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of ZnO [
102
]. They designed a novel double-sided CdS and CdSe quantum dot co-
sensitized ZnO nanowire arrays and used them for PEC hydrogen evolution. The
double-sided design represents a simple analog of tandem cell structure to maxi-
mize light absorption. This double-sided architecture enabled direct interaction
between quantum dot and nanowire, which could improve charge collection effi-
ciency, compared to single sided co-sensitized structure. A maximum photocurrent
density of *12 mA/cm
2
at 0.4 V versus Ag/AgCl was obtained on the double-
sided quantum dot co-sensitized ZnO nanowire arrayed photoanode [
102
].
Although enormous efforts have been devoted to develop more efficient ZnO-
and TiO
2
-based photoelectrodes, the overall efficiency is still limited by relatively
poor visible light absorption due to their large band-gap energies. Therefore, it is
still necessary and important to develop new method to modify their optical and
electronic properties and improve their performance for PEC water splitting.
3.2 WO
3
Nanomaterials for PEC Water Oxidation
In comparison to TiO
2
and ZnO, WO
3
has a relatively small band-gap
(2.5-2.8 eV) that allows it to capture about 12 % of the solar spectrum and is more
favorable for visible light absorption [
62
]. In addition, WO
3
also has a moderate
hole diffusion length (*150 nm) and good electron transport property. These
features make it a very promising photoanode material for PEC water oxidation
[
51
,
53
]. Considerable efforts have been devoted to develop various WO
3
films as
photoanodes for water oxidation [
10
,
29
,
54
,
78
,
84
,
85
,
99
], since Hodes et al. first
demonstrated the possibility of using WO
3
as a photoanode for water splitting [
32
].
However, WO
3
also has its own limitations. First, the energy level of its con-
duction band is more positive than the potential for water reduction. As a result,
water splitting can be only achieved with the application of additional external
bias. Second, WO
3
is an indirect band-gap semiconductor that requires a relatively
thick film for increasing light absorption. However, a thick layer of active elec-
trode material usually cause significant electron-hole recombination loss and
decrease photoactivity of WO
3
[
99
]. Nanostructured materials have large surface
area and short carrier diffusion distance, which could potentially address this issue.
Recently, various nanostrcutured WO
3
such as nanowire [
85
], nanoflake [
74
] and
nanoparticle film [
78
] have been reported for PEC water oxidation. For instance,
Cristineo et al. synthesized the wormlike WO
3
nanostructured photoanodes on
tungsten foils using potentiostatic anodization method in high dielectric constant
organic solvents [
10
]. The highest photocurrent density obtained was approxi-
mately 3.5 mA/cm
2
at 1.5 V (vs. SCE) in 1.0 M H
2
SO
4
solution under an incident
power of 150 mW/cm
2
and was more than 9 mA cm
-2
under incident light power
of 370 mW/cm
2
. Qin et al. fabricated vertically aligned WO
3
nanoparticulate
flakes that have both high surface area and sufficient film thickness for good light
absorption, using a simple surfactant-free hydrothermal growth in a mixture of
concentrated nitric acid and hydrochloric acid, followed by sintering at 500 Cin
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