Chemistry Reference
In-Depth Information
high-resolution transmission electron microscopy (HRTEM), nuclear magnetic
resonance (NMR) [
27
,
28
], mass spectrometry (MS) [
29
], electrochemistry [
30
],
and more accurate theoretical calculations [
31
-
33
].
2.1 Size Determination
Intensive studies showed that the optical, electronic, and catalytic properties of
metal nanoclusters are strongly dependent on the core size of metal nanoclusters.
For instance, for the coinage metal nanoparticles with core diameters
d
~2 nm,
obvious surface plasmon resonance (SPR) can be observed in the UV-Vis absorp-
tion spectra. This optical feature, however, is damped out for smaller-sized clusters.
Instead, single band or multiband UV-Vis absorptions are observed for the
sub-nanometer-sized metal clusters [
16
,
34
,
35
]. On the other hand, quantized
capacitance charging has been observed for monolayer-protected Au nanoclusters
(Au MPCs) with the diameters of ~1.5 nm
>
~2.5 nm in both scanning
tunneling microscope (STM) and electrochemical measurements, and this has
been attributed to single electron charging of MPC with a very small capacitance.
With even smaller metal core sizes (
d
d
<
<
~1.5 nm), the Au MPCs show
HOMO-LUMO energy gap and exhibit molecular-like rather than metallic
behaviors [
13
,
36
]. Besides, with Au nanoclusters as catalysts, much work has
been done to explore the size effect on the catalytic activity. Chen et al. [
37
]
prepared a series of Au nanoclusters with 11 to 140 gold atoms in their cores
(0.8-1.7 nm in diameter) and carried out detailed electrochemical studies in alka-
line media to evaluate the size effect on the electrocatalytic activity for the oxygen
reduction reaction. The results showed that the electrocatalytic activity for O
2
reduction increases with the core size decreasing and the Au
11
clusters exhibit the
highest catalytic activities. All these studies show that the core size has significant
effect on the properties of gold nanoclusters. Therefore, size determination is of
critical importance for the in-depth understanding the size-dependent properties of
nanoclusters. TEM is a very powerful technique in the size and surface structure
characterizations of nanomaterials. For metal nanoclusters with core size smaller
than 2 nm, especially for sub-nanometer clusters (
d
<
1 nm), traditional techniques
for the size characterization of large metal nanoparticles, including scanning
electron microscopy (SEM), low-resolution transmission electron microscopy
(TEM), and powder X-ray diffraction (XRD), will not be very reliable to get the
size information precisely. Along with the rapid development of electronic tech-
nology, the resolution of HRTEM has been improved to less than 1.0 nm. The size
and atom arrangement in the metal clusters can be directly observed by HRTEM
measurement. However, ultra-tiny clusters smaller than 1.0 nm may be barely
observable in HRTEM, and upon long electron beam irradiation, they can aggregate
to large nanoparticle [
38
]. Mass spectrometry is another effective technique to
analyze the exact core size of nanoclusters [
39
,
40
]. Meanwhile, NMR has also
been used as an analytical
tool
to estimate the size of thiol-stabilized gold
