Patent Application
20250019255
The invention is generally in the field of production of inorganic nanotubes, and relates to a method and system for molybdenum disulfide nanotubes production.
References considered to be relevant as background to the presently disclosed subject matter are listed below:
Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.
The discovery of carbon fullerenes (C60) and carbon nanotubes (CNTs) led to a new scientific domain of zero-dimensional (OD), one-dimensional (1D), and two-dimensional (2D) nanomaterials owing to their exclusive chemical and physical properties compared to their bulk counterparts. The particular interest of the scientific community was attracted by layered compounds, such as graphite, molybdenum disulphide (MoS2), tungsten disulphide (WS2), boron nitride (BN), etc. Thus, the discovery of graphene first time demonstrated the quantum Hall effect and massless Dirac fermions, suggesting a basis for faster and more powerful electronics [1]. 2D monolayers of MoS2 [2] and WS2 [3] reveal strong cathodoluminescence, control over chirality in the CNTs makes them metallic and semiconducting [4], h-boron nitride (BN) nanosheets show mechanical and thermal properties similar to graphene [5], cathodoluminescence [6], and electrical insulation and high thermal conductivity [7] were revealed in heterostructures with BN. Further exploration of 2D materials led to the wrapping and seaming of layered structures into chiral cylindrical tubes, resulting in the constitution of one-dimensional nanotubes [8-10].
Reducing the dimensionality of well-known materials from 3D (three-dimensional) bulk to 2D and, further, to 1D structures bring about polar nature and broken symmetry, bestowing them their properties, like ferroelectricity, piezoelectricity and photovoltaic effect, not existing in their 3D predecessor. This results in diverse applications in the fields such as electro-mechanics, optoelectronics, and memory devices [11]. In addition, WS2 and MoS2 1D inorganic nanotubes have been proven as consequential materials for strengthening polymers [112] due to their inherent strength and good thermal stability [13], proper dispersion and adhesion to polymer. The synthesis of these materials in pure phase and scalable amounts is not a trivial task, while investigation of their properties and materials strongly depends on their availability. Currently, WS2 INTs are in the superior stage compared to MoS2 INTs; the WS2 INTs have been synthesized on a large scale [14] and widely investigated for various applications such as bulk photovoltaic effect (BPVE) [15], superconductivity [16], optical properties [17], solid lubricants [18], etc. In addition, 2D layered transition metal dichalcogenides (TMDs) have been recognized as significant materials for electrocatalytic hydrogen evolution reaction (HER) due to their unique electronic configuration, layered structure, and electrochemical stability. However, their usage in electrocatalytic activity is limited due to their inactive basal planes [19], resulting in low conductivity and reduced activity. The required significant features to enhance the electrocatalytic hydrogen evolution reaction (HER) are lattice strain, higher basal surface defects, edge sites, and high surface area enriched in the currently prepared MoS2 nanotubes.
There is a need in the art for a novel technique for production of crystalline molybdenum disulfide (MoS2) inorganic nanotubes (INTs).
MoS2 inorganic nanotubes (INTs) are 40% lighter and are assumed to be 40% mechanically stronger than WS2 INTs, and the emission of MoS2 semiconductors INTs showed better results than WS2 INTs [20]. Hence, 1D MoS2 INTs are suitable to be used in significant optoelectronic applications.
In the early 1990s, the first MoS2 INTs were synthesized from MoO3 bulk powder using H2/H2S at an elevated temperature (850° C.) via gas-phase reactions. The obtained MoS2 INTs yield was little and poorly reproducible. Following this discovery, several attempts have been made to prepare MoS2 INTs. However, all the strategies were unsuccessful in producing pure-phase, good-yield, and highly crystalline MoS2 INTs. Remarkably, WS2 INTs were successfully synthesized on a large scale by the vapor-gas-solid (VGS) method only in 2009 [14].
Notwithstanding the similarity between the two compounds, until a recent study, the preparation and growth mechanism of MoS2 INTs were ambiguous, reducing the reliability of large-scale production. In a recent investigation [10], a difference between the growth mechanisms of WS2 and MoS2 INTs was elucidated.
In brief, the synthesis of WS2 INTs is a one-pot reaction process, where the reduction of precursor WO3 powder into volatile WO2.75 suboxide and non-volatile WO2 dioxide is followed by “chemical condensation” of these two oxide phases into stable W18O49 suboxide whiskers and their consequent sulfurization to WS2 INTs. All steps of this reaction are accomplished at constant temperature and flow of reactive gases (H2/H2S). Unfortunately, the synthesis process of WS2 INTs was unable to replicate the preparation of MoS2 INTs as the interim Mo4O11 suboxide (similar to W18O49) is prone to over-reduction.
Thus, a new reaction strategy (two-step reaction) was developed [10] for the production of MoS2 INTs in which (i) initial reduction of bulk MoO3 (micron-size) powder to interim Mo4O11 suboxide nanowhiskers under H2 gas flow was followed by subsequent (ii) sulfurization of the Mo4O11 nanowhiskers to MoS2 nanotubes by using H2S gas flow. Each step involved different parameters, including temperature and gas flows. Although this experimental strategy exhibited highly crystalline MoS2 nanotubes, precise control of the chemical condensation process for the production of interim Mo4O11 nanowhiskers could not be achieved, resulting in a maximum of 40% yield of MoS2 INTs [10].
The inventors of the present disclosure have found that in order to obtain exemplary MoS2 nanotubes, an intermediate precursor of MoO3 1D nanowhiskers can be used which also serves as a templating agent for 1D MoS2 nanotube preparation. However, the preparation of the high aspect ratio MoO3 nanowhiskers in a pure phase and their following sulfurization into MoS2 nanotubes reproducibly are challenging tasks due to the specific properties of MoO3. First, most probably, only specific hexagonal molybdenum oxide (h-MoO3) phase is ready to grow as a needle-like crystal and, second, MoO3 is used to sublime starting from 400° C. Once sublimed, the 1D morphology of MoO3 and its templating function is lost.
A few reports can be found in the literature attempting to prepare hollow MoS2 nanotubes. Based on a similar perception, nano-masks of co-shelled MoOx/MoS2 were prepared from MoO3 rods and chemically etched with conc. HCl (concentrated hydrochloric acid) to remove the inner oxide [22]. However, continuous chemical etching reduces the crystallinity of the nanotubes, which deteriorates the formation of pure-phase highly crystalline MoS2 nanotubes. Another study reports on chemical transport reaction elaborated for the preparation of multiwall MoS2 hollow nanotubes [23], however, the reaction product represented a mixture of three MoS2 phases: nanotubes, nanoribbons, and flakes.
In the present disclosure the inventors have developed a unique, and systematic two-step sulfurization process for the preparation of MoS2 nanotubes from MoO3 whiskers without compromising the 1D morphology. Further sulfurization according to the method of the present disclosure generates MoS2 INTs with distinct tubular morphology.
In principle, sulfurization is a diffusion process starting from the surface of the molybdenum oxide nanowhiskers and continuing inward using external sulfide layers as a template and the deeper oxide core as a “building material” for the INTs growth, resulting in the formation of a hollow core in the rim of molybdenum sulfide layers. The formation of a hollow core in the INTs is attributed to the significant difference in the densities between MoS2 and MoO3 precursors.
To produce the oxide nanowhiskers, the inventors take as a basis a recently reported synthesis of hexagonal molybdenum oxide (h-MoO3) nanorods with different aspect ratios (length/diameter). These nanorods were prepared via chemical precipitation method by acidification of an ammonium molybdate precursor with concentrated nitric acid (HNO3) at a temperature range of 90-110° C. [24, 25]. However, these studies reported MoO3 nanorods of limited aspect ratio with considerably large diameters and low length (D=50 nm-2 μm, L=1-30 μm and their aspect ratio is 10-60), unsuitable for producing large aspect ratio MoS2 nanotubes. The primary limitation of such large-diameter tubes is the absence of hollow cores even after considerable sulfurization.
In order to prepare high-aspect ratio MoO3 nanowhiskers (20-150 nm in diameter and up to 25 microns in length, with a maximum aspect ratio of 1250), the inventors of the present disclosure utilized the above-mentioned process, but significantly varied all the reaction parameters, such as a suitable set of reactants' concentrations, selection of suitable surfactant and its concentration, a unique way for heating, mixing and feeding the reactants into the reactor.
Generally, in facile synthesis processes, anionic surfactants have been proven to facilitate the growth of one-directional materials of different oxides, such as Fe2O3, ZnO, etc. Amongst anionic surfactants, sodium dodecyl sulfate (SDS) has been extensively used to prepare high aspect ratio 1D oxides by using an excess amount than micelle concentration and relying on their self-assembly crystallization. However, SDS has not been used as a structure-directing agent for synthesizing MoO3 nanorods to their full potential. SDS has been used for MoO3 microrod synthesis but resulted in a too large diameter, i.e., diameter ranging within 2-5 μm [26].
The inventors have found that SDS's diameter-restricting role towards other oxides is enticing for implementing them for MoO3 through optimized reaction protocols. In the present disclosure, a systematic, modified synthetic procedure for the production of MoO3 nanowhiskers is presented, comprising varying the reaction temperature, using an oil bath to maintain a uniform temperature, and adding nitric acid dropwise at a controlled rate using an automatic syringe pump.
Subsequent sulfurization of the h-MoO3 nanowhiskers into MoS2 nanotubes is an additional nontrivial task, and without the specific combination of parameters and reactor design, the tubular morphology of the nanotubes with a hollow core would be ambiguous.
The inventors have found that the sulfurizing process needs to be isolated in three exclusive steps. The first step renders the guiding initial sulfide layers on the surface of the h-MoO3 nanowhiskers, which saves their 1D morphology by avoiding sublimation of MoO3. This step is accompanied by the reduction of the inner core to MoO2. The second and third high-temperature steps ensure the complete conversion of remaining molybdenum oxides to sulphide (MoS2) accompanied by the formation of the hollow core and, finally, ensures straightening of the sulfide layers. The parameters influencing the morphology, the layer quality, and the hollow core are temperature, reaction time, and a combination and flow rates of hydrogen (H2), hydrogen sulfide (H2S), and nitrogen (N2) gases. These parameters are to be harmonized in specific stoichiometric ratios to achieve high crystallinity tubular structure without compromising the 1D morphology.
The influence of each parameter was investigated by the inventors until the best combination could be reached for a scalable yield of MoS2 nanotubes. The control over these parameters allows preparation of MoS2 nanotubes in pure phase 1D morphology, high aspect ratio (D ˜20-150 nm, L ˜15 μm, aspect ratio 5-750, while some length reduction was ascribed to the handling of the sample during multiple reactions and sample preparation) and 100% yield. The obtained MoS2 nanotubes can be of two crystallinity types; the first type (at times termed here as “type-I”) has tubes' walls including randomly oriented nanoplatelets, while the second type (so-called “type-II”) has layers substantially parallel to the tube axis (highly crystalline). The type-I nanotubes demonstrate superior performance in catalytic applications due to the vast density of the active sites on their surface, while the type-II nanotubes are the perfect material for optical and electronic applications.
In order to obtain the type-I MoS2 nanotubes, the technique of the present disclosure utilizes the second sulfurization stage performed under predetermined relatively high temperature conditions. In order to obtain the type-II MoS2 nanotubes, the technique of the present disclosure may further proceed towards the third stage of sulfurization under even higher temperature conditions and using the type-I MoS2 nanotubes as a precursor.
It should be noted that the type-I MoS2 INTs, having needle-like crystal with hollow core morphology and walls comprising the rolled MoS2 layers with abundant surface defects or large amount of active sites in addition to curvature and strain, are advantageous for catalytic activity, as compared to MoS2 bulk 2D structured material (platelets) comprising the plane layers with defect-free surface and relatively small amount of active sites at the layers' edges (as opposed to nanotube structure). This advantage of the type-I MoS2 INTs with their larger number of active sites (or defects) as compared to bulk 2D platelets is due to the larger total surface area of the nanosized tubes, their stable closed caged tubular morphology and surface defects, obtained due to unique synthetic process developed here. Thus, the large surface area type-I MoS2 INTs of the present disclosure, comprising abundant surface defects along the longitudinal axis of the MoS2 INTs, results in high-density arrangement of active sites enabling enhanced catalytic activity as compared to bulk 2D MoS2 platelets. Abundant active sites increase electron transport and electrical conductivity essential for effective catalytic reaction.
Thus, according to one broad aspect of the present disclosure, it provides a method of production of crystalline molybdenum disulfide (MoS2) inorganic nanotubes (INTs), the method comprising:
The initial synthesis of the pure phase h-MoO3 nanowhiskers can comprise a chemical precipitation process.
In some embodiments, the initial synthesis of MoO3 whiskers is performed in an oil bath reactor (instead of using a beaker on a hot plate) to maintain a 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.
For example, the initial synthesis comprises the following:
For example, the majority of the pure phase h-MoO3 nanowhiskers have an aspect ratio of up to 1250, e.g., have a diameter of 20-150 nm and a length of up to 25 microns.
In some embodiments, the majority and according to the discovered growth mechanism of the MoS2 INTs, their aspect ratio after the second stage (and possibly also third stage) of sulfurization is obtained up to 750, e.g., have a diameter of about 20-150 nm and a length of up to 15 microns. The length and aspect ratio of the NTs is partially shortened during the multiple reaction steps and intermediate product handling.
In some embodiments, the first predetermined temperature is in a range of about 380-400° C. (temperature condition T1).
In some embodiments, the partial reduction of h-MoO3 to MoOx comprises interacting the h-MoO3 with flows of the reducing agent H2 and carrier gas N2 at 5-20 ml/min and 100 ml/min flow rates, respectively, for about 10-20 min, to produce the MoOx-containing nanowhiskers being of MoOx/MoO3 phase comprising a mixture of MoOx suboxide phases and MoO3.
For example, the first stage sulfurization of h-MoO3 further comprises interacting the MoOx/MoO3 nanowhiskers with flows of reactive gases H2S and H2 and carrier gas N2, 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. By this, the nanostructures are produced comprising the initial MoS2 intermittent guiding layers on the surface of the MoOx/MoO3 nanowhiskers such that the mixture of suboxide phases comprising Mo4O11, Mo8O23 and MoO2 located inside the core of the MoS2 INTs at the initial stage of their growth, and the MoS2 intermittent guiding layers are located on the surface of the MoOx/MoO3 nanowhiskers providing protection against any one of sublimation or over-reduction of MoOx core at higher temperatures.
In some embodiments, the method comprises applying the second and third stages of sulfurization to different portions of said nanostructures under differently controlled conditions.
In some embodiments, the second predetermined temperature maintained during the second stage sulfurization is 750-820° C. (temperature condition T2). The MoS2 nanotubes being produced have a needle-like crystal with hollow core morphology characterized by walls comprising the randomly oriented nanoplatelets along a longitudinal axis of the MoS2 INTs, thereby forming a high-density arrangement of active sites enabling enhanced catalytic activity.
For example, the second stage sulfurization is carried out using flows of the reactive gas H2S and a carrier gas N2, at flow rates of 5-10 ml/min, and 80-100 ml/min, respectively, for a time period of 30-60 min.
For example, the MoS2 INTs have enhanced catalytic activity in electrocatalytic hydrogen evolution reaction (HER).
In some embodiments, the third predetermined high temperature is 950° C. (temperature condition T3). The MoS2 INTs being produced have the needle-like crystal with hollow core morphology, and have walls' highly crystalline continual MoS2 layers substantially parallel to a longitudinal axis of the INT.
For example, the third sulfurization stage is carried out using flows of H2S, H2 and N2 gases, at flow rates of 5-10 ml/min, 5-10 ml/min and 80-100 ml/min, respectively, for a time period of 30-60 min, using type-I MoS2 INTs as a precursor. The MoS2 INTs with such walls' structure may be operable as optically or electrically active elements.
According to another broad aspect of the present disclosure, it provides a product configured as a precursor and a templating agent for producing therefrom highly crystalline molybdenum disulfide (MoS2) inorganic nanotubes (INTs) via two-to-three successive stages of sulfurization, said product comprising pure phase hexagonal molybdenum oxide (h-MoO3) nanowhiskers having an aspect ratio of up to 1250.
In yet further broad aspect of the present disclosure, it provides a product comprising molybdenum disulfide (MoS2) inorganic nanotubes (INTs) having needle-like crystal with hollow core morphology and walls characterized by randomly oriented nanoplatelets along a longitudinal axis of the MoS2 INTs, defining a high-density arrangement of active sites enabling enhanced catalytic activity.
The present disclosure, in its yet other aspect provides a product comprising molybdenum disulfide (MoS2) inorganic nanotubes (INTs) having needle-like crystal with hollow core morphology and walls' structure characterized by highly crystalline continual MoS2 layers substantially parallel to a longitudinal axis of the INT. These MoS2 INTs may be operable as optically or electrically active elements. The highly crystalline structure of these nanotubes enables well-defined optical and electrical characteristics for optoelectronic 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.
In yet another aspect, the present disclosure provides a product configured as a templating agent (precursor) for producing therefrom highly crystalline molybdenum disulfide (MoS2) inorganic nanotubes (INTs) via two or three successive stages of sulfurization, said product being produced by the above-described synthetic method and comprising pure phase hexagonal molybdenum oxide (h-MoO3) nanowhiskers having an aspect ratio of up to 1250.
As described above, the MoS2 nanotubes having needle-like crystal with hollow core morphology and walls comprising the randomly oriented nanoplatelets along the longitudinal axis of the MoS2 INTs, are characterized by the high-density arrangement of active sites enabling enhanced catalytic activity. Such MoS2 nanotubes can be used to form an active surface of an electrode. To this end, the electrode (of any known suitable material- and geometrical characteristics) can be coated (using any known suitable technique) by the MoS2 nanotubes. The electrode can then be placed to enable interaction between the active surface thereof and any selected analyte-containing medium to enable the required reaction to proceed. Such electrodes can be part of an electrolyzer or a fuel cell.
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.
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:
Referring to
The chemical precipitation unit 12 is configured and operable to perform the initial synthesis of pure phase hexagonal molybdenum oxide (h-MoO3) nanowhiskers, which serves as a precursor and templating agent for further production of the MoS2 INTs.
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 N2, H2, H2S, 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 MoO3 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 MoS2 INTs of predetermined morphology and dimensions. As also shown in
Then, a first stage sulfurization of the precursor and templating agent, h-MoO3, is performed (step 104). This utilizes a solid-gas reaction of the h-MoO3 with reactive gases (H2, H2S) at the first predetermined temperature conditions Ti. This first stage of sulfurization includes partial reduction of h-MoO3 nanowhiskers to MoOx-containing nanowhiskers (2≤x<3), which is followed by the formation of nanostructures corresponding to an initial growth stage of MoS2 INTs. These nanostructures have cores comprising MoOx-containing nanowhiskers and initial MoS2 intermittent guiding layers at the surface of the MoOx-containing nanowhiskers. These initial MoS2 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 MoOx at higher temperatures.
The so-obtained nanostructures presenting the initial growth stage of MoS2 INTs proceed to second and third stages of sulfurization (step 106) performed via a solid-gas reaction at second and third predetermined temperature conditions T2, T3 such that T2 and T3 are relatively high as compared to the first temperature conditions Ti. This provides recrystallization of the MoS2 intermittent guiding layers to obtain highly crystalline substantially parallel layers, to sulfurize MoOx inside the cores to MoS2, and obtain pure phase and high aspect ratio MoS2 INTs of needle-like crystal with hollow core morphology and predetermined walls' structure and dimensions.
Reference is made to
The inventors found the conditions for obtaining very thin and long h-MoO3 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-MoO3 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.
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.
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).
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-MoO3 nanowhiskers.
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).
The final experimental parameters for obtaining MoO3 nanowhiskers were found as follows:
Increase in time or temperature results in a larger size distribution of MoO3 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.
Further, the inventors clarified that pure hexagonal phase of MoO3 with tunnelling structure along the whisker axis was essential to direct the growth of 1D needle-like whiskers (MoO3 could be orthorhombic or hexagonal). The inventors tuned the reaction to obtain h-phase. It should be noted that higher temperature synthesis of MoO3 whiskers results in orthorhombic phase and loss of 1D morphology.
The pre-prepared h-MoO3 nanowhiskers by method 200 are used as a precursor and templating material for preparing MoS2 INTs via a ‘solid-gas’ reaction, with two-step-temperature growth using H2S, H2 as reactive gases and N2 as a carrier gas, as described by the method 300 shown in
Since the MoO3 1D nanowhiskers are essential for 1D MoS2 nanotube preparation, being used as a templating agent, the dimensions of the resulting 1D MoS2 INTs are close to the sizes of MoO3 1D nanowhiskers. However, sulfurization of the MoO3 nanowhiskers into MoS2 nanotubes is challenging due to the specific properties of MoO3, which sublimes starting from 400° C. Once sublimed, the 1D morphology of MoO3 and its templating function for nanotubes' growth is lost.
In the present disclosure, the inventors developed a unique two-to-three step sulfurization process for preparing MoS2 nanotubes from MoO3 whiskers without compromising the 1D morphology. Moreover, the inventors can control the synthesis to prepare two types of these nanotubes: First-type (type-I) MoS2 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 MoS2 NTs; and second-type (type-II) MoS2 nanotubes with walls comprising highly crystalline parallel MoS2 layers, suitable for applications in optoelectronics and electro-mechanics.
The method 300 starts (step 302) by preparing h-MoO3 nanowhiskers by method 200, which, as mentioned above, are needed as a precursor and templating agent for 1D MoS2 nanotube preparation.
Short reduction of MoO3 to MoOx precedes the first sulfurization step (step 304). Reduction to MoOx 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: N2=100 ml/min, H2=5-20 ml/min, reaction time t=10-20 min.
Given the instability of h-MoO3 at elevated temperatures, the first stage sulfurization reaction (306) is executed at a lower temperature (380-400° C.), with additional process parameters to avoid MoO3 sublimation, using H2S, H2 as reactive gases and N2 as a carrier gas, resulting in Mo4O11/MoO2 core encapsulated inside the MoS2 shells. The experimental parameters of this step are: T=380-400° C., gas flows: N2=100 ml/min, H2=5-20 ml/min, H2S=5-10 ml/min, reaction time=30-60 min.
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 MoS2 guiding layers at the periphery of the nanotubes and Mo4O11/MoO2 inside the core (step 308). These guiding layers serve to protect against possible sublimation at higher temperatures.
It should be noted that, while the formation of the initial guiding MoS2 layers is essential to avoid MoO3 sublimation or collapse, the parameters of this reaction could vary in a wide range. However, all parameters, like temperature, timing, and H2S/H2 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 MoO3.
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 MoO3 is much faster. Therefore, the H2S 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.
For example, the parameters of such reactions are: T=750-920° C., gas flows: N2=80-90 ml/min, H2=10 ml/min, H2S=10-50 ml/min, reaction time=5 sec-5 min.
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) MoS2 INTs of the present disclosure.
The first stage sulfurization is followed by the high-temperature (T2, e.g., 750-820° C.) second stage sulfurization, and possibly further followed by the higher-temperature (T3, e.g., 950° C.) third stage sulfurization. Both, the second and third stages of sulfurization provide recrystallization of the MoS2 intermittent guiding layers to obtain highly crystalline parallel layers and sulfurizing the residual MoOx accompanied by the creation of a needle-like crystal with hollow core morphology, and to obtain pure phase and high aspect ratio MoS2 INTs of predetermined walls' structure and dimensions. The second stage sulfurization results in the type-I MoS2 INTs (step 312) having tubes' walls including randomly oriented nanoplatelets, and the third stage sulfurization results in the type-II MoS2 INTs having layers substantially parallel to the tube axis.
In order to obtain the type-I MoS2 INTs, the second stage sulfurization (step 310) may include the following reaction parameters: T2=750-820° C., N2=80-100 ml/min, H2S=5-10 ml/min, reaction time=30-60 min.
In order to obtain the type-II MoS2 INTs, the third stage sulfurization (step 314) may be performed using the following reaction parameters: T3=950° C., N2=80-100 ml/min, H2=5-10 ml/min, H2S=5-10 ml/min, and reaction time=30-60 min using type-I MoS2 INTs as a precursor. The type-II MoS2 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 MoS2 layers to the final nanotube.
The obtained MoS2 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 MoS2 layers on the periphery, and partial shortening of the whiskers was ascribed to the handling of the sample during multiple reactions and sample preparation.
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.
Reference is made to
As mentioned above, the inventors tuned the reaction to obtain the h-phase MoO3 since higher temperature synthesis of whiskers results in orthorhombic phase and loss of 1D morphology. The schematic representations of h-MoO3 and α-MoO3 crystal structures are shown in
The MoS2 INTs, according to the method of the present disclosure, can be prepared of two types in a controllable way.
The second and third sulfurization reactions at elevated temperatures (T2 and T3, respectively) form two types of MoS2 INTs. As described above, the process may include only first and second stages resulting in type-I MoS2 INTs, or may further proceed towards higher-temperature third stage of sulfurization resulting in type-II MoS2 INTs.
The type-I MoS2 INTs (second stage sulfurization under temperature conditions of T2=750-820° C.) comprise walls' structure defining randomly oriented nanoplatelets along a longitudinal axis of the MoS2 INT which may be also described as short intermittent MoS2 guiding layers at the periphery of the nanotubes and Mo4O11/MoO2 inside the core owing to defect-rich nanotubes' surface (
The type-II MoS2 INTs (third stage sulfurization under temperature conditions of T3=950° C.) results from the complete conversion of oxide to MoS2 providing walls' structure characterized by highly crystalline, continual MoS2 layers parallel to the tube axis (
X-ray diffraction (XRD) patterns of MoS2 nanotubes at the different stages of their synthesis are shown in
Thus, in the present disclosure, h-MoO3 nanowhiskers with a high-aspect-ratio have been synthesized using a chemical precipitation method. The obtained h-MoO3 nanowhiskers are crystalline and one-dimensional. The aspect ratios of h-MoO3 nanowhiskers are 20-125 times higher than previously reported. The as-prepared h-MoO3 nanowhiskers were used as a chemical precursor and template for the preparation of peerless MoS2 inorganic nanotubes by a two-to-three step sulfurization process using H2S/H2 as reactive gases. After the first sulfurization reaction of h-MoO3 whiskers, the intermittent MoS2 guiding layers are formed on the nanowhiskers' rim, with reduced Mo4O11 and MoO2 core. After the second sulfurization at 750-820° C., distorted MoS2 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 MoS2 INTs were obtained with continual layers parallel to the tube axis with a hollow core inside and some distorted MoS2 layers.
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.
In the following, the inventors demonstrate that the first-type MoS2 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 MoS2 INTs.
Layered transition metal dichalcogenides (TMD) such as MoS2 [28,29], MoSe2 [30,31], WS2 [32,33], and WSe2 [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 inactive—the 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 MoS2 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.
To demonstrate the activity towards the hydrogen evolution reaction, the inventors performed linear sweep voltammetry (LSV) measurements in acidic (0.5M H2SO4) and alkaline (0.5M KOH) media for a group of related layered materials. It is noted that the WS2 in this study have perfectly oriented molecular layers along the tubes' axis, demonstrating the significant advantage of the defected MoS2 nanotubes as catalysts.
Reference is made to
The activity trend is MoS2>WS2>VS2, as seen by the lower overpotential and smaller Tafel slopes. In addition, the MoS2 nanotubes offer low charge transfer resistance, facilitating the catalytic reaction, as revealed by electrochemical impedance spectroscopy (EIS) measurements. Specifically, the MoS2 nanotubes, corresponding to the first-type MoS2 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 MoS2 nanoflowers with their defective surfaces exhibited a lower ECSA, emphasizing the specific contribution of combining defects and crystallinity of the MoS2 nanotubes on the formation of active sites in the MoS2 nanotubes. Normalizing the overall activity to the ECSA provides a measure of the intrinsic activity of the catalysts, showing that MoS2 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 H2SO4. 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 MoS2, only slight surface oxidation of the catalyst was noticed.
Hence, the type-I MoS2 INTs of the present disclosure having high density arrangement of active sites therealong (i.e., MoS2 INTs of the needle-like crystal of hollow core morphology with the walls' structure of randomly oriented nanoplatelets along the longitudinal axis of the MoS2 INT), can be used to form a chemically active electrode. This is illustrated in
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17300330 | Jun 2023 | US |
| Child | 18352891 | US |
