and scanning tunneling microscopy have been used to characterize platinum
nanoparticles of various crystallographic faces [49b]. The surface morphology of
platinum on carbon used for enantioselective hydrogenation is related to the various sorption processes based on electrochemical data.
20.3.1
Monolayer-Protected Nanoclusters
Since the pioneering work of Schiffrin and co workers [50], the synthesis and
characterization of isolable, capped metallic clusters has gained considerable interest. Murray introduced the term ‘monolayer-protected clusters (MPCs)’, otherwise
known as 3-D self assembled monolayers (3-D SAMs) on metallic particles [51].
The synthesis procedure involves the phase transfer of a metal ion to an organic
phase where it is reduced in the presence of a capping agent. The solvent can then
be evaporated to obtain the solid particles that are soluble in various solvents. This
procedure has also been extended to alloy nanoparticles such as Au/Ag, Au/Cu,
Au/Ag/Cu, Au/Pt, Au/Pd, and Au/Ag/Cu/Pd stabilized with thiol molecules
[41b–d]. These clusters have been characterized by various techniques including UV–Vis absorbance, IR, NMR, DSC, TEM, STM/AFM and XRD. The MPCs
are soluble nanoelectrodes that exhibit various unusual electrochemical properties when compared to functionalized planar electrodes. Murray and co-workers
have contributed enormously to the understanding of the electrochemistry of ofunctionalized and unfunctionalized alkanethiolate stabilized Au-cluster compounds [52–59]. Functional groups such as anthraquinone (AQ), ferrocene (Fc),
phenothiazine, arenes, esters, acids, alcohols and dithiols of different chain lengths
have been incorporated into alkanethiolate modified Au-clusters using the place
exchange method [52]. The place exchange method offers an advantage in retaining the core size of the metallic particles while simultaneously allowing other
functionalities to be incorporated on the cluster. Clusters functionalized with redox
active substituents exhibit unusual multi-electron transfer reactions [52].
Ferrocene (Fc) functionalized MPCs show similar electrochemical activity in
both dissolved and adsorbed forms [52]. The voltammetry on glassy carbon electrodes shows symmetric peaks with a peak separation of 15 mV. The charge under
the peak reveals partial monolayer coverage of adsorbed clusters. Rotating disk
electrochemistry (RDE) experiments have been carried out in CH 2 Cl 2 solutions
containing Au clusters functionalized with mixed monolayers of octanethiolate and
o-ferrrocenyloctanethiolate ligands of different molar ratios. The voltammograms
exhibit an Fc oxidation wave with a limiting current that is mass transport controlled. Analysis of the RDE wave leads to the number of electrons involved in the
reaction (n) being equal to 1 (Figure 20.1). The NMR and the diffusion coefficient
measurements lead to the number of ferrocene units per cluster being largely dependent on the fraction of o-ferrrocenyloctanethiolate ligands used to prepare the
cluster. This implies that the Fc sites are not strongly electronically coupled. The
reaction can, however, be thought of as successive electron transfers, independent
20.3 Electrochemistry with Metallic Nanoparticles 651
nanoparticles of various crystallographic faces [49b]. The surface morphology of
platinum on carbon used for enantioselective hydrogenation is related to the various sorption processes based on electrochemical data.
20.3.1
Monolayer-Protected Nanoclusters
Since the pioneering work of Schiffrin and co workers [50], the synthesis and
characterization of isolable, capped metallic clusters has gained considerable interest. Murray introduced the term ‘monolayer-protected clusters (MPCs)’, otherwise
known as 3-D self assembled monolayers (3-D SAMs) on metallic particles [51].
The synthesis procedure involves the phase transfer of a metal ion to an organic
phase where it is reduced in the presence of a capping agent. The solvent can then
be evaporated to obtain the solid particles that are soluble in various solvents. This
procedure has also been extended to alloy nanoparticles such as Au/Ag, Au/Cu,
Au/Ag/Cu, Au/Pt, Au/Pd, and Au/Ag/Cu/Pd stabilized with thiol molecules
[41b–d]. These clusters have been characterized by various techniques including UV–Vis absorbance, IR, NMR, DSC, TEM, STM/AFM and XRD. The MPCs
are soluble nanoelectrodes that exhibit various unusual electrochemical properties when compared to functionalized planar electrodes. Murray and co-workers
have contributed enormously to the understanding of the electrochemistry of ofunctionalized and unfunctionalized alkanethiolate stabilized Au-cluster compounds [52–59]. Functional groups such as anthraquinone (AQ), ferrocene (Fc),
phenothiazine, arenes, esters, acids, alcohols and dithiols of different chain lengths
have been incorporated into alkanethiolate modified Au-clusters using the place
exchange method [52]. The place exchange method offers an advantage in retaining the core size of the metallic particles while simultaneously allowing other
functionalities to be incorporated on the cluster. Clusters functionalized with redox
active substituents exhibit unusual multi-electron transfer reactions [52].
Ferrocene (Fc) functionalized MPCs show similar electrochemical activity in
both dissolved and adsorbed forms [52]. The voltammetry on glassy carbon electrodes shows symmetric peaks with a peak separation of 15 mV. The charge under
the peak reveals partial monolayer coverage of adsorbed clusters. Rotating disk
electrochemistry (RDE) experiments have been carried out in CH 2 Cl 2 solutions
containing Au clusters functionalized with mixed monolayers of octanethiolate and
o-ferrrocenyloctanethiolate ligands of different molar ratios. The voltammograms
exhibit an Fc oxidation wave with a limiting current that is mass transport controlled. Analysis of the RDE wave leads to the number of electrons involved in the
reaction (n) being equal to 1 (Figure 20.1). The NMR and the diffusion coefficient
measurements lead to the number of ferrocene units per cluster being largely dependent on the fraction of o-ferrrocenyloctanethiolate ligands used to prepare the
cluster. This implies that the Fc sites are not strongly electronically coupled. The
reaction can, however, be thought of as successive electron transfers, independent
20.3 Electrochemistry with Metallic Nanoparticles 651