Multilayer Graphene Oxide Research Articles

Large scale electrochemical synthesis of high quality carbon nanodots and their photocatalytic property

High quality carbon nanodots (C-dots) with high purity were synthesized through a mild, one-step electrochemical approach, without the assistance of any chemicals but only pure water. This high productivity method makes the synthetic process of C-dots synthesis both economical as well as environment-friendly. The as prepared C-dots are predominantly multi-layer graphene oxide, with luminescence and high up-conversion photoluminescence (emission of light at shorter wavelengths than the excitation wavelength). Meanwhile, C-dots showed peroxidise mimetic function and visible-light-sensitive photocatalytic activity for methyl orange degradation. In addition, a novel photocatalyst (TiO(2)/C-dots) was obtained by combining C-dots with TiO(2) through an easy hydrothermal method. Remarkably, TiO(2)/C-dots exhibited an excellent visible-light photocatalytic activity.

Colors of graphene and graphene-oxide multilayers on various substrates

We investigated the colors of graphene and graphene-oxide multilayers that were deposited on various dielectric layers. In particular, the effects of the material thickness, the types of dielectric layers, and the existence of a back silicon substrate were analyzed. The colors of graphene-oxide layers on a SiO2/Si substrate were found to periodically change as the material thickness increased. However, the colors of graphene layers on the same substrate became saturated without a similar periodic change. The calculated colors corresponding to the material thicknesses were verified by optical microscopy and profilometry. We believe that these results demonstrate the possibility of utilizing color as a simple tool for detecting and estimating the thicknesses of graphene and graphene-oxide multilayers.

Enhanced Electrical Properties of Reduced Graphene Oxide Multilayer Films by In-Situ Insertion of a TiO2 Layer

Wrinkle-free reduced graphene oxide (rGO)/TiO(2) hybrid multilayer films were directly fabricated using an rGO solution stabilized by a TiO(2) precursor sol applied over a large area by an air spraying method without the use of additional reduction processes. In-situ insertion of the TiO(2) layer between rGO sheets dramatically increased the conductivity and carrier mobility despite the insulating properties of amorphous TiO(2). The TiO(2) situated between rGO sheets also induced significant hole doping. Electrical hysteresis caused by adsorbed water molecules and residual oxidative moieties in the rGO nanosheets vanished due to TiO(2)-assisted screening of charged impurities. These effects decreased the thermal carrier activation energy and increased the density of states at the Fermi level. Ambipolar transport properties were converted into unipolar-like hole transport characteristics by extensive hole doping in the TiO(2) layer.

Fabrication of free-standing graphene composite films as electrochemical biosensors

We report a simple fabrication method for large-scale free-standing graphene–gold nanoparticle and graphene-single wall carbon nanotube composite films by using a centrifugal vacuum evaporation followed by a thermal reduction process. The homogeneous mixture of a graphene oxide (GO) suspension with gold nanoparticle (Au NP) or single wall carbon nanotube (SWCNT) is self-assembled at the air/liquid interface, resulting in the multilayered GO–Au NP and GO–SWCNT composite films. The cross-sectional image reveals that the graphene layers are orderly stacked in the reduced GO–Au NP film, while the reduced GO–SWCNT film shows a randomly packed morphology due to the dominant π–π interaction between the side wall of SWCNTs and the GO surfaces. In particular, the reduced GO–Au NP film shows an increased electrode kinetics and cyclic voltammetric response in proportion to the amount of Au NPs, and 3-fold enhancement of anodic peak current was observed compared with that of the reduced GO films. We employed the reduced GO–Au NP film as a matrix to immobilize tyrosinase enzyme for phenol detection, and the phenol-induced electrochemical catalytic reaction can be monitored with 3-fold higher sensitivity than the reduced GO film, demonstrating great potential of graphene composite as an electrochemical enzyme biosensor for environmental pollutant screening.

Radiofrequency plasma assisted exfoliation and reduction of large-area graphene oxide platelets produced by a mechanical transfer process

We present a method to produce extended few layer flakes of reduced graphene oxide starting from bulk graphene oxide platelets using Ar plasma treatment at room temperature of mechanically exfoliated platelets. Multilayer graphene oxide platelets transferred to a silicon wafer by micromechanical cleavage were thinned in a controllable and reproducible way by plasma treatment to achieve few-layer reduced graphene sheets without the use of heating or wet chemistry approaches.

Electrochemically tunable ultrafast optical response of graphene oxide

We demonstrate reversible and irreversible changes in the ultrafast optical response of multilayer graphene oxide thin films upon electrical and optical stimulus. The reversible effects are due to electrochemical modification of graphene oxide, which allows tuning of the optical response by externally applied bias. Increasing the degree of reduction in graphene oxide causes excited state absorption to gradually switch to saturable absorption for shorter probe wavelengths. Spectral and temporal properties as well as the sign of the ultrafast response can be tuned either by changing the applied bias or exposing to high intensity femtosecond pulses.

Reversible Electrical Reduction and Oxidation of Graphene Oxide

We demonstrate that graphene oxide can be reversibly reduced and oxidized using electrical stimulus. Controlled reduction and oxidation in two-terminal devices containing multilayer graphene oxide films are shown to result in switching between partially reduced graphene oxide and graphene, a process which modifies the electronic and optical properties. High-resolution tunneling current and electrostatic force imaging reveal that graphene oxide islands are formed on multilayer graphene, turning graphene into a self-assembled heterostructure random nanomesh. Charge storage and resistive switching behavior is observed in two-terminal devices made of multilayer graphene oxide films, correlated with electrochromic effects. Tip-induced reduction and oxidation are also demonstrated. Results are discussed in terms of thermodynamics of oxidation and reduction reactions.

Highly controllable transparent and conducting thin films using layer-by-layer assembly of oppositely charged reduced graphene oxides

A new approach for the fabrication of reduced graphene oxide (rGO) multilayers which can be used for transparent and conducting thin films was developed. This was achieved by using layer-by-layer (LbL) assembly of positively and negatively charged rGO sheets, which could provide highly controllable thin films in terms of thickness, transmittance, and sheet resistance. In particular, the thickness of the multilayer thin films of rGO was able to be controlled precisely in the subnanometre scale by ∼0.46 nm via simply varying the number of stacking layers. Therefore, this method enabled an excellent control of the rGO multilayers over the optical and electrical properties, which are related to the thickness. Furthermore, we demonstrated the application of the rGO multilayers for an OLED device.

Hydrophobic properties of colloidal films coated with multi-wall carbon nanotubes/reduced graphene oxide multilayers

We introduce a novel and versatile approach for preparing superhydrophobic structures composed only of carbon nano-objects with tunable hydrophobicity can be realized via the LbL deposition of positively charged multiwall nanotubes (MWNTs) and negatively charged reduced grapheme oxide sheets (rGO) onto colloidal particles. Protonated MWNT (MWNT–NH3+) and anionic reduced graphene oxide (rGO–COO–) were used as building blocks for the layer-by-layer assembly of MWNT/rGO multilayer films onto silica (Si) colloids. After fluorination, it has been shown that the surface hydrophobicity, as measured by the water contact angle, can be readily controlled from 119 to 151° by controlling the nanoscale roughness of the colloidal particles, which, in turn, is controlled by the number of bilayers. SEM and ζ-potential analysis suggested the successful preparation of multilayer films on colloidal particles.

The Role of Intercalated Water in Multilayered Graphene Oxide

A detailed in situ infrared spectroscopy analysis of single layer and multilayered graphene oxide (GO) thin films reveals that the normalized infrared absorption in the carbonyl region is substantially higher in multilayered GO upon mild annealing. These results highlight the fact that the reduction chemistry of multilayered GO is dramatically different from the single layer GO due to the presence of water molecules confined in the ∼1 nm spacing between sheets. IR spectroscopy, XPS analysis, and DFT calculations all confirm that the water molecules play a significant role interacting with basal plane etch holes through passivation, via evolution of CO(2) leading to the formation of ketone and ester carbonyl groups. Displacement of water from intersheet spacing with alcohol significantly changes the chemistry of carbonyl formation with temperature.

A Controllable Self‐Assembly Method for Large‐Scale Synthesis of Graphene Sponges and Free‐Standing Graphene Films

AbstractA simple method to prepare large‐scale graphene sponges and free‐standing graphene films using a speed vacuum concentrator is presented. During the centrifugal evaporation process, the graphene oxide (GO) sheets in the aqueous suspension are assembled to generate network‐linked GO sponges or a series of multilayer GO films, depending on the temperature of a centrifugal vacuum chamber. While sponge‐like bulk GO materials (GO sponges) are produced at 40 °C, uniform free‐standing GO films of size up to 9 cm2 are generated at 80 °C. The thickness of GO films can be controlled from 200 nm to 1 µm based on the concentration of the GO colloidal suspension and evaporation temperature. The synthesized GO films exhibit excellent transparency, typical fluorescent emission signal, and high flexibility with a smooth surface and condensed density. Reduced GO sponges and films with less than 5 wt% oxygen are produced through a thermal annealing process at 800 °C with H2/Ar flow. The structural flexibility of the reduced GO sponges, which have a highly porous, interconnected, 3D network, as well as excellent electrochemical properties of the reduced GO film with respect to electrode kinetics for the [Fe(CN)6]3−/4− redox system, are demonstrated.

Good electrical and mechanical properties induced by the multilayer graphene oxide sheets incorporated to amorphous carbon films

Multilayer graphene oxide nanosheets were fabricated using commercially available expanded graphite by simple ultrasonic treatment and then were incorporated into the amorphous carbon matrix as fillers by electrochemical deposition. The electrical conductivity of the films was strongly improved due to the contribution of the multilayer graphene oxide sheets. Moreover, the Young’s modulus, hardness and elastic recovery of the composite films were measured to be about 171.1 GPa, 10.1 GPa and 81.4%, respectively, compared to 137.4 GPa, 5.1 GPa and 44.3% of undoped a-C:H films prepared at the same conditions. Additionally, the friction coefficient was tested to be 0.15 (0.5 N, 2 Hz) and the antiwear life was prolonged to about 200 s while the undoped DLC films obtained at the same condition were easy to be frazzled.