Environmental Engineering Reference
In-Depth Information
electrochemical properties of TiO
2
before and after hydrogen treatment. The
photocurrent densities of TiO
2
nanowire were significantly increased in the entire
potential windows after hydrogen treatment (Fig.
6
a). IPCE studies further showed
that the increased photocurrent density was mainly due to the enhanced photo-
activities in the UV region (Fig.
6
b). The results suggested that the enhanced
photocurrent was due to the improved charge collection efficiency and hydrogen
treatment did not narrow the band-gap of TiO
2
. STH conversion efficiency was
calculated to be around 1.1 %, by integrating IPCE spectra with standard solar
spectrum shown in Fig.
6
c; which was the highest efficiency obtained on TiO
2
nanostructure. Mott-Schottky studies confirmed the carrier densities of hydrogen
treated TiO
2
were significantly enhanced by orders of magnitude (Fig.
6
d). The
enhanced PEC performance was attributed to increased oxygen vacancies and
hydrogen impurities, which work as shallow donors for TiO
2
. The increased donor
density could facilitate charge transport in the semiconductor and charge separa-
tion at the interface between semiconductor and substrate.
Furthermore, other chemical modification methods such as element doping [
22
,
37
,
64
] and sensitization [
28
,
103
] have been used to increase visible light
photoactivity of TiO
2
. Element doping is a typical approach to extend light
absorption spectrum of TiO
2
into visible light region. Various impurity elements
such as Fe [
13
,
86
], C [
37
,
64
,
68
], P [
22
], and N [
9
,
31
,
61
] have been reported to
increase visible light absorption of TiO
2
. For instance, Hoang et al. reported
nitrogen-doped TiO
2
nanowire arrays and used them for PEC water oxidation [
31
].
Nitrogen doping was achieved by nitridation of TiO
2
in ammonia gas flow at
temperature of 500 C. Nitrogen-doped TiO
2
nanowire photoanode was light
yellow in color and IPCE studies showed that it exhibited obvious photoactivity in
the visible region from 400 to 500 nm (Fig.
7
). The visible light photoactivity is
attributed to the introduction of N impurity states in the electronic band structure
of TiO
2
, which narrows its band-gap. Hoang et al. further demonstrated that
hydrogenation of nitrogen-doped TiO
2
could further increase their performance of
TiO
2
for PEC water splitting, due to a synergistic interaction between nitrogen
dopant and the oxygen vacancy (Ti
3+
) in the TiO
2
[
30
].
Sensitization is another common method used to increase visible light photo-
activity of large band-gap semiconductor metal oxides. For example, TiO
2
sen-
sitized with small band-gap semiconductors such as CdS and CdSe have been
reported [
28
,
50
,
103
]. As shown in Fig.
8
, upon light illuminated, electron and
hole pairs will be generated in both TiO
2
and the sensitizer CdSe quantum dot. By
forming a type II heterojunction between TiO
2
and the sensitizer, the photogen-
erated electrons in the sensitizer can efficiently transfer to TiO
2
(Fig.
8
) In this
case, the sensitized TiO
2
composite structure can utilize visible solar light.
However, the major drawback of this approach is the instability of chalcogenides.
Hole scavengers such as sodium sulfide should be added into the electrolyte
solution to avoid the self-oxidation of CdS and CdSe, because the oxidation
potential of CdSe and CdS is much lower than water oxidation [
28
,
103
]. As a
result, although the excited electrons are used for water reduction to produce
hydrogen, the overall reaction is no longer water splitting.
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