Photocatalytic Energy Conversion over Anodized TiO ₂ Nanotube Arrays Decorated with Semiconductor Nanoparticles

Abstract

Materials research on a variety of well-controlled nanostructures based on widegap semiconductors have been widely performed for realizing efficient photocatalytic energy conversion such as water splitting and fuel production from carbon dioxide (CO 2 ), in terms of sustainability and cost effectiveness as well as in-situ resource utilization in the field of space engineering. Utilizing CO 2 and H 2 O for fuel and O 2 production on outer-Earth environments is indispensable for extraterrestrial survival. Meanwhile, the study on photocatalysis on the Earth is also significant for materials development of photocatalysts employable even in extraterrestrial conditions. In the past several years, we have performed research on synthesis and photocatalytic properties of nanostructured semiconductors such as anodized titanium dioxide (TiO 2 ) nanotube array (TNA). In this talk, our recent results on photocatalytic CO 2 reduction and hydrogen (H 2 ) production over nanocomposites of vertically aligned TNAs and other semiconductor nanoparticles will be presented. Monitoring photocatalytic reactions in high vacuum has an advantage that intermediate products during photocatalysis can be detected at a real-time scale. In our group, real-time observation of photocatalytic CO 2 reduction over TNA loaded with Cu 2 O nanoparticles (CNP) and platinum-loaded TNA (Pt/TNA) in high vacuum have been performed by utilizing a home-made apparatus, under ultraviolet-visible (UV-VIS) irradiation (300–600 nm) [1,2]. The real-time monitored partial pressure changes in various gas species indicated that CO 2 photoreduction arises through hydrogenation reaction in gas phase over CNP/TNA even in high vacuum, although CO 2 deoxygenation reaction is conventionally dominant for gas phase reactions as observed for Pt/TNA. In addition, photocatalytic H 2 production from water/methanol mixture over TNA under UV-VIS irradiation was enhanced by uniformly decorating TNA surface with sub 10-nm-size polymeric carbon nitride (g-C 3 N 4 ) nanoparticles with thermal chemical vapor deposition [3]. The direct Z-scheme mechanism can be included in the charge transfer process at the CNP/TNA and the g-C 3 N 4 /TNA interfaces. Nanoparticles of Cu 2 O and g-C 3 N 4 are effective as noble-metal-free co-catalysts for visible light responsive photocatalysis based on vertically aligned nanotube arrays. References 1. H. Goto, H. Masegi, S. B. Sadale, K. Noda, J. CO2 Util. 59 , 101964 (2022). 2. H. Goto, K. Ito, S. B. Sadale, K. Noda, Adv. Eng. Mater. 26 , 2302249 (2024). 3. K. Ito, S. Yoneyama, S. Yoneyama, P. Fons, K. Noda, ACS Mater. Au 5 , 299-307 (2025).