Manipulation of defect density and nitrogen doping on few-layer graphene sheets using the plasma methodology for electrochemical applications

In this study, we successfully manipulate the defect density and thickness (number of layers) of few-layer graphene sheets at low temperatures (≤300 °C) using microwave plasma torch (MPT) coupled with plasma-enhanced chemical vapor deposition (PECVD) method. The graphene defects and thickness of 2-10 layers can be controlled by variation in the argon flow rate and pressure during graphene growth step. The quality and thickness of graphene sheets are characterized using Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). The nitrogen plasma has been demonstrated to effectively dope N atoms onto graphene sheets in which the N-doping content varies monotonously from 4.7 to 13.3 at%. The distribution of the N-doping types can be tuned by control of the graphene defect density with the ID/IG ratio increasing gradually from 0.3 to 0.8 and the nitrogen plasma power to generate multiple functionalities of the resultant materials. For example, the medium-quality graphene doped by high-power nitrogen plasma exhibits the highest electrocatalytic activity toward the oxygen reduction reaction (ORR) with a mean electron-transfer number of 3.94 which is comparable with that of platinum. The high-quality graphene doped by low-power nitrogen plasma shows the high activity and selectivity for simultaneously detecting uric acid, ascorbic acid, and dopamine because of the high content of the pyridinic-N structure.