Ultralarge Graphene Oxide Sheets Research Articles

Microstructure Effects on the Electrochemical Properties of Graphene/Manganese Oxide Porous Electrodes

Manganese oxide (MnO2) nanowires as guest hosts with ultra-large reduced graphene oxide (RGO) sheets form a sandwich structure and may provide electrochemical properties that cannot be achieved with either material alone. Ultra-large graphene oxide (GO) sheets exhibit an order of magnitude larger length to diameter ratio compared to most studies with smaller GO sheets (~70,000 compared to ~2,000) and demonstrate potential for improved electrode design. Understanding the self-assembly properties of the combination of MnO2 nanowires and ultra-large RGO sheets was explored by two methods: a) mixing both dispersions that have been synthesized separately to form a mixture and b) growing the nanowires along the ultra-large GO sheets to form a hybrid material. The pure and multicomponent dispersions were characterized by spectroscopy and microscopy techniques to characterize the materials and gain insight into their dispersion microstructures. The dispersions were then freeze-dried into aerogels with 3D porous architectures. Tuning the initial dispersion parameters, including concentration, MnO2 content, and assembly results in different morphologies and physical and electrochemical properties of the aerogels. Results from this work convey that the hybrid exhibits a more uniform distribution of the nanowires than that of the mixture and the porous electrodes consist of mesopores (diameters between 2 and 50 nm) with interconnected pathways. Furthermore, the electrochemical performance of the hybrid and mixture electrodes suggests that there may be stronger interfacial contact between the RGO sheets and MnO2 nanowires in the hybrid due to the nanowires’ nucleation and growth along the sheets as opposed to the mixture. Comparison of the dispersion and aerogel morphologies as well as electrochemical performance provided insights into the relationships between material type, initial dispersion microstructure, and electrode properties.

Elastic three-dimensional graphene sponge fabricated by the liquid crystals of controlled large graphene oxide sheets

Three-dimensional graphene (3DG) sponge has attracted increasing attention because it combines the unique properties of cellular materials and the excellent performance of graphene. Preparation of 3DG sponge depends mainly on the self-assembly of graphene oxide sheets. In the case of using uniform large graphene oxide and ultralarge graphene oxide sheets, the nematic liquid crystals (LCs) phases are formed at low concentration. After chemical reduction, the LCs of GO solution are converted to 3DG sponges with a high degree of orientation, offering a new methodology to regulate the controlled large GO sheets. The orientation of GO solution can be inherited by 3DG sponge, making the sponge to have a large-scale ordered network structure. The 3D elastic graphene sponges have low density and good elasticity, promising for the applications in strain sensing, shock damping, and energy cushioning. Our work explores a novel strategy for organizing the ordered alignment of controlled large GO sheets and exploring the relationship between the microstructures and mechanical properties of 3DG sponge.

Binder-free reduced graphene oxide 3D structures based on ultra large graphene oxide sheets: High performance green micro-supercapacitor using NaCl electrolyte

Abstract We here report a breakthrough in the fabrication of green high performance symmetric supercapacitors. Our system is based on binder-free reduced graphene oxide foam electrodes using NaCl aqueous electrolyte (1 M). A liquid crystal ultra large graphene oxide sheets dispersion in water was used for the fabrication of foams through freeze drying. Proper initial self-assemblage/alignment of graphene oxide (GO) sheets of the aqueous mixture on the substrate and exposure to vacuum sublimation conditions resulted in a unique 3D open structure. A novel technique based on nitrogen adsorption has been applied for the first time to estimate the average wall thickness. The combination of the use of ultra large GO, freeze drying, and employment of NaCl electrolyte resulted in an exceptionally high capacitance of 618.15 F g−1 at a current density of 3.0 A g−1. This is the highest value ever reported for a pure carbonaceous electrode in a two electrode system. Although electrical double layer storage is dominant, our device exhibits a high capacitance typical of pseudo capacitive systems and does not suffer from a limited cycle life. Another excellent feature of the novel supercapacitor is the persistence of high capacitive behavior (132.13 F g−1) even at a high charge/discharge rate of 51.5 A g−1. This is mainly attributed to the amphiphilic character of the reduced graphene oxide sheets resulting from the high percentage of the carbonyl/carboxyl surface residual oxygenate groups which impart an enhanced wetting of the surface by the electrolyte in addition to inducing a higher concentration of Na+ and Cl- ions in the electrical double layers. Our optimum combination of energy and power densities is 42.92 W kg−1 and 1.35 kW h kg−1, respectively.

New insight of high temperature oxidation on self-exfoliation capability of graphene oxide

The preparation of graphene oxide (GO) via Hummers method is usually divided into two steps: low temperature oxidation at 35 °C (step I oxidation) and high temperature oxidation at 98 °C (step II oxidation). However, the effects of these two steps on the exfoliation capability and chemical structure of graphite oxide remain unclear. In this study, both the functional group content of graphite oxide and the entire evolution of interlayer spacing were investigated during the two steps. Step I oxidation is a slowly inhomogeneous oxidation step to remove unoxidized graphite flakes. The prepared graphite oxide can be easily self-exfoliated but contains a lot of organic sulfur. During the first 20 min of step II oxidation, the majority of organic sulfur can be efficiently removed and graphite oxide still remains a good exfoliation capability due to sharp increasing of carboxyl groups. However, with a longer oxidation time at step II oxidation, the decrease of organic sulfur content is slowed down apparently but without any carboxyl groups forming, then graphite oxide finally loses self-exfoliation capability. It is concluded that a short time of step II oxidation can produce purer and ultralarge GO sheets via self-exfoliation. The pure GO is possessed with better thermal stability and liquid crystal behavior. Besides, reduced GO films prepared from step II oxidation show better mechanical and electric properties after reducing compared with that obtained only via step I oxidation.

Sensor for volatile organic compounds using an interdigitated gold electrode modified with a nanocomposite made from poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) and ultra-large graphene oxide

A highly efficient gas sensor is described based on the use of a nanocomposite fabricated from poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) and ultra-large graphene oxide (UL-GO). The nanocomposite was placed by drop casting in high uniformity on interdigitated gold electrodes over a large area of silicon substrate and investigated for its response to volatile organic compounds (VOCs) at room temperature. Monolayers of UL-GOs were synthesized based on a novel solution-phase method involving pre-exfoliation of graphite flakes. The nanocomposite was optimized in terms of composition, and the resulting vapor sensor (containing 0.04 wt% of UL-GO) exhibits strong response to various VOC vapors. The improved gas-sensing performance is attributed to several effects, viz. (a) an enhanced transport of charge carriers, probably a result of the weakening of columbic attraction between PEDOT and PSS by the functional groups on the UL-GO sheets; (b) the increase in the specific surface area on adding UL-GO sheets; and (c) enhanced interactions between the sensing film and VOC molecules via the network of π-electrons. The sensitivity, response and recovery times of the PEDOT-PSS/UL-GO nanocomposite gas sensor with 0.04 wt% of UL-GO are 11.3 %, 3.2 s, and 16 s, respectively. At a methanol vapor concentration as low as 35 ppm, this is an improvement by factors of 110, 10, and 6 respectively, compared to a PEDOT-PSS reference gas sensor without UL-GO.

Effects of processing and material parameters on synthesis of monolayer ultralarge graphene oxide sheets

Chemically-derived ultralarge graphene oxide (UL-GO) sheets are synthesized from natural graphite (NG) flakes based on the modified Hummers method. Three different approaches are adopted and the effects of ultrasonication, thermal shock expansion, degree of oxidation and precursor NG flake size are specifically studied on the quality and size of GO sheets produced. Results show that the use of large-size NG flakes as precursors does not necessarily produce large GO sheets. Optimal processing conditions are identified to be thermal shock exfoliation with the addition of moderate oxidation, i.e. with an expanded graphite to KMnO4 weight ratio=1:7 for 24h, and avoiding ultrasonication during the oxidation process. The resulting UL-GO sheets have a maximum area over 10,000μm2 with a mean area 3400μm2 at a yield of 39.8% for GO sheets larger than 2500μm2, which are considered quite sufficient as precursors for many multifunctional applications, including transparent conductive films, optoelectronic devices and aligned graphene composites.

Graphene oxide dispersions: tuning rheology to enable fabrication

Ultra-large graphene oxide sheets exhibit unique viscoelastic properties, making them a new class of soft material. We provide fundamental insights enabling development of various fabrication techniques utilizing this 2D material.

Structure control of ultra-large graphene oxide sheets by the Langmuir–Blodgett method

As a unique type of soft building block, graphene oxide (GO) dispersions are easy to process to produce electronic devices. Developing methods to produce large sized GO with controllable structure is the key for these applications. Here, ultra-large graphene oxide (UL-GO) sheets (up to ∼100 μm in lateral size) were produced in aqueous media and deposited onto substrates with controllable structure, including flat, rippled, standing collapsed, folded, over folded, highly curved, and scrolled morphologies, via the Langmuir–Blodgett (L–B) technique. The MD simulations show that the GO becomes much softer than pristine graphene sheets due to the sp3-hybridization caused by oxygen containing functional groups and this is the reason why a series of distinct structures were observed. To demonstrate the fabrication of a transparent conductor, the close-packed flat UL-GO films on poly(ethylene terephthalate) (PET) were reduced with a low-temperature chemical reduction process in a hydriodic (HI) acid aqueous solution, achieving a sheet resistance as low as 8.1 kΩ/sq with 90% transparency.

Electrical and spectroscopic investigations on the reduction mechanism of graphene oxide

Ultra-large graphene oxide sheets of monolayer thickness were obtained by removing ultrasonication during the conventional oxidation process of graphite. In situ electrical conductivity measurement during the reduction by hydrazine monohydrate vapor and thermal annealing revealed the existence of an onset temperature at which electrical conduction started to occur. Corresponding X-ray photoelectron spectra indicated that an approximately 65% restoration of the sp2 network led to the occurrence of electrical conduction. Reduction by ring-opening of epoxide group by hydrazine treatment seemed to restore the sp2 network, while thermal annealing left a considerable number of defects in the graphene sheet through the formation of volatile gases.

Fabrication of Highly-Aligned, Conductive, and Strong Graphene Papers Using Ultralarge Graphene Oxide Sheets

This study demonstrates that large-size graphene oxide (GO) sheets can impart a tremendous positive impact on self-alignment, electrical conductivity, and mechanical properties of graphene papers. There is a remarkable, more than 3-fold improvement in electrical conductivity of the papers made from ultralarge GO sheets (with an average area of 272.2 μm(2)) compared to that of the small GO counterpart (with an average area of 1.1 μm(2)). The corresponding improvements in Young's modulus and tensile strength are equally notable, namely 320% and 280%, respectively. These improvements of bulk properties due to the large GO sheets are correlated to multiscale elemental and structural characteristics of GO sheets, such as the content of carboxyl groups on the GO edge, C/O ratio and Raman D/G-band intensity ratio of GO on the molecular-scale, and the degree of dispersion and stacking behavior of GO sheets on the microscale. The graphene papers made from larger GO sheets exhibit a closer-stacked structure and better alignment as confirmed by the fast Fourier transform analysis, to the benefits of their electrical conductivity and mechanical properties. The molecular dynamics simulation further elucidates that the enhanced intersheet interactions between large GO sheets play a key role in improving the Young's modulus of GO papers. The implication is that the said properties can be further improved by enhancing the intersheet stress transfer and electrical conduction especially through the thickness direction.

Langmuir-Blodgett assembly of ultra-large graphene oxide films for transparent electrodes

Monolayer ultra-large graphene oxide (UL-GO) sheets with diameter up to about 100 μm were synthesized based on a chemical method. Transparent conductive films were produced using the UL-GO sheets that were deposited layer-by-layer on a substrate by the Langmuir-Blodgett (L-B) assembly technique. The films produced from UL-GO sheets with a close-packed flat structure exhibit exceptionally high electrical conductivity and transparency after thermal reduction. A remarkable sheet resistance of 605 ωsq at 86% transparency is obtained, which outperforms the graphene films grown on a Ni substrate by chemical vapor deposition. The technique used to produce transparent conductive films is facile, inexpensive and tunable for mass production.

Transparent Conductive Films Consisting of Ultralarge Graphene Sheets Produced by Langmuir–Blodgett Assembly

Monolayer graphene oxide (GO) sheets with sizes ranging from a few to ∼200 μm are synthesized based on a chemical method and are sorted out to obtain four different grades having uniform sizes. Transparent conductive films are produced using the ultralarge graphene oxide (UL-GO) sheets that are deposited layer-by-layer on a substrate using the Langmuir-Blodgett (LB) assembly technique. The density and degree of wrinkling of the UL-GO monolayers are turned from dilute, close-packed flat UL-GO to graphene oxide wrinkles (GOWs) and concentrated graphene oxide wrinkles (CGOWs) by varying the LB processing conditions. The method demonstrated here opens up a new avenue for high-yield fabrication of GOWs or CGOWs that are considered promising materials for hydrogen storage, supercapacitors, and nanomechanical devices. The films produced from UL-GO sheets with a close-packed flat structure exhibit exceptionally high electrical conductivity and transparency after thermal reduction and chemical doping treatments. A remarkable sheet resistance of ∼500 Ω/sq at 90% transparency is obtained, which outperforms the graphene films grown on a Ni substrate by chemical vapor deposition. The technique used in this work to produce transparent conductive UL-GO thin films is facile, inexpensive, and tunable for mass production.

Spontaneous Formation of Liquid Crystals in Ultralarge Graphene Oxide Dispersions

AbstractA novel process is developed to synthesize graphene oxide sheets with an ultralarge size based on a solution‐phase method involving pre‐exfoliation of graphite flakes. Spontaneous formation of lyotropic nematic liquid crystals is identified upon the addition of the ultralarge graphene oxide sheets in water above a critical concentration of about 0.1 wt%. It is the lowest filler content ever reported for the formation of liquid crystals from any colloid, arising mainly from the ultrahigh aspect ratio of the graphene oxide sheets of over 30 000. It is proposed that the self‐assembled brick‐like graphene oxide nanostructure can be applied in many areas, such as energy‐storage devices and nanocomposites with a high degree of orientation.