Graphene Oxide Liquid Crystals Research Articles

Vertically Aligned and Interconnected Graphene Networks for High Thermal Conductivity of Epoxy Composites with Ultralow Loading

Efficient removal of heat via thermal interface materials has become one of the most critical challenges in the development of modern microelectronic devices. However, traditional polymer composites present limited thermal conductivity even when highly loaded with highly thermally conductive fillers due to the lack of efficient heat transfer channels. In this work, vertically aligned and interconnected graphene networks are first used as the filler, which is prepared by a controlled three-step procedure: formation of graphene oxide liquid crystals, oriented freeze casting, and high-temperature annealing reduction under Ar. The obtained composite, at an ultralow graphene loading of 0.92 vol %, exhibits a high thermal conductivity (2.13 W m–1 K–1) that is equivalent to a dramatic enhancement of 1231% compared to the pure matrix. Furthermore, the composite also presents a much reduced coefficient of thermal expansion (∼37.4 ppm K–1) and increased glass transition temperature (135.4 °C). This strategy provide...

Ultrasensitive Strain Sensor Produced by Direct Patterning of Liquid Crystals of Graphene Oxide on a Flexible Substrate.

Ultrasensitive flexible strain sensors were developed through the combination of shear alignment of a high concentration graphene oxide (GO) dispersion with fast and precise patterning of multiple rectangular features on a flexible substrate. Resistive changes in the reduced GO films were investigated under various uniaxial strain cycles ranging from 0.025 to 2%, controlled with a motorized nanopositioning stage. The devices uniquely combine a very small detection limit (0.025%) and a high gauge factor with a rapid fabrication process conducive to batch production.

Suppressed Polysulfide Crossover in Li-S Batteries through a High-Flux Graphene Oxide Membrane Supported on a Sulfur Cathode.

Utilization of permselective membranes holds tremendous promise for retention of the electrode-active material in electrochemical devices that suffer from electrode instability issues. In a rechargeable Li–S battery—a strong contender to outperform the Li-ion technology—migration of lithium polysulfides from the sulfur cathode has been linked to rapid capacity fading and lower Coulombic efficiency. However, the current approaches for configuring Li–S cells with permselective membranes suffer from large ohmic polarization, resulting in low capacity and poor rate capability. To overcome these issues, we report the facile fabrication of a high-flux graphene oxide membrane directly onto the sulfur cathode by shear alignment of discotic nematic liquid crystals of graphene oxide (GO). In conjunction with a carbon-coated separator, the highly ordered structure of the thin (∼0.75 μm) membrane and its inherent surface charge retain a majority of the polysulfides, enabling the cells to deliver very high initial dis...

Comparing effects of thermal annealing and chemical reduction treatments on properties of wet-spun graphene fibers

Graphene fibers, as one of the assembled ordered materials in the graphene field, have gained increasing attention in the recent years. Until now, the hot spot has still been focused on the methods to improve the mechanical properties of graphene fibers. Among many factors, the reduction processing has been found to be a very critical step influencing the final properties of graphene fibers. In this paper, to find out the effects and mechanism of the reduction treatment on the wet-spun graphene oxide fibers, the thermal annealing and chemical reduction methods have been applied and discussed parallelly. The results showed that the tensile strength and modulus of thermally reduced graphene oxide fibers can reach 820.2 MPa and 54.7 GPa respectively, which were much higher than the corresponding value of 431 MPa and 24.1 GPa obtained from the chemical reduction. The superior mechanical properties of thermally reduced graphene oxide fibers were attributed to the ordered graphite crystallites, stronger interlayer interactions, and fewer defects, which were analyzed from the microstructure characterization. Additionally, to give a clear evolution from the graphene oxide liquid crystal to long-range ordered graphene fibers, a structure evolution model has been built. It can be proved that thermal annealing is an efficient way to fabricate high-strength graphene fibers.

A High-Flux Polysulfide Blocking Membrane Fabricated By Shear Alignment of Discotic Nematic Liquid Crystals of Graphene Oxide on Sulfur Cathode

With consistent technological advances, demands for batteries with considerably higher specific energy than current Lithium ion devices are apparent. As an extremely promising alternative to Li-ion, the electrochemistry of Lithium-Sulfur (Li-S) has received strong attention due to its theoretical specific capacity of 1675 mAh g-1 and theoretical specific energy of 2500 Wh kg-1. Turning the principle of the Li-S concept into practice though faces a number of key challenges. Among these challenges, the continuous crossover of highly soluble polysulfides – the electrode active material from the cathode through the separator to the lithium anode and vice versa - is primarily responsible for rapid capacity fading of Li-S cells.[ 1 ] Accordingly, the focus of the research effort in this area has been devoted to the retention of sulfur and its reduction products on the cathode side of the battery. These studies have produced some striking improvements in both the sulfur cathode durability and the role that the separator plays in a Li-S battery.[ 2 , 3 ] The separator is a critical component in liquid electrolyte energy storage devices. Sitting between the positive and negative electrode to prevent physical contact between them, the membrane should show sufficient wettability, porosity, chemical, mechanical, thermal and dimensional stability while demonstrating high permeability and high flux. However, for electrochemical systems where redox-active species are dissolved or dispersed in the electrolyte, such as redox flow batteries, fuel cells, and Li-S batteries, permselectivity is also a crucial property. Little attention has been paid to the usage of permselective membranes in the Li-S cells while tremendous efforts have been devoted to the structure and composition of the cathode. A few recent articles have explored the use of permselective membrane separator in mitigating the issue of migration of polysulfides.[ 3 , 4 ] Despite partial success with retention of polysulfides, the presence of such permselective membranes results in a natural decrease in the ion flux across the membrane. This may potentially hinder the industrial use of permselective membranes in Li-S battery where the rate capability is already an issue due to the slow kinetics of the reactions. Accordingly it is highly desirable to achieve an ion-selective transport and mitigate the consequences of polysulfides whilst enabling unimpeded transport of ions. In here we report, for the first time, the integration of a high-selectivity, high-flux GO membrane directly supported onto the sulfur electrode in a Li-S battery. This simple approach dramatically resolves the issue of dissolution of electrode material in battery electrolyte and delivers properties close to those required for a practical rechargeable battery. Our shear-aligned GO membrane which is directly coated onto the electrode, is uniquely composed of well-ordered stacks of the graphene sheets in the membrane plane and we unambiguously demonstrate that its integration in the Li-S battery hardly interferes with free ionic flow in the device. For example, alongside with excellent capacity retention arising from the inherent surface charge of the GO, the highly ordered structure of the thin (~ 0.75 µm) membrane enables the cells to deliver unprecedented high initial discharge capacities of 1616, 1400 and 1170 mAh g-1 at 0.2 C, 0.5 C and 1 C rates, respectively. The remarkable rate capability results reported in this work provides a reliable foundation to challenge the long-held belief that integrating permselective membranes in electrochemical energy storage devises suffering from electrode instability issues, although critical for capacity retention are not suitable for power characteristics. [1] M. Barghamadi, A. S. Best, A. I. Bhatt, A. F. Hollenkamp, M. Musameh, R. J. Rees, T. Rüther, Energy & Environmental Science 2014. [2] J. Song, M. L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, J. Lu, Y. Ren, Y. Duan, D. Wang, Angewandte Chemie 2015. [3] J.-Q. Huang, T.-Z. Zhuang, Q. Zhang, H.-J. Peng, C.-M. Chen, F. Wei, ACS nano 2015, 9, 3002. [4] C. Li, A. L. Ward, S. E. Doris, T. A. Pascal, D. Prendergast, B. A. Helms, Nano letters 2015, 15, 5724.

ChemInform Abstract: Graphene Oxide Liquid Crystals: Discovery, Evolution and Applications

AbstractReview: 142 refs.

Ultrastiff and Strong Graphene Fibers via Full-Scale Synergetic Defect Engineering.

Kilometer-scale continuous graphene fibers (GFs) with outstanding mechanical properties and excellent electrical conductivity are produced by high-throughput wet-spinning of graphene oxide liquid crystals followed by graphitization through a full-scale synergetic defect-engineering strategy. GFs with superior performances promise wide applications in functional textiles, lightweight motors, microelectronic devices, and so on.

Deterioration and recovery of electro-optical performance of aqueous graphene-oxide liquid-crystal cells after prolonged storage

Electro-optical switching of graphene oxide (GO) liquid crystals is of great interest owing to its potential applications. However, a GO dispersion undergoes vinylogous carboxylic acidic reaction with time, accompanied by a decrease in pH, which deteriorates the electro-optical performance of a GO cell. We find that the vinylogous carboxylic reaction is hindered in high concentration GO dispersions, and also demonstrate that the deteriorating electro-optical performance can be recovered either by re-cleaning or by ionic neutralization treatment. The electro-optical performance is systematically analyzed in terms of the acidity of the neutralizing solvent, GO dispersion, and supernatant liquid. The best performance is obtained in a GO dispersion with a pH-7 supernatant liquid after neutralization by weakly alkaline water. The neutralization process decreases the bulk conductivity, and also improves the surface conductivity on GO. Approximately 56% of newly generated H+ ions reside near the GO surface, and the rest diffuse away into the bulk solvent. These findings may provide better understanding for the electro-optical switching phenomenon and its applicability.

Electro‐optic switching with liquid crystal graphene

Graphene oxide (GO) particles in aqueous dispersions can form liquid crystal (LC) phases at extremely low concentrations due to the extremely high aspect ratio of the flakes and noticeably, they possess an extremely large Kerr coefficient attractive for low power consumption electro‐optic devices. Reduced graphene does not easily form LC phases in water due to its hydrophobic nature but here we show that stable dispersions of reduced graphene oxide can be realized with surfactants and that they exhibit birefringence upon shearing as well as under application of electric fields. The performance of the system is largely superior to GO LC possessing longer time stability and drastically improved electro‐optic properties with an induced birefringence twice as large at the same field strength thanks to the almost recovery of graphene properties upon reduction. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Liquid Crystals: Graphene Oxide Liquid Crystals: Discovery, Evolution and Applications (Adv. Mater. 16/2016).

Graphene-oxide liquid crystals (GOLCs) have recently been discovered as a novel 2D material with remarkable properties. On page 3045, S. O. Kim and co-workers review the discovery of different GOLC mesophases and recent progress on fundamental studies and applications. The image displays the nematic schlieren texture (in the background) formed by flowing domains of graphene-oxide liquid crystals and their potential applications in energy storage, optoelectronics and wet-spun fibers.

Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide.

Graphene-based membranes demonstrating ultrafast water transport, precise molecular sieving of gas and solvated molecules shows great promise as novel separation platforms; however, scale-up of these membranes to large-areas remains an unresolved problem. Here we demonstrate that the discotic nematic phase of graphene oxide (GO) can be shear aligned to form highly ordered, continuous, thin films of multi-layered GO on a support membrane by an industrially adaptable method to produce large-area membranes (13 × 14 cm2) in <5 s. Pressure driven transport data demonstrate high retention (>90%) for charged and uncharged organic probe molecules with a hydrated radius above 5 Å as well as modest (30–40%) retention of monovalent and divalent salts. The highly ordered graphene sheets in the plane of the membrane make organized channels and enhance the permeability (71±5 l m−2 hr−1 bar−1 for 150±15 nm thick membranes).

Graphene Oxide Liquid Crystals: Discovery, Evolution and Applications.

The discovery and relevant research progress in graphene oxide liquid crystals (GOLCs), the latest class of 2D nanomaterials exhibiting colloidal liquid crystallinity arising from the intrinsic disc-like shape anisotropy, is highlighted. GOLC has conferred a versatile platform for the development of novel properties and applications based on the facile controllability of molecular scale alignment. The first part of this review offers a brief introduction to LCs, including the theoretical background. Particular attention has been paid to the different types of LC phases that have been reported thus far, such as nematic, lamellar and chiral phases. Several key parameters governing the ultimate stability of GOLC behavior, including pH and ionic strength of aqueous dispersions are highlighted. In a relatively short span of time since its discovery, GOLCs have proved their remarkable potential in a broad spectrum of applications, including highly oriented wet-spun fibers, self-assembled nanocomposites, and architectures for energy storage devices. The second part of this review is devoted to an exclusive overview of the relevant applications. Finally, an outlook is provided into this newly emerging research field, where two well established scientific communities for carbon nanomaterials and liquid crystals are ideally merged.

Base-Induced Liquid Crystals of Graphene Oxide for Preparing Elastic Graphene Foams with Long-Range Ordered Microstructures.

Base-induced graphene oxide (GO) liquid crystals form a highly ordered texture. This microstructure can be inherited by graphene foams prepared by hydrothermal reduction, showing a long-range ordered microstructure of graphene sheets in 3D. This provides an insightful understanding into the supramolecular chemistry of GO sheets.

Liquid‐Crystal‐Mediated Self‐Assembly of Porous α‐Fe2O3 Nanorods on PEDOT:PSS‐Functionalized Graphene as a Flexible Ternary Architecture for Capacitive Energy Storage

A novel aqueous‐based self‐assembly approach to a composite of iron oxide nanorods on conductive‐polymer (CP)‐functionalized, ultralarge graphene oxide (GO) liquid crystals (LCs) is demonstrated here for the fabrication of a flexible hybrid material for charge capacitive application. Uniform decoration of α‐Fe2O3 nanorods on a poly(3,4‐ethylene‐dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)‐functionalized, ultralarge GO scaffold results in a 3D interconnected layer‐by‐layer (LBL) architecture. This advanced interpenetrating network of ternary components is lightweight, foldable, and possesses highly conductive pathways for facile ion transportation and charge storage, making it promising for high‐performance energy‐storage applications. Having such structural merits and good synergistic effects, the flexible architecture exhibits a high specific discharge capacitance of 875 F g−1 and excellent volumetric specific capacitance of 868 F cm−3 at 5 mV s−1, as well as a promising energy density of 118 W h kg−1 (at 0.5 A g−1) and promising cyclability, with capacity retention of 100% after 5000 charge–discharge (CD) cycles. This synthesis method provides a simple, yet efficient approach for the solution‐processed LBL insertion of the hematite nanorods (HNR) into CP‐functionalized novel composite structure. It provides great promise for the fabrication of a variety of metal‐oxide (MO)‐nanomaterial‐based binder and current collector‐free flexible composite electrodes for high‐performance energy‐storage applications.

Graphene oxide liquid crystals: synthesis, phase transition, rheological property, and applications in optoelectronics and display.

Graphene oxide (GO) liquid crystals (LCs) are macroscopically ordered GO flakes dispersed in water or polar organic solvents. Since the first report in 2011, GO LCs have attracted considerable attention for their basic properties and potential device applications. In this review, we summarize recent developments and present a comprehensive understanding of GO LCs via many aspects ranging from the exfoliation of GO flakes from graphite, to phases and phase transitions under various conditions, the orientational responses of GO under external magnetic and electric fields, and finally Kerr effect and display applications. The emphasis is placed on the unique and basic properties of GO and their ordered assembly. We will also discuss challenges and issues that need to be overcome in order to gain a more fundamental understanding and exploit full device potentials of GO LCs.

Wet-spinning of continuous montmorillonite-graphene fibers for fire-resistant lightweight conductors.

All-inorganic fibers composed of neat 2D crystals possessing fascinating performance (e.g., alternately stacking layers, high mechanical strength, favorable electrical conductivity, and fire-resistance) are discussed in detail. We developed a wet-spinning assmebly strategy to achieve continuous all-inorganic fibers of montmorillonite (MMT) nanoplatelets by incorporation of a graphene oxide (GO) liquid crystal (LC) template at a rate of 9 cm/s, and the templating role of GO LC is confirmed by in situ confocal laser scanning microscopy and polarized optical microscopy inspections. After protofibers underwent thermal reduction, the obtained binary complex fibers composed of neat 2D crystals integrate the outstanding fire-retardance of MMT nanoplatelets and the excellent conductivity of graphene nanosheets. High-resolution transmission electron microscopy and scanning electron microscope observations reveal the microstructures of fibers with compactly stacking layers. MMT-graphene fibers show increaing tensile strengths (88-270 MPa) and electrical conductivities (130-10500 S/m) with increasing graphene fraction. MMT-graphene (10/90) fibers are used as fire-resistant (bearing temperature in air: 600-700 °C), lightweight (ρ < 1.62 g/cm(3)) conductors (conductivity: up to 1.04 × 10(4) S/m) in view of their superior performance in high-temperature air beyond commercial T700 carbon fibers. We attribute the fire-resistance of MMT-graphene fibers to the armor-like protection of MMT layers, which could shield graphene layers from the action of oxidative etching. The composite fibers worked well as fire-resistant conductors when being heated to glowing red by an alcohol lamp. Our GO LC-templating wet-spinning strategy may also inspire the continuous assembly of other layered crystals into high-performance composite fibers.

Liquid crystal graphene oxide with different layers: fabrication, characterization and applications

Graphene oxide liquid crystals with different layers were successfully preparedviasimple centrifugation. The self-assembled film demonstrates radial schlieren textures (in the background) and has a high electrical conductivity.

Graphene oxide liquid crystals for reflective displays without polarizing optics.

The recent emergence of liquid crystals of atomically thin two-dimensional (2D) materials not only has allowed us to explore novel phenomena of macroscopically aligned 2D nanomaterials but also has provided a route toward their controlled assembly into three-dimensional functional macrostructures. Using flow-induced mechanical alignment, we prepared flakes of graphene oxide (GO) in different orientational orders and demonstrated that GO liquid crystals (LC) can be used as rewritable media for reflective displays without polarizing optics. With a wire or stick as a pen, we can make the surface of GO LC reflective and bright, and we can then manually draw lines, curves, and any patterns with dark appearance. The contrast between bright and dark features is due to anisotropic optical responses of ordered GO flakes. Since optical anisotropy is an intrinsic property of 2D structures, our observations and demonstration represent one of many potential applications of macroscopically aligned 2D nanomaterials.

Graphene oxide liquid crystal Pickering emulsions and their assemblies

In this study, we report for the first time the fabrication of graphene-based liquid crystal Pickering emulsions (LCPEs) by promoting the localized alignment of GO nanosheets at the oil/water interfaces using poly(oxypropylene)diamine (D400) to stabilize as well as to cross-link the GO assembly around the oil droplets. The dispersed phase of the LCPEs serves as a soft template for making cross-linked graphene monoliths with the pores controlled by the size of the oil droplets, which is dependent on the ratio of oil to water. It is found that the optimal ratio of oil to water is 1:2 (v/v), under which more uniform size of oil droplets and thus resultant monolith pores is obtained with a narrow pore size distribution. Absence of D400 does not lead to a stable emulsion and a cross-linked structure. The key point of this study is the use of D400, which significantly reduces the interfacial tension at interface of oil and water and thus results in a stable emulsion.

Rheological properties of graphene oxide liquid crystal

We report the rheological properties of liquid crystalline graphene oxide (GO) aqueous dispersion. GO dispersions exhibit typical shear thinning behaviors of liquid crystals, which is described by power law or simple Curreau model. Irrespective of the shear rate, shear viscosity exhibits sudden decrease with the increase of GO composition around a critical volume fraction, ϕc=0.33%, demonstrating typical colloidal isotropic–nematic phase transition. Dynamic measurements reveal the liquid-like (isotropic phase, G′>G″) behavior at a low GO composition (ϕ∼0.08%) and solid-like (liquid crystalline) behavior at higher compositions (ϕ∼0.45%), where G′ exceeds over G″. Nematic gel-like phase is confirmed at a higher GO composition over ϕ>0.83%, where both G′ and G″ moduli are nearly independent of frequency (ω). Simple power law scaling arguments are introduced to model the dependence of yield stress and viscoelastic moduli on the GO composition. We also observed the yield stress and rigidity percolation transition above phase transition composition ϕc>0.33% with a percolation exponent of 1.3±0.1. These rheological insights provide valuable information for the liquid crystalline processing of GO based materials including fibers, sheets and other complex structures for electronic/optoelectronic and energy storage/conversion applications.