Multilayer Graphene Oxide Research Articles

Immediate fabrication of flower-like graphene oxide by ion beam bombardment

An effective and convenient method using ion beam bombardment (IBB) for separating a multi-layered compact graphene oxide (GO) piece into several small few-layered loose pieces was developed, and it was found that those small GO pieces had formed a flower-like structure. Therein, the main mechanism was proposed to be the etching and charge effects of IBB. This work could provide a facile and promising approach for improving the dispersion and the related properties of GO. Furthermore, X-ray diffraction and Raman spectrum determinations demonstrated that, with the increasing fluence, IBB could effectively decrease the chemical groups in the layers of GO, resulting in the decrease of the layer distance.

In-Situ ESEM and EELS Observation of Water Uptake and Ice Formation in Multilayer Graphene Oxide.

Graphene oxide (GO) is hydrophilic and swells significantly when in contact with water. Here, we investigate the change in thickness of multilayer graphene oxide membranes due to intercalation of water, via humidity-controlled observation in an environmental scanning electron microscope (ESEM). The thickness increases reproducibly with increasing relative humidity. Electron energy loss spectroscopy (EELS) reveals the existence of water ice under cryogenic conditions, even in high vacuum environment. Additionally, we demonstrate that freezing then thawing water trapped in the multilayer graphene oxide membrane leads to the opening up of micron-scale inter-lamellar voids due to the expansion of ice crystals.

Rolled graphene oxide foams as three-dimensional scaffolds for growth of neural fibers using electrical stimulation of stem cells

Graphene oxide foam (GOF) layers with thicknesses of ∼15–50μm and density of ∼10 graphene oxide (GO) sheets/μm were fabricated by precipitation of chemically exfoliated GO sheets in an aqueous suspension at ∼80°C under UV irradiation. Then, rolled GOFs with desirable scales were developed as electrically conductive 3D-scaffolds and applied in directional growth of neural fibers, through differentiation of human neural stem cells (hNSCs) into neurons under an electrical stimulation. X-ray photoelectron spectroscopy indicated that the UV irradiation resulted in partial deoxygenation of the layers. Scanning electron microscopy and Raman spectroscopy confirmed the presence of multilayer GO sheets in the foam structure. The electrical sheet resistance of the GOFs was found low enough to produce the electrical stimulation currents used in differentiation of the neural cells, under low voltages. Rolling the GOFs (with hydrophilic surfaces) resulted in formation of cross-sections with superhydrophilic characteristics, inducing effective proliferation and differentiation of the hNSCs throughout the pores and interfaces of the scaffold. The electrical stimulation induced more proliferation of the cells and acceleration of the differentiation into neurons (rather than glia). These results suggest the GOFs as flexible and conductive scaffolds for regeneration of nervous systems and tissue engineering.

A Novel Method of Fabricating Flexible Transparent Conductive Large Area Graphene Film**Supported by the Basic Research Program of Nanjing University of Posts and Telecommunications under Grant No NY212002, the Innovative Research Team in University under Grant No IRT1148, and the 2014 Shuangchuang Program of Jiangsu Province.

We fabricate flexible conductive and transparent graphene films on position-emission-tomography substrates and prepare large area graphene films by graphite oxide sheets with the new technical process. The multi-layer graphene oxide sheets can be chemically reduced by HNO3 and HI to form a highly conductive graphene film on a substrate at lower temperature. The reduced graphene oxide sheets show a high conductivity sheet with resistance of 476 Ω/sq and transmittance of 76% at 550 nm (6 layers). The technique used to produce the transparent conductive graphene thin film is facile, inexpensive, and can be tunable for a large area production applied for electronics or touch screens.

Nanoscale reduction of graphene oxide thin films and its characterization

In this paper, we report on a method to reduce thin films of graphene oxide (GO) to a spatial resolution better than 100 nm over several tens of micrometers by means of an electrochemical scanning probe based lithography. In situ tip-current measurements show that an edged drop in electrical resistance characterizes the reduced areas, and that the reduction process is, to a good approximation, proportional to the applied bias between the onset voltage and the saturation thresholds. An atomic force microscope (AFM) quantifies the drop of the surface height for the reduced profile due to the loss of oxygen. Complementarily, lateral force microscopy reveals a homogeneous friction coefficient of the reduced regions that is remarkably lower than that of native graphene oxide, confirming a chemical change in the patterned region. Micro Raman spectroscopy, which provides access to insights into the chemical process, allows one to quantify the restoration and de-oxidation of the graphitic network driven by the electrochemical reduction and to determine characteristic length scales. It also confirms the homogeneity of the process over wide areas. The results shown were obtained from accurate analysis of the shift, intensity and width of Raman peaks for the main vibrational bands of GO and reduced graphene oxide (rGO) mapped over large areas. Concerning multilayered GO thin films obtained by drop-casting we have demonstrated an unprecedented lateral resolution in ambient conditions as well as an improved control, characterization and understanding of the reduction process occurring in GO randomly folded multilayers, useful for large-scale processing of graphene-based material.

Graphene Wet-etching Kinetics as Time-reversed Crystallization

Abstract Wet-etching kinetics of single-layer graphene and multilayer graphene oxide by piranha solution under a mild condition was modeled as a time-reversed crystallization process and was analyzed by Avrami theory. The Avrami exponent is 1 at the early stage of the reaction for both materials, indicating that etching proceeds along reactive edges without forming additional defects. Then, the exponent for graphene changes to 2 whereas that of graphene oxide increases to 4. Stretched exponential behavior is also observed for highly defective samples.

Ultrasonic synthesis of high fluorescent C-dots and modified with CuWO4 nanocomposite for effective photocatalytic activity

High fluorescent C-dots were synthesized from dextrose via facile ultrasonic wave assisted reaction. Carbon dots (C-dots)/Copper tungstate (CuWO4) heterostructure has been prepared via a facile reflux approach. The novel C-dots/CuWO4 photocatalyst were characterized by using transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, UV–vis, photoluminescence and X-ray photoelectron spectroscopy. The photocatalytic property of heterostructure nanocomposite has also been investigated by using rhodamine B as a model organic pollutant. The obtained results revealed that as prepared C-dots are predominately multilayer graphene oxide with luminance properties. After incorporation of C-dots onto the CuWO4, it contributes to the improvement of charge separation and reduction of charge recombination and thereby enhancing the photocatalytic property, respectively. The C-dots in the nanocomposites can efficiently trap electrons, thus hindering the recombination of photogenerated electrons and holes. The heterostructure nanocomposite with 5.0 wt% C-dots shows the highest photocatalytic activity, which is about three times as that of pure CuWO4.

Adsorption of metals onto graphene oxide: Surface complexation modeling and linear free energy relationships

Abstract In this study, the adsorption behaviors of a range of metals onto multi-layered graphene oxide (GO) as a function of pH and metal:GO ratio were measured. The GO exhibited an affinity for the metals in the following order: Pb(II) > Cu(II) > > Cd(II) ≈ Ni(II) ≈ Zn(II) > Sr(II) ≈ Ca(II). A four discrete-site non-electrostatic surface complexation model was used to account for the observed adsorption behaviors. For most of the metals studied, we modeled the adsorption by invoking divalent cation binding onto two or more of the deprotonated GO surface sites. The stability constant for each metal-site complex was calculated from the modeling, and the resulting models in general yield good fits to the experimental data. The calculated stability constants exhibit a correlation to stability constants for the corresponding aqueous metal-acetate, or the corresponding metal-hydroxide complexes. The calculated stability constants and the derived relationships from this study enable preliminary but useful predictions of the extent of metal adsorption onto GO for metals which have yet to be studied experimentally, and we tested the accuracy of these predictions by estimating and measuring the extent of Nd(III) adsorption onto GO as a function of EDTA concentration. The predicted extent of Nd adsorption onto GO is within 5% of the measured extent of Nd adsorption in an EDTA-free solution, and the predicted Nd adsorption behavior with increasing EDTA concentration is also in reasonable agreement with the measurements. Our results demonstrate the high affinity that GO has for binding a wide range of aqueous metals, and the results can be used to estimate with a reasonable degree of certainty the distribution of metals in complex GO-bearing environmental or engineered systems.

Enhanced Ion Current Rectification in 2D Graphene-Based Nanofluidic Devices.

Furthering the promise of graphene‐based planar nanofluidic devices as flexible, robust, low cost, and facile large‐scale alternatives to conventional nanochannels for ion transport, we show how the nonlinear current–voltage (I–V) characteristics and ion current rectification in these platforms can be enhanced by increasing the system asymmetry. Asymmetric cuts made to the 2D multilayered graphene oxide film, for example, introduces further asymmetry to that natively inherent in the structurally symmetric system, which was recently shown to be responsible for its rectification behavior due to diffusion boundary layer fore–aft asymmetry. Supported by good agreement with theory, we attribute the enhancement to the decrease in the limiting current in the positive bias state in which counter‐ion trapping occurs within the negatively charged graphene oxide sheets due to increased film permselectivity as its cross‐section and surface charge distribution is altered on one end; these effects being shown to be sensitive to the electrolyte pH. Further, we show that an imbalance in the pH or concentration in the microreservoirs flanking the film can also increase asymmetry and hence rectification, in addition to displaying a host of other phenomena associated with the I–V characteristics of typical nanochannel electrokinetic systems.

A multilayered silicon-reduced graphene oxide electrode for high performance lithium-ion batteries.

A multilayered structural silicon-reduced graphene oxide electrode with superior electrochemical performance was synthesized from bulk Si particles through inexpensive electroless etching and graphene self-encapsulating approach. The prepared composite electrode presents a stable charge-discharge performance with high rate, showing a reversible capacity of 2787 mAh g(-1) at a charging rate of 100 mA g(-1), and a stable capacity over 1000 mAh g(-1) was retained at 1 A g(-1) after 50 cycles with a high columbic efficiency of 99% during the whole cycling process. This superior performance can be attributed to its novel multilayered structure with porous Si particles encapsulated, which can effectively accommodate the large volume change during the lithiation process and provide increased electrical conductivity. This facile low-cost approach offers a promising route to develop an optimized carbon encapsulated Si electrode for future industrial applications.

Wrinkled, wavelength-tunable graphene-based surface topographies for directing cell alignment and morphology

Textured surfaces with periodic topographical features and long-range order are highly attractive for directing cell-material interactions. They mimic physiological environments more accurately than planar surfaces and can fundamentally alter cell alignment, shape, gene expression, as well as multicellular organization into hierarchical tissue architectures. Here we demonstrate for the first time that wrinkled graphene-based surfaces are suitable as textured cell attachment substrates, and that engineered wrinkling can dramatically alter cell alignment and morphology. The wrinkled surfaces are fabricated by graphene oxide wet deposition onto pre-stretched elastomers followed by relaxation and mild thermal treatment to stabilize the films in cell culture medium. Multilayer graphene oxide films form periodic, delaminated buckle textures whose wavelengths and amplitudes can be systematically tuned by variation in the wet deposition process. Human and murine fibroblasts attach to these textured films and remain viable, while developing pronounced alignment and elongation relative to those on planar graphene controls. Compared to lithographic patterning of nanogratings, this method has advantages in the simplicity and scalability of fabrication, as well as the opportunity to couple the use of topographic cues with the unique conductive, adsorptive, or barrier properties of graphene materials for functional biomedical devices.

Heteroaggregation of graphene oxide with minerals in aqueous phase.

Upon release into waters, sediments, and soils, graphene oxide (GO) may interact with fine mineral particles. We investigated the heteroaggregation of GO with different minerals, including montmorillonite, kaolinite, and goethite, in aqueous phase. GO significantly enhanced the dispersion of positively charged goethite (>50%) via heteroaggregation, while there was no interaction between GO and negatively charged montmorillonite or kaolinite. Electrostatic attraction was the dominant force in the GO-goethite heteroaggregation (pH 4.0-8.5), and the dissolved Fe ions (<0.16 mg/L) from goethite were unable to destabilize GO suspension. The GO-goethite heteroaggregation was further quantitatively investigated through GO adsorption study. All adsorption isotherms of GO at different solution pH (4.0 and 6.5) followed the Linear model. The apparent intercept (1.0-6.9 mg/g) was observed for all the adsorption isotherms, indicating that this fraction of adsorbed GO was difficult to desorb from goethite (defined here as irreversible adsorption) under the tested conditions. Desorption hysteresis was observed, which could be explained by the formation of multilayered GO-goethite complex with high configurational stability. These findings are useful for understanding the interaction of GO with mineral surfaces, and potential fate and toxicity of GO under natural conditions in aquatic environments, as well as in soils and sediments.

Systematic study on the sensitivity enhancement in graphene plasmonic sensors based on layer-by-layer self-assembled graphene oxide multilayers and their reduced analogues.

The use of graphene in conventional plasmonic devices was suggested by several theoretic research studies. However, the existing theoretic studies are not consistent with one another and the experimental studies are still at the initial stage. To reveal the role of graphenes on the plasmonic sensors, we deposited graphene oxide (GO) and reduced graphene oxide (rGO) thin films on Au films and their refractive index (RI) sensitivity was compared for the first time in SPR-based sensors. The deposition of GO bilayers with number of deposition L from 1 to 5 was carried out by alternative dipping of Au substrate in positively- and negatively charged GO solutions. The fabrication of layer-by-layer self-assembly of the graphene films was monitored in terms of the SPR angle shift. GO-deposited Au film was treated with hydrazine to reduce the GO. For the rGO-Au sample, 1 bilayer sample showed a higher RI sensitivity than bare Au film, whereas increasing the rGO film from 2 to 5 layers reduced the RI sensitivity. In the case of GO-deposited Au film, the 3 bilayer sample showed the highest sensitivity. The biomolecular sensing was also performed for the graphene multilayer systems using BSA and anti-BSA antibody.

Targeted kinetic strategy for improving the thermal conductivity of epoxy composite containing percolating multi-layer graphene oxide chains

By adding 2 wt% multi-layer graphene oxide (MGO) to an epoxy resin, the thermal conductivity of the compos- ite reached a maximum, 2.03 times that of the epoxy. The presence of 2 wt%MGO percolating chains leads to an unprece- dentedly sharp rise in energy barrier at final curing stage, but an increased epoxy curing degree (!IR) is observed; however, this !IR difference nearly disappears after aging or thermal annealing. These results suggest that the steep concentration gra- dient of -OH, originated from the 2 wt%MGO percolating chains, exerts the vital driving force on the residual isolated/ trapped epoxy to conquer barrier for epoxy-MGO reaction. A modified Shrinking Core Model customized for the special layered-structure of MGO sheet was proposed to understand the resistance variation during the intercalative epoxy-MGO reaction. It shows that the promoted intercalative crosslinking is highly desirable for further improving the thermal conduc- tivity of the composite, but it meets with increased resistance. Guided by the kinetic studies, targeted optimization on the cure processing strategy was accordingly proposed to promote the intercalative crosslinking, a thermal conductivity, 2.96 times that of the epoxy, was got with only a small amount (30°C) increase of the post-heating temperature.

Facile identification of the critical content of multi-layer graphene oxide for epoxy composite with optimal thermal properties

A strategy to rapidly identify the critical content of multi-layer graphene oxide for epoxy composite with the optimal thermal properties.

Structure of graphene oxide membranes in solvents and solutions.

The change of distance between individual graphene oxide sheets due to swelling is the key parameter to explain and predict permeation of multilayered graphene oxide (GO) membranes by various solvents and solutions. In situ synchrotron X-ray diffraction study shows that swelling properties of GO membranes are distinctly different compared to precursor graphite oxide powder samples. Intercalation of liquid dioxolane, acetonitrile, acetone, and chloroform into the GO membrane structure occurs with maximum one monolayer insertion (Type I), in contrast with insertion of 2-3 layers of these solvents into the graphite oxide structure. However, the structure of GO membranes expands in liquid DMSO and DMF solvents similarly to precursor graphite oxide (Type II). It can be expected that Type II solvents will permeate GO membranes significantly faster compared to Type I solvents. The membranes are found to be stable in aqueous solutions of acidic and neutral salts, but dissolve slowly in some basic solutions of certain concentrations, e.g. in NaOH, NaHCO3 and LiF. Some larger organic molecules, alkylamines and alkylammonium cations are found to intercalate and expand the lattice of GO membranes significantly, e.g. up to ∼35 Å in octadecylamine/methanol solution. Intercalation of solutes into the GO structure is one of the limiting factors for nano-filtration of certain molecules but it also allows modification of the inter-layer distance of GO membranes and tuning of their permeation properties. For example, GO membranes functionalized with alkylammonium cations are hydrophobized and they swell in non-polar solvents.

High performance supercapacitor based on multilayer of polyaniline and graphene oxide

Multilayer films of polyaniline (PANI) and graphene oxide (GO) were deposited on indium tin oxide (ITO) electrode for supercapacitor application. The graphene oxide was synthesized using chemical method. The X-ray diffraction study confirms the formation of graphene oxide. The supercapacitive behavior of the PANI and PANI–GO electrodes were studied using cyclic voltammetry (CV) and galvanostatic charge–discharge techniques in 1M H2SO4 solution. The electrochemical analysis confirms that the multilayer film has improved specific capacitance compared to pure polyaniline film. The specific capacitance of 201F/g and 429F/g was achieved for PANI and PANI–GO multilayer electrode. High electrochemical performance of the PANI–GO electrode could be due to increasing active sites for the deposition of polyaniline provided by large surface areas of graphene oxide sheets and the synergistic effect between polyaniline and graphene oxide. These results demonstrated the importance and great potential of graphene oxide in the development of high-performance energy-storage system based on polyaniline.

Electrical conduction of palladium-decorated multi-layered graphene oxide effected by hydrogen dissociation

Multi-layered graphene oxide (MGO) and palladium-decorated MGO (PdGO) are electrically evaluated as a function of H2 pressure up to 20bar to elucidate an interaction between molecular hydrogen and PdGO. As H2 pressure increases, the electrical conductance (G) of MGO also increases while that of PdGO decreases. Using atomic force microscopy, X-ray photoelectron spectroscopy, and Fourier transform-infrared spectroscopy analysis, we found that MGO reduction due to H2 accounts for the increase in G of the MGO. For PdGO, an increase of OH groups was observed, which shows that PdGO was oxidized when exposed to high H2 pressure. The PdGO oxidation can be explained by a hydrogen spillover effect and the decrease of G may be due to this behavior.

Development of a solid-phase microextraction fiber by the chemical binding of graphene oxide on a silver-coated stainless-steel wire with an ionic liquid as the crosslinking agent.

Graphene oxide was bonded onto a silver-coated stainless-steel wire using an ionic liquid as the crosslinking agent by a layer-by-layer strategy. The novel solid-phase microextraction fiber was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and Raman microscopy. A multilayer graphene oxide layer was closely coated onto the supporting substrate. The thickness of the coating was about 4 μm. Coupled with gas chromatography, the fiber was evaluated using five polycyclic aromatic hydrocarbons (fluorene, anthracene, fluoranthene, 1,2-benzophenanthrene, and benzo(a)pyrene) as model analytes in direct-immersion mode. The main conditions (extraction time, extraction temperature, ionic strength, and desorption time) were optimized by a factor-by-factor optimization. The as-established method exhibited a wide linearity range (0.5-200 μg/L) and low limits of determination (0.05-0.10μg/L). It was applied to analyze environmental water samples of rain and river water. Three kinds of the model analytes were quantified and the recoveries of samples spiked at 10μg/L were in the range of 92.3-120 and 93.8-115%, respectively. The obtained results indicated the fiber was efficient for solid-phase microextraction analysis.

Graphene oxide nano-sheets wrapped Cu2O microspheres as improved performance anode materials for lithium ion batteries

Cu2O microspheres were successfully encapsulated by graphene oxide (GO) nano-sheets and used directly as the anode material for lithium ion batteries. The core–shell structured Cu2O@GO composite delivered a reversible capacity of 458mAhg−1 at a current density of 100mAg−1 after 50 cycles. Even at a high charge–discharge rate of 1000mAg−1, Cu2O@GO still demonstrated a reversible capacity of 240mAhg−1 after 200 cycles, significantly higher than those of the bare Cu2O microspheres (37mAhg−1) and GO nano-sheets (11mAhg−1). The rate capability evaluated by the ratio of capacity at 100mAg−1/1000mAg−1 current density was 49%, 25% and 9.8% for Cu2O@GO, bare Cu2O and GO, respectively. The greatly enhanced performance for GO nano-sheets wrapped Cu2O microspheres composite mainly resulted from the synergistic effect of Cu2O microspheres and GO nano-sheets core–shell composite: the flexible in-situ electrochemically reduced GO nano-sheet coating layer functioning as an efficient three dimensional (3D) conductive network and lithium storage active material; the Cu2O microsphere core functioning as a skeleton to support multilayer GO sheets and avoid GO nano-sheets aggregation.