Atomically Precise Gold Catalysis - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 109

reveals an equal number of spin-UP (alpha) and DOWN (beta) electrons.
While this predicted diamagnetism is in excellent agreement with the ex-
perimentally observed negative susceptibility and diamagnetism of bulk
gold, 78,79 it is interesting to note that a recent experimental X-ray magnetic
circular dichroism (XMCD) spectroscopic investigation reveals a Pauli
paramagnetism in bulk Au, which is masked by a large diamagnetic re-
sponse. 80 This Pauli paramagnetism predominantly emerges from the sur-
face atoms, whose spin polarization is stabilized by a large (

d n 9 r 4 n g | 7

B

30%) orbital
contribution from Au 5d electrons. This orbital contribution is sustained in
smaller gold nanoparticles and plays an important role in stabilizing
spontaneous spin polarization. The role of surface atoms is further mag-
nified in smaller gold nanoparticles because a decrease in the size of the
pure gold nanoparticles from 2.6 nm (Au 561 ) to 0.75 nm (Au 13 ), leads to a
two-fold increase (45% to 92%) in the ratio of surface atoms to the total
number of atoms.
To the best of our knowledge, there has been only a single investigation of
magnetism in pure uncapped icosahedral gold nanoparticles. 81 The spin
arrangements therein indicated that the core and surface moments pointing
in opposite directions. While the net magnetic moment 16 mB per particle
indicated ferrimagnetism, it is interesting to note that the net contribution
of the surface atoms was found to be much larger than that of the core
atoms. Nearly all the other investigations of magnetism in small gold
nanoparticles involved either polymer-capped or ligand-covered systems
(Table 4.1). This raises a very important question on whether the observed
magnetism is an intrinsic property of the gold atoms or the nature of the
ligands in the ligand-covered gold nanoparticle.
The presence of ligands has been shown to increase the number of holes
in the 5d band of gold nanoparticles. 70 The strong anity between the Au
surface atoms and the ligand atoms induces a charge transfer from the Au
surface atoms to the ligand atoms where the participation of 5d electrons
can also be implied leading to generation of unoccupied densities of d states
on Au atoms resulting in magnetism. Atomically precise gold clusters, unlike
gold nanoparticles, are made up of a specific number of atoms and ligands,
whose electronic configuration can be theoretically calculated, which in turn
could be used to predict their magnetic behavior. 82 Our group 83 has ex-
perimentally investigated the chemically induced magnetism in ligand-
capped atomically precise gold clusters, viz. Au 25 ,Au 38 and Au 55 using
SQUID. We found ferromagnetic behavior in ligand-capped Au 55 clusters.
Figure 4.16 shows their M vs. H curves obtained for four different gold
clusters at 5 K. The mixed ligand-stabilized Au 25 clusters (black) show typical
diamagnetic behavior. The thiol-stabilized Au 25 clusters (red) show para-
magnetic behavior with an experimentally observed saturation magnetic
moment of m B ¼ 0.0516/cluster. The thiol-stabilized Au 38 clusters (green)
show diamagnetic behavior and the phosphine-stabilized Au 55 clusters
(blue) show a ferromagnetic behavior with a saturation magnetic moment of
m B ¼ 0.0584/cluster.

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