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The level of oxidation (i.e. C/O ratio), however, is limited compared to strongly oxidizing chemical approaches which often require post-processing reduction steps to improve material properties. 5.3.1 Mechanicalexfoliation Mechanical exfoliation relies on the use of force to break the van der Waals attraction between the planes in graphite. There are essentially three ways to overcome this attraction 84: 1. Peel two graphite layers apart by applying a normal force to overcome the van der Waals attraction. 2. Application of a lateral, or shear force. Adjacent layers within graphite can slide in the lateral direction, when a suitable force is applied to overcome the van der Waals attraction. 3. Use exfoliation with fragmentation to break-down large graphite layers into smaller sizes. If the desired product does not require graphene of large area, then the method promotes easier exfoliation due to lower van der Waals interaction forces that are split across smaller sized graphite flakes. 5.3.2 Chemicalexfoliation Chemical exfoliation techniques utilise chemical approaches to turn the starting graphite into a graphite derivative. This can then be readily delaminated to give single or few layer sheets of chemically derivatized graphene. This derivative can receive subsequent treatment to convert it back to graphene. The most common of these techniques is the graphite oxide route, which capitalises on strongly oxidising graphite to produce graphite oxide. There are many approaches that have been developed for the preparation of graphite oxide. The Brodie method was developed using fuming HNO3 and KClO3 as intercalants and oxidants 85. Hummers and Offeman proposed a method in which H2SO4, NaNO3 and KMnO4 were used. This method eliminated some of the hazardous steps that were present. However, the key issues still limiting the adoptability of this method includes the formation 28PDF Image | graphene production via nonoxidizing liquid exfoliation
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