Alain Reiser1 Ramón Frey1 Volker Schnabel1 Ralph Spolenak1

1, ETH Zurich, Zurich, , Switzerland

Nanoporous coatings offer many exciting properties (e.g., for catalysis or structural colors) due to their exceptional internal geometry. An established way to prepare such coatings is by dealloying of co-deposited thin films made by physical vapor deposition. In contrast, electrodeposition is not typically employed for the deposition of the precursor films, although it is a simple and extensively used coating technique in both research and industry.
The reason for neglecting electrodeposition is straightforward: chemical dealloying asks for two alloying partners with very dissimilar nobility. In contrast, the electrochemical deposition of an alloy via co-deposition requires the two alloying elements to have similar reduction potentials. These contradictory premises make electrodeposition a less than ideal synthesis approach. Nevertheless, an electrochemical route would be favorable for some applications of nanoporous films, e.g., for the conformal coating of complex surfaces or low-cost applications.
Here, we demonstrate the electrochemical co-deposition of CuZn alloy films from a pyrophosphate bath [1] and their subsequent dealloying to fabricate nanoporous Cu films with pore and ligament sizes on the order of 50 – 100 nm. Films containing 0 – 50 at.% Zn can be synthesized from a single electrolyte bath, with the Zn percentage being a function of the deposition potential. This enables simple tuning of the Zn fraction during synthesis. We show that films with gradual or sharp gradients of Zn content along the growth direction can easily be achieved. These Zn-gradients translate in pore gradients upon dealloying. The microstructures and mechanical properties of the as deposited and de-alloyed thin films are characterized.
Interestingly, we observe a strong dependency of the dealloying behavior on the composition of the deposition solution, which is possibly related to the microstructure of the films. A closer study of the microstructures of these films will be presented. This study could help to elucidate generic dealloying principles of electroplated films and help define universal strategies for the electroplating of precursor coatings. Furthermore, we envision to utilize this fundamental understanding for microscale electrochemical additive manufacturing. Controlling the Zn content with voxel-by-voxel resolution could enable the synthesis of microstructures with 3D architectured porosity.

[1] L. F. Senna, S. L. Díaz, L. Sathler, J. Appl. Electrochem. 2003, 33, 1155.