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K. Kitsuka et al. / Electrochimica Acta 55 (2009) 31–36
electrons tunnel through the depletion layer and/or (ii) electrons
hop along the interfacial levels in the depletion layer. In both cases,
the electron transfer occurs at a more positive potential than the
oxidation–reduction potentials of oxygen and ozone. As a result,
the OER and/or EOP can take place. From the results shown in Fig. 3
and the above-mentioned tunnel oxidation of water, it should be
noted that the Si/TiOX/Pt/TiO2 electrode fabricated in this study is
efficient for the EOP probably because of its higher oxygen over-
potential, compared with that of the Pt electrode. The high oxygen
overpotential is considered to originate from the TiO2 band struc-
ture in which electronic transfer is possible at a high energy level
through a deep depletion layer of the TiO2 surface.
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4. Conclusions
EOP could be observed using the Si/TiOX/Pt/TiO2 electrodes cov-
ered with spin-coated TiO2 films of more than 440 nm in 0.01 M
HClO4 at 15 ◦C and the current efficiency of ca. 7% was achieved at
a current density of 26.7 mA cm−2. According to the electrochem-
ical measurements and EOP experiments, the electrodes in which
the exposure of the Pt substrate was not observed could gener-
ate ozone. The TiO2 films were found to be of anatase and have
the structures with TiO2 particles and apertures. The photoelec-
trochemical measurements revealed that the TiO2 films were of
n-type semiconductors. The band structure of the TiO2/HClO4 inter-
face accounts for electron transfer at a high bias potential, which is
profitable for EOP through the depletion layer of the n-type semi-
conductor TiO2, as tunneling current.
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Acknowledgement
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The authors would like to thank Drs. Tsutomu Minegishi,
Jun Kubota and Kazunari Domen at the University of Tokyo for
their skillful assistance with the photoelectrochemical measure-
ments.