Synthesis, characterisation and thermo-physical properties of highly stable graphene oxide-based aqueous nanofluids for potential low-temperature direct absorption solar applications

Two types of highly stable 0.1% graphene oxide-based aqueous nanofluids were synthesised and investigated. The first nanofluid (GO) was prepared under the influence of ultrasonic irradiation without surfactant. The second nanofluid was treated with tetra ethyl ammonium hydroxide to reduce the graphene oxide to form reduced graphene oxide (RGO) during ultrasonic irradiation. The GO and RGO powders were characterised by various techniques such as field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman. Also UV–visible absorption spectroscopy was carried out and band gap energies were determined. Optical band gap energies for indirect transitions ranged from 3.4 to 4.4 eV and for direct transitions they ranged between 2.2 and 3.7 eV. Thermal conductivity measurements of the GO-based aqueous nanofluid revealed an enhancement of 9.5% at 40 °C compared to pure water, while the RGO-based aqueous nanofluid at 40 °C had a value 9.23% lower than pure water. Furthermore, the photothermal response of the RGO-based aqueous nanofluid had a temperature increase of 13.5 °C, (enhancement of 60.2%) compared to pure water, the GO-based aqueous nanofluid only displayed a temperature rise of 10.9 °C, (enhancement of 46.6%) after 20 min exposure to a solar irradiance of 1000 W m⁻². Both nanofluid types displayed good long-term stability, with the GO-based aqueous nanofluid having a zeta potential of 30.3 mV and the RGO-based aqueous nanofluid having a value of 47.6 mV after 6 months. The good dispersion stability and photothermal performance makes both nanofluid types very promising working fluids for low-temperature direct absorption solar collectors.

Polypropylene Nanocomposite Filled with Spinel Ferrite NiFe2O4 Nanoparticles and In-Situ Thermally-Reduced Graphene Oxide for Electromagnetic Interference Shielding Application

Herein, we presented electromagnetic interference shielding characteristics of NiFe2O4 nanoparticles-in-situ thermally-reduced graphene oxide (RGO)-polypropylene nanocomposites with the variation of reduced graphene oxide content. The structural, morphological, magnetic, and electromagnetic parameters and mechanical characteristics of fabricated nanocomposites were investigated and studied in detail. The controllable composition of NiFe2O4-RGO-Polypropylene nanocomposites exhibited electromagnetic interference (EMI) shielding effectiveness (SE) with a value of 29.4 dB at a thickness of 2 mm. The enhanced EMI shielding properties of nanocomposites with the increase of RGO content could be assigned to enhanced attenuation ability, high conductivity, dipole and interfacial polarization, eddy current loss, and natural resonance. The fabricated lightweight NiFe2O4-RGO-Polypropylene nanocomposites have potential as a high performance electromagnetic interference shielding nanocomposite.

Graphene Derivative in Magnetically Recoverable Catalyst Determines Catalytic Properties in Transfer Hydrogenation of Nitroarenes to Anilines with 2-Propanol

Here, we report transfer hydrogenation of nitroarenes to aminoarenes using 2-propanol as a hydrogen source and Ag-containing magnetically recoverable catalysts based on partially reduced graphene oxide (pRGO) sheets. X-ray diffraction and X-ray photoelectron spectroscopy data demonstrated that, during the one-pot catalyst synthesis, formation of magnetite nanoparticles (NPs) is accompanied by the reduction of graphene oxide (GO) to pRGO. The formation of Ag⁰ NPs on top of magnetite nanoparticles does not change the pRGO structure. At the same time, the catalyst structure is further modified during the transfer hydrogenation, leading to a noticeable increase of sp² carbons. These carbons are responsible for the adsorption of substrate and intermediates, facilitating a hydrogen transfer from Ag NPs and creating synergy between the components of the catalyst. The nitroarenes with electron withdrawing and electron donating substituents allow for excellent yields of aniline derivatives with high regio and chemoselectivity, indicating that the reaction is not disfavored by these functionalities. The versatility of the catalyst synthetic protocol was demonstrated by a synthesis of an Ru-containing graphene derivative based catalyst, also allowing for efficient transfer hydrogenation. Easy magnetic separation and stable catalyst performance in the transfer hydrogenation make this catalyst promising for future applications.

Mechanically Strong Graphene/Aramid Nanofiber Composite Electrodes for Structural Energy and Power

Structural energy and power systems offer both mechanical and electrochemical performance in a single multifunctional platform. These are of growing interest because they potentially offer reduction in mass and/or volume for aircraft, satellites, and ground transportation. To this end, flexible graphene-based supercapacitors have attracted much attention due to their extraordinary mechanical and electrical properties, yet they suffer from poor strength. This problem may be exacerbated with the inclusion of functional guest materials, often yielding strengths of <15 MPa. Here, we show that graphene paper supercapacitor electrodes containing aramid nanofibers as guest materials exhibit extraordinarily high tensile strength (100.6 MPa) and excellent electrochemical stability. This is achieved by extensive hydrogen bonding and π-π interactions between the graphene sheets and aramid nanofibers. The trade-off between capacitance and mechanical properties is evaluated as a function of aramid nanofiber loading, where it is shown that these electrodes exhibit multifunctionality superior to that of other graphene-based supercapacitors, nearly rivaling those of graphene-based pseudocapacitors. We anticipate these composite electrodes to be a starting point for structural energy and power systems that harness the mechanical properties of aramid nanofibers.

Graphene oxide-carbon nanotube hybrid assemblies: cooperatively strengthened OH···O 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 C hydrogen bonds and the removal of chemisorbed water

The effective removal of chemisorbed water in graphene oxide by oxidized carbon nanotubes via cooperatively strengthened OH···OC hydrogen bonds.

Owing to their great significance for energy storage and sensing applications, multi-layer papers consisting of graphene oxide–carbon nanotube (GO–CNT) hybrid sheets were prepared by in situ exfoliation of graphite oxide in the presence of oxidized CNTs (oCNTs). For the first time we elucidate the influence of oCNTs on chemisorbed water (CW), i.e. the water molecules inherently bound to the oxygen functional groups (OFGs) of graphene oxide (GO) and responsible for irreversible structural damage upon thermal reduction processes. We show that oCNTs self-assemble onto GO sheets during the liquid phase processing steps by forming cooperatively strengthened OH···OC hydrogen bonds between the carboxylic groups of the oCNTs and OFGs of GO. At oCNT amounts of about 10 to 15 wt% this leads to the displacement of considerable amounts of CW without altering the original chemical composition of GO. The thermally reduced GO–CNT (rGO–CNT) papers reveal improved sp² character and an enhancement of the specific capacitance by 75% with respect to thermally reduced GO (rGO), largely due to the effective removal of CW by oxidized CNTs. These findings disclose the relevance of the cooperative hydrogen bonding phenomena in graphene oxide paper/film electrodes for the development of improved electrochemical energy storage and sensing devices.

Tunable Mixed Ionic/Electronic Conductivity and Permittivity of Graphene Oxide Paper for Electrochemical Energy Conversion

Graphene oxide (GO) is a two-dimensional graphitic carbon material functionalized with oxygen-containing surface functional groups. The material is of interest in energy conversion, sensing, chemical processing, gas barrier, and electronics applications. Multilayer GO paper has recently been applied as a new proton conducting membrane in low temperature fuel cells. However, a detailed understanding of the electrical/dielectric properties, including separation of the ionic vs electronic contributions under relevant operating conditions, has so far been lacking. Here, the electrical conductivity and dielectric permittivity of GO paper are investigated in situ from 30 to 120 °C, and from 0 to 100% relative humidity (RH) using impedance spectroscopy. These are related to the water content, measured by thermogravimetric analysis. With the aid of electron blocking measurements, GO is demonstrated to be a mixed electronic-protonic conductor, and the ion transference number is derived for the first time. For RH > 40%, conductivity is dominated by proton transport (with a maximum of 0.5 mS/cm at 90 °C and 100% RH). For RH < 40%, electronic conductivity dominates (with a maximum of 7.4 mS/cm at ∼80 °C and 0% RH). The relative permittivity of GO paper increases with decreasing humidity, from ∼10 at 100% RH to several 1000 at 10% RH. These results underline the potential of GO for application not only as a proton conducting electrolyte but also as a mixed conducting electrode material under appropriate conditions. Such materials are highly applicable in electrochemical energy conversion and storage devices such as fuel cells and electrolyzers.

Single-Step Process toward Achieving Superhydrophobic Reduced Graphene Oxide

We report the first use of spark plasma sintering (SPS) as a single-step process to achieve superhydrophobic reduced graphene oxide (rGO). It was found that SPS was capable of converting smooth and electrically insulating graphene oxide (GO) sheets into highly electrically conductive rGO with minimum residual oxygen and hierarchical roughness which could be well retained after prolonged ultrasonication. At a temperature of 500 °C, which is lower than the conventional critical temperature for GO exfoliation, GO was successfully exfoliated, reduced, and hierarchically roughened. rGO fabricated by only 1 min of treatment at 1050 °C was superhydrophobic with a surface roughness (Ra) 10 times as large as that of GO as well as an extraordinarily high C:O ratio of 83.03 (atom %) and water contact angle of 153°. This demonstrates that SPS is a superior GO reduction technique, which enabled superhydrophobic rGO to be quickly and effectively achieved in one single step. Moreover, the superhydrophobic rGO fabricated by SPS showed an impressive bacterial antifouling and inactivation effect against Escherichia coli in both aqueous solution and the solid state. It is envisioned that the superhydrophobic rGO obtained in this study can be potentially used for a wide range of industrial and biomedical applications, such as the fabrication of self-cleaning and antibacterial surfaces.

Enhanced UV detection by transparent graphene oxide/ZnO composite thin films

Highly transparent graphene oxide–ZnO composite films synthesized by simple chemical method inhibit electron–hole recombination, modulate carrier transport and enhance UV detection capability.

Structural Origin of the Activity in Mn3O4-Graphene Oxide Hybrid Electrocatalysts for the Oxygen Reduction Reaction

Non-precious metal oxide/carbon hybrid electrocatalysts are of increasing importance for the oxygen reduction reaction (ORR). A synergistic effect is commonly used to explain the superior ORR activity exerted by metal oxide/nanocarbon hybrids, and this effect is attributed to covalently coupled interfaces between the two materials. However, the origin of the high activity, the structure, and the electrocatalytic nature of the interface remain unclear. By combining X-ray photoelectron spectroscopy with synchrotron far-infrared spectroscopy, we resolved the interface structure between spinel manganese oxide nanocrystals and graphene oxide nanoribbons, and the role of this interface in the promoted ORR. Moreover, we demonstrated the excellent ORR activity by a functional synergism of the hybrid constituents through a series of comparative electrochemical experiments.

Preparation of a Three-Dimensional Reduced Graphene Oxide Film by Using the Langmuir-Blodgett Method

The Langmuir-Blodgett method has always been traditionally utilized in the deposition of two-dimensional structures. In this work, however, we employed the method to deposit three-dimensional reduced graphene oxide layers using an unconventional protocol for the first time. This was achieved by carrying out the dipping process after the collapse pressure or breaking point, which results in the formation of a highly porous three-dimensional surface topography. By varying the number of deposition layers, the porosity could be optimized from nanometer to micrometer dimensions. Employed as bioelectrodes, these three-dimensional reduced graphene oxide layers may allow improved adhesion and biocompatibility compared to the conventional two-dimensional surfaces. A larger number of pores also improves the mass transport of materials and therefore increases the charge-sustaining capacity and sensitivity. This could ultimately improve the performance of biofuel cells and other electrode-based systems.

In-Situ ESEM and EELS Observation of Water Uptake and Ice Formation in Multilayer Graphene Oxide

Graphene oxide (GO) is hydrophilic and swells significantly when in contact with water. Here, we investigate the change in thickness of multilayer graphene oxide membranes due to intercalation of water, via humidity-controlled observation in an environmental scanning electron microscope (ESEM). The thickness increases reproducibly with increasing relative humidity. Electron energy loss spectroscopy (EELS) reveals the existence of water ice under cryogenic conditions, even in high vacuum environment. Additionally, we demonstrate that freezing then thawing water trapped in the multilayer graphene oxide membrane leads to the opening up of micron-scale inter-lamellar voids due to the expansion of ice crystals.

Lateral assembly of oxidized graphene flakes into large-scale transparent conductive thin films with a three-dimensional surfactant 4-sulfocalix[4]arene

Graphene is a promising candidate material for transparent conductive films because of its excellent conductivity and one-carbon-atom thickness. Graphene oxide flakes prepared by Hummers method are typically several microns in size and must be pieced together in order to create macroscopic films. We report a macro-scale thin film fabrication method which employs a three-dimensional (3-D) surfactant, 4-sulfocalix[4]arene (SCX), as a lateral aggregating agent. After electrochemical exfoliation, the partially oxidized graphene (oGr) flakes are dispersed with SCX. The SCX forms micelles, which adsorb on the oGr flakes to enhance their dispersion, also promote aggregation into large-scale thin films under vacuum filtration. A thin oGr/SCX film can be shaved off from the aggregated oGr/SCX cake by immersing the cake in water. The oGr/SCX thin-film floating on the water can be subsequently lifted from the water surface with a substrate. The reduced oGr (red-oGr) films can be as thin as 10−20 nm with a transparency of >90% and sheet resistance of 890 ± 47 kΩ/sq. This method of electrochemical exfoliation followed by SCX-assisted suspension and hydrazine reduction, avoids using large amounts of strong acid (unlike Hummers method), is relatively simple and can easily form a large scale conductive and transparent film from oGr/SCX suspension.

Scalable thermal synthesis of a highly crumpled, highly exfoliated and N-doped graphene/Mn-oxide nanoparticle hybrid for high-performance supercapacitors

Schematic illustration for the synthesis of the highly crumpled, highly exfoliated, and N-doped graphene/Mn-oxide nanoparticle hybrid and its SEM image.

Graphene oxide nanoparticle attachment and its toxicity on living lung epithelial cells

Since its discovery graphene and its oxidized form graphene oxide have attracted interest in a wide range of applications, which calls for scrutinized studies about their possible toxicity.

A hybrid of holey graphene and Mn3O4 and its oxygen reduction reaction performance

A hybrid of holey graphene and Mn₃O₄ is prepared by a one-step process, in which the formation of a holey structure is accompanied with Mn₃O₄ nanoparticles through a high temperature reaction between graphene oxide sheets and KMnO₄.

All-optical graphene oxide humidity sensors

The optical characteristics of graphene oxide (GO) were explored to design and fabricate a GO-based optical humidity sensor. GO film was coated onto a SU8 polymer channel waveguide using the drop-casting technique. The proposed sensor shows a high TE-mode absorption at 1550 nm. Due to the dependence of the dielectric properties of the GO film on water content, this high TE-mode absorption decreases when the ambient relative humidity increases. The proposed sensor shows a rapid response (<1 s) to periodically interrupted humid air flow. The transmission of the proposed sensor shows a linear response of 0.553 dB/% RH in the range of 60% to 100% RH.

Self-assembly of graphene oxide at interfaces

Due to its amphiphilic property, graphene oxide (GO) can achieve a variety of nanostructures with different morphologies (for example membranes, hydrogel, crumpled particles, hollow spheres, sack-cargo particles, Pickering emulsions, and so on) by self-assembly. The self-assembly is mostly derived from the self-concentration of GO sheets at various interfaces, including liquid-air, liquid-liquid and liquid-solid interfaces. This paper gives a comprehensive review of these assembly phenomena of GO at the three types of interfaces, the derived interfacial self-assembly techniques, and the as-obtained assembled materials and their properties. The interfacial self-assembly of GO, enabled by its fantastic features including the amphiphilicity, the negatively charged nature, abundant oxygen-containing groups and two-dimensional flexibility, is highlighted as an easy and well-controlled strategy for the design and preparation of functionalized carbon materials, and the use of self-assembly for uniform hybridization is addressed for preparing hybrid carbon materials with various functions. A number of new exciting and potential applications are also presented for the assembled GO-based materials. This contribution concludes with some personal perspectives on future challenges before interfacial self-assembly may become a major strategy for the application-targeted design and preparation of functionalized carbon materials.

Self-powered humidity sensor based on graphene oxide composite film intercalated by poly(sodium 4-styrenesulfonate)

This Research Article reports self-powered humidity sensors based on graphene oxide (GO) and poly(sodium 4-styrenesulfonate) (PSS)-intercalated GO composite films used as the humidity-responsive dielectrics. A hydrophilic and electrically-insulating PSS polymer was used as an intercalant between the individual GO platelets to enhance the water permeation characteristics. Capacitive-type humidity sensors fabricated by forming metal electrodes on both sides of the GO and GO-PSS films were installed into the charge pumping system, which can produce a voltage output as a response to humidity sensing. While both sensors based on GO and GO-PSS dielectrics responded stably and reversibly to the changes in RH, the GO-PSS sensor showed enhanced sensing responses compared to the GO sensor, providing ∼5.6 times higher voltage output and 3 times faster responses in humidity sensing.

Construction of high-energy-density supercapacitors from pine-cone-derived high-surface-area carbons

Very high surface area activated carbons (AC) are synthesized from pine cone petals by a chemical activation process and subsequently evaluated as an electrode material for supercapacitor applications in a nonaqueous medium. The maximum specific surface area of ∼3950 m(2)  g(-1) is noted for the material treated with a 1:5 ratio of KOH to pine cone petals (PCC5), which is much higher than that reported for carbonaceous materials derived from various other biomass precursors. A symmetric supercapacitor is fabricated with PCC5 electrodes, and the results showed enhanced supercapacitive behavior with the highest energy density of ∼61 Wh kg(-1). Furthermore, outstanding cycling ability is evidenced for such a configuration, and ∼90 % of the initial specific capacitance after 20,000 cycles under harsh conditions was observed. This result revealed that the pine-cone-derived high-surface-area AC can be used effectively as a promising electrode material to construct high-energy-density supercapacitors.