Environmental Engineering Reference
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Fig. 8 The schematic
illustration of the band
structure of CdSe sensitized
TiO
2
. The photoexcited hole
in CdS transfer to the
interface between
semiconductor and
electrolyte to oxidize S
2-
to
S
2-
; the excited electron is
injected into TiO
2
Zinc oxide (ZnO) has similar band-gap and band-edge positions as TiO
2
, which
is also a suitable semiconductor material for PEC water splitting [
76
,
88
,
107
,
111
,
126
]. Although the chemical stability of ZnO is not as good as TiO
2
, due to the
dissolution in basic and acidic solution, the electron mobility of ZnO is typically
10-100 folds higher than that of TiO
2
[
27
,
111
], which is favorable for charge
transport and PEC application. Nanostructured ZnO such as nanoparticle film
[
107
], nanowire film [
111
], 3D nanotree film [
88
], and nanotube arrays [
8
] have
been developed and studied for PEC water splitting. For instance, Wolcott et al.
fabricated nanostructured ZnO thin film using three different deposition strategies:
normal pulsed laser deposition, pulse laser oblique-angle deposition and electron-
beam glancing-angle deposition [
107
]. They found that the thin film morphologies
played significant effect on their PEC properties. Normal pulsed laser deposition
produced densely packed particle films with *200 nm grain sizes; oblique-angle
deposition produced nanoplatelets with a fish scale morphology thin film; while
glancing-angle deposition generated a highly porous, interconnected nanoparticle
network. The highly porous thin film by glancing-angle deposition exhibited the
best PEC water splitting performance, among these three kinds of ZnO thin films.
The high PEC properties of ZnO by glancing-angle deposition were attributed to
the better crystallinity and higher surface area [
107
].
ZnO is also a large band-gap semiconductor, which limits the solar light
absorption and therefore STH conversion efficiency. Element doping and small
band-gap semiconductor sensitization have also been used to increase the light
absorption of ZnO [
38
,
44
,
63
,
73
,
79
,
102
,
111
]. For example, Yang et al. grew
single crystal ZnO nanowire arrays on ITO glass using a seed-mediated hydro-
thermal growth; nitrogen doping was carried out in a home-built CVD system with
ammonia as nitrogen source. XPS measurement revealed that nitrogen dopant
concentration can be controlled by varying the doping treatment time [
111
]. PEC
studies demonstrated that nitrogen doped ZnO show more than one magnitude
increase in photocurrent density, compared to pristine undoped ZnO nanowire
arrays. Importantly, IPCE studies shows the enhancement of photoactivity in vis-
ible light region, indicating nitrogen doping is an effective method to modify the
optical and photocatalytic properties of ZnO. Besides, Wang et al. demonstrated to
use quantum dot sensitization method to further increase the visible light absorption
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