followed and state of final product by different researchers.
Graphene Oxide (G.O)
It is graphene sheet to which oxide group attached to it.
Most cases it is synthesized by the reacting micro graphite powder with strong oxidising agent in strong acidic conditions.
The synthesis of graphene oxide made easy by the W. S. Hummers with acid oxidation method by reacting graphite with mixture of potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4).
KMnO4 + 3 H2SO4 K+ + MnO3- + H3O+ + HSO4–
MnO3+ + MnO4- Mn2O7
The dark green oil is formed from the reaction of potassium permanganate with sulfuric acid. The bimetallic heptoxide is far more reactive than its monometallic tetroxide.12 Trömel and Russ demonstrated the ability of Mn2O7 to selectively oxidize unsaturated aliphatic double bonds over aromatic double bonds, which may have important implications for the structure of graphite and reaction pathway occurring during the oxidation.13
Proposed structure of graphene oxides
Hofmann and Holst’s structure (Fig.2) consisted of epoxy groups spread across the basal planes of graphite, with a net molecular formula of C2O. 14
Hofmann and Holst’s structure
Ruess proposed a variation of this model in 1946 which incorporated hydroxyl groups into the basal plane, accounting for the hydrogen content of GO. 15
Certainly the most well-known model is the one by Lerf and Klinowski. 16, 17
The behavior of water in GO has also been characterized by neutron scattering, confirming the water is strongly bound to the basal plane of GO through hydrogen bonding interactions with the oxygen in the epoxides of the GO. 18
Proposed hydrogen bonding network between oxygen functionality on GO and water
While the studies mentioned above fundamental structural features of GO, a more refined picture of the complexity of the material was necessary.
Reduction of GO sheet is carried out to convert it in the form of graphene.
Reduction can be achieved by various methods like thermal reduction, chemical reduction (reduction with Hydrazine) etc.
Synthesis of Graphene
Many techniques have been available with time for synthesis of Graphene. There are mainly two types of synthesis techniques based on application of graphene. Both types are having their merits and demerits. First method which produce high purity graphene having strictly single layer of graphene with less defects. These method is useful for some fundamental study of graphene or applications in which single number of graphene is required with single layer structure. Electronic device fabrication is one of the examples for such applications. Limitation of these methods is that, they can produce one to few numbers of single layer Graphene. These methods are applicable where bulk amount is not basic criteria. On another side, second stream deals with some techniques which give final product having single to few layer Graphene having compromisingly inferiority in properties as compared to previous one because of defects and low purity. The better side of these techniques is that, it can produce bulk amount of graphene layers for fulfillment of the requirement for structural applications etc. Controlled synthesis of graphene with the desired number of layers is still a challenge. 19
By considering our lab setup and some other factors we decided to go with chemical reduction method of graphene oxide.