The fundamental basis of the sonoelectrochemical technique to form nanoparticles is massive nucleation using a high current density electrodeposition pulse
(ca. 150–300 mA cm
À2 ), followed by removal of the deposit from the sonoelectrode
by the sonic pulse. Removal of the particles from the electrode before the next
current pulse prevents crystal growth. Overall there are many experimental variables involved in sonoelectrochemical deposition: electrolyte composition and temperature, electrodeposition conditions including current density (I e ), pulse-on time
(t eðonÞ ) and ratio between pulse-on time and pulse-off time (t eðoff Þ ) (the duty cycle);
sonic probe conditions: sonic power (I s ), sonic pulse parameters, t sðonÞ and t sðoff Þ .
The effects of the various sonoelectrochemical parameters on crystal size can be
rationalized, in general, as follows:
. Sonic intensity. The greater the sonic intensity, the greater will be the efficiency
of removal of the deposit and therefore the less chance there will be for crystal
growth of existing nuclei. Typically above a certain intensity where all the deposit is removed, further increase in intensity is not expected to affect crystal
growth much (in the case of CdSe this maximum sonic intensity is ca. 60 W
cm
À2 ). Generally long sonic duration (t sðonÞ ) resulted in smaller crystal sizes,
however, it should be noted that sonoelectrodeposition under continuous ultrasound wave leads to the formation of amorphous materials.
. Deposition current pulse width. Quite separately from the sonic wave effects, pulse
electrodeposition is well known to result in a smaller crystal-sized deposit. This
is particularly pronounced for high current densities, where a high rate of nucleation occurs during each pulse. The shorter the pulse duration (t eðonÞ ), the
less chance there is of crystal growth occurring by deposition of new material on
a previous nucleus. In normal pulse plating, crystal size may or may not increase with the number of pulses, depending on whether each new pulse forms
a new nucleus or adds to pre-existing ones. In sonoelectrochemical deposition,
where the deposit is removed during each sonic pulse, only new nuclei should
be formed, and therefore the crystal size will be smaller with decreasing pulse
duration (t eðonÞ ).
. Temperature. Temperature can affect crystal growth in several ways, all of them
resulting in smaller crystal size at lower temperatures. The simplest is that
crystal growth is slower at lower temperatures. Within the time between sonic
pulses, growth can occur, either by coalescence during the deposition pulse or
by migration on the substrate and coalescence at any time. Another effect of
temperature is through the thermodynamic instability of very small nuclei below a certain critical size. These nuclei should re-dissolve, but may be stable for
long enough to grow larger than the critical size, after which they are thermodynamically stable.
In a series of papers Gedanken and co-workers described the use of a pulse sonoelectrochemical technique for the preparation of nanocrystalline materials. In the
first paper [151] in the series, the synthesis of silver nanoparticles of different
shapes: spheres, rods, and dendrites, is described. The nanocrystalline Ag de6.2 Sonoelectrochemistry 151
(ca. 150–300 mA cm
À2 ), followed by removal of the deposit from the sonoelectrode
by the sonic pulse. Removal of the particles from the electrode before the next
current pulse prevents crystal growth. Overall there are many experimental variables involved in sonoelectrochemical deposition: electrolyte composition and temperature, electrodeposition conditions including current density (I e ), pulse-on time
(t eðonÞ ) and ratio between pulse-on time and pulse-off time (t eðoff Þ ) (the duty cycle);
sonic probe conditions: sonic power (I s ), sonic pulse parameters, t sðonÞ and t sðoff Þ .
The effects of the various sonoelectrochemical parameters on crystal size can be
rationalized, in general, as follows:
. Sonic intensity. The greater the sonic intensity, the greater will be the efficiency
of removal of the deposit and therefore the less chance there will be for crystal
growth of existing nuclei. Typically above a certain intensity where all the deposit is removed, further increase in intensity is not expected to affect crystal
growth much (in the case of CdSe this maximum sonic intensity is ca. 60 W
cm
À2 ). Generally long sonic duration (t sðonÞ ) resulted in smaller crystal sizes,
however, it should be noted that sonoelectrodeposition under continuous ultrasound wave leads to the formation of amorphous materials.
. Deposition current pulse width. Quite separately from the sonic wave effects, pulse
electrodeposition is well known to result in a smaller crystal-sized deposit. This
is particularly pronounced for high current densities, where a high rate of nucleation occurs during each pulse. The shorter the pulse duration (t eðonÞ ), the
less chance there is of crystal growth occurring by deposition of new material on
a previous nucleus. In normal pulse plating, crystal size may or may not increase with the number of pulses, depending on whether each new pulse forms
a new nucleus or adds to pre-existing ones. In sonoelectrochemical deposition,
where the deposit is removed during each sonic pulse, only new nuclei should
be formed, and therefore the crystal size will be smaller with decreasing pulse
duration (t eðonÞ ).
. Temperature. Temperature can affect crystal growth in several ways, all of them
resulting in smaller crystal size at lower temperatures. The simplest is that
crystal growth is slower at lower temperatures. Within the time between sonic
pulses, growth can occur, either by coalescence during the deposition pulse or
by migration on the substrate and coalescence at any time. Another effect of
temperature is through the thermodynamic instability of very small nuclei below a certain critical size. These nuclei should re-dissolve, but may be stable for
long enough to grow larger than the critical size, after which they are thermodynamically stable.
In a series of papers Gedanken and co-workers described the use of a pulse sonoelectrochemical technique for the preparation of nanocrystalline materials. In the
first paper [151] in the series, the synthesis of silver nanoparticles of different
shapes: spheres, rods, and dendrites, is described. The nanocrystalline Ag de6.2 Sonoelectrochemistry 151