nanostructured films exhibit photon conversion efficiency of nearly 90% under
optimized light-harvesting conditions [163]. A detailed discussion on the semiconductor nanostructure based photochemical solar cells can be found elsewhere
[167–169].
19.5
Electrochemistry of Metal Nanostructures
Unlike bulk films, metal nanoparticles exhibit an unusual property of quantized
double layer charging effects [170]. However, relatively little work has been carried
out to date to elucidate the electrochemical behavior of metal nanoparticles. It is
evident from electrochemical [20, 134], photochemical [132], and spectroelectrochemical [171] experiments that metal nanoparticles possess a unique property of
storing electrons. For example, charge storage within the gold nanocore causes a
large build-up of capacitance of the Helmoltz and diffuse double layer. As demonstrated earlier [172], the excess electron density remains mainly on the metal islands because the Helmholtz capacitance of the metal–solution interface is much
higher than the space charge capacity of the semiconductor nanoparticle, even
under accumulation.
Engineering of the nanocluster surfaces with photoactive molecules has potential applications in designing electrochromic devices of nanometric dimension.
Controlled charging of an Au nanoassembly has enabled us to modulate the excited state interaction between the gold nanocore and a surface-bound fluorophore.
A bifunctional surface-linking molecule such as mercaptopropionic acid is useful
to link the gold nanoparticle to the TiO 2 surface (thiol group on to gold and carboxylic group on to TiO 2 ). Fluorophore functionalized electrodes can be prepared
by immersing a TiO 2 modified conducting electrode (referred to as OTE, optically transparent electrode) into the THF solution containing gold nanoparticles
that were previously functionalized with mercatopropionic acid and pyrene thiol
(AcaSa(Au)aSRaPy) [171]. The pyrene functionalized electrode (referred to as OTE/
TiO 2 /AcaSa(Au)aSaPy) exhibits weak emission with a maximum around 395 nm.
The decreased singlet lifetime as well as formation of the oxidation product, pyrene
cation radical in the AuaSRaPy assembly, indicates the ability of gold nanoparticles
to accept electrons from excited pyrene [132]. Since the oxidation potential of excited pyrene is around À1.5 V versus NHE one can expect an energetically favorable electron transfer of electrons to gold nanoparticles (E F ¼ 0:5 V).
Spectroelectrochemical experiments carried out using a thin layer electrochemical cell, show the emission spectra of OTE/TiO 2 /AcaSa(Au)aSaPy electrode at
different applied potentials (Figure 19.11). As the electrode is biased to negative
potentials one observes an increase in the emission yield. It is important to note
that the overall shape of the emission band essentially remains the same. This, in
turn, suggests that the photoactive species that contribute to the emission remain
unperturbed. At potentials more negative than À1.0 V, the quenching of the excited fluorophore by the gold nanocore is suppressed and the emission reaches
19.5 Electrochemistry of Metal Nanostructures 631
optimized light-harvesting conditions [163]. A detailed discussion on the semiconductor nanostructure based photochemical solar cells can be found elsewhere
[167–169].
19.5
Electrochemistry of Metal Nanostructures
Unlike bulk films, metal nanoparticles exhibit an unusual property of quantized
double layer charging effects [170]. However, relatively little work has been carried
out to date to elucidate the electrochemical behavior of metal nanoparticles. It is
evident from electrochemical [20, 134], photochemical [132], and spectroelectrochemical [171] experiments that metal nanoparticles possess a unique property of
storing electrons. For example, charge storage within the gold nanocore causes a
large build-up of capacitance of the Helmoltz and diffuse double layer. As demonstrated earlier [172], the excess electron density remains mainly on the metal islands because the Helmholtz capacitance of the metal–solution interface is much
higher than the space charge capacity of the semiconductor nanoparticle, even
under accumulation.
Engineering of the nanocluster surfaces with photoactive molecules has potential applications in designing electrochromic devices of nanometric dimension.
Controlled charging of an Au nanoassembly has enabled us to modulate the excited state interaction between the gold nanocore and a surface-bound fluorophore.
A bifunctional surface-linking molecule such as mercaptopropionic acid is useful
to link the gold nanoparticle to the TiO 2 surface (thiol group on to gold and carboxylic group on to TiO 2 ). Fluorophore functionalized electrodes can be prepared
by immersing a TiO 2 modified conducting electrode (referred to as OTE, optically transparent electrode) into the THF solution containing gold nanoparticles
that were previously functionalized with mercatopropionic acid and pyrene thiol
(AcaSa(Au)aSRaPy) [171]. The pyrene functionalized electrode (referred to as OTE/
TiO 2 /AcaSa(Au)aSaPy) exhibits weak emission with a maximum around 395 nm.
The decreased singlet lifetime as well as formation of the oxidation product, pyrene
cation radical in the AuaSRaPy assembly, indicates the ability of gold nanoparticles
to accept electrons from excited pyrene [132]. Since the oxidation potential of excited pyrene is around À1.5 V versus NHE one can expect an energetically favorable electron transfer of electrons to gold nanoparticles (E F ¼ 0:5 V).
Spectroelectrochemical experiments carried out using a thin layer electrochemical cell, show the emission spectra of OTE/TiO 2 /AcaSa(Au)aSaPy electrode at
different applied potentials (Figure 19.11). As the electrode is biased to negative
potentials one observes an increase in the emission yield. It is important to note
that the overall shape of the emission band essentially remains the same. This, in
turn, suggests that the photoactive species that contribute to the emission remain
unperturbed. At potentials more negative than À1.0 V, the quenching of the excited fluorophore by the gold nanocore is suppressed and the emission reaches
19.5 Electrochemistry of Metal Nanostructures 631