Nariman Yousefi

Highly Aligned Graphene/Polymer Nanocomposites with Excellent Dielectric Properties for High-Performance Electromagnetic Interference Shielding

Nanocomposites that contain reinforcements with preferred orientation have attracted significant attention because of their promising applications in a wide range of multifunctional fields. Many efforts have recently been focused on developing facile methods for preparing aligned graphene sheets in solvents and polymers because of their fascinating properties including liquid crystallinity and highly anisotropic characteristics. Self-aligned in situ reduced graphene oxide (rGO)/polymer nanocomposites are prepared using an all aqueous casting method. A remarkably low percolation threshold of 0.12 vol% is achieved in the rGO/epoxy system owing to the uniformly dispersed, monolayer graphene sheets with extremely high aspect ratios (>30000). The self-alignment into a layered structure at above a critical filler content induces a unique anisotropy in electrical and mechanical properties due to the preferential formation of conductive and reinforcing networks along the alignment direction. Accompanied by the anisotropic electrical conductivities are exceptionally high dielectric constants of over 14000 with 3 wt% of rGO at 1 kHz due to the charge accumulation at the highly-aligned conductive filler/insulating polymer interface according to the Maxwell-Wagner-Sillars polarization principle. The highly dielectric rGO/epoxy nanocomposites with the engineered structure and properties present high performance electromagnetic interference shielding with a remarkable shilding efficiency of 38 dB.

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.

Highly transparent conducting graphene films produced by langmuir blodgett assembly as flexible electrodes

This paper reports the development of an efficient method to produce transparent conductive graphene films layer-by-layer on a flexible substrate based on the Langmuir Blodgett (LB) assembly technique. Monolayer ultralarge graphene oxide (UL-GO) sheets of average lateral size greater than 300 µm2 are prepared by repeated centrifugation of as-prepared GO aqueous dispersion. GO films having different numbers of GO layers are fabricated using the LB method while controlling the LB trough surface pressure and pulling speed of the substrate from the dispersion. GO films are chemically reduced at 90°C using hydrogen iodide (HI) acid, followed by chemical doping treatments. The sheet resistance values of the graphene thin films on a PET film are 1.8 and 1.1 kΩ/sq for 2 and 4 graphene layers, respectively, with a transparency of higher than 90%, which are sufficient for many useful applications. It is found that the thicker the film, the higher the conductivity; and vice versa for the transparency of the graphene films.