12.1 Electrodeposition in Ultrathin Solution Layers Along the Edge of the Cell
425
The difference in the concentration along or between the growing dendrites
can lead to highly unexpected phenomena. Due to the concentration difference,
local concentration cells can form, and the local current density can be inverted
as compared to the mean one. This means that dissolution may happen locally much
behind the growth front (or, in other words, relatively far from the outer envelope
curve of the advancing deposit). This phenomenon was observed for Cu deposition
in a thin-layer cell by using the analysis of consecutive images of the growing deposit
[7]. This cannot be observed for thick deposits due to the lack of an appropriate in situ
cross-sectional imaging technique for bulk materials. The observation of the local
dissolution draws the attention to that the evolution of the entire deposit structure
and local density at a particular depth can change also after the outer deposition
front passes the depth of interest. If a disintegrated stand-alone nanowire (dendrite)
is studied in the solution in which it was deposited previously, a parallel deposition
and dissolution can be seen under appropriate conditions, due to which the wire
appears to migrate into the field direction [8]. Such electric field-driven phenomena
may occur in thin-layer cells because the potential difference across a nanoobject
can be large due to the large resistance of the solution.
Finally, it has to be noted that the information available for thin-layer electrodeposition cells are far from being complete. Since many of the studies were performed
with easy-to-handle and fairly reversible systems like the Cu
2+ /Cu, less is known
about irreversible deposition processes. Therefore, one has to be cautious by the
generalization of the result for all possible systems.
12.1.2 Deposition of Metal-Based Structures Under
Cryogenic Conditions
The big majority of the nanostructure preparation methods in thin-layer cells refer
to the deposition of Cu/Cu 2 O mixtures with varying oxide content [9–16]. In one
kind of the experimental setup, a horizontal thin-layer cell is built with a solution
layer thickness of about 40–500 µm. The schematic image of the rectangular cell
can be seen in Fig. 12.1. The anode is a sacrificial Cu wire at one edge of the cell,
which faces the cathode whose material is of less importance (e.g., carbon). The top
cover layer of the cell is the substrate. This is very unusual since the cathode and the
substrate are usually the same part of a cell, unlike here. The surprising experience
is that the deposit penetrating through the cell from the cathode exhibits a strong
enough adherence to the substrate, although it is not the electron-conducting part but
a boundary surface only. The bottom part of the cell, a glass plate, is attached to a
Peltier element that allows one to cool the system. The filling solution at the stage
of the cell assembly is a relatively dilute copper sulphate solution. Upon cooling the
cell, ice nucleation takes place at the chilled bottom face, which leads to the thinning
of the liquid layer. In the ideal case, the residual liquid layer is only 200–300 nm
thick as estimated from the thickness of the deposit that later formed during the
425
The difference in the concentration along or between the growing dendrites
can lead to highly unexpected phenomena. Due to the concentration difference,
local concentration cells can form, and the local current density can be inverted
as compared to the mean one. This means that dissolution may happen locally much
behind the growth front (or, in other words, relatively far from the outer envelope
curve of the advancing deposit). This phenomenon was observed for Cu deposition
in a thin-layer cell by using the analysis of consecutive images of the growing deposit
[7]. This cannot be observed for thick deposits due to the lack of an appropriate in situ
cross-sectional imaging technique for bulk materials. The observation of the local
dissolution draws the attention to that the evolution of the entire deposit structure
and local density at a particular depth can change also after the outer deposition
front passes the depth of interest. If a disintegrated stand-alone nanowire (dendrite)
is studied in the solution in which it was deposited previously, a parallel deposition
and dissolution can be seen under appropriate conditions, due to which the wire
appears to migrate into the field direction [8]. Such electric field-driven phenomena
may occur in thin-layer cells because the potential difference across a nanoobject
can be large due to the large resistance of the solution.
Finally, it has to be noted that the information available for thin-layer electrodeposition cells are far from being complete. Since many of the studies were performed
with easy-to-handle and fairly reversible systems like the Cu
2+ /Cu, less is known
about irreversible deposition processes. Therefore, one has to be cautious by the
generalization of the result for all possible systems.
12.1.2 Deposition of Metal-Based Structures Under
Cryogenic Conditions
The big majority of the nanostructure preparation methods in thin-layer cells refer
to the deposition of Cu/Cu 2 O mixtures with varying oxide content [9–16]. In one
kind of the experimental setup, a horizontal thin-layer cell is built with a solution
layer thickness of about 40–500 µm. The schematic image of the rectangular cell
can be seen in Fig. 12.1. The anode is a sacrificial Cu wire at one edge of the cell,
which faces the cathode whose material is of less importance (e.g., carbon). The top
cover layer of the cell is the substrate. This is very unusual since the cathode and the
substrate are usually the same part of a cell, unlike here. The surprising experience
is that the deposit penetrating through the cell from the cathode exhibits a strong
enough adherence to the substrate, although it is not the electron-conducting part but
a boundary surface only. The bottom part of the cell, a glass plate, is attached to a
Peltier element that allows one to cool the system. The filling solution at the stage
of the cell assembly is a relatively dilute copper sulphate solution. Upon cooling the
cell, ice nucleation takes place at the chilled bottom face, which leads to the thinning
of the liquid layer. In the ideal case, the residual liquid layer is only 200–300 nm
thick as estimated from the thickness of the deposit that later formed during the