Monolayer Graphene Oxide Research Articles

Detection of 2,4-dinitrotoluene by graphene oxide: first principles study

The surface of graphene oxide (GO) with different oxidation level is widely used in gas sensing applications. Otherwise, detection of 2,4-dinitrotoluene (DNT) have been extensively attend as a high explosive and environmental sources by various methods. Atomic level modelling are widely employed to explain the sensing mechanism at a microscopic level. The present work is an attempt to apply density functional theory (DFT) to investigate the structural and electronic properties of GO and adsorption of oxygen atom and hydroxyl on graphene surface. The focus is on the adsorption mechanisms of DNT molecule on the GO monolayer surface to detect DNT molecule. The calculated adsorption energy of DNT molecule on the GO surface indicates physisorption mechanism with −0.7 eV adsorption energy. Moreover, basis-set superposition errors correction based on off site orbitals consideration leads to −0.4 eV adsorption energy which it is more in the physisorption regime. Consequently, the results could shed more light to design and fabrication an efficient DNT sensor based on GO layers.

Understanding Self-Assembly of Porphyrin-Based SURMOFs: How Layered Minerals Can Be Useful.

Porphyrin-based metal-organic frameworks on surfaces are a new class of planar materials with promising features for applications in chemical sensing, catalysis, and organic optoelectronics at nanoscale. Herein, we studied systematically a series of the SURMOFs assembled from variously meso-carboxyphenyl/pyridyl-substituted porphyrins and zinc acetate on template monolayers of graphene oxide via layer-by-layer deposition. This microscopically flat template can initiate the growth of macroscopically uniform SURMOF films exhibiting well-resolved X-ray diffraction. By applying the D'yakonov method, which has been previously used for the extraction of self-convolution of electron density in clay minerals, to the analysis of the experimental diffraction patterns of the SURMOFs, we determined the relation between the structure of porphyrin linkers and the geometry of packing motives in the films. We showed that the packing of the SURMOFs differs significantly from that of bulk powders of similar composition because of steric limitations imposed on the assembly in 2D space. The results of microscopic examination of the SURMOFs suggest that the type of metal-to-linker chemical bonding dictates the morphology of the films. Our method provides an enlightening picture of the interplay between supramolecular ordering and surface-directed assembly in porphyrin-based SURMOFs and is useful for rationalizing the fabrication of various classes of layered metal-organic frameworks on solids.

Probing Electrochemical Structure-Property Relationships at Non-Porous Monolayer Electrodes of Exfoliated Graphene and MoS2 Single Layers

The electrochemically available surface area and porosity of 2D materials is difficult to control due to the strong aggregation and restacking phenomena that occur when single layers are processed into thin or thick film electrodes. Furthermore, the distributed resistance, the potential for pore-size related effects and the uncertainty in relating estimated surface area and porosity data to the actual electrochemically accessible surface area make measurements of the intrinsic electrochemical performance of these materials difficult to assess. Ideally, electrochemical measurements are carried out on flat, polished or atomically flat electrode surfaces for the evaluation of intrinsic double-layer capacitance and electrochemical rate constants. In this talk, we will discuss our efforts to create large-area monolayers of solution exfoliated 2D materials for electrochemical evaluation in aqueous and non-aqueous electrolytes. Using a modified Langmuir-Blodgett technique, we demonstrate the ability to create uniform and large area monolayers of graphene oxide, thermally reduced graphene oxide as well as 1T and 2H molybdenum disulfide which can be transferred to atomically flat electrode substrates such as highly oriented pyrolytic graphite. These films are found to be stable during electrochemical cycling in various electrolytes and allow us to probe electrochemical properties as a function of 2D material structure without porosity-related artifacts. This model system has been used to probe the changes in double-layer capacitance in both aqueous and non-aqueous electrolytes for graphene-based materials as a function of defect density. It has allowed us to estimate the theoretical limits of graphene-based supercapacitors as a function of graphene “type”. More recently, this analysis has been extended to probe the intrinsic capacitance of the metallic 1T and semiconducting 2H polymorphs of MoS2 as a function of layer number to probe the charging mechanisms at play. The system also offers the ability to probe the capacitance of graphene/MoS2 heterostructures. These and future studies are expected to provide valuable insight into how we might build more energy dense supercapacitors from the optimal combination of 2D materials.

Reducing ageing of thin PTMSP films by incorporating graphene and graphene oxide: Effect of thickness, gas type and temperature

It was previously proven that small loadings of few-layer graphene (G) and monolayer graphene oxide (GO) adjust the permselectivity and reduce the ageing of thick poly(trimethyl silyl propyne) (PTMSP) membranes, which currently inhibits their application in real separations. In this work, we extend the analysis to thin film composite membranes with top layers of PTMSP/G and PTMSP/GO between 1 and 7 µm. The good quality of the films, obtained by spin coating the polymer solution on porous flat supports, indicates that graphenic sheets are aligned parallel to the film surface, without defects, and that the large-scale production of such composite films is feasible. The addition of G and GO to PTMSP can improve the gas permeability and selectivity, and even change the behaviour from CO2-selective to He-selective. GO produces more repeatable effects than graphene, and it generally enhances the PTMSP permeability.The ageing was studied by tracking permeability of 4 gases with time: thin films age faster than thick ones, because ageing is due to free volume diffusion across the film. Incorporation of 1 wt% of G and GO visibly reduces the ageing of thin PTMSP films, and the effect is repeated on many different samples. The ageing can be correlated, with a single power law, to the ratio between time and squared film thickness. The exponent of such function, that quantifies the ageing rate, decreases when G and GO are added to PTMSP thin films, by factors as high as 40%.The temperature effect was studied, up to 60 °C, on annealed samples: the membranes, CO2-selective at room temperature, can become easily He-selective by raising the temperature. The He/CO2 selectivity increases with temperature: such effect could be exploited syngas purification and pre-combustion capture processes.

Synthesis, Characterization, and Light-Induced Spatial Charge Separation in Janus Graphene Oxide

Janus graphene oxides and Janus graphenes are materials with different functionalization on opposite faces of atomically thin carbon sheets. Owing to their monolayer nature, these Janus sheets show unique properties where the functionalization on one face can modulate the properties on the opposite face. However, few general procedures to create and characterize Janus graphene oxides or Janus graphenes have been reported, and as a consequence these intriguing materials remain largely unexplored. Here we report a general synthesis of Janus graphene oxide, where particles are deposited in situ from molecular precursors on opposite faces of monolayer graphene oxide (GO). We used a silicon wafer and a polymer film to successively expose and protect alternate graphene oxide faces for asymmetric deposition of Pt and TiO2 nanocrystals, thus producing Janus graphene oxide composites (Pt|GO|TiO2). We used electron microscopy of Janus graphene oxide cross-sections to conclusively show that Pt and TiO2 particles are...

Molecular dynamics based simulations to study the fracture strength of monolayer graphene oxide

The aim of this article is to study the effects of functional groups such as hydroxyl, epoxide and carboxyl on the fracture toughness of graphene. These functional groups form the backbone of the intrinsic atomic structure of graphene oxide (GO). Molecular dynamics based simulations were performed in conjunction with reactive force field parameters to capture the Mode-I fracture toughness of functionalised graphene. Simulations were performed in stages, to study the effect of these functional groups, individually as well as all together on the fracture toughness of GO nanosheets. The molecular dynamics based simulations performed in this article helps us to conclude that the spatial distribution and concentration of functional groups significantly affects the fracture behavior of GO nanosheets.

Graphene oxide activation with a constant magnetic field

The effect of constant magnetic field strength on activation of sensors modified with graphene oxide monolayers was investigated. The use of constant magnetic field resulted in improved electroanalytical properties of the sensors. It was proven that level of GO activation is clearly related to constant magnetic field strength. Moreover, it was demonstrated that observed phenomenon is stable in time.

Perpendicular Orientation of Diblock Copolymers Induced by Confinement between Graphene Oxide Sheets.

We have studied an orientation structure of self-assembled block copolymers (dPS-b-PMMA) of deuterated polystyrene (dPS) and poly(methyl methacrylate) (PMMA) confined between graphene oxide (GO) surfaces. The results of combination techniques, such as neutron reflectivity, time-of-flight secondary-ion mass spectrometry, grazing-incidence small-angle X-ray scattering, and scanning electron microscopy, show that self-assembled domains of the block copolymers in thin films near the GO sheets are oriented perpendicular to the surface of the GO monolayers, in contrast to the horizontal lamellar structure of the copolymer thin film in the absence of the GO monolayers. This is due to the amphiphilic nature of the GO, which leads to a nonpreferential interaction of both dPS and PMMA blocks. Double-sided confinement with the GO monolayers further extends the ordering behavior of the dPS-b-PMMA thin films. Continuous vertical orientation of the block copolymer thin films is also obtained in the presence of alternating GO layers within thick copolymer films.

Preparation of cellulose–graphene oxide aerogels with N‐methyl morpholine‐N‐oxide as a solvent

ABSTRACTCellulose–graphene oxide (GO) aerogel composites were successfully prepared from cellulose and GO dispersed in N‐methyl morpholine‐N‐oxide monohydrate, a nontoxic and environmentally friendly solvent, after a freeze‐drying process. Because of the strong interactions between the numerous oxygen‐containing groups located on the surface of GO and the functional groups of the cellulose molecules, the GO monolayers were well dispersed in the three‐dimensional porous structure of the cellulose aerogels. With the addition of 10 wt % GO, the swelling ratios and water contents of the composite cellulose–GO aerogels increased from 468 to 706% and from 82.4% to 87.6%, respectively. The corresponding maximum decomposition temperatures also increased from 335 to 353 °C with increasing GO content from 0 to 10%; this indicated that the thermal stability of the cellulose–GO aerogels was enhanced. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46152.

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.

Pulse-Driven Capacitive Lead Ion Detection with Reduced Graphene Oxide Field-Effect Transistor Integrated with an Analyzing Device for Rapid Water Quality Monitoring.

Rapid and real-time detection of heavy metals in water with a portable microsystem is a growing demand in the field of environmental monitoring, food safety, and future cyber-physical infrastructure. Here, we report a novel ultrasensitive pulse-driven capacitance-based lead ion sensor using self-assembled graphene oxide (GO) monolayer deposition strategy to recognize the heavy metal ions in water. The overall field-effect transistor (FET) structure consists of a thermally reduced graphene oxide (rGO) channel with a thin layer of Al2O3 passivation as a top gate combined with sputtered gold nanoparticles that link with the glutathione (GSH) probe to attract Pb2+ ions in water. Using a preprogrammed microcontroller, chemo-capacitance based detection of lead ions has been demonstrated with this FET sensor. With a rapid response (∼1-2 s) and negligible signal drift, a limit of detection (LOD) < 1 ppb and excellent selectivity (with a sensitivity to lead ions 1 order of magnitude higher than that of interfering ions) can be achieved for Pb2+ measurements. The overall assay time (∼10 s) for background water stabilization followed by lead ion testing and calculation is much shorter than common FET resistance/current measurements (∼minutes) and other conventional methods, such as optical and inductively coupled plasma methods (∼hours). An approximate linear operational range (5-20 ppb) around 15 ppb (the maximum contaminant limit by US Environmental Protection Agency (EPA) for lead in drinking water) makes it especially suitable for drinking water quality monitoring. The validity of the pulse method is confirmed by quantifying Pb2+ in various real water samples such as tap, lake, and river water with an accuracy ∼75%. This capacitance measurement strategy is promising and can be readily extended to various FET-based sensor devices for other targets.

Interface Strain Induced Hydrophobic Facet Suppression in Cellulose Nanocomposite Embedded with Highly Oxidized Monolayer Graphene Oxide

AbstractIn nature, cellulose is a unique lightweight biopolymer with outstanding mechanical and optical properties that is readily available. Rigorous investigations have been conducted to use cellulose as an ingredient in the advanced functional composite design. In this work, a hybrid film composed of homogeneous cellulose nanocrystals (CNC) and high oxidation graphene oxide (GO) is obtained by sufficient blending and vacuum filtration. For the first time through the X‐ray diffraction (XRD) spectrum, the suppression of the originally ordered hydrophobic (200) facet of CNC is observed with increasing concentrations of GO. Further, the originally ordered hydrophilic (110)/(1‐10) facets of CNC remain intact. Through systematic molecular dynamics simulations of a set of simplified CNC–GO sandwich structures, the mechanism behind this hydrophilic/hydrophobic facets manipulation is revealed. The strain induced by the hydrogen bonding between the CNC hydroxyl groups and the oxidation types on GO is the dominant reason to cause the suppression of the hydrophobic facet of CNC in CNC–GO hybrid film. This strain induced mechanism provides an understanding for intrinsically manipulating cellulose–matrix interface and potentially engineering the cellulose based nanocomposite material properties for future advanced materials development.

Wrinkling and Periodic Folding of Graphene Oxide Monolayers by Langmuir-Blodgett Compression.

Crumples, wrinkles, and other three-dimensional topographical features in graphene oxide (GO) have been of recent interest as these features have improved material performance for a variety of applications. However, wrinkling of monolayer GO films has yet to be reported. Herein, we demonstrate wrinkling and folding of monolayer GO using the Langmuir-Blodgett technique for the first time. First, cetyltrimethylammonium bromide (CTAB) and GO are deposited on the air-water interface and uniaxially compressed to form a monolayer. CTAB enhances in-plane rigidity of the monolayer through hydrophobic tail aggregation, preventing GO-GO in-plane sliding behavior. Overcompression of the GO monolayer results in the out-of-plane periodic nanoscale wrinkling and in turn generates folds that are stable during deposition onto a substrate and GO chemical reduction. Furthermore, we investigate one potential application of this material by constructing a 3D electrode of the stacked nanofolded GO-CTAB layers that exhibits superior volumetric capacitance compared to commercial devices and comparable volumetric capacitance compared to high-performing recently reported devices. The high volumetric capacitance is attributed to the electrolyte-accessible channels generated by the nanofolds which are similar in size to the hydrated ions.

Elastic–plastic properties of graphene engineered by oxygen functional groups

Using molecular dynamics (MD) simulations, we investigate the elastic–plastic mechanical performances of monolayer graphene oxide (GO) under uniaxial tension. The brittle–ductile–brittle transition and nonlinear–linear–nonlinear elastic transition is found in the uniaxial tension of GO, which displays strong correlations to the content, distribution and proportion of oxygen functional groups. In principle, the tensile behavior of graphene with epoxy groups exhibits ductile fracture features due to the unique epoxy-to-ether transformation in structural evolution. Our simulation results also reveal that wrinkling could cause a competing mechanism of strain-hardening or -softening, and in turn, the nonlinear–linear elasticity transition. Moreover, we propose a continuum mechanical model with a modified stress–strain relation to understand the unique deformation performances, which is consistent with the MD results. These findings might provide valuable insight and design guidelines for optimizing the specific mechanical properties and deformation behaviors of graphene and its derivatives.

Effect of the conditions of transfer on the structure and optical properties of Langmuir graphene oxide films during deposition on a substrate

The effect the solvent and transfer pressure of graphene oxide (SLGO) Langmuir–Blodgett films on the physicochemical properties of monolayers, and on their structural and optical properties, is studied. Examination of the physicochemical properties of SLGO monolayers on subphase surfaces that are formed from SLGO dispersions in different organic solvents reveals that monolayer behavior is virtually independent of the solvent. Electron microscope and optical studies show that the monolayers formed from SLGO dispersions in DMF and acetone have the highest transfer coefficients. It is concluded that the structural heterogeneity of the surfaces of graphene oxide films results from simultaneous effect of electrostatic interactions between graphene oxide particles and Van der Waals interactions with the solvation shell of the particles. Studies focusing on the effect the pressure of transferring a graphene oxide monolayer onto the surface of a solid substrate has on structural features of LB films show that films produced at low surface pressures have more homogeneous structures.

Improving the macro-scale tribology of monolayer graphene oxide coating on stainless steel by a silane bonding layer

This work aimed at improving the macro-scale tribology of single-layer graphene oxide (GO) coating on metallic surfaces. To this end, stainless steel substrate was modified by a silane prior to the self-assembly of a GO coating. Tribological tests, Raman spectroscopy and elemental analysis were performed. The results show that the pre-deposited silane intermediate layer effectively enhanced the tribological performance of the top GO coating in terms of very low friction (∼0.1), 10-fold reduction in wear and increased coating durability by 20times. The improvement could be mainly attributed to the enhanced bonding via the silane layer.

Controlling the Roughness of Langmuir-Blodgett Monolayers.

Controlling the surface roughness of thin films with nanoscale precision is of significant interest for the rational design of surface coatings. Although wrinkling and buckling of Langmuir monolayers under compression has been demonstrated for several years, there is currently no method to precisely control this behavior during compression and thereby modify the surface roughness of deposited films. Here, we combine conventional Langmuir phase analysis with a novel dynamic viscoelasticity measurement to simply and accurately observe the jamming transition of monolayers of silica spheres, graphene oxide, and surfactant. By overcompressing beyond this point, the surface roughness of the deposited monolayer can be precisely controlled. This technique could be used to tune the surface properties of a variety of materials from lipids to nanoparticles.

Enhancing the electrical conductivity of vacuum-ultraviolet-reduced graphene oxide by multilayered stacking

Vacuum-ultraviolet light irradiation under a high vacuum is a facile method to reduce graphene oxide (GO) sheets and therefore to enhance their electrical conductivity. The aim of this study was to investigate the local electrical properties of a reduced graphene oxide (rGO) monolayer and bilayer by using conductive atomic force microscopy (CAFM). Both the lateral and vertical CAFM measurements showed a higher current signal on the rGO bilayer than on the rGO monolayer. The enlargement of the tip contact area significantly affected the vertical CAFM measurements and enhanced the current signal in the bilayer regions. However, when performing the lateral CAFM measurements, the enlarged tip contact area had no obvious influence on the current signal. The increase in the current signal can be ascribed to the intrinsic enhancement of the electrical conductivity on the rGO bilayer. These results suggested that the stacked rGO sheets formed the new conductive paths for the carrier transportation in the lateral direction. This process can be further applied in producing microconductive patterns in multilayered GO.

Fast low-temperature plasma reduction of monolayer graphene oxide at atmospheric pressure

We report on an ultrafast plasma-based graphene oxide reduction method superior to conventional vacuum thermal annealing and/or chemical reduction. The method is based on the effect of non-equilibrium atmospheric-pressure plasma generated by the diffuse coplanar surface barrier discharge in proximity of the graphene oxide layer. As the reduction time is in the order of seconds, the presented method is applicable to the large-scale production of reduced graphene oxide layers. The short reduction times are achieved by the high-volume power density of plasma, which is of the order of 100 W cm−3. Monolayers of graphene oxide on silicon substrate were prepared by a modified Langmuir–Schaefer method and the efficient and rapid reduction by methane and/or hydrogen plasma was demonstrated. The best results were obtained for the graphene oxide reduction in hydrogen plasma, as verified by x-ray photoelectron spectroscopy and Raman spectroscopy.

Deposition of an Ultraflat Graphene Oxide Nanosheet on Atomically Flat Substrates

In this study, graphene oxide (GO) sheets produced in the form of stable aqueous dispersions were deposited on Au (111), freshly cleaved mica, and highly oriented pyrolytic graphite (HOPG) substrates. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to study the presence and distinct contact of GO sheets on the substrates. It was revealed from the topography images that high-quality ultraflat GO monolayer sheets formed on the substrates without distinct cracking/wrinkling or folding. GO sheets with apparent height variation observed by microscopy also indicate ultraflat deposition with clear underlying steps. It was observed that ultrasonication and centrifuge steps prior to deposition were very effective for getting oxidation debris (OD)-free ultraflat single monolayer GO nanosheets onto substrates and that the process depends on the concentration of supplied GO solutions.