METHOD OF MAKING ANHYDROUS METAL SULFIDE NANOCRYSTALS

Abstract

Methods of forming Li.sub.2S and other MS.sub.n nanocrystals are provided. The methods employ low-cost lithium salts as a reagent and utilizes one or more metathesis reactions that occur either in solution, preferably at or near ambient conditions, or in the solid-state at elevated temperatures.

Claims

1. A method of producing Li.sub.2S comprising: mixing a first sulfide salt and a first lithium salt; allowing sufficient time to form Li.sub.2S and a product salt; and recovering the Li.sub.2S.

2. The method of claim 1, wherein the first sulfide salt and the first lithium salt are mixed in a first solution, wherein the product salt is insoluble in or only sparingly soluble in the first solution, and wherein the product salt is separated from the first solution prior to recovering the Li.sub.2S.

3. The method of claim 2, wherein the first solution comprises a polar solvent having a boiling point of about 150° C. or less, and wherein the Li.sub.2S is recovered via solvent evaporation in an inert atmosphere or in the presence of H.sub.2S.

4. The method of claim 2, wherein the first sulfide salt and the first lithium salt spontaneously react to form the Li.sub.2S at room temperature.

5. The method of claim 1, wherein the first sulfide salt and the first lithium salt are mixed, without the addition of a solvent, and the mixture is heated to a temperature of at least 400° C., wherein the Li.sub.2S and the product salt are added to a first solution, wherein the product salt is insoluble in or only sparingly soluble in the first solution, and wherein the product salt is separated from the first solution prior to recovering the Li.sub.2S.

6. The method of claim 5, wherein the first solution comprises a polar solvent having a boiling point of about 150° C. or less, and wherein the Li.sub.2S is recovered via solvent evaporation in an inert atmosphere or in the presence of H.sub.2S.

7. The method of claim 1, further comprising annealing the recovered Li.sub.2S at a temperature of about 150° C. to about 300° C. in an inert atmosphere or in the presence of H.sub.2S.

8. The method of claim 1, wherein the Li.sub.2S is in the form of nanocrystals have a volume-averaged mean particle size (D.sub.50) from 5 nm to 50 nm.

9. The method of claim 1, wherein the first sulfide salt is selected from the group consisting of Na.sub.2S, K.sub.2S, Rb.sub.2S, Cs.sub.2S, Fr.sub.2S, (NH.sub.4).sub.2S, P.sub.2S.sub.5, NiS, and combinations thereof.

10. The method of claim 1, wherein the first lithium salt is selected from the group consisting of a lithium halide, lithium hydroxide (LiOH), lithium carbonate (Li.sub.2CO.sub.3), lithium sulfate (Li.sub.2SO.sub.4), lithium sulfite (Li.sub.2SO.sub.3), lithium amide (LiNH.sub.2), lithium nitride (LiN.sub.3), lithium nitrate (LiNO.sub.3), lithium phosphate (Li.sub.3PO.sub.4), and combinations thereof.

11. The method of claim 1, further comprising: mixing the recovered Li.sub.2S and a non-lithium containing salt; allowing sufficient time to form a metal or metalloid sulfide (MS.sub.n) and a second lithium salt; and recovering the MS.sub.n.

12. The method of claim 11, wherein the recovered Li.sub.2S and non-lithium containing salt are mixed in a second solution, wherein the MS.sub.n is insoluble in or only sparingly soluble in the second solution, wherein the MS.sub.n is separated from the second solution.

13. The method of claim 12, wherein the second solution comprises a polar aprotic solvent having a boiling point of about 150° C. or less.

14. The method of claim 11, wherein the recovered Li.sub.2S and non-lithium containing salt are mixed, without the addition of a solvent, and the mixture is heated to a temperature of at least 400° C., wherein the MS.sub.n and the product salt are added to a second solution, wherein the MS.sub.n is insoluble in or only sparingly soluble in the second solution, and wherein the MS.sub.n is separated from the second solution.

15. The method of claim 14, wherein the second solution comprises a polar aprotic solvent having a boiling point of about 150° C. or less.

16. The method of claim 11, wherein the non-lithium containing salt comprises a metal or metalloid cation and an anion selected from the group consisting of a halide, hydroxide, carbonate, sulfate, sulfite, nitrate, nitrite, phosphate, acetate, citrate, and combinations thereof.

17. The method of claim 11, wherein the MS.sub.n is selected from the group consisting of Cr.sub.2S.sub.3, M.sub.nS, ReS.sub.2, FeS.sub.2, RuS.sub.2, OsS.sub.2, CoS.sub.2, RhS.sub.2, IrS.sub.3, NiS.sub.2, PdS, PtS, HfS.sub.2, NbS.sub.2, TaS.sub.2, GeS.sub.2, SiS.sub.2, TiS.sub.2, SnS.sub.2, MoS.sub.2, ZrS.sub.2, CdS, ZnS, VS.sub.2, WS.sub.2, Al.sub.2S.sub.3, CaS, and MgS.

18. The method of claim 11, further comprising recovering the second lithium salt and recycling the second lithium salt second to be used as the first lithium salt.

19. A method of producing Li.sub.2S nanocrystals comprising: mixing a first sulfide salt and a first lithium salt in a first solution comprising a polar solvent; allowing sufficient time to form Li.sub.2S and a product salt precipitate; separating the product salt precipitate from the first solution; recovering the Li.sub.2S via solvent evaporation, and annealing the recovered Li.sub.2S at a temperature of about 150° C. to about 250° C., wherein the Li.sub.2S is in the form of nanocrystals have a volume-averaged mean particle size (D.sub.50) from 5 nm to 50 nm.

20. A method of producing metal or metalloid sulfide (MS.sub.n) nanocrystals comprising: mixing a first sulfide salt and a first lithium salt in a first solution comprising a polar solvent; allowing sufficient time to form Li.sub.2S and a product salt precipitate; separating the product salt precipitate from the first solution; recovering the Li.sub.2S from the first solution; mixing the recovered Li.sub.2S with a non-lithium containing salt in a second solution comprising a polar aprotic solvent; allowing sufficient time to form MS.sub.n nanocrystals and a second lithium salt; and recovering the MS.sub.n nanocrystals from the second solution, wherein the MS.sub.n nanocrystals are selected from the group consisting of Cr.sub.2S.sub.3, M.sub.nS, ReS.sub.2, FeS.sub.2, RuS.sub.2, OsS.sub.2, CoS.sub.2, RhS.sub.2, IrS.sub.3, NiS.sub.2, PdS, PtS, HfS.sub.2, NbS.sub.2, TaS.sub.2, GeS.sub.2, SiS.sub.2, TiS.sub.2, SnS.sub.2, MoS.sub.2, ZrS.sub.2, CdS, ZnS, VS.sub.2, WS.sub.2, Al.sub.2S.sub.3, CaS, and MgS.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the disclosed system and together with the general description of the disclosure given above and the detailed description of the drawings given below.

[0032] FIG. 1 is a schematic illustrating cascaded metathesis of metal or metalloid sulfide nanocrystals.

[0033] FIG. 2 is an XRD pattern (top) of precipitate recovered from solution using the solution-based metathesis approach described herein. The literature peaks for NaCl (bottom) are shown as well.

[0034] FIG. 3 is an XRD pattern (top) of the solids recovered from the supernatant using the solution-based metathesis approach described herein. The literature peaks for Li.sub.2S (bottom) are shown as well.

[0035] FIG. 4 is an XRD pattern of the solid-state metathesis products obtained as a function of temperature. The peaks associated with LiCl (•), Na.sub.2S (.diamond-solid.), NaCl (.box-tangle-solidup.), and Li.sub.2S (.square-solid.) are identified in legend.

[0036] FIG. 5 is an XRD pattern of the products recovered from solution after dissolving the solid-state products formed as a function of temperature. The reference pattern of Li.sub.2S (bottom) is provided, and the peaks associated with LiOH (.diamond-solid.) are identified in legend.

DETAILED DESCRIPTION

[0037] Disclosed herein are methods of producing Li.sub.2S and other metal or metalloid sulfide (MS.sub.n) nanocrystals. In some embodiments, Li.sub.2S is a final reaction product. In other embodiments, Li.sub.2S is an intermediate and is reacted to form other metal or metalloid sulfide nanocrystals.

[0038] An aspect of the invention is a method of producing Li.sub.2S nanocrystals. The process involves mixing a sulfide salt and a lithium salt in one or more solvents. The sulfide salt may be an alkali metal sulfide salt (M.sub.2S, where M=Na, K, Rb, Cs, and Fr) or another sulfide salt, such as, by way of non-limiting example, (NH.sub.4).sub.2S, P.sub.2S.sub.5, and NiS. The lithium salt may be a lithium halide such as LiF, LiCl, LiBr, and LiI, or another lithium salt, such as, by way of non-limiting example, LiOH, LiNH.sub.2, LiN.sub.3, and LiNO.sub.3. These salts are represented by the formula LiX, where X is an anion with a formal charge of −1. Other suitable lithium salts may be Li.sub.2CO.sub.3, Li.sub.2SO.sub.4, and Li.sub.2SO.sub.3. These salts are represented by the formula Li.sub.2X, where X is an anion with a formal charge of −2. Another suitable lithium salt may be Li.sub.3PO.sub.4. Li.sub.2S is formed via a metathesis or counter-ion exchange reaction in a suitable solvent. Two general reactions to produce Li.sub.2S are:

2LiX+M.sub.2.fwdarw.Li.sub.2S+2MX  (6); and

Li.sub.2X+M.sub.2.fwdarw.Li.sub.2S+M.sub.2X.  (7)

[0039] In an embodiment, the method comprises: preparing a solution comprising the sulfide salt; preparing a solution comprising the lithium salt; mixing the two solutions; and allowing sufficient time for the reaction to form Li.sub.2S and a product salt (MX or M.sub.2X). In another embodiment, the method comprises: preparing a solution of one of the sulfide salt or the lithium salt; adding the other of the sulfide salt or the lithium salt to the solution; and allowing sufficient time for the reaction to form Li.sub.2S and the product salt.

[0040] In embodiments, the formation of Li.sub.2S occurs spontaneously at low temperatures (i.e., temperatures of about 50° C. or less) and is ideally spontaneous under ambient or room temperatures (i.e., about 18° C. to about 28° C.). In some embodiments, the sulfide salt and the lithium salt spontaneously react to form Li.sub.2S at a temperature of about 50° C. or less, at a temperature of about 45° C. or less, at a temperature of about 40° C. or less, at a temperature of about 35° C. or less, at a temperature of about 30° C. or less, at a temperature of about 25° C. or less, at a temperature of about 20° C. or less, at a temperature of about 15° C. or less, at a temperature of about 10° C. or less, at a temperature of about 5° C. or less, at a temperature of about 0° C. or less, at a temperature of about −5° C. or less, at a temperature of about −10° C. or less, at a temperature of about −15° C. or less, at a temperature of about −20° C. or less, at a temperature of about −25° C. or less, at a temperature of about −30° C. or less, at a temperature of about −35° C. or less, at a temperature of about −40° C. or less, at a temperature of about −45° C. or less, at a temperature of about −50° C. or less, at a temperature of about −55° C. or less, at a temperature of about −60° C. or less, at a temperature of about −65° C. or less, or at a temperature of about −70° C. or less. In some embodiments, the reaction may be performed under ambient conditions (i.e., at room temperature). In some embodiments, the solution may be heated to a temperature of about 50° C., to a temperature of about 45° C., to a temperature of about 40° C., to a temperature of about 35° C., or to a temperature of about 30° C. In other embodiments, the solution may be cooled to a temperature of about 15° C., to a temperature of about 10° C., to a temperature of about 5° C., to a temperature of about 0° C., to a temperature of about −5° C., to a temperature of about −10° C., to a temperature of about −15° C., to a temperature of about −20° C., to a temperature of about −25° C., to a temperature of about −30° C., to a temperature of about −35° C., to a temperature of about −40° C., to a temperature of about −45° C., to a temperature of about −50° C., to a temperature of about −55° C., to a temperature of about −60° C., to a temperature of about −65° C., or to a temperature of about −70° C.

[0041] The reaction products, the Li.sub.2S and the product salt, may be separated from the solvent using standard chemical methods. For instance, depending upon the solvent or co-solvents, Li.sub.2S may either be precipitated from the solution upon formation or recovered via solvent evaporation. In some embodiments, the product salt is only sparingly soluble in the solvent and may be precipitated from the solution upon formation and then separated from the solvent by settling, centrifugation, filtration, decantation, or other suitable techniques. As used herein, “sparingly soluble” means that it requires about 1 to 2 L of the solvent to dissolve about 1 g the solute. The Li.sub.2S may then be recovered from the solvent via solvent evaporation once the product salt has been separated. The recovered Li.sub.2S may be in the form of nanocrystals. The evaporation process may be conducted at temperatures above the boiling point of the solvent and preferably less than about 150° C. The evaporation process may be conducted under an inert atmosphere, in the presence of H.sub.2S, or under reduced pressure. The solvent may be captured and reused in the process.

[0042] The sulfide salt is preferably in an anhydrous form. Examples of suitable sulfide salts include, but are not limited to Na.sub.2S, K.sub.2S, Rb.sub.2S, Cs.sub.2S, Fr.sub.2S, (NH.sub.4).sub.2S, P.sub.2S.sub.5, NiS, and combinations thereof. In an embodiment, the sulfide salt is anhydrous Na.sub.2S, which can be made in a number of ways. Sodium sulfide is typically sold commercially as hydrate flakes which may contain approximately 40% water by weight. The commercial Na.sub.2S.xH.sub.2O may be dehydrated before use in reaction or perhaps in situ through the addition of hygroscopic compounds (e.g., CaSO.sub.4, molecular sieves, etc.). The Na.sub.2S may also be purified by reaction at elevated temperature, for example by reaction with H.sub.2, carbon, H.sub.2S, and/or S.sub.8. Therefore, anion impurities other than sulfide S.sup.2− (e.g., SO.sub.3.sup.2−, S.sub.2O.sub.3.sup.2−, SO.sub.4.sup.2−, HS.sup.−, S.sub.x.sup.2−, OH.sup.−) can be removed.

[0043] The lithium salt is also preferably in an anhydrous form. The lithium salt may have a general formula of LiX, Li.sub.2X, or Li.sub.3X where X is a singly, doubly, or triply charged anion, respectively. Examples of suitable lithium salts include, but are not limited to, LiF, LiCl, LiBr, LiI, LiOH, LiNH.sub.2, LiN.sub.3, and LiNO.sub.3 Li.sub.2CO.sub.3, Li.sub.2SO.sub.3, Li.sub.2SO.sub.4, Li.sub.3PO.sub.4, and combinations thereof. In an embodiment, the lithium salt is anhydrous LiCl.

[0044] The one or more solutions may contain any suitable solvent or mixture of solvents to dissolve the reactants, facilitate the reaction to form the Li.sub.2S, and also their recovery. The solvent may contain one or more co-solvents. In some embodiments, the solvent may be chosen such that the product salt (2MX or M.sub.2X) is not soluble or is only sparingly soluble therein, while Li.sub.2S is soluble therein. In other embodiments, the solvent may be chosen such that Li.sub.2S is not soluble or are only sparingly soluble therein, while the product salt is soluble therein. In embodiments, the solvent comprises one or more volatile organic compounds that preferably have a boiling point of less than about 150° C. The solvent is preferably substantially free of water. In some embodiments, the solvent is selected from the group consisting of alcohols, ethers, esters, ketones, amides, and combinations thereof. Suitable solvents and co-solvents of the present invention include, by way of non-limiting example, C.sub.2-C.sub.5 alcohols, tetrahydrofuran (THF), dimethylformamide (DMF), and acetonitrile. In preferred embodiments, the solvent is a polar solvent such as an alcohol, preferably one or more of ethanol, isopropanol, propanol, butanol, and combinations thereof; these solvents may aid in the precipitation of the product salt from the solution.

[0045] The sulfide salt and the lithium salt are mixed together in solution and allowed to react for a sufficient amount of time such that the reaction produces Li.sub.2S and a product salt. In certain embodiments, the mixture contains approximately stoichiometric amounts of the sulfide salt and the lithium salt. In some embodiments, the mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 48 hours, between about 0 minutes to about 36 hours, between about 0 minutes to about 30 hours, between about 0 minutes to about 24 hours, between about 0 minutes to about 18 hours, between about 0 minutes to about 12 hours, between about 0 minutes to about 10 hours, between about 0 minutes to about 8 hours, between about 0 minutes to about 6 hours, between about 0 minutes to about 5 hours, between about 0 minutes to about 4 hours, between about 0 minutes to about 3 hours, between about 0 minutes to about 2 hours, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 30 to about 60 minutes. The reaction time is preferably sufficient such that the reaction proceeds to completion. In some embodiments, the yield of Li.sub.2S nanocrystals is greater than about 50%, greater than about 60%, greater than about 70%, greater than about 80%, greater than about 85%, greater than about 90%, greater than about 91%, greater than about 92%, greater than about 93%, greater than about 94%, greater than about 95%, greater than about 96%, greater than about 97%, greater than about 98%, or greater than about 99%.

[0046] In some embodiments, once Li.sub.2S has been recovered or separated from the solution, it is then annealed. Annealing removes any residual solvent and may also improve the crystallinity and particle size distribution of the particles. Annealing is performed by subjecting the Li.sub.2S powders to temperatures ranging from about 100° C. to about 350° C., preferably from about 150° C. to about 300° C., or more preferably from about 200° C. to about 250° C., for a sufficient period of time, for example, ranging from about 0 to 3 hours in an inert atmosphere, preferably 1 to 2 hours in an inert atmosphere.

[0047] An alternative approach to Li.sub.2S synthesis is through a solid-state reaction at elevated temperature. The reactions are as described in (6) and (7) above, but no solvent is present. In this case stoichiometric amounts of a metal sulfide salt (M.sub.nS) and a lithium salt (Li.sub.nX), which are disclosed above, are mixed together and heated in an inert environment to a sufficient temperature and for a sufficient amount of time. The reaction goes to completion, producing Li.sub.2S and the corresponding product salt. This solid-state mixture is separated by using a solvent that preferentially dissolves Li.sub.2S, such as for example a C.sub.2-C.sub.5 alcohol, while the product salt remains in solid form therein. After the product salt is separated from the solvent by centrifugation, filtration, settling, decantation, or other means, Li.sub.2S is recovered from solution by evaporating the solvent and it may also be annealed as described above.

[0048] The solution-free reaction between metal sulfide salt (M.sub.nS) and lithium salt (Li.sub.nX) occurs at elevated temperatures, generally above 400° C. In some embodiments, the reactants are heated to a temperature of about 450° C. or more, to a temperature of about 475° C. or more, to a temperature of about 500° C. or more, to a temperature of about 525° C. or more, to a temperature of about 550° C. or more, to a temperature of about 575° C. or more, to a temperature of about 600° C. or more, to a temperature of about 625° C. or more, to a temperature of about 650° C. or more, to a temperature of about 675° C. or more, to a temperature of about 700° C. or more, to a temperature of about 725° C. or more, to a temperature of about 750° C. or more, to a temperature of about 775° C. or more, or to a temperature of about 800° C. In some embodiments, the reactants are heated to a temperature of no more than about 800° C.

[0049] In some embodiments, the solution-free mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 180 minutes, between about 0 minutes to about 120 minutes, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 60 to about 120 minutes. The reaction time is preferably sufficient such that the reaction proceeds to completion.

[0050] The methods disclosed herein can be used to produced Li.sub.2S nanocrystals that have a well-defined morphology and particle size distribution. As used herein, unless otherwise specified, the term “particle size” refers to a volume-averaged mean particle size as defined by X-ray scattering and diffraction, also be referred to as “D.sub.50” values. The polydispersity index (PDI) is a measure of the heterogeneity of a sample based on size and is the mean size based on volume divided by the mean size based on number. In some embodiments, the Li.sub.2S nanocrystals have a particle size of less than 100 nm. In some embodiments, the Li.sub.2S nanocrystals have a particle size from 1 nm to 50 nm, from 1 nm to 45 nm, from 1 nm to 35 nm, from 1 nm to 30 nm, from 1 nm to 25 nm, from 1 nm to 20 nm, from 1 nm to 15 nm, from 1 nm to 10 nm, from 10 nm to 50 nm, from 15 nm to 50 nm, from 20 nm to 50 nm, from 25 nm to 50 nm, from 30 nm to 50 nm, from 35 nm to 50 nm, from 40 nm to 50 nm, from 15 nm to 45 nm, or from 20 nm to 40 nm. In some embodiments, the Li.sub.2S nanocrystals have a PDI of 2 or less, of 1.8 or less, of 1.6 or less, of 1.4 or less, of 1.2 or less, of 1.0 or less, of 0.8 or less, of 0.6 or less, or of 0.4 or less. In some embodiments, the Li.sub.2S nanocrystals are in the form of nanoflakes. In some embodiments the nanoflakes are arranged in cauliflower-like agglomerates.

[0051] While Li.sub.2S is a valuable reaction product, it may also be used a precursor to form other metal or metalloid sulfide (MS.sub.n) nanocrystals. Another aspect of the invention is the use of Li.sub.2S as intermediate in a series of reactions to form other metal or metalloid sulfide nanocrystals. The synthesis of these other metal or metalloid sulfide nanocrystals is performed by employing at least two metathesis or counter-ion exchange reactions. A first metathesis reaction results in the formation of Li.sub.2S, which is then used to synthesize MS.sub.n nanocrystals (such as those shown in Table 1) through a second metathesis reaction with an appropriate salt (MX.sub.2n). The second metathesis reaction may be represented by:

nLi.sub.2S+MX.sub.2n.fwdarw.MS.sub.n+2nLiX.  (8)

[0052] Here M is a metal or metalloid cation and n is an integer of 1 to 4, typically either 1 or 2. Non-limiting examples of M include, Cr, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Hf, Nb, Ta, Sn, Zr, V, Mo, W, Ge, Ti, Al, Ca, Mg, Cd, Zn, and Si. The MX.sub.2n salt does not comprise lithium. The non-lithium containing salt (MX.sub.2n) comprises an anion that may be selected from the group consisting of a halide, hydroxide, carbonate, sulfate, sulfite, nitrate, nitrite, phosphate, acetate, and citrate. The non-lithium containing salt preferably is chosen such that the reaction regenerates the same lithium salt that is used in the first metathesis reaction to form Li.sub.2S (e.g., reaction (6)).

[0053] Li.sub.2S may be prepared according to any of the methods described above. In some embodiments, Li.sub.2S and the non-lithium salt (MX.sub.2n) are mixed together in solution or suspension or a combination thereof and allowed to react for a sufficient amount of time such that the reaction produces MS.sub.n and a lithium salt. In an embodiment, the method comprises: preparing a solution comprising Li.sub.2S; preparing a solution comprising the non-lithium salt; mixing the two solutions; and allowing sufficient time for the reaction to form MS.sub.n nanocrystals and the product lithium salt. In another embodiment, the method comprises: preparing a solution of one of Li.sub.2S or the non-lithium salt; adding the other of the Li.sub.2S or the non-lithium salt to the solution; and allowing sufficient time for the reaction to form MS.sub.n and the product lithium salt.

[0054] In embodiments, the formation of MS.sub.n is spontaneous at low temperatures (i.e., temperatures of about 50° C. or less) and is ideally spontaneous under ambient or room temperature conditions. In some embodiments, Li.sub.2S and the non-lithium salt spontaneously react to form the MS.sub.n at a temperature of about 50° C. or less, at a temperature of about 45° C. or less, at a temperature of about 40° C. or less, at a temperature of about 35° C. or less, at a temperature of about 30° C. or less, at a temperature of about 25° C. or less, at a temperature of about 20° C. or less, at a temperature of about 15° C. or less, at a temperature of about 10° C. or less, at a temperature of about 5° C. or less, at a temperature of about 0° C. or less, at a temperature of about −5° C. or less, at a temperature of about −10° C. or less, at a temperature of about −15° C. or less, at a temperature of about −20° C. or less, at a temperature of about −25° C. or less, at a temperature of about −30° C. or less, at a temperature of about −35° C. or less, at a temperature of about −40° C. or less, at a temperature of about −45° C. or less, at a temperature of about −50° C. or less, at a temperature of about −55° C. or less, at a temperature of about −60° C. or less, at a temperature of about −65° C. or less, or at a temperature of about −70° C. or less. In some embodiments, the reaction may be performed under ambient conditions. In some embodiments, the solution may be heated to a temperature of about 50° C., to a temperature of about 45° C., to a temperature of about 40° C., to a temperature of about 35° C., or to a temperature of about 30° C. In other embodiments, the solution may be cooled to a temperature of about 15° C., to a temperature of about 10° C., to a temperature of about 5° C., to a temperature of about 0° C., to a temperature of about −5° C., to a temperature of about −10° C., to a temperature of about −15° C., to a temperature of about −20° C., to a temperature of about −25° C., to a temperature of about −30° C., to a temperature of about −35° C., to a temperature of about −40° C., to a temperature of about −45° C., to a temperature of about −50° C., to a temperature of about −55° C., to a temperature of about −60° C., to a temperature of about −65° C., or to a temperature of about −70° C.

[0055] In certain embodiments, the mixture contains approximately stoichiometric amounts of Li.sub.2S and the non-lithium salt. In certain embodiments, the mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 48 hours, between about 0 minutes to about 30 hours, between about 0 minutes to about 24 hours, between about 0 minutes to about 18 hours, between about 0 minutes to about 12 hours, between about 0 minutes to about 10 hours, between about 0 minutes to about 8 hours, between about 0 minutes to about 6 hours, between about 0 minutes to about 5 hours, between about 0 minutes to about 4 hours, between about 0 minutes to about 3 hours, between about 0 minutes to about 2 hours, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 30 to about 60 minutes. The reaction time is preferably sufficient such that the reaction proceeds to completion. In certain embodiments, the yield of MS.sub.n is greater than about 50%, greater than about 60%, greater than about 70%, greater than about 80%, greater than about 85%, greater than about 90%, greater than about 91%, greater than about 92%, greater than about 93%, greater than about 94%, greater than about 95%, greater than about 96%, greater than about 97%, greater than about 98%, or greater than about 99%.

[0056] The reaction products, MS.sub.n and a lithium salt, may be separated from the solvent using standard chemical methods. For instance, depending upon the solvent or co-solvents, MS.sub.n may either be precipitated from the solution or recovered via solvent evaporation. In some embodiments, MS.sub.n is only sparingly soluble in the solvent and may be precipitated from the solution upon formation and then separated from the solution by settling, centrifugation, filtration, or other suitable techniques. The lithium product salt may then be recovered from the solvent via solvent evaporation. The lithium product salt may be recycled back to the first metathesis reaction to be used a reactant to form Li.sub.2S (e.g., via equation (6) above). The solvent may be captured and reused in the process.

[0057] The one or more solutions for the second metathesis reaction may contain any suitable solvent or mixture of solvents to dissolve the reactants and facilitate the reaction to form and separate the MS.sub.n powders. The solvent may contain one or more co-solvents. In some embodiments, the solvent may be chosen such that the product lithium salt is not soluble in or is only sparingly soluble therein, while MS.sub.n is soluble therein. In other embodiments, the solvent may be chosen such that MS.sub.n is not soluble or is only sparingly soluble therein, while the lithium salt is soluble therein. In embodiments, the solvent comprises one or more volatile organic compounds that preferably have a boiling point of less than about 150° C. The solvent is preferably substantially free of water. In some embodiments, the solvent comprises alcohols, ethers, esters, ketones, amides, and combinations thereof. Suitable solvents and co-solvents of the present invention include, by way of non-limiting example, C.sub.2-C.sub.5 alcohols, tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, and acetone. In preferred embodiments, the solvent is a polar aprotic solvent, preferably tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, and acetone, and combinations thereof such solvents may aid in the precipitation of MS.sub.n from the solution upon formation.

[0058] In some embodiments, once the MS.sub.n powders have been recovered or separated from the solution, they are annealed. Annealing removes any residual solvent and may also improve the crystallinity and particle size distribution of the powders. Annealing is performed by subjecting MS.sub.n to temperatures ranging from about 100° C. to about 350° C., preferably from about 150° C. to about 300° C., or more preferably from about 200° C. to about 250° C., for a period of time, for example, ranging from about 0 to 3 hours in an inert atmosphere, preferably 1 to 2 hours in an inert atmosphere.

[0059] As an alternative to the solution based approach, MS.sub.n may also be formed through a solid-state or solvent-free reaction at elevated temperature. In this case stoichiometric amounts of Li.sub.2S and a suitable non-lithium containing salt (MX.sub.2n), which are disclosed above, are mixed together and heated in an inert environment to sufficient temperature and for a sufficient amount of time. The reaction goes to completion, producing MS.sub.n and the corresponding lithium containing product salt. This solid-state mixture is separated by using a solvent that preferentially dissolves lithium containing product salt, such as for example polar aprotic solvent, while MS.sub.n remains in solid form therein. MS.sub.n may then be separated from the solvent by centrifugation, filtration, settling, decantation, or other means and the MS.sub.n powders may be annealed as described above. The lithium containing product salt can be recovered from solution by evaporating the solvent.

[0060] The solvent-free reaction between metal or metalloid salt (MX.sub.2n) and lithium sulfide (Li.sub.2S) occurs at elevated temperatures, generally above 400° C. In some embodiments, the reactants are heated to a temperature of about 450° C. or more, to a temperature of about 475° C. or more, to a temperature of about 500° C. or more, to a temperature of about 525° C. or more, to a temperature of about 550° C. or more, to a temperature of about 575° C. or more, to a temperature of about 600° C. or more, to a temperature of about 625° C. or more, to a temperature of about 650° C. or more, to a temperature of about 675° C. or more, to a temperature of about 700° C. or more, to a temperature of about 725° C. or more, to a temperature of about 750° C. or more, to a temperature of about 775° C. or more, or to a temperature of about 800° C. In some embodiments, the reactants are heated to a temperature of no more than about 800° C.

[0061] In some embodiments, the solvent-free mixture is stirred or agitated for a portion of or all of the reaction time, which may be between about 0 minutes to about 180 minutes, between about 0 minutes to about 120 minutes, between about 0 minutes to about 60 minutes, between about 0 minutes to about 30 minutes, or between about 60 to about 120 minutes. In some embodiments, the reaction time is sufficient such that the reaction proceeds to completion.

[0062] The methods disclosed herein may be used to produce MS.sub.n nanocrystals that have a well-defined morphology and particle size distribution. In some embodiments, the MS.sub.n nanocrystals have a particle size of less than 100 nm. In some embodiments, the MS.sub.n nanocrystals have a particle size from 1 nm to 50 nm, from 1 nm to 45 nm, from 1 nm to 35 nm, from 1 nm to 30 nm, from 1 nm to 25 nm, from 1 nm to 20 nm, from 1 nm to 15 nm, from 1 nm to 10 nm, from 10 nm to 50 nm, from 15 nm to 50 nm, from 20 nm to 50 nm, from 25 nm to 50 nm, from 30 nm to 50 nm, from 35 nm to 50 nm, from 40 nm to 50 nm, from 15 nm to 45 nm, or from 20 nm to 40 nm. In some embodiments, the MS.sub.n nanocrystals have a PDI of 2 or less, of 1.8 or less, of 1.6 or less, of 1.4 or less, of 1.2 or less, of 1.0 or less, of 0.8 or less, of 0.6 or less, or of 0.4 or less.

[0063] The cascaded metathesis approach, may be used to produce a variety of MS.sub.n powders. In some embodiments, Li.sub.2S may be formed from the reaction of Na.sub.2S with LiCl, shown in reaction (9), in alcohol or another suitable solvent. This reaction is spontaneous at ambient temperature.

2LiCl.sub.(sol)+Na.sub.2S.sub.(sol).fwdarw.Li.sub.2S.sub.(sol)+2NaCl.sub.(s)ΔG°≈−90 kJ/mol  (9)

A first solution may comprise less than about 12 g/100 g of Na.sub.2S in ethanol, preferably from about 9 g/100 g to about 11.5 g/100 g of Na.sub.2S in ethanol, and a second solution may comprise less than about 25 g/100 g of LiCl in ethanol, preferably from about 15.5 g/100 g to about 25 g/100 g of LiCl in ethanol. Stoichiometric amounts of Na.sub.2S and LiCl may then be mixed after both have been dissolved in ethanol in the first and second solutions. The sodium chloride precipitates out of solution as it is sparingly soluble in ethanol, while Li.sub.2S remains dissolved. The NaCl precipitate is removed from the solution by standard techniques and Li.sub.2S is then recovered by solvent evaporation.

[0064] In other embodiments, Li.sub.2S may be formed from the reaction of Na.sub.2S with LiCl by heating in an inert environment. Stoichiometric amounts of Na.sub.2S and LiCl powders may be mixed and heated to temperatures of about 600° C. under inert atmosphere. The reaction products are a mixture of NaCl and Li.sub.2S which can be separated using ethanol, which dissolves Li.sub.2S while NaCl is sparingly soluble. The NaCl precipitate is removed from the solution by standard techniques and the Li.sub.2S powders can then recovered by solvent evaporation as described previously.

[0065] The recovered Li.sub.2S may then be reacted through a second metathesis reaction with a chloride salt, as shown in reaction (10):

[00001]$\begin{array}{cc}{n\mathrm{Li}}_2{S}_{\left(\mathrm{sol}\right)}+{\mathrm{MCl}}_{2n\left(\mathrm{sol}\right)}\stackrel{\mathrm{RT},1\mathrm{atm}}{.fwdarw.}{\mathrm{MS}}_{n\left(s\right)}+2{n\mathrm{LiCl}}_{\left(\mathrm{sol}\right)}\Delta G^o⪡0\mathrm{kJ}\text{/}\mathrm{mol}.& \left(10\right)\end{array}$

When the second metathesis reaction is conducted in a polar aprotic solvent (e.g., THF, DMF, acetonitrile, etc.) the MS.sub.n product precipitates from solution, while LiCl remains dissolved in the solution. Reaction (10) regenerates LiCl which can be recovered via solvent evaporation and used in reaction (9). Many examples of MS.sub.n synthesis via Reaction (10) have been demonstrated and can be found in R. R. Chianelli, M. B. Dines, “Low-Temperature Solution Preparation of Group 4B, 5B and 6B Transition-Metal Dichalcogenides”, Inorg. Chem. 17, 2758 (1978); R. R. Chianelli, E. B. Prestridge, T. A. Pecoraro, J. P. Deneufville, “Molybdenum Disulfide in the Poorly Crystalline “Rag” Structure”, Science 203, 1105 (1979); and T. Pecoraro, R. R. Chianelli, “Hydrodesulfurization catalysis by transition metal sulfides” J. of Catal. 67, 430 (1981) (each of which is incorporated herein in their entirety).

[0066] Alternatively, the second metathesis could be conducted thermally without solvent using the solid-state method described herein.

[0067] The central thesis of this approach is to couple reactions (9) and (10), or the analogous solid-state reactions, to enable the cascaded metathesis of innumerable metal sulfides using Li compounds as recycled intermediates. This reaction sequence is shown in FIG. 1. The net reaction is:

[00002]$\begin{array}{cc}n{\mathrm{Na}}_{2}S+{\mathrm{MCl}}_{2n}\stackrel{\mathrm{RT},1\mathrm{atm}}{.fwdarw.}{\mathrm{MS}}_n+2n\mathrm{NaCl}\Delta G^o⪡0\mathrm{kJ}\text{/}\mathrm{mol}.& \left(11\right)\end{array}$

[0068] The direct implementation of reaction (11) is commonly utilized to synthesize a number of metal sulfide compounds. The key benefits of the cascaded metathesis approach, rather than the direct approach, are threefold: First, the cascaded approach allows for the removal of the lithium salt byproduct from the product using a nonprotic or weakly protic solvent. In the direct approach, the salt byproduct is typically NaCl, which can only be removed by dissolving in strongly protic solvents such as water, methanol, or ethylene glycol. Many metal sulfides and/or their metal chloride precursors cannot tolerate strongly protic solvents such as those listed above because of their tendency to undergo hydrolysis. Therefore, in some cases, it is beneficial to form LiCl as the byproduct rather than NaCl due to the high solubility of LiCl in nonprotic or weakly protic polar solvents. These solvents, such as tetrahydrofuran or dimethylformamide, can be tolerated by many metal sulfides/chlorides. The second benefit is that the use of nonprotic or weakly protic solvents can impart a unique morphology or particle size distribution to the MS.sub.n product, which may be beneficial for certain applications. These morphologies are often not accessible when using the direct approach. The third benefit is that the net reaction achieves the previous two advantages with no net consumption of lithium, which could be prohibitively expensive.

[0069] The reagents involved are low cost salts and the reactions are preferably spontaneously, proceeding to completion under ambient conditions or near ambient conditions in solution. The only significant energy requirements are those associated with the vaporization of volatile organic solvents for the recovery of the Li intermediates. The solvents may be readily condensed and recycled and the MS.sub.n product may be directly recovered in nanocrystal form.

[0070] By coupling reactions (9) and (10), Li.sub.2S can be formed and then be used to facilitate the synthesis other metal or metalloid sulfides of interest. By way of non-limiting examples, the following metal or metalloid sulfides can be produced from Li.sub.2S:

2Li.sub.2S.sub.(s)+SnCl.sub.4(sol).fwdarw.SnS.sub.2(s)+4LiCl.sub.(sol)ΔG°=−382 kJ/mol;  (12)

2Li.sub.2S.sub.(s)+SiCl.sub.4(sol).fwdarw.SiS.sub.2(s)+4LiCl.sub.(sol)ΔG°=−248 kJ/mol;  (13)

3Li.sub.2S.sub.(s)+2AlCl.sub.3(sol).fwdarw.Al.sub.2S.sub.3(s)+6LiCl.sub.(sol)ΔG°=−449 kJ/mol;  (14)

Li.sub.2S.sub.(s)+CaCl.sub.2(sol).fwdarw.CaS.sub.(s)+2LiCl.sub.(sol)ΔG°=−56 kJ/mol  (15); and

Li.sub.2S.sub.(s)+MgCl.sub.2(sol).fwdarw.MgS.sub.(s)+2LiCl.sub.(sol)ΔG°=−77 kJ/mol.  (16)

These reactions are all highly thermodynamically favorable, driven by the exothermicity of salt precipitation. Examples of metal or metalloid sulfides that may be formed using the cascaded metathesis approach are Cr.sub.2S.sub.3, MnS, ReS.sub.2, FeS.sub.2, RuS.sub.2, OsS.sub.2, CoS.sub.2, RhS.sub.2, IrS.sub.3, NiS.sub.2, PdS, PtS, HfS.sub.2, NbS.sub.2, TaS.sub.2, GeS.sub.2, SiS.sub.2, TiS.sub.2, SnS.sub.2, MoS.sub.2, ZrS.sub.2, CdS, ZnS, VS.sub.2, WS.sub.2, Al.sub.2S.sub.3, CaS, and MgS.

[0071] While the above reaction sequence has been disclosed with some degree of specificity, one skilled in the art will understand that other reactants may be employed. In addition, the reaction conditions (e.g., temperature and pressure) may be varied to optimize the production of the Li.sub.2S and/or MS.sub.n nanocrystals. The reactions disclosed herein may conducted in a batch process or in a continuous process.

Example 1: Formation of Li.SUB.2.S Nanocrystals by Solution-Based Metathesis

[0072] Li.sub.2S nanocrystals were formed using the following solution-based method: [0073] (1) a first solution was prepared by dissolving anhydrous Na.sub.2S in ethanol at an approximate concentration of 10 g/100 g; [0074] (2) a second solution was prepared by dissolving anhydrous LiCl in ethanol at an approximate concentration of 20 g/100 g; [0075] (3) a stoichiometric amount of the first solution was added to the second solution and stirred for approximately 60 minutes, where the following reaction occurred: 2LiCl(sol)+Na.sub.2S(sol) Li.sub.2S(sol)+2NaCl(s); [0076] (4) the solution was centrifuged, and the supernatant was decanted; [0077] (5) the precipitate was dried at about T=150° C. under argon gas; [0078] (6) the supernatant was evaporated at about T=100° C. under a flow of argon gas to obtain a powder; and [0079] (7) the obtained powder was annealed at 250° C. for 2 hours under flow of argon gas.

[0080] The resulting powders obtained from the above method were analyzed using X-ray diffraction (XRD) using a Philips X′Pert X-ray diffractometer with Cu Kα radiation (λ=0.15405 nm) between 10 and 70° at a scan rate of 5° min.sup.−1. Samples were prepared on a glass slide with a piece of Scotch Magic Tape covering the material to prevent undesired reactions with ambient moisture. The contribution from the glass slide was background subtracted with a polynomial fit. FIG. 2 shows the reference XRD pattern of NaCl with the experimental diffraction pattern of the powder obtained from the precipitate after drying at 150° C. FIG. 3 shows the reference XRD pattern of anhydrous Li.sub.2S with the experimental diffraction pattern of the powder obtained from the supernatant after annealing at 250° C.

Example 2: Formation of Li.SUB.2.S Nanocrystals by Solid-State Metathesis

[0081] Li.sub.2S nanocrystals were formed using the following solid-state method: [0082] (1) a stoichiometric amount of anhydrous LiCl was mixed with anhydrous Na.sub.2S by mortar and pestle; [0083] (2) the solid mixture was heated to about 600° C. under argon gas flow for about 2 hours; [0084] (3) the resulting product mixture was ground with mortar and pestle and mixed with ethanol at a concentration of about 22 g solid/100 g ethanol. [0085] (4) the solution was centrifuged, and the supernatant was decanted; [0086] (5) the precipitate was dried at about T=150° C. under argon gas; [0087] (6) the supernatant was evaporated at about T=100° C. under a flow of argon gas to obtain a powder; and [0088] (7) the obtained powder was annealed at 250° C. for 2 hours under flow of argon gas.

[0089] The resulting powders obtained from the above method after step (2) were analyzed using XRD using the previously described method. FIG. 4 shows experimental XRD pattern of the solid product mixture after reaction at different temperatures, 400° C., 500° C., and 600° C. The peaks associated with LiCl, Na.sub.2S, NaCl, and Li.sub.2S are identified in legend. FIG. 5 shows the experimental XRD pattern of the products recovered from supernatant (step 7) after dissolving the solid-state products formed as a function of temperature. The reference pattern of Li.sub.2S is provided, and the peaks associated with LiOH are identified in legend.

[0090] Various modifications of the above-described invention will be evident to those skilled in the art. It is intended that such modifications are included within the scope of the following claims.