WO2017062736A1 - Structured molybdenum disulfide materials for electrocatalytic applications - Google Patents

Abstract

The present disclosure relates particularly to structured molybdenum disulfide materials disposed on a substrate, to methods for preparing such materials, and to methods for catalyzing a hydrogen evolution reaction with such materials. In one aspect, the disclosure provides a structured molybdenum disulfide material that includes a plurality of crystalline molybdenum disulfide particles disposed on a substrate, wherein the surface area of the structured molybdenum disulfide material is at least 10% MoS₂ crystalline particle edge surface; and/or at least 50% of the material is made of from crystalline particles having a minor aspect ratio of no more than 15.

Description

STRUCTURED MOLYBDENUM DISULFIDE MATERIALS FOR ELECTROCATALYTIC

APPLICATIONS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application no. 62/238,989, filed October 8, 2015, which is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

[0002] This disclosure relates generally to transition metal dichalcogenide materials. More particularly, the present disclosure relates to structured molybdenum disulfide materials disposed on a substrate, to methods for preparing such materials, and to methods for catalyzing a hydrogen evolution reaction with such materials.

Technical Background

[0003] Transition metal dichalcogenides (TMDCs) are a family of layered materials with the formula MX₂, where M is a transition metal (such as Mo or W) and X is a chalcogenide (S, Se, or Te). Molybdenum disulfide (MoS₂), a versatile and earth abundant member of this family, has shown great potential as a high performing and cost-effective catalyst for energy storage and generation. MoS₂ is widely used, in a broad range of applications, from electronics and optoelectronics to energy conversion and storage systems. Because of its direct and tunable bandgap, MoS₂ in the form of mono- and few-layer sheets is being extensively investigated for potential applications in low-dimensional logic transistors, photodetectors, and electroluminescent devices.

[0004] Crystalline MoS₂ has a layered structure, with individual S-Mo-S layers stacked along the c-axis by weak van der Waals interactions. Each charge-neutral layer consists of three covalently bonded atomic sheets, with a layer-to-layer distance of about 6 A. Two general types of surface sites are present on these crystals: terrace sites on the basal planes and edge sites on the side surfaces. Layered materials such as MoS₂ usually expose the basal planes, which have minimal roughness and few dangling bonds, as the terminating surface, which is ideal for electronic device applications.

[0005] Additionally, MoS₂ flakes have been identified as candidates for electrocatalytic reactions such as carbon dioxide reduction and the hydrogen evolution reaction (HER) . However, only the edges sites of crystalline MoS₂ materials are active for these catalytic reactions. Much work has been dedicated to produce MoS₂ materials with a high number of edge sites, e.g. , atomically thin two-dimensional (2D) MoS₂ sheets, synthetic structures such as MoS₂ nanoparticles on Au(1 1 1 ), vertically aligned MoS₂ nanoflakes, ordered double- gyroid MoS₂ bicontinuous networks, defect-rich MoS₂ nanosheets, and chemically synthesized thiomolybdate [Mo₃S₁₃]^2" clusters. However, MoS₂ materials produced for HER thus far have not had sufficiently high effectiveness (e.g. , as measured by turnover number and/or overpotential) .

[0006] Accordingly, there remains a need for a MoS₂ material that functions as an effective HER catalyst, that can be prepared on a variety of substrates by a scalable method.

SUMMARY OF THE DISCLOSURE

[0007] One aspect of the disclosure is a structured molybdenum disulfide material that includes a plurality of crystalline molybdenum disulfide particles disposed on a substrate, wherein the surface area of the structured molybdenum disulfide material is at least 10% MoS₂ crystalline particle edge surface; and/or at least 50% of the material is made of from crystalline particles having a minor aspect ratio of no more than 15.

[0008] Another aspect of the disclosure is a method for the synthesis of a structured molybdenum disulfide material, e.g . , the material as described above, the method including providing a substrate; and depositing molybdenum disulfide on the substrate by way of chemical vapor deposition to at least a thickness wherein the growth mode of the material transitions from two-dimensional molybdenum disulfide material growth to three-dimensional molybdenum disulfide particle growth, wherein the molybdenum disulfide particles are crystalline.

[0009] Another aspect of the disclosure is a structured molybdenum disulfide material disposed on a substrate, prepared according to the method described above.

[0010] Another aspect of the disclosure is a hydrogen evolution reaction (HER) catalyst comprising the structured molybdenum disulfide material as described herein; wherein the catalyst has an onset potential, and wherein the onset potential of the catalyst is less than 100 mV for the HER reaction.

[0011] Another aspect of the disclosure is a method for performing a hydrogen evolution reaction in an electrochemical cell, comprising contacting water with a catalyst comprising the structured molybdenum disulfide material or the HER catalyst as described herein, and applying a potential to the electrochemical cell sufficient to form hydrogen from the water.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a schematic of the structured MoS₂ material CVD setup as described in more detail in Example 1 , below. [0013] Figure 2 is a photograph of the structured MoS₂ material grown on an Si/Si0₂ substrate as described in more detail in Example 1 , below.

[0014] Figure 3 is a graph showing the temperature profiles for Zone 1 and Zone 2 of the CVD setup throughout the structured MoS₂ material synthesis procedure described in Example 1 , below.

[0015] Figure 4 is a set of detailed photographs (top) and scanning electron microscope (SEM) images (bottom) of the surface of the substrate shown in the photograph of Figure 2, as described in more detail in Example 2, below. Figure 4A corresponds to the rightmost region of the substrate shown in Figure 2, and Figure 4F corresponds to the leftmost region of end of the substrate. Scale bars for the photographs are 30 μηι. Scale bars for the SEM images are 2 μηι, 15 μηι, 15 μηι, 10 μηι, 4 μηι, and 4 μηι, for Figure 4Α, Β, C, D, E, and F, respectively.

[0016] Figure 5 is a set of atomic force microscopy (AFM) images of the surface of the substrate shown in the photograph of Figure 2, as described in more detail in Example 2, below. Figure 5A was collected from the (rightmost) low concentration region of the substrate, and Figure 5B was collected from the (leftmost) high concentration region of the substrate. The scale bars for Figure 5A and Figure 5B are 500 nm and 5 μηι, respectively.

[0017] Figure 6 is a representative Raman point spectrum obtained from a single layer MoS₂ flake (top) and a 3D MoS₂ particle (bottom) as described in more detail in Example 2, below.

[0018] Figure 7 is a low magnification SEM image of structured MoS₂ material comprising clusters of 3D MoS₂ particles with two different growth patterns: dense lines (left, dashed lines) and randomly dispersed (right, dashed circle), as described in more detail in Example 2, below. The scale bar is 15 μηι.

[0019] Figure 8 is a representative X-ray photoelectron spectroscopy (XPS) spectrum of a 3D MoS₂ particle, highlighting the sulfur peaks, as described in more detail in Example 2, below. Figure 8, inset, is the region of the spectrum that shows the molybdenum peaks.

[0020] Figure 9 is a low magnification annular dark field (ADF) scanning transmission electron microscopy (STEM) image of a cluster of 3D MoS₂ particles after dispersion of structured MoS₂ material in solution and drop casting onto a TEM grid, as described in more detail in Example 2, below. The scale bar is 1 μηι.

[0021] Figure 10 is a representative energy-dispersive X-ray spectroscopy (EDX) spectrum of a 3D MoS₂ particle from the denoted region of the cluster shown in Figure 9, as described in more detail in Example 4, below. Figure 10, inset, is a table listing the atomic percentages of molybdenum and sulfur that comprise the particle.

[0022] Figure 1 1 is an atomic resolution ADF STEM image of a 3D MoS₂ particle from the denoted region of the cluster shown in Figure 9, as described in more detail in Example 4, below. The scale bar is 3 nm. Figure 1 1 , inset, is a fast fourier transform (FFT) obtained from the particle.

[0023] Figure 12 is an SEM image of structured MoS₂ material obtained in a rapid cooling experiment described in more detail in Example 5, below. The scale bar is 0.5 μηι.

[0024] Figure 13 is a set of SEM images that highlight the growth modes in the structured MoS₂ material growth procedure, obtained through an extended growth procedure described in more detail in Example 5: initially, coalescence of 2D MoS₂ flakes to form sheets (A), next, nucleation of additional 2D flakes on the 2D sheets (B) , and finally, out-of- plane 3D MoS₂ particle growth (C). The scale bar shown in Figure 13A is 4 μηι.

[0025] Figure 14 is a set of representative Raman point spectra obtained from a MoS₂ triangular flake (labeled "SL"), a 2D MoS₂ sheet (labeled "ML"), and a 3D MoS₂ particle (labeled "3D"), as described in more detail in Example 6, below.

[0026] Figure 15 is a set of representative Raman point spectra corresponding to the spectra of Figure 14, expanded to show the silicon peak position.

[0027] Figure 16 is a set of images that highlight the growth modes of the structured MoS₂ material growth procedure, as described in more detail in Example 6, below. Figure 16A is a photograph of a 2D MoS₂ sheet on which 3D growth has not yet initiated. The scale bar is 15 μηι. Figure 16B is an SEM image of a 2D MoS₂ sheet on which 3D growth has just initiated, showing the formation of clusters. The scale bar is 15 μηι. Figure 16C is a SEM image of densely packed 3D MoS₂ particles, i.e. , a polycrystalline film. The scale bar is 1 0 μηι.

[0028] Figure 17 is an annotated photograph of a structured MoS₂ material grown uniformly on a Si/Si0₂ substrate, as described in more detail in Example 7, below.

[0029] Figure 18 is a set of images corresponding to the characterization of structured MoS₂ material grown on a glassy carbon (GC) substrate, as described in more detail in Example 7, below. Figure 18A is an SEM image of the material (the scale bar is 20 μηι), and Figure 18B is a set of representative XPS spectra of the material. Figure 18A, inset, shows a magnified image of the denoted area.

[0030] Figure 19 is an SEM image of a full-coverage film of graphene deposited onto a GC substrate by CVD, as descried in more detail in Example 7, below. [0031] Figure 20 is an SEM image of an 80% coverage film of graphene deposited onto a GC substrate by CVD, as described in more detail in Example 7, below.

[0032] Figure 21 is a set of images corresponding to the characterization of structured MoS₂ material grown on the substrate prepared by transferring the graphene film shown in Figure 20 and to a Si/Si0₂ substrate, as described in more detail in Example 7, below. Figure 21 A is an SEM image of the material (the scale bar is 20 μηι), and Figure 21 B is a representative XPS spectrum of the material. The insets of Figure 21 A are magnified images highlighting MoS₂ particle growth on Si/Si0₂ regions (left) and graphene regions (right).

[0033] Figure 22 is a set of cyclic voltammograms for different preparations of structured MoS₂ materials, as described in more detail in Example 8, below.

[0034] Figure 23 is comparative set of cyclic voltammograms, highlighting the stability of structured MoS₂ material disposed on a graphene substrate after 1000 continuous cyclic voltammetry (CV) cycles.

[0035] Figure 24 is a set of Tafel plots obtained for various catalytic structured MoS₂ materials, as described in more detail in Example 8, below.

[0036] Figure 25 is a set of extrapolations of the plots shown in Figure 24, highlighting the calculated exchange current density of different preparations of structured MoS₂ materials, as described in more detail in Example 8, below.

[0037] Figure 26 is a set of Nyquist plots collected from electrochemical impedance spectroscopy (EIS) experiments on structured MoS₂ materials disposed on graphene and glassy carbon substrates, as described in more detail in Example 8, below.

[0038] Figure 27 is a set of Nyquist plots collected from EIS experiments performed on structured MoS₂ materials disposed on a graphene substrate at a range of overpotentials, as described in more detail in Example 8, below.

[0039] Figure 28 is a set of Nyquist plots collected from EIS experiments performed on structured MoS₂ materials disposed on a glassy carbon substrate at a range of overpotentials, as described in more detail in Example 8, below.

[0040] Figure 29 is a plot showing current densities and turnover frequencies with respect to overpotential for different preparations of structured MoS₂ materials in an HER experiment as described in more detail in Example 8, below. [0041] Figure 30 is a set of cyclic voltammograms of a structured MoS₂ material disposed on graphene at a range of different scan rates, as described in more detail in Example 8, below.

[0042] Figure 31 is a graph showing current densities of the cyclic voltammograms of Figure 30 at an overpotential of +0.2V vs. reversible hydrogen electrode (RHE), as a function of scan rates.

[0043] Figure 32 is a plot showing the current density and turnover frequencies with respect to overpotential for different preparations of structured MoS₂ materials in an HER experiment as described in more detail in Example 8, below.

DETAILED DESCRIPTION

[0044] The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of various embodiments of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for the fundamental understanding of the invention, the description taken with the drawings and/or examples making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Thus, before the disclosed processes and devices are described, it is to be understood that the aspects described herein are not limited to specific embodiments, apparati, or configurations, and as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and, unless specifically defined herein, is not intended to be limiting.

[0045] The terms "a," "an," "the" and similar referents used in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0046] All methods described herein can be performed in any suitable order of steps unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0047] Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". Words using the singular or plural number also include the plural and singular number, respectively. Additionally, the words "herein," "above," and "below" and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of the application.

[0048] As will be understood by one of ordinary skill in the art, each embodiment disclosed herein can comprise, consist essentially of or consist of its particular stated element, step, ingredient or component. As used herein, the transition term "comprise" or "comprises" means includes, but is not limited to, and allows for the inclusion of unspecified elements, steps, ingredients, or components, even in major amounts. The transitional phrase "consisting of excludes any element, step, ingredient or component not specified. The transition phrase "consisting essentially of limits the scope of the embodiment to the specified elements, steps, ingredients or components and to those that do not materially affect the embodiment.

[0049] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. When further clarity is required, the term "about" has the meaning reasonably ascribed to it by a person skilled in the art when used in conjunction with a stated numerical value or range, i.e. denoting somewhat more or somewhat less than the stated value or range, to within a range of ±20% of the stated value; ±19% of the stated value; ±18% of the stated value; ±17% of the stated value; ±16% of the stated value; ±15% of the stated value; ±14% of the stated value; ±13% of the stated value; ±12% of the stated value; ±1 1 % of the stated value; ±10% of the stated value; ±9% of the stated value; ±8% of the stated value; ±7% of the stated value; ±6% of the stated value; ±5% of the stated value; ±4% of the stated value; ±3% of the stated value; ±2% of the stated value; or ±1 % of the stated value.

[0050] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0051] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member may be referred to and claimed individually or in any combination with other members of the group or other elements found herein. It is anticipated that one or more members of a group may be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

[0052] Some embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Of course, variations on these described embodiments will become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

[0053] Furthermore, numerous references have been made to patents and printed publications throughout this specification. Each of the cited references and printed publications are individually incorporated herein by reference in their entirety.

[0054] In closing, it is to be understood that the embodiments of the invention disclosed herein are illustrative of the principles of the present invention. Other modifications that may be employed are within the scope of the invention. Thus, by way of example, but not of limitation, alternative configurations of the present invention may be utilized in accordance with the teachings herein. Accordingly, the present invention is not limited to that precisely as shown and described.

[0055] In various aspects and embodiments, the disclosure relates to structured molybdenum disulfide materials (structured MoS₂ materials) comprising three-dimensional crystalline molybdenum disulfide particles (3D MoS₂ particles) disposed on a substrate. The disclosure demonstrates the synthesis of such materials to be scalable and adaptable to a variety of substrates. The disclosure further demonstrates hydrogen evolution reaction (HER) catalysts comprising such materials to have a low onset potential, high stability, and high turnover.

[0056] One aspect of the present disclosure is a structured molybdenum disulfide material including a plurality of MoS₂ crystalline particles disposed on a substrate. Each crystalline particle has a thickness and a basal plane surface having an major dimension and a minor dimension. As is familiar to the person of ordinary skill in the art, molybdenum disulfide crystallizes with alternating layers of molybdenum and sulfide; the basal plane surfaces are the surfaces generally parallel to these layers. The inventors have recognized than in certain applications, it is the edge atoms (i.e., not on the basal plane surfaces) that have the desired catalytic activity. Notably, the structured molybdenum disulfide material is formed from crystalline particles having, on average, a relatively low aspect ratio, which can provide a relatively high proportion of exposed edges in the material. The MoS₂ crystalline particles do not pack perfectly in the material, so that the material has a structured surface having a relatively high degree of exposed edge surfaces, especially as compared with conventional MoS₂ sheets, flakes, nanosheets and nanoflakes. Accordingly, in certain aspects of the materials described herein, the surface area of the structured molybdenum disulfide material is at least 10% MoS₂ crystalline particle edge surface (i.e., and no more than 90% MoS₂ crystalline particle basal plane surface). That is, of the total surface area of the structured material, at least 10% is formed from edges of the MoS₂ crystalline particles (i.e., and no more than 90% is formed from the basal plane surfaces of the MoS₂ crystalline particles). In certain embodiments of the materials as otherwise described herein, the surface area of the structured molybdenum disulfide material is at least 20% MoS₂ crystalline particle edge surface. In certain embodiments of the materials as otherwise described herein, the surface area of the structured molybdenum disulfide material is at least 30% MoS₂ crystalline particle edge surface. In certain embodiments of the materials as otherwise described herein, the surface area of the structured molybdenum disulfide material is at least 40% MoS₂ crystalline particle edge surface. In certain embodiments of the materials as otherwise described herein, the surface area of the structured molybdenum disulfide material is at least 50% MoS₂ crystalline particle edge surface.

[0057] In another aspect, a structured molybdenum disulfide material includes a plurality of MoS₂ crystalline particles disposed on a substrate. In this aspect, at least 50% (e.g., at least 75% or even at least 90%) of the material is made of from crystalline particles having a minor aspect ratio, defined as the ratio of the minor dimension of the particle (i.e., the minimum dimension in a plane parallel to the basal plane) to the thickness (i.e., the maximum dimension in a plane perpendicular to the basal plane) of no more than 15, e.g., no more than 10, or even no more than 5. In certain embodiments, at least 50% (e.g., at least 75% or even at least 90%) of the material is made of from crystalline particles having a major aspect ratio, defined as the ratio of the major dimension of the particle (i.e., the maximum dimension in a plane parallel to the basal plane) to the thickness (i.e., the maximum dimension in a plane perpendicular to the basal plane) is no more than 15, e.g., no more than 10, or even no more than 5. The chemical vapor deposition methods described herein can be used to make such particles.

[0058] The MoS₂ particles described herein are described as "crystalline." Each MoS₂ particle is monocrystalline, although a particle may have a minor degree of crystalline defects and still be considered to be monocrystalline to the person of ordinary skill in the art. The person of ordinary skill in the art will also appreciate, as described in more detail below, that a plurality of monocrystalline particles can be associated together in larger clusters of particles.

[0059] The MoS₂ crystalline particles described herein may appear granular in nature, such as those shown in Figure 5A (top) and described in Example 2, below. In order to provide a desired thicknes, the MoS₂ crystalline particles can have a relatively high number of layers of MoS₂ (i.e. of each of Mo and S). For example, in certain embodiments of the materials and methods as otherwise described herein, at least 50% (e.g., at least 75% or even at least 90%) of the structured molybdenum sulfide material is made of up MoS₂ crystalline particles having at least about 10 layers, or at least about 25 layers, or at least about 50 layers, or at least about 75 layers, or at least about 100 layers, or at least about 150 layers, or at least about 200 layers, or at least about 250 layers, or at least about 300 layers, or at least about 350 layers, or at least about 400 layers, or at least about 450 layers, or at least about 500 layers of MoS₂.

[0060] In some embodiments of the materials and methods as otherwise described herein, at least 50% (e.g., at least 75% or even at least 90%) of the material is made of from MoS₂ crystalline particles having a minor dimension of at least about 2 nm, or at least about 5 nm, or at least about 10 nm, or at least about 20 nm, or at least about 25 nm, or at least about 30 nm, or at least about 35 nm, or at least about 40 nm, or at least about 45 nm, or at least about 50 nm, or at least about 60 nm, or at least about 70 nm, or at least about 80 nm, or at least about 90 nm, or at least about 100 nm, or at least about 125 nm, or at least about 150 nm, or at least about 175 nm, or at least about 200 nm, or at least about 225 nm, or at least about 250 nm, or at least about 275 nm, or at least about 300 nm, or at least about 325 nm, or at least about 350 nm, or at least about 375 nm, or at least about 400 nm, or at least about 425 nm, or at least about 450 nm, or at least about 475 nm, or at least about 500 nm.

[0061] In some embodiments of the materials and methods as otherwise described herein, at least 50% (e.g., at least 75% or even at least 90%) of the material is made of from MoS₂ crystalline particles having a major dimension of at least about at least about 2 nm, or at least about 5 nm, or at least about 10 nm, or at least about 20 nm, or at least about 30 nm, or at least about 50 nm, or at least about 60 nm, or at least about 70 nm, or at least about 80 nm, or at least about 90 nm, or at least about 100 nm, or at least about 125 nm, or at least about 150 nm, or at least about 175 nm, or at least about 200 nm, or at least about 225 nm, or at least about 250 nm, or at least about 275 nm, or at least about 300 nm, or at least about 325 nm, or at least about 350 nm, or at least about 375 nm, or at least about 400 nm, or at least about 425 nm, or at least about 450 nm, or at least about 475 nm, or at least about 500 nm, or at least about 550 nm, or at least about 600 nm, or at least about 650 nm, or at least about 700 nm, or at least about 750 nm, or at least about 800 nm, or at least about 850 nm, or at least about 900 nm, or at least about 950 nm, or at least about 1 μηι.

[0062] In certain embodiments, however, in order to provide a desirably high surface area of edge surface, at least 50% (e.g., at least 75% or even at least 90%) of the material is made of MoS₂ crystalline particles having a product of minor dimension and major dimension that is no more than 50 μηι², e.g., no more than 30 μηι², no more than 20 μηι², no more than 10 μηι², no more than 5 μηι², no more than 2 μηι², or even no more than 1 μηι².

[0063] As noted above, in certain embodiments, individual MoS₂ crystalline particles may be granular, i.e., the major dimension and the minor dimension of the particle are similar. For example, in certain embodiments of the materials and methods as otherwise described herein, at least 50% (e.g., at least 75% or even at least 90%) of the material is made of MoS₂ crystalline particles having a major dimension/minor dimension value that is no more than 5, no more than 2, or even no more than 1 .5.

[0064] The person of ordinary skill in the art will appreciate that the materials described herein are differentiable from so-called two-dimensional (2D) MoS₂ materials, e.g., flakes, sheets, nanoflakes, nanosheets, other such materials in which the MoS₂ basal plane forms an overwhelmingly high amount of the surface area of the material. The materials described herein offer a surprising advantage over 2D MoS₂ materials, in that chemically active edge surfaces make up a significant portion of the surface of the material, i.e., the materials have a relatively high surface density of active edge atoms.

[0065] In some embodiments of the materials and methods as otherwise described herein, at least 50% (e.g., at least 75% or even at least 90%) of the material is made of MoS₂ crystalline particles in which both the major and minor dimension are within the range of about 2 nm to nm to about 5 μηι, e.g., about 2 nm to about 4 μηι, or about 2 nm to about 2 μηι, or about 2 nm to 1 μηι, or about 5 nm to about 5 μηι, or about 5 nm to about 4 μηι, or about 5 nm to about 2 μηι, or about 5 nm to 1 μηι, or about 10 nm to about 5 μηι, or about 10 nm to about 4 μηι, or about 10 nm to about 2 μηι, or about 10 nm to 1 μηι, or about 20 nm to about 5 μηι, or about 20 nm to about 4 μηι, or about 20 nm to about 2 μηι, or about 20 nm to 1 μηι, or about 50 nm to about 5 μηι, or about 50 nm to about 4 μηι, or about 50 nm to about 2 μηι, or about 50 nm to 1 μηι, or about 100 nm to about 5 μηι, or about 100 nm to about 4 μηι, or about 100 nm to about 2 μηι, or about 100 nm to 1 μηι, or about 200 nm to about 5 μηι, or about 200 nm to about 4 μηι, or about 200 nm to about 2 μηι, or about 200 nm to 1 μηι.

[0066] The person of ordinary skill in the art will appreciate that, based on the methods by which structured MoS₂ materials are prepared (e.g., the method of Example 1 , below), the structured MoS₂ material may comprise some proportion of 2D MoS₂ material (i.e., having a minor aspect ratio of 10 or greater, 50 or greater, or even 100 or greater). In some embodiments of the materials and methods as otherwise described herein, the structured MoS₂ material comprises less than about 50 wt% 2D MoS₂ material based on the total amount of MoS₂ in the structured MoS₂ material, e.g., less than about 45 wt%, or less than about 40 wt%, or less than about 35 wt%, or less than about 30 wt%, or less than about 25 wt%, or less than about 20 wt%, or less than about 15 wt%, or less than about 10 wt%, or less than about 7.5 wt%, or less than about 5 wt% 2D MoS₂ material based on the total amount of MoS₂ in the structured MoS₂ material.

[0067] In some embodiments of the materials and methods as otherwise described herein, the structured MoS₂ material comprises one or more clusters of contacted, unaligned MoS₂ crystalline particles (i.e., polycrystalline clusters). One example of such a cluster is shown in Figure 9. Another example of a structured MoS₂ material comprises a plurality of such clusters is shown in Figure 7. The person of ordinary skill in the art will appreciate that such clusters may form by the coalescence of individual MoS₂ crystalline particles during film growth according to, e.g., the method of Example 1 , below. In some embodiments, the minor dimension of the cluster (i.e., the shortest distance across the cluster) is at least 100 nm, e.g., at least 125 nm, or at least about 150 nm, or at least about 175 nm, or at least about 200 nm, or at least about 250 nm, or at least about 275 nm, or at least about 300 nm, or at least about 325 nm, or at least about 350 nm, or at least about 375 nm, or at least about 400 nm, or at least about 425 nm, or at least about 450 nm, or at least about 475 nm, or at least about 500 nm, or at least about 550 nm, or at least about 575 nm, or at least about 600 nm, or at least about 650 nm, or at least about 700 nm, or at least about 750 nm, or at least about 800 nm, or at least about 900 nm, or at least about 1 μηι. The person of ordinary skill in the art will appreciate that such a cluster may itself be granular, or may be extended in one dimension relative to the others. And as the density of clusters on a surface increases, multiple clusters may coalesce into larger clusters, and eventually into a substantially continuous film.

[0068] In some embodiments of the materials and methods as otherwise described herein, the structured MoS₂ material comprises a polycrystalline film of contacted MoS₂ crystalline particles (i.e., a polycrystalline film). The contacted particles can be substantially unaligned, e.g., not stacked with their basal planes parallel to one another. One example of such a film is shown in Figure 16C. The person of ordinary skill in the art will appreciate that such a film may form by the coalescence of individual MoS₂ crystalline particles, or of individual clusters of MoS₂ crystalline particles, during film growth according to, e.g., the method of Example 1 , below. The person of ordinary skill will further appreciate that such polycrystalline films may have a high surface roughness and special heterogeneity relative to, e.g., 2D MoS₂ materials. In some embodiments, the ratio of the total film surface area to the geometric planar surface area of the structured MoS₂ material (the roughness factor (RF)) of the polycrystalline film is at least about 3, e.g., is at least about 4, or is at least about 5, or is at least about 6, or is at least about 7, or is at least about 8, or is at least about 9, or is at least about 10, or is at least about 12.5, or is at least about 15, or is at least about 17.5, or is at least about 20, or is at least about 22.5, or is at least about 25, or is at least about 27.5, or is at least about 30, or is at least about 32.5, or is at least about 35. The polycrystalline film can be, for example, at least 100 nm in thickness (e.g., at least 200 nm, or even at least 500 nm). In certain such embodiments, the polycrystalline film is up to about 2 μηι, about 10 μηι, or even up to about 50 μηι in thickness. But the materials described herein can be made to any thickness desired, depending, for example, on the scale of the machinery (e.g., CVD apparatus) used to make the film.

[0069] The structured MoS₂ material is disposed on a substrate. In some embodiments of the materials and methods as otherwise described herein, the substrate comprises graphene, for example, a monolayer of graphene. The graphene (e.g., a monolayer thereof) may itself be disposed on glassy carbon. In some embodiments, the graphene (e.g., a monolayer thereof) may completely cover the glassy carbon. In other embodiments, the graphene (e.g., a monolayer thereof) may only partially cover the glassy carbon. Alternatively, the graphene (e.g., a monolayer thereof) may itself be disposed on Si/Si0₂. In some embodiments, the substrate comprises Si/Si0₂. The person of ordinary skill in the art will appreciate that the substrate may be any substrate known in the art capable of stably supporting the structured MoS₂ material. The person of ordinary skill in the art will appreciate that the substrate need not support growth of the material, e.g., by the method of Example 1 , below, but may still be useful as a substrate onto which structured MoS₂ material may be transferred, e.g., by the polymer-assisted method of Example 3, below.

[0070] Another aspect of the disclosure described herein is a method for synthesizing a structured MoS₂ material, such as a structured MoS₂ material as described herein, the method comprising providing a substrate and depositing MoS₂ on the substrate by way of chemical vapor deposition (CVD) to at least a thickness wherein the growth mode of the material transitions from 2D MoS₂ material growth to 3D MoS₂ particle growth. For example, the MoS2 can be deposited to a thickness of at least 50 nm, at least 100 nm, at least 200 nm, at least 300 nm, or at least 500 nm.

[0071] A non-limiting example of the progression of MoS₂ material growth of the method disclosed herein is provided in Figures 33-35, in both top-down (A) and side-on (B) views. The initial stage of growth upon the formation of molybdenum and sulfur vapor in a CVD chamber as described above involves the formation of roughly triangular MoS₂ flakes, i.e. , 2D MoS₂ material (Figure 33). As growth continues, the triangular MoS₂ flakes coalesce, forming another 2D MoS₂ material, MoS₂ sheets (Figure 34). The person of ordinary skill in the art will appreciate that, while these sheets may comprise several layers of MoS₂, these sheets are still considered 2D, because the exposed surfaces are still nearly entirely the crystalline MoS₂ basal plane. Growth continues further, as flakes continue to coalesce and the number of layers in the 2D MoS₂ materials continues to increase, until the thickness of the 2D materials reaches a critical level, at which point the growth mode transitions to out-of- plane, "3D" growth, forming crystalline particles rather than 2D sheets (Figure 35). After this point, defects in the MoS₂ materials and grain boundaries between unaligned MoS₂ materials serve as nucleation points for additional out-of-plane, 3D MoS₂ crystalline particle formation and growth (Figure 36). The person of ordinary skill in the art will appreciate that such a material provides not only 3D MoS₂ particles with surfaces that, as noted above, comprise a significant number of active edge atoms, but provides an arrangement of 3D MoS₂ particles such that the area of the surface comprising active edge atoms may be larger than the geometric surface area of the substrate itself. [0072] In some embodiments, the substrate of the method comprises graphene, e.g. , monolayer graphene. The graphene (e.g. , monolayer graphene) may be disposed on glassy carbon. In some embodiments, the graphene (e.g. , monolayer graphene) enhances the contact properties of structured MoS₂ materials. In some embodiments, the graphene (e.g. , monolayer graphene) may completely cover the glassy carbon. In other embodiments, the graphene (e.g. , monolayer graphene) may only partially cover the glassy carbon. The graphene (e.g. , monolayer graphene) may be disposed on Si/Si0₂. In some embodiments, the substrate comprises Si/Si0₂. The person of ordinary skill in the art will appreciate that the substrate may be any substrate known in the art capable of supporting the growth of a structured MoS₂ material.

[0073] In some embodiments, the method for synthesizing a structured MoS₂ material may further comprise using dual precursors in the CVD. In some embodiments of the method, one precursor comprises molybdenum and another precursor comprises sulfur. The dual precursors may independently be in a solid or liquid form. For example, in certain embodiments, the molybdenum precursor is an Mo(VI) species such as Mo0₃. In certain embodiments, the sulfur precursor is a S(0) species, such as a sulfur powder. In one example, the dual precursors are Mo0₃ powder and sulfur powder.

[0074] In some embodiments, the method for synthesizing a structured MoS₂ material may further comprise providing a CVD chamber, establishing two distinct heating zones comprising a first and a second heating zone within the chamber, disposing a sulfur precursor in the first heating zone, and disposing a molybdenum precursor in the second heating zone. The substrate may be disposed in the second heating zone. In one example, the CVD chamber is a dual zone tube furnace comprising a crucible of sulfur powder in the first heating zone and a crucible of Mo0₃ in the second heating zone, and the substrate is disposed in the second heating zone, positioned over the Mo0₃ crucible.

[0075] In some embodiments, the method for synthesizing a structured MoS₂ material may further comprise applying a two-stage temperature profile to the first heating zone. In some embodiments, the two-stage temperature profile of the first heating zone is independent of the temperature of the second heating zone. In some embodiments, the temperature of the first heating zone is lower than the temperature of the second heating zone.

[0076] The temperature profiles of the first and second heating zones may be controlled throughout the method such that the stoichiometric ratio of sulfur to molybdenum (S:Mo) vapor is about 2: 1 (e.g. , within 5%, within 1 %, or even within 0.5% of 2: 1 ). In one example, the first heating zone is held at a temperature higher than the vaporization temperature of a sulfur precursor for a period of time before the temperature of the second heating zone is increase to a temperature equal to or higher than the vaporization temperature of a molybdenum precursor. The person of ordinary skill in the art will appreciate that, in any embodiment of the method wherein the substrate is disposed in the second heating zone, such a temperature profile would allow sulfur vapor sufficient time to diffuse to the second heating zone and reach the concentration necessary in the second heating zone to achieve a desired stoichiometric ratio in the structured MoS₂ material. Suitable temperatues include temperatures in excess of 600 C, e.g. , in the range of about 600 °C to about 1000 °C.

[0077] In some embodiments of the materials and methods as otherwise described herein, a three-component electrochemical cell may be used. In a three-component cell a working electrode (WE) , counter electrode (CE) and a reference electrode (RE) are in contact with a solution comprising water. In certain methods of the disclosure, for example, the WE serves as a cathode and comprises the structured MoS₂ material. In a non-limiting example, Ag/AgCI may be used as the RE, platinum mesh may be used as the CE, and the WE may comprise the structured MoS₂ material.

[0078] When an electrochemical cell is used as an HER system, a voltage is applied to the cell, and the water reacts to form H₂ and 0₂. The applied potential can be held constant, e.g. , between about - 1 to about 0 V vs. reversible hydrogen electrode (V vs. RHE), or between about -0.7 to about 0 V vs. RHE. The electrical energy for the electrochemical production of hydrogen can come from a conventional energy source, including nuclear and alternatives (hydroelectric, wind, solar power, geothermal, etc.) , from a solar cell or other non-fossil fuel source of electricity. The minimum value for the applied potential will depend on the internal resistance of the cell employed and on other factors determinable by the person of ordinary skill in the art. In certain embodiments, at least 50 mV is applied across the cell.

[0079] Another aspect of the disclosure described herein is a hydrogen evolution reaction (HER) catalyst comprising a structured MoS₂ material disposed on a substrate as otherwise described herein. The HER catalyst has an onset potential. In some embodiments of the materials and methods as otherwise described herein, the onset potential of the HER catalyst is less than about 100 mV vs RHE when the HER catalyst is the WE in a three-component cell comprising a Pt mesh CE, an Ag/AgCI RE, and 0.5 M H₂S0₄ electrolyte. In some embodiments, the onset potential is less than about 95 mV, or less than about 90 mV, or less than about 85 mV, or less than about 80 mV, or less than about 75 mV. [0080] In some embodiments, the HER catalyst in a three-component cell as described above has a turnover frequency (TOF) of at least 2 s^"1 , e.g. , at least 2.5 s^"1 , or at least 3 s^"1 , or at least 3.5 s^"1 , or at least 4 s^"1. In some embodiments, the HER catalyst in a three- component cell as described above has a current density of at least 25 mA/cm², e.g. , at least 30 mA/cm², or at least 35 mA/cm², or at least 40 mA/cm², or at least 45 mA/cm², or at least 50 mA/cm².

[0081] Another aspect of the disclosure described herein is a method for performing a hydrogen evolution reaction in an electrochemical cell, the method comprising contacting water with an HER catalyst comprising a structured MoS₂ material disposed on a substrate as otherwise described herein, and applying a potential to the electrochemical cell sufficient to form hydrogen from water. In some embodiments, the electrochemical cell comprises a cathode, wherein the cathode is in contact with the HER catalyst. In some embodiments of the method, the applied potential is at least about 50 mV, e.g. , at least about 60 mV, or at least about 70 mV, or at least about 80 mV, or at least about 90 mV, or at least about 100 mV, or at least about 125 mV, or at least about 150 mV, or at least about 175 mV, or at least about 200 mV, or at least about 225 mV, or at least about 250 mV.

EXAMPLES

[0082] The Examples that follow are illustrative of specific embodiments of the invention, and various uses thereof. They are set forth for explanatory purposes only, and are not to be taken as limiting the invention.

Example 1. Synthesis of Structured MoS₂ Material

[0083] The 3D MoS₂ materials described in the Examples were synthesized through a dual precursor CVD method, with precise control over the temperature profile in the chamber. Figure 1 schematically shows the CVD setup, in which the sulfur (S) precursor is located upstream of the flow (Zone 1 ), and molybdenum trioxide (Mo0₃) powder is placed in another crucible in the center of the furnace (Zone 2) , with the target substrates mounted over it, upside-down.

[0084] In order to determine the role of Mo0₃ concentration in structured MoS₂ material formation, the Mo0₃ powder was deliberately loaded in the corner of the alumina crucible to produce different concentrations of Mo over the target substrate, while a sufficient amount of S powder was loaded to guarantee complete sulfurization. Figure 2 shows an optical image of structured MoS₂ material disposed on a Si/Si0₂ substrate. Variation in the color spectrum of the sample corresponds to different concentrations and morphologies of the structured MoS₂ material. [0085] Two alumina crucibles, one containing 2.5 milligrams of Mo03 powder (Sigma- Aldrich, 99.98%) and the other containing 1 gram of sulfur powder (Sigma-Aldrich, 99.98%) were used for MoS₂ growth. The sulfur crucible was placed in the upstream area of the furnace, where the maximum temperature reaches 300°C, while the Mo0₃ crucible was located in the center of the tube, where the maximum temperature reaches 850°C. The target substrates were loaded upside-down on top of the Mo0₃ crucible. Prior to running the temperature profile, the chamber was pumped down to 1 mtorr and then purged by argon flow to reach atmospheric pressure. A 200 standard cubic centimeter (seem) flow of argon was maintained in the chamber during the growth process, which was carried out at atmospheric pressure.

[0086] A two-stage temperature profile was applied to Zone 1 (location of the S crucible) independent of Zone 2 (location of the Mo0₃ crucible) in the CVD chamber in order to maintain a persistent vapor pressure ratio in the vicinity of the target substrate. The temperature of Zone 1 was increased to 100°C (close to the S melting point) at a rate of 1 °C/min, and Zone 2 was increased to 720°C, first at a rate of ~17°C/min to 550 °C, then at ~5 °C/min to 720°C. In the next stage, temperature of Zone 1 was sharply increased to 300°C at the rate of 10°C/min, while the ramping rate was unchanged for Zone 2 until it reached 850 °C, which allowed the S to melt 12 minutes sooner than the Mo0₃ (Figure 3). After reaching 850°C, the Zone 2 temperature was kept constant for 15 minutes.

[0087] Because the distance between the target substrate and the Mo0₃ source is small, a high evaporation rate of S was used to provide adequate time for the S vapor to reach suitable concentrations in order to maintain the desired stoichiometric ratio of S:Mo in the vapor. Otherwise, the local concentration of Mo vapor would become excessively high, which would result in the deposition of intermediate structures as noted below. Finally, the furnace was cooled down to room temperature by natural air convection. During the cool down process, sulfur evaporation continued for about 10 minutes after Mo0₃ evaporation ceased.

[0088] The CVD growth process of M0S2 is more complicated than the gas phase CVD growth process reported for other 2D materials such as graphene and hexagonal boron nitride. Specifically, since the precursors (S and M0O3) are initially in the solid-phase, the growth product is greatly affected by the initial amount of the loaded M0O3 and S powders, as well as the local temperature profile of Zones 1 and 2. In order to obtain highly crystalline M0S2, the spatial temperature profile of the chamber was precisely controlled to synchronize the evaporation rate of the S and M0O3 powders in order to maintain the stoichiometric ratio of S:Mo vapors at ~2: 1 throughout the process. The person of ordinary skill in the art, based on the present disclosure, will adjust temperatures to provide for substantial synchronization of the evaporation rate of If the chamber becomes S deficient in any phase of the growth process, intermediate structures such as oxysulfides (M0OS2) or combinations of molybdenum oxide and molybdenum disulfide (M0O2/M0S₂) will form on the substrate, affecting the purity of the final product. On the other hand, oversupply of S vapor results in the formation of an impermeable M0S2 layer on top of the M0O3 precursor, blocking further evaporation. The person of ordinary skill in the art, based on the present disclosure, will adjust temperatures to provide for substantial synchronization of the evaporation rate of the sulfur precursor and the molybdenum precursor, e.g., to within 5%, within 1 %, or even within 0.5% of a 2:1 S:Mo molar ratio).

Example 2. Morphological Characterization of Structured MoS₂ Materials

[0089] Detailed morphological characterization was performed by optical and scanning electron microscopy (SEM) imaging of several sections of the structured MoS₂ material shown in Figure 2, as shown in Figure 4, in which the MoS₂ concentration increases from left (A) to right (B). Starting from the low concentration side, small triangular MoS₂ monolayer flakes appeared on the substrate (Figure 4A). Then, as the concentration increased, the flakes grew to larger sizes, with some smaller MoS₂ islands growing on top of the flakes (Figure 4B). As shown in Figure 4C, the flakes kept growing in size, and smaller MoS₂ islands began to grow in the out-of-plane direction on top of the existing flakes. Next, the flakes merged together, making grain boundaries, and eventually forming numerous clusters of multilayer, 3D MoS₂ particles (Figure 4D). In higher concentration regions (Figure 4E), white lines representing thicker MoS₂ clusters appear on the polycrystalline film. The thicker MoS₂ clusters are believed to form due to higher local growth rate on the defect sites such as grain boundaries. Moving to even higher concentration regions (Figure 4F), the grown structures rapidly convert to a polycrystalline film with a high surface roughness and spatial heterogeneity.

[0090] As revealed by atomic force microscopy (AFM) imaging (Figure 5), the thickness of the MoS₂ structures on the substrate varies from one monolayer (~1 nm) in the lower concentration region (B) up to several hundred nanometers in the polycrystalline film region (A).

[0091] Figure 6 shows a Raman point spectra obtained from MoS₂ monolayer flakes (top), and 3D MoS₂ particles (bottom) up to 800 cm-i . Results indicate MoS₂ characteristic peaks associated with E_2g and A_1g vibrational modes for the 3D MoS₂ particles, without any intermediate crystals. [0092] Figure 7 shows a low magnification SEM image of the structural MoS₂ material comprising dense line patterns of clusters (dashed lines) as well as randomly grown clusters (dashed circle).

[0093] SEM images were acquired by the ln-lense detector of a Carl Ziess electron microscope integrated in a Raith e-LiNE plus electron beam lithography system at 20 kV acceleration voltage and 10 mm working distance. The AFM topography maps were acquired in tapping mode by an Icon Bruker system

[0094] X-ray photoelectron spectroscopy (XPS) was performed to determine the quantitative elemental composition of the synthesized 3D MoS₂ particles. The XPS results (Figure 8) show standard Mo 3d_5/2 (~229.0 eV) and S 2p_3/2 (~162 eV) peaks consistent with the presence of Mo(IV) and S^2" present in MoS₂ structures. The absence of Mo(VI) 3d₃/₂ peaks around ~236 eV (a characteristic for Mo(VI)) further suggests that molybdenum oxides are not present.

[0095] XPS was performed on a monochromatic Al Ka source instrument (Kratos, Axis 165, England) operating at 12 kV and 10 mA, for an X-ray power of 120 W. Spectra were collected with a photoelectron takeoff angle of 90° from the sample surface plane, energy steps of 0.1 eV, and a pass energy of 20 eV for all elements. All spectra were referenced to the C 1 s binding energy of 284.6 eV.

Example 3. PMMA Assisted Film Transfer

[0096] Clusters of 3D MoS₂ particles were then removed from the substrate by a poly(methyl methacrylate) (PMMA) assisted wet etching method. For this purpose, the MoS₂ material and substrate were spin-coated with PMMA (9% dissolved in anisole - A9) at 1000 RPM and baked at 100°C for 10 minutes. The film was then floated on a 0.3 ML potassium hydroxide (KOH) solution at elevated temperature (80°C) to detach the PMMA/MoS₂ material from the substrate. Next, the PMMA/MoS₂ material was moved through a series of deionized water baths and eventually scooped out by a thoroughly cleaned glassy carbon substrate. The sample temperature was then ramped up to 90°C (in 1 5 minutes) and was kept at that temperature for 15 more minutes to dry out the water and enhance the interaction of the film and substrate. The PMMA support layer was dissolved in an acetone bath and rinsed with IPA and dried with N2 gas.

Example 4. 3D MoS₂ Particle Cluster Characterization

[0097] The clusters were then deattached from the glassy carbon substrate into solution and drop cast onto transmission electron microscopy (TEM) grids to perform scanning TEM (STEM) and energy-dispersive x-ray spectroscopy (EDX) . Figure 9 shows the low- magnification STEM image of 3D MoS₂ particle clusters. EDX results obtained from the defined area in Figure 9 reveal an approximate composition of 32% Mo and 68% S, consistent with the stoichiometric ratio of MoS₂ (Figure 10). Figure 1 1 shows the atomic structure of a 3D MoS₂ particle and its edge terminating atoms using STEM. Multi-layered stacking of MoS₂ layers within a 3D MoS₂ particle, as shown in Figure 1 1 , with Mo- terminated edges is consistent with edge terminations reported previously for mechanically exfoliated MoS₂. See M . Asadi et al. , ACS Nano, 10, 2167-75 (2016). The corresponding fast Fourier transforms (FFTs) taken from a multi-layer particle area (Figure 1 1 , inset) show sharp hexagonal benzene-like patterns indicative of highly crystalline 3D structures with epitaxial stacking of the MoS₂ layers.

[0098] The present inventors have determined that the materials of the disclosure have edges that are terminated substantially by active molybdenum atoms, which can provide for a high degree of catalytic activity.

[0099] Scanning transmission electron microscopy (STEM) characterization was performed on a JEOL JEM-ARM200CF, operated at 200 kV, equipped with an Oxford X- Max^AN 100TLE silicon drift detector (SDD) for energy dispersive X-ray (EDX) analysis. EDX spectra were acquired from 0-10 keV with 2048 total channels.

Example 5. Structured MoS₂ Material Growth Mechanism

[00100] In order to further visualize the layered construction of the 3D MoS₂ particles, the CVD chamber of a 3D MoS₂ material synthesis procedure was quenched mid-run by force cooling to rapidly reduce the temperature and terminate the growth of the top layers, in order to examine the mechanism of growth. High magnification SEM images of the resulting structure (Figure 12) not only showed the layered construction of the 3D MoS₂ particles, but also revealed an epitaxial conformance between the layers, as evidenced by the parallel edges in the stacked layers. This is consistent with the FFT results of the atomic resolution images obtained from STEM, which shows a single diffraction pattern in the multilayer particle regions.

[00101] To gain insight into the growth mechanism of the 3D MoS₂ particles, another control experiment was performed by carrying out the synthesis procedure of Example 1 with an extended growth time at 850°C of one hour (vs. 15 minutes), after which the low concentration region was inspected for 3D growth. Interestingly, a transition from small triangular monolayers to 2D sheets, and finally to 3D MoS₂ structures was observed (see Figure 13) . Initially, the single flakes merged together, forming the 2D sheet observed in Figure 13A. After formation of the first layer, the density of nucleation sites increased as revealed by the growth of numerous MoS₂ triangular nanoflakes on top of the pre-existing 2D sheets (Figure 13B) . This is attributed to a higher density of structural defects such as vacancies, grain boundaries, impurities and sharp topological features that are more likely to occur on top of the growing structure rather than the bare substrate. As the number of layers increased, the patches of newly formed MoS₂ flakes served as additional nucleation sites, boosting the out-of-plane growth rate of the MoS₂ particles (Figure 13C) . However, this mechanism alone cannot explain the formation of a granular 3D particle with the high spatial heterogeneity of the observed 3D MoS₂ particles. This abrupt and localized transition from 2D to 3D growth resembles the Stranski-Krastanov (SK) growth mechanism of thin films, in which the growth mode suddenly changes as the layer thickness exceeds a critical level.

Example 6. Structured MoS₂ Material Raman Characterization

[00102] One possible explanation of such sudden transition can be the release of residual strain induced by thermal and intrinsic stresses between MoS₂ and the substrate. To test this hypothesis, Raman spectroscopy was performed on the MoS₂ flakes, 2D MoS₂ films, and 3D MoS₂ particles (Figure 14) of Example 5. The peak positions of the 2D sheets have a 3 cm^"1 positive shift relative to the bulk spectrum, which is indicative of an induced strain in the grown structure. The peak positions of the 3D MoS₂ particles shifted back to the peak positions of the bulk MoS₂ spectrum, which implies release of induced strain (and occurrence of the SK growth mode) . It should be noted that the difference in the positions of the E_2g and A_1g peaks, which change from ~20 cm^"1 in a single layer to ~25 cm^"1 in 2D sheets and 3D particles, is known to be dependent on the thickness of the MoS₂ structure. This is independent of the observed blue shift in the 2D sheet spectra, which is due to induced strain. The full Raman spectra are shown in Figure 15, which indicate that the silicon peak occurs at 520.5 cm^"1 for all experiments.

[00103] An Acton TriVista CRS confocal Raman with 1 1 mw power and a 0.5 μηι spot size was utilized to collect Raman spectra from M0S2 structures and graphene flakes.

[00104] Based on above discussions, and while not being bound by theory, the growth mode of 3D MoS₂ particles may be attributed to a dual-step SK mode (See, Figure 16). First, growth initiates from grain boundaries or ordered defects that have high surface energies following the oriented line patterns in Figure 7. Next, the 3D particles form on film regions with the thickness beyond the critical point in a compact manner (i.e. , clusters) the degree of which depends on the concentration of Mo and S (circle in Figure 7).

Example 7. Synthesis of Structured MoS₂ Material on Various Substrates

[00105] Structured MoS₂ material was grown on different substrates, e.g. , glassy carbon (GC) and monolayer graphene according to Example 1 , but using uniformly dispersed M0O3 powder in the crucible (vs. deliberately loaded in the corner), which provided films of uniform MoS₂ concentration (Figure 17). The glassy carbon and graphene substrates were selected due to their high conductivity, inert nature and wide applicability in various electrochemical systems. Figure 18A shows an SEM image of the structured MoS₂ material grown directly onto a GC substrate. The elemental composition of Mo:S (1 :2) was verified by XPS (Figure 18B). To reduce the electrical contact resistance between the substrate and the MoS₂ structure, structured MoS₂ material was grown directly onto a monolayer graphene sheet. The monolayer graphene sheet was prepared by first depositing graphene onto a glassy carbon substrate with CV to either full or 80% surface coverage, and subsequently transferred to a Si/Si0₂ substrate (Figures 19-20).

[00106] Structured MoS₂ material was initially synthesized on a partial coverage graphene film transferred to Si/Si0₂ as described above. The SEM image of the directly grown 3D MoS₂ particles (Figure 21A) illustrates that the growth is more compact on top of the hexagonally shaped graphene flakes compared to the bare Si/Si02 substrate. This is attributed to the presence of wrinkles and defects on graphene films, which act as nucleation sites and accelerate the growth rate of the 3D particles. The presence of the G and 2D characteristic peaks of graphene together with the A_1g and E_2g peaks in the Raman spectrum (Figure 21 B) confirmed the growth of 3D MoS₂ particles on top of the graphene flakes. The absence of a noticeable D-peak at ~1350 cm-¹ suggested that the underlying graphene film was not damaged during the growth process and can be effectively used as a conductive substrate to improve the charge carrier mobility of a structured MoS₂ material. This process was also extended to a full coverage graphene film transferred to GC, which was used for the electrochemical experiments of Example 8, below.

Example 8. Structured MoS₂ Material Electrochemical Characterization

[00107] Electrochemical experiments were performed inside a three-electrode electrochemical cell using 0.5 M H2SCU as an electrolyte. Synthesized catalysts, platinum (Pt) gauze (52 mesh, purchased via Alfa Aesar) and Ag/AgCI (3M KCI, purchased from BASF) were used as the working, counter, and reference electrodes, respectively. The reference electrode was calibrated with respect to the reference hydrogen electrode (RHE), using platinum mesh for both working and counter electrodes in the same electrolyte (0.5 M H₂S0₄) bubbled with pure H₂ (99.99%). The calibration resulted in a 0.164 V shift versus RHE. All CV experiments were obtained by sweeping the potential between +0.1 V to -0.7 V vs RHE with a scan rate of 5 mV s^"1 using a CHI-600D potentiostat.

[00108] The hydrogen evolution reaction (HER) performance of structured MoS₂ material synthesized on: (i) GC, (ii) fully covered graphene film (on GC), and (iii) Si/Si0₂ substrate (subsequently transferred to GC, according to Example 3). Cyclic voltammetry (CV) experiments were carried out with these structured 3D MoS₂ materials and compared to platinum, which is the most efficient catalyst known for HER. The current densities were normalized with respect to geometrical surface area and reported based on a RHE scale. As shown in Figure 22, both structured MoS₂ material transferred to GC and structured MoS₂ material grown directly on GC catalysts exhibited high onset potentials of 175 and 140 mV, respectively. Interestingly, the HER takes place at a much smaller onset potential (70 mV vs RHE) for the structured MoS₂ material grown on graphene. Particularly, a 10 mA/cm² current density is achieved at ~100 mV overpotential.

[00109] Notably, the structured MoS₂ material grown on graphene shows very stable performance over 1000 continuous CV cycles (Figure 22). The stability of the MoS₂/graphene catalyst was studied during 1000 continues cycles of CV experiments (Figure 23). The cycles were performed between 0.1 V and -0.2 V with a 100 mV/s scan rate. A magnetic stirring system and continuous bubbling of pure H₂ (99.99%) inside the solution were utilized to eliminate the mass transfer effect during the experiment. After 1000 cycles, the cathodic current density changed by less than 2 mA/cm² at the same overpotential (0.15 mV), which confirms the high stability of the structured MoS₂ material/graphene catalyst during HER.

[00110] Additionally, the linear part of Tafel plot (not iR corrected) for the three different catalysts mentioned above was studied to further explore the catalytic properties of structured MoS₂ materials (Figure 24). The Tafel slopes and exchange current densities obtained for Pt, structured MoS₂ material grown on graphene, structured MoS₂ material grown on GC, and structured MoS₂ material transferred to GC are shown in Figures 24-25. The calculated lower Tafel slope of 41 mV/dec and higher exchange current density of 18.2

2

μΑ/cm for the structured MoS₂ material grown on graphene shows that deposition on a graphene substrate results in a great improvement in the charge carrier mobility of the structured MoS₂ material, and consequently, superior HER performance. The calculated slope (~41 mV/dec) for structured MoS₂ material grown on graphene follows the Volmer- Heyrovsky mechanism, where desorption of the produced species from the catalyst surface is known to be the rate determining step for the reaction. The extracted exchange current densities of structured MoS₂ materials grown on graphene and GC, and structured MoS₂ materials transferred to GC, compared with Pt, are provided below in Table 1 : Table 1. Extracted Exchange Current Densities of 3D MoS₂ Materials

[00111] Electrochemical impedance spectroscopy (EIS) experiments were also performed above the HER onset potentials (150 mV) in order to study the charge transfer resistances (Ret) in the MoS₂ catalyst with and without the graphene layer. The Nyquist plot for different overpotentials e.g., 100, 150, 200, and 250 mV were recorded at a small (10 mV) AC voltage amplitude (to avoid nonlinearity) and over a frequency range of 1 to 105 Hz using a Voltalab PGZ100 potentiostat. An equivalent Randies circuit model was fit to the data to calculate R_ct for each catalyst system. The recorded Nyquist plots (Figure 26) exhibit much smaller R^ (~65Ω) for the MoS₂ grown on graphene sample compared to samples without graphene layers (>150Ω). EIS experiments at different overpotentials (Figures 27-28) verify smaller charge transfer resistance and faster electron transfer for structured MoS₂ material grown on graphene during HER. These results clearly show enhanced contact properties of structured MoS₂ material grown on graphene for HER.

[00112] Turn over frequency (TOF) was also calculated for different 3D MoS₂ material catalysts (See, Figure 29) using the roughness factor (RF) method. The RF number of the catalysts was determined by comparing the double layer capacitance (C_d|) of the catalyst with flat standard MoS₂ (60 [ F/cm). The CV experiment at different scan rates was performed to calculate the Cdl of the catalyst (Figure 30). The C_d| value (2.13 mF/cm) extracted from the slope of current density with respect to scan rate at the potential of +0.2 vs RHE (Figure 31) resulted in 35 RF for the catalyst. This is consistent with previously reported RF values for MoS₂ (Kibsgaard et al., Nat. Mater. 1 1 :963-9 (2012)). Thus, the number of active sites for the structural MoS₂ material catalyst is equal to 4.07x1016 sites/cm² calculated using equation 1 :

Density of active sites for catalyst (Sites/cm²) = Density of active sites for standard sample

(Sites/cm²) x RF (equation 1) [00113] Finally the TOFs at different overpotentials were calculated using equation 2:

TOF(s^"1) = io(A/cm²) /{[active sites density(sites/cm²)] [1 .602 χ 10^"19 (C/e^") ] [2e7H²]}

(equation 2)

[00114] In this case, structured MoS₂ material grown on graphene showed a 53.5 mA/cm² current density at 200 mV overpotential (frequently used for comparison of the HER performance). The calculated TOF is 4.10 (s^"1) , which represents a higher rate of H₂ formation per active site per second. The extended TOF with respect to overpotentials and current densities is also shown in Figure 32.

Claims

We claim:

1 . A structured molybdenum disulfide material comprising

a plurality of crystalline molybdenum disulfide particles disposed on a substrate, wherein

the surface area of the structured molybdenum disulfide material is at least 10%

MoS₂ crystalline particle edge surface; and/or

at least 50% of the material is made of from crystalline particles having a minor aspect ratio of no more than 15.

2. The structured molybdenum disulfide material of claim 1 , wherein the surface area of the structured molybdenum disulfide material is at least 20% MoS₂ crystalline particle edge surface.

3. The structured molybdenum disulfide material of claim 1 , wherein the surface area of the structured molybdenum disulfide material is at least 40% MoS₂ crystalline particle edge surface.

4. The structured molybdenum disulfide material of any of claims 1 -3, wherein at least 50% of the material is made of from crystalline particles having a minor aspect ratio of no more than 15.

5. The structured molybdenum disulfide material of any of claims 1 -3, wherein at least 75% of the material is made of from crystalline particles having a minor aspect ratio of no more than 15.

6. The structured molybdenum disulfide material of any of claims 1 -5, wherein at least 50% of the material is made of from crystalline particles having a minor aspect ratio of no more than 10.

7. The structured molybdenum disulfide material of any of claims 1 -5, wherein at least 75% of the material is made of from crystalline particles having a minor aspect ratio of no more than 10.

8. The structured molybdenum disulfide material of any of claims 1 -7, wherein at least 50% of the material is made of from crystalline particles having a major aspect ratio of no more than 15.

9. The structured molybdenum disulfide material of any of claims 1 -7, wherein at least 75% of the material is made of from crystalline particles having a minor aspect ratio of no more than 15.

10. The structured molybdenum disulfide material of any of claims 1 -9, wherein at least 50% (e.g. , at least 75% or even at least 90%) of the structured molybdenum sulfide material is made of up MoS₂ crystalline particles having at least about 50 layers of MoS₂.

1 1 . The structured molybdenum disulfide material of any of claims 1 -9, wherein at least 50% (e.g. , at least 75% or even at least 90%) of the structured molybdenum sulfide material is made of up MoS₂ crystalline particles having at least about 100 layers of MoS₂.

12. The structured molybdenum disulfide material of any of claims 1 -1 1 , wherein at least 50% (e.g. , at least 75% or even at least 90%) of the material is made of from MoS₂ crystalline particles having a minor dimension of at least about 20 nm.

13. The structured molybdenum disulfide material of any of claims 1 - 1 1 , wherein at least 50% (e.g. , at least 75% or even at least 90%) of the material is made of from MoS₂ crystalline particles having a minor dimension of at least about 150 nm.

14. The structured molybdenum disulfide material of any of claims 1 -13, wherein at least 50% (e.g. , at least 75% or even at least 90%) of the material is made of from MoS₂ crystalline particles having a major dimension of at least about 50 nm.

15. The structured molybdenum disulfide material of any of claims 1 -13, wherein at least 50% (e.g. , at least 75% or even at least 90%) of the material is made of from MoS₂ crystalline particles having a major dimension of at least about 200 nm.

16. The structured molybdenum disulfide material of any of claims 1 -15, wherein at least 50% (e.g. , at least 75% or even at least 90%) of the material is made of MoS₂ crystalline particles having a product of minor dimension and major dimension that is no more than 50 μηι².

17. The structured molybdenum disulfide material of any of claims 1 -15, wherein at least 50% (e.g. , at least 75% or even at least 90%) of the material is made of MoS₂ crystalline particles having a product of minor dimension and major dimension that is no more than 5 μηι².

18. The structured molybdenum disulfide material of any of claims 1 -17, wherein at least 50% (e.g. , at least 75% or even at least 90%) of the material is made of MoS₂ crystalline particles in which both the minor and major dimension are within the range of about 20 nm to about 5 μηι.

19. The structured molybdenum disulfide material of any of claims 1 - 18, comprising less than about 10 wt% 2D MoS₂ material.

20. The structured molybdenum disulfide material of any of claims 1 - 19, comprising one or more clusters of the MoS₂ crystalline particles, wherein the particles in each cluster are in contact with one another but are substantially unaligned.

21 . The structured molybdenum disulfide material according to claim 20, wherein the minor dimension of the cluster is at least 100 nm.

22. The structured molybdenum disulfide material of any one of claims 1 -21 , in the form of a polycrystalline film of contacted MoS₂ crystalline particles.

23. The structured molybdenum disulfide material according to claim 22, wherein the film is substantially continuous.

24. The structured molybdenum disulfide material according to claim 22 or claim 23, wherein the film has a thickness of at least 100 nm (e.g. , at least 200 nm, or at least 500 nm) .

25. The structured molybdenum disulfide material according to claim 24, having a thickness up to about 50 μηι (e.g. , up to about 10 μηι, or up to about 2 μηι).

26. The structured disulfide material according to any of claims 22-25, wherein the film has a roughness factor of at least about 3.

27. The structured disulfide material according to any of claims 22-25, wherein the film has a roughness factor of at least about 10.

28. The structured molybdenum disulfide film of any of claims 1 -27, wherein the substrate comprises graphene.

29. The structured molybdenum disulfide film of any of claims 1 -27, wherein the substrate comprises monolayer graphene.

30. The structured molybdenum disulfide film of any of claims 1 -27, wherein the substrate comprises glassy carbon covered with a monolayer graphene film.

31 . A method for the synthesis of a structured molybdenum disulfide material, e.g. , the material according to any of claims 1 -30, the method comprising:

providing a substrate; and

depositing molybdenum disulfide on the substrate by way of chemical vapor

deposition to at least a thickness wherein the growth mode of the material transitions from two-dimensional molybdenum disulfide material growth to three- dimensional molybdenum disulfide particle growth;

wherein the molybdenum disulfide particles are crystalline.

32. The method of claim 31 , wherein the substrate is monolayer graphene.

33. The method of claim 31 or 32, further comprising using dual precursors in the chemical vapor deposition.

34. The method of claim 33, further comprising providing a chemical vapor deposition chamber, establishing two heating zones within the chemical vapor deposition chamber comprising a first heating zone and a second heating zone, disposing a sulfur precursor in the first heating zone and disposing a molybdenum precursor in the second heating zone.

35. The method of claim 34, wherein the substrate is disposed in the second heating zone.

36. The method of claim 34 or 35, further comprising applying a two-stage temperature profile to the first heating zone, wherein the two-stage temperature profile of the first heating zone is independent of the temperature of the second heating zone.

37. The method of any one of claims 34-36, wherein the temperature of the first heating zone is lower than the temperature of the second heating zone.

38. The method of any one of claims 34-37 further comprising controlling the temperature zones to substantially synchronize evaporation rate of the sulfur precursor and the molybdenum precursor to maintain a stoichiometric ratio of S:Mo vapor of about 2: 1 .

39. A structured molybdenum disulfide material disposed on a substrate, prepared according to the method of any one of claims 34-38.

40. A hydrogen evolution reaction (HER) catalyst comprising the structured molybdenum disulfide material of any one of claims 1 -33 or 39; wherein the catalyst has an onset potential, and wherein the onset potential of the catalyst is less than 100 mV for the HER reaction.

41 . The HER catalyst of claim 40; wherein the onset potential is less than 75 mV.

42. A method for performing a hydrogen evolution reaction in an electrochemical cell, comprising contacting water with a catalyst comprising the structured molybdenum disulfide material of any one of claims 1 -33 or 39, or with the HER catalyst of any one of claims 40- 41 , and applying a potential to the electrochemical cell sufficient to form hydrogen from the water.

43. A method according to claim 42, wherein the electrochemical cell comprises a cathode, and wherein the cathode is in contact with the catalyst.