Dielectrophoretic integration of single- and few-layer graphenes

Iron tailings can be used as adsorbents to effectively remove graphene oxide (GO) in aqueous solutions. Experimental data show that pH, iron tailing quality, GO concentration and temperature have significant effects on the removal of GO, the adsorption of the tail involves a single layer adsorbed Langmuir model with exothermic reaction, and it compliance with the proposed dynamics model meets the requirements, the adsorption process is a spontaneous physical adsorption process. Combining scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) to analyze the iron tail according to the mechanisms of ore adsorption of GO, it was concluded that iron tailings might be a very promising material to effectively remove GO in aqueous solutions. The results of this research provide key information for the transportation and potential fate of GO in the natural environment.

Investigation of plasma-assisted functionalization of pristine single layer graphene

Graphene modification with gold nanoparticles using the gas aggregation technique

Supercapacitors, also known as electric double-layer capacitors (EDLCs), are able to store energy rapidly and reversibly through the formation of a double-layer of electronic and ionic charge, closely spaced, at the electrode/electrolyte interface.[1-2] Due to the combination of various advantageous properties, such as efficient operation at high power density, long cycle life and improved safety compared to Li-ion batteries,[3,4] supercapacitors are being increasingly used as alternative power sources to rechargeable batteries. However, the implementation of supercapacitors in practical application is still restricted by the limited energy density, typically 5-8 Wh/L,[5] which is much lower than that of lead-acid batteries which can achieve ~50-90 Wh/L.[6] Considering a symmetric configuration, the volumetric energy density (E V) of a supercapacitor is directly proportional to the volumetric capacitance (C V) of a single electrode and the square of operating voltage (U) following the equation, Ev = 1/8(C V·U 2). Therefore, in such a system, there are two ways to improve energy density: boosting capacitance and extending cell voltage window. The operating voltage of EDLCs is typically limited by the stability of electrolyte and thus room temperature ionic liquids (ILs) with large electrochemical stability windows (> 3-4 V) have become promising next-generation electrolytes. However, the relatively high viscosity of ILs results in lower ionic conductivity compared to traditional aqueous or organic electrolytes and also leads to challenges with pore wetting. On the electrode side, materials with a high intrinsic capacitance (C DL) per area and a large ion-accessible surface area (SSA) are needed to achieve high gravimetric capacitance (C G) since C G = C DL·SSA. The potentially high electrical conductivity, surface area, and chemical stability of graphene-based materials make them promising candidate electrode.[7,8] Theoretically, single layer graphene can exhibit SSA as high as 2675 m2/g. While pristine graphene is limited by its low quantum capacitance leading to C DL ~3-4 mF/cm2, more defective and functionalized graphene produced by the chemical or thermal reduction of graphene oxide (GO) have been shown to exhibit C DL > 17 mF/cm2 in non-aqueous electrolyte leading to theoretical gravimetric capacitance, C G,theoretical > 450 F/g if all of graphene’s surface area could be made ion-accessible. To prevent aggregation and restacking of graphene-based materials into lower SSA structures, we demonstrated an IL microemulsion system (Figure 1) that spontaneously assembles on the surface of GO, placing nanometer-sized droplets of a high-performance, hydrophobic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) directly onto the available surface of well-dispersed single GO layers. We first demonstrate that a common non-ionic surfactant, Tween 20 is capable of forming a stable microemulsion with EMImTFSI with a particle size on the order of several nanometers. These surfactant stabilized nano-droplets (microemulsion particles) spontaneously adsorb to GO sheets yielding a dispersion which can be cast directly onto current collectors leading to a dense nanocomposite of GO/IL/Tween 20. Tween 20 is then removed by evaporation, while the GO is thermally reduced leading to what we refer to as layered IL-mediated reduced GO (IM-rGO) electrodes. The approach is found to yield high ion-accessible SSA as evidenced by one of the highest C G ever reported (302 F/g) when the composite contains 80 wt% IL. These results indicate that the microemulsion particles formed were better able to deploy IL as a spacer to prevent rGO sheets from restacking. Reducing the IL content to 60 wt%, resulted in dense electrodes that exhibited a C V = 218 F/cm3, which is the highest value reported to date among all graphene-based supercapacitors leading to exceptional volumetric energy density. Reference (1) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. Springer Science & Business Media 2013. (2) Miller, J. R.; Simon, P. Electrochemical capacitors for energy management. Science 2008, 321, 651. (3) Du Pasquier, A.; Plitz, I.; Menocal, S.; Amatucci, G. A comparative study of Li-ion battery, supercapacitor and nonaqueous asymmetric hybrid devices for automotive applications. J. Power Sources 2003, 115, 171. (4) Khaligh, A.; Li, Z. Battery, ultracapacitor, fuel cell, and hybrid energy storage systems for electric, hybrid electric, fuel cell, and plug-in hybrid electric vehicles: State of the art. IEEE transactions on Vehicular Technology 2010, 59, 2806. (5) Burke, A. R&D considerations for the performance and application of electrochemical capacitors. Electrochim. Acta 2007, 53, 1083. (6) Linden, D. Handbook of batteries. Fuel and Energy s 1995, 265. (7) Pope, M. A.; Aksay, I. A. Four-Fold Increase in the Intrinsic Capacitance of Graphene through Functionalization and Lattice Disorder. J. Phys. Chem. C 2015, 119, 20369. (8) Stoller, M. D.; Park, S.; Ruoff, R. S. Graphene-based ultracapacitors. Nano Lett. 2008, 8, 3498. Figure 1

Adsorption properties and mechanism of sepiolite to graphene oxide in aqueous solution

An improved Hummers method was developed for the simple and efficient production of high-quality graphene oxide (GO), and the composite of GO and nickel foam (NF) (GO/NF) was fabricated by ultrasonication-vacuum-assisted deposition of an aqueous solution of GO on NF. After chemical or thermal reduction, the composite of reduced GO and nickel foam (rGO/NF) was obtained. The electrochemical capacitance performance of rGO/NF was investigated using cyclic voltammetry and galvanostatic charge/discharge measurements. The chemically reduced rGO/NF composite (C-rGO/NF) exhibited high specific capacitance of 379 F/g at 1.0 A/g and 266.5 F/g at 10 A/g. We also prepared thermally reduced graphene oxide at 473 K in order to illuminate the difference in effect between the chemical and low-temperature thermal reduction methods on electrochemical properties. The cycling performance of thermally reduced rGO/NF composite (T-rGO/NF) and C-rGO/NF had ~91% and ~95% capacitance retention after 2000 cycles in a 6 mol/L KOH electrolyte, respectively. Electrochemical experiments indicated that the obtained rGO/NF has very good capacitive performance and could be used as a potential application of electrochemical capacitors. Our work revealed high electrochemical capacitor performance of rGO/NF composite and provided a facile method of rGO/NF preparation.

Raman-assessed structural evolution of as-deposited few-layer graphene by He/H2 arc discharge during rapid-cooling thinning treatment

Contribution of oxygen functional groups in graphene to the mechanical and interfacial behaviour of nanocomposites: Molecular dynamics and micromechanics study

We investigate the effect of UV processing of graphene with different structural properties prepared by mechanical exfoliation and CVD growth. Depending on UV exposure time, we observe different effects like oxidation, doping, and etching. For bi-layered and few-layered graphene flakes, we do not observe significant etching even after 3 h exposure which indicates the high resistance of graphene to reactive oxygen species intercalation between graphene layers. Single-layer CVD-grown graphene is fully etched after 2 h of UV treatment. The crystalline size of exfoliated single layer graphene after UV exposure drops from 45 to 5 nm while for CVD graphene from just 10 to 2 nm. We investigate the effect of UV irradiation on field effect transistors, demonstrating sequential cleaning from polymer residuals, oxidation (doping), and final etching of graphene. After 30 minutes of UV irradiation, we observe the hole mobility of a CVD single layer graphene transistor increasing up to 400 cm2/V·s.

Removal of triclosan in a Fenton-like system mediated by graphene oxide: Reaction kinetics and ecotoxicity evaluation

In this thesis, I will mainly focus on using Raman spectroscopy and imaging to probe the electronic and crystal structures of graphene that modified both chemically and physically. This thesis is divided into five parts. The first part covers Chapter 1. It reviews the progress on modification of graphene's electronic structure, and introduces the relationship between Raman features and graphene's properties. For modification of graphene's electronic structure, I review some important and common methods, which include applying external electric and magnetic field, changing stacking status, defects, and strain. The second part (Chapter 2) introduces the investigation of graphene edge thermal dynamics. As controlling edge chirality of graphene nanoribbon can tune its electronic structure, and thus it is of great importance to understand the thermal annealing effect on graphene edges as it is one of the inevitable processes for nano device fabrication. In my studies, the armchair edges present much better thermal stability compared to zigzag edges, while the zigzag edges are not thermal stable as revealed by the activation of Raman D band after annealing treatment. The edge atomic structure evolution under annealed at different temperature (from 200 to 500°C) is also studied in detail by comparing the polarized Raman intensity ratio ofD and G bands. In the third part (Chapter 3), the edge dynamics ofbilayer graphene is presented. Motivated by one of the most influentially reported result that the AB-stacked bulk graphite is structurally modified to many AA-stacked bilayer graphene with closed edges at both armchair and zigzag edges after anneal at extremely high temperature (2000 °C) (Phys. Rev. Lett. 102, 015501, 2009), I carried out the work for low temperature edge dynamics study. Based on the conclusions I obtained from single layer graphene edge thermal dynamics, it is indirectly discovered by Raman image and spectroscopy that the zigzag-edge of bilayer graphene is preferred to form closed state when it anneals at low temperature. The formation of closed zigzag edges of bilayer graphene is also supported by the dramatically decreased system energy as revealed by DFT calculation. The armchair edges of bilayer graphene cannot form closed state because of geometrical incompatibility. Hence the thermal dynamic behaviour of the armchair edges of bilayer graphene can be considered as two independent armchair edges oftwo layers. The fourth part (Chapter 4 and 5) introduces the Raman studies ofFeCh-based full intercalation few-layer graphene compounds. Both evolutions of G and 2D bands are discussed in details for identification ofthe modified few-layer graphene crystal structure. The electronic properties of such compound are discussed by its 2D band features. The theoretical calculation results are used to assist the explanation of its Raman features and electronic properties. Followed by the preliminary characterization, various laser energies as excitation source to probe the Raman scattering features for this compound, and the laser dependent G band intensity for graphene layers doped in different level is observed. The mechanism for the enhancement of Raman G band for some specific graphene layers is also revealed theoretically. The fifth part (Chapter 6) introduces another kind of FeCh-based trilayer graphene intercalation compounds. In this type compound, only ·one interlayer space of the trilayer graphene is intercalated and the other interlayer space keeps unchanged. The intercalated FeCh separates the pristine trilayer graphene into two independent parts, which are single layer graphene and bilayer graphene, respectively. This kind of abnormal compound presents obvious different Raman features compared to the full intercalation case. The electronic band structure of this new type intercalation compounds also exhibits exotic property for its opened bandgap arising from inversion symmetry breaking of the separated bilayer graphene.

High values of Young’s modulus, excellent fracture strength, extremely high carrier mobility and optical transmittance make graphene films a promising and emerging candidate for next generation transparent and conductive electrodes for future optoelectronic devices. In this brief review, the research progress of graphene-based transparent and conductive electrode will be presented, with an emphasis on the preparation of single or few layers of graphene and their films as well as the latest developments of their applications in rapidly evolving optoelectronic devices, such as displays, solar cells and memory devices. Keywords: Graphene, graphene oxide, optoelectronic devices, transparent conductive films.

Few-layer graphene (FLG) samples prepared by two methods (chemical vapor deposition (CVD) followed by transfer onto SiO2/Si substrate and mechanical exfoliation) are characterized by combined optical contrast and micro-Raman mapping experiments. We examine the behavior of the integrated intensity ratio of the 2D and G bands (A2D/AG) and of the 2D band width (Γ2D) as a function of the number of layers (N). For our mechanically exfoliated FLG, A2D/AG decreases and Γ2D increases with N as expected for commensurately stacked FLG. For CVD FLG, both similar and opposite behaviors are observed and are ascribed to different stacking orders. For small (respectively, large) relative rotation angle between consecutive layers (θ), the values of the A2D/AG ratio is smaller (respectively, larger) and the 2D band is broader (respectively, narrower) than for single-layer graphene. Moreover, the A2D/AG ratio decreases (respectively, increases) and, conversely, Γ2D increases (respectively, decreases) as a function of N for small (respectively, large) θ. An intermediate behavior has also been found and is interpreted as the presence of both small and large θ within the studied area. These results confirm that neither A2D/AG nor Γ2D are definitive criteria to identify single-layer graphene, or to count N in FLG.

Graphene oxide (GO) forms persistent dispersions in aqueous solutions up to concentrations of 0.2 mg mL(-1). Addition of methylene blue (MB) to these aqueous dispersion of GO gives rise to the observation in optical spectroscopy of new absorption bands that are indicative of the formation of MB/GO conjugates. Four new absorption maxima have been characterized, and their intensity varies depending on the relative concentration of MB with respect to GO. Two of these bands appearing at 677 and 757 nm correspond to individual MB molecules adsorbed on neutral or acid sites of GO, respectively. Two other bands at 615 and 580 nm are attributable to adsorbed MB molecules showing interaction with other neighbor dye molecules at incomplete (615 nm) or complete (580 nm) surface coverage. Complete coverage of GO surface by MB causes the formation of a precipitate and the separation of the MB/GO conjugate. EDS mapping of carbon and sulfur atoms of MB/GO conjugate indicates the homogeneous distribution of MB molecules coating GO sheets. A simple and reliable protocol for surface area measurement and determination of the level of aggregation for GO dispersions in water has been proposed by determining the amount of MB that leads to the maximum intensity of the 580 nm band and precipitation of the MB/GO conjugate. Specific surface area as high as 736.6 m(2) g(-1) in the range of the theoretical value for GO has been experimentally measured for diluted GO solutions, but aggregation levels of 15% were estimated for GO concentration of 50 μg mL(-1).

Influence of the thermal reduction process on the tribological and conductive properties of single-layer graphene oxide