Atomically Precise Gold Nanoclusters: Synthesis and Catalytic Application - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 130

d n 9 r 4 n g | 3

Figure 5.6 (a) Thermogravimetric analysis of Au n (SR) m nanoclusters (under a N 2

atmosphere). (b) Isothermal stability analysis of pure Au 25 (SR) 18
nanoclusters (maintained at 150 1C, in air atmosphere for 60 min). No
discernible loss of ligands (starting: 100%) was observed. The inset
shows a zoom-in of the TGA curve at constant 150 1C.
Adapted with permission from ref. 25 and 26.

gold nanoclusters remain stable after the annealing process, Figure 5.6b.
The thermal tests confirm that the Au 25 (SR) 18 nanoclusters are robust and
thermally stable below the ligand loss temperature. 26

5.5 Reactivity and Catalytic Properties of Au n (SR) m
Nanoclusters
5.5.1 Reversible Conversion Between [Au 25 (SR) 18 ] 0 and
[Au 25 (SR) 18 ]
It is known that the electron transfer between metal nanocatalysts and
reactants is one of the key steps in catalytic reactions. The native Au 25 (SR) 18
nanoclusters from the synthesis are anionic (i.e., [Au 25 (SR) 18 ] , counter-
ion ¼ tetraoctylammonium (TOA 1 )). When a solution of [Au 25 (SR) 18 ] was
exposed to air, we found that the nanoclusters were gradually converted to
charge-neutral [Au 25 (SR) 18 ] 0 clusters. 27 During the process, a discernable
color change was observed, which was also reflected in the UV-visible
spectral changes (Figure 5.7). This result is quite interesting, as one would
normally expect that O 2 would first oxidize the thiolate ligands on the Au
surface. 28 But our experimental results revealed that the gold core, instead of
the thiolate ligands, first undergoes one-electron loss to yield neutral charge
[Au 25 (SCH 2 CH 2 Ph) 18 ] 0 nanoclusters. Single crystal X-ray crystallography re-
vealed that the [Au 25 (SCH 2 CH 2 Ph) 18 ] 0 structure 29 shares the same framework
as that of [Au 25 (SCH 2 CH 2 Ph) 18 ] . The negative charge of [Au 25 (SCH 2 CH 2 Ph) 18 ]
resides on the gold core of the cluster rather than on the ligands. The redox
process between [Au 25 (SCH 2 CH 2 Ph) 18 ] 0 and [Au 25 (SCH 2 CH 2 Ph) 18 ] is com-
pletely reversible. If peroxide is used as the oxidant, the anion-to-neutral
conversion is much faster than using O 2 . The oxidation by O 2 or peroxide was

.