Electrode Film Deposition
Ion Transport between Electrolyte and Electrode
The diffusion movement due to the thermal movement of the ions and molecules in the electrolyte as well as the drift movement of ions in the electric field of the electrodes is too low to maintain electro-plating deposition at the required rate when the region in the immediate vicinity of the electrode is already depleted of suitable ions. Therefore, a convection of the electrolyte is essential. Such a macroscopic circulation of all the particles of the electrolyte does not necessarily have to be dictated from the outside, but takes place by spatial temperature differences, and thus differences in the density of the electrolyte.
Diffusuion in the Boundary Layer
The ions or molecules directly on the electrode surface adhere to the electrode surface, which means they are bound to them physically by different interaction forces. The situation above the electrode can be visualized with a simple model: Stacked particle layers parallel to the electrode surface, each can move parallel to the previous layer with a particular difference in speed thus increasing with the distance to the electrode. From a certain distance, this diffusion layer passes into the convective region of the electrolyte. Within the diffusion layer, the concentration of the ions which are consumed at the electrode by growth (e.g. Cu+ + e- → Cu) or phase transformation
(e.g. 2 H+ + 2 e- → H2) decrease in the direction of the electrode which acts as "sink" for these ions. This concentration gradient as the driving force is all the greater, the thinner the diffusion layer, that is, the stronger the convection velocity over it. Strong convection therefore also promotes diffusion through the diffusion barrier layer
Adhesion to the Electrode
Cations are always hydrated in aqueous solutions, i.e. they are surrounded by a hydrate shell from the water molecules due to the ion-dipole interaction. This shell must first be stripped before the cation can attach to the cathode.
A Closer Look at the Diffusion Boundary Layer
Within the diffusion boundary layer two areas can be defined: The inner Helmholtz layer designates the monolayer adsorbed to the electrode of the solvent molecules (e.g. H2O) or other ions of the electrolyte. The outer Helmholtz layer consists of the hydrated ions of the electrolyte, which are attached to the inner Helmholtz layer. To get from the electrolyte to the surface of the electrode (or go into solution from the electrode), the ions must penetrate the Helmholtz layer (transient reaction). Subsequently, the ions strip off their hydration shells and are incorporated in the lattice of the solid. The passage through the boundary layer as well as the stripping of the hydration shell requires an activation energy, which must be applied to the external voltage source by an increased voltage (transient overvoltage).
After the cations have reached the cathode and have stripped off their hydration shells, they are initially loosely bound as adatoms to the surface of the solid. They can diff use thermally activated there until they are permanently incorporated into the crystal structure at an energetically favourable place.
The following simulations illustrate the mechanism of attachment and incorporation of a cation into the solid state in a simplified manner: Each particle in a cubic lattice has 26 neighbour sites (6 over the sides, 12 over the edges, and 8 over the corners of the central cell), each of which can be occupied or unoccupied. The more occupied neighbour sites a particle has (the neighbour sites contributing less to the edges and corners due to the greater distance), the greater its bonding energy at this location. The probability p for a thermally activated jump from a lattice location (energy E1) to an adjacent location (E2)) is thus:
where kT is the kinetic energy of the particle, and c is a constant. The series of images below shows the growth of a layer simulated exclusively by this model, monolayer by monolayer, at different stages:
Rate of Increase
The more time the deposited particles have for the search for an energetically best possible lattice site (for example, a lower deposition rate and/or higher temperature), the smoother the layer grows (see figure below).
Nucleation and grain growth
Not only the bonding energy to adjacent occupied lattice sites but also the local electric field contributes to the energy balance of a particle jump on the surface of the electrode: The smaller the radius of curvature of the surface, the greater the local electric field, and the more likely the incorporation of a particle at this site.
After the cations have reached the cathode and have stripped off their hydration shells, they are initially loosely bound as adatoms to the surface of the solid. They can diffuse thermally activated there until they are permanently incorporated into the crystal structure at an energetically favourable place.
… and Practice
The reality of electro-plating deposition is much more complex than the assumptions of the simulations treated here: Electrodes are usually not single crystals but amorphous or nano-crystalline solids, whereby the bonding conditions are locally different on their surface. Also ignored were the transport mechanisms within the diffusion boundary layer, in particular the passage through the inner and outer Helmholtz layer. The stripping of the hydration shell and chemical reactions also affect the energy balance of a particle between adsorption and incorporation into the crystal structure.