Edge Functional Groups Research Articles

MOLECULAR MECHANISM OF PHYSICAL AND MECHANICAL IMPROVEMENT IN GRAPHENE/GRAPHENE OXIDE-EPOXY COMPOSITE MATERIALS.

The performance provided by graphene (Gr) and graphene oxide (GO) additives can be improved by achieving strong adhesion and uniform dispersion in the epoxy resin matrix. In this study, molecular modeling and simulation of DGEBA/DETA based epoxy nanocomposites containing Gr and GO additives were performed. Density functional theory and molecular dynamics simulations were used to investigate interfacial interaction energies and Young's Modulus. Improvement in the interaction energies was studied by controlling the epoxy:hardener ratio, type and the number of oxygen-containing functional groups on the GO, the mass percentage of Gr/GO filler in the epoxy matrix, size and dispersion of GO in the cell. It was demonstrated that functional groups with up to 10% oxygen content in GO significantly increase interfacial interaction energy for large size Gr/GO. Increasing DETA type amine ratio in the preparation of epoxy polymers increases the interaction energy for high oxygen content while decreasing the interaction energy for low oxygen content in GO for small size GO with edge functional groups. The performance of material dramatically decreased even at high DETA hardener and high GO mass percentages when the aggregation factor of Gr/GO was included in simulations that explain lower Gr/GO percentages in the experimental studies.

Effect of temperature on interface debonding behavior of graphene/graphene-oxide on cement-based composites

Temperature effect easily affect the adhesion of graphene/graphene oxide (GO) on the cement-based composites originating from the weaker non-covalent bonding at the interface, leading to weakening of the cementitious composite properties. In this study, the interfacial debonding behavior of graphene/GO and cement-based composites was investigated for low to high temperatures (250∼550 K) using controlled molecular dynamics simulations. The maximum tensile force and work of adhesion both decrease with increasing temperature, and the value of GO is 5∼6 times that of graphene. Especially at the high temperature end, GO is still difficult to peel off (Fmax: 7.93×103 pN, Wmax:2129.60 Kcal/mol). Meanwhile, the insights into the distribution and concentration of functional groups have shown that high concentrations and edge functional groups can improve interface adhesion. Energy decomposition demonstrated that, compared to the Coulomb effect of charge and the prestress of surface wrinkles, the van der Waals effect plays a major role in interface adhesion. Entropy analysis further reveals the high temperature sensitivity of graphene and the low sensitivity of GO due to edge functional groups. We believe that the insight into the temperature effect of interface debonding behavior at nanoscale can provide theoretical support for the effective temperature working conditions of graphene/graphene-oxide on cement-based composites.

The NMR relaxivity of gadolinium (III) solutions as the function of oxidation level and flake size of graphene oxide

Chelating by graphene oxide (GO) results in dramatic shortening of the proton relaxation times of Gd3+ aqueous solutions. However, the factors, leading to the increase of relaxivity values, and the mechanisms behind the observed phenomena remain elusive. In this work, we investigated how the relaxation times of the GO/Gd3+ solutions depend on the oxidation level and the flake size of GO. The values of spin-lattice (r1) and spin-spin (r2) relaxivities increase with the oxidation degree of the used GO samples. The relaxivity also increases with decreasing the particle size of GO flakes by sonication. The observed phenomena are explained by the increase in the number of the edge functional groups, generated under the influence of both parameters. The content of these groups, their formation, and chelation of Gd3+ ions is discussed in the manuscript. Viscosity of solutions does not directly affect the relaxation times. With the use of high oxidation level and small particle size GO samples we have registered the record high relaxivity values: r2 = 114.7 mM−1 s−1, and r1 = 97.0 mM−1 s−1. This opens straight routes for developing the GO/Gd3+ based MRI contrast agents.

Cs+ conductance in graphene membranes with Ångström-scale pores: the role of pore entrance geometry.

Recently, 2D materials, such as graphene, have been successfully implemented as artificial conduits of molecular sizes. The extreme precision with which these structures can be fabricated provides an unprecedented framework for the development of highly specific and efficient devices. In this work, we study the electrophoretic transport of Cs+ ions in a graphene membrane with effective pore heights of 3.4 Å by conducting molecular dynamics simulations. The entrance of the pore is systematically modified to investigate the effect of pore geometry on ionic conductance. Simulation results suggest a significant correlation between ionic conductance and entrance geometry, with a variation of the conductance up to 100% across the studied cases. To explain the observed correlation, two mechanisms involving an intimate relationship between ion dehydration and edge functional groups are proposed. The present study provides theoretical insights that can aid the design of graphene-based membranes with tunable ionic transport properties.

Modulating the photooxidation selectivity on graphitic carbon nitride by tuning edge functional groups

Selective photooxidation reactions offer a promising route for transformation of organic pollutants into high-value products under mild conditions. In this work, we demonstrate that the edge functional groups in polymer carbon nitride (PCN) were key to the selectivity of methyl mercaptan (CH3SH) photocatalytic oxidation. The amino-decorated PCN favors the oxidation of the -SH group of CH3SH to forming CH3SO3H with selectivity up to 84.0%, while cyanamide-enriched PCN prefers to breaking the -CH3 end of CH3SH and produce H2SO4 with selectivity of 82.0%. The totally difference in reaction sites is ascribed to the functional-group-modulated electronic structure of PCN, which determines activity of photoinduced carriers and the molecular oxygen activation pathway. The amino- and cyanamide-decorated PCN leads to preferential generation of 1O2 and·O2-, respectively. This study highlights the significant role of edge groups engineering in modulation of the excitonic effects in PCN and the resultant selectivity control during photocatalytic reaction.

DFT studies on exposure of sulfur impregnated and sulfur functionalized activated carbon to Hg0 vapors

For removal of poisonous vapor emissions such as mercury, it is necessary to select suitable materials based on an understanding of their properties and interactions with the vapor. As mercury has a high affinity towards sulfur, it’s adsorption using sulfur-impregnated activated carbon was explored in this study. The impregnation of sulfur on activated carbon followed by the adsorption of Hg0 vapors was computationally investigated using DFT simulations. Sulfur adsorption was investigated on activated carbon with armchair edge, zigzag edge, and graphene surface. Sulfur adsorption was investigated on activated carbon with edge functional groups such as hydroxyl and carboxylic acid. Activated carbon with edge functional groups such as sulfonic acid, sulfenic acid, and sulfinic acid was further investigated for the adsorption of Hg0 vapors. Among the edge functional groups on the activated carbon, the hydroxyl group was most favored for sulfur adsorption and, subsequently, Hg0 vapors. This was quantified in terms of shortest bond lengths, strongest binding energies, and maximum charge transfer. Among the sulfur-containing functional groups on activated carbon, sulfenic acid was the most favored for the adsorption of Hg0 vapors. Transition state calculations were carried out, and a reaction pathway was proposed for the adsorption of Hg0 on sulfur-impregnated activated carbon.

Solid-State Phosphors from Coal-Derived Carbon Quantum Dots.

With unique optical and chemical properties, carbon quantum dots (CQDs) find tremendous applications in chemistry, biology, and materials science to medicine. To expand the applicability of coal-derived CQDs from the liquid to solid state, we herein report the sustainable synthesis of solid phosphors from coal-derived CQDs using poly(vinyl alcohol) (PVA) and silica (SiO2) as an organic and inorganic matrix. Two coal-derived CQDs were obtained using an eco-friendly ultrasonic-assisted wet oxidation method. The structural and chemical properties of the CQDs were extensively investigated and compared with commercial CQDs. The coal-derived CQDs exhibited blue fluorescence with 8.9 and 14.9% quantum yields. The CQDs were found to be self-co-doped with nitrogen and sulfur heteroatoms through surface and edge functional groups. Solid-state fluorescence of PVA/CQD composite films confirmed that the CQDs retained their excellent blue emission in a dry solid matrix. A facile one-pot sol-gel method was employed to fabricate SiO2/CQD phosphors with the unique fluorescence emission. Due to their special structural features, coal-derived CQDs favored the heterogeneous nucleation and rapid formation of SiO2/CQD phosphors. Further, coal-derived CQDs caused high-intensity white light emission with CIE coordinates of (0.312, 0.339) by endowing a suitable band gap structure in a SiO2/CQD solid phosphor for potential optical applications.

Light controls edge functional groups to enhance membrane permeability

Functionalized membranes have high potential value in a broad range of practical applications, and the functional groups at the membrane edge play a vital role in the permeability of the membranes. Here, based on an edge-functionalized graphene oxide (GO) membrane model, we theoretically report that high-frequency terahertz stimulation at the frequency near 44.0 THz can significantly enhance the water permeability of the membrane by nearly five times. Its mechanism was revealed to be that the stimulation amplified the C-O stretching of the functional groups and suppressed other vibration modes of the groups. As a result, the modulation of edge functional groups brings down the energy barricade of the membrane and allows water molecules to penetrate the GO membrane more easily. These findings provide a new perspective for enhancing membrane permeability by modulating particular functional groups such as the carboxyl on the edge of the GO membrane.

Butylamine functionalized graphene oxide: experimental and DFT studies on the corrosion inhibition efficiency of the MgAZ13 alloy in a 3.5% NaCl environment

Figure A depicts the electrostatic energy in edge functional groups with non-bonding electrons and suggests an interaction with the metal surface. Figure B shows protonated electrostatic energy that indicates the donation of a pair of electrons to the metal surface.

(Keynote, Digital Presentation) First-Principles Study of Optical and Electronic Properties of Carbon Quantum Dots

We study the optical and electronic properties of carbon-based quantum dots using first-principles calculations. Based on time-dependent density functional theory we calculated absorption and emission of carbon quantum dots, we focus on the influence of surface (edge) functional groups. We can show that nitrogen groups on the carbon edges are decisive for the emission properties of small carbon quantum dots and could give rise to charge-carrier multiplication which has been suggested by recent experiments. Moreover, we explain the stability, with regard to the absorption in the optical range, of carbon quantum dots under ambient conditions. Here, we demonstrate that surface oxidation does only influence the absorption in the visible light range but does give rise to variation in the UV absorption.

New insight on the interfacial behavior between graphene-based membranes and protonated silicon-dioxide via molecular dynamics simulations

Graphene, with many excellent properties, is often attached to silicon dioxide substrates in practical applications. In this paper, we used the molecular dynamics method to investigate the interfacial behavior between protonated silicon-dioxide and graphene-based membranes (GBM/pSiO2) with the different distribution and concentration of functional groups. The binding energy, free energy and entropy effect was obtained to get a detailed insight into the adhesive mechanism. Particularly, we laid stress on the size effect of 2D-square-sheet length on the interface behavior at nanoscales. Our results indicate that the high oxygen-concentration and edge functional groups in GBM enhance the interfacial adhesion, while the small sheets are more firmly adsorbed on the substrate. The analysis of energy decomposition proves that van der Waals energy still plays a major role on the interfacial behavior in the nanoscale contact system of GBM/pSiO2. We believe this nanoscale interfacial behavior simulation could be further extended for research on the adhesion of 2D materials to 3D materials.

Functionalized Reduced Graphene Oxide Thin Films for Ultrahigh CO2 Gas Sensing Performance at Room Temperature.

The demand for carbon dioxide (CO2) gas detection is increasing nowadays. However, its fast detection at room temperature (RT) is a major challenge. Graphene is found to be the most promising sensing material for RT detection, owing to its high surface area and electrical conductivity. In this work, we report a highly edge functionalized chemically synthesized reduced graphene oxide (rGO) thin films to achieve fast sensing response for CO2 gas at room temperature. The high amount of edge functional groups is prominent for the sorption of CO2 molecules. Initially, rGO is synthesized by reduction of GO using ascorbic acid (AA) as a reducing agent. Three different concentrations of rGO are prepared using three AA concentrations (25, 50, and 100 mg) to optimize the material properties such as functional groups and conductivity. Thin films of three different AA reduced rGO suspensions (AArGO25, AArGO50, AArGO100) are developed and later analyzed using standard FTIR, XRD, Raman, XPS, TEM, SEM, and four-point probe measurement techniques. We find that the highest edge functionality is achieved by the AArGO25 sample with a conductivity of ~1389 S/cm. The functionalized AArGO25 gas sensor shows recordable high sensing properties (response and recovery time) with good repeatability for CO2 at room temperature at 500 ppm and 50 ppm. Short response and recovery time of ~26 s and ~10 s, respectively, are achieved for 500 ppm CO2 gas with the sensitivity of ~50 Hz/µg. We believe that a highly functionalized AArGO CO2 gas sensor could be applicable for enhanced oil recovery, industrial and domestic safety applications.

Bionanocomposites based on a covalent network of chitosan and edge functionalized graphene layers.

In this study, carbon papers and aerogels were prepared from chitosan and graphene layers with aldehydic edge functional groups (G-CHO) able to form chemical bonds with chitosan and thus to form a crosslinked network. A high surface area graphite was edge functionalized with hydroxyl groups (G-OH) through the reaction with KOH. G-CHO, with 4.5 mmol/g of functional group, was prepared from G-OH by means of the Reimer-Tieman reaction. Characterization of the graphitic materials was performed with elemental analysis, titration, X-ray analysis, Raman spectroscopy and by estimating their Hansen solubility parameters. CS and G-CHO were mixed with mortar and pestle and carbon papers and aerogels were obtained from a stable acidic water suspension through casting and liophilization, respectively. Free standing and foldable carbon papers and monolithic aerogels based on a continuous covalent network between G-CHO and CS were prepared. G-CHO, which had about 22 stacked layers, was extensively exfoliated in the carbon paper, as confirmed by the absence of the 002 reflection of the graphitic crystallites in the XRD pattern. Carbon paper was found to be resistant to solvents and to be stable for pH ⩾ 7. Composites revealed electrical conductivity. The covalent network between the graphene layers and CS, suggested by the IR findings, accounts for these results. This work demonstrates the effectiveness of a continuous covalent network between chitosan and graphene layers edge functionalized with tailor made functional groups for the preparation of carbon papers and aerogels and paves the way for the scale up of such a type of composites.

Accurate large scale modelling of graphene oxide: Ion trapping and chaotropic potential at the interface

Graphene oxide (GO) shares many novel mechanical and electronic properties with graphene and has been applied extensively for uses in physics, engineering and medicine. Computational simulations of GO have widely neglected accurate characterisation by random functionalisation, forsaking steric strain and abandoning edge functional groups. Here, we show that molecular dynamics forcefield design using electronic structure calculations of hundreds of atoms of GO with accurate functionalisation shows good agreement with state-of-the-art ab initio molecular dynamics (AIMD) simulations. We find that the bespoke forcefield shows better agreement with previous AIMD and experimental results in terms of the interfacial water dynamics and ion adsorption. Namely, GO described by the bespoke forcefield is found to disrupt the hydrogen bonding network at the interface by playing a more dynamic role in accepting and donating hydrogen bonds from water. Furthermore, with the bespoke forcefield, we find preferential adsorption of ions to carboxyl functional groups and a similar mean adsorption half-life for Na+ and Cl− ions around GO. These findings are critical for future investigations of GO in complex environments in application ranging from desalination to protein adsorption for drug delivery.

Comparison of the effects of edge functionalized graphene oxide membranes on monovalent cation selectivity

Layer-by-layer graphene oxide (GO) membrane in principle has great potential in separating Li+ from monovalent cations, which is achieved by their functionalized interlayer entrance. The edge effects on different monovalent ions, however, are not fully understood. Therefore, molecular dynamics simulations were utilized in this study to separately elucidate the filtration effects of three typical edge functional groups, which were carboxyl (COOH), hydroxyl (OH), and hydrogen (H), on the LiCl, NaCl, and KCl solutions. The results revealed that the water permeance was dominantly influenced by the steric size of edge functional groups. It could also be affected by the ions blocked at the entrance. The drastic dehydration of the hydrated Na+ and K+ caused by the OH edge required more energy that led to higher ion rejection. The compressed-dehydrated hydration shell, which was tuned by the edge functional groups, introduced repulsion from Na+ and attraction from Li+ on Cl− when they were 3–5 Å away from each other. It would be strategic to use all three edge functional groups to retain NaCl in the retentate stream while allowing selective permeance of LiCl and the OH edge could additionally retain KCl.

High performance hierarchically nanostructured graphene oxide/covalent organic framework hybrid membranes for stable organic solvent nanofiltration

Stable nanofiltration membranes with excellent tolerance to organic solvents and harsh chemical environment are in high demand from petrochemical, pharmaceutical and oil refinery industries. As promising nanofiltration membrane candidates, graphene oxide lamellar membranes suffer from poor flux as well as instability under practical application conditions. Incorporating additional nanopores were believed to be an effective way to improve solvent transportation while enabling efficient molecular sieving. Herein, as a proof of concept, we designed and prepared a hierarchically nanostructured (i.e. with nanopores, micropores, mesopores) graphene oxide/covalent organic framework (GO/COF) hybrid membrane by intercalating imine-based COF nanoparticles. The imine-based COF nanoparticles with inside hollow structure and in-plane nanopores (1.8 nm) as well as abundant edge functional groups become superhydrophilic, providing additional fast transport nanochannels for water and organic solvents molecules in GO matrix. The hierarchically nanostructured GO/COF hybrid membrane shows excellent water (59 L m − 2h − 1 bar−1) and organic solvents (60 and 51 L m − 2h − 1 bar−1 of methanol and ethanol, respectively) permeate flux at 1 bar and superior organic dyes rejection rates for Methylene blue (320 g/mol) and Congo red (697 g/mol) in ethanol solvent of 99% and 99.82%, respectively. Additionally, intercalating COF could greatly reduce the GO matrix swelling to enhance membrane stability upon exposure to water as well as strong acidic/base solution, enabling a high filtration performance during long-term (48 h) operation.

Post-redox engineering electron configurations of atomic thick C3N4 nanosheets for enhanced photocatalytic hydrogen evolution

Polymeric carbon nitride (C3N4) with atomic thick layers has displayed ultrashort carrier transfer and ion/molecule migration path for enhancement of photocatalytic performance. However, rational modulation towards electron configurations of ultra-thin C3N4 nanosheets is still a challenge, because the existing exfoliation strategies suffer from uncontrollable atom-defects and edge functional groups, leading to poor solar-to-fuel conversion efficiency. Herein, a post-redox method is developed to form atomic thick C3N4 nanosheets with optimized electron configurations through liquid exfoliation and C-reduction. The as-prepared carbon-reduced atomic thick C3N4 (CRed-AT-C3N4) expose more active sites, greatly enhance charge carrier mobility and distinctly reduce the optical band gap, thus leading to a remarkably improved photocatalytic hydrogen evolution rate of 246.2 μmol h−1 under visible light irradiation (λ > 420 nm), which is 17-fold higher than that of the pristine counterpart. The apparent quantum efficiency reaches 18.52 % at 420 nm and surpasses most of existing C3N4-based photocatalysts.

Quantitative Understanding of Charge-Transfer-Mediated Fe3+ Sensing and Fast Photoresponse by N-Doped Graphene Quantum Dots Decorated on Plasmonic Au Nanoparticles.

The formation of a heterostructure with plasmonic nanoparticles drastically alters the optoelectronic properties of graphene quantum dots (GQDs), resulting in exceptional properties. In the present work, we prepare nitrogen-doped GQDs decorated on gold nanoparticles (Au@N-GQDs) by a one-step green reduction method and study its extraordinary fluorescence and photoresponse characteristics. The as-prepared Au@N-GQDs show more than one order of magnitude enhancement in the fluorescence intensity as compared to the bare N-GQDs, which is attributed to hot electron generation and improved absorption in N-GQDs by local field enhancement and the modification of the edge functional groups. Because of the selective coordination to Fe3+ ions, the Au@N-GQDs exhibit extraordinary quenching of fluorescence, with ultrahigh sensitivity for the detection of Fe3+ (<1 nM). A new model for the charge-transfer dynamics is developed involving the Langmuir's law of adsorption to explain the unusual quenching, which strongly deviates from the known models of static/dynamic quenching. The proposed sensor is successfully implemented for the ultrasensitive detection of Fe3+ ions in human serum and Brahmaputra river water samples, representing its high potential applications in clinical as well as environmental diagnosis. Additionally, because of its high absorption in the UV-vis-NIR region and high charge density with long life excitons, the Au@N-GQDs are utilized as photodetectors with ∼104 times faster response than that of bare N-GQDs. The Au@N-GQD-based photodetector possesses a high responsivity of ∼1.36 A/W and a remarkably high external quantum efficiency of ∼292.2%, which is much superior to the GQD-based photodetectors reported till date. The underlying mechanism of ultrafast photoresponse is ascribed to the transfer of hot electrons along with the tunneling of the electrons from Au NPs to N-GQDs as well as the defect reduction of N-GQDs by the incorporation of Au NPs. Without the use of any charge transporting layer, the outstanding performance of N-GQD-based plasmonic photodetector opens up unique opportunities for future high-speed optoelectronic devices.

Relationship between Desalination Performance of Graphene Oxide Membranes and Edge Functional Groups.

High desalination efficiency in principle could be achieved by layer-by-layer graphene oxide (GO) membranes, which benefits from their entrance-functionalized channels assembled by edge-functionalized GO nanosheets. The effects of these edge functional groups on desalination, however, are not fully understood yet. To study the isolated influence of three typical edge functional groups, namely, carboxyl (-COOH), hydroxyl (-OH), and hydrogen (-H), molecular dynamics simulation was used in this work. The results revealed that the edge volumetric blockage effect, resulting in ion permeability at G-H > G-OH > G-COOH membranes, was the dominant mechanistic effect inside the GO membranes with 7 Å interlayer channels. The OH edge has the same effect as the H edge in NaCl/water selectivity because of a unique "ion pulling" effect. Moreover, the OH and H edge-functionalized membranes with 7 Å interlayer channels showed preferential Na+ and Cl- rejections, respectively. This kind of preference leads to a cycle of charging and neutralization in the penetrant reservoir throughout the filtration process. The results from this work suggested that it would be strategic to keep the COOH and H edge functional groups, to maintain the size of interlayer channels in order to stimulate the effects of edge functional groups, and to increase the membrane porosity for designing higher desalination efficiency GO membranes.

Properties of nanographene in polymer nanocomposites through all-atom simulations: Shape fluctuations and rippling

Conformational transitions of graphene can have a strong impact on the overall properties of graphene-based nanocomposites. Here we present a detailed analysis of the behavior of nanographene sheets embedded in polymer matrices, through all-atom Molecular Dynamics simulations. Thermal fluctuations of nanographene, in terms of rippling, are examined both statically and dynamically. Several different factors are investigated: a) Different polymers matrices; b) Edge functionalization of graphene; c) Size of the nanographene flake and d) Temperature effect.Polymer-graphene interactions induce fluctuations in the overall shape of the sheet. Examining the morphological transitions of the sheets in the atomic level, the role of functional groups is found to be critical and prevalent of other factors in the rippling of nanographene. More and higher ripples are formed at the edges of the functionalized sheets. Functionalization of the nanographene sheet reduces the mean number of the formed crests and troughs in the central part of the flake, compared to pristine graphene, whereas a clear increase is observed at the edges of the carboxylated sheets. Edge functional groups suppress the amplitude, as well as the kinetics of rippling compared to pristine nanographene, providing an effective way to control the conformational changes of nanographene.