Few-layer Graphene Oxide Research Articles

Determination of High-Frequency Dielectric Constant and Surface Potential of Graphene Oxide and Influence of Humidity by Kelvin Probe Force Microscopy.

We use Kelvin probe force microscopy (KPFM) and capacitance coupling (dC/dz) to study the electrical properties of graphene oxide (GO). We propose using the dC/dz signal to probe the high frequency dielectric constant of mono- and few-layer GO. Our measurements suggest that the dynamic dielectric constant of GO is on the order of εGO ≅ 3.0 ε0, in the high frequency limit, and independent of the number of GO layers. The measurements are performed at a humidity controlled environment (5% of humidity). The effects of increasing humidity on both the dC/dz and KPFM measurements are analyzed.

Removal of 17β-estradiol by few-layered graphene oxide nanosheets from aqueous solutions: External influence and adsorption mechanism

This study demonstrates the use of few-layered graphene oxide nanosheets (GO) as efficient adsorbents for the removal of 17β-Estradiol (E2) from aqueous solutions via strong adsorptive interactions. The adsorption performance of GO was investigated by batch adsorption experiments. Further, adsorption experiments were carried out in the presence of other environmental pollutants to understand external influence on the adsorption of E2 by GO. The result indicated that the maximum adsorption capacity (qm) of GO for E2 obtained from the Langmuir isotherm was 149.4mg/g at 298K and it was the highest values of E2 adsorption compared to that of other adsorbents reported before. Thermodynamic study indicated that the adsorption was a spontaneous process. In addition, the result showed that E2 adsorption on GO was slight affected by the solution pH. The presence of NaCl in the solution facilitated the E2 adsorption and the optimum adsorption capacity was obtained when the NaCl concentration was 0.001M. Moreover, the effect of background electrolyte divalent cations (Mg2+ and Ca2+) was not similar with the monovalent cations (Na+ and K+). While the influence of background electrolyte anions (Cl−, NO3-, SO42-, and PO43-) were not significantly different. The presence of humic acid reduced E2 adsorption on GO at pH 7.0. GO still exhibited excellent adsorption capacity following numerous desorption/adsorption cycles. Besides, both π–π interactions and hydrogen bonds might be responsible for the adsorption of E2 onto GO.

Qualitative and Quantitative Analysis of Graphene Oxides by UV Spectroscopy

Graphene oxide(GO) was prepared by chemical oxidation- sonicated crushing method with flake graphite, sulfuric acid and potassium permanganate as raw materials. The prepared GO was characterized by means of spectroscopy SEM, laser granulometer, FT-IR, UV-vis and AFM. The results show that the few-layered GO could be differentiated from the multi-layered graphite oxide in a solution of the mixture of the two by observing the variation of the intensity and the profile of absorption peaks around 230 nm in the UV- vis spectroscopy. Based on this phenomenon, the qualitative and quantitative analysis of graphene oxides in the solution can be realized. Thereby the optimal conditions for the preparation of graphene oxide can be obtained. It is proposed that the appearence of multi absorption peaks nearby 230 nm in the UV spectra of GO may be attributed to the existence of a peculiar structure of discontinues p-p* conjugated system in the graphene oxide.

Multifunctional nanocomposites based on tetraethylenepentamine-modified graphene oxide/epoxy

Composites based on epoxy/graphene were investigated for thermal-mechanical performance. Initially, few-layer graphene oxide (GO) was modified with tetraethylenepentamine (GO-TEPA) in a reaction assisted by microwave radiation. GO and GO-TEPA samples were characterized for their structure and morphology. Composites containing 0.1, 0.3 and 0.5 wt.% of GO and GO-TEPA were prepared, and the effect of fillers on the morphology of cryofractured regions of epoxy matrix was observed through electron microscopy images. Dynamic mechanical thermal analysis (DMA) tests revealed increases of approximately 20 °C in glass transition. Moreover, when compared to neat polymer, composites containing 0.5 wt.% of GO-TEPA gained up to 103% in thermal conductivity (obtained by flash laser). Finally, nanoindentation analyses showed increases of 72% in Young's modulus and 143% in hardness for the same sample. The system is characterized as multifunctional nanocomposites because of the simultaneous gains in thermal and mechanical properties. The best results of the multifunctional composites were strongly associated with the chemical modification of the GO by TEPA.

Removal of Toxic Metal Ions from Sungun Acid Rock Drainage Using Mordenite Zeolite, Graphene Nanosheets, and a Novel Metal–Organic Framework

We evaluated the sorptive properties of mordenite zeolite (MOR), a copper terephthalate metal–organic framework (MOF), and graphene oxide (GO) and their potential use in treating acid rock drainage (ARD) containing Fe3+, Cu2+, Mn2+, Zn2+, Pb2+, and Cd2+ ions. MOR was prepared via a hydrothermal method, MOF was prepared via a solvothermal method, and few-layered GO nanosheets were synthesized using Hummers’ method. The aqueous contaminants were removed by chemico-physical sorption and ion exchange in batch tests. It proved possible to dramatically improve removal efficiency and sorptive capacity by optimizing experimental conditions. Magnetic MOF crystals were more efficient in removing metals than the MOR and GO. Sorption tests using ARD from the Sungun copper mine and a multi-component solution containing cationic metal species revealed that both GO nanosheets and magnetic MOF have great potential for ARD treatment.

Optical Properties of Silver Nanoparticles Integrated Graphene Oxide Thin Films on Glass and Silicon Substrates

We report the synthesis of graphene oxide colloid, and its transformation as a few-layer graphene oxide (GO) thin film on a glass and silicon substrates using a spin coating technique for plasmonic substrates. Annealed graphene oxide (AGO) thin films of variable thicknesses are used as substrates for silver nanoparticles (Ag NPs) formation. In surface morphological study, we observed modified Ag NPs growth by the presence of AGO thin film on glass and silicon substrates when compared to substrates without AGO film. Well-separated and worm-like Ag NPs formation was observed due to the high surface diffusion of Ag provided by the underlying AGO thin film; in contrast, dense and spheroid-like NPs formation was observed on uncoated substrates. Silver NPs prepared on AGO thin films are characterized by transmittance measurements on glass substrates and total reflectance measurements on silicon substrates from 200 to 1200 nm. We observed AGO film thickness dependent quadrupolar resonances of Ag NPs and total reflectance at longer wavelengths (>800 nm) region, while dipolar resonances are insensitive to the variation of AGO film thickness. The modified absorption/scattering profiles and intensities are explained by surface plasmon dipolar and quadrupolar resonance mode confinement variation at the AGO thin film-nanoparticles interface.

Triturating versatile carbon materials as saturable absorptive nano powders for ultrafast pulsating of erbium-doped fiber lasers

Five triturating nano-scale carbon materials based saturable absorbers are compared for passively mode-locked high-gain erbium-doped fiber lasers (EDFLs), including few-layer graphene nano-sheets, graphite, graphene oxide, carbon black and charcoal nano-particles. Nonlinear transmittance analyses show that these nano-scale carbon powders with saturation intensity increased from 0.8 to 6.1 MW/cm2 can reduce their modulation depth (MD) from 60% (few-layer graphene) to 23% (charcoal nano-powder). Except few-layer graphene, other carbon materials exhibit Raman scattering spectra with enlarged D-peaks and attenuated 2D-peaks. Even with the nonlinear absorbance slightly decreased from 5.4 × 10−2 (few-layer graphene) to 3.7 × 10−2 (charcoal), these carbon material powders still start the mode-locking of the EDFLs with <500-fs pulsewidth at L-band. The effect of saturable absorber induced self-amplitude modulation (SAM) becomes weakened, whereas the intense self-phase modulation (SPM) dominates the pulse shortening force afterwards. These observations elucidate that the carbon based saturable absorber plays a role like the mode-locking initiator and stabilizer instead of serving as a strong pulse compressor.

Facile Graphene Oxide Preparation by Microwave-Assisted Acid Method

Few-layered graphene oxide (GO) was prepared using a fast and energy-saving method by microwave-assisted acid technique. The oxygenated groups existing on the GO surface were determined using UV-Vis, X-ray photoelectron and Fourier-transform infrared spectroscopies. An oxygenated group percentage of 30% in mass for the GO was observed by thermogravimetric analysis. The reduced few-layered graphene oxide (rGO) film annealed at 110 °C deposited onto a silicon/silica wafer showed expanded graphite-like structure with 0.70 nm between the rGO sheets, as determined by X-ray diffraction. This rGO film exhibited a relatively high electrical conductivity value of 7.36 × 102 S m-1 confirming the good restoration of the π-conjugated system. The prepared GO sample exhibited good stability in water from pH 4 to 12, as determined by its zeta potential, and contained 5 to 9 layers, as determined by atomic force microscopy (AFM) and transmission electron microscopy (TEM).

Facile synthesis of graphene oxide in a Couette–Taylor flow reactor

A practical approach to bulk-scale graphene-based materials is critically important for their use in the industrial applications. Here, we describe a facile method to prepare graphite oxide (GO) using a Couette–Taylor flow reactor for the oxidation of bulk graphite flakes. We found that the turbulent Couette–Taylor flow in the reactor could be engineered to result in the efficient mixing and mass transfer of graphite and oxidizing agents (KMnO4 and H2SO4), thereby improving the efficiency of graphite into GO. As compared to the standard Hummers’ method, higher fraction of a single- and few-layer graphene oxide (G-O) can be yielded in a dramatically shortened reaction time, by optimizing the processing parameters, we have shown that ∼93% of G-O yield could be achieved within 60min of reaction time. This method also allowed for the in-situ functionalization of G-O with metal oxide nanoparticles to give a nanoparticle-decorated G-O hybrid material. Our method for facile and large-scale production of graphite oxide may find utility in a range of applications including energy storage, composites and supporting frameworks of catalyst.

On the oxidation degree of few-layer graphene oxide sheets obtained from chemically oxidized multiwall carbon nanotubes

Multiwall carbon nanotubes (MWCNT), generated by catalytic decomposition of methane using a Fe–Mo/MgO catalyst, were subjected to oxidation and exfoliation obtaining aqueous suspensions of few-layer graphene oxides (FLGO). FLGO with different oxidation ratios were obtained using a modified Hummers method by varying the amount of oxidizing agent (KMnO4), followed by ultrasonic exfoliation. The exhaustive characterization of the resulting materials clearly showed surface functional groups evolution during oxidation and structural and morphological changes. Oxidation caused expansion and separation of the graphene layers by oxygenated groups formation and water intercalation. The full opening of the MWCNT resulted in FLGO as carbon nanoribbons and sheets from a KMnO4/MWCNT ratio of 6.

Efficient removal of radionuclide U(VI) from aqueous solutions by using graphene oxide nanosheets

In this paper, few-layered graphene oxide nanosheets were synthesized by using modified Hummers’ method. Because of their high surface area and abundant functional groups for binding U(VI) ions, the graphene oxide nanosheets were applied as adsorbents to preconcentrate U(VI) ions from aqueous solutions by using batch technique. The results indicated that the sorption of U(VI) on graphene oxide nanosheets were strongly dependent on pH and independent of ionic strength, and the sorption process was spontaneous and endothermic. The sorption capacities of U(VI) on graphene oxides were much higher than any other reported values of todays’ materials, indicating that the graphene oxides may be a promising material for the preconcentration and solidification of U(VI) from large volumes of aqueous solutions in radionuclide pollution cleanup in near future.

Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods

The commercial and synthesised few-layer graphene oxide, prepared using oxidation reactions, and few-layer reduced graphene oxide samples were structurally and chemically investigated by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron spectroscopy methods, i.e. X-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS).The commercial graphene oxide (FL-GOc) shows a stacking nanostructure of about 22×6nm average diameter by height with the distance of 0.9nm between 6-7 graphene layers, whereas the respective reduced graphene oxide (FL-RGOc)—about 8×1nm average diameter by height stacking nanostructure with the distance of 0.4nm between 2-3 graphene layers (XRD). The REELS results are consistent with those by the XRD indicating 8 (FL-GOc) and 4 layers (FL-RGOc). In graphene oxide and reduced graphene oxide prepared from the graphite the REELS indicates 8–11 and 7–10 layers. All graphene oxide samples show the C/O ratio of 2.1–2.3, 26.5–32.1 at% of C sp3 bonds and high content of functional oxygen groups (hydroxyl—COH, epoxy—COC, carbonyl—CO, carboxyl—COOH, water) (XPS). Reduction increases the C/O ratio to 2.8–10.3, decreases C sp3 content to 11.4–20.3 at% and also the content of COC and CO groups, accompanied by increasing content of COH and COOH groups. Formation of additional amount of water due to functional oxygen group reduction leads to layer delamination. Removing of functional oxygen groups and water molecules results in decreasing the distance between the graphene layers.

Potential of laser-induced breakdown spectroscopy for discrimination of nano-sized carbon materials. Insights on the optical characterization of graphene

Since its invention in 2004, graphene has attracted considerable interest worldwide. Advances in the use of graphene in materials science and engineering require important increases in the quality of the final product for integration in photonic and electronic devices. To meet this demand, which will become increasingly strict in the future, analytical techniques capable of differentiating between the starting materials and graphene need to be developed. The interest in the use of laser-induced breakdown spectroscopy (LIBS) for this application rests on the rapid progress experienced by this technology for identification of carbon-based materials of close chemical composition. The potential of LIBS has been explored here by a careful investigation of the spectral properties of both multi-layer and few-layer graphene, graphite and graphene oxide. Results reveal significant differences in the specific optical emission responses of these materials, expressly reflected on the behavior of CN and C2 molecular emissions. These differences result from the particularities of the materials, such as the number of carbon layers and the carbon hybridization in the bonding structure, together with the post-ablation evolution of the concerned plasma plume. In short, this interconnection between ablation and emission events generated from each material allows its characterization and its differentiation from other materials with highly similar chemical composition.

Effects of humic acid on copper adsorption onto few-layer reduced graphene oxide and few-layer graphene oxide

The effects of humic acid (HA) on copper (Cu(II)) adsorption onto few-layer reduced graphene oxide (FRGO) and few-layer graphene oxide (FGO) were investigated using a batch equilibration method, micro-Fourier transform infrared spectroscopy, and extended X-ray absorption fine structure spectroscopy (EXAFS). The results showed that HA was adsorbed on FRGO through π–π interaction. The adsorbed HA introduced O-containing functional groups and negative charges to FRGO surfaces, increasing Cu(II) adsorption through chemical complexation and electrostatic attraction. In contrast, HA was adsorbed onto FGO mainly through polar interactions, due to its rich O-containing functional groups. The adsorbed HA had little effect on Cu(II) adsorption onto FGO because the shielding effect of HA on Cu(II) adsorption was offset by newly introduced adsorption sites of HA on FGO. EXAFS results suggested that Cu(II) was adsorbed on FRGO and FGO mainly through the coordination with their O-containing functional groups. When HA was added at pH 4.0 and 6.0, more Cu(II) was adsorbed on HA-coated FRGO. At pH 8.0, a portion of Cu(II) in solution precipitated on FRGO surface, while the presence of HA led to the formation of FRGO-HA-Cu ternary surface complexes instead of Cu(II) precipitation.

The synthesis of graphene oxide nanostructures for supercapacitors: a simple route

In this work, we report a simple strategy for synthesis of graphene oxide nanostructures with various morphologies including single-, few-layer, and three-dimensional networks. Morphology control is achieved by adding different amounts of Ni2+ into a one-step hydrothermal process. The involved growth mechanisms for the morphology control are discussed. A random arrangement of graphene oxide nanosheets is suggested to induce the networks’ formation. Ni2+ facilitates the formation of graphene oxide’s preferential face-to-face overlapping structure, and high Ni2+ concentrations render adjacent graphene oxide sheets to combine each other tightly to form closely packed, layered structures. Compared with single-, few-layer graphene oxide, the electrode prepared by three-dimensional networks has a mass specific capacitance of 352 F g−1 at v = 5 mV s−1, which is much higher than that of recently reported three-dimensional graphene oxide nanostructures (240 F g−1).

Exploring few-layer graphene and graphene oxide as fillers to enhance the oxygen-atom corrosion resistance of composites.

Few-layer graphene (FLG) and graphene oxide (GO) were explored to enhance the oxygen-atom corrosion resistance of composites. FLG flakes of two different average lateral sizes (large: ∼1.3 μm(2) and small: ∼0.23 μm(2)) were prepared by a centrifugation-based size selection route. After exposure to oxygen atoms, although all fillers could enhance the oxygen-atom corrosion resistance of the composites, we found a much greater enhancement using large FLG, i.e. adding 1 wt% large FLG can achieve a 42% decrease in the composites' mass loss. Bonding and barrier effects of the flaked fillers are responsible for the enhanced resistance. These preliminary yet intriguing results pave a novel way for resisting oxygen-atom corrosion.

Controlled assembly of layer-by-layer stacking continuous graphene oxide films and their application for actively modulated field electron emission cathodes

A featured "vapor transportation" assembly technique was developed to attain layer-by-layer stacking continuous graphene oxide (GO) films on both flat and concavo-concave surfaces. Few-layer (layer number < 10) GO sheets were "evaporated" (carried by water vapor) from the water-dispersed GO suspension and smoothly/uniformly tiled on the substrate surface. We have found evidence of the influence of the deposition time and substrate-liquid separation on the film thickness. A model was proposed for interpreting the assembly process. It was found that a current conditioning would induce a reduction of the GO surface and form an Ohmic contact between the GO-metal interfaces. Accordingly, an actively modulated GO cold cathode was fabricated by locally depositing continuous GO sheets on the drain electrode of a metal-oxide-semiconductor field effect transistor (MOSFET). The field emission current of the GO cathode can be precisely controlled by the MOSFET gate voltage (VGS). A current modulation range from 1 × 10(-10) A to 6.9 × 10(-6) A (4 orders of magnitude) was achieved by tuning the VGS from 0.812 V to 1.728 V. Due to the self-acting positive feedback of the MOSFET, the emission current fluctuation was dramatically reduced from 57.4% (non-control) to 3.4% (controlled). Furthermore, the integrated GO cathode was employed for a lab-prototype display pixel application demonstrating the active modulation of the phosphor luminance, i.e. from 0.01 cd m(-2) to 34.18 cd m(-2).

In situ Raman spectroelectrochemistry of graphene oxide

Electrochemical reduction of few-layer graphene oxide (FLGO) is a simple method for a partial restoration of sp2 network of the graphitic planes damaged by the previous oxidation/exfoliation process, and it is especially interesting for the in situ activation of FLGO in applications for energy conversion and storage. We present a detailed study of the structural evolution of FLGO and also non-oxidized graphene nanoplatelets (GNP) during electrochemical treatment. Two phases of the process can be traced tentatively in the case of FLGO by ex situ X-ray photoelectron spectroscopy and both ex situ and in situ Raman spectroscopy. The first phase is irreversible and dominated by a fast removal of oxygen-bearing functional groups accompanied by a structural ordering, while the second phase shows only a slow irreversible progressive reduction and the major changes in the Raman spectra caused by lattice expansion/contraction upon doping or a mild oxidation/reduction are reversible this time. In GNP, no irreversible reduction is observed, i.e. the first phase is absent, leaving only the reversible variations traceable in the Raman spectra.

Progressive In Situ Reduction of Graphene Oxide Studied by Raman Spectroelectrochemistry: Implications for a Spontaneous Activation of LiFePO4 (Olivine)

AbstractLiFePO4 olivine (LFP) with few‐layer graphene oxide (FLGO) was tested by repeated electrochemical charging/discharging. Here we report on a novel effect that the LFP/FLGO composite, which is electrochemically almost idle in a freshly made state, spontaneously enhances its activity, when the charge/discharge cycling progresses. Based on electrochemical, IR and Raman studies, we suggest that a gradual in situ reduction of FLGO is responsible for this activation.

Selective Gas Transport Through Few-Layered Graphene and Graphene Oxide Membranes

Graphene is a distinct two-dimensional material that offers a wide range of opportunities for membrane applications because of ultimate thinness, flexibility, chemical stability, and mechanical strength. We demonstrate that few- and several-layered graphene and graphene oxide (GO) sheets can be engineered to exhibit the desired gas separation characteristics. Selective gas diffusion can be achieved by controlling gas flow channels and pores via different stacking methods. For layered (3- to 10-nanometer) GO membranes, tunable gas transport behavior was strongly dependent on the degree of interlocking within the GO stacking structure. High carbon dioxide/nitrogen selectivity was achieved by well-interlocked GO membranes in high relative humidity, which is most suitable for postcombustion carbon dioxide capture processes, including a humidified feed stream.