Evaluation of graphene microstructural and optical properties affected by high-temperature annealing and rapid cooling in a nitrogen-rich environment

Abstract

Annealing at high temperatures holds the possibility of concentrating defects in the graphene. Moreover, a significant increase in annealing temperature destroys the surface properties. In this work, the reduced graphene oxide (RGO) was prepared by an electrochemical technique with a high voltage ([Formula: see text][Formula: see text]V, [Formula: see text][Formula: see text]A). Then, the potential effects of impact on thermal treatment in a temperature range of 800–1000∘C in nitrogen-rich environment on the microstructure and surface morphology, thermal stability, phase and crystallinity, structural disorder, absorption properties, and optical properties of RGO for optoelectronic applications were investigated. In addition, a link was established between the estimated crystallite sizes determined by X-ray diffraction (XRD) and Raman data. The microstructural data indicate that the annealing temperature has a significant effect on the microstructure and carbon–oxygen (C/O) ratio. The C/O ratio increases as a function of annealing temperature. Atomic force microscope (AFM) analysis revealed that the root mean square (RMS) roughness of annealed RGO increases with increasing annealing temperature indicating an increase in crystallite size during annealing. Since most organic compounds were removed from the surface of the annealed RGO, oxygen functionalities appear to have minimal effect on the thermal stability of RGO. The size of graphene crystallites increases with annealing temperature, as shown by XRD observations. The crystalline structure was restored by annealing. The Raman results show that in the “low” defect density zone, the [Formula: see text] values increase because a larger defect density causes a stronger elastic scattering. UV–Vis spectroscopy shows that the absorption of RGO is not affected by annealing temperatures between 800∘C and 900∘C. The optical bandgap of annealed RGOs decreases from 4.08 to 3.72[Formula: see text]eV upon annealing in the temperature range of 800–1000∘C.