Atomically Precise Gold Catalysis - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 103

d n 9 r 4 n g | 7

Figure 4.10 Au L 3 -edge XANES of the thiolate ligand protected Au 19 ,Au 25 ,Au 38 and
Au 144 nanoclusters compared with the bulk Au.
(Reproduced with permission from ref. 51 and 60, Copyright American
Chemical Society, 2010, and 2012).

nanoparticles to thiol. They also examined the Au-S interaction using S K-
and L 3,2 -edge XANES which confirmed the Au-S bonding. The Au L 3 -edge
XANES of Au 19 (SR) 13 , 60 Au 25 (SR) 18 , 39,51 Au 38 (SR) 24 32,51,61 and Au 144 (SR) 60 51
have been reported by different groups. As seen in Figure 4.10, 51,60 the
spectral features of all thiolate Au nanoclusters are broader in comparison
with the bulk and the edge is slightly shifted to higher energy. These are
related to the small particle size (or low Au coordination number) and
slightly oxidized Au. There is a very clear trend of increasing white line in-
tensity relative to the bulk Au as the cluster size reduces; which is consistent
with the work of Zhang and Sham. 50 The estimated gain of d-hole counts
relative to the bulk increased by 0.028 e for Au 144 , 0.082 e for Au 38 , and
0.089 e for Au 25 . It is known that the white line of bulk Au is contributed by
s-p-d hybridization. 62,63 For bare Au nanoparticles, 49,57 the d-d interaction
is very strong and s-d hybridization is relatively weak, so the corresponding
white line intensity is lower than bulk Au. However, in the case of thiolate
ligand protected Au nanoclusters, 51 the strong interaction between Au atoms
and thiolate ligands causes the d band electrons of Au atoms to transfer to
the thiolate ligands and thus result in increase in white line intensity. For
smaller Au nanoclusters, higher the fraction of 'staple' Au atoms, higher is
the observed d-band depletion. In addition to the Au L 3 -XANES, the S K-edge
XANES spectra of the nanoclusters (Figure 4.11) showed the lower energy
shift of Au-S features at around 2470-2471 eV accompanied by an increase of
Au-S relative peak in the order of Au 25 ,Au 38 and Au 144 . 51
The Au L 3 -edge XANES of phosphine ligand protected Au 55 nanoclusters
(Au 55 (PPh 3 ) 12 Cl 6 ) was reported by Benfield's group. 64 It was observed that the
white line intensity of Au 55 was much smaller relative to KAu III Cl 4 and
(PPh 3 )Au I Cl and closely resembles that of bulk Au. It is consistent with the
lower Au oxidization state in Au 55 ( þ 6/55). The estimation of densities of
unoccupied 5d states by the L 3 kL 2 calculations pointed out the relative
lower value for Au 55 (2.7) compared to bulk Au (4.1). The smaller size of Au 55
instead of the ligand effect was considered as dominant reason for it.

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