Nanoporous Structures Prepared by an Electrochemical Deposition Process

Abstract

Ramification of electrodeposited metals such as copper and tin has been a very intriguing topic for several decades from both the scientific and the technological point of view. [1,2] In particular, three-dimensional (3D) nanoramified metal deposits are ideally suited for electrodes in electrochemical devices such as fuel cells, batteries, and sensors. The open porous structure allows rapid transport of gas and liquid, while the extremely high surface area is desirable for electrochemical reactions. To date, however, most studies have focused just on avoiding the ramification of these metal deposits, as needed for the intended applications in electronic devices.[2±4] The potential applications of these unique structures in electrochemical devices are yet to be explored. While an electrodeposition process can easily create a 3D dendritic structure of metals, it is very difficult to control the microstructure with desired features. The trunks and branches in the porous dendritic structure are often unable to support the weight of numerous sub-branches, leading to collapses on itself in the electrolyte solution even with minimal convective forces. Furthermore, dendritic structures are easily overgrown to become a thick film with very low porosity, making it difficult for gas/liquid to transport through as required in electrochemical applications.In this communication, we report a unique method to overcome the above problems in order to make self-supported nanoramified (dendritic) deposits. This simple but very effective technique involves electrochemical deposition accompanying hydrogen evolution, which has been deliberately suppressed in typical electrodeposition processes to produce dense metallic components. In this process, hydrogen bubbles play a crucial role in the formation of the porous structure; they function as a dynamic negative template. The resulting structures are 3D free-standing foams of metals (such as copper and tin) with highly porous ramified (dendritic) walls. A recent study by Zach and Penner [5] has been also based upon a hydrogen co-evolution process. However, they have obtained just two-dimensional (2D) nanocrystalline nickel and the role of hydrogen evolution on electrochemical deposition (i.e., the size-selective deposition of nanoparticles) is quite different from the role of hydrogen evolution in this work. Shown in Figure 1 are some typical SEM (Hitachi S-800 FEG Scanning Electron Microscope) images of porous copper deposits created by electrodeposition for different periods of time. Clearly, the pore size of the 3D microfoam structure increased with the time of deposition (or the distance from the substrate). More importantly, the walls of the microfoam are also highly porous due to vigorous hydrogen evolution originated not only at the substrate but also at the deposited copper dendrites.Schematically shown in Figure 2 is a simplified description of the formation process of the metal foams. The hydrogen bubbles, originating from the cathodic reaction on copper substrate, create a c...

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