Reduced Graphene Film Research Articles

Preparation of Reduced Graphene Oxide Films with High and Uniform Thickness for Electromagnetic Interference Shielding

Reduced graphene films have attracted widespread commercial interest due to high electrical conductivity toward (EMI) shielding. At present, the preparation path of reduced graphene film is to use graphene oxide (GO) as the raw material through self-assembly and high-temperature heat treatment. However, the thickness of reduced graphene films is not high and uniform because of the higher mobility of the graphene oxide slurry, which destroys the reliability of the membrane in the field of electromagnetic interference shielding. Here, we propose the use of sodium carboxymethyl cellulose (CMC) to increase the viscosity of graphene oxide to prepare reduced graphene films with high and uniform thickness. After modification with sodium carboxymethyl cellulose, the EMI shielding effectiveness (EMI SE) of reduced graphene oxide films stabilized at 91–96 dB at 8–12 GHz, which is higher than pure graphene films. Meanwhile, the addition of CMC does not affect the structure of reduced graphene films. This work broadens the application of reduced graphene films in electromagnetic shielding.

Grafen Oksit Tabakalanma Derecesinin Grafen Film Özelliklerine Etkisi

Interest in graphene, which is a new material, is increasing day by day due to its unique mechanical, electrical, and thermal properties. Graphite oxide (GRO), which is synthesized as a result of a series of chemical processes of graphite, was used as the primary material in the production of graphene film. In this study, firstly, graphene oxide (GO) dispersions with different exfoliation degrees were prepared from de-ionized water (DI-water) and repeatedly washed GRO mixture at different ultrasound times. Then, GO films with the same film thickness were produced by casting of the prepared GO dispersions. GO films with different exfoliation degrees were thermally reduced to graphene at 1100 oC, and the change in reduced graphene film (RGO) properties was investigated. XRD, SEM, and FTIR measurements were made for the characterization of thermally reduced graphene and GO films. While the maximum degree of exfoliation was obtained for GO films in 8 hours of ultrasound, the maximum degree of exfoliation was obtained even in 3 hours of ultrasound duruationby thermal reduction of GO films at 1100 oC.

Reduction of Electrochemically Exfoliated Graphene Films for High-Performance Electromagnetic Interference Shielding.

Two-dimensional graphene is of great interest for electromagnetic interference (EMI) shielding owing to its inherent electrical conductivity, lightweight, and excellent mechanical flexibility even at minor thicknesses. However, the complex synthesis and quality-control difficulties limit its application. In this study, we demonstrate that electrochemically exfoliated graphene (EEG) with post-reduction treatment is a promising candidate for lightweight EMI shielding materials. A facile electrochemical exfoliation approach produces a high-quality multilayer graphene with a high electrical conductivity of ∼600 S cm-1, owing to its low degree of oxidation. The reduction of EEG by three different methods, including chemical, thermal, and microwave treatments, causes the removal of surface functional groups as well as significant changes in the microstructure of the final films. The reduced graphene films by microwaves, which are driven by the improved electrical conductivity and large volume expansion, exhibit an EMI shielding effectiveness of 108 dB at a thickness of 125 μm, one of the largest EMI shielding values ever reported for graphene at comparable thicknesses.

Enhancing the Efficiency of Graphene Oxide Reduction in Low-Power Digital Video Disc Drives by a Simple Precursor Heat Treatment.

Laser reduction of graphene oxide (GO) produces graphene effectively. As a low-power laser source, commercial digital video disc (DVD) drives provide a versatile platform to produce reduced graphene films in designed 2D patterns. However, research on how GO characteristics affect its laser reduction efficiency in DVD drives is rarely conducted. Here, we investigate how heating the GO dispersion affects the photoreduction process of GO films in a LightScribe DVD drive. Without noticeably changing the oxygen content, such mild heat treatment significantly improves GO's absorption in the visible region, resulting in significant enhancement on GO's laser reduction efficiency. We demonstrate that the laser reduction efficiency increases with the increasing treatment time. The enhanced reduction level greatly improves the performance of laser-scribed graphene electrodes in applications such as glucose sensors (with an optimal linear response range up to 2550 μM) and supercapacitors (with an optimal areal capacity of 1.37 mF cm-2 at the scan rate of 50 mV s-1). This proposed approach provides general insights into the production of laser-reduced graphene with low-power laser sources, for advanced device applications such as wearable electronics and flexible microelectronics.

Reinforced heat dissipation by simple graphene coating for phosphor-in-glass applied in high-power LED

A well dispersed graphene oxide sol was prepared by the improved Hummer method. The atomic force micrographs showed a monolayer structure and larger sheet area. Aiming at improving the poor heat dissipation ability of Phosphor-in-glass (PiG)for remote packaged LED, the reduced graphene oxide film was prepared by photoreduction that after the sol was coated on PiG. UV–Vis spectra showed that the degree of reduction of graphene oxide was related to the illumination time, while the reduced graphene film had high transmittance without affecting the luminescent properties of the fluorescent material. At the same time, the excellent thermal conductivity of the film significantly enhanced the cooling capacity of the PiG, which has a lower stable operating temperature and a higher luminous intensity under high-power excitation compared with the uncoated ones. Software simulation also showed similar results of heat dissipation.

Transparent and flexible ultraviolet photoconductors based on solution-processed graphene quantum dots on reduced graphene oxide films

A combination of graphene quantum dots and chemically reduced graphene oxide films was obtained by spray-coating their dispersions onto an indium tin oxide coated polyethylene terephthalate substrate. Herein, the graphene quantum dots played a major role in the absorption of ultraviolet light and generation of carriers, whereas the reduced graphene films acted as the transparent electron pathway network. Consequently, the fabricated structure showed good transparency and flexibility, and achieved noticeable ultraviolet sensing. The facile, inexpensive, and extremely low temperature techniques used can be easily upgraded to a larger scale and are compatible with other substrates. In addition, a combination of reduced graphene oxide and graphene quantum dots offers a potential graphene-based platform for foldable optoelectronic device applications.

Electrochemical Preparation of Polypyrrole/Graphene Films on Titanium Mesh as Active Materials for Supercapacitors

Films of polypyrrole/graphene on titanium mesh were prepared by electrochemical reduction of the fresh dried foam films of graphene oxide followed by an electrochemical polymerization of pyrrole. The as-obtained composite had highly surface area, conductivity, and could be used as the electrode for supercapacitors, especially directly used as the active materials in free of binders while the Ti mesh worked as the collector. Plenty of polypyrrole nanoparticles formed on the surface of reduced graphene film, and some fiber-like aggregates could be formed during the polymerization, which worked as the material for pseudo-capacitance. The specific capacitance of the supercapacitor reached 400 F/g and showed high stability with retaining capacitance of 82% after 5000 cycles, indicating that the nanocomposite is a suitable active material for supercapacitors.

Wet-Spun Continuous Graphene Films

Macroscopic assembled, self-standing graphene and graphene oxide (GO) films have been demonstrated as promising materials in many emerging fields, such as Li ion battery electrodes, supercapacitors, heat spreaders, gas separation, and water desalination. Such films were mainly available on centimeter-scale via the time- and energy-consuming vacuum-filtration method, which seriously impedes their progress and large-scale applications. Due to the incompatibility between large-scale and ordered assembly structures, it remains a big challenge to access large-area assembled graphene thick films. Here, we report for the first time a fast wet-spinning assembly strategy to produce continuous GO and graphene thick films. A 20 m long, 5 cm wide, well-defined GO film was readily achieved at a speed of 1 m min–1. The continuous, strong GO films were easily woven into bamboo-mat-like fabrics and scrolled into highly flexible continuous fibers. The reduced graphene films with high thermal and moderate electrical conduc...

Workfunction-Tunable, N-Doped Reduced Graphene Transparent Electrodes for High-Performance Polymer Light-Emitting Diodes

Graphene is a promising candidate to complement brittle and expensive transparent conducting oxides. Nevertheless, previous research efforts have paid little attention to reduced graphene, which can be of great benefit due to low-cost solution processing without substrate transfer. Here we demonstrate workfunction-tunable, highly conductive, N-doped reduced graphene film, which is obtainable from the spin-casting of graphene oxide dispersion and can be successfully employed as a transparent cathode for high-performance polymer light-emitting diodes (PLEDs) as an alternative to fluorine-doped tin oxide (FTO). The sheet resistance of N-doped reduced graphene attained 300 Ω/□ at 80% transmittance, one of the lowest values ever reported from the reduction of graphene oxide films. The optimal doping of quaternary nitrogen and the effective removal of oxygen functionalities via sequential hydrazine treatment and thermal reduction accomplished the low resistance. The PLEDs employing N-doped reduced graphene cathodes exhibited a maximum electroluminescence efficiency higher than those of FTO-based devices (4.0 cd/A for FTO and 7.0 cd/A for N-doped graphene at 17,000 cd/m(2)). The reduced barrier for electron injection from a workfunction-tunable, N-doped reduced graphene cathode offered this remarkable device performance.

Spin coated graphene films as the transparent electrode in organic photovoltaic devices

Many research efforts have been devoted to the replacement of the traditional indium–tin-oxide (ITO) electrode in organic photovoltaics. Solution-based graphene has been identified as a potential replacement, since it has less than two percent absorption per layer, relative high carrier mobility, and it offers the possibility of deposition on large area and flexible substrates, compatible with roll to roll manufacturing methods. In this work, soluble reduced graphene films with high electrical conductivity and transparency were fabricated and incorporated in poly(3-hexylthiophene) [6,6]-phenyl-C 61-butyric acid methyl ester photovoltaic devices, as the transparent electrode. The graphene films were spin coated on glass from an aqueous dispersion of functionalized graphene, followed by a reduction process combining hydrazine vapor and annealing under argon, in order to reduce the sheet resistance. The photovoltaic devices obtained from the graphene films showed lower performance than the reference devices with ITO, due to the higher sheet resistance (2 kΩ/sq) and the poor hydrophilicity of the spin coated graphene films.

Vertical ZnO nanowires/graphene hybrids for transparent and flexible field emission

We present a transparent and flexible optoelectronic material composed of vertically aligned ZnO NWs grown on reduced graphene/PDMS substrates. Large-area reduced graphene films were prepared on PDMS substrates by chemical exfoliation from natural graphitevia oxidative aqueous dispersion and subsequent thermal reduction. ZnO NWs were hydrothermally grown on the reduced graphene film substrate and maintained their structural uniformity even in highly deformed states. The electrical contact between semiconducting ZnO NWs and the metallic graphene film was straightforwardly measured by electric force microscopy (EFM). It shows a typical metal–semiconductor ohmic contact without a contact barrier. Owing to the mechanical flexibility, transparency, and low contact barrier, the ZnO NWs/graphene hybrids show excellent field emission properties. Low turn-on field values of 2.0 V μm−1, 2.4 V μm−1, and 2.8 V μm−1 were measured for convex, flat, and concave deformations, respectively. Such variation of field emission properties were attributed to the modification of ZnO NWs emitter density upon mechanical deformation.

Flexible Field Emission of Nitrogen‐Doped Carbon Nanotubes/Reduced Graphene Hybrid Films

The outstanding flexible field emission properties of carbon hybrid films made of vertically aligned N-doped carbon nanotubes grown on mechanically compliant reduced graphene films are demonstrated. The bottom-reduced graphene film substrate enables the conformal coating of the hybrid film on flexible device geometry and ensures robust mechanical and electrical contact even in a highly deformed state. The field emission properties are precisely examined in terms of the control of the bending radius, the N-doping level, and the length or wall-number of the carbon nanotubes and analyzed with electric field simulations. This high-performance flexible carbon field emitter is potentially useful for diverse, flexible field emission devices.

Surface Energy Modification by Spin-Cast, Large-Area Graphene Film for Block Copolymer Lithography

We demonstrate a surface energy modification method exploiting graphene film. Spin-cast, atomic layer thick, large-area reduced graphene film successfully played the role of surface energy modifier for arbitrary surfaces. The degree of reduction enabled the tuning of the surface energy. Sufficiently reduced graphene served as a neutral surface modifier to induce surface perpendicular lamellae or cylinders in a block copolymer nanotemplate. Our approach integrating large-area graphene film preparation with block copolymer lithography is potentially advantageous in creating semiconducting graphene nanoribbons and nanoporous graphene.

Flexible room-temperature NO2 gas sensors based on carbon nanotubes/reduced graphene hybrid films

We present a flexible room temperature NO2 gas sensor consisting of vertical carbon nanotubes (CNTs)/reduced graphene hybrid film supported by a polyimide substrate. The reduced graphene film alone showed a negligible sensor response, exhibiting abnormal N–P transitions during the initial NO2 injection. A hybrid film, formed by the growth of a vertically aligned CNT array (with CNTs 20 μm in length) on the reduced graphene film surface, exhibited remarkably enhanced sensitivities with weak N–P transitions. The increase in sensitivity was mainly attributed to the high sensitivity of the CNT arrays. The outstanding flexibility of the reduced graphene films ensured stable sensing performances in devices submitted to extreme bending stress.

Growth Kinetics of Wall-Number Controlled Carbon Nanotube Arrays

We present a simple model for the growth and termination of carbon nanotube (CNT) arrays. This model was developed by modifying several submodels and provides a comprehensive predictive tool for the CNT growth process as a function of CNT wall number. Wall number, diameter, and areal density-controlled CNT arrays were grown on a reduced graphene film by combining block copolymer lithography and plasma-enhanced chemical vapor deposition (PECVD) method. The number of carbonaceous platelets required for the termination of CNT growth process was measured by high-resolution transmission electron microscopy (HRTEM), and the effective exposed catalyst surface area was measured by high magnification scanning electron microscopy (SEM). A parametric study showed that our simple kinetic model successfully predicted the kinetics of wall number-dependent CNT growth and could be used to optimize the PECVD process for CNT production.