Qingbin Zheng

Transparent conductive films consisting of ultralarge graphene sheets produced by Langmuir-Blodgett assembly.

Monolayer graphene oxide (GO) sheets with sizes ranging from a few to ∼200 μm are synthesized based on a chemical method and are sorted out to obtain four different grades having uniform sizes. Transparent conductive films are produced using the ultralarge graphene oxide (UL-GO) sheets that are deposited layer-by-layer on a substrate using the Langmuir-Blodgett (LB) assembly technique. The density and degree of wrinkling of the UL-GO monolayers are turned from dilute, close-packed flat UL-GO to graphene oxide wrinkles (GOWs) and concentrated graphene oxide wrinkles (CGOWs) by varying the LB processing conditions. The method demonstrated here opens up a new avenue for high-yield fabrication of GOWs or CGOWs that are considered promising materials for hydrogen storage, supercapacitors, and nanomechanical devices. The films produced from UL-GO sheets with a close-packed flat structure exhibit exceptionally high electrical conductivity and transparency after thermal reduction and chemical doping treatments. A remarkable sheet resistance of ∼500 Ω/sq at 90% transparency is obtained, which outperforms the graphene films grown on a Ni substrate by chemical vapor deposition. The technique used in this work to produce transparent conductive UL-GO thin films is facile, inexpensive, and tunable for mass production.

Fabrication of Highly-aligned, Conductive, and Strong Graphene Papers Using Ultralarge Graphene Oxide Sheets

This study demonstrates that large-size graphene oxide (GO) sheets can impart a tremendous positive impact on self-alignment, electrical conductivity, and mechanical properties of graphene papers. There is a remarkable, more than 3-fold improvement in electrical conductivity of the papers made from ultralarge GO sheets (with an average area of 272.2 μm(2)) compared to that of the small GO counterpart (with an average area of 1.1 μm(2)). The corresponding improvements in Youngs modulus and tensile strength are equally notable, namely 320% and 280%, respectively. These improvements of bulk properties due to the large GO sheets are correlated to multiscale elemental and structural characteristics of GO sheets, such as the content of carboxyl groups on the GO edge, C/O ratio and Raman D/G-band intensity ratio of GO on the molecular-scale, and the degree of dispersion and stacking behavior of GO sheets on the microscale. The graphene papers made from larger GO sheets exhibit a closer-stacked structure and better alignment as confirmed by the fast Fourier transform analysis, to the benefits of their electrical conductivity and mechanical properties. The molecular dynamics simulation further elucidates that the enhanced intersheet interactions between large GO sheets play a key role in improving the Youngs modulus of GO papers. The implication is that the said properties can be further improved by enhancing the intersheet stress transfer and electrical conduction especially through the thickness direction.

Self-alignment and high electrical conductivity of ultralarge graphene oxide–polyurethane nanocomposites

Polyurethane (PU)-based composite films containing highly aligned graphene sheets are produced through an environmentally benign process. An aqueous liquid crystalline dispersion of graphene oxide (GO) is in situ reduced in PU, resulting in a fine dispersion and a high degree of orientation of graphene sheets. The PU particles are adsorbed onto the surface of the reduced graphene oxide (rGO), and the rGO sheets with a large aspect ratio of over 10 000 tend to self-align during the film formation when the graphene content is high enough, say more than 2 wt%. The resulting composites show excellent electrical conductivity with an extremely low percolation threshold of 0.078 vol%, which is considered one of the lowest values ever reported for polymer composites containing graphene. The electrical conductivity of the composites with high graphene contents presents significant anisotropy due to the preferential formation of conductive networks along the in-plane direction, another proof of the existence of the self-aligned, layered structure.

Self-assembled reduced graphene oxide/carbon nanotube thin films as electrodes for supercapacitors

Graphene oxide/carbon nanotube (GO/CNT) hybrid films are self-assembled on a Ti substrate via simple casting of aqueous dispersion. The amphiphilic nature of graphene oxide sheets allows adsorption of CNTs onto their surface in water, capable of forming a highly stable dispersion. Binder-free electrodes are prepared using the annealed GO/CNT films for high performance supercapacitors. The hybrid film electrodes with a moderate CNT content, typically 12.5 wt%, give rise to remarkable electrochemical performance with extremely high specific capacitances of 428 and 145 F g−1 at current densities of 0.5 and 100 A g−1, respectively, as well as a remarkable retention rate of 98% of the initial value after 10 000 charge/discharge cycles. The synergistic effects arising from (i) the enlarged surface area of electrodes due to the intercalation of CNTs between the stacked GO sheets with associated large electrochemical active sites and (ii) the improved conductivity through the formation of a 3D network aided by CNTs are mainly responsible for these findings.

Highly transparent and conducting ultralarge graphene oxide/single-walled carbon nanotube hybrid films produced by Langmuir–Blodgett assembly

Uniform, large-area hybrid transparent films composed of ultralarge graphene oxide (UL-GO) and functionalized single walled carbon nanotubes (SWNTs) are synthesized via a layer-by-layer Langmuir–Blodgett (L–B) assembly process. Before additional chemical doping, the GO/SWNT hybrid thin films deliver remarkable sheet resistance ranging 180–560 Ω sq−1 with optical transmittance ranging 77–86% depending on the number of hybrid layers. These optoelectrical properties are much better than the corresponding values of GO films prepared previously by the same technique, and the highest among all graphene, GO and/or carbon nanotube thin films reported in the literature, except graphene films synthesized by chemical vapor deposition on a Cu substrate. The L–B assembly technique developed here is capable of controlling the film composition, structure and thickness, highly suitable for fabrication of transparent conducting optoelectronic devices on a large scale without extra post-transfer processes.

The interface effect of the effective electrical conductivity of carbon nanotube composites

A model of the effective electrical conductivity for carbon nanotube (CNT) composites is presented by incorporating the interface effect with an average field theory. The dependence of the effective electrical conductivity on CNT length, diameter, concentration, and interface properties has been taken care of simultaneously in our treatment so that the model can describe well the interface effect of CNT composites. Predictions from the model are in good agreement with the experimental values of the effective electrical conductivity of CNT composites which the classical models have not been able to explain.

Ultralight Graphene Foam/Conductive Polymer Composites for Exceptional Electromagnetic Interference Shielding

Ultralight, high-performance electromagnetic interference (EMI) shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites are developed by drop coating of PEDOT:PSS on cellular-structured, freestanding GFs. To enhance the wettability and the interfacial bonds with PEDOT:PSS, GFs are functionalized with 4-dodecylbenzenesulfonic acid. The GF/PEDOT:PSS composites possess an ultralow density of 18.2 × 10-3 g/cm3 and a high porosity of 98.8%, as well as an enhanced electrical conductivity by almost 4 folds from 11.8 to 43.2 S/cm after the incorporation of the conductive PEDOT:PSS. Benefiting from the excellent electrical conductivity, ultralight porous structure, and effective charge delocalization, the composites deliver remarkable EMI shielding performance with a shielding effectiveness (SE) of 91.9 dB and a specific SE (SSE) of 3124 dB·cm3/g, both of which are the highest among those reported in the literature for carbon-based polymer composites. The excellent electrical conductivities of composites arising from both the GFs with three-dimensionally interconnected conductive networks and the conductive polymer coating, as well as the left-handed composites with absolute permittivity and/or permeability larger than one give rise to significant microwave attenuation by absorption.

Self-aligned graphene as anticorrosive barrier in waterborne polyurethane composite coatings

Graphene reinforced waterborne polyurethane (PU) composite coatings were fabricated on steel surfaces. When the filler content was 0.4 wt%, self-alignment of graphene was driven by the reduction of the total excluded volume. The superior anticorrosion properties were proven by electrochemical impedance spectroscopy (EIS) analysis for the PU matrix composite coating reinforced by 0.4 wt% of aligned graphene. The interaction mechanism between electrolyte and graphene layers was discussed for the three-dimensional randomly distributed graphene and the in-plane aligned graphene, respectively, to better understand their effects as anticorrosive barriers.

Influence of chirality on the interfacial bonding characteristics of carbon nanotube polymer composites

The influence of chirality on the interfacial bonding characteristics of single-walled nanotubes (SWNTs) reinforced polymer composites was investigated using molecular mechanics and molecular dynamics simulations. The simulations indicate that the interfacial bonding and shear stress between the SWNT and the poly(methyl methacrylate) depends on the chirality. For SWNTs with similar molecular weights, diameters, and lengths, nanotubes with larger chiral angles achieve higher bonding energy and the armchair nanotube may be the best nanotube type for reinforcement. The general conclusions derived from this work may be of importance in devising advanced nanotube reinforced composites.

A highly sensitive graphene woven fabric strain sensor for wearable wireless musical instruments

Highly flexible and sensitive strain sensors are essential components of wearable electronic devices. Herein, we present a novel graphene woven fabric (GWF)/polydimethylsiloxane (PDMS) composite as a highly flexible, sensitive strain sensor capable of detecting feeble human motions with an extremely high piezoresistive gauge factor of 223 at a strain of 3% and excellent durability. A wireless wearable musical instrument prototype made of the composite sensor demonstrates conversion of human motions to music of different instruments and sounds.