Graphene is an allotrope of carbon that possesses unique thermal, chemical, electrical, optical, physical, and mechanical properties. It has broad application prospects in high-frequency electronics, water purification systems, semiconductor materials, drug carriers, flexible energy storage, and biosensing devices [1]. Nowadays, thermal reduction of graphene oxide (GO) is one of the potential synthesis methods to obtain graphene in a simple, low-cost, high yield, and time-saving way. However, GO is attributed to a class of energetic materials due to its high enthalpy change of thermal decomposition ($\Delta\text{H}$). Thus, it could decompose violently if not properly stored and handled, and cause irreversible damage [2]. Furthermore, the mechanism of thermal reduction of GO is complex and not yet fully understood because of the consecutive stages of water evaporation, oxygen-containing functional groups removal, and basal-plane carbon decomposition occurring during the thermal exfoliation [3]. Therefore, it is necessary to focus on the kinetic analysis of GO thermal decomposition for a deeper understanding of the behaviour of the exothermic reduction reaction and ensuring safe manufacturing of graphene.
In this study, we present the impact of carbon suboxide (C3O2) on the kinetics of GO thermal decomposition. GO was prepared from natural graphite by the synthesis protocol reported by Yan et al. [4]. In a typical experiment, graphite powder was treated with concentrated H2SO4, K2S2O8, and P2O5. The obtained pre-oxidized graphite was subjected to oxidation by Hummers' method [5]. According to the literature, C3O2 could be synthesized by heating the mixture of malonic acid and phosphorus pentoxide [6]. To evaluate the effect of C3O2 on the kinetics of GO thermal reduction, the homogenous mixture of GO, malonic acid and phosphorus pentoxide was prepared. It was named GO_MA_P. The prepared samples were analysed by using thermogravimetry (TG) and differential scanning colorimetry (DSC) methods at three different heating rates: 2.5 °C min-1, 5 °C min-1, and 10 °C min-1. The kinetic and thermodynamic parameters of the thermal decomposition reaction of GO and GO_MA_P were determined by using Borchardt-Daniels, Kissinger, and Ozawa models [7].
DSC results revealed that the reduction temperature of GO is reduced (up to 125 °C) by using C3O2. Moreover, it was observed that C3O2 lowers the enthalpy $\Delta\text{H}$ and activation energy Ea but does not influence the reaction order $n$ of GO thermal decomposition. The values of Ea for the thermal deposition of GO and GO_MA_P were obtained similar by using all three methods (Kissinger, Ozawa and Borchardt-Daniels). Results obtained by Borchardt-Daniels method showed that the values of Ea increase with increasing the heating rate for both studied samples. The apparent reaction order $n$ for both GO and GO_MA_P equals 0.7. In fact, the mechanism of this process can be modelled including simultaneous zero-order and first-order stages. The zero-order process may originate in the case when the energy consumed in the thermal decomposition reaction comes through the basal graphene plane; then the reaction rate should not depend on the reagent concentration (i.e., the concentration of functional groups). The first-order process may occur in the case when the energy is absorbed directly by a functional group; this time the reaction rate depends only on the concentration of the reagent. The same reaction order for GO and GO_MA_P reduction reveals the same mechanism for both processes.