Flexible conductive graphene paper obtained by direct and gentle annealing of graphene oxide paper

We report the fabrication of flexible conductive graphene paper through a direct and gentle annealing process of graphene oxide paper. Thermal treatments at 700 °C under argon or hydrogen atmosphere directly applied to parent graphene oxide paper lead to a significant removal of disruptive oxygen-containing functional groups, and to a considerable recovery of the sp² network structure. Detailed comparison of chemical and combined chemical–thermal treatments by scanning electronic microscopy (SEM), Raman, X-photoelectron spectroscopy (XPS) and conductivity measurements underline the high efficiency of the direct annealing process. The resulting highly reduced graphene oxide paper exhibits electrical conductivities as high as 8100 S/m representing an increase of five orders of magnitude with respect to the parent graphene oxide paper, which significantly outperforms the results of chemical treatments. Moreover, our direct and gentle thermal reduction allows maintaining the structural integrity and mechanical flexibility of the parent graphene oxide paper thus overcoming problems of brittleness typically encountered in annealing processes. Our approach sets the base for an easy, cost-effective and environmentally friendly fabrication route for flexible conducting graphene paper of great application potential as flexible electrodes in various fields of technology.     

Surface modification of polyethylene and polypropylene by ion implantation

The creation of oxidized structures and double bonds in polyethylene (PE) and polypropylene (PP) samples implanted with P⁺ ions was studied. The surface polarity and the electrical conductivity of the ion-implanted polymers were also examined. As a result of the ion implantation, the polymer macromolecules are broken up and the material is degraded. An oxygen penetration into the radiation-damaged polymers is also observed, with PE being more vulnerable to the oxidation. The ion-implanted PP exhibits higher surface polarity and sheet conductivity compared to that of PE. © 1993 John Wiley & Sons, Inc.     

The enhanced anticoagulation for graphene induced by COOH+ ion implantation

Graphene may have attractive properties for some biomedical applications, but its potential adverse biological effects, in particular, possible modulation when it comes in contact with blood, require further investigation. Little is known about the influence of exposure to COOH⁺-implanted graphene (COOH⁺/graphene) interacting with red blood cells and platelets. In this paper, COOH⁺/graphene was prepared by modified Hummers'' method and implanted by COOH⁺ ions. The structure and surface chemical and physical properties of COOH⁺/graphene were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and contact angle measurement. Systematic evaluation of anticoagulation, including in vitro platelet adhesion assays and hemolytic assays, proved that COOH⁺/graphene has significant anticoagulation. In addition, at the dose of 5 × 10¹⁷ ions/cm², COOH⁺/graphene responded best on platelet adhesion, aggregation, and platelet activation.     

Exceptional negative thermal expansion and viscoelastic properties of graphene oxide paper

We report unusual negative thermal expansion and viscoelastic properties in graphene oxide paper. The paper was prepared from aqueous GO dispersions using a simple vacuum evaporation technique. From room temperature to 150 °C, this paper-like graphene oxide sheet exhibits a constant negative thermal expansion coefficient of ?67 × 10^?6/°C along the in-plane direction. Peculiar hysteresis loops of thermal expansion-temperature curves are observed, which are affected by the cooling rate and starting temperature of cooling. The tensile tests on GO paper show clear hysteresis loops, revealing the viscoelastic property of the paper. The viscoelastic properties show excellent frequency-stability in the range of 1.0–60 Hz. In the low temperature range ?150 to 25 °C, however, they show a strong temperature dependence. The storage modulus of the graphene oxide paper continuously increases with decreasing temperature.     

Molecular dynamics simulation of mechanical performance of graphene/graphene oxide paper based polymer composites

Highly ordered polymer composites of layered graphene/graphene oxide (GO) sheets, i.e. graphene/GO paper, are attractive candidates for novel structural and functional applications. Here, molecular dynamics simulations are employed to elucidate the structural and mechanical properties of the graphene/GO paper based polymer composites. We find that the large scale properties of these composites are controlled by the conformation and content of polymer molecules within the interlayer galleries. Polymer conformations affect the interlayer spacing, while the polymer content controls the layer–matrix interactions, thereby affecting the elastic modulus of the composites. Additionally, the chemical composition of individual GO sheets also plays a critical role in establishing the mechanical properties of the composites. Specifically, a higher density of oxygen-containing groups leads to the decrease of elastic modulus of individual GO sheets. However, the groups also lead to the increased hydrogen bonds between the GO sheets and polymer molecules, resulting in the corresponding increase in overall stiffness. Our studies suggest the possibility of tuning the properties of graphene/GO paper composites by altering the conformation and content of polymer, as well as the density of functional groups on individual GO sheets.     

Simultaneous enhancement of mechanical,electrical and thermal properties of graphene oxide paper by embedding dopamine

Reduced graphene oxide paper was fabricated by incorporating dopamine. By using adhesive property, reducing ability, high thermal stability, and high carbon-yielding characteristic of dopamine, the dopamine-embedded graphene oxide paper can be mechanically stronger, thermally more stable, and electrically more conductive compared to conventional graphene oxide paper. Herein, in order to preserve a planar structure of graphene sheets and obtain free-standing paper, a simple one-step fabrication method was introduced, in which vacuum-assisted filtration of a graphene oxide suspension and polymerization of dopamine proceed at the same time. The manufactured paper had sequentially layer-stacked and planar structure of graphene sheets and polydopamine. The paper showed increased tensile strength and elongation, and higher thermal stability. Electrical conductivity was recovered, which was not measurable in conventional graphene oxide paper. To investigate enhancement of reduction, thermal annealing was further carried out. As a result, a critical reduction temperature was lowered and higher electrical conductivity was recorded at all annealing temperatures.     

Electrocatalytic activation of tungsten and tungstic oxide doped with platinum by means of ion implantation

Minute quantities of platinum have been introduced into substrates of tungsten and tungstic oxide by means of ion implantation. In this way significant effects on the rate of the cathodic hydrogen evolution reaction have been demonstrated in which the electrocatalytic activity of the surface approaches that of platinum itself.     

Restoration of graphene from graphene oxide by defect repair

A simple and efficient method to repair defects in graphene oxide (GO) is reported, accompanied by a simultaneous reduction process by a methane plasma. The graphene after repair is of high quality. For a typical monolayer after repair and reduction, the minimum sheet resistance at the Dirac point and the Raman D/G peak intensity ratio are about 9.0 kΩ/□ and ～0.53, respectively.     

High-performance supercapacitor of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper

Although supercapacitors have higher power density than batteries, they are still limited by low energy density and low capacity retention. Here we report a high-performance supercapacitor electrode of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper (MnO₂–rGO/CFP). MnO₂–rGO nanocomposite was produced using a colloidal mixing of rGO nanosheets and 1.8 ± 0.2 nm MnO₂ nanoparticles. MnO₂–rGO nanocomposite was coated on CFP using a spray-coating technique. MnO₂–rGO/CFP exhibited ultrahigh specific capacitance and stability. The specific capacitance of MnO₂–rGO/CFP determined by a galvanostatic charge–discharge method at 0.1 A g⁻¹ is about 393 F g⁻¹, which is 1.6-, 2.2-, 2.5-, and 7.4-fold higher than those of MnO₂–GO/CFP, MnO₂/CFP, rGO/CFP, and GO/CFP, respectively. The capacity retention of MnO₂–rGO/CFP is over 98.5% of the original capacitance after 2000 cycles. This electrode has comparatively 6%, 11%, 13%, and 18% higher stability than MnO₂–GO/CFP, MnO₂/CFP, rGO/CFP, and GO/CFP, respectively. It is believed that the ultrahigh performance of MnO₂–rGO/CFP is possibly due to high conductivity of rGO, high active surface area of tiny MnO₂, and high porosity between each MnO₂–rGO nanosheet coated on porous CFP. An as-fabricated all-solid-state prototype MnO₂–rGO/CFP supercapacitor (2 × 14 cm) can spin up a 3 V motor for about 6 min.     

High quality graphene sheets from graphene oxide by hot-pressing

We report a simple and effective route to convert graphene oxide sheets to good quality graphene sheets using hot pressing. The reduced graphene oxide sheets obtained from graphene oxide by low temperature thermal exfoliation are annealed at 1500 °C and 40 MPa uniaxial pressures for 5 min in vacuum. No appreciable oxygen content was observed from X-ray photoelectron spectroscopy and no D peak was detected in the Raman spectrum. The graphene sheets produced had a much higher electron mobility (1000 cm² V⁻¹ S⁻¹) than other chemically modified graphenes.     

Deformation and failure mechanisms in graphene oxide paper using in situ nanomechanical tensile testing

Graphene oxide (GO) paper is a promising candidate for novel applications in energy storage systems such as electrical batteries, supercapacitors and multi-layered composites where the material undergoes deformation mechanisms. In particular, the strength of graphene oxide paper is critical in such applications and is defined by the interaction between the GO sheet constituents of the paper. The deformation behavior and tensile strength of focused ion beam (FIB) fabricated GO micro-beams was measured using in situ atomic force microscopy (AFM). GO sample deformation and failure was dependent on both the size of the micro-beams and the environmental testing conditions. Specifically, the failure stress of GO paper micro-beams tensile tested in air was found to increase when compared to testing in vacuum. This environmental dependent tensile strength of GO paper is attributed to water promoting stress transfer between GO sheets within the paper for higher strength during air testing while vacuum conditions remove water, leading to poor stress transfer between GO sheets for lower tensile strength results. A two-parameter Weibull distribution is introduced to quantify the micro-beam size dependent strength, which is attributed to interfacial defects determining GO paper failure strength.     

Surface attachment of horseradish peroxidase to nylon modified by plasma‐immersion ion implantation

The surface of polyamide (nylon 6) was modified by plasma‐immersion ion implantation (PIII) with nitrogen ions. Structural changes associated with carbonization, oxidation, and depolymerization were observed in the modified surface layer with Fourier transform infrared/attenuated total reflection (FTIR–ATR) spectroscopy, surface energy measurements, and X‐ray photoelectron spectroscopy (XPS). The enzyme activity of surface‐attached horseradish peroxidase was studied with a tetramethylbenzidine colorimetric activity assay. Compared to untreated controls, the PIII‐treated surface showed a higher level of the attached protein with increased longevity of bioactivity. Detection of the immobilized protein layer was made difficult by the presence of amide groups in nylon. Here we demonstrate the potential of combining FTIR–ATR spectroscopy with XPS measurements for this purpose. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120:2891–2903, 2011     

Xenon ion implantation induced defects and amorphization in 4H–SiC: Insights from MD simulation and Raman spectroscopy characterization

Xenon Focused Ion Beam (Xe-FIB) processing of 4H–SiC is an emerging technique with great potential for various applications. In this study, we investigate the evolution mechanism of damage caused by xenon ion implantation in 4H–SiC using a combination of molecular dynamics (MD) simulation and Raman spectroscopy. The study explores the microscopic mechanisms of damage processing and repairs using the proper potential function and the optimized simulation model. The MD simulation reveals that the vacancy and interstitial sites of silicon and carbon atoms, as identified by the Wigner-Seitz defect method, increase linearly with implanted dose until the dose reaches 2 × 10¹⁴ ions/cm². Subsequently, the growth rate of each defect site in the damaged area slows down and eventually comes to a saturation state with a continuous increase in dose. The growth rate of the amorphous region also slows down with the constant increase in dose, similar to the results obtained through variable temperature Raman spectroscopy characterization experiments on 4H–SiC (0001) nitrogen-doped substrates implanted with different doses of xenon ions. Furthermore, unlike light ions such as hydrogen and helium, Xe ions cause significant damage to the inside of 4H–SiC, resulting in the inability to produce structurally complete silicon vacancy defects. Our findings provide insights into the fundamental mechanism of Xe-FIB processing and have implications for future applications in semiconductor technology.     

Multiscale patterning of graphene oxide and reduced graphene oxide for flexible supercapacitors

A simple and facile method for multiscale, in-plane patterning of graphene oxide and reduced graphene oxide (GO–rGO) was developed by region-specific reduction of graphene oxide (GO) under a mild irradiation. The UV-induced reduction of graphene oxide was monitored by various spectroscopic techniques, including optical absorption, X-ray photoelectron spectroscopy (XPS), Raman, and X-ray diffraction (XRD), while the resultant GO–rGO patterned film morphology was studied on optical microscope, scanning electron microscope (SEM), and atomic force microscope (AFM). Flexible symmetric and in-plane supercapacitors were fabricated from the GO–rGO patterned polyethylene terephthalate (PET) electrodes to show capacitances up to 141.2 F/g.     

Additive-free hydrogelation of graphene oxide by ultrasonication

Graphene oxide hydrogels have been prepared by ultrasonication of precursor aqueous dispersions. The ultrasonication fractures the nanosheets, reducing their dimensions and exposing new sheet edges that do not possess the stabilizing carboxyl functional groups found along the edge of the as-prepared material. Ultrasonication does not affect the overall chemical functionality of the graphene oxide nanosheets, as spectra (carbon-13 nuclear magnetic resonance spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy) of samples before and after ultrasonication are nearly identical. Gelation is induced after only 30 min of ultrasonication to achieve a relatively weak gel with a shear modulus of 0.3 kPa; however, extension of ultrasonic treatment to 120 min yields a more robust hydrogel with a shear modulus of 1.6 kPa. Such enhancement in the gel’s physical properties can be attributed to the lack of stabilizing carboxyl groups on newly generated nanosheet fragments from the interior regions of the original nanosheets. As prepared, these hydrogels exhibit exceptionally low critical gelation concentrations ranging from ～0.050 to ～0.125 mg mL^?1 that can be tuned according to the extent of ultrasonic treatment.     

Preparation of graphene by the rapid and mild thermal reduction of graphene oxide induced by microwaves

Rapid and mild thermal reduction of graphene oxide (GO) to graphene was achieved with the assistance of microwaves in a mixed solution of N,N-dimethylacetamide and water (DMAc/H₂O). The mixed solution works as both a solvent for the produced graphene and a medium to control the temperature of the reactive system up to 165 °C. Fourier transform infrared spectrometry, X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and thermogravimetric analysis confirmed the formation of graphene under this mild thermal reduction condition. The reduction time is found to be in the scale of minutes. The as-prepared graphene can be well dispersed in DMAc to form an organic suspension, and the suspension is stable for months at room temperature. The conductivity of graphene paper prepared by the microwave reduced product is about 10⁴ times than that of GO paper.