Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 126

ranging from Au 25 to Au 102 was formed, and subsequently, size evolution led
to monodisperse Au 25 (SR) 18 nanoclusters (Figure 5.1b).
The case of Au 25 (SR) 18 synthesis has illustrated the basic principle of the
size-focusing growth process, which is based upon the stability property of
different sized nanoclusters. In order to attain atomic monodispersity,
synthetic control is needed so that only one specific size of nanoclusters
survives the size-focusing process. An important issue is how to render just
one size to survive the focusing process. In our work, we found that it is
critical to control the size distribution of the starting Au n (SR) m mixture.
Below, we shall elaborate on this with the case of Au 38 (SR) 24 nanocluster
synthesis as a typical example.

d n 9 r 4 n g | 3

5.2.2 The Case of Au 38 (SR) 24 Nanocluster
The Au 38 (SR) 24 formula was first unambiguously determined by Chaki
et al., 20 but the synthetic yield was quite low. Qian et al. 21 improved the
Au 38 (SR) 24 synthetic method and increased the yield of Au 38 (SC 12 H 25 ) 24 to
B

10% (Au atom basis) via a two-step approach, and the Au 38 (SC 12 H 25 ) 24
composition was verified by electrospray ionization (ESI) MS and other
characterizations. In this method, a crude mixture of glutathionate-protected
Au n (SG) m nanoclusters was first made; the mixed nanoclusters were then
subjected to a thermal thiol etching process (e.g., 80 1C) in a two-phase (water/
toluene) system. After the ligand exchange (from -SG to -SC 12 H 25 )onthe
nanoclusters was completed, the subsequent etching process (in neat dode-
canethiol) caused gold core etching, and eventually the starting polydisperse
nanoclusters were converted to monodisperse Au 38 (SC 12 H 25 ) 24 with high
purity.
The two-step method was further extended to the synthesis of
Au 38 (SC 2 H 4 Ph) 24 with a higher yield (

.

25% based on Au) after optimization
of some reaction parameters. 22 A detailed mechanistic investigation on the
conversion process clearly showed a size-focusing process, evidenced by
both optical spectroscopy and MALDI-MS analyses (Figure 5.2). The optical
spectrum of the starting Au n (SC 2 H 4 Ph) m showed a decaying curve, indicating
a mixture; with reaction going on, several distinct peaks started to emerge in
the spectrum, and the final product showed a distinctive optical spectrum
characteristic of Au 38 (SC 2 H 4 Ph) 24 nanoclusters (Figure 5.2a). 21,22 MALDI-MS
analysis reveals that, with the increase of reaction time, the relatively large
Au nanoclusters (n438) seem to decompose and convert to Au 38 (SC 2 H 4 Ph) 24
(Figure 5.2b). After

B

40 h, very pure Au 38 (SC 2 H 4 Ph) 24 nanoclusters
(MW, 10 780 Da) were obtained (Figure 5.2b); note that the fragment at
9342 Da was caused by the MALDI method.
As discussed above, a key condition to obtain single-sized nanoclusters is
to control the size distribution of the Au n (SG) m mixture prior to the size-
focusing step. We realized that the solvent might play an important role in
controlling the size range of the Au n (SG) m starting mixture. 22 In previous
work, the Au n (SG) m nanoclusters were typically prepared in methanol. 23

B