Graphene In Solution Research Articles

The role of solvent interfacial structural ordering in maintaining stable graphene dispersions

Liquid phase exfoliation is the most promising method for the low-cost, scalable production of two-dimensional nanosheets from their bulk counterparts. Extensive exfoliation occurs in most solvents due to the huge amount of energy introduced by sonication or shear mixing. However, the subsequent dispersion is not always stable, with extensive reaggregation occurring in some solvents. Identifying the optimal solvent for a particular layered material is difficult and requires a fundamental understanding of the mechanism involved in maintaining a stable dispersion. Here, we use molecular dynamics calculations to show that when graphene is immersed in a solvent, distinct solvation layers are formed irrespective of the choice of solvent and their formation is energetically favourable for all considered solvents. However, energetic considerations such as these do not explain the experimental solvent-dependence of the dispersion concentration. Instead, we find that solvents with high diffusion coefficients parallel to the graphene layer result in the lowest experimental concentration of graphene in solution. This can be explained by the enhanced ease of reaggregation in these solvents. Solvents with smaller diffusion coefficients result in higher experimental graphene concentrations as reaggregation is prevented. In the low diffusion limit, however, this relationship breaks down. We suggest that here the concentration of graphene in solution depends primarily on the separation efficiency of the initial exfoliation step. Based on this, we predict that the concentration of exfoliated graphene in solvents such as benzaldehyde and quinoline, which have low diffusion constants, can be increased dramatically by careful tuning of the experimental sonication parameters.

Graphene's effect on the mechanism of radical polymerization with in situ graphene composites

Polymerizing a monomer in the presence of graphene/graphite is a common approach to making composites. However, the role played by the graphitic carbon in the polymerization is often poorly understood or neglected. Specifically, the polymer formed at the interface between the graphite and filler is expected to be most affected by the presence of the filler and can have an outsized effect on the composite properties. Herein we study the impact of pristine graphene on the polymerization of styrene during the synthesis of styrene/graphene nanocomposites. Although there is reason to expect that pristine graphene will interact with radicals, the difficulty of suspending graphene in solution has meant that experimental investigations have been limited to graphene nanoplatelets produced from microcrystalline graphite. However, microcrystalline graphite is fundamentally different than flake graphite, possessing more defects and sheets of limited lateral dimension. We overcome this challenge and study graphene obtained from natural flake graphite using interfacial exfoliation and graphene trapping. We find that graphene plays an active role in radical polymerization, acting as a chain transfer agent. At high graphite/graphene loading, it reversibly couples growing chains, lowering the value of the dispersity (Ð) relative to controls without graphene by nearly an order of magnitude.

Improvement of anticorrosion properties of waterborne coatings by designing a new type of modified graphene with high dispersion and low conductivity

Gallic acid modified and reduced graphene coated by polydopamine (DM-GO) is synthesized. The results show that gallic acid can reduce graphene oxide and promote the stable dispersion of graphene in solution, and repair some defects of graphene surface through π-π interactions. Polydopamine can effectively mitigate serious corrosion caused by graphene due to its high electrical conductivity. Because of the barrier effect of well-dispersed graphene and the presence of polydopamine, the addition of DM-GO into waterborne coating leads to the significant improvement of the adhesion and long-term corrosion resistance of coatings.

Reliable fabrication of transparent conducting films by cascade centrifugation and Langmuir-Blodgett deposition of electrochemically exfoliated graphene.

Electrochemical exfoliation is an efficient and scalable method to obtain liquid-phase graphene. Graphene in solution, obtained through electrochemical exfoliation or other methods, is typically polydisperse, containing particles of various sizes, which is not optimal for applications. We employed cascade centrifugation to select specific particle sizes in solution and prepared thin films from those graphene particles using the Langmuir–Blodgett assembly. Employing centrifugation speeds of 3, 4, and 5 krpm, further diluting the solutions in different volumes of solvent, we reliably and consistently obtained films of tunable thickness. We show that there is a limit to how thin these films can be, which is imposed by the percolation threshold. The percolation threshold is quantitatively compared to results found in literature that are obtained using other, more complex graphene film fabrication methods, and is found to occur with a percolation exponent and percolative figure of merit that are of the same order as results in literature. A maximum optical transparency of 82.4% at a wavelength of 660 nm is obtained for these films, which is in agreement with earlier works on Langmuir–Blodgett assembled ultrasonic-assisted liquid-phase exfoliated graphene. Our work demonstrates that films that are in all respects on par with films of graphene obtained through other solution-based processes can be produced from inexpensive and widely available centrifugal post-processing of existing commercially available solutions of electrochemically exfoliated graphene. The demonstrated methodology will lower the entry barriers for new research and industrial uses, since it allows researchers with no exfoliation experience to make use of widely available graphene materials.

Alkalide-Assisted Direct Electron Injection for the Noninvasive n-Type Doping of Graphene.

Although the doping of graphene grown by chemical vapor deposition is crucial in graphene-based electronics, noninvasive methods of n-type doping have not been widely investigated in comparison with p-type doping methods. We developed a convenient and robust method for the noninvasive n-type doping of graphene, wherein electrons are directly injected from sodium anions into the graphene. This method involves immersing the graphene in solutions of [K(15-crown-5)2]Na prepared by dissolving a sodium-potassium (NaK) alloy in a 15-crown-5 solution. The n-type doping of the graphene was confirmed by downshifted G and 2D bands in Raman spectra and by the Dirac point shifting to a negative voltage. The electron-injected graphene showed no sign of structural damage, exhibited higher carrier mobilities than that of pristine graphene, and remained n-doped for over a month of storage in air. In addition, we demonstrated that electron injection enhances noncovalent interactions between graphene and metallomacrocycle molecules without requiring a linker, as used in previous studies, suggesting several potential applications of the method in modifying graphene with various functionalities.

PVP-assisted laser ablation growth of Ag nanocubes anchored on reduced graphene oxide (rGO) for efficient photocatalytic CO2 reduction

Metallic nanoparticles loaded graphene nanocomposites have been widely studied for various scientific and technological applications. In this study, a facile method was reported to realize a straightforward growth of shape and size-controllable of metallic nanoparticles, and the subsequent hybridization with graphene in solution by strategically coupling wet-chemical route and laser ablation. By mixing graphene oxide (GO) with a tunable concentration level of polyvinylpyrrolidone (PVP) in aqueous solution, Ag nanocubes with a face-centered cubic crystal structure were generated by pulsed laser ablation and then mounted on GO nanosheets with the assistance of PVP. The preferential adsorption of PVP to Ag (100) crystal face led to the production of Ag nanocubes with exposed (100) facet. The result showed that the morphological yield of spherical particles decreased with the increase in PVP concentration. X-ray diffraction (XRD) and UV–visible spectroscopy analyses confirmed that GO was partially reduced. In the reduction of CO2 gas, the photocatalytic conversion rate could achieve 133.1 μmol g−1 h−1 in 6 hrs for cubic Ag-loaded reduced GO composites.

Herbally Painted Biofunctional Scaffolds with Improved Osteoinductivity for Bone Tissue Engineering

In the bone tissue engineering composite scaffolds with osteogenic potential are emerging as the new tool. Here, we investigated the graphene (GP), graphene oxide (GO) andCissusquadrangularis(CQ) callus extract for their spontaneous osteoinductive potential. Electrospun poly ε-caprolactone (PCL) sheets were painted with varying combination GP, GO and CQ solutions as ink. The prepared PCL-GO, PCL-GO-CQ, PCL-GP and PCL-GP-CQ scaffolds were characterized for their physical, mechanical and biological properties. Addition of GO, GP, GO-CQ and GP-CQ to PCL enhanced roughness, wettability, Yield strength and tensile strength, biocompatibility .significantly. Presence of GO and CQ in PCL-GO-CQ scaffolds, while GP and CQ in PCL-GP-CQ scaffolds showed synergistic effect on the biocompatibility, Cell attachment,cell proliferation of human umbilical Wharton’s jelly derived mesenchymal stem cells (hUCMSCs) and their differentiation into osteoblasts by 21stday in culture without osteogenic differentiation media or any growth factors. Same is confirmed by the Alizarin red S staining and Von kossa staining. The combination of PCL-GO-CQ scaffold prepared by novel paint method was found to be the most potential in bone tissue engineering.

Human Immune Protein C1q Selectively Disaggregates Carbon Nanotubes.

We atomistically compute the change in free energy upon binding of the globular domain of the complement protein C1q to carbon nanotubes (CNTs) and graphene in solution. Our modeling results imply that C1q is able to disaggregate and disperse bundles of large diameter multiwalled CNTs but not those of thin single-walled CNTs, and we validate this prediction with experimental observations. The results support the view of a strong binding with potential implications for the understanding of the immune response and biomedical applications of graphitic nanomaterials.

Click and Patterned Functionalization of Graphene by Diels-Alder Reaction.

Chemical functionalization is a promising approach to controllably manipulate the characteristics of graphene. Here, we designed cis-dienes, featuring two dihydronaphthalene backbones, to decorate a graphene surface via Diels-Alder (DA) click reaction. The installation of a diene moiety into a nonplanar molecular structure to form cis-conformation enables a rapid (∼5 min) DA reaction between graphene and diene groups. Patterned graphene of sub-micrometer resolution can be obtained by easily soaking poly(methyl methacrylate)-masked graphene in solution of hydroxyl-substituted cis-diene at room temperature. The functionalization degree can be further controlled by carrying out the reaction at higher temperature. The present result gives important insight into the effect of molecular conformation on the graphene functionalization process, and provides an effective and facile method for graphene functionalization.

Acoustic carrier transportation induced by surface acoustic waves in graphene in solution

The acoustic charge transportation induced by surface acoustic wave (SAW) propagation in graphene in solution was investigated. The sign of acoustic current (IA) was found to switch when crossing the Dirac point because the major carrier was transitioned from holes to electrons by the change in electrolyte-gate voltage. IA also exhibited a peak value under conditions of both hole and electron conduction. These results can be explained on the basis of a change in the type of major carrier in graphene, as well as a change in the carrier mobility of graphene.

Synthesis and characterization of composite membranes made of graphene and polymers of intrinsic microporosity

Polymers of intrinsic microporosity (PIMs) are a group of polymers with molecular sieve behaviour due to their rigid, contorted macromolecular backbones. They show great potential in organophilic pervaporation, solvent-resistant nanofiltration and gas and vapour separations. However, they are susceptible to physical ageing, leading to a reduction in permeability over time. An improvement in membrane permeability, control over diffusion selectivity and a reduction of the effect of physical ageing is expected by adding graphene as a nanofiller.Little is experimentally known about how the material disperses in the polymer. Here we used Raman spectroscopy, scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to study the composite membrane's structure. Our results show that both STEM and Raman spectroscopy are able to identify the presence of graphene-based material in the composite. We show that STEM, through medium angle annular dark field (MAADF) or EELS imaging, can be exploited to obtain information on the morphology and the thickness of the flakes. Our results indicate that there is strong re-agglomeration of initially exfoliated graphene in solution when forming the composite. This is expected to produce strong changes in the mechanical properties and the physical ageing of the membrane.

Tracking graphene by fluorescence imaging: a tool for detecting multiple populations of graphene in solution.

Most methods used for the characterization of graphene produced by liquid phase exfoliation require the deposition of the liquid sample on a substrate and subsequent drying. Because of this or other post-synthetic treatments, the reliability of the data in describing the actual features of the graphene particles in the pristine solution becomes questionable. Hence there is a need for new methods that permit the study of graphene directly in solution. Fluorescence imaging is at present the most convenient and sensitive method to visualize nanosized objects in solution. Here we report the development of a new method for visualizing and tracking exfoliated graphene directly in solution using a conventional set-up for fluorescence microscopy. We functionalized a fluorescent surfactant typically used for exfoliating graphite in aqueous phase (Pluronic P123) with two different fluorophores, in order to make graphene detectable by fluorescence microscopy. The photophysical interactions between the fluorescent surfactant and graphene were investigated at the bulk level. Finally, fluorescence microscopy allowed us to track the carbon particles produced and to identify two different populations of particles with sizes of 265 ± 25 and 1100 ± 200 nm respectively. The correlation of these results with TEM and DLS data is discussed.

Magnetic Graphene-Dispersive Solid-Phase Extraction for Preconcentration and Determination of Lead and Cadmium in Dairy Products and Water Samples.

A magnetic-dispersive solid-phase extraction (MDSPE) was used for precocentration of Pb(II) and Cd(II) in milk, yoghurt and water samples. An appropriate amount of suspension containing the magnetic graphene and Triton X-114 was injected rapidly into the aqueous sample by a syringe. Triton X-114 was used to achieve stable suspension of graphene in solution. The structure of the resulting products was confirmed by Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) spectrometry. The effects of various parameters were studied. A detection limit of 0.16 and 0.50µgL(-1) for Cd(II) and Pb(II) was obtained, respectively. The relative standard deviations (RSDs, n=10) of 50µgL(-1) of Pb and Cd were 3.3% and 2.1%, respectively. The results indicated that the present method can be reliably used for determination of Pb(II) and Cd(II) in dairy products and water samples with good recoveries.

Fabrication of Potentiometric Enzymatic Glucose Biosensor Based on Graphene and Magnetic Beads

In this paper, ruthenium dioxide (RuO2) electrodes were fabricated by radio frequency sputtering and screen-printed technique, and RuO2 sensing films could be as pH sensor to measure different pH solutions. The average sensitivity and linearity of pH sensor were 51.847 mV/pH and 0.987, respectively. A glucose biosensor could be prepared through casting glucose oxidase (GOx) on RuO2 electrode, so the flexible arrayed glucose biosensors modified by magnetic beads (MBs) and graphene (GR) were developed in this paper. In preparation processes, GR solution was first cast on the RuO2, then we introduced carbodiimide hydrochloride (EDC) to connect glucose oxidase (GOx) and MB by covalent bond. Finally, MBs-GOx-nafion composite was dropped on RuO2 to complete preparation of the RuO2/GR/MB-GOx-Nafion glucose biosensor. The electrochemical impedance spectra (EIS) was used to investigate the capability of electron transfer of the different electrodes, and it was found that the additions of MBs and GR which had excellent electric conductivity could decrease the electron transfer resistance ( $R_{\rm ct}$ ) from the experiment results. Besides, EIS could also be used to demonstrate whether the glucose biosensors had already been completely modified by MBs and GR through the different resistances of the biosensors. Furthermore, the glucose biosensor modified by MBs and GR had the superior performance, the average sensitivity and linearity of RuO2/GR/MB-GOx-Nafion glucose biosensor were 10.628 mV/mM and 0.998, respectively. After 28 days, RuO2/GR/MB-GOx-Nafion glucose biosensor still had relative sensitivity of 82.2%.

Noncovalently Functionalized Graphene-Directed Synthesis of Ultralarge Graphene-Based TiO2 Nanosheet Composites: Tunable Morphology and Photocatalytic Applications

Ultralarge graphene-based TiO2 nanosheet composites are successfully fabricated by a noncovalent functionalization approach with use of benzyl alcohol as the linking agent. In the synthetic procedure, the aromatic molecules of benzyl alcohol direct themselves onto graphene (GR) surface via π–π interaction. Therefore, the basal planes of GR nanosheets are uniformly functionalized with hydroxyl groups derived from benzyl alcohol, which not only improves the dispersion of GR in solution but also induces a finely homogeneous coating of TiO2 nanocrystals onto the surface of GR nanosheets. The resulting GR@TiO2 nanocomposites, which feature unique ultralarge 2D sheet-like morphology with the lateral size far larger than the original GR and densely interfacial contact, are able to act as highly active photocatalysts toward selective reduction of aromatic nitro compounds to amines in water under ambient conditions. The higher photoactivity of GR@TiO2 than blank TiO2 is attributed to the efficient charge carriers separation and transfer by the GR platform. It is hoped that the facile synthesis strategy in this work could contribute to fabricating other ultralarge functional GR-based 2D sheet-onto-sheet composites with tunable morphology toward target photocatalytic applications.

Graphene-Sandwiched LiNi0.5Mn1.5O4 Cathode Composites for Enhanced High Voltage Performance in Li Ion Batteries

Graphene was used to form efficient conducting networks and to suppress the formation of a solid-electrolyte interface layer, in a sandwiched composite with LiNi0.5Mn1.5O4 (LNMO), which resulted in an improvement in rate capability and coulombic efficiency, respectively. The interaction between the LNMO particles and the residual oxygen functionalities on the basal plane of graphene in solution spontaneously produced LNMO particles that were sandwiched by the graphene. This configuration provided LNMO with less polarization due to higher electronic conductivity, which contributed to an increased rate capability. The graphene-sandwiched LNMO also enhanced a coulombic efficiency due to the suppression of continuous electrolyte decomposition, which eventually resulted in improved capacity retention. Unlike this graphene-sandwiched LNMO, a simple mixture of LNMO with graphene did not show noticeable improvements in rate capability due to a lack of efficient conducting network. A LiMn2O4/graphene composite prepared using the same procedure showed no improvement in coulombic efficiency and, thus, capacity retention was similar to graphene-free LiMn2O4, indicating that the graphene-sandwiching is an effective strategy for high-voltage cathode materials.

DFT modeling of the covalent functionalization of graphene: from ideal to realistic models

The production of multiple types of graphene, such as free standing, epitaxial graphene on silicon carbide and metals, graphene in solution, chemically grown graphene-like molecules, various graphene nanoribbons, and graphene oxide with different levels of reduction and various chemical composition, demonstrate the need for additional investigation beyond the basic principles of graphene functionalization for avoidance of occasional contradictions between the predictions from first-principles simulations and experimental results. Herein, we discuss the current state of modeling of the different types of graphene using density functional theory (DFT) methods. We focus on the static (substrate, shape, curvature, strain and doping) and dynamic (starting point of functionalization, migration barriers and stability of configurations) aspects that provide a more correct and selective modeling of the chemisorption of various chemical species on the graphene scaffold. Based on the recent modeling of experimentally realized functionalization of different types of graphene we can conclude that the formation of uniform one- or two-sided functionalized graphene discussed in earlier studies is an exception to the typical scenarios of graphene chemistry. The presence of different substrates, defects and lattice distortions, such as ripples and strain, results in the formation of clusters or lines from the functional groups. Several configurations of the chemical species on the graphene substrate have been found to exist with ideal models but are only stable for graphene functionalized under special conditions. The role of the technical parameters, such as the functionals and computational methods employed, and the important guidelines for the modeling of the chemical properties of various types of graphene are also discussed.

Solution-processed graphene materials and composites

Abstract

Deconstructing Graphite: Graphenide Solutions

Growing interest in graphene over past few years has prompted researchers to find new routes for producing this material other than mechanical exfoliation or growth from silicon carbide. Chemical vapor deposition on metallic substrates now allows researchers to produce continuous graphene films over large areas. In parallel, researchers will need liquid, large scale, formulations of graphene to produce functional graphene materials that take advantage of graphene's mechanical, electrical, and barrier properties. In this Account, we describe methods for creating graphene solutions from graphite. Graphite provides a cheap source of carbon, but graphite is insoluble. With extensive sonication, it can be dispersed in organic solvents or water with adequate additives. Nevertheless, this process usually creates cracks and defects in the graphite. On the other hand, graphite intercalation compounds (GICs) provide a means to dissolve rather than disperse graphite. GICS can be obtained through the reaction of alkali metals with graphite. These compounds are a source of graphenide salts and also serve as an excellent electronic model of graphene due to the decoupling between graphene layers. The graphenide macroions, negatively charged graphene sheets, form supple two-dimensional polyelectrolytes that spontaneously dissolve in some organic solvents. The entropic gain from the dissolution of counterions and the increased degrees of freedom of graphene in solution drives this process. Notably, we can obtain graphenide solutions in easily processable solvents with low boiling points such as tetrahydrofuran or cyclopentylmethylether. We performed a statistical analysis of high resolution transmission electronic micrographs of graphene sheets deposited on grids from GICs solution to show that the dissolved material has been fully exfoliated. The thickness distribution peaks with single layers and includes a few double- or triple-layer objects. Light scattering analysis of the solutions shows the presence of two-dimensional objects. The typical size of the dissolved flakes can be determined by either static or dynamic light scattering (DLS) using models available in the literature for disk-shape objects. A mean lateral size of ca. 1 μm is typically observed. We also used DLS to monitor the reaggregation that occurs as these sensitive solutions are exposed to air. The graphenide solutions reported in this Account can be used to deposit random arrays of graphene flakes and large single flakes of a lateral size of tens of micrometers onto different substrates. Using the graphenide solutions described in this Account, we foresee the large-scale production of graphene-based printings, coatings, and composites.

Single-step exfoliation and chemical functionalisation of graphene and hBN nanosheets with nickel phthalocyanine

Versatile routes to functionalise few-layer graphene and hBN with nickel phthalocyanine (Ni-Pc) were achieved using liquid phase exfoliation and sonication methods. EDX performed on the graphene//Ni-Pc specimen showed nickel on the flake surface whilst Raman spectroscopy revealed a prominent D peak indicating the presence of basal plane defects. New Raman active modes were also found in the hBN//Ni-Pc complex. X-ray photoelectron spectroscopy showed a charge transfer for both graphene and hBN confirming that the flake edges and basal-planes were covalently functionalised by the phthalocyanine molecules. Transmission electron microscopy confirmed the only presence of single and few-layer flakes of hBN and graphene in solution, demonstrating that the exfoliation yield was not affected by the functionalisation step. We therefore proved that tuning of the electronic and optical properties of graphene and hBN nanosheets is indeed conceivable. We used the Z-scan technique to prove the nonlinear optical (NLO) behaviour of the functionalised graphene sheets. Based on such optical response, we demonstrate an optical limiting effect for nanosecond laser pulses at 532 nm, proving these materials to be a suitable candidate for photonic and optoelectronic applications.