Chemically Modified Graphene Research Articles

Hygroscopic Behavior of Polypropylene Nanocomposites Filled with Graphene Functionalized by Alkylated Chains.

Owing to stringent international environmental and fuel efficiency requirements for lightweight automotive systems, polymer composites have attracted widespread attention. Polypropylene (PP) is a widely employed commercial polymer because of its lightweight and low cost. In this study, PP nanocomposites were fabricated to reduce the moisture absorption of PP composites in automotive headlamp housings. Alkylated chemically modified graphene (CMG-R) was synthesized to reduce the surface hydrophilicity of graphene and increase compatibility with the PP matrix. Fourier transform-infrared spectroscopy and scanning electron microscopy were performed to analyze the nanofillers. X-ray diffraction was performed to determine the interlayer spacing of the nanofiller resulting from surface treatment. Differential scanning calorimetry was used to analyze the crystallinity of the nanocomposites. The results indicated that the improved hydrophobicity of the nanofiller due to alkylation reduced the maximum moisture absorption of the PP nanocomposites by 15% compared to PP composites. The findings of this study are useful for reducing fogging in automotive headlamps.

Manipulating the elasticity of chemically modified graphene aerogel through water surface plasticization

Self-assembled chemically modified graphene (CMG) aerogels with high compressive elasticity are important materials because of their unique properties and broad applications, but there is a lack of guiding principles for the design of elastic CMG aerogels. In this work, we provide new insights into the elasticity of chemical modified graphene aerogels. We demonstrate that the self-assembled CMG aerogels have intrinsic compressive elasticity, and report a reversible transition of CMG aerogels from elasticity to plasticity caused by water adsorption. Experimental data and molecular dynamics simulations show that water molecules on the surface of the ultrathin CMG cell walls in the aerogel form a hydrogen bond network, which hinders the shape recovery of the bent cell walls and plasticize the aerogel. A general method is then developed to enhance the elasticity of CMG aerogel by reducing its surface hydrophilicity with thermal treatment. The method efficiently imparts good elasticity to aerogels prepared by conventional hydrothermal methods. This work clarifies the important influence of water surface plasticization on the elasticity of CMG aerogels, and provides principles for the design of highly elastic aerogels based on low dimensional nanomaterials.

Modeling of One-Side Surface Modifications of Graphene.

We model, with the use of the force field method, the dependence of mechanical conformations of graphene sheets, located on flat substrates, on the density of unilateral (one-side) attachment of hydrogen, fluorine or chlorine atoms to them. It is shown that a chemically-modified graphene sheet can take four main forms on a flat substrate: the form of a flat sheet located parallel to the surface of the substrate, the form of convex sheet partially detached from the substrate with bent edges adjacent to the substrate, and the form of a single and double roll on the substrate. On the surface of crystalline graphite, the flat form of the sheet is lowest in energy for hydrogenation density , fluorination density , and chlorination density . For higher attachment densities, the flat form of the graphene sheet becomes unstable. The surface of crystalline nickel has higher adsorption energy for graphene monolayer and the flat form of a chemically modified sheet on such a substrate is lowest in energy for hydrogenation density , fluorination density and chlorination density .

Paper-Based Flexible Electrode Using Chemically-Modified Graphene and Functionalized Multiwalled Carbon Nanotube Composites for Electrophysiological Signal Sensing

Flexible paper-based physiological sensor electrodes were developed using chemically-modified graphene (CG) and carboxylic-functionalized multiwalled carbon nanotube composites (f@MWCNTs). A solvothermal process with additional treatment was conducted to synthesize CG and f@MWCNTs to make CG-f@MWCNT composites. The composite was sonicated in an appropriate solvent to make a uniform suspension, and then it was drop cast on a nylon membrane in a vacuum filter. A number of batches (0%~35% f@MWCNTs) were prepared to investigate the performance of the physical characteristics. The 25% f@MWCNT-loaded composite showed the best adhesion on the paper substrate. The surface topography and chemical bonding of the proposed CG-f@MWCNT electrodes were characterized by scanning electron microscopy (SEM) and Raman spectroscopy, respectively. The average sheet resistance of the 25% CG-f@MWCNT electrode was determined to be 75 Ω/□, and it showed a skin contact impedance of 45.12 kΩ at 100 Hz. Electrocardiogram (ECG) signals were recorded from the chest and fingertips of healthy adults using the proposed electrodes. The CG-f@MWCNT electrodes demonstrated comfortability and a high sensitivity for electrocardiogram signal detection.

Photoluminescent graphene oxide porous particles in solution under environmental conditions produced by hydrothermal treatment

Photoluminescent graphene oxide particles (PGOPs) were synthesized by hydrothermal treatment. The relative transmission spectra of the PGOPs in the infrared region between 800 cm−1 and 4000 cm−1 showed frequencies that characterize chemical species such as ethers (CO), ketones (CO), carboxylic acids (COOH), double bonds of carbon (CC) and water (H2O). The emission wavelengths were 568 nm for graphene oxide with green photoluminescence (GOGP) and 634 nm for graphene oxide with orange photoluminescence (GOOP) when excited with a light source of 375 nm. Moreover, the detection of light emission at 600 nm in the photoluminescent excitation (PLE) spectrum is detected with more intensity in GOGP than GOOP due to the presence of groups CC and carboxylates ions (COO−) linked to graphene (GN) and the photon effective emission time according to the decay profile for each of them was 9.65 ns and 10.85 ns respectively. PGOPs films made by drop casting at room conditions were observed by Atomic Force Microscopy (AFM) and Scanning Electronic Microscopy (SEM). These films acquire a wrinkled and porous shapes with a pore diameter of 51 nm and roughness (Ra) of Chemically Modified Graphene (CMG) sheets equal to 0.14 nm for GOOP and 0.21 nm for GOGP.

Custom-Made Chemically Modified Graphene Oxide to Improve the Anti-Scratch Resistance of Urethane-Acrylate Transparent Coatings

In this work, a thermoset ultraviolet (UV)-cured polyurethane-acrylate resin was doped with different chemically-modified graphene obtained from a commercial graphene oxide (GO): as-received GO, chemically reduced GO (rGO), GO functionalized with vinyltriethoxysilane (VTES) (GOvtes), and GO functionalized with VTES and subsequently reduced with a chemical agent (rGOvtes). Modified graphene was introduced in the oligomer component via solvent-assisted process using acetone, which was recovered after completion of the process. Results indicate that the GO-doped oligomers produce cured coatings with improved anti-scratch resistance (above the resistance of conventional coatings), without surface defects and high transparency. The anti-scratch resistance was measured with atomic force microscopy (AFM). Additionally, results are presented in terms of Wolf–Wilburn scale, a straightforward method widely accepted and employed in the coating industry.

Seed-mediated growth of platinum nanoparticles anchored on chemically modified graphene and cationic polyelectrolyte composites for electrochemical multi-sensing applications

In this study, a hydrothermally treated chemically modified graphene (CG) and cationic polyelectrolyte (poly(diallyldimethylammonium chloride, PDDA) composite was successfully synthesized by using an ultrasonication technique. Several batches were prepared and tested for optimal loading of PDDA on the CG surface and for optimization of the electrochemical characteristics. The 0.2% of PDDA loading composite showed better response than the other composites and was used to fabricate a multi-sensing electrode. Platinum nanoparticles (PtNPs) were anchored on the composite-modified working electrode by using the seed-mediated growth method to fabricate a highly selective and sensitive sensor for ascorbic acid (AA), dopamine (DA) and uric acid (UA) detection. The as-fabricated hybrid electrode sensor showed excellent catalytic activity with high sensitivity of 200.65, 635.48 and 88.11 μA/mMcm2, wide detection range of 0.001–3.4, 0.0005-0.31 and 0.001-0.62 mM and short response time of 4, 4 and 5 s, for AA, DA and UA, respectively. The limits of detection, stability and reproducibility of the multi-sensors were also examined. In addition, biomolecule levels in human urine were investigated for the practical assessment of the proposed electrochemical sensor. The obtained results imply that the nanostructured electrode with high surface area and electrocatalytic activity offers great commitment for use in nonenzymatic electrochemical sensor applications.

Structural integrity versus lateral size: Enhancing graphene-based film materials by reducing planar defects rather than flake boundary

Chemically modified graphene (CMG) bulk films have displayed a large potential in a plethora of applications. Incorporation of large CMG sheets was applied as the major strategy to improve both the electrical and mechanical properties of resulted film materials, whereas the low fabrication yield of high quality large building blocks severely obstruct its scalability. Herein, we evaluated the individual impact of sheet size and structural integrity of CMG building blocks on the performance of film materials, and results show that low structural defects density rather than large size dominates the enhancement of performance. Compared with large building blocks (average area 37.2 μm2), small ones (average area 1.3 μm2) with optimized structural integrity and also higher preparation yield formed films with even higher strength and conductivity (increased by 55% and 120%, respectively). Furthermore, the influence of structural integrity on the material performance was studied via detailed characterization. These results unequivocally impart the importance of structural integrity, and may give instruction on the rational design of materials with excellent performance.

Self-Assembled Graphene-Based Architectures and Their Applications.

Due to unique planar structures and remarkable thermal, electronic, and mechanical properties, chemically modified graphenes (CMGs) such as graphene oxides, reduced graphene oxides, and the related derivatives are recognized as the attractive building blocks for “bottom‐up” nanotechnology, while self‐assembly of CMGs has emerged as one of the most promising approaches to construct advanced functional materials/systems based on graphene. By virtue of a variety of noncovalent forces like hydrogen bonding, van der Waals interaction, metal‐to‐ligand bonds, electrostatic attraction, hydrophobic–hydrophilic interactions, and π–π interactions, the CMGs bearing various functional groups are highly desirable for the assemblies with themselves and a variety of organic and/or inorganic species which can yield various hierarchical nanostructures and macroscopic composites endowed with unique structures, properties, and functions for widespread technological applications such as electronics, optoelectronics, electrocatalysis/photocatalysis, environment, and energy storage and conversion. In this review, significant recent advances concerning the self‐assembly of CMGs are summarized, and the broad applications of self‐assembled graphene‐based materials as well as some future opportunities and challenges in this vibrant area are elucidated.

Single-step self-assembly of multilayer graphene based dielectric nanostructures

We present high performance multilayer organic/inorganic nanostructured dielectric films including well dispersed chemically modified graphene (CMG) charge polarization layers based on single step solution self-assembly. Multilayer film structures of alternating aluminosilicate and amphiphilic block copolymer (BCP) lamellar layers intercalated with CMG in the hydrophilic aluminosilicate domains are prepared by simple solution casting process. The resultant composite dielectric film attains a high dielectric constant (∼30) along with low dielectric loss (<0.3). The increase of dielectric constant originates from the space charge polarization effect by high aspect ratio CMG, which is also precisely adjustable with the CMG composition in the composite films. Significantly, the dielectric loss rarely increased even with the injection of a large amount of conductive CMG fillers, as CMG plates are exclusively distributed in the hydrophilic lamellar domains without forming conductive connection in the film normal direction across hydrophobic domains. The composite dielectric film also demonstrates mechanically flexible and highly stable features while deposited on conventional flexible PEN substrates.

Fabrication of a gold nanocage/graphene nanoscale platform for electrocatalytic detection of hydrazine

In this study, we report a novel graphene decorated gold nanocage nanocomposite through thermal modification route as a hydrazine sensor. Gold nanocages were synthesized, using the galvanic replacement between silver nanocubes and aqueous gold solution. Graphene nanosheets were functionalized by reaction with N,N-dimethylformamide (DMF) at relatively high temperature to generate chemically modified graphene (CMG). Surface topography and thickness of CMG nanosheets were obtained by atomic force microscopy (AFM). Ultraviolet visible (UV–vis) spectroscopy, scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), Fourier transformed infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) were used to characterize the physicochemical properties of gold nanocage/CMG nanocomposite. The electrochemical behaviour of sensor was investigated using cyclic voltammetry (CV), amperometry and electrochemical impedance spectroscopy (EIS). It was found that the gold nanocage/CMG modified glassy carbon electrode (GCE) exhibits low oxidation potential (0.17V) with two linear ranges from 6μM to 30μM and 30μM to 1.7mM for sensing hydrazine with detection limit of 0.5μM. The modified electrode exhibits good selectivity for hydrazine amperometric response in the presence of common ions and some biological interfering species and it can be easily prepared which makes it reproducible. Furthermore, the present sensor exhibits high level of stability for the determination of hydrazine.

Development of High Performance Electrochemical and Physical Biosensors Based on Chemically Modified Graphene Nanostructured Electrodes

A solvothermal technique with extra treatment was performed to synthesize high quality chemically modified graphene (CG). An environmentally friendly reducing agent, glucose, was used to obtain biocompatible chemically modified graphene. After acidic treatment of glucose treated chemically modified graphene (GCG), contamination free CG sheet was produced. CG suspension was cast on the surface of the plain biosensor surface and nylon filter paper. Pt nanoparticles (PtNP) were deposited on the CG decorated surface by cyclic voltammogram technique. Chitosan - glucose oxidase (chit-GOx) composite and nafion were then integrated by dropped casting technique on the CG/PtNP modified surface. This as-prepared biosensor showed high electrochemical activity for the detection of glucose in phosphate buffered saline (PBS) solution. It exhibited excellent analytical properties including a short response time (4 s), high sensitivity (69.44 μA/mMcm−2), and wide detection range (0.002–12 mM) for glucose sensing. The repeatability, reproducibility, interference phenomena and the stability of the developed sensors were also investigated. The CG based electrode was also evaluated for use as a dry electrode for ECG signal monitoring. The performance of the dry electrode was excellent in comparison to the conventional wet electrodes.

Fluorinated Graphene Enables the Growth of Inorganic Thin Films by Chemical Bath Deposition on Otherwise Inert Substrates.

Chemically modified graphenes (CMGs) offer a means to tune a wide variety of graphene's exceptional properties. Critically, CMGs can be transferred onto a variety of substrates, thereby imparting functionalities to those substrates that would not be obtainable through conventional functionalization. One such application of CMGs is enabling and controlling the subsequent growth of inorganic thin films. In the current study, we demonstrated that CMGs enhance the growth of inorganic films on inert surfaces with poor growth properties. Fluorinated graphene transferred onto polyethylene enabled the dense and homogeneous deposition of a cadmium sulfide (CdS) film grown via chemical bath deposition. We showed that the coverage of the CdS film can be controlled by the degree of fluorination from less than 20% to complete coverage of the film. The approach can also be applied to other technologically important materials such as ZnO. Finally, we demonstrated that electron beam-generated plasma in a SF6-containing background could pattern fluorine onto a graphene/PE sample to selectively grow CdS films on the fluorinated region. Therefore, CMG coatings can tailor the surface properties of polymers and control the growth of inorganic thin films on polymers for the development of flexible electronics.

(Nanocarbons Division SES Young Investigator Award) Making Graphene Resist Aggregation

Graphene-based sheets have stimulated great interest in many scientific disciplines and shown promise for wide potential applications. Among various ways of creating single atomic layer carbon sheets, a promising route for bulk production is to first chemically exfoliate graphite powders to graphene oxide (GO) sheets, followed by reduction to form chemically modified graphene (CMG). Due to the strong van der Waals attraction between graphene sheets, CMG tends to aggregate. The restacking of sheets is largely uncontrollable and irreversible, thus it reduces their processability and compromises properties such as accessible surface area. Strategies based on colloidal chemistry have been applied to keep CMG dispersed in solvents by introducing electrostatic repulsion to overcome the van der Waals attraction or adding spacers to increase the inter-sheet spacing. In this dissertation, two very different ideas that can prevent CMG aggregation without extensively modifying the material or introducing foreign spacer materials are introduced. The van der Waals potential decreases with reduced overlapping area between sheets. For CMG, reducing the lateral dimension from micrometer to nanometer scale should greatly enhance their colloidal stability with additional advantages of increased charge density and decreased probability to interact. The enhanced colloidal stability of GO and CMG nanocolloids makes them especially promising for spectroscopy based bio-sensing applications. For potential applications in a compact bulk solid form, the sheets were converted into paper-ball like structure using capillary compression in evaporating aerosol droplets. The crumpled graphene balls are stabilized by locally folded π-π stacked ridges, and do not unfold or collapse during common processing steps. They can tightly pack without greatly reducing the surface area. This form of graphene leads to scalable performance in energy storage. For example, planer sheets tend to aggregate and lose surface area. So their capacitive performance tends to decrease with increased loading level. But the crumpled balls can resist aggregation and retain high capacitance at high loading level. The crumpled graphene balls can be also used as expandable shells for wrapping lithium ion battery anode materials such as Si nanoparticles, which can accommodate their expansion/contraction without facture, and thus greatly improve the coulombic efficiency of the anode.

Transfer of Chemically Modified Graphene with Retention of Functionality for Surface Engineering.

Single-layer graphene chemically reduced by the Birch process delaminates from a Si/SiOx substrate when exposed to an ethanol/water mixture, enabling transfer of chemically functionalized graphene to arbitrary substrates such as metals, dielectrics, and polymers. Unlike in previous reports, the graphene retains hydrogen, methyl, and aryl functional groups during the transfer process. This enables one to functionalize the receiving substrate with the properties of the chemically modified graphene (CMG). For instance, magnetic force microscopy shows that the previously reported magnetic properties of partially hydrogenated graphene remain after transfer. We also transfer hydrogenated graphene from its copper growth substrate to a Si/SiOx wafer and thermally dehydrogenate it to demonstrate a polymer- and etchant-free graphene transfer for potential use in transmission electron microscopy. Finally, we show that the Birch reduction facilitates delamination of CMG by weakening van der Waals forces between graphene and its substrate.

Chemically Modified Graphene and Sulfonic Acid-Doped Polyaniline Nanofiber Composites: Preparation Routes, Characterization, and Comparison of Direct DNA Detection

Recently, functional composites based on chemically modified graphenes (CMGs) and nanostructured conducting polymers have attracted wide interest in the field of electrochemical biosensing. However, comprehensive studies of the effects of various CMGs on the electrochemical properties and biosensing performance of the resulting composites are scarce. In this work, for the first time, we fabricated and deeply evaluated three composites composed of CMGs and sulfonic acid-doped polyaniline nanofiber (namely, CMG–SPAN composites). The CMGs (involving the unreduced form and reduced forms prepared by different reduction routes) were chosen to show the effects of reduction and different preparation routes on the morphologies, electrochemical properties, and DNA biosensing performances of the composites. Notably, the self-redox signals of SPAN in these composites were significantly enhanced and were used for rapid, direct, and label-free DNA detection. Moreover, a preliminary study of the capacitive characteristics of the thermally reduced graphene oxide–SPAN composite was conducted at the end of this work, owing to the potential benefits of the composite in a supercapacitor that were surprisingly observed in this research. The findings of this work will provide useful guides for better understanding of the interaction between CMG and SPAN and for the future development of high-performance functional materials for electrochemical sensors/biosensors and supercapacitors.

Finely tuning oxygen functional groups of graphene materials and optimizing oxygen levels for capacitors

Oxygen-containing, chemically modified graphene (CMG) systems have been intensively investigated for various applications. The development of methods that allow fine control of the oxygen functionality would help better understand the mechanisms associated with CMGs, facilitate optimization of the material properties, and provide standards for chemical characterization purposes. Here, the authors report a new method for finely controlling the levels of oxygen in CMG materials based on the refluxing of aqueous colloidal suspensions of graphene oxide for specific reflux times, which does not require additional reducing agents. Chemical analysis confirmed that the oxygen levels can be finely controlled and they can provide spectroscopic tools to monitor the oxygen levels of CMG-based systems. This system was applied to help provide a fundamental foundation for the correlation between the oxygen groups and capacitive features.

Synthesis of boron and nitrogen co-doped graphene nano-platelets using a two-step solution process and catalytic properties for oxygen reduction reaction

Chemically modified graphenes (CMGs) show great promise for various applications owing to the feasibility of their low-cost mass production and good solution processability. Recently, hetero-atom-doped CMGs have been suggested as good candidate materials for electrochemical catalysts in oxygen reduction reaction (ORR). In this study, we synthesized B, N co-doped graphene nano-platelets (BN-rG-O) using a two-step solution process with sequential reaction of graphene oxide with borane tetrahydrofuran and hydrazine monohydrate. In the ORR measured in a basic medium (0.1 M KOH), BN-rG-O exhibits an onset potential of 0.81 V (vs. reversible hydrogen electrode), follows near four electron pathway, and shows excellent stability against methanol poisoning and during durability tests.

Functional Gels Based on Chemically Modified Graphenes

Chemically modified graphene (CMG) materials have been extensively studied because of their unique structures, excellent properties, and potential applications in energy storage and conversion, catalysis, and environment remediation. However, the unique two-dimensional structure and amphiphilicity make CMG sheets easily restack into irregular aggregates, which greatly reduces their accessible surface area, and thereby deteriorates their performance in practical applications. To exploit their inherent properties fully, CMGs usually have to be fabricated or assembled into functional gels with desired three-dimensional (3D) interconnected porous microstructures. In this review, we summarize the recent achievements in the synthesis of CMG-based functional gels, including hydrogels, organogels, aerogels, and their composites. The mechanisms of gel formation and the applications of these functional gels will also be discussed.

Role of edge geometry and chemistry in the electronic properties of graphene nanostructures.

The geometry and chemistry of graphene nanostructures significantly affects their electronic properties. Despite a large number of experimental and theoretical studies dealing with the geometrical shape-dependent electronic properties of graphene nanostructures, experimental characterisation of their chemistry is clearly lacking. This is mostly due to the difficulties in preparing chemically-modified graphene nanostructures in a controlled manner and in identifying the exact chemistry of the graphene nanostructure on the atomic scale. Herein, we present scanning probe microscopic and first-principles characterisation of graphene nanostructures with different edge geometries and chemistry. Using the results of atomic scale electronic characterisation and theoretical simulation, we discuss the role of the edge geometry and chemistry on the electronic properties of graphene nanostructures with hydrogenated and oxidised linear edges at graphene boundaries and the internal edges of graphene vacancy defects. Atomic-scale details of the chemical composition have a strong impact on the electronic properties of graphene nanostructures, i.e., the presence or absence of non-bonding π states and the degree of resonance stability.