Martin 8. Dines Esso Research and Engineering Co. Linden, New Jersey 07036
Intercalation in Layered Compounds
In their solid state, some materials exist as two-dimensional molecular crystals having only van der Waals forces to hind the layers together into stacks. Such substances typically occur as cleavable platelets, a direct consequence of their microscopic properties. The sheets composing the structure may be one atom in thickness, as in the prototypal graphite, or there may he several, as is the case with many of the lamellar clays. Since the interlaver bonding is nominal, i t is frequently found that these cornpounds can serve as host matrices for various interstitial species. In the process of intercalation, one may actually observe the swelling of the solid, once again as a direct result of the microscopics of the system. The inclusion may or may not he reversible, depending on the nature of the bonding which occurs and the relative stabilities of the vacant and filled host. Occasionally a drastic deintercalation can he effected which affords an exfoliated host crystal-me from which the departure of the guest molecules occurred explosively from some of the interlayers. The resulting product may have a dimension expanded a hundred-fold over the initial material, with a vastly increased surface area as well. The inclusion process often may proceed in discreet stages, in which occupation of every nth interlayer occurs prior to further filling of the lattice. Stoichiometric products may or may not result, depending on the specifics of the interactions between guest and host, and the space fillinn- demands of the system. Figure 1 depicts this situation. A particularly appropriate analytical method in this area is X-ray diffraction powder patterns obtained on the solids. A dominating feature of most layered compound powder patterns is the series of reflections corresponding to the basal plane d spacing. Upon inclusion, this door series is glaringly shifted to lower values of 28 (corresponding to increasing dool) while the remaining spacings are relatively unaffected. To illustrate this phenomenon: In Figure 2 the situation of the layered transition metal dichalcogenide TaSz before and after treatment with ammonia is depicted. Initially, the three atom thick slabs of the compound have a spacing of nearly 6 A; subsequent to the reaction, in which a 1:l adduct forms, the interlayer T a distance becomes about 9 A, reflecting the spreading by ammonia molecules ( I ) . Chemists. ~hvsicists.and material scientists are actively studying'this-area f i r its wealth of theoretical and practical promise. Intercalated layer compounds serve as excellent for two-dimensional electronic. mametic. - - - ~ models -~~ and other phase behavior, obviating the necessity f i r sur: faces with all of their encumbering difficulties. Band theoreticians are busy explaining and predicting the various manifestations and perturbations induced by intercalation of layered semicond;ctors and metals. The nature of the bonding interactions between the guest and the layer are in general not well understood. Undoubtedly the entire range of ionic to covalent and very weak to strong binding is covered. Perhaps most investigated are the graphite inclusion compounds, which span the entire spectrum of interactions: vet even in this field many gaps persist and prediction is practically worthless. However the availability of the multitude of HOMO'S and ~~
~
Figure 1. A representation of staged intercalation. The mtermedafe is designated a second rtage compound
Figure 2. X-Ray diffraction powder patterns illustrating the effect of insertion of ammonia between the basal planes of 2H-TaS2.
LUMO's in graphite of moderate energies doubtless is the basis of its uniquely flexible ability to participate in bonding with both acceptors and donors of electrons. Analogous rationalizations can be put forth in the other cases of lamellar inclusion compounds. With the layered clays, even van der Waals bonding between guest and host will often suffice to drive the intercalation, and a broad spectrum of candidate guest species has been found. Many of these topics have been discussed by Barrer (2). The interstitial compounds of maphite have been treated by Henning (3), and the transition metal dichalcogenides are reviewed by Gamble and Gehalle (4) in a forthcoming chapter. The clay minerals have been covered by Grim (5). Properties
The properties, both physical and chemical, of the new materials resulting from the inclusion of a guest species within the layers of a host may he viewed as modifications of those of the starting materials, such modifications arising from the exchange of electron density between them, and the geometrical and spatial restrictions now imposed on them. The presence of molecules between the basal planes will have an obvious impact on certain bulk physical properties including density, lubricity, conductivity, and optical characteristics. The anisotropy often observed Volume 51, Number 4, April 1974
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in such nronerties is freouentlv found to be enhanced on intercalation, since this has the effect of exaggerating the two dimensional nature of the lattice. In snreadine the layers apart, any "shorts" or bridging defeds'in the Fnterlaver space are eliminated or mediated bv the nresumed insulating guest molecules; however, the picture may not be so simple, since new defects may be introduced in the process of inclusion, or the interlayer molecules themselves may facilitate the transfer of certain effects from layer to layer. It has been observed that upon inclusion of many species the initially semiconducting graphite undergoes a metallic transition. Molybdenum disulfide, also initially a semiconductor, becomes a superconducting metal when alkali metals lie between its nlanes. The sunerconductine transition temperature of 2 ~ - ; r a ~ ainitially'at , 0.8-K, can he shifted to neater than 5°K bv introduction of suitable host species. '?he anisotropic resistivity of TaSz is raised by a factor of 105 on intercalation of pyridine, a dramatic demonstration of the effect that can be induced. The chemistry of the modified guest and host is of particular interest to workers in the fields of catalysis and synthesis. One easily conceivable consequence of the spatial restriction imposed by the interlayer situation of a reagent or substrate molecule can be an unprecedented reeio-snecificitv of reaction. Furthermore, since some of t h i layered precursors are known to be he&ogeneous catalysts, it may be possible to tune them by intercalation, with its attendent nerturhation of the hand structure. resulting in a more highly active or selective agent. The t r a n s ~ o r nhenomena t occurrine in the interlaver regions is of wide-ranging interest to wkkers in man; areas including- maenetic resonance,. separations, and biolor)ical . sciences. Applications Interstitial potassium in graphite has been found to he an effective hvdroeenation (6) and reducine (7) aeent with specific oriekati& suggested. Graphite-chromic anhydride offers a selective oxidation method for the preparation of aldehydes from primary alcohols (7). According to Armand, certain graphite inclusion compounds constitute sunerior cathodic materials in the develonment of solidstate secondary batteries (8). The of placement of a reducible species such as CrOa between the conducting layers circumvents many of the problems encountered with other cathodic materials .such as efficient contact and ionic, as well as electronic transport. Another promising candidate for a cathode in a battery is the fluorinated graphite as described by Watanabe (9), with a lithium anode. The discharge of this battery may proceed via an intermediate step involving intercalation of lithium between the layers of (CF),. The forementioned ability of clays with a lamellar structure to absorb many salts and organic structures accounts for their applicability as gelling and emulsifying agents, ion-exchange media, and selective ahsorbants, as well as many other uses (5). The feasibility of inducing the nolvmerization of an interstitial monomer in montmor i ~ ~ o i i thas E heen demonstrated (10, l l , , and in so doing an "organized polymer" rould be shown to result. Lamellar liquid crystalline phases (smectic liquid c ~ s t a l s were ) shown to catalyze the hydrolysis of esters, possibly due to enhanced mobility of water to the reaction site (121. An English group has recently disclosed that silver ketenide crystallizes in a layered configuration, capable of intercalating pyndine. Removal of rhe organic moiety via deromposition under controlled conditions results in metallic silver whose magnetic and catalvric properties are strikingly different from those previously ohsewed. The ketenide or the silver metal derived from it have demonstrated superior catalytic capability in the air oxidation of olefins-a-process of~tremendouseconomic interest. The 222
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Layered Comoounds Found to Intercalate Host
Guests
Graphite
Alkali metals, transition metal halides and sulfides, ICI, CIS,Bm,HNOs, CrO,, AIClsCIx,many others Graphite oxide Water, acetone, ethers, acetic acid, nitric acid, slcohols Boron nitride Seems to behave much like graphite Lamellar clays (montmoril- Water, alcohols, acetone, lonite, smectite, kaolinite, ethers, amines, glycols, halloysite, vermiculite,~.etc.) amides, hvdrazine, sugars, carbohydrates, proteins, aromatics, many salts, etc. Zinc and copper hydroxides Similar metal nitro-phenolates, flavianates, benzaates, and salicylates Silver ketenide (and other Pyridine P Y P Ib) Transit~onmetal dichalcoAmides, amines. m i n e genides oxides, phosphines and their oxides, metals, salts MiscellaneousSmectic liquid crystals, some of the Werner complexes, metal phthalocyanines and Schiff base complexes, dititanates and other metal oxides mechanism of the oxidation is thought to depend on the layered structure in both circumstances (13). Some of the most intriguing possible applications are suggested by the behavior of the transition metal dichalcogenides which occur as layered crystals. Among the members of this group can be found metals, semiconductors, and insulators, and many have heen shown to undergo inclusion of a variety of species capable of donation of electronic density into the Fermi energy level, thus introducing perturbations which critically affect the many properties deriving from the electronic concentration in that reeion. The current BCS theorv of metallic behavior and otKer related conjectures on kperconductivity and related tunnelling phenomena indicate that such materials are promising high temperature superconductors and switching devices in future solid-state microminiaturization circuitry. Alloys derived from these materials which are in fact intercalation compounds, can be prepared such as to tailor certain desirable properties, or minimize unwanted properties; hence an entirely new family of compositions are available to fit the needs as they may present themselves. Molybdenum disulfide is a naturally occurring solid lubricant which presently enjoys wide utility; however, appropriately treated it may be possible to considerably extend its application to exotic situations posed by modern problems, such as outer space enpineering. Various mechanical devices can be envisioned whose operation depends on the swelling properties of these compounds. As in the field of liquid crystals, where it has been found that subtle structural changes can affect the phase transition in increments so that, for instance, a thermometer can be built, one may by intercalating a homologous series of guests be able to prepare materials having suitable ranges of a certain property (such as a phase change), which can act as indicators of conditions in small increments. Optical (or other electromagnetic) filters are conceivable based on this idea. In fact, anyone with a whit of imagination will have no trouble dreaming of exciting possibilities for this unique group of materials, whose characterization has and will continue to occupy physicists and chemists alike. A partial listing of the known layered inclusion compounds is shown in the table.
Literature Cited (1) Gamble, F. R., Osircki, J. H., Cais, M.. Piahamdy. R., DiSaluo, F. J.. and Geballe. T. H.. Science 174.493 (1971). (21 Bsner, R. M.. in "Non-Sfoiehiom&ie Compounds: (Editor: Mandclmm. Lyonl, Academic P-, h e . , New York. 1964, Chap. 6. (3) ncnning, G. R.. in "Pmgrcas in 1oo1genic Chemistry,'' Val. I, (Edilor: cotton. F. A l , Wiley-IntPrseisnce, New York. 1959. (4) Gamble. F.R.. and Geballe, T. H.. in "Treatise on Solid State Chemisttry," Vol. D. (Editor Hannay, N. B l , Perpmon Press bc..Elmsfard, N.Y., (inpresaJ. (51 Grim. R. E.. "Clay Minomloav." 2nd Ed., MeGraw-Hill Bmk Co., New Yirk.
tla").
(9) Wstanal*, N.. and Taknahima, M.. p-ntation at the 7th International S y m p sium onFlumine Chemistry. Santa Cruz. July 1975. (10) Fridlander, H. 2.. and Prink. C. R..PolymerLetf~rs,2.475 (19Ml. (11) zaitsev, Y. S.. R i d , N. G., Enal'ov. V. D., and Yunenko. A. I., Kdhid. Zh.. 2, 213 (19701. (12) Ahmad,S.I.,andFribeg, S.,JAmor. Chrm. Soc.. 94.5196(1912). (131 Blurs. E. T., Bryce-Smith. D., Hineh, H.. and Simona. M. J., Chsm. Commun, 6'3% 11970).lsoseeChom. .dEng. News, p.21,Nw.20. Wl21.
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