crossing is enhanced by quenching even when BN is used as a quencher. Since intersystem crossing is not enhanced by hydrogen bond formation but by exciplex formation,14 it is supposed that exciplex formation occurs simultaneously with hydrogen bond formation. Charge transfer from quencher to fluorescer is a prerequisite for both exciplex formation and hydrogen atom transfer. Therefore, such a supposition is consistent with the (12) Malaga, N.;
The dependence of the band bending of the oxide semiconductors on pH have been studied by measuring the flatband potential, photopotential, and rest dark potential in the solution without redox species. The rest dark potentials were almost constant with pH, while the flatband potential showed the 60 mV/pH dependence. This indicates that only the band in the space charge layer bends by pH with the 60 mV/pH dependence under the rest dark potential state. The flatband states were held at pH = -3.5, -4.5, 0, and -3.0 for TiO2, ZnO, SrTiO3, and Zn2TO4, respectively, under the rest dark potentials. The above phenomenon is based on the reaction of the oxide surface state with H § and/or OH-in the solution. This mechanism is described in detail. The enhancement of the water photolysis on TiO2 catalyst by the alkaline treatment is well explained by the large band bending in alkaline solution under the rest dark potential state.It is well known that the flatband potentials of oxide Semiconductors are dependent on pH of the solution (60 mV/pH at room temperature). This is explained by the basic or acidic character of the oxide surface which can interact with water by binding protons or hydroxyl ions. The following equilibria have to be considered for the protons H + ~-H § (ad)[1] ~tH + = ~H + (ad)[2]
A~H = ~s --~1 = const + (FIRT)ln (aH+laH+
Anodic characteristics of
La1−xSrxCoO3
(with
x=0.2 normaland 0.4
) electrodes were investigated in alkaline solutions. It was found that the catalytic activities for the oxygen evolution reaction are relatively high and that the activity of the electrode with
x=0.4
is higher than that of
x=0.2
. The secondary chemical step, i.e.,
normalCoOH+OH−→CoO−+H2O
, is rate‐controlling in the oxygen evolution reaction. Therefore, it is concluded that the rate of the secondary chemical step is more rapid at the electrode with the higher value of
x
because of the beneficial effect of the higher positive charge on the Co cation on this step.
La1−xSrxCoO3
electrodes are suitable for the anode material in alkaline solutions, because anodic dissolution scarcely occurs.
The electronic and photoelectrochemical properties of the n‐type spinel oxides consisting of Zn‐Ti‐Fe are studied. The composition systems that give the single phase of the spinel structure are
Znx2+Fe1−x3+false[Fe1−x2+Fe1+x3+false]O4
and
Zn2+false[Zn0.5x2+Ti0.5x4+Fe2−x3+false]O4
. The conductivities of the former oxides are governed by the hopping mechanism between Fe2+ and Fe3+ at the octahedral site, while those of the latter oxides will be governed by normal band conduction. The difference between the Fermi level and the bottom energy of the conduction band is determined from the activation energies of the conductivity and the Seebeck coefficient. Photocurrents are observed for almost all the samples but disappear in the composition region at
x<0.8
in the
Znx2+Fe1−x3+false[Fe1−x2+Fe1+x3+false]O4
system. This is caused by the disappearance of the space charge layer developed by the high carrier density in the hopping level. The flatband potential and the bandgap depend on the cations in the octahedral site, but not on those in the tetrahedral site. The flatband potential shifts to positive and the bandgap decreases when Fe increases in the octahedral site.
Zn2TiO4
is a useful anode material for the photoelectrolysis of water without bias, but it requires UV light. On the other hand,
ZnFe2O4
has a relatively small bandgap and a little more negative flatband potential than that of a reversible hydrogen electrode, but its onset potential of the anodic photocurrent is more positive. The band structures of the spinel oxides used in this study are also discussed.
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