(Invited) Ordered Nanoporous Semiconductors with Controlled Geometrical Structures By Anodization Processes

Abstract

For the preparation of functional nanodevices for efficient energy conversion, the use of the naturally occurring self-ordered structures is considered an effective process due to its capability of formation of precisely controlled fine nanostructures. To optimize the performance of the obtained devices, fabrication of precisely controlled geometrical structures of metals and semiconductors is essential. For example, the fabrication of highly ordered structures of metal and semiconductors contributes to the efficient absorption of the incident light and also enhance the charge separation in the interfaces. The anodization processes of metals have been widely applied to the fabrication of nanostructures of metal oxides. One advantageous point of this process is controllability of the geometrical structures of the obtained porous structures based on the anodization conditions. In addition, the ordered nanoporous structures can be obtained under the appropriate anodization conditions. In the present report, recent results concerning the preparation of nanoporous semiconductors with controlled geometrical structures and its applications are described. As a typical example, naturally ordered processes1 or pretexturing process2were applied to the preparation of nanoporous TiO2 with controlled geometrical structures. In addition, a high through-put process for the preparation of the through-hole membrane of TiO2 was also described. This process is based on the two-layer anodization used for the preparation of porous alumina through hole membranes3. The tow-layer anodization, which generates TiO2 layers with different solubility, and subsequent selective dissolution generate the through hole membrane of TiO2 4. Repetition of the process allows the high-throughput preparation of the ordered TiO2 through hole membranes. In addition to the self-ordered TiO2, preparation of the ideally ordered TiO2 through hole membranes is demonstrated. The obtained TiO2 through-hole membranes with controlled geometrical structures will be applied to various types of energy conversion devices. P. Roy, S. Berger, P. Schmuki, Angew. Chem. Int. Ed., 50, 2904 (2011).T. Kondo, S. Nagao, T. Yanagishita, N. T. Nguyen, K. Lee, P. Schmuki, H. Masuda, Electrochem. Commun., 50, 73 (2014).T. Yanagishita and H. Masuda, Electrochim. Acta, 184, 80 (2015).T. Yanagishita, H. Inada, T. Kondo, N. T. Nguyen, P. Schmuki, and H. Masuda, J. Electrochem. Soc., 165, E763 (2018).