TGA
Thermogravimetric analysis
UV-Vis
Ultraviolet-visible
WLED
White light emitting diode
XAFS
X-ray absorption fine structure
XEOL
X-ray-excited optical luminescence
XPS
X-ray photoelectron spectroscopy
XRD
X-ray diffraction
ZSM-5
Zeolite Socony Mobil-5
1 Confined Functional Metal Clusters
Metal clusters consisting of only several to tens of metal atoms possess unique
properties that are different from those found in metal nanoparticles and bulk metal.
Due to their electronic configuration and discrete energy levels, metal clusters
exhibit peculiar electronic and optical properties [1, 2], such as molecule-like energy
gaps [3, 4], strong photoluminescence [5, 6], and high catalytic activity [7–
9]. Recently, many efforts to study metal clusters with multifunctional properties
have been carried. Hence, these small structures might help to understand the
missing link between atomic and nanoparticle metal behavior [10]. However, their
direct experimental analysis is not always straightforward. Small metal clusters tend
to aggregate and thus rapidly form larger nanoparticles [11], whereas the typical
sub-nanometer size structures of such small metal clusters hinder their analysis
[12]. An alternative to overcome this problem is the stabilization of metal clusters
by different hosts. For instance, organic molecules such as thiolates [13, 14] or DNA
[15, 16] have been widely utilized, whereas template-based inorganic scaffolds such
as glasses [17] and zeolites [18, 19] are often used. However, the influence of the
confinement scaffolds on the physicochemical properties of the oligoatomic metal
clusters needs to be taken into consideration. It is well documented that the unique
properties of metal clusters are related to their size, charge, shape, and interaction
with their environment [20, 21]. For instance, the fluorescent emission of gold
clusters encapsulated in DNA strands could be adjusted from the near infrared
(IR) to the ultraviolet (UV) region by changing their core size. Gold clusters ranging
from Au 31 , Au 23 , Au 13 , Au 8 , and Au 5 showed near IR, red, green blue, and UV
emission, respectively [22, 23]. On the other hand, a relationship between gold metal
cluster size and catalytic activity in the oxidation of CO and thiophenol has been
demonstrated [9, 24]. It was found that gold clusters containing about 10 gold atoms
were the species with the most catalytic activity, whereas larger Au clusters showed
very low or no catalytic activity.
Next to functional gold cluster, a growing number of studies on sub-nanometer
sized copper and silver clusters have also appeared. For instance, the synthesis of
small oligoatomic copper clusters in microemulsions has been reported
[25]. Photoluminescence was observed in copper clusters with less than 10 Cu
Highly Luminescent Metal Clusters Confined in Zeolites
77
Thermogravimetric analysis
UV-Vis
Ultraviolet-visible
WLED
White light emitting diode
XAFS
X-ray absorption fine structure
XEOL
X-ray-excited optical luminescence
XPS
X-ray photoelectron spectroscopy
XRD
X-ray diffraction
ZSM-5
Zeolite Socony Mobil-5
1 Confined Functional Metal Clusters
Metal clusters consisting of only several to tens of metal atoms possess unique
properties that are different from those found in metal nanoparticles and bulk metal.
Due to their electronic configuration and discrete energy levels, metal clusters
exhibit peculiar electronic and optical properties [1, 2], such as molecule-like energy
gaps [3, 4], strong photoluminescence [5, 6], and high catalytic activity [7–
9]. Recently, many efforts to study metal clusters with multifunctional properties
have been carried. Hence, these small structures might help to understand the
missing link between atomic and nanoparticle metal behavior [10]. However, their
direct experimental analysis is not always straightforward. Small metal clusters tend
to aggregate and thus rapidly form larger nanoparticles [11], whereas the typical
sub-nanometer size structures of such small metal clusters hinder their analysis
[12]. An alternative to overcome this problem is the stabilization of metal clusters
by different hosts. For instance, organic molecules such as thiolates [13, 14] or DNA
[15, 16] have been widely utilized, whereas template-based inorganic scaffolds such
as glasses [17] and zeolites [18, 19] are often used. However, the influence of the
confinement scaffolds on the physicochemical properties of the oligoatomic metal
clusters needs to be taken into consideration. It is well documented that the unique
properties of metal clusters are related to their size, charge, shape, and interaction
with their environment [20, 21]. For instance, the fluorescent emission of gold
clusters encapsulated in DNA strands could be adjusted from the near infrared
(IR) to the ultraviolet (UV) region by changing their core size. Gold clusters ranging
from Au 31 , Au 23 , Au 13 , Au 8 , and Au 5 showed near IR, red, green blue, and UV
emission, respectively [22, 23]. On the other hand, a relationship between gold metal
cluster size and catalytic activity in the oxidation of CO and thiophenol has been
demonstrated [9, 24]. It was found that gold clusters containing about 10 gold atoms
were the species with the most catalytic activity, whereas larger Au clusters showed
very low or no catalytic activity.
Next to functional gold cluster, a growing number of studies on sub-nanometer
sized copper and silver clusters have also appeared. For instance, the synthesis of
small oligoatomic copper clusters in microemulsions has been reported
[25]. Photoluminescence was observed in copper clusters with less than 10 Cu
Highly Luminescent Metal Clusters Confined in Zeolites
77