Atomically Precise Gold Catalysis - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 90

Traditionally nanoparticle characterization for size determination has
been predominantly carried out using high-resolution transmission electron
microscopy (TEM). The advantage of using electron microscopy over other
techniques is that the particle size of the nanoparticles along with their size
distribution can be readily identified through a visual approach unlike
spectroscopy tools like UV-visible or FT-IR. The electron density differences
present in different elements also makes it convenient to distinguish
accurate sizes of organic-ligand capped nanoparticles, core-shell and
bimetallic/multi-metallic nanoparticles. However, due to the high-energy
electron beam source, the nanoparticles tend to interact with the electron
beam causing their aggregation and thus leading to size aberration. This
problem is often noticed during characterization of atomically precise gold
clusters. 20 Therefore, these clusters need a more precise and nondestructive
characterization tool that can accurately determine their size based on the
number of atoms present in them. Mass spectrometry appears to be the most
reliable alternative technique available to date for this purpose. Different
mass spectrometry techniques such as laser desorption ionization (LDI),
matrix-assisted laser desorption ionization (MALDI) and electrospray ion-
ization (ESI) have been utilized. Of the three, the LDI and MALDI are rela-
tively harsher techniques that tend to break the Au-S and S-C bonds of the
clusters during analysis. On the other hand, ESI is a much software and
more preferred technique.
In general, characterization of atomically precise gold cluster catalysts
involves three distinct stages: first, to determine the accuracy of their
atomic size of the as-synthesized clusters; second, characterization of the
clusters after supporting them on high-surface area catalytic supports such
as TiO 2 ,SiO 2 ,CeO 2 etc. and calcining to remove the organic capping lig-
ands; and finally, characterization of the supported catalysts after the
catalysis reaction is performed. However, the currently available mass
spectrometry techniques (such as MALDI and ESI) have only been helpful
in the first stage of characterization. For the second and third stages, the
catalytic support generally interferes with the mass spectral fragmentation
of the clusters thus making it dicult for their characterization through
mass spectroscopy. Therefore, there is still an unfulfilled need for de-
veloping mass spectrometry technique for characterizing supported
atomically-precise. Figure 4.2 gives a schematic of some of the character-
ization techniques that are currently used for atomically precise gold
cluster catalysts. More on the characterization techniques for exploring
electronic and magnetic structure (such as the X-ray absorption spec-
troscopy and SQUID measurements) and their correlation with catalysis
will be discussed in Section 4.3.
Recently, there have been significant advances in using single-crystal X-ray
techniques for characterisation of atomically-precise gold nanoclusters. For
example, Murray and co-workers have reported the formation and crystal
structure (Figure 4.3) of atomically precise anionic Au 25 clusters capped by

d n 9 r 4 n g | 7

.