Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 125

or convert to the stable one, and eventually only nanoclusters of the most
stable size in the distribution survive the 'focusing' process. By adjusting the
initial size range through kinetic control in the first step, the subsequent
size-focusing step can give rise to a series of size-discrete, robust Au n (SR) m
nanoclusters, such as Au 25 (SR) 18 , 9,11 Au 28 (SR) 20 , 12 Au 36 (SR) 24 , 13 Au 38 (SR) 24 , 14
Au 144 (SR) 60 , 15 Au 333 (SR) 79 , 16 etc. Herein we chose Au 25 (SR) 18 and Au 38 (SR) 24
nanoclusters as examples for a more detailed discussion.

d n 9 r 4 n g | 3

5.2.1 The Case of Au 25 (SR) 18 Nanoclusters
Size-focusing was first observed in the synthesis of Au 25 (SR) 18 nanoclusters
in our early work using the two-phase 17 and one-phase 18 synthetic methods.
In both systems, the first step consists of the reduction of Au( III ) to Au( I ) with
excess thiol to form a Au( I )/SR polymeric intermediate at a low temperature
(e.g., 0 1C or room temperature). The subsequent reduction of Au( I )/SR to
Au(0) by NaBH 4 gives rise to thiolate-protected nanoclusters. After prolonged
aging of the reaction product, we observed several prominent peaks in the
UV-visible spectrum of the crude product. A detailed investigation into this
aging process revealed a gradual growth of monodisperse Au 25 (SR) 18
nanoclusters from the early stage size-mixed product, Figure 5.1a. Three
absorption peaks (at 670, 450 and 400 nm) were observed in the spectrum, 9
which are characteristic of Au 25 (SR) 18 nanoclusters.
This size-focusing growth process was found to be quite common (e.g., not
limited by the type of thiol). Many types of thiols (HS-R) have been investi-
gated in the synthesis of Au 25 (SR) 18 (where R ¼ -C 2 H 4 Ph, -C 6 H 13 ,-C 12 H 25 ,-G
(glutathione), -C 10 H 22 COOH, etc.). 18 The size-focusing growth process of
Au 25 (SR) 18 in one-phase synthesis was later investigated further by Dass and
co-workers using matrix-assisted laser desorption ionization (MALDI) mass
spectrometry. 19

.

It was observed that at earlier reaction times, a mixture

Figure 5.1 UV-visible spectra (a) and mass spectrometric evidence (b) of the 'size
focusing' process of Au 25 (SR) 18 nanoclusters with aging time. Peaks
marked by asterisks are fragments caused by MALDI.
Adapted from ref. 18 and 19.