Atomically Precise Gold Catalysis - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 105

d n 9 r 4 n g | 7

Figure 4.12 Au L 3 -edge XANES of the BSA (left) and PVP (right) stabilized Au
nanoclusters compared with the bulk Au.
(Reproduced with permission from, ref. 54 and 58, Copyright AIP
Publishing LLC, 2009 and American Chemical Society, 2009).

nanoclusters 54 (shown in Figure 4.12) was found to possess lower d-electron
density than that of the bulk by 0.047 e and thus reflects higher d-band
depletion. This was consistent with the theoretical calculation of projected
local density of states (L-DOS). The Au-S bonding effect (charge transfer)
associated with the 'staple' motif structure analogous to Au 25 (SR) 18 was well
correlated with the increasing white line intensity in XANES and positive
edge shift in XANES and Au 4f XPS. 54
Interestingly, the characteristics of white line intensity in Au L 3 -edge
XANES (Figure 4.12) observed for 2-nm AuNPs capped with dendrimer was
very different from the thiolate and mixed ligands stabilized Au clustets. The
white line intensity is less intense compared to the bulk, indicating that the
Au atoms gain 5d electron relative to the bulk when capped with weakly
interacting dendrimers. 70 Similar phenomenon was reported in the case of
PVP polymer stabilized Au nanoparticles. 58 It is also worth mentioning
that the Au L 3 -XANES of the solvated Au 38 has shown noticeable lower
white line intensity in comparison with the solid-state Au 38 . 61 It indicates a
relatively higher 5d electron intensity in solvated Au 38 . It is likely due to the
charge transfer being more ecient in solid-state Au 38 having a shorter
Au-S bond.

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4.3.1.2 Photoemission Spectroscopy for Valence Band Study
The ultraviolet and valence band X-ray photoemission spectroscopy can be
used to directly study the valence band electronic structure including the 6s
and 5d bands density of states of Au. A series of ligand protected Au
nanoclusters have been investigated by the valence band XPS, as shown in
Figure 4.13. 50,51,60 The X-ray source was used in these experiments to more