Electroplating

    İzzet Aydın, General Director of Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti., explained electroplating in detail.

Atomistic View of Layer Formation

The deposition of metallic layers from aqueous electrolytes occurs when metal ions present in the electrolyte solution reach the cathode (base layer, substrate, workpiece, product, component), are discharged through electron absorption at the cathode/electrolyte interface, and are deposited as metal atoms on the cathode surface. The electrons required to discharge metal ions are either obtained from an external voltage source (electrochemical metal deposition) or provided by a reducing agent added to the electrolyte (chemical metal deposition). The electrons required to discharge metal ions are either obtained from an external voltage source (electrochemical metal deposition) or provided by a reducing agent added to the electrolyte (chemical metal deposition). The electrochemical metal deposition process is summarized in Figure 1. However, the metal deposition process is actually much more complex and involves several substeps. The most important of these substeps are: • Transport of hydrated metal ions or complexes from within the electrolyte solution to the cathode. • Removal of the hydration shell of metal ions at the cathode/electrolyte interface. • Charge transfer at the cathode surface and formation of adsorbed atoms (ad-atoms). • Formation of crystal nuclei through surface diffusion of ad-atoms on the cathode surface. • Growth of thermodynamically stable crystal nuclei to form a metallic layer. These partial steps are summarized in Figure 2. It is important to remember that this representation also reflects real conditions in a highly simplified manner. Transport of metal ions from within the electrolyte solution to the cathode occurs primarily through convection and diffusion. Discharge of ad-atoms through charge transfer takes place across the electrolytic double layer that spontaneously forms along the cathode/electrolyte interface. However, in the process, metal ions do not completely lose their charges but retain a certain residual charge and thus part of their hydration shell. After passing through the electrolytic double layer, they are adsorbed in this state on the cathode surface and form ad-atoms there. Layer formation now requires two further coupled processes called nucleation and crystal growth. Nucleation occurs through diffusion-controlled combination of ad-atoms on the cathode surface. The growth process begins the moment the resulting crystal nuclei reach a critical size.    

Metal Ion Structure

For deposition, metal ions are rarely found in the electrolyte solution or exist in only very small amounts as simple ions. As a rule, they are surrounded by other atoms, molecules or ions, but particularly by water molecules. In the latter case, the geometric structure of the water molecule, shown schematically in Figure 3, plays an important role.   The formation of a valence angle of 104.45° between the hydrogen atoms is the reason for the water molecule's strong dipole properties. The electrostatic attractive force between positively charged metal ions and water molecules leads to the formation of hydrated metal ions, also called solvated cations, protected by a hydration shell. Their composition can be symbolized by a general formula. Me^z+, where z represents a z-valent metal ion (cation) and x denotes the number of water molecules surrounding the metal ion. Figure 4 shows the structure of a hydrated metal ion schematically. The composition of a hydrated anion is also provided for comparison. This figure shows that cations and anions have different hydration behaviors due to their different diameters, and also because of the "triangular" structure and polar character of the water molecule. In positively charged cations, oxygen atoms are oriented toward the central ion; in negatively charged anions, hydrogen atoms are oriented toward the central ion. Furthermore, the extent of hydration in the case of cations is greater than in the case of anions. This is because cations have a smaller ionic radius than anions.   References Elektrochemie I' Chemie Weinheim Verlag Vom Ion zur Elektrode' VEW Deutscher Verlag Zur Theorie der diffusion Doppelschicht' Zeitschrift für Elektrochemie On the structure of charged interfaces' Proc. Roy. Soc. Theory and Principle of Electrode Processes' The Ronald Press Company Elektroorganische Chemie' Verlag Chemie Weinheim   İzzet Aydın General Director Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti.