Abstract:
Metal chalcogenide monolayer nanomaterials can be synthesized from metal alkoxide precursors by solution precipitation or solvothermal processing. The synthesis routes are more scalable, less complex and easier to implement than other synthesis routes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/968,182, filed Mar. 20, 2014, which is incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to metal chalcogenides and, in particular, to a method to synthesize metal chalcogenide monolayer nanomaterials. 
     BACKGROUND OF THE INVENTION 
     Synthetic routes to tungsten disulfide (WS 2 ) and molybdenum disulfide (MoS 2 ) nanomaterials (e.g., two-dimensional (2D) monolayers) are of interest for lubricants, catalyst, Li-ion batteries, semiconductors, and photodiodes. Previous efforts to synthesize bulk WS 2  nanomaterials involved chemical vapor deposition techniques, fluidized bed reactors, gas-solid reactions, laser ablation, and spray pyrolysis. For the production of 2D metal sulfide monolayer nanomaterials, synthesis have been limited to high temperature solid-state routes, sulfurization with H 2 S gas, vapor phase routes, exfoliation, and metathesis reaction routes with halide side products that can be detrimental to the application&#39;s performance properties. See Y. Feldman et al.,  Solid State Sciences  2(6), 663 (2000); C. Altavilla et al.,  Chem. Mater.  23, 3879 (2011); H. R. Gutierrez et al.,  Nano Lett.  13(8), 3447 (2013); and Y. Yu et al.,  Nature, Scientific Reports,  2013, 3:1866 DOI: 10.1038/srep01866. 
     Therefore, a need remains for a solution route for the synthesis of metal chalcogenide monolayer nanomaterials that is scalable, less complex and easier to implement than other synthesis routes. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a novel solution route to 2D metal sulfide monolayer nanomaterials from the reaction between metal alkoxides M(OR) x  and hexamethyldisilathiane (HTMS). According to the present invention, both solution precipitation and solvothermal processing of M(OR) x  and HTMS can be used at low temperatures to produce 2D metal sulfide monolayers. This general reaction route can be extended to produce other metal chalcogenide (ME, E=S, Se, Te) nanomaterials by varying the M(OR) x  and E precursors. The synthesis route yields air-stable layered nanomaterials. The method offers a scalable route that controls surface chemistry, but does not require additional heating or processing under H 2 S gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers. 
         FIGS. 1( a )-( d )  show general solution routes for the synthesis of WS 2 . 
         FIG. 2( a )  is a transmission electron microscope (TEM) image of as-prepared WS 2  particles.  FIG. 2( b )  is a TEM image of the monolayers making up a particle. 
         FIG. 3( a )  is a TEM image of a WS 2  monolayer.  FIG. 3( b )  is an energy-dispersive X-ray spectrum (EDS) of the WS 2  monolayer. 
         FIGS. 4( a )-( c )  show powder X-ray diffraction (PXRD) patterns of the tungsten disulfides as prepared. 
         FIG. 5  is a schematic illustration of the layered structure of WS 2 . 
         FIG. 6  shows thermogravimetric and differential temperature analyses (TGA-DTA) of the WS 2  as synthesized via the W(OEt) 5  route. 
         FIG. 7  shows PXRD patterns of WS 2  synthesized via the W(OEt) 5  route before and after annealing. 
         FIGS. 8( a ) and 8( b )  show TEMs of the WS 2  after annealing. 
         FIG. 9( a )  is a TEM of a MoSe 2  monolayer.  FIG. 9( b )  is an energy-dispersive X-ray spectrum of the monolayer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a novel solution route to 2D metal sulfide, selenide, or telluride monolayer nanomaterials from the reaction between metal alkoxides M(OR) x  and hexamethyldisilathiane (HTMS, or S(Si(CH 3 ) 3 ) 2 ) or trioctylphoshine selenide or telluride. For example, metal sulfide nanomaterials (MS x ) can be prepared from the reaction between M(OR) x  and HTMS in high boiling coordinating or non-coordinating solvents.  FIGS. 1( a )-( d )  show exemplary solution routes for the synthesis of WS 2 .  FIG. 1( a )  shows an exemplary solution route to a metal chalcogenide nanomaterial using a metal alkoxide precursor and HTMS. In this example, the production of the transition metal sulfide WS 2  can be achieved by the solvothermal processing route using tungsten (IV) ethoxide and HTMS.  FIGS. 1( b ) and 1( c )  show exemplary methods using a metal chloride precursor.  FIG. 1( d )  shows the chalcogenide precursor materials. These methods can be extended to WE x  and MoE x  more generally. Depending on the conditions used (e.g., solvent, time, temperature) the monolayer size and crystallinity can be controlled. For example, the as-prepared crystalline WS 2  monolayers can undergo formation of multilayer WS 2  structures after heat treatment under an inert atmosphere. 
     Eqs. (1) and (2) listed below show the general reaction routes used to produce metal chalcogenides from metal alkoxides: 
                                
To produce other metal chalcogenide (ME, E=S, Se, Te) nanomaterials, this general reaction route can also be extended by simply varying the M(OR) x  and E precursors. Therefore, crystalline nanoparticles of alkaline earth chalcogenides (A E E; A E =Ca, Sr, Ba; E=S, Se, Te) have been synthesized using similar procedures. For example, alkaline earth sulfides can be prepared by reacting calcium aryloxide (Ca(OAr) 2 ) or alkaline earth neopentoxides (A E (ONep) 2 , where A E =Sr, Ba), and HTMS in a stoichiometric ratio to produce CaS, SrS, and BaS with solvothermal processing, according to Eq. (2). The selenides and tellurides can be prepared by substituting HTMS with either trioctylphoshine selenide or telluride (TOP=E; E=Se, Te) with solution precipitation conditions, according to Eq. (1). CdE materials can also be prepared by this route using cadmium aryl and neopentoxide precursors. See S. D. Bunge et al.,  Inorg. Chem.  44(5), 1309 (2005). The size, shape, and surface chemistry of each of these material systems can be tailored by changing reaction solvent, reaction processing time and temperature used.
 
     As an example, under an inert atmosphere, a mixture of trioctylphosphine oxide (TOPO) (5.00 g, 12.9 mmol), and stoichiometric amounts of the appropriate M(OR) x  and S(Si(CH 3 ) 3 ) 2  or TOP=Se were added to the teflon liner of a 45 mL Parr Acid Digestion Bomb. The bomb was sealed in the glovebox, placed in an oven and heated to 200° C. for 48 h. After cooling to room temperature, the reaction was transferred into a glovebox and the nanoparticles were washed twice, by dissolving the reaction mixture with toluene, then precipitating the MEx nanoparticles with MeOH. The nanoparticles could be redispersed with toluene. Black powders were washed and isolated for both the WS 2  and MoSe 2  reactions. Each was dispersible in toluene. Once isolated, the powders were characterized with TEM, EDS, TGA/DTA, and XPS. These results are described below. 
       FIGS. 2( a )-( b )  are transmission electron microscopy (TEM) images of the as prepared tungsten disulfide.  FIG. 2( a )  shows that agglomerated particles are produced. Upon higher magnification of the agglomerated particles, the image in  FIG. 2( b )  shows the formation of layered rod structures, as expected from literature reports. See R. Tenne et al.,  Nature  360, 444 (1992). 
       FIG. 3( a )  shows energy-dispersive X-ray spectrum (EDS) of the WS 2  particle shown in  FIG. 3( b ) , as synthesized via the W(OEt) 5  route. The EDS spectrum of the particles shows peaks of tungsten and sulfur suggesting that WS 2  is the sole tungsten product, and that no oxides are formed during the synthesis. 
       FIGS. 4( a )-( c )  show powder XRD patterns of the tungsten disulfides as prepared. The patterns resemble those of previously reported tungsten disulfide nanomaterial diffraction patterns, with the WS 2  synthesized using the W(OEt) 5  route ( FIG. 4( a ) ) showing the most resemblance. See C. Altavilla et al.,  Chem. Mater.  23, 3879 (2011). 
       FIG. 5  shows the layered structure of tungsten disulfide. WS 2  adopts a layered structure similar to MoS 2 , wherein each W (IV) atom is situated in a trigonal prismatic coordination sphere bound by six sulfide ligands. Likewise, each sulfur atom is pyramidal and connected to three W centers. Therefore, the trigonal prisms are interconnected to provide a layered structure wherein the tungsten atoms are sandwiched between layers of sulfur atoms. The layers of sulfide atoms are bound by weak van der Waals interactions. The diffraction pattern shows the planes associated with these layers and the absence of other peaks associated with bulk WS 2  or tungstenite. 
     To help identify how these materials crystallize, thermal analysis was performed under argon. This was done as annealing leads to a more layered structure.  FIG. 6  shows thermogravimetric and differential temperature analyses (TGA-DTA) of the WS 2  as synthesized via the W(OEt) 5  route. The samples were ramped up to 850° C. and 10° C./min and held at 850° C. for 10 minutes under argon. A weight loss of 12% was observed in the 300-400° C. range resulting from the loss of organics or the TOPO surfactant. No decomposition of WS 2  occurred and as determined by PXRD.  FIG. 7  shows the patterns of WS 2  before and after annealing. The intensity of the pattern is stronger after annealing.  FIGS. 8( a ) and 8( b )  are TEMs of the WS 2  after annealing, revealing that the randomized monolayers are now stacking on top of each other to produce the standard layered WS 2 . 
     A similar approach can be used to synthesize the selenides. The TEM image in  FIGS. 9( a )-( b )  show similar monolayered materials were produced when TOP=Se under similar conditions. EDS also confirmed the presence of Mo and Se. 
     The present invention has been described as a method to synthesize metal chalcogenide monolayer nanomaterials. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.