Crystallographically-Oriented Macropores in Multi-Crystalline Zn
Authors: Gerngross Mark-Daniel , Carstensen Jürgen , Adelung Rainer 
Published: February 2020

In this abstract we report on the electrochemical formation of crystallographically-oriented macropores in multicrystalline Zn sheets. The formation of macropores by electrochemical etching is well-known from silicon and other semiconductors like InP or GaAs as model systems for the growth of crystallographic pores. Besides semiconductors pores have also been formed in specific metals like Al and Ti, where the resulting pore walls turned into metal oxides.
The substrate for the growth of crystallographic pores in Zn is a commercially available multicrystalline Zn sheet with a thickness of 0.1 mm and a purity of 99.95 %. The hardness degree is as-rolled. The pores are grown in a self-organized manner without any pre-structuring of the Zn substrate in pulsed-potential process in an adopted aqueous KCl electrolyte.
The crystallographic nature of the pores grown is easily detectable due to the multicrystalline Zn substrate, see Fig. 1. Here, pores form pore domains typically in the dimensions of the specific grain they are growing in. Inside a pore domain the neighboring pores grow parallel to each other into the depth - see the inset of Fig.1 - under a defined angle depending on the crystal orientation of the grain. These angles can vary from perpendicular to almost parallel to the sheet surface normal.
The pores exhibit a wide diameter distribution from around 100 nm up to several micrometers. This vast diameter range is a direct result of the pulsed-potential etching process causing frequent nucleations of new pores on the Zn surface. The microscale diameter pores originate from potential pulses at the beginning of the etching process. The shape of the pore openings transforms during the etching process. Young pores, i.e. late nucleated pores, typically exhibit a distorted hexagonal pore opening transforming into rhomboidal with continued etching time. This is a direct consequence of the design of the etching process itself, which is separated into a cathodic surface passivation process and an anodic dissolution process. With increasing pore length the pore wall passivation near the pore openings weakens resulting in a change in shape and a broadening of the pore openings. A renucleation of pores from the pore walls is also observed as one of the characteristic features of crystallographic pores found in III-V semiconductors.
XRD measurements of the porosified and untreated Zn sample show only peaks that can be assigned to crystalline Zn in its typical hcp crystal structure. The shape of the Zn peaks indicates rather large coherently scattering areas and thus, rather big Zn grain sizes. No additional peaks or amorphous halos appeared in the diffractogram that might be associated with ZnO, etc. EDX elemental mapping performed on the pore walls of pores growing into Zn under a low angle with respect to the sheet surface normal indicates that the pore walls consist of Zn with a native oxide coverage.
These Zn pore structures form a mechanical interlocking surface structure that could be employed to solve the common problem of poorly adhering paint layers on Zn plated metal surfaces.
Fig.1 SEM top view on the macroporous Zn surface with multiple pore domains, inset: high magnification of single pores inside a pore domain.

Figure 1