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d
n
3
r
4
n
g
|
4
Figure 8.3 Energy band diagram of a PEC cell for water splitting composed of an
n-type semiconductor and a metal counter electrode (a) before galvanic
contact, (b) after galvanic contact without illumination, (c) after galvanic
contact with illumination, and (d) after galvanic contact with illumin-
ation and an applied bias. E
C
: energy of conduction band minimum, E
V
:
energy of valence band maximum, E
F
: Fermi level, vacuum energy; E
C
,
energy of conduction band minimum; E
*
F,n
: quasi Fermi level for elec-
trons, E
*
F,p
: quasi Fermi level for holes, V
B
: surface potential, V
ph
:
internal photovoltage.
.
valence band edges of the semiconductor should straddle the electro-
chemical potentials of water reduction and oxidation, i.e. the conduction
band edge should be more negative than the H
2
evolution potential, E
0
(H
1
/
H
2
), and the valence band edge should be more positive than the O
2
evo-
lution potential, E
0
(O
2
/H
2
O), otherwise external potential should be applied,
(3) strong light absorption, (4) the electrode should be stable in the aqueous
electrolyte in the dark and under illumination, (5) ecient charge transport
in the photoelectrodes, (6) low cost. Though no material can cover all these
requirements, semiconductor metal oxides such as TiO
2
,WO
3
and Fe
2
O
3
are
good candidates since they are relatively inexpensive and are highly stable.
However, since the conduction band of most metal oxides is less negative
than the H
2
evolution potential energy as shown in Figure 8.5, external bias
needs to be applied. Please note that the vertical axis of the diagram is in-
dicated using the NHE (normal hydrogen electrode) as a reference and
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