Oxygen Functionalities Of Graphene Oxide Research Articles

Ionic liquid crystal – mediated preparation of reduced graphene oxide under microwave irradiation

Synthesis of pure graphene is still a highly challenging task and one of the common methods employed is, the oxidation of graphite to obtain graphene oxide (GO) followed by the reduction of oxygen functionalities in GO to yield reduced graphene oxide (RGO). By utilizing the dielectric properties of ionic liquid crystals (ILCs) to facilitate rapid microwave heating, we describe the preparation of reduced graphene oxide, starting from graphite. Besides, assisting the rapid microwave heating, the order and the anisotropic properties of the mesophase exhibited by ILCs are expected to influence the formation of the nanomaterial. The commercially obtained graphite flakes were oxidized to GO using modified Hummer’s method. The GO was reduced under the microwave (700 W, 23 s) in the presence of two structurally different ILCs to obtain RGO. The reduction of GO was also carried out in the presence of an ionic liquid (IL) and the absence of ILC and IL. The RGO was thoroughly characterized using standard analytical techniques. A higher percentage of reduction in the oxygen content was observed for RGO samples which were reduced in the presence of ILCs in comparison to RGO samples reduced in the presence of IL and the absence of ILC/IL.

Role of oxygen functionalities of GO in corrosion protection of metallic Fe

We demonstrated the mechanism that makes Graphene Oxide (GO) a superior material for corrosion protection over reduced graphene oxide (rGO) and nitrogen doped reduced graphene oxide (N-rGO). According to the results of our electrochemical study, supported by a host of characterization methods and density functional theory (DFT) based calculations, reduction of GO to synthesize rGO or doping of foreign heteroatoms like nitrogen to synthesize N-rGO, exposes and/or creates the defects/pores on the basal carbon plane of GO. Through these defects/pores of rGO and N-rGO, the corrodants react with the metal-surface and corrode it. A subnanometric layer of GO adhered to carbonyl iron (CI) surface through grafting by a thin Glycine (Gly) layer, repels the corrodants and shows robust corrosion protection performance in 1 M KCl solution, over a similar layer of rGO or N-rGO. Hence, our work demonstrate that a coating of GO on metallic Fe surface, grafted via a thin glycine layer, is the economical solution and has the robust performance for its direct industrial application for corrosion protection.

Charge Storage in Graphene Oxide: Impact of the Cation on Ion Permeability and Interfacial Capacitance.

The charge storage and membrane applications of graphene oxide (GO) materials are dictated by its intrinsic material properties. Structure-function relationships correlating periodic parameters, such as the hydrated ion radius and ion-GO interactions, are currently lacking yet are needed to provide insight on the charge storage and ion transport mechanism. We report the use of scanning ion conductance microscopy to measure the ion permeability of GO films and evaluate its relationship with the measured capacitance. We demonstrate that species (namely K+) with strong electrostatic interactions with the oxygen functionalities of GO provide the benefit of higher capacitance but suffer from inhibited ion mobility due to constriction of the GO interlayer spacing.

Probing structural variations of graphene oxide and reduced graphene oxide using methylene blue adsorption method

Recently, graphene oxide (GO) and reduced graphene oxide (rGO) have drawn significant attention in the development of gas sensors and nanocomposites. However, random distribution of oxygen functionalities in GO and rGO limits the use of these materials in the above application. In this study, methylene blue (MB) adsorption onto GO, synthesized under different oxidative conditions, and onto corresponding rGO was looked into in detail as both have high affinity towards MB. Our results show the amounts of MB adsorbed onto all GOs are the same irrespective of the time of oxidation. However, significant changes were observed in the amount of MB adsorbed on the corresponding rGOs that have been thermally reduced. Further, the FT-IR spectroscopic data show a blue shift for C=C stretching vibrations of MB adsorbed onto GO, whereas a red shift is observed for the bands assigned to stretching vibrations of C–N and CH3 bending upon adsorption on rGO. The above observations suggest complex structural geometry of GO and rGO, especially with respect to the spatial distribution of oxygen functionalities and their propagation during synthesis that influence MB adsorption, and its orientation. Thus, the information provided will be important in controllable synthesis of both GO and rGO with large surface area for the above applications as well as in applications such as water purification and photocatalysis.

Separation of europium using graphene oxide supported membranes

Spectroscopic and morphological properties of graphene oxide (GO)-europium (Eu(III)) composite sheets prepared at different Eu(III) concentrations and pH are examined to ascertain interaction mechanism of Eu(III) ions with diverse oxygen functionalities of GO. Incorporation of Eu(III) ions deteriorates absorption characteristics of GO, quenches GO luminescence, originates irregular wrinkles on GO sheets, which infer reorganization in structure of GO in composites. Preparation of composites at large Eu(III) concentrations and sufficiently ionized ambience results in formation of Eu(III) hydroxide and/or carboxylate complexes and consequent aggregation of composite sheets. Based on these inferences, conditions for optimum Eu(III) uptake together with perseverance of GO characteristics are derived. Further, membrane filters are prepared using different size/multiple layers of GO and their Eu(III) adsorption abilities are examined by percolation of aqueous Eu(III) solutions. These membranes displayed enhancement in Eu(III) filtration efficiency with increase in number of GO layers, attributed to formation of uninterrupted and multiple filtration layers for improved Eu(III) ion adsorption. These studies also emphasize significance of larger surface area and availability of un-aggregated GO sheets for superior adsorption of Eu(III) ions. In the present work, GO membranes showed an optimum Eu(III) removal efficiency of ∼35% and average Eu(III) adsorption capacity of ∼115 mg/g.

Novel and Highly Efficient Strategy for the Green Synthesis of Soluble Graphene by Aqueous Polyphenol Extracts of Eucalyptus Bark and Its Applications in High-Performance Supercapacitors

The sustainable synthesis of high-quality graphene sheets is one of the hottest and most inspiring topics in the fields of science and engineering. While the graphene oxide (GO) chemical reduction method is widely used to synthesize graphene sheets, this route commonly includes highly hazardous reducing agents that are dangerous to both humans and the environment. In this context, here we describe a green, effective, and economical strategy for the synthesis of soluble graphene by using a Eucalyptus polyphenol solution that is obtained from a Eucalyptus bark extract. The reducing ability of polyphenol compounds present in the Eucalyptus bark extract is responsible for the reduction of exfoliated GO to soluble graphene under reflux conditions in an aqueous medium. The XRD, FT-IR, XPS, and UV–vis results demonstrate the effective removal of the oxygen functionalities in GO. TEM and AFM images show straight corroboration for the development of 1–4 layers of graphene. The stable and homogeneous dispersion of the E-graphene in various solvents, both aqueous and nonaqueous, confirms the powerful interactions between Eucalyptus polyphenol compounds and graphene. The electrochemical performances are evaluated by cyclic voltametry (CV) and galvanostatic charge–discharge (GCD). GCD results show that the E-graphene supercapacitor has a high specific capacitance of 239 F g–1 and a high energy density of 71 W h kg–1 at a current density of 2 A g–1. These characteristics demonstrate that this green approach has an excellent prospective not only in the fabrication of high-performance supercapacitors but also in the synthesis of graphene-based materials.

Evolution of oxygen functionalities in graphene oxide and its impact on structure and exfoliation: An oxidation time based study

Control over oxygen functionalities is important for the realization of Graphene Oxide (GO) based applications. Evolution of oxygen functionalities with time of oxidation and its impact on GO structure, exfoliation and reduction is investigated using XRD, FTIR, XPS, TGA and AFM. The results indicate no net increase in oxygen bound carbon atoms compared to graphitic carbon within the samples studied. However the percentage contributions from each type of oxygen functionalities tend to vary as the oxidation progress. Changes in the thermogravimetric response were observed with continued oxidation that showed a change from one mass loss region to two mass loss regions. The interaction kinetics of the protective group and the spatial distribution of oxygen functionalities during graphite oxidation give key insight to the above observed properties in GO. It is also shown that the observed changes in the structure have a direct impact on the ability to exfoliate GO to single layers as well as on the reduction process to produce reduced graphene oxide (rGO). These results are significant in the scalable production of GO with controlled and consistent quality for applications in the areas such as chemical and biological sensing, water purification and development of nanocomposites.

Influence of (photo)bromination on the transformation, aggregation and sedimentation of graphene oxide

The rapidly increasing use of graphene oxide (GO) necessitates the immediate investigation of the environmental fate and effects of this unique material. To date, little is known about the transformation of GO in environmental media, especially in water and wastewater treatment systems. This study explored the changes in the physicochemical properties of GO that result from (photo)bromination by simulating the reactions experienced by GO during water and wastewater treatment. The surface oxygen functionalities of GO considerably changed upon treatment. Specifically, the COH and COOH groups of GO were lost under bromination, and the reduction of GO was enhanced under photobromination. GO sheets wrinkled and fragmented under bromination, and GO decomposition was enhanced by photobromination. The decreases in O/C ratio and GO decomposition in the presence of bromine were mainly attributed to the relatively high oxidation potential of bromine, which can oxidize carbon nanosheets. The oxidation of carbon nanosheets, in turn, resulted in the decomposition and formation of brominated disinfection by-products. Irradiation, hypobromous acid/hypobromite (HOBr/OBr−), and reactive radicals, such as OH, Br, BrO and Br2−, may all affect the physicochemical transformation of GO in the presence of bromine and simulated solar light. Bromination and photobromination drastically changed the aggregation and sedimentation behavior of GO. The findings of this study advance knowledge on the transformations of GO in water treatment systems and provide insight on the environmental fate of the transformation products of GO.

Effect of oxygen functionalities of graphene oxide on polymerization and thermal properties of reactive benzoxazine nanocomposites

Herein, we report the effect of oxygen functionalities of graphene oxide on thermal activated polymerization and thermal properties of reactive benzoxazine nanocomposites. The numbers of oxygen moieties of graphene oxide (GO) are controlled by hydrothermal reduction. The polymerization behavior of benzoxazine monomer (BA-a) is studied by Fourier transform infrared spectroscopy, differential scanning calorimetry and rheological analysis. It is hypothesized that the GO not only exhibits accelerated effect on the polymerization of the BA-a, but also the oxygen moieties such as carboxyl groups of GO interact with the benzoxazine polymers, leading to several orders of magnitude increase in the chemoviscosity and modulus of composite system. Thermal conductivity of poly(BA-a)/GO composite increases from an initial value of ∼0.27 W/mK to 0.47 W/mK as the loading increases from 1 wt% to 6 wt% (enhancement factor up to 176%). Moreover, the nanocomposites display enhanced initial decomposition temperature and char yields as the degree of GO reduction increases.

Photochlorination-induced transformation of graphene oxide: Mechanism and environmental fate

With the increasing production and wide utilization of graphene oxide (GO), the nanomaterials are expected to be released into the environment, and end up in surface waters, and/or wastewater treatment plants. This study explored the changes in the physicochemical properties of GO resulting from photochlorination— simulating the reactions that occur during water and wastewater treatment. Photochlorination resulted in significant changes in the surface oxygen-functionalities of the nanomaterials, and fragmenting of the graphenic carbon sheets was observed. We found that photochlorination can enhance the decomposition of GO through the formation of reduced GO. The changes in surface oxygen-functionalities of GO were attributed to the oxidation by chlorine of the nanomaterials' quinone groups, and further oxidation by ▪ and/or ▪ radicals. The surface charge of GO, measured by its zeta potential, increased in magnitude with chlorination but decreased in magnitude with photochlorination, leading to the decrease in the colloidal stability of the photochlorinated nanomaterials. The antibacterial effect of the nanomaterials increased with both chlorination and photochlorination. This study clearly shows how the physicochemical properties, and environmental fate and effect of GO are modified by photochlorination.

Biofabrication of polyphenols stabilized reduced graphene oxide and its anti-tuberculosis activity

A facile one step eco-friendly method for the reduction graphene oxide by Cinnamomumverum (C. verum) bark extract is reported in this work. This approach avoids the utilization of hazardous chemical reagents. The characterization results of various spectroscopic and microscopic techniques for the prepared graphene oxide (GO) and reduced graphene oxide (RGO) afford a strong indication of the removal of oxygen functionalities of GO after reduction and following stabilization by the oxidised polyphenols. Fourier transform infrared spectral results showed the capping of oxidised polyphenols onto the surface of reduced graphene oxide which further prevent their aggregation. Additionally, the prepared graphene nanosheets were tested for their antituberculosis activity against standard strain such as M. tuberculosis H37Ra. The obtained results suggested that the synthesized graphene acts as an effective growth inhibitors against M. tuberculosis H37Ra making it applicable for targeted drug delivery by combining with other chemical drugs as a therapeutic index.

Effect of TiO2/reduced graphene oxide composite thin film as a blocking layer on the efficiency of dye-sensitized solar cells

The objective of the present work was to study the change in efficiency of dye-sensitized solar cells due to introduction of a thin blocking composite layer of TiO2 and graphene. The reduced graphene oxide prepared by chemically exfoliation method was confirmed as few layer graphene through X-ray diffraction, Raman analysis and atomic force microscopy technique. X-ray photoelectron spectroscopy was used to study the oxygen functionalities in graphene oxide and in reduced graphene oxide. Graphene-doped TiO2 thin films were coated by sol-gel routed spin coating technique and characterized by ultraviolet-visible, photoluminescence analysis, Fourier transform infrared spectroscopy, X-ray diffraction studies, field emission scanning electron microscopy, and transmission electron microscopy. The effect of blocking layer on the efficiency of dye-sensitized solar cells was studied using the photocurrent-voltage characteristics and electrochemical impedance spectroscopy. The studies indicated that the titania-graphene composite thin film worked as both blocking layer and ohmic contact between porous TiO2 and fluorine-doped tin oxide glass improved the overall device performance by reducing the interfacial resistance which in turn depends on the doping concentration of graphene in TiO2. The device with blocking layer improved the photoconversion efficiency from 2.49 to 5.09 %.

Studies on synthesis of reduced graphene oxide (RGO) via green route and its electrical property

An environmentally friendly method has been applied for the preparation of reduced graphene oxide (RGO). This method was developed by using polyphenols that contained a phytoextract of Mangifera indica L. along with Solanum tuberosum L. as reducing agents since they are non-toxic and naturally available. The phytoextracts used in the production of RGO was set between 60 and 70°C. Graphene oxide (GO) was prepared by modified Hummer’s method as reported in earlier findings. Structural and morphological studies demonstrate that the part of the oxygen functionalities in GO can be removed by following green reduction. Characterizations of the resulting product have been done by X-ray diffraction, FTIR, UV–vis and Raman spectroscopy. FESEM, TEM, EDX spectrum, TGA, DLS and Zeta potential measurements of the samples have also been carried out to study the morphological, thermal and surface charge characteristics. Electrical conductivity was also measured to check the extent of reduction of GO to RGO.

Self‐Assembly of Graphene Oxide on Silicon Substrate via Covalent Interaction: Low Friction and Remarkable Wear‐Resistivity

Few‐layer graphene oxide (GO) is assembled on the silicon surface by a self‐assembly approach via covalent interaction using 3‐aminopropyltrimethoxysilane (APTMS) as a bifunctional chemical linker. X‐ray photoelectron spectroscopy results suggest chemical interactions between oxygen functionalities of GO and amino group of APTMS thin film. The oxygen functionalities of GO thin film are eliminated by vacuum ultraviolet (VUV) photon exposure. Topographic images reveal efficient grafting of GO on the silicon and suggest the presence of few layers in the GO thin film along with wrinkles and folds. Microtribological properties of VUV‐reduced GO (rGO) thin film are probed under the mean contact pressure of 0.3–0.6 GPa. The rGO thin film exhibits low and steady friction (0.12–0.15) compared to that of bare silicon (0.6). The rGO thin film could survive for ≈37 000 laps at 100 mN load, revealing its remarkable wear‐resistivity. Microscopic images and carbon mapping reveal the deposition of delaminated graphene lamellae on the counter steel ball surface. The low friction and excellent wear‐resistivity of rGO thin film are collectively attributed to low‐resistance to shear between the neighboring lamellae of rGO, full coverage and strong interaction of rGO thin film with silicon, and deposition of delaminated graphene lamellae on the counter steel surface.

Two-step process for programmable removal of oxygen functionalities of graphene oxide: functional, structural and electrical characteristics

Here we report a two-step programmable reduction of graphene oxide (GO) which was synthesized by oxidation of graphite.

Molecular pillar supported graphene oxide framework: conformational heterogeneity and tunable d-spacing.

n-Alkylamines were grafted on the basal plane oxygen functionalities of graphene oxide (GO), yielding molecular pillar supported graphene oxide frameworks (GOFs) with tunable interlayer spacing. A major fraction of n-alkylamines was found to covalently interact with the basal plane epoxy groups via nucleophilic substitution reactions. The d-spacing in GOFs could be tailored between 10.5 and 28.9 Å by varying the chain length of the n-alkylamines. (13)C SSNMR explicitly showed the coexistence of both trans and gauche conformation modes. The relative populations of these modes control the conformational heterogeneity and orientation of n-alkylamines in the GOFs. A plausible bilayer structural model of the GOFs was demonstrated. The terminal methyl and methylene units of the n-alkylamines grafted on the GO basal plane were interdigitated with the counter layer and afforded a double-layer structure of alkyl chain supported GOFs.

Graphene oxide supported molybdenum cluster: first heterogenized homogeneous catalyst for the synthesis of dimethylcarbonate from CO2 and methanol.

The octahedral molybdenum cluster-based compound, Cs2 Mo6 Br(i) 8 Br(a) 6 was immobilized on graphene oxide (GO) by using a facile approach. High resolution transmission electron microscopy results revealed that molybdenum clusters were uniformly distributed on the GO nanosheets. Cs2 Mo6 Br(i) 8 Br(a) 6 was attached to the GO support via chemical interaction between apical ligands of Mo6 Br(i) 8 Br(a) 6 cluster units and oxygen functionalities of GO, as revealed by XPS studies. The developed material was used for the synthesis of dimethyl carbonate by reduction of carbon dioxide. The synthesized catalyst, that is, GO-Cs2 Mo6 Br(i) 8 Br(a) x , exhibited higher catalytic efficiency than its homogeneous analogue without using dehydrating agent. The catalyst was found to be efficiently recyclable without significant loss of catalytic activity.

Synthesis and Characterization of Graphene and Graphene Oxide Based Palladium Nanocomposites and Their Catalytic Applications in Carbon-Carbon Cross-Coupling Reactions

We have developed an efficient method to generate highly active Pd and PdO nanoparticles (NPs) dispersed on graphene and graphene oxide (GO) by an impregnation method combined with thermal treatments in <TEX>$H_2$</TEX> and <TEX>$O_2$</TEX> gas flows, respectively. The Pd NPs supported on graphene (Pd/G) and the PdO NPs supported on GO (PdO/GO) demonstrated excellent carbon-carbon cross-coupling reactions under a solvent-free, environmentally-friendly condition. The morphological and chemical structures of PdO/GO and Pd/G were fully characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). We found that the remarkable reactivity of the Pd/G and PdO/GO catalysts toward the cross-coupling reaction is attributed to the high degree of dispersion of the Pd and PdO NPs while the oxidative states of Pd and the oxygen functionalities of graphene oxide are not critical for their catalytic performance.

Nacre-like graphene paper reinforced by polybenzimidazole

Nacre-like graphene paper reinforced by a high-performance aromatic polymer is reported for the first time. Polybenzimidazole (PBI) was selected as the organic phase to glue the graphene papers. The graphene/PBI composite papers were prepared by solution-casting of graphene oxide (GO)/PBI mixtures followed by in situ chemical reduction. SEM and XRD demonstrated the well-layered structures of the papers. It was found that the mechanical properties of both GO and reduced GO (RGO) papers were significantly improved by the incorporation of PBI due to the strong interactions between imidazole groups in polymer chains and oxygen functionalities in GO and RGO. The conductivity measurement also suggested the good electrical properties of the RGO composite papers. Moreover, as the excellent thermal stability of PBI, the thermal stabilities of the RGO composite papers were found to be much better than that of RGO paper. It is expected that the combination of good mechanical, electrical and thermal properties could make this novel composite paper a potential material for many applications.

Dispersion of alkylated graphene in organic solvents and its potential for lubrication applications

We report on the preparation of alkylated graphenes on a large scale followed by their dispersion in organic solvents. The alkylated graphenes with variable alkyl chain lengths (Cn = 8, 12, 18) are prepared by coupling of alkylamine with carboxylic groups of graphene oxide (GrO). The FTIR, UV-Visible, and TGA results reveal that, during alkylation, the oxygen functionalities of GrO are reduced significantly and the average size of the sp2 carbon domain increased, which is further supported by Raman characteristics. It is observed that the dispersibility of alkylated graphene in hydrocarbon solvents increases on increasing the chain length of (a) hydrocarbon solvents used for the dispersion and (b) alkyl groups attached to the graphene. The van der Waals interaction between methylene units associated with alkylated graphenes and hydrocarbon solvents plays a crucial role in determining their dispersion characteristics, and such an interaction increases with increasing methylene units. Octadecylamine functionalized graphene (ODA-Gr) dispersion in hexadecane is found to have long-term dispersion stability due to its high degree of cohesive interaction. The lubrication characteristics of hexadecane containing ODA-Gr were probed by evaluating its friction and wear properties. The results reveal that hexadecane doped with an optimized dose of 0.06 mg mL−1 ODA-Gr reduced friction and wear by 26% and 9%, respectively, compared to hexadecane. The lubricity enhancement could be attributed to uninterrupted supplies of graphene nanosheets under the rubbing surfaces, where these nanosheets prevent direct contact between the rubbing surfaces, providing low resistance to shear.