Free-standing Reduced Graphene Oxide Research Articles

Fabrication of free-standing reduced graphene oxide composite papers doped with different dyes and comparison of their electrochemical performance for electrocatalytical oxidation of nitrite

Free-standing graphene composite papers exhibit very high flexibility and stability. Therefore, these materials are preferred as modular electrodes for some electrochemical applications, e.g. electrochemical sensors, supercapacitors, and batteries. Graphene composite papers are usually doped with different substances to improve their electrical conductivity, electrochemical performance, and stability. This work describes the fabrication of flexible and free-standing reduced graphene oxide (rGO) papers doped with phenazine, phenothiazine, phenoxazine, xanthene, acridine, and thiazole class of dye molecules. Fabrication procedure was applied by two basic steps: vacuum filtration of a dispersion containing GO sheets and dye molecule through a membrane and reduction of free-standing GO/dye paper to rGO/dye paper by treatment with hydriodic acid. Characterization of these rGO/dye papers were achieved by using scanning electron microscopy, scanning tunneling microscopy, energy dispersive X-ray analysis, powder X-ray diffraction spectroscopy, Raman spectroscopy, fourier transform infrared spectroscopy, UV–vis. absorption spectroscopy, four-point probe conductivity measurement and cyclic voltammetry techniques. Electrical conductivity and electrochemical performance studies indicated that acriflavine (Acr) doped rGO paper electrode exhibited the best performance for electrocatalytical oxidation of nitrite. When rGO/dye paper was applied as amperometric sensor for nitrite, the sensor was very stable, flexible, and reproducible. rGO/Acr paper demonstrated good quality parameters with sensitivity of 0.40 μAμM−1cm−2, detection limit of 0.12 μM, and linear range of 0.40–3600 μM.

Highly conductive free-standing reduced graphene oxide thin films for fast photoelectric devices

Compared with mechanically exfoliated and chemical vapor deposited graphene, reduced graphene oxide (RGO) possesses unique advantages such as a wet process synthesis, high yield and the ability to assemble large-area thin films on various substrates. However, RGO is normally not recommended for advanced devices owing to its poor electrical conductivity. We report a new method to prepare highly conductive free-standing RGO thin films. The as-prepared RGO thin film possesses the highest conductivity of 87100 S m−1, the second-lowest sheet resistance of 21.2 Ω sq−1 and a medium-level mobility of 16.7 cm2 V−1 s−1 among all of the reported RGO films. To demonstrate the application potential of the free-standing RGO thin films in photoelectric devices, a fully suspended RGO photodetector is constructed using the free-standing RGO thin film, which exhibits the fastest (ca.100 ms) and broadest (from the ultraviolet to terahertz spectral range) photoresponse among all of the RGO film photodetectors that have been reported. The response speed is even comparable with those of CVD-grown graphene photodetectors and mechanically exfoliated graphene photodetectors. These results pave the way towards high-conductivity RGO thin films by a wet process assembly, thus facilitating applications of RGO in advanced electronic, optoelectronic and sensing devices.

Corrigendum: Lateral photovoltaic effect in flexible free-standing reduced graphene oxide film for self-powered position-sensitive detection.

Scientific Reports 6: Article number: 33525; published online: 16 September 2016; updated: 11 November 2016

Lateral photovoltaic effect in flexible free-standing reduced graphene oxide film for self-powered position-sensitive detection.

Lightweight, simple and flexible self-powered photodetectors are urgently required for the development and application of advanced optical systems for the future of wearable electronic technology. Here, using a low-temperature reduction process, we report a chemical approach for producing freestanding monolithic reduced graphene oxide papers with different gradients of the carbon/oxygen concentration ratio. We also demonstrate a novel type of freestanding monolithic reduced graphene oxide self-powered photodetector based on a symmetrical metal–semiconductor–metal structure. Upon illumination by a 633-nm continuous wave laser, the lateral photovoltage is observed to vary linfearly with the laser position between two electrodes on the reduced graphene oxide surface. This result may suggest that the lateral photovoltaic effect in the reduced graphene oxide film originates from the built-in electric field by the combination of both the photothermal electric effect and the gradient of the oxygen-to-carbon composition. These results represent substantial progress toward novel, chemically synthesized graphene-based photosensors and suggest one-step integration of graphene-based optoelectronics in the future.

A Free-standing Graphene-Polypyrrole Hybrid Paper via Electropolymerization with an Enhanced Areal Capacitance

Here we developed a free-standing reduced graphene oxide (rGO)-polypyrrole (PPy) hybrid paper via electropolymerization on a paper-like graphene gel. This flexible hybrid paper displayed a uniform layered structure with PPy coated onto the graphene layers. A high areal mass of 2.7mgcm−2 could be obtained. It delivered a greatly enhanced areal capacitance of 440mFcm−2 at 0.5Ag−1, in contrast to that 151∼198.5mFcm−2 previously reported for graphene paper or polypyrrole-graphene paper. It can retain ∼81% of the initial capacitance at a high current density of 6Ag−1. The combined high flexibility with outstanding electrochemical performance, makes such novel hybrid paper a promising electrode for flexible supercapacitors.

Steamed water engineering mechanically robust graphene films for high-performance electrochemical capacitive energy storage

Developing versatile methods to fabricate flexible graphene film electrodes with favorable mechanical strength and desirably tailored areal and volumetric capacitances are very challenging for high-performance capacitive energy storages. Here, we present a simple yet versatile method to regulate the structures of scalable free-standing reduced graphene oxide (rGO) films for high-performance flexible supercapacitors. Steamed water with a high pressure and a moderately high temperature in closed vessels was used to prepare reduced graphene oxide with regulated structures, and the resultant rGO films exhibited favorable mechanical robustness (with modulus and tensile strength higher than 0.28GPa and 5.9MPa respectively) as well as excellently controllable areal and volumetric capacitances (with a highest gravimetric specific capacitance, a highest areal specific capacitance, and a highest volumetric capacitance up to 340F/g, 915mF/cm2, and 326F/cm3, respectively), revealing the versatile behavior of this regulation technique for high-performance flexible energy storage. In addition, a typical assembled all-solid-state supercapacitor based on as-fabricated graphene films shows large gravimetric and areal specific capacitances, high energy density and power density, as well as excellent capacitance stability, highlighting its great potential for high-performance flexible energy storage devices.

Three-dimensional graphene/carbon nanotubes hybrid composites for exploring interaction between glucose oxidase and carbon based electrodes

This work presents a 3D carbon composite electrode matrix, consisting of reduced graphene oxide (RGO), carbon nanotubes, and carbon fiber felt. In this matrix, the edges of the RGO sheets are bounded to cup-stacked carbon nanotubes (CSCNTs) while the micrometer scale basal planes of the RGO sheets are suspended between the fibers. The presence of free-standing RGO sheets in which π orbitals of the basal planes do not interact with the electrode surface offers advantageous conditions to explore demanding bioelectrochemical reactions, such as the FADH2/FAD redox transition often claimed in the literature as originated from a direct electron communication between micro/nano carbon based electrodes and glucose oxidase (GOx). At least, three important questions rise from this system: (i) how the junction between the carbon nanostructures and the carbon microfibers affects the monitored electron transfer? (ii) are the monitored Faradaic peaks originated from native or denaturated GOx? (iii) is GOx still active for glucose sensing after suffering conformational changes due to the adsorption process on the surface of the electrode? It is intended here to contribute with the investigation of these questions. According to our results, the electronic interactions between CSCNTs and RGO sheets seem to modulate the performance of the aforementioned reaction, as improved kinetic parameters were observed on the 3D composite matrix rather than the obtained with 3D electrodes modified with CSCNTs or RGO sheets themselves. However, spectroscopic experiments suggested that GOx suffers significant conformational changes when adsorbed on the surface of the carbon nanostructures, leading to the denaturation of the protein. Even though the electrochemical experiments indicate that the denaturation of the enzyme does not seem to completely release the FAD moiety, the operational capability of the resulting glucose biosensor was greatly compromised.

Mesoporous graphene-layered double hydroxides free-standing films for enhanced flexible supercapacitors

Two-dimensional graphene materials as supercapacitors electrode have attracted tremendous attention owing to their high surface area and exceptionally electrical conductivity, while it is still a great challenge to achieve desired mesoporous graphene electrodes as the easy aggregation of graphene nanosheets. Here we report the design and fabrication of mesoporous free-standing reduced graphene oxide (rGO)-based films by introducing layered double hydroxides (LDHs) nanoplatelets into the interlayer of rGO nanosheets. The nanostructures regular evolutions of rGO/CoAl-LDH free-standing film due to the embedding of LDHs were rationally investigated with detail experiments. Moreover, the flexible all-solid-state asymmetric supercapacitor device fabricated by using rGO/CoAl-LDH as positive electrode demonstrates a superior performance compared to most reported rGO-based free-standing film electrodes. The results presented here provide valuable insights into exploring mesoporous graphene-based free-standing architectures for highly-efficient and stable energy storage devices.

Biocompatible, Free-Standing Film Composed of Bacterial Cellulose Nanofibers-Graphene Composite.

In recent years, graphene films have been used in a series of wide applications in the biomedical area, because of several advantageous characteristics. Currently, these films are derived from graphene oxide (GO) via chemical or physical reduction methods, which results in a significant decrease in surface hydrophilicity, although the electrical property could be greatly improved, because of the reduction process. Hence, the comprehensive performance of the graphene films showed practical limitations in the biomedical field, because of incompatibility of highly hydrophobic surfaces to support cell adhesion and growth. In this work, we present a novel fabrication of bacterial cellulose nanofibers/reduced graphene oxide (BC-RGO) film, using a bacterial reduction method. Thus-prepared BC-RGO films maintained excellent hydrophilicity, while electrical properties were improved by bacterial reduction of GO films in culture. Human marrow mesenchymal stem cells (hMSCs) cultured on these surfaces showed improved cellular response with higher cell proliferation on the BC-RGO film, compared to free-standing reduced graphene oxide film without the nanoscale fibrous structure. Furthermore, the cellular adhesion and proliferation were even comparable to that on the tissue culture plate, indicating that the bacterial cellulose nanofibers play a critically contructive role in supporting cellular activities. The novel fabrication method greatly enhanced the biochemical activity of the cells on the surface, which could aid in realizing several potential applications of graphene film in biomedical area, such as tissue engineering, bacterial devices, etc.

Bolometric response of free-standing reduced graphene oxide films

Free-standing films of reduced graphene oxide were prepared by evaporative drying of drop-cast graphene oxide followed by thermal reduction. The electrical resistance of reduced graphene oxide films showed a strong temperature dependence, reaching a temperature coefficient of resistance of 44×103 Ω/K at 60 K. The bolometric response under black body illumination was measured from 50 K to 300 K, reaching a voltage responsivity of up to 82 × 103 V/W at 50 K.

Free-Standing Reduced Graphene Oxide Paper with High Electrical Conductivity

With high carrier mobility and low cost, reduced graphene oxide (RGO) shows bright prospects for use in the field of thermoelectric materials. To investigate the intrinsic thermoelectric properties of RGO sheets, we prepared RGO papers which were reduced by HBr solution for 5 min, 20 min, and 60 min, respectively. Thermogravimetry analysis (TGA) and Raman analysis showed that the conjugated carbon network of graphene oxide (GO) was restored during the reduction process and the thermal stability of the RGO papers was much better than that of GO paper. The RGO paper that was reduced for 60 min and then annealed in Ar/H2 atmosphere exhibited the highest electrical conductivity of 3.22 × 105 S/m at 160°C. As the reduction degree of the RGO paper deepens, the Seebeck coefficient gradually transforms from positive to negative, indicating that the conduction type of RGO paper can be controlled by regulating the reduction degree.

Effects of reduction methods on the structure and thermal conductivity of free-standing reduced graphene oxide films

In this work, free-standing graphene oxide (GO) films obtained by vacuum filtration were reduced in HI acid or annealed in a confined space to form self-standing reduced graphene oxide (RGO) films. The effects of reduction methods on the reduction degree, microstructure and thermal conductivity of the RGO films were investigated. The RGO films obtained by thermal annealing have higher reduction degree than those treated with HI acid (55%) or treated with the combination of HI acid and 600°C annealing. The X-ray pole figure measurements show that the orientation degree of GO films after reduction is degraded, and the HI acid reduction results in the worst oriented structure. The in-plane thermal conductivities of the RGO films obtained by thermal reduction are higher than those of films reduced by other methods due to their higher reduction degree and the resulting unfolded structure. However, the differences in the thermal conductivity of the RGO films obtained by thermal annealing at different temperatures are minor, even their reduction degree is enhanced with the increasing of annealing temperature, which is probably due to the generated interspaces between the graphene layers during reduction that greatly limit thermal transport.

A general strategy for the fabrication of high performance microsupercapacitors

We propose a generic strategy for microsupercapacitor fabrication that integrates layers of reduced graphene oxide (rGO) and pseudocapacitive materials to create electrode heterostructures with significantly improved cycling stability and performance. Our approach involves a combination of photolithography and a simple transfer method of free-standing reduced graphene oxide film onto an Au/patterned photoresist bilayer. The resulting stack (rGO/Au/patterned resist/substrate) is then used for the electrochemical deposition of various pseudocapacitive materials before the final step of lift-off. To prove the viability of this method, we have successfully fabricated microsupercapacitors (MSCs) with the following interdigitated electrode heterostructures: MnO2/rGO, Co(OH)2/rGO and PANI/rGO. These MSCs show better performance and cycling stability compared to the single layer, (i.e., rGO-free) counterparts. The interdigitated electrode heterostructures result in MSCs with energy densities in the range of 3–12mWh/cm3 and power densities in the range of 400–1200mW/cm3, which is superior to the Li thin film batteries (E=10mWh/cm3), carbon, and metal oxide based MSCs (E=1–6mWh/cm3) while device energy densities are in the range of 1.3–5.3mWh/cm3, corresponding power densities are in the range of 178–533mW/cm3. These results can be explained by a facilitated nucleation model, where surface topology of the rGO film creates a favorable environment for the nucleation and growth of pseudocapacitive materials with strong interfacial contacts and enhanced surface area. This approach opens up a new avenue in fabricating MSCs involving a variety of heterostructures combining electrical double layer carbon type with Faradaic pseudocapacitive materials for enhanced electrochemical performance.

Synergistic effects of a novel free-standing reduced graphene oxide film and surface coating fibronectin on morphology, adhesion and proliferation of mesenchymal stem cells.

Graphene films have broad use in engineering, energy and biomedical applications. The cost-effective, eco-friendly and easy to scale-up fabrication methods of graphene films are always highly desired. In this work, we develop a novel fabrication method of free-standing reduced graphene oxide (RGO) films by vacuum filtration of graphene oxide aqueous solution through a nanofiber membrane in combination with chemical reduction. Instead of the smooth surface, the generated RGO films have nanoscale patterns transferred from the nanofiber membrane and controlled in a large range by varying the parameters of the electrospinning process. The cellular culture results of the human marrow mesenchymal stem cells (hMSCs) show that the fibronectin modified RGO films could exhibit excellent biocompatibility, which could be attributed to the synergistic effects of the RGO films including both surface morphology and fibronectin modification. The novel fabrication method greatly enhances the fabrication capability and the potential of graphene films for application in cell culture, tissue engineering as well as in other engineering and biomedical applications.

Synthesis of free-standing reduced graphene oxide membranes with different thicknesses and comparison of their electrochemical performance as anodes for lithium-ion batteries

Free-standing reduced graphene oxide membranes (rGOMs) with different thicknesses and carbon contents are prepared via a simple, low cost, scalable, and eco-friendly two-step process.

Ultralight free-standing reduced graphene oxide membranes for oil-in-water emulsion separation

Ultralight free-standing RGO membranes are capable of separating multiple types of surfactant stabilized oil-in-water emulsions with high separation efficiency.

Environmentally friendly synthesis of graphene–silver composites with surface-enhanced Raman scattering and antibacterial activity via reduction with l-ascorbic acid/water vapor

Free-standing reduced graphene oxide–Ag composite films with different Ag contents not only possess high Raman enhancement, but also have antibacterial activity.

Large-Area, Conductive and Flexible Reduced Graphene Oxide (RGO) Membrane Fabricated by Electrophoretic Deposition (EPD)

A large-area, conductive, and flexible membrane made from the stabilized aqueous solution of reduced graphene oxide (RGO) is successfully fabricated using an electrophoretic deposition (EPD) method. A low-voltage operation of EPD (∼3 volts) allows a robust consolidation of RGO layers desirably aligned in the in-plane direction through the cohesive electrophoretic squeezing force near the current collector. Transferring the deposited RGO layers to arbitrary substrates or achieving as a free-standing form, two methods of "chemical etching" and "electrochemical etching" are developed to detach the RGO layers from the EPD current collector without damaging the deposited RGO. Further reducing the free-standing RGO membrane by thermal annealing up to 1000 °C, a graphite-like architecture is restored (d-spacing at 3.42 Å with C/O ratio at 16.66) and the electrical conductivity increases as high as 5.51 × 10(5) S/m. The tightly-consolidated and securely-detached RGO membrane allows the free-standing and flexible features and highly conductive characteristics, which are further developed during thermal treatment. Because of the facile scale-up nature of the EPD process and RGO solution, the developed methodology has a considerable potential to be applied to various energy storage devices, flexible conductive coatings, and other electrochemical systems.

Free Standing Reduced Graphene Oxide Film Cathodes for Lithium Ion Batteries

We report the fabrication and electrochemical activity of free-standing reduced graphene oxide (RGO) films as cathode materials for lithium ion batteries. The conducting additive and binder-free RGO electrodes with different oxygen contents were assembled by a simple vacuum filtration process from aqueous RGO colloids prepared with the aid of cationic surfactants. The gravimetric capacity of RGO film cathodes showed clear dependence on the oxygen contents controlled by the thermal reduction process. The capacity increased with the increase of the amount of oxygen functional groups, indicating that the main lithium capturing mechanism of RGO cathodes is Li(+) ion interaction with the surface oxygen functionalities. The hydroxyl groups (C-OH) as well as carbon-oxygen double bonds have been identified as the lithiation-active species. The RGO cathodes achieved excellent rate capability due to the fast surface Faradaic reaction, suggesting that self-supported RGO films are promising cathodes for high power application. The graphene oxide (GO)/RGO composite films showed inferior performance to those of RGO only. The poor electronic conductivity of GO might result in inefficient utilization of redox active oxygen functional groups despite the higher oxygen content and higher theoretical capacity of GO/RGO composite films. Further optimization on the amount of oxygen functional groups for higher capacity and better electronic conductivity would lead to the development of RGO based high energy-high power cathodes.