Effect of Sonication on the Exfoliation of Graphite Nanoplatelets

Abstract

Since its discovery in 2004 graphene has generated huge interest in both academic institutions and industries due to its unrivalled physical properties. Graphene a 2-dimensional material is a oneatom- thick sheet of sp2-bonded carbon and has exceptionally good properties. Apart from the very high modulus of elasticity of ∼1 TPa and tensile strength of ∼125 GPa graphene also has a thermal conductivity of ∼3080-5300 W/mK which is 25 times that of Si. It has a carrier mobility at room temperature of 10,000 cm2/VS and a theoretical specific surface area is 2630 m2/g. Large scale economical production of high-quality graphene having few layers is essential for their real-world applications. Here, we report an effective and facile technique for the large-scale production of exfoliated graphite nanoplatelets. The processing route that has been adopted here for the synthesis of graphite nanoplatelets is based on the intercalation of the natural flake graphite (NFG) and solvent dispersion. Microcrystalline natural flake graphite is subjected to intercalation using concentrated sulfuric acid (H2SO4) along with hydrogen peroxide (H2O2) leading to the formation of a graphite intercalation compound (GIC). The GIC is later thermally exfoliated to obtain the graphite nanoplatelets. The graphite nanoplatelets are finally ultrasonicated in acetone for five different time periods to find the effect of vibrational mechanism on the exfoliation of the graphite nanoplatelets. Ultrasonication has been carried out at room temperature for 2, 5, 7, 12 and 20 h. Both HRTEM and AFM analysis confirm that the nanoplatelets obtained after ultrasonication for more than 12 h were well exfoliated and had a thickness less than 1 nm. The XPS analysis of various samples showed a maximum relative % of C (97.2 at. %) and a minimum relative % of O (2.8 at. %) in the case of 12 h sonicated sample. The surface area and pore size distribution were determined by the Brunauer-Emmet-Teller (BET) method. The extent of defects introduced in the nanoplatelets during ultrasonication was determined from the ID/IG intensity ratio using the Raman spectra obtained from the various ultrasonicated samples. Ultrasonication of the graphite nanoplatelets beyond 5 h shows a gradual increase in crystallinity and decrease in defect density as well as a decrease in the number of graphene layers in the graphite nanoplatelets. Beyond 12 h of ultrasonication both the intensity of the (002) X-ray diffraction peak and the ID/IG ratio of the Raman spectra come to a saturation suggesting that there is a limit to the extent of exfoliation possible by the ultrasonication process. Highly crystalline graphite nanoplatelets having low defect density and consisting of few layers of graphene could be synthesized by ultrasonication of the thermally exfoliated graphite. The process of thermal exfoliation followed by ultrasonication is very effective in producing few layer graphene (FLG) platelets in large quantities.