Liquid crystalline and shear-induced properties of an aqueous solution of graphene oxide sheets

Scalable microfluidic fabrication of vertically aligned two-dimensional nanosheets for superior thermal management

Two-dimensional (2D) nanosheets have been assembled into various macroscopic structures for wide engineering applications. To fully explore their exceptional thermal, mechanical, and electrical properties, 2D nanosheets must be aligned into highly ordered structures due to their strong structural anisotropy. Structures stacked layer by layer such as films and fibers have been readily assembled from 2D nanosheets due to their planar geometry. However, scalable manufacturing of macroscopic structures with vertically aligned 2D nanosheets remains challenging, given their large lateral size with a thickness of only a few nanometers. Herein, we report a scalable and efficient microfluidics-enabled sheet-aligning process to assemble 2D nanosheets into a large-area film with a highly ordered vertical alignment. By applying microchannels with a high aspect ratio, 2D nanosheets were well aligned vertically under strong channel size confinement and high flow shear stress. A vertically aligned graphene sheet film was obtained and applied to effectively improve the heat transfer of thermal interfacial materials (TIMs). Superior through-plane thermal conductivity of 82.7 W m-1 K-1 at a low graphene content of 11.8 vol% was measured for vertically aligned TIMs. Thus, they demonstrate exceptional thermal management performance for switching power supplies with high reliability.

Cytomembrane-Inspired MXene Ink with Amphiphilic Surfactant for 3D Printed Microsupercapacitors

Two-dimensional (2D) material-based hydrogels have been widely utilized as the ink for extrusion-based 3D printing in various electronics. However, the viscosity of the hydrogel ink is not high enough to maintain the self-supported structure without architectural deformation. It is also difficult to tune the microstructure of the printed devices using a low-viscosity hydrogel ink. Herein, by mimicking a phospholipid bilayer in a cytomembrane, the amphiphilic surfactant nonaethylene glycol monododecyl ether (C12E9) was incorporated into MXene hydrogel. The incorporation of C12E9 offers amphiphilicity to the MXene flakes and produces a 3D interlinked network of the MXene flakes. The 3D interlinked network offers a high-viscosity, homogenized flake distribution and enhanced printability to the ink. This ink facilitates the alignment of the MXene flakes during extrusion as well as the formation of the aligned micro- and sub-microsized porous structures, leading to the improved electrochemical performance of the printed microsupercapacitor. This study provides an example for the preparation of microelectronics with tunable microstructures.

Chiral Liquid Crystalline Properties of Cellulose Nanocrystals: Fundamentals and Applications

By using an independent self-assembly process that is occasionally controlled by evaporation, cellulose nanocrystals (CNCs) may create films (pure or in conjunction with other materials) that have iridescent structural colors. The self-forming chiral nematic structures and environmental safety of a new class of photonic liquid crystals (LCs), referred to as CNCs and CNC-embedded materials, make them simple to make and treat. The structure of the matrix interacts with light to give structural coloring, as opposed to other dye pigments, which interact with light by adsorption and reflection. Understanding how CNC self-assembly constructs structures is vital in several fields, including physics, science, and engineering. To constructure this review, the colloidal characteristics of CNC particles and their behavior during the formation of liquid crystals and gelling were studied. Then, some of the recognized applications for these naturally occurring nanoparticles were summarized. Different factors were considered, including the CNC aspect ratio, surface chemistry, concentration, the amount of time needed to produce an anisotropic phase, and the addition of additional substances to the suspension medium. The effects of alignment and the drying process conditions on structural changes are also covered. The focus of this study however is on the optical properties of the films as well as the impact of the aforementioned factors on the final transparency, iridescent colors, and versus the overall response of these bioinspired photonic materials. Control of the examined factors was found to be necessary to produce reliable materials for optoelectronics, intelligent inks and papers, transparent flexible support for electronics, and decorative coatings and films.

Crystallographically Textured Electrodes for Rechargeable Batteries: Symmetry, Fabrication, and Characterization

The vast of majority of battery electrode materials of contemporary interest are of a crystalline nature. Crystals are, by definition, anisotropic from an atomic-structure perspective. The inherent structural anisotropy may give rise to favored mesoscale orientations and anisotropic properties whether the material is in a rest state or subjected to an external stimulus. The overall perspective of this review is that intentional manipulation of crystallographic anisotropy of electrochemically active materials constitute an untapped parameter space in energy storage systems and thus provide new opportunities for materials innovations and design. To that end, we contend that crystallographically textured electrodes, as opposed to their textureless poly crystalline or single-crystalline analogs, are promising candidates for next-generation storage of electrical energy in rechargeable batteries relevant to commercial practice. This perspective is underpinned first by the fundamental─to a first approximation─uniaxial, rotation-invariant symmetry of electrochemical cells. On this basis, we show that a crystallographically textured electrode with the preferred orientation aligned out-of-plane toward the counter electrode represents an optimal strategy for utilization of the crystals' anisotropic properties. Detailed analyses of anisotropy of different types lead to a simple, but potentially useful general principle that "P_ec//P_c" textures are optimal for metal anodes, and "P_ec//S_c" textures are optimal for insertion-type electrodes.

Long-Term Aging of Concentrated Aqueous Graphene Oxide Suspensions Seen by Rheology and Raman Spectroscopy

Today, graphene oxide (GO) has gained well-deserved recognition, with its applications continuing to increase. Much of the processing of GO-based devices occurs in a dispersed form, which explains the commercialization of GO suspensions. Aging of these suspensions can, however, affect the shelf life and thus their application potential. Aging of GO preparations is often acknowledged, but no longer-term systematic study has been reported on the alteration of GO suspensions. This paper investigates high-concentration (10 mg/mL) aqueous GO suspensions over a 2-year time scale. In addition to steady shear tests, the dynamic behavior of the suspensions was studied in more detail by transient shear and frequency sweep measurements. Both the viscosity and the dynamic moduli increased with age, particularly within the first year. The results of the complementary Raman spectroscopic studies indicate that the change in the rheological behavior with aging results from a slow oxidation process occurring in the highly acidic aqueous medium during the relatively long-term storage. The (over)oxidized layers peel off spontaneously or are removed by high shear stress, resulting in increased viscosity, as it was corroborated by XRD and XPS.

Nanocomposites of Rigid Polyurethane Foam and Graphene Nanoplates Obtained by Exfoliation of Natural Graphite in Polymeric 4,4′-Diphenylmethane Diisocyanate

The influence of graphene nanoplates (GNPs) obtained by the ecofriendly exfoliation of natural graphite has been addressed on the mechanical and thermal insulating properties of rigid polyurethane foams (RPUFs). Few-layer GNPs with few defects were prepared in polymeric 4,4′-diphenylmethane diisocyanate (pMDI) under ultrasonication to obtain a GNP/pMDI dispersion. GNP/pMDI dispersions with different GNP concentrations were used to prepare RPUF nanocomposites via in situ polymerization. An important finding is that the GNP/pMDI dispersion exhibits lyotropic liquid crystalline behavior. It was found that the unique orientation of GNPs above the concentration of 0.1 wt% in the dispersion affected the mechanical and thermal insulation properties of the RPUF nanocomposites. GNP/RPUF nanocomposites with GNP concentrations at 0.2 wt% or more showed better thermal insulating properties than neat RPUF. The lyotropic liquid crystalline ordering of GNPs provides stable nucleation for bubble formation during foaming and prevents bubble coalescence. This decreases the average cell size and increases the closed cell content, producing GNP/RPUF nanocomposites with low thermal conductivity. Furthermore, GNPs incorporated into RPUF act as a barrier to radiant heat transfer through the cells, which effectively reduces the thermal conductivity of the resulting nanocomposites. It is expected that the nanocomposite of RPUF investigated in this study can be applied practically to improve the performance of thermal insulation foams.

Printing thermoelectric inks toward next-generation energy and thermal devices

The ability of thermoelectric (TE) materials to convert thermal energy to electricity and vice versa highlights them as a promising candidate for sustainable energy applications. Despite considerable increases in the figure of merit zT of thermoelectric materials in the past two decades, there is still a prominent need to develop scalable synthesis and flexible manufacturing processes to convert high-efficiency materials into high-performance devices. Scalable printing techniques provide a versatile solution to not only fabricate both inorganic and organic TE materials with fine control over the compositions and microstructures, but also manufacture thermoelectric devices with optimized geometric and structural designs that lead to improved efficiency and system-level performances. In this review, we aim to provide a comprehensive framework of printing thermoelectric materials and devices by including recent breakthroughs and relevant discussions on TE materials chemistry, ink formulation, flexible or conformable device design, and processing strategies, with an emphasis on additive manufacturing techniques. In addition, we review recent innovations in the flexible, conformal, and stretchable device architectures and highlight state-of-the-art applications of these TE devices in energy harvesting and thermal management. Perspectives of emerging research opportunities and future directions are also discussed. While this review centers on thermoelectrics, the fundamental ink chemistry and printing processes possess the potential for applications to a broad range of energy, thermal and electronic devices.

Preparation of graphene oxide liquid crystals with long-range highly-ordered flakes using a coat-hanger die

Graphene oxide (GO) was discovered as a liquid crystalline (LC) phase formation in its water dispersion and expanded to a large number of applications, such as highly ordered GO sheets papers, films, and foams. However, there are still few efficient ways to prepare graphene oxide liquid crystals (GOLCs) with long-range highly ordered flakes. In this work, after carefully studying the rheological properties of GO aqueous dispersions at different concentrations, we have provided a new method to prepare holistically-oriented GOLCs through a designed coat-hanger die. Further, by simulating the extrusion process in the slot of the coat-hanger die, the die's dimensional sizes were optimized to apply efficient shear force on GO dispersions. Then, GOLCs with long-range highly ordered flakes of different GO concentrations were prepared using this method. Finally, a GO foam with a highly ordered structure was prepared using a layer-by-layer method, which exhibited improved conductivity compared to that of normal disordered GO foams after chemical reduction.

Microfluidic tuning of linear and nonlinear absorption in graphene oxide liquid crystals

Manipulation of the nonlinear optical response of materials plays a significant role in photonics applications; however, it may be irreversible, untunable, and uncontrollable, which makes it difficult. In this Letter, we present a mechanical-hydrodynamical approach through a microchannel to tune the nonlinear absorption response of graphene oxide liquid crystals. In this material, the optical properties depend on the flake orientation. This feature has helped us to study empirically the dependency of the nonlinear absorption coefficients to external hydrodynamical force by employing the Z-scan technique. The experimental results show that increasing the flow rate in the microchannel enhances both linear and nonlinear absorption coefficients and, as a result, reduces the laser beam transmission through the sample. It has been observed that the percentage change in the nonlinear absorption coefficient of the sample is significant due to the flow rate.

A Review on Graphene Fibers: Expectations, Advances, and Prospects

Graphene fiber (GF) is a macroscopically assembled fibrous material made of individual units of graphene and its derivatives. Beyond traditional carbon fibers, graphene building blocks consisting of regulable sizes and regular orientations of GF are expected to generate extreme mechanical and transport properties, as well as multiple functions in smart electronic fibrous devices and textiles. Here, the features of GF are presented along four lines: preparation, morphology, structure-performance correlations, and state-of-the-art applications as flexible and wearable electronics. The principles, experiments, and keys of fabricating GF from graphite with different methods, focusing on the industrially viable mainstream strategy, wet spinning, are introduced. Then, the fundamental relationship between the mechanical and transport properties and the structure, including both highly condensed structures for high-performance and hierarchical structures for multiple functions, is presented. The advances of GF based on structure-performance formulas boost its functional applications, especially in electronic devices. Finally, the possible promotion methods and structural-functional integrated applications of GF are discussed.

Large Improvement in the Mechanical Properties of Polyurethane Nanocomposites Based on a Highly Concentrated Graphite Nanoplate/Polyol Masterbatch

In this study, a highly concentrated graphite nanoplate (GNP)/polyol masterbatch was prepared by the exfoliation of natural graphite in an aqueous system using cetyltrimethylammonium bromide and the replacement of aqueous solution with a polyol, viz. poly(tetramethylene ether glycol), and it was subsequently used to prepare polyurethane (PU) nanocomposites by simple dilution. The polyol in the masterbatch efficiently prevented the aggregation of GNPs during the preparation of PU nanocomposite. In addition, the dispersed GNPs in the masterbatch exhibited rheological behavior of lyotropic liquid crystalline materials. In this study, the manufacture and application methods of the GNP/polyol masterbatch were discussed, enabling the facile manufacture of the PU/GNP nanocomposites with excellent mechanical properties. In addition, the manner in which the GNP alignment affected the microphase separation of PU in the nanocomposites was investigated, which determined the improvement in the mechanical properties of the nanocomposites. High-performance PU/GNP nanocomposites are thought to be manufactured from the GNP/polyol masterbatch by the simple dilution to 0.1 wt% GNP in the nanocomposite.

Graphene oxide liquid crystals: a frontier 2D soft material for graphene-based functional materials

Graphene, despite being the best known strong and electrical/thermal conductive material, has found limited success in practical applications, mostly due to difficulties in the formation of desired large-scale highly organized structures. Our discovery of a liquid crystalline phase formation in graphene oxide dispersion has enabled a broad spectrum of highly aligned graphene-based structures, including films, fibers, membranes, and mesoscale structures. In this review, the current understanding of the structure-property relationship of graphene oxide liquid crystals (GOLCs) is overviewed. Various synthetic methods and parameters that can be optimized for GOLC phase formation are highlighted. Along with the results from different characterization methods for the identification of the GOLC phases, the typical characteristics of different types of GOLC phases introduced so far, including nematic, lamellar and chiral phases, are carefully discussed. Finally, various interesting applications of GOLCs are outlined together with the future prospects for their further developments.

Macroscopic assembly of flexible and strong green graphene fibres

The scalable production presented here facilitates flexible, strong and electrically conductive graphene fibres, which have emerged as promising graphene based electronic textiles and sensors.

Superflexibility of graphene oxide

Graphene oxide (GO), the main precursor of graphene-based materials made by solution processing, is known to be very stiff. Indeed, it has a Young's modulus comparable to steel, on the order of 300 GPa. Despite its very high stiffness, we show here that GO is superflexible. We quantitatively measure the GO bending rigidity by characterizing the flattening of thermal undulations in response to shear forces in solution. Characterizations are performed by the combination of synchrotron X-ray diffraction at small angles and in situ rheology (rheo-SAXS) experiments using the high X-ray flux of a synchrotron source. The bending modulus is found to be 1 kT, which is about two orders of magnitude lower than the bending rigidity of neat graphene. This superflexibility compares with the fluidity of self-assembled liquid bilayers. This behavior is discussed by considering the mechanisms at play in bending and stretching deformations of atomic monolayers. The superflexibility of GO is a unique feature to develop bendable electronics after reduction, films, coatings, and fibers. This unique combination of properties of GO allows for flexibility in processing and fabrication coupled with a robustness in the fabricated structure.

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.

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.

Impact of particle surface chemistry on the structure and rheological properties of graphene-based particle/polydimethylsiloxane composites

The inter-particle interaction of graphene-based particles has a key effect on the structure and rheological properties of graphene-based particle/polydimethylsiloxane composites.

Nematic order drives macroscopic patterns of graphene oxide in drying drops

We report on a series of experiments on large-area ordered patterns of graphene oxide on solid substrates deposited from aqueous dispersions by directed drop evaporation. The aqueous dispersion of graphene oxide exhibits phase transitions from isotropic to liquid crystalline nematic phases via a biphasic region with increasing concentration. In the single nematic phase, schlieren textures accompanied by oriented bands are frequent. Drying of drops in each phase results in deposition covering the whole drop base. The dynamic process of drop drying is analyzed based on the weight loss, radius change, and texture change over time. It is found that the radial bands develop in the nematic drops in the vicinity of the receding of the contact line and subsequently transform into birefringent stripes after drying. Study into the structure and morphology of the stripes reveals anisotropic wrinkling of graphene oxide sheets. The nature of stripe orientation is strongly dependent on the local nematic order at the dewetting water front. Various macroscopic patterns with different stripe orientations including radial spokes, spider webs, and parallel stripes have been generated by tuning the nematic order of drops.

Liquid crystalline behavior of graphene oxide in the formation and deformation of tough nanocomposite hydrogels

In this paper, we report the formation and transformation of graphene oxide (GO) liquid crystalline (LC) structures in the synthesis and deformation of tough GO nanocomposite hydrogels. GO aqueous dispersions form a nematic LC phase, while the addition of poly(N-vinylpyrrolidone) (PVP) and acrylamide (AAm), which are capable of forming hydrogen bonding with GO nanosheets, shifts the isotropic/nematic transition to a lower volume fraction of GO and enhances the formation of nematic droplets. During the gelation process, a phase separation of the polymers and GO nanosheets is accompanied by the directional assembly of GO nanosheets, forming large LC tactoids with a radial GO configuration. The shape of the large tactoids evolves from a sphere to a toroid as the tactoids increase in size. Interestingly, during cyclic uniaxial tensile deformation a reversible LC transition is observed in the very tough hydrogels. The isolated birefringent domains and the LC domains in the tactoids in the gels are highly oriented under a high tensile strain.

Supramolecular [60]fullerene liquid crystals formed by self-organized two-dimensional crystals

Fullerene-based liquid crystalline materials have both the excellent optical and electrical properties of fullerene and the self-organization and external-field-responsive properties of liquid crystals (LCs). Herein, we demonstrate a new family of thermotropic [60]fullerene supramolecular LCs with hierarchical structures. The [60]fullerene dyads undergo self-organization driven by π-π interactions to form triple-layer two-dimensional (2D) fullerene crystals sandwiched between layers of alkyl chains. The lamellar packing of 2D crystals gives rise to the formation of supramolecular LCs. This design strategy should be applicable to other molecules and lead to an enlarged family of 2D crystals and supramolecular liquid crystals.