Vapor-Phase Intercalation

Vapor-phase intercalation has its advantages and disadvantages. One advantage of vapor-phase intercalation is that the host is forced to retain its stoichiometry. In liquid-phase intercalation, the host can release atoms, or more likely, ions, to the liquid to which it is being exposed to. For example, when the layered semiconductor Cd2 2S6 is soaked in an aqueous solution of potassium chloride and ethylenediaminetetraacidic acid (EDTA), cadmium ions leave the lattice, and potassium ions enter the lattice (Clement et al., 1986)  

With vapor-phase intercalation, the host is much less likely to decompose as just described. The disadvantage of vapor-phase intercalation is that with far fewer molecules impinging on the surface of the host, reaction rates are much slower. The disadvantage can also be an advantage since the reduced reaction rate allows the intercalated material to remain more homogeneous with respect to the amount of material intercalated. 

In the example with aniline, the aniline vapor was provided by the equilibrium vapor liquid aniline. Vapor-phase intercalation can be done with compounds that are gases at room temperature and ambient pressure. The most common gas used for intercalation reactions is ammonia. Ammonia intercalation can be accomplished by exposing a host to the vapor generated by a concentrated aqueous ammonia solution. This multi-component vapor containing NH3(g), IfeOQj), 

A beautiful example of vapor-phase intercalation involves the reaction of lead(II) iodide with hydrazine (Ghorayeb et al., 1984). Lead(II) iodide is a layer structure (Cdl2 structure, P3m, no. 164) which forms large yellow transparent hexagonal crystals. When these crystals are exposed to hydrazine vapor, they turn colorless Similar chemistry is observed with ammonia (Cleary, unpublished results). In the ammonia case, when the colorless intercalated crystals are removed from the ammonia atmosphere, the ammonia deintercalates spontaneously and the crystal returns to its yellow color  

This rapid intercalation-deintercalation of ammonia can form the basis for a chemical sensing device. Thomas and Cleary (1996) used Mn2 2S6, a layered material similar to PM2, as the host material for ammonia intercalation and showed a rapid change in the dielectric properties of the host upon exposure to ppm concentrations of ammonia. 

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Vapor-phase intercalation can also be done with a guest material that is a solid at room temperature. For example, Harley and McNeil (2004) report the... 

The filling control approach has even been applied to some nanophase materials. For example, the onset of metallicity has been observed in individual alkali metal-doped single-walled zigzag carbon nanotubes. Zigzag nanotubes are semiconductors with a band gap around 0.6 eV. Using tubes that are (presumably) open on each end, it has been observed that upon vapor phase intercalation of potassium into the interior of the nanotube, electrons are donated to the empty conduction band, thereby raising the Fermi level and inducing metallic behavior (Bockrath, 1999). 

III) Synthesis in the vapor phase. Vapor-phase intercalation is slower (requiring several hours) and leads to better materials. It can be carried out in a two-bulb reactor (Fig. La) . 

Preparation of binary-alkali metal graphites by cathodic reduction of graphite in molten salt electrolytes is possible in principle but has no advantage over conventional vapor-phase intercalation, particularly because of difficulties related to the separation of the product from the melt. Cathodic intercalation of alkali cations from solid or organic polymer electrolyte is also possible . 

Intercalation via complexation in the vapor phase (S2i). Thus, for example, the nonvolatile C0CI2 can be intercalated in the presence of AICI3 at 500°C, because of the enhanced volatility of the former. Graphite-C0CI2 containing only traces of AICI3 is obtained. 

Another way to intercalate nonvolatile halides is through complexation in the vapor phase. By forming a volatile adduct, a nominally nonvolatile halide can be transported to graphite a molecule at a time, thereby allowing intercalation. The first example of this technique was with the C0CI2-AICI3 system , and it has also been used for the trichlorides of rare earth elements . 

Fig. 25.10 Experimental (//A 0) equatorial scattering data obtained in situ during (a) sodium and (b) cesium vapor doping (intercalation) of poly(p-phenylenevinylene) showing the structural progression HB 120° d-12()° of Fig. 25.9 for sodium and the HB stage 1 phase transformation for cesium. (A = 1.542 A.)...

Fullerenes have been reduced in the solid state, using the alkali metals Li, Na, K, Rb and Cs, which have the ability to intercalate in the vapor phase into solid fullerenes [133]. The discovery of the superconductivity of metal intercalated C o [136] attracted much attention to such materials therefore, the metal intercalation of... 

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