Free-standing Graphene Oxide Films Research Articles

Architectural engineering of vertically expanded graphene-CoMn2O4 compounds based interdigital electrode for in-plane micro-supercapacitor

Structurally engineered carbon allotropes and their composites are widely applicable in various electronic/electrochemical devices including energy storage. Herein, we develop an efficient method to fabricate graphene/ceramic hybrid nanostructure based in-plane micro-supercapacitors (MSCs). Via CO2-laser irradiation method, MSCs patterns consisting of CoMn2O4 nanoparticles incorporated in vertically structured laser-induced graphene (CMOLIG) were formed on a freestanding graphene oxide film. The process is extremely convenient, since the laser irradiated CMOLIG patterns serve as the in-plane MSCs electrode, while the untreated graphene oxide acts as the substrate. MSCs with enhanced performance can be fabricated with the CMOLIG hybrid electrode patterns that are made of CoMn2O4 nanoparticles with pseudocapacitive properties anchored on vertically structured high-quality graphene film with excellent electrical property and high surface area. Notably, the CMOLIG electrode patterns exhibit enhanced specific capacitance with fast redox kinetics. Using the technique, customized in-plane MSCs arrays are also fabricated, which demonstrated controllable output voltage and capacitance depending on the array configuration to suit the requirements of various wearable microelectronics.

Effective reduction and doping of graphene oxide films at near-room temperature by microwave-excited surface-wave plasma process

The effective reduction of graphene oxide (GO) and incorporation of dopant elements on flexible soft substrates, such as plastics, polymers, papers etc. are of significant importance for energy-related devices and flexible wearable electronics. Here, we demonstrate a novel microwave-excited surface-wave plasma (MW-SWP) process to reduce insulating GO to highly conducting reduced graphene oxide (rGO) on the polyethylene terephthalate (PET) substrate as well as achieving reduction of free-standing GO paper at a near-room temperature. It is also observed that the GO coated on the both top and back surface of a PET substrate reduce simultaneously to obtain conducting rGO films. Most effective reduction was achieved using an argon-hydrogen gas mixture, whereas addition of nitrogen in the gas mixture enabled to incorporate nitrogen dopant in the rGO layers. The GO coated PET and freestanding GO films showed a sheet resistance in the range of MΩ/□, which reduced to an average value of 1.075 kΩ/□ and 134 Ω/□, respectively after the plasma treatment. Thus, the developed MW-SWP process can be significant to produce highly conducting rGO-based graphene films on a soft substrate as well as enabling fabrication of highly conducting freestanding rGO papers.

Bioproduced Polymers Self-Assemble with Graphene Oxide into Nanocomposite Films with Enhanced Mechanical Performance.

Graphene oxide (GO) has recently been highlighted as a promising multipurpose two-dimensional material. However, free-standing graphene oxide films suffer from poor strength and flexibility, which limits scaling-up of production and lifetime structural robustness in applications. Inspired by the relationship between the organic and inorganic components of the hierarchical structure of nacre found in mollusk shells, we have fabricated self-assembled, layered graphene-based composite films. The organic phase of our composite is produced via environmentally friendly and economical methods based on bacterial production of γ-poly(glutamic acid) (PGA). Composite films made of GO, PGA, and divalent cations (Ca2+) were prepared through a slow solvent evaporation method at ambient temperature, resulting in a nacre-like layered structure. These biobased nanocomposite films showed impressive mechanical properties, which resulted from a synergistic combination of hydrogen bonding with the bacterially produced PGA and ionic bonding with calcium ions (Ca2+). The GO/PGA/Ca2+ composite films possessed a high strength of 150 ± 51.9 MPa and a high Young’s modulus of 21.4 ± 8.7 GPa, which represents an increase of 120% and over 70% with respect to pure GO films. We provide rational design strategies for the production of graphene-based films with improved mechanical performance, which can be applied in filtration purification of wastewater in the paper, food, beverage, pigment, and pharmaceuticals industries, as well as for manufacturing of functional membranes and surface coatings.

Fabrication of free-standing graphene oxide films using a facile approach toluene swollen paraffin peeling and green reduction of these films into highly conductive reduced graphene oxide films

Free-standing films of graphene oxide (GO) are regarded as potential materials for numerous applications in different fields. However, fabricating macroscopic free-standing GO films with high throughput is a bottleneck in large-scale application of these films. Here we introduce a method suitable for the mass-scale fabrication of free-standing GO films by exploring paraffin film (Parafilm M®) as a sacrificial substrate. The method involves deposition of GO on a paraffin film and peeling of the paraffin film after swelling it with toluene to obtain free-standing film of GO. Furthermore, the GO films were reduced into highly conductive RGO films using a green reducing agent ascorbic acid. A strategy of treating GO films with CuSO4 solution prior to reduction is also designed to avoid the possible disintegration of GO films in ascorbic acid solution. A detailed comparative study on the properties of RGO films prepared via thermal and chemical treatment and through the combination of both is also reported. An investigation of the variation in the electrical conductivity of the RGO films based on the reduction method is also carried out. The hydrophobic nature of the thermally reduced RGO films is explored for the oil/water separation.

DI Water Dispersed Graphene Oxide and Supercapacitors with Photoreduced Graphene Oxide Films

In recent years, graphene oxide (GO) has attracted a lot of attention as materials of an electrode for rechargeable batteries and supercapacitors. During the processes of GO conversion from a graphite, various oxygen containing functional groups and impurities such as sulfuric acid are engaged into the GO structure and GO flake, respectively, and they deteriorate the electrical properties and dispersivity of GO. In this study, we carried out washing process of GO flake using a centrifuge. Through the washing process, the remaining acid and impurities almost disappeared and the dispersivity of GO in DI water increased greatly. By using this GO dispersion, freestanding GO films of about 15μm thickness were prepared. Since these GO films have poor electrical conductivity and small specific surface area, they should be reduced in order to recover graphene like properties before being used as an electrode. For the reduction process, we employed an intense pulsed light (IPL) of Xe flash lamp and introduced a shadow mask between IPL and GO film during irradiation and the obtained photoreduced GO film showed improved properties compared to the photoreduced one without shadow mask. The photoreduced GO films were analyzed by Raman and infrared spectroscopy, and the electrochemical properties of supercapacitors with the GO electrodes were evaluated by cyclic voltammetry and impedance spectroscopy. The specific capacitance over 170 F/g was achieved.

Free Standing Graphene Oxide Films for Gas Sensing Applications

Free standing films of graphene oxide (GO) were synthesized using an improvisation of Tour’s method [ACS Nano, 4, 4806-4814 (2010)]. Reaction time and temperature were significantly modified to reduce the presence of graphitic residues in the as synthesized GO. These films were characterized for phase purity and we report their performance for ammonia sensors.

Ultrathin free-standing graphene oxide film based flexible touchless sensor * * Project supported by the National Natural Science Foundation of China (No. 61574163) and the Foundation Research Project of Jiangsu Province (Nos. BK20160392, BK20170008).

Ultrathin free-standing graphene oxide (GO) films were fabricated by vacuum filtration method assisted with Ni(OH)2 nanosheets as the sacrifice layer. The surface of the obtained GO film is very clean as the Ni(OH)2 nanosheets can be thoroughly etched by HCl. The thickness of the GO films can be well-controlled by changing the volume of GO dispersion, and the thinnest GO film reached ~12 nm. As a novel and transparent dielectric material, the GO film has been applied as the dielectric layer for the flexible touchless capacitive sensor which can effectively distinguish the approaching of an insulator or a conductor.

Solution-assisted ultrafast transfer of graphene-based thin films for solar cells and humidity sensors

Vacuum filtration enables the fabrication of large-area graphene-based membranes (GBMs), possessing a smoother surface than that by spray, spin coating or drop casting. However, due to the strong interaction with substrates, the separation of thin GBMs from the filter is problematic. Conventional stamping separation/transfer of graphene oxide (GO) thin films requires another substrate and pressing for >10 h, which may damage the delicate structure of the transfer substrates. Other methods require GO to be reduced on filters before separation, thus limiting the reduction methods. Inspired by a coagulation bath that enables rapid formation of ultrastrong GO fibers, we present an ultrafast (<1 min) and solution-assisted strategy to fabricate smooth and freestanding GO films. The diverse interfacial energy of hydrogen bonds also demonstrates another reason for the successful separation. The film thickness ranges from 45 nm to several micrometers. When used as a composite of counter electrodes in dye sensitized solar cells, it showed higher (8.58%) power conversion efficiency than its spin-(7.71%) and spray-coated (8.07%) counterparts. It also showed promising performance in capacitive humidity sensors. The capacitance varied by three orders of magnitude in the range of the relative humidity of 15%–95%. Therefore the strategy realizes an ultrafast and high-quality film production which is suitable for various applications.

Vacuum Assisted Graphene Oxide Membranes for Polymer Electrolyte Membrane Fuel Cells

Undisputedly hydrogen is the preferred and prominent fuel for polymer electrolyte membrane fuel cells (PEMFCs) because of its high electro reactivity for the anodic reaction, the lack of onsite green-house gas emissions (pure water is the only product) and higher cell efficiency. The presence of various oxygenated functional groups can facilitate the proton transportation through reformation of hydrogen bonds and makes graphene oxide (GO) itself as a good candidate as a proton exchange membrane in low temperature fuel cells. Besides ionic conductivity, GO can act as a physical barrier separating the fuels such as hydrogen and oxidant and prevents their cross-overs. In the recent times, significant attention has been shifted towards employing pristine/Virgin GO membranes as such in low temperature polymer electrolyte fuel cells. A number of preparation methods have been proposed to synthesise free-standing GO films in the recent past. However, the influence of method on the membrane properties and applications in fuel cells needs to be addressed and requires further exploration.In this work, graphene oxide membranes of different thickness will be prepared using two different methods: vacuum evaporation and hydrosol evaporation. The influence of the method of preparation, thickness, will be studied in relation to the membrane properties including proton conductivity, ion exchange capacity, and water uptake, swelling behavior, thermal stability and fuel cell performance. A comparative study made with the conventional membrane electrode assembly (Pt/C-Nafion-Pt/C) will be discussed in detail.

Free-standing dried foam films of graphene oxide for humidity sensing

Transparent free-standing films of graphene oxide (GO) have been prepared by a dried foam method using different oxygen-containing GO sheets as feedstocks. Then a humidity sensor was fabricated using the films as transparent and flexible sensing materials. The impedances of the films were measured at different relative humidity, and the results revealed that the relationship between impedance and relative humidity depends on both the oxygen content and the thickness of the GO films, and a linear relationship corresponds to an ultrathin film (in thickness of about 100nm) while a non-linear relationship for thick films. In addition, the thin film can also show a dynamic switch behavior for humidity sensing. The transparent free-standing GO film has potential application working as an optical window combing the humidity sensing property.

Tuning the grade of graphene: Gamma ray irradiation of free-standing graphene oxide films in gaseous phase

A direct approach to functionalize and reduce pre-shaped graphene oxide 3D architectures is demonstrated by gamma ray irradiation in gaseous phase under analytical grade air, N2 or H2. The formation of radicals upon gamma ray irradiation is shown to lead to surface functionalization of the graphene oxide sheets. The reduction degree of graphene oxide, which can be controlled through varying the γ-ray total dose irradiation, leads to the synthesis of highly crystalline and near defect-free graphene based materials. The crystalline structure of the graphene oxide and γ-ray reduced graphene oxide was investigated by x-ray diffraction and Raman spectroscopy. The results reveal no noticeable changes in the size of sp2 graphitic structures for the range of tested gases and total exposure doses suggesting that the irradiation in gaseous phase does not damage the graphene crystalline domains. As confirmed by X-ray photoemission spectroscopy, the C/O ratio of γ-ray reduced graphene oxide is increasing from 2.37 for graphene oxide to 6.25 upon irradiation in hydrogen gas. The removal of oxygen atoms with this reduction process in hydrogen results in a sharp 400 times increase of the electrical conductivity of γ-ray reduced graphene oxide from 0.05Scm−1 to as high as 23Scm−1. A significant increase of the contact angle of the γ-ray reduced graphene oxide bucky-papers and weakened oxygen rich groups characteristic peaks across the Fourier transform infrared spectra further illustrate the efficacy of the γ-ray reduction process. A mechanism correlating the interaction between hydrogen radicals formed upon γ-ray irradiation of hydrogen gas and the oxygen rich groups on the surface of the graphene oxide bucky-papers is proposed, in order to contribute to the synthesis of reduced graphene materials through solution-free chemistry routes.

Controllable fabrication of ultrathin free-standing graphene films

Graphene free-standing film-like or paper-like materials have attracted great attention due to their intriguing electronic, optical and mechanical properties and potential application in chemical filters, molecular storage and supercapacitors. Although significant progress has been made in fabricating graphene films or paper, there is still no effective method targeting ultrathin free-standing graphene films (UFGFs). Here, we present a modified filtration assembly method to prepare these ultrathin films. With this approach, we have fabricated a series of ultrathin free-standing graphene oxide films and UFGFs, up to 40 mm in diameter, with controllable thickness from micrometre to nanoscale (approx. 40 nm) dimensions. This method can be easily scaled up and the films display excellent optical, electrical and electrochemical properties. The ability to produce UFGFs from graphene oxide with a scalable, low-cost approach should take us a step closer to real-world applications of graphene.

Annealing a graphene oxide film to produce a free standing high conductive graphene film

A free-standing graphene oxide film (GOF) obtained by self-assembly at a liquid/air interface was annealed in a confined space between two stacked substrates to form a free-standing highly conductive graphene film. Characterization indicates that the oxygen-containing functional groups (e.g. epoxy, carboxyl, and carbonyl) were removed as small molecules (e.g. H 2O, CO 2, and CO) during the annealing, meanwhile the size of sp 2 domains in the film was decreased. When annealed between two stacked wafers, random interlayer expansion and fractional movement in the GOF were suppressed by the pressure-induced friction, which helps preserve the morphology of the film. The conjugation in the basal plane of graphene and π–π interactions between well stacked graphene sheets favor the transportation of charge carriers in the film, to produce a good electrical conductivity of the resulting free-standing reduced GOF (increased from 1.26 × 10 −5 to 272.3 S/cm).