Graphene oxide synthesis using modified Tour method

V O Kotsyubynsky, V M Boychuk, I M Budzulyak, B I Rachiy, M A Hodlevska, A I Kachmar and M A Hodlevsky

Abstract

Graphene oxide (GO) colloidal solution has been synthesised by the modified Tour method, in which the pH of the reaction medium has been increased to 2.0–2.2 at the final stage of graphite oxidation by adding NaOH solution. The solid-phase graphene oxide consists of multilayered graphene particles with a thickness of about 7.5 nm (9–10 layers of graphene) and an average size of about 7.7 nm. Reduced graphene oxide (rGO) has been prepared by hydrazine and microwave reduction. A comparative study of the structure, morphology, and electrical transport properties of rGO samples obtained by various methods has been carried out using XRD, SAXS, Raman spectroscopy, low-temperature nitrogen adsorption, and impedance spectroscopy. Structural analysis has shown the presence of two fractions of plate-like rGO particles for each reduction method, which consist of 4–6 layers of graphene in stacks and have a lateral size in the range of 7.1–7.6 nm. The BET specific surface area of the microwave-reduced rGO was higher than that of the chemically reduced one (296 and 237 m² g⁻¹ respectively). The frequency dependence (10⁻²–10⁵ Hz) of the AC conductivity of the GO and rGO samples has been analysed in the temperature range of 25 °C–175 °C. For the GO sample, the proton exchange conductivity mechanism dominates. A Drude-like response of electrical conductivity at frequencies above 10³ Hz with transition to Johnscher's law response at 175 °C has been observed for rGO samples obtained using both chemical and microwave routes. Changes in activation energies and relaxation times have been interpreted using a model of a thermally activated frequency-dependent electron hopping mechanism between randomly coupled conducting sp² rGO packages separated by disordered sp³ rGO regions with correspondingly higher resistivity.