2D MATERIALS

Abstract

The synthesis of 2D metal chalcogenide nanosheets and metal-ion or metalloid-ion doped 2D metal chalcogenide nanosheets by adding a metal complex to a hot dispersing medium. The mean lateral dimension of the nanosheets may be controlled by appropriate temperature selection.

Claims

1. A method for the synthesis of 2D metal chalcogenide nanosheets, the method comprising adding a metal complex to a dispersing medium which is at elevated temperature, wherein the metal complex comprises a metal ion and a ligand comprising at least two atoms selected from oxygen, sulfur, selenium, and tellurium, to form a dispersion of the 2D metal chalcogenide nanosheets in the dispersing medium.

2. A method for the synthesis of metal-ion or metalloid-ion doped 2D metal chalcogenide nanosheets, the method comprising adding a metal complex to a dispersing medium which is at elevated temperature, wherein the reaction is performed in the presence of a salt of said metal or metalloid ion, and wherein the complex comprises a metal ion and a ligand comprising at least two atoms selected from oxygen, sulfur, selenium and tellurium, to form a dispersion of the 2D metal chalcogenide nanosheets in the dispersing medium.

3. The method of claim 1, wherein the ligand comprises at least two atoms selected from sulfur and selenium.

4. The method of claim 1, wherein the metal complex comprises a transition metal ion, optionally wherein the metal complex comprises a molybdenum or tungsten ion.

5. The method of claim 1, wherein the method is a method for the synthesis of metal-ion doped 2D metal chalcogenide nanosheets, optionally wherein the metal ion is selected from manganese, iron, cobalt, nickel, copper, and zinc.

6. The method of claim 1, wherein the salt of said metal or metalloid ion is a halide, optionally wherein the salt is a chloride.

7. The method of claim 1, wherein the ligand is a chalcogenocarbamate or chalcogenocarbonate ion, optionally wherein the ligand is a dithiol-carbamate or a dithiol-carbonate or a diseleno-carbamate or diseleno-carbonate.

8. The method of claim 1, wherein the complex is a complex of formula (IV): ##STR00010## wherein each E is O, S, or Se, each X is S or Se, each Z is OR.sup.1 or NR.sup.2R.sup.3; R.sup.1, R.sup.2, and R.sup.3 are independently selected from optionally substituted alkyl, alkyenyl, cycloalkyl, cyclocalkyl-C.sub.1-6alkyl, cycloalkenyl, cycloalkenyl-C.sub.1-6alkyl, heterocyclyl, heterocyclyl-C.sub.1-6alkyl, aryl, aryl-C.sub.1-6alkyl, and heteroaryl-C.sub.1-6alkyl.

9. The method of claim 1, wherein the dispersing medium comprises at least one coordinating group selected from an amino group, a hydroxyl group, a carboxylic acid group, a phosphonic acid group, a phosphine group, and a phosphine oxide group.

10. The method of claim 1, wherein the 2D material is a binary 2D material.

11. The method of claim 1, wherein the nanosheets have a mean lateral dimension of from 4 to 10 nm with a size distribution no more than 20% of the mean lateral dimension, preferably no more than 15%.

12. The method of claim 1, the method further comprising a step of thermally annealing the nanosheets at a temperature of 350 C. or higher.

13. A composition comprising 2D metal chalcogenide nanosheets, wherein the variation in lateral dimension of the nanosheets is less than 20%, preferably less than 15%.

14. The composition of claim 13, wherein the 2D metal chalcogenide is MoS.sub.2.

15. The composition of claim 13, wherein the nanosheets have a mean lateral dimension of about 5 nm or wherein the nanosheets have a mean lateral dimension of about 7 nm or wherein the nanosheets have a mean lateral dimension of about 9 nm or wherein the nanosheets have a mean lateral dimension of about 11 nm.

16. A capacitor comprising nanosheets according to claim 13, wherein the nanosheets are provided as a composite with graphene.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0129] The invention will now be described with reference to the following figures in which:

[0130] FIG. 1 shows the typical nature of 1H-MoS.sub.2@oleylamine flocculates on holey carbon grids. Images were obtained from 1H-MoS.sub.2@oleylamine samples (a) 3, (b) 7 and (c) 15.

[0131] FIG. 2 shows TEM images of the 1H-MoS.sub.2@oleylamine flocculates, giving evidence for the presence of monolayer MoS.sub.2 nanosheets. The variation of the average nanosheet dimension from the reactions carried out at (a) 200 C. (sample 3; average size of 4.780.78 nm) and (b) 325 C. (sample 19; average size of 11.291.26 nm). The inserted images represent the SAED patterns, supporting the identification of the 1H-crystallites.

[0132] FIG. 3 shows the physical and spectroscopic properties of the MoS.sub.2 nanosheets within 1H-MoS.sub.2@oleylamine. (a) The lateral dimensions of the nanosheets produced (determined by statistical analyses of the TEM images obtained, with error bars) in relation to both reaction temperature and time. (b) A typical p-XRD diffraction pattern observed from the 1 H-MoS.sub.2@oleylamine products (datum from sample 15), accompanied by a reference spectrum of MoS.sub.2 (JCPDS card # 37-1492). (c) A typical Raman spectrum observed from the 1H-MoS.sub.2@oleylamine products (datum from sample 7). (d) The correlation between A.sub.1g-E.sub.2g Raman bands separation of all samples produced and its average nanosheet size determined by TEM analysis.

[0133] FIG. 4 shows atomic resolution ADF STEM images of the side-on MoS.sub.2 nanosheets in 1H-MoS.sub.2@oleylamine (sample 19). (a) A region where plan view flakes were atomically resolved (with resolution of 0.15 nm) and some side-on flakes (indicated by arrows) were also present. The fact that no basal plane interlayer spacings are observed demonstrates these side-on flakes were monolayer. (b) Another region containing multiple side-on flakes, again all were monolayer.

[0134] FIG. 5 shows atomic resolution ADF STEM of MoS.sub.2 nanosheets lying perpendicular to the electron beam. (a and b) Images showing that the flocculates in sample 19 were composed of a large number of nanosheets of a range of size and shapes, these sheets have lateral dimensions of only a few nanometres. Inset FTs show polycrystalline ring patterns, demonstrating that a wide range of crystallographic orientations were present within the scan area. (c and d) Enlarged areas (indicated by red boxes in a and b) allowing sheets' shape and crystallinity to be more easily observed.

[0135] FIG. 6 shows (a) ADF image of a MoS.sub.2 flocculate from sample 19, a STEM EDX spectrum image was acquired from the area indicated by the red box. (b and c) show the resulting Mo and S elemental maps extracted from the spectrum image (using the S K-series (2.31 keV) and Mo K-series (17.48 keV)), demonstrating uniform distributions of both elements.

[0136] FIG. 7 shows the proposed decomposition pathways of the molybdenum(V) complexes (Ia-c, IIb-c) to MoS.sub.2.

[0137] FIG. 8 shows a representative thermogram for the decomposition of 1 H-MoS.sub.2@oleylamine (sample 16) in air. The temperatures that initiate the decomposition of the components within the materials are included in red (vertical lines).

[0138] FIG. 9 shows a) Photograph of constructed coin cell (CR2032) showing an exploded schematic of the cell architecture. Photograph showing the MoS.sub.2/graphene composite on the flexible supporting membrane (i) along with optical microscope image (100) of the membrane surface (ii). The PVDF membranes are stacked back-to-back providing direct electrical contact between the active material and the current collector. The cells were filled with aqueous electrolyte (1M Na.sub.2SO.sub.4). b) Cyclic voltammograms with increasing scan rates for the MoS.sub.2/graphene composite symmetrical coin showing double-layer behaviour. Scan rates starting from the centre and moving outwards are 10, 20, 40, 80, 100, 150, 200, 250, and 300 mV/s. c) Galvanostatic discharge curves at different current densities. Inset shows the calculated specific capacitance as a function of current density. d) The measured specific capacitance.

[0139] FIG. 10 shows the Nyquist plot of the real (Z) and complex (Z) impedance of the coin cell. The semi-circle at the high frequency region is due to ion diffusion while at low frequencies more capacitive behaviour dominates. The equivalent series resistance (ESR) for the membrane is 1.39 .

[0140] FIG. 11 shows TEM images of WS.sub.2 nanosheets produced at 325 C. The image shows monolayer and bilayer, and the inserted diffraction lines indicate the (002) spacing in the bilayer sheets observed (0.68 nm).

[0141] FIG. 12 shows a TEM image of (Mo.sub.0.78W.sub.0.22)S.sub.2@oleylamine produced at 325 C.

[0142] FIG. 13 shows atomic resolution HAADF STEM images of a ternary (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine product. (a) shows a region containing multiple flakes, the ring pattern of the inset Fourier transform (FT) is consistent with multiple randomly oriented crystalline flakes. (b-d) show a higher magnification images of monolayer flakes, FTs show the flakes to be single crystals and the locations of bright atoms is consistent with W substitution into Mo lattice in the 1H-MoS.sub.2 lattice.

[0143] FIG. 14 shows HAADF STEM images of the (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine product of run 8 revealing an average W doping level of 25.98%. (a) and (c) show enlarged HAADF STEM images of regions of the flake. (b) shows HAADF intensity linescan extracted from the row of atoms indicated by the dashed box in (a), the high intensity of the final two atoms in the row are consistent with the W atoms while the intensity of the remaining atoms are assigned to Mo. Atomic identification based on HAADF intensity is illustrated in (c) and (d), with W atoms highlighted in by dark colouring and Mo atoms in brighter colouring.

[0144] FIG. 15 shows diffraction patterns for (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine produced.

[0145] FIG. 16 shows a stacked Raman spectra of (Mo.sub.xW.sub.1-x)S.sub.2 nanosheets (all in the 5-6 nm range) produced with differing compositions and (right) the band shifts of the E.sub.2g and A.sub.1g signals, with respect to composition, observed in the Raman spectra.

[0146] FIG. 17 shows high-resolution TEM images of (TM)-doped MoS.sub.2@oleylamine (Left) 12% Cu-doped MoS.sub.2@oleylamine (arrows highlight the presence of bi/multilayer domains. (Right) 13% Co-doped monolayer MoS.sub.2@oleylamine.

[0147] FIG. 18 shows Raman spectra of pure MoS.sub.2 and Co-doped MoS.sub.2. The observed A.sub.1g-E.sub.2g band separation versus dopant metal and dopant concentration (greyed area represent the range of separations measured for 10 samples of 1H-MoS.sub.2).

[0148] FIG. 19 shows XRD patterns of Ni-doped MoS.sub.2dataset smoothed for clarity.

DETAILED DESCRIPTION

[0149] The invention provides a one-pot synthetic route, based on hot injection-thermolysis, for the production of pure, high quality MoS.sub.2 nanosheets capped by oleylamine. Of course, other nanosheets as described herein are also envisioned. Nanometre-scale control over the lateral dimensions of 1H-MoS.sub.2 nanosheets (ranging from 4.5 to 11.5 nm), has been achieved by modulation of the reaction temperature (between 200 to 325 C.) whilst maintaining consistent levels of purity and oleylamine capping. In addition, the first atomic resolution STEM imaging of this class of materials gives new insights into the structure of MoS.sub.2 within the oleylamine matrix. Specifically, the inventors have shown that monolayer, highly crystalline and randomly oriented nanosheets were formed. The high purity of monolayer sheets, combined with small flake size was demonstrated to be ideal for energy storage applications such as supercapacitors. The calculated specific capacitance (of up to 50 mF/cm.sup.2) was significantly larger than previously reported from ultrasonication prepared MoS.sub.2, and can be maximised through further optimisation. These results indicate that composites of well-defined and thoroughly characterized 2D materials, such as MoS.sub.2 and graphene, show increasing promise for wide scale electrochemical energy storage applications.

[0150] The invention produces nanosheets. The term nanosheet as used in the art refers to two-dimensional nanostructures with a thickness on the nanometer scale. The thickness may be very small, with some monolayer nanosheets consisting of a single layer of atoms. For example, graphene is a nanosheet. Nanosheets are one type of nanomaterial. Other nanomaterials include nanotubes and nanorods (often referred to as 1D structures) and nanoparticles, for example quantum dots (sometimes referred to as 0D structures).

[0151] Nanosheets are typically described as having diameter:length aspect ratios close to about 1:1, although some variation in this is of course envisaged. By contrast, nanorods and nanowires typically have an aspect ratio of at least 1:10. Nanosheet, as used herein, may refer to a nanostructure having a diameter:length aspect ratio of 2:1 to 1:2, preferably 1.5:1 to 1:1.5, most preferably about 1:1.

[0152] The following relates to the complex [Mo.sub.2O.sub.2S.sub.2(S.sub.2COEt).sub.2] in the production of 1H-MoS.sub.2@oleylamine . It will be appreciated that other complexes as described herein may be used.

[0153] 1H-MoS.sub.2@oleylamine samples were prepared by the decomposition of [Mo.sub.2O.sub.2S.sub.2(S.sub.2COEt).sub.2] in oleylamine via a hot injection-thermolysis method..sup.[1] Reactions were carried out at temperatures ranging from 200 to 325 C. to produce black materials. Aliquots were taken at regular intervals and the reaction products isolated, by repeated ethanol washing and centrifugation steps. Upon injection, decomposition of the precursor occurs rapidly; there was no evidence of unreacted [Mo.sub.2O.sub.2S.sub.2(S.sub.2COEt).sub.2] within the products or the supernatants, even with the short reaction times used at most temperatures (e.g. 3 minutes at 250 C.). The only exception was at 3 minutes at the lowest temperature studied (200 C.; sample 1). The supernatant in this case contained a small amount of the unreacted precursor, giving it a brown hue. In methanolic suspensions, all 1H-MoS.sub.2@oleylamine samples consisted of black flocculates. Once isolated and dried most of the products were obtained as brittle solids, although the inventors found that a significant increase in both the reaction time and temperature could lead to the isolation of greasier materials (i.e. 16, 19 and 20; see Table 1).

[0154] The nature of oleylamine coordination in all 1H-MoS.sub.2@oleylamine products was determined by (ATR) FT-IR spectroscopy. A number of signals indicated the presence of oleylamine (2850-3000 cm.sup.1, 1647 cm.sup.1 and 1468 cm.sup. for v(CH), v(CC) and (CH) modes, respectively), but the absence of a signal at 3319 cm.sup. and the significantly reduced peak at 1560 cm.sup. (representative of v[NH] and [HNH] of free oleylamine, respectively) is noted.

[0155] These observations have previously been used as an indicator for oleylamine capping in a variety of nanoparticles,.sup.[2] as well as for MoS.sub.2 nanosheets,.sup.[3] and implies that the oleylamine present is chemically bound to the 1H-MoS.sub.2 nanosheet.

[0156] TEM analysis shows that all of the 1H-MoS.sub.2@oleylamine products consist of small MoS.sub.2 nanosheets which form highly disordered, aggregated structures. These flocculates typically have lateral dimensions from 100's to 1000's of nm and are commonly found to both adhere to and mould around the carbon film on lacey carbon TEM grids (FIG. 1). On performing high resolution TEM imaging of the flocculates (FIG. 2a-b), it is clear that the MoS.sub.2 nanosheets are randomly oriented; with the strongest phase contrast observed for nanosheets with their basal planes oriented parallel to the incident electron beam..sup.[3,4] The dimensions of the MoS.sub.2 nanosheets within each of the 1H-MoS.sub.2@oleylamine samples was estimated by statistical analysis of the basal plane dimensions observed for side-on monolayer nanosheets seen in the TEM images (sample size in each study: N=40). This analysis revealed that the lateral sizes of the nanosheets can be controlled by the selection of reaction temperature (Table 1 and FIG. 3a). The low temperature reactions at 200 and 250 C. produced MoS.sub.2 nanosheets within the 1H-MoS.sub.2@oleylamine with an approximate lateral size of 4.5-5 nm, whereas the gradual increase of the reaction temperature above 250 C. promoted the growth of larger nanosheets of up to an average of ca. 11.5 nm at 325 C. These observations suggest a non-classical crystal growth mechanism is prevalent in the formation of the MoS.sub.2 nanosheets..sup.[5] In all cases, the deviation of the nanosheets measured never exceeds 15% of the mean nanosheet length, showing a significantly increased level of control in the growth of the nanoscale-MoS.sub.2 monolayers, compared to other known processes where little-to-no control is observed.PA Nanosheet sizes appear to be unaffected by the reaction times employed; a survey of the aliquots obtained from the same hot injection reactions at 3 and 20 minutes intervals showed no significant size variations, suggesting that in all samples the nanosheet growth process is complete in under 3 minutes.

[0157] A probe side aberration-corrected STEM was used to perform high resolution annular dark field (ADF) imaging of the flocculate structure for sample 19 (synthesised at 325 C. for 12 minutes). The atomic resolution ADF images in FIG. 4 support the microstructures seen in the TEM images, showing structures comprised of large numbers of randomly oriented MoS.sub.2 nanosheets. STEM imaging of side-on MoS.sub.2 nanosheets allows precise determination of the number of layers in an individual flake,.sup.[6] the side-on flakes seen in our atomic resolution images show no multilayer structures. The Fourier transforms (FTs) of the atomic resolution images show the 0.27 nm spacing of the (100) planes (insert in FIG. 4a) but there is was no evidence of the considerably larger (002) interlayer spacing (0.62 nm) expected for bi- and multilayer structures. It is therefore believed that the flocculates are comprised exclusively of monolayer MoS.sub.2 nanosheets; multilayer flakes either are extremely rare or entirely absent from these samples. This observation is consistent with the TEM selected-area electron diffraction patterns (SAED) and the p-XRD patterns, which both display highly broadened bands for the (100) and (110) crystal planes of MoS.sub.2 in the 1H-phase (in addition to a broadened signal at approx. 20 for the reflections of the glass substrate in the p-XRD spectra; FIGS. 2a (insert), 2b (insert) and 3b). There were no discernible bands corresponding to the (002) reflection at ca. 14 from either diffraction experiment..sup.[7]

[0158] In ADF STEM images of sample 19, occasional flakes were favourably oriented with their basal planes normal to the optic axis allowing them to be imaged with atomic resolution. Even within relatively small scan areas (for example the 2525 nm area shown in FIG. 5) FTs of the atomic resolution images revealed ring like patterns characteristic of a polycrystalline material (with ring radius corresponding to the 0.27 nm d-spacing of the {100} planes), as opposed to the distinct spot patterns present when imaging individual isolated nanocrystals. Closer inspection of the images shows small nanosheets randomly oriented with respect to their neighbours and often overlapping one another. The lateral dimensions of the sheets seen in these images are consistent with the sizes determined from TEM imaging.

[0159] The STEM was also used to perform energy dispersive X-ray (EDX) spectrum imaging on flocculates, allowing chemical composition to be probed with nanometre resolution. FIG. 6 shows a spectrum image of a typical region of flocculate from sample 19. The resulting elemental maps reveal homogeneous distributions of Mo and S. It should be noted that the S K (2.31 keV) and Mo L (2.29 keV) peaks overlap making deconvoltion on a pixel by pixel basis challenging. The summed EDX spectra suggests that the MoS.sub.2 is pure, with all other elements seen in the spectrum associated with the TEM support (C, Si, O, Cu). Quantification of the summed spectra using a standardless Cliff-Lorimer approach supports the expected Mo:S stoichiometry of 1:2.

[0160] The only defined Raman-peaks in all samples were that of the A.sub.1g and E.sub.2g bands of MoS.sub.2; no other identifiable signals were observed in the 200-1000 cm.sup.1 range. This supports the expected decomposition mechanism of such xanthate-bearing complexes to MoS.sub.2, even in the presence of oxo-groups (FIG. 7)..sup.[8] Raman spectroscopy of large MoS.sub.2 nanosheets (lateral dimensions >100 nm) is regularly used to estimate nanosheet thicknesses of these materials, as the A.sub.1g and E.sub.2g bands are known to exhibit a well-defined dependence on layer thickness..sup.[9] However, Raman analysis of 1H-MoS.sub.2@oleylamine does not show the expected peak separation of 18 cm.sup. for single layer MoS.sub.2, instead showing band separations which depend upon the lateral sizes of nanosheets in the 1H-MoS.sub.2@oleylamine (FIG. 3c-d and Table 1). The peak separation from the samples obtained at 200 and 250 C. (average nanosheet size measured by TEM 4.8 nm) was approximately 24 cm.sup.1. This separation narrowed upon increasing reaction temperature, falling to ca. 22 cm.sup.1 for samples prepared at 325 C. (average nanosheet size measured by TEM 11.3 nm). The expansion of the A.sub.1g to E.sub.2g bands separation, as a consequence of the lateral dimensions of single-layer nanosheets being 100 nm, is thought to occur due to the quantum confinement of the crystal structure within the 2D-plane. This phenomenon has previously been observed in both MoS.sub.2 nanosheets and fullerene-like nanoparticles..sup.[10]

[0161] To confirm both the purities and the compositions of the products, the dried 1H-MoS.sub.2@oleylamine samples were subjected to TGA (10 C./min, up to 600 C. in 1 atm. air; an example thermogram is shown in FIG. 8). All the thermograms obtained display the same three stages of decomposition, previously described by Altavilla et al:.sup.[3] Stage 1 (30-360 C.)the oxidation of surface sulfur impurities on the 1H-MoS.sub.2@oleylamine, Stage 2 (360-475 C.)the decomposition of physisorbed oleylamine, Stage 3 (475-580 C.)the decomposition of chemisorbed oleylamine and the oxidation of MoS.sub.2. The remaining residue at the end of each stage (termed m.sub.Tn) were: 1H-MoS.sub.2@oleylamine and physisorbed oleylamine at 360 C. (m.sub.T1), 1H-MoS.sub.2@oleylamine at 475 C. (m.sup.T2) and MoO.sub.3 at 580 C. (m.sub.T3).

[0162] The inventors have devised a simplified set of calculations to approximate both the purities and the component ratios of the 1H-MoS.sub.2@oleylamine products from their TGA data. This is the first time this class of materials have been compositionally analysed to such a level. The purity of the isolated materials were determined simply from the residual mass of the residues at 475 C. (m.sub.T2) with respect to the initial mass, whereas to calculate the composition of 1H-MoS.sub.2@oleylamine the inventors have simplified the calculations to Equation 1 (detailed calculations shown in SI, the values obtained are in Table 1):

[00001]$\begin{array}{cc}1.Math.H-{\mathrm{MoS}}_2@{\mathrm{oleylamine}}_x,.Math.\mathrm{where}.Math..Math.x=0.545.Math.\frac{m_{T_2}}{m_{T_2}}-0.605& \left(1\right)\end{array}$

[0163] From the calculations, the 1H-MoS.sub.2@oleylamine products produced from the 200, 250 and 275 C. reactions were reasonably pure (in the region of 68-75%; the impurities consisting of surface sulfur adatoms and physisorbed oleylamine), with a composition of MoS.sub.2.Oleylamine.sub.0.28-0.32. Similar purities and compositions of the 1H-MoS.sub.2@oleylamine products were observed for the 300 and 325 C. reactions at the shorter reaction times, but prolonging the reactions was found to increase the amount of chemisorbed oleylamine, as demonstrated by samples 16, 19 and 20, probably contributing to the oily appearance of the products. These factors resulted in a significant decrease of overall purity due to an increase of both surface sulfur impurities and physisorbed oleylamine present within the greasier materials formed at longer reaction times.

[0164] To demonstrate the applicability of this material for use in electrochemical energy storage applications, symmetrical coin-cell type (CR2032) supercapacitors were constructed using a composite of the 1H-MoS2@oleylamine (flake size approx. 8 nm) combined with graphene as a conductive additive to overcome the inherent resistivity of the semiconducting MoS2 flakes, and analysed using best practice methods..sup.[11] The oleylamine was removed from the MoS.sub.2 first by thermal annealing (500 C.), the resulting crystals were re-dispersed in an organic solvent (N-methyl-2-pyrrolidone, NMP) and combined with a graphene dispersion, also prepared by liquid-exfoliation, in a 1:1 (w/w) ratio. This method of graphene production is known to produce large amounts of few layer flakes (1-5 layers) with lateral dimensions of 1-5 m..sup.[12] This composite dispersion was then filtered through a polyvinylidene fluoride (PVDF) filter to form a supported membrane without the need of any additional polymeric binders that are typically used..sup.[13] The mass of active material was approximately 1 mg (mass loading of 1 mg/cm.sup.2) which produces a mechanically flexible and stable thin film with a thickness of 5 m. These composite membranes were then stacked together in a symmetrical coin cell arrangement, as demonstrated previously for ultrasonication exfoliated Mos.sub.2..sup.[14,15]

[0165] FIG. 9 shows schematically the design of the coin cell as well as a photograph of the MoS.sub.2/composite membrane and electrochemical response of the membrane using an aqueous electrolyte (1 M Na.sub.2SO.sub.4). In the optical microscope image (FIG. 9a), several larger graphite flakes are visible, and with further optimisation of the exfoliation the capacitance values could be further improved. In FIG. 9b the cyclic voltammetry (CV) at differing scan rates is shown. At low scan rates the CV curves exhibit the expected square shape of an ideal electrochemical double-layer capacitor (EDLC) with no discernible pseudocapacitance peaks; however as the scan rate increases the curves deviate from the ideal shape and this indicates a change in the charge storage mechanism to surface mediated ion adsorption..sup.[16, 17] FIG. 9c shows the galvanostatic discharge curves for the cell with increasing current densities, along with the calculated specific capacitance (C.sub.sp, FIG. 9c inset). The non-linearity of the discharge curve at higher current density indicates a deviation from ideal EDLC behaviour and can be attributed to surface ion adsorption as an alternate charge storage mechanism in agreement with the CV results. The maximum value of C.sub.sp was calculated to be 50.65 mF/cm.sup.2 (current density of 0.37 Ng); this compares impressively with previously reported results from ultrasonication exfoliated MoS.sub.2 which range between 3-14 mF/cm.sup.2..sup.[14,16,18] This large increase is attributed to the small MoS.sub.2 flake dimensions used in this synthesis method compared to solution exfoliated material, whose dimensions ranges from several hundred nanometres to microns..sup.[19] The small flake dimensions lead to a maximum in the available surface area, providing a high density of highly reactive edge sites which can increase the available sites for ion adsorption and accumulation on the surface..sup.[20] Combined with the small lateral dimensions the synthesized MoS.sub.2 nanosheets are exclusively monolayer, as discussed previously. Despite some restacking that will occur during filtration, the monolayer nature of the flakes will maximise the available surface area and provide a maximum specific capacitance per unit area when compared to thicker less well defined material. The decrease in C.sub.spwith increasing current density indicates that the charge storage mechanism of the MoS.sub.2/graphene composite is not purely a double-layer effect due to the internal resistance of the membrane. This is in agreement with the measured impedance response of the cell at high frequencies (FIG. 10). However, by optimising the ratio of graphene to MoS.sub.2 it may be possible to overcome this and maximise the power density while still maintaining the high energy density that the MoS.sub.2 composite provides.

[0166] Impedance spectroscopy is a powerful tool as it allows the user to determine what processes are occurring at the electrode-electrolyte interface, which is crucial in understanding device performance. Supercapacitors oscillate between two states depending on the frequency, ideally exhibiting resistive behaviour at high frequencies and capacitance at low frequencies..sup.[21] At low frequency the imaginary component of the complex impedance sharply increases tending towards a vertical line with a phase of 90, indicative of ideal double-layer capacitive behaviour. In the middle frequency range the response is dominated by the electrode porosity and diffusion of the electrolyte ions; in this range the thickness of the electrode layer causes a shift towards more resistive behaviour for thicker active material. While all of the power is dissipated at high frequency, where the cell behaves like a pure resistor, matching the inventors' observations of the impedance response of the cell.

[0167] While the foregoing description has focussed on MoS.sub.2 as the produced TDC, as described herein the invention encompasses other metals.

[0168] For example, the inventors have demonstrated the production of WS.sub.2 nanosheets as follows. The complexes of WS(S.sub.2)(S.sub.2CNR.sub.2).sub.2 (R.sub.2=Et.sub.2[1], =.sup.iPr.sub.2 [2], =MeHex [3]) were used in the hot injection reaction as described herein (300 C., 10 mins). The sizes of the nanosheets produced were imaged by TEM: [1]7.610.98 nm, [2]6.781.24 nm, [3]7.501.19 nm. All show signs of some bilayer sheets, but a significant increase in those seen in [3].

[0169] The inventors have further demonstrated the synthesis of ReS.sub.2 nanosheets. The complexes of Re(S.sub.3CNEt.sub.2)(S.sub.2CNR.sub.2).sub.3 [1] and Re.sub.2O.sub.3(S.sub.2CNEt.sub.2).sub.4 [2] was used in the hot injection reaction (300 C., 10 mins), resulting in the production of nanosheet like shapes (seen by TEM). The sizes of the nanosheets produced were imaged by TEM: [1]4.490.67 nm, [2]5.80 0.77 nm. All appear to be monolayer sheets, with no sign of bi- or multilayers.

[0170] As described herein, the invention also provides ternary structures. The inventors have demonstrated the applicability of the method to ternary structures such as (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine. As described herein, these may be produced by using a mixture of precursors.

[0171] By way of example, (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine samples were prepared by hot injection thermolysis. A mixture of [Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2] and [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].H.sub.2O (total 0.50 mmol metal content) in oleylamine was injected into hot oleylamine (Table 2). Reactions were carried out at temperatures ranging from 250 to 325 C. to produce dark-coloured suspensions. The reaction was quenched after 10 minutes, before isolating and purifying by repeated ethanol washing and centrifugation steps. In the binary reactions (i.e. the reaction of solely [Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2] and [W.sub.2S.sub.4(S2CNEt2).sub.2].H.sub.2O) the decomposition of the precursors occurs rapidly; there was no evidence of unreacted materials within the products or the supernatants after reacting for 4 minutes. Most of the dried MoS.sub.2- and WS.sub.2@oleylamine products were obtained as brittle solids, the only exception was for the MoS.sub.2@oleylamine produced at 325 C., which yielded a greasy material, similar to those observed in the formation of MoS.sub.2@oleylamine. However, the WS.sub.2@oleylamine produced at the same temperature was found to be a non-greasy, brittle solid. In turn, the ternary (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine samples, prepared by the decomposition of mixtures of [Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2] and [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].H.sub.2O at 250-325 C., also gave brittle dark-coloured solids.

[0172] To determine the metal content in the (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine produced, inductively coupled plasma optical emission spectrometry (ICP-OES) was utilised. ICP-OES found that the metal content of the products had a Mo-to-W ratio that closely matches that of the initial precursor ratios used in the reaction, with a maximum variation of only x<0.05; Runs 1-4 and 17-20 showed exclusively the native metals employed, with the runs 5-8, 9-12 and 13-17 giving compositions of approximately 0.75:0.25, 0.50:0.50 and 0.25:0.75 (w.r.t. the Mo/W ratio), respectively. There appears to be a slight variation in the composition, depending on the temperature employed: at 250 C., the materials produced appeared to be slightly molybdenum richan indication that the tungsten precursor may not decompose completely in the reaction. On the other hand, at 325 C. the Mo/W ratios are the closest to the expected value, indicating a homogeneous decomposition process with the two precursors.

[0173] TEM analyses show that the binary MoS.sub.2@oleylamine (Runs 1-4) and WS.sub.2@oleylamine (Runs 17-20) materials consist of small MS.sub.2 nanosheets which form highly disordered, aggregated structures that are 100's to 1000's of nm in size. High resolution TEM imaging show the expected randomly oriented monolayer MoS.sub.2 and WS.sub.2 nanosheets within the aggregates; the strongest phase contrasts were observed for nanosheets with their basal planes oriented parallel to the incident electron beam.

[0174] The dimensions of the MS.sub.2 nanosheets within each of the MS.sub.2@oleylamine samples was estimated by statistical analysis of the basal plane dimensions observed for side-on monolayer nanosheets seen in the TEM images shown in FIGS. 11 and 12 (sample size in each study: N=40). The lateral sizes of the nanosheets produced is dictated by the reaction temperature, with higher temperatures producing larger MoS.sub.2 and WS.sub.2 nanosheets (7.72 and 10.56 nm, respectively at 325 C.) than those at 250 C. (4.03 and 4.17 nm, respectively). In general, the WS.sub.2 nanosheets are slightly larger than the MoS.sub.2 nanosheets, all other things being equal. The non-classical crystal growth process observed follows that seen in the hot-injection of Mo.sub.2O.sub.2S.sub.2(S.sub.2COEt).sub.2 In most cases, the deviation of the nanosheets measured never exceeds15% of the mean nanosheet length (in the case of [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].H.sub.2O at high temperatures (325 C.) the lateral dimensions deviates by a little more: up to 25%).

[0175] In addition, the images of the WS.sub.2@oleylamine prepared at 275, 300 and 325 C. (Runs 6, 7 and 8, respectively) show that an increasing amount of bilayer nanosheets present. The interlayer spacings of ca. 0.68 confirm that the bilayers (and any other multilayers) are stacked in the absence of an oleylamine intercalatant layer.

[0176] Statistical analyses of the dimensions in (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine (Runs 5-16) were also carried out. The materials produced in runs 5-8 (with a Mo:W precursor loadings of 0.75:0.25) follow the observations from the binary materials, with a gradual increase of nanosheet size when higher temperatures were employed. In the cases of runs 9-12 (Mo:W ratio 0.5:0.5) and 13-16 (Mo:W ratio 0.25:0.75) the growth of the nanosheets do not linearly increase with increasing reaction temperatures; the lateral dimensions of the nanosheets produced at 325 C. are smaller than those produced at 300 C. A small but non-negligible number of bilayer sheets was also observed in both ratios at higher temperature (325 C.).

[0177] Atomic resolution high angle annular dark field (HAADF) scanning transmission electron microscope (STEM) imaging shows crystalline monolayer flakes with W atoms directly substituted into Mo lattice sites in the 1H-MoS.sub.2 crystal structure (FIG. 13). The contrast mechanism in HAADF STEM imaging is strongly dependent on atomic number (Z). Consequently, in monolayer regions of 2D materials, atoms with different Z are distinguishable by atomic resolution HAADF STEM imaging. Due the significant difference in atomic numbers of Mo and W (Mo=42, W=74) the two elements can be clearly distinguished with W atoms appearing significantly brighter (FIG. 14). The bright W atoms appear to be randomly distributed across the flakes imaged, showing no evidence of clustering. Due to the contrast difference between Mo and W it is possible to determine the Mo:W ratio of individual flakes by atom counting. 10 regions of monolayer material were identified in images of sample 8 and their composition quantified by atom counting; in total 1501 atoms were counted revealing 25.98% W substitution, a value that is close to that found by bulk characterisation of the same sample (ca. 22%). Substitution levels show some inhomogeneity on a flake-by-flake basis, with the flakes measured ranging in composition from 18.5% to 32% W, such a spread in compositions is unsurprising given the small lateral dimensions of the flakes investigated. Quantitative energy dispersive X-ray (EDX) spectroscopy of the same sample reveals compositions in good agreement with the atom counting results, showing 25% W inclusion. EDX spectrum imaging of aggregated regions of flakes showing homogeneous co-localisation of Mo and Won the sub-10 nm level.

[0178] Thin films were prepared by drop-casting MS.sub.2@oleylamine dispersions onto glass substrates. Grazing incidence-XRD of films of all of the MS.sub.2@oleylamine samples, irrespective of the Mo/W ratio, displayed diffraction patterns that closely resemble each other: all spectra display highly broadened bands for the (100) and (110) crystal planes of the layered TMDC in the 1H-phase (FIG. 15). The spectra of runs 12, 16, 18, 19 and 20 show an additional, poorly-defined band at approx. 14, corresponding to the interlayer MS.sub.2 (002) band. This confirms the presence of some bilayer structures observed in these samples via TEM.

[0179] To compare the catalytic behaviour of the different compounds (Mo.sub.xW.sub.1-x)S.sub.2 dispersions were produced, after removal of the oleylamine by annealing and re-dispersion in NMP by ultrasonication. These different dispersions were then diluted in isopropanol before drop casting onto a glassy carbon electrode for hydrogen evolution reactions (HER). HER electrocatalysis was performed in constantly stirred and thoroughly degassed aqueous 1 M H.sub.2SO.sub.4 with differing catalyst loadings and compared to the performance of the bare glassy carbon and a platinum mesh. A silver/silver chloride reference electrode was used and the potentials have been corrected to the SHE, no iR compensation was used. To maximise the number of exposed catalytically active edge sites and to minimise flake restacking very low mass loadings were used (0.1 g/cm.sup.2). Changing of the mass loadings was done by taking 10 l aliquots of the diluted (Mo.sub.xW.sub.1-x)S.sub.2 dispersions and repeatedly drop casting onto the glassy carbon electrode and leaving to dry in air. The mass loadings used were determined from the absorbance spectroscopy of the starting dispersions and subsequent dilution. The bare glassy carbon electrode displayed poor catalytic performance with overpotential (q) of 400 mV, compared to the platinum mesh which is known to be an excellent HER catalyst with of 40 mV. After drop casting of the (Mo.sub.xW.sub.1-x)S.sub.2flakes there was a significant improvement in electrocatalytic performance compared to the bare glassy carbon, even for the low catalyst loadings. Of the deposited TMDC materials the lowest n was the pure MoS.sub.2, while the highest was the pure WS.sub.2, and each of the differing compositions were evenly spread between these depending on their Mo content. Table 3 shows the n values for each of the different (Mo.sub.xW.sub.1-x)S.sub.2dispersions, as well as the Tafel slopes, and the measured current densities at 0.6 V. At potentials much greater than the there is an increasing current density with Mo content, with the ratio of current increase matching closely to the stoichiometric ratio of the Mo determined earlier. The electrocatalytic activity of these alloyed materials is similar to recently demonstrated MoS.sub.2/WS.sub.2 heterostructures which were produced by a CVD process.

TABLE-US-00001 TABLE 3 Overpotential, calculated Tafel slope, and current density of the bare glassy carbon and platinum as well as for each of the nanoflake- modified electrodes. Current density Overpotential Tafel slope @ 600 mV Sample (, mV) (mV/dec) (A/cm.sup.2) Glassy carbon 400 290 9.44 3 (MoS.sub.2) 250 187 107.8 7 (Mo.sub.0.77W.sub.0.23S.sub.2) 270 200 93.7 10 (Mo.sub.0.55W.sub.0.45S.sub.2) 280 206 63.9 15 (Mo.sub.0.77W.sub.0.73S.sub.2) 290 223 56.6 17 (WS.sub.2) 300 198 43.2 Platinum 40 31

[0180] Before Raman spectroscopic analyses, restacked films of (Mo.sub.xW.sub.1-x)S.sub.2 were prepared by the annealing a small amount of MS.sub.2@oleylamine in N.sub.2 at 500 C., to remove the oleylamine ligand that often reduces the quality of the Raman spectrum. Raman spectroscopy of binary WS.sub.2 (at all temperatures) possess two major bands at ca. 353 and 419 cm.sup.1, corresponding to the E.sub.2g and A.sub.1g bands. Similarly, the Raman spectra for the MoS.sub.2 analogues gave two bands at ca. 381 and 405 cm.sup.1, which can be assigned to the E.sub.2g and and A.sub.1g optical modes, respectively. Raman spectroscopy was also used to investigate the ternary (Mo.sub.xW.sub.1-x)S.sub.2@oleylamine produced from mixtures of [Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2]and [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].H.sub.2O (FIG. 16). All of the ternary materials display a single band for the

[0181] A.sub.1g phonon, alongside two phonon bands of E.sub.2g symmetry. The dependence of the Raman shift for the three prominent bands in all films was plotted as a function of Mo content (mole fraction x), as found by ICP-OES (FIG. 16 right). The observation of these bands correlate well with the Raman modes observed for (Mo.sub.xW.sub.1-x)S.sub.2 thin films, produced by AACVD.

[0182] Metal or Metalloid Ion Doped Nanosheets The following representative example is directed to MoS.sub.2 nanosheets doped with transition metal ions (derived from the chloride salt). It will be appreciated that these are provided by way of illustration and are not intended to limit the invention or disclosure herein.

[0183] (TM)-doped MoS.sub.2@oleylamine samples were prepared by hot injection thermolysis, whereby a mixture of Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2 and the selected MCl.sub.2 dopant (total 0.75 mmol metal content) in oleylamine was injected into hot oleylamine. Reactions were carried out at the optimised temperature of 300 C. to produce dark-coloured suspensions which could be isolated as brittle solids. The reaction results in the formation of the target nanomaterials within a sulfur-rich environmentconditions which are thought to promote the substitutional doping of an Mo centre with a TM one. The inventors produced substitutional-doped MoS.sub.2 nanosheets (based on the information provided herein).

[0184] ICP-OES confirmed that the Mo-to-(TM) ratios in all of the (TM)-doped MoS.sub.2@oleylamine coincide with the initial precursor ratios used in the reaction. In addition, all of the samples were found to contain a metal-to-sulfur ratio of 1:2, supporting the MoS.sub.2-nature of the nanosheets.

[0185] TEM analyses show that all of the ca. 12% (TM)-doped MoS.sub.2@oleylamine samples consist of small MoS.sub.2 nanosheets which form highly disordered, aggregated structures that are 100's to 1000's of nm in size. In addition, there was no evidence of any other forms of nanomaterials, suggesting there are no (TM)S.sub.x-based nanomaterial impurities in the flocculates. High resolution TEM imaging shows that within these aggregates, the expected randomly oriented monolayer MoS.sub.2 nanosheets are prevalent (FIG. 17). Statistical analysis of the doped-MoS.sub.2 nanosheets within the samples (sample size in each study: N=40) found that in most cases, the nanosheets were monolayer and with lateral dimensions in the region of 5.5-6.0 nmconsistent with the results found in the assessment of undoped MoS.sub.2@oleylamine. The exception to the above was for the 12% Cu-doped MoS.sub.2@oleylamine, which found that the nanosheets were smaller (average lateral dimension of ca. 5.0 nm), but importantly found to contain significant amounts of bilayer and multilayer sheets. The interlayer separation in these sheets were found to be ca. 0.67 nm, consistent with the formation of an intercalatant-free multi-layered crystal.

[0186] 12% Co-doped MoS.sub.2@oleylamine was studied by high angle annular dark field (HAADF) scanning transmission electron microscope (STEM) imaging, and energy dispersive X-ray (EDX) spectrum imaging. Low magnification HAADF STEM images revealed aggregates of randomly oriented flakes, similar to those observed for un-doped MoS.sub.2@oleylamine. Flakes lying with their basal planes parallel to the electron beam appear bight, such flakes are found to be monolayers with lateral dimensions of 8 nm or less. Higher magnification HAADF STEM images of flakes lying with their basal plane's perpendicular to the electron bean showed the expected hexagonal 1H-MoS.sub.2crystal structure, the extent of organic contamination (deriving from oleylamine) limits the quality of atomic resolution images, this makes it challenging to distinguish Mo and Co atoms in such images. To confirm uniform Co alloying STEM EDX spectrum imaging was performed on the MoS.sub.2@Oleylamine aggregates, the resulting elemental maps demonstrate nm scale co-localisation of Co, Mo, and S, with no evidence of Co rich or deficient regions seen. These facts support the conclusion that Co-introduction into the MoS.sub.2 nanosheets produced a truly alloyed material, and not the formation of CoS.sub.x cluster or nanoparticles.

[0187] Before Raman spectroscopic analyses, restacked (TM)-doped-MoS.sub.2 was prepared by the annealing a small amount of the (TM)-doped-MoS.sub.2@oleylamine materials onto a Si substrate at 500 C. in a vacuum, to remove the oleylamine ligand that can often reduce the quality of the spectra obtained. Analyses of the (TM)-doped-MoS.sub.2 display the same E.sub.2g and A.sub.1g bands as seen in binary MoS.sub.2. However the band separation is dependent on both the metal dopant and dopant concentration; the largest separation was found to be over 30 cm.sup.1 with 12% Co-doping (FIG. 18). Reasoning for the increase in the band separations can be rationalised using the Co-doped MoS.sub.2 as an example; the shift of the E.sub.2g band thought to be as the composite E.sub.2g vibrational modes of the 1H-MoS.sub.2 and the structurally-confined 1 H-CoS.sub.2 (381 and 374 cm.sup.1, respectively).

[0188] Grazing incidence-XRD of the TM-doped MoS.sub.2@oleylamine thin films (prepared by the drop-casting of (TM)-doped MoS.sub.2@oleylamine dispersions onto a glass substrate) display diffraction patterns that closely resemble each other: Highly broadened bands for the (100) (accompanied by a shoulder corresponding to the (103) plane) and (110) crystal planes of the layered TMDC in the 1H-phase are seen. Closer inspection all of the (TM)-doped MoS.sub.2@oleylamine exhibits shifts in the (100) and (110) bands to lower 20 values, compared to the undoped MoS.sub.2@oleylamine (FIG. 19). These small but non-negligible changes suggests that the MoS.sub.2 crystal unit cell expands along the xy-plane. In general this unit cell expansion correlates with increasing dopant concentrations.

[0189] The magnetisation versus applied magnetic field curves of 12% TM-doped MoS.sub.2@oleylamne at 2K were investigated. All curves show typical ferromagnetic behaviour. The saturation magnetisation of pure MoS.sub.2@oleylamine was 0.056 emu/g: higher than previously reported values of freestanding MoS.sub.2 sheets (0.0025 and 0.0011 emu/g at 10 and 300 K). This higher saturation magnetisation is possibly due to the relatively smaller lateral sheet dimensions that have been shown to increase the ferromagnetism of few-layer

[0190] MoS.sub.2 sheets, or the generation of MoS.sub.2 nanosheets with a higher concentration of exposed zig-zag edges. Upon doping with various transition metals, the saturation magnetisation increases linearly with dopant concentration in Mn, Fe, Co and Ni whilst Cu and Zn doping has a negligible effect. Mn-doping had the highest saturation magnetisation (2.8 emu.g.sup.1@ 10%-doping), followed by Fe (0.75 emu.g.sup.1@14%), Ni (0.63 emu.g.sup.1@14%), Co (0.44 emu.g.sup.1@14%), Cu (0.12 emu.g.sup.1@ 12%) and Zn (0.04 emu.g.sup.1@10%); reflecting the trend of unpaired electrons, and hence total magnetic moment, of 2+ transition metals. Doping concentration studies in (TM)-doped MoS.sub.2 also found that the magnetisation of the materials linearly increased with increasing TM-content in the TM-doped MoS.sub.2@oleylamine. This suggests that the degree of magnetisation in the produced nanosheets can be controlled by the simple control of dopant concentration.

EXAMPLES

[0191] Methods: Elemental analyses were performed using a Thermo Scientific Flash 2000 Organic Elemental Analyser by the microanalytical laboratory at the University of Manchester.

[0192] Thermogravimetric analysis measurements were carried out by a Seiko SSC/S200 model under a heating rate of 10 C. min.sup. in both nitrogen and atmospheric conditions. Raman spectra were acquired on a Renshaw 1000 system, with a solid state (50 mW) 514.5 nm laser (operating at 10% power). The laser beam was focused onto the samples by a 50 objective lens. The scattered signal was detected by an air cooled CCD detector. Approximately 5 mg of the 1H-MoS.sub.2@oleylamine dispersed in toluene was drop cast onto a glass substrate for p-XRD studies, performed on a Bruker AXS D8-Advance diffractometer, using Cu K radiation. The thin film samples were mounted flat and scanned over the range of 10-80 . FT-IR spectra were obtained by a Thermo Fisher Nicolet iS5 spectrometer equipped with an ATR cell. Samples for transmission electron microscopy (TEM) were prepared from dilute 1H-MoS.sub.2@oleylamine dispersions in toluene (which were sonicated for 5 minutes) by drop casting onto holey carbon support films which were then washed with toluene and air dried. Bright field images and selected area electron diffraction (SAED) patterns were obtained using a Philips CM20 TEM equipped with a LaB6 electron source and operated at 200kV. STEM imaging and EDX analysis was performed in a probe-side aberration corrected FEI Titan G2 80-200 ChemiSTEM microscope operated at 200 kV equipped with the Super-X EDX detector with a total collection solid angle of 0.7 srad. For ADF imaging a probe current of 75 pA, convergence angle of 21 mrad and a detector inner angle of 28 mrad were used. EDX spectrum images were acquired with the sample at 0 tilt and with all four of the ChemiSTEM SDD detectors turned on. STEM images were recorded in FEI TIA software and EDX data was recorded and analysed using Bruker Esprit, quantification of EDX spectra was performed using the Cliff-Lorimer method (using the S K-series (2.31 keV) and Mo K-series (17.48 keV) and adsorption correction (assuming the flocculate has a density of bulk MoS.sub.2 (5.06 cm.sup.3) and thickness of 150 nm). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) were performed using a PGSTAT302N potentiostat (Metrohm Autolab, The Netherlands). All electrochemical measurements were performed in a sealed symmetrical coin cell (CR2032) using an aqueous electrolyte (1M Na.sub.2SO.sub.4). The membranes were stacked back-to-back within the coin cell with the active material making direct contact with the current collector. EIS was performed at a frequency range of 0.1 Hz to 100 kHz with a 10 mV (RMS) perturbation and 0 V dc bias. Specific capacitance was calculated using the established best practice..sup.[22]

[0193] Synthesis of [Mo.sub.2O.sub.4(S.sub.2CNEt.sub.2).sub.2]

[0194] The synthesis of [Mo.sub.2O.sub.4(S.sub.2CNEt.sub.2).sub.2] was modified from that described in literature..sup.[23] In a nitrogen environment, MoCl.sub.5 (5 g, 18 mmol) was carefully added to degassed H.sub.2O (80 mL). The resulting solution was cooled to 5 C. before the removal of volatile gases (mainly HCl) by vacuum evacuation for 1 hour. After the reintroduction of nitrogen, the reaction was warmed to room temperature before a solution of NaS.sub.2CNEt.sub.2.3H.sub.2O (4.1 g, 18.2 mmol) in degassed methanol (225 mL) was added slowly and heated to reflux for 30 minutes. The resulting yellow precipitate was filtered, washed with a H.sub.2O/EtOH solution (1:3, 275 mL) and dried in a vacuum overnight to give pure [Mo.sub.2O.sub.4(S.sub.2CNEt.sub.2).sub.2] as a yellow powder (6.75 g, 12.2 mmol, 68%). Anal. calcd for C.sub.10H.sub.20Mo.sub.2N.sub.2O.sub.4S.sub.4: C 21.74, H 3.65, N 5.07, S 23.17; found: C 21.97, H 3.51, N 5.05, S 23.30.

[0195] Synthesis of [Mo.sub.2O.sub.2S.sub.s(S.sub.2CNEt.sub.2).sub.21]

[0196] The synthesis of [Mo.sub.2O.sub.2S.sub.s(S.sub.2CNEt.sub.2).sub.2] follows the procedure described in literature..sup.[23] Yield 1.01 g (1.73 mmol, 80%) Anal. calcd for C.sub.10H.sub.20Mo.sub.2N.sub.2O.sub.2S.sub.6: C 20.57, H 3.45, N 3.45, S 32.85; found: C 20.69, H 3.48, N 4.74, S 32.85.

[0197] Synthesis of [Mo.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2]

[0198] Complex [Mo.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2] was synthesised by two separate routes:

[0199] The first method was modified from that described in the literature..sup.[24] In a dry nitrogen environment, [Mo2O4(S2CNEt2)2] (3 g, 5.44 mmol) and P4S10 (1.2 g, 2.72 mmol) were added to p-xylene (150 mL), before heating to reflux for 3 hours. The solution was then hot-filtered and the filtrate cooled to room temperature, yielding an orange-red microcrystalline powder. The powder was filtered and washed with cold toluene (230 mL) and dried in a vacuum overnight to give [Mo.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2] as an orange-red powder (1.31 g, 2.12 mmol, 39%). Anal. calcd for C.sub.10H.sub.20Mo.sub.2N.sub.2S.sub.8: C 19.50, H 3.27, N 4.55, S 41.53; found: C 19.33, H 3.11, N 4.61, s 41.09.

[0200] The second method follows the procedure described in literature..sup.[25] Yield2.9 g (4.7 mmol, 61%). Anal. calcd for C10H20Mo.sub.2N.sub.2Ss: C 19.50, H 3.27, N 4.55, S 41.53; found: C 19.61, H 3.31, N 4.53, S 41.98.

[0201] Synthesis of [Mo.sub.2O.sub.2S.sub.2(S.sub.2COEt).sub.2]

[0202] The procedure used was modified from that described in literature..sup.[26] In a dry nitrogen environment, a slow stream of H.sub.2S was bubbled through a solution of [Mo.sub.2O.sub.3(S.sub.2COEt).sub.4] (5.6 g, 7.7 mmol) in dry chloroform (250 mL) for two hours. The reaction was sealed in the H.sub.2S-rich environment and stirred overnight. After careful removal of volatile gases, the solvent was evaporated by vacuum to leave a dark brown powder. The by-products were removed from the solids by acetone extraction (2100 mL) and filtration to give an orange powder. The powder was washed with acetone (250 mL) and dried in a vacuum to give pure [Mo.sub.2O.sub.2S.sub.2(S.sub.2COEt).sub.2] as an orange powder (3.0 g, 5.6 mmol, 73%). Anal. Calcd. for C.sub.6H.sub.10MoO.sub.4S.sub.6: C 13.68, H 2.33, S 36.00; found: C 13.59, H 1.90, S 36.00.

[0203] Synthesis of [Mo.sub.2S.sub.4(S.sub.2COEt).sub.2]

[0204] The synthesis of [Mo.sub.2S.sub.4(S.sub.2COEt).sub.2] was modified from that described in literature..sup.[27] In a dry nitrogen environment, a slow stream of H.sub.2S was bubbled through a solution of [Mo.sub.2O.sub.3(S.sub.2COEt).sub.4] (10 g, 13.8 mmol) in a toluene-ethanol solvent mixture (4:1, 250 mL) for two hours. The reaction was sealed in the H.sub.2S-rich environment and stirred overnight. The dark-brown precipitate was filtered, washed with petroleum ether (3100 mL) and dried in a vacuum to give pure [Mo.sub.2S.sub.4(S.sub.2COEt).sub.2] as a dark brown solid (3.9 g, 7.0 mmol, 51%). Anal. calcd for C.sub.6H.sub.10MoO.sub.2S.sub.8: C 12.82, H 1.79, S 45.53; found: C 12.58, H 1.71, S 45.04.

[0205] 1H-MoS.sub.2@Oleylamine Synthesis by Hot Injection-Thermolysis

[0206] In a typical synthesis, a 200 mg solution of [Mo.sub.2O.sub.2S.sub.2(S.sub.2COEt).sub.2] in oleylamine (5 mL) was rapidly added to hot oleylamine (25 mL; reaction temperatures from 200 to 325 C.) under stirring. The solution turned a black colour and drops in reaction temperatures of 10-38 C. was observed; the reaction was kept at the lower temperature after addition. 9 mL aliquots were taken at regular intervals and added to methanol (35 mL), resulting in a flocculant-like precipitate. The black precipitate was separated by centrifugation (4,000 rpm for 20 minutes) and the supernatant removed. The precipitate was washed by repeated dispersion into 30 mL methanol and centrifugation before 1H-MoS.sub.2@oleylamine was finally dried in a vacuum for 16 hours.

[0207] Synthesis of [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].Monohydrate

[0208] An aqueous solution (300 mL) of [NH.sub.4].sub.2[WS.sub.4] (2.91 g, 8.36 mmol) and Na(S.sub.2CNEt.sub.2).3H.sub.2O (7.6 g, 33.77 mmol) was vigorously stirred whilst a 2M HCl solution was added dropwise until a pH2 solution was obtained. The addition initially produced a yellow precipitate, which eventually turned dark green with continual HCl addition. The resulting suspension was stirred for a further 30 minutes, before filtration, and the dark coloured precipitate was washed with water (3100 mL) and dried in a high-vacuum for an hour. The crude product was dissolved in acetone (250 mL), filtered and the precipitates washed with acetone (340 mL) to give a dark green solution and an orange-brown powder. The orange-brown powder was dried in a high vacuum to give pure W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2 (0.99 g, 1.25 mmol, 20.9%). In addition, the green solution can be stripped of its solvent by evaporation before drying in a high vacuum to give pure WS(S.sub.2)(S.sub.2CNEt.sub.2).sub.2 as a dark green powder (2.53 g, 4.39 mmol, 52.5%). Elemental analysis and other analytical data confirm purity, and cold storage (30 C.) prevented decomposition.

[0209] Thermogravimetric analysis (TGA) of [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].H.sub.2O showed that the hydrate ligand fully desorbs at 270 C. (trace not shown). The complex itself decomposes in three steps, from 316 to 421 C., with the final weight of the residues of 65.3% (at 600 C.), in close agreement to the predicted residual weights of two WS.sub.2 molecules (61.2%). The decomposition profile of [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].H.sub.2O is significantly cleaner than that of molybdenum analogue [Mo.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2]. [Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2] was selected as the molybdenum source for this experiment, as its decomposition profile was the best match. Naturally, other precursors (such as those described herein) may be used.

[0210] 1H-(Mo.sub.xW.sub.1-x), S.sub.2@Oleylamine Synthesis by Hot Injection-Thermolysis

[0211] In a typical synthesis, a 0.25 mmol of the total precursors (i.e. a mixture of x mmol [Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2] and [W.sub.2S.sub.4(S.sub.2CNEt.sub.2).sub.2].(H.sub.2O) in oleylamine (5 mL) was rapidly added to hot oleylamine (25 mL; reaction temperatures from 250 to 325 C.) under stirring. The solution turned a black colour and a drops in reaction temperature of 16-35 C. was typically observed; the reaction was kept at the lower temperature after addition. After 10 minutes the contents of the reactor was poured into 50 mL isopropanol and allowed to cool to room temperature, resulting in a flocculant-like precipitate. The resulting suspensions were diluted by half with methanol and the precipitates were separated by centrifugation (4,000 rpm for 20 minutes) and the supernatant removed. The precipitate was washed by twice dispersing into methanol (30 mL) and centrifugation and separation, followed by dispersion into acetone (30 mL) and a further centrifugation and separation step. The 1H-MoS.sub.2@oleylamine was finally dried in a vacuum for 16 hours.

[0212] The analogous synthesis of WS.sub.2 and ReS.sub.2 nanosheets is described earlier in the application.

[0213] Transition Metal Ion Doped Nanosheets

[0214] In a typical synthesis, an oleylamine solution (5 mL) containing a mixture of the metal complex (in this example, Mo.sub.2O.sub.2S.sub.2(S.sub.2CNEt.sub.2).sub.2) and (TM)Cl.sub.2 (TM=Mn, Fe, Co, Ni, Cu or Zn; in a 0.97:0.03, 0.94:0.06 or 0.88:0.12 molar ratio; total 0.75 mmol w.r.t metal atoms) was rapidly added to hot oleylamine (25 mL, 300 C.) under stirring. The solution turned a black colour and a drop in reaction temperatures of ca. 25 C. was observed; the reaction was kept at the lower temperature after addition. After 8 minutes the contents of the reactor was poured into 50 mL isopropanol and allowed to cool to room temperature, resulting in a flocculant-like precipitate. The resulting suspensions were diluted by half with methanol and the precipitates were separated by centrifugation (9,000 rpm for 20 minutes) and the supernatant removed. The precipitate was washed by twice dispersing into methanol (30 mL), centrifugation and separation, followed by dispersion into acetone (30 mL) and a further centrifugation and separation step. The (TM)-doped-MoS.sub.2@oleylamine was finally dried in a vacuum for 16 hours.

[0215] Electrochemistry

[0216] Graphene dispersions were produced by solution ultrasonication using previously reported methods..sup.[28] Briefly, graphite flakes were dispersed in N-methyl-2-pyrrolidone (10 mg/ml) and bath sonicated for 12 hours before centrifugation to remove any poorly exfoliated material. MoS.sub.2 dispersions were produced by first removing the oleylamine by thermal annealing (500 C., in N.sub.2), the resulting material was redispersed in NMP and combined with the graphene dispersion in a 1:1 ratio by weight. The concentration for the MoS.sub.2-NMP and graphene-NMP dispersions were determined by UV-Vis. Films of the MoS2 and graphene composite were synthesized by first diluting the NMP dispersions in isopropanol (IPA) by a factor of 20 followed by filtering through PVDF filters with 0.1 m pore size. The mass of active materials used on each membrane was 1 mg (1 mg/cm.sup.2).

[0217] It is to be understood that the examples and embodiments described herein are for illustrative purposes and that various modifications or changes in light thereof will be suggested to a person skilled in the art and are included in the spirit and scope of the invention and the appended claims.

[0218] The following references are cited in this application and are incorporated by reference for all purposes:

[0219] [1] C. De Mello Donega, P. Liljeroth, D. Vanmaekelbergh, Small 2005, 1, 1152.

[0220] [2] R. Huirache-Acua, F. Paraguay-Delgado, M. A. Albiter, J. Lara-Romero, R. Martnez-Snchez, Mater. Charact. 2009, 60, 932.

[0221] [3] C. Altavilla, M. Sarno, P. Ciambelli, Chem. Mater. 2011, 23, 3879.

[0222] [4] (a) L. Cheng, W. Huang, Q. Gong, C. Liu, Z. Liu, Y. Li, H. Dai, Angewandte Chemie 2014, 53, 7860. [14] E. Leite, C. Ribeiro, Crystallization and Growth of Colloidal Nanocrystals, Springer New York, 2012; (b) L. Cheng, C. Yuan, S. Shen, X. Yi, H. Gong, K. Yang and Z. Li, ACS Nano, 2015, 9, 11090.

[0223] [5] E. Leite, C. Ribeiro, Crystallization and Growth of Colloidal Nanocrystals, Springer New York, 2012.

[0224] [6] F. Withers, H. Yang, L. Britnell, A. P. Rooney, E. Lewis, A. Felten, C. R. Woods, V. Sanchez Romaguera, T. Georgiou, A. Eckmann, Y. J. Kim, S. G. Yeates, S. J. Haigh, A. K. Geim, K. S. Novoselov, C. Casiraghi, Nano letters 2014, 14, 3987; Y. Huafeng, W. Freddie, G. Elias, L. Edward, B. Liam, F. Alexandre, P. Vincenzo, H. Sarah, B. David, C. Cinzia, 2D Mater. 2014, 1, 011012.

[0225] [7] H. S. Matte, A. Gomathi, A. K. Manna, D. J. Late, R. Datta, S. K. Pati, C. N. Rao, Angewandte Chemie 2010, 49, 4059; K. H. Hu, X. G. Hu, Y. F. Xu, X. Z. Pan, React Kinet Mech Cat 2010, 100, 153.

[0226] [8] N. Savjani, J. R. Brent, P. O'brien, Chem. Vap. Depos. 2015, 21, 71.

[0227] [9] S. L. Li, H. Miyazaki, H. Song, H. Kuramochi, S. Nakaharai, K. Tsukagoshi, ACS nano 2012, 6, 7381.

[0228] [10] G. L. Frey, R. Tenne, M. J. Matthews, M. S. Dresselhaus, G. Dresselhaus, Phys. Rev. B 1999, 60, 2883.

[0229] [11] M. D. Stoller, R. S. Ruoff, Energ Environ Sci 2010, 3, 1294.

[0230] [12] Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. Mcgovern, B. Holland, M. Byrne, Y. K. Gun'ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari, J. N. Coleman, Nat. Nanotechnol. 2008, 3, 563.

[0231] [13] Z. N. Yu, L. Tetard, L. Zhai, J. Thomas, Energ Environ Sci 2015, 8, 702.

[0232] [14] M. A. Bissett, I. A. Kinloch, R. A. W. Dryfe, Adv. Energ. Mater. 2015.

[0233] [15] M. A. Bissett, I. A. Kinloch, R. A. W. Dryfe, ACS applied materials & interfaces 2015.

[0234] [16] J. M. Soon, K. P. Loh, Electrochem Solid St 2007, 10, A250; S. Patil, A. Harle, S. Sathaye, K. Patil, Crystengcomm 2014, 16, 10845, X. Cao, Y. Shi, W. Shi, X. Rui, Q. Yan, J. Kong, H. Zhang, Small 2013, 9, 3433.

[0235] [17] L. Cao, S. Yang, W. Gao, Z. Liu, Y. Gong, L. Ma, G. Shi, S. Lei, Y. Zhang, S. Zhang, R. Vajtai, P. M. Ajayan, Small 2013, 9, 2905; K. J. Huang, L. Wang, Y. J. Liu, Y. M. Liu, H. B. Wang, T. Gan, L. L. Wang, Int J Hydrogen Energ 2013, 38, 14027; E. G. Da Silveira Firmiano, A. C. Rabelo, C. J. Dalmaschio, A. N. Pinheiro, E. C. Pereira, W. H. Schreiner, E. R. Leite, Adv. Energ. Mater. 2014, 4, n/a.

[0236] [18] A. Winchester, S. Ghosh, S. Feng, A. L. Elias, T. Mallouk, M. Terrones, S. Talapatra, ACS applied materials & interfaces 2014, 6, 2125.

[0237] [19] J. N. Coleman, M. Lotya, A. O'neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. Mccomb, P. D. Nellist, V. Nicolosi, Science 2011, 331, 568.

[0238] [20] T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100.

[0239] [21] Taberna, P. L.; Simon, P.; Fauvarque , J. F. Electrochemical Characteristics and Impedance Spectroscopy Studies of Carbon-Carbon Supercapacitors. J. Electrochem. Soc. 2003, 150 (3), A292-A300.

[0240] [22] M. D. Stoller, R. S. Ruoff, Energ Environ Sci 2010, 3, 1294.

[0241] [23] A. Schultz, V. R. Ott, D. S. Rolison, D. C. Bravard, J. W. McDonald, W. E. Newton, Inorg. Chem. 1978, 17, 1758-1765.

[0242] [24] M. A. Malik, P. O'Brien, A. Adeogun, M. Helliwell, J. Raftery, J. Coord Chem. 2008, 61, 79-84.

[0243] [25] H. Coy Diaz, R. Addou, M. Batzill, Nanoscale 2014, 6, 1071-1078.

[0244] [26] W. E. Newton, J. L. Corbin, D. C. Bravard, J. E. Searles, J. W. Mcdonald, Inorg. Chem. 1974, 13, 1100.

[0245] [27] C. Gong, C. Huang, J. Miller, L. Cheng, Y. Hao, D. Cobden, J. Kim, R. S. Ruoff, R. M. Wallace, K. Cho, X. Xu, Y. J. Chabal, ACS Nano 2013, 7, 11350-11357.

[0246] [28] Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. Mcgovern, B. Holland, M. Byrne, Y. K. Gun'ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari, J. N. Coleman, Nature nanotechnology 2008, 3, 563.