Thermo-chemical metastability of multilayer epitaxial graphene oxide: Experiments and density functional theory calculat
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Thermo-chemical metastability of multilayer epitaxial graphene oxide: Experiments and density functional theory calculations Si Zhou1 , S. Kim2 , Y. Hu2 , C. Berger2 , W. de Heer2 , Elisa Riedo2 , and Angelo Bongiorno1 1
School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, GA 303320400, U.S.A. 2
School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430, U.S.A.
ABSTRACT Graphene oxide holds great promise for future applications in nano-technology. The chemistry of this material is not well understood. This understanding is crucial to enable future applications of graphene oxide. In this study, experiments and density functional theory calculations are combined to elucidate the chemical properties of multilayer graphene oxide obtained by oxidizing epitaxial graphene grown on silicon carbide via the Hummers method. This study shows that at room temperature as prepared graphene oxide films exhibit a uniform and homogeneous structure, include a minimal amount of edges and holes, and have an oxidation ratio of about 0.44. The comparison with density-functional calculations shows that graphene oxide includes a minimal amount of intercalated water molecules and well-defined fractions of epoxide and hydroxyl groups. INTRODUCTION Graphene oxide (GO) is a material of great interest for its potential applications in nanoelectronics [1,2], nano-electromechanical systems [3],sensors [4], and energy storage devices [5]. The traditional route to produce GO films involves several steps: oxidation of graphite via the Hummers method [6], exfoliation of graphite oxide, dissolution of GO layers in aqueous solution, and deposition of these layers onto a surface from the aqueous dispersion of GO [2]. The resulting material consists of a stack of oxidized graphene layers, with hydroxyl (C-OH) and epoxide (C-O-C) groups occupying the undamaged regions of the basal planes and carbonyl, carboxyl, and ketone species terminating the edges and holes of the oxidized carbon platelets [7]. In this study, we investigate the properties of GO films obtained by Hummers oxidation [6] of ultra-thin graphene films grown epitaxially on SiC [1]. This method differs from the traditional approach because it does not require the exfoliation/dissolution of the GO layers in aqueous solution and the filtration/deposition on a substrate. To investigate the chemical properties of this type of films, we combine density functional theory (DFT) calculations and experiments. Joint computational and experimental studies of GO have relied so far on molecular dynamics simulations based on reactive force fields and have addressed both thermal reduction mechanisms and mechanical properties of this material [8–10].
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EXPERIMENTS Graphene films are epitaxially grown on the C-surface of a SiC wafer and the details are reported elsewhere [11]. The Hummers method [6] is employed for the oxidation of the epitaxial graphene films. First, epitaxial graphene on SiC chips are dipped into a H2 SO4 /NaNO3 solution placed in an iced water bath. Seco
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