Sergey Dubin

Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors

Infrared Route to Graphene Electrodes Electrochemical capacitors can deliver large amounts of power quickly, but have limited energy storage because only the surface regions of electrodes can store charge. Graphene represents an alternative to activated carbon electrodes because of their high conductivity and surface area, but graphene sheets tend to reassociate and lose surface area. El-Kady et al. (p. 1326; see the Perspective by Miller) show that graphite oxide sheets can be converted by infrared laser irradiation into porous graphene sheets that are flexible, robust, and highly conductive. Infrared laser reduction of graphene oxide creates a strong porous electrode with both high surface area and high conductivity. Although electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, charge and discharge faster than batteries, they are still limited by low energy densities and slow rate capabilities. We used a standard LightScribe DVD optical drive to do the direct laser reduction of graphite oxide films to graphene. The produced films are mechanically robust, show high electrical conductivity (1738 siemens per meter) and specific surface area (1520 square meters per gram), and can thus be used directly as EC electrodes without the need for binders or current collectors, as is the case for conventional ECs. Devices made with these electrodes exhibit ultrahigh energy density values in different electrolytes while maintaining the high power density and excellent cycle stability of ECs. Moreover, these ECs maintain excellent electrochemical attributes under high mechanical stress and thus hold promise for high-power, flexible electronics.

A One-Step, Solvothermal Reduction Method for Producing Reduced Graphene Oxide Dispersions in Organic Solvents

Refluxing graphene oxide (GO) in N-methyl-2-pyrrolidinone (NMP) results in deoxygenation and reduction to yield a stable colloidal dispersion. The solvothermal reduction is accompanied by a color change from light brown to black. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images of the product confirm the presence of single sheets of the solvothermally reduced graphene oxide (SRGO). X-ray photoelectron spectroscopy (XPS) of SRGO indicates a significant increase in intensity of the C=C bond character, while the oxygen content decreases markedly after the reduction is complete. X-ray diffraction analysis of SRGO shows a single broad peak at 26.24 degrees 2theta (3.4 A), confirming the presence of graphitic stacking of reduced sheets. SRGO sheets are redispersible in a variety of organic solvents, which may hold promise as an acceptor material for bulk heterojunction photovoltaic cells, or electromagnetic interference shielding applications.

Graphene‐Supported Hemin as a Highly Active Biomimetic Oxidation Catalyst

Well supported: stable hemin-graphene conjugates formed by immobilization of monomeric hemin on graphene, showed excellent catalytic activity, more than 10 times better than that of the recently developed hemin-hydrogel system and 100 times better than that of unsupported hemin. The catalysts also showed excellent binding affinities and catalytic efficiencies approaching that of natural enzymes.

Patterning and Electronic Tuning of Laser Scribed Graphene for Flexible All-Carbon Devices

Engineering a low-cost graphene-based electronic device has proven difficult to accomplish via a single-step fabrication process. Here we introduce a facile, inexpensive, solid-state method for generating, patterning, and electronic tuning of graphene-based materials. Laser scribed graphene (LSG) is shown to be successfully produced and selectively patterned from the direct laser irradiation of graphite oxide films under ambient conditions. Circuits and complex designs are directly patterned onto various flexible substrates without masks, templates, post-processing, transferring techniques, or metal catalysts. In addition, by varying the laser intensity and laser irradiation treatments, the electrical properties of LSG can be precisely tuned over 5 orders of magnitude of conductivity, a feature that has proven difficult with other methods. This inexpensive method for generating LSG on thin flexible substrates provides a mode for fabricating a low-cost graphene-based NO(2) gas sensor and enables its use as a heterogeneous scaffold for the selective growth of Pt nanoparticles. The LSG also shows exceptional electrochemical activity that surpasses other carbon-based electrodes in electron charge transfer rate as demonstrated using a ferro-/ferricyanide redox couple.

Photothermal deoxygenation of graphene oxide for patterning and distributed ignition applications.

A xenon discharge tube, such as is used to produce a photographic flash, has been reported to cause the ignition of carbon nanotubes, silicon nanowires, and welding of nanofibers of the conducting polymer polyaniline. In these reactions, the high surface-to-volume ratio of the nanomaterials being irradiated, coupled with the inability of the small structures to efficiently dissipate the absorbed energy, leads to a rapid increase in temperature and subsequent ignition or welding of the materials. Although heating materials through the use of light energy is not a new phenomenon, achieving such a rapid and dramatic temperature change using only millisecond pulses of light demonstrates a tangible and technologically significant capability that is unique to nanoscale materials. Graphene oxide (GO) is a deeply colored, water dispersible, oxidized form of graphene obtained through the treatment of graphite powder with powerful oxidizing agents. Although GO has been known for over 150 years, only recently have scientists had access to the tools necessary to properly analyze its atomically thin sheet structure. This has rekindled interest in graphite oxide and has led to a number of recent discoveries, including: the stacking of GO platelets to form paper-like materials of high modulus and strength. Many studies have suggested that GO can be reduced to graphene-like carbon sheets by applying chemical reducing agents or by using thermal treatments. This has led to speculation that GO could find use as a precursor in a bulk route to dispersible graphene sheets. Already, several groups have succeeded in creating conducting polymer composites, transparent conducting films, and simple electronic devices based on reduced GO. In addition to the chemical reduction of GO, Aksay, et al. have reported the thermal deoxygenation of GO to create functionalized graphene sheets upon rapid heating to 1100 8C under an inert atmosphere. These organic solvent dispersible sheets have enabled the direct creation of polymer composites, without the need for surfactants. Thermal deoxygenation of GO to form graphitic carbon dates back to the 1960s when Boehm and Scholz first reported on the ignition and deflagration of graphite oxides prepared by different methods. Upon rapid heating to temperatures of 200 8C, GO decomposes to the most thermodynamically stable oxide of carbon, CO2. Along with the exothermic release of CO2, H2O, and CO also form as minor products. [31]

Integration of molecular and enzymatic catalysts on graphene for biomimetic generation of antithrombotic species

The integration of multiple synergistic catalytic systems can enable the creation of biocompatible enzymatic mimics for cascading reactions under physiologically relevant conditions. Here we report the design of a graphene-haemin-glucose oxidase conjugate as a tandem catalyst, in which graphene functions as a unique support to integrate molecular catalyst haemin and enzymatic catalyst glucose oxidase for biomimetic generation of antithrombotic species. Monomeric haemin can be conjugated with graphene through π-π interactions to function as an effective catalyst for the oxidation of endogenous L-arginine by hydrogen peroxide. Furthermore, glucose oxidase can be covalently linked onto graphene for local generation of hydrogen peroxide through the oxidation of blood glucose. Thus, the integrated graphene-haemin-glucose oxidase catalysts can readily enable the continuous generation of nitroxyl, an antithrombotic species, from physiologically abundant glucose and L-arginine. Finally, we demonstrate that the conjugates can be embedded within polyurethane to create a long-lasting antithrombotic coating for blood-contacting biomedical devices.