Tin Oxides with Nano and Micron-Sized Pores for Functional Electrochemical Devices

Metal oxides with one-dimensional (1-D) nano-channels have drawn much attention as potential electrode materials for high performance functional electrochemical devices. This is mainly because the large surface area of nanostructures and the 1-D pathway of reactants/products in channels result in low activation and concentration polarizations, respectively. In spite of such advantages, the expected high rate and high efficient operation of the devices adopting the electrodes with nano-channeled structure might not be quite sustainable particularly when the rate of surface reaction far exceeds that of mass transport through the channels. That is, the reactants/products would be seriously depleted/accumulated in the channel at the very high rate of surface reaction, possibly leading to the significant decrease in total reaction rate. One way to solve this problem is to prepare the hybrid porous structure composed of both nano-sized and micron-sized pores, where the latter efficiently provides the reactants to or receives the products from the nano-channels.

It has been known that an anodic oxidation process is a quite effective way to create the metal oxide films with well-defined 1-D nano-channeled structure. Among the anodic metal oxides with 1-D structure, tin oxide is worthy of attention due to its widespread use in various applications, such as catalysis, gas sensing and energy storage. In this work, meso- and macro- hybrid porous structure is created without a template by an anodic oxidation process. From the experimental results, it is suggested that the substrate, the anodizing time, the thickness of the pre-anodized tin have an important effect on the size and density of macro-pores in hybrid porous structure. For example, the well-defined meso/macro hybrid porous tin oxides can be created when silicon was used as a substrate to anodize tin thin film. On the basis of the mechanical analysis of the different substrates and different thicknesses of tin film on them, it is suggested that the substrate with high elastic modulus might result in high compressive stress to the anodic tin oxide and generate local cracks in the initial stage of anodizing process, which grow to macro-pores with semispherical shape in the later stage.

From two step anodizing process including the intervening selective etching of tin oxide, highly-open meso/macro hybrid porous tin oxide is successfully prepared. Anodic tin oxide formed under the macro-pores has meso-porous structure with radial shape, ideally suited for high rate and high efficient functional electrochemical devices.