On the other hand, it has a high boiling point and a surface tension suitable for the correct exfoliation. It is commonly applied as a reaction medium in fullerene chemistry and it is well known that it forms very stable dispersions due to its very efficient interactions with graphene via π-π stacking. O-Dichlorobenzene is an organic solvent that has excellent properties to be used as liquid phase for the process of graphite exfoliation. For this reason, it is very important to control the sonication times during the exfoliation process of the graphite and, above all, to choose the solvent that generates the least problems. For instance, these problems have been observed when DMF is used as an exfoliating liquid phase. On the contrary, this process can have negative effects, since prolonged sonication times can facilitate the existence of defects in the surface of the graphene sheets and the reduction of the layer size. This method of synthesis has different advantages with respect to conventional graphene production methods: strong oxidizing agents are not required, the synthesis time is reduced and unfunctionalized and non-oxidized graphenes are obtained in one step. By carrying out this operation it is possible to separate the non-exfoliated graphitic particles from the exfoliated graphene sheets. A very important parameter for obtaining few-layer graphene is to carry out a selective centrifugation.
The rest is a mixture of multilayer graphene or few-layer graphene. By means of the ultrasound method, a maximum of 15% of individual graphene sheets is obtained. For the successful exfoliation of graphite, different organic solvents have been used, such as N-methyl pyrrolidone (NMP), dimethylformamide (DMF), pyridine and o-dichlorobenzene (ODBC).
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In this way, the free energy is negative, and the structure is broken, giving rise to fragments of graphite interspersed with solvent molecules.
The colloidal graphene generated has the same surface energy as the solvent used. This method requires special experimental conditions at which very high pressures and temperatures are reached in very short periods of time. Graphene layers can be obtained by liquid-phase exfoliation of commercial graphite in different organic solvents using the ultrasonic technique. Many methods for obtaining graphene have been reported. Some of its most important properties are its high thermal and electrical conductivity, its high elasticity, its hardness and strength and its flexibility, in that it is more flexible than carbon fibre and equally lightweight. It is formed by a pattern of hexagonal rings of carbon atoms constituting a huge flat molecule. It is well known that graphene has excellent properties that provide a multitude of technological applications in different fields. The control of size during the synthesis of nanomaterials is decisive in various industrial sectors such as nanomedicine, nanofood, nanoenergy or nanocosmetics.
This parameter is essential since the synthesis at a small scale must be monitored for subsequent bulk production and for the control of nanotechnological products in the market. Particle size analysis is a key element because many properties of nanomaterials are size dependent. AF4 has shown to be a precise analytical technique for the separation of GNS of different sizes. NTA and AF4 gave higher resolution than DLS. The results provided by these techniques have been compared. An exhaustive study of the particle size distribution was carried out by different analytical techniques such as dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and asymmetric flow field flow fractionation (AF4). All of them revealed the formation of exfoliated graphene nanosheets with similar surface characteristics to the pristine graphite but with a decreased crystallite size and number of layers. Both materials were characterized by common techniques such as X-ray diffraction (XRD), Transmission Electronic Microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Graphene nanosheets (GNS) have been prepared by exfoliation of a commercial micrographite (MG) using an ultrasound probe. Sonochemical methods allow the production of low-defect graphene materials, which are preferred for certain uses. Graphene-based materials are highly interesting in virtue of their excellent chemical, physical and mechanical properties that make them extremely useful as privileged materials in different industrial applications.