The Nucleation Dependent Growth Layer: A Structure Element in Electrocrystallization - The 10th William Blum Lecture
Prof. Dr. Hellmuth Fischer
Recipient of the 1968 William Blum AES Scientific Achievement Award
Originally published as Plating, 56 (11), 1229-1233 (1968).
Editor’s Note: This paper is a re-publication of the 10th William Blum Lecture, presented at the 56th AES Annual Convention in Detroit, Michigan, on June 16, 1969. A printable PDF version is available by clicking HERE.
For the first time, it has been shown by Eichkorn that layer growth (not of growth-spirals) depends on continued nucleation of monoatomic layers building up growth layers. This has been done by determination of nucleation-overvoltage η and thickness of growth layers. During formation of growth layers, overvoltage must surpass η and time dependent adsorption of foreign substances must control the motion rate of monoatomic layers. Growth layers can develop to whiskers, columnar crystals, fiber textures, twinned or randomly dispersed structures.
The growth layer, a polyatomic macrostep, is a very important structure element for the construction of many polycrystalline aggregates of metal electrodeposits.
In general, we have to distinguish between two types of growth layers: self-perpetuating growth layers in spiral growth and nucleation-dependent growth layers.
Spiral growth layers do not need nucleation. They proceed from the penetration points of screw dislocations, which mostly are situated in the interior of crystal faces. The nucleation-dependent growth layers are generated at lattice faults preferentially situated at crystal or subcrystal boundaries, in the neighborhood of adsorbed foreign substances or at re-entrant grooves in twin formation.
The free energy for starting from a screw dislocation is much lower than that for formation of a nucleus. The nucleation work is decreased at lattice faults and may be further decreased by adsorbed foreign substances. Correspondingly, the overvoltage necessary for generation of a spiral growth layer is much lower than the nucleation overpotential.
Consequently, at overvoltages lower than the nucleation overvoltage, i.e., in the absence of strongly inhibiting adsorbates or with metals that are not sensitive to inhibition (e.g., like lead), electrodeposition occurs preferentially at screw dislocations forming self-perpetuating growth layers.
At overvoltages higher than the nucleation overvoltage, i.e., in presence of inhibiting adsorbates, growth layers will start by nucleation from the sites already mentioned. In these cases, the penetration points of screw dislocations may easily be blocked by inhibiting adsorbates.
As we shall see later, the nucleation-dependent growth layers may generate a very great variety of forms and aggregated structures. For that reason, our present knowledge of nucleation-dependent growth layers as a very important structure element for building up electrodeposits will be discussed in detail.
From extremely purified electrolytes neither growth spirals nor nucleation-dependent growth layers may develop, as Shanefield and Lighty1 have shown. The electrodeposits then grow by piling up atomic layers, but not growth layers. Growth layer formation needs the presence of adsorbable foreign substances, at least in traces.
If we assume that every growth layer consists of many crystal lattice planes and that every lattice plane is initiated by one two-dimensional nucleus, the thickness of the single growth layer must be proportional to the number of nucleation processes of the lattice planes building up the growth layer. Two years ago, my collaborator Eichkorn2 found that it is very likely that the growth layers develop by such continued nucleation. He showed this by x-ray analysis combined with a new method of measuring nucleation overpotentials.
The objectives of our investigation were electrodeposits of copper with the regular structure of the so called fiber-texture type. This type consists of compact aggregates of numerous crystal fibers all oriented perpendicular to the substrate (-texture). Every fiber is composed of numerous growth layers situated one upon another. Such a structure may be produced by electrodeposition of copper from acid CuSO4 solution with an addition agent (o-phenanthroline). The mean size of the growth layers may be determined by x-ray analysis and thus it was possible to count the mean number, X of growth-layers deposited per cm2 and also, mentioned before, this number must be proportional to the rate, J2 of two-dimensional nucleation. For J2 the formula of Volmer is valid:
where C and Khkl, are constants, ηn2 is the overpotential necessary for two-dimensional nucleation, R is the gas constant and T the absolute temperature. Thus, substituting X for J2 in the Volmer equation, a linear relation between ln X and 1/ηn2 should prove that two dimensional nucleation is occurring during the formation of atomic layers as the constituents of the growth layers. Indeed this linear relation can be found as shown by Fig. 1. There ln X is plotted against 1/ηn2. The activation energy for the formation of a two-dimensional nucleus can be calculated from the slope. Thus for ηn2 equal to 10 mV, one finds an energy of 8 × 10-13 ergs. This value agrees well with the corresponding energy value for a nucleation of silver determined by Budevski3 in an entirely different way.