Article

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

ABSTRACT

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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    • "As shown in Fig. 2, a UV-visible spectrum obtained for the GO aqueous dispersion (orange line) displays a plasmon peak at 231 nm which is related to the π → π* transitions due to the aromatic C − C bonds. Also, a hump can be detected around 300 nm approving the n → π* transitions of C = O bonds[31,32]. Gradually adding the lead ( ‫׀‬ ‫׀‬ ) aqueous ions into the GO dispersion resulted in producing a growing humpy pattern around 300 nm which can be attributed to the affinity between lead ( ‫׀‬ ‫׀‬ ) and C = O bonds relating to the carboxylic groups in the GO structure[33]. Fabricated Fe 3 O 4 @SiO 2 -GO was characterized with different techniques. "
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    ABSTRACT: Background: Magnetic graphene oxide (Fe3O4@SiO2-GO) nanocomposite was fabricated through a facile process and its application as an excellent adsorbent for lead (II) removal was also demonstrated by applying response surface methodology (RSM). Methods: Fe3O4@SiO2-GO nanocomposite was synthesized and characterized properly. The effects of four independent variables, initial pH of solution (3.5-8.5), nanocomposite dosage (1-60 mg L(-1)), contact time (2-30 min), and initial lead (II) ion concentration (0.5-5 mg L(-1)) on the lead (II) removal efficiency were investigated and the process was optimized using RSM. Using central composite design (CCD), 44 experiments were carried out and the process response was modeled using a quadratic equation as function of the variables. Results: The optimum values of the variables were found to be 6.9, 30.5 mg L(-1), 16 min, and 2.49 mg L(-1) for pH, adsorbent dosage, contact time, and lead (II) initial concentration, respectively. The amount of adsorbed lead (II) after 16 min was recorded as high as 505.81 mg g(-1) for 90 mg L(-1) initial lead (II) ion concentration. The Sips isotherm was found to provide a good fit with the adsorption data (KS = 256 L mg(-1), nS = 0.57, qm = 598.4 mg g(-1), and R(2) = 0.984). The mean free energy Eads was 9.901 kJ/mol which confirmed the chemisorption mechanism. The kinetic study determined an appropriate compliance of experimental data with the double exponential kinetic model (R(2) = 0.982). Conclusions: Quadratic and reduced models were examined to correlate the variables with the removal efficiency of Fe3O4@SiO2-GO. According to the analysis of variance, the most influential factors were identified as pH and contact time. At the optimum condition, the adsorption yield was achieved up to nearly 100 %.
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    • "The G band is related to the first-order scattering of the E 2g vibrational mode [52]. The D-band is typically associated with structural defects; thus, the intensity of the D-band can be used to evaluate the degree of disorder: the intensity ratio of the Dand G-bands (I D /I G ) increases with increasing defect density [51,53]. The Raman spectrum of the pristine graphite inFig. "
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    ABSTRACT: Flexible supercapacitors with large power and energy densities, long life cycles and good operational safety are necessary devices for various applications. In this work, we demonstrate the integration of a composite based on graphene nanosheets/multiwalled carbon nanotubes in an in-plane supercapacitor configuration by using a straightforward preparation involving the filtration of nanomaterials to produce an electrode film. Reduced graphene oxide (RGO) received 15 wt % carbon nanotubes to act as a conducting additive, which led to a flexible and transferable thin film (RGO/MW) with an average conductivity of 20.0 S cm-1. Three ionic liquids were tested as electrolytes for the supercapacitor, among which 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) was observed to exhibit the best performance. The specific capacitance of the supercapacitor based on RGO/MW-EMITFSI reached 153.7 F g-1 at a current density of 0.2 A g-1 and exhibited a capacitance retention of 88% after 2000 cycles. The maximum energy and power densities were calculated to be 41.3 Wh kg-1 and 3.5 kW kg-1, respectively, for the RGO/MW-EMITFSI supercapacitor.
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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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