12.05.2012
Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.
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a, XPS C 1s spectra of a multilayer GO film at increasing ageing times at room temperature: 1 (top), 40 (middle), and 70 (bottom) days after GO film synthesis. The spectra are fitted with three Gaussian–Lorentzian peaks. The PG (blue line) peak is assigned to C–C bonds and to a small fraction of possible C–H bonds and C vacancies; the PGO peak (orange line) is assigned to C species bound in epoxide and hydroxyl groups; the third peak (green line) is assigned to carbonyl species. The open circles show raw experimental data. The black line shows the result of the fit, it is the sum of the Shirley background (not shown) and three Gaussian–Lorentzian line shapes (blue, orange and green lines). b, PGO/PG in multilayer GO as a function of ageing time (black circles), and corresponding oxygen content in the same sample (red triangles). Symbols show experimental values and statistical errors are derived by analysing XPS spectra acquired from different regions of a given GO sample (see Supplementary Information). The black and red lines show the fits with an exponential decaying function. c, Table reporting oxygen content and structural changes with time in three different GO films prepared in similar conditions. In GOA, GOB and GOC, the number of layers, L, is as follows: L = 11 for GOA (Supplementary Figs S3a, S4), L = N/A for GOB (Supplementary Fig. S3b) and L = 11 for GOC (Supplementary Fig. S3c). The data presented in a and b are for sample GOA. N/A means data not available.
Left panel, ball-and-stick illustration of a model structure of GO showing the layered geometry and the complexity of the bonding network on the oxidized carbon basal planes. Right panel, a selected region of the model structure showing the predominant chemical species present in GO films obtained from graphene on SiC; clockwise from top and indicated by the green arrows: a hydrogen atom chemisorbed on the basal plane, an epoxide group, an intercalated water molecule, and a hydroxyl species. Grey, red and white are used to represent C, O and H atoms, respectively. The illustrations show regions of a 390-atom periodic model structure of GO presenting interlayer distances of about 4.22 Å and average intra-layer perpendicular buckling of 0.27 Å.
a, Selected measured and computed C 1s XPS spectra of GO. The experimental spectrum is obtained from a freshly synthesized—1 day old—GO film (open circles). The theoretical spectrum is extracted from a model structure of GO generated by using DFT simulations (filled circles). At the outset, composition and structure of the models (indicated in these figures) have been selected on the basis of experimental information. b, Selected measured (open circles) and computed (filled circles) C 1s XPS spectra of GO. The experimental spectrum is obtained from an aged—40 days old—GO film. c, Fraction of epoxide (purple) and hydroxyl (blue) species relative to the amount of C in GO as a function of the ageing time at room temperature. These fractions have been derived by obtaining PGO, Poxygen and Pcarbonyl from the experimental XPS spectra and by solving the equations (1) and (2). Error bars have been derived by analysing XPS spectra acquired from different regions of a given GO sample (see Supplementary Information). The data reported here are for sample GOA.
Schematic diagram showing the energies involved in the sequential reaction of an H species (C–H) with an epoxide (C–O–C) group (left) to form a hydroxyl (C–OH) species and then a second C–H species with the C–OH group (middle) to form a water molecule physisorbed on the basal plane (right). Reaction transition-state configurations and end product are shown as insets, and stable configurations of the C–H, C–O–C and C–OH species on a basal plane are shown on the right of the energy diagram. The cyan-coloured segments show the energy of, from left to right, two C–H and one C–O–C, one C–H and one C–OH, and one water molecule physisorbed on graphene; these energies are referred to that of the physisorbed water species. The kinetics of this reduction mechanism is controlled by diffusion processes. Our DFT calculations give activation energies of 0.55 eV, 0.81 eV and 0.35 eV for the diffusion of a C–H, C–O–C and C–OH species on the basal plane, respectively. For each reaction step, the magenta-coloured segments show the energy of the reacting species when one of them is at the diffusion transition state. C, O and H atoms are illustrated in grey, red and white, respectively.
