ficient value determined by the dynamic light scattering method. The OTTLE cell
thickness is only 75 mm and hence it is concluded that the electron transfer occurs
close to the diffusion-limited rate of encounters of the metallic particles with the
electrode surface. The potential window that is available for the study is limited by
hydrogen evolution on the negative side (À0.8 V) and the oxidation of silver particles on the positive side (0.4 V). The spectroelectrochemical shift leads to a double
layer capacitance value of (80 G 10) mF cm
À2 for the nanoparticles. Cottrell behaviour is satisfied for the equilibration of the particles with the planar electrode.
Based on the spectral shift, a value of 1790 has been calculated for the number of
electrons needed to shift the absorption peak from 404 nm to 392 nm in the case
of particles of 11 nm size. This is equivalent to a shift of electrochemical potential
from 0.4 V to À0.7 V that corresponds to a surface charge density increase of 6.2%.
The particle charging current is associated with the electron tunneling across the
particle and the electrode double layer and not by contact electrification.
Murray and co-workers reported similar observations on spectral shift by an
electrochemical stimulus, for the gold MPCs [147]. The electronic charging of gold
clusters stabilized by alkanethiolate monolayers effected by shifting the rest potential of À0.16 V to 0.82 V is found to result in a change of about 9 nm in the absorbance maximum. The observed change is compared to the predictions from the
theory based on a concentric sphere model [108] for cluster capacitance and the
Mie theory of scattering [2, 146]. The sensitivity of plasmon positions to the charge
associated with the metal particles opens up the possibility of tuning the absorption bands by electrochemical means. Nishihara and co-workers have demonstrated the electrochemical tuning of surface plasmon absorption for redox-active
Au-MPCs. The shift in the absorbance maximum as a function of applied potential
is reported to be predominantly due to the charging of the metal core. The contribution by the redox-active moiety, though small, should also be considered [62–64].
Kamat and co-workers demonstrated the electrochemical tuning of emission
properties of a fluorophore using functionalized gold nanoparticles on a transparent ITO electrode [148]. The schematics of the assembly and the suppression of
emission are given in Scheme 20.2. Spectroelectrochemical experiments using the
pyrene-modified gold nanostructured film show that the communication between
the fluorophore and the gold core is totally suppressed at À1.2 V versus SCE in
acetonitrile medium. The emission is restored to about 90% by simply charging
the electrode. The same authors have earlier shown that the nanocrystalline gold/
ITO electrode allows the flow of electrons following Fermi level equilibration [149].
Weaver and co-workers [150] carried out potential-dependent infrared spectroscopy to characterize adsorption of CO on nanosized platinum particles. Large particles of diameter 4 nm and above show CaO stretching frequencies similar to
those of platinum macroelectrodes. Small particles of diameter 2–4 nm, on the
other hand, show a red shift in CaO frequency, approaching that of platinum carbonyl clusters. The authors ascribe this observation to the changes in the platinum
surface coordination number, consistent with pseudo-spherical packing-density
considerations. A time-resolved IR absorption technique has also been used to
monitor electrocatalytic reactions using platinum nanoparticles [151].
20 Electrochemistry with Nanoparticles
668
thickness is only 75 mm and hence it is concluded that the electron transfer occurs
close to the diffusion-limited rate of encounters of the metallic particles with the
electrode surface. The potential window that is available for the study is limited by
hydrogen evolution on the negative side (À0.8 V) and the oxidation of silver particles on the positive side (0.4 V). The spectroelectrochemical shift leads to a double
layer capacitance value of (80 G 10) mF cm
À2 for the nanoparticles. Cottrell behaviour is satisfied for the equilibration of the particles with the planar electrode.
Based on the spectral shift, a value of 1790 has been calculated for the number of
electrons needed to shift the absorption peak from 404 nm to 392 nm in the case
of particles of 11 nm size. This is equivalent to a shift of electrochemical potential
from 0.4 V to À0.7 V that corresponds to a surface charge density increase of 6.2%.
The particle charging current is associated with the electron tunneling across the
particle and the electrode double layer and not by contact electrification.
Murray and co-workers reported similar observations on spectral shift by an
electrochemical stimulus, for the gold MPCs [147]. The electronic charging of gold
clusters stabilized by alkanethiolate monolayers effected by shifting the rest potential of À0.16 V to 0.82 V is found to result in a change of about 9 nm in the absorbance maximum. The observed change is compared to the predictions from the
theory based on a concentric sphere model [108] for cluster capacitance and the
Mie theory of scattering [2, 146]. The sensitivity of plasmon positions to the charge
associated with the metal particles opens up the possibility of tuning the absorption bands by electrochemical means. Nishihara and co-workers have demonstrated the electrochemical tuning of surface plasmon absorption for redox-active
Au-MPCs. The shift in the absorbance maximum as a function of applied potential
is reported to be predominantly due to the charging of the metal core. The contribution by the redox-active moiety, though small, should also be considered [62–64].
Kamat and co-workers demonstrated the electrochemical tuning of emission
properties of a fluorophore using functionalized gold nanoparticles on a transparent ITO electrode [148]. The schematics of the assembly and the suppression of
emission are given in Scheme 20.2. Spectroelectrochemical experiments using the
pyrene-modified gold nanostructured film show that the communication between
the fluorophore and the gold core is totally suppressed at À1.2 V versus SCE in
acetonitrile medium. The emission is restored to about 90% by simply charging
the electrode. The same authors have earlier shown that the nanocrystalline gold/
ITO electrode allows the flow of electrons following Fermi level equilibration [149].
Weaver and co-workers [150] carried out potential-dependent infrared spectroscopy to characterize adsorption of CO on nanosized platinum particles. Large particles of diameter 4 nm and above show CaO stretching frequencies similar to
those of platinum macroelectrodes. Small particles of diameter 2–4 nm, on the
other hand, show a red shift in CaO frequency, approaching that of platinum carbonyl clusters. The authors ascribe this observation to the changes in the platinum
surface coordination number, consistent with pseudo-spherical packing-density
considerations. A time-resolved IR absorption technique has also been used to
monitor electrocatalytic reactions using platinum nanoparticles [151].
20 Electrochemistry with Nanoparticles
668