Ultrathin Graphene Oxide Films Research Articles

A simple method for floating graphene oxide films facilitates nanoscale investigations of ion and water adsorption.

Graphene oxide (GO) is a promising material for separations. Nanoscale GO thin films at the air/water interface are excellent experimental models to understand molecular-scale interactions of ions and water with GO. However, the characteristics of GO, such as functional groups and flake size, also affect the thin film properties making it difficult to make systematic studies with GO thin films. This paper reports a simple, reliable, and quick method of preparing ultra-thin GO films, irrespective of their origin, and demonstrates the new opportunities possible with the utilization of this method. The total amount of GO used to form the thin film is significantly less compared to previous examples in the literature, minimizing the dissolved GO in the subphase. X-ray reflectivity (XR) studies show that the majority of the GO film has 1.5 nm thickness over a macroscopic area (∼100 cm2) with very small roughness. Sum frequency generation (SFG) spectroscopy measurements show that H2O and D2O interact differently with GO films, a property that was not observed before. SFG data show that functional groups vary significantly between different commercially available GO samples. The differences are also characterized with XR at high resolution. X-ray fluorescence near total reflection (XFNTR) measurements show that these differences strongly affect ion adsorption and interfacial water behavior near GO, which are vital properties in separation applications. The results pave the way for future studies to elucidate the complex separation mechanisms with GO.

Regulated Behavior in Living Cells with Highly Aligned Configurations on Nanowrinkled Graphene Oxide Substrates: Deep Learning Based on Interplay of Cellular Contact Guidance.

Micro-/nanotopographical cues have emerged as a practical and promising strategy for controlling cell fate and reprogramming, which play a key role as biophysical regulators in diverse cellular processes and behaviors. Extracellular biophysical factors can trigger intracellular physiological signaling via mechanotransduction and promote cellular responses such as cell adhesion, migration, proliferation, gene/protein expression, and differentiation. Here, we engineered a highly ordered nanowrinkled graphene oxide (GO) surface via the mechanical deformation of an ultrathin GO film on an elastomeric substrate to observe specific cellular responses based on surface-mediated topographical cues. The ultrathin GO film on the uniaxially prestrained elastomeric substrate through self-assembly and subsequent compressive force produced GO nanowrinkles with periodic amplitude. To examine the acute cellular behaviors on the GO-based cell interface with nanostructured arrays of wrinkles, we cultured L929 fibroblasts and HT22 hippocampal neuronal cells. As a result, our developed cell-culture substrate obviously provided a directional guidance effect. In addition, based on the observed results, we adapted a deep learning (DL)-based data processing technique to precisely interpret the cell behaviors on the nanowrinkled GO surfaces. According to the learning/transfer learning protocol of the DL network, we detected cell boundaries, elongation, and orientation and quantitatively evaluated cell velocity, traveling distance, displacement, and orientation. The presented experimental results have intriguing implications such that the nanotopographical microenvironment could engineer the living cells' morphological polarization to assemble them into useful tissue chips consisting of multiple cell types.

Surface-mediated high antioxidant and anti-inflammatory effects of astaxanthin-loaded ultrathin graphene oxide film that inhibits the overproduction of intracellular reactive oxygen species

BackgroundAstaxanthin (AST) is known as a powerful antioxidant that affects the removal of active oxygen and inhibits the production of lipid peroxide caused by ultraviolet light. However, it is easily decomposed by heat or light during production and storage because of the unsaturated compound nature with a structural double bond. The activity of AST can be reduced and lose its antioxidant capability. Graphene oxide (GO) is an ultrathin nanomaterial produced by oxidizing layered graphite. The chemical combination of AST with GO can improve the dispersion properties to maintain structural stability and antioxidant activity because of the tightly bonded functionalized GO surface.MethodsLayered GO films were used as nanocarriers for the AST molecule, which was produced via flow-enabled self-assembly and subsequent controlled solution deposition of RGD peptide and AST molecules. Synthesis of the GO-AST complex was also carried out for the optimized concentration. The characterization of prepared materials was analyzed through transmission electron microscopy (TEM), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), atomic force microscope (AFM), and Raman spectroscopy. Antioxidant activity was tested by 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 2.2-diphenyl-1-picrylhydrazyl (DPPH) assays. The antibacterial effect and antioxidant effects were monitored for the ultrathin GO/RGD/AST Film. Further, reactive oxygen species (ROS) assay was used to evaluate the anti-inflammatory effects on L-929 fibroblasts.ResultsCotreatment of GO-AST solution demonstrated a high antioxidant combined effect with a high ABTS and DPPH radicals scavenging activity. The GO/RGD/AST film was produced by the self-assembly process exhibited excellent antibacterial effects based on physicochemical damage against E. coli and S. aureus. In addition, the GO/RGD/AST film inhibited H2O2-induced intracellular ROS, suppressed the toxicity of lipopolysaccharide (LPS)-induced cells, and restored it, thereby exhibiting strong antioxidant and anti-inflammatory effects.ConclusionAs GO nanocarrier-assisted AST exerted promising antioxidant and antibacterial reactions, presented a new concept to expand basic research into the field of tissue engineering.

Precise tuning chemistry and tailoring defects of graphene oxide films by low energy ion beam irradiation

Precise tuning chemistry and tailoring nanopores of graphene oxide (GO) thin films are vital for their application for liquid and gas separation. In this work, ultra-thin GO films with thicknesses of about 150 nm were prepared and then modified by a low energy carbon ion beam with ion fluences ranging from 1 × 1015 ions·cm−2 to 1 × 1017 ions·cm−2. An ion fluence of 1 × 1016 ions·cm−2 is a threshold for the changes to the surface geometry (i.e. the chemical state and the consequent morphology) of the GO films. Moreover, X-ray photoelectron spectroscopy (XPS) reveals that oxygen loss in ion beam-induced reduction of GO films was mainly by the elimination of the unstable CO species. Raman spectroscopy indicates that a mass of defects with a mean defect distance of about 1.4 nm was generated in GO films by C+ irradiation. According to SRIM simulation, an average of 208 carbon vacancies were created in the GO film per impinging C+. These results suggest that low energy carbon ion beam irradiation is promising for simultaneously reducing and drilling nanoscale pores on GO surfaces in a controllable manner, which could be used for engineering GO-based separation membranes.

Ultra-thin graphene oxide films via contra-diffusion method: Fast fabrication for ion rejection

Membranes based on graphene or graphene oxide (GO) have received intensive studies recently for various separation processes. However, a great challenge lies in fast and facile fabrication of GO membranes with high salt rejection. Here we report for the first time that ultra-thin GO films with a good salt rejection can be fabricated via contra-diffusion-induced crosslinking based on cation-π and electrostatic interactions between cations and GO flakes. It only takes down to 5 min to construct a defect-free GO film. Various cations, including Na+, K+, Mg2+, Ca2+ and Fe3+, can be used as “crosslinkers” to endow the as-prepared GO membranes with highly tunable separation performance. The optimized GO membranes exhibit over 90% rejection of Na2SO4 with filtrate permeance of 4.3 L m−2 h−1 bar−1 during the nanofiltration process, which reaches to the performance upper limit of previously reported graphene-based membranes. Such a high performance should be attributed to the unique packing structures of the GO flakes crosslinked by cations and the thin and uniform GO selective layers.

A facile and fast transfer of ultrathin graphene oxide film on various substrates

We report a clean, facile, and fast transfer method for uniform and ultrathin graphene oxide (GO) films. Using vacuum filtration, we make the GO film with less 5 nm thicknesses transfer to the target substrates (silicon oxide, glass, PET, PI) and bend surfaces by vaporizing pressure of hot water. The quality of a transferred GO film is characterized by optical microscopy, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, and we demonstrate the successful transfer of GO films without defects on a film and residuals on a supporting membrane. We also demonstrated that GO films coated substrates exhibit improved anti-scratching properties and controlled hydrophilicity over non-coated substrates.

Efficient assembly of high-performance reduced graphene oxide/silver nanowire transparent conductive film based on in situ light-induced reduction technology

Silver nanowires (AgNWs) show great potential in the fabrication of high-flexible transparent conductive films (TCFs) and reduced graphene oxide (rGO) has been considered as one of the most promising overcoating materials for improving the long-term stability of AgNW TCFs. However, transmittance, conductivity, and flexibility of AgNW TCFs are often compromised during the assembly of rGO film. Herein, a highly efficient process of preparing a high-performance and highly stable AgNW TCF is proposed. Firstly, a water-based, self-assembled coating method is used to achieve GO/AgNW TCFs. Then, the light absorption property of AgNWs is utilized to assist the reduction of the graphene oxide (GO) layer through the use of a microsecond intense pulsed light (IPL) irradiation treatment, resulting in a high-performance rGO/AgNW TCF. The instantaneous high-temperature rising from AgNWs provides additional and abundant thermal energy during the irradiation to reduce the ultra-thin GO film without damage to the nanostructure of the GO layer or to the AgNW TCFs itself. The achieved rGO/AgNW TCFs have a low sheet resistance to 8 Ω/sq and a high transmittance of 86.2% owing to the in-situ reduction of rGO film from thin GO film and welding between the AgNWs. In addition, this technique presents a robust enhancement of AgNW TCFs, not only with regard to their electrical stability in high-temperature and high-humidity atmospheres but also to their overall mechanical stability, which significantly enhances the application prospects of AgNW TCFs for use in flexible optoelectronic devices.

Rapid self-assembly of ultrathin graphene oxide film and application to silver nanowire flexible transparent electrodes

A self-assembled ultrathin graphene oxide film was rapidly prepared within only 3 minutes to improve silver nanowire electrode performance.

An efficient polymer solar cell using graphene oxide interface assembled via layer-by-layer deposition

Graphene oxide (GO) is widely used as an interfacial material in applications such as organic light emitting diodes and photovoltaic devices. Herein we report a layer-by-layer (LbL) assembled GO thin film as an anode interfacial layer (AIL) for efficient polymer solar cells (PSCs). The GO thin film is fabricated by alternately depositing cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) and GO on ITO/glass substrate, which possesses controllable thickness by adjusting LbL deposition frequency. The presence of ultrathin GO films improves the work function of ITO, leading to a better contact between the active layer and ITO anode. With the optimized number of deposition times, the efficiency of 6.04% for the PSC with PDDA-GO bilayer (GO-2) as the AIL was achieved.

Effect of structure on the tribology of ultrathin graphene and graphene oxide films

The friction and wear properties of graphene and graphene oxide (GO) with varying C/O ratio were investigated using friction force microscopy. When applied as solid lubricants between a sliding contact of a silicon (Si) tip and a SiO2/Si substrate, graphene and ultrathin GO films (as thin as 1–2 atomic layers) were found to reduce friction by ∼6 times and ∼2 times respectively as compared to the unlubricated contact. The differences in measured friction were attributed to different interfacial shear strengths. Ultrathin films of GO with a low C/O ratio of ∼2 were found to wear easily under small normal load. The onset of wear, and the location of wear initiation, is attributed to differences in the local shear strength of the sliding interface as a result of the non-homogeneous surface structure of GO. While the exhibited low friction of GO as compared to SiO2 makes it an economically viable coating for micro/nano-electro-mechanical systems with the potential to extend the lifetime of devices, its higher propensity for wear may limit its usefulness. To address this limitation, the wear resistance of GO samples with a higher C/O ratio (∼4) was also studied. The higher C/O ratio GO was found to exhibit much improved wear resistance which approached that of the graphene samples. This demonstrates the potential of tailoring the structure of GO to achieve graphene-like tribological properties.

One-step synthesis of a suspended ultrathin graphene oxide film: Application in transmission electron microscopy

Ultrathin graphene films find their use as advantageous support for nano- and biomaterials investigations. Thin film causes a very slight deterioration to measured signals, thus providing more details of the object's structure at nanoscale. The ultimate thinness of graphene works in the best way for this purpose. However, obtaining suspended thin film of a large-area, which is convenient for applications, is often a relatively complicated and time-consuming task. Here we present a one-step 1-min technique for synthesis of an extremely thin (about 1–2nm) continuous film suspended over cells of a conventional copper grid (50–400μm mesh). This technique enables us to acquire a large-area film which is water-resistant, stable in organic solvents and can act as a support when studying nanoparticles or biomaterials. Moreover, the very mechanism of the film formation can be interesting from the point of view of other applications of ultrathin graphene oxide papers.

Formation of graphene oxide films at the liquid/liquid interface

A strategy of employing graphene oxide (GO) nanosheets to prepare large-area GO films is proposed in this work. The films are prepared by injecting GO suspension in a mixture of water and ethanol (H2O + EtOH) at the liquid/liquid (hexane/water, C6H14/H2O) interface and forming the GO films at the interface. After evaporation of C6H14, GO films at the air/(H2O + EtOH) interface are transferred onto solid substrates by the horizontal lifting deposition, and large area (2.5 cm × 2.5 cm) of homogeneous GO films with the thickness of 5.4 nm and the roughness of 3.6 nm is obtained. The formation scheme of the GO films at the C6H14/(H2O + EtOH) interface is discussed considering the density difference between the C6H14, (H2O + EtOH), and H2O, as well as the interaction between the GO nanosheets. This facile strategy paves a new path for preparation of large ultrathin GO films from GO nanosheets, which is of great importance for the preparation of GO-based materials to be used in supercapacitor electrodes, transparent conductive films and field-effect transistors.

Graphene-Based Environmental Barriers

Many environmental technologies rely on containment by engineered barriers that inhibit the release or transport of toxicants. Graphene is a new, atomically thin, two-dimensional sheet material, whose aspect ratio, chemical resistance, flexibility, and impermeability make it a promising candidate for inclusion in a next generation of engineered barriers. Here we show that ultrathin graphene oxide (GO) films can serve as effective barriers for both liquid and vapor permeants. First, GO deposition on porous substrates is shown to block convective flow at much lower mass loadings than other carbon nanomaterials, and can achieve hydraulic conductivities of 5 × 10(-12) cm/s or lower. Second we show that ultrathin GO films of only 20-nm thickness coated on polyethylene films reduce their vapor permeability by 90% using elemental mercury as a model vapor toxicant. The barrier performance of GO in this thin-film configuration is much better than the Nielsen model limit, which describes ideal behavior of flake-like fillers uniformly imbedded in a polymer. The Hg barrier performance of GO films is found to be sensitive to residual water in the films, which is consistent with molecular dynamics (MD) simulations that show lateral diffusion of Hg atoms in graphene interlayer spaces that have been expanded by hydration.

Thickness-dependent solar power conversion efficiencies of catalytic graphene oxide films in dye-sensitized solar cells

The catalytic effects of graphene oxide (GO) films were investigated for dye-sensitized solar cell (DSSC). The GO film consisted of a networked structure of two phases of GO with intercalated H 2O and exfoliated GO. A maximum solar power conversion efficiency ( η) of 2.30% was obtained for the ultrathin GO film. This η is higher than that of other reported types of DSSCs (0.84%) and organic solar cells (1.1%) with graphene electrodes. The catalytic effect of the GO film is attributed to the good ion permeability along the interlayer hydrophilic spaces; the electrons present in these spaces on the exfoliated GO were supplied to the iodide ions.