Exfoliated Graphene Oxide Sheets Research Articles

Hierarchical graphene oxide-Ni3S2 quantum dots nanocomposites modified glassy carbon electrode for electrochemical detection of dopamine and tyrosine

A facile synthetic strategy is demonstrated to generate nickel sulfide quantum dots (Ni3S2). The thus formed Ni3S2 quantum dots are assembled onto exfoliated graphene oxide sheets hydrothermally to form nickel sulfide-graphene oxide nanocomposite material (GO-Ni3S2). The microscopic and spectroscopic characterization of the GO-Ni3S2 nanocomposites revealed the shape, size, crystalline phases, and oxidation states (of elements) of the hybrid material. The GO-Ni3S2 nanocomposites are then coated onto the glassy carbon electrode by drop casting to form GO-Ni3S2@GCE. The modified electrode is then used to detect dopamine and tyrosine simultaneously. The effect of scan rate, analyte concentrations, pH, and interfering agents on the peak current are studied to establish a plausible mechanism for oxidizing dopamine and tyrosine at GO-Ni3S2@GCE. The GO-Ni3S2@GCE is stable for 3 weeks and ten cycles of washing with minimal loss in the peak current in each cycle. Dopamine with a concentration as low as 12 nM can be detected using the GO-Ni3S2@GCE system.

Hydrothermal-assisted green synthesis of reduced graphene oxide nanosheets (rGO) using lemon (Citrus Limon) peel extract

An economical, sustainable and environmentally friendly approach was employed to synthesise reduce graphene oxide from lemon (Citrus limon) peel extract. In the present study, highly exfoliated graphene oxide sheets were obtained from graphite flakes using the Tour method. Phytochemical extracts of lemon peel were adopted for the reduction of graphene oxide. The prepared graphene oxide and reduced graphene oxide were characterized using various techniques such as the X-ray diffraction (XRD) technique, Fourier transforms infrared (FTIR) spectroscopy, and UV–Visible spectroscopy. The morphology of synthesized graphene oxide and reduced graphene oxide were investigated using the scanning electron microscopy technique. UV–Vis absorption spectra show that the energy band gap reduces after the reduction of graphene oxide. According to the findings, lemon peel extract can function as a reducing agent, and hydrothermal treatment can start the reduction process, which results in the synthesis of high-quality reduced graphene oxide nanosheets.

Violet Laser Treatment of Nitrogen-Doped Reduced Graphene Oxide Electrodes and KOH Electrolytes Containing p-Phenylenediamine for High-Performance Supercapacitors

The electrical conductivity and capacitive behavior of supercapacitors (SCs) were found to be improved by the synergistic effects of the violet laser treatment (VLT) of nitrogen-doped reduced graphene oxide (N-rGO) as an electrode and 1.5% (w/v) p-phenylenediamine (PPD)-containing KOH as a redox electrolyte. The SC employing VLT treatment on the N-rGO electrode with 2 M KOH (N-rGO-V) represented a greater specific capacitance of up to 263.6 F g–1 compared to N-rGO in the same electrolyte, which was 232.2 F g–1 at 0.25 A g–1. To further improve the charge-storage ability, the SC using the VLT treatment on the N-rGO electrode with the PPD addition into 2 M KOH (N-rGO-V-PPD) revealed the highest specific capacitance, reaching 593.7 F g–1 at 0.25 A g–1 or 28.6% compared to N-rGO in the same redox electrolyte (N-rGO-PPD). Furthermore, the capacitance retention after 10,000 cycles of N-rGO-V-PPD remained at 62% (265 F g–1 at 100 mV s–1), which is still significantly higher than that of other samples, and the VLT technique provided a lower self-discharge rate. According to the physical characterizations, N-rGO-V-PPD has a rougher and larger surface area due to the exfoliated graphene oxide sheets and has a modest increase in the number of quinone/carbonyl functional groups in the graphene material, resulting in better electrical conductivity. All of these could be contributing factors to the performance improvement. Lastly, PPD provides redox mediators such as H+ and e– in the redox electrolyte, which are anticipated to improve the charge storage.

Enhanced surface and electrochemical properties of nitrogen-doped reduced graphene oxide by violet laser treatment for high charge storage and lower self-discharge supercapacitors

This work demonstrated that the performance improvement of nitrogen-doped reduced graphene oxide (N-rGO) electrodes over reduced graphene oxide (rGO) electrodes for supercapacitors could be further enhanced by violet laser treatment (VLT). A specific capacitance of 30 and 145 F g−1 was obtained for rGO and N-rGO electrodes, respectively. When using N-rGO with proper VLT, a high specific capacitance of 214 F g−1 could be achieved. In addition, the percentage of capacitive retention of the N-rGO with VLT remaining 94% after 10,000 cycles with a slower self-discharge rate at 0.55 V h−1, comparing to 0.91 V h−1 for N-rGO. Systematic investigations of N-rGO with VLT were carried out by using different film characterization techniques. The surfaces of N-rGO with VLT exhibit an increase in the number of exfoliated graphene oxide sheets with more roughness when increasing the fluence of laser. It was also found that there are the decrease in oxygen/nitrogen-containing functional groups contents and the increases in graphitic carbon phase with CC sp2-hybridization with the influence of laser. These may explain the improvements of the reported charge storage ability, electrode stability, and self-discharge rate.

Effect of graphite exfoliation degree on the corrosion protection for epoxy composite coating

Four kinds of graphite derivatives in different exfoliation degrees were synthesized via modified Hummers’ method by controlling the mass ration of graphite and KMnO4. The modified graphite derivatives (GUF) were synthesized through chemical modification with the prepolymer of urea–formaldehyde. Results of Raman spectroscopy (Raman) and Scanning electron microscopy (SEM) revealed that the layer exfoliation was affected by oxidation degree during the preparation of graphene oxide (GO). The highly exfoliated GO was fully modified by UF resin and presented excellent compatibility with epoxy resin. The data of electrochemical impedance spectra (EIS) verified the optimal corrosion resistance of GUF/EP composite coating with the highly exfoliated GO sheets. The data of localized electrochemical impedance spectroscopy (LEIS) demonstrated that the highly exfoliated modified GO exhibited slight galvanic corrosion in the defective coatings.

Drying-Time Study in Graphene Oxide.

Graphene oxide (GO) exhibits different properties from those found in free-standing graphene, which mainly depend on the type of defects induced by the preparation method and post-processing. Although defects in graphene oxide are widely studied, we report the effect of drying time in GO and how this modifies the presence or absence of edge-, basal-, and sp3-type defects. The effect of drying time is evaluated by Raman spectroscopy, UV-visible spectroscopy, and transmission electron microscopy (TEM). The traditional D, G, and 2D peaks are observed together with other less intense peaks called the D’, D*, D**, D+G, and G+D. Remarkably, the D* peak is activated/deactivated as a direct consequence of drying time. Furthermore, the broad region of the 2D peak is discussed as a function of its deconvoluted 2D1A, 2D2A, and D+G bands. The main peak in UV-visible absorption spectra undergoes a redshift as drying time increases. Finally, TEM measurements demonstrate the stacking of exfoliated GO sheets as the intercalated (water) molecules are removed.

Electrochemical sensing of pioglitazone hydrochloride on N-doped r-GO modified commercial electrodes.

In this paper, we explain the electrochemical sensing of commercially available pioglitazone hydrochloride (PIOZ) tablets on a nitrogen (N) doped r-GO (Nr-GO) modified commercial glassy carbon electrode (GCE) and a commercial screen printed graphite electrode (SPGE). Nr-GO is synthesized by the chemical reduction of graphene oxide (GO) and simultaneous insertion of an N-dopant by hydrazine monohydrate. Pristine GO itself is prepared by chemical exfoliation of bulk graphite. Upon chemical reduction, the exfoliated GO sheets restack together leaving behind the doped N-atom as evidenced by XRD and Raman spectroscopy. The N-atom exists in the pyrrolinic and pyridinic form at the edge of graphitic domains which is confirmed by XPS. The as-synthesized Nr-GO is used for the preparation of electro-active electrodes with the help of the GCE and SPGE. These electrodes have the capability to oxidize PIOZ by a diffusion dominated process as evidenced by the impedance spectroscopic technique. The differential pulse voltammetric responses of different concentrations of PIOZ are assessed over the Nr-GO modified GCE and SPGE, which exhibit better limits of detection (LODs) of 67 nM and 29 nM, respectively, compared to those from earlier reports. These assays exhibit non-interfering capability in the presence of various body interferents at pH = 7.0.

Development of amine-functionalized superparamagnetic iron oxide nanoparticles anchored graphene nanosheets as a possible theranostic agent in cancer metastasis.

The major objective of the present investigation was to assess the targeting potential of a designed system for breast cancer at metastatic phases with imaging ability. In a nutshell, we have developed surface-engineered graphene oxide (GO) nanosheets by covalent linking with amine-functionalized iron oxide nanoparticles (IONPs) (GOIOIs). Gefitinib (Gf) was selected as a model drug and entrapped in between exfoliated GO sheets (GOIGF) via π-π* stacking before functionalization with IONPs. Preliminary characterization of GO, IONPs, GOIOI, and GOIGF was performed using UV-visible and Fourier transform infrared spectroscopy. Scanning and transmission electron microscopy studies confirmed successful surface engineering of GO with IONPs. The in vitro drug release study demonstrated sustained release of Gf. The magnetic behavior of IONPs and GOIOI demonstrated a sigmoidal-shaped hysteresis loop with superparamagnetic properties. The in vitro cell cytotoxicity assay was carried out on MDA-MB-231 breast cancer adenocarcinoma cell lines. The cell cytotoxicity assay showed 61.18% inhibition of cell growth with 30ppm concentration containing 64% of the drug, whereas 100% of the pure drug revealed only 56% of inhibition. In the near future, GOIOI could be tailored further for theranostic research, especially for metastatic cancers. Graphical abstract.

Preparation of graphene oxide/natural rubber composites by latex co-coagulation: Relationship between microstructure and reinforcement

Abstract Graphene oxide (GO) has recently attracted substantial interest as a possible reinforcing agent for next generation rubber composite materials. In this research, GO was incorporated in natural rubber (NR) composites through latex co-coagulation technique. The microstructures of GO/NR composites were characterized through a combination of transmission electron microscope, scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and Differential scanning calorimeter. The results showed that highly exfoliated GO sheets were finely dispersed into NR rubber matrix with strong interface interaction between GO and NR. The mechanical properties of the GO/NR composites were further evaluated. The results showed that the tensile strength, tear strength and modulus can be significantly improved at a content of less than 2 phr. Especially, GO exhibited specific reinforce mechanism in NR due to the stress-induced crystallization effects of NR. The stress transfer from the NR to the GO sheets and the hindrance of GO sheets to the stress-induced crystallization of NR were further displayed in stress–strain behavior of GO/NR composites. These enhanced properties were attributed to the high surface area of GO sheets and highly exfoliated microstructures of GO sheets in NR.

Tailorable Synthesis of Highly Oxidized Graphene Oxides via an Environmentally-Friendly Electrochemical Process.

Graphene oxide (GO) is an attractive alternative to graphene for many applications due to its captivating optical, chemical, and electrical characteristics. In this work, GO powders with a different amount of surface groups were synthesized from graphite via an electrochemical two-stage process. Many synthesis conditions were tried to maximize the oxidation level, and comprehensive characterization of the resulting samples was carried out via elemental analysis, microscopies (TEM, SEM, AFM), X-ray diffraction, FT-IR and Raman spectroscopies as well as electrical resistance measurements. SEM and TEM images corroborate that the electrochemical process used herein preserves the integrity of the graphene flakes, enabling to obtain large, uniform and well exfoliated GO sheets. The GOs display a wide range of C/O ratios, determined by the voltage and time of each stage as well as the electrolyte concentration, and an unprecedented minimum C/O value was obtained for the optimal conditions. FT-IR evidences strong intermolecular interactions between neighbouring oxygenated groups. The intensity ratio of D/G bands in the Raman spectra is high for samples prepared using concentrated H2SO4 as an electrolyte, indicative of many defects. Furthermore, these GOs exhibit smaller interlayer spacing than that expected according to their oxygen content, which suggests predominant oxidation on the flake edges. Results point out that the electrical resistance is conditioned mostly by the interlayer distance and not simply by the C/O ratio. The tuning of the oxidation level is useful for the design of GOs with tailorable structural, electrical, optical, mechanical, and thermal properties.

Effective gas separation through graphene oxide containing mixed matrix membranes

ABSTRACTIn the present study, graphene oxide (GO) was incorporated into poly(vinylidene fluoride) (PVDF) and chemically modified PVDF (M‐PVDF) to prepare mixed matrix membranes (MMMs) for gas separation application. Performed analyses proved appropriate dispersion of exfoliated GO sheets in polymer matrices and sufficient compatibility at the interfacial phases. M‐PVDF based MMMs were thermally and mechanically more stable relative to the PVDF‐based MMMs. The oxygen containing functional groups in M‐PVDF was probably the main reason for this more stability. PVDF/GO MMMs rendered low gas permeability and high selectivity. Both impermeable GO sheets and crystalline phases of PVDF were responsible for such behavior. On the other hand, interestingly gas permeability of M‐PVDF/GO MMMs was enhanced while no substantial decline was recorded in gas selectivity. For instance, He and CO2 permeability was increased 12.46% and 25.89%, respectively, compared to the pure PVDF membrane. This behavior originated from functional groups of M‐PVDF and the interaction of these groups with GO sheets. Since GO often amplified gas barrier properties of polymers, such increscent would be appreciable. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46271.

Soluble Graphene Nanosheets from Recycled Graphite of Spent Lithium Ion Batteries

Soluble graphene nanosheets are fabricated from recycled graphite of spent lithium ion batteries through a modified Hammers process followed by deoxygenation with NaOH–KOH eutectic. Ultrasonic exfoliation in N-methyl-pyrrolidone indicates the loosened graphene layers in recycled graphite are prone to exfoliation. Reduction of the exfoliated graphene oxide sheets was conducted in molten NaOH–KOH eutectic at different temperatures. The results show that molten NaOH–KOH effectively eliminates the unsaturated oxygen-containing moieties from the exfoliated graphene oxide sheets while creating more hydroxyl functional groups. Higher temperature treatment is more prone to remove hydroxyls while producing the shrinkage on the surface of graphene sheets. Graphene sheet with a good solubility is produced when the graphene oxide is heat-treated at 220 °C for 10 h. After reduction, the graphene oxide sheets exhibit excellent dispersibility or solubility in water, ethanol and other polar solvents, therefore being highly desirable for solution processing of graphene materials. Such study not only identifies a high-quality stockpile to prepare soluble graphene but also paves a feasible alternative of graphite recycling from spent lithium batteries.

High-efficiency exfoliation of large-area mono-layer graphene oxide with controlled dimension

In this work, we introduce a novel and facile method of exfoliating large-area, single-layer graphene oxide using a shearing stress. The shearing stress reactor consists of two concentric cylinders, where the inner cylinder rotates at controlled speed while the outer cylinder is kept stationary. We found that the formation of Taylor vortex flow with shearing stress can effectively exfoliate the graphite oxide, resulting in large-area single- or few-layer graphene oxide (GO) platelets with high yields (>90%) within 60 min of reaction time. Moreover, the lateral size of exfoliated GO sheets was readily tunable by simply controlling the rotational speed of the reactor and reaction time. Our approach for high-efficiency exfoliation of GO with controlled dimension may find its utility in numerous industrial applications including energy storage, conducting composite, electronic device, and supporting frameworks of catalyst.

Uniform decoration of silver nanoparticle on exfoliated graphene oxide sheets and its ammonia gas detection

Silver nanoparticles (AgNP) decorated graphene oxide (GO) sheets were prepared by reducing AgNO3 with vitamin C in the presence of GO. Different molar concentration of AgNO3 played a significant role in the size of AgNP in GO. The synthesized AgNP–GO composites were characterized by powder X-ray diffraction, scanning electron microscopy, ultraviolet–visible–near infrared, Fourier transform infrared and micro Raman spectroscopy techniques. We demonstrated the fiber optic gas sensor using reduced graphene oxide (rGO) and AgNP–GO composites for 0–500ppm ammonia vapor concentrations at 27°C. Sensitivities of rGO, 0.1M AgNP–GO and 0.6M AgNP–GO composites coated fiber optic sensors were calculated as −0.08, −0.177, −0.12counts/ppm respectively. The agglomeration of AgNP at 0.6M concentration along with increased particle size significantly decreased the sensing performance.

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.

Simple Preparation of Exfoliated Graphene Oxide Sheets via Simplified Hummer’s Method

Today, research on graphene and other two-dimensional sp2-hybridized carbon nanomaterials has tremendously impacted the areas of modern chemistry, physics, and materials science and engineering. The significant attraction of these materials can be attributed to the outstanding electrical, optical, electrochemical, and mechanical properties of graphene-like materials, especially in comparison to other carbon materials. In this manner, graphene oxide as a substrate for graphene-like materials reduction process is getting more and more interesting. Although early routes to these materials were challenging, significant advances in synthetic and processing methods have enabled access to high-quality exfoliated graphene oxide sheets in appreciable quantities. Herein, we introduced a simple and efficient method for the high-conversion preparation of graphene oxide using a simplified hummer’s method from large graphite flakes (an average flake size of 100 μm). One-pot chemical oxidation of graphite was carried out at room temperature for the preparation purpose. It was found that different degree of oxidation of graphite flakes could be realized by stirring graphite in a mixture of sulphuric acid and potassium permanganate under different oxidation durations, resulting in exfoliated graphene oxide sheets with large lateral dimension and area. The simplified Hummer’s method provides a facile approach for the preparation of large-area exfoliated graphene oxide sheets.

Green preparation of reduced graphene oxide using a natural reducing agent

A simple and efficient method was introduced for the high-conversion preparation of graphene oxide (GO) from large graphite flakes (average flake size=100μm) using a simplified Hummer׳s method. Natural reducing agents such as lemon juice and vinegar were compared with hydrazine (N2H4) as potential reducing agents. Graphene was prepared by chemical reduction of GO because this method was low cost and could be used for large-scale graphene production. This one-pot graphene preparation was performed at room temperature. Different degrees of oxidation of graphite flakes were obtained by stirring graphite in a mixture of sulfuric acid and potassium permanganate at different oxidation times, and highly exfoliated GO sheets were produced. GO was subsequently reduced effectively by lemon juice, a new, green, and potential reducing agent with pH 2.3. This reduced GO exhibited a high electrical conductance of 24.6μS attributed to its higher C/O ratio (≈8:2) compared with other samples.

Electrochemically exfoliated graphene oxide/iron oxide composite foams for lithium storage, produced by simultaneous graphene reduction and Fe(OH)3 condensation

We describe the production of graphene-based composites for energy storage, obtained by a combination of electrochemical and solution processing techniques. Electrochemically exfoliated graphene oxide sheets (EGO) are produced using an original setup that allows fast expansion of graphite flakes and efficient exfoliation of expanded graphite via an electrochemical route. The sheets are deposited on a sacrificial nickel foam together with an iron hydroxide colloidal precursor. Calcination treatment simultaneously renders the EGO foam conductive and transforms Fe(OH)3 into hematite (α-Fe2O3), yielding a nanoporous Fe2O3 layer on the surface of the mesoporous EGO foam, creating an ideal structure for lithium storage. The obtained graphene/metal oxide hybrid is a continuous, electrically conductive three-dimensional (3D) composite featuring a hierarchical meso–nano porous structure. A systematic study of these composites, varying the Fe2O3:EGO ratio, is then performed to maximize their performance as nanostructured electrodes in standard coin cell batteries.

Donor–acceptor graphene-based hybrid materials facilitating photo-induced electron-transfer reactions

Graphene research and in particular the topic of chemical functionalization of graphene has exploded in the last decade. The main aim is to increase the solubility and thereby enhance the processability of the material, which is otherwise insoluble and inapplicable for technological applications when stacked in the form of graphite. To this end, initially, graphite was oxidized under harsh conditions to yield exfoliated graphene oxide sheets that are soluble in aqueous media and amenable to chemical modifications due to the presence of carboxylic acid groups at the edges of the lattice. However, it was obvious that the high-defect framework of graphene oxide cannot be readily utilized in applications that are governed by charge-transfer processes, for example, in solar cells. Alternatively, exfoliated graphene has been applied toward the realization of some donor–acceptor hybrid materials with photo- and/or electro-active components. The main body of research regarding obtaining donor–acceptor hybrid materials based on graphene to facilitate charge-transfer phenomena, which is reviewed here, concerns the incorporation of porphyrins and phthalocyanines onto graphene sheets. Through illustrative schemes, the preparation and most importantly the photophysical properties of such graphene-based ensembles will be described. Important parameters, such as the generation of the charge-separated state upon photoexcitation of the organic electron donor, the lifetimes of the charge-separation and charge-recombination as well as the incident-photon-to-current efficiency value for some donor–acceptor graphene-based hybrids, will be discussed.

Spongy graphene electrode in electrochemical detection of leukemia at single-cell levels

Mg2+-charged spongy graphene electrodes (SGEs) were fabricated by using electrophoretic deposition of chemically exfoliated graphene oxide sheets on graphite rods. The SGEs were able to present two distinguishable signals (originated from electrochemical oxidation of guanine) in differential pulse voltammetry (DPV) of leukemia and normal blood cells, in contrast to glassy carbon electrodes giving only one overlapped peak. Hence, the SGEs were applied in fast (60min) and ultra sensitive detection of leukemia (single abnormal cells in ∼109 normal cells) in a blood serum. The sensitivity obtained by the SGEs was three orders of magnitude better than that of the best available and current technologies (e.g., specific mutations by polymerase chain reaction with detection limit of one abnormal cell in ∼106 normal cells) which not only are expensive, but also require several days for incubation. Significant variations in DPV signals of the SGEs after the first electrochemical cycle indicated that the best performance of the SGEs can be achieved only at the first cycle. The linear dynamic detection behavior of the SGEs was investigated in wide concentration range of 1.0×105–0.1cell/mL. The lower detection limit was estimated ∼0.02cell/mL, based on the current resolution obtained by the SGEs.