Thermal Reduction of Graphene Oxide

Abstract

In a two-dimensional carbon system for graphene (GP), three carbon electrons in four hybridized bonding electrons (2s12px12py12pz1) form strong in-plane sp2 bonds consisting of a honeycomb structure, and a fourth electron spreads out over the top or bottom of the layer as a π electron. The π electrons play an important role in the coupling interactions of multilayered GPs. A two-dimensionally spread C=C resonance structure and the hybridized electrons confined in GP are directly related to graphene’s unique characteristics, such as its ballistic electron conduction, high thermal conduction, and high mechanical strength (Geim & Novoselov, 2007; Lee et al., 2008). These fundamental properties have been well studied using high-quality graphene produced by the “top-down” physical exfoliation (McAllister et al., 2007) and solvation-assisted exfoliation of graphite (Lotya et al., 2009) and GP thin films produced using chemical vapor deposition (Kim et al., 2009; Obraztsov, 2009). Meanwhile, thermally reduced GP from graphene oxide (GO) (Gao et al., 2009; Jeong et al., 2009), produced from graphite using various chemical oxidation routes (Brodie, 1859; Hummers & Offerman, 1958; Hirata et al., 2004), has attracted considerable attention as a potential material for use in various industrial applications such as photovoltaic cells, capacitors, sensors, and transparent electrodes (Geim & Novoselov, 2007; Stoller et al., 2008; Wang et al., 2008; Liu et al., 2009). This is because of not only its potential in significantly lowering the cost of mass-produced graphene but also the simple, nonchemical, thermal conversion of GO powder or film to GP powder or film, respectively (Titelman et al., 2005; (a) Jeong et al., 2009; (b)). However, the thermal reduction of GO is a very complex phenomenon because of the thermal-energy-induced multistep removal processes of intercalated H2O molecules and oxide groups of –COOH (carboxyl group), –OH (hydroxyl group), and >O (epoxy group). It should be noted that in chemical reduction, individual GO sheets in the solution phase are chemically reduced by the strong chemical base (Titelman et al., 2005; Ju et al., 2010). Therefore, the thermal reduction of GO and resultant GP needs to be studied in great detail. In this research, X-ray diffraction (XRD) was used to probe the temperature-dependent evolution of the interlayer distance (d002) of GO/GP films and powders within temperature ranges of room temperature (RT) to 1000°C and RT to 2000°C, respectively. XRD results show a detailed thermal reduction of GO with the removal of intercalated H2O molecules and oxide groups, defect formation, lattice contraction and exfoliation mechanisms of the GO/GP sheets, the folding and unfolding of the GO/GP layers, and a bottom-up layer stacking toward bulk graphite.