1 Ligand-Free Sub-Nanometer Metal Clusters in Catalysis
5
E =
E F
N
1 / 3
(1.1)
where N is the number of the atoms in the cluster, E F is the Fermi energy of the
metal (tabulated) and E is the band gap energy, which can be approximated to the
emission band in the photoluminescence experiments. With this formula in hand, it
is possible to calculate the number of atoms of an unknown cluster from a simple
photoluminescence spectrum.
1.2 Synthesis and Characterization of Ligand-Free Metal
Clusters
When Faraday treated (gold chloride) with phosphorus to generate particles, in what
he called “activated Au,” he had the intuition to propose that the red-colored Au
was in the form of very small particles and that the color may vary as a function
of the size, something later found to be correct. Faraday’s aggregates were in turn
Au nanoparticles, which are still prepared today from a similar reduction procedure.
Although speculative, it is possible that Faraday obtained clusters in his more diluted
experiments since Au clusters are able to persist in nanomolar aqueous solutions.
There are two paths to prepare metal clusters (Fig. 1.4). In the bottom–up way, the
metal clusters are prepared starting from small aggregate or single atoms, like metal
complexes or salts. On the contrary, in the bottom–down path, the starting materials
are bigger nanoparticles from which one can leach small aggregates of the metal.
The synthesis of large-scale metal clusters with atomic precision has experienced
great advances during the last years; [32] however, most of the methods still rely on
extremely expensive techniques that only recently start to be replaced by affordable
wet procedures, with potential industrial application.
TOP-DOWN
BOTTOM-UP
CLUSTERS
BULK
METAL
NANOPARTICLE
ATOM-IONS
Fig. 1.4 Top–down and bottom–up approaches for the synthesis of atomic metal clusters
5
E =
E F
N
1 / 3
(1.1)
where N is the number of the atoms in the cluster, E F is the Fermi energy of the
metal (tabulated) and E is the band gap energy, which can be approximated to the
emission band in the photoluminescence experiments. With this formula in hand, it
is possible to calculate the number of atoms of an unknown cluster from a simple
photoluminescence spectrum.
1.2 Synthesis and Characterization of Ligand-Free Metal
Clusters
When Faraday treated (gold chloride) with phosphorus to generate particles, in what
he called “activated Au,” he had the intuition to propose that the red-colored Au
was in the form of very small particles and that the color may vary as a function
of the size, something later found to be correct. Faraday’s aggregates were in turn
Au nanoparticles, which are still prepared today from a similar reduction procedure.
Although speculative, it is possible that Faraday obtained clusters in his more diluted
experiments since Au clusters are able to persist in nanomolar aqueous solutions.
There are two paths to prepare metal clusters (Fig. 1.4). In the bottom–up way, the
metal clusters are prepared starting from small aggregate or single atoms, like metal
complexes or salts. On the contrary, in the bottom–down path, the starting materials
are bigger nanoparticles from which one can leach small aggregates of the metal.
The synthesis of large-scale metal clusters with atomic precision has experienced
great advances during the last years; [32] however, most of the methods still rely on
extremely expensive techniques that only recently start to be replaced by affordable
wet procedures, with potential industrial application.
TOP-DOWN
BOTTOM-UP
CLUSTERS
BULK
METAL
NANOPARTICLE
ATOM-IONS
Fig. 1.4 Top–down and bottom–up approaches for the synthesis of atomic metal clusters