Knot Architecture for Biocompatible and Semiconducting 2D Electronic Fiber Transistors

Wearable devices have generally been rigid due to their reliance on silicon-based technologies, while future wearables will utilize flexible components for example transistors within microprocessors to manage data. Two-dimensional (2D) semiconducting flakes have yet to be investigated in fiber transistors but can offer a route toward high-mobility, biocompatible, and flexible fiber-based devices. Here, the electrochemical exfoliation of semiconducting 2D flakes of tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) is shown to achieve homogeneous coatings onto the surface of polyester fibers. The high aspect ratio (>100) of the flake yields aligned and conformal flake-to-flake junctions on polyester fibers enabling transistors with mobilities μ ≈1 cm2 V-1 s-1 and a current on/off ratio, Ion/Ioff ≈102-104. Furthermore, the cytotoxic effects of the MoS2 and WSe2 flakes with human keratinocyte cells are investigated and found to be biocompatible. As an additional step, a unique transistor 'knot' architecture is created by leveraging the fiber diameter to establish the length of the transistor channel, facilitating a route to scale down transistor channel dimensions (≈100 µm) and utilize it to make a MoS2 fiber transistor with a human hair that achieves mobilities as high as μ ≈15 cm2 V-1 s-1.

Near Room Temperature Production of Segregated Network Composites of Carbon Nanotubes and Regolith as Multifunctional, Extra-Terrestrial Building Materials

Constructing a semi-permanent base on the moon or Mars will require maximal use of materials found in situ and minimization of materials and equipment transported from Earth. This will mean a heavy reliance on regolith (Lunar or Marian soil) and water, supplemented by small quantities of additives fabricated on Earth. Here it is shown that SiO2-based powders, as well as Lunar and Martian regolith simulants, can be fabricated into building materials at near-ambient temperatures using only a few weight-percent of carbon nanotubes as a binder. These composites have compressive strength and toughness up to 100 MPa and 3 MPa respectively, higher than the best terrestrial concretes. They are electrically conductive (>20 S m-1) and display an extremely large piezoresistive response (gauge factor >600), allowing these composites to be used as internal sensors to monitor the structural health of extra-terrestrial buildings.

Mechanical Properties of Conducting Printed Nanosheet Network Thin Films Under Uniaxial Compression

Thin film networks of solution processed nanosheets show remarkable promise for use in a broad range of applications including strain sensors, energy storage, printed devices, textile electronics, and more. While it is known that their electronic properties rely heavily on their morphology, little is known of their mechanical nature, a glaring omission given the effect mechanical deformation has on the morphology of porous systems and the promise of mechanical post processing for tailored properties. Here, this work employs a recent advance in thin film mechanical testing called the Layer Compression Test to perform the first in situ analysis of printed nanosheet network compression. Due to the well-defined deformation geometry of this unique test, this work is able to explore the out-of-plane elastic, plastic, and creep deformation in these systems, extracting properties of elastic modulus, plastic yield, viscoelasticity, tensile failure and sheet bending vs. slippage under both out of plane uniaxial compression and tension. This work characterizes these for a range of networks of differing porosities and sheet sizes, for low and high compression, as well as the effect of chemical cross linking. This work explores graphene and MoS2 networks, from which the results can be extended to printed nanosheet networks as a whole.

Quantitative analysis of printed nanostructured networks using high-resolution 3D FIB-SEM nanotomography

Networks of solution-processed nanomaterials are becoming increasingly important across applications in electronics, sensing and energy storage/generation. Although the physical properties of these devices are often completely dominated by network morphology, the network structure itself remains difficult to interrogate. Here, we utilise focused ion beam - scanning electron microscopy nanotomography (FIB-SEM-NT) to quantitatively characterise the morphology of printed nanostructured networks and their devices using nanometre-resolution 3D images. The influence of nanosheet/nanowire size on network structure in printed films of graphene, WS2 and silver nanosheets (AgNSs), as well as networks of silver nanowires (AgNWs), is investigated. We present a comprehensive toolkit to extract morphological characteristics including network porosity, tortuosity, specific surface area, pore dimensions and nanosheet orientation, which we link to network resistivity. By extending this technique to interrogate the structure and interfaces within printed vertical heterostacks, we demonstrate the potential of this technique for device characterisation and optimisation.

Cobalt Oxide 2D Nanosheets Formed at a Polarized Liquid|Liquid Interface toward High-Performance Li-Ion and Na-Ion Battery Anodes

Cobalt oxide (Co3O4)-based nanostructures have the potential as low-cost materials for lithium-ion (Li-ion) and sodium-ion (Na-ion) battery anodes with a theoretical capacity of 890 mAh/g. Here, we demonstrate a novel method for the production of Co3O4 nanoplatelets. This involves the growth of flower-like cobalt oxyhydroxide (CoOOH) nanostructures at a polarized liquid|liquid interface, followed by conversion to flower-like Co3O4 via calcination. Finally, sonication is used to break up the flower-like Co3O4 nanostructures into two-dimensional (2D) nanoplatelets with lateral sizes of 20-100 nm. Nanoplatelets of Co3O4 can be easily mixed with carbon nanotubes to create nanocomposite anodes, which can be used for Li-ion and Na-ion battery anodes without any additional binder or conductive additive. The resultant electrodes display impressive low-rate capacities (at 125 mA/g) of 1108 and 1083 mAh/g, for Li-ion and Na-ion anodes, respectively, and stable cycling ability over >200 cycles. Detailed quantitative rate analysis clearly shows that Li-ion-storing anodes charge roughly five times faster than Na-ion-storing anodes.

High Selectivity and Sensitivity in Chemiresistive Sensing of Co(II) Ions with Liquid-Phase Exfoliated Functionalized MoS2 : A Supramolecular Approach

Chemical sensing of water contamination by heavy metal ions is key as it represents a most severe environmental problem. Liquid-phase exfoliated two-dimensional (2D) transition metal dichalcogenides (TMDs) are suitable candidates for chemical sensing thanks to their high surface-to-volume ratio, sensitivity, unique electrical characteristics, and scalability. However, TMDs lack selectivity due to nonspecific analyte-nanosheet interactions. To overcome this drawback, defect engineering enables controlled functionalization of 2D TMDs. Here, ultrasensitive and selective sensors of cobalt(II) ions via the covalent functionalization of defect-rich MoS2 flakes with a specific receptor, 2,2':6',2″-terpyridine-4'-thiol is developed. A continuous network is assembled by healing of MoS2 sulfur vacancies in a tailored microfluidic approach, enabling high control over the assembly of thin and large hybrid films. The Co2+ cations complexation represents a powerful gauge for low concentrations of cationic species which can be best monitored in a chemiresisitive ion sensor, featuring a 1 pm limit of detection, sensing in a broad concentration range (1 pm - 1 µm) and sensitivity as high as 0.308 ± 0.010 lg([Co2+ ])-1 combined with a high selectivity towards Co2+ over K+ , Ca2+ , Mn2+ , Cu2+ , Cr3+ , and Fe3+ cations. This supramolecular approach based on highly specific recognition can be adapted for sensing other analytes through specific ad-hoc receptors.

Solution-Processed Heterojunction Photodiodes Based on WSe2 Nanosheet Networks

Solution-processed photodetectors incorporating liquid-phase-exfoliated transition metal dichalcogenide nanosheets are widely reported. However, previous studies mainly focus on the fabrication of photoconductors, rather than photodiodes which tend to be based on heterojunctions and are harder to fabricate. Especially, there are rare reports on introducing commonly used transport layers into heterojunctions based on nanosheet networks. In this study, a reliable solution-processing method is reported to fabricate heterojunction diodes with tungsten selenide (WSe2 ) nanosheets as the optical absorbing material and PEDOT: PSS and ZnO as injection/transport-layer materials. By varying the transport layer combinations, the obtained heterojunctions show rectification ratios of up to ≈104 at ±1 V in the dark, without relying on heavily doped silicon substrates. Upon illumination, the heterojunction can be operated in both photoconductor and photodiode modes and displays self-powered behaviors at zero bias.

Transparent Conductors Printed from Grids of Highly Conductive Silver Nanosheets

Transparent conductors (TCs) represent key components in many applications from optoelectronic devices to electromagnetic shielding. While commercial applications typically use thin films of indium tin oxide, this material is brittle and increasingly scarce, meaning higher performing and cheaper alternatives are sought after. Solution-processible metals would be ideal owing to their high conductivities and printability. However, due to their opacity to visible light, such films need to be very thin to achieve transparency, thus limiting the minimum resistance achievable. One solution is to print metallic particles in a grid structure, which has the advantages of high tunable transparency and resistance at the cost of uniformity. Here, we report silver nanosheets that have been aerosol jet printed into grids as high-performance transparent conductors. We first investigate the effect of annealing on the silver nanosheets where we observe the onset of junction sintering at 160 °C after which the silver network becomes continuous. We then investigate the effect of line width and thickness on the electrical performance and the effect of varying the aperture dimensions on the optical performance. Using these data, we develop simple models, which allow us to optimize the grid and demonstrate a printed transparent conductor with a transmittance of 91% at a sheet resistance of 4.6 Ω/sq.

Unveiling Charge-Transport Mechanisms in Electronic Devices Based on Defect-Engineered MoS2 Covalent Networks

Device performance of solution-processed 2D semiconductors in printed electronics has been limited so far by structural defects and high interflake junction resistance. Covalently interconnected networks of transition metal dichalcogenides potentially represent an efficient strategy to overcome both limitations simultaneously. Yet, the charge-transport properties in such systems have not been systematically researched. Here, the charge-transport mechanisms of printed devices based on covalent MoS₂ networks are unveiled via multiscale analysis, comparing the effects of aromatic versus aliphatic dithiolated linkers. Temperature-dependent electrical measurements reveal hopping as the dominant transport mechanism: aliphatic systems lead to 3D variable range hopping, unlike the nearest neighbor hopping observed for aromatic linkers. The novel analysis based on percolation theory attributes the superior performance of devices functionalized with π-conjugated molecules to the improved interflake electronic connectivity and formation of additional percolation paths, as further corroborated by density functional calculations. Valuable guidelines for harnessing the charge-transport properties in MoS₂ devices based on covalent networks are provided.

High-Mobility Flexible Transistors with Low-Temperature Solution-Processed Tungsten Dichalcogenides

The investigation of high-mobility two-dimensional (2D) flakes beyond molybdenum disulfide (MoS₂) will be necessary to create a library of high-mobility solution-processed networks that conform to substrates and remain functional over thousands of bending cycles. Here we report electrochemical exfoliation of large-aspect-ratio (>100) semiconducting flakes of tungsten diselenide (WSe₂) and tungsten disulfide (WS₂) as well as MoS₂ as a comparison. We use Langmuir-Schaefer coating to achieve highly aligned and conformal flake networks, with minimal mesoporosity (∼2-5%), at low processing temperatures (120 °C) and without acid treatments. This allows us to fabricate electrochemical transistors in ambient air, achieving average mobilities of μ_MoS2 ≈ 11 cm² V^-1 s^-1, μ_WS2 ≈ 9 cm² V^-1 s^-1, and μ_WSe2 ≈ 2 cm² V^-1 s^-1 with a current on/off ratios of I_on/I_off ≈ 2.6 × 10³, 3.4 × 10³, and 4.2 × 10⁴ for MoS₂, WS₂, and WSe₂, respectively. Moreover, our transistors display threshold voltages near ∼0.4 V with subthreshold slopes as low as 182 mV/dec, which are essential factors in maintaining power efficiency and represent a 1 order of magnitude improvement in the state of the art. Furthermore, the performance of our WSe₂ transistors is maintained on polyethylene terephthalate (PET) even after 1000 bending cycles at 1% strain.

Solution processed, vertically stacked hetero-structured diodes based on liquid-exfoliated WS2 nanosheets: from electrode-limited to bulk-limited behavior

Vertically stacked metal-semiconductor-metal heterostructures, based on liquid-processed nanomaterials, hold great potential for various printed electronic applications. Here we describe the fabrication of such devices by spray-coating semiconducting tungsten disulfide (WS₂) nanosheets onto indium tin oxide (ITO) bottom electrodes, followed by spraying single-walled carbon nanotubes (SWNTs) as the top electrode. Depending on the formulation of the SWNTs ink, we could fabricate either Ohmic or Schottky contacts at the WS₂/SWNTs interface. Using isopropanol-dispersed SWNTs led to Ohmic contacts and bulk-limited devices, characterized by out-of-plane conductivities of ∼10^-4 S m^-1. However, when aqueous SWNTs inks were used, rectification was observed, due to the formation of a doping-induced Schottky barrier at the WS₂/SWNTs interface. For thin WS₂ layers, such devices were characterized by a barrier height of ∼0.56 eV. However, increasing the WS₂ film thickness led to increased series resistance, leading to a change-over from electrode-limited to bulk-limited behavior at a transition thickness of ∼2.6 μm. This work demonstrates that Ohmic/Schottky behavior is tunable and lays the foundation for fabricating large-area 2D nanosheet-based solution-deposited devices and stacks.

Liquid Processing of Interfacially Grown Iron-Oxide Flowers into 2D-Platelets Yields Lithium-Ion Battery Anodes with Capacities of Twice the Theoretical Value

Iron oxide (Fe₂ O₃ ) is an abundant and potentially low-cost material for fabricating lithium-ion battery anodes. Here, the growth of α-Fe₂ O₃ nano-flowers at an electrified liquid-liquid interface is demonstrated. Sonication is used to convert these flowers into quasi-2D platelets with lateral sizes in the range of hundreds of nanometers and thicknesses in the range of tens of nanometers. These nanoplatelets can be combined with carbon nanotubes to form porous, conductive composites which can be used as electrodes in lithium-ion batteries. Using a standard activation process, these anodes display good cycling stability, reasonable rate performance and low-rate capacities approaching 1500 mAh g^-1 , consistent with the current state-of-the-art for Fe₂ O₃ . However, by using an extended activation process, it is found that the morphology of these composites can be significantly changed, rendering the iron oxide amorphous and significantly increasing the porosity and internal surface area. These morphological changes yield anodes with very good cycling stability and low-rate capacity exceeding 2000 mAh g^-1 , which is competitive with the best anode materials in the literature. However, the data implies that, after activation, the iron oxide displays a reduced solid-state lithium-ion diffusion coefficient resulting in somewhat degraded rate performance.

Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets

For nearly 15 years, researchers have been using liquid-phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution-processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials is enabled by the very large bonding anisotropy of layered materials where the strength of intralayer chemical bonds is very much larger than that of interlayer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of nonlayered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here the progress in this unexpected new research area is examined. The structure and properties of nanoplatelets produced by LPE of nonlayered materials are reviewed. A number of unexplained trends are found, not least the preponderance of isotropic materials that have been exfoliated to give high-aspect-ratio nanoplatelets. Finally, the applications potential of this new class of 2D materials are considered.

Highly Conductive Networks of Silver Nanosheets

Although printed networks of semiconducting nanosheets have found success in a range of applications, conductive nanosheet networks are limited by low conductivities (<10⁶ S m^-1 ). Here, dispersions of silver nanosheets (AgNS) that can be printed into highly conductive networks are described. Using a commercial thermal inkjet printer, AgNS patterns with unannealed conductivities of up to (6.0 ± 1.1) × 10⁶  S m^-1 are printed. These networks can form electromagnetic interference shields with record shielding effectiveness of >60 dB in the microwave region at thicknesses <200 nm. High resolution patterns with line widths down to 10 µm are also printed using an aerosol-jet printer which, when annealed at 200 °C, display conductivity >10⁷  S m^-1 . Unlike conventional Ag-nanoparticle inks, the 2D geometry of AgNS yields smooth, short-free interfaces between electrode and active layer when used as the top electrode in vertical nanosheet heterostructures. This shows that all-printed vertical heterostructures of AgNS/WS₂ /AgNS, where the top electrode is a mesh grid, function as photodetectors demonstrating that such structures can be used in optoelectronic applications that usually require transparent conductors.

Quantifying the Piezoresistive Mechanism in High-Performance Printed Graphene Strain Sensors

Printed strain sensors will be important in applications such as wearable devices, which monitor breathing and heart function. Such sensors need to combine high sensitivity and low resistance with other factors such as cyclability, low hysteresis, and minimal frequency/strain-rate dependence. Although nanocomposite sensors can display a high gauge factor (G), they often perform poorly in the other areas. Recently, evidence has been growing that printed, polymer-free networks of nanoparticles, such as graphene nanosheets, display very good all-round sensing performance, although the details of the sensing mechanism are poorly understood. Here, we perform a detailed characterization of the thickness dependence of piezoresistive sensors based on printed networks of graphene nanosheets. We find both conductivity and gauge factor to display percolative behavior at low network thickness but bulk-like behavior for networks above ∼100 nm thick. We use percolation theory to derive an equation for gauge factor as a function of network thickness, which well-describes the observed thickness dependence, including the divergence in gauge factor as the percolation threshold is approached. Our analysis shows that the dominant contributor to the sensor performance is not the effect of strain on internanosheet junctions but the strain-induced modification of the network structure. Finally, we find these networks display excellent cyclability, hysteresis, and frequency/strain-rate dependence as well as gauge factors as high as 350.

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.

Highly Sensitive Composite Foam Bodily Sensors Based on the g-Putty Ink Soaking Procedure

Electrically conductive composite materials are highlighted as a potential tech path toward future flexible devices for wearable health technologies. To be commercially viable, these materials must not only be mechanically soft, highly sensitive to deformation, and report a sustainable signal but also utilize manufacturing methods that facilitate large-scale production. An ideal candidate for these envisioned technologies is the viscous, electromechanically sensitive composite material g-putty. Inks based on g-putty here are shown to transform a commercial polymer foam into a sensitive strain sensing material through a simple, scalable soaking procedure. Foam composites reported here have sensitives as high as ∼20 in terms of compressive strain and ∼0.4 kPa^-1 with respect to applied compressive stress; both values being comparable to the parent g-putty material. Through g-putty's self-adhering nature, the foams used acted as an elastic scaffolding that aided in overcoming many of the hysteresis effects associated with g-putty without the need for further encapsulation methods. From this, these composite foams were demonstrated to have a sustainable signal that allowed for effective impact and vital sign sensing.

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.

Covalently interconnected transition metal dichalcogenide networks via defect engineering for high-performance electronic devices

Solution-processed semiconducting transition metal dichalcogenides are at the centre of an ever-increasing research effort in printed (opto)electronics. However, device performance is limited by structural defects resulting from the exfoliation process and poor inter-flake electronic connectivity. Here, we report a new molecular strategy to boost the electrical performance of transition metal dichalcogenide-based devices via the use of dithiolated conjugated molecules, to simultaneously heal sulfur vacancies in solution-processed transition metal disulfides and covalently bridge adjacent flakes, thereby promoting percolation pathways for the charge transport. We achieve a reproducible increase by one order of magnitude in field-effect mobility (µ_FE), current ratio (I_ON/I_OFF) and switching time (τ_S) for liquid-gated transistors, reaching 10^-2 cm² V^-1 s^-1, 10⁴ and 18 ms, respectively. Our functionalization strategy is a universal route to simultaneously enhance the electronic connectivity in transition metal disulfide networks and tailor on demand their physicochemical properties according to the envisioned applications.

A Simple Model Relating Gauge Factor to Filler Loading in Nanocomposite Strain Sensors

Conductive nanocomposites are often piezoresistive, displaying significant changes in resistance upon deformation, making them ideal for use as strain and pressure sensors. Such composites typically consist of ductile polymers filled with conductive nanomaterials, such as graphene nanosheets or carbon nanotubes, and can display sensitivities, or gauge factors, which are much higher than those of traditional metal strain gauges. However, their development has been hampered by the absence of physical models that could be used to fit data or to optimize sensor performance. Here we develop a simple model which results in equations for nanocomposite gauge factors as a function of both filler volume fraction and composite conductivity. These equations can be used to fit experimental data, outputting figures of merit, or predict experimental data once certain physical parameters are known. We have found these equations to match experimental data, both measured here and extracted from the literature, extremely well. Importantly, the model shows the response of composite strain sensors to be more complex than previously thought and shows factors other than the effect of strain on the interparticle resistance to be performance limiting.

Label-free screening of biochemical changes in macrophage-like cells following MoS2 exposure using Raman micro-spectroscopy

The emergence of large scale production techniques for 2D particulate materials has dramatically increased their applications potential. Understanding the interactions of biological cells with such particulate material is therefore of paramount importance, both for toxicological assessment and potential biomedical applications. Conventional in-vitro cytological assays commonly record only a single colorimetric end-point, and do not provide an in-depth analysis of how such materials are uptaken and processed within cells. To demonstrate its potential as an alternative, label free approach, confocal Raman micro-spectroscopy has been used to profile the cellular response of macrophage-like immune cells as a result of exposure to a sub-lethal dose of particulate MoS₂, as an example novel 2D material. Particles were seen to be uptaken and trafficked in sub-cellular vesicles, and this sensitive technique allows differences in the biochemical composition of the vesicles to be assessed and monitored as a function of time. Untreated macrophage-like cells contain lipidic vesicles which are found to be relatively rich in the membrane lipid sphingomyelin, key to the process of cell membrane regeneration. After exposure to MoS₂, the particulate material is seen to be invaginated in similar vesicles, the most prominent of which now, however, have spectroscopic signatures which are dominated by those of phosphatidyl family lipids, consistent with the phagocytotic pathway. The lipidic content of cells is seen to increase at all time-points (4, 24 and 72 h). although vesicles composed of sphingomyelin become more prominent again following a prolonged incubation of 72 h to a sub-lethal dose of MoS₂, as the immune cell has processed the particulate material and initiates recovery to a normal/untreated state. This study reveals Raman micro-spectroscopy is an effective method for monitoring cellular responses and evolution of organelle compositions in response to MoS₂ exposure. The additional benefit of using this technique is that cells can be monitored as a function of time, while it can also be used for screening other micro/nano materials for toxicology and/or establishing cell responses.

Tuning the Photo-electrochemical Performance of RuII -Sensitized Two-Dimensional MoS2

Covalently tethering photosensitizers to catalytically active 1T-MoS₂ surfaces holds great promise for the solar-driven hydrogen evolution reaction (HER). Herein, we report the preparation of two new Ru^II -complex-functionalized MoS₂ hybrids [Ru^II (bpy)₂ (phen)]-MoS₂ and [Ru^II (bpy)₂ (py)Cl]-MoS₂ . The influence of covalent functionalization of chemically exfoliated 1T-MoS₂ with coordinating ligands and Ru^II complexes on the HER activity and photo-electrochemical performance of this dye-sensitized system was studied systematically. We find that the photo-electrochemical performance of this Ru^II -complex-sensitized MoS₂ system is highly dependent on the surface extent of photosensitizers and the catalytic activity of functionalized MoS₂ . The latter was strongly affected by the number and the kind of functional groups. Our results underline the tunability of the photovoltage generation in this dye-sensitized MoS₂ system by manipulation of the surface functionalities, which provides a practical guidance for smart design of future dye-sensitized MoS₂ hydrogen production devices towards improved the photofuel conversion efficiency.

Production of Quasi-2D Platelets of Nonlayered Iron Pyrite (FeS2) by Liquid-Phase Exfoliation for High Performance Battery Electrodes

Over the past 15 years, two-dimensional (2D) materials have been studied and exploited for many applications. In many cases, 2D materials are formed by the exfoliation of layered crystals such as transition-metal disulfides. However, it has recently become clear that it is possible to exfoliate nonlayered materials so long as they have a nonisotropic bonding arrangement. Here, we report the synthesis of 2D-platelets from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS₂), using liquid-phase exfoliation. The resultant 2D platelets exhibit the same crystal structure as bulk pyrite but are surface passivated with a density of 14 × 10¹⁸ groups/m². They form stable suspensions in common solvents and can be size-selected and liquid processed. Although the platelets have relatively low aspect ratios (∼5), this is in line with the anisotropic cleavage energy of bulk FeS₂. We observe size-dependent changes to optical properties leading to spectroscopic metrics that can be used to estimate the dimensions of platelets. These platelets can be used to produce lithium ion battery anodes with capacities approaching 1000 mAh/g.

Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene

Liquid- phase exfoliation (LPE) is the principal method of producing two-dimensional (2D) materials such as graphene in large quantities with a good balance between quality and cost and is now widely adopted by both the academic and industrial sectors. The fragmentation and exfoliation mechanisms involved have usually been simply attributed to the force induced by ultrasound and the interaction with the solvent molecules. Nonetheless, little is known about how they actually occur, i.e., how thick and large graphite crystals can be exfoliated into thin and small graphene flakes. Here, we demonstrate that during ultrasonic LPE the transition from graphite flakes to graphene takes place in three distinct stages. First, sonication leads to the rupture of large flakes and the formation of kink band striations on the flake surfaces, primarily along zigzag directions. Second, cracks form along these striations, and together with intercalation of solvent, lead to the unzipping and peeling off of thin graphite strips that in the final stage are exfoliated into graphene. The findings will be of great value in the quest to optimize the lateral dimensions, thickness, and yield of graphene and other 2D materials in large-scale LPE for various applications.

Mechanochromic and Thermochromic Sensors Based on Graphene Infused Polymer Opals

High quality opal-like photonic crystals containing graphene are fabricated using evaporation-driven self-assembly of soft polymer colloids. A miniscule amount of pristine graphene within a colloidal crystal lattice results in the formation of colloidal crystals with a strong angle-dependent structural color and a stop band that can be reversibly shifted across the visible spectrum. The crystals can be mechanically deformed or can reversibly change color as a function of their temperature, hence their sensitive mechanochromic and thermochromic response make them attractive candidates for a wide range of visual sensing applications. In particular, it is shown that the crystals are excellent candidates for visual strain sensors or integrated time-temperature indicators which act over large temperature windows. Given the versatility of these crystals, this method represents a simple, inexpensive, and scalable approach to produce multifunctional graphene infused synthetic opals and opens up exciting applications for novel solution-processable nanomaterial based photonics.

Selective electrochemical production of hydrogen peroxide at zigzag edges of exfoliated molybdenum telluride nanoflakes

The two-electron reduction of molecular oxygen represents an effective strategy to enable the green, mild and on-demand synthesis of hydrogen peroxide. Its practical viability, however, hinges on the development of advanced electrocatalysts, preferably composed of non-precious elements, to selectively expedite this reaction, particularly in acidic medium. Our study here introduces 2H-MoTe₂ for the first time as the efficient non-precious-metal-based electrocatalyst for the electrochemical production of hydrogen peroxide in acids. We show that exfoliated 2H-MoTe₂ nanoflakes have high activity (onset overpotential ∼140 mV and large mass activity of 27 A g⁻¹ at 0.4 V versus reversible hydrogen electrode), great selectivity (H₂O₂ percentage up to 93%) and decent stability in 0.5 M H₂SO₄. Theoretical simulations evidence that the high activity and selectivity of 2H-MoTe₂ arise from the proper binding energies of HOO^* and O^* at its zigzag edges that jointly favor the two-electron reduction instead of the four-electron reduction of molecular oxygen.

The Rate Performance of Two-Dimensional Material-Based Battery Electrodes May Not Be as Good as Commonly Believed

Two-dimensional (2D) materials show great potential for use in battery electrodes and are believed to be particularly promising for high-rate applications. However, there does not seem to be much hard evidence for the superior rate performance of 2D materials compared to non-2D materials. To examine this point, we have analyzed published rate-performance data for a wide range of 2D materials as well as non-2D materials for comparison. For each capacity-rate curve, we extract parameters that quantify performance which can then be analyzed using a simple mechanistic model. Contrary to expectations, by comparing a previously proposed figure of merit, we find 2D-based electrodes to be on average ∼40 times poorer in terms of rate performance than non-2D materials. This is not due to differences in solid-state diffusion times which were similarly distributed for 2D and non-2D materials. In fact, we found the main difference between 2D and non-2D materials is that ion mobility within the electrolyte-filled pores of the electrodes is significantly lower for 2D materials, a situation which we attribute to their high aspect ratios.

High Performance Na-O2 Batteries and Printed Microsupercapacitors Based on Water-Processable, Biomolecule-Assisted Anodic Graphene

Integrated approaches that expedite the production and processing of graphene into useful structures and devices, particularly through simple and environmentally friendly strategies, are highly desirable in the efforts to implement this two-dimensional material in state-of-the-art electrochemical energy storage technologies. Here, we introduce natural nucleotides (e.g., adenosine monophosphate) as bifunctional agents for the electrochemical exfoliation and dispersion of graphene nanosheets in water. Acting both as exfoliating electrolytes and colloidal stabilizers, these biomolecules facilitated access to aqueous graphene bio-inks that could be readily processed into aerogels and inkjet-printed interdigitated patterns. Na-O₂ batteries assembled with the graphene-derived aerogels as the cathode and a glyme-based electrolyte exhibited a full discharge capacity of ∼3.8 mAh cm^-2 at a current density of 0.2 mA cm^-2. Moreover, shallow cycling experiments (0.5 mAh cm^-2) boasted a capacity retention of 94% after 50 cycles, which outperformed the cycle life of prior graphene-based cathodes for this type of battery. The positive effect of the nucleotide-adsorbed nanosheets on the battery performance is discussed and related to the presence of the phosphate group in these biomolecules. Microsupercapacitors made from the interdigitated graphene patterns as the electrodes also displayed a competitive performance, affording areal and volumetric energy densities of 0.03 μWh cm^-2 and 1.2 mWh cm^-3 at power densities of 0.003 mW cm^-2 and 0.1 W cm^-3, respectively. Taken together, by offering a green and straightforward route to different types of functional graphene-based materials, the present results are expected to ease the development of novel energy storage technologies that exploit the attractions of graphene.

Negative Gauge Factor Piezoresistive Composites Based on Polymers Filled with MoS2 Nanosheets

Nanocomposite strain sensors, particularly those consisting of polymer-graphene composites, are increasingly common and are of great interest in the area of wearable sensors. In such sensors, application of strain yields an increase in resistance due to the effect of deformation on interparticle junctions. Typically, widening of interparticle separation is thought to increase the junction resistance by reducing the probability of tunnelling between conducting particles. However, an alternative approach would be to use piezoresistive fillers, where an applied strain modifies the intrinsic filler resistance and so the overall composite resistance. Such an approach would broaden sensing capabilities, as using negative piezoresistive fillers could yield strain-induced resistance reductions rather than the usual resistance increases. Here, we introduce nanocomposites based on polyethylene oxide (PEO) filled with MoS₂ nanosheets. Doping of the MoS₂ by the PEO yields nanocomposites which are conductive enough to act as sensors, while efficient stress transfer leads to nanosheet deformation in response to an external strain. The intrinsic negative piezoresistance of the MoS₂ leads to a reduction of the composite resistance on the application of small tensile strains. However, at higher strain the resistance grows due to increases in junction resistance. MoS₂-PEO composite gauge factors are approximately -25 but fall to -12 for WS₂-PEO composites and roughly -2 for PEO filled with MoSe₂ or WSe₂. We develop a simple model, which describes all these observations. Finally, we show that these composites can be used as dynamic strain sensors.

Equipartition of Energy Defines the Size-Thickness Relationship in Liquid-Exfoliated Nanosheets

Liquid phase exfoliation is a commonly used method to produce 2D nanosheets from a range of layered crystals. However, such nanosheets display broad size and thickness distributions and correlations between area and thickness, issues that limit nanosheet application potential. To understand the factors controlling the exfoliation process, we have liquid-exfoliated 11 different layered materials, size-selecting each into fractions before using AFM to measure the nanosheet length, width, and thickness distributions for each fraction. The resultant data show a clear power-law scaling of nanosheet area with thickness for each material. We have developed a simple nonequilibrium thermodynamics-based model predicting that the power-law prefactor is proportional to both the ratios of in-plane-tearing/out-of-plane-peeling energies and in-plane/out-of-plane moduli. By comparing the experimental data with the modulus ratio calculated from first-principles, we find close agreement between experiment and theory. This supports our hypothesis that energy equipartition holds between nanosheet tearing and peeling during sonication-assisted exfoliation.

A qualitative study of the feasibility and acceptability of a smoking cessation program for people living with HIV and emotional dysregulation

Despite high rates of co-occurring tobacco use and anxiety among persons living with HIV, evidence-based interventions for these individuals are limited. An existing cognitive-behavioral treatment protocol for smoking cessation and anxiety (Norton, P. J., & Barrera, T. L. (2012). Transdiagnostic versus diagnosis-specific CBT for anxiety disorders: A preliminary randomized controlled noninferiority trial. Depression and Anxiety, 29(10), 874-882. https://doi.org/10.1002/da.21974) was modified to address transdiagnostic constructs, such as anxiety sensitivity, distress tolerance, and depressive symptomatology (Labbe, A. K., Wilner, J. G., Kosiba, J. D., Gonzalez, A., Smits, J. A., Zvolensky, M. J., … O'Cleirigh, C. (2017). Demonstration of an Integrated Treatment for Smoking Cessation and Anxiety Symptoms in People with HIV: A Clinical Case Study. Cognitive and Behavioral Practice, 24(2), 200-214. https://doi.org/10.1016/j.cbpra.2016.03.009). This study examines the feasibility and acceptability of the intervention as determined from qualitative data from structured exit interviews from 10 participants who completed treatment. Results demonstrated that participants were very motivated to quit smoking and enrolled in the program for health-related reasons and to be able to quit. Participants found nearly all the treatment components to be useful for reaching their smoking cessation goal and in managing emotional dysregulation. Last, all participants stated that they would strongly recommend the treatment program. This qualitative study provides initial evidence for the feasibility and acceptability of a modified smoking cessation treatment protocol for HIV+ individuals with anxiety and emotional dysregulation. Future research will focus on evaluating the efficacy of the protocol in a full-scale randomized controlled trial, as well as working to collect qualitative data from participants who discontinue treatment to better understand reasons for treatment attrition.

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.

Liquid phase exfoliation of MoO2 nanosheets for lithium ion battery applications

Molybdenum dioxide (MoO₂) is a layered material which shows promise for a number of applications in the electrochemical energy storage arena. Mostly studied as a bulk layered material, MoO₂ has not previously been exfoliated in large quantities. Here we demonstrate the liquid phase exfoliation of MoO₂ in the solvent isopropanol, yielding reasonable amounts of good quality nanosheets. However, we found that, when dispersed in isopropanol under ambient conditions, MoO₂ nanosheets are gradually oxidized to higher oxides such as MoO₃ over a period of days. Conversely, if the nanosheets are processed into dried films immediately after exfoliation, and before oxidation has had a chance to progress, the nanosheets are relatively stable under ambient conditions, remaining unoxidised unless the films are heated. We also found that MoO₂ nanosheets can be size selected by controlled centrifugation and show size-dependent optical properties. This allows us to propose spectroscopic metrics which allow concentration- and size-estimation from extinction spectra. Finally, we found that liquid-exfoliated MoO₂ nanosheets could be used to produce lithium ion battery anodes with capacities of up to 1140 mA h g^-1.

Solvent exfoliation stabilizes TiS2 nanosheets against oxidation, facilitating lithium storage applications

Titanium disulfide is a promising material for a range of applications, including lithium-ion battery (LIB) anodes. However, its application potential has been severely hindered by the tendency of exfoliated TiS2 to rapidly oxidize under ambient conditions. Herein, we confirm that, although layered TiS2 powder can be exfoliated by sonication in aqueous surfactant solutions, the resultant nanosheets oxidise almost completely within hours. However, we find that upon performing the exfoliation in the solvent cyclohexyl-pyrrolidone (CHP), the oxidation is almost completely suppressed. TiS2 nanosheets dispersed in CHP and stored at 4 °C in an open atmosphere for 90 days remained up to 95% intact. In addition, CHP-exfoliated nanosheets did not show any evidence of oxidation for at least 30 days after being transformed into dry films even when stored under ambient conditions. This stability, probably a result of a residual CHP coating, allows TiS2 nanosheets to be deployed in applications. To demonstrate this, we prepared lithium ion battery anodes from nano : nano composites of TiS2 nanosheets mixed with carbon nanotubes. These anodes displayed reversible capacities (920 mA h g-1) close to the theoretical value and showed good rate performance and cycling capability.

Liquid phase exfoliation of carbonate-intercalated layered double hydroxides

Direct exfoliation of a carbonate layered double hydroxide (LDH) has been achieved by using a novel horn-probe sonic tip, avoiding the development of time-consuming anion-exchange reactions. The most suitable solvents were chosen based on the Hildebrand solubility parameters and the thickness of the exfoliated nanosheets confirmed unambiguously the successful delamination.

Percolation Effects in Electrolytically Gated WS2/Graphene Nano:Nano Composites

Mixed networks of conducting and nonconducting nanoparticles show promise in a range of applications where fast charge transport is important. While the dependence of network conductivity on the loading level of conductive additive is well understood, little is known about the loading dependence of mobility and carrier density. This is particularly important as the addition of graphene might lead to increases in the mobility of semiconducting nanosheet network transistors. Here, we use electrolytic gating to investigate the transport properties of spray-coated composite networks of graphene and WS₂ nanosheets. As the graphene loading is increased, we find that both conductivity and carrier density increase in line with the percolation theory with percolation thresholds (∼8 vol %) and exponents (∼2.5) consistent with previous reporting. Perhaps surprisingly, we find the mobility increases modestly from ∼0.1 cm²/V s (for a WS₂ network) to ∼0.3 cm²/V s (for a graphene network) which we attribute to the similarity between WS₂-WS₂ and graphene-graphene junction resistances. In addition, we find both the transistor on- and off-currents to scale with loading according to the percolation theory, changing sharply at the percolation threshold. Through fitting, we show that only the current in the WS₂ network changes significantly upon gating. As a result, the on-off ratio falls sharply at the percolation threshold from ∼10⁴ to ∼2 at higher loading. Reflecting on these results, we conclude that the addition of graphene to a semiconducting network is not a viable strategy to improve transistor performance as it reduces the on:off ratio far more than it improves the mobility.

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.

Non-resonant light scattering in dispersions of 2D nanosheets

Extinction spectra of nanomaterial suspensions can be dominated by light scattering, hampering quantitative spectral analysis. No simple models exist for the wavelength-dependence of the scattering coefficients in suspensions of arbitrary-sized, high-aspect-ratio nanoparticles. Here, suspensions of BN, talc, GaS, Ni(OH)₂, Mg(OH)₂ and Cu(OH)₂ nanosheets are used to explore non-resonant scattering in wide-bandgap 2D nanomaterials. Using an integrating sphere, scattering coefficient (σ) spectra were measured for a number of size-selected fractions for each nanosheet type. Generally, σ scales as a power-law with wavelength in the non-resonant regime: σ(λ)∝[λ/〈L〉]^−m, where 〈L〉 is the mean nanosheet length. For all materials, the scattering exponent, m, forms a master-curve, transitioning from m = 4 to m = 2, as the characteristic nanosheet area increases, indicating a transition from Rayleigh to van der Hulst scattering. In addition, once material density and refractive index are factored out, the proportionality constant relating σ to [λ/〈L〉]^−m, also forms a master-curve when plotted versus 〈L〉.

Quantitative analysis of the extinction spectra of dispersions of 2D materials is complicated by light scattering. Here, the authors investigate non-resonant scattering in suspensions of wide-bandgap nanosheets, and develop a general model which allows the scattering spectra to be used as metrics for particle size in nanosheet dispersions.

Electroconductive Biohybrid Collagen/Pristine Graphene Composite Biomaterials with Enhanced Biological Activity

Electroconductive substrates are emerging as promising functional materials for biomedical applications. Here, the development of biohybrids of collagen and pristine graphene that effectively harness both the biofunctionality of the protein component and the increased stiffness and enhanced electrical conductivity (matching native cardiac tissue) obtainable with pristine graphene is reported. As well as improving substrate physical properties, the addition of pristine graphene also enhances human cardiac fibroblast growth while simultaneously inhibiting bacterial attachment (Staphylococcus aureus). When embryonic-stem-cell-derived cardiomyocytes (ESC-CMs) are cultured on the substrates, biohybrids containing 32 wt% graphene significantly increase metabolic activity and cross-striated sarcomeric structures, indicative of the improved substrate suitability. By then applying electrical stimulation to these conductive biohybrid substrates, an enhancement of the alignment and maturation of the ESC-CMs is achieved. While this in vitro work has clearly shown the potential of these materials to be translated for cardiac applications, it is proposed that these graphene-based biohybrid platforms have potential for a myriad of other applications-particularly in electrically sensitive tissues, such as neural and neural and musculoskeletal tissues.

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.

RuII Photosensitizer-Functionalized Two-Dimensional MoS2 for Light-Driven Hydrogen Evolution

Metallic-phase molybdenum disulfide (1T-MoS₂ ) nanosheets have proven to be highly active in the hydrogen evolution reaction (HER). We describe construction of photosensitizer functionalized 1T-MoS₂ by covalently tethering the molecular photosensitizer [Ru^II (bpy)₃ ]²⁺ (bpy=2,2'-bipyridine) on 1T-MoS₂ nanosheets. This was achieved by covalently tethering the bpy ligand to 1T-MoS₂ nanosheets, and subsequent complexation with [Ru^II (bpy)₂ Cl₂ ] to yield [Ru^II (bpy)₃ ]-MoS₂ . The obtained [Ru^II (bpy)₃ ]-MoS₂ nanosheets were characterized using infra-red, electronic absorption, X-ray photoelectron, and Raman spectroscopies, X-ray powder diffraction and electron microscopy. The fabricated material exhibited a significant improvement of photocurrent and HER performance, demonstrating the potential of such two-dimensional [Ru^II (bpy)₃ ]-MoS₂ constructs in photosensitized HER.

Surface coatings of silver nanowires lead to effective, high conductivity, high-strain, ultrathin sensors

Integrated sensors for bodily measurements require a sensing material that is highly conductive, flexible, thin and sensitive. It is important that these materials are non-invasive in application but robust in nature to allow for effective, continuous measurement. Herein, we report a comparative study of two simple, scalable methods to produce silver nanowire (AgNW) polyurethane (PU) composite materials: layer-by-layer (LBL) and mixed filtration. Both types of composites formed were ultrathin (∼50 μm) and highly conductive (10⁴ S m^-1), with the LBL method ultimately found to be superior due to its low percolation threshold. Electrical resistance of the LBL composites was found to vary with strain, making these materials suitable for strain sensing. LBL composites displayed a working strain up to ∼250% and a high gauge factor (G), with values of G ∼70 reported. The sensors reported here were ∼10⁹-times more conductive and ∼10⁴-times thinner than their carbon-based composite sensor counterparts with similar gauge factor. This made the strain sensors presented here among one of the most flexible, highly sensitive, thinnest, conductive materials in literature. We demonstrated that with these properties, the LBL composites formed were ideal for bodily motion detection.

Light scattering and random lasing in aqueous suspensions of hexagonal boron nitride nanoflakes

Liquid phase exfoliation allows large scale production of 2D materials in solution. The particles are highly anisotropic and strongly scatter light. While spherical particles can be accurately and precisely described by a single parameter-the radius, 2D nanoflakes, however, cannot be so easily described. We investigate light scattering in aqueous solutions of 2D hexagonal boron nitride nanoflakes in the single and multiple scattering regimes. In the single scattering regime, the anisotropic 2D materials show a much stronger depolarization of light when compared to spherical particles of similar size. In the multiple scattering regime, the scattering as a function of optical path for hexagonal boron nitride nanoflakes of a given lateral length was found to be qualitatively equivalent to scattering from spheres with the same diameter. We also report the presence of random lasing in high concentration suspensions of aqueous h-BN mixed with Rhodamine B dye. The h-BN works as a scattering agent and Rhodamine B as a gain medium for the process. We observed random lasing at 587 nm with a threshold energy of 0.8 mJ.

Cobalt hydroxide nanoflakes and their application as supercapacitors and oxygen evolution catalysts

Finding alternative routes to access and store energy has become a major issue recently. Transition metal oxides have shown promising behaviour as catalysts and supercapacitors. Recently, liquid exfoliation of bulk metal oxides appears to be an effective route which provides access to two-dimensional (2D) nano-flakes, the size of which can be easily selected. These 2D materials exhibit excellent electrochemical charge storage and catalytic activity for the oxygen evolution reaction. In this study, various sized selected cobalt hydroxide nano-flake materials are fabricated by this time efficient and highly reproducible process. Subsquently, the electrochemical properties of the standard size Co(OH)₂ nanoflakes were investigated. The oxide modified electrodes were prepared by spraying the metal oxide flake suspension onto a porous conductive support electrode foam, either glassy carbon or nickel. The cobalt hydroxide/nickel foam system was found to have an overpotential value at 10 mA cm^-2 in 1 M NaOH as low as 280 mV and an associated redox capacitance exhibiting numerical values up to 1500 F g^-1, thereby making it a viable dual use electrode.

Transparent, Flexible, and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance

2D transition-metal carbides and nitrides, known as MXenes, have displayed promising properties in numerous applications, such as energy storage, electromagnetic interference shielding, and catalysis. Titanium carbide MXene (Ti₃ C₂ T_x ), in particular, has shown significant energy-storage capability. However, previously, only micrometer-thick, nontransparent films were studied. Here, highly transparent and conductive Ti₃ C₂ T_x films and their application as transparent, solid-state supercapacitors are reported. Transparent films are fabricated via spin-casting of Ti₃ C₂ T_x nanosheet colloidal solutions, followed by vacuum annealing at 200 °C. Films with transmittance of 93% (≈4 nm) and 29% (≈88 nm) demonstrate DC conductivity of ≈5736 and ≈9880 S cm^-1 , respectively. Such highly transparent, conductive Ti₃ C₂ T_x films display impressive volumetric capacitance (676 F cm^-3 ) combined with fast response. Transparent solid-state, asymmetric supercapacitors (72% transmittance) based on Ti₃ C₂ T_x and single-walled carbon nanotube (SWCNT) films are also fabricated. These electrodes exhibit high capacitance (1.6 mF cm^-2 ) and energy density (0.05 µW h cm^-2 ), and long lifetime (no capacitance decay over 20 000 cycles), exceeding that of graphene or SWCNT-based transparent supercapacitor devices. Collectively, the Ti₃ C₂ T_x films are among the state-of-the-art for future transparent, conductive, capacitive electrodes, and translate into technologically viable devices for next-generation wearable, portable electronics.

Enabling Flexible Heterostructures for Li-Ion Battery Anodes Based on Nanotube and Liquid-Phase Exfoliated 2D Gallium Chalcogenide Nanosheet Colloidal Solutions

2D metal chalcogenide (MC) nanosheets (NS) have displayed high capacities as lithium-ion battery (LiB) anodes. Nevertheless, their complicated synthesis routes coupled with low electronic conductivity greatly limit them as promising LiB electrode material. Here, this work reports a facile single-walled carbon nanotube (SWCNT) percolating strategy for efficiently maximizing the electrochemical performances of gallium chalcogenide (GaX, X = S or Se). Multiscaled flexible GaX NS/SWCNT heterostructures with abundant voids for Li⁺ diffusion are fabricated by embedding the liquid-exfoliated GaX NS matrix within a SWCNT-percolated network; the latter improves the electron transport and ion diffusion kinetics as well as maintains the mechanical flexibility. Consequently, high capacities (i.e., 838 mAh g^-1 per gallium (II) sulfide (GaS) NS/SWCNT mass and 1107 mAh g^-1 per GaS mass; the latter is close to the theoretical value) and good rate capabilities are achieved, which can be majorly attributed to the alloying processes of disordered Ga formed after the first irreversible GaX conversion reaction, as monitored by in situ X-ray diffraction. The presented approach, colloidal solution processing of SWCNT and liquid-exfoliated MC NS to produce flexible paper-based electrode, could be generalized for wearable energy storage devices with promising performances.

Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS2 and WS2 shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

Revealing the nature of excitons in liquid exfoliated monolayer tungsten disulphide

Transition metal dichalcogenides (TMD) hold promise for applications in novel optoelectronic devices. There is therefore a need for materials that can be obtained in large quantities and with well understood optical properties. In this report, we present thorough photoluminescence (PL) investigations of monolayer tungsten disulphide obtained via liquid phase exfoliation. As shown by microscopy studies, the exfoliated nanosheets have dimensions of tens of nanometers and thickness of 2.5 monolayers on average. The monolayer content is about 20%. Our studies show that at low temperature the PL is dominated by excitons localized on nanosheet edges. As a consequence, the PL is strongly sensitive to the environment and exhibits an enhanced splitting in magnetic field. As the temperature is increased, the excitons are thermally excited out of the defect states and the dominant transition is that of the negatively charged exciton. Furthermore, upon excitation with a circularly polarized light, the PL retains a degree of polarization reaching 50% and inherited from the valley polarized photoexcited excitons. The studies of PL dynamics reveal that the PL lifetime is on the order of 10 ps, which is probably limited by non-radiative processes. Our results underline the potential of liquid exfoliated TMD monolayers in large scale optoelectronic devices.