The oxygen photoevolution reaction on TiO₂ and related metal oxides has attracted strong attention from the point of view of solar water splitting. In our previous study, we have proposed a new reaction model in which generated holes are consumed not only by the O₂ evolution reaction but also by three competitive paths possibly occur at different surface sites; photoluminescence, surface roughening (i.e. photocorrosion), and nonradiative recombination [1,2]. Actually, we found that the branch ratio for these paths on TiO₂ (110) changed by the density and the direction of steps (i.e. atomic leveled surface structure) [3]. On the other hand, it is expected that the nano-sized structure of the TiO₂ surface also affect the branch ratio of 4 competitive photooxidation reactions. Thus, in this study, we investigated oxygen photoevolution efficiency on a nanostructured TiO₂ electrode that exposes different facets.

A Pt wire and an Ag/AgCl/sat.KCl electrode were used as a counter and a reference electrode, respectively. An n-type TiO₂ (110) doped with 0.05wt% Nb was used as a working electrode. The j−U curves were measured in 0.1 M HClO₄ (pH 1.1) under UV irradiation.

Nanostructured TiO₂(110) surfaces were prepared by photoetching in 0.05 M H₂SO₄ [4]. We obtained the nanostructured surfaces on which shallow grooves are regularly arranged. We should note that formed surface of the nanostructure is composed by (100) facet faces [4]. Photocurrent generated along with the O₂ evolution from H₂O on the nanostructured TiO₂ (110) was apparently different from that on the flat TiO₂ (100). The onset potential of nanostructured TiO₂ electrode was shifted toward negative voltage, indicating that the activation energy of oxygen evolution reaction was smaller than that on the flat surface. In addition, the consumption ratio of the photogenerated holes by above mentioned 4 competitive reactions was largely different from that on the atomically flat (100) surface. Considering the fact that the photoetched TiO₂(110) surface is composed by (100) microfacets[4], this result can be attributed to the nanostructuring effect. We also carried out the same experiment using the some other TiO₂ electrode those have different nanostructure on their surfaces, and found that the nanostructuring effect (1. Branch ratio of 4 competitive photooxidation reactions, 2. Activation energy of O₂ evolution reaction) depends on the nano-sized surface morphology of the TiO₂ electrode.

[1] A. Imanishi, T. Okamura, N. Ohashi, R. Nakamura, Y. Nakato, J. Am. Chem. Soc., 129 (2007) 11569.

[2] A. Imanishi, K. Fukui, J. Phys. Chem. Lett., 5 (2014) 2108.

[3] M. Kadono, Master’s thesis, Osaka Univ. (2013).

[4] A. Imanishi, H. Suzuki, K. Murakoshi, Y. Nakato, J. Phys. Chem. B, 110 (2006) 21050.