Page 271 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

528 Metal Particles and Cluster Compounds

FIGURE 10 Drawings of closed (sub) shell clusters in the proposed growth sequence from n = 83−147. Black denotes
primary NH 3 adsorption sites; gray, third shell atoms; and white, second shell atoms.

way to elucidate how the classic silver-silver bromide metal particles, say 2–100 nm in diameter, or M 100 –
photographic process works. Woste and coworkers soft- M 1,000,000 . Although one million metal atoms sounds like
landed Ag 2 ,Ag 3 , and Ag 4 clusters (positively or nega- a larger number, it is still small compared to Avogadro’s
tively charged) on a surface of silver bromide. It was number. This size range is in the nanometer regime where
found that only the Ag 4 cluster was capable of catalyt- it has been found that numerous physical and chemi-
ically enhancing the photographic development process cal properties change and are dependent on the exact
size in this range. Optical properties (including color)
subsequently carried out. These data suggest that Ag 4
is a key species that is formed when silver-bromide- change with size, as do melting points, magnetic prop-
containing photographic film is exposed to light. In- erties, and surface chemistry. For example, gold nanopar-
deed, it has been estimated that about 10 photons are ticles suspended in solution can be blue, red, or purple
needed to produce one Ag 4 species (according to the pro- depending on the size due to changes in plasmon reso-
posed mechanism, four would be the minimum number nance (a collective phenomenon where the particle gen-
required). erates its own molecular orbitals and, in a sense, be-
Based on these results, it has been proposed that at de- haves like one giant atom). Another interesting point is
fect sites on the AgBr surface photons caused free elec- that a 5-nm gold particle melts 50 C lower than bulk
◦
tions to be formed, which migrate to the surface and reduce gold.
0
0
Ag to Ag . These Ag atoms aggregate to form Ag 4 , and A unique development here was the synthesis of a class
+
this is a thermodynamically favored species. The presence of ligand-stabilized metal clusters by Schmid and cowork-
of Ag 4 then can later serve as a catalyst for further silver ers. These represent giant clusters when considered in the
ion reduction to silver metal in the photographic developer light of normal ligand-stabilized clusters, which are cov-
solution. ered in Section III. For example, Au 55 (PPh 3 ) 12 Cl 6 and
Thus, it is again clear that the study of small metal Pd 561 (Phen) 36 O 190−200 exemplify better than any others
clusters can yield a great deal of useful information. the crossover between molecular metal clusters and metal
particles. Thus, they are soluble molecular species.
Although these represent “giants” in molecular clus-
4. Larger Particles (M 100 –M 1,000,000 )
ters, they are small compared to what has been prepared
In recent years unprecedented progress has been made in the nanoscale metal particle area, which will now be
in synthesis and characterization of comparatively large discussed.