Chemistry Reference
In-Depth Information
stabilising cationic gold species [
214
] and surface hydroxyl groups appear to play
an important role at the active site. The chapter contributed by Freud et al. reports
the growth of Au on ultrathin films and thicker, more bulk-like substrates by
physical vapour deposition. Their recent studies which have combined imaging
and spectroscopic techniques has have resulted in a detailed picture of how clusters
form from single Au atoms to clusters and further on to nanoparticles. Ultrathin
MgO and alumina lead to electron transfer to the supported Au, whereas iron oxide
leads to electron transfer from the Au to the support [
215
]. These important
developments in catalysis using gold have encouraged the study of nanoalloys of
gold with other metals. Recent theoretical developments in this field have been
discussed by Johnston [
216
,
217
].
Interestingly this renaissance has been matched by important developments in
the use of gold complexes as homogeneous catalysts in the transformation of
organic molecules. Recent literature compilations by Hashmi provide an excellent
introduction to the recent developments in this field [
218
]. The emergence of this
important area was initiated in 1986 by Ito and Hayashi's discovery of the asym-
metric aldol reaction [
219
], and sustained by Teles' [
220
,
221
] 1998 finding of
highly active catalysts for the addition of heteronucleophiles to alkynes and the
C-C-bond formation reactions developed by Hashmi et al. in 2000 [
222
-
224
]. The
importance of these developments in the context of organic chemistry is
summarised in [
225
].
4.6 Metal Insulator Transition
Frohlich and Kubo have argued that the electrical conductivity within a metal
particle is expected to decrease rapidly when the diameter of the metal particle
approaches the de Broglie wavelength of the conduction electrons. Chemically this
corresponds to the localisation of electrons as valence electrons become highly
confined and quantum effects predominate. This transition has attracted consider-
able theoretical interest, but experimental verification has been limited by the
absence of reliable synthetic routes to monodispersed gold particles with well-
defined dimensions. The Schriffen-Brust method for synthesising large gold clus-
ters described above has provided a method for making well-defined large clusters
which may be characterised by single-crystal X-ray techniques. They provide
individual mesoscopic conductors well separated from each other by a sheath of
insulating organic ligands and provide ideal systems for studying the size confine-
ment of metallic conduction electrons. Using microwave absorption techniques, it
has recently been shown that individual gold particles with diameters approxi-
mately 4 nm have electrical conductivities 10
7
smaller than that of bulk gold.
Edwards and Thomas have suggested that this is a dramatic example of size-
induced metal-insulator transition [
2
,
3
] and have given a full account of the
relevant experimental and theoretical work.
