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Chapter 1-Section 1.4
1.4 Intercalation Chemistry
Intercalation is the insertion
of atomic or molecular species into the structure of a host material.19 In layered materials, intercalation typically takes places
where there is strong binding forces within a layer, but only weak
interlayer binding forces. This behavior is prevalent in graphite,
which is made up of sp2 bonded carbon layers separated by
van der Waals gaps. While the covalently bonded carbon layers will
not easily allow another atom or molecule to be inserted within the
layer, species can easily be inserted between these layers. A schematic
diagram of intercalation can be seen in Illustration 1.7.
Intercalation can be accomplished
by a variety of methods.20 Vapor- phase intercalation can yield the highest quality
materials since it is easier to separate the host from the species
that is being inserted. This is often accomplished by placing the
intercalation species at one end of a tube and the host at the other. This
method requires that the intercalation species be volatile and stable
enough at a given temperature to be transferred into the host lattice. Another
method is to accomplish intercalation by a liquid or solution route. Exposing
the host to the liquid of the intercalation species can be a rapid
method of intercalation. Due to the rate of intercalation, this method
can introduce a large number of defects into the host sample.
Illustration 1.7: Schematic drawing of a layered material
where a) is the lattice before intercalation, and b) is the lattice after
intercalation of an atomic species such as K+ or Cs+.
While lowering the concentration by using a dilute solution
of the intercalating species may overcome this problem, the intercalation
of unwanted solvent molecules can occur. A solution method can also
utilize electrochemical methods for intercalation. Finally, solid-phase
reaction of an intercalation species can be employed. This method
may exploit the application of pressure to facilitate intercalation. Removal
of excess intercalation species could prove difficult. All three of
these methods are usually carried out with an excess of intercalation
species to insure the reaction goes to completion.
Three types
of intercalation species can be inserted into the lattice of a host
material. The first are electron donor species, which donate electrons
to the lattice of the host material. Metals such as Li+,
K+, and Hg+2 are typical donors that have been
intercalated. Alloys of metals can also be intercalated. Next, acceptor
species can also be intercalated. Iodine is a common acceptor used
for intercalation. Vapor of I2 can enter a host lattice
as I3-. Other examples of acceptors that can
be used for intercalation are: Cl2, Br2, FeCl3,
and MgCl2. Last of all neutral intercalation species can
be intercalated. Benzene is one good example. Because there is not
a strong driving force for the insertion of neutral species, only a
small number of these have been intercalated. Often neutral species
accompany donor or acceptor species from solution reactions.
