Chemistry Reference
In-Depth Information
Fig. 13 Positive MALDI mass spectra of the meta-stable species observed in the core-size
conversion reactions of Au
67
,Au
103-105
, and
Au
67
clusters
<
5.1.3 MS Data-Based Mechanism of Core-Size Conversion
Au
38
and Au
40
were observed in the core-size conversion reactions for a long time
now, but the mechanism of formation of these nanomolecules is not completely
understood. For the formation of smaller gold nanomolecules by core conversion,
the larger nanoclusters need to break down and decrease in size. During the etching
reactions, there are several meta-stable species observed in the MALDI-MS data of
the samples. These peaks are well resolved in the MALDI-MS data acquired for the
samples collected at different time intervals from the etching reactions.
To understand the mechanism in the formation of Au
38
and Au
40
during core-
size conversion reactions, the transient species observed in the etching samples
were studied in detail. Figure
13
shows the assignment of the meta-stable species
observed during these reactions.
In the etching of Au
67
(black curve in Fig.
13
)
,
the composition of the meta-
stable species is indicated on top of each peak. Of the several species observed,
Au
57
(SR)
28
,Au
55
(SR)
30
,Au
56
(SR)
27
, and Au
51
(SR)
27
are the most dominant peaks
in the mass spectra. Other species include Au
60
(SR)
27,
Au
49
(SR)
29,
Au
47
(SR)
27
,
Au
46
(SR)
26,
and Au
41
(SR)
24
. Looking at the composition of these meta-stable
species, it is reasonable to assume that there is a systematic loss of gold atoms
and ligands to form Au
40
(SR)
24
in the etching of Au
67.
Au
103-105
fraction, when etched, converts to Au
40
through core-size conversion
process. As the final product in both of the reactions is Au
40
, it is highly likely that
these reactions proceed via the same or closely related intermediate species. For
both Au
67
and Au
103-105
etching, Au
51
(SR)
27
,Au
49
(SR)
29
, and Au
41
(SR)
24
were
