Functionalized Graphene Sheets Research Articles

Multiscale Modeling and Characterization of Graphene Epoxy Nanocomposite.

This study aims to characterize graphene epoxy nanocomposite properties using multiscale modeling. Molecular dynamics was used to study the nanocomposite at the nanoscale and finite element analysis at the macroscale to complete the multiscale modeling. The coupling of these two scales was carried out using the Irving-Kirkwood averaging method. First, the functionalization of graphene was carried and 6% grafted graphene was selected based on Young's modulus and the tensile strength of the grafted graphene sheet. Functionalized graphene with weight fractions of 1.8, 3.7, and 5.6 wt.% were reinforced with epoxy polymer to form a graphene epoxy nanocomposite. The results showed that the graphene with 3.7 wt.% achieved the highest modulus. Subsequently, a functionalized graphene sheet with an epoxy matrix was developed to obtain the interphase properties using the MD modeling technique. The normal and shear forces at the interphase region of the graphene epoxy nanocomposite were investigated using a traction-separation test to analyze the mechanical properties including Young's modulus and traction forces. The mean stiffness of numerically tested samples with 1.8, 3.7, and 5.6 wt.% graphene and the stiffness obtained from experimental results from the literature were compared. The experimental results are lower than the multiscale model results because the experiments cannot replicate the molecular-scale behavior. However, a similar trend could be observed for the addition of up to 3.7 wt.% graphene. This demonstrated that the graphene with 3.7 wt.% shows improved interphase properties. The macroscale properties of the graphene epoxy nanocomposite models with 1.8 and 3.7 wt.% were comparatively higher.

Spin-polarized electronic/transport properties of iron-benzene complex-adsorbed graphene sheets

The electronic and transport properties of a series of covalently functionalized graphene sheets m(FeBz)n-G (m, n = 1, 2) are calculated by density functional theory (DFT) and the nonequilibrium Green's function (NEGF) methods. All the studied systems are chemically stable due to the d–π interaction between Fe and graphene. Addition of (FeBz)n (n = 1, 2) introduces a half-metallic behavior: the up-spin state degradates into a semiconductor channel while the down-spin state behaves as a metallic channel. Particularly, two-fold anisotropy magnetism could be obtained, with the ferromagnetic ordering extending either parallel to the graphene plane originated from the Fe-d up-spin state or perpendicular due to the d-electron coupling along the Fe-Bz-Fe-Bz chain in the down-spin state. Because of the trapping effect, attaching (FeBz)n decreases the conductivity. Anisotropy inherited from graphene after functionalized with (FeBz)n (n = 1, 2): zigzag direction has higher conductivity than armchair direction. The number and alignment style of (FeBz)n (n = 1, 2), and the spin-polarized character, all have impacts on the transport property. Our findings provide useful guidelines for achieving graphene-based electronic and spintronic nanodevices by attaching other similar long multi-decker metal-arene systems.

Atomistic approach for predicting mechanical properties and creep behavior of graphene reinforced natural rubber composites

Graphene has proven to be one of the finest nanomaterials for rubber. In the present study the effect of volume fraction and functionalization of graphene sheets on the mechanical properties and creep behavior of natural rubber (NR) composites has been predicted using the molecular dynamics approach. The properties of NR nanocomposite with the graphene sheet volume fractions of 2.5%, 4.72%, and 11.35% have been compared with the properties of pristine NR. It was observed that the values of Young’s modulus, bulk modulus, and shear modulus were increased up to 104%, 68.40%, and 17.5% respectively for the NR nanocomposite with the reinforcement of 11.35% of graphene compared to pristine NR. The ultimate tensile strength was also increased up to 46% with a higher volume fraction of the graphene sheet. The carboxyl and ester functional groups were used for the functionalization of the graphene sheet. Among the two groups, it was observed that the carboxyl group functionalized graphene sheet provided a better result than the ester group functionalized graphene sheet reinforced NR nanocomposite. The results showed that the introduction of graphene sheets improved the creep resistance of NR nanocomposites significantly. The pristine NR reached the failure stage in just 600 ps for the stress level of 140 MPa, whereas the NR nanocomposite with an 11.35% volume fraction sustained a higher stress level of 160 MPa and has not shown any sign of failure, even after the simulation time of 1000 ps.

Investigation of unified impact of Ti adatom and N doping on hydrogen gas adsorption capabilities of defected graphene sheets

In this work, we studied the hydrogen adsorption capabilities of functionalized graphene sheets containing a variety of defects (D-G) via molecular dynamics (MD) simulations that govern the mechanisms involved in hydrogen adsorption. Specifically, the graphene sheets containing monovacancy (MV), Stone-Wales (SW), and multiple double vacancy (DV) defects were functionalized with Ti and N atoms to enhance their hydrogen adsorption capacity. We measured the adsorption capacities of the N-/D-G sheets with varying concentrations of Ti adatoms at 300 K and 77 K temperatures and various pressures. Our study revealed that the increasing concentration of Ti adatoms on the D-G sheets led to a significant improvement in the hydrogen adsorption capacity of the graphene sheets. The DV(III)-G sheets showed the maximum adsorption capacity at 300 K because the DV(III)-G sheets had a small number of large-sized pores that bind hydrogen with high binding energy. Thus, hydrogen remained adsorbed even at higher temperatures (300 K). The N doping on the D-G sheets initially reduced their hydrogen adsorption capabilities; however, the N-D-G sheets enhanced their hydrogen adsorption capacity with the increasing concentrations of Ti adatoms. Compared to all other defect types, the Ti–N-DV(III)-G sheet with a Ti concentration of 10.5% showed a hydrogen uptake of 5.5 wt% at 300 K and 100 bar pressure. Thus, the N doping and Ti implantations improved the hydrogen storage capabilities of the graphene sheets, and these findings helped design solid-state hydrogen storage systems operating at ambient conditions and moderate pressure ranges.

A New Support Film for Cryo Electron Microscopy Protein Structure Analysis Based on Covalently Functionalized Graphene.

Protein adsorption at the air-water interface is a serious problem in cryogenic electron microscopy (cryoEM) as it restricts particle orientations in the vitrified ice-film and promotes protein denaturation. To address this issue, the preparation of a graphene-based modified support film for coverage of conventional holey carbon transmission electron microscopy (TEM) grids is presented. The chemical modification of graphene sheets enables the universal covalent anchoring of unmodified proteins via inherent surface-exposed lysine or cysteine residues in a one-step reaction. Langmuir-Blodgett (LB) trough approach is applied for deposition of functionalized graphene sheets onto commercially available holey carbon TEM grids. The application of the modified TEM grids in single particle analysis (SPA) shows high protein binding to the surface of the graphene-based support film. Suitability for high resolution structure determination is confirmed by SPA of apoferritin. Prevention of protein denaturation at the air-water interface and improvement of particle orientations is shown using human 20S proteasome, demonstrating the potential of the support film for structural biology.

Synthesis and characterization of the two magnetic nanocomposites based on polyaniline and doped with cerium and terbium

In this study, two new quadripartite nanocomposites, containing conductive polymer, expanded and functionalized graphene sheets and magnetic parts were synthesized in a simple procedure at room temperature. These compounds were synthesized to investigate the efficacy of contiguity of magnetic oxide, conductive polymer, and rare earth metal oxide. The synthesized compounds were identified by Fourier transform infrared (FTIR), X‐ray diffraction (XRD), field emission scanning electron microscopy (FE‐SEM), and energy‐dispersive X‐ray spectroscopy (EDX) and their electrochemical and magnetic properties were also investigated. The results showed that the presence of cerium oxide instead of terbium oxide reduces the magnetic property and increases the current density.

Molecular Dynamics Simulation of the Interfacial Shear Properties between Thermoplastic Polyurethane and Functionalized Graphene Sheet.

In this study, the effect of the type and content of functional groups on the interfacial shear properties of a functionalized graphene sheet (FGS)/thermoplastic polyurethane (TPU) nanocomposite are investigated by molecular dynamics (MD) simulations. The maximum pull-out force and separation energy were used to characterize the interfacial strength of the FGS/TPU nanocomposite in sliding mode. To find out how the type and content of functional groups affect the interfacial shear properties of the TPU/FGS system from an atomic view, the details of interactions between FGS and TPU were characterized. Based on the results, stronger interfacial shear properties of the TPU/FGS system can be achieved by adding the carboxyl group or hydroxyl group on the surface of graphene than that between TPU and FGS modified by the amine group or epoxy group, because of the strong interaction of electrostatic forces and H-bonds. In addition, interfacial shear properties can also be enhanced by increasing the content of functional groups modified on the surface of graphene.

Infrared and Raman spectroscopic analysis of functionalized graphene

AbstractIn this mini‐review, we discuss our work in the analysis of material properties of electrochemically functionalized graphene using infrared (IR) and Raman techniques as thin film sensitive vibrational spectroscopies. Multiscale characterization is demonstrated using a combination of IR spectroscopic ellipsometry (IRSE), confocal Raman spectroscopy (RS) and photothermal atomic force microscopy coupled IR analysis (AFM‐IR). IRSE is used for spots with dimensions of a few mm, RS for spots with a diameter of a few micrometer and AFM‐IR at the sub‐100 nm scale. In the first part of the article, functionalized large‐area graphene sheets electrochemically modified by ultrathin oligomer films of maleimidophenyl or 4‐aminophenyl acetic acid are studied. Characteristic molecular vibrations of the functional layers are analyzed by the IR methods, while the graphene‐related phonons are characterized by RS. This dual approach allowed for the determination of the attached functional groups as well as the identification of the nature of chemical coupling of the functional moieties to graphene. The thickness of the deposited layers extracted from IRSE data correlates very well with AFM measurements. In the second part of the article, functionalized graphene sheets are characterized by correlative vibrational optical spectroscopy directly at a device level.

Efficient functionalization of reduced graphene oxide by 3-(pyridin-2-yl)-1,2,4,5-tetrazine and design of hybrids with silver nanoparticles for electrocatalysis

The efficiency of graphene functionalization by 3-(pyridin-2-yl)-1,2,4,5-tetrazine using inverse demand Diels-Alder reaction was investigated in comparison with the symmetrical 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine. Elemental compositions on the surface of functionalized graphene were analyzed by X-ray photoelectron spectroscopy (XPS), evidencing the higher amount of nitrogen when using the H-substituted tetrazine derivative. In a second step, silver nanoparticles were electrodeposited on these functionalized graphene sheets to enhance the electrocatalytic properties of graphene. The amount of silver determined by XPS increases with the amount of nitrogen introduced on graphene as it was intended for. The electrocatalytic properties have been assessed on the Oxygen Reduction Reaction (ORR), showing good performances in terms of ORR potential even with very small amounts of silver compared to previously reported data.

The fate of aggregated graphene oxide upon the increasing of pH: An experimental and molecular dynamic study

Given the possible ecological dangers of graphene oxide (GO), a thorough understanding of its aggregation behavior is essential. During industrial applications, GOs may be used as multi-layered, and there is some possibility that GOs are released into the water environment in the aggregated state. Thus, elucidating the fate of aggregated GO is valuable for evaluating their environmental fate. In this work, the effect of pH on the fate of aggregated graphene oxide (GO) was explored using experimental measurements and molecular dynamic simulations and promoted aggregation of GO upon the increase of pH was observed. Additional investigations show that the presence of oxidation debris (ODs) on GO served as the primary driver of the unanticipated trend in aggregation behavior. GO consists of lightly oxidized functionalized graphene sheets and highly oxidized ODs. Upon the increase of pH and the deprotonation of functional groups, ODs are stripped from GO due to electrostatic repulsions and steric hindrance of water molecules. The stripping of ODs decreased the zeta potential and increased the hydrophobicity of GO, thus accelerating the aggregation. Additionally, the stripped ODs may recombine to GO edges and bridged GOs, which also contribute to further aggregation. Functional group deprotonation, ODs stripping, OD bridging, double layer compression, and charge neutralization all worked together to promote aggregation, resulting in the formation of FG-water-OD aggregates. Overall, the presence of ODs complicates the structures and properties of GO and should be considered during the development of GO-related nanomaterials and the evaluation of their environmental impact.

Greatly enhanced corrosion/wear resistances of epoxy coating for Mg alloy through a synergistic effect between functionalized graphene and insulated blocking layer

The poor corrosion and wear resistances of Mg alloys seriously limit their potential applications in various industries. The conventional epoxy coating easily forms many intrinsic defects during the solidification process, which cannot provide sufficient protection. In the current study, we design a double-layer epoxy composite coating on Mg alloy with enhanced anti-corrosion/wear properties, via the spin-assisted assembly technique. The outer layer is functionalized graphene (FG) in waterborne epoxy resin (WEP) and the inner layer is Ce-based conversion (Ce) film. The FG sheets can be homogeneously dispersed within the epoxy matrix to fill the intrinsic defects and improve the barrier capability. The Ce film connects the outer layer with the substrate, showing the transition effect. The corrosion rate of Ce/WEP/FG composite coating is 2131 times lower than that of bare Mg alloy, and the wear rate is decreased by ∼90%. The improved corrosion resistance is attributed to the labyrinth effect (hindering the penetration of corrosive medium) and the obstruction of galvanic coupling behavior. The synergistic effect derived from the FG sheet and blocking layer exhibits great potential in realizing the improvement of multi-functional integration, which will open up a new avenue for the development of novel composite protection coatings of Mg alloys.

Functionalization Enhancement on Interfacial Properties Between Graphene and ZnO NW/Epoxy: A Molecular Dynamics Simulation Study

AbstractThis study presents the atomistic characterization of the fiber/matrix interfacial properties in the hybrid functionalized carbon fiber reinforced polymer composites. A molecular model is developed containing ZnO nanowire (NW) aligned vertically on functionalized graphene sheet (FGS) and embedded in the cross‐linked epoxy matrix. The molecular dynamics simulation technique is employed to evaluate the traction‐separation behavior of FGS in both opening and sliding modes. Three different chemical groups of carboxyl (COOH), carbonyl, and hydroxyl with multiple grafting densities are explored as functional groups. The results reveal that the interfacial strength and the cohesive energy are considerably enhanced in the functionalized model compared with the pristine structure. COOH has the most impact on the traction‐separation response with a 112% improvement of interfacial strength. The enhanced effect of functionalization is more intense in the shear separation than the normal one. However, comparing the graphene/ZnO NW with graphene/no ZnO NW structures show that adding ZnO NW can compensate for the slight effect of grafting in the normal direction. The results indicate that combining the growth of ZnO NW and functionalization enhances the interfacial strength up to two‐fold in the opening and up to eight‐fold in the sliding mode of carbon fiber composites.

Fine-tuning the functionality of reduced graphene oxide via bipolar electrochemistry in freestanding 2D reaction layers

Graphene has unique characteristics that are appealing for energy-related applications such as ultra-lightweight and high surface-area/electrical-conductivity. However, generating functional graphene sheets is still a very challenging task. Here, a novel approach based on an original bipolar electrochemistry set-up, using a quasi-2D reaction layer, is suggested, which allows a precise control of dispersibility and conductivity of graphene sheets. In this system, a freestanding 2D layer of aqueous solution, containing 2D graphene oxide (GO) sheets, is placed between two platinum feeder electrodes, which are used to apply an electric field. As a result, the GO sheets experience a sufficiently high polarization to cause their transformation into reduced GO (rGO). The degree of reduction can be readily controlled by the field strength and exposure time, resulting in a wide range of rGO with different conductivity/dispersibility features. The partially reduced GO (prGO) sheets with engineered conductivity/dispersibility are used to prepare aqueous composites with a redox-polymer for organic battery applications. Additionally, at higher potentials, Pt nanoparticles are released from the feeder electrodes and attached to rGO sheets. The sheets were used for catalyzing hydrogen evolution reaction with a performance comparable to bulk Pt.

Decomposition Mechanism of Isoprenoid Hydrocarbon p-Menthane in the Presence of Pt@FGS Nanoparticles: A ReaxFF-MD Study.

Pt@FGS nanoparticles have shown effective enhancement in the decomposition of hydrocarbon fuels. To further explore the potential enhancing mechanisms of Pt@FGS nanoparticles, the catalytic decomposition of p-menthane, a bioderived isoprenoid "drop-in" fuel with great promise, is investigated here using the reactive force-field molecular dynamics (ReaxFF-MD) simulations. The results show that the Pt@FGS nanoparticles exhibit good catalytic reactivity with a reduction of the activation energy by nearly 62%. Possible initial reactions of enhanced p-menthane (PMT) decomposition are discussed, which suggests that the supported Pt-cluster plays a key role in the dehydrogenation of PMT, as does the oxygen-containing functional group of the functionalized graphene sheets (FGS). It is also interesting to note that the presence of Pt@FGS causes the initial reactions, which are dominated by H-abstraction, favorable in both kinetics and thermodynamics.

Adsorption of acetic acid and benzoic acid on pristine and defect containing graphene: A DFT study

The modification of graphene with acetic acid (AA) and benzoic acid (BA) was performed through computational simulations using the density functional theory (DFT). Pristine graphene, graphene with vacancies and Stone-Wales defects were considered. Adsorption results on pristine and Stone-Wales defected grapehene show weak physical interactions with both molecules, as confirmed by the non-covalent interaction index. Functionalized graphene sheets have similar structural and electronic properties. On the other hand, by placing vacancies in its structure, graphene promotes strong molecule adsorption on the surface due to the dangling bonds in these systems. Chemical adsorption is present by having a stronger interaction between the acids and surface. Single vacancy graphene becomes metallic. However, upon functionalizing it with acetic acid and benzoic acid, the electronic properties become pristine-like, demonstrating that functionalization with weak acids does not change the electronic characteristics of graphene.

Mechanisms of the Aggregation of Graphene Oxide at High pH: Roles of Oxidation Debris and Metal Adsorption.

In this work, aggregation of graphene oxide (GO) in synthetic surface water at high pH was elaborated, and experimental characterizations and molecular dynamics simulations were employed to uncover the mechanisms. According to previous studies, aggregation of GO is supposed to be impossible at high pH considering the deprotonation of functional groups on GO and the increased electrostatic repulsions. However, significant aggregations and a reversed trend in zeta potential at high pH were observed. One of the mechanisms was that the promoted metal adsorption at high pH can offset the negative charges generated by the deprotonation. Additionally, the stripping of oxidation debris (OD) on GO also contributes to the unexpected trend in the aggregation behavior and zeta potential. GO consists of lightly oxidized functionalized graphene (FG) sheets and highly oxidized OD. Upon the increase of pH and the deprotonation of functional groups on FG and OD, OD was stripped from FG, which decreased the electrostatic repulsions between FG sheets and accelerated the aggregation. The stripped ODs may recombine to FG edges and bridged FG sheets, which also contribute to the aggregation. Upon the stripping of OD and microstructure transformation of FG, FG-water-OD aggregates formed. According to this study, the aggregation of GO was accompanied by deprotonation of functional groups, metal adsorption, and surface property transformation triggered by the stripping of ODs and should be considered during the development of GO-related nanomaterials and the evaluation of its environmental impact.

Competitive adsorption of methanol co-solvent and dioctyl phthalate on functionalized graphene sheet: Integrated investigation by molecular dynamics simulations and quantum chemical calculations

Hypothesis.Organic co-solvents, which are universally employed in adsorption studies of hydrophobic organic chemicals (HOCs), can inhibit HOC adsorption by competing for active sites on the adsorbent. The adsorbent structure can influence co-solvent interference of HOC adsorption; however, this effect remains unclear, leading to an incomplete understanding of the adsorption mechanism. Experiments.In this study, dioctyl phthalate (DOP) was used to investigate competitive adsorption on functionalized graphene sheet in a water–methanol co-solvent system through molecular dynamics simulations and quantum chemical calculations. Findings.The simulations showed that the functional groups in the graphene defects had a strong adsorption affinity for methanol. The adsorbed methanol occupied a large number of active sites at the graphene center, thereby weakening DOP adsorption. However, the methanol adsorbed at the graphene edges could not compete with DOP for the active sites. –COOH had the strongest binding affinity for methanol among the functional groups and thus predominantly controlled the interaction between graphene and methanol. This study makes an innovative contribution toward understanding the competitive adsorption of methanol and DOP on functionalized graphene sheet, especially in visualizing the competition for active sites, and provides theoretical guidance for the removal of HOCs and practical application of graphene.

An atomistic study on the thermomechanical properties of graphene and functionalized graphene sheets modified asphalt

Functionalized graphene is potential to become the next generation of modifier for asphalt pavements which require long-term durability in service. However, the reinforcing mechanism of graphene-based sheet towards mechanical performance of asphalt and the effect from functional groups remains unclear. In this study, the thermomechanical properties of graphene and functionalized graphene modified asphalt nanocomposites are investigated using molecular dynamics simulations. The shear capacity of asphalt nanocomposites at different temperature levels is evaluated through pullout test, and the details of interactions between functionalized graphene and asphalt matrix are characterized. Results show that hydrogen bond plays an important role in the reinforcement of asphalt nanocomposites. Polar components of asphalt are easier to interact with functional groups on graphene sheet, while nonpolar components interact with modifier by π−π stacking and mechanical entanglement, respectively. Graphene sheet with hydroxyl functional groups presents more effective reinforcement due to stronger hydrogen bond interactions. The shear resistance of asphalt nanocomposites decreases with increasing temperature due to the increment of intermolecular distance and free volume size. The in-depth understanding of reinforcing mechanism helps to provide information on more favorable functional groups, which is beneficial for manipulation and design of durable asphalt pavement materials using nano-engineering.

Functionalized Graphene/Nickel/Polyaniline Ternary Nanocomposites: Fabrication and Application as Electromagnetic Wave Absorbers.

The evolution of high electromagnetic absorption materials is essential in the fast growing electronic industry in overcoming electromagnetic pollution. In view of this, a series of Ni nanoparticle-decorated functionalized graphene sheets (FG/Ni) are synthesized by a solvothermal method using different ratios of FG/Ni precursors. Subsequently, FG/Ni is subjected to in situ polymerization of aniline to form FG/Ni/PANI ternary composites and characterized. The total electromagnetic interference shielding efficiency (SET) measurements on FG/Ni/PANI with an optimized FG/Ni ratio (50 mg:600 mg NiCl2·6H2O) exhibit enhanced performance, i.e., ∼47-65 dB (2-3.8 GHz) and ∼65-45 dB (3.8-8 GHz), following absorption as the dominant mechanism due to the matching of dielectric loss and magnetic loss. It is anticipated that such excellent performance of robust FG/Ni/PANI ternary composites at a very low thickness (0.5 mm) has great potential in the application of microwave-absorbing materials.

DFT analysis of nitrogen and Boron doped Graphene sheet as lead detector

In the last few decades, the heavy metal contamination in water has affected the human health in a big way and hence an area of great scientific concern. Among these heavy metals, lead (Pb) is highly toxic and leads to the adverse effect on human body, causing severe health problems like anemia, hypertension, renal impairment, immunotoxicity, toxicity to the reproductive organs, cancer etc. here, the Boron and Nitrogen functionalized graphene sheets (B-Graphene and N-Graphene), have been analysed for its suitability to detect the lead, by computing the band structure, density of state, charge transfer and Current-Voltage characteristics, using the Density Functional Theory based ab-initio approach. The computed results demonstrate that the N-doped Graphene is relatively better sensor for lead in aqueous environment, with a sensitivity of ~ 67% at lower bias voltage, high detection range and short recovery time. However, the Pb sensitivity of the N-Graphene sheet at low bias is very high 329% in absence of water environment.