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.

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.

Electrical, Mechanical, and Capacity Percolation Leads to High-Performance MoS2/Nanotube Composite Lithium Ion Battery Electrodes.

Advances in lithium ion batteries would facilitate technological developments in areas from electrical vehicles to mobile communications. While two-dimensional systems like MoS2 are promising electrode materials due to their potentially high capacity, their poor rate capability and low cycle stability are severe handicaps. Here, we study the electrical, mechanical, and lithium storage properties of solution-processed MoS2/carbon nanotube anodes. Nanotube addition gives up to 10(10)-fold and 40-fold increases in electrical conductivity and mechanical toughness, respectively. The increased conductivity results in up to a 100× capacity enhancement to ∼1200 mAh/g (∼3000 mAh/cm(3)) at 0.1 A/g, while the improved toughness significantly boosts cycle stability. Composites with 20 wt % nanotubes combine high reversible capacity with excellent cycling stability (e.g., ∼950 mAh/g after 500 cycles at 2 A/g) and high rate capability (∼600 mAh/g at 20 A/g). The conductivity, toughness, and capacity scale with nanotube content according to percolation theory, while the stability increases sharply at the mechanical percolation threshold. We believe that the improvements in conductivity and toughness obtained after addition of nanotubes can be transferred to other electrode materials, such as silicon nanoparticles.

Mapping of Low-Frequency Raman Modes in CVD-Grown Transition Metal Dichalcogenides: Layer Number, Stacking Orientation and Resonant Effects.

Layered inorganic materials, such as the transition metal dichalcogenides (TMDs), have attracted much attention due to their exceptional electronic and optical properties. Reliable synthesis and characterization of these materials must be developed if these properties are to be exploited. Herein, we present low-frequency Raman analysis of MoS2, MoSe2, WSe2 and WS2 grown by chemical vapour deposition (CVD). Raman spectra are acquired over large areas allowing changes in the position and intensity of the shear and layer-breathing modes to be visualized in maps. This allows detailed characterization of mono- and few-layered TMDs which is complementary to well-established (high-frequency) Raman and photoluminescence spectroscopy. This study presents a major stepping stone in fundamental understanding of layered materials as mapping the low-frequency modes allows the quality, symmetry, stacking configuration and layer number of 2D materials to be probed over large areas. In addition, we report on anomalous resonance effects in the low-frequency region of the WS2 Raman spectrum.

Liquid Phase Exfoliated MoS2 Nanosheets Percolated with Carbon Nanotubes for High Volumetric/Areal Capacity Sodium-Ion Batteries.

The search for high-capacity, low-cost electrode materials for sodium-ion batteries is a significant challenge in energy research. Among the many potential candidates, layered compounds such as MoS2 have attracted increasing attention. However, such materials have not yet fulfilled their true potential. Here, we show that networks of liquid phase exfoliated MoS2 nanosheets, reinforced with 20 wt % single-wall carbon nanotubes (SWNTs), can be formed into sodium-ion battery electrodes with large gravimetric, volumetric, and areal capacity. The MoS2/SWNT composite films are highly porous, electrically conductive, and mechanically robust due to its percolating carbon nanotube network. When directly employed as the working electrode, they exhibit a specific capacity of >400 mAh/g and volumetric capacity of ∼650 mAh/cm(3). Their mechanical stability allows them to be processed into free-standing films with tunable thickness up to ∼100 µm, corresponding to an areal loading of 15 mg/cm(2). Their high electrical conductivity allows the high volumetric capacity to be retained, even at high thickness, resulting in state-of-the-art areal capacities of >4.0 mAh/cm(2). Such values are competitive with their lithium-ion counterparts.

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.

Spectroscopic metrics allow in situ measurement of mean size and thickness of liquid-exfoliated few-layer graphene nanosheets.

Liquid phase exfoliation is a powerful and scalable technique to produce defect-free mono- and few-layer graphene. However, samples are typically polydisperse and control over size and thickness is challenging. Notably, high throughput techniques to measure size and thickness are lacking. In this work, we have measured the extinction, absorption, scattering and Raman spectra for liquid phase exfoliated graphene nanosheets of various lateral sizes (90 ≤ 〈L〉 ≤ 810 nm) and thicknesses (2.7 ≤ 〈N〉 ≤ 10.4). We found all spectra to show well-defined dependences on nanosheet dimensions. Measurements of extinction and absorption spectra of nanosheet dispersions showed both peak position and spectral shape to vary with nanosheet thickness in a manner consistent with theoretical calculations. This allows the development of empirical metrics to extract the mean thickness of liquid dispersed nanosheets from an extinction (or absorption) spectrum. While the scattering spectra depended on nanosheet length, poor signal to noise ratios made the resultant length metric unreliable. By analyzing Raman spectra measured on graphene nanosheet networks, we found both the D/G intensity ratio and the width of the G-band to scale with mean nanosheet length allowing us to establish quantitative relationships. In addition, we elucidate the variation of 2D/G band intensities and 2D-band shape with the mean nanosheet thickness, allowing us to establish quantitative metrics for mean nanosheet thickness from Raman spectra.

Slow and fast absorption saturation of black phosphorus: experiment and modelling.

According to the excited-state decay time, a slow saturable absorber model was employed to obtain the ground-state absorption and excited-state absorption cross sections σgs and σes of black phosphorus (BP) nanosheets, which are (1.25 ± 0.07) × 10-16 cm2 and (2.97 ± 0.21) × 10-17 cm2 at 515 nm, and (5.95 ± 0.17) × 10-18 cm2 and (5.19 ± 0.71) × 10-19 cm2 at 1030 nm, respectively. In comparison, the σgs and σes of MoS2 and graphene were also obtained with the same model. The ratio σes/σgs of BP is the smallest (only 0.24 ± 0.03 at 515 nm and 0.09 ± 0.01 at 1030 nm) among the three two-dimensional layered materials both at 515 nm and 1030 nm. Relatively large ground-state transition probability and weak loss of excited-state absorption result in good saturable absorption performance. In comparison with MoS2 and graphene, the much lower saturation intensity of BP can be well explained from this perspective.

Comparison of liquid exfoliated transition metal dichalcogenides reveals MoSe2 to be the most effective hydrogen evolution catalyst.

While 2D transition metal dichalcogenides are known to be promising materials for electrocatalysis of hydrogen production, it is not clear which member of this family of materials is the most effective catalyst. Here we perform a comprehensive study, comparing the catalytic performance of electrodes consisting of porous arrays of liquid exfoliated MX2 nanosheets (M = Mo, W; X = S, Se, Te). We find a clear hierarchy with selenides > sulphides > tellurides with MoSe2 clearly out-performing the other materials. In all cases the performance, as characterised by current density at a given potential, can be improved by increasing the number of active sites (via control of the electrode thickness) and/or by adding carbon nanotubes to the electrode (i.e. increasing the electrode conductivity). While all materials formed reasonably stable electrodes, addition of nanotubes tended to improve mechanical cohesion. In an attempt to maximise performance, we prepared thick (∼15 µm), free standing MoSe2/SWNT composite electrodes which displayed Tafel slopes of ∼77 mV per decade and exchange current densities of ∼0.1 mA cm(-2). These electrodes had low onset potentials, reaching -2 mA cm(-2) at -41 mV (vs. RHE) and generated high current densities of -35 mA cm(-2) at -200 mV (vs. RHE).

Production of Highly Monolayer Enriched Dispersions of Liquid-Exfoliated Nanosheets by Liquid Cascade Centrifugation.

While liquid exfoliation is a powerful technique to produce defect-free nanosheets in large quantities, its usefulness is limited by broad nanosheet thickness distributions and low monolayer contents. Here we demonstrate liquid processing techniques, based on iterative centrifugation cascades, which can be designed to achieve either highly efficient nanosheet size-selection and/or monolayer enrichment. The resultant size-selected dispersions were used to establish quantitative metrics to determine monolayer volume fraction, as well as mean nanosheet size and thickness, from standard spectroscopic measurements. Such metrics allowed us to design and optimize centrifugation cascades to enrich liquid exfoliated WS2 dispersions up to monolayer contents of 75%. Monolayer-rich dispersions show relatively bright photoluminescence with narrow line widths (<35 meV) indicating the high quality of the nanosheets. The enriched dispersions display extinction spectra with distinct features, which also allow the direct estimation of monolayer contents.

Large variations in both dark- and photoconductivity in nanosheet networks as nanomaterial is varied from MoS2 to WTe2.

We have used solution processing techniques to fabricate thin-film networks of nanosheets of six different transition metal dichalcogenides; MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2. We have measured both the dark conductivity and the photoconductivity under broad band illumination in the intensity range from 0-1500 W m(-2). The dark conductivity varied from ∼ 10(-6) S m(-1) for MoS2 to ∼ 1 S m(-1) for WTe2, with an apparent exponential dependence on bandgap. All materials studied show photocurrents which rise slowly with time and depend sub-linearly on light intensity, both hallmarks of trap limited processes. Because the photoresponse depends relatively weakly on bandgap, the ratio of photo- to dark conductivity is largest for the sulphides because of their lower dark conductivities. As such, MoS2 and WS2 may be best suited to photo-detection applications. However, due to their lower bandgap and superior conductivity, WSe2 and WTe2 might prove more effective in other applications, for example in photovoltaic cells.

Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics.

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

Transition metal dichalcogenide growth via close proximity precursor supply.

Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components.

Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets.

Two-dimensional nanomaterials such as MoS2 are of great interest both because of their novel physical properties and their applications potential. Liquid exfoliation, an important production method, is limited by our inability to quickly and easily measure nanosheet size, thickness or concentration. Here we demonstrate a method to simultaneously determine mean values of these properties from an optical extinction spectrum measured on a liquid dispersion of MoS2 nanosheets. The concentration measurement is based on the size-independence of the low-wavelength extinction coefficient, while the size and thickness measurements rely on the effect of edges and quantum confinement on the optical spectra. The resultant controllability of concentration, size and thickness facilitates the preparation of dispersions with pre-determined properties such as high monolayer-content, leading to first measurement of A-exciton MoS2 luminescence in liquid suspensions. These techniques are general and can be applied to a range of two-dimensional materials including WS2, MoSe2 and WSe2.