The elastic response of graphene oxide gels as a crumpling phenomenon

The broad spectrum of chemical and electronic properties of 2D nanomaterials makes them attractive in a wide range of applications, especially in the context of printed electronics. Therefore, understanding the rheological properties of nanosheet suspensions is crucial for many additive manufacturing techniques. Here, we study the viscoelastic properties of aqueous suspensions of graphene oxide nanosheets. We show that in the gel phase, the magnitude of the elastic response and its scaling with volume fraction is independent of the lateral size of the particles and the interaction strength between them. We explain this behavior by modelling the elasticity of these gels as a crumpling phenomenon where the magnitude of the response is determined by the bending stiffness and thickness of the sheets. Due to their low bending stiffness these nanosheets crumple upon deformation and may therefore be considered soft colloids. Furthermore, we provide an explanation why the yield strain decreases with packing fraction for these gels.

Additive Manufacturing of Ti3 C2 -MXene-Functionalized Conductive Polymer Hydrogels for Electromagnetic-Interference Shielding

The ongoing miniaturization of devices and development of wireless and implantable technologies demand electromagnetic interference (EMI)-shielding materials with customizability. Additive manufacturing of conductive polymer hydrogels with favorable conductivity and biocompatibility can offer new opportunities for EMI-shielding applications. However, simultaneously achieving high conductivity, design freedom, and shape fidelity in 3D printing of conductive polymer hydrogels is still very challenging. Here, an aqueous Ti₃ C₂ -MXene-functionalized poly(3,4-ethylenedioxythiophene):polystyrene sulfonate ink is developed for extrusion printing to create 3D objects with arbitrary geometries, and a freeze-thawing protocol is proposed to transform the printed objects directly into highly conductive and robust hydrogels with high shape fidelity on both the macro- and microscale. The as-obtained hydrogel exhibits a high conductivity of 1525.8 S m^-1 at water content up to 96.6 wt% and also satisfactory mechanical properties with flexibility, stretchability, and fatigue resistance. Furthermore, the use of the printed hydrogel for customizable EMI-shielding applications is demonstrated. The proposed easy-to-manufacture approach, along with the highlighted superior properties, expands the potential of conductive polymer hydrogels in future customizable applications and represents a real breakthrough from the current state of the art.

Printable G-Putty for Frequency- and Rate-Independent, High-Performance Strain Sensors

While nanocomposite electromechanical sensors are expected to display reasonable conductivity and high sensitivity, little consideration is given to eliminating hysteresis and strain rate/frequency dependence from their response. For example, while G-putty, a composite of graphene and polysiloxane, has very high electromechanical sensitivity, its extreme viscoelasticity renders it completely unsuitable for real sensors due to hysteretic and rate-/frequency-dependent effects. Here it is shown that G-putty can be converted to an ink and printed into patterned thin films on elastic substrates. A partial graphene-polymer phase segregation during printing increases the thin-film conductivity by ×10⁶ compared to bulk, while the mechanical effects of the substrate largely suppress hysteresis and completely remove strain rate and frequency dependence. This allows the fabrication of practical, high-gauge-factor, wearable sensors for pulse measurements as well as patterned sensors for low-signal vibration sensing.

Additive-free MXene inks and direct printing of micro-supercapacitors

Direct printing of functional inks is critical for applications in diverse areas including electrochemical energy storage, smart electronics and healthcare. However, the available printable ink formulations are far from ideal. Either surfactants/additives are typically involved or the ink concentration is low, which add complexity to the manufacturing and compromises the printing resolution. Here, we demonstrate two types of two-dimensional titanium carbide (Ti₃C₂T_x) MXene inks, aqueous and organic in the absence of any additive or binary-solvent systems, for extrusion printing and inkjet printing, respectively. We show examples of all-MXene-printed structures, such as micro-supercapacitors, conductive tracks and ohmic resistors on untreated plastic and paper substrates, with high printing resolution and spatial uniformity. The volumetric capacitance and energy density of the all-MXene-printed micro-supercapacitors are orders of magnitude greater than existing inkjet/extrusion-printed active materials. The versatile direct-ink-printing technique highlights the promise of additive-free MXene inks for scalable fabrication of easy-to-integrate components of printable electronics.

Printing functional inks is attractive for applications in electrochemical energy storage and smart electronics, among others. Here the authors report highly concentrated, additive-free, aqueous and organic MXene-based inks that can be used for high-resolution extrusion and inkjet printing.

High capacity silicon anodes enabled by MXene viscous aqueous ink

The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated the quest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a critical thickness, solution-processed films typically encounter electrical/mechanical problems, limiting the achievable areal capacity and rate performance as a result. Herein, we show that two-dimensional titanium carbide or carbonitride nanosheets, known as MXenes, can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives. The nanosheets form a continuous metallic network, enable fast charge transport and provide good mechanical reinforcement for the thick electrode (up to 450 µm). Consequently, very high areal capacity anodes (up to 23.3 mAh cm⁻²) have been demonstrated.

Developing thick electrodes could enable high-energy-density Li-ion batteries, however, above a critical thickness, the mass transport issues become dominating. Here the authors show that MXene can serve as a conductive binder leading to thick silicon anodes (up to 450 µm) with high areal capacity.

Graphene-coated polymer foams as tuneable impact sensors

The use of graphene-based nanocomposites as electromechanical sensors has been broadly explored in recent times with a number of papers describing porous, foam-like composites. However, there are no reported foam-based materials that are capable of large dynamic compressive load measurements and very few studies on composite impact sensing. In this work, we describe a simple method of infusing commercially-available foams with pristine graphene to form conductive composites, which we refer to as G-foam. Displaying a strain-dependent electrical response, G-foam was found to be a reasonably effective pressure sensing material. More interestingly, G-foam is a sensitive impact-sensing material. Through the addition of various amounts of polymer filler, the mechanical properties of the composites can be tuned leading to the controllable variation of the impact sensing range. We have developed a simple model which quantitatively explains all our impact sensing data.

Yielding and flow of highly concentrated, few-layer graphene suspensions

For a wide range of applications of graphene suspensions, a thorough understanding of their rheological properties is crucial. We probe the microstructure of dense suspensions of micron-sized, few-layer, defect-free graphene platelets by measuring their viscoelastic properties at various concentrations up to 39 mg ml(-1). We propose a model to relate the yield strain to the mesh size of the microstructure as a function of volume fraction ϕ. From the yield stress measurements we infer the typical bond energy (≈20 kBT) and ϕ dependence of the bond number density. These results allow us to express the steady shear viscosity for Peclet number Pe < 10 in terms of the platelet dimensions, bond energy and ϕ using a relaxation ansatz.

Boron nitride nanosheets as barrier enhancing fillers in melt processed composites

In this work we have used melt-processing to mix liquid-exfoliated boron-nitride nanosheets with PET to produce composites for gas barrier applications. Sonication of h-BN powder, followed by centrifugation-based size-selection, was used to prepare suspensions of nanosheets with aspect ratio >1000. The solvent was removed to give a weakly aggregated powder which could easily be mixed into PET, giving a composite containing well-dispersed nanosheets. These composites showed very good barrier performance with oxygen permeability reductions of 42% by adding just 0.017 vol% nanosheets. At low loading levels the composites were almost completely transparent. At higher loading levels, while some haze was introduced, the permeability fell by ∼70% on addition of 3 vol% nanosheets.

Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids

To progress from the laboratory to commercial applications, it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene. Here we show that high-shear mixing of graphite in suitable stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. X-ray photoelectron spectroscopy and Raman spectroscopy show the exfoliated flakes to be unoxidized and free of basal-plane defects. We have developed a simple model that shows exfoliation to occur once the local shear rate exceeds 10(4) s(-1). By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from hundreds of millilitres up to hundreds of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals.