Reduced Graphene Oxide Films Research Articles

Transport and Durability of Energy Storage Materials Operating at High Temperatures.

Temperature is a state variable that significantly affects thermodynamic and kinetic performances and performance degradation of energy storage materials. In this Perspective, we address our recent progress in the energy storage performance and transporting phenomena of supercapacitors when temperatures are elevated to >100 °C. Electrodes include reduced graphene oxide film and foam and conductive metal organic frameworks; electrolytes include phosphoric-acid-doped polybenzimidazole and double networked ionogels. The electrochemical, thermal, and mechanical properties of electrodes and electrolytes are correlated with energy storage performance and degradation at high temperatures. We also address the fundamental understanding of ion transport of polymeric electrolytes and the emergence of nanoscale-confined fast mobile protons at elevated temperatures.

Low resistance, high mobility reduced graphene oxide films prepared with commercial antioxidant supplements

In this work, conductive reduced graphene oxide films onto glass substrates were prepared by immersion of the substrate at room temperature in a suspension of graphene oxide and antioxidant commercial supplements such as resveratrol, omega-3, triple-dosed omega-3 and astaxanthin. The films prepared with triple-dosed omega-3 and astaxanthin showed a defined diffraction peak at the graphite 2 theta position and the lower oxygen contents. Raman spectra indicate the restoring of the sp2 character and increase in the crystallite size, which increased the electron mobility from 0.009 cm2V−1s−1 in the GO film to 68 cm2V−1s−1 and decreased sheet resistance from 4.6 × 1010 ohm/sq in the graphene oxide film to the lowest achieved sheet resistance of 92.5 O/sq in the film reduced with astaxhantin.

Electrochemically deposited graphene oxide thin film supercapacitors: Comparing liquid and solid electrolytes

In the present work, graphene oxide (GO) thin films were prepared on ITO substrates by electrochemical deposition for different voltages and deposition times and were further electrochemically reduced by cyclic voltammetry (CV). Reduced GO films (rGO) were used in the manufacturing of supercapacitors with solid-state electrolyte based on a freeze-thawed PVA-Na2SO4 hydrogel. GO and rGO films were tested using SEM, XRD, RAMAN and XPS techniques for comparative reasons. The specific capacitance of the electrodes and supercapacitors were estimated by electrochemical methods. XRD analysis revealed increase of the rGO interlayer spacing after multiple CV cycles that was assigned to incorporation of the electrolytic agents such as sodium ions of the electrolyte. Raman spectra showed that the ID/IG ratio associated with the graphene matrix defects remained fairly constant even after several CV cycles, meaning that no new defects were added. It was the XPS analysis that confirmed the reduction of GO to rGO as well as the presence of sodium in the rGO films. The optimum value of the electrode capacitance of the constructed model supercapacitor with thin rGO films and PVA-20%Na2SO4 electrolyte was 8.55 mF/cm2 that approaches the electrode capacitance value of 10.67 mF/cm2 measured with liquid Na2SO4 electrolyte.

Fabrication, structural evolutions and properties of large-area orientation reduced graphene oxide films by self-assembly at the air–water interface and thermal treatment

A very facile and efficient method was supplied to fabricate the large-area orientation RGO films by the interface self-assembly and low-temperature thermal reduction. The structural evolutions of RGO films were systematically investigated by AFM, FT-IR, Raman scattering and XPS spectra. The conductivity and wettability of RGO films with temperature changes were quantitatively evaluated by the four-probe measurement and contacted angle testing. As a result, the RGO films could be well reduced in the 200 ~ 300 °C for 2 h under air atmosphere.

Temperature and environment effects on the graphene oxide reduction via electrical conductivity studies

In this article, we report a study carried out to understand how changes in environment and temperature, in which the graphene oxide (GO) film reduction occurs, alter the electrical properties of the resulting films. In fact, it is possible to improve the GO reduction efficiency by choosing suitable environment and temperature. To this purpose, three different environments (high vacuum, air and nitrogen) and three dissimilar temperatures (177 °C, 300 °C and 600 °C) have been chosen, as suggested by differential scanning calorimetry (DSC) measurements. Among the investigated thermally reduced graphene oxide films, the ones treated at 600 °C in nitrogen have shown to possess higher electrical conductivity. Such an enhanced electrical conductivity can be attributed to reduction of oxygen functional groups and restoration of sp2 carbon network, as confirmed by Raman spectra.

Poorly conductive electrochemically reduced graphene oxide films modified with alkyne chains to avoid the corrosion-promoting effect of graphene-based materials on carbon steel

Recently, the discovery of the corrosion-promoting effect of graphene has hindered the progress in the use of graphene-based materials as anticorrosive coatings. This work proposes to avoid the corrosion-promoting effect by the introduction of favorable defects to electrochemically reduced graphene oxide (ErGO), which consisted in the hydrogenation of sp2 domains during the removal of O-groups in graphene oxide, favoring sp3 domains and low conductivity. The ErGO films were subsequently modified with 4-pentyonate chains to obtain films with lower conductivity and enhanced hydrophobicity (HrGO). The alkyne chains were bonded to the remaining sp2 domains in ErGO by a simple electrochemical process. Both films were characterized by FT-IR, XPS, Raman spectroscopy and electrochemical methods. As anticorrosive coating, the HrGO film achieved a 4-times decrease in the corrosion current and showed a longer lifetime in comparison to the ErGO film, and an 18-times decrease compared to the bare carbon steel. According to the galvanic current measurements, neither of the films presented a corrosion-promoting effect, which was attributed to their low conductivity, 13 and 1.3 S cm−1 for ErGO and HrGO, respectively. These results show that the corrosion promoting effect of graphene can be avoided using correctly engineering ErGO with adequate defects.

Graphene-Coated Porous PDMS As a Gas Sensor for Potential Epidermal Electronics Applications

Introduction Epidermal electronics that can intimately integrate with human skin are getting much attention due to ultrathin, soft, and light-weight [1]. These capabilities create interesting opportunities for diverse wearable sensor applications such as sweat monitoring, multiple electrophysiological signal monitoring, subtle motion sensing, ultraviolet sensing, gas monitoring, etc [2]. The authors have been developed graphene-based gas sensors having various forms including electrospun fiber [3], commercially-available yarn [4], stretchable yarn [5], and fabric [6]. In this paper, we present the stretchable porous e-skin based gas sensor comprising of porous polydimethysiloxane (PDMS) as a stretchable substrate and reduced graphene oxide (RGO) as gas sensing materials showed high NO2 sensitivity under 30% strain at room temperature. Method Porous PDMS thin films with the average pore size of 10 um were achieved by breath figure method [7]. The porous PDMS film were functionalized with 3-aminopropyltriethoxysilane (APTES) via chemical vapor treatment, which induced positive charges on the surface of porous PDMS film. Spray coating was performed with a commercial spray coater system (Jinsung, Korea). GO dispersed solution of 1.0 mg/mL was deposited onto the amine functionalized periodic porous PDMS films. The prepared GO deposited porous PDMS films were chemically reduced by immersing them in a reduction agent of hydriodic acid solution at 50 °C for 20 min. To neutralize and remove any residual on the RGO porous PDMS films, the samples were repeatedly rinsed with a sodium bicarbonate solution and deionized water, and then dried in a chemical hood. The stretchable gas sensors were fabricated by deposition silver paste onto the tops of the RGO films to make electrodes by silk screening method. Results and Conclusions The RGO porous PDMS films exhibited distinctive wrinkles and ripples with high density on its surface, which was attributed to the successful coating of RGO sheets on the surface of porous PDMS film. We believe that numerous wrinkles make the RGO porous PDMS films more stable against several deformations. The Raman spectrum of RGO porous PDMS films exhibited typical RGO spectral features, such as the D peak at 1340 cm-1 (defects in graphene) and the G peak at 1580 cm-1 (a stretching mode of C-C bond of the sp2 structure of graphene).We also studied the electrical properties of porous PDMS films coated with RGO sheets using four probe method. A GO coated porous PDMS film yielded an electrical conductivity of about 10-8 S/cm. After chemical reduction, the conductivity of RGO porous PDMS increased by approximately seven orders of magnitude. Our RGO porous PDMS films could maintain stable electrical conductivity values upon about 30% strain deformation. The mechanical stability of the RGO porous PDMS films was evaluated by measuring the electrical conductivity under repeated stretching and releasing. The electrical resistance of the samples remains unchanged even after 5000 cycles. The as-prepared RGO porous films are mechanically robust and can be deformed without device performance degradation, making them very attractive candidates for epidermal electronic devices.Porous PDMS coated with reduced graphene oxide responded to NO2 gas with sensitive and selective manner. When we compare the response (Ra/Rg) of porous PDMS with that of non-porous PDMS, porous PDMS shows 20% stronger response to NO2 compared to non-porous PDMS. The porous nature is considered to contribute to strong response due to increase of reaction possibility with reduced graphene oxide and NO2 gas. We found that the response of gas sensor was increased around 20% upon elongation. We believe that the hidden area of pores in the PDMS matrix can be exposed to the gas molecules upon elongation. Finally, we found that the sensor shows reliability in terms of the response up to 5,000 times elongation-relaxation cycles. In the future, we expect that the thin porous PDMS can be attached to the skin and find the applications in healthcare by sensing gas molecules emitted from skin. Acknowledgement This research was supported the R&D Program (Development of a mobile diet-monitoring technology based on multiple biomarkers by the National Research Foundation of Korea under research projects, NRF-2017M3A9F1033056).

Flexible Metal‐Free Memory Electronic Made of π‐Conjugation‐Interrupted Hyperbranched Polymer Switch and Reduced Graphene Oxide Electrodes

AbstractA flexible metal‐free flash memory device with reduced graphene oxide films as electrodes and diarylfluorene‐based π‐conjugation‐interrupted hyperbranched polymers (CIHPs) as active switches is reported. Two CIHPs, named PCzPF and PCzPF‐PF, are synthesized via BF3•Et2O‐catalyzed fluorenol’s Friedel–Crafts reaction, where the PCzPF‐PF is designed by the decoration of hindrance functional groups, i.e., bulky 9‐phenyl‐fluorenyl (PF) moieties, at the end‐groups of PCzPF. Both of the two CIHPs exhibit excellent solubility and thermal stability. The PCzPF‐based device exhibits no switching effect while the PCzPF‐PF‐based device shows a flash type memory effect, indicating the incorporation of PF groups plays a critical role in realizing electrically bistable behaviors. In particular, the optimized memory exhibits a high ON/OFF ratio of > 3.0 × 103, long retention time of up to 1.2 × 104 s, and high mechanical stability. This work opens a new avenue for metal‐free memory electronics through a low‐cost and full‐solution process approach.

High-speed femtosecond laser plasmonic lithography and reduction of graphene oxide for anisotropic photoresponse

Micro/nanoprocessing of graphene surfaces has attracted significant interest for both science and applications due to its effective modulation of material properties, which, however, is usually restricted by the disadvantages of the current fabrication methods. Here, by exploiting cylindrical focusing of a femtosecond laser on graphene oxide (GO) films, we successfully produce uniform subwavelength grating structures at high speed along with a simultaneous in situ photoreduction process. Strikingly, the well-defined structures feature orientations parallel to the laser polarization and significant robustness against distinct perturbations. The proposed model and simulations reveal that the structure formation is based on the transverse electric (TE) surface plasmons triggered by the gradient reduction of the GO film from its surface to the interior, which eventually results in interference intensity fringes and spatially periodic interactions. Further experiments prove that such a regular structured surface can cause enhanced optical absorption (>20%) and an anisotropic photoresponse (~0.46 ratio) for the reduced GO film. Our work not only provides new insights into understanding the laser-GO interaction but also lays a solid foundation for practical usage of femtosecond laser plasmonic lithography, with the prospect of expansion to other two-dimensional materials for novel device applications.

Dense Reduced Graphene Oxide Films Obtained by Pressing Create Stable and Compact Capacitive Energy Storage

AbstractIt is generally believed that the re‐stacking of reduced graphene oxide (rGO) sheets limits the electrochemical energy storage. Therefore, rGO based electrodes are usually designed to be non‐compact porous and fluffy structures. However, fluffy structures could reduce the conductance, structure stability, and cycling stability of electrodes. Here, to balance the contradiction, three electrodes with different compactness were studied. Amazingly, dense rGO hydrogel films (DrGO‐HF) achieved by simply pressing filtrated rGO films, show highly stable (92 % capacitance retention after 250 000 cycles) and high‐performance compact capacitive energy storage, which can be attributed to the structural stability and electrical conductivity enhanced by the close contact between graphene sheets. This study clarifies the scientific fact that the re‐stacking of rGO sheets would not degrade the electrochemical energy storage performances, which is of great significance for the design and preparation of supercapacitor electrodes with high volumetric energy density.

Extremely Foldable and Highly Porous Reduced Graphene Oxide Films for Shape-Adaptive Triboelectric Nanogenerators.

Here, a thin and foldable porous reduced graphene oxide (rGO) fabricated by a solvent casting method (SC-rGO) is introduced. The SC-rGO is superior to aluminum as a positive triboelectric material in triboelectric nanogenerators (TENGs), significantly enhancing TENG output performance. The film shows extremely foldable features, where it could be folded by 1/16 size. The electrical properties and device performance of SC-rGO are optimized varying thicknesses from 5 to 30 µm. A 30 µm thick TENG with a non-annealed SC-rGO film (STENG) shows the highest output of about 255 µW cm-2 due to its high carrier concentration, low work function, and high surface area. After annealing, STENG performance is optimized with a 10 µm thick SC-rGO because their work functions decreases, while the corresponding carrier concentrations decrease according to the thickness of the SC-rGO films. The SC-rGO films are highly durable and stable, where their output and conductivity show negligible changes after 100 000 cycles of mechanical deformation. A large SC-rGO with a size of 13 × 3 cm2 is fabricated and is attached inside a person's arm to demonstrate the shape-adaptive characteristics. Consequently, 170 V is obtained and it turns on 19 green light emitting diodes by simply touching the STENG.

Highly Ordered and Dense Thermally Conductive Graphitic Films from a Graphene Oxide/Reduced Graphene Oxide Mixture

Summary We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1.

Multifunctional reduced graphene oxide coating on laminated composites

Carbon Fibre Reinforced Plastics (CFRPs) are commonly used for structural applications due to their high specific mechanical properties. CFRPs can also be functionalized exploiting a combination of several materials that introduce multifunctional features to the global performance of a structure and widen their range of operations. This work investigates the use of a Reduced Graphene Oxide (RGO) Film as multifunctional coating on CFRP laminates. Exploiting the inherent properties of these films, surface properties of composite structures such as electrical conductivity and wettability can be improved. Moreover, potential built-in functions, as live strain sensing and DC-biased thermography, are studied. Three point bending tests demonstrated a negligible influence of the RGO films on the flexural properties of the CFRP laminates and confirmed a satisfying adhesion between the coating and the structure.

Wafer-scale fabrication of high-purity reduced graphene oxide films as ultrahigh-frequency capacitors with minimal self-discharge

Convenient integration of high-purity graphene film on industrial-sized silicon wafer is highly desired for on-chip electronics such as pulse energy harvesting capacitors. However, current graphene materials usually need costly additional purification procedures to remove metallic impurities before use. Herein, a purification-integration technique is explored for graphene oxide (GO) by using the reverse migrations of metal ions and negatively charged GO sheets in water under electric fields, and corresponding films are successfully deposited on 6-inch anodic silicon wafers in one step. Nearly ~90% of metallic impurities (e.g., Fe, Mn, and Cr) could be removed for the electric-field purified reduced graphene oxide film (EPGF) with the content of Cr as low as ~10 ppm. Owing to the high purity and the robust interface contact to silicon substrates, the EPGF as capacitor electrodes delivered the minimal self-discharging current of ~0.11 μA and the highest working frequency of 18420.7 Hz (i.e., up to 10 kHz-level at −73.5°) for graphene capacitors reported to date, as well as maintaining normal areal capacitance. In contrast to the common strategy by increasing ion rate, this study focuses on the improvement of electron rate by optimizing electronic interface contact, which provides new insights for fabricating kilohertz graphene capacitors.

Chemical Changes of Graphene Oxide Thin Films Induced by Thermal Treatment under Vacuum Conditions

Reduction of graphene oxide is one of the most promising strategies for obtaining bulk quantities of graphene-like materials. In this study, graphene oxide was deposited on SiO2 and reduced by annealing at 500 K under vacuum conditions (5 × 10−1 Pa). Here, graphene oxide films as well as their chemical changes upon heating were characterized in depth by X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron and atomic force microscopies. From the chemical point of view, the as prepared graphene oxide films presented a large quantity of oxidized functional groups that were reduced to a large extent upon heating. Moreover, residual oxidized sulfur species that originated during the synthesis of graphene oxide (GO) were almost completely removed by heating while nitrogen traces were integrated into the carbon framework. On the other hand, regarding structural considerations, reduced graphene oxide films showed more homogeneity and lower roughness than graphene oxide films.

Broadband photoresponse of flexible textured reduced graphene oxide films

Herein, we fabricated flexible and transparent textured graphene films based on reduced graphene oxide flakes on polyethylene terephthalate substrates using a spray coating technique and investigated their photoresponsivity over a broadband range of wavelengths from the ultraviolet to infrared regions. The fabricated films exhibited a broadband photoresponse, with responsivity values reaching 2.5 mA/W at 365 nm, 2.4 mA/W at 530 nm, and 1.7 mA/W at 940 nm. In addition, the photoresponsivity was studied under bending deformation. Our flexible and textured reduced graphene oxide films could maintain a stable photoresponse upon bending for the radii of curvature larger than ~ 0.35 cm with a photocurrent deviation of <10% in the infrared range. The broadband photoresponse with good flexibility and transparency suggests that the reduced graphene oxide films can have immense potential for applications in flexible optoelectronic technologies.

Electricity generation by sliding an ionic solution droplet on a self-assembled reduced graphene oxide film

A voltage at the millivolt level could be generated by moving ionic droplets on a partially reduced, smooth graphene oxide film.

The role of iodine in the enhancement of the supercapacitance properties of HI-treated flexible reduced graphene oxide film: an experimental study with insights from DFT simulations

Iodine on graphene frameworks enhances the specific capacitance towards supercapacitor applications.

Adhesion of Bacteria to a Graphene Oxide Film.

Graphene oxide (GO)-based materials have recently received increasing attention in antibacterial fields due to their unique two-dimensional structure, high specific surface area, water solubility, easy functionality, low cost, etc. In this work, we investigated the interaction of GO films with several bacteria, including Escherichia coli, Staphylococcus aureus, Bacillus, and Shewanella-MR 1. Surprisingly, it was found that GO films had no influence on the growth of bacteria; bacteria could selectively attach on the GO films, and the adhesion efficiency was dependent on the genus of bacteria and the quality of GO films, as confirmed by quartz crystal microbalance (QCM) measurement. X-ray photoelectron spectroscopy, Ramam spectra analyses, and current-voltage curves further indicated that the bacteria could reduce GO to reductive GO (rGO) or graphite. Through constant current measurements, a possible mechanism that involved electron transfer from bacteria to the GO film was proposed for the adherence of bacteria to GO films and reduction of GO films by bacteria. The study suggested that GO films could act as biocompatible sites for adhesion of bacteria on their surfaces, providing a paradigm for exploring the interaction of GO-related materials with bacteria.

Wavelength effect of ns-pulsed radiation on the reduction of graphene oxide

Laser-based methods to reduce of Graphene Oxide (GO) have been pointed out as a promising methodology to produce reduced graphene (rGO) due to its potential for scalable production without the use of chemicals and the processing taking place in ambient conditions. Within this context, this study presents the results of a systematic investigation of the reduction of graphene oxide films by means of an Nd:YAG pulsed laser radiation using wavelengths ranging from Ultra-Violet (UV) to Infra-Red (IR). Our results demonstrated that the reduction carried out using infrared (λ = 1064 nm) and visible radiation (λ = 532 nm) yield higher sp3-sp2 conversion as a consequence of photothermal reduction. On the other hand, UV radiation is more efficient in removing oxygen groups due to an enhanced photochemical effect. Furthermore, our results demonstrate that rGO films with C/O ratio greater than 100 can be produced if both photochemical and photothermal effects are present in the reduction process.