Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 124

thiolate (-SR) ligands (m), denoted as Au n (SR) m , with n ranging up to a few
hundred atoms (e.g., o 200, equivalent size up to 2 nm). These thiolate-
protected nanoclusters are well-defind to the atomic level (i.e., of molecular
purity), rather than the usual nanometer precision in conventional nano-
particles. The Au n (SR) m nanoclusters are particularly robust 8

d n 9 r 4 n g | 3

(discussed

below).
In this chapter, we first briefly discuss the synthesis of Au n (SR) m
nanoclusters, and then focus our discussion on the catalytic reactions of
such nanoclusters. We note that research on the catalytic application of
Au n (SR) m nanoclusters is still in its infancy, but the reported work has shown
great promise of such nanoclusters. We use Au 25 (SR) 18 nanoclusters as a
typical example to illustrate its promising catalytic properties, as Au 25 (SR) 18
has been widely investigated among all the gold nanoclusters.
Compared to conventional metallic Au nanoparticle catalysts, Au n (SR) m
nanoclusters possess several distinct features that are of particular interest
to catalysis. First of all, metallic gold nanoparticles (2 to 100 nm) adopt a
face-centered cubic (fcc) structure, but Au n (SR) m nanoclusters (1 to 2 nm)
often adopt different atom-packing structures; for example, an icosahedral
structure was found in Au 25 (SR) 18 nanoclusters ( 1.27 nm metal core, see

detailed discussion in Section 5.2). 9 Second, the ultra-small size induces
strong electron-energy quantization in nanoclusters, as opposed to the
continuous conduction band in metallic gold nanoparticles. 9,10 Thus,
Au n (SR) m nanoclusters become semiconductors and possess a sizable
bandgap (e.g.,

B

0.9 eV for Au 38 (SR) 24 ).
The atomically precise Au n (SR) m nanoclusters are expected to become a
promising class of model catalysts, although much work remains to be
carried out in future. These well-defined nanoclusters will provide new
opportunities for achieving fundamental understanding of metal nano-
catalysis, such as the insight into the size dependence and deep under-
standing of the molecular activation, active centers, and catalytic
mechanism by correlation with the structures of nanoclusters. Future
research on atomically precise nanocluster catalysts will contribute to the
fundamental catalysis and the new design of highly selective catalysts for
specific chemical processes.

B 1.3 eV for Au 25 (SR) 18 and B

.

5.2 Synthesis of Atomically Precise Gold
Nanoclusters: Size-focusing Method
We briefly introduce the 'size focusing' methodology for synthesizing
atomically precise Au n (SR) m nanoclusters. 6 In this method, a proper distri-
bution of size-mixed nanoclusters is first made by kinetically controlling the
reduction reaction of gold precursor (typically Au( I )) with NaBH 4 , which is
the key step for the final product. Then the size-mixed nanoclusters are
subjected to size-focusing under harsh conditions (e.g., at 80 1C and in the
presence of excess thiol), under which the unstable nanoclusters decompose