Presentation on graphene nanomaterials and their applications

Nanotechnology and nanomaterials

1. Graphene Chemical synthesis of graphene Oxide Hummers method Oxidation C:O= 1.5-2.5 Stage I: Graphite+ H 2 SO 4 + KNO 3 Stirred in ice bath for half an hour Stage II : KMnO 4 added within one hr. at temperature below 5 C & stirred for 1 more hr. Stage III : Heated to 35C and stirred for 2 h Stage IV : Heated to 98C, water added and stirred for 15 min Stage V : Added water and 30% H 2 O 2

2. Staudenmair method : HNO 3 , H 2 SO 4 , and KClO 3 Brodie method : HNO 3 , KClO 3 Modified Hummers method (Hummers and Offeman ) : H 2 SO 4 : KMnO 4 = 9 : 1 Stage VI : Filtered, washed with dil. HCl and finally with water and dried in oven. ***Hummers method discarded because N 2 O 4 and NO 2 are produced

3. Modified Hummers method : (graphite+ H 2 SO 4 + KMnO 4 ) Step I H 2 SO 4 reacts with graphite to form graphite bisulfate (C x HSO 4 - )– This ion intercalates graphite KMnO 4 oxidises graphene to form graphene oxide Step II A ddition of H 2 O 2 Residual permanganate and manganese dioxide are converted to colourless soluble manganese sulfate Color turns from dark brown to yellow

4. Step III Filter and Wash with dil. HCl Metal ions are removed Step IV Wash with water till neutral At low pH, the carboxyl groups are protonated and graphene oxide becomes less hydrophilic and form agglomerate At high pH deprotonated carboxyl groups are very hydrophilic and dissolve in water Step V Dispersion into water by ultrasonication The paste dispersed in water is exfoliated graphene oxide

5. Step VI Centrifugation The suspension is centrifuged to remove any unexfoliated graphite oxide , and to make the graphene oxide sediment

6. Reduction of graphene oxide 2. Reduction by nascent hydrogen Reduction by hydrazine hydrate, N 2 H 4 , H 2 O ( toxic & explosive ) Oxygen functionalities attached to the interior of an aromatic domain in GO are removed more easily

7. 3. Stepwise Thermal reduction Dried GO powder Vaporization of free and intercalated H 2 O +Exfoliation of GO +Reduction of COOH + Exit of CO 2 and some hydrocarbon+ Macropores Reduction of OH+ Exit of CO+ partial defolding and thinning of Graphene Removal of residual OH, >O + Defect formation + Defolding and flat graphene

8. Thermal reduction atmosphere (N 2 / Ar ) Ar /H 2 mix is better than pure Ar , because: Thermally dissociated hydrogen atoms from H 2 molecules could transform the epoxies into hydroxyls, which further form H 2 O molecules This transformation is significant as the outlet of oxygen in the form of CO or CO 2 through epoxies is related to the loss of carbon and implantation of oxygen However, oxygen released via H 2 O does not introduce any oxygen into the carbon backbone, and the exterior hydrogen atoms serve to transform these epoxies into hydroxyls