Free-standing Reduced Graphene Oxide Films Research Articles

Patterning Large Free-Standing Reduced Graphene Oxide Films Assisted by Open-Area Copper Substrates for Graphene Drums

The growth of large free-standing reduced graphene oxide films induced by open-area copper substrates is analyzed. This phenomenon, which is initiated by the self-assembly and reduction of graphene...

Ultra-broadband self-powered reduced graphene oxide photodetectors with annealing temperature-dependent responsivity

Self-powered ultra-broadband photothermoelectric photodetectors based on free-standing reduced graphene oxide (RGO) films annealed under various temperatures (200–1000 °C) are developed for the first time. The preparation process of the free-standing RGO films is simple, low-cost, and scalable. An interesting phenomenon was found that with increasing annealing temperature, responsivities of RGO-based photodetectors decreased across the ultra-broadband range from ultraviolet (375 nm) to terahertz (118.8 μm) regions. The dependence of device responsivity on annealing temperature is derived from differences in the oxygen functional groups present on the RGO films. The negative sheet Hall coefficients of the annealed RGO films confirmed that they behaved as n-type semiconductors, indicating that electron-donating groups dominated in all the annealed free-standing RGO films. The content of electron-donating groups gradually decrease as the annealing temperature rose. The response times of our devices (<50 ms) are approximately one to three orders of magnitude faster than the reported ultra-broadband photodetectors based on supported RGO films and silicon-based heterojunctions. Since our photodetectors are macro-electronic devices, the response time could be further decreased by reducing the channel length. Considering the properties of self-powered, low cost, easy preparation, and ultra-broadband response, our RGO photodetectors have potential applications for integrated optoelectronics.

The Influence of Dispersion State of Graphene Sheets on the Microstructure and Thermal Conductivity of Free-Standing Reduced Graphene Oxide Films

Reduced graphene oxide (rGO) aqueous dispersions were synthesized from GO suspensions with hydrazine as reducing agent via a microwave-hydrothermal process in ammonia solution. The rGO films were fabricated by vacuum filtration of above dispersions. When the weight ratio of hydrazine to GO (R[Formula: see text]) increases from 0.125 to 9, the dispersion stability of rGO nanosheets in hydrazine/ammonia solution degrades gradually, the surfaces of resultant films become rough from smooth and large interspaces turn up in the transverse section. With the increasing of R[Formula: see text], the C/O atomic ratio and orientation degree of (002) plane of rGO films increase, and the in-plane thermal conductivity also increases, achieving a maximum value of 1056[Formula: see text]W/m K when R[Formula: see text] is 3.375, then decreases because large interspaces in the rGO film with higher reduction degree greatly limit the thermal transport due to the severe phonon scatterings at the sheets boundaries.

Comparative study on chemical reduction of free-standing flexible GO films and their cyclic voltammetry performance

Chemical reduction of graphene oxide (GO) is one of the most practical methods to recover the properties of graphene. However, these properties vary with the reduction agents. Understanding these differences is fundamental to applications related to reduced graphene oxide (RGO). In this work, six different reduction agents were used to make flexible and free-standing RGO films, resulting in different properties such as surface morphology, inner structure, surface C/O ratio, electrical conductivity, and wettability. We found that not all the six reduction agents were suitable for making flexible and free-standing films. Then, we further studied the electrochemical behavior of these samples and found that the electrochemical properties of the RGO films depend on all the properties but electrical conductivity played the most important role in determining the pseudocapacitance behavior.

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.

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.

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.

Reduction of graphene oxide films on Al foil for hybrid transparent conductive film applications

We report the reduction of graphene oxide (G-O) films on Al foil using hydrogen as a reducing agent generated during the etching of Al foil in an aqueous solution of hydrochloric acid (HCl). Complete etching of the Al substrate results in simultaneous reduction and a free standing film composed of stacked and overlapped reduced graphene oxide (RG-O) platelets. Generation of hydrogen at the G-O/Al interface increases the reduction efficiency of this method that is demonstrated in better electrical conductivity of the obtained films compared to the RG-O films reduced by the similar method but using remote Al foil in HCl solution and hydrazine reduced RG-O films. By transferring the free standing RG-O films onto Ag NW films, hybrid transparent conductive films (TCFs) with opto-electrical properties comparable to that of ITO films were obtained.