Defect-free Graphene Research Articles

Graphene Nanoplatelets-Based Advanced Materials and Recent Progress in Sustainable Applications

Graphene is the first 2D crystal ever isolated by mankind. It consists of a single graphite layer, and its exceptional properties are revolutionizing material science. However, there is still a lack of convenient mass-production methods to obtain defect-free monolayer graphene. In contrast, graphene nanoplatelets, hybrids between graphene and graphite, are already industrially available. Such nanomaterials are attractive, considering their planar structure, light weight, high aspect ratio, electrical conductivity, low cost, and mechanical toughness. These diverse features enable applications ranging from energy harvesting and electronic skin to reinforced plastic materials. This review presents progress in composite materials with graphene nanoplatelets applied, among others, in the field of flexible electronics and motion and structural sensing. Particular emphasis is given to applications such as antennas, flexible electrodes for energy devices, and strain sensors. A separate discussion is included on advanced biodegradable materials reinforced with graphene nanoplatelets. A discussion of the necessary steps for the further spread of graphene nanoplatelets is provided for each revised field.

Graphene-based drug delivery systems in tissue engineering and nanomedicine.

The time and dosage form of graphene derivatives have been found to determine therapeutic and toxic windows in several cell lines and preclinical models. The enhanced biological action of graphene derivatives is made possible by altering the chemistry of native materials via surface conjugation, or by changing the oxidation state. The high level of chemical reactivity vested in the planar structure of graphene can be used to load various drugs and biomolecules with maximum radical scavenging effect. The integration of graphene and polymers brings electrical conductivity to scaffolds, making them ideal for cardiac or neuronal tissue engineering. Drawbacks associated with graphene-based materials for biomedical applications include defect-free graphene formation and heteroatom contamination during synthesis process; reduced availability of sp2 hybridized carbon centers due to serum proteins masking; and poor availability of data pertaining to in vivo clearance of graphene-based formulations. Personalized medicine is an emerging area of alternative treatments, which in combination with graphene-based nanobiomaterials, has revolutionary potential for the development of individualized nanocarriers to treat highly challenging diseases.

Large area ultra-thin graphene films for functional photovoltaic devices

Abstract

Sonication Exfoliation of Defect-Free Graphene in Aqueous Silk Nanofiber Solutions

Green processes to exfoliate graphene would be advantageous but is challenged by issues such as stability and dispersibility. Here, stable silk fibroin nanofibers (SFNs) with high crystallinity (β-sheet content) and high net negative charge were endowed with hydrophobic features yet excellent water dispersibility. These nanofibers were used to exfoliate graphite into graphene in aqueous solutions. A high concentration (1.92 mg mL–1) and yield (approaching 20%) were achieved for the exfoliated graphene dispersions using low initial graphite concentrations via sonication treatment. This approach was superior to most protein-assisted exfoliation processes reported to date, and the graphene obtained in the process exhibited fewer defects, good conductivity, and cell compatibility. The graphene prepared by this new process provided an option for SF-graphene composite materials and devices where defect-free graphene with improved performance through the introduction of SFNs into the preparation strategies will ...

Gold nanorod embedded novel 3D graphene nanocomposite for selective bio-capture in rapid detection of Mycobacterium tuberculosis

Tuberculosis (TB) is a chronic and infectious airborne disease which requires a diagnosing system with high sensitivity and specificity. However, the traditional gold standard method for TB detection remains unreliable with low specificity and sensitivity. Nanostructured composite materials coupled with impedimetric sensing utilised in this study offered a feasible solution. Herein, novel gold (Au) nanorods were synthesized on 3D graphene grown by chemical vapour deposition. The irregularly spaced and rippled morphology of 3D graphene provided a path for Au nanoparticles to self-assemble and form rod-like structures on the surface of the 3D graphene. The formation of Au nanorods were showcased through scanning electron microscopy which revealed the evolution of Au nanoparticle into Au islets. Eventually, it formed nanorods possessing lengths of ~ 150 nm and diameters of ~ 30 nm. The X-ray diffractogram displayed appropriate peaks suitable to defect-free and high crystalline graphene with face centered cubic Au. The strong optical interrelation between Au nanorod and 3D graphene was elucidated by Raman spectroscopy analysis. Furthermore, the anchored Au nanorods on 3D graphene nanocomposite enables feasible bio-capturing on the exposed Au surface on defect free graphene. The impedimetric sensing of DNA sequence from TB on 3D graphene/Au nanocomposite revealed a remarkable wide detection linear range from 10 fM to 0.1 µM, displays the capability of detecting femtomolar DNA concentration. Overall, the novel 3D graphene/Au nanocomposite demonstrated here offers high-performance bio-sensing and opens a new avenue for TB detection.

Defect-induced oxygen adsorption on graphene films

Although defects on graphene can degrade electron transport and its ability for use as a protection layer, they can also be helpful to tailor the local properties or activate new sites for particular adsorbates. Here, carbon vacancy defects are formed in graphene films on Ru(0001) using low energy Ar+ bombardment and the materials are then reacted at room temperature with oxygen (O2). Helium low energy ion scattering shows that no oxygen attaches to the intact graphene layer. When isolated single carbon vacancy defects are present, oxygen adsorbs molecularly at the defect sites and intercalates beneath the graphene overlayer after post-annealing at 600 K. When the defects are large enough to consist of open areas of bare substrate, the oxygen dissociatively chemisorbs to the Ru. This work shows that the adsorption depends on the size of the surface vacancies, and that it is important to have defect-free graphene when using it as a protection layer.

High surface graphene nanoflakes as sensitive sensing platform for simultaneous electrochemical detection of metronidazole and chloramphenicol

We demonstrate a mediator-free electrochemical sensitive sensing for detection of metronidazole (MNZ) and chloramphenicol (CAP) in real samples. A few-layered defect-free graphene nanoflakes (GNFs) were synthesized by surfactant-assisted exfoliation method, which was characterized by several methods like XRD, Raman spectroscopy, FE-SEM and TEM images. Cyclic voltammetry and electrochemical impedance spectroscopy were studied to understand the electron transfer behavior of GNFs modified glassy carbon electrode (GCE). Electrochemical behavior of MNZ and CAP was investigated at GNFs/GCE in PBS and observed peak potentials at −0.32 V and − 0.51 V vs. Ag/AgCl. A wider reduction potential window of MNZ and CAP were observed by differential pulse voltammetry which was found to be 280 mV at GNFs/GCE. Amperometry method was employed for detection of the unknown concentration of analytes under hydrodynamic condition. A linear calibration plot was obtained by plotting the concentrations of MNZ and CAP in ranging from 0.5 to 5.5 nM with R2 of 0.9983 and 0.9980, respectively. The detection limit of MNZ and CAP were found to be 0.15 nM and 0.38 nM, this system have shown good stability, excellent reproducibility and free interference from GNFs/GCE. The present method was successfully applied for the determination of MNZ and CAP in drugs and urine samples.

Investigation of tip sonication effects on structural quality of graphene nanoplatelets (GNPs) for superior solvent dispersion

The exceptional properties of graphene and its structural uniqueness can improve the performance of nanocomposites if it can attain the uniform dispersion. Tip sonication assisted graphene solvent dispersion has been emerged as an efficient approach but it can cause significant degradation of graphene structure. This study aimed to evaluate the parametric influence of tip sonication on the characteristics of sp2 carbon structure in graphene nanoplatelets by varying the sonication time and respective energy at three different amplitudes (60%, 80% and 100%). The study is essential to identify appropriate parameters so as to achieve high-quality and defect-free graphene with a highly desirable aspect ratio after solvent dispersion for composite reinforcement. Quantitative approach via Raman spectroscopy is used to find the defect ratio and lateral size of graphene evolved under the effect of tip sonication parameters. Results imply that the defect ratio is steady and increases continually with GNPs, along with the transformation to the nano-crystalline stage I up to 60 min sonication at all amplitudes. Exfoliation was clearly observed at all amplitudes together with sheet re-stacking due to considerable size reduction of sheets with large quantity. Finally, considerable GNPs fragmentation occurred during sonication with increased amplitude and time as confirmed by the reduction of sp2 domain (La) and flake size. This also validates the formation of edge-type defect in graphene. Convincingly, lower amplitude and time (up to 60 min) produce better results for a low defect content and larger particle size as quantified by Raman analysis.

Sensitive Raman Probe of Electronic Interactions between Monolayer Graphene and Substrate under Electrochemical Potential Control

In situ electrochemical Raman spectroscopic measurements of defect-free monolayer graphene on various substrates were performed under electrochemical potential control. The G and 2D Raman band wavenumbers (ωG, ω2D) of graphene were found to depend upon the electrochemical potential, i.e., the charge density of graphene. The values of ωG and ω2D also varied depending on the choice of substrates. On metal substrates where graphene was synthesized by chemical vapor deposition, a strong blue shift of ω2D was induced, which could not account for the strain and charge doping. We attributed the blue shift of ω2D to a change in the electronic properties of graphene induced by distinct electronic interactions with the metal substrates. To explain the unique characteristics in the Raman spectrum of graphene on various substrates, a novel mechanism is proposed considering reduction of the Fermi velocity in graphene owing to dielectric screening from the metal substrates.

Defect-Free Graphene Synthesized Directly at 150 °C via Chemical Vapor Deposition with No Transfer.

Direct graphene synthesis on substrates via chemical vapor deposition (CVD) is an attractive approach for manufacturing flexible electronic devices. The temperature for graphene synthesis must be below ∼200 °C to prevent substrate deformation while fabricating flexible devices on plastic substrates. Herein, we report a process whereby defect-free graphene is directly synthesized on a variety of substrates via the introduction of an ultrathin Ti catalytic layer, due to the strong affinity of Ti to carbon. Ti with a thickness of 10 nm was naturally oxidized by exposure to air before and after the graphene synthesis, and the various functions of neither the substrates nor the graphene were influenced. This report offers experimental evidence of high-quality graphene synthesis on Ti-coated substrates at 150 °C via CVD. The proposed methodology was applied to the fabrication of flexible and transparent thin-film capacitors with top electrodes of high-quality graphene.

Efficient large-scale preparation of defect-free few-layer graphene using a conjugated ionic liquid as green media and its polyetherimide composite

It is very important to obtain large-scale and high-quality graphene for its application. However, the efficient preparation of large-scale and defect-free graphene is difficult by using a general method. In this report, a conjugated ionic liquid (IL) is designed and synthesized, which is used as a green medium to easily exfoliate graphite into graphene with the assistance of microwave irradiation. The as-prepared IL-functionized graphene nanoplatelets (GNPIL) is few-layer, large-size and defect-free, which is attributed to the strong π-π and cation-π interactions between conjugated IL and graphene layers. And the results show that a relatively high yield of 40% is obtained. The high-quality GNPIL could be dispersed in organic solvent homogeneously and stably with a high content, which is beneficial for its further applications. Furthermore, this GNPIL is used to composite with polyetherimide (PEI) to improve the conductivity of polymer. As compared with pure PEI, a sharp decrease of 11 orders of magnitude in volume resistivity is obtained for PEI composite with only 0.75 wt% GNPIL, which keeps a good mechanical property as well.

Graphene–Solvent Interactions in Nonaqueous Dispersions: 2D ROESY NMR Measurements and Molecular Dynamics Simulations

The liquid phase exfoliation of graphite by sonication in nonaqueous solvents like N-methyl-2-pyrrolidone (NMP) provides a simple and scalable route to dispersions of defect-free graphene sheets. The role of the solvent is crucial to the process; it is the interactions of the solvent with the graphene sheets that prevent agglomeration and stabilize the dispersion. Here, we show that the two-dimensional solution nuclear magnetic resonance (NMR) technique, rotating frame Overhauser effect spectroscopy (ROESY), provides a molecular signature of these interactions in graphene–NMP dispersions. Significant differences are observed in the spectra of the dispersions as compared to the pure solvent. Using classical molecular dynamics simulations, we show that these differences arise because of the induced layering of solvent molecules with reduced rotational mobility in the vicinity of the graphene sheets. The reduced mobility of solvent molecules in the dispersion as compared to the bulk solvent are reflected as differences in their two-dimensional ROESY NMR.

Cobalt phthalocyanine tagged graphene nanoflakes for enhanced electrocatalytic detection of N-acetylcysteine by amperometry method

We have employed surfactant-assisted exfoliation method to prepare defect-free few-layer graphene flakes on a large scale. Various instrumental techniques like FT-IR, Raman, XRD, and FE-SEM were used to characterize the GNF. It can be used as a substrate to immobilize CoPc-treated GNF-based CPE that exhibits an enhanced electrocatalytic oxidation of NAC in 0.1 M PBS (pH 7.0). An enhanced oxidative peak current response was noted at + 0.33 V vs. Ag/AgCl for CoPc@GNF/CPE whereas the poor oxidation behavior was observed in the case of CoPc@rGO/CPE and CoPc/CPE. Electrochemical oxidation of NAC is a diffusion-controlled irreversible process which involves 2H+/2e− redox reaction. DPV and amperometry were employed for the determination of NAC and its linear ranges from 0.03–0.96 and 0.02–0.38 μM with a detection limit of 11 and 6.5 nM, respectively for CoPc@GNF/CPE. The present system can be exploited for quantitative detection of NAC in drug formulations and urine samples.

Effect of high donor number solvent and cathode morphology on interfacial processes in Li-air batteries

The work is focused on the investigation of the effect of solvent and carbon cathode morphology on the performance of Li-air batteries. Molecular dynamics simulation was used to explore the interfacial behavior of the main reactants (O2 and Li+) of the oxygen reduction reaction in high donor number solvent dimethyl sulfoxide (DMSO) at the following carbon surfaces: graphene plane, graphene edge, nanotube. It was shown that the adsorption barrier of O2 molecules decreases in the order graphene plane > nanotube > graphene edge, leading to the fastest adsorption kinetics on graphene edges. Strong solvation of Li+ in DMSO prevents ions adsorption on defect-free graphene planes and nanotubes, which is qualitatively different from low donor number solvents, such as acetonitrile. It can be concluded from these results, that nucleation and growth of discharge products in DMSO is shifted from the surface towards the solvent bulk that, in turn, leads to capacity increase of Li-air batteries.

Carbon Precursor Structures and Graphene on Palladium Nanoparticles

Due to their stable catalytic properties graphene-encapsulated metal nanoparticles (NPs) have come into the focus of heterogeneous catalysis. Research on graphene’s atomic structure is a prerequisite for understanding and controlling catalytic processes; however, it is still at its very beginning considering metal NPs. This atomic-scale scanning tunnelling microscopy study documents preliminary growth stages and the structural properties of graphene on highly oriented pyrolytic graphite (HOPG) supported palladium NPs. It is shown that an annealing of the NPs at ∼650 °C in a few tens of Langmuir of ethylene (ethene) leads first to a carbon precursor structure covering the NP’s (111) facets. At higher ethylene exposures, the precursor structure turns into single-layer graphene encapsulating the NPs. Graphene-encapsulated palladium NPs with a size larger than ∼30 nm exhibit mostly one single and an almost defect-free graphene sheet on a facet. In most of the cases, such a perfect growth does not take place o...

Chemistry below graphene: Decoupling epitaxial graphene from metals by potential-controlled electrochemical oxidation

While high-quality defect-free epitaxial graphene can be efficiently grown on metal substrates, strong interaction with the supporting metal quenches its outstanding properties. Thus, protocols to transfer graphene to insulating substrates are obligatory, and these often severely impair graphene properties by the introduction of structural or chemical defects. Here we describe a simple and easily scalable general methodology to structurally and electronically decouple epitaxial graphene from Pt(111) and Ir(111) metal surfaces. A multi-technique characterization combined with ab-initio calculations was employed to fully explain the different steps involved in the process. It was shown that, after a controlled electrochemical oxidation process, a single-atom thick metal-hydroxide layer intercalates below graphene, decoupling it from the metal substrate. This decoupling process occurs without disrupting the morphology and electronic properties of graphene. The results suggest that suitably optimized electrochemical treatments may provide effective alternatives to current transfer protocols for graphene and other 2D materials on diverse metal surfaces.

Role of Cu foil in-situ annealing in controlling the size and thickness of CVD graphene domains

Producing graphene with minimal crystalline defects and a controllable number of layers is highly desirable for its integration in advanced technological applications. Here we show how the chemical vapor deposition (CVD) protocol can be adjusted in order to control the size, the shape and the thickness of graphene domains. More particularly, this work focuses on the correlation between the conditions employed during the Cu foil in-situ thermal treatment and the subsequent graphene growth. The influence of Cu pre-growth treatments has been systematically investigated by considering two different pressure regimes (1 and 800 mbar) and various gas compositions (pure Ar, Ar/H2 and Ar/O2 mixtures) while using identical graphene growth conditions. We show that exposing the Cu foil to oxygen, either present as residuals in the Ar feedstock or precisely dosed using the Ar/O2 gas line, effectively contributes to reduce the catalyst carbon content prior to graphene growth and hence significantly decreases the nucleation site density. It is also found that annealing the Cu catalyst in a hydrogen-free environment favors the formation of defect-free multi-layer graphene branches that extend underneath the millimeter-size single-crystalline graphene top layer. Our results provide clear guidelines to control the formation of graphene nuclei and additional layers.

Long-Term Corrosion Protection of a Cupro-Nickel Alloy Due to Graphene Coating

This study demonstrates the corrosion resistance of a Cu-Ni alloy coated with a multi-layer graphene. This is one of the first demonstrations of long-term corrosion resistance due to graphene coating, which is crucial since earlier studies have suggested graphene-coated copper to be considerably inferior to bare copper in terms of corrosion during long-term exposure to a corrosive environment. The inferior corrosion resistance of graphene-coated copper arises due to defects and poor surface coverage by graphene. Since it is prohibitively difficult to develop defect-free graphene on metals at a commercially feasible scale, this study investigated the hypothesis for an alternative approach, i.e., to develop multi-layer graphene with a reasonable assumption that the areas of defects/poor coverage of a layer will be masked by the subsequent over-layer(s). This study has validated this hypothesis. Electrochemical investigations have demonstrated multi-layer graphene to improve the corrosion resistance of a Cu-Ni alloy by an order of magnitude. However, the most striking finding of this study is that the improvement in corrosion resistance due to the multi-layer graphene coating sustained the entire duration of a long-term test (~350 h).

Top-Down, Scalable Graphene Sheets Production: It Is All about the Precipitate

Graphene production has been widely explored and developed in the past decade. Most research has aimed at developing scalable, environmentally friendly and cheap procedures to produce defect-free graphene that can be used in a variety of applications such as mechanical properties enhancement and energy storage. Top-down graphene production approaches (from graphite) in liquid, which include high-shear mixing and sonication, were recently scaled up. Nevertheless, their production yields have remained low (<5%) due to the need for stabilization of the graphene in dispersion (supernatant), while the precipitate has usually not been considered as graphene product. In this study, we focus on the liquid-based approach of sonication, from lab to industrial scale, in an attempt to improve the graphene yield. Unlike previous studies, we explore and characterize the product both in the supernatant and in the precipitate. We found that above a certain critical energy all graphite flakes were exfoliated into graphene...

Stress concentrations in nanoscale defective graphene

Defect-free graphene nanosheets are the strongest material known but manufactured graphene tends to contain flaws of different forms and dimensions, leading to the degradation of mechanical performance. Here we report a quantitative mechanical approach to quantitatively evaluate the influence of defects within exfoliated pristine graphene sheets. Results indicate stress concentrations around defects within graphene sheets that lower strength. The description of stress concentration broadly follows a Griffith strength approach for continuum materials, despite the non-continuum structure of graphene, but has little impact on the Young’s modulus.