The morphology, structure and electrocatalytic ability of graphene prepared with different drying methods

Abstract

Reduction in an aqueous suspension of exfoliated graphene oxide sheets with ammonia water results in the formation of graphene. Here, vacuum drying and freeze drying were used to prepare graphene, and then the morphology and characteristics of the two samples were mainly compared via electron microscopy, spectra analysis, and electrochemical measurements. The changes in the oxygen-containing functional groups, confirmed using Fourier Transform Infrared Spectroscopy (FTIR), showed the successful preparation of graphene oxide and graphene. The characteristic peak at 11° disappeared, and a peak appeared at 24° in the X-ray diffraction (XRD) spectra. This confirmed that the formation of graphene from graphene oxide required a reaction time of 15 h. The results of X-ray photoelectron spectroscopy (XPS) revealed that the freeze-dried graphene contained more nitrogen and less oxygen, indicating a more complete reduction in the freeze dried graphene, this finding echoes the results of the Raman spectra. The value of the zeta potential of graphene prepared via freeze drying was about −32.6 mV, while for the graphene prepared via vacuum drying it was −21.6 mV. The results suggested that an aqueous solution of freeze-dried graphene has a stronger dispersion and stability. The BET surface areas of vacuum-dried graphene and freeze-dried graphene were 473.8 and 558.4 m² g⁻¹, respectively. The peak currents of graphene-modified glass carbon electrode samples prepared via these two methods of drying were 83.95 μA and 59.6 μA, respectively. The nearly vanished semicircle shown from electrochemical impedance spectroscopy highlights that the graphene had excellent electrocatalytic ability. Rotating disk electrode results in systems of [Fe(CN)₆]^3−/4− freeze-dried graphene modified GCEs showed lower electrocatalytic activity, while in a uric acid system it was more active towards the electrooxidation of uric acid. A steeper slope demonstrates better oxygen reduction reaction (ORR) activity, so freeze-dried graphene has more ORR activity. Above all, we found that the Gr2 had a relatively better capacity to promote charge transfer.