Controlled high temperature stability of microwave plasma synthesized graphene nanosheets

High temperature stability of nanomaterials plays an important role for their application in the field of nanocomposites, batteries, and sensors. Few-layer graphene nanosheets prepared by microwave plasma based decomposition of ethanol exhibited high thermal stability in the oxidation atmosphere in dependence on controlled formation of structural disorder. Analysis of differential thermogravimetry (DTG) curve profile showed three temperature regions, around 345 °C, 570 °C and above 700 °C, related to amorphous phase with a carbon-oxygen functional groups, small defective nanostructures and highly crystalline structure of graphene nanosheets, respectively. Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) analysis of the nanosheets showed an increase of D/G Raman band ratio as well as increasing of sp3 phase content, from 6.1 at% to 15.2 at%, for highly crystalline and highly disordered structure of the nanosheets. Thermal annealing under synthetic air was used to investigate the variation in D/G and 2D/G Raman band ratio of the samples and to estimate activation energy of oxidation and disintegration process of graphene nanosheets. The highest oxidation resistance exhibited sample with high 2D/G band ratio (1.54) and lowest oxygen content of 1.7 at%. The synthesis process led to stabilization of nanosheet structure by formation of curved edges and elimination of free dangling bonds. The nanosheets prepared in microwave plasma exhibited high surface area, over 350 m2 g−1, and superior thermal stability with defect activation energy in an oxidation atmosphere higher than 2 eV. Heat release rate during the oxidation process was in correlation with the amount of disorder in the samples. Fast and easy to use technique based on high power Raman spectroscopy was developed for assessment of nanomaterial oxidation resistance.