Kesong Hu

Ultra-Robust Graphene Oxide-Silk Fibroin Nanocomposite Membranes

Nanocomposite materials in forms of membranes, fi lms, and coatings are gaining surging interests in structural and functional applications, because they are more effi cient in loading transfer than conventional composites and can substantially eliminate catastrophic failure caused by poor loading transfer between components. To enhance the mechanical properties of polymeric nanocomposites, carbon nanotubes, intercalated clay, graphene, and graphene oxide are added as high-performance reinforcing nanofi llers. For example, ultrahigh toughness was reported for polyvinyl alcohol nanocomposite fi lms fi lled with single-walled carbon nanotubes; [ 1 ] and ultrahigh modulus was reported for crosslinked nanoclay containing nanocomposites. [ 2 ] However, improving toughness is usually achieved by increasing the ultimate strain and compromising the strength, which is not desired for high-performance applications. [ 3 ]

Ultrarobust Transparent Cellulose Nanocrystal‐Graphene Membranes with High Electrical Conductivity

Ultra-robust nanomembranes possessing high mechanical strength combined with excellent stiffness and toughness rarely achieved in nanocomposite materials are presented. These are fabricated by alternately depositing 1D cellulose nanocrystals and 2D graphene oxide nanosheets by using a spin assisted layer-by-layer assembly technique. Such a unique combination of 1D and 2D reinforcing nanostructures results in layered nanomaterials.

Written-in conductive patterns on robust graphene oxide biopaper by electrochemical microstamping.

The silk road: By employing silk fibroin as a binder between graphene oxide films and aluminum foil for a facile, highly localized reduction process, conductive paper is reinvented. The flexible, robust biographene papers have high toughness and electrical conductivity. This electrochemical written-in approach is readily applicable for the fabrication of conductive patterned papers with complex circuitries.

Self-Powered Electronic Skin with Biotactile Selectivity

Power-generating flexible thin films for facile detection of biotactile events are fabricated from patterned metal-graphene oxide biopaper. These tactile materials are mechanically robust with a consistent output of 1 V and high response rate of 20 Hz. It is demonstrated that the simple quadruple electronic skin sensitively and selectively recognizes nine spatial biotactile positions and can readily be expanded.

Chemical Reduction of Individual Graphene Oxide Sheets as Revealed by Electrostatic Force Microscopy

We report continuous monitoring of heterogeneously distributed oxygenated functionalities on the entire surface of the individual graphene oxide flake during the chemical reduction process. The charge densities over the surface with mixed oxidized and graphitic domains were observed for the same flake after a step-by-step chemical reduction process using electrostatic force microscopy. Quantitative analysis revealed heavily oxidized nanoscale domains (50-100 nm across) on the graphene oxide surface and a complex reduction mechanism involving leaching of sharp oxidized asperities from the surface followed by gradual thinning and formation of uniformly mixed oxidized and graphitic domains across the entire flake.

Biopolymeric Nanocomposites with Enhanced Interphases

Ultrathin and robust nanocomposite membranes were fabricated by incorporating graphene oxide (GO) sheets into a silk fibroin (SF) matrix by a dynamic spin-assisted layer-by-layer assembly (dSA-LbL). We observed that in contrast to traditional SA-LbL reported earlier fast solution removal during dropping of solution on constantly spinning substrates resulted in largely unfolded biomacromolecules with enhanced surface interactions and suppressed nanofibril formation. The resulting laminated nanocomposites possess outstanding mechanical properties, significantly exceeding those previously reported for conventional LbL films with similar composition. The tensile modulus reached extremely high values of 170 GPa, which have never been reported for graphene oxide-based nanocomposites, the ultimate strength was close to 300 MPa, and the toughness was above 3.4 MJ m(-3). The failure modes observed for these membranes suggested the self-reinforcing mechanism of adjacent graphene oxide sheets with strong 2 nm thick silk interphase composed mostly from individual backbones. This interphase reinforcement leads to the effective load transfer between the graphene oxide components in reinforced laminated nanocomposite materials with excellent mechanical strength that surpasses those known today for conventional flexible laminated carbon nanocomposites from graphene oxide and biopolymer components.

Ultrastrong Freestanding Graphene Oxide Nanomembranes with Surface-Enhanced Raman Scattering Functionality by Solvent-Assisted Single-Component Layer-by-Layer Assembly

We report single-component ultrathin reduced graphene oxide (rGO) nanomembranes fabricated via nonconventional layer-by-layer assembly (LbL) of graphene oxide flakes, using organic solvent instead of water to provide strong complementary interactions and to ensure the uniform layered growth. This unique approach does not require regular polymeric from the assembly process or intermediate surface chemical modification. The resulting ultrastrong freestanding graphene oxide (rGO) LbL nanomembranes with a very low thickness of 3 nm (three GO monolayers) can be transferred over a large surface area across tens of square centimeters by using a facile surface-tension-assisted release technique. These uniform and ultrasmooth nanomembranes with high transparency (up to 93% at 550 nm) and high electrical conductivity (up to 3000 S/m) also exhibit outstanding mechanical strength of 0.5 GPa and a Youngs modulus of 120 GPa, which are several times higher than that of other reported regular rGO films. Furthermore, up to 94 wt % of silver nanoplates can be sandwiched between 5 nm GO layers to construct a flexible freestanding protected noble metal monolayer with surface-enhanced Raman scattering properties. These flexible rGO/Ag/rGO nanomembranes can be transferred and conformally coat complex surfaces and show a cleaner Raman signature, enhanced wet stability, and lower oxidation compared to bare Ag nanostructures.

Biotactile Sensors: Self‐Powered Electronic Skin with Biotactile Selectivity (Adv. Mater. 18/2016)

On page 3549, V. V. Tsukruk and co-workers develop self-powered ultrathin flexible films for bio-tactile detection. Graphene oxide materials are engineered for robust self-powered tactile sensing applications harnessing their electrochemical reactivity. The simple quadruple electronic skin sensor can recognize nine spatial bio-tactile positions with high sensitivity and selectivity-an approach that can be expanded towards large-area flexible skin arrays.

Highly Conductive and Transparent Reduced Graphene Oxide Nanoscale Films via Thermal Conversion of Polymer-Encapsulated Graphene Oxide Sheets

Despite noteworthy progress in the fabrication of large-area graphene sheetlike nanomaterials, the vapor-based processing still requires sophisticated equipment and a multistage handling of the material. An alternative approach to manufacturing functional graphene-based films includes the employment of graphene oxide (GO) micrometer-scale sheets as precursors. However, search for a scalable manufacturing technique for the production of high-quality GO nanoscale films with high uniformity and high electrical conductivity is still continuing. Here we show that conventional dip-coating technique can offer fabrication of high quality mono- and bilayered films made of GO sheets. The method is based on our recent discovery that encapsulating individual GO sheets in a nanometer thick molecular brush copolymer layer allows for the nearly perfect formation of the GO layers via dip coating from water. By thermal reduction the bilayers (cemented by a carbon-forming polymer linker) are converted into highly conductive and transparent reduced GO films with a high conductivity up to 104 S/cm and optical transparency on the level of 90%. The value is the highest electrical conductivity reported for thermally reduced nanoscale GO films and is close to the conductivity of indium tin oxide currently in use for transparent electronic devices, thus making these layers intriguing candidates for replacement of ITO films.