Atomic Force and Scanning Tunneling Microscopy Imaging of Graphene Nanosheets Derived from Graphite Oxide

Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain.
Langmuir (Impact Factor: 4.46). 05/2009; 25(10):5957-68. DOI: 10.1021/la804216z
Source: PubMed


Graphene nanosheets produced in the form of stable aqueous dispersions by chemical reduction of graphene oxide and deposited onto graphite substrates have been investigated by atomic force and scanning tunneling microscopy (AFM/STM). The chemically reduced graphene oxide nanosheets were hardly distinguishable from their unreduced counterparts in the topographic AFM images. However, they could be readily discriminated through phase imaging in the attractive regime of tapping-mode AFM, probably because of differences in hydrophilicity arising from their distinct oxygen contents. The chemically reduced nanosheets displayed a smoothly undulated, globular morphology on the nanometer scale, with typical vertical variations in the subnanometer range and lateral feature sizes of approximately 5-10 nm. Such morphology was attributed to be the result of significant structural disorder in the carbon skeleton, which originates during the strong oxidation that leads to graphene oxide and remains after chemical reduction. Direct evidence of structural disorder was provided by atomic-scale STM imaging, which revealed an absence of long-range periodicity in the graphene nanosheets. Only structured domains a few nanometers large were observed instead. Likewise, the nanosheet edges appeared atomically rough and ill-defined, though smooth on the nanometer scale. The unreduced graphene oxide nanosheets could only be imaged by STM at very low tunneling currents (approximately 1 pA), being visualized in some cases with inverted contrast relative to the graphite substrate, a result that was attributed to their extremely low conductivity. Complementary characterization of the unreduced and chemically reduced nanosheets was carried out by thermogravimetric analysis as well as UV-visible absorption and X-ray photoelectron and Raman spectroscopies. In particular, the somewhat puzzling Raman results were interpreted to be the result of an amorphous character of the graphene oxide material.

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    • "(A colour version of this figure can be viewed online.) at 287.2 eV, COOH at 288.6 eV and pep* shake up satellite of the 284.4 eV peak at 289.6 eV [28] [29]. The peak positions along with the relative integrated intensities of various de-convoluted components are shown in Table S1 of Supplementary Information. "
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    ABSTRACT: Large area graphene oxide (GO) monolayer sheets were transferred on Si and SiO 2 /Si substrates by LangmuireBlodgett technique and reduced by heat treatment in the presence of graphite powder, inside an evacuated and sealed enclosure. Reduction at 1000 C results in a decrease of sheet thickness to ~0.5 nm without any morphological changes, and de-oxygenation (O/C ratio 5%) accompanied by enhancement in sp 2-C to 84%. Red-shift of G-peak to ~1585 cm À1 , decrease in I(D)/I(G) ratio and appearance of an intense G 0-peak as a single Lorentzian, are indicative of substantial reduction in defects and restoration of graphitic network. Ultraviolet photoelectron spectroscopy (UPS) studies show large increase in density of states (DOS) in the immediate vicinity of Fermi level and decrease in work function after reduction. Bottom gated field effect transistors fabricated with isolated RGO sheets display ambi-polar behavior, with charge neutrality point at a positive gate voltage, indicating p-type nature, consistent with UPS results. RGO sheets exhibit conductivity of (2e3) Â 10 3 S/cm and field effect mobility of (20e45) cm 2 /Vs, which are substantially higher than the values usually reported for RGO sheets, particularly those obtained by chemical reduction followed by heat treatment.
    Carbon 09/2015; 95:843. DOI:10.1016/j.carbon.2015.08.067 · 6.20 Impact Factor
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    • "The thickness of GO was indicated to be around 1 nm, which agrees well with the thickness range of 1.0–1.2 nm, determined for singlelayered GO [30] "
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    ABSTRACT: Au nanoparticles (NPs) with a radius ranging from 4 ∼ 18 nm (mostly 11 nm) were stably and uniformly hybridized on the surface of reduced graphene oxide (RGO) after co-reduction of Au precursor ions and graphene oxide (GO), to Au atoms and RGO, respectively. The hybridization was provided by dry plasma reduction (DPR) operated under atmospheric pressure and at a low temperature without any toxic chemicals. The structure of the AuNP/RGO nanohybrid was characterized by SEM, TEM, XPS, XRD and Raman spectroscopy. Raman spectra indicated that DPR induced an increase in the degree of clustering of the sp2 phase in addition to sp2 bond restoration. Increasing the number of AuNP/RGO layers yielded a decrease in the transmittance and sheet resistance of the AuNP/RGO nanohybrid. A developed electrode based on the AuNP/RGO nanohybrid showed high electrochemical catalytic activity and high conductivity in comparison with the AuNP and GO electrodes. Thus, the AuNP/RGO nanohybrid fabricated by DPR could be an excellent material for a low-cost counter electrode for dye-sensitized solar cells using Co2+/Co3+ redox electrolyte.
    Electrochimica Acta 01/2015; 156. DOI:10.1016/j.electacta.2014.12.109 · 4.50 Impact Factor
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    • "Th e control experiment with the same GO solution reduced with hydrazine (the process proceeds without EB) gave the identical values of λ max , A RedGO and A RedGO /A GO . Th e ratio A RedGO / A GO ϭ 1.6 – 1.7 was calculated also from the data presented in (Li et al. 2008) and (Paredes et al. 2009). If the ratio of A RedGO /A GO in the range of 1.6 – 1.7 is taken as a simple, but reliable spectroscopic criterion to judge the degree of GO reduction in aqueous solutions, EB reduction provides the same high quality of RedGO as the reference process with hydrazine. "
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    ABSTRACT: Purpose: The aim of the work was to investigate mechanistic details of the preparation of graphene-like materials (GLM) via reduction of graphene oxide (GO) in aqueous dispersions by electron beam (EB) generated reducing free radicals. Materials and methods: A 10 MeV linear accelerator was employed to irradiate aqueous GO dispersions at ambient temperatures. The kinetics of GO reduction was followed using UV-Vis spectroscopy. The resulting GLM were characterized by X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), Raman spectroscopy and conductivity measurements. Results: The reduction of GO could be afforded with high efficiency within minutes at room temperature via the reaction of GO with reducing radicals generated by EB irradiation. The detailed investigation of the reduction mechanism allowed a selection of the best reducing free radicals in terms of both their efficiency and environmental impact of their precursors and final products. Conclusions: The EB-treatment of aqueous GO dispersions is a highly efficient, environmentally friendly, cost-effective and easily up-scalable method for the preparation of GLM. The efficiency of the new reduction approach is comparable with the best existing methods.
    International Journal of Radiation Biology 03/2014; 90(6). DOI:10.3109/09553002.2014.907934 · 1.69 Impact Factor
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