Method and system for producing molybdenum disulfide inorganic nanotubes

Abstract

Method is presented for crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs) production. Initial synthesis of pure phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers is performed forming precursor and templating agent for MoS.sub.2 INTs production. First-stage sulfurization of h-MoO.sub.3 is performed via a solid-gas reaction at first temperature conditions T.sub.1 producing MoO.sub.x-containing nanowhiskers (2x<3) followed by formation of initial growth stage of MoS.sub.2 INTs being nanostructures having cores with MoO.sub.x-containing nanowhiskers and initial MoS.sub.2 intermittent guiding layers being randomly oriented nanoplatelets or partially distorted layers at surface of MoO.sub.x-containing nanowhiskers. Second or successive second and third stages of sulfurization of said nanostructures is/are performed providing recrystallization of MoS.sub.2 intermittent guiding layers to obtain highly crystalline layers and complete sulfurization of MoO.sub.x inside the cores to MoS.sub.2, and obtain pure phase and high aspect ratio MoS.sub.2 INTs of needle-like crystal with hollow core morphology, and predetermined walls' structure.

Claims

1. A method of production of crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs), the method comprising: performing initial synthesis of pure phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers, said h-MoO.sub.3 nanowhiskers presenting a precursor and templating agent for producing the MoS.sub.2 INTs; performing a first-stage sulfurization of the precursor and templating agent, h-MoO.sub.3, via a solid-gas reaction of the h-MoO.sub.3 with reactive gases at first predetermined temperature conditions, said first stage sulfurization comprising partial reduction of h-MoO.sub.3 nanowhiskers to MoO.sub.x-containing nanowhiskers (2x<3), followed by formation of nanostructures corresponding to an initial growth stage of MoS.sub.2 INTs having cores comprising MoO.sub.x-containing nanowhiskers and initial MoS.sub.2 intermittent guiding layers, configured as randomly oriented nanoplatelets or partially distorted layers, at the surface of the MoO.sub.x-containing nanowhiskers, such that said MoS.sub.2 intermittent guiding layers provide protection against any one of sublimation or over-reduction of the MoO.sub.x at higher temperatures; performing at least a second stage sulfurization of said nanostructures via a solid-gas reaction thereof with reactive gases at second predetermined temperature conditions being relatively high as compared to the first temperature conditions, thereby providing recrystallization of said MoS.sub.2 intermittent guiding layers to obtain highly crystalline layers and complete sulfurization of MoO.sub.x inside the cores to MoS.sub.2, and to obtain pure phase and high aspect ratio MoS.sub.2 INTs of needle-like crystal with hollow core morphology, and predetermined walls' structure.

2. The synthetic method according to claim 1, wherein said initial synthesis of the pure phase h-MoO.sub.3 nanowhiskers is characterized by at least one of the following: said initial synthesis comprises a chemical precipitation process; said initial synthesis is performed in an oil bath reactor to maintain uniform temperature throughout a reaction time of the initial synthesis with a predetermined amount of double distilled water being heated, to thereby providing uniform temperature condition of the initial synthesis.

3. The synthetic method according to claim 1, characterized by at least one of the following: said at least second stage sulfurization is applied to different portions of said nanostructures under differently controlled conditions; a majority of the MoS.sub.2 INTs resulting from the at least second stage sulfurization have an aspect ratio of a value in a range of about 25-1250; said second predetermined temperature maintained during the second stage sulfurization is 750-820 C. producing a first type of said MoS.sub.2 INTs of the needle-like crystal with hollow core morphology with the walls' structure characterized by the randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT, thereby forming a high density arrangement of active sites enabling enhanced catalytic activity.

4. The synthetic method according to claim 3, wherein the MoS.sub.2 INTs resulting from the at least second stage sulfurization have a diameter of about 20-150 nm and a length of up to 15 microns.

5. The synthetic method according to claim 1, wherein said second predetermined temperature maintained during the second stage sulfurization is 750-820 C. producing a first type of said MoS.sub.2 INTs of the needle-like crystal with hollow core morphology with the walls' structure characterized by the randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT, thereby forming a high density arrangement of active sites enabling enhanced catalytic activity, said second stage sulfurization being carried out using flows of a reactive gas H.sub.2S and a carrier gas N.sub.2, at predetermined flow rates for a predetermined time period.

6. The synthetic method according to claim 5, wherein said second stage sulfurization is carried out using the flows of the reactive gas H.sub.2S and the carrier gas N.sub.2, at the flow rates of 5-10 ml/min, and 80-100 ml/min, respectively, for the time period of 30-60 min.

7. The synthetic method according to claim 1, wherein said second predetermined temperature maintained during the second stage sulfurization is 750-820 C. producing a first type of said MoS.sub.2 INTs of the needle-like crystal with hollow core morphology with the walls' structure characterized by the randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT, thereby forming a high density arrangement of active sites enabling enhanced catalytic activity, said MoS.sub.2 INTs of the first type have the enhanced catalytic activity in electrocatalytic hydrogen evolution reaction (HER).

8. The synthetic method according to claim 7, wherein said second predetermined temperature maintained during the second stage sulfurization is 750-820 C. producing a first type of said MoS.sub.2 INTs of the needle-like crystal with hollow core morphology with the walls' structure characterized by the randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT, thereby forming a high density arrangement of active sites enabling enhanced catalytic activity, said method further comprising performing a third sulfurization stage following said second stage, said third sulfurization stage being performed under predetermined third high temperature higher than said temperature of the second stage and using said MoS.sub.2 INTs of the first type as a precursor, thereby producing said MoS.sub.2 INTs of a second type having the needle-like crystal and hollow core morphology having the walls' structure characterized by highly crystalline continual MoS.sub.2 layers substantially parallel to a longitudinal axis of the INT.

9. The synthetic method according to claim 8, characterized by at least one of the following: said predetermined third high temperature is about 950 C.; said third sulfurization is carried out using flows of H.sub.2S, H.sub.2 and N.sub.2 gases, at predetermined flow rates for a predetermined time period producing said MoS.sub.2 INTs of the second type; said MoS.sub.2 INTs are operable as optically or electrically active elements.

10. The synthetic method according to claim 8, wherein said third sulfurization is carried out using flows of H.sub.2S, H.sub.2 and N.sub.2 gases, at predetermined flow rates for a predetermined time period producing said MoS.sub.2 INTs of the second type, said third sulfurization stage being carried out using the flows of H.sub.2S, H.sub.2 and N.sub.2 gases, at the flow rates of 5-10 ml/min, 5-10 ml/min and 80-100 ml/min, respectively, for the time period of 30-60 min.

11. The synthetic method according to claim 3, wherein said initial synthesis comprises: (i) heating the double distilled water to provide a reaction temperature of about 75-80 C. in said oil bath reactor; (ii) adding ammonium molybdate tetrahydrate ((NH.sub.4).sub.6Mo.sub.7O.sub.24.Math.4H.sub.2O) and sodium dodecyl sulphate (CH.sub.3(CH.sub.2).sub.11OSO.sub.3Na) and stirring a mixture; (iii) adding, to said mixture, nitric acid (HNO.sub.3) in a dropwise fashion at a predetermined drop rate to ensure uniform dropping, thereby obtaining a reaction solution; (iv) stirring the reaction solution to obtain a white milky precipitate, thereby enabling to obtain a clean precipitate of pure phase h-MoO.sub.3 nanowhiskers.

12. The synthetic method according to claim 11, characterized by at least one of the following: said double distilled water in the oil bath reactor is in amount of 8-12 ml; said ammonium molybdate tetrahydrate ((NH.sub.4).sub.6Mo.sub.7O.sub.24.Math.4H.sub.2O) being added is in amount of about 0.57-0.63 g, and said sodium dodecyl sulphate (CH.sub.3(CH.sub.2).sub.11OSO.sub.3Na) being added is in amount of about 0.32-0.36 g; said mixture is stirring for about 10-15 min; said nitric acid (HNO.sub.3) being added is 69%-concentrated nitric acid (HNO.sub.3) in amount of about 18-22 ml; said stirring of the reaction solution is during a time period of about 15-30 min; a majority of said pure phase h-MoO.sub.3 nanowhiskers have an aspect ratio of up to 1250.

13. The synthetic method according to claim 11, wherein said nitric acid (HNO.sub.3) being added is 69%-concentrated nitric acid (HNO.sub.3) in amount of about 18-22 ml, the drop rate being about 2.4 ml/min.

14. The synthetic method according to claim 13, wherein said adding in the dropwise fashion to ensure uniform dropping comprises use of an automated syringe pump controller.

15. The synthetic method according to claim 11, wherein a majority of said pure phase h-MoO.sub.3 nanowhiskers have an aspect ratio of up to 1250, a diameter of 20-150 nm, and a length of up to 25 microns.

16. The synthetic method according to claim 1, characterized by at least one of the following: said first predetermined temperature is in a range of about 380-400 C.; said first stage sulfurization of h-MoO.sub.3 comprises: the partial reduction of h-MoO.sub.3 to MoO.sub.x comprising interacting the h-MoO.sub.3 with flows of the reducing gas H.sub.2 and carrier gas N.sub.2 at 5-20 ml/min and 100 ml/min flow rates, respectively, for about 10-20 min, to produce the MoO.sub.x-containing nanowhiskers being MoO.sub.x/MoO.sub.3 nanowhiskers comprising a mixture of MoO.sub.x suboxide phases and MoO.sub.3; and interaction of the MoO.sub.x/MoO.sub.3 nanowhiskers with flows of the reactive gases H.sub.2S and H.sub.2 and carrier gas N.sub.2, at flow rates of 5-10 ml/min, 5-20 ml/min, and 100 ml/min, respectively, for a time period of 30-60 min, to thereby produce said nanostructures comprising said initial MoS.sub.2 intermittent guiding layers on the surface of the MoO.sub.x/MoO.sub.3 nanowhiskers such that said mixture of suboxide phases comprising Mo.sub.4O.sub.11, Mo.sub.8O.sub.23 and MoO.sub.2, is located inside the core of said MoS.sub.2 INTs at the initial stage of their growth, and said MoS.sub.2 intermittent guiding layers are located on the surface of the MoO.sub.x/MoO.sub.3 nanowhiskers providing protection against any one of sublimation or over-reduction of MoO.sub.x core at higher temperatures.

17. A product configured as a precursor and a templating agent for producing therefrom highly crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs) via at least two successive stages of sulfurization, said product comprising pure phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers having an aspect ratio of up to 1250.

18. A product comprising molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs) having needle-like crystal with hollow core morphology and walls' structure, said walls structure having one of the following configurations: characterized by randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT, defining a high density arrangement of active sites enabling enhanced catalytic activity; and comprising highly crystalline continual MoS.sub.2 layers substantially parallel to a longitudinal axis of the INT.

19. A product configured as a templating agent for producing therefrom highly crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs) via two successive stages of sulfurization, said product being produced by the synthetic method according to claim 11 and comprising pure phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers having an aspect ratio of up to 1250.

20. A product comprising molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs) produced by the synthetic method according to claim 1, said MoS.sub.2 INTs having needle-like crystal with hollow core morphology and walls' structure, said walls' structure having one of the following configurations: characterized by randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT, defining a high-density arrangement of active sites enabling enhanced catalytic activity; and characterized by highly crystalline continual MoS.sub.2 layers substantially parallel to a longitudinal axis of the INT.

21. A chemically active electrode comprising: an electrically conductive substrate having a surface carrying the product of claim 18, where the walls' structure of the product is characterized by randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT, defining a high density arrangement of active sites enabling enhanced catalytic activity.

22. An optical or electro-optical device comprising the product according to claim 18, where the walls' structure of the product is characterized by the highly crystalline continual MoS.sub.2 layers substantially parallel to a longitudinal axis of the TNT.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0091] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

[0092] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0093] FIG. 1A is a schematic illustration of a reaction setup for performing the method of the present disclosure for the production of crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs).

[0094] FIG. 1B is a flow diagram of the main steps/stages in the method of the present disclosure for producing crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs).

[0095] FIGS. 2A to 2B are flow diagrams showing the method for production of crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs) according to the present disclosure, wherein FIG. 2A shows the method of the initial synthesis of pure phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers, and FIG. 2B shows the method of the first, second and third sulfurization steps/stages for preparing two types of pure phase and high aspect ratio MoS.sub.2 INTs of predetermined morphology and dimensions;

[0096] FIGS. 2C and 2D illustrate, respectively, the exemplary process of the preparation of h-MoO.sub.3 nanowhiskers, the production of crystalline MoS.sub.2 INTs, and the structures of the resulting type-I and type-II MoS.sub.2 INTs according to the technique of the present disclosure;

[0097] FIGS. 3A to 3B show SEM images of the top view (FIG. 3A) and lateral view (FIG. 3B) of h-MoO.sub.3 nanorods, where the inset image in FIG. 3B depicts the sea-urchin shaped h-MoO.sub.3 rods on the Si-substrate, from Ref. [21];

[0098] FIGS. 4A to 4B show SEM images of low magnification (FIG. 4A) and high magnification (FIG. 4B) of h-MoO.sub.3 nanowhiskers with a high size distribution from 20 to up to 400 nm in diameter, from Ref. [10];

[0099] FIG. 5 is a schematic illustration of crystal structures of tunnel-shaped h-MoO.sub.3 and -MoO.sub.3, from Ref. [27], responsible for the needle-like morphology of MoO.sub.3 nanocrystals;

[0100] FIGS. 6A to 6D show SEM images of h-MoO.sub.3 nanowhiskers prepared according to the method of the present disclosure, at low-to-high magnification, with low size distribution from 20 to up to 150 nm in diameter;

[0101] FIG. 6E shows XRD spectra the h-MoO.sub.3 nanowhiskers obtained with the technique of the present disclosure, and, for comparison, also shows also the XRD pattern of MoO.sub.3 standard;

[0102] FIGS. 7A to 7C show TEM images of MoS.sub.2 INTs prepared at 380-400 C. (FIG. 7A), 750-820 C. (FIG. 7B), and 950 C. (FIG. 7C);

[0103] FIGS. 8A to 8C show XRD spectra of MoS.sub.2 INTs prepared at 380-400 C. (FIG. 8A), 750-820 C. (FIG. 8B), and 950 C. (FIG. 8C);

[0104] FIGS. 9A to 9D are SEM images showing the hollow core morphology of the MoS.sub.2 INTs obtained by the technique of the present disclosure;

[0105] FIGS. 10A to 10C show SEM images of nanobelts, nanoparticles, and nanorods, respectively (These morphologies are obtained when the specific reaction parameters of the present disclosure are not strictly followed);

[0106] FIGS. 11A to 11D show results of linear sweep voltammetry (LSV) measurements performed with different structures of VS.sub.2, WS.sub.2 and MoS.sub.2 (VS.sub.2 tubes, WS.sub.2 tubes, MoS.sub.2 tubes, VS.sub.2 flowers WS.sub.2 triangles and MoS.sub.2 flowers) in acidic (FIGS. 11A and 11B) and alkaline (FIGS. 11C and 11D) media, wherein FIG. 11A shows HER polarization curves in 0.5 M H.sub.2SO.sub.4; FIG. 11B shows corresponding Tafel plots obtained from the polarization curves of FIG. 11A; FIG. 11C shows HER polarization curves in 0.5 M KOH; and FIG. 11D shows corresponding Tafel plots obtained from the polarization curves of FIG. 11C; and

[0107] FIG. 12 shows a chemically activated electrode configured according to the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0108] Referring to FIG. 1A, there is schematically illustrated a system 10 configured and operable to implement the technique of the present disclosure for producing crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs). The system 10 includes a list of reactants, a chemical precipitation unit 12 (oil bath reactor), and a reaction setup 14 configured as a horizontal tubular quartz reactor having a region thereof passing through a high temperature furnace 16.

[0109] The chemical precipitation unit 12 is configured and operable to perform the initial synthesis of pure phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers, which serves as a precursor and templating agent for further production of the MoS.sub.2 INTs.

[0110] The reaction setup 14 has a tubular furnace 16, tubular reactor (quartz tube) 14A, which has a gas input 14B for entering the carrier/reactive gases such as N.sub.2, H.sub.2, H.sub.2S, and quartz reaction cell 14E made of porous quartz filter. The cell is connected to quartz handle 14D with a small magnet 14C embedded into the handle's end to introduce the cell into the reactor, and a reaction zone 18 for as-prepared precursor MoO.sub.3 nanowhiskers to be loaded towards the sulfurization reaction. As will be described further below, the reactor is configured and operable to perform the sequential first, second and third stages of sulfurization to obtain pure 1D crystalline phase and high aspect ratio MoS.sub.2 INTs of predetermined morphology and dimensions. As also shown in FIG. 1A, there is the temperature profile of the furnace which confirms the constant temperature along the region of reaction zone 18 passing through the furnace.

[0111] FIG. 1B shows a flow diagram 100 of the main steps/stages in the technique of the present disclosure to produce crystalline MoS.sub.2 INTs. In step 102, the initial stage is performed including the initial synthesis of pure-phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers. These h-MoO.sub.3 nanowhiskers present a precursor and templating agent for further production of the MoS.sub.2 INTs.

[0112] Then, a first stage sulfurization of the precursor and templating agent, h-MoO.sub.3, is performed (step 104). This utilizes a solid-gas reaction of the h-MoO.sub.3 with reactive gases (H.sub.2, H.sub.2S) at the first predetermined temperature conditions Ti. This first stage of sulfurization includes partial reduction of h-MoO.sub.3 nanowhiskers to MoO.sub.x-containing nanowhiskers (2x<3), which is followed by the formation of nanostructures corresponding to an initial growth stage of MoS.sub.2 INTs. These nanostructures have cores comprising MoO.sub.x-containing nanowhiskers and initial MoS.sub.2 intermittent guiding layers at the surface of the MoO.sub.x-containing nanowhiskers. These initial MoS.sub.2 intermittent guiding layers are configured as randomly oriented nanoplatelets or as partially distorted layers, and they provide protection against sublimation and/or over-reduction of the MoO.sub.x at higher temperatures.

[0113] The so-obtained nanostructures presenting the initial growth stage of MoS.sub.2 INTs proceed to second and third stages of sulfurization (step 106) performed via a solid-gas reaction at second and third predetermined temperature conditions T.sub.2, T.sub.3 such that T.sub.2 and T.sub.3 are relatively high as compared to the first temperature conditions Ti. This provides recrystallization of the MoS.sub.2 intermittent guiding layers to obtain highly crystalline substantially parallel layers, to sulfurize MoO.sub.x inside the cores to MoS.sub.2, and obtain pure phase and high aspect ratio MoS.sub.2 INTs of needle-like crystal with hollow core morphology and predetermined walls' structure and dimensions.

[0114] Reference is made to FIGS. 2A and 2B showing more specifically the exemplary implementations of the above-described steps/stages for the production of crystalline molybdenum disulfide (MoS.sub.2) inorganic nanotubes (INTs) according to the present disclosure.

[0115] FIG. 2A shows a flow diagram 200 of the initial synthesis of pure phase hexagonal molybdenum oxide (h-MoO.sub.3) nanowhiskers, where the h-MoO.sub.3 nanowhiskers present a precursor and templating agent for producing MoS.sub.2 INTs.

[0116] The inventors found the conditions for obtaining very thin and long h-MoO.sub.3 nanowhiskers by controlling the uniform and unidirectional growth of the whiskers in contrast to the reported literature where rods were prepared with a low aspect ratio (10), wide diameter (500-700 nm), flower and sea-urchin agglomerates morphology. Specifically, the inventors found the optimal (or even only suitable) set of reactants concentrations and a unique way for heating, mixing and feeding the reactants into the reactor for the preparation of h-MoO.sub.3 nanowhiskers to thereby obtain them with the controlled sizes, with a high aspect ratio (up to 1250) and narrow size distribution of diameter (20-150 nm), contrary to the previous report [15] where diameters varied between 40-400 nm.

[0117] The initial synthesis of oxide nanowhiskers is performed in an oil bath reactor (instead of using a beaker directly on hotplate) to maintain uniform temperature throughout the reaction time of the initial synthesis. In a typical synthetic procedure, before the reaction, 8-12 ml of double distilled water was taken in the round-bottom flask and heated in an oil bath at 75-80 C. for 30-60 min on a hot plate integrated with a magnetic stirrer (step 202) to optimize the water temperature throughout the reaction time. The inventors optimized the reaction temperatures to 75-80 C. instead of the reported 90-110 C. and others (including timing, concentrations of reactants and surfactant, feeding of reactants, and mixing protocol). The reported temperatures of 90-110 C. result in a smaller aspect ratio or not in a 1D morphology.

[0118] In a typical synthetic procedure, 0.46-0.51 mM (0.57-0.63 g) ammonium molybdate tetrahydrate (AMT) and 1.11-1.25 mM (0.32-0.36 g) SDS were added to the pre-heated double distilled water and stirred for 10-15 min to make a homogeneous solution mixture with constant stirring (step 204).

[0119] Following this, 18-22 ml concentrated nitric acid (69%) was added dropwise to the above solution mixture at a specific rate of 2.4 ml/min using an automated syringe pump controller to ensure uniform dropping (step 206). This controlled feeding rate of the nitric acid was shown by the inventors to be critical for obtaining the desired morphology of the h-MoO.sub.3 nanowhiskers.

[0120] At the same time, the stirring was continued for a further 15-30 min (step 208) to obtain a white milky precipitate. After the desired precipitate was obtained, the flask was taken out of the oil bath and cooled down to room temperature naturally (step 210), washed with distilled water (three times) and ethanol (two times) (step 212) to remove the reaction residuals, centrifuged, and then dried in an open atmosphere at room temperature for further analysis (step 214).

[0121] The final experimental parameters for obtaining MoO.sub.3 nanowhiskers were found as follows: [0122] Ammonium molybdate tetrahydrate: (NH.sub.4).sub.6Mo.sub.7O.sub.24.Math.4H.sub.2O0.57-0.63 g, [0123] Sodium dodecyl sulphate (SDS, surfactant): CH.sub.3(CH.sub.2).sub.11OSO.sub.3Na0.32-0.36 g, [0124] Water8-12 ml and HNO.sub.3-18-22 ml (2.4 ml/min) [0125] Reaction temperature: 75-80 C. and time: 15-30 mins.
The chemical reaction for obtaining MoO.sub.3 nanowhiskers follows in Eq. (1):

[00001]$\begin{array}{cc}{\left({\mathrm{NH}}_4\right)}_6{\mathrm{Mo}}_7{O}_{24}.Math.4H_{2}O+H_{2}O+6{\mathrm{HNO}}_3\frac{\mathrm{SDS}}{75-80C}=7{\mathrm{MoO}}_3+6{\mathrm{NH}}_4{\mathrm{NO}}_3+8H_{2}O& \left(1\right)\end{array}$

[0126] Increase in time or temperature results in a larger size distribution of MoO.sub.3 whiskers (20-400 nm); a lower or higher concentration of SDS surfactant results in a short length (few microns) and large diameter (400-700 nm) rods; direct addition of nitric acid results in the formation of nanobelts (instead of nanowhiskers); change in concentrations of all reactants and rate of dropping results in different morphology such as nanoparticles or rods formation instead of uniform (low size distribution) and high aspect ratio nanowhiskers. FIGS. 10A to 10C show various morphologies obtained when the specific reaction parameters of the present disclosure are not strictly followed.

[0127] Further, the inventors clarified that pure hexagonal phase of MoO.sub.3 with tunnelling structure along the whisker axis was essential to direct the growth of 1D needle-like whiskers (MoO.sub.3 could be orthorhombic or hexagonal). The inventors tuned the reaction to obtain h-phase. It should be noted that higher temperature synthesis of MoO.sub.3 whiskers results in orthorhombic phase and loss of 1D morphology.

[0128] The pre-prepared h-MoO.sub.3 nanowhiskers by method 200 are used as a precursor and templating material for preparing MoS.sub.2 INTs via a solid-gas reaction, with two-step-temperature growth using H.sub.2S, H.sub.2 as reactive gases and N.sub.2 as a carrier gas, as described by the method 300 shown in FIG. 2B.

[0129] Since the MoO.sub.3 1D nanowhiskers are essential for 1D MoS.sub.2 nanotube preparation, being used as a templating agent, the dimensions of the resulting 1D MoS.sub.2 INTs are close to the sizes of MoO.sub.3 1D nanowhiskers. However, sulfurization of the MoO.sub.3 nanowhiskers into MoS.sub.2 nanotubes is challenging due to the specific properties of MoO.sub.3, which sublimes starting from 400 C. Once sublimed, the 1D morphology of MoO.sub.3 and its templating function for nanotubes' growth is lost.

[0130] In the present disclosure, the inventors developed a unique two-to-three step sulfurization process for preparing MoS.sub.2 nanotubes from MoO.sub.3 whiskers without compromising the 1D morphology. Moreover, the inventors can control the synthesis to prepare two types of these nanotubes: First-type (type-I) MoS.sub.2 nanotubes with walls comprising small, randomly oriented layers resulting in defect-rich nanotubes' surface, but without compromising morphology, this property enhancing the catalytic activity of MoS.sub.2 NTs; and second-type (type-II) MoS.sub.2 nanotubes with walls comprising highly crystalline parallel MoS.sub.2 layers, suitable for applications in optoelectronics and electro-mechanics.

[0131] The method 300 starts (step 302) by preparing h-MoO.sub.3 nanowhiskers by method 200, which, as mentioned above, are needed as a precursor and templating agent for 1D MoS.sub.2 nanotube preparation.

[0132] Short reduction of MoO.sub.3 to MoO.sub.x precedes the first sulfurization step (step 304). Reduction to MoO.sub.x is one of the process parameters used to avoid sublimation and conserves 1D morphology. The parameters for this short reduction reaction are T=380-400 C., gas flows: N.sub.2=100 ml/min, H.sub.2=5-20 ml/min, reaction time t=10-20 min.

[0133] Given the instability of h-MoO.sub.3 at elevated temperatures, the first stage sulfurization reaction (306) is executed at a lower temperature (380-400 C.), with additional process parameters to avoid MoO.sub.3 sublimation, using H.sub.2S, H.sub.2 as reactive gases and N.sub.2 as a carrier gas, resulting in Mo.sub.4O.sub.11/MoO.sub.2 core encapsulated inside the MoS.sub.2 shells. The experimental parameters of this step are: T=380-400 C., gas flows: N.sub.2=100 ml/min, H.sub.2=5-20 ml/min, H.sub.2S=5-10 ml/min, reaction time=30-60 min.

[0134] It should be noted that these parameters are extremely important, as over-reduction/sulfurization will destroy nanowhiskers and result in their collapse. The first sulfurization renders randomly oriented intermittent MoS.sub.2 guiding layers at the periphery of the nanotubes and Mo.sub.4O.sub.11/MoO.sub.2 inside the core (step 308). These guiding layers serve to protect against possible sublimation at higher temperatures.

[0135] It should be noted that, while the formation of the initial guiding MoS.sub.2 layers is essential to avoid MoO.sub.3 sublimation or collapse, the parameters of this reaction could vary in a wide range. However, all parameters, like temperature, timing, and H.sub.2S/H.sub.2 flows, are to be properly selected to provide very quick sulfurization of the full surface layer of the whiskers, such that this is performed quicker than the sublimation of MoO.sub.3.

[0136] Thus, the inventors demonstrated that similar results could also be obtained if the first sulfurization reaction is carried at higher temperatures like 750-820 C. Obviously, at such temperatures, the kinetics of both sublimation as well as sulfurization of MoO.sub.3 is much faster. Therefore, the H.sub.2S gas flow should be increased in parallel with temperature. In such a way, the morphology of the nanotubes is maintained and the creation of the guiding layers is achieved.

[0137] For example, the parameters of such reactions are: T=750-920 C., gas flows: N.sub.2=80-90 ml/min, H.sub.2=10 ml/min, H.sub.2S=10-50 ml/min, reaction time=5 sec-5 min.

[0138] It is noted that the protocol for the first stage sulfurization is common for production of both first-type (type-I) and second-type (type-II) MoS.sub.2 INTs of the present disclosure.

[0139] The first stage sulfurization is followed by the high-temperature (T.sub.2, e.g., 750-820 C.) second stage sulfurization, and possibly further followed by the higher-temperature (T.sub.3, e.g., 950 C.) third stage sulfurization. Both, the second and third stages of sulfurization provide recrystallization of the MoS.sub.2 intermittent guiding layers to obtain highly crystalline parallel layers and sulfurizing the residual MoO.sub.x accompanied by the creation of a needle-like crystal with hollow core morphology, and to obtain pure phase and high aspect ratio MoS.sub.2 INTs of predetermined walls' structure and dimensions. The second stage sulfurization results in the type-I MoS.sub.2 INTs (step 312) having tubes' walls including randomly oriented nanoplatelets, and the third stage sulfurization results in the type-II MoS.sub.2 INTs having layers substantially parallel to the tube axis.

[0140] In order to obtain the type-I MoS.sub.2 INTs, the second stage sulfurization (step 310) may include the following reaction parameters: T.sub.2=750-820 C., N.sub.2=80-100 ml/min, H.sub.2S=5-10 ml/min, reaction time=30-60 min.

[0141] In order to obtain the type-II MoS.sub.2 INTs, the third stage sulfurization (step 314) may be performed using the following reaction parameters: T.sub.3=950 C., N.sub.2=80-100 ml/min, H.sub.2=5-10 ml/min, H.sub.2S=5-10 ml/min, and reaction time=30-60 min using type-I MoS.sub.2 INTs as a precursor. The type-II MoS.sub.2 INTs are characterized by walls' structure comprising highly crystalline continual layers parallel to tube axis with a hollow core (316). The chosen reaction parameters cause recrystallization of the disordered layers and render continual parallel MoS.sub.2 layers to the final nanotube.

[0142] The obtained MoS.sub.2 nanotubes of both types are 20-150 nm in diameter and up to 15 microns in length, exhibiting a high aspect ratio of up to 750 (step 318). It is noted that nanotubes' diameter of 120 nm renders more distorted MoS.sub.2 layers on the periphery, and partial shortening of the whiskers was ascribed to the handling of the sample during multiple reactions and sample preparation.

[0143] FIGS. 2C and 2D exemplify, in a self-explanatory manner, the above-described chemical processes (initial synthesis and first, second and third sulfurization stages) and the resulting MoS.sub.2 INTs.

[0144] In the following, the method of the present disclosure is described in more detail with reference to analytical techniques used to characterize the various synthesis stages.

[0145] Reference is made to FIG. 3A to 3B showing the SEM images of h-MoO.sub.3 nanorods prepared by Ramana et al. [21], which reaction approach has been used as a reference by the inventors to prepare high-aspect-ratio h-MoO.sub.3 nanowhiskers by substantially modifying the parameters used in the present disclosure. A straight hexagonal rod-shaped morphology (FIGS. 3A and 3B) of 500-700 nm in diameter was reported, and their calculated aspect ratio was about 10-60. The first attempts of the inventors to modify this synthesis [10] resulted in a reduction of the whisker's diameter, with a majority of the whisker's diameters being 80-150 nm. However, the size distribution of the h-MoO.sub.3 nanowhiskers obtained by that process was still wide, ranging from 40 nm to 400 nm; the agglomeration morphology appeared like a flower, a property which could impede the dispersion of the agglomerates into individual nanotubes; their lengths were up to a few microns only as shown in FIGS. 4A to 4B, and a few hexagonal-shaped rods could be seen as well. Using the novel technique of the present disclosure, the inventors succeeded in preparing a pure hexagonal h-MoO.sub.3 phase (MoO.sub.3 could be orthorhombic or hexagonal) with a structure along the whisker axis which is essential for the growth of 1D needle-like whiskers.

[0146] As mentioned above, the inventors tuned the reaction to obtain the h-phase MoO.sub.3 since higher temperature synthesis of whiskers results in orthorhombic phase and loss of 1D morphology. The schematic representations of h-MoO.sub.3 and -MoO.sub.3 crystal structures are shown in FIG. 5.

[0147] FIGS. 6A to 6D show low-high magnification SEM images of h-MoO.sub.3 nanowhiskers with a high-aspect-ratio prepared by the method of the present disclosure and FIG. 6E shows XRD spectra the h-MoO.sub.3 nanowhiskers obtained with the technique of the present disclosure. For comparison, FIG. 6E shows also the XRD pattern of MoO.sub.3 standard. The obtained nanowhiskers are thin, long, isolated (not flower-like), and formed as typical nanowhiskers morphology. Moreover, owing to their longer lengths and needle-like morphology (not tangled spaghetti-like wires), they are piled up and create a network-like structure that can be easily dispersed by a simple sonication process. The diameter of the as-prepared whiskers is 20-150 nm, and their lengths are up to 25 m. The overall aspect ratio of the obtained whiskers is 25-1250 (taking D 20-150 nm and L up to 25 m), which is (at the maximum achievable aspect ratio) at least 20 times higher than the reported literature (literature aspect ratio is 10-60) and may reach even two orders of magnitude higher aspect ratios than reported in the literature. The modified synthesis strategy of the present disclosure nurtures the growth and nucleation of crystals uniformly in one direction and leads to the formation of desired h-MoO.sub.3 nanowhiskers. In addition, the inventors found that an increase in time or temperature results in the large size distribution of whiskers (20-400 nm); a low or high concentration of SDS surfactant results in a short length (few microns) and large diameter (400-700 nm) rods; direct addition of nitric acid (instead of dropping) results in the formation of nanobelts (instead of nanowhiskers); change in concentrations of all reactants and rate of nitric acid dropping results in different morphology nanoparticles/rods formation instead of nanowhiskers.

[0148] The MoS.sub.2 INTs, according to the method of the present disclosure, can be prepared of two types in a controllable way. FIGS. 7A to 7C depict the TEM images of MoS.sub.2 INTs obtained after the first, second and third sulfurization stages of MoO.sub.3 nanowhiskers. As shown in FIG. 7A, the first sulfurization reaction of h-MoO.sub.3 at 380-400 C. under H.sub.2S/H.sub.2 flow renders the morphological features, namely, (i) formation of the partially distorted MoS.sub.2 guiding layers at the periphery of the nanotubes and (ii) quick reduction of MoO.sub.3 into a mixture of suboxide phases of Mo.sub.4O.sub.11/MoO.sub.2 inside the guiding layers. Before the sulfurization, a short reduction of MoO.sub.3 to MoO.sub.x (380-400 C., N.sub.2 gas=100 ml/min, H.sub.2=5-20 ml/min, time=10-20 min) precedes the first sulfurization step. Reduction to MoO.sub.x is one of the parameters to avoid sublimation and conserve 1D morphology (XRD patterns of reduced Mo.sub.4O.sub.11/MoO.sub.2 phases are shown in FIG. 8A).

[0149] The second and third sulfurization reactions at elevated temperatures (T.sub.2 and T.sub.3, respectively) form two types of MoS.sub.2 INTs. As described above, the process may include only first and second stages resulting in type-I MoS.sub.2 INTs, or may further proceed towards higher-temperature third stage of sulfurization resulting in type-II MoS.sub.2 INTs.

[0150] The type-I MoS.sub.2 INTs (second stage sulfurization under temperature conditions of T.sub.2=750-820 C.) comprise walls' structure defining randomly oriented nanoplatelets along a longitudinal axis of the MoS.sub.2 INT which may be also described as short intermittent MoS.sub.2 guiding layers at the periphery of the nanotubes and Mo.sub.4O.sub.11/MoO.sub.2 inside the core owing to defect-rich nanotubes' surface (FIG. 7B). The characteristic features such as distorted MoS.sub.2 layers on the periphery (higher defects, edge sites), lattice strain (generated due to bending), and high surface area (formed due to a high-aspect-ratio and nanosize) of these nanotubes have been generated exclusively for their applications in electrocatalytic hydrogen evolution reaction (HER) which act as active sites and enhance the catalytic activity. Nanotubes' diameter of 120 nm renders more distorted MoS.sub.2 layers on the periphery.

[0151] The type-II MoS.sub.2 INTs (third stage sulfurization under temperature conditions of T.sub.3=950 C.) results from the complete conversion of oxide to MoS.sub.2 providing walls' structure characterized by highly crystalline, continual MoS.sub.2 layers parallel to the tube axis (FIG. 7C). Concurrently, a hollow core is formed due to the density differences between the MoO.sub.2 and MoS.sub.2. In addition, some randomly oriented MoS.sub.2 platelets are also observed in some tubes. These highly crystalline nanotubes are suitable for applications in optoelectronics and electro-mechanics.

[0152] X-ray diffraction (XRD) patterns of MoS.sub.2 nanotubes at the different stages of their synthesis are shown in FIGS. 8A to 8C. FIG. 8A depicts the XRD of NTs prepared at 380-400 C. with abroad (002) peak, characteristic to distorted layers, at 14.3 corresponding to the characteristic feature of the hexagonal MoS.sub.2 phase with a JCPDS No. 6-97 and space group P6.sub.3/mmc (no. 194). In addition, peaks corresponding to residual Mo.sub.4O.sub.11 suboxide and MoO.sub.2 after the first low-temperature sulfurization are clearly observed. The obtained Mo.sub.4O.sub.11 suboxide signifies the orthorhombic phase (JCPDS No. 5-337) and MoO.sub.2 to the monoclinic phase (JCPDS No. 32-671). In contrast, FIGS. 8B and 8C show the diffraction patterns of MoS.sub.2 INTs obtained after the second and third sulfurization stages at 750-820 C. and 950 C., respectively, producing 100% MoS.sub.2 indicating comprehensive conversion of the core oxide into sulfide and, finally, the straightening of the layers, respectively. The thin (002) peak on FIG. 8C points on highly crystalline MoS.sub.2 INTs obtained at 950 C.

[0153] FIGS. 9A to 9D are SEM images showing the hollow core morphology of the MoS.sub.2 INTs obtained by the technique of the present disclosure.

[0154] Thus, in the present disclosure, h-MoO.sub.3 nanowhiskers with a high-aspect-ratio have been synthesized using a chemical precipitation method. The obtained h-MoO.sub.3 nanowhiskers are crystalline and one-dimensional. The aspect ratios of h-MoO.sub.3 nanowhiskers are 20-125 times higher than previously reported. The as-prepared h-MoO.sub.3 nanowhiskers were used as a chemical precursor and template for the preparation of peerless MoS.sub.2 inorganic nanotubes by a two-to-three step sulfurization process using H.sub.2S/H.sub.2 as reactive gases. After the first sulfurization reaction of h-MoO.sub.3 whiskers, the intermittent MoS.sub.2 guiding layers are formed on the nanowhiskers' rim, with reduced Mo.sub.4O.sub.11 and MoO.sub.2 core. After the second sulfurization at 750-820 C., distorted MoS.sub.2 layers on the periphery were generated exclusively for applications in catalysis, where they act as active catalytic sites and enhance the catalytic activity. After third stage of sulfurization at 950 C., pure-phase, highly crystalline MoS.sub.2 INTs were obtained with continual layers parallel to the tube axis with a hollow core inside and some distorted MoS.sub.2 layers.

[0155] The highly crystalline structure of the type-II nanotubes enables well-defined optical and electrical characteristics for opto-electronic applications, such as photovoltaic cells, piezoresistive sensors, infrared and visible range detectors, lithium and sodium batteries, memory devices, such as PV-RAM (photovoltaic random access memory), etc.

[0156] In the following, the inventors demonstrate that the first-type MoS.sub.2 INTs of the present disclosure are advantageous in catalytic reactions, e.g., in hydrogen evolution reaction (HER), showing the best intrinsic activity when compared to other metal dichalcogenides, the activity being enhanced by an extensive electrochemical surface area of the first-type MoS.sub.2 INTs.

[0157] Layered transition metal dichalcogenides (TMD) such as MoS.sub.2 [28,29], MoSe.sub.2 [30,31], WS.sub.2 [32,33], and WSe.sub.2 [34] are well-known electrocatalysts towards the hydrogen evolution reaction (HER) due to their layered structure, unique electronic configuration, and electrochemical stability. The highly active catalytic sites in these materials are located at the layers' edges, while the van der Walls basal planes are practically inactivethe main limitation for the further improvement of the electrocatalytic activity [35]. Similar characteristics for the nanoflowers morphology compared to the bulk structure were previously reported by Bar Sadan, which were attributed to the defective basal plane and abundant edges, inherent to the nanoflowers morphology [29,34]. In addition, the effect of strain was also reported, showing that strain can modulate the hydrogen binding properties [36-38]. However, the nanoflowers are produced in small batches with heavy organic solids, that are retained on the structures as ligands and residues, and hamper the catalytic activity. Moreover, these heavy organic mixtures are expensive and therefore this production protocol has less commercial potential. In contrast, solid gas production of MoS.sub.2 structures provides clean surfaces. In addition, the formation of defect-rich interfaces between the solid catalyst and the liquid electrolyte is especially beneficial for the electro-catalytic activity, making these specific nanotubes of unique value.

[0158] To demonstrate the activity towards the hydrogen evolution reaction, the inventors performed linear sweep voltammetry (LSV) measurements in acidic (0.5M H.sub.2SO.sub.4) and alkaline (0.5M KOH) media for a group of related layered materials. It is noted that the WS.sub.2 in this study have perfectly oriented molecular layers along the tubes' axis, demonstrating the significant advantage of the defected MoS.sub.2 nanotubes as catalysts.

[0159] Reference is made to FIGS. 11A to 11D showing the results of linear sweep voltammetry (LSV) measurements performed with different structures of VS.sub.2, WS.sub.2 and MoS.sub.2 (VS.sub.2 tubes, WS.sub.2 tubes, MoS.sub.2 tubes, VS.sub.2 flowers WS.sub.2 triangles and MoS.sub.2 flowers) in acidic (FIGS. 11A and 11B) and alkaline (FIGS. 11C and 11D) media, wherein FIG. 11A shows HER polarization curves in 0.5 M H.sub.2SO.sub.4; FIG. 11B shows corresponding Tafel plots obtained from the polarization curves of FIG. 11A; FIG. 11C shows HER polarization curves in 0.5 M KOH; and FIG. 11D shows corresponding Tafel plots obtained from the polarization curves of FIG. 11C. Table 1 summarizes the electrochemical HER activity of the various VS.sub.2, WS.sub.2 and MoS.sub.2 structures.

TABLE-US-00001 TABLE 1 Over Potential R.sub.ct from at 10 mA/cm.sup.2 Tafel slope Impedance at (mV) (mV/dec) 400 mV () ECSA Catalyst Acid Base Acid Base Acid Base (cm.sup.2) MoS.sub.2 nanotubes 0.223 0.270 84 139 13 34 276 MoS.sub.2 nanoflowers 0.239 0.260 76 113 17 20 174 WS.sub.2 nanotubes 0.355 0.381 115 134 102 316 32 WS.sub.2 nanotriangles 0.289 0.420 73 179 98 206 154 VS.sub.2 nanotubes 0.588 0.599 154 174 1695 2109 22 VS.sub.2 nanoflowers 0.398 0.558 95 129 187 890 37

[0160] The activity trend is MoS.sub.2>WS.sub.2>VS.sub.2, as seen by the lower overpotential and smaller Tafel slopes. In addition, the MoS.sub.2 nanotubes offer low charge transfer resistance, facilitating the catalytic reaction, as revealed by electrochemical impedance spectroscopy (EIS) measurements. Specifically, the MoS.sub.2 nanotubes, corresponding to the first-type MoS.sub.2 INTs described above, exhibited the highest electrochemical surface area (ECSA), showing that the surface of the nanotubes is strongly activated by both the bending and high density of defects. In contrast, the MoS.sub.2 nanoflowers with their defective surfaces exhibited a lower ECSA, emphasizing the specific contribution of combining defects and crystallinity of the MoS.sub.2 nanotubes on the formation of active sites in the MoS.sub.2 nanotubes. Normalizing the overall activity to the ECSA provides a measure of the intrinsic activity of the catalysts, showing that MoS.sub.2 nanotubes of the first type described in the present disclosure, exhibit a combination of intrinsically active sites with abundance of such sites. The electrocatalytic stability was confirmed by performing continuous 3000 cyclic voltammetry (CV) cycles in 0.5 M H.sub.2SO.sub.4. Overall, all the samples maintained the catalytic activity even after 3000 CV cycles with slight change in nature of the plot. In the case of MoS.sub.2, only slight surface oxidation of the catalyst was noticed.

[0161] Hence, the type-I MoS.sub.2 INTs of the present disclosure having high density arrangement of active sites therealong (i.e., MoS.sub.2 INTs of the needle-like crystal of hollow core morphology with the walls' structure of randomly oriented nanoplatelets along the longitudinal axis of the MoS.sub.2 INT), can be used to form a chemically active electrode. This is illustrated in FIG. 12: an electrode E (generally, an electrically-conductive substrate) has a surface S carrying (coated by) a layer L of the first-type MoS.sub.2 INTs. The surface S with the MoS.sub.2 INTs thereon is therefore configured and operable as the active surface characterized by enhanced catalytic activity.