Chia-Cheng Tai

Electrochemical Preparation of Porous Copper Surfaces in Zinc Chloride-1-ethyl-3-methyl Imidazolium Chloride Ionic Liquid

The preparation of porous copper or copper-zinc surfaces by electrochemical formation of binary Cu-Zn alloys on Cu substrate and subsequent electrochemical etching of the zinc was investigated in a zinc chloride-1-ethyl-3-methylimidazolium chloride ionic liquid at 120°C. Cyclic voltammetry and X-ray diffraction measurements suggested that phase transformation from -t o- Cu-Zn alloy occurred during constant potential dealloying. Essentially all the Zn content in the Cu-Zn could be removed from the alloy with dealloying at a sufficiently positive potential. Dealloyed materials exhibited well-developed bicontinuous porous structure. The dependence of the surface morphology of the porous Cu film on several experimental parameters, including Porous copper is an interesting electrode material for the elec- trolysis process such as hydrogenation of CO2 into methanol. 1-3 Po- rous copper electrodes have been prepared by selective dissolution of aluminum from CuAl2 alloy in NaOH solutions. This process is known as dealloying and has also been applied for the preparation of other porous metals, and the mechanism for formation of porous metal surfaces by the dealloying process has been well discussed. 4-12 In brief, the formation of porous structure is a result of a competi- tion between the selective dissolution roughening process of the less noble metal and the surface diffusion smoothing process of the more noble metal. While thermal casting or sputter deposition methods are often employed for the preparation of the precursor alloy CuAl2, the electrodeposition method was not used because it is difficult to find a suitable electrolyte for the electrodeposition of aluminum. To overcome this, Cu-Zn, which can be formed electrochemically, may be used in place of CuAl2. Zinc chloride reacts with 1-ethyl-3-methylimidazolium chloride ZnCl2-EMIC, forming ionic liquids that are liquids near or below ambient temperature. 13 This ionic liquid system has been investi- gated for electrodeposition applications. 14-16 ZnCl2-EMIC is an aprotic medium and provides a wide working temperature covering from room temperature to above 150°C. The high working tempera- ture may be advantageous for electrodeposition of alloys. Recently, the fabrication of nanostructured platinum, gold, and silver films on the surface of corresponding electrodes by electrochemical alloying/ dealloying in the ZnCl2-EMIC without using hazardous acids or bases has been demonstrated. 17-19 This method provides an easy way to form porous metal films, and because the zincII species that was consumed during the electrodeposition step was redissolved into the ZnCl2-EMIC ionic liquid during the electrochemical dealloying step, the ZnCl2-EMIC ionic liquid is reusable. In previous studies, the effects of the experimental factors such as deposition current, potential, and temperature on the structure of the porous metal films have been examined. To verify the previous findings and further explore this electrochemical alloying/dealloying approach, this present paper examines the fabrication of porous copper films by electrochemical formation and electrochemical dealloying of Cu-Zn alloys in a 50-50 mol % ZnCl2-EMIC ionic liquid. TheCu-Zn alloy phase change during the anodic dealloying step is noticed.