Chemistry Reference
In-Depth Information
anatomy of Au
25
(SR)
18
, the formula of Au
38
(SR)
24
may be written as Au
23
@[Au
(SR)
2
]
3
[Au
2
(SR)
3
]
6
, which indicates that Au
38
nanocluster has a 23-gold-atom
kernel protected by three monomeric staples and six dimeric staples.
The Au
38
(SR)
24
structure is chiral, as reflected from the rotating arrangement of
the dimeric staples. The unit cell of Au
38
(SR)
24
nanoclusters indeed contains a pair
of enantiomers. The dimeric staples can be arranged in a left-handed or right-
handed way, forming two enantiomers of the Au
38
nanocluster.
5.1.3 The Case of Au
102
(SPh-COOH)
44
Au
102
(SR)
44
was the first reported structure of thiolate-protected gold nanoclusters
[
7
]. Its formula may be written as Au
79
@[Au(SR)
2
]
19
[Au
2
(SR)
3
]
2
. The Au
102
nanocluster possesses a 79-gold-atom kernel. To understand the Au
79
kernel, we
first briefly discuss Marks' decahedron and rhombicosidodecahedron - which are
important noncrystalline shapes.
The Marks' decahedron was discovered in early research on the multi-twinned
nanoparticles. L. D. Marks found that a special form of decahedron is one of the
common low-energy (i.e., stable) shapes for small nanoparticles [
90
,
91
]. A deca-
hedron can be viewed as five tetrahedrons assembled together by sharing a common
edge (as the central axis), with each tetrahedron sharing two facets with neighbor-
ing tetrahedrons, hence forming a fivefold twinned structure (Fig.
13a
). The
remaining two facets of each tetrahedron are exposed, composing the total 10 tri-
angular {111} surfaces of the decahedron. By truncating through the five horizontal
edges of a decahedron, one obtains an Ino decahedron with additional five {100}
facets (Fig.
13b
). A Marks' decahedron is a derivative of Ino's decahedron. Marks
found that when introducing a reentrant {111} surface at each of the five vertical
edges of Ino's decahedron, a more stable decahedron structure can be obtained
(Fig.
13c
). A Marks' decahedron is denoted by (m, n, p), with m indicating the
number of atoms on the equatorial edges, n the number of atoms on the vertical
edge, and p the number of atoms on the reentrant edge (Fig.
13d
)[
44
].
The central portion of the Au
79
kernel of Au
102
(SR)
44
is actually a 49-atom
Marks' decahedron of (2,1,2). The 49 atoms can be counted as 4
1+3
5+2
49, indicating that there are four atoms at the central axis,
3 atoms at the first pentagon column, 2 atoms on the second, and 1 atom on the
third (Fig.
13e
).
The Au
49
Marks' decahedron has
ten
extended {111} facets. In order to reduce
the exposed surface area, nature chooses to cover each {111} facet with
three
more
atoms (Fig.
14a
), giving rise to Au
79
. The three additional atoms adopt close
packing onto the Au
9
triangular {111} facets; hence, there are additional 15 gold
atoms on the top of the Au
49
decahedron and another 15 on the bottom (Fig.
14b
),
composing an Au
79
kernel (Fig.
14c
). The 15 atoms actually connect together to
10 + 1
10
ΒΌ
