CA2005826C - Sheets of transition metal dichalcogenides - Google Patents

Abstract

Compositions of the formula MX2:Y, wherein MX2 is a layer type transition metal dichalcogenide, M is a metal selected from the group consisting of niobium, tantalum, molybdenum and tungsten, X is a chalcogen selected from the group consisting of sulfur and selenium and Y is a material located between layers of MX2. The compositions may be prepared by the steps of first forming a suspension of the MX2 in a body of water. A liquid which is immiscible with water is added to the suspension. The suspension and immiscible liquid are agitated together to form a temporary emulsion. The emulsion is allowed to rest until the water and the liquid separate with an interface therebetween. A sheet-like composition of MX2:Y
forms at the interface. Preferably, the MX2 is in exfoliated, singular molecular thickness layers suspended in the water. The MX2:Y compositions may be used to coat objects or provide films for applications requiring the special properties of MX2:Y, including selective optical filters and energy selective x-ray focusing devices.

Description

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FIELD OF THE INVENTION
This invention relates to sheets or films of transition metal dichalcogenides, particularly molybdenum disulfide.
BACKGROUND OF THE INVENTION
It has recently been found that singular molecular layers of layer-type transition metal dichalcogenides, such as MoS2, TaS2 and WS2, can be prepared by intercalating such compounds with lithium and then reacting the intercalated compound with water. This gives rise to a suspension of single molecular layers of the transition metal dichalcogenides in water.
Attempts have been made in the past to produce sheet-like forms of metal dichalcogenides as revealed, for example, in United States Patent No. 4,299,892 to Dines and Chianelli. Here, an amorphous transition metal dichalcogenide product is prepared by low temperature, non-aqueous precipitation of the compound from mixtures of the metal salt. The amorphous products are converted into sheets of metal dichalcogenides referred to in the patent as having a "rag-like" structure by controlled heating at temperatures between 250° C and 400° C: Ho~rever, neither the end product, nor the intermediate product are oriented films or sheets, that is films or sheets wherein the crystalline c-axes of single layers of the metal dichalcogenide are aligned.

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United States Patent No. 4,647,386 to Jamieson discloses an intercalated transition metal based solid lubricating composition. A transition metal dichalcogenide is intercalated with a metal, preferably a coinage metal.
SUMMARY OF THE INVENTION
The invention provides a process for forming sheet-like compositions of the formula MX2:Y, wherein MX2 is a layer-type transition metal dichalcogenide, M is a metal group consisting of niobium, tantalum, molybdenum and tungsten, X is a chalcogen selected from sulfur and selenium and Y is a material located between layers of MX2. The process includes the steps of forming a suspension of the MX2 in water and adding a liquid which is immiscible with water to the suspension to form a mixture. The mixture is agitated to form a temporary emulsion. The emulsion is allowed to rest until the water and the liquid separate with an interface therebetween. A
sheet-like composition of MXZ:Y forms at the interface, MX2 has a crystalline structure with c-axes aligned in a direction perpendicular to the plane of the layers., Preferably, the MX2 suspension comprises ~~~D~~~~

liquid, water molecules or molecules of a foreign substance dissolved in the liquid.
The invention also relates to compositions prepared according to the processes described above. The compositions may be used to coat objects.
The invention provides thin. aligned sheets or films and coatings of transition metal dichalcogenides by a relatively expeditious process which can be readily scaled to provide large area films and coatings. These coatings or films have unique qualities. For example, such thin, oriented films of molybdenum disulfide have unique optical qualities and can be used as selective filters.
These films are believed to have advantageous lubrication qualities when compared with non-oriented films. Because thin, highly oriented films of MoS2 can be deposited on curved surfaces, the invention may be used to make relatively inexpensive, large area energy selective X-ray focusing devices. The invention also provides unique layered compositions which may be utilized for the composition of the transition'metal dichalcogenide, for the properties of other substances between layers of the metal dichalcogenides or combinations of the two. For example, the optical properties of MoS2 in combination with the other organic molecules between MoS2 layers can be used as selective optical filters.

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BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Figure la is a diagrammatic representation of a process for making a thin, oriented film of a transition metal dichalcogenide according to an embodiment of the invention and employing a water immiscible liquid having a specific gravity less than 1;
.. Figure lb is a view similar to Figure la. showing the process using a water immiscible liquid with a specific gravity greater than 1;
Figure 2 is an enlarged view of the portion within the circle in Figure lb.;
Figure 3a is a diagrammatic representation of the coating of an object with a film produced by the process of Figure la.;
Figure 3b is a diagrammatic representation of the process of coating of a wet glass slide employing the.film produced by the process of Figure lb:;

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Figure 4 is an enlarged, diagrammatic representation of a portion of an object coated with a film of transition metal dichalcogenide incorporating water immiscible liquid and water molecules produced by the processes Figures 3a or 3b;
Figure 5 is a view similar to Figure 4 showing the object and film after heating;
Figure 6 is a diagrammatic side view of the metal w dichalcogenide film shown after drying;
Figure 7 is an enlarged diagrammatic view of the portion within a circle of Figure 6 showing ferrocene between the layers of metal dichalcogenide;
Figure 8 is an enlarged diagrammatic view of an object coated with a film of transition metal di.chalcogenide incorporating ferrocene shown after baking;
Figure 9 i5 a diagrammatic view of a method for.
coating a hydrophobic substrate with molybdenum disulfide film;
Figure 10 is a diagrammatic view of a prpcess for transferring a molybdenum disulfide film from one substrate to another;

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Figure lla is a proposed model of single layers of molybdenum disulfide in water; and Figure llb is a view similar to Figure lla and showing the layers as modified by the presence of a water immiscible liquid.
1~ DESCRIPTION OF THE PREFERRED EMBODIMENTS
A known process for preparing single layers of molybdenum disulfide in water involves exfoliating a lithium intercalated layered transition metal dichalcogenide, such as MoS2, TaS2, NbS2 or WS2 by immersion in water. Selenium may replace the sulfur as the dichalcogen. ,_, In one example molybdenum disulfide powder was soaked in a solution of n-butyl lithium in hexane for about 48 hours in a dry box containing an argon atmosphere. Once the molybdenum disulfide was fully intercalated with lithium ions between the layers of molybdenum disulfide, the product was removed and washed repeatedly in hexane, dried and sealed in a vial while still in the dry box under argon atmosphere. The vial was then removed from the dry box, immersed in water, and the cap removed from the vial. Upon contact of the contents of the vial with liquid water,'copious'gas evolution followed and the molybdenum disulfide powder formed a ':

~~Cl~~~,~i highly opaque suspension in the water. The suspension was agitated, in this case ultrasonicated, during the reaction to assist in the exfoliation. The hydrogen gas produced by the reaction between the lithium and the water pushed the layers of molybdenum disulfide apart until they were completely separated. The suspension was repeatedly centrifuged and washed with distilled water. A final concentration of molybdenum disulfide of about 5 mg/cc of water was produced.
The formation of films or sheet-like forms of transition metal dichalcogenides according to this invention begins with the single layer suspension of molybdenum disulfide in water obtained above. Referring Fig, la,, a liquid 21 is immiscible with water and, in this .' case, has a density less than water. Various organic solvents were tested including alkanes, alkenes and butyl alcohol (n-butyl and iso-butyl). The liquid was added to the suspension 21 of molybdenum disulfide in water to form a two-phase liquid 23. The suspended molybdenum disulfide stays in the water.
The mixture was then shaken as indicated by arrows 25 and formed an unstable emulsion containing globules of water in the liquid. The molybdenum disulfide layers placed themselves at the surfaces of the resulting globules of water. When the shaking ceased, these globules _ ~~~i~~~~~o _8_ gradually migrated towards the water/liquid interface 27 where they coalesced with each other. The molybdenum disulfide molecular layers formed a film at the horizontal interface 27 between the two liquids. With moderate shaking, both the water and the water immiscible liquid became clear. Therefore. all of the molybdenum disulfide was in the interface film 32 as shown to the right of Fig.
la. Clear water 20.1 remained below the film.
It was found, qualitatively, that the lower the pH of the water, the more rapid the accumulation of the molybdenum disulfide at the interface. The accumulation of molybdenum disulfide at the interface occurred faster with alcohol than with alkanes or alkenes. After the interface is formed, it is possible to further transfer the molybdenum disulfide layers to the alcohol but not the alkanes or alkenes. This was done by lowering the pH of the water. The transfer occurred at a pH between 2 and 3 in the case of n-butyl alcohol when dilute nitric acid was gradually added while shaking. With alkanes and alkenes, ' attempts at such transfers resulted only in flocculation of molybdenum disulfide at the water/liquid interface.
The following are organic solvents with a specific gravity less than l were tested:
alkanes (n = 5 to 12) hexene benzene cyclohexane n-butyl alcohol styrene iso - butyl alcohol tertiary butyl benzene 1, 3-5, trimethylbenzene g -Fig. lb illustrates a variation of the process of Fig. la wherein the specific gravity of the water immiscible liquid is greater than 1. Suitable solvents tested were 1,2-dichloroethane, carbon tetrachloride, dimethoxybenzene, 1-chloronaphthelene, and iron pentacarbonyl. However, it was found that mercury does not work. In this process, the solvent 22 was mixed with the molybdenum disulfide suspension 20 to form a two-phase liquid 24 which is agitated as indicated by arrows 26.
Film accumulation 32 occurred both at water/liquid interface 28 as well as at water/air interface 30.
Referring to Fig. lla, this shows one of the single layers of molybdenum disulfide 32 contained within the water suspension 20 of Figures la and lb. While we do not wish to be bound to this theory, it is believed that each of the single layers 20 possesses a net negative charge due to surface hydration represented in Fig. lla by the OH'ions.
Referring Fig. llb, this shows the result of mixing an aqueous suspension of exfoliated molybdenum disulfide with a liquid which is immiscible with water and agitating the resulting mixture as described above.
As described above; the first configuration that forms after agitating the two-phase mixture is that of globules encased by molybdenum disulfide layers. These ~v~LD;~~~G
- to -globules are inherently unstable and, in time, coalesce, presumably because the free energy associated with the globules is much higher (proportional to interfacial area of the globules) than that of the final phase-separated mixture with a single horizontal interface between the water and the water immiscible liquid. With the coalescence of globules, the modified single layers of Fig. llb form a multi-layer membrane at the interface.
Referring to Fig. 2, this shows in diagramatic form an enlarged section through the membrane. The layers 32 of molybdenum disulfide are stacked on each other with water molecules 34, identified by while ovals, and liquid molecules 36, identified by black ovals, trapped between them. Based on the above discussion, it can be expected that the immiscible liquid will be non-polar or weakly polar.
2~,y. It was observed that the suspensions of exfoliated molybdenum disulfide are completely cleared by shaking the mixture, leaving no layers in the water phase.
This appears to indicate that all of the hydroxyl graups on the basal planes are replaced by the liquid molecules:
If the hydroxyl groups were still present in any significant amount, some degree of dispersion of molybdenum disulfide in water would be expected after shaking.

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It has been found that the molybdenum disulfide film at the interface has a tendency to spread. As shown in Figures la and 1b, the film 32 tends to creep along the walls of a glass container holding the mixture.
It has also been found that the thin film of molybdenum disulfide spreads on a thin layer on wetted surfaces. In particular, referring to Fig. 3b, a pre-cleaned glass slide 38 was wetted after a brief wash with dilute hydrofluoric acid and then dipped into the phase separated mixture such that the lower end just touched the accumulated molybdenum disulfide at the interface of water 20.1 and solvent 21. This resulted in a rapid spreading of a film 32 of molybdenum disulfide up both sides of the slide as illustrated, covering the entire wetted area. After the motion of the film stopped, the slide was withdrawn from solution and was kept hanging vertically in room air for drying. Although the film was apparently dry after a few minutes, it was not completely devoid of water at this stage as seen in Fig: 4 where molecules 20.1 of water and molecules 21 of solvent are trapped between layers of MoS2 32. However, the resulting film was optically very uniform and highly oriented ds determined by x-ray diffraction. The film is shown after drying on slide 38 in Fig. 5.
Films of molybdenum disulfide were grown on various hydrophilic substrates using this method including glass, ceramics, oxidized copper, oxidized silicone, tungsten trioxide, glass and even cardboard.
A slight variation of the above method is to dip a dry hydrophilic substrate into the phase-separated mixture past the accumulated molybdenum disulfide into the water 20.1. The film was riot formed when inserting the glass substrate through the interface 27 into the water 20.1, but a film of molybdenum disulfide was deposited when the substrate, now wet. was withdrawn.
When a hydrophobic substrate. such as PTFE was inserted, a film formed on it, but was restored to the water/organic interface when the substrate was withdrawn.
Another method of depositing films tested was to allow the water immiscible liquid to evaporate after the process of Fig. lb., leaving the molybdenum disulfide film at the air/water interface as shown in Fig. 9. The resulting mufti-layer film 32 can be transferred to a hydrophobic substrate 33 by simply touching the film with the substrate oriented horizontally, alternatively from above the film 32 disulfide interface and from below the film 32.
Fig: 10 illustrates another method used for coating hydrophobic surfaces with film. following formation of the film using the spreading technique of o~~~a~~~

Fig. 3b, there is a water layer between the film and the substrate. When the substrate was reimmersed into deionized (resistivity greater than 10 Mohm cm) water, the film 52 disengaged itself from the glass slide and re-spread on the air/water interface 54. The hydrophobic substrate 56 was placed on a support 58 in a horizontal position just under the air/water interface 54. The water level was then lowered past the substrate and the film is deposited on a non-polar substrate as shown to the right of the figure. The proportion of the film transferred was directly related to the fraction of the slide which was -. immersed in water. The substrate 56 with the newly transferred film of molybdenum disulfide 60 was then removed from the container and let dry in room air.
0 ..
This method of transfer was used with such substrates as PTFE, polystyrene, copper, titanium, aluminum, brass and silicon. Relatively thin films z5 (approximately 30 angstroms) as well as relatively thick films (350 angstroms), have been transferred in this manner. Dilute hydrofluoric acid (approximate 5 - 10 percent in water) can be used instead of the deionized 30 water as the intermediate medium when transferring films of molybdenum disulfide onto silicon. Keeping the silicon substrate in dilute hydrofluoric. acid ensures that the native oxide of silicon is removed, thereby enabling intimate contact between the molybdenum disulfide film and the silicon. The fact that molybdenum disulfide does not react with hydrofluoric acid is of importance here.

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Fig. 3a illustrates a further method of transferring films when the water immiscible liquid has a specific gravity greater than 1 as previously used in the process of Fig. la. In Fig. 3a, the film was deposited directly on to a metal substrate. Freshly etched metals including aluminum, copper or steel 64 were inserted into the water/air interface 30. A coating of molybdenum disulfide was deposited on them. This method could be useful for preparing lubricant coatings on metal surfaces, for example.
In one example styrene was used as the water immiscible liquid to obtain styrene molecules between the MoS2, layers giving a c-spacing of 11.5 angstroms. The composition was subsequently heated to about 60°C in an inert argon atmosphere to polymerize the styrene into polystyrene. This may be of use in protecting the MoS2, layers.
In another example a mixture of the water immiscible liquid iron pentacarbonyl, Fe(CO)5, and a suspension of single layer MoS2 in water was shaken to produce a composition of MoS2 with iron pentecarbonyl between the MoS2 layers with a c-spacing of 12.0 angstroms. The resulting composition was heated in an inert atmosphere until the iron pentacarbonyl decomposed, releasing carbon monoxide and forming iron between the MoS2 layers. Other metal carbonyls can also be used as the immiscible liquid:

_. o~~~ ~~a~s~a X-ray diffraction measurements confirmed that iron between the layers gave a c-spacing of 6.33 angstroms as compared with 6.13 angstroms for the unexfoliated MoS2. Novel compositions of Fex MX2, wherein M is Nb, Mo,Ta or W and X is S or Se with varying x, can be obtained using this method. This technique could potentially be extended to obtain compositions of MX2, with many other metals between the layers.
Instead of using merely a water immiscible liquid as described above, it is possible to dissolve other solutes in the liquid. For example,.referring to Figs. 7 and 8, ferrocene was dissolved in benzene and, alternatively in carbon tetrachloride, to produce films that included ferrocene 76 between the molybdenum disulfide layers 74.
To produce a powered material with ferrocene included between the Mo52 layers, with reference to Fig, la, the water 20.1 was decanted and the water immiscible liquid 22 was pipetted. In the case of Fig.
lb., the liquid 21 was evaporated and the water 20.1 pipetted. The resultant composition was dried in room air.
The result was a powder 72 of molybdenum disulfide as shown in Fig. 6. The enlarged view of Fig. 7 shows that the powder includes a plurality of layers 74 of MoS2 with molecules 76 of ferrocene, represented by x's, between the layers. The resultant composition can be identified by the ~~C~;~~'~.'~

formula MX2:Y, where Y is the ferrocene. This was in powder form. Fig. 9 shows a molybdenum disulfide film incorporating ferrocene molecules coating a substrate 78 via the processes described above. It should be noted that there is only a single layer of ferrocene molecules between the layers of molybdenum disulfide instead of two layers of liquid molecules as shown in Fig. 2.
The resulting films are highly oriented as shown by x-ray diffraction. The orientation is much better than obtained with organic solvent alone. Additionally, the presence of ferrocene between the layers makes the resultant films on substrates electrically more conducting. The insertion of ferrocene into the interlayer 0 spacing of molybdenum disulfide is by itself novel and cannot be done with conventional intercalating techniques.
Inclusions such as other metallocenes, dyes, monomers and liquid crystals can be included in layered transition metal dichalcogenide films by this technique.
zntercalcated complexes of NbS2 and TaS2 existed prior to the invention, but not MoS2 and WS2 intercalated with organic materials.
Besides ferrocene, other possible solutes are copper phthalocyanine (CuPc), phthalocyanine (Pc), stearamide. and chrysene. Below is a table of films produced with molybdenum disulfide using a particular solvent and a solute dissolved in the solvent. Large area films can be produced using the above method.

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Solvent Solute Properties and Uses 1. benzene, ferrocene Highly oriented, and carbon better electrical tetrachloride conductivity. Powder may have applications as a catalytic material.

2. benzene, PhthalocyanineBlue colour films, hexene dyes, CuPc, possible gas sensing Pc etc. ability.

1-chlorona-Pthalene 3, benzene, chrysene whitish film, fluorescent hexene film, purple fluorescence upon exposure to ultra-violet radiation.

4. hexene stearamide widely separated I~oS2 layers.

. .,.

Claims (26)

1. A process for forming sheet-like compositions of the formula MX2: Y
wherein MX2 is a layer-type transition metal dichalcogenide, MX2 having a crystalline structure with c-axes aligned in a direction perpendicular to the plane of the layers, M is a metal selected from the group consisting of niobium, tantalum, molybdenum and tungsten, X is a chalcogen selected from the group consisting of sulfur and selenium and Y is a material located between layers of MX2, the MX2, which comprises the steps of:

(a) forming a suspension of MX2 in a body of water;
(b) adding a non-metallic liquid which is immiscible with water to the suspension;
(c) agitating the mixture of the MX2 suspension and the liquid to form a temporary emulsion;
(d) allowing the emulsion to rest until the water and the immiscible liquid separate with an interface therebetween and a sheet-like composition of MX2:Y forms at said interface.

2. A process as claimed in Claim 1, further comprising the steps of removing the MX2:Y from the body of water and the immiscible liquid.

3. A process as claimed in Claim 1, wherein the MX2 is provided in the form of platelets.

4. A process as claimed in Claim 1, wherein the MX2 is provided in layers having a single molecular thickness.

5. A process as claimed in Claim 1, wherein the immiscible liquid is selected with a density less than the density of water.

6. A process as claimed in Claim 5, wherein the water immiscible liquid is selected from the group consisting of hexene; alkanes (n = 5 to 12); 1, 2-dichloroethane;
isobutyl alcohol; n-butyl alcohol; benzene; cyclohexane; 1, 3, 5 trimethylbenzene; and tertiary butyl benzene.

7. A process as claimed in Claim 1, wherein the immiscible liquid is selected with a density greater than the density of water.

8. A process as claimed in Claim 7, wherein the immiscible liquid is selected from the group consisting of 1,2-dichloroethane; carbon tetrachloride; dimethoxy benzene;
1-chloronaphthalene; and iron pentacarbonyl.

9. A process as claimed in Claim 1, further comprising the step of adding acidic material to the water so the pH is between 2 and 7.

10. A process as claimed in Claim 1, wherein Y includes water and the immiscible liquid between layers of the MX2.

11. A process as claimed in Claim 10, wherein Y further includes a solute.

12. A process as claimed in Claim 11, wherein the solute is selected from a group consisting of ferrocene, 1, 2-diferrocenylethane; chrysene; stearamide; phthalocyanine and copper phthalocyanine.

13. A process as claimed in Claim 10, further comprising the step of drying the MX2:Y to remove the Y from the MX2.

14. A process as claimed in Claim 11, wherein the MX2:Y is heated in an inert atmosphere to remove the water and immiscible liquid and leave the solute between the layers of MX2.

15. A process as claimed in Claim 6, wherein the immiscible liquid selected is styrene, the process further comprising the steps of heating the MX2:Y to polymerize the styrene between the layers of MX2.

16. A process as claimed in Claim 6, wherein the immiscible liquid is a liquid metal carbonyl.

17. A process as claimed in Claim 16, wherein the immiscible liquid is iron pentacarbonyl, the process further comprising the steps of heating the MX2:Y to decompose the iron pentacarbonyl to form iron between the layers of MX2.

18. A method of coating an object, comprising:
(a) forming a sheet-like composition at an interface between water and a non-metallic liquid which is immiscible with water, the composition having the formula:
MX2:Y

wherein MX2 is a layer-type transition metal dichalcogenide, MX2 having a crystalline structure with c-axes aligned in a direction perpendicular to the plane of the layers, the M is selected from the group consisting of niobium, tantalum, molybdenum and tungsten; X is selected from the group consisting of sulfur and selenium, and Y is a material located between layers of MX2; and bringing the object into contact with the sheet-like composition and allowing the composition to spread over the object.

19. A method as claimed in Claim 18, wherein the MX2 is crystalline with c-axes perpendicular to the substrate.

20. A composition having the formula:~
MX2: Y

wherein MX2 is a layer-type transition metal dichalcogenide selected from the group consisting of molybdenum disulfide and tungsten disulfide MX2 having a crystalline structure with c-axes aligned in a direction perpendicular to the plane of the layers; and Y is an organic material located between layers of MX2.

21. A composition as claimed in Claim 20, wherein the MX2 is in the form of single molecular thickness sheets.

22. A composition as claimed in Claim 20, wherein Y is selected from a group consisting of hexene, alkanes (n = 5 to 12); 1,2-dichloroethane; iso-butyl alcohol; n-butyl alcohol; cyclohexane; benzene; styrene; 1, 3-5 trimethylbenzene; tertiary butyl benzene; 1, 2-dichloroethane; carbon tetrachloride; dimethoxy benzene;
1-chloronaphalene; chrysene; stearamide; phthalocyanine;
copper phthalocyanine and iron pentacarbonyl.

23. A composition as claimed in Claim 21, wherein Y is a monolayer of a metallocene molecules.

24. A composition as claimed in Claim 23, wherein the metallocene is selected from the group consisting of ferrocene and 1, 2 diferroceylenthane.

25. An object having a surface coated with a composition having the formula:

MX2:Y

wherein MX2 is a layer-type transition metal dichalcogenide, M is a metal selected from the group consisting of niobium, tantalum, molybdenum and tungsten, X is a chalcogen selected from the group consisting of sulfur and selenium, and Y is an organic substance located between layers of MX2, the MX2 being crystalline with c-axes perpendicular to the surface of the object.

26. An object as claimed in Claim 25, wherein the MX2 is in the form of single molecular layers.