1 An Overview of Metal Clusters and Their Reactivity
3
size dependence of the reactivity where cluster anions containing a certain number of
Al atoms were unreactive towards oxygen while the other species were etched away
[104]. Following that, ongoing efforts are devoted to studying metal cluster stability
by measuring the relative mass abundances of clusters formed in comparable conditions using mass spectrometry, by further reacting with oxygen or other appropriate
reactants, or by evaluating the size-dependent reactivity of mass-selected clusters
[105, 106]. Based on near-free electron gas (NFEG) theory, a “jellium model” was
introduced to account for the enhanced stability of these specific metal clusters [107–
112]. Within this model, metal clusters with closed electronic shells often exhibit a
large HOMO–LUMO gap hence enhanced chemical stability and reduced reactivity.
However, not all metal clusters are subject to the same fundamental constraints,
and those of favorable geometry within Mackay icosahedrons often find prominent
stability [113]. In general, magically stable metal clusters are anticipated to be associated with both geometric and/or electronic shell closures [101, 102]. In comparison,
superatoms are expected to retain their integrity when they undergo reactions or are
used as building blocks for macro materials, as an atom retains its integrity when it
bounds into a molecule. Therefore, superatoms do not have to possess closed electronic shell; instead, fruitful superatomic species may ultimately enable to establish
a 3D periodic table of elements [2, 9, 101, 114]. Investigations of superatomic metal
clusters could lead to the finding of new stable species with unique chemical activity
and highly tunable properties that pave the way to design aimed catalysts, develop
new materials with promising applications [5, 68, 115, 116].
In addition to the application in elucidating molecular details of condensed matter,
studies on the kinetics of association reactions are important in the subject of phase
transitions where progress is impeded because of rare fundamental data for comparison with general theories. Clusters comprised predominantly of free-electron metals
often demonstrate different properties and reactivity with the addition or removal of
a single atom, distinguishing them from bulk materials in this regard. The studies of
metal cluster reactivity under a variety of experimental conditions have provided a
wealth of information concerning the evolution of solid state with respect to that of
clusters in the gas phase [117]. There is a fact that the metal-metal bonds are weaker
than ionic bonds and covalent bonds, and the valence electrons of metal often occupy
the higher energy levels of the cluster. It is unequivocally significant for insightful
studies of naked metal clusters and their reactivities to bridge the knowledge obtained
from gas-phase and soft-landing deposition on diverse supports, pertaining to practical applications in metal catalysis and genetic materials [68] which consist of metal
clusters instead of metal atoms as building blocks [118].
In order to determine the binding energies of electrons in a substance, researchers
performed energy measurement of electrons emitted from solids, gases or liquids by
the photoelectric effect, known as photoelectron spectroscopy (PES) or photoemission spectroscopy. As bulk metals generally exhibit essentially free electrons, the
metal clusters in a vast size regime between a single atom and the condensed phase
offer opportunities for better understanding the electronic properties of metal aggregates generally. Among the metal clusters studied via photoelectron spectroscopy,
3
size dependence of the reactivity where cluster anions containing a certain number of
Al atoms were unreactive towards oxygen while the other species were etched away
[104]. Following that, ongoing efforts are devoted to studying metal cluster stability
by measuring the relative mass abundances of clusters formed in comparable conditions using mass spectrometry, by further reacting with oxygen or other appropriate
reactants, or by evaluating the size-dependent reactivity of mass-selected clusters
[105, 106]. Based on near-free electron gas (NFEG) theory, a “jellium model” was
introduced to account for the enhanced stability of these specific metal clusters [107–
112]. Within this model, metal clusters with closed electronic shells often exhibit a
large HOMO–LUMO gap hence enhanced chemical stability and reduced reactivity.
However, not all metal clusters are subject to the same fundamental constraints,
and those of favorable geometry within Mackay icosahedrons often find prominent
stability [113]. In general, magically stable metal clusters are anticipated to be associated with both geometric and/or electronic shell closures [101, 102]. In comparison,
superatoms are expected to retain their integrity when they undergo reactions or are
used as building blocks for macro materials, as an atom retains its integrity when it
bounds into a molecule. Therefore, superatoms do not have to possess closed electronic shell; instead, fruitful superatomic species may ultimately enable to establish
a 3D periodic table of elements [2, 9, 101, 114]. Investigations of superatomic metal
clusters could lead to the finding of new stable species with unique chemical activity
and highly tunable properties that pave the way to design aimed catalysts, develop
new materials with promising applications [5, 68, 115, 116].
In addition to the application in elucidating molecular details of condensed matter,
studies on the kinetics of association reactions are important in the subject of phase
transitions where progress is impeded because of rare fundamental data for comparison with general theories. Clusters comprised predominantly of free-electron metals
often demonstrate different properties and reactivity with the addition or removal of
a single atom, distinguishing them from bulk materials in this regard. The studies of
metal cluster reactivity under a variety of experimental conditions have provided a
wealth of information concerning the evolution of solid state with respect to that of
clusters in the gas phase [117]. There is a fact that the metal-metal bonds are weaker
than ionic bonds and covalent bonds, and the valence electrons of metal often occupy
the higher energy levels of the cluster. It is unequivocally significant for insightful
studies of naked metal clusters and their reactivities to bridge the knowledge obtained
from gas-phase and soft-landing deposition on diverse supports, pertaining to practical applications in metal catalysis and genetic materials [68] which consist of metal
clusters instead of metal atoms as building blocks [118].
In order to determine the binding energies of electrons in a substance, researchers
performed energy measurement of electrons emitted from solids, gases or liquids by
the photoelectric effect, known as photoelectron spectroscopy (PES) or photoemission spectroscopy. As bulk metals generally exhibit essentially free electrons, the
metal clusters in a vast size regime between a single atom and the condensed phase
offer opportunities for better understanding the electronic properties of metal aggregates generally. Among the metal clusters studied via photoelectron spectroscopy,