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Electrowetting is a simple way to induce the spreading and retraction of electrolyte droplets. This method is widely used in “device” applications, where a dielectric layer is applied between the...

Carbon-fibre-reinforced polymer (CFRP) composites are promising materials for non-metallic pipe applications in the oil and gas industry owing to their high corrosion resistance, specific strength and stiffness. However, CFRP has poor gas barrier performance meaning that a liner has to be inserted. Graphene-based nanomaterials have been demonstrated to improve gas barrier properties in thermoplastic polymers, and thus, a CFRP–graphene hybrid composite could provide an alternative to lined pipes. In this work, a method combining spray coating with vacuum-assisted resin infusion was developed to fabricate CFRP hybrid composites with preferred in-plane aligned graphene nanoplatelets. Tensile and flexural properties, as well as CO2 gas permeability, were evaluated. It was illustrated that both tensile and flexural properties performed better under relatively low GNP loadings ( Graphical abstract

Aqueous zinc‐ion hybrid supercapacitors are a promising energy storage technology, owing to their high safety, low cost, and long‐term stability. At present, however, there is a lack of understanding of the potential window and self‐discharge of this aqueous energy storage technology. This study concerns a systematic investigation of the potential window of this device by cyclic voltammetry and galvanostatic charge–discharge. Hybrid supercapacitors based on commercial activated carbon (AC) demonstrate a wide and stable potential window (0.2 V to 1.8 V), high specific capacitances (308 F g−1 at 0.5 A g−1 and 110 F g−1 at 30 A g−1), good cycling stability (10000 cycles with 95.1 % capacitance retention), and a high energy density (104.8 Wh kg−1 at 383.5 W kg−1), based on the active materials. The mechanism involves simultaneous adsorption–desorption of ions on the AC cathode and zinc ion plating/stripping on the Zn anode. This work leads to better understanding of such devices and will aid future development of practical high‐performance aqueous zinc‐ion hybrid supercapacitors based on commercial carbon materials, thus accelerating the deployment of these hybrid supercapacitors and filling the gap between supercapacitors and batteries., A window into self‐discharge: A systematic investigation of the potential window and self‐discharge in aqueous zinc‐ion hybrid supercapacitors based on commercial activated carbon (AC) and Zn foil with nontoxic electrolyte is carried out by cyclic voltammetry and galvanostatic charge–discharge methods, with a view to filling the gap between supercapacitors and batteries.

Most polymers are long-lasting and produced from monomers derived from fossil fuel sources. Bio-based and/or biodegradable plastics have been proposed as a sustainable alternative. Amongst those available, polyhydroxyalkanoate (PHA) shows great potential across a large variety of applications but is currently limited to packaging, cosmetics and tissue engineering due to its relatively poor physical properties. An expansion of its uses can be accomplished by developing nanocomposites where PHAs are used as the polymer matrix. Herein, a PHA biopolyester was melt blended with graphene nanoplatelets (GNPs) or with a 1:1 hybrid mixture of GNPs and carbon nanofibers (CNFs). The resulting nanocomposites exhibited enhanced thermal stability while their Young's modulus roughly doubled compared to pure PHA. The hybrid nanocomposites percolated electrically at lower nanofiller loadings compared to the GNP-PHA system. The electrical conductivity at 15 wt.% loading was ~ 6 times higher than the GNP-based sample. As a result, the electromagnetic interference shielding performance of the hybrid material was around 50% better than the pure GNPs nanocomposites, exhibiting shielding effectiveness above 20 dB, which is the threshold for common commercial applications. The thermal conductivity increased significantly for both types of bio-nanocomposites and reached values around 5 W K-1 m-1 with the hybrid-based material displaying the best performance. Considering the solvent-free and industrially compatible production method, the proposed multifunctional materials are promising to expand the range of application of PHAs and increase the environmental sustainability of the plastic and plastic electronics industry., 26 pages

Poor cycling stability and mechanistic controversies have hindered the wider application of rechargeable aqueous Zn–MnO2 batteries. Herein, direct evidence was provided of the importance of Mn2+ in this type of battery by using a bespoke cell. Without pre‐addition of Mn2+, the cell exhibited an abnormal discharge–charge profile, meaning it functioned as a primary battery. By adjusting the Mn2+ content in the electrolyte, the cell recovered its charging ability through electrodeposition of MnO2. Additionally, a dynamic pH variation was observed during the discharge–charge process, with a precipitation of Zn4(OH)6(SO4)⋅5H2O buffering the pH of the electrolyte. Contrary to the conventional Zn2+ intercalation mechanism, MnO2 was first converted into MnOOH, which reverted to MnO2 through disproportionation, resulting in the dissolution of Mn2+. The charging process occurred by the electrodeposition of MnO2, thus improving the reversibility through the availability of Mn2+ ions in the solution., Taking charge: This work provides direct evidence of the importance of Mn2+ in rechargeable aqueous Zn–MnO2 batteries. The charging process is implemented by the electrodeposition of MnO2, thus improving the reversibility by the availability of Mn2+ in the solution. A conversion reaction between MnO2 and MnOOH occurs, rather than Zn2+ intercalation into MnO2.

A significant challenge for graphene nanoplatelet (GNP) suppliers is the characterisation of platelet morphology in industrial environments. This challenge is further exacerbated to platelet surface chemistry when scalable functionalisation processes, such as plasma treatment, are used to modify the GNPs to improve the filler-matrix interphase in nanocomposites. The costly and complex suite of analytical equipment necessary for a complete material description makes quality control and process optimisation difficult. Raman spectroscopy is a facile and accessible characterisation technique, with recent advancements unlocking fast mapping for rapid data collection. In this study, we develop novel techniques to better characterise GNP morphology and changes in surface chemistry using Raman maps of bulk powders. Providing a bespoke algorithmic framework for the analysis of these advanced materials. An unsupervised peak fitting and processing algorithm was used to extract crystallinity data and correlate it with laser-diffraction-derived lateral size values for a commercial set of GNPs rapidly and accurately. Classical machine learning was used to identify the most informative Raman features for classifying the plasma-functionalised GNPs. The initial material properties were found to affect the peak features that were the most useful for classification. In low defect density and low specific surface area GNPs, the D peak full width at half maximum is found to be the most useful, whereas the I 2D / I G ratio is the most useful in the opposite case. Finally, a convolutional neural network was trained to discern between different GNP grades with 86% accuracy. This work demonstrates how computer vision could be deployed for rapid and accurate quality control on the factory floor.

Graphene oxide (GO) and reduced graphene oxide (rGO) membranes have attracted significant attention as a potential technology for energy storage, gas separation, and water purification applications. However, these membranes have a significant drawback as they became swollen, hence unstable, after exposure to aqueous solutions. Here, we describe membranes produced from graphene prepared by liquid phase exfoliation, possessing a low oxygen content, unlike the GO/rGO systems typically used, and demonstrate their applicability for ion sieving in aqueous solutions. These low oxygen content graphene membranes formed from flakes of varying size were used to determine the effect of flake morphology on ion transport. Interestingly decreasing flake length and thickness leads to an increase in the number and tortuosity of nanochannels between the layers, resulting in a significant reduction of ion transport. The smaller flakes show an increased surface charge, due to the level of defects, which impedes chloride mobility allowing for both physical sieving and charge repulsion. Moreover, the graphene membranes reported here exhibit excellent Na+ rejection properties (∼97%) with water permeance ∼10 times higher than those reported for GO membranes, while demonstrating high stability in aqueous solutions with no observed swelling. These materials are therefore extremely promising for future applications in water purification.

MoS2 and MoO3 have attracted attention due to their interesting properties in energy storage applications, however, the operative charge storage mechanism, whether based on surface ion adsorption and intercalation, is not yet fully understood. In this work, the intercalation of non-hydrated cations into free-standing MoO3-MoS2 electrodes, prepared as composites with graphene, was studied. The oxide material is formed during the solution phase exfoliation process. It is found that tetramethylammonium chloride (TMACl) provides twice the capacitance of tetraethylammonium chloride (TEACl) and tetrapropylammonium chloride (TPACl) solutions. This is attributed to the interlayer spacing of MoS2 (0.615 nm) and MoO3 (0.690 nm), which are greater than the crystallographic diameter of TMA+ (0.558 nm). In contrast, the crystallographic diameter of TEA+ (0.674 nm) and TPA+ (0.758 nm), being larger than the interlayer spacing of MoS2, leads to storage of charge only on the surface of the materials through ion adsorption. Moreover, we have found that use of the TPA+ ion leads to the partial re-exfoliation of the as-prepared materials, which can enhance the capacitance retention during cycling. These results improve the understanding of charge storage mechanism of layered 2D materials.

A green and low-cost method is presented to fabricate graphene fibre electrodes by electrochemical exfoliation of thin strips of graphite foil. When assembled into micro-supercapacitors, the graphene fibres achieved an excellent electrochemical capacitance (∼247.6 mF cm−2 at ∼2 mA cm−2) and low equivalent series resistance (∼2.4 Ω), a significant improvement over the typically used graphene oxide based fibres. This excellent capacitive performance indicates these graphene-based energy storage devices could be ideal for microelectronics applications. Additionally, an all-solid-state flexible micro-supercapacitor was fabricated using a gel electrolyte (H3PO4/PVA) and exhibited a high areal capacitance of 70.6 mF cm−2 and a superior cycling stability of ∼96% capacitance retention after 2400 cycles, suggesting possible applications for flexible energy storage. The easily scalable and facile single step strategy presented here outperforms the conventionally used graphene oxide based methods, which typically require harmful chemicals and involve a more complex synthesis procedure.

Stress transfer has been investigated for exfoliated hexagonal boron nitride (hBN) nanosheets (BNNSs) through the use of Raman spectroscopy. Single BNNSs of different thicknesses of up to 100 nm (300 layers) were deposited upon a poly(methyl methacrylate) (PMMA) substrate and deformed in unixial tension. The Raman spectra from the BNNSs were relatively weak compared to graphene, but the in-plane E2g Raman mode (the G band) could be distinguished from the spectrum of the PMMA substrate. It was found that G band down-shifted during tensile deformation and that the rate of band shift per unit strain decreased as the thickness of the BNNSs increased, as is found for multi-layer graphene. The efficiency of internal stress transfer between the different hBN layers was found to be of the order of 99% compared to 60%–80% for graphene, as a result of the stronger bonding between the hBN layers in the BNNSs. The reduction in bandshift rate can be related to the effective Young’s modulus of the 2D material in a nanocomposites and the findings show that it would be expected that even 100 layer BNNSs should have a Young’s modulus of more than half that of hBN monolayer. Interfacial stress transfer between a single hBN nanosheet and the PMMA substrate has been evaluated using shear lag theory. It is found that the interfacial shear stress between the BNNS and the substrate is of the order of 10 MPa, a factor of around 4 higher than that for a graphene monolayer. These findings imply that BNNSs should give better mechanical reinforcement than graphene in polymer-based nanocomposites as a result of good internal interlayer stress transfer within the nanosheets and better interfacial stress transfer to the polymer matrix.

2D transition metal carbides and nitrides (MXenes) are electrochemically active materials capable of exhibiting pseudocapacitance. Multilayer MXenes are similar to graphite, but with larger interlayer spacing and surface functionalities which allow them to readily disperse in water and undergo a range of reactions without compromising their electrical conductivity. The large interlayer spacing enables MXenes to readily intercalate large ions, and form composites with materials such as graphene, metal oxides, transition metal dichalcogenides and silicon, with which they make electrodes able to deliver exceptional capacities at high power rates over thousands of cycles. Research into MXenes for energy storage has grown exponentially since 2011, and it is now necessary, especially for readers new to the field, to review progress made in more specific areas. This critical review will therefore analyse the progress made in developing MXene‐based batteries, focusing solely on anodes developed for metal‐ion batteries such as Li‐ion, Na‐ion and K‐ion.

There has been a massive growth in the study of transition metal dichalcogenides (TMDs) over the past decade, based upon their interesting and unusual electronic, optical and mechanical properties, such as tuneable and strain-dependent bandgaps. Tungsten disulphide (WS2), as a typical example of TMDs, has considerable potential in applications such as strain engineered devices and the next generation multifunctional polymer nanocomposites. However, controlling the strain, or more practically, monitoring the strain in WS2 and the associated micromechanics have not been so well studied. Both photoluminescence (PL) spectroscopy and Raman spectroscopy have been proved to be effective but PL cannot be employed to characterise multilayer TMDs while it is difficult for Raman spectroscopy to reveal the band structure. In this present study, PL and Raman spectroscopy have been combined to monitor the strain distribution and stress transfer of monolayer WS2 on a flexible polymer substrate and in polymer nanocomposites. It is demonstrated that WS2 still follows continuum mechanics on the microscale and that strain generates a non-uniform bandgap distribution even in a single WS2 flake through a simple strain engineering. It is shown that these flakes could be useful in optoelectronic applications as they become micron-sized PL emitters with a band gap that can be tuned by the application of external strain to the substrate. The analysis of strain distributions using Raman spectroscopy is further extended to thin-film few-layer WS2 polymer nanocomposites where it is demonstrated that the stress can be transferred effectively to WS2 flakes. The relationship between the mechanical behaviour of single monolayer WS2 flakes and that of few-layer flakes in bulk composites is investigated.

Materials for electronics that function as electrical and/or thermal conductors are often rigid, expensive, difficult to be sourced and sometimes toxic. An electrically and thermally conductive nanocomposite that is lightweight, flexible and eco-friendly could improve the environmental friendliness of the electronics sector and enable new applications. Considering this, we have fabricated electrically and thermally conductive flexible materials by functionalizing paper with nanocarbon conductive inks. Carnauba wax is emulsified in isopropyl alcohol and mixed with graphene nanoplatelets (GNPs) or with hybrids of GNPs and carbon nanofibers (CNFs). The percolation threshold of the hybrid samples is lowered compared with the pure GNPs composites, due to their increased filler aspect ratio. The hybrid samples also exhibit superior bending and folding stability. Densification of the coating to decrease their sheet resistance enables them to achieve as low as ~ 50 {\Omega} sq-1 for the GNP-based paper. The densification procedure improves the bending stability, the abrasion resistance, and the electromagnetic interference shielding of the paper-based conductors. Finally, the compressed samples show an impressive enhancement of their thermal diffusivity. The flexible and multifunctional nanocarbon coated paper is a promising electronic conductor and thermally dissipative material and, at the same time, can increase the environmental sustainability of the electronics sector., Comment: 27 pages, 4 figures

A systematic study has been undertaken of the reinforcement of poly (vinyl alcohol) (PVA) by hexagonal boron nitride (hBN) nanosheets (BNNSs) produced by the liquid-phase exfoliation of hBN crystals. Three types of BNNSs with different geometries were prepared, two of which had similar lateral dimensions and two that had a similar aspect ratio (length/thickness). PVA nanocomposites with different loadings of the three types of BNNSs were prepared and this enabled the effect of BNNS volume fraction and geometry upon the mechanical properties such as Young's modulus, yield stress and breaking strength, to be determined. Although the Raman scattering from hBN is relatively weak compared with that from graphene, it was shown that Raman spectroscopy could be used to both evaluate the distribution of the BNNSs in the nanocomposites and follow stress transfer from the polymer matrix to the BNNSs. It was found that the reinforcement of the polymer could be modelled using a combinations of the rule of mixtures and modified shear lag theory. The highest level of reinforcement was found for the BNNSs with the largest aspect ratio although there was evidence of a decline in the level of reinforcement at the highest loadings of all types of BNNSs, as the result of agglomeration of the nanosheets.

Thermolysis of molecular precursors followed by liquid phase exfoliation accesses 2-D IV–VI semiconductor nanomaterials., Solventless thermolysis of molecular precursors followed by liquid phase exfoliation allows access to two-dimensional IV–VI semiconductor nanomaterials hitherto unreachable by a scalable processing pathway. Firstly, the use of metal dithiocarbamate precursors to produce bulk alloys in the series Pb1–xSnxS (0 ≤ x ≤ 1) by thermolysis is demonstrated. The bulk powders are characterised by powder X-ray diffraction (pXRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. It was found that there is a transition from cubic structures for the Pb-rich alloys including the end compound, PbS (0 ≤ x ≤ 0.4) to layered orthorhombic structures for Sn-rich alloys and the end compound SnS (0.5 ≤ x ≤ 1.0). A smooth elemental progression from lead-rich to tin-rich monochalcogenides across the series of materials is observed. Liquid phase exfoliation was applied to produce two dimensional (2D) nanosheets for a mixed Pb1–xSnxS alloy (where x = 0.8) in 1-methyl-2-pyrrolidone (NMP) using the synthetic bulk powder as starting material. The nanosheet products were characterized by SEM, atomic force microscopy (AFM) and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). First principle calculations of Pb1–xSnxS alloys show that the Sn content x modifies the size of the band gap by several 100 meV and that x changes the gap type from indirect in SnS to direct in Pb0.2Sn0.8S. These results are supported by UV-Vis spectroscopy of exfoliated Pb0.2Sn0.8S. The method employed demonstrates a new, scalable, processing pathway which can potentially be used to synthesize a range of synthetic layered structures that can be exfoliated to as-yet unaccessed 2D materials with tunable electronic properties.

A base wash procedure was used to partially remove the oxygen functional groups from the graphene oxide (GO) flakes to prepare the base-washed GO (BwGO). It is found that the base wash treatment does not alter the physical properties (size, thickness etc.) of the GO significantly but the chemical composition is changed. Nanocomposites were prepared by incorporating the BwGO flakes obtained in a poly(vinyl alcohol) (PVA) matrix. It was found the storage modulus of the nanocomposites is enhanced from 4.4 GPa to 6.5 GPa with 5 wt% of BwGO. This is in agreement with the micromechanical estimation obtained by using Raman spectroscopy that follows the interfacial stress transfer from the matrix to the BwGO fillers. The lower effective modulus of BwGO than GO as calculated using the classical ‘rule of mixtures’ is due to the loss of functional groups on GO that serve as a surfactant to prevent the flakes from re-agglomerating. An agglomeration factor ηa is therefore proposed and a concept of ‘effective volume fraction’ is introduced to quantify and evaluate the level of agglomeration of fillers in nanocomposites, which can be otherwise difficult to visualize optically. It is found that the removal of the functional groups causes the flakes to re-agglomerate, and reduces the ‘effective volume fraction’ by about 10–20%.

Organic–inorganic composites show great potential for organic rechargeable lithium-ion batteries. In this work, two-dimensional phthalocyanine molecules were converted into hybrid nanoparticles with a porous structure and bound to a conductive graphene layer to act as a cathode material. The conductivity of this reduced graphene oxide/Fe-phthalocyanine (rGO/FePc) composite is improved through good interfacial connections and internal polymerization. The FePc spheres were shaped with the assistance of Fe3O4 and immobilized between the layers of reduced graphene oxide (rGO). The electrochemical properties of the organic–inorganic composites were investigated by testing in a lithium-ion cell. A high discharge capacity of 186 mAh g−1 was maintained after 100 cycles at 300 mA g−1, which demonstrates a significant improvement in the cycle life compared to previous reports of phthalocyanine-based electrochemical energy storage behaviour.

Safe and low-cost zinc-ion hybrid supercapacitors using neutral aqueous electrolytes are promising for large scale and high power energy storage. A key challenge for Zn-ion hybrid supercapacitors is to increase their energy density without sacrificing the high power performance. Herein, we report a Zn-ion hybrid supercapacitor using a polypyrrole/electrochemical graphene oxide (PPy/EGO) composite cathode and aqueous 1 M ZnCl2 or ZnBr2 electrolytes. The Zn-PPy/EGO system with an operating cell voltage from 0.5 to 1.5 V exhibited high energy and high power densities of 117.7 and 72.1 Wh kg−1 at 0.34 and 12.4 kW kg−1, respectively, outperforming the Zn-ion hybrid supercapacitors using carbon cathodes. This high performance is because of the facilitated electronic and ionic transport in the PPy/EGO composite. In brief, the EGO prepared via the electrochemical method we recently reported has good structure integrity and is easily reduced; the porous PPy/EGO composite from co-electrodeposition benefits fast ion diffusion in pores; the small, monovalent halides anions in the zinc halides electrolytes are highly mobile in bulk PPy for fast solid-phase anion insertion/de-insertion. Electrochemical characterization and ex-situ X-ray photoelectron spectroscopy confirmed the anion-dominated charge storage mechanism of PPy/EGO cathode in Zn-PPy/EGO system.

A green and facile molten-salt (MS) route for the scalable synthesis of free-standing, long-range oriented and corrugated graphene-like sheets from a copper phthalocyanine (CuPc) precursor is reported. Their unique arrangement and transformation behavior in molten potassium chloride (KCl) play a key role in promoting the successful synthesis of the anisotropic nanostrucure.

Laminar MoS2 membranes show outstanding potential for practical applications in energy conversion/storage, sensing, and as nanofluidic devices. The re-stacking of exfoliated MoS2 creates nanocapillaries between the layers of MoS2 nanosheets. These MoS2 membranes have been shown to possess a unique combination of ionic rejection properties, high water permeation rates, and long-term solvent stability, with no significant swelling when exposed to aqueous or organic solvents. Chemical modification of MoS2 membranes has been shown to improve their ionic rejection properties, however the mechanism behind this improvement is not well understood. In this work, we elucidate the ion-sieving mechanism by the study of potential-dependent ion transport through functionalized MoS2 membranes. The ionic permeability of the MoS2 membrane is transformed by chemical functionalization with a simple naphthalene sulfonate dye (sunset yellow) and with a resultant attenuation of permeability by at least an order of magnitude, compared to the pristine MoS2 membranes and permeability reported for graphene oxide and Ti3C2T x (MXene) membranes. The effects of pH, solute concentration, and ionic size/charge on the ionic selectivity of the functionalized MoS2 membranes are also reported. Understanding the mechanism of ionic sieving within functionalized MoS2 membranes will enable future applications in electro-dialysis and ion exchange for water treatment technologies.

Lithium (Li) and sodium (Na) intercalation into molybdenum disulfide (MoS2) flakes with layer thicknesses of 2.2 nm (3 layers) and 51 nm (ca. 82 layers) was investigated in situ under potential control via a combination of Raman spectroscopy and optical microscopy. A Raman frequency shift indicative of reduced strain along the MoS2 sheet during Na intercalation compared with Li intercalation is observed, despite the atomic radii of Na being larger than Li, r(Na+) 1.02 A > r(Li+) 0.76 A. Overall, the shift of Raman bands exhibited similar trends in trilayer and multilayer flakes during lithiation. A combination of strain and electron doping was used to explain the observed Raman frequency shifts. The differences between lithiation and sodiation in MoS2 flake were also observed visually by optical microscopy, whereby Li inserted into MoS2 via a pushed-atom-by-atom behaviour and Na via a layer-by-layer behaviour. Variation of the insertion behaviour between lithiation and sodiation in MoS2 was further investigated via galvanostatic intermittent titration technique, in which the diffusion coefficient as a function of x in MxMoS2 (M = Li or Na) suggested a stable intermediate phase existed in NaxMoS2 during sodiation, whereas this stable intermediate phase was absent in LixMoS2.

Additive manufacturing (AM) technologies appear as a paradigm for scalable manufacture of electrochemical energy storage (EES) devices, where complex 3D architectures are typically required but are hard to achieve using conventional techniques. The combination of these technologies and innovative material formulations that maximize surface area accessibility and ion transport within electrodes while minimizing space are of growing interest. Herein, aqueous inks composed of atomically thin (1-3 nm) 2D Ti3 C2 Tx with large lateral size of about 8 µm possessing ideal viscoelastic properties are formulated for extrusion-based 3D printing of freestanding, high specific surface area architectures to determine the viability of manufacturing energy storage devices. The 3D-printed device achieves a high areal capacitance of 2.1 F cm-2 at 1.7 mA cm-2 and a gravimetric capacitance of 242.5 F g-1 at 0.2 A g-1 with a retention of above 90% capacitance for 10 000 cycles. It also exhibits a high energy density of 0.0244 mWh cm-2 and a power density of 0.64 mW cm-2 at 4.3 mA cm-2 . It is anticipated that the sustainable printing and design approach developed in this work can be applied to fabricate high-performance bespoke multiscale and multidimensional architectures of functional and structural materials for integrated devices in various applications.

Correction for ‘Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties from a potentially scalable reaction pathway’ by Kane Norton et al., Chem. Sci., 2019, 10, 1035–1045.

With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed.

Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS2 structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH3)6]3+/2+ and LiCl, respectively). Our work shows a strong dependence of the electrochemical properties on the number of MoS2 layers and illumination intensity and proves that an effective interlayer charge transport occurs in bulk MoS2. This highlights the exciting opportunities for tuning of the electrochemical performance of MoS2 through modification of its structure, external environment, and illumination.

Electrically conductive, polymeric materials that maintain their conductivity even when under significant mechanical deformation are needed for actuator electrodes, conformable electromagnetic shielding, stretchable tactile sensors and flexible energy storage. The challenge for these materials is that the percolated, electrically conductive networks tend to separate even at low strains, leading to significant piezoresistance. Herein, deformable conductors were fabricated by spray-coating a nitrile substrate with a graphene-elastomer solution. The coatings showed only slight increase in electrical resistance after thousands of bending cycles and repeated folding-unfolding events. The deformable conductors doubled their electrical resistance at 12% strain and were washable without changing their electrical properties. The conductivity-strain behaviour was modelled by considering the nanofiller separation upon deformation. To boost the conductivity at higher strains, the production process was adapted by stretching the nitrile substrate before spraying, after which it was released. This adaption meant that the electrical resistance doubled at 25 % strain. The electrical resistance was found sufficiently low to give a 1.9 dB/{\mu}m shielding in the 8-12 GHz electromagnetic band. The physical and electrical properties, including the EM screening, of the flexible conductors, were found to deteriorate upon cycling but could be recovered through reheating the coating., Comment: 29 pages, 6 figures

The demand for efficient electrochemical energy storage technology, such as supercapacitors, continues to increase as both the energy and power demands of devices grow. Graphene has attracted wide interest in addressing this energy challenge due to its high conductivity and specific surface area. However, in reality the hydrophobic properties and the restacking of the graphene sheets during device manufacture leads to significantly lower storage performance than that theoretically predicted for isolated sheets. Herein, functionalized graphene was prepared by a convenient one-pot process, where graphene was functionalized with aryl diazonium salts (4-nitrobenzenediazonium tetrafluoroborate (NBD) and 4-bromobenzenediazonium tetrafluoroborate (BBD)) simultaneously during oxidative electrochemical exfoliation of graphite. It was found that the specific capacitance for functionalized graphene was significantly improved compared to pristine graphene due to the introduction of pseudocapacitance by the aryl diazonium salts. The dispersibility of functionalized graphene in water was also found to be improved, implying a better hydrophilicity. NBD functionalized graphene which had been exfoliated/functionalized for a total of 30 minutes exhibited the best energy storage properties with a 5 times increase in specific capacitance (17 mF cm-2) compared to pristine graphene (3 mF cm-2).

Carbon materials are ubiquitous in energy storage; however, many of the fundamental electrochemical properties of carbons are still not fully understood. In this work, we studied the capacitance of highly ordered pyrolytic graphite (HOPG), with the aim of investigating specific ion effects seen in the capacitance of the basal plane and edge-oriented planes of the material. A series of alkali metal cations, from Li+, Na+, K+, Rb+, and Cs+ with chloride as the counterion, were used at a fixed electrolyte concentration. The basal plane capacitance at a fixed potential relative to the potential of zero charge was found to increase from 4.72 to 9.39 μF cm-2 proceeding down Group 1. In contrast, the edge-orientated samples display capacitance ca. 100 times higher than those of the basal plane, attributed to pseudocapacitance processes associated with the presence of oxygen groups and largely independent of cation identity. This work improves understanding of capacitive properties of carbonaceous materials, leading to their continued development for use in energy storage.

In situ electrochemical atomic force microscopy (ec-AFM) is utilised for the first time to probe the initial stages of metal-organic framework (MOF) coating growth via anodic dissolution. Using the example of the Cu MOF HKUST-1, real time surface analysis is obtained that supports and verifies many of the reaction steps in a previously proposed mechanism for this type of coating growth. No evidence is observed however for the presence or formation of Cu2O, which has previously been suggested to be both key for the formation of the coating and a potential explanation for the anomalously high adhesion strength of coatings obtained via this methodology. Supporting in situ electrochemical Raman spectroscopy also fails to detect the presence of any significant amount of Cu2O before or during the coating's growth process.

Black phosphorus is a two-dimensional material that has potential applications in energy storage, high frequency electronics and sensing, yet it suffers from instability in oxygenated and/or aqueous systems. Here we present the use of a polymeric stabilizer which prevents the degradation of nearly 68% of the material in aqueous media over the course of ca. 1 month.

A new wet chemistry approach, based on the hot-injection–thermolytic decomposition of the single-source precursor [Mo2O2S2(S2COEt)2] in oleylamine, is described for the production of nanodimensional 1H-MoS2@oleylamine. High quality freestanding MoS2 nanosheets capped with oleylamine have been prepared and subjected to detailed compositional analyses for the first time. The selection of the appropriate reaction temperatures (200–325 °C) in the simple yet robust procedure allows control of the lateral nanosheet dimensions which range from 4.5 to 11.5 nm, as 1H-MoS2@oleylamine entities which maintain a consistent chemical composition (MoS2·oleylamine0.28–0.33). This work provides the first example of atomic resolution STEM imaging of these fine-scale nanosheet materials, providing new insights into their morphology and demonstrating that those freestanding MoS2 nanosheets are pure, highly crystalline, randomly oriented monolayers. The 1H-MoS2@oleylamine samples were analyzed by attenuated total reflectance F...

The increasing demand for portable and wearable electronics requires high reliability of devices for continuous miniaturization, and provides challenges for the dimensionally-confined bottom-up assembly methods which typically begin with materials such as graphene oxide (GO). However, GO based devices suffer from complicated synthesis procedures and lower device performance due to the presence of oxygen functionalities on the electrode materials, as well as the need to ensure a good interface between the electrode material and the electrolyte used. Here, a facile, two-step, top-down strategy was used to fabricate thin, all-in-one (PVA/H3PO4)/graphene/graphite paper-like electrodes. The assembled flexible, all-solid-state micro-supercapacitor devices exhibit excellent performance compared to previously reported values, such as a high volumetric capacitance of ∼3.6 F·cm−3 even after 20000 cycles. Notably, the devices also showed excellent rate performance with extremely high specific capacitance retention of up to 94% as the current density increased from 0.5 to 5 A·cm−3, indicating promise for high power applications. The paper-like electrodes were tailored to less than 0.5 mm width for further miniaturization and suffered from minimal capacitance attenuation. The excellent mechanical flexibility, capacity, and reliability indicate their promising application in energy storage devices.

We demonstrate the controlled tuning of the electronic band structure of large-arrays of graphene nanoribbons (GNRs) by chemical functionalization. The GNR arrays are synthesized by substrate-controlled metal-assisted etching of graphene in H2 at high temperature, and functionalized with different molecules. From Raman spectroscopy and carrier transport measurements, we found that 4-nitrobenzenediazonium (4-NBD) and diethylene triamine (DETA) molecules can tune the doping level of the GNR arrays to p- and n-type, respectively. In both cases, the doping effects induced in the GNRs were found to be higher than for a pristine graphene sheet, due to the presence of a large quantity of edges. Effects of chemical doping on the Raman spectrum of sp(2) carbon materials are also discussed. Our findings offer an effective way to control the electronic structure of GNRs by chemical functionalization, and are expected to facilitate the production of nanoribbon-based p-n junctions for future implementation into electronic circuits.

Laminar membranes of two-dimensional materials are excellent candidates for applications in water filtration due to the formation of nanocapillaries between individual crystals that can exhibit a molecular and ionic sieving effect, while allowing high water flux. This approach has been exemplified previously with graphene oxide, however these membranes suffer from swelling when exposed to liquid water, leading to low salt rejection and reducing their applicability for desalination applications. Here, we demonstrate that by producing thin (∼5 μm) laminar membranes of exfoliated molybdenum disulfide (MoS

The hydrogen evolution reaction (HER) plays a crucial role in clean energy production in hydrogen fuel cells. In order to utilise this process effectively, new catalysts are required that are cheap, non-toxic and efficient. In this context, 2D materials such as transition metal dichalcogenides (e.g. MoS2) should offer the desired properties but have so far proven difficult to manufacture into useful devices. In this work, liquid|liquid interfaces are used for the assembly and testing of the catalytic efficiency of a number of 2D materials and their composites, exploiting the ability of the materials to self-assemble at these interfaces and be tested electrochemically in situ. MoS2, WS2, and graphene were developed for hydrogen evolution at the water|1,2-dichlorobenzene (DCB) interface. The exfoliation process was carried out in DCB and resulted in multi-layer MoS2, few layer WS2 and graphene: when assembled at the water|DCB interface, these materials acted as efficient HER catalysts. HER was investigated using voltammetry, with bulk reaction kinetics monitored by in-situ UV-visible spectroscopy at a constant potential. MoS2 exhibited the highest performance of the catalysts examined, with an average rate constant of 0.0132 ± 0.063 min-1 at an applied Galvani potential of +0.5 V. This is ascribed to the sulphur edge sites of MoS2, which are known to be active for hydrogen evolution predominantly.