Preparation and Characterization of Nanoporous Copper Films By (electro)Chemical Dealloying
Authors: Hecker Burkhard , Dosche Carsten , Knake Jonas , Oezaslan Mehtap 
Published: February 2020

Nanostructured materials are of great interest in different fields such as solar cells, batteries, fuel cells and sensors.1-3 The chemical and physical properties of nanostructured materials strongly differ from those of the corresponding bulk materials. For instance, with decreasing size, quantum-physical effects get more relevant.2 Nanoporous materials offer not only new chemical and physical properties by nano-sizing but also very high surface area and good mass transport properties through the pores.3

A promising synthetic approach to prepare nanoporous copper (np-Cu) materials is the (electro)chemical dealloying process.4 Dealloying is the dissolution of the less noble metal (Zn) from a Cu-based alloy. The Cu-based alloys are prepared by electrodeposition of Zn on a Cu surface followed by thermal annealing step in an inert atmosphere. We showed that by modifying the dealloying conditions, np-Cu films with tuneable structure (e.g. ligament size and shape) and chemical composition (e.g. the residual content of the foreign atom and surface oxides) can be produced. For the characterization, a broad combination of microscopic and spectroscopic techniques was used: XPS, XRD, EDX, SEM and XANES/EXAFS.

Our study shows the relationship between the dealloying parameters such as temperature, electrolyte, concentration etc. and the material properties of the np-Cu such as ligament size and shape, pore distribution and content of the foreign atom. First, the annealing temperature controls the crystal phase and the degree of alloying of the Zn-Cu film. The crystalline phase and chemical surface composition of the master alloy have a strong influence on the resulting structure after the dealloying step. In particular, with higher annealing temperature, the dealloying proceeds slower and the ligament-pore size ratio increases. Furthermore, the variation of the dealloying conditions (electrolyte concentration and time) leads to different structures of the np-Cu films, indicating that the mechanism and kinetics of the dealloying significantly change. One of the critical parameters is the effect of the supporting electrolyte. For instance, in HCl, a homogeneous ligament structure is formed, whereas in NaOH solution the dealloying mainly proceeds at the grain boundaries of the Zn-Cu alloy and forms cracks. Large ligament sizes and strong oxidized surfaces with very low content of Zn was observed. Moreover, the temperature during the dealloying can be used to tune the ligament and pore size. For example, using a HCl solution with the same concentration, the ligament size can be adjusted from 30 to 300 nm in a temperature range between 10 and 70 °C. Another observation is that by increasing dealloying time, the Zn content further decreases. In basic solutions, the absence of Zn leads to an increased formation of copper oxide at the surface.

The aim of our work is to better understand the formation mechanism and kinetics of the np-Cu. The next step is the rational design of various np-Cu materials as electrocatalysts for the electrochemical carbon dioxide reduction reaction (CO2RR).

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