Atomic Force and Scanning Tunneling Microscopy Imaging of Graphene Nanosheets Derived from Graphite Oxide
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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- "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  "
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. "
ABSTRACT: Abstract Purpose: The aim of the work is 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 can 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 allows 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 · 1.84 Impact Factor
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- "This reduction can enlarge the graphitic domains to form a percolated network, resulting in partial restoration of electrical conductivity. However, the harsh chemical oxidation reaction and the loss of carbon atoms during reduction can introduce carbon vacancies in the basal plane, which can be observed directly under scanning tunneling microscopy . These vacancies can provide more adsorption sites for gas molecules, which makes rGO a promising material for chemical sensors . "
ABSTRACT: Patterning oriented reduced graphene oxide (rGO) into functional structures is significant for its application in electronics and sensors. A large array of highly oriented rGO microbelts are prepared by a soft lithography process. These rGO microbelts have a uniform structure that enables the massive production of graphene electronics using a simple mask shielding process. A high performance NH3 sensor array which was fabricated from rGO microbelts exhibits a reproducible performance with the relative resistance response (ΔR/R0) reaching 0.35, whilst offering a large concentration range response of 10 ppm ~38%, showing these sensors to be both highly sensitive and responsive. The impact of working temperature on the response to NH3 in low and high concentration ranges of NH3 is also discussed.Journal of Micromechanics and Microengineering 08/2013; 23(9):095031. DOI:10.1088/0960-1317/23/9/095031 · 1.73 Impact Factor