Chapter 26
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Solid-State Metathesis Routes to Layered Transition-Metal Dichalcogenides and Refractory Materials John B. Wiley, Philippe R. Bonneau, Randolph E. Treece, Robert F. Jarvis, Jr., Edward G. Gillan, Lin Rao, and Richard B. Kaner Department of Chemistry and Biochemistry and Solid State Science Center, University of California—Los Angeles, Los Angeles, CA 90024-1569 A large number of materials can be prepared via new metathetical (exchange) solid-state precursor reactions. This synthetic route is extremely rapid (often < 1 sec.), can be initiated at low temperature, and is potentially useful for controlling particle size and for preparing both cationic and anionic solid solutions. The often self-propagating and sometimes explosive behavior observed in these reactions is related to their exothermicity. Consequently, thermodynamic considerations can be employed to help select the best set of precursors as judged from reaction enthalpies. The control of particle size and product yield through manipulations of reaction temperature and the factors that influence reaction initiation in these systems are discussed.
Synthetic routes derived from molecular and non-molecular precursors have expedited the development of technologically important 2- and 3- dimensional materials. Such approaches have often proved superior to conventional ceramic techniques in that high purity bulk samples or thin films can be prepared at lower temperatures much more rapidly. Predominant among the precursor methods are those based on decomposition reactions. These either involve gaseous species, such as those used in chemical vapor deposition ( C V D ) , or solids. Examples include the pyrolysis of the gas-phase precursor [(CH ) A1(NH )]3 to produce aluminum nitride (7) and the thermal decomposition of solid state carbonate precursors of calcium and manganese ( C a . M n C 0 , 0 < χ < 1) to produce several of the known ternary compounds in the Ca-Mn-O system (2). Single-displacement reactions are also common as precursor methods. These approaches usually involve gas-phase reactions and are also used in C V D techniques. Examples here include the formation oi 3
1
x
2
2
x
3
0097-6156/92/0499-0369$06.00/0 © 1992 American Chemical Society
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SUPRAMOLECULAR ARCHITECTURE
elemental silicon (5) by hydrogen reduction of S i C l and the preparation of gallium arsenide in hydrogen from A s C l and gallium metal (4). Metathetical reaction pathways are not as well studied as other precursor methods. This approach, though requiring the use of two precursors, can be performed in the liquid, gas or solid state. Chianelli and Dines (J) have carried out a series of investigations on the solution preparation of transition-metal dichalcogenides. Titanium tetrachloride, for example, was found to react with lithium sulfide in nonaqueous solvents to produce metal disulfide and lithium chloride (equation 1). 4
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3
TiCl
4
+ 2Li S - - > 2
TiS
+ 4 LiCl
2
(1)
The metal dichalcogenides formed are usually amorphous but can be crystallized on heating (> 400°C). Gas-phase exchange reactions are exemplified by the preparation of silicon nitride from silicon tetrachloride and ammonia (equation 2) (6). 3SiCl
4
+ 4NH
3
—-> S i N 3
+ 12 HC1
4
(2)
Very few examples of solid state metathesis reactions are known. In 1932 Hilpert and Wille (7) reported the preparation of mixed metal ferrites at moderate temperatures (400 - 500°C) by the reaction: MC1
2
+ Li Fe 0 2
2
4
—>
MFe 0 2
4
+ 2 LiCl
(3).
The approach has not been fully explored as a synthetic technique (8). The investigational void associated with metathesis reactions in the solid state suggested to us the need to explore this precursor method as a viable synthetic approach. We have found solid state metathesis reactions to be an extremely powerful route. Surprisingly, these reactions can often selfinitiate at room temperature, internally produce enough heat to sustain themselves, and yield crystalline products in seconds. These reactions are also applicable to a very broad range of compounds. We have successfully synthesized transition-metal, main-group, and rare-earth chalcogenides (S, Se, and Te) and pnictides (Ν, P, As, and Sb), as well as, selected carbides, oxides, and silicides. This list of compounds includes 2- and 3- dimensional materials with important magnetic, electronic, catalytic, and refractory properties. Additional features of this synthetic technique, not typically seen in other precursor methods, are the facile control of crystallinity and the ability to prepare both cationic and anionic solid solutions. Molybdenum Disulfide The preparation of layered transition-metal dichalcogenides has been one of the areas of emphasis in this research (9). Molybdenum disulfide, an
In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
26.
WILEY ET AL.
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Solid-State Metathesis Routes
important lubricant, battery cathode and catalytic material, is readily prepared from the appropriate molybdenum halide and alkali-metal sulfide precursors. Though many different pairs of transition-metal halide alkalimetal sulfide precursors are available, the combination of M o C l and N ^ S was found to be most effective (equation 4). 5
M o C l + 5 / 2 N a ^ — > M o S + 5 N a C l + 1/2 S
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5
(4)
2
Typically these metathesis reactions are initiated in one of three ways: 1) by self-initiation on light mixing or grinding, 2) local heating with a hot filament, or 3) low-temperature heating of the sample in a sealed evacuated glass tube. The particular method used depends both on the activation energy and the facility of the reaction. In the case of M o C l and N a 2 S , these precursors will self-initiate on light mixing at room temperature. The reaction, carried out in a helium-filled dry box, produces a bright white intense flash of light, a small mushroom cloud (due to the volatilization of the sulfur byproduct), and a dark product mixture. Pure highly crystalline molybdenum disulfide can be isolated simply by washing the product mixture with methanol to remove unreacted M o C l and water to remove the halide salt byproduct and any unreacted N a ^ . (Excess sulfur can be removed with chloroform or carbon disulfide, though this is often not necessary because it essentially all boils away during the reaction.) The percent yield for the reaction is typically 80% of theoretical. The X-ray powder diffraction pattern of the M o S (Figure 1) shows its high crystallinity. Thermogravimetric analysis ( T G A ) of this material heated in hydrogen ( 5 % in nitrogen) to 1000°C (10°C/min) showed the sulfur content to be within 0.1% of the theoretical value. 5
5
2
W A R N I N G ; These metathesis reactions are often very exothermic and sometimes explosive. Some materials self-initiate and many can be activated with a drop of water. Reactions are typically carried out under inert atmosphere with the use of covered nonairtight sealed stainless steel bombs to contain fulminating mixtures. Before carrying out reactions in sealed glass tubes, gas law calculations should be done to determine the maximum pressure based on all the possible volatile reactants and products. Sample sizes should then be adjusted so as not to exceed the pressure limitations of the glass container (typically < 5 a t m . ) . Further caution should be exercised in the choice of the reaction container. When the reactions are quite exothermic, the heat produced can exceed the melting point of the glass and result in an implosion. In these instances, sealed quartz tubes (m.p. « 1200°C) may be more appropriate than Pyrex (m.p. « 550°C). However, metal or ceramic containers are preferred. Many of the precursors and reaction byproducts, especially the pnictides, are toxic and potentially hazardous. O n exposure to air or on washing with water, the product mixtures can evolve toxic gases from some of the unreacted starting materials e.g. H S and P H from Na S and Na P, respectively. Workups should, at a minimum, be done in a fume hood. 2
3
2
In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
3
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PQ
