(Invited) Surface Local Structure Dependence of Photoinduced Reaction Process on TiO2 Single Crystal Electrode in Aqueous Solution
Authors: Imanishi Akihito 
Published: December 2024

Metal oxides such as TiO2 are widely used as electrocatalysts and photo-responsive electrodes in various electrochemical devices, and many researchers have studied them. However, the reaction processes at their interfaces are generally complex, and there are many aspects that remain to be elucidated. Even for TiO2 electrodes, which have been the most widely studied, the mechanism of interfacial reactions still remains unclear, especially the effects of crystal faces and atomic- to nano-level surface structures on electrode reactions. Our group has been investigated how the atomic- to nano-level structure of the TiO2 electrode surface affects the reaction at the TiO2 electrode interface.
In our previous study, we revealed that the water photooxidation reaction (i.e. oxygen evolution reaction) on TiO2 surface accompanies with other three kinds of side reactions (photoluminescence (PL), surface roughening and non-radiative recombination) [1]. These reactions are competitive with each other and the ratio of their quantum efficiencies strongly depends on the atomic-scale surface local structure. Thus, we investigated the surface local structure dependence of those competitive reactions on TiO2 surface using atomically flat TiO2 (rutile) single crystal electrode those step-terrace structure was strictly controlled. It was revealed that the ratio of the quantum efficiency of four kinds of reactions strongly depends on the surface local structure such as step-terrace structure. On the other hand, the surface roughening process, which is one of the four competitive reactions, induced the drastic change of the surface local structure and also induced the change in the ratio of quantum efficiency of four kinds of competitive reactions, leading to the increase of the photocatalytic activity for oxygen photoevolution [2]. Such an investigation would be very important because some recent studies indicate that the surface local structure changes during the photooxidation process even in the case of practical catalyst. We also investigated the influence of nanostructure formed on the TiO2 electrode on the branch ratio of 4 competitive photooxidation reactions. We found that the overvoltage for O2 evolution on nanostructured electrode was decreased comparing with that on atomically flat (100) surface. We can explain these results by the spatial configuration of edge and facet sites which depends on the surface nanostructure.
The organic synthesis by using of photooxidation reaction on photocatalysis has been investigated by many researchers. Although the formation of phenol from benzene on TiO2 surface by photo-induced oxidation reaction is one of the representative reactions, its molecular mechanism is still unclear. We can assumed that the quantum efficiencies of phenol generation depends on the atomic-scale surface local structure. We investigated the atomic leveled surface local structure dependence of the efficiency of photooxidation reaction of benzene (in HClO4aq solution) on Nb-doped rutile TiO2(110) single crystal electrode. It was found that the efficiency of the photooxidation reaction of benzene is largely affected by the step structure on the TiO2 surface [3]. Detailed analysis of the dependence on surface structure revealed that the photooxidation reaction of benzene and the photoinduced decomposition of water (Oxygen evolution reaction) proceed competitively via the same intermediate, and that the ratio of their efficiencies is strongly affected by the step direction and step density.
[1] A. Imanishi, T. Okamura, N. Ohashi, R. Nakamura, Y. Nakato, J. Am. Chem. Soc., 129, 11569(2007)., A. Imanishi, K. Fukui, J. Phys. Chem. Lett. 5, 2108(2014) [2] E. Tsuji, K. Fukui, A. Imanishi, J. Phys. Chem. C, 118, 5406(2014). [3] S. Nagaoka, S. Kadono, K. Fukui, A. Imanishi, Abstract of the 87th ECSJ Spring Meeting (2020).