Abstract
TiO2 nanotube (TNT) photoanodes exhibit activity and stability for the oxygen evolution reaction (OER) by photoelectrochemical water oxidation. However, the location of the OER site by the photogenerated holes has not been clarified for the TNT photoanodes, unlike well-studied macrocrystalline photocatalysts. In this study, we performed reactions of TNT photoanodes in a 0.1 M Pb(NO3)2 under UV irradiation. The photoelectrochemically deposited PbO2 particles were observed through scanning electron microscopy in the backscattered electron mode. We found that β-PbO2 was deposited on the nanotubes with photocurrent decay and that the reaction site was located on the upper part (∼1 μm) of the TNT array with ∼3 μm length. The photocurrent decay implies the selective deposition of PbO2 on the catalytic site for water oxidation. The PbO2 nanoparticles were deposited on the inner and outer surfaces of the tube walls. This result is consistent with the mechanism of charge separation at the space charge layers formed on both surfaces of the walls. We also confirmed that the OER site changes depending on the wavelength of the incident light due to the change in the light penetration depth.
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